📄 rfc2524.txt
字号:
Network Working Group M. Banan
Request for Comments: 2524 Neda Communications, Inc.
Category: Informational February 1999
Neda's
Efficient Mail Submission and Delivery (EMSD)
Protocol Specification Version 1.3
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
IESG Note
The protocol specified in this document may be satisfactory for
limited use in private wireless IP networks. However, it is
unsuitable for general-purpose message transfer or for transfer of
messages over the public Internet, because of limitations that
include the following:
- Lack of congestion control
EMSD is layered on ESRO [RFC 2188], which does not provide
congestion control. This makes EMSD completely unsuitable for
end-to-end use across the public Internet. EMSD should be
considered for use in a wireless network only if all EMSD email
exchanged between the wireless network and the public Internet
will transit an EMSD<->SMTP gateway between the two regions.
- Inadequate security
The document specifies only clear-text passwords for
authentication. EMSD should be used across a wireless network
only if sufficiently strong encryption is in use to protect the
clear-text password.
- Lack of character set internationalization
EMSD has no provision for representation of characters outside of
the ASCII repertoire or for language tags.
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RFC 2524 EMSD February 1999
- Poorly defined gatewaying to and from Internet Mail
Because Internet Mail and EMSD have somewhat different and
conflicting service models and different data models, mapping
between them may provide good service only in limited cases, and
this may cause operational problems.
The IESG therefore recommends that EMSD deployment be limited to
narrow circumstances, i.e., only to communicate with devices that
have inherent limitations on the length and format of a message (no
more than a few hundred bytes of ASCII text), using either:
a. wireless links with adequate link-layer encryption and gatewayed
to the public Internet, or
b. a private IP network that is either very over-provisioned or has
some means of congestion control.
In the near future, the IESG may charter a working group to define an
Internet standards-track protocol for efficient transmission of
electronic mail messages, which will be highly compatible with
existing Internet mail protocols, and which wil be suitable for
operation over the global Internet, including both wireless and wired
links.
ABSTRACT
This document specifies the protocol and format encodings for
Efficient Mail Submission and Delivery (EMSD). EMSD is a messaging
protocol that is highly optimized for submission and delivery of
short Internet mail messages. EMSD is designed to be a companion to
existing Internet mail protocols.
This specification narrowly focuses on submission and delivery of
short mail messages with a clear emphasis on efficiency. EMSD is
designed specifically with wireless network (e.g., CDPD, Wireless-IP,
Mobile-IP) usage in mind. EMSD is designed to be a natural
enhancement to the mainstream of Internet mail protocols when
efficiency in mail submission and mail delivery are important. As
such, EMSD is anticipated to become an initial basis for convergence
of Internet Mail and IP-based Two-Way Paging.
The reliability requirement for message submission and message
delivery in EMSD are the same as existing email protocols. EMSD
protocol accomplishes reliable connectionless mail submission and
delivery services on top of Efficient Short Remote Operations (ESRO)
protocols as specified in RFC-2188 [1].
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RFC 2524 EMSD February 1999
Most existing Internet mail protocols are not efficient. Most
existing Internet mail protocols are designed with simplicity and
continuity with SMTP traditions as two primary requirements. EMSD is
designed with efficiency as a primary requirement.
The early use of EMSD in the wireless environment is manifested as
IP-based Two-Way Paging services. The efficiency of this protocol
also presents significant benefits for large centrally operated
Internet mail service providers.
