One document matched: draft-ietf-p2psip-base-04.xml
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<rfc category="std" docName="draft-ietf-p2psip-base-04"
ipr="pre5378Trust200902">
<front>
<title abbrev="RELOAD Base">REsource LOcation And Discovery (RELOAD) Base
Protocol</title>
<author fullname="Cullen Jennings" initials="C." surname="Jennings">
<organization>Cisco</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<street>MS: SJC-21/2</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>USA</country>
</postal>
<phone>+1 408 421-9990</phone>
<email>fluffy@cisco.com</email>
</address>
</author>
<author fullname="Bruce B. Lowekamp" initials="B. B." role="editor"
surname="Lowekamp">
<organization>MYMIC LLC</organization>
<address>
<postal>
<street>1040 University Blvd., Suite 100</street>
<city>Portsmouth</city>
<region>VA</region>
<code>23703</code>
<country>USA</country>
</postal>
<email>bbl@lowekamp.net</email>
</address>
</author>
<author fullname="Eric Rescorla" initials="E.K." surname="Rescorla">
<organization>Network Resonance</organization>
<address>
<postal>
<street>2064 Edgewood Drive</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94303</code>
<country>USA</country>
</postal>
<phone>+1 650 320-8549</phone>
<email>ekr@networkresonance.com</email>
</address>
</author>
<author fullname="Salman A. Baset" initials="S.A." surname="Baset">
<organization>Columbia University</organization>
<address>
<postal>
<street>1214 Amsterdam Avenue</street>
<city>New York</city>
<region>NY</region>
<country>USA</country>
</postal>
<email>salman@cs.columbia.edu</email>
</address>
</author>
<author fullname="Henning Schulzrinne" initials="H.G."
surname="Schulzrinne">
<organization>Columbia University</organization>
<address>
<postal>
<street>1214 Amsterdam Avenue</street>
<city>New York</city>
<region>NY</region>
<country>USA</country>
</postal>
<email>hgs@cs.columbia.edu</email>
</address>
</author>
<date day="6" month="October" year="2009" />
<area>RAI</area>
<workgroup>P2PSIP</workgroup>
<abstract>
<t>In this document the term BCP 78 and BCP 79 refer to RFC 3978 and RFC
3979 respectively. They refer only to those RFCs and not any documents
that update or supersede them. </t>
<t>This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. A P2P
signaling protocol provides its clients with an abstract storage and
messaging service between a set of cooperating peers that form the
overlay network. RELOAD is designed to support a P2P Session Initiation
Protocol (P2PSIP) network, but can be utilized by other applications
with similar requirements by defining new usages that specify the kinds
of data that must be stored for a particular application. RELOAD defines
a security model based on a certificate enrollment service that provides
unique identities. NAT traversal is a fundamental service of the
protocol. RELOAD also allows access from "client" nodes that do not need
to route traffic or store data for others.</t>
</abstract>
<note title="Legal">
<t>This documents and the information contained therein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION THEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.</t>
</note>
</front>
<middle>
<section title="Introduction">
<!-- TODO EKR: compare this against the glossary so that each
term is introduced -->
<t>This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. It
provides a generic, self-organizing overlay network service, allowing
nodes to efficiently route messages to other nodes and to efficiently
store and retrieve data in the overlay. RELOAD provides several features
that are critical for a successful P2P protocol for the Internet:</t>
<t><list style="hanging">
<t></t>
<t hangText="Security Framework:">A P2P network will often be
established among a set of peers that do not trust each other.
RELOAD leverages a central enrollment server to provide credentials
for each peer which can then be used to authenticate each operation.
This greatly reduces the possible attack surface.</t>
<t></t>
<t hangText="Usage Model:">RELOAD is designed to support a variety
of applications, including P2P multimedia communications with the
Session Initiation Protocol <xref
target="I-D.ietf-p2psip-sip"></xref>. RELOAD allows the definition
of new application usages, each of which can define its own data
types, along with the rules for their use. This allows RELOAD to be
used with new applications through a simple documentation process
that supplies the details for each application.</t>
<t></t>
<t hangText="NAT Traversal:">RELOAD is designed to function in
environments where many if not most of the nodes are behind NATs or
firewalls. Operations for NAT traversal are part of the base design,
including using ICE to establish new RELOAD or application protocol
connections.</t>
<t></t>
<t hangText="High Performance Routing:">The very nature of overlay
algorithms introduces a requirement that peers participating in the
P2P network route requests on behalf of other peers in the network.
This introduces a load on those other peers, in the form of
bandwidth and processing power. RELOAD has been defined with a
simple, lightweight forwarding header, thus minimizing the amount of
effort required by intermediate peers.</t>
<t></t>
<t hangText="Pluggable Overlay Algorithms:">RELOAD has been designed
with an abstract interface to the overlay layer to simplify
implementing a variety of structured (DHT) and unstructured overlay
algorithms. This specification also defines how RELOAD is used with
Chord, which is mandatory to implement. Specifying a default "must
implement" overlay algorithm promotes interoperability, while
extensibility allows selection of overlay algorithms optimized for a
particular application.</t>
</list></t>
<t>These properties were designed specifically to meet the requirements
for a P2P protocol to support SIP. This document defines the base
protocol for the distributed storage and location service, as well as
critical usages for NAT traversal and security. The SIP Usage itself is
described separately in <xref target="I-D.ietf-p2psip-sip"></xref>.
RELOAD is not limited to usage by SIP and could serve as a tool for
supporting other P2P applications with similar needs. RELOAD is also
based on the concepts introduced in <xref
target="I-D.ietf-p2psip-concepts"></xref>.</t>
<section title="Basic Setting">
<t>In this section, we provide a brief overview of the operational
setting for RELOAD. See the concepts document for more details. A
RELOAD Overlay Instance consists of a set of nodes arranged in a
partly connected graph. Each node in the overlay is assigned a numeric
Node-ID which, together with the specific overlay algorithm in use,
determines its position in the graph and the set of nodes it connects
to. The figure below shows a trivial example which isn't drawn from
any particular overlay algorithm, but was chosen for convenience of
representation.</t>
<figure>
<artwork><![CDATA[
+--------+ +--------+ +--------+
| Node 10|--------------| Node 20|--------------| Node 30|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 40|--------------| Node 50|--------------| Node 60|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 70|--------------| Node 80|--------------| Node 90|
+--------+ +--------+ +--------+
|
|
+--------+
| Node 85|
|(Client)|
+--------+
]]></artwork>
</figure>
<t>Because the graph is not fully connected, when a node wants to send
a message to another node, it may need to route it through the
network. For instance, Node 10 can talk directly to nodes 20 and 40,
but not to Node 70. In order to send a message to Node 70, it would
first send it to Node 40 with instructions to pass it along to Node
70. Different overlay algorithms will have different connectivity
graphs, but the general idea behind all of them is to allow any node
in the graph to efficiently reach every other node within a small
number of hops.</t>
<t>The RELOAD network is not only a messaging network. It is also a
storage network. Records are stored under numeric addresses which
occupy the same space as node identifiers. Peers are responsible for
storing the data associated with some set of addresses as determined
by their Node-ID. For instance, we might say that every peer is
responsible for storing any data value which has an address less than
or equal to its own Node-ID, but greater than the next lowest Node-ID.
Thus, Node-20 would be responsible for storing values 11-20.</t>
<t>RELOAD also supports clients. These are nodes which have Node-IDs
but do not participate in routing or storage. For instance, in the
figure above Node 85 is a client. It can route to the rest of the
RELOAD network via Node 80, but no other node will route through it
and Node 90 is still responsible for all addresses between 81-90. We
refer to non-client nodes as peers.</t>
<t>Other applications (for instance, SIP) can be defined on top of
RELOAD and use these two basic RELOAD services to provide their own
services.</t>
</section>
<section title="Architecture">
<t>RELOAD is fundamentally an overlay network. Therefore, it can be
divided into components that mimic the layering of the Internet
model<xref target="RFC1122"></xref>.</t>
<figure>
<artwork><![CDATA[
Application
+-------+ +-------+
| SIP | | XMPP | ...
| Usage | | Usage |
+-------+ +-------+
-------------------------------------- Messaging API
+------------------+ +---------+
| Message |<--->| Storage |
| Transport | +---------+
+------------------+ ^
^ ^ |
| v v
| +-------------------+
| | Topology |
| | Plugin |
| +-------------------+
| ^
v v
+------------------+
| Forwarding & |
| Link Management |
+------------------+
-------------------------------------- Overlay Link API
+-------+ +------+
|TLS | |DTLS | ...
+-------+ +------+
]]></artwork>
</figure>
<t>The major components of RELOAD are:</t>
<t><list style="hanging">
<t></t>
<t hangText="Usage Layer:">Each application defines a RELOAD
usage; a set of data kinds and behaviors which describe how to use
the services provided by RELOAD. These usages all talk to RELOAD
through a common Message Transport API.</t>
<t></t>
<t hangText="Message Transport:">Handles end-to-end reliability,
manages request state for the usages, and forwards Store and Fetch
operations to the Storage component. Delivers message responses to
the component initiating the request.</t>
<t></t>
<t hangText="Storage:">The Storage component is responsible for
processing messages relating to the storage and retrieval of data.
It talks directly to the Topology Plugin to manage data
replication and migration, and it talks to the Message Transport
component to send and receive messages.</t>
<t></t>
<t hangText="Topology Plugin:">The Topology Plugin is responsible
for implementing the specific overlay algorithm being used. It
uses the Message Transport component to send and receive overlay
management messages, to the Storage component to manage data
replication, and directly to the Forwarding Layer to control
hop-by-hop message forwarding. This component closely parallels
conventional routing algorithms, but is more tightly coupled to
the Forwarding Layer because there is no single "routing table"
equivalent used by all overlay algorithms.</t>
<t></t>
<t hangText="Forwarding and Link Management Layer:">Stores and
implements the routing table by providing packet forwarding
services between nodes. It also handles establishing new links
between nodes, including setting up connections across NATs using
ICE.</t>
<t></t>
<t hangText="Overlay Link Layer:">TLS <xref
target="RFC5246"></xref> and DTLS <xref target="RFC4347"></xref>
are the "link layer" protocols used by RELOAD for hop-by-hop
communication. Each such protocol includes the appropriate
provisions for per-hop framing or hop-by-hop ACKs required by
unreliable transports.</t>
</list></t>
<t>To further clarify the roles of the various layer, this figure
parallels the architecture with each layer's role from an overlay
perspective and implementation layer in the internet:</t>
<figure>
<artwork><![CDATA[
| Internet Model |
Real | Equivalent | Reload
Internet | in Overlay | Architecture
--------------+-----------------+------------------------------------
| | +-------+ +-------+
| Application | | SIP | | XMPP | ...
| | | Usage | | Usage |
| | +-------+ +-------+
| | ----------------------------------
| |+------------------+ +---------+
| Transport || Message |<--->| Storage |
| || Transport | +---------+
| |+------------------+ ^
| | ^ ^ |
| | | v v
Application | | | +-------------------+
| (Routing) | | | Topology |
| | | | Plugin |
| | | +-------------------+
| | | ^
| | v v
| Network | +------------------+
| | | Forwarding & |
| | | Link Management |
| | +------------------+
| | ----------------------------------
Transport | Link | +-------+ +------+
| | |TLS | |DTLS | ...
| | +-------+ +------+
--------------+-----------------+------------------------------------
Network |
|
Link |
]]></artwork>
</figure>
<section title="Usage Layer">
<t>The top layer, called the Usage Layer, has application usages,
such as the SIP Location Usage, that use the abstract Message
Transport API provided by RELOAD. The goal of this layer is to
implement application-specific usages of the generic overlay
services provided by RELOAD. The usage defines how a specific
application maps its data into something that can be stored in the
overlay, where to store the data, how to secure the data, and
finally how applications can retrieve and use the data.</t>
<t>The architecture diagram shows both a SIP usage and an XMPP
usage. A single application may require multiple usages, for example
a SIP application may also require a voicemail usage. A usage may
define multiple kinds of data that are stored in the overlay and may
also rely on kinds originally defined by other usages.</t>
<t>Because the security and storage policies for each kind are
dictated by the usage defining the kind, the usages may be coupled
with the Storage component to provide security policy enforcement
and to implement appropriate storage strategies according to the
needs of the usage. The exact implementation of such an interface is
outside the scope of this draft.</t>
</section>
<section title="Message Transport">
<t>The Message Transport component provides a generic message
routing service for the overlay. The Message Transport layer is
responsible for end-to-end message transactions, including
retransmissions. Each peer is identified by its location in the
overlay as determined by its Node-ID. A component that is a client
of the Message Transport can perform two basic functions:</t>
<t><list style="symbols">
<t>Send a message to a given peer specified by Node-ID or to the
peer responsible for a particular Resource-ID.</t>
<t>Receive messages that other peers sent to a Node-ID or
Resource-ID for which this peer is responsible.</t>
</list></t>
<t>All usages rely on the Message Transport component to send and
receive messages from peers. For instance, when a usage wants to
store data, it does so by sending Store requests. Note that the
Storage component and the Topology Plugin are themselves clients of
the Message Transport, because they need to send and receive
messages from other peers.</t>
<t>The Message Transport API is similar to those described as
providing "Key based routing" (KBR), although as RELOAD supports
different overlay algorithms (including non-DHT overlay algorithms)
that calculate keys in different ways, the actual interface must
accept Resource Names rather than actual keys.</t>
</section>
<section title="Storage">
<t>One of the major functions of RELOAD is to allow nodes to store
data in the overlay and to retrieve data stored by other nodes or by
themselves. The Storage component is responsible for processing data
storage and retrieval messages. For instance, the Storage component
might receive a Store request for a given resource from the Message
Transport. It would then query the appropriate usage before storing
the data value(s) in its local data store and sends a response to
the Message Transport for delivery to the requesting peer.
Typically, these messages will come from other nodes, but depending
on the overlay topology, a node might be responsible for storing
data for itself as well, especially if the overlay is small.</t>
<t>A peer's Node-ID determines the set of resources that it will be
responsible for storing. However, the exact mapping between these is
determined by the overlay algorithm in use. The Storage component
will only receive a Store request from the Message Transport if this
peer is responsible for that Resource-ID. The Storage component is
notified by the Topology Plugin when the Resource-IDs for which it
is responsible change, and the Storage component is then responsible
for migrating resources to other peers, as required.</t>
</section>
<section title="Topology Plugin">
<t>RELOAD is explicitly designed to work with a variety of overlay
algorithms. In order to facilitate this, the overlay algorithm
implementation is provided by a Topology Plugin so that each overlay
can select an appropriate overlay algorithm that relies on the
common RELOAD core protocols and code.</t>
<t>The Topology Plugin is responsible for maintaining the overlay
algorithm Routing Table, which is consulted by the Forwarding and
Link Management Layer before routing a message. When connections are
made or broken, the Forwarding and Link Management Layer notifies
the Topology Plugin, which adjusts the routing table as appropriate.
The Topology Plugin will also instruct the Forwarding and Link
Management Layer to form new connections as dictated by the
requirements of the overlay algorithm Topology. The Topology Plugin
issues periodic update requests through Message Transport to
maintain and update its Routing Table.</t>
<t>As peers enter and leave, resources may be stored on different
peers, so the Topology Plugin also keeps track of which peers are
responsible for which resources. As peers join and leave, the
Topology Plugin instructs the Storage component to issue resource
migration requests as appropriate, in order to ensure that other
peers have whatever resources they are now responsible for. The
Topology Plugin is also responsible for providing for redundant data
storage to protect against loss of information in the event of a
peer failure and to protect against compromised or subversive
peers.</t>
</section>
<section title="Forwarding and Link Management Layer">
<t>The Forwarding and Link Management Layer is responsible for
getting a packet to the next peer, as determined by the Topology
Plugin. This Layer establishes and maintains the network connections
as required by the Topology Plugin. This layer is also responsible
for setting up connections to other peers through NATs and firewalls
using ICE, and it can elect to forward traffic using relays for NAT
and firewall traversal.</t>
<t>This layer provides a fairly generic interface that allows the
topology plugin control the overlay and resource operations and
messages. Since each overlay algorithm is defined and functions
differently, we generically refer to the table of other peers that
the overlay algorithm maintains and uses to route requests
(neighbors) as a Routing Table. The Topology Plugin actually owns
the Routing Table, and forwarding decisions are made by querying the
Topology Plugin for the next hop for a particular Node-ID or
Resource-ID. If this node is the destination of the message, the
message is delivered to the Message Transport.</t>
<t>The Forwarding and Link Management Layer sits on top of the
Overlay Link Layer protocols that carry the actual traffic. This
specification defines how to use DTLS and TLS protocols to carry
RELOAD messages.</t>
</section>
</section>
<section title="Security">
<t>RELOAD's security model is based on each node having one or more
public key certificates. In general, these certificates will be
assigned by a central server which also assigns Node-IDs, although
self-signed certificates can be used in closed networks. These
credentials can be leveraged to provide communications security for
RELOAD messages. RELOAD provides communications security at three
levels:</t>
<t><list style="hanging">
<t hangText="Connection Level: ">Connections between peers are
secured with TLS or DTLS.</t>
<t hangText="Message Level: ">Each RELOAD message must be
signed.</t>
<t hangText="Object Level: ">Stored objects must be signed by the
storing peer.</t>
</list></t>
<t>These three levels of security work together to allow peers to
verify the origin and correctness of data they receive from other
peers, even in the face of malicious activity by other peers in the
overlay. RELOAD also provides access control built on top of these
communications security features. Because the peer responsible for
storing a piece of data can validate the signature on the data being
stored, the responsible peer can determine whether a given operation
is permitted or not.</t>
<t>RELOAD also provides a shared secret based admission control
feature using shared secrets and TLS-PSK. In order to form a TLS
connection to any node in the overlay, a new node needs to know the
shared overlay key, thus restricting access to authorized users.</t>
</section>
<section title="Structure of This Document">
<t>The remainder of this document is structured as follows.</t>
<t><list style="symbols">
<t><xref target="sec-term"></xref> provides definitions of terms
used in this document.</t>
<t><xref target="sec-overlay-overview"></xref> provides an
overview of the mechanisms used to establish and maintain the
overlay.</t>
<t><xref target="sec-app-support"></xref> provides an overview of
the mechanism RELOAD provides to support other applications.</t>
<t><xref target="sec-overlay-protocol"></xref> defines the
protocol messages that RELOAD uses to establish and maintain the
overlay.</t>
<t><xref target="sec-data-protocol"></xref> defines the protocol
messages that are used to store and retrieve data using
RELOAD.</t>
<t><xref target="sec-store-usage"></xref> defines the Certificate
Store Usage that is fundamental to RELOAD security.</t>
<t><xref target="sec-turn-server"></xref> defines the TURN Server
Usage needed to locate TURN servers for NAT traversal.</t>
<t><xref target="sec-chord-algorithm"></xref> defines a specific
Topology Plugin using Chord.</t>
<t><xref target="sec-enrollment"></xref> defines the mechanisms
that new RELOAD nodes use to join the overlay for the first
time.</t>
<t><xref target="sec-msgflow"></xref> provides an extended
example.</t>
</list></t>
</section>
</section>
<section anchor="sec-term" title="Terminology">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
<t>We use the terminology and definitions from the <xref
target="I-D.ietf-p2psip-concepts">Concepts and Terminology for Peer to
Peer SIP</xref> draft extensively in this document. Other terms used in
this document are defined inline when used and are also defined below
for reference. Terms which are new to this document (and perhaps should
be added to the concepts document) are marked with a (*).</t>
<t><list style="hanging">
<t></t>
<t hangText="DHT:">A distributed hash table. A DHT is an abstract
hash table service realized by storing the contents of the hash
table across a set of peers.</t>
<t></t>
<t hangText="Overlay Algorithm:">An overlay algorithm defines the
rules for determining which peers in an overlay store a particular
piece of data and for determining a topology of interconnections
amongst peers in order to find a piece of data.</t>
<t></t>
<t hangText="Overlay Instance:">A specific overlay algorithm and the
collection of peers that are collaborating to provide read and write
access to it. There can be any number of overlay instances running
in an IP network at a time, and each operates in isolation of the
others.</t>
<t></t>
<t hangText="Peer:">A host that is participating in the overlay.
Peers are responsible for holding some portion of the data that has
been stored in the overlay and also route messages on behalf of
other hosts as required by the Overlay Algorithm.</t>
<t></t>
<t hangText="Client:">A host that is able to store data in and
retrieve data from the overlay but which is not participating in
routing or data storage for the overlay.</t>
<t></t>
<t hangText="Node:">We use the term "Node" to refer to a host that
may be either a Peer or a Client. Because RELOAD uses the same
protocol for both clients and peers, much of the text applies
equally to both. Therefore we use "Node" when the text applies to
both Clients and Peers and the more specific term when the text
applies only to Clients or only to Peers.</t>
<t></t>
<t hangText="Node-ID:">A 128-bit value that uniquely identifies a
node. Node-IDs 0 and 2^128 - 1 are reserved and are invalid
Node-IDs. A value of zero is not used in the wire protocol but can
be used to indicate an invalid node in implementations and APIs. The
Node-ID of 2^128-1 is used on the wire protocol as a wildcard.
(*)</t>
<t></t>
<t hangText="Resource:">An object or group of objects associated
with a string identifier see "Resource Name" below.</t>
<t></t>
<t hangText="Resource Name:">The potentially human readable name by
which a resource is identified. In unstructured P2P networks, the
resource name is sometimes used directly as a Resource-ID. In
structured P2P networks the resource name is typically mapped into a
Resource-ID by using the string as the input to hash function. A SIP
resource, for example, is often identified by its AOR which is an
example of a Resource Name.(*)</t>
<t></t>
<t hangText="Resource-ID:">A value that identifies some resources
and which is used as a key for storing and retrieving the resource.
Often this is not human friendly/readable. One way to generate a
Resource-ID is by applying a mapping function to some other unique
name (e.g., user name or service name) for the resource. The
Resource-ID is used by the distributed database algorithm to
determine the peer or peers that are responsible for storing the
data for the overlay. In structured P2P networks, Resource-IDs are
generally fixed length and are formed by hashing the resource name.
In unstructured networks, resource names may be used directly as
Resource-IDs and may have variable length.</t>
<t></t>
<t hangText="Connection Table:">The set of peers to which a node is
directly connected. This includes nodes with which Attach handshakes
have been done but which have not sent any Updates.</t>
<t></t>
<t hangText="Routing Table:">The set of peers which a node can use
to route overlay messages. In general, these peers will all be on
the connection table but not vice versa, because some peers will
have Attached but not sent updates. Peers may send messages directly
to peers that are in the connection table but may only route
messages to other peers through peers that are in the routing table.
(*)</t>
<t></t>
<t hangText="Destination List:">A list of IDs through which a
message is to be routed. A single Node-ID is a trivial form of
destination list. (*)</t>
<t></t>
<t hangText="Usage:">A usage is an application that wishes to use
the overlay for some purpose. Each application wishing to use the
overlay defines a set of data kinds that it wishes to use. The SIP
usage defines the location data kind. (*)</t>
</list></t>
<t>The term numBitsInNodeId is used to mean 128 which is the number of
bits in a Node-ID. The term "maximum request lifetime" is maximum a
request will wait for a response and defaults to 15 seconds. The term
"successor replacement hold-down time" is the amount of time to wait
before starting replication when a new successor is found and defaults
to 30 seconds.</t>
</section>
<section anchor="sec-overlay-overview" title="Overlay Management Overview">
<t>The most basic function of RELOAD is as a generic overlay network.
Nodes need to be able to join the overlay, form connections to other
nodes, and route messages through the overlay to nodes to which they are
not directly connected. This section provides an overview of the
mechanisms that perform these functions.</t>
<section anchor="sec.overview.security"
title="Security and Identification">
<t>Every node in the RELOAD overlay is identified by a Node-ID. The
Node-ID is used for three major purposes:</t>
<t><list style="symbols">
<t>To address the node itself.</t>
<t>To determine its position in the overlay topology when the
overlay is structured.</t>
<t>To determine the set of resources for which the node is
responsible.</t>
</list></t>
<t>Each node has a certificate <xref target="RFC5280"></xref>
containing a Node-ID, which is globally unique.</t>
<t>The certificate serves multiple purposes:</t>
<t><list style="symbols">
<t>It entitles the user to store data at specific locations in the
Overlay Instance. Each data kind defines the specific rules for
determining which certificates can access each Resource-ID/Kind-ID
pair. For instance, some kinds might allow anyone to write at a
given location, whereas others might restrict writes to the owner
of a single certificate.</t>
<t>It entitles the user to operate a node that has a Node-ID found
in the certificate. When the node forms a connection to another
peer, it can use this certificate so that a node connecting to it
knows it is connected to the correct node. In addition, the node
can sign messages, thus providing integrity and authentication for
messages which are sent from the node.</t>
<t>It entitles the user to use the user name found in the
certificate.</t>
</list></t>
<t>If a user has more than one device, typically they would get one
certificate for each device. This allows each device to act as a
separate peer.</t>
<t>RELOAD supports two certificate issuance models. The first is based
on a central enrollment process which allocates a unique name and
Node-ID to the node a certificate for a public/private key pair for
the user. All peers in a particular Overlay Instance have the
enrollment server as a trust anchor and so can verify any other peer's
certificate.</t>
<t>In some settings, a group of users want to set up an overlay
network but are not concerned about attack by other users in the
network. For instance, users on a LAN might want to set up a short
term ad hoc network without going to the trouble of setting up an
enrollment server. RELOAD supports the use of self-generated and
self-signed certificates. When self-signed certificates are used, the
node also generates its own Node-ID and username. The Node-ID is
computed as a digest of the public key, to prevent Node-ID theft,
however this model is still subject to a number of known attacks (most
notably Sybil attacks <xref target="Sybil"></xref>) and can only be
safely used in closed networks where users are mutually trusting.</t>
<t>The general principle here is that the security mechanisms (TLS and
message signatures) are always used, even if the certificates are
self-signed. This allows for a single set of code paths in the systems
with the only difference being whether certificate verification is
required to chain to a single root of trust.</t>
<section anchor="sec-shared-key" title="Shared-Key Security">
<t>RELOAD also provides an admission control system based on shared
keys. In this model, the peers all share a single key which is used
to authenticate the peer-to-peer connections via
TLS-PSK/TLS-SRP.</t>
</section>
</section>
<section title="Clients">
<t>RELOAD defines a single protocol that is used both as the peer
protocol and the client protocol for the overlay. This simplifies
implementation, particularly for devices that may act in either role,
and allows clients to inject messages directly into the overlay.</t>
<t>We use the term "peer" to identify a node in the overlay that
routes messages for nodes other than those to which it is directly
connected. Peers typically also have storage responsibilities. We use
the term "client" to refer to nodes that do not have routing or
storage responsibilities. When text applies to both peers and clients,
we will simply refer to such a device as a "node."</t>
<t>RELOAD's client support allows nodes that are not participating in
the overlay as peers to utilize the same implementation and to benefit
from the same security mechanisms as the peers. Clients possess and
use certificates that authorize the user to store data at its
locations in the overlay. The Node-ID in the certificate is used to
identify the particular client as a member of the overlay and to
authenticate its messages.</t>
<t>In RELOAD, unlike some other designs, clients are not a first-class
concept. From the perspective of a peer, a client is simply a node
which has not yet sent any Updates or Joins. It might never do so (if
it's a client) or it might eventually do so (if it's just a node
that's taking a long time to join). The routing and storage rules for
RELOAD provide for correct behavior by peers regardless of whether
other nodes Attached to them are clients or peers. Of course, a client
implementation must know that it intends to be a client, but this
localizes complexity only to that node.</t>
<t>For more discussion of the motivation for RELOAD's client support,
see <xref target="sec-why-clients"></xref>.</t>
<section title="Client Routing">
<t>There are two routing options by which a client may be located in
an overlay.</t>
<t><list style="symbols">
<t>Establish a connection to the peer responsible for the
client's Node-ID in the overlay. Then requests may be sent
from/to the client using its Node-ID in the same manner as if it
were a peer, because the responsible peer in the overlay will
handle the final step of routing to the client. This will not
work in overlays where NAT or firewall do not allow all clients
to form connections with any other peer. Note that clients that
choose this option MUST process Update messages from the peer
and if the topology shifts so that it is no longer responsible
form a connection to the appropriate peer. Failure to do so will
result in the client no longer receiving messages.</t>
<t>Establish a connection with an arbitrary peer in the overlay
(perhaps based on network proximity or an inability to establish
a direct connection with the responsible peer). In this case,
the client will rely on RELOAD's Destination List feature to
ensure reachability. The client can initiate requests, and any
node in the overlay that knows the Destination List to its
current location can reach it, but the client is not directly
reachable directly using only its Node-ID. The Destination List
required to reach it must be learnable via other mechanisms,
such as being stored in the overlay by a usage, if the client is
to receive incoming requests from other members of the
overlay.</t>
</list></t>
</section>
<section title="Minimum Functionality Requirements for Clients">
<t>A node may act as a client simply because it does not have the
resources or even an implementation of the topology plugin required
to acts as a peer in the overlay. In order to exchange RELOAD
messages with a peer, a client must meet a minimum level of
functionality. Such a client must:</t>
<!-- TODO add xrefs to relevant sections-->
<t><list style="symbols">
<t>Implement RELOAD's connection-management connections that are
used to establish the connection with the peer.</t>
<t>Implement RELOAD's data retrieval methods (with client
functionality).</t>
<t>Be able to calculate Resource-IDs used by the overlay.</t>
<t>Possess security credentials required by the overlay it is
implementing.</t>
</list></t>
<t>A client speaks the same protocol as the peers, knows how to
calculate Resource-IDs, and signs its requests in the same manner as
peers. While a client does not necessarily require a full
implementation of the overlay algorithm, calculating the Resource-ID
requires an implementation of the appropriate algorithm for the
overlay.</t>
<!--
<t>RELOAD does not support a separate protocol for clients that do
not meet these functionality requirements. Any such extension would
either entail compromises on the features of RELOAD or require an
entirely new protocol to reimplement the core features of RELOAD.
Furthermore, for SIP and many other applications, a native
application-level protocol already exists that is sufficient for
such a client to interact with a member of the RELOAD overlay.</t>
-->
</section>
</section>
<section title="Routing">
<t>This section will discuss the requirements RELOAD's routing
capabilities must meet, then describe the routing features in the
protocol, and provide a brief overview of how they are used. <xref
target="sec-route-alt"></xref> discusses some alternative designs and
the tradeoffs that would be necessary to support them.</t>
<t>RELOAD's routing capabilities must meet the following
requirements:</t>
<t><list style="hanging">
<t hangText="NAT Traversal: ">RELOAD must support establishing and
using connections between nodes separated by one or more NATs,
including locating peers behind NATs for those overlays
allowing/requiring it.</t>
<t hangText="Clients: ">RELOAD must support requests from and to
clients that do not participate in overlay routing.</t>
<t hangText="Client promotion:">RELOAD must support clients that
become peers at a later point as determined by the overlay
algorithm and deployment.</t>
<t hangText="Low state: ">RELOAD's routing algorithms must not
require significant state to be stored on intermediate peers.</t>
<t hangText="Return routability in unstable topologies: ">At some
points in times, different nodes may have inconsistent information
about the connectivity of the routing graph. In all cases, the
response to a request needs to delivered to the node that sent the
request and not to some other node.</t>
</list></t>
<t>To meet these requirements, RELOAD's routing relies on two basic
mechanisms:</t>
<t><list style="hanging">
<t hangText="Via Lists: ">The forwarding header used by all RELOAD
messages contains both a Via List (built hop-by-hop as the message
is routed through the overlay) and a Destination List (providing
source-routing capabilities for requests and return-path routing
for responses).</t>
<t hangText="Route_Query: ">The Route_Query method allows a node
to query a peer for the next hop it will use to route a message.
This method is useful for diagnostics and for iterative
routing.</t>
</list></t>
<t>The basic routing mechanism used by RELOAD is Symmetric Recursive.
We will first describe symmetric routing and then discuss its
advantages in terms of the requirements discussed above.</t>
<t>Symmetric recursive routing requires a message follow the path
through the overlay to the destination without returning to the
originating node: each peer forwards the message closer to its
destination. The return path of the response is then the same path
followed in reverse. For example, a message following a route from A
to Z through B and X:</t>
<figure>
<artwork><![CDATA[
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Via=A, B
Dest=Z
<----------
Dest=X, B, A
<----------
Dest=B, A
<----------
Dest=A
]]></artwork>
</figure>
<t>Note that the preceding Figure does not indicate whether A is a
client or peer, A forwards its request to B and the response is
returned to A in the same manner regardless of A's role in the
overlay.</t>
<t>This figure shows use of full via-lists by intermediate peers B and
X. However, if B and/or X are willing to store state, then they may
elect to truncate the lists, save that information internally (keyed
by the transaction id), and return the response message along the path
from which it was received when the response is received. This option
requires greater state on intermediate peers but saves a small amount
of bandwidth and reduces the need for modifying the message in route.
