Re: [Sipping] Current status of response to 3gpp Liaison Statement on offer/answer procedures

[Paul Kyzivat <[e-mail]>]

Rockson Li (zhengyli) wrote:
>  Paul,
> 
> Comments inline.
> 
> Thanks
> Regards,
> -Rockson
> 
> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]] On Behalf Of Paul Kyzivat (pkyzivat)
> Sent: Thursday, July 03, 2008 10:57 PM
> To: sipping
> Cc: Mary Barnes; Gonzalo Camarillo
> Subject: Re: [Sipping] Current status of response to 3gpp Liaison Statement on offer/answer procedures
> 
> I have been thinking about how we might solve the UPDATE ambiguity issue which the MULTI-OA-TRANSACTION has. I have a potential solution, which involves the following new restrictions:
> 
> 1) A UAS for a reINVITE MUST NOT send UPDATE requests
>     within the scope of that INVITE. It must refrain from sending
>     UPDATE until it has received an ACK for the INVITE.
> 
>     Note that this isn't much of a restriction, since the same things
>     can be accomplished with reliable provisional responses before
>     the INVITE completes, and reINVITE can be used after sending a
>     final response. The only limitation I can see is if reliable
>     provisionals are not being used and yet a change is desired
>     before completion of the INVITE. But I doubt that is a realistic
>     case.
> 
> 2) A UAC for an INVITE or reINVITE MUST NOT send an UPDATE request
>     immediately *after* the completion of the INVITE. It must refrain
>     until the timer has expired on the ACK. (I forget which timer that
>     is.)
> 
>     This also isn't much of a restriction. Anything that can be done
>     by the UPDATE can also be done with a reINVITE.
> 
> With these restrictions, the recipient of an UPDATE never has any question of whether it should be part of the prior INVITE or not. To be sure, lets cover the cases:
> 
> UAC (for the INVITE):
> 
> - An UPDATE that was legally sent by the UAS will arrive after the
>    final response for the INVITE is received and the ACK sent.
>    It will be unaffected by failure of the prior reINVITE.
> 
> - There is no possibility that a legally sent UPDATE will arrive
>    before the final response. If one arrives it must have been sent
>    by a UAS not compliant to these new rules. If one does arrive,
>    I propose that it be assumed to have been sent within the
>    INVITE, and hence be rolled back if the INVITE eventually fails.
> [Rockson] if UAS sends UPDATE after final response, but first final response is lost, 
> So UPDATE will arrives to UAC before retransmission of final response. Roll back this UPDATE??

My proposal was that the UAS may not send an update until it has 
received the ACK from the invite. The ACK won't be sent until the final 
response is received. So this case doesn't happen.

> - There is no possibility that a legally sent UPDATE will arrive
>    after the receipt of a failing final response, and before any
>    ACK has been sent. If one arrives it must have been sent
>    by a UAS not compliant to these new rules. If one does arrive,
>    I propose that it be assumed to have been sent after the
>    final response, and hence not be subject to rollback.
> [Rockson] Same issue here, if UPDATE is sent before final response, but it's lost in the middle, retransmission of 
> UPDATE arrives to UAC after final response, not rollback??

Same answer.

> UAS (for the INVITE):
> 
> - An UPDATE that is received before the final response to the
>    INVITE has been sent is assumed to belong within the INVITE.
>    If the final response is a failure, then any o/a effects of
>    the UPDATE will be rolled back.
> [Rockson] Cseq could be used to identify if UPDATE is sent before or after INIVITE, looks no request-lost-in-the-middle issue here.

Hmm. I didn't discuss the case of an UDATE being sent, and then a 
reINVITE. You are right that CSeq can be used to verify the order of 
those, but it may not be needed.

Consider the cases:

- UPDATE w/offer, followed by reINVITE w/offer.

   This is clearly invalid, since the earlier offer hasn't been answered.

- UPDATE w/offer, followed by offerless reINVITE:

   Seems like a stupid thing to do. But if the messages arrive in order
   it should work without any difficulty.

   If the messages arrive out of order, then the UPDATE will get
   a 500 response because of the CSeq. Nothing special required for
   that to happen.

Its at the other end of the reINVITE that there is a problem, and that 
is covered by the following:

> - An UPDATE that is received after the final response to the
>    INVITE has been sent, but before the ACK has been received,
>    is assumed to have been sent before the final response was
>    received. Hence it is subject to rollback if the final response
>    was failure. Since it hasn't yet been processed, and is to be
>    rolled back, the response to it should be an error - perhaps
>    487.
> 
> [Rockson] I do not understand this case, why UPDATE should be assumed being sent before the final response was received?

