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Rfc Message Header Format For Essay

RFC # 822 Obsoletes: RFC #733 (NIC #41952)

RFC822: Standard for ARPA Internet Text Messages

Revised by David H. Crocker

Dept. of Electrical Engineering

University of Delaware, Newark, DE 19711

Network: DCrocker @ UDel-Relay

Partial Hypertext conversion by Tim Berners-Lee/CERN

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 3.1. General Description 3.2. Header Field Definitions 3.3. Lexical Tokens 3.4. Clarifications 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

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, participated 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 recoding the BNF into an augmented BNF that makes the representation smaller and easier to understand.

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.

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.

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.

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.

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 characters. 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 representation 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 structured fields, such as those containing addresses, limit folding to higher-level syntactic breaks. For address fields, it is recommended that such folding occur 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 composed 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 structured 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 structuring 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 composing the body of the field as a sequence of lexical symbols.

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 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 structured information to other systems, such as mail protocol 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.

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>

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) "]" 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 characters 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 particular, 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

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 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 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 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.

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:
Transformation Reversal
The "current" network's idiosyncracies are removed and the message is returned to the canonical form specified in this standard.
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 ------------------

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 ";" 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 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 fields whose names do not contain this prefix.

4.3. TRACE FIELDS

Trace information is used to provide an audit trail of message 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 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. 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. 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 message. The uniqueness of the message identifier is guaranteed by the host which generates it. This identifier is intended to be machine readable and not necessarily meaningful to humans. A message identifier pertains to exactly one instantiation of a particular message; subsequent revisions to the message should each receive new message identifiers.

4.6.2. IN-REPLY-TO

The contents of this field identify previous correspondence which this message answers. Note that if message identifiers are used in this field, they must use the msg-id specification format.

4.6.3. REFERENCES

The contents of this field identify other correspondence which this message references. Note that if message identifiers are used, they must use the msg-id specification format.

4.6.4. KEYWORDS

This field contains keywords or phrases, separated by commas.

4.7. Other Fields

4.7.1. SUBJECT

This is intended to provide a summary, or indicate the nature, of the message.

4.7.2. COMMENTS

Permits adding text comments onto the message without disturbing the contents of the message's body.

4.7.3. ENCRYPTED

Sometimes, data encryption is used to increase the privacy of message contents. If the body of a message has been encrypted, to keep its contents private, the "Encrypted" field can be used to note the fact and to indicate the nature of the encryption. The first <word> parameter indicates the software used to encrypt the body, and the second, optional <word> is intended to aid the recipient in selecting the proper decryption key. This code word may be viewed as an index to a table of keys held by the recipient. Note: Unfortunately, headers must contain envelope, as well as contents, information. Consequently, it is neces- sary that they remain unencrypted, so that mail tran- sport services may access them. Since names, addresses, and "Subject" field contents may contain sensitive information, this requirement limits total message privacy. Names of encryption software are registered with the Net- work Information Center, SRI International, Menlo Park, Cali- fornia.

4.7.4. EXTENSION-FIELD

A limited number of common fields have been defined in this document. As network mail requirements dictate, addi- tional fields may be standardized. To provide user-defined fields with a measure of safety, in name selection, such extension-fields will never have names that begin with the string "X-". Names of Extension-fields are registered with the Network Information Center, SRI International, Menlo Park, California.

4.7.5. USER-DEFINED-FIELD

Individual users of network mail are free to define and use additional header fields. Such fields must have names which are not already used in the current specification or in any definitions of extension-fields, and the overall syntax of these user-defined-fields must conform to this specification's rules for delimiting and folding fields. Due to the extension-field publishing process, the name of a user- defined-field may be pre-empted Note: The prefatory string "X-" will never be used in the names of Extension-fields. This provides user-defined fields with a protected set of names.

5. Date and Time Specification

5.1. SYNTAX

date-time = [ day "," ] date time ; dd mm yy ; hh:mm:ss zzz day = "Mon" / "Tue" / "Wed" / "Thu" / "Fri" / "Sat" / "Sun" date = 1*2DIGIT month 2DIGIT ; day month year ; e.g. 20 Jun 82 month = "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" / "Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec" time = hour zone ; ANSI and Military hour = 2DIGIT ":" 2DIGIT [":" 2DIGIT] ; 00:00:00 - 23:59:59 zone = "UT" / "GMT" ; Universal Time ; North American : UT / "EST" / "EDT" ; Eastern: - 5/ - 4 / "CST" / "CDT" ; Central: - 6/ - 5 / "MST" / "MDT" ; Mountain: - 7/ - 6 / "PST" / "PDT" ; Pacific: - 8/ - 7 / 1ALPHA ; Military: Z = UT; ; A:-1; (J not used) ; M:-12; N:+1; Y:+12 / ( ("+" / "-") 4DIGIT ) ; Local differential ; hours+min. (HHMM)

5.2. SEMANTICS

If included, day-of-week must be the day implied by the date specification.

Time zone may be indicated in several ways. "UT" is Universal Time (formerly called "Greenwich Mean Time"); "GMT" is permitted as a reference to Universal Time. The military standard uses a single character for each zone. "Z" is Universal Time. "A" indicates one hour earlier, and "M" indicates 12 hours earlier; "N" is one hour later, and "Y" is 12 hours later. The letter "J" is not used. The other remaining two forms are taken from ANSI standard X3.51-1975. One allows explicit indication of the amount of offset from UT; the other uses common 3-character strings for indicating time zones in North America.

6. Address Specification

6.1. Syntax

address = mailbox ; one addressee / group ; named list group = phrase ":" [#mailbox] ";" mailbox = addr-spec ; simple address / phrase route-addr ; name & addr-spec route-addr = "<" [route] addr-spec ">" route = 1#("@" domain) ":" ; path-relative addr-spec = local-part "@" domain ; global address local-part = word *("." word) ; uninterpreted ; case-preserved domain = sub-domain *("." sub-domain) sub-domain = domain-ref / domain-literal domain-ref = atom ; symbolic reference

6.2. SEMANTICS

A mailbox receives mail. It is a conceptual entity which does not necessarily pertain to file storage. For example, some sites may choose to print mail on their line printer and deliver the output to the addressee's desk.

A mailbox specification comprises a person, system or process name reference, a domain-dependent string, and a name-domain reference. The name reference is optional and is usually used to indicate the human name of a recipient. The name-domain reference specifies a sequence of sub-domains. The domain-dependent string is uninterpreted, except by the final sub-domain; the rest of the mail service merely transmits it as a literal string.

