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P2PSIP                                                       C. Jennings
Internet-Draft                                                     Cisco
Intended status:  Standards Track                            B. Lowekamp
Expires:  August 27, 2008                      SIPeerior; William & Mary
                                                             E. Rescorla
                                                       Network Resonance
                                                            J. Rosenberg
                                                                S. Baset
                                                          H. Schulzrinne
                                                     Columbia University
                                                       February 24, 2008

                REsource LOcation And Discovery (RELOAD)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on August 27, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

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   This document defines REsource LOcation And Discovery (RELOAD), a
   peer-to-peer (P2P) binary signaling protocol for use on the Internet.
   A P2P signaling protocol provides its clients with an abstract hash
   table 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 data kinds 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  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.1.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   7
       1.1.1.  Usage Layer . . . . . . . . . . . . . . . . . . . . .   8
       1.1.2.  Overlay Routing and Storage Layer . . . . . . . . . .   9
       1.1.3.  Forwarding Layer  . . . . . . . . . . . . . . . . . .  10
     1.2.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  10
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  11
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.1.  Distributed Storage Layer . . . . . . . . . . . . . . . .  13
       3.1.1.  DHT Concepts  . . . . . . . . . . . . . . . . . . . .  13
       3.1.2.  DHT Topology  . . . . . . . . . . . . . . . . . . . .  14
       3.1.3.  Routing . . . . . . . . . . . . . . . . . . . . . . .  14
       3.1.4.  Storing and Retrieving Structured Data  . . . . . . .  15
       3.1.5.  Joining, Leaving, and Maintenance . . . . . . . . . .  16
     3.2.  Forwarding Layer  . . . . . . . . . . . . . . . . . . . .  17
       3.2.1.  Forming Direct Connections  . . . . . . . . . . . . .  17
       3.2.2.  Via Lists . . . . . . . . . . . . . . . . . . . . . .  18
       3.2.3.  Clients . . . . . . . . . . . . . . . . . . . . . . .  19
     3.3.  Transport Layer . . . . . . . . . . . . . . . . . . . . .  20
     3.4.  Enrollment  . . . . . . . . . . . . . . . . . . . . . . .  20
       3.4.1.  Certificate Issuance  . . . . . . . . . . . . . . . .  21
       3.4.2.  Bootstrap . . . . . . . . . . . . . . . . . . . . . .  21
     3.5.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  21
       3.5.1.  Certificate-Based Security  . . . . . . . . . . . . .  21
       3.5.2.  Shared-Key Security . . . . . . . . . . . . . . . . .  23
     3.6.  Migration . . . . . . . . . . . . . . . . . . . . . . . .  24
     3.7.  Usages Layer  . . . . . . . . . . . . . . . . . . . . . .  24
       3.7.1.  SIP Usage . . . . . . . . . . . . . . . . . . . . . .  25
       3.7.2.  Certificate Store Usage . . . . . . . . . . . . . . .  27

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       3.7.3.  TURN Usage  . . . . . . . . . . . . . . . . . . . . .  27
       3.7.4.  Diagnostic Usage  . . . . . . . . . . . . . . . . . .  27
       3.7.5.  HIP Tunnel Usage  . . . . . . . . . . . . . . . . . .  27
   4.  Base Protocol . . . . . . . . . . . . . . . . . . . . . . . .  28
     4.1.  Forwarding Header . . . . . . . . . . . . . . . . . . . .  28
       4.1.1.  Changes to Forwarding Header  . . . . . . . . . . . .  33
       4.1.2.  Message Routing . . . . . . . . . . . . . . . . . . .  33
       4.1.3.  Fragmentation and Reassembly  . . . . . . . . . . . .  35
       4.1.4.  Route Logging . . . . . . . . . . . . . . . . . . . .  36
     4.2.  Message Contents Format . . . . . . . . . . . . . . . . .  38
       4.2.1.  Common Header . . . . . . . . . . . . . . . . . . . .  38
       4.2.2.  Payload . . . . . . . . . . . . . . . . . . . . . . .  39
       4.2.3.  Signature . . . . . . . . . . . . . . . . . . . . . .  39
     4.3.  Response Codes and Response Errors  . . . . . . . . . . .  42
   5.  End-to-End Timeout and Retransmission . . . . . . . . . . . .  43
   6.  Transports  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     6.1.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .  44
     6.2.  DTLS  . . . . . . . . . . . . . . . . . . . . . . . . . .  44
       6.2.1.  Reliability for Unreliable Transports . . . . . . . .  44
     6.3.  HIP . . . . . . . . . . . . . . . . . . . . . . . . . . .  45
   7.  Method Definitions  . . . . . . . . . . . . . . . . . . . . .  46
     7.1.  Connection Management . . . . . . . . . . . . . . . . . .  46
       7.1.1.  PING  . . . . . . . . . . . . . . . . . . . . . . . .  46
       7.1.2.  CONNECT . . . . . . . . . . . . . . . . . . . . . . .  48
       7.1.3.  TUNNEL  . . . . . . . . . . . . . . . . . . . . . . .  51
     7.2.  Data Storage and Retrieval  . . . . . . . . . . . . . . .  53
       7.2.1.  STORE . . . . . . . . . . . . . . . . . . . . . . . .  53
       7.2.2.  FETCH . . . . . . . . . . . . . . . . . . . . . . . .  60
       7.2.3.  REMOVE  . . . . . . . . . . . . . . . . . . . . . . .  64
       7.2.4.  FIND  . . . . . . . . . . . . . . . . . . . . . . . .  65
     7.3.  DHT Maintenance . . . . . . . . . . . . . . . . . . . . .  67
       7.3.1.  JOIN  . . . . . . . . . . . . . . . . . . . . . . . .  67
       7.3.2.  LEAVE . . . . . . . . . . . . . . . . . . . . . . . .  68
       7.3.3.  UPDATE  . . . . . . . . . . . . . . . . . . . . . . .  68
       7.3.4.  ROUTE_QUERY . . . . . . . . . . . . . . . . . . . . .  69
   8.  ICE and Connection Formation  . . . . . . . . . . . . . . . .  70
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  71
     8.2.  Collecting STUN Servers . . . . . . . . . . . . . . . . .  72
     8.3.  Gathering Candidates  . . . . . . . . . . . . . . . . . .  73
     8.4.  Encoding the CONNECT Message  . . . . . . . . . . . . . .  73
     8.5.  Verifying ICE Support . . . . . . . . . . . . . . . . . .  74
     8.6.  Role Determination  . . . . . . . . . . . . . . . . . . .  74
     8.7.  Connectivity Checks . . . . . . . . . . . . . . . . . . .  75
     8.8.  Concluding ICE  . . . . . . . . . . . . . . . . . . . . .  75
     8.9.  Subsequent Offers and Answers . . . . . . . . . . . . . .  75
     8.10. Media Keepalives  . . . . . . . . . . . . . . . . . . . .  75
     8.11. Sending Media . . . . . . . . . . . . . . . . . . . . . .  75
     8.12. Receiving Media . . . . . . . . . . . . . . . . . . . . .  76

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   9.  DHT Algorithms  . . . . . . . . . . . . . . . . . . . . . . .  76
     9.1.  Generic Algorithm Requirements  . . . . . . . . . . . . .  76
     9.2.  Chord Algorithm . . . . . . . . . . . . . . . . . . . . .  76
       9.2.1.  Overview  . . . . . . . . . . . . . . . . . . . . . .  76
       9.2.2.  Routing . . . . . . . . . . . . . . . . . . . . . . .  77
       9.2.3.  Redundancy  . . . . . . . . . . . . . . . . . . . . .  77
       9.2.4.  Joining . . . . . . . . . . . . . . . . . . . . . . .  77
       9.2.5.  Routing CONNECTs  . . . . . . . . . . . . . . . . . .  78
       9.2.6.  UPDATEs . . . . . . . . . . . . . . . . . . . . . . .  79
       9.2.7.  Leaving . . . . . . . . . . . . . . . . . . . . . . .  82
   10. Enrollment and Bootstrap  . . . . . . . . . . . . . . . . . .  82
     10.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . .  82
     10.2. Overlay Configuration . . . . . . . . . . . . . . . . . .  83
     10.3. Credentials . . . . . . . . . . . . . . . . . . . . . . .  84
       10.3.1. Credentials for HIP . . . . . . . . . . . . . . . . .  85
     10.4. Locating a Peer . . . . . . . . . . . . . . . . . . . . .  85
   11. Usages  . . . . . . . . . . . . . . . . . . . . . . . . . . .  86
     11.1. Generic Usage Requirements  . . . . . . . . . . . . . . .  86
     11.2. SIP Usage . . . . . . . . . . . . . . . . . . . . . . . .  87
       11.2.1. SIP-REGISTRATION kind . . . . . . . . . . . . . . . .  87
       11.2.2. GRUUs . . . . . . . . . . . . . . . . . . . . . . . .  89
       11.2.3. SIP Connect . . . . . . . . . . . . . . . . . . . . .  89
       11.2.4. SIP Tunnel  . . . . . . . . . . . . . . . . . . . . .  89
     11.3. TURN Usage  . . . . . . . . . . . . . . . . . . . . . . .  90
     11.4. Certificate Store Usage . . . . . . . . . . . . . . . . .  93
     11.5. HIP Tunnel  . . . . . . . . . . . . . . . . . . . . . . .  94
     11.6. Diagnostic Usage  . . . . . . . . . . . . . . . . . . . .  94
       11.6.1. State Acquisition Mechanisms  . . . . . . . . . . . .  95
       11.6.2. Black-box diagnostics . . . . . . . . . . . . . . . .  96
       11.6.3. Diagnostic Metrics for a P2PSIP Deployment  . . . . .  98
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  99
     12.1. Overview  . . . . . . . . . . . . . . . . . . . . . . . .  99
     12.2. Attacks on P2P Overlays . . . . . . . . . . . . . . . . .  99
     12.3. Certificate-based Security  . . . . . . . . . . . . . . . 100
     12.4. Shared-Secret Security  . . . . . . . . . . . . . . . . . 101
     12.5. Storage Security  . . . . . . . . . . . . . . . . . . . . 101
       12.5.1. Authorization . . . . . . . . . . . . . . . . . . . . 102
       12.5.2. Distributed Quota . . . . . . . . . . . . . . . . . . 102
       12.5.3. Correctness . . . . . . . . . . . . . . . . . . . . . 103
       12.5.4. Residual Attacks  . . . . . . . . . . . . . . . . . . 103
     12.6. Routing Security  . . . . . . . . . . . . . . . . . . . . 104
       12.6.1. Background  . . . . . . . . . . . . . . . . . . . . . 104
       12.6.2. Admissions Control  . . . . . . . . . . . . . . . . . 104
       12.6.3. Peer Identification and Authentication  . . . . . . . 105
       12.6.4. Protecting the Signaling  . . . . . . . . . . . . . . 105
       12.6.5. Residual Attacks  . . . . . . . . . . . . . . . . . . 106
     12.7. SIP-Specific Issues . . . . . . . . . . . . . . . . . . . 106
       12.7.1. Fork Explosion  . . . . . . . . . . . . . . . . . . . 106

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       12.7.2. Malicious Retargeting . . . . . . . . . . . . . . . . 106
       12.7.3. Privacy Issues  . . . . . . . . . . . . . . . . . . . 107
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 107
     13.1. Overlay Algorithm Types . . . . . . . . . . . . . . . . . 107
     13.2. Data Kind-Id  . . . . . . . . . . . . . . . . . . . . . . 107
     13.3. Data Model  . . . . . . . . . . . . . . . . . . . . . . . 108
     13.4. Message Codes . . . . . . . . . . . . . . . . . . . . . . 108
   14. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 109
   15. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . 109
   16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 109
   17. Appendix: Operation with SIP clients outside the DHT domain . 110
   18. Appendix: Notes on DHT Algorithm Selection  . . . . . . . . . 110
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . . 110
     19.1. Normative References  . . . . . . . . . . . . . . . . . . 110
     19.2. Informative References  . . . . . . . . . . . . . . . . . 111
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 113
   Intellectual Property and Copyright Statements  . . . . . . . . . 115

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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 Distributed Hash Table (DHT) service, which allows
   participating nodes to read and write entries into a hash table that
   is stored collectively among the participants.  RELOAD is a
   lightweight, binary protocol.  It provides several functions that are
   critical for a successful P2P protocol for the Internet.  These are:

   Security Framework:  Security is one of the most challenging problems
      in a P2P protocol.  A P2P network will often be established among
      a set of peers that do not trust each other.  Yet, despite this
      lack of trust, the network must operate reliably to allow storage
      and retrieval of data.  RELOAD defines an abstract enrollment
      server, which all entities trust to generate unique identifiers
      for each user.  Using that small amount of trust as an anchor,
      RELOAD defines a security framework that allows for authorization
      of P2P protocol functions and authentication of data stored in the
      overlay.  This does not remove all attacks but greatly reduces the
      possible attack space.
   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].
      Consequently, RELOAD has the notion of a usage, one of which is
      defined to support each application (this document also defines
      the SIP usage for multimedia communications).  Each usage
      identifies a set of data kind that need to be stored and retrieved
      from the DHT.  Each kind defines a data structure, authorization
      policies, size quota, and information required for storage and
      retrieval in the DHT.  The usage concept allows RELOAD to be used
      with new applications through a simple documentation process that
      supplies the details for each application.
   NAT Traversal:  Operations for NAT traversal are part of the base
      design, including establishing new RELOAD connections and
      tunneling SIP or other application protocols required by P2PSIP.
      RELOAD makes use of Interactive Connectivity Establishment (ICE)
      [I-D.ietf-mmusic-ice] to facilitate the creation of the P2P
      network and the establishment of channels for use by the
      application protocol (SIP and RTP, for example).  RELOAD also
      defines how peers in the P2P network act as STUN and TURN servers
      and how those resources can be discovered through the DHT.  With
      these features, RELOAD can run in modes in which nearly all the
      peers are behind NATs, yet are able to fully participate without
      imposing any constraints on the actual DHT algorithm.

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   High Performance Routing:  The very nature of DHT 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 to reduce
      the amount of bandwidth and processing required of peers.  It does
      so by using a very lightweight binary protocol, and furthermore,
      by defining a packet structure that facilitates low-complexity
      forwarding, including hardware-based forwarding.  In particular, a
      generic transport header is used for routing the message through
      the overlay without the contents needing to be parsed by (or even
      visible to) intermediate peers.  The header includes no
      information about specific IP addresses because none are needed to
      route along an overlay.  The header only includes lists of peers
      which the message should be routed through/too, as well as some
      minor options and version flags.  Clearly separating the header
      components necessary for routing from the message contents
      simplifies processing and increases security.
   Transport Flexibility:  RELOAD has native support for both DTLS and
      TLS for the underlying transport protocol, with support for DTLS
      over UDP as mandatory to implement.  TLS over TCP is preferred
      because it has better bulk data performance and connection
      stability, but UDP is more likely to provide direct connections
      between peers in the presence of NATs.  Explicit support for
      fragmentation is provided and required when using UDP.  Because
      there is no single universally available and suitable transport
      protocol, the peer protocol must be flexible in this regard.  New
      transports can be supported trivially.
   Pluggable DHT 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" DHT
      will allow interoperability, while the extensibility allows
      selection of DHTs optimized for a particular application.

   These properties were designed specifically to meet the requirements
   for a P2P protocol to support SIP.  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].

1.1.  Architecture

   Architecturally this specification is divided into several layers, as
   shown in the following figure.

