Internet Engineering Task Force (IETF) A. Melnikov
Request for Comments: 6710 Isode Ltd
Category: Standards Track K. Carlberg
ISSN: 2070-1721 G11
August 2012
Simple Mail Transfer Protocol Extension for Message Transfer Priorities
Abstract
This memo defines an extension to the SMTP (Simple Mail Transfer
Protocol) service whereby messages are given a label to indicate
preferential handling, to enable mail handling nodes to take this
information into account for onward processing.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6710.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6710 Message Transfer Priority SMTP Extension August 2012
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Definition of the Priority SMTP Extension . . . . . . . . . . 4
4. Handling of Messages Received via SMTP . . . . . . . . . . . . 5
4.1. Handling of the MT-PRIORITY Parameter by the Receiving
SMTP Server . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Relay of Messages to Other Conforming SMTP/LMTP Servers . 6
4.3. Relay of Messages to Non-Conforming SMTP/LMTP Servers . . 7
4.4. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 7
4.5. Gatewaying a Message into a Foreign Environment . . . . . 7
4.6. Interaction with the DSN SMTP Extension . . . . . . . . . 7
5. The Priority Service Extension . . . . . . . . . . . . . . . . 8
5.1. Expedited Transfer . . . . . . . . . . . . . . . . . . . . 9
5.2. Timely Delivery . . . . . . . . . . . . . . . . . . . . . 10
6. Use of MT-PRIORITY with LMTP . . . . . . . . . . . . . . . . . 10
7. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 14
9.1. Multiple MX Records . . . . . . . . . . . . . . . . . . . 14
9.2. Priority Assignment Policies . . . . . . . . . . . . . . . 15
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10.1. Requirements on Priority Assignment Policy
Registrations . . . . . . . . . . . . . . . . . . . . . . 17
10.2. Initial Priority Assignment Policy Registrations . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Priority Assignment Policy for Military Messaging . . 22
Appendix B. Priority Assignment Policy for MIXER . . . . . . . . 23
Appendix C. Priority Assignment Policy for National Security
/ Emergency Preparedness (NS/EP) . . . . . . . . . . 24
Appendix D. Possible Implementation Strategies . . . . . . . . . 25
D.1. Probability . . . . . . . . . . . . . . . . . . . . . . . 25
D.2. Preemption of Sessions or Transactions . . . . . . . . . . 25
D.3. Resource Allocation Models . . . . . . . . . . . . . . . . 26
Appendix E. Background on Design Choices . . . . . . . . . . . . 26
Appendix F. Acknowledgements . . . . . . . . . . . . . . . . . . 27
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1. Introduction
Where resources for switching or transferring messages are
constrained (e.g., bandwidth, round trip time, transition storage, or
processing capability), it is desirable to give preferential handling
to some messages over others, according to their labeled priority.
This is particularly important during emergencies for first
responders (Appendix C) and for environments such as military
(Appendix A) and aviation (Appendix B) messaging, where messages have
high operational significance, and the consequences of extraneous
delay can be significant.
In order for an SMTP receiver to be able to relay higher-priority
messages first, there needs to be a mechanism to communicate (during
both Message Submission [RFC6409] and Message Transfer [RFC5321]) the
priority of each message. This specification defines this mechanism
by specification of an SMTP [RFC5321] extension.
In order to permit end-to-end use of this extension across an email
infrastructure that does not support it, a companion tunneling
mechanism is defined in [PRIORITY-TUNNELING] that uses a new message
header field [RFC5322].
This extension provides services to some classes of users in networks
with limited available bandwidth or long round trip times, when the
actual message transfer over the network can create a significant
portion of the overall message delivery time from a sender to a
recipient, for example, over a satellite or high-frequency radio
link. It is also useful in case of a Mail Transfer Agent (MTA) queue
buildup due to the rate of incoming messages being higher than the
rate of outgoing messages. When neither of the two conditions
mentioned above is true, the use of the MT-PRIORITY SMTP extension
will not result in better SMTP service to any user. Also note that
while this SMTP extension can help in improving delivery speed for
higher-priority messages, it does not provide any guarantees that for
two given messages with priorities M and N (M > N) submitted
simultaneously, the message with priority M will arrive earlier than
the message with priority N. That is, this extension calls for best
effort to provide preferential processing.
Besides the actions taken at the application level, it can thus be
important to deploy priority or precedence mechanisms offered by the
network itself to ensure timely delivery of the emails. Examples
would be the use of DiffServ [RFC2474], RSVP [RFC2205], and [RFC6401]
(an extension to RSVP that prioritizes reservations).
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2. Conventions Used in This Document
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 [RFC2119] when they
appear in ALL CAPS. These words also appear in this document in
lower case as plain English words, absent their normative meanings.
The formal syntax uses the Augmented Backus-Naur Form (ABNF)
[RFC5234] notation including the core rules defined in Appendix B of
RFC 5234 [RFC5234].
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively. Line breaks that do not start with a new "C:"
or "S:" exist for editorial reasons and are not a part of the
protocol.
This document uses the term "priority" specifically in relation to
the internal treatment of a message by the server. Messages with
higher priorities may be given expedited handling, and those with
lower priorities may be handled only as resources become available.
