RFC 1873

RFC 1873: Message/External-Body Content-ID Access Type 🎀

okay bestie, this is RFC 1873, and it's giving Experimental vibes 🧪 It came out of the Network Working Group in December 1995, written by E. Levinson of Accurate Information Systems, Inc. and J. Clark 💖 The header doesn't list anything it obsoletes or updates, so it's doing its own thing ✨

Status of this Memo 📜

ngl this part is the official disclaimer 🙈 This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested, and distribution of this memo is unlimited 🦋

Abstract 🌸

okay so picture this: you're using MIME [MIME] to wrap up a structured object made of lots of elements, like an SGML [SGML] document 📄 A single element can show up several times, but normal encapsulation maps each element to its own MIME entity. It's actually super useful to include the repeated elements exactly once 🙌 and to keep the object's structure intact, which means you want a way to unambiguously point at another body part in the same message 🫶

The existing Message/External-Body access-types already let a MIME entity (body-part) refer to something outside the message by describing how to access it 🌐 The Content-ID access method in this document is the one that lets you point at an object inside the message itself 💅

1. Introduction 🧚

Imagine a MIME multipart entity where several body parts carry the same data (body) but have different parameters or Content-* headers 🎀 Repeating that data in every single part is wasteful, so the efficient move is to store it once and save bandwidth and storage space 💾 To get those benefits, this doc defines an access-type that lets one message part refer to another part in the same message ✨

2. The Content-ID Access Type 💅

2.1 Registration Information 📝

Here's the registration form, verbatim:

MIME access-type name:   content-id

Required parameters:     none

Optional parameters:     none

Published specification: this document

Person & email address
to contact for further
information:             Ed Levinson <ELevinson@accurate.com>

The additional requirements are the real tea 🍵 The content-id header of the access-type=content-id MIME entity must match (be identical to) exactly one content-id in the same message, excluding other access-type=content-id entities. So the content-id access type can only occur within a multipart message, and it can refer to another body part anywhere in that same message 🔥

A MIME User Agent (MUA) builds the resulting MIME body part as described below. We call the access-type=content-id MIME entity the referring body part, and the MIME body part it refers to (the one with the matching content-id) the referenced body part 🦋 The MIME entity that results from the content-id access type consists of:

• (a) the referenced body part's content-type header,
• (b) the referring body part's headers except its content-type header,
• (c) any headers in the referenced body part not in the referring one,
• (d) the line separating the headers from the body, and
• (e) the referenced body part's body.

2.2 Example Usage 🍓

This example shows a message with two identical images, and it's so cute 🥹

MIME-Version: 1.0
Content-Type: Multipart/Mixed;
   boundary=tiger-lily

--tiger-lily
Content-Type: image/jpeg
Content-ID: <950323.1552@XIson.com>

AAAcdb...
--tiger-lily
Content-type: Message/External-Body;
   access-type=content-id
Content-ID: <950323.1552@XIson.com>
Content-Description:
   This body part is duplicated by reference

--tiger-lily--

The equivalent MIME entity for the second body part is:

--tiger-lily
Content-Type: image/jpeg
Content-ID: <950323.1552@XIson.com>
Content-Description:
   This body part is duplicated by reference

AAAcdb...
--tiger-lily

3. Security Considerations 🔐

Okay, this section is important, so listen up 👑 The content-id access-type does not impact the security of messages or systems. The referenced MIME entity may have security implications, though, so don't skip past that part 😭

4. References 📚

• [822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822, UDEL, August 1982.
• [SGML] ISO 8879:1988, Information processing -- Text and office systems -- Standard Generalized Markup Language (SGML).
• [MIME] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, Bellcore, Innosoft, September 1993.

5. Authors' Addresses 🫶

Edward Levinson
Accurate Information Systems, Inc.
2 Industrial Way
Eatontown, NJ  07724-2265
USA

Phone: +1 908 389 5550
EMail: <ELevinson@Accurate.com>


James Clark
90 Clarendon Road
London W11 2HR
UK

EMail: <jjc@jclark.com>