update message with g729 varient

kashif_abbasi
Joined: Mon 14 of Nov, 2016

update message with g729 varient

Posted:Mon 14 of Nov, 2016 (19:13 UTC)
I have an invite from A to B side with SDP as below;

m=audio 60144 RTP/AVP 8 18 101
a=rtpmap:8 PCMA/8000
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=ptime:20
a=sendrecv

the B side responds 18x with SDP as below (reliable means also requesting for PRACK which comes successfully from A side)

m=audio 57290 RTP/AVP 8 101
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=ptime:20

Now B side sends an UPDATE message probably to change codecs again with below SDP which A side immediately fails with "488 Unsupported media".

m=audio 57290 RTP/AVP 18 8
a=rtpmap:18 G729/8000/1
a=rtpmap:8 PCMA/8000
a=ptime:20

Now my question is that if A party doesn't like G729 variant in the UPDATE as it had earlier sent "G729 annexb=no" in the offer? Is this 488 response due to difference in variants?

secondly, if A party fails this UPDATE request with 488 due to G729 variant, but thing is that this UPDATE request also has PCMA in the SDP which was also present in the initial offer (INVITE) from A party side. So if in this UPDATE, A party doesn't like G729 variat, shouldn't it accept PCMA at second place? Can someone please comment on it?