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Network Working Group J. Palme
Request for Comments: 2557 Stockholm University/KTH
Obsoletes: 2110 A. Hopmann
Category: Standards Track Microsoft Corporation
N. Shelness
Lotus Development Corporation
March 1999
MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)
Status of this Memo
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.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
HTML [RFC 1866] defines a powerful means of specifying multimedia
documents. These multimedia documents consist of a text/html root
resource (object) and other subsidiary resources (image, video clip,
applet, etc. objects) referenced by Uniform Resource Identifiers
(URIs) within the text/html root resource. When an HTML multimedia
document is retrieved by a browser, each of these component resources
is individually retrieved in real time from a location, and using a
protocol, specified by each URI.
In order to transfer a complete HTML multimedia document in a single
e-mail message, it is necessary to: a) aggregate a text/html root
resource and all of the subsidiary resources it references into a
single composite message structure, and b) define a means by which
URIs in the text/html root can reference subsidiary resources within
that composite message structure.
This document a) defines the use of a MIME multipart/related
structure to aggregate a text/html root resource and the subsidiary
resources it references, and b) specifies a MIME content-header
(Content-Location) that allow URIs in a multipart/related text/html
root body part to reference subsidiary resources in other body parts
of the same multipart/related structure.
Palme, et al. Standards Track [Page 1]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
While initially designed to support e-mail transfer of complete
multi-resource HTML multimedia documents, these conventions can also
be employed to resources retrieved by other transfer protocols such
as HTTP and FTP to retrieve a complete multi-resource HTML multimedia
document in a single transfer or for storage and archiving of
complete HTML-documents.
Differences between this and a previous version of this standard,
which was published as RFC 2110, are summarized in chapter 12.
Table of Contents
1. Introduction ................................................. 3
2. Terminology ................................................. 4
2.1 Conformance requirement terminology ...................... 4
2.2 Other terminology ........................................ 4
3. Overview ..................................................... 6
4. The Content-Location MIME Content Header ..................... 6
4.1 MIME content headers ..................................... 6
4.2 The Content-Location Header .............................. 7
4.3 URIs of MHTML aggregates ................................. 8
4.4 Encoding and decoding of URIs in MIME header fields ...... 8
5. Base URIs for resolution of relative URIs .................... 9
6. Sending documents without linked objects ..................... 10
7. Use of the Content-Type "multipart/related" .................. 11
8. Usage of Links to Other Body Parts ........................... 13
8.1 General principle ........................................ 13
8.2 Resolution of URIs in text/html body parts ............... 13
8.3 Use of the Content-ID header and CID URLs ................ 14
9. Examples ..................................................... 14
9.1 Example of a HTML body without included linked objects ... 15
9.2 Example with an absolute URI to an embedded GIF picture .. 15
9.3 Example with relative URIs to embedded GIF pictures ...... 16
9.4 Example with a relative URI and no BASE available ........ 17
9.5 Example using CID URL and Content-ID header to an embedded
GIF picture .............................................. 18
9.6 Example showing permitted and forbidden references between
nested body parts ........................................ 19
10. Character encoding issues and end-of-line issues ............ 21
11. Security Considerations ..................................... 22
11.1 Security considerations not related to caching .......... 22
11.2 Security considerations related to caching .............. 23
12. Differences as compared to the previous version of this
proposed standard in RFC 2110 ............................... 24
13. Acknowledgments ............................................. 24
14. References .................................................. 25
15. Authors' Addresses .......................................... 27
16. Full Copyright Statement .................................... 28
Palme, et al. Standards Track [Page 2]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
1. Introduction
There are a number of document formats (Hypertext Markup Language
[HTML2], Extended Markup Language [XML], Portable Document format
[PDF] and Virtual Reality Markup Language [VRML]) that specify
documents consisting of a root resource and a number of distinct
subsidiary resources referenced by URIs within that root resource.
There is an obvious need to be able to send such multi-resource
documents in e-mail [SMTP], [RFC822] messages.
The standard defined in this document specifies how to aggregate such
multi-resource documents in MIME-formatted [MIME1 to MIME5] messages
for precisely this purpose.
While this specification was developed to satisfy the specific
aggregation requirements of multi-resource HTML documents, it may
also be applicable to other multi-resource document representations
linked by URIs. While this is the case, there is no requirement that
implementations claiming conformance to this standard be able to
handle any URI linked document representations other than those whose
root is HTML.
This aggregation into a single message of a root resource and the
subsidiary resources it references may also be applicable to
resources retrieved by other protocols such as HTTP or FTP, or to the
archiving of complete web pages as they appeared at a particular
point in time.
An informational RFC will be published as a supplement to this
standard. The informational RFC will discuss implementation methods
and some implementation problems. Implementers are strongly
recommended to read this informational RFC when developing
implementations of this standard. You can find it through URL
http://www.dsv.su.se/~jpalme/ietf/mhtml.html.
