<pre>Internet Engineering Task Force (IETF)                          K. Drage
Request for Comments: 6050                                Alcatel-Lucent
Category: Informational                                    November 2010
ISSN: 2070-1721


             <span class="h1">A Session Initiation Protocol (SIP) Extension</span>
                   <span class="h1">for the Identification of Services</span>

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 for use in the Internet at large.

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

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; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6050">http://www.rfc-editor.org/info/rfc6050</a>.










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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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 Contents

   <a href="#section-1">1</a>.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
   <a href="#section-2">2</a>.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
   <a href="#section-3">3</a>.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
   <a href="#section-4">4</a>.  Syntax of the Header Fields  . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
     <a href="#section-4.1">4.1</a>.  The P-Asserted-Service Header  . . . . . . . . . . . . . .  <a href="#page-6">6</a>
     <a href="#section-4.2">4.2</a>.  The P-Preferred-Service Header . . . . . . . . . . . . . .  <a href="#page-7">7</a>
     <a href="#section-4.3">4.3</a>.  Service and Application Definition . . . . . . . . . . . .  <a href="#page-8">8</a>
     <a href="#section-4.4">4.4</a>.  Registration Template  . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
   5.  Usage of the P-Preferred-Service and P-Asserted-Service
       Header Fields  . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
     5.1.  Usage of the P-Preferred-Service and
           P-Asserted-Service Header Fields in Requests . . . . . . . <a href="#page-10">10</a>
       <a href="#section-5.1.1">5.1.1</a>.  Procedures at User Agent Clients (UAC) . . . . . . . . <a href="#page-10">10</a>
       <a href="#section-5.1.2">5.1.2</a>.  Procedures at Intermediate Proxies . . . . . . . . . . <a href="#page-11">11</a>
       <a href="#section-5.1.3">5.1.3</a>.  Procedures at User Agent Servers . . . . . . . . . . . <a href="#page-12">12</a>
     5.2.  Usage of the P-Preferred-Service and
           P-Asserted-Service Header Fields in Responses  . . . . . . <a href="#page-12">12</a>
   <a href="#section-6">6</a>.  Examples of Usage  . . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
   <a href="#section-7">7</a>.  Security Considerations  . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
   <a href="#section-8">8</a>.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
     8.1.  P-Asserted-Service and P-Preferred-Service Header
           Fields . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
     <a href="#section-8.2">8.2</a>.  Definition of Service-ID Values  . . . . . . . . . . . . . <a href="#page-16">16</a>
   <a href="#section-9">9</a>.  References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
     <a href="#section-9.1">9.1</a>.  Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
     <a href="#section-9.2">9.2</a>.  Informative References . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>








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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   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 for use in the Internet at large.

   The concept of "service" within SIP has no hard and fast rules.  <a href="./rfc5897">RFC</a>
   <a href="./rfc5897">5897</a> [<a href="./rfc5897" title="&quot;Identification of Communications Services in the Session Initiation Protocol (SIP)&quot;">RFC5897</a>] 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 <a href="./rfc5897">RFC 5897</a> [<a href="./rfc5897" title="&quot;Identification of Communications Services in the Session Initiation Protocol (SIP)&quot;">RFC5897</a>].

   It should be noted that <a href="./rfc5897">RFC 5897</a> [<a href="./rfc5897" title="&quot;Identification of Communications Services in the Session Initiation Protocol (SIP)&quot;">RFC5897</a>] clearly states that
   declarative service identification -- the process by which a user
   agent inserts a moniker into a message that defines the desired
   service, separate from explicit and well-defined protocol mechanisms
   -- is harmful.

   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 Session Description Protocol
   (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 signaling to capture the missing element.  <a href="./rfc5897">RFC 5897</a>
   [<a href="./rfc5897" title="&quot;Identification of Communications Services in the Session Initiation Protocol (SIP)&quot;">RFC5897</a>] provides further explanation.





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   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
   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 signaled service identification exists, and the session
   proceeds for each node's definition of the service in use, on the
   basis of information contained in the SDP and in other SIP header
   fields.