The assumption is in accord with the proposal I made initially.

> The UPDATE could be sent before or after ACK is sent , but all after final response is received, ACK might be lost in the middle.
> You cannot tell if UPDATE is sent before or after ACK , Cseq does not work here.

The restriction on when UPDATE can be used to precisely to deal with this.

If the UAC doesn't adhere to this new restriction, then the problem 
could occur. The "assumption" in that case may be wrong. If so, things 
will be messed up. That is just the recognition of there being a bug in 
the spec.

	Thanks,
	Paul

> I think the above will resolve the issue and be interoperable with current implementations except in cases of message reordering. I doubt we can do any better than that.
> 
> 	Thanks,
> 	Paul
> 
> Paul Kyzivat wrote:
>> Some time ago 3gpp requested liaison regarding offer/answer procedures. 
>> The liaison document may be found at:
>>
>> https://datatracker.ietf.org/liaison/444/
>>
>> Information about the discussion can be found at:
>>
>> http://www.ietf.org/mail-archive/web/sipping/current/msg15771.html
>>
>> Some of us (especially Christer and I) have been discussing this 
>> privately. Mary has asked for a clarification of the current status to 
>> the group. This is my attempt to do so:
>>
>> To summarize the issue:
>>
>> - Assume one issues a re-INVITE,
>> - and that results in multiple offer/answer exchanges
>>   (via PRACK and UPDATE) prior to the completion of
>>   the re-INVITE,
>> - and then the re-INVITE *fails* (response >= 300)
>>
>> Then in what state is the session left, with regard to SDP and media 
>> sessions?
>>
>> None of the RFCs clearly cover this case. The offer/answer draft 
>> touched on it, but is not normative and so could not resolve it.
>>
>> We have concluded that there are two plausible ways of treating this:
>>
>> MULTI-OA-TRANSACTION:
>>    The re-INVITE, and all the offers/answers that take place within
>>    its scope, are treated as a transaction. All succeed or fail
>>    together based on the outcome of the re-INVITE. So, if the
>>    re-INVITE fails, then the media state reverts to what it had been
>>    before the re-INVITE began.
>>
>> SINGLE-OA-TRANSACTION:
>>    Each time an answer is transmitted *reliably*, that is considered
>>    final, regardless of what happens subsequently. A failure of the
>>    re-INVITE only rolls back an offer that offer that was not reliably
>>    answered prior to the failure response.
>>
>> The merits of MULTI-OA-TRANSACTION:
>>
>> The advantage of the MULTI-OA-TRANSACTION approach is that it aligns 
>> with a real need. In some cases it is necessary to do multiple o/a 
>> exchanges to transition from one stable state to another.
>>
>> A clear example of this is when preconditions are used. Multiple 
>> exchanges are required to resolve the preconditions, and the 
>> intermediate states may not be useful for exchanging media. The 
>> ultimate failure is likely an indication that the preconditions could 
>> not be resolved. Rolling back to the state prior to the re-INVITE 
>> cleanly resolves this.
>>
>> A key to making this work is, when a re-INVITE failure occurs, the UAC 
>> and UAS must agree on on which offers and answers were part of the 
>> re-INVITE and hence must be rolled back. Those carried in the 
>> re-INVITE itself, its responses, in PRACKs, and in the ACK, are 
>> clearly within the scope of the re-INVITE. The UPDATEs that are sent 
>> within the scope of the re-INVITE also must be included, but in that 
>> case there is a problem. When an UPDATE is sent near the time when the 
>> re-INVITE fails, the recipient of it cannot clearly determine if it 
>> was sent before or after the re-INVITE failed. This case is discussed in section xxx of yyy.
>>
>> Adopting answer (1) requires that we find a resolution to this 
>> ambiguity. The need to solve this problem is a disadvantage of 
>> MULTI-OA-TRANSACTIONs.
>>
>> Another possible disadvantage is that this requires the UAC and UAS to 
>> maintain enough state to accomplish the rollback.
>>
>> The merits of SINGLE-OA-TRANSACTION:
>>
>> These are, unsurprisingly, pretty much the inverse of 
>> MULTI-OA-TRANSACTIONs.
>>
>> One advantage is that less state need be kept. Once an answer is 
>> received reliably, or the confirmation of an answer sent reliably is 
