Network Working Group J. Palme

Network Working Group J. Palme
Request for Comments: 2110 Stockholm University/KTH
Category: Standards Track A. Hopmann
Microsoft Corporation
March 1997

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

Status of this Document

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

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".

Table of Contents

1. Introduction.............................................. 2
2. Terminology............................................... 3
2.1 Conformance requirement terminology................... 3
2.2 Other terminology..................................... 4
3. Overview.................................................. 5
4. The Content-Location and Content-Base MIME Content Headers 6
4.1 MIME content headers.................................. 6
4.2 The Content-Base header............................... 7
4.3 The Content-Location Header........................... 7
4.4 Encoding of URIs in e-mail headers.................... 8
5. Base URIs for resolution of relative URIs................. 8
6. Sending documents without linked objects.................. 9
7. Use of the Content-Type: Multipart/related................ 9
8. Format of Links to Other Body Parts....................... 11

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RFC 2110 MHTML March 1997

8.1 General principle..................................... 11
8.2 Use of the Content-Location header.................... 11
8.3 Use of the Content-ID header and CID URLs............. 12
9 Examples................................................... 12
9.1 Example of a HTML body without included linked objects 12
9.2 Example with absolute URIs to an embedded GIF picture 13
9.3 Example with relative URIs to an embedded GIF picture 13
9.4 Example using CID URL and Content-ID header to an
embedded GIF picture.................................. 14
10. Content-Disposition header............................... 15
11. Character encoding issues and end-of-line issues......... 15
12. Security Considerations.................................. 16
13. Acknowledgments.......................................... 17
14. References............................................... 18
15. Author's Address......................................... 19

Mailing List Information

Further discussion on this document should be done through the
mailing list [email protected].

To subscribe to this list, send a message to
[email protected]
which contains the text
SUB MHTML <your name (not your e-mail address)>

Archives of this list are available by anonymous ftp from
FTP://SEGATE.SUNET.SE/lists/mHTML/
The archives are also available by e-mail. Send a message to
[email protected] with the text "INDEX MHTML" to get a list
of the archive files, and then a new message "GET <file name>" to
retrieve the archive files.

Comments on less important details may also be sent to the editor,
Jacob Palme <[email protected]>.

More information may also be available at URL:
HTTP://www.dsv.su.se/~jpalme/ietf/jp-ietf-home.HTML

1. Introduction

There are a number of document formats, HTML [HTML2], PDF [PDF] and
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 [RFC821=SMTP, RFC822]. This document gives
additional specifications on how to send such documents in MIME [RFC
1521=MIME1] e-mail messages. This version of this standard was based
on full consideration only of the needs for objects with links in the

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Text/HTML media type (as defined in RFC 1866 [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 published.

2. Terminology

2.1 Conformance requirement terminology

This specification uses the same words as RFC 1123 [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.

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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
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 RFC 1808 [RELURL].
AbsoluteURI

CID See [MIDCID].

Content-Base See section 4.2 below.

Content-ID See [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
ding specified in [MIME1].

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.

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

HTML See RFC 1866 [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 "<" 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 RFC 1521 [MIME1], [MIME2].

MUA Messaging User Agent.

PDF Portable Document Format, see [PDF].

Relative URI, See RFC 1866 [HTML2] and RFC 1808[RELURL].
RelativeURI

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

URL See RFC 1738 [URL].

URL, relative See [RELURL].

VRML Virtual Reality Markup Language.

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

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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 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 content.

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 at present (June 1996) restricted to the syntax for URLs
as defined in RFC 1738 [URL].

These two headers are valid only for exactly the content heading or
message heading where they occurs and its text. They are thus not
valid for the parts inside multipart headings, and are thus
meaningless in multipart headings.

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These two headers may occur both inside and outside of a
multipart/related part.

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*[email protected]
; A Content-Base header cannot be placed here, since this is a
; multipart MIME object.

