It is frequently desirable, in sending mail, to encapsulate another mail message. For this common operation, a special Content-Type, "message", is defined. The primary subtype, message/rfc822, has no required parameters in the Content- Type field. Additional subtypes, "partial" and "External- body", do have required parameters. These subtypes are explained below.
NOTE: It has been suggested that subtypes of message might be defined for forwarded or rejected messages. However, forwarded and rejected messages can be handled as multipart messages in which the first part contains any control or descriptive information, and a second part, of type message/rfc822, is the forwarded or rejected message. Composing rejection and forwarding messages in this manner will preserve the type information on the original message and allow it to be correctly presented to the recipient, and hence is strongly encouraged.
As stated in the definition of the Content-Transfer-Encoding field, no encoding other than "7bit", "8bit", or "binary" is permitted for messages or parts of type "message". The message header fields are always US-ASCII in any case, and data within the body can still be encoded, in which case the Content-Transfer-Encoding header field in the encapsulated message will reflect this. Non-ASCII text in the headers of an encapsulated message can be specified using the mechanisms described in [RFC-1342].
Mail gateways, relays, and other mail handling agents are commonly known to alter the top-level header of an RFC 822 message. In particular, they frequently add, remove, or reorder header fields. Such alterations are explicitly forbidden for the encapsulated headers embedded in the bodies of messages of type "message."
Three parameters must be specified in the Content-Type field of type message/partial: The first, "id", is a unique identifier, as close to a world-unique identifier as possible, to be used to match the parts together. (In general, the identifier is essentially a message-id; if placed in double quotes, it can be any message-id, in accordance with the BNF for "parameter" given earlier in this specification.) The second, "number", an integer, is the part number, which indicates where this part fits into the sequence of fragments. The third, "total", another integer, is the total number of parts. This third subfield is required on the final part, and is optional on the earlier parts. Note also that these parameters may be given in any order.
Thus, part 2 of a 3-part message may have either of the following header fields:
When the parts of a message broken up in this manner are put together, the result is a complete RFC 822 format message, which may have its own Content-Type header field, and thus may contain any other data type.
Message fragmentation and reassembly: The semantics of a reassembled partial message must be those of the "inner" message, rather than of a message containing the inner message. This makes it possible, for example, to send a large audio message as several partial messages, and still have it appear to the recipient as a simple audio message rather than as an encapsulated message containing an audio message. That is, the encapsulation of the message is considered to be "transparent".
When generating and reassembling the parts of a message/partial message, the headers of the encapsulated message must be merged with the headers of the enclosing entities. In this process the following rules must be observed:
It should also be noted that the inclusion of a "References" field in the headers of the second and subsequent pieces of a fragmented message that references the Message-Id on the previous piece may be of benefit to mail readers that understand and track references. However, the generation of such "References" fields is entirely optional.
The external-body subtype indicates that the actual body data are not included, but merely referenced. In this case, the parameters describe a mechanism for accessing the external data.
When a message body or body part is of type "message/external-body", it consists of a header, two consecutive CRLFs, and the message header for the encapsulated message. If another pair of consecutive CRLFs appears, this of course ends the message header for the encapsulated message. However, since the encapsulated message's body is itself external, it does NOT appear in the area that follows. For example, consider the following message:
The only always-mandatory parameter for message/external- body is "access-type"; all of the other parameters may be mandatory or optional depending on the value of access-type.
An access-type of FTP or TFTP indicates that the message body is accessible as a file using the FTP [RFC-959] or TFTP [RFC-783] protocols, respectively. For these access-types, the following additional parameters are mandatory:
In addition, the following optional parameters may also appear when the access-type is FTP or ANON-FTP:
The "anon-ftp" access-type is identical to the "ftp" access type, except that the user need not be asked to provide a name and password for the specified site. Instead, the ftp protocol will be used with login "anonymous" and a password that corresponds to the user's email address.
7.3.3.3 The "local-file" and "afs" access-types
An access-type of "local-file" indicates that the actual body is accessible as a file on the local machine. An access-type of "afs" indicates that the file is accessible via the global AFS file system. In both cases, only a single parameter is required:
NAME -- The name of the file that contains the actual body data.
The following optional parameter may be used to describe the locality of reference for the data, that is, the site or sites at which the file is expected to be visible:
SITE -- A domain specifier for a machine or set of machines that are known to have access to the data file. Asterisks may be used for wildcard matching to a part of a domain name, such as "*.bellcore.com", to indicate a set of machines on which the data should be directly visible, while a single asterisk may be used to indicate a file that is expected to be universally available, e.g., via a global file system.
7.3.3.4 The "mail-server" access-type
The "mail-server" access-type indicates that the actual body is available from a mail server. The mandatory parameter for this access-type is:
SERVER -- The email address of the mail server from which the actual body data can be obtained.
Because mail servers accept a variety of syntax, some of which is multiline, the full command to be sent to a mail server is not included as a parameter on the content-type line. Instead, it may be provided as the "phantom body" when the content-type is message/external-body and the access-type is mail-server.
