Subject: [SIP] Question on Tag for To and From headers

From: "William Lee" <[email-masked]>
Date: Thu, 15 Jun 2000 11:12:17 -0500

The syntax for the To and the From headers is: ( name-addr | addr-spec )
*(";"addr-params) where
addr-params = tag-param | addr-extension

Does this mean that you can have zero or more tag-params followed or
inter-mingled among zero or more
addr-extensions? This is very unclear.

There is a similar situation with the Server and User-Agent headers 1*(
product | comment ).  Is there a
one to one relationship between product and comment, or can there be say one
product followed by
several comments, etc. ?

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From: Henning Schulzrinne <[email-masked]>
Date: Thu, 15 Jun 2000 12:24:40 -0400

> William Lee wrote:
> 
> The syntax for the To and the From headers is: ( name-addr | addr-spec
> ) *(";"addr-params) where

Yes, that's what the BNF says. As has been discussed here in different
contexts, BNFs are not good at (and not meant for) conveying semantic
restrictions such as "no duplication". That said, since there's only one
tag, it makes sense to label it as 

[ tag-param ] *addr-extension

> 
> There is a similar situation with the Server and User-Agent headers
> 1*( product | comment ).  Is there a
> one to one relationship between product and comment, or can there be
> say one product followed by
> several comments, etc. ?

Yes.

-- 
Henning Schulzrinne   http://www.cs.columbia.edu/~hgs

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From: "Rick Workman" <[email-masked]>
Date: Thu, 15 Jun 2000 13:19:49 -0400

I'd even argue for taking it one step further. Since tag-param fits into the
syntactic defintion of addr-extension, why not:

( name-addr | addr-spec ) *(";"addr-extension)

and leave it up to the semantic desription of what address parameters are
allowed, what are the semantics of duplicates, and what they actually mean
at the application level (as opposed to what they look like). I generally
think that applying semantic restrictions in formal syntax specifications is
not a good idea, even though it's sometimes possible.

Rick Workman
Nortel Networks