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Versions: 00 01 02 03 04 05 06 07 08 RFC 8494

Network Working Group                                          D. Wilson
Internet-Draft                                          A. Melnikov, Ed.
Intended status: Informational                                 Isode Ltd
Expires: March 5, 2019                                 September 1, 2018


Multicast Email (MULE) over Allied Communications Publication (ACP) 142
                   draft-melnikov-email-over-pmul-08

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 an application
   protocol called MULE (Multicast Email) for transferring of Internet
   Mail messages (as described in RFC 5322) over P_MUL (as defined in
   ACP 142A).  MULE enables Message Transfer Agent (MTA) to MTA transfer
   and 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
   Simple Mail Transfer Protocol (SMTP, RFC 5321), including some common
   SMTP extensions, to provide an alternate MTA to MTA transfer
   mechanism.

   This is not an IETF specification, but describes an existing
   implementation.  It is provided in order to facilitate interoperable
   implementations and third-party diagnostics.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 5, 2019.





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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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   3.  MULE  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  BSMTP-like Payload construction . . . . . . . . . . . . .   5
     3.2.  Payload compression . . . . . . . . . . . . . . . . . . .   7
     3.3.  Error handling  . . . . . . . . . . . . . . . . . . . . .   9
   4.  Gatewaying from Internet Mail to MULE . . . . . . . . . . . .   9
     4.1.  Use of BDAT . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Gatewaying from MULE to Internet Mail . . . . . . . . . . . .  10
     5.1.  Handling of ESMTP extensions and Error handling . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  SMTP Extension Support in MULE  . . . . . . . . . . . . .  11
     6.2.  SMTP Extension Support in MULE  . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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 an application protocol
   called MULE for transferring Internet Mail messages [RFC5322] over
   ACP 142 P_MUL.  The objectives of MULE are first to take advantage of
   the bandwidth saving feature of using the multicast service as
   supported by modern computer networks and second to allow message
   transfer under EMCON (Emission Control) conditions.  EMCON or "Radio




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   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 - which
   means dialogue oriented - communication condition and under EMCON
   condition.  EMCON condition means that a receiving node is able to
   receive messages, but it cannot - for a relatively long time (hours
   or even days) - acknowledge the received messages.

   This 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 ---------------->|


   Typical MULE Deployment.  The gateway (G/W) and Router might or might
   not be running on the same system.

                                 Figure 1

   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 4 as required with unicast.  This saves the
   transmision of 3 message transactions and thus network bandwidth
   utilisation.  Depending on the network bandwidth (in some radio
   networks less than 9.6 Kb/s) this saving can be of vital importance.
   The saving in bandwidth utilisation becomes 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 those cases, when for a certain period of
   time, one or more of the receiving MTAs are not able or allowed to
   acknowledge completely received messages.




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   This protocol specification requires fixed multicast groups and a
   well known knowledge at each participating node (MTA) about the group
   memberships in one or more multicast groups of each participating
   node.  Membership in multicast groups needs to be established before
   MULE messages can be sent.

   MULE enables Message Transfer Agents (MTA) to MTA transfer and
   doesn't provide service similar to SMTP Submission.

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 messages [RFC5322]
   over ACP 142 P_MUL network.  It provides service similar to MTA-to-
   MTA SMTP [RFC5321].  This document doesn't define a service similar
   to Message 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 mutually-ACP142-accessible nodes.  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 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 and CHUNKING [RFC3030].  (As the 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 compressedContent field and uncompressed as
       specified in Section 3.2.  If the contentType-ShortForm value is



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       not 25, it is handled as described in [ACP142A].  This document
       doesn't discuss further any cases where 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 inderectly) for any of the recipients, the message is
       discarded and no further processing is done.

   4.  The relay adds trace header fields, for example the Received
       header field.  See Section 4.4. of [RFC5321] and [RFC7601].

   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
       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, which 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
           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
           corresponding ACP 142 PDUs, including Address_PDU, DATA_PDUs,
           DISCARD_MESSAGE_PDU.  Here MappedPriority(x) is defined as "6
           - x".

3.1.  BSMTP-like Payload construction

   MULE uses BSMTP-like payload which 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 eluded in order to save a few bytes.

