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<div class="gmail_quote">On Mon, Jan 4, 2010 at 1:46 PM, Kevin P. Fleming <span dir="ltr"><<a href="mailto:kpfleming@digium.com">kpfleming@digium.com</a>></span> wrote:<br>
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<div class="im">hadi motamedi wrote:<br><br>> Sorry . I didn't get the point clearly . In the SIP Invite message , it<br>> says "my audio endpoint is IP x.x.x.x port x, and I can use codecs<br>> A,B,C". The remote endpoint responds with a 200 OK, saying "my audio<br>
> stream is at IP y.y.y.y port y, and I choose codec B". Can you please do<br>> me favor and let me know if my understanding is right or not ?<br>> Thank you<br><br></div>No, you are not understanding the SDP offer/answer model properly. If<br>
one endpoint offers codecs A, B and C in its SDP, it is willing to<br>*receive* media in those formats. The receiver of that offer can choose<br>to send media to the offerer in any of those formats, at any time. If<br>the answering endpoint includes only codec B in its SDP, then it is<br>
willing to *receive* only codec B. In that scenario, it is possible for<br>media to flow from endpoint 1 to endpoint 2 using codec B, and from<br>endpoint 2 to endpoint 1 using codec A (or C), but this will not happen<br>
if Asterisk is an endpoint in this scenario.<br><br>When Asterisk receives a media frame, if the format of that frame is not<br>the format that it is currently sending to the other endpoint, it will<br>switch to that format automatically. If it cannot do so because the<br>
other endpoint did not offer to receive that format, then the call's<br>audio will probably fail. This is the reason why I responded before that<br>Asterisk does not support asymmetric formats in a media session.<br><br>
In reality, it is extremely uncommon for a SIP endpoint to want to send<br>media in a format that it is not also willing to receive; in fact, I<br>can't say I've ever seen this situation arise in any testing I've done<br>
or in any issues reported in our issue tracker.<br><font color="#888888"><br>--<br></font>
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<div class="h5">Kevin P. Fleming<br>Digium, Inc. | Director of Software Technologies<br>445 Jan Davis Drive NW - Huntsville, AL 35806 - USA<br>skype: kpfleming | jabber: <a href="mailto:kpfleming@digium.com">kpfleming@digium.com</a><br>
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<div>Thank you very much for correcting me .</div>
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