RFC 2234 section 3.8 indicates meaning of the brackets within ABNF:
Square brackets enclose an optional element sequence:
[foo bar]
is equivalent to
*1(foo bar).
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On
> Behalf Of Iñaki Baz Castillo
> Sent: Thursday, December 11, 2008 3:28 PM
> To: [email protected]
> Subject: Re: [Sip-implementors] Empty Proxy-Authorization Header
>
> El Jueves, 11 de Diciembre de 2008, Brett Tate escribió:
>
> > I agree that the header value is not optional within
> > Proxy-Authorization. However an empty header value is valid within
> > some headers such as Supported.
>
> "Supported" header does NOT allow empty value:
>
> --------
> Supported = ( "Supported" / "k" ) HCOLON [option-tag
> *(COMMA option-tag)]
>
> option-tag = token
>
> token = 1*( alphanum
> / "-" / "." / "!" / "%" / "*"
> / "_" / "+" / "`" / "'" / "~" )
> ---------
>
> It requires, at least, one value.
>
>
> --
> Iñaki Baz Castillo
>
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>
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