>> received, prior state may be discarded.
>>
>> Another advantage is that the ordering of an UPDATE relative to the 
>> completion of the prior re-INVITE need not be of concern.
>>
>> The main disadvantage of this approach arises when multiple o/a 
>> exchanges are required to achieve a stable state, such as with 
>> preconditions. With this approach, each o/a exchange is locked in as 
>> it occurs. If the re-INVITE subsequently fails, there may be wreckage 
>> to clean up. Until it is cleaned up, the state of the media session(s) 
>> may be problematic.
>>
>> General discussion:
>>
>> While I have used preconditions as an example of the need for multiple 
>> o/a exchanges, they are not the only example. While I don't recall 
>> seeing them in any of our use-case documents, I have definitely seem 
>> them in the wild. For instance there are cases where initial offers 
>> are made with a=inactive, and later revised to a=sendrecv, not because 
>> the call is initially on hold, but because the caller is waiting to 
>> see how things come out. This may be "poor man's preconditions". These 
>> aren't always done within the re-INVITE, but could be.
>>
>> Either approach will require some normative change, since the existing 
>> text seems ambiguous as to which of these is the "correct"
>> interpretation. The MULTI-OA-TRANSACTION requires additional work to 
>> define a mechanism for determining of an UPDATE near the end of an 
>> INVITE transaction falls within it, or beyond it. So far there has 
>> been no proposal for how to do this. It seems likely that it will 
>> require that something new be placed into some messages. And this may 
>> present backward compatibility issues.
>>
>> Many UAs will never experience a re-INVITE containing multiple O/A 
>> exchanges. But even those are impacted by this issue. If a re-INVITE 
>> has an offer, and it is answered in a reliable provisional response, 
>> and then the re-INVITE fails, we still have the issue. If one side 
>> assumes the O/A is rolled back, and the other assumes it remains in 
>> effect, then we have an interoperability error. So it is important to 
>> come to some conclusion.
>>
>> NOTE: There is a related issue which we have agreed to rule out of 
>> scope for the current discussion. This is whether a change of Contact 
>> address during a re-INVITE is rolled back if the re-INVITE fails. We 
>> concluded that the two issues should not be constrained to have the same answer.
>> This latter issue is left for another day.
>>
>>     Thanks,
>>     Paul
>>
>> Paul Kyzivat wrote:
>>>
>>> Robert Sparks wrote:
>>>> (off-list reply)
>>>>
>>>> I'm mostly comfortable with that approach. Let me ask a question or 
>>>> two to see if I can remove some of the dangling bits of discomfort.
>>>> The conversation so far has been described to me (I haven't been 
>>>> following it closely - sorry)  as  focusing _only_ on the impact on 
>>>> the session(s) being negotiated.
>>>> It is _not_ attempting to answer some of the gnarly dialog-state 
>>>> questions we've uncovered for these failed requests (specifically 
>>>> what happens to a failed attempt to update a remote target (by 
>>>> changing the Contact in new requests), correct?
>>> Early in the thread I proposed separating the concerns. The remainder 
>>> of the thread had indeed focused on the o/a issues.
>>>
>>> I do think that the other dialog state, notably the contact, issues 
>>> need to be addressed. But I think we must not constrain them to have 
>>> the same answer that works for o/a. We can start a separate thread to 
>>> discuss that now, or we can wait for the current o/a discussion to 
>>> settle first to avoid losing focus.
>>>
>>>> If I misunderstand and the second thing's in scope for this effort, 
>>>> then my comfort is much lower.
>>>>
>>>> Paul - you responded separately that you think this touches 3261 as 
>>>> well - roughly what is the character of those touches?
>>> Hopefully it touches only slightly. The current text regarding 
>>> rolling back state to where it was prior to reinvite *may* need some 
>>> tweaking depending on what solution we come to.
>>>
>>> *If* we agree on the solution that does indeed cause rollback even if 
>>> there have been PRACKs and/or UPDATEs along the way, then maybe it 
>>> won't need to be changed at all.
>>>
>>> I am personally still undecided on which is the better solution. They 
