One document matched: draft-ietf-p2psip-base-11.xml
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
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Check if the qualicom IPR issueed and if anything can be done
Say which of
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are mandatory to implements
Add pad data option to PING ... and update section 5.7 to mention this
in NODE-MULTIPLE, you need to gen and text all i which seems lame
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<rfc category="std" docName="draft-ietf-p2psip-base-11"
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>Skype</organization>
<address>
<postal>
<!--<street>8000 Marina Blvd., 3rd Floor</street>-->
<street></street>
<city>Palo Alto</city>
<!--<code>V7X1M3</code>-->
<region>CA</region>
<country>USA</country>
</postal>
<email>bbl@lowekamp.net</email>
</address>
</author>
<author fullname="Eric Rescorla" initials="E.K." surname="Rescorla">
<organization>Skype</organization>
<address>
<postal>
<street>8000 Marina Blvd</street>
<city>Brisbane</city>
<region>CA</region>
<code>94005</code>
<country>USA</country>
</postal>
<phone>+1 650 678 2350</phone>
<email>ekr@skype.net</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="12" month="Oct" year="2010" />
<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 to any
documents that update or supersede them.</t>
<t>This specification 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">
<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 (e.g., distributed hash tables)
and unstructured overlay algorithms. This specification also defines
how RELOAD is used with the Chord DHT algorithm, 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<xref
target="I-D.ietf-p2psip-concepts"></xref> for more details. A RELOAD
Overlay Instance consists of a set of nodes arranged in a 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. The following figure
shows the layered RELOAD architecture.</t>
<figure>
<artwork><![CDATA[
Application
+-------+ +-------+
| SIP | | XMPP | ...
| Usage | | Usage |
+-------+ +-------+
------------------------------------ Messaging Service Boundary
+------------------+ +---------+
| Message |<--->| Storage |
| Transport | +---------+
+------------------+ ^
^ ^ |
| v v
| +-------------------+
| | Topology |
| | Plugin |
| +-------------------+
| ^
v v
+------------------+
| Forwarding & |
| Link Management |
+------------------+
------------------------------------ Overlay Link Service Boundary
+-------+ +------+
|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 Service.</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:">Responsible for actually
transporting traffic directly between nodes. Each such protocol
includes the appropriate provisions for per-hop framing or
hop-by-hop ACKs required by unreliable transports. TLS <xref
target="RFC5246"></xref> and DTLS <xref target="RFC4347"></xref>
are the currently defined "link layer" protocols used by RELOAD
for hop-by-hop communication. New protocols MAY be defined, as
described in <xref target="sec.future-link"></xref> and <xref
target="sec-configuration"></xref>. As this document defines only
TLS and DTLS, we use those terms throughout the remainder of the
document with the understanding that some future specification may
add new overlay link layers.</t>
</list></t>
<t>To further clarify the roles of the various layers, 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 <xref
target="I-D.ietf-p2psip-sip"></xref>, that use the abstract Message
Transport Service 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 softphone 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 specification.</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 the receiving 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 Service 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 sending a response to
the Message Transport for delivery to the requesting node.
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 message 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 generic interface that allows the topology
plugin to 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>This layer also utilizes a framing header to encapsulate messages
as they are forwarding along each hop. This header aids reliability
congestion control, flow control, etc. It has meaning only in the
context of that individual link.</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, DTLS, or potentially some to be defined future
protocol.</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 an optional 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
only. This feature is used together with certificate-based access
control, not as a replacement for it. It is typically used when
self-signed certificates are being used but would generally not be
used when the certificates were all signed by an enrollment
server.</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.</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="Kind:">A kind defines a particular type of data that
can be stored in the overlay. Applications define new Kinds to story
the data they use. Each Kind is identified with a unique IANA
assigned integer called a Kind-ID.</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 (i.e. client or
peer) when the text applies only to Clients or only to Peers.</t>
<t></t>
<t hangText="Node-ID:">A fixed-length value that uniquely identifies
a node. Node-IDs of all 0s and all 1s 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 all 1s 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 be variable lengths.</t>
<t></t>
<t hangText="Connection Table:">The set of nodes 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 "maximum request lifetime" is the maximum time a request
will wait for a response; it 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; it 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 unique within an overlay instance.</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 uses this certificate so that a node connecting to it
knows it is connected to the correct node (technically: a (D)TLS
association with client authentication is formed.) 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 multiple certificate issuance models. The first is
based on a central enrollment process which allocates a unique name
and Node-ID and puts them in a certificate 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,
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 as 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 such devices as "nodes."</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 certain
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>Clients may insert themselves in the overlay in two ways:</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 may require
a TURN relay in cases where NATs or firewalls prevent a client
from forming a direct connections with its responsible peer.
Note that clients that choose this option MUST process Update
messages from the peer. Those updates can indicate that the peer
no longer is responsible for the Client's Node-ID. The client
then MUST 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 using only its Node-ID. If the client is to receive
incoming requests from other members of the overlay, the
Destination List required to reach it must be learnable via
other mechanisms, such as being stored in the overlay by a
usage.</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 act 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>
<t><list style="symbols">
<t>Implement RELOAD's connection-management operations 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>
</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 then 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>RELOAD's routing provides three mechanisms designed to assist in
meeting these needs:</t>
<t><list style="hanging">
<t hangText="Destination Lists: ">While in principle it is
possible to just inject a message into the overlay with a bare
NodeID as the destination, RELOAD provides a source routing
capability in the form of "Destination Lists". A "Destination List
provides a list of the nodes through which a message must
flow.</t>
<t hangText="Via Lists: ">In order to allow responses to follow
the same path as requests, each message also contains a "Via
List", which is added to by each node a message traverses. This
via list can then be inverted and used as a destination list for
the response.</t>
<t hangText="RouteQuery: ">The RouteQuery 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 recursive routing and then discuss
its advantages in terms of the requirements discussed above.</t>
<t>Symmetric recursive routing requires that a message follow a 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 to be stored on intermediate peers but saves a
small amount of bandwidth and reduces the need for modifying the
message en 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 to X1 and saving the state internally.</t>
<figure>
<artwork><![CDATA[
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Dest=Z, X1
<----------
Dest=X,X1
<----------
Dest=B, A
<----------
Dest=A
]]></artwork>
</figure>
<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 RouteQuery method, which
requests this behavior. Note that iterative routing is selected only
by the initiating node.</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="RFC5245"></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"
typically 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 specification 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>
</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 and a set of credentials which
match that Node-ID. When an enrollment server is used that Node-ID
will be in the certificate the node 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 to form a connection to
some "bootstrap node". Because this is the first connection the peer
makes, these nodes must have public IP addresses so that they 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 other 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 perform 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 node 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.</t>
<t>If a node already has the valid configuration document that it
received by some out of band method, this step can be skipped.</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 without using the overlay.</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 undetectably
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 limit 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 needs to
specify 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 of the 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 as if 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 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 simplistic
TURN-specific discovery mechanism is provided. A variety of service
discovery algorithms can be implemented as extensions to the base
protocol, such as the service discovery algorithm 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, it is the overlay itself -
rather than an external authority such as DNS - which is used to
establish a connection. RELOAD provides connectivity to applications
using the AppAttach method. For example, if a P2PSIP node wishes to
establish a SIP dialog with another P2PSIP node, it will use AppAttach
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
AppAttach.</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 to be 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 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 (note that private IDs can be distinguished from NodeIDs and
Resource IDs on the wire; see <xref
target="sec.dest-via-list"></xref>).</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 an ID for which the
node is responsible, there are several sub-cases to consider.</t>
<t><list style="symbols">
<t></t>
<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 equals this node's Node-ID,
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
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 before the message is further
processed.</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 three 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, the peer consults 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. It 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>No matter what mechanism for storing via list state is used, 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>
<t>Note that if an intermediate peer retains per-transaction state
instead of modifying the via list, it needs some mechanism for
timing out that state, otherwise its state database will grow
without bound. Whatever algorithm is used, state MUST be maintained
for at least the value of the overlay reliability timer (3 seconds)
and MAY be kept longer.</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
Node-ID 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 using this routing
algorithm, 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 are opaque, however, they are interpreted by the
higher layers.</t>
<t></t>
<t hangText="Security Block:">A security block containing
certificates and a digital signature over the ""Message Contents".
