RFC 2112

RFC 2112: The MIME Multipart/Related Content-type 💖

Okay bestie, look at this title block first 🎀 This doc is from the Network Working Group and it's Request for Comments: 2112. It was written by E. Levinson from XIson, Inc. ✨ It's a Standards Track doc, dated March 1997, and it obsoletes RFC 1872 🦋 It doesn't update anything else.

Status of this Memo ✨

This doc is an Internet standards track protocol for the Internet community, and it's basically asking for discussion and suggestions for improvements 🫶 For the current standardization state and status, check the "Internet Official Protocol Standards" (STD 1) 💅 Distribution of this memo is unlimited, so share away 🌈

Abstract 🎀

The Multipart/Related content-type is the main character for representing objects that are made of related MIME body parts 💜 This doc defines Multipart/Related and gives examples of how to use it ✨

1. Introduction 🦋

Okay so a bunch of MIME stuff, like MIME-PEM, MIME-Macintosh, and other proposals, all need multiple body parts that only make sense when you look at them together 🧚 The old way of handling this was to make a new multipart subtype for every new object 😭 But MIME is all about having one mechanism for lots of different purposes, so this doc gives us a single mechanism for these aggregate or compound objects 💅

Multipart/Related is how we represent compound objects in MIME 🔥 The object gets labeled with a "type" parameter. There are extra parameters for pointing to a specific starting body part (the root) and for auxiliary info that might be needed when unpacking or processing the object 🌸

Multipart/Related MIME entities can have Content-Disposition headers that suggest how a body part should be stored and displayed 🎀 But Multipart/Related processing wins over Content-Disposition, and section 4 explains how they work together ✨

Whoever builds the compound object is also the one responsible for displaying or processing its parts 👑

2. Multipart/Related Registration Information 💖

This form is copied from RFC 1590, Appendix A 💅

To:  IANA@isi.edu Subject:  Registration of new Media Type content-
type/subtype

Media Type name:           Multipart

Media subtype name:        Related

Required parameters:       Type, a media type/subtype.

Optional parameters:       Start
                  Start-info

Encoding considerations:   Multipart content-types cannot have
                           encodings.

Security considerations:   Depends solely on the referenced type.

Published specification:   RFC-REL (this document).

Person & email address to contact for further information:
                  Edward Levinson
                  47 Clive Street
                  Metuchen, NJ 08840-1060
                  +1 908 494 1606
                  XIson@cnj.digex.net

3. Intended usage 🧚

Multipart/Related is for compound objects that have several inter-related body parts 🦋 You literally can't display a Multipart/Related object by showing each body part on its own, it's giving "the whole is more than the parts" 😭 The content-type of the whole object is set by the type parameter. The "start" parameter (if you include it) uses a content-ID to point at the body part with the object root 💖 If you don't include it, the default root is the first body part in the Multipart/Related body 🌈

What makes a compound object different from other object types is how its body parts relate to each other 💜 Those relationships are often links inside the components that point at the other components 🫶 Inside one operating environment those links are often file names, and in a MIME message they can be represented with content-IDs or the value of some other "Content-" headers ✨

3.1. The Type Parameter 🎀

The type parameter is required, and its value is the MIME media type of the "root" body part 💅 That lets a MIME user agent figure out the content-type without having to look inside the enclosed body part ✨ If the type parameter's value and the root body part's content-type are different, the User Agent's behavior is undefined 🙈

3.2. The Start Parameter 🦋

The start parameter (if given) is the content-ID of the compound object's "root" 💖 If it's not there, the "root" is the first body part in the Multipart/Related entity 🌸 The "root" is the element the applications process first 👑

3.3. The Start-Info Parameter 🌈

The start-info parameter gives extra info to an application 🧚 It holds either a string, or a content-ID that points to another MIME entity in the message 💜 A typical use would be extra command line parameters or a MIME entity with auxiliary info for processing the compound object 🔥

Applications that use Multipart/Related must spell out how they interpret start-info ✨ User Agents shall pass the parameter's value along to the processing application 💅 Processes can tell a start-info reference apart from a token or quoted-string by checking the first non-white-space character: a "<" means it's a reference 🎀

3.4. Syntax 💖

  related-param    := [ ";" "start" "=" cid ]
                      [ ";" "start-info"  "="
                  ( cid-list / value ) ]
                      [ ";" "type"  "=" type "/" subtype ]
                ; order independent

  cid-list        := cid cid-list
  cid             := msg-id     ; c.f. [822]

  value           := token / quoted-string    ; c.f. [MIME]
                 ; value cannot begin with "<"