Selection of this mode of operation is a choice for the individual
peer, the techniques are interoperable even on a single message. The
figure below shows B using full via lists but X truncating them and
saving the state internally.</t>
<figure>
<artwork><![CDATA[
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Dest=Z
<----------
Dest=X
<----------
Dest=B, A
<----------
Dest=A
]]></artwork>
</figure>
<!-- <t>For debugging purposes, a Route Log attribute is available that -->
<!-- stores information about each peer as the message is forwarded.</t> -->
<t>RELOAD also supports a basic Iterative routing mode (where the
intermediate peers merely return a response indicating the next hop,
but do not actually forward the message to that next hop themselves).
Iterative routing is implemented using the Route_Query method, which
requests this behavior. Note that iterative routing is selected only
by the initiating node. RELOAD does not support an intermediate peer
returning a response that it will not recursively route a normal
request. The willingness to perform that operation is implicit in its
role as a peer in the overlay.</t>
</section>
<section title="Connectivity Management">
<t>In order to provide efficient routing, a peer needs to maintain a
set of direct connections to other peers in the Overlay Instance. Due
to the presence of NATs, these connections often cannot be formed
directly. Instead, we use the Attach request to establish a
connection. Attach uses ICE <xref target="I-D.ietf-mmusic-ice"></xref>
to establish the connection. It is assumed that the reader is familiar
with ICE.</t>
<t>Say that peer A wishes to form a direct connection to peer B. It
gathers ICE candidates and packages them up in an Attach request which
it sends to B through usual overlay routing procedures. B does its own
candidate gathering and sends back a response with its candidates. A
and B then do ICE connectivity checks on the candidate pairs. The
result is a connection between A and B. At this point, A and B can add
each other to their routing tables and send messages directly between
themselves without going through other overlay peers.</t>
<t>There is one special case in which Attach cannot be used: when a
peer is joining the overlay and is not connected to any peers. In
order to support this case, some small number of "bootstrap nodes"
need to be publicly accessible so that new peers can directly connect
to them. <xref target="sec-enrollment"></xref> contains more detail on
this.</t>
<t>In general, a peer needs to maintain connections to all of the
peers near it in the Overlay Instance and to enough other peers to
have efficient routing (the details depend on the specific overlay).
If a peer cannot form a connection to some other peer, this isn't
necessarily a disaster; overlays can route correctly even without
fully connected links. However, a peer should try to maintain the
specified link set and if it detects that it has fewer direct
connections, should form more as required. This also implies that
peers need to periodically verify that the connected peers are still
alive and if not try to reform the connection or form an alternate
one.</t>
</section>
<section title="Overlay Algorithm Support">
<t>The Topology Plugin allows RELOAD to support a variety of overlay
algorithms. This draft defines a DHT based on Chord <xref
target="Chord"></xref>, which is mandatory to implement, but the base
RELOAD protocol is designed to support a variety of overlay
algorithms.</t>
<section title="Support for Pluggable Overlay Algorithms">
<t>RELOAD defines three methods for overlay maintenance: Join,
Update, and Leave. However, the contents of those messages, when
they are sent, and their precise semantics are specified by the
actual overlay algorithm; RELOAD merely provides a framework of
commonly-needed methods that provides uniformity of notation (and
ease of debugging) for a variety of overlay algorithms.</t>
<!--
<t>An implementation should provide a suitable interface to
the topology plugin to allow it full freedom to implement
whatever routing, maintenance, and resource
allocation/replication strategies it desires. Specifying an
API for this purpose is beyond the scope of this draft, but
we recommend implementing at least two different overlay
algorithms to confirm that the interface is sufficiently
flexible.</t>
-->
</section>
<section anchor="sec-join-leave-maint"
title="Joining, Leaving, and Maintenance Overview">
<t>When a new peer wishes to join the Overlay Instance, it must have
a Node-ID that it is allowed to use. It uses the Node-ID in the
certificate it received from the enrollment server. The details of
the joining procedure are defined by the overlay algorithm, but the
general steps for joining an Overlay Instance are:</t>
<t><list style="symbols">
<t>Forming connections to some other peers.</t>
<t>Acquiring the data values this peer is responsible for
storing.</t>
<t>Informing the other peers which were previously responsible
for that data that this peer has taken over responsibility.</t>
</list></t>
<t>The first thing the peer needs to do is form a connection to some
"bootstrap node". Because this is the first connection the peer
makes, these nodes must have public IP addresses and therefore can
be connected to directly. Once a peer has connected to one or more
bootstrap nodes, it can form connections in the usual way by routing
Attach messages through the overlay to other nodes. Once a peer has
connected to the overlay for the first time, it can cache the set of
nodes it has connected to with public IP addresses for use as future
bootstrap nodes.</t>
<t>Once a peer has connected to a bootstrap node, it then needs to
take up its appropriate place in the overlay. This requires two
major operations:</t>
<t><list style="symbols">
<t>Forming connections to other peers in the overlay to populate
its Routing Table.</t>
<t>Getting a copy of the data it is now responsible for storing
and assuming responsibility for that data.</t>
</list></t>
<t>The second operation is performed by contacting the Admitting
Peer (AP), the node which is currently responsible for that section
of the overlay.</t>
<t>The details of this operation depend mostly on the overlay
algorithm involved, but a typical case would be:</t>
<t><list style="numbers">
<t>JP (Joining Peer) sends a Join request to AP (Admitting Peer)
announcing its intention to join.</t>
<t>AP sends a Join response.</t>
<t>AP does a sequence of Stores to JP to give it the data it
will need.</t>
<t>AP does Updates to JP and to other peers to tell it about its
own routing table. At this point, both JP and AP consider JP
responsible for some section of the Overlay Instance.</t>
<t>JP makes its own connections to the appropriate peers in the
Overlay Instance.</t>
</list></t>
<t>After this process is completed, JP is a full member of the
Overlay Instance and can process Store/Fetch requests.</t>
<t>Note that the first node is a special case. When ordinary nodes
cannot form connections to the bootstrap nodes, then they are not
part of the overlay. However, the first node in the overlay can
obviously not connect to others nodes. In order to support this
case, potential first nodes (which must also serve as bootstrap
nodes initially) must somehow be instructed (perhaps by
configuration settings) that they are the entire overlay, rather
than not part of it.</t>
<t>Note that clients do not perfom either of these operations.</t>
</section>
</section>
<section title="First-Time Setup">
<t>Previous sections addressed how RELOAD works once a node has
connected. This section provides an overview of how users get
connected to the overlay for the first time. RELOAD is designed so
that users can start with the name of the overlay they wish to join
and perhaps a username and password, and leverage that into having a
working peer with minimal user intervention. This helps avoid the
problems that have been experienced with conventional SIP clients
where users are required to manually configure a large number of
settings.</t>
<section title="Initial Configuration">
<t>In the first phase of the process, the user starts out with the
name of the overlay and uses this to download an initial set of
overlay configuration parameters. The user does a DNS SRV lookup on
the overlay name to get the address of a configuration server. It can
then connect to this server with HTTPS to download a configuration
document which contains the basic overlay configuration parameters as
well as a set of bootstrap nodes which can be used to join the
overlay.<!-- Note that one or more of the peers can serve as a
configuration server. For instance, the first peer to join the overlay
might take on that role. --></t>
</section>
<section title="Enrollment">
<t>If the overlay is using centralized enrollment, then a user needs
to acquire a certificate before joining the overlay. The certificate
attests both to the user's name within the overlay and to the
Node-IDs which they are permitted to operate. In that case, the
configuration document will contain the address of an enrollment
server which can be used to obtain such a certificate. The
enrollment server may (and probably will) require some sort of
username and password before issuing the certificate. The enrollment
server's ability to restrict attackers' access to certificates in
the overlay is one of the cornerstones of RELOAD's security.</t>
</section>
</section>
</section>
<section anchor="sec-app-support" title="Application Support Overview">
<t>RELOAD is not intended to be used alone, but rather as a substrate
for other applications. These applications can use RELOAD for a variety
of purposes:</t>
<t><list style="symbols">
<t>To store data in the overlay and retrieve data stored by other
nodes.</t>
<t>As a discovery mechanism for services such as TURN.</t>
<t>To form direct connections which can be used to transmit
application-level messages.</t>
</list></t>
<t>This section provides an overview of these services.</t>
<section title="Data Storage">
<t>RELOAD provides operations to Store and Fetch data. Each location
in the Overlay Instance is referenced by a Resource-ID. However, each
location may contain data elements corresponding to multiple kinds
(e.g., certificate, SIP registration). Similarly, there may be
multiple elements of a given kind, as shown below:</t>
<figure>
<artwork><![CDATA[
+--------------------------------+
| Resource-ID |
| |
| +------------+ +------------+ |
| | Kind 1 | | Kind 2 | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | +------------+ |
| | +--------+ | |
| | | Value | | |
| | +--------+ | |
| +------------+ |
+--------------------------------+
]]></artwork>
</figure>
<t>Each kind is identified by a Kind-ID, which is a code point
assigned by IANA. As part of the kind definition, protocol designers
may define constraints, such as limits on size, on the values which
may be stored. For many kinds, the set may be restricted to a single
value; some sets may be allowed to contain multiple identical items
while others may only have unique items. Note that a kind may be
employed by multiple usages and new usages are encouraged to use
previously defined kinds where possible. We define the following data
models in this document, though other usages can define their own
structures:</t>
<t><list style="hanging">
<t></t>
<t hangText="single value:">There can be at most one item in the
set and any value overwrites the previous item.</t>
<t></t>
<t hangText="array:">Many values can be stored and addressed by a
numeric index.</t>
<t></t>
<t hangText="dictionary:">The values stored are indexed by a key.
Often this key is one of the values from the certificate of the
peer sending the Store request.</t>
</list></t>
<t>In order to protect stored data from tampering, by other nodes,
each stored value is digitally signed by the node which created it.
When a value is retrieved, the digital signature can be verified to
detect tampering.</t>
<section title="Storage Permissions">
<t>A major issue in peer-to-peer storage networks is minimizing the
burden of becoming a peer, and in particular minimizing the amount
of data which any peer is required to store for other nodes. RELOAD
addresses this issue by only allowing any given node to store data
at a small number of locations in the overlay, with those locations
being determined by the node's certificate. When a peer uses a Store
request to place data at a location authorized by its certificate,
it signs that data with the private key that corresponds to its
certificate. Then the peer responsible for storing the data is able
to verify that the peer issuing the request is authorized to make
that request. Each data kind defines the exact rules for determining
what certificate is appropriate.</t>
<t>The most natural rule is that a certificate authorizes a user to
store data keyed with their user name X. This rule is used for all
the kinds defined in this specification. Thus, only a user with a
certificate for "alice@example.org" could write to that location in
the overlay. However, other usages can define any rules they choose,
including publicly writable values.</t>
<t>The digital signature over the data serves two purposes. First,
it allows the peer responsible for storing the data to verify that
this Store is authorized. Second, it provides integrity for the
data. The signature is saved along with the data value (or values)
so that any reader can verify the integrity of the data. Of course,
the responsible peer can "lose" the value but it cannot undetectable
modify it.</t>
<t>The size requirements of the data being stored in the overlay are
variable. For instance, a SIP AoR and voicemail differ widely in the
storage size. RELOAD leaves it to the Usage and overlay
configuration to address the size imbalance of various kinds.</t>
</section>
<section anchor="sec-usages" title="Usages">
<t>By itself, the distributed storage layer just provides
infrastructure on which applications are built. In order to do
anything useful, a usage must be defined. Each Usage specifies
several things:</t>
<t><list style="symbols">
<t>Registers Kind-ID code points for any kinds that the Usage
defines.</t>
<t>Defines the data structure for each of the kinds.</t>
<t>Defines access control rules for each kinds.</t>
<t>Defines how the Resource Name is formed that is hashed to
form the Resource-ID where each kind is stored.</t>
<t>Describes how values will be merged after a network
partition. Unless otherwise specified, the default merging rule
is to act as if all the values that need to be merged were
stored and that the order they were stored in corresponds to the
stored time values associated with (and carried in) their
values. Because the stored time values are those associated with
the peer which did the writing, clock skew is generally not an
issue. If two nodes are on different partitions, write to the
same location, and have clock skew, this can create merge
conflicts. However because RELOAD deliberately segregates
storage so that data from different users and peers is stored in
different locations, and a single peer will typically only be in
a single network partition, this case will generally not
arise.</t>
</list></t>
<t>The kinds defined by a usage may also be applied to other usages.
However, a need for different parameters, such as different size
limits, would imply the need to create a new kind.</t>
</section>
<section title="Replication">
<t>Replication in P2P overlays can be used to provide:</t>
<t><list style="hanging">
<t hangText="persistence: ">if the responsible peer crashes
and/or if the storing peer leaves the overlay</t>
<t hangText="security: ">to guard against DoS attacks by the
responsible peer or routing attacks to that responsible peer</t>
<t hangText="load balancing: ">to balance the load of queries
for popular resources.</t>
</list></t>
<t>A variety of schemes are used in P2P overlays to achieve some of
these goals. Common techniques include replicating on neighbors of
the responsible peer, randomly locating replicas around the overlay,
or replicating along the path to the responsible peer.</t>
<t>The core RELOAD specification does not specify a particular
replication strategy. Instead, the first level of replication
strategies are determined by the overlay algorithm, which can base
the replication strategy on the its particular topology. For
example, Chord places replicas on successor peers, which will take
over responsibility should the responsible peer fail <xref
target="Chord"></xref>.</t>
<t>If additional replication is needed, for example if data
persistence is particularly important for a particular usage, then
that usage may specify additional replication, such as implementing
random replications by inserting a different well known constant
into the Resource Name used to store each replicated copy of the
resource. Such replication strategies can be added independent of
the underlying algorithm, and their usage can be determined based on
the needs of the particular usage.</t>
</section>
</section>
<section title="Service Discovery">
<t>RELOAD does not currently define a generic service discovery
algorithm as part of the base protocol; although a TURN-specific
discovery mechanism is provided. A variety of service discovery
algorithm can be implemented as extensions to the base protocol, such
as ReDIR <xref target="opendht-sigcomm05"></xref>.</t>
</section>
<section title="Application Connectivity">
<t>There is no requirement that a RELOAD usage must use RELOAD's
primitives for establishing its own communication if it already
possesses its own means of establishing connections. For example, one
could design a RELOAD-based resource discovery protocol which used
HTTP to retrieve the actual data.</t>
<t>For more common situations, however, the overlay itself is used to
establish a connection rather than an external authority such as DNS,
RELOAD provides connectivity to applications using the same Attach
method as is used for the overlay maintenance. For example, if a
P2PSIP node wishes to establish a SIP dialog with another P2PSIP node,
it will use Attach to establish a direct connection with the other
node. This new connection is separate from the peer protocol
connection, it is a dedicated UDP or TCP flow used only for the SIP
dialog. Each usage specifies which types of connections can be
initiated using Attach.</t>
</section>
</section>
<section anchor="sec-overlay-protocol" title="Overlay Management Protocol">
<t>This section defines the basic protocols used to create, maintain,
and use the RELOAD overlay network. We start by defining the basic
concept of how message destinations are interpreted when routing
messages. We then describe the symmetric recursive routing model, which
is RELOAD's default routing algorithm. We then define the message
structure and then finally define the messages used to join and maintain
the overlay.</t>
<section anchor="sec-message-forwarding"
title="Message Receipt and Forwarding">
<t>When a peer receives a message, it first examines the overlay,
version, and other header fields to determine whether the message is
one it can process. If any of these are incorrect (e.g., the message
is for an overlay in which the peer does not participate) it is an
error. The peer SHOULD generate an appropriate error but local policy
can override this and cause the messages is silently dropped.</t>
<t>Once the peer has determined that the message is correctly
formatted, it examines the first entry on the destination list. There
are three possible cases here:</t>
<t><list style="symbols">
<t>The first entry on the destination list is an id for which the
peer is responsible.</t>
<t>The first entry on the destination list is a an id for which
another peer is responsible.</t>
<t>The first entry on the destination list is a private id that is
being used for destination list compression. This is described
later.</t>
</list></t>
<t>These cases are handled as discussed below.</t>
<section anchor="sec-responsible-id" title="Responsible ID">
<t>If the first entry on the destination list is a ID for which the
node is responsible, there are several sub-cases. <list
style="symbols">
<t>If the entry is a Resource-ID, then it MUST be the only entry
on the destination list. If there are other entries, the message
MUST be silently dropped. Otherwise, the message is destined for
this node and it passes it up to the upper layers.</t>
<t>If the entry is a Node-ID which belongs to this node, then
the message is destined for this node. If this is the only entry
on the destination list, the message is destined for this node
and is passed up to the upper layers. Otherwise the entry is
removed from the destination list and the message is passed it
to the Message Transport. If the message is a response and there
is state for the transaction ID, the state is reinserted into
the destination list first.</t>
<t>If the entry is a Node-ID which is not equal to this node,
then the node MUST drop the message silently unless the Node-ID
corresponds to a node which is directly connected to this node
(i.e., a client). In that case, it MUST forward the message to
the destination node as described in the next section.</t>
</list></t>
<t>Note that this implies that in order to address a message to "the
peer that controls region X", a sender sends to Resource-ID X, not
Node-ID X.</t>
</section>
<section anchor="sec-other-id" title="Other ID">
<t>If neither of the other two cases applies, then the peer MUST
forward the message towards the first entry on the destination list.
This means that it MUST select one of the peers to which it is
connected and which is likely to be responsible for the first entry
on the destination list. If the first entry on the destination list
is in the peer's connection table, then it SHOULD forward the
message to that peer directly. Otherwise, it consult the routing
table to forward the message.</t>
<t>Any intermediate peer which forwards a RELOAD message MUST
arrange that if it receives a response to that message the response
can be routed back through the set of nodes through which the
request passed. This may be arranged in one of two ways:</t>
<t><list style="symbols">
<t>The peer MAY add an entry to the via list in the forwarding
header that will enable it to determine the correct node.</t>
<t>The peer MAY keep per-transaction state which will allow it
to determine the correct node.</t>
</list></t>
<t>As an example of the first strategy, if node D receives a message
from node C with via list (A, B), then D would forward to the next
node (E) with via list (A, B, C). Now, if E wants to respond to the
message, it reverses the via list to produce the destination list,
resulting in (D, C, B, A). When D forwards the response to C, the
destination list will contain (C, B, A).</t>
<t>As an example of the second strategy, if node D receives a
message from node C with transaction ID X and via list (A, B), it
could store (X, C) in its state database and forward the message
with the via list unchanged. When D receives the response, it
consults its state database for transaction id X, determines that
the request came from C, and forwards the response to C.</t>
<t>Intermediate peers which modify the via list are not required to
simply add entries. The only requirement is that the peer be able to
reconstruct the correct destination list on the return route. RELOAD
provides explicit support for this functionality in the form of
private IDs, which can replace any number of via list entries. For
instance, in the above example, Node D might send E a via list
containing only the private ID (I). E would then use the destination
list (D, I) to send its return message. When D processes this
destination list, it would detect that I is a private ID, recover
the via list (A, B, C), and reverse that to produce the correct
destination list (C, B, A) before sending it to C. This feature is
called List Compression. I MAY either be a compressed version of the
original via list or an index into a state database containing the
original via list.</t>
<t>Note that if an intermediate peer exits the overlay, then on the
return trip the message cannot be forwarded and will be dropped. The
ordinary timeout and retransmission mechanisms provide stability
over this type of failure.</t>
</section>
<section anchor="sec-private-Node-ID" title="Private ID">
<t>If the first entry in the destination list is a private id (e.g.,
a compressed via list), the peer MUST replace that entry with the
original via list that it replaced and then re-examine the
destination list to determine which of the above cases now
applies.</t>
</section>
</section>
<section title="Symmetric Recursive Routing">
<t>This Section defines RELOAD's symmetric recursive routing
algorithm, which is the default algorithm used by nodes to route
messages through the overlay. All implementations MUST implement this
routing algorithm. An overlay may be configured to use alternative
routing algorithms, and alternative routing algorithms may be selected
on a per-message basis.</t>
<section anchor="sec-request-origination" title="Request Origination">
<t>In order to originate a message to a given Node-ID or
Resource-ID, a node constructs an appropriate destination list. The
simplest such destination list is a single entry containing the peer
or Resource-ID. The resulting message will use the normal overlay
routing mechanisms to forward the message to that destination. The
node can also construct a more complicated destination list for
source routing.</t>
<t>Once the message is constructed, the node sends the message to
some adjacent peer. If the first entry on the destination list is
directly connected, then the message MUST be routed down that
connection. Otherwise, the topology plugin MUST be consulted to
determine the appropriate next hop.</t>
<t>Parallel searches for the resource are a common solution to
improve reliability in the face of churn or of subversive peers.
Parallel searches for usage-specified replicas are managed by the
usage layer. However, a single request can also be routed through
multiple adjacent peers, even when known to be sub-optimal, to
improve reliability <xref target="vulnerabilities-acsac04"></xref>.
Such parallel searches MAY BE specified by the topology plugin.</t>
<t>Because messages may be lost in transit through the overlay,
RELOAD incorporates an end-to-end reliability mechanism. When an
originating node transmits a request it MUST set a 3 second timer.
If a response has not been received when the timer fires, the
request is retransmitted with the same transaction identifier. The
request MAY be retransmitted up to 4 times (for a total of 5
messages). After the timer for the fifth transmission fires, the
message SHALL be considered to have failed. Note that this
retransmission procedure is not followed by intermediate nodes. They
follow the hop-by-hop reliability procedure described in <xref
target="sec-reliability"></xref>.</t>
<t>The above algorithm can result in multiple requests being
delivered to a node. Receiving nodes MUST generate semantically
equivalent responses to retransmissions of the same request (this
can be determined by transaction id) if the request is received
within the maximum request lifetime (15 seconds). For some requests
(e.g., Fetch) this can be accomplished merely by processing the
request again. For other requests, (e.g., Store) it may be necessary
to maintain state for the duration of the request lifetime.</t>
</section>
<section anchor="sec-response-origination"
title="Response Origination">
<t>When a peer sends a response to a request, it MUST construct the
destination list by reversing the order of the entries on the via
list. This has the result that the response traverses the same peers
as the request traversed, except in reverse order (symmetric
routing).</t>
</section>
</section>
<section title="Message Structure">
<t>RELOAD is a message-oriented request/response protocol. The
messages are encoded using binary fields. All integers are represented
in network byte order. The general philosophy behind the design was to
use Type, Length, Value fields to allow for extensibility. However,
for the parts of a structure that were required in all messages, we
just define these in a fixed position as adding a type and length for
them is unnecessary and would simply increase bandwidth and introduces
new potential for interoperability issues.</t>
<t>Each message has three parts, concatenated as shown below:</t>
<figure>
<artwork><![CDATA[
+-------------------------+
| Forwarding Header |
+-------------------------+
| Message Contents |
+-------------------------+
| Security Block |
+-------------------------+
]]></artwork>
</figure>
<t>The contents of these parts are as follows: <list style="hanging">
<t></t>
<t hangText="Forwarding Header:">Each message has a generic header
which is used to forward the message between peers and to its
final destination. This header is the only information that an
intermediate peer (i.e., one that is not the target of a message)
needs to examine.</t>
<t></t>
<t hangText="Message Contents:">The message being delivered
between the peers. From the perspective of the forwarding layer,
the contents is opaque, however, it is interpreted by the higher
layers.</t>
<t></t>
<t hangText="Security Block:">A security block containing
certificates and a digital signature over the message. Note that
this signature can be computed without parsing the message
contents. All messages MUST be signed by their originator.</t>
</list></t>
<t>The following sections describe the format of each part of the
message.</t>
<section anchor="sec-presentation-language"
title="Presentation Language">
<t>The structures defined in this document are defined using a
C-like syntax based on the presentation language used to define TLS.
Advantages of this style include:</t>
<t><list style="symbols">
<t>It is easy to write and familiar enough looking that most
readers can grasp it quickly.</t>
<t>The ability to define nested structures allows a separation
between high-level and low level message structures.</t>
<t>It has a straightforward wire encoding that allows quick
implementation, but the structures can be comprehended without
knowing the encoding.</t>
<t>The ability to mechanically (compile) encoders and
decoders.</t>
</list></t>
<t>This presentation is to some extent a placeholder. We consider it
an open question what the final protocol definition method and
encodings use. We expect this to be a question for the WG to
decide.</t>
<t>Several idiosyncrasies of this language are worth noting.</t>
<t><list style="symbols">
<t>All lengths are denoted in bytes, not objects.</t>
<t>Variable length values are denoted like arrays with angle
brackets.</t>
<t>"select" is used to indicate variant structures.</t>
</list></t>
<t>For instance, "uint16 array<0..2^8-2>;" represents up to
254 bytes but only up to 127 values of two bytes (16 bits)
each..</t>
<section anchor="sec-definitions" title="Common Definitions">
<t>The following definitions are used throughout RELOAD and so are
defined here. They also provide a convenient introduction to how
to read the presentation language.</t>
<t>An enum represents an enumerated type. The values associated
with each possibility are represented in parentheses and the
maximum value is represented as a nameless value, for purposes of
describing the width of the containing integral type. For
instance, Boolean represents a true or false:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { false (0), true(1), (255)} Boolean;
]]></artwork>
</figure>
<t>A boolean value is either a 1 or a 0 and is represented as a
single byte on the wire.</t>
<t>The NodeId, shown below, represents a single Node-ID.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
typedef opaque NodeId[16];
]]></artwork>
</figure>
<t>A NodeId is a fixed-length 128-bit structure represented as a
series of bytes, most significant byte first. Note: the use of
"typedef" here is an extension to the TLS language, but its
meaning should be relatively obvious.</t>
<t>A ResourceId, shown below, represents a single Resource-ID.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
typedef opaque ResourceId<0..2^8-1>;
]]></artwork>
</figure>
<t>Like a NodeId, a Resource-ID is an opaque string of bytes, but
unlike Node-IDs, Resource-IDs are variable length, up to 255 bytes
(2048 bits) in length. On the wire, each ResourceId is preceded by
a single length byte (allowing lengths up to 255). Thus, the
3-byte value "Foo" would be encoded as: 03 46 4f 4f.</t>
<t>A more complicated example is IpAddressPort, which represents a
network address and can be used to carry either an IPv6 or IPv4
address:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {reserved_addr(0), ipv4_address (1), ipv6_address (2),
(255)} AddressType;
struct {
uint32 addr;
uint16 port;
} IPv4AddrPort;
struct {
uint128 addr;
uint16 port;
} IPv6AddrPort;
struct {
AddressType type;
uint8 length;
select (type) {
case ipv4_address:
IPv4AddrPort v4addr_port;
case ipv6_address:
IPv6AddrPort v6addr_port;
/* This structure can be extended */
} IpAddressPort;
]]></artwork>
</figure>
<t>The first two fields in the structure are the same no matter
what kind of address is being represented:</t>
<t><list style="hanging">
<t hangText="type:">the type of address (v4 or v6).</t>
<t hangText="length:">the length of the rest of the
structure.</t>
</list></t>
<t>By having the type and the length appear at the beginning of
the structure regardless of the kind of address being represented,
an implementation which does not understand new address type X can
still parse the IpAddressPort field and then discard it if it is
not needed.</t>
<t>The rest of the IpAddressPort structure is either an
IPv4AddrPort or an IPv6AddrPort. Both of these simply consist of
an address represented as an integer and a 16-bit port. As an
example, here is the wire representation of the IPv4 address
"192.0.2.1" with port "6100".</t>
<figure>
<artwork><![CDATA[
01 ; type = IPv4
06 ; length = 6
c0 00 02 01 ; address = 192.0.2.1
17 d4 ; port = 6100
]]></artwork>
</figure>
<t>Unless a given structure that uses a select explicitly allows
for unknown types in the select, any unknown type SHOULD be
treated as an parsing error and the whole message discarded with
no response.</t>
</section>
</section>
<section anchor="sec-forwarding-header" title="Forwarding Header">
<t>The forwarding header is defined as a ForwardingHeader structure,
as shown below.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint32 relo_token;
uint32 overlay;
uint16 configuration_sequence;
uint8 version;
uint8 ttl;
uint32 fragment;
uint32 length;
uint64 transaction_id;
uint32 max_response_length;
uint16 via_list_length;
uint16 destination_list_length;
uint16 options_length;
Destination via_list[via_list_length];
Destination destination_list
[destination_list_length];
ForwardingOptions options[options_length];
} ForwardingHeader;
]]></artwork>
</figure>
<t>The contents of the structure are:</t>
<t><list style="hanging">
<t></t>
<t hangText="relo_token:">The first four bytes identify this
message as a RELOAD message. The message is easy to demultiplex
from STUN messages by looking at the first bit. This field MUST
contain the value 0xc2454c4f (the string 'RELO' with the high
bit of the first byte set.).</t>
<t></t>
<t hangText="overlay:">The 32 bit checksum/hash of the overlay
being used. The variable length string representing the overlay
name is hashed with SHA-1 and the low order 32 bits are used.
The purpose of this field is to allow nodes to participate in
multiple overlays and to detect accidental misconfiguration.
This is not a security critical function.</t>
<t></t>
<t hangText="configuration_sequence:">The sequence number of the
configuration file.</t>
<t></t>
<t hangText="version:">The version of the RELOAD protocol being
used. This document describes version 0.1, with a value of
0x01.</t>
<t></t>
<t hangText="ttl:">An 8 bit field indicating the number of
iterations, or hops, a message can experience before it is
discarded. The TTL value MUST be decremented by one at every hop
along the route the message traverses. If the TTL is 0, the
message MUST NOT be propagated further and MUST be discarded,
and a "Error_TTL_Exceeded" error should be generated. The
initial value of the TTL SHOULD be 100 unless defined otherwise
by the overlay configuration.</t>
<t></t>
<t hangText="fragment:">This field is used to handle
fragmentation. The high order two bits are used to indicate the
fragmentation status: If the high bit (0x80000000) is set, it
indicates that the message is a fragment. If the next bit
(0x40000000) is set, it indicates that this is the last
fragment.</t>
<t>The remainder of the field is used to indicate the fragment
offset; see <xref target="sec-frag-reass"></xref></t>
<t></t>
<t hangText="length:">The count in bytes of the size of the
message, including the header.</t>
<t></t>
<t hangText="transaction_id:">A unique 64 bit number that
identifies this transaction and also allows receivers to
disambiguate transactions which are otherwise identical.
Responses use the same Transaction ID as the request they
correspond to. Transaction IDs are also used for fragment
reassembly.</t>
<t></t>
<t hangText="max_response_length:">The maximum size in bytes of
a response. Used by requesting nodes to avoid receiving
(unexpected) very large responses. If this value is non-zero,
responding peers MUST check that any response would not exceed
it and if so generate an Error_Response_Too_Large value. This
value SHOULD be set to zero for responses.</t>
<t></t>
<!--
<t hangText="flags:">The flags word contains control flags. Which are
ORed together. There is two currently defined flags: ROUTE-LOG (0x1) and
RESPONSE-ROUTE-LOG (0x2). These flags indicate that the route log should
be included (see <xref target="sec-route-log"></xref>.).</t> -->
<t hangText="via_list_length:">The length of the via list in
bytes. Note that in this field and the following two length
fields we depart from the usual variable-length convention of
having the length immediately precede the value in order to make
it easier for hardware decoding engines to quickly determine the
length of the header.</t>
<t></t>
<t hangText="destination_list_length:">The length of the
destination list in bytes.</t>
<t></t>
<!-- <t hangText="route_log_length:">The length of the route log
in bytes.</t> -->
<t hangText="options_length:">The length of the header options
in bytes.</t>
<t></t>
<t hangText="via_list:">The via_list contains the sequence of
destinations through which the message has passed. The via_list
starts out empty and grows as the message traverses each
peer.</t>
<t></t>
<t hangText="destination_list:">The destination_list contains a
sequence of destinations which the message should pass through.
The destination list is constructed by the message originator.
The first element in the destination list is where the message
goes next. The list shrinks as the message traverses each listed
peer.</t>
<t></t>
<!-- <t hangText="route_log:">Contains a series of route log entries. -->
<!-- See <xref target="sec-route-log"></xref>.</t> -->
<t hangText="options:">Contains a series of ForwardingOptions
entries. See <xref target="sec-forwarding-options"></xref>.</t>
</list></t>
<section anchor="sec.config-seq"
title="Processing Configuration Sequence Numbers">
<t>In order to be part of the overlay, a node MUST have a copy of
the overlay configuration document. In order to allow for
configuration document changes, each version of the configuration
document has a sequence number which is monotonically increasing mod
65536. Because the sequence number may in principle wrap, greater
than or less than are interpreted by modulo arithmetic as in
TCP.<!-- TODO: EKR: Can this cause odd livelock problems with
wraparound? --></t>
<t>When a destination node receives a request, it MUST check that
the configuration_sequence field is equal to its own configuration
sequence number. If they do not match, it MUST generate an error,
either Error_Config_Too_Old or Error_Config_Too_New. In addition,
if the configuration file in the request is too old, it MUST
generate a Config_Update message to update the requesting node.
