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Keywords: P_MUL
Independent Submission D. Wilson
Request for Comments: 8494 A. Melnikov, Ed.
Category: Informational Isode Ltd
ISSN: 2070-1721 November 2018
Multicast Email (MULE) over Allied Communications Publication (ACP) 142
Abstract
Allied Communications Publication (ACP) 142 defines P_MUL, which is a
protocol for reliable multicast suitable for bandwidth-constrained
and delayed acknowledgement (Emissions Control or "EMCON")
environments running over UDP. This document defines MULE (Multicast
Email), an application protocol for transferring Internet Mail
messages (as described in RFC 5322) over P_MUL (as defined in ACP
142). MULE enables transfer between Message Transfer Agents (MTAs).
It doesn't provide a service similar to SMTP Submission (as described
in RFC 6409).
This document explains how MULE can be used in conjunction with SMTP
(RFC 5321), including some common SMTP extensions, to provide an
alternate MTA-to-MTA transfer mechanism.
This is not an IETF specification; it describes an existing
implementation. It is provided in order to facilitate interoperable
implementations and third-party diagnostics.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8494.
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Copyright Notice
Copyright (c) 2018 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
(https://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.
Table of Contents
1. Introduction ....................................................3
2. Conventions Used in This Document ...............................4
3. MULE ............................................................4
3.1. BSMTP-Like Payload Construction ............................6
3.2. Payload Compression ........................................7
3.3. Error Handling .............................................9
4. Gatewaying from Internet Mail to MULE ...........................9
4.1. Use of BDAT ...............................................10
5. Gatewaying from MULE to Internet Mail ..........................10
5.1. Handling of ESMTP Extensions and Errors ...................10
6. IANA Considerations ............................................11
6.1. Instructions for Designated Experts .......................11
6.2. SMTP Extension Support in MULE ............................12
7. Security Considerations ........................................14
8. References .....................................................15
8.1. Normative References ......................................15
8.2. Informative References ....................................17
Acknowledgements ..................................................19
Authors' Addresses ................................................19
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1. Introduction
P_MUL [ACP142A] is a transport protocol for reliable multicast in
bandwidth-constrained and delayed acknowledgement environments
running on top of UDP. This document defines MULE, an application
protocol for transferring Internet Mail messages [RFC5322] over ACP
142 P_MUL. The objectives of MULE are 1) to take advantage of the
bandwidth-saving feature of using the multicast service as supported
by modern computer networks and 2) to allow message transfer under
EMCON (Emissions Control) conditions. EMCON or "radio silence" means
that although receiving nodes are able to receive messages, they are
not able to acknowledge the receipt of messages.
The objective of this protocol is to take advantage of multicast
communication for the transfer of messages between MTAs (Message
Transfer Agents) on a single multicast network under normal (i.e.,
dialog-oriented) communication conditions and under EMCON conditions.
An "EMCON condition" means that a receiving node is able to receive
messages but cannot acknowledge the received messages for a
relatively long time (hours or even days).
Figure 1 illustrates a simple multicast scenario, where the same
message has to be sent from MTA A (through G/W) to MTA 1, MTA 2, MTA
3, and MTA 4.
+-------+ +-------+
| MTA 1 |<-\ /->| MTA 3 |
+-------+ +-----+ +-------+ \ +-------+ / +-------+
| MTA A |<--->| G/W |<---------------->| Router|<
+-------+ +-----+ +-------+ / +-------+ \ +-------+
| MTA 2 |<-/ \->| MTA 4 |
+-------+ +-------+
|< -------------- MULE ---------------->|
Note: The gateway (G/W) and Router might or might not be running on
the same system.
Figure 1: Typical MULE Deployment
Due to multicast use (instead of a unicast communication service) in
the above MTA configuration, only one message transmission from the
gateway to the Router is required in order to reach MTA 1, MTA 2, MTA
3, and MTA 4, instead of four as required with unicast. This saves
the transmission three message transactions and thus results in
savings in bandwidth utilization. Depending on the network bandwidth
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(in some radio networks, it is less than 9.6 Kb/s), this savings can
be of vital importance. The savings in bandwidth utilization become
even greater with every additional receiving MTA.
