REsource LOcation And Discovery (RELOAD)
draft-ietf-p2psip-reload-00

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Abstract

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.

Table of Contents

1.  Introduction
    1.1.  Basic Setting
    1.2.  Architecture
        1.2.1.  Usage Layer
        1.2.2.  Routing Layer
        1.2.3.  Storage
        1.2.4.  Topology Plugin
        1.2.5.  Forwarding Layer
    1.3.  SIP Usage
    1.4.  Security
    1.5.  Structure of This Document
2.  Terminology
3.  Overlay Management Overview
    3.1.  Security and Identification
        3.1.1.  Shared-Key Security
    3.2.  Clients
        3.2.1.  Client Routing
        3.2.2.  Client Behavior
            3.2.2.1.  Why Not Only Peers?
            3.2.2.2.  Minimum Functionality Requirements for Clients
            3.2.2.3.  Clients as Application-Level Agents
    3.3.  Routing
        3.3.1.  Routing Alternatives
            3.3.1.1.  Iterative vs Recursive
            3.3.1.2.  Symmetric vs Forward response
            3.3.1.3.  Direct Response
            3.3.1.4.  Relay Peers
            3.3.1.5.  Symmetric Route Stability
    3.4.  Connectivity Management
    3.5.  Overlay Algorithm Support
        3.5.1.  Support for Pluggable Overlay Algorithms
        3.5.2.  Joining, Leaving, and Maintenance Overview
    3.6.  First-Time Setup
        3.6.1.  Initial Configuration
        3.6.2.  Enrollment
4.  Application Support Overview
    4.1.  Data Storage
        4.1.1.  Storage Permissions
        4.1.2.  Usages
        4.1.3.  Replication
    4.2.  Service Discovery
    4.3.  Application Connectivity
5.  P2PSIP Integration Overview
6.  Overlay Management Protocol
    6.1.  Message Routing
        6.1.1.  Request Origination
        6.1.2.  Message Receipt and Forwarding
            6.1.2.1.  Responsible ID
            6.1.2.2.  Other ID
            6.1.2.3.  Private ID
        6.1.3.  Response Origination
    6.2.  Message Structure
        6.2.1.  Presentation Language
            6.2.1.1.  Common Definitions
        6.2.2.  Forwarding Header
            6.2.2.1.  Destination and Via Lists
            6.2.2.2.  Route Logging
            6.2.2.3.  Forwarding Options
        6.2.3.  Message Contents Format
            6.2.3.1.  Response Codes and Response Errors
        6.2.4.  Signature
    6.3.  Overlay Topology
        6.3.1.  Topology Plugin Requirements
        6.3.2.  Methods and types for use by topology plugins
            6.3.2.1.  Join
            6.3.2.2.  Leave
            6.3.2.3.  Update
            6.3.2.4.  Route_Query
    6.4.  Forwarding Layer
        6.4.1.  Transports
            6.4.1.1.  Future Support for HIP
            6.4.1.2.  Reliability for Unreliable Transports
            6.4.1.3.  Fragmentation and Reassembly
        6.4.2.  Connection Management Methods
            6.4.2.1.  Attach
            6.4.2.2.  Ping
            6.4.2.3.  Tunnel
7.  Data Storage Protocol
    7.1.  Data Signature Computation
    7.2.  Data Models
        7.2.1.  Single Value
        7.2.2.  Array
        7.2.3.  Dictionary
    7.3.  Data Storage Methods
        7.3.1.  Store
            7.3.1.1.  Request Definition
            7.3.1.2.  Response Definition
        7.3.2.  Fetch
            7.3.2.1.  Request Definition
            7.3.2.2.  Response Definition
        7.3.3.  Remove
            7.3.3.1.  Single Value
            7.3.3.2.  Array
            7.3.3.3.  Dictionary
            7.3.3.4.  Response Definition
        7.3.4.  Find
            7.3.4.1.  Request Definition
            7.3.4.2.  Response Definition
            7.3.4.3.  Defining New Kinds
8.  Certificate Store Usage
9.  TURN Server Usage
10.  SIP Usage
    10.1.  Registering AORs
    10.2.  Looking up an AOR
    10.3.  Forming a Direct Connection
    10.4.  GRUUs
    10.5.  SIP-REGISTRATION Kind Definition
11.  Diagnostic Usage
    11.1.  Diagnostic Metrics for a P2PSIP Deployment
12.  Chord Algorithm
    12.1.  Overview
    12.2.  Routing
    12.3.  Redundancy
    12.4.  Joining
    12.5.  Routing Attaches
    12.6.  Updates
        12.6.1.  Sending Updates
        12.6.2.  Receiving Updates
        12.6.3.  Stabilization
    12.7.  Route Query
    12.8.  Leaving
13.  Enrollment and Bootstrap
    13.1.  Discovery
    13.2.  Overlay Configuration
    13.3.  Credentials
        13.3.1.  Self-Generated Credentials
    13.4.  Joining the Overlay Peer
14.  Message Flow Example
15.  Security Considerations
    15.1.  Overview
    15.2.  Attacks on P2P Overlays
    15.3.  Certificate-based Security
    15.4.  Shared-Secret Security
    15.5.  Storage Security
        15.5.1.  Authorization
        15.5.2.  Distributed Quota
        15.5.3.  Correctness
        15.5.4.  Residual Attacks
    15.6.  Routing Security
        15.6.1.  Background
        15.6.2.  Admissions Control
        15.6.3.  Peer Identification and Authentication
        15.6.4.  Protecting the Signaling
        15.6.5.  Residual Attacks
    15.7.  SIP-Specific Issues
        15.7.1.  Fork Explosion
        15.7.2.  Malicious Retargeting
        15.7.3.  Privacy Issues
16.  IANA Considerations
    16.1.  Overlay Algorithm Types
    16.2.  Data Kind-Id
    16.3.  Data Model
    16.4.  Message Codes
    16.5.  Error Codes
    16.6.  Route Log Extension Types
    16.7.  Transport Types
    16.8.  Forwarding Options
    16.9.  Ping Information Types
    16.10.  reload: URI Scheme
        16.10.1.  URI Registration
17.  Acknowledgments
18.  References
    18.1.  Normative References
    18.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

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:

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.

Usage Model:

RELOAD is designed to support a variety of applications, including P2P multimedia communications with the Session Initiation Protocol [I‑D.ietf‑p2psip‑concepts] (Bryan, D., “Concepts and Terminology for Peer to Peer SIP,” July 2007.). 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.

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.

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.

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.

These properties were designed specifically to meet the requirements for a P2P protocol to support SIP, and this document defines a SIP Usage of RELOAD. However, 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 [I‑D.ietf‑p2psip‑concepts] (Bryan, D., “Concepts and Terminology for Peer to Peer SIP,” July 2007.).

1.1.  Basic Setting

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.

          +--------+              +--------+              +--------+
          | Node 10|--------------| Node 20|--------------| Node 30|
          +--------+              +--------+              +--------+
              |                       |                       |
              |                       |                       |
          +--------+              +--------+              +--------+
          | Node 40|--------------| Node 50|--------------| Node 60|
          +--------+              +--------+              +--------+
              |                       |                       |
              |                       |                       |
          +--------+              +--------+              +--------+
          | Node 70|--------------| Node 80|--------------| Node 90|
          +--------+              +--------+              +--------+
                                      |
                                      |
                                  +--------+
                                  | Node 85|
                                  |(Client)|
                                  +--------+

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.

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.

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.

Other applications (for instance, SIP) can be defined on top of RELOAD and use these two basic RELOAD services to provide their own services.

1.2.  Architecture

Architecturally RELOAD is divided into several layers, as shown in the following figure:

                 Application

            +-------+  +-------+
            | SIP   |  | XMPP  |  ...
            | Usage |  | Usage |
            +-------+  +-------+
          -------------------------------------- Message Routing API
            +------------------+   +---------+
            |                  |<->| Storage |
            |                  |   +---------+
            |      Routing     |        ^
            |       Layer      |        v
            |                  |   +---------+
            |                  |<->|Topology |
            |                  |   | Plugin  |
            +------------------+   +---------+
                      ^                 ^
                      v                 |
            +------------------+ <------+
            |    Forwarding    |
            |       Layer      |
            +------------------+
          -------------------------------------- Transport API
             +-------+  +------+
             |TLS    |  |DTLS  |  ...
             +-------+  +------+

The major components of RELOAD are:

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.

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.

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.

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.

Forwarding Layer:

The Forwarding Layer provides packet forwarding services between nodes. It also handles setting up connections across NATs using ICE.

1.2.1.  Usage Layer

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.

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.

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.

1.2.2.  Routing Layer

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:

• Send a message to a given peer, specified by Node-Id or Resource-Id.
• Receive messages that other peers sent to a Node-Id or Resource-Id for which this peer is responsible.

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.

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.

1.2.3.  Storage

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 store 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.

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.

1.2.4.  Topology Plugin

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.

The Topology Plugin is responsible for maintaining the overlay algorithm Routing Table, which is consulted by the Routing 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.

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.

1.2.5.  Forwarding Layer

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.

The Forwarding Layer sits on top of transport layer protocols which carry the actual traffic. This specification defines how to use DTLS and TLS to carry RELOAD messages.

1.3.  SIP Usage

The SIP Usage of RELOAD allows SIP user agents to provide a peer-to-peer telephony service without the requirement for permanent proxy or registration servers. In such a network, the RELOAD overlay itself performs the registration and rendezvous functions ordinarily associated with such servers.

The SIP Usage involves two basic functions:

Registration:

SIP UAs can use the RELOAD data storage functionality to store a mapping from their AOR to their Node-Id in the overlay, and to retrieve the Node-Id of other UAs.

Rendezvous:

Once a SIP UA has identified the Node-Id for an AOR it wishes to call, it can use the RELOAD message routing system to set up a direct connection which can be used to exchange SIP messages.

