Hello Isshed, the problem may be solved by negotiating already the preferred configurations at call establishment, - and T.38 would be then negotiated as latent configuration. The re-INVITE would then just trigger the switch from the audio configuration to the T.38 configuration.
-Albrecht PS SDP for FoIP T.38 is defined in Annex D of T.38 (09/2010). These procedures are normative for T.38. SDP for VBDoIP V.152 is defined in V.152 (09/2010). Both Recommendations provide examples with configurations which contain audio VoIP, FoIP T.38 and VBD. > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On > Behalf Of isshed > Sent: Dienstag, 19. Juli 2011 12:23 > To: sip-implementors > Subject: [Sip-implementors] SDP negotiation > > 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;T38F axRateManagement=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;T38F axRateManagement=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 > _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
