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Versions: (draft-holmberg-sipcore-proxy-feature) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 6809

SIPCORE Working Group                                        C. Holmberg
Internet-Draft                                               I. Sedlacek
Intended status: Standards Track                                Ericsson
Expires: December 16, 2012                                     H. Kaplan
                                                             Acme Packet
                                                           June 14, 2012


   Mechanism to indicate support of features and capabilities in the
                   Session Initiation Protocol (SIP)
                draft-ietf-sipcore-proxy-feature-04.txt

Abstract

   This specification creates a new IANA registry, "Proxy-Feature
   Feature Caps Trees", for registering "feature caps", which are used
   by SIP entities not represented by the URI of the Contact header
   field to indicate support of features and capabilities, where media
   feature tags cannot be used to indicate the support.

   This specification also defines a new SIP header field, Feature-Caps,
   to convey feature caps in SIP messages.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 16, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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



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   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

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Feature Caps . . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Registration Trees . . . . . . . . . . . . . . . . . . . .  5
       4.2.1.  General  . . . . . . . . . . . . . . . . . . . . . . .  5
       4.2.2.  Global Tree  . . . . . . . . . . . . . . . . . . . . .  5
       4.2.3.  SIP Tree . . . . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Registration Template  . . . . . . . . . . . . . . . . . .  6
     4.4.  Feature Cap Specification Requirements . . . . . . . . . .  8
       4.4.1.  General  . . . . . . . . . . . . . . . . . . . . . . .  8
       4.4.2.  Overall Description  . . . . . . . . . . . . . . . . .  8
       4.4.3.  Feature Cap Values . . . . . . . . . . . . . . . . . .  8
       4.4.4.  Usage Restrictions . . . . . . . . . . . . . . . . . .  9
       4.4.5.  Examples . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Feature-Caps Header Field  . . . . . . . . . . . . . . . . . .  9
     5.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  User Agent and Proxy Behavior  . . . . . . . . . . . . . . 10
       5.2.1.  General  . . . . . . . . . . . . . . . . . . . . . . . 10
       5.2.2.  B2BUA Behavior . . . . . . . . . . . . . . . . . . . . 10
       5.2.3.  Registrar Behavior . . . . . . . . . . . . . . . . . . 11
       5.2.4.  Proxy behavior . . . . . . . . . . . . . . . . . . . . 11
     5.3.  SIP Message Type and Response Code Semantics . . . . . . . 11
       5.3.1.  General  . . . . . . . . . . . . . . . . . . . . . . . 11
       5.3.2.  SIP Dialog . . . . . . . . . . . . . . . . . . . . . . 12
       5.3.3.  SIP Registration (REGISTER)  . . . . . . . . . . . . . 12
       5.3.4.  SIP Stand-Alone Transactions . . . . . . . . . . . . . 13
   6.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.2.  Syntax: feature cap  . . . . . . . . . . . . . . . . . . . 13
       6.2.1.  General  . . . . . . . . . . . . . . . . . . . . . . . 13
       6.2.2.  ABNF . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.3.  Syntax: Feature-Caps header field  . . . . . . . . . . . . 14
       6.3.1.  ABNF . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Registration of the Feature-Caps header field  . . . . . . 14
     7.2.  Proxy-Feature Feature Caps Trees . . . . . . . . . . . . . 15



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       7.2.1.  Introduction . . . . . . . . . . . . . . . . . . . . . 15
       7.2.2.  Global Feature Cap Registration Tree . . . . . . . . . 15
       7.2.3.  SIP Feature Cap Registration Tree  . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     11.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18









































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1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] "Caller Preferences"
   extension, defined in RFC 3840 [RFC3840], provides a mechanism that
   allows a SIP message to convey information relating to the
   originator's features and capabilities, using the Contact header
   field.

   This specification creates a new IANA registry, "Proxy-Feature
   Feature Caps Trees", for registering "feature caps", which are used
   by SIP entities not represented by the URI of the Contact header
   field to indicate support of features and capabilities, where media
   feature tags cannot be used to indicate the support.  Such cases are:

   o  - The SIP entity acts as a SIP proxy.
   o  - The SIP entity acts as a SIP registrar.
   o  - The SIP entity acts as a B2BUA, where the Contact header field
      URI represents another SIP entity.

   This specification also defines a new SIP header field, Feature-Caps,
   to convey feature caps in SIP messages.

