SIPPING WG                                                     J. Elwell
Internet-Draft                         Siemens Enterprise Communications
Updates:  RFC 3325                                          GmbH & Co KG
(if approved)                                              June 27,                                            August 15, 2008
Intended status:  Informational
Expires:  December 29, 2008  February 16, 2009

 Updates to Asserted Identity in the Session Initiation Protocol (SIP)
                  draft-ietf-sipping-update-pai-04.txt
                  draft-ietf-sipping-update-pai-05.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 29, 2008. February 16, 2009.

Abstract

   SIP has a mechanism for conveying the asserted identity of the
   originator of a request by means of the P-Asserted-Identity header
   field.  This header field is specified for use in requests using a
   number of SIP methods, in particular the INVITE method.  However, RFC
   3325 does not specify the insertion of this header field by a trusted
   UAC, does not specify the use of this header field with certain SIP
   methods such as UPDATE, REGISTER, MESSAGE, PUBLISH and ACK, and is
   unclear on the use of this header field in responses.  This document
   extends RFC 3325 to cover these situations.

   This work is being discussed on the sipping@ietf.org mailing list.

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  3  4
     3.1.  Inclusion of P-Asserted-Identity by a UAC  . . . . . . . .  4
     3.2.  Inclusion of P-Asserted-Identity in any request  . . . . .  4  5
     3.3.  Inclusion of P-Asserted-Identity or
           P-Preferred-Identity in a response . . . . . . . . . . . .  6
     3.4.  Dialog implications  . . . . . . . . . . . . . . . . . . .  7  8
   4.  Behaviour  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  UAC Behaviour  . . . . . . . . . . . . . . . . . . . . . .  8
       4.1.1.  Request handling . . . . . . . . . . . . . . . . . . .  8
       4.1.2.  Response handling  . . . . . . . . . . . . . . . . . .  8
     4.2.  Proxy Behaviour  . . . . . . . . . . . . . . . . . . . . .  9
       4.2.1.  Request handling . . . . . . . . . . . . . . . . . . .  9
       4.2.2.  Response handling  . . . . . . . . . . . . . . . . . .  9
     4.3.  Registrar Behaviour  . . . . . . . . . . . . . . . . . . . 10
     4.4.  UAS Behaviour  . . . . . . . . . . . . . . . . . . . . . . 10
       4.4.1.  Request handling . . . . . . . . . . . . . . . . . . . 10
       4.4.2.  Response handling  . . . . . . . . . . . . . . . . . . 10
     4.5.  General handling . . . . . . . . . . . . . . . . . . . . . 10
   5.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 10 11
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 10 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 13 14

1.  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 [RFC2119].

   This document uses the concepts of Trust Domain and Spec(T), as
   specified in section 2.3 of RFC 3324 [RFC3324].

2.  Introduction

   The Session Initiation Protocol (SIP) is specified in RFC 3261
   [RFC3261].  RFC 3325 [RFC3325] specifies a mechanism for conveying
   within a Trust Domain the asserted identity of the originator of a
   SIP request.  This is achieved by means of the P-Asserted-Identity
   header field, which is specified for use in requests using a number
   of SIP methods, in particular the INVITE method.

   RFC 3325 does not specify the insertion of the P-Asserted-Identity
   header field by a UAC in the same Trust Domain as the first proxy.
   Also RFC 3325 does not specify the use of the P-Asserted-Identity
   header field with certain SIP methods such as UPDATE [RFC3311],
   REGISTER, MESSAGE [RFC3428], PUBLISH [RFC3903] and ACK.  Finally, RFC
   3325 is unclear on the use of this header field in responses.  There
   are similar omissions concerning the P-Preferred-Identity header
   field.

   This document extends RFC 3325 by allowing inclusion of the
   P-Asserted-Identity header field by a UAC in the same Trust Domain as
   the first proxy, allowing use of this header field in any request
   and, under certain conditions, allowing use of this header field in
   SIP responses.  This document also allows the use of the P-Preferred-
   Identity header field in some of these situations.

   RFC 3325 allows the P-Asserted-Identity and P-Preferred-Identity
   header fields each to contain at most two URIs, where one is a SIP or
   SIPS URI [RFC3261] and the other is a TEL URI [RFC3966].  This may be
   unduly restrictive in future, for example if there is a need to allow
   other URI schemes, if there is a need to allow both a SIP and a SIPS
   URI or if there is a need to allow more than one URI with the same
   scheme (e.g., a SIP URI based on a telephone number and a SIP URI
   that is not based on a telephone number).  This document therefore
   provides forwards compatibility by mandating tolerance to the receipt
   of unexpected URIs.

