Ian Elz
System Manager
DUCI LDC UK
(Lucid Duck)
Office: + 44 24 764 35256
gsm: +44 7801723668
ian.elz at ericsson.com
-----Original Message-----
From: sipping-bounces at ietf.org [mailto:sipping-bounces at ietf.org] 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: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
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_______________________________________________
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
_______________________________________________
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