Subject: [SIP] Few queries after reading 2543 rfc...

From: "T.Sunil Kumar" <[email-masked]>
Date: Mon, 09 Apr 2001 19:54:45 +0530

Hi guys,

We're seeking explanations for the following queries that raised after
going thru SIP rfc 2543. Can anyone of you please respond....

1) What is the importance of 'tag' parameter in 'To' header field in
response. RFC says that it is useful to differentiate responses when an
intermediate proxy forks the request. How does a proxy or User agent
client maps a response to its host using 'tag' parameter when that 'tag'
was not sent in the request.

2) What is the significance of 'Cseq' header field. RFC says that ACK
request should contain Cseq same as corresponding INVITE request but BYE
should increment the sequence number in Cseq - why?

3)  When the callee responds (in its 200 response) with a new URI in
Contact header field, the caller would send ACK request to the new URI
mentioned by the callee. How does the session description bargain happen
between caller and new callee  if there is no session information passed
in ACK request - pl. refer to example 16.2

4) When does the bearer path/voice path become through between callee
and caller - is it after caller receives 200 response OR after caller
send ACK request for 200 response.

Thank you so much for your time and help.

Best regards,
--Sunil

T.Sunil Kumar
Net Brahma Technologies
Internext Networking Software
A Microland Group Venture
[email-masked]
http://www.netbrahma.com
Tel : 91.80. 552 1451/5011/5012/5014 ext 269/270
Fax : 91.80. 553 7233

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---

From: "Mayank Sharma" <[email-masked]>
Date: Tue, 10 Apr 2001 15:44:27 +0530

> was not sent in the request.

When the UAS responds to a request it puts a tag parameter in the To header 
of the response that it sends. In case the Request was forked by a proxy
server
The proxy server will receive two responses to its request with different Tag

parameter in it's To field. Hence the Proxy server will know that response
with
such an such tag came from such and such UAS.

> should increment the sequence number in Cseq - why?

In a particular sequence of transactions with same Call-ID, Cseq
differentiates
the transactions (read the rfc for the defination of transaction). ACK
contains the
CSeq of the response to which it is ACKing. in case of response to INVITE, it
will
have the same Cseq as that of INVITE.

> between caller and new callee  if there is no session 

the Contact that is send with a 200 OK response tells the caller the current
location
of the callee. so the ACK need not travel through all the proxies through
which INVITE and 200 OK passed. Thus the ACK is sent to the same person
bypassing the proxy. 

RFC is very clear regarding these doubts.

Regards,
Maynk Sharma
Mindtree Consultancy Pvt. Ltd.