This standard specifies that body parts to be referenced can be
identified either by a Content-ID (containing a Message-ID value) or
by a Content-Location (containing an arbitrary URL). The reason why
this standard does not only recommend the use of Content-ID-s is that
it should be possible to forward existing web pages via e-mail
without having to rewrite the source text of the web pages. Such
rewriting has several disadvantages, one of them that security
checksums will probably be invalidated.
Palme, et al. Standards Track [Page 3]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
2. Terminology
2.1 Conformance requirement terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [IETF-TERMS].
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 Relative Uniform Resource Locators
AbsoluteURI [RELURL].
CID See Message/External Body Content-ID [MIDCID].
Content-Base This header was specified in RFC 2110, but has
been removed in this new version of the MHTML
standard.
Content-ID See Message/External Body Content-ID [MIDCID].
Content-Location MIME message or content part header with one
URI of the MIME message or content part body,
defined in section 4.2 below.
Content-Transfer- Conversion of a text into 7-bit octets as
Encoding specified in [MIME1] chapter 6.
CR See [RFC822].
CRLF See [RFC822].
Displayed text The text shown to the user reading a document
with a web browser. This may be different from
the HTML markup, see the definition of HTML
markup below.
Palme, et al. Standards Track [Page 4]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
Header Field in a message or content heading
specifying the value of one attribute.
Heading Part of a message or content before the first
CRLFCRLF, containing formatted fields with
attributes of the message or content.
HTML See HTML 2 specification [HTML2].
HTML Aggregate HTML objects together with some or all objects,
objects to which the HTML object contains hyperlinks,
directly or indirectly.
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 the MIME specifications [MIME1 to MIME5].
MUA Messaging User Agent.
PDF Portable Document Format, see [PDF].
Relative URI, See HTML 2 [HTML2] and RFC 1808 [RELURL].
RelativeURI
URI, absolute and See RFC 1866 [HTML2].
relative
URL See RFC 1738 [URL].
URL, relative See Relative Uniform Resource Locators [RELURL].
VRML See Virtual Reality Markup Language [VRML].
Palme, et al. Standards Track [Page 5]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
3. Overview
An aggregate document is a MIME-encoded message that contains a root
resource (object) as well as other resources linked to it via URIs.
These other resources may be required to display a multimedia
document based on the root resource (inline pictures, style sheets,
applets, etc.), or be the root resources of other multimedia
documents. It is important to keep in mind that aggregate documents
need to satisfy the differing needs of several audiences.
Mail sending agents might send aggregate documents as an encoding of
normal day-to-day electronic mail. Mail sending agents might also
send aggregate documents when a user wishes to mail a particular
document from the web to someone else. Finally mail sending agents
might send aggregate documents as automatic responders, providing
access to WWW resources for non-IP connected clients. Also with other
protocols such as HTTP or FTP, there may sometimes be a need to
retrieve aggregate documents. Receiving agents also have several
differing needs. Some 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 transmitted and displayed without breaking the message
integrity (MIC) checksum that is part of the signature.
4. The Content-Location MIME Content Header
4.1 MIME content headers
In order to resolve URI references to resources in other body parts,
one MIME content header is defined, Content-Location. This header can
occur in any message or content heading.
The syntax for this header is, using the syntax definition tools from
[ABNF]:
quoted-pair = ("\" text)
text = %d1-9 / ; Characters excluding CR and LF
%d11-12 /
%d14-127
WSP = SP / HTAB ; Whitespace characters
Palme, et al. Standards Track [Page 6]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
FWS = ([*WSP CRLF] 1*WSP) ; Folding white-space
ctext = NO-WS-CTL / ; Non-white-space controls
%d33-39 / ; The rest of the US-ASCII
%d42-91 / ; characters not including "(",
%d93-127 ; ")", or "\"
comment = "(" *([FWS] (ctext / quoted-pair / comment))
[FWS] ")"
CFWS = *([FWS] comment) (([FWS] comment) / FWS)
content-location = "Content-Location:" [CFWS] URI [CFWS]
URI = absoluteURI | relativeURI
where URI is restricted to the syntax for URLs as defined in Uniform
Resource Locators [URL] until IETF specifies other kinds of URIs.
4.2 The Content-Location Header
A Content-Location header specifies an URI that labels the content of
a body part in whose heading it is placed. Its value CAN be an
absolute or a 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.
An URI in a Content-Location header need not refer to an resource
which is globally available 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.
A Content-Location header can thus be used to label a resource which
is not retrievable by some or all recipients of a message. For
example a Content-Location header may label an object which is only
retrievable using this URI in a restricted domain, such as within a
company-internal web space. A Content-Location header can even
contain a fictitious URI. Such an URI need not be globally unique.
