Subject: [SIP] REFER feature with 'replaces' header

From: Suheel Hussain <[email-masked]>
Date: Mon, 12 Mar 2001 12:21:51 -0500

I am in the process of implementin REFER feature on a user-agent. I 
plan to use 'replaces' header (draft-biggs-sip-replaces-00) in REFER 
for smooth call-transfer. (Transfer target quitely replaces, its 
connection with Transferror, with INVITE from Transferee.)

I have following questions regarding REFER with replaces:
1. Is there proposed IETF activity for draft-biggs-sip-replaces-00?
    Or, above draft has been replaced by another equivalent draft?

2. 'replaces' header specifies only Transferror<->Target call-id to
     identify call-leg to be replaced on transfer target. According to
     SIP spec,
     From, To, and call-id specifies a complete call.
     Thus, more than call-id mandatory field are required to specify
     transferror-target call.

I would appreciate if anyone can throw a light on this.

Thanks.
-- 

Suheel Hussain
[email-masked]
Work # (919) 392-2312

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---

From: Billy Biggs <[email-masked]>
Date: Mon, 12 Mar 2001 12:53:26 -0500

Suheel Hussain ([email-masked]):

> [...]
>    SIP spec, From, To, and call-id specifies a complete call.  Thus,
>    more than call-id mandatory field are required to specify
>    transferror-target call.

  There was some debate internally when we wrote the draft on what
exactly should be required in the header such that it would be easiest
to implement and easiest to use.

  The call-id as a unique key is attractive because it is "very
unlikely" that two established call legs will share the same call-id.
Escaping the full to and from headers is a pain, and performing the
unescaping and compare is easy to screw up (especially if there are
parameters in the URIs etc).  We wanted to avoid interoperability
difficulties, even at the cost of messing up transfers in the extremely
unlikely case.

  Personally, I think the best compromise is to place in the Replaces
header the Call-ID, near-tag and far-tag for the call leg.  This cuts
down on message size and avoids dangerous compares while still uniquely
identifying the call.

  Thoughts?

-- 
Billy Biggs                     [email-masked]
http://www.billybiggs.com/      [email-masked]

---

From: Suheel Hussain <[email-masked]>
Date: Mon, 12 Mar 2001 13:44:17 -0500

inline:

> header the Call-ID, near-tag and far-tag for the call leg.  This cuts

how do you define near-tag and far-tag being?

-- 

Suheel Hussain
[email-masked]
Work # (919) 392-2312