This document is obsolete. Please
refer to RFC 2557.
Network Working Group J. Palme Request for Comments: 2110 Stockholm University/KTH Category: Standards Track A. Hopmann Microsoft Corporation March 1997
MIME E-mail Encapsulation of Aggregate Documents, such as HTML (MHTML)
Status of this Document
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Abstract
Although HTML [RFC 1866] was designed within the context of MIME, more than the specification of HTML as defined in RFC 1866 is needed for two electronic mail user agents to be able to interoperate using HTML as a document format. These issues include the naming of objects that are normally referred to by URIs, and the means of aggregating objects that go together. This document describes a set of guidelines that will allow conforming mail user agents to be able to send, deliver and display these objects, such as HTML objects, that can contain links represented by URIs. In order to be able to handle inter-linked objects, the document uses the MIME type multipart/related and specifies the MIME content-headers "Content- Location" and "Content-Base".
Table of Contents
1. Introduction.............................................. 2 2. Terminology............................................... 3 2.1 Conformance requirement terminology................... 3 2.2 Other terminology..................................... 4 3. Overview.................................................. 5 4. The Content-Location and Content-Base MIME Content Headers 6 4.1 MIME content headers.................................. 6 4.2 The Content-Base header............................... 7 4.3 The Content-Location Header........................... 7 4.4 Encoding of URIs in e-mail headers.................... 8 5. Base URIs for resolution of relative URIs................. 8 6. Sending documents without linked objects.................. 9 7. Use of the Content-Type: Multipart/related................ 9 8. Format of Links to Other Body Parts....................... 11
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8.1 General principle..................................... 11 8.2 Use of the Content-Location header.................... 11 8.3 Use of the Content-ID header and CID URLs............. 12 9 Examples................................................... 12 9.1 Example of a HTML body without included linked objects 12 9.2 Example with absolute URIs to an embedded GIF picture 13 9.3 Example with relative URIs to an embedded GIF picture 13 9.4 Example using CID URL and Content-ID header to an embedded GIF picture.................................. 14 10. Content-Disposition header............................... 15 11. Character encoding issues and end-of-line issues......... 15 12. Security Considerations.................................. 16 13. Acknowledgments.......................................... 17 14. References............................................... 18 15. Author's Address......................................... 19
Mailing List Information
Further discussion on this document should be done through the mailing list 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 editor, Jacob Palme <jpalme@dsv.su.se>.
More information may also be available at URL: HTTP://www.dsv.su.se/~jpalme/ietf/jp-ietf-home.HTML
There are a number of document formats, HTML [HTML2], PDF [PDF] and VRML for example, which provide links using URIs for their resolution. There is an obvious need to be able to send documents in these formats in e-mail [RFC821=SMTP, RFC822]. This document gives additional specifications on how to send such documents in MIME [RFC 1521=MIME1] e-mail messages. This version of this standard was based on full consideration only of the needs for objects with links in the
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Text/HTML media type (as defined in RFC 1866 [HTML2]), but the standard may still be applicable also to other formats for sets of interlinked objects, linked by URIs. There is no conformance requirement that implementations claiming conformance to this standard are able to handle URI-s in other document formats than HTML.
URIs in documents in HTML and other similar formats reference other objects and resources, either embedded or directly accessible through hypertext links. When mailing such a document, it is often desirable to also mail all of the additional resources that are referenced in it; those elements are necessary for the complete interpretation of the primary object.
An alternative way for sending an HTML document or other object containing URIs in e-mail is to only send the URL, and let the recipient look up the document using HTTP. That method is described in [URLBODY] and is not described in this document.
An informational RFC will at a later time be published as a supplement to this standard. The informational RFC will discuss implementation methods and some implementation problems. Implementors are recommended to read this informational RFC when developing implementations of the MHTML standard. This informational RFC is, when this RFC is published, still in IETF draft status, and will stay that way for at least six months in order to gain more implementation experience before it is published.