3. Definition of the Priority SMTP Extension
The Priority SMTP service extension is defined as follows:
1. The textual name of this extension is "Priority Message
Handling".
2. The EHLO keyword value associated with this extension is
"MT-PRIORITY".
3. The EHLO keyword has an OPTIONAL parameter that conveys the name
of the Priority Assignment Policy (see Section 9.2) used by the
server. (See the <mt-priority-ehlo> ABNF non-terminal in
Section 7 for details of its syntax.) Absence of the parameter
means that the server is unwilling to disclose its Priority
Assignment Policy. Clients can choose to use the MT-PRIORITY
SMTP extension even if they don't recognize a particular Priority
Assignment Policy name advertised by a server.
4. No additional SMTP verbs are defined by this extension.
5. One optional parameter ("MT-PRIORITY") is added to the MAIL FROM
command. The value associated with this parameter is a decimal
integer number from -9 to 9 (inclusive) indicating the priority
of the email message (see Appendix E for more details on why this
range was selected). The syntax of the MT-PRIORITY parameter is
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described by the <priority-value> ABNF non-terminal defined in
Section 7. Higher numbers mean higher priority.
6. The maximum length of a MAIL FROM command line is increased by 15
octets by the possible addition of a space, the MT-PRIORITY
keyword, and a priority value.
7. The MT-PRIORITY extension is valid for the submission service
[RFC6409] and the Local Mail Transfer Protocol (LMTP) [RFC2033].
4. Handling of Messages Received via SMTP
This section describes how a conforming SMTP server should handle any
messages received via SMTP.
4.1. Handling of the MT-PRIORITY Parameter by the Receiving SMTP Server
The following rules apply to SMTP transactions in a server that
supports the MT-PRIORITY parameter:
1. If any of the associated <esmtp-value>s (as defined in Section
4.1.2 of [RFC5321]) are not syntactically valid, or if there is
more than one MT-PRIORITY parameter in a particular MAIL FROM
command, the server MUST return an error, for example "501 syntax
error in parameter" (with the 5.5.2 Enhanced Status Code
[RFC2034] [RFC5248]).
2. When inserting a Received header field as specified in Section
4.4 of [RFC5321], the compliant MTA/MSA (Mail Submission Agent)
SHOULD include the "PRIORITY" clause whose syntax is specified in
Section 7.
3. The received MT-PRIORITY parameter value SHOULD be logged as part
of any logging of message transactions.
4. If the sending SMTP client specified the MT-PRIORITY parameter to
the MAIL FROM command, then the value of this parameter is the
message priority.
5. If no priority has been determined by the above, the server may
use its normal policies to set the message's priority. By
default, each message has priority 0.
The SMTP server MUST NOT allow "upgraded" (positive) priorities from
untrusted (e.g., unauthenticated) or unauthorized sources. (One
example of an "unauthorized source" might be an SMTP sender that
successfully authenticated using SMTP AUTH, but that is not
explicitly authorized to use the SMTP MT-PRIORITY service. In case
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of MTA-to-MTA transfer, such authorization will usually be done as a
bilateral agreement between two domains to honor priorities from each
other.) The server MAY, however, allow an untrusted source to lower
its own message's priorities -- consider, for example, an email
marketer that voluntarily sends its marketing messages at a negative
priority.
The SMTP server MAY also alter the message priority (to lower or to
raise it) in order to enforce some other site policy. (Note that
this also includes the case in which the priority is not explicitly
specified.) For example, an MSA might have a mapping table that
assigns priorities to messages based on authentication credentials.
If the SMTP server changes (lowers or raises) the priority of a
message, it SHOULD use the X.3.6 Enhanced Status Code [RFC2034] in
its response to the MAIL FROM or in the final response to the DATA
(or similar) command. The human readable text part after the status
code contains the new priority, followed by SP (ASCII space) and
explanatory human readable text.
Alternatively, an SMTP server that is an MSA MAY reject a message
based on the determined priority. In such cases, the MSA SHOULD use
the 450 or 550 reply code. The corresponding Enhanced Status Code
MUST be X.7.15 [RFC2034] if the determined priority level is below
the lowest priority currently acceptable for the receiving SMTP
server. Note that this condition might be temporary. In some
environments, operational policies might permit periods of operation
that relay only higher-priority messages and reject lower priority
ones. Such handling choices need to be specified for that
operational environment.
4.2. Relay of Messages to Other Conforming SMTP/LMTP Servers
The following rules govern the behavior of a conforming MTA (in the
role of an SMTP/LMTP client) when relaying a message that was
received via the SMTP protocol to an SMTP/LMTP server that supports
the MT-PRIORITY extension:
1. An MT-PRIORITY parameter with the value determined by the
procedure from Section 4.1 MUST appear in the MAIL FROM command
issued when the message is relayed to an MTA/MDA (Mail Delivery
Agent) that also supports the MT-PRIORITY extension. (Note that
due to site policy, this value might be different from the value
received from the SMTP client. See Section 4.1 for details.
Also note that this value might be different than the priority
level at which the MTA actually handles the request, due to the
rounding described in Section 5.)
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2. Further processing of the MT-PRIORITY parameter is described in
Section 5.
