IETF fax WG G. Klyne, MIMEsweeper Group
Internet draft D. Crocker, Brandenburg Consulting
5 November 2001
Expires: May 2002
Timely Completion for Internet Messaging Services
<draft-ietf-fax-timely-delivery-05.txt
Status of this memo
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Copyright Notice
Copyright (C) The Internet Society 2001. All Rights Reserved.
Abstract
This specification provides a way to request timely completion for
Internet mail delivery, for services such as facsimile and voice
messaging. Traditional Internet mail uses a _postal_ mail model,
with normal delivery having an indeterminate gap between delivery
into a mailbox and processing by the recipient. Timely completion
adds a timelines service feature and extends delivery processing
all the way to the recipient. This specification provides a
deterministic service quality response, while preserving most of
the traditional roles and responsibilities of the agents involved
in email transfers.
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It is essentially a profile of the Delivery Service Notification
(DSN) and DELIVERBY extensions for ESMTP, along with a new TIMELY
option to DELIVERBY with a new deterministic service quality
response.
Discussion of this document
Please send comments to: <ietf-fax@imc.org>.
To subscribe: send a message with the body 'subscribe' to
<ietf-fax-request@imc.org>. The mailing list archive is at
<http://www.imc.org/ietf-fax/>.
Table of contents
1. Introduction.............................................4
1.1 Structure of this document ...........................4
1.2 Document conventions .................................5
1.3 Terminology ..........................................5
2. Background and goals.....................................6
2.1 Background ...........................................7
2.2 Basis for timely completion ..........................7
2.2.1 The SMTP "contract"..............................8
2.2.2 Framework for timely delivery....................8
2.3 What does the TIMELY option add? .....................9
2.3.1 DELIVERBY and timely delivery....................10
2.4 Goals for timely completion ..........................10
3. Mechanisms for timely completion.........................11
3.1 Transmitting a message for timely completion .........11
3.2 Relaying a message ...................................12
3.3 Delivery MTA message acceptance ......................14
3.3.1 Timing of final receipt..........................14
3.4 Reporting failures ...................................15
3.5 Timely confirmation ..................................16
4. Timely extension to ESMTP Deliver By extension...........17
4.1 Framework for TIMELY extension to DELIVERBY.............18
4.2 Extension to EHLO DELIVERBY keyword.....................18
4.3 MAIL FROM: TIMELY parameter.............................18
5. DSN reporting extensions.................................19
5.1 New extended mail system status codes ................19
5.2 'Retry-count' per-recipient DSN header ...............19
6. Implementation notes.....................................20
6.1 Message state management .............................20
6.2 Retransmission timing issues .........................21
6.3 Delivery timing granularity ..........................22
6.4 Partial success ......................................23
6.5 Routing TIMELY and non-TIMELY messages ...............23
6.6 Expediting message handling ..........................24
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7. Examples.................................................24
7.1 Timely delivery and confirmation .....................24
7.2 Received by delivery MTA and timed out ...............26
7.3 Timed out with delivery in progress ..................28
7.4 Timed out before receipt by delivery MTA .............29
7.5 Timely delivery feature not supported ................31
8. IANA Considerations......................................33
9. Internationalization considerations......................33
10. Security considerations.................................33
11. Acknowledgements........................................33
12. References..............................................34
13. Authors' addresses......................................35
Appendix A: Amendment history...............................36
Full copyright statement....................................39
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1. Introduction
Traditional Internet mail uses a _postal_ mail model, with normal
delivery placing a message into the recipient's mailbox and the
recipient retrieving and processing the message sometime later.
For traditional mail, delivery responsibility stops at the mailbox.
However some uses of messaging require a service model that
confirms receipt by the actual recipient, not just delivery into
their mailbox. Also, some uses require delivery within a specified
period of time.
Timely completion adds this timeliness service feature and extends
delivery processing all the way to the recipient. This
specification provides a deterministic service quality response,
while preserving most of the traditional roles and responsibilities
of the agents involved in email transfers.
RFC 1891 [4] defines an ESMTP extension for Delivery Service
Notification (DSN), and RFC 2852 [5] defines one for requesting
delivery of a message within a given interval. Timely Completion
is essentially a profile of the DSN and DELIVERBY extensions for
ESMTP, along with a new TIMELY option to DELIVERBY with a new
deterministic service quality response This memo describes how to
use those specifications, along with some small extensions, to
achieve timely completion of message delivery.
1.1 Structure of this document
Section 2 gives the background, principal ideas and goals of this
specification.
Section 3 describes the mechanisms used, and how they are combined
to achieve the timely delivery goals.
Section 4 describes an addition to the ESMTP "Deliver by" extension
which is one of the mechanisms used to achieve timely delivery.
Section 5 describes extensions to the DSN reporting format and
status codes used to report conditions related to timely delivery
requests.
Section 6 contains some non-normative discussion of implementation
issues related to this specification.
Section 7 contains some examples uses of this specification.
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1.2 Document conventions
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 [11].
NOTE: Comments like this provide additional nonessential
information about the rationale behind this document.
Such information is not needed for building a conformant
implementation, but may help those who wish to understand
the design in greater depth.
1.3 Terminology
Delivery
is the process performed by a delivery MTA in attempt to
achieve final receipt of a message.
Final receipt
means a message is accepted by a receiving agent, and is
immediately available for disposition (i.e. received by a
disposing agent). For example, if mail is received using a
protocol like POP or IMAP, final receipt is not deemed to have
occurred until the message has been transferred to the
recipient's system. The difference between delivery and final
receipt is due to the ability have an MTA store a message in a
user mailbox, but not be able to notify recipient software of
the delivery. Hence there can be arbitrary delay between the
time the message is delivered into the mailbox and recipient
user's software agent does any processing.
Disposition
is receipt and processing of a message by a recipient. Timely
notification of disposition is problematic to achieve because
it is not always possible to determine that disposition has
occurred, and in any case may be undesirable for privacy
reasons.
Best effort
indicates that a system will ensure that an assigned task is
successfully completed under all but the most catastrophic of
failure circumstances. Common failure modes, such as power
failures, SHOULD NOT prevent eventual completion of a task.
Reasonable effort
indicates that while a system will try to complete an assigned
task, it MAY also indulge in behaviours, or make operational
decisions, that significantly reduce the certainty of an
action's being completed in the face of disruptive
circumstances.
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Delivery interval
a period of time, measured in seconds, allowed for completion
of message delivery and/or receipt.
Timely delivery
means a message is delivered to a recipient's MTA within a
specified time interval.
Timely delivery
means a message is delivered by a recipient's mail transfer
agent (MTA) -_ that is, typically, handed to the user's
mailbox -- within a specified time interval.
Timely receipt
means final receipt of a message, by active recipient user
software, within a specified time interval.
Timely completion
means that notification of a requested timely receipt is
received by the sender of a message within a determined time
interval. Non-receipt of notification within that interval is
indicative of failure.
MTA, Mail Transfer Agent
is an email system component with the roles of receiving,
transferring, and delivering messages.