A single Content-Location header field is allowed in any message or
content heading, in addition to a Content-ID header (as specified in
[MIME1]) and, in Message headings, a Message-ID (as specified in
[RFC822]). All of these constitute different, equally valid body part
labels, and any of them may be used to satisfy a reference to a body
part. Multiple Content-Location header fields in the same message
heading are not allowed.
Palme, et al. Standards Track [Page 7]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
Example of a multipart/related structure containing body parts with
both Content-Location and Content-ID labels:
Content-Type: multipart/related; boundary="boundary-example";
type="text/html"
--boundary-example
Content-Type: text/html; charset="US-ASCII"
... ... <IMG SRC="fiction1/fiction2"> ... ...
... ... <IMG SRC="cid:97116092811xyz@foo.bar.net"> ... ...
--boundary-example
Content-Type: image/gif
Content-ID: <97116092511xyz@foo.bar.net>
Content-Location: fiction1/fiction2
--boundary-example
Content-Type: image/gif
Content-ID: <97116092811xyz@foo.bar.net>
Content-Location: fiction1/fiction3
--boundary-example--
4.3 URIs of MHTML aggregates
The URI of an MHTML aggregate is not the same as the URI of its root.
The URI of its root will directly retrieve only the root resource
itself, even if it may cause a web browser to separately retrieve
in-line linked resources. If a Content-Location header field is used
in the heading of a multipart/related, this Content-Location SHOULD
apply to the whole aggregate, not to its root part.
When an URI referring to an MHTML aggregate is used to retrieve this
aggregate, the set of resources retrieved can be different from the
set of resources retrieved using the Content-Locations of its parts.
For example, retrieving an MHTML aggregate may return an old version,
while retrieving the root URI and its in-line linked objects may
return a newer version.
4.4 Encoding and decoding of URIs in MIME header fields
4.4.1 Encoding of URIs containing inappropriate characters
Some documents may contain URIs with characters that are
inappropriate for an RFC 822 header, either because the URI itself
has an incorrect syntax according to [URL] or the URI syntax standard
Palme, et al. Standards Track [Page 8]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
has been changed to allow characters not previously allowed in MIME
headers. These URIs cannot be sent directly in a message header. If
such a URI occurs, all spaces and other illegal characters in it must
be encoded using one of the methods described in [MIME3] section 4.
This encoding MUST only be done in the header, not in the HTML text.
Receiving clients MUST decode the [MIME3] encoding in the heading
before comparing URIs in body text to URIs in Content-Location
headers.
The charset parameter value "US-ASCII" SHOULD be used if the URI
contains no octets outside of the 7-bit range. If such octets are
present, the correct charset parameter value (derived e.g. from
information about the HTML document the URI was found in) SHOULD be
used. If this cannot be safely established, the value "UNKNOWN-8BIT"
[RFC 1428] MUST be used.
Note, that for the matching of URIs in text/html body parts to URIs
in Content-Location headers, the value of the charset parameter is
irrelevant, but that it may be relevant for other purposes, and that
incorrect labeling MUST, therefore, be avoided. Warning: Irrelevance
of the charset parameter may not be true in the future, if different
character encodings of the same non-English filename are used in
HTML.
4.4.2 Folding of long URIs
Since MIME header fields have a limited length and long URIs can
result in Content-Location headers that exceed this length, Content-
Location headers may have to be folded.
Encoding as discussed in clause 4.4.1 MUST be done before such
folding. After that, the folding can be done, using the algorithm
defined in [URLBODY] section 3.1.
4.4.3 Unfolding and decoding of received URLs in MIME header fields
Upon receipt, folded MIME header fields should be unfolded, and then
any MIME encoding should be removed, to retrieve the original URI.
5. Base URIs for resolution of relative URIs
Relative URIs inside the contents of MIME body parts are resolved
relative to a base URI using the methods for resolving relative URIs
described in [RELURL]. In order to determine this base URI, the
first-applicable method in the following list applies.
Palme, et al. Standards Track [Page 9]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
(a) There is a base specification inside the MIME body part
containing the relative URI which resolves relative URIs into
absolute URIs. For example, HTML provides the BASE element for
this purpose.
(b) There is a Content-Location header in the immediately surrounding
heading of the body part and it contains an absolute URI. This
URI can serve as a base in the same way as a requested URI can
serve as a base for relative URIs within a file retrieved via
HTTP [HTTP].
(c) If necessary, step (b) can be repeated recursively to find a
suitable Content-Location header in a surrounding multi-part or
message heading.
(d) If the MIME object is returned in a HTTP response, use the URI
used to initiate the request
(e) When the methods above do not yield an absolute URI, a base URL
of "thismessage:/" MUST be employed. This base URL has been
defined for the sole purpose of resolving relative references
within a multipart/related structure when no other base URI is
specified.