Note that MIME does not define a mail server syntax. Rather, it allows the inclusion of arbitrary mail server commands in the phantom body. Implementations should include the phantom body in the body of the message it sends to the mail server address to retrieve the relevant data. 7.3.3.5 Examples and Further Explanations
With the emerging possibility of very wide-area file systems, it becomes very hard to know in advance the set of machines where a file will and will not be accessible directly from the file system. Therefore it may make sense to provide both a file name, to be tried directly, and the name of one or more sites from which the file is known to be accessible. An implementation can try to retrieve remote files using FTP or any other protocol, using anonymous file retrieval or prompting the user for the necessary name and password. If an external body is accessible via multiple mechanisms, the sender may include multiple parts of type message/external-body within an entity of type multipart/alternative.
However, the external-body mechanism is not intended to be limited to file retrieval, as shown by the mail-server access-type. Beyond this, one can imagine, for example, using a video server for external references to video clips.
If an entity is of type "message/external-body", then the body of the entity will contain the header fields of the encapsulated message. The body itself is to be found in the external location. This means that if the body of the "message/external-body" message contains two consecutive CRLFs, everything after those pairs is NOT part of the message itself. For most message/external-body messages, this trailing area must simply be ignored. However, it is a convenient place for additional data that cannot be included in the content-type header field. In particular, if the "access-type" value is "mail-server", then the trailing area must contain commands to be sent to the mail server at the address given by NAME@SITE, where NAME and SITE are the values of the NAME and SITE parameters, respectively.
The embedded message header fields which appear in the body of the message/external-body data can be used to declare the Content-type of the external body. Thus a complete message/external-body message, referring to a document in PostScript format, might look like this:
From: Whomever Subject: whatever MIME-Version: 1.0 Message-ID: id1@host.com Content-Type: multipart/alternative; boundary=42
--42 Content-Type: message/external-body; name="BodyFormats.ps";
site="thumper.bellcore.com"; access-type=ANON-FTP; directory="pub"; mode="image"; expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
--42 Content-Type: message/external-body; name="/u/nsb/writing/rfcs/RFC-XXXX.ps"; site="thumper.bellcore.com"; access-type=AFS expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
--42 Content-Type: message/external-body; access-type=mail-server server="listserv@bogus.bitnet"; expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
get rfc-xxxx doc
--42--
Like the message/partial type, the message/external-body type is intended to be transparent, that is, to convey the data type in the external body rather than to convey a message with a body of that type. Thus the headers on the outer and inner parts must be merged using the same rules as for message/partial. In particular, this means that the Content-type header is overridden, but the From and Subject headers are preserved.
Note that since the external bodies are not transported as mail, they need not conform to the 7-bit and line length requirements, but might in fact be binary files. Thus a Content-Transfer-Encoding is not generally necessary, though it is permitted.
Note that the body of a message of type "message/external- body" is governed by the basic syntax for an RFC 822 message. In particular, anything before the first consecutive pair of CRLFs is header information, while anything after it is body information, which is ignored for most access-types.
7.4 The Application Content-Type
The "application" Content-Type is to be used for data which do not fit in any of the other categories, and particularly for data to be processed by mail-based uses of application programs. This is information which must be processed by an application before it is viewable or usable to a user. Expected uses for Content-Type application include mail- based file transfer, spreadsheets, data for mail-based scheduling systems, and languages for "active" (computational) email. (The latter, in particular, can pose security problems which should be understood by implementors, and are considered in detail in the discussion of the application/PostScript content-type.)
For example, a meeting scheduler might define a standard representation for information about proposed meeting dates. An intelligent user agent would use this information to conduct a dialog with the user, and might then send further mail based on that dialog. More generally, there have been several "active" messaging languages developed in which programs in a suitably specialized language are sent through the mail and automatically run in the recipient's environment.
Such applications may be defined as subtypes of the "application" Content-Type. This document defines three subtypes: octet-stream, ODA, and PostScript.
In general, the subtype of application will often be the name of the application for which the data are intended. This does not mean, however, that any application program name may be used freely as a subtype of application. Such usages must be registered with IANA, as described in Appendix F.
7.4.1 The Application/Octet-Stream (primary) subtype
The primary subtype of application, "octet-stream", may be used to indicate that a body contains binary data. The set of possible parameters includes, but is not limited to:
NAME -- a suggested name for the binary data if stored as a file.
TYPE -- the general type or category of binary data. This is intended as information for the human recipient rather than for any automatic processing.
CONVERSIONS -- the set of operations that have been performed on the data before putting it in the mail (and before any Content-Transfer-Encoding that might have been applied). If multiple
conversions have occurred, they must be separated by commas and specified in the order they were applied -- that is, the leftmost conversion must have occurred first, and conversions are undone from right to left. Note that NO conversion values are defined by this document. Any conversion values that that do not begin with "X-" must be preceded by a published specification and by registration with IANA, as described in Appendix F.
PADDING -- the number of bits of padding that were appended to the bitstream comprising the actual contents to produce the enclosed byte-oriented data. This is useful for enclosing a bitstream in a body when the total number of bits is not a multiple of the byte size.