For instance, Bob could register his Node-Id, "1234", under his AOR, "sip:bob@dht.example.com". When Alice wants to call Bob, she queries the overlay for "sip:bob@dht.example.com" and gets back Node-Id 1234. She then uses the overlay to establish a direct connection with Bob and can use that direct connection to perform a standard SIP INVITE.

1.4.  Security

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:

Connection Level:

Connections between peers are secured with TLS or DTLS.

Message Level:

Each RELOAD message must be signed.

Object Level:

Stored objects must be signed by the storing peer.

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.

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.

1.5.  Structure of This Document

The remainder of this document is structured as follows.

• Section 2 provides definitions of terms used in this document.
• Section 3 provides an overview of the mechanisms used to establish and maintain the overlay.
• Section 4 provides an overview of the mechanism RELOAD provides to support other applications.
• Section 5 provides an overview of the SIP usage for RELOAD.
• Section 6 defines the protocol messages that RELOAD uses to establish and maintain the overlay.
• Section 7 defines the protocol messages that are used to store and retrieve data using RELOAD.
• Sections 8-10 define three Usages of RELOAD that provide certificate storage, SIP, and Diagnostics.
• Section 11 defines a specific Topology Plugin using Chord.
• Section 12 defines the mechanisms that new RELOAD nodes use to join the overlay for the first time.
• Section 13 provides an extended example.
• Sections 14 and 15 provide Security and IANA considerations.

2.  Terminology

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 RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

We use the terminology and definitions from the Concepts and Terminology for Peer to Peer SIP (Bryan, D., “Concepts and Terminology for Peer to Peer SIP,” July 2007.) [I‑D.ietf‑p2psip‑concepts] 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 (*).

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.

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.

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.

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.

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.

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.

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. (*)

Resource:

An object or group of objects associated with a string identifier see "Resource Name" below.

Resource Name:

The (potentially) human readable name by which a resource is identified. In unstructured P2P networks, the resource name is used directly as a Resource-Id. In structured P2P networks the resource name can be 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 (see Resource Name below).(*)

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 identifier. In unstructured networks, resource identifiers may be used directly as resource-IDs and may have variable length.

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.

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. (*)

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. (*)

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. (*)

3.  Overlay Management Overview

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.

3.1.  Security and Identification

Every node in the RELOAD overlay is identified by a Node-ID. The Node-ID is used for three major purposes:

• To address the node itself.
• To determine its position in the overlay topology when the overlay is structured.
• To determine the set of resources for which the node is responsible.

Each node has a certificate [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.) containing a Node-ID, which is globally unique.

The certificate serves multiple purposes:

• 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.
• 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.
• It entitles the user to use the user name found in the certificate.

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.

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.

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 [Sybil] (Douceur, J., “The Sybil Attack,” March 2002.)) and can only be safely used in closed networks where users are mutually trusting.

3.1.1.  Shared-Key Security

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.

3.2.  Clients

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.

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."

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.

The remainder of this section discusses how RELOAD supports clients in terms of routing issues specific to clients, minimum functionality requirements for clients, and alternatives for devices not capable of meeting those requirements.

3.2.1.  Client Routing

There are two routing options by which a client may be located in an overlay.

• 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.
• 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.

3.2.2.  Client Behavior

There are a wide variety of reasons a node may act as a client rather than as a peer [I‑D.pascual‑p2psip‑clients] (Pascual, V., Matuszewski, M., Shim, E., Zhang, H., and S. Yongchao, “P2PSIP Clients,” February 2008.). This section outlines some of those scenarios and how the client's behavior changes based on its capabilities.

3.2.2.1.  Why Not Only Peers?

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:

• The node does not have appropriate network connectivity---typically because it is behind an overly restrictive NAT, or it has a low-bandwidth network connection.
• The node may not have sufficient resources, such as computing power, storage space, or battery power.
• 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.

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.

3.2.2.2.  Minimum Functionality Requirements for Clients

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:

• Implement RELOAD's connection-management connections that are used to establish the connection with the peer.
• Implement RELOAD's data storage and retrieval methods (with client functionality).
• Be able to calculate Resource-IDs used by the overlay.
• Possess security credentials required by the overlay it is implementing.

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.

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 P2PSIP and many other applications, a native application-level protocol already exists that is sufficient for such a client, as described in the next section.

3.2.2.3.  Clients as Application-Level Agents

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.

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.

3.3.  Routing

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. The section will conclude by discussing some alternative designs and the tradeoffs that would be necessary to support them.

RELOAD's routing capabilities must meet the following requirements:

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.

Clients:

RELOAD must support requests from and to clients that do not participate in overlay routing.

Client promotion:

RELOAD must support clients that become peers at a later point as determined by the overlay algorithm and deployment.

Low state:

RELOAD's routing algorithms must not require significant state to be stored on intermediate peers.

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.

To meet these requirements, RELOAD's routing relies on two basic mechanisms:

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).

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.

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.

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:

A         B         X         Z
-------------------------------

---------->
Dest=Z
          ---------->
          Via=A
          Dest=Z
                    ---------->
                    Via=A, B
                    Dest=Z


                    <----------
                   Dest=X, B, A
          <----------
            Dest=B, A
<----------
     Dest=A

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.

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 enroute. 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.

A         B         X         Z
-------------------------------

---------->
Dest=Z
          ---------->
          Via=A
          Dest=Z
                    ---------->
                    Dest=Z

                    <----------
                         Dest=X
            <----------
            Dest=B, A
<----------
     Dest=A

For debugging purposes, a Route Log attribute is available that stores information about each peer as the message is forwarded.

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.

3.3.1.  Routing Alternatives

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.

3.3.1.1.  Iterative vs Recursive

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 [stoica‑non‑transitive‑worlds05] (Freedman, M., Lakshminarayanan, K., Rhea, S., and I. Stoica, “Non-Transitive Connectivity and DHTs,” .). However, in the presence of NATs iterative routing is intolerably expensive because a new connection must be established for each hop (using ICE) [bryan‑design‑hotp2p08] (Bryan, D., Lowekamp, B., and M. Zangrilli, “The Design of a Versatile, Secure P2PSIP Communications Architecture for the Public Internet,” .).

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.

3.3.1.2.  Symmetric vs Forward response

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.

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).

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) [stoica‑non‑transitive‑worlds05] (Freedman, M., Lakshminarayanan, K., Rhea, S., and I. Stoica, “Non-Transitive Connectivity and DHTs,” .).

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.

3.3.1.3.  Direct Response

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.

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.

• 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 [stoica‑non‑transitive‑worlds05] (Freedman, M., Lakshminarayanan, K., Rhea, S., and I. Stoica, “Non-Transitive Connectivity and DHTs,” .). 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.
• 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.
• 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.
• 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 [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.), and no clear recommendation is available, the prevalence of this feature in future devices remains unclear.

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.

3.3.1.4.  Relay Peers

SEP (Jiang, X. and H. Zhang, “Service Extensible P2P Peer Protocol,” February 2008.) [I‑D.jiang‑p2psip‑sep] 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.

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.

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.

A concept similar to relay peers, essentially choosing a relay peer at random, has previously been suggested to solve problems of pairwise non-transitivity [stoica‑non‑transitive‑worlds05] (Freedman, M., Lakshminarayanan, K., Rhea, S., and I. Stoica, “Non-Transitive Connectivity and DHTs,” .), but deterministic filtering provided by NATs make random relay peers no more likely to work than the responding peer.

3.3.1.5.  Symmetric Route Stability

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.

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.

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 [ng‑analytical‑churn‑ieeep2p06] (Wu, D., Tian, Y., and K. Ng, “Analytical Study on Improving DHT Lookup Performance under Churn,” .) [stoica‑non‑transitive‑worlds05] (Freedman, M., Lakshminarayanan, K., Rhea, S., and I. Stoica, “Non-Transitive Connectivity and DHTs,” .)

Finally, because RELOAD retries the end-to-end request, that retry will address the issues of churn that remain.

3.4.  Connectivity Management

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 [I‑D.ietf‑mmusic‑ice‑tcp] (Rosenberg, J., “TCP Candidates with Interactive Connectivity Establishment (ICE,” March 2007.) to establish the connection. It is assumed that the reader is familiar with ICE.

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.

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. Section 13 (Enrollment and Bootstrap) contains more detail on this.

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.

3.5.  Overlay Algorithm Support

The Topology Plugin allows RELOAD to support a variety of overlay algorithms. This draft defines a DHT based on Chord [Chord] (Stoica, I., Morris, R., Liben-Nowell, D., Karger, D., Kaashoek, M., Dabek, F., and H. Balakrishnan, “Chord: A Scalable Peer-to-peer Lookup Service for Internet Applications,” .), which is mandatory to implement, but the base RELOAD protocol is designed to support a variety of overlay algorithms.

3.5.1.  Support for Pluggable Overlay Algorithms

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.

3.5.2.  Joining, Leaving, and Maintenance Overview

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:

• Forming connections to some other peers.
• Acquiring the data values this peer is responsible for storing.
• Informing the other peers which were previously responsible for that data that this peer has taken over responsibility.

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.

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:

• Forming connections to other peers in the overlay to populate its Routing Table.
• Getting a copy of the data it is now responsible for storing and assuming responsibility for that data.

The second operation is performed by contacting the Admitting Peer (AP), the node which is currently responsible for that section of the overlay.

The details of this operation depend mostly on the overlay algorithm involved, but a typical case would be:

1. JP (Joining Peer) sends a Join request to AP (Admitting Peer) announcing its intention to join.
2. AP sends a Join response.
3. AP does a sequence of Stores to JP to give it the data it will need.
4. 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.
5. JP makes its own connections to the appropriate peers in the Overlay Instance.

After this process is completed, JP is a full member of the Overlay Instance and can process Store/Fetch requests.

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.

3.6.  First-Time Setup

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.

3.6.1.  Initial Configuration

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.

3.6.2.  Enrollment

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.

4.  Application Support Overview

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:

• To store data in the overlay and retrieve data stored by other nodes.
• As a discovery mechanism for services such as TURN.
• To form direct connections which can be used to transmit application-level messages.

