Network Working Group
Independent Submission D. Wilson
Internet-Draft
Request for Comments: 8494 A. Melnikov, Ed.
Intended status:
Category: Informational Isode Ltd
Expires: March 5, 2019 September 1,
ISSN: 2070-1721 November 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 bandwidth-constrained
and delayed acknowledgement (Emissions Control or "EMCON")
environments running over UDP. This document defines MULE (Multicast
Email), 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).
142). MULE enables transfer between Message Transfer Agent (MTA) to MTA transfer
and 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
Simple Mail Transfer Protocol (SMTP, RFC SMTP
(RFC 5321), including some common SMTP extensions, to provide an
alternate MTA to MTA MTA-to-MTA transfer mechanism.
This is not an IETF specification, but specification; it describes an existing
implementation. It is provided in order to facilitate interoperable
implementations and third-party diagnostics.
Status of This Memo
This Internet-Draft document is submitted in full conformance with not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the
provisions RFC Series, independently of BCP 78 any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and BCP 79.
Internet-Drafts makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are working documents not candidates for any level of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list Standard;
see Section 2 of RFC 7841.
Information about the current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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.
https://www.rfc-editor.org/info/rfc8494.
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 ....................................................3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4 ...............................4
3. MULE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 ............................................................4
3.1. BSMTP-like BSMTP-Like Payload construction . . . . . . . . . . . . . 5 Construction ............................6
3.2. Payload compression . . . . . . . . . . . . . . . . . . . 7 Compression ........................................7
3.3. Error handling . . . . . . . . . . . . . . . . . . . . . 9 Handling .............................................9
4. Gatewaying from Internet Mail to MULE . . . . . . . . . . . . 9 ...........................9
4.1. Use of BDAT . . . . . . . . . . . . . . . . . . . . . . . 9 ...............................................10
5. Gatewaying from MULE to Internet Mail . . . . . . . . . . . . 10 ..........................10
5.1. Handling of ESMTP extensions Extensions and Error handling . . . . . 10 Errors ...................10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 ............................................11
6.1. SMTP Extension Support in MULE . . . . . . . . . . . . . 11 Instructions for Designated Experts .......................11
6.2. SMTP Extension Support in MULE . . . . . . . . . . . . . 11 ............................12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13 ........................................14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 .....................................................15
8.1. Normative References . . . . . . . . . . . . . . . . . . 14 ......................................15
8.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. ....................................17
Acknowledgements . . . . . . . . . . . . . . . . . . 18 ..................................................19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 Addresses ................................................19
1. Introduction
P_MUL [ACP142A] is a transport protocol for reliable multicast in
bandwidth constrained
bandwidth-constrained and delayed acknowledgement environments
running on top of UDP. This document defines MULE, an application
protocol
called MULE for transferring Internet Mail messages [RFC5322] over ACP
142 P_MUL. The objectives of MULE are first 1) to take advantage of the bandwidth saving
bandwidth-saving feature of using the multicast service as supported
by modern computer networks and second 2) to allow message transfer under
EMCON (Emission (Emissions Control) conditions. EMCON or "Radio
Silence" "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 - which
means dialogue oriented - (i.e.,
dialog-oriented) communication condition conditions and under EMCON
condition. EMCON condition conditions.
An "EMCON condition" means that a receiving node is able to receive messages,
messages but it cannot - acknowledge the received messages for a
relatively long time (hours or even days) - acknowledge the received messages.
This 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
3, and MTA 4.
+-------+ +-------+
| MTA 1 |<-\ /->| MTA 3 |
+-------+ +-----+ +-------+ \ +-------+ / +-------+
| MTA A |<--->| G/W |<---------------->| Router|<
+-------+ +-----+ +-------+ / +-------+ \ +-------+
| MTA 2 |<-/ \->| MTA 4 |
+-------+ +-------+
|< -------------- MULE ---------------->|
Typical MULE Deployment.
Note: The gateway (G/W) and Router might or might not be running on
the same system.
