RFC1830 - SMTP Service Extensions for Transmission of Large and Binary MIME Mess

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　　Network Working Group G. Vaudreuil
Request for Comments: 1830 Octel Network Services
Category: Experimental August 1995

SMTP Service Extensions
for Transmission of Large
and Binary MIME Messages

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

1. Abstract

This memo defines two extensions to the SMTP service. The first
service enables a SMTP client and server to negotiate the use of an
alternate DATA command "BDAT" for efficiently sending large MIME
messages. The second extension takes advantage of the BDAT command
to permit the negotiated sending of unencoded binary data.

2. Introduction

The MIME extensions to the Internet message protocol provides for the
transmission of many kinds of data which were previously unsupported
in Internet mail. Anticipating the need to more efficiently
transport the new media made possible with MIME, the SMTP protocol
has been extended to provide transport for new message types. RFC
1426 defines one such extension for the transmission of unencoded 8
bit MIME messages [8BIT]. This service extension permits the
receiver SMTP to declare support for 8 bit body parts and the sender
to request 8 bit transmission of a particular message.

One expected result of the use of MIME is that the Internet mail
system will be expected to carry very large mail messages. In such
transactions, there is a need to eliminate the requirement that the
message be scanned for "CR LF . CR LF" sequences upon sending and
receiving to detect the end of message.

Independent of the need to send large messages, Internet mail is
increasingly multi-media there is a need to avoid the overhead of
base64 and quoted-printable encoding of binary objects sent using the
MIME message format over SMTP between hosts which support binary
message processing.

This memo uses the mechanism defined in [ESMTP] to define two
extensions to the SMTP service whereby a client ("sender-SMTP") may
declare support for the message chunking transmission mode using the
BDAT command and support for the sending of Binary messages.

3. Framework for the Large Message Extensions

The following service extension is hereby defined:

1) The name of the data chunking service extension is
"CHUNKING".

2) The EHLO keyword value associated with this extension is
"CHUNKING".

3) A new SMTP verb is defined "BDAT" as an alternative to
the "DATA" command of [RFC821]. The BDAT verb takes two
arguments. The first argument indicates the length of the
binary data packet. The second optional argument indicates
that the data packet is the last.

bdat-cmd ::= "BDAT" SP chunk-size
[ SP end-marker ] CR LF
chunk-size ::= 1*DIGIT
end-marker ::= "LAST"

The CHUNKING service extension enables the use of the BDAT
alternative to the DATA command. This extension can be used for any
message, whether 7 bit, 8BITMIME or BINARYMIME.

When a client SMTP wishes to submit (using the MAIL command) a large
message using the CHUNKING extension, it first issues the EHLO
command to the server SMTP. If the server SMTP responds with code
250 to the EHLO command, and the response includes the EHLO keyword
value CHUNKING, then the server SMTP is indicating that it supports
the BDAT command and will accept the sending of messages in chunks.

After all MAIL FROM and RCPT TO responses are collected and
processed, the message is sent using a series of BDAT commands. The
BDAT command takes one argument, the exact length of the data segment
in octets. The message data is sent immediately after the BDAT
command. Once the receiver-SMTP receives the specified number of
octets, it will return a 250 reply code.

The LAST parameter on the BDAT command indicates that this is the
last chunk of message data to be sent. Any BDAT command sent after
the BDAT LAST is illegal and must be replied to with a 503 "Bad

sequence of commands" reply code. The state resulting from this error
is indeterminate. A RSET command must be sent to clear the
transaction before continuing.

A 250 response should be sent to each BDAT data block. If a 5XX code
is sent in response to a BDAT chunk the message should be considered
failed and, the sender SMTP must not send any additional BDAT
segments. If using the ESMTP pipelining extensions [PIPE], the
sender SMTP must complete the sending of the current segment and not
send any more BDATs. When streaming, the receiver SMTP must accept
and discard additional BDAT chunks after the failed BDAT. After
receiving a 5XX error in response to a BDAT command, the resulting
state is indeterminate. A RSET command must be issued to clear the
transaction before additional commands may be sent.

Note that an error on the receiver SMTP such as disk full or
imminent shutdown can only be reported after the BDAT segment has
been sent. It is therefore important to choose a reasonable chunk
size given the expected end to end bandwidth.

