RE: [Sip] Never can a proxy modify SIP message body?

["Jain, Rajnish" <rajnishjain@xl.com>]

ISUP encapsulation w/in SIP is relevant to PSTN backhaul over 
an intermediary SIP network (not the other way round) such as 
the following:

         PSTN <-----> SIP Network <-----> PSTN 
    
The PSTN/SIP gateways at the two ends here are UAs. The SIP 
network in the middle is perhaps comprised of a chain of proxy 
servers. Your requirements may lead you to a B2BUA approach
which are entitled to do anything to the SIP messages at their
discretion.

- Rajnish

-----Original Message-----
From: Chen Zaifeng,BISC R&D SA(BJ)
[mailto:zaifeng.chen@BISC.SIEMENS.COM.CN]
Sent: Sunday, June 15, 2003 9:19 PM
To: 'sip@ietf.org'
Cc: 'Jonathan Rosenberg'
Subject: [Sip] Never can a proxy modify SIP message body?


Dear All:
I found two statements in rfc3261 which seem to conflict each other:
1. In page 23, in the definition of "Proxy, Proxy Server", it says that "A proxy interprets, and, if necessary, rewrites 
    specific parts of a request message before forwarding it."
2. In page 100, under the title of "1. Copy request", it says that "The proxy MUST NOT add to, modify, or remove the 
    message body."

On the other hand, there are scenarios that need a proxy to modify a SIP message body, e.g. for SIP<-->PSTN/ISDN
calls, it is dangerous to delivery ISUP MIME content encapsulated in SIP message body to SIP user client, or receive
such ISUP content from SIP user client (in such cases the ISUP content might be mostly for malicious usage) and
forward it to somewhere like SIP-PSTN gateways, then a proxy might have to screen/remove ISUP MIME content
encapsulated in SIP message body.

Any of your ideas or clarifications to my puzzles are highly appreciated. Thanks!

Sincerely yours, 
Chen Zaifeng
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This list is for NEW development of the core SIP Protocol
Use sip-implementors@cs.columbia.edu for questions on current sip
Use sipping@ietf.org for new developments on the application of sip