SMTP Service Extensions for Transmission of Large and Binary MIME Messages
draft-ietf-mailext-smtp-binary-06

Network Working Group                         Greg Vaudreuil
     Internet Draft                        Octel Network Services
     Expires: 2/20/95                              March 22, 1995

                          SMTP Service Extensions
                          for Transmission of Large
                          and Binary MIME Messages

                    <draft-mailext-smtp-binary-06.txt>

     Changes from Previous Draft

     1) Numerous typo's corrected.

     1.Status of this Memo

     This document is an Internet Draft.  Internet Drafts are working
     documents of the Internet Engineering Task Force (IETF), its
     Areas, and its Working Groups.  Note that other groups may also
     distribute working documents as Internet Drafts.

     Internet Drafts are valid for a maximum of six months and may be
     updated, replaced, or obsoleted by other documents at any time.
     It is inappropriate to use Internet Drafts as reference material
     or to cite them other than as a "work in progress".

     2.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.

     3.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.  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.


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     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 [4] 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.

     4.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 [1]. 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.

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     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 streaming, 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 afer 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 ``
     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.

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     5.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 [5]
          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 [3]) with arbitrary octet content being
          sent. The revised syntax of the value is as follows, using
          the ABNF notation of [2]:

               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
     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 [1], except that a BODY parameter must appear after
     the address.  The complete syntax of this extended command is
     defined in [4]. The ESMTP-keyword is BODY and the syntax for
     ESMTP-value is given by the syntax for body-value in [4].

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     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:

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

        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 [3].

     6.Examples

     6.1. 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

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     6.2. Streaming Binarymime

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

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     7.Security Considerations

     This RFC does 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 [1], or
     otherwise made possible by [3].

     8.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.

     9.References

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

     [2] Crocker, D., "Standard for the Format of ARPA Internet Text
        Messages", STD 11, RFC 822, UDEL, August 1982.

     [3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
        Extensions", RFC 1341, Bellcore, Innosoft, June 1992.

     [4] Klensin, J., WG Chair, Freed, N., Editor, Rose, M.,
        Stefferud, E., and D. Crocker, "SMTP Service Extensions" RFC
        1425,

     [5] Klensin, J., WG Chair, Freed, N., Editor, Rose, M.,
        Stefferud, E., and D. Crocker, "SMTP Service Extension for
        8bit-MIMEtransport" RFC 1426, United Nations University,
        Innosoft International, Inc., Dover Beach Consulting, Inc.,
        Network Management Associates, Inc., The Branch Office,
        February 1993.

     10.  Author's Address

     Gregory M. Vaudreuil
     Octel Network Services
     17060 Dallas Parkway
     Suite 214
     Dallas, TX 75248-1905
     Greg.Vaudreuil@ons.octel.com

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