6.2.1. DOMAINS

A name-domain is a set of registered (mail) names. A name- domain specification resolves to a subordinate name-domain specification or to a terminal domain-dependent string. Hence, domain specification is extensible, permitting any number of registration levels. Name-domains model a global, logical, hierarchical addressing scheme. The model is logical, in that an address specifica- tion is related to name registration and is not necessarily tied to transmission path. The model's hierarchy is a directed graph, called an in-tree, such that there is a single path from the root of the tree to any node in the hierarchy. If more than one path actually exists, they are considered to be different addresses. The root node is common to all addresses; consequently, it is not referenced. Its children constitute "top-level" name- domains. Usually, a service has access to its own full domain specification and to the names of all top-level name-domains. The "top" of the domain addressing hierarchy -- a child of the root -- is indicated by the right-most field, in a domain specification. Its child is specified to the left, its child to the left, and so on. Some groups provide formal registration services; these con- stitute name-domains that are independent logically of specific machines. In addition, networks and machines impli- citly compose name-domains, since their membership usually is registered in name tables. In the case of formal registration, an organization implements a (distributed) data base which provides an address-to-route mapping service for addresses of the form: person@registry.organization Note that "organization" is a logical entity, separate from any particular communication network. A mechanism for accessing "organization" is universally avail- able. That mechanism, in turn, seeks an instantiation of the registry; its location is not indicated in the address specif- ication. It is assumed that the system which operates under the name "organization" knows how to find a subordinate regis- try. The registry will then use the "person" string to deter- mine where to send the mail specification. The latter, network-oriented case permits simple, direct, attachment-related address specification, such as: user@host.network Once the network is accessed, it is expected that a message will go directly to the host and that the host will resolve the user name, placing the message in the user's mailbox.

6.2.2. ABBREVIATED DOMAIN SPECIFICATION

Since any number of levels is possible within the domain hierarchy, specification of a fully qualified address can become inconvenient. This standard permits abbreviated domain specification, in a special case: For the address of the sender, call the left-most sub-domain Level N. In a header address, if all of the sub-domains above (i.e., to the right of) Level N are the same as those of the sender, then they do not have to appear in the specification. Otherwise, the address must be fully qualified. This feature is subject to approval by local sub- domains. Individual sub-domains may require their member systems, which originate mail, to provide full domain specification only. When permitted, abbrevia- tions may be present only while the message stays within the sub-domain of the sender. Use of this mechanism requires the sender's sub-domain to reserve the names of all top-level domains, so that full specifications can be distinguished from abbrevi- ated specifications. For example, if a sender's address is: sender@registry-A.registry-1.organization-X and one recipient's address is: recipient@registry-B.registry-1.organization-X and another's is: recipient@registry-C.registry-2.organization-X then ".registry-1.organization-X" need not be specified in the the message, but "registry-C.registry-2" DOES have to be specified. That is, the first two addresses may be abbrevi- ated, but the third address must be fully specified. When a message crosses a domain boundary, all addresses must be specified in the full format, ending with the top-level name-domain in the right-most field. It is the responsibility of mail forwarding services to ensure that addresses conform with this requirement. In the case of abbreviated addresses, the relaying service must make the necessary expansions. It should be noted that it often is difficult for such a service to locate all occurrences of address abbreviations. For exam- ple, it will not be possible to find such abbreviations within the body of the message. The "Return-Path" field can aid recipients in recovering from these errors. Note: When passing any portion of an addr-spec onto a process which does not interpret data according to this stan- dard (e.g., mail protocol servers). There must be NO LWSP-chars preceding or following the at-sign or any delimiting period ("."), such as shown in the above examples, and only ONE SPACE between contiguous <word>s.

6.2.3. DOMAIN TERMS

A domain-ref must be THE official name of a registry, network, or host. It is a symbolic reference, within a name sub- domain. At times, it is necessary to bypass standard mechan- isms for resolving such references, using more primitive information, such as a network host address rather than its associated host name. To permit such references, this standard provides the domain- literal construct. Its contents must conform with the needs of the sub-domain in which it is interpreted. Domain-literals which refer to domains within the ARPA Inter- net specify 32-bit Internet addresses, in four 8-bit fields noted in decimal, as described in Request for Comments #820, "Assigned Numbers." For example: [10.0.3.19] Note: THE USE OF DOMAIN-LITERALS IS STRONGLY DISCOURAGED. It is permitted only as a means of bypassing temporary system limitations, such as name tables which are not complete. The names of "top-level" domains, and the names of domains under in the ARPA Internet, are registered with the Network Information Center, SRI International, Menlo Park, California.

6.2.4. DOMAIN-DEPENDENT LOCAL STRING

The local-part of an addr-spec in a mailbox specification (i.e., the host's name for the mailbox) is understood to be whatever the receiving mail protocol server allows. For exam- ple, some systems do not understand mailbox references of the form "P. D. Q. Bach", but others do. This specification treats periods (".") as lexical separators. Hence, their presence in local-parts which are not quoted- strings, is detected. However, such occurrences carry NO semantics. That is, if a local-part has periods within it, an address parser will divide the local-part into several tokens, but the sequence of tokens will be treated as one uninter- preted unit. The sequence will be re-assembled, when the address is passed outside of the system such as to a mail pro- tocol service. For example, the address: First.Last@Registry.Org is legal and does not require the local-part to be surrounded with quotation-marks. (However, "First Last" DOES require quoting.) The local-part of the address, when passed outside of the mail system, within the Registry.Org domain, is "First.Last", again without quotation marks.

6.2.5. BALANCING LOCAL-PART AND DOMAIN

In some cases, the boundary between local-part and domain can be flexible. The local-part may be a simple string, which is used for the final determination of the recipient's mailbox. All other levels of reference are, therefore, part of the domain. For some systems, in the case of abbreviated reference to the local and subordinate sub-domains, it may be possible to specify only one reference within the domain part and place the other, subordinate name-domain references within the local-part. This would appear as: mailbox.sub1.sub2@this-domain Such a specification would be acceptable to address parsers which conform to RFC #733, but do not support this newer Internet standard. While contrary to the intent of this stan- dard, the form is legal. Also, some sub-domains have a specification syntax which does not conform to this standard. For example: sub-net.mailbox@sub-domain.domain uses a different parsing sequence for local-part than for domain. Note: As a rule, the domain specification should contain fields which are encoded according to the syntax of this standard and which contain generally-standardized information. The local-part specification should con- tain only that portion of the address which deviates from the form or intention of the domain field.

6.2.6. MULTIPLE MAILBOXES

An individual may have several mailboxes and wish to receive mail at whatever mailbox is convenient for the sender to access. This standard does not provide a means of specifying "any member of" a list of mailboxes. A set of individuals may wish to receive mail as a single unit (i.e., a distribution list). The <group> construct permits specification of such a list. Recipient mailboxes are speci- fied within the bracketed part (":" - ";"). A copy of the transmitted message is to be sent to each mailbox listed. This standard does not permit recursive specification of groups within groups. While a list must be named, it is not required that the con- tents of the list be included. In this case, the <address> serves only as an indication of group distribution and would appear in the form: name:; Some mail services may provide a group-list distribution facility, accepting a single mailbox reference, expanding it to the full distribution list, and relaying the mail to the list's members. This standard provides no additional syntax for indicating such a service. Using the <group> address alternative, while listing one mailbox in it, can mean either that the mailbox reference will be expanded to a list or that there is a group with one member.