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          -------------------------------------- Usage-defined API
            +-------+  +-------+
Usage       | SIP   |  | XMPP  |  ...
Layer       | Usage |  | Usage |
            +-------+  +-------+
          -------------------------------------- Distributed Storage API

Overlay     Overlay              +-------------+
Routing &   Routing &    +----+  | +-----+     |
Storage     Replication  | DB |  | |Chord| ... | Topology
Layer       Logic        +----+  | |     |     | Plugins
                                 | +-----+     |
                               +------+ +-----+
Forwarding     Forwarding &    | STUN | | ICE |
Layer          Encoding Logic  +------+ +-----+
          -------------------------------------- Common Packet Encoding
Transport     +-------+  +------+
Layer         |TLS    |  |DTLS  |
              +-------+  +------+

   The three layers defined by RELOAD include:

   Usage Layer:  Provides an application-specific interface that maps an
      application's requirements onto the generic services of the DHT.
   Overlay Routing & Storage Layer:  Implements the overlay.  Chooses
      what links to establish to form the DHT's overlay network, manages
      the storage and migration of data for this peer and on behalf of
      other peers, and performs searches for requested data across the
   Forwarding Layer:  Provides packet forwarding services between nodes.
      Also handles setting up connections across NATs using ICE.

1.1.1.  Usage Layer

   The top layer, called the Usage Layer, has application usages, such
   as the SIP Location Usage, that use the abstract distributed storage
   API to store and retrieve data from the DHT.  The goal of this layer
   is to implement application-specific usages of the Overlay Routing
   and Storage Layer below it.  The Usage defines how a specific
   application maps its data into something that can be stored in the
   DHT, 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

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   single application may require multiple usages.  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.  A usage is not
   itself encoded on the wire, only the kind-ids and data models are,
   but is rather a specification of the functionality that is required
   for a given application.  That specification typically specifies
   semantics, access control rules, and the format and size of the data
   which may be stored.

   One usage may depend on another.  For example, the SIP usage depends
   on a Certificate Store usage (not shown in the diagram) to obtain the
   certificates required to authenticate messages.  Because certificates
   are stored in standard X.509 form, there is no reason for each usage
   to specify this service independently.

1.1.2.  Overlay Routing and Storage Layer

   The Overlay Routing and Storage Layer stores and retrieves
   information, performs maintenance of the DHT as peers join and leave
   the DHT, and routes messages on the overlay.  The DHT implementation
   is provided by a pluggable component so that each overlay can select
   an appropriate DHT that relies on the common RELOAD core code.

   The Overlay Routing and Replication Logic provides a fairly generic
   interface that allows the DHT implementation to control the overlay
   and resource operations and messages.  Since each DHT is defined and
   functions differently, we generically refer to the table of other
   peers that the DHT maintains and uses to route requests (neighbors)
   as a Routing Table.  The Logic component makes queries to the DHT's
   Routing Table to determine the next hop, then encodes and sends the
   message itself.  Similarly, the DHT issues periodic update requests
   through the logic component to maintain and update its Routing Table.

   The DHT shown in the illustration is Chord, but a variety of DHT
   algorithms are possible through a pluggable interface.  A single node
   could be functioning in multiple overlays simultaneously, each using
   its own DHT algorithm.  Each peer is identified by and its location
   in the overlay determined by its Peer-ID that is assigned by the
   enrollment server when the user or peer first enrolls in the overlay.
   The Peer-ID also determines the set of resources which it will be
   responsible for storing.  The exact mapping between these is
   determined by the DHT algorithm used by the overlay, therefore the
   logic component always queries the DHT to determine where a
   particular resource should be stored.

   As peers enter and leave, resources may be stored on different peers,
   so the information related to them is exchanged as peers enter and
   leave.  Redundancy is used to protect against loss of information in

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   the event of a peer failure and to protect against compromised or
   subversive peers.  The Logic component notifies the DHT as neighbors
   join and leave, and the DHT updates its Routing Table and issues
   resource migration requests as appropriate.

1.1.3.  Forwarding Layer

   This 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 required by the
   DHT's Routing Table.  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

1.2.  Security

   RELOAD provides two security mechanisms, one based on public key
   certificates and one based on a globally shared key.

   RELOAD's preferred security framework is built upon an enrollment
   server.  The enrollment server issues each new peer a certificate
   that assigns it a Peer-ID.  By generating the Peer-IDs randomly and
   controlling what peers are issued certificates, the enrollment server
   protects against many of the attacks on the overlay network.
   Similarly, all users are issued certificates for their identities by
   the enrollment server.  All resources stored on the overlay must be
   signed by their creator, thus ensuring that an attacker cannot forge
   data belonging to another user.  The enrollment process is only
   required to join the overlay (and perhaps to refresh an expired
   certificate).  The peers and users do not need to have real-time
   access to the enrollment server.

   TLS or DTLS are used for communication between peers.  In combination
   with the certificates, this provides both confidentiality and
   authentication for communication across the overlay.  Applications
   such as P2PSIP can also make use of the users' certificates to
   achieve secure end-to-end connections at the application layer.

   RELOAD also provides for an authorization framework based on
   certificates.  Each usage defines the access control rules for which
   users/peers are allowed to read/write each Resource-ID.  This
   protects data belonging to one user from being written by another, as
   well as allowing for a distributed quota mechanism based on the
   maximum size of each kind of data.

   In addition to the enrollment server model, RELOAD offers a security
   model using a pre-shared-key.  Although this provides significantly

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   less security than is provided through an enrollment server, it
   allows ad hoc or ephemeral overlays to be set up with minimal effort
   on the part of the users.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   We use the terminology and definitions from the Concepts and
   Terminology for Peer to Peer SIP [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.

   The following important terms from the Concepts document are defined
   below for reference.

   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.
   DHT Algorithm:  A DHT algorithm defines the rules for determining
      which peers in a DHT store a particular piece of data and for
      determining a topology of interconnections amongst peers in order
      to find a piece of data.
   DHT Instance:  A specific hash table and the collection of peers that
      are collaborating to provide read and write access to it.  There
      can be any number of DHT instances running in an IP network at a
      time, and each operates in isolation of the others.
   P2P Network:  Another name for a DHT instance.
   P2P Network Name:  A string that identifies a unique P2P network.
      P2P network names are DNS names - for example, "example.org".
      Lookup of such a name in DNS returns services associated with the
      DHT, such as enrollment servers, bootstrap peers, or gateways (for
      example, a SIP gateway between a traditional SIP and a P2P SIP
      network called "example.com").
   Resource-ID:  A value that is not human friendly to read and
      identifies some resources and which is used as a key for storing
      and retrieving the resource.  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.

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   Peer:  A host that is participating in the DHT.  By virtue of its
      participation it can store data and is responsible for some
      portion of the overlay.
   Peer-ID:  A value that uniquely identifies a peer.  Peer-IDs 0 and
      2^N - 1 are reserved and are invalid peer-IDs.  A value of zero is
      not used in the wire protocol but can be used to indicate an
      invalid peer in implementations and APIs.  The peer-id of 2^N-1 is
      used on the wire protocol as a wildcard.
   Resource:  An object associated with a string identifier.  In
      unstructured P2P networks, the identifier is used directly as a
      Resource-Id.  In structured P2P networks the identifier can be
      mapped into a Resource-ID by using the string as the input to the
      hash function.  A SIP resource, for example, is identified by its
   User:  A human being.

   We also introduce the following important new terms.

   Connection Table:  The set of peers to which a peer is directly
      connected.  This includes peers with which CONNECT handshakes have
      been done but which have not sent any UPDATEs.
   Routing Table:  The set of peers which a peer can use to route DHT
      messages.  In general, these peers will all be on the connection
      table but not vice versa, because some peers will have CONNECTed
      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.
   Hashed-ID:  The generic term for an identifier in the hash space of
      the DHT.  Examples of Hashed-IDs include Resource-IDs and Peer-
      IDs.  This only applies to structured overlays.
   Unhashed-ID:  An Unhashed-ID is a string used as an input to a hash
      function, the result of which is a Hashed-ID.  This only applies
      to structured overlays.
   Usage:  A usage is an application that wishes to use the DHT for some
      purpose.  Each application wishing to use the DHT 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.
   Destination List:  A list of IDs through which a message is to be
      routed.  This allows for request/response source routing.  A
      single ID is a trivial form of destination list.

3.  Overview

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3.1.  Distributed Storage Layer

   RELOAD is designed to be extensible to both structured and
   unstructured overlays.  However, this version is only completely
   worked out for structured overlays such as DHTs.  The following text
   assumes structured overlays; in particular Resource-IDs are assumed
   to be fixed length for any given overlay, although the protocol
   allows them to be variable length to allow extension to unstructured

   Each logical address in the DHT where data can be stored is referred
   to as a Resource-ID.  A given peer will be responsible for storing
   data from many Resource-ID locations.  Typically literature on DHTs
   uses the term "key" to refer to a location in the DHT; however, in
   this specification the term key is used to refer to public or private
   keys used for cryptographic operations and the term Resource-ID is
   used to refer to a location for storage in the DHT.

3.1.1.  DHT Concepts

   While very early P2P systems used flood based techniques, some newer
   P2P systems locate resources using a Distributed Hash Table, or DHT
   to improve efficiency.  Peers are organized using a Distributed Hash
   Table (DHT) structure.  In such a system, every resource has a
   Resource-ID, which is obtained by hashing some keyword or value (an
   Unhashed-ID) that uniquely identifies the resource.  Resources can be
   thought of as being stored in a hash table at the entry corresponding
   to their Resource-ID.  The peers that make up the overlay network are
   also assigned an ID, called a Peer-ID, in the same hash space as the
   Resource-IDs.  A peer is responsible for storing all resources that
   have Resource-IDs near the peer's Peer-ID.  The hash space is divided
   up so that all of the hash space is always the responsibility of some
   particular peer, although as peers enter and leave the system a
   particular peer's area may change.  Messages are exchanged between
   the peers in the DHT as the peers enter and leave to preserve the
   structure of the DHT and exchange stored entries.  Various DHT
   implementations may visualize the hash space as a grid, circle, line,
   or hypercube.

   Peers keep information about the location of other peers in the hash
   space and typically know about many peers nearby in the hash space,
   and progressively fewer more distant peers.  We refer to this table
   of other peers as a Routing Table.  When a peer wishes to operate on
   a resource it consults the list of peers it is aware of and contacts
   the peer with the Peer-ID nearest the desired Resource-ID.  If that
   peer does not know how to find the resource, it either returns
   information about a closer peer it knows about, or forwards the
   request to a closer peer.  In this fashion, the request eventually

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   reaches the peer responsible for the resource, which then replies to
   the requester.

3.1.2.  DHT Topology

   Each DHT will have a somewhat different structure, but many of the
   concepts are common.  The DHT defines a large space of Resource-IDs,
   which can be thought of as addresses.  In many DHTs, the Resource-IDs
   are simply 128- or 160-bit integers.  Each DHT also has a distance
   metric such that we can say that Resource-ID A is closer to
   Resource-ID B than to Resource-ID C.

   Each peer in the DHT is assigned a Peer-ID and is "responsible" for
   the nearby space of Resource-IDs.  So, for instance, if we have a
   peer P, then it could also be responsible for storing data associated
   with Resource-ID P+epsilon as long as no other peer P was closer.
   The DHT Resource-ID space is divided so that some peer is responsible
   for each Resource-ID.

3.1.3.  Routing

   The way routing works in a DHT is specified by the specific DHT
   algorithm but the basic concepts are common to most systems.  Each
   peer maintains connections to some other set of peers N. There need
   not be anything special about the peers in N, except that the peer
   has a direct connection to them:  it can reach them without going
   through any other peer.  When it wishes to deliver a message to some
   peer P, it selects some member of N, N_i that is closer to P than
   itself (as a degenerate case, P may be in N).  The peer sends the
   message to N_i.  At this point two things can happen:

   Recursive Routing:  N_i repeats the same process as P, sending the
      message to one of its peers N_j.  This same process repeats until
      the message is delivered to N.
   Iterative Routing:  N_i consults its table of direct connections and
      selects a new peer N_j which is closer to N. It responds to the
      original sending peer with a redirect to N_j.  The original peer
      then sends the message to N_j, where the process repeats until the
      sending peer is redirected to N.

   The advantage of iterative routing is that it consumes less resources
   for the intermediate peers; they only have to send redirect messages
   rather than forwarding requests and responses.  The advantage of
   recursive routing is that it does not require the sending or
   receiving peer to have a rich set of connections to other nodes in
   the overlay.  Thus, iterative routing is problematic in NATed
   networks because there is no way to guarantee that a peer will be
   able to form a connection to whatever peer it is redirected to.  In

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   RELOAD, iterative routing is supported using the ROUTE-QUERY request.

   In most DHTs, the peers in N are selected in a particular way.  One
   common strategy is to have them arranged exponentially further away
   from yourself so that any message can be routed in a O(log(N)) steps.
   The details of the routing structure depend on the DHT algorithm,
   however, since it defines the distance metric and the structure of
   the connection table.

   In RELOAD, messages may either be REQUESTS or RESPONSES to REQUESTS.
   Requests are routed as described above.  In principle, responses
   could be routed independently from requests.  This is called
   "Asymmetric" routing because requests and responses will generally
   follow different paths through the network.  Asymmetric routing makes
   diagnosis of errors difficult because you need to be able to acquire
   debugging information at multiple locations.  In the alternative
   strategy, called "Symmetric" routing, as requests travel through the
   network they accumulate a history of the peers they passed through
   and responses are routed in the opposite direction so that they
   follow the same path in reverse.  RELOAD supports both flavors of

   Symmetric routing is easier to debug.  Symmetric routing is also
   required when the overlay topology is changing.  For example, when a
   new peer is joining the overlay, asymmetric routing cannot work
   because the response would not be able to reach the new peer until it
   has completed the joining process.  Symmetric routing solves this
   situation because the response is routed from the admitting peer
   through the bootstrap peer, thus relying on a path that is already
   known and established.  In order to implement symmetric routing,
   RELOAD provides the Via List (Section 3.2.2) feature.  Asymmetric
   routing, however, requires no state to be stored in the message (as a
   Via List) or in on-path peers.

   [[TODO:  again, this is a topic that needs WG discussion.  It seems
   like there are situations where symmetric is very desirable (e.g.,
   startup).  It's less clear that asymmetric will have a performance/
   state difference that will be significant.]]

   [[TODO:  it's not clear that symmetric/asymmetric is really that
   evocative a terminology.  Henning suggested key-based/trace-based but
   EKR doesn't like that.  Other suggestions?]]

3.1.4.  Storing and Retrieving Structured Data

   The Data Storage Layer provides operations to STORE, FETCH, and
   REMOVE data.  Each location in the DHT is referenced by a single
   integer Resource-ID.  However, each location may contain data

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   elements corresponding to multiple kinds (e.g., certificate, SIP
   registration).  Similarly, there may be multiple elements of a given

                       |            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.  Note that a kind may be employed by multiple usages and new
   usages are encouraged to use previously defined kinds where possible.
   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.  We define the following data
   models in this document, though other usages can define their own

   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 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.

3.1.5.  Joining, Leaving, and Maintenance

   When a new peer wishes to join the DHT, it must have a peer-id that
   it is allowed to use.  It uses one of the peer-ids in the certificate
   it received from the enrollment server.  The main steps in joining
   the DHT are:

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

   First, the peer ("JP," for Joining Peer) uses the bootstrap
   procedures to find some (any) peer in the DHT.  It then typically
   contacts the peer which would have formerly been responsible for the
   peer's Resource-ID (since that is where in the DHT the peer will be
   joining), the Admitting Peer (AP).  It copies the other peer's state,
   including the data values it is now responsible for and the
   identities of the peers with which the other peer has direct

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

   1.  JP sends a JOIN request to AP announcing its intention to join.
   2.  AP sends an JOIN response.
   3.  AP does a sequence of STOREs to JP to give it the data it will
   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 DHT.
   5.  JP makes its own connections to the appropriate peers in the DHT.

