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

   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

   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.