The values for these attributes are left undefined at present, but may require specification in the future. An example of a common (though UNIX-specific) usage might be:
Content-Type: application/octet-stream; name=foo.tar.Z; type=tar; conversions="x-encrypt,x-compress"
However, it should be noted that the use of such conversions is explicitly discouraged due to a lack of portability and standardization. The use of uuencode is particularly discouraged, in favor of the Content-Transfer-Encoding mechanism, which is both more standardized and more portable across mail boundaries.
The recommended action for an implementation that receives application/octet-stream mail is to simply offer to put the data in a file, with any Content-Transfer-Encoding undone, or perhaps to use it as input to a user-specified process.
To reduce the danger of transmitting rogue programs through the mail, it is strongly recommended that implementations NOT implement a path-search mechanism whereby an arbitrary program named in the Content-Type parameter (e.g., an "interpreter=" parameter) is found and executed using the mail body as input.
7.4.2 The Application/PostScript subtype
A Content-Type of "application/postscript" indicates a PostScript program. The language is defined in [POSTSCRIPT]. It is recommended that Postscript as sent through email should use Postscript document structuring conventions if at all possible, and correctly.
The execution of general-purpose PostScript interpreters entails serious security risks, and implementors are discouraged from simply sending PostScript email bodies to "off-the-shelf" interpreters. While it is usually safe to send PostScript to a printer, where the potential for harm is greatly constrained, implementors should consider all of the following before they add interactive display of PostScript bodies to their mail readers.
The remainder of this section outlines some, though probably not all, of the possible problems with sending PostScript through the mail.
Dangerous operations in the PostScript language include, but may not be limited to, the PostScript operators deletefile, renamefile, filenameforall, and file. File is only dangerous when applied to something other than standard input or output. Implementations may also define additional nonstandard file operators; these may also pose a threat to security. Filenameforall, the wildcard file search operator, may appear at first glance to be harmless. Note, however, that this operator has the potential to reveal information about what files the recipient has access to, and this information may itself be sensitive. Message senders should avoid the use of potentially dangerous file operators, since these operators are quite likely to be unavailable in secure PostScript implementations. Message- receiving and -displaying software should either completely disable all potentially dangerous file operators or take special care not to delegate any special authority to their operation. These operators should be viewed as being done by an outside agency when interpreting PostScript documents. Such disabling and/or checking should be done completely outside of the reach of the PostScript language itself; care should be taken to insure that no method exists for reenabling full-function versions of these operators.
The PostScript language provides facilities for exiting the normal interpreter, or server, loop. Changes made in this "outer" environment are customarily retained across documents, and may in some cases be retained semipermanently in nonvolatile memory. The operators associated with exiting the interpreter loop have the potential to interfere with subsequent document processing. As such, their unrestrained use constitutes a threat of service denial. PostScript operators that exit the interpreter loop include, but may not be limited to, the exitserver and startjob operators. Message-sending software should not generate PostScript that depends on exiting the interpreter loop to operate. The ability to exit will probably be unavailable in secure PostScript implementations. Message-receiving and -displaying software should, if possible, disable the ability to make retained changes to the PostScript environment. Eliminate the startjob and exitserver commands.
If these commands cannot be eliminated, at least set the password associated with them to a hard-to-guess value.
PostScript provides operators for setting system-wide and device-specific parameters. These parameter settings may be retained across jobs and may potentially pose a threat to the correct operation of the interpreter. The PostScript operators that set system and device parameters include, but may not be limited to, the setsystemparams and setdevparams operators. Message-sending software should not generate PostScript that depends on the setting of system or device parameters to operate correctly. The ability to set these parameters will probably be unavailable in secure PostScript implementations. Message-receiving and -displaying software should, if possible, disable the ability to change system and device parameters. If these operators cannot be disabled, at least set the password associated with them to a hard-to-guess value.
Some PostScript implementations provide nonstandard facilities for the direct loading and execution of machine code. Such facilities are quite obviously open to substantial abuse. Message-sending software should not make use of such features. Besides being totally hardware- specific, they are also likely to be unavailable in secure implementations of PostScript. Message-receiving and -displaying software should not allow such operators to be used if they exist.
PostScript is an extensible language, and many, if not most, implementations of it provide a number of their own extensions. This document does not deal with such extensions explicitly since they constitute an unknown factor. Message-sending software should not make use of nonstandard extensions; they are likely to be missing from some implementations. Message-receiving and -displaying software should make sure that any nonstandard PostScript operators are secure and don't present any kind of threat.
It is possible to write PostScript that consumes huge amounts of various system resources. It is also possible to write PostScript programs that loop infinitely. Both types of programs have the potential to cause damage if sent to unsuspecting recipients. Message-sending software should avoid the construction and dissemination of such programs, which is antisocial. Message-receiving and -displaying software should provide appropriate mechanisms to abort processing of a document after a reasonable amount of time has elapsed. In addition, PostScript interpreters should be limited to the consumption of only a reasonable amount of any given system resource.
Finally, bugs may exist in some PostScript interpreters which could possibly be exploited to gain unauthorized
access to a recipient's system. Apart from noting this possibility, there is no specific action to take to prevent this, apart from the timely correction of such bugs if any are found.
