This document is the official problem statement related to the work of the SIP Forum FoIP SIP-SDP subgroup.

While the T.38 protocol, approved by the ITU-T in 1998, was designed to allow fax machines and computer-based fax to carry forward in a transitioning communications infrastructure of both IP- and TDM-based telephony, in 2009 there are enough problems and confusion among vendors, enterprises, and service providers to significantly slow the use of IP as a real-time fax transport. The issues surrounding IP-based fax in general and the use of T.38 make it difficult for users to determine if T.38 can or will work reliably and thus offer an alternative to traditional TDM-based fax transport. To address these problems and offer solutions, the SIP Forum has chartered the FoIP Task Group (TG).

This document details a number of SDP offer/answer interoperability issues found while implementing and connecting T.38 compliant endpoints together, primarily over the SIP signaling mechanism. However, many of the issues presented here are not specific to SIP or SDP, but are in fact problems that would occur over any T.38-capable signaling mechanism.