P_MUL employs a connectionless transport protocol to transmit
messages. This guarantees reliable message transfer (through ACP 142
retransmissions) even in cases where one or more of the receiving
MTAs are not able or allowed to acknowledge completely received
messages for a certain period of time.
This protocol specification requires fixed multicast groups and
knowledge of the group memberships in one or more multicast groups of
each participating node (MTA). Membership in multicast groups needs
to be established before MULE messages can be sent.
MULE enables MTA-to-MTA transfer. It doesn't provide service similar
to SMTP Submission [RFC6409].
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document also uses terminology from [RFC5321] and [RFC5598].
3. MULE
MULE is an electronic mail transport of Internet Mail messages
[RFC5322] over an ACP 142 P_MUL network. It provides service similar
to MTA-to-MTA SMTP [RFC5321]. This document doesn't define a service
similar to SMTP Submission [RFC6409].
An important feature of MULE is its capability to transport mail
across multiple networks, referred to as "MULE mail relaying". A
network consists of the nodes that are mutually accessible by ACP
142. Using MULE, a process can transfer mail to another process on
the same ACP 142 network or to some other ACP 142 network via a relay
or gateway process accessible to both networks.
MULE reuses the ESMTP extension framework defined in [RFC5321]. MULE
servers MUST support the following ESMTP extensions: DSN [RFC3461],
SIZE [RFC1870], 8BITMIME [RFC6152], MT-PRIORITY [RFC6710], DELIVERBY
[RFC2852], BINARYMIME [RFC3030], and CHUNKING [RFC3030]. (As the
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message content size can always be determined from the compression
wrapper and the size of the envelope, no special handling is needed
for binary messages.)
Relaying a message using MULE is performed as follows:
1. The message is reassembled from one or more DATA_PDUs [ACP142A].
2. If the contentType-ShortForm value is 25, the BSMTP-like payload
is extracted from the compressedContent field and uncompressed
(the reverse of the compression process specified in
Section 3.2). If the contentType-ShortForm value is not 25, it
is handled as described in [ACP142A]. This document doesn't
further discuss any cases where the contentType-ShortForm value
is not 25.
3. The list of recipients is extracted from RCPT-lines (see
Section 3.1). If the receiving node is not responsible (directly
or indirectly) for any of the recipients, the message is
discarded and no further processing is done.
4. The relay adds trace header fields, e.g., the Received header
field. See [RFC7601] and Section 4.4 of [RFC5321].
5. The set of ACP 142 destinations for the message is created by
extracting right-hand sides (hostnames) of each RCPT-line,
eliminating duplicates, and then converting each hostname into
the next ACP 142 destination using static configuration.
6. For each unique ACP 142 destination, the following steps are
performed:
A. A new BSMTP-like payload is formed, as described in
Section 3.1, that only contains RCPT-lines that correspond to
recipients that can receive mail through the ACP 142
destination.
B. The created payload is compressed and encoded as specified in
Section 3.2.
C. The compressed payload is sent by P_MUL as a series of an
Address_PDU and one or more DATA_PDUs. When the message has
an associated MT-PRIORITY value [RFC6710], the
MappedPriority(value) is included as the Priority field of
the corresponding ACP 142 PDUs, including Address_PDUs,
DATA_PDUs, and DISCARD_MESSAGE_PDUs. Here, MappedPriority(x)
is defined as "6 - x".
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3.1. BSMTP-Like Payload Construction
MULE uses a BSMTP-like payload that differs from Batch SMTP (BSMTP)
[RFC2442] in that it eliminates unnecessary information. As with
BSMTP, ESMTP capability negotiation is not used, since receiver EMCON
restrictions prohibit such real-time interaction. For that reason,
there is no point in including EHLO capabilities. "MAIL FROM:" and
"RCPT TO:" prefixes are also excluded in order to save a few bytes.
