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Internet Engineering Task Force (IETF)
Request for Comments: 6477
Category: Informational
ISSN: 2070-1721
A. Melnikov
Isode Ltd
G. Lunt
SMHS Ltd
January 2012

Registration of Military Message Handling System (MMHS)
  Header Fields for Use in Internet Mail

Abstract

A Military Message Handling System (MMHS) processes formal messages ensuring release, distribution, security, and timely delivery across national and international strategic and tactical networks. The MMHS Elements of Service are defined as a set of extensions to the ITU-T X.400 (1992) international standards and are specified in STANAG 4406 Edition 2 and ACP 123. This document specifies message header fields and associated processing for RFC 5322 (Internet Message Format) to provide a comparable messaging service. In addition, this document provides for a STANAG 4406 / Internet Email Gateway that supports message conversion.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6477.

Copyright Notice

Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................3
   3. Registration Templates ..........................................3
      3.1. Header Field: MMHS-Exempted-Address ........................5
      3.2. Header Field: MMHS-Extended-Authorisation-Info .............5
      3.3. Header Field: MMHS-Subject-Indicator-Codes .................6
      3.4. Header Field: MMHS-Handling-Instructions ...................6
      3.5. Header Field: MMHS-Message-Instructions ....................7
      3.6. Header Field: MMHS-Codress-Message-Indicator ...............8
      3.7. Header Field: MMHS-Originator-Reference ....................8
      3.8. Header Field: MMHS-Primary-Precedence ......................9
      3.9. Header Field: MMHS-Copy-Precedence ........................10
      3.10. Header Field: MMHS-Message-Type ..........................10
      3.11. Header Field: MMHS-Other-Recipients-Indicator-To .........11
      3.12. Header Field: MMHS-Other-Recipients-Indicator-CC .........12
      3.13. Header Field: MMHS-Acp127-Message-Identifier .............13
      3.14. Header Field: MMHS-Originator-PLAD .......................13
   4. Formal Syntax ..................................................14
   5. Service in Comparison to ACP 123 / STANAG 4406 .................16
   6. Gatewaying with ACP 123 / STANAG 4406 ..........................16
   7. Gatewaying with ACP 127 ........................................18
   8. IANA Considerations ............................................18
   9. Security Considerations ........................................18
   10. References ....................................................19
      10.1. Normative References .....................................19
      10.2. Informative References ...................................19
   Appendix A. Acknowledgements ......................................21

1. Introduction

[RFC5322] defines a protocol for the format of electronic messages exchanged on the Internet. MMHS is a military specification defined in ACP 123 [ACP123] (also specified in STANAG 4406 [STANAG-4406]), which defines a number of extensions to the basic X.400 (1992) protocol for the services required by military messaging.

This document supports translating most of the Elements of Service defined in ACP 123 [ACP123] to Internet message header fields (see Section 5 for more details). This specification is written to extend the Mime Internet X.400 Enhanced Relay (MIXER) specification [RFC2156] to enable inter-conversion in a MIXER gateway with the X.400 Interpersonal Messaging System (IPMS) heading extensions defined in ACP 123 / STANAG 4406, Annex A.

The document is aimed at the ability to represent MMHS messages as RFC 5322 messages. All RFC 5322 header fields defined in this document are prefixed with the string "MMHS-" to distinguish them from any other header fields.

Unless stated otherwise, all header fields described in this document are OPTIONAL in an Internet Message.

This document is structured as follows: Section 3 and its subsections formally define new Internet header fields and show some examples. Section 4 provides Augmented Backus-Naur Form (ABNF) syntax for them. Section 5 provides some background information about which features of ACP 123 / STANAG 4406 were not implemented in this specification. Subsequent sections talk about additional requirements for gatewaying to/from ACP 123 / STANAG 4406 and ACP 127 [ACP127] environments, respectively.

2. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The formal syntax uses the ABNF [RFC5234] notation including the core rules defined in Appendix B of RFC 5234 [RFC5234].

3. Registration Templates

Header field entries are summarized below in tabular form for convenience of reference and presented in full in the following subsections.

Any header field specified in this document MUST NOT appear more than once in message headers.