6.2.7. EXPLICIT PATH SPECIFICATION

At times, a message originator may wish to indicate the transmission path that a message should follow. This is called source routing. The normal addressing scheme, used in an addr-spec, is carefully separated from such information; the <route> portion of a route-addr is provided for such occa- sions. It specifies the sequence of hosts and/or transmission services that are to be traversed. Both domain-refs and domain-literals may be used. Note: The use of source routing is discouraged. Unless the sender has special need of path restriction, the choice of transmission route should be left to the mail tran- sport service.

6.3. RESERVED ADDRESS

It often is necessary to send mail to a site, without knowing any of its valid addresses. For example, there may be mail system dysfunctions, or a user may wish to find out a person's correct address, at that site.

This standard specifies a single, reserved mailbox address (local-part) which is to be valid at each site. Mail sent to that address is to be routed to a person responsible for the site's mail system or to a person with responsibility for general site operation. The name of the reserved local-part address is:

Postmaster so that "Postmaster@domain" is required to be valid.

Note: This reserved local-part must be matched without sensitivity to alphabetic case, so that "POSTMASTER", "postmaster", and even "poStmASteR" is to be accepted.

7. Bibliography

ANSI. "USA Standard Code for Information Interchange," X3.4. American National Standards Institute: New York (1968). Also in: Feinler, E. and J. Postel, eds., "ARPANET Protocol Hand- book", NIC 7104. ANSI. "Representations of Universal Time, Local Time Differen- tials, and United States Time Zone References for Information Interchange," X3.51-1975. American National Standards Insti- tute: New York (1975). Bemer, R.W., "Time and the Computer." In: Interface Age (Feb. 1979). Bennett, C.J. "JNT Mail Protocol". Joint Network Team, Ruther- ford and Appleton Laboratory: Didcot, England. Bhushan, A.K., Pogran, K.T., Tomlinson, R.S., and White, J.E. "Standardizing Network Mail Headers," ARPANET Request for Comments No. 561, Network Information Center No. 18516; SRI International: Menlo Park (September 1973). Birrell, A.D., Levin, R., Needham, R.M., and Schroeder, M.D. "Grapevine: An Exercise in Distributed Computing," Communica- tions of the ACM 25, 4 (April 1982), 260-274. Crocker, D.H., Vittal, J.J., Pogran, K.T., Henderson, D.A. "Standard for the Format of ARPA Network Text Message," ARPANET Request for Comments No. 733, Network Information Center No. 41952. SRI International: Menlo Park (November 1977). Feinler, E.J. and Postel, J.B. ARPANET Protocol Handbook, Net- work Information Center No. 7104 (NTIS AD A003890). SRI International: Menlo Park (April 1976). Harary, F. "Graph Theory". Addison-Wesley: Reading, Mass. (1969). Levin, R. and Schroeder, M. "Transport of Electronic Messages through a Network," TeleInformatics 79, pp. 29-33. North Holland (1979). Also as Xerox Palo Alto Research Center Technical Report CSL-79-4. Myer, T.H. and Henderson, D.A. "Message Transmission Protocol," ARPANET Request for Comments, No. 680, Network Information Center No. 32116. SRI International: Menlo Park (1975). NBS. "Specification of Message Format for Computer Based Message Systems, Recommended Federal Information Processing Standard." National Bureau of Standards: Gaithersburg, Maryland (October 1981). NIC. Internet Protocol Transition Workbook. Network Information Center, SRI-International, Menlo Park, California (March 1982). Oppen, D.C. and Dalal, Y.K. "The Clearinghouse: A Decentralized Agent for Locating Named Objects in a Distributed Environ- ment," OPD-T8103. Xerox Office Products Division: Palo Alto, CA. (October 1981). Postel, J.B. "Assigned Numbers," ARPANET Request for Comments, No. 820. SRI International: Menlo Park (August 1982). Postel, J.B. "Simple Mail Transfer Protocol," ARPANET Request for Comments, No. 821. SRI International: Menlo Park (August 1982). Shoch, J.F. "Internetwork naming, addressing and routing," in Proc. 17th IEEE Computer Society International Conference, pp. 72-79, Sept. 1978, IEEE Cat. No. 78 CH 1388-8C. Su, Z. and Postel, J. "The Domain Naming Convention for Internet User Applications," ARPANET Request for Comments, No. 819. SRI International: Menlo Park (August 1982).

APPENDIX A. EXAMPLES

A.1. ADDRESSES

A.1.1. Alfred Neuman <Neuman@BBN-TENEXA>

A.1.2. Neuman@BBN-TENEXA

These two "Alfred Neuman" examples have identical seman- tics, as far as the operation of the local host's mail sending (distribution) program (also sometimes called its "mailer") and the remote host's mail protocol server are concerned. In the first example, the "Alfred Neuman" is ignored by the mailer, as "Neuman@BBN-TENEXA" completely specifies the reci- pient. The second example contains no superfluous informa- tion, and, again, "Neuman@BBN-TENEXA" is the intended reci- pient. Note: When the message crosses name-domain boundaries, then these specifications must be changed, so as to indicate the remainder of the hierarchy, starting with the top level.

A.1.3. "George, Ted" <Shared@Group.Arpanet>

This form might be used to indicate that a single mailbox is shared by several users. The quoted string is ignored by the originating host's mailer, because "Shared@Group.Arpanet" completely specifies the destination mailbox.

A.1.4. Wilt . (the Stilt) Chamberlain@NBA.US

The "(the Stilt)" is a comment, which is NOT included in the destination mailbox address handed to the originating system's mailer. The local-part of the address is the string "Wilt.Chamberlain", with NO space between the first and second words.

A.1.5. Address Lists

Gourmets: Pompous Person <WhoZiWhatZit@Cordon-Bleu>, Childs@WGBH.Boston, Galloping Gourmet@ ANT.Down-Under (Australian National Television), Cheapie@Discount-Liquors;, Cruisers: Port@Portugal, Jones@SEA;, Another@Somewhere.SomeOrg This group list example points out the use of comments and the mixing of addresses and groups.

A.2. ORIGINATOR ITEMS

A.2.1. Author-sent

George Jones logs into his host as "Jones". He sends mail himself. From: Jones@Group.Org or From: George Jones <Jones@Group.Org>

A.2.2. Secretary-sent

George Jones logs in as Jones on his host. His secre- tary, who logs in as Secy sends mail for him. Replies to the mail should go to George. From: George Jones <Jones@Group> Sender: Secy@Other-Group

A.2.3. Secretary-sent, for user of shared directory

George Jones' secretary sends mail for George. Replies should go to George. From: George Jones<Shared@Group.Org> Sender: Secy@Other-Group Note that there need not be a space between "Jones" and the "<", but adding a space enhances readability (as is the case in other examples.

