Network Working Group                                       Jacob Palme
Internet Draft                                 Stockholm University/KTH
Category-to-be: Informational
Expires: February 1997                                      August 1996

Sending HTML in E-mail, an informational supplement to RFC ???:
MIME E-mail Encapsulation of Aggregate HTML Documents (MHTML)

Status of this Memo

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This memo provides information for the Internet community. This' memo
does not specify an Internet standard of any kind. Distribution of this
memo is unlimited.


The memo "MIME E-mail Encapsulation of Aggregate HTML Documents (MHTML)"
(draft-ietf-mhtml-spec-03.txt) specifies how to send packaged aggregate
HTML objects in MIME e-mail. This memo is an accompanying informational
document, intended to be an aid to developers. This document is not an
Internet standard.

Issues discussed are implementation methods, caching strategies, making
messages suitable both for mailers which can and which cannot handle
Multipart/related and handling recipients which do not have full
Internet connectivity.

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draft-ietf-mhtml-info-03.txt                                August 1996

Table of Contents

1. Introduction
2. Implementation methods
2.1 Method 1: Combining web browser and e-mail program
2.2 Method 2: Rewriting the HTML
2.3 Method 3: Using a translation table
2.4 Method 4: Using a proxy HTTP server do retrieve referenced body parts
2.5 Method 5: Putting the mail client into a proxy HTTP server
2.6 Other methods
2.7 Communication between web browser mail client
3. Caching of body parts
4. Recipients which cannot handle the Multipart/related Content-type
5. Use of the Content-Type: Multipart/alternative
6. Recipient may not have full Internet connectivity
7. Encoding of non-ascii characters
8. Conversion from HTTP to e-mail
9. Acknowledgments
10. References
11. Author's Address

Mailing List Information

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1. Introduction

[MHTML] specifies how to send packaged aggregate HTML objects in MIME e-
mail. This memo is an accompanying informational document, intended to
be an aid to developers. This document is not an Internet standard.

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2. Implementation methods

The [MHTML] standard has been intentionally written to be implementable
both in cases where the web browser and e-mail program is combined, and
when they are separate programs. Implementation is of course no problem
if the web browser is combined with the e-mail client.

2.1 Method 1: Combining web browser and e-mail program

This is the architecturally simplest approach. A web-browser with a
built in e-mail program will be able to use its own web browser
capabilities to display HTML-formatted messages.

2.2 Method 2: Rewriting the HTML

    +---------+                           +--------+
    | Web     |                           | Mail   |
    | browser |                           | client |
    +-------+-+                           +-+------+
            |                               |
         | +----------+  +--+  +--+            |
         | | Start    |  |  |  |  | Related    |        Figure 1
         | | HTML     |  |  |  |  | body part  |
         | | document |  |  |  |  | parts      |
         | +----------+  +--+  +--+            |

If the web browser is separate from the e-mail client, the e-mail client
might turn over the HTML body part to the web browser and ask it to
display it (Figure 1). One way of doing this is to store the HTML body
part in a file, and ask the web browser to display this file. If
multipart/related is used, this can be implemented by storing all the
body parts within the multipart/related in an otherwise empty

The mail client may have to rewrite the HTML, replacing URL-s with
shorter relative URL-s which the Web browser can resolve as file names
in the same directory/folder where the HTML document itself is stored
when turning it over to the Web browser.

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2.3 Method 3: Using a translation table

    +---------+                          +--------+
    | Web     |                          | Mail   |
    | browser |                          | client |
    +-------+-+                          +-+------+
            |                              |
         | +--------+  +--+  +--+            |
         | | Trans- |  |  |  |  | Related    |        Figure 2
         | | lation |  |  |  |  | body part  |
         | | table  |  |  |  |  | parts      |
         | +--------+  +--+  +--+            |

An alternative to rewriting the HTML file before turning it over to the
Web browser may be to use a translation table, in case the Web browser
has the capability to use such a table to rewrite URL-s on the fly while
displaying the document (Figure 2). This requires that the Web browser
is capable of receiving CID: URL-s and resolving them using this
translation table in the same way as for other URL-s.

2.4 Method 4: Using a proxy HTTP server do retrieve referenced body

    +--------+       +-----------+       +--------+
    | Proxy  |       | Data base |       | Mail   |
    | web    |-------| of cached |-------| server |
    | server |       | objects   |       |        |
    +----+---+       +-----------+       +----+---+
         |                                    |
    +----+----+                          +----+---+   Figure 3
    | Web     |                          | Mail   |
    | browser |                          | client |
    +-------+-+                          +-+------+
            |                              |
         |         Start HTML object         |

Yet another method is to use a proxy web server, to which the web
browser requests are sent, and which will then use the cached body parts
instead of normal web retrieval from the network (Figure 3). If the Web
browser is set to use this proxy server for all URL-s, also for CID URL-
s, no rewriting of the HTML will be necessary.

