Hi all, please find below my interpretation of how renegotiation must be done (using REINVITE) for setting up a fax call, when first a normal voice call was setup. The below procedure is based on ITU-T T.38 and RFC for SDP and offer/answer. I like your comments on this description : Fax transport in SIP networks, where receiving GW supports T38 on top of the VBD. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />A call is setup from Emitting to receiving Residential GW. First a voice call is setup, the SDPs do not contain t38 capability. Somewhat later, a fax call is detected in receiving GW. The Receiving GW sends a RE-INVITE, which contains an SDP offer with : (mandatory) m= line that was present in previous SDP used for voice call (per RFC 3264); the port is set to 0 to indicate this media is rejected/no longer used m= line for t38 using UDPTL m= line for t38 using TCP m= line for t38 using RTP m= line for G711 (VBD) At least one of the previous m=lines for t38 or VBD is mandatory. This depends on local configuration/capabilities. The emitting GW sends an SDP answer with (all m lines that were present in received offer): m= line used for voice call with the port is set to 0 m= line for t38 using UDPTL; port is set to 0 if rejected m= line for t38 using TCP; port is set to 0 if rejected m= line for t38 using RTP; port is set to 0 if rejected m= line for G711 (VBD) ; port is set to 0 if rejected The sequence of m=lines must be maintained. Only one media is accepted by the answerer (i.e. all but one m lines have the port set to 0) Do you see any interop issues in this procedure? _______________________________________________ Sip-implementors mailing list [email protected] http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
