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Versions: 00 01 02 03 04 05 RFC 6050

Network Working Group                                           K. Drage
Internet-Draft                                            Alcatel-Lucent
Intended status: Informational                          October 20, 2008
Expires: April 23, 2009


A Session Initiation Protocol (SIP) Extension for the Identification of
                                Services
             draft-drage-sipping-service-identification-02

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
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 23, 2009.

















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Abstract

   This document describes private extensions to the Session Initiation
   Protocol (SIP) that enable a network of trusted SIP servers to assert
   the service of authenticated users.  The use of these extensions is
   only applicable inside an administrative domain with previously
   agreed-upon policies for generation, transport and usage of such
   information.  This document does NOT offer a general service
   identification model suitable for use between different trust
   domains, or use in the Internet at large.

   The document also defines a URN to identify both services and UA
   applications.  This URN can be used to identify services within the
   SIP header fields defined in this document, and also within the
   framework defined for caller preferences and callee capabilities in
   to identify usage of both services and applications between end UAs.



































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1.  Introduction

   This document describes private extensions to the Session Initiation
   Protocol (SIP) that enable a network of trusted SIP servers to assert
   the service possibly subject to the user being entitled to that
   service.  The use of these extensions is only applicable inside an
   administrative domain with previously agreed-upon policies for
   generation, transport and usage of such information.  This document
   does NOT offer a general service model suitable for use between
   different trust domains, or use in the Internet at large.

   The concept of "service" within SIP has no hard and fast rules.
   draft-ietf-sipping-service-identification
   [I-D.ietf-sipping-service-identification] provides general guidance
   on what constitutes a service within SIP and what does not.

   This document also makes use of the terms "derived service
   identification" and "declarative service identification" as defined
   in draft-ietf-sipping-service-identification
   [I-D.ietf-sipping-service-identification].

   During a session setup proxies may need to understand what service
   the request is related to in order to know what application server to
   contact or other service logic to invoke.  The SIP INVITE request
   contains all of the information necessary to determine the service.
   However, the calculation of the service may be computational and
   database intensive.  For example, a given trust domain's definition
   of a service might include request authorization.  Moreover the
   analysis may require examination of the SDP.

   For example, an INVITE request with video SDP directed to a video-on-
   demand Request-URI could be marked as an IPTV session.  An INVITE
   request with push-to-talk over cellular (PoC) routes could be marked
   as a PoC session.  An INVITE request with a Require header field
   containing an option tag of "foogame" could be marked as a foogame
   session.

   NOTE: If the information contained within the SIP INVITE request is
   not sufficient to uniquely identify a service, the remedy is to
   extend the SIP signalling to capture the missing element.
   draft-ietf-sipping-service-identification
   [I-D.ietf-sipping-service-identification] provides further
   explanation.

   By providing a mechanism to compute and store the results of the
   domain specific service calculation, i.e. the derived service
   identification, this optimization allows a single trusted proxy to
   perform an analysis of the request and authorize the requestor's



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   permission to request such a service.  The proxy may then include a
   service identifier that relieves other trusted proxies and trusted
   UAs from performing further duplicate analysis of the request for
   their service identification purposes.  In addition, this extension
   allows user agent clients outside the trust domain to provide a hint
   of the requested service.

   This extension does not provide for the dialog or transaction to be
   rejected if the service is not supported end-to-end.  SIP provides
   other mechanisms, such as the option-tag and use of the Require and
   Proxy-Require header fields, where such functionality is required.
   No explicitly signalled service identification exists and the session
   proceeds for each nodes definition of the service in use, on the
   basis of information contained in SDP and in other SIP header fields.

   This mechanism is specifically a mechanism to manage the information
   needs of intermediate routeing devices between the calling user and
   the user represented by the Request-URI.  In support of this
   mechanism, a URN is defined to identify the services.  This URN has
   wider applicability to additionally identify services and terminal
   applications.  Between end users, caller preferences and callee
   capabilities as specified in RFC 3840 [RFC3840] and RFC 3841
   [RFC3841] provide an appropriate mechanism for indicating such
   service and application identification.  These mechanisms have been
   extended by draft-rosenberg-sip-app-media-tag
   [I-D.rosenberg-sip-app-media-tag] to provide further capabilities in
   this area.