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.
<xref target="RFC5246"></xref> Advantages of this style include:</t>
<t><list style="symbols">
<t>It 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>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. The max value of 255
indicates this 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[NodeIdLength];
]]></artwork>
</figure>
<t>A NodeId is a fixed-length structure represented as a series of
bytes, with the most significant byte first. The length is set on
a per-overlay basis within the range of 16-20 bytes (128 to 160
bits). (See <xref target="sec-configuration"></xref> for how
NodeIdLength is set.) Note: the use of "typedef" here is an
extension to the TLS language, but its meaning should be
relatively obvious. Note the [ size ] syntax defines a fixed
length element that does not include the length of the element in
the on the wire encoding.</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 ResourceId is an opaque string of bytes, but
unlike NodeIds, ResourceIds 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. Note the <
range > syntax defines a variable length element that does
include the length of the element in the on the wire encoding. The
number of bytes to encode the length on the wire is derived by
range; i.e., it is the minimum number of bytes which can encode
the largest range value.</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 {reservedAddr(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 0xd2454c4f (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 is a fixed point integer between 0.1 and 25.4. This
document describes version 0.1, with a value of 0x01. [[ Note to
RFC Editor: Please update this to version 1.0 with value of 0x0a
and remove this note. ]]</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. The next six bits (0x20000000 to 0x01000000) are
reserved and SHOULD be set to zero. 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. In
order to provide a high probability that transaction IDs are
unique, they MUST be randomly generated. 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="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="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="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.</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 ConfigUpdate 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 ConfigUpdate, then the message MUST be accepted
regardless of the receiving node's configuration sequence number.
Since 65535 is a special value, peers sending a new configuration
when the configuration sequence is currently 65534 MUST set the
configuration sequence number to 0 when they send out a new
configuration.</t>
</section>
<section anchor="sec.dest-via-list"
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 a 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. If it is a 16 bit
integer, it is treated as if it were 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 the destination
structure 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 Payload Data Unit (PDU).
This may be one of "node", "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="node"></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 as 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 uses of 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-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 { reservedForwarding(0),
directResponseForwarding(1), (255) } ForwardingOptionsType;
struct {
ForwardingOptionsType type;
uint8 flags;
uint16 length;
select (type) {
case directResponseForwarding:
DirectResponseForwardingOption directResponseForwardingOption;
/* This type may be extended */
} 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 that 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 except that
the RESPONSE_COPY, FORWARD_CRITICAL and DESTINATION_CRITICAL
flags must be cleared.</t>
<t></t>
<t hangText="option"></t>
<t>The option value.</t>
</list></t>
</section>
<section anchor="drr-forwarding-options"
title="Direct Return Response Forwarding Options">
<t>This section defines an OPTIONAL forwarding option that allows
the originator of a request to signal that the node responding to
the request should try to route the response directly to the node
that made the request instead of having the responses traverse the
overlay. :</t>
<figure>
<!-- begin-pdu-->
<artwork><![CDATA[
struct {
AttachReqAns connection_information;
NodeID requesting_node;
} DirectResponseForwardingOption;
]]></artwork>
</figure>
<t>Each ForwardingOption consists of the following values:</t>
<t><list style="hanging">
<t></t>
<t hangText="connection_information"></t>
<t>All of the information needed to initiate a new connection
to the requesting node. This type is defined in <xref
target="sec-connect-request"></xref>.</t>
<t></t>
<t hangText="requesting_node"></t>
<t>The NodeID of the node that originated the request. This is
used to match the TLS certificate.</t>
<t></t>
</list></t>
<t>This option can only be used if the
direct-return-response-permitted flag in the configuration for the
overlay is set to TRUE. The RESPONSE_COPY flag SHOULD be set to
false while the FORWARD_CRITICAL and DESTINATION_CRITICAL MUST be
set to true. When a node that supports this forwarding options
receives a request with it, it acts as if it had send an Attach
request to the requesting_node and it had received the
connection_information in the answer. This causes it to form a new
connection directly to that node. Once that is complete the
response to this request is sent over that connection. If a
connection already exists directly to that node, it is used
instead of a new connection being formed. The connection MAY be
closed at any point but is typically kept open until it has not
been used for a significant period of time or one of the nodes
needs to reclaim resources.</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 { reservedMessagesExtension(0), (2^16-1) } MessageExtensionType;
struct {
MessageExtensionType type;
Boolean critical;
opaque extension_contents<0..2^32-1>;
} MessageExtension;
struct {
uint16 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>The message codes are defined in <xref
target="sec-iana-messages-codes"></xref></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 can 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 MAY 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 has 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 UTF-8 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 node 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 node 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_Data_Too_Old:">A store cannot be completed
because the storage_time precedes the existing value.</t>
<t></t>
<t hangText="Error_Data_Too_Large:">A store cannot be
completed because the requested object exceeds the size limits
for that kind.</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, overlay
algorithm, 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. A node which generates this response MUST then generate a
ConfigUpdate message containing the correct configuration.</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
ConfigUpdate 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 ConfigUpdate message which contains the
appropriate kind definition (assuming that in fact a kind was
used which was defined in the configuration document).</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 the 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 to be sent. 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 because
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 the
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 { reservedSignerIdentity(0),
cert_hash(1), (255)} SignerIdentityType;
select (identity_type) {
case cert_hash;
HashAlgorithm hash_alg; // From TLS
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; // From TLS
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. The SignerIdentity structure is
typed purely to allow for future (unanticipated) extensibility.</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>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. 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 maintenance procedures that
send Updates to clients and peers that have established connections
to the peer responsible for a particular ID when the responsibility
for that ID changes. Because tracking this information is difficult,
overlay algorithms MAY simply specify that an Update is sent to all
members of the Connection Table whenever the range of IDs for which
the peer is responsible changes.</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 one that 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>
<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 that 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>
<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 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 newly
responsible node already 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
for the node that the receiving peer would next route to in order
to get to X. A RouteQuery can also request that the receiving peer
initiate an Update request to transfer the receiving peer's
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 use the Attach method to attach to that peer(s), and repeat
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>
<section title="Request Definition">
<t>A RouteQueryReq message indicates the peer or resource that
the requesting node is interested in. It also contains a
"send_update" option allowing the requesting node 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 primitive "exploration" services: it allows node
to determine which resources another node is responsible for; and
it allows some discovery services using multicast, anycast, or
broadcast. 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 { reservedProbeInformation(0), responsible_set(1),
num_resources(2), uptime(3), (255)}
ProbeInformationType;
struct {
ProbeInformationType requested_info<0..2^8-1>;
} ProbeReq
]]></artwork>
</figure>
<t>The 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 option
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 node 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. Attach also allows two nodes to
connect via No-ICE instead of full ICE.</t>
<t hangText="AppAttach: ">used to form application layer
connections between nodes.</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 purpose 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 (unlike peers) can
simply Attach without sending Join or Update.</t>
<section anchor="sec-connect-request" title="Request Definition">
<t>An Attach request message contains the requesting node ICE
connection parameters formatted into a binary structure.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { reservedOverlayLink(0), DTLS-UDP-SR(1),
DTLS-UDP-SR-NO-ICE(3), TLS-TCP-FH-NO-ICE(4),
(255) } OverlayLinkType;
enum { reservedCand(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;
OverlayLinkType overlay_link;
opaque foundation<0..255>;
uint32 priority;
CandType type;
select (type){
case host:
; /* Nothing */
case srflx:
case prflx:
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>;
Boolean send_update;
} AttachReqAns;
]]></artwork>
</figure>
<t>The values contained in AttachReqAns 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>
<t>An active/passive/actpass attribute from RFC 4145 <xref
target="RFC4145"></xref>. This value MUST be 'passive' for the
offerer (the peer sending the Attach request) and 'active' for
the answerer (the peer sending the Attach response).</t>
<t></t>
<t hangText="candidates "></t>
<t>One or more ICE candidate values, as described below.</t>
<t hangText="send_update "></t>
<t>Has the same meaning as the send_update field in
RouteQueryReq.</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, and thus left out of
the PDU. 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="overlay_link"></t>
<t>corresponds to the OverlayLinkType production, Overlay Link
protocols used with No-ICE MUST specify "No-ICE" in their
description. Future overlay link values can be added be
defining new OverlayLinkType values in the IANA registry in
<xref target="sec-iana-overlay-link"></xref>. Future
extensions to the encapsulation or framing that provide for
backward compatibility with that specified by a previously
defined OverlayLinkType values MUST use that previous value.