   +------------------------------------+----------+-------------------+
   | Header name                        | Protocol | Reference         |
   +------------------------------------+----------+-------------------+
   | MMHS-Exempted-Address              | mail     | [ACP123],         |
   |                                    |          | Appendices A1.1   |
   |                                    |          | and B.105         |
   | MMHS-Extended-Authorisation-Info   | mail     | [ACP123],         |
   |                                    |          | Appendices A1.2   |
   |                                    |          | and B.106         |
   | MMHS-Subject-Indicator-Codes       | mail     | [ACP123],         |
   |                                    |          | Appendices A1.3   |
   |                                    |          | and B.107         |
   | MMHS-Handling-Instructions         | mail     | [ACP123][ACP123], |
   |                                    |          | Appendices A1.4   |
   |                                    |          | and B.108         |
   | MMHS-Message-Instructions          | mail     | [ACP123],         |
   |                                    |          | Appendices A1.5   |
   |                                    |          | and B.109         |
   | MMHS-Codress-Message-Indicator     | mail     | [ACP123],         |
   |                                    |          | Appendices A1.6   |
   |                                    |          | and B.110         |
   | MMHS-Originator-Reference          | mail     | [ACP123],         |
   |                                    |          | Appendices A1.7   |
   |                                    |          | and  B.111        |
   | MMHS-Primary-Precedence            | mail     | [ACP123],         |
   |                                    |          | Appendices A1.8   |
   |                                    |          | and B.101         |
   | MMHS-Copy-Precedence               | mail     | [ACP123],         |
   |                                    |          | Appendices A1.9   |
   |                                    |          | and B.102         |
   | MMHS-Message-Type                  | mail     | [ACP123],         |
   |                                    |          | Appendices A1.10  |
   |                                    |          | and B.103         |
   | MMHS-Other-Recipients-Indicator-To | mail     | [ACP123],         |
   |                                    |          | Appendices A1.12  |
   |                                    |          | and B.113         |
   | MMHS-Other-Recipients-Indicator-CC | mail     | [ACP123],         |
   |                                    |          | Appendices A1.12  |
   |                                    |          | and B.113         |
   | MMHS-Acp127-Message-Identifier     | mail     | [ACP123],         |
   |                                    |          | Appendices A1.14  |
   |                                    |          | and B.116         |
   | MMHS-Originator-PLAD               | mail     | [ACP123],         |
   |                                    |          | Appendices A1.15  |
   |                                    |          | and B.117         |
   +------------------------------------+----------+-------------------+

3.1. Header Field: MMHS-Exempted-Address

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Exempted-Address header field, by its presence, indicates the addresses of members in an Address List (AL) that should not receive the message. If this header field is absent from the message, all members of an AL will be considered to be valid recipients of the message.

Note: there is no guarantee that the exempted addresses will not receive the message as the result of redirection, Distribution List (DL) expansion, etc.

   Example:
   MMHS-Exempted-Address:
    UK SHL CGT Samuals G <graham.samuals@shl.example.com>,
    UK SHL Duty Officer <duty@shl.example.com>

3.2. Header Field: MMHS-Extended-Authorisation-Info

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]]

The MMHS-Extended-Authorisation-Info header field, by its presence, indicates either the date and the time when the message was officially released by the releasing officer or the date and time when the message was initially submitted to a communication facility for transmission.

This header field SHOULD always be present in an email message that complies with this specification.

   Example:
   MMHS-Extended-Authorisation-Info:
     Mon, 09 Aug 2010 15:27:40 +0100

The example above demonstrates use of folding white space (FWS [RFC5322]).

3.3. Header Field: MMHS-Subject-Indicator-Codes

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

A Subject Indicator Code (SIC) is a mechanism for formally identifying the topic of a message. SICs are nested codes that provide information for message distribution after delivery to the recipient organization. SICs are usually three letters or three letters and digits, but may be up to eight characters long. Nations and organizations using SICs usually maintain a central registry.

When present, an MMHS-Subject-Indicator-Codes header field contains one or more SICs, which indicates distribution information to a recipient or a recipient's User Agent. This information can be used to perform automatic or manual local distribution of a message. If the MMHS-Subject-Indicator-Codes header field is absent, then the local distribution will be in accordance with the message handling policy of the recipient's domain.

[ACP123] specifies two optional components of the Distribution Code Element of Service. The MMHS-Subject-Indicator-Codes header field covers only the SIC code component of distribution codes.

