Network Working Group Jacob Palme Internet Draft Stockholm draft-palme-mhtml-info-01.txt University/KTH December 2001 Category-to-be: Informational Expires: June 2001 Sending HTML in MIME, an informational supplement to the RFC: MIME Encapsulation of Aggregate Documents, such as HTML (MHTML) Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright (C) The Internet Society 2001. All Rights Reserved. 1. Abstract The memo "MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)" [RFC 2557] specifies how to send packaged aggregate HTML objects in MIME format. This memo is an accompanying informational document, intended to be an aid to developers. This document is not an Internet standard. Issues discussed are implementation methods, caching strategies, problems with rewriting of URIs, making messages suitable both for mailers which can and which cannot handle Multipart/related and handling recipients which do not have full Internet connectivity. 2. Table of Contents 1. Abstract 2. Table of Contents 3. Introduction 4. Implementation Methods 4.1 Method 1: Combining Viewer And MIME Receiving Program 4.2 Method 2: Rewriting The HTML 4.3 Method 3: Using A Translation Table 4.4 Method 4: Using A Proxy HTTP Server To Retrieve Referenced Body Parts 4.5 Method 5: Putting The Mail Client Into A Proxy HTTP Server 4.6 Other Methods 4.7 Combined Methods 4.8 Communication Between Document Viewer And Mail Client 5. Problems with Rewriting URIs when Copying HTML Documents 6. Caching of Body Parts 7. "Save as" Command 8. Recipients which cannot Handle the Multipart/related Content-Type 9. Use of the Content-Type: Multipart/alternative 9.1 Multipart/alternative inside Multipart/related 9.2 Multipart/alternative outside Multipart/related 9.3 Comparing the Two Methods 9.4 Reducing the Download Time 10. Writing Readable HTML 11. Textual Alternatives to HTML Forms 11.1 Form in HTML Format 11.2 The same Form In Textual Format 12. Recipient may not have Full Internet Connectivity 13. Encoding of Non-Ascii Characters 14. Conversion from HTTP to MIME 15. Default Font Size 16. Copyright and Disclaimer 17. Acknowledgments 18. References 19. Author's Address 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 email address)> Archives of this list are available by anonymous ftp from FTP://SEGATE.SUNET.SE/lists/MHTML/ The archives are also available by email. 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://dsv.su.se/jpalme/ietf/mhtml.html 3. Introduction [MHTML] specifies how to send packaged aggregate HTML objects in MIME multipart format. This memo is an accompanying informational document, intended to be an aid to developers. This document is not an Internet standard. The latest revised version of this document can be find in plain text and HTML format at http://dsv.su.se/jpalme/ietf/mhtml.html#info. 4. Implementation Methods The [MHTML] standard has been intentionally written to be implementable both in cases where a HTML document viewer (web browser) and a program receiving MIME objects, such as an email program, are combined, and when they are separate programs. Implementation is of course easier if the document viewer is combined with the MIME receiving client. Below are described different implementation methods. Real implementations may sometimes combine ideas from more than one of the different methods described below. Note: Some document viewers can take a whole document of "Content-Type: message" or "Content-Type: multipart" as one single file to be displayed. When such viewers are known to be used, the problems described below become much easier to handle, just submit the whole combined MIME message as a single file to the viewer. 4.1 Method 1: Combining Viewer And MIME Receiving Program This is the architecturally simplest approach. A web-browser with a built in MIME receiving program (such as an email program) will be able to use its own document viewer capabilities to display HTML-formatted messages. Since it is the same program, that program will more easily be able to connect a URL in the HTML text to a body part in the message. 4.2 Method 2: Rewriting The HTML +----------+ +--------+ | Document | | Mail | | viewer | | client | +-------+--+ +-+------+ | | +--+-------------------------------+--+ | +----------+ +--+ +--+ | | | Start | | | | | Related | Figure 1 | | HTML | | | | | body part | | | document | | | | | parts | | +----------+ +--+ +--+ | +-------------------------------------+ If the document viewer is separate from the MIME receiving client, the MIME client might turn over the HTML body part to the document viewer and ask it to display it (Figure 1). One way of doing this is to store the HTML body part in a file, and ask the document viewer to display this file. If multipart/related is used, this can be implemented by storing all the body parts within the multipart/related in an otherwise empty folder/directory. The mail client may have to rewrite the HTML, replacing URI-s with (possibly relative) URL-s which the Document viewer can resolve as file names in the same directory/folder where the HTML document itself is stored when turning it over to the Document viewer. Problems with such rewriting of URIs is discussed in section 5 below. 