brian@sdcsvax.UCSD.EDU (Brian Kantor) (09/01/87)
-
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.
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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.
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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.
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2. NOTATIONAL CONVENTIONS
This specification uses an augmented Backus-Naur Form (BNF)
notation. The differences from standard BNF involve naming rules
and indicating repetition and "local" alternatives.
2.1. RULE NAMING
Angle brackets ("<", ">") are not used, in general. The
name of a rule is simply the name itself, rather than "<name>".
Quotation-marks enclose literal text (which may be upper and/or
lower case). Certain basic rules are in uppercase, such as
SPACE, TAB, CRLF, DIGIT, ALPHA, etc. Angle brackets are used in
rule definitions, and in the rest of this document, whenever
their presence will facilitate discerning the use of rule names.
2.2. RULE1 / RULE2: ALTERNATIVES
Elements separated by slash ("/") are alternatives. There-
fore "foo / bar" will accept foo or bar.
2.3. (RULE1 RULE2): LOCAL ALTERNATIVES
Elements enclosed in parentheses are treated as a single
element. Thus, "(elem (foo / bar) elem)" allows the token
sequences "elem foo elem" and "elem bar elem".
2.4. *RULE: REPETITION
The character "*" preceding an element indicates repetition.
The full form is:
<l>*<m>element
indicating at least <l> and at most <m> occurrences of element.
Default values are 0 and infinity so that "*(element)" allows any
number, including zero; "1*element" requires at least one; and
"1*2element" allows one or two.
2.5. [RULE]: OPTIONAL
Square brackets enclose optional elements; "[foo bar]" is
equivalent to "*1(foo bar)".
2.6. NRULE: SPECIFIC REPETITION
"<n>(element)" is equivalent to "<n>*<n>(element)"; that is,
exactly <n> occurrences of (element). Thus 2DIGIT is a 2-digit
number, and 3ALPHA is a string of three alphabetic characters.
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2.7. #RULE: LISTS
A construct "#" is defined, similar to "*", as follows:
<l>#<m>element
indicating at least <l> and at most <m> elements, each separated
by one or more commas (","). This makes the usual form of lists
very easy; a rule such as '(element *("," element))' can be shown
as "1#element". Wherever this construct is used, null elements
are allowed, but do not contribute to the count of elements
present. That is, "(element),,(element)" is permitted, but
counts as only two elements. Therefore, where at least one ele-
ment is required, at least one non-null element must be present.
Default values are 0 and infinity so that "#(element)" allows any
number, including zero; "1#element" requires at least one; and
"1#2element" allows one or two.
2.8. ; COMMENTS
A semi-colon, set off some distance to the right of rule
text, starts a comment that continues to the end of line. This
is a simple way of including useful notes in parallel with the
specifications.
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3. LEXICAL ANALYSIS OF MESSAGES
3.1. GENERAL DESCRIPTION
A message consists of header fields and, optionally, a body.
The body is simply a sequence of lines containing ASCII charac-
ters. It is separated from the headers by a null line (i.e., a
line with nothing preceding the CRLF).
3.1.1. LONG HEADER FIELDS
Each header field can be viewed as a single, logical line of
ASCII characters, comprising a field-name and a field-body.
For convenience, the field-body portion of this conceptual
entity can be split into a multiple-line representation; this
is called "folding". The general rule is that wherever there
may be linear-white-space (NOT simply LWSP-chars), a CRLF
immediately followed by AT LEAST one LWSP-char may instead be
inserted. Thus, the single line
To: "Joe & J. Harvey" <ddd @Org>, JJV @ BBN
can be represented as:
To: "Joe & J. Harvey" <ddd @ Org>,
JJV@BBN
and
To: "Joe & J. Harvey"
<ddd@ Org>, JJV
@BBN
and
To: "Joe &
J. Harvey" <ddd @ Org>, JJV @ BBN
The process of moving from this folded multiple-line
representation of a header field to its single line represen-
tation is called "unfolding". Unfolding is accomplished by
regarding CRLF immediately followed by a LWSP-char as
equivalent to the LWSP-char.
Note: While the standard permits folding wherever linear-
white-space is permitted, it is recommended that struc-
tured fields, such as those containing addresses, limit
folding to higher-level syntactic breaks. For address
fields, it is recommended that such folding occur
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between addresses, after the separating comma.