   The mechanism proposed in this document relies on a new header field
   called 'P-Asserted-Service' that contains a URN.  This is supported
   by a further new header field called 'P-Preferred-Service' that also
   contains a URN, and which allows the UA to express a preferences to
   the decisions made on service within the trust domain.

   P-Asserted-Service: urn-xxx:exampletelephony.version1

   A proxy server which handles a request can, after authenticating the
   originating user in some way (for example: Digest authentication), to
   ensure that the user is entitled to that service, insert such a
   P-Asserted-Service header field into the request and forward it to
   other trusted proxies.  A proxy that is about to forward a request to
   a proxy server or UA that it does not trust removes all the
   P-Asserted-Service header field values.

   This document labels services by means of an informal URN.  This
   provides a hierarchical structure for defining services and
   subservices, and provides an address that can be resolvable for
   various purposes outside the scope of this document, e.g. to obtain



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   information about the service so described.


















































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2.  Applicability Statement

   This document describes private extensions to SIP (see RFC 3261
   [RFC3261]) that enable a network of trusted SIP servers to assert the
   service of end users or end systems.  The use of these extensions is
   only applicable inside a 'Trust Domain' as defined in Short term
   requirements for Network Asserted Identity (see RFC 3324 [RFC3324]).
   Nodes in such a Trust Domain are explicitly trusted by its users and
   end-systems to publicly assert the service of each party, and that
   they have common and agreed upon definitions of services and
   homogeneous service offerings.  The means by which the network
   determines the service to assert is outside the scope of this
   document (though it commonly entails some form of authentication).

   The mechanism for defining a trust domain is to provide a certain set
   of specifications known as 'Spec(T)', and they specify compliance to
   that set of specifications.  Spec(T) MUST specify behavior as
   documented in RFC 3324 [RFC3324].

   This document does NOT offer a general service model suitable for
   inter-domain use or use in the Internet at large.  Its assumptions
   about the trust relationship between the user and the network may not
   apply in many applications.  For example, these extensions do not
   accommodate a model whereby end users can independently assert their
   service by use of the extensions defined here.  End users assert
   their service by including the SIP and SDP parameters that correspond
   to the service they require.  Furthermore, since the asserted
   services are not cryptographically certified, they are subject to
   forgery, replay, and falsification in any architecture that does not
   meet the requirements of RFC 3324 [RFC3324].

   The asserted services also lack an indication of who specifically is
   asserting the service, and so it must be assumed that a member of the
   Trust Domain is asserting the service.  Therefore, the information is
   only meaningful when securely received from a node known to be a
   member of the Trust Domain.

   Despite these limitations, there are sufficiently useful specialized
   deployments that meet the assumptions described above, and can accept
   the limitations that result, to warrant informational publication of
   this mechanism.










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3.  Conventions

   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 BCP 14, RFC 2119
   [RFC2119].

   Throughout this document requirements for or references to proxy
   servers or proxy behavior apply similarly to other intermediaries
   within a Trust Domain (ex: B2BUAs).

   The term Trust Domain in this document has the meaning as defined in
   RFC 3324 [RFC3324].






































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4.  Syntax of the Header Fields

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC 4234 [RFC5234].

4.1.  The P-Asserted-Service Header

   The P-Asserted-Service header field is used among trusted SIP
   entities (typically intermediaries) to carry the service information
   of the user sending a SIP message.

   The P-Asserted-Service header field carries information that is
   derived service identification.  While a declarative service
   identification can assist in deriving the value transferred in this
   header, this should be in the form of streamlining the correct
   derived service identification.

         PAssertedService = "P-Asserted-Service"
                                HCOLON PAssertedService-value
         PAssertedService-value = Service-ID *(COMMA Service-ID)

   See Section 4.4 for the definition of Service-ID in ABNF.

   Proxies can (and will) add and remove this header field.