OverlayLinkType protocols are defined in <xref
target="sec-overlay-link"></xref></t>
<t>A single AttachReqAns MUST NOT include both candidates
whose OverlayLinkType protocols use ICE (the default) and
candidates that specify "No-ICE".</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 MUST process the
request and SHOULD generate its own response with a AttachReqAns.
A peer which is overloaded or detects some other kind of error may
of course generate an error instead of an 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.</t>
<t>In ICE as defined by <xref target="RFC5245"></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>
<t>An agent follows the ICE specification as described in <xref
target="RFC5245"></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 nodes. These nodes 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 learn 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
MUST be selected unless there are no others servers in the group in
which case some other peer MAY be used. </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 application signaling, it follows the
process of gathering candidates as described in Section 4 of ICE
<xref target="RFC5245"></xref>. RELOAD utilizes a single
component. Consequently, gathering for these "streams" requires a
single component. In the case where a node has not yet found a
TURN server, the agent would not include a relayed candidate.</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 default candidate selection described in Section 4.1.4 of
ICE is ignored; defaults are not signaled or utilized by
RELOAD.</t>
<t>An alternative to using the full ICE supported by the Attach
request is to use No-ICE mechanism by providing candidates with
"No-ICE" Overlay Link protocols. Configuration for the overlay
indicates whether or not these Overlay Link protocols can be used.
An overlay MUST be either all ICE or all No-ICE.</t>
<t>No-ICE 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. Therefore it is RECOMMENDED that full ICE
be used even for a node that has a public, unfiltered IP address,
to take advantage of STUN connectivity checks, etc.</t>
</section>
<section title="Prioritizing Candidates">
<t>At the time of writing, UDP is the only transport protocol
specified to work with ICE. However, standardization of additional
protocols for use with ICE is expected, including TCP and
datagram-oriented protocols such as SCTP and DCCP. In particular,
UDP encapsulations for SCTP and DCCP are expected to be
standardized in the near future, greatly expanding the available
Overlay Link protocols available for RELOAD. When additional
protocols are available, the following prioritization is
RECOMMENDED:</t>
<t><list style="symbols">
<t>Highest priority is assigned to message-oriented protocols
that offer well-understood congestion and flow control without
head of line blocking. For example, SCTP without message
ordering, DCCP, or those protocols encapsulated using UDP.</t>
<t>Second highest priority is assigned to stream-oriented
protocols, e.g. TCP.</t>
<t>Lowest priority is assigned to protocols encapsulated over
UDP that do not implement well-established congestion control
algorithms. The DTLS/UDP with SR overlay link protocol is an
example of such a protocol.</t>
</list></t>
</section>
<section title="Encoding the Attach Message">
<t>Section 4.3 of ICE describes procedures for encoding the SDP
for conveying RELOAD candidates. Instead of actually encoding an
SDP, the candidate information (IP address and port and transport
protocol, priority, foundation, 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="Full ICE">
<t>When neither side has provided an No-ICE candidate,
connectivity checks and nominations are used as in regular
ICE.</t>
<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 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="Media Keepalives">
<t>STUN MUST be utilized for the keepalives described in Section
10 of ICE.</t>
</section>
</section>
<section title="No-ICE">
<t>No-ICE is selected when either side has provided "no ICE"
Overlay Link candidates. STUN is not used for connectivity checks
when doing No-ICE; instead the DTLS or TLS handshake (or similar
security layer of future overlay link protocols) forms the
connectivity check. The certificate exchanged during the (D)TLS
handshake must match the node that sent the AttachReqAns and if it
does not, the connection MUST be closed.</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="Sending Media">
<t>The procedures of Section 11 of ICE 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 server MUST request TLS client
authentication. 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.</t>
</section>
</section>
<section anchor="sec-appattach-details" title="AppAttach">
<t>A node sends an AppAttach request when it wishes to establish a
direct 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 an application attribute,
which indicates what protocol is to be run over the connection.</t>
<section anchor="sec-appattach-request" title="Request Definition">
<t>An AppAttachReq message contains the requesting node'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>;
} AppAttachReq;
]]></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 application-id as defined in the <xref
target="sec.iana.app"></xref>. This number represents the IANA
registered application that is going to send data on this
connection. For SIP, this is 5060 or 5061.</t>
<t></t>
<t hangText="role "></t>
<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 process the
request and generate its own response with a AppAttachAns. 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>
<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>;
} AppAttachAns;
]]></artwork>
</figure>
<t>The meaning of the fields is the same as in the
AppAttachReq.</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, to 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
UNIX style time, represented in milliseconds since midnight
Jan 1, 1970 and not counting leap seconds.</t>
</list></t>
</section>
</section>
<section title="ConfigUpdate">
<t>The ConfigUpdate 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 ConfigUpdate request.
