Network Working Group Neil Joffe
Internet Draft Dan Wing
October 15, 1997 Cisco Systems, Inc.
Expires March 1998 Larry Masinter
Xerox Corporation
SMTP Service Extension for Immediate Delivery
draft-ietf-fax-smtp-session-00.txt
Status of this memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Note: although this work has been discussed in the
IETF-FAX working group, it does not purport to represent
the consensus of the group.
0. Administrivia
0.1. Open design issues
0.1.1. Server's response to STAT
The authors feel that message/delivery-status allows for all sorts of
delivery information to be conveyed, including necessary transformations
which might be necessary for some deliveries, such as fax.
It was suggested to us in private email to expand this to be
multipart/report, which allows non-parsable data to be sent to the SMTP
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client. As the examples below show, this data can be a audio or a fax
(tiff-f) image. However, this may be too unwieldy for many uses of
Session.
Another idea is a one-line response that indicates Session delivery
status as in-progress or completed, which makes combining responses
easier as well.
0.1.2. STAT for each recipient or all recipients
The authors have been asked to create a single command to query delivery
status for all recipients (instead of requiring a separate STAT command
for each recipient, as this memo is currently written).
If we use DSNs, this is not possible unless we send multiple
multipart/reports (or multiple message/delivery-status) because DSNs
aren't designed to be coalesced.
0.2. Notes and unresolved issues
More verbage needed for security section
Add example showing DSN in conjunction with SESSION
Need to register new enhanced mail system status codes?
0.3. Changes since previous versions
Changes from -01 to -02:
Corrected diagram in section 6.
Changes from -00 to -01:
Server's reply to STAT is now a DSN
Language clarifications
Require immediate SMTP server reply after client sends "."
Specify SMTP server must respond to STAT within 30 seconds
1. Abstract
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This memo defines an extension to SMTP which provides a mechanism for
requesting and verifying immediate message delivery over SMTP.
2. Introduction
Historically, SMTP [SMTP] has been used for store and forward delivery
of messages. This memo describes a new SMTP extension called SESSION.
This new extension allows an SMTP client to request immediate delivery
by the SMTP server.
The SESSION extension allows for immediate delivery of mail; it presumes
either a direct connection between sender and recipient or a chain of
session-enabled servers in which each supports the SESSION extension.
If an MTA in the SMTP "path" does not support SESSION, delivery
automatically falls back to normal store and forward, and such fallback
is communicated to the SMTP client, as described in section 4.1.
Unlike the deprecated SAML, SOML and SEND commands (documented in [SMTP]
and deprecated in [DRUMS]) the SESSION extension allows for a mix of
immediate and store & forward delivery recipients.
The SESSION extension was motivated by an analysis of the requirements
for using the Internet to deliver fax messages, and, coupled with a
mechanism for exchanging capabilities and preferences of sender and
recipient, can be used by email<->fax gateway applications. In
addition, the SESSION extension may be useful for other messaging
applications where immediate delivery and confirmation are requested.
This memo uses the mechanism described in [SMTP-EXT] to define an
extension to the SMTP protocol for immediate delivery.
2.1. Discussion of this draft
This draft is being discussed on the "ietf-fax" mailing list. To
subscribe, send a message to <ietf-fax-request@imc.org> with the line
"subscribe" in the body of the message. Archives are available from
http://www.imc.org/ietf-fax.
2.2. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and server
respectively. If such lines are wrapped without a new "C:" or "S:"
label, then the wrapping is for editorial clarity and is not part of the
command.
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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 [REQ].
3. Framework for immediate delivery support
The immediate message delivery is defined as follows:
(1) The name of the immediate extension is Session;
(2) the EHLO keyword value associated with the immediate extension
is SESSION;
(3) no parameter is used with the SESSION EHLO keyword;
(4) one new SMTP verb, STAT (used to retreive an in-line DSN), is
defined with this extension, and is described in section 6;
(5) one optional parameter is added to the RCPT command, using the
esmtp-keyword SESSION, and is described in section 5,
no parameters are added to the MAIL FROM command;
(6) the maximum length of a RCPT TO is increased by 8 characters.
4. Esmtp-keyword SESSION
Upon receiving a RCPT command with the esmtp-keyword SESSION, a
session-enabled server will normally send either a positive (2xx) or
negative (5xx) reply to the SMTP client.
