[SIPForum-discussion] Re: Query regarding offer-answewr

setf2006 at yahoo.co.jp setf2006 at yahoo.co.jp
Fri Jul 13 06:45:22 UTC 2007

  >Suppose A party sends offer with 3 'm' line. then my question is that 
>1. B party can send 2 out of the 3 received 'm' line or not.
  >2. if yes ,then which wud be used for opening RTP connection. 
  The SDP answer from B party must include three "m=" lines, corresponding
  ones in the SDP offer received from A party. 
  (RFC3264 P.9)
     For each "m=" line in the offer, there MUST be a corresponding "m="
   line in the answer.  The answer MUST contain exactly the same number
   of "m=" lines as the offer.  This allows for streams to be matched up
   based on their order.  This implies that if the offer contained zero
   "m=" lines, the answer MUST contain zero "m=" lines.

  If B party want to reject one stream out of three, 
  the port number in the corresponding "m=" line in the answer
  must be set to zero.
  (RFC3264 P.9)
     An offered stream MAY be rejected in the answer, for any reason.  If
   a stream is rejected, the offerer and answerer MUST NOT generate
   media (or RTCP packets) for that stream.  To reject an offered
   stream, the port number in the corresponding stream in the answer
   MUST be set to zero.  Any media formats listed are ignored.  At least
   one MUST be present, as specified by SDP.

  An example.
   o=alice 2890844526 2890844526 IN IP4 host.anywhere.com
   c=IN IP4 host.anywhere.com
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   m=video 51372 RTP/AVP 31
   a=rtpmap:31 H261/90000
   m=video 53000 RTP/AVP 32
   a=rtpmap:32 MPV/90000

   o=bob 2890844730 2890844730 IN IP4 host.example.com
   c=IN IP4 host.example.com
   t=0 0
   m=audio 49920 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   m=video 0 RTP/AVP 31
   m=video 53000 RTP/AVP 32
   a=rtpmap:32 MPV/90000


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