4.3 Method 3: Using A Translation Table +----------+ +--------+ | Document | | Mail | | viewer | | client | +-------+--+ +-+------+ | | +--+------------------------------+-+ | +--------+ +--+ +--+ | | | Trans- | | | | | Related | Figure 2 | | lation | | | | | body part | | | table | | | | | parts | | +--------+ +--+ +--+ | +-----------------------------------+ An alternative to rewriting the HTML file before turning it over to the Document viewer may be to use a translation table, in case the Document viewer has the capability to use such a table to rewrite URL-s on the fly while displaying the document (Figure 2). This requires that the Document viewer is capable of receiving CID: URL-s and resolving them using this translation table in the same way as for other URL-s. 4.4 Method 4: Using A Proxy HTTP Server To Retrieve Referenced Body Parts +--------+ +-----------+ +--------+ | Proxy | | Data base | | Mail | | web |-------| of cached |-------| server | | server | | objects | | | +----+---+ +-----------+ +----+---+ | | +----+-----+ +----+---+ | Document | | Mail | | viewer | | client | +-------+--+ +-+------+ | | +--+------------------------------+-+ | Start HTML object | Figure 3 +-----------------------------------+ Yet another method is to use a proxy web server, to which the document viewer requests are sent, and which will then use the cached body parts instead of normal web retrieval from the network (Figure 3). If the Document viewer is set to use this proxy server for all URL-s, including CID URL-s, no rewriting of the HTML will be necessary. 4.5 Method 5: Putting The Mail Client Into A Proxy HTTP Server +--------+--------+ | Proxy | Mail | | HTTP | client | | server | | +--------+--------+ | HTTP protocol Figure 4 | +----+-----+ | Document | | Viewer | +----------+ A mail client can also be included in an HTTP server (Figure 4). The user will then not have to install any mail client software in his personal computer; all the mail functionality is mapped on HTTP and HTML elements. 4.6 Other Methods The mail client and the document viewer can of course communicate in other ways, such as using inter-process communication. 4.7 Combined Methods Several of the methods described above can also be combined. The mailer might for example display simpler HTML documents itself, but automatically or manually transfer the HTML documents to a separate HTML viewer for more complex documents. A common practice in HTML viewers is to simply ignore all markups which the viewer does not understand. This practice, if implemented in a mailer with limited HTML viewing capabilities, might mean that the user is shown a very incomplete message without any warning that information is missing. In this case, it is better to give the user some kind of warning, combined with a command to view the letter with a separate HTML viewer, or turn the document over automatically to a separate viewer when the document contains markup which the mailer cannot render itself. 4.8 Communication Between Document Viewer And Mail Client Many document viewers (web browsers) have API-s to allow other programs to communicate with them. There is however no accepted real or de-facto standard for such API-s, which means that a mail program which relies on such API-s will only be able to use those document viewers, whose API they support. Note however, that most of the methods described above can be implemented with a very minimal such API. The only API function needed is to be able to tell a document viewer, when it is started, to open a particular file. And this API function is a standardized part of the operating system on most platforms. In particular, method 1 and 3 above uses the functionality that a relative URL is resolved with the location of the base document as base. This means that if the base document is a file, relative URL-s will be resolved as FILE URL-s in the same directory/folder where the HTML document itself is placed. There is a need for buttons in the Web page which the user can use to get back to the mail program again after reading the mail with the document viewer. A common technique to achieve this is to define a new MIME data type for this button. The document viewer is then configured to transfer control to the mail client when the user pushes this button; i.e. downloads a file of this new MIME type. 5. Problems with Rewriting URIs when Copying HTML Documents Sending of HTML-formatted messages is based on the assumption that an HTML documents, together with in-line objects like images, applets and frames, can be copied into a MIME message. Such copying may require rewriting of URIs containing references between the different message parts. The MHTML standard [MHTML] has been carefully prepared to allow existing web pages to be copied without such rewriting, through the use