   Table 1 extends the headers defined in this document to Table 2 in
   SIP [RFC3261], Section 7.1 of the SIP-specific event notification
   [RFC3265], tables 1 and 2 in the SIP INFO method [RFC2976], tables 1
   and 2 in Reliability of provisional responses in SIP [RFC3262],
   tables 1 and 2 in the SIP UPDATE method [RFC3311], tables 1 and 2 in
   the SIP extension for Instant Messaging [RFC3428], table 1 in the SIP
   REFER method [RFC3515] and tables 2 and 3 in the SIP PUBLISH method
   [RFC3903]:

         Header field          where  proxy  ACK BYE CAN INV OPT REG SUB
         _______________________________________________________________
         P-Asserted-Service      R     admr   -   -   -   o   o   -   o

         Header field                        NOT PRA INF UPD MSG REF PUB
         _______________________________________________________________
         P-Asserted-Service                   -   -   -   -   o   o   o

                              Table 1

   There may be multiple P-Asserted-Service header fields.  The
   semantics of multiple P-Asserted-Service header fields appearing in
   the same request is not defined.




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4.2.  The P-Preferred-Service Header

   The P-Preferred-Service header field is used from a user agent to a
   trusted proxy to carry the preferred service of the user sending the
   SIP request that the user wishes to be used for the P-Asserted-
   Service field value that the trusted element will insert.

   The P-Preferred-Service header field carries information that is
   declarative service identification.  Such information should only be
   used to assist in deriving a derived service identification at the
   recipient entity.

         PPreferredService = "P-Preferred-Service"
                                 HCOLON PPreferredService-value
         PPreferredService-value = Service-ID *(COMMA Service-ID)

   See Section 4.4 for the definition of Service-ID in ABNF.

   Table 2 extends the headers defined in this document to Table 2 in
   SIP [RFC3261], Section 7.1 of the SIP-specific event notification
   [RFC3265], tables 1 and 2 in the SIP INFO method [RFC2976], tables 1
   and 2 in Reliability of provisional responses in SIP [RFC3262],
   tables 1 and 2 in the SIP UPDATE method [RFC3311], tables 1 and 2 in
   the SIP extension for Instant Messaging [RFC3428], table 1 in the SIP
   REFER method [RFC3515] and tables 2 and 3 in the SIP PUBLISH method
   [RFC3903]:

         Header field          where  proxy  ACK BYE CAN INV OPT REG SUB
         _______________________________________________________________
         P-Preferred-Service     R      dr    -   -   -   o   o   -   o

         Header field                        NOT PRA INF UPD MSG REF PUB
         _______________________________________________________________
         P-Preferred-Service                  -   -   -   -   o   o   o

                              Table 2

   There may be multiple P-Preferred-Service header fields.  The
   semantics of multiple P-Preferred-Service header fields appearing in
   the same request is not defined.

4.3.  Service and Application Definition

   Definition of services and their characteristics is outside the scope
   of this document.  Other standards organizations, vendors and
   operators may define their own services and register them.

   A hierarchical structure is defined consisting of service identifiers



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   or application identitifiers, subservice identifiers.

   The service and subservice identifiers identify the service as
   described in Section 1.  The URN may also be used to identify a
   service or an application between end users for use within the
   context of RFC 3841 [RFC3841].

   IANA maintains a registry of service identifier values that have been
   assigned.  This registry is created by the actions of Section 8.2 of
   this document.

   Subservice identifiers are not managed by IANA.  It is the
   responsibility of the organisation that registered the service to
   manage the subservices.

4.4.  Registration Template

   Below, we include the registration template for the URN scheme
   according to RFC 3406 [RFC3406].  The URN scheme is defined as an
   informal NID.

   Namespace ID:  urn-xxx

   Registration Information:  Registration version: 1; registration
      date: 2007-04-21

   Declared registrant of the namespace:  TBD

   Declaration of syntactic structure:  The URN consists of a
      hierarchical service identifier or application identifier, with a
      sequence of labels separated by periods.  The left-most label is
      the most significant one and is called 'top-level service
      identifier', while names to the right are called 'sub-services' or
      'sub-applications'.  The set of allowable characters is the same
      as that for domain names (see RFC 1123 [RFC1123]) and a subset of
      the labels allowed in RFC 3958 [RFC3958].  Labels are case-
      insensitive and MUST be specified in all lower-case.  For any
      given service identifier, labels can be removed right-to-left and
      the resulting URN is still valid, referring a more generic
      service, with the except of the top-level service identifier and
      possibly the first sub-service or sub-application identifier.  In
      other words, if a service identifier 'w.x.y.z' exists, the URNs
      'w.x' and 'w.x.y' are also valid service identifiers.