4.3. Relay of Messages to Non-Conforming SMTP/LMTP Servers
The following rules govern the behavior of a conforming MTA (in the
role of an SMTP/LMTP client) when relaying a message that was
received via the SMTP protocol to an SMTP/LMTP server that does not
support the MT-PRIORITY extension:
1. The MTA relays the message without including the MT-PRIORITY
parameter in the MAIL FROM command.
4.4. Mailing Lists and Aliases
Several types of mechanisms exist to redirect or forward messages to
alternative or multiple addresses [RFC5598]. Examples for this are
aliases and mailing lists [RFC5321].
If a message is subject to such processing, the Mediator node
(Section 2.1 of [RFC5598]) SHOULD retain the MT-PRIORITY parameter
value for all expanded and/or translated addresses.
4.5. Gatewaying a Message into a Foreign Environment
The following rules govern the behavior of a conforming MTA when
gatewaying a message that was received via the SMTP protocol into a
foreign (non-SMTP) environment:
1. If the destination environment is unable to provide an equivalent
of the MT-PRIORITY parameter, the conforming MTA SHOULD behave as
if it is relaying to a non-conformant SMTP server (Section 4.3).
2. If the destination environment is capable of providing an
equivalent of the MT-PRIORITY parameter, the conforming MTA
SHOULD behave as if it is relaying to a conformant SMTP server
(Section 4.2), converting the MT-PRIORITY value to the equivalent
in the destination environment.
4.6. Interaction with the DSN SMTP Extension
An MTA that needs to generate a delivery report (whether for
successful delivery or delayed/failed delivery) for a message it is
processing SHOULD use the priority value of the message as the
priority of the generated delivery report. In particular, this
requirement applies to MTAs that also implement [RFC3461].
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For delivery reports (DSNs) received by an MTA for relay, processing
rules specified in Section 4.1 apply -- there is no special
processing for relayed DSNs. It might seem tempting to try to detect
DSNs and process them at an elevated priority under the assumption
that failure notices need to get through quickly, even or perhaps
especially if the DSN came from an untrusted source. But such a
policy can create an exposure to fake DSN attacks by giving untrusted
systems a way to inject high-priority messages. Implementation of
such a policy also assumes that DSNs can be detected reliably, which
may not be the case since some systems use nonstandard DSN formats.
5. The Priority Service Extension
The priorities of messages affect the order in which messages are
transferred from the client to the server. This is largely
independent from the order in which they were originally received by
the server.
A message priority is a decimal integer in the range from -9 to 9
(inclusive). SMTP servers compliant with this specification are not
required to support all 19 distinct priority levels (i.e., to treat
each priority value as a separate priority), but they MUST implement
all distinct priority levels specified in the Priority Assignment
Policy (see Section 9.2) implemented by the server. That is, an
implementation that only supports N priority levels (where N < 19)
will internally round up a syntactically valid priority value that
isn't supported to the next higher supported number (or to the
highest supported priority, if the value is higher than any supported
priority). For example, an implementation can treat priority values
below and including -4 as priority -4, priority -3 as priority -2,
and all priorities starting from 5 can be treated as priority 6.
(See Section 9.2 for implementation/deployment considerations related
to Priority Assignment Policy.)
Irrespective of the number of distinct priority levels supported by
the SMTP server, when relaying the message to the next hop or
delivering it over LMTP, the SMTP server MUST communicate the
priority value as determined in Section 4.1.
Note: 19 possible priority levels are defined by this specification
for extensibility. For example, a particular implementation or
deployment environment might need to provide finer-grained control
over message transfer priorities. See Appendix E for more details on
why the range from -9 to 9 was selected.
As per the Priority Assignment Policy, some SMTP servers MAY impose
additional maximum message size constraints for different message
transfer priorities; for example, messages with priority 6 might not
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be larger than 4 Kb. If an SMTP server chooses to reject a message
because it is too big for the determined priority, it SHOULD use 552
reply codes together with the X.7.16 Enhanced Status Code [RFC2034].
Implementation Note: If the SMTP server also supports the SMTP SIZE
extension [RFC1870], then an SMTP client can use both SIZE= and
MT-PRIORITY= parameters on the MAIL FROM command. This allows the
server to perform early rejection of a message in case the message
size is too big for the specified priority, thus avoiding wasting
bandwidth by transferring the message first and then rejecting it due
to its size.
The Priority Service Extension can be combined with the DELIVERBY
[RFC2852] SMTP service extension; however, there is no requirement
that both extensions always be implemented together.
5.1. Expedited Transfer
The main service provided by the Priority Message Handling SMTP
Service Extension is expedited transfer of emails with a higher
priority. Therefore, an SMTP client that has more than one email to
send at a given time sends those with a higher priority before those
with a lower one. Additionally, the retry interval and/or default
timeout before a non-delivery report is generated MAY be lower (more
aggressive) for messages of higher priority. Lower retry intervals/
default timeouts are controlled by the local MTA policy.
Note that as this SMTP extension requires some sort of trust
relationship between a sender and a receiver and thus some form of
authentication (whether using SMTP AUTH, TLS, IP address whitelist,
etc.), so senders using this SMTP extension will not be subject to
greylisting [RFC6647], unless they are unauthorized to use this SMTP
extension due to an explicit policy decision or a misconfiguration
error. However, note that in case of connection-level or SMTP EHLO/
HELO greylisting, SMTP AUTH or TLS authentication options are not
available to the server.