For each received message, the corresponding BSMTP-like payload is
constructed as follows. Note that lines are terminated using CR LF.
1. The first line is what would be used for the data following "MAIL
FROM:" in the SMTP dialog, i.e., it contains the return-path
address (including the angle brackets -- "<" and ">") followed by
any ESMTP extension parameters to the MAIL FROM command.
2. After that, there is a separate line for each recipient of the
message. The value is what would follow "RCPT TO:" in the SMTP
dialog, i.e., the recipient address (including the angle brackets
-- "<" and ">") followed by any ESMTP extension parameters to the
corresponding RCPT TO command.
3. The list of recipients is terminated by an empty line (i.e., just
CR LF).
4. The message content follows the empty line. There is no need for
transparency ("dot stuffing") or terminating with a sequence "CR
LF . CR LF", as the end of the message content is indicated by
the end of the data (see Section 3.2 for more details).
The following is an example of a BSMTP-like payload:
<from@example.com> MT-PRIORITY=4 BODY=8BITMIME RET=HDRS ENVID=QQ314159
<to1@example.net> NOTIFY=FAILURE ORCPT=rfc822;Bob@ent.example.net
<to2@example.net> NOTIFY=SUCCESS,FAILURE
From: from@example.com
To: To1 <to1@example.net>, To2 <to2@example.net>
Date: 27 Apr 2017 16:17 +0100
Subject: a test
MIME-Version: 1.0
Content-type: text/plain; charset=utf-8
Content-transfer-encoding: 8bit
This is worth <poundsign>100
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ABNF [RFC5234] for the BSMTP-like payload is:
bsmtp-like-payload = envelope CRLF payload
envelope = FROM-line 1*RCPT-line
FROM-line = reverse-path [SP mail-parameters] CRLF
RCPT-line = forward-path [SP rcpt-parameters] CRLF
payload = *OCTET
; Conforms to message syntax as defined in RFC 5322
; and extended in MIME
OCTET = <any 0-255 octet value>
reverse-path = <as defined in RFC 5321>
forward-path = <as defined in RFC 5321>
mail-parameters = <as defined in RFC 5321>
rcpt-parameters = <as defined in RFC 5321>
3.2. Payload Compression
A BSMTP-like payload (Section 3.1) is first compressed using
zlibCompress [RFC1950]. This compressed payload is placed in the
compressedContent field of the CompressedContentInfo element defined
in Section 4.2.6 of [STANAG-4406]. This is then encoded as BER
encoding [ITU.X690.2002] of the CompressedData ASN.1 structure. For
convenience, the original definition of the CompressedData ASN.1
structure is included below. The contentType-ShortForm value used by
MULE MUST be 25. (The contentType-OID alternative is never used by
MULE.)
The above procedure is similar to how X.400 messages are sent using
Annex E of [STANAG-4406]. This makes it easier to implement MTAs
that support both Internet messages and X.400 messages in the same
code base.
The Compressed Data Type (CDT) consists of content of any type that
is compressed using a specified algorithm. The following object
identifier identifies the CDT:
id-mmhs-CDT ID ::= { iso(1) identified-organization(3) nato(26)
stanags(0) mmhs(4406) object-identifiers(0)
id-mcont(4) 2 }
The CDT is defined by the following ASN.1 type. Note that this
definition is copied from [STANAG-4406] and is only reproduced here
for the reader's convenience.
