Network Working Group                                       Jacob Palme
Internet Draft                                 Stockholm University/KTH
draft-ietf-mhtml-rev-02.txt
draft-ietf-mhtml-rev-03.txt                           Alexander Hopmann
IETF status to be: Proposed standard              Microsoft Corporation
Revises: RFC 2110
Expires: March May 1998                                        October                                         November 1997

MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)

Status of this Document

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 groups may also distribute working
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Abstract

   Although HTML [RFC 1866] was designed within the context of MIME,
   more than the specification of HTML as defined in RFC 1866 is needed
   for two electronic mail user agents to be able to interoperate using
   HTML as a document format. These issues include the naming of
   objects that are normally referred to by URIs, and the means of
   aggregating objects that go together. This document describes a set
   of guidelines that will allow conforming mail user agents to be able
   to send, deliver and display these objects, such as HTML objects,
   that can contain links represented by URIs. In order to be able to
   handle inter-linked objects, the document uses the MIME type
   ''multipart/related'' and specifies the MIME content-headers
   ''Content-Location'' and ''Content-Base''. The guidelines in this
   document can also be used when sending aggregate HTML objects in
   other forms than e-mail, such as through HTTP or FTP.

   Differences compared to the previous version of this proposed
   standard, published in RFC 2110, are summarized in chapter 13.

Table of Contents

1. Introduction
2. Terminology
   2.1 Conformance requirement terminology
   2.2 Other terminology
3. Overview
4. The Content-Location and Content-Base MIME Content Headers
   4.1 MIME content headers
   4.2 The Content-Location Header
   4.3 The Content-Base header
   4.4 Encoding of URIs in MIME headers
5. Base URIs for resolution of relative URIs
6. Sending documents without linked objects
7. Use of the Content-Type: Content-Type "multipart/related"
8. Usage of Links to Other Body Parts
   8.1 General principle
   8.2 Resolution of hyperlinks in text/html body parts
   8.3 Use of the Content-ID header and CID URLs
   8.4 Conformance requirement on receipt
9. Examples
   9.1 Example of a HTML body without included linked objects
   9.2 Example with absolute URIs to an embedded GIF picture
   9.3 Example with relative URIs to an embedded GIF picture
   9.4 Example with relative URIs and no BASE available
   9.5 Example using a BASE on the Multipart
   9.6 Example using CID URL and Content-ID header to an embedded GIF
        picture
10. Content-Disposition header
11. Character encoding issues and end-of-line issues
12. Security Considerations
13. Differences as compared to the previous version of this proposed
   standard in RFC 2110
14. Copyright
15. Acknowledgments
16. References
17. Author's Addresses

Mailing List Information

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     Further discussion on this document should be done through the
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More information

     Information about the IETF work in developing this standard may
     also be available at URL:
     http://www.dsv.su.se/~jpalme/ietf/mhtml.html

     A collection of test messages is available at
     http://www.dsv.su.se/~jpalme/mimetest/MHTML-test-messages.html

1.    Introduction

There are a number of document formats, Hypertext Markup Language
[HTML2], Portable Document format [PDF] and Virtual Reality Markup
Language [VRML] for example, which provide links using URIs for their
resolution. There is an obvious need to be able to send documents in
these formats in email [SMTP], [RFC822]. This document gives additional
specifications on how to send such documents in MIME-formatted [MIME1
to MIME5] messages. This version of this standard was based on full
consideration only of the needs for objects with links in the text/html
media type (as defined in [HTML2]), but the standard may still be
applicable also to other formats for sets of interlinked objects,
linked by URIs. There is no conformance requirement that
implementations claiming conformance to this standard are able to
handle URI-s in other document formats than HTML.

URIs in documents in HTML and other similar formats reference other
objects and resources, either embedded or directly accessible through
hypertext links. When mailing such a document, it is often desirable to
also mail all of the additional resources that are referenced in it;
those elements are necessary for the complete interpretation of the
primary object. Also with other protocols such as HTTP or FTP, it can
sometimes be desirable to send several documents in one aggregate
document.

Since the formats specified in this standard specifies a way of saving
a complete web page with all in-line objects copied into one single
file, the formats might also be useful for archiving of complete web
page as they looked at a particular moment of time.

An alternative way for sending an HTML document or other object
containing URIs in email is to only send the URI, and let the recipient
look up the document using HTTP. That method is described in [URLBODY]
and is not described in this document.

An informational RFC will be published as a supplement to this
standard. The informational RFC will discuss implementation methods and
some implementation problems. Implementors are recommended to read this
informational RFC when developing implementations of the MHTML
standard. This informational RFC is, when this RFC is published, still
in IETF draft status.

2.    Terminology

2.1   Conformance requirement terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [IETF-TERMS].

An implementation is not compliant if it fails to satisfy one or more
of the MUST requirements for the protocols it implements. An
implementation that satisfies all the MUST and all the SHOULD
requirements for its protocols is said to be "unconditionally
compliant"; one that satisfies all the MUST requirements but not all
the SHOULD requirements for its protocols is said to be "conditionally
compliant."

2.2   Other terminology

Most of the terms used in this document are defined in other RFCs.