Example: MMHS-Subject-Indicator-Codes: SDM; KKZ ; BRL

The example above includes three SIC codes: "SDM" (GROUND/LAND REQUIREMENTS), "KKZ" (HELICOPTER PUBLICATIONS/MANUALS), and "BRL" (HILEX INCIDENTS).

3.4. Header Field: MMHS-Handling-Instructions

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Handling-Instructions header field, by its presence, indicates human-readable local handling instructions that require some manual handling by a traffic operator. If this header field is absent, the message will be considered as not requiring manual handling by a traffic operator. Handling instructions (also called "transmission instructions") are a part of format line 4 as defined in ACP 127 [ACP127] and concern the sending of the message, e.g., that a particular system shall be used for transfer of the message.

This header field is used to support interoperability with ACP 127 systems.

Example: MMHS-Handling-Instructions: RXFPA ZOV MINDEF

The example above includes one ACP 131(F) handling instruction: "RXFPA ZOV MINDEF". The "ZOV MINDEF" indicates that MINDEF rerouted the message for some reason, and the correct routing is via RXFPA.

3.5. Header Field: MMHS-Message-Instructions

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Message-Instructions header field, by its presence, indicates message instructions (also known as "remarks") accompanying the message (e.g., similar to the operating signals specified in ACP 131 [ACP131]). If this header field is absent, the message will be considered received without message instructions.

The difference between handling instructions and message instructions is that the former is only for manual handling by traffic operators, while the latter also contains information of interest to the persons reading the message.

Example: MMHS-Message-Instructions: MINIMIZE CONSIDERED; NO DISTRIBUTION

The example above includes two message instructions defined by ACP123(B) [ACP123]: "MINIMIZE CONSIDERED" indicating that the originating user has considered the Minimize status of the recipients and "NO DISTRIBUTION" indicating that the recipients should not distribute the message further without the originating user's approval.

3.6. Header Field: MMHS-Codress-Message-Indicator

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Codress-Message-Indicator header field, by its presence, indicates that the message is in Codress format. If this header field is absent, the message will be considered received without the Codress format.

A Codress message is one in which all addresses, i.e., the sender and all recipients, are encrypted within the ACP 127 text (body) [ACP127]. The heading of any Codress message contains only the minimum amount of information that will enable a receiving station to deal properly and expeditiously with the particular transmission. The general rules for the preparation and transmission of Codress messages are given in ACP 121 [ACP121].

This header field is used only to support interoperability with ACP 127 systems.

Example: MMHS-Codress-Message-Indicator: 23

3.7. Header Field: MMHS-Originator-Reference

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Originator-Reference header field, by its presence, indicates a user-defined reference called the "originator's number". If this header field is absent, then the message will be considered received without any user-defined reference.

The originator's number is used by the originating organizational unit and is further qualified within national policy.

Note: trailing and leading spaces in an originator reference are not allowed by syntax.

Example: MMHS-Originator-Reference: IMSCOM-JIC-612-78

3.8. Header Field: MMHS-Primary-Precedence

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Primary-Precedence header field, by its presence, indicates the precedence level of the primary ("action") recipients. The message originating domain MUST ensure that this header field is always present if the message contains "To:" ("action") addresses.

The MMHS Primary Precedence Element of Service indicates the relative order in which Military Messages are to be handled for primary (action) recipients, i.e., a Military Message with a higher MMHS- Primary-Precedence header field value SHOULD be handled before a Military Message with a lower MMHS-Primary-Precedence header field value.

The header field value is a non-negative integer, or one of the six predefined case-insensitive labels: "deferred" (same as "0"), "routine" (same as "1"), "priority" (same as "2"), "immediate" (same as "3"), "flash" (same as "4"), or "override" (same as "5"), optionally followed by a comment. Note that, according to ACP 123, values in the range from 0 to 15 are reserved for NATO-defined precedence levels, and values in the range from 16 to 31 are reserved for national users.

Example 1: MMHS-Primary-Precedence: 0 (Deferred)

Example 2: MMHS-Primary-Precedence: FLASH

Example 3: MMHS-Primary-Precedence: 7

3.9. Header Field: MMHS-Copy-Precedence

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Copy-Precedence header field, by its presence, indicates the precedence level of the copy ("information") recipients. The message originating domain MUST ensure that this header field is always present if the message contains "Cc:" or "Bcc:" ("information") addresses.

