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P2PSIP                                                       C. Jennings
Internet-Draft                                                     Cisco
Intended status:  Standards Track                            B. Lowekamp
Expires:  July 3, 2013                                             Skype
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                                S. Baset
                                                          H. Schulzrinne
                                                     Columbia University
                                                       T C. Schmidt, Ed.
                                                             HAW Hamburg
                                                       December 30, 2012


                         A SIP Usage for RELOAD
                        draft-ietf-p2psip-sip-08

Abstract

   This document defines a SIP Usage for REsource LOcation And Discovery
   (RELOAD).  The SIP Usage provides the functionality of a SIP proxy or
   registrar in a fully-distributed system and includes a lookup service
   for Address of Records (AoRs) stored in the overlay.  It also defines
   Globally Routable User Agent Uris (GRUUs) that allow the
   registrations to map an AoR to a specific node reachable through the
   overlay.  After such initial contact of a peer, the AppAttach method
   is used to establish a direct connection between nodes through which
   SIP messages are exchanged.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 3, 2013.

Copyright Notice




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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.



























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Registering AORs in the Overlay  . . . . . . . . . . . . . . .  6
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Data Structure . . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Access Control . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Overlay Configuration Document Extension . . . . . . . . .  9
   4.  Looking up an AOR  . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Finding a Route to an AOR  . . . . . . . . . . . . . . . . 10
     4.2.  Resolving an AOR . . . . . . . . . . . . . . . . . . . . . 11
   5.  Forming a Direct Connection  . . . . . . . . . . . . . . . . . 11
   6.  Using GRUUs  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  SIP-REGISTRATION Kind Definition . . . . . . . . . . . . . . . 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     8.1.  RELOAD-Specific Issues . . . . . . . . . . . . . . . . . . 13
     8.2.  SIP-Specific Issues  . . . . . . . . . . . . . . . . . . . 13
       8.2.1.  Fork Explosion . . . . . . . . . . . . . . . . . . . . 13
       8.2.2.  Malicious Retargeting  . . . . . . . . . . . . . . . . 13
       8.2.3.  Misuse of AORs . . . . . . . . . . . . . . . . . . . . 13
       8.2.4.  Privacy Issues . . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     11.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Third Party Registration  . . . . . . . . . . . . . . 15
   Appendix B.  Change Log  . . . . . . . . . . . . . . . . . . . . . 16
     B.1.  Changes since draft-ietf-p2psip-sip-07 . . . . . . . . . . 16
     B.2.  Changes since draft-ietf-p2psip-sip-06 . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16



















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1.  Introduction

   The REsource LOcation And Discovery (RELOAD) [I-D.ietf-p2psip-base]
   specifies a peer-to-peer (P2P) signaling protocol for the general use
   on the Internet.  This document defines a SIP Usage of RELOAD that
   allows SIP [RFC3261] user agents (UAs) to establish peer-to-peer SIP
   sessions without the requirement for permanent proxy or registration
   servers, , e.g., a fully distributed telephony service.  In such a
   network, the RELOAD overlay itself performs the registration and
   rendezvous functions ordinarily associated with such servers.

   The SIP Usage involves two basic functions.

   Registration:  SIP UAs can use the RELOAD data storage functionality
      to store a mapping from their address-of-record (AOR) to their
      Node-ID in the overlay, and to retrieve the Node-ID of other UAs.
   Rendezvous:  Once a SIP UA has identified the Node-ID for an AOR it
      wishes to call, it can use the RELOAD message routing system to
      set up a direct connection for exchanging SIP messages.

   Mappings are stored in the SipRegistration Resource Record defined in
   this document.  All operations required to perform a SIP registration
   or rendezvous are standard RELOAD protocol methods.

   For example, Bob registers his AOR, "bob@dht.example.com", for his
   Node-ID "1234".  When Alice wants to call Bob, she queries the
   overlay for "bob@dht.example.com" and receives Node-ID 1234 in
   return.  She then uses the overlay routing to establish a direct
   connection with Bob and can directly transmit a standard SIP INVITE.
   In detail, this works along the following steps.

