# A content-id URL scheme

Ed Levinson (elevinso@Accurate.COM)
Fri, 03 Feb 1995 16:29:35 -0500

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    Message-Id: <9502032130.AA07352@Accurate.COM>

    To: uri@bunyip.com

    Subject: A content-id URL scheme

    Date: Fri, 03 Feb 1995 16:29:35 -0500

    From: Ed Levinson <elevinso@Accurate.COM>

--tiger-lily

Attached is an internet-draft describing a URL scheme for MIME
content-ids. Given that it's brief, 3 pages, I've taken the liberty of
distributing it to this list and I solicit your comments.

Thanks.../Ed

--tiger-lily
Content-Description: <draft-levinson-cid-00.txt>

Network Working Group E. Levinson
Internet Draft: URL:cid ACCURATE Info. Sys.
<draft-levinson-cid-00.txt> January 31, 1995

CID: The Content-ID Uniform Resource Locator

This draft document is being circulated for comment. Please
send your comments to the authors or to the uri-wg mail list
<uri-wg@bunyip.com>. If consensus is reached, this URL may
be registered as a scheme with IANA and this document may be
submitted to the RFC editor as an Informational protocol
specification.

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 Working Groups. Note that other
groups may also distribute working documents as Internet
Drafts.

Internet Drafts are draft documents valid for a maximum of
six months. They may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use
Internet Drafts as reference material or to cite them other
than as a "working draft" or "work in progress".

Please check the abstract listing in each Internet Draft
directory for the current status of this or any other
Internet Draft.

Abstract

The Uniform Resource Locator (URL) scheme, "cid", allows
compound or aggregate objects in a multipart mail message to
refer to one another by their body part labels.

1. Introduction

Consider an object that consists of a set of distinct parts
containing internal references to one another. The
structure of such references will be called an entity
structure after [SGML]. Within a given computing system,
the entity structure usually exists as file references.
When transferring the object between systems, the sending
system's file names may be problematic because of special
characters or differing interpretations of those characters.

The encapsulation of these objects within [MIME] requires
preserving the entity structure while providing a neutral
representation for the references. A Content-ID Uniform
Resource Locator (cid URL) serves that purpose.

A companion document, [REL], contains an example of cid

Levinson Expires July 31, 1995 [Page 1]
Internet Draft URL Scheme: cid

usage.

2. CID

RFC1738 [URL] reserves the scheme "cid" for Content-ID.
This memorandum defines the syntax for the cid URL.

A cid URL takes the form

cidurl = "cid" ":" cid-spec

where cid-spec is a restricted form of "addr-spec" as
defined in [RFC822]. The purpose of the restriction is to
eliminate special characters from the cid URL. Such
characters can be problematical in many environments (e.g.,
HTML and SGML) in which the cid URL may be used. Cid URLs
are a subset of MIME content-IDs and RFC822 message-IDs

cid-spec = local-part "@" host ; globally unique

local-part = atom ; as per RFC822

domain-ref = atom ; as per RFC822

host = ; RFC1738, sec 3.1

A cid differs from a MIME content-id in the ommission of the
leading and trailing brackets, "<" and ">", and the
restricted character set. To transform a cid into a valid
content-id, surround the cid-spec with the enclosing
brackets, i.e.,

content-id = "<" cid-spec ">".

Cids are globally unique [MIME, p.19]. A common technique
for generating them is to use a time and date stamp with the
local host's domain name, e.g., 950124.162336@XIson.com.

3. Security

Security issues are not addressed in this memorandum.

4. References

[822] Crocker, D., Standard for the Format of ARPA Internet Text
Messages, August 1982, University of Delaware, RFC 822.

[MIME] Borenstein, N. and Freed, N., MIME (Multipurpose Internet
Mail Extensions): Mechanisms for Specifying and Describing
the Format of Internet Message Bodies, June 1992, RFC 1341.

[REL] Alvestrand, H. T., Levinson, E., The MIME Multipart/Related
Content-Type, work in progress,
ftp://ds.internic.net/internet-drafts/draft-levinson-

Levinson Expires July 31, 1995 [Page 2]
Internet Draft URL Scheme: cid

multipart-related-00.txt.

[SGML] ISO 8879:1988, Information processing -- Text and office
systems -- Standard Generalized Markup Language (SGML).

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

5. Acknowledgements

This work reflects the ideas freely provided to the author by many peo-
ple, including Tim Berners-Lee who pointed me at the idea of using a URL
"scheme" in the SGML encapsulation proposal.

6. Author's Address

Edward Levinson
Accurate Information Systems, Inc.
2 Industrial Way
Eatontown, NJ 07724-2265
USA
+1 908 389 5550
<ELevinson@Accurate.com>

Levinson Expires July 31, 1995 [Page 3]
--tiger-lily--

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