MUA, Mail User Agent, disposing agent
is an email system component with the role of preparing and
sending, and/or receiving and processing, messages. MUAs are
the endpoints between which emails are sent; MTAs are relays
on the path between a sending MUA and a receiving MUA.
Delivery MTA
is the final MTA in an MTA relay sequence; it accepts a
message and passes it to the receiving MUA.
2. Background and goals
RFC 2852 [5] provides a mechanism to request timely delivery of a
message using SMTP. While this is helpful, it falls short for some
usage profiles, such as timely processing of fax messages. These
profiles are determined, in part, by the capabilities of
traditional facsimile [8].
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2.1 Background
Traditional email [2] is open-loop. The sender of a message
normally has no certainty if or when a message is delivered. (A
separate memo [6] contains a discussion of some open- and closed-
loop issues in email.)
To be more than just a hint to the message transfer system, timely
completion requires a deterministic confirmation mechanism, to
close the loop. This is provided by DSN [4].
Some different levels of timeliness can be identified:
(a) timely delivery to the recipient's MTA
(b) timely final receipt by the recipient's disposing agent
(c) timely disposition (receipt and processing) by the recipient
(d) timely notification to the sender of delivery
(e) timely notification to the sender of final receipt
(f) timely notification to the sender of disposition by the
recipient's user agent
From the sender's point of view, timely confirmation of disposition
is the most desirable requirement. As noted previously this can be
problematic, but timely notification of final receipt is
practically as useful.
2.2 Basis for timely completion
A premise of the service specified here is that timeliness CAN be
achieved using existing protocols, with appropriate software design
and operational management. But the sender and receiver do not
control all the relays used:
o The real issue is lack of determinism: a message might be
delivered quickly, or it might take hours or even days, or it
might not be delivered at all; the sender has little knowledge
and no control.
o A second issue is post-delivery handling: will the recipient's
user agent receive the message in timely fashion?
Then, assuming that the infrastructure is generally capable of
achieving the desired timely completion, the main thrust of this
memo is provide protocol enhancements that put the sender in
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complete control on those occasions when timeliness is not
achieved.
One challenge to achieving this is dealing with uncertain transit
times of the confirmation message over the return path (which is
not necessarily the same as the forward path).
2.2.1 The SMTP "contract"
On accepting a message, a normal SMTP message transfer agent (MTA)
accepts responsibility to:
(a) Use a best effort to ensure ultimate delivery of the message,
or
(b) Attempt to provide notification that delivery could not be
achieved.
This memo introduces mechanisms to allow this contract to be
modified. A timely-completion MTA accepts responsibility to:
(a) Use a reasonable effort to ensure delivery within a specified
time, and to provide timely confirmation of this, or
(b) Provide timely notification that delivery was not achieved as
requested.
The sender can then decide a recovery strategy
2.2.2 Framework for timely delivery
The diagram shows typical SMTP message delivery and delivery status
notification (DSN) paths. (The confirmation path is not
necessarily the same as the message delivery path.)
Outbound message -->
+-------+ +-----+ +-----+ +---------+
|Sending|-->--|Relay| >>> |Relay|-->--|Receiving|
| MTA | | MTA | | MTA | | MTA |
+-------+ +-----+ +-----+ +---------+
| | |
^ | v
| | |
+-------+ | +---------+
|Sending| | |Receiving|
| UA |-<------------- <<< ------------- | UA |
+-------+ +---------+
<-- Return confirmation (disposing
agent)
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As well as requesting timely delivery of a message, this
specification needs to take account of the possibly varying
characteristics of relays of the outbound and return message paths.
Practically, it is possible to require that every relay on the
outbound path recognizes timely completion semantics (using the
ESMTP extension framework), but it is not possible to require this
of every relay on the return path. Thus, it may be necessary to
make some assumptions about the confirmation return path.
NOTE: The uncertainty about return path characteristics
might be removed by requiring an MTA to send any timely
delivery notification to the MTA from which it was
received, but this goes against trends in SMTP design and
deployment. This might also raise state maintenance and
hence scalability concerns.
The other issue apparent from the diagram is that providing timely
delivery through the SMTP message relays does not ensure that the
receiving UA will receive the message in a timely fashion. If the
receiving MTA delivers to a POP mailbox, there is currently no way
that it can guarantee timely receipt by the disposing agent.
2.3 What does the TIMELY option add?
The TIMELY option adds three elements to the DELIVERBY extension:
o It modifies the SMTP contract, permitting an MTA to commute its
responsibility for delivering the message from "best effort" to
"reasonable effort", with notification of outcome.
o It extends the reach of the timeliness constraint to cover final
receipt: i.e. hand-off to a disposing agent (see below).
o It establishes a basis for determining the allowable time
interval for certain behaviours initiated by the receiver of a
message (i.e. DELIVERBY interval for delivery status responses,
and how long to wait for possible duplicate message
transmissions).
This is all consistent with the fundamental strategy of seeking to
give the sender control over the whole message transfer process:
if an MTA or the receiving agent cannot communicate the required
guarantee, delivery is not completed and the sender is duly
notified.
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2.3.1 DELIVERBY and timely delivery
Timely completion of delivery is handled by the DELIVERBY ESMTP
extension base specification [5]. However its scope ends with
final delivery by SMTP, not covering final receipt by the disposing
agent. The TIMELY option modifies the DELIVERBY semantics to cover
the additional step needed for the message to reach the recipient.
Consider the following scenario:
+------+ +-----+ +---------+ ------- +---------+
|Sender|-->--|Relay|-->--|Receiving|->-(Message)->-|Disposing|
| MTA | | MTA | | MTA | ( store ) | agent |
+------+ +-----+ +---------+ ------- +---------+
<------SMTP-------> <-------?------->
The base DELIVERBY specification concerns itself with only
participants in the SMTP transfers. But for the purposes of timely
completion of final receipt, the sender must be able to specify the
timeliness constraint to include this extra step.
The TIMELY option requires that the receiving MTA communicate with
the disposing agent (in some unspecified way), and that it confirm
final delivery of the message only if the disposing agent confirms
that it will deal with the message in timely fashion, or that it
will return an indication to the receiving MTA if it fails to do
so. Simply putting the message into a POP mailbox would not meet
this criterion.
2.4 Goals for timely completion
The primary goal is to allow consenting parties to establish a
relationship that carries a guarantee of final receipt within a
specified time, or timely notification that it was not achieved.
Further goals:
o Provide "while-you-wait" delivery of messages by email, where
available infrastructure and connectivity permit.
o Deterministic behaviour, whereby sender who requests timely
completion is able to determine with reasonable certainty, and in
reasonable time, whether that request was successful.
o If the message cannot be delivered as requested, it SHOULD NOT be
delivered at all. This means that a sender can choose other
strategies for message delivery (e.g. if timely delivery by email
does not succeed, to resend the message as a traditional
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facsimile; in such circumstances it is preferable that multiple
copies of the message are not delivered).
o Operate within the existing ESMTP extension framework [3], using
existing facilities where available.
