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Versions: (draft-bryan-sipping-p2p) 00

P2PSIP                                                          D. Bryan
Internet-Draft                              SIPeerior Technologies, Inc.
Intended status: Standards Track                             B. Lowekamp
Expires: August 29, 2007                       SIPeerior; William & Mary
                                                             C. Jennings
                                                           Cisco Systems
                                                       February 25, 2007


     dSIP: A P2P Approach to SIP Registration and Resource Location
                       draft-bryan-p2psip-dsip-00

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
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   This Internet-Draft will expire on August 29, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document outlines the motivation, requirements, and
   architectural design for a distributed Session Initiation Protocol
   (dSIP). dSIP is a Peer-to-Peer (P2P) based approach for SIP
   registration and resource discovery using distributed hash tables
   maintained with SIP messages.  This design removes the need for



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   central servers from SIP, while offering full backward compatibility
   with SIP, allowing reuse of existing clients, and allowing P2P
   enabled peers to communicate with conventional SIP entities.  A basic
   introduction to the concepts of P2P is presented, backward
   compatibility issues addressed, and security considerations are
   discussed.

   dSIP is one possible implementation of the protocols being discussed
   for creation in the P2PSIP WG.  In the context of the work being
   proposed, this draft represents a concrete proposal for the P2PSIP
   Peer Protocol, using SIP with extensions as the underlying protocol.
   In this architecture, no P2PSIP Client Protocol is needed, rather
   unmodified SIP is used for access by non-peers.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Peer-to-Peer Fundamentals  . . . . . . . . . . . . . . . .  6
     3.2.  DHTs and Overlay Structure . . . . . . . . . . . . . . . .  7
     3.3.  P2PSIP . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  The Architecture of dSIP . . . . . . . . . . . . . . . . . . .  8
     4.1.  Peer Functions and          Behavior in dSIP . . . . . . .  9
     4.2.  P2P Overlay Structure  . . . . . . . . . . . . . . . . . . 10
     4.3.  Use of SIP Messages in dSIP  . . . . . . . . . . . . . . . 11
     4.4.  Routing in dSIP  . . . . . . . . . . . . . . . . . . . . . 12
       4.4.1.  dSIP Operations  . . . . . . . . . . . . . . . . . . . 13
     4.5.  Bootstrapping  . . . . . . . . . . . . . . . . . . . . . . 15
   5.  Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Option Tags  . . . . . . . . . . . . . . . . . . . . . . . 16
     5.2.  Hash Algorithms and Identifiers  . . . . . . . . . . . . . 16
       5.2.1.  Peer-IDs . . . . . . . . . . . . . . . . . . . . . . . 16
       5.2.2.  Resource-IDs and the Replication . . . . . . . . . . . 17
     5.3.  P2PSIP URIs  . . . . . . . . . . . . . . . . . . . . . . . 17
       5.3.1.  Peer URIs  . . . . . . . . . . . . . . . . . . . . . . 17
       5.3.2.  Resource URIs and the resource-ID URI Parameter  . . . 18
     5.4.  The DHT-PeerID Header and Overlay Parameters . . . . . . . 19
       5.4.1.  Hash Algorithms and the algorithm Parameter  . . . . . 19
       5.4.2.  Overlay Names and the overlay Parameter  . . . . . . . 20
       5.4.3.  DHT Algorithms and the dht Parameter . . . . . . . . . 21
       5.4.4.  PeerID Expires header parameter  . . . . . . . . . . . 21
     5.5.  The DHT-Link Header  . . . . . . . . . . . . . . . . . . . 21
       5.5.1.  Expires Processing . . . . . . . . . . . . . . . . . . 22
   6.  Message Routing  . . . . . . . . . . . . . . . . . . . . . . . 22
     6.1.  Peer Registration  . . . . . . . . . . . . . . . . . . . . 23



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     6.2.  Resource Registration  . . . . . . . . . . . . . . . . . . 23
     6.3.  Session Establishment  . . . . . . . . . . . . . . . . . . 24
     6.4.  DHT Maintenance  . . . . . . . . . . . . . . . . . . . . . 24
   7.  Peer/DHT Operations  . . . . . . . . . . . . . . . . . . . . . 25
     7.1.  Peer Registration  . . . . . . . . . . . . . . . . . . . . 25
       7.1.1.  Constructing a Peer Registration . . . . . . . . . . . 25
       7.1.2.  Processing the Peer Registration . . . . . . . . . . . 27
     7.2.  Peer Query . . . . . . . . . . . . . . . . . . . . . . . . 29
       7.2.1.  Constructing a Peer Query Message  . . . . . . . . . . 29
       7.2.2.  Processing Peer Query Message  . . . . . . . . . . . . 30
     7.3.  Populating the Joining Peer's Routing Table  . . . . . . . 31
     7.4.  Transfering User Registrations . . . . . . . . . . . . . . 31
     7.5.  Peers Leaving the Overlay Gracefully . . . . . . . . . . . 31
     7.6.  NAT and Firewall Traversal . . . . . . . . . . . . . . . . 32
     7.7.  Handling Failed Requests . . . . . . . . . . . . . . . . . 32
   8.  Resource Operations  . . . . . . . . . . . . . . . . . . . . . 32
     8.1.  Resource Registrations . . . . . . . . . . . . . . . . . . 32
     8.2.  Refreshing Resource Registrations  . . . . . . . . . . . . 33
     8.3.  Removing Resource Registrations  . . . . . . . . . . . . . 34
     8.4.  Querying Resource Registrations  . . . . . . . . . . . . . 34
     8.5.  Session Establishment  . . . . . . . . . . . . . . . . . . 34
     8.6.  Presence . . . . . . . . . . . . . . . . . . . . . . . . . 35
     8.7.  Offline Storage  . . . . . . . . . . . . . . . . . . . . . 35
   9.  Pluggable DHT Algorithm         Requirements . . . . . . . . . 35
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 36
     10.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . . 36
     10.2. Protecting the ID Namespace  . . . . . . . . . . . . . . . 37
       10.2.1. Protection Using ID Hashing  . . . . . . . . . . . . . 37
       10.2.2. Cryptographic Protection . . . . . . . . . . . . . . . 38
     10.3. Protecting the resource namespace  . . . . . . . . . . . . 38
     10.4. Protecting the Routing . . . . . . . . . . . . . . . . . . 39
     10.5. Protecting the Signaling . . . . . . . . . . . . . . . . . 39
     10.6. Protecting the Media . . . . . . . . . . . . . . . . . . . 39
     10.7. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 39
   11. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 39
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 40
   14. Changes to this Version  . . . . . . . . . . . . . . . . . . . 40
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     15.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43
   Intellectual Property and Copyright Statements . . . . . . . . . . 44








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1.  Introduction

   As SIP [1] and SIMPLE based Voice over IP (VoIP) and Instant
   Messaging (IM) systems have increased in popularity, situations have
   emerged where centralized servers are either inconvenient or
   undesirable.  For example, a group of users wishing to communicate
   between each other, but using machines that are not consistently
   connected to the network, are often forced to use a central server
   that is outside the control of the group.  Similarly, groups wishing
   to establish ephemeral networks for use in meetings, conferences, or
   classes often do not wish to configure a centralized server.
   Organizations may also want to allow their members to communicate
   with each other without traffic flowing to third parties, but may not
   have the staff or equipment to maintain a server.

   Peer-to-Peer (P2P) computing has emerged as a mechanism for
   completely decentralized, server-free implementations of various
   applications.  In particular, many recent efforts have focused on
   applying P2P to SIP within the IETF, starting with the forerunners of
   this document submitted by the authors.  Since then a substantial
   usecases document [15] has emerged and, most recently, a concepts and
   terminology [2] document has helped define a common set of terms.
   This iteration of this document incorporates the terminology from
   that draft.

   This draft presents dSIP, a SIP based system that uses P2P mechanisms
   to remove the need for central servers in SIP and SIMPLE based
   communications systems.  While this draft evolved from early work
   done on the SoSIMPLE [16] P2PSIP project, it has changed extensively.
   This works reflects experience gained in actually building
   commercially available P2PSIP products based on this draft, as well
   as from extensive work/insight gleaned from the P2PSIP mailing list.


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [3].

   Terminology defined in RFC 3261 [1] is used without definition.

   We use the terminology and definitions from the Concepts and
   Terminology for Peer to Peer SIP [2] draft extensively in this
   document without further definition.  Other terms used in this
   document are defined inline when used and are also defined below for
   reference.




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   In this illustrative purposes in this document we sometimes use 10
   hexadecimal digit values for SHA-1 hashes.  In reality, SHA-1
   produces 40 digit values.  They are shortened in this document for
   clarity and typographical considerations only.

2.1.  Definitions
   Peer-to-Peer (P2P) Architecture:  An architecture in which peer nodes
      cooperate together to perform tasks.  Each peer has essentially
      equal importance and performs the same tasks within the network.
      Additionally, peers communicate directly with one another to
      perform tasks.  Contrast this to a Client-Server architecture.
   Client-Server Architecture:  An architecture in which some small
      number of nodes (servers) provide services to a larger number of
      nodes (clients).  Client nodes initiate connections to servers,
      but typically do not communicate among themselves.
   Conventional SIP:  The architecture used by SIP as defined by
      RFC3261, RFC3263, and many others.  Conventional SIP centralizes
      certain roles, such as registrar, but allows for direct end-to-end
      establishment of dialogs and media connections.
   Distributed Hash Table (DHT):  A mechanism in which resources are
      given a unique key produced by hashing some attribute of the
      resource, locating them in a hash space (see below).  Peers
      located in this hash space also have a unique ID within the hash
      space.  Peers store information about resources with keys that are
      numerically similar to the peer's ID in the hash space.
   Namespace or hash space:  The range of values that valid results from
      the hash algorithm fall into.  For example, using the SHA-1
      algorithm, the namespace is all 40 digit hexadecimal identifiers.
      This namespace forms the set of valid values for Peer-IDs and
      Resource-IDs (see below).
   Routing Table:  The list of peers that a peer uses to send messages
      to when routing.  The structure and makeup of this table varies
      depending on the particular DHT selected.
   Connection Table:  A list of peers that the peer currently is
      maintaining open connections to.  In general, this is a superset
      of the Routing Table.  The extra entries may be cached entries for
      efficiency or additional entries needed for NAT traversal
      purposes.
   Neighbors:  A collection of peers that a particular peer can reach in
      one hop.  In general, note that a peer's set of neighbors is
      equivalent to the entries in that peer's Routing Table.  However,
      neighbors may include one or more peers that immediately precede
      the peer (predecessors) and one or more peers that immediately
      follow the peer in the namespace (successor peers).  Note that
      neighbor relations do NOT have to be symmetric.






