Assertion when offering SDP media with port zero but answered with port non-zero

[nanang]

When SDP media in the offer uses port zero, it means that the media is not wanted and must be answered with port zero as well (RFC 3264 Section 5.1 and 8.2). However, some implementations may disobey it (we found one!), and unfortunately it may lead to assertion in the SDP negotiation process (when there is dynamic payload type involved).

Sample SDP offer/answer that trigger assertion:

• Offer (may be after some offer/answer cycles):

  ...
  m=audio 4000 RTP/AVP 0
  m=video 0 RTP/AVP 100
  ...
  

• Answer:

  ...
  m=audio 6000 RTP/AVP 0
  m=video 7000 RTP/AVP 100
  a=rtpmap:100 H261/90000
  ...