3. Mechanisms for timely completion
Deterministic timely completion is achieved through a number of
ESMTP extensions used in concert:
o Delivery Status Notification ("DSN"), per RFC 1891 [4].
o Deliver-by ("DELIVERBY"), per RFC 2852 [5]
o A new TIMELY extension to DELIVERBY, that serves to modify the
SMTP contract and also to establish that the receiving user agent
can process the message in timely fashion as required, or provide
timely notification of its failure to do so
The confirmation loop for successful delivery looks something like
this:
+-----------+ +--------+ +--------+ +---------+
|Originating|-->--|Relaying| ... |Relaying|-->--|Receiving|
| MTA | | MTA | | MTA | --| MTA |
+-----------+ +--------+ +--------+ | +---------+
| | |
+-------------+ | +---------+
| Originating |--<-- ... .... ... --<-- |Receiving|
| MUA | | MUA |
+-------------+ +---------+
The path through MTAs taken by the confirmation response is not
defined, and may be different than the forward path of the original
message.
3.1 Transmitting a message for timely completion
A transmitted message for which timely completion is required MUST
include the following:
o An 'ENVID' parameter on the MAIL FROM command, per DSN [4]
o An 'ORCPT' parameter on the corresponding RCPT TO command(s), per
DSN [4]. (This is to allow the sender to tell exactly which
recipients were successfully delivered.)
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o A 'NOTIFY=SUCCESS,FAILURE' parameter on the corresponding RCPT TO
command(s), per DSN [4]
o A 'BY' parameter on the MAIL FROM command, per [5], with a 'by-
mode' value of 'R'.
o A 'TIMELY' parameter on the MAIL FROM command, as described
below, initially having the same time interval as specified for
'BY'.
The message MUST NOT be transmitted to any MTA that does not
indicate support for all of these extensions, in its response to
the EHLO command. In this case, a negative delivery status report
MUST be generated, which SHOULD indicate the non-compliant MTA, the
extensions that it does not support, and the name of the reporting
MTA (per DSN, using the non-compliance reporting extensions noted
later).
Standard DNS MX-based message routing, per RFC 974, SHOULD be used
when sending or relaying the message.
NOTE: Any strategies that vary standard MX routing
should be used with care, and only with the goal of
improving network transit times and timing consistency.
These comments about mail routing apply especially to the
handling of DSN responses.
Ideally, there will be no intermediate relay between the
sending and receiving MTAs, and in any case the number of
such relays should be minimized to reduce timing
variability on the transfer path.
3.2 Relaying a message
An MTA that relays a message for timely completion MUST support all
of the ESMTP extensions noted above; otherwise it MUST NOT be
given the message in the first place. When a relaying MTA accepts
a message (by its 2xx status response to receipt of the message
data), it becomes responsible for its onward delivery, including
satisfying all of the options associated with the message.
In order to relay such a message, an MTA MUST note when the message
was received, and the time when the attempt to transmit the message
to the next MTA is initiated, and reduce accordingly the time
interval used for the BY parameter. (The time interval SHOULD be
taken to start with receipt of the MAIL FROM command.)
If the DELIVERBY time interval is reduced to zero or less (or less
than some system-configurable value indicating that delivery within
the indicated interval is unlikely to be achieved) then the message
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MUST NOT be relayed. Instead, a negative delivery status report
MUST be generated indicating that the time for delivery of the
message has expired, and the reporting MTA (per DSN, using the
deliver-by extensions and/or non-compliance reporting extensions
noted below).
The above behaviour is as specified for the DELIVERBY ESMTP
extension; see RFC 2852 [5] for a definitive description of how to
handle relaying of such messages. The following additional
considerations are applicable when the TIMELY option is used:
The TIMELY parameter in the MAIL FROM command of a message in
transit is copied unchanged when the message is retransmitted.
Thus, any originally specified time interval is conveyed to the
final MTA, to be used as a basis for selecting a delivery interval
for returning a timely notification.
Standard DNS MX-based message routing, per RFC 974, SHOULD be used
when relaying the message. (See note at end of previous section.)
If the first attempt to relay a message fails, the relaying MTA MAY
assume that delivery within the desired time will not be achieved,
and immediately indicate a delivery failure, indicating the name of
the next-hop MTA. Alternatively, the relaying MTA MAY wait and
retry the transmission, provided that the retry attempt will be
performed within the remaining delivery period; if the
transmission cannot be completed after one or more such retries
then a negative DSN MUST be generated as noted above.
Any negative DSN generated SHOULD indicate the number of retries
attempted (where 0 means no retries).
The choice to retry or not to retry is installation dependent.
Effectively, when a relay does not retry, any responsibility for
overcoming the delivery failure is passed back to the original
sender. This strategy may be appropriate for cases where very
rapid delivery is required or expected.
NOTE: The presence of a 'TIMELY' option may cause a
relay to abandon a message that it would otherwise retry
(even given a 'by-mode' value of 'R'). One purpose of
this option is to establish that responsiveness to the
sender is more important than getting the message
through. An effect of this may be to severely constrain
the number and frequency of retry attempts.
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3.3 Delivery MTA message acceptance
The MTA that performs final delivery of a message has
responsibility for passing the message to a Mail User Agent. The
exact mechanism by which this is achieved is a local matter, and is
not defined here or by the Internet mail specifications. The
delivery MTA, or its agent, is also responsible for generating any
successful DSN message.
Before generating a success DSN message, the final MTA MUST ensure
that all of the conditions for timely completion of the message
have been achieved. Specifically, when the TIMELY option is used,
it MUST ensure that final delivery to the disposing agent will be
completed within the delivery interval indicated as the value of
the BY parameter of the received MAIL FROM command.
The time interval for completion of final receipt SHOULD be taken
to start with receipt of the MAIL FROM command.
NOTE: Final receipt by an MUA is expected to include some
guarantee of timely processing. Exactly what this
constitutes may depend on the circumstances: in a simple
case, depositing the message in a local mailbox and
immediately notifying the recipient possibly constitutes
final receipt. A more complex case would be that of a
fax offramp, where final receipt may be completion of a
successful outdial and transmission of the fax.
3.3.1 Timing of final receipt
In the presence of a TIMELY option, final receipt SHOULD NOT be
indicated unless the delivery MTA can establish that the receiving
MUA will deal with the message promptly. Here "promptly" means a
reasonable waiting time for a human; e.g. that the message (or at
least the start of the message) will be available to its intended
final recipient within a period of, say, 30 seconds.
The relationship between the delivery MTA and receiving MUA can
work in one of two ways:
o The MUA always processes the message promptly, barring
exceptional circumstances. Queuing a message to a network
printer would constitute such processing -- normally the message
will be printed within seconds, even though it might be delayed
if the printer runs out of paper. The delivery MTA can generate
the final DSN when the MUA has accepted the message.
o The MUA attempts to process the message promptly and reports the
outcome within the remaining DELIVERBY period. If processing is
not performed within the stated period, the message is abandoned
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and failure is signalled back to the delivery MTA. The delivery
MTA must hold off generating the final DSN until the MUA has
provided a status report; if no such report is provided within
the remaining DELIVERBY interval, it SHOULD report failure.