The ConfigUpdate 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 { reservedConfigUpdate(0), config(1), kind(2), (255) }
ConfigUpdateType;
typedef opaque KindDescription<2^16-1>;
struct {
ConfigUpdateType type;
uint32 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*/
};
} ConfigUpdateReq;
]]></artwork>
</figure>
<t>The ConfigUpdateReq 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. This is
included to preserve backward compatibility and is 32 bits
instead of 24 to facilitate easy conversion between network
and host byte order.</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>). These MUST be encoded
with UTF-8 and assume a default namespace of
"urn:ietf:params:xml:ns:p2p:config-base".</t>
</list></t>
</section>
<section title="Response Definition">
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
} ConfigUpdateRsp
]]></artwork>
</figure>
<t>If the ConfigUpdateReq 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 ConfigUpdateReq is of type "kind" it MUST only be
processed if it is correctly digitally signed by an acceptable
kind signer as specified in the configuration file. Details on
kind-signer field in the configuration file is described in <xref
target="sec-configuration"></xref>. 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 ConfigUpdate 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 them. Any link protocol needs to be able to
establish a secure, authenticated connection and to provide data
origin authentication and message integrity for individual data
elements. RELOAD currently supports three Overlay Link protocols:</t>
<t><list style="symbols">
<t>DTLS <xref target="RFC4347"></xref> over UDP with Simple
Reliability (SR)</t>
<t>TLS <xref target="RFC5246"></xref> over TCP with Framing
Header, No-ICE</t>
<t>DTLS <xref target="RFC4347"></xref> over UDP with SR,
No-ICE</t>
</list></t>
<t>Note that although UDP does not properly have "connections", both
TLS and DTLS have a handshake which establishes a similar, stateful
association, 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 and 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>The Framing Header (FH) is used to frame messages and provide
timing when used on a reliable stream-based transport protocol. Simple
Reliability (SR) makes use of the FH to provide congestion control and
semi-reliability when using unreliable message-oriented transport
protocols. We will first define each of these algorithms, then define
overlay link protocols that use them.</t>
<t>Note: We expect future Overlay Link protocols to define
replacements for all components of these protocols, including the
framing header. These protocols have been chosen for simplicity of
implementation and reasonable performance.</t>
<t>Note to implementers: There are inherent tradeoffs in utilizing
short timeouts to determine when a link has failed. To balance the
tradeoffs, an implementation should be able to quickly act to remove
entries from the routing table when there is reason to suspect the
link has failed. For example, in a Chord-derived overlay algorithm, a
closer finger table entry could be substituted for an entry in the
finger table that has experienced a timeout. That entry can be
restored if it proves to resume functioning, or replaced at some point
in the future if necessary. End-to-end retransmissions will handle any
lost messages, but only if the failing entries do not remain in the
finger table for subsequent retransmissions.</t>
<section anchor="sec.future-link"
title="Future Overlay Link Protocols">
<t>The only currently defined overlay link protocols are TLS and
DTLS. It is possible to define new link-layer protocols and apply
them to a new overlay using the "overlay-link-protocol"
configuration directive (see <xref
target="sec-configuration"></xref>.). However, any new protocols
MUST meet the following requirements.</t>
<t><list style="hanging">
<t hangText="Endpoint authentication">When a node forms an
association with another endpoint, it MUST be possible to
cryptographically verify that the endpoint has a given
NodeId.</t>
<t></t>
<t hangText="Traffic origin authentication and integrity">When a
node receives traffic from another endpoint, it MUST be possible
to cryptographically verify that the traffic came from a given
association and that it has not been modified in transit from
the other endpoint in the association. The overlay link protocol
MUST also provide replay prevention/detection.</t>
<t></t>
<t hangText="Traffic confidentiality">When a node sends traffic
to another endpoint, it MUST NOT be possible for a third party
not involved in the association to determine the contents of
that traffic.</t>
</list></t>
<t>Any new overlay protocol MUST be defined via RFC 5226 Standards
Action; see <xref
target="sec.iana-overlay-link-protocols"></xref>.</t>
<section title="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>
</section>
<section title="ICE-TCP">
<t>The ICE-TCP draft <xref
target="I-D.ietf-mmusic-ice-tcp"></xref> should allow TCP to be
supported as an Overlay Link protocol that can be added using
ICE.</t>
</section>
<section title="Message-oriented Transports">
<t>Modern message-oriented transports offer high performance, good
congestion control, and avoid head of line blocking in case of
lost data. These characteristics make them preferable as
underlying transport protocols for RELOAD links. SCTP without
message ordering and DCCP are two examples of such protocols.
However, currently they are not well-supported by commonly
available NATs, and specifications for ICE session establishment
are not available.</t>
</section>
<section title="Tunneled Transports">
<t>As of the time of this writing, there is significant interest
in the IETF community in tunneling other transports over UDP,
motivated by the situation that UDP is well-supported by modern
NAT hardware, and similar performance can be achieved to native
implementation. Currently SCTP, DCCP, and a generic tunneling
extension are being proposed for message-oriented protocols. Baset
et al. have proposed tunneling TCP over UDP for similar reasons
<xref target="I-D.baset-tsvwg-tcp-over-udp"></xref>. Once ICE
traversal has been specified for these tunneled protocols, they
should be straightforward to support as overlay link
protocols.</t>
</section>
</section>
<section anchor="sec-framing-header" title="Framing Header">
<t>In order to support unreliable links and to allow for quick
detection of link failures when using reliable end-to-end
transports, 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. The same header
is used for both reliable and unreliable transports for simplicity
of implementation - not all aspects of the header apply to both
types of transports.</t>
<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>
<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 receives 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 has been among the 32 packets most
recently received 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. 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. Note that a bit being set to one
indicates positively that a particular packet was received, but
a bit being set to zero means only that it is unknown whether or
not the packet has been received, because it might have been
received before the 32 most recently received packets.</t>
</list></t>
<t>The received field bits in the ACK provide a high degree of
redundancy so that the sender can figure out which packets the
receiver has received and can then estimate packet loss rates. If
the sender also keeps track of the time at which recent sequence
numbers have been sent, the RTT can be estimated.</t>
</section>
<section anchor="sec-reliability" title="Simple Reliability">
<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 whether or not
it carries a request or response, 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>
<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 alternative
would be TFRC-SP <xref target="RFC4828"></xref> and use the
received bitmask to allow the sender to compute packet loss event
rates.</t>
<section title="Trivial Retransmission">
<t>A peer SHOULD retransmit a message if it has not received an
ACK after an interval of RTO ("Retransmission TimeOut"). The
peer MUST double the time to wait 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 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 exception that the 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 the time when 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 <xref target="RFC1122"></xref> or a TLS alert.