This is also described in other words in section 8.2 below.
6. Sending documents without linked objects
If a text/html resource (object) is sent without subsidiary
resources, to which it refers, it MAY be sent by itself. In this
case, embedding it in a multipart/related structure is not necessary.
Such a text/html resource may either contain no URIs, or URIs which
the recipient is expected to retrieve (if possible) via a URI
specified protocol. A text/html resource may also be sent with
unresolvable links in special cases, such as when two authors
exchange drafts of unfinished resources.
Inclusion of URIs referencing resources which the recipient has to
retrieve via an URI specified protocol may not work for some
recipients. This is because not all e-mail recipients have full
Internet connectivity, or because URIs which work for a sender will
not work for a recipient. This occurs, for example, when an URI
refers to a resource within a company-internal network that is not
accessible from outside the company.
Palme, et al. Standards Track [Page 10]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
7. Use of the Content-Type "multipart/related"
If a message contains one or more MIME body parts containing URIs and
also contains as separate body parts, resources, to which these URIs
(as defined, for example, in HTML 2.0 [HTML2]) refer, then this whole
set of body parts (referring body parts and referred-to body parts)
SHOULD be sent within a multipart/related structure as defined in
[REL].
Even though headers can occur in a message that lacks an associated
multipart/related structure, this standard only covers their use for
resolution of URIs between body parts inside a multipart/related
structure. This standard does cover the case where a resource in a
nested multipart/related structure contains URIs that reference MIME
body parts in another multipart/related structure, in which it is
enclosed. This standard does not cover the case where a resource in a
multipart/related structure contains URIs that reference MIME body
parts in another parallel or nested multipart/related structure, or
in another MIME message, even if methods similar to those described
in this standard are used. Implementers who employ such URIs are
warned that receiving agents implementing this standard may not be
able to process such references.
When the start body part of a multipart/related structure is an
atomic object, such as a text/html resource, it SHOULD be employed as
the root resource of that multipart/related structure. When the start
body part of a multipart/related structure is a multipart/alternative
structure, and that structure contains at least one alternative body
part which is a suitable atomic object, such as a text/html resource,
then that body part SHOULD be employed as the root resource of the
aggregate document. Implementers are warned, however, that some
receiving agents treat multipart/alternative as if it had been
multipart/mixed (even though MIME [MIME1] requires support for
multipart/alternative).
[REL] specifies that a type parameter is mandatory in a "Content-
Type: multipart/related" header, and requires that it be employed to
specify the type of the multipart/related start object. Thus, the
type parameter value shall be "multipart/alternative", when the start
part is of "Content-type multipart/alternative", even if the actual
root resource is of type "text/html". In addition, if the
multipart/related start object is not the first body part in a
multipart/related structure, [REL] further requires that its
Content-ID MUST be specified as the value of a start parameter in the
"Content-Type: multipart/related" header.
Palme, et al. Standards Track [Page 11]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
When rendering a resource in a multipart/related structure, URI
references within that resource can be satisfied by body parts within
the same multipart/related structure (see section 8.2 below). This is
useful:
(a) For those recipients who only have email 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 URI referenced
resources via URI specified protocols.
Note, that this means that you can, via e-mail, send text/html
objects which includes URIs which the recipient cannot resolve
via HTTP or other connectivity-requiring URIs.
(c) To send a document whose content is preserved even if the
resources to which embedded URIs refer are later changed or
deleted.
(d) For resources which are not available for protocol based
retrieval.
(e) To speed up access.
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 message, as
well as verify the documents against their WWW counterpoints, if this
is appropriate.
In certain cases this will not work - for example, if a resource
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.
Within a multipart/related structure, each body part MUST have, if
assigned, a different Content-ID header value and a Content-Location
header field values which resolve to a different URI.
Two body parts in the same multipart/related structure can have the
same relative Content-Location header value, only if when resolved to
absolute URIs they become different.
Palme, et al. Standards Track [Page 12]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
8. Usage of Links to Other Body Parts
8.1 General principle
A body part, such as a text/html body part, may contain URIs that
reference resources which are included as body parts in the same
message -- in detail, as body parts within the same multipart/related
structure. Often such URI linked resources are meant to be displayed
inline to the viewer of the referencing body part; for example,
objects referenced with the SRC attribute of the IMG element in HTML
2.0 [HTML2]. New elements and attributes with this property are
proposed in the ongoing development of HTML (examples: applet, frame,
profile, OBJECT, classid, codebase, data, SCRIPT). A sender might
also want to send a set of HTML documents which the reader can
traverse, and which are related with the attribute href of the A
element.
If a user retrieves and displays a web page formed from a text/html
resource, and the subsidiary resources it references, and merely
saves the text/html resource, that user may not at a later time be
able to retrieve and display the web page as it appeared when saved.
