[sip-comm-dev] SIP Communicator uses wrong RTP payload type on asymmetric mapping


#1

Hi,

I was using SIP Communicator to make an outbound call and it resulted in one sided audio: I could hear the remote side but they could not hear me. Attached is the traffic dump. The remote side offered speex/16000 with PT 100 as the preferred codec but SC used PT 97 which is what speex/16000 was mapped to in the offer. I believe this violates RFC 3264 which says:

  For recvonly RTP streams, the payload type numbers indicate the value
    of the payload type field in RTP packets the offerer is expecting to
    receive for that codec. For sendonly RTP streams, the payload type
    numbers indicate the value of the payload type field in RTP packets
    the offerer is planning to send for that codec. For sendrecv RTP
    streams, the payload type numbers indicate the value of the payload
    type field the offerer expects to receive, and would prefer to send.
    However, for sendonly and sendrecv streams, the answer might indicate
    different payload type numbers for the same codecs, in which case,
    the offerer MUST send with the payload type numbers from the answer.

Best regards,

sip-communicator-asymmetric-rtp-payload-bug.pcap (5.65 KB)

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Dmitry Panov