Definition of the URL MIME External-Body Access-Type
draft-ietf-mailext-acc-url-01
The information below is for an old version of the document that is already published as an RFC.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 2017.
|
|
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Authors | Ned Freed , Keith Moore | ||
Last updated | 2013-03-02 (Latest revision 1995-11-20) | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Intended RFC status | (None) | ||
Formats | |||
Stream | WG state | (None) | |
Document shepherd | (None) | ||
IESG | IESG state | Became RFC 2017 (Proposed Standard) | |
Consensus boilerplate | Unknown | ||
Telechat date | (None) | ||
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draft-ietf-mailext-acc-url-01
Network Working Group Ned Freed
Internet Draft Keith Moore
<draft-ietf-mailext-acc-url-01.txt> Allan Cargille, WG Chair
Definition of the URL
MIME External-Body Access-Type
November 1995
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
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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
Internet Draft URL Access-Type November 1995
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".
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.
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Internet Draft URL Access-Type November 1995
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 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
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Internet Draft URL Access-Type November 1995
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 follow 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
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Internet Draft URL Access-Type November 1995
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, RFC 822, UDEL, August 1982.
[RFC-1521]
Borenstein, N. and Freed, N., "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., McCahill, M., "Uniform
Resource Locators (URL)", December 1994.
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Internet Draft URL Access-Type November 1995
7. Author Addresses
Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
tel: +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
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