   1.  Bob, operating Node-ID 1234, stores a mapping from his URI to his
       Node-ID in the overlay by applying a Store request for
       "bob@dht.example.com -> 1234".
   2.  Alice, operating Node-ID 5678, decides to call Bob. She retrieves
       Node-ID "1234" by performing a Fetch request on
       "bob@dht.example.com".
   3.  Alice uses the overlay to route an AppAttach message to Bob's
       peer (ID 1234).  Bob responds with his own AppAttach and they set
       up a direct connection, as shown in Figure 1.  Note that mutual
       ICE checks are invoked automatically from AppAttach message
       exchange.









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

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


      Figure 1: Connection setup in P2P SIP using the RELOAD overlay

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

   In addition to mappings from AORs to Node-IDs, the SIP Usage also
   allows mappings from AORs to other AORs.  This enables an indirection
   useful for call forwarding.  For instance, if Bob wants his phone
   calls temporarily forwarded to Charlie, he can store the mapping
   "bob@dht.example.com -> charlie@dht.example.com".  When Alice wants
   to call Bob, she retrieves this mapping and can then fetch Charlie's
   AOR to retrieve his Node-ID.  These mechanisms are described in
   Section 3.

   Alternatively, Globally Routable User Agent URIs (GRUUs) can be used
   for directly accessing peers.  They are handled via a separate
   mechanism, as described in Section 6.

   The SIP Usage for RELOAD addresses a fully distributed deployment of
   session-based services among overlay peers.  Two opposite scenarios
   of deploying P2P SIP services are in the focus of this document:  A



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   highly regulated environment of a "single provider" that admits
   parties using AORs with domains from controlled namespace(s), only,
   and an open, multi-party infrastructure that liberally allows a
   registration and rendezvous for various or any domain namespace.  It
   is noteworthy in this context that - in contrast to regular SIP -
   domain names play no role in routing to a proxy server.  Once
   connectivity to an overlay is given, any name registration can be
   technically processed.


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 [RFC2119].

   We use the terminology and definitions from Concepts and Terminology
   for Peer to Peer SIP [I-D.ietf-p2psip-concepts] and the RELOAD Base
   Protocol [I-D.ietf-p2psip-base] extensively in this document.

   In addition, term definitions from SIP [RFC3261] apply to this memo.
   The term AOR is the SIP "Address of Record" used to identify a user
   in SIP.  For example, alice@example.com could be the AOR for Alice.
   For the purposes of this specification, an AOR is considered not to
   include the scheme (e.g sip:) as the AOR needs to match the
   rfc822Name in the X509v3 certificates.


3.  Registering AORs in the Overlay

3.1.  Overview

   In ordinary SIP, a UA registers its AOR and location with a
   registrar.  In RELOAD, this registrar function is provided by the
   overlay as a whole.  To register its location, a RELOAD peer stores a
   SipRegistration Resource Record under its own AOR using the SIP-
   REGISTRATION Kind, which is formally defined in Section 7.  A RELOAD
   overlay MAY restrict the storage of AORs.  Namespaces (i.e., the
   right hand side of the AOR) that are supported for registration and
   lookup can be configured for each RELOAD deployment as described in
   Section 3.4.

   As a simple example, consider Alice with AOR "alice@dht.example.org"
   at Node-ID "1234".  She might store the mapping
   "alice@dht.example.org -> 1234" telling anyone who wants to call her
   to contact node "1234".

   RELOAD peers MAY store two kinds of SIP mappings,



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   o  from an AOR to a destination list (a single Node-ID is just a
      trivial destination list), or
   o  from an AOR to another AOR.

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

3.2.  Data Structure

   This section defines the SipRegistration Resource Record as follows:


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

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

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

            /* This type can be extended */

          } SipRegistrationData;

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



   The contents of the SipRegistration PDU are:


   type
      the type of the registration







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   length
      the length of the rest of the PDU

   data
      the registration data

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

   The encoding of contact_prefs - the callee's contact preferences -
   follows the media feature set syntax of [RFC2533].  As an example, a
   voicemail server that is a UA that supports audio and video media
   types and is not mobile would carry the following feature set
   description in its contact_prefs attribute:

   (& (sip.audio=TRUE)
         (sip.video=TRUE)
         (sip.actor=msg-taker)
         (sip.automata=TRUE)
         (sip.mobility=fixed)
         (| (sip.methods=INVITE) (sip.methods=BYE) (sip.methods=OPTIONS)
            (sip.methods=ACK) (sip.methods=CANCEL)))

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

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

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

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

   Note that this structure explicitly allows one Node-ID to forward to
   another Node-ID.  For instance, Alice could set calls to her desk
   phone to ring at her cell phone by storing a SipRegistration of type
   "sip_registration_route" with dictionary key "1234" and value "5678".




