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

[Maxim Sobolev]

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. 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 
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.

Regards,
-- 
Maksym Sobolyev
Sippy Software, Inc.
Internet Telephony (VoIP) Experts
T/F: +1-646-651-1110
Web: http://www.sippysoft.com