First, IMO it is entirely valid to send an offer where all media lines
have the port set to zero. It is however somewhat of a strain to find a
good reason why someone would do this. (But I expect we can come up with
something.)
Its also quite ok to send an answer with all the ports set to zero, and
if the offer had them that way then the answer must.
While an answer may want to refuse an INVITE if all media have been
rejected, I don't believe it is required to do so. Its quite all right
to have a call with no media. I don't see why it should matter how you
get into that state:
- an initial offer with no m-lines
- because the answerer rejected all the m-lines in the offer
- because the offerer started out with port=0 in all the
m-lines of the initial offer
- because the offerer in a re-INVITE sets the port=0 in
all the m-lines.
Some of these may make more sense than others, but they all seem ok, and
likely to eventually find a use.
Retesh Chadha wrote:
> RFC doesnt put any restriction regarding this.
>
> It doesnt make much sense apart from the following scenario i can think of.
While the following is possible, it might make more sense to achieve the
intended results by using a non-zero port number and c=0.0.0.0 (a "black
holed" media address.)
Paul
> Say, UAC just wants to check if the UAS is available or not, and then
> when UAS responds with 200 ok, UAC negotiates the capabilities; ie UAC
> doesnt want to negotiate media unless it knows for sure UAS is
> available. Here we may think that INVITE without sdp may do the same
> but not actually because, then UAS may send sdp in 200 ok which will
> force UAC to negotiate.
>
> Dont know if the above is a practical scenario.
>
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