of the Content-Location MIME content heading field. There is however a problem if the source HTML document contains relative URIs in parameters to objects and applets, such as in the example below: 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 Content-Base: "http://www.ietf.cnri.reston.va.us" --boundary-example 1 Content-Type: Text/HTML; charset=US-ASCII ... text of the HTML document... <OBJECT CLASSID = "clsid:5220cb21-c88d-11cf-b347-00aa00a28331"> <PARAM NAME="imageurl" VALUE="image.gif"> </OBJECT> ...etc... --boundary-example-1 Content-Location: "image.gif" Content-Type: IMAGE/GIF Content-Transfer-Encoding: BASE64 R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk 5 ..etc... --boundary-example-1-- 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 Content-Base: "http://www.ietf.cnri.reston.va.us" --boundary-example 1 Content-Type: Text/HTML; charset=US-ASCII ... text of the HTML document... <OBJECT CLASSID = "clsid:5220cb21-c88d-11cf-b347-00aa00a28331"> <PARAM NAME="imageurl" VALUE="image.gif"> </OBJECT> ...etc... --boundary-example-1 Content-Location: "image.gif" Content-Type: IMAGE/GIF Content-Transfer-Encoding: BASE64 R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk 5 ..etc... --boundary-example-1-- Only the object might know that the imageurl parameter is a relative URI. It's nearly impossible for the HTML parser to understand that the parameter is a relative URI. Simply searching for "image.gif" is not robust, as the string "image.gif" may be used elsewhere. URIs in scripts can also have similar problems. One might envisage even more difficult cases, an applet might take a parameter "subject" and another parameter "range" and when subject="auto" and range="1-5" it could compute, and try to use auto1.gif, auto2.gif ... auto5.gif as relative URLs. Some implementation methods described in section 4 above, for example method 2 described in section 4.2, may require rewriting of the URIs in the HTML document. There is no perfect solution to this problem. One way of alleviating the problem is to produce the original document using only absolute URIs, preferably of the CID type, since they are more easily identifiable. Another way of alleviating the problem is to make all URIs and Content-Locations into simple relative URIs containing file names only (without paths, preferably using a file name format common to most platforms, i.e. 1-6 ascii letters or digits, a period, and 1-3 extension ascii letters or digits). An implementation using method 2 described in section 4.2 above can then just store the parts as files in an empty directory on the recipient computer with the Content-Locations as file names. It can then turn the start HTML file over to a document viewer, and need not rewrite the URIs at all. This simple variant of use of the MHTML standard is probably most robust, and those implementors who can control the production of the HTML documents to be sent are thus recommended to use this variant. 6. Caching of Body Parts Suppose a message contains body parts with the Content-Location header as defined in [MHTML]. A receiving agent might then put this body part into a web cache, with the URI in the Content-Location as its name, so that later retrievals of this URI use the cached body parts. There is however no guarantee that such a cached item is correct. Such caching is thus not recommended for use in other ways than for resolution of links within one particular MIME message. The MHTML standard does not cover links between different messages, but if you want to implement this, use of Content- ID and/or Message-ID, rather than Content-Location, is recommended. If incoming messages are stored in a store where messages can be automatically deleted (purged), purging of body parts should not occur before purging of the whole message, to which they belong. If an incoming message contains a body part which is linked via Content-Location, then no HTTP lookup should be performed to check if the body part is recent. The message should thus still contain the old HTML document, even if the HTTP-available document has been revised. (Example: "Here is the weather map of October 29, 1997"). Exception from this is: (a) If the linked document is not enclosed in the message, but referred to via Content-Type: message/external-body, then the latest version should be shown using ordinary HTTP caching conventions. (b) If a new message is sent with a Supersedes reference to the old message, the old message should still show the old version of all the body parts, but it might be wise to inform the user that a superseding message is available. 