Network Working Group N. Borenstein, Bellcore
Request for Comments: 1344 June 1992
Implications of MIME for Internet Mail Gateways
Status of This Memo
This is an informational memo for the Internet community,
and requests discussion and suggestions for improvements.
This memo does not specify an Internet standard.
Distribution of this memo is unlimited.
Abstract
The recent development of MIME (Multipurpose Internet Mail
Extensions) offers a wide range of new opportunities for
electronic mail system systems. Most of these opportunites
are relevant only to user agents, the programs that interact
with human users when they send and receive mail. However,
some opportunities are also opened up for mail transport
systems. While MIME was carefully designed so that it does
not require any changes to Internet electronic message
transport facilities, there are several ways in which
message transport systems may want to take advantage of
MIME. These opportunities are the subject of this memo.
Background -- The MIME Format
Recently, a new standardized format has been defined for
enhanced electronic mail messages on the Internet. This
format, known as MIME, permits messages to include, in a
standardized manner, non-ASCII text, images, audio, and a
variety of other kinds of interesting data.
The MIME effort was explicitly focused on requiring
absolutely no changes at the message transport level.
Because of this fact, MIME-format mail runs transparently on
all known Internet or Internet-style mail systems. This
means that those concerned solely with the maintenance and
development of message transport services can safely ignore
MIME completely, if they so choose.
However, the fact that MIME can be ignored, for the purpose
of message transport, does not necessarily mean that it
should be ignored. In particular, MIME offers several
features that should be of interest to those responsible for
message transport services. By exploiting these features,
transport systems can provide certain additional kinds of
service that are currently unavailable, and can alleviate a
few existing problems.
The remainder of this document is an attempt to briefly
point out and summarize some important ways in which MIME
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may be of use for message transport systems. This document
makes no attempt to present a complete technical description
of MIME, however. For that, the reader is refered to the
MIME document itself [RFC-1341].
Mail Transport and Gateway Services: A Key Distinction
Before implementing any of the mechanisms discussed in this
memo, one should be familiar with the distinction between
mail transport service and mail gateway service. Basically,
mail transport software is responsible for moving a message
within a homogeneous electronic mail service network. Mail
gateways, on the other hand, exchange mail between two
significantly different mail environments, including via
non-electronic services, such as postal mail.
In general, it is widely considered unacceptable for mail
transport services to alter the contents of messages. In
the case of mail gateways, however, such alteration is often
inevitable. Thus, strictly speaking, many of the mechanisms
described here apply only to gateways, and should not be
used in simple mail transport systems. However, it is
possible that some very special situations -- e.g., an SMTP
relay that transports mail across extremely expensive
intercontinental network links -- might need to modify
messages, in order to provide appropriate service for those
situations, and hence must redefine its role to be that of a
gateway.
In this memo, it is assumed that transformations which alter
a message's contents will be performed only by gateways, but
it is recognized that some existing mail transport agents
may choose to reclassify themselves as gateways in order to
perform the functions described here.
Rejected Messages
An unfortunately frequent duty of message transport services
is the rejection of mail to the sender. This may happen
because the mail was undeliverable, or because it did not
conform to the requirements of a gateway (e.g., it was too
large).
There has never been a standard format for rejected messages
in the past. This has been an annoyance, but not a major
problem for text messages. For non-text messages, however,
the lack of a standard rejection format is more crucial,
because rejected messages typically appear to be text, and
the user who finds himself viewing images or audio as if
they were text is rarely happy with the result.
MIME makes it very easy to encapsulate messages in such a
way that their semantics are completely preserved. The
simplest way to do this is to make each rejection notice a
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MIME "multipart/mixed" message. That multipart message
would contain two parts, a text part explaining the reason
for the rejection, and an encapsulated message part that
contained the rejected message itself.
