Re: [Sip] Record-Route, Path, Service Route, and Multi-Homed Prox ies

[Gonzalo Camarillo <Gonzalo.Camarillo@lmf.ericsson.se>]

Dean,

inline:

> >
> >  Now quoting the SIGCOMP drafr from Gonzalo revision 01
> >
> >  5) Sending a Response to a Client
> >
> >    A response is sent to the host in the sent-by parameter of the Via
> >    header field. If the topmost Via header field contains the
> > parameter
> >    comp=sigcomp, the response SHOULD be compressed. Otherwise, the
> >    response MUST NOT be compressed.
> >
> >    A proxy performing Record-Route inspects the Record-Route header
> >    field in the response and the Contact header field in the request
> >    that triggered this response (see example in Section 8).
> > It looks for
> >    the URI of the next upstream (closer to the user agent
> > client) hop in
> >    the route set. If this URI contains the parameter comp=sigcomp, the
> >    proxy SHOULD add comp=sigcomp to its entry in the
> > Record-Route header
> >    field. If this URI does not contain the parameter comp=sigcomp, the
> >    proxy SHOULD remove comp=sigcomp (if it is present) from
> > its entry in
> >    the Record-Route header field.
> >
> > The above implies that we may to need to do rewrite of RR in
> > the response.
> 
> Hmm. Could this be accounted for by having "in" and "out" RR heaader
> field values as proposed for sips and network adaptation? The one on the
> compressed side would list sigcomp, the other one wouldn't. Gonzalo,
> what do you think?

When SigComp is used, you need to re-write your RR header when the last
proxy that RRed added comp=SigComp and the next one will not use it.
 
Previous P           P                   Next P
   |                 |                     |
   |   compression   |  no compression     |

Your RR in the INVITE will be used for "Next P" to send you requests, so
it should not contain comp=sigcomp. Your RR in the 200 OK will be used
by "Previous P" to send you requests, so it should have comp=sigcomp.

If you use your double-RR trick, you could add comp=sigcomp to RR1
already in the INVITE and leave RR2 without comp=sigcomp. Everything
will work fine. However, you would be sending messages to yourself using
compression.

We could add a section about this in the SigComp-SIP draft so that
everything is clear. If a proxy is routing a message to itself, it would
be safe to ignore comp=sigcomp and "send" the request uncompressed.

Thanks,

Gonzalo
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