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DEFINITIONS ::=
BEGIN
CompressedData ::= SEQUENCE {
compressionAlgorithm CompressionAlgorithmIdentifier,
compressedContentInfo CompressedContentInfo
}
CompressionAlgorithmIdentifier ::= CHOICE {
algorithmID-ShortForm [0] AlgorithmID-ShortForm,
algorithmID-OID [1] OBJECT IDENTIFIER
}
AlgorithmID-ShortForm ::= INTEGER { zlibCompress (0) }
CompressedContentInfo ::= SEQUENCE {
CHOICE {
contentType-ShortForm [0] ContentType-ShortForm,
contentType-OID [1] OBJECT IDENTIFIER
},
compressedContent [0] EXPLICIT OCTET STRING
}
ContentType-ShortForm ::= INTEGER {
unidentified (0),
external (1), -- identified by the
-- object-identifier
-- of the EXTERNAL content
p1 (2),
p3 (3),
p7 (4)
}
END
This document effectively adds another enumeration choice to the
ContentType-ShortForm definition. The updated definition looks like
this:
ContentType-ShortForm ::= INTEGER {
unidentified (0),
external (1), -- identified by the
-- object-identifier
-- of the EXTERNAL content
p1 (2),
p3 (3),
p7 (4),
mule (25)
}
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3.3. Error Handling
MULE doesn't allow a next-hop Message Transfer Agent / Mail Delivery
Agent (MTA/MDA) to return immediate Response Codes for the FROM-line
or any of the recipients in the RCPT-line. Therefore, when MTAs/MDAs
that are compliant with this specification receive a message that
can't be relayed further or delivered, they MUST generate a non-
delivery DSN report [RFC6522] message that includes the message/
delivery-status body part [RFC3464] and submit it using MULE to the
FROM-line return-path address.
MULE relays (unlike MULE MDAs) don't need to verify that they
understand all FROM-line and/or RCPT-line parameters. This keeps
relay-only implementations simpler and avoids the need to upgrade
them when MULE MDAs are updated to support extra SMTP extensions.
4. Gatewaying from Internet Mail to MULE
A gateway from Internet Mail to MULE acts as an SMTP server on the
receiving side and as a MULE client on the sending side.
When the content type for a message is an Internet message content
type (which may be 7-bit, 8-bit, or binary MIME), this is transported
using ACP 142 [ACP142A] as follows:
1. For each mail message, a BSMTP-like payload is formed, as
described in Section 3.1.
2. The created payload is compressed and encoded, as specified in
Section 3.2.
3. The compressed payload is sent by P_MUL as a series of an
Address_PDU and one or more DATA_PDUs. When the message has an
associated MT-PRIORITY value [RFC6710], the MappedPriority(value)
is included as the Priority field of the corresponding ACP 142
PDUs, including Address_PDUs, DATA_PDUs, and
DISCARD_MESSAGE_PDUs. Here, MappedPriority(x) is defined as "6 -
x".
The set of ACP 142 destinations for the message is derived from the
next-hop MTAs for each of the recipients.
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4.1. Use of BDAT
If a message is received by a gateway through SMTP transfers using
the CHUNKING [RFC3030] extension, the message is rebuilt by the
receiving MTA into its complete form and is then used as a single
MULE message payload. Use of the BINARYMIME [RFC3030] extension is
conveyed by inclusion of the BODY=BINARY parameter in the FROM-line.
5. Gatewaying from MULE to Internet Mail
A gateway from MULE to Internet Mail acts as a MULE server on the
receiving side and as an SMTP client on the sending side.
Gatewaying from an ACP 142 environment to Internet Email is the
reverse of the process specified in Section 4.
1. The ACP 142 message is reassembled from one or more DATA_PDUs.
2. If the contentType-ShortForm value is 25, the BSMTP-like payload
is extracted from the compressedContent field and uncompressed
(the reverse of the compression process specified in
Section 3.2). If the contentType-ShortForm value is not 25, it
is handled as described in [ACP142A].
3. The BSMTP-like payload is converted to an SMTP transaction (see
Section 3.1). (The first line of the BSMTP-like payload is
prepended with "MAIL FROM:", and each following line (until the
empty line is encountered) is prepended with "RCPT TO:". After
skipping the empty delimiting line, the rest of the payload is
the message body. This can be sent using either DATA or a series
of BDAT commands, depending on the capabilities of the receiving
SMTP system. For example, the presence of the BODY=BINARY
parameter in the FROM-line would necessitate the use of BDAT or
down-conversion of the message to 7-bit compatible
representation.)