It should be stressed that the transport software does not
need to understand the structure of the rejected message at
all. It merely needs to encapsulate it properly. The
following, for example, shows how any MIME message may be
encapsulated in a rejection message in such a way that all
information will be immediately visible in the correct form
if the recipient reads it with a MIME-conformant mail
reader:
From: Mailer-Daemon
Subject: Rejected Message
Content-type: multipart/mixed; boundary=unique-boundary
--unique-boundary
Content-type: text/plain; charset=us-ascii
A mail message you sent was rejected. The details of
the rejected message are as follows:
From: Nathainel Borenstein
Message-ID: <12345@bellcore.com>
To: bush@whitehouse.gov
Subject: I know my rights!
Rejection-reason: No mail from libertarians is
accepted.
The original message follows below.
--unique-boundary
Content-type: message/rfc822
The ENTIRE REJECTED MESSAGE, starting with the headers,
goes here.
--unique-boundary--
In the above example, the ONLY thing that is not
'boilerplate" is the choice of boundary string. The phrase
"unique-boundary" should be replaced by a string that does
not appear (prefixed by two hyphens) in any of the body
parts.
Encapsulating a message in this manner is very easily done,
and will constitute a significant service that message
transport services can perform for MIME users.
IMPORTANT NOTE: The format given above is simply one of
many possible ways to format a rejection message using MIME.
Independent IETF efforts are needed in order to standardize
the format of rejections and acknowledgements.
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RFC 1344 MIME and Mail Gateways June 1992
Fragmenting and Reassembling Large Messages
One problem that occurs with increasing frequency in
Internet mail is the rejection of messages because of size
limitations. This problem can be expected to grow
substantially more severe with the acceptance of MIME, as
MIME invites the use of very large objects such as images
and audio clips. Fortunately, MIME also provides mechanisms
that can help alleviate the problem.
One particularly relevant MIME type is "message/partial",
which can be used for the automatic fragmentation and
reassembly of large mail messages. The message/partial type
can be handled entirely at the user agent level, but message
transport services can also make use of this type to provide
more intelligent behavior at gateways.
In particular, when gatewaying mail to or from a system or
network known to enforce size limitations that are more or
less stringent than are enforced locally, message transport
services might choose either to break a large message into
fragments, or (perhaps less likely) to reassemble fragments
into a larger message. The combination of these two
behaviors can make the overall Internet mail environment
appear more complete and seamless than it actually is.
Details on the message/partial format may be found in the
MIME document. What follows is an example of how a simple
short message might be broken into two message/partial
messages. In practice, of course, the message/partial
facility would only be likely to be used for much longer
messages.
The following initial message:
From: Nathaniel Borenstein
To: Ned Freed:
Subject: a test message
Content-type: image/gif
Content-Transfer-Encoding: base64
R0lGODdhQAGMAbMAAAAAAP/u7swzIu6ZiLsiEd1EM+5VRGaI3WYAAO67qkRV
uwARd6q7/ywAAAAAQAGMAUME/hDISau9OOvNu/9gKI6kRJwoUa5s675wLM90l
XW5YKxqPyKRygxv2dr4czwlMCZrQLFTYHBJ2hlyQYFiaz+i0WWBou7fOq1x8vXWfU
qU1fJ2qEhYaHGjhZQmJ2QT1xBW1ak1xUdV0/VjtsbpUEDaEJCQOIpqeoNV+LXo5W
fVN3dZKceAQPvgyhwQ2lqcXGxx5wja59eJIGUNCszF90sYp50CoqFZ4DoqMMo6M
can be transformed, invertibly, into the following two
message/partial messages:
From: Nathaniel Borenstein
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RFC 1344 MIME and Mail Gateways June 1992
To: Ned Freed
Subject: a test message
Content-type: message/partial; id="xyx@host.com";
number=1; total=2
Content-type: image/gif
Content-Transfer-Encoding: base64
R0lGODdhQAGMAbMAAAAAAP/u7swzIu6ZiLsiEd1EM+5VRGaI3WYAAO67qkRV
and
From: Nathaniel Borenstein
To: Ned Freed
Subject: a test message
Content-type: message/partial; id="xyx@host.com";
number=2; total=2
uwARd6q7/ywAAAAAQAGMAUME/hDISau9OOvNu/9gKI6kRJwoUa5s675wLM90l
XW5YKxqPyKRygxv2dr4czwlMCZrQLFTYHBJ2hlyQYFiaz+i0WWBou7fOq1x8vXWfU