This section provides an overview of these services.

4.1.  Data Storage

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:

                    +--------------------------------+
                    |            Resource-ID         |
                    |                                |
                    | +------------+  +------------+ |
                    | |   Kind 1   |  |   Kind 2   | |
                    | |            |  |            | |
                    | | +--------+ |  | +--------+ | |
                    | | | Value  | |  | | Value  | | |
                    | | +--------+ |  | +--------+ | |
                    | |            |  |            | |
                    | | +--------+ |  | +--------+ | |
                    | | | Value  | |  | | Value  | | |
                    | | +--------+ |  | +--------+ | |
                    | |            |  +------------+ |
                    | | +--------+ |                 |
                    | | | Value  | |                 |
                    | | +--------+ |                 |
                    | +------------+                 |
                    +--------------------------------+

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:

single value:

There can be at most one item in the set and any value overwrites the previous item.

array:

Many values can be stored and addressed by a numeric index.

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.

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.

4.1.1.  Storage Permissions

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.

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.

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.

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.

4.1.2.  Usages

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:

• Registers kind-id code points for any kinds that the Usage defines.
• Defines the data structure for each of the kinds.
• Defines access control rules for each kinds.
• Defines how the Resource Name is formed that is hashed to form the Resource-ID where each kind is stored.
• 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.

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.

4.1.3.  Replication

Replication in P2P overlays can be used to provide:

persistence:

if the responsible peer crashes and/or if the storing peer leaves the overlay

security:

to guard against DoS attacks by the responsible peer or routing attacks to that responsible peer

load balancing:

to balance the load of queries for popular resources.

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.

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 [Chord] (Stoica, I., Morris, R., Liben-Nowell, D., Karger, D., Kaashoek, M., Dabek, F., and H. Balakrishnan, “Chord: A Scalable Peer-to-peer Lookup Service for Internet Applications,” .).

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.

4.2.  Service Discovery

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 [opendht‑sigcomm05] (Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J., Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu, “OpenDHT: A Public DHT and its Uses,” .).

4.3.  Application Connectivity

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.

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.

5.  P2PSIP Integration Overview

The SIP Usage of RELOAD allows SIP user agents to provide a peer-to-peer telephony service without the requirement for permanent proxy or registration servers. In such a network, the RELOAD overlay itself performs the registration and rendezvous functions ordinarily associated with such servers.

The basic function of the SIP usage is to allow Alice to start with a SIP URI (e.g., "bob@dht.example.com") and end up with a connection which Alice's SIP UA can use to pass SIP messages back and forth to Bob's SIP UA. The way this works is as follows:

1. Bob, operating Node-ID 1234, stores a mapping from his URI to his Node-ID in the overlay. I.e., "sip:bob@dht.example.com -> 1234".
2. Alice, operating Node-ID 5678, decides to call Bob. She looks up "sip:bob@dht.example.com" in the overlay and retrieves "1234".
3. Alice uses the overlay to route an Attach message to Bob's peer. Bob responds with his own Attach and they set up a direct connection, as shown below.

Alice       Peer1      Overlay     PeerN      Bob
(5678)                                     (1234)
-------------------------------------------------
Attach ->
          Attach ->
                      Attach ->
                                   Attach ->
                                       <- Attach
                                <- Attach
                   <- Attach
         <- Attach

<------------------ ICE Checks ----------------->
INVITE ----------------------------------------->
<--------------------------------------------- OK
ACK -------------------------------------------->
<------------ ICE Checks for media ------------->
<-------------------- RTP ---------------------->

It is important to note that RELOAD's only role here is to set up the direct connection between Alice and Bob. As soon as the ICE checks complete and the connection is established, then ordinary SIP is used. In particular, the establishment of the media channel for the phone call happens via the usual SIP mechanisms, and RELOAD is not involved. Media never goes over the overlay. After the successful exchange of SIP messages, call peers run ICE connectivity checks for media.

As well as allowing mappings from AORs to Node-IDs, the SIP Usage also allows mappings from AORs to other AORs. For instance, if Bob wanted his phone calls temporarily forwarded to Charlie, he could store the mapping "sip:bob@dht.example.com -> sip:charlie@dht.example.com". When Alice wants to call Bob, she retrieves this mapping and can then fetch Charlie's AOR to retrieve his Node-ID.

6.  Overlay Management Protocol

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.

6.1.  Message Routing

This section describes procedures used by nodes to route messages through the overlay.

6.1.1.  Request Origination

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.

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.

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 [vulnerabilities‑acsac04] (Srivatsa, M. and L. Liu, “Vulnerabilities and Security Threats in Structured Peer-to-Peer Systems: A Quantitative Analysis,” .). Such parallel searches MAY BE specified by the topology plugin.

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 Section 6.4.1.2 (Reliability for Unreliable Transports).

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.

6.1.2.  Message Receipt and Forwarding

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.

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:

• The first entry on the destination list is an id for which the peer is responsible.
• The first entry on the destination list is a an id for which another peer is responsible.
• The first entry on the destination list is a private id which is being used for destination list compression.

These cases are handled as discussed below.

6.1.2.1.  Responsible ID

If the first entry on the destination list is a ID for which the node is responsible, there are several sub-cases.

• 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.
• 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.
• 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.

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.

6.1.2.2.  Other ID

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.

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:

• The peer MAY add an entry to the via list in the forwarding header that will enable it to determine the correct node.
• The peer MAY keep per-transaction state which will allow it to determine the correct node.

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).

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.

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.

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.

6.1.2.3.  Private ID

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.

6.1.3.  Response Origination

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.

6.2.  Message Structure

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.

Each message has three parts, concatenated as shown below:

   +-------------------------+
   |    Forwarding Header    |
   +-------------------------+
   |    Message Contents     |
   +-------------------------+
   |       Signature         |
   +-------------------------+

The contents of these parts are as follows:

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.

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.

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.

The following sections describe the format of each part of the message.

6.2.1.  Presentation Language

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:

• It is easy to write and familiar enough looking that most readers can grasp it quickly.
• The ability to define nested structures allows a separation between high-level and low level message structures.
• It has a straightforward wire encoding that allows quick implementation, but the structures can be comprehended without knowing the encoding.
• The ability to mechanically (compile) encoders and decoders.

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.

Several idiosyncrasies of this language are worth noting.

• All lengths are denoted in bytes, not objects.
• Variable length values are denoted like arrays with angle brackets.
• "select" is used to indicate variant structures.

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..

6.2.1.1.  Common Definitions

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.

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:

       enum { false (0), true(1), (255)} Boolean;

A boolean value is either a 1 or a 0 and is represented as a single byte on the wire.

The NodeId, shown below, represents a single Node-ID.

           typedef opaque       NodeId[16];

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.

A ResourceId, shown below, represents a single resource-id.

           typedef opaque       ResourceId<0..2^8-1>;

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.

A more complicated example is IpAddressPort, which represents a network address and can be used to carry either an IPv6 or IPv4 address:

      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;

The first two fields in the structure are the same no matter what kind of address is being represented:

type

the type of address (v4 or v6).

length

the length of the rest of the structure.

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.

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".

           01           ; type    = IPv4
           06           ; length  = 6
           c0 00 02 01  ; address = 192.0.2.1
           17 d4        ; port    = 6100

6.2.2.  Forwarding Header

The forwarding header is defined as a ForwardingHeader structure, as shown below.

      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;

The contents of the structure are:

relo_token

The first four bytes identify this message as a RELOAD message. The message is easy to demultiplex from STUN messages by looking at the first bit. This field MUST contain the value 0xc2454c4f (the string 'RELO' with the high bit of the first byte set.).

overlay

The 32 bit checksum/hash of the overlay being used. The variable length string representing the overlay name is hashed with SHA-1 and the low order 32 bits are used. The purpose of this field is to allow nodes to participate in multiple overlays and to detect accidental misconfiguration. This is not a security critical function.

ttl

An 8 bit field indicating the number of iterations, or hops, a message can experience before it is discarded. The TTL value MUST be decremented by one at every hop along the route the message traverses. If the TTL is 0, the message MUST NOT be propagated further and MUST be discarded. The initial value of the TTL should be TBD.

fragment

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.

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.]]

version

The version of the RELOAD protocol being used. This document describes version 0.1, with a value of 0x01.

length

The count in bytes of the size of the message, including the header.

transaction_id

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.

flags

The flags word contains control flags. Which are ORed together. There is two currently defined flags: ROUTE-LOG (0x1) and RESPONSE-ROUTE-LOG (0x2). These flags indicate that the route log should be included (see Section 6.2.2.2 (Route Logging).).

via_list_length

The length of the via list in bytes. Note that in this field and the following two length fields we depart from the usual variable-length convention of having the length immediately precede the value in order to make it easier for hardware decoding engines to quickly determine the length of the header.

destination_list_length

The length of the destination list in bytes.

route_log_length

The length of the route log in bytes.

options_length

The length of the header options in bytes.

via_list

The via_list contains the sequence of destinations through which the message has passed. The via_list starts out empty and grows as the message traverses each peer.

destination_list

The destination_list contains a sequence of destinations which the message should pass through. The destination list is constructed by the message originator. The first element in the destination list is where the message goes next. The list shrinks as the message traverses each listed peer.

route_log

Contains a series of route log entries. See Section 6.2.2.2 (Route Logging).

options

Contains a series of ForwardingOptions entries. See Section 6.2.2.3 (Forwarding Options).

6.2.2.1.  Destination and Via Lists

The destination list and via lists are sequences of Destination values:

      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;

This is a TLV structure with the following contents:

type

The type of the DestinationData PDU. This may be one of "peer", "resource", or "compressed".

length

The length of the destination_data.

destination_value

The destination value itself, which is an encoded DestinationData structure, depending on the value of "type".

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.

A DestinationData can be one of three types:

peer

A Node-ID.

compressed

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.

resource

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.

6.2.2.2.  Route Logging

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 SIP's (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., and J. Peterson, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] 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.