In order to make implementations of this extension easier, this SMTP
extension only allows a single priority for all recipients of the
same message.
Within a priority level, the MTA uses its normal algorithm (the
algorithm used in absence of this SMTP extension) for determining
message processing order.
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Several possible ways of implementing expedited transfer are
described in more details in Appendix D. Note that these sections
don't describe all details and pitfalls for each implementation
strategy.
5.2. Timely Delivery
An important constraint (usually associated with higher-priority
levels) in some environments is that messages with high-priority
values have some delivery time constraints. In some cases, higher
priorities mean a shorter maximum time allowed for delivery.
Unextended SMTP does not offer a service for timely delivery, i.e.,
"deliver this message within X seconds from submission" service. The
"Deliver By SMTP Service Extension" (DELIVERBY Extension) defined in
[RFC2852] is an example of an SMTP extension providing a service that
can be used to implement timely delivery. Note that SMTP DELIVERBY
and SMTP MT-PRIORITY extensions are complimentary and can be used
together (assuming the SMTP server they are talking to advertises
support for both). However, note that use of the DELIVERBY extension
alone does not guarantee any priority processing. If the client is
using both SMTP DELIVERBY and SMTP MT-PRIORITY at the same time, the
client can consider using smaller DELIVERBY timeouts for higher-
priority messages.
6. Use of MT-PRIORITY with LMTP
An LMTP server can advertise support for the MT-PRIORITY extension if
it supports any combination of the following features:
1. The LMTP server is architected in such a way that it can deliver
higher-priority messages quicker than lower-priority messages.
2. The LMTP server logs that the MT-PRIORITY extension was used by
the previous SMTP hop.
3. The LMTP server is exposing information about the MT-PRIORITY
extension to a delivery-time filtering engine such as Sieve
[RFC5228].
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7. Syntax
priority-value = (["-"] NZDIGIT) / "0"
; Allowed values are from -9 to 9 inclusive
NZDIGIT = %x31-39
; "1"-"9"
CFWS = <defined in RFC 5322>
; New "clause" that can be used in the Received header field
Pri = CFWS "PRIORITY" FWS priority-value
; Complies with the <Additional-Registered-Clauses>
; non-terminal syntax from RFC 5321.
mt-priority-ehlo = "MT-PRIORITY" [SP priority-profile]
; Complies with the <ehlo-line> ABNF production
; from RFC 5321.
priority-profile = 1*20(ALPHA / DIGIT / "-" / "_" / ".")
; name of the Priority Assignment Profile advertized in
; the MT-PRIORITY EHLO response.
ALPHA = <Defined in RFC 5234>
DIGIT = <Defined in RFC 5234>
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8. Example
The original submission (from MUA (Mail User Agent) to MSA) might
appear as shown below. Note that the example is also making use of
the STARTTLS [RFC3207], DELIVERBY [RFC2852], and DSN [RFC3461] SMTP
extensions, even though there is no requirement that these other
extensions be supported when the MT-PRIORITY SMTP extension is
implemented.
S: 220 example.com SMTP server here
C: EHLO mua.example.com
S: 250-example.com
S: 250-STARTTLS
S: 250-AUTH SCRAM-SHA-1 DIGEST-MD5
S: 250-DSN
S: 250-DELIVERBY
S: 250-ENHANCEDSTATUSCODES
S: 250 MT-PRIORITY MIXER
C: AUTH SCRAM-SHA-1
[...authentication exchange...]
S: 235 2.7.0 Authentication successful
C: MAIL FROM:<eljefe@example.com> BY=125;R ENVID=QQ314159
MT-PRIORITY=3
S: 250 2.1.0 <eljefe@example.com> sender ok
C: RCPT TO:<topbanana@example.net>
S: 250 2.1.5 <topbanana@example.net> recipient ok
C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
ORCPT=rfc822;Dana@Ivory.example.net
S: 250 2.1.5 <Dana@Ivory.example.net> recipient ok
C: DATA
S: 354 okay, send message
C: (message goes here)
C: .
S: 250 2.1.0 message accepted
C: QUIT
S: 221 2.0.0 goodbye
In the above example, the MUA has specified the priority 3 and the
server has accepted it. The server is advertising the MIXER Priority
Assignment Policy (the default). Another variant of the initial
submission might look like:
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S: 220 example.com SMTP server here
C: EHLO mua.example.com
S: 250-example.com
S: 250-STARTTLS
S: 250-AUTH SCRAM-SHA-1 DIGEST-MD5
S: 250-DSN
S: 250-DELIVERBY
S: 250-ENHANCEDSTATUSCODES
S: 250 MT-PRIORITY
C: AUTH SCRAM-SHA-1
[...authentication exchange...]
S: 235 2.7.0 Authentication successful
C: MAIL FROM:<eljefe@example.com> BY=125;R ENVID=QQ314159
S: 250 2.1.0 <eljefe@example.com> sender ok
C: RCPT TO:<topbanana@example.net>
S: 250 2.1.5 <topbanana@example.net> recipient ok
C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
ORCPT=rfc822;Dana@Ivory.example.net
S: 250 2.1.5 <Dana@Ivory.example.net> recipient ok
C: DATA
S: 354 okay, send message
C: (message goes here)
C: .
