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Versions: 00 01 02 03 04 05 RFC 2110

Network Working Group                                       Jacob Palme
Internet Draft                                 Stockholm University/KTH
draft-ietf-mhtml-spec-00.txt                          Alexander Hopmann
Category-to-be: Standard                         ResNova Software, Inc.
Expires: October 1996                                        April 1996




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



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 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).

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

Although HTML 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 memo describes a set of guidelines that will allow
conforming mail user agents to be able to send, deliver and display
these HTML objects. In addition it is hoped that these techniques will
also apply to the wider category of URI-enabled objects. In order to do
this, the memo introduces two new MIME content-headers with the names
"Content-Location" and "Content-Base".






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Differences from draft-palme-text-html-02.txt and from draft-hopmann-
html-email-packaging-00.txt:

This document is based on two previous ietf drafts, draft-palme-text-
html-02.txt and draft-hopmann-html-email-packaging-00.txt. About one
third of this memo is taken from each of these previous Internet drafts,
and about one third is new text.

This draft is based on the discussions during the Los Angeles IETF
meeting in March 1996. Where decisions were taken at that meeting, the
document reflects what was decided. Where decisions were not taken, the
draft reflects suggestions from the editor for resolving such issues.

The most important decision taken at that meeting was to choose two
methods for linking of HTML documents to body parts as described in
sections 8.2 and 8.3 of this memo.

Who did it: Because of lack of time, Alex Hopmann has not had time to
check this draft before its submission to IETF, so Jacob Palme alone is
responsible. But many important sections are copied from Hopmann's
earlier draft, and hopefully Hopmann will have time to approve the
document so that we can both co-author it when finally published.
































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Table of Contents

1. Introduction
2. Terminology
3. Purpose
4. The Content-Location and Content-Base MIME Content Headers
     4.1 New MIME content headers
     4.2 The Content-Base header
     4.3 The Content-Location Header
     4.3 Encoding of URIs in e-mail headers
5. Use of Relative URL-s in Text/HTML Contents
6. Sending HTML documents without linked documents
7. Use of the Content-Type: Multipart/related
     7.1 How to use the Multipart/related Content-Type
     7.2 The includes parameter to 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
     8.4 Catalogs
9 Examples
     9.1 Example of a HTML body without included linked objects
     9.3 Example with relative URI-s to an embedded GIF picture:
     9.4 Example using CID URL and Content-ID header to an
     embedded GIF picture:
10. Content-Disposition header
11. Encoding Considerations for HTML bodies
     11.1 Character set issues
     11.2 Line break characters
12. Security Considerations
13. Conformance
14. Acknowledgments
15. References
16. 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.

Comments on less important details may also be sent to the main editor,
Jacob Palme <jpalme@dsv.su.se>. See also URL:
http://www.dsv.su.se/~jpalme/ietf/jp-ietf-home.html>

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1. Introduction

The HTML format is a very common format for documents in the Internet,
and there is an obvious need to be able to send documents in this format
in e-mail [RFC821=SMTP, RFC822]. The "text/html; version=2.0" media type
is defined in RFC 1866 [HTML2]. This memo gives additional
specifications on how to use the text/html media type as a Content-Type
in MIME [RFC 1521=MIME1] e-mail messages. In particular, the document
discusses sending of HTML documents with embedded links to images and
other data in separate documents which are to be displayed inline to the
recipient.

An alternative way for sending HTML documents 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 memo.


2. Terminology

Most of the terms used in this memo are defined in other RFC-s.

Absolute URI          See RFC 1866 [HTML2]

CID                   See [MIDCID]

Content-Base          See [RELURL] and section 4.2 below.

Content-ID            See [MIME1].

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.

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 RFC 1866 [HTML2]

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

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

MIME                  See RFC 1521 [MIME1], [MIME2]

MUA                   Messaging User Agent

MUST                  See RFC 1123 [HOSTS]


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Relative URI          See RFC 1866 [HTML2]

Relative URL          See [RELURL]

SHOULD                See RFC 1123 [HOSTS]

URI, absolute and     See RFC 1866 [HTML2]
relative

URL                   See RFC 1738 [URL]

URL, relative         See [RELURL]


3. Purpose

Although HTML [RFC 1866=HTML2] is a valid MIME [RFC1521=MIME1, MIME2]
type, RFC 1866 [HTML2] does not provide enough specification in order
for two electronic mail user agents to be able to interoperate using
HTML as a document format. This draft describes a set of guidelines that
will allow conforming mail user agents to be able to send, deliver and
display HTML objects. This standard only covers HTML objects containing
URI-s [RFC 1738=URL], but it is hoped that these techniques can also be
used for other object formats containing URI-s.