The MMHS Copy Precedence Element of Service indicates the relative order in which Military Messages are to be handled for copy (information) recipients. i.e. a Military Message with higher MMHS- Copy-Precedence header field value SHOULD be handled before a Military Message with a lower MMHS-Copy-Precedence header field value.

The header field value is a non-negative integer, or one of the 6 predefined case-insensitive labels: "deferred" (same as "0"), "routine" (same as "1"), "priority" (same as "2"), "immediate" (same as "3"), "flash" (same as "4"), or "override" (same as "5"), optionally followed by a comment. Note that according to ACP 123, values in the range from 0 to 15 are reserved for NATO-defined precedence levels and values in the range from 16 to 31 are reserved for national users.

Example 1: MMHS-Copy-Precedence: 2 (priority)

Example 2: MMHS-Copy-Precedence: Priority

3.10. Header Field: MMHS-Message-Type

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Message-Type heading extension, by its presence, indicates whether the message is to be considered as an exercise, an operation, a project, or a drill. (Note that the list of types is extensible, and other types can be specified using the numeric form, see below). It may include an optional parameter specifying the name of the exercise, operation, project, or drill. If this extension is absent, the message will be considered to be of an undefined type.

The header field value is a non-negative integer, or one of the four predefined case-insensitive labels: "exercise" (same as "0"), "operation" (same as "1"), "project" (same as "2"), "drill" (same as "3"). Note that according to ACP 123, values in the range from 0 to 127 are reserved for NATO-defined Message Type identifiers and values in the range from 128 to 255 are not defined by NATO and may be used nationally or bilaterally.

Example 1: MMHS-Message-Type: 0(exercise); identifier="CANDLE FISH"

Example 2: MMHS-Message-Type: 3

Example 3: MMHS-Message-Type: 2 (projet)

Example 4: MMHS-Message-Type: project

Note that some of the examples above demonstrate use of optional comments. See Section 4 for the exact syntax of this header field.

3.11. Header Field: MMHS-Other-Recipients-Indicator-To

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Other-Recipients-Indicator-To header field, by its presence, indicates the names of primary ("action") recipients that are intended to receive, or have received, the message via means other than MMHS. Note that the absence of both this header field and the MMHS-Other-Recipients-Indicator-CC header field (see Section 3.12) does not guarantee that all recipients are within the MMHS.

This header field enables a recipient to determine all action recipients of a Military Message. This header field is derived from the Other Recipient Indicator Element of Service. There are several reasons as to why a recipient of a Military Message may be identified by this header:

1. The recipient is not part of the MMHS.

   2.  The path to the recipient through the MMHS may not be secure;
       therefore, the originator has used alternative mechanisms to
       distribute the Military Message.
   3.  The recipient was already in receipt of the Military Message
       prior to the Military Message being inserted into the MMHS.

Example: MMHS-Other-Recipients-Indicator-To: UK SHL COS; UK SHL IM

The example above includes names of two primary recipients that received the message via means other than MMHS.

3.12. Header Field: MMHS-Other-Recipients-Indicator-CC

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Other-Recipients-Indicator-CC header field, by its presence, indicates the names of copy ("information") recipients that are intended to receive, or have received, the message via means other than MMHS. Note that the absence of both this header field and the MMHS-Other-Recipients-Indicator-To header field (see Section 3.11) does not guarantee that all recipients are within the MMHS.

This header field enables a recipient to determine all copy recipients of a Military Message. This header field is derived from the Other Recipient Indicator Element of Service.

There are several reasons as to why a recipient of a Military Message may be identified by this header:

1. The recipient is not part of the MMHS.

   2.  The path to the recipient through the MMHS may not be secure;
       therefore, the originator has used alternative mechanisms to
       distribute the Military Message.
   3.  The recipient was already in receipt of the Military Message
       prior to it being inserted into the MMHS.
Example: MMHS-Other-Recipients-Indicator-CC: UK SHL LEGAD

The example above includes 1 copy (information) recipient that received the message via means other than MMHS.

3.13. Header Field: MMHS-Acp127-Message-Identifier

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Acp127-Message-Identifier header field, by its presence, indicates an ACP 127 message identifier [ACP127] for a message that originated from an ACP 127 domain. If this extension is absent, then the message did not encounter an ACP 127 domain.

The MMHS-Acp127-Message-Identifier contains the contents of ACP 127 format line 3, which consists of three space-separated fields: the Calling Station (DERI), Station Serial Number (SSN), and Filing Time (JFT) [ACP127].

