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rfc:rfc2017

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 RFC 1738.

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 RFC 1521.  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 RFC 1521 defines an initial set of access types: "FTP",
 "ANON-FTP", "TFTP", "LOCAL-FILE", and "MAIL-SERVER".

Freed, et. al. Standards Track [Page 1] RFC 2017 URL Access-Type October 1996

 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!

Freed, et. al. Standards Track [Page 2] 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 [Page 3] RFC 2017 URL Access-Type October 1996

 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.

Freed, et. al. Standards Track [Page 4] RFC 2017 URL Access-Type October 1996

6. References

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

Freed, et. al. Standards Track [Page 5]

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