An HTML aggregate object is a MIME-encoded message that contains an HTML
document as well as other data that is required in order to represent
that object (inline pictures, style sheets, applets, etc.). HTML
aggregate objects can also include additional HTML documents that are
linked to the first object, as well as other arbitrary MIME content.

In designing HTML capabilities for electronic mail user agents (UAs), it
is important to keep in mind the differing needs of several audiences.
Mail sending agents might send aggregate HTML objects as an encoding of
normal day-to-day electronic mail. Mail sending agents might also send
aggregate HTML objects when a user wishes to mail a particular document
from the web to someone else. Finally mail sending agents might send
aggregate HTML documents as automatic responders (=mail servers),
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 HTML document and
display it just as any other text content type would be displayed.
Others might have to pass this aggregate HTML document to an HTML
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 an HTML 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.



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

4.1 New MIME content headers

In order to resolve URI references to other body parts, two new MIME
content headers are defined, Content-Location and Content-Base. Both the
new 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 URI-s which are URL-s are used, but
it is anticipated that other forms of URI-s will in the future be used.

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

     content-location ::= "Content-Location:" URI-parameter

     content-base ::= "Content-Base:" URI-parameter

where URI is at present (April 1996) restricted to the syntax for URL-s
as defined in RFC 1738 [URL]. This syntax may be widened when the
definition of the URI syntax becomes more stable.

4.2 The Content-Base header

The Content-Base gives a base for relative URL-s occuring in other
heading fields and in HTML contents which do not have any BASE element
in their HTML code. Its value MUST be an absolute URI.

A Content-Base header is valid within the content or message heading
where it occurs and in body parts within that message or content part.
If several Content-Base headers apply to a content part, the innermost
is valid.

Example showing which Content-Base is valid where:

   Content-Base: "http://www.ietf.cnri.reston.va.us/"
   Content-Type: Multipart/related; boundary="boundary-example-1";
                 type=Text/HTML; start=foo2*foo3@bar2.net

   --boundary-example-1

   Part 1:
   Content-Type: Text/HTML
   Content-ID: foo2*foo3@bar2.net
   Content-Location: "foo1.bar1" ; The Content-Base above applies to
                                    ; this relative URL

   --boundary-example-1






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   Part 2:
   Content-Type: Text/HTML
   Content-ID: foo4*foo5@bar2.net
   Content-Location: "foo1.bar1" ; The Content-Base below applies to
                                 ; this relative URL
   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
object present in whose heading the header is placed. Its value CAN be
an absolute or relative URI.

IF a Content-Location header contains a relative URI, then there MUST
also be a Content-Base header specifying the base for the relative URI,
in the same or in a surrounding heading.

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. Thus, 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 a document which is only
retrievable using this URI in a restricted domain, such as within a
company-internal web space. The header MUST, even in this case, after
transformation to an absolute URI, just like any other absolute URI, be
globally unique.

4.3 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. When the lines are folded, only
white-space, no additional non-white space characters, may be
introduced. Receivers can then just remove all white-space within the
URI to get back the original URI.

IETF may in the future separately specify in more detail how URIs are to
be encoded in e-mail headers. Such a separate specification will then
replace the paragraph above.


5. Use of Relative URL-s in Text/HTML Contents

Relative URL-s inside contents with the Content-Type: Text/HTML
SHOULD never be used except in one of the following three
cases (in order of priority, if more than one of them are present,
the first-listed applies):






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(a) There is a BASE element in the HTML document which resolves the
    relative URL into a non-relative URL.

(b) There is a Content-Base header (as defined in [RELURL]), giving
    the base to be used.

(c) There is a Content-Location header in the heading of the Text/HTML
    body which can then serve as the base in the same way as the URL
    of a HTML document itself can serve as a base for relative URL-s
    within the document.