This header field is used only to support interoperability with ACP 127 systems, it should be treated as opaque by a pure MMHS system.

Example: MMHS-Acp127-Message-Identifier: RPDLE 1234 0341215

3.14. Header Field: MMHS-Originator-PLAD

Applicable protocol: mail [RFC5322] Status: informational Author/change controller: IESG (iesg@ietf.org) on behalf of the IETF Specification document(s): [RFC6477]

The MMHS-Originator-PLAD (PLAD: Plain Language Address Designator) header field, by its presence, indicates the plain language address associated with an originator for cross-referencing purposes. If this header field is absent, then the message will be considered not to have an originator PLAD cross-reference between the MMHS and ACP 127 domains.

This header field is used only to support interoperability with ACP 127 systems. This header field and the MMHS-Extended-Authorisation-Info header field provide a cross-reference for message identification in both ACP 127 and MMHS domains.

Example: MMHS-Originator-PLAD: SACLANT

4. Formal Syntax

The following syntax specification uses the Augmented Backus-Naur Form (ABNF) as described in [RFC5234]. Terms not defined here are taken from [RFC5322], [RFC5234], and [RFC2156].
   NZ-DIGIT       =  %x31-39
                     ; "1".."9"
   nonneg-integer = "0" / (NZ-DIGIT *DIGIT)
   military-string = 1*69( ps-char )
   quoted-military-string = DQUOTE military-string DQUOTE
   military-string-sequence = military-string
                       *( [FWS] ";" [FWS] military-string )
   Exempted-Address = "MMHS-Exempted-Address:"
                      [FWS] address-list [FWS] CRLF
   Extended-Authorisation-Info = "MMHS-Extended-Authorisation-Info:"
                                 [FWS] date-time CRLF
   Subject-Indicator-Codes = "MMHS-Subject-Indicator-Codes:"
                             [FWS] sic-sequence [FWS] CRLF
   sic-sequence = sic *( [FWS] ";" [FWS] sic )
                  ; ACP 123 specifies that the maximum number of
                  ; SICs is 8. Use of more than 8 SICs is
                  ; permitted, but additional SICs might not
                  ; be transferred to ACP 123 system.
   sic = 3*8( ps-char )
   Handling-Instructions = "MMHS-Handling-Instructions:"
                           [FWS] military-string-sequence [FWS] CRLF
   Message-Instructions = "MMHS-Message-Instructions:"
                          [FWS] military-string-sequence [FWS] CRLF
   Codress-Message-Indicator = "MMHS-Codress-Message-Indicator:"
                               [FWS] nonneg-integer [FWS] CRLF
   Originator-Reference = "MMHS-Originator-Reference:"
                          [FWS] military-string [FWS] CRLF

PrimaryPrecedence = "MMHS-Primary-Precedence:" [FWS] precedence CRLF

CopyPrecedence = "MMHS-Copy-Precedence:" [FWS] precedence CRLF

   precedence = (nonneg-integer / std-precedence) [CFWS]
   std-precedence = "deferred" / "routine" / "priority" /
                    "immediate" / "flash" / "override"
                    ; deferred == 0
                    ; routine == 1
                    ; priority == 2
                    ; immediate == 3
                    ; flash == 4
                    ; override == 5
   MessageType = "MMHS-Message-Type:" [FWS] message-type [CFWS]
                 [";" [FWS] MessageTypeParam [FWS] ] CRLF
   message-type = nonneg-integer / std-message-type
   std-message-type = "exercise" / "operation" / "project" /  "drill"
                      ; exercise  == 0
                      ; operation == 1
                      ; project == 2
                      ; drill == 3
   MessageTypeParam = "identifier" [FWS] "=" [FWS]
                      quoted-military-string
   Designator = military-string
   OtherRecipIndicatorPrimary = "MMHS-Other-Recipients-Indicator-To:"
                        [FWS] Designator *([FWS] ";" [FWS] Designator)
                        [FWS] CRLF
   OtherRecipIndicatorCopy = "MMHS-Other-Recipients-Indicator-CC:"
                        [FWS] Designator *([FWS] ";" [FWS] Designator)
                        [FWS] CRLF
   Acp127MessageIdentifier = "MMHS-Acp127-Message-Identifier:"
                             [FWS] military-string [FWS] CRLF
   OriginatorPLAD = "MMHS-Originator-PLAD:" [FWS] military-string [FWS]
                    CRLF
   address-list = <Defined in RFC 5322>

5. Service in Comparison to ACP 123 / STANAG 4406

The service specified in this document is a subset of the functionality set out in Annex A1 "Military Heading Extensions" of [ACP123]. The majority of this functionality is supported in this document. A few capabilities have been left out, which would have significantly increased the complexity of this specification.