   This document does not alter the fact that the asserted identity
   mechanism has limited applicability, i.e., within a Trust Domain.

   For general applicability, including operation outside a Trust Domain
   (e.g., over the public Internet) or between different Trust Domains,
   a different mechanism is needed.  RFC 4474 [RFC4474] specifies the
   Identity header field, in conjunction with the From header field, for
   providing authenticated identity in such circumstances.

3.  Discussion

3.1.  Inclusion of P-Asserted-Identity by a UAC

   RFC 3325 does not include procedures for a UAC to include the
   P-Asserted-Identity header field in a request.  This can be
   meaningful if the UAC is in the same Trust Domain as the first
   downstream SIP entity.  Examples of types of UAC that are often
   suitable for inclusion in a Trust Domain are:

   o  PSTN gateways;

   o  media servers;

   o  application servers (or B2BUAs) that act as URI list servers
      [I-D.ietf-sipping-uri-services];

   o  application servers (or B2BUAs) that perform third party call
      control.

   In the particular case of a PSTN gateway, the PSTN gateway might be
   able to assert an identity received from the PSTN, the proxy itself
   having no means to authenticate such an identity.  Likewise, in the
   case of certain application server or B2BUA arrangements, the
   application server or B2BUA may be in a position to assert an
   identity of a user on the other side of that application server or
   B2BUA.

   In accordance with RFC 3325, nodes within a Trust Domain must behave
   in accordance with a Spec(T), and this principle needs to apply
   between a UAC and its proxy as part of the condition for considering
   the UAC to be within the same Trust Domain.  Normal proxy procedures
   of RFC 3325 ensure that the header field is removed or replaced if
   the first proxy considers the UAC to be outside the Trust Domain.

   This update to RFC 3325 clarifies that a UAC may include a
   P-Asserted-Identity header field in a request in certain
   circumstances.

3.2.  Inclusion of P-Asserted-Identity in any request

   There are several use cases that would benefit from the use of the
   P-Asserted-Identity header field in an UPDATE request.  These use
   cases apply within a Trust Domain where the use of asserted identity
   is appropriate (see RFC 3325).

   In one example, an established call passes through a gateway to the
   PSTN.  The gateway becomes aware that the remote party in the PSTN
   has changed, e.g., due to call transfer.  By including the
   P-Asserted-Identity header field in an UPDATE request, the gateway
   can convey the identity of the new remote party to the peer SIP UA.

      Note that the (re-)INVITE method could be used in this situation.
      However, this forces an offer-answer exchange, which typically is
      not required in this situation.  Also it involves 3 messages
      rather than 2.

   In another example, a B2BUA that provides third party call control
   (3PCC) [RFC3725] wishes to join two calls together, one of which is
   still waiting to be answered and potentially is forked to different
   UAs.  At this point in time it is not possible to trigger the normal
   offer-answer exchange between the two joined parties, because of the
   mismatch between a single dialog on the one side and potentially
   multiple early dialogs on the other side, so this action must wait
   until one of the called UAs answers.  However, it would be useful to
   give an early indication to each user concerned of the identity of
   the user to which they will become connected when the call is
   answered.  In other words, it would provide the new calling UA with
   the identity of the new called user and provide the new called UA(s)
   with the identity of the new calling user.  This can be achieved by
   the B2BUA sending an UPDATE request with a P-Asserted-Identity header
   field on the dialogs concerned.

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in a REGISTER request between an edge proxy
   that has authenticated the source of the request and the registrar.

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in a MESSAGE request to assert the source of a
   page mode instant message.  This would complement its use in an
   INVITE request to assert the source of an instant message session or
   any other form of session.  Similarly, between a UAC and first proxy
   that are not within the same Trust Domain, a P-Preferred-Identity
   header field could be used in a MESSAGE request to express a
   preference when the user has several identities.

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in a PUBLISH request to assert the source of
   published state information.  This would complement its use in
   SUBSCRIBE and NOTIFY requests.  Similarly, between a UAC and first
   proxy that are not within the same Trust Domain, a P-Preferred-
   Identity header field could be used in a PUBLISH request to express a
   preference when the user has several identities.

