One document matched: draft-ietf-p2psip-base-00.xml
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<rfc category="std" docName="draft-ietf-p2psip-base-00" ipr="full3978">
<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>SIPeerior Technologies</organization>
<address>
<postal>
<street>5251-18 John Tyler Highway #330</street>
<city>Williamsburg</city>
<region>VA</region>
<code>23185</code>
<country>USA</country>
</postal>
<phone>+1 757 565 0101</phone>
<email>bbl@lowekamp.net</email>
</address>
</author>
<author fullname="Eric Rescorla" initials="E.K." surname="Rescorla">
<organization>Network Resonance</organization>
<address>
<postal>
<street>2064 Edgewood Drive</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94303</code>
<country>USA</country>
</postal>
<phone>+1 650 320-8549</phone>
<email>ekr@networkresonance.com</email>
</address>
</author>
<author fullname="Salman A. Baset" initials="S.A." surname="Baset">
<organization>Columbia University</organization>
<address>
<postal>
<street>1214 Amsterdam Avenue</street>
<city>New York</city>
<region>NY</region>
<country>USA</country>
</postal>
<email>salman@cs.columbia.edu</email>
</address>
</author>
<author fullname="Henning Schulzrinne" initials="H.G."
surname="Schulzrinne">
<organization>Columbia University</organization>
<address>
<postal>
<street>1214 Amsterdam Avenue</street>
<city>New York</city>
<region>NY</region>
<country>USA</country>
</postal>
<email>hgs@cs.columbia.edu</email>
</address>
</author>
<date day="27" month="October" year="2008" />
<area>RAI</area>
<workgroup>P2PSIP</workgroup>
<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 that do not need
to route traffic or store data for others.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<!-- TODO EKR: compare this against the glossary so that each
term is introduced -->
<t>This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. It
provides a generic, self-organizing overlay network service, allowing
nodes to efficiently route messages to other nodes and to efficiently
store and retrieve data in the overlay. RELOAD provides several features
that are critical for a successful P2P protocol for the Internet:</t>
<t><list style="hanging">
<t></t>
<t hangText="Security Framework:">A P2P network will often be
established among a set of peers that do not trust each other.
RELOAD leverages a central enrollment server to provide credentials
for each peer which can then be used to authenticate each operation.
This greatly reduces the possible attack surface.</t>
<t></t>
<t hangText="Usage Model:">RELOAD is designed to support a variety
of applications, including P2P multimedia communications with the
Session Initiation Protocol <xref
target="I-D.ietf-p2psip-sip"></xref>. RELOAD allows the definition
of new application usages, each of which can define its own data
types, along with the rules for their use. This allows RELOAD to be
used with new applications through a simple documentation process
that supplies the details for each application.</t>
<t></t>
<t hangText="NAT Traversal:">RELOAD is designed to function in
environments where many if not most of the nodes are behind NATs or
firewalls. Operations for NAT traversal are part of the base design,
including using ICE to establish new RELOAD or application protocol
connections as well as tunneling application protocols across the
overlay.</t>
<t></t>
<t hangText="High Performance Routing:">The very nature of overlay
algorithms introduces a requirement that peers participating in the
P2P network route requests on behalf of other peers in the network.
This introduces a load on those other peers, in the form of
bandwidth and processing power. RELOAD has been defined with a
simple, lightweight forwarding header, thus minimizing the amount of
effort required by intermediate peers.</t>
<t></t>
<t hangText="Pluggable Overlay Algorithms:">RELOAD has been designed
with an abstract interface to the overlay layer to simplify
implementing a variety of structured (DHT) and unstructured overlay
algorithms. This specification also defines how RELOAD is used with
Chord, which is mandatory to implement. Specifying a default "must
implement" overlay algorithm will allow interoperability, while the
extensibility allows selection of overlay algorithms optimized for a
particular application.</t>
</list></t>
<t>These properties were designed specifically to meet the requirements
for a P2P protocol to support SIP. This document defines the base
protocol for the distributed storage and location service, as well as
critical usages for NAT traversal and security. The SIP Usage itself is
described separately in <xref target="I-D.ietf-p2psip-sip"></xref>.
RELOAD is not limited to usage by SIP and could serve as a tool for
supporting other P2P applications with similar needs. RELOAD is also
based on the concepts introduced in <xref
target="I-D.ietf-p2psip-concepts"></xref>.</t>
<section title="Basic Setting">
<t>In this section, we provide a brief overview of the operational
setting for RELOAD. See the concepts document for more details. A
RELOAD Overlay Instance consists of a set of nodes arranged in a
partly connected graph. Each node in the overlay is assigned a numeric
Node-ID which, together with the specific overlay algorithm in use,
determines its position in the graph and the set of nodes it connects
to. The figure below shows a trivial example which isn't drawn from
any particular overlay algorithm, but was chosen for convenience of
representation.</t>
<figure>
<artwork><![CDATA[
+--------+ +--------+ +--------+
| Node 10|--------------| Node 20|--------------| Node 30|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 40|--------------| Node 50|--------------| Node 60|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 70|--------------| Node 80|--------------| Node 90|
+--------+ +--------+ +--------+
|
|
+--------+
| Node 85|
|(Client)|
+--------+
]]></artwork>
</figure>
<t>Because the graph is not fully connected, when a node wants to send
a message to another node, it may need to route it through the
network. For instance, Node 10 can talk directly to nodes 20 and 40,
but not to Node 70. In order to send a message to Node 70, it would
first send it to Node 40 with instructions to pass it along to Node
70. Different overlay algorithms will have different connectivity
graphs, but the general idea behind all of them is to allow any node
in the graph to efficiently reach every other node within a small
number of hops.</t>
<t>The RELOAD network is not only a messaging network. It is also a
storage network. Records are stored under numeric addresses which
occupy the same space as node identifiers. Nodes 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 node is
responsible for storing any data value which has an address less than
or equal to its own Node-ID, but greater than the next lowest Node-ID.
Thus, Node-20 would be responsible for storing values 11-20.</t>
<t>RELOAD also supports clients. These are nodes which have Node-IDs
but do not participate in routing or storage. For instance, in the
figure above Node 85 is a client. It can route to the rest of the
RELOAD network via Node 80, but no other node will route through it
and Node 90 is still responsible for all addresses between 81-90. We
refer to non-client nodes as peers.</t>
<t>Other applications (for instance, SIP) can be defined on top of
RELOAD and use these two basic RELOAD services to provide their own
services.</t>
</section>
<section title="Architecture">
<t>
RELOAD is fundamentally an overlay network. Therefore,
there are two possible perspectives for viewing RELOAD's
architecture. The first (and what has been presented in
previous revisions of this document) is an implementer's
view, with the bottom of the stack being a "transport layer"
because it comprises TCP/TLS, UDP/DTLS, etc. The second is
the overlay network's perspective, in which the bottom layer
of the RELOAD stack is labeled a "link layer" because it is
providing link-like hop-by-hop services from the overlay's
point of view.
</t>
<t>This revision of the document presents both perspectives.
The authors will attempt to determine WG consensus for the
preferred layering perspective and adopt that terminology in a
future revision of this draft.
</t>
<t>From an implementer's perspective, architecturally RELOAD is divided into several layers, as shown in
the following figure:</t>
<figure>
<artwork><![CDATA[
Application
+-------+ +-------+
| SIP | | XMPP | ...
| Usage | | Usage |
+-------+ +-------+
-------------------------------------- Message Routing API
+------------------+ +---------+
| |<->| Storage |
| | +---------+
| Routing | ^
| Layer | v
| | +---------+
| |<->|Topology |
| | | Plugin |
+------------------+ +---------+
^ ^
v |
+------------------+ <------+
| Forwarding |
| Layer |
+------------------+
-------------------------------------- Transport API
+-------+ +------+
|TLS | |DTLS | ...
+-------+ +------+
]]></artwork>
</figure>
<t>The major components of RELOAD are:</t>
<t><list style="hanging">
<t></t>
<t hangText="Usage Layer:">Each application defines a RELOAD
usage; a set of data kinds and behaviors which describe how to use
the services provided by RELOAD. These usages all talk to RELOAD
through a common Message Routing API.</t>
<t></t>
<t hangText="Routing Layer:">The Routing Layer is responsible for
routing messages through the overlay. It also manages request
state for the usages and forwards Store and Fetch operations to
the Storage component. It talks directly to the Topology Plugin,
which is responsible for implementing the specific topology
defined by the overlay algorithm being used.</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 and the routing layer in
order to send and receive messages and manage data replication and
migration.</t>
<t></t>
<t hangText="Topology Plugin:">The Topology Plugin is responsible
for implementing the specific overlay algorithm being used. It
talks directly to the Routing Layer 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.</t>
<t></t>
<t hangText="Forwarding Layer:">The Forwarding Layer provides
packet forwarding services between nodes. It also handles setting
up connections across NATs using ICE.</t>
</list></t>
<t>From the overlay's perspective, the architecture can be
viewed as:</t>
<figure>
<artwork><![CDATA[
Application
+-------+ +-------+
| SIP | | XMPP | ...
| Usage | | Usage |
+-------+ +-------+
-------------------------------------- Messaging API
+------------------+ +---------+
| Message |<--->| Storage |
| Transport | +---------+
+------------------+ ^
^ ^ |
| v v
| +-------------------+
| | Topology |
| | Plugin |
| +-------------------+
v ^
v
+------------------+
| Forwarding & |
| Link Management |
+------------------+
-------------------------------------- Overlay Link API
+-------+ +------+
|TLS | |DTLS | ...
+-------+ +------+
]]></artwork>
</figure>
<t>
Other than labeling, there is a slight difference in
responsibilities between these components.
</t>
<t><list style="hanging">
<t></t>
<t hangText="Message Transport:">Handles the 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="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="Link Layer:">TLS and DTLS
are the "link layer" protocols used by RELOAD for
inter-node communication. Each such protocol includes the
appropriate provisions for per-hop framing or hop-by-hop
ACKs required by unreliable transports.</t>
</list>
</t>
<t>To further clarify the roles of the various layer, this
figure parallels the architecture with each layer's role from
an overlay perspective and implementation layer in the
internet:</t>
<figure>
<artwork><![CDATA[
| Internet Model |
Real | Equivalent | Reload
Internet | in Overlay | Architecture
---------------+-----------------+------------------------------------
| | +-------+ +-------+
| Application | | SIP | | XMPP | ...
| | | Usage | | Usage |
| | +-------+ +-------+
| | ----------------------------------
| |+------------------+ +---------+
| Transport || Message |<--->| Storage |
| || Transport | +---------+
| |+------------------+ ^
| | ^ ^ |
| | | v v
Application | | | +-------------------+
| (Routing) | | | Topology |
| | | | Plugin |
| | | +-------------------+
| | v ^
| | v
| Network | +------------------+
| | | Forwarding & |
| | | Link Management |
| | +------------------+
| | ----------------------------------
Transport | Link | +-------+ +------+
| | |TLS | |DTLS | ...
| | +-------+ +------+
----------------+-----------------+------------------------------------
Network |
|
Link |
]]></artwork>
</figure>
<section title="Usage Layer">
<t>The top layer, called the Usage Layer, has application usages,
such as the SIP Location Usage, that use the abstract Message
Routing API provided by RELOAD. The goal of this layer is to
implement application-specific usages of the generic overlay
services provided by RELOAD. The usage defines how a specific
application maps its data into something that can be stored in the
overlay, where to store the data, how to secure the data, and
finally how applications can retrieve and use the data.</t>
<t>The architecture diagram shows both a SIP usage and an XMPP
usage. A single application may require multiple usages, for example
a SIP application may also require a voicemail usage. A usage may
define multiple kinds of data that are stored in the overlay and may
also rely on kinds originally defined by other usages.</t>
<t>Because the security and storage policies for each kind
are dictated by the usage defining the kind, the usages may
be coupled with the Storage component to provide security
policy enforcement and to implement appropriate storage
strategies according to the needs of the usage. The exact
implementation of such an interface is outside the scope of
this draft.</t>
<t>This draft also defines a Diagnostics Usage, which can be used to
obtain diagnostic information about a peer in the overlay. The
Diagnostics Usage is interesting both to administrators monitoring
the overlay as well as to some overlay algorithms that base their
decisions on capabilities and current load of nodes in the
overlay.</t>
</section>
<section title="Routing Layer">
<t>The Routing Layer provides a generic message routing service for
the overlay. Each peer is identified by its location in the overlay
as determined by its Node-ID. A component which is a client of the
Routing Layer can perform two basic functions:</t>
<t><list style="symbols">
<t>Send a message to a given peer, specified by Node-ID or
Resource-ID.</t>
<t>Receive messages that other peers sent to a Node-ID or
Resource-ID for which this peer is responsible.</t>
</list></t>
<t>All usages are clients of the Routing Layer and use RELOAD's
services by sending and receiving 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 Routing Layer, because they need to
send and receive messages from other peers.</t>
<t>The Routing Layer provides a fairly generic interface that allows
the topology plugin control the overlay and resource operations and
messages. Since each overlay algorithm is defined and functions
differently, we generically refer to the table of other peers that
the overlay algorithm maintains and uses to route requests
(neighbors) as a Routing Table. The Routing Layer component makes
queries to the overlay algorithm to determine the next hop, then
encodes and sends the message itself. Similarly, the overlay
algorithm issues periodic update requests through the logic
component to maintain and update its Routing Table.</t>
<t>The "Overlay Perspective" architecture diagram replaces
this component with the "Message Transport" component, which
responsible for end-to-end responsibilities for a message
and not the hop-by-hop forwarding, which is clearly assigned
to the forwarding layer.</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 Routing
Layer. It would then query the appropriate usage before storing the data value(s) in its local data store
and sends a response to the Routing Layer for delivery to the
requesting peer. Typically, these messages will come for 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>The node's Node-ID determines the set of resources which it will
be responsible for storing. However, the exact mapping between these
is determined by the overlay algorithm used by the overlay,
therefore the Storage component always the queries the topology
plugin to determine where a particular resource should be
stored.</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 Routing
Layer (replace with Forwarding Layer)
before routing a message. When connections are made or broken, the
Forwarding Layer notifies the Topology Plugin, which adjusts the
routing table as appropriate. The Topology Plugin will also instruct
the Forwarding Layer to form new connections as dictated by the
requirements of the overlay algorithm Topology.</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 issues 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 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 Layer">
<t>The Forwarding Layer is responsible for getting a packet to the
next peer, as determined by the Routing and Storage Layer. The
Forwarding 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>The "Overlay Perspective" assigns this layer the
responsibility of performing hop-by-hop forwarding based on
the routing table, or delivering a message bound for this
node to the Message Transport component as appropriate.</t>
<t>The Forwarding Layer sits on top of transport layer protocols
which carry the actual traffic. This specification defines how to
use DTLS and TLS protocols to carry RELOAD messages.</t>
</section>
</section>
<section title="Security">
<t>RELOAD's security model is based on each node having one or more
public key certificates. In general, these certificates will be
assigned by a central server which also assigns Node-IDs, although
self-signed certificates can be used in closed networks. These
credentials can be leveraged to provide communications security for
RELOAD messages. RELOAD provides communications security at three
levels:</t>
<t><list style="hanging">
<t hangText="Connection Level: ">Connections between peers are
secured with TLS or DTLS.</t>
<t hangText="Message Level: ">Each RELOAD message must be
signed.</t>
<t hangText="Object Level: ">Stored objects must be signed by the
storing peer.</t>
</list></t>
<t>These three levels of security work together to allow peers to
verify the origin and correctness of data they receive from other
peers, even in the face of malicious activity by other peers in the
overlay. RELOAD also provides access control built on top of these
communications security features. Because the peer responsible for
storing a piece of data can validate the signature on the data being
stored, the responsible peer can determine whether a given operation
is permitted or not.</t>
<t>RELOAD also provides a shared secret based admission control
feature using shared secrets and TLS-PSK. In order to form a TLS
connection to any node in the overlay, a new node needs to know the
shared overlay key, thus restricting access to authorized users.</t>
</section>
<section title="Structure of This Document">
<t>The remainder of this document is structured as follows.</t>
<t><list style="symbols">
<t><xref target="sec-term"></xref> provides definitions of terms
used in this document.</t>
<t><xref target="sec-overlay-overview"></xref> provides an
overview of the mechanisms used to establish and maintain the
overlay.</t>
<t><xref target="sec-app-support"></xref> provides an overview of
the mechanism RELOAD provides to support other applications.</t>
<t><xref target="sec-overlay-protocol"></xref> defines the
protocol messages that RELOAD uses to establish and maintain the
overlay.</t>
<t><xref target="sec-data-protocol"></xref> defines the protocol
messages that are used to store and retrieve data using
RELOAD.</t>
<t><xref target="sec-store-usage"></xref> defines the Certificate
Store Usage that is fundamental to RELOAD security.</t>
<t><xref target="sec-turn-server"></xref> defines the TURN Server
Usage needed to locate TURN servers for NAT traversal.</t>
<t><xref target="sec-diagnostic"></xref> defines a diagnostic
usage for obtaining information about node performance.</t>
<t><xref target="sec-chord-algorithm"></xref> defines a specific
Topology Plugin using Chord.</t>
<t><xref target="sec-enrollment"></xref> defines the mechanisms
that new RELOAD nodes use to join the overlay for the first
time.</t>
<t><xref target="sec-msgflow"></xref> provides an extended
example.</t>
</list></t>
</section>
</section>
<section anchor="sec-term" title="Terminology">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
<t>We use the terminology and definitions from the <xref
target="I-D.ietf-p2psip-concepts">Concepts and Terminology for Peer to
Peer SIP</xref> draft extensively in this document. Other terms used in
this document are defined inline when used and are also defined below
for reference. Terms which are new to this document (and perhaps should
be added to the concepts document) are marked with a (*).</t>
<t><list style="hanging">
<t></t>
<t hangText="DHT:">A distributed hash table. A DHT is an abstract
hash table service realized by storing the contents of the hash
table across a set of peers.</t>
<t></t>
<t hangText="Overlay Algorithm:">An overlay algorithm defines the
rules for determining which peers in an overlay store a particular
piece of data and for determining a topology of interconnections
amongst peers in order to find a piece of data.</t>
<t></t>
<t hangText="Overlay Instance:">A specific overlay algorithm and the
collection of peers that are collaborating to provide read and write
access to it. There can be any number of overlay instances running
in an IP network at a time, and each operates in isolation of the
others.</t>
<t></t>
<t hangText="Peer:">A host that is participating in the overlay.
Peers are responsible for holding some portion of the data that has
been stored in the overlay and also route messages on behalf of
other hosts as required by the Overlay Algorithm.</t>
<t></t>
<t hangText="Client:">A host that is able to store data in and
retrieve data from the overlay but which is not participating in
routing or data storage for the overlay.</t>
<t></t>
<t hangText="Node:">We use the term "Node" to refer to a host that
may be either a Peer or a Client. Because RELOAD uses the same
protocol for both clients and peers, much of the text applies
equally to both. Therefore we use "Node" when the text applies to
both Clients and Peers and the more specific term when the text
applies only to Clients or only to Peers.</t>
<t></t>
<t hangText="Node-ID:">A 128-bit value that uniquely identifies a
node. Node-IDs 0 and 2^128 - 1 are reserved and are invalid
Node-IDs. A value of zero is not used in the wire protocol but can
be used to indicate an invalid node in implementations and APIs. The
Node-ID of 2^128-1 is used on the wire protocol as a wildcard.
(*)</t>
<t></t>
<t hangText="Resource:">An object or group of objects associated
with a string identifier see "Resource Name" below.</t>
<t></t>
<t hangText="Resource Name:">The potentially human readable name by
which a resource is identified. In unstructured P2P networks, the
resource name is sometimes used directly as a Resource-ID. In
structured P2P networks the resource name is typically mapped into a
Resource-ID by using the string as the input to hash function. A SIP
resource, for example, is often identified by its AOR which is an
example of a Resource Name.(*)</t>
<t></t>
<t hangText="Resource-ID:">A value that identifies some resources
and which is used as a key for storing and retrieving the resource.
Often this is not human friendly/readable. One way to generate a
Resource-ID is by applying a mapping function to some other unique
name (e.g., user name or service name) for the resource. The
Resource-ID is used by the distributed database algorithm to
determine the peer or peers that are responsible for storing the
data for the overlay. In structured P2P networks, Resource-IDs are
generally fixed length and are formed by hashing the resource name.
In unstructured networks, resource names may be used directly as
Resource-IDs and may have variable length.</t>
<t></t>
<t hangText="Connection Table:">The set of peers to which a node is
directly connected. This includes nodes with which Attach handshakes
have been done but which have not sent any Updates.</t>
<t></t>
<t hangText="Routing Table:">The set of peers which a node can use
to route overlay messages. In general, these peers will all be on
the connection table but not vice versa, because some peers will
have Attached but not sent updates. Peers may send messages directly
to peers which are on the connection table but may only route
messages to other peers through peers which are on 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, certificate, STUN server and TURN server
data kinds. (*)</t>
</list></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="RFC3280"></xref>
containing a Node-ID, which is globally unique.</t>
<t>The certificate serves multiple purposes:</t>
<t><list style="symbols">
<t>It entitles the user to store data at specific locations in the
Overlay Instance. Each data kind defines the specific rules for
determining which certificates can access each Resource-ID/Kind-ID
pair. For instance, some kinds might allow anyone to write at a
given location, whereas others might restrict writes to the owner
of a single certificate.</t>
<t>It entitles the user to operate a node that has a Node-ID found
in the certificate. When the node forms a connection to another
peer, it can use this certificate so that a node connecting to it
knows it is connected to the correct node. In addition, the node
can sign messages, thus providing integrity and authentication for
messages which are sent from the node.</t>
<t>It entitles the user to use the user name found in the
certificate.</t>
</list></t>
<t>If a user has more than one device, typically they would get one
certificate for each device. This allows each device to act as a
separate peer.</t>
<t>RELOAD supports two certificate issuance models. The first is based
on a central enrollment process which allocates a unique name and
Node-ID to the node a certificate for a public/private key pair for
the user. All peers in a particular Overlay Instance have the
enrollment server as a trust anchor and so can verify any other peer's
certificate.</t>
<t>In some settings, a group of users want to set up an overlay
network but are not concerned about attack by other users in the
network. For instance, users on a LAN might want to set up a short
term ad hoc network without going to the trouble of setting up an
enrollment server. RELOAD supports the use of self-generated and
self-signed certificates. When self-signed certificates are used, the
node also generates its own Node-ID and username. The Node-ID is
computed as a digest of the public key, to prevent Node-ID theft,
however this model is still subject to a number of known attacks (most
notably Sybil attacks <xref target="Sybil"></xref>) and can only be
safely used in closed networks where users are mutually trusting.</t>
<section anchor="sec-shared-key" title="Shared-Key Security">
<t>RELOAD also provides an admission control system based on shared
keys. In this model, the peers all share a single key which is used
to authenticate the peer-to-peer connections via
TLS-PSK/TLS-SRP.</t>
</section>
</section>
<section title="Clients">
<t>RELOAD defines a single protocol that is used both as the peer
protocol and the client protocol for the overlay. This simplifies
implementation, particularly for devices that may act in either role,
and allows clients to inject messages directly into the overlay.</t>
<t>We use the term "peer" to identify a node in the overlay that
routes messages for nodes other than those to which it is directly
connected. Peers typically also have storage responsibilities. We use
the term "client" to refer to nodes that do not have routing or
storage responsibilities. When text applies to both peers and clients,
we will simply refer to such a device as a "node."</t>
<t>RELOAD's client support allows nodes that are not participating in
the overlay as peers to utilize the same implementation and to benefit
from the same security mechanisms as the peers. Clients possess and
use certificates that authorize the user to store data at its
locations in the overlay. The Node-ID in the certificate is used to
identify the particular client as a member of the overlay and to
authenticate its messages.</t>
<t>For more discussion of the motivation for RELOAD's client support,
see <xref target="sec-why-clients"></xref>.</t>
<section title="Client Routing">
<t>There are two routing options by which a client may be located in
an overlay.</t>
<t><list style="symbols">
<t>Establish a connection to the peer responsible for the
client's Node-ID in the overlay. Then requests may be sent
from/to the client using its Node-ID in the same manner as if it
were a peer, because the responsible peer in the overlay will
handle the final step of routing to the client. This will not
work in overlays where NAT or firewall do not allow all clients
to form connections with any other peer.</t>
<t>Establish a connection with an arbitrary peer in the overlay
(perhaps based on network proximity or an inability to establish
a direct connection with the responsible peer). In this case,
the client will rely on RELOAD's Destination List feature to
ensure reachability. The client can initiate requests, and any
node in the overlay that knows the Destination List to its
current location can reach it, but the client is not directly
reachable directly using only its Node-ID. The Destination List
required to reach it must be learnable via other mechanisms,
such as being stored in the overlay by a usage, if the client is
to receive incoming requests from other members of the
overlay.</t>
</list></t>
</section>
<section title="Minimum Functionality Requirements for Clients">
<t>A node may act as a client simply because it does not have the
resources or even an implementation of the topology plugin required
to acts as a peer in the overlay. In order to exchange RELOAD
messages with a peer, a client must meet a minimum level of
functionality. Such a client must:</t>
<!-- TODO add xrefs to relevant sections-->
<t><list style="symbols">
<t>Implement RELOAD's connection-management connections that are
used to establish the connection with the peer.</t>
<t>Implement RELOAD's data retrieval methods (with client
functionality).</t>
<t>Be able to calculate Resource-IDs used by the overlay.</t>
<t>Possess security credentials required by the overlay it is
implementing.</t>
</list></t>
<t>A client speaks the same protocol as the peers, knows how to
calculate Resource-IDs, and signs its requests in the same manner as
peers. While a client does not necessarily require a full
implementation of the overlay algorithm, calculating the Resource-ID
requires an implementation of the appropriate algorithm for the
overlay.</t>
<t>RELOAD does not support a separate protocol for clients that do
not meet these functionality requirements. Any such extension would
either entail compromises on the features of RELOAD or require an
entirely new protocol to reimplement the core features of RELOAD.
