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Versions: 00 01 02 03 04 RFC 5360

SIPPING                                                     J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: January 6, 2008                               G. Camarillo, Ed.
                                                                Ericsson
                                                               D. Willis
                                                            Unaffiliated
                                                            July 5, 2007


 A Framework for Consent-Based Communications in the Session Initiation
                             Protocol (SIP)
                draft-ietf-sip-consent-framework-02.txt

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   This Internet-Draft will expire on January 6, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Session Initiation Protocol (SIP) supports communications across
   many media types, including real-time audio, video, text, instant
   messaging, and presence.  In its current form, it allows session
   invitations, instant messages, and other requests to be delivered



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   from one party to another without requiring explicit consent of the
   recipient.  Without such consent, it is possible for SIP to be used
   for malicious purposes, including amplification, and DoS (Denial of
   Service) attacks.  This document identifies a framework for consent-
   based communications in SIP.














































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Relays and Translations  . . . . . . . . . . . . . . . . . . .  5
   4.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Permissions at a Relay . . . . . . . . . . . . . . . . . .  7
     4.2.  Consenting Manipulations on a Relay's Transaction Logic  .  8
     4.3.  Store-and-forward Servers  . . . . . . . . . . . . . . . .  9
     4.4.  Recipients Grant Permissions . . . . . . . . . . . . . . . 10
     4.5.  Entities Implementing this Framework . . . . . . . . . . . 10
   5.  Framework Operations . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Amplification Avoidance  . . . . . . . . . . . . . . . . . 12
       5.1.1.  Relay's Behavior . . . . . . . . . . . . . . . . . . . 12
     5.2.  Subscription to the Permission Status  . . . . . . . . . . 12
       5.2.1.  Relay's Behavior . . . . . . . . . . . . . . . . . . . 13
     5.3.  Request for Permission . . . . . . . . . . . . . . . . . . 13
       5.3.1.  Relay's Behavior . . . . . . . . . . . . . . . . . . . 13
     5.4.  Permission Document Structure  . . . . . . . . . . . . . . 14
     5.5.  Permission Requested Notification  . . . . . . . . . . . . 15
     5.6.  Permission Grant . . . . . . . . . . . . . . . . . . . . . 15
       5.6.1.  Relay's Behavior . . . . . . . . . . . . . . . . . . . 16
     5.7.  Permission Granted Notification  . . . . . . . . . . . . . 17
     5.8.  Permission Revocation  . . . . . . . . . . . . . . . . . . 18
     5.9.  Request-contained URI Lists  . . . . . . . . . . . . . . . 19
       5.9.1.  Relay's Behavior . . . . . . . . . . . . . . . . . . . 19
       5.9.2.  Definition of the 470 Response Code  . . . . . . . . . 19
       5.9.3.  Definition of the Permission-Missing Header Field  . . 20
     5.10. Registrations  . . . . . . . . . . . . . . . . . . . . . . 21
     5.11. Relays Generating Traffic towards Recipients . . . . . . . 23
       5.11.1. Relay's Behavior . . . . . . . . . . . . . . . . . . . 23
       5.11.2. Definition of the Trigger-Consent Header Field . . . . 23
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     6.1.  Registration of the 470 Response Code  . . . . . . . . . . 24
     6.2.  Registration of the Trigger-Consent Header Field . . . . . 24
     6.3.  Registration of the target-uri Header Field Parameter  . . 24
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 29








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1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] supports
   communications across many media types, including real-time audio,
   video, text, instant messaging, and presence.  This communication is
   established by the transmission of various SIP requests (such as
   INVITE and MESSAGE [RFC3428]) from an initiator to the recipient with
   whom communication is desired.  Although a recipient of such a SIP
   request can reject the request, and therefore decline the session, a
   network of SIP proxy servers will deliver a SIP request to its
   recipients without their explicit consent.

   Receipt of these requests without explicit consent can cause a number
   of problems.  These include amplification and DoS (Denial of Service)
   attacks.  These problems are described in more detail in a companion
   requirements document [RFC4453].

   This specification defines a basic framework for adding consent-based
   communication to SIP.


2.  Definitions and 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].

   Recipient URI:  The Request-URI of an outgoing request sent by an
      entity (e.g., a user agent or a proxy).  The sending of such
      request can have been the result of a translation operation.

   Relay:  Any SIP server, be it a proxy, B2BUA (Back-to-Back User
      Agent), or some hybrid, that receives a request, translates its
      Request-URI into one or more next-hop URIs (i.e., recipient URIs),
      and delivers the request to those URIs.

   Target URI:  The Request-URI of an incoming request that arrives to a
      relay that will perform a translation operation.

   Translation logic:  The logic that defines a translation operation at
      a relay.  This logic includes the translation's target and
      recipient URIs.

   Translation operation:  Operation by which a relay translates the
      Request-URI of an incoming request (i.e., the target URI) into one
      or more URIs (i.e., recipient URIs) which are used as the Request-
      URIs of one or more outgoing requests.




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3.  Relays and Translations

   Relays play a key role in this framework.  A relay is defined as any
   SIP server, be it a proxy, B2BUA (Back-to-Back User Agent), or some
   hybrid, which receives a request, translates its Request-URI into one
   or more next hop URIs, and delivers the request to those URIs.  The
   Request-URI of the incoming request is referred to as 'target URI'
   and the destination URIs of the outgoing requests are referred to as
   'recipient URIs', as shown in Figure 1.


                       +---------------+
                       |               |  recipient URI
                       |               |---------------->
           target URI  |  Translation  |
        -------------->|   Operation   |  recipient URI
                       |               |---------------->
                       |               |
                       +---------------+

                      Figure 1: Translation operation

   Thus, an essential aspect of a relay is that of translation.  When a
   relay receives a request, it translates the Request-URI (target URI)
   into one or more additional URIs (recipient URIs).  Through this
   translation operation, the relay can create outgoing requests to one
   or more additional recipient URIs, thus creating the consent problem.