   After this process is completed, JP is a full member of the DHT and
   can process STORE/FETCH requests.

3.2.  Forwarding Layer

   The forwarding layer is responsible for looking at message and doing
   one of three things:

   o  Deciding the message was destined for this peer and passing the
      message up to the layer above this.
   o  Looking at the peer-id that represents the next peer to send the
      message too and if there is an existing connection, sending the
      message over the connection.
   o  Requesting the DHT Routing logic to tell the forwarding layer
      which peer the message needs to be forwarded to (based on the
      target peer-id or resource-id), and then sending the message.

3.2.1.  Forming Direct Connections

   As described in Section 3.1.3, a peer maintains a set of direct
   connections to other peers in the DHT.  Consider the case of a peer
   JP just joining the DHT.  It communicates with the admitting peer AP

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   and gets the list of the peers in AP's routing table.  Naively, it
   could simply connect to the IP address listed for each peer, but this
   works poorly if some of those peers are behind a NAT or firewall.
   Instead, we use the CONNECT request to establish a connection.

   Say that peer A wishes to form a direct connection to peer B. It
   gathers ICE candidates and packages them up in a CONNECT request
   which it sends to B through usual DHT 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 DHT peers.

   In general, a peer needs to maintain connections to all of the peers
   near it in the DHT and to enough other peers to have efficient
   routing (the details depend on the specific DHT).  If a peer cannot
   form a connection to some other peer, this isn't necessarily a
   disaster; DHTs 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.

3.2.2.  Via Lists

   In a general messaging system, messages need a source and a
   destination and peers need to be able to send a response to the peer
   that sent the request.  This can be particularly tricky in overlay
   networks when a new peer is joining, or the overlay network is
   stabilizing and different peers have different ideas on what the
   overlay topology is.  A simple and reliable way to make sure that a
   response can reach the node that sent the request in these situations
   is to have the response traverse the reverse path of the request.

   The approach used here is to have each node the request traverses add
   its peer-id to the "via list" in the request.  Then the response is
   routed by looking at the list and using it as list of peers that the
   response will be routed thorough.  To support this, each message has
   a destination list of nodes it needs to be routed through as well as
   a via list of what nodes it has traversed.

   When a peer receives a message from the Transport Layer, it adds the
   peer-id of the node it received the message from to the end of the
   via list.  When a peer goes to transmit a message to the Transport
   Layer, it looks at the first entry on the destination list.  If the
   entry is this peer, it removes this entry from the list and looks at
   the next entry and if the entry is not this peer, it sends the
   message to the first peer on the destination list.

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   When a peer goes to send a response to a request, it can simply copy
   the via list in reverse to form the destination list for the response
   if it wishes to route the response along the reverse path as the

   Peers that are willing to maintain state may do list compression for
   privacy reason and to reduce the message size.  They do this by
   taking some number of entries off the via list and replacing them
   with a unique entry that this peer can later identify.  Later, if the
   peer sees the unique entry in a destination list, it removes the
   unique entry and replaces it with the all the entries removed from
   the original via list (and reverses the order of these entries).
   Note that this technique will generally require storing some per-
   message state on the intermediate peer, so this is a bandwidth/
   per-peer state tradeoff.  The exception is if the list is not
   compressed but rather the peer-ids are simply encrypted.

   The via list approach provides several features.  First it allows a
   response to follow the same path as the request.  This is
   particularly important for peers that are sending requests while they
   are joining and before other peers can route to them as well as
   situations where message are being exchanged to stabilize the overlay
   network.  It also makes it easier to diagnose and manage the system
   when all peers see the response to any request they forward.

3.2.3.  Clients

   RELOAD also allows for the possibility of client nodes.  A client is
   a node with a peer-id which connects to an admitting peer (or peers)
   like an ordinary peer but never sends a JOIN or an UPDATE.  It is
   therefore in the AP's connection table but not its routing table and
   never is used to store any DHT data.  However, because it is
   reachable through the AP, it can still send and receive messages.
   The client MUST still have the usual credentials.  Also, because it
   never sends JOINs it is never responsible for storing data.

   Because the client may only have a connection to a single AP, which,
   due to topology shifts may no longer be the responsible peer, clients
   SHOULD use symmetric routing and should advertise route lists that
   contain both the AP to which they are connected and themselves.
   E.g., if the client has peer-id X and the AP has peer-id Y, the
   client should advertise the destination list (Y, X).  This guarantees

   Note that clients MAY also contact APs which are not in fact
   responsible for the client's peer-id.

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3.3.  Transport Layer

   The transport layer sends and receives messages over TLS and DTLS.
   For TLS it simply pushes the messages into the stream.  For DTLS it
   takes care of fragmentation issues.  The reason for including TLS is
   the improved performance it can offer for bulk transport of data.
   The reason for including DTLS is that the percentage of the time that
   two devices behind NATs can form a direct connection without a relay
   is much higher for DTLS than for TLS.  If all NATs were
   [I-D.ietf-behave-tcp] compliant, then TLS over TCP would be

3.4.  Enrollment

   Before a new user can join the DHT for the first time, they must
   enroll in the P2P Network for the DHT they want to join.  Enrollment
   will typically be done by contacting a centralized enrollment server.
   Other approaches (for instance static out of band configuration) are
   possible but are outside the scope of this specification.  During
   enrollment a new node learns about a particular overlay, sets up a
   names and credentials, and discovers the bootstrap nodes.  This would
   typically be done when a new peer joined an overlay for the very
   first time.  Bootstrap is the process that happens each time a node
   boots and is how the peer finds an node that can be used to join the

   Before a node can join an overlay, it needs to be provided with a
   name for the overlay.  Some examples are "example.com", "example",
   and "example.local".  This name is resolved via a DNS SRV lookup for
   service name p2p_enroll and a protocol of tcp.  If the TLD for the
   name is .local, then this DNS SRV lookup is done using
   [I-D.cheshire-dnsext-multicastdns] and the service name p2p_menroll.
   The intention here is to support ad hoc/local overlays.  The
   resulting DNS lookup will provide the address of a enrollment server.
   Once this server is found, HTTPS is used to retrieve a XML file that
   contains the parameters for the overlay.  These include things such
   as:  what algorithms the overlay uses, overlay parameters, what
   usages are a peer on this overlay is required to support, the type of
   credentials required, addresses of credentials servers, the root
   certificate for the DHT, information about the DHT algorithm that is
   being used, a P2P-Network-Id that uniquely identifies this ring, and
   any other parameters it may need to connect to the DHT.  The DHT also
   informs the peers what Usages it is required to support to be a peer
   on this P2P Network.  It also provides an initial list of bootstrap
   nodes that consists of multiple bootstrap entries that each have the
   IP address and port for contacting a bootstrap peer.  Some of the
   address may be multicast addresses.  In the case of multicast DNS,
   every peer may also act as an enrollment server.

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   If shared-key security (Section 3.5.2) is being used, then the peer
   can proceed directly to bootstrap.  If certificate-based security
   (Section 3.5.1 is being used, the peer MUST contact the credential
   server to obtain a certificate.

3.4.1.  Certificate Issuance

   Once the peer has the XML file that identifies if credentials are
   needed, it can contact the credential server.  The user establishes
   his identity to the server's satisfaction and provides the server
   with its public key.  The centralized server then returns a
   certificate binding the user's user name to his public key.  The
   properties of the certificate are discussed in Section 3.5.  The
   amount of authentication performed here can vary radically depending
   on the DHT network being joined.  Some networks may do no
   verification at all and some may require extensive identity
   verification (e.g., checking a driver's license) before issuing
   credentials for a given user name.  The only invariant that the
   enrollment server needs to ensure is that no two users may have the
   same identity.

3.4.2.  Bootstrap

   The above steps are only done the first time a peer joins a new
   overlay or when the overlay parameters are close to their expiration
   time (as listed in the configuration document) and need to be
   refreshed.  The next step is the bootstrap step which is done every
   time the peer boots.

   Bootstrapping consists of looking at the list of cached nodes and
   bootstraps nodes and sending a RELOAD PING to them to see if they
   respond.  Once a node responds, it can be used to join the overlay.
   After a node has joined, it keeps track of a small number of peers to
   which it could directly connect.  Theses are saved as the cached
   nodes and used next time the peer boots.  The point of the cached
   nodes is to reduce the load on the bootstrap nodes.

3.5.  Security

3.5.1.  Certificate-Based Security

   The certificate-based security model revolves around the enrollment
   process allocating a unique name to the user and issuing a
   certificate [RFC3280] for a public/private key pair for the user.
   All peers in a particular DHT can verify these certificates.  A given
   peer acts on behalf of a user, and that user is responsible for its

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   The certificate serves two purposes:

   o  It entitles the user to store data at specific locations in the
      DHT.  Each usage defines the specific rules for determining which
      certificates can access each Hashed-ID/kind-id pair.  For
      instance, some usages might allow anyone to write at a given
      location, whereas others might restrict writes to a single
   o  It entitles the user to operate a peer that has a peer-id found in
      the certificate.  When the peer is acting as a DTLS or TLS server,
      it can use this certificate so that a client connecting to it
      knows it is connected to the correct server.

   When a user enrolls, or enrolls a device with no keying material, the
   user is given a certificate.  This certificate contains information
   that identifies the user and the device they are using.  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.

   The contents of the certificate include:

   o  A public key provided by the user.
   o  Zero or more user names that the DHT is allowing this user to use.
      For example, "alice@example.org".  Typically a certificate will
      have one name.  In the SIP usage, this name corresponds to the
   o  Zero or more peer-ids.  Typically there will be one peer-id.  Each
      device will use a different peer-id, even if two devices belong to
      the same user.  Peer-IDs should be chosen randomly by the
      enrollment server.
   o  A serial number that is unique to this certificate across all the
      certificates issued for this DHT.
   o  An expiration time for the certificate.

   Note that because peer-IDs are chosen randomly, they will be randomly
   distributed with respect to the user name.  This has the result that
   any given peer is highly unlikely to be responsible for storing data
   corresponding to its own user, which promotes high availability.  Storage Permissions

   When a peer uses a STORE request to place data at a particular
   location X, it must sign with the private key that corresponds to a
   certificate that is suitable for storing at location X. Each data
   kind in a usage defines the exact rules for determining what
   certificate is appropriate.

   The most natural rule is that a certificate with user name X "owns"

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   data located at Hash(X) (X is the Unhashed-ID and Hash(X) is the
   Hashed-ID) and only he can write there.  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 DHT.  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 undetectably
   modify it.  Peer Permissions

   The second purpose of a certificate is to allow the device to act as
   a peer with the specified peer-ID.  When a peer wishes to connect to
   peer X, it forms a TLS/DTLS connection to the peer and then performs
   TLS mutual authentication and verifies that the presented certificate
   contains peer-ID X.

   Note that because the formation of a connection between two nodes
   generally requires traversing other nodes in the DHT, as specified in
   Section 3.2.1, those nodes can interfere with connection initiation.
   However, if they attempt to impersonate the target peer they will be
   unable to complete the TLS mutual authentication:  therefore such
   attacks can be detected.  Expiry and Renewal

   At some point before the certificate expires, the user will need to
   get a new certificate from the enrollment server.

3.5.2.  Shared-Key Security

   RELOAD also defines a shared-key security model which can be used in
   closed networks where the peers are not mutually suspicious.  In this
   model, the peers all share a single key which is used to authenticate
   the peer-to-peer DTLS connections via TLS-PSK/TLS-SRP.  If shared-key
   security mode is in use, a shared-key capable cipher suite such as
   TLS-PSK or TLS-SRP MUST be used.  This is useful for admission
   control, but is completely unsafe in any setting where peers are not
   mutually trusted, since it allows any peer to impersonate any other

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3.6.  Migration

   At some point in time, a given P2P Network may want to migrate from
   one underlying DHT algorithm to another or update to a later
   extension of the protocol.  This can also be used for crypto agility
   issues.  The migration approach is done by having peers initializing
   algorithm A. When the clients go to periodically renew their
   credentials, they find out that the P2P Network now requires them to
   use algorithm A but also to store all the data with algorithm B. At
   this point there are effectively two DHT rings in use, rings A and B.
   All data is written to both but queries only go to A. At some point
   when the clients periodically renew their credentials, they learn
   that the P2P Network has moved to storing to both A and B but that
   FETCH requests are done with P2P Network B and that any SEND should
   first be attempted on P2P Network B and if that fails, retried on P2P
   Network A. In the final stage when clients renew credentials, they
   find out that P2P Network A is no longer required and only P2P
   Network B is in use.  Some types of usages and environments may be
   able to migrate very quickly and do all of these steps in under a
   week, depending on how quickly software that supports both A and B is
   deployed and how often credentials are renewed.  On the other hand,
   some very ad-hoc environments involving software from many different
   providers may take years to migrate.

   [[TODO:  This needs more filling out]]

3.7.  Usages Layer

   By itself, the distributed storage layer just provides infrastructure
   on which applications are built.  In order to do anything useful, a
   usage must be defined.  Each Usage needs to specify several things:

   o  Register kind-id code points for any kinds that the Usage defines.
   o  Define the data structure for each of the kinds.
   o  Define access control rules for each kinds.
   o  Provide a size limit for each kinds.
   o  Define how the Unhashed-ID is formed that is hashed to form the
      Resource-ID where each kind is stored.
   o  Describe 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 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

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      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.

3.7.1.  SIP Usage

   From the perspective of P2PSIP, the most important usage is the SIP
   Usage.  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 Bob's SIP UA can use to pass SIP messages back and
   forth to Alice's SIP UA.

   This is done using three key operations that are provided by the SIP
   Usage.  They are:

   o  Mapping SIP URIs that are not GRUUs to other SIP URIs or to the
      DHT peer responsible for the SIP UA.
   o  Mapping SIP GRUUs to the DHT peer responsible for the SIP UA.
   o  Forming a connection directly to a DHT peer that is used to send
      SIP messages to the SIP UA.

   All SIP URIs for a given overlay MUST be constructed so that they
   terminate in the domain name of the overlay.  For instance, if the
   overlay name is "example.com", then all AORs must be of the form
   {sip,sips}:username@example.com.  Accordingly, to dereference a URI,
   a P2PSIP implementation MUST check to see if the domain matches an
   overlay which it is a member of.  If so, it uses the following
   procedures.  Otherwise, it MUST follow [RFC3263] procedures.  Note
   that unless the P2PSIP overlay provides some kind of gateway to
   ordinary SIP (e.g., a publicly accessible SIP server) this is likely
   to be only partially successful, since, for instance, the callee may
   not be able to call back.  SIP Location

   A peer acting as a SIP UA stores their registration information in
   the DHT by storing either another URI (for retargeting) or a
   destination lists to reach them at a Resource-ID in the DHT formed
   from the user's SIP AOR.  When another peer wishes to find a peer
   that is registered for a SIP URI, the lookup of the user's name is
   done by taking the user's SIP Address or Record (AOR) and using it as
   the Unhashed-ID that is hashed to get a Resource-ID.  When the
   Unhashed-ID is dereferenced, the result is a set of values.  Each
   value is either another SIP URI or a destination list.  If the value
   is a SIP URI, the calling peer looks up that URI and continues the

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   process until it gets a destination list.

   If the value is a destination list, then it is used to reach a peer
   that represents a SIP UA registered for that AOR.  Typically this
   destination list will have just one entry but in the case of peers or
   clients that can not be directly reached (for instance via a strict
   NAT or firewall), a destination list with more than one entry may
   need to be used.