7. "Save as" Command Many HTML viewers have a "Save as" command to save an HTML document in a local file. Usually, this command has two variants, "Save as text" which converts the HTML document to plain text before saving it, and "Save as source" which saves the HTML document as an HTML-formatted document. These two variants may not be enough in the case of MHTML documents. There is a third option, which might be named "Save as aggregate". This option would save the HTML plus all related parts in a file with the Content-Type: Multipart/related. The file would thus begin with the heading of the Multipart/related body part. There are two variants of this: Saving the document as it looked like when you got it, or saving the document including all inline body parts, even those you had to retrieve from the Internet when showing the message to the user. The second format is of special value, because it provides an archiving format of the full document, allowing the user to view it in the future as it looked like at one particular time, even though web content may change in the future. Finally, a user may also want to save the e-mail or http heading fields of an incoming message. This is sometimes the same as "Save as aggregate", but may include additional body parts before or outside of the mulitpart/related aggregate. To indicate whether such a saved document was received by e- mail or http, it might be saved with an additional surrounding body part of content-type message/rfc822 or message/http. Example, suppose you receive by e-mail the following message: MAIL FROM:<alice@bar.net> RCPT TO:<bob@foo.net> DATA From: Alice <alice@bar.net> To: Bob <bob2@foo.net> Date: 23 Jan 1998 10:51 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: text/html;charset=US-ASCII Content-ID: <foo3@foo1@bar.net> Here is the IETF logo with white background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo.gif" ALT="IETF logo with white background"> And here is the IETF logo with transparent background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo2e.gif" <ALT="IETF logo with transparent background"> --boundary-example-1 Content-Location: ietflogo.gif Content-Base: http://www.ietf.cnri.reston.va.us/images/ Content-Type: IMAGE/GIF Content-Transfer-Encoding: BASE64 R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5 NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A etc... --boundary-example-1-- Saving the above message as text might give the following file: From: Alice <alice@bar.net> To: Bob <bob2@foo.net> Date: 23 Jan 1998 10:51 Subject: A simple example Here is the IETF logo with white background: IETF logo with white background And here is the IETF logo with transparent background: IETF logo with transparent background Saving the same text as html source might give the following file: Here is the IETF logo with white background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ietflogo.gif" ALT="IETF logo with white background"> And here is the IETF logo with transparent background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo2e.gif" <ALT="IETF logo with transparent background"> Saving the same text as aggregate might give the following file From: Alice <alice@bar.net> To: Bob <bob2@foo.net> Date: 23 Jan 1998 10:51 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: text/html;charset=US-ASCII Content-ID: <foo3@foo1@bar.net> Here is the IETF logo with white background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo.gif" ALT="IETF logo with white background"> And here is the IETF logo with transparent background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo2e.gif" <ALT="IETF logo with transparent background"> --boundary-example-1 Content-Location: ietflogo.gif Content-Base: http://www.ietf.cnri.reston.va.us/images/ Content-Type: IMAGE/GIF Content-Transfer-Encoding: BASE64 R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5 NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A etc... --boundary-example-1-- Saving the same text as archiving aggregate might give the following file (where the missing body part is fetched through http and added to the saved file): From: Alice <alice@bar.net> To: Bob <bob2@foo.net> Date: 23 Jan 1998 10:51 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: text/html;charset=US-ASCII Content-ID: <foo3@foo1@bar.net> Here is the IETF logo with white background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo.gif" ALT="IETF logo with white background"> And here is the IETF logo with transparent background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo2e.gif" <ALT="IETF logo with transparent background"> --boundary-example-1 Content-Location: ietflogo.gif Content-Base: http://www.ietf.cnri.reston.va.us/images/ Content-Type: IMAGE/GIF Content-Transfer-Encoding: BASE64 R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5 NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A etc... --boundary-example-1 Content-Location: ietflogo2e.gif Content-Base: http://www.ietf.cnri.reston.va.us/images/ Content-Type: IMAGE/GIF Content-Transfer-Encoding: BASE64 R0lGODlhGAGgANX/ACkpKTExMTk5OUJCQkpKSlJSUlpaWmNjY2tra3Nzc3t7e4 SEhIyMjJSUlJycnKWlpa2trbW1tcDAwM7Ozv/eQnNzjHNzlGtrjGNjhFpae1pa etc... --boundary-example-1-- Saving the same message as message might give the following file: from:<alice@bar.net> To:<bob@foo.net> Mime-Version: 1.0 Content-Type: Message/rfc822; boundary="boundary-example-2" --boundary-example-2 