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INFORMATIONAL

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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   [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>.



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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>.




























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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





















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