   The Unhashed-ID for this usage is a user's SIP AOR, such as
   "sip:alice@example.com".  This allows the set to store many values in
   a dictionary structure.  The authorization policy is that STORE
   requests are only allowed if the user name in the signing
   certificate, when turned into a SIP URL and hashed, matches the
   Resource-ID.  This policy ensures that only a user with the
   certificate with the user name "alice@example.com" can write to the
   Resource-ID that will be used to look up calls to

   [[Open Issue:  Should the Unhashed-ID be "sip:alice@example.com",
   "alice@example.com", or a string that includes the code point defined
   for the kind?  The issue here is determining whether different usages
   that store data at a Unhashed-ID that is primarily formed from
   "alice@example.com" should hash to the same Resource-ID as the SIP
   Usage.  For example, if a buddy list had a Unhashed-ID that was
   roughly the same, would we want the buddy list information to end up
   on the same peers that stored the SIP location data or on different
   peers?]]  SIP GRUUs

   GRUUs that refer to peers in the P2P network are constructed by
   simply forming a GRUU, where the value of gr URI parameter contains a
   base64 encoded version of the destination list that will reach the
   peer.  Typically the destination list is just a single entry with the
   peer-id of peer.  SIP Connect

   This usage allows two clients to form a new TLS or DTLS connection
   between them and then use this connection for sending SIP messages to
   one another.  This does not store any information in the DHT, but it
   allows the CONNECT request to be used to set up a TLS or DTLS
   connection between two peers and then use that connection to send SIP
   messages back and forth.

   The CONNECT request will ensure that the connection is formed to a
   peer that has a certificate which includes the user that the

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   connection is being formed to.  SIP Tunnel

   This TUNNEL request allows two peers to exchange SIP messages across
   the overlay using the TUNNEL method without first setting up a direct
   connection using CONNECT.  This allows a SIP message to be sent
   immediately, without the delay associated with CONNECT and for a
   simple SIP exchange, it may result in fewer messages being sent.

3.7.2.  Certificate Store Usage

   This usage allows each user to store their certificate in the DHT so
   that it can be retrieved to be checked by various peers and
   applications.  Peers acting on behalf of a particular user store that
   user's certificate in the DHT, and any peer that needs the
   certificate can do a FETCH to retrieve the certificate.  Typically it
   is retrieved to check a signature on a request or the signature on a
   chunk of data that the DHT has received.

3.7.3.  TURN Usage

   This usage defines a new kind for finding STUN-Relay servers.  Any
   peer that supports this usage saves a pointer to the IP address and
   port of the TURN server in the DHT.  When a peer wishes to discover a
   TURN server, it picks a random Resource-ID and performs a FIND at
   that Resource-ID for the appropriate type for the service.  If
   nothing is found, this can be repeated until an appropriate set of
   servers are found.

3.7.4.  Diagnostic Usage

   This usage defines several new kinds that be queried to find
   information about the peer that may be useful for monitoring and
   diagnostics.  This includes information such as software version,
   neighbor information, and performance statistics.

3.7.5.  HIP Tunnel Usage

   This usage allows two peers running HIP to tunnel HIP messages across
   the overlay.  This allows the HIP peers to use the overlay as a
   rendezvous system to set up a direct path using HIP NAT traversal

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4.  Base Protocol

   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, just
   define theses in a fixed position as adding a type and length for
   them is unnecessary and would simply increase bandwidth and
   introduces new potentials for interoperability issues.

   Each message has three parts:

   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

4.1.  Forwarding Header

   The layout of the forwarding header is shown below

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |1|     R       |       E       |       L       |       O       |
   4  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Overlay                             |
   8  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |               |F|L|                           |
      |     TTL       |   Reserved    |R|F|      Fragment Offset      |
      |               |               |A|R|                           |
      |               |               |G|G|                           |
   12 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |                                               |
      |    Version    |                    Length                     |
      |               |                                               |
   16 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Transaction ID                         |
      +                                                               +
      |                                                               |
   24 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Via       |     Route     |                               |
      |     List      |     List      |            Flags              |
      |     Length    |     Length    |                               |
   28 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //                           Via List                          //
      |                                                               |
      |                                                               |
      //                          Destination List                   //
      |                                                               |
      |                                                               |
      //                          Route Log                          //
      |                                                               |

   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.

   The Overlay field is 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.

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   TTL (time-to-live) is 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.

   FRAG is a 1 bit field used to specify if this message is a fragment.

   NOT-FRAGMENT    : 0x0
   FRAGMENT        : 0x1

   LFRG is a 1 bit field used to specify whether this is the last
   fragment in a complete message.

   LAST-FRAGMENT        : 0x1

   [[Open Issue:  How should 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 is a 7 bit field that indicates the version of the RELOAD
   protocol being used.

   Version1.0       : 0x1

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

   The Transaction ID is 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

   The Destination List Length and the Via List Length contain the
   lengths of the route and via lists respectively, in the number of

   [[Open Issue:  How should we handle peer-id lengths?  This basically
   assumes they're fixed length per DHT algorithm (but not fixed-length
   for RELOAD) so that you can unambiguously parse things.  Should we
   have a length byte?]]

   The flags word contains control flags.  There is one currently
   defined flag.

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   ROUTE-LOG       : 0x1

   The ROUTE-LOG flag indicates that the route log should be included
   (see Section 4.1.4

   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.  Destinations are defined at the end of
   this section.

   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.

   If a message was being sent thought the sequences of peers A,B,C,D,
   the message from A to B would have a empty via list and a route of
   list of B,C,D. The message from B to C would have a via list of A
   then route of C,D and so on.  This means that when the route list is
   followed exactly, all that is needed to update these lists is to
   change their lengths.  This avoids the need to change or move any of
   the other list entries.  In other cases, some entries may need to be
   copied or moved.

   The destination list and via lists a list of objects of type

   STRUCTURE: destination_object
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                                                               |
       /                                                               /
       |                              Info                             |
       /                                                               /
       |                                                               |

   STRUCTURE: peer
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x00     |                                               |
   004 +-+-+-+-+-+-+-+-+                                               +
       |                              Peer                             |
   008 +                                                               +

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       |                                                               |
   012 +                                                               +
       |                                                               |
   016 +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               |
   017 +-+-+-+-+-+-+-+-+

   STRUCTURE: compressed
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x01     | Compressed Len|                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                           Compressed                          |
       /                                                               /
       |                                                               |

   STRUCTURE: resource
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x02     |  Resource Len |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                            Resource                           |
       /                                                               /
       |                                                               |

   A destination_object can have one of three types (this is

   peer:  A peer-id.  All peer-ids are of fixed length for a given
      overlay and therefore peer-ids have no length.
   compressed:  A compressed list of peer-ids and/or resources.  This
      value is variable length but because it was compressed by one of
      the peers, it is only meaningful to that peer and cannot be
      decoded by other peers.
   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.

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4.1.1.  Changes to Forwarding Header

   The RELOAD-01 forwarding header was completely fixed, whereas this
   header includes lists that change en-route.  However, this type of
   operation is easily accomplished in both software and hardware,
   therefore we still view it as a low-overhead header.  The changes
   include the following.

   o  Rearranged fields to have a cleaner separation between payload and
   o  Removed DHT, Hash, and Security parameters.  These are now in the
      overlay bootstrap system rather than per-message.
   o  Source and destination IDs are now destination lists to
      accommodate source routing and recursion without state on
      intermediate peers.
   o  Added route log to header to allow payload/header separation.

4.1.2.  Message Routing  Request Origination

   In order to send a message to a given peer-id or resource-id, a peer
   must construct an appropriate destination list.  The most common such
   destination list is a single entry containing the peer/resource-id.
   This simply uses the normal DHT routing mechanisms to forward the
   message to that destination.

   Messages can also be source routed.  In order to construct a source
   route, the originator provides a destination list containing a
   sequence of resource-ids.  The semantics of this destination list are
   that the message is to traverse in order (potentially with
   intermediate hops) each entry on the destination list.  As each peer
   is traversed, that entry is removed from the destination list.  This
   makes it possible to address a peer which is potentially behind a NAT
   or a firewall in such a way that it cannot be connected to directly
   under any circumstances.

   [[TODO:  Salman has suggested the originator doing parallel requests/
   responses.  This is an open issue.]  Response Origination

   When a peer sends a response to a request, it SHOULD construct the
   destination list by reversing the order of the entries on the via
   list.  This has the result that the response traverses (at least) the
   same peers as the request traversed, except in reverse order
   (symmetric routing).  For asymmetric routing, the peer MAY simply use
   the first entry on the via list.

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   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 MAY simply
   drop the message.

   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:

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

   These cases are handled separately.  Private ID

   If the first entry on the destination list is a private id, the peer
   replaces that entry with the store local value that it indexes and
   then re-examines the destination list to determine which case now
   applies.  Responsible ID

   If the first entry on the destination list is a Hashed-ID for which
   the peer is responsible, the peer strips the entry off the route
   list.  If there are remaining entries on the destination list, the
   peer then re-examines the destination list to determine which case
   now applies.  If the destination list is now empty, then the message
   was destined for this peer and it MUST pass it to the next layer up.  Other Hashed-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 in its route table which
   is closer to the first entry than to itself and send the message to
   that peer.  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.

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   When forwarding a message, the peer MUST:

   o  Decrement the TTL value
   o  Update the via list.

   The natural way to update the via list is simply to add the peer-id
   of the peer from which the message was received to the end of the
   list.  However, peers may use any algorithm of their choice provided
   that if the peer received a destination list constructed by reversing
   the via list it would be able to route the outgoing message
   correctly, enabling symmetric routing.

   For instance, if node D receives a message from node C with via list
   (A, B), the simple approach is simply to 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 (B, A).  However, node D could also list
   compression and send E the via list (X).  E would then use the
   destination list (D, X).  When D processes this destination list, it
   MUST detect that X is a compressed entry, recover the via list (A, B,
   C), and reverse that to produce the correct destination list (C, B,
   A) before sending it to C.

   Note that if a peer is using list compression and then exits the
   overlay, the message cannot be forwarded and will be dropped.  The
   ordinary timeout and retransmission networks provide stability over
   this type of failure.

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 prevent denial of
   service attacks, receivers SHOULD time out incomplete fragments.
   [[TODO:  Describe algorithm]]

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4.1.4.  Route Logging

   The route logging feature provides diagnostic information about the
   path taken by the request so far and in this manner it is similar in
   function to SIP's [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.  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 in a request, the route log MUST
   be copied into the response and the ROUTE-LOG flag set so that the
   originator receives the ROUTE-LOG data.

   If the responder wishes to have a route log in the reverse direction,
   it MAY set the ROUTE-LOG flag in its response as well.  Note,
   however, that this 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.

   STRUCTURE: route_log
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |          Entries Len          |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                            Entries                            |
       /                                                               /
       |                                                               |

   The route log is simply a variable length list of route log entries.
   The first two bytes are the length, followed by a sequence of route
   leg entries, each of which may be individually parsed.

  STRUCTURE: route_log_entry
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |          Version Len          |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                            Version                            |
      /                                                               /
      |                                                               |

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      |   Transport   |                                               |
      +-+-+-+-+-+-+-+-+                                               +
      |                               Id                              |
      +                                                               +
      |                                                               |
      +                                                               +
      |                                                               |
      +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |                     Uptime                    |
      |               |        Certificate Len        |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
      |                          Certificate                          |
      /                                                               /
      |                                                               |
      |                                                               |
      /                                                               /
      |                            Address                            |
      /                                                               /
      |                                                               |

  STRUCTURE: ip4_address_type
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |      0x01     |                      Addr                     |
  004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |              Port             |
  007 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  STRUCTURE: ip6_address_type
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |      0x02     |                                               |
  004 +-+-+-+-+-+-+-+-+                                               +
      |                              Addr                             |
  008 +                                                               +
      |                                                               |
  012 +                                                               +
      |                                                               |
  016 +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |              Port             |
  019 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   Each route log entry consists of the following values:

   Version -  A textual representation of the software version
   Transport -  The transport type, 1 for TLS, 2 for DTLS
   Id -  The peer-id of the peer.
   Uptime -  The uptime of the peer in seconds.
   Certificate -  The peer's certificate.  Note that this may be omitted
      by setting the length to zero.
   Address -  The address and port of the peer.  This can be either an
      IPv4 or IPv6 address.

4.2.  Message Contents Format

   Although from the perspective of the forwarding layer the content is
   opaque, all RELOAD messages share a common content structure
   consisting of two parts:

   Common Header:  A common header containing the request method/
      response code, and a transaction ID.
   Payload:  The actual body of the request/response.  These are
      dependent on whether this is a request or response and the type of
      request being carried.

4.2.1.  Common Header

   The layout of the common header is shown below:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |                                 |                             |
      |        Message Code             |           Reserved          |
      |                                 |                             |
    4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message Code is a 16 bit field that indicates which message this is.
   This field 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_Q (the PING request)
      has value 1 and PING_A (the PING response) has value 2

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   0x8000 .. 0xfffe  Reserved
   0xffff  Error

4.2.2.  Payload

   Payload is a simple string of uninterpreted bytes preceded by a
   length field indicating the length of the data, not including the
   length field.  The bytes themselves are dependent on the code value.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |                                                               |
      |                             Length                            |
      |                                                               |
    4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //                     Length bytes of data                    //
      |                                                               |

4.2.3.  Signature

   The Signature element is used to attach signatures to messages and or
   stored data elements.  All signatures are formatted using this
   element.  However, the input structure to the signature computation
   varies depending on the data element being signed.

   STRUCTURE: signature
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |   Algorithm   |      Signature Value Len      |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                        Signature Value                        |
       /                                                               /
       |                                                               |
       |                                                               |
       /                                                               /
       |                            Identity                           |
       /                                                               /
       |                                                               |

   The signature construct is just a container for the signature.  It
   contains the following values:

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   Algorithm -  The signature algorithm in use.  This may have the
      values RSA-SHA1 (0x01) or RSA-SHA-256 (0x02).
   Value -   The signature value itself.  This is just the string of
      bytes emitted by the signature algorithm.
   Identity -  The identity or certificate used to form the signature

   [[TODO:  Should we convert all of this to CMS?]]

   A number of possible identity formats are permitted, as shown below.
   The peer may indicate any of:

   o  Peer-id
   o  User name
   o  The certificate itself.

   The first byte of the identity field is a type indicating the type of
   identity in use.

   STRUCTURE: signer_object
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                                                               |
       /                                                               /
       |                             Signer                            |
       /                                                               /
       |                                                               |

   STRUCTURE: signer_identity_peer
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x01     |                                               |
   004 +-+-+-+-+-+-+-+-+                                               +
       |                      Signer Identity Peer                     |
   008 +                                                               +
       |                                                               |
   012 +                                                               +
       |                                                               |
   016 +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               |
   017 +-+-+-+-+-+-+-+-+

   STRUCTURE: signer_identity_name
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2

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       |      0x02     |    Signer Identity Name Len   |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                      Signer Identity Name                     |
       /                                                               /
       |                                                               |

   STRUCTURE: signer_identity_certificate
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x03     |Signer Identity Certificate Len|               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                  Signer Identity Certificate                  |
       /                                                               /
       |                                                               |

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

   STRUCTURE: message_signature_input
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                            Overlay                            |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
   008 +                                                               +
       |                              Xid                              |
   012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                                                               /
       |                        Signer Identity                        |
       /                                                               /
       |                                                               |
       |                                                               |
       /                                                               /
       |                        Message Contents                       |
       /                                                               /
       |                                                               |

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   The contents of this structure are as follows:

   Overlay -  The overlay identifier from the message.
   Xid -  The transaction id from the message.
   Signer Identity -  The identify of the signer (from the signature
   Message Contents -  The contents section of the message.

