Network Working Group               N. Borenstein, Bellcore
            Request for Comments: 1341               N. Freed, Innosoft
                                                              June 1992

MIME (Multipurpose Internet Mail Extensions):

Mechanisms for Specifying and Describing the Format of Internet Message Bodies

Status of this Memo

This RFC specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of

            the    "IAB    Official    Protocol   Standards"   for   the
            standardization  state  and   status   of   this   protocol.
            Distribution of this memo is unlimited.


RFC 822 defines a message representation protocol which specifies considerable detail about message headers, but which leaves the message content, or message body, as flat ASCII text. This document redefines the format of message bodies to allow multi-part textual and non-textual message bodies to be represented and exchanged without loss of information. This is based on earlier work documented in RFC 934 and RFC 1049, but extends and revises that work. Because RFC 822 said so little about message bodies, this document is largely orthogonal to (rather than a revision of) RFC 822.

            In  particular,  this  document  is  designed   to   provide
            facilities  to include multiple objects in a single message,
            to represent body text in  character  sets  other  than  US-
            ASCII,  to  represent formatted multi-font text messages, to
            represent non-textual material  such  as  images  and  audio
            fragments,  and  generally  to  facilitate  later extensions
            defining new types of Internet mail for use  by  cooperating
            mail agents.

This document does NOT extend Internet mail header fields to permit anything other than US-ASCII text data. It is recognized that such extensions are necessary, and they are the subject of a companion document [RFC -1342].

A table of contents appears at the end of this document.

            Borenstein & Freed                                  [Page i]

Since its publication in 1982, RFC 822 [RFC-822] has defined
            the   standard  format  of  textual  mail  messages  on  the
            Internet.  Its success has been such that the RFC 822 format
            has  been  adopted,  wholly  or  partially,  well beyond the
            confines of the Internet and  the  Internet  SMTP  transport
            defined  by RFC 821 [RFC-821].  As the format has seen wider
            use,  a  number  of  limitations  have  proven  increasingly
            restrictive for the user community.

RFC 822 was intended to specify a format for text messages. As such, non-text messages, such as multimedia messages that might include audio or images, are simply not mentioned. Even in the case of text, however, RFC 822 is inadequate for the needs of mail users whose languages require the use of character sets richer than US ASCII [US-ASCII]. Since RFC 822 does not specify mechanisms for mail containing audio, video, Asian language text, or even text in most European languages, additional specifications are needed

One of the notable limitations of RFC 821/822 based mail systems is the fact that they limit the contents of electronic mail messages to relatively short lines of seven-bit ASCII. This forces users to convert any non- textual data that they may wish to send into seven-bit bytes representable as printable ASCII characters before invoking a local mail UA (User Agent, a program with which human users send and receive mail). Examples of such encodings currently used in the Internet include pure hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in RFC 1113, the Andrew Toolkit Representation [ATK], and many others.

The limitations of RFC 822 mail become even more apparent as gateways are designed to allow for the exchange of mail messages between RFC 822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the inclusion of non-textual body

            parts  within  electronic  mail   messages.    The   current
            standards  for  the  mapping  of  X.400  messages to RFC 822
            messages specify that either X.400  non-textual  body  parts
            should  be converted to (not encoded in) an ASCII format, or
            that they should be discarded, notifying the  RFC  822  user
            that  discarding has occurred.  This is clearly undesirable,
            as information that a user may  wish  to  receive  is  lost.
            Even  though  a  user's  UA  may  not have the capability of
            dealing with the non-textual body part, the user might  have
            some  mechanism  external  to the UA that can extract useful
            information from the body part.  Moreover, it does not allow
            for  the  fact  that the message may eventually be gatewayed
            back into an X.400 message handling system (i.e., the  X.400
            message  is  "tunneled"  through  Internet  mail), where the
            non-textual  information  would  definitely  become   useful

            Borenstein & Freed                                  [Page 1]

This document describes several mechanisms that combine to solve most of these problems without introducing any serious incompatibilities with the existing world of RFC 822 mail. In particular, it describes:

1. A MIME-Version header field, which uses a version number
to declare a message to be conformant with this specification and allows mail processing agents to distinguish between such messages and those generated by older or non-conformant software, which is presumed to lack such a field.

2. A Content-Type header field, generalized from RFC 1049
[RFC-1049], which can be used to specify the type and subtype of data in the body of a message and to fully specify the native representation (encoding) of such data.

2.a. A "text" Content-Type value, which can be used to
represent textual information in a number of character sets and formatted text description languages in a standardized manner.

2.b. A "multipart" Content-Type value, which can be
used to combine several body parts, possibly of differing types of data, into a single message.

2.c. An "application" Content-Type value, which can be
used to transmit application data or binary data, and hence, among other uses, to implement an electronic mail file transfer service.

2.d. A "message" Content-Type value, for encapsulating
a mail message.

2.e An "image" Content-Type value, for transmitting
still image (picture) data.

2.f. An "audio" Content-Type value, for transmitting
audio or voice data.

2.g. A "video" Content-Type value, for transmitting
video or moving image data, possibly with audio as part of the composite video data format.

3. A Content-Transfer-Encoding header field, which can be
used to specify an auxiliary encoding that was applied to the data in order to allow it to pass through mail transport mechanisms which may have data or character set limitations.

4. Two optional header fields that can be used to further
describe the data in a message body, the Content-ID and Content-Description header fields.

            Borenstein & Freed                                  [Page 2]

MIME has been carefully designed as an extensible mechanism, 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
            including character set names, 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.   Appendix  F provides
            details about how IANA registration is accomplished.

Finally, to specify and promote interoperability, Appendix A of this document provides a basic applicability statement for a subset of the above mechanisms that defines a minimal level of "conformance" with this document.

HISTORICAL NOTE: Several of the mechanisms described in this document may seem somewhat strange or even baroque at first reading. It is important to note that compatibility with existing standards AND robustness across existing practice were two of the highest priorities of the working

            group   that   developed   this  document.   In  particular,
            compatibility was always favored over elegance.

            2    Notations, Conventions, and Generic BNF Grammar

This document is being published in two versions, one as plain ASCII text and one as PostScript. The latter is recommended, though the textual contents are identical. An Andrew-format copy of this document is also available from the first author (Borenstein).

Although the mechanisms specified in this document are all described in prose, most are also described formally in the modified BNF notation of RFC 822. Implementors will need to be familiar with this notation in order to understand this specification, and are referred to RFC 822 for a complete explanation of the modified BNF notation.

Some of the modified BNF in this document makes reference to syntactic entities that are defined in RFC 822 and not in this document. A complete formal grammar, then, is obtained by combining the collected grammar appendix of this document with that of RFC 822.

The term CRLF, in this document, refers to the sequence of the two ASCII characters CR (13) and LF (10) which, taken together, in this order, denote a line break in RFC 822 mail.

The term "character set", wherever it is used in this document, refers to a coded character set, in the sense of ISO character set standardization work, and must not be

            Borenstein & Freed                                  [Page 3]

misinterpreted as meaning "a set of characters."

The term "message", when not further qualified, means either the (complete or "top-level") message being transferred on a network, or a message encapsulated in a body of type "message".

The term "body part", in this document, means one of the parts of the body of a multipart entity. A body part has a header and a body, so it makes sense to speak about the body of a body part.

The term "entity", in this document, means either a message or a body part. All kinds of entities share the property that they have a header and a body.

The term "body", when not further qualified, means the body of an entity, that is the body of either a message or of a body part.

Note : the previous four definitions are clearly circular. This is unavoidable, since the overal structure of a MIME message is indeed recursive.

In this document, all numeric and octet values are given in decimal notation.

It must be noted that Content-Type values, subtypes, and parameter names as defined in this document are case- insensitive. However, parameter values are case-sensitive unless otherwise specified for the specific parameter.

FORMATTING NOTE: This document has been carefully formatted

            for   ease  of  reading.  The  PostScript  version  of  this
            document, in particular, places notes like this  one,  which
            may  be  skipped  by  the  reader, in a smaller, italicized,
            font, and indents it as well.  In the text version, only the
            indentation  is  preserved,  so  if you are reading the text
            version of this you  might  consider  using  the  PostScript
            version  instead.  However,  all such notes will be indented
            and preceded by "NOTE:" or some similar  introduction,  even
            in the text version.

The primary purpose of these non-essential notes is to convey information about the rationale of this document, or

            to  place  this  document  in  the  proper   historical   or
            evolutionary  context.   Such  information may be skipped by
            those who are  focused  entirely  on  building  a  compliant
            implementation,  but  may  be  of  use  to those who wish to
            understand why this document is written as it is.

For ease of recognition, all BNF definitions have been placed in a fixed-width font in the PostScript version of this document.

            Borenstein & Freed                                  [Page 4]

            3    The MIME-Version Header Field

Since RFC 822 was published in 1982, there has really been only one format standard for Internet messages, and there has been little perceived need to declare the format standard in use. This document is an independent document that complements RFC 822. Although the extensions in this document have been defined in such a way as to be compatible with RFC 822, there are still circumstances in which it might be desirable for a mail-processing agent to know whether a message was composed with the new standard in mind.

Therefore, this document defines a new header field, "MIME- Version", which is to be used to declare the version of the Internet message body format standard in use.

Messages composed in accordance with this document MUST include such a header field, with the following verbatim text:

MIME-Version: 1.0

The presence of this header field is an assertion that the message has been composed in compliance with this document.

Since it is possible that a future document might extend the message format standard again, a formal BNF is given for the content of the MIME-Version field:

            MIME-Version := text

Thus, future format specifiers, which might replace or extend "1.0", are (minimally) constrained by the definition of "text", which appears in RFC 822.

Note that the MIME-Version header field is required at the top level of a message. It is not required for each body part of a multipart entity. It is required for the embedded headers of a body of type "message" if and only if the embedded message is itself claimed to be MIME-compliant.

            Borenstein & Freed                                  [Page 5]

            4    The Content-Type Header Field

The purpose of the Content-Type field is to describe the data contained in the body fully enough that the receiving user agent can pick an appropriate agent or mechanism to present the data to the user, or otherwise deal with the data in an appropriate manner.

HISTORICAL NOTE: The Content-Type header field was first defined in RFC 1049. RFC 1049 Content-types used a simpler and less powerful syntax, but one that is largely compatible with the mechanism given here.

The Content-Type header field is used to specify the nature of the data in the body of an entity, by giving type and subtype identifiers, and by providing auxiliary information that may be required for certain types. After the type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute/value notation. The set of meaningful parameters differs for the different types. The ordering of parameters is not significant. Among the defined parameters is a "charset" parameter by which the character set used in the body may be declared. Comments are allowed in accordance with RFC 822 rules for structured header fields.

