Andreas Byström wrote:
> Hi all!
>
> I have a question regarding how a B2BUA should handle session timer. The
> current case is that a call between A and B goes via a B2BUA, the B2BUA uses
> session timers to be able to have somewhat secure billing,
There is no reason for a UA to *request* session timer. If UA (including
a B2BUA) wants to verify the sip signaling path periodically it is free
to do so without the use of session timer.
(OTOH, it is good for the UA/B2BUA to *support* session timer, in case
some proxy along the path needs it. But it needn't propose the use.)
> but A and B does
> not support Session Timer. So what will happen is that after time x is that
> B2BUA must refresh the session timer towards B (I consider it enough to have
> session timer on one call leg). The problem is how to handle SDP data.
If you don't use session timer, then after time x (any x you choose) you
can send any convenient message to A and/or to B. It doesn't have to be
reINVITE. For instance OPTIONS comes to mind. It will check continuity
just as well as reINVITE without the offer/answer issues.
OTOH, if you insist on using session timer...
> 1) Only reInvite and Update is allowed to be used to refresh session timers
> (as according to RFC4028), right?
Yes.
> If B2BUA would use reInvite, if it send a reInvite to B without any SDP
> data, B migth add a SDP offer in the 200 OK response
There is no *might*. B MUST include offer in the first reliable response.
> which means I have to
> have a SDP response in the ACK... and I cant really do this without
> including A... which I dont want to do (cant send the 200 OK to A to get SDP
> response since A did not trigger reInvite)
You send a reINVITE to A with the offer from B. Wait for the answer from
A and send that in the ACK to B. This is the standard 3pcc ploy. It
works fine as long as it is only *re*INVITEs and they are responded
promptly. (No alerting for the reinvite.)
> 2) Is there some way to send an reInvite, and indicate to the recipent (in
> this case B), that B should not insert a SDP offer in the 200 OK?
No. The offer is required.
> The other option would be for B2BUA to refreash the session by generating an
> Update to B when timer x triggers.
> 3) Is it allowed to send an UPDATE request without a SDP body?
Yes. This is ideal for session timers, when they are needed. But it only
works if the peer supports UPDATE.
> Since there
> is no ACK to 200 OK of a Update I guess it is not valid for B to add a SDP
> offer in 200 OK, which will prevent me from having the same problems as in
> 2)...
That is true.
> 4) What would be the expected behavior of a UA that receives an update
> without a SDP body? Just to keep the current voice/video session as usuall?
Yup.
Paul
> Thanks in advance!
> // Andreas
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