>>> have complementary pros and cons. It really is a matter of picking 
>>> your poison. The precondition stuff really does cause nasty problems.
>>> I just posted another response in the thread about that.
>>>
>>> I earlier suggested splitting off the precondition issues as their 
>>> own problem, and solving the rest. But apparently 3gpp wants this 
>>> resolved precisely *because* they want to know how it impacts 
>>> preconditions. So my suggestion wasn't helpful.
>>>
>>> It would be helpful to get some additional perspectives into this 
>>> discussion. So far there have been very few participants.
>>>
>>>     Paul
>>>
>>>> RjS
>>>>
>>>> On Jun 9, 2008, at 5:18 AM, Gonzalo Camarillo wrote:
>>>>
>>>>> Hi Paul,
>>>>>
>>>>> yes, it may make more sense to update RFCs 3262 and 3311 than to 
>>>>> update RFC 3264... do people agree that the way to document the 
>>>>> resolution of this issue would be to write a new RFC that would 
>>>>> clarify how offer/answer works with re-INVITEs, PRACKs, and 
>>>>> UPDATEs, and would include discussions on preconditions?
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Gonzalo
>>>>>
>>>>>
>>>>> Paul Kyzivat wrote:
>>>>>> Gonzalo,
>>>>>>
>>>>>> I generally agree with your characterization below. But as I see 
>>>>>> it there likely are no changes needed to 3264. It is intentionally 
>>>>>> focused on the SDP, and not the conveyance of the SDP in some 
>>>>>> containing protocol. The following is about the extent of it in 
>>>>>> 3264:
>>>>>>
>>>>>>   Protocol operation begins when one agent sends an initial offer to
>>>>>>   another agent.  An offer is initial if it is outside of any context
>>>>>>   that may have already been established through the higher layer
>>>>>>   protocol.  It is assumed that the higher layer protocol provides
>>>>>>   maintenance of some kind of context which allows the various SDP
>>>>>>   exchanges to be associated together.
>>>>>>
>>>>>>   The agent receiving the offer MAY generate an answer, or it MAY
>>>>>>   reject the offer.  The means for rejecting an offer are dependent on
>>>>>>   the higher layer protocol.  The offer/answer exchange is atomic; if
>>>>>>   the answer is rejected, the session reverts to the state prior 
>>>>>> to the
>>>>>>   offer (which may be absence of a session).
>>>>>>
>>>>>> SIP messed this up somewhat with the offerless-invite, and more 
>>>>>> when it introduced PRACK and UPDATE. The offerless-invite creates 
>>>>>> a case when it is impossible to reject an offer. But we aren't 
>>>>>> discussing that case here. Without PRACK and UPDATE, and with an 
>>>>>> offer in the INVITE, it the success or failure of the INVITE that 
>>>>>> determines the acceptance or rejection of the offer. (With an 
>>>>>> offerless invite, the ACK always accepts the offer, for better or 
>>>>>> worse.)
>>>>>>
>>>>>> The use of PRACK and UPDATE while an INVITE transaction is is 
>>>>>> progress creates an ambiguous situation due to the following from 
>>>>>> section
>>>>>> 14.1 of
>>>>>> 3261:
>>>>>>
>>>>>>   If a UA receives a non-2xx final response to a re-INVITE, the 
>>>>>> session
>>>>>>   parameters MUST remain unchanged, as if no re-INVITE had been 
>>>>>> issued.
>>>>>>
>>>>>> This implies that changes made via PRACK and UPDATE during the 
>>>>>> INVITE transaction must be rolled back. Since the problem created 
>>>>>> by 3262 and 3311, in conjunction with 3261, I think the fixes will 
>>>>>> have to apply to those, not to 3264.
>>>>>>
>>>>>> Also, the issue about changing Contact addresses clearly has 
>>>>>> nothing to do with 3264. And I am becoming increasingly convinced 
>>>>>> that the rules for "committing" a change of Contact address ought 
>>>>>> to be decoupled from the rules for "committing" a change to media 
>>>>>> sessions.
>>>>>>
>>>>>> Before we get into the specifics, does the above make sense?
>>>>>>
>>>>>>    Thanks,
>>>>>>    Paul
>>>>>>
>>>>>>
>>>>>>
>>>>>> Gonzalo Camarillo wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> we should be providing 3GPP with an answer to their liaison soon:
>>>>>>>
>>>>>>> https://datatracker.ietf.org/liaison/444/
>>>>>>>
>>>>>>> The thing is that when working on the offer/answer usage draft 
>>>>>>> below, we kept from making normative changes to offer/answer:
>>>>>>>
>>>>>>> http://www.ietf.org/internet-drafts/draft-ietf-sipping-sip-offera