The format described in this standard can be used to archive and
retrieve all of the resources required to display the web page, as it
originally appeared at a certain moment of time, in one aggregate
file.
In order to send or store complete such messages, there is a need to
specify how a URI in one body part can reference a resource in
another body part.
8.2 Resolution of URIs in text/html body parts
The resolution of inline, retrieval and other kinds of URIs in
text/html body parts is performed in the following way:
(a) Unfold multiple line header values according to [URLBODY]. Do NOT
however translate character encodings of the kind described in
[URL]. Example: Do not transform "a%2eb/c%20d" into "a/b/c d".
(b) Remove all MIME encodings, such as content-transfer encoding and
header encodings as defined in MIME part 3 [MIME3] Do NOT however
translate character encodings of the kind described in [URL].
Example: Do not transform "a%2eb/c%20d" into "a/b/c d".
(c) Try to resolve all relative URIs in the HTML content and in
Content-Location headers using the procedure described in chapter
5 above. The result of this resolution can be an absolute URI, or
an absolute URI with the base "thismessage:/" as specified in
Palme, et al. Standards Track [Page 13]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
chapter 5.
(d) For each referencing URI in a text/html body part, compare the
value of the referencing URI after resolution as described in (a)
and (b), with the URI derived from Content-ID and Content-
Location headers for other body parts within the same or a
surrounding Multipart/related structure. If the strings are
identical, octet by octet, then the referencing URI references
that body part. This comparison will only succeed if the two URIs
are identical. This means that if one of the two URIs to be
compared was a fictitious absolute URI with the base
"thismessage:/", the other must also be such a fictitious
absolute URI, and not resolvable to a real absolute URI.
(e) If (d) fails, try to retrieve the URI referenced resource
hyperlink through ordinary Internet lookup. Resolution of URIs of
the URL-types "mid" or "cid" to other content-parts, outside the
same multipart/related structure, or in other separately sent
messages, is not covered by this standard, and is thus neither
encouraged nor forbidden.
8.3 Use of the Content-ID header and CID URLs
When URIs employing a CID (Content-ID) scheme as defined in [URL] and
[MIDCID] are used to reference other body parts in an MHTML
multipart/related structure, they MUST only be matched against
Content-ID header values, and not against Content-Location header
with CID: values. Thus, even though the following two headers are
identical in meaning, only the Content-ID value will be matched, and
the Content-Location value will be ignored.
Content-ID: <foo@bar.net>
Content-Location: CID: foo@bar.net
Note: Content-IDs MUST be globally unique [MIME1]. It is thus not
permitted to make them unique only within a message or within a
single multipart/related structure.
9. Examples
Warning: The examples are provided for illustrative purposes only. If
there is a contradiction between the explanatory text and the
examples in this standard, then the explanatory text is normative.
Notation: The examples contain indentation to show the structure, the
real objects should not be indented in this way.
Palme, et al. Standards Track [Page 14]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
9.1 Example of a HTML body without included linked objects
The first example is the simplest form of an HTML email message. This
message does not contain an aggregate HTML object, but simply a
message with a single HTML body part. This body part contains a URI
but the messages does not contain the resource referenced by that
URI. To retrieve the resource referenced by the URI the receiving
client would need either IP access to the Internet, or an electronic
mail web gateway.
From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: 8bit
<HTML>
<head></head>
<body>
<h1>Acute accent</h1>
The following two lines look have the same screen rendering:<p>
E with acute accent becomes �.<br>
E with acute accent becomes É.<p>
Try clicking <a href="http://www.ietf.cnri.reston.va.us/">
here.</a><p>
</body></HTML>
9.2 Example with an absolute URI to an embedded GIF picture
The second example is an HTML message which includes a single image,
referenced using the Content-Location mechanism.
From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: multipart/related; boundary="boundary-example";
type="text/html"; start="<foo3@foo1@bar.net>"
--boundary-example
Content-Type: text/html;charset="US-ASCII"
Content-ID: <foo3@foo1@bar.net>
... text of the HTML document, which might contain a URI
referencing a resource in another body part, for example
through a statement such as:
<IMG SRC="http://www.ietf.cnri.reston.va.us/images/ietflogo.gif"
Palme, et al. Standards Track [Page 15]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
ALT="IETF logo">
--boundary-example
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--
9.3 Example with relative URIs to embedded GIF pictures
In this example, a Content-Location header field in the outermost
heading will be a base to all relative URLs, also inside the HTML
text being sent.