   This mechanism is specifically for managing the information needs of
   intermediate routing 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 <a href="./rfc3840">RFC 3840</a> [<a href="./rfc3840" title="&quot;Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)&quot;">RFC3840</a>] and <a href="./rfc3841">RFC 3841</a>
   [<a href="./rfc3841" title="&quot;Caller Preferences for the Session Initiation Protocol (SIP)&quot;">RFC3841</a>] provide an appropriate mechanism for indicating such
   service and application identification.  These mechanisms have been
   extended by <a href="./rfc5688">RFC 5688</a> [<a href="./rfc5688" title="&quot;A Session Initiation Protocol (SIP) Media Feature Tag for MIME Application Subtypes&quot;">RFC5688</a>] 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 that allows the UA to express preferences
   regarding the decisions made on service within the trust domain.

   An example of the P-Asserted-Service header field is:

   P-Asserted-Service: urn:urn-7:3gpp-service.exampletelephony.version1

   A proxy server that 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





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   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
   information about the service so described.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Applicability Statement</span>

   This document describes private extensions to SIP (see <a href="./rfc3261">RFC 3261</a>
   [<a href="./rfc3261" title="&quot;SIP: Session Initiation Protocol&quot;">RFC3261</a>]) 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 <a href="./rfc3324">RFC 3324</a> [<a href="./rfc3324" title="&quot;Short Term Requirements for Network Asserted Identity&quot;">RFC3324</a>]).
   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 then specify compliance to
   that set of specifications.  Spec(T) MUST specify behavior as
   documented in <a href="./rfc3324">RFC 3324</a> [<a href="./rfc3324" title="&quot;Short Term Requirements for Network Asserted Identity&quot;">RFC3324</a>].

   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 <a href="./rfc3324">RFC 3324</a> [<a href="./rfc3324" title="&quot;Short Term Requirements for Network Asserted Identity&quot;">RFC3324</a>].

   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
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   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.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Conventions</span>

   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 <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>
   [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

   Throughout this document, requirements for or references to proxy
   servers or proxy behavior apply similarly to other intermediaries
   within a trust domain (for example, back-to-back user agents
   (B2BUAs)).

   The term trust domain in this document has the meaning as defined in
   <a href="./rfc3324">RFC 3324</a> [<a href="./rfc3324" title="&quot;Short Term Requirements for Network Asserted Identity&quot;">RFC3324</a>].

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Syntax of the Header Fields</span>

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in <a href="./rfc5234">RFC 5234</a> [<a href="./rfc5234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC5234</a>].

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  The P-Asserted-Service Header</span>

   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 field, 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 <a href="#section-4.4">Section 4.4</a> for the definition of Service-ID in ABNF.

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






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   Table 1 adds the header fields defined in this document to Table 2 in
   SIP <a href="./rfc3261#section-7.1">[RFC3261], Section&nbsp;7.1</a> of the SIP-specific event notification
   [<a href="./rfc3265" title="&quot;Session Initiation Protocol (SIP)-Specific Event Notification&quot;">RFC3265</a>], Tables 1 and 2 in the SIP INFO method [<a href="./rfc2976" title="&quot;The SIP INFO Method&quot;">RFC2976</a>], Tables 1
   and 2 in the reliability of provisional responses in SIP [<a href="./rfc3262" title="&quot;Reliability of Provisional Responses in Session Initiation Protocol (SIP)&quot;">RFC3262</a>],
   Tables 1 and 2 in the SIP UPDATE method [<a href="./rfc3311" title="&quot;The Session Initiation Protocol (SIP) UPDATE Method&quot;">RFC3311</a>], Tables 1 and 2 in
   the SIP extension for instant messaging [<a href="./rfc3428" title="&quot;Session Initiation Protocol (SIP) Extension for Instant Messaging&quot;">RFC3428</a>], Table 1 in the SIP
   REFER method [<a href="./rfc3515" title="&quot;The Session Initiation Protocol (SIP) Refer Method&quot;">RFC3515</a>], and Tables 2 and 3 in the SIP PUBLISH method
   [<a href="./rfc3903" title="&quot;Session Initiation Protocol (SIP) Extension for Event State Publication&quot;">RFC3903</a>]:

         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

   Syntactically, there may be multiple P-Asserted-Service header fields
   in a request.  The semantics of multiple P-Asserted-Service header
   fields appearing in the same request is not defined at this time.
   Implementations of this specification MUST provide only one
   P-Asserted-Service header field value.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  The P-Preferred-Service Header</span>

   The P-Preferred-Service header field is used by a user agent sending
   the SIP request to provide a hint to a trusted proxy of the preferred
   service 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 <a href="#section-4.4">Section 4.4</a> for the definition of Service-ID in ABNF.