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.

The route log is defined as follows:

    enum { (255) } RouteLogExtensionType;

    struct {
      RouteLogExtensionType     type;
      uint16                    length;

      select (type){
        /* Extension values go here */
      } extension;
    } RouteLogExtension;

    enum { reserved(0), tcp_tls(1),  udp_dtls(2),  (255)}  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;

The route log consists of an arbitrary number of RouteLogEntry values, each representing one node through which the message has passed.

Each RouteLogEntry consists of the following values:

version

A textual representation of the software version

transport

The transport type, currently either "tcp_tls" or "udp_dtls".

id

The Node-ID of the peer.

uptime

The uptime of the peer in seconds.

address

The address and port of the peer.

certificate

The peer's certificate. Note that this may be omitted by setting the length to zero.

extensions

Extensions, if any.

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:

type

The type of the extension.

length

The length of the rest of the structure.

extension

The extension value.

6.2.2.3.  Forwarding Options

The Forwarding header can be extended with forwarding header options, which are a series of ForwardingOptions structures:

    enum { (255) } ForwardingOptionsType;

    struct {
      ForwardingOptionsType     type;
      uint8                     flags;
      uint16                    length;
      select (type) {
        /* Option values go here */
      } option;
    } ForwardingOption;

Each ForwardingOption consists of the following values:

type

The type of the option.

length

The length of the rest of the structure.

flags

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 resonse. 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 resonse. 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.

option

The option value.

6.2.3.  Message Contents Format

The second major part of a RELOAD message is the contents part, which is defined by MessageContents:

       struct {
         MessageCode            message_code;
         opaque                 payload<0..2^24-1>;
       } MessageContents;

The contents of this structure are as follows:

message_code

This indicates the message that is being sent. The code space is broken up as follows.

0

Reserved

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, "ping_request" (the Ping request) has value 1 and "ping_answer" (the Ping response) has value 2

0xffff

Error

message_body

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.

6.2.3.1.  Response Codes and Response Errors

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.

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.

When the message code is 0xffff, the payload MUST be an ErrorResponse.

       public struct {
         uint16             error_code;
         opaque             reason_phrase<0..2^8-1>;  /* String*/
         opaque             error_info<0..2^16-1>;
       } ErrorResponse;

The contents of this structure are as follows:

error_code

A numeric error code indicating the error that occurred.

reason_phrase

A free form text string indicating the reason for the response. The reason phrase SHOULD BE as indicated in the error code list below (e.g., "Moved Temporarily"). [[Open Issue: These reason phrases are pretty useless. Like the rest of this error system, They're a holdover from SIP. Should we remove?]]

error_info

Payload specific error information. This MUST be empty (zero length) except as specified below.

The following error code values are defined. [[TODO: These are currently semi-aligned with SIP codes. that's probably bad and we need to fix.]

302 (Moved Temporarily):

The requesting peer SHOULD retry the request at the new address specified in the 302 response message.

401 (Unauthorized):

The requesting peer needs to sign and provide a certificate. [[TODO: The semantics here don't seem quite right.]]

403 (Forbidden):

The requesting peer does not have permission to make this request.

404 (Not Found):

The resource or peer cannot be found or does not exist.

408 (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.

412 (Precondition Failed):

A request can't be completed because some precondition was incorrect. For instance, the wrong generation counter was provided

498 (Incompatible with Overlay)

A peer receiving the request is using a different overlay, overlay algorithm, or hash algorithm. [[Open Issue: What is the best error number and reason phrase to use?]]

757 (Unsupported Forwarding Option)

A peer receiving the request with a forwarnding options flaged as critical but the peer does not support this option. See section Section 6.2.2.3 (Forwarding Options). [[Open Issue: What is the best error number and reason phrase to use?]]

6.2.4.  Signature

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.

       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;

The signature construct contains the following values:

algorithm

The signature algorithm in use. The algorithm definitions are found in the IANA TLS SignatureAlgorithm Registry.

identity

The identity or certificate used to form the signature

signature_value

The value of the signature

A number of possible identity formats are permitted. The current possibilities are: a Node-ID, a user name, and a certificate.

For signatures over messages the input to the signature is computed over:

overlay + transaction_id + MessageContents + SignerIdentity

Where overlay and transaction_id come from the forwarding header and + indicates concatenation.

[[TODO: Check the inputs to this carefully.]]

The input to signatures over data values is different, and is described in Section 7.1 (Data Signature Computation).

6.3.  Overlay Topology

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.

6.3.1.  Topology Plugin Requirements

When specifying a new overlay algorithm, at least the following need to be described:

• Joining procedures, including the contents of the Join message.
• 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.
• Exit procedures, including the contents of the Leave message.
• The length of the Resource-IDs and Node-IDs. For DHTs, the hash algorithm to compute the hash of an identifier.
• The procedures that peers use to route messages.
• The replication strategy used to ensure data redundancy.

6.3.2.  Methods and types for use by topology plugins

This section describes the methods that topology plugins use to join, leave, and maintain the overlay.

6.3.2.1.  Join

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.

       struct {
          NodeId                joining_peer_id;
          opaque                overlay_specific_data<0..2^16-1>;
       } JoinReq;

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.

If the request succeeds, the responding peer responds with a JoinAns message, as defined below:

       struct {
          opaque                overlay_specific_data<0..2^16-1>;
       } JoinAns;

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.

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.

6.3.2.2.  Leave

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.

       public struct {
          NodeId                leaving_peer_id;
          opaque                overlay_specific_data<0..2^16-1>;
       } LeaveReq;

LeaveReq contains only the Node-ID of the leaving peer. Overlay algorithms MAY specify other data to appear in this request.

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.

6.3.2.3.  Update

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.

The contents of the UpdateReq message are completely overlay-specific. The UpdateAns response is expected to be either success or an error.

6.3.2.4.  Route_Query

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.

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.

6.3.2.4.1.  Request Definition

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.

       struct {
         Boolean                send_update;
         Destination            destination;
         opaque                 overlay_specific_data<0..2^16-1>;
       } RouteQueryReq;

The contents of the RouteQueryReq message are as follows:

send_update

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".

destination

The destination which the requester is interested in. This may be any valid destination object, including a Node-ID, compressed ids, or resource-id.

overlay_specific_data

Other data as appropriate for the overlay.

6.3.2.4.2.  Response Definition

A response to a successful RouteQueryReq request is a RouteQueryAns message. This is completely overlay specific.

6.4.  Forwarding Layer

Each node maintains connections to a set of other nodes defined by the topology plugin.

6.4.1.  Transports

RELOAD can use multiple transports to send its messages. Because ICE is used to establish connections (see Section 6.4.2.1.3 (Using ICE With RELOAD)), 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:

• TLS [REF] over TCP
• DTLS [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) over UDP

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.

6.4.1.1.  Future Support for HIP

The P2PSIP Working Group has expressed interest in supporting a HIP-based transport. Such support would require specifying such details as:

• 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.
• How to carry the HIP I1 and I2 messages. We anticipate that this would require defining a HIP Tunnel usage.
• How to carry RELOAD messages over HIP.

We leave this work as a topic for another draft.

6.4.1.2.  Reliability for Unreliable Transports

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.

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.

6.4.1.2.1.  Framed Message Format

The definition of FramedMessage is:

      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;

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.

If the message is of type "data", then the remainder of the PDU is as follows:

sequence

the sequence number

message

the original message that is being transmitted.

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.

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.

If the PDU is of type "ack", the contents are as follows:

ack_sequence

The sequence number of the message being acknowledged.

received

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.

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.

6.4.1.2.2.  Retransmission and Flow Control

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 [RFC4828] (Floyd, S. and E. Kohler, “TCP Friendly Rate Control (TFRC): The Small-Packet (SP) Variant,” April 2007.) 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.

6.4.1.3.  Fragmentation and Reassembly

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.

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]]

6.4.2.  Connection Management Methods

This section defines the methods RELOAD uses to form and maintain connections between nodes in the overlay. Three methods are defined:

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.

Ping:

is a simple request/response which is used to verify connectivity (analogous to the UNIX ping command) along a path and to gather a small amount of information about the resources held by the target peer

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.

6.4.2.1.  Attach

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 Section 6.4.2.1 (Attach).

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.

6.4.2.1.1.  Request Definition

An AttachReq message contains the requesting peer's ICE connection parameters formatted into a binary structure.

      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;

The values contained in AttachReq and AttachAns are:

ufrag

The username fragment (from ICE)

password

The ICE password.

application

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.

role

An active/passive/actpass attribute from RFC 4145 [RFC4145] (Yon, D. and G. Camarillo, “TCP-Based Media Transport in the Session Description Protocol (SDP),” September 2005.).

candidates

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.

These values should be generated using the procedures described in Section 6.4.2.1.3 (Using ICE With RELOAD).

6.4.2.1.2.  Response Definition

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.

6.4.2.1.3.  Using ICE With RELOAD

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.

In ICE as defined by [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” June 2007.), 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:

• Only a single media stream is supported.
• 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.
• 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.

An agent follows the ICE specification as described in [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” June 2007.) and [I‑D.ietf‑mmusic‑ice‑tcp] (Rosenberg, J., “TCP Candidates with Interactive Connectivity Establishment (ICE,” March 2007.) with the changes and additional procedures described in the subsections below.

6.4.2.1.4.  Collecting STUN Servers

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 [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., “Session Traversal Utilities for (NAT) (STUN),” March 2007.). Accordingly, any peer a node knows will be willing to be a STUN server -- though of course it may be behind a NAT.

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.

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.

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.

6.4.2.1.5.  Gathering Candidates

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 [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” June 2007.). RELOAD utilizes a single component, as does SIP. Consequently, gathering for these "streams" requires a single component.

An agent MUST implement ICE-tcp [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” June 2007.), and MUST gather at least one UDP and one TCP host candidate for RELOAD and for SIP.

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 Section 6.4.2.1.4 (Collecting STUN Servers) and Section 9 (TURN Server Usage).