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in an ACK request.  Considering the 3PCC
   scenario in Flow I of [RFC3725], the asserted identity of user B may
   not be known when the B2BUA (controller) sends the initial INVITE
   request to UA A, but might be known when the B2BUA sends the ACK
   request to UA A (having received it in the 200 response from UA B).

   Thus there are several examples where P-Asserted-Identity could be
   used in requests with methods that are not provided for in RFC 3325
   or any other RFC.  This leaves a few methods for which use cases are
   less obvious, but the inclusion of P-Asserted Identity would not
   cause any harm.  In any requests, the header field would simply
   assert the source of that request, whether or not this is of any use
   to the UAS.  Similarly there are examples where P-Preferred-Identity
   could be used in requests with methods that are not provided for in
   RFC 3325 or any other RFC.

   This update to RFC 3325 allows a P-Asserted-Identity or P-Preferred-
   Identity header field to be included in any request.

3.3.  Inclusion of P-Asserted-Identity or P-Preferred-Identity in a
      response

   There are cases where the inclusion of the P-Asserted-Identity header
   field in responses would be useful.  Retargeting of a request can
   result in the responding entity having a different identity from that
   placed in the To URI of the request.  Inclusion of asserted identity
   in a response would provide the UAC with the identity of the
   responder.  Some examples of the benefits to be gained include:

   o  Asserted identity in a 2xx response to an INVITE request would
      indicate the identity of the connected user.

   o  Asserted identity in a provisional response to an INVITE request
      would indicate the contacted (e.g., alerted) user.

   o  Asserted identity in a 2xx response to a MESSAGE request would
      provide confirmation of where the message was delivered to.

   o  Asserted identity in certain 4xx/5xx/6xx responses would provide
      an indication of where the response originated.

   In the case of a request that results in the formation of a dialog, a
   mid-dialog request (e.g., UPDATE) in the reverse direction can
   provide the identity of the user at the destination end of that
   dialog, and therefore the need to include asserted identity in a
   response to the dialog-forming request to identify the connected user
   is debatable.  There can be some benefits in terms of ease of
   interworking with PSTN, where such information is placed in the
   response to a call establishment request.  For other responses,
   including successful responses to requests such as MESSAGE and
   PUBLISH and unsuccessful responses, the use of a request in the
   reverse direction is unsuitable.

      Note that when the authenticated identity of the connected user is
      to be provided using the From and Identity header fields (as
      opposed to providing asserted identity using the P-Asserted-
      Identity header field), RFC 4916 [RFC4916] requires this to be
      done in a mid-dialog request (e.g., UPDATE) in the reverse
      direction.  This is because the Identity header field is defined
      only for use in requests.

   RFC 3325 is ambiguous on inclusion of P-Asserted-Identity in a
   response.  For example, section 4 of RFC 3325 talks about inclusion
   of the header field in messages, as opposed to requests.  Moreover
   section 5 explicitly mentions "message (request or response)".
   However, there are other places (e.g., sections 6, 7 and 8) that only
   mention requests.

   Section 5 of RFC 3325 requires a proxy to authenticate the originator
   of a message before adding a P-Asserted-Identity header field to the
   forwarded message.  In practice there is no SIP means to authenticate
   the sender of a SIP response message.  However, authentication may be
   possible by other means.  For example, if the proxy has TLS
   connectivity with the originator of the response and has previously
   authenticated the connected entity (e.g., using SIP digest
   authentication at registration time), then the originator of the
   response can be considered to be authenticated.  In such
   circumstances it is permissible for a proxy to insert a P-Asserted-
   Identity header field in a SIP response.

   It should also be permissible for a UAS to insert a P-Asserted-
   Identity header field into a response if it is within the same Trust
   Domain as the SIP entity from which the request was received.

   Between a UAS and a SIP entity that are not within the same Trust
   Domain, a P-Preferred-Identity header field could be used in a
   response, in order to express a preference when the authenticated
   user has several identities.

   This update to RFC 3325 allows a P-Asserted-Identity or P-Preferred-
   Identity header field to be included in a response in certain
   circumstances.

