RFC 6809 PROPOSED STANDARD SIP · CAPABILITIES · REGISTRIES
SIP Feature-Caps: Mechanism to Indicate Support of Features and Capabilities in the Session Initiation Protocol (SIP)
A SIP header that lets proxies, registrars and B2BUAs advertise what they support.
RFC EDITOR
Holmberg, C., Sedlacek, I., and H. Kaplan, "Mechanism to Indicate Support of Features and Capabilities in the Session Initiation Protocol (SIP)", RFC 6809, DOI 10.17487/RFC6809, November 2012, <https://www.rfc-editor.org/info/rfc6809>.
BIBTEX
@misc{rfc6809,
series = {Request for Comments},
number = 6809,
howpublished = {RFC 6809},
publisher = {RFC Editor},
doi = {10.17487/RFC6809},
url = {https://www.rfc-editor.org/info/rfc6809},
author = {C. Holmberg and I. Sedlacek and H. Kaplan},
title = {{Mechanism to Indicate Support of Features and Capabilities in the Session Initiation Protocol (SIP)}},
pagetotal = 19,
year = 2012,
month = nov,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Holmberg
Request for Comments: 6809 I. Sedlacek
Category: Standards Track Ericsson
ISSN: 2070-1721 H. Kaplan
Acme Packet
November 2012
Mechanism to Indicate Support of Features and Capabilities in
the Session Initiation Protocol (SIP)
Abstract
This specification defines a new SIP header field, Feature-Caps. The
Feature-Caps header field conveys feature-capability indicators that
are used to indicate support of features and capabilities for SIP
entities that are not represented by the Uniform Resource Identifier
(URI) of the Contact header field.
SIP entities that are represented by the URI of the SIP Contact
header field can convey media feature tags in the Contact header
field to indicate support of features and capabilities.
This specification also defines feature-capability indicators and
creates a new IANA registry, "Proxy-Feature Feature-Capability
Indicator Trees", for registering feature-capability indicators.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc6809.
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
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 ....................................................3
2. Conventions .....................................................4
3. Definitions .....................................................4
4. Feature-Caps Header Field .......................................4
4.1. Introduction ...............................................4
4.2. User Agent and Proxy Behavior ..............................4
4.2.1. General .............................................4
4.2.2. B2BUA Behavior ......................................5
4.2.3. Registrar Behavior ..................................6
4.2.4. Proxy Behavior ......................................6
4.3. SIP Message Type and Response Code Semantics ...............7
4.3.1. General .............................................7
4.3.2. SIP Dialog ..........................................7
4.3.3. SIP Registration (REGISTER) .........................7
4.3.4. SIP Standalone Transactions .........................8
5. Feature-Capability Indicators ...................................8
5.1. Introduction ...............................................8
5.2. Registration Trees .........................................9
5.2.1. General .............................................9
5.2.2. Global Tree .........................................9
5.2.3. SIP Tree ............................................9
5.3. Feature-Capability Indicator Specification Requirements ...10
5.3.1. General ............................................10
5.3.2. Overall Description ................................10
5.3.3. Feature-Capability Indicator Values ................10
5.3.4. Usage Restrictions .................................11
5.3.5. Interoperability Considerations ....................11
5.3.6. Security Considerations ............................11
5.3.7. Examples ...........................................12
5.3.8. Other Information ..................................12
6. Syntax .........................................................12
6.1. General ...................................................12
6.2. Syntax: Feature-Caps Header Field .........................12
6.2.1. ABNF ...............................................12
6.3. Syntax: Feature-Capability Indicator ......................12
6.3.1. General ............................................12
6.3.2. ABNF ...............................................13
7. IANA Considerations ............................................13
7.1. Registration of the Feature-Caps Header Field .............13
7.2. Registration of the Feature-Caps Header Field Parameter ...13
7.3. Proxy-Feature Feature-Capability Indicator Trees ..........14
7.3.1. Introduction .......................................14
7.3.2. Global Feature-Capability Indicator
Registration Tree ..................................14
7.3.3. SIP Feature-Capability Indicator
Registration Tree ..................................15
8. Feature-Capability Indicator Registration Template .............16
9. Security Considerations ........................................17
10. Acknowledgements ..............................................17
11. References ....................................................18
11.1. Normative References .....................................18
11.2. Informative References ...................................18
1. Introduction
The Session Initiation Protocol (SIP) [RFC3261] extension for
indicating User Agent (UA) capabilities, 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 defines a new SIP header field, Feature-Caps. The
Feature-Caps header field conveys feature-capability indicators that
are used to indicate support of features and capabilities for SIP
entities that are not represented by the Uniform Resource Identifier
(URI) of the Contact header field. 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 Back-to-Back User Agent (B2BUA)
[RFC3261], where the Contact header field URI represents another
SIP entity.