Absolute URI,         See Relative Uniform Resource Locators [RELURL].
AbsoluteURI

CID                   See Message/External Body Content-ID [MIDCID].

Content-Base          See section 4.2 below.

Content-ID            See Message/External Body Content-ID [MIDCID].

Content-Location      MIME message or content part header with the URI of
                      the MIME message or content part body, defined in
                      section 4.3 below.

Content-Transfer-     Conversion of a text into 7-bit octets as specified
Encoding              in [MIME1] chapter 6.

CR                    See [RFC822].

CRLF                  See [RFC822].

Displayed text        The text shown to the user reading a document with
                      a web browser. This may be different from the HTML
                      markup, see the definition of HTML markup below.

Header                Field in a message or content heading specifying
                      the value of one attribute.

Heading               Part of a message or content before the first
                      CRLFCRLF, containing formatted fields with
                      attributes of the message or content.

HTML                  See HTML 2 specification [HTML2].

HTML Aggregate        HTML objects together with some or all objects, to
objects               which the HTML object contains hyperlinks.

HTML markup           A file containing HTML encodings as specified in
                      [HTML] which may be different from the displayed
                      text which a person using a web browser sees. For
                      example, the HTML markup may contain "&lt;" where
                      the displayed text contains the character "<".

LF                    See [RFC822].

MIC                   Message Integrity Codes, codes use to verify that a
                      message has not been modified.

MIME                  See the MIME specifications [MIME1 to MIME5].

MUA                   Messaging User Agent.

PDF                   Portable Document Format, see [PDF].

Relative URI,         See HTML 2 [HTML2] and RFC 1808[RELURL].
RelativeURI

URI, absolute and     See RFC 1866 [HTML2].
relative

URL                   See RFC 1738 [URL].

URL, relative         See Relative Uniform Resource Locators [RELURL].

VRML                  See Virtual Reality Markup Language [VRML].

3.    Overview

An aggregate document is a MIME-encoded message that contains a root
document as well as other data that is required in order to represent
that document (inline pictures, style sheets, applets, etc.). Aggregate
documents can also include additional elements that are linked to the
first object.  It is important to keep in mind the differing needs of
several audiences. Mail sending agents might send aggregate documents
as an encoding of normal day-to-day electronic mail. Mail sending
agents might also send aggregate documents when a user wishes to mail a
particular document from the web to someone else. Finally mail sending
agents might send aggregate documents as automatic responders,
providing access to WWW resources for non-IP connected clients. Also
with other protocols such as HTTP or FTP, there may sometimes be a need
to send aggregate documents in MIME multipart format.

Receiving agents also have several differing needs. Some receiving
agents might be able to receive an aggregate document and display it
just as any other text content type would be displayed. Others might
have to pass this aggregate document to a browsing program, and
provisions need to be made to make this possible.

Finally several other constraints on the problem arise. It is important
that it be possible for a document to be signed and for it to be able
to be transmitted to a client and displayed with a minimum risk of
breaking the message integrity (MIC) check that is part of the
signature.

4.    The Content-Location and Content-Base MIME Content Headers

4.1   MIME content headers

In order to resolve URI references to other body parts, two MIME
content headers are defined, Content-Location and Content-Base. Both
these headers can occur in any message or content heading, and will
then be valid within this heading and for its immediate content.

These two headers are valid for the content heading or message heading
where they occur and its text. If they occur in multipart headings,
they apply to its body parts only in that they can be used to derive a
base for relative URIs in the body parts, but and only if no such base is
provided in the body part itself. itself or in headings closer to the body.

These two headers may occur on any message or content heading, but
their usage for handling hyperlinks between body parts in a message
SHOULD only occur inside the same "multipart/related".

In practice, at present only those URIs which are URLs are used, but it
is anticipated that other forms of URIs will in the future be used.

The syntax for these headers is, using the syntax definition tools from
[RFC822]:

    content-location = "Content-Location:"
                       ( absoluteURI | relativeURI )

    content-base = "Content-Base:" absoluteURI

where URI is restricted to the syntax for URLs as defined in Unform
Resource Locators [URL] until IETF specifies other kinds of URIs.

4.2   The Content-Location Header

The Content-Location header specifies the URI that corresponds to the
content of the body part in whose heading the header is placed. Its
value CAN be an absolute or relative URI. Any URI or URL scheme may be
used, but use of non-standardized URI or URL schemes might entail some
risk that recipients cannot handle them correctly.

The Content-Location header can be used to indicate that the data sent
under this heading is also retrievable, in identical format, through
normal use of this URI. If used for this purpose, it must contain an
absolute URI or be resolvable, through a Content-Base header, into an
absolute URI. In this case, the information sent in the message can be
seen as a cached version of the original data.

The URI in the Content-Location header may, but need not refer to an
object which is actually available globally for retrieval using this
URI (after resolution of relative URIs). However, URI-s in
Content-Location headers (if absolute, or resolvable to absolute URIs)
SHOULD still be globally unique.

The header can also be used for data which is not available to some or
all recipients of the message, for example if the header refers to an
object which is only retrievable using this URI in a restricted domain,
such as within a company-internal web space. The header can even
contain a fictious URI and need in that case not be globally unique.