3.4 Reporting failures
When a relay or receiving MTA determines that a message cannot be
delivered as requested to any recipient, a DSN report MUST BE sent
back to the sender.
The following status codes indicated that message delivery has been
abandoned, are used with DSN "Action: failed" for reporting
conditions that are specific to timely delivery:
4.4.7: Delivery time expired -- failed.
Message delivery could not be completed within the specified
time interval. This code is also used when the final MTA has
accepted the message but has been unable to achieve final
receipt within the requested interval.
5.4.9: Protocol required for timely delivery not supported.
A relay MTA was encountered that did not support the range of
capabilities required for timely completion. Defined by [15].
4.4.1: Next MTA not accepting messages.
A relay MTA has been unable to contact a next-hop MTA, and has
decided to abandon delivery. (See note in section 3.2 about a
relay's options with respect to retries.) This code SHOULD be
accompanied by a 'Retry-Count' DSN field.
4.3.3: Receiving MTA cannot honour required timely receipt.
A message has been delivered to a receiving MTA within the
required delivery interval, but that MTA is unable to ensure
timely receipt or timely notification of failure to do so.
The following status code is used with DSN "Action: delayed" for
reporting delayed message receipt following delivery:
4.4.7: Delivery time expired -- continuing.
The message has been received by the delivery MTA and is in
the process of final delivery, but that final delivery has not
yet been completed.
A subsequent DSN SHOULD be sent when the final delivery
succeeds or fails.
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This status code is defined for situations where a receiving
MTA has handed off the message to another agent for final
delivery, and has therefore committed to provide a timely
confirmation response, but the delivery agent has not
signalled completion. For example, a fax dial-out gateway may
have been invoked assuming that the outdial leg would complete
within a given period, but has failed to do so.
3.5 Timely confirmation
Fully deterministic behaviour requires that the round-trip time to
deliver a message and receive a confirmation response be within a
known time interval.
As noted above, it cannot be guaranteed that confirmation of
delivery or non-delivery will be transferred in timely fashion,
although return path transit times often are comparable with
forward path times. Use of the DELIVERBY extension for a message
confirmation MAY serve to expedite its forwarding (noting that this
is not a required behaviour; see implementation notes section 6.1
for further discussion).
Further, it is likely that perfect determinism can never be
achieved using SMTP; e.g. see RFC 1047 [9]. Repeat deliveries are
considered less harmful than lost messages, but even these should
be minimized.
The following behaviour is followed to achieve near-deterministic
timely confirmation:
o Always fail forward delivery if a non-TIMELY MTA is encountered.
o A return DSN does not itself request delivery notification and
has an empty return path (as required for DSN).
o Do NOT use the TIMELY option on any DSN return, so that
notification delivery does not fail if a non-DELIVERBY or
non-TIMELY MTA is encountered on the return path.
o Use the DELIVERBY option to request timely delivery for any DSN
return, using a delivery interval 2 times the original forward
path DELIVERBY time (taken from the received TIMELY parameter),
specifying a 'by-mode' value of 'R'. (The lack of a return path
on the DSN response will mean that neither success or failure
notification will be generated: if the DSN cannot be returned
within the time given, it is silently dropped.)
o The sender MAY assume that the message is lost after 3 times the
original DELIVERBY interval has passed without notification.
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NOTE: The above timings are based on a working assumption
that normal transit times do not vary by more than a
factor of two. There is nothing scientific about this
choice of value, but laying down an assumption provides a
basis for defining some operational parameters used by
cooperating parties, which in turn provides some basis
for deterministic behaviour.
The purpose of this specification is to give the sender control
over the recovery strategy to be used if timely delivery does not
succeed. It is therefore beyond its scope to set out exactly what
recovery action the sender should take. One possible action is to
retry the transmission, in which case the following additional
considerations apply:
o Retries should be used very sparingly, as the likely cause of
failure is either a permanent network condition or network
congestion. In the case of congestion, retries are likely to
make things worse. (The design of the TCP protocol takes account
of many lessons about network behaviour that have been learned
over the years. A particularly important strategy used is
exponential back-off when retransmitting.)
o The sender is required to provide envelope ID with message. If
it re-tries, it MUST use same envelope ID and SHOULD do so within
a reasonable period of determining the original message has not
been delivered.
o The receiver of a TIMELY message SHOULD keep note of the received
envelope ID for some period, for the purpose of weeding out
duplicates.
4. Timely extension to ESMTP Deliver By extension
The purpose of this extension is to allow a message sender to
require that timely delivery semantics, described in this memo, be
supported all along the path from message sender to receiving
agent, in addition to the existing semantics of DELIVERBY.
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4.1 Framework for TIMELY extension to DELIVERBY
This extends the framework template for DELIVERBY, given in RFC
2852 [5]:
(1) ESMTP extension name:
"Deliver by", extended for "timely completion"
(2) EHLO keyword:
DELIVERBY, extended as described below
(3) EHLO keyword parameters:
TIMELY (see 4.2 below)
(4) SMTP command parameters:
MAIL FROM: TIMELY (see 4.3 below)
(5) The maximum length of a MAIL FROM command line is increased by
a further 17 characters for the TIMELY parameter (this being in
addition to the 17 character extension for the basic DELIVERBY
extension.
(6) Additional SMTP commands:
(none)
4.2 Extension to EHLO DELIVERBY keyword
This specification defines an extension token for timely
completion. The extension token syntax (from RFC 2852 [5]) is
extended thus:
extension-token /= "TIMELY"
An ESMTP server that supports this timely completion extension MUST
also support the delivery status notification (DSN) ESMTP
extension.
Support for the timely completion extension indicates support for
the MAIL FROM: TIMELY parameter, described below, and for all the
associated processing semantics.
4.3 MAIL FROM: TIMELY parameter
The MAIL FROM command TIMELY parameter MUST be used in conjunction
with a BY parameter. Its use imposes requirements on the receiving
server's handling of the message that are in addition to those
imposed by the BY parameter.
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TIMELY parameter syntax:
timely-parameter = "TIMELY=" interval
interval = 1*9DIGIT
The 'interval' is specified by the original sender of a message to
be the same as the corresponding BY parameter value.
A mail relay copies the received TIMELY value to the retransmitted
message, unchanged. In this way, the originally specified delivery
interval is available to all MTAs that handle the message. The
TIMELY value is used when generating a DSN response.
The effect of a TIMELY parameter is to require that message
processing be performed in accordance with the timely completion
mechanisms described in section 3 above.
5. DSN reporting extensions
This specification defines some DSN reporting extensions to allow
additional status information to be returned, which a sending
system might use in choosing a recovery strategy.
5.1 New extended mail system status codes
This specification uses the following additional enhanced mail
system status codes, extending the range of those defined by RFC
1893 [13]:
5.4.9: Protocol required for timely delivery not supported.
(Defined by [15].)
See section 3.4 for a more detailed description.
5.2 'Retry-count' per-recipient DSN header
This memo defines an additional per-recipient DSN report field
'Retry-count':
retry-count-field = "Retry-Count" ":" 1*3DIGIT
This field is used in conjunction with status code 4.4.1 to
indicate the number of retries attempted before delivery was
abandoned. A value of "0" means that no retries were attempted.
