Re: [MMUSIC] Simplifying draft-westerlund-avtcore-rtp-simulcast

[Bernard Aboba <[e-mail]>]

Christer said: 
 
"If the answerer does NOT support simulcast, it will see an Offer with multiple uni-directional streams. The behavior is difficult to predict. So, even with a mechanism based on grouping, the question is still: should the support of the simulcast feature be negotiated between the endpoints first, before it is enabled? Most likely it would require 2 O/A transactions."
 
[BA] If the answerer does NOT support simulcast, it would not respond with an a=group:SC line, so the Offerer will know that simulcast is not supported.  If the Answerer accepted more than one of the m= lines offered by the Offerer, but didn't include an a=group:SC line, then the Offerer could send a modified offer in order to avoid confusion on the part of the Answerer.   A similar scenario can occur in draft-westerlund-avtcore-rtp-simulcast, since the Answerer may not understand the a=sim-send-cap or a=sim-recv-cap lines.  
 
In an O/A exchange where both the Offerer and Answerer support simulcast, the exchange would look like this: 
 
Offer: 
 
v=0
o=alice 2362969037 2362969040 IN IP4 192.0.2.156
s=Simulcast Enabled Client
t=0 0
c=IN IP4 192.0.2.156
b=AS:665
a=group:SC S1 S2
m=video 49300 RTP/AVP 97
a=sendonly
b=AS:520
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=42c01e
a=imageattr:97 send [x=640,y=360]
a=mid:S1
m=video 49302 RTP/AVP 98
a=sendonly
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42c01e
a=imageattr:98 send [x=320,y=180]
a=mid:S2
m=video 49304 RTP/AVP 100 99
a=recvonly
a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42c01e
a=imageattr:100 recv [x=640,y=360] 
a=rtpmap:99 H264/90000
a=fmtp:99 profile-level-id=42c01e
a=imageattr:99 recv [x=320,y=180]
a=mid:S3
 
Answer: 
 
v=0
o=server 823479283 1209384938 IN IP4 192.0.2.2
s=Answer to Simulcast Enabled Client
t=0 0
c=IN IP4 192.0.2.43
b=AS:665
a=group:SC S1 S2
m=video 48000 RTP/AVP 97
a=recvonly
b=AS:520
a=rtpmap:97 H264/90000
a=fmtp:97 profile-level-id=42c01e
a=imageattr:97 recv [x=640,y=360]
a=mid:S1 
m=video 48002 RTP/AVP 98
a=recvonly
a=rtpmap:98 H264/90000
a=fmtp:98 profile-level-id=42c01e
a=imageattr:98 recv [x=320,y=180]
a=mid:S2
m=video 48004 RTP/AVP 100 99
a=sendonly
a=rtpmap:100 H264/90000
a=fmtp:100 profile-level-id=42c01e
a=imageattr:100 send [x=640,y=360] 
a=rtpmap:99 H264/90000
a=fmtp:99 profile-level-id=42c01e
a=imageattr:99 send [x=320,y=180]
a=mid:S3