The sender knows a response 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, then the sending node SHOULD close the connection routing
the message.</t>
<t>To determine when a link may be failing without waiting for
the final timeout, observe when no ACKs have been received for
an entire RTO interval, and then wait for three retransmissions
to occur beyond that point. If no ACKs have been received by the
time the third retransmission occurs, it is RECOMMENDED that the
link be removed from the routing table. The link MAY be restored
to the routing table if ACKs resume before the connection is
closed, as described above.</t>
<t>Once an ACK has been received for a message, the next message
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 than 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>
<section anchor="sec-dtls-udp-sr" title="DTLS/UDP with SR">
<t>This overlay link protocol consists of DTLS over UDP while
implementing the Simple Reliability protocol. STUN Connectivity
checks and keepalives are used.</t>
</section>
<section anchor="sec-tls-tcp-fs-no-ice"
title="TLS/TCP with FH, No-ICE">
<t>This overlay link protocol consists of TLS over TCP with the
framing header. Because ICE is not used, STUN connectivity checks
are not used upon establishing the TCP connection, nor are they used
for keepalives.</t>
<t>Because the TCP layer's application-level timeout is too slow to
be useful for overlay routing, the Overlay Link implementation MUST
use the framing header to measure the RTT of the connection and
calculate an RTO as specified in Section 2 of <xref
target="RFC2988"></xref>. The resulting RTO is not used for
retransmissions, but as a timeout to indicate when the link SHOULD
be removed from the routing table. It is RECOMMENDED that such a
connection be retained for 30s to determine if the failure was
transient before concluding the link has failed permanently.</t>
<t>When sending candidates for TLS/TCP with FH, No-ICE, a passive
candidate MUST be provided. The following table shows which side of
the exchange initiates the connection depending on whether they
provided ICE or No-ICE candidates. Note that the active TCP role
does not alter the TLS server/client determination.</t>
<texttable anchor="table-oa-role"
title="Determining Active Role for No-ICE">
<ttcol width="33%">Offeror</ttcol>
<ttcol>Answerer</ttcol>
<ttcol>TCP Active Role</ttcol>
<c>ICE</c>
<c>No-ICE</c>
<c>Offeror</c>
<c>No-ICE</c>
<c>ICE</c>
<c>Answerer</c>
<c>No-ICE</c>
<c>No-ICE</c>
<c>Offeror</c>
</texttable>
</section>
<section anchor="sec-dtls-udp-sr-no-ice"
title="DTLS/UDP with SR, No-ICE">
<t>This overlay link protocol consists of DTLS over UDP while
implementing the Simple Reliability protocol. Because ICE is not
used, no STUN connectivity checks or keepalives are used.</t>
</section>
</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>
<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 destination 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 it
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 as many fragments
as can arrive in the maximum request lifetime. 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 between the end of the forwarding
header and the start of the data. The first fragment therefore 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 bits are set to 1 and the offset is 0 resulting in a fragment
value of 0xC0000000.</t>
</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 size of the StoredData structure in octets excluding the size
of length itself.</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 and not
counting leap seconds. 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>A node that is attempting to store new data in response to a user
request (rather than as an overlay maintenance operation such as
occurs during unpartitioning) is rejected with an Error_Data_Too_Old
error, the node MAY elect to perform its store using a storage_time
that increments the value used with the previous store. This
situation may occur when the clocks of nodes storing to this
location are not properly synchronized.</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 as defined in <xref
target="sec-data-sig"></xref>.</t>
</list></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 + storage_time + StoredDataValue +
SignerIdentity</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="storage_time "></t>
<t></t>
<t>The contents of the storage_time data value.</t>
<t hangText="StoredDataValue "></t>
<t>The contents of the stored data value, as described in the
previous sections.</t>
<t></t>
<t hangText="SignerIdentity "></t>
<t>The signer identity as defined in <xref
target="sec-signature"></xref>.</t>
</list></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>
<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 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 of i. When this policy is in use, the maximum value of i MUST
be specified in the kind definition.</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>
<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 MUST reject requests
corresponding to unknown kinds.</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>
<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>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>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>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 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 by the node processing the request..</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 have in
fact been stored. It does this verification by using the Stat
method. 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 an
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 the replica list, which will
be empty. 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<0..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 is 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 node 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 model is single value, the specifier is
empty.</t>
<t>If the data model is array, the specifier contains 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 first integer must be less than the second. While multiple
ranges MAY be specified, they MUST NOT overlap.</t>
<t>If the data model is 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 node that 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 the array is set to zero before the signature is computed. This
implies that malicious storing nodes can reorder array entries
without being detected.<!-- [[ 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 file system. 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 closest 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 Resource-ID and a series of
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>
<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, and
if not the request MUST be rejected with an error.</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 RELOAD error code.</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 a 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 kinds:</t>
<t><list style="hanging">
<t></t>
<t hangText="Name:">CERTIFICATE_BY_NODE</t>
<t></t>
<t hangText="Data Model:">The data model for CERTIFICATE_BY_NODE
data is array.</t>
<t></t>
<t hangText="Access Control:">NODE-MATCH.</t>
</list></t>
<t><list style="hanging">
<t></t>
<t hangText="Name:">CERTIFICATE_BY_USER</t>
<t></t>
<t hangText="Data Model:">The data model for CERTIFICATE_BY_USER
data is array.</t>
<t></t>
<t hangText="Access Control:">USER-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="RFC5766"></xref>. When a node starts up, it joins the overlay
network and forms several connections in the process. If the ICE stage
in any of these connections returns a reflexive address that is not the
same as the peer's perceived address, then the peer is behind a NAT and
not a candidate for a TURN server. Additionally, if the peer's IP
address is in the private address space range, then it is also 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 turn-density 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. If the turn-density is not set to zero, for each value, called d,
between 1 and turn-density, 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 turn-density be an accurate estimate of the
true density of TURN servers. If turn-density 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 that provide this service need to support the TURN extensions
to STUN for media relay as defined in <xref
target="RFC5766"></xref>.</t>
<t>This usage defines the following kind to indicate that 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 and implementation experiences, and to adapt it to the
RELOAD protocol. A short list of differences:</t>
<t><list style="symbols">
<t>The original Chord algorithm specified that a single predecessor
and a successor list be stored. The chord-reload algorithm attempts
to have more than one predecessor and successor. 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>RELOAD uses a 128 bit hash instead of a 160 bit hash, as RELOAD
is not designed to be used in networks with close to or more than
2^128 nodes (and it is hard to see how one would assemble such a
network).</t>
<t>RELOAD uses randomized finger entries as described in <xref
target="sec-finger-refresh"></xref>.</t>
<t>This algorithm allows the use of either reactive or periodic
recovery. The original Chord paper used periodic recovery. Reactive
recovery provides better performance in small overlays, but is
believed to be unstable in large (>1000) overlays with high
levels of churn <xref target="handling-churn-usenix04"></xref>. The
overlay configuration file specifies a "chord-reload-reactive"
element that indicates whether reactive recovery should be used.</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 contains at least the three peers before and
after this peer in the DHT ring. There may not be three entries in all
cases such as small rings or while the ring topology is changing. 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 the
insertion 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="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 peer's 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>Managing replicas as the overlay changes is described in <xref
target="sec-processing-updates"></xref>.</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
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 has 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>The AP MUST send an Update to all of its neighbors with the new
values of its neighbor set (including JP).</t>
<t>The JP MUST send Updates to all the peers in its neighbor
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 finger table entry for peer i, JP simply
sends an Attach to peer (n+2^(128-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>
</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 "uptime" field contains the time this peer has been up in
seconds.</t>
<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="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 of 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, even though this will not be
possible if the ring is very small. It is RECOMMENDED that O(log(N))
predecessors and successors be maintained in the neighbor set.</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 the 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 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>If the neighbor failure effects the peer's range of responsible
IDs, then the Update MUST be sent to all nodes in its Connection
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 successor peers in the neighbor
table, 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 anchor="sec-processing-updates" 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 two
major cases:</t>
<t><list style="symbols">
<t>The UpdateReq contains peers that match N's neighbor table,
so no change is needed to the neighbor set.</t>
<t>The UpdateReq contains peers N does not know about that
should be in N's neighbor table, i.e. they are closer than
entries in the neighbor table.</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>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. These Update
requests are what end 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 is responsible for a Resource-ID R, and N 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>
<t>When a peer discovers that its range of responsible IDs have
changed, it MUST send an UPDATE to all entries in its connection
table.</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. 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-interval" 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^(128-i),
n+2^(128-(i-1))-2^(128-(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-interval", 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>
<t>Alternative 1: A peer selects one entry in the finger table
from among the invalid entries. It 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, send a Ping to replace that entry with
probability 0.5.</t>
<t>Alternative 2: A peer monitors the Update messages received
from its connections to observe when an Update indicates a peer
that would be used to replace in invalid finger table entry, i,
and flags that entry in the finger table. Every
"chord-reload-ping-interval" seconds, the peer selects from among
those flagged 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 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
maintaining connectivity to the overlay even with strange network
partitions.</t>
<t>For many overlays, 16 finger table entries will be enough, but
as an overlay grows very large, 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 128 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 finger table larger 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 node, B. 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 an 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_peer;
} 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
in order to deliver it to the destination listed in the
request.</t>
</list></t>
<t>If the requester has set the send_update flag, the responder SHOULD
initiate an Update immediately after sending the RouteQueryAns.</t>
</section>
<section title="Leaving">
<t>To support extensions, such as <xref
target="I-D.maenpaa-p2psip-self-tuning"></xref>, Peers SHOULD send a
Leave request to all members of their neighbor table prior to exiting
the Overlay Instance. The overlay_specific_data field MUST contain the
ChordLeaveData structure defined below: <figure>
<artwork><![CDATA[
enum { reserved (0),
from_succ(1), from_pred(2), (255) }
ChordLeaveType;
struct {
ChordLeaveType type;
select(type) {
case from_succ:
NodeId successors<0..2^16-1>;
case from_pred:
NodeId predecessors<0..2^16-1>;
};
} ChordLeaveData;
]]></artwork>
</figure> <figure>
<artwork><![CDATA[
The 'type' field indicates whether the Leave request was sent by a
predecessor or a successor of the recipient:
from_succ
The Leave request was sent by a successor.