The purpose of this is to provide information to the sender that
can be used in deciding a recovery strategy.
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NOTE: It is in the nature of timely completion that
retries, if performed, need to be more closely spaced
than is typical for SMTP retries; thus it may be
necessary to reduce the number of retries to avoid
overloading a relay. Some relays may choose to not
attempt any retries for messages with the TIMELY option.
In such circumstances, a sender may wish to retry before
attempting transmission by alternative means.
6. Implementation notes
This section is not a normative part of this specification.
The timely completion mechanism is a response to requests for
improved performance in certain uses of email.
Ultimately, achieving the desired performance levels is dependent
on quality of implementation and operational deployment factors.
If a system capable of handling 1000 messages-per-hour is subjected
to periods of demand for 2000 messages-per-hour throughput, then
the performance goals are bound to be substantially under-achieved,
whatever the protocol specification may demand.
The rest of this section discusses some of the implementation
issues and choices raised by this memo, and indicates some ways in
which the performance goals can be addressed.
6.1 Message state management
All requirements for extended-term state retention are in the
sending and receiving MTAs -- at or close to the edge of the
network. Ideally, these would be the only MTAs involved, so
provisioning of the service would be entirely under the control of
the organizations that use (or sell) it.
Where intermediate relays are used, there is no requirement to
maintain information about a message after it has been relayed.
Thus there are no scalability problems created by a need for state
maintenance; performance comes down to message throughput. The
requirement for "reasonable effort" rather than "best effort"
delivery for TIMELY messages means that some message handling
requirements can be relaxed. Rather than copying message data to
disk for re-transmission, it can be held in memory -- it might even
be streamed through to the next relay; loss of message data is not
critical because reporting failure back to the sender is an allowed
option.
When a TIMELY MTA is subjected to high load factors, it needs a
strategy for dealing with this.
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The design for timely confirmation to the sender depends on
reasonably consistent transit times on the forward and return
message paths. Delays on the forward path are picked up and
responses can be generated. Delays on the return path will result
in loss of confirmation; losing failure responses should not be
too damaging as the sender will time out and invoke a recovery
strategy. Losing success responses is more harmful, as it may
cause unnecessary additional network traffic.
In view of the above, the following message handling strategy is
suggested:
o Give top priority to forwarding timely status notifications;
i.e. messages with a BY parameter and no return path address.
o Give next priority to receiving new messages.
o Give next priority to processing accepted messages using the
TIMELY option.
o Give next priority to forwarding messages using the DELIVERBY
option.
o Finally, forward ordinary messages
If confirmation for a message sent using the TIMELY option is not
received within the expected interval, the sender should be very
conservative about simply retrying. The reason for non-receipt of
confirmation is probably:
(a) Because of mail system congestion, in which case
retransmission will just make things worse, or
(b) Some other network problem, in which case a retry won't help.
Since the motivation for this specification is to provide message
delivery while the sender waits, a reasonable approach would be to
give the sender an option to retry later, send by regular email or
use some other delivery mechanism.
6.2 Retransmission timing issues
Even allowing for the caution stated above about the problems of
simply retransmitting a failed message, it may be that some limited
retransmission by the original sender is appropriate as part of a
recovery strategy.
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NOTE: this section draws on some well-known TCP
strategies, but the primary intent is different. TCP
specifies a retransmission strategy to achieve
reliability. This specification aims for deterministic
behaviour as far as the sender is concerned, and limits
on retransmission to reduce congestion and duplicate
delivery.
In order not to exacerbate congestion of intermediate relays, the
following approach is suggested:
o A first retry should not be attempted before 4x the original
DELIVERBY interval has expired.
o Subsequent retry attempts should be attempted at exponentially
increasing intervals; e.g. 8x original interval for the 2nd
retry, 16x for the 3rd retry, etc.
o The requested delivery interval should be increased exponentially
for each retry.
o The total number of retries attempted should be kept reasonably
small; e.g. a maximum of 3-4 retries. If a timely delivery is
not achieved within a few attempts, it is probably not achievable
at all within a reasonable time.
o The receiver of a message should keep a record of the received
message identifier for some period of time, at least 8 times the
original DELIVERBY interval, for the purpose of weeding out
duplicates. It is not possible to state an absolute upper bound
on this period, but it should be as long as the receiver can
reasonably manage, but probably no more than a few days.
More specific recommendations for retransmission strategies may
emerge from deployment experience with this protocol. The basic
approach outlined above uses lessons learned from TCP, notably that
exponential back-off is important to avoid exacerbating congestion
conditions that may be the reason for failure in the first place.
6.3 Delivery timing granularity
This specification uses seconds for its time interval values. The
best possible timing resolution for each relay is a whole number of
seconds. Careless handling of these time intervals could lead to
timing errors of a second or worse at each relay.
In general, it is expected that delivery time intervals will be of
the order of 10s of seconds, not less than 10 seconds. The effects
of cumulative timing errors should not be significant if the number
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of MTAs involved is kept small (e.g. no more than 2 intermediate
relays).
The following procedure is suggested for dealing with timing
through relay MTAs:
o On receipt of a MAIL FROM command, note the time at which it was
received, preferably with sub-second granularity.
o When the message is subsequently forwarded, note the time
immediately prior to generating the new MAIL FROM command, and
use the difference from time of receipt to calculate the transit
delay. The calculated transit delay should be rounded up to a
whole number of seconds.
o Generate a new MAIL FROM command with the BY parameter 'by-time'
value decreased by the transit delay value.
Rounding up the transit delay should mean that the BY interval is
always decreased by at least 1 when passing through a relay. This
should mean that if many relays are involved, the overall timing
becomes more conservative. This is consistent with the idea that
responsiveness to the sender is considered more important than
actually achieving delivery.
6.4 Partial success
Messages sent to more than one recipient using the TIMELY option
may succeed or fail independently.
Systems must be designed to handle this possibility. E.g. a
sending agent that gives the user an option to resend, or send by
another route, should be capable of recognizing (and reporting)
that some messages have been transferred successfully, and only
attempt an alternative transfer for those that did not (unless, of
course, the user directs otherwise).
6.5 Routing TIMELY and non-TIMELY messages
The use of MX mail routing means that TIMELY and non-TIMELY
messages to the same domain will be routed via the same servers.
It may be desirable to use separate servers for TIMELY messages.
One way to achieve this operationally would be to use a different
email domain for TIMELY messages, but this may not be ideal from
the users' view of the service.
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6.6 Expediting message handling
Invoking the DELIVERBY extension for a given message may be used by
an MTA as a signal to expedite message delivery. But note that
status reports are part of the timely completion cycle, and while
these are sent using the DELIVERBY extension, they do not use the
TIMELY option. Unlike forward-path delays, any delays on the
return path may directly result in the silent loss of a message
status report.
This means that return path messages should be processed at least
as expeditiously as the original message. Hence messages sent
using the TIMELY option should not be given a higher priority than
messages that use just the DELIVERBY option.