   For each received message, the corresponding BSMTP-like payload is
   constructed as follows (Lines are terminated using CR LF).:




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      The first line is what would be used for the data following "MAIL
      FROM:" in the SMTP dialogue.  I.e. it contains the return-path
      address, within <>'s followed by any ESMTP extension parameters to
      the MAIL FROM command.

      After that, there is a separate line for each recipient of the
      message.  The value is what would follow "RCPT TO:" in the SMTP
      dialogue, i.e. the recipient address within <>'s followed by any
      ESMTP extension parameters to the corresponding RCPT TO command.

      The list of recipients is terminated by an empty line (i.e. just
      CR LF)

      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).

   An example of BSMTP-like payload follows


<from@example.com> MT-PRIORITY=4 BODY=8BITMIME RET=HDRS ENVID=QQ314159
<to1@example.net> NOTIFY=SUCCESS,FAILURE ORCPT=rfc822;Bob@enterprise.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] and the 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 ASN.1 of the CompressedData
   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 Ed 2.  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 Compressed Data Type:

            id-mmhs-CDT ID ::= { iso(1) identified-organization(3) nato(26) stanags(0)
                                  mmhs(4406) object-identifiers(0) id-mcont(4) 2 }












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   The Compressed Data Type is defined by the following ASN.1 type (Note
   that this definition is copied from [STANAG-4406] and only reproduced
   here for reader's convenience):

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

   As MULE doesn't allow next hop MTA/MDA to return immediate Response
   Codes for FROM-line or any of the recipients in RCPT-line, MTAs/MDAs
   that are compliant with this specification that receive a message
   that can't be relayed further or delivered MUST generate a non
   delivery DSN report [RFC6522] message which includes 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 7bit, 8bit 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
       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 corresponding ACP 142 PDUs,
       including Address_PDU, DATA_PDUs, DISCARD_MESSAGE_PDU.  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.

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 BINARYMIME [RFC3030] extension is
   conveyed by inclusion of BODY=BINARY parameter in the FROM-line.




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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 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 compressedContent field and uncompressed as
       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 either sent using DATA or a series
       of BDAT commands, depending on capabilities of the receiving SMTP
       system.  For example, the presence of BODY=BINARY parameter in
       FROM-line would necessitate use of BDAT or downconversion of the
       message to 7-bit compatible representation.)

5.1.  Handling of ESMTP extensions and Error handling

   ESMTP extension parameters to MAIL FROM and RCPT TO SMTP commands
   obtained from BSMTP-like payload are processed according to
   specifications of the corresponding ESMTP extensions, including
   dealing with absence of support for ESMTP extensions that correspond
   to MAIL FROM/RCPT TO parameters found in the BSMTP-like payload.

   Failures to extract or uncompress BSMTP-like payload should result in
   the receiver discarding such payloads.

6.  IANA Considerations

   IANA is requested to create a new registry "Multicast Email SMTP
   extensions".  Registration procedure for the new registry is
   "Specification Required" [RFC8126], but 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 good knowledge of SMTP protocol
   (and its extensions/extensibility mechanisms), and ACP 142 and its
   limitations.  Subsections of this section provide more details.  In
   particular, Section 6.1 specifies instructions for the Designated



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   Expert(s) and Section 6.2 defines the initial content of the
   registry.

6.1.  SMTP Extension Support in MULE

   The Designated Expert for the new "Multicast Email SMTP extensions"
   registry verifies that

   1.  the requested SMTP extension is already registered in the "SMTP
       Service Extensions" registry in the "Mail Parameters" section of
       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:

       1.  If the SMTP extension only affects commands other than MAIL
           FROM/RCPT TO, then the status should be "N/A".

       2.  If the SMTP extension only applies to SMTP submission (and
           not to SMTP relay or final SMTP delivery), then the status
           should be "N/A".

       3.  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/BDAT to be a part of SMTP transaction),
           then the status should be "Disallowed" or "Special".

       4.  If the SMTP extension adds extra round trips during SMTP
           transaction, then the status should be "Disallowed" or
           "Special".

