On 3/22/12 11:13 PM, Alexey Entsov wrote:
> Hi Everyone,
>
> I was faced with the following question, while working on issues related to
> inconsistency of SDP "o=" line and call forking.
> E.g. say we have Alice and Bob, both are UAs.
>
> Alice initiates a call to Bob by sending of INVITE w/SDP Offer, but Alice
> doesn't support UPDATE&PRACK (doesn't matter why).
> Bob would like to provide a kind of early media, so it sends media
> capabilities with forked 18x (180/183) responses.
>
> So, we have the following call flow:
>
> Alice Bob
> INVITE w/SDP0
> ---------------------->
> ....
>
> 180 w/SDP1, to-tag=1
> <----------------------
> 183 w/SDP2, to-tag=2
> <----------------------
> ....
> 200 OK w/SDPn, to-tag=n
> <----------------------
>
> So, here we have multiple SDP Answers sent within different SIP dialog.
> The question is: What should be the "o=" line of each SDP Answer? Should it
> be unique for each dialog?
It doesn't really matter, because the two dialogs are independent. The
SDPs of the two dialogs should never be compared to one another. It
could be that they are sent by independent entities that don't
coordinate in any way, so there can be no requirements on how they
relate to one another.
Thanks,
Paul
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