7. Examples
In the following examples, 'C:' prefixes commands sent from the
SMTP client to the server in a mail transaction, and 'S:' prefixes
responses from the server back to the client.
The notation '\' at the end of a command example indicates that it
continues on the next line. The actual SMTP command must be
presented on a single line.
7.1 Timely delivery and confirmation
This example is of a successful timely delivery and confirmation.
+----------+ +----------+ +----------+
|Lemas.com |-->--|Benden.net|-->--|Harper.org|
+----------+ +----------+ +----------+
First hop transfer:
S: 220 Benden.net ESMTP server
C: EHLO Lemas.com
S: 250-Benden.net
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<Asgenar@Lemas.com> BY=20;R TIMELY=20 \
ENVID=EE271828 RET=HDRS
S: 250 OK to attempt delivery within 20 seconds
C: RCPT TO:<Robinton@Harper.org> NOTIFY=SUCCESS,FALURE \
ORCPT=rfc822;Robinton@Harper.org
S: 250 OK
C: DATA
S: 354 Send data
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C: (message data goes here)
:
.
S: 250 Message received
At this point, the receiving server Benden.net has accepted
responsibility to deliver the message to its destination or send a
failure report back to the sender. Assuming that the next hop is
initiated after a delay of 4 seconds, it may look like this:
S: 220 Harper.org ESMTP server
C: EHLO Benden.net
S: 250-Harper.org
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<Asgenar@Lemas.com> BY=16;R TIMELY=20 \
ENVID=EE271828 RET=HDRS
S: 250 OK
C: RCPT TO:<Robinton@Harper.org> NOTIFY=SUCCESS,FALURE \
ORCPT=rfc822;Robinton@Harper.org
S: 250 OK
C: DATA
S: 354 Send data
C: (message data goes here)
:
.
S: 250 Message received
At this point, the delivery MTA Harper.org has accepted
responsibility to achieve message delivery and report success or to
report a failure within 16 seconds of receiving the MAIL FROM
command. This will depend on some kind of cooperation with the
receiving user agent. When delivery is completed within the
specified interval, a DSN report is sent in the following fashion:
S: 220 Benden.net ESMTP server
C: EHLO Harper.org
S: 250-Benden.net
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<> BY=40;R
S: 250 OK
C: RCPT TO:<Asgenar@Lemas.com> NOTIFY=NEVER
S: 250 OK
C: DATA
S: 354 Send data
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C: To: Asgenar@Lemas.com
From: Message-handling@Harper.org
Subject: Disposition OK for Robinton@Harper.org
Content-type: multipart/report; boundary=next;
report-type=delivery-status
MIME-version: 1.0
--next
Content-type: text/plain; charset=US-ASCII
Your message (EE271828) to <Robinton@Harper.org> was processed
--next
Content-type: message/delivery-status
reporting-MTA: dns; mail-receiver.Harper.org
Original-Envelope-ID: EE271828
Original-recipient: rfc822;Robinton@Harper.org
Final-recipient: rfc822;Robinton@Harper.org
Action: delivered
Status: 2.0.0
--next--
.
S: 250 Message received
On receipt of this confirmation message, the sender's user agent
will be able to correlate with the original using the 'Original-
Envelope-ID' and 'Original-recipient' values, and confirm to the
sender that the message has been delivered and processed.
7.2 Received by delivery MTA and timed out
This example follows the same sequence as the previous one, up to
the point that the delivery MTA Harper.org has accepted
responsibility to achieve message delivery or to report a failure.
In this case, having accepted the message, final delivery cannot be
achieved in the desired interval so a failure DSN must be sent:
S: 220 Benden.net ESMTP server
C: EHLO Harper.org
S: 250-Benden.net
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<> BY=40;R
S: 250 OK
C: RCPT TO:<Asgenar@Lemas.com> NOTIFY=NEVER
S: 250 OK
C: DATA
S: 354 Send data
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C: To: Asgenar@Lemas.com
From: Message-handling@Harper.org
Subject: Disposition failed for Robinton@Harper.org
Content-type: multipart/report; boundary=next;
report-type=delivery-status
MIME-version: 1.0
--next
Content-type: text/plain; charset=US-ASCII
Your message (EE271828) to <Robinton@Harper.org> could not
be processed within the requested time.
--next
Content-type: message/delivery-status
reporting-MTA: dns; mail-receiver.Harper.org
Original-Envelope-ID: EE271828
Original-recipient: rfc822;Robinton@Harper.org
Final-recipient: rfc822;Robinton@Harper.org
Action: failed
Status: 4.4.7 (Timed out during delivery)
--next--
.
S: 250 Message received
Because this is a specific failure condition being sent to a source
that has used the timely delivery extension, and the message can be
correlated with the original by means of the 'Original-Envelope-ID'
and 'Original-Recipient' values, no part of the original message is
returned with the DSN report.
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7.3 Timed out with delivery in progress
This example is similar to the previous one, except that final
delivery is in progress but its completion is delayed. In this
case, the message cannot be recalled, so a notification report is
sent to the sender, within the requested delivery period,
indicating that the message is delivered and in delivery. Later, a
final delivery status message will be sent.
S: 220 Benden.net ESMTP server
C: EHLO Harper.org
S: 250-Benden.net
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<> BY=40;R
S: 250 OK
C: RCPT TO:<Asgenar@Lemas.com> NOTIFY=NEVER
S: 250 OK
C: DATA
S: 354 Send data
C: To: Asgenar@Lemas.com
From: Message-handling@Harper.org
Subject: Disposition delayed for Robinton@Harper.org
Content-type: multipart/report; boundary=next;
report-type=delivery-status
MIME-version: 1.0
--next
Content-type: text/plain; charset=US-ASCII
Your message (EE271828) to <Robinton@Harper.org> is being
delivered but not completed within the requested time.
--next
Content-type: message/delivery-status
reporting-MTA: dns; mail-receiver.Harper.org
Original-Envelope-ID: EE271828
Original-recipient: rfc822;Robinton@Harper.org
Final-recipient: rfc822;Robinton@Harper.org
Action: delayed
Status: 4.4.7 (Timed out during delivery)
--next--
.
S: 250 Message received
The difference between this and the example in section 7.2 is in
the "Action:" field of the delivery status message.
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7.4 Timed out before receipt by delivery MTA
This example is of a failed attempt to achieve timely delivery
because the message could not be forwarded within the requested
interval.
+----------+ +----------+ +----------+
|Ruatha.com|-->--|Fort.net |-->--|Harper.org|
+----------+ +----------+ +----------+
First hop transfer:
S: 220 Fort.net ESMTP server
C: EHLO Ruatha.com
S: 250-Fort.net
S: 250-DELIVERBY 15,TIMELY
S: 250 DSN
C: MAIL FROM:<Jaxom@Ruatha.com> BY=20;R TIMELY=20 \
ENVID=EE271828 RET=HDRS
S: 250 OK to attempt delivery within 20 seconds
C: RCPT TO:<Sebell@Harper.org> NOTIFY=SUCCESS,FALURE \
ORCPT=rfc822;Sebell@Harper.org
S: 250 OK
C: DATA
S: 354 Send data
C: (message data goes here)
:
.