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   Adapter Peer:  An adapter peer is a peer in the overlay that acts as
      an adapter for other non-P2P enabled SIP entities, allowing them
      to access the resources of the overlay.  The adapter peer
      participates actively in the overlay network, while the non-P2P
      enabled SIP entities it provides service to DO NOT participate
      directly in the overlay.  Compare these to the term "super peer"
      in the P2P community, although adapter peers may be thin software
      shims intended for only one client.
   Peer Admission:  The act of a peer joining the overlay.  Registration
      allows a peer to communicate with other peers, and requires
      (allows?) it to take on some server-like responsibilities such as
      maintaining resource location information.  It DOES NOT register
      the user so that they can receive phone calls, which is the
      conventional SIP use of the word registration.  We refer to
      conventional SIP registration as "user registration".
   User Registration:  The act of a user registering themselves with a
      SIP network.  User registration creates a mapping between a SIP
      URI and a contact for a user.  This is the conventional meaning of
      registration in SIP.  For a dSIP peer, this action MUST occur
      after peer registration.  User or resource registration are terms
      used in this draft to refer to P2PSIP Resource Record Insertion,
      with the additional requirement that the resource's (user's) peer
      must first be admitted.
   Joining Peer:  During the peer admission process, this is the peer
      that is attempting to register -- that is, the peer that is
      attempting to join the overlay network.
   Bootstrap Peer:  During the process of peer registration, the
      bootstrap peer is the peer that the joining peer contacts.  This
      peer may be a well-known peer, a peer located using a broadcast
      method, a peer that the joining peer previously knew about, or a
      peer that another bootstrap peer referred the joining peer to.
      The bootstrap peer MAY validate the joining peer's credentials and
      help the joining peer in opening connections to the admitting
      peer, but its primary purpose is to direct the joining peer to the
      admitting peer.
   Admitting Peer:  During the process of peer registration, this is the
      peer that is currently responsible for the portion of the
      namespace the new peer will eventually reside in.  This peer is
      responsible for generating many of the messages exchanged during
      peer registration.


3.  Background

3.1.  Peer-to-Peer Fundamentals

   The fundamental principle behind Peer-to-Peer (P2P) Architectures is
   that applications are provided by number of entities, called peers or



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   nodes working together with each other to accomplish tasks.  Each and
   every peer is responsible for contributing to serving some of the
   transactions that take place on the network.  Contrast this with the
   more traditional Client-Server Architecture in which a large number
   of clients communicate only with a small number of central servers
   responsible for performing tasks.

   Each peer provides server-like functionality and services as well as
   being a client within the system.  In this way, the services or
   resources that would be provided by a centralized entity are instead
   available in a distributed fashion from the peers of the system.
   Note that a particular peer may or may not provide a particular
   service, but some peer does, ensuring that collectively the peers can
   provide that particular service.  The peers form a logical cluster of
   peers called an overlay or overlay network.  The services provided
   are often said to be provided by the overlay, since collectively the
   members provide the services.  The overlay is so named because they
   form a new, small sub-network at a higher logical level than lower
   level network connections.

   In many P2P systems peers are assumed to be ephemeral in nature.  A
   peer may join or leave the overlay at any time.  The design of
   algorithms for P2P architectures take this into account.  Information
   is replicated, and the topology of the overlay can be quickly adapted
   as peers enter and leave.

3.2.  DHTs and Overlay Structure

   While very early P2P systems used flood based techniques, most 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 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, or
   line.

   Peers keep information about the location of other peers in the hash



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   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 user wishes to search,
   they consult the list of peers they are aware of and contact 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 reaches the
   peer responsible for the resource, which then replies to the
   requester.

3.3.  P2PSIP

   Unlike a conventional SIP architecture, P2PSIP systems require no
   central servers.  In a conventional SIP architecture many UAs connect
   to one or more central servers which play a number of roles,
   including proxy server, registrar, presence server, and redirect
   server.  In a P2PSIP architecture, the peers participating in the
   overlay not only act as conventional SIP UAs, allowing their users to
   place and receive calls, but, when viewed collectively with the other
   peers, perform the roles normally provided by a central server.  Each
   participating peer will maintain some fraction of the information
   that would normally be maintained by the central servers in a
   conventional SIP network.

   P2PSIP peers provide many functions, more than any single entity in a
   conventional SIP architecture.  Minimally, a participating peer must
   be an active member of the overlay, participating in storage of
   resources, routing and providing some SIP "server-like" behaviors as
   well.  In the terminology used in the concepts draft, these peers
   speak the P2PSIP Peer Protocol to organize among themselves.

   The general concepts are more fully explained in the Concepts and
   Terminology for Peer to Peer SIP [2] draft.


4.  The Architecture of dSIP

   In this section we provide an overview of the architecture of dSIP
   and explain how it works in an informative way.  Protocol details and
   syntax are provided in a normative form in the remainder of the
   document.

   dSIP is a specific proposal for the P2PSIP Peer Protocol proposed in
   the Concepts and Terminology for Peer to Peer SIP [2] draft, using
   SIP messages as the syntax for encoding the protocol.  The function
   of the P2PSIP Peer Protocol is to provide for mechanisms to maintain
   the overlay, as well as to store and retrieve information, and to



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   route messages when needed. dSIP's syntax is SIP with a number of
   newly defined headers, however no new methods are added to SIP in
   dSIP.

   Because dSIP uses conventional SIP messages, the mechanisms used for
   NAT traversal in SIP, including STUN [4], TURN [17], and ICE [5] are
   reused, as explained in NAT Traversal for dSIP [6].  As a
   consequence, many peers are able to participate in the overlay even
   when behind NATs.  For those that cannot for some reason,
   conventional SIP can be used, and these peers can connect using
   adapter peers, as described below.  Since conventional SIP is used
   for this, there is no need for a P2PSIP Client Protocol, and
   therefore dSIP defines no such protocol.

   dSIP is modular, allowing for the use of multiple DHTs, including
   those defined later.  DHTs can be negotiated among the peers in much
   the same way as codecs or features are negotiated in conventional
   SIP.  For compatibility, support for one basic DHT algorithm, Chord,
   is required.  Additional DHTs can be added and supported.  We detail
   the Chord algorithm for dSIP [7] and provide an alternate DHT
   algorithm for dSIP, based on Bamboo [8].  Note that this document
   does not specify the details of the DHTs, including Chord.  These are
   defined in their own documents, which describe how the basic dSIP
   operations and syntax are used to implement that specific DHT
   algorithms.  Our intention and hope is that others will design other
   overlay algorithms that rely on the same basic operations so that
   compatibility can be maintained.

4.1.  Peer Functions and          Behavior in dSIP

   dSIP peers provide many functions, more than any single entity in a
   conventional SIP architecture.  Minimally, a participating peer must
   be an active member of the overlay and must provide some SIP "server-
   like" behaviors as well.  The code that implements the additional
   server-like and DHT behavior can be located in several places in the
   network.  The simplest is to have peers that are endpoints directly
   joining the overlay as peers.  In this case, these peers provide the
   basic functionality of any SIP endpoint, but additionally implement
   the operations described in this document to enable self-organization
   and provide SIP server-like functionality.

   The behavior can also be located in an adapter peer, which allows one
   or more non-P2P aware SIP UAs (UAs that do not speak dSIP) to
   interact with the P2P overlay network.  These adapters perform the
   additional self-organizing and SIP server-like behavior on behalf of
   the UA or UAs they support.  In this case, only the adapter peer is a
   peer in the overlay, the UAs are not peers themselves.  In this
   approach, the adaptors speak the P2PSIP Peer Protocol (dSIP in this



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   case), where the UAs speak conventional SIP.  All interaction with
   the P2P overlay is carried out by the adapter peer.  The adapter
   essentially acts as a proxy server for the unmodified SIP UAs.  The
   adapter can take the form of a small software shim or may be code
   within a conventional RFC 3261 server.

   In most places in this document, which type of peer we are discussing
   won't affect the discussion.  In those cases where it will, we have
   noted the differences.

4.2.  P2P Overlay Structure

   The P2P overlay in dSIP consists of peers, which collectively serve
   as a directory service for locating resources (users, voicemail
   messages, etc.).  Peers are organized using a supported Distributed
   Hash Table (DHT) P2P structure. dSIP allows for pluggable DHT
   algorithms the exact form of which is defined in the DHT algorithm
   definition.

   Each peer is assigned a Peer-ID, and each resource that is stored in
   the overlay is assigned a Resource-ID.  These values must map to the
   same name space. dSIP provides for various algorithms to be used to
   produce these values, although all members of the overlay must use
   the same algorithm.  For example, in the Chord DHT implementation,
   SHA-1 is used to produce 160 bit values for both the Peer-ID and
   Resource-ID.

   The Peer-ID assigned to each peer determines the peer's location in
   the DHT and the range of Resource-IDs for which it will be
   responsible.  The exact mapping between these is determined by the
   DHT algorithm used by the overlay.  The mechanism for selecting these
   Peer-IDs depends on the security mechanism being used by the overlay.
   For example, a simple SHA-1 hash of the IP address and the port of
   the peer could be used to generate the Peer-IDs or alternatively, a
   certificate based system in which CAs assign Peer-IDs could be used
   to obtain the Peer-IDs [9].

   Every resource has a Resource-ID, obtained by hashing some keyword
   that identifies the resource.  The Resource-IDs map to the same space
   as the Peer-IDs.  In the case of users, the unique keyword is the
   userid and the resource is the registration -- a mapping between the
   user name and a contact.  Resources can be thought of as being stored
   in the distributed hash table at a location corresponding to their
   Resource-ID.  Because entities searching for resources must be able
   to locate them given the unique keyword, Resource-IDs are produced by
   hashing, and are never assigned, regardless of the DHT and security
   algorithms being used.




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   A resource with Resource-ID k will be stored by the peer with Peer-ID
   closest to the Resource-ID, as defined by the particular pluggable
   DHT algorithm being used.  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 the event of a peer failure and to
   protect against compromised or subversive peers.

   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. dSIP defines the
   syntax for the headers used to exchange these entries, but leaves the
   exact form of the data each DHT stores in the table as a decision for
   the DHT implementation.  Peers may additionally maintain a list of
   peers to which they maintain connections for purposes other than
   routing, for example NAT traversal or caching.  This larger table
   (usually a superset of the routing table) is referred to as the
   connection table in dSIP.  In this draft, we refer to routing
   decisions being made from the entries in the routing table, although
   a peer might choose an entry from the connection table if it is a
   better match.