In general, the top-level Content-Type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Thus, a Content-Type of "image/xyz" is enough to tell a user agent that the data is an image, even if the user agent has no knowledge of the specific image format "xyz". Such information can be used, for example, to decide whether or not to show a user the raw data from an unrecognized subtype -- such an action might be reasonable for unrecognized subtypes of text, but not for unrecognized subtypes of image or audio. For this reason, registered subtypes of audio, image, text, and video, should not contain embedded information that is really of a different type. Such compound types should be represented using the "multipart" or "application" types.

Parameters are modifiers of the content-subtype, and do not fundamentally affect the requirements of the host system. Although most parameters make sense only with certain content-types, others are "global" in the sense that they might apply to any subtype. For example, the "boundary" parameter makes sense only for the "multipart" content-type, but the "charset" parameter might make sense with several content-types.

An initial set of seven Content-Types is defined by this document. This set of top-level names is intended to be substantially complete. It is expected that additions to

            the   larger   set  of  supported  types  can  generally  be

            Borenstein & Freed                                  [Page 6]

accomplished by the creation of new subtypes of these initial types. In the future, more top-level types may be defined only by an extension to this standard. If another primary type is to be used for any reason, it must be given a name starting with "X-" to indicate its non-standard status and to avoid a potential conflict with a future official name.

In the Extended BNF notation of RFC 822, a Content-Type header field value is defined as follows:

            Content-Type := type "/" subtype *[";" parameter]

            type :=          "application"     / "audio"
                      / "image"           / "message"
                      / "multipart"  / "text"
                      / "video"           / x-token

            x-token := <The two characters "X-" followed, with no
                       intervening white space, by any token>

            subtype := token

            parameter := attribute "=" value

            attribute := token

            value := token / quoted-string

            token := 1*<any CHAR except SPACE, CTLs, or tspecials>

            tspecials :=  "(" / ")" / "<" / ">" / "@"  ; Must be in
                       /  "," / ";" / ":" / "\" / <">  ; quoted-string,
                       /  "/" / "[" / "]" / "?" / "."  ; to use within
                       /  "="                        ; parameter values

Note that the definition of "tspecials" is the same as the RFC 822 definition of "specials" with the addition of the three characters "/", "?", and "=".

Note also that a subtype specification is MANDATORY. There are no default subtypes.

            The  type,  subtype,  and  parameter  names  are  not   case
            sensitive.   For  example,  TEXT,  Text,  and  TeXt  are all
            equivalent.  Parameter values are normally  case  sensitive,
            but   certain   parameters   are  interpreted  to  be  case-
            insensitive, depending on the intended use.   (For  example,
            multipart  boundaries  are  case-sensitive, but the "access-
            type" for message/External-body is not case-sensitive.)

Beyond this syntax, the only constraint on the definition of subtype names is the desire that their uses must not conflict. That is, it would be undesirable to have two

            Borenstein & Freed                                  [Page 7]

            different       communities       using       "Content-Type:
            application/foobar"  to  mean  two  different  things.   The
            process  of  defining  new  content-subtypes,  then,  is not
            intended to be a mechanism for  imposing  restrictions,  but
            simply  a  mechanism  for publicizing the usages. There are,
            therefore,  two  acceptable  mechanisms  for  defining   new
            Content-Type subtypes:

1. Private values (starting with "X-") may be
defined bilaterally between two cooperating
                      agents  without   outside   registration   or

2. New standard values must be documented,
registered with, and approved by IANA, as described in Appendix F. Where intended for public use, the formats they refer to must also be defined by a published specification, and possibly offered for standardization.

The seven standard initial predefined Content-Types are detailed in the bulk of this document. They are:

text -- textual information. The primary subtype, "plain", indicates plain (unformatted) text. No special software is required to get the full meaning of the text, aside from support for the indicated character set. Subtypes are to be used for enriched text in forms where application software may enhance the appearance of the text, but such software must not be required in order to get the general idea of the content. Possible subtypes thus include any readable word processor format. A very simple and portable subtype, richtext, is defined in this document. multipart -- data consisting of multiple parts of independent data types. Four initial subtypes

                      are  defined,  including   the   primary   "mixed"
                      subtype,  "alternative"  for representing the same
                      data in multiple  formats,  "parallel"  for  parts
                      intended to be viewed simultaneously, and "digest"
                      for multipart entities in which each  part  is  of
                      type "message".
                 message  --  an  encapsulated  message.   A   body   of
                      Content-Type "message" is itself a fully formatted
                      RFC 822 conformant message which may  contain  its
                      own  different  Content-Type  header  field.   The
                      primary  subtype  is  "rfc822".    The   "partial"
                      subtype is defined for partial messages, to permit
                      the fragmented transmission  of  bodies  that  are
                      thought  to be too large to be passed through mail
                      transport    facilities.      Another     subtype,
                      "External-body",  is  defined for specifying large
                      bodies by reference to an external data source.

            Borenstein & Freed                                  [Page 8]

image -- image data. Image requires a display device (such as a graphical display, a printer, or a FAX
                      machine)  to  view   the   information.    Initial
                      subtypes  are  defined  for  two widely-used image
                      formats, jpeg and gif.
                 audio --  audio data,  with  initial  subtype  "basic".
                      Audio  requires  an audio output device (such as a
                      speaker or a telephone) to "display" the contents.
                 video --  video data.  Video requires the capability to
                      display   moving   images,   typically   including
                      specialized hardware and  software.   The  initial
                      subtype is "mpeg".
                 application --  some  other  kind  of  data,  typically
                      either uninterpreted binary data or information to
                      be processed by  a  mail-based  application.   The
                      primary  subtype, "octet-stream", is to be used in
                      the case of uninterpreted binary  data,  in  which
                      case  the  simplest recommended action is to offer
                      to write the information into a file for the user.
                      Two  additional  subtypes, "ODA" and "PostScript",
                      are defined for transporting  ODA  and  PostScript
                      documents  in  bodies.   Other  expected  uses for
                      "application"  include  spreadsheets,   data   for
                      mail-based  scheduling  systems, and languages for
                      "active" (computational) email.  (Note that active
                      email   entails   several  securityconsiderations,
                      which  are   discussed   later   in   this   memo,
                      particularly      in      the      context      of

Default RFC 822 messages are typed by this protocol as plain text in the US-ASCII character set, which can be explicitly specified as "Content-type: text/plain; charset=us-ascii". If no Content-Type is specified, either by error or by an older user agent, this default is assumed. In the presence of a MIME-Version header field, a receiving User Agent can also assume that plain US-ASCII text was the sender's intent. In the absence of a MIME-Version specification, plain US-ASCII text must still be assumed, but the sender's intent might have been otherwise.

RATIONALE: In the absence of any Content-Type header field or MIME-Version header field, it is impossible to be certain that a message is actually text in the US-ASCII character set, since it might well be a message that, using the conventions that predate this document, includes text in another character set or non-textual data in a manner that

            cannot  be  automatically  recognized  (e.g.,  a   uuencoded
            compressed  UNIX  tar  file).  Although  there  is  no fully
            acceptable alternative to treating such untyped messages  as
            "text/plain;  charset=us-ascii",  implementors should remain
            aware that if a message lacks both the MIME-Version and  the
            Content-Type  header  fields,  it  may  in  practice contain
            almost anything.

            Borenstein & Freed                                  [Page 9]

It should be noted that the list of Content-Type values given here may be augmented in time, via the mechanisms described above, and that the set of subtypes is expected to grow substantially.

When a mail reader encounters mail with an unknown Content- type value, it should generally treat it as equivalent to

            "application/octet-stream",  as  described  later  in   this

            5    The Content-Transfer-Encoding Header Field

Many Content-Types which could usefully be transported via email are represented, in their "natural" format, as 8-bit character or binary data. Such data cannot be transmitted

            over   some  transport  protocols.   For  example,  RFC  821
            restricts mail messages to 7-bit  US-ASCII  data  with  1000
            character lines.

It is necessary, therefore, to define a standard mechanism for re-encoding such data into a 7-bit short-line format.

            This  document  specifies  that  such  encodings   will   be
            indicated by a new "Content-Transfer-Encoding" header field.
            The Content-Transfer-Encoding field is used to indicate  the
            type  of  transformation  that  has  been  used  in order to
            represent the body in an acceptable manner for transport.

Unlike Content-Types, a proliferation of Content-Transfer- Encoding values is undesirable and unnecessary. However,

            establishing   only   a   single   Content-Transfer-Encoding
            mechanism  does  not  seem  possible.    There is a tradeoff
            between the desire for a compact and efficient  encoding  of
            largely-binary  data  and the desire for a readable encoding
            of data that is mostly, but not entirely, 7-bit  data.   For
            this reason, at least two encoding mechanisms are necessary:
            a "readable" encoding and a "dense" encoding.

The Content-Transfer-Encoding field is designed to specify an invertible mapping between the "native" representation of a type of data and a representation that can be readily exchanged using 7 bit mail transport protocols, such as those defined by RFC 821 (SMTP). This field has not been defined by any previous standard. The field's value is a single token specifying the type of encoding, as enumerated below. Formally:

            Content-Transfer-Encoding := "BASE64" / "QUOTED-PRINTABLE" /
                                         "8BIT"   / "7BIT" /
                                         "BINARY" / x-token

These values are not case sensitive. That is, Base64 and BASE64 and bAsE64 are all equivalent. An encoding type of 7BIT requires that the body is already in a seven-bit mail- ready representation. This is the default value -- that is,

            Borenstein & Freed                                 [Page 10]

            "Content-Transfer-Encoding:  7BIT"   is   assumed   if   the
            Content-Transfer-Encoding header field is not present.

The values "8bit", "7bit", and "binary" all imply that NO encoding has been performed. However, they are potentially useful as indications of the kind of data contained in the object, and therefore of the kind of encoding that might need to be performed for transmission in a given transport system. "7bit" means that the data is all represented as short lines of US-ASCII data. "8bit" means that the lines are short, but there may be non-ASCII characters (octets with the high-order bit set). "Binary" means that not only may non-ASCII characters be present, but also that the lines are not necessarily short enough for SMTP transport.

The difference between "8bit" (or any other conceivable bit-width token) and the "binary" token is that "binary" does not require adherence to any limits on line length or to the SMTP CRLF semantics, while the bit-width tokens do require such adherence. If the body contains data in any

            bit-width   other  than  7-bit,  the  appropriate  bit-width
            Content-Transfer-Encoding token must be used  (e.g.,  "8bit"
            for unencoded 8 bit wide data).  If the body contains binary
            data, the "binary" Content-Transfer-Encoding token  must  be

NOTE: The distinction between the Content-Transfer-Encoding values of "binary," "8bit," etc. may seem unimportant, in that all of them really mean "none" -- that is, there has been no encoding of the data for transport. However, clear labeling will be of enormous value to gateways between future mail transport systems with differing capabilities in transporting data that do not meet the restrictions of RFC 821 transport.

