[OpenSIPS-Users] What is the role of t_check_trans at line 253 of opensips.cfg in SVN trunk

[Bogdan-Andrei Iancu]

Hi Alex,

To detect a retransmission you need to have the transaction created by 
the initial request - otherwise you cannot detect a retransmission. So, 
if you create the transaction very late in the script (t_relay() time), 
whatever retransmission you might get in the mean while will not be 
detected as retransmissions.

Ex:

Process 1 (timeline):
  --recv R1 ------start processing 
--------------------------------------------------t_relay (create T for 
R1)---------------- >
Process 2 (timeline):
  ------------------------recv R2 (retransmission)------start processing 
---------------------------------------------- t_relay( retransmission 
is detected)

As you see above, if the retransmission is received while you do the 
processing of initial request, you will do the processing for the 
retransmitted request as for a normal one....only at t_relay() time you 
will figure out that it is a retransmission and you do not have to do 
any processing (script) for it.

Second Option:

Process 1 (timeline):
  --recv R1 ------create T ----start processing 
--------------------------------------------------t_relay 
()---------------- >
Process 2 (timeline):
  ---------------------------recv R2 (retransmission)----R2 matches T -> 
retransmission -> stop script....

here , for the retransmission, there is no processing done as we found 
the transaction, so we can say it;s a retransmission.


Hope this will help.

Regards,
bogdan




Alex Balashov wrote:
> Thanks Bogdan. Appreciate your followup.
>
> So let me put the question this way:
>
> What is the benefit of creating a new transaction on top of the 
> retrans checks?  Why would I not just want to wait until I call 
> t_relay(), which will also create a transaction if it does not already 
> exist.  Why it would be beneficial to have it exist beforehand?It 
> seems that retransmission detection works the same way regardless.
>
> -- 
> Sent from mobile device
>
> On Jul 16, 2009, at 11:54 AM, Bogdan-Andrei Iancu 
> <bogdan at voice-system.ro> wrote:
>
>> Hi Alex,
>>
>> No, t_check_trans() will NOT create a new transaction. Both function 
>> will check (for non-ACK and non-CANCEL) if it is retransmission and 
>> if so, it will sent (via TM) the last sent reply and stop the script 
>> exectution. If it is not a retransmission, t_check_trans() will not 
>> do anything else, but t_newtran() will create a new transaction.
>>
>> I added this function in 1.0 (?!?) as it was mainly intended for 
>> proper CANCEL and ACK routing.
>>
>> Regards,
>> Bogdan
>>
>> Alex Balashov wrote:
>>> Bogdan,
>>>
>>> Are you saying that t_check_trans() will create a new transaction 
>>> for a non-ACK/CANCEL retransmission too?   Or that it retransmits 
>>> the last reply sent "statelessly" somehow?
>>>
>>> -- 
>>> Sent from mobile device
>>>
>>> On Jul 14, 2009, at 9:10 AM, Bogdan-Andrei Iancu 
>>> <bogdan at voice-system.ro> wrote:
>>>
>>>> Hi Stan,
>>>>
>>>> when comes to handling retransmissions  (and not CANCELs and ACKs
>>>> belonging to an INVITE transaction), both function do more or less the
>>>> same - handle the retransmission (by retransmitting the last sent 
>>>> reply)
>>>> and breaking the script execution - of course, the difference is if no
>>>> retransmission, t_newtran() will create a new transaction for the 
>>>> request.
>>>>
>>>> So :
>>>>
>>>> t_check_trans();
>>>> t_new_trans();
>>>>
>>>>
>>>> is a bit redundant. Only:
>>>>
>>>> t_new_trans();
>>>>
>>>>
>>>> will do exactly the same job.
>>>>
>>>> Again, this is true only in the context of non-CANCEL  and non-ACK 
>>>> requests!
>>>>
>>>> Regards,
>>>> Bogdan
>>>>
>>>> Stanisław Pitucha wrote:
>>>>> 2009/7/14 Alex Balashov <abalashov at evaristesys.com>:
>>>>>
>>>>>> http://www.opensips.org/html/docs/modules/1.5.x/tm.html#id272150
>>>>>>
>>>>>
>>>>> A bit related question. Since the docs mention:
>>>>> "If the processing of requests may take long time (e.g. DB lookups)
>>>>> and the retransmission arrives before t_relay() is called, you can 
>>>>> use
>>>>> the t_newtran() function to manually create a transaction."
>>>>>
>>>>> Is there any situation where:
>>>>>
>>>>> t_check_trans();
>>>>> t_new_trans();
>>>>>
>>>>> after all cancel / ack checks is a bad thing to do? Or maybe even:
>>>>>
>>>>> t_check_trans();
>>>>> if (is_method('INVITE|UPDATE|REFER')) t_new_trans();
>>>>>
>>>>> since everything else can be safely duplicated / is rather light 
>>>>> in processing.
>>>>>
>>>>> _______________________________________________
>>>>> Users mailing list
>>>>> Users at lists.opensips.org
>>>>> http://lists.opensips.org/cgi-bin/mailman/listinfo/users
>>>>>
>>>>>
>>>>
>>>>
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>>>
>>
>