Hi all,
we saw a SIP implementation violating the SDP offer/answer procedure when
answering a Re-Invite.
Here is the signalling:
Server Client
+----------------INVITE--------------------->
m=audio 18410 RTP/AVP 8 100 118
b=AS:87
b=RS:1087
b=RR:3262
a=rtpmap:8 PCMA/8000
a=fmtp:8 vad=no
a=rtpmap:100 telephone-event/8000
a=fmtp:100 0-15
a=rtpmap:118 PCMA/8000
a=gpmd:118 vbd=yes
a=sendrecv
<----------------200 OK---------------------+
m=audio 9158 RTP/AVP 8 118 100
a=rtpmap:8 PCMA/8000
a=fmtp:8 vad=no
a=rtpmap:118 PCMA/8000
a=rtpmap:100 telephone-event/8000
a=fmtp:100 0-15
a=sendrecv
+-----------------Re-Invite----------------->
m=audio 18410 RTP/AVP 8
a=rtpmap:8 PCMA/8000
a=gpmd:8 vbd=yes
a=silenceSupp:off - - - -
<----------------200 OK---------------------+
m=audio 9158 RTP/AVP 118
a=rtpmap:118 PCMA/8000
a=sendrecv
In the last 200OK Client is sending a Payload type (118) which was not
offered in the Re-Invite. The Re-Invite is generated when the SIP Server is
detecting a fax transmission and applies ITU V.152 (Voice Band Data), this
is also how gpmd Attribute is introduced.
The SIP Client vendor commented this on the case:
*"after call is answered (media session is established), provider
renegotiates codec set and changes PCMA codec description bound to payload
8 with the description which was bound to payload 118 in initial
negotiation of media session.*
*It causes conflict, because new description of PCMA codec is already bound
to payload 118 in initial offer/answer.*
*Voip provider can configure own side of trunk and avoid usage of
non-standard extensions in payload description (remove unnecessary
duplication of codecs). Also, provider need to preserve payload definitions
when renegotiate media session."*
So is the vendor right by saying that payload description have to be
preserved during a session? It did not find this in the SDP offer/answer
RFCs.
We never saw any other vendor having issues with this signalling.
Best Regards
Philipp Schöning
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