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

Maxim Sobolev sobomax at sippysoft.com
Fri Mar 7 19:00:45 CET 2008


Andrei Pelinescu-Onciul wrote:
> On Mar 06, 2008 at 10:43, Maxim Sobolev <sobomax at sippysoft.com> 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. 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.
>>
> 
> OTOH if you wait for the timeout and you have some unreachable branches
> (I think this is quite common due, e.g. changing ip address), you'll
>  delay possible 6xx answers and use more memory (even a 2xx replied
>  transaction could be kept longer in memory if it has a not responing
>  branch).
> 
> I'm also not sure if this would  be rfc conformant (although after a
> quick look I cannot find anything for or against it).
> 
> I would rather send CANCEL even on branches with no provisional response
> (although the rfc seems to deny this due some possible race conditions
> (?)).
> 
> 
> I guess we can have yet another tm config param. for specifying cancel
> non-pending branches behaviour, but what should be its default value
> (present way, keep retransmitting INV, or send CANCEL)?

There are three issues, really:

1. According to the RFC CANCEL on non-pending branches should be 
deferred until provisional reply comes. Neither SER nor OpenSER do that. 
They simply don't relay CANCEL if provisional reply comes later on. It 
looks like a clear bug to me and it needs to be fixed unconditionally.

2. The questionable issue is whether or not UAC should keep 
retransmitting INVITE when waiting for (1) to happen. This could be made 
an option.

3. Another questionable issue is whether or not UAS should be sending 
487 immediately in the presence of such non-pending branches, it could 
also be made an option.

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



More information about the Devel mailing list