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Versions: 00 01 02 03 04 05 06 07 RFC 2557

Network Working Group                                       Jacob Palme
Internet Draft                                 Stockholm University/KTH
draft-ietf-mhtml-rev-00.txt                           Alexander Hopmann
IETF status: Standards track                      Microsoft Corporation
Expires: January 1998                                         July 1997

MIME E-mail 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
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as ``work in progress.''

To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).


   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".

Temporary note

This is a revision of RFC 2110 to take into account problems which have
cropped up by developers when developing software adhering to RFC 2110.
RFC 2110 is an IETF Proposed Standard, and the intention is that this
document, possibly after more revisions, will either be submitted as a
revised Proposed Standard or as a Draft Standard.

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-Base header
   4.3 The Content-Location Header
   4.4 Encoding of URIs in e-mail headers
5. Base URIs for resolution of relative URIs
6. Sending documents without linked objects
7. Use of the Content-Type: Multipart/related
8. Format of Links to Other Body Parts
   8.1 General principle
   8.2 Use of the Content-Location header
   8.3 Use of the Content-ID header and CID URLs
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 using CID URL and Content-ID header to an embedded GIF
10. Content-Disposition header
11. Character encoding issues and end-of-line issues
12. Security Considerations
13. Robustness Principle
   13.1 Content of the "type" parameter to Content-Type:
   13.2 Quoting of the "type" parameter to Content-Type:
   13.3 Quoting of the "start" parameter to Content-Type:
        Multipart/related and the value of the Message-ID and Content-
        ID header
   13.4 Content-Base and Content-Location on Multipart Content
14. Acknowledgments
15. References
16. Author's Addresses

Mailing List Information

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     Comments on less important details may also be sent to the editor,
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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 e-mail [SMTP], [RFC822]. This document gives
additional specifications on how to send such documents in MIME [MIME1
to MIME5] e-mail 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.

An alternative way for sending an HTML document or other object
containing URIs in e-mail is to only send the URL, 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 at a later time 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, and will stay that way for at least six
months in order to gain more implementation experience before it is

2.    Terminology

2.1   Conformance requirement terminology

This specification uses the same words as the Requirement for Internet
Hosts [HOSTS] for defining the significance of each particular
requirement. These words are:

MUST    This word or the adjective "required" means that the item is
        an absolute requirement of the specification.

SHOULD  This word or the adjective "recommended" means that there may
        exist valid reasons in particular circumstances to ignore this
        item, but the full implications should be understood and the
        case carefully weighed before choosing a different course.

MAY     This word or the adjective "optional" means that this item is
        truly optional. One vendor may choose to include the item
        because a particular marketplace requires it or because it
        enhances the product, for example; another vendor may omit the
        same item.

An implementation is not compliant if it fails to satisfy one or more
the MUST requirements for the protocols it implements. An
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].

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-Enco Conversion of a text into 7-bit octets as specified
ding                  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].

URI, absolute and     See RFC 1866 [HTML2].

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.

Mail receiving agents also have several differing needs. Some mail
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

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 only for exactly the content heading or
message heading where they occur and its text. They are thus not valid
for the parts inside multipart headings. They are allowed, but cannot
be used for resolution, when they occur in multipart headings.

These two headers may occur both inside and outside of a
Multipart/related part, but their usage for handling HTML links between
body parts in a message SHOULD only occur inside 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

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

    content-base ::= "Content-Base:" absoluteURI

where URI is at present (June 1996) restricted to the syntax for URLs
as defined in Unform Resource Locators [URL].

4.2   The Content-Base header

The Content-Base gives a base for relative URIs occurring in other
heading fields and in HTML documents which do 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, but is not valid
   ; for resolution of relative URL-s in Part 1 and Part 2.
   ; A Content-Base header here would thus be rather meaningless.


   Part 1:
   Content-Type: Text/HTML; charset=US-ASCII
   Content-ID: <foo2*foo3@bar2.net>
   Content-Location: http://www.ietf.cnir.reston.va.us/foo1.bar1
   ;  This Content-Location must contain an absolute URI, since no base
   ;  is valid here. A combination of Content-Base with an absolute
   ;  URL and a Content-Location with a relative URL would also be
   ;  allowed here.

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


   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.cnir.reston.va.us/foo1.bar1">
   To top window </A>


Note: If there is both a Content-ID and a Content-Location header on
the same body parts, then these will indicate two different, equally
valid references for this body part, and any of them may be used in
other body parts within the Multipart/related to refer to such a body

4.3   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 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.


Content-Type: Multipart/related; boundary="boundary-example-1";


   Part 1:
   Content-Type: Text/HTML; charset=US-ASCII

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


   Part 2:
   Content-Type: Text/HTML; charset=US-ASCII
   Content-Location: fiction1/fiction2


4.4   Encoding of URIs in e-mail headers

Since MIME header fields have a limited length and URIs can get quite
long, these lines may have to be folded. If such folding is done, the
algorithm defined in [URLBODY] section 3.1 should be employed.

5.    Base URIs for resolution of relative URIs

Relative URIs inside contents of MIME body parts are resolved relative
to a base URI. 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 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].

When the methods above do not yield an absolute URI the procedure in
section 8.2 for matching relative URIs MUST be followed.

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 e-mail 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: 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].

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 many mail
programs 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" headers, and requires that the 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", 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" 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 e-mail 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 e-mail, send HTML which
        includes URIs which the recipient cannot resolve via HTTPor
        other connectivity-requiring URIs.

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

    (d) For any recipient to speed up access.

The type parameter of the "Content-Type: 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 email 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 mailers implementing this standard may not
be able to resolve such links.

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

8.    Format 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 IMG tag in HTML 2.0 [HTML2].
New tags with this property are proposed in the ongoing development of
HTML (example: applet, frame).

