RFC 1341
MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies
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
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
specification.
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
document.
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
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.
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
underlined.
Center -- causes the subsequent text to be centered.
FlushLeft -- causes the subsequent text to be left
justified.
FlushRight -- causes the subsequent text to be right
justified.
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.
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
message.
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
allowed.
np -- <np> causes a page break. No balancing </np> is
allowed.
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:
<bold><italic>the-text</italic></bold>
or, alternately,
<italic><bold>the-text</bold></italic>
but, in particular, the following is illegal
richtext:
<bold><italic>the-text</bold></italic>
NOTE: The nesting requirement for formatting commands
imposes a slightly higher burden upon the composers of
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
characters.)
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
<nl>
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
beloved
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).
NOTE ON THE RELATIONSHIP OF RICHTEXT TO SGML: Richtext is
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,
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.
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,
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;
boundary=gc0p4Jq0M2Yt08jU534c0p
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
--gc0p4Jq0M2Yt08jU534c0p
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
boundary.
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
entities.
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:
--gc0p4Jq0M2Yt08jU534c0p--
There appears to be room for additional information prior to
the first encapsulation boundary and following the final
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 <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
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
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
field.
; when content-type is
multipart
; There must be no space
; between "--" and boundary.
close-delimiter := delimiter "--" ; Again, no space before
"--"
preamble := *text ; to be ignored upon
receipt.
epilogue := *text ; to be ignored upon
receipt.
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
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
recognized.
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
anywhere:
From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Subject: Formatted text mail
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=boundary42
--boundary42
Content-Type: text/plain; charset=us-ascii
...plain text version of message goes here....
--boundary42
Content-Type: text/richtext
.... richtext version of same message goes here ...
--boundary42
Content-Type: text/x-whatever
.... fanciest formatted version of same message goes here
...
--boundary42--
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
case.
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.
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 event.
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
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;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
Content-Type: Message/Partial;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
number=2
But part 3 MUST specify the total number of parts:
Content-Type: Message/Partial;
number=3; total=3;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
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
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
From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail
Message-ID: id1@host.com
MIME-Version: 1.0
Content-type: message/partial;
id="ABC@host.com";
number=1; total=2
X-Weird-Header-1: Bar
X-Weird-Header-2: Hello
Message-ID: anotherid@foo.com
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:
From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail
MIME-Version: 1.0
Message-ID: id2@host.com
Content-type: message/partial;
id="ABC@host.com"; 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:
X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail
Message-ID: anotherid@foo.com
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
message:
Content-type: message/external-body; access-
type=local-file;
name=/u/nsb/Me.gif
Content-type: image/gif
THIS IS NOT REALLY THE BODY!
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
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
ignored.
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.
7.3.3.1 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:
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".
7.3.3.2 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.
7.3.3.3 The "local-file" and "afs" access-types
An access-type of "local-file" indicates that the actual
body is accessible as a file on the local machine. An
access-type of "afs" indicates that the file is accessible
via the global AFS file system. In both cases, only a
single parameter is required:
NAME -- The name of the file that contains the
actual body data.
The following optional parameter may be used to describe the
locality of reference for the data, that is, the site or
sites at which the file is expected to be visible:
SITE -- A domain specifier for a machine or set of
machines that are known to have access to the data
file. Asterisks may be used for wildcard matching
to a part of a domain name, such as
"*.bellcore.com", to indicate a set of machines on
which the data should be directly visible, while a
single asterisk may be used to indicate a file
that is expected to be universally available,
e.g., via a global file system.
7.3.3.4 The "mail-server" access-type
The "mail-server" access-type indicates that the actual body
is available from a mail server. The mandatory parameter
for this access-type is:
SERVER -- The email address of the mail server
from which the actual body data can be obtained.
Because mail servers accept a variety of syntax, some of
which is multiline, the full command to be sent to a mail
server is not included as a parameter on the content-type
line. Instead, it may be provided as the "phantom body"
when the content-type is message/external-body and the
access-type is mail-server.
Note that MIME does not define a mail server syntax.
Rather, it allows the inclusion of arbitrary mail server
commands in the phantom body. Implementations should
include the phantom body in the body of the message it sends
to the mail server address to retrieve the relevant data.
7.3.3.5 Examples and Further Explanations With the emerging possibility of very wide-area file systems, it becomes very hard to know in advance the set of machines where a file will and will not be accessible directly from the file system. Therefore it may make sense to provide both a file name, to be tried directly, and the name of one or more sites from which the file is known to be accessible. An implementation can try to retrieve remote files using FTP or any other protocol, using anonymous file retrieval or prompting the user for the necessary name and password. If an external body is accessible via multiple mechanisms, the sender may include multiple parts of type message/external-body within an entity of type multipart/alternative. However, the external-body mechanism is not intended to be limited to file retrieval, as shown by the mail-server access-type. Beyond this, one can imagine, for example, using a video server for external references to video clips. If an entity is of type "message/external-body", then the body of the entity will contain the header fields of the encapsulated message. The body itself is to be found in the external location. This means that if the body of the "message/external-body" message contains two consecutive CRLFs, everything after those pairs is NOT part of the message itself. For most message/external-body messages, this trailing area must simply be ignored. However, it is a convenient place for additional data that cannot be included in the content-type header field. In particular, if the "access-type" value is "mail-server", then the trailing area must contain commands to be sent to the mail server at the address given by NAME@SITE, where NAME and SITE are the values of the NAME and SITE parameters, respectively. The embedded message header fields which appear in the body of the message/external-body data can be used to declare the Content-type of the external body. Thus a complete message/external-body message, referring to a document in PostScript format, might look like this: From: Whomever Subject: whatever MIME-Version: 1.0 Message-ID: id1@host.com Content-Type: multipart/alternative; boundary=42 --42 Content-Type: message/external-body; name="BodyFormats.ps";
site="thumper.bellcore.com";
access-type=ANON-FTP;
directory="pub";
mode="image";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
--42
Content-Type: message/external-body;
name="/u/nsb/writing/rfcs/RFC-XXXX.ps";
site="thumper.bellcore.com";
access-type=AFS
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
--42
Content-Type: message/external-body;
access-type=mail-server
server="listserv@bogus.bitnet";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
get rfc-xxxx doc
--42--
Like the message/partial type, the message/external-body
type is intended to be transparent, that is, to convey the
data type in the external body rather than to convey a
message with a body of that type. Thus the headers on the
outer and inner parts must be merged using the same rules as
for message/partial. In particular, this means that the
Content-type header is overridden, but the From and Subject
headers are preserved.
Note that since the external bodies are not transported as
mail, they need not conform to the 7-bit and line length
requirements, but might in fact be binary files. Thus a
Content-Transfer-Encoding is not generally necessary, though
it is permitted.
Note that the body of a message of type "message/external-
body" is governed by the basic syntax for an RFC 822
message. In particular, anything before the first
consecutive pair of CRLFs is header information, while
anything after it is body information, which is ignored for
most access-types.
(next page on part 3)
