Comments below > I am analyzing the trace of an unsuccessful SIP conversation > to find the possible problem. The SIP message flow was ok > (INVITE/200/ACK), media streams flowed in both directions, > but I could not hear my partner. > > During analysis two questions come up: > > 1. I recognized (from RTP stream analysis), that both parties > use a different codec to send. My user agent (MS Messenger > 5.0) sends with PT=3 (GSM/8000) and the other user agent > (X-PRO) sends with PT=97 (speex/8000). Both payload types are > valid, because both are included in offer and answer SDP. But > isn't it a bit strange, that the answerer does not use the first > (common) media stream of the answer, which would have been PT=3? > The offerer follows the recommendation of RFC 3264 ( ... it > SHOULD use the first media format listed in the answer when > it does send).
The SDP answer is not correct. Matching of codecs should be done based on the format name, i.e. "speex" != "red", even though PT=97 in boot cases. The only common codec in the offer and answer if GSM (apart from telephone-event) which is the one that must be selected. / Christian Jansson, Hotsip > > Below are the the offer ans answer details. I have marked the > sent and received streams with s-> and r->. > > The SDP in the offer (Messenger 5.0): > v=0 > o=- 0 0 IN IP4 212.152.200.96 > s=session > c=IN IP4 212.152.200.96 > b=CT:44 > t=0 0 > m=audio 28376 RTP/AVP 97 111 112 4 3 101 > k=base64:A4wdo2GTiv2T8pRGMqQgG3+3UZD1UodEC4weTCZrRs0 > r->a=rtpmap:97 red/8000 > a=rtpmap:111 SIREN/16000 > a=fmtp:111 bitrate=16000 > a=rtpmap:112 G7221/16000 > a=fmtp:112 bitrate=24000 > a=rtpmap:4 G723/8000 > s->a=rtpmap:3 GSM/8000 > a=rtpmap:101 telephone-event/8000 > a=fmtp:101 0-16 > a=encryption:optional > > > The SDP in the answer (X-PRO): > v=0 > o=X 41745451 41745451 IN IP4 65.39.205.114 > s=X-PRO > c=IN IP4 65.39.205.114 > t=0 0 > m=audio 10916 RTP/AVP 0 8 3 4 98 97 101 > a=rtpmap:0 pcmu/8000 > a=rtpmap:8 pcma/8000 > r->a=rtpmap:3 gsm/8000 > a=rtpmap:98 iLBC/8000 > s->a=rtpmap:97 speex/8000 > a=rtpmap:101 telephone-event/8000 > a=fmtp:101 0-15 > > > 2. The dynamic payload type 97 has a different description on > both SDP-parts. > - Messenger 5.0 description is "a=rtpmap: 97 red/8000" > - X-PRO description is "a=rtpmap: 97 speex/8000" > If X-PRO sends with "speex"-codec and Messenger receives as > "red" it might not be decodeable, as I suggest, because these > are different codecs. Maybe this is the cause auf the > unsuccessful call. > > > Any comments highly appreciated. > > > Franz > > _______________________________________________ > Sip-implementors mailing list > [EMAIL PROTECTED] > http://lists.cs.columbia.edu/mailman/listinfo/> sip-implementors > _______________________________________________ Sip-implementors mailing list [EMAIL PROTECTED] http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