As of the publication of this document, there are no standardized Internet transports for which it is legitimate to include unencoded 8-bit or binary data in mail bodies. Thus there are no circumstances in which the "8bit" or "binary" Content-Transfer-Encoding is actually legal on the Internet. However, in the event that 8-bit or binary mail transport becomes a reality in Internet mail, or when this document is used in conjunction with any other 8-bit or binary-capable transport mechanism, 8-bit or binary bodies should be labeled as such using this mechanism.

NOTE: The five values defined for the Content-Transfer- Encoding field imply nothing about the Content-Type other than the algorithm by which it was encoded or the transport system requirements if unencoded.

            Implementors  may,  if  necessary,   define   new   Content-
            Transfer-Encoding  values, but must use an x-token, which is
            a name prefixed by "X-" to indicate its non-standard status,

            Borenstein & Freed                                 [Page 11]

            e.g.,    "Content-Transfer-Encoding:     x-my-new-encoding".
            However, unlike Content-Types and subtypes, the creation  of
            new   Content-Transfer-Encoding  values  is  explicitly  and
            strongly  discouraged,  as  it  seems   likely   to   hinder
            interoperability  with  little potential benefit.  Their use
            is allowed only  as  the  result  of  an  agreement  between
            cooperating user agents.

If a Content-Transfer-Encoding header field appears as part of a message header, it applies to the entire body of that

            message.   If  a  Content-Transfer-Encoding   header   field
            appears as part of a body part's headers, it applies only to
            the body of that  body  part.   If  an  entity  is  of  type
            "multipart"  or  "message", the Content-Transfer-Encoding is
            not permitted to have any  value  other  than  a  bit  width
            (e.g., "7bit", "8bit", etc.) or "binary".

It should be noted that email is character-oriented, so that the mechanisms described here are mechanisms for encoding arbitrary byte streams, not bit streams. If a bit stream is to be encoded via one of these mechanisms, it must first be converted to an 8-bit byte stream using the network standard bit order ("big-endian"), in which the earlier bits in a stream become the higher-order bits in a byte. A bit stream not ending at an 8-bit boundary must be padded with zeroes. This document provides a mechanism for noting the addition of such padding in the case of the application Content-Type, which has a "padding" parameter.

The encoding mechanisms defined here explicitly encode all data in ASCII. Thus, for example, suppose an entity has header fields such as:

Content-Type: text/plain; charset=ISO-8859-1 Content-transfer-encoding: base64

This should be interpreted to mean that the body is a base64 ASCII encoding of data that was originally in ISO-8859-1, and will be in that character set again after decoding.

The following sections will define the two standard encoding

            mechanisms.    The   definition   of  new  content-transfer-
            encodings is explicitly discouraged and  should  only  occur
            when  absolutely  necessary.   All content-transfer-encoding
            namespace except that  beginning  with  "X-"  is  explicitly
            reserved  to  the  IANA  for future use.  Private agreements
            about   content-transfer-encodings   are   also   explicitly

Certain Content-Transfer-Encoding values may only be used on certain Content-Types. In particular, it is expressly forbidden to use any encodings other than "7bit", "8bit", or "binary" with any Content-Type that recursively includes

            other Content-Type  fields,   notably  the  "multipart"  and

            Borenstein & Freed                                 [Page 12]

"message" Content-Types. All encodings that are desired for bodies of type multipart or message must be done at the innermost level, by encoding the actual body that needs to be encoded.

            NOTE  ON  ENCODING  RESTRICTIONS:   Though  the  prohibition
            against  using  content-transfer-encodings  on  data of type
            multipart or message may  seem  overly  restrictive,  it  is
            necessary  to  prevent  nested  encodings, in which data are
            passed through an encoding  algorithm  multiple  times,  and
            must  be  decoded  multiple  times  in  order to be properly
            viewed.  Nested encodings  add  considerable  complexity  to
            user  agents:   aside  from  the obvious efficiency problems
            with such multiple encodings, they  can  obscure  the  basic
            structure  of a message.  In particular, they can imply that
            several decoding operations are necessary simply to find out
            what  types  of  objects a message contains.  Banning nested
            encodings may complicate the job of certain  mail  gateways,
            but  this  seems less of a problem than the effect of nested
            encodings on user agents.


            TRANSFER-ENCODING:   It  may seem that the Content-Transfer-
            Encoding could be inferred from the characteristics  of  the
            Content-Type  that  is to be encoded, or, at the very least,
            that certain Content-Transfer-Encodings  could  be  mandated
            for  use  with  specific  Content-Types.  There  are several
            reasons why this is not the case. First, given  the  varying
            types  of  transports  used  for mail, some encodings may be
            appropriate for some Content-Type/transport combinations and
            not  for  others.  (For  example, in an  8-bit transport, no
            encoding would be required for  text  in  certain  character
            sets,  while  such  encodings are clearly required for 7-bit
            SMTP.)  Second, certain Content-Types may require  different
            types  of  transfer  encoding under different circumstances.
            For example, many PostScript bodies might  consist  entirely
            of  short lines of 7-bit data and hence require little or no
            encoding. Other PostScript bodies  (especially  those  using
            Level  2 PostScript's binary encoding mechanism) may only be
            reasonably represented using a  binary  transport  encoding.
            Finally,  since Content-Type is intended to be an open-ended
            specification  mechanism,   strict   specification   of   an
            association  between Content-Types and encodings effectively
            couples the specification of an application protocol with  a
            specific  lower-level transport. This is not desirable since
            the developers of a Content-Type should not have to be aware
            of all the transports in use and what their limitations are.

            NOTE ON TRANSLATING  ENCODINGS:   The  quoted-printable  and
            base64  encodings  are  designed  so that conversion between
            them is possible. The only  issue  that  arises  in  such  a
            conversion  is  the handling of line breaks. When converting
            from  quoted-printable  to  base64  a  line  break  must  be
            converted  into  a CRLF sequence. Similarly, a CRLF sequence

            Borenstein & Freed                                 [Page 13]

in base64 data should be converted to a quoted-printable line break, but ONLY when converting text data.

            NOTE  ON  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  as
            Appendix H.

5.1 Quoted-Printable Content-Transfer-Encoding

The Quoted-Printable encoding is intended to represent data that largely consists of octets that correspond to printable characters in the ASCII character set. It encodes the data in such a way that the resulting octets are unlikely to be modified by mail transport. If the data being encoded are mostly ASCII text, the encoded form of the data remains largely recognizable by humans. A body which is entirely ASCII may also be encoded in Quoted-Printable to ensure the integrity of the data should the message pass through a character-translating, and/or line-wrapping gateway.

In this encoding, octets are to be represented as determined by the following rules:

Rule #1: (General 8-bit representation) Any octet, except those indicating a line break according to the newline convention of the canonical form of the data being encoded, may be represented by an "=" followed by a two digit hexadecimal representation of the octet's value. The digits of the hexadecimal alphabet, for this purpose, are "0123456789ABCDEF". Uppercase letters must be
used when sending hexadecimal data, though a robust

                 implementation   may   choose  to  recognize  lowercase
                 letters on receipt. Thus, for  example,  the  value  12
                 (ASCII  form feed) can be represented by "=0C", and the
                 value 61 (ASCII  EQUAL  SIGN)  can  be  represented  by
                 "=3D".   Except  when  the  following  rules  allow  an
                 alternative encoding, this rule is mandatory.

Rule #2: (Literal representation) Octets with decimal values of 33 through 60 inclusive, and 62 through 126, inclusive, MAY be represented as the ASCII characters which correspond to those octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN through TILDE, respectively).

Rule #3: (White Space): Octets with values of 9 and 32 MAY be represented as ASCII TAB (HT) and SPACE

                 characters,  respectively,   but   MUST   NOT   be   so

            Borenstein & Freed                                 [Page 14]

represented at the end of an encoded line. Any TAB (HT) or SPACE characters on an encoded line MUST thus be followed on that line by a printable character. In particular, an "=" at the end of an encoded line, indicating a soft line break (see rule #5) may follow one or more TAB (HT) or SPACE characters. It follows that an octet with value 9 or 32 appearing at the end of an encoded line must be represented according to Rule #1. This rule is necessary because some MTAs (Message Transport Agents, programs which transport messages from one user to another, or perform a part of such transfers) are known to pad lines of text with SPACEs, and others are known to remove "white space" characters from the end of a line. Therefore, when decoding a Quoted-Printable body, any trailing white space on a line must be deleted, as it will necessarily have been added by intermediate transport agents.

Rule #4 (Line Breaks): A line break in a text body

                 part,   independent   of  what  its  representation  is
                 following the  canonical  representation  of  the  data
                 being  encoded, must be represented by a (RFC 822) line
                 break,  which  is  a  CRLF  sequence,  in  the  Quoted-
                 Printable  encoding.  If isolated CRs and LFs, or LF CR
                 and CR LF sequences are allowed  to  appear  in  binary
                 data  according  to  the  canonical  form, they must be
                 represented   using  the  "=0D",  "=0A",  "=0A=0D"  and
                 "=0D=0A" notations respectively.

Note that many implementation may elect to encode the local representation of various content types directly. In particular, this may apply to plain text material on systems that use newline conventions other than CRLF delimiters. Such an implementation is permissible, but the generation of line breaks must be generalized to account for the case where alternate representations of newline sequences are used.

                 Rule  #5  (Soft  Line  Breaks):  The   Quoted-Printable
                 encoding REQUIRES that encoded lines be no more than 76
                 characters long. If longer lines are to be encoded with
                 the  Quoted-Printable encoding, 'soft' line breaks must
                 be used. An equal sign  as  the  last  character  on  a
                 encoded  line indicates such a non-significant ('soft')
                 line break in the encoded text. Thus if the "raw"  form
                 of the line is a single unencoded line that says:

Now's the time for all folk to come to the aid of their country.

                 This  can  be  represented,  in  the   Quoted-Printable
                 encoding, as

            Borenstein & Freed                                 [Page 15]

Now's the time =
for all folk to come=
to the aid of their country.

This provides a mechanism with which long lines are encoded in such a way as to be restored by the user agent. The 76 character limit does not count the

                 trailing   CRLF,   but  counts  all  other  characters,
                 including any equal signs.

