Re: [Sip] Issue: Expiration of temp-gruu

[Miguel Garcia <[e-mail]>]

For issue 3, I would say that the solution is to avoid the situation, 
e.g., by registering way in advance before the registration expires. 
Doesn't RFC 3261 recommends to re-register half time before the 
registration timer expires.

Issues 4, 5, and 6 are solved by the reg event. So, shouldn't the GRUU 
draft have a recommendation on the usage of the reg event to solve them?

/Miguel

Paul Kyzivat wrote:
> After some study, I have encountered a potential difficulty in 
> determining when temp-gruus expire. After discussing this with Jonathan 
> we decided to bring it to the list for discussion.
> 
> As now defined, when you first register you may get a temp-gruu, which 
> will remain valid as long as the registration does not expire or be 
> removed. (Removal is a special case of expiration.) Each time the UA 
> refreshes the registration it may get a new temp-gruu which also remains 
> valid for the life of the registration. The UA can accumulate all these 
> temp-gruus and use them as it sees fit until they expire.
> 
> The problem is: how does the UA know when the registration has ended and 
> the temp-gruus associated with it rendered invalid?
> 
> This seems like a simple question, and in some cases it is simple, but 
> not in all cases. The following are at least some of the ways that the 
> registration can be ended, causing the invalidation of temp-gruus:
> 1) the UA that originally registered may explicitly deregister
> 2) the UA the originally registered may simply permit the
>    registration to expire without attempting to refresh it
> 3) the UA may attempt to refresh the registration, but be too slow,
>    so that the reregistration arrives after the prior one expires
> 4) some other UA may deregister all the registrations, including
>    the one by this UA. (When the UA decides it should refresh the
>    registration it will actually create a new one.)
> 5) the registrar may remove the registration administratively
> 6) the registrar may crash and restart, losing all its registrations
> 
> In each of these cases the previously assigned temp-gruus become 
> invalid. If the UA continues to use them, bad things will happen. This 
> is true both for calls that are active when the deregistration occurs, 
> and calls that are established later. What kinds of bad things happen?
> - requests addressed to the gruu, both in-dialog and out-of-dialog
>   will be undeliverable. (For instance a reINVITE or BYE in an
>   existing call, or an out-of-dialog INVITE/Replaces for a transfer.)
> 
> - request originated by the UA using the invalid gruu will fail
>   immediately because the gruu is invalid.
> 
> - if the UA subsequently reregisters, receives an incoming call, and
>   uses one of the old temp-gruus as its contact, then the call will
>   be established, but subsequent in- and out-of-dialog requests to
>   the contact will fail.
> 
> So clearly it is important for the UA to stop using temp-gruus that have 
> become invalid. How can it know to do that in all of the above cases?
> 
> 1) is straightforward. The UA knows it is deregistering, and should
>    discard any cached temp-gruus. If it has a dialog with one of the
>    temp-gruus as a contact, then it should do a target refresh and
>    supply some working contact. (It really should do this *before*
>    deregistering, and for a pub-gruu as well.)
> 
> 2) the UA should take all the actions mentioned for (1) at or before
>    the expiration time. This should be done a bit early to guard
>    against clock skew problems.
> 
> 3) in this case, it may be impossible for the UA to know whether
>    the REGISTER arrived in time to be a reREGISTER or if it arrived
>    late and was treated as a new REGISTER after the expiration of
>    the old one. (The response to the REGISTER includes nothing that
>    distinguishes the two cases.) About the best that can be done in
>    this case is to check the time at which the response to the
>    REGISTER is received. If it falls after the expected expiration
>    time for the registration, act as if the old registration had
>    expired.
> 
> 4) This is a major problem. Of course, regardless of GRUU, the UA
>    won't be able to receive incoming calls until it decides it is
>    time to refresh the registration. In addition, until the
>    registration reestablished, calls originated with one of its
>    existing gruus will fail. Once the registration is reestablished,
>    it still won't know its old temp-gruus are invalid. The only
>    remedy for this is to subscribe to the "reg" event package,
>    and use the resulting notifications to identify when it has
>    been unregistered. If it does that, it can invalidate all its
>    old temp-gruus, and then presumably reregister.
> 
> 5) this is functionally equivalent to (4), except that perhaps
>    attempts to reregister may fail.
> 
> 6) this is also functionally equivalent to (4).
> 
> The "reg" event does provide a fairly complete solution to this problem, 
> for those that are willing and able to use it. It is not however a 100% 
> reliable solution. Consider case (3), where the UA has a reg event 
> subscription. All is well if the registration expires and a notification 
> is sent indicating that, and then the reregistration occurs and a 
> notification is sent about that. But if they happen very closely in 
> time, it may be that only one notification is sent, reflecting the 
> result of the reregistration. In that case the UA will not be aware that 
> it had been unregistered for a period of time.
> 
> A *complete* solution would, IMO, involve changing REGISTER so that it 
> returns an indication of whether this was a new registration, or a 
> refresh. This could take many forms. For instance the timestamp of the 
> initial registration could always be returned with each contact, as well 
> as the expiration time.
> 
> The big question is whether this is a big enough issue to require a 
> solution now, as part of GRUU, or if it can be ignored as insignificant 
> or postponed to future work.
> 
> I'd appreciate hearing what others think about this. At the moment I am 
> leaning toward doing nothing, at least for now.
> 
>     Thanks,
>     Paul
> 
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-- 
Miguel A. Garcia           tel:[phone]
sip:[e-mail]
Nokia Research Center      Helsinki, Finland

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