The agent SHOULD prioritize its TCP-based candidates over its UDP-based candidates in the prioritization described in Section 4.1.2 of ICE [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” June 2007.).

The default candidate selection described in Section 4.1.3 of ICE is ignored; defaults are not signaled or utilized by RELOAD.

6.4.2.1.6.  Encoding the Attach Message

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. Section 6.4.2.1 (Attach) 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.

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.

6.4.2.1.7.  Verifying ICE Support

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.

6.4.2.1.8.  Role Determination

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.

6.4.2.1.9.  Connectivity Checks

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.

6.4.2.1.10.  Concluding ICE

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.

The procedures in Section 8 of ICE are followed to conclude ICE, with the following exceptions:

• The controlling agent MUST NOT attempt to send an updated offer once the state of its single media stream reaches Completed.
• 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.

6.4.2.1.11.  Subsequent Offers and Answers

An agent MUST NOT send a subsequent offer or answer. Thus, the procedures in Section 9 of ICE MUST be ignored.

6.4.2.1.12.  Media Keepalives

STUN MUST be utilized for the keepalives described in Section 10 of ICE.

6.4.2.1.13.  Sending Media

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.

The concept of a previous selected pair for a component does not apply to RELOAD, since ICE restarts are not possible with RELOAD.

6.4.2.1.14.  Receiving Media

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.

6.4.2.2.  Ping

Ping provides a number of primitive "exploration" services: (1) it is used to test connectivity along a path (2) it allows node to determine which resources another node is responsible for (3) it allows some discovery services in multicast settings. 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). In either case, the target Node-IDs respond with a simple response containing some status information.

6.4.2.2.1.  Request Definition

The PingReq message contains a list (potentially empty) of the pieces of status information that the requester would like the responder to provide.

      enum { responsible_set(1), num_resources(2), (255)}
           PingInformationType;

      struct {
        PingInformationType     requested_info<0..2^8-1>;
      } PingReq

The two currently defined values for PingInformation are:

responsible_set

indicates that the peer should Respond with the fraction of the overlay for which the responding peer is responsible.

num_resources

indicates that the peer should Respond with the number of resources currently being stored by the peer.

6.4.2.2.2.  Response Definition

A successful PingAns response contains the information elements requested by the peer.

       struct {
         PingInformationType    type;

         select (type) {
           case responsible_set:
             uint32             responsible_ppb;

           case num_resources:
             uint32             num_resources;

           /* This type may be extended */

         };
       } PingInformation;

       struct {
         uint64                 response_id;
         PingInformation        ping_info<0..2^16-1>;
       } PingAns;

A PingAns message contains the following elements:

response_id

A randomly generated 64-bit response ID. This is used to distinguish Ping responses in cases where the Ping request is multicast.

ping_info

A sequence of PingInformation structures, as shown below.

Each of the current possible Ping 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.

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.

6.4.2.3.  Tunnel

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.

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.

When a usage requires the Tunnel method, it must specify the specific application protocol(s) that will be Tunneled and for each protocol, specify:

• An application attribute that indicates the protocol being tunneled. This the IANA-registered port of the application protocol.
• The conditions under which the application will be Tunneled over the overlay rather than using a direct Attach.
• A mechanism for moving future application-level communication from Tunneling on the overlay to a direct Connection, or an explanation why this is unnecessary.
• A means of associating messages together as required for dialog-oriented or request/response-oriented protocols.
• 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.

6.4.2.3.1.  Request Definition

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.

       struct  {
         uint16                 application;
         opaque                 dialog_id<0..2^8-1>;
         opaque                 application_pdu<0..2^24-1>;
       } TunnelReq;

The values contained in the TunnelReq are:

application

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.

dialog_id

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.

application_pdu

An application PDU in the format specified by the application.

6.4.2.3.2.  Response Definition

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)

7.  Data Storage Protocol

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.

The basic unit of stored data is a single StoredData structure:

      struct {
        uint32                  length;
        uint64                  storage_time;
        uint32                  lifetime;
        StoredDataValue         value;
        Signature               signature;
      } StoredData;

The contents of this structure are as follows:

length

The length of the rest of the structure in octets.

storage_time

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.

lifetime

The validity period for the data, in seconds, starting from the time of store.

value

The data value itself, as described in Section 7.2 (Data Models).

signature

A signature over the data value. Section 7.1 (Data Signature Computation) describes the signature computation. The element is formatted as described in Section 6.2.4 (Signature)

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 Section 7.2 (Data Models) 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.

7.1.  Data Signature Computation

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:

resource_id + kind + StoredData

Where these values are:

resource

The resource ID where this data is stored.

kind

The kind-id for this data.

StoredData

The contents of the stored data value, as described in the previous sections.

[OPEN ISSUE: Should we include the identity in the string that forms the input to the signature algorithm?.]

Once the signature has been computed, the signature is represented using a signature element, as described in Section 6.2.4 (Signature).

7.2.  Data Models

The protocol currently defines the following data models:

• single value
• array
• dictionary

These are represented with the StoredDataValue structure:

      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;

We now discuss the properties of each data model in turn:

7.2.1.  Single Value

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.

A single value element is represented as a DataValue, which contains the following two elements:

exists

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.

value

The stored data.

7.2.2.  Array

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.

A array element is represented as an ArrayEntry:

       struct {
         uint32                  index;
         DataValue               value;
       } ArrayEntry;

The contents of this structure are:

index

The index of the data element in the array.

value

The stored data.

7.2.3.  Dictionary

A dictionary is a set of opaque values indexed by an opaque key with one value for each key. single dictionary entry is represented as follows

A dictionary element is represented as a DictionaryEntry:

       typedef opaque           DictionaryKey<0..2^16-1>;

       struct {
         DictionaryKey          key;
         DataValue              value;
       } DictionaryEntry;

The contents of this structure are:

key

The dictionary key for this value.

value

The stored data.

7.3.  Data Storage Methods

RELOAD provides several methods for storing and retrieving data:

• Store values in the overlay
• Fetch values from the overlay
• Remove values from the overlay
• Find the values stored at an individual peer

These methods are each described in the following sections.

7.3.1.  Store

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.

7.3.1.1.  Request Definition

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.

The store request is defined by the StoreReq structure:

     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;

A single Store request stores data of a number of kinds to a single resource location. The contents of the structure are:

resource

The resource to store at.

replica_number

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.

kind_data

A series of elements, one for each kind of data to be stored.

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.

Each StoreKindData element represents the data to be stored for a single kind-id. The contents of the element are:

kind

The kind-id. Implementations SHOULD reject requests corresponding to unknown kinds unless specifically configured otherwise.

data_model

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.

generation

The expected current state of the generation counter (approximately the number of times this object has been written, see below for details).

values

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.

The peer MUST perform the following checks:

• The kind_id is known and supported.
• The data_model matches the kind_id.
• The signatures over each individual data element (if any) are valid.
• Each element is signed by a credential which is authorized to write this kind at this resource-id
• 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.
• 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?]]

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.

The properties of stores for each data model are as follows:

Single-value:

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.

Array:

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 the array. The resulting StoredData has the correct index value when it is subsequently fetched.

Dictionary:

A store of a dictionary entry replaces (or inserts) the given value at the location specified by the dictionary key.

The following figure shows the relationship between these structures for an example store which stores the following values at resource "1234"

• The value "abc" in the single value slot for kind X
• The value "foo" at index 0 in the array for kind Y
• The value "bar" at index 1 in the array for kind Y

                                  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"

7.3.1.2.  Response Definition

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.

      struct {
        KindId                  kind;
        uint64                  generation_counter;
        NodeId                  replicas<0..2^16-1>;
      } StoreKindResponse;


      struct {
        StoreKindResponse       kind_responses<0..2^16-1>;
      } StoreAns;

The contents of each StoreKindResponse are:

kind

The kind-id being represented.

generation

The current value of the generation counter for that kind-id.

replicas

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.

The response itself is just StoreKindResponse values packed end-to-end.

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.

7.3.2.  Fetch

The Fetch request retrieves one or more data elements stored at a given resource-id. A single Fetch request can retrieve multiple different kinds.

7.3.2.1.  Request Definition

      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;

The contents of the Fetch requests are as follows:

resource

The resource ID to fetch from.

specifiers

A sequence of StoredDataSpecifier values, each specifying some of the data values to retrieve.

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:

kind

The kind-id of the data being fetched. Implementations SHOULD reject requests corresponding to unknown kinds unless specifically configured otherwise.

model

The data model of the data. This must be checked against the kind-id.

generation

The last generation counter that the requesting peer saw. This may be used to avoid unnecessary fetches or it may be set to zero.

length

The length of the rest of the structure, thus allowing extensibility.

model_specifier

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.

The model_specifier is as follows:

• If the data is of data model single value, the specifier is empty.
• If the data is of data model array, the specifier contains of a list of ArrayRange elements, each of which contains two integers. 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.
• 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.]]

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.

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.

7.3.2.2.  Response Definition

The response to a successful Fetch request is a FetchAns message containing the data requested by the requester.

       struct {
         KindId                 kind;
         uint64                 generation;
         StoredData             values<0..2^32-1>;
       } FetchKindResponse;

       struct {
         FetchKindResponse      kind_responses<0..2^32-1>;
       } FetchAns;

The FetchAns structure contains a series of FetchKindResponse structures. There MUST be one FetchKindResponse element for each kind-id in the request.

The contents of the FetchKindResponse structure are as follows:

kind

the kind that this structure is for.

generation

the generation counter for this kind.

values

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.

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.]]

7.3.3.  Remove

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.

      struct {
        ResourceId              resource;
        StoredDataSpecifier     specifiers<0..2^16-1>;
      } RemoveReq;

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.

Before processing the Remove request, the peer MUST perform the following checks.

• The kind-id is known.
• The signature over the message is valid or (depending on overlay policy) no signature is required.
• 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.
• 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.

Assuming that the request is permitted, the operations proceed as follows.