   The consent problem is created by two types of translations:
   translations based on local data and translations that involve
   amplifications.

   Translation operations based on local policy or local data (such as
   registrations) are the vehicle by which a request is delivered
   directly to an endpoint, when it would not otherwise be possible to.
   In other words, if a spammer has the address of a user,
   'sip:user@example.com', it cannot deliver a MESSAGE request to the UA
   (User Agent) of that user without having access to the registration
   data that maps 'sip:user@example.com' to the user agent on which that
   user is present.  Thus, it is the usage of this registration data,
   and more generally, the translation logic, which is expected to be
   authorized in order to prevent undesired communications.  Of course,
   if the spammer knows the address of the user agent, it will be able
   to deliver requests directly to it.

   Translation operations that result in more than one recipient URI are
   a source of amplification.  Servers that do not perform translations,
   such as outbound proxy servers, do not cause amplification.



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   Figure 2 shows a relay that performs translations.  The user agent
   client in the figure sends a SIP request to a URI representing a
   resource in the domain 'example.com' (sip:resource@example.com).
   This request can pass through a local outbound proxy (not shown), but
   eventually arrives at a server authoritative for the domain
   'example.com'.  This server, which acts as a relay, performs a
   translation operation, translating the target URI into one or more
   recipient URIs, which can but need not belong to the domain
   'example.com'.  This relay can be, for instance, a proxy server or a
   URI-list service [I-D.ietf-sipping-uri-services].


                                                    +-------+
                                                    |       |
                                                   >|  UA   |
                                                  / |       |
                                                 /  +-------+
                                                /
                                               /
                  +-----------------------+   /
                  |                       |  /
    +-----+       |         Relay         | /       +-------+
    |     |       |                       |/        |       |
    | UA  |------>|                       |-------->| Proxy |
    |     |       |+---------------------+|\        |       |
    +-----+       ||     Translation     || \       +-------+
                  ||        Logic        ||  \
                  |+---------------------+|   \       [...]
                  +-----------------------+    \
                                                \
                                                 \  +-------+
                                                  \ |       |
                                                   >| B2BUA |
                                                    |       |
                                                    +-------+

                 Figure 2: Relay performing a translation

   This framework allows potential recipients of a translation to agree
   to be actual recipients by giving the relay performing the
   translation permission to send them traffic.


4.  Architecture

   Figure 3 shows the architectural elements of this framework.  The
   manipulation of a relay's translation logic typically causes the
   relay to send a permission request, which in turn causes the



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   recipient to grant or deny the relay permissions for the translation.
   Section 4.1 describes the role of permissions at a relay.
   Section 4.2 discusses the actions taken by a relay when its
   translation logic is manipulated by a client.  Section 4.3 discusses
   store-and-forward servers and their functionality.  Section 4.4
   describes how potential recipients can grant a relay permissions to
   add them to the relay's translation logic.  Section 4.5 discusses
   which entities need to implement this framework.


                  +-----------------------+ Permission +-------------+
                  |                       |  Request   |             |
   +--------+     |         Relay         |----------->| Store & Fwd |
   |        |     |                       |            |   Server    |
   | Client |     |                       |            |             |
   |        |     |+-------+ +-----------+|            +-------------+
   +--------+     ||Transl.| |Permissions||                   |
       |          ||Logic  | |           ||        Permission |
       |          |+-------+ +-----------+|         Request   |
       |          +-----------------------+                   V
       |               ^           ^                   +-------------+
       | Manipulation  |           |  Permission Grant |             |
       +---------------+           +-------------------|  Recipient  |
                                                       |             |
                                                       +-------------+

                     Figure 3: Reference Architecture

4.1.  Permissions at a Relay

   Relays implementing this framework obtain and store permissions
   associated to their translation logic.  These permissions indicate if
   a particular recipient has agreed to receive traffic or not at any
   given time.  Recipients that have not given the relay permission to
   send them traffic are simply ignored by the relay when performing a
   translation.

   In principle, permissions are valid as long as the context where they
   were granted is valid or until they are revoked.  For example, the
   permissions obtained by a URI-list SIP service that distributes
   MESSAGE requests to a set of recipients will be valid as long as the
   URI-list SIP service exists or until the permissions are revoked.

   Additionally, if a recipient is removed from a relay's translation
   logic, the relay SHOULD delete the permissions related to that
   recipient.  For example, if the registration of a contact URI expires
   or is otherwise terminated, the registrar deletes the permissions
   related to that contact address.



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   It is also RECOMMENDED that relays request recipients to refresh
   their permissions periodically.  If a recipient fails to refresh its
   permissions for a given period of time, the relay SHOULD delete the
   permissions related to that recipient.

      This framework does not provide any guidance for the values of the
      refreshment intervals because different applications can have
      different requirements to set those values.

4.2.  Consenting Manipulations on a Relay's Transaction Logic

   This framework aims to ensure that any particular relay only performs
   translations towards destinations that have given the relay
   permission to perform such a translation.  Consequently, when the
   translation logic of a relay is manipulated (e.g., a new recipient
   URI is added), the relay obtains permission from the new recipient in
   order to install the new translation logic.  Relays ask recipients
   for permission using MESSAGE [RFC3428] requests.

   For example, the relay hosting the URI-list service at
   'sip:friends@example.com' performs a translation from that target URI
   to a set of recipient URIs.  When a client (e.g., the administrator
   of that URI-list service) adds 'bob@example.org' as a new recipient
   URI, the relay sends a MESSAGE request to 'sip:bob@example.org'
   asking whether or not it is OK to perform the translation from
   'sip:friends@example.com' to 'sip:bob@example.org'.  The MESSAGE
   request carries in its message body a permission document that
   describes the translation for which permissions are being requested
   and a human readable part that also describes the translation.  If
   the answer is positive, the new translation logic is installed at the
   relay.  That is, the new recipient URI is added.

