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Re: [Sipping] Liaison Statement on offer/answer procedures
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:pkyzivat at cisco.com]
>>> 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: sipping-bounces at ietf.org
>>>>> [mailto:sipping-bounces at ietf.org] 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 sip-implementors at cs.columbia.edu for questions on current sip
>>>> Use sip at ietf.org 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 sip-implementors at cs.columbia.edu for questions on current sip
>>> Use sip at ietf.org for new developments of core SIP
>>>
>>
>>
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