A 250 reply code indicates that the session-enabled server believes the
message will be sent immediately -- that is, that the request for
SESSION delivery will be honored. Note that SESSION delivery can
subsequently fail due to disk space exhaustion, disk quotas, failure of
a remote MTA, loss of connectivity to a remote MTA, or other reasons.
The MTA that detected the failure may attempt to deliver the message via
store-and-forward -- the SMTP client may use the STAT command to
determine the delivery status of the message.
If a session-enabled server is unable to send a message immediately
(that is, the request for SESSION will not be honored), but the
session-enabled server is willing to send the message via
store-and-forward, it MUST respond with a 252 reply code. The SMTP
client can use this information to inform the user that immediate
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delivery isn't available, and the SMTP client (or the user) may decide
to abort the SMTP transaction.
5. New SMTP verb STAT
One new SMTP verb is introduced with the Session extension. The STAT
verb allows a client to query the session delivery status for all
recipients in this mail transaction.
The STAT command, and the server's reply to STAT, are sent along the
SMTP "path". Using the diagram in section 4, the SMTP client (A) would
send a STAT command to (B), and (B) would forward the STAT command to
(C), which forwards it to (D). (D) generates the reply to STAT which
is echoed back through the MTAs until it is finally sent by (B) to (A).
The syntax of the STAT verb, using the notation described in [ABNF], is:
stat-cmd = "STAT" SP <forward-path> CR LF
The reply to the STAT command is called an ``in-line Delivery Status
Notification'', or ``in-line DSN''.
The SMTP server MUST respond to a STAT command no later than 60
seconds after a STAT command is received. After 120 seconds an SMTP
client MAY assume the connection to the SMTP server is broken.
5.1. Format of in-line Delivery Status Notification
The reply to STAT is a multipart/report as defined in [MIME-RPT].
However, note that this Session extension does NOT require the
session-enabled SMTP server also implement Delivery Status Notifications
[SMTP-DSN].
Each line of the multipart/report is sent following the rules for
SMTP replies. In the notation described in [ABNF]:
inline-dsn = *( "250-" [status-code SP] dsn-line CR LF )
"250 " [status-code SP] dsn-line CR LF
dsn-line = <each line of the multipart/report from [MIME-RPT]>
status-code = <This is <status-code> of section 4 of [SMTP-ENH-ERR]
if SMTP server implements [SMTP-ENH-ERR]>
text = <any CHAR except CTL>
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5.2. Per-recipient DSN extension
[DSN] permits extension fields to be defined. The following new
per-recipient DSN extension is defined for each recipient that had the
esmtp-keyword SESSION. In the notation described in [ABNF]:
extension-field = "Session-Delivery" *WSP ":" *WSP session-status
session-status = "delivered" / "in-progress" / "queued"
The <extension-field> and <session-status> can be spelled in any
combination of uppercase and lowercase letters.
"delivered" Session delivery was successful. Message was delivered
to the recipient immediately. This is a terminal
value.
"in-progress" Session delivery has not yet completed. A STAT command
issued later will show final status of this message.
This is the only non-terminal value.
"queued" Session delivery failed for some reason, but the MTA was
able to successfully queue the message using normal SMTP
store-and-forward. One cause of this status is when the
session-enabled server forwards the message to a
non-session-enabled server. This is a terminal value.
Once a terminal value in-line DSN has been sent for a specific
forward-path, the SMTP server does not need to include that forward-path
in subsequent STAT replies. If an SMTP client sends a STAT command and
the SMTP server has already informed the SMTP client that all recipients
had a terminal values, the SMTP server should return a 503 reply.
5.3. Delivery notifications and STAT
During normal store and forward operation and in the absence of special
RFC822 headers (such as Return-Receipt-To, Generate-Delivery-Report
described in [HEADERS]), only a delivery failure causes a message
to be generated (commonly called a `bounce').
If an SMTP server implments [SMTP-DSN], the SMTP client may request
a delivery status notification, and such a request may be for actions
other than delivery failure (such as SUCCESS or DELAY).
[A session-enabled server SHOULD NOT send such a `bounce' (or DSN)
[message if the recipient included the esmtp-keyword SESSION, and the
[SMTP server can be positive that the SMTP client has already received
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[the same information from the in-line delivery status notification.
[
[For example, if the SMTP client sends a QUIT command after the SMTP
[server has sent the in-line delivery status notification, the SMTP
[server knows the SMTP client has successfully received the in-line
[delivery status notification.