SIP entities that are represented by the URI of the SIP Contact
header field can convey media feature tags in the Contact header
field to indicate support of features and capabilities.
Unlike media feature tags, feature-capability indicators are intended
to only be used with SIP.
This specification also defines feature-capability indicators and
creates a new IANA registry, "Proxy-Feature Feature-Capability
Indicator Trees", for registering feature-capability indicators.
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 Header Field
4.1. Introduction
The Feature-Caps header field is used by SIP entities to convey
support of features and capabilities, by setting feature-capability
indicators. A feature-capability indicator conveyed in a
Feature-Caps header field indicates that a SIP entity in the SIP
message signaling path supports the associated feature and
capability.
4.2. User Agent and Proxy Behavior
4.2.1. 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-capability
indicators in the header field inform the entity about the features
and capabilities supported by entities in the SIP message signaling
path. The procedure by which features and capabilities are invoked
are outside the scope of this specification and MUST be described by
individual feature-capability indicator specifications.
A Feature-Caps header field value cannot 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-capability
indicator.
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-capability
indicators in the top-most Feature-Caps header field will represent
the supported features and capabilities "closest", from a SIP
signaling point of view, to the entity.
Based on features and policies, a SIP entity MAY remove a
Feature-Caps header field from a SIP message. Also, a SIP entity MAY
remove a feature-capability indicator from a Feature-Caps header
field within a SIP message. A SIP entity SHOULD NOT re-order the
Feature-Caps header fields within a SIP message.
For a given fc-value, as defined in Section 6.2.1, the order in which
feature-capability indicators are listed has no significance. For
example, "foo;bar" and "bar;foo" have the same meaning (i.e., that
the SIP entity that inserted the feature-capability indicator
supports the features and capabilities associated with the "foo" and
"bar" feature-capability indicators).
4.2.2. B2BUA Behavior
The procedures in this section apply to User Agents (UAs) [RFC3261]
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 such 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 in the request,
containing one or more feature-capability indicators associated with
the supported features and 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 in the SIP
request.
When such 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 in the response,
containing one or more feature-capability indicators 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
fields by copying them to the outgoing SIP response, similar to a SIP
proxy, before it inserts its own Feature-Caps header field in the SIP
response.
4.2.3. 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-capability
indicators associated with the supported features and capabilities,
in a REGISTER response.
4.2.4. 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 in the request,
containing one or more SIP feature-capability indicators 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 in the response,
containing one or more SIP feature-capability indicators associated
with the supported features and capabilities, before it forwards the
response.
4.3. SIP Message Type and Response Code Semantics
4.3.1. General
This section describes the general usage and semantics of the
Feature-Caps header field for different SIP message types and
response codes.
Section 6.2.1 defines the Feature-Caps header field ABNF.
4.3.2. 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-capability indicator is inserted in a Feature-Caps
header field of an initial request for a dialog, or within a response
of such a request, it indicates to the receivers of the request (or
response) that the feature associated with the feature-capability
indicator is supported for the duration of the dialog, until a target
refresh request is sent for the dialog, or until the dialog is
terminated.