6. Sending HTML documents without linked documents

If an HTML document is sent without other documents, to which it is
linked, it CAN be sent as a Text/HTML body part which need not be
included in any Multipart/related body part.

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
SHOULD only be done if all the recipients has the necessary Internet
connections.

Note that it is PERMITTED, although usually NOT RECOMMENDED, to send
documents with links that the recipient cannot resolve. (Example: Two
persons developing a new HTML page may send incomplete versions back and
forward.)


7. Use of the Content-Type: Multipart/related

7.1 How to use the Multipart/related Content-Type

The use of URI references creates some additional issues for aggregate
HTML objects. Normal URI references can of course be used, however it is
likely that many user agents may not be able to retrieve those objects
referred to. This document provides a means for these additional objects
to be transmitted with the HTML and for the links between these objects
to be properly resolved.

If a message contains one or more Text/HTML body parts and also contains
as separate body parts, data, to which hyperlinks (as defined in RFC
1866 [HTML2]) in the Text/HTML body parts refers, then this set of
documents SHOULD be sent within a Multipart/Related body part as
defined in [REL].

The root of the Multipart/related SHOULD be of the Content-Type:
Text/HTML, or of the Content-Type Multipart/Alternative which CAN be
resolved to Text/HTML.



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If the root is not the first body part within the Multipart/related, 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 URL-s which the recipient cannot resolve via HTTPor
other connectivity-requiring URL-s.

(c) 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 URLs. It SHOULD not transform these URLs into
some other URL 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.

It is permitted, but NOT RECOMMENDED, that the Text/HTML body contains
links to MIME body parts outside of the Multipart/Related or in other
messages. Implementors are reminded that many receiving mailers will not
be able to resolve such links.

Within such a Multipart/related, no two different parts may have the
same Content-Location value.

7.2 The includes parameter to multipart/related

*** New text:

A new parameter is added to the Multipart/related header, with the name
"includes". The value of this parameter can be either
"includes=complete" or "includes=incomplete". If this is done,
"complete" means that all in-line embedded information is contained
within this Multipart/related, while "incomplete" means that som in-line
embedded information is not included and may have to be retrieved by
other means in order to display the document to the user.


8. Format of Links to Other Body Parts

8.1 General principle

A Text/HTML body part may contain hyperlinks to documents which are
included as other body parts in the same message and within the same

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multipart/related content. Often such linked documents are meant to be
displayed inline to the reader of the main document. HTML version 2.0
[RFC 1866=HTML2] has only one way of specifying hyperlinks to such
inline embedded content, the IMG tag. New tags with this property are
however 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 Text/HTML body parts.
This is done in the following way: For each distinct URI in the
Text/HTML document, which refers to data which is sent in the same MIME
message, there SHOULD be a separate body part within the
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).

*** Question: Only IETF-defined URI schemes? Why not allow any privately
defined scheme also? Since it is not meant to be used for actual
retrieval, any kind or URI or URL scheme might be allowed.

8.2 Use of the Content-Location header

When a linked body part has a Content-Location header, the string in
this field SHOULD be identical to the URI as used in the Text/HTML body
part referring to it.

Note: By identical string is not meant equivalent URI-s after resolution
of relative URI-s to absolute URIs, but actually identical URI strings,
except for added white-space as specified in 4.3 above.

The URI in the Content-Location header need not refer to a document
which is actually available globally for retrieval using this URI (afer
resolution of relative URI-s). The URI (after resolution of relative URI-
s) SHOULD however still be globally unique.

8.3 Use of the Content-ID header and CID URLs

When CID (Content-ID) URL-s as defined in RFC 1738 [URL] and RFC 1873
[MIDCID] is 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-ID-s MUST be globally unique [MIME1]. It is thus not
permitted to make them unique only within this message or within this
multipart/related.

8.4 Catalogs

*** Controversial

The Multipart/related MAY contain as its first body part a catalog

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body part, containing a list of the body parts with size and type
information for each body part. Such a catalogue can be used by
receiving agents to provide better progressive display of the document
before it has been fully downloaded. This standard does not specify a
format for such catalogues, such format may become specifed in other
IETF standards. The Content-Type of such catalogues MAY be
Application/parts-directory.

A receiving mail agent can ignore body parts of this type, the only loss
may be delayed progressive rendering in some cases.


