Internet Draft E. Allman
draft-ietf-msgtrk-smtpext-01.txt Sendmail, Inc.
Valid for six months T. Hansen
Updates: RFC 1891 AT&T Laboratories
March 20, 2001
SMTP Service Extension
for Message Tracking
<draft-ietf-msgtrk-smtpext-01.txt>
Status of This Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also dis-
tribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
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This document is a submission by the MSGTRK Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the msgtrk@imc.org mailing list. An archive of the mailing list
may be found at
http://www.ietf.org/archive/msgtrk
Distribution of this memo is unlimited.
1. Abstract
This memo defines an extension to the SMTP service whereby a
client may mark a message for future tracking.
Internet Draft Message Tracking ESMTP Extension March 20, 2001
2. Other Documents and Conformance
The model used for Message Tracking is described in [DRAFT-
MTRK-MODEL].
Doing a Message Tracking query is intended as a "last resort"
mechanism. Normally, Delivery Status Notifications (DSNs) [RFC-
DSN-SMTP] and Message Disposition Notifications (MDNs) [RFC-MDN]
would provide the primary delivery status. Only if the message is
not received, or there is no response from either of these mecha-
nisms should a Message Tracking query be issued.
The definition of the base64 token is imported from section
6.8 of [RFC-MIME].
Syntax notation in this document conforms to [RFC-ABNF].
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
[RFC-KEYWORDS].
3. SMTP Extension Overview
The Message Tracking SMTP service extension uses the SMTP ser-
vice extension mechanism described in [RFC-ESMTP]. The following
service extension is hereby defined:
(1) The name of the SMTP service extension is "Message Track-
ing".
(2) The EHLO keyword value associated with this extension is
"MTRK".
(3) No parameters are allowed with this EHLO keyword value.
Future documents may extend this specification by specifying
options.
(4) One optional parameter using the keyword "MTRK" is added to
the MAIL FROM command. In addition, the ENVID and ORCPT
parameters (as defined in RFC 1891 sections 5.4 and 5.2
respectively) MUST be supported, with extensions as
described below.
(5) The maximum length of a MAIL FROM command line is increased
by 40 characters by the possible addition of the MTRK key-
word and value. Note that a further extension of 614 char-
acters for the ORCPT and ENVID parameters is required by
RFC-DSN-EXT].
(6) No SMTP verbs are defined by this extension.
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Internet Draft Message Tracking ESMTP Extension March 20, 2001
4. The Extended MAIL FROM Command
The extended MAIL FROM command is issued by an SMTP client
when it wishes to inform an SMTP server that message tracking
information should be retained for future querying. The extended
MAIL FROM command is identical to the MAIL FROM command as defined
in [RFC-SMTP], except that MTRK, ORCPT, and ENVID parameters appear
after the address.
4.1. The MTRK parameter to the ESMTP MAIL command
Any sender wishing to track a message must first tag that
message as trackable by creating two values A and B:
A = some-large-random-number
B = SHA1(A)
The large random number A is calculated on a host-dependent
basis. See [RFC-RANDOM] for a discussion of choosing good ran-
dom numbers. This random number MUST be at least 128 bits but
MUST NOT be more than 1024 bits.
The 128-bit hash B of A is then computed using the SHA-1
algorithm as described in [NIST-SHA1].
The sender then base64 encodes value B and passes that
value as the mtrk-certifier on the MAIL FROM command:
mtrk-parameter = "MTRK=" mtrk-certifier [ ":" mtrk-timeout ]
mtrk-certifier = base64 ; authenticator
mtrk-timeout = 1*9digit; seconds until timeout
A is stored in the originator's tracking database to vali-
date future tracking requests as described in [DRAFT-MTRK-MTQP].
B is stored in tracking tracking databases of compliant MTAs and
used to authenticate future tracking requests.
The mtrk-timeout field indicates the number of seconds that
the client requests that this tracking information be retained
on intermediate servers, as measured from the initial receipt of
the message at that server. Servers MAY ignore this value if it
violates local policy. In particular, servers MAY silently
enforce an upper limit to how long they will retain tracking
data; this limit MUST be at least one day.
If no mtrk-timeout field is specified then the server
should use a local default. This default SHOULD be 8-10 days
and MUST be at least one day. Notwithstanding this clause, the
information MUST NOT be expired while the message remains in the
queue for this server: that is, an MTQP server MUST NOT deny
knowledge of a message while that same message sits in the MTA
queue.
If the message is relayed to another compliant SMTP server,
the MTA acting as the client SHOULD pass an mtrk-timeout field
Allman & Hansen [Page 3]
Internet Draft Message Tracking ESMTP Extension March 20, 2001
equal to the remaining life of that message tracking informa-
tion. Specifically, the tracking timeout is decremented by the
number of seconds the message has lingered at this MTA and then
passed to the next MTA. If the decremented tracking timeout is
less than or equal to zero, the entire MTRK parameter MUST NOT
be passed to the next MTA; essentially, the entire tracking path
is considered to be lost at that point.
See [RFC-DELIVERYBY] section 4 for an explanation of why a
timeout is used instead of an absolute time.