Parameter values usually need quoting, bestie 🙈 Msg-id has special characters like "<", ">", and "@", plus maybe others 🫶 If msg-id has quoted-strings in it, those quote marks must be escaped 💅 The type parameter has a special character too, the "/" 🦋

4. Handling Content-Disposition Headers 🔥

Content-Disposition Headers [DISP] suggest presentation styles for MIME body parts ✨ [DISP] describes two presentation styles, called the disposition types INLINE and ATTACHMENT 💜 Inside a multipart entity, these let the sender suggest how things should be presented 🌸 [DISP] also allows an optional storage (file) name 🧚 Content-Disposition headers can show up in one or more body parts inside a Multipart/Related entity 💖

Using Content-Disposition headers alongside Multipart/Related gives presentation info to User Agents that don't recognize Multipart/Related 🎀 Those agents will treat the multipart as Multipart/Mixed, and they might find the Content-Disposition info useful 🌈

With Multipart/Related, though, the application that processes the compound object decides the presentation style for all of the contained parts 👑 In that case the Content-Disposition header info is redundant, or even misleading 😭 So User Agents that understand Multipart/Related shall ignore the disposition type within a Multipart/Related body part 🙈

A User Agent that handles both Multipart/Related and Content-Disposition headers could pass the invoked application the Content-Disposition header's optional filename parameter, as the Multipart/Related's filename 🦋 How that info gets used depends on the specific application, and it should be specified when describing how the matching compound object is handled ✨ Those descriptions would fit in an RFC that registers that object's media type 💅

5. Examples 💜

5.1 Application/X-FixedRecord ✨

The X-FixedRecord content-type is made of one or more octet-streams plus a list of the lengths of each record 🧚 The root lists the record lengths of each record within the streams 🫶 The record length list (type Application/X-FixedRecord) is a set of INTEGERs written in ASCII format, one per line 🌸 Each INTEGER gives the number of octets from the octet-stream body part that make up the next "record" 🔥

The example below uses a single data block 🎀

  Content-Type: Multipart/Related; boundary=example-1
          start="<950120.aaCC@XIson.com>";
          type="Application/X-FixedRecord"
       start-info="-o ps"

  --example-1
  Content-Type: Application/X-FixedRecord
  Content-ID: <950120.aaCC@XIson.com>

  25
  10
  34
  10
  25
  21
  26
  10
  --example-1
  Content-Type: Application/octet-stream
  Content-Description: The fixed length records
  Content-Transfer-Encoding: base64
  Content-ID: <950120.aaCB@XIson.com>

  T2xkIE1hY0RvbmFsZCBoYWQgYSBmYXJtCkUgSS
  BFIEkgTwpBbmQgb24gaGlzIGZhcm0gaGUgaGFk
  IHNvbWUgZHVja3MKRSBJIEUgSSBPCldpdGggYS
  BxdWFjayBxdWFjayBoZXJlLAphIHF1YWNrIHF1
  YWNrIHRoZXJlLApldmVyeSB3aGVyZSBhIHF1YW
  NrIHF1YWNrCkUgSSBFIEkgTwo=

  --example-1--

5.2 Text/X-Okie 💖

Text/X-Okie is a made-up markup language that lets you include images with text 🦋 The cool thing about this example is that it has two extra body parts, both pictures 🌈 The encapsulated document refers to them internally using each picture's body part content-ID 💅 Using "cid:" like this might be handy for lots of compound objects 🧚 But it's not part of the Multipart/Related spec, so don't get it twisted 🙈

  Content-Type: Multipart/Related; boundary=example-2;
          start="<950118.AEBH@XIson.com>"
       type="Text/x-Okie"

  --example-2
  Content-Type: Text/x-Okie; charset=iso-8859-1;
          declaration="<950118.AEB0@XIson.com>"
  Content-ID: <950118.AEBH@XIson.com>
  Content-Description: Document

  {doc}
  This picture was taken by an automatic camera mounted ...
  {image file=cid:<950118.AECB@XIson.com>}
  {para}
  Now this is an enlargement of the area ...
  {image file=cid:<950118:AFDH@XIson.com>}
  {/doc}
  --example-2
  Content-Type: image/jpeg
  Content-ID: <950118.AFDH@XIson.com>
  Content-Transfer-Encoding: BASE64
  Content-Description: Picture A

  [encoded jpeg image]
  --example-2
  Content-Type: image/jpeg
  Content-ID: <950118.AECB@XIson.com>
  Content-Transfer-Encoding: BASE64
  Content-Description: Picture B