Since the hyphen character ("-") is represented as itself in the Quoted-Printable encoding, care must be taken, when encapsulating a quoted-printable encoded body in a multipart entity, to ensure that the encapsulation boundary does not appear anywhere in the encoded body. (A good strategy is to choose a boundary that includes a character sequence such as "=_" which can never appear in a quoted-printable body. See

            the   definition   of   multipart  messages  later  in  this

NOTE: The quoted-printable encoding represents something of

            a   compromise   between   readability  and  reliability  in
            transport.   Bodies  encoded   with   the   quoted-printable
            encoding will work reliably over most mail gateways, but may
            not work  perfectly  over  a  few  gateways,  notably  those
            involving  translation  into  EBCDIC.  (In theory, an EBCDIC
            gateway could decode a quoted-printable body  and  re-encode
            it  using  base64,  but  such gateways do not yet exist.)  A
            higher  level  of  confidence  is  offered  by  the   base64
            Content-Transfer-Encoding.  A way to get reasonably reliable
            transport through EBCDIC gateways is to also quote the ASCII


according to rule #1. See Appendix B for more information.

Because quoted-printable data is generally assumed to be line-oriented, it is to be expected that the breaks between the lines of quoted printable data may be altered in transport, in the same manner that plain text mail has always been altered in Internet mail when passing between

            systems   with   differing  newline  conventions.   If  such
            alterations are likely to constitute  a  corruption  of  the
            data,  it  is  probably  more  sensible  to  use  the base64
            encoding rather than the quoted-printable encoding.

            Borenstein & Freed                                 [Page 16]

5.2 Base64 Content-Transfer-Encoding

            The  Base64   Content-Transfer-Encoding   is   designed   to
            represent  arbitrary  sequences  of octets in a form that is
            not humanly readable.  The encoding and decoding  algorithms
            are simple, but the encoded data are consistently only about
            33 percent larger than the unencoded data.  This encoding is
            based on the one used in Privacy Enhanced Mail applications,
            as defined in RFC 1113.   The  base64  encoding  is  adapted
            from  RFC  1113, with one change:  base64 eliminates the "*"
            mechanism for embedded clear text.

A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, "=", is used to signify a special processing function.)

NOTE: This subset has the important property that it is

            represented   identically   in  all  versions  of  ISO  646,
            including US ASCII, and all characters  in  the  subset  are
            also  represented  identically  in  all  versions of EBCDIC.
            Other popular encodings, such as the encoding  used  by  the
            UUENCODE  utility  and the base85 encoding specified as part
            of Level 2 PostScript, do not share  these  properties,  and
            thus  do  not  fulfill the portability requirements a binary
            transport encoding for mail must meet.

The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from

            left  to  right,  a  24-bit  input  group   is   formed   by
            concatenating  3  8-bit input groups. These 24 bits are then
            treated as 4 concatenated 6-bit groups,  each  of  which  is
            translated  into a single digit in the base64 alphabet. When
            encoding a bit stream  via  the  base64  encoding,  the  bit
            stream  must  be  presumed  to  be  ordered  with  the most-
            significant-bit first.  That is, the first bit in the stream
            will be the high-order bit in the first byte, and the eighth
            bit will be the low-order bit in the first byte, and so on.

Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string. These characters, identified in Table 1, below, are selected so as to be universally

            representable,  and  the  set   excludes   characters   with
            particular  significance to SMTP (e.g., ".", "CR", "LF") and
            to the encapsulation boundaries  defined  in  this  document
            (e.g., "-").

            Borenstein & Freed                                 [Page 17]

Table 1: The Base64 Alphabet

               Value Encoding  Value  Encoding   Value  Encoding   Value
                   0 A            17 R            34 i            51 z
                   1 B            18 S            35 j            52 0
                   2 C            19 T            36 k            53 1
                   3 D            20 U            37 l            54 2
                   4 E            21 V            38 m            55 3
                   5 F            22 W            39 n            56 4
                   6 G            23 X            40 o            57 5
                   7 H            24 Y            41 p            58 6
                   8 I            25 Z            42 q            59 7
                   9 J            26 a            43 r            60 8
                  10 K            27 b            44 s            61 9
                  11 L            28 c            45 t            62 +
                  12 M            29 d            46 u            63 /
                  13 N            30 e            47 v
                  14 O            31 f            48 w         (pad) =
                  15 P            32 g            49 x
                  16 Q            33 h            50 y

The output stream (encoded bytes) must be represented in lines of no more than 76 characters each. All line breaks or other characters not found in Table 1 must be ignored by decoding software. In base64 data, characters other than those in Table 1, line breaks, and other white space probably indicate a transmission error, about which a warning message or even a message rejection might be appropriate under some circumstances.

Special processing is performed if fewer than 24 bits are available at the end of the data being encoded. A full encoding quantum is always completed at the end of a body. When fewer than 24 input bits are available in an input group, zero bits are added (on the right) to form an integral number of 6-bit groups. Output character positions which are not required to represent actual input data are set to the character "=". Since all base64 input is an integral number of octets, only the following cases can arise: (1) the final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output will be an integral multiple of 4 characters with no "=" padding, (2) the final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output

            will  be  two  characters  followed  by  two   "="   padding
            characters,  or  (3)  the final quantum of encoding input is
            exactly 16 bits; here, the final unit of encoded output will
            be three characters followed by one "=" padding character.

Care must be taken to use the proper octets for line breaks if base64 encoding is applied directly to text material that has not been converted to canonical form. In particular, text line breaks should be converted into CRLF sequences

            Borenstein & Freed                                 [Page 18]

prior to base64 encoding. The important thing to note is that this may be done directly by the encoder rather than in a prior canonicalization step in some implementations.

NOTE: There is no need to worry about quoting apparent encapsulation boundaries within base64-encoded parts of multipart entities because no hyphen characters are used in the base64 encoding.

            6    Additional Optional Content- Header Fields

6.1 Optional Content-ID Header Field

In constructing a high-level user agent, it may be desirable

            to   allow   one   body   to   make  reference  to  another.
            Accordingly, bodies may be labeled  using  the  "Content-ID"
            header  field,  which  is  syntactically  identical  to  the
            "Message-ID" header field:

            Content-ID := msg-id

Like the Message-ID values, Content-ID values must be generated to be as unique as possible.

6.2 Optional Content-Description Header Field

The ability to associate some descriptive information with a given body is often desirable. For example, it may be useful to mark an "image" body as "a picture of the Space Shuttle

            Endeavor."    Such  text  may  be  placed  in  the  Content-
            Description header field.

            Content-Description := *text

The description is presumed to be given in the US-ASCII character set, although the mechanism specified in [RFC- 1342] may be used for non-US-ASCII Content-Description values.

            Borenstein & Freed                                 [Page 19]

            7    The Predefined Content-Type Values

This document defines seven initial Content-Type values and an extension mechanism for private or experimental types. Further standard types must be defined by new published specifications. It is expected that most innovation in new types of mail will take place as subtypes of the seven types defined here. The most essential characteristics of the seven content-types are summarized in Appendix G.

7.1 The Text Content-Type

The text Content-Type is intended for sending material which is principally textual in form. It is the default Content- Type. A "charset" parameter may be used to indicate the character set of the body text. The primary subtype of text is "plain". This indicates plain (unformatted) text. The default Content-Type for Internet mail is "text/plain; charset=us-ascii".

Beyond plain text, there are many formats for representing what might be known as "extended text" -- text with embedded formatting and presentation information. An interesting characteristic of many such representations is that they are to some extent readable even without the software that interprets them. It is useful, then, to distinguish them, at the highest level, from such unreadable data as images, audio, or text represented in an unreadable form. In the

            absence  of  appropriate  interpretation  software,  it   is
            reasonable to show subtypes of text to the user, while it is
            not reasonable to do so with most nontextual data.

Such formatted textual data should be represented using subtypes of text. Plausible subtypes of text are typically given by the common name of the representation format, e.g., "text/richtext".

            7.1.1     The charset parameter

A critical parameter that may be specified in the Content- Type field for text data is the character set. This is specified with a "charset" parameter, as in:

Content-type: text/plain; charset=us-ascii

Unlike some other parameter values, the values of the charset parameter are NOT case sensitive. The default character set, which must be assumed in the absence of a charset parameter, is US-ASCII.

An initial list of predefined character set names can be found at the end of this section. Additional character sets may be registered with IANA as described in Appendix F, although the standardization of their use requires the usual

            Borenstein & Freed                                 [Page 20]

            IAB  review  and  approval.  Note  that  if  the   specified
            character  set  includes  8-bit  data,  a  Content-Transfer-
            Encoding header field and a corresponding  encoding  on  the
            data  are  required  in  order to transmit the body via some
            mail transfer protocols, such as SMTP.

The default character set, US-ASCII, has been the subject of some confusion and ambiguity in the past. Not only were there some ambiguities in the definition, there have been wide variations in practice. In order to eliminate such ambiguity and variations in the future, it is strongly recommended that new user agents explicitly specify a character set via the Content-Type header field. "US-ASCII" does not indicate an arbitrary seven-bit character code, but specifies that the body uses character coding that uses the exact correspondence of codes to characters specified in ASCII. National use variations of ISO 646 [ISO-646] are NOT

            ASCII   and   their  use  in  Internet  mail  is  explicitly
            discouraged. The omission of the ISO 646  character  set  is
            deliberate  in  this regard.  The character set name of "US-
            ASCII" explicitly refers  to ANSI X3.4-1986 [US-ASCII] only.
            The  character  set name "ASCII" is reserved and must not be
            used for any purpose.

NOTE: RFC 821 explicitly specifies "ASCII", and references an earlier version of the American Standard. Insofar as one of the purposes of specifying a Content-Type and character set is to permit the receiver to unambiguously determine how the sender intended the coded message to be interpreted, assuming anything other than "strict ASCII" as the default would risk unintentional and incompatible changes to the

            semantics  of  messages  now being transmitted.    This also
            implies that messages containing characters coded  according
            to  national  variations on ISO 646, or using code-switching
            procedures (e.g., those of ISO 2022), as well  as  8-bit  or
            multiple   octet character encodings MUST use an appropriate
            character set  specification  to  be  consistent  with  this

The complete US-ASCII character set is listed in [US-ASCII]. Note that the control characters including DEL (0-31, 127) have no defined meaning apart from the combination CRLF (ASCII values 13 and 10) indicating a new line. Two of the characters have de facto meanings in wide use: FF (12) often means "start subsequent text on the beginning of a new page"; and TAB or HT (9) often (though not always) means "move the cursor to the next available column after the current position where the column number is a multiple of 8 (counting the first column as column 0)." Apart from this, any use of the control characters or DEL in a body must be

            part   of   a  private  agreement  between  the  sender  and
            recipient.  Such  private  agreements  are  discouraged  and
            should  be  replaced  by  the  other  capabilities  of  this

            Borenstein & Freed                                 [Page 21]

            NOTE:   Beyond  US-ASCII,  an  enormous   proliferation   of
            character  sets  is  possible. It is the opinion of the IETF
            working group that a large number of character sets is NOT a
            good  thing.   We would prefer to specify a single character
            set that can be used universally for representing all of the
            world's   languages   in  electronic  mail.   Unfortunately,
            existing practice in several communities seems to  point  to
            the  continued  use  of  multiple character sets in the near
            future.  For this reason, we define names for a small number
            of  character  sets  for  which  a  strong  constituent base
            exists.    It is our hope  that  ISO  10646  or  some  other
            effort  will  eventually define a single world character set
            which can then be specified for use in Internet mail, but in
            the  advance of that definition we cannot specify the use of
            ISO  10646,  Unicode,  or  any  other  character  set  whose
            definition is, as of this writing, incomplete.