3.4.  Dialog implications

   A P-Asserted-Identity header field in a received request or response
   asserts the identity of the source of that request or response and
   says nothing about the source of subsequent received requests or
   responses claiming to relate to the same dialog.  The recipient can
   make its own deductions about the source of subsequent requests or
   responses not containing a P-Asserted-Identity header field.  This
   document does not change RFC 3325 in this respect.

4.  Behaviour

   This document updates RFC 3325 by allowing a P-Asserted-Identity
   header field to be included by a UAC within the same Trust Domain, by
   allowing a P-Asserted-Identity or P-Preferred-Identity header field
   to appear in any request, and by allowing a P-Asserted-Identity
   header field to appear in a response in certain circumstances.

4.1.  UAC Behaviour

4.1.1.  Request handling

   A UAC MAY include a P-Asserted-Identity header field in a request to
   report the identity of the user on behalf of which the UAC is acting
   and whose identity the UAC is in a position to assert.  A UAC SHOULD
   do so only in cases where it believes it is in the same Trust Domain
   as the SIP entity to which it sends the request and is connected to
   that SIP entity in accordance with the security requirements of RFC
   3325.  A UAC SHOULD NOT do so in other circumstances and might
   instead use the P-Preferred-Identity header field.  A UAC MUST NOT
   include both header fields.

   A UAC MAY include a P-Asserted-Identity or P-Preferred-Identity
   header field in any request, i.e., not limited to the methods allowed
   in RFC 3325.

4.1.2.  Response handling

   Typically a UA renders the value of a P-Asserted-Identity header
   field that it receives in a response to its user.  It may consider
   the identity provided by a Trust Domain to be privileged, or
   intrinsically more trustworthy than other information in the
   response.  However, any particular behaviour is specific to
   implementations or services.  This document also does not mandate any
   UA handling for multiple P-Asserted-Identity header field values that
   happen to appear in a response (such as a SIP URI alongside a tel
   URL).

   However, if a UAC receives a response from a previous element it does
   not trust, it MUST NOT use the P-Asserted-Identity header field in
   any way.

   If a UA is part of the Trust Domain from which it received a response
   containing a P-Asserted-Identity header field, then it can use the
   value internally but it MUST ensure that it does not forward the
   information to any element that is not part of the Trust Domain if
   the responding user has requested that asserted identity information
   be kept private.

4.2.  Proxy Behaviour

4.2.1.  Request handling

   If a proxy receives a request from a UAC within the Trust Domain it
   MUST behave as for a request from any other node within the Trust
   Domain, in accordance with the rules of RFC 3325 for a proxy.

      Note that this implies that the proxy must have authenticated the
      sender of the request in accordance with the Spec(T) in force for
      the Trust Domain and determined that the sender is indeed part of
      the Trust Domain.

   If a proxy receives a request containing a P-Asserted-Identity or
   P-Preferred-Identity header field, it MUST behave in accordance with
   the rules of RFC 3325 for a proxy, even if the method is not one for
   which RFC 3325 specifies use of that header field.

4.2.2.  Response handling

   The proxy behaviour specified in RFC 3325 is applicable to responses
   with the following qualifications.  A proxy that receives a response
   from a node outside the Trust Domain cannot directly authenticate the
   UAS by SIP means.  Therefore it MUST NOT include a P-Asserted-
   Identity header field when forwarding the response unless it has
   authenticated the UAS by other means.

      One possible circumstance in which a proxy can include a
      P-Asserted-Identity header field when forwarding a response from a
      node outside the Trust Domain is when the proxy has direct TLS
      connectivity with the UAS and has authenticated the UA by some
      other means (e.g., SIP digest authentication) during that same TLS
      session.

   The proxy behaviour specified in RFC 3325 applies for handling a
   P-Asserted-Identity header field in a response from a node within the
   Trust Domain.

   The proxy behaviour specified in RFC 3325 for handling a received
   P-Preferred-Identity header field is applicable also to responses,
   subject to the qualification above concerning authentication of the
   UAS as a pre-requisite for inserting a P-Asserted-Identity header
   field.

4.3.  Registrar Behaviour

   If a registrar receives a REGISTER request containing a P-Asserted-
   Identity header field, it MUST disregard the asserted identity unless
   received over a secure transport from a node within the Trust Domain.
   Otherwise it MAY use this as evidence that the registering UA has
   been authenticated as representing the identity asserted in the
   header field.

