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Internet Engineering Task Force (IETF)                         J. ElwellRequest for Comments: 5876             Siemens Enterprise CommunicationsUpdates:3325                                                 April 2010Category: InformationalISSN: 2070-1721Updates to Asserted Identity in the Session Initiation Protocol (SIP)Abstract   The Session Initiation Protocol (SIP) has a mechanism for conveying   the identity of the originator of a request by means of the   P-Asserted-Identity and P-Preferred-Identity header fields.  These   header fields are 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 the P-Asserted-Identity header field by a   trusted User Agent Client (UAC), does not specify the use of   P-Asserted-Identity and P-Preferred-Identity header fields with   certain SIP methods such as UPDATE, REGISTER, MESSAGE, and PUBLISH,   and does not specify how to handle an unexpected number of URIs or   unexpected URI schemes in these header fields.  This document extendsRFC 3325 to cover these situations.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5876.Elwell                        Informational                     [Page 1]

RFC 5876            Updates to SIP Asserted Identity          April 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1. Introduction ....................................................22. Terminology .....................................................43. Discussion ......................................................43.1. Inclusion of P-Asserted-Identity by a UAC ..................43.2. Inclusion of P-Asserted-Identity in Any Request ............53.3. Dialog Implications ........................................64. Behaviour .......................................................64.1. UAC Behaviour ..............................................74.2. Proxy Behaviour ............................................74.3. Registrar Behaviour ........................................74.4. UAS Behaviour ..............................................84.5. General Handling ...........................................85. Security Considerations .........................................96. Acknowledgements ...............................................107. References .....................................................107.1. Normative References ......................................107.2. Informative References ....................................111.  Introduction   The Session Initiation Protocol (SIP) is specified inRFC 3261   [RFC3261].RFC 3325 [RFC3325] specifies a mechanism for conveying   the asserted identity of the originator of a SIP request within a   Trust Domain.  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.  In addition, the   P-Preferred-Identity header field can be used to indicate a   preference for which identity should be asserted when there is a   choice.Elwell                        Informational                     [Page 2]

RFC 5876            Updates to SIP Asserted Identity          April 2010RFC 3325 does not specify the insertion of the P-Asserted-Identity   header field by a User Agent Client (UAC) in the same Trust Domain as   the first proxy.  Also,RFC 3325 does not specify the use of the   P-Asserted-Identity and P-Preferred-Identity header fields with   certain SIP methods such as UPDATE [RFC3311], REGISTER, MESSAGE   [RFC3428], and PUBLISH [RFC3903].  This document extendsRFC 3325 by   allowing inclusion of the P-Asserted-Identity header field by a UAC   in the same Trust Domain as the first proxy and allowing use of   P-Asserted-Identity and P-Preferred-Identity header fields in any   request except ACK and CANCEL.  The reason for these two exceptions   is that ACK and CANCEL requests cannot be challenged for digest   authentication.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 the 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, to   provide authenticated identity in such circumstances.RFC 4916   [RFC4916] specifies the use ofRFC 4474 in mid-dialog requests, in   particular, in requests in the reverse direction to the dialog-   forming request as a means of providing authenticated connected   identity.RFC 3325 is unclear on the use of P-Asserted-Identity in responses.   In contrast to requests, there is no means in SIP to challenge a User   Agent Server (UAS) to provide SIP digest authentication in a   response.  As a result, there is currently no standardised mechanism   whereby a proxy can authenticate a UAS.  Since authenticating the   source of a message is a prerequisite for asserting an identity, this   document does not specify the use of the P-Asserted-Identity header   field in responses.  This may be the subject of a future update toRFC 3325.  Also, this document does not specify the use of the   P-Preferred-Identity header field in responses, as this would serve   no purpose in the absence of the ability for a proxy to insert the   P-Asserted-Identity header field.Elwell                        Informational                     [Page 3]

RFC 5876            Updates to SIP Asserted Identity          April 20102.  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 insection 2.3 of RFC 3324 [RFC3324].3.  Discussion3.1.  Inclusion of P-Asserted-Identity by a UACRFC 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 UACs that are often   suitable for inclusion in a Trust Domain are:   o  Public Switched Telephone Network (PSTN) gateways;   o  media servers;   o  application servers (or Back-to-Back User Agents (B2BUAs)) that      act as URI list servers [RFC5363];   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 withRFC 3325, nodes within a Trust Domain must behave   in accordance with a Spec(T), and this principle needs to be applied   between a UAC and its proxy as part of the condition to consider the   UAC to be within the same Trust Domain.  The normal proxy procedures   ofRFC 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 toRFC 3325 clarifies that a UAC may include a   P-Asserted-Identity header field in a request in certain   circumstances.Elwell                        Informational                     [Page 4]

RFC 5876            Updates to SIP Asserted Identity          April 20103.2.  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 (seeRFC 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 User   Agent (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 three messages      rather than two.   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.Elwell                        Informational                     [Page 5]