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3.3.  Access Control

   In order to prevent hijacking or other misuse, registrations are
   subject to access control rules.  Two kinds of restrictions apply:

   o  A Store is permitted only for AORs with domain names that fall
      into the namespaces supported by the RELOAD overlay instance.
   o  Storing requests are performed according to the USER-NODE-MATCH
      access control policy of RELOAD.

   Before issuing a Store request to the overlay, any peer SHOULD verify
   that the AOR of the request is a valid Resource Name with respect to
   its domain name and the namespaces defined in the overlay
   configuration document (see Section 3.4).

   Before a Store is permitted, the storing peer MUST check that:

   o  The AOR of the request is a valid Resource Name with respect to
      the namespaces defined in the overlay configuration document.
   o  The certificate contains a username that is a SIP AOR which hashes
      to the Resource-ID it is being stored at.
   o  The certificate contains a Node-ID that is the same as the
      dictionary key it is being stored at.

   Note that these rules permit Alice to forward calls to Bob without
   his permission.  However, they do not permit Alice to forward Bob's
   calls to her.  See Section 8.2.2 for additional descriptions.

3.4.  Overlay Configuration Document Extension

   The use of a SIP-enabled overlay MAY be restricted to users with AORs
   from specific domains.  When deploying an overlay service, providers
   can decide about these use case scenarios by defining a set of
   namespaces for admissible domain names.  This section extends the
   overlay configuration document by defining new elements for patterns
   that describe a corresponding domain name syntax.

   A RELOAD overlay can be configured to accept store requests for any
   AOR, or to apply domain name restrictions.  For the latter, an
   enumeration of admissible domain names including wildcarded name
   patterns of the following form MAY be configured.

   Example of Domain Patterns:
   dht.example.com
   .*.my.name

   In this example, any AOR will be accepted that is either of the form
   <user>@dht.example.com, or contains the domain suffix "my.name".



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   When restrictions apply and in the absence of domain patterns, the
   default behavior is to accept only AORs that exactly match the domain
   name of the overlay.  Otherwise, i.e., when restrictions are not
   configured (attribute enable not set), the default behavior is to
   accept any AOR.  Encoding of the domain name complies to the
   restricted ASCII character set without character escaping as defined
   in Section 19.1 of [RFC3261].

   The Relax NG Grammar for the AOR Domain Restriction reads:

   <!-- AOR DOMAIN RESTRICTION URN SUB-NAMESPACE -->

   namespace sip = "urn:ietf:params:xml:ns:p2p:config-base:sip"

   <!--  AOR DOMAIN RESTRICTION ELEMENT -->

   kind-parameter &= element sip:domain-restriction {

       attribute enable { xsd:boolean }

       <!-- PATTERN ELEMENT -->

       element pattern { xsd:string }*
   }?


4.  Looking up an AOR

4.1.  Finding a Route to an AOR

   A RELOAD user, member of an overlay, who wishes to call another user
   with given AOR SHALL proceed in the following way.

   AOR is GRUU?  If the AOR is a GRUU for this overlay, the callee can
      be contacted directly as described in Section 6.
   AOR domain is hosted in overlay?  If the domain part of the AOR
      matches a domain pattern configured in the overlay, the user can
      continue to resolve the AOR in this overlay.  The user MAY choose
      to query the DNS service records to search for additional support
      of this domain name.
   AOR domain not supported by overlay?  If the domain part of the AOR
      is not supported in the current overlay, the user SHOULD query the
      DNS (or other discovery services at hand) to search for an
      alternative overlay that services the AOR under request.
      Alternatively, standard SIP procedures for contacting the callee
      SHOULD be used.





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   AOR inaccessible?  If all of the above contact attempts fail, the
      call fails.

   The procedures described above likewise apply when nodes are
   simultaneously connected to several overlays.

