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+Network Working Group                                   Craig Partridge
+Request for Comments: 1047                              CIC at BBN Labs
+                                                          February 1988
+
+
+                      DUPLICATE MESSAGES AND SMTP
+
+
+STATUS OF THIS MEMO
+
+   An examination of a synchronization problem in the Simple Mail
+   Transfer Protocol (SMTP) is presented.  This synchronization problem
+   can cause a message to be delivered multiple times.  A method for
+   avoiding this problem is suggested.  Nodding familiarity with the
+   SMTP specification, RFC-821, is required.  Distribution of this memo
+   is unlimited.
+
+INTRODUCTION
+
+   Over the past few years, the staff of the CSNET Coordination and
+   Information Center (CIC) has often been asked to help determine why a
+   single mail message is being delivered multiple times to its
+   recipients.  In the process of tracing the problems of multiple
+   delivery, we have discovered that many duplicate messages are the
+   result of a synchronization problem in SMTP.  There is a point in the
+   process of delivering a message where the receiving mailer knows it
+   has accepted the message but the sending mailer is still not sure the
+   message has been reliably delivered.  If the SMTP conversation is
+   broken at this point, the sending mailer will be forced to re-deliver
+   the message, even though the message has already been received and
+   delivered by the receiving mailer.
+
+DESCRIPTION OF THE PROBLEM
+
+   The synchronization problem occurs at the end of delivering a
+   message.  When the sending mailer has finished sending the text of a
+   message, it is required to send a line containing a single dot or
+   period.  When the receiving mailer receives this final dot, it is
+   expected to do its final message processing and either confirm
+   receipt of the message (with a 250 reply) or reject the message with
+   any one of several error codes.
+
+   Observe that there is a potential synchronization gap here.  During
+   the period between the time the receiving mailer has determined that
+   it will accept the message, and the time that sending mailer gets the
+   250 reply, the message is active at both the sending and receiving
+   mailer.  Until the sending mailer gets the 250 reply, it must assume
+   the message was not delivered.  After the receiving mailer has
+
+
+
+Partridge                                                       [Page 1]
+
+RFC 1047              DUPLICATE MESSAGES AND SMTP          February 1988
+
+
+   decided to accept the message, it must assume the message has been
+   delivered to it.  If the communications link fails during this
+   synchronization gap, then the message has been duplicated.  Both
+   mailers have active copies of the message that they will try to
+   deliver.
+
+   It may be hard to believe that this problem is the cause of many
+   duplicate messages.  Intuitively, one might expect that the time
+   spent in the state between the final dot and its accepting 250 reply
+   is quite small.  In practice, however, this period is often quite
+   long; long enough that timeouts by the sending mailer (or possibly
+   network failures) are quite common.  Observations by the author
+   suggest that this synchronization problem may be the second leading
+   cause of duplicate messages on the Internet (second to mail loops).
+
+   Many mailers delay responding to the final dot because they are doing
+   sophisticated processing of the message, in an attempt to confirm
+   that they can deliver the message.  For example, the mailers may
+   expand an entire mailing list to confirm that it can reach all
+   addressees or may attempt to physically deposit the message into the
+   mailboxes of local users, before confirming receipt of the final dot.
+   These practices are not unreasonable, but they often cause the
+   synchronization gap to continue for several minutes, and increase the
+   likelihood that the sending mailer will timeout or the network will
+   fail before the accepting 250 reply is sent.
+
+AVOIDING SYNCHRONIZATION PROBLEMS
+
+   The best way to avoid the synchronization problem is to minimize the
+   length of the synchronization gap.  In other words, receiving mailers
+   should acknowledge the final dot as soon as possible and do more
+   complex processing of the message later.
+
+   RFC-821 (on page 22) states that unless the receiving mailer is
+   completely unable to process a message it should accept the message
+   and acknowledge any errors in processing in a separate message or
+   messages sent back to the originator of the message.  As a result,
+   receiving mailers should be able to acknowledge the final dot as soon
+   as the message has been safely put in a non-volatile (e.g., disk)
+   queue for further processing.  Fast acceptance of a message does not
+   violate RFC-821.
+
+   Some mailers can be configured to do more or less processing upon
+   receipt of the final dot.  In such situations, the mailer should
+   always be configured to do less processing.
+
+   Finally, some mailers allow remote mailers only a minute or two to
+   acknowledge the final dot before timing out and trying again.  Given
+
+
+
+Partridge                                                       [Page 2]
+
+RFC 1047              DUPLICATE MESSAGES AND SMTP          February 1988
+
+
+   the increasing round-trip times on the Internet, and that some
+   processing after the final dot is required, the timeout for reply to
+   the final dot should probably be at least 5 minutes and a timeout of
+   10 minutes would not be unreasonable.
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+Partridge                                                       [Page 3]
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