      The human-readable part is included so that user agents that do
      not understand permission documents can still process the request
      and display it in a sensible way to the user.

   The mechanism to be used to manipulate the translation logic of a
   particular relay depends on the relay.  Two existing mechanisms to
   manipulate translation logic are XCAP [RFC4825] and REGISTER
   transactions.

      Section 5 uses a URI-list service whose translation logic is
      manipulated with XCAP as an example of a translation in order to
      specify this framework.  Section 5.10 discusses how to apply this
      framework to registrations, which are a different type of
      translation.

   In any case, relays implementing this framework SHOULD have a means



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   to indicate that a particular recipient URI is in the states
   specified in [I-D.ietf-sipping-pending-additions] (i.e., pending,
   waiting, error, denied, or granted).

4.3.  Store-and-forward Servers

   When a MESSAGE request with a permission document arrives to the
   recipient URI to which it was sent by the relay, the receiving user
   can grant or deny the permission needed to perform the translation.
   However, users are not on-line all the time and, so, sometimes are
   not able to receive MESSAGE requests.

   This issue is also found in presence, where a user's status is
   reported by a presence server instead of by the user's user agents,
   which can go on and off line.  Similarly, store-and-forward servers
   are elements that act as SIP user agents and handle MESSAGE requests
   for a user.  A store-and-forward server stores incoming MESSAGE
   requests when the user is unavailable and delivers them when the user
   is available again.

   There are several mechanisms to implement store-and-forward message
   services (e.g., with an instant message to email gateway).  Any of
   these mechanisms can be used between a user agent and its store-and-
   forward server as long as they agree on which mechanism to use.
   Therefore, this framework does not make any provision for the
   interface between user agents and their store-and-forward servers.

      Note that the same store-and-forward message service can handle
      all incoming MESSAGE requests for a user while this is off line,
      not only those MESSAGE requests with a permission document in
      their bodies.

   Even though store-and-forward servers perform a useful function and
   they are expected to be deployed in most domains, some domains will
   not deploy them from day one.  However, user agents and relays in
   domains without store-and-forward servers can still use this consent
   framework.

   When a relay requests permissions from an off-line user agent that
   does not have an associated store-and-forward server, the relay will
   obtain an error response indicating that its MESSAGE request could
   not be delivered.  The client that attempted to add the off-line user
   to the relay's translation logic will be notified about the error
   (e.g., using the Pending Additions event package
   [I-D.ietf-sipping-pending-additions]).  This client MAY attempt to
   add the same user at a later point, hopefully when the user is on-
   line.  Clients can discover whether or not a user is on-line by using
   a presence service, for instance.



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4.4.  Recipients Grant Permissions

   Relays include in the permission documents they generate URIs that
   can be used by the recipient of the document to grant or deny the
   relay the permission described in the document.  Relays always
   include SIP URIs and can include HTTP [RFC2616] URIs for this
   purpose.  Consequently, recipients provide relays with permissions
   using SIP PUBLISH requests or HTTP GET requests.

4.5.  Entities Implementing this Framework

   The goal of this framework is to keep relays from executing
   translations towards unwilling recipients.  Therefore, it is
   RECOMMENDED that all relays implement this framework in order to
   avoid being used to perform attacks (e.g., amplification attacks).

   This framework has been designed with backwards compatibility in mind
   so that legacy user agents (i.e., user agents that do not implement
   this framework) can act both as clients and recipients with an
   acceptable level of functionality.  However, it is RECOMMENDED that
   user agents implement this framework, which includes supporting the
   Pending Additions event package specified in
   [I-D.ietf-sipping-pending-additions], the format for permission
   documents specified in [I-D.ietf-sipping-consent-format], and the
   header fields and response code specified in this document, in order
   to achieve full functionality.

   The only requirement that this framework places on store-and-forward
   servers is that they need to be able to deliver encrypted and
   integrity- protected messages to their user agents, as discussed in
   Section 7.  However, this is not a requirement specific to this
   framework but a general requirement for store-and-forward servers.


5.  Framework Operations

   This section specifies this consent framework using an example of the
   prototypical call flow.  The elements described in Section 4 (i.e.,
   relays, translations, and store-and-forward servers) play an
   essential role in this call flow.

   Figure 4 shows the complete process to add a recipient URI
   ('sip:B@example.com') to the translation logic of a relay.  User A
   attempts to add 'sip:B@example.com' as a new recipient URI to the
   translation logic of the relay (1).  User A uses XCAP [RFC4825] and
   the XML (Extensible Markup Language) format for representing resource
   lists [RFC4826] to perform this addition.  Since the relay does not
   have permission from 'sip:B@example.com' to perform translations



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   towards that URI, the relay places 'sip:B@example.com' in the pending
   state, as specified in [I-D.ietf-sipping-pending-additions].