5.1. Handling of ESMTP Extensions and Errors
ESMTP extension parameters to MAIL FROM and RCPT TO SMTP commands
obtained from a BSMTP-like payload are processed according to
specifications of the corresponding ESMTP extensions. This includes
dealing with the absence of support for ESMTP extensions that
correspond to MAIL FROM and RCPT TO parameters found in the BSMTP-
like payload.
Failures to extract or uncompress BSMTP-like payloads should result
in the receiver discarding such payloads.
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6. IANA Considerations
IANA has created a new "Multicast Email SMTP Extensions" registry
under the "MAIL Parameters" registry. The registration procedure for
this new registry is "Specification Required" [RFC8126]. The
designated expert(s) will be appointed and managed by the editors of
this document together with the Independent Submissions Editor.
Selected designated expert(s) should (collectively) have a good
knowledge of SMTP (and its extensions and extensibility mechanisms),
as well as ACP 142 and its limitations. The subsections below
provide more details: Section 6.1 specifies instructions for the
designated expert(s), and Section 6.2 defines the initial content of
the registry.
6.1. Instructions for Designated Experts
The designated expert(s) for the new "Multicast Email SMTP
Extensions" registry verifies that:
1. The requested SMTP extension is already registered in the "SMTP
Service Extensions" registry under the "MAIL Parameters" registry
on the IANA website or is well documented on a stable, publicly
accessible web page.
2. The requested SMTP extension has the correct status as specified
in Section 6.2. When deciding on status, the designated
expert(s) is provided with the following guidelines:
A. If the SMTP extension only affects commands other than MAIL
FROM and RCPT TO, then the status should be "N/A".
B. If the SMTP extension only applies to SMTP Submission
[RFC6409] (and not to SMTP relay or final SMTP delivery),
then the status should be "N/A".
C. If the SMTP extension changes which commands are allowed
during an SMTP transaction (e.g., if it adds commands
alternative to DATA or declares commands other than MAIL
FROM, RCPT TO, DATA, and BDAT to be a part of SMTP
transaction), then the status should be "Disallowed" or
"Special".
D. If the SMTP extension adds extra round trips during SMTP
transaction, then the status should be "Disallowed" or
"Special".
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Registration requests should include the SMTP extension name, status
(see Section 6.2), and specification reference. They may also
include an optional note.
6.2. SMTP Extension Support in MULE
The following table summarizes how different SMTP extensions can be
used with MULE. Each extension has one of the following statuses:
o Required - support by MULE relays, SMTP-to-MULE gateway, or MULE-
to-SMTP gateway is required.
o Disallowed - incompatible with MULE.
o N/A - not relevant because the extension affects commands other
than MAIL FROM and/or RCPT TO or is only defined for SMTP
Submission [RFC6409]. Such extensions can still be used on the
receiving SMTP side of an SMTP-to-MULE gateway.
o Supported - can be used with MULE but requires bilateral agreement
between sender and receiver.
o Special - needs to be accompanied by an explanation.
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+------------------------+---------------+-----------+
| SMTP Extension Keyword | Status | Reference |
+------------------------+---------------+-----------+
| SIZE | Required | [RFC1870] |
| | | |
| 8BITMIME | Required | [RFC6152] |
| | | |
| DSN | Required | [RFC3461] |
| | | |
| MT-PRIORITY | Required | [RFC6710] |
| | | |
| DELIVERBY | Required | [RFC2852] |
| | | |
| BINARYMIME | Required | [RFC3030] |
| | | |
| CHUNKING | Special (*) | [RFC3030] |
| | | |
| ENHANCEDSTATUSCODES | Special (**) | [RFC2034] |
| | | |
| RRVS | Supported | [RFC7293] |
| | | |
| SUBMITTER | Supported | [RFC4405] |
| | | |
| PIPELINING | N/A | [RFC2920] |
| | | |
| STARTTLS | N/A | [RFC3207] |
| | | |
| AUTH | Special (***) | [RFC4954] |
| | | |
| BURL | N/A | [RFC4468] |
| | | |
| NO-SOLICITING | N/A | [RFC3865] |
| | | |
| CHECKPOINT | Disallowed | [RFC1845] |
| | | |
| CONNEG | Disallowed | [RFC4141] |
+------------------------+---------------+-----------+
Table 1: Initial Content of Multicast Email SMTP Extensions Registry
(*) - SMTP CHUNKING MUST be supported on the receiving SMTP side of
an SMTP-to-MULE gateway and MAY be used on the sending side of a
MULE-to-SMTP gateway. A MULE relay doesn't need to do anything
special for this extension.