[
[XXX - bzzt: pipelining allows the "." and QUIT commands to be sent at
[ the same time. How should we resolve this problem?
If the RCPT command included the esmtp-keyword SESSION, any DSN
generated must include the DSN per-recipient extension defined in
section 5.2 -- omission of this extension implies "Session-Delivery:
queued" (because the message was sent through an MTA that doesn't
support SESSION).
5.4. Usage of [DSN] fields
For Session recipients, the <will-retry-until-field>, described in
[DSN], can be used to indicate when the SMTP server will stop attempting
to deliver the Session message. Note it is not possible to determine if
the Delivery-Status will be "queued" or "delivered" except by querying
via another STAT command.
XXX - Is this going to be sufficient for our FAX needs? We could
add more things, like bytes, pages, seconds-to-completion for
in-progress. Would another extension be useful to indicate
how long it took to send the fax (for billing)?
6. Behavior with multiple hop delivery
Both the esmtp-keyword SESSION and the new SMTP verb STAT require
special behavior when dealing with multiple-hop delivery. Multiple-hop
delivery occurs when the sending mailer isn't communicating directly
with the receiving mailer -- that is, there are intervening mail
transfer agents (MTAs) between the sending mailer and the receiving
mailer.
6.1. esmtp-keyword SESSION
The following sections describe the behavior of the SMTP server when
a RCPT command contains the esmtp-keyword SESSION.
6.1.1. SESSION local delivery
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In the case of a local delivery, the SESSION keyword causes the
session-enabled server to attempt immediate delivery of the message to
the local user's mailbox.
6.1.2. SESSION relayed delivery
The following diagram is used to help describe the relayed delivery
behavior. The diagram depicts a user agent sending mail to two
recipients with the esmtp-keyword SESSION, and each recipient has
a different MTA.
+-------+ +-----------+
-=> | MTA-1 | => | receiving | user@host-x
/ | | | MTA-1 |
+-----+ +--------+ / +-------+ +-----------+
| user| => |Original| =< (C) (D)
|agent| | MTA | \
|-----| +--------+ \ +-------+ +-----------+
(A) (B) -=> | MTA-2 | => | receiving | user@host-y
| | | MTA-2 |
+-------+ +-----------+
(E) (F)
In the case of several MTAs in the SMTP 'path', as in the above diagram,
the RCPT command with the esmtp-keyword SESSION is forwarded to the next
MTA (before the current MTA responds to the RCPT command) until finally
the RCPT command is sent to the 'last' MTA which actually sends a reply.
The reply is then echoed back along the SMTP 'path'.
Thus in the above diagram, if (A) issued two RCPT commands, the order
of processing is as follows:
(1) A>B: ... normal SMTP initiation sequence...
(2) A>B: RCPT TO:<user@host-x> SESSION
(3) B>C: ... normal SMTP initiation sequence...
(4) B>C: RCPT TO:<user@host-x> SESSION
(5) C>D: ... normal SMTP initiation sequence...
(6) C>D: RCPT TO:<user@host-x> SESSION
(7) D>C: 220 <user@host-x> okay
(8) C>B: 220 <user@host-x> okay
(9) B>A: 220 <user@host-x> okay
(10) A>B: RCPT TO:<user@host-y> SESSION
(11) B>E: ... normal SMTP initiation sequence...
(12) B>E: RCPT TO:<user@host-y> SESSION
(13) E>F: ... normal SMTP initiation sequence...
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(14) E>F: RCPT TO:<user@host-y> SESSION
(15) F>E: 220 <user@host-y> okay
(16) E>B: 220 <user@host-y> okay
(17) B>A: 220 <user@host-y> okay
The DATA command is also handled in a parallel function, such that a
single DATA command from (A) is transmitted to (B), which then sends
a DATA command to both (C) and (E). During mail data transfer, each MTA
is expected to stream the network data to the next MTA (or to the local
user's mailbox, as appropriate) -- that is, the data should be sent from
the MTA while it is being received.
When (B) receives the end-of-mail indicator, it must reply to (A) as
soon as possible to prevent duplicate messages as described in
[DUPLICATE].
The MTA at (C) and (D) must likewise reply to the end-of-mail indicator
as soon as possible.
6.1.3. Excessive delays with multiple MTAs
The cumulative delays of going through many MTAs will cause Session
delivery to fail (by falling back to normal store-and-forward). Proper
configuration and deployment of SMTP servers will prevent this
problem.
