SIP Support for Real-time Fax: Call Flow Examples And Best Current Practices

Document Type Expired Internet-Draft (sipping WG)
Authors Jean-Francois Mule  , Jian Li 
Last updated 2003-03-05
Stream Internet Engineering Task Force (IETF)
Intended RFC status (None)
Expired & archived
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Stream WG state WG Document
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IESG IESG state Expired
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


The Session Initiation Protocol (SIP) allows the establishment of real-time Internet fax communications. Real-time facsimile communications over IP may follow 2 modes of operation: T.38 fax relay as defined by the ITU-T T.38 recommendation or fax pass- through. This document clarifies the options available to Internet telephony gateway vendors to handle real-time fax calls using SIP. While our primary focus is to address the more reliable real-time T.38 Group 3 fax mode, we briefly cover the fax pass-through mode to enable fallback operations and super G3 fax communications using SIP. We also give examples of SIP call flows for real-time Internet fax gateways or SIP proxy redirect servers. Elements in these call flows include SIP User Agents, SIP Proxy Servers, and Gateways to the PSTN (Public Switch Telephone Network). This document introduces best current practices for SIP T.38 fax and SIP fax pass-through sessions. A session starts with audio capabilities, and, upon fax tone detection, T.38 fax capabilities are negotiated; upon successful negotiation, the session continues with fax capabilities and the media termination hosts exchange T.38 Internet fax packets. The T.38 fax call scenarios include various aspects of the call sequence: the detection of fax transmission, the usage of the T.38 session description attributes, the optional fallback into fax pass-through mode and the session termination. The fax pass-through call scenarios involve some specific SDP media attributes to enable proper fax transmission. Fax transmission can be detected by the receiving side, the emitting side or both (in the latter case, a 'glare' effect may appear). This document only covers the case when the fax transmission is detected by the receiving side: it is the most common practice and the other cases do not represent any particular challenges and are therefore left for future discussions). Call flow diagrams and message details are shown. A list of IANA defined SDP attribute names for T.38 is summarized in section 7.


Jean-Francois Mule (
Jian Li (

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)