Hi Everyone, I am implementing a sip client. I have a basic doubt on SDP negotation. When client A and B are connected. Basically A wants to send FAx to B. So first voice call is connected. now they are switching to fax. In this process the SDPs as negotiated as follows. A supports t38 and VBD both but B supports only t38 (does not support VBD). In reinvite A sends dynamic codec 104. Since B does not support should it remove 104 from media line? It already removes the a:gpmd:104 vbd=yes line. All the 4 SDPs are listed below please help me understand it.
UAC(A)----------------------------------------------------------------------------------------------------------------UAS(B) Initial Invite from A has SDP as follows m=audio 23508 RTP/AVP 0 8 101 13 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:101 telephone-event/8000 a=fmtp:101 0-15 a=rtpmap:13 CN/8000 B responds to A with SDP as follows m=audio 11986 RTP/AVP 0 8 101 13 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:101 telephone-event/8000 a=fmtp:101 0-15 a=rtpmap:13 CN/8000 now it switches on FAX A sends reinvite with SDP a=pmft:T38 m=audio 23508 RTP/AVP 0 8 99 104 103 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:99 RED/8000 a=rtpmap:104 PCMA/8000 a=rtpmap:103 t38/8000 a=fmtp:103 T38FaxVersion=0;T38MaxBitrate=14400;T38FaxMaxDatagram=173;T38FaxRateManagement=transferredTCF a=fmtp:99 104/104 a=fmtp:99 103/103 a=gpmd:104 vbd=yes And B responds with SDP a=pmft:T38 m=audio 11986 RTP/AVP 0 8 99 104 103 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:99 RED/8000 a=rtpmap:104 PCMA/8000 a=rtpmap:103 t38/8000 a=fmtp:103 T38FaxVersion=0;T38MaxBitrate=14400;T38FaxMaxDatagram=173;T38FaxRateManagement=transferredTCF a=fmtp:99 104/104 a=fmtp:99 103/103 Thanks, Isshed. _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
