Internet Engineering Task Force (IETF)
Request for Comments: 6050
Category: Informational
ISSN: 2070-1721

K. Drage
Alcatel-Lucent
November 2010

A Session Initiation Protocol (SIP) Extension

for the Identification of Services

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 Section 2 of RFC 5741.

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

Copyright Notice

This document is subject to BCP 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.

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

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 for use in the Internet at large.

The concept of "service" within SIP has no hard and fast rules. RFC 5897 [RFC5897] 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 RFC 5897 [RFC5897].

It should be noted that RFC 5897 [RFC5897] 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. RFC 5897 [RFC5897] 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 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 RFC 3840 [RFC3840] and RFC 3841 [RFC3841] provide an appropriate mechanism for indicating such service and application identification. These mechanisms have been extended by RFC 5688 [RFC5688] 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 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.

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 then 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.

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 (for example, back-to-back user agents (B2BUAs)).

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

4. Syntax of the Header Fields

The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC 5234 [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 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 Section 4.4 for the definition of Service-ID in ABNF.

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

Table 1 adds the header fields 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 the 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

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.

4.2. The P-Preferred-Service Header

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

Table 2 adds the header fields 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

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.

4.3. Service and Application Definition

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 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 3840 [RFC3840] and RFC 3841 [RFC3841].

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

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 Namespace ID (NID).

   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 RFC 1123 [RFC1123]) and a subset of
      the labels allowed in RFC 3958 [RFC3958].  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 (Section 8)).  Uniqueness is guaranteed by the IANA
      registration.
   
   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 (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.

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 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.

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). 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 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.

5.1.3. Procedures at User Agent Servers

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.

5.2. Usage of the P-Preferred-Service and P-Asserted-Service Header

Fields in Responses

There is no usage of these header fields in responses.

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, 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.

      * 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
      
      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 -> 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"
      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 -> 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: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
    * 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: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

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 where the calling user is entitled to use that service. RFC 5897 [RFC5897] 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.

8. IANA Considerations

8.1. P-Asserted-Service and P-Preferred-Service Header Fields

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

        Header Name        compact    Reference
        -----------------  -------    ---------
        P-Asserted-Service            RFC 6050
        P-Preferred-Service           RFC 6050

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