Hi Gotham,
No need of second offer/ans for codec negotiation. Becuase of preferred
list first one is selected
Regards,
Praveen
On Thu, Jun 24, 2010 at 2:37 PM, goutam kumar <[email protected]> wrote:
> 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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>
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