(**) - The ENHANCEDSTATUSCODES extension is supported by including
relevant status codes in DSN [RFC3461] reports.
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(***) - The AUTH parameter to the MAIL FROM command is "Supported",
but the rest of the AUTH extension is not applicable to MULE.
Note that the above table is not exhaustive. Future RFCs can define
how SMTP extensions not listed above can be used in MULE.
7. Security Considerations
As MULE provides a service similar to SMTP, many of the security
considerations from [RFC5321] apply to MULE as well; in particular,
Sections 7.1, 7.2, 7.4, 7.6, 7.7, and 7.9 of [RFC5321] apply to MULE.
As MULE doesn't support capability negotiation or the SMTP HELP
command, Section 7.5 of [RFC5321] ("Information Disclosure in
Announcements") doesn't apply to MULE.
As MULE doesn't support the VRFY or EXPN SMTP commands, Section 7.3
of [RFC5321] ("VRFY, EXPN, and Security"), which discusses email
harvesting, doesn't apply to MULE.
Arguably, it is more difficult to cause an application-layer denial-
of-service attack on a MULE server than on an SMTP server. This is
partially due to the fact that ACP 142 is used in radio/wireless
networks with relatively low bandwidth and very long round-trip time
(especially if EMCON is in force). However, as MULE is using
multicast, multiple MULE nodes can receive the same message and spend
CPU resources processing it, even if the message is addressed to
recipients that are not going to be handled by such nodes. As MULE
lacks transport-layer source authentication, this can be abused by
malicious senders.
For security considerations related to use of zlib compression, see
[RFC6713].
Due to the multicast nature of MULE, it cannot use TLS or DTLS.
Accordingly, it does not support STARTTLS [RFC3207]. Users should
not depend on hop-by-hop confidentiality or integrity protection of
mail transferred among MULE MTAs (in the same way they can't
generally rely on the use of STARTTLS on SMTP MTA-to-MTA links) and
should consider the use of end-to-end protection, such as S/MIME
[RFC5750] [RFC5751].
S/MIME signatures and/or encryption survive gatewaying between MULE
and SMTP environments.
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8. References
8.1. Normative References
[ACP142A] CCEB, "P_Mul - A Protocol for Reliable Multicast in
Bandwidth Constrained and Delayed Acknowledgement (EMCON)
Environments", ACP 142(A), October 2008.
[ITU.X690.2002]
ITU-T, "Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, August 2015.
[RFC1870] Klensin, J., Freed, N., and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10, RFC 1870,
DOI 10.17487/RFC1870, November 1995,
<https://www.rfc-editor.org/info/rfc1870>.
[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950,
DOI 10.17487/RFC1950, May 1996,
<https://www.rfc-editor.org/info/rfc1950>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2852] Newman, D., "Deliver By SMTP Service Extension", RFC 2852,
DOI 10.17487/RFC2852, June 2000,
<https://www.rfc-editor.org/info/rfc2852>.
[RFC3030] Vaudreuil, G., "SMTP Service Extensions for Transmission
of Large and Binary MIME Messages", RFC 3030,
DOI 10.17487/RFC3030, December 2000,
<https://www.rfc-editor.org/info/rfc3030>.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, DOI 10.17487/RFC3461, January 2003,
<https://www.rfc-editor.org/info/rfc3461>.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
DOI 10.17487/RFC3464, January 2003,
<https://www.rfc-editor.org/info/rfc3464>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed.,
"SMTP Service Extension for 8-bit MIME Transport", STD 71,
RFC 6152, DOI 10.17487/RFC6152, March 2011,
<https://www.rfc-editor.org/info/rfc6152>.