Unless a feature-capability indicator is inserted in a Feature-Caps
header field of a target refresh request, or within a response of
such a request, it indicates to the receivers of the request (or
response) that the feature is no longer supported for the dialog.
For a given dialog, a SIP entity MUST insert the same feature-
capability indicators in all 18x and 2xx responses associated with a
given transaction.
As it cannot be guaranteed that 2xx responses associated with SIP
SUBSCRIBE requests will reach the User Agent Client (UAC) [RFC3261],
due to forking of the request, entities need to indicate supported
features and capabilities in the SIP NOTIFY request that will be sent
for each of the created subscription dialogs.
4.3.3. 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 a
request.
If a feature-capability indicator 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-capability indicator 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-capability indicator associated
with the feature.
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 requests sent to the contact(s)
that were explicitly conveyed in the associated REGISTER request.
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 are
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 in 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.
4.3.4. SIP Standalone Transactions
The Feature-Caps header field can be used within a standalone SIP
request and within any 2xx response associated with such a request.
If a feature-capability indicator is inserted in a Feature-Caps
header field of a standalone request, or within a response of such a
request, it indicates to the receivers of the request (or response)
that the feature associated with the feature-capability indicator is
supported for the duration of the standalone transaction.
5. Feature-Capability Indicators
5.1. Introduction
Feature-capability indicators 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 such support.
A value, or a list of values, that provides additional information
about the supported feature or capability can be associated with a
feature-capability indicator.
5.2. Registration Trees
5.2.1. 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-Capability Indicator Trees" registry.
The trees defined herein are similar to the global tree and SIP tree
defined for media feature tags, in RFCs 2506 [RFC2506] and 3840
[RFC3840]. Other registration trees are outside the scope of this
specification.
In contrast to RFCs 2506 and 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-capability indicator is registered in any registration
tree, no leading "+" is used in the registration.
5.2.2. Global Tree
The global feature-capability indicator tree is similar to the media
feature tag global tree defined in RFC 2506 [RFC2506].
A feature-capability indicator 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").
5.2.3. SIP Tree
The SIP feature-capability indicator tree is similar to the media
feature tag SIP tree defined in RFC 3840.
A feature-capability indicator in the SIP tree will be distinguished
by the leading facet "sip.".
5.3. Feature-Capability Indicator Specification Requirements
5.3.1. General
A feature-capability indicator specification MUST address the issues
defined in the following subsections or document why an issue is not
applicable for the specific feature-capability indicator. A
reference to the specification MUST be provided when the feature-
capability indicator is registered with IANA (see Section 8).
It is bad practice for feature-capability indicator specifications to
repeat procedures (e.g., general procedures on the usage of the
Feature-Caps header field and feature-capability indicators) defined
in this specification, unless needed for clarification or emphasis
purposes. A feature-capability indicator specification MUST NOT
modify the Feature-Caps header field rules and semantics defined in
Section 4.
A feature-capability indicator 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 feature-capability indicator
require it.
5.3.2. Overall Description
The feature-capability indicator specification MUST contain an
overall description of the feature-capability indicator: how it is
used to indicate support of a feature, a description of the feature
associated with the feature-capability indicator, a description of
any additional information (conveyed using one or more feature-
capability indicator values) that can be conveyed together with the
feature-capability indicator, and a description of how the associated
feature MAY be exercised/invoked.
5.3.3. Feature-Capability Indicator Values
A feature-capability indicator can have an associated value, or a
list of values. The feature-capability indicator specification MUST
define the syntax and semantics of any value defined for the feature-
capability indicator, including possible restrictions related to the
usage of a specific value. The feature-capability indicator
specification MUST define the value(s) in accordance with the ABNF
defined in Section 6.3.2. The feature-capability indicator
specification MUST define whether the feature-capability indicator
has a default value.
