Conveying Feature Tags with the Session Initiation Protocol (SIP) REFER Method
updated by
- rfc-8217 — Clarifications for When to Use the name-addr Production in SIP Messages
Extracted elements (12)
Feature tag information is especially useful prior to session establishment. For example, knowing a remote UA supports video allows the calling UA to offer video in SDP; knowing a UA is a conference focus allows subscribing to the conference URI before sending an INVITE. This motivates extending REFER to carry feature tags about the REFER-Target.
If any URI parameters are present in the Refer-To header field, the entire URI must be enclosed in "<" and ">". Without the angle brackets, all parameters after the URI are interpreted as header parameters, not URI parameters.
A feature tag included in a REFER SHOULD be discovered in an authenticated and secure method (such as an OPTIONS response or from a remote target URI in a dialog) directly from the REFER-Target.
A REFER-Issuer MUST NOT create or guess feature tags to include in a REFER request.
It is RECOMMENDED that the REFER-Issuer includes in the Refer-To header field all feature tags that were listed in the most recent Contact header field of the REFER-Target.
The REFER-Recipient MUST treat feature tags provided in a REFER request as a hint, since they cannot be authenticated or certified directly from the REFER request.
Audio and video feature tags can be included in the Refer-To header to indicate media capabilities of the REFER-Target. Example: Refer-To: "Alice's Videophone" <sip:alice@videophone.example.com>;audio;video
Multiple feature tags (e.g., actor, automata, audio) can be combined in the Refer-To header alongside URI parameters. When URI parameters like transport are present, the URI must be enclosed in angle brackets. Example: Refer-To: <sip:alice-vm@example.com;transport=tcp>;actor="msg-taker";automata;audio
The 'isfocus' feature tag can be included in a Refer-To header to signal that the REFER-Target is a conference focus, indicating that sending an INVITE will bring the REFER-Recipient into the conference. Example: Refer-To: sip:conf44@example.com;isfocus
The Refer-To header field BNF is extended to allow feature parameters (feature-param) in addition to generic parameters. The extended grammar is: Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec ) *(SEMI refer-param), where refer-param = generic-param / feature-param.
Feature tags can provide sensitive information about a user or UA. Once this information is conveyed in a REFER, it may be relayed to third parties. RFC 3840 cautions against providing sensitive information to another party, and this specification extends that concern to the REFER context.
To verify feature tags received in a REFER, the REFER-Recipient can send an OPTIONS query directly to the REFER-Target over a secure (mutually authenticated and integrity-protected) connection, protecting against incorrect or malicious feature tags.