[SR-Users] re-SUBSCRIBE and Record-Route

[Klaus Darilion]

Not so easy .....

16.4 Route Information Preprocessing


   The proxy MUST inspect the Request-URI of the request.  If the
   Request-URI of the request contains a value this proxy previously
   placed into a Record-Route header field (see Section 16.6 item 4),
   the proxy MUST replace the Request-URI in the request with the last
   value from the Route header field, and remove that value from the
   Route header field.  The proxy MUST then proceed as if it received
   this modified request.

      This will only happen when the element sending the request to the
      proxy (which may have been an endpoint) is a strict router.  This
      rewrite on receive is necessary to enable backwards compatibility
      with those elements.  It also allows elements following this
      specification to preserve the Request-URI through strict-routing
      proxies (see Section 12.2.1.1).


Thus, Kamailio should somehow detect if the URI is inserted by himself
or not. This may only happen if the previous hop is a strict-router. As
Kamailio is a loose-router and always adds the ;lr parameter, the check
could be extended to check if the RURI also cotains lr parameter. If the
lr parameter is not present, this means for sure that the RURI was not
generated by Kamailio.

Of course this would require code changes.

A dirty hack would be to manipulate the Route headers manually and set $du.


regards
Klaus



Am 04.03.2011 15:25, schrieb marius zbihlei:
> Hello list,
> 
> An UAC sends a resubscribe in the form
> 
> SUBSCRIBE sip:Alice at example.com SIP/2.0
> Route: <sip:sip.example.com;lr=on>
> Route:<sip:sip.next-hop.example.com;lr=on>
> ....
> 
> The request has a to tag, from tag... Basic resubscribe(R-URI is the
> same as original SUBSCRIBE) The proxy that handles it is a Kamailio (1.5
> but this refers to 3.1 as well). The problem is that loose_route()
> treats this request as coming from a strict routing router, not from the
> UAC, so it applies the strict-to-loose rules and rewrites the request as:
> 
> SUBSCRIBE sip:sip.next-hop.example.com;lr=on SIP/2.0
> Route: <sip:sip.example.com;lr=on>
> ....
> 
> The segment that handles this is pretty standard:
> 
> if (loose_route()) {
>                 # sequential request withing a dialog should take the path
>                 # determined by record-routing
>                 if(has_totag()) {
>                         forward();
>                         exit;
> }
> 
> It sees that message has Route set, it finds the to tag (so the routes
> were not a preset), and then , the important thing, sees that the domain
> from the RURI matches itself (kamailio has aliases configured for
> example.com), thus considering the request as coming from a strict
> router. Then it takes the bottom Route header and overwrites the RURI.
> 
> What is the best way to ensure that loose routing is applied to the
> resubscribe as described above?
> 
> Cheers,
> Marius
> 
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