Table of Contents
1 PRELIMINARIES 4
1.1 Internet Mail Submission and Delivery . . . . 4
1.2 Relationship Of EMSD To Other Mail Protocols . . . 5
1.3 EMSD Requirements and Goals . . . . . . . 7
1.4 Anticipated Uses Of EMSD . . . . . . . . 8
1.5 Definitions of Terms Used in this Specification . . 9
1.6 Conventions Used In This Specification . . . . 9
1.7 About This Specification . . . . . . . . 10
2 EFFICIENT MAIL SUBMISSION AND DELIVERY OVERVIEW 10
3 EFFICIENT MAIL SUBMISSION AND DELIVERY PROTOCOL 11
3.1 Use Of Lower Layers . . . . . . . . . 13
3.1.1 Use of ESROS . . . . . . . . . 13
3.1.2 Use Of UDP . . . . . . . . . . 13
3.1.3 Encoding Rules . . . . . . . . . 13
3.1.4 Presentation Context . . . . . . . 14
3.2 EMSD-UA Invoked Operations . . . . . . . 14
3.2.1 submit . . . . . . . . . . . 14
3.2.2 deliveryControl . . . . . . . . 17
3.2.3 deliveryVerify . . . . . . . . . 21
3.3 EMSD-SA Invoked Operations . . . . . . . 23
3.3.1 deliver . . . . . . . . . . 23
3.3.2 submissionControl . . . . . . . . 25
3.3.3 submissionVerify . . . . . . . . 28
3.4 EMSD Common Information Objects . . . . . . 30
3.4.1 SecurityElements . . . . . . . . 30
3.4.2 Message Segmentation and Reassembly . . . 30
3.4.3 Common Errors . . . . . . . . . 33
3.4.4 ContentType . . . . . . . . . 35
3.4.5 EMSDMessageId . . . . . . . . . 35
3.4.6 EMSDORAddress . . . . . . . . . 36
3.4.7 EMSDAddress . . . . . . . . . 36
3.4.8 DateTime . . . . . . . . . . 36
3.4.9 AsciiPrintableString . . . . . . . 37
3.4.10 ProtocolVersionNumber . . . . . . . 37
3.5 Submission and Delivery Procedures . . . . . 38
4 DUPLICATE OPERATION DETECTION SUPPORT 40
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RFC 2524 EMSD February 1999
4.1 Duplicate Operation Detection Support Overview . . 40
4.1.1 Operation Value . . . . . . . . 40
4.1.2 Operation Instance Identifier . . . . . 41
5 EMSD PROCEDURE FOR OPERATIONS 42
5.1 MTS Behavior . . . . . . . . . . . 43
5.1.1 MTS Performer . . . . . . . . . 43
5.1.2 Message-submission . . . . . . . . 44
5.1.3 Delivery-control . . . . . . . . 46
5.1.4 Delivery-verify . . . . . . . . 46
5.1.5 MTS Invoker . . . . . . . . . 46
5.2 UA Behavior . . . . . . . . . . . 49
5.2.1 UA Performer . . . . . . . . . 49
5.2.2 UA Invoker . . . . . . . . . . 52
6 EMSD FORMAT STANDARDS 54
6.1 Format Standard Overview . . . . . . . . 54
6.2 Interpersonal Messages . . . . . . . . 54
6.2.1 Heading fields . . . . . . . . . 55
6.2.2 Body part types . . . . . . . . 61
7 ACKNOWLEDGMENTS 62
8 SECURITY CONSIDERATIONS 62
9 AUTHOR'S ADDRESS 62
A EMSD-P ASN.1 MODULE 63
B EMSD-IPM ASN.1 MODULE 74
C RATIONALE FOR KEY DESIGN DECISIONS 78
C.1 Deviation From The SMTP Model . . . . . . 78
C.1.1 Comparison of SMTP and EMSD Efficiency . . . 78
C.2 Use of ESRO Instead of TCP . . . . . . . 79
C.3 Use Of Remote Procedure Call (RPC) Model . . . . 79
C.4 Use Of ASN.1 . . . . . . . . . . . 80
D FURTHER DEVELOPMENT 81
E REFERENCES 82
F FULL COPYRIGHT STATEMENT 83
1 PRELIMINARIES
Mail in the Internet was not a well-planned enterprise, but instead
arose in more of an "organic" way.
This introductory section is not intended to be a reference model and
concept vocabulary for mail in the Internet. Instead, it only
provides the necessary preliminaries for the concepts and terms that
are essential to this specification.
1.1 Internet Mail Submission and Delivery
For the purposes of this specification, mail submission is the
process of putting mail into the mail transfer system (MTS).
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RFC 2524 EMSD February 1999
For the purposes of this specification, mail delivery is the process
of the MTS putting mail into a user's final mail-box.