This allows new configuration documents to propagate quickly
throughout the system. The one exception to this rule is that if
the configuration_sequence field is equal to 0xffff, and the
message type is Config_Update, then the message MUST be accepted
regardless of the receiving node's configuration sequence
number.</t>
</section>
<section title="Destination and Via Lists">
<t>The destination list and via lists are sequences of Destination
values:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {reserved(0), node(1), resource(2), compressed(3),
/* 128-255 not allowed */ (255) }
DestinationType;
select (destination_type) {
case node:
NodeId node_id;
case resource:
ResourceId resource_id;
case compressed:
opaque compressed_id<0..2^8-1>;
/* This structure may be extended with new types */
} DestinationData;
struct {
DestinationType type;
uint8 length;
DestinationData destination_data;
} Destination;
struct {
uint16 compressed_id; /* top bit MUST be 1 */
} Destination;
]]></artwork>
</figure>
<t>If destination structure has its first bit set to 1, then it is a
16 bit integer. If the first bit is not set, then it is a structure
starting with DestinationType. In the case it is a 16 bit integer,
it is treated the same as it had been a full structure with a
DestinationType of compressed and a compressed_id that was 2 bytes
long with the value of the 16 bit integer. When it is not a 16 bit
integer, it is the TLV structure with the following contents: <list
style="hanging"> <t></t>
<t hangText="type "></t>
<t>The type of the DestinationData PDU. This may be one of
"peer", "resource", or "compressed".</t>
<t></t>
<t hangText="length "></t>
<t>The length of the destination_data.</t>
<t></t>
<t hangText="destination_value "></t>
<t>The destination value itself, which is an encoded
DestinationData structure, depending on the value of
"type".</t>
</list></t>
<t><list style="hanging">
<t hangText="Note:">This structure encodes a type, length,
value. The length field specifies the length of the
DestinationData values, which allows the addition of new
DestinationTypes. This allows an implementation which does not
understand a given DestinationType to skip over it.</t>
</list></t>
<t>A DestinationData can be one of three types: <list
style="hanging">
<t></t>
<t hangText="peer"></t>
<t>A Node-ID.</t>
<t></t>
<t hangText="compressed"></t>
<t>A compressed list of Node-IDs and/or resources. Because
this value was compressed by one of the peers, it is only
meaningful to that peer and cannot be decoded by other peers.
Thus, it is represented as an opaque string.</t>
<t></t>
<t hangText="resource"></t>
<t>The Resource-ID of the resource which is desired. This type
MUST only appear in the final location of a destination list
and MUST NOT appear in a via list. It is meaningless to try to
route through a resource.</t>
</list></t>
<t>One possible encoding of the 16 bit integer version is an
opaque identifier is to encode an index into a connection table.
To avoid misrouting responses in the event a response is delayed
and the connection table entry has changed, the identifier should
be split between an index and a generation counter for that index.
At startup, the generation counters should be initialized to
random values. An implementation could use 12 bits for the
connection table index and 3 bits for the generation counter.
(Note that this does not suggest a 4096 entry connection table for
every node, only the ability to encode for a larger connection
table.) When a connection table slot is used for a new connection,
the generation counter is incremented (with wrapping). Connection
table slots are used on a rotating basis to maximize the time
interval between using the same slot for different connections.
When routing a message to an entry in the destination list
encoding a connection table entry, the node confirms that the
generation counter matches the current generation counter of that
index before forwarding the message. If it does not match, the
message is silently dropped.</t>
<t>Regardless of how the 16 bit integer field or opaque
DestinationType is used, the encoding MUST include a generation
counter designed to prevent misrouting of responses due to the
connection table entry having changed since the request message
was originally forwarded.</t>
</section>
<!-- <section anchor="sec-route-log" title="Route Logging"> -->
<!-- <t>The route logging feature provides diagnostic information about -->
<!-- the path taken by the message so far and in this manner it is -->
<!-- similar in function to <xref target="RFC3261">SIP's</xref> Via -->
<!-- header field. If the ROUTE-LOG flag is set in the Flags word, at -->
<!-- each hop peers MUST append a route log entry to the route log -->
<!-- element in the header or reject the request. The order of the -->
<!-- route log entry elements in the message is determined by the order -->
<!-- of the peers were traversed along the path. The first route log -->
<!-- entry corresponds to the peer at the first hop along the path, and -->
<!-- each subsequent entry corresponds to the peer at the next hop -->
<!-- along the path. If the ROUTE-LOG flag is set, the route log -->
<!-- entries in the request MUST be copied to the response or the -->
<!-- request rejected. If, and only if, the ROUTE-LOG-RESPONSE flag is -->
<!-- set in a request, the ROUTE-LOG flag MUST be set in the -->
<!-- response.</t> -->
<!-- <t>Note that use of the ROUTE-LOG-RESPONSE flag means that the -->
<!-- response will grow on the return path, which may potentially mean -->
<!-- that it gets dropped due to becoming too large for some -->
<!-- intermediate hop. Thus, this option must be used with care.</t> -->
<!-- <t>The route log is defined as follows:</t> -->
<!-- <figure> -->
<!-- begin-pdu-->
<!-- <artwork><![CDATA[ -->
<!-- enum { (255) } RouteLogExtensionType; -->
<!-- struct { -->
<!-- RouteLogExtensionType type; -->
<!-- uint16 length; -->
<!-- select (type){ -->
<!-- /* Extension values go here */ -->
<!-- } extension; -->
<!-- } RouteLogExtension; -->
<!-- enum { -->
<!-- reserved(0), -->
<!-- tcp_tls(1), -->
<!-- udp_dtls(2), -->
<!-- (255) -->
<!-- } OverlayLink; -->
<!-- struct { -->
<!-- opaque version<0..2^8-1>; /* A string */ -->
<!-- OverlayLink linkProtocol; /* TCP or UDP */ -->
<!-- NodeId id; -->
<!-- uint32 uptime; -->
<!-- IpAddressPort address; -->
<!-- opaque certificate<0..2^16-1>; -->
<!-- RouteLogExtension extensions<0..2^16-1>; -->
<!-- } RouteLogEntry; -->
<!-- struct { -->
<!-- RouteLogEntry entries<0..2^16-1>; -->
<!-- } RouteLog; -->
<!-- ]]></artwork> -->
<!-- </figure> -->
<!-- <t>The route log consists of an arbitrary number of RouteLogEntry -->
<!-- values, each representing one node through which the message has -->
<!-- passed.</t> -->
<!-- <t>Each RouteLogEntry consists of the following values:</t> -->
<!-- <t><list style="hanging"> -->
<!-- <t></t> -->
<!-- <t hangText="version "></t> -->
<!-- <t>A textual representation of the software version</t> -->
<!-- <t></t> -->
<!-- <t hangText="linkProtocol "></t> -->
<!-- <t>The Overlay Link Layer protocol, currently either "tcp_tls" -->
<!-- or "udp_dtls".</t> -->
<!-- <t></t> -->
<!-- <t hangText="id "></t> -->
<!-- <t>The Node-ID of the peer.</t> -->
<!-- <t></t> -->
<!-- <t hangText="uptime "></t> -->
<!-- <t>The uptime of the peer in seconds.</t> -->
<!-- <t></t> -->
<!-- <t hangText="address "></t> -->
<!-- <t>The address and port of the peer.</t> -->
<!-- <t></t> -->
<!-- <t hangText="certificate "></t> -->
<!-- <t>The peer's certificate. Note that this may be omitted by -->
<!-- setting the length to zero.</t> -->
<!-- <t></t> -->
<!-- <t hangText="extensions "></t> -->
<!-- <t>Extensions, if any.</t> -->
<!-- </list></t> -->
<!-- <t>Extensions are defined using a RouteLogExtension structure. New -->
<!-- extensions are defined by defining a new code point for -->
<!-- RouteLogExtensionType and adding a new arm to the -->
<!-- RouteLogExtension structure. The contents of that structure -->
<!-- are:</t> -->
<!-- <t><list style="hanging"> -->
<!-- <t></t> -->
<!-- <t hangText="type"></t> -->
<!-- <t>The type of the extension.</t> -->
<!-- <t></t> -->
<!-- <t hangText="length"></t> -->
<!-- <t>The length of the rest of the structure.</t> -->
<!-- <t></t> -->
<!-- <t hangText="extension"></t> -->
<!-- <t>The extension value.</t> -->
<!-- </list></t> -->
<!-- </section> -->
<section anchor="sec-forwarding-options" title="Forwarding Options">
<t>The Forwarding header can be extended with forwarding header
options, which are a series of ForwardingOptions structures:</t>
<figure>
<!-- begin-pdu-->
<artwork><![CDATA[
enum { (255) } ForwardingOptionsType;
struct {
ForwardingOptionsType type;
uint8 flags;
uint16 length;
select (type) {
/* Option values go here */
} option;
} ForwardingOption;
]]></artwork>
</figure>
<t>Each ForwardingOption consists of the following values:</t>
<t><list style="hanging">
<t></t>
<t hangText="type"></t>
<t>The type of the option. This structure allows for unknown
options types.</t>
<t></t>
<t hangText="length"></t>
<t>The length of the rest of the structure.</t>
<t></t>
<t hangText="flags"></t>
<t>Three flags are defined FORWARD_CRITICAL(0x01),
DESTINATION_CRITICAL(0x02), and RESPONSE_COPY(0x04). These
flags MUST NOT be set in a response. If the FORWARD_CRITICAL
flag is set, any node that would forward the message but does
not understand this options MUST reject the request with an
Error_Unsupported_Forwarding_Option error response. If the
DESTINATION_CRITICAL flag is set, any node generates a
response to the message but does not understand the forwarding
option MUST reject the request with an
Error_Unsupported_Forwarding_Option error response. If the
RESPONSE_COPY flag is set, any node generating a response MUST
copy the option from the request to the response and clear the
RESPONSE_COPY, FORWARD_CRITICAL and DESTINATION_CRITICAL
flags.</t>
<t></t>
<t hangText="option"></t>
<t>The option value.</t>
</list></t>
</section>
</section>
<section anchor="sec-contents" title="Message Contents Format">
<t>The second major part of a RELOAD message is the contents part,
which is defined by MessageContents:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { (2^16-1) } MessageExtensionType;
struct {
MessageExtensionType type;
Boolean critical;
opaque extension_contents<0..2^32-1>;
} MessageExtension;
struct {
MessageCode message_code;
opaque message_body<0..2^32-1>;
MessageExtensions extensions<0..2^32-1>;
} MessageContents;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list
style="hanging">
<t></t>
<t hangText="message_code "></t>
<t>This indicates the message that is being sent. The code space
is broken up as follows. <list style="hanging">
<t></t>
<t hangText="0">Reserved</t>
<t></t>
<t hangText="1 .. 0x7fff">Requests and responses. These code
points are always paired, with requests being odd and the
corresponding response being the request code plus 1. Thus,
"probe_request" (the Probe request) has value 1 and
"probe_answer" (the Probe response) has value 2</t>
<t></t>
<t hangText="0xffff">Error</t>
</list></t>
<t></t>
<t hangText="message_body "></t>
<t>The message body itself, represented as a variable-length
string of bytes. The bytes themselves are dependent on the code
value. See the sections describing the various RELOAD methods
(Join, Update, Attach, Store, Fetch, etc.) for the definitions
of the payload contents.</t>
<t hangText="extensions "></t>
<t>Extensions to the message. Currently no extensions are
defined, but new extensions may be defined by the process
described in <xref target="sec-message-extensions"></xref>.</t>
</list></t>
<t>All extensions have the following form:</t>
<t><list style="hanging">
<t></t>
<t hangText="type "></t>
<t>The extension type.</t>
<t></t>
<t hangText="critical "></t>
<t>Whether this extension must be understood in order to process
the message. If critical = True and the recipient does not
understand the message, it MUST generate an
Error_Unknown_Extension error. If critical = False, the
recipient SHOULD choose to process the message even if it does
not understand the extension.</t>
<t></t>
<t hangText="extension_contents "></t>
<t>The contents of the extension (extension-dependent).</t>
</list></t>
<section anchor="sec-response-code"
title="Response Codes and Response Errors">
<t>A peer processing a request returns its status in the
message_code field. If the request was a success, then the message
code is the response code that matches the request (i.e., the next
code up). The response payload is then as defined in the
request/response descriptions.</t>
<t>If the request failed, then the message code is set to 0xffff
(error) and the payload MUST be an error_response PDU, as shown
below.</t>
<t>When the message code is 0xffff, the payload MUST be an
ErrorResponse.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
public struct {
uint16 error_code;
opaque error_info<0..2^16-1>;
} ErrorResponse;
]]></artwork>
</figure>
<t>The contents of this structure are as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="error_code "></t>
<t>A numeric error code indicating the error that
occurred.</t>
<t></t>
<t hangText="error_info "></t>
<t>An optional arbitrary byte string. Unless otherwise
specified, this will be a text string providing further
information about what went wrong.</t>
</list></t>
<t>The following error code values are defined. The numeric values
for these are defined in <xref
target="sec-iana-error-codes"></xref>.</t>
<t><list style="hanging">
<t></t>
<t hangText="Error_Forbidden:">The requesting peer does not
have permission to make this request.</t>
<t></t>
<t hangText="Error_Not_Found:">The resource or peer cannot be
found or does not exist.</t>
<t></t>
<t hangText="Error_Request_Timeout:">A response to the request
has not been received in a suitable amount of time. The
requesting peer MAY resend the request at a later time.</t>
<t></t>
<t hangText="Error_Data_Too_Old:">A store cannot be completed
because the storage_time precedes the existing value.</t>
<t></t>
<t hangText="Error_Generation_Counter_Too_Low:">A store cannot
be completed because the generation counter precedes the
existing value.</t>
<t></t>
<t hangText="Error_Incompatible_with_Overlay:">A peer
receiving the request is using a different overlay,
overlayalgorithm, or hash algorithm.</t>
<t></t>
<t hangText="Error_Unsupported_Forwarding_Option:">A peer
receiving the request with a forwarding options flagged as
critical but the peer does not support this option. See
section <xref target="sec-forwarding-options"></xref>.</t>
<t></t>
<t hangText="Error_TTL_Exceeded:">A peer receiving the request
where the TTL got decremented to zero. See section <xref
target="sec-forwarding-header"></xref>.</t>
<t></t>
<t hangText="Error_Message_Too_Large:">A peer receiving the
request that was too large. See section <xref
target="sec-overlay-link"></xref>.</t>
<t></t>
<t hangText="Error_Response_Too_Large:">A peer would have
generated a response that is too large per the
max_response_length field.</t>
<t></t>
<t hangText="Error_Config_Too_Old:">A destination peer
received a request with a configuration sequence that's too
old.</t>
<t></t>
<t hangText="Error_Config_Too_New:">A destination node
received a request with a configuration sequence that's too
new. A node which receives this error MUST generate a
Config_Update message to send a new copy of the configuration
document to the node which generated the error.</t>
<t></t>
<t hangText="Error_Unknown_Kind:">A destination node received
a request with an unknown kind-id. A node which receives this
error MUST generate a Config_Update message which contains the
appropriate kind deinition.</t>
<t hangText="Error_Unknown_Extension:">A destination node
received a request with an unknown extension.</t>
</list></t>
</section>
</section>
<section anchor="sec-signature" title="Security Block">
<t>The third part of a RELOAD message is the security block. The
security block is represented by a SecurityBlock structure:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { x509(0), (255) } certificate_type;
struct {
certificate_type type;
opaque certificate<0..2^16-1>;
} GenericCertificate;
struct {
GenericCertificate certificates<0..2^16-1>;
Signature signature;
} SecurityBlock;
]]></artwork>
</figure>
<t>The contents of this structure are:</t>
<t><list style="hanging">
<t></t>
<t hangText="certificates"></t>
<t>A bucket of certificates.</t>
<t></t>
<t hangText="signature"></t>
<t>A signature over the message contents.</t>
</list></t>
<t>The certificates bucket SHOULD contain all certificates necessary
to verify every signature in both the message and the internal
message objects. This is the only location in the message which
contains certificates, thus allowing for only a single copy of each
certificate. In systems which have some alternate certificate
distribution mechanism, some certificates MAY be omitted. However,
implementors should note that this creates the possibility that
messages may not be immediately verifiable upon receipt of the
certificates must first be retrieved.</t>
<t>Each certificate is represented by a GenericCertificate
structure, which has the following contents:</t>
<t><list style="hanging">
<t></t>
<t hangText="type"></t>
<t>The type of the certificate. Only one type is defined: x509
representing an X.509 certificate</t>
<t></t>
<t hangText="certificate"></t>
<t>The encoded version of the certificate. For X.509
certificates, it is the DER form.</t>
</list></t>
<t>The signature is computed over the payload and parts of
forwarding header. The payload, in case of a Store, may contain an
additional signature computed over a StoreReq structure. All
signatures are formatted using the Signature element. This element
is also used in other contexts where signatures are needed. The
input structure to the signature computation varies depending on the
data element being signed.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {reserved(0), cert_hash(1), (255)} SignerIdentityType;
select (identity_type) {
case cert_hash;
HashAlgorithm hash_alg;
opaque certificate_hash<0..2^8-1>;
/* This structure may be extended with new types if necessary*/
} SignerIdentityValue;
struct {
SignerIdentityType identity_type;
uint16 length;
SignerIdentityValue identity[SignerIdentity.length];
} SignerIdentity;
struct {
SignatureAndHashAlgorithm algorithm;
SignerIdentity identity;
opaque signature_value<0..2^16-1>;
} Signature;
]]></artwork>
</figure>
<t>The signature construct contains the following values:</t>
<t><list style="hanging">
<t></t>
<t hangText="algorithm "></t>
<t>The signature algorithm in use. The algorithm definitions are
found in the IANA TLS SignatureAlgorithm Registry.</t>
<t></t>
<t hangText="identity "></t>
<t>The identity used to form the signature</t>
<t></t>
<t hangText="signature_value "></t>
<t>The value of the signature</t>
</list></t>
<t>The only currently permitted identity format is a hash of the
signer's certificate. The hash_alg field is used to indicate the
algorithm used to produce the hash. The certificate_hash contains the
hash of the certificate object as represented in the certificates
structure. The SignerIdentity structure is typed purely to allow for
future (unanticipated) extensibility. [Open Issue: Should we remove
this extensibility point?]</t>
<t>For signatures over messages the input to the signature is
computed over:</t>
<t><list>
<t>overlay + transaction_id + MessageContents +
SignerIdentity</t>
</list></t>
<t>Where overlay and transaction_id come from the forwarding header
and + indicates concatenation.</t>
<!-- <t>[[TODO: Check the inputs to this carefully.]]</t> -->
<t>The input to signatures over data values is different, and is
described in <xref target="sec-data-sig"></xref>.</t>
<t>All RELOAD messages MUST be signed. Upon receipt, the receiving
node MUST verify the signature and the authorizing certificate. This
check provides a minimal level of assurance that the sending node is
a valid part of the overlay as well as cryptographic authentication
of the sending node. In addition, responses MUST be checked as
follows:</t>
<t><list style="numbers">
<t>The response to a message sent to a specific Node-Id MUST
have been sent by that Node-Id.</t>
<t>The response to a message sent to a Resource-Id MUST have
been sent by a Node-Id which is as close to or closer to the
target Resource-Id than any node in the requesting node's
neighbor table.</t>
</list></t>
<t>The second condition serves as a primitive check for responses
from wildly wrong nodes but is not a complete check. Note that in
periods of churn, it is possible for the requesting node to obtain a
closer neighbor while the request is outstanding. This will cause
the response to be rejected and the request to be retransmitted.</t>
<t>In addition, some methods (especially Store) have additional
authentication requirements, which are described in the sections
covering those methods.</t>
</section>
</section>
<section anchor="sec-overlay-topology" title="Overlay Topology">
<t>As discussed in previous sections, RELOAD does not itself implement
any overlay topology. Rather, it relies on Topology Plugins, which
allow a variety of overlay algorithms to be used while maintaining the
same RELOAD core. This section describes the requirements for new
topology plugins and the methods that RELOAD provides for overlay
topology maintenance.</t>
<section title="Topology Plugin Requirements">
<t>When specifying a new overlay algorithm, at least the following
need to be described:</t>
<t><list style="symbols">
<t>Joining procedures, including the contents of the Join
message.</t>
<t>Stabilization procedures, including the contents of the
Update message, the frequency of topology probes and keepalives,
and the mechanism used to detect when peers have
disconnected.</t>
<t>Exit procedures, including the contents of the Leave
message.</t>
<t>The length of the Resource-IDs and Node-IDs. For DHTs, the
hash algorithm to compute the hash of an identifier.</t>
<t>The procedures that peers use to route messages.</t>
<t>The replication strategy used to ensure data redundancy.</t>
</list></t>
<t>All overlay algorithms MUST specify maintainence procedures the
send Updates to all members of the Connection Table whenever the
range of IDs for which the peer is responsible changes. This Update
allows clients and peers that have established connections to the
peer responsible for a particular ID to update that connection as
appropriate.</t>
</section>
<section title="Methods and types for use by topology plugins">
<t>This section describes the methods that topology plugins use to
join, leave, and maintain the overlay.</t>
<section title="Join">
<t>A new peer (but which already has credentials) uses the JoinReq
message to join the overlay. The JoinReq is sent to the
responsible peer depending on the routing mechanism described in
the topology plugin. This notifies the responsible peer that the
new peer is taking over some of the overlay and it needs to
synchronize its state.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
NodeId joining_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} JoinReq;
]]></artwork>
</figure>
<!-- join-request = Node-ID -->
<t>The minimal JoinReq contains only the Node-ID which the sending
peer wishes to assume. Overlay algorithms MAY specify other data
to appear in this request.</t>
<t>If the request succeeds, the responding peer responds with a
JoinAns message, as defined below:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
opaque overlay_specific_data<0..2^16-1>;
} JoinAns;
]]></artwork>
</figure>
<t>If the request succeeds, the responding peer MUST follow up by
executing the right sequence of Stores and Updates to transfer the
appropriate section of the overlay space to the joining peer. In
addition, overlay algorithms MAY define data to appear in the
response payload that provides additional info.</t>
<t>In general, nodes which cannot form connections SHOULD report an
error. However, implementations MUST provide some mechanism whereby
nodes can determine they are potentially the first node and take
responsibility for the overlay. This specification does not mandate
any particular mechanism, but a configuration flag or setting seems
appropriate.</t>
</section>
<section title="Leave">
<t>The LeaveReq message is used to indicate that a node is exiting
the overlay. A node SHOULD send this message to each peer with
which it is directly connected prior to exiting the overlay.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
public struct {
NodeId leaving_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} LeaveReq;
]]></artwork>
</figure>
<!-- leave-request = Node-ID -->
<t>LeaveReq contains only the Node-ID of the leaving peer. Overlay
algorithms MAY specify other data to appear in this request.</t>
<t>Upon receiving a Leave request, a peer MUST update its own
routing table, and send the appropriate Store/Update sequences to
re-stabilize the overlay.</t>
</section>
<section title="Update">
<t>Update is the primary overlay-specific maintenance message. It is
used by the sender to notify the recipient of the sender's view of
the current state of the overlay (its routing state) and it is up to
the recipient to take whatever actions are appropriate to deal with
the state change. In general, peers SHOULD send Update messages to
all their adjacencies whenever they detect a topology shift.</t>
<t>When a peer detects through an Update that it is no longer
responsible for any data value it is storing, it MUST attempt to
Store a copy to the correct node unless it knows the the newly
responsible node allready has a copy of the data. This prevents data
loss during large-scale topology shifts such as the merging of
partitioned overlays.</t>
<t>The contents of the UpdateReq message are completely
overlay-specific. The UpdateAns response is expected to be either
success or an error.</t>
</section>
<section anchor="sec-route-query" title="Route_Query">
<t>The Route_Query request allows the sender to ask a peer where
they would route a message directed to a given destination. In other
words, a RouteQuery for a destination X requests the Node-ID where
the receiving peer would next route to get to X. A RouteQuery can
also request that the receiving peer initiate an Update request to
transfer his routing table.</t>
<t>One important use of the RouteQuery request is to support
iterative routing. The sender selects one of the peers in its
routing table and sends it a RouteQuery message with the
destination_object set to the Node-ID or Resource-ID it wishes to
route to. The receiving peer responds with information about the
peers to which the request would be routed. The sending peer MAY
then Attaches to that peer(s), and repeats the RouteQuery.
Eventually, the sender gets a response from a peer that is closest
to the identifier in the destination_object as determined by the
topology plugin. At that point, the sender can send messages
directly to that peer.</t>
<!--
<t>Note that this procedure only works well if all the peers are
mutually directly reachable either by all having public IP
addresses or at least by all being behind the same NAT. Accordingly,
peers MUST only use this method if permitted by the overlay
configuration (see [XREF]).</t> -->
<section title="Request Definition">
<t>A RouteQueryReq message indicates the peer or resource that
the requesting peer is interested in. It also contains a
"send_update" option allowing the requesting peer to request a
full copy of the other peer's routing table.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
Boolean send_update;
Destination destination;
opaque overlay_specific_data<0..2^16-1>;
} RouteQueryReq;
]]></artwork>
</figure>
<t>The contents of the RouteQueryReq message are as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="send_update "></t>
<t>A single byte. This may be set to "true" to indicate that
the requester wishes the responder to initiate an Update
request immediately. Otherwise, this value MUST be set to
"false".</t>
<t></t>
<t hangText="destination "></t>
<t>The destination which the requester is interested in.
This may be any valid destination object, including a
Node-ID, compressed ids, or Resource-ID.</t>
<t></t>
<t hangText="overlay_specific_data "></t>
<t>Other data as appropriate for the overlay.</t>
</list></t>
</section>
<section title="Response Definition">
<t>A response to a successful RouteQueryReq request is a
RouteQueryAns message. This is completely overlay specific.</t>
</section>
</section>
<section title="Probe">
<t>Probe provides a number of primitive "exploration" services:
(1) it allows node to determine which resources another node is
responsible for (2) it allows some discovery services in multicast
settings. A probe can be addressed to a specific Node-ID, or the
peer controlling a given location (by using a resource ID). In
either case, the target Node-IDs respond with a simple response
containing some status information.</t>
<section title="Request Definition">
<t>The ProbeReq message contains a list (potentially empty) of
the pieces of status information that the requester would like
the responder to provide.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { responsible_set(1), num_resources(2), uptime(3), (255)}
ProbeInformationType;
struct {
ProbeInformationType requested_info<0..2^8-1>;
} ProbeReq
]]></artwork>
</figure>
<t>The two currently defined values for ProbeInformation
are:</t>
<t><list style="hanging">
<t></t>
<t hangText="responsible_set"></t>
<t>indicates that the peer should Respond with the fraction
of the overlay for which the responding peer is
responsible.</t>
<t></t>
<t hangText="num_resources"></t>
<t>indicates that the peer should Respond with the number of
resources currently being stored by the peer.</t>
<t></t>
<t hangText="uptime"></t>
<t>indicates that the peer should Respond with how long the
peer has been up in seconds.</t>
</list></t>
</section>
<section title="Response Definition">
<t>A successful ProbeAns response contains the information
elements requested by the peer.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
select (type) {
case responsible_set:
uint32 responsible_ppb;
case num_resources:
uint32 num_resources;
case uptime:
uint32 uptime;
/* This type may be extended */
};
} ProbeInformationData;
struct {
ProbeInformationType type;
uint8 length;
ProbeInformationData value;
} ProbeInformation;
struct {
ProbeInformation probe_info<0..2^16-1>;
} ProbeAns;
]]></artwork>
</figure>
<t>A ProbeAns message contains a sequence of ProbeInformation
structures. Each has a "length" indicating the length of the
following value field. This structure allows for unknown options
types.</t>
<t>Each of the current possible Probe information types is a
32-bit unsigned integer. For type "responsible_ppb", it is the
fraction of the overlay for which the peer is responsible in
parts per billion. For type "num_resources", it is the number of
resources the peer is storing. For the type "uptime" it is the
number of seconds the peer has been up.</t>
<t>The responding peer SHOULD include any values that the
requesting peer requested and that it recognizes. They SHOULD be
returned in the requested order. Any other values MUST NOT be
returned.</t>
</section>
</section>
</section>
</section>
<section title="Forwarding and Link Management Layer">
<t>Each node maintains connections to a set of other nodes defined by
the topology plugin. This section defines the methods RELOAD uses to
form and maintain connections between nodes in the overlay. Three
methods are defined:</t>
<t><list style="hanging">
<t hangText="Attach: ">used to form RELOAD connections between
nodes. When node A wants to connect to node B, it sends an Attach
message to node B through the overlay. The Attach contains A's ICE
parameters. B responds with its ICE parameters and the two nodes
perform ICE to form connection.</t>
<t hangText="AttachLite: ">like attach, it is used to form
connections between nodes but instead of using full ICE, it only
uses a subset known as ICE-Lite.</t>
<t hangText="AppAttach: ">used to form application layer
connections between nodes.</t>
<t hangText="AppAttachLite: ">like AppAttach but uses
ICE-Lite.</t>
<t hangText="Ping: ">is a simple request/response which is used to
verify connectivity of the target peer.</t>
</list></t>
<section anchor="sec-connect-details" title="Attach">
<t>A node sends an Attach request when it wishes to establish a
direct TCP or UDP connection to another node for the purposes of
sending RELOAD messages.</t>
<t>As described in <xref target="sec-message-forwarding"></xref>, an
Attach may be routed to either a Node-ID or to a Resource-ID. An
Attach routed to a specific Node-ID will fail if that node is not
reached. An Attach routed to a Resource-ID will establish a connection
with the peer currently responsible for that Resource-ID, which may be
useful in establishing a direct connection to the responsible peer for
use with frequent or large resource updates.</t>
<t>An Attach in and of itself does not result in updating the routing
table of either node. That function is performed by Updates. If node
A has Attached to node B, but not received any Updates from B, it MAY
route messages which are directly addressed to B through that channel
but MUST NOT route messages through B to other peers via that
channel. The process of Attaching is separate from the process of
becoming a peer (using Join and Update) to prevent half-open states
where a node has started to form connections but is not really ready
to act as a peer. Thus, clients can simply Attach without sending Join
or Update.</t>
<section anchor="sec-connect-request" title="Request Definition">
<t>An AttachReq message contains the requesting peer's ICE
connection parameters formatted into a binary structure.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { reserved(0), UDP(1), TCP(2), (255) } Transport;
enum { reserved(0), host(1), srflx(2), prflx(3), relay(4),
(255) } CandType;
struct {
opaque name<2^16-1>;
opaque value<2^16-1>;
} IceExtension;
struct {
IpAddressPort addr_port;
Transport transport;
opaque foundation<0..255>;
uint32 priority;
CandType type;
select (type){
case host:
case srflx:
case prflx:
; /* Nothing */
case relay:
IpAddressPort rel_addr_port;
}
IceExtension extensions<0..2^16-1>;
} IceCandidate;
struct {
opaque ufrag<0..2^8-1>;
opaque password<0..2^8-1>;
opaque role<0..2^8-1>;
IceCandidate candidates<0..2^16-1>;
} AttachReqAns;
]]></artwork>
</figure>
<t>The values contained in AttachReq and AttachAns are: <list
style="hanging">
<t></t>
<t hangText="ufrag "></t>
<t>The username fragment (from ICE)</t>
<t></t>
<t hangText="password "></t>
<t>The ICE password.</t>
<t></t>
<t hangText="role "></t>
<!-- TODO: EKR: answering party does TLS client -->
<t>An active/passive/actpass attribute from RFC 4145 <xref
target="RFC4145"></xref>.</t>
<t></t>
<t hangText="candidates "></t>
<t>One or more ICE candidate values, as described below</t>
</list></t>
<t>Each ICE candidate is represented as an IceCandidate structure,
which is a direct translation of the information from the ICE string
structures, with the exception of the component ID. Since there is
only one component, it is always 1. The remaining values are
specified as follows:</t>
<t><list style="hanging">
<t hangText="addr_port"></t>
<t>corresponds to the connection-address and port
productions.</t>
<t></t>
<t hangText="transport"></t>
<t>corresponds to the transport production. New transports
such as SCTP or <xref
target="I-D.baset-tsvwg-tcp-over-udp"></xref> can be added be
defining new Transport values in the IANA registry in <xref
target="sec-iana-transport"></xref>.</t>
<t></t>
<t hangText="foundation"></t>
<t>corresponds to the foundation production.</t>
<t></t>
<t hangText="priority"></t>
<t>corresponds to the priority production.</t>
<t></t>
<t hangText="type"></t>
<t>corresponds to the cand-type production.</t>
<t></t>
<t hangText="rel_addr_port"></t>
<t>corresponds to the rel-addr and rel-port productions. Only
present for type "relay".</t>
<t></t>
<t hangText="extensions"></t>
<t>ICE extensions. The name and value fields correspond to
binary translations of the equivalent fields in the ICE
extensions.</t>
</list></t>
<t>These values should be generated using the procedures described
in <xref target="sec-ice-reload"></xref>.</t>
</section>
<section anchor="sec-connect-response" title="Response Definition">
<t>If a peer receives an Attach request, it SHOULD follow the
process the request and generate its own response with a
AttachReqAns. It should then begin ICE checks. When a peer
receives an Attach response, it SHOULD parse the response and
begin its own ICE checks.</t>
</section>
<section anchor="sec-ice-reload" title="Using ICE With RELOAD">
<t>This section describes the profile of ICE that is used with
RELOAD. RELOAD implementations MUST implement full ICE. Because
RELOAD always tries to use TCP and then UDP as a fallback, there
will be multiple candidates of the same IP version, which requires
full ICE.</t>
<t>In ICE as defined by <xref
target="I-D.ietf-mmusic-ice"></xref>, SDP is used to carry the ICE
parameters. In RELOAD, this function is performed by a binary
encoding in the Attach method. This encoding is more restricted
than the SDP encoding because the RELOAD environment is
simpler:</t>
<t><list style="symbols">
<t>Only a single media stream is supported.</t>
<t>In this case, the "stream" refers not to RTP or other types
of media, but rather to a connection for RELOAD itself or for
SIP signaling.</t>
<t>RELOAD only allows for a single offer/answer exchange.