From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Location: http://www.ietf.cnri.reston.va.us/
Content-Type: multipart/related; boundary="boundary-example";
type="text/html"
--boundary-example
Content-Type: text/html; charset="ISO-8859-1"
Content-Transfer-Encoding: QUOTED-PRINTABLE
... text of the HTML document, which might contain URIs
referencing resources in other body parts, for example through
statements such as:
<IMG SRC="images/ietflogo1.gif" ALT="IETF logo1">
<IMG SRC="images/ietflogo2.gif" ALT="IETF logo2">
<IMG SRC="images/ietflogo3.gif" ALT="IETF logo3">
Example of a copyright sign encoded with Quoted-Printable: =A9
Example of a copyright sign mapped onto HTML markup: ¨
--boundary-example
Content-Location:
http://www.ietf.cnri.reston.va.us/images/ietflogo1.gif
; Note - Absolute Content-Location does not require a
; base
Palme, et al. Standards Track [Page 16]
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Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...
--boundary-example
Content-Location: images/ietflogo2.gif
; Note - Relative Content-Location is resolved by base
; specified in the Multipart/Related Content-Location heading
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...
--boundary-example
Content-Location:
http://www.ietf.cnri.reston.va.us/images/ietflogo3.gif
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...
--boundary-example--
9.4 Example with a relative URI and no BASE available
From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: multipart/related; boundary="boundary-example";
type="text/html"
--boundary-example
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: QUOTED-PRINTABLE
... text of the HTML document, which might contain a URI
referencing a resource in another body part, for example
through a statement such as:
<IMG SRC="ietflogo.gif" ALT="IETF logo">
Example of a copyright sign encoded with Quoted-Printable: =A9
Example of a copyright sign mapped onto HTML markup: ¨
Palme, et al. Standards Track [Page 17]
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--boundary-example
Content-Location: ietflogo.gif
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...
--boundary-example--
9.5 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";
type="text/html"
--boundary-example
Content-Type: text/html; charset="US-ASCII"
... text of the HTML document, which might contain a URI
referencing a resource in another body part, for example
through a statement such as:
<IMG SRC="cid:foo4@foo1@bar.net" ALT="IETF logo">
--boundary-example
Content-Location: CID:something@else ; this header is disregarded
Content-ID: <foo4@foo1@bar.net>
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...
--boundary-example--
Palme, et al. Standards Track [Page 18]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
9.6 Example showing permitted and forbidden references between nested
body parts
This example shows in which cases references are allowed between
multiple multipart/related body parts in a message.
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;charset="US-ASCII"
Content-ID: <foo3@foo1@bar.net>
The image reference below will be resolved with the image
in the next body part.
<IMG SRC="http://www.ietf.cnri.reston.va.us/images/ietflogo.gif"
ALT="IETF logo with white background">
The image reference below cannot be resolved within this
MIME message, since it contains a reference from an outside
body part to an inside body part, which is not supported
by this standard.
<IMG SRC=images/ietflogo2e.gif"
ALT="IETF logo with transparent background">
The anchor reference immediately below will be resolved with
the nested text/html body part below:
<A HREF="http://www.ietf.cnri.reston.va.us/more-info>
More info</A>
The anchor reference immediately below will be resolved with
the nested text/html body part below:
<A HREF="http://www.ietf.cnri.reston.va.us/even-more-info>
Even more info</A>
--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...
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RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
--boundary-example-1
Content-Location:
http://www.ietf.cnri.reston.va.us/more-info
Content-Type: multipart/related; boundary="boundary-example-2";
type="text/html"
--boundary-example-2
Content-Type: text/html;charset="US-ASCII"
Content-ID: <foo4@foo1@bar.net>
The image reference below will be resolved with the image
in the surrounding multipart/related above.
<IMG SRC="images/ietflogo.gif"
ALT="IETF logo with white background">
The image reference below will be resolved with the image
inside the current nested multipart/related below.
<IMG SRC=images/ietflogo2e.gif"
ALT="IETF logo with transparent background">
--boundary-example-2
Content-Location: http:images/ietflogo2.gif
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgANX/ACkpKTExMTk5OUJCQkpKSlJSUlpaWmNjY2tra3Nzc3t7e4
SEhIyMjJSUlJycnKWlpa2trbW1tcDAwM7Ozv/eQnNzjHNzlGtrjGNjhFpae1pa
etc...
--boundary-example-2--
--boundary-example-1
Content-Location:
http://www.ietf.cnri.reston.va.us/even-more-info
Content-Type: multipart/related; boundary="boundary-example-3";
type="text/html"
--boundary-example-3
Content-Type: text/html;charset="US-ASCII"
Content-ID: <4@foo@bar.net>
The image reference below will be resolved with the image
inside the current nested multipart/related below.
<IMG SRC=images/ietflogo2d.gif"
ALT="IETF logo with shadows">
The image reference below cannot be resolved according to
this standard since references between parallel multipart/
related structures are not supported.
