[Kamailio-Devel] How does Kamailio match an ACK for a stateless [3456]XX replied by Kamailio itself?

Alex Balashov abalashov at evaristesys.com
Mon Dec 15 12:29:55 CET 2008

Klaus Darilion wrote:
> Iñaki Baz Castillo wrote:
>> El Lunes, 15 de Diciembre de 2008, Klaus Darilion escribió:
>>> AFAIK the sl module registers a callback. It can detect ACK
>>> corresponding to stateless responses by certain parameter - I think it
>>> is encoded in the totag.
>> Wouldn't be better the Via "branch" parameter? (the ACK for a negative reply 
>> has the same "branch" since it's part of the same transaction).
>> But there can be multiple's ACK with same To_tag (due to re-INVITE's during a 
>> dialog).
> Maybe it is the branch parameter. I do not know. I only know that it 
> uses one of these parameters.

I believe it does use the Via branch parameter, along with host and 
port.  In tags.h:calc_crc_suffix():

/*! \brief
  * Generate variable part of to-tag for a request;
  * it will have length of CRC16_LEN, sufficiently
  * long buffer must be passed to the function */
static inline void calc_crc_suffix( struct sip_msg *msg, char *tag_suffix)
         int ss_nr;
         str suffix_source[3];

         if (msg->via1==0) return; /* no via, bad message */
         if (msg->via1->branch)
         crcitt_string_array( tag_suffix, suffix_source, ss_nr );

I would opine that this function should be renamed to be a little 
clearer that it applies to the to-tag specifically.  Otherwise, 
calculate CRC suffix of what?

Alex Balashov
Evariste Systems
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