   When locating a resource with a particular Resource-ID, the peer will
   send the request to the routing table entry with the Peer-ID closest
   to the desired Resource-ID, as defined by the particular DHT in use.
   Since DHTs must converge on the resource, the peer receiving the
   request is assumed to know of a peer with a Peer-ID closer to the
   Resource-ID, and responds by suggesting or forwarding the message to
   this peer, depending on the routing mechanism being used.

4.3.  Use of SIP Messages in dSIP

   dSIP uses SIP messages to implement the P2PSIP Peer Protocol.  This
   was done for a number of reasons.  In order to properly implement a
   P2PSIP protocol, it is necessary to have mechanisms to store,
   retrieve and query the locations of resources, as well as to route
   information.  NAT traversal and security considerations require
   several techniques for routing information, as discussed below.
   Pluggable hashing techniques and DHT algorithms require capabilities
   to negotiate the use of these pluggable modules.  We have found SIP
   offers mechanisms that meet all of these requirements today, has well
   defined security mechanisms, and additionally works well with the
   IETF suite of NAT traversal techniques: STUN, TURN and ICE.  Because
   all this work would need to be redefined in a new P2PSIP protocol,
   and because all P2PSIP devices must, by definition, implement SIP
   anyway, we feel the only reasonable syntax choice for the P2PSIP Peer
   Protocol is SIP.




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   Our motivation throughout has been to preserve the semantics of
   conventional SIP messages to the extent possible.  All of the
   messages that are needed to maintain the DHT, as well as those needed
   to query for information, are implemented using SIP messages.
   Fundamentally, messages are being exchanged for two purposes.  The
   purpose of the first class of messages is to maintain the DHT, such
   as the messages needed to join or leave the overlay, and to transfer
   information between peers.  The second type of message is the type
   most SIP users will be familiar with -- registering users, inviting
   other users to a session, etc. -- basic session establishment.  As
   the DHT is used as a distributed registrar, the registration and
   other searches are performed within the DHT.  Once the target
   resource has been located, further communication proceeds directly
   between the UAs (or designated adapter peers) as with conventional
   SIP communications.

   The messages used to manipulate the DHT are SIP REGISTER messages.
   RFC 3261, Section 10.2, specifies that REGISTER messages are used to
   "add, remove, and query bindings."  Accordingly, we have selected
   REGISTER methods to use to add, remove, and query bindings.  We use
   REGISTER both for the bindings of hosts as neighbors (entries in the
   routing table) in DHT maintenance operations as well as the bindings
   of resource names to locations that are commonly maintained by SIP
   registrars.  The only fundamental difference is that these operations
   occur within the overlay, rather than on the conventional server.

4.4.  Routing in dSIP

   When a peer sends a message within the DHT, it begins by calculating
   the target ID it is attempting to locate, using the particular
   algorithm used by the overlay.  The target could be another user, a
   particular resource, or a peer (including itself) for DHT maintenance
   purposes.  It then consults its routing table, and its other neighbor
   peers, for the closest peer it is aware of to the target ID, as
   defined by the closeness metric of the DHT in use.

   In discussions of P2PSIP, several mechanisms have been discussed for
   routing.  In each case, the initial message is sent from the
   requester to the peer in the routing table most likely to route
   correctly, as defined by the DHT algorithm in use.  Subsequently,
   that peer may provide further routing using one of three mechanisms.
   These three types of routing are:
   o  Iterative: If the contacted peer is not responsible for the target
      ID, then the contacted peer issues a 302 redirect response
      pointing the search peer toward the best match the contacted peer
      has for the target ID.  The searching peer then contact the peer
      to which it has been redirected and the process iterates until the
      responsible peer is located.



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   o  Recursive: If the contacted peer is not responsible for the target
      ID, it will forward the query to the nearest peer to the target
      that it knows, and the process repeats until the target is
      reached.  The response unwinds and follows the same path on the
      message return.  Because dSIP uses SIP messages for transport,
      SIP's proxy behavior is used to enable recursive routing.
   o  Semi-Recursive: Semi-Recursive is the same as Recursive routing on
      the outbound leg, but the reply "shortcuts" and is directly sent
      back to the requester.  When discussing these techniques, we often
      just refer to Iterative and Recursive, because of the similarity
      between recursive and semi-recursive routing.

   Various mechanisms may be used within the same overlay and even
   within the same search.  For example, a search may start as
   iterative, but if a particular peer receiving the request knows that
   the requester cannot reach the next hop directly (perhaps due to NAT
   issues), the search may have recursive and iterative portions.

   In general, the messages can be routed using any of these mechanisms,
   and this draft does not specify which mechanism will be used.  The
   decision as to which mechanism is appropriate may be a factor of
   security, NAT traversal, or even the properties of the particular DHT
   being used.  We generally refer to the message as being routed
   through the overlay.

4.4.1.  dSIP Operations

   dSIP provides mechanisms that are used for a number of operations.

4.4.1.1.  Peer Registration

   When a peer (called the joining peer) wishes to join the overlay, it
   determines its Peer-ID and sends a REGISTER message to a bootstrap
   peer already in the overlay, requesting to join.  Any peer in the DHT
   may serve as a bootstrap peer.  The mechanism for selecting bootstrap
   peers is application dependent, and discussed in Bootstrapping
   (Section 4.5).

   Following the iterative routing scheme, the bootstrap peer looks up
   the peer it knows nearest to the Peer-ID of the joining peer and
   responds with 302 redirect to this closer peer.  The joining peer
   will repeat this process until it reaches the peer currently
   responsible for the space it will occupy.

   If recursive routing is being used, the bootstrap peer looks up the
   peer it knows nearest to the Peer-ID of the joining peer and forwards
   the REGISTER message to that peer.  This process of forwarding the
   message repeats until the peer currently responsible for the space



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   the joining peer will occupy is found.

   Once the peer responsible for the joining peer's portion of the
   namespace is located, the joining peer then exchanges DHT state
   information with this peer, called the admitting peer, to allow the
   joining peer to learn about other peers in the overlay (neighbors)
   and to obtain information about resources the joining peer will be
   responsible for maintaining.  Other DHT maintenance messages will be
   exchanged later to maintain the overlay as other peers enter and
   leave, as well as to periodically verify the information about the
   overlay, but once the initial messages are exchanged, a peer has
   joined the overlay.

4.4.1.2.  Resource Registration

   The peer registration does not register the peer's user(s) or other
   resources with the P2PSIP network -- it has only allowed the peer to
   join the overlay.  Once a peer has joined the overlay, the user that
   peer hosts must be registered with the system.  This process is
   referred to as resource registration.  This registration is analogous
   to the conventional SIP registration, in which a message is sent to
   the registrar creating a mapping between a SIP URI and a user's
   contact.  The only difference is that since there is no central
   registrar, some peer in the overlay will maintain the registration on
   the users behalf.

   Resource registrations are routed similarly to peer registrations.
   The resource's peer calculates the resource-ID and contacts the peer
   it is aware of nearest to the resource-ID.  This search process
   continues in either an iterative or recursive manner until the
   responsible peer is located.  This peer then stores the registration
   for that user and returns a 200 response.

   For redundancy, resources should also be registered at additional
   peers within the overlay.  These replicas are located by adding a
   replica number to the resource name and hashing to identify a new
   resource-ID for each replica.  In this way, replicas are located at
   unrelated points around the DHT, minimizing the risk of an attacker
   compromising more than one registration for a single resource.

4.4.1.3.  Session Establishment

   Sessions are established by contacting the UA identified by the
   registration in the DHT.  The first step in establishing a session is
   locating this peer, which is done by searching for a resource in the
   DHT.  The name of the target resource is used to calculate a
   resource-ID and a REGISTER message with no Contact information (a
   conventional SIP search) is sent to the closest known peer to that



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   resource-ID.  The search iterates until the responsible peer is
   located.  The responsible peer then returns either a 200 OK with the
   Contact information for the resource or a 404 Not Found.  The session
   is then initiated directly with the resource's UA.

4.4.1.4.  DHT Maintenance

   In order to keep the overlay stable, peers must periodically perform
   book keeping operation to take into account peer failures.  These DHT
   maintenance messages are sent using REGISTER messages and the overlay
   algorithm being used will dictate how often and where these messages
   are sent.

   DHT maintenance messages are routed similarly to peer registrations
   and resource registrations.  The peer calculates the Peer-ID of the
   peer it wants to exchange DHT information with and contacts the peer
   it is aware of closest to that Peer-ID.  This search process
   continues in either an iterative or recursive manner until the peer
   is located at which point the peers exchange DHT maintenance
   information.

4.5.  Bootstrapping

   When a peer wishes to join an existing overlay, it must first locate
   some peer that is already participating in the overlay, referred to
   as the bootstrap peer.  Peers may use any method they choose to
   locate the initial bootstrap peer --- the decision is outside the
   scope of this specification.  The following are a few of the many
   methods that may be used:
   Static Locations:  Some number of peers in the overlay may be
      persistent and have well know addresses.  These addresses could be
      configured into the peer application or obtained using an out-of-
      band mechanism such as a web page.
   Cached Peers:  While this mechanism cannot be used the first time
      that a peer runs, on subsequent attempts to join the overlay a
      peer might attempt to use a previously contacted peer as a
      bootstrap peer.
   Broadcast mechanisms:  Peers can use a broadcast mechanism to locate
      the initial peer, for example by sending the first REGISTER
      message to the SIP multicast address.


5.  Message Syntax

   This section provides normative text explaining the syntax of the
   extensions we use for SIP messages.





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5.1.  Option Tags

   We create a new option tag "dht" as described in RFC 3261.  This
   option tag indicates support for DHT based P2PSIP.  Peers MUST
   include a Require and Supported header with the option tag dht for
   all messages that are intended to be processed using dSIP or include
   P2P extensions.  Clients supporting P2P and contacting another SIP
   entity using a non-P2P mechanism for a transaction that may or may
   later be P2P SHOULD include a Supported header with dht.  For a
   typical session establishment the search within the DHT MUST specify
   Require dht, whereas the actual contact with the resource's UA SHOULD
   include a Supported header with dht but SHOULD NOT include a Require
   header with dht.

5.2.  Hash Algorithms and Identifiers

   All IDs used for an overlay must be calculated using the same
   algorithm.  Implementations MUST support the SHA-1 algorithm, which
   produces a 160 bit hash value.  The hash algorithm used is specified
   in the DHT-PeerID header, described below.  An implementation MAY
   rely on a secret initialization vector, key, or other shared secret
   to use the identifier as an HMAC, from RFC 2104 [10] such that no
   peer may join the overlay without knowledge of the shared secret,
   however this technique by itself does not protect the overlay against
   replay attacks.  See Security Extensions to the Distributed Session
   Initiation Protocol (dSIP) [9]for information on how to protect
   against replay attacks.