Figure 1 1: Typical MULE Deployment
Due to multicast use (instead of a unicast communication service) in
the above MTA configuration configuration, only one message transmission from the
gateway to the Router is required in order to reach MTA 1, MTA 2, MTA
3
3, and MTA 4, instead of 4 four as required with unicast. This saves
the
transmision of 3 transmission three message transactions and thus network results in
savings in bandwidth
utilisation. utilization. Depending on the network bandwidth
(in some radio
networks networks, it is less than 9.6 Kb/s) Kb/s), this saving savings can
be of vital importance. The saving savings in bandwidth utilisation becomes utilization become
even greater with every additional receiving MTA.
P_MUL employs a connectionless transport protocol to transmit
messages: this
messages. This guarantees reliable message transfer (through ACP 142
retransmissions),
retransmissions) even in those cases, when for a certain period of
time, cases where one or more of the receiving
MTAs are not able or allowed to acknowledge completely received messages.
messages for a certain period of time.
This protocol specification requires fixed multicast groups and a
well known
knowledge at each participating node (MTA) about of the group memberships in one or more multicast groups of
each participating
node. node (MTA). Membership in multicast groups needs
to be established before MULE messages can be sent.
MULE enables Message Transfer Agents (MTA) to MTA transfer and MTA-to-MTA transfer. It doesn't provide service similar
to SMTP Submission. 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 MTA-to-MTA SMTP [RFC5321]. This document doesn't define a service
similar to Message SMTP Submission ([RFC6409]). [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. 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
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 as
(the reverse of the compression process specified in
Section 3.2. 3.2). If the contentType-ShortForm value is not 25, it
is handled as described in [ACP142A]. This document doesn't discuss
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 inderectly) indirectly) for any of the recipients, the message is
discarded and no further processing is done.
4. The relay adds trace header fields, for example e.g., the Received header
field. See [RFC7601] and Section 4.4. 4.4 of [RFC5321] and [RFC7601]. [RFC5321].
5. The set of ACP 142 destinations for the message is created by
extracting right hand right-hand sides (hostnames) of each RCPT-line,
eliminating duplicates 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, which 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_PDU, Address_PDUs,
DATA_PDUs,
DISCARD_MESSAGE_PDU. Here and DISCARD_MESSAGE_PDUs. Here, MappedPriority(x)
is defined as "6 - x".
3.1. BSMTP-like BSMTP-Like Payload construction Construction
MULE uses a BSMTP-like payload which that differs from Batch SMTP (BSMTP,
[RFC2442]) (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 excluded in order to save a few bytes.
For each received message, the corresponding BSMTP-like payload is
constructed as follows (Lines follows. Note that lines are terminated using CR LF).: LF.
1. The first line is what would be used for the data following "MAIL
FROM:" in the SMTP dialogue. I.e. dialog, i.e., it contains the return-path
address, within <>'s
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
dialogue, i.e.
dialog, i.e., the recipient address within <>'s (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. (i.e., just
CR LF) 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 (see Section 3.2 for more details).
An
The following is an example of a BSMTP-like payload follows 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 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
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 Compression
A BSMTP-like payload (Section 3.1) is first compressed using
zlibCompress [RFC1950] and the [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 ASN.1 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 Ed 2. [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 Compressed Data Type: CDT:
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 CDT is defined by the following ASN.1 type (Note type. Note that this
definition is copied from [STANAG-4406] and is only reproduced here
for the reader's convenience): 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)
}
3.3. Error handling
As Handling
MULE doesn't allow next hop MTA/MDA 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 RCPT-line, the RCPT-line. Therefore, when MTAs/MDAs
that are compliant with this specification that receive a message that
can't be relayed further or delivered delivered, they MUST generate a non non-
delivery DSN report [RFC6522] message which 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 7bit, 8bit 7-bit, 8-bit, or binary MIME), this is transported
using ACP 142 [ACP142A] as follows:
1. For each mail message message, a BSMTP-like payload is formed, as
described in Section 3.1.