From: Alice <alice@bar.net> To: Bob <bob2@foo.net> Date: 23 Jan 1998 10:51 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: text/html;charset=US-ASCII Content-ID: <foo3@foo1@bar.net> Here is the IETF logo with white background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo.gif" ALT="IETF logo with white background"> And here is the IETF logo with transparent background: <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ ietflogo2e.gif" <ALT="IETF logo with transparent background"> --boundary-example-1 Content-Location: ietflogo.gif Content-Base: http://www.ietf.cnri.reston.va.us/images/ Content-Type: IMAGE/GIF Content-Transfer-Encoding: BASE64 R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5 NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A etc... --boundary-example-1-- --boundary-example-2-- 8. Recipients which cannot Handle the Multipart/related Content-Type A message sent according to the specifications in [MHTML] may have recipients, whose mailers cannot handle the Multipart/related Content-Type in the way specified in [MHTML]. According to [MIME1] a mailer which encounters an unknown subtype to Multipart, should handle this as Multipart/mixed. To improve this, Multipart/alternative can be used as discussed in section 9 of this memo. Content-Disposition, as specified in [CONDISP] and in [MHTML], section 10, can also be used as an aid to mailers which do not understand Multipart/related. Captions on images, which are included in the HTML text, might for non-HTML-capable recipients be found in the Content-Description header [CONDISP]. Do not assume, however, that HTML-capable user agents will display the Content-Description header, they may assume that this information is included in the HTML text instead. 9. Use of the Content-Type: Multipart/alternative If the message is sent to recipients, all of which may not have mailers capable of handling the Text/HTML content-type, then the "Content-Type: Multipart/Alternative" [MIME1] can be used in two ways: 9.1 Multipart/alternative inside Multipart/related The Multipart/alternative is put inside the "Content-Type Multipart/related", body parts can be specified with "Content-Type: Text/plain" as the first choice, and "Content-Type: Text/HTML" as the second choice. Example: Content-Type: Multipart/related; boundary="boundary-example-1"; type=MULTIPART/ALTERNATIVE --boundary-example 1 Content-Type: MULTIPART/ALTERNATIVE Boundary: boundary-example-2 --boundary-example-2 Content-Type: Text/plain ... plain text version of the document for recipients whose mailers cannot handle Text/HTML ... --boundary-example-2 Content-Type: Text/HTML; charset=US-ASCII Content-ID: content-id-example@example.host ... text of the HTML document ... --boundary-example-2-- --boundary-example-1 Content-Type: Image/GIF ... a body part, to which the HTML document has a link ... --boundary-example-1-- Note that the type parameter of Multipart/related in this case should be Multipart/alternative and not Text/HTML. 9.2 Multipart/alternative outside Multipart/related The multipart/alternative is put outside the Multipart/Related, with Multipart/Related as one alternative and Multipart/Mixed as the other alternative. Note however that the [MHTML] does not recommend links from inside Multipart/Related to objects outside of the Multipart/Related, so putting inline images outside the Multipart/Related is not suitable. Instead, such inline images may have to repeated in both branches of the multipart/alternative with this method. Example: Content-Type: MULTIPART/ALTERNATIVE Boundary: boundary-example-1 --boundary-example-1 Content-Type: Multipart/mixed; boundary="boundary-example-3" --boundary-example-3 Content-Type: Text/plain; charset=US-ASCII ... plain text version of the message for recipients whose mailers cannot handle Text/HTML ... --boundary-example-3 Content-Type: Image/GIF ... A picture associated with the plain text message ... --boundary-example-3-- --boundary-example-1 Content-Type: Multipart/related; boundary="boundary-example-1"; type=Text/HTML --boundary-example 2 Content-Type: Text/HTML; charset=US-ASCII Content-ID: content-id-example@example.host ... text of the HTML document ... --boundary-example-2 Content-Type: Image/GIF ... a body part, to which the HTML document has a link ... --boundary-example-2-- --boundary-example-1-- 9.3 Comparing the Two Methods When choosing between these two methods of employing multipart/alternative, note the following: (1) Clients which do not support Multipart/related, and which thus will interpret it as Multipart/mixed, will with choice 9.1 display the inline objects. Thus, a recipient whose mailer can handle image/gif but not multipart/related will still be shown the images, they will not be suppressed by being inside a suppressed branch of the Multipart/alternative. (2) Choice 9.2 will not show inline images in the Multipart/Related, unless this information is repeated in both branches of the Multipart/Alternative. A general warning: Some mailers do not support "Content-Type: Multipart/alternative", and may then interpret it as Multipart/mixed, even though support of multipart/alternative is required for MIME conformance. 