Subject: [SIP] "dedicated" proxy servers and user application custom SIP headers ?

From: Daniel Gross <[email-masked]>
Date: Mon, 8 Jan 2001 15:03:20 -0800

Dear all,

I was referred to the SIP list and started to browse its archives. I 
thought to also post some specific questions i have in mind in the hope 
someone might be able to give me some quick feedback on them ...

I have started read about SIP as an infrastructure for flexible services 
creation, and have stumbled over an interesting paper by Jonathan Rosenberg 
et. al., where the authors describes the use of a SIP CGI interface for 
programming services. An admin (a trusted user) can provide SIP CGI scripts 
to SIP proxy servers, which are executed in an event driven manner when the 
server processes SIP protocol commands (quite similar to http processing). 
You can find the paper at: 
http://www.cs.columbia.edu/~jdrosen/papers/program_report.ps  .

The authors do not elaborate where the SIP proxy server that processes the 
SIP CGI scripts would reside in the "SIP network". I was wondering, for 
service applications that need user states over long periods of time 
(beyond single SIP transactions -- perhaps over the lifetime of the user 
being a customer to an operator), would there be a "dedicated" stateful 
user proxy servers "close" to the user agents, that would take care of the 
users preferences/policies/data, when he/she makes or receives a "call" (by 
invoking those CGI scripts)? Would infrastructure operators create such an 
application proxy server for each one of their customers?

I was also wondering what is the current state of the art in SIP, are such 
SIP CGI scripts utilized, or are perhaps java servlets used or other 
mechanisms (if at all)?

And perhaps if i can ask one more question. In the paper the authors 
describe how custom header information can be passed back and forth between 
the SIP proxy server and the CGI script. Is it possible in SIP to have 
custom headers generated by the users agent application (and/or by a 
"dedicated" user application proxy server), similar to user defined 
"X-headers" in Mime, which could then be utilized by the CGI scripts (or 
other 3rd party call control "plug-in" functions)?  Or does SIP not allow 
for custom header tags to be specified to be delivered to such 3rd call 
control "plug-ins", unless they are official header extension made by IETF 
( does SIP perhaps expects extensions to be placed in the body as, for 
example a mime message, thus making call control extension "media" 
specific).

appreciating any response

Daniel

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