*RFC 2017 3.1.  Syntax and Use of the URL parameter

   Using the ANBF notations and definitions of RFC 822 and RFC 1521, the
   syntax of the URL parameter Is as follows:

     URL-parameter := <"> URL-word *(*LWSP-char URL-word) <">

     URL-word := token
                 ; Must not exceed 40 characters in length

   The syntax of an actual URL string is given in RFC 1738.  URL strings
   can be of any length and can contain arbitrary character content.
   This presents problems when URLs are embedded in MIME body part
   headers that are wrapped according to RFC 822 rules. For this reason
   they are transformed into a URL-parameter for inclusion in a
   message/external-body content-type specification as follows:

    (1)   A check is made to make sure that all occurrences of
          SPACE, CTLs, double quotes, backslashes, and 8-bit
          characters in the URL string are already encoded using
          the URL encoding scheme specified in RFC 1738. Any
          unencoded occurrences of these characters must be
          encoded.  Note that the result of this operation is
          nothing more than a different representation of the
          original URL.

    (2)   The resulting URL string is broken up into substrings
          of 40 characters or less.

    (3)   Each substring is placed in a URL-parameter string as a
          URL-word, separated by one or more spaces.  Note that
          the enclosing quotes are always required since all URLs
          contain one or more colons, and colons are tspecial
          characters [RFC 1521].

   Extraction of the URL string from the URL-parameter is even simpler:
   The enclosing quotes and any linear whitespace are removed and the
   remaining material is the URL string.

   The following example shows how a long URL is handled:

     Content-type: message/external-body; access-type=URL;
                   URL="ftp://ftp.deepdirs.org/1/2/3/4/5/6/7/
                        8/9/10/11/12/13/14/15/16/17/18/20/21/
                        file.html"

     Content-type: text/html
     Content-Transfer-Encoding: binary

     THIS IS NOT REALLY THE BODY!

   Some URLs may provide access to multiple versions of the same object
   in different formats. The HTTP URL mechanism has this capability, for
   example.  However, applications may not expect to receive something
   whose type doesn't agree with that expressed in the
   message/external-body, and may in fact have already made irrevocable
   choices based on this information.

   Due to these considerations, the following restriction is imposed:
   When URLs are used in the context of an access-type only those
   versions of an object whose content-type agrees with that specified
   by the inner message/external-body header can be retrieved and used.

*実装について

URL-word の定義たる token とは何か。 [[RFC1521]] <urn:ietf:rfc:1521>
の名が挙がっているので [[MIME-token]] のように思えますけど、
URL では token に含まれない文字も使えたはずです。

*SEE ALSO
-[[MIME]]
--[[Internet媒体型]]
--[[message/external-body]]
--[[RFC2047]]
--[[RFC2231]]
-[[RFC822と仲間達の頭領域名]]
--[[Content-Base:領域]]
--[[Content-Location:領域]]
--[[Content-Type:領域]]
-[[URI]]
--[[URL]]

*LICENSE

See [[RFCのライセンス]]