Throughout the Internet, presently most of mail submission and
delivery is done through SMTP.
SMTP was defined as a message *transfer* protocol, that is, a means
to route (if needed) and deliver mail by putting finished (complete)
messages in a mail-box. Originally, users connected to servers from
terminals, and all processing occurred on the server. Now, a split-
MUA (Mail User Agent) model is common, with MUA functionality
occurring on both the user's own system and the server.
In the split-MUA model, getting the messages to the user is
accomplished through access to a mail-box on the server through such
protocols as POP and IMAP. In the split-MUA model, user's access to
its message is a "Message Pull" paradigm where the user is required
to poll his mailbox. Proper message delivery based on a "Message
Push" paradigm is presently not supported. The EMSD protocol
addresses this shortcoming with an emphasis on efficiency.
In the split-MUA model, message submission is often accomplished
through SMTP. SMTP is widely used as a message *submission* protocol.
Widespread use of SMTP for submission is a reality, regardless of
whether this is good or bad. EMSD protocol provides an alternative
mechanism for message submission which emphasizes efficiency.
1.2 Relationship Of EMSD To Other Mail Protocols
Various Internet mail protocols facilitate accomplishment of various
functions in mail processing.
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RFC 2524 EMSD February 1999
Figure 1, categorizes the capabilities of SMTP, IMAP, POP and EMSD
based on the following functions:
+------------------+------+-------+-----+------+
| Protocols| SMTP | IMAP | POP | EMSD |
|Functions | | | | |
|------------------|------|-------|-----|------|
|Submission | XX | | | XXX |
|------------------|------|-------|-----|------|
|Delivery | XXX | | | XXX |
|------------------|------|-------|-----|------|
|Relay (Routing) | XXX | | | |
|------------------|------|-------|-----|------|
|Retrieval | | XXX | XXX | XX |
|------------------|------|-------|-----|------|
|Mailbox Access | | XXX | X | |
|------------------|------|-------|-----|------|
|Mailbox Synch. | | XXX | | |
+------------------+------+-------+-----+------+
Figure 1: Messaging Protocols vs. Supported Functions
o Mail Submission
o Mail Delivery
o Mail Routing (Relay)
o Mail Retrieval
o Mail-box Access
o Mail-box Synchronization
In Figure 1, the number of "X"es in each box denotes the extent to
which a particular function is supported by a particular protocol.
Figure 1 clearly shows that combinations of these protocols can be
used to complement each other in providing rich functionality to the
user. For example, a user interested in highly mobile messaging
functionalities can use EMSD for "submission and delivery of time
critical and important messages" and use IMAP for comprehensive
access to his/her mail-box.
For mail submission and delivery of short messages EMSD is up to 5
times more efficient than SMTP both in terms of the number of packets
transmitted and in terms of number of bytes transmitted. Even with
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RFC 2524 EMSD February 1999
PIPELINING and other possible optimizations of SMTP, EMSD is up to 3
times more efficient than SMTP both in terms of the number of packets
transmitted and in terms of number of bytes transmitted. Various
efficiency studies comparing EMSD with SMTP, POP and IMAP are
available. See Section C.1.1 for more information about comparison
of SMTP and EMSD's efficiency.
1.3 EMSD Requirements and Goals
The requirements and goals driving design of EMSD protocol are
enumerated below.
1. Provide for submission of short mail messages with the same level
of functionality (or higher) that the existing Internet mail
protocols provide.
2. Provide for delivery of short mail messages with the same level
of functionality (or higher) that the existing Internet mail
protocols provide.
3. Function as an extension of the existing mainstream Internet
mail.
4. Minimize the number of transmissions.
5. Minimize the number of bytes transmitted.
6. Be quick: minimize latency of message submission and delivery.
7. Provide the same level of reliability (or higher) that the
existing email protocols provide.
8. Accommodate varying sizes of messages: the size of a message may
determine how the system deals with the message, but the system
must accommodate it.
9. Be power efficient and respect mobile platform resources:
including memory and CPU levels, as well as battery power
longevity (i.e. client-light and server-heavy).
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