   Both Peer-IDs and Resource-IDs MUST have the same range of values
   (map to the same space).  Formally:

   P2PID = token

   When using SHA-1:

   P2PID = 40LHEX

5.2.1.  Peer-IDs

   The particular DHT algorithm being used MAY specify an alternate
   mechanism for determining Peer-ID.  Similarly, some security models
   may assign Peer-IDs from a central authority.  In the event that
   neither of these mechanisms are being used, the Peer-ID MUST be
   formed by taking the IP address of the peer, without the colon or
   port, and with no leading zeros, and hashing this string with the
   hash algorithm.  Then the least significant sixteen bits of the hash
   are replaced by the port used by the peer.  For peers behind a NAT
   participating in an overlay on the public Internet, they must



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   identify their address on the public Internet through a protocol such
   as STUN [4] and use this address for their Peer-ID.

   The string hashed to obtain the PeerID is formally defined below as
   ipaddress.

   ipaddress         =  IPV4address / IPv6reference

   PeerID is formally defined as:

   PeerID = P2PID

5.2.2.  Resource-IDs and the Replication

   Resource-IDs MUST be formed by hashing the resource URI after
   converting it to canonical form.  To do that, all URI parameters MUST
   be removed (including the user-param) except for the replica URI
   parameter, Any escaped characters MUST be converted to their
   unescaped form.  Formally:

   ResourceID = P2PID

5.3.  P2PSIP URIs

   Because hashing URIs to produce identifiers is a non-trivial cost,
   dSIP messages are constructed including these values already
   calculated.  This is strictly as a courtesy to peers processing
   messages for this peer, as it prevents them from having to hash the
   URI again before routing.  Identifiers provided in a message are a
   courtesy only and MUST NOT be used when making any changes to the
   data stored in an overlay, as they may be spoofed or incorrect.  If
   the hash parameter is used incorrectly for routing, this only affects
   the transmitting peer's user.  If it is used to insert or modify
   stored information, it can affect the system's integrity.  Peers MUST
   verify the hash of all URIs before making changes that affect the
   overlay.

5.3.1.  Peer URIs

   A P2PSIP peer is represented by constructing a SIP-URI (or SIPS-URI)
   with the keyword "peer" or a short form of "P" for the userinfo
   portion.  The URI parameter "peer-ID", or the short form "pID" MUST
   be used.

   PeerURI        =   ("peer@" / "P@") hostport ";" PeerID-Param ";"
                      uri-parameters

   Formally, the peerID uri-parameter is defined as type other-param



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   from RFC 3261 with a pname of "peerID" or "pID" for short form, and a
   pvalue which is of type PeerID.  A peer receiving a PeerURI MUST
   verify the hash value of the PeerID-Param before using it to update
   its routing table.

   PeerID-Param        =   ("peer-ID" / "pID") EQUAL PeerID

   For search operations, where an identifier is being searched for, but
   the host responsible for that identifier is unknown, hostport MUST be
   set to "0.0.0.0".  All non-search operations MUST specify a valid
   hostport.

   P2P Peer URIs MUST NOT include the resource-ID URI parameter (below),
   as it is intended to define information about resources that are
   stored in the overlay, not information about the peers making up the
   overlay.  P2P Peer URIs used in name-addr SHOULD NOT include any
   display-name information, and peers receiving name-addrs for peers
   with display-name information MUST ignore the information.

   Examples, using a shortened hash for clarity:
   The URI for a peer using the SHA-1 hash           algorithm, with
   hashed ID ed57487add matching an IP address           10.6.5.5 used
   in a To header. Uses the short forms:

   To: <sip:P@10.6.5.5;pID=ed57487add>

   The URI for a peer using the SHA-1 hash           algorithm, with
   hashed ID ed57487add matching an IP address           10.6.5.5 used
   in a To header. Uses the long forms:

   To: <sip:peer@10.6.5.5;peer-ID=ed57487add>

5.3.2.  Resource URIs and the resource-ID URI Parameter

   Resource URIs are no different for P2PSIP resources than for non-P2P
   SIP applications.  However, because calculating the ResourceID is a
   significant expense, the optional URI parameter resource-
   ID=<Resource-ID> or the short form rID=<Resource-ID> SHOULD be
   provided.  This parameter is a courtesy only and MUST NOT be used
   when making any changes to the data stored in an overlay without
   being recalculated, as it may be spoofed or incorrect.  The
   resource-ID URI parameter is of type other-param as defined in RFC
   3261.

   resourceID-param  =  ("resource-ID" \ "rID") EQUAL ResourceID

   P2P Resource URIs MUST NOT include the PeerID-Param URI parameter,
   because this indicates that the target of the URI is a peer.  P2P



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   Resource URIs MAY include other user-parameters such as user=phone.

   Examples (again using shortened hashes for clarity):
   The URI for a user resource with username     bob@p2psip.org using
   the SHA-1 hash     algorithm, with hashed Resource-ID     723fedaab1.
   The optional resource-ID URI     parameter is included, using the
   long form:

   sip:bob@p2psip.org;resource-ID=723fedaab1

   The URI for a user resource with username     bob@p2psip.org using
   the SHA-1 hash     algorithm, with hashed Resource-ID     723fedaab1.
   The optional resource-ID URI     parameter is included, using the
   short form:

   sip:bob@p2psip.org;rID=723fedaab1

   The URI, used in a To header for user Alice White, with username
   alice@p2psip.org. This example omits the optional resource-ID URI
   parameter:

   To: "Alice White" <sip:alice@p2psip.org>

5.4.  The DHT-PeerID Header and Overlay Parameters

   We introduce a new SIP header called the DHT-PeerID header.  This
   header is used to express the Peer-ID of the sending peer as well as
   to identify the name and parameters of the overlay.  The format of
   the DHT-PeerID header is as follows:

   DHT-PeerID  = "DHT-PeerID" HCOLON PeerURI SEMI algorithm SEMI
                 dht-param SEMI overlay-param *(SEMI generic-param)

   Examples:
   A peer with an SHA-1 hashed Peer-ID of a04d371e on IP 192.168.1.1.
   We include the PeerURI, algorithm, dht-param, and overlay as well as
   the optional expires header parameter. In this example, the overlay
   name is chat and the DHT algorithm being used is dhtalg1.0


   DHT-PeerID: <sip:peer@192.168.1.1;peer-ID=a04d371e>;algorithm=sha1;
               dht=dhtalg1.0;overlay=chat;expires=600

5.4.1.  Hash Algorithms and the algorithm Parameter

   The hash algorithm used for the overlay is specified as a parameter
   of the DHT-PeerID header.  This parameter MUST appear in the DHT-
   PeerID header.  It MUST be the algorithm used to calculate all PeerID



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   and ResourceID values used in the message.  It SHOULD NOT appear in
   other headers in the message, but if it does it MUST match the value
   in the DHT-PeerID header.

   The hash algorithm is specified using the algorithm parameter from
   RFC3261.  The tokens used to identify the algorithm MUST be the same
   as those used in other SIP documents such as RFC4474. [11] Currently,
   those consist of 'sha1', indicating SHA-1 as defined in RFC 3174 [12]
   and 'hmac-sha1', indicating HMAC-SHA1 as defined in RFC2104 [10].
   Implementations MUST support the SHA-1 algorithm.

   A peer MUST reject a message with 488 Not Acceptable here if it
   specifies a different hash algorithm than that used by the peer's
   overlay.  An initial contact to a bootstrap peer may specify the hash
   algorithm as the wildcard "*", in which case the joining peer
   indicates its willingness to use whatever hash algorithm the
   bootstrap peer identifies in its response.  A peer responding to such
   a request MUST route the message according to the rules described in
   the Message Routing Section (Section 6) if all other elements of the
   message are correct and the routing algorithm indicates such a
   response is appropriate.  If the normal response would be to allow
   the join with a 200 OK, the receiving peer MAY respond with a 302
   redirect to itself and specifying the algorithm used in this overlay,
   in which case the joining peer should reissue the message with the
   proper hash algorithm specification.

5.4.2.  Overlay Names and the overlay Parameter

   Each overlay is named using a string, which SHOULD be unique to a
   particular deployment environment.  Peers will use this value to
   identify messages in cases where they may belong to multiple overlays
   simultaneously.  These are defined formally simply as a token:

   overlay-name       = "*" / token

   The overlay-param parameter MUST appear in the DHT-PeerID header.  It
   SHOULD NOT appear in other headers in the message, but if it does it
   MUST match the value in the DHT-PeerID header.  This parameter is
   defined formally as:

   overlay-param      = "overlay" EQUAL overlay-name

   A peer MUST reject a message with 488 Not Acceptable here if it
   specifies an overlay in which the peer is not participating.  An
   initial contact to a bootstrap peer MAY specify overlay-name as the
   wildcard "*", in which case the joining peer indicates its
   willingness to join whatever overlay the bootstrap peer identifies in
   its response.  A peer responding to such a request MUST route the



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   message according to the rules described in the Message Routing
   Section (Section 6) if all other elements of the message are correct
   and the routing algorithm indicates such a response is appropriate.
   If the normal response would be to allow the join with a 200 OK, the
   receiving peer MAY respond with a 302 redirect to itself, in which
   case the joining peer should reissue the message with the proper
   overlay specification.

5.4.3.  DHT Algorithms and the dht Parameter

   The routing algorithm used to implement the overlay is specified
   using a dht-param in the DHT-PeerID header.  It SHOULD NOT appear in
   other headers in the message, but if it does it MUST match the value
   in the DHT-PeerID header.  This parameter is defined formally as:

   dht-name           = token
   dht-param          = "dht" EQUAL dht-name

   The behavior of a peer receiving a message with a dht-param
   specifying a routing algorithm other than that which it is following
   is dependent on the routing algorithm.  An initial contact of a
   bootstrap peer MAY specify dht-param as the wildcard "*", in which
   case the joining peer indicates its willingness to use whatever DHT
   algorithm the bootstrap peer identifies in its response.  A peer
   responding to such a request MUST route the message according to the
   rules described in the Message Routing Section (Section 6) if all
   other elements of the message are correct and the routing algorithm
   indicates such a response is appropriate.  New routing algorithms
   SHOULD be designed to maintain backward compatibility with previous
   algorithms where possible.  If the routing algorithm specified is
   incompatible, a 488 Not Acceptable Here response MUST be returned.