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 one by itself. 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

   <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>

*** Temporary note: The example below includes a parts directory to
allow for progressive display of messages downloaded via slow IMAP or
POP connections as defined in 8.4. Whether to provide for this has
not yet been decided.

9.2 Example with absolute URI-s to an embedded GIF picture:

This example also includes a parts-directory as specified in section 8.4
above.

   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

      --boundary-example 1
      Content-Type: Application/parts-directory

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      Part 1:
      ... the parts directory ...

      Part 2:
      Content-Location:
            "http://www.ietf.cnri.reston.va.us/images/ietflogo.gif"
      Content-Type: IMAGE/GIF; name="ietflogo.gif"

      --boundary-example 1
      Content-Type: Text/HTML
      Content-ID: foo3*foo1@bar.net

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

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

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

      --boundary-example-1--

9.3 Example with relative URI-s to an embedded GIF picture:

   From: foo1@bar.net
   To: foo2@bar.net
   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

      --boundary-example 1
      Content-Type: Text/HTML

      ... 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">

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

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

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      --boundary-example-1--

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

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

      --boundary-example 1
      Content-Type: Text/HTML

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

      --boundary-example-1
      Content-ID: foo4*foo1@bar.net
      Content-Type: IMAGE/GIF; name="ietflogo.gif"
      Content-Transfer-Encoding: BASE64

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

      --boundary-example-1--


10. Content-Disposition header

Information in a Content-Disposition header (as defined in RFC 1806
[CONDISP]) on individual body parts within a multipart/related SHOULD be
ignored by a receiving mailer which can handle Multipart/related and
Text/html, since corresponding information is defined by tags in the
HTML text itself.

Receiving mailers which are not capable of handling the
multipart/related header, and which thus by default handles the
multipart/related header as if it was multipart/mixed, CAN however make
use of information in a Content-Disposition header.


11. Encoding Considerations for HTML bodies

11.1 Character set issues

A mail user agent that wishes to send a content-type of HTML can just do
so, so long as the normal data encoding issues are taken care of as
specified in RFC 1521 [MIME1]. However at a basic level there are some
differences between HTML being transferred by HTTP and HTML being
transferred through Internet email. When transferred through HTTP, HTML

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by default uses the document character set ISO-8859-1 [HTML2]. Within
electronic mail, the default character set is US-ASCII [MIME1].

There are two recommended ways to encode 8-bit characters in Text/HTML
contents:

(1) Let the charset of the content part be iso-8859-1 or some other
    non-US-ASCII character set, and encode the content with the QUOTED-
    PRINTABLE encoding method.

(2) Let the charset of the content part be US-ASCII, and encode
    non-US-ASCII characters in the text using the data character
    encoding defined in RFC 1866 [HTML2].

Both these encoding methods are PERMITTED, and they CAN also be mixed in
the same document. Recipients MUST be capable of handling both encoding
alternatives. However, it is RECOMMENDED that encoding method (2) above
is used when sending Text/HTML messages.

If only method (2) is used, the charset parameter SHOULD be "us-ascii".

If method (1), or a mixture of method (1) and method (2) is used, the
charset parameter SHOULD be the character set used in the HTML text, for
example "iso-8859-1".

11.2 Line break characters

*** Controversial issue:

Line breaks in HTML documents SHOULD be of the CRLF format (not bare LF
or bare CR), but receiving systems SHOULD be able to handle receipt of
Content-Type Text/html documents which use bare LF or bare CR for line
breaks.


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 URL 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.

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 turns over 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

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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.

In addition, by allowing people to mail aggregate HTML 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.


13. Conformance

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).

An e-mail system which claims conformance to this standard SHOULD be
able to send Multipart/related (as defined in section 7) with at least
one URI or URL scheme. Either the Content-Location method or the Content-
ID method MUST be supported, but both need not be supported.

Support of the include parameter (section 7.2) or for body parts
catalogs (section 8.4) is not required for conformance.


14. Acknowledgments

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


15. 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.








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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.

[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.


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

Alex Hopmann
President
ResNova Software, Inc.               E-mail: alex.hopmann@resnova.com
5011 Argosy Dr. #13
Huntington Beach, CA 92649

Working group chairman:

Einar Stefferud <stef@nma.com>


































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