There MUST only be a single Content-Location header in each message or
content-heading, and its value is a single URI. Note however, that both
one Content-Location and one Content-ID or Message-ID header are
allowed. In such a case, these will indicate two different, equally
valid references for this body part, and any of them may be used in
other body parts within one "multipart/related" to refer to this body
part.

Example:

Content-Type: "multipart/related"; boundary="boundary-example-1";
                 type="text/html"

   --boundary-example-1

   Part 1:
   Content-Type: text/html; charset=US-ASCII

   ... ... <IMG SRC="fiction1/fiction2"> ... ...

   --boundary-example-1

   Part 2:
   Content-Type: text/html; charset=US-ASCII
   Content-Location: fiction1/fiction2

   --boundary-example-1--

4.3   The Content-Base header

The Content-Base gives a base for relative URIs occurring in other
fields in the same content heading and in its content, if the text is a
HTML document which does not have any BASE element in its HTML code.
Its value MUST be an absolute URI.

Example showing which Content-Base is valid where:

   Content-Type: "multipart/related"; boundary="boundary-example-1";
                 type="text/html"; start=<foo2*foo3@bar2.net>
   ; A Content-Base header is allowed here, and can be used
   ; for resolution of relative URL-s in Part 1 and Part 2,
   ; if these did not have any absolute base of their own.
   ; However, both part 1 and part 2 below have an absolute
   ; base, in part 1 through an absolute Content-Location header,
   ; in part 2 through a Content-Base header, and thus a Content-
   ; base up here would not be used for resoultion of relative
   ; URLs within the body parts 1 and 2.

   --boundary-example-1

   Part 1:
   Content-Type: text/html; charset=US-ASCII
   Content-ID: <foo2*foo3@bar2.net>
   Content-Location: http://www.ietf.cnri.reston.va.us/foo1.bar1
   ;  Since this Content-Location contains an absolute URL, it
   ;  does not need to be resolved using any Content-Base header.
   ;  A combination of a Content-Location with a relative URL
   ;  and a Content-Base with an absolute URL would also be valid,
   ;  as well as only a Content-Location with a relative URL
   ;  and resolved through the Content-Base in the surrounding
   ;  multipart heading.

   <FRAME NAME=topwindow src="/frames/foo2.bar2">

   --boundary-example-1

   Part 2:
   Content-Type: text/html; charset=US-ASCII
   Content-ID: <foo4*foo5@bar2.net>
   Content-Location: foo2.bar2   ; The Content-Base below applies to
                                 ; this relative URI
   Content-Base: http://www.ietf.cnri.reston.va.us/frames/

   <A HREF="http://www.ietf.cnri.reston.va.us/foo1.bar1">
   To top window </A>

   --boundary-example-1--

4.4   Encoding of URIs in MIME headers

4.4.1 Handling of URIs containing inappropriate characters

Some documents may contain URIs with characters that are inappropriate
for an RFC 822 header, either because the URI itself has an incorrect
syntax according to [URL] or the URI syntax standard has been changed
to allow characters not previously allowed in MIME headers. These URIs
cannot be sent directly in a message header. There are two approaches
that can be taken when encountering such a URI as the text to be placed
in a Content-Location or Content-Base header:

a)

(a) In some situations, an implementation might be able to replace the
    URI with one that can be sent directly. This might be accomplished,
    for example, by using the encoding method of [URL] to replace
    inappropriate characters within the URI with ones encoded using the
    "%nn" encoding. This replacement MUST in that case be done both in
    the header and in the HTML text which has a hyperlink which is to
    match the header. Since the change is done in both places, a
    receiving agent need not decode it, and MUST NOT decode [URL]-encoding [URL]-
   encoding before matching hyperlinks to body parts.

b)

(b) The URI might be encoded using the method described in [MIME3].
    This replacement MUST only be done in the header, not in the HTML
    text.  Receiving clients must decode the [MIME3] encoding in the
    heading before comparing hyperlinks in body text to URIs in
    Content-Location headers.

With method (b), the charset parameter value "US-ASCII" SHOULD be used
if the URI contains no octets outside of the 7-bit range. If such
octets are present, the correct charset parameter value (derived e.g.
from information about the HTML document the URI was found in) SHOULD
be used. If this cannot be safely established, the value "UKNOWN-8BIT"
[RFC 1428] MUST be used.

Note that for the MHTML processing of matching URIs in body text to URI
in Content-Location headers the value of the charset parameter is
irrelevant, but it may be relevant for other purposes, and incorrect
labeling MUST therefore be avoided. Warning: Irrelevance of the charset
parameter may not be true in the future, if different character
encodings of the same non-English filename is used in HTML.

Caution should be taken in using method (a), since, in general, this
encoding can not be applied safely to characters that are used for
reserved purposes within the URI scheme. In addition, changing the HTML
body which contains the URI might invalidate a message integrity check.
Because of these problems, this method SHOULD only be used if it is
performed in cooperation with the author/owner of the documents
involved.

4.4.2 Folding of long URIs

Since MIME header fields have a limited length and URIs can get quite
long, these lines may have to be folded.

Encoding as discussed in clause 4.4.1 MUST be done before such folding.
This MUST include encoding of space characters, if any. After that, the
folding can be done, using the algorithm defined in [URLBODY] section
3.1.