Furthermore, for SIP and many other applications, a native
application-level protocol already exists that is sufficient for
such a client to interact with a member of the RELOAD overlay.</t>
</section>
</section>
<section title="Routing">
<t>This section will discuss the requirements RELOAD's routing
capabilities must meet, then describe the routing features in the
protocol, and provide a brief overview of how they are used. <xref
target="sec-route-alt"></xref> discusses some alternative designs and
the tradeoffs that would be necessary to support them.</t>
<t>RELOAD's routing capabilities must meet the following
requirements:</t>
<t><list style="hanging">
<t hangText="NAT Traversal: ">RELOAD must support establishing and
using connections between nodes separated by one or more NATs,
including locating peers behind NATs for those overlays
allowing/requiring it.</t>
<t hangText="Clients: ">RELOAD must support requests from and to
clients that do not participate in overlay routing.</t>
<t hangText="Client promotion:">RELOAD must support clients that
become peers at a later point as determined by the overlay
algorithm and deployment.</t>
<t hangText="Low state: ">RELOAD's routing algorithms must not
require significant state to be stored on intermediate peers.</t>
<t hangText="Return routability in unstable topologies: ">At some
points in times, different nodes may have inconsistent information
about the connectivity of the routing graph. In all cases, the
response to a request needs to delivered to the node that sent the
request and not to some other node.</t>
</list></t>
<t>To meet these requirements, RELOAD's routing relies on two basic
mechanisms:</t>
<t><list style="hanging">
<t hangText="Via Lists: ">The forwarding header used by all RELOAD
messages contains both a Via List (built hop-by-hop as the message
is routed through the overlay) and a Destination List (providing
source-routing capabilities for requests and return-path routing
for responses).</t>
<t hangText="Route_Query: ">The Route_Query method allows a node
to query a peer for the next hop it will use to route a message.
This method is useful for diagnostics and for iterative
routing.</t>
</list></t>
<t>The basic routing mechanism used by RELOAD is Symmetric Recursive.
We will first describe symmetric routing and then discuss its
advantages in terms of the requirements discussed above.</t>
<t>Symmetric recursive routing requires a message follow the path
through the overlay to the destination without returning to the
originating node: each peer forwards the message closer to its
destination. The return path of the response is then the same path
followed in reverse. For example, a message following a route from A
to Z through B and X:</t>
<figure>
<artwork><![CDATA[
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Via=A, B
Dest=Z
<----------
Dest=X, B, A
<----------
Dest=B, A
<----------
Dest=A
]]></artwork>
</figure>
<t>Note that the preceding Figure does not indicate whether A is a
client or peer, A forwards its request to B and the response is
returned to A in the same manner regardless of A's role in the
overlay.</t>
<t>This figure shows use of full via-lists by intermediate peers B and
X. However, if B and/or X are willing to store state, then they may
elect to truncate the lists, save that information internally (keyed
by the transaction id), and return the response message along the path
from which it was received when the response is received. This option
requires greater state on intermediate peers but saves a small amount
of bandwidth and reduces the need for modifying the message in route.
Selection of this mode of operation is a choice for the individual
peer, the techniques are mutually interoperable even on a single
message. The figure below shows B using full via lists but X
truncating them and saving the state internally.</t>
<figure>
<artwork><![CDATA[
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Dest=Z
<----------
Dest=X
<----------
Dest=B, A
<----------
Dest=A
]]></artwork>
</figure>
<t>For debugging purposes, a Route Log attribute is available that
stores information about each peer as the message is forwarded.</t>
<t>RELOAD also supports a basic Iterative routing mode (where the
intermediate peers merely return a response indicating the next hop,
but do not actually forward the message to that next hop themselves).
Iterative routing is implemented using the Route_Query method, which
requests this behavior. Note that iterative routing is selected only
by the initiating node. RELOAD does not support an intermediate peer
returning a response that it will not recursively route a normal
request. The willingness to perform that operation is implicit in its
role as a peer in the overlay.</t>
</section>
<section title="Connectivity Management">
<t>In order to provide efficient routing, a peer needs to maintain a
set of direct connections to other peers in the Overlay Instance. Due
to the presence of NATs, these connections often cannot be formed
directly. Instead, we use the Attach request to establish a
connection. Attach uses ICE <xref
target="I-D.ietf-mmusic-ice-tcp"></xref> to establish the connection.
It is assumed that the reader is familiar with ICE.</t>
<t>Say that peer A wishes to form a direct connection to peer B. It
gathers ICE candidates and packages them up in an Attach request which
it sends to B through usual overlay routing procedures. B does its own
candidate gathering and sends back a response with its candidates. A
and B then do ICE connectivity checks on the candidate pairs. The
result is a connection between A and B. At this point, A and B can add
each other to their routing tables and send messages directly between
themselves without going through other overlay peers.</t>
<t>There is one special case in which Attach cannot be used: when a
peer is joining the overlay and is not connected to any peers. In
order to support this case, some small number of "bootstrap nodes"
need to be publicly accessible so that new peers can directly connect
to them. <xref target="sec-enrollment"></xref> contains more detail on
this.</t>
<t>In general, a peer needs to maintain connections to all of the
peers near it in the Overlay Instance and to enough other peers to
have efficient routing (the details depend on the specific overlay).
If a peer cannot form a connection to some other peer, this isn't
necessarily a disaster; overlays can route correctly even without
fully connected links. However, a peer should try to maintain the
specified link set and if it detects that it has fewer direct
connections, should form more as required. This also implies that
peers need to periodically verify that the connected peers are still
alive and if not try to reform the connection or form an alternate
one.</t>
</section>
<section title="Overlay Algorithm Support">
<t>The Topology Plugin allows RELOAD to support a variety of overlay
algorithms. This draft defines a DHT based on Chord <xref
target="Chord"></xref>, which is mandatory to implement, but the base
RELOAD protocol is designed to support a variety of overlay
algorithms.</t>
<section title="Support for Pluggable Overlay Algorithms">
<t>RELOAD defines three methods for overlay maintenance: Join,
Update, and Leave. However, the contents of those messages, when
they are sent, and their precise semantics are specified by the
actual overlay algorithm; RELOAD merely provides a framework of
commonly-needed methods that provides uniformity of notation (and
ease of debugging) for a variety of overlay algorithms.</t>
<!--
<t>An implementation should provide a suitable interface to
the topology plugin to allow it full freedom to implement
whatever routing, maintenance, and resource
allocation/replication strategies it desires. Specifying an
API for this purpose is beyond the scope of this draft, but
we recommend implementing at least two different overlay
algorithms to confirm that the interface is sufficiently
flexible.</t>
-->
</section>
<section anchor="sec-join-leave-maint"
title="Joining, Leaving, and Maintenance Overview">
<t>When a new peer wishes to join the Overlay Instance, it must have
a Node-ID that it is allowed to use. It uses the Node-ID in the
certificate it received from the enrollment server. The details of
the joining procedure are defined by the overlay algorithm, but the
general steps for joining an Overlay Instance are:</t>
<t><list style="symbols">
<t>Forming connections to some other peers.</t>
<t>Acquiring the data values this peer is responsible for
storing.</t>
<t>Informing the other peers which were previously responsible
for that data that this peer has taken over responsibility.</t>
</list></t>
<t>The first thing the peer needs to do is form a connection to some
"bootstrap node". Because this is the first connection the peer
makes, these nodes must have public IP addresses and therefore can
be connected to directly. Once a peer has connected to one or more
bootstrap nodes, it can form connections in the usual way by routing
Attach messages through the overlay to other nodes. Once a peer has
connected to the overlay for the first time, it can cache the set of
nodes it has connected to with public IP addresses for use as future
bootstrap nodes.</t>
<t>Once the peer has connected to a bootstrap node, it then needs to
take up its appropriate place in the overlay. This requires two
major operations:</t>
<t><list style="symbols">
<t>Forming connections to other peers in the overlay to populate
its Routing Table.</t>
<t>Getting a copy of the data it is now responsible for storing
and assuming responsibility for that data.</t>
</list></t>
<t>The second operation is performed by contacting the Admitting
Peer (AP), the node which is currently responsible for that section
of the overlay.</t>
<t>The details of this operation depend mostly on the overlay
algorithm involved, but a typical case would be:</t>
<t><list style="numbers">
<t>JP (Joining Peer) sends a Join request to AP (Admitting Peer)
announcing its intention to join.</t>
<t>AP sends a Join response.</t>
<t>AP does a sequence of Stores to JP to give it the data it
will need.</t>
<t>AP does Updates to JP and to other peers to tell it about its
own routing table. At this point, both JP and AP consider JP
responsible for some section of the Overlay Instance.</t>
<t>JP makes its own connections to the appropriate peers in the
Overlay Instance.</t>
</list></t>
<t>After this process is completed, JP is a full member of the
Overlay Instance and can process Store/Fetch requests.</t>
<t>Note that the first node is a special case. When ordinary nodes
cannot form connections to the bootstrap nodes, then they are not
part of the overlay. However, the first node in the overlay can
obviously not connect to others nodes. In order to support this
case, potential first nodes (which must also serve as bootstrap
nodes initially) must somehow be instructed (perhaps by
configuration settings) that they are the entire overlay, rather
than not part of it.</t>
</section>
</section>
<section title="First-Time Setup">
<t>Previous sections addressed how RELOAD works once a node has
connected. This section provides an overview of how users get
connected to the overlay for the first time. RELOAD is designed so
that users can start with the name of the overlay they wish to join
and perhaps a username and password, and leverage that into having a
working peer with minimal user intervention. This helps avoid the
problems that have been experienced with conventional SIP clients
where users are required to manually configure a large number of
settings.</t>
<section title="Initial Configuration">
<t>In the first phase of the process, the user starts out with the
name of the overlay and uses this to download an initial set of
overlay configuration parameters. The user does a DNS SRV lookup on
the overlay name to get the address of a configuration server. It
can then connect to this server with HTTPS to download a
configuration document which contains the basic overlay
configuration parameters as well as a set of bootstrap nodes which
can be used to join the overlay.<!-- Note that one or more of the peers can serve as
a configuration server. For instance, the
first peer to join the overlay might take on that role. --></t>
</section>
<section title="Enrollment">
<t>If the overlay is using centralized enrollment, then a user needs
to acquire a certificate before joining the overlay. The certificate
attests both to the user's name within the overlay and to the
Node-IDs which they are permitted to operate. In that case, the
configuration document will contain the address of an enrollment
server which can be used to obtain such a certificate. The
enrollment server may (and probably will) require some sort of
username and password before issuing the certificate. The enrollment
server's ability to restrict attackers' access to certificates in
the overlay is one of the cornerstones of RELOAD's security.</t>
</section>
</section>
<!-- usages are not being covered in the overview section
<section title="Diagnostics">
<t>The Diagnostic Usage allows to retrieve diagnostic and performance
statistics of nodes in the overlay. This is useful during debugging,
deployment, and maintenance of the overlay. Examples of diagnostic
information are routing table size, bytes sent and received per second, RTT, and
size of the instances stored. In addition, this usage allows to
retrieve kinds defined by other usages.</t>
<t>The access control of the kinds is defined by the peer and/or by the
overlay policy. The peer may have a list of users (such as "admin") that
it is willing to return the information for and restrict access to users
with that name.</t>
<t>During debugging and initial deployment, the diagnostic usage allows
to retrieve the routing and storage of peers, thereby allowing to
verify the routing state of the overlay. In a large deployment, the usage
allows the overlay 'admin' to query the state of a few peers and
construct a connectivity graph to identify routing inconsistencies.
Among the various candidates for a routing table entry, a peer may
select a node which has the smallest RTT, and the largest uptime.</t>
</section>
-->
<!-- EKR: RemoveD: TMI
<section title="Enrollment"> <t>Before a new user can join the Overlay
Instance for the first time, they must enroll in the P2P Network for
the Overlay Instance they want to join. Enrollment will typically be
done by contacting a centralized enrollment server. Other approaches
(for instance static out of band configuration) are possible but are
outside the scope of this specification. During enrollment a new node
learns about a particular overlay, sets up a names and credentials, and
discovers the bootstrap nodes. This would typically be done when a new
peer joined an overlay for the very first time. Bootstrap is the
process that happens each time a node boots and is how the peer finds
an node that can be used to join the overlay.</t>
<t>Before a node can join an overlay, it needs to be provided with a
name for the overlay. Some examples are "example.com", "example", and
"example.local". This name is resolved via a DNS SRV lookup for
service name p2p_enroll and a protocol of tcp. If the TLD for the
name is .local, then this DNS SRV lookup is done using <xref
target="I-D.cheshire-dnsext-multicastdns"></xref> and the service
name p2p_menroll. The intention here is to support ad hoc/local
overlays. The resulting DNS lookup will provide the address of a
enrollment server. Once this server is found, HTTPS is used to
retrieve a XML file that contains the parameters for the
overlay. These include things such as: what algorithms the overlay
uses, overlay parameters, what usages are a peer on this overlay is
required to support, the type of credentials required, addresses of
credentials servers, the root certificate for the Overlay Instance,
information about the overlay algorithm that is being used, a
P2P-Network-ID that uniquely identifies this ring, and any other
parameters it may need to connect to the Overlay Instance. The
overlay also informs the peers what Usages it is required to support
to be a peer on this P2P Network. It also provides an initial list of
bootstrap nodes that consists of multiple bootstrap entries that each
have the IP address and port for contacting a bootstrap peer. Some of
the address may be multicast addresses. In the case of multicast DNS,
every peer may also act as an enrollment server.</t>
<t>If shared-key security (<xref target="sec-shared-key"></xref>) is
being used, then the peer can proceed directly to bootstrap. If
certificate-based security (<xref target="sec-cert-security"></xref>
is being used, the peer MUST contact the credential server to obtain a
certificate.</t>
<section title="Certificate Issuance">
<t>Once the peer has the XML file that identifies if credentials are
needed, it can contact the credential server. The user establishes
his identity to the server's satisfaction and provides the server
with its public key. The centralized server then returns a
certificate binding the user's user name to his public key. The
properties of the certificate are discussed in [XREF].
The amount of authentication
performed here can vary radically depending on the overlay network being
joined. Some networks may do no verification at all and some may
require extensive identity verification (e.g., checking a driver's
license) before issuing credentials for a given user name. The only
invariant that the enrollment server needs to ensure is that no two
users may have the same identity.</t>
</section>
<section title="Bootstrap">
<t>The above steps are only done the first time a peer joins a new
overlay or when the overlay parameters are close to their expiration
time (as listed in the configuration document) and need to be
refreshed. The next step is the bootstrap step which is done every
time the peer boots.</t>
<t>Bootstrapping consists of looking at the list of cached nodes and
bootstraps nodes and sending a RELOAD Probe to them to see if they
respond. Once a node responds, it can be used to join the overlay.
After a node has joined, it keeps track of a small number of peers
to which it could directly connect. Theses are saved as the cached
nodes and used next time the peer boots. The point of the cached
nodes is to reduce the load on the bootstrap nodes.</t>
</section>
</section>
-->
</section>
<section anchor="sec-app-support" title="Application Support Overview">
<t>RELOAD is not intended to be used alone, but rather as a substrate
for other applications. These applications can use RELOAD for a variety
of purposes:</t>
<t><list style="symbols">
<t>To store data in the overlay and retrieve data stored by other
nodes.</t>
<t>As a discovery mechanism for services such as TURN.</t>
<t>To form direct connections which can be used to transmit
application-level messages.</t>
</list></t>
<t>This section provides an overview of these services.</t>
<section title="Data Storage">
<t>RELOAD provides operations to Store, Fetch, and Remove 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 rules is used for all
the kinds defined in this specification. Thus, only a user with a
certificate for "alice@example.org" could write to that location in
the overlay. However, other usages can define any rules they choose,
including publicly writable values.</t>
<t>The digital signature over the data serves two purposes. First,
it allows the peer responsible for storing the data to verify that
this Store is authorized. Second, it provides integrity for the
data. The signature is saved along with the data value (or values)
so that any reader can verify the integrity of the data. Of course,
the responsible peer can "lose" the value but it cannot undetectable
modify it.</t>
<t>The size requirements of the data being stored in the overlay are
variable. For instance, a SIP AoR and voicemail differ widely in the
storage size. RELOAD leaves it to the Usage and overlay
configuration to address the size imbalance of various kinds.</t>
</section>
<section anchor="sec-usages" title="Usages">
<t>By itself, the distributed storage layer just provides
infrastructure on which applications are built. In order to do
anything useful, a usage must be defined. Each Usage specifies
several things:</t>
<t><list style="symbols">
<t>Registers Kind-ID code points for any kinds that the Usage
defines.</t>
<t>Defines the data structure for each of the kinds.</t>
<t>Defines access control rules for each kinds.</t>
<t>Defines how the Resource Name is formed that is hashed to
form the Resource-ID where each kind is stored.</t>
<t>Describes how values will be merged after a network
partition. Unless otherwise specified, the default merging rule
is to act as if all the values that need to be merged were
stored and that the order they were stored in corresponds to the
stored time values associated with (and carried in) their
values. Because the stored time values are those associated with
the peer which did the writing, clock skew is generally not an
issue. If two nodes are on different partitions, clocks, this
can create merge conflicts. However because RELOAD deliberately
segregates storage so that data from different users and peers
is stored in different locations, and a single peer will
typically only be in a single network partition, this case will
generally not arise.</t>
</list></t>
<t>The kinds defined by a usage may also be applied to other usages.
However, a need for different parameters, such as different size
limits, would imply the need to create a new kind.</t>
</section>
<section title="Replication">
<t>Replication in P2P overlays can be used to provide:</t>
<t><list style="hanging">
<t hangText="persistence: ">if the responsible peer crashes
and/or if the storing peer leaves the overlay</t>
<t hangText="security: ">to guard against DoS attacks by the
responsible peer or routing attacks to that responsible peer</t>
<t hangText="load balancing: ">to balance the load of queries
for popular resources.</t>
</list></t>
<t>A variety of schemes are used in P2P overlays to achieve some of
these goals. Common techniques include replicating on neighbors of
the responsible peer, randomly locating replicas around the overlay,
or replicating along the path to the responsible peer.</t>
<t>The core RELOAD specification does not specify a particular
replication strategy. Instead, the first level of replication
strategies are determined by the overlay algorithm, which can base
the replication strategy on the its particular topology. For
example, Chord places replicas on successor peers, which will take
over responsibility should the responsible peer fail <xref
target="Chord"></xref>.</t>
<t>If additional replication is needed, for example if data
persistence is particularly important for a particular usage, then
that usage may specify additional replication, such as implementing
random replications by inserting a different well known constant
into the Resource Name used to store each replicated copy of the
resource. Such replication strategies can be added independent of
the underlying algorithm, and their usage can be determined based on
the needs of the particular usage.</t>
</section>
</section>
<section title="Service Discovery">
<t>RELOAD does not currently define a generic service discovery
algorithm as part of the base protocol; although a TURN-specific
discovery mechanism is provided. A variety of service discovery
algorithm can be implemented as extensions to the base protocol, such
as ReDIR <xref target="opendht-sigcomm05"></xref>.</t>
</section>
<section title="Application Connectivity">
<t>There is no requirement that a RELOAD usage must use RELOAD's
primitives for establishing its own communication if it already
possesses its own means of establishing connections. For example, one
could design a RELOAD-based resource discovery protocol which used
HTTP to retrieve the actual data.</t>
<t>For more common situations, however, the overlay itself is used to
establish a connection rather than an external authority such as DNS,
RELOAD provides connectivity to applications using the same Attach
method as is used for the overlay maintenance. For example, if a
P2PSIP node wishes to establish a SIP dialog with another P2PSIP node,
it will use Attach to establish a direct connection with the other
node. This new connection is separate from the peer protocol
connection, it is a dedicated UDP or TCP flow used only for the SIP
dialog. Each usage specifies which types of connections can be
initiated using Attach.</t>
</section>
</section>
<!-- EKR: RemoveD
<section title="Overview">
<section title="Distributed Storage Layer">
<t>RELOAD is designed to be extensible to both structured and
unstructured overlays. However, this version is only completely worked
out for structured overlays such as DHTs. The following text assumes
structured overlays; in particular Resource-IDs are assumed to be
fixed length for any given overlay, although the protocol allows them
to be variable length to allow extension to unstructured overlays.</t>
<t>Each logical address in the DHT where data can be stored is
referred to as a Resource-ID. A given peer will be responsible for
storing data from many Resource-ID locations. Typically literature on
DHTs uses the term "key" to refer to a location in the DHT; however,
in this specification the term key is used to refer to public or
private keys used for cryptographic operations and the term
Resource-ID is used to refer to a location for storage in the DHT.</t>
<section title="DHT Concepts">
<t>While very early P2P systems used flood based techniques, some
newer P2P systems locate resources using a Distributed Hash Table,
or DHT to improve efficiency. Peers are organized using a
Distributed Hash Table (DHT) structure. In such a system, every
resource has a Resource-ID, which is obtained by hashing some
keyword or value (an Resource Name) that uniquely identifies the
resource. Resources can be thought of as being stored in a hash
table at the entry corresponding to their Resource-ID. The peers
that make up the overlay network are also assigned an ID, called a
Node-ID, in the same hash space as the Resource-IDs. A peer is
responsible for storing all resources that have Resource-IDs near
the peer's Node-ID. The hash space is divided up so that all of the
hash space is always the responsibility of some particular peer,
although as peers enter and leave the system a particular peer's
area may change. Messages are exchanged between the peers in the DHT
as the peers enter and leave to preserve the structure of the DHT
and exchange stored entries. Various DHT implementations may
visualize the hash space as a grid, circle, line, or hypercube.</t>
<t>Peers keep information about the location of other peers in the
hash space and typically know about many peers nearby in the hash
space, and progressively fewer more distant peers. We refer to this
table of other peers as a Routing Table. When a peer wishes to
operate on a resource it consults the list of peers it is aware of
and contacts the peer with the Node-ID nearest the desired
Resource-ID. If that peer does not know how to find the resource, it
either returns information about a closer peer it knows about, or
forwards the request to a closer peer. In this fashion, the request
eventually reaches the peer responsible for the resource, which then
replies to the requester.</t>
</section>
<section title="DHT Topology">
<t>Each DHT will have a somewhat different structure, but many of
the concepts are common. The DHT defines a large space of
Resource-IDs, which can be thought of as addresses. In many DHTs,
the Resource-IDs are simply 128- or 160-bit integers. Each DHT also
has a distance metric such that we can say that Resource-ID A is
closer to Resource-ID B than to Resource-ID C.</t>
<t>Each peer in the DHT is assigned a Node-ID and is "responsible"
for the nearby space of Resource-IDs. So, for instance, if we have a
peer P, then it could also be responsible for storing data
associated with Resource-ID P+epsilon as long as no other peer P was
closer. The DHT Resource-ID space is divided so that some peer is
responsible for each Resource-ID.</t>
</section>
<section title="Migration">
<t>At some point in time, a given P2P Network may want to migrate from
one underlying DHT algorithm to another or update to a later extension
of the protocol. This can also be used for crypto agility issues. The
migration approach is done by having peers initializing algorithm A.
When the clients go to periodically renew their credentials, they find
out that the P2P Network now requires them to use algorithm A but also
to store all the data with algorithm B. At this point there are
effectively two DHT rings in use, rings A and B. All data is written
to both but queries only go to A. At some point when the clients
periodically renew their credentials, they learn that the P2P Network
has moved to storing to both A and B but that Fetch requests are done
with P2P Network B and that any SEND should first be attempted on P2P
Network B and if that fails, retried on P2P Network A. In the final
stage when clients renew credentials, they find out that P2P Network A
is no longer required and only P2P Network B is in use. Some types of
usages and environments may be able to migrate very quickly and do all
of these steps in under a week, depending on how quickly software that
supports both A and B is deployed and how often credentials are
renewed. On the other hand, some very ad-hoc environments involving
software from many different providers may take years to migrate.</t>
<t>[[TODO: This needs more filling out]]</t>
</section>
<section title="Usages Layer">
<section title="Certificate Store Usage">
<t>This usage allows each user to store their certificate in the overlay
so that it can be retrieved to be checked by various peers and
applications. Peers acting on behalf of a particular user store that
user's certificate in the overlay, and any peer that needs the
certificate can do a Fetch to retrieve the certificate. Typically it
is retrieved to check a signature on a request or the signature on a
chunk of data that the overlay has received.</t>
</section>
<section title="TURN Usage">
<t>This usage defines a new kind for finding STUN-Relay servers. Any
peer that supports this usage saves a pointer to the IP address and
port of the TURN server in the overlay. When a peer wishes to discover a
TURN server, it picks a random Resource-ID and performs a Find at
that Resource-ID for the appropriate type for the service. If
nothing is found, this can be repeated until an appropriate set of
servers are found.</t>
</section>
<section title="Diagnostic Usage">
<t>This usage defines several new kinds that be queried to find
information about the peer that may be useful for monitoring and
diagnostics. This includes information such as software version,
neighbor information, and performance statistics.</t>
</section>
<section title="HIP Tunnel Usage">
<t>This usage allows two peers running HIP to tunnel HIP messages
across the overlay. This allows the HIP peers to use the overlay as
a rendezvous system to set up a direct path using HIP NAT traversal
mechanisms.</t>
</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 how messages
are transmitted, received, and routed in an existing overlay, then
define the message structure, and then finally define the messages used
to join and maintain the overlay.</t>
<section title="Message Routing">
<t>This section describes procedures used by nodes to route messages
through the overlay.</t>
<!-- EKR RemoveD. Repetitive
<t>
Regardless of which overlay algorithm is used, a RELOAD overlay
is a partly connected (incomplete) graph of nodes, each identified by
Node-ID. Each node maintains a set of connections to some
other set of nodes in the overlay. If a node is directly
connected to the destination of a message, it can send it
directly. However, in general, any two nodes will probably not be
directly connected; when node A wants to send a message to
node B, the message must traverse some set of other peers
in the graph,
with the precise set of intermediate peers traversed
depending on the overlay algorithm.
</t>
<t>
RELOAD intentionally separates the generic mechanisms for
routing messages from the precise overlay topology.
The topology
plugin (see [XREF]) should be thought of as providing a
routing table. When a node wishes to transmit a message to
a given Node-ID to which it is not connected, it consults
the routing table which tells
it which of its existing connections to forward the message
down. However, the procedures for sending, receiving, and
forwarding the messages are the same regardless of the
topology and contents of the routing table.
</t>
<t>
RELOAD also incorporates a loose source routing feature using
DESTINATION LISTS. When a node transmits a message it
can provide a set of Node-IDs which it wishes the message
to be routed through. Each intermediate peer examines
the first entry on the destination list and routes
the message to that node. When that node is reached,
it removes itself from the destination list and routes
based on the next entry. This repeats until the
message arrives at its final destination.