S: 250 Message received
After a delay of 12 seconds (with 8 seconds of the original
delivery interval remaining), the server Fort.net attempts to relay
the message:
S: 220 Harper.org ESMTP server
C: EHLO Fort.net
S: 250-Harper.org
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: QUIT
S: 221 <Harper.org> closing channel
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The minimum delivery interval declared by the server Harper.org is
greater than the time remaining to complete delivery, so Fort.net
does not even attempt to send the message. Instead, it returns a
failure report back to Ruatha.com:
S: 220 Ruatha.com ESMTP server
C: EHLO Fort.net
S: 250-Ruatha.com
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<> BY=40;R
S: 250 OK
C: RCPT TO:<Jaxom@Ruatha.com> NOTIFY=NEVER
S: 250 OK
C: DATA
S: 354 Send data
C: To: Jaxom@Ruatha.com
From: Message-handling@Fort.net
Subject: Delivery failed for Sebell@Harper.org
Content-type: multipart/report; boundary=next;
report-type=delivery-status
MIME-version: 1.0
--next
Content-type: text/plain; charset=US-ASCII
Your message (EE271828) to <Sebell@Harper.org> could not be
delivered within the requested time.
--next
Content-type: message/delivery-status
reporting-MTA: dns; mail-relay.Fort.net
Original-Envelope-ID: EE271828
Original-recipient: rfc822;Sebell@Harper.org
Final-recipient: rfc822;Sebell@Harper.org
Action: failed
Status: 4.4.7 (Timed out during message transfer)
Retry-count: 0
--next--
.
S: 250 Message received
From the sender's perspective, this is pretty much the same
condition as reported by example 7.2, the difference being that the
time-out has occurred before the message reaches the delivery MTA.
The status code is the same but the reporting MTA is different.
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The retry count value is returned to give the sender an indication
about whether it might retry this path before switching to an
alternative delivery strategy.
7.5 Timely delivery feature not supported
This final example shows failure of a timely delivery request
because a receiving MTA does not support the capability:
+----------+ +----------+ +----------+
|Lemas.com |-->--|Benden.net|-->--|Miners.org|
+----------+ +----------+ +----------+
First hop transfer:
S: 220 Benden.net ESMTP server
C: EHLO Lemas.com
S: 250-Benden.net
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<Asgenar@Lemas.com> BY=20;R TIMELY=20 \
ENVID=EE271828 RET=HDRS
S: 250 OK to attempt delivery within 20 seconds
C: RCPT TO:<Nicat@Miners.org> NOTIFY=SUCCESS,FALURE \
ORCPT=rfc822;Nicat@Miners.org
S: 250 OK
C: DATA
S: 354 Send data
C: (message data goes here)
:
.
S: 250 Message received
Five seconds later, Benden.net attempts to forward the message:
S: 220 Miners.org ESMTP server
C: EHLO Benden.net
S: 250-Harper.org
S: 250-DELIVERBY 60
S: 250 DSN
C: QUIT
S: 221 <Miners.org> closing channel
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The Miners.org server does not support timely delivery, so
Benden.net does not attempt to send the message. Instead, it sends
a failure report back to Lemas.com:
S: 220 Lemas.com ESMTP server
C: EHLO Benden.net
S: 250-Lemas.com
S: 250-DELIVERBY 10,TIMELY
S: 250 DSN
C: MAIL FROM:<> BY=40;R
S: 250 OK
C: RCPT TO:<Asgenar@Lemas.com> NOTIFY=NEVER
S: 250 OK
C: DATA
S: 354 Send data
C: To: Asgenar@Lemas.com
From: Message-handling@Benden.net
Subject: Delivery failed for Nicat@Miners.org
Content-type: multipart/report; boundary=next;
report-type=delivery-status
MIME-version: 1.0
--next
Content-type: text/plain; charset=US-ASCII
Your message (EE271828) to <Nicat@Miners.org> could not be
delivered within the requested time.
--next
Content-type: message/delivery-status
reporting-MTA: dns; mail-relay.Benden.net
Original-Envelope-ID: EE271828
Original-recipient: rfc822;Nicat@Miners.org
Final-recipient: rfc822;Nicat@Miners.org
Action: failed
Status: 5.4.9 (Timely delivery not supported by Miners.org)
--next--
.
S: 250 Message received
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8. IANA Considerations
This specification introduces some new protocol elements for which
IANA registration be required or desirable:
o Extension to DELIVERBY ESMTP extension: see section 4.
o New extended mail system status codes: see section 5.1.
o New DSN report per-recipient field: see section 5.2.
9. Internationalization considerations
This specification introduces no new internationalization
considerations other than those already present in DSN, which,
through MIME, provides for charset identification and language
tagging of the human readable part of a DSN report.
10. Security considerations
See also RFC 1894 [12], RFC 2852 [5].
To offer timely handling of messages may require some dedication of
resource. It is conceivable that systems supporting this feature
may be more susceptible to denial of service attacks from a flood
of messages requesting timely completion. (See also section 6.1.)
There is a distant possibility that responses to time-sensitive
requests may disclose information about the loading or topology of
the network accessed. This is unlikely to be any worse than for
web access protocols (but note that HTTP has been shown to allow
certain kinds of timing attack on private information about a
client's network activities.).
Systems that depend on the physical presence of a user to achieve
timely receipt SHOULD NOT accept a message for such disposition
without the user's explicit permission (c.f. automated generation
of MDN responses in RFC 2998 [14]).
11. Acknowledgements
The authors thank Hiroshi Tamura-san for undertaking the task of
reviewing a very rough, early draft and making several pertinent
observations. The authors also acknowledge helpful comments by Dan
Wing, Ned Freed and Greg Vaudreuil.
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12. References
[1] RFC 2542, "Terminology and Goals for Internet Fax"
L. Masinter, Xerox Corporation
March 1999.
[2] RFC 821, "Simple Mail Transfer Protocol"
Jonathan B. Postel, ISI/USC
August 1982.
[3] RFC 1651, "SMTP Service Extensions"
J. Klensin, MCI
N. Freed, Innosoft
M. Rose, Dover Beach Consulting, Inc.
E. Stefferud, Network Management Associates, Inc.
D. Crocker, Silicon Graphics, Inc.
July 1994.
[4] RFC 1891, "SMTP Service Extension for Delivery Status
Notifications"
K. Moore, University of Tennessee
January 1996.
[5] RFC 2852, "Deliver By SMTP Service Extension"
D. Newman, Sun Microsystems
June 2000
[6] "Content Negotiation for Internet Messaging Services"
G. Klyne, Baltimore Technologies
R. Iwazaki, Toshiba TEC
D. Crocker, Brandenburg Consulting
Internet draft: draft-ietf-fax-content-negotiation-05.txt
Work in progress: May 2001.
[7] RFC 2234, "Augmented BNF for Syntax Specifications: ABNF"
D. Crocker (editor), Internet Mail Consortium
P. Overell, Demon Internet Ltd.
November 1997.