5.4.4.  PeerID Expires header parameter

   The DHT-PeerID header MAY include an Expires parameter indicating how
   long a recipient may keep knowledge of this peer.  If not present, a
   default of 3600 is assumed.  Mobile peers may wish to specify a
   shorter interval.

5.5.  The DHT-Link Header

   We introduce a new SIP header called the DHT-Link header.  The DHT-
   Link header is used to transfer information about where in the DHT
   other peers are located.  In particular, it is used by peers to pass
   information about its neighbor peers and routing table information
   stored by a peer.





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   DHT-Link    = "DHT-Link" HCOLON PeerURI SEMI link-param SEMI
                      expires-param *(SEMI generic-param)
   link-param      = "link" EQUAL link-value
   expires-param   = "expires" EQUAL delta-seconds

   The value of linkvalue -- that is, how you represent what type of
   link this is, is defined by the DHT algorithm specification.  The
   generic-param leaves flexibility for an algorithm to add additional
   parameters if needed.

   As an example, the header might look like (using a shortened 10 digit
   Peer-ID for clarity).  The value *** here is intended to represent a
   value determined by the particular DHT:

DHT-Link: <sip:peer@192.168.0.1;Peer-ID=671a65bf22>;link=***;expires=600

5.5.1.  Expires Processing

   Each DHT-Link header MUST contain an expires parameter.  Each peer
   maintains an expiration time for each of its neighbor and routing
   table entries.  These expiration times are updated whenever the peer
   receives a response with a longer expiration time than it currently
   maintains, most commonly in the PeerID header of a response to a join
   or search.  A peer MUST NOT report an expired entry in a DHT-Link
   header.  A peer MUST update the expires parameter with the current
   value, adjusted for passed time, each time it generates a DHT-Link
   header.


6.  Message Routing

   When a peer sends a message within the DHT, it begins by calculating
   the target ID it is attempting to locate, which might be its own
   location in the DHT, or a user's registration, for which it hashes
   the user's URI to obtain the appropriate Resource-ID.  It then
   consults its routing table, and its other neighbor peers, for the
   closest peer it is aware of to the target ID.

   The messages in the overlay MAY be routed either iteratively or
   recursively.  The Request-Disposition header as described in [13]
   SHOULD be used to indicate if the next node should process the
   message using a recursive or iterative mechanism.  If the header is
   omitted, the receiving node may process the message either
   recursively or iteratively.

   If the Request-Disposition header is iterative, the contacted peer
   MUST determine if it is responsible for that target ID.  If it is
   not, then the contacted peer MUST issue a 302 redirect pointing the



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   search peer toward the best match the contacted peer has for the
   target ID.  The searching peer then contact the peer to which it has
   been redirected and the process iterates until the responsible peer
   is located.

   In recursive routing, the peer sends a message to the peer it knows
   that is nearest to the target.  If the contacted peer is not
   responsible for the target ID, it MUST forward the query to the
   nearest peer to the target that it knows, and the process repeats
   until the target is reached.  This process follows standard proxy
   behavior in RFC 3261.

6.1.  Peer Registration

   When a peer (the joining peer) wishes to join the overlay, it creates
   its Peer-ID and sends a REGISTER message to a bootstrap peer already
   in the overlay, requesting to join.  Any peer in the DHT may serve as
   a bootstrap peer, although we expect that most UAs will be configured
   with a small number of well-known peers.

   Following the iterative routing scheme, the bootstrap peer looks up
   the peer it knows nearest to the Peer-ID of the joining peer and
   responds with 302 redirect to this nearer peer.  The joining peer
   will repeat this process until it reaches the peer currently
   responsible for the space it will occupy.

   If recursive routing is being used, the bootstrap peer looks up the
   peer it knows nearest to the Peer-ID of the joining peer and forwards
   the REGISTER message to that peer.  This process of forwarding the
   message repeats until the peer currently responsible for the space
   the joining peer will occupy is found.

   Once the peer responsible for the joining peer's portion of the
   namespace is located, the joining peer then exchanges DHT state
   information with this peer, called the admitting peer, to allow the
   joining peer to learn about other peers in the overlay (neighbors)
   and to obtain information about resources the joining peer will be
   responsible for maintaining.  Other DHT maintenance messages will be
   exchanged later to maintain the overlay as other peers enter and
   leave, as well as to periodically verify the information about the
   overlay, but once the initial messages are exchanged, a peer has
   joined the overlay.

6.2.  Resource Registration

   The peer registration does not register the peer's user(s) or other
   resources with the P2PSIP network -- it has only allowed the peer to
   join the overlay.  Once a peer has joined the overlay, the user that



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   peer hosts must be registered with the system.  This process is
   referred to as resource registration.  This registration is analogous
   to the conventional SIP registration, in which a message is sent to
   the registrar creating a mapping between a SIP URI and a user's
   contact.  The only difference is that since there is no central
   registrar, some peer in the overlay will maintain the registration on
   the users behalf.

   Resource registrations are routed similarly to peer registrations.
   The resource's peer calculates the resource-ID and contacts the peer
   it is aware of closest to the resource-ID.  This search process
   continues in either an iterative or recursive manner until the
   responsible peer is located.  This peer then stores the registration
   for that user and returns a 200 response.

   For redundancy, resources should also be registered at additional
   peers within the overlay.  These replicas are located by adding a
   replica number to the resource name and hashing to identify a new
   resource-ID for each replica.  In this way, replicas are located at
   unrelated points around the DHT, minimizing the risk of an attacker
   compromising more than one registration for a single resource.

6.3.  Session Establishment

   Sessions are established by contacting the UA identified by the
   registration in the DHT.  The first step in establishing a session is
   locating this peer, which is done by searching for a resource in the
   DHT.  The name of the target resource is used to calculate a
   resource-ID and a REGISTER message with no Contact information (a
   conventional SIP search) is sent to the closest known peer to that
   resource-ID.  The search iterates until the responsible peer is
   located.  The responsible peer then returns either a 200 OK with the
   Contact information for the resource or a 404 Not Found.  The session
   is then initiated directly with the resource's UA.

   If the peer needs to have the session establishment routed through
   the overlay, it MAY use the Request-Disposition header with a value
   of proxy to request that intermediate nodes proxy the invite over the
   overlay on their behalf.  This is particular critical for NAT
   traversal [6].

6.4.  DHT Maintenance

   In order to keep the overlay stable, peers must periodically perform
   book keeping operations to take into account peer failures.  These
   DHT maintenance messages are sent using REGISTER messages and the
   overlay algorithm being used will dictate how often and where these
   messages are sent.



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   DHT maintenance messages are routed similarly to peer registrations
   and resource registrations.  The peer calculates the Peer-ID of the
   peer it wants to exchange DHT information with and contacts the peer
   it is aware of closest to that Peer-ID.  This search process
   continues until the current closest peer to the target Peer-ID is
   located at which point the peers exchange DHT maintenance
   information.


7.  Peer/DHT Operations

   The SIP REGISTER message is used extensively in this system.
   REGISTER is used to register users, as in conventional SIP systems,
   and we discuss this further in the Resource Registration
   (Section 8.1) section of this document.  Additionally, SIP REGISTER
   messages are used to register a new peer with the DHT and to transmit
   the information needed to maintain the DHT.

7.1.  Peer Registration

   After a peer has located an initial bootstrap peer, the process of
   joining the overlay is started by constructing a REGISTER message and
   sending it to the bootstrap peer.  Third party registration MAY NOT
   be used for registering peers into the overlay, and attempts to do so
   MUST be rejected by the peer receiving such a request (although third
   party registrations are used for other purposes, as described below).
   The peer MUST construct a SIP REGISTER message following the
   instructions in RFC3261, Section 10, with the exceptions/rules
   outlined below.

7.1.1.  Constructing a Peer Registration

   The Request-URI MUST include only the IP address of the peer that is
   being contacted (initially the bootstrap peer).  This URI MUST NOT
   include any of the P2P defined parameters.  For example, a request
   intended for peer 10.3.44.2 should look like: "REGISTER sip:10.3.44.2
   SIP/2.0".

   The To and From fields of the REGISTER message MUST contain the URI
   of the registering peer constructed according to the rules in the
   subsection Peer URIs (Section 5.3.1) in the Message Syntax section.

   While allowing the IP address of the sender for To and From is
   different than conventional SIP registers, there are two reasons for
   this.  First, in a P2P network, which peer the request is sent to,
   and thus the domain for which the registration is intended, is not
   important.  Any peer can process the information, and the user name
   is not associated with a particular IP address or DNS domain, but



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   rather with the overlay name, which is encoded elsewhere.  In that
   sense, the IP address used is irrelevant.  Choosing the domain of the
   sender ensures that if a request is sent to a non-P2P aware RFC 3261
   compliant registrar, it will be rejected.  RFC 3261 (section 10.3)
   states that a registrar should examine the To header to determine if
   it presents a valid address-of-record for the domain it serves.
   Since the IP address of the sending peer is unlikely to be a valid
   address for a non-P2P aware registrar, the message will be rejected,
   eliminating possibly erroneous handling by the registrar.

   The registering peer MUST also list its PeerURI in the contact field
   when registering so that this may be identified as a registration/
   update, rather than a query.  The peer MUST provide an expires
   parameter or expires header with a non-zero value.  As in standard
   SIP registrations, Expire headers with a value of zero will be used
   to remove registrations.

   The registering peer MUST provide a DHT-PeerID header field.  It MAY
   leave the overlay parameter set to "*" for its initial registration
   message, but MUST set this parameter to the name of the overlay it is
   joining as soon as it receives a response from the bootstrap peer.

   The registering peer MUST include Require and Supported headers with
   the option tag "dht".