5.    Base URIs for resolution of relative URIs

Relative URIs inside contents of MIME body parts are resolved relative
to a base URI using the methods for resolving relative URIs described
in [RELURL]. In order to determine this base URI, the first-applicable
method in the following list applies.

(a) There is a base specification inside the MIME body part containing
    the link which resolves relative URIs into absolute URIs. For
    example, HTML provides the BASE element for this.

(b) There is a Content-Base header (as defined in section 4.2), in the
    immediately surrounding content heading, specifying the base to be
    used.

(c) There is a Content-Location header in the immediately surrounding
    heading of the body part which contains an absolute URI and can
    then serve as the base in the same way as the requested URI can
    serve as a base for relative URIs within a file retrieved via HTTP
    [HTTP].

(d) Step (b) and (c) can be repeated recursively on Content-Base and
    Content-Location headers in surrounding multi-part headings.
    However, a base from an absolute Content-Location in an inner
    heading takes precedence over a base from a Content-Base or a
    Content-Location in a surrounding heading.

When the methods above do not yield an absolute URI, matching of two
relative URIs against each other can still be done for matches within a
multipart/related. This matching is done as if they had been given as
base an imaginary URL "this_message:/", which exists for the sole
purpose of resolving relative references within a multipart/related
entitity.

This is also described in other words in section 8.2 below.

6.    Sending documents without linked objects

If a document, such as an HTML object, is sent without other objects,
to which it is linked, it MAY be sent as a text/html body part by
itself. In this case, "multipart/related" need not be used.

Such a document may either not include any links, or contain links
which the recipient resolves via ordinary net look up, or contain links
which the recipient cannot resolve.

Inclusion of links which the recipient has to look up through the net
may not work for some recipients, since all email recipients do not
have full internet connectivity. Also, such links may work for the
sender but not for the recipient, for example when the link refers to
an URI within a company-internal network not accessible from outside
the company.

Note that documents with links that the recipient cannot resolve MAY be
sent, although this is discouraged. For example, two persons developing
a new HTML page may exchange incomplete versions.

7.    Use of the Content-Type: Content-Type "multipart/related"

If a message contains one or more MIME body parts containing links and
also contains as separate body parts, data, to which these links (as
defined, for example, in HTML 2.0 [HTML2]) refers, then this whole set
of body parts (referring body parts and referred-to body parts) SHOULD
be sent within a "multipart/related" body part as defined in [REL].

Even though Content-Location and Content-Base can occur without
multipart/related, this standard only covers their use for resolution
of links between body parts inside one multipart/related. This standard
does not cover links from one multipart/related to another
multipart/related in a message containing multiple multipart/related
objects.

The root body part of the "multipart/related" SHOULD be the start
object for rendering the object, such as a text/html object, and which
contains links to objects in other body parts, or a
multipart/alternative of which at least one alternative resolves to
such a start object. Implementors are warned, however, that some
receiving agents treat multipart/alternative as if it had been
multipart/mixed (even though MIME [MIME1] requires support for
multipart/alternative).

[REL] specifies that the type attribute is mandatory in Content-Type:
"multipart/related" "Content-Type:
multipart/related" headers, and requires that this attribute be the
type of the root object, and this value shall thus for example be
"multipart/alternative", if the root part is of Content-type
"multipart/alternative", "Content-type
multipart/alternative", even if one of the subparts of the
"multipart/alternative" is of type "text/html". If the root is not the
first body part within the "multipart/related", [REL] further requires
that its Content-ID MUST be given in a start parameter to the
"Content-Type: "multipart/related" multipart/related" header.

When presenting the root body part to the user, the additional body
parts within the "multipart/related" can be used:

(a) For those recipients who only have email but not full Internet
    access.

(b) For those recipients who for other reasons, such as firewalls or
    the use of company-internal links, cannot retrieve the linked body
    parts through the net.

    Note that this means that you can, via email, send HTML which
    includes URIs which the recipient cannot resolve via HTTPor other
    connectivity-requiring URIs.

(c) To send a document in a format which is preserved even if the
    object to which the hyperlinks refer through HTTP is later changed
    or deleted.

(d) For items which are not available on the web.

(e) For any recipient to speed up access.

The type parameter of the "Content-Type: "multipart/related" multipart/related" MUST be the
same as the Content-Type of its root.

When a sending MUA sends objects which were retrieved from the WWW, it
SHOULD maintain their WWW URIs. It SHOULD not transform these URIs into
some other URI form prior to transmitting them. This will allow the
receiving MUA to both verify MICs included with the message, as well as
verify the documents against their WWW counterpoints.

In certain special cases this will not work if the original HTML
document contains URIs as parameters to objects and applets. In such a
case, it might be better to rewrite the document before sending it.
This problem is discussed in more detail in the informational RFC which
will be published as a supplement to this standard.

This standard does not cover the case where a "multipart/related"
contains links to MIME body parts outside of the current
"multipart/related" or in other MIME messages, even if methods similar
to those described in this standard are used. Implementors who provide
such links are warned that receiving agents implementing this standard
may not be able to resolve such links.

Within a "multipart/related", ALL different parts MUST have different
Content-ID values or Content-Location headers which resolve to
different URIs.