The defined charset values are:

US-ASCII -- as defined in [US-ASCII].

ISO-8859-X -- where "X" is to be replaced, as necessary, for the parts of ISO-8859 [ISO- 8859]. Note that the ISO 646 character sets have deliberately been omitted in favor of

                      their  8859  replacements,  which   are   the
                      designated  character sets for Internet mail.
                      As of the publication of this  document,  the
                      legitimate  values  for  "X" are the digits 1
                      through 9.

Note that the character set used, if anything other than

            US-ASCII,   must  always  be  explicitly  specified  in  the
            Content-Type field.

No other character set name may be used in Internet mail without the publication of a formal specification and its registration with IANA as described in Appendix F, or by private agreement, in which case the character set name must begin with "X-".

Implementors are discouraged from defining new character sets for mail use unless absolutely necessary.

The "charset" parameter has been defined primarily for the purpose of textual data, and is described in this section for that reason. However, it is conceivable that non- textual data might also wish to specify a charset value for some purpose, in which case the same syntax and values should be used.

In general, mail-sending software should always use the "lowest common denominator" character set possible. For example, if a body contains only US-ASCII characters, it

            Borenstein & Freed                                 [Page 22]

should be marked as being in the US-ASCII character set, not ISO-8859-1, which, like all the ISO-8859 family of character sets, is a superset of US-ASCII. More generally, if a widely-used character set is a subset of another character set, and a body contains only characters in the widely-used subset, it should be labeled as being in that subset. This will increase the chances that the recipient will be able to view the mail correctly.

            7.1.2     The Text/plain subtype

            The primary subtype of text   is  "plain".   This  indicates
            plain  (unformatted)  text.  The  default  Content-Type  for
            Internet  mail,  "text/plain;  charset=us-ascii",  describes
            existing  Internet practice, that is, it is the type of body
            defined by RFC 822.

7.1.3 The Text/richtext subtype

In order to promote the wider interoperability of simple formatted text, this document defines an extremely simple subtype of "text", the "richtext" subtype. This subtype was designed to meet the following criteria:

1. The syntax must be extremely simple to parse,
so that even teletype-oriented mail systems can easily strip away the formatting information and leave only the readable text.

2. The syntax must be extensible to allow for new
formatting commands that are deemed essential.

3. The capabilities must be extremely limited, to
ensure that it can represent no more than is likely to be representable by the user's primary word processor. While this limits what can be sent, it increases the likelihood that what is sent can be properly displayed.

4. The syntax must be compatible with SGML, so
that, with an appropriate DTD (Document Type Definition, the standard mechanism for defining a document type using SGML), a general SGML parser could be made to parse richtext. However, despite this compatibility, the syntax should be far simpler than full SGML, so that no SGML knowledge is required in order to implement it.

The syntax of "richtext" is very simple. It is assumed, at the top-level, to be in the US-ASCII character set, unless of course a different charset parameter was specified in the Content-type field. All characters represent themselves, with the exception of the "<" character (ASCII 60), which is

            used   to  mark  the  beginning  of  a  formatting  command.

            Borenstein & Freed                                 [Page 23]

            Formatting  instructions  consist  of  formatting   commands
            surrounded  by angle brackets ("<>", ASCII 60 and 62).  Each
            formatting command may be no  more  than  40  characters  in
            length,  all in US-ASCII, restricted to the alphanumeric and
            hyphen ("-") characters. Formatting commands may be preceded
            by  a  forward slash or solidus ("/", ASCII 47), making them
            negations, and such negations must always exist  to  balance
            the  initial opening commands, except as noted below.  Thus,
            if the formatting command "<bold>" appears  at  some  point,
            there  must  later  be a "</bold>" to balance it.  There are
            only three exceptions to this "balancing" rule:  First,  the
            command "<lt>" is used to represent a literal "<" character.
            Second, the command "<nl>" is used to represent  a  required
            line  break.   (Otherwise,  CRLFs in the data are treated as
            equivalent to  a  single  SPACE  character.)   Finally,  the
            command  "<np>"  is  used to represent a page break.  (NOTE:
            The 40 character  limit  on  formatting  commands  does  not
            include  the  "<",  ">",  or  "/"  characters  that might be
            attached to such commands.)

Initially defined formatting commands, not all of which will be implemented by all richtext implementations, include:

Bold -- causes the subsequent text to be in a bold font.
Italic -- causes the subsequent text to be in an italic font.
Fixed -- causes the subsequent text to be in a fixed width font.
Smaller -- causes the subsequent text to be in a smaller font.
Bigger -- causes the subsequent text to be in a bigger font.

                 Underline  --  causes  the  subsequent   text   to   be
                 Center -- causes the subsequent text to be centered.
                 FlushLeft -- causes the  subsequent  text  to  be  left
                 FlushRight -- causes the subsequent text  to  be  right
                 Indent -- causes the subsequent text to be indented  at
                      the left margin.
                 IndentRight  --  causes  the  subsequent  text  to   be
                      indented at the right margin.
                 Outdent -- causes the subsequent text to  be  outdented
                      at the left margin.
                 OutdentRight  --  causes  the  subsequent  text  to  be
                      outdented at the right margin.
                 SamePage -- causes the subsequent text to  be  grouped,
                      if possible, on one page.
                 Subscript  --  causes  the  subsequent   text   to   be
                      interpreted as a subscript.

            Borenstein & Freed                                 [Page 24]

                 Superscript  --  causes  the  subsequent  text  to   be
                      interpreted as a superscript.
                 Heading -- causes the subsequent text to be interpreted
                      as a page heading.
                 Footing -- causes the subsequent text to be interpreted
                      as a page footing.
                 ISO-8859-X  (for any value of X  that  is  legal  as  a
                      "charset" parameter) -- causes the subsequent text
                      to be  interpreted  as  text  in  the  appropriate
                      character set.
                 US-ASCII  --  causes  the   subsequent   text   to   be
                      interpreted as text in the US-ASCII character set.
                 Excerpt -- causes the subsequent text to be interpreted
                      as   a   textual   excerpt  from  another  source.
                      Typically this will be displayed using indentation
                      and  an  alternate font, but such decisions are up
                      to the viewer.
                 Paragraph  --  causes  the  subsequent   text   to   be
                      interpreted    as   a   single   paragraph,   with
                      appropriate  paragraph  breaks  (typically   blank
                      space) before and after.
                 Signature  --  causes  the  subsequent   text   to   be
                      interpreted  as  a  "signature".  Some systems may
                      wish to display signatures in a  smaller  font  or
                      otherwise set them apart from the main text of the
                 Comment -- causes the subsequent text to be interpreted
                      as a comment, and hence not shown to the reader.
                 No-op -- has no effect on the subsequent text.
                 lt -- <lt> is replaced by a literal "<" character.   No
                      balancing </lt> is allowed.
                 nl -- <nl> causes a line break.  No balancing </nl>  is
                 np -- <np> causes a page break.  No balancing </np>  is

Each positive formatting command affects all subsequent text until the matching negative formatting command. Such pairs of formatting commands must be properly balanced and nested. Thus, a proper way to describe text in bold italics is:


or, alternately,


but, in particular, the following is illegal richtext:


            NOTE:   The  nesting  requirement  for  formatting  commands
            imposes  a  slightly  higher  burden  upon  the composers of

            Borenstein & Freed                                 [Page 25]

            richtext  bodies,  but   potentially   simplifies   richtext
            displayers  by  allowing  them  to be stack-based.  The main
            goal of richtext is to be simple enough to  make  multifont,
            formatted  email  widely  readable,  so  that those with the
            capability of  sending  it  will  be  able  to  do  so  with
            confidence.   Thus  slightly  increased  complexity  in  the
            composing software was  deemed  a  reasonable  tradeoff  for
            simplified  reading  software.  Nonetheless, implementors of
            richtext  readers  are  encouraged  to  follow  the  general
            Internet  guidelines  of being conservative in what you send
            and liberal in what you accept.  Those implementations  that
            can  do so are encouraged to deal reasonably with improperly
            nested richtext.

Implementations must regard any unrecognized formatting command as equivalent to "No-op", thus facilitating future extensions to "richtext". Private extensions may be defined using formatting commands that begin with "X-", by analogy to Internet mail header field names.

It is worth noting that no special behavior is required for the TAB (HT) character. It is recommended, however, that, at

            least  when  fixed-width  fonts  are  in  use,  the   common
            semantics  of  the  TAB  (HT)  character should be observed,
            namely that it moves to the next column position that  is  a
            multiple  of  8.   (In  other words, if a TAB (HT) occurs in
            column n, where the leftmost column is column 0,  then  that
            TAB   (HT)   should   be  replaced  by  8-(n  mod  8)  SPACE

Richtext also differentiates between "hard" and "soft" line breaks. A line break (CRLF) in the richtext data stream is interpreted as a "soft" line break, one that is included only for purposes of mail transport, and is to be treated as white space by richtext interpreters. To include a "hard" line break (one that must be displayed as such), the "<nl>" or "<paragraph> formatting constructs should be used. In general, a soft line break should be treated as white space, but when soft line breaks immediately follow a <nl> or a

            </paragraph>  tag they should be ignored rather than treated
            as white space.

Putting all this together, the following "text/richtext" body fragment:

                      <bold>Now</bold> is the time for
                      <italic>all</italic> good men
                       <smaller>(and <lt>women>)</smaller> to
                      <ignoreme></ignoreme> come

to the aid of their


            Borenstein & Freed                                 [Page 26]

beloved <nl><nl>country. <comment> Stupid quote! </comment> -- the end

represents the following formatted text (which will, no doubt, look cryptic in the text-only version of this document):

Now is the time for all good men (and <women>) to come to the aid of their

country. -- the end

Richtext conformance: A minimal richtext implementation is one that simply converts "<lt>" to "<", converts CRLFs to SPACE, converts <nl> to a newline according to local newline convention, removes everything between a <comment> command and the next balancing </comment> command, and removes all other formatting commands (all text enclosed in angle brackets).

            decidedly  not  SGML,  and  must  not  be  used to transport
            arbitrary SGML  documents.   Those  who  wish  to  use  SGML
            document  types as a mail transport format must define a new
            text or application subtype, e.g.,  "text/sgml-dtd-whatever"
            or   "application/sgml-dtd-whatever",   depending   on   the
            perceived readability  of  the  DTD  in  use.   Richtext  is
            designed  to  be  compatible  with SGML, and specifically so
            that it will be possible to define a richtext DTD if one  is
            needed.   However,  this  does not imply that arbitrary SGML
            can be called richtext, nor that richtext implementors  have
            any  need  to  understand  SGML;  the  description  in  this
            document is a complete definition of richtext, which is  far
            simpler than complete SGML.