7.4.3 The Application/ODA subtype
The "ODA" subtype of application is used to indicate that a body contains information encoded according to the Office Document Architecture [ODA] standards, using the ODIF representation format. For application/oda, the Content- Type line should also specify an attribute/value pair that indicates the document application profile (DAP), using the key word "profile". Thus an appropriate header field might look like this:
Content-Type: application/oda; profile=Q112
Consult the ODA standard [ODA] for further information.
7.5 The Image Content-Type
A Content-Type of "image" indicates that the bodycontains an image. The subtype names the specific image format. These names are case insensitive. Two initial subtypes are "jpeg" for the JPEG format, JFIF encoding, and "gif" for GIF format [GIF].
The list of image subtypes given here is neither exclusive nor exhaustive, and is expected to grow as more types are registered with IANA, as described in Appendix F.
7.6 The Audio Content-Type
A Content-Type of "audio" indicates that the body contains audio data. Although there is not yet a consensus on an "ideal" audio format for use with computers, there is a pressing need for a format capable of providing interoperable behavior.
The initial subtype of "basic" is specified to meet this requirement by providing an absolutely minimal lowest common denominator audio format. It is expected that richer formats for higher quality and/or lower bandwidth audio will be defined by a later document.
The content of the "audio/basic" subtype is audio encoded using 8-bit ISDN u-law [PCM]. When this subtype is present, a sample rate of 8000 Hz and a single channel is assumed.
7.7 The Video Content-Type
A Content-Type of "video" indicates that the body contains a time-varying-picture image, possibly with color and coordinated sound. The term "video" is used extremely generically, rather than with reference to any particular technology or format, and is not meant to preclude subtypes such as animated drawings encoded compactly. The subtype "mpeg" refers to video coded according to the MPEG standard [MPEG].
Note that although in general this document strongly discourages the mixing of multiple media in a single body, it is recognized that many so-called "video" formats include a representation for synchronized audio, and this is explicitly permitted for subtypes of "video".
7.8 Experimental Content-Type Values
A Content-Type value beginning with the characters "X-" is a private value, to be used by consenting mail systems by mutual agreement. Any format without a rigorous and public definition must be named with an "X-" prefix, and publicly specified values shall never begin with "X-". (Older
versions of the widely-used Andrew system use the "X-BE2" name, so new systems should probably choose a different name.)
In general, the use of "X-" top-level types is strongly discouraged. Implementors should invent subtypes of the existing types whenever possible. The invention of new types is intended to be restricted primarily to the development of new media types for email, such as digital odors or holography, and not for new data formats in general. In many cases, a subtype of application will be more appropriate than a new top-level type.
Summary
Using the MIME-Version, Content-Type, and Content-Transfer- Encoding header fields, it is possible to include, in a standardized way, arbitrary types of data objects with RFC 822 conformant mail messages. No restrictions imposed by either RFC 821 or RFC 822 are violated, and care has been taken to avoid problems caused by additional restrictions imposed by the characteristics of some Internet mail transport mechanisms (see Appendix B). The "multipart" and "message" Content-Types allow mixing and hierarchical structuring of objects of different types in a single message. Further Content-Types provide a standardized mechanism for tagging messages or body parts as audio, image, or several other kinds of data. A distinguished parameter syntax allows further specification of data format details, particularly the specification of alternate character sets. Additional optional header fields provide mechanisms for certain extensions deemed desirable by many implementors. Finally, a number of useful Content-Types are defined for general use by consenting user agents, notably text/richtext, message/partial, and message/external-body.
Acknowledgements
This document is the result of the collective effort of a large number of people, at several IETF meetings, on the IETF-SMTP and IETF-822 mailing lists, and elsewhere. Although any enumeration seems doomed to suffer from egregious omissions, the following are among the many contributors to this effort:
Harald Tveit Alvestrand Timo Lehtinen Randall Atkinson John R. MacMillan Philippe Brandon Rick McGowan Kevin Carosso Leo Mclaughlin Uhhyung Choi Goli Montaser-Kohsari Cristian Constantinof Keith Moore Mark Crispin Tom Moore Dave Crocker Erik Naggum Terry Crowley Mark Needleman Walt Daniels John Noerenberg Frank Dawson Mats Ohrman Hitoshi Doi Julian Onions Kevin Donnelly Michael Patton Keith Edwards David J. Pepper Chris Eich Blake C. Ramsdell Johnny Eriksson Luc Rooijakkers Craig Everhart Marshall T. Rose Patrik Faeltstroem Jonathan Rosenberg Erik E. Fair Jan Rynning Roger Fajman Harri Salminen Alain Fontaine Michael Sanderson James M. Galvin Masahiro Sekiguchi Philip Gladstone Mark Sherman Thomas Gordon Keld Simonsen Phill Gross Bob Smart James Hamilton Peter Speck Steve Hardcastle-Kille Henry Spencer David Herron Einar Stefferud Bruce Howard Michael Stein Bill Janssen Klaus Steinberger Olle Jaernefors Peter Svanberg Risto Kankkunen James Thompson Phil Karn Steve Uhler Alan Katz Stuart Vance Tim Kehres Erik van der Poel Neil Katin Guido van Rossum Kyuho Kim Peter Vanderbilt Anders Klemets Greg Vaudreuil John Klensin Ed Vielmetti Valdis Kletniek Ryan Waldron Jim Knowles Wally Wedel Stev Knowles Sven-Ove Westberg Bob Kummerfeld Brian Wideen
Pekka Kytolaakso John Wobus Stellan Lagerstr.m Glenn Wright Vincent Lau Rayan Zachariassen Donald Lindsay David Zimmerman The authors apologize for any omissions from this list, which are certainly unintentional.