This specification uses the same words as RFC 1123 [HOSTS] for defining the significance of each particular requirement. These words are:
MUST This word or the adjective "required" means that the item is an absolute requirement of the specification.
SHOULD This word or the adjective "recommended" means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course.
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MAY This word or the adjective "optional" means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item.
An implementation is not compliant if it fails to satisfy one or more of the MUST requirements for the protocols it implements. An implementation that satisfies all the MUST and all the SHOULD requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST requirements but not all the SHOULD requirements for its protocols is said to be "conditionally compliant."
Displayed text The text shown to the user reading a document with a web browser. This may be different from the HTML markup, see the definition of HTML markup below.
Header Field in a message or content heading specifying the value of one attribute.
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Heading Part of a message or content before the first CRLFCRLF, containing formatted fields with attributes of the message or content.
HTML Aggregate HTML objects together with some or all objects, to objects which the HTML object contains hyperlinks.
HTML markup A file containing HTML encodings as specified in [HTML] which may be different from the displayed text which a person using a web browser sees. For example, the HTML markup may contain "<" where the displayed text contains the character "<".
An aggregate document is a MIME-encoded message that contains a root document as well as other data that is required in order to represent that document (inline pictures, style sheets, applets, etc.). Aggregate documents can also include additional elements that are linked to the first object. It is important to keep in mind the differing needs of several audiences. Mail sending agents might send
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aggregate documents as an encoding of normal day-to-day electronic mail. Mail sending agents might also send aggregate documents when a user wishes to mail a particular document from the web to someone else. Finally mail sending agents might send aggregate documents as automatic responders, providing access to WWW resources for non-IP connected clients.
Mail receiving agents also have several differing needs. Some mail receiving agents might be able to receive an aggregate document and display it just as any other text content type would be displayed. Others might have to pass this aggregate document to a browsing program, and provisions need to be made to make this possible.
Finally several other constraints on the problem arise. It is important that it be possible for a document to be signed and for it to be able to be transmitted to a client and displayed with a minimum risk of breaking the message integrity (MIC) check that is part of the signature.
4. The Content-Location and Content-Base MIME Content Headers
In order to resolve URI references to other body parts, two MIME content headers are defined, Content-Location and Content-Base. Both these headers can occur in any message or content heading, and will then be valid within this heading and for its content.
In practice, at present only those URIs which are URLs are used, but it is anticipated that other forms of URIs will in the future be used.
The syntax for these headers is, using the syntax definition tools from [RFC822]:
where URI is at present (June 1996) restricted to the syntax for URLs as defined in RFC 1738 [URL].
These two headers are valid only for exactly the content heading or message heading where they occurs and its text. They are thus not valid for the parts inside multipart headings, and are thus meaningless in multipart headings.
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These two headers may occur both inside and outside of a multipart/related part.
The Content-Base gives a base for relative URIs occurring in other heading fields and in HTML documents which do not have any BASE element in its HTML code. Its value MUST be an absolute URI.
Example showing which Content-Base is valid where:
Content-Type: Multipart/related; boundary="boundary-example-1"; type=Text/HTML; start=foo2*foo3@bar2.net ; A Content-Base header cannot be placed here, since this is a ; multipart MIME object.
--boundary-example-1
Part 1: Content-Type: Text/HTML; charset=US-ASCII Content-ID: <foo2*foo3@bar2.net> Content-Location: http://www.ietf.cnir.reston.va.us/images/foo1.bar1 ; This Content-Location must contain an absolute URI, since no base ; is valid here.
--boundary-example-1
Part 2: Content-Type: Text/HTML; charset=US-ASCII Content-ID: <foo4*foo5@bar2.net> Content-Location: foo1.bar1 ; The Content-Base below applies to ; this relative URI Content-Base: http://www.ietf.cnri.reston.va.us/images/
The Content-Location header specifies the URI that corresponds to the content of the body part in whose heading the header is placed. Its value CAN be an absolute or relative URI. Any URI or URL scheme may be used, but use of non-standardized URI or URL schemes might entail some risk that recipients cannot handle them correctly.
