Internet DRAFT - draft-atkins-smtp-traffic-control
draft-atkins-smtp-traffic-control
Internet Engineering Task Force S. Atkins
Internet-Draft Word to the Wise
Intended status: Experimental October 23, 2011
Expires: April 25, 2012
Next-Hop SMTP Traffic Control
draft-atkins-smtp-traffic-control-00
Abstract
This document defines an extended SMTP response for mail servers to
assist clients in adjusting the rate at which they send mail. Use of
this mechanism can reduce wasteful SMTP connections and reduce mail
delivery delays; at scale the aggregate benefit to clients and
servers can be significant.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 25, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Response Structure . . . . . . . . . . . . . . . . . . . . . . 4
3. Server Recommendations . . . . . . . . . . . . . . . . . . . . 4
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Requirements Language . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Ideally a mail server would deliver all mails to recipients'
mailboxes immediately, but that is not always possible due to limited
system resources.
For active sites, mail volume can be very high. Clients sometimes
attempt to deliver large quantities of mail in a very short time
period, often mail which has low perceived value to the recipient.
Those spikes in mail volume can exceed server capacity for short
periods and delay processing of other mail with a higher perceived
value. Such servers therefore attempt to throttle traffic from
clients with a lower perceived value.
Servers will also throttle or defer mail from senders for other
reasons, including reputation or behaviour based spam filters that
prefer to wait for more data before they make a delivery decision.
There are two approaches currently used to throttle mail traffic.
One is to limit the bandwidth of the transport the SMTP session is
running over. By delaying the delivery of each message while keeping
the SMTP session open it causes each connection to be held open for
longer, significantly increasing resource usage at both ends of the
connection.
The other approach is to defer accepting a message with an RFC 5321
[RFC5321] transient negative completion reply, typically "421",
relying on the client to resend the message later. This can cause
many repeated, failed delivery attempts when the client retries
delivery of the same message rapidly, or attempted delivery of many
(thousands) of messages, all of which are deferred. That pattern of
high volumes of deferred connections can have an aggregate effect
that is expensive and wasteful for both the client and server.
Many clients will retry delivery only after a fairly long period
(RFC 5321 [RFC5321] suggests 30 minutes) which can cause mail
delivery to be significantly delayed. Long delivery delays are an
obvious problem for time-sensitive messages, bit they can also cause
problems indirectly by causing messages to be delivered out of
chronological order.
If the server is only temporarily overloaded, and so needs to reduce
traffic for just a short period (seconds to minutes), then a shorter
retry delay reduces those problems.
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2. Response Structure
This specification defines a new extended response that SMTP server
can include in the text section of any SMTP response - separated by
white space from the rest of the text section. It's intended use is
to refine the second mechanism described above, to guide future
transfer attempts by the client.
response = "wait:" delay
delay = 1*5DIGIT ; an integer in the range 0 to 99999
DIGIT = 0x30-0x39
The delay value is the length of time, in seconds, that the server is
asking the client to wait before sending further mail. A value of
"0" is valid, with the meaning that the client SHOULD gracefully
terminate the current connection, but need not wait before attempting
future connections. Disconnecting in this way, rather than keeping
the connection open for further messages, moves the client to the
back of the connection queue and gives competing clients an
opportunity to connect.
If a client sees this additional response in any SMTP response from a
server then it SHOULD gracefully close the SMTP connection by sending
a QUIT command as soon as possible. It SHOULD NOT make any new
connection to the same server for the duration of the delay. If
possible it SHOULD gracefully close any other connections to the same
server. If the implementation of the client makes it difficult to
implement some of these behaviours, it can still usefully implement
the other requirements.
Other than this, it makes no changes in the semantics of the SMTP
transaction.
3. Server Recommendations
A server can add a wait: additional response to any permanent or
transient negative completion reply ("5xx or 4xx error") to ask the
client to disconnect and wait before reconnecting.
A server can also usefully add a wait: additional response to a 250
positive completion response at the end of the DATA section to ask
the server to disconnect and wait before reconnecting. At that point
in the transaction a "250 wait:" asks the client to disconnect rather
than attempting to send further messages in the same transaction.
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4. Examples
"421 4.3.2 wait:15 example.com Service not available" means the
client should retry delivery of the mail, after at least 15 seconds
and not attempt any other deliveries for at least 15 seconds.
"554 wait:60 Transaction failed" means the client should not retry
delivery of this message, and should not attempt any other deliveries
for at least 60 seconds.
"250 OK wait:90" in response to end of data means that the mail has
been accepted for delivery and that the client should send a QUIT
command, and not attempt delivery of any other messages for 90
seconds.
5. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
This document describes an additional response code. Use of this
response code may disclose additional information about how a mail
system is implemented beyond that currently available.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
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Author's Address
Steve Atkins
Word to the Wise
Palo Alto, CA
US
Email: steve@wordtothewise.com
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