Hi Richard, Why does A require the 200 OK to be able to decode the packets? The 200 OK contains information about what B wishes to receive. A should already have opened its decoders as soon as it send the INVITE outwards. It knows the PayloadTypes that B is going to send it. So it can also detect which codec is being sent.
Are you of the opinion that the 200 OK is needed to inform which codec has been selected? -Kamath -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Richard Sent: Tuesday, December 16, 2008 7:21 AM To: [email protected] Subject: [Sip-implementors] Send media in video call Vs audio call in SDPAnswer/Offer model 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This message (including any attachments) is for the named addressee(s)'s use only. It may contain sensitive, confidential, private proprietary or legally privileged information intended for a specific individual and purpose, and is protected by law. If you are not the intended recipient, please immediately delete it and all copies of it from your system, destroy any hard copies of it and notify the sender. Any use, disclosure, copying, or distribution of this message and/or any attachments is strictly prohibited. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