The Content-Location header can be used to indicate that the data sent under this heading is also retrievable, in identical format, through normal use of this URI. If used for this purpose, it must contain an absolute URI or be resolvable, through a Content-Base
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header, into an absolute URI. In this case, the information sent in the message can be seen as a cached version of the original data.
The header can also be used for data which is not available to some or all recipients of the message, for example if the header refers to an object which is only retrievable using this URI in a restricted domain, such as within a company-internal web space. The header can even contain a fictious URI and need in that case not be globally unique.
Since MIME header fields have a limited length and URIs can get quite long, these lines may have to be folded. If such folding is done, the algorithm defined in [URLBODY] section 3.1 should be employed.
Relative URIs inside contents of MIME body parts are resolved relative to a base URI. In order to determine this base URI, the first-applicable method in the following list applies.
(a) There is a base specification inside the MIME body part containing the link which resolves relative URIs into absolute URIs. For example, HTML provides the BASE element for this.
(b) There is a Content-Base header (as defined in section 4.2), specifying the base to be used.
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(c) There is a Content-Location header in the heading of the body part which can then serve as the base in the same way as the requested URI can serve as a base for relative URIs within a file retrieved via HTTP [HTTP].
When the methods above do not yield an absolute URI the procedure in section 8.2 for matching relative URIs MUST be followed.
If a document, such as an HTML object, is sent without other objects, to which it is linked, it MAY be sent as a Text/HTML body part by itself. In this case, multipart/related need not be used.
Such a document may either not include any links, or contain links which the recipient resolves via ordinary net look up, or contain links which the recipient cannot resolve.
Inclusion of links which the recipient has to look up through the net may not work for some recipients, since all e-mail recipients do not have full internet connectivity. Also, such links may work for the sender but not for the recipient, for example when the link refers to an URI within a company-internal network not accessible from outside the company.
Note that documents with links that the recipient cannot resolve MAY be sent, although this is discouraged. For example, two persons developing a new HTML page may exchange incomplete versions.
If a message contains one or more MIME body parts containing links and also contains as separate body parts, data, to which these links (as defined, for example, in RFC 1866 [HTML2]) refers, then this whole set of body parts (referring body parts and referred-to body parts) SHOULD be sent within a multipart/related body part as defined in [REL].
The root body part of the multipart/related SHOULD be the start object for rendering the object, such as a text/html object, and which contains links to objects in other body parts, or a multipart/alternative of which at least one alternative resolves to such a start object. Implementors are warned, however, that many mail programs treat multipart/alternative as if it had been multipart/mixed (even though MIME [MIME1] requires support for multipart/alternative).
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[REL] requires that the type attribute of the "Content-Type: Multipart/related" statement be the type of the root object, and this value can thus be "multipart/alternative". If the root is not the first body part within the multipart/related, [REL] further requires that its Content-ID MUST be given in a start parameter to the "Content-Type: Multipart/related" header.
When presenting the root body part to the user, the additional body parts within the multipart/related can be used:
(a) For those recipients who only have e-mail but not full Internet access.
(b) For those recipients who for other reasons, such as firewalls or the use of company-internal links, cannot retrieve the linked body parts through the net.
Note that this means that you can, via e-mail, send HTML which includes URIs which the recipient cannot resolve via HTTPor other connectivity-requiring URIs.
(c) For items which are not available on the web.
(d) For any recipient to speed up access.
The type parameter of the "Content-Type: Multipart/related" MUST be the same as the Content-Type of its root.
When a sending MUA sends objects which were retrieved from the WWW, it SHOULD maintain their WWW URIs. It SHOULD not transform these URIs into some other URI form prior to transmitting them. This will allow the receiving MUA to both verify MICs included with the email message, as well as verify the documents against their WWW counterpoints.