Appendix A -- Minimal MIME-Conformance
The mechanisms described in this document are open-ended. It is definitely not expected that all implementations will support all of the Content-Types described, nor that they will all share the same extensions. In order to promote interoperability, however, it is useful to define the concept of "MIME-conformance" to define a certain level of implementation that allows the useful interworking of messages with content that differs from US ASCII text. In this section, we specify the requirements for such conformance.
A mail user agent that is MIME-conformant MUST:
1. Always generate a "MIME-Version: 1.0" header field.
2. Recognize the Content-Transfer-Encoding header field, and decode all received data encoded with either the quoted-printable or base64 implementations. Encode any data sent that is not in seven-bit mail-ready representation using one of these transformations and include the appropriate Content-Transfer-Encoding header field, unless the underlying transport mechanism supports non-seven-bit data, as SMTP does not.
3. Recognize and interpret the Content-Type header field, and avoid showing users raw data with a Content-Type field other than text. Be able to send at least text/plain messages, with the character set specified as a parameter if it is not US-ASCII.
4. Explicitly handle the following Content-Type values, to at least the following extents:
Text: -- Recognize and display "text" mail with the character set "US-ASCII." -- Recognize other character sets at least to the extent of being able to inform the user about what character set the message uses. -- Recognize the "ISO-8859-*" character sets to the extent of being able to display those characters that are common to ISO-8859-* and US-ASCII, namely all characters represented by octet values 0-127. -- For unrecognized subtypes, show or offer to show the user the "raw" version of the data. An ability at
least to convert "text/richtext" to plain text, as shown in Appendix D, is encouraged, but not required for conformance. Message: --Recognize and display at least the primary (822) encapsulation. Multipart: -- Recognize the primary (mixed) subtype. Display all relevant information on the message level and the body part header level and then display or offer to display each of the body parts individually. -- Recognize the "alternative" subtype, and avoid showing the user redundant parts of multipart/alternative mail. -- Treat any unrecognized subtypes as if they were "mixed". Application: -- Offer the ability to remove either of the two types of Content-Transfer- Encoding defined in this document and put the resulting information in a user file.
5. Upon encountering any unrecognized Content- Type, an implementation must treat it as if it had a Content-Type of "application/octet-stream" with no parameter sub-arguments. How such data are handled is up to an implementation, but likely options for handling such unrecognized data include offering the user to write it into a file (decoded from its mail transport format) or offering the user to name a program to which the decoded data should be passed as input. Unrecognized predefined types, which in a MIME- conformant mailer might still include audio, image, or video, should also be treated in this way.
A user agent that meets the above conditions is said to be MIME-conformant. The meaning of this phrase is that it is assumed to be "safe" to send virtually any kind of properly-marked data to users of such mail systems, because such systems will at least be able to treat the data as undifferentiated binary, and will not simply splash it onto the screen of unsuspecting users. There is another sense in which it is always "safe" to send data in a format that is MIME-conformant, which is that such data will not break or be broken by any known systems that are conformant with RFC 821 and RFC 822. User agents that are MIME-conformant have the additional guarantee that the user will not be shown data that were never intended to be viewed as text.
Appendix B -- General Guidelines For Sending Email Data
Internet email is not a perfect, homogeneous system. Mail may become corrupted at several stages in its travel to a final destination. Specifically, email sent throughout the Internet may travel across many networking technologies. Many networking and mail technologies do not support the full functionality possible in the SMTP transport environment. Mail traversing these systems is likely to be modified in such a way that it can be transported.
There exist many widely-deployed non-conformant MTAs in the Internet. These MTAs, speaking the SMTP protocol, alter messages on the fly to take advantage of the internal data structure of the hosts they are implemented on, or are just plain broken.
The following guidelines may be useful to anyone devising a data format (Content-Type) that will survive the widest range of networking technologies and known broken MTAs unscathed. Note that anything encoded in the base64 encoding will satisfy these rules, but that some well-known mechanisms, notably the UNIX uuencode facility, will not. Note also that anything encoded in the Quoted-Printable encoding will survive most gateways intact, but possibly not some gateways to systems that use the EBCDIC character set.
(1) Under some circumstances the encoding used for data may change as part of normal gateway or user agent operation. In particular, conversion from base64 to quoted-printable and vice versa may be necessary. This may result in the confusion of CRLF sequences with line breaks in text body parts. As such, the persistence of CRLF as something other than a line break should not be relied on.