This makes it possible to address a node which
is potentially behind a NAT or a firewall in such a way that it
cannot be connected to directly under any circumstances
</t>
-->
<section anchor="sec-request-origination" title="Request Origination">
<t>In order to originate a message to a given Node-ID or
Resource-ID, a node constructs an appropriate destination list. The
simplest such destination list is a single entry containing the peer
or Resource-ID. The resulting message will use the normal overlay
routing mechanisms to forward the message to that destination. The
node can also construct a more complicated destination list for
source routing.</t>
<t>Once the message is constructed, the node sends the message to
some adjacent peer. If the first entry on the destination list is
directly connected, then the message MUST be routed down that
connection. Otherwise, the topology plugin MUST be consulted to
determine the appropriate next hop.</t>
<t>Parallel searches for the resource are a common solution to
improve reliability in the face of churn or of subversive peers.
Parallel searches for usage-specified replicas are managed by the
usage layer. However, a single request can also be routed through
multiple adjacent peers, even when known to be sub-optimal, to
improve reliability <xref target="vulnerabilities-acsac04"></xref>.
Such parallel searches MAY BE specified by the topology plugin.</t>
<t>Because messages may be lost in transit through the overlay,
RELOAD incorporates an end-to-end reliability mechanism. When an
originating node transmits a request it MUST set a 3 second timer.
If a response has not been received when the timer fires, the
request is retransmitted with the same transaction identifier. The
request MAY be retransmitted up to 4 times (for a total of 5
messages). After the timer for the fifth transmission fires, the
message SHALL be considered to have failed. Note that this
retransmission procedure is not followed by intermediate nodes. They
follow the hop-by-hop reliability procedure described in <xref
target="sec-reliability"></xref>.</t>
<t>The above algorithm can result in multiple requests being
delivered to a node. Receiving nodes MUST generate semantically
equivalent responses to retransmissions of the same request (this
can be determined by transaction id) if the request is received
within the maximum request lifetime (15 seconds). For some requests
(e.g., FETCH) this can be accomplished merely by processing the
request again. For other requests, (e.g., STORE) it may be necessary
to maintain state for the duration of the request lifetime.</t>
</section>
<section anchor="sec-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 if local
policy can override this in which case the messages is silently
dropped.</t>
<t>Once the peer has determined that the message is correctly
formatted, it examines the first entry on the destination list.
There are three possible cases here:</t>
<t><list style="symbols">
<t>The first entry on the destination list is an id for which
the peer is responsible.</t>
<t>The first entry on the destination list is a an id for which
another peer is responsible.</t>
<t>The first entry on the destination list is a private id which
is being used for destination list compression.</t>
</list></t>
<t>These cases are handled as discussed below.</t>
<section anchor="sec-responsible-id" title="Responsible ID">
<t>If the first entry on the destination list is a ID for which
the node is responsible, there are several sub-cases. <list
style="symbols">
<t>If the entry is a Resource-ID, then it MUST be the only
entry on the destination list. If there are other entries, the
message MUST be silently dropped. Otherwise, the message is
destined for this node and it passes it up to the upper
layers.</t>
<t>If the entry is a Node-ID which belongs to this node, then
the message is destined for this node. If this is the only
entry on the destination list, the message is destined for
this node and is passed up to the upper layers. Otherwise the
entry is removed from the destination list and the message is
passed it to the routing layer. If the message is a response
and there is state for the transaction ID, the state is
reinserted into the destination list first.</t>
<t>If the entry is a Node-ID which is not equal to this node,
then the node MUST drop the message silently unless the
Node-ID corresponds to a node which is directly connected to
this node (i.e., a client). In that case, it MUST forward the
message to the destination node as described in the next
section.</t>
</list></t>
<t>Note that this implies that in order to address a message to
"the peer that controls region X", a sender sends to Resource-ID
X, not Node-ID X.</t>
</section>
<section anchor="sec-other-id" title="Other ID">
<t>If neither of the other two cases applies, then the peer MUST
forward the message towards the first entry on the destination
list. This means that it MUST select one of the peers to which it
is connected and which is likely to be responsible for the first
entry on the destination list. If the first entry on the
destination list is in the peer's connection table, then it SHOULD
forward the message to that peer directly. Otherwise, it consult
the routing table to forward the message.</t>
<t>Any intermediate peer which forwards a RELOAD message MUST
arrange that if it receives a response to that message the
response can be routed back through the set of nodes through which
the request passed. This may be arranged in one of two ways:</t>
<t><list style="symbols">
<t>The peer MAY add an entry to the via list in the forwarding
header that will enable it to determine the correct node.</t>
<t>The peer MAY keep per-transaction state which will allow it
to determine the correct node.</t>
</list></t>
<t>As an example of the first strategy, if node D receives a
message from node C with via list (A, B), then D would forward to
the next node (E) with via list (A, B, C). Now, if E wants to
respond to the message, it reverses the via list to produce the
destination list, resulting in (D, C, B, A). When D forwards the
response to C, the destination list will contain (C, B, A).</t>
<t>As an example of the second strategy, if node D receives a
message from node C with transaction ID X and via list (A, B), it
could store (X, C) in its state database and forward the message
with the via list unchanged. When D receives the response, it
consults its state database for transaction id X, determines that
the request came from C, and forwards the response to C.</t>
<t>Intermediate peer which modify the via list are not required to
simply add entries. The only requirement is that the peer be able
to reconstruct the correct destination list on the return route.
RELOAD provides explicit support for this functionality in the
form of private IDs, which can replace any number of via list
entries. For instance, in the above example, Node D might send E a
via list containing only the private ID (I). E would then use the
destination list (D, I) to send its return message. When D
processes this destination list, it would detect that I is a
private ID, recover the via list (A, B, C), and reverse that to
produce the correct destination list (C, B, A) before sending it
to C. This feature is called List Compression. I MAY either be a
compressed version of the original via list or an index into a
state database containing the original via list.</t>
<t>Note that if an intermediate peer exits the overlay, then on
the return trip the message cannot be forwarded and will be
dropped. The ordinary timeout and retransmission mechanisms
provide stability over this type of failure.</t>
</section>
<section anchor="sec-private-Node-ID" title="Private ID">
<t>If the first entry on the destination list is a private id
(e.g., a compressed via list), the peer MUST that entry with the
original via list that it replaced indexes and then re-examine the
destination list to determine which case now applies.</t>
</section>
</section>
<section anchor="sec-response-origination"
title="Response Origination">
<t>When a peer sends a response to a request, it MUST construct the
destination list by reversing the order of the entries on the via
list. This has the result that the response traverses the same peers
as the request traversed, except in reverse order (symmetric
routing). Note that this rule will need to be relaxed if other
routing algorithms are supported.</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 |
+-------------------------+
| Signature |
+-------------------------+
]]></artwork>
</figure>
<t>The contents of these parts are as follows: <list style="hanging">
<t></t>
<t hangText="Forwarding Header:">Each message has a generic header
which is used to forward the message between peers and to its
final destination. This header is the only information that an
intermediate peer (i.e., one that is not the target of a message)
needs to examine.</t>
<t></t>
<t hangText="Message Contents:">The message being delivered
between the peers. From the perspective of the forwarding layer,
the contents is opaque, however, it is interpreted by the higher
layers.</t>
<t></t>
<t hangText="Signature:">A digital signature over the message
contents and parts of the header of the message. Note that this
signature can be computed without parsing the message
contents.</t>
</list></t>
<t>The following sections describe the format of each part of the
message.</t>
<section anchor="sec-presentation-language"
title="Presentation Language">
<t>The structures defined in this document are defined using a
C-like syntax based on the presentation language used to define TLS.
Advantages of this style include:</t>
<t><list style="symbols">
<t>It is easy to write and familiar enough looking that most
readers can grasp it quickly.</t>
<t>The ability to define nested structures allows a separation
between high-level and low level message structures.</t>
<t>It has a straightforward wire encoding that allows quick
implementation, but the structures can be comprehended without
knowing the encoding.</t>
<t>The ability to mechanically (compile) encoders and
decoders.</t>
</list></t>
<t>This presentation is to some extent a placeholder. We consider it
an open question what the final protocol definition method and
encodings use. We expect this to be a question for the WG to
decide.</t>
<t>Several idiosyncrasies of this language are worth noting.</t>
<t><list style="symbols">
<t>All lengths are denoted in bytes, not objects.</t>
<t>Variable length values are denoted like arrays with angle
brackets.</t>
<t>"select" is used to indicate variant structures.</t>
</list></t>
<t>For instance, "uint16 array<0..2^8-2>;" represents up to
254 bytes but only up to 127 values of two bytes (16 bits)
each..</t>
<section anchor="sec-definitions" title="Common Definitions">
<t>The following definitions are used throughout RELOAD and so are
defined here. They also provide a convenient introduction to how
to read the presentation language.</t>
<t>An enum represents an enumerated type. The values associated
with each possibility are represented in parentheses and the
maximum value is represented as a nameless value, for purposes of
describing the width of the containing integral type. For
instance, Boolean represents a true or false:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { false (0), true(1), (255)} Boolean;
]]></artwork>
</figure>
<t>A boolean value is either a 1 or a 0 and is represented as a
single byte on the wire.</t>
<t>The NodeId, shown below, represents a single Node-ID.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
typedef opaque NodeId[16];
]]></artwork>
</figure>
<t>A NodeId is a fixed-length 128-bit structure represented as a
series of bytes, most significant byte first. Note: the use of
"typedef" here is an extension to the TLS language, but its
meaning should be relatively obvious.</t>
<t>A ResourceId, shown below, represents a single Resource-ID.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
typedef opaque ResourceId<0..2^8-1>;
]]></artwork>
</figure>
<t>Like a NodeId, a Resource-ID is an opaque string of bytes, but
unlike Node-IDs, Resource-IDs are variable length, up to 255 bytes
(2048 bits) in length. On the wire, each ResourceId is preceded by
a single length byte (allowing lengths up to 255). Thus, the
3-byte value "Foo" would be encoded as: 03 46 4f 4f.</t>
<t>A more complicated example is IpAddressPort, which represents a
network address and can be used to carry either an IPv6 or IPv4
address:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {reserved_addr(0), ipv4_address (1), ipv6_address (2),
(255)} AddressType;
struct {
uint32 addr;
uint16 port;
} IPv4AddrPort;
struct {
uint128 addr;
uint16 port;
} IPv6AddrPort;
struct {
AddressType type;
uint8 length;
select (type) {
case ipv4_address:
IPv4AddrPort v4addr_port;
case ipv6_address:
IPv6AddrPort v6addr_port;
/* This structure can be extended */
} IpAddressPort;
]]></artwork>
</figure>
<t>The first two fields in the structure are the same no matter
what kind of address is being represented:</t>
<t><list style="hanging">
<t></t>
<t hangText="type "></t>
<t>the type of address (v4 or v6).</t>
<t></t>
<t hangText="length "></t>
<t>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>
</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;
uint8 ttl;
uint8 reserved;
uint16 fragment;
uint8 version;
uint24 length;
uint64 transaction_id;
uint16 flags;
uint16 via_list_length;
uint16 destination_list_length;
uint16 route_log_length;
uint16 options_length;
Destination via_list[via_list_length];
Destination destination_list
[destination_list_length];
RouteLogEntry route_log[route_log_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"></t>
<t>The first four bytes identify this message as a RELOAD
message. The message is easy to demultiplex from STUN messages
by looking at the first bit. This field MUST contain the value
0xc2454c4f (the string 'RELO' with the high bit of the first
byte set.).</t>
<t></t>
<t hangText="overlay"></t>
<t>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="ttl"></t>
<t>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. The initial value of
the TTL should be TBD.</t>
<t></t>
<t hangText="fragment"></t>
<t>This field is used to handle fragmentation. The high order
two bits are used to indicate the fragmentation status: If the
high bit (0x8000) is set, it indicates that the message is a
fragment. If the next bit (0x4000) is set, it indicates that
this is the last fragment.</t>
<t>The remainder of the field is used to indicate the fragment
offset. [[Open Issue: This is conceptually clear, but the
details are still lacking. Need to define the fragment offset
and total length be encoded in the header. Right now we have 14
bits reserved with the intention that they be used for
fragmenting, though additional bytes in the header might be
needed for fragmentation.]]</t>
<t></t>
<t hangText="version"></t>
<t>The version of the RELOAD protocol being used. This document
describes version 0.1, with a value of 0x01.</t>
<t></t>
<t hangText="length"></t>
<t>The count in bytes of the size of the message, including the
header.</t>
<t></t>
<t hangText="transaction_id"></t>
<t>A unique 64 bit number that identifies this transaction and
also serves as a salt to randomize the request and the response.
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="flags"></t>
<t>The flags word contains control flags. Which are ORed
together. There is two currently defined flags: ROUTE-LOG (0x1)
and RESPONSE-ROUTE-LOG (0x2). These flags indicate that the
route log should be included (see <xref
target="sec-route-log"></xref>.).</t>
<t></t>
<t hangText="via_list_length"></t>
<t>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"></t>
<t>The length of the destination list in bytes.</t>
<t></t>
<t hangText="route_log_length"></t>
<t>The length of the route log in bytes.</t>
<t></t>
<t hangText="options_length"></t>
<t>The length of the header options in bytes.</t>
<t></t>
<t hangText="via_list"></t>
<t>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"></t>
<t>The destination_list contains a sequence of destinations
which the message should pass through. The destination list is
constructed by the message originator. The first element in the
destination list is where the message goes next. The list
shrinks as the message traverses each listed peer.</t>
<t></t>
<t hangText="route_log"></t>
<t>Contains a series of route log entries. See <xref
target="sec-route-log"></xref>.</t>
<t></t>
<t hangText="options"></t>
<t>Contains a series of ForwardingOptions entries. See <xref
target="sec-forwarding-options"></xref>.</t>
</list></t>
<section title="Destination and Via Lists">
<t>The destination list and via lists are sequences of Destination
values:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {reserved(0), peer(1), resource(2), compressed(3), (255) }
DestinationType;
select (destination_type) {
case peer:
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;
]]></artwork>
</figure>
<t>This is a TLV structure with the following contents: <list
style="hanging">
<t></t>
<t hangText="type "></t>
<t>The type of the DestinationData PDU. This may be one of
"peer", "resource", or "compressed".</t>
<t></t>
<t hangText="length "></t>
<t>The length of the destination_data.</t>
<t></t>
<t hangText="destination_value "></t>
<t>The destination value itself, which is an encoded
DestinationData structure, depending on the value of
"type".</t>
</list></t>
<t><list style="hanging">
<t hangText="Note:">This structure encodes a type, length,
value. The length field specifies the length of the
DestinationData values, which allows the addition of new
DestinationTypes. This allows an implementation which does not
understand a given DestinationType to skip over it.</t>
</list></t>
<t>A DestinationData can be one of three types: <list
style="hanging">
<t></t>
<t hangText="peer"></t>
<t>A Node-ID.</t>
<t></t>
<t hangText="compressed"></t>
<t>A compressed list of Node-IDs and/or resources. Because
this value was compressed by one of the peers, it is only
meaningful to that peer and cannot be decoded by other peers.
Thus, it is represented as an opaque string.</t>
<t></t>
<t hangText="resource"></t>
<t>The Resource-ID of the resource which is desired. This type
MUST only appear in the final location of a destination list
and MUST NOT appear in a via list. It is meaningless to try to
route through a resource.</t>
</list></t>
</section>
<section anchor="sec-route-log" title="Route Logging">
<t>The route logging feature provides diagnostic information about
the path taken by the message so far and in this manner it is
similar in function to <xref target="RFC3261">SIP's</xref> Via
header field. If the ROUTE-LOG flag is set in the Flags word, at
each hop peers MUST append a route log entry to the route log
element in the header or reject the request. The order of the
route log entry elements in the message is determined by the order
of the peers were traversed along the path. The first route log
entry corresponds to the peer at the first hop along the path, and
each subsequent entry corresponds to the peer at the next hop
along the path. If the ROUTE-LOG flag is set, the route log
entries in the request MUST be copied to the response or the
request rejected. If, and only if, the ROUTE-LOG-RESPONSE flag is
set in a request, the ROUTE-LOG flag MUST be set in the
response.</t>
<t>Note that use of the ROUTE-LOG-RESPONSE flag means that the
response will grow on the return path, which may potentially mean
that it gets dropped due to becoming too large for some
intermediate hop. Thus, this option must be used with care.</t>
<t>The route log is defined as follows:</t>
<figure>
<!-- begin-pdu-->
<artwork><![CDATA[
enum { (255) } RouteLogExtensionType;
struct {
RouteLogExtensionType type;
uint16 length;
select (type){
/* Extension values go here */
} extension;
} RouteLogExtension;
enum {
reserved(0),
tcp_tls(1),
udp_dtls(2),
<!-- tcp_test_mode(3), -->
(255)
} Transport;
struct {
opaque version<0..2^8-1>; /* A string */
Transport transport; /* TCP or UDP */
NodeId id;
uint32 uptime;
IpAddressPort address;
opaque certificate<0..2^16-1>;
RouteLogExtension extensions<0..2^16-1>;
} RouteLogEntry;
struct {
RouteLogEntry entries<0..2^16-1>;
} RouteLog;
]]></artwork>
</figure>
<t>The route log consists of an arbitrary number of RouteLogEntry
values, each representing one node through which the message has
passed.</t>
<t>Each RouteLogEntry consists of the following values:</t>
<t><list style="hanging">
<t></t>
<t hangText="version "></t>
<t>A textual representation of the software version</t>
<t></t>
<t hangText="transport "></t>
<t>The transport type, currently either "tcp_tls" or
"udp_dtls".</t>
<t></t>
<t hangText="id "></t>
<t>The Node-ID of the peer.</t>
<t></t>
<t hangText="uptime "></t>
<t>The uptime of the peer in seconds.</t>
<t></t>
<t hangText="address "></t>
<t>The address and port of the peer.</t>
<t></t>
<t hangText="certificate "></t>
<t>The peer's certificate. Note that this may be omitted by
setting the length to zero.</t>
<t></t>
<t hangText="extensions "></t>
<t>Extensions, if any.</t>
</list></t>
<t>Extensions are defined using a RouteLogExtension structure. New
extensions are defined by defining a new code point for
RouteLogExtensionType and adding a new arm to the
RouteLogExtension structure. The contents of that structure
are:</t>
<t><list style="hanging">
<t></t>
<t hangText="type"></t>
<t>The type of the extension.</t>
<t></t>
<t hangText="length"></t>
<t>The length of the rest of the structure.</t>
<t></t>
<t hangText="extension"></t>
<t>The extension value.</t>
</list></t>
</section>
<section anchor="sec-forwarding-options" title="Forwarding Options">
<t>The Forwarding header can be extended with forwarding header
options, which are a series of ForwardingOptions structures:</t>
<figure>
<!-- begin-pdu-->
<artwork><![CDATA[
enum { (255) } ForwardingOptionsType;
struct {
ForwardingOptionsType type;
uint8 flags;
uint16 length;
select (type) {
/* Option values go here */
} option;
} ForwardingOption;
]]></artwork>
</figure>
<t>Each ForwardingOption consists of the following values:</t>
<t><list style="hanging">
<t></t>
<t hangText="type"></t>
<t>The type of the option.</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
757 error response. If the DESTINATION_CRITICAL flag is set,
any node generates a response to the message but does not
understand the forwarding option MUST reject the request with
an 757 error response. If the RESPONSE_COPY flag is set, any
node generating a response MUST copy the option from the
request to the response and clear the RESPONSE_COPY,
FORWARD_CRITICAL and DESTINATION_CRITICAL flags.</t>
<t></t>
<t hangText="option"></t>
<t>The option value.</t>
</list></t>
</section>
</section>
<section anchor="sec-contents" title="Message Contents Format">
<t>The second major part of a RELOAD message is the contents part,
which is defined by MessageContents:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
MessageCode message_code;
opaque payload<0..2^24-1>;
} MessageContents;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list
style="hanging">
<t></t>
<t hangText="message_code "></t>
<t>This indicates the message that is being sent. The code space
is broken up as follows. <list style="hanging">
<t></t>
<t hangText="0">Reserved</t>
<t></t>
<t hangText="1 .. 0x7fff">Requests and responses. These code
points are always paired, with requests being odd and the
corresponding response being the request code plus 1. Thus,
"probe_request" (the Probe request) has value 1 and
"probe_answer" (the Probe response) has value 2</t>
<t></t>
<t hangText="0xffff">Error</t>
</list></t>
<t></t>
<t hangText="message_body "></t>
<t>The message body itself, represented as a variable-length
string of bytes. The bytes themselves are dependent on the code
value. See the sections describing the various RELOAD methods
(Join, Update, Attach, Store, Fetch, etc.) for the definitions
of the payload contents.</t>
</list></t>
<section anchor="sec-response-code"
title="Response Codes and Response Errors">
<t>A peer processing a request returns its status in the
message_code field. If the request was a success, then the message
code is the response code that matches the request (i.e., the next
code up). The response payload is then as defined in the
request/response descriptions.</t>
<t>If the request failed, then the message code is set to 0xffff
(error) and the payload MUST be an error_response PDU, as shown
below.</t>
<t>When the message code is 0xffff, the payload MUST be an
ErrorResponse.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
public struct {
uint16 error_code;
opaque error_info<0..2^16-1>;
} ErrorResponse;
]]></artwork>
</figure>
<t>The contents of this structure are as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="error_code "></t>
<t>A numeric error code indicating the error that
occurred.</t>
<t></t>
<t hangText="error_info "></t>
<t>A free form text string indicating the reason for the
response for diagnostic purposes.</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_Unauthorized:">The requesting peer needs to
sign and provide a certificate. [[TODO: The semantics here
don't seem quite right.]]</t>
<t></t>
<t hangText="Error_Forbidden:">The requesting peer does not
have permission to make this request.</t>
<t></t>
<t hangText="Error_Not_Found:">The resource or peer cannot be
found or does not exist.</t>
<t></t>
<t hangText="Error_Request_Timeout:">A response to the request
has not been received in a suitable amount of time. The
requesting peer MAY resend the request at a later time.</t>
<t></t>
<t hangText="Error_Precondition_Failed:">A request can't be
completed because some precondition was incorrect. For
instance, the wrong generation counter was provided</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>
</list></t>
</section>
</section>
<section anchor="sec-signature" title="Signature">
<t>The third part of a RELOAD message is the signature, represented
by a Signature structure. The message signature is computed over the
payload and parts of forwarding header. The payload, in case of a
Store, may contain an additional signature computed over a StoreReq
structure. All signatures are formatted using the Signature element.
This element is also used in other contexts where signatures are
needed. The input structure to the signature computation varies
depending on the data element being signed.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {reserved(0), signer_identity_peer (1),
signer_identity_name (2), signer_identity_certificate (3),
(255)} SignerIdentityType;
select (identity_type) {
case signer_identity_peer:
NodeId id;
case signer_identity_name:
opaque name<0..2^16-1>;
case signer_identity_certificate:
opaque certificate<0..2^16-1>;
/* This structure may be extended with new types */
} SignerIdentityValue;
struct {
SignerIdentityType identity_type;
uint16 length;
SignerIdentityValue identity[SignerIdentity.length];
} SignerIdentity;
struct {
SignatureAndHashAlgorithm algorithm;
SignerIdentity identity;
opaque signature_value<0..2^16-1>;
} Signature;
]]></artwork>
</figure>
<t>The signature construct contains the following values:</t>
<t><list style="hanging">
<t></t>
<t hangText="algorithm "></t>
<t>The signature algorithm in use. The algorithm definitions are
found in the IANA TLS SignatureAlgorithm Registry.</t>
<t></t>
<t hangText="identity "></t>
<t>The identity or certificate used to form the signature</t>
<t></t>
<t hangText="signature_value "></t>
<t>The value of the signature</t>
</list></t>
<t>A number of possible identity formats are permitted. The current
possibilities are: a Node-ID, a user name, and a certificate.</t>
<t>For signatures over messages the input to the signature is
computed over:</t>
<t><list>
<t>overlay + transaction_id + MessageContents +
SignerIdentity</t>
</list></t>
<t>Where overlay and transaction_id come from the forwarding header
and + indicates concatenation.</t>
<t>[[TODO: Check the inputs to this carefully.]]</t>
<t>The input to signatures over data values is different, and is
described in <xref target="sec-data-sig"></xref>.</t>
</section>
</section>
<section anchor="sec-overlay-topology" title="Overlay Topology">
<t>As discussed in previous sections, RELOAD does not itself implement
any overlay topology. Rather, it relies on Topology Plugins, which
allow a variety of overlay algorithms to be used while maintaining the
same RELOAD core. This section describes the requirements for new
topology plugins and the methods that RELOAD provides for overlay
topology maintenance.</t>
<section title="Topology Plugin Requirements">
<t>When specifying a new overlay algorithm, at least the following
need to be described:</t>
<t><list style="symbols">
<t>Joining procedures, including the contents of the Join
message.</t>
<t>Stabilization procedures, including the contents of the
Update message, the frequency of topology probes and keepalives,
and the mechanism used to detect when peers have
disconnected.</t>
<t>Exit procedures, including the contents of the Leave
message.</t>
<t>The length of the Resource-IDs and Node-IDs. For DHTs, the
hash algorithm to compute the hash of an identifier.</t>
<t>The procedures that peers use to route messages.</t>
<t>The replication strategy used to ensure data redundancy.</t>
</list></t>
</section>
<section title="Methods and types for use by topology plugins">
<t>This section describes the methods that topology plugins use to
join, leave, and maintain the overlay.</t>
<section title="Join">
<t>A new peer (but which already has credentials) uses the JoinReq
message to join the overlay. The JoinReq is sent to the
responsible peer depending on the routing mechanism described in
the topology plugin. This notifies the responsible peer that the
new peer is taking over some of the overlay and it needs to
synchronize its state.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
NodeId joining_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} JoinReq;
]]></artwork>
</figure>
<!-- join-request = Node-ID -->
<t>The minimal JoinReq contains only the Node-ID which the sending
peer wishes to assume. Overlay algorithms MAY specify other data
to appear in this request.</t>
<t>If the request succeeds, the responding peer responds with a
JoinAns message, as defined below:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
opaque overlay_specific_data<0..2^16-1>;
} JoinAns;
]]></artwork>
</figure>
<t>If the request succeeds, the responding peer MUST follow up by
executing the right sequence of Stores and Updates to transfer the
appropriate section of the overlay space to the joining peer. In
addition, overlay algorithms MAY define data to appear in the
response payload that provides additional info.</t>
<t>In general, nodes which cannot form connections SHOULD report
an error. However, implementations MUST provide some mechanism
whereby nodes can determine they are potentially the first node
and take responsibility for the overlay. This specification does
not mandate any particular mechanism, but a configuration flag or
setting seems appropriate.</t>
</section>
<section title="Leave">
<t>The LeaveReq message is used to indicate that a node is exiting
the overlay. A node SHOULD send this message to each peer with
which it is directly connected prior to exiting the overlay.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
public struct {
NodeId leaving_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} LeaveReq;
]]></artwork>
</figure>
<!-- leave-request = Node-ID -->
<t>LeaveReq contains only the Node-ID of the leaving peer. Overlay
algorithms MAY specify other data to appear in this request.</t>
<t>Upon receiving a Leave request, a peer MUST update its own
routing table, and send the appropriate Store/Update sequences to
re-stabilize the overlay.</t>
</section>
<section title="Update">
<t>Update is the primary overlay-specific maintenance message. It
is used by the sender to notify the recipient of the sender's view
of the current state of the overlay (its routing state) and it is
up to the recipient to take whatever actions are appropriate to
deal with the state change.</t>
<t>The contents of the UpdateReq message are completely
overlay-specific. The UpdateAns response is expected to be either
success or an error.</t>
</section>
<section anchor="sec-route-query" title="Route_Query">
<t>The Route_Query request allows the sender to ask a peer where
they would route a message directed to a given destination. In
other words, a RouteQuery for a destination X requests the Node-ID
where the receiving peer would next route to get to X. A
RouteQuery can also request that the receiving peer initiate an
Update request to transfer his routing table.</t>
<t>One important use of the RouteQuery request is to support
iterative routing. The sender selects one of the peers in its
routing table and sends it a RouteQuery message with the
destination_object set to the Node-ID or Resource-ID it wishes to
route to. The receiving peer responds with information about the
peers to which the request would be routed. The sending peer MAY
then Attaches to that peer(s), and repeats the RouteQuery.