--boundary-example-1

Part 1:
Content-Type: Text/HTML; charset=US-ASCII
Content-ID: <foo2*[email protected]>
Content-Location: http://www.ietf.cnir.reston.va.us/images/foo1.bar1
; This Content-Location must contain an absolute URI, since no base
; is valid here.

--boundary-example-1

Part 2:
Content-Type: Text/HTML; charset=US-ASCII
Content-ID: <foo4*[email protected]>
Content-Location: foo1.bar1 ; The Content-Base below applies to
; this relative URI
Content-Base: http://www.ietf.cnri.reston.va.us/images/

--boundary-example-1--

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

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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.

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--

4.4 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),
specifying the base to be used.

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(c) There is a Content-Location header in the 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 RFC 1866 [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).

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[REL] requires that the type attribute of the "Content-Type:
Multipart/related" statement be the type of the root object, and this
value can thus be "multipart/alternative". 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.

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RFC 2110 MHTML March 1997

Within such a multipart/related, ALL different parts MUST have
different Content-Location or Content-ID values.

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
[RFC 1866=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

If there is a Content-Base header, then the recipient MUST employ
relative to absolute resolution as defined in RFC 1808 [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, and the HTML markup contains a BASE element so that relative
URIs in the HTML markup can be resolved.

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

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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 BASE specifications, such as the BASE element in HTML. Note
that this only applies for matching Content-Location headers, not for
URL-s in the HTML document which are resolved through network look up
at read time.

The URI in the Content-Location header 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 RFC 1738 [URL] and RFC 1873
[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: [email protected]
Content-Location: CID: [email protected]

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.

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: [email protected]
To: [email protected]
Subject: A simple example
Mime-Version: 1.0
Content-Type: Text/HTML; charset=US-ASCII

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RFC 2110 MHTML March 1997

<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: [email protected]
To: [email protected]
Subject: A simple example
Mime-Version: 1.0
Content-Type: Multipart/related; boundary="boundary-example-1";
type=Text/HTML; start=foo3*[email protected]

--boundary-example-1
Content-Type: Text/HTML;charset=US-ASCII
Content-ID: <foo3*[email protected]>

... 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
Content-Transfer-Encoding: BASE64

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

--boundary-example-1--

9.3 Example with relative URIs to an embedded GIF picture

From: [email protected]
To: [email protected]
Subject: A simple example
Mime-Version: 1.0
Content-Base: http://www.ietf.cnri.reston.va.us
Content-Type: Multipart/related; boundary="boundary-example-1";
type=Text/HTML

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RFC 2110 MHTML March 1997

--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="/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: ¨

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

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

--boundary-example-1--

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

From: [email protected]
To: [email protected]
Subject: A simple example
Mime-Version: 1.0
Content-Type: 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*[email protected]" ALT="IETF logo">

--boundary-example-1
Content-ID: <foo4*[email protected]>
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64

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

--boundary-example-1--

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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 "á" or "á") 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
[MIME-IMB section 4.2].

Note that the "charset" parameter refers only to the MIME
character encoding. For example, the string "á" 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 5].

- The MIME standard [MIME1] 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.

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RFC 2110 MHTML March 1997

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
default.

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

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RFC 2110 MHTML March 1997

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. Acknowledgments

Harald T. Alvestrand, Richard Baker, Dave Crocker, Martin J. Duerst,
Lewis Geer, Roy Fielding, Al Gilman, Paul Hoffman, 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.

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RFC 2110 MHTML March 1997

14. 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: "Content-ID and Message-ID Uniform
Resource Locators". RFC 2111, February 1997.

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

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

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

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

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RFC 2110 MHTML March 1997

[PDF] Bienz, T., Cohn, R. and Meehan, J.: "Portable Document
Format Reference Manual, Version 1.1", Adboe Systems
Inc.

[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.

15. Author's Address

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: [email protected]
S-164 40 Kista, Sweden

Alex Hopmann E-mail: [email protected]
Microsoft Corporation
3590 North First Street
Suite 300
San Jose
CA 95134
Working group chairman:

Einar Stefferud <[email protected]>

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