   Registration requests should include SMTP extension name, status (see
   Section 6.2), specification reference and may 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:
   "Required" (required to be supported by MULE relays, SMTP-to-MULE
   gateway or MULE-to-SMTP gateway), "Disallowed" (incompatible with
   MULE), "N/A" (not relevant, because they affect commands other than
   MAIL FROM and/or RCPT TO, or only defined for SMTP Submission.  Such
   extensions can still be used on the receiving SMTP side of SMTP-to-
   MULE gateway) "Supported" (can be used with MULE, but requires



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   bilateral agreement between sender and receiver), or "Special".
   "Special" needs to be accompanied by an explanation.

                      SMTP Extension Support in MULE:

          +------------------------+-----------+---------------+
          | SMTP Extension Keyword | Reference | Status        |
          +------------------------+-----------+---------------+
          | SIZE                   | [RFC1870] | Required      |
          |                        |           |               |
          | 8BITMIME               | [RFC6152] | Required      |
          |                        |           |               |
          | DSN                    | [RFC3461] | Required      |
          |                        |           |               |
          | MT-PRIORITY            | [RFC6710] | Required      |
          |                        |           |               |
          | DELIVERBY              | [RFC2852] | Required      |
          |                        |           |               |
          | BINARYMIME             | [RFC3030] | Required      |
          |                        |           |               |
          | CHUNKING               | [RFC3030] | Special (*)   |
          |                        |           |               |
          | ENHANCEDSTATUSCODES    | [RFC2034] | Special (**)  |
          |                        |           |               |
          | RRVS                   | [RFC7293] | Supported     |
          |                        |           |               |
          | SUBMITTER              | [RFC4405] | Supported     |
          |                        |           |               |
          | PIPELINING             | [RFC2920] | N/A           |
          |                        |           |               |
          | STARTTLS               | [RFC3207] | N/A           |
          |                        |           |               |
          | AUTH                   | [RFC4954] | Special (***) |
          |                        |           |               |
          | BURL                   | [RFC4468] | N/A           |
          |                        |           |               |
          | NO-SOLICITING          | [RFC3865] | N/A           |
          |                        |           |               |
          | CHECKPOINT             | [RFC1845] | Disallowed    |
          |                        |           |               |
          | CONNEG                 | [RFC4141] | Disallowed    |
          +------------------------+-----------+---------------+

   (*) - SMTP CHUNKING MUST be supported on the receiving SMTP side of a
   SMTP-to-MULE gateway and MAY be used on the sending side of MULE-to-
   SMTP gateway.  A MULE relay doesn't need to do anything special for
   this extension.




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   (**) - ENHANCEDSTATUSCODES extension is supported by including
   relevant status codes in DSN [RFC3461] reports.

   (***) - The AUTH parameter to MAIL FROM command is "supported", but
   the rest of 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 Security
   Considerations from [RFC5321] apply to MULE as well, in particular
   Sections 7.1, 7.2, 7.4, 7.6, 7.7, 7.9 of [RFC5321] still apply to
   MULE.

   As MULE doesn't support capability negotiation or SMTP HELP command,
   Section 7.5 of [RFC5321] ("Information Disclosure in Announcements")
   doesn't apply to MULE.

   As MULE doesn't support VRFY or EXPN SMTP commands, Section 7.3 of
   [RFC5321] ("VRFY, EXPN, and Security") that talks about 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 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 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 transfered among MULE MTAs (in the same way they can't generally
   rely on 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].






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8.  References

8.1.  Normative References

   [ACP142A]  "Common Messaging strategy and procedures", ACP 142(A),
              August 2008.

   [ITU.X690.2002]
              International Telecommunications Union, "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, July 2002.

   [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]
              "STANAG 4406 Edition 2: 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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Appendix A.  Acknowledgements

   Thank you to Steve Kille for suggestions, comments and corrections on
   this document.  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 draft-riechmann-multicast-mail-00, thus
   work of authors of that document is greatefully acknowledged.

Authors' Addresses

   David Wilson
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   UK

   EMail: David.Wilson@isode.com


   Alexey Melnikov (editor)
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   UK

   EMail: Alexey.Melnikov@isode.com























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