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Network Working Group                                       Jacob Palme
Internet Draft                                 Stockholm University/KTH
draft-ietf-mhtml-info-00.txt                                   May 1996
Category-to-be: Informational
Expires: November 1996




Sending HTML in E-mail



Status of this Memo


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
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Internet-Drafts are draft documents valid for a maximum of six
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To learn the current status of any Internet-Draft, please check
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ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).

This memo provides information for the Internet community. This'
memo does not specify an Internet standard of any kind, since
this document is mainly a compilation of information taken from
other RFC-s. Distribution of this memo is unlimited.


Abstract

The memo "MIME E-mail Encapsulation of Aggregate HTML Documents (MHTML)"
(draft-ietf-mhtml-spec-00.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-00.txt       Sending HTML in E-mail       May 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. Acknowledgments
8. References
9. Author's Address


Mailing List Information:

Further discussion on this memo should be done through the mailing list
MHTML@SEGATE.SUNET.SE.

To subscribe to this list, send a message to
    LISTSERV@SEGATE.SUNET.SE
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
LISTSERV@SEGATE.SUNET.SE 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.
or
GET MHTML DIGEST


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
capaibilities 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
folder/directory.

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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draft-ietf-mhtml-info-00.txt       Sending HTML in E-mail       May 1996

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
parts

    +--------+       +-----------+       +--------+
    | 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
elements.

 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 witha 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 standardised 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 specified in
section 8 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
Multipart/related.


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, for example with Content-
Type: Text/plain as the first choice, and Content-Type: Text/HTML as the
second choice.

Both the Content-type: Multipart/related, as defined in chapter 6 above
and the Content-Type: Multipart/alternative, as defined in chapter 7
above can be combined in the same message. It is then recommended to put
the Multipart/alternative inside the Multipart/related.














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Example:

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

      --boundary-example 1
      Content-Type: MULTIPART/ALTERNATIVE
      Boundary: boundary-example-2

         --boundary-example-2
         Content-Type: Text/plain

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

         --boundary-example-2
         Content-Type: Text/HTML
         Content-ID: content-id-example@example.host

         ... text of the HTML document ...

         --boundary-example-2--
      --boundary-example-1
      Content-Type: Image/GIF

      ... a body part, to which the HTML document has a link  ...
      --boundary-example-1--

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

There are two reasons for the above recommendation:

(a) All related objects will be inside the multipart/related, as
    specified in [MHTML] and [REL].

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


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

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URL-s in the message. This may of course not always be possible, in some
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.


7. 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 Sawinski 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.


8. 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.

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.

[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", <draft-ietf-http-v10-
                spec-04.txt>, April 1996.

[MHTML]         J. Palme & A. Hopmann: "Packaging Aggregate HTML
                Objects in MIME E-mail", <draft-ietf-mhtml-spec-
                00.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.


9. 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: jpalme@dsv.su.se
S-164 40 Kista, Sweden













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