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RFC 1830 - SMTP Service Extensions for Transmission of Large and


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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 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 [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, RFC 822, UDEL, August 1982.

     [MIME] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
        Extensions", RFC 1521, 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" RFC 1426, 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
   EMail: Greg.Vaudreuil@Octel.com

 

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