   NOTE: Unlike media feature tags, feature caps are intended to only be
   used with the SIP protocol.


2.  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].


3.  Definitions

   Downstream SIP entity: SIP entity in the direction towards which a
   SIP request is sent.

   Upstream SIP entity: SIP entity in the direction from which a SIP
   request is received.


4.  Feature Caps







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4.1.  Introduction

   Feature caps are used by SIP entities not represented by the URI of
   the Contact header field to indicate support of features and
   capabilities, where media feature tags cannot be used to indicate the
   support.

   A value, or a list of values, that provides additional information
   about the supported feature or capability, can be associated with a
   feature cap.

   Section 5 defines how feature caps are conveyed using the Feature-
   Caps header field.

   The feature cap ABNF is defined in Section 6.2.2.

4.2.  Registration Trees

4.2.1.  General

   The following subsections define registration trees, distinguished by
   the use of faceted names (e.g., names of the form "tree.feature-
   name").  The registration trees are defined in the IANA "Proxy-
   Feature Feature Caps Trees" registry.

   The trees defined herein are similar to the global tree and sip tree
   defined for media feature tags, in RFC 2506 [RFC2506] and RFC 3840
   [RFC3840].  Other registration trees are outside the scope of this
   specification.

   NOTE: In contrast to RFC 2506 and RFC 3840, this specification only
   defines a global tree and a sip tree, as they are the only trees
   defined in those RFCs that have been used for defining SIP-specific
   media feature tags.

   When a feature cap is registered in any registration tree, no leading
   "+" is used in the registration.

4.2.2.  Global Tree

   The global feature cap tree is similar to the media feature tag
   global tree defined in RFC 2506 [RFC2506].

   A feature cap for the global tree will be registered by the IANA
   after review by a designated expert.  That review will serve to
   ensure that the feature cap meets the technical requirements of this
   specification.




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   A feature cap in the global tree will be distinguished by the leading
   facet "g.".  An organization can propose either a designation
   indicative of the feature, (e.g., "g.blinktags") or a faceted
   designation including the organization name (e.g.,
   "g.organization.blinktags").

   When a feature cap is registered in the global tree, it needs to meet
   the "Expert Review" policies defined in RFC 5226 [RFC5226].  A
   designated area expert will review the proposed feature cap, and
   consult with members of related mailing lists.

4.2.3.  SIP Tree

   The sip feature cap tree is similar to the media feature tag sip tree
   defined in RFC 3840 [RFC3840].

   A feature cap in the sip tree will be distinguished by the leading
   facet "sip.".

   When a feature cap is registered in the sip tree, it needs to meet
   the "IETF Consensus" policies defined in RFC 5226 [RFC5226].  An RFC,
   which contains the registration of the feature cap, MUST be
   published.

4.3.  Registration Template



   To: sip-feature-caps@apps.ietf.org (feature caps mailing list)
   Subject: Registration of feature cap XXXX

   | Instructions are preceded by '|'.  Some fields are optional.

   Feature cap name:

   Summary of feature indicated by this feature cap:

   | The summary should be no longer than 4 lines. More
   | detailed information can be provided in the SIP feature
   | cap specification.

   Feature cap specification reference:

   | The referenced specification MUST contain the information
   | listed in section XX of XXXX (IANA: Replace XXXX with
   | assigned RFC number of this specification.

   Values appropriate for use with this feature cap:



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   | If no values are defined for the feature cap,
   | indicate "N/A". Details about feature cap values
   | MUST be defined in the feature cap specification.

   The feature cap is intended primarily for
   use in the following applications, protocols,
   services, or negotiation mechanisms:                    [optional]

   | For applications, also specify the number of the
   | first version which will use the feature cap,
   | if applicable.

   Examples of typical use:                                [optional]

   Considerations particular to use in individual
   applications, protocols, services, or negotiation
   mechanisms:                                             [optional]

   Interoperability considerations:                        [optional]

   Security considerations:

   Privacy concerns, related to exposure of personal
   information:

   Denial of service concerns related to consequences
   of specifying incorrect values:

   Other:

       Additional information:                         [optional]

       Keywords:                                       [optional]

       Related feature caps:                       [optional]

       Name(s) & email address(es) of person(s) to
       contact for further information:

       Intended usage:

       | one of COMMON, LIMITED USE or OBSOLETE

       Author/Change controller:

       Other information:                               [optional]

       | Any other information that the author deems



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       | interesting may be added here.