   A@example.com        Relay       B's Store & Fwd   B@example.com
                                         Server

         |(1) Add Recipient                |                |
         |    sip:B@example.com            |                |
         |--------------->|                |                |
         |(2) HTTP 202 (Accepted)          |                |
         |<---------------|                |                |
         |                |(3) MESSAGE sip:B@example        |
         |                |    Permission Document          |
         |                |--------------->|                |
         |                |(4) 202 Accepted|                |
         |                |<---------------|                |
         |(5) SUBSCRIBE   |                |                |
         |    Event: pending-additions     |                |
         |--------------->|                |                |
         |(6) 200 OK      |                |                |
         |<---------------|                |                |
         |(7) NOTIFY      |                |                |
         |<---------------|                |                |
         |(8) 200 OK      |                |                |
         |--------------->|                |                |
         |                |                |                |User B goes
         |                |                |                |  on line
         |                |                |(9) Request for |
         |                |                |  stored messages
         |                |                |<---------------|
         |                |                |(10) Delivery of|
         |                |                |  stored messages
         |                |                |--------------->|
         |                |(11) PUBLISH uri-up              |
         |                |    Permission Document          |
         |                |<--------------------------------|
         |                |(12) 200 OK     |                |
         |                |-------------------------------->|
         |(13) NOTIFY     |                |                |
         |<---------------|                |                |
         |(14) 200 OK     |                |                |
         |--------------->|                |                |

                     Figure 4: Prototypical call flow






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5.1.  Amplification Avoidance

   Once 'sip:B@example.com' is in the pending state, the relay needs to
   ask user B for permission by sending a MESSAGE request to
   'sip:B@example.com'.  However, the relay needs to ensure that it is
   not used as an amplifier to launch amplification attacks.

   In such an attack, the attacker would add a large number of recipient
   URIs to the translation logic of a relay.  The relay would then send
   a MESSAGE request to each of those recipient URIs.  The bandwidth
   generated by the relay would be much higher than the bandwidth used
   by the attacker to add those recipient URIs to the translation logic
   of the relay.

   This framework uses a credit-based authorization mechanism to avoid
   the attack just described.  It requires users adding new recipient
   URIs to a translation to generate an amount of bandwidth that is
   comparable to the bandwidth the relay will generate when sending
   MESSAGE requests towards those recipient URIs.  When XCAP is used,
   this requirement is met by not allowing clients to add more than one
   URI per HTTP transaction.  When a REGISTER transaction is used, this
   requirement is met by not allowing clients to register more than one
   contact per REGISTER transaction.

5.1.1.  Relay's Behavior

   Relays implementing this framework MUST NOT allow clients to add more
   than one recipient URI per transaction.  If a client using XCAP
   attempts to add more than one recipient URI in a single HTTP
   transaction, the XCAP server SHOULD return an HTTP 409 (Conflict)
   response.  The XCAP server SHOULD describe the reason for the refusal
   in an XML body using the <constraint-failure> element, as described
   in [RFC4825].  If a client attempts to register more than one contact
   in a single REGISTER transaction, the registrar SHOULD return a SIP
   403 response and explain the reason for the refusal in its reason
   phrase (e.g., Maximum one contact per registration).

5.2.  Subscription to the Permission Status

   Clients need a way to be informed about the status of the operations
   they requested.  Otherwise, users can be waiting for an operation to
   succeed when it has actually already failed.  In particular, if the
   target of the request for consent was not reachable and did not have
   an associated store-and-forward server, the client needs to know to
   retry the request later.  The Pending Additions SIP event package
   [I-D.ietf-sipping-pending-additions] is a way to provide clients with
   that information.




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   Clients can use the Pending Additions SIP event package to be
   informed about the status of the operations they requested.  That is,
   the client will be informed when an operation (e.g., the addition of
   a recipient URI to a relay's translation logic) is authorized (and
   thus executed) or rejected.  Clients use the target URI of the SIP
   translation being manipulated to subscribe to the 'pending-additions'
   event package.

   In our example, after receiving the response from the relay (2), user
   A subscribes to the Pending Additions event package at the relay (5).
   This subscription keeps user A informed about the status of the
   permissions (e.g., granted or denied) the relay will obtain.

5.2.1.  Relay's Behavior

   Relays SHOULD support the Pending Additions SIP event package
   specified in [I-D.ietf-sipping-pending-additions].

5.3.  Request for Permission

   A relay requests permissions from potential recipients to add them to
   its translation logic using MESSAGE requests.  In our example, on
   receiving the request to add User B to the translation logic of the
   relay (1), the relay generates a MESSAGE request (3) towards
   'sip:B@example.com'.  This MESSAGE request carries a permission
   document, which describes the translation that needs to be authorized
   and carries a set of URIs to be used by the recipient to grant or to
   deny the relay permission to perform that translation.  User B will
   authorize the translation by using one of those URIs, as described in
   Section 5.6.  The MESSAGE request also carries a body part that
   contains the same information as the permission document but in a
   human-readable format.

   When User B uses one of the URIs in the permission document to grant
   or deny permissions, the relay needs to make sure that it was
   actually User B the one using that URI, and not an attacker.  The
   relay can use any of the methods described in Section 5.6 to
   authenticate the permission document.

5.3.1.  Relay's Behavior

   Relays that implement this framework MUST obtain permissions from
   potential recipients before adding them to their translation logic.
   Relays request permissions from potential recipients using MESSAGE
   requests.

   Section 5.6 describes the methods a relay can use to authenticate
   recipients giving the relay permission to perform a particular



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   translation.  These methods are SIP identity [RFC4474], P-Asserted-
   Identity [RFC3325], a return routability test, or SIP digest.  Relays
   that use the method consisting of a return routability test have to
   send their MESSAGE requests to a SIPS URI, as specified in
   Section 5.6.

   MESSAGE requests sent to request permissions MUST include a
   permission document and SHOULD include a human-readable part in their
   bodies.  The human-readable part contains the same information as the
   permission document (but in a human-readable format), including the
   URIs to grant and deny permissions.  User agents that do not
   understand permission documents can still process the request and
   display it in a sensible way to the user, as they would display any
   other instant message.  This way, even if the user agent does not
   implement this framework, the (human) user will be able to manually
   click on the correct URI in order to grant or deny permissions.

5.4.  Permission Document Structure

   A permission document is the XML representation of a permission.  A
   permission document contains several pieces of data:

   Identity of the Sender:  A URI representing the identity of the
      sender for whom permissions are granted.

   Identity of the Original Recipient:  A URI representing the identity
      of the original recipient, which is used as the input for the
      translation operation.  This is also called the target URI.