S: 250 X.3.6 3 is the new priority assigned to the message
C: QUIT
S: 221 2.0.0 goodbye
In the above example, the MUA has not specified any priority, but the
MSA has assigned priority 3 to the message. Also note that the
server is unwilling to adverte the Priority Assignment Policy it
supports in the EHLO response.
The MSA relays the message to the next MTA.
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S: 220 example.net SMTP server here
C: EHLO example.com
S: 250-example.net
S: 250-DSN
S: 250-DELIVERBY
S: 250 MT-PRIORITY STANAG4406
C: MAIL FROM:<eljefe@example.com> BY=120;R ENVID=QQ314159
MT-PRIORITY=3
S: 250 <eljefe@example.com> sender ok
C: RCPT TO:<topbanana@example.net>
S: 250 <topbanana@example.net> recipient ok
C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
ORCPT=rfc822;Dana@Ivory.example.net
S: 250 <Dana@Ivory.example.net> recipient ok
C: DATA
S: 354 okay, send message
C: (message goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
The receiving SMTP server advertises support for the "STANAG4406"
Priority Assignment Policy, which supports 6 priority levels as
described in Appendix A. This means that the server will use the
priority value 4 internally (the next supported priority higher or
equal to 3) and will communicate the priority value 3 when relaying
it to the next hop (if necessary).
9. Deployment Considerations
9.1. Multiple MX Records
If multiple DNS MX records are used to specify multiple servers for a
domain in Section 5 of [RFC5321], it is strongly advised that all of
them support the MT-PRIORITY extension and handle priorities in
exactly the same way. If one or more servers behave differently in
this respect, then it is strongly suggested that none of the servers
support the MT-PRIORITY extension. Otherwise, unexpected differences
in message delivery speed or even rejections can happen during
temporary or permanent failures, which users might perceive as
serious reliability issues.
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9.2. Priority Assignment Policies
This document allows up to 19 distinct priority values. In a
particular operating environment, independent originators need to
assign priority values according to, roughly, the same criteria, so
that the same "high priority message" doesn't get associated with the
value 3 for one sender and with the value 5 for another, as such
messages might unintentionally receive different preferential
treatment.
In order to achieve consistent behavior in an operating environment,
the Priority Assignment Policy (together with possible associated
restrictions on maximum message sizes for each priority (if any),
default timeouts, etc.) should be documented for the environment.
Each SMTP/LMTP server supports a Priority Assignment Policy, whether
explicit (advertised in the MT-PRIORITY EHLO response) or implicit
(not advertised). The default Priority Assignment Policy (assumed by
the client when no Priority Assignment Policy name is advertised in
the MT-PRIORITY EHLO response) is specified in Appendix B. Two other
policies are specified in Appendix A and Appendix C. Additional
policies SHOULD be registered with IANA as specified in Section 10.1.
Moreover, all MSAs/MTAs/MDAs within any given Administrative
Management Domain has to be configured to use the same Priority
Assignment Policy. Otherwise, a differently configured MSA/MTA/MDA
can expose the whole domain to possible attacks, like injection of a
high-priority fake DSN.
When this SMTP extension is deployed across multiple cooperating
Administrative Domains, such Administrative Domains need to use the
same or at least compatible policies. Again, differences in policies
(for example, differences in how users are authenticated or
differences in how priorities are handled) can expose an
Administrative Domain to weaknesses in a partner domain.
10. IANA Considerations
IANA has added the MT-PRIORITY SMTP extension to the "SMTP Service
Extensions" registry
(http://www.iana.org/assignments/mail-parameters). This extension is
suitable for the Submit port.
IANA has added the following new Received header field clause to the
"Additional-registered-clauses" sub-registry
(http://www.iana.org/assignments/mail-parameters) to help with
tracing email messages delivered using the MT-PRIORITY SMTP
extension:
Melnikov & Carlberg Standards Track [Page 15]
RFC 6710 Message Transfer Priority SMTP Extension August 2012
Clause name: PRIORITY
Description: Records the value of the MT-PRIORITY parameter specified
in the MAIL FROM command
Syntax of the value: See Section 7 of RFC 6710
Reference: RFC 6710
IANA has added the following Enumerated Status Codes to the "Simple
Mail Transfer Protocol (SMTP) Enhanced Status Codes" registry
(http://www.iana.org/assignments/smtp-enhanced-status-codes)
established by [RFC5248]:
1) Code: X.7.15
Sample Text: Priority Level is too low
Associated basic status code: 450, 550 (other 4XX or 5XX codes
are allowed)
Description: The specified priority level is below the lowest
priority acceptable for the receiving SMTP server. This
condition might be temporary, for example the server is
operating in a mode where only higher-priority messages are
accepted for transfer and delivery, while lower-priority
messages are rejected.
Reference: RFC 6710
Submitter: A. Melnikov
Change controller: IESG
2) Code: X.7.16
Sample Text: Message is too big for the specified priority
Associated basic status code: 552 (other 4XX or 5XX codes are
allowed)
Description: The message is too big for the specified priority.
This condition might be temporary, for example the server is
operating in a mode where only higher-priority messages below a
certain size are accepted for transfer and delivery.
