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Versions: 00 01 02                                                      
SIPPING WG                                                     J. Elwell
Internet-Draft                         Siemens Enterprise Communications
Updates: RFC 3325                                           GmbH & Co KG
(if approved)                                          November 16, 2007
Intended status: Informational
Expires: May 19, 2008


 Updates to Asserted Identity in the Session Initiation Protocol (SIP)
                 draft-elwell-sipping-update-pai-02.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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 the SIP



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   UPDATE, MESSAGE or PUBLISH methods, 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
     3.1.  Inclusion of P-Asserted-Identity by a UAC . . . . . . . . . 3
     3.2.  Inclusion of P-Asserted-Identity in an UPDATE request . . . 3
     3.3.  Inclusion of P-Asserted-Identity in a MESSAGE request . . . 4
     3.4.  Inclusion of P-Asserted-Identity in a PUBLISH request . . . 4
     3.5.  Inclusion of P-Asserted-Identity in a response  . . . . . . 4
   4.  Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     4.1.  UAC Behaviour . . . . . . . . . . . . . . . . . . . . . . . 5
       4.1.1.  Request handling  . . . . . . . . . . . . . . . . . . . 5
       4.1.2.  Response handling . . . . . . . . . . . . . . . . . . . 5
     4.2.  Proxy Behaviour . . . . . . . . . . . . . . . . . . . . . . 6
       4.2.1.  Request handling  . . . . . . . . . . . . . . . . . . . 6
       4.2.2.  Response handling . . . . . . . . . . . . . . . . . . . 6
     4.3.  UAS Behaviour . . . . . . . . . . . . . . . . . . . . . . . 7
       4.3.1.  Request handling  . . . . . . . . . . . . . . . . . . . 7
       4.3.2.  Response handling . . . . . . . . . . . . . . . . . . . 7
   5.  IANA considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9



















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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 RFC-2119 [2].


2.  Introduction

   SIP (RFC 3261 [1]) has a mechanism for conveying the asserted
   identity of the originator of a request by means of the P-Asserted-
   Identity header field (RFC 3325 [4]).  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 the SIP UPDATE method (RFC 3311 [3]), the SIP
   MESSAGE method (RFC 3428 [5]) or the SIP PUBLISH method (RFC 3903
   [6]), and is unclear on the use of this header field in responses.
   This document extends RFC 3325 by allowing inclusion of the
   P-Asserted-Identity header field by trusted UACs, allowing use of
   this header field in UPDATE, MESSAGE and PUBLISH requests and, under
   certain conditions, allowing use of this header field in SIP
   responses.


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 can be trusted by the first proxy, as is often
   the case for PSTN gateways, for example.  Indeed it is common
   practice to do so.  Normal proxy procedures would ensure that the
   header field is removed or replaced if the UAC cannot be trusted and
   the asserted identity cannot be authenticated.

3.2.  Inclusion of P-Asserted-Identity in an UPDATE 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 trusted environment 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



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   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) 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.  This can be achieved by the B2BUA sending an UPDATE
   request with a P-Asserted-Identity header field on the dialogs
   concerned.

3.3.  Inclusion of P-Asserted-Identity in a MESSAGE request

   Within a trusted environment, 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.

3.4.  Inclusion of P-Asserted-Identity in a PUBLISH request

   Within a trusted environment, 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.

3.5.  Inclusion of P-Asserted-Identity in a response

   There are cases where the inclusion of the P-Asserted-Identity header
   field in responses would be useful.  RFC 3325 is ambiguous on this.
   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 talk only about
   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



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   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.


4.  Behaviour

   This updates RFC 3325 by allowing a P-Asserted-Identity header field
   to be included by a trusted UAC, by allowing a P-Asserted-Identity
   header field to appear in an UPDATE, MESSAGE or PUBLISH 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 can expect to be trusted by the first
   proxy.  A UAC SHOULD NOT do so in other circumstances and might
   instead use the P-Preferred-Identity header field.

      Note that a proxy might trust a UAC such as a known PSTN gateway.
      The proxy might trust the PSTN gateway to assert an identity
      received from the PSTN, the proxy itself having no means to
      authenticate such an identity.

   A UAC MAY include a P-Asserted-Identity header field in an UPDATE
   request to report a changed identity mid-dialog.  This can be an
   UPDATE request sent specially for this purpose or an UPDATE request
   sent for some other purpose.

   A UAC MAY include a P-Asserted-Identity header field in a MESSAGE or
   PUBLISH request.

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



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   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 that 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 freely but it MUST ensure that it does not forward the
   information to any element that is not part of the Trust Domain if
   the user has requested that asserted identity information be kept
   private.

   If a UA is not part of the Trust Domain from which it received a
   response containing a P-Asserted-Identity header field, then it can
   assume this information does not need to be kept private.

4.2.  Proxy Behaviour

4.2.1.  Request handling

   If a proxy receives an UPDATE, MESSAGE or PUBLISH 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 proxy.

4.2.2.  Response handling

   The proxy behaviour specified in RFC 3325 is applicable to responses
   with the following qualification.  A proxy that receives a response
   from a node that it does not trust 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 that it does not trust 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.






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4.3.  UAS Behaviour

4.3.1.  Request handling

   If a UAS receives an UPDATE, MESSAGE or PUBLISH 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.

4.3.2.  Response handling

   A UAS MAY include a P-Asserted-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
   do so only in cases where it can expect to be trusted by the first
   proxy that handles the response.

      Note that a proxy might trust a UAS such as a known PSTN gateway.
      The proxy might trust the PSTN gateway to assert an identity
      received from the PSTN, the proxy itself having no means to
      authenticate such an identity.


5.  IANA considerations

   None


6.  Security considerations

   The use of asserted identity raises a number of security
   considerations, which are discussed fully in RFC 3325 [4].  This
   document raises the following additional security considerations.

   When receiving a request or response containing a P-Asserted-Identity
   header field directly from a UA (rather than from another proxy), a
   proxy may trust the UA in certain circumstances.  This might occur if
   the UA is a known PSTN gateway and the request or response has been
   secured by TLS.  The proxy might trust the PSTN gateway to assert an
   identity received from the PSTN, the proxy itself having no means to
   authenticate such an identity.  In other circumstances a proxy should
   not trust an identity asserted by a UA.

   When receiving a response from an untrusted node, 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.



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   In other circumstances a proxy must not assert identity for a
   responding user.


7.  Normative References

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

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

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

   [4]  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.

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

   [6]  Niemi, A., "Session Initiation Protocol (SIP) Extension for
        Event State Publication", RFC 3903, November 2002.


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













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