7.3.3.1.  Single Value

A Remove of a single value element causes it not to exist. If no such element exists, then this is a silent success.

7.3.3.2.  Array

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"].

7.3.3.3.  Dictionary

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.

7.3.3.4.  Response Definition

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.

      struct {
        StoreKindResponse          kind_responses<0..2^16-1>;
      } RemoveAns;

7.3.4.  Find

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).

7.3.4.1.  Request Definition

The FindReq message contains a series of resource-IDs and kind-ids identifying the resource the peer is interested in.

   struct {
     ResourceID                 resource;
     KindId                     kinds<0..2^8-1>;
   } FindReq;

The request contains a list of kind-ids which the Find is for, as indicated below:

resource

The desired resource-id

kinds

The desired kind-ids. Each value MUST only appear once.

7.3.4.2.  Response Definition

A response to a successful Find request is a FindAns message containing the closest resource-id for each kind specified in the request.

  struct {
    KindId                      kind;
    ResourceID                  closest;
  } FindKindData;

  struct {
    FindKindData                results<0..2^16-1>;
  } FindAns;

If the processing peer is not responsible for the specified resource-id, it SHOULD return a 404 error.

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.

The response is simply a series of FindKindData elements, one per kind, concatenated end-to-end. The contents of each element are:

kind

The kind-id.

closest

The closest resource ID to the specified resource ID. This is 0 if no resource ID is known.

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.

7.3.4.3.  Defining New Kinds

A new kind MUST define:

• The meaning of the data to be stored.
• The kind-id.
• The data model (single value, array, dictionary, etc.)
• Access control rules for indicating what credentials are allowed to read and write that kind-id at a given location.

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.

8.  Certificate Store Usage

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.

A user/peer MUST store its certificate at resource-ids derived from two Resource Names:

• The user name in the certificate.
• The Node-ID in the certificate.

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.

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.

Kind IDs

This usage defines the CERTIFICATE kind-id to store a peer or user's certificate.

Data Model

The data model for CERTIFICATE data is array.

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.

9.  TURN Server Usage

The TURN server usage allows a RELOAD peer to advertise that it is prepared to be a TURN server as defined in [I‑D.ietf‑behave‑turn] (Rosenberg, J., “Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN),” March 2007.). 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.

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:

      struct {
        uint8                   iteration;
        IpAddressAndPort        server_address;
      } TurnServer;

The contents of this structure are as follows:

iteration

the d value

server_address

the address at which the TURN server can be contacted.

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.

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 [I-D.ietf-behave-turn] and [I-D.ietf-behave-tcp].

[[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.]]

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.

Data Model

The TURN-SERVICE stores a single value for each resource-id.

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.

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.

10.  SIP Usage

The SIP usage allows a RELOAD overlay to be used as a distributed SIP registrar/proxy network augmenting the functionality of [RFC3263] (Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” June 2002.). This entails three primary operations:

• Registering one's own AOR with the overlay.
• Looking up a given AOR in the overlay.
• Forming a direct connection to a given peer.

10.1.  Registering AORs

In ordinary SIP, a UA registers its AOR and location with a registrar. In RELOAD, this registrar function is provided by the overlay as a whole. To register its location, a RELOAD peer stores a SipRegistration structure under its own AOR. This uses the SIP-REGISTRATION kind-id, which is formally defined in Section 10.5 (SIP-REGISTRATION Kind Definition).

Note:

GRUUs are handled via a separate mechanism, as described in Section 10.4 (GRUUs).

As a simple example, if Alice's AOR were "sip:alice@dht.example.com" and her Node-ID were "1234", she might store the mapping "sip:alice@example.org -> 1234". This would tell anyone who wanted to call Alice to contact node "1234".

RELOAD peers MAY store two kinds of SIP mappings:

• From AORs to destination lists (a single Node-ID is just a trivial destination list.)
• From AORs to other AORs.

The meaning of the first kind of mapping is "in order to contact me, form a connection with this peer." The meaning of the second kind of mapping is "in order to contact me, dereference this AOR". This allows for forwarding. For instance, if Alice wants calls to her to be forwarded to her secretary, Sam, she might insert the following mapping "sip:alice@dht.example.org -> sip:sam@dht.example.org".

The contents of a SipRegistration structure are as follows:

       enum {sip_registration_uri (1), sip_registration_route (2),
          (255)} SipRegistrationType;

       select (SipRegistration.type) {
         case sip_registration_uri:
           opaque               uri<0..2^16-1>;

         case sip_registration_route:
           opaque               contact_prefs<0..2^16-1>;
           Destination          destination_list<0..2^16-1>;

         /* This type can be extended */

       } SipRegistrationData;

       struct {
          SipRegistrationType   type;
          uint16                length;
          SipRegistrationData   data;
      } SipRegistration;

The contents of the SipRegistration PDU are:

type

the type of the registration

length

the length of the rest of the PDU

data

the registration data

• If the registration is of type "sip_registration_uri", then the contents are an opaque string containing the URI.
• If the registration is of type "sip_registration_route", then the contents are an opaque string containing the callee's contact preferences and a destination list for the peer.

RELOAD explicitly supports multiple registrations for a single AOR. The registrations are stored in a Dictionary with the dictionary keys being Node-IDs. Consider, for instance, the case where Alice has two peers:

• her desk phone (1234)
• her cell phone (5678)

Alice might store the following in the overlay at resource "sip:alice@dht.example.com".

• A SipRegistration of type "sip_registration_route" with dictionary key "1234" and value "1234".
• A SipRegistration of type "sip_registration_route" with dictionary key "5678" and value "5678".

Note that this structure explicitly allows one Node-ID to forward to another Node-ID. For instance, Alice could set calls to her desk phone to ring at her cell phone. It's not clear that this is useful in this case, but may be useful if Alice has two AORs.

In order to prevent hijacking, registrations are subject to access control rules. Before a Store is permitted, the storing peer MUST check that:

• The certificate contains a username that is a SIP AOR that hashes to the resource-id being stored at.
• The certificate contains a Node-ID that is the same as the dictionary key being stored at.

Note that these rules permit Alice to forward calls to Bob without his permission. However, they do not permit Alice to forward Bob's calls to her. See Section 15.7.2 (Malicious Retargeting) for more on this point.

10.2.  Looking up an AOR

When a RELOAD user wishes to call another user, starting with a non-GRUU AOR, he follows the following procedure. (GRUUs are discussed in Section 10.4 (GRUUs)).

1. Check to see if the domain part of the AOR matches the domain name of an overlay of which he is a member. If not, then this is an external AOR, and he MUST do one of the following:
   • Fail the call.
   • Use ordinary SIP procedures.
   • Attempt to become a member of the overlay indicated by the domain part (only possible if the enrollment procedure defined in Section 13.1 (Discovery) indicates that this is a RELOAD overlay.)
2. Perform a Fetch for kind SIP-REGISTRATION at the resource-id corresponding to the AOR. This Fetch SHOULD NOT indicate any dictionary keys, which will result in fetching all the stored values.
3. If any of the results of the Fetch are non-GRUU AORs, then repeat step 1 for that AOR.
4. Once only GRUUs and destination lists remain, the peer removes duplicate destination lists and GRUUs from the list and forms a SIP connection to the appropriate peers as described in the following sections. If there are also external AORs, the peer follows the appropriate procedure for contacting them as well.

10.3.  Forming a Direct Connection

Once the peer has translated the AOR into a set of destination lists, it then uses the overlay to route Attach messages to each of those peers. The "application" field MUST be 5060 to indicate SIP. If certificate-based authentication is in use, the responding peer MUST present a certificate with a Node-ID matching the terminal entry in the route list. Note that it is possible that the peers already have a RELOAD connection between them. This MUST NOT be used for SIP messages. However, if a SIP connection already exists, that MAY be used. Once the Attach succeeds, the peer sends SIP messages over the connection as in normal SIP.

10.4.  GRUUs

GRUUs do not require storing data in the Overlay Instance. Rather, they are constructed by embedding a base64-encoded destination list in the gr URI parameter of the GRUU. The base64 encoding is done with the alphabet specified in table 1 of RFC 4648 with the exception that ~ is used in place of =. An example GRUU is "sip:alice@example.com;gr=MDEyMzQ1Njc4OTAxMjM0NTY3ODk~". When a peer needs to route a message to a GRUU in the same P2P network, it simply uses the destination list and connects to that peer.

Because a GRUU contains a destination list, it MAY have the same contents as a destination list stored elsewhere in the resource dictionary.

Anonymous GRUUs are done in roughly the same way but require either that the enrollment server issue a different Node-ID for each anonymous GRUU required or that a destination list be used that includes a peer that compresses the destination list to stop the Node-ID from being revealed.

10.5.  SIP-REGISTRATION Kind Definition

The first mapping is provided using the SIP-REGISTRATION kind-id:

Kind IDs

The Resource Name for the SIP-REGISTRATION kind-id is the AOR of the user. The data stored is a SipRegistrationData, which can contain either another URI or a destination list to the peer which is acting for the user.

Data Model

The data model for the SIP-REGISTRATION kind-id is dictionary. The dictionary key is the Node-ID of the storing peer. This allows each peer (presumably corresponding to a single device) to store a single route mapping.

Access Control

If certificate-based access control is being used, stored data of kind-id SIP-REGISTRATION must be signed by a certificate which (1) contains user name matching the storing URI used as the Resource Name for the resource-id and (2) contains a Node-ID matching the storing dictionary key.

Data stored under the SIP-REGISTRATION kind is of type SipRegistration. This comes in two varieties:

sip_registration_uri

a URI which the user can be reached at.

sip_registration_route

a destination list which can be used to reach the user's peer.

11.  Diagnostic Usage

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.

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.

The following kinds are defined:

ROUTING_TABLE_SIZE

A single value element containing an unsigned 32-bit integer representing the number of peers in the peer's routing table.

SOFTWARE_VERSION

A single value element containing a US-ASCII string that identifies the manufacture, model, and version of the software.