        Service-ID      = "urn-xxx:" urn-service-id
        urn-service-id  = top-level *("." sub-service-id)
        top-level       = let-dig [ *26let-dig ]
        sub-service-id  = let-dig [ *let-dig ]



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        let-dig         = ALPHA / DIGIT / "-"

      While the naming convention above uses the term "service" all the
      constructs are equally applicable to identifying applications
      within the UA.

      Note to RFC editor: replace xxx with the assigned 3 numeric digit
      identifier.

   Relevant ancillary documentation:  None

   Identifier uniqueness considerations:  A service identifier
      identifies a service, and an application identifier an application
      indicated in the service or application registration (see IANA
      Considerations (Section 8)).  Uniqueness is guaranteed by the IANA
      registration.

   Identifier persistence considerations:  The service or application
      identifier for the same service is or application expected to be
      persistent, although there naturally cannot be a guarantee that a
      particular service will continue to be available globally or at
      all times.

   Process of identifier assignment:  The process of identifier
      assignment is described in the IANA Considerations (Section 8).

   Process for identifier resolution:  There is no single global
      resolution service for service identifiers or application
      identifiers.

   Rules for Lexical Equivalence:  'service' identifiers are compared
      according to case-insensitive string equality.

   Conformance with URN Syntax:  The BNF in the 'Declaration of
      syntactic structure' above constrains the syntax for this URN
      scheme.

   Validation mechanism:  Validation determines whether a given string
      is currently a validly-assigned URN (see RFC 3406 [RFC3406]).  Due
      to the distributed nature of usage and since not all services are
      available everywhere, validation in this sense is not possible

   Scope:  The scope for this URN can be local to a single domain, or
      may be more widely used.







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5.  Usage of the P-Preferred-Service and P-Asserted-Service header
    fields

5.1.  Usage of the P-Preferred-Service and P-Asserted-Service header
      fields in Requests

5.1.1.  Procedures at User Agent Clients (UAC)

   The UAC MAY insert a P-Preferred-Service in a request that creates a
   dialog, or a request outside of a dialog.  This information can
   assist the proxies in identifying appropriate service capabilities to
   apply to the call.  This information MUST NOT conflict with other SIP
   or SDP information included in the request.  Furthermore, the SIP or
   SDP information needed to signal functionality of this service MUST
   be present.  Thus if a service requires a video component, then the
   SDP has to include the media line associated with that video
   component; it cannot be assumed from the P-Preferred-Service header
   field value.  Similarly if the service requires particular SIP
   functionality for which a SIP extension and a Require header field
   value is defined, then the request has to include that SIP signalling
   as well as the P-Preferred-Service header field value.

5.1.2.  Procedures at Intermediate Proxies

   A proxy in a Trust Domain can receive a request from a node that it
   trusts, or a node that it does not trust.  When a proxy receives a
   request from a node it does not trust and it wishes to add a
   P-Asserted-Service header field, the proxy MUST identify the service
   appropriate to the capabilities (e.g.  SDP) in the request, MAY
   authenticate the originator of the request (in order to determine
   whether the user is subscribed for that service), and use the
   identity which results from this checking and authentication to
   insert a P-Asserted-Service header field into the request.

   If the proxy receives a request from a node that it trusts, it can
   use the information in the P-Asserted-Service header field, if any,
   as if it had authenticated the user itself.

   If there is no P-Asserted-Service header field present, or it is not
   possible to match the request to a specific service as identified by
   the service identifier, a proxy MAY add one containing it using its
   own analysis of the information contained in the SIP request.  If the
   proxy received the request from an element that it does not trust and
   there is a P-Asserted-Service header present, the proxy MUST replace
   that header field contents with a new analysis or remove this header
   field.

