Hi,
I am trying to implement the offer-answer model as per RFC 3264. When I went
through RFC 4317 to see the various scenarios of this negotiation I found
this:
" 2.2. Audio and Video 2
Alice can support PCMU, PCMA, and iLBC codecs, but not more than one
at the same time. Alice offers all three to maximize chances of a
successful exchange, and Bob accepts two of them. An audio-only
session is established in the initial exchange between Alice and Bob,
using either PCMU or PCMA codecs (payload type in RTP packet tells
which is being used). Since Alice only supports one audio codec at a
time, a second offer is made with just that one codec, to limit the
codec choice to just one.
[Offer]
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 8 97
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 51372 RTP/AVP 31 32
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
[Answer]
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 0 8
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
m=video 0 RTP/AVP 31
a=rtpmap:31 H261/90000
[Second-Offer]
v=0
o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 51372 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31
a=rtpmap:31 H261/90000
[Second-Answer]
v=0
o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
s=
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 0 RTP/AVP 31
a=rtpmap:31 H261/90000
" ----RFC 4317
My doubt is, whether its mandatory to send the second offer to ensure that
media is exchanged using that codec. Can't we start sending media using PCMU
(in this case) after receiving the first answer, since the list of codecs is
already arranged according to preference in the offer and answer?? In case
the second offer is not sent, what problems may arise during the call??
Thanks,
Gotham
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