4.2.  Resolving an AOR

   A RELOAD user that has discovered a route to an AOR in the current
   overlay SHALL execute the following steps.

   1.  Perform a Fetch for kind SIP-REGISTRATION at the Resource-ID
       corresponding to the AOR.  This Fetch SHOULD NOT indicate any
       dictionary keys, so that it will fetch all the stored values.
   2.  If any of the results of the Fetch are non-GRUU AORs, then repeat
       step 1 for that AOR.
   3.  Once only GRUUs and destination lists remain, the peer removes
       duplicate destination lists and GRUUs from the list and initiates
       SIP connections to the appropriate peers as described in the
       following sections.  If there are also external AORs, the peer
       follows the appropriate procedure for contacting them as well.


5.  Forming a Direct Connection

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


6.  Using GRUUs

   Globally Routable User Agent Uris (GRUUs) [RFC5627] have been
   designed to allow direct routing without the indirection of a SIP
   proxy function.  The concept is transferred to RELOAD overlays as
   follows.  GRUUs in RELOAD are constructed by embedding a base64-
   encoded destination list in the gr URI parameter of the GRUU.  The
   base64 encoding is done with the alphabet specified in table 1 of
   [RFC4648] with the exception that ~ is used in place of =.




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   Example of a RELOAD GRUU:
   alice@example.com;gr=MDEyMzQ1Njc4OTAxMjM0NTY3ODk~

   GRUUs do not require to store data in the Overlay Instance.  Rather
   when a peer needs to route a message to a GRUU in the same P2P
   overlay, it simply uses the destination list and connects to that
   peer.  Because a GRUU contains a destination list, it MAY have the
   same contents as a destination list stored elsewhere in the resource
   dictionary.

   Anonymous GRUUs [RFC5767] are constructed analogously, but require
   either that the enrollment server issues a different Node-ID for each
   anonymous GRUU required, or that a destination list be used that
   includes a peer that compresses the destination list to stop the
   Node-ID from being revealed.


7.  SIP-REGISTRATION Kind Definition

   This section defines the SIP-REGISTRATION kind.


   Name  SIP-REGISTRATION

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

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

   Access Control  USER-NODE-MATCH.  Note that this matches the SIP AOR
      against the rfc822Name in the X509v3 certificate.  The rfc822Name
      does not include the scheme so that the "sip:" prefix needs to be
      removed from the SIP AOR before matching.

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

   sip_registration_uri
      a URI which the user can be reached at.







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


8.  Security Considerations

8.1.  RELOAD-Specific Issues

   This Usage for RELOAD does not define new protocol elements or
   operations.  Hence no new threats arrive from message exchanges in
   RELOAD.

   This document introduces an AOR domain restriction function that must
   be surveyed by the storing peer.  A misconfigured or malicious peer
   could cause frequent rejects of illegitimate storing requests.
   However, domain name control relies on a lightweight pattern matching
   and can be processed prior to validating certificates.  Hence no
   extra burden is introduced for RELOAD peers beyond loads already
   present in the base protocol.

8.2.  SIP-Specific Issues

8.2.1.  Fork Explosion

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

8.2.2.  Malicious Retargeting

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

8.2.3.  Misuse of AORs

   A RELOAD overlay and enrollment service that liberally accept
   registrations for AORs of domain names unrelated to the overlay
   instance and without further justification, eventually store presence
   state for misused AORs.  An attacker could hijack names, register a



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   bogus presence and attract calls dedicated to a victim that resides
   within or outside the Overlay Instance.

   A hijacking of AORs can be mitigated by restricting the name spaces
   admissible in the Overlay Instance, or by additional verification
   actions of the enrollment service.  To prevent an (exclusive) routing
   to a bogus registration, a caller can in addition query the DNS (or
   other discovery services at hand) to search for an alternative
   presence of the callee in another overlay or a normal SIP
   infrastructure.

8.2.4.  Privacy Issues

   All RELOAD SIP registration data is public.  Methods of providing
   location and identity privacy are still being studied.  Location
   privacy can be gained from using anonymous GRUUs.