3.1.2. STRUCTURE OF HEADER FIELDS
Once a field has been unfolded, it may be viewed as being com-
posed of a field-name followed by a colon (":"), followed by a
field-body, and terminated by a carriage-return/line-feed.
The field-name must be composed of printable ASCII characters
(i.e., characters that have values between 33. and 126.,
decimal, except colon). The field-body may be composed of any
ASCII characters, except CR or LF. (While CR and/or LF may be
present in the actual text, they are removed by the action of
unfolding the field.)
Certain field-bodies of headers may be interpreted according
to an internal syntax that some systems may wish to parse.
These fields are called "structured fields". Examples
include fields containing dates and addresses. Other fields,
such as "Subject" and "Comments", are regarded simply as
strings of text.
Note: Any field which has a field-body that is defined as
other than simply <text> is to be treated as a struc-
tured field.
Field-names, unstructured field bodies and structured
field bodies each are scanned by their own, independent
"lexical" analyzers.
3.1.3. UNSTRUCTURED FIELD BODIES
For some fields, such as "Subject" and "Comments", no struc-
turing is assumed, and they are treated simply as <text>s, as
in the message body. Rules of folding apply to these fields,
so that such field bodies which occupy several lines must
therefore have the second and successive lines indented by at
least one LWSP-char.
3.1.4. STRUCTURED FIELD BODIES
To aid in the creation and reading of structured fields, the
free insertion of linear-white-space (which permits folding
by inclusion of CRLFs) is allowed between lexical tokens.
Rather than obscuring the syntax specifications for these
structured fields with explicit syntax for this linear-white-
space, the existence of another "lexical" analyzer is assumed.
This analyzer does not apply for unstructured field bodies
that are simply strings of text, as described above. The
analyzer provides an interpretation of the unfolded text
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composing the body of the field as a sequence of lexical sym-
bols.
These symbols are:
- individual special characters
- quoted-strings
- domain-literals
- comments
- atoms
The first four of these symbols are self-delimiting. Atoms
are not; they are delimited by the self-delimiting symbols and
by linear-white-space. For the purposes of regenerating
sequences of atoms and quoted-strings, exactly one SPACE is
assumed to exist, and should be used, between them. (Also, in
the "Clarifications" section on "White Space", below, note the
rules about treatment of multiple contiguous LWSP-chars.)
So, for example, the folded body of an address field
":sysmail"@ Some-Group. Some-Org,
Muhammed.(I am the greatest) Ali @(the)Vegas.WBA
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is analyzed into the following lexical symbols and types:
:sysmail quoted string
@ special
Some-Group atom
. special
Some-Org atom
, special
Muhammed atom
. special
(I am the greatest) comment
Ali atom
@ atom
(the) comment
Vegas atom
. special
WBA atom
The canonical representations for the data in these addresses
are the following strings:
":sysmail"@Some-Group.Some-Org
and
Muhammed.Ali@Vegas.WBA
Note: For purposes of display, and when passing such struc-
tured information to other systems, such as mail proto-
col services, there must be NO linear-white-space
between <word>s that are separated by period (".") or
at-sign ("@") and exactly one SPACE between all other
<word>s. Also, headers should be in a folded form.
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3.2. HEADER FIELD DEFINITIONS
These rules show a field meta-syntax, without regard for the
particular type or internal syntax. Their purpose is to permit
detection of fields; also, they present to higher-level parsers
an image of each field as fitting on one line.
field = field-name ":" [ field-body ] CRLF
field-name = 1*<any CHAR, excluding CTLs, SPACE, and ":">
field-body = field-body-contents
[CRLF LWSP-char field-body]
field-body-contents =
<the ASCII characters making up the field-body, as
defined in the following sections, and consisting
of combinations of atom, quoted-string, and
specials tokens, or else consisting of texts>
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3.3. LEXICAL TOKENS
The following rules are used to define an underlying lexical
analyzer, which feeds tokens to higher level parsers. See the
ANSI references, in the Bibliography.
; ( Octal, Decimal.)
CHAR = <any ASCII character> ; ( 0-177, 0.-127.)
ALPHA = <any ASCII alphabetic character>
; (101-132, 65.- 90.)
; (141-172, 97.-122.)
DIGIT = <any ASCII decimal digit> ; ( 60- 71, 48.- 57.)
CTL = <any ASCII control ; ( 0- 37, 0.- 31.)
character and DEL> ; ( 177, 127.)