from_pred
The Leave request was sent by a predecessor.
If the type of the request is 'from_succ', the contents will be:
successors
The sender's successor list.
If the type of the request is 'from_pred', the contents will be:
predecessors
The sender's predecessor list.
]]></artwork>
</figure></t>
<t>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">
<t>The section defines the format of the configuration data as well the
process to join a new overlay.</t>
<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">
<configuration instance-name="overlay.example.org" sequence="22"
expiration="2002-10-10T07:00:00Z">
<direct-return-response-permitted>false
</direct-return-response-permitted>
<bootstrap-node address="192.0.0.1" port="5678" />
<bootstrap-node address="192.0.2.2" port="6789" />
<initial-ttl> 30 </initial-ttl>
<overlay-link-protocol>TLS</overlay-link-protocol>
<clients-permitted>false</clients-permitted>
<turn-density> 10 </turn-density>
<max-message-size>4000</max-message-size>
<enrollment-server>https://example.org</enrollment-server>
<ext:example-extension> foo </ext:example-extension>
<chord:chord-ping-interval>300</chord:chord-ping-interval>
<chord:chord-update-interval>400</chord:chord-update-interval>
<self-signed-permitted
digest="sha1">false</self-signed-permitted>
<shared-secret>asecret</shared-secret>
<topology-plugin>chord</topology-plugin>
<node-id-length>16</node-id-length>
<root-cert>DATA GOES HERE</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>
VGhpcyBpcyBub3QgcmlnaHQhCg==
</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-extension> 1
</ext:example-kind-extension>
</kind>
<kind-signature>
VGhpcyBpcyBub3QgcmlnaHQhCg==
</kind-signature>
</kind-block>
</required-kinds>
<kind-signer> 47112162e84c69ba </kind-signer>
<kind-signer> 6eba45d31a900c06 </kind-signer>
<bad-node> 6ebc45d31a900c06 </bad-node>
</configuration>
<signature> VGhpcyBpcyBub3QgcmlnaHQhCg==</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".</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 no 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 defines the overlay
algorithm being used.</t>
<t hangText="node-id-length">This element contains the length of a
NodeId (NodeIdLength) in bytes. This value MUST be between 16 (128
bits) and 20 (160 bits). If this element is not present, the
default of 16 is used.</t>
<t hangText="root-cert ">This element contains a PEM encoded
X.509v3 certificate that 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="enrollment-server ">This element contains the URL at
which the enrollment server can be reached in a "url" element.
This URL MUST be of type "https:". More than one enrollment-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 enrollment-server and root-cert elements may be absent.
Otherwise, it SHOULD be absent, but MAY be set to "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="direct-return-response-permitted ">This element
indicates whether direct return routed responses as described in
<xref target="drr-forwarding-options"></xref> are permitted. If it
is set to "true", they MAY be used. Otherwise, it SHOULD be
absent, but MAY be set to "false". Implementations MAY support
direct return routed response.</t>
<t hangText="bootstrap-node ">This element represents the address
of one of the bootstrap nodes. 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. The IP address is in typical hexadecimal
form using standard period and colon separators as specified in
<xref target="I-D.ietf-6man-text-addr-representation"></xref>.
More than one bootstrap-peer element may be present.</t>
<t hangText="turn-density ">This element is a positive integer
that represents the approximate reciprocal of density of nodes
that can act as TURN servers. For example, if 5% of the nodes can
act as TURN servers, this would be set to 20. If it is not
present, the default value is 1. If there are no TURN servers in the overlay, it is set to zero.</t>
<t hangText="multicast-bootstrap ">This element represents the
address of a multicast, broadcast, or anycast address and port
that may be used for bootstrap. Nodes SHOULD listen on the
address. 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="no-ice ">This element represents whether nodes are
required to use the "No-ICE" Overlay Link protocols in this
overlay. If it is absent, it is treated as if it were set to
"FALSE".</t>
<t hangText="chord-update-interval ">The update frequency for the
Chord-reload topology plugin (see <xref
target="sec-chord-algorithm"></xref>).</t>
<t hangText="chord-ping-interval ">The ping frequency for the
Chord-reload topology plugin (see <xref
target="sec-chord-algorithm"></xref>).</t>
<t hangText="chord-reload-reactive ">Whether reactive recovery
should be used for this overlay. (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>
<t hangText="overlay-link-protocol">Indicates a permissible
overlay link protocol (see <xref target="sec.future-link"></xref>
for requirements for such protocols). An arbitrary number of these
elements may appear. If none appear, then this implies the default
value, "TLS", which refers to the use of TLS and DTLS. If one or
more elements appear, then no default value applies.</t>
<t hangText="kind-signer ">This contains a single Node-ID in
hexadecimal and indicates that the certificate with this Node-ID
is allowed to sign kinds. Identifying kind-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="bad-node ">This contains a single Node-ID in
hexadecimal and indicates that the certificate with this Node-ID
MUST NOT be considered valid. This allows certificate revocation.
An arbitrary number of these elements can be provided. Note that
because certificates may expire, bad-node entries need only be
present for the lifetime of the certificate. Technically speaking,
bad node-ids may be reused once their certificates have expired,
the requirement for node-ids to be pseudo randomly generated gives
this event a vanishing probability.</t>
</list></t>
<t>Inside each overlay element, the required-kinds elements can also
occur. This element indicates the kinds that members must support and
contains multiple kind-block elements that each define a single kind
that MUST be supported by nodes in the overlay. Each kind-block
consists of a single kind element and a kind-signature. The kind
element defines the kind. The kind-signature is the signature computed
over the kind element.</t>
<t>Each kind has either an ID attribute or a name attribute. The name
attribute is a string representing the kind (the name registered to
IANA) while the ID is an integer kind-id allocated out of private
space.</t>
<t>In addition, the kind element contains the following elements:
<list style="hanging">
<t hangText="max-count:">the maximum number of values which
members of the overlay must support.</t>
<t hangText="data-model:">the data model to be used.</t>
<t hangText="max-size:">the maximum size of individual values.</t>
<t hangText="access-control:">the access control model to be
used.</t>
<t hangText="max-node-multiple:">This is optional and 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></t>
<t>All of the non optional 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>The kind-block element also MUST contain a "kind-signature"
element. This signature is computed across the kind from the beginning
of the first < of the kind to the end of the last > of the kind
in the same way as the "signature element described later in this
section.</t>
<t>The configuration file is a binary file and cannot be changed -
including whitespace changes - or the signature will break. The
signature is computed by taking each configuration element and
starting from, and including, the first < at the start of
<configuration> up to and including the > in
</configuration> and treating this as a binary blob that is
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. Note that forcing an avalanche restart of the overlay
with a configuration change that requires re-joining the overlay may
result in serious performance problems, including total collapse of
the network if configuration parameters are not properly considered.