4.2. Use of ENVID
To function properly, Message Tracking requires that each
message have a unique identifier that is never reused by any
other message. For that purpose, if the MTRK parameter is
given, an ENVID parameter MUST be included, and the syntax of
ENVID from RFC 1891 section 5.4 is extended as follows:
envid-parameter = "ENVID=" unique-envid
unique-envid = local-envid "@" fqhn
local-envid = xtext
fqhn = xtext
The unique-envid MUST be chosen in such a way that the same
ENVID will never be used by any other message sent from this
system or any other system. In most cases, this means setting
fqhn to be the fully qualified host name of the system generat-
ing this ENVID, and local-envid to an identifier that is never
re-used by that host.
Any retransmissions of this message MUST assign a new
ENVID. In this context, "retransmission" includes forwarding or
resending a message.
4.3. Forwarding Tracking Certifiers
MTAs SHOULD forward unexpired tracking certifiers to com-
pliant mailers as the mail is transferred during regular hop-to-
hop transfers. If the "downstream" MTA is not MTRK-compliant,
then the MTRK= parameter MUST be deleted. If the downstream MTA
is DSN-compliant, then the ENVID and ORCPT parameters MUST NOT
be deleted.
If aliasing, forwarding, or other redirection of messages
to a single recipient occurs, then the MTA SHOULD treat this as
an ordinary hop-to-hop transfer and forward the MTRK=, ENVID=,
and ORCPT= values; these values MUST NOT be modified.
MTAs MUST NOT copy MTRK certifiers when relaying a message
to multiple recipients. An MTA MAY designate one recipient in a
multi-recipient alias as the "primary" recipient to which track-
ing requests shall be forwarded; other addresses SHALL NOT
receive tracking certifiers. MTAs MUST NOT forward MTRK certi-
fiers when doing mailing list expansion.
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Internet Draft Message Tracking ESMTP Extension March 20, 2001
5. Security Issues
5.1. Denial of service
An attacker could attempt to flood the database of a server
by submitting large numbers of small, tracked messages. In this
case, a site may elect to lower its maximum retention period
retroactively.
5.2. Confidentiality
The mtrk-authenticator value (``A'') must be hard to pre-
dict and not reused.
The originating client must take reasonable precautions to
protect the secret. For example, if the secret is stored in a
message store (e.g., a "Sent" folder), the client must make sure
the secret isn't accessible by attackers, particularly on a
shared store.
MTAs SHOULD take precautions to make certain that message
tracking cannot be used to explore internal topologies of net-
works.
6. References
[DRAFT-MTRK-MODEL]
T. Hansen, ``Message Tracking Model and Requirements.''
draft-ietf-msgtrk-model-03.txt. November 2000.
[DRAFT-MTRK-MTQP]
T. Hansen, ``Message Tracking Query Protocol.'' draft-ietf-
msgtrk-mtqp-01.txt. November 2000.
[RFC-ABNF]
Crocker, D., Editor, and P. Overell, ``Augmented BNF for Syn-
tax Specifications: ABNF'', RFC 2234, November 1997.
[RFC-DELIVERYBY]
D. Newman, ``Deliver By SMTP Service Extension.'' RFC 2852.
June 2000.
[RFC-DSN-REPT]
G. Vaudreuil, ``The Multipart/Report Content Type for the
Reporting of Mail System Administrative Messages.'' RFC 1892.
January 1996.
[RFC-DSN-SMTP]
K. Moore, ``SMTP Service Extension for Delivery Status Notifi-
cations.'' RFC 1891. January 1996.
[RFC-DSN-STAT]
K. Moore and G. Vaudreuil, ``An Extensible Message Format for
Delivery Status Notifications.'' RFC 1894. January 1996.
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Internet Draft Message Tracking ESMTP Extension March 20, 2001
[RFC-EMSSC]
G. Vaudreuil, ``Enhanced Mail System Status Codes.'' RFC
1893. January 1996.
[RFC-ESMTP]
Rose, M., Stefferud, E., Crocker, D., Klensin, J. and N.
Freed, ``SMTP Service Extensions.'' STD 10, RFC 1869. Novem-
ber 1995.
[RFC-KEYWORDS]
S. Bradner, ``Key words for use in RFCs to Indicate Require-
ment Levels.'' RFC 2119. March 1997.
[RFC-MDN]
R. Fajman, ``An Extensible Message Format for Message Disposi-
tion Notifications.'' RFC 2298. March 1998.
[RFC-MIME]
N. Freed and N. Borenstein, ``Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bod-
ies.'' RFC 2045. November 1996.
[RFC-MSGFMT]
D. Crocker, ``Standard for the Format of ARPA Internet Text
Messages.'' RFC 822. August 1982.
[RFC-RANDOM]
[RFC-RELATED]
E. Levinson, ``The MIME Multipart/Related Content-type.'' RFC
2387. August 1998.
[NIST-SHA1]
NIST FIPS PUB 180-1, ``Secure Hash Standard.'' National
Institute of Standards and Technology, U.S. Department of Com-
merce. May 1994. DRAFT.
[RFC-SMTP]
J. Postel, ``Simple Mail Transport Protocol.'' RFC 821.
August 1982.
7. Authors' Addresses
Eric Allman
Sendmail, Inc.
6603 Shellmound
Emeryville, CA 94608
U.S.A.
E-Mail: eric@Sendmail.COM
Phone: +1 510 594 5501
Fax: +1 510 594 5411
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Internet Draft Message Tracking ESMTP Extension March 20, 2001
Tony Hansen
AT&T Laboratories
Lincroft, NJ 07738
U.S.A.
Phone: +1 732 576 3207
E-Mail: tony@att.com
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