[8] "Procedures for document facsimile transmission in the general
switched telephone network"
ITU-T Recommendation T.30 (1996), including Amendment 1 (1997),
Amendment 2 (1997), Amendment 3 (1998) and Amendment 4 (1999)
International Telecommunications Union.
[9] RFC 1047, "Duplicate messages and SMTP"
C. Partridge
February 1988.
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[10] RFC 974, "Mail routing and the domain system"
C. Partridge
January 1986.
[11] RFC 2119, "Key words for use in RFCs to Indicate Requirement
Levels"
S. Bradner, Harvard University
March 1997.
[12] RFC 1894, "An Extensible Format for Delivery Status
Notifications"
K. Moore, University of Tennessee
G. Vaudreuil, Octel Network Services
January 1996.
[13] RFC 1893, "Enhanced Mail System Status Codes"
G. Vaudreuil, Octel Network Services
January 1996.
[14] RFC 2298, "An Extensible Message Format for Message Disposition
Notifications"
R. Fajman, National Institutes of Health
March 1998.
[15] G. Vaudreuil, Lucent Technologies,
Extensions to Mail System Status Codes
Internet draft: draft-vaudreuil-1983ext-01.txt
Work-in-progress, November 2001.
13. Authors' addresses
Graham Klyne (editor)
MIMEsweeper Group,
1310 Waterside,
Arlington Business Park
Theale
Reading, RG7 4SA
United Kingdom.
Telephone: +44 118 903 8000
Facsimile: +44 118 903 9000
Email: Graham.Klyne@MIMEsweeper.com
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David H. Crocker
Brandenburg Consulting
675 Spruce Drive
Sunnyvale
CA 94086
USA.
Telephone: +1 408 246 8253
Facsimile: +1 408 273 6464
Email: dcrocker@brandenburg.com
Appendix A: Amendment history
[[[RFC editor: please remove this appendix on publication]]]
00a 22-Oct-1999 Memo initially created.
01a 13-Sep-1999 Incorporate review comments. Update references.
Changed title. Incorporate material from IETF
meeting presentations.
02a 25-Jan-2001 Update author details. Simplify COMPLIANCE
extension to a TIMELY extension of DELIVERBY. Add
original interval parameter to TIMELY option.
Strengthen description of mechanism for timely
confirmation. Add template decsription for TIMELY
extension. Refer to the goal of this
specification as "timely completion" rather than
just "timely delivery" (to clearly distinguish
from basic DELIVERBY). Added subsection dealing
with final MTA/MUA interaction. Defined DSN
extension header and status codes for reporting
timely delivery failures. Drafted some
implementation notes.
02b 30-Jan-2001 Add examples. Update some references. Other
editorial drafting.
02c 31-Jan-2001 Fold in review comments. Added implementation
note about using DELIVERBY to expedite message
handling (6.5).
02d 01-Feb-2001 More editorial changes.
02e 01-Feb-2001 Revised text dealing with time-out; move
discussion of retries to implementation notes.
03a 16-Feb-2001 Editorial changes. Added some clarifying text to
introductory section 2.2.
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03b 13-Jun-2001 Editorial changes.
04a 12-Sep-2001 Rework some descriptions and status codes to be
clear that this document describes an extension of
the mail delivery semantics rather than some
aspect of disposition semantics. As a result,
some status codes have been removed. Removed text
in section 6.1 about immediate rejection of a
message to help avoid exacerbating congestion
conditions. Changed terminology to focus on the
goal of achieving "final receipt" (rather than
"disposition"). Added reference [15].
04b 13-Sep-2001 Change contact details. Editorial corrections and
refinements.
04c 13-Sep-2001 Changed some section titles; revised examples
commentary and status codes in line with other
changes.
05a 17-Sep-2001 Reworked abstact and introduction to more clearly
place this work in context. Various minor
editorial changes.
06a 05-Nov-2001 Prepare for WG last call. Fixed extended status
code for "protocol not supported". Removed
editorial notes. Fix some spelling errors.
REVIEW CHECKLIST:
(Points to be checked or considered more widely on or before final
review.)
o Are there any deployed mechanisms that MTAs may use to recognize
expedited message relay?
o Possible minor revision to DELIVERBY spec? If a DELIVERBY MTA
fails message delivery because the delivery time has expired, AND
the message has an empty SMTP sender address/return path, the
message should be silently discarded (c.f. RFC 1891, section 6.2;
I think the considerations noted there seem less applicable.).
If this doesn't work, try next...
o Consider addition of new 'by-mode' value for return DSNs; e.g.
'E' for expedite: try to deliver within interval given, or
abandon delivery, but don't notify success or failure.
(Currently specify 'R' without return-path.) A notification
should not be abandoned if a non-DELIVERBY MTA is encountered.
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o Try to model system behaviour under high-load/backlog conditions.
Especially w.r.t. section 3.5.
o What lessons to learn from IP QoS efforts?
o Query use of enhanced status codes 4.x.x and 5.x.x; Use by
DELIVERBY seems at odds with RFC 1893.
o Note use of status 5.4.1 not in line with expectations of RFC
1893.
o Use new code instead of 5.3.3?
o Special considerations for fax offramp gateay? How to deal with
uncertain dial-out times.
o Apparently, DSN extension fields must be registered with IANA,
but there appears to be no registry for them.
o Use of alternative port? (e.g. like message submission).
o Allow MTAs to impose size limit on messages for timely delivery?
o Operational issues surrounding selection of delivery interval?
o DISCUSS: In environments where the timing of final delivery of
the message is outside the control of the final MTA (e.g. the
time required for an outdial, or waiting for a client to collect
the message), an interim DSN report may be generated indicating
that the message has been received pending final delivery. This
report should be clear whether final delivery is dependent on the
receiving user (e.g. mail collection) or some other unknown
infrastructure delay (e.g. fax out-dial or external e-mail
environment).
This is covered somewhat by section 3.3.1: is this adequate?
o MX configuration -- uniform routing for TIMELY/non-TIMELY. Is a
differential routing option required; e.g. SRV records?
o Can use of ORCPT be relaxed? If partial success occurs for
multiple recipients, it is important to be able to tell which
were successful and which were not.
o When a timely-delivery failure message is sent back, it is
addressed to the sender of the original message; thus it becomes
the sender UA responsibility to handle the failure of timely
delivery -- does this cause any problems?
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o Check examples. (Should relays declare mail-domain or host name?
Does it matter? Should the From: header for DSNs always be
'postmaster', or is any appropriate mailbox OK?)
Full copyright statement
Copyright (C) The Internet Society 2001. All Rights Reserved.
This document and translations of it may be copied and furnished to
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it or assist in its implementation may be prepared, copied,
published and distributed, in whole or in part, without restriction
of any kind, provided that the above copyright notice and this
paragraph are included on all such copies and derivative works.
However, this document itself may not be modified in any way, such
as by removing the copyright notice or references to the Internet
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must be followed, or as required to translate it into languages
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Klyne & Crocker Internet draft [Page 39]