Eventually, the sender gets a response from a peer that is closest
to the identifier in the destination_object as determined by the
topology plugin. At that point, the sender can send messages
directly to that peer.</t>
<!--
<t>Note that this procedure only works well if all the peers are
mutually directly reachable either by all having public IP
addresses or at least by all being behind the same NAT. Accordingly,
peers MUST only use this method if permitted by the overlay
configuration (see [XREF]).</t> -->
<section title="Request Definition">
<t>A RouteQueryReq message indicates the peer or resource that
the requesting peer is interested in. It also contains a
"send_update" option allowing the requesting peer to request a
full copy of the other peer's routing table.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
Boolean send_update;
Destination destination;
opaque overlay_specific_data<0..2^16-1>;
} RouteQueryReq;
]]></artwork>
</figure>
<t>The contents of the RouteQueryReq message are as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="send_update "></t>
<t>A single byte. This may be set to "true" to indicate that
the requester wishes the responder to initiate an Update
request immediately. Otherwise, this value MUST be set to
"false".</t>
<t></t>
<t hangText="destination "></t>
<t>The destination which the requester is interested in.
This may be any valid destination object, including a
Node-ID, compressed ids, or Resource-ID.</t>
<t></t>
<t hangText="overlay_specific_data "></t>
<t>Other data as appropriate for the overlay.</t>
</list></t>
</section>
<section title="Response Definition">
<t>A response to a successful RouteQueryReq request is a
RouteQueryAns message. This is completely overlay specific.</t>
</section>
</section>
<section title="Probe">
<t>Probe provides a number of primitive "exploration" services:
(1) it allows
node to determine which resources another node is responsible for
(2) it allows some discovery services in multicast settings. A
probe can be addressed to a specific Node-ID, or the peer
controlling a given location (by using a resource ID). In either
case, the target Node-IDs respond with a simple response
containing some status information.</t>
<section title="Request Definition">
<t>The ProbeReq message contains a list (potentially empty) of
the pieces of status information that the requester would like
the responder to provide.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { responsible_set(1), num_resources(2), (255)}
ProbeInformationType;
struct {
ProbeInformationType requested_info<0..2^8-1>;
} ProbeReq
]]></artwork>
</figure>
<t>The two currently defined values for ProbeInformation
are:</t>
<t><list style="hanging">
<t></t>
<t hangText="responsible_set"></t>
<t>indicates that the peer should Respond with the fraction
of the overlay for which the responding peer is
responsible.</t>
<t></t>
<t hangText="num_resources"></t>
<t>indicates that the peer should Respond with the number of
resources currently being stored by the peer.</t>
</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 {
ProbeInformationType type;
select (type) {
case responsible_set:
uint32 responsible_ppb;
case num_resources:
uint32 num_resources;
/* This type may be extended */
};
} ProbeInformation;
struct {
ProbeInformation probe_info<0..2^16-1>;
} ProbeAns;
]]></artwork>
</figure>
<t>A ProbeAns message contains the following elements: <list
style="hanging">
<t></t>
<t hangText="probe_info "></t>
<t>A sequence of ProbeInformation structures, as shown
below.</t>
</list></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.</t>
<t>The responding peer SHOULD include any values that the
requesting peer requested and that it recognizes. They SHOULD be
returned in the requested order. Any other values MUST NOT be
returned.</t>
</section>
</section>
<!--
<section title="Diagnostic" anchor="sec-diagnostic">
<t>Diagnostic is an overlay specific message that a sender can
use to query the routing or storage state of a peer. In a small
scale overlay, a sender may send a Diagnostic message to all nodes
in the overlay to retrieve the appropriate node state. The contents
of the DiagnosticReq message are completely overlay specific.</t>
</section>
-->
</section>
</section>
<section title="Forwarding Layer">
<t>Each node maintains connections to a set of other nodes defined by
the topology plugin.<!--For instance, in the Chord topology
described in <xref target="sec-chord-algorithm"/>, a node would maintain
connections to 16 nodes in its finger table and to 6 neighbors.--></t>
<section title="Transports">
<t>RELOAD can use multiple transports 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 transports are offered by other nodes and establish
connections between each other. Any transport protocol needs to be
able to establish a secure, authenticated connection, and provide
data origin authentication and message integrity for individual data
elements. RELOAD currently supports two transport protocols:</t>
<t><list style="symbols">
<t>TLS [TODO REF] over TCP</t>
<t>DTLS <xref target="RFC4347"></xref> over UDP</t>
<!--
<t>TCP Test Mode defined in <xref
target="sec-tcp-test"></xref></t>
-->
</list></t>
<t> Open Issue: Consider adding TCP Test mode. This would be a TCP
based transport that had no security. </t>
<t>Note that although UDP does not properly have "connections", both
TLS and DTLS have a handshake which establishes a stateful
association, a similar stateful construct, and we simply refer to
these as "connections" for the purposes of this document.</t>
<section title="Future Support for HIP">
<t>The P2PSIP Working Group has expressed interest in supporting a
HIP-based transport. Such support would require specifying such
details as:</t>
<t><list style="symbols">
<t>How to issue certificates which provided identities
meaningful to the HIP base exchange. We anticipate that this
would require a mapping between ORCHIDs and NodeIds.</t>
<t>How to carry the HIP I1 and I2 messages. We anticipate that
this would require defining a HIP Tunnel usage.</t>
<t>How to carry RELOAD messages over HIP.</t>
</list></t>
<t>We leave this work as a topic for another draft.</t>
</section>
<!--
<section anchor="sec-tcp-test" title="TCP Test Mode Transport">
<t>TCP Test Mode is a transport based on TCP but no security
layer. It SHOULD NOT be used in any production environment as it
has many security vulnerabilities. It is meant only as simple test
mode to facilitate testing and interoperability before moving to
full TLS. When a new TCP session of this type is formed, both ends
of the connection MUST write their binary Node-ID to the wire
before sending any other messages over the session. This allows
both sides to discover the Node-ID of the other side and use this
in a similar way to the Node-ID discovered when using TLS or DTLS
from the certificate in the TLS handshake. This mode MUST not be
used unless the configuration for the overlay instance
specifically allows it. </t>
</section>
-->
<section anchor="sec-reliability"
title="Reliability for Unreliable Transports">
<t>When RELOAD is carried over DTLS or another unreliable
transport, it needs to be used with a reliability and congestion
control mechanism, which is provided on a hop-by-hop basis,
matching the semantics if TCP were used. The basic principle is
that each message, regardless of if it carries a request or
responses, will get an ACK and be reliably retransmitted. The
receiver's job is very simple, limited to just sending ACKs. All
the complexity is at the sender side. This allows the sending
implementation to trade off performance versus implementation
complexity without affecting the wire protocol.</t>
<t>In order to support unreliable transport, each message is
wrapped in a very simple framing layer (FramedMessage) which is
only used for each hop. This layer contains a sequence number
which can then be used for ACKs.</t>
<section title="Framed Message Format">
<t>The definition of FramedMessage is:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum {data (128), ack (129), (255)} FramedMessageType;
struct {
FramedMessageType type;
select (type) {
case data:
uint24 sequence;
opaque message<0..2^24-1>;
case ack:
uint24 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. Note that these values
have been set to force the first bit to be high, thus allowing
easy demultiplexing with STUN. All FramedMessageType values must
be > 128.</t>
<t>If the message is of type "data", then the remainder of the
PDU is as follows: <list style="hanging">
<t></t>
<t hangText="sequence "></t>
<t>the sequence number</t>
<t></t>
<t hangText="message "></t>
<t>the original message that is being transmitted.</t>
</list></t>
<t>Each connection has it own sequence number. Initially the
value is zero and it increments by exactly one for each message
sent over that connection.</t>
<t>When the receiver receive a message, it SHOULD immediately
send an ACK message. The receiver MUST keep track of the 32 most
recent sequence numbers received on this association in order to
generate the appropriate ack.</t>
<t>If the PDU is of type "ack", the contents are as follows:
<list style="hanging">
<t></t>
<t hangText="ack_sequence "></t>
<t>The sequence number of the message being
acknowledged.</t>
<t></t>
<t hangText="received "></t>
<t>A bitmask indicating whether or not each of the previous
32 packets has been received before the sequence number in
ack_sequence. The high order bit represents the first packet
in the sequence space.</t>
</list></t>
<t>The received field bits in the ACK provide a very high degree
of redundancy for the sender to figure out which packets the
receiver received and can then estimate packet loss rates. If
the sender also keeps track of the time at which recent sequence
numbers were sent, the RTT can be estimated.</t>
</section>
<section 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. It is RECOMMENDED that senders
implement TFRC-SP <xref target="RFC4828"></xref> and use the
received bitmask to allow the sender to compute packet loss
event rates. Senders MUST implement a retransmission and
congestion control scheme no more aggressive then TFRC-SP.</t>
</section>
</section>
<!-- EKR RemoveD
<section title="HIP">
<t>RELOAD MAY also be used with a HIP transport using the architecture
for HIP BONE described in <xref
target="I-D.camarillo-hip-bone"></xref>. From the perspective of the
P2P layer, HIP looks very much like normal IP. Either TLS (over TCP)
or DTLS (over UDP) is run over top of the HIP. Thus the reliability
and congestion control schemes are the same for DTLS section. If an
overlay is configured such that HIP is the only transport that it will
use, then it may make sense to configure the p2p layer to only offer
the ORCHID when gather candidate addresses for ICE. This will
effectively disable ICE at the p2p layer.</t>
<t>For overlays that use HIP, the enrollment server MUST provide each
peer with a unique ORCHID and use that ORCHID to generate the Node-ID
for the peer (see <xref target="sec-credentials"></xref>. Later when
the HIP layer wishes to tunnel a message (such as an I1 message)
through the overlay, the HIP layer can use the ORCHID to generate the
Node-ID, and then use the Tunnel message with the HIP to route the
message to that the peer that owns that ORCHID.</t>
</section>
<section title="TLS">
<t>TLS runs on top of TCP which offers the best performance from a
data transfer point of view and does not require as frequent keep
alive messages.</t>
</section>
-->
<section anchor="sec-frag-reass"
title="Fragmentation and Reassembly">
<t>In order to allow transport over datagram protocols, RELOAD
messages may be fragmented. If a message is too large for a peer
to transmit to the next peer it MUST fragment the message. Note
that this implies that intermediate peers may re-fragment messages
if the incoming and outgoing paths have different maximum datagram
sizes. Intermediate peers SHOULD NOT reassemble fragments.</t>
<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 unfragmented message and processed. In order to mitigate
denial of service attacks, receivers SHOULD time out incomplete
fragments. [[TODO: Describe algorithm]]</t>
</section>
</section>
<section title="Connection Management Methods">
<t>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 connections between nodes.
When node A wants to connect to node B, it sends an Attach
message to node B through the overlay. The Attach contains A's
ICE parameters. B responds with its ICE parameters and the two
nodes perform ICE to form connection.</t>
<t hangText="AttachLite: ">like attach, it is used to form
connections between nodes but instead of using full ICE, it only
uses a subset known as ICE-Lite.</t>
<t hangText="Ping: ">is a simple request/response which is used
to verify connectivity of the target peer.</t>
<t hangText="Tunnel: ">in some cases, it will be too expensive
for an application layer protocol to set up a connection in
order to send a small number of messages. The Tunnel message
allows applications to route individual application layer
protocol messages through the overlay.</t>
</list></t>
<section anchor="sec-connect-details" title="Attach">
<t>A node sends an Attach request when it wishes to establish a
direct TCP or UDP connection to another node for the purposes of
sending RELOAD messages or application layer protocol messages,
such as SIP. Detailed procedures for the Attach and its response
are described in <xref target="sec-connect-details"></xref>.</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 Update) to prevent
half-open states where a node has started to form connections but
is not really ready to act as a peer.</t>
<section anchor="sec-connect-request" title="Request Definition">
<t>An AttachReq message contains the requesting peer's ICE
connection parameters formatted into a binary structure.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
typedef opaque IceCandidate<0..2^16-1>;
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>;
} AttachReqAns;
]]></artwork>
</figure>
<t>The values contained in AttachReq and AttachAns are: <list
style="hanging">
<t></t>
<t hangText="ufrag "></t>
<t>The username fragment (from ICE)</t>
<t></t>
<t hangText="password "></t>
<t>The ICE password.</t>
<t></t>
<t hangText="application "></t>
<t>A 16-bit port number. This port number represents the
IANA registered port of the protocol that is going to be
sent on this connection. For SIP, this is 5060 or 5061, and
for RELOAD is TBD. By using the IANA registered port, we
avoid the need for an additional registry and allow RELOAD
to be used to set up connections for any existing or future
application protocol.</t>
<t></t>
<t hangText="role "></t>
<!-- TODO: EKR: answering party does TLS client -->
<t>An active/passive/actpass attribute from RFC 4145 <xref
target="RFC4145"></xref>.</t>
<t></t>
<t hangText="candidates "></t>
<t>One or more ICE candidate values in the string
representation used in ordinary ICE. [[OPEN ISSUE: This is
convenient for stacks, but unaesthetic.]] Each candidate has
an IP address, IP address family, port, transport protocol,
priority, foundation, component ID, STUN type and related
address. The candidate_list is a list of string candidate
values from ICE.</t>
</list></t>
<t>These values should be generated using the procedures
described in <xref target="sec-ice-reload"></xref>.</t>
</section>
<section anchor="sec-connect-response" title="Response Definition">
<t>If a peer receives an Attach request, it SHOULD follow the
process the request and generate its own response with a
AttachReqAns. It should then begin ICE checks. When a peer
receives an Attach response, it SHOULD parse the response and
begin its own ICE checks.</t>
</section>
<section anchor="sec-ice-reload" title="Using ICE With RELOAD">
<t>This section describes the profile of ICE that is used with
RELOAD. RELOAD implementations MUST implement full ICE. Because
RELOAD always tries to use TCP and then UDP as a fallback, there
will be multiple candidates of the same IP version, which
requires full ICE.</t>
<t>In ICE as defined by <xref
target="I-D.ietf-mmusic-ice"></xref>, SDP is used to carry the
ICE parameters. In RELOAD, this function is performed by a
binary encoding in the Attach method. This encoding is more
restricted than the SDP encoding because the RELOAD environment
is simpler:</t>
<t><list style="symbols">
<t>Only a single media stream is supported.</t>
<t>In this case, the "stream" refers not to RTP or other
types of media, but rather to a connection for RELOAD itself
or for SIP signaling.</t>
<t>RELOAD only allows for a single offer/answer exchange.
Unlike the usage of ICE within SIP, there is never a need to
send a subsequent offer to update the default candidates to
match the ones selected by ICE.</t>
</list></t>
<!-- EKR RemoveD: not appropriate here.
<t>RELOAD and SIP always run over TLS for TCP connections and DTLS
<xref target="RFC4347"></xref> for UDP "connections". Consequently,
once ICE processing has completed, both agents will begin TLS and DTLS
procedures to establish a secure link. Its important to note that, had
a TURN server been utilized for the TCP or UDP stream, the TURN server
will transparently relay the TLS messaging and the encrypted TLS
content, and thus will not have access to the contents of the
connection once it is established. Any attack by the TURN server to
insert itself as a man-in-the-middle are thwarted verifying that
the certificates used to establish the secure connection match the
identties of the connecting peers.</t>
-->
<t>An agent follows the ICE specification as described in <xref
target="I-D.ietf-mmusic-ice"></xref> and <xref
target="I-D.ietf-mmusic-ice-tcp"></xref> with the changes and
additional procedures described in the subsections below.</t>
</section>
<section anchor="sec-collect" title="Collecting STUN Servers">
<t>ICE relies on the node having one or more STUN servers to
use. In conventional ICE, it is assumed that nodes are
configured with one or more STUN servers through some
out-of-band mechanism. This is still possible in RELOAD but
RELOAD also learns STUN servers as it connects to other peers.
Because all RELOAD peers implement ICE and use STUN keepalives,
every peer is a STUN server <xref target="RFC5389"></xref>.
Accordingly, any peer a node knows will be willing to be a STUN
server -- though of course it may be behind a NAT.</t>
<t>A peer on a well-provisioned wide-area overlay will be
configured with one or more bootstrap peers. These peers make an
initial list of STUN servers. However, as the peer forms
connections with additional peers, it builds more peers it can
use as STUN servers.</t>
<t>Because complicated NAT topologies are possible, a peer may
need more than one STUN server. Specifically, a peer that is
behind a single NAT will typically observe only two IP addresses
in its STUN checks: its local address and its server reflexive
address from a STUN server outside its NAT. However, if there
are more NATs involved, it may discover that it learns
additional server reflexive addresses (which vary based on where
in the topology the STUN server is). To maximize the chance of
achieving a direct connection, a peer SHOULD group other peers
by the peer-reflexive addresses it discovers through them. It
SHOULD then select one peer from each group to use as a STUN
server for future connections.</t>
<t>Only peers to which the peer currently has connections may be
used. If the connection to that host is lost, it MUST be removed
from the list of stun servers and a new server from the same
group SHOULD be selected.</t>
<!-- EKR RemoveD. TMI
<t>OPEN ISSUE: should the peer try to keep at least one peer in each
group, even if it has no other reason for the connection? Need to
specify when to stop adding new groups if the peer is behind a really
bad NAT.</t>
<t>OPEN ISSUE: RELOAD-01 had a Peer-Info structure that allowed peers
to exchange information such as a "default" IP-port pair in Updates.
This structure could be expanded to include the candidate list for a
peer, thus allowing ICE negotiation to begin or even direct
communication before an Attach request has been received. (The
candidate pairs for the P2P port are fixed because the same source
port is used for all connections.) However, because this would require
significant changes to the ICE algorithm, we have not introduced such
an extension at this point.</t>
-->
</section>
<section anchor="sec-gather" title="Gathering Candidates">
<t>When a node wishes to establish a connection for the purposes
of RELOAD signaling or SIP signaling (or any other application
protocol for that matter), it follows the process of gathering
candidates as described in Section 4 of ICE <xref
target="I-D.ietf-mmusic-ice"></xref>. RELOAD utilizes a single
component, as does SIP. Consequently, gathering for these
"streams" requires a single component.</t>
<t>An agent MUST implement ICE-tcp <xref
target="I-D.ietf-mmusic-ice"></xref>, and MUST gather at least
one UDP and one TCP host candidate for RELOAD and for SIP.</t>
<t>The ICE specification assumes that an ICE agent is configured
with, or somehow knows of, TURN and STUN servers. RELOAD
provides a way for an agent to learn these by querying the
overlay, as described in <xref target="sec-collect"></xref> and
<xref target="sec-turn-server"></xref>.</t>
<t>The agent SHOULD prioritize its TCP-based candidates over its
UDP-based candidates in the prioritization described in Section
4.1.2 of ICE <xref target="I-D.ietf-mmusic-ice"></xref>.</t>
<t>The default candidate selection described in Section 4.1.3 of
ICE is ignored; defaults are not signaled or utilized by
RELOAD.</t>
</section>
<section title="Encoding the Attach Message">
<t>Section 4.3 of ICE describes procedures for encoding the SDP
for conveying RELOAD or SIP ICE candidates. Instead of actually
encoding an SDP, the candidate information (IP address and port
and transport protocol, priority, foundation, component ID, type
and related address) is carried within the attributes of the
Attach request or its response. Similarly, the username fragment
and password are carried in the Attach message or its response.
<xref target="sec-connect-details"></xref> describes the
detailed attribute encoding for Attach. The Attach request and
its response do not contain any default candidates or the
ice-lite attribute, as these features of ICE are not used by
RELOAD. The Attach request and its response also contain a
application attribute, with a value of SIP or RELOAD, which
indicates what protocol is to be run over the connection. The
RELOAD Attach request MUST only be utilized to set up
connections for application protocols that can be multiplexed
with STUN.</t>
<t>Since the Attach request contains the candidate information
and short term credentials, it is considered as an offer for a
single media stream that happens to be encoded in a format
different than SDP, but is otherwise considered a valid offer
for the purposes of following the ICE specification. Similarly,
the Attach response is considered a valid answer for the
purposes of following the ICE specification.</t>
</section>
<section title="Verifying ICE Support">
<t>An agent MUST skip the verification procedures in Section 5.1
and 6.1 of ICE. Since RELOAD requires full ICE from all agents,
this check is not required.</t>
</section>
<section title="Role Determination">
<t>The roles of controlling and controlled as described in
Section 5.2 of ICE are still utilized with RELOAD. However, the
offerer (the entity sending the Attach request) will always be
controlling, and the answerer (the entity sending the Attach
response) will always be controlled. The connectivity checks
MUST still contain the ICE-CONTROLLED and ICE-CONTROLLING
attributes, however, even though the role reversal capability
for which they are defined will never be needed with RELOAD.
This is to allow for a common codebase between ICE for RELOAD
and ICE for SDP.</t>
</section>
<section title="Connectivity Checks">
<t>The processes of forming check lists in Section 5.7 of ICE,
scheduling checks in Section 5.8, and checking connectivity
checks in Section 7 are used with RELOAD without change.</t>
</section>
<section title="Concluding ICE">
<t>The controlling agent MUST utilize regular nomination. This
is to ensure consistent state on the final selected pairs
without the need for an updated offer, as RELOAD does not
generate additional offer/answer exchanges.</t>
<t>The procedures in Section 8 of ICE are followed to conclude
ICE, with the following exceptions:</t>
<t><list style="symbols">
<t>The controlling agent MUST NOT attempt to send an updated
offer once the state of its single media stream reaches
Completed.</t>
<t>Once the state of ICE reaches Completed, the agent can
immediately free all unused candidates. This is because
RELOAD does not have the concept of forking, and thus the
three second delay in Section 8.3 of ICE does not apply.</t>
</list></t>
</section>
<section title="Subsequent Offers and Answers">
<t>An agent MUST NOT send a subsequent offer or answer. Thus,
the procedures in Section 9 of ICE MUST be ignored.</t>
</section>
<section title="Media Keepalives">
<t>STUN MUST be utilized for the keepalives described in Section
10 of ICE. [[ TODO - this does not define what happens for TCP
]]</t>
</section>
<section title="Sending Media">
<t>The procedures of Section 11 apply to RELOAD as well.
However, in this case, the "media" takes the form of application
layer protocols (RELOAD or SIP for example) over TLS or DTLS.
Consequently, once ICE processing completes, the agent will
begin TLS or DTLS procedures to establish a secure connection.
The node which sent the Attach request MUST be the TLS server.
The other node MUST be the TLS client. The nodes MUST verify
that the certificate presented in the handshake matches the
identity of the other peer as found in the Attach message. Once
the TLS or DTLS signaling is complete, the application protocol
is free to use the connection.</t>
<t>The concept of a previous selected pair for a component does
not apply to RELOAD, since ICE restarts are not possible with
RELOAD.</t>
</section>
<section title="Receiving Media">
<t>An agent MUST be prepared to receive packets for the
application protocol (TLS or DTLS carrying RELOAD, SIP or
anything else) at any time. The jitter and RTP considerations in
Section 11 of ICE do not apply to RELOAD or SIP.</t>
</section>
</section>
<section title="AttachLite">
<t>An alternative to using the full ICE supported by the Attach
request is to use ICE-Lite with the AttachLite request. This will
not work in all of the scenarios where ICE would work, but in some
cases, particularly those with no NATs or firewalls, it will work.
Configuration for the overlay indicates if this can be used or
not.</t>
<t>OPEN ISSUE: We originally envisioned adding support for
ICE-Lite directly to the regular Attach method. However,
we found that both the parameters and processing were
completely different, resulting in almost no overlap
between the two methods. Therefore we chose to separate
this out for overlays where the complexities of ICE are
not needed. Note that it is still possible for a node
with a public unfiltered address intending to interoperate
to implement Attach without the candidate gathering phases
of ICE and achieve essentially the same result. If
simpler behavior or a better encoding of ICE-Lite in
Attach is developed, such an approach would be
preferable.</t>
<section title="Request Definition">
<t>An AttachLiteReq message contains the requesting peer's
ICE-Lite connection parameters formatted into a binary
structure. When using the AttachLite request, both sides act as
ICE-Lite hosts.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
IpAddressPort addr_port;
Transport transport;
uint32 priority;
} IceLiteCandidate;
struct {
uint16 application;
IceLiteCandidate candidates<0..2^16-1>;
} AttachLiteReqs;
]]></artwork>
</figure>
<t>The values contained in AttachLiteReq are: <list
style="hanging">
<t></t>
<t hangText="application "></t>
<t>A 16-bit port number used in the same was as in the
Attach request. This port number represents the IANA
registered port of the protocol that is going to be sent on
this connection.</t>
<t></t>
<t hangText="candidates "></t>
<t>One or more ICE candidate values. Each one contains an IP
address and family, transport protocol, and port to connect
to as well as a priority.</t>
</list></t>
<t>These values should be generated using the procedures
described in <xref target="sec-ice-reload"></xref>.</t>
</section>
<section title="Attach-Lite Connectivity Checks">
<t>STUN is not used for connectivity checks when doing ICE-Lite,
instead the DTLS or TLS handshake forms the connectivity check.