The RSET command when issued during after the first BDAT and before
the BDAT LAST clears all segments sent during that transaction and
resets the session.

DATA and BDAT commands cannot be used in the same transaction. If a
DATA statement is issued after a BDAT for the current transaction, a
503 "Bad sequence of commands" must be issued. The state resulting
from this error is indeterminate. A RSET command must be sent to
clear the transaction before continuing. There is no prohibition on
using DATA and BDAT in the same session, so long as they are not
mixed in the same transaction.

The local storage size of a message may not accurately reflect the
actual size of the message sent due to local storage conventions. In
particular, text messages sent with the BDAT command must be sent in
the canonical MIME format with lines delimited with a <CR><LF>. It
may not be possible to convert the entire message to the canonical
format at once. Chunking provides a mechanism to convert the message
to canonical form, accurately count the bytes, and send the message a
single chunk at a time.

Note that correct byte counting is essential. If too many bytes
are indicated by the sender SMTP, the receiver SMTP will continue
to wait for the remainder of the data or will read the subsequent
command as additional message data. In the case where a portion
of the previous command was read as data, the parser will return a
syntax error when the incomplete command is read.

If too few bytes are indicated by the sender SMTP, the receiver
SMTP will interpret the remainder of the message data as invalid
commands. Note that the remainder of the message data may be
binary and as such lexigraphical parsers must be prepared to
receive, process, and reject lines of arbitrary octets.

4. Framework for the Binary Service Extension

The following service extension is hereby defined:

1) The name of the binary service extension is "BINARYMIME".

2) The EHLO keyword value associated with this extension is
"BINARYMIME".

3) The BINARYMIME service extension can only be used with
the "CHUNKING" service extension.

4) No parameter is used with the BINARYMIME keyword.

5) One additional parameter to the BODY keyword defined
[8BIT] for the MAIL FROM command is defined, "BINARYMIME".
The value "BINARYMIME" associated with this parameter
indicates that this message is a Binary MIME message (in
strict compliance with [MIME]) with arbitrary octet content
being sent. The revised syntax of the value is as follows,
using the ABNF notation of [RFC822]:

body-value ::= "7BIT" / "8BITMIME" / "BINARYMIME"

6) No new verbs are defined for the BINARYMIME extension.

A sender SMTP may request that a binary MIME message be sent without
transport encoding by sending a BINARYMIME parameter with the MAIL
FROM command. When the receiver SMTP accepts a MAIL FROM command
with the BINARYMIME body type requested, it agrees to preserve all
bits in each octet passed using the BDAT command.

BINARYMIME cannot be used with the DATA command. If a DATA command
is issued after a MAIL FROM command containing the body-value of
"BINARYMIME", a 501 response should be sent. The resulting state
from this error condition is indeterminate and the transaction should
be reset with the RSET command.

It is important to note that when using BINARYMIME, it is
especially important to ensure that the MIME message itself is
properly formed. In particular, it is essential that text be
canonically encoded with each line properly terminated with <CR>

<LF>. Any transformation of text into non-canonical MIME to
observe local storage conventions must be reversed before sending
as BINARYMIME. The usual line-oriented shortcuts will break if
used with BINARYMIME.

The syntax of the extended MAIL command is identical to the MAIL
command in [RFC821], except that a BODY parameter must appear after
the address. The complete syntax of this extended command is defined
in [ESMTP]. The ESMTP-keyword is BODY and the syntax for ESMTP-value
is given by the syntax for body-value in [ESMTP].

If a receiver SMTP does not support the BINARYMIME message format
(either by not responding with code 250 to the EHLO command, or by
rejecting the BINARYMIME parameter to the MAIL FROM command, then the
client SMTP must not, under any circumstances, send binary data using
the DATA or BDAT commands.

If the receiver-SMTP does not support BINARYMIME and the message
content is a MIME object with a binary encoding, a client SMTP has
two options in this case: first, it may implement a gateway
transformation to convert the message into valid 7bit encoded MIME,
or second, it may treat this as a permanent error and handle it in
the usual manner for delivery failures. The specifics of the
transformation from Binary MIME to 7bit MIME are not described by
this RFC; the conversion is nevertheless constrained in the following
ways:

o The conversion must cause no loss of information; MIME
transport encodings must be employed as needed to insure this
is the case.

o The resulting message must be valid 7bit MIME.