(2) Many systems may elect to represent and store text data using local newline conventions. Local newline conventions may not match the RFC822 CRLF convention -- systems are known that use plain CR, plain LF, CRLF, or counted records. The result is that isolated CR and LF characters are not well tolerated in general; they may be lost or converted to delimiters on some systems, and hence should not be relied on.
(3) TAB (HT) characters may be misinterpreted or may be automatically converted to variable numbers of spaces. This is unavoidable in some environments, notably those not based on the ASCII character set. Such conversion is STRONGLY DISCOURAGED, but it may occur, and mail formats should not rely on the persistence of TAB (HT)
characters.
(4) Lines longer than 76 characters may be wrapped or truncated in some environments. Line wrapping and line truncation are STRONGLY DISCOURAGED, but unavoidable in some cases. Applications which require long lines should somehow differentiate between soft and hard line breaks. (A simple way to do this is to use the quoted-printable encoding.)
(5) Trailing "white space" characters (SPACE, TAB (HT)) on a line may be discarded by some transport agents, while other transport agents may pad lines with these characters so that all lines in a mail file are of equal length. The persistence of trailing white space, therefore, should not be relied on.
(6) Many mail domains use variations on the ASCII character set, or use character sets such as EBCDIC which contain most but not all of the US- ASCII characters. The correct translation of characters not in the "invariant" set cannot be depended on across character converting gateways. For example, this situation is a problem when sending uuencoded information across BITNET, an EBCDIC system. Similar problems can occur without crossing a gateway, since many Internet hosts use character sets other than ASCII internally. The definition of Printable Strings in X.400 adds further restrictions in certain special cases. In particular, the only characters that are known to be consistent across all gateways are the 73 characters that correspond to the upper and lower case letters A-Z and a-z, the 10 digits 0-9, and the following eleven special characters:
"'" (ASCII code 39) "(" (ASCII code 40) ")" (ASCII code 41) "+" (ASCII code 43) "," (ASCII code 44) "-" (ASCII code 45) "." (ASCII code 46) "/" (ASCII code 47) ":" (ASCII code 58) "=" (ASCII code 61) "?" (ASCII code 63)
A maximally portable mail representation, such as the base64 encoding, will confine itself to relatively short lines of text in which the only meaningful characters are taken from this set of
73 characters.
Please note that the above list is NOT a list of recommended practices for MTAs. RFC 821 MTAs are prohibited from altering the character of white space or wrapping long lines. These BAD and illegal practices are known to occur on established networks, and implementions should be robust in dealing with the bad effects they can cause. Appendix C -- A Complex Multipart Example
What follows is the outline of a complex multipart message. This message has five parts to be displayed serially: two introductory plain text parts, an embedded multipart message, a richtext part, and a closing encapsulated text message in a non-ASCII character set. The embedded multipart message has two parts to be displayed in parallel, a picture and an audio fragment.
MIME-Version: 1.0
From: Nathaniel Borenstein
This is the preamble area of a multipart message.
Mail readers that understand multipart format
should ignore this preamble.
If you are reading this text, you might want to
consider changing to a mail reader that understands
how to properly display multipart messages.
--unique-boundary-1
...Some text appears here...
[Note that the preceding blank line means
no header fields were given and this is text,
with charset US ASCII. It could have been
done with explicit typing as in the next part.]
--unique-boundary-1
Content-type: text/plain; charset=US-ASCII
This could have been part of the previous part,
but illustrates explicit versus implicit
typing of body parts.
--unique-boundary-1
Content-Type: multipart/parallel;
boundary=unique-boundary-2
--unique-boundary-2
Content-Type: audio/basic
Content-Transfer-Encoding: base64
... base64-encoded 8000 Hz single-channel
u-law-format audio data goes here....
--unique-boundary-2
Content-Type: image/gif
Content-Transfer-Encoding: Base64
... base64-encoded image data goes here....
--unique-boundary-2--
--unique-boundary-1
Content-type: text/richtext
This is
--unique-boundary-1
Content-Type: message/rfc822
From: (name in US-ASCII)
Subject: (subject in US-ASCII)
Content-Type: Text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: Quoted-printable
... Additional text in ISO-8859-1 goes here ...
--unique-boundary-1--
Appendix D -- A Simple Richtext-to-Text Translator in C
One of the major goals in the design of the richtext subtype
of the text Content-Type is to make formatted text so simple
that even text-only mailers will implement richtext-to-
plain-text translators, thus increasing the likelihood that
multifont text will become "safe" to use very widely. To
demonstrate this simplicity, what follows is an extremely
simple 44-line C program that converts richtext input into
plain text output:
#include
while((c = getc(stdin)) != EOF) {
if (c == '<') {
for (i=0; (i<49 && (c = getc(stdin)) != '>'
&& c != EOF); ++i) {
token[i] = isupper(c) ? tolower(c) : c;
}
if (c == EOF) break;
if (c != '>') while ((c = getc(stdin)) !=
'>'
&& c != EOF) {;}
if (c == EOF) break;
token[i] = '\0';
if (!strcmp(token, "lt")) {
putc('<', stdout);
} else if (!strcmp(token, "nl")) {
putc('\n', stdout);
} else if (!strcmp(token, "/paragraph")) {
fputs("\n\n", stdout);
} else if (!strcmp(token, "comment")) {
int commct=1;
while (commct > 0) {
while ((c = getc(stdin)) != '<'
&& c != EOF) ;
if (c == EOF) break;
for (i=0; (c = getc(stdin)) != '>'
&& c != EOF; ++i) {
token[i] = isupper(c) ?