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

   The input to signatures over data values is different, and is
   described in Section

4.3.  Response Codes and Response Errors

   A peer processing a request returns its status in the Message Code
   field of the common header.  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.

   For any code other than 200, the payload should be as defined below:

   STRUCTURE: error_response
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |           Error Code          |       Reason Phrase Len       |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                                                               /
       |                         Reason Phrase                         |
       /                                                               /
       |                                                               |
       |         Error Info Len        |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                           Error Info                          |
       /                                                               /
       |                                                               |

   The contents of this payload are:

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   Error Code -   A numeric error code indicating the error that
   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 (e.g., "Moved Temporarily).
   Error Info -  Payload specific error information.  This MUST be empty
      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
   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
   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, DHT algorithm, or hash algorithm.
      [[Open Issue:  What is the best error number and reason phrase to
   499 (UnWilling To Proxy)  A peer receiving the request is unwilling
      to support the Routing mechanism specified in the Routing field of
      the message header.  [[Open Issue:  What is the best error number
      and reason phrase to use?]]

5.  End-to-End Timeout and Retransmission

   Timeout and retransmission are handled on an end-to-end basis as well
   as the transports providing a hop by hop reliability mechanism.  For
   end-to-end reliability, the requesting node retransmits a requests
   every 3 seconds until it receives a response or after it has send the
   request 5 times.  Retransmissions MUST use the same transaction ID.

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6.  Transports

   Currently multiple transport protocols are specified and more may be
   defined in the future.  Implementation MUST implement TLS and DTLS.
   A given overlay can choose which protocols it uses.

6.1.  TLS

   TLS runs on top of TCP which offers the best performance from a data
   transfer point of view and does not require as frequent keep alive

6.2.  DTLS

   DTLS runs on top of UDP which offers the highest probability of
   direct connectivity in the face of the current generation of consumer

6.2.1.  Reliability for Unreliable Transports

   When RELOAD is carried over DTLS or another unreliable transport, it
   needs to be used with a reliability and flow 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.  Message Format

   Each message being sent is prepended with a header that indicates the
   24 bit sequence number.

   STRUCTURE: header
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x01     |                      Seq                      |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Seq  The sequence number of the message.

   Each DTLS session has it own sequence number.  Initially the value is
   zero and it increments by exactly one for each message sent over that
   DTLS session.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x02     |                    Ack Seq                    |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Received                           |
   008 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 DTLS flow.  The contents of this
   packet are:

   Ack Seq -  The sequence number of the message being acknowledged.
   Received -  A bitmask indicating whether or not each of the previous
      32 packets has been received.  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.  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 use TFRC-
   SP[RFC4828] and use the received bitmask to allow the sender to
   compute packer loss event rates.  Senders MUST implement a
   retransmission and congestion control scheme no more aggressive then

6.3.  HIP

   RELOAD MAY also be used with a HIP transport using the architecture
   for HIP BONE described in [I-D.camarillo-hip-bone].  From the
   perspective of the P2P layer, HIP looks very much like normal IP.
   Either TLS (over TCP) or DTLS (over UDP) is run over top of the HIP.
   Thus the reliability and congestion control schemes are the same for
   DTLS section.  If an overlay is configured such that HIP is the only
   transport that it will use, then it may make sense to configure the
   p2p layer to only offer the ORCHID when gather candidate addresses

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   for ICE.  This will effectively disable ICE at the p2p layer.

   For overlays that use HIP, the enrollment server MUST provide each
   peer with a unique ORCHID and use that ORCHID to generate the peer-id
   for the peer (see Section 10.3.  Later when the HIP layer wishes to
   tunnel a message (such as an I1 message) through the overlay, the HIP
   layer can use the ORCHID to generate the peer-id, and then use the
   TUNNEL message with the HIP to route the message to that the peer
   that owns that ORCHID.

7.  Method Definitions

   In this section, we define the initial set of methods supported by
   RELOAD.  New methods are defined by adding new method codes.  Each
   method defines the contents of the payload element (see
   Section 4.2.2).

   PDUs are named using the following convention.  For method type FOO,
   the request PDU is named FOO_Q and the response PDU is named FOO_A
   (as are the method codes).  When discussing the PDU itself, we use
   these terms.  Throughout the rest of the document we refer to the FOO
   method or the FOO request/response for easier readability.

7.1.  Connection Management

7.1.1.  PING

   PING is used to test connectivity along a path.  A ping can be
   addressed to a specific peer-id or to the broadcast peer-id (all 1s).
   In either case, the target peer-ids respond with a simple response
   containing some status information.  Request Definition

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

   STRUCTURE: ping_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       | Ping Info Len |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                           Ping Info                           |
       /                                                               /
       |                                                               |

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   The two currently defined types are:

   NUM-RESOURCES   : 0x02

   RESPONSIBLE-SET indicates that the peer should Respond with the
   fraction of the overlay for which the responding peer is responsible
   (in parts per billion).

   NUM-RESOURCES indicates that the peer should Respond with the number
   of resources currently being stored by the peer.  Response Definition

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

   STRUCTURE: ping_a
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                                                               |
   004 +                                                               +
       |                          Response Id                          |
   008 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Infos Len           |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                             Infos                             |
       /                                                               /
       |                                                               |

   A PING_A 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
   Infos -  A sequence of ping info data structures, as shown below.

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  STRUCTURE: ping_info_data
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |                                                               |
      /                                                               /
      |                         Ping Info Data                        |
      /                                                               /
      |                                                               |

  STRUCTURE: info_responsible_type
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |      0x01     |             Info Responsible Type             |
  004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |
  005 +-+-+-+-+-+-+-+-+

  STRUCTURE: info_num_resources_type
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |      0x02     |            Info Num Resources Type            |
  004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |
  005 +-+-+-+-+-+-+-+-+

   The ping info data elements are simple typed elements, with a type
   identifier as the leading 16 bits and then arbitrary (type-specific)
   text following.  In the case of the two defined types, the responses
   are 32-bit integers.

   The responding peer SHOULD include any values that the requesting
   peer requested and that it recognizes.  They SHOULD be returned in
   the requested order.

7.1.2.  CONNECT

   A node sends a CONNECT 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 CONNECT and its response are described in
   Section 8.

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   Note:  A CONNECT does not result in updating the routing table of
      either node.  That function is performed by UPDATEs.  If node A
      has CONNECTed to node 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.  Request Definition

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

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   STRUCTURE: connect_data
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |           Ufrag Len           |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                             Ufrag                             |
       /                                                               /
       |                                                               |
       |          Password Len         |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                            Password                           |
       /                                                               /
       |                                                               |
       |          Application          |        Fingerprint Len        |
       |                                                               |
       /                                                               /
       |                          Fingerprint                          |
       /                                                               /
       |                                                               |
       |            Role Len           |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                              Role                             |
       /                                                               /
       |                                                               |
       |       Candidate List Len      |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                         Candidate List                        |
       /                                                               /
       |                                                               |

   STRUCTURE: candidate
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      Candidate String Len     |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                        Candidate String                       |
       /                                                               /
       |                                                               |

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   The values contained in connect-request 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.
   Fingerprint -  One fingerprint attribute (from RFC 4572 [RFC4572]).
   Role -  An active/passive/actpass attribute from RFC 4145 [RFC4145].
   Candidate -  One or more ICE candidate values.  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.

   These values should be generated using the procedures of Section 8.  Response Definition

   If a peer receives a CONNECT request, it SHOULD follow the procedures
   of Section 8 to process the request and generate its own response (a
   CONNECT_A) containing a connect_data object.  It should then begin
   ICE checks.  When a peer receives a CONNECT response, it SHOULD parse
   the response and begin its own ICE checks.

7.1.3.  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 CONNECT or when ICE is unable to establish a direct
   connection via CONNECT 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 CONNECT, 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.

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

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

   The TUNNEL_Q message contains the application PDU that the requesting
   peer wishes to transmit, along with some control information
   identifying the handling of the PDU.

   STRUCTURE: tunnel_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |          Application          |         Dialog Id Len         |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                                                               /
       |                           Dialog Id                           |
       /                                                               /
       |                                                               |
       |      Application Pdu Len      |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                        Application Pdu                        |
       /                                                               /
       |                                                               |

   The values contained in the TUNNEL_Q are:

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   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
   Application PDU -  An application PDU in the format specified by the
      application.  Response Definition

   A TUNNEL_A 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

7.2.  Data Storage and Retrieval

   The STORE, FETCH, and REMOVE methods are used to manipulate
   information in the DHT.  They form an instantiation of the abstract
   GET and PUT operations described in [I-D.ietf-p2psip-concepts].

7.2.1.  STORE

   The STORE method is used to store data in the overlay.  As described
   in Section 3.1.4, each location may contain data of multiple kinds.
   Each kind-id is a code point assigned to a specific application usage
   by IANA.  As part of the Usage 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
   item; some sets may be allowed to contain multiple identical items
   while others may only have unique items.  The protocol currently
   defines the following data models:

   o  single value
   o  array
   o  dictionary

   Each kind MUST specify the appropriate data model for that kind.  The
   format of the STORE request depends on the data model.

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   A STORE_Q message is a sequence of kind-data pairs, 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.

   STRUCTURE: store_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |  Resource Len |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                            Resource                           |
       /                                                               /
       |                                                               |
       |                         Store Data Len                        |
       |                                                               |
       /                                                               /
       |                           Store Data                          |
       /                                                               /
       |                                                               |

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

   Resource -  The resource to store at.
   Store Kind Data -  A series of elements, one for each kind of data to
      be stored.

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   STRUCTURE: store_kind_data
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                              Kind                             |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Data Model  |                                               |
   008 +-+-+-+-+-+-+-+-+                                               +
       |                           Generation                          |
   012 +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               |           Values Len          |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                             Values                            |
       /                                                               /
       |                                                               |

   Each store kind data 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
   Data Model -  The data model of the data.
   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.

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   STRUCTURE: stored_data
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                             Length                            |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
   008 +                                                               +
       |                          Storage Time                         |
   012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Lifetime                           |
   016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                                                               /
       |                           Data-value                          |
       /                                                               /
       |                                                               |
       |                                                               |
       /                                                               /
       |                           Signature                           |
       /                                                               /
       |                                                               |

   Each stored_data element represents a single stored data value.
   These elements are individually signed.  The contents of the element
   are as follows:

   Length -  The length of the stored data element.
   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 known to
      the receiving peer 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.
   Signature -  A signature over the data value.  Section
      describes the signature computation.  The element is formatted as
      described in Section 4.2.3
   Data Value -  The data value itself, as described below.

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   STRUCTURE: single_value_entry
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |           Value Len           |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                             Value                             |
       /                                                               /
       |                                                               |

   STRUCTURE: array_entry
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                             Index                             |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Value Len           |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                             Value                             |
       /                                                               /
       |                                                               |

   STRUCTURE: dictionary_entry
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |            Key Len            |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                              Key                              |
       /                                                               /
       |                                                               |
       |           Value Len           |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                             Value                             |
       /                                                               /
       |                                                               |

   The responsible peer MUST perform the following checks:

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

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   o  The signatures over each individual data element (if any) are
   o  Each element is signed by a credential which is authorized to
      write this kind at this resource-id
   o  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.
   o  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.  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 store_kind_data, 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.

   We now discuss each data model:  Single Value

   There may be only one single-value element for each resource-id,
   kind-id pair.  A store of a new single-value element MUST overwrite
   the current value.  Array

   A store of an array entry replaces (or inserts) the given value at
   the location specified by the index.  Arrays are zero-based.  Note
   that arrays can be sparse.  Thus, 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 empty strings.
   If the index value is -1, then the value is placed at the end of the
   array.  Dictionary

   A stored dictionary entry has a dictionary-key used as a lookup key
   and a dictionary-value containing the data.  There may be only one
   value for any given dictionary-key and therefore a write to a
   dictionary-key overwrites whatever is there.  Response Definition

   In response to a successful STORE request the peer MUST return a
   STORE_A message containing a series of store_kind_response elements

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   containing the current value of the generation counter for each
   kind-id, as well as a list of the peers where the data was

   STRUCTURE: store_kind_response
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                              Kind                             |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
   008 +                                                               +
       |                           Generation                          |
   012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Replicas Len         |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                            Replicas                           |
       /                                                               /
       |                                                               |

   The contents of each element are:

   Kind -  The kind-id being represented.
   Generation -  The current value of the generation counter for that
   Replicas -  The list of other peers at which the data was/will-be
      replicated.  In DHTs 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

   The response itself is just the store_kind_response values packed

   If the request was rejected because of an invalid generation counter,
   then the store-response MUST also be returned, but with a response
   code of 412.  Otherwise, the response MAY contain a response-error-
   reason production or MAY be empty.  [[TODO:  The generation counter
   may need more thinking for uniqueness.]]  Data Signature Computation

   Each stored-data 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 stored value.
   The data signed is defined as:

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   STRUCTURE: stored_data_to_be_signed
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |  Resource Len |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                            Resource                           |
       /                                                               /
       |                                                               |
       |                              Kind                             |
       |                                                               |
       /                                                               /
       |                          Stored Data                          |
       /                                                               /
       |                                                               |

   The contents of this value are as follows:

   Resource -  The resource ID where this data is stored.
   Type -  The kind-id for this data.
   Stored Data -  The contents of the stored data value, as described in
      the stored_data PDU of Section

   [[TODO:  Should we include the identity?.]]

   Once the signature has been computed, the signature is represented
   using a signature element, as described in Section 4.2.3.

7.2.2.  FETCH

   The FETCH request retrieves one or more data elements stored at a
   given resource-id.  Request Definition

   A FETCH_Q message consists of a single fetch_request element followed
   by a series of fetch_kind_data elements.

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   STRUCTURE: fetch_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |  Resource Len |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                            Resource                           |
       /                                                               /
       |                                                               |
       |         Fetch Data Len        |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                           Fetch Data                          |
       /                                                               /
       |                                                               |

   The contents of the request are as follows:

   Resource -  The resource ID to fetch from.
   Fetch Data -  A sequence of data specifiers, one for each desired

   Each fetch_kind_data element is specified as follows.

   STRUCTURE: fetch_kind_data
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                              Kind                             |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Data Model  |                                               |
   008 +-+-+-+-+-+-+-+-+                                               +
       |                           Generation                          |
   012 +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                           Reference                           |
       /                                                               /
       |                                                               |

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   Kind -  The kind-id of the data being fetched.  Implementations
      SHOULD reject requests corresponding to unknown kinds unless
      specifically configured otherwise.
   Data Model -  The data model of the data.
   Generation -  The last generation counter that the requesting peer
      saw.  This is used to avoid unnecessary fetches.
   Reference -  A reference to the data value being requested within the
      data model specified for the kind, as specified below.

   STRUCTURE: fetch_array_reference
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                             First                             |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              Last                             |
   008 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   STRUCTURE: fetch_dictionary_reference
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      Dictionary Keys Len      |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                        Dictionary Keys                        |
       /                                                               /
       |                                                               |

   STRUCTURE: dictionary_key
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |         Key Value Len         |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                           Key Value                           |
       /                                                               /
       |                                                               |

   As with STORE, the FETCH_Q contains a list of kind-ids and associated
   references.  The reference encoding depends on the kind of value
   being stored.

   o  If the data is of data model single value, the reference is empty.

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   o  If the data is of data model array, the reference contains two
      integers.  The first integer is the beginning of the range and the
      second is the end of the range. 0 is used to indicate the first
      element and -1 is used to indicate the final element.  The
      beginning of the range MUST be earlier in the array then the end.
   o  If the data is of data model dictionary then the reference
      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 cache hit
   indicator rather than the stored data.

   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.  Response Definition

   The response to a successful FETCH request is a FETCH_A message
   containing the data requested by the requester.