                      Figure 1: Registration Template

4.4.  Feature Cap Specification Requirements

4.4.1.  General

   A feature cap specification MUST address the issues defined in the
   following subsections, or document why an issue is not applicable for
   the specific feature cap.  A reference to the specification MUST be
   provided when the feature cap is registered with IANA (see
   Section 4.3).

   It is bad practice for feature cap specifications to repeat
   procedures (e.g. general procedures on the usage of the Feature-Caps
   header field and feature caps) defined in this specification, unless
   needed for clarification or emphasis purpose.

   A feature cap specification MUST NOT weaken any behavior designated
   with "SHOULD" or "MUST" in this specification.  However, a
   specification MAY strengthen "SHOULD", "MAY", or "RECOMMENDED"
   requirements to "MUST" strength if features and capabilities
   associated with the SIP feature cap require it.

4.4.2.  Overall Description

   The feature cap specification MUST contain an overall description of
   the feature cap: how it is used to indicate support of a feature, a
   description of the feature associated with the SIP feature cap, a
   description of any additional information (conveyed using one or more
   feature cap values) that can be conveyed together with the feature
   cap, and a description of how the associated feature may be
   exercised/invoked.

4.4.3.  Feature Cap Values

   A feature cap can have an associated value, or a list of values.

   The feature cap specification MUST define the syntax and semantics of
   any value defined for the feature cap, including possible
   restrictions related to the usage of a specific value.  The feature
   cap specification MUST define the value(s) in accordance with the
   syntax defined in section 6.2.2.

   A feature cap value is only applicable for the feature cap for which
   it has been defined.  For other feature caps, the value has to be



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   defined explicitly, even if the semantics are identical.

   It is STRONGLY RECOMMENDED to not re-use a value that already has
   been defined for another feature cap, unless the semantics of the
   values are the same.

4.4.4.  Usage Restrictions

   If there are restrictions on how SIP entities can insert a SIP
   feature cap, the feature cap specification MUST document such
   restrictions.

   There might be restrictions related to whether entities are allowed
   to insert a feature cap in registration related messages, standalone
   transaction messages, or dialog related messages, whether entities
   are allowed to insert a feature cap in requests or responses, whether
   entities also need to support other features and capabilities in
   order to insert a feature cap, and whether entities are allowed to
   indicate support of a feature in conjunction with another feature.

4.4.5.  Examples

   It is RECOMMENDED that the feature cap specification provide
   demonstrative message flow diagrams, paired with complete messages
   and message descriptions.

   Note that example message flows are by definition informative, and do
   not replace normative text.


5.  Feature-Caps Header Field

5.1.  Introduction

   The Feature-Caps header field is used by SIP entities to convey
   support of features and capabilities, by setting feature caps.
   Feature caps conveyed in a Feature-Caps header field indicate that
   the SIP entity that inserted the header field supports the associated
   features and capabilities.

   NOTE: It is not possible to, as a Feature-Caps header field value,
   convey the address of the SIP entity that inserted the Feature-Caps
   header field.  If additional data about a supported feature needs to
   be conveyed, such as the address of the SIP entity that indicated
   support of the feature, then the feature definition needs to define a
   way to convey that information as a value of the associated feature
   cap.




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   The feature cap specification MUST specify for which SIP methods and
   message types, and the associated semantics, the feature cap is
   applicable.  See Section 4 for more information.  No semantics is
   defined for feature caps present in SIP methods and message types not
   covered by the associated feature cap specification.

   Within a given Feature-Caps header field, feature caps are listed in
   a non-priority order, and for a given header field any order of
   listed SIP feature caps have the same meaning.  For example,
   "foo;bar" and "bar;foo" have the same meaning (i.e. that the SIP
   entity that inserted the feature caps supports the features and
   capabilities associated with the "foo" and "bar" feature caps.

5.2.  User Agent and Proxy Behavior

5.2.1.  General

   If the URI in a Contact header field of a request or response
   represents a SIP entity, the entity MUST NOT indicate supported
   features and capabilities using a Feature-Caps header field within
   that request or response.

   When a SIP entity receives a SIP request, or response, that contains
   one or more Feature-Caps header fields, the feature caps in the
   header field inform the entity about the features and capabilities
   supported by the entities that inserted the header fields.
   Procedures how features and capabilities are invoked are outside the
   scope of this specification, and MUST be described by individual
   feature cap specifications.