Such an event may be necessary in case of a compromised CA or similar
problem, but for large overlays should be avoided in almost all
circumstances.</t>
<section title="Relax NG Grammar">
<t>The grammar for the configuration data is:</t>
<figure>
<artwork><![CDATA[
namespace chord = "urn:ietf:params:xml:ns:p2p:config-chord"
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 }*
foreign-attributes = attribute * - (p2pcf:*|local:*|chord:*)
{ text }*
foreign-nodes = (foreign-attributes | foreign-elements)*
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"
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 { kind-block* }
parameter &= element enrollment-server { xsd:anyURI }?
parameter &= element kind-signer { text }*
parameter &= element bad-node { text }*
parameter &= element no-ice { xsd:boolean }?
parameter &=
element direct-return-response-permitted { xsd:boolean }?
parameter &= element shared-secret { xsd:string }?
parameter &= element overlay-link-protocol { xsd:string }*
parameter &= element clients-permitted { xsd:boolean }?
parameter &= element turn-density { xsd:int }?
parameter &= element node-id-length { xsd:int }?
parameter &= foreign-elements*
parameter &=
element self-signed-permitted {
attribute digest { self-signed-digest-type },
xsd:boolean
}?
self-signed-digest-type |= "sha1"
parameter &=
element bootstrap-node {
attribute address { xsd:string },
attribute port { xsd:int }
}+
hostPort = text
parameter &=
element multicast-bootstrap { hostPort
}*
kind-block = element kind-block {
element kind {
(attribute name { kind-names }
| attribute id { xsd:int }),
kind-paramter
} &
element kind-signature {
attribute algorithm { signature-algorithm-type }?,
xsd:base64Binary
}?
}
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-multiple { 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"
kind-names |= "sip-registration"
kind-names |= "turn-service"
parameter &= element chord:chord-ping-interval { xsd:int }?
parameter &= element chord:chord-update-interval { 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.</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 for the enrollment
server. If a URL for 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 "/.well-known/p2psip-enroll" to the overlay name.
This uses the "well known URI" framework defined in <xref
target="RFC5785"></xref>. For example, if the overlay name was
example.com, the URL would be
"https://example.com//.well-known/p2psip-enroll".</t>
<t>Once an address and URL for the enrollment server 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 XML configuration file sent over email.</t>
</section>
<section anchor="sec-credentials" title="Credentials">
<t>If the configuration document contains a enrollment-server element,
credentials are required to join the Overlay Instance. A peer which
does not yet have credentials MUST contact the enrollment server to
acquire them.</t>
<t>RELOAD defines its own trivial certificate request protocol. We
would have liked to have used 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 enrollment 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 generates 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>. Each MUST be chosen by the
enrollment server in such a way that they are unpredictable to the
requesting user. E.g., the user MUST NOT be informed of potential
(random) Node-IDs prior to authenticating. Each is placed in the
subjectAltName using the uniformResourceIdentifier type 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>A single name 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 status 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="Self-Generated Credentials">
<t>If the "self-signed-permitted" element is present in the
configuration 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 Node">
<t>If no cached bootstrap nodes are available and the config file has
an multicast-bootstrap element, then the node SHOULD send a Ping
request over UDP to the address and port found to each
multicast-bootstrap element found in the configuration document. This
MAY be a multicast, broadcast, or anycast address. The Ping should use
the wildcard Node-ID as the destination Node-ID.</t>
<t>The responder node 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 Node">
<t>In order to join the overlay, the joining node MUST contact a node
in the overlay. Typically this means contacting the bootstrap nodes,
since they are reachable by the local peer or have public IP
addresses. If the joining node 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 nodes before
falling back to the bootstrap nodes listed in the configuration
file.</t>
<t>When contacting a bootstrap node, the joining node first forms the
DTLS or TLS connection to the bootstrap node and then sends an Attach
request over this connection with the destination Node-ID set to the
joining node's Node-ID.</t>
<t>When the requester node finally does receive a response from some
responding node, 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 node has successfully joined the overlay network, it will
have direct connections to several peers. Some MAY be added to the
cached bootstrap nodes 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>The following abbreviation are used in the message flow diagrams: JP
= joining peer, AP = admitting peer, NP = next peer after the AP, NNP =
next next peer which is the peer after NP, PP = previous peer before the
AP, PPP = previous previous peer which is the peer before the PP, BP =
bootstrap peer.</t>
<t>The following abbreviation are used in the message flow diagrams:</t>
<t>In the following example, we assume that JP has formed a connection
to one of the bootstrap nodes. 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>
<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 an
Attach to a variety of locations around the overlay. The diagram below
shows it sending an Attach halfway around the Chord ring to 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 it is ready to take its
place in the DHT. It 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 it did not yet know their
addresses. However, now that it has received an Update from AP, it has
AP's predecessors, which are also its own, so it sends Attaches to them.
Below it is shown connecting to AP's 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. More background
information can be found in <xref target="RFC5765"></xref>.</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 third peer to fetch this stored data. Other 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 specification 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 Node-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 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 an 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 allow 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 with the same private key that was used to authorize
the 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 an Overlay Instance carries with it the
responsibility to store data for a given region of the Overlay
Instance. However, allowing clients to store unlimited amounts of
data would create unacceptable burdens on peers and would also
enable 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 provides 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
has 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>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 effects 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 that misroutes 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 one 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 a 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 will match only 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>Replay is typically prevented for messages that impact the
topology of the overlay by having the information come directly, or
be verified by, the nodes that claimed to have generated the update.