The host that received the AttachLiteReq MUST initiate TLS or
DTLS connections to candidates provided in the request. When a
connection forms, the node MUST check the certificate is for the
node that send AttachLiteReq and if is not, MUST close the
connection.</t>
<t>Since TLS provides the connectivity check, there is no need
for the <xref target="RFC4571">RFC 4571</xref> style framing
shim for STUN when using TLS and this is not used for this
protocol.</t>
</section>
<section title="Implementation Notes for Attach-Lite">
<t>This is a non normative section to help implementors.</t>
<t>At times ICE can seem a bit daunting to gets one head around.
For a simple IPv4 only peer, a simple implementation of
Attach-Lite could be done be doing the following: <list
style="symbols">
<t>When sending an AttachLiteReq, form one with a candidate
with a priority value of
(2^24)*(126)+(2^8)*(65535)+(2^0)*(256-1) that specifies the
UDP port being listened to and another one with the TCP
port.</t>
<t>When receiving an AttachLiteReq, try to form a connection
to each candidate in the request. Check the certificate
receive in the TLS handshake has the correct Node-ID as the
node that send the AttchLiteReq. If multiple connection
succeed, close all but the one with highest priority.</t>
<t>Do normal TLS and DTLS with no need for any special
framing or STUN processing.</t>
</list></t>
</section>
</section>
<section title="Ping">
<t>Ping is used to test connectivity along a path. A ping can be
addressed to a specific Node-ID, the peer controlling a given
location (by using a resource ID), or to the broadcast Node-ID
(all 1s).</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;
} 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 in cases where the Ping request
is multicast.</t>
</list></t>
</section>
</section>
<section anchor="sec-tunnel-details" title="Tunnel">
<t>A node sends a Tunnel request when it wishes to exchange
application-layer protocol messages without the expense of
establishing a direct connection via Attach or when ICE is unable
to establish a direct connection via Attach and a TURN relay is
not available. The application-level protocols that are routed via
the Tunnel request are defined by that application's usage.</t>
<t><list style="hanging">
<t></t>
<t hangText="Note:">The decision of whether to route
application-level traffic across the overlay or to open a
direct connection requires careful consideration of the
overhead involved in each transaction. Establishing a direct
connection requires greater initial setup costs, but after
setup, communication is faster and imposes no overhead on the
overlay. For example, for the SIP usage, an INVITE request to
establish a voice call might be routed over the overlay, a
SUBSCRIBE with regular updates would be better used with a
Attach, and media would both impose too great a load on the
overlay and likely receive unacceptable performance. However,
there may be a tradeoff between locating TURN servers and
relying on Tunnel for packet routing.</t>
</list></t>
<t>When a usage requires the Tunnel method, it must specify the
specific application protocol(s) that will be Tunneled and for
each protocol, specify:</t>
<t><list style="symbols">
<t>An application attribute that indicates the protocol being
tunneled. This the IANA-registered port of the application
protocol.</t>
<t>The conditions under which the application will be Tunneled
over the overlay rather than using a direct Attach.</t>
<t>A mechanism for moving future application-level
communication from Tunneling on the overlay to a direct
Connection, or an explanation why this is unnecessary.</t>
<t>A means of associating messages together as required for
dialog-oriented or request/response-oriented protocols.</t>
<t>How the Tunneled message (and associated responses) will be
delivered to the correct application. This is particularly
important if there might be multiple instances of the
application on or behind a single peer.</t>
</list></t>
<section anchor="sec-tunnel-request" title="Request Definition">
<t>The TunnelReq message contains the application PDU that the
requesting peer wishes to transmit, along with some control
information identifying the handling of the PDU.</t>
<figure>
<!-- begin-pdu -->
<artwork><![CDATA[
struct {
uint16 application;
opaque dialog_id<0..2^8-1>;
opaque application_pdu<0..2^24-1>;
} TunnelReq;
]]></artwork>
</figure>
<t>The values contained in the TunnelReq are:</t>
<t><list style="hanging">
<t></t>
<t hangText="application "></t>
<t>A 16-bit port number. This port number represents the
IANA registered port of the protocol that is going to be
sent on this connection. For SIP, this is 5060 or 5061, and
for RELOAD is TBD. By using the IANA registered port, we
avoid the need for an additional registry and allow RELOAD
to be used to set up connections for any existing or future
application protocol.</t>
<t></t>
<t hangText="dialog_id "></t>
<t>An arbitrary string providing an application-defined way
of associating related Tunneled messages. This attribute may
also encode sequence information as required by the
application protocol.</t>
<t></t>
<t hangText="application_pdu "></t>
<t>An application PDU in the format specified by the
application.</t>
</list></t>
</section>
<section anchor="sec-tunnel-response" title="Response Definition">
<t>A TunnelAns message serves as confirmation that the message
was received by the destination peer. It implies nothing about
the processing of the application. If the application protocol
specifies an acknowledgement or confirmation, that must be sent
with a separate Tunnel request. The TunnelAns message is empty
(has a zero length payload)</t>
</section>
</section>
</section>
</section>
</section>
<section anchor="sec-data-protocol" title="Data Storage Protocol">
<t>RELOAD provides a set of generic mechanisms for storing and
retrieving data in the Overlay Instance. These mechanisms can be used
for new applications simply by defining new code points and a small set
of rules. No new protocol mechanisms are required.</t>
<t>The basic unit of stored data is a single StoredData structure:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint32 length;
uint64 storage_time;
uint32 lifetime;
StoredDataValue value;
Signature signature;
} StoredData;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list style="hanging">
<t></t>
<t hangText="length "></t>
<t>The length of the rest of the structure in octets.</t>
<t></t>
<t hangText="storage_time "></t>
<t>The time when the data was stored in absolute time, represented
in seconds since the Unix epoch. Any attempt to store a data value
with a storage time before that of a value already stored at this
location MUST generate a 412 error. This prevents rollback attacks.
Note that this does not require synchronized clocks: the receiving
peer uses the storage time in the previous store, not its own
clock.</t>
<t></t>
<t hangText="lifetime "></t>
<t>The validity period for the data, in seconds, starting from the
time of store.</t>
<t></t>
<t hangText="value "></t>
<t>The data value itself, as described in <xref
target="sec-kind-model"></xref>.</t>
<t></t>
<t hangText="signature "></t>
<t>A signature over the data value. <xref
target="sec-data-sig"></xref> describes the signature computation.
The element is formatted as described in <xref
target="sec-signature"></xref></t>
</list></t>
<!--
<t>
[TODO: this doesn't include the resource ID and kind to
avoid duplicating it in each value. It would make things
more self-contained, though.]
</t>
-->
<t>Each Resource-ID specifies a single location in the Overlay Instance.
However, each location may contain multiple StoredData values
distinguished by Kind-ID. The definition of a kind describes both the
data values which may be stored and the data model of the data. Some
data models allow multiple values to be stored under the same Kind-ID.
Section <xref target="sec-kind-model"></xref> describes the available
data models. Thus, for instance, a given Resource-ID might contain a
single-value element stored under Kind-ID X and an array containing
multiple values stored under Kind-ID Y.</t>
<section anchor="sec-data-sig" title="Data Signature Computation">
<t>Each StoredData element is individually signed. However, the
signature also must be self-contained and cover the Kind-ID and
Resource-ID even though they are not present in the StoredData
structure. The input to the signature algorithm is:</t>
<t><list>
<t>resource_id + kind + StoredData</t>
</list></t>
<t>Where these values are: <list style="hanging">
<t></t>
<t hangText="resource "></t>
<t>The resource ID where this data is stored.</t>
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID for this data.</t>
<t></t>
<t hangText="StoredData "></t>
<t>The contents of the stored data value, as described in the
previous sections.</t>
</list></t>
<t>[OPEN ISSUE: Should we include the identity in the string that
forms the input to the signature algorithm?.]</t>
<t>Once the signature has been computed, the signature is represented
using a signature element, as described in <xref
target="sec-signature"></xref>.</t>
</section>
<section anchor="sec-kind-model" title="Data Models">
<t>The protocol currently defines the following data models:</t>
<t><list style="symbols">
<t>single value</t>
<t>array</t>
<t>dictionary</t>
</list></t>
<!-- TODO: EKR; add a kind in here? -->
<t>These are represented with the StoredDataValue structure:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { reserved(0), single_value(1), array(2),
dictionary(3), (255)} DataModel;
struct {
Boolean exists;
opaque value<0..2^32-1>;
} DataValue;
struct {
DataModel model;
select (model) {
case single_value:
DataValue single_value_entry;
case array:
ArrayEntry array_entry;
case dictionary:
DictionaryEntry dictionary_entry;
/* This structure may be extended */
} ;
} StoredDataValue;
]]></artwork>
</figure>
<t>We now discuss the properties of each data model in turn:</t>
<section title="Single Value">
<t>A single-value element is a simple, opaque sequence of bytes.
There may be only one single-value element for each Resource-ID,
Kind-ID pair.</t>
<t>A single value element is represented as a DataValue, which
contains the following two elements:</t>
<t><list style="hanging">
<t hangText="exists"></t>
<t>This value indicates whether the value exists at all. If it
is set to False, it means that no value is present. If it is
True, that means that a value is present. This gives the
protocol a mechanism for indicating nonexistence as opposed to
emptiness.</t>
<t></t>
<t hangText="value"></t>
<t>The stored data.</t>
</list></t>
</section>
<section title="Array">
<t>An array is a set of opaque values addressed by an integer index.
Arrays are zero based. Note that arrays can be sparse. For instance,
a Store of "X" at index 2 in an empty array produces an array with
the values [ NA, NA, "X"]. Future attempts to fetch elements at
index 0 or 1 will return values with "exists" set to False.</t>
<t>A array element is represented as an ArrayEntry:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint32 index;
DataValue value;
} ArrayEntry;
]]></artwork>
</figure>
<t>The contents of this structure are: <list style="hanging">
<t></t>
<t hangText="index"></t>
<t>The index of the data element in the array.</t>
<t></t>
<t hangText="value"></t>
<t>The stored data.</t>
</list></t>
<!-- EKR: do something about (-1) and array stores. -->
</section>
<section title="Dictionary">
<t>A dictionary is a set of opaque values indexed by an opaque key
with one value for each key. A single dictionary entry is
represented as follows</t>
<t>A dictionary element is represented as a DictionaryEntry:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
typedef opaque DictionaryKey<0..2^16-1>;
struct {
DictionaryKey key;
DataValue value;
} DictionaryEntry;
]]></artwork>
</figure>
<t>The contents of this structure are: <list style="hanging">
<t></t>
<t hangText="key"></t>
<t>The dictionary key for this value.</t>
<t></t>
<t hangText="value"></t>
<t>The stored data.</t>
</list></t>
</section>
</section>
<section 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>Remove values from 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;
DataModel data_model;
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 if not reject the
request.</t>
<t>Each StoreKindData element represents the data to be stored for
a single Kind-ID. The contents of the element are: <list
style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID. Implementations SHOULD reject requests
corresponding to unknown kinds unless specifically configured
otherwise.</t>
<t></t>
<t hangText="data_model "></t>
<t>The data model of the data. The kind defines what this has
to be so this is redundant in the case where the software
interpreting the messages understands the kind.</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 data_model matches the kind_id.</t>
<!-- EKR: REMOVED. this doesn't allow conflict resolution.
<t>The signature over the message is valid or (depending on
overlay policy) no signature is required.</t> -->
<t>The signatures over each individual data element (if any)
are valid.</t>
<t>Each element is signed by a credential which is authorized
to write this kind at this Resource-ID</t>
<t>For original (non-replica) stores, the peer MUST check that
if the generation-counter is non-zero, it equals the current
value of the generation-counter for this kind. This feature
allows the generation counter to be used in a way similar to
the HTTP Etag feature.</t>
<t>The storage time values are greater than that of any value
which would be replaced by this Store. [[OPEN ISSUE: do peers
need to save the storage time of Removes to prevent
reinsertion?]]</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
by other nodes to indicate that they are indifferent to the
generation counter's current value. For replica Stores, the peer
MUST set the generation counter to match the generation_counter in
the message. Replica Stores MUST NOT use a generation counter of
0.</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 slot 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
/ \
/ \
StoreKindData StoreKindData
kind=X kind=Y
model=Single-Value model=Array
| /\
| / \
StoredData / \
| / \
| StoredData StoredData
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
was replicated.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
KindId kind;
uint64 generation_counter;
NodeId replicas<0..2^16-1>;
} StoreKindResponse;
struct {
StoreKindResponse kind_responses<0..2^16-1>;
} StoreAns;
]]></artwork>
</figure>
<t>The contents of each StoreKindResponse are:</t>
<t><list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID being represented.</t>
<t></t>
<t hangText="generation "></t>
<t>The current value of the generation counter for that
Kind-ID.</t>
<t></t>
<t hangText="replicas "></t>
<t>The list of other peers at which the data was/will-be
replicated. In overlays and applications where the responsible
peer is intended to store redundant copies, this allows the
storing peer to independently verify that the replicas were in
fact stored by doing its own Fetch.</t>
</list></t>
<t>The response itself is just StoreKindResponse values packed
end-to-end.</t>
<t>If any of the generation counters in the request precede the
corresponding stored generation counter, then the peer MUST fail
the entire request and respond with a 412 error. The error_info in
the ErrorResponse MUST be a StoreAns response containing the
correct generation counter for each kind and empty replicas lists.
[[ TODO need to fix this up in better way ]]</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;
DataModel model;
uint64 generation;
uint16 length;
select (model) {
case single_value: ; /* Empty */
case array:
ArrayRange indices<0..2^16-1>;
case dictionary:
DictionaryKey keys<0..2^16-1>;
/* This structure may be extended */
} model_specifier;
} StoredDataSpecifier;
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} FetchReq;
]]></artwork>
</figure>
<t>The contents of the Fetch requests are as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="resource "></t>
<t>The resource ID to fetch from.</t>
<t></t>
<t hangText="specifiers "></t>
<t>A sequence of StoredDataSpecifier values, each specifying
some of the data values to retrieve.</t>
</list></t>
<t>Each StoredDataSpecifier specifies a single kind of data to
retrieve and (if appropriate) the subset of values that are to be
retrieved. The contents of the StoredDataSpecifier structure are
as follows:</t>
<t><list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID of the data being fetched. Implementations
SHOULD reject requests corresponding to unknown kinds unless
specifically configured otherwise.</t>
<t></t>
<t hangText="model "></t>
<t>The data model of the data. This must be checked against
the Kind-ID.</t>
<t></t>
<t hangText="generation "></t>
<t>The last generation counter that the requesting peer saw.
This may be used to avoid unnecessary fetches or it may be set
to zero.</t>
<t></t>
<t hangText="length "></t>
<t>The length of the rest of the structure, thus allowing
extensibility.</t>
<t></t>
<t hangText="model_specifier "></t>
<t>A reference to the data value being requested within the
data model specified for the kind. For instance, if the data
model is "array", it might specify some subset of the
values.</t>
</list></t>
<t>The model_specifier is as follows:</t>
<t><list style="symbols">
<t>If the data is of data model single value, the specifier is
empty.</t>
<t>If the data is of data model array, the specifier contains
of a list of ArrayRange elements, each of which contains two
integers. The first integer is the beginning of the range and
the second is the end of the range. 0 is used to indicate the
first element and 0xffffffff is used to indicate the final
element. The beginning of the range MUST be earlier in the
array then the end. The ranges MUST be non-overlapping.</t>
<t>If the data is of data model dictionary then the specifier
contains a list of the dictionary keys being requested. If no
keys are specified, than this is a wildcard fetch and all
key-value pairs are returned. [[TODO: We really need a way to
return only the keys. We'll need to modify this.]]</t>
</list></t>
<t>The generation-counter is used to indicate the requester's
expected state of the storing peer. If the generation-counter in
the request matches the stored counter, then the storing peer
returns a response with no StoredData values.</t>
<t>Note that because the certificate for a user is typically
stored at the same location as any data stored for that user, a
requesting peer which does not already have the user's certificate
should request the certificate in the Fetch as an
optimization.</t>
</section>
<section title="Response Definition">
<t>The response to a successful Fetch request is a FetchAns
message containing the data requested by the requester.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
KindId kind;
uint64 generation;
StoredData values<0..2^32-1>;
} FetchKindResponse;
struct {
FetchKindResponse kind_responses<0..2^32-1>;
} FetchAns;
]]></artwork>
</figure>
<t>The FetchAns structure contains a series of FetchKindResponse
structures. There MUST be one FetchKindResponse element for each
Kind-ID in the request.</t>
<t>The contents of the FetchKindResponse structure are as follows:
<list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>the kind that this structure is for.</t>
<t></t>
<t hangText="generation "></t>
<t>the generation counter for this kind.</t>
<t></t>
<t hangText="values "></t>
<t>the relevant values. If the generation counter in the
request matches the generation-counter in the stored data,
then no StoredData values are returned. Otherwise, all
relevant data values MUST be returned. A nonexistent value is
represented with "exists" set to False.</t>
</list></t>
<t>There is one subtle point about signature computation on
arrays. If the storing node uses the append feature (where the
index=0xffffffff), then the index in the StoredData that is
returned will not match that used by the storing node, which would
break the signature. In order to avoid this issue, the index value
in array is set to zero before the signature is computed. This
implies that malicious storing nodes can reorder array entries
without being detected. [[OPEN ISSUE: We've considered a number of
alternate designs here that would preserve security against this
attack if the storing node did not use the append feature.
However, they are more complicated for one or both sides. If this
attack is considered serious, we can introduce one of them.]]</t>
</section>
</section>
<section title="Stat">
<t>The Stat request is used to get metadata (length, generation
counter, digest, etc.) for a stored element without retrieving the
element itself. The name is from the UNIX stat(2) system call which
performs a similar function for files in a filesystem. It also
allows the requesting node to get a list of matching elements
without requesting the entire element.</t>
<section title="Request Definition">
<t>The Stat request is identical to the Fetch request. It simply
specifies the elements to get metadata about.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} StatReq;
]]></artwork>
</figure>
</section>
<section title="Response Definition">
<t>The Stat response contains the same sort of entries that a
Fetch response would contain, however instead of containing the
element data it contains metadata.</t>
<!--begin-pdu-->
<figure>
<artwork><![CDATA[
struct {
Boolean exists;
uint32 value_length;
HashAlgorithm hash_algorithm;
opaque hash_value<0..255>;
} MetaData;
struct {
uint32 index;
MetaData value;
} ArrayEntryMeta;
struct {
DictionaryKey key;
MetaData value;
} DictionaryEntryMeta;
struct {
DataModel model;
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 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="Remove">
<t>The Remove request is used to remove a stored element or elements
from the storing peer. Any successful remove of an existing element
for a given kind MUST increment the generation counter by at least
1.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} RemoveReq;
]]></artwork>
</figure>
<t>A RemoveReq has exactly the same syntax as a Fetch request except
that each entry represents a set of values to be removed rather than
returned. The same Kind-ID MUST NOT be used twice in a given
RemoveReq. Each specifier 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>Before processing the Remove request, the peer MUST perform the
following checks.</t>
<t><list style="symbols">
<t>The Kind-ID is known.</t>
<t>The signature over the message is valid or (depending on
overlay policy) no signature is required.</t>
<t>The signer of the message has permissions which permit him to
remove this kind of data. Although each kind defines its own
access control requirements, in general only the original signer
of the data should be allowed to remove it.</t>
<t>If the generation-counter is non-zero, it must equal 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>
</list></t>
<t>Assuming that the request is permitted, the operations proceed as
follows.</t>
<section title="Single Value">
<t>A Remove of a single value element causes it not to exist. If
no such element exists, then this is a silent success.</t>
</section>
<section title="Array">
<t>A Remove of an array element (or element range) replaces those
elements with null elements. Note that this does not cause the
array to be packed. An array which contains ["A", "B", "C"] and
then has element 0 removed produces an array containing [NA, "B",
"C"]. Note, however, that the removal of the final element of the
array shortens the array, so in the above case, the removal of
element 2 makes the array ["A", "B"].</t>
</section>
<section title="Dictionary">
<t>A Remove of a dictionary element (or elements) replaces those
elements with null elements. If no such elements exist, then this
is a silent success.</t>
</section>
<section title="Response Definition">
<t>The response to a successful Remove simply contains a list of
the new generation counters for each Kind-ID, using the same
syntax as the response to a Store request. Note that if the
generation counter does not change, that means that the requested
items did not exist. However, if the generation counter does
change, that does not mean that the items existed.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
StoreKindResponse kind_responses<0..2^16-1>;
} RemoveAns;
]]></artwork>
</figure>
</section>
</section>
<section title="Find">
<t>The Find request can be used to explore the Overlay Instance. A
Find request for a Resource-ID R and a Kind-ID T retrieves the
Resource-ID (if any) of the resource of kind T known to the target
peer which is closes to R. This method can be used to walk the
Overlay Instance by interactively fetching R_n+1=nearest(1 +
R_n).</t>
<section title="Request Definition">
<t>The FindReq message contains a series of Resource-IDs and
Kind-IDs identifying the resource the peer is interested in.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
ResourceID resource;
KindId kinds<0..2^8-1>;
} FindReq;
]]></artwork>
</figure>
<!-- find-request = Resource-ID 1*type-id -->
<t>The request contains a list of Kind-IDs which the Find is for,
as indicated below: <list style="hanging">
<t></t>
<t hangText="resource "></t>
<t>The desired Resource-ID</t>
<t></t>
<t hangText="kinds "></t>
<t>The desired Kind-IDs. Each value MUST only appear once.</t>
</list></t>
</section>
<section title="Response Definition">
<t>A response to a successful Find request is a FindAns message
containing the closest Resource-ID for each kind specified in the
request.</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
KindId kind;
ResourceID closest;
} FindKindData;
struct {
FindKindData results<0..2^16-1>;
} FindAns;
]]></artwork>
</figure>
<t>If the processing peer is not responsible for the specified
Resource-ID, it SHOULD return a 404 error.</t>
<!-- EKR removed. It is confusing here.
<t>When each kind is defined, it can indicate if the kind is not
allowed to be used in a Find request. This would be done to help
achieve some types of security properties for the data stored in
that kind.</t>
-->
<t>For each Kind-ID in the request the response MUST contain a
FindKindData indicating the closest Resource-ID for that Kind-ID
unless the kind is not allowed to be used with Find in which case
a FindKindData for that Kind-ID MUST NOT be included in the
response. If a Kind-ID is not known, then the corresponding
Resource-ID MUST be 0. Note that different Kind-IDs may have
different closest Resource-IDs.</t>
<t>The response is simply a series of FindKindData elements, one
per kind, concatenated end-to-end. The contents of each element
are:</t>
<t><list style="hanging">
<t></t>
<t hangText="kind "></t>
<t>The Kind-ID.</t>
<t></t>
<t hangText="closest "></t>
<t>The closest resource ID to the specified resource ID. This
is 0 if no resource ID is known.</t>
</list></t>
<t>Note that the response does not contain the contents of the
data stored at these Resource-IDs. If the requester wants this, it
must retrieve it using Fetch.</t>
</section>
<section title="Defining New Kinds">
<t>A new kind MUST define:</t>
<t><list style="symbols">
<t>The meaning of the data to be stored.</t>
<t>The Kind-ID.</t>
<t>The data model (single value, array, dictionary, etc.)</t>
<t>Access control rules for indicating what credentials are
allowed to read and write that Kind-ID at a given
location.</t>
</list></t>
<t>While each kind MUST 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 built-in data models.
However, they MAY define any new required data models. The
intention is that the basic data model set be sufficient for most
applications/usages.</t>
</section>
</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 and old and new certificate the
both have the same Node-ID which allows for migration of certificates
when they are renewed.</t>
<!-- <t>when joining and redo the check about every 24 hours after that.
Certificate data should be stored with an expiry time of 60 days. When
a client is checking the existence of data, if the expiry is less than
30 days, it should be refreshed to have an expiry of 60 days. The
certificate information is frequently used for many operations, and
peers should cache it for 8 hours.</t>-->
<t><list style="hanging">
<t></t>
<t hangText="Kind IDs">This usage defines the CERTIFICATE Kind-ID to
store a peer or user's certificate.</t>
<t></t>
<t hangText="Data Model">The data model for CERTIFICATE data is
array.</t>
<t></t>
<t hangText="Access Control">The CERTIFICATE MUST contain a Node-ID
or user name which, when hashed, maps to the Resource-ID at which
the value is being stored.</t>
</list></t>
</section>
<section anchor="sec-turn-server" title="TURN Server Usage">
<t>The TURN server usage allows a RELOAD peer to advertise that it is
prepared to be a TURN server as defined in <xref
target="I-D.ietf-behave-turn"></xref>. When a node starts up, it joins
the overlay network and forms several connection in the process. If the
ICE stage in any of these connection return a reflexive address that is
not the same as the peers perceived address, then the peers is behind a
NAT and not an candidate for a TURN server. Additionally, if the peers
IP address is in the private address space range, then it is not a
candidate for a TURN server. Otherwise, the peer SHOULD assume it is a
potential TURN server and follow the procedures below.</t>
<t>If the node is a candidate for a TURN server it will insert some
pointers in the overlay so that other peers can find it. The overlay
configuration file specifies a turnDensity parameter that indicates how
many times each TURN server should record itself in the overlay.