Unlike the usage of ICE within SIP, there is never a need to
send a subsequent offer to update the default candidates to
match the ones selected by ICE.</t>
</list></t>
<!-- EKR removed: not appropriate here.
<t>RELOAD and SIP always run over TLS for TCP connections and DTLS <xref
target="RFC4347"></xref> for UDP "connections". Consequently, once ICE
processing has completed, both agents will begin TLS and DTLS procedures
to establish a secure link. Its important to note that, had a TURN server
been utilized for the TCP or UDP stream, the TURN server will
transparently relay the TLS messaging and the encrypted TLS content, and
thus will not have access to the contents of the connection once it is
established. Any attack by the TURN server to insert itself as a
man-in-the-middle are thwarted verifying that the certificates used to
establish the secure connection match the identties of the connecting
peers.</t>
-->
<t>An agent follows the ICE specification as described in <xref
target="I-D.ietf-mmusic-ice"></xref> and <xref
target="I-D.ietf-mmusic-ice-tcp"></xref> with the changes and
additional procedures described in the subsections below.</t>
</section>
<section anchor="sec-collect" title="Collecting STUN Servers">
<t>ICE relies on the node having one or more STUN servers to use.
In conventional ICE, it is assumed that nodes are configured with
one or more STUN servers through some out-of-band mechanism. This is
still possible in RELOAD but RELOAD also learns STUN servers as it
connects to other peers. Because all RELOAD peers implement ICE and
use STUN keepalives, every peer is a STUN server <xref
target="RFC5389"></xref>. Accordingly, any peer a node knows will be
willing to be a STUN server -- though of course it may be behind a
NAT.</t>
<t>A peer on a well-provisioned wide-area overlay will be
configured with one or more bootstrap peers. These peers make an
initial list of STUN servers. However, as the peer forms
connections with additional peers, it builds more peers it can use
as STUN servers.</t>
<t>Because complicated NAT topologies are possible, a peer may need
more than one STUN server. Specifically, a peer that is behind a
single NAT will typically observe only two IP addresses in its STUN
checks: its local address and its server reflexive address from a
STUN server outside its NAT. However, if there are more NATs
involved, it may discover that it learns additional server reflexive
addresses (which vary based on where in the topology the STUN server
is). To maximize the chance of achieving a direct connection, a peer
SHOULD group other peers by the peer-reflexive addresses it
discovers through them. It SHOULD then select one peer from each
group to use as a STUN server for future connections.</t>
<t>Only peers to which the peer currently has connections may be
used. If the connection to that host is lost, it MUST be removed
from the list of stun servers and a new server from the same group
SHOULD be selected.</t>
<!-- EKR removed. TMI
<t>OPEN ISSUE: should the peer try to keep at least one peer in each
group, even if it has no other reason for the connection? Need to
specify when to stop adding new groups if the peer is behind a really
bad NAT.</t>
<t>OPEN ISSUE: RELOAD-01 had a Peer-Info structure that allowed peers
to exchange information such as a "default" IP-port pair in Updates.
This structure could be expanded to include the candidate list for a
peer, thus allowing ICE negotiation to begin or even direct
communication before an Attach request has been received. (The
candidate pairs for the P2P port are fixed because the same source
port is used for all connections.) However, because this would require
significant changes to the ICE algorithm, we have not introduced such
an extension at this point.</t>
-->
</section>
<section anchor="sec-gather" title="Gathering Candidates">
<t>When a node wishes to establish a connection for the purposes
of RELOAD signaling or SIP signaling (or any other application
protocol for that matter), it follows the process of gathering
candidates as described in Section 4 of ICE <xref
target="I-D.ietf-mmusic-ice"></xref>. RELOAD utilizes a single
component, as does SIP. Consequently, gathering for these
"streams" requires a single component.</t>
<t>An agent MUST implement ICE-tcp <xref
target="I-D.ietf-mmusic-ice"></xref>, and MUST gather at least one
UDP and one TCP host candidate for RELOAD and for SIP.</t>
<t>The ICE specification assumes that an ICE agent is configured
with, or somehow knows of, TURN and STUN servers. RELOAD provides
a way for an agent to learn these by querying the overlay, as
described in <xref target="sec-collect"></xref> and <xref
target="sec-turn-server"></xref>.</t>
<t>The agent SHOULD prioritize its TCP-based candidates over its
UDP-based candidates in the prioritization described in Section
4.1.2 of ICE <xref target="I-D.ietf-mmusic-ice"></xref>.</t>
<t>The default candidate selection described in Section 4.1.3 of
ICE is ignored; defaults are not signaled or utilized by
RELOAD.</t>
</section>
<section title="Encoding the Attach Message">
<t>Section 4.3 of ICE describes procedures for encoding the SDP for
conveying RELOAD or SIP ICE candidates. Instead of actually encoding
an SDP, the candidate information (IP address and port and transport
protocol, priority, foundation, component ID, type and related
address) is carried within the attributes of the Attach request or
its response. Similarly, the username fragment and password are
carried in the Attach message or its response. <xref
target="sec-connect-details"></xref> describes the detailed
attribute encoding for Attach. The Attach request and its response
do not contain any default candidates or the ice-lite attribute, as
these features of ICE are not used by RELOAD.</t>
<t>Since the Attach request contains the candidate information and
short term credentials, it is considered as an offer for a single
media stream that happens to be encoded in a format different than
SDP, but is otherwise considered a valid offer for the purposes of
following the ICE specification. Similarly, the Attach response is
considered a valid answer for the purposes of following the ICE
specification.</t>
</section>
<section title="Verifying ICE Support">
<t>An agent MUST skip the verification procedures in Section 5.1
and 6.1 of ICE. Since RELOAD requires full ICE from all agents,
this check is not required.</t>
</section>
<section title="Role Determination">
<t>The roles of controlling and controlled as described in Section
5.2 of ICE are still utilized with RELOAD. However, the offerer
(the entity sending the Attach request) will always be
controlling, and the answerer (the entity sending the Attach
response) will always be controlled. The connectivity checks MUST
still contain the ICE-CONTROLLED and ICE-CONTROLLING attributes,
however, even though the role reversal capability for which they
are defined will never be needed with RELOAD. This is to allow for
a common codebase between ICE for RELOAD and ICE for SDP.</t>
</section>
<section title="Connectivity Checks">
<t>The processes of forming check lists in Section 5.7 of ICE,
scheduling checks in Section 5.8, and checking connectivity checks
in Section 7 are used with RELOAD without change.</t>
</section>
<section title="Concluding ICE">
<t>The controlling agent MUST utilize regular nomination. This is
to ensure consistent state on the final selected pairs without the
need for an updated offer, as RELOAD does not generate additional
offer/answer exchanges.</t>
<t>The procedures in Section 8 of ICE are followed to conclude
ICE, with the following exceptions:</t>
<t><list style="symbols">
<t>The controlling agent MUST NOT attempt to send an updated
offer once the state of its single media stream reaches
Completed.</t>
<t>Once the state of ICE reaches Completed, the agent can
immediately free all unused candidates. This is because RELOAD
does not have the concept of forking, and thus the three
second delay in Section 8.3 of ICE does not apply.</t>
</list></t>
</section>
<section title="Subsequent Offers and Answers">
<t>An agent MUST NOT send a subsequent offer or answer. Thus, the
procedures in Section 9 of ICE MUST be ignored.</t>
</section>
<section title="Media Keepalives">
<t>STUN MUST be utilized for the keepalives described in Section
10 of ICE.</t>
</section>
<section title="Sending Media">
<t>The procedures of Section 11 apply to RELOAD as well. However,
in this case, the "media" takes the form of application layer
protocols (RELOAD or SIP for example) over TLS or DTLS.
Consequently, once ICE processing completes, the agent will begin
TLS or DTLS procedures to establish a secure connection. The node
which sent the Attach request MUST be the TLS server. The other
node MUST be the TLS client. The nodes MUST verify that the
certificate presented in the handshake matches the identity of the
other peer as found in the Attach message. Once the TLS or DTLS
signaling is complete, the application protocol is free to use the
connection.</t>
<t>The concept of a previous selected pair for a component does
not apply to RELOAD, since ICE restarts are not possible with
RELOAD.</t>
</section>
<section title="Receiving Media">
<t>An agent MUST be prepared to receive packets for the
application protocol (TLS or DTLS carrying RELOAD, SIP or anything
else) at any time. The jitter and RTP considerations in Section 11
of ICE do not apply to RELOAD or SIP.</t>
</section>
</section>
<section title="AttachLite">
<t>An alternative to using the full ICE supported by the Attach
request is to use ICE-Lite with the AttachLite request. This will not
work in all of the scenarios where ICE would work, but in some cases,
particularly those with no NATs or firewalls, it will work.
Configuration for the overlay indicates if this can be used or
not.</t>
<!-- <t>OPEN ISSUE: We originally envisioned adding support for
ICE-Lite directly to the regular Attach method. However, we found
that both the parameters and processing were completely
different, resulting in almost no overlap between the two
methods. Therefore we chose to separate this out for overlays
where the complexities of ICE are not needed. Note that it is
still possible for a node with a public unfiltered address
intending to interoperate to implement Attach without the
candidate gathering phases of ICE and achieve essentially the
same result. If simpler behavior or a better encoding of ICE-Lite
in Attach is developed, such an approach would be
preferable.</t>-->
<section title="Request Definition">
<t>An AttachLiteReqAns message contains the requesting peer's
ICE-Lite connection parameters formatted into a binary structure.
When using the AttachLite request, both sides act as ICE-Lite
hosts.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
IpAddressPort addr_port;
Transport transport;
uint32 priority;
} IceLiteCandidate;
struct {
IceLiteCandidate candidates<0..2^16-1>;
} AttachLiteReqAns;
]]></artwork>
</figure>
<t>The values contained in AttachLiteReqAns are: <list
style="hanging">
<t></t>
<t hangText="candidates "></t>
<t>One or more ICE candidate values. Each one contains an IP
address and family, transport protocol, and port to connect to
as well as a priority.</t>
</list></t>
<t>These values should be generated using the procedures described
in <xref target="sec-ice-reload"></xref>.</t>
</section>
<section title="Response Definition">
<t>If a peer receives an AttachLite request, it SHOULD follow the
process the request and generate its own response with a
AttachLiteReqAns. It should then initiate connections as described
below. When a peer receives an AttachLite response, it SHOULD
parse the response and handle any received connections.</t>
</section>
<section title="Attach-Lite Connectivity Checks">
<t>STUN is not used for connectivity checks when doing ICE-Lite,
instead the DTLS or TLS handshake forms the connectivity check. The
host that received the AttachLiteReqAns MUST initiate TLS or DTLS
connections to candidates provided in the request. When a connection
forms, the node MUST check the certificate is for the node that send
AttachLiteReqAns and if is not, MUST close the connection.</t>
<t>Since TLS provides the connectivity check, there is no need for
the <xref target="RFC4571">RFC 4571</xref> style framing shim for
STUN when using TLS and this is not used for this protocol.</t>
</section>
<section title="Implementation Notes for Attach-Lite">
<t>This is a non normative section to help implementors.</t>
<t>At times ICE can seem a bit daunting to gets one head around.
For a simple IPv4 only peer, a simple implementation of
Attach-Lite could be done be doing the following: <list
style="symbols">
<t>When sending an AttachLiteReqAns, form one candidate with a
priority value of (2^24)*(126)+(2^8)*(65535)+(2^0)*(256-1)
that specifies the UDP port being listened to and another one
with the TCP port.</t>
<t>When receiving an AttachLiteReqAns, try to form a
connection to each candidate in the request. Check the
certificate receive in the TLS handshake has the correct
Node-ID as the node that send the AttchLiteReq. If multiple
connection succeed, close all but the one with highest
priority.</t>
<t>Do normal TLS and DTLS with no need for any special framing
or STUN processing.</t>
</list></t>
</section>
</section>
<section anchor="sec-appattach-details" title="AppAttach">
<t>A node sends an AppAttach request when it wishes to establish a
direct TCP or UDP connection to another node for the purposes of
sending application layer messages. AppAttach is basically like
Attach, except for the purpose of the connection. A separate request
is used to avoid implementor confusion between the two methods (this
was found to be a real problem with initial implementations). The
AppAttach request and its response contain a application attribute,
with a value of SIP or RELOAD, which indicates what protocol is to be
run over the connection.</t>
<section anchor="sec-appattach-request" title="Request Definition">
<t>An AttachReq message contains the requesting peer's ICE
connection parameters formatted into a binary structure.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
opaque ufrag<0..2^8-1>;
opaque password<0..2^8-1>;
uint16 application;
opaque role<0..2^8-1>;
IceCandidate candidates<0..2^16-1>;
} AppAttachReqAns;
]]></artwork>
</figure>
<t>The values contained in AppAttachReq and AppAttachAns are:
<list style="hanging">
<t></t>
<t hangText="ufrag "></t>
<t>The username fragment (from ICE)</t>
<t></t>
<t hangText="password "></t>
<t>The ICE password.</t>
<t></t>
<t hangText="application "></t>
<t>A 16-bit port number. This port number represents the IANA
registered port of the protocol that is going to be sent on
this connection. For SIP, this is 5060 or 5061. By using the
IANA registered port, we avoid the need for an additional
registry and allow RELOAD to be used to set up connections for
any existing or future application protocol.</t>
<t></t>
<t hangText="role "></t>
<!-- TODO: EKR: answering party does TLS client -->
<t>An active/passive/actpass attribute from RFC 4145 <xref
target="RFC4145"></xref>.</t>
<t></t>
<t hangText="candidates "></t>
<t>One or more ICE candidate values</t>
</list></t>
</section>
<section anchor="sec-appattach-response" title="Response Definition">
<t>If a peer receives an AppAttach request, it SHOULD follow the
process the request and generate its own response with a
AppAttachReqAns. It should then begin ICE checks. When a peer
receives an AppAttach response, it SHOULD parse the response and
begin its own ICE checks.</t>
</section>
</section>
<section title="AppAttachLite">
<t>Similar to the AttachLite method, RELOAD provides an
AppAttachLite to allow application connections when full ICE is not
needed.</t>
<section title="Request Definition">
<t>An AppAttachLiteReqAns message contains the requesting peer's
ICE-Lite connection parameters formatted into a binary structure.
When using the AppAttachLite request, both sides act as ICE-Lite
hosts.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint16 application;
IceLiteCandidate candidates<0..2^16-1>;
} AppAttachLiteReqAns;
]]></artwork>
</figure>
<t>The values contained in AppAttachLiteReqAns are: <list
style="hanging">
<t></t>
<t hangText="application "></t>
<t>A 16-bit port number used in the same was as in the
AppAttach request. This port number represents the IANA
registered port of the protocol that is going to be sent on
this connection.</t>
<t></t>
<t hangText="candidates "></t>
<t>One or more ICE candidate values. Each one contains an IP
address and family, transport protocol, and port to connect to
as well as a priority.</t>
</list></t>
<t>These values should be generated using the procedures described
in <xref target="sec-ice-reload"></xref>.</t>
</section>
<section title="Response Definition">
<t>If a peer receives an AppAttachLite request, it SHOULD follow
the process the request and generate its own response with a
AppAttachLiteReqAns as described in the AttachLite section.</t>
</section>
</section>
<section title="Ping">
<t>Ping is used to test connectivity along a path. A ping can be
addressed to a specific Node-ID, the peer controlling a given
location (by using a resource ID), or to the broadcast Node-ID
(2^128-1).</t>
<section title="Request Definition">
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
} PingReq
]]></artwork>
</figure>
</section>
<section title="Response Definition">
<t>A successful PingAns response contains the information elements
requested by the peer.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint64 response_id;
uint64 time;
} PingAns;
]]></artwork>
</figure>
<t>A PingAns message contains the following elements: <list
style="hanging">
<t></t>
<t hangText="response_id "></t>
<t>A randomly generated 64-bit response ID. This is used to
distinguish Ping responses.</t>
<t></t>
<t hangText="time "></t>
<t>The time when the ping responses was created in absolute
time, represented in milliseconds since midnight Jan 1, 1970
which is the UNIX epoch.</t>
</list></t>
</section>
</section>
<section title="Config_Update">
<t>The Config_Update method is used to push updated configuration
data across the overlay. Whenever a node detects that another node
has old configuration data, it MUST generate a Config_Update
request. The Config_Update request allows updating of two kinds of
data: the configuration data (<xref target="sec.config-seq"></xref>)
and kind information (<xref target="sec-store-req"></xref>).</t>
<section title="Request Definition">
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { reserved(0), config(1), kind(2), (255) }
Config_UpdateType;
typedef opaque KindDescription<2^16-1>;
struct {
Config_UpdateType type;
uint24 length;
select (type) {
case config:
opaque config_data<2^24-1>;
case kind:
KindDescription kinds<2^24-1>;
/* This structure may be extended with new types*/
};
} Config_UpdateReq;
]]></artwork>
</figure>
<t>The Config_UpdateReq message contains the following
elements:</t>
<t><list style="hanging">
<t></t>
<t hangText="type"></t>
<t>The type of the contents of the message. This structure
allows for unknown content types.</t>
<t hangText="length"></t>
<t>The length of the remainder of the message (included to
preserve backward compatibility.)</t>
<t hangText="config_data (type==config)"></t>
<t>The contents of the configuration document</t>
<t hangText="kinds (type==kind)"></t>
<t>One or more XML kind-block productions (see <xref
target="sec-configuration"></xref>).</t>
</list></t>
</section>
<section title="Response Definition">
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
} Config_UpdateReq
]]></artwork>
</figure>
<t>If the Config_UpdateReq is of type "config" it MUST only be
processed if all the following are true: <list style="symbols">
<t>The sequence number in the document is greater than the
current configuration sequence number.</t>
<t>The configuration document is correctly digitally signed
(see <xref target="sec-enrollment"></xref> for details on
signatures.</t>
</list> Otherwise appropriate errors MUST be generated.</t>
<t>If the Config_UpdateReq is of type "kind" it MUST only be
processed if it is correctly digitally signed by an acceptable
kind signer. In addition, if the kind update conflicts with an
existing known kind (i.e., it is signed by a different signer),
then it should be rejected with "Error_Forbidden". This should not
happen in correctly functioning overlays.</t>
<t>If the update is acceptable, then the node MUST reconfigure
itself to match the new information. This may include adding
permissions for new kinds, deleting old kinds, or even, in extreme
circumstances, exiting and reentering the overlay, if, for
instance, the DHT algorithm has changed.</t>
<t>The response for Config_Update is empty.</t>
</section>
</section>
</section>
<section anchor="sec-overlay-link" title="Overlay Link Layer">
<t>RELOAD can use multiple Overlay Link protocols to send its
messages. Because ICE is used to establish connections (see <xref
target="sec-ice-reload"></xref>), RELOAD nodes are able to detect which
Overlay Link protocols are offered by other nodes and establish
connections between each other. Any link protocol needs to be able to
establish a secure, authenticated connection, and provide data origin
authentication and message integrity for individual data elements.
RELOAD currently supports two Overlay Link protocols:</t>
<t><list style="symbols">
<t>TLS <xref target="RFC5246"></xref> over TCP</t>
<t>DTLS <xref target="RFC4347"></xref> over UDP</t>
</list></t>
<t>Note that although UDP does not properly have "connections", both
TLS and DTLS have a handshake which establishes a stateful
association, a similar stateful construct, and we simply refer to
these as "connections" for the purposes of this document.</t>
<t>If a peer receives a message that is larger than value of
max-message-size defined in the overlay configuration, the peer SHOULD
send an Error_Message_Too_Large error then close the TLS or DTLS
session from which the message was received. Note that this error can
be sent and the session closed before receiving the complete message.
If the forwarding header is larger than the max-message-size, the
receiver SHOULD close the TLS or DTLS session without sending an
error.</t>
<!--
<t>TODO: need to clarify that we use STUN for keepalives
(inherit from ice). Need to change and specify that wrapping
header is used between peers when using TCP (b/c protocol
timeouts are shorter than TCP. Need to specify how to detect
link failure.</t>
-->
<section title="Future Support for HIP">
<t>The P2PSIP Working Group has expressed interest in supporting a
HIP-based link protocol <xref target="RFC5201"></xref>. Such support
would require specifying such details as:</t>
<t><list style="symbols">
<t>How to issue certificates which provided identities
meaningful to the HIP base exchange. We anticipate that this
would require a mapping between ORCHIDs and NodeIds.</t>
<t>How to carry the HIP I1 and I2 messages. We anticipate that
this would require defining a HIP Tunnel usage.</t>
<t>How to carry RELOAD messages over HIP.</t>
</list></t>
<t>We leave this work as a topic for another draft.</t>
</section>
<section anchor="sec-reliability"
title="Reliability for Unreliable Links">
<t>When RELOAD is carried over DTLS or another unreliable link
protocol, it needs to be used with a reliability and congestion
control mechanism, which is provided on a hop-by-hop basis. The
basic principle is that each message, regardless of if it carries a
request or responses, will get an ACK and be reliably retransmitted.
The receiver's job is very simple, limited to just sending ACKs. All
the complexity is at the sender side. This allows the sending
implementation to trade off performance versus implementation
complexity without affecting the wire protocol.</t>
<t>In order to support unreliable links, each message is wrapped in
a very simple framing layer (FramedMessage) which is only used for
each hop. This layer contains a sequence number which can then be
used for ACKs.</t>
<section title="Framed Message Format">
<t>The definition of FramedMessage is:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {data (128), ack (129), (255)} FramedMessageType;
struct {
FramedMessageType type;
select (type) {
case data:
uint32 sequence;
opaque message<0..2^24-1>;
case ack:
uint32 ack_sequence;
uint32 received;
};
} FramedMessage;
]]></artwork>
</figure>
<t>The type field of the PDU is set to indicate whether the
message is data or an acknowledgement.</t>
<!-- This isn't relevant because we're using TLS here
Note that these values have
been set to force the first bit to be high, thus allowing easy
demultiplexing with STUN. All FramedMessageType values must be
> 128.</t> -->
<t>If the message is of type "data", then the remainder of the PDU
is as follows: <list style="hanging">
<t></t>
<t hangText="sequence "></t>
<t>the sequence number. This increments by 1 for each framed
message sent over this transport session.</t>
<t></t>
<t hangText="message "></t>
<t>the message that is being transmitted.</t>
</list></t>
<t>Each connection has it own sequence number space. Initially the
value is zero and it increments by exactly one for each message
sent over that connection.</t>
<t>When the receiver receive a message, it SHOULD immediately send
an ACK message. The receiver MUST keep track of the 32 most recent
sequence numbers received on this association in order to generate
the appropriate ack.</t>
<t>If the PDU is of type "ack", the contents are as follows: <list
style="hanging">
<t></t>
<t hangText="ack_sequence "></t>
<t>The sequence number of the message being acknowledged.</t>
<t></t>
<t hangText="received "></t>
<t>A bitmask indicating if each of the previous 32 sequence
numbers before this packet had been received as one of the most
recently received 32 packets on this connection. When a packet
is received with a sequence number N, the receiver looks at the
sequence number of the previously 32 packets received on this
connection. Call the previously received packet number M. And
for each of the previous 32 packets, if the sequence number M is
less than N but greater than N-32, the N-M bit of the received
bitmask is set to one otherwise it is zero.</t>
<t>Note that a bit being set indicates a particular packet was
received but if the bit is set to zero it only means it is
unknown if it was received or not. It might have been received
but not in the 32 most recently received window.</t>
</list></t>
<t>The received field bits in the ACK provide a very high degree
of redundancy for the sender to figure out which packets the
receiver received and can then estimate packet loss rates. If the
sender also keeps track of the time at which recent sequence
numbers were sent, the RTT can be estimated.</t>
</section>
<section anchor="sec-retran-stop-n-wait"
title="Retransmission and Flow Control">
<t>Because the receiver's role is limited to providing packet
acknowledgements, a wide variety of congestion control algorithms
can be implemented on the sender side while using the same basic
wire protocol. Senders MUST implement a retransmission and
congestion control scheme no more aggressive then TFRC<xref
target="RFC5348"></xref>. One way to do that is for senders to
implement the scheme in the following section. Another would be to
implement the scheme described in <xref
target="sec-retran-aimd"></xref>. Another alternative would be
TFRC-SP <xref target="RFC4828"></xref> and use the received bitmask
to allow the sender to compute packet loss event rates. An
alternative</t>
<section title="Trivial Retransmission">
<!--
<t>An algorithm which will not perform as well as TFRC-SP but is
easy to implement is described in this section and can be used
if implementations don't use a more advanced techniques such as
TFRC-SP.</t>
<t>A peer SHOULD retransmit a message if it has not received an
ACK for that messages starting with an interval of RTO
("Retransmission TimeOut"), doubling after each retransmission.
In each retransmission, the sequence number is incremented. The
RTO is an estimate of the round-trip time (RTT), and is computed
as described in RFC 2988 [RFC2988], with two exceptions. First,
the initial value for RTO SHOULD be configurable (rather than the
3 s recommended in RFC 2988) and SHOULD be equal to or greater
than 500 ms. The exception cases for this "SHOULD" are when other
mechanisms are used to derive congestion thresholds, or when this
is used in non- Internet environments with known network
capacities. In fixed-line access links, a value of 500 ms is
SHOULD be used. Second, the value of RTO SHOULD NOT be rounded up
to the nearest second. Rather, a 1 ms accuracy SHOULD be
maintained. As with TCP, the Karn's [TODO REF KARN87] algorithm
SHOULD be used which means that RTT estimates SHOULD NOT be
computed from transactions that result in the retransmission of a
request. The value for RTO is calculated separately for each DTLS
session.</t>
-->
<t>A peer SHOULD retransmit a message if it has not received an
ACK for that messages starting with an interval of RTO
("Retransmission TimeOut"), and then doubling after each
retransmission. In each retransmission, the sequence number is
incremented.</t>
<t>The RTO is an estimate of the round-trip time (RTT).
Implementations can use a static value for RTO or a dynamic
estimate which will result in better performance. For
implementations that use a static value, the default value for RTO
is 500 ms. Nodes MAY use smaller values of RTO if it is known that
all nodes are are within the local network. The default RTO MAY be
chosen larger, and this is RECOMMENDED if it is known in advance
(such as on high latency access links) that the round-trip time is
larger.</t>
<t>Implementations that use a dynamic estimate to compute the RTO
MUST use the algorithm described in RFC 2988<xref
target="RFC2988"></xref>, with the exceptions that value of RTO
SHOULD NOT be rounded up to the nearest second but instead rounded
up to the nearest millisecond. The RTT of a successful STUN
transaction from the ICE stage is used as the initial measurement
for formula 2.2 of RFC 2988. The sender keeps track of the time
each message was sent for all recently sent messages. Any time an
ACK is received, the sender can compute the RTT for that message
by looking at the time the ACK was received and when time the
message was sent. This is used as a subsequent RTT measurement for
formula 2.3 of RFC 2988 to update the RTO estimate. (Note that
because retransmissions receive new sequence numbers, all received
ACKs are used.)</t>
<t>The value for RTO is calculated separately for each DTLS
session.</t>
<t>Retransmissions continue until a response is received, or until
a total of 5 requests have been sent or there has been a hard ICMP
error [RFC1122]. The sender knows a responses was received when it
receives an ACK with a sequence number that indicates it is a
response to one of the transmissions of this messages. For
example, assuming an RTO of 500 ms, requests would be sent at
times 0 ms, 500 ms, 1500 ms, 3500 ms, and 7500 ms. If all
retransmissions for a message fail, the link</t>
<t>Once an ACK has been received for a message, the next messages
can be sent but the peer SHOULD ensure that there is at least 10
ms between sending any two messages. The only time a value less 10
ms can be used is when it is known that all nodes are on a network
that can support retransmissions faster than 10 ms with no
congestion issues.</t>
</section>
</section>
</section>
<!-- EKR removed <section title="HIP"> <t>RELOAD MAY also be used with a
HIP transport using the architecture for HIP BONE described in
<xref target="I-D.camarillo-hip-bone"></xref>. From the perspective
of the P2P layer, HIP looks very much like normal IP. Either TLS
(over TCP) or DTLS (over UDP) is run over top of the HIP. Thus the
reliability and congestion control schemes are the same for DTLS
section. If an overlay is configured such that HIP is the only
transport that it will use, then it may make sense to configure the
p2p layer to only offer the ORCHID when gather candidate addresses
for ICE. This will effectively disable ICE at the p2p layer.</t>
<t>For overlays that use HIP, the enrollment server MUST provide each
peer with a unique ORCHID and use that ORCHID to generate the Node-ID
for the peer (see <xref target="sec-credentials"></xref>. Later when
the HIP layer wishes to tunnel a message (such as an I1 message)
through the overlay, the HIP layer can use the ORCHID to generate the
Node-ID, and then use the Tunnel message with the HIP to route the
message to that the peer that owns that ORCHID.</t>
</section>
<section title="TLS">
<t>TLS runs on top of TCP which offers the best performance from a
data transfer point of view and does not require as frequent keep
alive messages.</t>
</section>
-->
<section anchor="sec-frag-reass" title="Fragmentation and Reassembly">
<t>In order to allow transmission over datagram protocols such as
DTLS, RELOAD messages may be fragmented.</t>
<!-- If a message is too large
for a peer to transmit to the next peer it MUST fragment the
message. Note that this implies that intermediate peers may
re-fragment messages if the incoming and outgoing paths have
different maximum datagram sizes. Intermediate peers SHOULD NOT
reassemble fragments.
-->
<t>Any node along the path can fragment the message but only the final
destination reassembles the fragments. When a node takes a packet and
fragments it, each fragment has a full copy of the Forwarding Header
but the data after the Forwarding Header is broken up in appropriate
sized chunks. The size of the payload chunks needs to take into
account space to allow the via and lists to grow. Each fragment MUST
contain a full copy of the via and destination list and MUST contain
at least 256 bytes of the message body. If the via and destination
list are so large that this is not possible, RELOAD fragmentation is
not performed and IP-layer fragmentation is allowed to occur. When a
message must be fragmented, it SHOULD be split into equal-sized
fragments that are no larger than the PMTU of the next overlay link
minus 32 bytes. This is to allow the via list to grow before further
fragmentation is required.</t>
<t>Note that this fragmentation is not optimal for the end-to-end path
- a message may be refragmented multiple times as it traverses the
overlay. This option has been chosen as it is far easier to implement
than e2e PMTU discovery across an ever-changing overlay, and
effectively addresses the reliability issues of relying on IP-layer
fragmentation. However, PING can be used to allow e2e PMTU to be
implemented if desired.</t>
<t>Upon receipt of a fragmented message by the intended peer, the peer
holds the fragments in a holding buffer until the entire message has
been received. The message is then reassembled into a single message
and processed. In order to mitigate denial of service attacks,
receivers SHOULD time out incomplete fragments after maximum request
lifetime (15 seconds). Note this time was derived from looking at the
end to end retransmission time and saving fragments long enough for
the full end to end retransmissions to take place. Ideally the
receiver would have enough buffer space to deal with maximum request
lifetime worth of fragments at whatever rate it receives messages on
it's interfaces, however, if the receiver runs out of buffer space to
reassemble the messages it MUST drop the message.</t>
<t>When a message is fragmented, the fragment offset value is stored
in the lower 24 bits of the fragment field of the forwarding header.
The offset is the number of bytes of the start of data from the end
of the forwarding header so the first fragment has an offset of 0.
The first and last bit indicators MUST be appropriately set. If the
message is not fragmented, then both the first and last fragment are
set to 1 and the offset is 0 resulting in a fragment value of
0xC0000000.</t>
</section>
</section>
</section>
<section anchor="sec-data-protocol" title="Data Storage Protocol">
<t>RELOAD provides a set of generic mechanisms for storing and retrieving
data in the Overlay Instance. These mechanisms can be used for new
applications simply by defining new code points and a small set of
rules. No new protocol mechanisms are required.</t>
<t>The basic unit of stored data is a single StoredData structure:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint32 length;
uint64 storage_time;
uint32 lifetime;
StoredDataValue value;
Signature signature;
} StoredData;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list style="hanging">
<t></t>
<t hangText="length "></t>
<t>The length of the rest of the structure in octets.</t>
<t></t>
<t hangText="storage_time "></t>
<t>The time when the data was stored in absolute time, represented
in milliseconds since the Unix epoch of midnight Jan 1, 1970. Any
attempt to store a data value with a storage time before that of a
value already stored at this location MUST generate a
Error_Data_Too_Old error. This prevents rollback attacks. Note that
this does not require synchronized clocks: the receiving peer uses
the storage time in the previous store, not its own clock.</t>
<t></t>
<t hangText="lifetime "></t>
<t>The validity period for the data, in seconds, starting from the
time of store.</t>
<t></t>
<t hangText="value "></t>
<t>The data value itself, as described in <xref
target="sec-kind-model"></xref>.</t>
<t></t>
<t hangText="signature "></t>
<t>A signature over the data value. <xref
target="sec-data-sig"></xref> describes the signature computation.
