Mechanism to indicate support of features and capabilities in the Session Initiation Protocol (SIP)

The Session Initiation Protocol (SIP) "Caller Preferences" extension, defined in RFC 3840 , 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, to convey feature capability indicators, 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: - The SIP entity acts as a SIP proxy. - The SIP entity acts as a SIP registrar. - The SIP entity acts as a B2BUA, where the Contact header field URI represents another SIP entity. NOTE: Unlike media feature tags, feature capability indicators are intended to only be used with the SIP protocol. This specification also defines feature capability indicators, and creates a new IANA registry, "Proxy-Feature Feature Capability Indicator Trees", for registering feature capability indicators.

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 .

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.

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 signalling path supports the associated feature and capability. For a given fc-value, as defined in section 5.3.1, feature capability indicators are listed in a non-priority order, and any order of the listed SIP feature capability indicators 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 capability indicator supports the features and capabilities associated with the "foo" and "bar" feature capability indicators.

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 signalling path. Procedures how features and capabilities are invoked are outside the scope of this specification, and MUST be described by individual feature capability indicator specifications. 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 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" to the entity.

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 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 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 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 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.

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, to a REGISTER response.

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 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 to the response, containing one or more SIP feature capability indicators associated with the supported features and capabilities, before it forwards the response.

This Section describes the general usage and semantics of the Feature-Caps header field for different SIP message types and response codes. 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. Section 6.2.1 defines the Feature-Caps header field ABNF.

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 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 the dialog is terminated. Unless a feature capability indicator 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 capability indicators in all 18x and 2xx responses associated with a given transaction.

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

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 capability indicator 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 capability indicator is supported for the duration of the standalone transaction.

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 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 capability indicator. Section 4 defines how feature capability indicators are conveyed using the Feature-Caps header field. Section 6.3.2 defines the feature capability indicator ABNF. Section 8 provides a template for registering feature capability indicators.

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 RFC 2506 and RFC 3840 . 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 capability indicator is registered in any registration tree, no leading "+" is used in the registration.

The global feature capability indicator tree is similar to the media feature tag global tree defined in RFC 2506 . A feature capability indicator 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 capability indicator meets the technical requirements of this specification. 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"). When a feature capability indicator is registered in the global tree, it needs to meet the "Expert Review" policies defined in RFC 5226 . A designated area expert will review the proposed feature capability indicator, and consult with members of related mailing lists. This policy overrides the policy defined for registering new header field parameters.

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.". When a feature capability indicator is registered in the sip tree, it needs to meet the "IETF Consensus" policies defined in RFC 5226 . An RFC, which contains the registration of the feature capability indicator, MUST be published. This policy overrides the policy defined for registering new header field parameters.

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 ). 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 purpose. 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 SIP feature capability indicator require it.

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 SIP feature cap, 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.

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 cap specification MUST define the value(s) in accordance with the ABNF defined in Section 6.3.2. 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.

If there are restrictions on how SIP entities can insert a SIP feature cap, the feature capability indicator specification MUST document such restrictions. There might be restrictions related to whether entities are allowed to insert a feature capability indicator in registration related messages, standalone transaction messages, or dialog related messages, whether entities are allowed to insert a feature capability indicator in requests or responses, whether entities also need to support other features and capabilities in order to insert a feature capability indicator, and whether entities are allowed to indicate support of a feature in conjunction with another feature.

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.

This Section defines the ABNF for the Feature-Caps header field, and for the feature capability indicators.

The ABNF for the Feature-Caps header fields is:

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

In a feature capability indicator 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.

The ABNF for the feature capability indicator:

This specification registers a new SIP header field, Feature-Caps, according to the process of RFC 3261 . The following is the registration for the Feature-Caps header field: RFC Number: RFC XXX Header Field Name: Feature-Caps

This specification adds the Feature-Caps header field to the IANA "Header Field Parameters and Parameter Values" registry, according to the process of RFC 3968

* No [xxx] * denotes parameter names conforming to the syntax defined in [xxx]. Valid feature capability indicators are registered in [reference to the new Proxy-Feature Feature Capability Indicator Trees registry]. ]]> (IANA: please sort the "Feature-Caps" line into the table and place the remainder of the above as a footnote to the table.)

This specification creates a new sub registry to the IANA "Session Initiation Protocol (SIP) Parameters" Protocol Registry, according to the process of RFC 5226 . The name of the sub registry is "Proxy-Feature Feature Capability Indicator Trees".

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. 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 capability indicator, and consult with members of related mailing lists. The information required in the registration is defined in Section 5.3 of RFC XXX. 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.

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. 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 5.3 of RFC XXX. 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 security issues for feature capability indicators are similar to the ones defined in RFC 3840 for media feature tags. However, as feature capability indicators will typically not be used to convey capability information of end-user devices, those aspects of RFC 3840 do not apply to feature capability indicators. In addition, the RFC 3840 security issue regarding an attacker using the SIP caller preferences extension in order to affect routing decisions does not apply, as the mechanism is not defined to be used with feature capability indicators. 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 capability indicators might provide address information as feature capability indicator values. Therefore, mechanisms for guaranteeing confidentiality and authenticity SHOULD be provided.

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

[RFC EDITOR NOTE: Please remove this Section when publishing] Changes from draft-holmberg-sipcore-proxy-feature-04 WGLC comments from Keith Drage 'feature cap' name changed to 'feature capability indicator'. Feature-Caps header field added to IANA Header Field Parameters and Parameter Values registry. Editorial modifications. Changes from draft-ietf-sipcore-proxy-feature-03 Additional Security Considerations text added. IANA Considerations modified. Editorial corrections. Changes from draft-ietf-sipcore-proxy-feature-02 Changes based on WGLC comments from Shida Schubert. - Document title changed - Terminology alignment - Note text clarifications Changes based on WGLC comments from Lili Yang. Changes from draft-ietf-sipcore-proxy-feature-01 Changes based on comments from Paul Kyzivat. IANA Considerations text added. Changes from draft-holmberg-sipcore-proxy-feature-04/draft-ietf-sipcore-proxy-feature-00 Media feature tags replaced with feature caps, based on SIPCORE consensus at IETF#83 (Paris). Editorial corrections and modifications. Changes from draft-holmberg-sipcore-proxy-feature-03 Hadriel Kaplan added as co-author. 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). Clarification regarding the usage of the header field in 18x/2xx responses (http://www.ietf.org/mail-archive/web/sipcore/current/msg04449.html). Specifying that feature support can also be indicated in target refresh requests (http://www.ietf.org/mail-archive/web/sipcore/current/msg04454.html). Feature Cap specification registration information added. Changes from draft-holmberg-sipcore-proxy-feature-02 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 Requirement section added Use-cases and examples updated based on work in 3GPP Changes from draft-holmberg-sipcore-proxy-feature-00 Additional use-cases added Direction section added