NOTE ON THE INTENDED USE OF RICHTEXT: It is recognized that implementors of future mail systems will want rich text

            functionality  far  beyond  that   currently   defined   for
            richtext.   The  intent  of  richtext is to provide a common
            format for expressing that functionality in a form in  which
            much  of  it, at least, will be understood by interoperating
            software.  Thus,  in  particular,  software  with  a  richer
            notion  of  formatted  text  than  richtext  can  still  use
            richtext as its basic representation, but can extend it with
            new  formatting  commands and by hiding information specific
            to that software  system  in  richtext  comments.   As  such
            systems  evolve,  it  is  expected  that  the  definition of
            richtext  will  be  further  refined  by  future   published
            specifications,  but  richtext  as  defined  here provides a
            platform on which evolutionary refinements can be based.

IMPLEMENTATION NOTE: In some environments, it might be impossible to combine certain richtext formatting commands,

            Borenstein & Freed                                 [Page 27]

whereas in others they might be combined easily. For example, the combination of <bold> and <italic> might produce bold italics on systems that support such fonts, but there exist systems that can make text bold or italicized, but not both. In such cases, the most recently issued recognized formatting command should be preferred.

One of the major goals in the design of richtext was to make it 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,  an  extremely
            simple  35-line  C program that converts richtext input into
            plain text output is included in Appendix D.

            Borenstein & Freed                                 [Page 28]

7.2 The Multipart Content-Type

In the case of multiple part messages, in which one or more different sets of data are combined in a single body, a "multipart" Content-Type field must appear in the entity's header. The body must then contain one or more "body parts," each preceded by an encapsulation boundary, and the last one followed by a closing boundary. Each part starts with an encapsulation boundary, and then contains a body part

            consisting  of   header area, a blank line, and a body area.
            Thus a body part is similar to an RFC 822 message in syntax,
            but different in meaning.

A body part is NOT to be interpreted as actually being an RFC 822 message. To begin with, NO header fields are actually required in body parts. A body part that starts with a blank line, therefore, is allowed and is a body part for which all default values are to be assumed. In such a case, the absence of a Content-Type header field implies that the encapsulation is plain US-ASCII text. The only header fields that have defined meaning for body parts are those the names of which begin with "Content-". All other header fields are generally to be ignored in body parts.

            Although  they  should  generally  be   retained   in   mail
            processing,  they may be discarded by gateways if necessary.
            Such other fields are permitted to appear in body parts  but
            should  not  be  depended on. "X-" fields may be created for
            experimental or private purposes, with the recognition  that
            the information they contain may be lost at some gateways.

The distinction between an RFC 822 message and a body part is subtle, but important. A gateway between Internet and X.400 mail, for example, must be able to tell the difference between a body part that contains an image and a body part that contains an encapsulated message, the body of which is an image. In order to represent the latter, the body part must have "Content-Type: message", and its body (after the blank line) must be the encapsulated message, with its own "Content-Type: image" header field. The use of similar syntax facilitates the conversion of messages to body parts, and vice versa, but the distinction between the two must be understood by implementors. (For the special case in which all parts actually are messages, a "digest" subtype is also defined.)

As stated previously, each body part is preceded by an encapsulation boundary. The encapsulation boundary MUST NOT appear inside any of the encapsulated parts. Thus, it is crucial that the composing agent be able to choose and specify the unique boundary that will separate the parts.

All present and future subtypes of the "multipart" type must use an identical syntax. Subtypes may differ in their semantics, and may impose additional restrictions on syntax,

            Borenstein & Freed                                 [Page 29]

but must conform to the required syntax for the multipart type. This requirement ensures that all conformant user agents will at least be able to recognize and separate the parts of any multipart entity, even of an unrecognized subtype.

As stated in the definition of the Content-Transfer-Encoding field, no encoding other than "7bit", "8bit", or "binary" is permitted for entities of type "multipart". The multipart delimiters and header fields are always 7-bit ASCII in any case, and data within the body parts can be encoded on a part-by-part basis, with Content-Transfer-Encoding fields for each appropriate body part.

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 body part headers embedded in the bodies of messages of type "multipart."

            7.2.1     Multipart:  The common syntax

All subtypes of "multipart" share a common syntax, defined in this section. A simple example of a multipart message also appears in this section. An example of a more complex multipart message is given in Appendix C.

The Content-Type field for multipart entities requires one

            parameter,   "boundary",   which  is  used  to  specify  the
            encapsulation  boundary.   The  encapsulation  boundary   is
            defined   as  a  line  consisting  entirely  of  two  hyphen
            characters ("-", decimal code 45) followed by  the  boundary
            parameter value from the Content-Type header field.

NOTE: The hyphens are for rough compatibility with the earlier RFC 934 method of message encapsulation, and for

            ease   of   searching   for   the   boundaries    in    some
            implementations.  However, it should be noted that multipart
            messages  are  NOT  completely  compatible  with   RFC   934
            encapsulations;  in  particular,  they  do  not obey RFC 934
            quoting conventions  for  embedded  lines  that  begin  with
            hyphens.   This  mechanism  was  chosen  over  the  RFC  934
            mechanism because the latter causes lines to grow with  each
            level  of  quoting.  The combination of this growth with the
            fact that SMTP implementations  sometimes  wrap  long  lines
            made  the  RFC 934 mechanism unsuitable for use in the event
            that deeply-nested multipart structuring is ever desired.

Thus, a typical multipart Content-Type header field might look like this:

Content-Type: multipart/mixed;

            Borenstein & Freed                                 [Page 30]


This indicates that the entity consists of several parts, each itself with a structure that is syntactically identical to an RFC 822 message, except that the header area might be completely empty, and that the parts are each preceded by the line


Note that the encapsulation boundary must occur at the beginning of a line, i.e., following a CRLF, and that that initial CRLF is considered to be part of the encapsulation

            boundary  rather  than  part  of  the preceding part.    The
            boundary must be followed immediately either by another CRLF
            and the header fields for the next part, or by two CRLFs, in
            which case there are no header fields for the next part (and
            it is therefore assumed to be of Content-Type text/plain).

            NOTE:   The  CRLF  preceding  the  encapsulation   line   is
            considered  part  of  the boundary so that it is possible to
            have a part that does not end with  a  CRLF  (line   break).
            Body  parts that must be considered to end with line breaks,
            therefore, should have two CRLFs preceding the encapsulation
            line, the first of which is part of the preceding body part,
            and the  second  of  which  is  part  of  the  encapsulation

The requirement that the encapsulation boundary begins with a CRLF implies that the body of a multipart entity must itself begin with a CRLF before the first encapsulation line

            --  that  is, if the "preamble" area is not used, the entity
            headers must be followed by TWO CRLFs.  This is  indeed  how
            such  entities  should be composed.  A tolerant mail reading
            program, however, may interpret a  body  of  type  multipart
            that  begins  with  an encapsulation line NOT initiated by a
            CRLF  as  also  being  an  encapsulation  boundary,  but   a
            compliant  mail  sending  program  must  not  generate  such

            Encapsulation  boundaries  must  not   appear   within   the
            encapsulations,  and  must  be no longer than 70 characters,
            not counting the two leading hyphens.

The encapsulation boundary following the last body part is a distinguished delimiter that indicates that no further body parts will follow. Such a delimiter is identical to the previous delimiters, with the addition of two more hyphens at the end of the line:


There appears to be room for additional information prior to the first encapsulation boundary and following the final

            Borenstein & Freed                                 [Page 31]

boundary. These areas should generally be left blank, and implementations should ignore anything that appears before the first boundary or after the last one.

NOTE: These "preamble" and "epilogue" areas are not used because of the lack of proper typing of these parts and the

            lack  of  clear  semantics  for  handling  these  areas   at
            gateways, particularly X.400 gateways.

NOTE: Because encapsulation boundaries must not appear in the body parts being encapsulated, a user agent must exercise care to choose a unique boundary. The boundary in the example above could have been the result of an algorithm designed to produce boundaries with a very low probability of already existing in the data to be encapsulated without having to prescan the data. Alternate algorithms might result in more 'readable' boundaries for a recipient with an old user agent, but would require more attention to the

            possibility   that   the   boundary   might  appear  in  the
            encapsulated  part.   The  simplest  boundary  possible   is
            something like "---", with a closing boundary of "-----".

As a very simple example, the following multipart message has two parts, both of them plain text, one of them explicitly typed and one of them implicitly typed:

From: Nathaniel Borenstein <> To: Ned Freed <>
Subject: Sample message
MIME-Version: 1.0
Content-type: multipart/mixed; boundary="simple boundary"

This is the preamble. It is to be ignored, though it is a handy place for mail composers to include an explanatory note to non-MIME compliant readers.

                 --simple boundary

This is implicitly typed plain ASCII text.
It does NOT end with a linebreak.

                 --simple boundary
                 Content-type: text/plain; charset=us-ascii

This is explicitly typed plain ASCII text.
It DOES end with a linebreak.

                 --simple boundary--
                 This is the epilogue.  It is also to be ignored.

The use of a Content-Type of multipart in a body part within another multipart entity is explicitly allowed. In such cases, for obvious reasons, care must be taken to ensure that each nested multipart entity must use a different boundary delimiter. See Appendix C for an example of nested

            Borenstein & Freed                                 [Page 32]

multipart entities.

The use of the multipart Content-Type with only a single body part may be useful in certain contexts, and is explicitly permitted.

The only mandatory parameter for the multipart Content-Type is the boundary parameter, which consists of 1 to 70 characters from a set of characters known to be very robust through email gateways, and NOT ending with white space. (If a boundary appears to end with white space, the white space must be presumed to have been added by a gateway, and

            should  be  deleted.)   It  is  formally  specified  by  the
            following BNF:

            boundary := 0*69<bchars> bcharsnospace

            bchars := bcharsnospace / " "

            bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+"  /
                           / "," / "-" / "." / "/" / ":" / "=" / "?"