Implementors must carefully design session-enabled MTAs to respond
quickly when Session recipients are present to minimize timing
problems.
7. Implementation notes
7.1. SMTP server reply to STAT
To prevent excessive network activity by an SMTP client querying
delivery status "too often", the SMTP server may delay responding to a
client's STAT command. Such a delay MUST NOT exceed 10 seconds.
7.2. PIPELINING implementation recommendation
Due to the delays inherent with establishing connections with each MTA
in the SMTP "path", SMTP servers that implement this SESSION extension
SHOULD also implement [SMTP-PIPE]. This permits the SMTP client to send
the STAT command immediately after the end-of-mail-data indicator,
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without waiting for the SMTP server's reply to the end-of-mail-data
indicator.
8. Security Considerations
Denial of service attacks are possible with SESSION.
XXX - more verbage
9. Examples
9.1. Successful Session delivery to two recipients
This example shows a successful Session delivery with two recipients.
The first recipient, masinter@parc.xerox.com, was still being queued
when the first STAT command was sent by the client, but a subsequent
STAT command shows the final status.
Note also that the in-line DSN for njoffe@cisco.com includes a both a
TIFF (fax format) and audio reply, which may be used directly by some
mail user agents. This is merely an example to show how in-line DSNs
might be useful for low-end SMTP clients; a good SMTP server
implementation would only special content-types if it had reason to
believe the SMTP client could use them.
S: 220 mailer.cisco.com ESMTP service ready
C: EHLO pc.cisco.com
S: 250-mailer.cisco.com says hello
S: 250 SESSION
C: MAIL FROM:<dwing@cisco.com>
S: 250 <dwing@cisco.com> Sender ok
C: RCPT TO:<masinter@parc.xerox.com> SESSION
S: 250 <masinter@parc.xerox.com> and options ok
C: RCPT TO:<njoffe@cisco.com> SESSION
S: 250 <njoffe@cisco.com> and options ok
C: DATA
S: 354 Enter your data
C: From: Dan Wing <dwing@cisco.com>
C: To: njoffe@cisco.com, masinter@parc.xerox.com
C: Date: Mon, 6 Oct 1997 12:42:32 -0700
C: Subject: Palo Alto Coffee shops
C:
C: Please let me know your favorite coffee shop in Palo Alto.
C: .
S: 250 message accepted
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C: STAT <masinter@parc.xerox.com>
S: 250-Content-type: multipart/report;boundary="MessageBoundary"
S: 250-
S: 250---MessageBoundary
S: 250-Content-type: text/plain
S: 250-
S: 250-Your message is still being sent to:
S: 250-
S: 250-Recipient address: masinter@parc.xerox.com
S: 250-Reason: Still writing 32 bytes to remote system
S: 250-
S: 250---MessageBoundary
S: 250-Content-type: message/delivery-status
S: 250-
S: 250-Original-envelope-id: 01IOHGLWJLHE8X2IDT@Cisco.COM
S: 250-Reporting-MTA: dns; parc-1.xerox.com
S: 250-
S: 250-Action: delivered
S: 250-Status: 2.0.0
S: 250-Final-recipient: rfc822;masinter
S: 250-Session-Delivery: in-progress
S: 250-
S: 250 --MessageBoundary--
C: STAT <njoffe@cisco.com>
S: 250-Content-type: multipart/report;boundary="MessageBoundary"
S: 250-
S: 250---MessageBoundary
S: 250-Content-type: multipart/alternative;boundary="AltBoundary"
S: 250-
S: 250---AltBoundary
S: 250-Content-type: text/plain
S: 250-
S: 250-Your message was successfully delivered to these recipients:
S: 250-
S: 250-Recipient address: njoffe@cisco.com
S: 250-Reason: Successfully delivered immediately to local user.