Typically this should be set to the reciprocal of the estimate of what
percentage of peers will act as TURN servers. For each value, called d,
between 1 and turnDensity, the peer forms a Resource Name by
concatenating its Peer-ID and the value d. This Resource Name is hashed
to form a Resource-ID. The address of the peer is stored at that
Resource-ID using type TURN-SERVICE and the TurnServer object:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
uint8 iteration;
IpAddressAndPort server_address;
} TurnServer;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list style="hanging">
<t></t>
<t hangText="iteration"></t>
<t>the d value</t>
<t></t>
<t hangText="server_address"></t>
<t>the address at which the TURN server can be contacted.</t>
</list></t>
<t><list style="hanging">
<t hangText="Note:">Correct functioning of this algorithm depends
critically on having turnDensity be an accurate estimate of the true
density of TURN servers. If turnDensity is too high, then the
process of finding TURN servers becomes extremely expensive as
multiple candidate Resource-IDs must be probed.</t>
</list></t>
<t>Peers peers that provide this service need to support the TURN
extensions to STUN for media relay of both UDP and TCP traffic as
defined in <xref target="I-D.ietf-behave-turn"></xref> and <xref
target="RFC5382"></xref>.</t>
<t>[[OPEN ISSUE: This structure only works for TURN servers that have
public addresses. It may be possible to use TURN servers that are behind
well-behaved NATs by first ICE connecting to them. If we decide we want
to enable that, this structure will need to change to either be a
Peer-ID or include that as an option.]]</t>
<t><list style="hanging">
<t hangText="Kind IDs">This usage defines the TURN-SERVICE Kind-ID
to indicate that a peer is willing to act as a TURN server. The Find
command MUST return results for the TURN-SERVICE Kind-ID.</t>
<t hangText="Data Model">The TURN-SERVICE stores a single value for
each Resource-ID.</t>
<t hangText="Access Control">If certificate-based access control is
being used, stored data of kind TURN-SERVICE MUST be authenticated
by a certificate which contains a Peer-ID which when hashed with the
iteration counter produces the Resource-ID being stored at.</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-diagnostic" title="Diagnostic Usage" toc="default">
<!--
<t>[[TODO: reduce text of motivation description in the next
version]]</t>
<t>The development and deployment of a peer-to-peer system is a
continuous process. The developers write code which is tested on a
scale that may be smaller than the actual deployment size. After this
local testing, the code is deployed in a real environment. Bugs arise
during development and deployment phases. The designers of the
peer-to-peer system need mechanisms which can help identify problems
and bugs in a peer-to-peer system during development and deployment
phases. Peer-to-peer systems are an example of a distributed system
and it is not a trivial task to provide protocol mechanisms, tools and
techniques to identify problems that may arise in such systems.</t>
<t>The diagnostic mechanisms can broadly be classified into online and
offline mechanisms. The online mechanisms attempt to identify faults
in a running system where as offline mechanisms try to infer faults by
gathering the log files of machines participating in a distributed
system.</t>
<t>In a peer-to-peer system, a peer maintains routing state to forward
messages according to the overlay protocol being used. In addition, a
peer stores information published by other peers. The routing and
storage of resources consume network, space (memory), and CPU
resources. A peer also needs to keep track of how long the P2PSIP
application has been running and the last time peers in the routing
table were last contacted. During development and deployment phase, an
overlay designer needs mechanisms to query some or all of the above
mentioned information.</t>
<t>The overlay designer may also treat overlay as a black box and
determine if the routing mechanisms are working correctly under
various levels of churn.</t>
<t>Thus, there are at least two types of online diagnostic mechanisms:
1) state acquisition 2) black-box diagnostics</t>
<section title="State Acquisition Mechanisms">
<t>The protocol provides a DIAGNOSTIC method [TODO] which queries
the peer for its routing state, average bandwidth, CPU utilization,
and storage state. The state acquisition mechanism can be used to
construct a local view of the connectivity state of the system. It
can also be used to construct a geographical map of the system.</t>
<t>Below, we identify potential issues with the state acquisition
mechanisms.</t>
<t>Security: If any peer can query the routing or storage state of
any other peer, then clearly privacy and security concerns arise. To
address this, the state acquisition mechanisms need an access list
like mechanism so that only the overlay implementer can query the
state of all the nodes. Alternatively, the state acquisition
mechanisms are only enabled during the development phase or are only
enabled for 'admin' users.</t>
<t>Scalability: It is possible to query the state of few hundred or
a few thousand nodes (as it is currently done in our live system on
Planet lab); however, a serial state acquisition of a million node
is a non starter. In large scale networks, one option is to query
the state of few hundred nodes and to construct an high level
connectivity map. CAIDA [ref] collects data at a few vantage points
to construct BGP maps.</t>
<t>Instantaneous vs. long term state: Another issue with these state
acquisitions mechanisms is whether they acquire the instantaneous
state snapshot or an exponential moving average or a list of
snapshots over a period of time. For diagnostic metrics such as CPU
utilization, an exponential moving average metric is also helpful in
addition to the instantaneous snapshot.</t>
<t>Pull vs. push: The state acquisition mechanisms can either be
pull-based or push-based or a combination of both. In pull-based
mechanisms, peer explicitly request state of another peer. This may
not be sufficient because pull-based mechanisms require a to
periodically poll a peer for any change state. In a push-based
mechanism, peers advertise any change in certain metrics to their
routing or neighbor peers. As an example of push-based mechanism, a
peer which starts to relay a call may indicate a change in its
bandwidth to its routing or neighbor peers in a Probe message.</t>
<t>Development vs. deployment: A hard problem is to decide which
diagnostics are absolutely necessary during deployment and which are
needed during development.</t>
<t>Clearly, complete state acquisition has security concerns in a
deployed system. The other option an overlay implementer can use is
to run a few peers and have complete control over the functionality
of these peers. These peers are same as other peers with the
difference that an overlay implementer can explicitly query the
state of these peers. It can then use this information to 'crawl'
the overlay network and construct a local map of the network.</t>
</section>
<section title="Black-box diagnostics">
<t>[[TODO: a better name for this section]]</t>
<t>Black-box diagnostics: DHTs are examples of structured
peer-to-peer networks and they allow nodes to store key/value pairs
in the overlay. A simple diagnostic mechanism is to treat the
overlay as a black-box: publish several key/value pairs at one peer
and then look them up from another peer. For this kind of diagnostic
mechanism, clients are more suitable as they do not provide any
routing or storage services to the overlay and can connect to an
arbitrary peer.</t>
</section>
-->
<t>The Diagnostic Usage allows a node to report various statistics about
itself that may be useful for diagnostics or performance management. It
can be used to discover information such as the software version,
uptime, routing table, stored resource-objects, and performance
statistics of a peer. The usage defines several new kinds which can be
retrieved to get the statistics and also allows to retrieve other kinds
that a node stores. In essence, the usage allows querying a node's state
such as storage and network to obtain the relevant information.
Additional diagnostic capabilities have been proposed in <xref
target="I-D.zheng-p2psip-diagnose"></xref>.</t>
<t>The access control model for all kinds is a local policy defined by
the peer or the overlay policy. The peer may be configured with a list
of users that it is willing to return the information for and restrict
access to users with that name. Unless specific policy overrides it,
data SHOULD NOT be returned for users not on the list. The access
control can also be determined on a per kind basis - for example, a node
may be willing to return the software version to any users while
specific information about performance may not be returned.</t>
<!--
<t>A peer may query other peers for kinds defined in this usage using
a Diagnostic message <xref target="sec-diagnostic"></xref>. A peer MAY
request multiple kinds in a single Diagnostic request. </t>
-->
<!--
A peer can use this information to choose an appropriate
peer for its routing or connection table. It may also maintain kinds defined
in this usage for peers in its routing and connection table.
-->
<t>TODO - need to explain how this is addressed to node-id. [TODO: Do we
need a DIAGNOSTIC method? Access control mechanisms for DIAGNOSTIC may
be different from a Fetch.]</t>
<!--
<texttable>
<ttcol align="left">Kind</ttcol>
<ttcol align="left">Data Model</ttcol>
<ttcol align="left">Access Control</ttcol>
<ttcol align="left">Data size</ttcol>
<c>ROUTING_TABLE_SIZE</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 32-bit integer</c>
<c>SOFTWARE_VERSION</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>US-ASCII string up to 255 characters</c>
<c>MACHINE_UPTIME</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 64-bit integer</c>
<c>APP_UPTIME</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 64-bit integer</c>
<c>MEMORY_FOOTPRINT</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 32-bit integer</c>
<c>DATASIZE_STORED</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 64-bit integer</c>
<c>INSTANCES_STORED</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 64-bit integer</c>
<c>MESSAGES_SENT_RCVD</c> <c>array (indexed by method code)</c> <c>admin, cert, shared-key</c> <c>each
array entry is a pair of unsigned 64-bit integers representing messages sent and received</c>
<c>EWMA_BYTES_SENT</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 32-bit integer</c>
<c>EWMA_BYTES_RCVD</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 32-bit integer</c>
<c>LAST_CONTACT</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 32-bit integer</c>
<c>RTT</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 32-bit integer</c>
<c>SPARE_LINK_CAPACITY</c> <c>single value</c> <c>admin, cert, shared-key</c> <c>unsigned 32-bit integer</c>
</texttable>
-->
<t>The following kinds are defined:</t>
<t><list style="hanging">
<t></t>
<t hangText="ROUTING_TABLE_SIZE">A single value element containing
an unsigned 32-bit integer representing the number of peers in the
peer's routing table.</t>
<t></t>
<t hangText="SOFTWARE_VERSION">A single value element containing a
US-ASCII string that identifies the manufacture, model, and version
of the software.</t>
<t></t>
<t hangText="MACHINE_UPTIME">A single value element containing an
unsigned 64-bit integer specifying the time the nodes has been up in
seconds.</t>
<t></t>
<t hangText="APP_UPTIME">A single value element containing an
unsigned 64-bit integer specifying the time the p2p application has
been up in seconds.</t>
<t></t>
<t hangText="MEMORY_FOOTPRINT">A single value element containing an
unsigned 32-bit integer representing the memory footprint of the
peer program in kilo bytes.</t>
<t><list style="hanging">
<t hangText="Note:">What's a kilo byte? 1000 or 1024? --
Cullen</t>
<t hangText="Note:">Good question. 1000 seems like not quite
enough room but 1024 is too much? -- EKR</t>
</list></t>
<t></t>
<t hangText="DATASIZE_STORED">An unsigned 64-bit integer
representing the number of bytes of data being stored by this
node.</t>
<t></t>
<t hangText="INSTANCES_STORED">An array element containing the
number of instances of each kind stored. The array is index by
Kind-ID. Each entry is an unsigned 64-bit integer.</t>
<t></t>
<t hangText="MESSAGES_SENT_RCVD">An array element containing the
number of messages sent and received. The array is indexed by method
code. Each entry in the array is a pair of unsigned 64-bit integers
(packed end to end) representing sent and received.</t>
<t></t>
<t hangText="EWMA_BYTES_SENT">A single value element containing an
unsigned 32-bit integer representing an exponential weighted average
of bytes sent per second by this peer.</t>
<t>sent = alpha x sent_present + (1 - alpha) x sent</t>
<t>where sent_present represents the bytes sent per second since the
last calculation and sent represents the last calculation of bytes
sent per second. A suitable value for alpha is 0.8. This value is
calculated every five seconds.</t>
<t></t>
<t hangText="EWMA_BYTES_RCVD">A single value element containing an
unsigned 32-bit integer representing an exponential weighted average
of bytes received per second by this peer. Same calculation as
above.</t>
<!-- This need to be indexed by something so I commented them out till we get them fixed
<t></t><t hangText="LAST_CONTACT">A single value element containing an
unsigned 32-bit integer specifying the time in number of seconds
the node was last contacted. A peer typically stores this element
for each entry in its routing and connection table.</t>
<t></t><t hangText="RTT">A single value element containing an unsigned
32-bit integer specifying the recent RTT estimate in ms between
two peers. A peer typically stores this element for each entry in its
routing table.</t>
-->
<!--
<t></t><t hangText="AS_NUMBER">A single value element containing the
Autonomous System [TODO REF] number as an unsigned 32-bit integer.
Zero is returned if the AS number is unknown.</t>
<t>(OPEN ISSUES: How to determine a AS number? This metric is
primarily used for advertising and locating STUN/TURN servers. A
TURN server is inserted and looked up under H(AS). What if there are
no TURN servers in the same AS? ) [[TODO: I propose we remove this
unless we can say how to compute it. I note most the software I have
seen just uses the table lookup on IP address - if this is the case
it is probably better just to return IP address of NAT. ]]</t>
<t></t><t hangText="CPU_UTILIZATION">A single value element containing an
unsigned 8-bit integer representing the percentage CPU load from 1
to 100.</t>
<t>(OPEN ISSUE: It is not a very precise metric.)</t>
<t></t><t hangText="NEIGHBOR_TABLE_SIZE">A single value element
containing an unsigned 32-bit integer representing the number of
peers in the node's neighbor table.</t>
-->
</list></t>
<t>[[TODO: We would like some sort of bandwidth measurement, but we're
kind of unclear on the units and representation.]]</t>
<section title="Diagnostic Metrics for a P2PSIP Deployment">
<!--
<t>Clearly, all diagnostic metrics are useful during development and
testing. The hard question is which metrics are absolutely necessary
for a deployed P2PSIP system. We attempt to identify these metrics
and classify them under 'resource' and 'peer' metrics.</t>
<t>For 'resource' metric, we identify CPU_UTILIZATION,
EWMA_BYTES_SENT, EWMA_BYTES_RCVD, and MEMORY_FOOTPRINT as the key
metrics and for 'peer' metric we identify UPTIME, LAST_CONTACT, and
RTT as the metrics that are crucial for a deployed P2PSIP
system.</t>
-->
<t>(OPEN QUESTION: any other metrics?)</t>
<t>Below, we sketch how these metrics can be used. A peer can use
EWMA_BYTES_SENT and EWMA_BYTES_RCVD of another peer to infer whether
it is acting as a media relay. It may then choose not to forward any
requests for media relay to this peer. Similarly, among the various
candidates for filling up routing table, a peer may prefer a peer with
a large UPTIME value, small RTT, and small LAST_CONTACT value.</t>
</section>
<!-- <t>[[TODO: Why not MIB]]</t> -->
</section>
<section anchor="sec-chord-algorithm" title="Chord Algorithm ">
<t>This algorithm is assigned the name chord-128-2-16+ to indicate it is
based on Chord, uses SHA-1 then truncates that to 128 bit for the hash
function, stores 2 redundant copies of all data, and has finger tables
with at least 16 entries.</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 of 16 entries and a
neighborhood table of 6 entries. The neighborhood table contains the 3
peers before this peer and the 3 peers after it in the DHT ring. The
first entry in the finger table contains the peer half-way around the
ring from this peer; the second entry contains the peer that is 1/4 of
the way around; the third entry contains the peer that is 1/8th of the
way around, and so on. Fundamentally, the chord data structure can be
thought of a doubly-linked list formed by knowing the successors and
predecessor peers in the neighborhood table, sorted by the Node-ID. As
long as the successor peers are correct, the DHT will return the
correct result. The pointers to the prior peers are kept to enable
inserting of new peers into the list structure. Keeping multiple
predecessor and successor pointers makes it possible to maintain the
integrity of the data structure even when consecutive peers
simultaneously fail. The finger table forms a skip list, so that
entries in the linked list can be found in O(log(N)) time instead of
the typical O(N) time that a linked list would provide.</t>
<t>A peer, n, is responsible for a particular Resource-ID k if k is
less than or equal to n and k is greater than p, where p is the peer
id of the previous peer in the neighborhood table. Care must be taken
when computing to note that all math is modulo 2^128.</t>
</section>
<section title="Reactive vs Periodic Recovery">
<t>Open Issue: The algorithm currently presented in this section uses
reactive recovery---when a neighbor is lost, that
information is immediately propagated. Research in DHT
performance by Rhea et al. indicates that this is not
optimal in large-scale networks with churn
<xref target="handling-churn-usenix04"/>. Addressing this
issue, however, needs to take into account the requirements
placed on this algorithm.
Because it is the mandatory DHT for RELOAD, the algorithm
described here is designed to meet two primary challenges:
<list style="symbols">
<t>
Scale from small (ten or fewer) overlays on a LAN
to global overlays with millions of nodes
</t>
<t>
Simple to implement
</t>
</list>
</t>
<t>
One of the challenges these requirements entail is achieving
reasonable performance as the overlay scales without undue
complexity. We have two possibly conflicting concerns:
<list style="symbols">
<t>
A small-scale overlay may not be stable without reactive
recovery, because a single peer represents a large
portion of the overlay.
</t>
<t>
A large-scale overlay with significant churn may perform
poorly, both in terms of traffic volume and success
rates, when using reactive recovery.
</t>
</list>
As a result, multiple solutions have been proposed:
<list style="symbols">
<t>
Identify one set of behaviors that achieves adequate
functionality as the overlay scales.
</t>
<t>
Add a parameter dictating the type of recovery used by
peers in the overlay, configuring the peers
appropriately as they join the overlay.
</t>
<t>
Make the algorithm adaptive, according to the size of
the overlay or the churn rates observed.
</t>
</list>
</t>
<t>
At IETF 72, the WG elected to defer a decision on the final
choice until data could be collected on the effectiveness of
the strategies. This section, therefore, retains the
reactive recovery model until evidence supporting a decision
is available.
</t>
</section>
<section title="Routing">
<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.</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. [[Open Issue: should it delay sending this success
until it has successfully stored the redundant copies?]]. It then
sends a Store request to its successor in the neighborhood table and
to that peers successor. Note that these Store requests are addressed
to those specific peers, even though the Resource-ID they are being
asked to store is outside the range that they are responsible for. The
peers receiving these check they came from an appropriate predecessor
in their neighborhood 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>Note that a malicious node can return a success response but not
store the data locally or in the replica set. Requesting peers that
wish to ensure that the replication actually occurred SHOULD
[[Open Issue: SHOULD or MAY?]] contact
each peer listed in the replicas field of the Store response and
retrieve a copy of the data. [[TODO: Do we want to have some
optimization in Fetch where they can retrieve just a digest instead of
the data values?]]</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 connects to its chosen bootstrap node.</t>
<t>JP uses a series of Probes to populate its routing table.</t>
<t>JP sends Attach requests to initiate connections to each of the
peers in the connection 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 enters all the peers it contacted into its routing
table.</t>
<t>JP sends 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 does a series of Store requests to JP to store the data that
JP will be responsible for.</t>
<t>AP 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>
<t>AP sends an Update to all of its neighbors with the new values
of its neighbor set (including JP).</t>
<t>JP sends UpdateS to all the peers in its routing table.</t>
</list></t>
<t>In order to populate its routing table, JP sends a Probe 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 probe to p0+1 to discover its successor (p1).
This process can be repeated to discover as many successors as
desired. The values for the two peers before p will be found at a
later stage when n receives an Update.</t>
<t>In order to set up its neighbor table entry for peer i, JP simply
sends an Attach to peer (n+2^(numBitsInNodeId-i). This will be routed
to a peer in approximately the right location around the ring.</t>
</section>
<section title="Routing Attaches">
<t>When a peer needs to Attach to a new peer in its neighborhood
table, it MUST source-route the Attach request through the peer from
which it learned the new peer's Node-ID. Source-routing these requests
allows the overlay to recover from instability.</t>
<t>All other Attach requests, such as those for new finger table
entries, are routed conventionally through the overlay.</t>
<t>If a peer is unable to successfully Attach with a peer that should
be in its neighborhood, it MUST locate either a TURN server or another
peer in the overlay, but not in its neighborhood, through which it can
exchange messages with its neighbor peer</t>
</section>
<section title="Updates">
<t>A chord Update is defined as</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
enum { reserved (0),
peer_ready(1), neighbors(2), full(3), (255) }
ChordUpdateType;
struct {
ChordUpdateType type;
select(type){
case peer_ready: /* Empty */
;
case neighbors:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
case full:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
NodeId fingers<0..2^16-1>;
};
} ChordUpdate;
]]></artwork>
</figure>
<t>The "type" field contains the type of the update, which depends on
the reason the update was sent.</t>
<t><list style="hanging">
<t hangText="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 pers.</t>
<t hangText="neighbors: ">this version is sent to members of the
Chord neighbor table.</t>
<t hangText="full: ">this version is sent to peers which request
an Update with a RouteQueryReq.</t>
</list></t>
<t>If the message is of type "neighbors", then the contents of the
message will be:</t>
<t><list style="hanging">
<t></t>
<t hangText="predecessors "></t>
<t>The predecessor set of the Updating peer.</t>
<t></t>
<t hangText="successors "></t>
<t>The successor set of the Updating peer.</t>
</list></t>
<t>If the message is of type "full", then the contents of the message
will be:</t>
<t><list style="hanging">
<t></t>
<t hangText="predecessors "></t>
<t>The predecessor set of the Updating peer.</t>
<t></t>
<t hangText="successors "></t>
<t>The successor set of the Updating peer.</t>
<t></t>
<t hangText="fingers "></t>
<t>The finger table if the Updating peer, in numerically ascending
order.</t>
</list></t>
<t>A peer MUST maintain an association (via Attach) to every member of
its neighbor set. A peer MUST attempt to maintain at least three
predecessors and three successors. However, it MUST send its entire
set in any Update message sent to neighbors.</t>
<section title="Sending Updates">
<t>Every time a connection to a peer in the neighborhood set is lost
(as determined by connectivity probes or failure of some request),
the peer should remove the entry from its neighborhood table and
replace it with the best match it has from the other peers in its
routing table. It then sends an Update to all its remaining
neighbors. 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 30 seconds after removing the failed entry from
its neighborhood table in order to allow a triggered update to
inform it of a better match for its neighborhood table.</t>
<t>If connectivity is lost to all three of the peers that succeed
this peer in the ring, then this peer should behave as if it is
joining the network and use Probes 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="Receiving Updates">
<t>When a peer, N, receives an Update request, it examines the
Node-IDs in the UpdateReq and at its neighborhood table and decides
if this UpdateReq would change its neighborhood table. This is done
by taking the set of peers currently in the neighborhood table and
comparing them to the peers in the update request. There are three
major cases:</t>
<t><list style="symbols">
<t>The UpdateReq contains peers that would not change the
neighbor set because they match the neighborhood table.</t>
<t>The UpdateReq contains peers closer to N than those in its
neighborhood table.</t>
<t>The UpdateReq defines peers that indicate a neighborhood
table further away from N than some of its neighborhood table.
Note that merely receiving peers further away does not
demonstrate this, since the update could be from a node far away
from N. Rather, the peers would need to bracket N.</t>
</list></t>
<t>In the first case, no change is needed.</t>
<t>In the second case, N MUST attempt to Attach to the new peers and
if it is successful it MUST adjust its neighbor set accordingly.
Note that it can maintain the now inferior peers as neighbors, but
it MUST remember the closer ones.</t>
<t>The third case implies that a neighbor has disappeared, most
likely because it has simply been disconnected but perhaps because
of overlay instability. N MUST Probe the questionable peers to
discover if they are indeed missing and if so, remove them from its
neighborhood table.</t>
<t>After any Probes and Attaches are done, if the neighborhood table
changes, the peer sends an Update request to each of its neighbors
that was in either the old table or the new table. These Update
requests are what ends up filling in the predecessor/successor
tables of peers that this peer is a neighbor to. A peer MUST NOT
enter itself in its successor or predecessor table and instead
should leave the entries empty.</t>
<t>If peer N which is responsible for a Resource-ID R discovers that
the replica set for R (the next two nodes in its successor set) has
changed, it MUST send a Store for any data associated with R to any
new node in the replica set. It SHOULD NOT delete data from peers
which have left the replica set.</t>
<t>When a peer N detects that it is no longer in the replica set for
a resource R (i.e., there are three predecessors between N and R),
it SHOULD delete all data associated with R from its local
store.</t>
</section>
<section title="Stabilization">
<t>There are four components to stabilization: <list style="numbers">
<t>exchange Updates with all peers in its routing 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>
<t>A peer MUST periodically send an Update request to every peer in
its routing table. The purpose of this is to keep the predecessor
and successor lists up to date and to detect connection failures.
The default time is about every ten minutes, but the enrollment
server SHOULD set this in the configuration document using the
"chord-128-2-16+-update-frequency" element (denominated in seconds.)
A peer SHOULD randomly offset these Update requests so they do not
occur all at once. If an Update request fails or times out, the peer
MUST mark that entry in the neighbor table invalid and attempt to
reestablish a connection. If no connection can be established, the
peer MUST attempt to establish a new peer as its neighbor and do
whatever replica set adjustments are required. If a finger
table entry is found to have failed, the peer MUST search
for a random replacement as directed below.</t>
<!-- BBL: strict finger table maintenance is deprecated
<t>Periodically a peer should select a random entry i from the
finger table and do a Probe to resource (n+2^(numBitsInNodeId-i).
The purpose of this is to find a more accurate finger table entry if
there is one. This is done less frequently than the connectivity
checks in the previous section because forming new connections is
somewhat expensive and the cost needs to be balanced against the
cost of not having the most optimal finger table entries. The
default time is about every hour, but the enrollment server SHOULD
set this in the configuration document using the
"chord-128-2-16+-probe-frequency" element (denominated in seconds).
If this returns a different peer than the one currently in this
entry of the peer table, then a new connection should be formed to
this peer and it should replace the old peer in the finger
table.</t>-->
<t>Periodically a peer should select a random entry i from the
finger table. If the i'th entry is a member of the range
[n+2^(numBitsInNodeId-i),n+2^(numBitsInNodeId-(i-1))],
i.e. its Node-ID is in the range for that finger table
entry, no stabilization is performed. [[Open issues: need to
specify smaller range for entries at start of finger table.]] However, if the
finger table entry is outside that range (which typically
occurs when no peer is found in a given range during an
earlier search), the peer should do a Probe to a random
Resource-ID in the range. The
default time is about every hour, but the enrollment server SHOULD
set this in the configuration document using the
"chord-128-2-16+-probe-frequency" element (denominated in seconds).
If this returns a different peer than the one currently in this
entry of the peer table, then a new connection should be formed to
this peer and it should replace the old peer in the finger
table.</t>
<t>As an overlay grows, more than 16 entries may be required in the
finger table for efficient routing. To determine if its finger table
is sufficiently large, one an hour the peer should perform a Probe
to determine whether growing its finger table by four entries would
result in it learning at least two peers that it does not already
have in its neighbor table. If so, then the finger table SHOULD be
grown by four entries. Similarly, if the peer observes that its
closest finger table entries are also in its neighbor table, it MAY
shrink its finger table to the minimum size of 16 entries. [[OPEN
ISSUE: there are a variety of algorithms to gauge the population of
the overlay and select an appropriate finger table size. Need to
consider which is the best combination of effectiveness and
simplicity.]]</t>
<t>To detect that a partitioning has occurred and to heal the
overlay, a peer P MUST periodically repeat the discovery process
used in the initial join for the overlay to locate an appropriate
bootstrap peer, B. If an overlay has multiple mechanisms for
discovery it should randomly select a method to locate a bootstrap
peer. P should then send a Probe for its own Node-ID routed through
B. If a response is received from a peer S', which is not P's
successor, then the overlay is partitioned and P should send a
Attach to S' routed through B, followed by an Update sent to S'.