   When a SIP entity adds a Feature-Caps header field to a SIP message,
   it MUST place the header field before any existing Feature-Caps
   header field in the message to be forwarded, so that the added header
   field becomes the top-most one.  Then, when another SIP entity
   receives a SIP request or the response, the SIP feature caps in the
   top-most Feature-Caps header field will represent the supported
   features and capabilities "closest" to the entity.

5.2.2.  B2BUA Behavior

   The procedures in this Section applies to UAs that are part of B2BUAs
   that are referenced in the message by a Record-Route header field
   rather than by the URI of the Contact header field.

   When a UA sends a SIP request, if the UA wants to indicate support of
   features and capabilities towards its downstream SIP entities, it
   inserts a Feature-Caps header field to the request, containing one or
   more feature caps associated with the supported features and



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   capabilities, before it forwards the request.

   If the SIP request is triggered by another SIP request that the B2BUA
   has received, the UA MAY forward received Feature-Caps header fields
   by copying them to the outgoing SIP request, similar to a SIP proxy,
   before it inserts its own Feature-Caps header field to the SIP
   request.

   When a UA receives a SIP response, if the UA wants to indicate
   support of features and capabilities towards its upstream SIP
   entities, it inserts a Feature-Caps header field to the response,
   containing one or more feature caps associated with the supported
   features and capabilities, before it forwards the response.

   If the SIP response is triggered by another SIP response that the
   B2BUA has received, the UA MAY forward received Feature-Caps header
   field by copying them to the outgoing SIP response, similar to a SIP
   proxy, before it inserts its own Feature-Caps header field to the SIP
   response.

5.2.3.  Registrar Behavior

   If a SIP registrar wants to indicate support of features and
   capabilities towards its upstream SIP entities, it inserts a Feature-
   Caps header field, containing one or more feature caps associated
   with the supported features and capabilities, to a REGISTER response.

5.2.4.  Proxy behavior

   When a SIP proxy receives a SIP request, if the proxy wants to
   indicate support of features and capabilities towards its downstream
   SIP entities, it inserts a Feature-Caps header field to the request,
   containing one or more SIP feature caps associated with the supported
   features and capabilities, before it forwards the request.

   When a proxy receives a SIP response, if the proxy wants to indicate
   support of features and capabilities towards its upstream SIP
   entities, it inserts a Feature-Caps header field to the response,
   containing one or more SIP feature caps associated with the supported
   features and capabilities, before it forwards the response.

5.3.  SIP Message Type and Response Code Semantics

5.3.1.  General

   This Section describes the general usage and semantics of the
   Feature-Caps header field for different SIP message types and
   response codes.  The usage and semantics of a specific feature cap



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   MUST be described in the associated feature cap specification.

   NOTE: Future specifications can define usage and semantics of the
   Feature-Caps header field for SIP methods, response codes and request
   types not specified in this specification.

   The Feature-Caps header field ABNF is defined in Section 6.3.1.

5.3.2.  SIP Dialog

   The Feature-Caps header field can be used within an initial SIP
   request for a dialog, within a target refresh SIP request, and within
   any 18x or 2xx response associated with such requests.

   If a feature cap is inserted in a Feature-Caps header field of an
   initial request for a dialog, or within a response of such request,
   it indicates to the receivers of the request (or response) that the
   feature associated with the feature cap is supported for the duration
   of the dialog, until a target refresh request is sent for the dialog,
   or the dialog is terminated.

   Unless a feature cap is inserted in a Feature-Caps header field or a
   target refresh request, or within a response of such request, it
   indicates to the receivers of the request (or response) that the
   feature is no long supported for the dialog.

   For a given dialog a SIP entity MUST insert the same feature caps in
   all 18x and 2xx responses associated with a given transaction.

5.3.3.  SIP Registration (REGISTER)

   The Feature-Caps header field can be used within a SIP REGISTER
   request, and within the 200 (OK) response associated with such
   request.

   If a feature cap is conveyed in a Feature-Caps header field of a
   REGISTER request, or within an associated response, it indicates to
   the receivers of the message that the feature associated with the
   feature cap is supported for the registration, until the registration
   of the contact that was explicitly conveyed in the REGISTER request
   expires, or until the registered contact is explicitly refreshed and
   the refresh REGISTER request does not contain the feature cap
   associated with the feature.