CR = <ASCII CR, carriage return> ; ( 15, 13.)
LF = <ASCII LF, linefeed> ; ( 12, 10.)
SPACE = <ASCII SP, space> ; ( 40, 32.)
HTAB = <ASCII HT, horizontal-tab> ; ( 11, 9.)
<"> = <ASCII quote mark> ; ( 42, 34.)
CRLF = CR LF
LWSP-char = SPACE / HTAB ; semantics = SPACE
linear-white-space = 1*([CRLF] LWSP-char) ; semantics = SPACE
; CRLF => folding
specials = "(" / ")" / "<" / ">" / "@" ; Must be in quoted-
/ "," / ";" / ":" / "\" / <"> ; string, to use
/ "." / "[" / "]" ; within a word.
delimiters = specials / linear-white-space / comment
text = <any CHAR, including bare ; => atoms, specials,
CR & bare LF, but NOT ; comments and
including CRLF> ; quoted-strings are
; NOT recognized.
atom = 1*<any CHAR except specials, SPACE and CTLs>
quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or
; quoted chars.
qtext = <any CHAR excepting <">, ; => may be folded
"\" & CR, and including
linear-white-space>
domain-literal = "[" *(dtext / quoted-pair) "]"
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dtext = <any CHAR excluding "[", ; => may be folded
"]", "\" & CR, & including
linear-white-space>
comment = "(" *(ctext / quoted-pair / comment) ")"
ctext = <any CHAR excluding "(", ; => may be folded
")", "\" & CR, & including
linear-white-space>
quoted-pair = "\" CHAR ; may quote any char
phrase = 1*word ; Sequence of words
word = atom / quoted-string
3.4. CLARIFICATIONS
3.4.1. QUOTING
Some characters are reserved for special interpretation, such
as delimiting lexical tokens. To permit use of these charac-
ters as uninterpreted data, a quoting mechanism is provided.
To quote a character, precede it with a backslash ("\").
This mechanism is not fully general. Characters may be quoted
only within a subset of the lexical constructs. In particu-
lar, quoting is limited to use within:
- quoted-string
- domain-literal
- comment
Within these constructs, quoting is REQUIRED for CR and "\"
and for the character(s) that delimit the token (e.g., "(" and
")" for a comment). However, quoting is PERMITTED for any
character.
Note: In particular, quoting is NOT permitted within atoms.
For example when the local-part of an addr-spec must
contain a special character, a quoted string must be
used. Therefore, a specification such as:
Full\ Name@Domain
is not legal and must be specified as:
"Full Name"@Domain
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3.4.2. WHITE SPACE
Note: In structured field bodies, multiple linear space ASCII
characters (namely HTABs and SPACEs) are treated as
single spaces and may freely surround any symbol. In
all header fields, the only place in which at least one
LWSP-char is REQUIRED is at the beginning of continua-
tion lines in a folded field.
When passing text to processes that do not interpret text
according to this standard (e.g., mail protocol servers), then
NO linear-white-space characters should occur between a period
(".") or at-sign ("@") and a <word>. Exactly ONE SPACE should
be used in place of arbitrary linear-white-space and comment
sequences.
Note: Within systems conforming to this standard, wherever a
member of the list of delimiters is allowed, LWSP-chars
may also occur before and/or after it.
Writers of mail-sending (i.e., header-generating) programs
should realize that there is no network-wide definition of the
effect of ASCII HT (horizontal-tab) characters on the appear-
ance of text at another network host; therefore, the use of
tabs in message headers, though permitted, is discouraged.
3.4.3. COMMENTS
A comment is a set of ASCII characters, which is enclosed in
matching parentheses and which is not within a quoted-string
The comment construct permits message originators to add text
which will be useful for human readers, but which will be
ignored by the formal semantics. Comments should be retained
while the message is subject to interpretation according to
this standard. However, comments must NOT be included in
other cases, such as during protocol exchanges with mail
servers.
Comments nest, so that if an unquoted left parenthesis occurs
in a comment string, there must also be a matching right
parenthesis. When a comment acts as the delimiter between a
sequence of two lexical symbols, such as two atoms, it is lex-
ically equivalent with a single SPACE, for the purposes of
regenerating the sequence, such as when passing the sequence
onto a mail protocol server. Comments are detected as such
only within field-bodies of structured fields.