(Note that S' may not be in P's neighborhood table once the overlay
is healed, but the connection will allow S' to discover appropriate
neighbor entries for itself via its own stabilization.)</t>
</section>
</section>
<section title="Route Query">
<t>For this topology plugin, the RouteQueryReq contains no additional
information. The RouteQueryAns contains the single node ID of the next
peer to which the responding peer would have routed the request
message in recursive routing:</t>
<figure>
<!--begin-pdu-->
<artwork><![CDATA[
struct {
NodeId next_id;
} ChordRouteQueryAns;
]]></artwork>
</figure>
<t>The contents of this structure are as follows: <list
style="hanging">
<t></t>
<t hangText="next_peer "></t>
<t>The peer to which the responding peer would route the message
to in order to deliver it to the destination listed in the
request.</t>
</list></t>
<t>If the requester set the send_update flag, the responder SHOULD
initiate an Update immediately after sending the RouteQueryAns.</t>
</section>
<section title="Leaving">
<t>Peers SHOULD send a Leave request prior to exiting the Overlay
Instance. Any peer which receives a Leave for a peer n in its neighbor
set must remove it from the neighbor set, update its replica sets as
appropriate (including Stores of data to new members of the replica
set) and send Updates containing its new predecessor and successor
tables.</t>
</section>
</section>
<section anchor="sec-enrollment" title="Enrollment and Bootstrap">
<section anchor="sec-discovery" title="Discovery">
<t>When a peer first joins a new overlay, it starts with a discovery
process to find an enrollment server. Related work to the approach
used here is described in <xref
target="I-D.garcia-p2psip-dns-sd-bootstrapping"></xref> and <xref
target="I-D.matthews-p2psip-bootstrap-mechanisms"></xref>. Another
scheme for referencing overlays is described in <xref
target="I-D.hardie-p2poverlay-pointers"></xref>. The peer first
determines the overlay name. This value is provided by the user or
some other out of band provisioning mechanism. If the name is an IP
address, that is directly used otherwise the peer MUST do a DNS SRV
query using a Service name of "p2p_enroll" and a protocol of tcp to
find an enrollment server.</t>
<!--
<t>If the overlay name ends in .local, then a DNS SRV lookup using
implement <xref target="I-D.cheshire-dnsext-dns-sd"></xref> with a
Service name of "p2p_menroll" can also be tried to find an enrollment
server. If they implement this, the user name MAY be used as the
Instance Identifier label.</t>
-->
<t>Once an address for the enrollment servers is determined, the peer
forms an HTTPS connection to that IP address. The certificate MUST
match the overlay name as described in <xref target="RFC2818"></xref>.
The peer then performs a GET to the URL formed by appending a path of
"/p2psip/enroll" to the overlay name. For example, if the overlay name
was example.com, the URL would be
"https://example.com/p2psip/enroll".</t>
<t>The result is an XML configuration file with the syntax described
in the following section.</t>
</section>
<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. This information is fetched from the enrollment server,
as described above. An example document is shown below.</t>
<figure>
<artwork><![CDATA[
<?xml version="1.0" encoding="UTF-8"?>
<overlay instance-name="chord.example.com" expiration="86400">
<toplogy-plugin algorithm-name="chord-128-2-16+"/>
<root-cert>[PEM encoded certificate here]</root-cert>
<required-kind name="SIP-REGISTRATION" max-values="10"
max-size="1000"/>
<credential-server url="https://www.example.com/csr"/>
<bootstrap-peer address="192.0.2.2" port="5678"/>
<bootstrap-peer address="192.0.2.3" port="5678"/>
<bootstrap-peer address="192.0.2.4" port="5678"/>
<multicast-bootstrap="192.0.2.99" port="5678"/>
</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 conferment 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:overlay.</t>
<t>The file can contain multiple "overlay" elements where each one
contains the configuration information for a different overlay. Each
"overlay" has the following attributes:</t>
<t><list style="hanging">
<t></t>
<t hangText="instance-name:">name of the overlay</t>
<t></t>
<t hangText="expiration:">time in future at which this overlay
configuration is not longer valid and need to be retrieved again.
This is expressed in seconds from the current time.</t>
</list></t>
<t>Inside each overlay element, the following elements can occur:</t>
<t><list style="hanging">
<t hangText="topology-plugin">This element has an attribute called
algorithm-name that describes the overlay-algorithm being
used.</t>
<t hangText="root-cert ">This element contains a PEM encoded
X.509v3 certificate that is the root trust store used to sign all
certificates in this overlay. There can be more than one of
these.</t>
<t hangText="required-kinds ">This element indicates the kinds
that members must support. It has three attributes: <list
style="symbols">
<t>name: a string representing the kind.</t>
<t>max-count: the maximum number of values which members of
the overlay must support.</t>
<t>max-size: the maximum size of individual values.</t>
</list> For instance, the example above indicates that members
must support SIP-REGISTRATION with a maximum of 10 values of up to
1000 bytes each. Multiple required-kinds elements MAY be
present.</t>
<t hangText="credential-server ">This element contains the URL at
which the credential server can be reached in a "url" element.
This URL MUST be of type "https:". More than one credential-server
element may be present.</t>
<t hangText="self-signed-permitted ">This element indicates
whether self-signed certificates are permitted. If it is set to
"TRUE", then self-signed certificates are allowed, in which case
the credential-server and root-cert elements may be absent.
Otherwise, it SHOULD be absent, but MAY be set "FALSE". This
element also contains an attribute "digest" which indicates the
digest to be used to compute the Node-ID. Valid values for this
parameter are "SHA-1" and "SHA-256".</t>
<t hangText="bootstrap-peer ">This elements represents the address
of one of the bootstrap peers. It has an attribute called
"address" that represents the IP address (either IPv4 or IPv6,
since they can be distinguished) and an attribute called "port"
that represents the port. More than one bootstrap-peer element may
be present.</t>
<t hangText="multicast-bootstrap ">This element represents the
address of a multicast address and port that may be used for
bootstrap and that peers SHOULD listen on to enable bootstrap. It
has an attributed called "address" that represents the IP address
and an attribute called "port" that represents the port. More than
one "multicast-bootstrap" element may be present.</t>
<t hangText="clients-permitted ">This element represents whether
clients are permitted or whether all nodes must be peers. If it is
set to "TRUE" or absent, this indicates that clients are
permitted. If it is set to "FALSE" then nodes MUST join as
peers.</t>
<!-- tcp test mode code <t hangText="tcp-test-mode-permitted ">This element represents
whether nodes are allowed to use the TCP-Test-Mode transport in
this overlay. If it is absent, it is treated as if it was set to
"FALSE".</t>-->
<t hangText="attach-lite-permitted ">This element represents
whether nodes are allowed to use the AttachLite request in this
overlay. If it is absent, it is treated as if it was set to
"FALSE".</t>
<t hangText="chord-128-2-16+-update-frequency ">The update
frequency for the Chord-128-2-16+ topology plugin (see <xref
target="sec-chord-algorithm"></xref>).</t>
<t hangText="chord-128-2-16+-probe-frequency ">The probe frequency
for the Chord-128-2-16+ topology plugin (see <xref
target="sec-chord-algorithm"></xref>).</t>
<t hangText="credential-server">Base URL for credential
server.</t>
<t hangText="shared-secret">If shared secret mode is used, this
contains the shared secret.</t>
</list></t>
<t>[[TODO: Do a RelaxNG grammar.]]</t>
</section>
<section anchor="sec-credentials" title="Credentials">
<t>If the configuration document contains a credential-server element,
credentials are required to join the Overlay Instance. A peer which
does not yet have credentials MUST contact the credential server to
acquire them.</t>
<t>In order to acquire credentials, the peer generates an asymmetric
key pair and then generates a "Simple Enrollment Request" (as defined
in <xref target="RFC5272"></xref>) and sends this over
HTTPS as defined in <xref target="RFC5273"></xref> to
the URL in the credential-server element. The subjectAltName in the
request MUST contain the required user name.</t>
<t>The credential server MUST authenticate the request using the
provided user name and password. If the authentication succeeds and
the requested user name is acceptable, the server and returns a
certificate. The SubjectAltName field in the certificate contains the
following values:</t>
<t><list style="symbols">
<t>One or more Node-IDs which MUST be cryptographically random
<xref target="RFC4086"></xref>. These MUST be chosen by the
credential server in such a way that they are unpredictable to the
requesting user. These are of type URI and MUST contain RELOAD
URIs as described in <xref target="sec-reload-uri"></xref> and
MUST contain a Destination list with a single entry of type
"node_id".</t>
<t>The names this user is allowed to use in the overlay, using
type rfc822Name.</t>
</list></t>
<t>The certificate is returned in a "Simple Enrollment Response".
[[TODO: REF]]</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 peer then reads the certificate to
find the Node-IDs it can use.</t>
<!-- <section title="Credentials for HIP">
<t>When RELOAD is used with HIP, the certificates MUST be generated
so that: <list style="symbols">
<t>Each node is assigned a unique ORCHID.</t>
<t>The Node-ID can be uniquely determined from the ORCHID.</t>
</list> Because in general, ORCHIDs are shorter than Node-IDs,
this means that the ORCHIDS MUST be generated first and MUST be
cryptographically random in order to make the Node-IDs
cryptographically random. The mapping function used to produce the
Node-ID from the ORCHID MUST be the same as that used by the Overlay Instance to
produce Resource-IDs from Resource Names.</t>
<t>In addition to the usual attributes, when HIP is in use
certificates MUST contain a subjectAltName with an iPAddress value
containing the HIP ORCHID. This allows these certificates to be used
by the HIP peers during the HIP base exchange.</t>
</section>-->
<section title="Self-Generated Credentials">
<t>If the "self-signed-permitted" element is present and set to
"TRUE", then a node MUST generate its own self-signed certificate to
join the overlay. The self-signed certificate MAY contain any user
name of the users choice. Users SHOULD make some attempt to make it
unique but this document does not specify any mechanisms for
that.</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. 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="Joining the Overlay Peer">
<t>In order to join the overlay, the peer MUST contact a peer.
Typically this means contacting the bootstrap peers, since they are
guaranteed to have public IP addresses (the system should not
advertise them as bootstrap peers otherwise). If the peer has cached
peers it SHOULD contact them first by sending a Probe request to the
known peer address with the destination Node-ID set to that peer's
Node-ID.</t>
<t>If no cached peers are available, then the peer SHOULD send a Probe
request to the address and port found in the broadcast-peers element
in the configuration document. This MAY be a multicast or anycast
address. The Probe should use the wildcard Node-ID as the destination
Node-ID.</t>
<t>The responder peer that receives the Probe 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 Probe request even if the response is only an
error.</t>
<t>When the requester peer finally does receive a response from some
responding peer, it can note the Node-ID in the response and use this
Node-ID to start sending requests to join the Overlay Instance as
described in <xref target="sec-overlay-topology"></xref>.</t>
<t>After a peer has successfully joined the overlay network, it SHOULD
periodically look at any peers to which it has managed to form direct
connections. Some of these peers MAY be added to the cached-peers list
and used in future boots. Peers that are not directly connected MUST
NOT be cached. The RECOMMENDED number of peers to cache is 10.</t>
</section>
</section>
<section anchor="sec-msgflow" title="Message Flow Example">
<t>In the following example, we assume that JP has formed a connection
to one of the bootstrap peers. JP then sends an Attach through that peer
to the admitting peer (AP) to initiate a connection. When AP responds,
JP and AP use ICE to set up a connection and then set up TLS.</t>
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach Dest=JP | | | | |
|---------------------------------------------------------->|
| | | | | | |
| | | | | | |
| | |Attach Dest=JP | | |
| | |<--------------------------------------|
| | | | | | |
| | | | | | |
| | |Attach Dest=JP | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
| | |AttachAns | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |AttachAns | | |
| | |-------------------------------------->|
| | | | | | |
| | | | | | |
|AttachAns | | | | |
|<----------------------------------------------------------|
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.............................| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>Once JP has connected to AP, it needs to populate its Routing Table.
In Chord, this means that it needs to populate its neighbor table and
its finger table. To populate its neighbor table, it needs the successor
of AP, NP. It sends an Attach to the Resource-IP AP+1, which gets routed
to NP. When NP responds, JP and NP use ICE and TLS to set up a
connection.</t>
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach AP+1 | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | |Attach AP+1 | |
| | | |-------->| | |
| | | | | | |
| | | | | | |
| | | |AttachAns | |
| | | |<--------| | |
| | | | | | |
| | | | | | |
|AttachAns | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|Attach | | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.......................................| | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>JP also needs to populate its finger table (for Chord). It issues a
Attach to a variety of locations around the overlay. The diagram below
shows it sending an Attach halfway around the Chord ring the JP +
2^127.</t>
<figure>
<artwork><![CDATA[
JP NP XX TP
| | | |
| | | |
| | | |
|Attach JP+2<<126 | |
|-------->| | |
| | | |
| | | |
| |Attach JP+2<<126 |
| |-------->| |
| | | |
| | | |
| | |Attach JP+2<<126
| | |-------->|
| | | |
| | | |
| | |AttachAns|
| | |<--------|
| | | |
| | | |
| |AttachAns| |
| |<--------| |
| | | |
| | | |
|AttachAns| | |
|<--------| | |
| | | |
| | | |
|TLS | | |
|.............................|
| | | |
| | | |
| | | |
| | | |
]]></artwork>
</figure>
<t>Once JP has a reasonable set of connections he is ready to take his
place in the DHT. He does this by sending a Join to AP. AP does a series
of Store requests to JP to store the data that JP will be responsible
for. AP then sends JP an Update explicitly labeling JP as its
predecessor. At this point, JP is part of the ring and responsible for a
section of the overlay. AP can now forget any data which is assigned to
JP and not AP.</t>
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|JoinReq | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|JoinAns | | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreReq Data A | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|StoreReq Data B | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>In Chord, JP's neighbor table needs to contain its own predecessors.
It couldn't connect to them previously because Chord has no way to route
immediately to your predecessors. However, now that it has received an
Update from AP, it has APs predecessors, which are also its own, so it
sends Attaches to them. Below it is shown connecting to its closest
predecessor, PP.</t>
<figure>
<artwork><![CDATA[
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach Dest=PP | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | |Attach Dest=PP | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |AttachAns| | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
|AttachAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|...................| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|------------------>| | | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<------------------| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<--------------------------------------| | |
| | | | | | |
| | | | | | |
]]></artwork>
</figure>
<t>Finally, now that JP has a copy of all the data and is ready to route
messages and receive requests, it sends Updates to everyone in its
Routing Table to tell them it is ready to go. Below, it is shown sending
such an update to TP.</t>
<figure>
<artwork><![CDATA[
JP NP XX TP
| | | |
| | | |
| | | |
|Update | | |
|---------------------------->|
| | | |
| | | |
|UpdateAns| | |
|<----------------------------|
| | | |
| | | |
| | | |
| | | |
]]></artwork>
</figure>
</section>
<section title="Security Considerations">
<section title="Overview">
<t>RELOAD provides a generic storage service, albeit one designed to
be useful for P2PSIP. In this section we discuss security issues that
are likely to be relevant to any usage of RELOAD.</t>
<t>In any Overlay Instance, any given user depends on a number of
peers with which they have no well-defined relationship except that
they are fellow members of the Overlay Instance. In practice, these
other nodes may be friendly, lazy, curious, or outright malicious. No
security system can provide complete protection in an environment
where most nodes are malicious. The goal of security in RELOAD is to
provide strong security guarantees of some properties even in the face
of a large number of malicious nodes and to allow the overlay to
function correctly in the face of a modest number of malicious
nodes.</t>
<t>P2PSIP deployments require the ability to authenticate both peers
and resources (users) without the active presence of a trusted entity
in the system. We describe two mechanisms. The first mechanism is
based on public key certificates and is suitable for general
deployments. The second is an admission control mechanism based on an
overlay-wide shared symmetric key.</t>
</section>
<section title="Attacks on P2P Overlays">
<t>The two basic functions provided by overlay nodes are storage and
routing: some node is responsible for storing a peer's data and for
allowing a peer to fetch other peer's data. Some other set of nodes
are responsible for routing messages to and from the storing nodes.
Each of these issues is covered in the following sections.</t>
<t>P2P overlays are subject to attacks by subversive nodes that may
attempt to disrupt routing, corrupt or remove user registrations, or
eavesdrop on signaling. The certificate-based security algorithms we
describe in this draft are intended to protect overlay routing and
user registration information in RELOAD messages.</t>
<t>To protect the signaling from attackers pretending to be valid
peers (or peers other than themselves), the first requirement is to
ensure that all messages are received from authorized members of the
overlay. For this reason, RELOAD transports all messages over a secure
channel (TLS and DTLS are defined in this document) which provides
message integrity and authentication of the directly communicating
peer. In addition, messages and data are digitally signed with the
sender's private key, providing end-to-end security for
communications.</t>
</section>
<section title="Certificate-based Security">
<t>This specification stores users' registrations and possibly other
data in an overlay network. This requires a solution to securing this
data as well as securing, as well as possible, the routing in the
overlay. Both types of security are based on requiring that every
entity in the system (whether user or peer) authenticate
cryptographically using an asymmetric key pair tied to a
certificate.</t>
<t>When a user enrolls in the Overlay Instance, they request or are
assigned a unique name, such as "alice@dht.example.net". These names
are unique and are meant to be chosen and used by humans much like a
SIP Address of Record (AOR) or an email address. The user is also
assigned one or more Node-IDs by the central enrollment authority.
Both the name and the peer ID are placed in the certificate, along
with the user's public key.</t>
<t>Each certificate enables an entity to act in two sorts of
roles:</t>
<t><list style="symbols">
<t>As a user, storing data at specific Resource-IDs in the Overlay
Instance corresponding to the user name.</t>
<t>As a overlay peer with the peer ID(s) listed in the
certificate.</t>
</list></t>
<t>Note that since only users of this Overlay Instance need to
validate a certificate, this usage does not require a global PKI.
Instead, certificates are signed by require a central enrollment
authority which acts as the certificate authority for the Overlay
Instance. This authority signs each peer's certificate. Because each
peer possesses the CA's certificate (which they receive on enrollment)
they can verify the certificates of the other entities in the overlay
without further communication. Because the certificates contain the
user/peer's public key, communications from the user/peer can be
verified in turn.</t>
<t>If self-signed certificates are used, then the security provided is
significantly decreased, since attackers can mount Sybil attacks. In
addition, attackers cannot trust the user names in certificates
(though they can trust the Node-IDs because they are cryptographically
verifiable). This scheme is only appropriate for small deployments,
such as a small office or ad hoc overlay set up among participants in
a meeting. Some additional security can be provided by using the
shared secret admission control scheme as well.</t>
<t>Because all stored data is signed by the owner of the data the
storing peer can verify that the storer is authorized to perform a
store at that Resource-ID and also allows any consumer of the data to
verify the provenance and integrity of the data when it retrieves
it.</t>
<t>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 (a slot) is bound to some small set of
certificates. In order to write data in a slot, the writer must prove
possession of the private key for one of those certificates. Moreover,
all data is stored signed by the certificate which authorized its
storage. This set of rules makes questions of authorization and data
integrity - which have historically been thorny for overlays -
relatively simple.</t>
<section title="Authorization">
<t>When a client wants to store some value in a slot, 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 in this slot. In
order to do so, it executes the Resource Name construction algorithm
for the specified kind based on the user's certificate information.
It then computes the Resource-ID from the Resource Name and verifies
that it matches the slot which the user is requesting to write to.
If it does, the user is authorized to write to this slot, 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. When a
peer receives a request to store a record at Resource-ID X, it takes
the signing certificate and recomputes the Resource Name, in this
case "alice@dht.example.com". If H("alice@dht.example.com")=X then
the Store is authorized. Otherwise it is not. Note that the Resource
Name construction algorithm may be different for other kinds.</t>
</section>
<section title="Distributed Quota">
<t>Being a peer in a Overlay Instance carries with it the
responsibility to store data for a given region of the Overlay
Instance. However, if clients were allowed to store unlimited
amounts of data, this would create unacceptable burdens on peers, as
well as enabling trivial denial of service attacks. RELOAD addresses
this issue by requiring configurations to define maximum sizes for
each kind of stored data. Attempts to store values exceeding this
size MUST be rejected (if peers are inconsistent about this, then
strange artifacts will happen when the zone of responsibility shifts
and a different peer becomes responsible for overlarge data).
Because each slot 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 title="Residual Attacks">
<t>The mechanisms described here provide a high degree of security,
but some attacks remain possible. Most simply, it is possible for
storing nodes to refuse to store a value (i.e., reject any request).
In addition, a storing node can deny knowledge of values which it
previously accepted. To some extent these attacks can be ameliorated
by attempting to store to/retrieve from replicas, but a retrieving
client does not know whether it should try this or not, since there
is a cost to doing so.</t>
<t>Although the certificate-based authentication scheme prevents a
single peer from being able to forge data owned by other peers.
Furthermore, although a subversive peer can refuse to return data
resources for which it is responsible it cannot return forged data
because it cannot provide authentication for such registrations.
Therefore parallel searches for redundant registrations can mitigate
most of the affects of a compromised peer. The ultimate reliability
of such an overlay is a statistical question based on the
replication factor and the percentage of compromised peers.</t>
<t>In addition, when a kind is multivalued (e.g., an array data
model), the storing node can return only some subset of the values,
thus biasing its responses. This can be countered by using single
values rather than sets, but that makes coordination between
multiple storing agents much more difficult. This is a tradeoff that
must be made when designing any usage.</t>
</section>
</section>
<section title="Routing Security">
<t>Because the storage security system guarantees (within limits) the
integrity of the stored data, routing security focuses on stopping the
attacker from performing a DOS attack on the system by misrouting
requests in the overlay. There are a few obvious observations to make
about this. First, it is easy to ensure that an attacker is at least a
valid peer in the Overlay Instance. Second, this is a DOS attack only.
Third, if a large percentage of the peers on the Overlay Instance are
controlled by the attacker, it is probably impossible to perfectly
secure against this.</t>
<section title="Background">
<t>In general, attacks on DHT routing are mounted by the attacker
arranging to route traffic through or two nodes it controls. In the
Eclipse attack <xref target="Eclipse"></xref> the attacker tampers
with messages to and from nodes for which it is on-path with respect
to a given victim node. This allows it to pretend to be all the
nodes that are reachable through it. In the Sybil attack <xref
target="Sybil"></xref>, the attacker registers a large number of
nodes and is therefore able to capture a large amount of the traffic
through the DHT.</t>
<t>Both the Eclipse and Sybil attacks require the attacker to be
able to exercise control over her peer IDs. The Sybil attack
requires the creation of a large number of peers. The Eclipse attack
requires that the attacker be able to impersonate specific peers. In
both cases, these attacks are limited by the use of centralized,
certificate-based admission control.</t>
</section>
<section title="Admissions Control">
<t>Admission to an RELOAD Overlay Instance is controlled by
requiring that each peer have a certificate containing its peer ID.
The requirement to have a certificate is enforced by using
certificate-based mutual authentication on each connection. Thus,
whenever a peer connects to another peer, each side automatically
checks that the other has a suitable certificate. These peer IDs are
randomly assigned by the central enrollment server. This has two
benefits:</t>
<t><list style="symbols">
<t>It allows the enrollment server to limit the number of peer
IDs issued to any individual user.</t>
<t>It prevents the attacker from choosing specific peer IDs.</t>
</list></t>
<t>The first property allows protection against Sybil attacks
(provided the enrollment server uses strict rate limiting policies).
The second property deters but does not completely prevent Eclipse
attacks. Because an Eclipse attacker must impersonate peers on the
other side of the attacker, he must have a certificate for suitable
peer IDs, which requires him to repeatedly query the enrollment
server for new certificates which only will match by chance. From
the attacker's perspective, the difficulty is that if he only has a
small number of certificates the region of the Overlay Instance he
is impersonating appears to be very sparsely populated by comparison
to the victim's local region.</t>
</section>
<section title="Peer Identification and Authentication">
<t>In general, whenever a peer engages in overlay activity that
might affect the routing table it must establish its identity. This
happens in two ways. First, whenever a peer establishes a direct
connection to another peer it authenticates via certificate-based
mutual authentication. All messages between peers are sent over this
protected channel and therefore the peers can verify the data origin
of the last hop peer for requests and responses without further
cryptography.</t>
<t>In some situations, however, it is desirable to be able to
establish the identity of a peer with whom one is not directly
connected. The most natural case is when a peer Updates its state.