If no values are defined for the feature-capability indicator, it
MUST be indicated in the feature-capability indicator specification.
A feature-capability indicator value is only applicable for the
feature-capability indicator for which it has been defined. For
other feature-capability indicators, the value has to be 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-capability indicator, unless the
semantics of the values are the same.
5.3.4. Usage Restrictions
If there are restrictions on how SIP entities can insert a feature-
capability indicator, the feature-capability indicator specification
MUST document such restrictions.
There might be restrictions related to whether or not entities
o are allowed to insert a feature-capability indicator in
registration-related messages, standalone transaction messages, or
dialog-related messages,
o are allowed to insert a feature-capability indicator in requests
or responses,
o also need to support other features and capabilities in order to
insert a feature-capability indicator, and
o are allowed to indicate support of a feature in conjunction with
another feature.
5.3.5. Interoperability Considerations
The feature-capability indicator specification MUST document any
specific interoperability considerations that apply to the feature-
capability indicator.
Interoperability considerations can, e.g., include procedures related
to cases where an expected feature-capability indicator is not
present or where it contains an unexpected value.
5.3.6. Security Considerations
The feature-capability indicator specification MUST document any
specific security considerations that apply to the feature-capability
indicator.
5.3.7. Examples
It is recommended that the feature-capability indicator 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.
5.3.8. Other Information
If there is additional information about the feature-capability
indicator, it is recommended to describe such information. It can
include, for example, names of related feature-capability indicators.
6. Syntax
6.1. General
This section defines the ABNF for the Feature-Caps header field and
for the feature-capability indicators. The ABNF defined in this
specification is conformant to RFC 5234 [RFC5234].
6.2. Syntax: Feature-Caps Header Field
6.2.1. 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)
NOTE: The "*" value is present in order to follow the guidelines for
syntax in RFC 4485 [RFC4485] and to maintain a consistent format with
RFCs 3840 [RFC3840] and 3841 [RFC3841].
6.3. Syntax: Feature-Capability Indicator
6.3.1. General
In a feature-capability indicator name (ABNF: fcap-name), dots can be
used to implement a feature-capability indicator tree hierarchy
(e.g., tree.feature.subfeature). The description of usage of such a
tree hierarchy must be described when registered.
6.3.2. ABNF
The ABNF for the feature-capability indicator is:
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-capability indicator name is mandatory.
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 defined in RFC 3261 [RFC3261].
The following is the registration for Feature-Caps in the "Header
Fields" registry:
RFC Number: RFC 6809
Header Field Name: Feature-Caps
7.2. Registration of the Feature-Caps Header Field Parameter
This specification adds the Feature-Caps header field to the IANA
"Header Field Parameters and Parameter Values" registry, according to
the process described in RFC 3968 [RFC3968].
Predefined
Header Field Parameter Name Values Reference
--------------------------------------------------------------------
Feature-Caps +<fcap-name> * No [RFC6809]
* <fcap-name> denotes parameter names conforming to the
syntax <fcap-name> defined in RFC 6809. Valid
feature-capability indicators are registered in the
Proxy-Feature Feature-Capability Indicator Trees registry.
7.3. Proxy-Feature Feature-Capability Indicator Trees
7.3.1. Introduction
This specification creates a new sub-registry to the IANA "Session
Initiation Protocol (SIP) Parameters" registry, according to the
process defined in RFC 5226. The name of the sub-registry is
"Proxy-Feature Feature-Capability Indicator Trees".
Feature-capability indicators are categorized by the "leading facet"
of their name. The leading facet is a prefix of the name consisting
of all characters up to and including the first ".". Feature-
capability indicator names that contain no "." characters are
considered to have an empty ("") leading facet.
The "Proxy-Feature Feature-Capability Indicator Trees" registry
contains sub-registries for subsets (called 'trees') of feature-
capability indicators sharing the same leading facet. Each feature-
capability indicator is registered within the tree that matches its
leading facet. If no tree matches its leading facet, then the
feature-capability indicator cannot be registered.