9.  IANA Considerations

   IANA shall register the following code point in the "RELOAD Data
   Kind-ID" Registry (cf., [I-D.ietf-p2psip-base]) to represent the SIP-
   REGISTRATION kind, as described in Section 7.  [NOTE TO IANA/
   RFC-EDITOR:  Please replace RFC-AAAA with the RFC number for this
   specification in the following list.]

              +---------------------+------------+----------+
              | Kind                |    Kind-ID |      RFC |
              +---------------------+------------+----------+
              | SIP-REGISTRATION    |          1 | RFC-AAAA |
              +---------------------+------------+----------+


10.  Acknowledgments

   This document was generated in parts from initial drafts and
   discussions in the early specification phase of the P2PSIP base
   protocol.  Significant contributions (in alphabetical order) were
   from David A. Bryan, James Deverick, Marcin Matuszewski, Jonathan
   Rosenberg, and Marcia Zangrilli, which is gratefully acknowledged.

   Additional thanks go to all those who helped with ideas, discussions,
   and reviews, in particular (in alphabetical order) Michael Chen, Marc
   Petit-Huguenin, Brian Rosen, and Matthias Waehlisch.


11.  References




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11.1.  Normative References

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

   [I-D.ietf-p2psip-base]
              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
              Base Protocol", draft-ietf-p2psip-base-23 (work in
              progress), November 2012.

   [RFC3261]  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.

   [RFC2533]  Klyne, G., "A Syntax for Describing Media Feature Sets",
              RFC 2533, March 1999.

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

11.2.  Informative References

   [I-D.ietf-p2psip-concepts]
              Bryan, D., Willis, D., Shim, E., Matthews, P., and S.
              Dawkins, "Concepts and Terminology for Peer to Peer SIP",
              draft-ietf-p2psip-concepts-04 (work in progress),
              October 2011.

   [RFC5767]  Munakata, M., Schubert, S., and T. Ohba, "User-Agent-
              Driven Privacy Mechanism for SIP", RFC 5767, April 2010.

   [I-D.ietf-p2psip-share]
              Knauf, A., Schmidt, T., Hege, G., and M. Waehlisch, "A
              Usage for Shared Resources in RELOAD (ShaRe)",
              draft-ietf-p2psip-share-00 (work in progress),
              October 2012.


Appendix A.  Third Party Registration

   In traditional SIP, the mechanism of a third party registration
   (i.e., an assistant acting for a boss, changing users register a



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   role-based AOR, ...) is defined in Section 10.2 of [RFC3261].  This
   is a REGISTER which uses the URI of the third-party in its From
   header and cannot be translated directly into a P2PSIP registration,
   because only the owner of the certificate can store a SIP-
   REGISTRATION in a RELOAD overlay.

   A way to implement third party registration is by using the extended
   access control mechanism USER-CHAIN-ACL defined in
   [I-D.ietf-p2psip-share].  Creating a new kind "SIP-3P-REGISTRATION"
   that is ruled by USER-CHAIN-ACL allows the owner of the certificate
   to delegate the right for registration to individual third parties.
   In this way, original SIP functionality can be regained without
   weakening the security control of RELOAD.


Appendix B.  Change Log

B.1.  Changes since draft-ietf-p2psip-sip-07

   o  Cleared open issues
   o  Clarified use cases after WG discussion
   o  Added configuration document extensions for configurable domain
      names
   o  Specified format of contact_prefs
   o  Clarified routing to AORs
   o  Extended security section
   o  Added Appendix on Third Party Registration
   o  Added IANA code points
   o  Editorial polishing
   o  Updated and extended references

B.2.  Changes since draft-ietf-p2psip-sip-06

   o  Added Open Issue


Authors' Addresses

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

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




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   Bruce B. Lowekamp
   Skype
   Palo Alto, CA
   USA

   Email:  bbl@lowekamp.net


   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   Phone:  +1 650 678 2350
   Email:  ekr@rtfm.com


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

   Email:  salman@cs.columbia.edu


   Henning Schulzrinne
   Columbia University
   1214 Amsterdam Avenue
   New York, NY
   USA

   Email:  hgs@cs.columbia.edu


   Thomas C. Schmidt (editor)
   HAW Hamburg
   Berliner Tor 7
   Hamburg  20099
   Germany

   Email:  schmidt@informatik.haw-hamburg.de








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