[SIPForum-discussion] forwarding more than one 2xx in a proxy server

[Robert Sparks]

On Feb 2, 2007, at 9:42 PM, Young-Geun Park wrote:

>              Thanks for you answer.
>
>
>              For a proxy to forward the second and any subsequent  
> 2xx responses,
>
> should it work in both stateful and stateless mode ?
>
>
> anyway, while a proxy can forward the second and any subsequent 2xx  
> responses,
>
> a UAC that sent the INVITE can not receive these responses  
> subsequently
>
> because the INVITE client transaction transition to ‘terminated’  
> already.
>
> This is right?
No - the specification split transactions and transaction users  
(search for "transaction users" and "cores").
The specification of the endpoints' UAC core behavior talks about  
handling multiple 200s.

In a nutshell, the endpoint can end up in multiple dialogs (multiple  
calls) by sending one INVITE.
It is local policy how it handles that.

Most things deal with it by treating the first 200 normally, and  
entering whatever session it negotiated.
The 2nd and subsequent are ACKed, and then BYEs are sent on the  
dialogs they create.

>
> In that case the following service scenario is impossible. Is that  
> right?
>
> A UAC sends a INVITE that searches for any service to a proxy and  
> want to receive available all services.
>
> The proxy will fork the INVITE to many targets(UAS) and several  
> UASs return a response that service is available.
>
> Then the UAC receives those responses and then determine one service.
This is possible - the endpoint _could_ collect all the 200s (there  
are some subtle things in determining how long to wait, and some other
not-so-subtle tradeoffs it would have to make with respect to media  
clipping) and then choose the "best" one it likes.

It's also legal for it to enter into calls with every leg. This might  
make more sense if the UAC is an automaton than if it were a phone being
used by a human.

RjS

>
> Best regards,
>
> Park
>
>
>
> -----Original Message-----
> From: Robert Sparks [mailto:rjsparks at nostrum.com]
> Sent: Saturday, February 03, 2007 12:39 AM
> To: Young-Geun Park
> Cc: discussion at sipforum.org
> Subject: Re: [SIPForum-discussion] forwarding more than one 2xx in  
> a proxy server
>
>
> The text as written was intended to cause the proxy to forward the  
> second and any subsequent 2xx responses statelessly
>
> (search for the word statelessly in that section).
>
>
> MANY proxy implementors have written code that does the correct  
> thing here (if you are not sure what the correct thing
>
> to do is, we can walk through it off-list).
>
>
> Now, I have to point out that there is a known bug with moving that  
> transition to terminated (see http://bugs.sipit.net/show_bug.cgi? 
> id=769).
>
> From that bug report:
>
>
>> The current text in 3261 instructs a UAS to destroy an INVITE  
>> transaction the
>>
>> instant it sends a 200.
>>
>>
>> This has the unintended consequence that any retransmissions of  
>> the INVITE
>>
>> request that arrive after that destruction will be treated as a  
>> new request.
>>
>>
>> This interacts both with endpoints and proxies (the deletion of  
>> the server
>>
>> transaction combined with stateless forwarding of responses  
>> without matching
>>
>> transactions provided forwarding of multiple 200s to forked INVITES).
>>
>>
>> The fix will involve having the server transaction continue to  
>> exist long
>>
>> enough to drain any retransmissions of the INVITE and related  
>> changes to
>>
>> the UAS handling of retransmitting the 200s, and proxies handling  
>> multiple
>>
>> 200s/stray responses.
>
> RjS
>
>
> On Feb 2, 2007, at 6:21 AM, Young-Geun Park wrote:
>
>
>
>
> Hi, all
>
>
> The following is described in 16.7.5, RFC3261(p110):
>
> After a final response has been sent on the server transaction,
>
> the following responses MUST be forwarded immediately:
>
> - Any 2xx response to an INVITE request
>
> (………….)
>
> This step, combined with the next, ensures that a stateful
>
> proxy will forward exactly one final response to a non-INVITE
>
> request, and either exactly one non-2xx response or *one or more*
>
> 2xx responses to an INVITE request.
>
> --
>
> The server tx’s state would transition to ‘terminated’ state  
> because of first 200 OK.
>
> So I think that it is impossible for second 200Ok to be forwarded  
> by the same server tx.
>
>
> I don’t know why it is possible.
>
>
> Best regards,
>
> Park
>
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