A.2.4. Committee activity, with one author

George is a member of a committee. He wishes to have any replies to his message go to all committee members. From: George Jones <Jones@Host.Net> Sender: Jones@Host Reply-To: The Committee: Jones@Host.Net, Smith@Other.Org, Doe@Somewhere-Else; Note that if George had not included himself in the enumeration of The Committee, he would not have gotten an implicit reply; the presence of the "Reply-to" field SUPER- SEDES the sending of a reply to the person named in the "From" field.

A.2.5. Secretary acting as full agent of author

George Jones asks his secretary (Secy@Host) to send a message for him in his capacity as Group. He wants his secre- tary to handle all replies. From: George Jones <Group@Host> Sender: Secy@Host Reply-To: Secy@Host

A.2.6. Agent for user without online mailbox

A friend of George's, Sarah, is visiting. George's secretary sends some mail to a friend of Sarah in computer- land. Replies should go to George, whose mailbox is Jones at Registry. From: Sarah Friendly <Secy@Registry> Sender: Secy-Name <Secy@Registry> Reply-To: Jones@Registry.

A.2.7. Agent for member of a committee

George's secretary sends out a message which was authored jointly by all the members of a committee. Note that the name of the committee cannot be specified, since <group> names are not permitted in the From field. From: Jones@Host, Smith@Other-Host, Doe@Somewhere-Else Sender: Secy@SHost

A.3. COMPLETE HEADERS

A.3.1. Minimum required

Date: 26 Aug 76 1429 EDT Date: 26 Aug 76 1429 EDT From: Jones@Registry.Org or From: Jones@Registry.Org Bcc: To: Smith@Registry.Org Note that the "Bcc" field may be empty, while the "To" field is required to have at least one address.

A.3.2. Using some of the additional fields

Date: 26 Aug 76 1430 EDT From: George Jones<Group@Host> Sender: Secy@SHOST To: "Al Neuman"@Mad-Host, Sam.Irving@Other-Host Message-ID: <some.string@SHOST>

A.3.3. About as complex as you're going to get

Date : 27 Aug 76 0932 PDT From : Ken Davis <KDavis@This-Host.This-net> Subject : Re: The Syntax in the RFC Sender : KSecy@Other-Host Reply-To : Sam.Irving@Reg.Organization To : George Jones <Group@Some-Reg.An-Org>, Al.Neuman@MAD.Publisher cc : Important folk: Tom Softwood <Balsa@Tree.Root>, "Sam Irving"@Other-Host;, Standard Distribution: /main/davis/people/standard@Other-Host, "<Jones>standard.dist.3"@Tops-20-Host>; Comment : Sam is away on business. He asked me to handle his mail for him. He'll be able to provide a more accurate explanation when he returns next week. In-Reply-To: <some.string@DBM.Group>, George's message X-Special-action: This is a sample of user-defined field- names. There could also be a field-name "Special-action", but its name might later be preempted Message-ID: <4231.629.XYzi-What@Other-Host>

Appendix B. Simple Field Parsing

Some mail-reading software systems may wish to perform only minimal processing, ignoring the internal syntax of structured field-bodies and treating them the same as unstructured-field-bodies. Such software will need only to distinguish:
  • Header fields from the message body,
  • Beginnings of fields from lines which continue fields,
  • Field-names from field-contents.
The abbreviated set of syntactic rules which follows will suffice for this purpose. It describes a limited view of messages and is a subset of the syntactic rules provided in the main part of this specification. One small exception is that the contents of field-bodies consist only of text:

B.1. SYNTAX

message = *field *(CRLF *text) field = field-name ":" [field-body] CRLF field-name = 1*<any CHAR, excluding CTLs, SPACE, and ":"> field-body = *text [CRLF LWSP-char field-body]

B.2. SEMANTICS

Headers occur before the message body and are terminated by a null line (i.e., two contiguous CRLFs). A line which continues a header field begins with a SPACE or HTAB character, while a line beginning a field starts with a printable character which is not a colon. A field-name consists of one or more printable characters (excluding colon, space, and control-characters). A field-name MUST be contained on one line. Upper and lower case are not dis- tinguished when comparing field-names.

Apppendix C. DIFFERENCES FROM RFC #733

The following summarizes the differences between this stan- dard and the one specified in Arpanet Request for Comments #733, "Standard for the Format of ARPA Network Text Messages". The differences are listed in the order of their occurrence in the current specification. C.1. FIELD DEFINITIONS C.1.1. FIELD NAMES These now must be a sequence of printable characters. They may not contain any LWSP-chars. C.2. LEXICAL TOKENS C.2.1. SPECIALS The characters period ("."), left-square bracket ("["), and right-square bracket ("]") have been added. For presentation purposes, and when passing a specification to a system that does not conform to this standard, periods are to be contigu- ous with their surrounding lexical tokens. No linear-white- space is permitted between them. The presence of one LWSP- char between other tokens is still directed. C.2.2. ATOM Atoms may not contain SPACE. C.2.3. SPECIAL TEXT ctext and qtext have had backslash ("\") added to the list of prohibited characters. C.2.4. DOMAINS The lexical tokens <domain-literal> and <dtext> have been added. C.3. MESSAGE SPECIFICATION C.3.1. TRACE The "Return-path:" and "Received:" fields have been specified. C.3.2. FROM The "From" field must contain machine-usable addresses (addr- spec). Multiple addresses may be specified, but named-lists (groups) may not. C.3.3. RESENT The meta-construct of prefacing field names with the string "Resent-" has been added, to indicate that a message has been forwarded by an intermediate recipient. C.3.4. DESTINATION A message must contain at least one destination address field. "To" and "CC" are required to contain at least one address. C.3.5. IN-REPLY-TO The field-body is no longer a comma-separated list, although a sequence is still permitted. C.3.6. REFERENCE The field-body is no longer a comma-separated list, although a sequence is still permitted. C.3.7. ENCRYPTED A field has been specified that permits senders to indicate that the body of a message has been encrypted. C.3.8. EXTENSION-FIELD Extension fields are prohibited from beginning with the char- acters "X-".

C.4. DATE AND TIME SPECIFICATION

C.4.1. SIMPLIFICATION Fewer optional forms are permitted and the list of three- letter time zones has been shortened.

C.5. ADDRESS SPECIFICATION

C.5.1. ADDRESS

The use of quoted-string, and the ":"-atom-":" construct, have been removed. An address now is either a single mailbox reference or is a named list of addresses. The latter indi- cates a group distribution. C.5.2. GROUPS Group lists are now required to to have a name. Group lists may not be nested.

C.5.3. MAILBOX

A mailbox specification may indicate a person's name, as before. Such a named list no longer may specify multiple mailboxes and may not be nested.

C.5.4. ROUTE ADDRESSING

Addresses now are taken to be absolute, global specifications, independent of transmission paths. The <route> construct has been provided, to permit explicit specification of transmis- sion path. RFC #733's use of multiple at-signs ("@") was intended as a general syntax for indicating routing and/or hierarchical addressing. The current standard separates these specifications and only one at-sign is permitted.

C.5.5. AT-SIGN

The string " at " no longer is used as an address delimiter. Only at-sign ("@") serves the function.