The element is formatted as described in <xref
target="sec-signature"></xref></t>
</list></t>
<!--
<t>
[TODO: this doesn't include the resource ID and kind to
avoid duplicating it in each value. It would make things
more self-contained, though.]
</t>
-->
<t>Each Resource-ID specifies a single location in the Overlay Instance.
However, each location may contain multiple StoredData values
distinguished by Kind-ID. The definition of a kind describes both the data
values which may be stored and the data model of the data. Some data
models allow multiple values to be stored under the same Kind-ID. Section
<xref target="sec-kind-model"></xref> describes the available data
models. Thus, for instance, a given Resource-ID might contain a
single-value element stored under Kind-ID X and an array containing
multiple values stored under Kind-ID Y.</t>
<section anchor="sec-data-sig" title="Data Signature Computation">
<t>Each StoredData element is individually signed. However, the
signature also must be self-contained and cover the Kind-ID and
Resource-ID even though they are not present in the StoredData
structure. The input to the signature algorithm is:</t>
<t><list>
<t>resource_id + kind + StoredData</t>
</list></t>
<t>Where these values are: <list style="hanging">
<t></t>
<t hangText="resource "></t>
<t>The resource ID where this data is stored.</t>
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID for this data.</t>
<t></t>
<t hangText="StoredData "></t>
<t>The contents of the stored data value, as described in the
previous sections, with the lifetime set to 0.</t>
</list></t>
<t>[OPEN ISSUE: Should we include the identity in the string that
forms the input to the signature algorithm?.]</t>
<t>Once the signature has been computed, the signature is represented
using a signature element, as described in <xref
target="sec-signature"></xref>.</t>
</section>
<section anchor="sec-kind-model" title="Data Models">
<t>The protocol currently defines the following data models:</t>
<t><list style="symbols">
<t>single value</t>
<t>array</t>
<t>dictionary</t>
</list></t>
<!-- TODO: EKR; add a kind in here? -->
<t>These are represented with the StoredDataValue structure:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { reserved(0), single_value(1), array(2),
dictionary(3), (255)} DataModel;
struct {
Boolean exists;
opaque value<0..2^32-1>;
} DataValue;
struct {
select (DataModel) {
case single_value:
DataValue single_value_entry;
case array:
ArrayEntry array_entry;
case dictionary:
DictionaryEntry dictionary_entry;
/* This structure may be extended */
} ;
} StoredDataValue;
]]></artwork>
</figure>
<t>We now discuss the properties of each data model in turn:</t>
<section title="Single Value">
<t>A single-value element is a simple, opaque sequence of bytes.
There may be only one single-value element for each Resource-ID,
Kind-ID pair.</t>
<t>A single value element is represented as a DataValue, which
contains the following two elements:</t>
<t><list style="hanging">
<t hangText="exists"></t>
<t>This value indicates whether the value exists at all. If it
is set to False, it means that no value is present. If it is
True, that means that a value is present. This gives the
protocol a mechanism for indicating nonexistence as opposed to
emptiness.</t>
<t></t>
<t hangText="value"></t>
<t>The stored data.</t>
</list></t>
</section>
<section title="Array">
<t>An array is a set of opaque values addressed by an integer index.
Arrays are zero based. Note that arrays can be sparse. For instance, a
Store of "X" at index 2 in an empty array produces an array with the
values [ NA, NA, "X"]. Future attempts to fetch elements at index 0 or
1 will return values with "exists" set to False.</t>
<t>A array element is represented as an ArrayEntry:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint32 index;
DataValue value;
} ArrayEntry;
]]></artwork>
</figure>
<t>The contents of this structure are: <list style="hanging">
<t></t>
<t hangText="index"></t>
<t>The index of the data element in the array.</t>
<t></t>
<t hangText="value"></t>
<t>The stored data.</t>
</list></t>
<!-- EKR: do something about (-1) and array stores. -->
</section>
<section title="Dictionary">
<t>A dictionary is a set of opaque values indexed by an opaque key
with one value for each key. A single dictionary entry is
represented as follows</t>
<t>A dictionary element is represented as a DictionaryEntry:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
typedef opaque DictionaryKey<0..2^16-1>;
struct {
DictionaryKey key;
DataValue value;
} DictionaryEntry;
]]></artwork>
</figure>
<t>The contents of this structure are: <list style="hanging">
<t></t>
<t hangText="key"></t>
<t>The dictionary key for this value.</t>
<t></t>
<t hangText="value"></t>
<t>The stored data.</t>
</list></t>
</section>
</section>
<section anchor="sec.access_control" title="Access Control Policies">
<t>Every kind which is storable in an overlay MUST be associated with
an access control policy. This policy defines whether a request from a
given node to operate on a given value should succeed or fail. It is
anticipated that only a small number of generic access control
policies are required. To that end, this section describes a small set
of such policies and <xref target="sec.iana.access_control"></xref>
establishes a registry for new policies if required. Each policy has a
short string identifier which is used to reference it in the
configuration document.</t>
<section title="USER-MATCH">
<t>In the USER-MATCH policy, a given value MUST be written (or
overwritten) if and only if the request is signed with a key
associated with a certificate whose user name hashes (using the hash
function for the overlay) to the Resource-ID for the resource.
Recall that the certificate may, depending on the overlay
configuration, be self-signed.</t>
</section>
<section title="NODE-MATCH">
<t>In the NODE-MATCH policy, a given value MUST be written (or
overwritten) if and only if the request is signed with a key
associated with a certificate whose Node-ID hashes (using the hash
function for the overlay) to the Resource-ID for the resource.</t>
</section>
<section title="USER-NODE-MATCH">
<t>The USER-NODE-MATCH policy may only be used with dictionary
types. In the USER-NODE-MATCH policy, a given value MUST be written
(or overwritten) if and only if the request is signed with a key
associated with a certificate whose user name hashes (using the hash
function for the overlay) to the Resource-ID for the resource. In
addition, the dictionary key MUST be equal to the Node-ID in the
certificate.</t>
</section>
<section title="NODE-MULTIPLE">
<t>In the NODE-MULTIPLE policy, a given value MUST be written (or
overwritten) if and only if the request is signed with a key
associated with a certificate containing a Node-ID such that
H(Node-ID || i) is equal to the Resource-ID for some small integer
value if i. When this policy is in use, the maximum value of i MUST
be specified in the XML that defines the kind.</t>
</section>
<!--
<section title="USER-MATCH-WITH-ANONYMOUS-CREATE">
<t>The USER-MATCH-WITH-ANONYMOUS-CREATE policy is like the
USER-MATCH policy except that any user can create a new value in a
given location. However, only a user matching the USER-MATCH
criteria may overwrite an existing value. This allows the creation
of an anonymous "drop box" which may be useful for applications like
voice mail.</t>
</section>
-->
<!--
<section title="FIRST-USER-WITH-ANONYMOUS-CREATE">
<t>
The FIRST-USER-WITH-ANONYMOUS-CREATE policy allows any user to store
data but once a given user has stored a value, only that user can
overwrite the value. Upon receiving a request to store, a node
checks if it already has an value present for the particular kind
and Resource ID. If it does not have a value with this Resource-ID
and Kind-ID, the store is allowed. In the case that it does have a
value, the overwrite MUST be allowed if, and only if, the request is
signed with a certificate which contains a user name that matches
the user name that stored the existing value.
</t>
</section>
<section title="FIRST-NODE-WITH-ANONYMOUS-CREATE">
<t>
The FIRST-NODE-WITH-ANONYMOUS-CREATE policy allows any node to store
data but once a given node has stored a value, only that node can
overwrite the value. Upon receiving a request to store, a node
checks if it already has an value present for the particular kind
and Resource ID. If it does not have a value with this Resource-ID
and Kind-ID, the store is allowed. In the case that it does have a
value, the overwrite MUST be allowed if, and only if, the request is
signed with a certificate which contains a Node-ID that matches
the Node-ID that stored the existing value.
</t>
</section>
-->
<!--
<t> <section title="USER-KEY-MATCH">
<t>
This policy can only be used for data models that have a key such as
Dictionary. In the USER-KEY-MATCH policy, a given value MUST be
written (or overwritten) if and only if the request is signed using
a certificate whose user name hashes (using the hash function for
the overlay) to the key used in the store.
</t>
</section>
<section title="NODE-KEY-MATCH">
<t>
This policy can only be used for data models that have a key such as
Dictionary. In the NODE-KEY-MATCH policy, a given value MUST be
written (or overwritten) if and only if the request is signed using
a certificate whose Node-ID hashes (using the hash function for the
overlay) to the key used in the store.
</t>
</section>
-->
</section>
<section title="Data Storage Methods">
<t>RELOAD provides several methods for storing and retrieving
data:</t>
<t><list style="symbols">
<t>Store values in the overlay</t>
<t>Fetch values from the overlay</t>
<t>Stat: get metadata about values in the overlay</t>
<t>Find the values stored at an individual peer</t>
</list></t>
<t>These methods are each described in the following sections.</t>
<section anchor="sec-store" title="Store">
<t>The Store method is used to store data in the overlay. The format
of the Store request depends on the data model which is determined
by the kind.</t>
<section anchor="sec-store-req" title="Request Definition">
<t>A StoreReq message is a sequence of StoreKindData values, each
of which represents a sequence of stored values for a given kind.
The same Kind-ID MUST NOT be used twice in a given store request.
Each value is then processed in turn. These operations MUST be
atomic. If any operation fails, the state MUST be rolled back to
before the request was received.</t>
<t>The store request is defined by the StoreReq structure:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
KindId kind;
uint64 generation_counter;
StoredData values<0..2^32-1>;
} StoreKindData;
struct {
ResourceId resource;
uint8 replica_number;
StoreKindData kind_data<0..2^32-1>;
} StoreReq;
]]></artwork>
</figure>
<t>A single Store request stores data of a number of kinds to a
single resource location. The contents of the structure are: <list
style="hanging">
<t></t>
<t hangText="resource "></t>
<t>The resource to store at.</t>
<t></t>
<t hangText="replica_number "></t>
<t>The number of this replica. When a storing peer saves
replicas to other peers each peer is assigned a replica number
starting from 1 and sent in the Store message. This field is
set to 0 when a node is storing its own data. This allows
peers to distinguish replica writes from original writes.</t>
<t></t>
<t hangText="kind_data "></t>
<t>A series of elements, one for each kind of data to be
stored.</t>
</list></t>
<t>If the replica number is zero, then the peer MUST check that it
is responsible for the resource and if not reject the request. If
the replica number is nonzero, then the peer MUST check that it
expects to be a replica for the resource and that the request
sender is consistent with being the responsible node (i.e., that
the receiving peer does not know of a better node) and if not
reject the request.</t>
<t>Each StoreKindData element represents the data to be stored for
a single Kind-ID. The contents of the element are: <list
style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID. Implementations SHOULD reject requests
corresponding to unknown kinds unless specifically configured
otherwise.</t>
<t></t>
<t hangText="generation "></t>
<t>The expected current state of the generation counter
(approximately the number of times this object has been
written, see below for details).</t>
<t></t>
<t hangText="values "></t>
<t>The value or values to be stored. This may contain one or
more stored_data values depending on the data model associated
with each kind.</t>
</list></t>
<t>The peer MUST perform the following checks:</t>
<t><list style="symbols">
<t>The kind_id is known and supported.</t>
<!-- EKR: REMOVED. this doesn't allow conflict resolution.
<t>The signature over the message is valid or (depending on
overlay policy) no signature is required.</t> -->
<t>The signatures over each individual data element (if any)
are valid. If this check fails, the request MUST be rejected
with an Error_Forbidden error.</t>
<t>Each element is signed by a credential which is authorized
to write this kind at this Resource-ID. If this check fails,
the request MUST be rejected with an Error_Forbidden
error.</t>
<t>For original (non-replica) stores, the peer MUST check that
if the generation-counter is non-zero, it equals the current
value of the generation-counter for this kind. This feature
allows the generation counter to be used in a way similar to
the HTTP Etag feature.</t>
<t>The storage time values are greater than that of any value
which would be replaced by this Store.</t>
<t>The size and number of the stored values is consistent with
the limits specified in the overlay configuration.</t>
</list></t>
<t>If all these checks succeed, the peer MUST attempt to store the
data values. For non-replica stores, if the store succeeds and the
data is changed, then the peer must increase the generation counter
by at least one. If there are multiple stored values in a single
StoreKindData, it is permissible for the peer to increase the
generation counter by only 1 for the entire Kind-ID, or by 1 or more
than one for each value. Accordingly, all stored data values must
have a generation counter of 1 or greater. 0 is used in the Store
request to indicate that the generation counter should be ignored
for processing this request, however the responsible peer should
increase the stored generation counter, and should return the
correct generation counter in the response.</t>
<t>For replica Stores, the peer MUST set the generation counter to
match the generation_counter in the message, and MUST NOT check
the generation counter against the current value. Replica Stores
MUST NOT use a generation counter of 0.</t>
<t>When a peer stores data previously stored by another node
(e.g., for replicas or topology shifts) it MUST adjust the
lifetime value downward to reflect the amount of time the value
was stored at the peer.</t>
<t>Unless otherwise specified by the usage, if a peer attempts to
store data previously stored by another node (e.g., for replicas
or topology shifts) and that store fails with either an
Error_Generation_Counter_Too_Low or an Error_Data_Too old error,
the peer MUST fetch the newer data from the the peer generating
the error and use that to replace its own copy. This rule allows
resynchronization after partitions heal.</t>
<t>The properties of stores for each data model are as follows:
<list style="hanging">
<t></t>
<t hangText="Single-value:"></t>
<t>A store of a new single-value element creates the element
if it does not exist and overwrites any existing value with
the new value.</t>
<t></t>
<t hangText="Array:"></t>
<t>A store of an array entry replaces (or inserts) the given
value at the location specified by the index. Because arrays are
sparse, a store past the end of the array extends it with
nonexistent values (exists=False) as required. A store at index
0xffffffff places the new value at the end of the array
regardless of the length of the array. The resulting StoredData
has the correct index value when it is subsequently fetched.</t>
<t></t>
<t hangText="Dictionary:"></t>
<t>A store of a dictionary entry replaces (or inserts) the
given value at the location specified by the dictionary
key.</t>
</list></t>
<t>The following figure shows the relationship between these
structures for an example store which stores the following values
at resource "1234"</t>
<t><list style="symbols">
<t>The value "abc" in the single value location for kind X</t>
<t>The value "foo" at index 0 in the array for kind Y</t>
<t>The value "bar" at index 1 in the array for kind Y</t>
</list></t>
<figure>
<artwork><![CDATA[
Store
resource=1234
replica_number = 0
/ \
/ \
StoreKindData StoreKindData
kind=X (Single-Value) kind=Y (Array)
generation_counter = 99 generation_counter = 107
| /\
| / \
StoredData / \
storage_time = xxxxxxx / \
lifetime = 86400 / \
signature = XXXX / \
| | |
| StoredData StoredData
| storage_time = storage_time =
| yyyyyyyy zzzzzzz
| lifetime = 86400 lifetime = 33200
| signature = YYYY signature = ZZZZ
| | |
StoredDataValue | |
value="abc" | |
| |
StoredDataValue StoredDataValue
index=0 index=1
value="foo" value="bar"
]]></artwork>
</figure>
</section>
<section title="Response Definition">
<t>In response to a successful Store request the peer MUST return
a StoreAns message containing a series of StoreKindResponse
elements containing the current value of the generation counter
for each Kind-ID, as well as a list of the peers where the data
will be replicated.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
KindId kind;
uint64 generation_counter;
NodeId replicas<0..2^16-1>;
} StoreKindResponse;
struct {
StoreKindResponse kind_responses<0..2^16-1>;
} StoreAns;
]]></artwork>
</figure>
<t>The contents of each StoreKindResponse are:</t>
<t><list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID being represented.</t>
<t></t>
<t hangText="generation "></t>
<t>The current value of the generation counter for that
Kind-ID.</t>
<t></t>
<t hangText="replicas "></t>
<t>The list of other peers at which the data was/will-be
replicated. In overlays and applications where the responsible
peer is intended to store redundant copies, this allows the
storing peer to independently verify that the replicas were in
fact stored by doing its own Stat. Note that the storing peer
is not require to perform this verification.</t>
</list></t>
<t>The response itself is just StoreKindResponse values packed
end-to-end.</t>
<t>If any of the generation counters in the request precede the
corresponding stored generation counter, then the peer MUST fail the
entire request and respond with a Error_Generation_Counter_Too_Low
error. The error_info in the ErrorResponse MUST be a StoreAns
response containing the correct generation counter for each kind and
empty replicas lists. For original (non-replica) stores, a node
which receives such an error SHOULD attempt to fetch the data and,
if the storage_time value is newer, replace its own data with that
newer data. This rule improves data consistency in the case of
partitions and merges.</t>
<t>If the data being stored is too large for the allowed limit by
the given usage, then the peer MUST fail the request and generate
an Error_Data_Too_Large error.</t>
<t>If any type of request tries to access a data kind that the
node does not know about, an Error_Unknown_Kind MUST be generated.
The error_info in the Error_Response is:</t>
<figure>
<artwork><![CDATA[
KindId unknown_kinds<2^8-1>;
]]></artwork>
</figure>
<t>Which lists all the kinds that were unrecognized.</t>
</section>
<section title="Removing Values">
<t>This version of RELOAD (unlike previous versions) does not have
an explicit Remove operation. Rather, values are Removed by
storing "nonexistent" values in their place. Each DataValue
contains a boolean value called "exists" which indicates whether a
value is present at that location. In order to effectively remove
a value, the owner stores a new DataValue with:</t>
<t><list>
<t>exists = false</t>
<t>value = {} (0 length)</t>
</list></t>
<t>Storing nodes MUST treat these nonexistent values the same way
they treat any other stored value, including overwriting the
existing value, replicating them, and aging them out as necessary
when lifetime expires. When a stored nonexistent value's lifetime
expires, it is simply removed from the storing node like any other
stored value expiration. Note that in the case of arrays and
dictionaries, this may create an implicit, unsigned "nonexistent"
value to represent a gap in the data structure. However, this value
isn't persistent nor is it replicated, it's simply synthesized by
the storing node.</t>
</section>
</section>
<section title="Fetch">
<t>The Fetch request retrieves one or more data elements stored at a
given Resource-ID. A single Fetch request can retrieve multiple
different kinds.</t>
<section title="Request Definition">
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
int32 first;
int32 last;
} ArrayRange;
struct {
KindId kind;
uint64 generation;
uint16 length;
select (model) {
case single_value: ; /* Empty */
case array:
ArrayRange indices<0..2^16-1>;
case dictionary:
DictionaryKey keys<0..2^16-1>;
/* This structure may be extended */
} model_specifier;
} StoredDataSpecifier;
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} FetchReq;
]]></artwork>
</figure>
<t>The contents of the Fetch requests are as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="resource "></t>
<t>The resource ID to fetch from.</t>
<t></t>
<t hangText="specifiers "></t>
<t>A sequence of StoredDataSpecifier values, each specifying
some of the data values to retrieve.</t>
</list></t>
<t>Each StoredDataSpecifier specifies a single kind of data to
retrieve and (if appropriate) the subset of values that are to be
retrieved. The contents of the StoredDataSpecifier structure are
as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID of the data being fetched. Implementations
SHOULD reject requests corresponding to unknown kinds unless
specifically configured otherwise.</t>
<t></t>
<t hangText="model "></t>
<t>The data model of the data. This must be checked against
the Kind-ID.</t>
<t></t>
<t hangText="generation "></t>
<t>The last generation counter that the requesting peer saw.
This may be used to avoid unnecessary fetches or it may be set
to zero.</t>
<t></t>
<t hangText="length "></t>
<t>The length of the rest of the structure, thus allowing
extensibility.</t>
<t></t>
<t hangText="model_specifier "></t>
<t>A reference to the data value being requested within the
data model specified for the kind. For instance, if the data
model is "array", it might specify some subset of the
values.</t>
</list></t>
<t>The model_specifier is as follows:</t>
<t><list style="symbols">
<t>If the data is of data model single value, the specifier is
empty.</t>
<t>If the data is of data model array, the specifier contains
of a list of ArrayRange elements, each of which contains two
integers. The first integer is the beginning of the range and
the second is the end of the range. 0 is used to indicate the
first element and 0xffffffff is used to indicate the final
element. The beginning of the range MUST be earlier in the
array then the end. The ranges MUST be non-overlapping.</t>
<t>If the data is of data model dictionary then the specifier
contains a list of the dictionary keys being requested. If no
keys are specified, than this is a wildcard fetch and all
key-value pairs are returned.</t>
</list></t>
<t>The generation-counter is used to indicate the requester's
expected state of the storing peer. If the generation-counter in
the request matches the stored counter, then the storing peer
returns a response with no StoredData values.</t>
<t>Note that because the certificate for a user is typically
stored at the same location as any data stored for that user, a
requesting peer which does not already have the user's certificate
should request the certificate in the Fetch as an
optimization.</t>
</section>
<section title="Response Definition">
<t>The response to a successful Fetch request is a FetchAns
message containing the data requested by the requester.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
KindId kind;
uint64 generation;
StoredData values<0..2^32-1>;
} FetchKindResponse;
struct {
FetchKindResponse kind_responses<0..2^32-1>;
} FetchAns;
]]></artwork>
</figure>
<t>The FetchAns structure contains a series of FetchKindResponse
structures. There MUST be one FetchKindResponse element for each
Kind-ID in the request.</t>
<t>The contents of the FetchKindResponse structure are as follows:
<list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>the kind that this structure is for.</t>
<t></t>
<t hangText="generation "></t>
<t>the generation counter for this kind.</t>
<t></t>
<t hangText="values "></t>
<t>the relevant values. If the generation counter in the request
matches the generation-counter in the stored data, then no
StoredData values are returned. Otherwise, all relevant data
values MUST be returned. A nonexistent value is represented with
"exists" set to False.</t>
</list></t>
<t>There is one subtle point about signature computation on
arrays. If the storing node uses the append feature (where the
index=0xffffffff), then the index in the StoredData that is returned
will not match that used by the storing node, which would break the
signature. In order to avoid this issue, the index value in array is
set to zero before the signature is computed. This implies that
malicious storing nodes can reorder array entries without being
detected. <!-- [[OPEN ISSUE: We've considered a number of alternate
designs here that would preserve security against this attack if the
storing node did not use the append feature. However, they are more
complicated for one or both sides. If this attack is considered
serious, we can introduce one of them.]] --></t>
</section>
</section>
<section title="Stat">
<t>The Stat request is used to get metadata (length, generation
counter, digest, etc.) for a stored element without retrieving the
element itself. The name is from the UNIX stat(2) system call which
performs a similar function for files in a filesystem. It also
allows the requesting node to get a list of matching elements
without requesting the entire element.</t>
<section title="Request Definition">
<t>The Stat request is identical to the Fetch request. It simply
specifies the elements to get metadata about.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} StatReq;
]]></artwork>
</figure>
</section>
<section title="Response Definition">
<t>The Stat response contains the same sort of entries that a
Fetch response would contain, however instead of containing the
element data it contains metadata.</t>
<!--begin-pdu-->
<figure>
<artwork><![CDATA[
struct {
Boolean exists;
uint32 value_length;
HashAlgorithm hash_algorithm;
opaque hash_value<0..255>;
} MetaData;
struct {
uint32 index;
MetaData value;
} ArrayEntryMeta;
struct {
DictionaryKey key;
MetaData value;
} DictionaryEntryMeta;
struct {
select (model) {
case single_value:
MetaData single_value_entry;
case array:
ArrayEntryMeta array_entry;
case dictionary:
DictionaryEntryMeta dictionary_entry;
/* This structure may be extended */
} ;
} MetaDataValue;
struct {
uint32 value_length;
uint64 storage_time;
uint32 lifetime;
MetaDataValue metadata;
} StoredMetaData;
struct {
KindId kind;
uint64 generation;
StoredMetaData values<0..2^32-1>;
} StatKindResponse;
struct {
StatKindResponse kind_responses<0..2^32-1>;
} StatAns;
]]></artwork>
</figure>
<t>The structures used in StatAns parallel those used in FetchAns: a
response consists of multiple StatKindResponse values, one for each
kind that was in the request. The contents of the StatKindResponse
are the same as those in the FetchKindResponse, except that the
values list contains StoredMetaData entries instead of StoredData
entries.</t>
<t>The contents of the StoredMetaData structure are the same as
the corresponding fields in StoredData except that there is no
signature field and the value is a MetaDataValue rather than a
StoredDataValue.</t>
<t>A MetaDataValue is a variant structure, like a StoredDataValue,
except for the types of each arm, which replace DataValue with
MetaData.</t>
<t>The only really new structure is MetaData, which has the
following contents: <list style="hanging">
<t></t>
<t hangText="exists"></t>
<t>Same as in DataValue</t>
<t></t>
<t hangText="value_length"></t>
<t>The length of the stored value.</t>
<t></t>
<t hangText="hash_algorithm"></t>
<t>The hash algorithm used to perform the digest of the
value.</t>
<t></t>
<t hangText="hash_value"></t>
<t>A digest of the value using hash_algorithm.</t>
</list></t>
</section>
</section>
<section title="Find">
<t>The Find request can be used to explore the Overlay Instance. A
Find request for a Resource-ID R and a Kind-ID T retrieves the
Resource-ID (if any) of the resource of kind T known to the target
peer which is closes to R. This method can be used to walk the
Overlay Instance by interactively fetching R_n+1=nearest(1 +
R_n).</t>
<section title="Request Definition">
<t>The FindReq message contains a series of Resource-IDs and
Kind-IDs identifying the resource the peer is interested in.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
ResourceID resource;
KindId kinds<0..2^8-1>;
} FindReq;
]]></artwork>
</figure>
<!-- find-request = Resource-ID 1*type-id -->
<t>The request contains a list of Kind-IDs which the Find is for,
as indicated below: <list style="hanging">
<t></t>
<t hangText="resource "></t>
<t>The desired Resource-ID</t>
<t></t>
<t hangText="kinds "></t>
<t>The desired Kind-IDs. Each value MUST only appear once.</t>
</list></t>
</section>
<section title="Response Definition">
<t>A response to a successful Find request is a FindAns message
containing the closest Resource-ID on the peer for each kind
specified in the request.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
KindId kind;
ResourceID closest;
} FindKindData;
struct {
FindKindData results<0..2^16-1>;
} FindAns;
]]></artwork>
</figure>
<t>If the processing peer is not responsible for the specified
Resource-ID, it SHOULD return a 404 error.</t>
<!-- EKR removed. It is confusing here.
<t>When each kind is defined, it can indicate if the kind is not
allowed to be used in a Find request. This would be done to help
achieve some types of security properties for the data stored in
that kind.</t>
-->
<t>For each Kind-ID in the request the response MUST contain a
FindKindData indicating the closest Resource-ID for that Kind-ID
unless the kind is not allowed to be used with Find in which case
a FindKindData for that Kind-ID MUST NOT be included in the
response. If a Kind-ID is not known, then the corresponding
Resource-ID MUST be 0. Note that different Kind-IDs may have
different closest Resource-IDs.</t>
<t>The response is simply a series of FindKindData elements, one
per kind, concatenated end-to-end. The contents of each element
are:</t>
<t><list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID.</t>
<t></t>
<t hangText="closest "></t>
<t>The closest resource ID to the specified resource ID. This
is 0 if no resource ID is known.</t>
</list></t>
<t>Note that the response does not contain the contents of the
data stored at these Resource-IDs. If the requester wants this, it
must retrieve it using Fetch.</t>
</section>
</section>
<section title="Defining New Kinds">
<t>There are two ways to define a new kind. The first is by writing
a document and registering the kind-id with IANA. This is the
preferred method for kinds which may be widely used and reused. The
second method is to simply define the kind and its parameters in the
configuration document using the section of kind-id space set aside
for private use. This method MAY be used to define ad hoc kinds in
new overlays.</t>
<t>However a kind is defined, the definition must include:</t>
<t><list style="symbols">
<t>The meaning of the data to be stored (in some textual
form).</t>
<t>The Kind-ID.</t>
<t>The data model (single value, array, dictionary, etc.)</t>
<t>The access control model.</t>
</list></t>
<t>In addition, when kinds are registered with IANA, each kind is
assigned a short string name which is used to refer to it in
configuration documents.</t>
<t>While each kind needs to define what data model is used for its
data, that does not mean that it must define new data models. Where
practical, kinds should use the existing data models. The intention
is that the basic data model set be sufficient for most
applications/usages.</t>
</section>
</section>
</section>
<section anchor="sec-store-usage" title="Certificate Store Usage">
<t>The Certificate Store usage allows a peer to store its certificate in
the overlay, thus avoiding the need to send a certificate in each
message - a reference may be sent instead.</t>
<t>A user/peer MUST store its certificate at Resource-IDs derived from
two Resource Names:</t>
<t><list style="symbols">
<t>The user name in the certificate.</t>
<t>The Node-ID in the certificate.</t>
</list></t>
<t>Note that in the second case the certificate is not stored at the
peer's Node-ID but rather at a hash of the peer's Node-ID. The intention
here (as is common throughout RELOAD) is to avoid making a peer
responsible for its own data.</t>
<t>A peer MUST ensure that the user's certificates are stored in the
Overlay Instance. New certificates are stored at the end of the list.
This structure allows users to store an old and new certificate that
both have the same Node-ID which allows for migration of certificates
when they are renewed.</t>
<t>This usage defines the following kind:</t>
<t><list style="hanging">
<t></t>
<t hangText="Name:">CERTIFICATE</t>
<t></t>
<t hangText="Data Model:">The data model for CERTIFICATE data is
array.</t>
<t></t>
<t hangText="Access Control:">NODE-MATCH.</t>
</list></t>
</section>
<section anchor="sec-turn-server" title="TURN Server Usage">
<t>The TURN server usage allows a RELOAD peer to advertise that it is
prepared to be a TURN server as defined in <xref
target="I-D.ietf-behave-turn"></xref>. When a node starts up, it joins
the overlay network and forms several connection in the process. If the
ICE stage in any of these connection return a reflexive address that is
not the same as the peers perceived address, then the peers is behind a
NAT and not an candidate for a TURN server. Additionally, if the peers
IP address is in the private address space range, then it is not a
candidate for a TURN server. Otherwise, the peer SHOULD assume it is a
potential TURN server and follow the procedures below.</t>
<t>If the node is a candidate for a TURN server it will insert some
pointers in the overlay so that other peers can find it. The overlay
configuration file specifies a turnDensity parameter that indicates how
many times each TURN server should record itself in the overlay.
Typically this should be set to the reciprocal of the estimate of what
percentage of peers will act as TURN servers. For each value, called d,
between 1 and turnDensity, the peer forms a Resource Name by
concatenating its Peer-ID and the value d. This Resource Name is hashed
to form a Resource-ID. The address of the peer is stored at that
Resource-ID using type TURN-SERVICE and the TurnServer object:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint8 iteration;
IpAddressAndPort server_address;
} TurnServer;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list style="hanging">
<t></t>
<t hangText="iteration"></t>
<t>the d value</t>
<t></t>
<t hangText="server_address"></t>
<t>the address at which the TURN server can be contacted.</t>
</list></t>
<t><list style="hanging">
<t hangText="Note:">Correct functioning of this algorithm depends
critically on having turnDensity be an accurate estimate of the true
density of TURN servers. If turnDensity is too high, then the
process of finding TURN servers becomes extremely expensive as
multiple candidate Resource-IDs must be probed.</t>
</list></t>
<t>Peers peers that provide this service need to support the TURN
extensions to STUN for media relay of both UDP and TCP traffic as
defined in <xref target="I-D.ietf-behave-turn"></xref> and <xref
target="RFC5382"></xref>.</t>
<t>[[OPEN ISSUE: This structure only works for TURN servers that have
public addresses. It may be possible to use TURN servers that are behind
well-behaved NATs by first ICE connecting to them. If we decide we want
to enable that, this structure will need to change to either be a
Peer-ID or include that as an option.]]</t>
<t>This usage defines the following kind to indicate that the a peer is
willing to act as a TURN server:</t>
<t><list style="hanging">
<t hangText="Name">TURN-SERVICE</t>
<t hangText="Data Model">The TURN-SERVICE kind stores a single value
for each Resource-ID.</t>
<t hangText="Access Control">NODE-MULTIPLE, with maximum iteration
counter 20.</t>
</list></t>
<t>Peers can find other servers by selecting a random Resource-ID and then
doing a Find request for the appropriate server type with that
Resource-ID. The Find request gets routed to a random peer based on the
Resource-ID. If that peer knows of any servers, they will be returned.
