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ITU-T T.38 SIP/SDP Negotiation Improvements

This document proposes changes to ITU-T Recommendation T.38 as a means to improve interoperability between implementations and provide improved clarity and understanding among implementers of the recommendation. The proposed changes represent a year of analysis and discussion by the SIP Forum’s Fax over IP working group.

The changes proposed include additional session parameters to be declared or negotiated between the endpoints, and because the SIP Forum’s working group addresses only usage of ITU-T T.38 over SIP signalling, these changes affect Annex D and the SIP-related portions of Annex H only. However, since the equivalent rationale and motivations for these changes also apply to H.248.1 and H.245, and it is recommended that the changes should also be applied to Annexes E and G, and to the H.248.1 and H.245 sections of Annex H.

In addition, this document proposes that the currently Optional behaviour by Receiving Gateways of muting the voice channel during T.38 session negotiation be modified to Recommended, and in addition provides details on timing and specifics of the implementation that would likely result in the highest transaction success rates.