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 often
has links to other objects, which might be included in other body parts
in the same MIME message. The referencing of other body parts is done
in the following way: For each body part containing links and each
distinct URI within it, which refers to data which is sent in the same
MIME message, there SHOULD be a separate body part within the current
multipart/related part of the message containing this data. Each such
body part SHOULD contain a Content-Location header (see section 8.2) or
a Content-ID header (see section 8.3).

An e-mail 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).

8.2   Use of the Content-Location header

8.2.1 Matching of URL-s which can be resolved to absolute URL-s

If there is a Content-Base header, then the recipient MUST employ
relative to absolute resolution as defined in Relative Uniform Resource
Locators [RELURL] of relative URIs in both the HTML markup and the
Content-Location header before matching a hyperlink in the HTML markup
to a Content-Location header. The same applies if the Content-Location
contains an absolute URI, or if the HTML markup contains a <BASE>
element so that relative URIs in the HTML markup can be resolved.
<BASE> elements inside HTML markup MUST not be used to resolve URI-s in
the Content-Heading which contains this HTML markup.

8.2.2 Matching of URL-s which cannot be resolved to absolute URL-s

If there is NO Content-Base header, and the Content-Location header
contains a relative URI, then NO relative to absolute resolution SHOULD
be performed. Matching the relative URI in the Content-Location header
to a hyperlink in an HTML markup text is in this case a two step
process. First remove any LWSP from the relative URI which may have
been introduced as described in section 4.4. Then perform an exact
textual match against the HTML URIs. For this matching process, ignore
any <BASE> element in the HTML markup. By "exact textual match" means
case sensitive matching and no resolution of encodings like
"file%20name" to "file name". (Note that the string "file name" is an
illegal URL, since unquoted spaces are not allowed in URLs.)

Note: If there are two body parts, one with a base, one with only a
relative URL and no base, then one of them cannot refer to the other,
since a non-resolved relative URI cannot match an absolute URI.

8.2.3 Must the URL refer to an existing WWW object?

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.

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-Location statement will
normally be replaced by a Content-ID header. Thus, the following two
headers are identical in meaning:

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

9.    Examples

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

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

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; boundary="boundary-example-1";
                 type="Text/HTML"; start=<foo3*foo1@bar.net>

      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">

      Content-Type: IMAGE/GIF
      Content-Transfer-Encoding: BASE64



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; boundary="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;

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



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

   From: foo1@bar.net
   To: foo2@bar.net
   Subject: A simple example
   Mime-Version: 1.0
   Content-Type: Multipart/related; boundary="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">

      Content-ID: <foo4*foo1@bar.net>
      Content-Type: IMAGE/GIF
      Content-Transfer-Encoding: BASE64



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", 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 via e-mail. Receiving mail 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

- The MIME standard [MIME2] requires that 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" 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

12.   Security Considerations

Some Security Considerations include the potential to mail 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 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.

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

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 e-mail 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 URLs. If a person gets such a document, and forwards it via
e-mail, the person may inadvertently disclose sensitive information.

13.   Robustness Principle

The Internet Hosts requirements [HOSTS] section 1.2.2 states the very
important Internet Standards Robustness Principle:

                "Be liberal in what you accept, and
                 conservative in what you send"

This principle is of special importance when working with HTML, since
accepted practice is that HTML readers should accept all kinds of
faulty or illegal HTML codes and make the best possible use of them.

Here is a (not complete) list of ways in which this principle SHOULD be
implemented as applied to this standard.

13.1  Content of the "type" parameter to Content-Type:

What you send: Always include the "type" parameter in the "Content-
type: Multipart/relative" header, and always make it identical to the
Content-type of the root as specified in RFC 2112.

What you accept: Regard the "type" parameter only as a hint, whose
value may be wrong. Also accept input where this parameter is omitted.

13.2  Quoting of the "type" parameter to Content-Type:

What you send: Always quote this parameter if it contains any of the
characters  "(" / ")" / "<" / ">" / "@" /, "," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "=" as required by [MIME1] section 5.1.

What you accept: Accept this parameter, even if it contains these
characters without quoting.

13.3  Quoting of the "start" parameter to Content-Type:
Multipart/related and the value of the Message-ID and Content-ID header

What you send: Always surround the Message-ID in the Message-ID and
Content-ID value and in the start parameter of Content-Type
Multipart/related with "<" and ">" as specified in  [REL] and [RFC822].

What you accept: Accept these values without surrounding "<" ">", and
treat them as if they had been surrounded by angle brackets.

13.4  Content-Base and Content-Location on Multipart Content headings

What you send: Do not use the Content-Base or the Content-Location
header on a Multipart/related if you expect that this Content-Base or
Content-Location is to be used for any URI resolution. These headers
are meant to convey information only for this particular body parts,
not for its subparts, and thus cannot be used for resolution of URLs
inside the subparts of the multipart.

What you accept: If a message you receive has such a Content-Base or
Content-Location, and lacks this information on a subpart, so that you
cannot resolve URIs in the subpart,  you might try to use the Content-
Base and Content-Location to resolve URIs in the subpart.

14.   Acknowledgments

Harald T. Alvestrand, Richard Baker, Isaac Chan, Dave Crocker,
Martin J. Duerst, Lewis Geer, Roy Fielding, 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, Pete Resnick,
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.

15.   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 and
                Message-ID Uniform Resource Locators", RFC 2111,
                February 1997.

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

[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

[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 Content-
                Type", RFC 2112, February 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,

16.   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                         E-mail: jpalme@dsv.su.se
S-164 40 Kista, Sweden

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

Working group chairman:

Einar Stefferud <stef@nma.com>

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