As of present there are no mechanisms for converting a binary MIME
object into a 8 bit-MIME object. Such a transformation will require
the specification of a new MIME content-transfer-encoding, the
standardization of which is discouraged by [MIME].

5. Examples

5.1 Simple Chunking

The following simple dialogue illustrates the use of the large
message extension to send a short psudo-RFC822 message to one
recipient using the CHUNKING extension:

R: <wait for connection on TCP port 25>
S: <open connection to server>
R: 220 cnri.reston.va.us SMTP service ready
S: EHLO ymir.claremont.edu
R: 250-cnri.reston.va.us says hello
R: 250 CHUNKING
S: MAIL FROM:<Sam@Random.com>
R: 250 <Sam@Random.com>... Sender ok
S: RCPT TO:<Susan@Random.com>
R: 250 <Susan@random.com>... Recipient ok
S: BDAT 69 LAST
S: To: Susan@random.com<CR><LF>
S: From: Sam@random.com<CR><LF>
S: Subject: This is a bodyless test message<CR><LF>
R: 250 Message OK, 69 octets received
S: QUIT
R: 221 Goodbye

5.2 Pipelining Binarymime

The following dialogue illustrates the use of the large message
extension to send a BINARYMIME object to two recipients using the
CHUNKING and PIPELINING extensions:

R: <wait for connection on TCP port 25>
S: <open connection to server>
R: 220 cnri.reston.va.us SMTP service ready
S: EHLO ymir.claremont.edu
R: 250-cnri.reston.va.us says hello
R: 250-PIPELINING
R: 250-BINARYMIME
R: 250 CHUNKING
S: MAIL FROM:<ned@ymir.claremont.edu> BODY=BINARYMIME
S: RCPT TO:<gvaudre@cnri.reston.va.us>
S: RCPT TO:<jstewart@cnri.reston.va.us>
R: 250 <ned@ymir.claremont.edu>... Sender and BINARYMIME ok
R: 250 <gvaudre@cnri.reston.va.us>... Recipient ok
R: 250 <jstewart@cnri.reston.va.us>... Recipient ok
S: BDAT 100000

S: (First 10000 octets of canonical MIME message data)
S: BDAT 324 LAST
S: (Remaining 324 octets of canonical MIME message data)
R: 250 100000 bytes received
R: 250 Message OK, 100324 octets received
S: QUIT
R: 221 Goodbye

6. Security Considerations

This RFCdoes not discuss security issues and is not believed to
raise any security issues not already endemic in electronic mail and
present in fully conforming implementations of [RFC821], or otherwise
made possible by [MIME].

7. Acknowledgments

This document is the result of numerous discussions in the IETF SMTP
Extensions Working Group and in particular due to the continued
advocacy of "chunking" by Neil Katin.

8. References

[RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, USC/Information Sciences Institute, August 1982.

[RFC822] Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC822, UDEL, August 1982.

[MIME] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions", RFC1521, Bellcore, Innosoft, June 1992.

[ESMTP] Klensin, J., WG Chair, Freed, N., Editor, Rose, M.,
Stefferud, E., and D. Crocker, "SMTP Service Extensions" RFC
1425, United Nations University, Innosoft International,
Inc., Dover Beach Consulting, Inc., Network Management
Associates, Inc., The Branch Office, February 1993.

[8BIT] Klensin, J., WG Chair, Freed, N., Editor, Rose, M.,
Stefferud, E., and D. Crocker, "SMTP Service Extension for
8bit-MIMEtransport" RFC1426, United Nations University,
Innosoft International, Inc., Dover Beach Consulting, Inc.,
Network Management Associates, Inc., The Branch Office,
February 1993.

[PIPE] Freed, N., "SMTP Service Extensions for Command
Pipelining", Innosoft International, Work in Progress.

9. Author's Address

Gregory M. Vaudreuil
Octel Network Services
17060 Dallas Parkway
Suite 214
Dallas, TX 75248-1905

Voice/Fax: 214-733-2722

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