Two body parts in the same multipart/related can have the same relative
URI as value of their Content-Location headers only if there are
headers containing a different Content-Base header, so that the
absolute URI after resolution against the Content-Base header is
different.

8.    Usage of Links to Other Body Parts

8.1   General principle

A body part, such as a text/html body part, may contain hyperlinks to
objects which are included as other body parts in the same message and
within the same "multipart/related" content. Often such linked objects
are meant to be displayed inline to the reader of the main document;
for example, objects referenced with the src attribute of the IMG
element in HTML 2.0 [HTML2]. New elements and attributes with this
property are proposed in the ongoing development of HTML
(example: (examples:
applet, frame, profile, OBJECT, classid, codebase, data, SCRIPT). A
sender might also want to send a set of HTML documents which the reader
can traverse, and which are related with the attribute href of the A
element.

In order to send such messages, there is a need to indicate which other
body parts are referred to by the links in the body parts containing
such links. For example, a body part of Content-Type: text/html "Content-Type: text/html" often
has links to other objects, which might be included in other body parts
in the same MIME message.

8.2   Resolution of hyperlinks in text/html body parts

The resolution of hyperlinks in text/html body parts is performed in
the following way:

(a) Unfold multiple line header values according to [URLBODY]. Do NOT
    however translate character encodings of the kind described in
    [URL]. Example: Do not transform "a%2eb/c%20d" into "a/b/c d".

(b) Remove all MIME encodings, such as content-transfer encoding and
    header encodings as defined in MIME part 3 [MIME3] Do NOT however
    translate character encodings of the kind described in [URL].
    Example: Do not transform "a%2eb/c%20d" into "a/b/c d".

(c) Try to resolve all relative URIs in the HTML content and in
    Content-Location headers using the procedure described in chapter 5
    above. The result of this resolution can be an absolute URI, or a
    fictiuous absolute URI with the base "this_message:/" as specified
    in chapter 5.

(d) For each hyperlink in any HTML body, compare the value of the
    hyperlink after resolution as described in (a) and (b), with the
    URI derived from Content-ID and Content-Location headers for other
    body parts within the same Multipart/related. If the strings are
    identical, octet by octet, then this hyperlink is resolved by the
    body part with the same URI. This comparison will only succeed if
    the two URIs are identical. This means that if one of the two URIs
    to be compared was a fictituous absolute URI with the base
    "this_message:/", the other must also be such a fictituous absolute
    URI, and not resolvable to a real absolute URI.

(e) If (c) fails, try to resolve the hyperlink through ordinary
    Internet lookup. Resolution of hyperlinks of the URL-types "mid" or
    "cid" to other content-parts, outside multipart/related, or in
    other separately sent messages, is not covered by this standard,
    and is thus neither encouraged nor forbidden.

8.3   Use of the Content-ID header and CID URLs

When CID (Content-ID) URLs as defined in [URL] and [MIDCID] are used
for links between body parts, the Content-ID header MUST be used
instead of the Content-Location header. Thus, even though the following
two headers are identical in meaning, only the Content-ID variant MUST
be used, and all "Content-Location: CID:" should be ignored.

Content-ID: <foo@bar.net>
Content-Location: CID: foo@bar.net

Note: Content-IDs MUST be globally unique [MIME1]. It is thus not
permitted to make them unique only within this message or within this
"multipart/related".

8.4   Conformance requirement on receipt

An email system which claims conformance to this standard MUST support
receipt of "multipart/related" (as defined in section 7) with links
between body parts using both the Content-Location (as defined in
section 8.2) and the Content-ID method (as defined in section 8.3).

9.    Examples

Warning: If there is a contradiction between the explanatory text and
the examples in this standard, then the explanatory text, not the
examples are normative.

9.1   Example of a HTML body without included linked objects

The first example is the simplest form of an HTML email message. This
is not an aggregate HTML object, but simply a message with a single
HTML body part. This message contains a hyperlink but does not provide
the ability to resolve the hyperlink. To resolve the hyperlink the
receiving client would need either IP access to the Internet, or an
electronic mail web gateway.

   From: foo1@bar.net
   To: foo2@bar.net
   Subject: A simple example
   Mime-Version: 1.0
   Content-Type: text/html; charset=US-ASCII

   <HTML>
   <head></head>
   <body>
   <h1>Hi there!</h1>
   An example of an HTML message.<p>
   Try clicking <a href="http://www.resnova.com/">here.</a><p>
   </body></HTML>

9.2   Example with absolute URIs to an embedded GIF picture

   From: foo1@bar.net
   To: foo2@bar.net
   Subject: A simple example
   Mime-Version: 1.0
   Content-Type: "multipart/related"; multipart/related; boundary="boundary-example-1";
                 type="text/html"; start=<foo3*foo1@bar.net>

   --boundary-example-1
      Content-Type: text/html;charset=US-ASCII
      Content-ID: <foo3*foo1@bar.net>

      ... text of the HTML document, which might contain a hyperlink
      to the other body part, for example through a statement such as:
      <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ietflogo.gif"
       ALT="IETF logo">

   --boundary-example-1
      Content-Location:
            http://www.ietf.cnri.reston.va.us/images/ietflogo.gif
      Content-Type: "IMAGE/GIF" IMAGE/GIF
      Content-Transfer-Encoding: BASE64

      R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
      NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
      etc...