Reference: RFC 6710
Submitter: A. Melnikov
Change controller: IESG
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3) Code: X.3.6
Sample Text: Requested priority was changed
Associated basic status code: 250 or 251
Description: The message was accepted for relay/delivery, but
the requested priority (possibly the implied default) was not
honored. The human readable text after the status code
contains the new priority, followed by SP (space) and
explanatory human readable text.
Reference: RFC 6710
Submitter: A. Melnikov
Change controller: IESG
IANA has created a new IANA registry called "SMTP PRIORITY Extension
Priority Assignment Policy". Future registrations in this registry
are governed by the "Specification Required" [RFC5226] IANA
registration policy. Requirements on registrations (to be verified
by the Designated Expert) are specified in Section 10.1. Changes to
registrations undergo the same process as initial registrations. In
cases of significant changes to registrations (other than editorial
clarifications), the Designated Expert MAY require registration of a
Priority Assignment Policy with a new name instead of updating the
existing one.
10.1. Requirements on Priority Assignment Policy Registrations
Priority Assignment Policy registrations with IANA are accompanied by
a policy specification document that MUST specify the following
information:
1. The Priority Assignment Policy name, which is a case-insensitive
string of 1 to 20 US-ASCII characters to be advertised as the
MT-PRIORITY EHLO parameter. Allowed characters are: ALPHA,
DIGIT, "-", "_", and "."
2. Number of distinct priority levels supported by all servers
implementing the policy and their respective values.
3. For each supported priority level: default retry timeouts (how
often to retry sending a message if there is a temporary error to
transfer/deliver it). The policy specification can also
explicitly define such information as implementation and/or
deployment specific.
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4. For each supported priority level: default expiration timeouts
(how long to attempt transfer/delivery before the message expires
and causes a non-delivery report to be generated). The policy
specification can also explicitly define such information as
implementation and/or deployment specific. Note that a client
can override such default when it uses additional SMTP extensions
(such as the one mentioned in Section 5.2).
5. Maximum message size associated with each priority level. The
policy specification can also explicitly define such information
as implementation and/or deployment specific.
6. Any requirements/restrictions on the kind of SMTP client
authentication required in order for an SMTP server implementing
this policy to accept priority values specified by an SMTP
client. For example, this can limit which Simple Authentication
and Security Layer (SASL) [RFC4422] authentication mechanisms are
to be used, require TLS, etc.
7. Any other information that might affect processing of messages
with different priorities.
8. Note that the policy specification document is not allowed to
redefine the allowed range of priorities specified in Section 5
and other aspects of handling of different priorities, unless
explicitly specified by this document.
10.2. Initial Priority Assignment Policy Registrations
IANA has registered the following initial values in the "SMTP
PRIORITY Extension Priority Assignment Policy" registry:
Initial Priority Assignment Policy Registrations
+-------------+------------------------+----------------+
| Policy Name | Reference | Comment |
+-------------+------------------------+----------------+
| MIXER | Appendix B of RFC 6710 | Default policy |
| STANAG4406 | Appendix A of RFC 6710 | |
| NSEP | Appendix C of RFC 6710 | |
+-------------+------------------------+----------------+
11. Security Considerations
Message Submission Agents ought to only accept message transfer
priorities from users (or only certain groups of such users) who are
authenticated and authorized in some way that's acceptable to the
MSA. As part of this policy, they can also restrict maximum priority
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values that different groups of users can request, and can override
the priority values specified by MUAs.
Similarly, MTAs ought to only accept message transfer priorities from
senders (or only certain groups of such senders) who are
authenticated and authorized in some way that's acceptable to the
MTA. As part of this policy, they can also restrict maximum priority
values that different groups of senders can request, and can override
the priority values specified by them.
In the absence of the policy enforcement mentioned above, an SMTP
server (whether an MSA or an MTA) implementing this SMTP extension
might be susceptible to a denial-of-service attack. For example,
malicious clients (MUAs/MSAs/MTAs) can try to abuse this feature by
always requesting priority 9.
12. References
12.1. Normative References
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248, June 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
Melnikov & Carlberg Standards Track [Page 19]
RFC 6710 Message Transfer Priority SMTP Extension August 2012
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
12.2. Informative References
[ACP123] CCEB, "Common Messaging strategy and procedures", ACP 123,
May 2009.
[PRIORITY-TUNNELING]
Melnikov, A. and K. Carlberg, "Tunneling of SMTP Message
Transfer Priorities", Work in Progress, July 2012.
[RFC1845] Crocker, D., Freed, N., and A. Cargille, "SMTP Service
Extension for Checkpoint/Restart", RFC 1845,
September 1995.
[RFC1870] Klensin, J., Freed, N., and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10, RFC 1870,
November 1995.
[RFC2156] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
Mapping between X.400 and RFC 822/MIME", RFC 2156,
January 1998.
[RFC2205] Braden, B., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC2852] Newman, D., "Deliver By SMTP Service Extension", RFC 2852,
June 2000.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC4125] Le Faucheur, F. and W. Lai, "Maximum Allocation Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering", RFC 4125, June 2005.
Melnikov & Carlberg Standards Track [Page 20]
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[RFC4127] Le Faucheur, F., Ed., "Russian Dolls Bandwidth Constraints
Model for Diffserv-aware MPLS Traffic Engineering",
RFC 4127, June 2005.
