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rfc:rfc8494

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.

Wilson & Melnikov Informational [Page 1] RFC 8494 Email over ACP 142 November 2018

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

Wilson & Melnikov Informational [Page 2] RFC 8494 Email over ACP 142 November 2018

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

Wilson & Melnikov Informational [Page 3] RFC 8494 Email over ACP 142 November 2018

 (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

Wilson & Melnikov Informational [Page 4] RFC 8494 Email over ACP 142 November 2018

 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".

Wilson & Melnikov Informational [Page 5] RFC 8494 Email over ACP 142 November 2018

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.

Wilson & Melnikov Informational [Page 7] RFC 8494 Email over ACP 142 November 2018

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)
                    }

Wilson & Melnikov Informational [Page 8] RFC 8494 Email over ACP 142 November 2018

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.

Wilson & Melnikov Informational [Page 10] RFC 8494 Email over ACP 142 November 2018

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".

Wilson & Melnikov Informational [Page 11] RFC 8494 Email over ACP 142 November 2018

 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.

Wilson & Melnikov Informational [Page 12] RFC 8494 Email over ACP 142 November 2018

        +------------------------+---------------+-----------+
        | 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.

Wilson & Melnikov Informational [Page 13] RFC 8494 Email over ACP 142 November 2018

 (***) - 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.

Wilson & Melnikov Informational [Page 14] RFC 8494 Email over ACP 142 November 2018

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>.

Wilson & Melnikov Informational [Page 15] RFC 8494 Email over ACP 142 November 2018

 [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>.

Wilson & Melnikov Informational [Page 16] 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] RFC 8494 Email over ACP 142 November 2018

 [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] RFC 8494 Email over ACP 142 November 2018

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]

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