Subject: [Sip] Questions on the draft on the Replaces header

From: [email-masked]
Date: Thu, 24 Oct 2002 17:03:44 +0200

The Replaces header draft 
(http://www.ietf.org/internet-drafts/draft-ietf-sip-replaces-02.txt),
states that this header is only defined for INVITE requests (section 6.1). 
The Overview
section (section 2), however, states that it is frequently used in 
combination with the 
REFER method as used in call transfer. Although the latter does not imply 
that a Replaces
header could be used in a REFER request, it does seem to hint in that 
direction.

Especially in the case of call transfer, it would be useful if it were 
allowed to use the
Replaces header in a REFER request. The reasoning behind this is the 
following. 

Receiving a REFER request, is a request to _also_ contact the destination 
contained in the
Refer-To header. It has no implications for the Dialog (or the Session) on 
which the REFER
was sent. This means that it is up to the recipient of the REFER request, 
to decide on the 
action(s) it is going to take after having contacted the indicated 
destination. I.e. it is
neither explicitely, nor implicitely conveyed that a transfer should take 
place. If this
does take place, this is up to the endpoint that received the REFER 
request. This is 
outside the control of the endpoint that sent the REFER request.

If it were allowed to use the Replaces header in a REFER request, it would 
become possible
to explicitely effect a transfer. Assuming that the endpoint sending the 
REFER request has
a Session with both parties that it wishes to transfer the call to, such a 
REFER would be
used as follows. The REFER request will contain a Refer-To header to 
indicate the endpoint
to which the Session should be transferred. I.e. the endpoint that should 
be contacted by
the recipient of the REFER request. The Replaces header contains details 
about the Session
that the sender of the REFER request has with the party to which that 
Session should be
transferred. The recipient of the REFER copies this header into the INVITE 
request that
it sends to establish the transfer call. The latter is, what I believe, 
described in the
current Replaces draft.

Any opinions on this?

Regards,

Frank

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---

From: Rohan Mahy <[email-masked]>
Date: Thu, 24 Oct 2002 15:14:17 -0700

Hi,

If I understand your message correctly, I believe you are trying to add 
additional semantics to Replaces which are not necessary to accomplish 
what I believe you want to achieve.

If A has an existing session with C using a call-id of foo, I think you 
are asking for A to send a REFER that causes B to send an INVITE to C, 
such that the INVITE contains a Replaces header.  This would cause a 
new session from B to C to replace the old session from A to C.

To do this A sends a REFER with the following Refer-To header:

REFER sip:B SIP/2.0
Refer-To: <sip:C?Replaces=foo>

This causes B to generate an INVITE of the following form:

INVITE sip:C SIP/2.0
Replaces: foo

Headers in the REFER request are orthogonal to headers in the triggered 
request.  Headers  intended for inclusion in the triggered request are 
included in the Refer-To URI.

I hope this answers your question.

thanks,
-rohan