RFC 1047 (rfc1047) - Page 2 of 3
Duplicate messages and SMTP
Alternative Format: Original Text Document
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