2. The created payload is compressed and encoded 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_PDU, Address_PDUs, DATA_PDUs, DISCARD_MESSAGE_PDU. Here 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
next-hop MTAs for each of the recipients.
4.1. Use of BDAT
If a message is received by a gateway, 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 as
(the reverse of the compression process specified in
Section 3.2. 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:" 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 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 downconversion
down-conversion of the message to 7-bit compatible
representation.)
5.1. Handling of ESMTP extensions Extensions and Error handling 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, including extensions. This includes
dealing with the absence of support for ESMTP extensions that
correspond to MAIL FROM/RCPT FROM and RCPT TO parameters found in the BSMTP-like BSMTP-
like payload.
Failures to extract or uncompress BSMTP-like payload payloads should result
in the receiver discarding such payloads.
6. IANA Considerations
IANA is requested to create has created a new registry "Multicast Email SMTP
extensions". Registration Extensions" registry
under the "MAIL Parameters" registry. The registration procedure for the
this new registry is "Specification Required" [RFC8126], but the Designated Expert(s) [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) designated expert(s) should (collectively) have a good
knowledge of SMTP protocol (and its extensions/extensibility mechanisms), extensions and extensibility mechanisms),
as well as ACP 142 and its limitations. Subsections of this section The subsections below
provide more details. In
particular, details: Section 6.1 specifies instructions for the Designated
Expert(s)
designated expert(s), and Section 6.2 defines the initial content of
the registry.
6.1. SMTP Extension Support in MULE
The Instructions for Designated Expert Experts
The designated expert(s) for the new "Multicast Email SMTP extensions"
Extensions" registry verifies that that:
1. the The requested SMTP extension is already registered in the "SMTP
Service Extensions" registry in under the "Mail "MAIL Parameters" section of registry
on the IANA Website website or is well documented on a stable, publicly
accessible web page.
2. the The requested SMTP extension has the correct status as specified
in Section 6.2. When deciding on status, the Designated
Expert(s) designated
expert(s) is provided with the following guidelines:
1.
A. If the SMTP extension only affects commands other than MAIL
FROM/RCPT
FROM and RCPT TO, then the status should be "N/A".
2.
B. If the SMTP extension only applies to SMTP submission Submission
[RFC6409] (and not to SMTP relay or final SMTP delivery),
then the status should be "N/A".
3.
C. If the SMTP extension changes which commands are allowed
during an SMTP transaction (e.g. (e.g., if it adds commands
alternative to DATA or declares commands other than MAIL
FROM/RCPT TO/DATA/BDAT
FROM, RCPT TO, DATA, and BDAT to be a part of SMTP
transaction), then the status should be "Disallowed" or
"Special".
4.
D. If the SMTP extension adds extra round trips during SMTP
transaction, then the status should be "Disallowed" or
"Special".
Registration requests should include the SMTP extension name, status
(see Section 6.2), specification reference 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:
"Required" (required to be supported
o Required - support by MULE relays, SMTP-to-MULE
gateway gateway, or MULE-to-SMTP gateway), "Disallowed" (incompatible MULE-
to-SMTP gateway is required.
o Disallowed - incompatible with
MULE), "N/A" (not relevant, MULE.
o N/A - not relevant because they affect the extension affects commands other
than MAIL FROM and/or RCPT TO, TO or is only defined for SMTP Submission.
Submission [RFC6409]. Such extensions can still be used on the
receiving SMTP side of SMTP-to-
MULE gateway) "Supported" (can an SMTP-to-MULE gateway.
o Supported - can be used with MULE, MULE but requires bilateral agreement
between sender and receiver), or "Special".
"Special" receiver.
o Special - needs to be accompanied by an explanation.