The returned response may be empty if the peer does not know of any
servers, in which case the process gets repeated with some other random
Resource-ID. As long as the ratio of servers relative to peers is not too
low, this approach will result in finding a server relatively quickly.</t>
</section>
<section anchor="sec-chord-algorithm" title="Chord Algorithm ">
<t>This algorithm is assigned the name chord-reload to indicate it is an
adaptation of the basic Chord DHT algorithm.</t>
<t>This algorithm differs from the originally presented Chord algorithm
<xref target="Chord"></xref>. It has been updated based on more recent
research results, implementation experience, and to adapt it to the
RELOAD protocol. A short list of differences: <list style="symbols">
<t>The original Chord algorithm specified a single predecessor and a
successor list be stored. chord-reload specifies at least three of
each. The predecessor sets help other neighbors learn their successor
list.</t>
<t>The original Chord specification and analysis called for
iterative routing. RELOAD specifies recursive routing. In addition
to the performance implications, the cost of NAT traversal dictates
recursive routing.</t>
<t>Finger table entries are indexed in opposite order. Original Chord
specifies finger[0] as the immediate successor of the peer.
chord-reload specifies finger[0] as the peer 180 degrees around the
ring from the peer. This change was made to simplify discussion and
implementation of variable sized finger tables. However, with either
approach no more than O(log N) entries should typically be stored in a
finger table.</t>
<t>The stabilize() and fix_fingers() algorithms in the original
Chord algorithm are merged into a single periodic process.
Stabilization is implemented slightly differently because of the
larger neighborhood, and fix_fingers is not as aggressive to reduce
load, nor does it search for optimal matches of the finger table
entries.</t>
<t>RELAOD uses a 128 bit hash instead of 160 bit hash as RELOAD is
not desinged to be used in networks with close to or more thatn
2^128 nodes.</t>
<t>RELOAD uses randomized finger entries as described in <xref
target="sec-finger-refresh"></xref>.</t>
</list></t>
<section title="Overview">
<t>The algorithm described here is a modified version of the Chord
algorithm. Each peer keeps track of a finger table and a neighbor
table. The neighbor table typically contains the 3 peers before this
peer and the 3 peers after it in the DHT ring. The first entry in the
finger table contains the peer half-way around the ring from this peer;
the second entry contains the peer that is 1/4 of the way around; the
third entry contains the peer that is 1/8th of the way around, and so
on. Fundamentally, the chord data structure can be thought of a
doubly-linked list formed by knowing the successors and predecessor
peers in the neighbor table, sorted by the Node-ID. As long as the
successor peers are correct, the DHT will return the correct result. The
pointers to the prior peers are kept to enable inserting of new peers
into the list structure. Keeping multiple predecessor and successor
pointers makes it possible to maintain the integrity of the data
structure even when consecutive peers simultaneously fail. The finger
table forms a skip list, so that entries in the linked list can be found
in O(log(N)) time instead of the typical O(N) time that a linked list
would provide.</t>
<t>A peer, n, is responsible for a particular Resource-ID k if k is
less than or equal to n and k is greater than p, where p is the peer
id of the previous peer in the neighbor table. Care must be taken when
computing to note that all math is modulo 2^128.</t>
</section>
<section title="Reactive vs Periodic Recovery">
<t>Open Issue: The algorithm currently presented in this section uses
reactive recovery when a neighbor is lost, that information is
immediately propagated. Research in DHT performance by Rhea et al.
indicates that this is not optimal in large-scale networks with churn
<xref target="handling-churn-usenix04"></xref>. Addressing this issue,
however, needs to take into account the small-scale requirements
placed on this algorithm. Because it is the mandatory DHT for RELOAD,
the algorithm described here is designed to meet two primary
challenges: <list style="symbols">
<t>Scale from small (ten or fewer) overlays on a LAN to global
overlays with millions of nodes</t>
<t>Simple to implement</t>
</list></t>
<t>One of the challenges these requirements entail is achieving
reasonable performance as the overlay scales without undue complexity.
We have two possibly conflicting concerns: <list style="symbols">
<t>A small-scale overlay may not be stable without reactive
recovery, because a single peer represents a large portion of the
overlay.</t>
<t>A large-scale overlay with significant churn may perform
poorly, both in terms of traffic volume and success rates, when
using reactive recovery.</t>
</list> As a result, multiple solutions have been proposed: <list
style="symbols">
<t>Identify one set of behaviors that achieves adequate
functionality as the overlay scales.</t>
<t>Add a parameter dictating the type of recovery used by peers in
the overlay, configuring the peers appropriately as they join the
overlay.</t>
<t>Make the algorithm adaptive, according to the size of the
overlay or the churn rates observed.</t>
</list></t>
<t>At IETF 72, the WG elected to defer a decision on the final choice
until data could be collected on the effectiveness of the strategies.
This section now presents options for both approaches. Further work is
needed to decide whether a single solution is appropriate or to
specify a method for choosing the appropriate option.</t>
</section>
<section title="Routing">
<t>The routing table is the union of the neighbor table and the finger
table.</t>
<t>If a peer is not responsible for a Resource-ID k, but is directly
connected to a node with Node-ID k, then it routes the message to that
node. Otherwise, it routes the request to the peer in the routing
table that has the largest Node-ID that is in the interval between the
peer and k. If no such node is found, it finds the smallest node id
that is greater than k and routes the message to that node.</t>
</section>
<section title="Redundancy ">
<t>When a peer receives a Store request for Resource-ID k, and it is
responsible for Resource-ID k, it stores the data and returns a
success response. It then sends a Store request to its successor in
the neighbor table and to that peers successor. Note that these Store
requests are addressed to those specific peers, even though the
Resource-ID they are being asked to store is outside the range that
they are responsible for. The peers receiving these check they came
from an appropriate predecessor in their neighbor table and that they
are in a range that this predecessor is responsible for, and then they
store the data. They do not themselves perform further Stores because
they can determine that they are not responsible for the
Resource-ID.</t>
<t>The sequential replicas used in this overlay algorithm protect
against peer failure but not against malicious peers. Additional
replication from the Usage is required to protect resources from such
attacks, as discussed in <xref
target="sec-residual-attacks"></xref>.</t>
</section>
<section title="Joining">
<t>The join process for a joining party (JP) with Node-ID n is as
follows.</t>
<t><list style="numbers">
<t>JP MUST connect to its chosen bootstrap node.</t>
<t>JP SHOULD use a series of Pings to populate its routing
table.</t>
<t>JP SHOULD send Attach requests to initiate connections to each
of the peers in the neighbor table as well as to the desired 16
finger table entries. Note that this does not populate their
routing tables, but only their connection tables, so JP will not
get messages that it is expected to route to other nodes.</t>
<t>JP MUST enter all the peers it contacted into its routing
table.</t>
<t>JP SHOULD send a Join to its immediate successor, the admitting
peer (AP) for Node-ID n. The AP sends the response to the
Join.</t>
<t>AP MUST do a series of Store requests to JP to store the data
that JP will be responsible for.</t>
<t>AP MUST send JP an Update explicitly labeling JP as its
predecessor. At this point, JP is part of the ring and responsible
for a section of the overlay. AP can now forget any data which is
assigned to JP and not AP.</t>
<t>AP MUST send an Update to all of its neighbors with the new
values of its neighbor set (including JP).</t>
<t>JP SHOULD sends Updates to all the peers in its routing
table.</t>
</list></t>
<t>In order to populate its neighbor table, JP sends a Ping via the
bootstrap node directed at Resource-ID n+1 (directly after its own
Resource-ID). This allows it to discover its own successor. Call that
node p0. It then sends a ping to p0+1 to discover its successor (p1).
This process can be repeated to discover as many successors as
desired. The values for the two peers before p will be found at a later
stage when n receives an Update.</t>
<t>In order to set up its neighbor table entry for peer i, JP simply
sends an Attach to peer (n+2^(numBitsInNodeId-i). This will be routed
to a peer in approximately the right location around the ring.</t>
<t>The joining peer MUST NOT send any Update message placing itself in
the overlay until it has successfully completed an Attach with each
peer that should be in its neighbor table.</t>
</section>
<section title="Routing Attaches">
<t>When a peer needs to Attach to a new peer in its neighbor table, it
MUST source-route the Attach request through the peer from which it
learned the new peer's Node-ID. Source-routing these requests allows
the overlay to recover from instability.</t>
<t>All other Attach requests, such as those for new finger table
entries, are routed conventionally through the overlay.</t>
<t>If a peer is unable to successfully Attach with a peer that should
be in its neighborhood, it MUST locate either a TURN server or another
peer in the overlay, but not in its neighborhood, through which it can
exchange messages with its neighbor peer.</t>
</section>
<section title="Updates">
<t>A chord Update is defined as</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { reserved (0),
peer_ready(1), neighbors(2), full(3), (255) }
ChordUpdateType;
struct {
uint32 uptime;
ChordUpdateType type;
select(type){
case peer_ready: /* Empty */
;
case neighbors:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
case full:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
NodeId fingers<0..2^16-1>;
};
} ChordUpdate;
]]></artwork>
</figure>
<t>The "type" field contains the type of the update, which depends on
the reason the update was sent.</t>
<t><list style="hanging">
<t hangText="uptime: ">time this peer has been up in seconds.</t>
<t></t>
<t hangText="peer_ready: ">this peer is ready to receive messages.
This message is used to indicate that a node which has Attached is
a peer and can be routed through. It is also used as a
connectivity check to non-neighbor peers.</t>
<t></t>
<t hangText="neighbors: ">this version is sent to members of the
Chord neighbor table.</t>
<t></t>
<t hangText="full: ">this version is sent to peers which request
an Update with a RouteQueryReq.</t>
</list></t>
<t>If the message is of type "neighbors", then the contents of the
message will be:</t>
<t><list style="hanging">
<t></t>
<t hangText="predecessors "></t>
<t>The predecessor set of the Updating peer.</t>
<t></t>
<t hangText="successors "></t>
<t>The successor set of the Updating peer.</t>
</list></t>
<t>If the message is of type "full", then the contents of the message
will be:</t>
<t><list style="hanging">
<t></t>
<t hangText="predecessors "></t>
<t>The predecessor set of the Updating peer.</t>
<t></t>
<t hangText="successors "></t>
<t>The successor set of the Updating peer.</t>
<t></t>
<t hangText="fingers "></t>
<t>The finger table if the Updating peer, in numerically ascending
order.</t>
</list></t>
<t>A peer MUST maintain an association (via Attach) to every member of
its neighbor set. A peer MUST attempt to maintain at least three
predecessors and three successors. However, it MUST send its entire
set in any Update message sent to neighbors.</t>
<section anchor="sec-neighbor-failure"
title="Handling Neighbor Failures">
<t>Every time a connection to a peer in the neighbor table is lost (as
determined by connectivity pings or failure of some request), the peer
MUST remove the entry from its neighbor table and replace it with the
best match it has from the other peers in its routing table. If using
reactive recovery, it then sends an immediate Update to all nodes in
its Connection Table<!--neighbor table-->. The update will contain all
the Node-IDs of the current entries of the table (after the failed one
has been removed). Note that when replacing a successor the peer
SHOULD delay the creation of new replicas for successor replacement
hold-down time (30 seconds) after removing the failed entry from its
neighbor table in order to allow a triggered update to inform it of a
better match for its neighbor table.</t>
<t>A peer MAY attempt to reestablish connectivity with a lost neighbor
either by waiting additional time to see if connectivity returns or by
actively routing a new ATTACH to the lost peer. Details for these
procedures are beyond the scope of this document. In no event does an
attempt to reestablish connectivity with a lost neighbor allow the
peer to remain in the neighbor table. Such a peer is returned to the
neighbor table once connectivity is reestablished.</t>
<t>If connectivity is lost to all three of the peers that follow this
peer in the ring, then this peer should behave as if it is joining the
network and use Pings to find a peer and send it a Join. If
connectivity is lost to all the peers in the finger table, this peer
should assume that it has been disconnected from the rest of the
network, and it should periodically try to join the DHT.</t>
</section>
<section title="Handling Finger Table Entry Failure">
<t>If a finger table entry is found to have failed, all references
to the failed peer are removed from the finger table and replaced
with the closest preceding peer from the finger table or neighbor
table.</t>
<t>If using reactive recovery, the peer initiates a search for a new
finger table entry as described below.</t>
</section>
<section title="Receiving Updates">
<t>When a peer, N, receives an Update request, it examines the
Node-IDs in the UpdateReq and at its neighbor table and decides if
this UpdateReq would change its neighbor table. This is done by
taking the set of peers currently in the neighbor table and
comparing them to the peers in the update request. There are three
major cases:</t>
<t><list style="symbols">
<t>The UpdateReq contains peers that would not change the
neighbor set because they match the neighbor table.</t>
<t>The UpdateReq contains peers closer to N than those in its
neighbor table.</t>
<t>The UpdateReq defines peers that indicate a neighbor table
further away from N than some of its neighbor table. Note that
merely receiving peers further away does not demonstrate this,
since the update could be from a node far away from N. Rather,
the peers would need to bracket N.</t>
</list></t>
<t>In the first case, no change is needed.</t>
<t>In the second case, N MUST attempt to Attach to the new peers and
if it is successful it MUST adjust its neighbor set accordingly.
Note that it can maintain the now inferior peers as neighbors, but
it MUST remember the closer ones.</t>
<t>The third case implies that a neighbor has disappeared, most
likely because it has simply been disconnected but perhaps because
of overlay instability. N MUST Ping the questionable peers to
discover if they are indeed missing and if so, remove them from its
neighbor table.</t>
<t>After any Pings and Attaches are done, if the neighbor table
changes and the peer is using reactive recovery, the peer sends an
Update request to each member of its Connection Table<!-- of its neighbors
that was in either the old table or the new table-->. These Update
requests are what ends up filling in the predecessor/successor
tables of peers that this peer is a neighbor to. A peer MUST NOT
enter itself in its successor or predecessor table and instead
should leave the entries empty.</t>
<t>If peer N which is responsible for a Resource-ID R discovers that
the replica set for R (the next two nodes in its successor set) has
changed, it MUST send a Store for any data associated with R to any
new node in the replica set. It SHOULD NOT delete data from peers
which have left the replica set.</t>
<t>When a peer N detects that it is no longer in the replica set for
a resource R (i.e., there are three predecessors between N and R),
it SHOULD delete all data associated with R from its local
store.</t>
</section>
<section title="Stabilization">
<t>There are four components to stabilization: <list style="numbers">
<t>exchange Updates with all peers in its neighbor table to
exchange state</t>
<t>search for better peers to place in its finger table</t>
<t>search to determine if the current finger table size is
sufficiently large</t>
<t>search to determine if the overlay has partitioned and needs
to recover</t>
</list></t>
<section title="Updating neighbor table">
<t>A peer MUST periodically send an Update request to every peer
in its Connection Table<!--neighbor table-->. The purpose of this
is to keep the predecessor and successor lists up to date and to
detect failed peers. The default time is about every ten minutes,
but the enrollment server SHOULD set this in the configuration
document using the "chord-reload-update-frequency" element
(denominated in seconds.) A peer SHOULD randomly offset these
Update requests so they do not occur all at once.</t>
</section>
<section anchor="sec-finger-refresh" title="Refreshing finger table">
<t>A peer MUST periodically search for new peers to replace
invalid (repeated) entries in the finger table. A finger table
entry i is valid if it is in the range [n+2^(numBitsInNodeId-i),
n+2^(numBitsInNodeId-(i-1))-2^(numBitsInNodeId-(i+1))]. Invalid
entries occur in the finger table when a previous finger table
entry has failed or when no peer has been found in that range.</t>
<t>A peer SHOULD NOT send Ping requests looking for new finger
table entries more often than the configuration element
"chord-reload-ping-frequency", which defaults to 3600 seconds (one
per hour).</t>
<t>Two possible methods for searching for new peers for the finger
table entries are presented:</t>
<!-- TODO: Is chord responsibility the smallest node
greater than X or greater than or equal to X? I.e.,
if there is a peer X, does it store values with
Resource-ID X?-->
<t>Alternative 1: A peer selects one entry in the finger table
from among the invalid entries Pings for a new peer for that
finger table entry. The selection SHOULD be exponentially weighted
to attempt to replace earlier (lower i) entries in the finger
table. A simple way to implement this selection is to search
through the finger table entries from i=0 and each time an invalid
entry is encountered sending a Ping to replace that entry with
probability 0.5.</t>
<t>Alternative 2: Every "chord-reload-ping-frequency" seconds, the
peer scans through its finger table and for each invalid finger
table entry i, sends a RouteQuery request for the ID
n+2^(numBitsInNodeId-i) to the closest preceding peer to that ID
in the routing table. The responses to these route queries are
used to identify the set of entries for which a new Ping is likely
to result in a valid entry: the responses that contain a peer not
currently in the finger table indicate a Ping may result in a new
valid entry for the finger table. The peer then selects from among
those candidates using an exponentially weighted probability as
above.</t>
<t>When searching for a better entry, the peer SHOULD send the
Ping to a Node-ID selected randomly from that range. Random
selection is preferred over a search for strictly spaced entries
to minimize the effect of churn on overlay routing <xref
target="minimizing-churn-sigcomm06"></xref>. An implementation or
subsequent specification MAY choose a method for selecting finger
table entries other than choosing randomly within the range. Any
such alternate methods SHOULD be employed only on finger table
stabilization and not for the selection of initial finger table
entries unless the alternative method is faster and imposes less
overhead on the overlay.</t>
<t>A peer MAY choose to keep connections to multiple peers that
can act for a a given finger table entry.</t>
</section>
<section title="Adjusting finger table size">
<t>If the finger table has less than 16 entries, the node SHOULD
attempt to discover more fingers to grow the size of the table to
16. The value 16 was chosen to ensure high odds of a node
maintaing connectivity to the overlay even with strange network
portions.</t>
<t>For many overlays, 16 finger table entreis will be enough but
for very large overlays, as an overlay grows, more than 16 entries
may be required in the finger table for efficient routing. An
implementation SHOULD be capable of increasing the number of
entries in the finger table to numBitsInNodeId entries.</t>
<t>Note to implementers: Although log(N) entries are all that are
required for optimal performance, careful implementation of
stabilization will result in no additional traffic being generated
when maintaining a larger finger table than log(N) entries.
Implementers are encouraged to make use of RouteQuery and algorithms
for determining where new finger table entries may be
found. Complete details of possible implementations are outside the
scope of this specification.</t>
<t>A simple approach to sizing the finger table is to ensure the
finger table is large enough to contain at least the final
successor in the peer's neighbor table.</t>
</section>
<section title="Detecting partitioning">
<t>To detect that a partitioning has occurred and to heal the
overlay, a peer P MUST periodically repeat the discovery process
used in the initial join for the overlay to locate an appropriate
bootstrap peer, B. <!-- If an overlay has multiple mechanisms for
discovery it should randomly select a method to locate a bootstrap
peer. --> P should then send a Ping for its own Node-ID routed
through B. If a response is received from a peer S', which is not
P's successor, then the overlay is partitioned and P should send a
Attach to S' routed through B, followed by an Update sent to S'.
(Note that S' may not be in P's neighbor table once the overlay is
healed, but the connection will allow S' to discover appropriate
neighbor entries for itself via its own stabilization.)</t>
<t>Future specifications may describe alternative mechanisms for
determining when to repeat the discovery process.</t>
</section>
</section>
</section>
<section title="Route Query">
<t>For this topology plugin, the RouteQueryReq contains no additional
information. The RouteQueryAns contains the single node ID of the next
peer to which the responding peer would have routed the request
message in recursive routing:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
NodeId next_id;
} ChordRouteQueryAns;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list
style="hanging">
<t></t>
<t hangText="next_peer "></t>
<t>The peer to which the responding peer would route the message
to in order to deliver it to the destination listed in the
request.</t>
</list></t>
<t>If the requester set the send_update flag, the responder SHOULD
initiate an Update immediately after sending the RouteQueryAns.</t>
</section>
<section title="Leaving">
<t>Peers SHOULD send a Leave request to all members of their neighbor
table prior to exiting the Overlay Instance. Any peer which receives a
Leave for a peer n in its neighbor set follows procedures as if it had
detected a peer failure as described in <xref
target="sec-neighbor-failure"></xref>.</t>
</section>
</section>
<section anchor="sec-enrollment" title="Enrollment and Bootstrap">
<section anchor="sec-configuration" title="Overlay Configuration">
<t>This specification defines a new content type
"application/p2p-overlay+xml" for an MIME entity that contains overlay
information. An example document is shown below.</t>
<figure>
<artwork><![CDATA[
<?xml version="1.0" encoding="UTF-8"?>
<overlay xmlns="urn:ietf:params:xml:ns:p2p:config-base"
xmlns:ext="urn:ietf:params:xml:ns:p2p:config-ext1"
xmlns:chord="urn:ietf:params:xml:ns:p2p:config-chord-128-2">
<configuration instance-name="overlay.example.org" sequence="22"
expiration="2002-10-10T07:00:00Z">
<attach-lite-permitted>false</attach-lite-permitted>
<bootstrap-peer>192.0.0.1:5678</bootstrap-peer>
<bootstrap-peer>192.0.2.2:6789</bootstrap-peer>
<initial-ttl> 30 </initial-ttl>
<clients-permitted>false</clients-permitted>
<turn-density> 10 </turn-density>
<max-message-size>4000</max-message-size>
<credential-server>https://example.org</credential-server>
<ext:example-extention> foo </ext:example-extention>
<multicast-bootstrap>192.0.0.3:5678</multicast-bootstrap>
<chord:ping-frequency>300</chord:ping-frequency>
<chord:update-frequency>400</chord:update-frequency>
<self-signed-permitted
digest="sha1">false</self-signed-permitted>
<shared-secret>asecret</shared-secret>
<topology-plugin>Chord-128-2-16</topology-plugin>
<root-cert>TODO</root-cert>
<required-kinds>
<kind-block>
<kind name="sip-registration">
<data-model>single</data-model>
<access-control>user-match</access-control>
<max-count>1</max-count>
<max-size>100</max-size>
</kind>
<kind-signature>
TODO BASE 64 encoded signature block
</kind-signature>
</kind-block>
<kind-block>
<kind id="2000">
<data-model>array</data-model>
<access-control>NODE-MULTIPLE</access-control>
<max-node-multiple>3</max-node-multiple>
<max-count>22</max-count>
<max-size>4</max-size>
<ext:example-kind-extention> 1
</ext:example-kind-extention>
</kind>
<kind-signature>
TODO BASE 64 encoded signature block
</kind-signature>
</kind-block>
</required-kinds>
<kind-signer> 47112162e84c69ba </kind-signer>
<kind-signer> 6eba45d31a900c06 </kind-signer>
</configuration>
<signature>TODO BASE 64 encoded signature block</signature>
</overlay>
]]></artwork>
</figure>
<t>The file MUST be a well formed XML document and it SHOULD contain
an encoding declaration in the XML declaration. If the charset
parameter of the MIME content type declaration is present and it is
different from the encoding declaration, the charset parameter takes
precedence. Every application conforming to this specification MUST
accept the UTF-8 character encoding to ensure minimal
interoperability. The namespace for the elements defined in this
specification is urn:ietf:params:xml:ns:p2p:config-base and
urn:ietf:params:xml:ns:p2p:config-chord-128-2".</t>
<t>The file can contain multiple "configuration" elements where each
one contains the configuration information for a different overlay.
Each "configuration" has the following attributes:</t>
<t><list style="hanging">
<t hangText="instance-name:">name of the overlay</t>
<t hangText="expiration:">time in future at which this overlay
configuration is not longer valid and needs to be retrieved
again</t>
<t hangText="sequence:">a monotonically increasing sequence number
between 1 and 2^32</t>
</list></t>
<t>Inside each overlay element, the following elements can occur:</t>
<t><list style="hanging">
<t hangText="topology-plugin">This element has an attribute called
algorithm-name that describes the overlay-algorithm being
used.</t>
<t hangText="root-cert ">This element contains a PEM encoded
X.509v3 certificates is a root trust anchor used to sign all
certificates in this overlay. There can be more than one root-cert
element.</t>
<t hangText="required-kinds ">This element indicates the kinds
that members must support. It has three attributes: <list
style="symbols">
<t>kind: either a string representing the kind (the name
registered to IANA) or an integer kind-id allocated out of
private space</t>
<t>max-count: the maximum number of values which members of
the overlay must support.</t>
<t>data-model: the data model to be used.</t>
<t>max-size: the maximum size of individual values.</t>
<t>access-control: the access control model to be used.</t>
<t>max-node-multiple: Only used when the access control is
NODE-MULTIPLE. This indicates the maximum value for the i
counter. This is an integer greater than 0.</t>
</list> All of these values MUST be provided. If the kind is
registered with IANA, the data-model and access-control attributes
MUST match those in the kind registration. For instance, the
example above indicates that members must support SIP-REGISTRATION
with a maximum of 10 values of up to 1000 bytes each. Multiple
required-kinds elements MAY be present.</t>
<t hangText="kinds-signature ">This signature is computed across
the prevision kind from the beginning of the first < of the
kind to the end of the last > of the kind in the same was as
the "signature element described later in this section.</t>
<t hangText="kinds-signer ">This contains a single Node-Id in
hexadecimal and indicates that the certificate with this Node-ID
is allowed to sign kinds. Identifying kinds-signer by Node-Id
instead of certificate allows the use of short lived certificates
without constantly having to provide an updated configuration
file.</t>
<t hangText="credential-server ">This element contains the URL at
which the credential server can be reached in a "url" element.
This URL MUST be of type "https:". More than one credential-server
element may be present.</t>
<t hangText="self-signed-permitted ">This element indicates
whether self-signed certificates are permitted. If it is set to
"true", then self-signed certificates are allowed, in which case
the credential-server and root-cert elements may be absent.
Otherwise, it SHOULD be absent, but MAY be set "false". This
element also contains an attribute "digest" which indicates the
digest to be used to compute the Node-ID. Valid values for this
parameter are "SHA-1" and "SHA-256". Implementations MUST support
both of these algorithms.</t>
<t hangText="bootstrap-peer ">This elements represents the address
of one of the bootstrap peers. It has an attribute called
"address" that represents the IP address (either IPv4 or IPv6,
since they can be distinguished) and an attribute called "port"
that represents the port. More than one bootstrap-peer element may
be present.</t>
<t hangText="turn-density ">This elements is a positive integer
that represents the approximate reciprocal of density of nodes
that can act as TURN servers. For example, if 10% of the nodes can
act as TURN servers, this would be set to 10. If it is not
present, the default value is 1.</t>
<t hangText="multicast-bootstrap ">This element represents the
address of a multicast address and port that may be used for
bootstrap and that peers SHOULD listen on to enable bootstrap. It
has an attributed called "address" that represents the IP address
and an attribute called "port" that represents the port. More than
one "multicast-bootstrap" element may be present.</t>
<t hangText="clients-permitted ">This element represents whether
clients are permitted or whether all nodes must be peers. If it is
set to "TRUE" or absent, this indicates that clients are
permitted. If it is set to "FALSE" then nodes MUST join as
peers.</t>
<t hangText="attach-lite-permitted ">This element represents
whether nodes are allowed to use the AttachLite request in this
overlay. If it is absent, it is treated as if it was set to
"FALSE".</t>
<t hangText="chord-reload-update-frequency ">The update frequency
for the Chord-reload topology plugin (see <xref
target="sec-chord-algorithm"></xref>).</t>
<t hangText="chord-reload-ping-frequency ">The ping frequency for
the Chord-reload topology plugin (see <xref
target="sec-chord-algorithm"></xref>).</t>
<t hangText="shared-secret">If shared secret mode is used, this
contains the shared secret.</t>
<t hangText="max-message-size">Maximum size in bytes of any
message in the overlay. If this value is not present, the default
is 5000.</t>
<t hangText="initial-ttl">Initial default TTL (time to live, see
<xref target="sec-forwarding-header"></xref>) for messages. If
this value is not present, the default is 100.</t>
</list></t>
<t>The configuration file is a binary file and can not be changed,
including whitespace changes or the signature will break. The
signature is computed by taking each configuration element and
starting form, and including, the first < at the start of
<configuration> up to and including the > in
</configuration> and treating this as a binary blob thats signed
using the standard SecurityBlock defined in <xref
target="sec-signature"></xref>. The SecurityBlock is base 64 encoded
using the base64 alphabet from RFC<xref target="RFC4648"></xref> and
put in the signature element following the configuration object in the
config file.</t>
<t>When a node receives a new configuration file, it MUST change its
configuration to meet the new requirements. This may require the node
to exit the DHT and re-join. If a node is not capable of supporting
the new requirements, it MUST exit the overlay. If some information
about a particular kind changes from what the node previously knew
about the kind (for example the max size), the new information in the
configuration files overrides any previously learned information. If
any kind data was signed by a node that is no longer allowed to sign
kinds, that kind MUST be discarded along with any stored information
of that kind.</t>
<section title="Relax NG Grammars">
<t>The grammar for the configuration data is:</t>
<t>TODO: Need to update to match the new XML layout for kinds.</t>
<figure>
<artwork><![CDATA[
namespace chord = "urn:ietf:params:xml:ns:p2p:config-chord-128-2"
namespace local = ""
default namespace p2pcf = "urn:ietf:params:xml:ns:p2p:config-base"
namespace rng = "http://relaxng.org/ns/structure/1.0"
anything =
(element * { anything }
| attribute * { text }
| text)*
foreign-elements = element *
- (p2pcf:* | local:* | chord:*) { anything }*
hostPort = text
start =
element p2pcf:overlay {
element configuration {
attribute instance-name { text },
attribute expiration { xsd:dateTime },
attribute sequence { xsd:long },
parameter
},
element signature {
attribute algorithm { signature-algorithm-type }?,
xsd:base64Binary
}?
}
signature-algorithm-type |= "rsa-sha1"
signature-algorithm-type |= "rsa-sha256"
parameter &= element topology-plugin { topology-plugin-type }
parameter &= element max-message-size { xsd:int }?
parameter &= element initial-ttl { xsd:int }?
parameter &= element root-cert { text }?
parameter &= element required-kinds { kinds* }
parameter &= element credential-server { xsd:anyURI }?
parameter &=
element self-signed-permitted {
attribute digest { self-signed-digest-type },
xsd:boolean
}?
self-signed-digest-type |= "sha1"
self-signed-digest-type |= "sha256"
parameter &=
element bootstrap-peer { hostPort
}+
parameter &=
element multicast-bootstrap { hostPort
}*
parameter &= element clients-permitted { xsd:boolean }?
parameter &= element attach-lite-permitted { xsd:boolean }?
parameter &= element shared-secret { xsd:string }?
parameter &= element turn-density { xsd:int }?
parameter &= foreign-elements*
kinds =
element kind {
(attribute name { kind-names }
| attribute id { xsd:int }),
kind-paramter
}
kind-paramter &= element max-count { xsd:int }
kind-paramter &= element max-size { xsd:int }
kind-paramter &= element data-model { data-model-type }
data-model-type |= "single"
data-model-type |= "array"
data-model-type |= "dictionary"
kind-paramter &= element access-control { access-control-type }
kind-paramter &= element max-node-muliple { xsd:int }
access-control-type |= "user-match"
access-control-type |= "node-match"
access-control-type |= "user-node-match"
access-control-type |= "node-multiple"
access-control-type |= "user-match-with-anon-create"
kind-paramter &= foreign-elements*
# Chord specific paramters
topology-plugin-type |= "Chord-128-2-16"
kind-names |= "sip-registration"
kind-names |= "turn-service"
parameter &= element chord:update-frequency { xsd:int }?
parameter &= element chord:ping-frequency { xsd:int }?
]]></artwork>
</figure>
</section>
</section>
<section anchor="sec-discovery"
title="Discovery Through Enrollment Server">
<t>When a node first enrolls in a new overlay, it starts with a
discovery process to find an enrollment server. Related work to the
approach used here is described in <xref
target="I-D.garcia-p2psip-dns-sd-bootstrapping"></xref> and <xref
target="I-D.matthews-p2psip-bootstrap-mechanisms"></xref>. Another
scheme for referencing overlays is described in <xref
target="I-D.hardie-p2poverlay-pointers"></xref>.</t>
<t>The node first determines the overlay name. This value is provided
by the user or some other out of band provisioning mechanism. The out
of band mechanisms may also provide an optional URL to the enrollment
server. If a URL to the enrollment server is not provided, the node
MUST do a DNS SRV query using a Service name of "p2psip_enroll" and a
protocol of tcp to find an enrollment server and form the URL by
appending a path of "/p2psip/ enroll" to the overlay name. For
example, if the overlay name was example.com, the URL would be
"https://example.com/p2psip/enroll".</t>
<t>Once an address and URL for the enrollment servers is determined,
the peer forms an HTTPS connection to that IP address. The certificate
MUST match the overlay name as described in <xref
target="RFC2818"></xref>. Then the node MUST fetch a new copy of the
configuration file. To do this, the peer performs a GET to the URL.