If a comment is to be "folded" onto multiple lines, then the
syntax for folding must be adhered to. (See the "Lexical
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Analysis of Messages" section on "Folding Long Header Fields"
above, and the section on "Case Independence" below.) Note
that the official semantics therefore do not "see" any
unquoted CRLFs that are in comments, although particular pars-
ing programs may wish to note their presence. For these pro-
grams, it would be reasonable to interpret a "CRLF LWSP-char"
as being a CRLF that is part of the comment; i.e., the CRLF is
kept and the LWSP-char is discarded. Quoted CRLFs (i.e., a
backslash followed by a CR followed by a LF) still must be
followed by at least one LWSP-char.
3.4.4. DELIMITING AND QUOTING CHARACTERS
The quote character (backslash) and characters that delimit
syntactic units are not, generally, to be taken as data that
are part of the delimited or quoted unit(s). In particular,
the quotation-marks that define a quoted-string, the
parentheses that define a comment and the backslash that
quotes a following character are NOT part of the quoted-
string, comment or quoted character. A quotation-mark that is
to be part of a quoted-string, a parenthesis that is to be
part of a comment and a backslash that is to be part of either
must each be preceded by the quote-character backslash ("\").
Note that the syntax allows any character to be quoted within
a quoted-string or comment; however only certain characters
MUST be quoted to be included as data. These characters are
the ones that are not part of the alternate text group (i.e.,
ctext or qtext).
The one exception to this rule is that a single SPACE is
assumed to exist between contiguous words in a phrase, and
this interpretation is independent of the actual number of
LWSP-chars that the creator places between the words. To
include more than one SPACE, the creator must make the LWSP-
chars be part of a quoted-string.
Quotation marks that delimit a quoted string and backslashes
that quote the following character should NOT accompany the
quoted-string when the string is passed to processes that do
not interpret data according to this specification (e.g., mail
protocol servers).
3.4.5. QUOTED-STRINGS
Where permitted (i.e., in words in structured fields) quoted-
strings are treated as a single symbol. That is, a quoted-
string is equivalent to an atom, syntactically. If a quoted-
string is to be "folded" onto multiple lines, then the syntax
for folding must be adhered to. (See the "Lexical Analysis of
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Messages" section on "Folding Long Header Fields" above, and
the section on "Case Independence" below.) Therefore, the
official semantics do not "see" any bare CRLFs that are in
quoted-strings; however particular parsing programs may wish
to note their presence. For such programs, it would be rea-
sonable to interpret a "CRLF LWSP-char" as being a CRLF which
is part of the quoted-string; i.e., the CRLF is kept and the
LWSP-char is discarded. Quoted CRLFs (i.e., a backslash fol-
lowed by a CR followed by a LF) are also subject to rules of
folding, but the presence of the quoting character (backslash)
explicitly indicates that the CRLF is data to the quoted
string. Stripping off the first following LWSP-char is also
appropriate when parsing quoted CRLFs.
3.4.6. BRACKETING CHARACTERS
There is one type of bracket which must occur in matched pairs
and may have pairs nested within each other:
o Parentheses ("(" and ")") are used to indicate com-
ments.
There are three types of brackets which must occur in matched
pairs, and which may NOT be nested:
o Colon/semi-colon (":" and ";") are used in address
specifications to indicate that the included list of
addresses are to be treated as a group.
o Angle brackets ("<" and ">") are generally used to
indicate the presence of a one machine-usable refer-
ence (e.g., delimiting mailboxes), possibly including
source-routing to the machine.
o Square brackets ("[" and "]") are used to indicate the
presence of a domain-literal, which the appropriate
name-domain is to use directly, bypassing normal
name-resolution mechanisms.
3.4.7. CASE INDEPENDENCE
Except as noted, alphabetic strings may be represented in any
combination of upper and lower case. The only syntactic units
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which requires preservation of case information are:
- text
- qtext
- dtext
- ctext
- quoted-pair
- local-part, except "Postmaster"
When matching any other syntactic unit, case is to be ignored.
For example, the field-names "From", "FROM", "from", and even
"FroM" are semantically equal and should all be treated ident-
ically.
When generating these units, any mix of upper and lower case
alphabetic characters may be used. The case shown in this
specification is suggested for message-creating processes.
Note: The reserved local-part address unit, "Postmaster", is
an exception. When the value "Postmaster" is being
interpreted, it must be accepted in any mixture of
case, including "POSTMASTER", and "postmaster".