>>>>>>> nswer-08.txt
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> However, it seems that there are a few cases that would require 
>>>>>>> normative updates to RFC 3264. In this thread, two cases have 
>>>>>>> been
>>>>>>> identified: roll back and address changes during ongoing 
>>>>>>> transactions.
>>>>>>> I would like to see a list of such pending updates in order to 
>>>>>>> decide whether we need to revise RFC 3264 at this point or 
>>>>>>> document the current issues (like we are doing with RFC 3261) for a future update.
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Gonzalo
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Christer Holmberg wrote:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> I do NOT think John's case is connected to the rollback issue.
>>>>>>>>
>>>>>>>> The rollback issue is: what happens to data that has been 
>>>>>>>> updated between the re-INVITE request and failure response? It 
>>>>>>>> of course included the target, but is not related to where responses are sent.
>>>>>>>>
>>>>>>>> Responses are, afaik, always sent to where the request came 
>>>>>>>> from, so if one updates the local target he has to make sure 
>>>>>>>> that he listens to the "old" port if there are ongoing transactions.
>>>>>>>>
>>>>>>>> Regards,
>>>>>>>>
>>>>>>>> Christer
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> ________________________________
>>>>>>>>
>>>>>>>> Lähettäjä: Paul Kyzivat [mailto:[e-mail]]
>>>>>>>> Lähetetty: pe 16.5.2008 14:38
>>>>>>>> Vastaanottaja: Elwell, John
>>>>>>>> Kopio: Christer Holmberg; sipping List
>>>>>>>> Aihe: Re: [Sipping] Liaison Statement on offer/answer procedures
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> John,
>>>>>>>>
>>>>>>>> This is a good point.
>>>>>>>>
>>>>>>>> It does expose a potentially long window when address changes 
>>>>>>>> are problematic. I guess if a quick address change is necessary 
>>>>>>>> then the INVITE, or reINVITE, can be CANCELed.
>>>>>>>>
>>>>>>>> IMO this is starting to identify an area that could stand to 
>>>>>>>> have more specification. I guess this sounds like a best 
>>>>>>>> practices draft, but its still a little fuzzy to me. And I am 
>>>>>>>> far from clear whether this is tightly connected to the o/a 
>>>>>>>> rollback issue.
>>>>>>>>
>>>>>>>>        Thanks,
>>>>>>>>        Paul
>>>>>>>>
>>>>>>>> Elwell, John wrote:
>>>>>>>>> Paul,
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> -----Original Message-----
>>>>>>>>>> From: [e-mail] 
>>>>>>>>>> [mailto:[e-mail]] On Behalf Of Paul Kyzivat
>>>>>>>>>> Sent: 15 May 2008 14:48
>>>>>>>>>> To: Christer Holmberg
>>>>>>>>>> Cc: sipping List
>>>>>>>>>> Subject: Re: [Sipping] Liaison Statement on offer/answer 
>>>>>>>>>> procedures
>>>>>>>>>>
>>>>>>>>>> Christer,
>>>>>>>>>>
>>>>>>>>>> Saying "you shouldn't do it" to changing contact address or 
>>>>>>>>>> media address ignores facts of life that may require doing it. 
>>>>>>>>>> This overlaps strongly with the session mobility discussion 
>>>>>>>>>> that is currently going on.
>>>>>>>>>>
>>>>>>>>>> Specifically, if a UA is losing possession of its address, or 
>>>>>>>>>> connectivity via that address, then it will have to do 
>>>>>>>>>> *something*. If we are going to say that you shouldn't change 
>>>>>>>>>> the contact address in a dialog, and shouldn't change the 
>>>>>>>>>> media address in a media session, then we need to specify some 
>>>>>>>>>> alternative.
>>>>>>>>>>
>>>>>>>>>> Clearly there are at least two distinct cases here:
>>>>>>>>>>
>>>>>>>>>> - there is a desire to switch to a new address, but the old 
>>>>>>>>>> address
>>>>>>>>>>   can continue to be supported until and unless use of the new one
>>>>>>>>>>   can be established
>>>>>>>>> [JRE] So if the contact address changes and we successfully 
>>>>>>>>> conclude the UPDATE transaction, and then the old contact 
>>>>>>>>> address disappears, it is likely that the Via list on the 
>>>>>>>>> re-INVITE request will have become invalidated too, so the 
>>>>>>>>> final response will not reach the UAC.
>>>>>>>>> Correct?
>>>>>>>>>
>>>>>>>>> John
>>>>>>>>> _______________________________________________
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>>>>>>>>>
>>>>>>>>
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>>>>>>>
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