Subject: [Sip] Comments on draft-ietf-sip-multiple-refer-01.txt

From: Neil Hampton <[email-masked]>
Date: Wed, 14 Feb 2007 18:35:52 +1300

I have just come against a call centre application that requires a REFER 
to multiple destinations.  In my investigations, it appears that 
draft-ietf-sip-multiple-refer-01.txt meets what I require from the message.

After reading the draft, however, I am a little disappointed that it 
does not use a SIP header to specify the destinations but instead an XML 
body.  This seems to me to go a little against the grain - why do we 
need another format to specify a call control mechanism?  Surely the 
current SIP syntax can handle it!

The addition of XML means that implementations that wish to use REFER in 
this way, and previously contained purely a SIP stack, now require an 
XML parser with the implications for performance etc.  This also makes 
this request more difficult to implement on resource limited devices.

Are there any alternatives in discussion?
A Refer-To header with multiple values seems more appropriate to me; and 
if this violates the REFER specification, perhaps add a Refer-Cc header?

-- 
Regards,
Neil Hampton.
Hampton Software Ltd.
Tel: +64 9 974 3601
Mobile: +64 2102 344909
http://www.hamptonsoftware.co.uk

Hampton Software Limited is a company incorporated in England and Wales (Company Number 5563994)
and has its registered office at Hatherley House, Bisley Green, Bisley, Surrey.  GU24 9EW.

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---

From: Miguel Garcia <[email-masked]>
Date: Thu, 15 Feb 2007 15:35:27 +0200

Hi Neil:

The decision of using an XML document to convey the list of recipients 
was taking long time ago. At that time, we wanted to re-use as much as 
possible existing available building blocks, and XML documents for 
URI-lists were already there. Especially, the first method that was 
adopted to use a URI-list was INVITE, and there is no Refer-To header in 
an INVITE request, so the Refer-To wasn't an option. Additionally, we 
can reuse the same list format for other non-REFER URI-list methods, 
such as MESSAGE, INVITE, and SUBSCRIBE. These methods do not contain a 
Refer-To header, as I said.

Using XML URI-lists has other advantages, such as the possibility to add 
attributes, for example to qualify a recipient as a copied (similar to 
e-mail cc:).

The kind of lists that we are managing are fairly simple, so you don't 
really need to build a general XML parser, if you don't want to do it, 
but in any case, there are a few open source XML parsers out there. 
Whether we like it or not, XML is more and more present in our SIPpish 
lifes.

/Miguel

-- 
Miguel A. Garcia           tel:+358-50-4804586
sip:[email-masked]
Nokia Research Center      Helsinki, Finland