   Assume that a peer running on IP address 10.4.1.2 on port 5060
   attempts to join the network by contacting a bootstrap peer at
   address 10.7.8.129.  Further assume that 10.4.1.2 hashes to
   463ac4b449 under SHA-1 (using a 10 digit hash for example
   simplicity), and the least significant bits are replaced with the
   port number, yielding 463ac413c4 and that the overlay name is chat
   and the dht-param is dhtalg1.0.  An example message would look like
   this (neglecting tags):

   REGISTER sip:10.7.8.129 SIP/2.0
   To: <sip:peer@10.4.1.2;peer-ID=463ac413c4>
   From: <sip:peer@10.4.1.2;peer-ID=463ac413c4>
   Contact: <sip:peer@10.4.1.2;peer-ID=463ac413c4>
   Expires: 600
   DHT-PeerID: <sip:peer@10.4.1.2;peer-ID=463ac413c4>;algorithm=sha1;
               dht=dhtalg1.0;overlay=chat;expires=600
   Require: dht
   Supported: dht








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7.1.2.  Processing the Peer Registration

   The receiving peer determines that this is a P2PSIP message based on
   the presence of the dht Require and Supported fields.  In the event
   that the peer does not support P2P extensions, it MUST reply with a
   420 Bad Extension response.  If the peer examines the overlay
   parameters and determines that this is not an overlay the peer
   participates in, the peer MUST reject the message with a 488 Not
   Acceptable Here response.  Likewise if the peer examines the dht-
   param and determines that the algorithm specified is not compatible
   with its algorithm, the peer MUST reject the message with a 488 Not
   Acceptable Here response.  If a P2P peer receives a non-P2P request
   it MAY reject it with a message such as 421 Extension Required or it
   MAY process it as a conventional SIP message.

   An implementation may support both P2P and conventional SIP messages.
   In that case, it MAY include the dht Supported field with all
   messages but MUST NOT include it with messages intended for
   conventional nodes.

7.1.2.1.  Routing the Peer Registration

   The presence of peer-ID URI parameter in the To and Contact headers
   and a valid expiration time indicate that this message is a peer
   registration and the receiving peer MUST process this as a DHT level
   request.  The bootstrap peer SHOULD verify that the Peer-ID
   corresponds to peer listed in the URI by validating the hash or the
   peers credentials.  If these do not match, the message SHOULD be
   rejected with a response of 493 Undecipherable.  The bootstrap peer
   examines the Peer-ID to determine if it corresponds to the portion of
   the overlay the bootstrap peer is responsible for.  If it does, the
   peer will handle the REGISTER request itself.  If not, the bootstrap
   peer will either provide the joining peer with information about a
   peer closer to the area of the overlay where the joining peers
   Peer-ID is stored (iterative routing) or forward the request along
   the closest peer it knows about (recursive routing).  If a Request-
   Disposition header is present and set to proxy, the peer MUST use a
   recursive routing mechanism, and if it is present and set to
   redirect, the peer MUST use an iterative routing mechanism.  In the
   event that the Request-Disposition header is not present, the peer
   may choose either mechanism.

   In the case of iterative routing, if the receiving peer is not
   responsible for the area of the hash table where Peer-ID should be
   stored, the peer SHOULD generate a 302 message.  The 302 is
   constructed according the rules of RFC 3261 with the following rules.
   The receiving peer MUST look in its list of neighbors or in the
   routing table to find the peer with Peer-ID nearest the to joining



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   peer's Peer-ID, and use it to create a contact field in the form of a
   peer URI, as specified in the P2P Peer URIs (Section 5.3.1) section
   of this document, including appropriate URI parameters.  The response
   MUST contain a valid DHT-PeerID header.  This response is sent to the
   joining peer.

   In the case of recursive routing, if the receiving peer is not
   responsible for the area of the hash table where the Peer-ID should
   be stored, the receiving peer should forward the request to the peer
   it knows about that is closest to the Peer-ID.

   A peer MUST NOT add a new peer to its routing table or redirect
   requests to that new peer until it has successfully contacted that
   peer itself.  By redirecting a message to another peer, the contacted
   peer indicates that it believes that peer to be alive and that it is
   willing to route messages to it for NAT and Firewall traversal
   purposes.

   Using our example register from the previous section, assume that
   iterative routing is being used and that the bootstrap peer
   10.7.8.129 receives the message, determines it is not responsible for
   that area of the overlay, and redirects the joining peer to a peer
   with Peer-ID 47e46fa2cd at IP address 10.3.1.7.  The 302 response,
   again neglecting tags, is shown below.  Note that the peer creating
   the response uses its information to construct the DHT-PeerID header.

   SIP/2.0 302 Moved Temporarily
   To: <sip:peer@10.4.1.2;peerId=463ac413c4>
   From: <sip:peer@10.4.1.2;peerId=463ac413c4>
   Contact: <sip:peer@10.3.1.7;peerId=47e46fa2cd>
   Expires: 600
   DHT-PeerID: <sip:@10.7.8.129;peerId=084d299ff2>;algorithm=sha1;
               dht-param=dhtalg1.0;overlay=chat;expires=600
   Require: dht
   Supported: dht

   Upon receiving the 302, the joining peer uses the contact address as
   the new bootstrap peer.  The process is repeated until the peer
   contacted is currently responsible for the area of the DHT in which
   the new peer will reside.  The receiving peer that is responsible for
   that portion of the overlay is referred to as the admitting peer.

   TODO: we should have example of how to forward request in a recursive
   routing case







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7.1.2.2.  Admitting the Joining Peer

   The admitting peer MUST verify that the Peer-ID is valid, as
   described above.  If these do not match, the message MUST be rejected
   with a response of 493 Undecipherable.  The admitting peer recognizes
   that it is presently responsible for this region of the hash space --
   that is, it is currently the peer storing the information that this
   Peer-Id will eventually be responsible for.  The admitting peer knows
   this because the joining peer's Peer-ID is closest to its own
   Peer-ID.  The admitting peer is responsible for helping the joining
   peer become a member of the overlay.  In addition to verifying that
   the Peer-ID was properly calculated, the admitting peer MAY perform
   additional security checks [9].  Once any challenge has been met, the
   admitting will reply with a 200 OK message to the joining peer.  As
   in a conventional registration, the Contact in the 200 OK will be the
   same as in the request, and the expiry time MUST be provided.

   The admitting peer MUST reply with a 200 response if the admitting
   peer's Peer-ID is the closest to the joining peer's Peer-ID.  Each
   DHT algorithm MAY choose to define closest however they want, but the
   DHT algorithm MUST be able to deterministically find the closest
   Peer-ID.  The admitting peer must populate the DHT-Link headers with
   all values required by the DHT routing protocol so that the joining
   peer can initialize its neighbors and routing table entries.
   Additionally, the admitting peer MUST include its DHT-PeerID header
   containing the admitting peer's Peer-ID and IP.

7.2.  Peer Query

   As with conventional SIP, REGISTER messages that are sent without a
   Contact: header are assumed to be queries, as described in Section 10
   of RFC3261.

7.2.1.  Constructing a Peer Query Message

   The peer looks for the routing table entry or neighbor peer that is
   closest to the ID they are searching for.  If the routing table has
   not yet been filled, then the peer may send the request to any peer
   it has available, including their other neighbor peers or even some
   bootstrap peer.  While these initial searches may be less efficient,
   they will succeed.  The Request-URI MUST include only the IP address
   of the peer that the search is intended for.  This URI MUST NOT
   include any of the P2P defined parameters.  For example, a request
   intended for peer 10.3.44.2 should look like: "REGISTER sip:10.3.44.2
   SIP/2.0".

   Because this is a query, the sending peer MUST NOT include a contact
   header.  The sender MUST NOT include an expires header.



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   The peer MUST provide a DHT-PeerID header.

   The peer MUST include Require and Supported headers with the option
   tag "dht".

   Assume that a peer running on IP address 10.4.1.2 on port 5060 wants
   to determine who is responsible for Peer-ID 4823affe45, and asks the
   peer with IP address 10.5.6.211 Further assume that the peer uses
   SHA-1 (using a 10 digit hash for example simplicity), and that the
   overlay name is chat.  An example message would look like this
   (neglecting tags):

   REGISTER sip:10.5.6.211 SIP/2.0
   To: <sip:peer@0.0.0.0;peerId=4823affe45>
   From: <sip:peer@10.4.1.2;peerId=463ac413c4>
   DHT-PeerID: <sip:peer@10.4.1.2;peerId=463av413c4>;algorithm=sha1;
               dht-param=dhtalg1.0;overlay=chat;expires=600
   Require: dht
   Supported: dht

   The To field of the REGISTER message MUST contain the PeerURI of the
   identifier being search for, constructed according to the rules in
   the subsection P2P peer URIs (Section 5.3.1) in the Message Syntax
   section.  If a specific peer is being sought, the PeerURI must
   specify that hostport.  If only the identifier is being searched for,
   then hostport MUST be set to "0.0.0.0".  The From URI MUST use the
   searching peer's PeerURI.

7.2.2.  Processing Peer Query Message

   The receiving peer determines that this is a P2PSIP message based on
   the presence of the dht Require and Supported fields.  In the event
   that the peer does not support P2P extensions, it MUST reply with a
   5xx class response such as 501 Not Implemented.  If the peer examines
   the overlay parameters and determines that this is not an overlay the
   peer participates in, the peer MUST reject the message with a 488 Not
   Acceptable Here response.  In the event a P2P peer receives a non-P2P
   request, it SHOULD reject it with a message such as 421 Extension
   Required.

7.2.2.1.  Routing the Peer Query Message

   The presence of a PeerURI and lack of an expiration time indicate
   that this message is a peer query and the receiving peer MUST process
   this as a DHT level request.  The receiving peer SHOULD NOT alter any
   of its internal values such as successor or predecessor in response
   to this message, since it is a query.  Otherwise, the message is
   processed and routed as a peer registration (Section 7.1.2.1) until



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   the responsible peer is reached.

7.2.2.2.  Responding to the Peer Query Message

   If the receiving peer is responsible for the region that the search
   key lies within, it MUST respond to the query.  If the receiving
   peer's Peer-ID exactly matches the search key, it MUST respond with a
   200 OK message.  If it is responsible for that region, but its
   Peer-ID is not the search key, it MUST respond with a 404 Not Found
   message.  The peer MAY verify the Peer-ID and IP address presented by
   the querying peer in the message.  If these do not match, the message
   should be rejected with a response of 493 Undecipherable.

7.3.  Populating the Joining Peer's Routing Table

   Once admitted, the joining peer SHOULD populate its routing table and
   locate neighbors by issuing queries for peers with the appropriate
   identifiers.  If the admitting peer provided neighbor or routing
   table information in its response, the joining peer MAY use this
   information to construct a temporary routing table and neighbor
   information and use this temporary table in the queries to populate
   the table.

7.4.  Transfering User Registrations

   When a new peer joins, it splits the area in the hash space the
   admitting peer is responsible for.  Some portion of the user
   registrations the admitting peer was responsible for may now be the
   responsibility of the joining peer, and these user registrations are
   handed to the joining peer by means of third party user
   registrations.  Third party registrations are allowed for user
   registrations and arbitrary searches, but are not allowed for peer
   registrations.  These registrations are exactly the same as those
   discussed in Registering and Removing User Registrations
   (Section 8.1), except that as they are third party registration from
   a peer, that is, the From header contains the PeerURI of the
   admitting peer.