Data storage replay detection is done by signing time of the node
that generated the signature on the store request thus providing a
time based replay protection but the time synchronization is only
needed between peers that can write to the same location.</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 is unlikely to be able to
observe the activities of a specific individual 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 an
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 anchor="sec.iana-cons" 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="Well-Known URI Registration">
<t>IANA will make the following "Well Known URI" registration as
described in <xref target="RFC5785"></xref>:</t>
<t>[[Note to RFC Editor - this paragraph can be removed before
publication. ]] A review request was sent to
wellknown-uri-review@ietf.org on October 12, 2010.</t>
<texttable>
<ttcol></ttcol>
<ttcol></ttcol>
<c>URI suffix:</c>
<c>p2psip-enroll</c>
<c>Change controller:</c>
<c>IETF <iesg@ietf.org></c>
<c>Specification document(s):</c>
<c>[RFC-AAAA]</c>
<c>Related information:</c>
<c>None</c>
</texttable>
</section>
<section title="Port Registrations">
<t>[[Note to RFC Editor - this paragraph can be removed before
publication. ]] IANA has already allocated a TCP port for the main
peer to peer protocol. This port has the name p2p-sip and the port
number of 6084. IANA will update this registration to be defined for
UDP as well as TCP.</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</c>
<c>Port Number</c>
<c>TBD</c>
<c>Service Name</c>
<c>p2psip-enroll</c>
<c>Description</c>
<c>Peer to Peer Infrastructure Enrollment</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>
<texttable>
<ttcol align="left">Access Policy</ttcol>
<ttcol align="right">RFC</ttcol>
<c>USER-MATCH</c>
<c>RFC-AAAA</c>
<c>NODE-MATCH</c>
<c>RFC-AAAA</c>
<c>USER-NODE-MATCH</c>
<c>RFC-AAAA</c>
<c>NODE-MULTIPLE</c>
<c>RFC-AAAA</c>
<!--
<t>USER-MATCH-WITH-ANONYMOUS-CREATE</t>
<t>USER-KEY-MATCH</t>
<t>NODE-KEY-MATCH</t>
-->
</texttable>
</section>
<section anchor="sec.iana.app" title="Application-ID">
<t>IANA SHALL create a "RELOAD Application-ID" Registry. Entries in
this registry are 16-bit integers denoting application kinds. Code
points in the range 0x0001 to 0x7fff SHALL be registered via RFC 5226
Standards Action. Code points in the range 0x8000 to 0xf000 SHALL be
registered via RFC 5226 Expert Review. Code points in the range 0xf001
to 0xfffe are reserved for private use. The initial contents of this
registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Application</ttcol>
<ttcol align="right">Application-ID</ttcol>
<ttcol align="right">Specification</ttcol>
<c>INVALID</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>RELOAD</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>SIP</c>
<c>5060</c>
<c>Reserved for use by SIP Usage</c>
<c>SIP</c>
<c>5061</c>
<c>Reserved for use by SIP Usage</c>
<c>Reserved</c>
<c>0xffff</c>
<c>RFC-AAAA</c>
</texttable>
</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
0xfffffffe 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_BY_NODE</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>CERTIFICATE_BY_USER</c>
<c>16</c>
<c>RFC-AAAA</c>
<c>Reserved</c>
<c>0x7fffffff</c>
<c>RFC-AAAA</c>
<c>Reserved</c>
<c>0xfffffffe</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 anchor="sec-iana-messages-codes" 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>unused (was attachlite_req)</c>
<c>27</c>
<c>RFC-AAAA</c>
<c>unused (was attachlite_ans)</c>
<c>28</c>
<c>RFC-AAAA</c>
<c>app_attach_req</c>
<c>29</c>
<c>RFC-AAAA</c>
<c>app_attach_ans</c>
<c>30</c>
<c>RFC-AAAA</c>
<c>unused (was app_attachlite_req)</c>
<c>31</c>
<c>RFC-AAAA</c>
<c>unused (was app_attachlite_ans)</c>
<c>32</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>Error_Response_Too_Large</c>
<c>14</c>
<c>RFC-AAAA</c>
<c>Error_Config_Too_Old</c>
<c>15</c>
<c>RFC-AAAA</c>
<c>Error_Config_Too_New</c>
<c>16</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>0x8000..0xfffe</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section anchor="sec-iana-overlay-link" title="Overlay Link Types">
<t>IANA shall create a "RELOAD Overlay Link." 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>DTLS-UDP-SR</c>
<c>1</c>
<c>RFC-AAAA</c>
<!-- <c>TLS-TCP-FH</c> -->
<!-- <c>2</c> -->
<!-- <c>RFC-AAAA</c> -->
<c>DTLS-UDP-SR-NO-ICE</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>TLS-TCP-FH-NO-ICE</c>
<c>4</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section anchor="sec.iana-overlay-link-protocols"
title="Overlay Link Protocols">
<t>IANA shall create an "Overlay Link Protocol Registry". Entries in
this registry SHALL be defined via RFC 5226 Standards Action. This
registry SHALL be initially populated with the following value:
"TLS".</t>
</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 then 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>[[ Note to RFC Editor - please remove this paragraph before
publication. ]] Review request was sent to uri-review@ietf.org on
Oct 7, 2010.</t>
<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> of RFC-AAAA</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, so it is encoded entirely in
US-ASCII.</t>
<t
hangText="Applications/protocols that use this URI scheme:">The
RELOAD protocol described in RFC-AAAA.</t>
<t hangText="Interoperability considerations:">See RFC-AAAA.</t>
<t hangText="Security considerations:">See RFC-AAAA</t>
<t hangText="Contact:">Cullen Jennings
<fluffy@cisco.com></t>
<t hangText="Author/Change controller:">IESG</t>
<t hangText="References:">RFC-AAAA</t>
</list></t>
</section>
</section>
</section>
<section title="Acknowledgments">
<t>This specification 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 improvements.</t>
<t>The ideas and text for the Chord specific extension data to the Leave
mechanisms was provided by J. Maenpaa, G. Camarillo, and J.
Hautakorpi.</t>
<t>Thanks to the many people who contributed including Ted Hardie,
Michael Chen, Dan York, Das Saumitra, Lyndsay Campbell, Brian Rosen,
David Bryan, Dave Craig, and Julian Cain. Extensive working last call
comments were provided by: Jouni Maenpaa, Roni Even, Ari Keranen, John
Buford, Michael Chen, Frederic-Philippe Met, and David Bryan.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.5245"?>
<?rfc include="reference.RFC.5389"?>
<?rfc include="reference.RFC.5766"?>
<?rfc include="reference.RFC.5273"?>
<?rfc include="reference.RFC.5272"?>
<?rfc include="reference.RFC.4279"?>
<?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"?>
<?rfc include="reference.RFC.2818"?>
<!--
TODO this is the 6man ref
include="reference.RFC.5952"
-->
<?rfc include="reference.I-D.ietf-6man-text-addr-representation"?>
</references>
<references title="Informative References">
<reference anchor="I-D.ietf-mmusic-ice-tcp">
<front>
<title>TCP Candidates with Interactive Connectivity Establishment
(ICE)</title>
<author fullname="Jonathan Rosenberg" initials="J"
surname="Rosenberg">
<organization></organization>
</author>
<date day="14" month="July" year="2008" />
<abstract>
<t>Interactive Connectivity Establishment (ICE) defines a
mechanism for NAT traversal for multimedia communication protocols
based on the offer/answer model of session negotiation. ICE works
by providing a set of candidate transport addresses for each media
stream, which are then validated with peer-to-peer connectivity
checks based on Session Traversal Utilities for NAT (STUN). ICE
provides a general framework for describing candidates, but only
defines UDP-based transport protocols. This specification extends
ICE to TCP-based media, including the ability to offer a mix of
TCP and UDP-based candidates for a single stream.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-mmusic-ice-tcp-07" />
<format target="http://www.ietf.org/internet-drafts/draft-ietf-mmusic-ice-tcp-07.txt"
type="TXT" />
</reference>
<?rfc include="reference.I-D.maenpaa-p2psip-self-tuning"?>
<?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.4145"?>
<?rfc include="reference.RFC.4086"?>
<?rfc include="reference.RFC.5054"?>
<?rfc include="reference.RFC.5280"?>
<?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.RFC.5785"?>
<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>
<?rfc include="reference.RFC.5765"?>
<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="In Proc. of the USENIX Annual Technical Conference June 2004"
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>
<!--
<section anchor="sec-changes" title="Change Log">
<section title="Changes since draft-ietf-p2psip-reload-10">
<t><list style="symbols">
<t>TODO</t>
</list></t>
</section>
</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
selecting 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 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 were a new message initiated 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 ones are, because the forward
path is more likely to have a failed peer than is 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, it is
absolutely necessary to establish a secure connection to
authenticate A before delivering the response. 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 both be 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 makes 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 participating in the overlay to
determine trustworthiness; controlling the number of peers in the
overlay to reduce overly-long routing paths; or ensuring 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 Attach 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>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 10:05:29 |