   Table 2 adds the header fields defined in this document to Table 2 in
   SIP <a href="./rfc3261#section-7.1">[RFC3261], Section&nbsp;7.1</a> of the SIP-specific event notification
   [<a href="./rfc3265" title="&quot;Session Initiation Protocol (SIP)-Specific Event Notification&quot;">RFC3265</a>], Tables 1 and 2 in the SIP INFO method [<a href="./rfc2976" title="&quot;The SIP INFO Method&quot;">RFC2976</a>], Tables 1
   and 2 in Reliability of provisional responses in SIP [<a href="./rfc3262" title="&quot;Reliability of Provisional Responses in Session Initiation Protocol (SIP)&quot;">RFC3262</a>],



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   Tables 1 and 2 in the SIP UPDATE method [<a href="./rfc3311" title="&quot;The Session Initiation Protocol (SIP) UPDATE Method&quot;">RFC3311</a>], Tables 1 and 2 in
   the SIP extension for Instant Messaging [<a href="./rfc3428" title="&quot;Session Initiation Protocol (SIP) Extension for Instant Messaging&quot;">RFC3428</a>], Table 1 in the SIP
   REFER method [<a href="./rfc3515" title="&quot;The Session Initiation Protocol (SIP) Refer Method&quot;">RFC3515</a>], and Tables 2 and 3 in the SIP PUBLISH method
   [<a href="./rfc3903" title="&quot;Session Initiation Protocol (SIP) Extension for Event State Publication&quot;">RFC3903</a>]:

         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

   Syntactically, there may be multiple P-Preferred-Service header
   fields in a request.  The semantics of multiple P-Preferred-Service
   header fields appearing in the same request is not defined at this
   time.  Implementations of this specification MUST only provide one
   P-Preferred-Service header field value.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Service and Application Definition</span>

   Service definitions and characteristics are 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
   or application identifiers, and subservice identifiers.

   The service and subservice identifiers are as described in <a href="#section-1">Section 1</a>.
   The URN may also be used to identify a service or an application
   between end users for use within the context of <a href="./rfc3840">RFC 3840</a> [<a href="./rfc3840" title="&quot;Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)&quot;">RFC3840</a>]
   and <a href="./rfc3841">RFC 3841</a> [<a href="./rfc3841" title="&quot;Caller Preferences for the Session Initiation Protocol (SIP)&quot;">RFC3841</a>].

   IANA maintains a registry of service identifier values that have been
   assigned.  This registry has been created by the actions of <a href="#section-8.2">Section</a>
   <a href="#section-8.2">8.2</a> of this document.

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

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Registration Template</span>

   Below, we include the registration template for the URN scheme
   according to <a href="./rfc3406">RFC 3406</a> [<a href="./rfc3406" title="&quot;Uniform Resource Names (URN) Namespace Definition Mechanisms&quot;">RFC3406</a>].  The URN scheme is defined as an
   informal Namespace ID (NID).



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   Namespace ID:  urn-7

   Registration Information:
      Registration version: 1; registration date: 2009-03-22

   Declared registrant of the namespace:  3GPP Specifications Manager
      (3gppContact@etsi.org) (+33 (0)492944200)

