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Re: Ambiguities in header-field rules (p1-messaging)

From: Amos Jeffries <squid3@treenet.co.nz>
Date: Thu, 18 Aug 2011 15:16:38 +1200
To: <ietf-http-wg@w3.org>
Message-ID: <88b489507e504d9eef318438194f929e@treenet.co.nz>
 On Wed, 17 Aug 2011 19:58:21 +0200, Frank Mertens wrote:
> Hi,
>
> I played around with the ABNF published by this WG and stumbled
> over some rough edges.
>
> Current rules:
>
> OWS = *( [ obs-fold ] WSP )
> header-field = field-name ":" OWS [ field-value ] OWS
> field-value = *( field-content / OWS )
> field-content = *( WSP / VCHAR / obs-text )
>
> Problems:
>
>  - field-value and field-content match the empty symbol,
>    which requires searching for the longest match, which is costly
>    (and confusing for the human reader)
>  - because field-value matches the empty symbol claiming it optional
>    in header-field allows ambiguous productions of same length
>    (with or without field-value of zero length?)
>
> Suggested improvement:
>
> field-value = 1*( field-content OWS )
> field-content = 1*( VCHAR / WSP / obs-text )
>
> Best Regards,
> Frank Mertens.


 The OWS on header-field remains ambiguous as well.

 Also, with WSP being in field-content there is the possibility of 
 header-field matching:

   field-name ":" [ obs-fold ] 1*( WSP OWS ) OWS

 Nasty. But section 3.2 comes to the rescue:
  "The field value does not include any leading or trailing white space"
 and
  "HTTP/1.1 senders MUST NOT produce messages that include line folding"

 So OWS in the field-value ABNF appears to be invalid in several ways 
 going by the text.


 Perhapse this would be better:

  header-field = field-name ":" [ WSP ] BWS [ field-value ]
  field-value = 1*( field-content BWS )
  field-content = 1*( VCHAR / WSP / obs-text )




 Nit: section 1.2.2 currently says:

 "Multiple OWS octets that occur within field-content
    SHOULD be replaced with a single SP before interpreting the field
    value or forwarding the message downstream."
 ...
 "Multiple RWS octets that occur within field-content SHOULD be
    replaced with a single SP before interpreting the field value or
    forwarding the message downstream.
 "

 When there is no OWS or RWS in the field-content ABNF.

 I think both should say header-field instead of field-content. Or maybe 
 drop the "within field-content" condition to make it general.


 AYJ
Received on Thursday, 18 August 2011 03:17:15 GMT

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