For Distribution Codes (A.1.3) only Subject Indicator Codes are supported and Distribution Extensions are omitted. Authors of this document believe that distribution extensions are not widely used.

Address List Indication (A.1.11) is not supported. This complex extension is deprecated in [ACP123].

Pilot Forwarding Information (A.1.13) is not supported.

Security Information Labels (A.1.16) is not supported. This extension is deprecated in favor of Annex A of [ACP123], which uses Enhanced Security Services (ESS) Labels [RFC2634] that can be supported in a directly compatible manner in S/MIME [RFC5751].

ACP 127 Notification Requests (see Annex A.2.1 of [ACP123) and Responses (see Annex A.3.1 of [ACP123]) are not supported. These extensions are used to request and return notifications from ACP 127 gateways, and are not relevant to an SMTP gateway.

6. Gatewaying with ACP 123 / STANAG 4406

The header fields defined in this specification are designed to be mapped with ACP 123 Annex A1 heading extensions as part of a MIXER mapping according to [RFC2156]. The syntax of these headings is defined such that mapping is mechanical. OR Names SHOULD be mapped with Internet Email addresses according to [RFC2156].

This section summarizes how a gateway between [ACP123] and [RFC5322] conformant to this specification operates.

If an incoming X.400 message is encoded as P772, [RFC5322] header fields MUST be generated according to this specification for all ACP 123 heading extensions where an equivalent header is defined in this specification. For the three heading extensions where no mapping is defined, the heading extension MAY be discarded or mapped in a proprietary manner. If a Distribution Extension is encoded, this MAY be discarded or represented as a comment (<CFWS>). The whole message MAY be signed according to [RFC5652]. These rules also apply to heading extensions in forwarded messages. MM-Message MUST be treated as a forwarded message for the purposes of MIXER mapping. If an ACP 127 Notification Request is present, this MAY be discarded or represented as a comment (<CFWS>).

Incoming X.400 notifications are encoded according to [RFC2156]. If an ACP 127 Notification Response is present, this MAY be discarded or mapped in a proprietary manner.

If an incoming SMTP message contains any of the header fields defined in this specification, the outgoing X.400 message MUST be encoded as P772. The outgoing message MAY be encoded as P772 for other reasons, for instance, policy or characteristics such as the message containing a military body part. The X.400 message might be signed according to ACP 123 Annex B [ACP123] or STANAG 4406 Annex G [STANAG-4406]. message/rfc822 body parts included in the message SHOULD be mapped to MM-Message and the heading mapping rules applied.

Generated P772 messages SHOULD follow the following rules, generating heading extensions if needed.

   a.  Extended Authorization is required.  If the MMHS-Extended-
       Authorization-Info header field is absent, then the default value
       is taken from the Date header field.
   b.  Primary Precedence is required if the To header field is present.
       If the MMHS-Primary-Precedence header field is absent, the
       message need not be considered a Military Message and can be
       handled according to a local policy.
   c.  Copy Precedence is required if the Cc header field is present and
       there is an MMHS-Copy-Precedence header field that is different
       from the MMHS-Primary-Precedence header field.
   d.  For Message-ID fields, ACP 123 applies additional constraints
       over X.400, leading to the following rules in addition to
       [RFC2156], which SHOULD be followed by a gateway following this
       specification.
       1.  The local identifier MUST be at least 15 characters long.  If
           the [RFC2156] generated value is shorter than this, then it
           is padded with spaces to 15 characters.  This value will
           correctly reverse map.
       2.  The OR Address part is required, and it is not usually
           generated by an [RFC2156] mapping.  It is mandatory in ACP
           123.  The gateway SHOULD generate an OR Address in a manner
           that can be reverse mapped.  It MAY use the OR Address to
           encode long message ids that cannot be encoded in the local
           identifier.