3.4.8. FOLDING LONG HEADER FIELDS
Each header field may be represented on exactly one line con-
sisting of the name of the field and its body, and terminated
by a CRLF; this is what the parser sees. For readability, the
field-body portion of long header fields may be "folded" onto
multiple lines of the actual field. "Long" is commonly inter-
preted to mean greater than 65 or 72 characters. The former
length serves as a limit, when the message is to be viewed on
most simple terminals which use simple display software; how-
ever, the limit is not imposed by this standard.
Note: Some display software often can selectively fold lines,
to suit the display terminal. In such cases, sender-
provided folding can interfere with the display
software.
3.4.9. BACKSPACE CHARACTERS
ASCII BS characters (Backspace, decimal 8) may be included in
texts and quoted-strings to effect overstriking. However, any
use of backspaces which effects an overstrike to the left of
the beginning of the text or quoted-string is prohibited.
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3.4.10. NETWORK-SPECIFIC TRANSFORMATIONS
During transmission through heterogeneous networks, it may be
necessary to force data to conform to a network's local con-
ventions. For example, it may be required that a CR be fol-
lowed either by LF, making a CRLF, or by <null>, if the CR is
to stand alone). Such transformations are reversed, when the
message exits that network.
When crossing network boundaries, the message should be
treated as passing through two modules. It will enter the
first module containing whatever network-specific transforma-
tions that were necessary to permit migration through the
"current" network. It then passes through the modules:
o Transformation Reversal
The "current" network's idiosyncracies are removed and
the message is returned to the canonical form speci-
fied in this standard.
o Transformation
The "next" network's local idiosyncracies are imposed
on the message.
------------------
From ==> | Remove Net-A |
Net-A | idiosyncracies |
------------------
||
\/
Conformance
with standard
||
\/
------------------
| Impose Net-B | ==> To
| idiosyncracies | Net-B
------------------
August 13, 1982 - 16 - RFC #822
Standard for ARPA Internet Text Messages
4. MESSAGE SPECIFICATION
4.1. SYNTAX
Note: Due to an artifact of the notational conventions, the syn-
tax indicates that, when present, some fields, must be in
a particular order. Header fields are NOT required to
occur in any particular order, except that the message
body must occur AFTER the headers. It is recommended
that, if present, headers be sent in the order "Return-
Path", "Received", "Date", "From", "Subject", "Sender",
"To", "cc", etc.
This specification permits multiple occurrences of most
fields. Except as noted, their interpretation is not
specified here, and their use is discouraged.
The following syntax for the bodies of various fields should
be thought of as describing each field body as a single long
string (or line). The "Lexical Analysis of Message" section on
"Long Header Fields", above, indicates how such long strings can
be represented on more than one line in the actual transmitted
message.
message = fields *( CRLF *text ) ; Everything after
; first null line
; is message body
fields = dates ; Creation time,
source ; author id & one
1*destination ; address required
*optional-field ; others optional
source = [ trace ] ; net traversals
originator ; original mail
[ resent ] ; forwarded
trace = return ; path to sender
1*received ; receipt tags
return = "Return-path" ":" route-addr ; return address
received = "Received" ":" ; one per relay
["from" domain] ; sending host
["by" domain] ; receiving host
["via" atom] ; physical path
*("with" atom) ; link/mail protocol
["id" msg-id] ; receiver msg id
["for" addr-spec] ; initial form
August 13, 1982 - 17 - RFC #822
Standard for ARPA Internet Text Messages
";" date-time ; time received
originator = authentic ; authenticated addr
[ "Reply-To" ":" 1#address] )
authentic = "From" ":" mailbox ; Single author
/ ( "Sender" ":" mailbox ; Actual submittor
"From" ":" 1#mailbox) ; Multiple authors
; or not sender
resent = resent-authentic
[ "Resent-Reply-To" ":" 1#address] )
resent-authentic =
= "Resent-From" ":" mailbox
/ ( "Resent-Sender" ":" mailbox
"Resent-From" ":" 1#mailbox )
dates = orig-date ; Original
[ resent-date ] ; Forwarded
orig-date = "Date" ":" date-time
resent-date = "Resent-Date" ":" date-time
destination = "To" ":" 1#address ; Primary
/ "Resent-To" ":" 1#address
/ "cc" ":" 1#address ; Secondary
/ "Resent-cc" ":" 1#address
/ "bcc" ":" #address ; Blind carbon
/ "Resent-bcc" ":" #address
optional-field =
/ "Message-ID" ":" msg-id
/ "Resent-Message-ID" ":" msg-id
/ "In-Reply-To" ":" *(phrase / msg-id)
/ "References" ":" *(phrase / msg-id)
/ "Keywords" ":" #phrase
/ "Subject" ":" *text
/ "Comments" ":" *text
/ "Encrypted" ":" 1#2word
/ extension-field ; To be defined
/ user-defined-field ; May be pre-empted
msg-id = "<" addr-spec ">" ; Unique message id
August 13, 1982 - 18 - RFC #822
Standard for ARPA Internet Text Messages
extension-field =
<Any field which is defined in a document
published as a formal extension to this
specification; none will have names beginning
with the string "X-">
user-defined-field =
<Any field which has not been defined
in this specification or published as an
extension to this specification; names for
such fields must be unique and may be
pre-empted by published extensions>
4.2. FORWARDING
Some systems permit mail recipients to forward a message,
retaining the original headers, by adding some new fields. This
standard supports such a service, through the "Resent-" prefix to
field names.