9.4 Reducing the Download Time If a message is sent as multipart/alternative, this would normally mean that the mail client downloads both variants, and then shows only one of the to the user. This will thus increase the download time. A way of avoiding this problem is to use the FETCH command of IMAP, which allows a client to download only certain body parts from a multipart message. 10. Writing Readable HTML An alternative to use of multipart/alternative is to produce HTML code which is is easy to read as it is. This alternative only works if you restrict the use of HTML features, for example if you have a special program to generate the HTML text. Below is an example of such a readable HTML text: <p> This is an example of HTML code, which is written so as to be readable for people who read it as plain text. <p> Here is the second paragraph, which contains a <a href="cid:456*foo@bar.net"> link </a> to a separate body part. <p> Here is an embedded picture: <p> <img src="cid:123*foo@bar.net" width="13" height="13"> <p> End of this HTML-formatted message. 11. Textual Alternatives to HTML Forms One important usage of HTML in e-mail is to send forms, which the recipients fill in and return. It is then problematic how to handle recipients whose mailers do not support HTML. One way is to use textual encoding of the forms. This encoding is done so that the user action needed to send in the form is made simple also for those who have only textual e-mail systems. Important is that the textual users are not forced to write complex commands in special command languages. Instead, the form should be written so that the user need only make simple changes to the form before sending it back, like deleting or adding single characters. Below is an example which shows how this can be done. The main principle is that every line beginning with ";" is an explanation for the reader, and every line beginning with "!" is a text, which the user can convert into a command by just deleting the "!" in front of the line. The users will thus have to learn a very simple rule of filling in forms: Just delete the "!" in front of your selections. Technically, the recipient of a filled-in textual form should regard all lines beginning with ";" or "!" as comment, and interpret all other lines as commands. 11.1 Form in HTML Format <FORM action="mailto:meeting-scheduling@ietf.org" method="POST"> <P>Which meeting date do you prefer? <P>1 December 1997 <SELECT NAME="19971201"> <OPTION>Very good <OPTION>Good <OPTION>Acceptable <OPTION>Bad <OPTION>Very bad </SELECT> <P>7 December 1997 <SELECT NAME="19971207"> <OPTION>Very good <OPTION>Good <OPTION>Acceptable <OPTION>Bad <OPTION>Very bad </SELECT> <P>14 December 1997 <SELECT NAME="19971214"> <OPTION>Very good <OPTION>Good <OPTION>Acceptable <OPTION>Bad <OPTION>Very bad </SELECT> <P>21 December 1997 <SELECT NAME="19971221"> <OPTION>Very good <OPTION>Good <OPTION>Acceptable <OPTION>Bad <OPTION>Very bad </SELECT> <P>Who should be the chairman? <P><INPUT TYPE="radio" NAME="chairman" VALUE="Mary">Mary <P><INPUT TYPE="radio" NAME="chairman" VALUE="John">John <P>Do you want simultaneous translation during the meeting? <P><INPUT TYPE="checkbox" NAME="translation" VALUE="English">To and from English <P><INPUT TYPE="checkbox" NAME="translation" VALUE="French">To and from French <P><INPUT TYPE="checkbox" NAME="translation" VALUE="Japanese">To and from Japanese <P>Please propose issues to discuss during the meeting: <P><TEXTAREA NAME="issues" ROWS=7 COLS=66></TEXTAREA> <P><INPUT TYPE="submit" NAME="Submit" VALUE="Submit"><INPUT TYPE="reset" VALUE="Reset"> 11.2 The same Form In Textual Format ; This is a computer-generated form. Please fill it in and return it ; to meeting-scheduler@ietf.org. To fill in the form, just copy its ; text into your reply and remove the exclamation mark (!) in front ; of your choices. ; If your mailer adds ">" or "> " in front of lines, you can keep ; these or remove them as you prefer. Question 1: Which meeting date do you prefer? Option 1.1: 1 December 1997 ! Very good ! Good ! Acceptable ! Bad ! Very bad Option 1.2: 7 December 1997 ! Very good ! Good ! Acceptable ! Bad ! Very bad Option 1.3: 14 December 1997 ! Very good ! Good ! Acceptable ! Bad ! Very bad Option 1.4: 21 December 1997 ! Very good ! Good ! Acceptable ! Bad ! Very bad Question 2: Who should be the chairman? ! Mary ! John Question 3: Do you want simultaneous translation during the meeting? Option 3.1: To and from English ! Yes ! No Option 3.2: To and from French ! Yes ! No Option 3.3: To and from Japanese ! Yes ! No Question 4: Please propose issues to discuss during the meeting. Write your proposal on the empty lines below. -- End of Question 4 12. Recipient may not have Full Internet Connectivity The recipient of a message sent by email may not always have full Internet connectivity. The