<IMG SRC=images/ietflogo2e.gif"
ALT="IETF logo with transparent background">
Palme, et al. Standards Track [Page 20]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
--boundary-example-3
Content-Location: http:images/ietflogo2d.gif
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgANX/AMDAwCkpKTExMTk5OUJCQkpKSlJSUlpaWmNjY2tra3Nz
c3t7e4SEhIyMjJSUlJycnKWlpa2trbW1tb29vcbGxs7OztbW1t7e3ufn5+/v
etc...
--boundary-example-3--
--boundary-example-1--
10. 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
[MIME2] chapter 4.
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 in MIME format. Receiving 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,
Palme, et al. Standards Track [Page 21]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
chapter 5].
- The MIME standard [MIME2] requires that e-mailed documents of
"Content-Type: Text/ MUST be in canonical form before a Content-
Transfer-Encoding is applied, i.e. that line breaks are encoded as
CRLFs, not as bare CRs or bare LFs or something else. This is in
contrast to [HTTP] where section 3.6.1 allows other
representations of line breaks.
Note that this might cause problems with integrity checks based on
checksums, which might not be preserved when moving a document from
the HTTP to the MIME environment. If a document has to be converted
in such a way that a checksum based message 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 character sets 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.
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 e-mail, the
charset parameter SHOULD be included, rather than relying on the
default.
11. Security Considerations
11.1 Security considerations not related to caching
It is possible for a message sender to misrepresent the source of a
multipart/related body part to a message recipient by labeling it
with a Content-Location URI that references another resource.
Therefore, message recipients should only interpret Content-Location
URIs as labeling a body part for the resolution of references from
body parts in the same multipart/related message structure, and not
as the source of a resource, unless this can be verified by other
means.
URIs, especially File URIs, if used without change in a message, may
inadvertently reveal information that was not intended to be revealed
outside a particular security context. Message senders should take
care when constructing messages containing the new header fields,
defined in this standard, that they are not revealing information
outside of any security contexts to which they belong.
Palme, et al. Standards Track [Page 22]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
Some resource servers hide passwords and tickets (access tokens to
information which should not be reveled to others) and other
sensitive information in non-visible fields or URIs within a
text/html resource. If such a text/html resource is forwarded in an
email message, this sensitive information may be inadvertently
revealed to others.
Since HTML documents can either directly contain executable content
(i.e., JavaScript) or indirectly reference executable content (The
"INSERT" specification, Java). It is exceedingly dangerous for a
receiving User Agent to execute content received in a mail message
without careful attention to restrictions on the capabilities of that
executable content.
HTML-formatted messages can be used to investigate user behaviour,
for example to break anonymity, in ways which invade the privacy of
individuals. If you send a message with a inline link to an object
which is not itself included in the message, the recipients mailer or
browser may request that object through HTTP. The HTTP transaction
will then reveal who is reading the message. Example: A person who
wants to find out who is behind an anonymous user identity, or from
which workstation a user is reading his mail, can do this by sending
a message with an inline link and then observe from where this link
is used to request the object.
11.2 Security considerations related to caching
There is a well-known problem with the caching of directly retrieved
web resources. A resource retrieved from a cache may differ from that
re-retrieved from its source. This problem, also manifests itself
when a copy of a resource is delivered in a multipart/related
structure.
When processing (rendering) a text/html body part in an MHTML
multipart/related structure, all URIs in that text/html body part
which reference subsidiary resources within the same
multipart/related structure SHALL be satisfied by those resources and
not by resources from any another local or remote source.
Therefore, if a sender wishes a recipient to always retrieve an URI
referenced resource from its source, an URI labeled copy of that
resource MUST NOT be included in the same multipart/related
structure.
In addition, since the source of a resource received in a
multipart/related structure can be misrepresented (see 11.1 above),
if a resource received in multipart/related structure is stored in a
cache, it MUST NOT be retrieved from that cache other than by a
Palme, et al. Standards Track [Page 23]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
reference contained in a body part of the same multipart/related
structure. Failure to honor this directive will allow a
multipart/related structure to be employed as a Trojan Horse. For
example, to inject bogus resources (i.e. a misrepresentation of a
competitor's Web site) into a recipient's generally accessible Web
cache.
12. Differences as compared to the previous version of this proposed
standard in RFC 2110
The specification has been changed to show that the formats described
do not only apply to multipart MIME in email, but also to multipart
MIME transferred through other protocols such as HTTP or FTP.
In order to agree with [RELURL], Content-Location headers in
multipart Content-Headings can now be used as a base to resolve
relative URIs in their component parts, but only if no base URI can
be derived from the component part itself. Base URIs in Content-
Location header fields in inner headings have precedence over base
URIs in outer multipart headings.
The Content-Base header, which was present in RFC 2110, has been
removed. A conservative implementor may choose to accept this header
in input for compatibility with implementations of RFC 2110, but MUST
never send any Content-Base header, since this header is not any more
a part of this standard.
A section 4.4.1 has been added, specifying how to handle the case of
sending a body part whose URI does not agree with the correct URI
syntax.