New feature-capability indicator sub-registries (trees) can be
registered. The registration must meet the "Standards Action"
policies defined in RFC 5226 [RFC5226]. A new name, unique leading
facet, and registration policies (as defined in RFC 5226) for
feature-capability indicators within this tree need to be provided.
This document defines the first two feature-capability indicator
trees ("g." and "sip."). It does not define a tree for the empty
leading facet.
7.3.2. Global Feature-Capability Indicator Registration Tree
This specification creates a new feature-capability indicator tree in
the IANA "Proxy-Feature Feature-Capability Indicator Trees" registry.
The name of the tree is "Global Feature-Capability Indicator
Registration Tree", and its leading facet is "g.". It is used for
the registration of feature-capability indicators.
When a feature-capability indicator is registered in the global tree,
it needs to meet the "Specification Required" policies defined in
RFC 5226. A designated area expert will review the proposed feature-
capability indicator and consult with members of related mailing
lists. The information required in the registration is defined in
Section 5.3 of this document.
Note that all feature-capability indicators 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-capability indicator
registration trees.
The format of the global tree is as described below:
Name Description Reference
------------------------------
Name - contains the Feature-Capability Indicator Name, provided in
the registration feature-capability indication registration template.
Description - provided in the registration feature-capability
indication registration template.
Reference - contains the Feature-Capability Indicator specification
reference provided in the registration feature-capability indication
registration template.
No initial values are registered in the global tree.
7.3.3. SIP Feature-Capability Indicator Registration Tree
This specification creates a new feature-capability indicator tree in
the IANA "Proxy-Feature Feature-Capability Indicator Trees" registry.
The name of the tree is "SIP Feature-Capability Indicator
Registration Tree", and its leading facet is "sip.". It is used for
the registration of feature-capability indicators.
When a feature-capability indicator is registered in the SIP tree, it
needs to meet the "IETF Review" policies defined in RFC 5226. The
information required in the registration is defined in Section 5.3 of
this document.
Note that all feature-capability indicators 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-capability indicator
registration trees.
The format of the SIP tree is as described below:
Name Description Reference
------------------------------
Name - contains the Feature-Capability Indicator Name, provided in
the registration feature-capability indication registration template.
Description - provided in the registration feature-capability
indication registration template.
Reference - contains the Feature-Capability Indicator specification
reference provided in the registration feature-capability indication
registration template.
No initial values are registered in the SIP tree.
8. Feature-Capability Indicator Registration Template
Registration requests for the global tree are submitted by email to
iana@iana.org.
Registration requests for the SIP tree requires submitting an
Internet-Draft to the IESG.
| Instructions are preceded by '|'. All fields are mandatory.
Feature-capability indicator name:
Description:
| The description should be no longer than 4 lines. More
| detailed information can be provided in the feature
| capability indicator specification.
Feature-capability indicator specification reference:
| The referenced specification must contain the information
| listed in Section 5.3 of RFC 6809.
Contact:
| Name(s) & email address(es) of person(s) to
| contact for further information.
9. Security Considerations
The security issues for feature-capability indicators are similar to
the ones defined in RFC 3840 for media feature tags. Media feature
tags can reveal information about end users and end-user equipment,
which can be used for industrial espionage. The knowledge about end-
user equipment capabilities can also be used to influence application
behavior. As feature-capability indicators are not intended to
convey capability information of end-user devices, such end-user
security aspects of RFC 3840 do not apply to feature-capability
indicators.
In addition, the security issue discussed in RFC 3840 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-capability indicators.
Feature-capability indicators can, however, provide capability and
characteristics information about the SIP entity, some of which might
be sensitive. Malicious elements viewing the indicators may be able
to discern application deployment details or identify elements with
exploitable feature implementation weaknesses. The Feature-Caps
header field does not convey address information about SIP entities.