RFC 5876            Updates to SIP Asserted Identity          April 2010   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.   Thus, there are several examples where P-Asserted-Identity could be   used in requests with methods for which there is no provision inRFC3325.  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.   Inclusion of P-Asserted-Identity in a request requires that the   original asserter of an identity be able to authenticate the source   of the request.  This implies the ability to challenge a request for   SIP digest authentication, which is not possible with ACK and CANCEL   requests.  Therefore, ACK and CANCEL requests need to be excluded.   Similarly, there are examples where P-Preferred-Identity could be   used in requests with methods for which there is no provision inRFC3325 or any other RFC (with the exception of ACK and CANCEL).   This update toRFC 3325 allows a P-Asserted-Identity or P-Preferred-   Identity header field to be included in any request except ACK and   CANCEL.3.3.  Dialog Implications   A P-Asserted-Identity header field in a received request asserts the   identity of the source of that request and says nothing about the   source of subsequent received requests claiming to relate to the same   dialog.  The recipient can make its own deductions about the source   of subsequent requests not containing a P-Asserted-Identity header   field.  This document does not changeRFC 3325 in this respect.4.  Behaviour   This document updatesRFC 3325 by allowing a P-Asserted-Identity   header field to be included by a UAC within the same Trust Domain and   by allowing a P-Asserted-Identity or P-Preferred-Identity header   field to appear in any request except ACK or CANCEL.Elwell                        Informational                     [Page 6]

RFC 5876            Updates to SIP Asserted Identity          April 20104.1.  UAC Behaviour   A UAC MAY include a P-Asserted-Identity header field in any request   except ACK and CANCEL 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 where it is connected to that SIP entity in accordance   with the security requirements ofRFC 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-Preferred-Identity header field in any request   except ACK or CANCEL.   Inclusion of a P-Asserted-Identity or P-Preferred-Identity header   field in a request is not limited to the methods allowed inRFC 3325.4.2.  Proxy Behaviour   If a proxy receives a request containing a P-Asserted-Identity header   field from a UAC within the Trust Domain, it MUST behave as it would   for a request from any other node within the Trust Domain, in   accordance with the rules ofRFC 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 (other than ACK or CANCEL) containing a   P-Asserted-Identity or P-Preferred-Identity header field, it MUST   behave in accordance with the rules ofRFC 3325 for a proxy, even if   the method is not one for whichRFC 3325 specifies use of that 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   it is received from a node within the Trust Domain.  If the node is   within the Trust Domain (the node having been authenticated by some   means), the registrar MAY use this as evidence that the registering   UA has been authenticated and is represented by the identity asserted   in the header field.Elwell                        Informational                     [Page 7]

RFC 5876            Updates to SIP Asserted Identity          April 20104.4.  UAS Behaviour   If a UAS receives any request (other than ACK or CANCEL) containing a   P-Asserted-Identity header field, it MUST behave in accordance with   the rules ofRFC 3325 for a UAS, even if the method is not one for   whichRFC 3325 specifies use of that header field.4.5.  General Handling   An entity receiving a P-Asserted-Identity or P-Preferred-Identity   header field can expect the number of URIs and the combination of URI   schemes in the header field to be in accordance withRFC 3325, any   updates toRFC 3325, or any Spec(T) that states otherwise.  If an   entity receives a request 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.   In the absence of a Spec(T) determining otherwise, this document does   not change theRFC 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 or a   Spec(T) determining otherwise, theRFC 3325 requirement means that an   entity receiving a request 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 request if that URI   is to be ignored in accordance with the requirement above.Elwell                        Informational                     [Page 8]

RFC 5876            Updates to SIP Asserted Identity          April 2010   When a UAC or a proxy sends a request containing a P-Asserted-   Identity header field to another node in the Trust Domain, if that   other node complies withRFC 3325 but not with this specification,   and if the method is not one for whichRFC 3325 specifies use of the   P-Asserted-Identity header field, and if the request also contains a   Privacy header field with value 'id', as specified inRFC 3325, the   other node might not handle the Privacy header field correctly.  To   prevent incorrect handling of the Privacy header field with value   'id', the Spec(T) in force for the Trust Domain SHOULD require all   nodes to comply with this specification.  If this is not the case, a   UAC or a proxy SHOULD NOT include a P-Asserted-Identity header field   in a request if the method is not one for whichRFC 3325 specifies   use of the P-Asserted-Identity header field and if the request also   contains a Privacy header field with value 'id'.5.  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 adding a P-Asserted-Identity header field to a message, an   entity must have authenticated the source of the message by some   means.  One means is to challenge the sender of a message to provide   SIP digest authentication.  Responses cannot be challenged, and also   ACK and CANCEL requests cannot be challenged.  Therefore, this   document limits the use of P-Asserted-Identity to requests other than   ACK and CANCEL.   When sending a request containing the P-Asserted-Identity header   field and also the Privacy header field with value 'id' to a node   within the Trust Domain, special considerations apply if that node   does not support this specification.Section 4.5 makes a special   provision for this case.   When receiving a request 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.Elwell                        Informational                     [Page 9]

RFC 5876            Updates to SIP Asserted Identity          April 2010   When receiving a response from a node outside the Trust Domain, a   proxy has no standardised SIP means to authenticate the source of the   response.  For this reason, this document does not specify the use of   P-Asserted-Identity or P-Preferred-Identity in responses.6.  Acknowledgements   Useful comments were received from Francois Audet, John-Luc Bakker,   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.7.  References7.1.  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.Elwell                        Informational                    [Page 10]

RFC 5876            Updates to SIP Asserted Identity          April 20107.2.  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.   [RFC5363]  Camarillo, G. and A. Roach, "Framework and Security              Considerations for Session Initiation Protocol (SIP) URI-              List Services",RFC 5363, October 2008.Author's Address   John Elwell   Siemens Enterprise Communications   Phone: +44 115 943 4989   EMail: john.elwell@siemens-enterprise.comElwell                        Informational                    [Page 11]