C.5.6. DOMAINS

Hierarchical, logical name-domains have been added.

C.6. RESERVED ADDRESS

The local-part "Postmaster" has been reserved, so that users can be guaranteed at least one valid address at a site.

Appendix D. Alphabetical Listing of Syntax Rules

address = mailbox ; one addressee / group ; named list addr-spec = local-part "@" domain ; global address ALPHA = <any ASCII alphabetic character> ; (101-132, 65.- 90.) ; (141-172, 97.-122.) atom = 1*<any CHAR except specials, SPACE and CTLs> authentic = "From" ":" mailbox ; Single author / ( "Sender" ":" mailbox ; Actual submittor "From" ":" 1#mailbox) ; Multiple authors ; or not sender CHAR = <any ASCII character> ; ( 0-177, 0.-127.) comment = "(" *(ctext / quoted-pair / comment) ")" CR = <ASCII CR, carriage return> ; ( 15, 13.) CRLF = CR LF ctext = <any CHAR excluding "(", ; => may be folded ")", "\" & CR, & including linear-white-space> CTL = <any ASCII control ; ( 0- 37, 0.- 31.) character and DEL> ; ( 177, 127.) date = 1*2DIGIT month 2DIGIT ; day month year ; e.g. 20 Jun 82 dates = orig-date ; Original [ resent-date ] ; Forwarded date-time = [ day "," ] date time ; dd mm yy ; hh:mm:ss zzz day = "Mon" / "Tue" / "Wed" / "Thu" / "Fri" / "Sat" / "Sun" delimiters = specials / linear-white-space / comment destination = "To" ":" 1#address ; Primary / "Resent-To" ":" 1#address / "cc" ":" 1#address ; Secondary / "Resent-cc" ":" 1#address / "bcc" ":" #address ; Blind carbon / "Resent-bcc" ":" #address DIGIT = <any ASCII decimal digit> ; ( 60- 71, 48.- 57.) domain = sub-domain *("." sub-domain) domain-literal = "[" *(dtext / quoted-pair) "]" domain-ref = atom ; symbolic reference dtext = <any CHAR excluding "[", ; => may be folded "]", "\" & CR, & including linear-white-space> 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-"> field = field-name ":" [ field-body ] CRLF fields = dates ; Creation time, source ; author id & one 1*destination ; address required *optional-field ; others optional 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> field-name = 1*<any CHAR, excluding CTLs, SPACE, and ":"> group = phrase ":" [#mailbox] ";" hour = 2DIGIT ":" 2DIGIT [":" 2DIGIT] ; 00:00:00 - 23:59:59 HTAB = <ASCII HT, horizontal-tab> ; ( 11, 9.) LF = <ASCII LF, linefeed> ; ( 12, 10.) linear-white-space = 1*([CRLF] LWSP-char) ; semantics = SPACE ; CRLF => folding local-part = word *("." word) ; uninterpreted ; case-preserved LWSP-char = SPACE / HTAB ; semantics = SPACE mailbox = addr-spec ; simple address / phrase route-addr ; name & addr-spec message = fields *( CRLF *text ) ; Everything after ; first null line ; is message body month = "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" / "Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec" msg-id = "<" addr-spec ">" ; Unique message id 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 orig-date = "Date" ":" date-time originator = authentic ; authenticated addr [ "Reply-To" ":" 1#address] ) phrase = 1*word ; Sequence of words qtext = <any CHAR excepting <">, ; => may be folded "\" & CR, and including linear-white-space> quoted-pair = "\" CHAR ; may quote any char quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or ; quoted chars. 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 ";" date-time ; time received resent = resent-authentic [ "Resent-Reply-To" ":" 1#address] ) resent-authentic = = "Resent-From" ":" mailbox / ( "Resent-Sender" ":" mailbox "Resent-From" ":" 1#mailbox ) resent-date = "Resent-Date" ":" date-time return = "Return-path" ":" route-addr ; return address route = 1#("@" domain) ":" ; path-relative route-addr = "<" [route] addr-spec ">" source = [ trace ] ; net traversals originator ; original mail [ resent ] ; forwarded SPACE = <ASCII SP, space> ; ( 40, 32.) specials = "(" / ")" / "<" / ">" / "@" ; Must be in quoted- / "," / ";" / ":" / "\" / <"> ; string, to use / "." / "[" / "]" ; within a word. sub-domain = domain-ref / domain-literal text = <any CHAR, including bare ; => atoms, specials, CR & bare LF, but NOT ; comments and including CRLF> ; quoted-strings are ; NOT recognized. time = hour zone ; ANSI and Military trace = return ; path to sender 1*received ; receipt tags 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> word = atom / quoted-string zone = "UT" / "GMT" ; Universal Time ; North American : UT / "EST" / "EDT" ; Eastern: - 5/ - 4 / "CST" / "CDT" ; Central: - 6/ - 5 / "MST" / "MDT" ; Mountain: - 7/ - 6 / "PST" / "PDT" ; Pacific: - 8/ - 7 / 1ALPHA ; Military: Z = UT; <"> = <ASCII quote mark> ; ( 42, 34.)
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BEST CURRENT PRACTICE