   NOTE: While a REGISTER response can contain contacts that have been
   registered as part of other registration transactions, support of any
   indicated feature only applies to the contact(s) that were explicitly
   conveyed in the associated REGISTER request.



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   This specification does not define any semantics for usage of the
   Feature-Caps header field in pure registration binding fetching
   messages (see Section 10.2.3 of RFC 3261), where the REGISTER request
   does not contain a Contact header field.  Unless such semantics is
   defined in a future extension, fetching messages will not have any
   impact on previously indicated support of features and capabilities,
   and SIP entities MUST NOT insert a Feature-Caps header field to such
   messages.

   If SIP Outbound [RFC5626] is used, the rules above apply.  However,
   supported features and capabilities only apply for the registration
   flow on which support has been explicitly indicated.

5.3.4.  SIP Stand-Alone Transactions

   The Feature-Caps header field can be used within a standalone SIP
   request, and within any 18x or 2xx response associated with such
   request.

   If a feature cap is inserted in a Feature-Caps header field of a
   standalone request, or within a response of such request, it
   indicates to the receivers of the request (or response) that the
   feature associated with the feature cap is supported for the duration
   of the standalone transaction.


6.  Syntax

6.1.  General

   This Section defines the ABNF for Feature-Caps, and for the Feature-
   Cap header field.

6.2.  Syntax: feature cap

6.2.1.  General

   In a feature cap name (ABNF: fcap-name), dots can be used to
   implement a SIP feature cap tree hierarchy (e.g.
   tree.feature.subfeature).  The description of usage of such tree
   hierarchy must be described when registered.

6.2.2.  ABNF

   The ABNF for the feature cap:






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   feature-cap       =  "+" fcap-name [EQUAL LDQUOT (fcap-value-list
                            / fcap-string-value ) RDQUOT]
   fcap-name         =  ftag-name
   fcap-value-list   =  tag-value-list
   fcap-string-value =  string-value
   ;; ftag-name, tag-value-list, string-value defined in RFC 3840

   NOTE: In comparison with media feature tags, the "+" sign in front
   of the feature cap name is mandatory.


                              Figure 2: ABNF

6.3.  Syntax: Feature-Caps header field

6.3.1.  ABNF

   The ABNF for the Feature-Caps header fields is:


   Feature-Caps = "Feature-Caps" HCOLON fc-value
                   *(COMMA fc-value)
   fc-value     = "*" *(SEMI feature-cap)


                              Figure 3: ABNF

   NOTE: A "*" value means that no information regarding which SIP
   entity, or domain, that indicate support of features and capabilities
   is provided.


7.  IANA Considerations

7.1.  Registration of the Feature-Caps header field

   This specification registers a new SIP header field, Feature-Caps,
   according to the process of RFC 3261 [RFC3261].

   The following is the registration for the Feature-Caps header field:

   RFC Number: RFC XXX

   Header Field Name: Feature-Caps







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7.2.  Proxy-Feature Feature Caps Trees

7.2.1.  Introduction

   This specification creates a new sub registry to the IANA "Session
   Initiation Protocol (SIP) Parameters" Protocol Registry, per the
   guidelines in RFC 5226 [RFC5226].  The name of the sub registry is
   "Proxy-Feature Feature Caps Trees".

7.2.2.  Global Feature Cap Registration Tree

   This specification creates a new feature cap tree in the IANA "Proxy-
   Feature Feature Caps Trees" registry.  The name of the tree is
   "Global Feature Cap Registration Tree", and its leading facet is
   "g.".  It is used for the registration of feature caps.

   The addition of entries into this tree occurs through the Expert
   Review policies, as defined in RFC 5226.  A designated area expert
   will review the proposed feature cap, and consult with members of
   related mailing lists.  The information required in the registration
   is defined in Section 4.3 of RFC XXX.

   Note that all feature caps registered in the global tree will have
   names with a leading facet "g.".  No leading "+" is used in the
   registrations in any of the feature cap registration trees.

7.2.3.  SIP Feature Cap Registration Tree

   This specification creates a new feature cap tree in the IANA "Proxy-
   Feature Feature Caps Trees" registry.  The name of the tree is "SIP
   Feature Cap Registration Tree", and its leading facet is "sip.".  It
   is used for the registration of feature caps.