In certain special cases this will not work if the original HTML document contains URIs as parameters to objects and applets. In such a case, it might be better to rewrite the document before sending it. This problem is discussed in more detail in the informational RFC which will be published as a supplement to this standard.
This standard does not cover the case where a multipart/related contains links to MIME body parts outside of the current multipart/related or in other MIME messages, even if methods similar to those described in this standard are used. Implementors who provide such links are warned that mailers implementing this standard may not be able to resolve such links.
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Within such a multipart/related, ALL different parts MUST have different Content-Location or Content-ID values.
A body part, such as a text/HTML body part, may contain hyperlinks to objects which are included as other body parts in the same message and within the same multipart/related content. Often such linked objects are meant to be displayed inline to the reader of the main document; for example, objects referenced with the IMG tag in HTML [RFC 1866=HTML2]. New tags with this property are proposed in the ongoing development of HTML (example: applet, frame).
In order to send such messages, there is a need to indicate which other body parts are referred to by the links in the body parts containing such links. For example, a body part of Content-Type: Text/HTML often has links to other objects, which might be included in other body parts in the same MIME message. The referencing of other body parts is done in the following way: For each body part containing links and each distinct URI within it, which refers to data which is sent in the same MIME message, there SHOULD be a separate body part within the current multipart/related part of the message containing this data. Each such body part SHOULD contain a Content-Location header (see section 8.2) or a Content-ID header (see section 8.3).
An e-mail system which claims conformance to this standard MUST support receipt of multipart/related (as defined in section 7) with links between body parts using both the Content-Location (as defined in section 8.2) and the Content-ID method (as defined in section 8.3).
If there is a Content-Base header, then the recipient MUST employ relative to absolute resolution as defined in RFC 1808 [RELURL] of relative URIs in both the HTML markup and the Content-Location header before matching a hyperlink in the HTML markup to a Content-Location header. The same applies if the Content-Location contains an absolute URI, and the HTML markup contains a BASE element so that relative URIs in the HTML markup can be resolved.
If there is NO Content-Base header, and the Content-Location header contains a relative URI, then NO relative to absolute resolution SHOULD be performed. Matching the relative URI in the Content- Location header to a hyperlink in an HTML markup text is in this case
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a two step process. First remove any LWSP from the relative URI which may have been introduced as described in section 4.4. Then perform an exact textual match against the HTML URIs. For this matching process, ignore BASE specifications, such as the BASE element in HTML. Note that this only applies for matching Content-Location headers, not for URL-s in the HTML document which are resolved through network look up at read time.
The URI in the Content-Location header need not refer to an object which is actually available globally for retrieval using this URI (after resolution of relative URIs). However, URI-s in Content- Location headers (if absolute, or resolvable to absolute URIs) SHOULD still be globally unique.
When CID (Content-ID) URLs as defined in RFC 1738 [URL] and RFC 1873 [MIDCID] are used for links between body parts, the Content-Location statement will normally be replaced by a Content-ID header. Thus, the following two headers are identical in meaning:
Note: Content-IDs MUST be globally unique [MIME1]. It is thus not permitted to make them unique only within this message or within this multipart/related.
9.1 Example of a HTML body without included linked objects
The first example is the simplest form of an HTML email message. This is not an aggregate HTML object, but simply a message with a single HTML body part. This message contains a hyperlink but does not provide the ability to resolve the hyperlink. To resolve the hyperlink the receiving client would need either IP access to the Internet, or an electronic mail web gateway.
From: foo1@bar.net To: foo2@bar.net Subject: A simple example Mime-Version: 1.0 Content-Type: Text/HTML; charset=US-ASCII
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<HTML> <head></head> <body> <h1>Hi there!</h1> An example of an HTML message.<p> Try clicking <a href="http://www.resnova.com/">here.</a><p> </body></HTML>
9.2 Example with absolute URIs to an embedded GIF picture
From: 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
... text of the HTML document, which might contain a hyperlink to the other body part, for example through a statement such as: <IMG SRC="/images/ietflogo.gif" ALT="IETF logo"> Example of a copyright sign encoded with Quoted-Printable: =A9 Example of a copyright sign mapped onto HTML markup: ¨
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
... 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">
Note the specification in [REL] on the relations between Content- Disposition and multipart/related.
