Network Working Group N. Freed
Request for Comments: 2017 Innosoft International
Category: Standards Track K. Moore
University of Tennessee
A. Cargille, WG Chair
October 1996

Definition of the URL
MIME External-Body Access-Type

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

1. Abstract

This memo defines a new access-type for message/external-body MIME
parts for Uniform Resource Locators (URLs). URLs provide schemes to
access external objects via a growing number of protocols, including
HTTP, Gopher, and TELNET. An initial set of URL schemes are defined
in RFC1738.

2. IntrodUCtion

The Multipurpose Internet Message Extensions (MIME) define a facility
whereby an object can contain a reference or pointer to some form of
data rather than the actual data itself. This facility is embodied in
the message/external-body media type defined in RFC1521. Use of
this facility is growing as a means of conserving bandwidth when
large objects are sent to large mailing lists.

Each message/external-body reference must specify a mechanism whereby
the actual data can be retrieved. These mechanisms are called access
types, and RFC1521 defines an initial set of access types: "FTP",
"ANON-FTP", "TFTP", "LOCAL-FILE", and "MAIL-SERVER".

Uniform Resource Locators, or URLs, also provide a means by which
remote data can be retrieved automatically. Each URL string begins
with a scheme specification, which in turn specifies how the
remaining string is to be used in conjunction with some protocol to
retrieve the data. However, URL schemes exist for protocol operations
that have no corresponding MIME message/external-body access type.
Registering an access type for URLs therefore provides
message/external-body with access to the retrieval mechanisms of URLs
that are not currently available as access types. It also provides
access to any future mechanisms for which URL schemes are developed.

This access type is only intended for use with URLs that actually
retreive something. Other URL mechansisms, e.g. mailto, may not be
used in this context.

3. Definition of the URL Access-Type

The URL access-type is defined as follows:

(1) The name of the access-type is URL.

(2) A new message/external-body content-type parameter is
used to actually store the URL string. The name of the
parameter is also "URL", and this parameter is
mandatory for this access-type. The syntax and use of
this parameter is specified in the next section.

(3) The phantom body area of the message/external-body is
not used and should be left blank.

For example, the following message illustrates how the URL access-
type is used:

Content-type: message/external-body; access-type=URL;
URL="http://www.foo.com/file"

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

THIS IS NOT REALLY THE BODY!

3.1. Syntax and Use of the URL parameter

Using the ANBF notations and definitions of RFC822 and RFC1521, 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 RFC1738. 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 RFC822 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 RFC1738. 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 [RFC1521].

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.

4. Security Considerations

The security considerations of using URLs in the context of a MIME
access-type are no different from the concerns that arise from their
use in other contexts. The specific security considerations
associated with each type of URL are discussed in the URL's defining
document.

Note that the Content-MD5 field can be used in conjunction with any
message/external-body access-type to provide an integrity check. This
insures that the referenced object really is what the message
originator intended it to be. This is not a signature service and
should not be confused with one, but nevetheless is quite useful in
many situations.

5. Acknowledgements

The authors are grateful for the feedback and review provided by John
Beck and John Klensin.

6. References

[RFC-822]
Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC822, UDEL, August 1982.

[RFC-1521]
Borenstein, N. and N. Freed, "MIME (Multipurpose
Internet Mail Extensions): Mechanisms for Specifying and
Describing the Format of Internet Message Bodies", RFC
1521, Bellcore, Innosoft, September, 1993.

[RFC-1590]
Postel, J., "Media Type Registration Procedure", RFC
1590, USC/Information Sciences Institute, March 1994.

[RFC-1738]
Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", December 1994.

7. Authors' Addresses

Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA

Phone: +1 818 919 3600
Fax: +1 818 919 3614
EMail: ned@innosoft.com

Keith Moore
Computer Science Dept.
University of Tennessee
107 Ayres Hall
Knoxville, TN 37996-1301
USA

EMail: moore@cs.utk.edu