Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)
Extracted elements (11)
Fax calls in SIP are handled in a 'voice first' manner: the call is set up as a voice call first, and only after connection does a switchover to T.38 occur. This means fax calls cannot be distinguished from voice calls at initiation, leading to misrouting to non-fax-capable UAs.
While SDP is generally preferred for capability negotiation in SIP, fax is unique in that the ultimate intention of the call is not accurately signaled in the initial SDP exchange—T.38 or other fax transport protocols are not known at INVITE time. This justifies a media feature tag approach for pre-call fax capability advertisement.
A calling UA that requires 'sip.fax' should be prepared to relax that requirement to a preference if it receives a failure response indicating the requirement mechanism itself is not supported by the called UAs, their proxies, or other SIP network elements.
A calling UA SHOULD include the 'sip.fax' media feature tag in the Accept-Contact header of an INVITE request to express its desire for routing to a fax-capable UA; without this tag, fax call determination is not possible until after the call is connected.
A calling UA SHOULD NOT require the 'sip.fax' media type (as opposed to expressing a preference), as doing so could result in call failure if no fax-capable UAs are registered or if SIP network elements do not support RFC 3840/3841 processing.
A receiving UA MUST include the 'sip.fax' media feature tag in the Contact header field of REGISTER messages to indicate fax capability, as per RFC 3840.
The 'passthrough' value of 'sip.fax' indicates the device supports 'audio/pcmu' and 'audio/pcma' media types for transporting T.30 and T.4 fax data using G.711; additional recommendations are in ITU-T V.152 Sections 6 and 6.1.
The 'sip.fax' media feature tag is a new string parameter that allows a call to indicate a fax preference. It has two allowed values: 't38' (for ITU-T T.38 fax protocol) and 'passthrough' (for G.711 codec-based fax transport).
The 't38' value of 'sip.fax' indicates the device supports 'image/t38' media type and implements ITU-T T.38 for transporting T.30 and T.4 fax data over IP.
This specification adds the 'sip.fax' media feature tag to the SIP Media Feature Tag Registration Tree per RFC 2506 and RFC 3840 procedures, with ASN.1 identifier 1.3.6.1.8.4.25.
The security considerations related to the use of media feature tags from Section 11.1 of RFC 3840 apply to the 'sip.fax' feature tag.