MACHINE_UPTIME

A single value element containing an unsigned 64-bit integer specifying the time the nodes has been up in seconds.

APP_UPTIME

A single value element containing an unsigned 64-bit integer specifying the time the p2p application has been up in seconds.

MEMORY_FOOTPRINT

A single value element containing an unsigned 32-bit integer representing the memory footprint of the peer program in kilo bytes.

Note:

What's a kilo byte? 1000 or 1024? -- Cullen

Note:

Good question. 1000 seems like not quite enough room but 1024 is too much? -- EKR

DATASIZE_STORED

An unsigned 64-bit integer representing the number of bytes of data being stored by this node.

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.

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.

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.

sent = alpha x sent_present + (1 - alpha) x sent

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.

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.

[[TODO: We would like some sort of bandwidth measurement, but we're kind of unclear on the units and representation.]]

11.1.  Diagnostic Metrics for a P2PSIP Deployment

(OPEN QUESTION: any other metrics?)

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.

12.  Chord Algorithm

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.

12.1.  Overview

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.

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.

12.2.  Routing

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.

12.3.  Redundancy

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.

Note that a malicious node can return a success response but not store the data locally or in the replica set. Requesting peers which wish to ensure that the replication actually occurred SHOULD 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?]]

12.4.  Joining

The join process for a joining party (JP) with Node-ID n is as follows.

1. JP connects to its chosen bootstrap node.
2. JP uses a series of Pings to populate its routing table.
3. 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.
4. JP enters all the peers it contacted into its routing table.
5. JP sends a Join to its immediate successor, the admitting peer (AP) for Node-ID n. The AP sends the response to the Join.
6. AP does a series of Store requests to JP to store the data that JP will be responsible for.
7. 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.
8. AP sends an Update to all of its neighbors with the new values of its neighbor set (including JP).
9. JP sends UpdateS to all the peers in its routing table.

In order to populate its routing table, JP sends a Ping via the bootstrap node directed at resource-id n+1 (directly after its own resource-id). This allows it to discover its own successor. Call that node p0. It then sends a ping to p0+1 to discover its successor (p1). This process can be repeated to discover as many successors as desired. The values for the two peers before p will be found at a later stage when n receives an Update.

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.

12.5.  Routing Attaches

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.

All other Attach requests, such as those for new finger table entries, are routed conventionally through the overlay.

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

12.6.  Updates

A chord Update is defined as

      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;

The "type" field contains the type of the update, which depends on the reason the update was sent.

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.

neighbors:

this version is sent to members of the Chord neighbor table.

full:

this version is sent to peers which request an Update with a RouteQueryReq.

If the message is of type "neighbors", then the contents of the message will be:

predecessors

The predecessor set of the Updating peer.

successors

The successor set of the Updating peer.

If the message is of type "full", then the contents of the message will be:

predecessors

The predecessor set of the Updating peer.

successors

The successor set of the Updating peer.

fingers

The finger table if the Updating peer, in numerically ascending order.

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.

12.6.1.  Sending Updates

Every time a connection to a peer in the neighborhood set is lost (as determined by connectivity pings 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.

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 Pings to find a peer and send it a Join. If connectivity is lost to all the peers in the finger table, this peer should assume that it has been disconnected from the rest of the network, and it should periodically try to join the DHT.

12.6.2.  Receiving Updates

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:

• The UpdateReq contains peers that would not change the neighbor set because they match the neighborhood table.
• The UpdateReq contains peers closer to N than those in its neighborhood table.
• 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.

In the first case, no change is needed.

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.

The third case implies that a neighbor has disappeared, most likely because it has simply been disconnected but perhaps because of overlay instability. N MUST Ping the questionable peers to discover if they are indeed missing and if so, remove them from its neighborhood table.

After any Pings 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.

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.

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.

12.6.3.  Stabilization

There are four components to stabilization:

1. exchange Updates with all peers in its routing table to exchange state
2. search for better peers to place in its finger table
3. search to determine if the current finger table size is sufficiently large
4. search to determine if the overlay has partitioned and needs to recover

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.

Periodically a peer should select a random entry i from the finger table and do a Ping 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+-ping-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.

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 Ping 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.]]

To detect that a partitioning has occurred and to heal the overlay, a peer P MUST periodically repeat the discovery process used in the initial join for the overlay to locate an appropriate bootstrap peer, B. If an overlay has multiple mechanisms for discovery it should randomly select a method to locate a bootstrap peer. P should then send a Ping for its own Node-ID routed through B. If a response is received from a peer S', which is not P's successor, then the overlay is partitioned and P should send a Attach to S' routed through B, followed by an Update sent to S'. (Note that S' may not be in P's neighborhood table once the overlay is healed, but the connection will allow S' to discover appropriate neighbor entries for itself via its own stabilization.)

12.7.  Route Query

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:

    struct {
       NodeId                  next_id;
    } ChordRouteQueryAns;

The contents of this structure are as follows:

next_peer

The peer to which the responding peer would route the message to in order to deliver it to the destination listed in the request.

If the requester set the send_update flag, the responder SHOULD initiate an Update immediately after sending the RouteQueryAns.

12.8.  Leaving

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.

13.  Enrollment and Bootstrap

13.1.  Discovery

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 [I‑D.garcia‑p2psip‑dns‑sd‑bootstrapping] (Garcia, G., “P2PSIP bootstrapping using DNS-SD,” October 2007.) and [I‑D.matthews‑p2psip‑bootstrap‑mechanisms] (Cooper, E., “Bootstrap Mechanisms for P2PSIP,” February 2007.). 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.

If the overlay name ends in .local, then a DNS SRV lookup using implement [I‑D.cheshire‑dnsext‑dns‑sd] (Krochmal, M. and S. Cheshire, “DNS-Based Service Discovery,” August 2006.) 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.

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 [RFC2818] (Rescorla, E., “HTTP Over TLS,” May 2000.). 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".

The result is an XML configuration file with the syntax described in the following section.

13.2.  Overlay Configuration

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.

   <?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>

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.

The file can contain multiple "overlay" elements where each one contains the configuration information for a different overlay. Each "overlay" has the following attributes:

instance-name:

name of the overlay

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.

Inside each overlay element, the following elements can occur:

topology-plugin

This element has an attribute called algorithm-name that describes the overlay-algorithm being used.

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.

required-kinds

This element indicates the kinds that members must support. It has three attributes:

• name: a string representing the kind.
• max-count: the maximum number of values which members of the overlay must support.
• max-size: the maximum size of individual values.

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.

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.

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".

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.

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.

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.

chord-128-2-16+-update-frequency

The update frequency for the Chord-128-2-16+ topology plugin (see Section 12 (Chord Algorithm )).

chord-128-2-16+-ping-frequency

The ping frequency for the Chord-128-2-16+ topology plugin (see Section 12 (Chord Algorithm )).

credential-server

Base URL for credential server.

shared-secret

If shared secret mode is used, this contains the shared secret.

[[TODO: Do a RelaxNG grammar.]]

13.3.  Credentials

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.

In order to acquire credentials, the peer generates an asymmetric key pair and then generates a "Simple Enrollment Request" (as defined in [I‑D.ietf‑pkix‑2797‑bis] (Myers, M. and J. Schaad, “Certificate Management Messages over CMS,” March 2006.)) and sends this over HTTPS as defined in [I‑D.ietf‑pkix‑cmc‑trans] (Schaad, J. and M. Myers, “Certificate Management over CMS (CMC) Transport Protocols,” May 2006.) to the URL in the credential-server element. The subjectAltName in the request MUST contain the required user name.

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:

• One or more Node-IDs which MUST be cryptographically random [RFC4086] (Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” June 2005.). 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 Section 16.10 (reload: URI Scheme) and MUST contain a Destination list with a single entry of type "node_id".
• The names this user is allowed to use in the overlay, using type rfc822Name.

The certificate is returned in a "Simple Enrollment Response". [[TODO: REF]]

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.

13.3.1.  Self-Generated Credentials

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.

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.

Once the node has constructed a self-signed certificate, it MAY join the overlay. Before storing its certificate in the overlay (Section 8 (Certificate Store Usage)) 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.

13.4.  Joining the Overlay Peer

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 Ping request to the known peer address with the destination Node-ID set to that peer's Node-ID.

If no cached peers are available, then the peer SHOULD send a Ping request to the address and port found in the broadcast-peers element in the configuration document. This MAY be a multicast or anycast address. The Ping should use the wildcard Node-ID as the destination Node-ID.

The responder peer that receives the Ping request SHOULD check that the overlay name is correct and that the requester peer sending the request has appropriate credentials for the overlay before responding to the Ping request even if the response is only an error.

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 Section 6.3 (Overlay Topology).

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.

14.  Message Flow Example

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.

         JP        PPP       PP        AP        NP        NNP       BP
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |Attach Dest=JP     |         |         |         |         |
          |---------------------------------------------------------->|
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |Attach Dest=JP     |         |         |
          |         |         |<--------------------------------------|
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |Attach Dest=JP     |         |         |
          |         |         |-------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |AttachAns          |         |         |
          |         |         |<--------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |AttachAns          |         |         |
          |         |         |-------------------------------------->|
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |AttachAns          |         |         |         |         |
          |<----------------------------------------------------------|
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |TLS      |         |         |         |         |         |
          |.............................|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |

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.

         JP        PPP       PP        AP        NP        NNP       BP
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |Attach AP+1        |         |         |         |         |
          |---------------------------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |Attach AP+1        |         |
          |         |         |         |-------->|         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |AttachAns          |         |
          |         |         |         |<--------|         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |AttachAns          |         |         |         |         |
          |<----------------------------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |Attach   |         |         |         |         |         |
          |-------------------------------------->|         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |TLS      |         |         |         |         |         |
          |.......................................|         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |

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.