qU1fJ2qEhYaHGjhZQmJ2QT1xBW1ak1xUdV0/VjtsbpUEDaEJCQOIpqeoNV+LXo5W
fVN3dZKceAQPvgyhwQ2lqcXGxx5wja59eJIGUNCszF90sYp50CoqFZ4DoqMMo6M
Fragmenting such messages rather than rejecting them might
be a reasonable option for some gateway services, at least
for a certain range of message sizes. Of course, it is
often difficult for a gateway to know what size limitations
will be encountered "downstream", but intelligent guesses
are often possible. Moreover, an IETF working group on SMTP
extensions has proposed augmenting SMTP with a "SIZE" verb
that would facilitate this process, thereby possibly
requiring fragmentation on the fly during message
transmission.
Note also that fragmentation or reassembly might reasonably
be performed, in differing circumstances, by either the
sending or receiving gateway systems, depending on which
system knew more about the capabilities of the other.
Using or Removing External-Body Pointers
Another MIME type oriented to extremely large messages is
the "message/external-body" type. In this type of message,
all or part of the body data is not included in the actual
message itself. Instead, the Content-Type header field
includes information that tells how the body data can be
retrieved -- either via a file system, via anonymous ftp, or
via other mechanisms.
The message/external-body type provides a new option for
mail transport services that wishes to optimize the way
bandwidth resources are used in a given environment. For
example, the basic use of message/external-body is to reduce
bandwidth in email traffic. However, when email crosses a
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slow and expensive boundary -- e.g., a satellite link across
the Pacific -- it might make sense to retrieve the data
itself and transform the external-body reference into the
actual data. Alternately, it might make sense to copy the
data itself to a new location, closer to the message
recipients, and change the location pointed to in the
message. Because the external-body specification can
include an expiration date, message transport services can
trade off storage and bandwidth capabilities to try to
optimize the overall use of resources for very large
messages.
Such behaviors by a gateway require careful analysis of
cost/benefit tradeoffs and would be a dramatic departure
from typical mail transport services. However, the
potential benefits are quite significant, so that such the
appropriate use of these service options should be explored.
For example, the following message includes PostScript data
by external reference:
From: Nathaniel Borenstein
To: Ned Freed
Subject: The latest MIME draft
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
A gateway to Australia might choose to copy the file to an
Australian FTP archive, changing the relevant parameters on
the Content-type header field. Alternately, it might choose
simply to transform the message into one in which all the
data were included:
From: Nathaniel Borenstein
To: Ned Freed
Subject: The latest MIME draft
Content-type: application/postscript
%!PS-Adobe-1.0
%%Creator: greenbush:nsb (Nathaniel Borenstein,MRE 2A-
274,4270,9938586,21462)
etc...
This is an example which suggests both the benefits and the
dangers. There is considerable benefit to having a copy of
the data immediately available, but there also may be
considerable expense involved in transporting it to all of
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a the members of a list, if only a few will use the data
anytime soon.
Alternatively, instead of replacing an external-body message
with its real contents, it might make sense to transform it
into a "multipart/alternative" message containing both the
external body reference and the expanded version. This
means that only the external body part can be forwarded if
desired, and the recipient doesn't lose the information as
to where the data was fetched from, if they want to fetch an
up-to-date version in the future. Such information could be
represented, in MIME, in the following form:
From: Nathaniel Borenstein
To: Ned Freed
Subject: The latest MIME draft
Content-type: multipart/alternative; boundary=foo
--foo
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
--foo
Content-type: application/postscript
%!PS-Adobe-1.0
%%Creator: greenbush:nsb (Nathaniel Borenstein,MRE 2A-
274,4270,9938586,21462)
etc...