   Identity of the Final Recipient:  A URI representing the result of
      the translation.  The permission grants ability for the sender to
      send requests to the target URI, and for a relay receiving those
      requests to forward them to this URI.  This is also called the
      recipient URI.

   URIs to Grant Permission:  URIs that recipients can use to grant the
      relay permission to perform the translation described in the
      document.  At least one of these URIs MUST be a SIP or SIPS URI.
      HTTP and HTTPS URIs MAY also be used.

   URIs to Deny Permission:  URIs that recipients can use to deny the
      relay permission to perform the translation described in the
      document.  At least one of these URIs MUST be a SIP or SIPS URI.
      HTTP and HTTPS URIs MAY also be used.

   Permission documents can contain wildcards.  For example, a
   permission document can request permission for any relay to forward
   requests coming from a particular sender to a particular recipient.



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   Such a permission document would apply to any target URI.  That is,
   the field containing the identity of the original recipient would
   match any URI.  However, the recipient URI MUST NOT be wildcarded.

   Entities implementing this framework MUST support the format for
   permission documents defined in [I-D.ietf-sipping-consent-format] and
   MAY support other formats.

   In our example, the permission document in the MESSAGE request (3)
   sent by the relay contains the following values:

   Identity of the Sender:  Any.

   Identity of the Original Recipient:  sip:friends@example.com

   Identity of the Final Recipient:  sip:B@example.com

   URI to Grant Permission:  sips:grant-1awdch5Fasddfce34@example.com

   URI to Grant Permission:  https://example.com/grant-1awdch5Fasddfce34

   URI to Deny Permission:  sips:deny-23rCsdfgvdT5sdfgye@example.com

   URI to Deny Permission:  https://example.com/deny-23rCsdfgvdT5sdfgye

   It is expected that the Sender field often contains a wildcard.
   However, scenarios involving request-contained URI lists, such as the
   one described in Section 5.9, can require permission documents that
   apply to a specific sender.  In cases where the identity of the
   sender matters, relays MUST authenticate senders.

5.5.  Permission Requested Notification

   On receiving the MESSAGE request (3), User B's store-and-forward
   server stores it because User B is off line at that point.  When User
   B goes on line, User B fetches all the requests its store-and-forward
   server has stored (9).

5.6.  Permission Grant

   A client gives a relay permission to execute the translation
   described in a permission document by sending a SIP PUBLISH or an
   HTTP GET request to one of the URIs to grant permissions contained in
   the document.  Similarly, a client denies a relay permission to
   execute the translation described in a permission document by sending
   a SIP PUBLISH or an HTTP GET request to one of the URIs to deny
   permissions contained in the document.  Requests to grant or deny
   permissions contain an empty body.



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   In our example, User B obtains the permission document (10) that was
   received earlier by its store-and-forward server in the MESSAGE
   request (3).  User B authorizes the translation described in the
   permission document received by sending a PUBLISH request (11) to the
   SIP URI to grant permissions contained in the permission document.

5.6.1.  Relay's Behavior

   Relays MUST ensure that the SIP PUBLISH or the HTTP GET request
   received was generated by the recipient of the translation and not by
   an attacker.  Relays can use four methods to authenticate those
   requests.  SIP identity, P-Asserted-Identity [RFC3325], a return
   routability test, or SIP digest.  While return routability tests can
   be used to authenticate both SIP PUBLISH and HTTP GET requests, SIP
   identity, P-Asserted-Identity, and SIP digest can only be used to
   authenticate SIP PUBLISH requests.  SIP digest can only be used to
   authenticate clients that share a secret with the relay (e.g.,
   clients that are in the same domain as the relay).

5.6.1.1.  SIP Identity

   The SIP identity [RFC4474] mechanism can be used to authenticate the
   sender of a PUBLISH request.  The relay MUST check that the
   originator of the PUBLISH request is the owner of the recipient URI
   in the permission document.  Otherwise, the PUBLISH request SHOULD be
   responded with a 401 (Unauthorized) response and MUST NOT be
   processed further.

5.6.1.2.  P-Asserted-Identity

   The P-Asserted-Identity [RFC3325] mechanism can also be used to
   authenticate the sender of a PUBLISH request.  However, as discussed
   in [RFC3325], this mechanism is intended to be used only within
   networks of trusted SIP servers.  That is, the use of this mechanism
   is only applicable inside an administrative domain with previously
   agreed-upon policies.

   The relay MUST check that the originator of the PUBLISH request is
   the owner of the recipient URI in the permission document.
   Otherwise, the PUBLISH request SHOULD be responded with a 401
   (Unauthorized) response and MUST NOT be processed further.

5.6.1.3.  Return Routability

   SIP identity provides a good authentication mechanism for incoming
   PUBLISH requests.  Nevertheless, SIP identity is not widely available
   on the public Internet yet.  That is why an authentication mechanism
   that can already be used at this point is needed.



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   Return routability tests do not provide the same level of security as
   SIP identity, but they provide a better-than-nothing security level
   in architectures where the SIP identity mechanism is not available
   (e.g., the current Internet).  The relay generates an unguessable URI
   (i.e., with a cryptographically random user part) and places it in
   the permission document in the MESSAGE request (3).  The recipient
   needs to send a SIP PUBLISH request or an HTTP GET request to that
   URI.  Any incoming request sent to that URI SHOULD be considered
   authenticated by the relay.

      Note that the return routability method is the only one that
      allows the use of HTTP URIs in permission documents.  The other
      methods require the use of SIP URIs.

