RFC 2017 (rfc2017) - Page 3 of 5
Definition of the URL MIME External-Body Access-Type
Alternative Format: Original Text Document
RFC 2017 URL Access-Type October 1996
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.
Freed, et. al. Standards Track