Overall, the body of a multipart entity may be specified as follows:

            multipart-body := preamble 1*encapsulation
                           close-delimiter epilogue

            encapsulation := delimiter CRLF body-part

            delimiter := CRLF "--" boundary   ; taken from  Content-Type
                                           ;   when   content-type    is
                                         ; There must be no space
                                         ; between "--" and boundary.

            close-delimiter := delimiter "--" ; Again, no  space  before

            preamble :=  *text                  ;  to  be  ignored  upon

            epilogue :=  *text                  ;  to  be  ignored  upon

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. Note that the

            Borenstein & Freed                                 [Page 33]

semantics of a part differ from the semantics of a message, as described in the text.>

NOTE: Conspicuously missing from the multipart type is a notion of structured, related body parts. In general, it seems premature to try to standardize interpart structure yet. It is recommended that those wishing to provide a more structured or integrated multipart messaging facility should

            define   a   subtype  of  multipart  that  is  syntactically
            identical, but  that  always  expects  the  inclusion  of  a
            distinguished part that can be used to specify the structure
            and integration of the other parts,  probably  referring  to
            them  by  their Content-ID field.  If this approach is used,
            other implementations will not recognize  the  new  subtype,
            but  will  treat it as the primary subtype (multipart/mixed)
            and will thus be able to show the user the  parts  that  are

7.2.2 The Multipart/mixed (primary) subtype

The primary subtype for multipart, "mixed", is intended for use when the body parts are independent and intended to be

            displayed  serially.   Any  multipart   subtypes   that   an
            implementation does not recognize should be treated as being
            of subtype "mixed".

7.2.3 The Multipart/alternative subtype

The multipart/alternative type is syntactically identical to

            multipart/mixed,   but  the  semantics  are  different.   In
            particular, each of the parts is an "alternative" version of
            the same information.  User agents should recognize that the
            content of the various parts are interchangeable.  The  user
            agent  should  either  choose  the  "best" type based on the
            user's environment and preferences, or offer  the  user  the
            available  alternatives.  In general, choosing the best type
            means displaying only the LAST part that can  be  displayed.
            This  may be used, for example, to send mail in a fancy text
            format in such  a  way  that  it  can  easily  be  displayed

From: Nathaniel Borenstein <>
To: Ned Freed <>
Subject: Formatted text mail
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=boundary42

            Content-Type: text/plain; charset=us-ascii

..plain text version of message goes here

            Borenstein & Freed                                 [Page 34]

            Content-Type: text/richtext

            Content-Type: text/x-whatever



In this example, users whose mail system understood the "text/x-whatever" format would see only the fancy version, while other users would see only the richtext or plain text version, depending on the capabilities of their system.

In general, user agents that compose multipart/alternative entities should place the body parts in increasing order of preference, that is, with the preferred format last. For fancy text, the sending user agent should put the plainest format first and the richest format last. Receiving user agents should pick and display the last format they are capable of displaying. In the case where one of the alternatives is itself of type "multipart" and contains unrecognized sub-parts, the user agent may choose either to show that alternative, an earlier alternative, or both.

NOTE: From an implementor's perspective, it might seem more sensible to reverse this ordering, and have the plainest alternative last. However, placing the plainest alternative

            first    is    the    friendliest   possible   option   when
            mutlipart/alternative entities are viewed using a  non-MIME-
            compliant mail reader.  While this approach does impose some
            burden on  compliant  mail  readers,  interoperability  with
            older  mail  readers was deemed to be more important in this

It may be the case that some user agents, if they can recognize more than one of the formats, will prefer to offer the user the choice of which format to view. This makes sense, for example, if mail includes both a nicely-formatted image version and an easily-edited text version. What is most critical, however, is that the user not automatically be shown multiple versions of the same data. Either the user should be shown the last recognized version or should explicitly be given the choice.

            Borenstein & Freed                                 [Page 35]

            7.2.4     The Multipart/digest subtype

This document defines a "digest" subtype of the multipart Content-Type. This type is syntactically identical to

            multipart/mixed,  but  the  semantics  are  different.    In
            particular,  in a digest, the default Content-Type value for
            a   body   part   is   changed    from    "text/plain"    to
            "message/rfc822".   This  is  done  to allow a more readable
            digest format that is largely  compatible  (except  for  the
            quoting convention) with RFC 934.

A digest in this format might, then, look something like this:

From: Moderator-Address
MIME-Version: 1.0
Subject: Internet Digest, volume 42
Content-Type: multipart/digest;
boundary="---- next message ----"

            ------ next message ----

From: someone-else
Subject: my opinion

...body goes here ...

            ------ next message ----

From: someone-else-again
Subject: my different opinion

... another body goes here...

            ------ next message ------

7.2.5 The Multipart/parallel subtype

This document defines a "parallel" subtype of the multipart Content-Type. This type is syntactically identical to

            multipart/mixed,  but  the  semantics  are  different.    In
            particular,  in  a  parallel  entity,  all  of the parts are
            intended to be presented in parallel, i.e.,  simultaneously,
            on  hardware  and  software  that  are  capable of doing so.
            Composing agents should be aware that many mail readers will
            lack this capability and will show the parts serially in any

            Borenstein & Freed                                 [Page 36]

7.3 The Message Content-Type

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."

            7.3.1     The Message/rfc822 (primary) subtype

A Content-Type of "message/rfc822" indicates that the body contains an encapsulated message, with the syntax of an RFC 822 message.

            7.3.2     The Message/Partial subtype

A subtype of message, "partial", is defined in order to allow large objects to be delivered as several separate

            pieces  of  mail  and  automatically  reassembled   by   the
            receiving  user  agent.   (The  concept  is  similar  to  IP
            fragmentation/reassembly in the basic  Internet  Protocols.)
            This  mechanism  can  be  used  when  intermediate transport
            agents limit the size of individual  messages  that  can  be
            sent.   Content-Type  "message/partial"  thus indicates that

            Borenstein & Freed                                 [Page 37]

the body contains a fragment of a larger 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:

Content-Type: Message/Partial;
number=2; total=3;

Content-Type: Message/Partial;
id=""; number=2

But part 3 MUST specify the total number of parts:

Content-Type: Message/Partial;
number=3; total=3;

Note that part numbering begins with 1, not 0.

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

            Borenstein & Freed                                 [Page 38]

entities. In this process the following rules must be observed:

(1) All of the headers from the initial enclosing entity (part one), except those that start with "Content-" and "Message-ID", must be copied, in order, to the new message.

(2) Only those headers in the enclosed message which start with "Content-" and "Message-ID" must be appended, in order, to the headers of the new message. Any headers in the enclosed message which do not start with "Content-" (except for "Message-ID") will be ignored.

(3) All of the headers from the second and any subsequent messages will be ignored.

For example, if an audio message is broken into two parts, the first part might look something like this:

X-Weird-Header-1: Foo
Subject: Audio mail
MIME-Version: 1.0
Content-type: message/partial;
number=1; total=2

X-Weird-Header-1: Bar
X-Weird-Header-2: Hello
Content-type: audio/basic
Content-transfer-encoding: base64

... first half of encoded audio data goes here...

and the second half might look something like this:

Subject: Audio mail
MIME-Version: 1.0
Content-type: message/partial;
id=""; number=2; total=2

... second half of encoded audio data goes here...

Then, when the fragmented message is reassembled, the resulting message to be displayed to the user should look something like this:

            Borenstein & Freed                                 [Page 39]

X-Weird-Header-1: Foo
Subject: Audio mail
MIME-Version: 1.0
Content-type: audio/basic
Content-transfer-encoding: base64

... first half of encoded audio data goes here... ... second half of encoded audio data goes here...

It should be noted that, because some message transfer agents may choose to automatically fragment large messages, and because such agents may use different fragmentation thresholds, it is possible that the pieces of a partial message, upon reassembly, may prove themselves to comprise a partial message. This is explicitly permitted.

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.

            7.3.3     The Message/External-Body subtype

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

Content-type: message/external-body; access- type=local-file;

Content-type: image/gif


The area at the end, which might be called the "phantom body", is ignored for most external-body messages. However, it may be used to contain auxilliary information for some

            Borenstein & Freed                                 [Page 40]

such messages, as indeed it is when the access-type is
            "mail-server".   Of  the  access-types   defined   by   this
            document, the phantom body is used only when the access-type
            is "mail-server".  In all other cases, the phantom  body  is

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.

ACCESS-TYPE -- One or more case-insensitive words,

                 comma-separated,   indicating   supported   access
                 mechanisms by  which  the  file  or  data  may  be
                 obtained.  Values include, but are not limited to,
                 "FTP", "ANON-FTP",  "TFTP",  "AFS",  "LOCAL-FILE",
                 and   "MAIL-SERVER".  Future  values,  except  for
                 experimental values beginning with "X-",  must  be
                 registered with IANA, as described in Appendix F .

In addition, the following two parameters are optional for ALL access-types:

EXPIRATION -- The date (in the RFC 822 "date-time" syntax, as extended by RFC 1123 to permit 4 digits in the date field) after which the existence of the external data is not guaranteed.

SIZE -- The size (in octets) of the data. The intent of this parameter is to help the recipient decide whether or not to expend the necessary resources to retrieve the external data.

PERMISSION -- A field that indicates whether or not it is expected that clients might also attempt

                 to  overwrite  the  data.   By  default,   or   if
                 permission  is "read", the assumption is that they
                 are not, and that if the data is  retrieved  once,
                 it  is never needed again. If PERMISSION is "read-
                 write", this assumption is invalid, and any  local
                 copy  must  be  considered  no  more than a cache.
                 "Read"  and  "Read-write"  are  the  only  defined
                 values of permission.

The precise semantics of the access-types defined here are described in the sections that follow. The "ftp" and "tftp" access-types

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:

            Borenstein & Freed                                 [Page 41]

NAME -- The name of the file that contains the actual body data.

SITE -- A machine from which the file may be obtained, using the given protocol

Before the data is retrieved, using these protocols, the user will generally need to be asked to provide a login id and a password for the machine named by the site parameter.

In addition, the following optional parameters may also appear when the access-type is FTP or ANON-FTP:

DIRECTORY -- A directory from which the data named by NAME should be retrieved.

                 MODE  --  A  transfer  mode  for  retrieving   the
                 information, e.g. "image". The "anon-ftp" access-type

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. 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
                 "*", 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. The "mail-server" access-type

            Borenstein & Freed                                 [Page 42]

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.

            Borenstein & Freed                                 [Page 43] 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

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
Content-Type: multipart/alternative; boundary=42

                 Content-Type: message/external-body;

            Borenstein & Freed                                 [Page 44]

expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

Content-type: application/postscript

                 Content-Type: message/external-body;
                      expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

Content-type: application/postscript

                 Content-Type: message/external-body;
                      expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

Content-type: application/postscript

get rfc-xxxx doc


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.

            Borenstein & Freed                                 [Page 45]

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

            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

            Borenstein & Freed                                 [Page 46]

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;

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.

            Borenstein & Freed                                 [Page 47]

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.