   Relays using a return routability test to perform this authentication
   MUST send the MESSAGE request with the permission document to a SIPS
   URI.  This ensures that attackers do not get access to the
   (unguessable) URI.  Thus, the only user able to use the (unguessable)
   URI is the receiver of the MESSAGE request.  Similarly, permission
   documents sent by relays using a return routability test MUST only
   contain secure URIs (i.e., SIPS and HTTPS) to grant and deny
   permissions.  The user part of these URIs MUST be cryptographically
   random with at least 32 bits of randomness.

   Relays can transition from return routability tests to SIP identity
   by simply requiring the use of SIP identity for incoming PUBLISH
   requests.  That is, such a relay would reject PUBLISH requests that
   did not use SIP identity.

5.6.1.4.  SIP Digest

   The SIP digest mechanism can be used to authenticate the sender of a
   PUBLISH request as long as that sender shares a secret with the
   relay.  The relay MUST check that the originator of the PUBLISH
   request is the owner of the recipient URI in the permission document.
   Otherwise, the PUBLISH request SHOULD be responded with a 401
   (Unauthorized) response and MUST NOT be processed further.

5.7.  Permission Granted Notification

   On receiving the PUBLISH request (11), the relay sends a NOTIFY
   request (13) to inform user A that the permission for the translation
   has been received and that the translation logic at the relay has
   been updated.  That is, 'sip:B@example.com' has been added as a
   recipient URI.






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5.8.  Permission Revocation

   At any time, if a recipient wants to revoke any permission, it uses
   the URI it received in the permission document to deny the
   permissions it previously granted.  If a recipient loses this URI for
   some reason, it needs to wait until it receives a new request
   produced by the translation.  Such a request will contain a Trigger-
   Consent header field with a URI.  That Trigger-Consent header field
   will have a target-uri header field parameter identifying the target
   URI of the translation.  The recipient needs to send a PUBLISH
   request with an empty body to the URI in the Trigger-Consent header
   field in order to receive a MESSAGE request from the relay.  Such a
   MESSAGE request will contain a permission document with a URI to
   revoke the permission that was previously granted.

   Figure 5 shows an example of how a user that lost the URI to revoke
   permissions at a relay can obtain a new URI using the Trigger-Consent
   header field of an incoming request.  The user rejects an incoming
   INVITE (1) request, which contains a Trigger-Consent header field.
   Using the URI in the that header field, the user sends a PUBLISH
   request (4) to the relay.  On receiving the PUBLISH request (4), the
   relay generates a MESSAGE request (6) towards the user.  Finally, the
   user revokes the permissions by sending a PUBLISH request (8) to the
   relay.


           Relay                     B@example.com
             |(1) INVITE                   |
             |    Trigger-Consent: sip:123@relay.example.com
             |     ;target-uri="sip:friends@relay.example.com"
             |---------------------------->|
             |(2) 603 Decline              |
             |<----------------------------|
             |(3) ACK                      |
             |---------------------------->|
             |(4) PUBLISH sip:123@relay.example.com
             |<----------------------------|
             |(5) 200 OK                   |
             |---------------------------->|
             |(6) MESSAGE sip:B@example    |
             |    Permission Document      |
             |---------------------------->|
             |(7) 200 OK                   |
             |<----------------------------|
             |(8) PUBLISH uri-deny         |
             |<----------------------------|
             |(9) 200 OK                   |
             |---------------------------->|



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                      Figure 5: Permission Revocation

5.9.  Request-contained URI Lists

   In the scenarios described so far, a user adds recipient URIs to the
   translation logic of a relay.  However, the relay does not perform
   translations towards those recipient URIs until permissions are
   obtained.

   URI-list services using request-contained URI lists are a special
   case because the selection of recipient URIs is performed at the same
   time as the communication attempt.  A user places a set of recipient
   URIs in a request and sends it to a relay so that the relay sends a
   similar request to all those recipient URIs.

   Relays implementing this framework and providing this type of URI-
   list services behave in a slightly different way as the relays
   described so far.  This type of relay also maintains a list of
   recipient URIs for which permissions have been received.  Clients
   also manipulate this list using a manipulation mechanism (e.g.,
   XCAP).  Nevertheless, this list does not represent the recipient URIs
   of every translation performed by the relay.  This list just
   represents all the recipient URIs for which permissions have been
   received.  That is, the set of URIs that will be accepted if a
   request containing a URI-list arrives to the relay.  This set of URIs
   is a super set of the recipient URIs of any particular translation
   the relay performs.

5.9.1.  Relay's Behavior

   On receiving a request-contained URI-list, the relay checks whether
   or not it has permissions for all the URIs contained in the incoming
   URI-list.  If it does, the relay performs the translation.  If it
   lacks permissions for one of more URIs, the relay does not perform
   the translation and returns an error response.

   A relay that receives a request-contained URI-list with a URI for
   which the relay has no permissions SHOULD return a 470 (Consent
   Needed) response.  The relay SHOULD add a Permission-Missing header
   field with the URIs for which the relay has no permissions.

5.9.2.  Definition of the 470 Response Code

   A 470 (Consent Needed) response indicates that the request that
   triggered the response contained a URI-list with at least a URI for
   which the relay had no permissions.  The URI or URIs for which the
   relay had not permissions are listed in a Permission-Missing header
   field, should the response carry one.



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   A client receiving a 470 (Consent Needed) response uses a
   manipulation mechanism (e.g., XCAP) to add those URIs to the relay's
   list of URIs.  The relay will obtain permissions for those URIs as
   usual.

5.9.3.  Definition of the Permission-Missing Header Field

   Permission-Missing header fields carry URIs for which a relay did not
   have permissions.  The following is the augmented Backus-Naur Form
   (BNF) [RFC4234] syntax of the Permission-Missing header field.  Some
   of its elements are defined in [RFC3261].