At this point, other peers may need to update their view of the
overlay structure, but they need to verify that the Update message
came from the actual peer rather than from an attacker. To prevent
this, all overlay routing messages are signed by the peer that
generated them.</t>
<t>[OPEN ISSUE: this allows for replay attacks on requests. There
are two basic defenses here. The first is global clocks and loose
anti-replay. The second is to refuse to take any action unless you
verify the data with the relevant node. This issue is
undecided.]</t>
<t>[TODO: I think we are probably going to end up with generic
signatures or at least optional signatures on all overlay
messages.]</t>
</section>
<section title="Protecting the Signaling">
<t>The goal here is to stop an attacker from knowing who is
signaling what to whom. An attacker being able to observe the
activities of a specific individual is unlikely given the
randomization of IDs and routing based on the present peers
discussed above. Furthermore, because messages can be routed using
only the header information, the actual body of the RELOAD message
can be encrypted during transmission.</t>
<t>There are two lines of defense here. The first is the use of TLS
or DTLS for each communications link between peers. This provides
protection against attackers who are not members of the overlay. The
second line of defense, if certificate-based security is used, is to
digitally sign each message. This prevents adversarial peers from
modifying messages in flight, even if they are on the routing
path.</t>
</section>
<section title="Residual Attacks">
<t>The routing security mechanisms in RELOAD are designed to contain
rather than eliminate attacks on routing. It is still possible for
an attacker to mount a variety of attacks. In particular, if an
attacker is able to take up a position on the overlay routing
between A and B it can make it appear as if B does not exist or is
disconnected. It can also advertise false network metrics in attempt
to reroute traffic. However, these are primarily DoS attacks.</t>
<t>The certificate-based security scheme secures the namespace, but
if an individual peer is compromised or if an attacker obtains a
certificate from the CA, then a number of subversive peers can still
appear in the overlay. While these peers cannot falsify responses to
resource queries, they can respond with error messages, effecting a
DoS attack on the resource registration. They can also subvert
routing to other compromised peers. To defend against such attacks,
a resource search must still consist of parallel searches for
replicated registrations.</t>
</section>
</section>
</section>
<section title="IANA Considerations">
<t>This section contains the new code points registered by this
document. [NOTE TO IANA/RFC-EDITOR: Please replace RFC-AAAA with the RFC
number for this specification in the following list.]</t>
<t>[[TODO - add IANA registration for p2p_enroll SRV and
p2p_menroll]]</t>
<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-128-2-16+</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 0xfffffffe SHALL be registered via
RFC 5226 Expert Review. The initial contents of this registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Kind</ttcol>
<ttcol align="right">Kind-ID</ttcol>
<ttcol align="right">RFC</ttcol>
<c>INVALID</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>SIP-REGISTRATION</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>TURN_SERVICE</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>CERTIFICATE</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>ROUTING_TABLE_SIZE</c>
<c>4</c>
<c>RFC-AAAA</c>
<c>SOFTWARE_VERSION</c>
<c>5</c>
<c>RFC-AAAA</c>
<c>MACHINE_UPTIME</c>
<c>6</c>
<c>RFC-AAAA</c>
<c>APP_UPTIME</c>
<c>7</c>
<c>RFC-AAAA</c>
<c>MEMORY_FOOTPRINT</c>
<c>8</c>
<c>RFC-AAAA</c>
<c>DATASIZE_StoreD</c>
<c>9</c>
<c>RFC-AAAA</c>
<c>INSTANCES_StoreD</c>
<c>10</c>
<c>RFC-AAAA</c>
<c>MESSAGES_SENT_RCVD</c>
<c>11</c>
<c>RFC-AAAA</c>
<c>EWMA_BYTES_SENT</c>
<c>12</c>
<c>RFC-AAAA</c>
<c>EWMA_BYTES_RCVD</c>
<c>13</c>
<c>RFC-AAAA</c>
<c>LAST_CONTACT</c>
<c>14</c>
<c>RFC-AAAA</c>
<c>RTT</c>
<c>15</c>
<c>RFC-AAAA</c>
<c>Reserved</c>
<c>0x7fffffff</c>
<c>RFC-AAAA</c>
<c>Reserved</c>
<c>0xffffffff</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Data Model">
<t>IANA SHALL create a "RELOAD Data Model" Registry. Entries in this
registry are 8-bit integers denoting data models, as described in
<xref target="sec-kind-model"></xref>. Code points in this registry
SHALL be registered via RFC 5226 IETF Review. The initial contents of
this registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Data Model</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">RFC</ttcol>
<c>INVALID</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>SINGLE_VALUE</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>ARRAY</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>DICTIONARY</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>RESERVED</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Message Codes">
<t>IANA SHALL create a "RELOAD Message Code" Registry. Entries in this
registry are 16-bit integers denoting method codes as described in
<xref target="sec-contents"></xref>. These codes SHALL be registered
via RFC 5226 Standards Action. The initial contents of this registry
are:</t>
<t></t>
<texttable>
<ttcol align="left">Message Code Name</ttcol>
<ttcol align="right">Code Value</ttcol>
<ttcol align="right">RFC</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>probe_req</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>probe_ans</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>attach_req</c>
<c>3</c>
<c>RFC-AAAA</c>
<c>attach_ans</c>
<c>4</c>
<c>RFC-AAAA</c>
<c>tunnel_req</c>
<c>5</c>
<c>RFC-AAAA</c>
<c>tunnel_ans</c>
<c>6</c>
<c>RFC-AAAA</c>
<c>store_req</c>
<c>7</c>
<c>RFC-AAAA</c>
<c>store_ans</c>
<c>8</c>
<c>RFC-AAAA</c>
<c>fetch_req</c>
<c>9</c>
<c>RFC-AAAA</c>
<c>fetch_ans</c>
<c>10</c>
<c>RFC-AAAA</c>
<c>remove_req</c>
<c>11</c>
<c>RFC-AAAA</c>
<c>remove_ans</c>
<c>12</c>
<c>RFC-AAAA</c>
<c>find_req</c>
<c>13</c>
<c>RFC-AAAA</c>
<c>find_ans</c>
<c>14</c>
<c>RFC-AAAA</c>
<c>join_req</c>
<c>15</c>
<c>RFC-AAAA</c>
<c>join_ans</c>
<c>16</c>
<c>RFC-AAAA</c>
<c>leave_req</c>
<c>17</c>
<c>RFC-AAAA</c>
<c>leave_ans</c>
<c>18</c>
<c>RFC-AAAA</c>
<c>update_req</c>
<c>19</c>
<c>RFC-AAAA</c>
<c>update_ans</c>
<c>20</c>
<c>RFC-AAAA</c>
<c>route_query_req</c>
<c>21</c>
<c>RFC-AAAA</c>
<c>route_query_ans</c>
<c>22</c>
<c>RFC-AAAA</c>
<c>ping_req</c>
<c>23</c>
<c>RFC-AAAA</c>
<c>ping_ans</c>
<c>24</c>
<c>RFC-AAAA</c>
<c>stat_req</c>
<c>25</c>
<c>RFC-AAAA</c>
<c>stat_ans</c>
<c>26</c>
<c>RFC-AAAA</c>
<c>attachlite_req</c>
<c>27</c>
<c>RFC-AAAA</c>
<c>attachlite_ans</c>
<c>28</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>0x8000..0xfffe</c>
<c>RFC-AAAA</c>
<c>error</c>
<c>0xffff</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section anchor="sec-iana-error-codes" title="Error Codes">
<t>IANA SHALL create a "RELOAD Error Code" Registry. Entries in this
registry are 16-bit integers denoting error codes. New entries SHALL
be defined via RFC 5226 Standards Action. The initial contents of this
registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Error Code Name</ttcol>
<ttcol align="right">Code Value</ttcol>
<ttcol align="right">RFC</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>Error_Unauthorized</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_Precondition_Failed</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>reserved</c>
<c>0x8000..0xfffe</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Route Log Extension Types">
<t>IANA SHALL create a "RELOAD Route Log Extension Type Registry." New
entries SHALL be defined via RFC 5226 Specification Required. The
initial contents of this registry are:</t>
<t></t>
<texttable>
<ttcol align="left">Route Log Extension Name</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">Specification</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Transport Types">
<t>IANA shall create a "RELOAD Transport Type Registry." 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>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>tcp_tls</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>udp_dtls</c>
<c>2</c>
<c>RFC-AAAA</c>
<!--
<c>tcp_test_mode</c>
<c>3</c>
<c>RFC-AAAA</c>
-->
<c>reserved</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Forwarding Options">
<t>IANA shall create a "Forwarding Option Registry". Entries in this
registry between 1 and 127 SHALL be defined via RFC 5226 Standards
Action. Entries in this registry between 128 and 254 SHALL be defined
via RFC 5226 Specification Required. This registry SHALL be initially
populated with the following values:</t>
<t></t>
<texttable>
<ttcol align="left">Forwarding Option</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">Specification</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>255</c>
<c>RFC-AAAA</c>
</texttable>
</section>
<section title="Probe Information Types">
<t>IANA shall create a "RELOAD Probe Information Type Registry".
Entries in this registry SHALL be defined via RFC 5226 Standards
Action. This registry SHALL be initially populated with the following
values:</t>
<t></t>
<texttable>
<ttcol align="left">Probe Option</ttcol>
<ttcol align="right">Code</ttcol>
<ttcol align="right">Specification</ttcol>
<c>invalid</c>
<c>0</c>
<c>RFC-AAAA</c>
<c>responsible_set</c>
<c>1</c>
<c>RFC-AAAA</c>
<c>requested_info</c>
<c>2</c>
<c>RFC-AAAA</c>
<c>reserved</c>
<c>255</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 [REF] and the associated URI
Guidelines [REF: RFC4395] per the following ABNF syntax:</t>
<figure>
<artwork><![CDATA[
RELOAD-URI = "reload://" destination "@" overlay "/"
[specifier]
destination = 1 * HEXDIG
overlay = reg-name
specifier = 1*HEXDIG
]]></artwork>
</figure>
<t>The definitions of these productions are as follows:</t>
<t><list style="hanging">
<t hangText="destination: ">a hex-encoded Destination List
object.</t>
<t></t>
<t hangText="overlay: ">the name of the overlay.</t>
<t></t>
<t hangText="specifier :">a hex-encoded StoredDataSpecifier
indicating the data element.</t>
</list></t>
<t>If no specifier is present than this URI addresses the peer which
can be reached via the indicated destination list at the indicated
overlay name. If a specifier is present, then the URI addresses the
data value.</t>
<section title="URI Registration">
<t>The following summarizes the information necessary to register
the reload: URI.</t>
<t><list style="hanging">
<t hangText="URI Scheme Name: ">reload</t>
<t hangText="Status: ">permanent</t>
<t hangText="URI Scheme Syntax: ">see <xref
target="sec-reload-uri"></xref>.</t>
<t hangText="URI Scheme Semantics: ">The reload: URI is intended
to be used as a reference to a RELOAD peer or resource.</t>
<t hangText="Encoding Considerations: ">The reload: URI is not
intended to be human-readable text, therefore they are encoded
entirely in US-ASCII.</t>
<t
hangText="Applications/protocols that use this URI scheme: ">The
RELOAD protocol described in RFC-AAAA.</t>
<t>TBD for the rest of this template.</t>
</list></t>
</section>
</section>
</section>
<section title="Acknowledgments">
<t>This draft is a merge of the "REsource LOcation And Discovery
(RELOAD)" draft by David A. Bryan, Marcia Zangrilli and Bruce B.
Lowekamp, the "Address Settlement by Peer to Peer" draft by Cullen
Jennings, Jonathan Rosenberg, and Eric Rescorla, the "Security
Extensions for RELOAD" draft by Bruce B. Lowekamp and James Deverick,
the "A Chord-based DHT for Resource Lookup in P2PSIP" by Marcia
Zangrilli and David A. Bryan, and the Peer-to-Peer Protocol (P2PP) draft
by Salman A. Baset, Henning Schulzrinne, and Marcin Matuszewski.</t>
<t>Thanks to the many people who contributed including: Michael Chen,
TODO - fill in.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.I-D.ietf-mmusic-ice"?>
<?rfc include="reference.RFC.5389"?>
<?rfc include="reference.I-D.ietf-behave-turn"?>
<?rfc include="reference.RFC.5273"?>
<?rfc include="reference.RFC.5272"?>
<?rfc include="reference.RFC.4279"?>
<?rfc include="reference.I-D.ietf-mmusic-ice-tcp"?>
<?rfc include="reference.RFC.4347"?>
<?rfc include="reference.RFC.4828"?>
</references>
<references title="Informative References">
<?rfc include="reference.I-D.ietf-p2psip-concepts"?>
<?rfc include="reference.RFC.3261"?>
<?rfc include="reference.RFC.5382"?>
<?rfc include="reference.RFC.4145"?>
<?rfc include="reference.RFC.4571"?>
<?rfc include="reference.RFC.2818"?>
<?rfc include="reference.RFC.4086"?>
<?rfc include="reference.RFC.5054"?>
<?rfc include="reference.RFC.3280"?>
<?rfc include="reference.I-D.matthews-p2psip-bootstrap-mechanisms"?>
<?rfc include="reference.I-D.garcia-p2psip-dns-sd-bootstrapping"?>
<?rfc include="reference.I-D.pascual-p2psip-clients"?>
<?rfc include="reference.RFC.4787"?>
<?rfc include="reference.I-D.jiang-p2psip-sep"?>
<?rfc include="reference.I-D.zheng-p2psip-diagnose"?>
<?rfc include="reference.I-D.hardie-p2poverlay-pointers"?>
<reference anchor="I-D.ietf-p2psip-sip">
<front>
<title>A SIP Usage for RELOAD</title>
<author fullname="Cullen Jennings" initials="C" surname="Jennings">
<organization></organization>
</author>
<author fullname="Bruce Lowekamp" initials="B" surname="Lowekamp">
<organization></organization>
</author>
<author fullname="Eric Rescorla" initials="E" surname="Rescorla">
<organization></organization>
</author>
<author fullname="Salman Baset" initials="S" surname="Baset">
<organization></organization>
</author>
<author fullname="Henning Schulzrinne" initials="H"
surname="Schulzrinne">
<organization></organization>
</author>
<date day="27" month="October" year="2008" />
<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-00" />
<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 Service for Internet
Applications</title>
<author fullname="Ian Stoica" initials="I." surname="Stoica">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="Robert Morris" initials="R." surname="Morris">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="David Liben-Nowell" initials="D."
surname="Liben-Nowell">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="David Karger" initials="D." surname="Karger">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="M. Frans Kaashoek" initials="M. Frans"
surname="Kaashoek">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="Frank Dabek" initials="F." surname="Dabek">
<organization>MIT Laboratory for Computer Science</organization>
</author>
<author fullname="Hari Balakrishnan" initials="H."
surname="Balakrishnan">
<organization>MIT Laboratory for Computer Science</organization>
</author>
</front>
<seriesInfo name="IEEE/ACM Transactions on Networking"
value="Volume 11, Issue 1, 17-32, Feb 2003" />
<format target="http://pdos.csail.mit.edu/chord/papers/paper-ton.pdf"
type="PDF" />
</reference>
<reference anchor="vulnerabilities-acsac04">
<front>
<title>Vulnerabilities and Security Threats in Structured
Peer-to-Peer Systems: A Quantitative Analysis</title>
<author initials="M." surname="Srivatsa">
<organization />
</author>
<author initials="L." surname="Liu">
<organization />
</author>
</front>
<seriesInfo name="" value="ACSAC 2004" />
</reference>
<reference anchor="handling-churn-usenix04">
<front>
<title>Handling Churn in a DHT</title>
<author initials="S." surname="Rhea">
<organization />
</author>
<author initials="D." surname="Geels">
<organization />
</author>
<author initials="T." surname="Roscoe">
<organization />
</author>
<author initials="J." surname="Kubiatowicz">
<organization />
</author>
</front>
<seriesInfo name="" value="USENIX 2004" />
<format target="http://www.srhea.net/papers/bamboo-usenix.pdf"
type="PDF" />
</reference>
</references>
<!--
<reference anchor="I-D.cheshire-dnsext-multicastdns">
<front>
<title>Multicast DNS</title>
<author fullname="Stuart Cheshire" initials="S" surname="Cheshire">
<organization></organization>
</author>
<author fullname="Marc Krochmal" initials="M" surname="Krochmal">
<organization></organization>
</author>
<date day="25" month="August" year="2006" />
</front>
<seriesInfo name="Internet-Draft"
value="draft-cheshire-dnsext-multicastdns-06" />
<format target="http://www.ietf.org/internet-drafts/draft-cheshire-dnsext-multicastdns-06.txt"
type="TXT" />
</reference>
<reference anchor="I-D.cheshire-dnsext-dns-sd">
<front>
<title>DNS-Based Service Discovery</title>
<author fullname="Marc Krochmal" initials="M" surname="Krochmal">
<organization></organization>
</author>
<author fullname="Stuart Cheshire" initials="S" surname="Cheshire">
<organization></organization>
</author>
<date day="28" month="August" year="2006" />
<abstract>
<t>This document describes a convention for naming and structuring
DNS resource records. Given a type of service that a client is
looking for, and a domain in which the client is looking for that
service, this convention allows clients to discover a list of
named instances of that desired service, using only standard DNS
queries. In short, this is referred to as DNS-based Service
Discovery, or DNS-SD.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft"
value="draft-cheshire-dnsext-dns-sd-04" />
<format target="http://www.ietf.org/internet-drafts/draft-cheshire-dnsext-dns-sd-04.txt"
type="TXT" />
</reference>
-->
<section anchor="sec-changes" title="Change Log">
<section title="Changes since draft-ietf-p2psip-reload-00">
<t><list style="symbols">
<t>Split base protocol from combined draft into new
draft.</t>
<t>Update architecture discussion to address concerns
raised about clarity of roles.</t>
<t>Moved extensive discussion of routing and client
behaviors to appendix.</t>
<t>Split Ping into Ping and Probe</t>
<t>Added AttachLite to provide way to implement ICE-Lite</t>
<t>added Stat call for retrieving meta-data</t>
<t>added discussion of periodic vs reactive recovery
issue</t>
<t>changed finger table stabilization to prefer long-lived
over best-match</t>
<t>removed mDNS discovery method</t>
<t>updated IANA considerations to be more complete</t>
<t>changed error codes from http-based</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
selection of symmetric recursive routing for RELOAD and describes the
extensions that would be required to support additional routing
algorithms.</t>
<section title="Iterative vs Recursive">
<t>Iterative routing has a number of advantages. It is easier to
debug, consumes fewer resources on intermediate peers, and allows the
querying peer to identify and route around misbehaving peers <xref
target="non-transitive-dhts-worlds05"></xref>. However, in the
presence of NATs iterative routing is intolerably expensive because a
new connection must be established for each hop (using ICE) <xref
target="bryan-design-hotp2p08"></xref>.</t>
<t>Iterative routing is supported through the Route_Query mechanism
and is primarily intended for debugging. It is also allows the
querying peer to evaluate the routing decisions made by the peers at
each hop, consider alternatives, and perhaps detect at what point the
forwarding path fails.</t>
</section>
<section title="Symmetric vs Forward response">
<t>An alternative to the symmetric recursive routing method used by
RELOAD is Forward-Only routing, where the response is routed to the
requester as if it is a new message initiating by the responder (in
the previous example, Z sends the response to A as if it were sending
a request). Forward-only routing requires no state in either the
message or intermediate peers.</t>
<t>The drawback of forward-only routing is that it does not work when
the overlay is unstable. For example, if A is in the process of
joining the overlay and is sending a Join request to Z, it is not yet
reachable via forward routing. Even if it is established in the
overlay, if network failures produce temporary instability, A may not
be reachable (and may be trying to stabilize its network connectivity
via Attach messages).</t>
<t>Furthermore, forward-only responses are less likely to reach the
querying peer than symmetric recursive because the forward path is
more likely to have a failed peer than the request path (which was
just tested to route the request) <xref
target="non-transitive-dhts-worlds05"></xref>.</t>
<t>An extension to RELOAD that supports forward-only routing but
relies on symmetric responses as a fallback would be possible, but due
to the complexities of determining when to use forward-only and when
to fallback to symmetric, we have chosen not to include it as an
option at this point.</t>
</section>
<section title="Direct Response">
<t>Another routing option is Direct Response routing, in which the
response is returned directly to the querying node. In the previous
example, if A encodes its IP address in the request, then Z can simply
deliver the response directly to A. In the absence of NATs or other
connectivity issues, this is the optimal routing technique.</t>
<t>The challenge of implementing direct response is the presence of
NATs. There are a number of complexities that must be addressed. In
this discussion, we will continue our assumption that A issued the
request and Z is generating the response.</t>
<t><list style="symbols">
<t>The IP address listed by A may be unreachable, either due to
NAT or firewall rules. Therefore, a direct response technique must
fallback to symmetric response <xref
target="non-transitive-dhts-worlds05"></xref>. The hop-by-hop ACKs
used by RELOAD allow Z to determine when A has received the
message (and the TLS negotiation will provide earlier confirmation
that A is reachable), but this fallback requires a timeout that
will increase the response latency whenever A is not reachable
from Z.</t>
<t>Whenever A is behind a NAT it will have multiple candidate IP
addresses, each of which must be advertised to ensure
connectivity, therefore Z will need to attempt multiple
connections to deliver the response.</t>
<t>One (or all) of A's candidate addresses may route from Z to a
different device on the Internet. In the worst case these nodes
may actually be running RELOAD on the same port. Therefore,
establishing a secure connection to authenticate A before
delivering the response is absolutely necessary. This step
diminishes the efficiency of direct response because multiple
roundtrips are required before the message can be delivered.</t>
<t>If A is behind a NAT and does not have a connection already
established with Z, there are only two ways the direct response
will work. The first is that A and Z are both behind the same NAT,
in which case the NAT is not involved. In the more common case,
when Z is outside A's NAT, the response will only be received if
A's NAT implements endpoint-independent filtering. As the choice
of filtering mode conflates application transparency with security
<xref target="RFC4787"></xref>, and no clear recommendation is
available, the prevalence of this feature in future devices
remains unclear.</t>
</list></t>
<t>An extension to RELOAD that supports direct response routing but
relies on symmetric responses as a fallback would be possible, but due
to the complexities of determining when to use direct response and
when to fallback to symmetric, and the reduced performance for
responses to peers behind restrictive NATs, we have chosen not to
include it as an option at this point.</t>
</section>
<section title="Relay Peers">
<t><xref target="I-D.jiang-p2psip-sep">SEP</xref> has proposed
implementing a form of direct response by having A identify a peer, Q,
that will be directly reachable by any other peer. A uses Attach to
establish a connection with Q and advertises Q's IP address in the
request sent to Z. Z sends the response to Q, which relays it to A.
This then reduces the latency to two hops, plus Z negotiating a secure
connection to Q.</t>
<t>This technique relies on the relative population of nodes such as A
that require relay peers and peers such as Q that are capable of
serving as a relay peer. It also requires nodes to be able to identify
which category they are in. This identification problem has turned out
to be hard to solve and is still an open area of exploration.</t>
<t>An extension to RELOAD that supports relay peers is possible, but
due to the complexities of implementing such an alternative, we have
not added such a feature to RELOAD at this point.</t>
<t>A concept similar to relay peers, essentially choosing a relay peer
at random, has previously been suggested to solve problems of pairwise
non-transitivity <xref target="non-transitive-dhts-worlds05"></xref>,
but deterministic filtering provided by NATs make random relay peers
no more likely to work than the responding peer.</t>
</section>
<section title="Symmetric Route Stability">
<t>A common concern about symmetric recursive routing has been that
one or more peers along the request path may fail before the response
is received. The significance of this problem essentially depends on
the response latency of the overlay. An overlay that produces slow
responses will be vulnerable to churn, whereas responses that are
delivered very quickly are vulnerable only to failures that occur over
that small interval.</t>
<t>The other aspect of this issue is whether the request itself can be
successfully delivered. Assuming typical connection maintenance
intervals, the time period between the last maintenance and the
request being sent will be orders of magnitude greater than the delay
between the request being forwarded and the response being received.
Therefore, if the path was stable enough to be available to route the
request, it is almost certainly going to remain available to route the
response.</t>
<t>An overlay that is unstable enough to suffer this type of failure
frequently is unlikely to be able to support reliable functionality
regardless of the routing mechanism. However, regardless of the
stability of the return path, studies show that in the event of high
churn, iterative routing is a better solution to ensure request
completion <xref target="lookups-churn-p2p06"></xref> <xref
target="non-transitive-dhts-worlds05"></xref></t>
<t>Finally, because RELOAD retries the end-to-end request, that retry
will address the issues of churn that remain.</t>
</section>
</section>
<section anchor="sec-why-clients" title="Why Clients?">
<t>There are a wide variety of reasons a node may act as a client rather
than as a peer <xref target="I-D.pascual-p2psip-clients"></xref>. This
section outlines some of those scenarios and how the client's behavior
changes based on its capabilities.</t>
<section title="Why Not Only Peers?">
<t>For a number of reasons, a particular node may be forced to act as
a client even though it is willing to act as a peer. These
include:</t>
<t><list style="symbols">
<t>The node does not have appropriate network connectivity,
typically because it has a low-bandwidth network connection.</t>
<t>The node may not have sufficient resources, such as computing
power, storage space, or battery power.</t>
<t>The overlay algorithm may dictate specific requirements for
peer selection. These may include participation in the overlay to
determine trustworthiness, control the number of peers in the
overlay to reduce overly-long routing paths, or ensure minimum
application uptime before a node can join as a peer.</t>
</list></t>
<t>The ultimate criteria for a node to become a peer are determined by
the overlay algorithm and specific deployment. A node acting as a
client that has a full implementation of RELOAD and the appropriate
overlay algorithm is capable of locating its responsible peer in the
overlay and using CONNECT to establish a direct connection to that
peer. In that way, it may elect to be reachable under either of the
routing approaches listed above. Particularly for overlay algorithms
that elect nodes to serve as peers based on trustworthiness or
population, the overlay algorithm may require such a client to locate
itself at a particular place in the overlay.</t>
</section>
<section title="Clients as Application-Level Agents">
<t>SIP defines an extensive protocol for registration and security
between a client and its registrar/proxy server(s). Any SIP device can
act as a client of a RELOAD-based P2PSIP overlay if it contacts a peer
that implements the server-side functionality required by the SIP
protocol. In this case, the peer would be acting as if it were the
user's peer, and would need the appropriate credentials for that
user.</t>
<t>Application-level support for clients is defined by a usage. A
usage offering support for application-level clients should specify
how the security of the system is maintained when the data is moved
between the application and RELOAD layers.</t>
</section>
</section>
<!--
<t>The forwarding layer is responsible for looking at message and
doing one of three things:</t>
<t><list style="symbols">
<t>Deciding the message was destined for this peer and passing the
message up to the layer above this.</t>
<t>Looking at the Node-ID that represents the next peer to send
the message too and if there is an existing connection, sending
the message over the connection.</t>
<t>Requesting the overlay Routing logic to tell the forwarding layer
which peer the message needs to be forwarded to (based on the
target Node-ID or Resource-ID), and then sending the message.</t>
</list></t>
-->
<!--
<section anchor="sec-via-list" title="Via Lists">
<t>
[TODO: BBL; I would merge this into the symmetric section
and try to trim a bit.]
</t>
<t>In a general messaging system, messages need a source and a
destination and peers need to be able to send a response to the peer
that sent the request. This can be particularly tricky in overlay
networks when a new peer is joining, or the overlay network is
stabilizing and different peers have different ideas on what the
overlay topology is. A simple and reliable way to make sure that a
response can reach the node that sent the request in these
situations is to have the response traverse the reverse path of the
request.</t>
<t>The approach used here is to have each node the request traverses
add its Node-ID to the "via list" in the request. Then the response
is routed by looking at the list and using it as list of peers that
the response will be routed thorough. To support this, each message
has a destination list of nodes it needs to be routed through as
well as a via list of what nodes it has traversed.</t>
<t>When a peer receives a message from the Transport Layer, it adds
the Node-ID of the node it received the message from to the end of
the via list. When a peer goes to transmit a message to the
Transport Layer, it looks at the first entry on the destination
list. If the entry is this peer, it removes this entry from the list
and looks at the next entry and if the entry is not this peer, it
sends the message to the first peer on the destination list.</t>
<t>When a peer goes to send a response to a request, it can simply
copy the via list in reverse to form the destination list for the
response if it wishes to route the response along the reverse path
as the request.</t>
<t>Peers that are willing to maintain state may do list compression
for privacy reason and to reduce the message size. They do this by
taking some number of entries off the via list and replacing them
with a unique entry that this peer can later identify. Later, if the
peer sees the unique entry in a destination list, it removes the
unique entry and replaces it with the all the entries removed from
the original via list (and reverses the order of these entries).
Note that this technique will generally require storing some
per-message state on the intermediate peer, so this is a
bandwidth/per-peer state tradeoff. The exception is if the list is
not compressed but rather the Node-IDs are simply encrypted.</t>
<t>The via list approach provides several features. First it allows
a response to follow the same path as the request. This is
particularly important for peers that are sending requests while
they are joining and before other peers can route to them as well as
situations where message are being exchanged to stabilize the
overlay network. It also makes it easier to diagnose and manage the
system when all peers see the response to any request they
forward.</t>
</section>
-->
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 15:31:28 |