Subject: [rohc] Question: NOT include comp=sigcomp when sending uncompressed msg?

From: "Carrie Zhang" <[email-masked]>
Date: Thu, 10 Aug 2006 16:40:34 +0800

Hi experts,

I have a question about whether the comp=sigcomp should be included
when the message is sent uncompressed.

RFC3486 says:
"The presence of this parameter (comp=sigcomp) in a URI indicates
that the request has to be compressed using SigComp, as defined
in [2]. The presence of comp=sigcomp in a Via header field indicates
that the response has to be compressed using SigComp."

and, "draft-ietf-rohc-sigcomp-sip-02.txt" says:
"Applications MUST NOT mix SIP messages and SigComp messages on a
single TCP connection."

But when a user does send message over TCP with comp=sigcomp
in the request-URI or topmost Route URI, and comp=sigcomp in the
topmost Via, however, the message is not actually compressed,
then the response message becomes compressed over the same TCP
connection because comp=sigcomp is in the topmost Via. This breaks
the rule in "draft-ietf-rohc-sigcomp-sip-02.txt" above.

So, my question would be:
should this draft specify that if a message is to be sent uncompressed
without shim header, comp=sigcomp MUST NOT be included in request-URI,
topmost Route URI, and topmost Via. So that SigComp and non-SigComp
message won't mix in the same TCP connection?

Thanks in advance for your time and help.

Regards,
Carrie

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From: Carsten Bormann <[email-masked]>
Date: Thu, 10 Aug 2006 12:32:03 +0200

On Aug 10 2006, at 10:40, Carrie Zhang wrote:

> connection because comp=sigcomp is in the topmost Via.

Non sequitur:
For sending a compressed message, you need to open a new TCP  
connection when all you have is an uncompressed one.

Gruesse, Carsten

---

From: "Carrie Zhang" <[email-masked]>
Date: Fri, 11 Aug 2006 10:26:29 +0800

Hi Carsten,

Thanks a lot for your quick response.

The issue here is that the comp=sigcomp was included in the
URI of the incoming message when the incoming message is actually
not compressed. When we send the response, we think the incoming
message is compressed based on the comp=sigcomp in the URIs,
then send the response compressed based on comp=sigcomp in topmost
Via. Both directions are compressed, so no new TCP connection is
opened.

To be able to know a new TCP connection is needed, we have to
know exactly whether the incoming message is compressed or not
regardless the comp=sigcomp presence in the URIs, since the
comp=sigcomp in URIs may deliver wrong message. Do we need to
do so?

Regards,
Carrie

Carsten Bormann wrote:
> For sending a compressed message, you need to open a new TCP  connection 
> when all you have is an uncompressed one.
> 
> Gruesse, Carsten
>