7. Gatewaying with ACP 127

The header fields defined in this specification include fields to carry Elements of Service specific to ACP 127 [ACP127]. This specification does not define a mapping of these header fields to ACP 127. In the absence of this mapping, it is recommended that these headings be mapped to ACP 123 and hence into ACP 127 following the Annex D (Gateway Translation) of [STANAG-4406].

8. IANA Considerations

IANA has added the list of header fields specified in subsections of Section 3 to the "Permanent Message Header Field Names" registry defined by "Registration Procedures for Message Header Fields" [RFC3864].

9. Security Considerations

Annex B of [ACP123] describes how MMHS messages can be protected in an X.400 environment. Similar protection can be provided using S/MIME [RFC5751] and/or DKIM [RFC6376]. In particular, DKIM can be used to protect against alteration, deletion, or insertion of header fields specified in this document that can affect disposition and quality of service applied to processing of the protected Internet message by receiving gateways/endpoints that support this specification. (Note that most of the header fields defined in this document might affect processing of the message by the receiving gateway/end system, MMHS-Subject-Indicator-Codes and MMHS-Primary- Precedence/MMHS-Copy-Precedence header fields being the most important examples. For example, alteration of the MMHS-Primary- Precedence header field value might affect processing speed of the message by the recipient Message Transfer Agent (MTA).)

When the original message header fields are digitally signed, the act of gatewaying messages with such header fields to/from an Internet environment from/to an ACP 123 environment breaks digital signatures. The gateway can sign the translated message itself (e.g., with DKIM), but a message recipient would be unable to verify that the message was generated by the original sender.

10. References

10.1. Normative References

   [ACP123]    CCEB, "Common Messaging Strategy and Procedures", ACP 123
               (B), May 2009, http://jcs.dtic.mil/j6/cceb/acps/acp123/.
   [ACP127]    CCEB, "Communication Instructions - Tape Relay
               Procedures", ACP 127 (G), November 1988,
               http://jcs.dtic.mil/j6/cceb/acps/acp127/.
   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC2156]   Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
               Mapping between X.400 and RFC 822/MIME", RFC 2156,
               January 1998.
   [RFC3864]   Klyne, G., Nottingham, M., and J. Mogul, "Registration
               Procedures for Message Header Fields", BCP 90, RFC 3864,
               September 2004.
   [RFC5234]   Crocker, D., Ed., and P. Overell, "Augmented BNF for
               Syntax Specifications: ABNF", STD 68, RFC 5234, January
               2008.
   [RFC5322]   Resnick, P., Ed., "Internet Message Format", RFC 5322,
               October 2008.
   [RFC5652]   Housley, R., "Cryptographic Message Syntax (CMS)", STD
               70, RFC 5652, September 2009.
   [RFC6376]   Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
               "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
               September 2011.

10.2. Informative References

   [ACP121]    CCEB, "Comms Instructions - General", ACP 121 (I),
               October 2010, http://jcs.dtic.mil/j6/cceb/acps/acp121/.
   [ACP131]    CCEB, "Comms Instructions - Operating Signals", ACP 131
               (F), April 2009,
               http://jcs.dtic.mil/j6/cceb/acps/acp131/.
   [RFC2634]   Hoffman, P., Ed., "Enhanced Security Services for
               S/MIME", RFC 2634, June 1999.
   [RFC5751]   Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Message
               Specification", RFC 5751, January 2010.
   [STANAG-4406]
               NATO, "STANAG 4406 Edition 2: Military Message Handling
               System", STANAG 4406, March 2005.

Appendix A. Acknowledgements

This document copies a lot of text from the "Mapping between X.400 P772 and RFC-822" by Julian Onions and Graeme Lunt and STANAG 4406 (2nd Edition). So the authors of this document would like to acknowledge contributions made by the authors of these documents.

Many thanks for reviews and text provided by Steve Kille, Alan Ross, David Wilson, James Usmar, Kathy Nuckles, Andy Trayler, Ken Carlberg, Chris Bonatti, Oeyvind Jonsson, Mykyta Yevstifeyev, Sean Turner, Stephen Farrell, Adrian Farrel, and Peter Saint-Andre.

Authors' Addresses

   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex, TW12 2BX
   UK
   EMail: Alexey.Melnikov@isode.com
   Graeme Lunt
   SMHS Ltd
   Bescar Moss Farm
   Bescar Lane
   Ormskirk  L40 9QN
   UK
   EMail: graeme.lunt@smhs.co.uk