   The analysis performed to identify such service identifiers is



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   outside the scope of this document.  However, it is perfectly valid
   as a result of the analysis to not include any service identifier in
   the forwarded required, and thus not include a P-Asserted-Service
   header.

   If a proxy forwards a request to a node outside the proxy's trust
   domain, there MUST NOT be a P-Asserted-Service header field in the
   forwarded request.

5.1.3.  Procedures at User Agent Servers (UAS)

   For a UAS outside the trust domain, the P-Asserted-Service header is
   removed before it reaches this entity, therefore there are no
   procedures for such a device.

   However, if a User Agent Server receives a request from a previous
   element that it does not trust, it MUST NOT use the P-Asserted-
   Service header field in any way.

   If a UA is part of the Trust Domain from which it received a request
   containing a P-Asserted-Service 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.

5.2.  Usage of the P-Preferred-Service and P-Asserted-Service header
      fields in Responses

   There is no usage of these header field in responses.























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6.  Examples of Usage

   In this example, proxy.example.com creates a P-Asserted-Service
   header field from the user identity it discovered from SIP Digest
   authentication, and the list of services appropriate to that user,
   and the services that correspond to the SDP information included in
   the request.  It forwards this information to a trusted proxy which
   forwards it to a trusted gateway.  Note that these examples consist
   of partial SIP messages that illustrate only those headers relevant
   to the authenticated identity problem.

      * F1   useragent.example.com -> proxy.example.com

      INVITE sip:+14085551212@example.com SIP/2.0
      Via: SIP/2.0/TCP useragent.example.com;branch=z9hG4bK-123
      To: <sip:+14085551212@example.com>
      From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748
      Call-ID: 245780247857024504
      CSeq: 1 INVITE
      Max-Forwards: 70


      * F2   proxy.example.com -> useragent.example.com

      SIP/2.0 407 Proxy Authorization
      Via: SIP/2.0/TCP useragent.example.com;branch=z9hG4bK-123
      To: <sip:+14085551212@example.com>;tag=123456
      From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748
      Call-ID: 245780247857024504
      CSeq: 1 INVITE
      Proxy-Authenticate: .... realm="sip.example.com"


      * F3   useragent.example.com -> proxy.example.com

      INVITE sip:+14085551212@example.com SIP/2.0
      Via: SIP/2.0/TCP useragent.example.com;branch=z9hG4bK-124
      To: <sip:+14085551212@example.com>
      From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748
      Call-ID: 245780247857024504
      CSeq: 2 INVITE
      Max-Forwards: 70
      Proxy-Authorization: .... realm="sip.example.com" user="fluffy"








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      * F4   proxy.example.com -> proxy.pstn.example (trusted)

      INVITE sip:+14085551212@proxy. pstn.example SIP/2.0
      Via: SIP/2.0/TCP useragent.example.com;branch=z9hG4bK-124
      Via: SIP/2.0/TCP proxy.example.com;branch=z9hG4bK-abc
      To: <sip:+14085551212@example.com>
      From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748
      Call-ID: 245780247857024504
      CSeq: 2 INVITE
      Max-Forwards: 69
      P-Asserted-Service: "urn-xxx:example-telephony.version1"


      * F5   proxy.pstn.example -> gw.pstn.example (trusted)

      INVITE sip:+14085551212@gw.pstn.example SIP/2.0
      Via: SIP/2.0/TCP useragent.example.com;branch=z9hG4bK-124
      Via: SIP/2.0/TCP proxy.example.com;branch=z9hG4bK-abc
      Via: SIP/2.0/TCP proxy.pstn.example;branch=z9hG4bK-a1b2
      To: <sip:+14085551212@example.com>
      From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748
      Call-ID: 245780247857024504
      CSeq: 2 INVITE
      Max-Forwards: 68
      P-Asserted-Service: urn-xxx:exampletelephony.version1


























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7.  Security considerations

   The mechanism provided in this document is a partial consideration of
   the problem of service identification in SIP.  For example, these
   mechanisms provide no means by which end users can securely share
   service information end-to-end without a trusted service provider.
   This information is secured by transitive trust, which is only as
   reliable as the weakest link in the chain of trust.