11. Character encoding issues and end-of-line issues
For the encoding of characters in HTML documents and other text documents into a MIME-compatible octet stream, the following mechanisms are relevant:
- HTML [HTML2, HTML-I18N] as an application of SGML [SGML] allows characters to be denoted by character entities as well as by numeric character references (e.g. "Latin small letter a with acute accent" may be represented by "á" or "á") in the HTML markup.
- HTML documents, in common with other documents of the MIME "Content-Type text", can be represented in MIME using one of several character encodings. The MIME Content-Type "charset" parameter value indicates the particular encoding used. For the exact meaning and use of the "charset" parameter, please see [MIME-IMB section 4.2].
Note that the "charset" parameter refers only to the MIME character encoding. For example, the string "á" can be sent in MIME with "charset=US-ASCII", while the raw character "Latin small letter a with acute accent" cannot.
The above mechanisms are well defined and documented, and therefore not further explained here. In sending a message, all the above mentioned mechanisms MAY be used, and any mixture of them MAY occur when sending the document via e-mail. Receiving mail user agents (together with any Web browser they may use to display the document) MUST be capable of handling any combinations of these mechanisms.
Also note that:
- Any documents including HTML documents that contain octet values outside the 7-bit range need a content-transfer-encoding applied before transmission over certain transport protocols [MIME1, chapter 5].
- The MIME standard [MIME1] requires that documents of "Content-Type: Text MUST be in canonical form before Content-Transfer-Encoding, i.e. that line breaks are encoded as CRLFs, not as bare CRs or bare LFs or something else. This is in contrast to [HTTP] where section 3.6.1 allows other representations of line breaks.
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Note that this might cause problems with integrity checks based on checksums, which might not be preserved when moving a document from the HTTP to the MIME environment. If a document has to be converted in such a way that a checksum integrity check becomes invalid, then this integrity check header SHOULD be removed from the document.
Other sources of problems are Content-Encoding used in HTTP but not allowed in MIME, and charsets that are not able to represent line breaks as CRLF. A good overview of the differences between HTTP and MIME with regards to "Content-Type: Text" can be found in [HTTP], appendix C.
If the original document has line breaks in the canonical form (CRLF), then the document SHOULD remain unconverted so that integrity check sums are not invalidated.
A provider of HTML documents who wants his documents to be transferable via both HTTP and SMTP without invalidating checksum integrity checks, should always provide original documents in the canonical form with CRLF for line breaks.
Some transport mechanisms may specify a default "charset" parameter if none is supplied [HTTP, MIME1]. Because the default differs for different mechanisms, when HTML is transferred through mail, the charset parameter SHOULD be included, rather than relying on the default.
Some Security Considerations include the potential to mail someone an object, and claim that it is represented by a particular URI (by giving it a Content-Location header). There can be no assurance that a WWW request for that same URI would normally result in that same object. It might be unsuitable to cache the data in such a way that the cached data can be used for retrieval of this URI from other messages or message parts than those included in the same message as the Content-Location header. Because of this problem, receiving User Agents SHOULD not cache this data in the same way that data that was retrieved through an HTTP or FTP request might be cached.
URLs, especially File URLs, may in their name contain company- internal information, which may then inadvertently be revealed to recipients of documents containing such URLs.
One way of implementing messages with linked body parts is to handle the linked body parts in a combined mail and WWW proxy server. The mail client is only given the start body part, which it passes to a web browser. This web browser requests the linked parts from the
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proxy server. If this method is used, and if the combined server is used by more than one user, then methods must be employed to ensure that body parts of a message to one person is not retrievable by another person. Use of passwords (also known as tickets or magic cookies) is one way of achieving this. Note that some caching WWW proxy servers may not distinguish between cached objects from e-mail and HTTP, which may be a security risk.