   Declaration of syntactic structure:  The URN consists of a
      hierarchical service identifier or application identifier, with a
      sequence of labels separated by periods.  The leftmost label is
      the most significant one and is called 'top-level service
      identifier', while names to the right are called 'subservices' or
      'sub-applications'.  The set of allowable characters is the same
      as that for domain names (see <a href="./rfc1123">RFC 1123</a> [<a href="./rfc1123" title="&quot;Requirements for Internet Hosts - Application and Support&quot;">RFC1123</a>]) and a subset of
      the labels allowed in <a href="./rfc3958">RFC 3958</a> [<a href="./rfc3958" title="&quot;Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS)&quot;">RFC3958</a>].  Labels are case-
      insensitive and MUST be specified in all lowercase.  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 exception of the top-level service identifier and
      possibly the first subservice or sub-application identifier.
      Labels cannot be removed beyond a defined basic service; for
      example, the label w.x may define a service, but the label w may
      only define an assignment authority for assigning subsequent
      values and not define a service in its own right.  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, but w may not be a
      valid service identifier if it merely defines who is responsible
      for defining x.

        Service-ID      = "urn:urn-7:" urn-service-id
        urn-service-id  = top-level *("." sub-service-id)
        top-level       = let-dig [ *26let-dig ]
        sub-service-id  = let-dig [ *let-dig ]
        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.

   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 (<a href="#section-8">Section 8</a>)).  Uniqueness is guaranteed by the IANA
      registration.




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   Identifier persistence considerations:  The service or application
      identifier for the same service or application is 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 (<a href="#section-8">Section 8</a>).

   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 <a href="./rfc3406">RFC 3406</a> [<a href="./rfc3406" title="&quot;Uniform Resource Names (URN) Namespace Definition Mechanisms&quot;">RFC3406</a>]).  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.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Usage of the P-Preferred-Service and P-Asserted-Service Header</span>
<span class="h2">    Fields</span>

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Usage of the P-Preferred-Service and P-Asserted-Service Header</span>
<span class="h3">      Fields in Requests</span>

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  Procedures at User Agent Clients (UAC)</span>

   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





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   functionality for which a SIP extension and a Require header field
   value is defined, then the request has to include that SIP signaling
   as well as the P-Preferred-Service header field value.

   A UAC that is within the same trust domain as the proxy to which it
   sends a request (e.g., a media gateway or application server) MAY
   insert a P-Asserted-Service header field in a request that creates a
   dialog, or a request outside of a dialog.  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.

<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>.  Procedures at Intermediate Proxies</span>

   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).  Where the
   originator of the request is authenticated, the proxy MUST use the
   identity that results from this checking and authentication to insert
   a P-Asserted-Service header field into the request.

   When a proxy receives a request containing a P-Preferred-Service
   header field, the Proxy MAY use the contents of that header field to
   assist in determining the service to be included in a P-Asserted-
   Service header field (for instance, to prioritize the order of
   comparison of filter criteria for potential services that the request
   could match).  The proxy MUST NOT use the contents of the
   P-Preferred-Service header field to identify the service without
   first checking against the capabilities (e.g., SDP) contained in the
   request.  If the proxy inserts a P-Asserted-Service header field in
   the request, the proxy MUST remove the P-Preferred-Service header
   field before forwarding the request; otherwise, the Proxy SHOULD
   include the P-Preferred-Service header field when forwarding 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



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   there is a P-Asserted-Service header present, the proxy MUST replace
   that header field's contents with a new analysis or remove that
   header field.

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

   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.

<span class="h4"><a class="selflink" id="section-5.1.3" href="#section-5.1.3">5.1.3</a>.  Procedures at User Agent Servers</span>

   For a User Agent Server (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 UAS 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.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Usage of the P-Preferred-Service and P-Asserted-Service Header</span>
<span class="h3">      Fields in Responses</span>

   There is no usage of these header fields in responses.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Examples of Usage</span>

   In this example, proxy.example.com creates a P-Asserted-Service
   header field from the user identity it discovered from SIP digest
   authentication, the list of services appropriate to that user, and
   the services that correspond to the SDP information included in the
   request.  Note that F1 and F2 are about identifying the user and do
   not directly form part of the capability provided in this document.
   It forwards this information to a trusted proxy that forwards it to a
   trusted gateway.  Note that these examples consist of partial SIP
   messages that illustrate only those header fields relevant to the
   authenticated identity problem.