   The trust domain provides a set of servers where the characteristics
   of the service are agreed for that service identifier value, and that
   the calling user is entitled to use that service.
   draft-ietf-sipping-service-identification
   [I-D.ietf-sipping-service-identification] identifies the impact of
   allowing such service identifier values to "leak" outside of the
   trust domain, including implications on fraud, interoperability and
   stifling of service innovation.


































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8.  IANA considerations

8.1.  P-Asserted-Service and P-Preferred-Service header fields

   This document specifies two new SIP headers: P-Asserted-Service and
   P-Preferred-Service.  Their syntax is given in Section 3.  These
   headers are defined by the following information, which has been
   added to the header sub-registry under
   http://www.iana.org/assignments/sip-parameters.

        Header Name        compact    Reference
        -----------------  -------    ---------
        P-Asserted-Service            [RFCxxxx]
        P-Preferred-Service           [RFCxxxx]

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

8.2.  Definition of Service-ID values

   top-level identifiers are identified by labels managed by IANA,
   according to the processes outlined in RFC 5226 [RFC5226] in a new
   registry called "Service-ID/Application-ID Labels".  Thus, creating a
   new service at the top-level requires IANA action.  The policy for
   adding service labels is 'specification required'.  The following two
   identifiers are initially defined:

   3gpp-service

   3gpp-application

   Subservice identifiers are not managed by IANA.  It is the
   responsibility of the organisation that registered the service to
   manage the subservices.

   Application identifiers are not managed by IANA.  It is the
   responsibility of the organisation that registered the service to
   manage the applicable applications.

   Entries in the registration table have the following format:











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    Service/Application   Reference  Description
    --------------------------------------------------------------------
    3gpp-service          RFCxxx     Communication services defined by
                                     3GPP for use by the IM CN subsystem
                                     and its attached UAs

    3gpp-application      RFCxxx     Applications defined by 3GPP for
                                     use by UAs attached to the IM CN
                                     subsystem










































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9.  APPENDIX: Changes history

   Note to RFC Editor: Please remove this entire appendix before
   publication

9.1.  Changes between version -01 and version -02

   1.  Incorporation of terms "derived service identification" and
       "declarative service identification" from
       draft-ietf-sipping-service-identification.

   2.  Correction of the URN syntax in examples.

   3.  Appropriate introduction to table 1 and table 2 placing these in
       a normative context to those in other tables in other RFCs.

   4.  Addition to security considerations section to clarify trust
       domain concept.

   5.  References to RFC 3325 changed to RFC 3324 for definition of
       trust domain.

   6.  Reference to RFC 2234 updated to RFC 5234 because the later
       revision applies.  No consequential technical change.

   7.  Reference to RFC 2434 updated to RFC 5226 because the later
       revision applies.  No consequential technical change.

   8.  References updated to symbolic.  Remove of reference identifiers
       from abstract.

   9.  Numerous editorial changes and minor clarifications.



















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10.  References

10.1.  Normative References

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [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.

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

   [RFC3406]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "Uniform Resource Names (URN) Namespace Definition
              Mechanisms", BCP 66, RFC 3406, October 2002.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, January 2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

10.2.  Informative References

   [RFC2976]  Donovan, S., "The SIP INFO Method", RFC 2976,
              October 2000.

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, June 2002.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

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




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

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

   [RFC3841]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, August 2004.

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

   [I-D.ietf-sipping-service-identification]
              Rosenberg, J., "Identification of Communications Services
              in the Session Initiation Protocol  (SIP)",
              draft-ietf-sipping-service-identification-03 (work in
              progress), August 2008.

   [I-D.rosenberg-sip-app-media-tag]
              Rosenberg, J., "A Session Initiation Protocol (SIP) Media
              Feature Tag for MIME Application  Sub-Types",
              draft-rosenberg-sip-app-media-tag-02 (work in progress),
              November 2007.






















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Author's Address

   Keith Drage
   Alcatel-Lucent
   Quadrant, Stonehill Green, Westlea
   Swindon, Wilts
   UK

   Email: drage@alcatel-lucent.com










































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