Mail Transfer Protocol over ACP 142
draft-melnikov-email-over-pmul-01

Abstract

ACP 142 defines P_Mul, which is a protocol for reliable multicast in bandwidth constrained and delayed acknowledgement (EMCON) environments running over UDP. This document is a specification of the basic protocol for electronic mail transfer over P_MUL. It also described how to gateway this basic protocol to/from Simple Mail Transfer Protocol (RFC 5321).

Status of This Memo

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This Internet-Draft will expire on February 11, 2018.

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Table of Contents

• 1. Introduction
• 2. Conventions Used in This Document
• 3. Gatewaying from Internet Mail
• 3.1. Sending Internet Messages
• 3.1.1. BSMTP-like Payload construction
• 3.1.2. Payload compression
• 3.2. Error handling
• 3.3. Use of BDAT
• 4. Gatewaying to Internet Mail
• 4.1. Error handling
• 5. IANA Considerations
• 5.1. Mapping of existing SMTP extensions to MULE
• 6. Security Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Acknowledgements
• Authors' Addresses

1. Introduction

P_MUL [ACP142A] is a protocol for reliable multicast in bandwidth constrained and delayed acknowledgement environments running over UDP. The objectives of this protocol 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 conditions. EMCON (Emission Control) 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 EMCON conditions. EMCON condition means that a receiving node is able to receive messages, but it cannot - for a relitive 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 1 to MTA 2 and to MTA 3.

                                         +-------+
                                      /->| MTA 2 |
           +-------+       +-------+ /   +-------+
           | MTA 1 |<----->| Router|<
           +-------+       +-------+ \   +-------+
                                      \->| MTA 3 |
                                         +-------+

Typical MTA Configuration

Figure 1

Using a multicast instead of an unicast communication service in the above MTA configuration only one message transmission from MTA 1 to the Router is required, instead of two as required with unicast. This saves the transmision of one message 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.

As P_Mul employs a connectionless transport protocol to transmit messages, the reliable message transfer is guaranteed 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.

This protocol specification requires fixed multicast groups and a well known knowledge at each participating node(MTA) about the group memberships to one or more multicast groups of each participating node.

This document defines application protocol MULE (Multicast Email) for transferring Internet Mail messages [RFC5322] over ACP 142 P_Mul.

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].

This document is also using terminology from [RFC5321].

3. Gatewaying from Internet Mail

3.1. Sending Internet Messages

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]. Firstly, for each mail message a BSMTP-like payload is formed, as described in Section 3.1.1. Secondly, the created payload is compressed and encoded as specified in Section 3.1.2. Thirdly, 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 "-1 * x + 6".

The sender of the message can assume that the receiver supports the following ESMTP extensions: DSN [RFC3461], SIZE [RFC1870], 8BITMIME [RFC6152], MT-PRIORITY [RFC6710], DELIVERBY [RFC2852].

The receiver MAY accept BINARYMIME [RFC3030] SMTP extension. This is a matter for bilateral agreement. 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.

The set of ACP 142 destinations for the message is derived from the next hop MTAs for each of the recipients.

3.1.1. BSMTP-like Payload construction

MULE uses BSMTP-like payload which differs from BSMTP [RFC2442]. ESMTP is using capability negotiation, which is not practical when EMCON is used, as there is no way for a recipient to return capabilities before a message can be sent. For that reason, there is no point in including EHLO capabilities. "MAIL FROM:" and "RCPT TO:" prefixes can also be 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):

• 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 CRLF.CRLF as the end of the message content is indicated by the end of the data (See Section 3.1.2 for more details).

Example of a BSMTP-like payload

                
<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
              
              

ABNF 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>
              

3.1.2. Payload compression

BSMTP-like payload (Section 3.1.1) is compressed using zlibCompress [RFC1951] and 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, ASN.1 of the CompressedData structure is included below. The contentType-ShortForm value used by MULE is 25.

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 }
              

The Compressed Data Type are defined by the following ASN.1 type:

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
              

3.2. Error handling

As MULE doesn't allow next hop MTA to return immediate Response Codes for FROM-line or any of recipients in RCPT-line, MTAs that are compliant with this specification that receive a message that can't be 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.

TBC: Also need to describe how to handle FROM-line or RCPT-line parameters that we don't understand. Probably, they can be rejected on receipt or be relayed to the final destination/gateway, which can decide what to do with them.

3.3. Use of BDAT

If SMTP CHUNKING extension is used by the SMTP side, data from all BDAT blocks is concatenated into continuous block and is used as the message payload. Use of BINARYMIME [RFC3030] might need to be enabled in the FROM-line.

BURL [RFC4468] SMTP extension only applies to SMTP Submission, so it is already mapped to BDAT/DATA by the time it reaches MULE.

4. Gatewaying to Internet Mail

Gatewaying from ACP 142 environment to Internet Email is the reverse of the process specified in Section 3.1. Firstly, the message is reassembled from one or more DATA_PDUs. Secondly, if the contentType-ShortForm value is 25, the BSMTP-like payload is extracted from compressedContent field and uncompressed as specified in Section 3.1.2. Thirdly, the BSMTP-like payload is converted to SMTP transaction (see Section 3.1.1). (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.)

4.1. 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.

Failures to extract or uncompres BSMTP-like payload are handled according to ACP 142.

5. IANA Considerations

IANA is requested to create a new registry "Multicast Email SMTP extensions". SMTP extensions registered in the "SMTP Service Extensions" IANA registry can be registered in this new registry. Registration procedure for the new registry is "Specification Required". Registration requests should include SMTP extension name, status (see Section 5.1) and specification reference.

5.1. Mapping of existing SMTP extensions to MULE

The following table summarizes how different SMTP extensions can be gatewayed to/from MULE. Each extension has one of the following statuses: "required" (required to be supported on the SMTP side), "disallowed" (incompatible with this protocol), "N/A" (not relevant, because they affect commands other than MAIL FROM and RCPT TO. Can be used on the SMTP side) or "supported".

6. Security Considerations

TBD.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Acknowledgements

Thank you to Steve Kille for suggestions, comments and corrections 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