SMTP Extension Support in MULE:
+------------------------+-----------+---------------+
+------------------------+---------------+-----------+
| SMTP Extension Keyword | Reference | Status |
+------------------------+-----------+---------------+ Reference | SIZE
+------------------------+---------------+-----------+
| [RFC1870] SIZE | Required | [RFC1870] |
| | | | 8BITMIME
| [RFC6152] 8BITMIME | Required | [RFC6152] |
| | | | DSN
| [RFC3461] DSN | Required | [RFC3461] |
| | | | MT-PRIORITY
| [RFC6710] MT-PRIORITY | Required | [RFC6710] |
| | | | DELIVERBY
| [RFC2852] DELIVERBY | Required | [RFC2852] |
| | | | BINARYMIME
| [RFC3030] BINARYMIME | Required | [RFC3030] |
| | | | CHUNKING
| [RFC3030] CHUNKING | Special (*) | [RFC3030] |
| | | | ENHANCEDSTATUSCODES
| [RFC2034] ENHANCEDSTATUSCODES | Special (**) | [RFC2034] |
| | | | RRVS
| [RFC7293] RRVS | Supported | [RFC7293] |
| | | | SUBMITTER
| [RFC4405] SUBMITTER | Supported | [RFC4405] |
| | | | PIPELINING
| [RFC2920] PIPELINING | N/A | [RFC2920] |
| | | | STARTTLS
| [RFC3207] STARTTLS | N/A | [RFC3207] |
| | | | AUTH
| [RFC4954] AUTH | Special (***) | [RFC4954] |
| | | | BURL
| [RFC4468] BURL | N/A | [RFC4468] |
| | | | NO-SOLICITING
| [RFC3865] NO-SOLICITING | N/A | [RFC3865] |
| | | | CHECKPOINT
| [RFC1845] CHECKPOINT | Disallowed | [RFC1845] |
| | | | CONNEG
| [RFC4141] CONNEG | Disallowed |
+------------------------+-----------+---------------+ [RFC4141] |
+------------------------+---------------+-----------+
Table 1: Initial Content of Multicast Email SMTP Extensions Registry
(*) - SMTP CHUNKING MUST be supported on the receiving SMTP side of a
an SMTP-to-MULE gateway and MAY be used on the sending side of MULE-to-
SMTP 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.
(***) - The AUTH parameter to the MAIL FROM command is "supported", "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 extensions not listed above can be used in MULE.
7. Security Considerations
As MULE provides a service similar to SMTP, many of Security
Considerations the security
considerations from [RFC5321] apply to MULE as well, well; in particular particular,
Sections 7.1, 7.2, 7.4, 7.6, 7.7, and 7.9 of [RFC5321] still 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") that talks about Security"), which discusses email
harvesting
harvesting, doesn't apply to MULE.
Arguably, it is more difficult to cause an application layer Denial-
of-Service 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 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 transport-layer source authentication, this can be abused by
malicious senders.
For Security Considerations security considerations related to use of zlib compression 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 transferred among MULE MTAs (in the same way they can't
generally rely on the use of STARTTLS on SMTP MTA-to-MTA links), 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.
8. References
8.1. Normative References
[ACP142A] "Common Messaging strategy CCEB, "P_Mul - A Protocol for Reliable Multicast in
Bandwidth Constrained and procedures", Delayed Acknowledgement (EMCON)
Environments", ACP 142(A),
August October 2008.
[ITU.X690.2002]
International Telecommunications Union,
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, July 2002. 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>.
[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>.
[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
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>.
[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>.
Appendix A.
Acknowledgements
Thank you to Steve Kille for suggestions, comments comments, and corrections
on this document. Additional An additional thank you goes to Barry Leiba, Sean
Turner, Dave Crocker Crocker, and Nick Hudson for reviews and comments on
this document.
Some text was borrowed from draft-riechmann-multicast-mail-00, thus "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 is greatefully acknowledged. document.
Authors' Addresses
David Wilson
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail:
United Kingdom
Email: David.Wilson@isode.com
Alexey Melnikov (editor)
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail:
United Kingdom
Email: Alexey.Melnikov@isode.com