Network Working Group G. Klyne Request for Comments: 3864 Nine by Nine BCP: 90 M. Nottingham Category: Best Current Practice BEA J. Mogul HP Labs September 2004 Registration Procedures for Message Header Fields Status of this Memo This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2004). Abstract This specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. Klyne, et al. Best Current Practice [Page 1]
RFC 3864 Header Field Registration September 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 21.1. Structure of this Document . . . . . . . . . . . . . . . 31.2. Document Terminology and Conventions . . . . . . . . . . 42. Message Header Fields. . . . . . . . . . . . . . . . . . . . . 42.1. Permanent and Provisional Header Fields. . . . . . . . . 42.2. Definitions of Message Header Fields . . . . . . . . . . 52.2.1. Application-specific Message Header Fields. . . . 52.2.2. MIME Header Fields. . . . . . . . . . . . . . . . 63. Registry Usage Requirements. . . . . . . . . . . . . . . . . . 64. Registration Procedure . . . . . . . . . . . . . . . . . . . . 64.1. Header Field Specification . . . . . . . . . . . . . . . 74.2. Registration Templates . . . . . . . . . . . . . . . . . 7 4.2.1. Permanent Message Header Field Registration Template. . . . . . . . . . . . . . . . . . . . . 7 4.2.2. Provisional Message Header Field Submission Template. . . . . . . . . . . . . . . . . . . . . 84.3. Submission of Registration . . . . . . . . . . . . . . . 104.4. Objections to Registration . . . . . . . . . . . . . . . 104.5. Change Control . . . . . . . . . . . . . . . . . . . . . 114.6. Comments on Header Definitions . . . . . . . . . . . . . 124.7. Location of Header Field Registry. . . . . . . . . . . . 125. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 126. Security Considerations. . . . . . . . . . . . . . . . . . . . 127. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 138. References . . . . . . . . . . . . . . . . . . . . . . . . . . 138.1. Normative References . . . . . . . . . . . . . . . . . . 138.2. Informative References . . . . . . . . . . . . . . . . . 139. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 1610. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 171. Introduction This specification defines registration procedures for the message header field names used by Internet mail, HTTP, newsgroup feeds and other Internet applications. It is not intended to be a replacement for protocol-specific registries, such as the SIP registry [30]. Benefits of a central registry for message header field names include: o providing a single point of reference for standardized and widely-used header field names; o providing a central point of discovery for established header fields, and easy location of their defining documents; Klyne, et al. Best Current Practice [Page 2]
RFC 3864 Header Field Registration September 2004 o discouraging multiple definitions of a header field name for different purposes; o helping those proposing new header fields discern established trends and conventions, and avoid names that might be confused with existing ones; o encouraging convergence of header field name usage across multiple applications and protocols. The primary specification for Internet message header fields in email is the Internet mail message format specification, RFC 2822 [4]. HTTP/1.0 [10] and HTTP/1.1 [24] define message header fields (respectively, the HTTP-header and message-header protocol elements) for use with HTTP. RFC 1036 [5] defines message header elements for use with Netnews feeds. These specifications also define a number of header fields, and provide for extension through the use of new field-names. There are many other Internet standards track documents that define additional header fields for use within the same namespaces, notably MIME [11] and related specifications. Other Internet applications that use MIME, such as SIP (RFC 3261 [30]) may also use many of the same header fields (but note that IANA maintains a separate registry of header fields used with SIP). Although in principle each application defines its own set of valid header fields, exchange of messages between applications (e.g., mail to Netnews gateways), common use of MIME encapsulation, and the possibility of common processing for various message types (e.g., a common message archive and retrieval facility) makes it desirable to have a common point of reference for standardized and proposed header fields. Listing header fields together reduces the chance of an accidental collision, and helps implementers find relevant information. The message header field registries defined here serve that purpose. 1.1. Structure of this DocumentSection 2 discusses the purpose of this specification, and indicates some sources of information about defined message header fields. Section 4 defines the message header field name repositories, and sets out requirements and procedures for creating entries in them. Klyne, et al. Best Current Practice [Page 3]
RFC 3864 Header Field Registration September 20041.2. Document Terminology and Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [2]. 2. Message Header Fields2.1. Permanent and Provisional Header Fields Many message header fields are defined in standards-track documents, which means they have been subjected to a process of community review and achieved consensus that they provide a useful and well-founded capability, or represent a widespread use of which developers should be aware. Some are defined for experimental use, typically indicating consensus regarding their purpose but not necessarily concerning their technical details. Many others have been defined and adopted ad-hoc to address a locally occurring requirement; some of these have found widespread use. The catalogues defined here are intended to cater for all of these header fields, while maintaining a clear distinction and status for those which have community consensus. To this end, two repositories are defined: o A Permanent Message Header Field Registry, intended for headers defined in IETF standards-track documents, those that have achieved a comparable level of community review, or are generally recognized to be in widespread use. The assignment policy for such registration is "Specification Required", as defined by RFC2434 [3], where the specification must be published in an RFC (standards-track, experimental, informational or historic), or as an "Open Standard" in the sense of RFC 2026, section 7 [1]. o A Provisional Message Header Field Repository, intended for any header field proposed by any developer, without making any claim about its usefulness or the quality of its definition. The policy for recording these is "Private Use", per RFC 2434 [3]. Neither repository tracks the syntax, semantics or type of field- values. Only the field-names, applicable protocols and status are registered; all other details are specified in the defining documents referenced by repository entries. Significant updates to such references (e.g., the replacement of a Proposed Standard RFC by a Draft Standard RFC, but not necessarily the revision of an Internet- draft) SHOULD be accompanied by updates to the corresponding repository entries. Klyne, et al. Best Current Practice [Page 4]
RFC 3864 Header Field Registration September 20042.2. Definitions of Message Header FieldsRFC 2822 [4] defines a general syntax for message headers, and also defines a number of fields for use with Internet mail. HTTP/1.0 [10] and HTTP/1.1 [24] do likewise for HTTP. Additional field names are defined in a variety of standards-track RFC documents, including: RFC1036 [5], RFC 1496 [6], RFC 1505 [7], RFC 1864 [9], RFC 2156 [14], RFC 2183 [15], RFC 2045 [11], RFC 2046 [12], RFC 2557 [23], RFC 2227 [16], RFC 2231 [17], RFC 2298 [18], RFC 2369 [19], RFC 2421 [21], RFC2518 [22], RFC 2617 [25], RFC 2821 [26], RFC 2912 [27], RFC 2919 [28], RFC 2965 [29], and RFC 3282 [31]. 2.2.1. Application-specific Message Header Fields Internet applications that use similar message headers include Internet mail [26] [4], NNTP newsgroup feeds [5], HTTP web access [24] and any other that uses MIME [11] encapsulation of message content. In some cases (notably HTTP [24]), the header syntax and usage is redefined for the specific application. This registration is concerned only with the allocation and specification of field names, and not with the details of header implementation in specific protocols. In some cases, the same field name may be specified differently (by different documents) for use with different application protocols; e.g., The Date: header field used with HTTP has a different syntax than the Date: used with Internet mail. In other cases, a field name may have a common specification across multiple protocols (ignoring protocol-specific lexical and character set conventions); e.g., this is generally the case for MIME header fields with names of the form 'Content-*'. Thus, we need to accommodate application-specific fields, while wishing to recognize and promote (where appropriate) commonality of other fields across multiple applications. Common repositories are used for all applications, and each registered header field specifies the application protocol for which the corresponding definition applies. A given field name may have multiple registry entries for different protocols; in the Permanent Message Header Field registry, a given header field name may be registered only once for any given protocol. (In some cases, the registration may reference several defining documents.) Klyne, et al. Best Current Practice [Page 5]