recipient may be behind a gateway or firewall which prohibits or restricts Internet connectivity. This means that the recipient may not be able to resolve URI-s in an email message, unless the referred-to documents are included in the email message itself. Thus, it is often suitable to include in an email message all documents which are referred to (directly or indirectly) by URI-s in the message. This may of course not always be possible, in some cases the set of referred-to documents (directly or indirectly) may be the whole WWW document space, i.e. millions of documents. A choice must then be made how much to include. Of course, it is most important to include all inline objects, i.e. objects linked by such hyperlinks as IMG, etc., which specify that the linked objects are to be shown to the user immediately. In the case of ACTION elements in HTML forms, by making these ACTION elements of the "mailto:" URL type, rather than the "http:" URL type, you will enable also recipients without full Internet connectivity to fill in and send in your forms. The HTML specification [HTML2] allows default action when no ACTION element is included, but this default action may not be suitable when sending the HTML document via email. Thus, it is better to always put an explicit ACTION element into HTML forms sent by email. A disadvantage with the "mailto:" URL as ACTION, however, is that this may not work if the user has not specified his e-mail address in the preferences of this HTML viewer. This is common for multi-user workstations. Including URLs, which have to resolved over the Internet, typically referring to 1x1 pixel transparent images, is a known method for spammers to get information about the recipient of e-mail. It is possible that some mailers will restrict or disallow this in order to stop this method of information-gathering by spammers. Encoding of Non-Ascii Characters Displayed text Displayed text | ^ V | +-------------+ +----------------+ | HTML editor | | HTML viewer | | | | or Web browser | +-------------+ +----------------+ | ^ V | HTML markup HTML markup | ^ V | +---------+ +---------------+ +-------------+ +---------------+ | MIME | | MIME content- | | MIME | | MIME content- | | encap- | | transfer- | | heading | | transfer- | | sulator | | encoder | | interpreter | | decoder | +---------+ +---------------+ +-------------+ +---------------+ | | ^ ^ V V +-----------+ | | MIME heading + MIME content->| Transport |->MIME heading + MIME content +-----------+ Figure 5 Definitions (see Figure 5): Displayed A visual representation of the intended text text. HTML markup A sequence of characters formatted according to the HTML specification [HTML2]. MIME content A sequence of octets physically forwarded via email, may use MIME content-transfer- encoding as specified in [MIME1]. HTML editor Software used to produce HTML markup. MIME content- Software used to encode non-US-ASCII transfer- charactersr as specified in [MIME1]. encoder MIME content- Software used to decode non-US-ASCII transfer- characters as specified in [MIME1]. decoder MIME heading Software used to interpret the information interpreter in MIME headings. HTML viewer Software used to display HTML documents to recipients. Some implementations may have a choice of whether to represent non-ascii characters at the HTML layer (using "&" entity references or numeric character references as defined in [HTML2] section 3.2.1) or at the MIME layer (using Content-Transfer-Encoding as defined in [MIME1] section 5). In choosing between these two representation methods, note the following effects: (1) Modifying HTML markup may disrupt security content integrity checksums. If the checksums are computed between the HTML editor and the MIME encapsulator, then making the encoding in the MIME encapsulator will not break the checksums. (2) The choice of modifying HTML markup may be more suitable for recipients whose mailers do not support MIME. (3) Using MIME Content-Transfer-Encoding may be more suitable for recipients who have MIME-compliant mailers but do pass the text over to a document viewer (web browser). 14. Conversion from HTTP to MIME Information received or retrieved using HTTP cannot always be sent unchanged as email using the "Content-Type: Text/HTML", because of the restrictions which MIME places on the format of "Content-Type: Text/HTML". The same problem may occur for documents retrieved via HTTP, which are in other textual formats than HTML. In particular, note the following: (a) Content-encodings allowed in HTTP, but not allowed in MIME, must be removed. (b) HTTP allows line breaks as bare CRs or bare LFs or something else, while MIME only allows line breaks as CRLF in subtypes of the Text content-type. (c) HTTP allows character sets like Unicode-1-1, which do not represent line breaks as CRLFs, such text may have to be rewritten to character sets like Unicode-1-1-UTF-7 in which line breaks are represented as CRLFs. A good overview of the differences, with regard to the use of "Content-Type: Text", between MIME and HTTP, can be found in [HTTP] appendix C. If you want to provide web documents, which can be sent through e-mail without modification (which might break integrity checksums), then you SHOULD provide them up in the canonical form, with line breaks as CRLF, and avoid lines longer than 76 characters/line. If you want to send HTTP unchanged via email, you might consider using the "Content-Type: Message/HTTP" instead of the "Content-Type: Text/HTML". Note that with this Content- Type, the whole object, as sent through HTTP, can be encoded as a single object with, for example, BASE64 encoding. After decoding of the BASE64, the resulting object can have HTTP peculiar formats, like single LF or single CR between lines. However, some mailers may not be capable of handling the Message/HTTP Content-Type. Example, the binary part of the following message Content-Type: message/http Content-Transfer-Encoding: base64 SFRUUC8xLjEgMjAwIE9LDURhdGU6IFNhdCwgMTQgRmViIDE5OTggMTM6MDM 6MzggR01U DVNlcnZlcjogQXBhY2hlLzEuMi40DUxhc3QtTW9kaWZpZWQ6IFdlZCwgMjM gSnVsIDE5 ... ... ... might, when the base64 encoding above is decoded, yield: HTTP/1.1 200 OK Date: Sat, 14 Feb 1998 13:03:38 GMT ETag: "43788-124-33d658c5" Content-Length: 292 Accept-Ranges: bytes Content-Type: text/html ... <HTML data with only LF between lines> ... 15. Default Font Size Many HTML editors and viewers allow the user to specify the size of the default font (<FONT SIZE=3> or <FONT SIZE="+0"> according to personal wishes, for example 10 pt or 12 pt or 14 pt depending on eye sight and screen distance. This setting should *not* cause a change in the FONT SIZE= value in the generated HTML which is produced and sent. The reason for this is that otherwise users may inadvertently send whole letters with the text in <FONT SIZE=1> or <FONT SIZE=2>, which may be easy to read for the sender but difficult to read for some recipients. Similarly, a user choice of default FONT, to for example GENEVA or ARIAL, should not cause <FONT FACE=GENEVA> or <FONT FACE=ARIAL> to be sent. User who wish to send e-mail with <FONT SIZE=2> or <FONT FACE=GENEVA> must explicitly specify this, for example using a FONT command in their HTML editor or e-mail text editor. 16. Copyright and Disclaimer The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat." The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Copyright (C) The Internet Society (date). 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 implmentation 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. 17. Acknowledgments Harald Tveit Alvestrand, Richard Baker, Dave Crocker, Martin J. Duerst, Roy Fielding, Lewis Geer, Al Gilman, Paul Hoffman, Alexander Hopmann, Mark K. Joseph, Greg Herlihy, Valdis Kletnieks, Daniel LaLiberte, Ed Levinson, Jay Levitt, Albert Lunde, Larry Masinter, Keith Moore, Gavin Nicol, Pete Resnick, Jon Smirl, Einar Stefferud, Jamie Zawinski and several other people have helped us with preparing this memo. I alone take responsibility for any errors which may still be in the memo. 18. References Temporary note: This list contains some references to Internet drafts. It is anticipated that these Internet drafts will become RFC-s before this memo. The references will then in this memo be changed to refer to the corresponding RFC instead. This list also includes some RFC-s which are not up to date, and which will be replaced by new memos presently in ietf draft status. Ref. Author, title --- ------------- [CONDISP] R. Troost, S. Dorner: "Communicating Presentation Information in Internet Messages: The Content- Disposition Header", RFC 1806, June 1995. [HOSTS] R. Braden (editor): "Requirements for Internet Hosts -- Application and Support", STD-3, RFC 1123, October 1989. [HTML2] T. Berners-Lee, D. Connolly: "Hypertext Markup Language - 2.0", RFC 1866, November 1995. [HTTP] J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee: 2616 Hypertext Transfer Protocol -- HTTP/1.1. R. Fielding, J. Gettys, RFC 2616, June 1999. [MHTML] J. Palme, A. Hopmann, N. Shelness: MIME Encapsulation of Aggregate Documents, such as HTML (MHTML). RFC 2557, March 1999. [MIDCID] E. Levinson.: Content-ID and Message-ID Uniform Resource Locators. RFC 2392, August 1998. [MIME1] N. Freed & N. Borenstein: "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 2045, November 1996. [MIME2] N. Freed & N. Borenstein: "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. [REL] E. Levinson.: The MIME Multipart/Related Content- type. RFC 2387, August 1998. [RELURL] R. Fielding: "Relative Uniform Resource Locators", RFC 1808, June 1995. [MSGFMT] P. Resnick: "Internet Message Format" STD 11, RFC 2822, April 2001. [SMTP] J. Klensin: "Simple Mail Transfer Protocol", RFC 2821, April 2001. [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. 19. Author's Address Jacob Palme Phone: +46-8-16 16 67 Stockholm University and KTH Fax: +46-8-783 08 29 Electrum 230 Email: jpalme@dsv.su.se S-164 40 Kista, Sweden Working group chairman: Einar Stefferud <stef@nma.com>