   --boundary-example-1--

9.3   Example with relative URIs to an embedded GIF picture

   From: foo1@bar.net
   To: foo2@bar.net
   Subject: A simple example
   Mime-Version: 1.0
   Content-Type: "multipart/related"; multipart/related; boundary="boundary-example-1";
                 type="text/html"

   --boundary-example-1
      Content-Base: http://www.ietf.cnri.reston.va.us/
      Content-Type: text/html; charset=ISO-8859-1
      Content-Transfer-Encoding: QUOTED-PRINTABLE

      ... text of the HTML document, which might contain a hyperlink
      to the other body part, for example through a statement such as:
      <IMG SRC="/images/ietflogo.gif" ALT="IETF logo">
      Example of a copyright sign encoded with Quoted-Printable: =A9
      Example of a copyright sign mapped onto HTML markup: &#168;

   --boundary-example-1
      Content-Location: ietflogo.gif
      Content-Base: http://www.ietf.cnri.reston.va.us/images/
      ; Note that the fact that the Content-Base comes after the
      ; Content-Location within the same Content-Heading will not
      ; influence their interpretation
      Content-Type: "IMAGE/GIF" IMAGE/GIF
      Content-Transfer-Encoding: BASE64

      R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
      NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
      etc...

   --boundary-example-1--

9.4   Example with relative URIs and no BASE available

   From: foo1@bar.net
   To: foo2@bar.net
   Subject: A simple example
   Mime-Version: 1.0
   Content-Type: "multipart/related"; multipart/related; boundary="boundary-example-1";
                 type="text/html"

   --boundary-example-1
      Content-Type: text/html; charset=ISO-8859-1
      Content-Transfer-Encoding: QUOTED-PRINTABLE

      ... text of the HTML document, which might contain a hyperlink
      to the other body part, for example through a statement such as:
      <IMG SRC="ietflogo.gif" ALT="IETF logo">
      Example of a copyright sign encoded with Quoted-Printable: =A9
      Example of a copyright sign mapped onto HTML markup: &#168;

   --boundary-example-1
      Content-Location: ietflogo.gif
      Content-Type: "IMAGE/GIF" IMAGE/GIF
      Content-Transfer-Encoding: BASE64

      R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
      NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
      etc...

   --boundary-example-1--

9.5   Example using a BASE on the Multipart

   From: foo1@bar.net
   To: foo2@bar.net
   Subject: A simple example
   Mime-Version: 1.0
   Content-Type: "multipart/related"; multipart/related; boundary="boundary-example-1";
                 type="text/html"
   Content-Base: http://www.ietf.cnri.reston.va.us/

   --boundary-example-1
      Content-Type: text/html; charset=ISO-8859-1
      Content-Transfer-Encoding: QUOTED-PRINTABLE

      ... text of the HTML document, which might contain a hyperlink
      to the other body part, for example through a statement such as:
      <IMG SRC="ietflogo.gif" ALT="IETF logo">
      Example of a copyright sign encoded with Quoted-Printable: =A9
      Example of a copyright sign mapped onto HTML markup: &#168;

   --boundary-example-1
      Content-Location: http://www.ietf.cnri.reston.va.us/ietflogo.gif
      Content-Type: "IMAGE/GIF" IMAGE/GIF
      Content-Transfer-Encoding: BASE64

      R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
      NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
      etc...

   --boundary-example-1--

9.6   Example using CID URL and Content-ID header to an embedded GIF
picture

   From: foo1@bar.net
   To: foo2@bar.net
   Subject: A simple example
   Mime-Version: 1.0
   Content-Type: "multipart/related"; multipart/related; boundary="boundary-example-1";
                 type="text/html"

   --boundary-example-1
      Content-Type: text/html; charset=US-ASCII

      ... text of the HTML document, which might contain a hyperlink
      to the other body part, for example through a statement such as:
      <IMG SRC="cid:foo4*foo1@bar.net" ALT="IETF logo">

   --boundary-example-1
      Content-Location: CID:something@else ; this header is disregarded
      Content-ID: <foo4*foo1@bar.net>
      Content-Type: "IMAGE/GIF" IMAGE/GIF
      Content-Transfer-Encoding: BASE64

      R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
      NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
      etc...

   --boundary-example-1--

10.   Content-Disposition header

Note the specification in [REL] on the relations between
Content-Disposition and multipart/related.

11.   Character encoding issues and end-of-line issues

For the encoding of characters in HTML documents and other text
documents into a MIME-compatible octet stream, the following mechanisms
are relevant:

-  HTML [HTML2], [HTML-I18N] as an application of SGML [SGML] allows
   characters to be denoted by character entities as well as by numeric
   character references (e.g. "Latin small letter a with acute accent"
   may be represented by "&aacute;" or "&#225;") in the HTML markup.

-  HTML documents, in common with other documents of the MIME
   "Content-Type text",
   Content-Type "text", can be represented in MIME using one of several
   character encodings. The MIME Content-Type "charset" parameter value
   indicates the particular encoding used. For the exact meaning and
   use of the "charset" parameter, please see [MIME2] chapter 4.