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<span class="grey"><a href="./rfc6050">RFC 6050</a>               SIP Service Identification          November 2010</span>


      * F1   useragent.example.com -&gt; proxy.example.com

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

      v=0
      o=- 2987933615 2987933615 IN IP6 5555::aaa:bbb:ccc:ddd
      s=-
      c=IN IP6 5555::aaa:bbb:ccc:ddd
      t=0 0
      m=audio 3456 RTP/AVPF 97 96
      b=AS:25.4
      a=curr:qos local sendrecv
      a=curr:qos remote none
      a=des:qos mandatory local sendrecv
      a=des:qos mandatory remote sendrecv
      a=sendrecv
      a=rtpmap:97 AMR
      a=fmtp:97 mode-set=0,2,5,7; maxframes


      * F2   proxy.example.com -&gt; useragent.example.com

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


      * F3   useragent.example.com -&gt; proxy.example.com

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




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      v=0
      o=- 2987933615 2987933615 IN IP6 5555::aaa:bbb:ccc:ddd
      s=-
      c=IN IP6 5555::aaa:bbb:ccc:ddd
      t=0 0
      m=audio 3456 RTP/AVPF 97 96
      b=AS:25.4
      a=curr:qos local sendrecv
      a=curr:qos remote none
      a=des:qos mandatory local sendrecv
      a=des:qos mandatory remote sendrecv
      a=sendrecv
      a=rtpmap:97 AMR
      a=fmtp:97 mode-set=0,2,5,7; maxframes


    * F4   proxy.example.com -&gt; 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: &lt;sip:+14085551212@example.com&gt;
    From: "Anonymous" &lt;sip:anonymous@anonymous.invalid&gt;;tag=9802748
    Call-ID: 245780247857024504
    CSeq: 2 INVITE
    Max-Forwards: 69
    P-Asserted-Service: urn:urn-7:3gpp-service.exampletelephony.version1

    v=0
    o=- 2987933615 2987933615 IN IP6 5555::aaa:bbb:ccc:ddd
    s=-
    c=IN IP6 5555::aaa:bbb:ccc:ddd
    t=0 0
    m=audio 3456 RTP/AVPF 97 96
    b=AS:25.4
    a=curr:qos local sendrecv
    a=curr:qos remote none
    a=des:qos mandatory local sendrecv
    a=des:qos mandatory remote sendrecv
    a=sendrecv
    a=rtpmap:97 AMR
    a=fmtp:97 mode-set=0,2,5,7; maxframes









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    * F5   proxy.pstn.example -&gt; 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: &lt;sip:+14085551212@example.com&gt;
    From: "Anonymous" &lt;sip:anonymous@anonymous.invalid&gt;;tag=9802748
    Call-ID: 245780247857024504
    CSeq: 2 INVITE
    Max-Forwards: 68
    P-Asserted-Service: urn:urn-7:3gpp-service.exampletelephony.version1

    v=0
    o=- 2987933615 2987933615 IN IP6 5555::aaa:bbb:ccc:ddd
    s=-
    c=IN IP6 5555::aaa:bbb:ccc:ddd
    t=0 0
    m=audio 3456 RTP/AVPF 97 96
    b=AS:25.4
    a=curr:qos local sendrecv
    a=curr:qos remote none
    a=des:qos mandatory local sendrecv
    a=des:qos mandatory remote sendrecv
    a=sendrecv
    a=rtpmap:97 AMR
    a=fmtp:97 mode-set=0,2,5,7; maxframes

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   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
   where the calling user is entitled to use that service.  <a href="./rfc5897">RFC 5897</a>
   [<a href="./rfc5897" title="&quot;Identification of Communications Services in the Session Initiation Protocol (SIP)&quot;">RFC5897</a>] 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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<span class="grey"><a href="./rfc6050">RFC 6050</a>               SIP Service Identification          November 2010</span>


<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  P-Asserted-Service and P-Preferred-Service Header Fields</span>

   This document specifies two new SIP header fields: P-Asserted-Service
   and P-Preferred-Service.  Their syntax is given in <a href="#section-3">Section 3</a>.  These
   header fields are defined by the following information, which has
   been added to the header sub-registry under <a href="http://www.iana.org">http://www.iana.org</a>.