   STRUCTURE: fetch_a
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                              Kind                             |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
   008 +                                                               +
       |                           Generation                          |
   012 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Stored Data Len        |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                          Stored Data                          |
       /                                                               /
       |                                                               |

   There MUST be one fetch_kind_data element for each kind-id in the
   request.  If the generation-counter in the request matches the
   generation-counter in the stored data, then the count of stored data
   elements MUST be zero.  Otherwise, all relevant data values MUST be

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   returned.  A nonexistent value is represented as a value with an
   empty data value portion and no signature.  In particular, if a
   dictionary key that does not exist is requested, then there must be a
   dictionary entry with that key but an empty value.

7.2.3.  REMOVE

   The REMOVE request is used to remove a stored element or elements
   from the storing peer.  Although each kind defines its own access
   control requirements, in general only the original signer of the data
   should be allowed to remove it.  Any successful remove of an existing
   element for a given kind MUST increment the generation counter by at
   least one.

   A remove-request 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 remove-request.  Each fetch_kind_data 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.

   o  The kind-id is known.
   o  The signature over the message is valid or (depending on overlay
      policy) no signature is required.
   o  The signer of the message has permissions which permit him to
      remove this kind of data.
   o  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.  Single Value

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

   A REMOVE of an array element (or element range) replaces those
   elements with empty 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"].

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   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"].  Dictionary

   A REMOVE of a dictionary element (or elements) replaces those
   elements with empty elements.  If no such elements exist, then this
   is a silent success.  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.

7.2.4.  FIND

   The FIND request is used to explore the DHT.  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 DHT by interactively fetching
   R_n+1=nearest(1 + R_n).  Request Definition

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

   STRUCTURE: find_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |  Resource Len |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                            Resource                           |
       /                                                               /
       |                                                               |
       |    Ids Len    |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                              Ids                              |
       /                                                               /
       |                                                               |

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

   Resource -  The desired resource-id
   Ids -  The desired kind-ids.  Each value MUST only appear once.  Response Definition

   A response to a successful FIND request is a FIND_A message
   containing the closest resource-ID for each kind specified in the

   STRUCTURE: find_kind_data
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                              Kind                             |
   004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Closest Len  |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                            Closest                            |
       /                                                               /
       |                                                               |

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

   When each kind is defined, it can indicate if the kind is not allowed
   to be used in a FIND request.  This would be done to help achieve
   some types of security properties for the data stored in that kind.

   For each kind-id in the request the response MUST contain a
   find_response_value indicating the closest resource-id for that
   kind-id unless the kind is not allowed to be used with FIND in which
   case a find_kind_data 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 find_kind_data elements, one per
   kind, concatenated end-to-end.  The contents of each element are:

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   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.  DHT Maintenance

   This section describes methods that are expected to be useful for all
   DHTs.  These methods have generic semantics (join, leave, update) and
   some common fields, but where appropriate allow room for DHT-specific

7.3.1.  JOIN

   A new peer (but which already has credentials) uses the JOIN_Q
   message to join the DHT.  The JOIN_Q is sent to the peer which
   previously was responsible for the resource-id corresponding to the
   peer-id which the new peer has.  This notifies the responsible peer
   that the new peer is taking over some of the overlay and it needs to
   synchronize its state.

   STRUCTURE: join_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                                                               |
   004 +                                                               +
       |                        Desired Peer Id                        |
   008 +                                                               +
       |                                                               |
   012 +                                                               +
       |                                                               |
   016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                                                               /
       |                       Dht Specific Data                       |
       /                                                               /
       |                                                               |

   The default JOIN_Q contains only the peer-id which the sending peer
   wishes to assume.  DHTs MAY specific other data to appear in this

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   The responding peer responds with success or failure.  However, if it
   is success it 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, DHTs MAY define data to
   appear in the response payload.

7.3.2.  LEAVE

   The LEAVE_Q message is used to indicate that a peer is exiting the
   overlay.  The peer SHOULD send this message to each peer with which
   it is directly connected prior to exiting the overlay.

   STRUCTURE: leave_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                                                               |
   004 +                                                               +
       |                        Leaving Peer Id                        |
   008 +                                                               +
       |                                                               |
   012 +                                                               +
       |                                                               |
   016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                                                               /
       |                       Dht Specific Data                       |
       /                                                               /
       |                                                               |

   The default LEAVE_Q contains only the peer-id of the leaving peer.
   DHTs MAY specific other data to appear in this request.

   Upon receiving a LEAVE request, a peer MUST update its own routing
   and routing table, and send the appropriate STORE/UPDATE sequences to
   re-stabilize the overlay.

7.3.3.  UPDATE

   Update is the primary DHT-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 and it is up to the recipient to take
   whatever actions are appropriate to deal with the state change.

   The contents of the UPDATE_Q message are completely DHT-specific.
   The UPDATE_A response is expected to be either success or an error.

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   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 ROUTE-QUERY for destination X requests the peer-id where the
   receiving peer would next route to get to X. A ROUTE-QUERY can also
   request that the receiving peer initiate an UPDATE request to
   transfer his routing table.

   One important use of the ROUTE-QUERY request is to support iterative
   routing.  The way that his works is that the sender selects one of
   the peers in its neighbor table and sends it a ROUTE-QUERY message
   with the destination_object set to the peer-id/resource-id it wishes
   to route to.  The neighbor responds with the next peer-id to send to.
   The sending peer then CONNECTs to that peer and repeats the ROUTE-
   QUERY.  Eventually, the sender gets a response from a peer containing
   a peer-id that is the same as that peer.  At that point, the sender
   can send whatever request is needed directly to that peer.

   Note that this procedure only works well if all the peers are
   mutually directly reachable--either by all having public IP addresses
   or at least by all being behind the same NAT.  Accordingly, peers
   MUST only use this method if permitted by the overlay configuration
   (see Section 10.2).  Request Definition

   A ROUTE_QUERY_Q 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.

   STRUCTURE: route_query_q
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |  Send Update  |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                       Destination Object                      |
       /                                                               /
       |                                                               |

   The contents of the ROUTE_QUERY_Q message are as follows:

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   send_update:  A single byte.  This may be set to 1 to indicate that
      the requester wishes the responder to initiate an UPDATE request
      immediately.  Otherwise, this value MUST be set to zero.
   destination_object:  The destination which the requester is
      interested in.  This may be any valid destination object,
      including a peer-id, compressed ids, or resource-id  Response Definition

   A response to a successful ROUTE_QUERY request is a ROUTE_QUERY_A
   message containing the address of the peer to which the responding
   peer would have routed the request message in recursive routing.

   STRUCTURE: route_query_a
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                                                               |
   004 +                                                               +
       |                           Next Peer                           |
   008 +                                                               +
       |                                                               |
   012 +                                                               +
       |                                                               |
   016 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                                                               /
       |                           Next Addr                           |
       /                                                               /
       |                                                               |

   The contents of the ROUTE_QUERY_A 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
   next_addr:  The address of the next peer.

   If the requester set the send_update flag, the responder SHOULD
   initiate an UPDATE immediately after.

8.  ICE and Connection Formation

   At numerous times during the operation of RELOAD, a node will need to
   establish a connection to another node.  This may be for the purposes

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   of building finger tables when the node joins the P2P network, or
   when the node learns of a new neighbor through an UPDATE and needs to
   establish a connection to that neighbor.

   In addition, a node may need to connect to another node for the
   purposes of an application connection.  In the case of SIP, when a
   node has looked up the target AOR in the DHT, it will obtain a
   Node-ID that identifies that peer.  The next step will be to
   establish a "direct" connection for the purposes of performing SIP

   In both of these cases, the node starts with a destination Node-ID,
   and its objective is to create a connection (ideally using TCP, but
   falling back to UDP when it is not available) to the node with that
   given Node-ID.  The establishment of this connection is done using
   the CONNECT request in conjunction with ICE.  It is assumed that the
   reader has familiarity with ICE.

   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.

8.1.  Overview

   To utilize ICE, the CONNECT method provides a basic offer/answer
   operation that exchanges a set of candidates for a single "stream".
   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.
   The CONNECT request contains the candidates for this stream, and the
   CONNECT response contains the corresponding answer with candidates
   for that stream.  Though CONNECT provides an offer/answer exchange,
   it does not actually carry or utilize Session Description Protocol
   (SDP) messages.  Rather, it carries the raw ICE parameters required
   for ICE operation, and the ICE spec is utilized as if these
   parameters had actually been used in an SDP offer or answer.  In
   essence, ICE is utilized by mapping the CONNECT parameters into an
   SDP for the purposes of following the details of ICE itself.  That
   avoids the need for RELOAD to respecify ICE, yet allows it to operate
   without the baggage that SDP would bring.

   In addition, 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.

   RELOAD and SIP always run over TLS for TCP connections and DTLS
   [RFC4347] for UDP "connections".  Consequently, once ICE processing
   has completed, both agents will begin TLS and DTLS procedures to

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   establish a secure link.  Its important to note that, had a TURN
   server been utilized for the TCP or UDP stream, the TURN server will
   transparently relay the TLS messaging and the encrypted TLS content,
   and thus will not have access to the contents of the connection once
   it is established.  Any attack by the TURN server to insert itself as
   a man-in-the-middle are thwarted by the usage of the fingerprint
   mechanism of RFC 4572 [RFC4572], which will reveal that the TLS and
   DTLS certificates do not match the ones used to sign the RELOAD

   An agent follows the ICE specification as described in
   [I-D.ietf-mmusic-ice] and [I-D.ietf-mmusic-ice-tcp] with the changes
   and additional procedures described in the subsections below.

8.2.  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].
   Accordingly, any peer you know about will be willing to be a STUN
   server for you -- 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

   OPEN ISSUE:  should the peer try to keep at least one peer in each

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   group, even if it has no other reason for the connection?  Need to
   specify when to stop adding new groups if the peer is behind a really
   bad NAT.

   OPEN ISSUE:  RELOAD-01 had a Peer-Info structure that allowed peers
   to exchange information such as a "default" IP-port pair in UPDATEs.
   This structure could be expanded to include the candidate list for a
   peer, thus allowing ICE negotiation to begin or even direct
   communication before a CONNECT request has been received.  (The
   candidate pairs for the P2P port are fixed because the same source
   port is used for all connections.)  However, because this would
   require significant changes to the ICE algorithm, we have not
   introduced such an extension at this point.

8.3.  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].  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], and MUST
   gather at least one UDP and one TCP host candidate for RELOAD and for

   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 ring, as described in
   Section 8.2 and Section 11.3.

   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].

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

8.4.  Encoding the CONNECT Message

   Section 4.3 of ICE describes procedures for encoding the SDP.
   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 CONNECT request or its response.  Similarly, the
   username fragment and password are carried in the CONNECT message or
   its response.  Section 7.1.2 describes the detailed attribute

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   encoding for CONNECT.  The CONNECT 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 CONNECT request and its
   response also contain a Next-Protocol attribute, with a value of SIP
   or RELOAD, which indicates what protocol is to be run over the
   connection.  The RELOAD CONNECT request MUST only be utilized to set
   up connections for application protocols that can be multiplexed with
   STUN and RELOAD itself.

   Since the CONNECT 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 CONNECT response is
   considered a valid answer for the purposes of following the ICE

   Since all messages with RELOAD are secured between nodes, the node
   MUST implement the fingerprint attribute of RFC 4572 [RFC4572], and
   encode it into the CONNECT request and response as described in
   Section 7.1.2.  This fingerprint will be matched with the
   certificates utilized to authenticate the RELOAD CONNECT request and
   its response.

   Similarly, the node MUST implement the active, passive, and actpass
   attributes from RFC 4145 [RFC4145].  However, here they refer
   strictly to the role of active or passive for the purposes of TLS
   handshaking.  The TCP connection directions are signaled as part of
   the ICE candidate attribute.

8.5.  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.

8.6.  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 CONNECT request) will always be controlling, and
   the answerer (the entity sending the CONNECT 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.

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8.7.  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.

8.8.  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:

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

8.9.  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.

8.10.  Media Keepalives

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

8.11.  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 fingerprint from the CONNECT
   request and its response are used as described in RFC 4572 [RFC4572],
   to ensure that another node in the P2P network, acting as a TURN
   server, has not inserted itself as a man-in-the-middle.  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.

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8.12.  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.

9.  DHT Algorithms

9.1.  Generic Algorithm Requirements

   When specifying a new DHT, at least the following need to be

   o  Joining procedures, including the contents of the JOIN message.
   o  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.
   o  Exit procedures, including the contents of the LEAVE message.
   o  The hash algorithm used to go from a Unhashed-ID, such as a user
      name, to a Resource-ID.  This also includes the length of the
      Resource-IDs and Peer-IDs
   o  The procedures that peers use to route messages.
   o  The replication strategy used to ensure data redundancy.

9.2.  Chord Algorithm

   This algorithm is assigned the name chord-128-2-16+ to indicate it is
   based on Chord, uses a 128 bit hash function, stores 2 redundant
   copies of all data, and has finger tables with at least 16 entries.

9.2.1.  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 peer-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

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   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 rapidly be found - it
   needs to be there so that peers 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.

9.2.2.  Routing

   If a peer is not responsible for a Resource-ID k, then it routes a
   request to that location by routing it to the peer in either the
   neighborhood or finger table that has the largest peer-id that is in
   the interval between the peer and k.

9.2.3.  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.

   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?]]

9.2.4.  Joining

   The join process for a joining party (JP) with peer-id n is as

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   1.  JP connects to its chosen bootstrap node.
   2.  JP uses a series of PINGs to populate its routing table.
   3.  JP sends CONNECT 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 peer-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 a CONNECT to peer (n+2^(numBitsInPeerId-i).  This will be
   routed to a peer in approximately the right location around the ring.

9.2.5.  Routing CONNECTs

   When a peer needs to CONNECT with a new peer in its neighborhood
   table, it MUST source-route the CONNECT request through the peer from
   which it learned the new peer's peer-id.  Source-routing these
   requests allows the overlay to recover from instability.

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

   If a peer is unable to successfully CONNECT 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

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9.2.6.  UPDATEs

   An UPDATE is defined as

   STRUCTURE: chord_update
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |Predecessors Le|                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                          Predecessors                         |
       /                                                               /
       |                                                               |
       | Successors Len|                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                           Successors                          |
       /                                                               /
       |                                                               |

   The contents of this message are:

   Predecessors -  The predecessor set of the UPDATEing peer.
   Successors -  The successor set of the UPDATEing peer.

   A peer MUST maintain an association (via CONNECT) 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.  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 peer-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

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   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.  Receiving UPDATEs

   When a peer, N, receives an UPDATE request, it examines the peer-ids
   in the UPDATE_Q and at its neighborhood table and decides if this
   UPDATE_Q 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

   o  The UPDATE_Q contains peers that would not change the neighbor set
      because they match the neighborhood table.
   o  The UPDATE_Q contains peers closer to N than those in its
      neighborhood table.
   o  The UPDATE_Q 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 CONNECT 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 CONNECTs 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.

   A 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

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   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.  Stabilization

   There are four components to stabilization:
   1.  exchange UPDATES will all peers in its routing table to exchange
   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

   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 peer (n+2^(numBitsInPeerId-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

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   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

   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 overay 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 peer-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 CONNECT 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.)

9.2.7.  Leaving

   Peers SHOULD send a LEAVE request prior to exiting the DHT.  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.

10.  Enrollment and Bootstrap

10.1.  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]
   and [I-D.matthews-p2psip-bootstrap-mechanisms].  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 the DNS SRV lookup is done

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   using implement [I-D.cheshire-dnsext-dns-sd] with a Service name of
   "p2p_menroll" can also be tried to find an enrollment server.  If
   they implement this, the user name can 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].  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.

