4 jan 2010 kl. 14.46 skrev Kevin P. Fleming: > hadi motamedi wrote: > >> Sorry . I didn't get the point clearly . In the SIP Invite message , it >> says "my audio endpoint is IP x.x.x.x port x, and I can use codecs >> A,B,C". The remote endpoint responds with a 200 OK, saying "my audio >> stream is at IP y.y.y.y port y, and I choose codec B". Can you please do >> me favor and let me know if my understanding is right or not ? >> Thank you > > No, you are not understanding the SDP offer/answer model properly. If > one endpoint offers codecs A, B and C in its SDP, it is willing to > *receive* media in those formats. The receiver of that offer can choose > to send media to the offerer in any of those formats, at any time. If > the answering endpoint includes only codec B in its SDP, then it is > willing to *receive* only codec B. In that scenario, it is possible for > media to flow from endpoint 1 to endpoint 2 using codec B, and from > endpoint 2 to endpoint 1 using codec A (or C), but this will not happen > if Asterisk is an endpoint in this scenario. > > When Asterisk receives a media frame, if the format of that frame is not > the format that it is currently sending to the other endpoint, it will > switch to that format automatically. If it cannot do so because the > other endpoint did not offer to receive that format, then the call's > audio will probably fail. This is the reason why I responded before that > Asterisk does not support asymmetric formats in a media session. > > In reality, it is extremely uncommon for a SIP endpoint to want to send > media in a format that it is not also willing to receive; in fact, I > can't say I've ever seen this situation arise in any testing I've done > or in any issues reported in our issue tracker.
But it's fairly common to have asymmetric media in the call. If the caller offers A, B and C and the callee responds with B, the caller sends B but the callee might send A. /O _______________________________________________ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