7.5.  Peers Leaving the Overlay Gracefully

   Peers MUST send their registrations to the closest peer before
   leaving the overlay, as described in the section above.
   Additionally, peers MUST unregister themselves with their symmetric
   neighbors (if the DHT routing algorithm uses symmetric neighbors in
   any form).  These graceful exit REGISTER messages are constructed
   exactly the same as one used to join, with the following exceptions.
   The expires parameter or header MUST be provided, and MUST be set to
   0.  DHT-Link headers must be provided, as specified in DHT routing



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   algorithm

7.6.  NAT and Firewall Traversal

   The filtering properties of NATs and firewalls can lead to non-
   transitive connectivity.  Typically this will manifest itself in a
   peer receiving a 302 redirecting it to another peer that it cannot
   contact, most likely because address dependent filtering is
   occurring.  We discuss mechanisms to address these problems in [6].

7.7.  Handling Failed Requests

   When a request sent to another peer fails, the peer MUST perform
   searches to update its pointers.  If the failed request was sent to a
   peer in the routing table or a neighbor peer, then the searches
   discussed in Populating the Joining Peer's Routing Table
   (Section 7.3) should be performed.


8.  Resource Operations

   The most important element of resource operations within the P2PSIP
   DHT is that they are performed exactly as if using a conventional SIP
   registrar, except that the registrar responsibilities are distributed
   among the DHT members.

8.1.  Resource Registrations

   When a peer is in the overlay, it must register the contacts for
   users and other resources for which it is responsible into the
   overlay.  This differs from the registrations described above in that
   these registrations are responsible for entering a URI name to URI
   location mapping into the overlay as data, rather than joining a peer
   into the overlay.  These registrations are very similar to those
   outlined in section 10 of RFC3261.

   The Request-URI that is constructed for the REGISTER MUST be
   addressed to the peer the request is sent to.  The To and From fields
   of the REGISTER message MUST contain the Resource URI of the resource
   being registered, as described in Resource URIs (Section 5.3.2).  The
   request MUST include the value dht in Require and Supported headers.
   The request MUST include a DHT-PeerID header and MAY include one or
   more DHT-Link headers.

   The resource registration MUST include at least one Contact header
   containing a location of the resource and allowing this to be
   identified as a registration/update, rather than a query.  The peer
   MUST provide an expires parameter or an Expire header with a non-zero



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   value.  As in standard SIP registrations, Expires parameters with a
   value of zero will be used to remove registrations.  Any valid
   Contact for RFC 3261 is valid Contact for P2PSIP.  Most users will
   register a Contact with the address of the user's UA (which may or
   may not be the IP address of the peer, since the peer could be an
   adaptor peer).  The Contact URI does not need to include the
   ResourceID or other P2PSIP parameters as it is stored in the DHT but
   not processed or routed by it in any way.

   The message is routed in a fashion exactly analogous to that
   described in the section on peer registration (Section 7.1).  In
   iterative routing algorithms, 302 messages are sent to indicate that
   the message is to be redirected to another Peer URI.  In recursive
   routing algorithms, the receiving peer SHOULD forward the request to
   the peer in its connection table that is closest to the ResourceID.
   Once the message arrives at a destination that is responsible for
   that portion of the hash namespace, the peer recognizes it as a
   resource registration, rather than a peer wishing to join the system,
   based upon the fact that the To and From fields do not contain a Peer
   URI.  The peer responds with a 200 indicating a successful
   registration.  The response is constructed as dictated by RFC3261.

   The registering peer SHOULD construct and register replica
   registrations using the same Contact headers, but with the replica
   URI parameter used in the To and From headers.

8.2.  Refreshing Resource Registrations

   Resource registrations are refreshed exactly as described in RFC
   3261, Section 10.  Responsible peers should send a new registration
   with a valid expiration time prior to the time that the registration
   is set to expire.

   Agents MAY cache the address where they previously registered and
   attempt to send refreshes to this peer, but they are not guaranteed
   success, as a new peer may have registered and may now be responsible
   for this area of the space.  In such a case if iterative routing is
   being used, the peer will receive a 302 from the peer with which they
   previously registered, and should follow the same procedure for
   locating the peer they used in the initial registration.

   As with initial registrations, the sending peer should use the
   neighbor peer or routing table information provided in the 200 to
   send these updates to the redundant peers as well.







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8.3.  Removing Resource Registrations

   Resource registrations are removed exactly as described in RFC 3261,
   Section 10.  Responsible peers MUST send a registration with
   expiration time of zero.

   As with initial registrations, the sending peer MUST construct
   replica unregister messages and use these to unregister the replicas.

8.4.  Querying Resource Registrations

   Resource queries are constructed as described in RFC 3261, Section 
   10.  Querying peers should send a REGISTER message with no contact
   header.  As described in Peer Search (Section 7.2.1), this mechanism
   can also be used to locate the peer responsible for a particular
   Resource-ID.

   A P2P environment can do little to protect against an individual peer
   compromising the registrations it is responsible for.  Accordingly, a
   UA cannot trust a response from a single peer, whether it indicates a
   successful search or an error.  In the absence of other methods of
   verifying the response (such as having a certificate of the user
   being searched for and a signed registration that can be verified
   with the certificate) a UA should search for the primary registration
   and at least one replica.  Because the locations the replicas are
   stored are unrelated to the location of the primary registration, a
   single attacker is unlikely to be able to compromise both entries.
   As the overlay gains more peers and more replicas are searched for,
   the odds of a compromise are reduced.  Better protection for
   registrations is discussed in [9].

8.5.  Session Establishment

   When a caller wishes to send a SIP message (such as an INVITE,
   MESSAGE or SUBSCRIBE), the caller must first locate the peer where
   this callee's information resides using the resource search procedure
   described in the section titled Resource Location. (Section 8.4)

   Establishing a session is done entirely in the normal SIP fashion
   after the user is located using the P2P resource query.  Once the
   peer responsible for the Resource-ID is located, it will provide
   either a 200, providing a contact for the users UA, or will provide a
   404 if the user is not registered.  If a 200 with a valid contact is
   received, the call will then be initiated directly with the UAS of
   the called using the standard RFC 3261 fashion for methods such as
   INVITE or MESSAGE, or the INVITE can be processed by routing it
   through the overlay if necessary for NAT traversal [6].




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8.6.  Presence

   We use SUBSCRIBE/NOTIFY for this.  We subscribe to every user on our
   friend list when we come online.  If the friends are online, that
   means that we know exactly where they are.  Peers MAY use the PeerIDs
   of their friends' peers as additional routing table entries or
   neighbor peers (essentially, cached values), consulting these first,
   as connections are likely to be made to people on the user's friend
   list.  These should also be periodically checked, as described in the
   DHT Maintenance (Section 6.4).

   If friends are offline, one should periodically try to make the
   connection.  However, if a UA receives a SUBSCRIBE from a friend that
   it believes to be offline, it SHOULD attempt to subscribe to that
   friend.  This will allow people that are reciprocally on each other's
   friend lists to rapidly be notified when one or the other comes
   online, therefore the retry interval for subscribing to offline
   friends can be fairly long because it is only necessary in the case
   of race conditions or other temporary failures in resource location.

8.7.  Offline Storage

   Delivery of messages to offline users, or voicemail for voice
   applications, requires storing that information for later retrieval.
   Storing user configuration information in a format accessible from
   the network also will allow a user to retrieve their profile from any
   computer.  Cao et al. [18] describe an approach that separates the
   storage of resource location information from the actual storage of
   the offline research.  We believe that this approach is in agreement
   with the approach taken by the rest of this document, which relies on
   the DHT overlay to store the registrar's location information, but
   relies on external, conventional methods for the actual connection.
   For offline storage, it also allows the use of other standard
   protocols to store and retrieve the offline information, keeping the
   P2PSIP scope restricted to storing resource mappings.


9.  Pluggable DHT Algorithm         Requirements

   All dSIP peers MUST support the Chord pluggable DHT algorithm for
   compatibility.  They MAY support additional pluggable algorithms.
   The requirements for new pluggable algorithms are defined in this
   section.

   Pluggable algorithm MUST use Peer-IDs and Resource-IDs as defined in
   Hash Algorithms and Identifiers (Section 5.2) Pluggable algorithms
   are free to define what hash algorithms they support, but they MUST
   clearly specify what they are.



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   A resource with Resource-ID k will be stored by the peer with Peer-ID
   closest to the Resource-ID.  The definition of closeness may vary in
   different DHT algorithms, but each DHT algorithm MUST guarantee
   Resource-ID searches converge to exactly one peer responsible for
   that portion of the namespace.  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 the event of a peer failure.

   Each new DHT algorithm MUST define a value for the dht-name parameter
   to be used in the dht-param parameter of the DHT-PeerID header, as
   defined in DHT Algorithms and the dht parameter (Section 5.4.3).

   Each new DHT algorithm MUST define the valid BNF for the link-value
   used in the DHT-Link header, as defined in The DHT-Link header
   (Section 5.5).


10.  Security Considerations

   The goal of P2PSIP is to scale gracefully from ad hoc groups of a few
   people to an overlay of millions of peers across the globe.  As such,
   there is no one security model that fits the needs of all envisioned
   environments; for the small network establishing a certificate chain
   is ludicrously difficult, while for a global network the unrestricted
   ability to insert resources and devise useful Peer IDs is a clear
   invitation to insecurity.  Any P2PSIP protocol must offer a range of
   security models that can be selected according to the needs of the
   overlay.

10.1.  Threat Model

   Without other security, the attacker is able to generate an ID and
   become a valid peer in the system.  They can see other peers and
   process certain queries.  Attackers may wish to receive
   communications intended for other participants, prevent other users
   from receiving their messages, prevent large portions of the users
   from receiving messages, or send messages that appear to be from
   others.  Users would like to be sure they are communicating with the
   same person they have previously talked to, to be able to verify
   identity via some out of band mechanism.  Attackers may try to squat
   on all the good names.  Users would like names that are meaningful to
   them.  Attackers may have computers that are many times faster than
   the average user's.  Attackers may be able to DOS other particular
   peers and make them fail.  To make a robust DHT, many peers need to
   store information on behalf of the community.  Peers may lie about
   this and not store the information.  Attackers may wish to see who is
   communicating with whom and how much data is getting communicated.



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   Many of the threats to P2P SIP are also threats to conventional SIP.
   As such, P2P SIP imports much of its security from conventional SIP.
   However, because conventional SIP generally relies on secure servers
   to maintain the integrity of the system, modifications to those
   techniques are required to maintain the same level of security.

