Hi all,
I have a SIP UA implementation which seems to choose the codec to use
for rx and tx at the point at which the INVITE is accepted. This
confuses me because the SDP offer and response contain multiple codecs,
and I can't see how the UA knows which one the other end will start
sending!
Can anybody explain this to me?
Here's the detail:
The first SIP UA sends the second the following SDP offer in a SIP
INVITE:
v=0
o=vssip 123456 654321 IN IP4 10.0.0.228
s=A conversation
c=IN IP4 10.0.0.228
t=0 0
m=audio 7078 RTP/AVP 0 3 8 101
a=rtpmap:0 PCMU/8000/1
a=rtpmap:3 GSM/8000/1
a=rtpmap:8 PCMA/8000/1
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-11
Then the second sends the first the following SDP response in it's SIP
200 OK message:
v=0
o=vssip 123456 654321 IN IP4 10.0.0.107
s=A conversation
c=IN IP4 10.0.0.107
t=0 0
m=audio 7078 RTP/AVP 0 3 8 101
a=rtpmap:0 PCMU/8000/1
a=rtpmap:3 GSM/8000/1
a=rtpmap:8 PCMA/8000/1
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-11
So both know that they both support ulaw (0), gsm (3), and alaw(8). But
how do they know which codec the other UA will start sending?
Regards,
Alex
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