   Note that the "charset" parameter refers only to the MIME character
   encoding. For example, the string "&aacute;" can be sent in MIME
   with "charset=US-ASCII", while the raw character "Latin small letter
   a with acute accent" cannot.

The above mechanisms are well defined and documented, and therefore not
further explained here. In sending a message, all the above mentioned
mechanisms MAY be used, and any mixture of them MAY occur when sending
the document in MIME format. Receiving user agents (together with any
Web browser they may use to display the document) MUST be capable of
handling any combinations of these mechanisms.

Also note that:

-  Any documents including HTML documents that contain octet values
   outside the 7-bit range need a content-transfer-encoding applied
   before transmission over certain transport protocols [MIME1,
   chapter 5].

-  The MIME standard [MIME2] requires that emailed documents of
   "Content-Type: Text MUST be in canonical form before
   Content-Transfer-Encoding, i.e. that line breaks are encoded as
   CRLFs, not as bare CRs or bare LFs or something else. This is in
   contrast to [HTTP] where section 3.6.1 allows other representations
   of line breaks.

Note that this might cause problems with integrity checks based on
checksums, which might not be preserved when moving a document from the
HTTP to the MIME environment. If a document has to be converted in such
a way that a checksum integrity check becomes invalid, then this
integrity check header SHOULD be removed from the document.

Other sources of problems are Content-Encoding used in HTTP but not
allowed in MIME, and charsets that are not able to represent line
breaks as CRLF. A good overview of the differences between HTTP and
MIME with regards to "Content-Type: Text" Content-Type: "text" can be found in [HTTP],
appendix C.

If the original document has line breaks in the canonical form (CRLF),
then the document SHOULD remain unconverted so that integrity check
sums are not invalidated.

A provider of HTML documents who wants his documents to be transferable
via both HTTP and SMTP without invalidating checksum integrity checks,
should always provide original documents in the canonical form with
CRLF for line breaks.

Some transport mechanisms may specify a default "charset" parameter if
none is supplied [HTTP, MIME1]. Because the default differs for
different mechanisms, when HTML is transferred through mail, the
charset parameter SHOULD be included, rather than relying on the
default.

12.   Security Considerations

Some Security Considerations include the potential to send someone an
object, and claim that it is represented by a particular URI (by giving
it a Content-Location header). There can be no assurance that a WWW
request (like HTTP or FTP) for that same URI would normally result in
that same object. It might be unsuitable to cache the data in such a
way that the cached data can be used for retrieval of this URI from
other messages or message parts than those included in the same message
as the Content-Location header. Because of this problem, receiving User
Agents SHOULD not cache this data in the same way that data that was
retrieved through an HTTP or FTP request might be cached.

URIs, especially File URIs, may in their name contain company-internal
information, which may then inadvertently be revealed to recipients of
documents containing such URIs.

One way of implementing messages with linked body parts is to handle
the linked body parts in a combined mail and WWW proxy server. The mail
client is only given the start body part, which it passes to a web
browser. This web browser requests the linked parts from the proxy
server. If this method is used, and if the combined server is used by
more than one user, then methods must be employed to ensure that body
parts of a message to one person is not retrievable by another person.
Use of passwords (also known as tickets or magic cookies) is one way of
achieving this. Note that some caching WWW proxy servers may not
distinguish between cached objects from email and HTTP, which may be a
security risk.

In addition, by allowing people to mail aggregate objects, we are
opening the door to other potential security problems that until now
were only problems for WWW users. For example, some HTML documents now
either themselves contain executable content (JavaScript) or contain
links to executable content (The "INSERT" specification, Java). It
would be exceedingly dangerous for a receiving User Agent to execute
content received through a mail message without careful attention to
restrictions on the capabilities of that executable content.

Some WWW applications hide passwords and tickets (access tokens to
information which may not be available to anyone) and other sensitive
information in hidden fields in the web documents or in on-the-fly
constructed URIs. If a person gets such a document, and forwards it via
email, the person may inadvertently disclose sensitive information.

13.   Differences as compared to the previous version of this proposed
standard in RFC 2110

The specification has been changed to show that the formats described
do not only apply to multipart MIME in email, but also to multipart
MIME transferred through other protocols such as HTTP or FTP.

In order to agree with [RELURL], Content-Base headers in multipart
Content-Headings can now be used to resolve relative URIs in their
component parts, but only if no base URI can be derived from the
component part itself. Base URIs in inner headings, both in Content-
Base and Content-Location headers, have precedence over base URIs in
outer multipart headings.

Specification has been added that a Content-Heading cannot contain more
than one Content-Location header.

A section 4.4.1 has been added, specifying how to handle the case of
sending a body part whose URI does not agree with the correct URI
syntax.

The handling of relative and absolute URIs for matching between body
parts have been merged into a single description, by specifying that
relative URIs which cannot be resolved otherwise should be handled as
if they had been given imaginary URL "this_message:/".

14.   Copyright

Copyright (C) The Internet Society (date). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights defined
in the Internet Standards process must be followed, or as required to
translate it into languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL
NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE.