     Permission-Missing  =  "Permission-Missing" HCOLON per-miss-spec
                            *( COMMA per-miss-spec )
     per-miss-spec       =  ( name-addr / addr-spec )
                           *( SEMI generic-param )

   The following is an example of a Permission-Missing header field:


     Permission-Missing: sip:C@example.com

   Figure 6 shows a relay that receives a request (1) that contains URIs
   for which the relay does not have permission.  The relay rejects the
   request with a 470 (Consent Needed) response (2).  That response
   contains a Permission-Missing header field with the URIs for which
   there was no permission.


       A@example.com               Relay

             |(1) INVITE             |
             |    sip:B@example.com  |
             |    sip:C@example.com  |
             |---------------------->|
             |(2) 470 Consent Needed |
             |    Permission-Missing: sip:C@example.com
             |<----------------------|
             |(3) ACK                |
             |---------------------->|

               Figure 6: INVITE with a URI list in its body








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5.10.  Registrations

   Even though the example used to specify this framework has been a
   URI-list service, this framework applies to any type of translation
   (i.e., not only to URI-list services).  Registrations are a different
   type of translations that deserve discussion.

   Registrations are a special type of translations.  The user
   registering has a trust relationship with the registrar in its home
   domain.  This is not the case when a user gives any type of
   permissions to a relay in a different domain.

   Traditionally, REGISTER transactions have performed two operations at
   the same time: setting up a translation and authorizing the use of
   that translation.  For example, a user registering its current
   contact URI is giving permission to the registrar to forward traffic
   sent to the user's AoR (Address of Records) to the registered contact
   URI.  This works fine when the entity registering is the same as the
   one that will be receiving traffic at a later point (e.g., the entity
   receives traffic over the same connection used for the registration
   as described in [I-D.ietf-sip-outbound]).  However, this schema
   creates some potential attacks which relate to third-party
   registrations.

   An attacker binds, via a registration, his or her AoR with the
   contact URI of a victim.  Now, the victim will receive unsolicited
   traffic that was originally addressed to the attacker.

   The process of authorizing a registration is shown in Figure 7.  User
   A performs a third-party registration (1) and receives a 202
   (Accepted) response (2).

   Since the relay does not have permission from
   'sip:a@ws123.example.com' to perform translations towards that
   recipient URI, the relay places 'sip:a@ws123.example.com' in the
   'pending' state.  Once 'sip:a@ws123.example.com' is in the
   'Permission Pending' state, the registrar needs to ask
   'sip:a@ws123.example.com' for permission by sending a MESSAGE request
   (3).

   After receiving the response from the relay (2), user A subscribes to
   the Pending Additions event package at the registrar (4).  This
   subscription keeps the user informed about the status of the
   permissions (e.g., granted or denied) the registrar will obtain.  The
   rest of the process is similar to the one described in Section 5.






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       A@example.com         Registrar      a@ws123.example.com

             |(1) REGISTER       |                   |
             |    Contact: sip:a@ws123.example.com   |
             |------------------>|                   |
             |(2) 202 Accepted OK|                   |
             |<------------------|                   |
             |                   |(3) MESSAGE sip:a@ws123.example
             |                   |    Permission Document
             |                   |------------------>|
             |                   |(4) 200 OK         |
             |                   |<------------------|
             |(5) SUBSCRIBE      |                   |
             |    Event: pending-additions           |
             |------------------>|                   |
             |(6) 200 OK         |                   |
             |<------------------|                   |
             |(7) NOTIFY         |                   |
             |<------------------|                   |
             |(8) 200 OK         |                   |
             |------------------>|                   |
             |                   |(9) PUBLISH uri-up |
             |                   |<------------------|
             |                   |(10) 200 OK        |
             |                   |------------------>|
             |(11) NOTIFY        |                   |
             |<------------------|                   |
             |(12) 200 OK        |                   |
             |------------------>|                   |

                          Figure 7: Registration

   Permission documents generated by registrars are typically very
   general.  For example, in one such document a registrar can ask a
   recipient for permission to forward any request from any sender to
   the recipient's URI.  This is the type of granularity that this
   framework intends to provide for registrations.  Users who want to
   define how incoming requests are treated with a finer granularity
   (e.g., requests from user A are only accepted between 9:00 and 11:00)
   will have to use other mechanisms such as CPL [RFC3880].

      Note that, as indicated previously, user agents using the same
      connection to register and to receive traffic from the registrar,
      as described in [I-D.ietf-sip-outbound] do not need to use the
      mechanism described in this section.

   A user agent being registered by a third party can be unable to use
   the SIP Identity, P-Asserted-Identity, or SIP digest mechanisms to



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   prove to the registrar that the user agent is the owner of the URI
   being registered (e.g., sip:user@192.0.2.1), which is the recipient
   URI of the translation.  In this case, return routability MUST be
   used.

5.11.  Relays Generating Traffic towards Recipients

   Relays generating traffic towards recipient need to make sure that
   those recipients can revoke the permissions they gave at any time.
   The Trigger-Consent helps achieve this.

5.11.1.  Relay's Behavior

   A relay executing a translation that involves sending a request to a
   URI from which permissions were obtained previously SHOULD add a
   Trigger-Consent header field to the request.  The URI in the Trigger-
   Consent header field MUST have a target-uri header field parameter
   identifying the target URI of the translation.

   On receiving a PUBLISH request addressed to the URI a relay placed in
   a Trigger-Consent header field, the relay SHOULD send a MESSAGE
   request to corresponding recipient URI with a permission document.
   Therefore, the relay needs to be able to correlate the URI it places
   in the Trigger-Consent header field with the recipient URI of the
   translation.

5.11.2.  Definition of the Trigger-Consent Header Field

   The following is the augmented Backus-Naur Form (BNF) [RFC4234]
   syntax of the Trigger-Consent header field.  Some of its elements are
   defined in [RFC3261].


