Hi all,
Suppose caller A wants to initiate a video call with B. He sends an
INVITE to B and B accepts the call and then sends back 200 OK with SDP.
According to RFC 3264, practically caller B will send the audio and
video RTP packet to caller A immediately. Since SIP messages and RTP
packets take different paths to the destination, the media data usually
arrive to caller A first before the 200OK. It's okay for audio data
because voice decoder can decode individual RTP payloads even the first
few packets are skipped (e.g. G.711, G.729). However, it's not case in
video. Take H.263 as an example. Usually the first encoded video frame
is I-frame, and the next few frames are P-frames. If the first I-frame
are skipped, caller A cannot decode the video packet properly until he
gets next I-frames. There would cause significant delays for caller A to
see the caller B. How to solve it? Thanks in advance.
Caller A Caller B
Invite w SDP(audio & video)
|----------------------------------->|
200OK w SDP(audio & video)
|<-----------------------------------|
<--B start sending audio and
video data
ACK
|----------------------------------->|
Richard Wu
ASTRI
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