However, individual feature-capability indicators might provide
address information as feature-capability indicator values.
Therefore, if the feature-capability indicators provide information
that requires data integrity or origin authentication, mechanisms for
providing those MUST be provided. If confidentiality is required,
then the specification MUST call for the use of Transport Layer
Security (TLS) [RFC5246] at all hops. Since there are no
satisfactory middle-to-end or middle-to-middle SIP confidentiality
mechanisms, TLS is as good as it gets, and specifications SHOULD NOT
define feature-capability indicators that need confidentiality that
is better than the hop-by-hop confidentiality provided by TLS.
10. Acknowledgements
The authors wish to thank everyone in the SIP community that provided
input and feedback on the work of this specification.
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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
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
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[RFC3968] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Header Field Parameter Registry for the Session
Initiation Protocol (SIP)", BCP 98, RFC 3968,
December 2004.
[RFC4485] Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors
of Extensions to the Session Initiation Protocol (SIP)",
RFC 4485, May 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
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
Hadriel Kaplan
Acme Packet
71 Third Ave.
Burlington, MA 01803
USA
EMail: hkaplan@acmepacket.com
AI ELI5
On this page
PROBLEMSIP user agents can announce features with media feature tags in the Contact header field, but proxies, registrars and some B2BUAs are not represented by the Contact URI and so had no way to do the same.
MECHANISMRFC 6809 defines the Feature-Caps header field, which carries feature-capability indicators, and creates an IANA registry with a global tree (g.) and a SIP tree (sip.) for registering them.
WHY IT MATTERSEntities on the signaling path can now announce supported features, while each individual indicator's specification defines how the feature is used. The RFC is a Proposed Standard.
§1Why Another Capability Header
The SIP extension in RFC 3840 lets a message carry information about the originator's features and capabilities in the Contact header field. That only works for an entity represented by the Contact URI.
RFC 6809 adds a header field, Feature-Caps, for entities that are not represented that way. The RFC lists three cases: the entity acts as a SIP proxy, it acts as a SIP registrar, or it acts as a Back-to-Back User Agent (B2BUA) where the Contact URI represents another entity. The values it carries are called feature-capability indicators. Unlike media feature tags, they are intended to be used only with SIP.
Some helpers in a phone call do not have their own name tag. This gives them a way to wear a tag listing what they can do.
§2How Entities Use Feature-Caps
A feature-capability indicator in a Feature-Caps header field says that some entity in the SIP message signaling path supports the associated feature. An entity whose own URI is in the Contact header field MUST NOT use Feature-Caps for its support. The header field cannot carry the address of the entity that inserted it. If that address matters, the feature's own definition has to put it in an indicator value.
A proxy that wants to advertise support towards downstream entities inserts the header in the request before forwarding. To advertise towards upstream entities, it inserts it in the response. A B2BUA that is referenced by a Record-Route header field does the same, and MAY also copy received Feature-Caps fields, like a proxy. A registrar inserts the header in a REGISTER response.
An entity that adds a Feature-Caps header field MUST place it before any existing one, so the top-most one is the closest from a signaling point of view. Entities MAY remove a whole header field or single indicators, and SHOULD NOT re-order the header fields. Order within one field has no significance, so foo;bar and bar;foo mean the same.
Each helper adds its tag to the top of the pile as a message passes through, so the helper nearest to you is always on top.
§3Dialogs, Registrations and Standalone Requests
How long an indicator applies depends on the message type. In a dialog, the header can appear in an initial request, a target refresh request and any 18x or 2xx response to them. An indicator in an initial request applies for the dialog until a target refresh request is sent or the dialog ends. If a later target refresh request or its response lacks the indicator, the feature is no longer supported for the dialog. An entity MUST insert the same indicators in all 18x and 2xx responses of one transaction. For SUBSCRIBE, forking means the 2xx may not reach the UAC, so support is indicated in the NOTIFY sent for each subscription dialog.