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2.5 Method 5: Putting the mail client into a proxy HTTP server

     | Proxy  | Mail   |
     |  HTTP  | client |
     | server |        |
        HTTP protocol              Figure 4
         | Web     |
         | browser |

A mail client can also be included in an HTTP server (Figure 4). The
user will then not have to install any mail client software in his
personal computer, all the mail functionality is mapped on HTTP and HTML

 2.6 Other methods

The mail client and the web browser can of course communicate in other
ways, such as using inter-process communication.

 2.7 Communication between web browser mail client

Many web browsers have API-s to allow other programs to communicate with
them. There is however no accepted real or de-facto standard for such
API-s, which means that a mail program which relies on such API-s will
only be able to use those Web browser, whose API they support.

Note however, that most of the methods described above can be
implemented with a very minimal such API. The only API function needed
is to be able to tell a Web browser, when it is started, to open a
particular file. And this API function is a standardized part of the
operating system on most platforms. In particular, method 1 and 3 above
uses the functionality that a relative URL is resolved with the location
of the base document as base. This means that if the base document is a
file, relative URL-s will be resolved as FILE URL-s in the same
directory/folder where the HTML document itself is placed.

There is a need for buttons in the Web page which the user can use to
get back to the mail program again after reading the mail with the Web
browser. A common technique to achieve this is to define a new MIME data
type for this button. The Web browser is then configured to transfer
control to the mail client when the user pushes this button, i.e.
downloads a file of this new MIME type.

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3. Caching of body parts

Suppose a message contains body parts with the Content-Location header
as defined in [MHTML]. A receiving agent might then put this body part
into a web cache, with the URL in the Content-Location as its name, so
that later retrievals of this URL use the cached body parts. There is
however no guarantee that such a cached item is correct. Such caching is
thus not recommended for use in other ways than for resolution of links
within this e-mail message.

4. Recipients which cannot handle the Multipart/related Content-Type

A message sent according to the specifications in [MHTML] may have
recipients, whose mailers cannot handle the Multipart/related Content-
Type in the way specified in [MHTML].

According to [MIME1] a mailer which encounters an unknown subtype to
Multipart, should handle this as Multipart/mixed.

To improve this, Multipart/alternative can be used as discussed in
section 5 of this memo.

Content-Disposition, as specified in [CONDISP] and in [MHTML], section
10, can also be used as an aid to mailers which do not understand

Captions on images, which are included in the HTML text, might for non-
HTML-capable recipient been found in the Content-Description header. Do
not assume that HTML-capable user agents will display the Content-
Description header, they may assume that this information is included in
the HTML text instead.

5. Use of the Content-Type: Multipart/alternative

If the message is sent to recipients, all of which may not have mailers
capable of handling the Text/HTML content-type, then the Content-Type:
Multipart/Alternative [MIME1] can be used in two ways:

(a) Inside the Content-Type Multipart/related, body parts can be
specified with Content-Type: Text/plain as the first choice, and Content-
Type: Text/HTML as the second choice.


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

      --boundary-example 1
      Boundary: boundary-example-2

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         Content-Type: Text/plain

         ... plain text version of the document for recipients
         whose mailers cannot handle Text/HTML ...

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

         ... text of the HTML document ...

      Content-Type: Image/GIF

      ... a body part, to which the HTML document has a link  ...

Note that the type parameter of Multipart/related in this case should be
Multipart/alternative and not Text/HTML.

(b) Outside the Multipart/Related, with Multipart/Related as one
alternative and Multipart/Mixed as the other alternative.

When choosing between these two methods of employing
multipart/alternative, note the following:

 (1) Clients which do not support Multipart/related, and which thus will
     interpret it as multipart/mixed, will with choice (a) display
     the inline objects. Thus, a recipient whose mailer can handle Image/gif
     but not multipart/related will still be shown the images, they
     will not be suppressed by being inside a suppressed branch of the

 (2) Choice (b) will not show inline images in the Multipart/Related,
     unless this information is repeated in both branches of the

A general warning: Many mailers do not support Content-Type:
Multipart/alternative, and may then interpret it as Multipart/mixed.

6. Recipient may not have full Internet connectivity

The recipient of a message sent by e-mail may not always have full
Internet connectivity. The recipient may be behind a gateway or firewall
which prohibits or restricts Internet connectivity.

This means that the recipient may not be able to resolve URL-s in an e-
mail message, unless the referred-to documents are included in the e-
mail message itself. Thus, it is often suitable to include in an e-mail
message all documents which are referred to (directly or indirectly) by
URL-s in the message. This may of course not always be possible, in some

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draft-ietf-mhtml-info-03.txt                                August 1996

cases the set of referred-to documents (directly or indirectly) may be
the whole WWW document space, i.e. millions of documents. A choice must
then be made how much to include. Of course, it is most important to
include all inline objects, i.e. objects linked by such hyperlinks as
IMG, etc., which specify that the linked objects are to be shown to the
user immediately.

In the case of ACTION elements in HTML forms, by making these ACTION
elements of the "mailto:" URL type, rather than the "http:" URL type,
you will enable also recipients without full Internet connectivity to
fill in and send in your forms. The HTML specification [HTML2] allows
default action when no ACTION element is included, but this default
action may not be suitable when sending the HTML document via e-mail.
Thus, it is better to always put an explicit ACTION element into HTML
forms sent by e-mail.