The result of the HTTP GET is an XML configuration file described
above, which replaces any previously learned configuration file for
this overlay.</t>
<t>For overlays that do not use an enrollment server, nodes obtain the
configuration information needed to join the overlay through some out
of band approach such an an XML configuration file sent over
email.</t>
</section>
<section anchor="sec-credentials" title="Credentials">
<t>If the configuration document contains a credential-server element,
credentials are required to join the Overlay Instance. A peer which
does not yet have credentials MUST contact the credential server to
acquire them.</t>
<t>RELOAD defines its own trivial certificate request protocol. We would
have liked to use an existing protocol, but were concerned about the
implementation burden of even the simplest of those protocols, such as
<xref target="RFC5272"></xref>) and <xref target="RFC5273"></xref>. Our
objective was to have a protocol which could be easily implemented in a
Web server which the operator did not control (e.g., in a hosted
service) and was compatible with the existing certificate handling
tooling as used with the Web certificate infrastructure. This means
accepting bare PKCS#10 requests and returning a single bare X.509
certificate. Although the MIME types for these objects are defined, none
of the existing protocols support exactly this model.</t>
<t>The certificate request protocol is performed over HTTPS. The
request is an HTTP POST with the following properties:</t>
<t><list style="symbols">
<t>If authentication is required, there is a URL parameter of
"password" and "username" containing the user's name and password
in the clear (hence the need for HTTPS)</t>
<t>The body is of content type "application/pkcs10", as defined in
<xref target="RFC2311"></xref>.</t>
<t>The Accept header contains the type "application/pkix-cert",
indicating the type that is expected in the response.</t>
</list></t>
<t>The credential server MUST authenticate the request using the
provided user name and password. If the authentication succeeds and
the requested user name is acceptable, the server and returns a
certificate. The SubjectAltName field in the certificate contains the
following values:</t>
<t><list style="symbols">
<t>One or more Node-IDs which MUST be cryptographically random
<xref target="RFC4086"></xref>. These MUST be chosen by the
credential server in such a way that they are unpredictable to the
requesting user. These are of type URI and MUST contain RELOAD
URIs as described in <xref target="sec-reload-uri"></xref> and
MUST contain a Destination list with a single entry of type
"node_id".</t>
<t>The names this user is allowed to use in the overlay, using
type rfc822Name.</t>
</list></t>
<t>The certificate is returned as type "application/pkix-cert", with
an HTTP status code of 200 OK. Certificate processing errors should be
treated as HTTP errors and have appropriate HTTP stats codes.</t>
<t>The client MUST check that the certificate returned was signed by
one of the certificates received in the "root-cert" list of the
overlay configuration data. The node then reads the certificate to
find the Node-IDs it can use.</t>
<!-- <section title="Credentials for HIP">
<t>When RELOAD is used with HIP, the certificates MUST be generated
so that: <list style="symbols">
<t>Each node is assigned a unique ORCHID.</t>
<t>The Node-ID can be uniquely determined from the ORCHID.</t>
</list> Because in general, ORCHIDs are shorter than Node-IDs,
this means that the ORCHIDS MUST be generated first and MUST be
cryptographically random in order to make the Node-IDs
cryptographically random. The mapping function used to produce the
Node-ID from the ORCHID MUST be the same as that used by the Overlay Instance to
produce Resource-IDs from Resource Names.</t>
<t>In addition to the usual attributes, when HIP is in use
certificates MUST contain a subjectAltName with an iPAddress value
containing the HIP ORCHID. This allows these certificates to be used
by the HIP peers during the HIP base exchange.</t>
</section>-->
<section title="Self-Generated Credentials">
<t>If the "self-signed-permitted" element is present and set to
"TRUE", then a node MUST generate its own self-signed certificate to
join the overlay. The self-signed certificate MAY contain any user
name of the users choice.</t>
<t>The Node-ID MUST be computed by applying the digest specified in
the self-signed-permitted element to the DER representation of the
user's public key (more specifically the subjectPublicKeyInfo) and
taking the high order bits. When accepting a self-signed
certificate, nodes MUST check that the Node-ID and public keys
match. This prevents Node-ID theft.</t>
<t>Once the node has constructed a self-signed certificate, it MAY
join the overlay. Before storing its certificate in the overlay
(<xref target="sec-store-usage"></xref>) it SHOULD look to see if
the user name is already taken and if so choose another user name.
Note that this only provides protection against accidental name
collisions. Name theft is still possible. If protection against name
theft is desired, then the enrollment service must be used.</t>
</section>
</section>
<section title="Searching for a Bootstrap Peer">
<t>If no cached bootstrap peers are available and the config file has
an broadcast-peers element, then the peer SHOULD send a Ping request
to the address and port found in the broadcast-peers element in the
configuration document. This MAY be a multicast or anycast address.
The Ping should use the wildcard Node-ID as the destination
Node-ID.</t>
<t>The responder peer that receives the Ping request SHOULD check that
the overlay name is correct and that the requester peer sending the
request has appropriate credentials for the overlay before responding
to the Ping request even if the response is only an error.</t>
</section>
<section title="Contacting a Bootstrap Peer">
<t>In order to join the overlay, the joining peer MUST contact a peer in
the overlay. Typically this means contacting the bootstrap peers, since
they are guaranteed to have public IP addresses (the overlay
configuration should not advertise them as bootstrap peers otherwise).
If the joining peer has cached a list of peers it has previously been
connected with in this overlay, as an optimization it MAY attempt to use
one or more of them as bootstrap peers before falling back to the
bootstrap peers listed in the configuration file.</t>
<t>When contacting a bootstrap peer, the joining peer sends a Ping
request to the bootstrap peer's known IP address with the destination
Node-ID set to the joining peer's Node-ID.</t>
<t>When the requester peer finally does receive a response from some
responding peer, it can note the Node-ID in the response and use this
Node-ID to start sending requests to join the Overlay Instance as
described in <xref target="sec-overlay-topology"></xref>.</t>
<t>After a peer has successfully joined the overlay network, it will
have direct connections to several peers. Some MAY be added to the
cached bootstrap peers list and used in future boots. Peers that are
not directly connected MUST NOT be cached. The suggested number of
peers to cache is 10. Algorithms for determining which peers to cache
are beyond the scope of this specification.</t>
</section>
</section>
<section anchor="sec-msgflow" title="Message Flow Example">
<t>In the following example, we assume that JP has formed a connection
to one of the bootstrap peers. JP then sends an Attach through that peer
to the admitting peer (AP) to initiate a connection. When AP responds,
JP and AP use ICE to set up a connection and then set up TLS.</t>
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach Dest=JP | | | | |
|---------------------------------------------------------->|
| | | | | | |
| | | | | | |
| | |Attach Dest=JP | | |
| | |<--------------------------------------|
| | | | | | |
| | | | | | |
| | |Attach Dest=JP | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
| | |AttachAns | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |AttachAns | | |
| | |-------------------------------------->|
| | | | | | |
| | | | | | |
|AttachAns | | | | |
|<----------------------------------------------------------|
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.............................| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>Once JP has connected to AP, it needs to populate its Routing Table.
In Chord, this means that it needs to populate its neighbor table and
its finger table. To populate its neighbor table, it needs the successor
of AP, NP. It sends an Attach to the Resource-IP AP+1, which gets routed
to NP. When NP responds, JP and NP use ICE and TLS to set up a
connection.</t>
<!--
I don't think this is right that we need a Ping - CJ
<t>[[TODO: there should be a Ping here before populating]]</t>
-->
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach AP+1 | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | |Attach AP+1 | |
| | | |-------->| | |
| | | | | | |
| | | | | | |
| | | |AttachAns | |
| | | |<--------| | |
| | | | | | |
| | | | | | |
|AttachAns | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|Attach | | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.......................................| | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>JP also needs to populate its finger table (for Chord). It issues a
Attach to a variety of locations around the overlay. The diagram below
shows it sending an Attach halfway around the Chord ring the JP +
2^127.</t>
<figure>
<artwork><![CDATA[
JP NP XX TP
| | | |
| | | |
| | | |
|Attach JP+2<<126 | |
|-------->| | |
| | | |
| | | |
| |Attach JP+2<<126 |
| |-------->| |
| | | |
| | | |
| | |Attach JP+2<<126
| | |-------->|
| | | |
| | | |
| | |AttachAns|
| | |<--------|
| | | |
| | | |
| |AttachAns| |
| |<--------| |
| | | |
| | | |
|AttachAns| | |
|<--------| | |
| | | |
| | | |
|TLS | | |
|.............................|
| | | |
| | | |
| | | |
| | | |
]]></artwork>
</figure>
<t>Once JP has a reasonable set of connections he is ready to take his
place in the DHT. He does this by sending a Join to AP. AP does a series
of Store requests to JP to store the data that JP will be responsible
for. AP then sends JP an Update explicitly labeling JP as its
predecessor. At this point, JP is part of the ring and responsible for a
section of the overlay. AP can now forget any data which is assigned to
JP and not AP.</t>
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|JoinReq | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|JoinAns | | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreReq Data A | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|StoreReq Data B | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>In Chord, JP's neighbor table needs to contain its own predecessors.
It couldn't connect to them previously because Chord has no way to route
immediately to your predecessors. However, now that it has received an
Update from AP, it has APs predecessors, which are also its own, so it
sends Attaches to them. Below it is shown connecting to its closest
predecessor, PP.</t>
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach Dest=PP | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | |Attach Dest=PP | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |AttachAns| | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
|AttachAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|...................| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|------------------>| | | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<------------------| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<--------------------------------------| | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>Finally, now that JP has a copy of all the data and is ready to route
messages and receive requests, it sends Updates to everyone in its
Routing Table to tell them it is ready to go. Below, it is shown sending
such an update to TP.</t>
<figure>
<artwork><![CDATA[
JP NP XX TP
| | | |
| | | |
| | | |
|Update | | |
|---------------------------->|
| | | |
| | | |
|UpdateAns| | |
|<----------------------------|
| | | |
| | | |
| | | |
| | | |
]]></artwork>
</figure>
</section>
<section title="Security Considerations">
<section title="Overview">
<t>RELOAD provides a generic storage service, albeit one designed to
be useful for P2PSIP. In this section we discuss security issues that
are likely to be relevant to any usage of RELOAD.</t>
<t>In any Overlay Instance, any given user depends on a number of
peers with which they have no well-defined relationship except that
they are fellow members of the Overlay Instance. In practice, these
other nodes may be friendly, lazy, curious, or outright malicious. No
security system can provide complete protection in an environment
where most nodes are malicious. The goal of security in RELOAD is to
provide strong security guarantees of some properties even in the face
of a large number of malicious nodes and to allow the overlay to
function correctly in the face of a modest number of malicious
nodes.</t>
<t>P2PSIP deployments require the ability to authenticate both peers
and resources (users) without the active presence of a trusted entity
in the system. We describe two mechanisms. The first mechanism is
based on public key certificates and is suitable for general
deployments. The second is an admission control mechanism based on an
overlay-wide shared symmetric key.</t>
</section>
<section title="Attacks on P2P Overlays">
<t>The two basic functions provided by overlay nodes are storage and
routing: some node is responsible for storing a peer's data and for
allowing a peer to fetch other peer's data. Some other set of nodes
are responsible for routing messages to and from the storing nodes.
Each of these issues is covered in the following sections.</t>
<t>P2P overlays are subject to attacks by subversive nodes that may
attempt to disrupt routing, corrupt or remove user registrations, or
eavesdrop on signaling. The certificate-based security algorithms we
describe in this draft are intended to protect overlay routing and
user registration information in RELOAD messages.</t>
<t>To protect the signaling from attackers pretending to be valid
peers (or peers other than themselves), the first requirement is to
ensure that all messages are received from authorized members of the
overlay. For this reason, RELOAD transports all messages over a secure
channel (TLS and DTLS are defined in this document) which provides
message integrity and authentication of the directly communicating
peer. In addition, messages and data are digitally signed with the
sender's private key, providing end-to-end security for
communications.</t>
</section>
<section title="Certificate-based Security">
<t>This specification stores users' registrations and possibly other
data in an overlay network. This requires a solution to securing this
data as well as securing, as well as possible, the routing in the
overlay. Both types of security are based on requiring that every
entity in the system (whether user or peer) authenticate
cryptographically using an asymmetric key pair tied to a
certificate.</t>
<t>When a user enrolls in the Overlay Instance, they request or are
assigned a unique name, such as "alice@dht.example.net". These names
are unique and are meant to be chosen and used by humans much like a
SIP Address of Record (AOR) or an email address. The user is also
assigned one or more Node-IDs by the central enrollment authority.
Both the name and the peer ID are placed in the certificate, along
with the user's public key.</t>
<t>Each certificate enables an entity to act in two sorts of
roles:</t>
<t><list style="symbols">
<t>As a user, storing data at specific Resource-IDs in the Overlay
Instance corresponding to the user name.</t>
<t>As a overlay peer with the peer ID(s) listed in the
certificate.</t>
</list></t>
<t>Note that since only users of this Overlay Instance need to
validate a certificate, this usage does not require a global PKI.
Instead, certificates are signed by require a central enrollment
authority which acts as the certificate authority for the Overlay
Instance. This authority signs each peer's certificate. Because each
peer possesses the CA's certificate (which they receive on enrollment)
they can verify the certificates of the other entities in the overlay
without further communication. Because the certificates contain the
user/peer's public key, communications from the user/peer can be
verified in turn.</t>
<t>If self-signed certificates are used, then the security provided is
significantly decreased, since attackers can mount Sybil attacks. In
addition, attackers cannot trust the user names in certificates
(though they can trust the Node-IDs because they are cryptographically
verifiable). This scheme may be appropriate for some small
deployments, such as a small office or ad hoc overlay set up among
participants in a meeting where all hosts on the network are trusted.
Some additional security can be provided by using the shared secret
admission control scheme as well.</t>
<t>Because all stored data is signed by the owner of the data the
storing peer can verify that the storer is authorized to perform a
store at that Resource-ID and also allows any consumer of the data to
verify the provenance and integrity of the data when it retrieves
it.</t>
<t>Note that RELOAD does not itself provide a revocation/status
mechanism (though certificates may of course include OCSP responder
information). Thus, certificate lifetimes should be chosen to balance
the compromise window versus the cost of certificate renewal. Because
RELOAD is already designed to operate in the face of some fraction of
malicious peers, this form of compromise is not fatal.</t>
<t>All implementations MUST implement certificate-based security.</t>
</section>
<section title="Shared-Secret Security">
<t>RELOAD also supports a shared secret admission control scheme that
relies on a single key that is shared among all members of the
overlay. It is appropriate for small groups that wish to form a
private network without complexity. In shared secret mode, all the
peers share a single symmetric key which is used to key TLS-PSK <xref
target="RFC4279"></xref> or TLS-SRP <xref target="RFC5054"></xref>
mode. A peer which does not know the key cannot form TLS connections
with any other peer and therefore cannot join the overlay.</t>
<t>One natural approach to a shared-secret scheme is to use a
user-entered password as the key. The difficulty with this is that in
TLS-PSK mode, such keys are very susceptible to dictionary attacks. If
passwords are used as the source of shared-keys, then TLS-SRP is a
superior choice because it is not subject to dictionary attacks.</t>
</section>
<section title="Storage Security">
<t>When certificate-based security is used in RELOAD, any given
Resource-ID/Kind-ID pair is bound to some small set of certificates.
In order to write data, the writer must prove possession of the
private key for one of those certificates. Moreover, all data is
stored signed by the certificate which authorized its storage. This
set of rules makes questions of authorization and data integrity -
which have historically been thorny for overlays - relatively
simple.</t>
<section title="Authorization">
<t>When a client wants to store some value, it first digitally signs
the value with its own private key. It then sends a Store request
that contains both the value and the signature towards the storing
peer (which is defined by the Resource Name construction algorithm
for that particular kind of value).</t>
<t>When the storing peer receives the request, it must determine
whether the storing client is authorized to store at this
Resource-ID/Kind-ID pair. Determining this requires comparing the
user's identity to the requirements of the access control model (see
<xref target="sec.access_control"></xref>). If it satisfies those
requirements the user is authorized to write, pending quota checks
as described in the next section.</t>
<t>For example, consider the certificate with the following
properties:</t>
<figure>
<artwork><![CDATA[
User name: alice@dht.example.com
Node-ID: 013456789abcdef
Serial: 1234
]]></artwork>
</figure>
<t>If Alice wishes to Store a value of the "SIP Location" kind, the
Resource Name will be the SIP AOR "sip:alice@dht.example.com". The
Resource-ID will be determined by hashing the Resource Name. Because
SIP Location uses the USER-NODE-MATCH policy, it first verifies that
the user name in the certificate hashes to the requested
Resource-ID. It then verifies that the node-id in the certificate
matches the dictionary key being used for the store. If both of these
checks succeed, the Store is authorized. Note that because the access
control model is different for different kinds, the exact set of
checks will vary.</t>
</section>
<section title="Distributed Quota">
<t>Being a peer in a Overlay Instance carries with it the
responsibility to store data for a given region of the Overlay
Instance. However, if clients were allowed to store unlimited amounts
of data, this would create unacceptable burdens on peers, as well as
enabling trivial denial of service attacks. RELOAD addresses this
issue by requiring configurations to define maximum sizes for each
kind of stored data. Attempts to store values exceeding this size MUST
be rejected (if peers are inconsistent about this, then strange
artifacts will happen when the zone of responsibility shifts and a
different peer becomes responsible for overlarge data). Because each
Resource-ID/Kind-ID pair is bound to a small set of certificates,
these size restrictions also create a distributed quota mechanism,
with the quotas administered by the central enrollment server.</t>
<t>Allowing different kinds of data to have different size
restrictions allows new usages the flexibility to define limits that
fit their needs without requiring all usages to have expansive
limits.</t>
</section>
<section title="Correctness">
<t>Because each stored value is signed, it is trivial for any
retrieving peer to verify the integrity of the stored value. Some more
care needs to be taken to prevent version rollback attacks. Rollback
attacks on storage are prevented by the use of store times and
lifetime values in each store. A lifetime represents the latest time
at which the data is valid and thus limits (though does not completely
prevent) the ability of the storing node to perform a rollback attack
on retrievers. In order to prevent a rollback attack at the time of
the Store request, we require that storage times be monotonically
increasing. Storing peers MUST reject Store requests with storage
times smaller than or equal to those they are currently storing. In
addition, a fetching node which receives a data value with a storage
time older than the result of the previous fetch knows a rollback has
occurred.</t>
</section>
<section anchor="sec-residual-attacks" title="Residual Attacks">
<t>The mechanisms described here provide a high degree of security,
but some attacks remain possible. Most simply, it is possible for
storing nodes to refuse to store a value (i.e., reject any request).
In addition, a storing node can deny knowledge of values which it
previously accepted. To some extent these attacks can be ameliorated
by attempting to store to/retrieve from replicas, but a retrieving
client does not know whether it should try this or not, since there is
a cost to doing so.</t>
<t>Although the certificate-based authentication scheme prevents a
single peer from being able to forge data owned by other peers.
Furthermore, although a subversive peer can refuse to return data
resources for which it is responsible it cannot return forged data
because it cannot provide authentication for such registrations.
Therefore parallel searches for redundant registrations can mitigate
most of the affects of a compromised peer. The ultimate reliability of
such an overlay is a statistical question based on the replication
factor and the percentage of compromised peers.</t>
<t>In addition, when a kind is multivalued (e.g., an array data
model), the storing node can return only some subset of the values,
thus biasing its responses. This can be countered by using single
values rather than sets, but that makes coordination between multiple
storing agents much more difficult. This is a trade off that must be
made when designing any usage.</t>
</section>
</section>
<section title="Routing Security">
<t>Because the storage security system guarantees (within limits) the
integrity of the stored data, routing security focuses on stopping the
attacker from performing a DOS attack on the system by misrouting
requests in the overlay. There are a few obvious observations to make
about this. First, it is easy to ensure that an attacker is at least a
valid peer in the Overlay Instance. Second, this is a DOS attack only.
Third, if a large percentage of the peers on the Overlay Instance are
controlled by the attacker, it is probably impossible to perfectly
secure against this.</t>
<section title="Background">
<t>In general, attacks on DHT routing are mounted by the attacker
arranging to route traffic through or two nodes it controls. In the
Eclipse attack <xref target="Eclipse"></xref> the attacker tampers
with messages to and from nodes for which it is on-path with respect
to a given victim node. This allows it to pretend to be all the nodes
that are reachable through it. In the Sybil attack <xref
target="Sybil"></xref>, the attacker registers a large number of nodes
and is therefore able to capture a large amount of the traffic through
the DHT.</t>
<t>Both the Eclipse and Sybil attacks require the attacker to be able
to exercise control over her peer IDs. The Sybil attack requires the
creation of a large number of peers. The Eclipse attack requires that
the attacker be able to impersonate specific peers. In both cases,
these attacks are limited by the use of centralized, certificate-based
admission control.</t>
</section>
<section title="Admissions Control">
<t>Admission to an RELOAD Overlay Instance is controlled by requiring
that each peer have a certificate containing its peer ID. The
requirement to have a certificate is enforced by using
certificate-based mutual authentication on each connection. (Note: the
following only applies when self-signed certificates are not used.)
Whenever a peer connects to another peer, each side automatically
checks that the other has a suitable certificate. These peer IDs are
randomly assigned by the central enrollment server. This has two
benefits:</t>
<t><list style="symbols">
<t>It allows the enrollment server to limit the number of peer
IDs issued to any individual user.</t>
<t>It prevents the attacker from choosing specific peer IDs.</t>
</list></t>
<t>The first property allows protection against Sybil attacks
(provided the enrollment server uses strict rate limiting policies).
The second property deters but does not completely prevent Eclipse
attacks. Because an Eclipse attacker must impersonate peers on the
other side of the attacker, he must have a certificate for suitable
peer IDs, which requires him to repeatedly query the enrollment server
for new certificates which only will match by chance. From the
attacker's perspective, the difficulty is that if he only has a small
number of certificates the region of the Overlay Instance he is
impersonating appears to be very sparsely populated by comparison to
the victim's local region.</t>
</section>
<section title="Peer Identification and Authentication">
<t>In general, whenever a peer engages in overlay activity that might
affect the routing table it must establish its identity. This happens
in two ways. First, whenever a peer establishes a direct connection to
another peer it authenticates via certificate-based mutual
authentication. All messages between peers are sent over this
protected channel and therefore the peers can verify the data origin
of the last hop peer for requests and responses without further
cryptography.</t>
<t>In some situations, however, it is desirable to be able to
establish the identity of a peer with whom one is not directly
connected. The most natural case is when a peer Updates its state. At
this point, other peers may need to update their view of the overlay
structure, but they need to verify that the Update message came from
the actual peer rather than from an attacker. To prevent this, all
overlay routing messages are signed by the peer that generated
them.</t>
<t>[OPEN ISSUE: this allows for replay attacks on requests. There are
two basic defenses here. The first is global clocks and loose
anti-replay. The second is to refuse to take any action unless you
verify the data with the relevant node. This issue is undecided.]</t>
</section>
<section title="Protecting the Signaling">
<t>The goal here is to stop an attacker from knowing who is signaling
what to whom. An attacker being able to observe the activities of a
specific individual is unlikely given the randomization of IDs and
routing based on the present peers discussed above. Furthermore,
because messages can be routed using only the header information, the
actual body of the RELOAD message can be encrypted during
transmission.</t>
<t>There are two lines of defense here. The first is the use of TLS or
DTLS for each communications link between peers. This provides
protection against attackers who are not members of the overlay. The
second line of defense is to digitally sign each message. This
prevents adversarial peers from modifying messages in flight, even if
they are on the routing path.</t>
</section>
<section title="Residual Attacks">
<t>The routing security mechanisms in RELOAD are designed to contain
rather than eliminate attacks on routing. It is still possible for an
attacker to mount a variety of attacks. In particular, if an attacker
is able to take up a position on the overlay routing between A and B
it can make it appear as if B does not exist or is disconnected. It
can also advertise false network metrics in attempt to reroute
traffic. However, these are primarily DoS attacks.</t>
<t>The certificate-based security scheme secures the namespace, but if
an individual peer is compromised or if an attacker obtains a
certificate from the CA, then a number of subversive peers can still
appear in the overlay. While these peers cannot falsify responses to
resource queries, they can respond with error messages, effecting a
DoS attack on the resource registration. They can also subvert routing
to other compromised peers. To defend against such attacks, a resource
search must still consist of parallel searches for replicated
registrations.</t>
</section>
</section>
</section>
<section title="IANA Considerations">
<t>This section contains the new code points registered by this
document. [NOTE TO IANA/RFC-EDITOR: Please replace RFC-AAAA with the RFC
number for this specification in the following list.]</t>
<section title="Port Registrations">
<t>[[Note to RFC Editor - this paragraph can be removed before
publication. ]] IANA has already allocated a port for the main peer to
peer protocol. This port has the name p2p-sip and the port number of
6084. The names of this port may need to be changed as this draft
progresses and if it does careful instructions will be needed to IANA to
ensure the final RFC and IANA registrations are in sync.</t>
<t>IANA will make the following port registration:</t>
<texttable>
<ttcol></ttcol>
<ttcol></ttcol>
<c>Registration Technical Contact</c>
<c>Cullen Jennings <fluffy@cisco.com></c>
<c>Registration Owner</c>
<c>IETF <iesg@ietf.org></c>
<c>Transport Protocol</c>
<c>TCP, UDP</c>
<c>Port Number</c>
<c>6084</c>
<c>Service Name</c>
<c>p2psip_enroll</c>
<c>Description</c>
<c>RELOAD P2P Protcol</c>
<c>Reference</c>
<c>[RFC-AAAA]</c>
</texttable>
</section>
<section title="Overlay Algorithm Types">
<t>IANA SHALL create a "RELOAD Overlay Algorithm Type" Registry.
Entries in this registry are strings denoting the names of overlay
algorithms. The registration policy for this registry is RFC 5226 IETF
Review. The initial contents of this registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Algorithm Name</ttcol>
<ttcol align="right">RFC</ttcol>
<c>chord-reload</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section anchor="sec.iana.access_control"
title="Access Control Policies">
<t>IANA SHALL create a "RELOAD Access Control Policy" Registry.
Entries in this registry are strings denoting access control policies,
as described in <xref target="sec.access_control"></xref>. New entries
in this registry SHALL be registered via RFC 5226 Standards Action.
The initial contents of this registry are:</t>
<t><list>
<t>USER-MATCH</t>
<t>NODE-MATCH</t>
<t>USER-NODE-MATCH</t>
<t>NODE-MULTIPLE</t>
<!--
<t>USER-MATCH-WITH-ANONYMOUS-CREATE</t>
<t>USER-KEY-MATCH</t>
<t>NODE-KEY-MATCH</t>
-->
</list></t>
</section>
<section title="Data Kind-ID">
<t>IANA SHALL create a "RELOAD Data Kind-ID" Registry. Entries in this
registry are 32-bit integers denoting data kinds, as described in
<xref target="sec-usages"></xref>. Code points in the range 0x00000001
to 0x7fffffff SHALL be registered via RFC 5226 Standards Action. Code
points in the range 0x8000000 to 0xf0000000 SHALL be registered via
RFC 5226 Expert Review. Code points in the range 0xf0000001 to
0xffffffff are reserved for private use via the kind description
mechanism described in <xref target="sec-enrollment"></xref>. The
initial contents of this registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Kind</ttcol>
<ttcol align="right">Kind-ID</ttcol>
<ttcol align="right">RFC</ttcol>
<c>INVALID</c>
<c>0</c>
<c>RFC-AAAA</c>
<!--
<c>SIP-REGISTRATION</c>
<c>1</c>
<c>RFC-AAAA</c>
-->
<c>TURN_SERVICE</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>CERTIFICATE</c>
<c>3</c>
<c>RFC-AAAA</c>
<!--
<c>ROUTING_TABLE_SIZE</c>
<c>4</c>
<c>RFC-AAAA</c>
<c>SOFTWARE_VERSION</c>
<c>5</c>
<c>RFC-AAAA</c>
<c>MACHINE_UPTIME</c>
<c>6</c>
<c>RFC-AAAA</c>
<c>APP_UPTIME</c>
<c>7</c>
<c>RFC-AAAA</c>
<c>MEMORY_FOOTPRINT</c>
<c>8</c>
<c>RFC-AAAA</c>
<c>DATASIZE_StoreD</c>
<c>9</c>
<c>RFC-AAAA</c>
<c>INSTANCES_StoreD</c>
<c>10</c>
<c>RFC-AAAA</c>
<c>MESSAGES_SENT_RCVD</c>
<c>11</c>
<c>RFC-AAAA</c>
<c>EWMA_BYTES_SENT</c>
<c>12</c>
<c>RFC-AAAA</c>
<c>EWMA_BYTES_RCVD</c>
<c>13</c>
<c>RFC-AAAA</c>
<c>LAST_CONTACT</c>
<c>14</c>
<c>RFC-AAAA</c>
<c>RTT</c>
<c>15</c>
<c>RFC-AAAA</c>
-->
<c>Reserved</c>
<c>0x7fffffff</c>
<c>RFC-AAAA</c>
<c>Reserved</c>
<c>0xffffffff</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Data Model">
<t>IANA SHALL create a "RELOAD Data Model" Registry. Entries in this
registry are 8-bit integers denoting data models, as described in
<xref target="sec-kind-model"></xref>. Code points in this registry
SHALL be registered via RFC 5226 Standards Action. The initial
contents of this registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Data Model</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">RFC</ttcol>
<c>INVALID</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>SINGLE_VALUE</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>ARRAY</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>DICTIONARY</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>RESERVED</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Message Codes">
<t>IANA SHALL create a "RELOAD Message Code" Registry. Entries in this
registry are 16-bit integers denoting method codes as described in
<xref target="sec-contents"></xref>. These codes SHALL be registered
via RFC 5226 Standards Action. The initial contents of this registry
are:</t>
<t></t>
<texttable>
<ttcol align="left">Message Code Name</ttcol>
<ttcol align="right">Code Value</ttcol>
<ttcol align="right">RFC</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>probe_req</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>probe_ans</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>attach_req</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>attach_ans</c>
<c>4</c>
<c>RFC-AAAA</c>
<c>unused</c>
<c>5</c>
<c></c>
<c>unused</c>
<c>6</c>
<c></c>
<c>store_req</c>
<c>7</c>
<c>RFC-AAAA</c>
<c>store_ans</c>
<c>8</c>
<c>RFC-AAAA</c>
<c>fetch_req</c>
<c>9</c>
<c>RFC-AAAA</c>
<c>fetch_ans</c>
<c>10</c>
<c>RFC-AAAA</c>
<c>remove_req</c>
<c>11</c>
<c>RFC-AAAA</c>
<c>remove_ans</c>
<c>12</c>
<c>RFC-AAAA</c>
<c>find_req</c>
<c>13</c>
<c>RFC-AAAA</c>
<c>find_ans</c>
<c>14</c>
<c>RFC-AAAA</c>
<c>join_req</c>
<c>15</c>
<c>RFC-AAAA</c>
<c>join_ans</c>
<c>16</c>
<c>RFC-AAAA</c>
<c>leave_req</c>
<c>17</c>
<c>RFC-AAAA</c>
<c>leave_ans</c>
<c>18</c>
<c>RFC-AAAA</c>
<c>update_req</c>
<c>19</c>
<c>RFC-AAAA</c>
<c>update_ans</c>
<c>20</c>
<c>RFC-AAAA</c>
<c>route_query_req</c>
<c>21</c>
<c>RFC-AAAA</c>
<c>route_query_ans</c>
<c>22</c>
<c>RFC-AAAA</c>
<c>ping_req</c>
<c>23</c>
<c>RFC-AAAA</c>
<c>ping_ans</c>
<c>24</c>
<c>RFC-AAAA</c>
<c>stat_req</c>
<c>25</c>
<c>RFC-AAAA</c>
<c>stat_ans</c>
<c>26</c>
<c>RFC-AAAA</c>
<c>attachlite_req</c>
<c>27</c>
<c>RFC-AAAA</c>
<c>attachlite_ans</c>
<c>28</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>0x8000..0xfffe</c>
<c>RFC-AAAA</c>
<c>error</c>
<c>0xffff</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section anchor="sec-iana-error-codes" title="Error Codes">
<t>IANA SHALL create a "RELOAD Error Code" Registry. Entries in this
registry are 16-bit integers denoting error codes. New entries SHALL
be defined via RFC 5226 Standards Action. The initial contents of this
registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Error Code Name</ttcol>
<ttcol align="right">Code Value</ttcol>
<ttcol align="right">RFC</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>Unused</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>Error_Forbidden</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>Error_Not_Found</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>Error_Request_Timeout</c>
<c>4</c>
<c>RFC-AAAA</c>
<c>Error_Generation_Counter_Too_Low</c>
<c>5</c>
<c>RFC-AAAA</c>
<c>Error_Incompatible_with_Overlay</c>
<c>6</c>
<c>RFC-AAAA</c>
<c>Error_Unsupported_Forwarding_Option</c>
<c>7</c>
<c>RFC-AAAA</c>
<c>Error_Data_Too_Large</c>
<c>8</c>
<c>RFC-AAAA</c>
<c>Error_Data_Too_Old</c>
<c>9</c>
<c>RFC-AAAA</c>
<c>Error_TTL_Exceeded</c>
<c>10</c>
<c>RFC-AAAA</c>
<c>Error_Message_Too_Large</c>
<c>11</c>
<c>RFC-AAAA</c>
<c>Error_Unknown_Kind</c>
<c>12</c>
<c>RFC-AAAA</c>
<c>Error_Unknown_Extension</c>
<c>13</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>0x8000..0xfffe</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<!-- <section title="Route Log Extension Types"> -->
<!-- <t>IANA SHALL create a "RELOAD Route Log Extension Type Registry." New -->
<!-- entries SHALL be defined via RFC 5226 Specification Required. The -->
<!-- initial contents of this registry are:</t> -->
<!-- <t></t> -->
<!-- <texttable> -->
<!-- <ttcol align="left">Route Log Extension Name</ttcol> -->
<!-- <ttcol align="right">Code</ttcol> -->
<!-- <ttcol align="right">Specification</ttcol> -->
<!-- <c>invalid</c> -->
<!-- <c>0</c> -->
<!-- <c>RFC-AAAA</c> -->
<!-- <c>reserved</c> -->
<!-- <c>255</c> -->
<!-- <c>RFC-AAAA</c> -->
<!-- </texttable> -->
<!-- </section> -->
<section anchor="sec-iana-transport" title="Transport Types">
<t>IANA shall create a "RELOAD Transport." New entries SHALL be
defined via RFC 5226 Standards Action. This registry SHALL be
initially populated with the following values:</t>
<t></t>
<texttable>
<ttcol align="left">Protocol</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">Specification</ttcol>
<c>reserved</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>UDP (DTLS over UDP)</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>TCP (TLS over TCP)</c>
<c>2</c>
<c>RFC-AAAA</c>
<!--
<c>tcp_test_mode</c>
<c>3</c>
<c>RFC-AAAA</c>
-->
<c>reserved</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Forwarding Options">
<t>IANA shall create a "Forwarding Option Registry". Entries in this
registry between 1 and 127 SHALL be defined via RFC 5226 Standards
Action. Entries in this registry between 128 and 254 SHALL be defined
via RFC 5226 Specification Required. This registry SHALL be initially
populated with the following values:</t>
<t></t>
<texttable>
<ttcol align="left">Forwarding Option</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">Specification</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Probe Information Types">
<t>IANA shall create a "RELOAD Probe Information Type Registry".
