NOTE: It has been suggested that subtypes of message might be defined for forwarded or rejected messages. However, forwarded and rejected messages can be handled as multipart messages in which the first part contains any control or descriptive information, and a second part, of type message/rfc822, is the forwarded or rejected message. Composing rejection and forwarding messages in this manner will preserve the type information on the original message and allow it to be correctly presented to the recipient, and hence is strongly encouraged.
As stated in the definition of the Content-Transfer-Encoding field, no encoding other than "7bit", "8bit", or "binary" is permitted for messages or parts of type "message". The message header fields are always US-ASCII in any case, and data within the body can still be encoded, in which case the Content-Transfer-Encoding header field in the encapsulated message will reflect this. Non-ASCII text in the headers of an encapsulated message can be specified using the mechanisms described in [RFC-1342].
Mail gateways, relays, and other mail handling agents are commonly known to alter the top-level header of an RFC 822 message. In particular, they frequently add, remove, or reorder header fields. Such alterations are explicitly forbidden for the encapsulated headers embedded in the bodies of messages of type "message."
Three parameters must be specified in the Content-Type field of type message/partial: The first, "id", is a unique identifier, as close to a world-unique identifier as possible, to be used to match the parts together. (In general, the identifier is essentially a message-id; if placed in double quotes, it can be any message-id, in accordance with the BNF for "parameter" given earlier in this specification.) The second, "number", an integer, is the part number, which indicates where this part fits into the sequence of fragments. The third, "total", another integer, is the total number of parts. This third subfield is required on the final part, and is optional on the earlier parts. Note also that these parameters may be given in any order.
Thus, part 2 of a 3-part message may have either of the following header fields:
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:
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.
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.
In addition, the following optional parameters may also appear when the access-type is FTP or ANON-FTP:
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.
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.