S: 250-
S: 250---AltBoundary
S: 250-Content-type: audio/basic
S: 250-Content-Transfer-Encoding: base64
S: 250-
S: 250-ABCDEF01234567890ABCDEF01234567890ABCDEF01234567890ABCDEF
S: 250-ABCDEF01234567890ABCDEF01234567890ABCDEF01234567890ABCDEF
S: 250-ABCDEF01234567890ABCDEF01234567890ABCDEF01234567890ABCDEF
S: 250-
S: 250---AltBoundary
S: 250-Content-type: tiff; application=f
S: 250-Content-Transfer-Encoding: base64
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S: 250-
S: 250-01234567890ABCDEF01234567890ABCDEF01234567890ABCDEF012345
S: 250-01234567890ABCDEF01234567890ABCDEF01234567890ABCDEF012345
S: 250-01234567890ABCDEF01234567890ABCDEF01234567890ABCDEF012345
S: 250-
S: 250---AltBoundary--
S: 250-
S: 250---MessageBoundary
S: 250-Content-type: message/delivery-status
S: 250-
S: 250-Original-envelope-id: 01IOHGLWJLHE8X2IDT@Cisco.COM
S: 250-Reporting-MTA: dns; lint.cisco.com
S: 250-
S: 250-Action: delivered
S: 250-Status: 2.0.0 (Delivered immediately to local user)
S: 250-Final-recipient: rfc822;njoffe
S: 250-Session-Delivery: delivered
S: 250-
S: 250 --MessageBoundary--
C: STAT <masinter@parc.xerox.com>
S: 250-Content-type: MULTIPART/REPORT;BOUNDARY="MessageBoundary"
S: 250-
S: 250---MessageBoundary
S: 250-Content-type: text/plain
S: 250-
S: 250-Your message was successfully delivered to these recipients:
S: 250-
S: 250-Recipient address: masinter@parc.xerox.com
S: 250-Reason: Successfully delivered immediately to local user.
S: 250-
S: 250---MessageBoundary
S: 250-Content-type: message/delivery-status
S: 250-
S: 250-Original-envelope-id: 01IOHGLWJLHE8X2IDT@Cisco.COM
S: 250-Reporting-MTA: dns; parc-1.xerox.com
S: 250-
S: 250-Action: delivered
S: 250-Status: 2.0.0
S: 250-Final-recipient: rfc822;masinter
S: 250-Session-Delivery: delivered
S: 250-
S: 250 --MessageBoundary--
C: QUIT
S: 221 Goodbye
9.2 XXX - other examples?
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10. Acknowledgments
Much of this document was produced by work begun in the Internet FAX
Working Group of the IETF.
The authors would like to thank Graham Klyne (Integralis Ltd.) for his
contributions to this work.
11. References
[ABNF]
D. Crocker, P. Overell, "Augmented BNF for Syntax Specifications:
ABNF", Internet Draft, Work in Progress,
draft-ietf-drums-abnf-??.txt.
[CHUNKING]
G. Vaudreuil, "SMTP Service Extensions for Transmission of Large
and Binary MIME Messages", RFC 1830 (Experimental), August 1995.
[DSN]
K. Moore, G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 1894, January 1996.
[DRUMS]
J. Klensin, D. Mann, "Simple Mail Transfer Protocol", Internet
Draft, Work in Progress, draft-ietf-drums-smtpupd-??.txt.
[DUPLICATE]
C. Partridge, "DUPLICATE MESSAGES AND SMTP", RFC 1047, February
1988.
[]
J. Palme, "Common Internet Message Headers", RFC 2076, February
1997.
[MIME-RPT]
G. Vaudreuil, "The Multipart/Report Content Type for the
Reporting of Mail System Administrative Messages", RFC 1892,
January 1996.
[REQ]
S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP-14, RFC 2119, March 1997.
[SMTP]
J. Postel, "Simple Mail Transfer Protocol", STD-10, RFC 821,
August 1982.
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[SMTP-DSN]
K. Moore, "SMTP Service Extension for Delivery Status
Notifications", RFC 1891, January 1996.
[SMTP-ENH-ERR]
N. Freed, "SMTP Service Extension for Returning Enhanced Error
Codes", RFC 2034, October 1996.
[SMTP-EXT]
J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker, "SMTP
Service Extensions", STD-10, RFC 1869, November 1995.
[SMTP-PIPE]
N. Freed, "SMTP Service Extension for Command Pipelining",
RFC 2197, September 1997.
12. Authors' Addresses
Dan Wing
Cisco Systems, Inc.
101 Cooper Street
Santa Cruz, CA 95060 USA
Phone: +1 408 457 5200
Fax: +1 408 457 5208
EMail: dwing@cisco.com
Neil Joffe
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706 USA
Phone: +1 408 526 4000
Email: njoffe@cisco.com
Larry Masinter
Xerox Palo Alto Research Center
3333 Coyote Hill Road
Palo Alto, CA 94304 USA
Phone: +1 415 812 4365
Fax: +1 415 812 4333
EMail: masinter@parc.xerox.com
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