Entries in this registry SHALL be defined via RFC 5226 Standards
Action. This registry SHALL be initially populated with the following
values:</t>
<t></t>
<texttable>
<ttcol align="left">Probe Option</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">Specification</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>responsible_set</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>num_resources</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>uptime</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section anchor="sec-message-extensions" title="Message Extensions">
<t>IANA shall create a "RELOAD Extensions Registry". Entries in this
registry SHALL be defined via RFC 5226 Specification Required. This
registry SHALL be initially populated with the following values:</t>
<t></t>
<texttable>
<ttcol align="left">Extensions Name</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">Specification</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>0xFFFF</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section anchor="sec-reload-uri" title="reload: URI Scheme">
<t>This section describes the scheme for a reload: URI, which can be
used to refer to either:</t>
<t><list style="symbols">
<t>A peer.</t>
<t>A resource inside a peer.</t>
</list></t>
<t>The reload: URI is defined using a subset of the URI schema
specified in Appendix A of RFC 3986 <xref target="RFC3986"></xref> and
the associated URI Guidelines <xref target="RFC4395"></xref> per the
following ABNF syntax:</t>
<figure>
<artwork><![CDATA[
RELOAD-URI = "reload://" destination "@" overlay "/"
[specifier]
destination = 1 * HEXDIG
overlay = reg-name
specifier = 1*HEXDIG
]]></artwork>
</figure>
<t>The definitions of these productions are as follows:</t>
<t><list style="hanging">
<t hangText="destination: ">a hex-encoded Destination List
object.</t>
<t></t>
<t hangText="overlay: ">the name of the overlay.</t>
<t></t>
<t hangText="specifier :">a hex-encoded StoredDataSpecifier
indicating the data element.</t>
</list></t>
<t>If no specifier is present than this URI addresses the peer which
can be reached via the indicated destination list at the indicated
overlay name. If a specifier is present, then the URI addresses the
data value.</t>
<section title="URI Registration">
<t>The following summarizes the information necessary to register
the reload: URI.</t>
<t><list style="hanging">
<t hangText="URI Scheme Name: ">reload</t>
<t hangText="Status: ">permanent</t>
<t hangText="URI Scheme Syntax: ">see <xref
target="sec-reload-uri"></xref>.</t>
<t hangText="URI Scheme Semantics: ">The reload: URI is intended
to be used as a reference to a RELOAD peer or resource.</t>
<t hangText="Encoding Considerations: ">The reload: URI is not
intended to be human-readable text, therefore they are encoded
entirely in US-ASCII.</t>
<t
hangText="Applications/protocols that use this URI scheme: ">The
RELOAD protocol described in RFC-AAAA.</t>
<t>TBD for the rest of this template.</t>
</list></t>
</section>
</section>
</section>
<section title="Acknowledgments">
<t>This draft is a merge of the "REsource LOcation And Discovery
(RELOAD)" draft by David A. Bryan, Marcia Zangrilli and Bruce B.
Lowekamp, the "Address Settlement by Peer to Peer" draft by Cullen
Jennings, Jonathan Rosenberg, and Eric Rescorla, the "Security
Extensions for RELOAD" draft by Bruce B. Lowekamp and James Deverick,
the "A Chord-based DHT for Resource Lookup in P2PSIP" by Marcia
Zangrilli and David A. Bryan, and the Peer-to-Peer Protocol (P2PP) draft
by Salman A. Baset, Henning Schulzrinne, and Marcin Matuszewski. Thanks
to the authors of RFC 5389 for text included from that. Vidya Narayanan
provided many comments and imporvements.</t>
<t>Thanks to the many people who contributed including: Ted Hardie,
Michael Chen, Dan York, Das Saumitra, Brian Rosen, David Bryan, Michael
Chen, Dave Craig, Julian Cain.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.I-D.ietf-mmusic-ice"?>
<?rfc include="reference.RFC.5389"?>
<?rfc include="reference.I-D.ietf-behave-turn"?>
<?rfc include="reference.RFC.5273"?>
<?rfc include="reference.RFC.5272"?>
<?rfc include="reference.RFC.4279"?>
<?rfc include="reference.I-D.ietf-mmusic-ice-tcp"?>
<?rfc include="reference.RFC.5246"?>
<?rfc include="reference.RFC.4347"?>
<?rfc include="reference.RFC.5348"?>
<?rfc include="reference.RFC.4648"?>
<?rfc include="reference.RFC.2988"?>
<?rfc include="reference.RFC.3986"?>
<?rfc include="reference.RFC.4395"?>
</references>
<references title="Informative References">
<?rfc include="reference.I-D.baset-tsvwg-tcp-over-udp"?>
<?rfc include="reference.RFC.5201"?>
<?rfc include="reference.RFC.4828"?>
<?rfc include="reference.I-D.ietf-p2psip-concepts"?>
<?rfc include="reference.RFC.1122"?>
<?rfc include="reference.RFC.5382"?>
<?rfc include="reference.RFC.4145"?>
<?rfc include="reference.RFC.4571"?>
<?rfc include="reference.RFC.2818"?>
<?rfc include="reference.RFC.4086"?>
<?rfc include="reference.RFC.5054"?>
<?rfc include="reference.RFC.5280"?>
<?rfc include="reference.I-D.matthews-p2psip-bootstrap-mechanisms"?>
<?rfc include="reference.I-D.garcia-p2psip-dns-sd-bootstrapping"?>
<?rfc include="reference.I-D.pascual-p2psip-clients"?>
<?rfc include="reference.RFC.4787"?>
<?rfc include="reference.RFC.2311"?>
<?rfc include="reference.I-D.jiang-p2psip-sep"?>
<?rfc include="reference.I-D.hardie-p2poverlay-pointers"?>
<reference anchor="I-D.ietf-p2psip-sip">
<front>
<title>A SIP Usage for RELOAD</title>
<author fullname="Cullen Jennings" initials="C" surname="Jennings">
<organization></organization>
</author>
<author fullname="Bruce Lowekamp" initials="B" surname="Lowekamp">
<organization></organization>
</author>
<author fullname="Eric Rescorla" initials="E" surname="Rescorla">
<organization></organization>
</author>
<author fullname="Salman Baset" initials="S" surname="Baset">
<organization></organization>
</author>
<author fullname="Henning Schulzrinne" initials="H"
surname="Schulzrinne">
<organization></organization>
</author>
<date day="07" month="March" year="2009" />
<abstract>
<t>This document defines REsource LOcation And Discovery (RELOAD),
a peer-to-peer (P2P) signaling protocol for use on the Internet. A
P2P signaling protocol provides its clients with an abstract
storage and messaging service between a set of cooperating peers
that form the overlay network. RELOAD is designed to support a P2P
Session Initiation Protocol (P2PSIP) network, but can be utilized
by other applications with similar requirements by defining new
usages that specify the kinds of data that must be stored for a
particular application. RELOAD defines a security model based on a
certificate enrollment service that provides unique identities.
NAT traversal is a fundamental service of the protocol. RELOAD
also allows access from "client" nodes which do not need to route
traffic or store data for others.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-p2psip-sip-01" />
<format target="http://www.ietf.org/internet-drafts/draft-ietf-p2psip-sip-00.txt"
type="TXT" />
</reference>
<reference anchor="Sybil">
<front>
<title>The Sybil Attack</title>
<author fullname="John R. Douceur" initials="J. R."
surname="Douceur">
<organization>Microsoft Research</organization>
</author>
<date month="March" year="2002" />
</front>
<seriesInfo name="IPTPS" value="02" />
<format target="http://www.cs.rice.edu/Conferences/IPTPS02/101.pdf"
type="PDF" />
</reference>
<reference anchor="Eclipse">
<front>
<title>Eclipse Attacks on Overlay Networks: Threats and
Defenses</title>
<author fullname="Atul Singh" initials="A." surname="Singh">
<organization></organization>
</author>
<author fullname="Tsuen-Wan Ngan" initials="T. W." surname="Ngan">
<organization></organization>
</author>
<author fullname="Peter Druschel" initials="T." surname="Druschel">
<organization></organization>
</author>
<author fullname="Dan S. Wallach" initials="D. S." surname="Wallach">
<organization></organization>
</author>
<date month="April" year="2006" />
</front>
<seriesInfo name="INFOCOM" value="2006" />
</reference>
<reference anchor="non-transitive-dhts-worlds05">
<front>
<title>Non-Transitive Connectivity and DHTs</title>
<author initials="M.J." surname="Freedman">
<organization />
</author>
<author initials="K." surname="Lakshminarayanan">
<organization />
</author>
<author initials="S." surname="Rhea">
<organization />
</author>
<author initials="I." surname="Stoica">
<organization />
</author>
</front>
<seriesInfo name="" value="WORLDS'05" />
</reference>
<reference anchor="lookups-churn-p2p06">
<front>
<title>Analytical Study on Improving DHT Lookup Performance under
Churn</title>
<author initials="D." surname="Wu">
<organization />
</author>
<author initials="Y." surname="Tian">
<organization />
</author>
<author initials="K.-W." surname="Ng">
<organization />
</author>
</front>
<seriesInfo name="" value="IEEE P2P'06" />
</reference>
<reference anchor="bryan-design-hotp2p08">
<front>
<title>The Design of a Versatile, Secure P2PSIP Communications
Architecture for the Public Internet</title>
<author initials="D." surname="Bryan">
<organization />
</author>
<author initials="B." surname="Lowekamp">
<organization />
</author>
<author initials="M." surname="Zangrilli">
<organization />
</author>
</front>
<seriesInfo name="" value="Hot-P2P'08" />
</reference>
<reference anchor="opendht-sigcomm05">
<front>
<title>OpenDHT: A Public DHT and its Uses</title>
<author initials="S." surname="Rhea">
<organization />
</author>
<author initials="B." surname="Godfrey">
<organization />
</author>
<author initials="B." surname="Karp">
<organization />
</author>
<author initials="J." surname="Kubiatowicz">
<organization />
</author>
<author initials="S." surname="Ratnasamy">
<organization />
</author>
<author initials="S." surname="Shenker">
<organization />
</author>
<author initials="I." surname="Stoica">
<organization />
</author>
<author initials="H." surname="Yu">
<organization />
</author>
</front>
<seriesInfo name="" value="SIGCOMM'05" />
</reference>
<reference anchor="Chord">
<front>
<title>Chord: A Scalable Peer-to-peer Lookup Protocol for Internet
Applications</title>
<author fullname="Ian Stoica" initials="I." surname="Stoica">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="Robert Morris" initials="R." surname="Morris">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="David Liben-Nowell" initials="D."
surname="Liben-Nowell">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="David Karger" initials="D." surname="Karger">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="M. Frans Kaashoek" initials="M. Frans"
surname="Kaashoek">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="Frank Dabek" initials="F." surname="Dabek">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="Hari Balakrishnan" initials="H."
surname="Balakrishnan">
<organization>MIT Laboratory for Computer Science</organization>
</author>
</front>
<seriesInfo name="IEEE/ACM Transactions on Networking"
value="Volume 11, Issue 1, 17-32, Feb 2003" />
<format target="http://pdos.csail.mit.edu/chord/papers/paper-ton.pdf"
type="PDF" />
</reference>
<reference anchor="vulnerabilities-acsac04">
<front>
<title>Vulnerabilities and Security Threats in Structured
Peer-to-Peer Systems: A Quantitative Analysis</title>
<author initials="M." surname="Srivatsa">
<organization />
</author>
<author initials="L." surname="Liu">
<organization />
</author>
</front>
<seriesInfo name="" value="ACSAC 2004" />
</reference>
<reference anchor="handling-churn-usenix04">
<front>
<title>Handling Churn in a DHT</title>
<author initials="S." surname="Rhea">
<organization />
</author>
<author initials="D." surname="Geels">
<organization />
</author>
<author initials="T." surname="Roscoe">
<organization />
</author>
<author initials="J." surname="Kubiatowicz">
<organization />
</author>
</front>
<seriesInfo name="" value="USENIX 2004" />
<format target="http://www.srhea.net/papers/bamboo-usenix.pdf"
type="PDF" />
</reference>
<reference anchor="minimizing-churn-sigcomm06">
<front>
<title>Minimizing Churn in Distributed Systems</title>
<author initials="P. B. " surname="Godfrey">
<organization />
</author>
<author initials="S." surname="Shenker">
<organization />
</author>
<author initials="I." surname="Stoica">
<organization />
</author>
</front>
<seriesInfo name="" value="SIGCOMM 2006" />
<format target="http://www.cs.berkeley.edu/~pbg/churn.pdf" type="PDF" />
</reference>
</references>
<!--
<reference anchor="I-D.cheshire-dnsext-multicastdns">
<front>
<title>Multicast DNS</title>
<author fullname="Stuart Cheshire" initials="S" surname="Cheshire">
<organization></organization>
</author>
<author fullname="Marc Krochmal" initials="M" surname="Krochmal">
<organization></organization>
</author>
<date day="25" month="August" year="2006" />
</front>
<seriesInfo name="Internet-Draft"
value="draft-cheshire-dnsext-multicastdns-06" />
<format target="http://www.ietf.org/internet-drafts/draft-cheshire-dnsext-multicastdns-06.txt"
type="TXT" />
</reference>
<reference anchor="I-D.cheshire-dnsext-dns-sd">
<front>
<title>DNS-Based Service Discovery</title>
<author fullname="Marc Krochmal" initials="M" surname="Krochmal">
<organization></organization>
</author>
<author fullname="Stuart Cheshire" initials="S" surname="Cheshire">
<organization></organization>
</author>
<date day="28" month="August" year="2006" />
<abstract>
<t>This document describes a convention for naming and structuring
DNS resource records. Given a type of service that a client is
looking for, and a domain in which the client is looking for that
service, this convention allows clients to discover a list of
named instances of that desired service, using only standard DNS
queries. In short, this is referred to as DNS-based Service
Discovery, or DNS-SD.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft"
value="draft-cheshire-dnsext-dns-sd-04" />
<format target="http://www.ietf.org/internet-drafts/draft-cheshire-dnsext-dns-sd-04.txt"
type="TXT" />
</reference>
-->
<section anchor="sec-changes" title="Change Log">
<section title="Changes since draft-ietf-p2psip-reload-01">
<t><list style="symbols">
<t>Added the ability to introduce new kinds dynamically.</t>
<t>Added configuration file updating.</t>
<t>Major revisions to reliability and flow control algorithms.</t>
<t>Moved diagnostics out--they no go in a separate draft.</t>
<t>Removed REMOVE: you now store a "nonexistent" element.</t>
</list></t>
</section>
<section title="Changes since draft-ietf-p2psip-reload-00">
<t><list style="symbols">
<t>Split base protocol from combined draft into new draft.</t>
<t>Update architecture discussion to address concerns raised about
clarity of roles.</t>
<t>Moved extensive discussion of routing and client behaviors to
appendix.</t>
<t>Split Ping into Ping and Probe</t>
<t>Added AttachLite to provide way to implement ICE-Lite</t>
<t>added Stat call for retrieving meta-data</t>
<t>added discussion of periodic vs reactive recovery issue</t>
<t>changed finger table stabilization to prefer long-lived over
best-match</t>
<t>updated IANA considerations to be more complete</t>
<t>changed error codes from http-based</t>
</list></t>
</section>
<section title="Changes since draft-ietf-p2psip-base-00">
<t><list style="symbols">
<t>removed TUNNEL method</t>
<t>allow implementations more flexibility in picking finger table
entry and revise random range</t>
<t>decouple overlay configuration from enrollment server</t>
<t>add error for data too large</t>
<t>change architecture to overlay perspective from previous
revision and update terminology in document to match</t>
</list></t>
</section>
<section title="Changes since draft-ietf-p2psip-base-01">
<t><list style="symbols">
<t>reordered message routing section to clarify that other routing
algorithms are possible besides symmetric recursive.</t>
<t>clarified document IPR terms</t>
</list></t>
</section>
<section title="Changes since draft-ietf-p2psip-base-01a">
<t><list style="symbols">
<t>Fragment offset was too small to hold 2^24 bit messages so
fixed this from 16 bits to 32 bits.</t>
<t>Changed absolute times from seconds to milliseconds</t>
<t>Added error for messages over max size</t>
<t>Added error for TTL expired</t>
<t>Add time in response to PING</t>
<t>Clarified retransmission and fragmentation algorithm</t>
<t>Clarified acknowledgement tracking for congestion control</t>
</list></t>
</section>
<section title="Changes since draft-ietf-p2psip-base-02">
<t><list style="symbols">
<t>Rearranged forwarding header to fix alignment among other
issues</t>
<t>Removed route logging</t>
<t>Switched to binary ICE for Attach</t>
<t>ConfigUpdate improved</t>
<t>Change from close DTLS session on fragmentation attack to drop
fragments, indirect attack</t>
<t>Updates to trivial sender/receiver text</t>
<t>Updates to data model based on list discussion</t>
<t>Updates to chord overlay algorithm section</t>
<t>Added AppAttach and removed port number from Attach</t>
<t>Changed via-list to use shorter structure</t>
<t>Rewrote fragmentation</t>
<t>Moved AIMD and TFRC congestion control algorithms to appendix
until further wg effort decides direction there.</t>
</list></t>
</section>
</section>
<section anchor="sec-retran-aimd" title="AIMD Retransmission Scheme">
<t>This Appendix specifies an optional sender retransmission algorithm
with better performance that senders MAY implement and is based on the
theAIMD algorithm in TCP. The algorithm here is only the AIMD portion of
TCP. All other features are restricted to simplify the implementation,
i.e. no slow start (initial window is 1), no fast retransmission, and no
fast recovery.</t>
<t>AIMD extends stop and wait defined in <xref
target="sec-retran-stop-n-wait"></xref> by allowing multiple fragments to
be pending at the same time. The sender allows w unacknowledged fragments
to be outstanding at any given time. w is initially set to one. Every RTO
interval that no retransmissions occur, w is increased by one. When a loss
occurs, w is halved. After halving w, if there are more than w fragments
for which an ACK is pending, no further retransmissions of the most
recently initiated fragments are performed until they fit in the window w,
at which point they begin the retransmission algorithm again. w is held
fixed for one RTO. After that point, if additional retransmissions occur,
it will be halved again, otherwise it may be incremented after an
additional RTO without loss.</t>
<t>If w drops to one and the one pending fragment is not ACKed by the
other side after 5 requests are sent, the link is considered to have
failed. Otherwise, unACKed fragments are simply dropped.</t>
</section>
<section anchor="sec-retran-tfrc" title="TFRC Retransmission Scheme">
<t>This Appendix specifies an option TFRC (RFC5348) based scheme that can
be implemented in the Sender-Based Variant format with the same receiver
algorithm. This implementation requires the sender to maintain precise
timestamps in ms of the transmission time of each sequence number as well
as the segment sizes. That, combined with the ACKs (and that the ACKs are
not delayed) allows the sender to calculate the performance required by
TFRC, including information calculated by the receiver in the conventional
form of TFRC.</t>
<t>TFRC is used for congestion control. For reliability, an individual
fragment is retransmitted up to twice at RTO intervals, pending the
availability of room in the congestion window. If it is not ACKed after
another RTO following its last retransmission, it is dropped.</t>
</section>
<section anchor="sec-route-alt" title="Routing Alternatives">
<t>Significant discussion has been focused on the selection of a routing
algorithm for P2PSIP. This section discusses the motivations for
selection of symmetric recursive routing for RELOAD and describes the
extensions that would be required to support additional routing
algorithms.</t>
<section title="Iterative vs Recursive">
<t>Iterative routing has a number of advantages. It is easier to debug,
consumes fewer resources on intermediate peers, and allows the querying
peer to identify and route around misbehaving peers <xref
target="non-transitive-dhts-worlds05"></xref>. However, in the presence
of NATs iterative routing is intolerably expensive because a new
connection must be established for each hop (using ICE) <xref
target="bryan-design-hotp2p08"></xref>.</t>
<t>Iterative routing is supported through the Route_Query mechanism and
is primarily intended for debugging. It is also allows the querying peer
to evaluate the routing decisions made by the peers at each hop,
consider alternatives, and perhaps detect at what point the forwarding
path fails.</t>
</section>
<section title="Symmetric vs Forward response">
<t>An alternative to the symmetric recursive routing method used by
RELOAD is Forward-Only routing, where the response is routed to the
requester as if it is a new message initiating by the responder (in the
previous example, Z sends the response to A as if it were sending a
request). Forward-only routing requires no state in either the message
or intermediate peers.</t>
<t>The drawback of forward-only routing is that it does not work when
the overlay is unstable. For example, if A is in the process of joining
the overlay and is sending a Join request to Z, it is not yet reachable
via forward routing. Even if it is established in the overlay, if
network failures produce temporary instability, A may not be reachable
(and may be trying to stabilize its network connectivity via Attach
messages).</t>
<t>Furthermore, forward-only responses are less likely to reach the
querying peer than symmetric recursive because the forward path is
more likely to have a failed peer than the request path (which was
just tested to route the request) <xref
target="non-transitive-dhts-worlds05"></xref>.</t>
<t>An extension to RELOAD that supports forward-only routing but relies
on symmetric responses as a fallback would be possible, but due to the
complexities of determining when to use forward-only and when to
fallback to symmetric, we have chosen not to include it as an option at
this point.</t>
</section>
<section title="Direct Response">
<t>Another routing option is Direct Response routing, in which the
response is returned directly to the querying node. In the previous
example, if A encodes its IP address in the request, then Z can simply
deliver the response directly to A. In the absence of NATs or other
connectivity issues, this is the optimal routing technique.</t>
<t>The challenge of implementing direct response is the presence of
NATs. There are a number of complexities that must be addressed. In
this discussion, we will continue our assumption that A issued the
request and Z is generating the response.</t>
<t><list style="symbols">
<t>The IP address listed by A may be unreachable, either due to NAT
or firewall rules. Therefore, a direct response technique must
fallback to symmetric response <xref
target="non-transitive-dhts-worlds05"></xref>. The hop-by-hop ACKs
used by RELOAD allow Z to determine when A has received the message
(and the TLS negotiation will provide earlier confirmation that A is
reachable), but this fallback requires a timeout that will increase
the response latency whenever A is not reachable from Z.</t>
<t>Whenever A is behind a NAT it will have multiple candidate IP
addresses, each of which must be advertised to ensure connectivity,
therefore Z will need to attempt multiple connections to deliver the
response.</t>
<t>One (or all) of A's candidate addresses may route from Z to a
different device on the Internet. In the worst case these nodes may
actually be running RELOAD on the same port. Therefore, establishing
a secure connection to authenticate A before delivering the response
is absolutely necessary. This step diminishes the efficiency of
direct response because multiple roundtrips are required before the
message can be delivered.</t>
<t>If A is behind a NAT and does not have a connection already
established with Z, there are only two ways the direct response will
work. The first is that A and Z are both behind the same NAT, in
which case the NAT is not involved. In the more common case, when Z
is outside A's NAT, the response will only be received if A's NAT
implements endpoint-independent filtering. As the choice of
filtering mode conflates application transparency with security
<xref target="RFC4787"></xref>, and no clear recommendation is
available, the prevalence of this feature in future devices remains
unclear.</t>
</list></t>
<t>An extension to RELOAD that supports direct response routing but
relies on symmetric responses as a fallback would be possible, but due
to the complexities of determining when to use direct response and when
to fallback to symmetric, and the reduced performance for responses to
peers behind restrictive NATs, we have chosen not to include it as an
option at this point.</t>
</section>
<section title="Relay Peers">
<t><xref target="I-D.jiang-p2psip-sep">SEP</xref> has proposed
implementing a form of direct response by having A identify a peer, Q,
that will be directly reachable by any other peer. A uses Attach to
establish a connection with Q and advertises Q's IP address in the
request sent to Z. Z sends the response to Q, which relays it to A.
This then reduces the latency to two hops, plus Z negotiating a secure
connection to Q.</t>
<t>This technique relies on the relative population of nodes such as A
that require relay peers and peers such as Q that are capable of
serving as a relay peer. It also requires nodes to be able to identify
which category they are in. This identification problem has turned out
to be hard to solve and is still an open area of exploration.</t>
<t>An extension to RELOAD that supports relay peers is possible, but
due to the complexities of implementing such an alternative, we have
not added such a feature to RELOAD at this point.</t>
<t>A concept similar to relay peers, essentially choosing a relay peer
at random, has previously been suggested to solve problems of pairwise
non-transitivity <xref target="non-transitive-dhts-worlds05"></xref>,
but deterministic filtering provided by NATs make random relay peers
no more likely to work than the responding peer.</t>
</section>
<section title="Symmetric Route Stability">
<t>A common concern about symmetric recursive routing has been that one
or more peers along the request path may fail before the response is
received. The significance of this problem essentially depends on the
response latency of the overlay. An overlay that produces slow responses
will be vulnerable to churn, whereas responses that are delivered very
quickly are vulnerable only to failures that occur over that small
interval.</t>
<t>The other aspect of this issue is whether the request itself can be
successfully delivered. Assuming typical connection maintenance
intervals, the time period between the last maintenance and the request
being sent will be orders of magnitude greater than the delay between
the request being forwarded and the response being received. Therefore,
if the path was stable enough to be available to route the request, it
is almost certainly going to remain available to route the response.</t>
<t>An overlay that is unstable enough to suffer this type of failure
frequently is unlikely to be able to support reliable functionality
regardless of the routing mechanism. However, regardless of the
stability of the return path, studies show that in the event of high
churn, iterative routing is a better solution to ensure request
completion <xref target="lookups-churn-p2p06"></xref> <xref
target="non-transitive-dhts-worlds05"></xref></t>
<t>Finally, because RELOAD retries the end-to-end request, that retry
will address the issues of churn that remain.</t>
</section>
</section>
<section anchor="sec-why-clients" title="Why Clients?">
<t>There are a wide variety of reasons a node may act as a client rather
than as a peer <xref target="I-D.pascual-p2psip-clients"></xref>. This
section outlines some of those scenarios and how the client's behavior
changes based on its capabilities.</t>
<section title="Why Not Only Peers?">
<t>For a number of reasons, a particular node may be forced to act as a
client even though it is willing to act as a peer. These include:</t>
<t><list style="symbols">
<t>The node does not have appropriate network connectivity,
typically because it has a low-bandwidth network connection.</t>
<t>The node may not have sufficient resources, such as computing
power, storage space, or battery power.</t>
<t>The overlay algorithm may dictate specific requirements for peer
selection. These may include participation in the overlay to
determine trustworthiness, control the number of peers in the
overlay to reduce overly-long routing paths, or ensure minimum
application uptime before a node can join as a peer.</t>
</list></t>
<t>The ultimate criteria for a node to become a peer are determined by
the overlay algorithm and specific deployment. A node acting as a client
that has a full implementation of RELOAD and the appropriate overlay
algorithm is capable of locating its responsible peer in the overlay and
using CONNECT to establish a direct connection to that peer. In that
way, it may elect to be reachable under either of the routing approaches
listed above. Particularly for overlay algorithms that elect nodes to
serve as peers based on trustworthiness or population, the overlay
algorithm may require such a client to locate itself at a particular
place in the overlay.</t>
</section>
<section title="Clients as Application-Level Agents">
<t>SIP defines an extensive protocol for registration and security
between a client and its registrar/proxy server(s). Any SIP device can
act as a client of a RELOAD-based P2PSIP overlay if it contacts a peer
that implements the server-side functionality required by the SIP
protocol. In this case, the peer would be acting as if it were the
user's peer, and would need the appropriate credentials for that
user.</t>
<t>Application-level support for clients is defined by a usage. A usage
offering support for application-level clients should specify how the
security of the system is maintained when the data is moved between the
application and RELOAD layers.</t>
</section>
</section>
<!--
<t>The forwarding layer is responsible for looking at message and
doing one of three things:</t>
<t><list style="symbols">
<t>Deciding the message was destined for this peer and passing the
message up to the layer above this.</t>
<t>Looking at the Node-ID that represents the next peer to send
the message too and if there is an existing connection, sending
the message over the connection.</t>
<t>Requesting the overlay Routing logic to tell the forwarding layer
which peer the message needs to be forwarded to (based on the
target Node-ID or Resource-ID), and then sending the message.</t>
</list></t>
-->
<!--
<section anchor="sec-via-list" title="Via Lists">
<t>
[TODO: BBL; I would merge this into the symmetric section
and try to trim a bit.]
</t>
<t>In a general messaging system, messages need a source and a
destination and peers need to be able to send a response to the peer
that sent the request. This can be particularly tricky in overlay
networks when a new peer is joining, or the overlay network is
stabilizing and different peers have different ideas on what the
overlay topology is. A simple and reliable way to make sure that a
response can reach the node that sent the request in these
situations is to have the response traverse the reverse path of the
request.</t>
<t>The approach used here is to have each node the request traverses
add its Node-ID to the "via list" in the request. Then the response
is routed by looking at the list and using it as list of peers that
the response will be routed thorough. To support this, each message
has a destination list of nodes it needs to be routed through as
well as a via list of what nodes it has traversed.</t>
<t>When a peer receives a message from the Transport Layer, it adds
the Node-ID of the node it received the message from to the end of
the via list. When a peer goes to transmit a message to the
Transport Layer, it looks at the first entry on the destination
list. If the entry is this peer, it removes this entry from the list
and looks at the next entry and if the entry is not this peer, it
sends the message to the first peer on the destination list.</t>
<t>When a peer goes to send a response to a request, it can simply
copy the via list in reverse to form the destination list for the
response if it wishes to route the response along the reverse path
as the request.</t>
<t>Peers that are willing to maintain state may do list compression
for privacy reason and to reduce the message size. They do this by
taking some number of entries off the via list and replacing them
with a unique entry that this peer can later identify. Later, if the
peer sees the unique entry in a destination list, it removes the
unique entry and replaces it with the all the entries removed from
the original via list (and reverses the order of these entries).
Note that this technique will generally require storing some
per-message state on the intermediate peer, so this is a
bandwidth/per-peer state tradeoff. The exception is if the list is
not compressed but rather the Node-IDs are simply encrypted.</t>
<t>The via list approach provides several features. First it allows
a response to follow the same path as the request. This is
particularly important for peers that are sending requests while
they are joining and before other peers can route to them as well as
situations where message are being exchanged to stabilize the
overlay network. It also makes it easier to diagnose and manage the
system when all peers see the response to any request they
forward.</t>
</section>
-->
<!-- CHANGES
* Remove Route Log
* Removed all DataModel fields redundant
* Added a clarification about clients and topology shifts
* Mandatory SHA-1
* Binary ICE encoding
* HArmonized all message lengths on 2^32-1
* Added AppAttach
-->
<!-- TODO: Extension field on every message -->
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 20:34:49 |