[OpenSER-Devel] Possible bug in the tm module in the presence of packet loss/branches

[Dan Pascu]

On Thursday 06 March 2008, Maxim Sobolev wrote:
> Bogdan-Andrei Iancu wrote:
> > Hi Maxim,
> >
> > You stated:
> >
> > <quote>
> > The correct behavior of the tm module in this case would be to
> > continue with INVITE re-transmits until we get provisional response
> > and immediate CANCEL once that response comes in.
> > </quote>
> >
> > Is this based on RFC indication or a personal opinion? If RFC based,
> > could you please point me out the relevant section?
> >
> > I'm asking mainly because, following my own logic, I would rather say
> > that once the transaction is cancelled on UAS side, no further
> > attempts (read retransmissions) should be done on UAC side.
>
> Bogdan,
>
> It's based on common sense.

I have to disagree. It may make sense to you when you try to operate it in 
a network with a "high magnitude of packet loss", however it doesn't make 
sense to me to hack things into the SIP protocol behavior to address 
networks with high packet loss issues. Retransmissions deal nicely with 
occasional packet loss and that's where it should end.

> Unless UAC does number of retransmits 
> specified by the RFC it can never be sure whether absence of
> provisional reply has been caused by the dead destination or network
> packet loss issue and the destination is in fact ringing. In the tm
> module you always assume "dead destination", which is IMHO wrong. In my
> situation this problem has been aggravated by the magnitude of packet
> loss, but in general I've seen this issue before once in a while on a
> network with close to zero packet loss rate.
>
> Another bad decision is to generate 487 locally in the presence of
> unconfirmed active branches. SIP proxy should not do it unless it is

It's not active anymore. It did timeout after the timer has expired, so it 
has generated an implicit 408.

> prepared to generate BYE if 200 OK comes from any of those branches
> (i.e. proxy provides some kind of dialog functionality). Again, in the
> real world, where packets are getting lost from time to time this could
> lead to 200 OK coming from the branch even if you do stop INVITE
> retransmitions. You will get yourself in the situation with originating
> UA already received fake final negative 487 from proxy, while
> terminating UA having dialog established, so that the only way to "fix"
> the issue is to send BYE from the proxy to the terminating UA.

Not true. The dialog is not final until the ACK comes and it'll never come 
as the originating side has already received a final 487 reply. So a RFC 
compliant UA on the callee side should discard the dialog itself without 
any BYE if the ACK is not received after a certain amount of time.

-- 
Dan