Whenever the string "Resent-" begins a field name, the field
has the same semantics as a field whose name does not have the
prefix. However, the message is assumed to have been forwarded
by an original recipient who attached the "Resent-" field. This
new field is treated as being more recent than the equivalent,
original field. For example, the "Resent-From", indicates the
person that forwarded the message, whereas the "From" field indi-
cates the original author.
Use of such precedence information depends upon partici-
pants' communication needs. For example, this standard does not
dictate when a "Resent-From:" address should receive replies, in
lieu of sending them to the "From:" address.
Note: In general, the "Resent-" fields should be treated as con-
taining a set of information that is independent of the
set of original fields. Information for one set should
not automatically be taken from the other. The interpre-
tation of multiple "Resent-" fields, of the same type, is
undefined.
In the remainder of this specification, occurrence of legal
"Resent-" fields are treated identically with the occurrence of
August 13, 1982 - 19 - RFC #822
Standard for ARPA Internet Text Messages
fields whose names do not contain this prefix.
4.3. TRACE FIELDS
Trace information is used to provide an audit trail of mes-
sage handling. In addition, it indicates a route back to the
sender of the message.
The list of known "via" and "with" values are registered
with the Network Information Center, SRI International, Menlo
Park, California.
4.3.1. RETURN-PATH
This field is added by the final transport system that
delivers the message to its recipient. The field is intended
to contain definitive information about the address and route
back to the message's originator.
Note: The "Reply-To" field is added by the originator and
serves to direct replies, whereas the "Return-Path"
field is used to identify a path back to the origina-
tor.
While the syntax indicates that a route specification is
optional, every attempt should be made to provide that infor-
mation in this field.
4.3.2. RECEIVED
A copy of this field is added by each transport service that
relays the message. The information in the field can be quite
useful for tracing transport problems.
The names of the sending and receiving hosts and time-of-
receipt may be specified. The "via" parameter may be used, to
indicate what physical mechanism the message was sent over,
such as Arpanet or Phonenet, and the "with" parameter may be
used to indicate the mail-, or connection-, level protocol
that was used, such as the SMTP mail protocol, or X.25 tran-
sport protocol.
Note: Several "with" parameters may be included, to fully
specify the set of protocols that were used.
Some transport services queue mail; the internal message iden-
tifier that is assigned to the message may be noted, using the
"id" parameter. When the sending host uses a destination
address specification that the receiving host reinterprets, by
August 13, 1982 - 20 - RFC #822
Standard for ARPA Internet Text Messages
expansion or transformation, the receiving host may wish to
record the original specification, using the "for" parameter.
For example, when a copy of mail is sent to the member of a
distribution list, this parameter may be used to record the
original address that was used to specify the list.
4.4. ORIGINATOR FIELDS
The standard allows only a subset of the combinations possi-
ble with the From, Sender, Reply-To, Resent-From, Resent-Sender,
and Resent-Reply-To fields. The limitation is intentional.
4.4.1. FROM / RESENT-FROM
This field contains the identity of the person(s) who wished
this message to be sent. The message-creation process should
default this field to be a single, authenticated machine
address, indicating the AGENT (person, system or process)
entering the message. If this is not done, the "Sender" field
MUST be present. If the "From" field IS defaulted this way,
the "Sender" field is optional and is redundant with the
"From" field. In all cases, addresses in the "From" field
must be machine-usable (addr-specs) and may not contain named
lists (groups).