The handling of relative and absolute URIs for matching between body
parts have been merged into a single description, by specifying that
relative URIs, which cannot be resolved otherwise, should be handled
as if they had been given the URL "thismessage:/".
13. Acknowledgments
Harald T. Alvestrand, Richard Baker, Isaac Chan, Dave Crocker, Martin
J. Duerst, Lewis Geer, Roy Fielding, Ned Freed, Al Gilman, Paul
Hoffman, Andy Jacobs, Richard W. Jesmajian, Mark K. Joseph, Greg
Herlihy, Valdis Kletnieks, Daniel LaLiberte, Ed Levinson, Jay Levitt,
Albert Lunde, Larry Masinter, Keith Moore, Gavin Nicol, Martyn W.
Peck, Pete Resnick, Jon Smirl, Einar Stefferud, Jamie Zawinski, Steve
Zilles and several other people have helped us with preparing this
document. We alone take responsibility for any errors which may still
be in the document.
Palme, et al. Standards Track [Page 24]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
14. References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[CONDISP] Troost, R. and S. Dorner, "Communicating Presentation
Information in Internet Messages: The Content-
Disposition Header", RFC 2183, August 1997.
[HOSTS] Braden, R., Ed., "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, October
1989.
[HTML-I18N] Yergeau, F., Nicol, G. Adams, G. and M. Duerst:
"Internationalization of the Hypertext Markup
Language", RFC 2070, January 1997.
[HTML2] Berners-Lee, T. and D. Connolly: "Hypertext Markup
Language - 2.0", RFC 1866, November 1995.
[HTML3.2] Dave Raggett: HTML 3.2 Reference Specification, W3C
Recommendation, January 1997, at URL
http://www.w3.org/TR/REC-html32.html
[HTTP] Berners-Lee, T., Fielding, R. and H. Frystyk,
"Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945,
May 1996.
[IETF-TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirements Levels", BCP 14, RFC 2119, March 1997.
[INFO] J. Palme: Sending HTML in MIME, an informational
supplement to the RFC: MIME Encapsulation of
Aggregate Documents, such as HTML (MHTML), Work in
Progress.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC
1321, April 1992.
[MIDCID] Levinson, E., "Content-ID and Message-ID Uniform
Resource Locators", RFC 2387, August 1998.
[MIME1] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, December 1996.
Palme, et al. Standards Track [Page 25]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
[MIME2] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types", RFC
2046, December 1996.
[MIME3] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, December 1996.
[MIME4] Freed, N., Klensin, J. and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four:
Registration Procedures", RFC 2048, January 1997.
[MIME5] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Five: Conformance
Criteria and Examples", RFC 2049, November 1996.
[NEWS] Horton, M. and R. Adams: "Standard for interchange of
USENET messages", RFC 1036, December 1987.
[PDF] Tim Bienz and Richar Cohn: "Portable Document Format
Reference Manual", Addison-Wesley, Reading, MA, USA,
1993, ISBN 0-201-62628-4.
[REL] Levinson, E., "The MIME Multipart/Related Content-
Type", RFC 2389, August 1998.
[RELURL] Fielding, R., "Relative Uniform Resource Locators",
RFC 1808, June 1995.
[RFC822] Crocker, D., "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] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[URL] Berners-Lee, T., Masinter, L. and M. McCahill,
"Uniform Resource Locators (URL)", RFC 1738, December
1994.
[URLBODY] Freed, N. and K. Moore, "Definition of the URL MIME
External-Body Access-Type", RFC 2017, October 1996.
Palme, et al. Standards Track [Page 26]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
[VRML] Gavin Bell, Anthony Parisi, Mark Pesce: "Virtual
Reality Modeling Language (VRML) Version 1.0 Language
Specification." May 1995,
http://www.vrml.org/Specifications/.
[XML] Extensible Markup Language, published by the World
Wide Web Consortium, URL http://www.w3.org/XML/
15. Authors' Addresses
For contacting the editors, preferably write to Jacob Palme.
Jacob Palme
Stockholm University and KTH
Electrum 230
S-164 40 Kista, Sweden
Phone: +46-8-16 16 67
Fax: +46-8-783 08 29
EMail: jpalme@dsv.su.se
Alex Hopmann
Microsoft Corporation
One Microsoft Way
Redmond WA 98052
Phone: +1-425-703-8238
EMail: alexhop@microsoft.com
Nick Shelness
Lotus Development Corporation
55 Cambridge Parkway
Cambridge MA 02142-1295
EMail: Shelness@lotus.com
Working group chairman:
Einar Stefferud
EMail: stef@nma.com
Palme, et al. Standards Track [Page 27]
RFC 2557 MIME Encapsulation of Aggregate Documents March 1999
16. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Palme, et al. Standards Track [Page 28]