4.4.  UAS Behaviour

4.4.1.  Request handling

   If a UAS receives any request containing a P-Asserted-Identity header
   field, it MUST behave as for any other request in accordance with the
   rules of RFC 3325 for a UAS, even if the method is not one for which
   RFC 3325 specifies use of that header field.

4.4.2.  Response handling

   A UAS MAY include a P-Asserted-Identity or P-Preferred-Identity
   header field in a response to report the identity of the user on
   behalf of which the UAS is acting and whose identity the UAS is in a
   position to assert.  A UAS SHOULD include a P-Asserted-Identity
   header field only in cases where it believes it is in the same Trust
   Domain as the SIP entity from which it received the request and is
   connected to that SIP entity in accordance with the security
   requirements of RFC 3325.

4.5.  General handling

   If an entity receives a request or response containing a P-Asserted-
   Identity or P-Preferred-Identity header field containing an
   unexpected number of URIs or unexpected URI schemes it MUST act as
   follows:

   o  ignore any URI with an unexpected URI scheme;

   o  ignore any URI for which the expected maximum number of URIs with
      the same scheme occurred earlier in the header field; and

   o  ignore any URI whose scheme is not expected to occur in
      combination with a scheme that occurred earlier in the header
      field.

   This document does not change the RFC 3325 requirement that allows
   each of these header fields to contain at most two URIs, where one is
   a SIP or SIPS URI and the other is a TEL URI, but future updates to
   this document may relax that requirement.  In the absence of such a
   relaxation, the requirement above means that an entity receiving a
   request or response containing a P-Asserted-Identity or P-Preferred-
   Identity header field must act as follows:

   o  ignore any URI with a scheme other than SIP, SIPS or TEL;

   o  ignore a second or subsequent SIP URI, a second or subsequent SIPS
      URI or a second or subsequent TEL URI; and

   o  ignore a SIP URI if a SIPS URI occurred earlier in the header
      field and vice versa.

   A proxy MUST NOT forward a URI when forwarding a message if that URI
   is to be ignored in accordance with the requirement above.

5.  IANA considerations

   This document requires no IANA actions.

6.  Security considerations

   The use of asserted identity raises a number of security
   considerations, which are discussed fully in [RFC3325].  This
   document raises the following additional security considerations.

   When receiving a request or response containing a P-Asserted-Identity
   header field, a proxy will trust the assertion only if the source is
   known to be within the Trust Domain and behaves in accordance with a
   Spec(T), which defines the security requirements.  This applies
   regardless of the nature of the resource (UA or proxy).  One example
   where a trusted source might be a UA is a PSTN gateway.  In this case
   the UA can assert an identity received from the PSTN, the proxy
   itself having no means to authenticate such an identity.  A SIP
   entity must not trust an identity asserted by a source outside the
   Trust Domain.  Typically a UA under the control of an individual user
   (such as a desk phone or mobile phone) should not be considered part
   of a Trust Domain.

   When receiving a response from a node outside the Trust Domain, a
   proxy has no direct SIP means to authenticate the node.  However, if
   authentication has taken place by other means (e.g., an earlier use
   of SIP digest authentication) and the entity sending the response is
   known to be the same entity (e.g., connected via the same TLS
   session) this can be sufficient grounds for asserting an identity.
   In other circumstances a proxy must not assert identity for a
   responding user.

   When receiving a REGISTER request containing a P-Asserted-Identity
   header field, a proxy will trust the asserted identity only if
   received over a secure connection from a proxy within the Trust
   Domain.

7.  Acknowledgements

   Useful comments were received from Francois Audet, Jeroen van Bemmel,
   Hans Erik van Elburg, Vijay Gurbani, Cullen Jennings, Hadriel Kaplan,
   Paul Kyzivat, Jonathan Rosenberg, Thomas Stach and Brett Tate during
   drafting and review.

8.  References

8.1.  Normative References

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

   [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.

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, October 2002.

   [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted
              Identity", RFC 3324, November 2002.

   [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.

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

   [RFC3903]  Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", RFC 3903, October 2004.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

   [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-07 (work in progress),
              November 2007.

8.2.  Informative References

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, April 2004.

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

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.

Author's Address

   John Elwell
   Siemens Enterprise Communications GmbH & Co KG
   Hofmannstrasse 51
   D-81379 Munich
   Germany

   Phone:  +44 115 943 4989
   Email:  john.elwell@siemens.com

Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.