tolower(c) : c;
}
if (c== EOF) break;
token[i] = NULL;
if (!strcmp(token, "/comment")) --
commct;
if (!strcmp(token, "comment"))
++commct;
}
} /* Ignore all other tokens */
} else if (c != '\n') putc(c, stdout);
}
putc('\n', stdout); /* for good measure */
}
It should be noted that one can do considerably better than
this in displaying richtext data on a dumb terminal. In
particular, one can replace font information such as "bold"
with textual emphasis (like *this* or _T_H_I_S_). One can
also properly handle the richtext formatting commands
regarding indentation, justification, and others. However,
the above program is all that is necessary in order to
present richtext on a dumb terminal.
Appendix E -- Collected Grammar
This appendix contains the complete BNF grammar for all the
syntax specified by this document.
By itself, however, this grammar is incomplete. It refers
to several entities that are defined by RFC 822. Rather
than reproduce those definitions here, and risk
unintentional differences between the two, this document
simply refers the reader to RFC 822 for the remaining
definitions. Wherever a term is undefined, it refers to the
RFC 822 definition.
attribute := token
body-part = <"message" as defined in RFC 822,
with all header fields optional, and with the
specified delimiter not occurring anywhere in
the message body, either on a line by itself
or as a substring anywhere.>
boundary := 0*69
bchars := bcharsnospace / " "
bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" /
"_"
/ "," / "-" / "." / "/" / ":" / "=" / "?"
close-delimiter := delimiter "--"
Content-Description := *text
Content-ID := msg-id
Content-Transfer-Encoding := "BASE64" / "QUOTED-
PRINTABLE" /
"8BIT" / "7BIT" /
"BINARY" / x-token
Content-Type := type "/" subtype *[";" parameter]
delimiter := CRLF "--" boundary ; taken from Content-Type
field.
; when content-type is
multipart
; There should be no space
; between "--" and boundary.
encapsulation := delimiter CRLF body-part
epilogue := *text ; to be ignored upon
receipt.
MIME-Version := 1*text
multipart-body := preamble 1*encapsulation close-delimiter
epilogue
parameter := attribute "=" value
preamble := *text ; to be ignored upon
receipt.
subtype := token
token := 1*
tspecials := "(" / ")" / "<" / ">" / "@" ; Must be in
/ "," / ";" / ":" / "\" / <"> ; quoted-string,
/ "/" / "[" / "]" / "?" / "." ; to use within
/ "=" ; parameter values
type := "application" / "audio" ; case-
insensitive
/ "image" / "message"
/ "multipart" / "text"
/ "video" / x-token
value := token / quoted-string
x-token :=
Appendix F -- IANA Registration Procedures
MIME has been carefully designed to have extensible
mechanisms, and it is expected that the set of content-
type/subtype pairs and their associated parameters will grow
significantly with time. Several other MIME fields, notably
character set names, access-type parameters for the
message/external-body type, conversions parameters for the
application type, and possibly even Content-Transfer-
Encoding values, are likely to have new values defined over
time. In order to ensure that the set of such values is
developed in an orderly, well-specified, and public manner,
MIME defines a registration process which uses the Internet
Assigned Numbers Authority (IANA) as a central registry for
such values.
In general, parameters in the content-type header field are
used to convey supplemental information for various content
types, and their use is defined when the content-type and
subtype are defined. New parameters should not be defined
as a way to introduce new functionality.
In order to simplify and standardize the registration
process, this appendix gives templates for the registration
of new values with IANA. Each of these is given in the form
of an email message template, to be filled in by the
registering party.
F.1 Registration of New Content-type/subtype Values
Note that MIME is generally expected to be extended by
subtypes. If a new fundamental top-level type is needed,
its specification should be published as an RFC or
submitted in a form suitable to become an RFC, and be
subject to the Internet standards process.
To: IANA@isi.edu
Subject: Registration of new MIME content-type/subtype
MIME type name:
(If the above is not an existing top-level MIME type,
please explain why an existing type cannot be used.)
MIME subtype name:
Required parameters:
Optional parameters:
Encoding considerations:
Security considerations:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be if a new top-level type is being defined, and
must be a publicly available specification in any
case.)
Person & email address to contact for further
information:
F.2 Registration of New Character Set Values
To: IANA@isi.edu
Subject: Registration of new MIME character set value
MIME character set name:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be or an international standard.)
Person & email address to contact for further
information:
F.3 Registration of New Access-type Values for
Message/external-body
To: IANA@isi.edu
Subject: Registration of new MIME Access-type for
Message/external-body content-type
MIME access-type name:
Required parameters:
Optional parameters:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be.)
