IETF fax WG G. Klyne, Content Technologies
Internet draft [[[et al]]]
13 September 2000
Expires: March 2001
Timely Delivery for Internet Messaging Services
<draft-ietf-fax-timely-delivery-01.txt>
Status of this memo
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Copyright Notice
Copyright (C) The Internet Society 2000. All Rights Reserved.
Abstract
This proposal describes a way to request timely delivery for
facsimile, voice and other messaging services that use Internet
e-mail. It provides a deterministic service quality response,
while preserving the traditional roles and responsibiltiies of the
agents involved in e-mail transfers.
It is essentially a profile of the DSN and DELIVERBY extentions for
ESMTP, and a new COMPLIANCE extension for establishing the
deterministic service quality response.
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NOTE: This is an early, preliminary version of specification,
indicating a possible way to achieve the timely delivery
requirement for full mode Internet fax. The content is rough, and
the intent at this time is to indicate the outline of a mechanism.
Please address comments to major structural and semantic issues.
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.............................................3
1.1 Structure of this document ...........................3
1.2 Document terminology and conventions .................3
2. Background and goals.....................................3
2.1 Background ...........................................3
2.2 Basis for timely delivery ............................4
2.2.1 The SMTP "contract"..............................4
2.2.2 Framework for timely delivery....................5
2.3 Why is 'compliance required' included? ...............6
2.4 Goals for timely delivery ............................6
3. Mechanisms for timely delivery...........................7
3.1 Transmitting a message for timely delivery ...........7
3.2 Relaying a message ...................................8
3.3 Accepting a message by the final MTA .................9
3.4 Timely confirmation ..................................10
4. Compliance-required ESMTP extension......................11
5. DSN reporting extensions.................................11
6. Notes....................................................11
7. Examples.................................................12
8. IANA Considerations......................................12
9. Internationalization considerations......................12
10. Security considerations.................................12
11. Acknowledgements........................................12
12. References..............................................12
13. Authors' addresses......................................13
Appendix A: Amendment history...............................14
Full copyright statement....................................14
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1. Introduction
1.1 Structure of this document
[[[TBD]]]
1.2 Document terminology and conventions
[[[TBD]]]
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.
[[[Editorial comments and questions about outstanding issues are
provided in triple brackets like this. These working comments
should be resolved and removed prior to final publication.]]]
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 of the
goals for timely delivery of fax messages. These goals are
determined, in part, by the capabilities of traditional facsimile
[8].
2.1 Background
Traditional e-mail [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 e-mail.)
To be more than just a hint to the message transfer system, timely
delivery requires a deterministic confirmation mechanism, to close
the loop. This is provided by DSN [4].
Three kinds of timeliness can be identified:
(a) timely delivery to the receipient
(b) timely notification to the sender of delivery
(c) timely notification to the sender that the message has been
received and processed
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From the sender's point of view, timely confirmation of receipt and
processing is the most desirable requirement.
2.2 Basis for timely delivery
A key premise of this proposal 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 secondary issue is post-delivery handling: will the receiving
user agent process the message in timely fashion?
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 best efforts to ensure delivery of the message, or
(b) provide notification that delivery could not be achieved.
This memo introduces mechanisms to allow this contract to be
modified. A timely-delivery MTA accepts responsibility to:
(a) use reasonable efforts to ensure delivery within a specified
time, and to provide timely confirmation of delivery, or
(b) provide timely notification that delivery was not achieved.
The sender can then decide a recovery strategy
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2.2.2 Framework for timely delivery
The diagram shows typical SMTP message delivery and delivery status
notification (DSN) paths. Note that 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
As well as requesting timely delivery of a message, this proposal
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 delivery 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 notifcation 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 MTA will process the message in a timely fashion. If the
receiving MTA delivers to a POP mailbox, thereis no way that it can
guarantee timely delivery.
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2.3 Why is 'compliance required' included?
For the purposes of SMTP transfer, the "compliance required"
feature is not required and there is a strong case for pursuing
timely delivery without that part.
But consider the following scenario:
+--------+ +-------+ +-----------+ +-----------+
| Sender |-->--| Relay |-->--| Receiving |-->--| Disposing |
| MTA | | MTA | | MTA | | agent |
+--------+ +-------+ +-----------+ +-----------+
<------SMTP-------> <-?->
The DELIVER-BY specification concerns itself only with the SMTP
transfers. But for the purposes of timely delivery, the goal is
not satisfied from a user perspective unless the disposition also
completes in "timely" fashion.