[RFC4190] Carlberg, K., Brown, I., and C. Beard, "Framework for
Supporting Emergency Telecommunications Service (ETS) in
IP Telephony", RFC 4190, November 2005.
[RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource
Priority for the Session Initiation Protocol (SIP)",
RFC 4412, February 2006.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
June 2006.
[RFC5228] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
Filtering Language", RFC 5228, January 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[RFC6401] Le Faucheur, F., Polk, J., and K. Carlberg, "RSVP
Extensions for Admission Priority", RFC 6401,
October 2011.
[RFC6647] Kucherawy, M. and D. Crocker, "Email Greylisting: An
Applicability Statement for SMTP", RFC 6647, June 2012.
[SMTP-PRI-OLD]
Schmeing, M., Brendecke, J., and K. Carlberg, "SMTP
Service Extension for Priority Message Handling", Work
in Progress, August 2006.
[STANAG-4406]
NATO, "STANAG 4406 Edition 2: Military Message Handling
System", STANAG 4406, March 2005.
Melnikov & Carlberg Standards Track [Page 21]
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Appendix A. Priority Assignment Policy for Military Messaging
Military Messaging as specified in ACP 123 [ACP123] (also specified
in STANAG 4406 [STANAG-4406]) defines 6 priority ("precedence")
values. While ACP 123/STANAG 4406 allow for 32 different priority
levels (16 levels are reserved for NATO and an additional 16 are
reserved for national use), only 6 are in use in practice. This
section specifies the Priority Assignment Policy for Military
Messaging and how the MT-PRIORITY parameter can be mapped when
gatewaying between SMTP and ACP 123/STANAG 4406 environments.
Where SMTP is used to support military messaging, the following
mappings SHOULD be used.
Recommended Mapping of MT-PRIORITY Values for MMHS
+-------------------+----------------------+
| MT-PRIORITY value | MMHS Precedence name |
+-------------------+----------------------+
| -4 | Deferred |
| -2 | Routine |
| 0 | Priority |
| 2 | Immediate |
| 4 | Flash |
| 6 | Override |
+-------------------+----------------------+
Table 1
The Priority Assignment Policy registration for Military Messaging is
as follows:
1. The Priority Assignment Policy name is "STANAG4406".
2. Number of distinct priority levels: 6, as specified in the table
above.
3. Default retry timeouts for each priority level are implementation
and/or deployment specific.
4. Default expiration timeouts for each priority level are
implementation and/or deployment specific.
5. Maximum message size associated with each priority level is
implementation and/or deployment specific.
6. No restrictions on what kind of SMTP client authentication is
required.
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Appendix B. Priority Assignment Policy for MIXER
MIXER [RFC2156] defines the Priority header field with 3 values.
This section specifies the Priority Assignment Policy for MIXER and
how the MT-PRIORITY parameter can be mapped when used with MIXER.
Where SMTP is used to support MIXER messaging, the following mappings
SHOULD be used.
Recommended Mapping of MT-PRIORITY Values for MIXER
+-------------------+----------------------+
| MT-PRIORITY value | MIXER Priority value |
+-------------------+----------------------+
| -4 | non-urgent |
| 0 | normal |
| 4 | urgent |
+-------------------+----------------------+
Table 2
The Priority Assignment Policy registration for MIXER is as follows:
1. The Priority Assignment Policy name is "MIXER".
2. Number of distinct priority levels: 3, as specified in the table
above.
3. Default retry timeouts for each priority level are implementation
and/or deployment specific.
4. Default expiration timeouts for each priority level are
implementation and/or deployment specific.
5. Maximum message size associated with each priority level is
implementation and/or deployment specific.
6. No restrictions on what kind of SMTP client authentication is
required.
Appendix C. Priority Assignment Policy for National Security /
Emergency Preparedness (NS/EP)
There are several forms of communication systems used during an
emergency or disaster. The most well known form involves the many-
to-one model of the general public contacting a public safety access
point via 911/999/112 calls through the public telephone network.
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Typically, these calls do not require authorization, nor do they
invoke any prioritization.
Another form of emergency communications involves a set of authorized
users or nodes that use prioritized services to help establish and
continue communication given limited available resources. [RFC4190]
includes descriptions of several systems that have been developed to
support National Security / Emergency Preparedness (NS/EP). These
deployed systems require a form of authentication and have focused on
prioritization of telephony-based services. They have also been
designed as a binary form (on/off) of signaled priority
communications.
[RFC4412] includes examples of a more expansive view of NS/EP
communications in which priority migrates from a single on/off bit
value to one that comprises 5 priority values. This is shown in the
cases of the Emergency Telecommunications Service (ETS) and Wireless
Priority Service (WPS) Namespaces. Given a lack of pre-existing
NS/EP values assigned for email, we follow the paradigm of the ETS
and WPS Namespaces and recommend the 5 ascending values shown in the
table below.
+-------------------+------------------+
| MT-PRIORITY value | Relational Order |
+-------------------+------------------+
| -2 | Lowest Priority |
| 0 | ---------- |
| 2 | ---------- |
| 4 | ---------- |
| 6 | Highest Priority |
+-------------------+------------------+
The Priority Assignment Policy registration for NS/EP is as follows:
1. The Priority Assignment Policy name is "NSEP".
2. Number of distinct priority levels: 5, as specified in the table
above.