10.2.  Protecting the ID Namespace

   The fundamental protection that P2PSIP relies on is protecting the ID
   namespace.  In particular, many of the attacks on P2PSIP require
   identifying a particular portion of the ID space and acquiring
   control of that space.  This is a common vector both for attacks on a
   particular user, by obtaining control of the location in the overlay
   where the user is registered, and on the overlay itself, by means of
   a Sybil [19] attack when one is able to insert multiple identities at
   different locations on the ring.

   The P2PSIP ID Namespace is considered protected when an attacker is
   not able to select an arbitrary Peer-ID and insert a peer at the
   location by convincing other peers to route traffic to them.  This
   protects against hijacking and DoS attacks.  The ID Namespace may
   also be protected by restricting admission to the overlay to some
   authorized (and trusted) set of individuals.

10.2.1.  Protection Using ID Hashing

   The default base security for P2PSIP determines Peer-IDs by hashing
   the peer's IP address and appending the port number.  The security of
   this scheme depends on the ease with which an attacker can choose
   their own Peer-ID.  Because the port number is only appended to the
   Peer-ID, an attacker gains nothing by selecting different ports on
   the same node.  Assuming that the SHA1 hash used to calculate the
   Peer-ID is reliably random, the attacker's ability to succeed depends
   on the number of separate IP addresses that they are able to obtain
   from which to launch their attacks.

   In the current predominantly IPV4 Internet, few attackers have access
   to more than a handful of IP addresses, perhaps a few hundred at
   worst.  For a large-scale P2P system, this is unlikely to provide the
   ability to hijack a particular user ID or control a sufficient
   portion of the network to affect other peers, in particular when
   registrations are replicated at independent peers.  Ultimately,
   however, a sufficiently skilled and provisioned attacker can
   compromise this scheme.

   As the Internet migrates to IPV6, however, it is unclear that the
   assumption that few attackers have access to a significant range of
   IP addresses will remain true.  Therefore, hashing IP addresses to



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   Peer-IDs is assumed to provide a diminishing amount of security in
   the future.

10.2.2.  Cryptographic Protection

   Stronger protection guarantees are possible by relying on
   cryptographic techniques to restrict the generation of peer IDs,
   either through requiring knowledge of a shared secret to calculate a
   valid hash or by issuing certificates through a central authority.
   These techniques are further described in Security for dSIP [9].

10.3.  Protecting the resource namespace

   The two primary vectors of attacks on resources in a P2PSIP overlay
   are inserting illegitimate resources into the overlay and corrupting
   the registrations for which a compromised peer is responsible.

   For overlays that do not rely on certificates, once a peer has joined
   the overlay there are no restrictions on its ability to register
   resources.  In an unsecured network, multiple peers can register the
   same resource (username) in the overlay.  However, self-signed
   certificates [14] can be used to authenticate a user as the same user
   previously contacted with that certificate.  Unless a conventional
   SIP authentication server is available, however, establishing
   identity upon initial contact is still a problem.  One potential
   solution is for an overlay that is expected to persist over long
   time-frames to store the credentials of previous users for
   verification of a new registration.  These techniques are beyond the
   scope of this document.

   The second form of resource attack, which is really an ID attack,
   concerns the attacks that are possible when a peer has legitimately
   inserted itself into the overlay and is now responsible for storing
   resource registrations.  Such an attack could occur through a
   corrupted peer or by an attacker who convinces the CA to issue them a
   certificate for a Peer-ID.  In this case, the peer can corrupt any
   resource that is assigned to it.  In the absence of certificates, the
   primary means of defense of such attacks is relying on the
   replication described in Section Section 5.2.2.  By storing replicas
   of each registration on multiple peers and performing parallel
   searches for resource lookup, the searching peer protects itself from
   a single peer trying to corrupt the namespace.

   Further protection from each attack vector is achieved by relying on
   certificates for resource authentication [9].






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10.4.  Protecting the Routing

   The DHT forms a complex routing table.  When a peer joins, it may
   contact a subversive peer that lies about the finger table
   information it provides.  The subversive peer could do this to try to
   trick the joining peer to route all the traffic to a subversive group
   of peers.  Prevention of this attack relies on protecting the
   namespace and (for hashed namespaces) identifying trusted bootstrap
   peers to use when joining.

   Resource searches are protected from a single subversive peer through
   the use of parallel searches on replicated registrations.  Similar
   protection could be achieved through performing parallel searches
   using multiple bootstrap peers for initial join, but such
   specification is beyond the scope of this draft.  When possible,
   securing the namespace is a better solution.

10.5.  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.

10.6.  Protecting the Media

   As with conventional SIP, all the media SHOULD be encrypted.
   Negotiating encryption for an end-to-end media session should be
   performed in the same manner for P2PSIP communications.

10.7.  Replay Attacks

   Defense against replay attacks is discussed in [9].


11.  Open Issues

   There are certainly many open issues.  Here are a few.

   Still to be worked out are details of how P2PSIP names are
   disambiguated from conventional names that use DNS based routing.


12.  Acknowledgments

   A team of people have worked on the various drafts related to the
   dSIP protocol and extensions thereof.  The team consists of: David
   Bryan, Eric Cooper, James Deverick, Cullen Jennings, Bruce Lowekamp,



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   Philip Matthews, and Marcia Zangrilli.

   Thanks to all who have been actively participating in the P2PSIP
   efforts.  Special thanks to Spencer Dawkins, Enrico Marocco, and
   Jean-Francois Wauthy for providing editorial feedback, and Henry
   Sinnreich, Eric Rescorla, and Alan Johnston for various discussions
   related to this work.


13.  IANA Considerations

   This document would require registering the following:
   o  Option tag "DHT"
   o  "DHT-Link" as a Header Field
   o  "DHT-PeerID" as a Header Field
   o  "peer" as a valid value for parameter user (?)
   o  "Resource-ID" as a valid URI parameter (?)
   o  "hmac-sha1" as an Identity-Info 'alg' parameter

   [ToDo: This section needs to be revamped to include all the new BNF
   introduced]


14.  Changes to this Version

   While this is a -00 document, it has grown from the earlier drafts
   draft-bryan-sipping-p2p-xx.  As such, we discuss the changes from the
   most recent version of that draft, -03.
   1.   The earlier draft has been split into a number of drafts:
        1.  This draft, providing the background and overall concept,
            basic terminology for encoding P2P messages in SIP,
   2.   We have removed "-" from a number of headers and parameter names
        to shorten the overall length of the messages.  Additionally, we
        have provided short versions for some strings in the syntax to
        help reduce message size.
   3.   We have attempted to use the new terminology defined in [2]
        wherever possible, and have attempted not to replicate
        definitions here.  In particular, we have substituted the use of
        the term "peer" for "node"
   4.   As a consequence of the above, NodeID has been replaced with
        PeerID, both in text and in the actual defined messages sent
        over the wire.
   5.   We have made many changes to include details essential to using
        this in real deployed systems or clarifying difficult concepts;
        lessons learned from building a commercial application based on
        this draft.





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   6.   Large parts of the description of how an initial overlay is
        formed were quite confusing as our description did not
        explicitly embrace the NULL predecessor concept of Chord.  We
        have corrected this in the sections describing the algorithms.
   7.   A full and detailed example showing the startup of a 3 node
        system has been inserted into the examples section.
   8.   A new section has been added detailing early work on
        incorporating SIP identity into a P2P environment.  This work is
        then used in the security section.
   9.   The security section has been thoroughly rewritten to reflect
        changes both in our thoughts and the thoughts of the P2PSIP
        working group as a whole.
   10.  We corrected a number of outright errors and typos pointed by a
        number of individuals, as mentioned in the acknowledgments.


15.  References

15.1.  Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Willis, D., "Concepts and Terminology for Peer to Peer SIP",
         draft-willis-p2psip-concepts-03 (work in progress),
         October 2006.

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

   [4]   Rosenberg, J., "Simple Traversal Underneath Network Address
         Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05
         (work in progress), October 2006.

   [5]   Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-13 (work in
         progress), January 2007.

   [6]   Cooper, E., Matthews, P., Bryan, D., and B. Lowekamp, "NAT
         Traversal for dSIP", Internet
         Draft draft-matthews-p2psip-dsip-nat-traversal-00,
         February 2007.

   [7]   Zangrilli, M. and D. Bryan, "A Chord-based DHT for Resource
         Lookup in P2PSIP", Internet
         Draft draft-zangrilli-p2psip-dsip-dhtchord-00, February 2007.



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   [8]   Zangrilli, M. and D. Bryan, "A Bamboo-based DHT for Resource
         Lookup in P2PSIP", Internet
         Draft draft-zangrilli-p2psip-dsip-dhtbamboo-00, February 2007.

   [9]   Lowekamp, B. and J. Deverick, "Authenticated Identity
         Extensions to dSIP", Internet
         Draft draft-lowekamp-p2psip-dsip-security-00, February 2007.

   [10]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [11]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
         Identity Management in the Session Initiation Protocol (SIP)",
         RFC 4474, August 2006.

   [12]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
         RFC 3174, September 2001.

   [13]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
         Preferences for the Session Initiation Protocol (SIP)",
         RFC 3841, August 2004.

   [14]  Jennings, C., "Certificate Management Service for The Session
         Initiation Protocol (SIP)", draft-ietf-sip-certs-02 (work in
         progress), October 2006.

15.2.  Informative References

   [15]  Bryan, D., Shim, E., and B. Lowekamp, "Use Cases for Peer-to-
         Peer Session Initiation Protocol (P2PSIP)", Internet
         Draft draft-bryan-sipping-p2p-usecases-00, November 2005.

   [16]  Bryan, D., Jennings, C., and B. Lowekamp, "SOSIMPLE: A
         Serverless, Standards-based, P2P SIP Communication System",
         Proceedings of the 2005 International Workshop on Advanced
         Architectures and Algorithms for Internet Delivery and
         Applications (AAA-IDEA) '05, June 2005.

   [17]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in
         progress), October 2006.

   [18]  Cao, F., Bryan, D., and B. Lowekamp, "Providing Secure Services
         in Peer-to-Peer Communications Networks with Central Security
         Server", Internation Conference on Internet and Web
         Applications and Services (ICIW) '06, February 2006.

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



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Authors' Addresses

   David A. Bryan
   SIPeerior Technologies, Inc.
   3000 Easter Circle
   Williamsburg, VA  23188
   USA

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


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

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


   Cullen Jennings
   Cisco Systems
   170 West Tasman Drive
   MS: SJC-21/3
   San Jose, CA  95134
   USA

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




















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