The "compliance required" option expects that the receiving MTA
communicates with the disposing agent (in some unspecified way),
and confirms final delivery of the message only if the disposing
agent confirms that it will deal with the message in timely
fashion. Simply putting the message into a POP mailbox would not
meet this criterion. It also permits an MTA to commute its
responsibility for delivering the message from "best effort" to
"reasonable effort".
This is all consistent with the fundamental strategy of giving the
sender control over the whole process: if the disposing agent
cannot communicate the required guarantee, delivery is not
completed and the sender is duly notified.
2.4 Goals for timely delivery
The primary goal is to provide a mechanism that allows consenting
parties to establish a relationship with guaranteed delivery within
a specified time, or notification that the delivery was not
achieved.
Further goals are:
o Provide "while-you-wait" delivery of facsimile messages by e-mail
(where available infrastructure and connectivity permits).
o Deterministic behaviour. A sender who requests timely delivery
should be able to determine with reasonable certainty whether or
not that request was successful.
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o If the message cannot be delivered as requested, it should not be
delivered at all. This means that a sender can take control over
alternative strategies for message delivery (e.g. if timely
delivery by e-mail does not succeed, to resend the message as a
traditional facsimile; in such circumstances it is preferable
that multiple copies of the message are not delivered.
o Operates within the existing ESMTP extension framework [3],using
existing facilities where available.
3. Mechanisms for timely delivery
Timely delivery 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 "Compliance-required" extension, 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.
The confirmation loop for succesful delivery looks something like
this. The path through MTAs taken by the confirmation response is
not defined, and may be different than the forward path of the
original message.
+-----------+ +--------+ +--------+ +---------+
|Originating|-->--|Relaying| ... |Relaying|-->--|Receiving|
| MTA | | MTA | | MTA | --| MTA |
+-----------+ +--------+ +--------+ | +---------+
| | |
+-------------+ | +---------+
| Originating |--<-- ... .... ... --<-- |Receiving|
| MUA | | MUA |
+-------------+ +---------+
3.1 Transmitting a message for timely delivery
A transmitted message for which timely delivery is required MUST
include the following:
o an 'ENVID' parameter on the MAIL command, per DSN [4]
o a 'NOTIFY=SUCCESS,FAILURE' parameter on the corresponding RCPT
command, per DSN [4]
o an 'ORCPT' parameter on the MAIL command, per DSN [4]
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o a 'BY' parameter on the corresponding RCPT command, per [5], with
a 'by-mode' value of 'R'.
o a 'COMPLIANCE=TIMELY' parameter on the corresponding RCPT, as
described below.
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 indicating 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 below).
3.2 Relaying a message
An MTA that relays a message for timely delivery MUST support all
of the ESMTP extensions noted above, otherwise it should not
receive 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 a message, an MTA must note when the message was
received, note the time when the attempt to transmit the message to
the next MTA is initiated, and reduce accordingly the time interval
used for the 'deliver-by' parameter (see note below on handling
fine-grained timing requirements).
If the deliver-by interval is reduced to less than zero, (or less
than some system-configurable value indicating that delivery within
the indicated interval is unlikely to be achieved) then the message
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).
[[[Remove duplication between above and DELIVERBY spec]]]
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 deliver-by period; if the
transmission cannot be completed after one or more such retries
then a negative DSN should be generated as noted above.
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In all cases, any DSN generated should indicate the number of
retries attempted (where 0 means no retries).
The choice to retry or not retry is installation dependent.
Effectively, when a relay does not retry, any reposibility 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 'COMPLIANCE=TIMELY' option may
cause a relay to abandon a message that it would
otherwise retry (even given a 'by-mode' value of 'R').
The intent of this option is to establish that
responsiveness to the sender is more important than
getting the message through. One effect of this may be
to severely constrain the number and frequency of retry
attempts.
3.3 Accepting a message by the final MTA
The MTA that accepts 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 not defined here
or by the Internet e-mail specifications. The final MTA is also
responsible for generating any successful DSN message.
Before generating a DSN message, the final MTA must ensure that all
of the conditions for delivery of the message have been achieved.