[RFC6522] Kucherawy, M., Ed., "The Multipart/Report Media Type for
the Reporting of Mail System Administrative Messages",
STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012,
<https://www.rfc-editor.org/info/rfc6522>.
[RFC6710] Melnikov, A. and K. Carlberg, "Simple Mail Transfer
Protocol Extension for Message Transfer Priorities",
RFC 6710, DOI 10.17487/RFC6710, August 2012,
<https://www.rfc-editor.org/info/rfc6710>.
[RFC6713] Levine, J., "The 'application/zlib' and 'application/gzip'
Media Types", RFC 6713, DOI 10.17487/RFC6713, August 2012,
<https://www.rfc-editor.org/info/rfc6713>.
[RFC7601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7601,
DOI 10.17487/RFC7601, August 2015,
<https://www.rfc-editor.org/info/rfc7601>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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RFC 8494 Email over ACP 142 November 2018
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[STANAG-4406]
NATO, "Military Message Handling System", STANAG 4406 Ed.
2, March 2005.
8.2. Informative References
[RFC1845] Crocker, D., Freed, N., and A. Cargille, "SMTP Service
Extension for Checkpoint/Restart", RFC 1845,
DOI 10.17487/RFC1845, September 1995,
<https://www.rfc-editor.org/info/rfc1845>.
[RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, DOI 10.17487/RFC2034, October
1996, <https://www.rfc-editor.org/info/rfc2034>.
[RFC2442] Freed, N., Newman, D., Belissent, J., and M. Hoy, "The
Batch SMTP Media Type", RFC 2442, DOI 10.17487/RFC2442,
November 1998, <https://www.rfc-editor.org/info/rfc2442>.
[RFC2920] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
September 2000, <https://www.rfc-editor.org/info/rfc2920>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[RFC3865] Malamud, C., "A No Soliciting Simple Mail Transfer
Protocol (SMTP) Service Extension", RFC 3865,
DOI 10.17487/RFC3865, September 2004,
<https://www.rfc-editor.org/info/rfc3865>.
[RFC4141] Toyoda, K. and D. Crocker, "SMTP and MIME Extensions for
Content Conversion", RFC 4141, DOI 10.17487/RFC4141,
November 2005, <https://www.rfc-editor.org/info/rfc4141>.
[RFC4405] Allman, E. and H. Katz, "SMTP Service Extension for
Indicating the Responsible Submitter of an E-Mail
Message", RFC 4405, DOI 10.17487/RFC4405, April 2006,
<https://www.rfc-editor.org/info/rfc4405>.
[RFC4468] Newman, C., "Message Submission BURL Extension", RFC 4468,
DOI 10.17487/RFC4468, May 2006,
<https://www.rfc-editor.org/info/rfc4468>.
Wilson & Melnikov Informational [Page 17]
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[RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954,
DOI 10.17487/RFC4954, July 2007,
<https://www.rfc-editor.org/info/rfc4954>.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
<https://www.rfc-editor.org/info/rfc5750>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>.
[RFC7293] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid-
Since Header Field and SMTP Service Extension", RFC 7293,
DOI 10.17487/RFC7293, July 2014,
<https://www.rfc-editor.org/info/rfc7293>.
Wilson & Melnikov Informational [Page 18]
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Acknowledgements
Thank you to Steve Kille for suggestions, comments, and corrections
on this document. An additional thank you goes to Barry Leiba, Sean
Turner, Dave Crocker, and Nick Hudson for reviews and comments on
this document.
Some text was borrowed from "P_Mul: An Application Protocol for the
Transfer of Messages over Multicast Subnetworks and under EMCON
Restrictions" (September 1997); we gratefully acknowledge the work of
the authors of that document.
Authors' Addresses
David Wilson
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
United Kingdom
Email: David.Wilson@isode.com
Alexey Melnikov (editor)
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
United Kingdom
Email: Alexey.Melnikov@isode.com
Wilson & Melnikov Informational [Page 19]