10.2.  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 name="chord.example.com" expiration="86400">
          <dht name="chord-128-2-8"/>
          <root-cert>[DER certificate here]</root-cert>
          <required-usage name="SIP"/>
          <credential-server url="https://www.example.com/csr"/>
          <bootstrap-peer address="" port="5678"/>
          <bootstrap-peer address="" port="5678"/>
          <bootstrap-peer address="" port="5678"/>
          <multicast-bootstrap="" port="5678"/>

   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:

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   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:

   dht -  This element has an attribute called name that describes which
      DHT algorithm is being used.
   root-cert -  This element contains a DER 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-usage -  This element has an attribute called "name" that
      describes a usage that peers in this overlay are required to
      support.  More than one required-usage element 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.
   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.
   iterative-permitted -  This element indicates that iterative routing
      (see Section 7.3.4) MAY be used.  If iterative routing is
      permitted, then this value MUST be set to "TRUE".  Otherwise, it
      SHOULD be absent, but MAY be set to "FALSE".

   [[TODO:  Do a RelaxNG grammar.]]

10.3.  Credentials

   If the configuration document contains a credential-server element,
   credentials are required to use the DHT.  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]) and sends this over HTTPS as defined in
   [I-D.ietf-pkix-cmc-trans] to the URL in the credential-server
   element.  The subjectAltName in the request MUST contain the required

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   user name(s).

   The credential server MUST authenticate the request using HTTP digest
   [RFC2617].  If the authentication succeeds and the requested user
   name(s) is acceptable, the server and returns a certificate.  The
   SubjectAltName field in the certificate contains the following

   o  One or more Peer-IDs which MUST be cryptographically random
      [RFC4086].  These MUST be chosen by the credential server in such
      a way that they are unpredictable to the requesting user.
   o  The names this user is allowed to use in the overlay

   The certificate is returned in a "Simple Enrollment Response".

   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
   Peer-IDs it can use.

10.3.1.  Credentials for HIP

   When RELOAD is used with HIP, the certificates MUST be generated so
   o  Each node is assigned a unique ORCHID.
   o  The peer-id can be uniquely determined from the ORCHID.
   Because in general, ORCHIDs are shorter than peer-ids, this means
   that the ORCHIDS MUST be generated first and MUST be
   cryptographically random in order to make the peer-ids
   cryptographically random.  The mapping function used to produce the
   peer-id from the ORCHID MUST be the same as that used by the DHT to
   produce resource-ids from Unhashed-IDs.

   In addition to the usual attributes, when HIP is in use certificates
   MUST contain a subjectAltName with an iPAddress value containing the
   HIP ORCHID.  This allows these certificates to be used by the HIP
   peers during the HIP base exchange.

10.4.  Locating a 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 peer-id set to that peer's

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   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 peer-id as the destination

   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 peer-id in the response and use this
   peer-id to start sending requests to join the DHT as described in
   Section 3.1.5 and Section 7.3.

   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.

11.  Usages

11.1.  Generic Usage Requirements

   A new usage MUST specify the following information:

   o  The kind-ids which the usage defines and what each kind means.
   o  The data model for the data being stored (single value, array,
      dictionary, etc.) for each kind
   o  Access control rules for each kind, indicating what credentials
      are allowed to read and write that kind-id at a given location.
   o  The minimum amounts of data of each kind that a conformant
      implementation MUST store.

   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, kind SHOULD use the build-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.

   Note:  New usages MAY reuse existing kind-ids.  New kind-ids only
      need to be defined where different data is stored or different
      behavior is required.

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11.2.  SIP Usage

   The SIP usage allows a RELOAD overlay to be used as a distributed SIP
   registrar/proxy network.  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 Bob's SIP UA can use to pass SIP
   messages back and forth to Alice's SIP UA.  Provides the following
   three functions:

   o  Mapping SIP URIs that are not GRUUs to the overlay peer
      responsible for the SIP UA.
   o  Mapping SIP GRUUs to the DHT peer responsible for the SIP UA.
   o  Forming a connection directly to a DHT peer that is used to send
      SIP messages to the SIP UA.

   Section 3.7.1 provides an overview of how these fit together.

11.2.1.  SIP-REGISTRATION kind

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

   Kind IDs  The Unhashed-ID for the SIP-REGISTRATION kind-id is a URI,
      typically the AOR for the user.  The data stored is a sip-
      registration-data, which can contain either another URI or a
      destination list to the peer which is acting for the user.
      [[TODO:  we want to somehow put caller-prefs in here along with
      the route list, but I'm not sure how to do it yet.]]
   Data Model  The data model for the SIP-REGISTRATION kind-id is
      dictionary.  The dictionary key is the peer-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 Unhashed-ID for the resource-id and (2) contains a
      peer-id matching the storing dictionary key.
   Data Sizes  Peers MUST be prepared to store SIP-REGISTRATION values
      of up to 10 kilobytes and must be prepared to store up to 10
      values for each user name.

   The contents of the SIP-REGISTRATION kind are

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   STRUCTURE: sip_registration
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |                                                               |
       /                                                               /
       |                       Registration Data                       |
       /                                                               /
       |                                                               |

   STRUCTURE: sip_registration_uri_type
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x01     | Sip Registration Uri Type Len |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                   Sip Registration Uri Type                   |
       /                                                               /
       |                                                               |

   STRUCTURE: sip_registration_route_type
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
       |      0x02     |       Contact Prefs Len       |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                         Contact Prefs                         |
       /                                                               /
       |                                                               |
       |      Destination List Len     |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                        Destination List                       |
       /                                                               /
       |                                                               |

   A registration may contain either a URI (type code 0x01) or a contact
   preferences structure and a destination list (type code 0x01).  The
   leading byte indicates the type.

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11.2.2.  GRUUs

   GRUUs do not require storing data in the DHT.  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.

   Anonymous GRUUs are done in roughly the same way but require either
   that the enrollment server issue a different peer-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
   peer-id from being revealed.

11.2.3.  SIP Connect

   Once the destination list for a user has been identified, the calling
   peer uses the CONNECT request to form a connection to the peer
   identified by the destination list.  The CONNECT request MUST contain
   the connect-application value of 5160 (SIP).  If certificate-based
   authentication is in use, the responding peer MUST present a
   certificate with a peer-id matching the terminal entry in the route

   [[TODO:  Note that this constrains destination lists from hiding the
   last peer-id when used here.  I think that's OK, but we should take a

   Once the association has been formed, the calling peer sends generic
   SIP messages down the new association and ordinary SIP procedures are

11.2.4.  SIP Tunnel

   This usage allows two peers to exchange SIP messages across the
   overlay using the TUNNEL method.  TUNNEL is provided as an
   alternative to using CONNECT because it allows a SIP message to be
   sent immediately, without the delay associated with CONNECT.  For a
   simple SIP exchange, it may result in fewer messages being sent.

   An implementation SHOULD use CONNECT for a dialog that is expected to
   endure for sufficient time and exchange significant numbers of
   messages.  An implementation MAY establish an initial dialog using
   TUNNELing and then migrate it to a direct dialog opened with CONNECT
   once that negotiation is complete.

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   As an application of TUNNEL, this usage defines the following items:

   o  For SIP, the application attribute is 5060.
   o  The application MAY establish any dialog using TUNNEL if it
      expects to replace it once a CONNECT request completes.  The
      application SHOULD NOT exchange messages with another SIP UA
      repeatedly using a TUNNEL unless it is unable to complete a
   o  The Replaces header should be used to migrate dialogs established
      via TUNNEL to a direct connection.
   o  The dialogid is the GRUU of the destination of the request.
   o  By using the GRUU of the destination as the dialogid, the
      receiving peer is able to deliver the message to the appropriate
      process without parsing the SIP message.

   In constructing the message, the SIP UA forms the message as if it
   were being routed directly to the GRUU of the destination.  The SIP
   stack hands the message to RELOAD for delivery.  Although the message
   is passed through a sequence of untrusted peers, it is not subject to
   modification by those peers because of the message's signature.

   OPEN ISSUE:  should specify how to request encryption of the message

   Note:  The easiest implementation of TUNNEL is likely to default to
      sending all messages across a TUNNEL when the first message is
      sent to a new destination GRUU and simultaneously issuing a
      CONNECT.  Messages then continue through the TUNNEL until the
      CONNECT completes, at which point they are delivered via the new

   OPEN ISSUE:  If the tunneling vs direct decision can be made
   equivalently to a link-layer decision, it may not be necessary to
   modify the dialog or inform the SIP UA in any way that it has now
   obtained a direct route.

11.3.  TURN Usage

   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

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   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 Unhashed-ID by
   concatenating its peer-ID and the value d.  This Unhashed-ID is
   hashed to form a Resource-ID.  The address of the peer is stored at
   that Resource-ID using type TURN-SERVICE and the turn-server

   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 the STUN-Relay server type 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

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  STRUCTURE: turn_server
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |   Iteration   |                                               |
      +-+-+-+-+-+-+-+-+                                               +
      |                            Address                            |
      /                                                               /
      |                                                               |

  STRUCTURE: ip4_address_type
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |      0x01     |                      Addr                     |
  004 +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |              Port             |
  007 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  STRUCTURE: ip6_address_type
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      |      0x02     |                                               |
  004 +-+-+-+-+-+-+-+-+                                               +
      |                              Addr                             |
  008 +                                                               +
      |                                                               |
  012 +                                                               +
      |                                                               |
  016 +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |              Port             |
  019 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   [[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.

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   Data Model  The TURN-SERVICE stores a single value for each
   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.
   Data Sizes  TURN-SERVICE values are of fixed size.  Peers MUST be
      prepared to store values with iteration counter of up to 100.

   The data is stored in a data structure with the IP address of the
   server and an indication whether the address is an IPv4 or IPv6
   address.  The Unhashed-ID used to form the storage Resource-ID is
   simply the peer-id.  The access control rule is that the certificate
   used to sign the request must contain a peer-id that when hashed
   would match the Resource-ID where the data is being stored.

   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.

   Open issues:  Should there be low and high bandwidth version of STUN-
   Relay one can find?  Low would be usable for signaling type things
   and high would be usable for audio, video, and others.

11.4.  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 Unhashed-IDs:

   o  The user names in the certificate.
   o  The peer-ids in the certificate.

   Note that in the second case the certificate is not stored at the
   peer's peer-id but rather at a hash of the peer's peer-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 DHT

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   when joining and redo the check about every 24 hours after that.
   Certificate data should be stored with an expiry time of 60 days.
   When a client is checking the existence of data, if the expiry is
   less than 30 days, it should be refreshed to have an expiry of 60
   days.  The certificate information is frequently used for many
   operations, and peers should cache it for 8 hours.

   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 peer-id or user name
      which, when hashed, maps the resource-id at which the value is
      being stored.
   Data Sizes  Peers MUST be prepared to store at least 10 certificates
      of sizes up to 1K each.

11.5.  HIP Tunnel

   This usage allows two peers to exchange HIP messages across the
   overlay using the TUNNEL method.  This is meant to be used as part of
   the HIP BONE architecture described in [I-D.camarillo-hip-bone].

   As an application of TUNNEL, this usage defines the following items:

   o  For HIP, the application attribute is TBD (IANA port number).
   o  The dialogid is empty and set to zero length.

   In order to route HIP messages correctly, there needs to be an
   unambiguous mapping between the ORCHID assigned to each HIP node and
   the peer-id assigned to that node.  The ORCHID MUST be used as the
   Unhashed-ID to generate the peer-id.  [TODO:  We need a general
   scheme for mapping Unhashed-IDs to IDs so they don't collide.  This
   isn't the place to define it.]  Messages to a given ORCHID are then
   routed to the mapped peer-id.  Section 10.3 describes more
   considerations for the generation of ORCHIDs for use with RELOAD.

   TODO - should discuss interaction of HIP and P2P retransmission

11.6.  Diagnostic Usage

   [[TODO:  reduce text of motivation description in the next version]]

   The development and deployment of a peer-to-peer system is a
   continuous process.  The developers write code which is tested on a
   scale that may be smaller than the actual deployment size.  After
   this local testing, the code is deployed in a real environment.  Bugs
   arise during development and deployment phases.  The designers of the

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   peer-to-peer system need mechanisms which can help identify problems
   and bugs in a peer-to-peer system during development and deployment
   phases.  Peer-to-peer systems are an example of a distributed system
   and it is not a trivial task to provide protocol mechanisms, tools
   and techniques to identify problems that may arise in such systems.

   The diagnostic mechanisms can broadly be classified into online and
   offline mechanisms.  The online mechanisms attempt to identify faults
   in a running system where as offline mechanisms try to infer faults
   by gathering the log files of machines participating in a distributed

   In a peer-to-peer system, a peer maintains routing state to forward
   messages according to the overlay protocol being used.  In addition,
   a peer stores information published by other peers.  The routing and
   storage of resources consume network, space (memory), and CPU
   resources.  A peer also needs to keep track of how long the P2PSIP
   application has been running and the last time peers in the routing
   table were last contacted.  During development and deployment phase,
   an overlay designer needs mechanisms to query some or all of the
   above mentioned information.

   The overlay designer may also treat overlay as a black box and
   determine if the routing mechanisms are working correctly under
   various levels of churn.

   Thus, there are at least two types of online diagnostic mechanisms:
   1) state acquisition 2) black-box diagnostics

11.6.1.  State Acquisition Mechanisms

   The protocol provides a DIAGNOSTIC method [TODO] which queries the
   peer for its routing state, average bandwidth, CPU utilization, and
   storage state.  The DIAGNOSTIC request should typically be sent over
   a reliable transport protocol as the response will likely exceed UDP
   MTU size.  The state acquisition mechanism can be used to construct a
   local view of the connectivity state of the system.  It can also be
   used to construct a geographical map of the system.

   Below, we identify potential issues with the state acquisition

   Security:  If any peer can query the routing or storage state of any
   other peer, then clearly privacy and security concerns arise.  To
   address this, the state acquisition mechanisms need an access list
   like mechanism so that only the overlay implementer can query the
   state of all the nodes.  Alternatively, the state acquisition
   mechanisms are only enabled during the development phase or are only

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   enabled for 'admin' users.

   Scalability:  It is possible to query the state of few hundred or a
   few thousand nodes (as it is currently done in our live system on
   Planet lab); however, a serial state acquisition of a million node is
   a non starter.  In large scale networks, one option is to query the
   state of few hundred nodes and to construct an high level
   connectivity map.  CAIDA [ref] collects data at a few vantage points
   to construct BGP maps.

   Instantaneous vs. long term state:  Another issue with these state
   acquisitions mechanisms is whether they acquire the instantaneous
   state snapshot or an exponential moving average or a list of
   snapshots over a period of time.  For diagnostic metrics such as CPU
   utilization, an exponential moving average metric is also helpful in
   addition to the instantaneous snapshot.

   Pull vs. push:  The state acquisition mechanisms can either be pull-
   based or push-based or a combination of both.  In pull-based
   mechanisms, peer explicitly request state of another peer.  This may
   not be sufficient because pull-based mechanisms require a to
   periodically poll a peer for any change state.  In a push-based
   mechanism, peers advertise any change in certain metrics to their
   routing or neighbor peers.  As an example of push-based mechanism, a
   peer which starts to relay a call may indicate a change in its
   bandwidth to its routing or neighbor peers in a PING message.

   Development vs. deployment:  A hard problem is to decide which
   diagnostics are absolutely necessary during deployment and which are
   needed during development.

   Clearly, complete state acquisition has security concerns in a
   deployed system.  The other option an overlay implementer can use is
   to run a few peers and have complete control over the functionality
   of these peers.  These peers are same as other peers with the
   difference that an overlay implementer can explicitly query the state
   of these peers.  It can then use this information to 'crawl' the
   overlay network and construct a local map of the network.