15.   Acknowledgments

Harald T. Alvestrand, Richard Baker, Isaac Chan, Dave Crocker, Martin
J. Duerst, Lewis Geer, Roy Fielding, Ned Freed, Al Gilman, Paul
Hoffman, Andy Jacobs, Richard W. Jesmajian, Mark K. Joseph, Greg
Herlihy, Valdis Kletnieks, Daniel LaLiberte, Ed Levinson, Jay Levitt,
Albert Lunde, Larry Masinter, Keith Moore, Gavin Nicol, Martyn W. Peck,
Pete Resnick, Nick Shelness, Jon Smirl, Einar Stefferud, Jamie
Zawinski, Steve Zilles and several other people have helped us with
preparing this document. I alone take responsibility for any errors
which may still be in the document.

16.   References

Ref.            Author, title
---------       --------------------------------------------------------

[CONDISP]       R. Troost, S. Dorner: "Communicating Presentation
                Information in Internet Messages: The
                Content-Disposition Header", RFC 1806, June 1995.

[HOSTS]         R. Braden (editor): "Requirements for Internet Hosts --
                Application and Support", STD-3, RFC 1123, October 1989.

[HTML-I18N]     F. Yergeau, G. Nicol, G. Adams, & M. Duerst:
                "Internationalization  of the Hypertext Markup
                Language". RFC 2070, January 1997.

[HTML2]         T. Berners-Lee, D. Connolly: "Hypertext Markup Language
                - 2.0", RFC 1866, November 1995.

[HTTP]          T. Berners-Lee, R. Fielding, H. Frystyk: Hypertext
                Transfer Protocol -- HTTP/1.0. RFC 1945, May 1996.

[MD5]           R. Rivest: "The MD5 Message-Digest Algorithm", RFC 1321,
                April 1992.

[MIDCID]        E. Levinson: Message/External-Body Content-ID
                Access"Message/External-Body Content-ID and Message-ID
                Uniform Resource Locators", draft-ietf-mhtml-cid-v2-
                00.txt, July 1997.

[MIME1]         N. Freed, N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", RFC 2045, December 1996.
                .
[MIME-IMB]      N. Freed & N. Borenstein
                ::
                "Multipurpose Internet Mail Extensions (MIME) Part
                One: Format of Internet Message Bedies". RFC 2045,
                November 1996.

[MIME2]         N. Freed, N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part Two:  Media Types", RFC 2046,
                December 1996.

[MIME3]         K. Moore, "MIME (Multipurpose Internet Mail Extensions)
                Part Three:  Message Header Extensions for Non-ASCII
                Text", RFC 2047, December 1996.

[MIME1]         N. Borenstein & N. Freed: "MIME (Multipurpose Internet
                Mail Extensions) Part One: Mechanisms for Specifying and
                ing and Describing the Format of Internet Message
                Bodies", RFC 1521, Sept 1993.

[MIME4]          N. Freed, J. Klensin, J. Postel, "Multipurpose Internet
                Mail Extensions (MIME) Part Four:  Registration
                Procedures", RFC 2048, January 1997.

[MIME5]         "Multipurpose Internet Mail Extensions (MIME) Part Five:
                Conformance Criteria and Examples", RFC 2049, December
                1996.

[NEWS]          M.R. Horton, R. Adams: "Standard for interchange of
                USENET messages", RFC 1036, December 1987.

[PDF]           Tim Bienz and Richar Cohn: "Portable Document Format
                Reference Manual", Addison-Wesley, Reading, MA, USA,
                1993, ISBN 0-201-62628-4.

[REL]           Edward Levinson: "The MIME
                Multipart/Related"multipart/related" Content-Type",
                draft-ietf-mhtml-re-v2-00.txt, September 1997.
[RELURL]        R. Fielding: "Relative Uniform Resource Locators", RFC
                1808, June 1995.

[RFC822]        D. Crocker: "Standard for the format of ARPA Internet
                text messages." STD 11, RFC 822, August 1982.

[SGML]          ISO 8879. Information Processing -- Text and Office  -
                Standard Generalized Markup Language (SGML),
                1986. <URL:http://www.iso.ch/cate/d16387.html>

[SMTP]          J. Postel: "Simple Mail Transfer Protocol", STD 10, RFC
                821, August 1982.

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

[URLBODY]       N. Freed and Keith Moore: "Definition of the URL MIME
                External-Body Access-Type", RFC 2017, October 1996.

[VRML]          Gavin Bell, Anthony Parisi, Mark Pesce: "Virtual Reality
                Modeling Language (VRML) Version 1.0 Language
                Specification." May 1995,
                http://www.vrml.org/Specifications/.

[IETF-TERMS]    S. Bradner: Key words for use in RFCs to Indicate
                Requirements Levels. RFC 2119, March 1997.

17.   Author's Addresses

For contacting the editors, preferably write to Jacob Palme rather than
Alex Hopmann.

Jacob Palme                          Phone: +46-8-16 16 67
Stockholm University and KTH         Fax: +46-8-783 08 29
Electrum 230                         Email: jpalme@dsv.su.se
S-164 40 Kista, Sweden

Alex Hopmann                         Email: alexhop@microsoft.com
Microsoft Corporation
3590 North First Street
Suite 300
San Jose
CA 95134

Working group chairman:

Einar Stefferud <stef@nma.com>
I.