Specifically, it should ensure that final delivery to the MTA will
be completed within the DELIVER-BY interval indicated. Exactly
what constitutes final delivery to the MTA may depend somewhat on
the nature of the MTA: in the simplest case, depositing the
message in a local mailbox probably constitutes final delivery; a
more complex case would be that of a fax offramp: in this case it
may be reasonable for final delivery to be completion of a
successful outdial and transmission of the fax.
In the presence of a 'COMPLIANCE=TIMELY' option, final delivery
should not be completed 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 in a time comparable with
that taken to establish a telephone call.
[[[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
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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).]]]
[[[I think the above is verging on trying to be too clever, getting
too far into MDN teritory]]]
3.4 Timely confirmation
Fully deterministic behaviour requires that the round-trip time to
deliver a message and receive a response is completed 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. It
seems reasonable to assume that return path transit times will
normally be comparable with forward path times.
Further, it is likely that perfect determinism can never be
achieved using SMTP; e.g. see RFC 1047 [9]. Repeat deliveries are
probably less harmful than lost messages, but even these should be
minimized.
The following behaviour is proposed for achieving nearly
deterministic behaviour:
o Always fail forward delivery if non-TIMELY MTA is encountered.
o Use the DELIVER-BY option to request timely delivery for DSN
return, using same total time (x2?) as forward path, but
specifying a 'by-mode' value of 'N' rather than 'R'.
o Use a high priority option for the DSN return (for MTAs that may
take note of such things).
o Do NOT fail notification delivery if non-TIMELY or non-DELIVER-BY
MTA is encountered on the return path.
o The return DSN does not itself request delivery-notification (as
required by DSN).
o Sender may assume that message is lost after some multiple of
original DELIVER-BY interval has passed without notificaton (x4
is suggested).
o Sender is required to provide envelope ID with message. If it
re-tries, it should use same envelope ID and should do so within
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some multiple of DELIVER-BY interval (x6 suggested) since the
last retry.
o The receiver of a TIMELY message is strongly encouraged to keep
note of received envelope ID for some multiple of the DELIVER-BY
interval (x8 suggested) for the purpose of weeding out
duplicates.
4. Compliance-required ESMTP extension
[[[TBD]]]
Essentially, the semantics will be to REQUIRE conformance to any
SMTP extensions used for delivery to be successfully completed.
Unlike declared ESMTP extensions, this option requires the
extensions to be applied for a specific message.
[[[A simpler approach might be to simply define an extension and
parameter called TIMELY whose semantics are similar to those
attributed to COMPLIANCE=TIMELY.]]]
5. DSN reporting extensions
[[[Details TBD]]]
- Extension not supported
- Delivery time exceeded
- Delivered for further transmission: final confirmation pending
[[[???]]]
- Delivered for collection by user: final confimation pending
[[[???]]]
6. Notes
[[[These are placeholders for further discussion]]]
- Use of alternative port (e.g. like message submission).
- Scalability analysis. Required state information -- all at the
edges?
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- Handling fine-grained timing requirements (deliver-by
modification and implementation techniques). Must assume deliver-
by interval is large relative to normal network transit times.
- Partial non-delivery: failure to some recipients. Must be
handled, since all-or-nothing cannot be imposed within the SMTP
transfer environment.
7. Examples
[[[TBD]]]
8. IANA Considerations
[[[TBD: ESMTP and DSN extension registrations]]]
9. Internationalization considerations
[[[TBD?]]]
10. Security considerations
[[[TBD]]]
11. Acknowledgements
The authors thank Hiroshi Tamura-san for undertaking the task of
reviewing a very rough, early draft and making several pertinent
observations.
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.
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[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] <draft-ietf-fax-content-negotiation-03.txt>
[7] RFC 2234, "Augmented BNF for Syntax Specifications: ABNF"
D. Crocker (editor), Internet Mail Consortium
P. Overell, Demon Internet Ltd.
November 1997.
[8] T.30
[9] RFC 1047
13. Authors' addresses
Graham Klyne (editor)
Content Technologies Ltd.
1220 Parkview,
Arlington Business Park
Theale
Reading, RG7 4SA
United Kingdom.
Telephone: +44 118 930 1300
Facsimile: +44 118 930 1301
E-mail: GK@ACM.ORG
[[[et. al. TBD]]]
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Appendix A: Amendment history
00a 22-Oct-1999 Memo initially created.
01a 13-Sep-1999 Incorporate review comments. Update references.
Changed title. Incorporate material from IETF
meeting presentations.
TODO:
Full copyright statement
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