Subject: [Sip] Caller Preferences (05) - Clarification on ABNF syntax for cp-par ams.

From: "Padhye, Chinmay" <[email-masked]>
Date: Mon, 17 Dec 2001 12:01:35 -0800

Hello,

	I have a question about the ABNF definition for cp-params as defined
in version - 05 of the caller preferences draft. I am taking 'feature-param'
as an example.

	The ABNF for 'feature-param' is defined as -

---- (From the draft)
	feature-param   =  "feature" "=" <"> [<!>] 1#feature-conjunction <">
	feature-conjunction    =  feature-value ["&" feature-conjunction]
   	feature-value          =  "voice-mail" | "attendant" | other-value
----
   
	This is an example from the draft -

---- (From the draft)
	However, the following are illegal Contact headers:
      Contact: sip:user@host;feature="voicemail&attendant"'
      Contact: sip:[email-masked];mobility="!fixed"

      But the same two are valid Accept-Contact headers:
      Accept-Contact: sip:user@host;feature="voicemail&attendant"
      Accept-Contact: sip:[email-masked];mobility="!fixed"
----

	Hence ',' is used a separator for 'feature-conjunction' when
'feature-param' appears in the 'Contact' header and  '&' is used a separator
for 'feature-conjunction' when 'feature-param' appears in the
'Accept-Contact' header. Is this correct? 

	What is the ABNF for 'feature-param' if someone decides to use in
some other method. Specifically what should the separator value be? 

	IMO there should be a consistent ABNF definition for a parameter
irrespective of the header in which appears, especially for things like the
separator value. The description of a parameter should not depend on the
header in which the parameter is present. 

	Any comments?

Regards,

Chinmay
(The opinions expressed here are mine alone)

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 18 Dec 2001 10:08:04 -0500

> 'Accept-Contact' header. Is this correct? 

No. When used in Contact, the feature-conjunction can only have one 
element. Thus, the comma separates single-valued conjunctions.

> header in which the parameter is present. 

It is not. The header its present in does introduces constraints which 
are not described by the BNF; that is a frequent thing. For example, a 
SIP URL isn't supposed to have a ttl if the address isn't multicast. 
Good luck representing that with BNF.

-Jonathan R.
-- 
Jonathan Rosenberg, PhD                 72 Eagle Rock Avenue
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