[Serusers] branch parameter

[Michal Matyska]

It takes me ages too. I've to check the RFC due to the fact, that my UAS
was rejecting ACK with branch=0 and the result of the digg is, that I'm
not sure whether is it correct to use it at all.

If I skip the requirement in 8.1.1.7 Via:

   The branch ID inserted by an element compliant with this
   specification MUST always begin with the characters "z9hG4bK".  These
   7 characters are used as a magic cookie (7 is deemed sufficient to
   ensure that an older RFC 2543 implementation would not pick such a
   value), so that servers receiving the request can determine that the
   branch ID was constructed in the fashion described by this
   specification (that is, globally unique).  Beyond this requirement,
   the precise format of the branch token is implementation-defined.

then in 17.2.3 Matching Requests to Server Transactions we can read:

   If the branch parameter in the top Via header field is not present,
   or does not contain the magic cookie, the following procedures are
   used.  These exist to handle backwards compatibility with RFC 2543
   compliant implementations.

   The INVITE request matches a transaction if the Request-URI, To tag,
   From tag, Call-ID, CSeq, and top Via header field match those of the
   INVITE request which created the transaction.  In this case, the
   INVITE is a retransmission of the original one that created the
   transaction.  The ACK request matches a transaction if the Request-
   URI, From tag, Call-ID, CSeq number (not the method), and top Via
*  header field match those of the INVITE request which created the
   transaction, and the To tag of the ACK matches the To tag of the
   response sent by the server transaction.  Matching is done based on
   the matching rules defined for each of those header fields.
   Inclusion of the tag in the To header field in the ACK matching
   process helps disambiguate ACK for 2xx from ACK for other responses
   at a proxy, which may have forwarded both responses (This can occur
   in unusual conditions.  Specifically, when a proxy forked a request,
   and then crashes, the responses may be delivered to another proxy,
   which might end up forwarding multiple responses upstream).  An ACK
   request that matches an INVITE transaction matched by a previous ACK
   is considered a retransmission of that previous ACK.

Note the sentence marked by * (hope the formatting will sustain) - if
the branch is not present os does not contain the cookie (true for
branch=0).... top Via header field match those of the INVITE.... we have
branch id which does not start with the cookie - shall we use the branch
id for the matching or not? As I understand the text, we use the Via
header with the branch id, so it won't ever match the INVITE.

I'm looking forward to your comments. 
OTOH, setting sync_branch=0 has helped (CVS head version).

Michal


On Wed, 2007-02-07 at 14:41 +0100, Jiri Kuthan wrote:
> At 16:58 18/12/2006, Martin Hoffmann wrote:
> >Jiri Kuthan wrote:
> >> At 17:05 17/12/2006, Kapil Dhawan wrote:
> >> 
> >> > I am using RTC/1.2 library and its not generating branch parameter.
> >> 
> >> too bad, most implementation can deal with it in backw2ards compatibility mode
> >> but clients should no longer generate such branch. I would be looking for
> >> a more up-to-date stack if I was you.
> >
> ><evil>
> >Ever used forward() in SER?
> ></evil>
> 
> 
> 
> sorry -- it takes me ages to answer. -jiri
> 
> forward.c:
> 
>     /* calculate branch for outbound request;  if syn_branch is turned off,
>        calculate is from transaction key, i.e., as an md5 of From/To/CallID/
>        CSeq exactly the same way as TM does; good for reboot -- than messages
>        belonging to transaction lost due to reboot will still be forwarded
>        with the same branch parameter and will be match-able downstream
> 
>        if it is turned on, we don't care about reboot; we simply put a simple
>        value in there; better for performance
>     */
>     if (syn_branch ) {
>         *msg->add_to_branch_s='0';
>         msg->add_to_branch_len=1;
>     } else {
> 
> main.c
> 
> /* shall use stateful synonym branches? faster but not reboot-safe */
> int syn_branch = 1;
> 
> 
> 
> --
> Jiri Kuthan            http://iptel.org/~jiri/ 
> 
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