3. Default retry timeouts for each priority level are implementation
and/or deployment specific.
4. Default expiration timeouts for each priority level are
implementation and/or deployment specific.
5. Maximum message size associated with each priority level is
implementation and/or deployment specific.
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6. No restrictions on what kind of SMTP client authentication is
required.
Appendix D. Possible Implementation Strategies
This appendix suggests some strategies to implement the SMTP
extension defined in this document. The list is not exhaustive.
This appendix and its subsections are Informative.
D.1. Probability
As the name suggests, probability involves increasing the chances of
obtaining resources without adversely affecting previously
established connections. One example would involve requesting
resources set aside for specific priority levels. If these
additional resources are exhausted, then the desired connection is
denied. Queues, new timers, or combinations thereof can be used to
facilitate the higher-priority requests, but the key is that
mechanisms focus on increasing the probability of message transfer.
D.2. Preemption of Sessions or Transactions
Preemption is a type of action that focuses only on a comparison of
priorities to determine if previously established transactions need
to be displaced in favor of higher-priority requests. If no
additional connection is possible, the client aborts a running
session for emails with lower priority no later than directly after
the current transaction. The client can even interrupt an active
transaction, and ought to do so if other constraints, such as
delivery time (as specified in the DELIVERBY SMTP extension
[RFC2852]), would be violated for the email with higher priority.
When interrupting an active transaction, the client ought to take the
total message size and the size of the transferred portion of the
message being interrupted into consideration. This preliminary
termination of sessions or transactions is called preemption.
If preemption of running transactions occurs, the client needs to
choose a transaction with the lowest priority currently processed.
If the client has an option (i.e., it is supported by the next-hop
MTA) to interrupt transactions in a way that allows them to be
restarted at the interruption point later, it ought to deploy it. An
example for a mechanism providing such a service is the "SMTP Service
Extension for Checkpoint/Restart" defined in [RFC1845].
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If a client opts for the preemption of sessions instead of
transactions, it needs to preempt the next session that reaches the
end of a transaction.
D.3. Resource Allocation Models
Adding prioritization to a design moves the subject away from a
strictly best effort (and a first-come-first-served) model to one
that includes admission control and resource allocation models. Over
the years, a variety of work has been done within the IETF to specify
resource allocations models. Examples include the Maximum Allocation
Model [RFC4125], the Russian Dolls Model [RFC4127], and the Priority
Bypass Model (Appendix A.3 of [RFC6401]).
While we recognize that these various models have been designed for
other protocols (i.e., MPLS and RSVP), an understanding of their
design characteristics may be beneficial in considering future
implementations of a priority SMTP service.
In cases where the processing of high-priority messages by an MTA is
not considered negligible and exceeds engineered expectations, then
operators managing that MTA may be notified in some form (e.g.,
pushed alarm, polled status).
Appendix E. Background on Design Choices
This section provides some background on design choices made during
development of the MT-PRIORITY SMTP extension.
The priority applies per message, rather than per recipient, in order
to keep the protocol simpler and because of the expectation that it
will be uncommon to need different priorities for different
recipients on the same message. In cases where that is necessary, it
can always be achieved by sending separate messages with the same
content, segregating the recipients by desired message priority.
The choice of the priority range -9 to 9 (as opposed to, say, 1 to 6,
or 0 to 9) was made after taking the following into consideration:
1. Clearly, having multiple priority levels is the whole point of
this extension. Existing implementations of similar
functionality in MTAs are already using 3 levels. One of the use
cases motivating this extension requires 6 levels, so at least 6
different values are required.
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2. During discussions of this extension, several different use cases
were suggested that required differing numbers of priority
levels. Defining just the 6 priority levels needed in item 1,
above, would limit the extensibility for possible future use
cases. Therefore, this document is defining a wider range, which
allows implementations and deployments to add higher or lower
priority levels and to insert additional priority levels between
the recommended set of 6. This avoids the need to further extend
this extension just to have a few more priority levels.
3. It seems natural to use zero for the "normal" or default
priority, rather than picking some non-zero number and having the
priorities go up or down from there. This way, negative numbers
always represent priorities that are lower than normal, with
positive numbers as higher priorities.
Appendix F. Acknowledgements
This document copies lots of text from "SMTP Service Extension for
Priority Message Handling" [SMTP-PRI-OLD]. Therefore, the authors of
this document would like to acknowledge contributions made by the
authors of that document: Michael Schmeing and Jan-Wilhelm Brendecke.
Many thanks for input provided by Steve Kille, David Wilson, John
Klensin, Dave Crocker, Graeme Lunt, Alessandro Vesely, Barry Leiba,
Bill McQuillan, Murray Kucherawy, SM, Glenn Parsons, Pete Resnick,
Chris Newman, Ned Freed, and Claudio Allocchio.
Special thanks to Barry Leiba for agreeing to shepherd this document.
Melnikov & Carlberg Standards Track [Page 27]
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Authors' Addresses
Alexey Melnikov
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
Ken Carlberg
G11
1601 Clarendon Blvd, #203
Arlington, VA 22209
USA
EMail: carlberg@g11.org.uk
Melnikov & Carlberg Standards Track [Page 28]