7. Encoding of non-ascii characters

Displayed text                        Displayed text
               |                                     ^
               V                                     |
         +-------------+                       +----------------+
         | HTML editor |                       | HTML viewer    |
         |             |                       | or Web browser |
         +-------------+                       +----------------+
             |                                       ^
             V                                       |
         HTML markup                             HTML markup
             |                                       ^
             V                                       |
  +---------+ +---------------+       +-------------+ +---------------+
  | MIME    | | MIME content- |       | MIME        | | MIME content- |
  | encap-  | | transfer-     |       | heading     | | transfer-     |
  | sulator | | encoder       |       | interpreter | | decoder       |
  +---------+ +---------------+       +-------------+ +---------------+
    |              |                            ^              ^
    V              V         +-----------+      |              |
MIME heading + MIME content->| Transport |->MIME heading + MIME content

                               Figure 5

Definitions (see Figure 5):

Displayed text   A visual representation of the intended text.

HTML markup      A sequence of characters formatted according to the
                 HTML specification [HTML2].

MIME content     A sequence of octets physically forwarded via e-mail,
                 may use MIME content-transfer-encoding as specified
                 in [MIME1].

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HTML editor      Software used to produce HTML markup.

MIME content-    Software used to encode non-US-ASCII characters
transfer-encoder as specified in [MIME1].

MIME content-    Software used to decode non-US-ASCII characters
transfer-decoder as specified in [MIME1].

MIME heading     Software used to interpret the information in MIME
interpreter      headings.

HTML viewer      Software used to display HTML documents to recipients.

Some implementations may have a choice of whether to represent non-ascii
characters at the HTML layer (using "&" entity references or numeric
character references as defined in [HTML2] section 3.2.1) or at the MIME
layer (using Content-Transfer-Encoding as defined in [MIME1] section 5).

In choosing between these two representation methods, note the following

(1) Modifying HTML markup may disrupt security content integrity checks.

(2) The choice of modifying HTML markup may be more suitable for
    recipients whose mailers do not support MIME.

(3) Using MIME Content-Transfer-Encoding may be more suitable for
    recipients who have MIME-compliant mailers but do pass the text over
    to a web browser.

8.  Conversion from HTTP to e-mail

Information received or retrieved using HTTP cannot always be sent
unchanged as e-mail using the Content-Type: Text/HTML, because of the
restrictions which MIME places no the format of Content-Type: Text/HTML.
The same problem may occur for documents retrieved via HTTP, which are
in other textual formats than HTML. In particular, note the following:

(a) Content-encodings allowed in HTTP, but not allowed in MIME, must be

(b) HTTP allows line breaks as bare CRs or bare LFs or something else,
while MIME only allows line breaks as CRLF in subtypes of the Text

(c) HTTP allows character sets like Unicode-1-1, which do not represent
line breaks as CRLFs, such text may have to be rewritten to character
sets like Unicode-1-1-UTF-7 in which line breaks are represented as

A good overview of the differences, with regard to the use of Content-
Type:text, between MIME and HTTP, can be found in [HTTP] appendix C.

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If you want to send HTTP unchanged via e-mail, you might consider using
the content-type message/HTTP instead of the content-type Text/HTML.

8. Representation of line breaks

Implementors should note that while HTML allows bare CRs, bare LFs and
CRLFs as line breaks, MIME requires that all line breaks must be CRLF.
If you want the same HTML text to be returned after decoding that was
sent before encoding, then you can use BASE64 or Quoted-Printable
encoding of bare CRs and bare LFs.

9. Acknowledgments

Harald Tveit Alvestrand, Richard Baker, Dave Crocker, Martin J. Duerst,
Roy Fielding, Al Gilman, Paul Hoffman, Alexander Hopmann, 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 and several other
people have helped us with preparing this memo. I alone take
responsibility for any errors which may still be in the memo.

10. References

Temporary note: This list contains some references to Internet drafts. It is anticipated that these Internet drafts will become RFC-s before this memo. The references will then in this memo be changed to refer to the corresponding RFC instead. This list also includes some RFC-s which are not up to date, and which will be replaced by new memos presently in ietf draft status.

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

[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.

[MHTML]         J. Palme & A. Hopmann: "Packaging Aggregate HTML
                Objects in MIME E-mail", <draft-ietf-mhtml-spec-
                03.txt>, April 1996.

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[MIDCID]        E. Levinson: "Message/External-Body Content-ID Access
                Type", RFC 1873, December 1995.

[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". draft-ietf-
                822ext-mime-imt-02.txt, December 1995.

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

[REL]           Harald Tveit Alvestrand, Edward Levinson: "The MIME
                Multipart/Related Content-type", <draft-levinson-
                multipart-related-00.txt>, January 1995.

[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.

[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", draft-ietf-mailext-acc-url-
                01.txt, November 1995.

11. Author's Address

Jacob Palme                          Phone: +46-8-16 16 67
Stockholm University and KTH         Fax: +46-8-783 08 29
Electrum 230                         E-mail:
S-164 40 Kista, Sweden

Working group chairman:

Einar Stefferud <>

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