For REGISTER, the header can appear in the request and in the 200 (OK) response. Support lasts until the explicitly registered contact expires, or until a refresh REGISTER omits the indicator, and applies only to contacts conveyed in that request. Pure binding-fetch REGISTERs, with no Contact header field, MUST NOT carry the header. With SIP outbound (RFC 5626), support applies only to the flow on which it was indicated.
In a standalone request, an indicator applies for the duration of that transaction.
A tag is only good for as long as the conversation it was attached to. If the next message does not repeat it, the helper no longer promises that skill.
§4Syntax of the Header and Indicators
The ABNF follows RFC 5234. The header field is Feature-Caps followed by one or more fc-value items separated by commas. Each fc-value starts with an asterisk, then any number of semicolon-separated feature-capability indicators. The asterisk is there to follow the syntax guidelines of RFC 4485 and to stay consistent with RFCs 3840 and 3841.
Each indicator starts with a mandatory +, then a name, and optionally an equals sign and a quoted value list or string value. Unlike media feature tags, the + is always required. Dots in a name can build a tree hierarchy such as tree.feature.subfeature, and its use must be described at registration.
It is a form with fixed boxes. Each skill gets a plus sign and a name, and may have a quoted detail after it.
Feature-Caps and feature-capability indicator
Feature-Caps = "Feature-Caps" HCOLON fc-value
*(COMMA fc-value)
fc-value = "*" *(SEMI feature-cap)
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§5Defining and Registering Indicators
The RFC creates the IANA sub-registry "Proxy-Feature Feature-Capability Indicator Trees". Indicators are sorted into trees by their leading facet, the prefix up to and including the first dot. The RFC defines two trees. The global tree uses the leading facet g. and needs "Specification Required" with a designated expert. The SIP tree uses sip. and needs "IETF Review". Names with no dot have an empty leading facet and have no tree. No initial values are registered in either tree, and no leading + is used in registrations.
An indicator's own specification MUST cover an overall description, values and their syntax, usage restrictions, interoperability considerations and security considerations, and it is recommended to include examples. It MUST NOT change the Feature-Caps rules in this RFC or weaken any SHOULD or MUST, though it MAY strengthen a SHOULD, MAY or RECOMMENDED to MUST. The registration template asks for the name, a description of no more than 4 lines, the specification reference and a contact.
There are two official lists where new skill names can be filed, one easier to get into and one run by the standards group, and every new skill has to come with its own instructions.
| Tree | Leading facet | Registration policy |
|---|---|---|
| Global | g. | Specification Required |
| SIP | sip. | IETF Review |
§6Security Considerations
The end-user privacy concerns of RFC 3840 do not apply, because these indicators are not meant to describe end-user devices, and the caller-preferences routing concern does not apply because the mechanism is not defined for use with indicators.
Indicators can still reveal sensitive details about SIP entities. Malicious elements that see them may learn deployment details or spot elements with exploitable weaknesses. The header does not carry addresses, but individual indicator values might. If an indicator needs integrity or origin authentication, its specification MUST provide mechanisms for it. If it needs confidentiality, the specification MUST call for TLS at all hops, and specifications SHOULD NOT define indicators that need better confidentiality than hop-by-hop TLS gives.
A skill list can give away secrets about a helper, so anything private should only be sent over a locked connection.
§7References
[RFC3261]RFC 3261Rosenberg, J., et al., "SIP: Session Initiation Protocol", RFC 3261, June 2002.July 2002[RFC3840]RFC 3840Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, August 2004.August 2004[RFC3841]RFC 3841Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller Preferences for the Session Initiation Protocol (SIP)", RFC 3841, August 2004.August 2004[RFC5626]RFC 5626Jennings, C., Mahy, R., and F. Audet, "Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)", RFC 5626, October 2009.October 2009[RFC5226]RFC 5226Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.May 2008[RFC5234]RFC 5234Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.January 2008