In addition, by allowing people to mail aggregate objects, we are opening the door to other potential security problems that until now were only problems for WWW users. For example, some HTML documents now either themselves contain executable content (JavaScript) or contain links to executable content (The "INSERT" specification, Java). It would be exceedingly dangerous for a receiving User Agent to execute content received through a mail message without careful attention to restrictions on the capabilities of that executable content.
Some WWW applications hide passwords and tickets (access tokens to information which may not be available to anyone) and other sensitive information in hidden fields in the web documents or in on-the-fly constructed URLs. If a person gets such a document, and forwards it via e-mail, the person may inadvertently disclose sensitive information.
Harald T. Alvestrand, Richard Baker, Dave Crocker, Martin J. Duerst, Lewis Geer, Roy Fielding, Al Gilman, Paul Hoffman, Richard W. Jesmajian, Mark K. Joseph, Greg Herlihy, Valdis Kletnieks, Daniel LaLiberte, Ed Levinson, Jay Levitt, Albert Lunde, Larry Masinter, Keith Moore, Gavin Nicol, Pete Resnick, Jon Smirl, Einar Stefferud, Jamie Zawinski, Steve Zilles and several other people have helped us with preparing this document. I alone take responsibility for any errors which may still be in the document.
Ref. Author, title --------- --------------------------------------------------------
[CONDISP] R. Troost, S. Dorner: "Communicating Presentation Information in Internet Messages: The Content-Disposition Header", RFC 1806, June 1995.
[HOSTS] R. Braden (editor): "Requirements for Internet Hosts -- Application and Support", STD-3, RFC 1123, October 1989.
[HTML-I18N] F. Yergeau, G. Nicol, G. Adams, & M. Duerst: "Internationalization of the Hypertext Markup Language". RFC 2070, January 1997.
[HTML2] T. Berners-Lee, D. Connolly: "Hypertext Markup Language - 2.0", RFC 1866, November 1995.
[HTTP] T. Berners-Lee, R. Fielding, H. Frystyk: Hypertext Transfer Protocol -- HTTP/1.0. RFC 1945, May 1996.
[MD5] R. Rivest: "The MD5 Message-Digest Algorithm", RFC 1321, April 1992.
[MIDCID] E. Levinson: "Content-ID and Message-ID Uniform Resource Locators". RFC 2111, February 1997.
[MIME-IMB] N. Freed & N. Borenstein: "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bedies". RFC 2045, November 1996.
[MIME1] N. Borenstein & N. Freed: "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, Sept 1993.
[MIME2] N. Borenstein & N. Freed: "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types". RFC 2046, November 1996.
[NEWS] M.R. Horton, R. Adams: "Standard for interchange of USENET messages", RFC 1036, December 1987.
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[PDF] Bienz, T., Cohn, R. and Meehan, J.: "Portable Document Format Reference Manual, Version 1.1", Adboe Systems Inc.
[REL] Edward Levinson: "The MIME Multipart/Related Content- Type". RFC 2112, February 1997.
[RELURL] R. Fielding: "Relative Uniform Resource Locators", RFC 1808, June 1995.
[RFC822] D. Crocker: "Standard for the format of ARPA Internet text messages." STD 11, RFC 822, August 1982.
[SGML] ISO 8879. Information Processing -- Text and Office - Standard Generalized Markup Language (SGML), 1986. <URL:http://www.iso.ch/cate/d16387.html>
[SMTP] J. Postel: "Simple Mail Transfer Protocol", STD 10, RFC 821, August 1982.
[URL] T. Berners-Lee, L. Masinter, M. McCahill: "Uniform Resource Locators (URL)", RFC 1738, December 1994.
[URLBODY] N. Freed and Keith Moore: "Definition of the URL MIME External-Body Access-Type", RFC 2017, October 1996.