            Borenstein & Freed                                 [Page 48]

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

            Borenstein & Freed                                 [Page 49]

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.

            Borenstein & Freed                                 [Page 50]

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

            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

            Borenstein & Freed                                 [Page 51]

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.

            Borenstein & Freed                                 [Page 52]


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.

            Borenstein & Freed                                 [Page 53]


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

            Borenstein & Freed                                 [Page 54]

            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.

            Borenstein & Freed                                 [Page 55]

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

A mail user agent that is MIME-conformant MUST:

1. Always generate a "MIME-Version: 1.0" header

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:


                      -- 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

            Borenstein & Freed                                 [Page 56]

least to convert "text/richtext" to plain text, as shown in Appendix D, is encouraged, but not required for conformance.
                      --Recognize and  display  at  least  the
                           primary (822) encapsulation.
                      --   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
                      -- Recognize the "alternative"  subtype,
                           and    avoid   showing   the   user
                           redundant         parts          of
                           multipart/alternative mail.
                      -- Treat any unrecognized subtypes as if
                           they were "mixed".
                      -- 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

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

            Borenstein & Freed                                 [Page 57]

have the additional guarantee that the user will not be shown data that were never intended to be viewed as text.

            Borenstein & Freed                                 [Page 58]

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)

            Borenstein & Freed                                 [Page 59]


(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

(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

            Borenstein & Freed                                 [Page 60]

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.

            Borenstein & Freed                                 [Page 61]

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 <> Subject: A multipart example
Content-Type: multipart/mixed;

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.


...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.]

                 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.

                 Content-Type: multipart/parallel;

                 Content-Type: audio/basic
                 Content-Transfer-Encoding: base64

... base64-encoded 8000 Hz single-channel
u-law-format audio data goes here....

                 Content-Type: image/gif
                 Content-Transfer-Encoding: Base64

            Borenstein & Freed                                 [Page 62]

.. base64-encoded image data goes here

                 Content-type: text/richtext

This is <bold><italic>richtext.</italic></bold>

                 <nl><nl>Isn't it

                 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 ...


            Borenstein & Freed                                 [Page 63]

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 <stdio.h>
#include <ctype.h>
main() {
int c, i;
char token[50];

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"))


            Borenstein & Freed                                 [Page 64]

} /* 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.

            Borenstein & Freed                                 [Page 65]

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

            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
                                           ;   when   content-type    is
                                         ; There should be no space
                                         ; between "--" and boundary.

            encapsulation := delimiter CRLF body-part

            epilogue :=  *text                  ;  to  be  ignored  upon

            Borenstein & Freed                                 [Page 66]

            MIME-Version := 1*text

            multipart-body := preamble  1*encapsulation  close-delimiter

            parameter := attribute "=" value

            preamble :=  *text                  ;  to  be  ignored  upon

            subtype := token

            token := 1*<any CHAR except SPACE, CTLs, or tspecials>

            tspecials :=  "(" / ")" / "<" / ">" / "@"  ; Must be in
                       /  "," / ";" / ":" / "\" / <">  ; quoted-string,
                       /  "/" / "[" / "]" / "?" / "."  ; to use within
                       /  "="                        ; parameter values

            type :=            "application"     /  "audio"     ;  case-
                      / "image"           / "message"
                      / "multipart"  / "text"
                      / "video"           / x-token

            value := token / quoted-string

            x-token := <The two characters "X-" followed, with no
                       intervening white space, by any token>

            Borenstein & Freed                                 [Page 67]

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.

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:

            Borenstein & Freed                                 [Page 68]

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

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

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

Subject: Registration of new MIME Conversions value for Application content-type

MIME Conversions name:

            Borenstein & Freed                                 [Page 69]

Published specification:

(The published specification must be an Internet RFC or RFC-to-be.)

Person & email address to contact for further information:

            Borenstein & Freed                                 [Page 70]

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

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,

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

            Borenstein & Freed                                 [Page 71]

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

            Borenstein & Freed                                 [Page 72]

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.

            Borenstein & Freed                                 [Page 73]

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.

            Borenstein & Freed                                 [Page 74]


[US-ASCII] Coded Character Set--7-Bit American Standard Code for Information Interchange, ANSI X3.4-1986.

[ATK] Borenstein, Nathaniel S., Multimedia Applications Development with the Andrew Toolkit, Prentice-Hall, 1990.

[GIF] Graphics Interchange Format (Version 89a), Compuserve, Inc., Columbus, Ohio, 1990.

[ISO-2022] International Standard--Information Processing-- ISO 7-bit and 8-bit coded character sets--Code extension techniques, ISO 2022:1986.

[ISO-8859] Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2, ISO 8859-2, 1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin alphabet No. 4, ISO 8859-4, 1988. Part 5:

            Latin/Cyrillic   alphabet,  ISO  8859-5,  1988.     Part  6:
            Latin/Arabic  alphabet,  ISO  8859-6,   1987.      Part   7:
            Latin/Greek   alphabet,   ISO   8859-7,   1987.     Part  8:
            Latin/Hebrew alphabet, ISO 8859-8, 1988.     Part  9:  Latin
            alphabet No. 5, ISO 8859-9, 1990.

[ISO-646] International Standard--Information Processing-- ISO 7-bit coded character set for information interchange, ISO 646:1983.

[MPEG] Video Coding Draft Standard ISO 11172 CD, ISO IEC/TJC1/SC2/WG11 (Motion Picture Experts Group), May, 1991.

[ODA] ISO 8613; Information Processing: Text and Office System; Office Document Architecture (ODA) and Interchange Format (ODIF), Part 1-8, 1989.

[PCM] CCITT, Fascicle III.4 - Recommendation G.711, Geneva, 1972, "Pulse Code Modulation (PCM) of Voice Frequencies".

            [POSTSCRIPT]  Adobe  Systems,  Inc.,   PostScript   Language
            Reference Manual,  Addison-Wesley, 1985.

[X400] Schicker, Pietro, "Message Handling Systems, X.400", Message Handling Systems and Distributed Applications, E. Stefferud, O-j. Jacobsen, and P. Schicker, eds., North- Holland, 1989, pp. 3-41.

[RFC-783] Sollins, K.R. TFTP Protocol (revision 2). June, 1981, MIT, RFC-783.

[RFC-821] Postel, J.B. Simple Mail Transfer Protocol. August, 1982, USC/Information Sciences Institute, RFC-821.

            Borenstein & Freed                                 [Page 75]

            [RFC-822]   Crocker, D.  Standard for  the  format  of  ARPA
            Internet  text  messages. August, 1982, UDEL, RFC-822.

            [RFC-934]   Rose, M.T.; Stefferud, E.A.   Proposed  standard
            for    message     encapsulation.  January,   1985, Delaware
            and NMA, RFC-934.

            [RFC-959]   Postel,  J.B.;  Reynolds,  J.K.   File  Transfer
            Protocol.      October,   1985,   USC/Information   Sciences
            Institute, RFC-959.

            [RFC-1049]   Sirbu,  M.A.   Content-Type  header  field  for
            Internet messages.  March, 1988, CMU,  RFC-1049.

            [RFC-1113]   Linn,  J.   Privacy  enhancement  for  Internet
            electronic    mail:  Part    I  -  message  encipherment and
            authentication procedures.   August,  1989, IAB Privacy Task
            Force, RFC-1113.

[RFC-1154] Robinson, D.; Ullmann, R. Encoding header field

            for   Internet   messages.  April,   1990,   Prime Computer,
            Inc., RFC-1154.

[RFC-1342] Moore, Keith, Representation of Non-Ascii Text in

            Internet   Message   Headers.   June,  1992,  University  of
            Tennessee, RFC-1342.

Security Considerations

Security issues are discussed in Section 7.4.2 and in Appendix G. Implementors should pay special attention to the security implications of any mail content-types that can cause the remote execution of any actions in the recipient's

            environment.   In  such  cases,  the   discussion   of   the
            applicaton/postscript   content-type  in  Section  7.4.2 may
            serve as a model for considering  other  content-types  with
            remote execution capabilities.

            Borenstein & Freed                                 [Page 76]

Authors' Addresses

For more information, the authors of this document may be contacted via Internet mail:

Nathaniel S. Borenstein
MRE 2D-296, Bellcore
445 South St.
Morristown, NJ 07962-1910

Phone: +1 201 829 4270
Fax: +1 201 829 7019

Ned Freed
Innosoft International, Inc.
250 West First Street
Suite 240
Claremont, CA 91711

Phone: +1 714 624 7907
Fax: +1 714 621 5319

            Borenstein & Freed                                 [Page 77]


Please discard this page and place the following table of contents after the title page.

            Borenstein & Freed                                  [Page i]

1 Introduction
2 Notations, Conventions, and Generic BNF Grammar
3 The MIME-Version Header Field
4 The Content-Type Header Field
5 The Content-Transfer-Encoding Header Field
5.1 Quoted-Printable Content-Transfer-Encoding
5.2 Base64 Content-Transfer-Encoding
6 Additional Optional Content- Header Fields
6.1 Optional Content-ID Header Field
6.2 Optional Content-Description Header Field
7 The Predefined Content-Type Values
7.1 The Text Content-Type
7.1.1 The charset parameter
7.1.2 The Text/plain subtype
7.1.3 The Text/richtext subtype
7.2 The Multipart Content-Type
7.2.1 Multipart: The common syntax
7.2.2 The Multipart/mixed (primary) subtype
7.2.3 The Multipart/alternative subtype
7.2.4 The Multipart/digest subtype
7.2.5 The Multipart/parallel subtype
7.3 The Message Content-Type
7.3.1 The Message/rfc822 (primary) subtype
7.3.2 The Message/Partial subtype
7.3.3 The Message/External-Body subtype
7.4 The Application Content-Type
7.4.1 The Application/Octet-Stream (primary) subtype
7.4.2 The Application/PostScript subtype
7.4.3 The Application/ODA subtype
7.5 The Image Content-Type
7.6 The Audio Content-Type
7.7 The Video Content-Type
7.8 Experimental Content-Type Values
Appendix A -- Minimal MIME-Conformance
Appendix B -- General Guidelines For Sending Email Data59 Appendix C -- A Complex Multipart Example
Appendix D -- A Simple Richtext-to-Text Translator in C64 Appendix E -- Collected Grammar
Appendix F -- IANA Registration Procedures
F.1 Registration of New Content-type/subtype Values..68 F.2 Registration of New Character Set Values
F.3 Registration of New Access-type Values for Message/external-body69 F.4 Registration of New Conversions Values for Application69 Appendix G -- Summary of the Seven Content-types... 71 Appendix H -- Canonical Encoding Model
Security Considerations
Authors' Addresses

            Borenstein & Freed                                 [Page ii]