RFC 3864 Header Field Registration September 20042.2.2. MIME Header Fields Some header fields with names of the form Content-* are associated with the MIME data object encapsulation and labelling framework. These header fields can meaningfully be applied to a data object separately from the protocol used to carry it. MIME is used with email messages and other protocols that specify a MIME-based data object format. MIME header fields used with such protocols are defined in the registry with the protocol "mime", and as such are presumed to be usable in conjunction with any protocol that conveys MIME objects. Other protocols do not convey MIME objects, but define a number of header fields with similar names and functions to MIME. Notably, HTTP defines a number of entity header fields that serve a purpose in HTTP similar to MIME header fields in email. Some of these header fields have the same names and similar functions to their MIME counterparts (though there are some variations). Such header fields must be registered separately for any non-MIME-carrying protocol with which they may be used. It is poor practice to reuse a header field name from another protocol simply because the fields have similar (even "very similar") meanings. Protocols should share header field names only when their meanings are identical in all foreseeable circumstances. In particular, new header field names of the form Content-* should not be defined for non-MIME-carrying protocols unless their specification is exactly the same as in MIME. 3. Registry Usage Requirements RFCs defining new header fields for Internet mail, HTTP, or MIME MUST include appropriate header registration template(s) (as given in Section 4.2) for all headers defined in the document in their IANA considerations section. Use of the header registry MAY be mandated by other protocol specifications, however, in the absence of such a mandate use of the registry is not required. 4. Registration Procedure The procedure for registering a message header field is: 1. Construct a header field specification 2. Prepare a registration template 3. Submit the registration template Klyne, et al. Best Current Practice [Page 6]
RFC 3864 Header Field Registration September 20044.1. Header Field Specification Registration of a new message header field starts with construction of a proposal that describes the syntax, semantics and intended use of the field. For entries in the Permanent Message Header Field Registry, this proposal MUST be published as an RFC, or as an Open Standard in the sense described by RFC 2026, section 7 [1]. A registered field name SHOULD conform at least to the syntax defined by RFC 2822 [4], section 3.6.8. Further, the "." character is reserved to indicate a naming sub- structure and MUST NOT be included in any registered field name. Currently, no specific sub-structure is defined; if used, any such structure MUST be defined by a standards track RFC document. Header field names may sometimes be used in URIs, URNs and/or XML. To comply with the syntactic constraints of these forms, it is recommended that characters in a registered field name are restricted to those that can be used without escaping in a URI [20] or URN [13], and that are also legal in XML [32] element names. Thus, for maximum flexibility, header field names SHOULD further be restricted to just letters, digits, hyphen ('-') and underscore ('_') characters, with the first character being a letter or underscore. 4.2. Registration Templates The registration template for a message header field may be contained in the defining document, or prepared separately. 4.2.1. Permanent Message Header Field Registration Template A header registered in the Permanent Message Header Field Registry MUST be published as an RFC or as an "Open Standard" in the sense described by RFC 2026, section 7 [1], and MUST have a name which is unique among all the registered permanent field names that may be used with the same application protocol. The registration template has the following form. PERMANENT MESSAGE HEADER FIELD REGISTRATION TEMPLATE: Header field name: The name requested for the new header field. This MUST conform to the header field specification details noted in Section 4.1. Klyne, et al. Best Current Practice [Page 7]
RFC 3864 Header Field Registration September 2004 Applicable protocol: Specify "mail" (RFC 2822), "mime" (RFC 2045), "http" (RFC 2616), "netnews" (RFC 1036), or cite any other standards-track RFC defining the protocol with which the header is intended to be used. Status: Specify "standard", "experimental", "informational", "historic", "obsoleted", or some other appropriate value according to the type and status of the primary document in which it is defined. For non-IETF specifications, those formally approved by other standards bodies should be labelled as "standard"; others may be "informational" or "deprecated" depending on the reason for registration. Author/Change controller: For Internet standards-track, state "IETF". For other open standards, give the name of the publishing body (e.g., ANSI, ISO, ITU, W3C, etc.). For other specifications, give the name, email address, and organization name of the primary specification author. A postal address, home page URI, telephone and fax numbers may also be included. Specification document(s): Reference to document that specifies the header for use with the indicated protocol, preferably including a URI that can be used to retrieve a copy of the document. An indication of the relevant sections MAY also be included, but is not required. Related information: Optionally, citations to additional documents containing further relevant information. (This part of the registry may also be used for IESG comments.) Where a primary specification refers to another document for substantial technical detail, the referenced document is usefully mentioned here. 4.2.2. Provisional Message Header Field Submission Template Registration as a Provisional Message Header Field does not imply any kind of endorsement by the IETF, IANA or any other body. The main requirements for a header field to be included in the provisional repository are that it MUST have a citable specification, and there MUST NOT be a corresponding entry (with same field name and protocol) in the permanent header field registry. Klyne, et al. Best Current Practice [Page 8]
RFC 3864 Header Field Registration September 2004 The specification SHOULD indicate an email address for sending technical comments and discussion of the proposed message header. The submission template has the following form. PROVISIONAL MESSAGE HEADER FIELD SUBMISSION TEMPLATE: Header field name: The name proposed for the new header field. This SHOULD conform to the field name specification details noted in Section 4.1. Applicable protocol: Specify "mail" (RFC 2822), "mime" (RFC 2045), "http" (RFC 2616), "netnews" (RFC 1036), or cite any other standards-track RFC defining the protocol with which the header is intended to be used. Status: Specify: "provisional". This will be updated if and when the header registration is subsequently moved to the permanent registry. Author/Change controller: The name, email address, and organization name of the submission author, who may authorize changes to or retraction of the repository entry. A postal address, home page URI, telephone and fax numbers may also be included. If the proposal comes from a standards body working group, give the name and home page URI of the working group, and an email address for discussion of or comments on the specification. Specification document(s): Reference to document that specifies the header for use with the indicated protocol. The document MUST be an RFC, a current Internet-draft or the URL of a publicly accessible document (so IANA can verify availability of the specification). An indication of the relevant sections MAY also be included, but is not required. NOTE: if the specification is available in printed form only, then an Internet draft containing full reference to the paper document should be published and cited in the registration template. The paper specification MAY be cited under related information. Related information: Optionally, citations to additional documents containing further relevant information. Klyne, et al. Best Current Practice [Page 9]
RFC 3864 Header Field Registration September 20044.3. Submission of Registration The registration template is submitted for incorporation in one of the IANA message header field repositories by one of the following methods: o An IANA considerations section in a defining RFC, calling for registration of the message header and referencing information as required by the registration template within the same document. Registration of the header is then processed as part of the RFC publication process. o Send a copy of the template to the designated email discussion list [33] [34]. Allow a reasonable period - at least 2 weeks - for discussion and comments, then send the template to IANA at the designated email address [35]. IANA will publish the template information if the requested name and the specification document meet the criteria noted in Section 4.1 and Section 4.2.2, unless the IESG or their designated expert have requested that it not be published (see Section 4.4). IESG's designated expert should confirm to IANA that the registration criteria have been satisfied. When a new entry is recorded in the permanent message header field registry, IANA will remove any corresponding entries (with the same field name and protocol) from the provisional registry. 4.4. Objections to Registration Listing of an entry in the provisional repository should not be lightly refused. An entry MAY be refused if there is some credible reason to believe that such registration will be harmful. In the absence of such objection, IANA SHOULD allow any registration that meets the criteria set out in Section 4.1 and Section 4.2.2. Some reasonable grounds for refusal might be: o There is IETF consensus that publication is considered likely to harm the Internet technical infrastructure in some way. o Disreputable or frivolous use of the registration facilities. o The proposal is sufficiently lacking in purpose, or misleading about its purpose, that it can be held to be a waste of time and effort. o Conflict with some current IETF activity. Klyne, et al. Best Current Practice [Page 10]