         JP        NP        XX        TP
          |         |         |         |
          |         |         |         |
          |         |         |         |
          |Attach JP+2<<126   |         |
          |-------->|         |         |
          |         |         |         |
          |         |         |         |
          |         |Attach JP+2<<126   |
          |         |-------->|         |
          |         |         |         |
          |         |         |         |
          |         |         |Attach JP+2<<126
          |         |         |-------->|
          |         |         |         |
          |         |         |         |
          |         |         |AttachAns|
          |         |         |<--------|
          |         |         |         |
          |         |         |         |
          |         |AttachAns|         |
          |         |<--------|         |
          |         |         |         |
          |         |         |         |
          |AttachAns|         |         |
          |<--------|         |         |
          |         |         |         |
          |         |         |         |
          |TLS      |         |         |
          |.............................|
          |         |         |         |
          |         |         |         |
          |         |         |         |
          |         |         |         |

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.

         JP        PPP       PP        AP        NP        NNP       BP
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |JoinReq  |         |         |         |         |         |
          |---------------------------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |JoinAns  |         |         |         |         |         |
          |<----------------------------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |StoreReq Data A    |         |         |         |         |
          |<----------------------------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |StoreAns |         |         |         |         |         |
          |---------------------------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |StoreReq Data B    |         |         |         |         |
          |<----------------------------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |StoreAns |         |         |         |         |         |
          |---------------------------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateReq|         |         |         |         |         |
          |<----------------------------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateAns|         |         |         |         |         |
          |---------------------------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |

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.

         JP        PPP       PP        AP        NP        NNP       BP
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |Attach Dest=PP     |         |         |         |         |
          |---------------------------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |Attach Dest=PP     |         |         |
          |         |         |<--------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |AttachAns|         |         |         |
          |         |         |-------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |AttachAns|         |         |         |         |         |
          |<----------------------------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |TLS      |         |         |         |         |         |
          |...................|         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateReq|         |         |         |         |         |
          |------------------>|         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateAns|         |         |         |         |         |
          |<------------------|         |         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateReq|         |         |         |         |         |
          |---------------------------->|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateAns|         |         |         |         |         |
          |<----------------------------|         |         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateReq|         |         |         |         |         |
          |-------------------------------------->|         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |
          |UpdateAns|         |         |         |         |         |
          |<--------------------------------------|         |         |
          |         |         |         |         |         |         |
          |         |         |         |         |         |         |

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.

         JP        NP        XX        TP
          |         |         |         |
          |         |         |         |
          |         |         |         |
          |Update   |         |         |
          |---------------------------->|
          |         |         |         |
          |         |         |         |
          |UpdateAns|         |         |
          |<----------------------------|
          |         |         |         |
          |         |         |         |
          |         |         |         |
          |         |         |         |

15.  Security Considerations

15.1.  Overview

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. In Section 15.7 (SIP-Specific Issues) we describe issues that are specific to SIP.

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.

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.

15.2.  Attacks on P2P Overlays

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.

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.

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.

15.3.  Certificate-based Security

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.

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.

Each certificate enables an entity to act in two sorts of roles:

• As a user, storing data at specific Resource-IDs in the Overlay Instance corresponding to the user name.
• As a overlay peer with the peer ID(s) listed in the certificate.

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.

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.

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.

All implementations MUST implement certificate-based security.

15.4.  Shared-Secret Security

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 [RFC4279] (Eronen, P. and H. Tschofenig, “Pre-Shared Key Ciphersuites for Transport Layer Security (TLS),” December 2005.) or TLS-SRP [I‑D.ietf‑tls‑srp] (Taylor, D., “Using SRP for TLS Authentication,” June 2007.) mode. A peer which does not know the key cannot form TLS connections with any other peer and therefore cannot join the overlay.

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.

15.5.  Storage Security

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.

15.5.1.  Authorization

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).

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.

For example, consider the certificate with the following properties:

        User name: alice@dht.example.com
        Node-ID:   013456789abcdef
        Serial:    1234

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.

15.5.2.  Distributed Quota

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.

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.

15.5.3.  Correctness

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.

15.5.4.  Residual Attacks

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.

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.

In addition, when a kind is 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.

15.6.  Routing Security

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.

15.6.1.  Background

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 [Eclipse] (Singh, A., Ngan, T., Druschel, T., and D. Wallach, “Eclipse Attacks on Overlay Networks: Threats and Defenses,” April 2006.) 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 [Sybil] (Douceur, J., “The Sybil Attack,” March 2002.), the attacker registers a large number of nodes and is therefore able to capture a large amount of the traffic through the DHT.

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.

15.6.2.  Admissions Control

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:

• It allows the enrollment server to limit the number of peer IDs issued to any individual user.
• It prevents the attacker from choosing specific peer IDs.

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.

15.6.3.  Peer Identification and Authentication

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.

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.

[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.]

[TODO: I think we are probably going to end up with generic signatures or at least optional signatures on all overlay messages.]

15.6.4.  Protecting the Signaling

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.

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.

15.6.5.  Residual Attacks

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.

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.

15.7.  SIP-Specific Issues

15.7.1.  Fork Explosion

Because SIP includes a forking capability (the ability to retarget to multiple recipients), fork bombs are a potential DoS concern. However, in the SIP usage of RELOAD, fork bombs are a much lower concern because the calling party is involved in each retargeting event and can therefore directly measure the number of forks and throttle at some reasonable number.

15.7.2.  Malicious Retargeting

Another potential DoS attack is for the owner of an attractive number to retarget all calls to some victim. This attack is difficult to ameliorate without requiring the target of a SIP registration to authorize all stores. The overhead of that requirement would be excessive and in addition there are good use cases for retargeting to a peer without there explicit cooperation.

15.7.3.  Privacy Issues

All RELOAD SIP registration data is public. Methods of providing location and identity privacy are still being studied.

16.  IANA Considerations

This section contains the new code points registered by this document.

[[TODO - add IANA registration for p2p_enroll SRV and p2p_menroll]]

16.1.  Overlay Algorithm Types

IANA SHALL create/(has created) 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:

chord-128-2-16+

The algorithm defined in Section 12 (Chord Algorithm ) of this document.

16.2.  Data Kind-Id

IANA SHALL create/(has created) a "RELOAD Data Kind-Id" Registry. Entries in this registry are 32-bit integers denoting data kinds, as described in Section 4.1.2 (Usages). Code points in the range 0x00000000 to 0x7fffffff SHALL be registered via RFC 5226 Standards Action. Code points in the range 0x8000000 to 0xffffffff SHALL be registered via RFC 5226 Expert Review.

The initial contents of this registry are:

16.3.  Data Model

IANA SHALL create/(has created) a "RELOAD Data Model" Registry. Entries in this registry are 8-bit integers denoting data models, as described in Section 7.2 (Data Models). Code points in this registry SHALL be registered via RFC 5226 IETF Review.

16.4.  Message Codes

IANA SHALL create/(has created) a "RELOAD Message Code" Registry. Entries in this registry are 16-bit integers denoting method codes as described in Section 6.2.3 (Message Contents Format). These codes SHALL be registred via RFC 5226 Standards Track.

The initial contents of this registry are:

16.5.  Error Codes

IANA SHALL create/(has created) 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 Track.

16.6.  Route Log Extension Types

IANA SHALL create/(has created) a "RELOAD Route Log Extension Type Registry. This registry is currently empty. New entries SHALL be defined via RFC 5226 Expert Review.

16.7.  Transport Types

IANA shall create/(has created) a "RELOAD Transport Type Registry." This registry SHALL be initially populated with the following values:

            reserved     0
            tcp_tls      1
            udp_dtls     2

New entries SHALL be defined via RFC 5226 Standards Action.

16.8.  Forwarding Options

IANA shall create/(has created) a "RELOAD Forwarding Option Registry". Entries in this registry between 0 and 127 SHALL be defined via RFC 5226 Standards Track. Entries in this registry between 128 and 255 SHALL be defined via RFC 5226 Specification Required.

16.9.  Ping Information Types

IANA shall create/(has created) a "RELOAD Ping Information Type Registry". This registry SHALL be initially populated with the following values:

            responsible_set  1
            requested_info   2

Entries in this registry SHALL be defined via RFC 5226 Standards Track.

16.10.  reload: URI Scheme

This section describes the scheme for a reload: URI, which can be used to refer to either:

• A peer.
• A resource inside a peer.

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:

    RELOAD-URI = "reload://" destination "@" overlay "/"
             [specifier]

          destination = 1 * HEXDIG
    overlay = reg-name
    specifier = 1*HEXDIG

The definitions of these productions are as follows:

destination:

a hex-encoded Destination List object.

overlay:

the name of the overlay.

specifier :

a hex-encoded StoredDataSpecifier indicating the data element.

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.

16.10.1.  URI Registration

The following summarizes the information necessary to register the reload: URI. [NOTE TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number for this specification in the following list.]

URI Scheme Name:

reload

Status:

permanent

URI Scheme Syntax:

see Section 16.10 (reload: URI Scheme).

URI Scheme Semantics:

The reload: URI is intended to be used as a reference to a RELOAD peer or resource.

Encoding Considerations:

The reload: URI is not intended to be human-readable text, therefore they are encoded entirely in US-ASCII.

Applications/protocols that use this URI scheme:

The RELOAD protocol described in RFC XXXX.

TBD for the rest of this template.

17.  Acknowledgments

This draft is a merge of the "REsource LOcation And Discovery (RELOAD)" draft by David A. Bryan, Marcia Zangrilli and Bruce B. Lowekamp, the "Address Settlement by Peer to Peer" draft by Cullen Jennings, Jonathan Rosenberg, and Eric Rescorla, the "Security Extensions for RELOAD" draft by Bruce B. Lowekamp and James Deverick, the "A Chord-based DHT for Resource Lookup in P2PSIP" by Marcia Zangrilli and David A. Bryan, and the Peer-to-Peer Protocol (P2PP) draft by Salman A. Baset, Henning Schulzrinne, and Marcin Matuszewski.

Thanks to the many people who contributed including: Michael Chen, TODO - fill in.

18.  References

18.1. Normative References

18.2. Informative References

Authors' Addresses

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