        Header Name        compact    Reference
        -----------------  -------    ---------
        P-Asserted-Service            <a href="./rfc6050">RFC 6050</a>
        P-Preferred-Service           <a href="./rfc6050">RFC 6050</a>

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Definition of Service-ID Values</span>

   Top-level identifiers are identified by labels managed by IANA,
   according to the processes outlined in <a href="./rfc5226">RFC 5226</a> [<a href="./rfc5226" title="">RFC5226</a>], 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 organization that registered the service to
   manage the subservices.

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

















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<span class="grey"><a href="./rfc6050">RFC 6050</a>               SIP Service Identification          November 2010</span>


   Entries in the registration table have the following format:

   Service/Application   Description                          Reference
   --------------------------------------------------------------------
   3gpp-service          Communication services defined by    <a href="./rfc6050">RFC 6050</a>
                         3GPP for use by the IM CN subsystem
                         and its attached UAs.  This value
                         in itself does not define a service
                         and requires subsequent labels to
                         define the service.

   3gpp-application      Applications defined by 3GPP for     <a href="./rfc6050">RFC 6050</a>
                         use by UAs attached to the IM CN
                         subsystem. This value in itself
                         does not define a service and
                         requires subsequent labels to define
                         the service.

   Here, the IM CN subsystem stands for the IP Multimedia Core Network
   subsystem.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  References</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Normative References</span>

   [<a id="ref-RFC1123">RFC1123</a>]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, <a href="./rfc1123">RFC 1123</a>, October 1989.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC3261">RFC3261</a>]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>,
              June 2002.

   [<a id="ref-RFC3324">RFC3324</a>]  Watson, M., "Short Term Requirements for Network Asserted
              Identity", <a href="./rfc3324">RFC 3324</a>, November 2002.

   [<a id="ref-RFC3406">RFC3406</a>]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "Uniform Resource Names (URN) Namespace Definition
              Mechanisms", <a href="https://www.rfc-editor.org/bcp/bcp66">BCP 66</a>, <a href="./rfc3406">RFC 3406</a>, October 2002.

   [<a id="ref-RFC3958">RFC3958</a>]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", <a href="./rfc3958">RFC 3958</a>, January 2005.





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   [<a id="ref-RFC5226">RFC5226</a>]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>,
              May 2008.

   [<a id="ref-RFC5234">RFC5234</a>]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>, January 2008.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Informative References</span>

   [<a id="ref-RFC2976">RFC2976</a>]  Donovan, S., "The SIP INFO Method", <a href="./rfc2976">RFC 2976</a>,
              October 2000.

   [<a id="ref-RFC3262">RFC3262</a>]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", <a href="./rfc3262">RFC 3262</a>, June 2002.

   [<a id="ref-RFC3265">RFC3265</a>]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", <a href="./rfc3265">RFC 3265</a>, June 2002.

   [<a id="ref-RFC3311">RFC3311</a>]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", <a href="./rfc3311">RFC 3311</a>, October 2002.

   [<a id="ref-RFC3428">RFC3428</a>]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
              and D. Gurle, "Session Initiation Protocol (SIP) Extension
              for Instant Messaging", <a href="./rfc3428">RFC 3428</a>, December 2002.

   [<a id="ref-RFC3515">RFC3515</a>]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", <a href="./rfc3515">RFC 3515</a>, April 2003.

   [<a id="ref-RFC3840">RFC3840</a>]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", <a href="./rfc3840">RFC 3840</a>, August 2004.

   [<a id="ref-RFC3841">RFC3841</a>]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              <a href="./rfc3841">RFC 3841</a>, August 2004.

   [<a id="ref-RFC3903">RFC3903</a>]  Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", <a href="./rfc3903">RFC 3903</a>, October 2004.

   [<a id="ref-RFC5688">RFC5688</a>]  Rosenberg, J., "A Session Initiation Protocol (SIP) Media
              Feature Tag for MIME Application Subtypes", <a href="./rfc5688">RFC 5688</a>,
              January 2010.

   [<a id="ref-RFC5897">RFC5897</a>]  Rosenberg, J., "Identification of Communications Services
              in the Session Initiation Protocol (SIP)", <a href="./rfc5897">RFC 5897</a>,
              June 2010.




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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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