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

[Robert Sparks <[e-mail]>]

One comment inline:

On Jul 25, 2008, at 3:58 PM, Paul Kyzivat wrote:

> Copying Robert. I think his input on this would be invaluable.
>
> This is exactly the sort of response I was hoping to get to my  
> proposal. It really needs to be carefully vetted.
>
> Comments inline.
>
> 	Paul
>
> Ian Elz wrote:
>> Paul,
>> I have two potential issues with the rules you have mentioned below.
>> Sorry for the delay in replying to your email but I have taken some  
>> vacation and one of the possible issues has just been noticed.
>> 1. Regarding rule 2. I am assuming that in this rule the UAC not  
>> sending an UPDATE also applies after any retransmission of the ACK  
>> due to receipt of a 200 OK retransmission. Perhaps this should be  
>> explicitly stated.
>
> What I had in mind was that UPDATE couldn't be used until all  
> retransmissions of the ACK has completed. So it would  be 32 seconds.
>
> Since it is quite possible that there will be need of a new O/A  
> within that period (e.g. because the UAC goes on hold quickly), the  
> UAC will have to be prepared to use alternative means (reINVITE).  
> The net effect is probably that the UAC will *never* use UPDATE for  
> O/A outside of an invite transaction.
>
>> I will preface by stating that this is an unlikely scenario but it  
>> can and will occur. The UAC is not allowed to send an UPDATE until  
>> after sending an ACK until the timer has expired. In the UAS the  
>> timer for receipt of an ACK is based upon T1 with the 200 OK being  
>> resent at 2 * T1, 4 * T1, 8 * T1 etc until 64 * T1. With the  
>> recommended value of T1 (0.5 s) then the timer is 32 s.
>> The setting of T1 however is only recommended and the value of T1  
>> on the UAS may be different than as set on the UAC. If the UAS has  
>> its T1 set 4 X or greater than the T1 of the UAC then there may be  
>> an issue.
>> I will explain by showing the 200 OK retransmission times from the  
>> UAS.
>> T1 value                   0.5        1          2         4
>> 1st Retransmission          1         2          4         8
>> 2nd retransmission          2         4          8        16
>> 3rd retransmission          4         8         16        32
>> 4th retransmission          8        16         32        64
>> 5th retransmission         16        32         64       128 If the  
>> UAC has T1 set to 0.5 seconds then there are retransmission  
>> intervals which are equal to or greater than the ACK timer when the  
>> UAS T1 value is 2 seconds or greater. This could cause an issue.
>> This may be prevented by making recommendations about timer setting.
>
> I don't know much about the timers. It has always baffled me that we  
> allow them to be changed. Its my impression that all bets are off if  
> the two ends of a transaction don't have the same timer values.

It is a hard requirement that any pair of transaction state machines  
(the client and server machines for any given hop) have the same timer  
values. Without that, behavior becomes very incorrect. And for INVITE/ 
200/ACK the TU timers (which  are all based on T1) have to be the same  
or you get into broken territory. The result is that if you change a  
timer somewhere in the network, you either change it across the whole  
network or you put in a node that's essentially playing the role of a  
SIP-SIP gateway that deals with complexity of having timers different  
on each side.

>
>
>> 2. Regarding rule 1
>> This prevents the UAS from sending an UPDATE as part of a re-INVITE  
>> transaction.
>> We have come across a case for an initial INVITE where it becomes  
>> necessary for the UAS to send an early dialog UPDATE which may also  
>> be applicable in a re-INVITE case. I am not sure if it is  
>> applicable for a re-INVITE but others may be able to advise.
>> When a SIP INVITE passes via an MGC to an ISDN connection the IP  
>> address and port to be used can be controlled by the ISDN PBX. When  
>> the ISDN bearer identity is provided by the ISDN terminal, in an  
>> Alerting signal, the MGC may change the ephemeral (& possibly H.248  
>> context). If we have an INVITE sequence involving preconditions the  
>> extended INVITE sequence occurs with an UPDATE from the UAC to  
>> confirm reservation of resources. As an immediate response is  
>> required to this UPDATE the UAS can only respond with the IP  
>> address and port of the H.248 ephemeral that it selected initially.  
>> In the ALERTING response to the SETUP request the ISDN terminal  
>> specifies a bearer which may result in a different the IP address  
>> and/or port. This requires that the UAS sends a further UPDATE with  
>> a modified sdp offer to change the IP address and/or port prior to  
>> sending the 200 OK.
>> While this scenario can occur for an initial INVITE it may also be  
>> possible for a re-INVITE; e.g. when a new media type is added to an  
>> existing session.
>> I am uncertain as to whether this will occur in this situation and  
>> I would welcome input from the list to show that it cannot.
>
> My point was that while an invite/reinvite is in progress, any offer  
> the UAS might want to send in an UPDATE can instead be sent in a  
> reliable provisional response.
>
> This *does* present a problem if the UAC supports UPDATE but doesn't  
> support reliable provisional responses. Is that an important case?  
> Does anybody do that?
>
> If we must deal with the case where a UAC doesn't support 100rel,  
> but does support UPDATE, then this proposal won't work. If so, I  
> don't currently have any alternative.
>
> 	Paul
>
>> Ian Elz
>> System Manager
>> DUCI LDC UK
>> (Lucid Duck)
>> Office: + 44 24 764 35256
>> gsm: [phone]
>> [e-mail]
>> -----Original Message-----
>> From: [e-mail] [mailto:[e-mail]] On  
>> Behalf Of Paul Kyzivat
>> Sent: 03 July 2008 15:57
>> 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.
>> - 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.
>> 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.
>> - 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.
>> 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-offeranswer-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
>>>>>>>>>> _______________________________________________
>>>>>>>>>> Sipping mailing list  https://www.ietf.org/mailman/listinfo/sipping
>>>>>>>>>> This list is for NEW development of the application of SIP
>>>>>>>>>> Use [e-mail] for questions on  
>>>>>>>>>> current sip
>>>>>>>>>> Use [e-mail] for new developments of core SIP
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> Sipping mailing list  https://www.ietf.org/mailman/listinfo/sipping
>>>>>>>>> This list is for NEW development of the application of SIP
>>>>>>>>> Use [e-mail] for questions on  
>>>>>>>>> current sip
>>>>>>>>> Use [e-mail] for new developments of core SIP
>>>>>>>>>
>>>>>>>>
>>>>>> _______________________________________________
>>>>>> Sipping mailing list  https://www.ietf.org/mailman/listinfo/sipping
>>>>>> This list is for NEW development of the application of SIP
>>>>>> Use [e-mail] for questions on current sip
>>>>>> Use [e-mail] for new developments of core SIP
>>>>>
>>> _______________________________________________
>>> Sipping mailing list  https://www.ietf.org/mailman/listinfo/sipping
>>> This list is for NEW development of the application of SIP
>>> Use [e-mail] for questions on current sip
>>> Use [e-mail] for new developments of core SIP
>>>
>> _______________________________________________
>> Sipping mailing list  https://www.ietf.org/mailman/listinfo/sipping
>> This list is for NEW development of the application of SIP
>> Use [e-mail] for questions on current sip
>> Use [e-mail] for new developments of core SIP

_______________________________________________
Sipping mailing list  https://www.ietf.org/mailman/listinfo/sipping
This list is for NEW development of the application of SIP
Use [e-mail] for questions on current sip
Use [e-mail] for new developments of core SIP