   The addition of entries into this tree occurs through the IETF
   Consensus, as defined in RFC 5226.  This requires the publication of
   an RFC that contains the registration.  The information required in
   the registration is defined in Section 4.3 of RFC XXX.

   Note that all feature caps registered in the SIP tree will have names
   with a leading facet "sip.".  No leading "+" is used in the
   registrations in any of the feature cap registration trees.


8.  Security Considerations

   The security issues for feature caps are similar to the ones defined
   in RFC 3840 for media feature tags.  However, as feature caps will
   typically not be used to convey capability information of end-user



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   devices, those aspects of RFC 3840 do not apply to feature caps.

   In addition, the RFC 3840 security issue regarding an attacker using
   the SIP caller preferences extension [RFC3841] in order to affect
   routing decisions does not apply, as the mechanism is not defined to
   be used with feature caps.

   Feature caps can provide capability and characteristics information
   about the SIP entity, some of which might be sensitive.  The Feature-
   Caps header field does not convey address information about SIP
   entities.  However, individual feature caps might provide address
   information as feature cap values.  Therefore, mechanisms for
   guaranteeing confidentiality and authenticity SHOULD be provided.


9.  Acknowledgements

   The authors wish to thank everyone in the SIP community that provided
   input and feedback on the work of this specification.


10.  Change Log

   [RFC EDITOR NOTE: Please remove this Section when publishing]

   Changes from draft-ietf-sipcore-proxy-feature-03
   o  Additional Security Considerations text added.
   o  IANA Considerations modified.
   o  Editorial corrections.

   Changes from draft-ietf-sipcore-proxy-feature-02
   o  Changes based on WGLC comments from Shida Schubert.
   o  - Document title changed
   o  - Terminology alignment
   o  - Note text clarifications
   o  Changes based on WGLC comments from Lili Yang.

   Changes from draft-ietf-sipcore-proxy-feature-01
   o  Changes based on comments from Paul Kyzivat.
   o  IANA Considerations text added.

   Changes from draft-holmberg-sipcore-proxy-feature-04/
   draft-ietf-sipcore-proxy-feature-00
   o  Media feature tags replaced with feature caps, based on SIPCORE
      consensus at IETF#83 (Paris).
   o  Editorial corrections and modifications.

   Changes from draft-holmberg-sipcore-proxy-feature-03



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   o  Hadriel Kaplan added as co-author.
   o  Terminology change: instead of talking of proxies, talk about
      entities which are not represented by the URI in a Contact header
      field (http://www.ietf.org/mail-archive/web/sipcore/current/
      msg04449.html).
   o  Clarification regarding the usage of the header field in 18x/2xx
      responses (http://www.ietf.org/mail-archive/web/sipcore/current/
      msg04449.html).
   o  Specifying that feature support can also be indicated in target
      refresh requests (http://www.ietf.org/mail-archive/web/sipcore/
      current/msg04454.html).
   o  Feature Cap specification registration information added.

   Changes from draft-holmberg-sipcore-proxy-feature-02
   o  Definition, and usage of, a new header field, instead of Path,
      Record-Route, Route and Service-Route.

   Changes from draft-holmberg-sipcore-proxy-feature-01
   o  Requirement section added
   o  Use-cases and examples updated based on work in 3GPP

   Changes from draft-holmberg-sipcore-proxy-feature-00
   o  Additional use-cases added
   o  Direction section added


11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

11.2.  Informative References

   [RFC2506]  Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
              Registration Procedure", BCP 31, RFC 2506, March 1999.

   [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



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              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, August 2004.

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

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

   [3GPP.23.237]
              3GPP, "IP Multimedia Subsystem (IMS) Service Continuity;
              Stage 2", 3GPP TS 23.237 10.9.0, March 2012.

   [3GPP.24.837]
              3GPP, "IP Multimedia (IM) Core Network (CN) subsystem
              inter-UE transfer enhancements; Stage 3", 3GPP TR 24.837
              10.0.0, April 2011.


Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com


   Ivo Sedlacek
   Ericsson
   Scheelevaegen 19C
   Lund  22363
   Sweden

   Email: ivo.sedlacek@ericsson.com












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   Hadriel Kaplan
   Acme Packet
   71 Third Ave.
   Burlington, MA  01803
   USA

   Email: hkaplan@acmepacket.com












































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