chozhan A wrote:
> Hi,
> So i think i can assume that when a useragent receives a offer with
> port set to 0 in all the m= lines they may not generate a bye but try
> re-notiating the media and continue with the call.
>
> I have another question related to this, will the answerer generate a
> BYE on receiving a offer with the ip in c= line set to 0 or can they again
> re-negotiate and continue the call.
It would be *very* unwise to reject such a call. THis is a form of being
called "on hold". The preferred way is with a valid port and a=sendonly
or a=inactive. But the use of c=0 is also allowed for this, and in some
cases its the only practical way to get started.
In this case the callee should just proceed without media, in the
expectation that something will eventually be changed. If the user
doesn't like it, he can hang up.
One place this can arise is with a 3pcc call controller trying to place
a call between two parties. It has to call one of them first. A typical
way is to start with an offerless invite, but for some scenarios that
doesn't work well. If the call controller feels a need to make an offer,
but has no media termination resources of its own, then using c=0 is
about all it can do. In this case it will be using the answer to make a
call to the other party, and shortly there will be a reinvite with
working media.
Paul
> Thanks in advance.
> AC
>
> Paul Kyzivat <[EMAIL PROTECTED]> wrote:
> 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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