11.6.2.  Black-box diagnostics

   [[TODO:  a better name for this section]]

   Black-box diagnostics:  DHTs are examples of structured peer-to-peer
   networks and they allow nodes to store key/value pairs in the
   overlay.  A simple diagnostic mechanism is to treat the overlay as a
   black-box:  publish several key/value pairs at one peer and then look
   them up from another peer.  For this kind of diagnostic mechanism,

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   clients are more suitable as they do not provide any routing or
   storage services to the overlay and can connect to an arbitrary peer.

   The Diagnostic Usage allow a peer 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, and performance statistics of a peer.  The usage defines
   several new kinds which can be retrieved to get the statistics.  The
   peer-id is directly used when retrieving data so no Unhashed-ID is
   defined.  The access control model for all of these is local policy
   defined by the peer.  The peer MAY have a list of users (such as
   "admin") that it is willing to return the information for and
   restrict access to users with that name.  The access control can 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:

   SOFTWARE_VERSION  A single value element containing a US-ASCII string
      that identifies the manufacture, model, and version of the
   UPTIME  A single value element containing an unsigned 64-bit integer
      specifying the time the nodes has been up in seconds.
   AS_NUMBER  A single value element containing the Autonomous System
      [TODO REF] number as an unsigned 32-bit integer.  Zero is returned
      if the AS number is unknown.
      (OPEN ISSUES:  How to determine a AS number?  This metric is
      primarily used for advertising and locating STUN/TURN servers.  A
      TURN server is inserted and looked up under H(AS).  What if there
      are no TURN servers in the same AS? )
   CPU_UTILIZATION  A single value element containing an unsigned 8-bit
      integer representing the percentage CPU load from 1 to 100.
      (OPEN ISSUE:  It is not a very precise metric.)
   DATA_STORED  A single value element containing an unsigned 64-bit
      integer representing the number of bytes of data being stored by
      this node.
   MESSAGES_SENT  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.
   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.

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   ROUTING_TABLE_SIZE  A single value element containing an unsigned 32-
      bit integer representing the number of peers in the node's routing
   NEIGHBOR_TABLE_SIZE  A single value element containing an unsigned
      32-bit integer representing the number of peers in the node's
      neighbor table.
   EWMA_BYTES_SENT  A single value element containing an unsigned 32-bit
      integer representing an exponential weighted average of bytes sent
      by this peer.
   EWMA_BYTES_RCVD  A single value element containing an unsigned 32-bit
      integer representing an exponential weighted average of bytes
      received by this peer.
   LAST_CONTACT  A single value element containing an unsigned 32-bit
      integer specifying the time in number of seconds the node was last
   MEMORY_FOOTPRINT  A single value element containing an unsigned 32-
      bit integer representing the memory footprint of the peer program
      in kilo bytes.
   RTT  A single value element containing an unsigned 32-bit integer
      specifying the recent RTT estimate in ms between two peers.

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

11.6.3.  Diagnostic Metrics for a P2PSIP Deployment

   Clearly, all diagnostic metrics are useful during development and
   testing.  The hard question is which metrics are absolutely necessary
   for a deployed P2PSIP system.  We attempt to identify these metrics
   and classify them under 'resource' and 'peer' metrics.

   For 'resource' metric, we identify CPU_UTILIZATION, EWMA_BYTES_SENT,
   EWMA_BYTES_RCVD, and MEMORY_FOOTPRINT as the key metrics and for
   'peer' metric we identify UPTIME, LAST_CONTACT, and RTT as the
   metrics that are crucial for a deployed P2PSIP system.

   (OPEN QUESTION:  any other metrics?)

   (OPEN:  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. )

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12.  Security Considerations

12.1.  Overview

   RELOAD provides a generic storage service, albeit one designed to be
   useful for P2P SIP.  In this section we discuss security issues that
   are likely to be relevant to any usage of RELOAD.  In Section 12.7 we
   describe issues that are specific to SIP.

   In any DHT, 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 DHT.  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 DHT 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 based on an overlay-wide shared symmetric
   key and is suitable only for limited deployments in which the
   relationship between admitted peers is not adversarial.

12.2.  Attacks on P2P Overlays

   The two basic functions provided by DHT 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 DHT 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 DTLS
   or TLS, which provides message integrity and authentication of the
   directly communicating peer.  In addition, when the certificate-based
   security system is used, messages and data are digitally signed with

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   the sender's private key, providing end-to-end security for

12.3.  Certificate-based Security

   This specification stores users' registrations and possibly other
   data in a Distributed Hash table (DHT).  This requires a solution to
   securing this data as well as securing, as well as possible, the
   routing in the DHT.  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 DHT, 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 peer-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:

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

   Note that since only users of this DHT need to validate a
   certificate, this usage does not require a global PKI.  It does,
   however, require a central enrollment authority which acts as the
   certificate authority for the DHT.  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.

   In order to protect data storage, in the certificate-based security
   scheme, all stored data is signed by the owner of the data.  This
   allows the storing peer to 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.

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12.4.  Shared-Secret Security

   For small environments where deployment of the PKI necessary to use a
   certificate-based model is impractical, RELOAD supports a shared
   secret security 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] or TLS-SRP [I-D.ietf-tls-srp] mode.  A peer which does
   not know the key cannot form TLS connections with any other peer and
   therefore cannot join the overlay.

   The shared-secret scheme prohibits unauthorized peers from joining
   the overlay, but it provides no protection from a compromised peer
   inserting arbitrary resource registrations, performing a Sybil
   attack[Sybil], or performing other attacks on the resources or
   routing.  Thus, it is only safe to use in limited settings in which
   peers are not adversarial.  In addition, because the messages and
   data are not authenticated, each intermediate peer MUST take care to
   use TLS and check the other peer's knowledge of the shared secret, or
   message insertion is possible.

   If the shared secret key for the shared-key security scheme is
   discovered by an attacker, then most of the security of the scheme is
   lost:  an attacker can impersonate any peer to any other peer.  Thus,
   the shared-secret scheme is only appropriate for small deployments,
   such as a small office or ad hoc overlay set up among participants in
   a meeting.

   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.

12.5.  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 DHTs - relatively simple.

   When shared-secret security is used, then all peers trust all other
   peers, provided that they have demonstrated that they have the

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   credentials to join the overlay at all.  The following text therefore
   applies only to certificate-based security.

12.5.1.  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 Unhashed-ID 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 Unhashed-ID construction algorithm for the
   specified kind based on the user's certificate information.  It then
   computes the Resource-ID from the Unhashed-ID 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
           Peer-Id:   013456789abcdef
           Serial:    1234

   If Alice wishes to STORE a value of the "SIP Location" kind, the
   Unhashed-ID will be the SIP AOR "sip:alice@dht.example.com".  The
   Resource-ID will be determined by hashing the Unhashed-ID.  When a
   peer receives a request to store a record at Resource-ID X, it takes
   the signing certificate and recomputes the Unhashed-ID, 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 Unhashed-ID
   construction algorithm may be different for other kinds.

12.5.2.  Distributed Quota

   Being a peer in a DHT carries with it the responsibility to store
   data for a given region of the DHT.  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 each usage 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

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   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.

12.5.3.  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.

12.5.4.  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

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   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.

12.6.  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 DHT.  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 DHT.  Second, this is a DOS attack only.  Third,
   if a large percentage of the peers on the DHT are controlled by the
   attacker, it is probably impossible to perfectly secure against this.

12.6.1.  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] 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], 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.

12.6.2.  Admissions Control

   Admission to an RELOAD DHT is controlled by requiring that each peer
   have a certificate containing its peer ID.  The requirement to have a
   certificate is enforced by using TLS 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:

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

   The first property allows protection against Sybil attacks (provided

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   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 DHT he is
   impersonating appears to be very sparsely populated by comparison to
   the victim's local region.

12.6.3.  Peer Identification and Authentication

   In general, whenever a peer engages in DHT 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 TLS 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 DHT structure, but
   they need to verify that the UPDATE message came from the actual peer
   rather than from an attacker.  To prevent this, all DHT routing
   messages are signed by the peer that generated them.

   [TODO:  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 DHT messages.]

12.6.4.  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

   There are two lines of defense here.  The first is the use of TLS or

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   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.

12.6.5.  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 DHT 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.

12.7.  SIP-Specific Issues

12.7.1.  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.

12.7.2.  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.

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12.7.3.  Privacy Issues

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

13.  IANA Considerations

   This section contains the new code points registered by this
   document.  The IANA policies are TBD.

13.1.  Overlay Algorithm Types

   IANA SHALL create/(has created) a "RELOAD Overlay Algorithm Type"
   Registry.  Entries in this registry are strings denoting the names of
   DHT algorithms.  The registration policy for this registry is TBD.

   The initial contents of this registry are:

   chord-128-2-8 -  The algorithm defined in Section 9.2 of this

13.2.  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 11.1.  The registration policy for this registry
   is TBD.

   The initial contents of this registry are:

                     | Kind                | Kind-Id |
                     | SIP-REGISTRATION    |     TBD |
                     | TURN_SERVICE        |     TBD |
                     | CERTIFICATE         |     TBD |
                     | SOFTWARE_VERSION    |     TBD |
                     | UPTIME              |     TBD |
                     | AS_NUMBER           |     TBD |
                     | CPU_UTILIZATION     |     TBD |
                     | DATA_STORED         |     TBD |
                     | MESSAGES_SENT       |     TBD |
                     | INSTANCES_STORED    |     TBD |
                     | ROUTING_TABLE_SIZE  |     TBD |
                     | NEIGHBOR_TABLE_SIZE |     TBD |

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13.3.  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 3.1.4.  The registration policy for this
   registry is TBD.

                       | Data Model   | Identifier |
                       | SINGLE_VALUE |        TBD |
                       | ARRAY        |        TBD |
                       | DICTIONARY   |        TBD |

13.4.  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 4.2.1 The registration policy for this registry
   is TBD.

   The initial contents of this registry are:

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                   | Message Code Name |   Code Value |
                   | RESERVED          |            0 |
                   | PING_Q            |          TBD |
                   | PING_A            |          TBD |
                   | CONNECT_Q         |          TBD |
                   | CONNECT_A         |          TBD |
                   | TUNNEL_Q          |          TBD |
                   | TUNNEL_A          |          TBD |
                   | STORE_Q           |          TBD |
                   | STORE_A           |          TBD |
                   | FETCH_Q           |          TBD |
                   | FETCH_A           |          TBD |
                   | REMOVE_Q          |          TBD |
                   | REMOVE_A          |          TBD |
                   | FIND_Q            |          TBD |
                   | FIND_A            |          TBD |
                   | JOIN_Q            |          TBD |
                   | JOIN_A            |          TBD |
                   | LEAVE_Q           |          TBD |
                   | LEAVE_A           |          TBD |
                   | UPDATE_Q          |          TBD |
                   | UPDATE_A          |          TBD |
                   | ROUTE_QUERY_Q     |          TBD |
                   | ROUTE_QUERY_A     |          TBD |
                   | RESERVED          | 0x800..0xffe |
                   | ERROR             |       0xffff |

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

14.  Error Codes

   IANA SHALL create/(has created) a "RELOAD Error Code" Registry.
   Entries in this registry are 16-bit integers denoting error codes.
   [[TODO:  Complete this once we decide on error code strategy.

15.  Examples

   See draft [TODO add ref] for message flow examples.

16.  Acknowledgments

   This draft is a merge of the "REsource LOcation And Discovery

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   (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

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

17.  Appendix: Operation with SIP clients outside the DHT domain

18.  Appendix: Notes on DHT Algorithm Selection

   An important point:  if you assume NATs are doing ICE to set up
   connections, you want a lot fewer connections than you might have on
   a very open network - this might push towards something like Chord
   with fewer connections than, say, bamboo.

   TODO - ref draft-irtf-p2prg-survey-search

19.  References

19.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-16 (work in progress), June 2007.

              Rosenberg, J., "Session Traversal Utilities for (NAT)
              (STUN)", draft-ietf-behave-rfc3489bis-06 (work in
              progress), March 2007.

              Rosenberg, J., "Obtaining Relay Addresses from Simple
              Traversal Underneath NAT (STUN)",
              draft-ietf-behave-turn-03 (work in progress), March 2007.

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              Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC) Transport Protocols", draft-ietf-pkix-cmc-trans-05
              (work in progress), May 2006.

              Myers, M. and J. Schaad, "Certificate Management Messages
              over CMS", draft-ietf-pkix-2797-bis-04 (work in progress),
              March 2006.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

              Taylor, D., "Using SRP for TLS Authentication",
              draft-ietf-tls-srp-14 (work in progress), June 2007.

              Rosenberg, J., "TCP Candidates with Interactive
              Connectivity Establishment (ICE",
              draft-ietf-mmusic-ice-tcp-03 (work in progress),
              March 2007.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., and J. Peterson, "SIP: Session Initiation Protocol",
              RFC 3261, June 2002.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC4828]  Floyd, S. and E. Kohler, "TCP Friendly Rate Control
              (TFRC): The Small-Packet (SP) Variant", RFC 4828,
              April 2007.

19.2.  Informative References

              Guha, S., "NAT Behavioral Requirements for TCP",
              draft-ietf-behave-tcp-07 (work in progress), April 2007.

              Bryan, D., "Concepts and Terminology for Peer to Peer
              SIP", draft-ietf-p2psip-concepts-00 (work in progress),

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              July 2007.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572, July 2006.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              and P. Leach, "HTTP Authentication: Basic and Digest
              Access Authentication", RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [Sybil]    Douceur, J., "The Sybil Attack", IPTPS 02, March 2002.

   [Eclipse]  Singh, A., Ngan, T., Druschel, T., and D. Wallach,
              "Eclipse Attacks on Overlay Networks: Threats and
              Defenses", INFOCOM 2006, April 2006.

              Cheshire, S. and M. Krochmal, "Multicast DNS",
              draft-cheshire-dnsext-multicastdns-06 (work in progress),
              August 2006.

              Krochmal, M. and S. Cheshire, "DNS-Based Service
              Discovery", draft-cheshire-dnsext-dns-sd-04 (work in
              progress), August 2006.

              Cooper, E., "Bootstrap Mechanisms for P2PSIP",
              draft-matthews-p2psip-bootstrap-mechanisms-00 (work in
              progress), February 2007.

              Garcia, G., "P2PSIP bootstrapping using DNS-SD",
              draft-garcia-p2psip-dns-sd-bootstrapping-00 (work in

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              progress), October 2007.

              Camarillo, G., Nikander, P., and J. Hautakorpi, "HIP BONE:
              Host Identity Protocol (HIP) Based Overlay Networking
              Environment", draft-camarillo-hip-bone-00 (work in
              progress), December 2007.

Authors' Addresses

   Cullen Jennings
   170 West Tasman Drive
   MS: SJC-21/2
   San Jose, CA  95134

   Phone:  +1 408 421-9990
   Email:  fluffy@cisco.com

   Bruce B. Lowekamp
   SIPeerior; William & Mary
   3000 Easter Circle
   Williamsburg, VA  23188

   Phone:  +1 757 565 0101
   Email:  lowekamp@sipeerior.com

   Eric Rescorla
   Network Resonance
   2064 Edgewood Drive
   Palo Alto, CA  94303

   Phone:  +1 650 320-8549
   Email:  ekr@networkresonance.com

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   Jonathan Rosenberg
   Edison, NJ

   Email:  jdrosen@cisco.com

   Salman A. Baset
   Columbia University
   1214 Amsterdam Avenue
   New York, NY

   Email:  salman@cs.columbia.edu

   Henning Schulzrinne
   Columbia University
   1214 Amsterdam Avenue
   New York, NY

   Email:  hgs@cs.columbia.edu

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