--foo--
Similarly for the case where a message is copied to a local
FTP site, one could offer two external body parts as the
alternatives, allowing the user agent to choose which FTP
site is preferred.
Image and other Format Conversions
MIME currently defines two image formats, image/gif and
image/jpeg. The former is much more convenient for many
users, and can be displayed more quickly on many systems.
The latter is a much more compact representation, and
therfore places less stress on mail transport facilities.
Message transport services can optimize both transport
bandwidth and user convenience by intelligent translation
between these formats (and other formats that might be added
later). When a message of type image/gif is submitted for
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long-haul delivery, it might reasonably be translated to
image/jpeg. Conversely, when image/jpeg data is received
for final delivery on a system with adequate storage
resources, it might be translated to image/gif for the
convenience of the recipient. Software to perform these
translations is widely available. It should be noted,
however, that performance of such conversions presumes
support for the new format by the recipient.
Although MIME currently only defines one audio format, more
are likely to be defined and registered with IANA in the
future. In that case, similar format conversion facilities
might be appropriate for audio.
If format conversion is done, it is STRONGLY RECOMMENDED
that some kind of trace information (probably in the form of
a Received header field) should be added to a message to
document the conversion that has been performed.
Some people have expressed concerns, or even the opinion
that conversions should never be done. To accomodate the
desires of those who dislike the idea of automatic format
conversion. For this reason, it is suggested that such
transformations be generally restricted to gateways rather
than general message transport services, and that services
which perform such conversions should be sensitive to a
header field that indicates that the sender does not wish to
have any such conversions performed. A suggested value for
this header field is:
Content-Conversion: prohibited
User agents that wish to explicitly indicate a willingness
for such conversions to be performed may use:
Content-Conversion: permitted
However, this will be the default assumption of many
gateways, so this header field is not strictly necessary.
It also should be noted that such control of conversion
would only be available to the sender, rather than to any of
the recipients.
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RFC 1344 MIME and Mail Gateways June 1992
Robust Encoding of Data
In addition to all the reasons given above for possible
transformation of body data, it will sometimes be the case
that a gateway can tell that the body data, as given, will
not robustly survive the next step of transport. For
example, mail crossing an ASCII-to-EBCDIC gateway will lose
information if certain characters are used. In such cases,
a gateway can make the data more robust simply by applying
one of the MIME Content-Transfer-Encoding algorithms (base64
or quoted-printable) to the body or body part. This will
generally be a loss-less transformation, but care must be
taken to ensure that the resulting message is MIME-
conformant if the inital message was not. (For example, a
MIME-Version header field may need to be added.)
User-oriented concerns
If a gateway is going to perform major transformations on a
mail message, such as translating image formats or mapping
between included data and external-reference data, it seems
inevitable that there will be situations in which users will
object to these transformations. This is, in large part, an
implementation issue, but it seems advisable, wherever
possible, to provide a mechanism whereby users can specify,
to the transport system, whether or not they want such
services performed automatically on their behalf. The use of
the "Content-Conversion" header field, as mentioned above,
is suggested for this purpose, since it it least provides
some control by the sender, if not the recipient.
References
[RFC-1341] Borenstein, N., and N. Freed, "MIME
(Multipurpose Internet Mail Extensions): Mechanisms for
Specifying and Describing the Format of Internet Message
Bodies", RFC 1341, Bellcore, June, 1992.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Nathaniel S. Borenstein
MRE 2D-296, Bellcore
445 South St.
Morristown, NJ 07962-1910
Email: nsb@bellcore.com
Phone: +1 201 829 4270
Fax: +1 201 829 7019
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RFC, FYI, BCP