4.4.2. SENDER / RESENT-SENDER
This field contains the authenticated identity of the AGENT
(person, system or process) that sends the message. It is
intended for use when the sender is not the author of the mes-
sage, or to indicate who among a group of authors actually
sent the message. If the contents of the "Sender" field would
be completely redundant with the "From" field, then the
"Sender" field need not be present and its use is discouraged
(though still legal). In particular, the "Sender" field MUST
be present if it is NOT the same as the "From" Field.
The Sender mailbox specification includes a word sequence
which must correspond to a specific agent (i.e., a human user
or a computer program) rather than a standard address. This
indicates the expectation that the field will identify the
single AGENT (person, system, or process) responsible for
sending the mail and not simply include the name of a mailbox
from which the mail was sent. For example in the case of a
shared login name, the name, by itself, would not be adequate.
The local-part address unit, which refers to this agent, is
expected to be a computer system term, and not (for example) a
generalized person reference which can be used outside the
network text message context.
August 13, 1982 - 21 - RFC #822
Standard for ARPA Internet Text Messages
Since the critical function served by the "Sender" field is
identification of the agent responsible for sending mail and
since computer programs cannot be held accountable for their
behavior, it is strongly recommended that when a computer pro-
gram generates a message, the HUMAN who is responsible for
that program be referenced as part of the "Sender" field mail-
box specification.
4.4.3. REPLY-TO / RESENT-REPLY-TO
This field provides a general mechanism for indicating any
mailbox(es) to which responses are to be sent. Three typical
uses for this feature can be distinguished. In the first
case, the author(s) may not have regular machine-based mail-
boxes and therefore wish(es) to indicate an alternate machine
address. In the second case, an author may wish additional
persons to be made aware of, or responsible for, replies. A
somewhat different use may be of some help to "text message
teleconferencing" groups equipped with automatic distribution
services: include the address of that service in the "Reply-
To" field of all messages submitted to the teleconference;
then participants can "reply" to conference submissions to
guarantee the correct distribution of any submission of their
own.
Note: The "Return-Path" field is added by the mail transport
service, at the time of final deliver. It is intended
to identify a path back to the orginator of the mes-
sage. The "Reply-To" field is added by the message
originator and is intended to direct replies.
4.4.4. AUTOMATIC USE OF FROM / SENDER / REPLY-TO
For systems which automatically generate address lists for
replies to messages, the following recommendations are made:
o The "Sender" field mailbox should be sent notices of
any problems in transport or delivery of the original
messages. If there is no "Sender" field, then the
"From" field mailbox should be used.
o The "Sender" field mailbox should NEVER be used
automatically, in a recipient's reply message.
o If the "Reply-To" field exists, then the reply should
go to the addresses indicated in that field and not to
the address(es) indicated in the "From" field.
August 13, 1982 - 22 - RFC #822
Standard for ARPA Internet Text Messages
o If there is a "From" field, but no "Reply-To" field,
the reply should be sent to the address(es) indicated
in the "From" field.
Sometimes, a recipient may actually wish to communicate with
the person that initiated the message transfer. In such
cases, it is reasonable to use the "Sender" address.
This recommendation is intended only for automated use of
originator-fields and is not intended to suggest that replies
may not also be sent to other recipients of messages. It is
up to the respective mail-handling programs to decide what
additional facilities will be provided.
Examples are provided in Appendix A.
4.5. RECEIVER FIELDS
4.5.1. TO / RESENT-TO
This field contains the identity of the primary recipients of
the message.
4.5.2. CC / RESENT-CC
This field contains the identity of the secondary (informa-
tional) recipients of the message.
4.5.3. BCC / RESENT-BCC
This field contains the identity of additional recipients of
the message. The contents of this field are not included in
copies of the message sent to the primary and secondary reci-
pients. Some systems may choose to include the text of the
"Bcc" field only in the author(s)'s copy, while others may
also include it in the text sent to all those indicated in the
"Bcc" list.
4.6. REFERENCE FIELDS
4.6.1. MESSAGE-ID / RESENT-MESSAGE-ID
This field contains a unique identifier (the local-part
address unit) which refers to THIS version of THIS an, in Lpa