RFC 3864 Header Field Registration September 2004 Note that objections or disagreements about technical detail are not, of themselves, considered grounds to refuse listing in the provisional repository. After all, one of its purposes is to allow developers to communicate with a view to combining their ideas, expertise and energy to the maximum benefit of the Internet community. Publication in an RFC or other form of Open Standard document (per RFC 2026 [1], section 7) is sufficient grounds for publication in the permanent registry. To assist IANA in determining whether or not there is a sustainable objection to any registration, IESG nominates a designated expert to liaise with IANA about new registrations. For the most part, the designated expert's role is to confirm to IANA that the registration criteria have been satisfied. The IESG or their designated expert MAY require any change or commentary to be attached to any registry entry. The IESG is the final arbiter of any objection. 4.5. Change Control Change control of a header field registration is subject to the same condition as the initial registration; i.e., publication (or reclassification) of an Open Standards specification for a Permanent Message Header Field, or on request of the indicated author/change controller for a Provisional Message Header (like the original submission, subject to review on the designated email discussion list [33].) A change to a permanent message header field registration MAY be requested by the IESG. A change to or retraction of any Provisional Message Header Field Repository entry MAY be requested by the IESG or designated expert. IANA MAY remove any Provisional Message Header Field Repository entry whose corresponding specification document is no longer available (e.g., expired Internet-draft, or URL not resolvable). Anyone may notify IANA of any such cases by sending an email to the designated email address [35]. Before removing an entry for this reason, IANA SHOULD contact the registered Author/Change controller to determine whether a replacement for the specification document (consistent with the requirements of section Section 4.2.2) is available. Klyne, et al. Best Current Practice [Page 11]
RFC 3864 Header Field Registration September 2004 It is intended that entries in the Permanent Message Header Field Registry may be used in the construction of URNs (per RFC 2141 [13]) which have particular requirements for uniqueness and persistence (per RFC 1737 [8]). Therefore, once an entry is made in the Permanent Message Header Registry, the combination of the header name and applicable protocol MUST NOT subsequently be registered for any other purpose. (This is not to preclude revision of the applicable specification(s) within the appropriate IETF Consensus rules, and corresponding updates to the specification citation in the header registration.) 4.6. Comments on Header Definitions Comments on proposed registrations should be sent to the designated email discussion list [33]. 4.7. Location of Header Field Registry The message header field registry is accessible from IANA's web site http://www.iana.org/assignments/message-headers/message-header-index.html5. IANA Considerations This specification calls for: o A new IANA registry for permanent message header fields per Section 4 of this document. The policy for inclusion in this registry is described in Section 4.1 and Section 4.2.1. o A new IANA repository listing provisional message header fields per Section 4 of this document. The policy for inclusion in this registry is described in Section 4.1 and Section 4.2.2. o IESG appoints a designated expert to advise IANA whether registration criteria for proposed registrations have been satisfied. No initial registry entries are provided. 6. Security Considerations No security considerations are introduced by this specification beyond those already inherent in the use of message headers. Klyne, et al. Best Current Practice [Page 12]
RFC 3864 Header Field Registration September 20047. Acknowledgements The shape of the registries described here owes much to energetic discussion of previous versions by many denizens of the IETF-822 mailing list. The authors also gratefully acknowledge the contribution of those who provided valuable feedback on earlier versions of this memo: Charles Lindsey, Dave Crocker, Pete Resnick, Jacob Palme, Ned Freed, Michelle Cotton. 8. References8.1. Normative References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [4] Resnick, P., Ed., "Internet Message Format", RFC 2822, April 2001. 8.2. Informative References [5] Horton, M. and R. Adams, "Standard for interchange of USENET messages", RFC 1036, December 1987. [6] Alvestrand, H., Jordan, K., and J. Romaguera, "Rules for downgrading messages from X.400/88 to X.400/84 when MIME content-types are present in the messages", RFC 1496, August 1993. [7] Costanzo, A., Robinson, D., and R. Ullmann, "Encoding Header Field for Internet Messages", RFC 1505, August 1993. [8] Sollins, K. and L. Masinter, "Functional Requirements for Uniform Resource Names", RFC 1737, December 1994. [9] Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864, October 1995. [10] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. Klyne, et al. Best Current Practice [Page 13]
RFC 3864 Header Field Registration September 2004 [11] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [12] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [13] Moats, R., "URN Syntax", RFC 2141, May 1997. [14] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping between X.400 and RFC 822/MIME", RFC 2156, January 1998. [15] Troost, R., Dorner, S., and K. Moore, "Communicating Presentation Information in Internet Messages: The Content- Disposition Header Field", RFC 2183, August 1997. [16] Mogul, J. and P. Leach, "Simple Hit-Metering and Usage-Limiting for HTTP", RFC 2227, October 1997. [17] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC2231, November 1997. [18] Hansen, T. and G. Vaudreuil, Eds., "Message Disposition Notification", RFC 3798, May 2004. [19] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields", RFC 2369, July 1998. [20] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. [21] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet Mail - version 2 (VPIMv2)", RFC 3801, June 2004. [22] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, "HTTP Extensions for Distributed Authoring -- WEBDAV", RFC 2518, February 1999. [23] Palme, F., Hopmann, A., Shelness, N., and E. Stefferud, "MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)", RFC2557, March 1999. Klyne, et al. Best Current Practice [Page 14]
RFC 3864 Header Field Registration September 2004 [24] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [25] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, June 1999. [26] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [27] Klyne, G., "Indicating Media Features for MIME Content", RFC2912, September 2000. [28] Chandhok, R. and G. Wenger, "List-Id: A Structured Field and Namespace for the Identification of Mailing Lists", RFC 2919, March 2001. [29] Kristol, D. and L. Montulli, "HTTP State Management Mechanism", RFC 2965, October 2000. [30] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [31] Alvestrand, H., "Content Language Headers", RFC 3282, May 2002. [32] Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C Recommendation xml, October 2000, <http://www.w3.org/TR/2000/REC-xml-20001006>. [33] "Mail address for announcement of new header field submissions", Mail address: ietf-message-headers@lists.ietf.org [34] "Mail address for subscription to ietf-message- headers@lists.ietf.org. (DO NOT SEND SUBSCRIPTION REQUESTS TO THE MAILING LIST ITSELF)", Mail address: ietf-message-headers- request@lists.ietf.org [35] "Mail address for submission of new header field templates", Mail address: iana@iana.org Klyne, et al. Best Current Practice [Page 15]
RFC 3864 Header Field Registration September 20049. Authors' Addresses Graham Klyne Nine by Nine EMail: GK-IETF@ninebynine.org URI: http://www.ninebynine.net/ Mark Nottingham BEA Systems 235 Montgomery St. Level 15 San Francisco, CA 94104 USA EMail: mnot@pobox.com Jeffrey C. Mogul HP Labs 1501 Page Mill Road Palo Alto, CA 94304 US EMail: JeffMogul@acm.org Klyne, et al. Best Current Practice [Page 16]
RFC 3864 Header Field Registration September 200410. Full Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Klyne, et al. Best Current Practice [Page 17]
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