  [encoded jpeg image]
  --example-2--

5.3 Content-Disposition 🌸

In the example above, each image body part could also have a Content-Disposition header ✨ Here's what that looks like 👑

  ...
  --example-2
  Content-Type: image/jpeg
  Content-ID: <950118.AECB@XIson.com>
  Content-Transfer-Encoding: BASE64
  Content-Description: Picture B
  Content-Disposition: INLINE

  [encoded jpeg image]
  --example-2--

User Agents that recognize Multipart/Related will ignore the Content-Disposition header's disposition type 🙈 Other User Agents will process the Multipart/Related as Multipart/Mixed, and they may make use of that header's info 🫶

6. User Agent Requirements ✨

okay so bestie, this part is all about what user agents have to do, and the first thing is what happens when a client doesn't know Multipart/Related at all 👀. Those clients shall, following [MIME], treat the whole entity as Multipart/Mixed. 🫶 Then there's the in-between case: MIME User Agents that do recognize Multipart/Related but can't process the particular type should give the user the option of suppressing the entire Multipart/Related body part shall be. 💅 The user gets the final say!!

Existing MIME-capable mail user agents (MUAs) already handle the existing media types in a super straightforward way ✨. For discrete media types (e.g. text, image, etc.), the body of the entity can go straight to a display process. The composite subtypes can basically be broken down into handling one or more discrete types. Multipart/Related is different though, because you can't just handle each entity on its own 😭. The processing is all about how the parts relate to each other, so it's a whole vibe.

Next up are the details the processing application needs to know 🌸.

It's possible that an application-specific "receiving agent" will tweak the entities for display before the actual application process gets them 🦋. Okie, above, is an example of this: it may need a receiving agent to parse the document and swap in local file names for the originator's file names. Other applications might only need a table showing how local file names match up with the originator's. The receiving agent takes responsibility for all of that processing 💜.

6.1 Data Requirements

MIME-capable mail user agents (MUAs) are required to provide the application:

• (a) the bodies of the MIME entities and the entity Content-* headers,
• (b) the parameters of the Multipart/Related Content-type header, and
• (c) the correspondence between each body's local file name, that body's header data, and, if present, the body part's content-ID.

Basically the app needs all the receipts 🧾🎀.

6.2 Storing Multipart/Related Entities

Multipart/Related is the media type for objects with internal linkages between body parts 🔗. When these objects get stored, those linkages may need processing by the application or its receiving agent. Keep that in mind, bestie 🌈.

6.3 Recursion

MIME is recursive, so you've got to expect a Multipart/Related entity to contain other Multipart/Related entities 🪆. When a Multipart/Related entity is being processed for display or storage, any enclosed Multipart/Related entities shall be processed as though they were being stored. Nesting is no cap a whole mood 😭🔥.

6.4 Configuration Considerations

It's suggested that MUAs using configuration mechanisms (see [CFG] for an example) refer to Multipart/Related as Multipart/Related/<type>, where <type> is the value of the "type" parameter 🧚. Cute and organized, periodt ✨.

---

7. Security considerations 🔐

Security considerations relevant to Multipart/Related are identical to those of the underlying content-type. So no sneaky new threats here, but you still have to take the underlying content-type's security seriously, bestie 💀. Don't skip it!!

8. Acknowledgments 🫶

This proposal came out of conversations the author had with many people 💖. In particular, Harald A. Alvestrand, James Clark, Charles Goldfarb, Gary Houston, Ned Freed, Ray Moody, and Don Stinchfield gave both encouragement and invaluable help 👑. The author, however, takes full responsibility for all errors contained in this document 🙈.

9. References 📚

• [822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", August 1982, University of Delaware, RFC 822.
• [CID] E. Levinson, J. Clark, "Message/External-Body Content-ID Access Type", 12/26/1995, RFC 1873 Levinson, E., "Message/External-Body Content-ID Access Type", February 1997, RFC 2111.
• [CFG] Borenstein, N., "A User Agent Configuration Mechanism For Multimedia Mail Format Information", September 23, 1993, RFC 1524.
• [DISP] R. Troost, S. Dorner, "Communicating Presentation Information in Internet Messages: The Content-Disposition Header", June 7, 1995, RFC 1806.
• [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.

9. Author's Address 🌷

Edward Levinson
XIson, Inc.
47 Clive Street
Metuchen, NJ  08840-1060
USA
+1 908 549 3716
<XIson@cnj.digex.com>

That's the whole RFC, bestie! 🌈💖 Edward really brought the receipts, and you now know exactly how Multipart/Related works. Iconic, honestly 👑✨