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rfc 822 文档,邮件协议
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RFC # 822
Obsoletes: RFC #733 (NIC #41952)
STANDARD FOR THE FORMAT OF
ARPA INTERNET TEXT MESSAGES
August 13, 1982
Revised by
David H. Crocker
Dept. of Electrical Engineering
University of Delaware, Newark, DE 19711
Network: DCrocker @ UDel-Relay
Standard for ARPA Internet Text Messages
TABLE OF CONTENTS
PREFACE .................................................... ii
1. INTRODUCTION ........................................... 1
1.1. Scope ............................................ 1
1.2. Communication Framework .......................... 2
2. NOTATIONAL CONVENTIONS ................................. 3
3. LEXICAL ANALYSIS OF MESSAGES ........................... 5
3.1. General Description .............................. 5
3.2. Header Field Definitions ......................... 9
3.3. Lexical Tokens ................................... 10
3.4. Clarifications ................................... 11
4. MESSAGE SPECIFICATION .................................. 17
4.1. Syntax ........................................... 17
4.2. Forwarding ....................................... 19
4.3. Trace Fields ..................................... 20
4.4. Originator Fields ................................ 21
4.5. Receiver Fields .................................. 23
4.6. Reference Fields ................................. 23
4.7. Other Fields ..................................... 24
5. DATE AND TIME SPECIFICATION ............................ 26
5.1. Syntax ........................................... 26
5.2. Semantics ........................................ 26
6. ADDRESS SPECIFICATION .................................. 27
6.1. Syntax ........................................... 27
6.2. Semantics ........................................ 27
6.3. Reserved Address ................................. 33
7. BIBLIOGRAPHY ........................................... 34
APPENDIX
A. EXAMPLES ............................................... 36
B. SIMPLE FIELD PARSING ................................... 40
C. DIFFERENCES FROM RFC #733 .............................. 41
D. ALPHABETICAL LISTING OF SYNTAX RULES ................... 44
August 13, 1982 - i - RFC #822
Standard for ARPA Internet Text Messages
PREFACE
By 1977, the Arpanet employed several informal standards for
the text messages (mail) sent among its host computers. It was
felt necessary to codify these practices and provide for those
features that seemed imminent. The result of that effort was
Request for Comments (RFC) #733, "Standard for the Format of ARPA
Network Text Message", by Crocker, Vittal, Pogran, and Henderson.
The specification attempted to avoid major changes in existing
software, while permitting several new features.
This document revises the specifications in RFC #733, in
order to serve the needs of the larger and more complex ARPA
Internet. Some of RFC #733’s features failed to gain adequate
acceptance. In order to simplify the standard and the software
that follows it, these features have been removed. A different
addressing scheme is used, to handle the case of inter-network
mail; and the concept of re-transmission has been introduced.
This specification is intended for use in the ARPA Internet.
However, an attempt has been made to free it of any dependence on
that environment, so that it can be applied to other network text
message systems.
The specification of RFC #733 took place over the course of
one year, using the ARPANET mail environment, itself, to provide
an on-going forum for discussing the capabilities to be included.
More than twenty individuals, from across the country, partici-
pated in the original discussion. The development of this
revised specification has, similarly, utilized network mail-based
group discussion. Both specification efforts greatly benefited
from the comments and ideas of the participants.
The syntax of the standard, in RFC #733, was originally
specified in the Backus-Naur Form (BNF) meta-language. Ken L.
Harrenstien, of SRI International, was responsible for re-coding
the BNF into an augmented BNF that makes the representation
smaller and easier to understand.
August 13, 1982 - ii - RFC #822
Standard for ARPA Internet Text Messages
1. INTRODUCTION
1.1. SCOPE
This standard specifies a syntax for text messages that are
sent among computer users, within the framework of "electronic
mail". The standard supersedes the one specified in ARPANET
Request for Comments #733, "Standard for the Format of ARPA Net-
work Text Messages".
In this context, messages are viewed as having an envelope
and contents. The envelope contains whatever information is
needed to accomplish transmission and delivery. The contents
compose the object to be delivered to the recipient. This stan-
dard applies only to the format and some of the semantics of mes-
sage contents. It contains no specification of the information
in the envelope.
However, some message systems may use information from the
contents to create the envelope. It is intended that this stan-
dard facilitate the acquisition of such information by programs.
Some message systems may store messages in formats that
differ from the one specified in this standard. This specifica-
tion is intended strictly as a definition of what message content
format is to be passed BETWEEN hosts.
Note: This standard is NOT intended to dictate the internal for-
mats used by sites, the specific message system features
that they are expected to support, or any of the charac-
teristics of user interface programs that create or read
messages.
A distinction should be made between what the specification
REQUIRES and what it ALLOWS. Messages can be made complex and
rich with formally-structured components of information or can be
kept small and simple, with a minimum of such information. Also,
the standard simplifies the interpretation of differing visual
formats in messages; only the visual aspect of a message is
affected and not the interpretation of information within it.
Implementors may choose to retain such visual distinctions.
The formal definition is divided into four levels. The bot-
tom level describes the meta-notation used in this document. The
second level describes basic lexical analyzers that feed tokens
to higher-level parsers. Next is an overall specification for
messages; it permits distinguishing individual fields. Finally,
there is definition of the contents of several structured fields.
August 13, 1982 - 1 - RFC #822
Standard for ARPA Internet Text Messages
1.2. COMMUNICATION FRAMEWORK
Messages consist of lines of text. No special provisions
are made for encoding drawings, facsimile, speech, or structured
text. No significant consideration has been given to questions
of data compression or to transmission and storage efficiency,
and the standard tends to be free with the number of bits con-
sumed. For example, field names are specified as free text,
rather than special terse codes.
A general "memo" framework is used. That is, a message con-
sists of some information in a rigid format, followed by the main
part of the message, with a format that is not specified in this
document. The syntax of several fields of the rigidly-formated
("headers") section is defined in this specification; some of
these fields must be included in all messages.
The syntax that distinguishes between header fields is
specified separately from the internal syntax for particular
fields. This separation is intended to allow simple parsers to
operate on the general structure of messages, without concern for
the detailed structure of individual header fields. Appendix B
is provided to facilitate construction of these parsers.
In addition to the fields specified in this document, it is
expected that other fields will gain common use. As necessary,
the specifications for these "extension-fields" will be published
through the same mechanism used to publish this document. Users
may also wish to extend the set of fields that they use
privately. Such "user-defined fields" are permitted.
The framework severely constrains document tone and appear-
ance and is primarily useful for most intra-organization communi-
cations and well-structured inter-organization communication.
It also can be used for some types of inter-process communica-
tion, such as simple file transfer and remote job entry. A more
robust framework might allow for multi-font, multi-color, multi-
dimension encoding of information. A less robust one, as is
present in most single-machine message systems, would more
severely constrain the ability to add fields and the decision to
include specific fields. In contrast with paper-based communica-
tion, it is interesting to note that the RECEIVER of a message
can exercise an extraordinary amount of control over the
message’s appearance. The amount of actual control available to
message receivers is contingent upon the capabilities of their
individual message systems.
August 13, 1982 - 2 - RFC #822
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