Person & email address to contact for further
information:
F.4 Registration of New Conversions Values for Application
To: IANA@isi.edu
Subject: Registration of new MIME Conversions value
for Application content-type
MIME Conversions name:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be.)
Person & email address to contact for further
information:
Appendix G -- Summary of the Seven Content-types
Content-type: text
Subtypes defined by this document: plain, richtext
Important Parameters: charset
Encoding notes: quoted-printable generally preferred if an
encoding is needed and the character set is mostly an
ASCII superset.
Security considerations: Rich text formats such as TeX and
Troff often contain mechanisms for executing arbitrary
commands or file system operations, and should not be
used automatically unless these security problems have
been addressed. Even plain text may contain control
characters that can be used to exploit the capabilities
of "intelligent" terminals and cause security
violations. User interfaces designed to run on such
terminals should be aware of and try to prevent such
problems.
________________________________________________________________
Content-type: multipart
Subtypes defined by this document: mixed, alternative,
digest, parallel.
Important Parameters: boundary
Encoding notes: No content-transfer-encoding is permitted.
________________________________________________________________
Content-type: message
Subtypes defined by this document: rfc822, partial,
external-body
Important Parameters: id, number, total
Encoding notes: No content-transfer-encoding is permitted.
________________________________________________________________
Content-type: application
Subtypes defined by this document: octet-stream,
postscript, oda
Important Parameters: profile
Encoding notes: base64 generally preferred for octet-stream
or other unreadable subtypes.
Security considerations: This type is intended for the
transmission of data to be interpreted by locally-installed
programs. If used, for example, to transmit executable
binary programs or programs in general-purpose interpreted
languages, such as LISP programs or shell scripts, severe
security problems could result. In general, authors of
mail-reading agents are cautioned against giving their
systems the power to execute mail-based application data
without carefully considering the security implications.
While it is certainly possible to define safe application
formats and even safe interpreters for unsafe formats, each
interpreter should be evaluated separately for possible
security problems.
________________________________________________________________
Content-type: image
Subtypes defined by this document: jpeg, gif
Important Parameters: none
Encoding notes: base64 generally preferred
________________________________________________________________
Content-type: audio
Subtypes defined by this document: basic
Important Parameters: none
Encoding notes: base64 generally preferred
________________________________________________________________
Content-type: video
Subtypes defined by this document: mpeg
Important Parameters: none
Encoding notes: base64 generally preferred
Appendix H -- Canonical Encoding Model
There was some confusion, in earlier drafts of this memo,
regarding the model for when email data was to be converted
to canonical form and encoded, and in particular how this
process would affect the treatment of CRLFs, given that the
representation of newlines varies greatly from system to
system. For this reason, a canonical model for encoding is
presented below.
The process of composing a MIME message part can be modelled
as being done in a number of steps. Note that these steps
are roughly similar to those steps used in RFC1113:
Step 1. Creation of local form.
The body part to be transmitted is created in the system's
native format. The native character set is used, and where
appropriate local end of line conventions are used as well.
The may be a UNIX-style text file, or a Sun raster image, or
a VMS indexed file, or audio data in a system-dependent
format stored only in memory, or anything else that
corresponds to the local model for the representation of
some form of information.
Step 2. Conversion to canonical form.
The entire body part, including "out-of-band" information
such as record lengths and possibly file attribute
information, is converted to a universal canonical form.
The specific content type of the body part as well as its
associated attributes dictate the nature of the canonical
form that is used. Conversion to the proper canonical form
may involve character set conversion, transformation of
audio data, compression, or various other operations
specific to the various content types.
For example, in the case of text/plain data, the text must
be converted to a supported character set and lines must be
delimited with CRLF delimiters in accordance with RFC822.
Note that the restriction on line lengths implied by RFC822
is eliminated if the next step employs either quoted-
printable or base64 encoding.
Step 3. Apply transfer encoding.
A Content-Transfer-Encoding appropriate for this body part
is applied. Note that there is no fixed relationship
between the content type and the transfer encoding. In
particular, it may be appropriate to base the choice of
base64 or quoted-printable on character frequency counts
which are specific to a given instance of body part.
Step 4. Insertion into message.
The encoded object is inserted into a MIME message with
appropriate body part headers and boundary markers.
It is vital to note that these steps are only a model; they
are specifically NOT a blueprint for how an actual system
would be built. In particular, the model fails to account
for two common designs:
1. In many cases the conversion to a canonical
form prior to encoding will be subsumed into the
encoder itself, which understands local formats
directly. For example, the local newline
convention for text bodyparts might be carried
through to the encoder itself along with knowledge
of what that format is.
2. The output of the encoders may have to pass
through one or more additional steps prior to
being transmitted as a message. As such, the
output of the encoder may not be compliant with
the formats specified by RFC822. In particular,
once again it may be appropriate for the
converter's output to be expressed using local
newline conventions rather than using the standard
RFC822 CRLF delimiters.
Other implementation variations are conceivable as well.
The only important aspect of this discussion is that the
resulting messages are consistent with those produced by the
model described here.