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Re: non-hierarchical URIs and square brackets

From: Al Gilman <asgilman@iamdigex.net>
Date: Thu, 19 Jun 2003 14:35:07 -0400
Message-Id: <>
To: uri@w3.org

At 05:45 AM 2003-06-19, Graham Klyne wrote:

>or mid:, defined by RFC 2392, which is clearly non-hierarchical, but:
>      mid-url       = "mid" ":" message-id [ "/" content-id ]
>-- http://www.faqs.org/rfcs/rfc2392.html

Please explain in what terms you find the 'mid' scheme to be "clearly 

MIME multiparts use the header-list form of metadata wrapper 
hierarchically, and this
reference is a reference based on that hierarchy.

This construction  uses the '/' delimiter to indicate a part of an RFC-2822
object and it identifies it both by the ID of the outer message/multipart
package and the ID of the inner object or MIME part.




  Universal Resource Locators: cid scheme

There is some serious mis-information.

The uniqueness of 'cid' values is global to the Internet, with similar
schemes and reliability as regards 'mid' values.  Also, the syntax for
referencing Content-ID values together with their Message-ID contexts is
well defined as quoted above.


    With the appended "content-id", it refers to a body part within a
    message, as does a "cid" URL.  The Content-ID of a MIME body part is
    required to be globally unique.  However, in many systems that store
    messages, body parts are not indexed independently their context
    (message).  The "mid" URL long form was designed to supply the
    context needed to support interoperability with such systems.


It's too bad the w3.org URL is delivering disinformation about MIME.
This incorrect page comes up first in a Google search for "'cid' scheme".

>This message is prompted by some questions/comments I received from 
>someone recently.  I've not yet read the -03 draft in detail, so I may 
>have overlooked some material.
>(1) hierarchical and non-hierarchical URIs
>I notice that in -03 the opaque-part syntax distinction has been 
>dropped.  My concern is that it may now not be clear when 
>relative-to-absolute URI conversion should take part of hierarchical '/' 
>characters in the URIs.  Previously, my understanding was that an 
>algorithm would look at the path component of the base URI and, if it 
>starts with a '/', assume the base and relative URIs are hierarchical and 
>apply the path-merging logic;  otherwise, the opaque-part in the base URI 
>is used in all-or-nothing fashion depending on what is present in the 
>"relative" URI.
>Section 3 says "a non-hierarchical path will be treated as opaque data by 
>a generic URI parser", but it's not clear at this point what constitutes a 
>"non-hierarchical path".
>Section 3.3 says " A path is always defined for a URI, though the defined 
>path may be empty (zero length) or opaque (not containing any "/" 
>delimiters)", which suggests that an opaque path may not contain *any* 
>un-escaped '/' characters.  This seems like an onerous restriction, and in 
>conflict with existing URI scheme usage; e.g. news: according to IANA is 
>currently specified by RFC 1738, and has:
>    A <newsgroup-name> is a period-delimited hierarchical name, such as
>    "comp.infosystems.www.misc". A <message-id> corresponds to the
>    Message-ID of section 2.1.5 of RFC 1036, without the enclosing "<"
>    and ">"; it takes the form <unique>@<full_domain_name>.  A message
>    identifier may be distinguished from a news group name by the
>    presence of the commercial at "@" character. No additional characters
>    are reserved within the components of a news URL.
>-- http://www.faqs.org/rfcs/rfc1738.html
>or mid:, defined by RFC 2392, which is clearly non-hierarchical, but:
>      mid-url       = "mid" ":" message-id [ "/" content-id ]
>-- http://www.faqs.org/rfcs/rfc2392.html
>Other non-hier URI schemes using '/' are:
>service:  http://www.faqs.org/rfcs/rfc2609.html
>My suggestion would be to add a brief comment in section 3 clarifying the 
>intent (as to what constitutes a hierarchical URI).  My preference is that 
>an absolute URI with net-path or abs-path form is hierarchical, otherwise not.
>(2) square brackets
>Is it necessary for square brackets to be reserved outside the net-path 
>component?  I personally use them quite often in fragment identifiers for 
>references.  My correspondent had another use for them in the path 
>component of a URI scheme.  I think there are several instances of them 
>occurring (unescaped) in message-IDs, and the mid: spec doesn't require 
>them to be escaped.  I think this also applies to the ldap: scheme.
>-- http://www.faqs.org/rfcs/rfc2392.html
>-- http://www.faqs.org/rfcs/rfc2255.html
>Graham Klyne
>PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E
Received on Thursday, 19 June 2003 16:30:08 UTC

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