Dale Worley wrote:
On Mon, 2009-03-23 at 08:18 -0700, Brett Tate wrote:
Everyone: Is REFER within dialog part of INVITE usage, SUBSCRIPTION
usage, or neither?
I'm asking mainly because of REFER 481 impacts; however the answer
also has potential implications concerning draft-ietf-sip-info-events.
RFC 5057 appears to imply that it is part of the subscription usage;
however I'm not positive if my interpretation and/or RFC 5057 is
correct.
If REFER isn't considered part of the INVITE usage, how should REFER
481 be interpreted? More specifically, should receiver of REFER 481
send BYE?
I would argue that the REFER is part of the INVITE usage, because REFER
in that context is intimately related to the INVITE usage -- it is
intended to transfer the INVITE usage/dialog transferred to a different
destination.
Well, I guess there is that implied association. (Part of the "do what
*I* had in mind" approach that REFER has.) Since this was never spelled
out, it could also be simply "while this isn't part of an INVITE usage,
if there happens to be an INVITE usage, please refer that one."
I guess the question is: would it be valid to send a REFER within an
existing dialog that had no INVITE usage? If it were valid, presumably
it would have the same meaning as sending one outside a dialog, except
that the resulting subscription would share the dialog. I expect that is
a usage it would be difficult to find in the wild.
Ah, but now I want to reconsider. Remember that REFER can take a method
name. So, rather than referring an INVITE, I could be referring MESSAGE,
OPTIONS, BYE, or maybe (????) SUBSCRIBE. In some of those cases, the
association to the INVITE usage is probably meaningless. (Of course its
meaningful for BYE.)
And it seems quite safe that if the REFER receives a 481 response, the
INVITE usage must not be functional any more, since the far end of that
usage has denied that it can act on the REFER.
IIRC, 5057 decided that each dialog usage must get its own 481 before it
goes away. I don't think there are any cases where a single 481 response
can make multiple usages go away.
So the important question is whether the REFER is part of the INVITE
usage and so its 481 takes down the INVITE usage or not. That gets
especially interesting if the dialog already had both an INVITE usage
and a SUBSCRIBE usage. Does it make sense that the refer take down the
INVITE usage and leave up the SUBSCRIBE usage?
Now it's possible that the REFER, if it is successful, is the first
transaction of the subscription usage. But I don't think that has any
paradoxical consequences, as by hypothesis, the REFER succeeded.
Yeah, its already so weird, why not a little more weird?
Also, I think it has recently been clarified that the 200 of a SUBSCRIBE
should not be considered to create a dialog - that its the NOTIFY that
creates the dialog. If so, then I suppose the same should be true of REFER.
Bottom line: I can live with it either way, but think it should be
documented one way or the other.
Thanks,
Paul
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