     Trigger-Consent     =  "Trigger-Consent" HCOLON trigger-cons-spec
                            *( COMMA trigger-cons-spec )
     trigger-cons-spec   =  ( SIP-URI / SIPS-URI )
                            *( SEMI trigger-param )
     trigger-param       =  target-uri / generic-param
     target-uri          =  "target-uri" EQUAL
                                 LDQUOT *( qdtext / quoted-pair ) RDQUOT

   The target-uri header field parameter MUST contain a URI.

   The following is an example of a Trigger-Consent header field:


     Trigger-Consent: sip:123@relay.example.com
                      ;target-uri="sip:friends@relay.example.com"



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6.  IANA Considerations

   The following sections request the IANA to register a SIP response
   code, two SIP header fields, and a SIP header field parameter.

6.1.  Registration of the 470 Response Code

   The IANA is requested to add the following new response code to the
   Methods and Response Codes subregistry under the SIP Parameters
   registry.


     Response Code Number:   470
     Default Reason Phrase:  Consent Needed
     Reference:              [RFCxxxx]

   Note to the RFC editor: substitute xxxx with the RFC number of this
   document.

6.2.  Registration of the Trigger-Consent Header Field

   The IANA is requested to add the following new SIP header field to
   the Header Fields subregistry under the SIP Parameters registry.


     Header Name:   Trigger-Consent
     Compact Form:  (none)
     Reference:     [RFCxxxx]

   Note to the RFC editor: substitute xxxx with the RFC number of this
   document.

6.3.  Registration of the target-uri Header Field Parameter

   The IANA is requested to register the 'target-uri' Trigger-Consent
   header field parameter under the Header Field Parameters and
   Parameter Values subregistry within the SIP Parameters registry:


                                                  Predefined
   Header Field                  Parameter Name     Values     Reference
   ----------------------------  ---------------   ---------   ---------
   Trigger-Consent               target-uri           No       [RFCxxxx]

   Note to the RFC editor: substitute xxxx with the RFC number of this
   document.





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7.  Security Considerations

   Security has been discussed throughout the whole document.  However,
   there are some issues that deserve special attention.

   The specifications of mechanisms to manipulate translation logic at
   relays usually stress the importance of client authentication and
   authorization.  Having relays authenticate and authorize clients
   manipulating their translation logic keeps unauthorized clients from
   adding recipients to a translation.  However, this does not prevent
   authorized clients to add recipients to a translation without their
   consent.  Additionally, some relays provide web interfaces for any
   client to add new recipients to the translation (e.g., many email
   mailing lists are operated in this way).  In this situation, every
   client is considered authorized to manipulate the translation logic
   at the relay.  This makes the use of this framework even more
   important.  Therefore, it is RECOMMENDED that relays performing
   translations implement this framework.

   As pointed out in Section 5.6.1.3, when return routability tests are
   used to authenticate recipients granting or denying permissions, the
   URIs used to grant or deny permissions need to be protected from
   attackers.  SIPS URIs provide a good tool to meet this requirement,
   as described in [I-D.ietf-sipping-consent-format].  When store-and-
   forward servers are used, the interface between a user agent and its
   store-and-forward server is frequently not based on SIP.  In such
   case, SIPS cannot be used to secure those URIs.  Implementations of
   store-and-forward servers MUST provide a mechanism for delivering
   encrypted and integrity-protected messages to their user agents.

   The information provided by the Pending Additions event package can
   be sensitive.  For this reason, as described in
   [I-D.ietf-sipping-pending-additions], relays need to use strong means
   for authentication and information confidentiality.  SIPS URIs are a
   good mechanism to meet this requirement.


8.  Acknowledgments

   Henning Schulzrinne, Jon Peterson, and Cullen Jennings provided
   useful ideas on this document.  Ben Campbell, AC Mahendran, Keith
   Drage, and Mary Barnes performed a thorough review of this document.


9.  References






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9.1.  Normative References

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [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.

   [RFC3428]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
              and D. Gurle, "Session Initiation Protocol (SIP) Extension
              for Instant Messaging", RFC 3428, December 2002.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [I-D.ietf-sipping-uri-services]
              Camarillo, G. and A. Roach, "Framework and Security
              Considerations for Session Initiation Protocol (SIP)
              Uniform Resource Identifier (URI)-List Services",
              draft-ietf-sipping-uri-services-06 (work in progress),
              September 2006.

   [I-D.ietf-sipping-consent-format]
              Camarillo, G., "A Document Format for Requesting Consent",
              draft-ietf-sipping-consent-format-03 (work in progress),
              April 2007.

   [I-D.ietf-sipping-pending-additions]
              Camarillo, G., "The Session Initiation Protocol (SIP)
              Pending Additions Event Package",
              draft-ietf-sipping-pending-additions-02 (work in
              progress), April 2007.

9.2.  Informative References

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3880]  Lennox, J., Wu, X., and H. Schulzrinne, "Call Processing
              Language (CPL): A Language for User Control of Internet



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              Telephony Services", RFC 3880, October 2004.

   [RFC4453]  Rosenberg, J., Camarillo, G., and D. Willis, "Requirements
              for Consent-Based Communications in the Session Initiation
              Protocol (SIP)", RFC 4453, April 2006.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
              Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

   [RFC4826]  Rosenberg, J., "Extensible Markup Language (XML) Formats
              for Representing Resource Lists", RFC 4826, May 2007.

   [I-D.ietf-sip-outbound]
              Jennings, C. and R. Mahy, "Managing Client Initiated
              Connections in the Session Initiation Protocol  (SIP)",
              draft-ietf-sip-outbound-08 (work in progress), March 2007.


Authors' Addresses

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net


   Gonzalo Camarillo (editor)
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com









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   Dean Willis
   Unaffiliated
   3100 Independence Pkwy #311-164
   Plano, TX  75075
   USA

   Email: dean.willis@softarmor.com












































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Full Copyright Statement

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