Browse Prior Art Database

Duplicate messages and SMTP (RFC1047)

IP.com Disclosure Number: IPCOM000001853D
Original Publication Date: 1988-Feb-01
Included in the Prior Art Database: 2000-Sep-12
Document File: 2 page(s) / 5K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

C. Partridge: AUTHOR

Abstract

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.

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 56% of the total text.

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. Observa...