Hypertext Transfer Protocol -- HTTP/1.1
draft-lafon-rfc2616bis-latest

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Copyright © The IETF Trust (2007). All Rights Reserved.

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and distributed object management systems, through extension of its request methods, error codes and headers [RFC2324]. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.

HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC2616.

Editorial Note (To be removed by RFC Editor before publication)

Distribution of this document is unlimited. Please send comments to the Hypertext Transfer Protocol (HTTP) mailing list at ietf-http-wg@w3.org, which may be joined by sending a message with subject "subscribe" to ietf-http-wg-request@w3.org. Discussions of the HTTP working group are archived at <http://lists.w3.org/Archives/Public/ietf-http-wg/>. XML versions, latest edits and the issues list for this document are available from <http://www.w3.org/Protocols/HTTP/1.1/rfc2616bis/>.

The purpose of this document is to revise [RFC2616], doing only minimal corrections. For now, it is not planned to advance the standards level of HTTP, thus - if published - the specification will still be a "Proposed Standard" (see [RFC2026]).

The current plan is to incorporate known errata, and to update the specification text according to the current IETF publication guidelines. In particular:

• Incorporate the corrections collected in the RFC2616 errata document (<http://purl.org/NET/http-errata>) (most of the suggested fixes have been applied to draft 01).
• Incorporate corrections for newly discovered and agreed-upon problems, using the HTTP WG mailing list as forum and <http://www.w3.org/Protocols/HTTP/1.1/rfc2616bis/issues/> as issues list.
• Update references, and re-classify them into "Normative" and "Informative", based on the prior work done by Jim Gettys in <http://tools.ietf.org/html/draft-gettys-http-v11-spec-rev-00>.

This document is based on a variant of the original RFC2616 specification formatted using Marshall T. Rose's "xml2rfc" tool (see <http://xml.resource.org>) and therefore deviates from the original text in word wrapping, page breaks, list formatting, reference formatting, whitespace usage and appendix numbering. Otherwise, it is supposed to contain an accurate copy of the original specification text. See <http://www.w3.org/Protocols/HTTP/1.1/rfc2616bis-00-from-rfc2616.diff.html> for a comparison between both documents, as generated by "rfcdiff" (<http://tools.ietf.org/tools/rfcdiff/>).

Table of Contents

• 1.   Introduction
  • 1.1   Purpose
  • 1.2   Requirements
  • 1.3   Terminology
  • 1.4   Overall Operation
• 2.   Notational Conventions and Generic Grammar
  • 2.1   Augmented BNF
  • 2.2   Basic Rules
• 3.   Protocol Parameters
  • 3.1   HTTP Version
  • 3.2   Uniform Resource Identifiers
    • 3.2.1   General Syntax
    • 3.2.2   http URL
    • 3.2.3   URI Comparison
  • 3.3   Date/Time Formats
    • 3.3.1   Full Date
    • 3.3.2   Delta Seconds
  • 3.4   Character Sets
    • 3.4.1   Missing Charset
  • 3.5   Content Codings
  • 3.6   Transfer Codings
    • 3.6.1   Chunked Transfer Coding
  • 3.7   Media Types
    • 3.7.1   Canonicalization and Text Defaults
    • 3.7.2   Multipart Types
  • 3.8   Product Tokens
  • 3.9   Quality Values
  • 3.10   Language Tags
  • 3.11   Entity Tags
  • 3.12   Range Units
• 4.   HTTP Message
  • 4.1   Message Types
  • 4.2   Message Headers
  • 4.3   Message Body
  • 4.4   Message Length
  • 4.5   General Header Fields
• 5.   Request
  • 5.1   Request-Line
    • 5.1.1   Method
    • 5.1.2   Request-URI
  • 5.2   The Resource Identified by a Request
  • 5.3   Request Header Fields
• 6.   Response
  • 6.1   Status-Line
    • 6.1.1   Status Code and Reason Phrase
  • 6.2   Response Header Fields
• 7.   Entity
  • 7.1   Entity Header Fields
  • 7.2   Entity Body
    • 7.2.1   Type
    • 7.2.2   Entity Length
• 8.   Connections
  • 8.1   Persistent Connections
    • 8.1.1   Purpose
    • 8.1.2   Overall Operation
      • 8.1.2.1   Negotiation
      • 8.1.2.2   Pipelining
    • 8.1.3   Proxy Servers
    • 8.1.4   Practical Considerations
  • 8.2   Message Transmission Requirements
    • 8.2.1   Persistent Connections and Flow Control
    • 8.2.2   Monitoring Connections for Error Status Messages
    • 8.2.3   Use of the 100 (Continue) Status
    • 8.2.4   Client Behavior if Server Prematurely Closes Connection
• 9.   Method Definitions
  • 9.1   Safe and Idempotent Methods
    • 9.1.1   Safe Methods
    • 9.1.2   Idempotent Methods
  • 9.2   OPTIONS
  • 9.3   GET
  • 9.4   HEAD
  • 9.5   POST
  • 9.6   PUT
  • 9.7   DELETE
  • 9.8   TRACE
  • 9.9   CONNECT
• 10.   Status Code Definitions
  • 10.1   Informational 1xx
    • 10.1.1   100 Continue
    • 10.1.2   101 Switching Protocols
  • 10.2   Successful 2xx
    • 10.2.1   200 OK
    • 10.2.2   201 Created
    • 10.2.3   202 Accepted
    • 10.2.4   203 Non-Authoritative Information
    • 10.2.5   204 No Content
    • 10.2.6   205 Reset Content
    • 10.2.7   206 Partial Content
  • 10.3   Redirection 3xx
    • 10.3.1   300 Multiple Choices
    • 10.3.2   301 Moved Permanently
    • 10.3.3   302 Found
    • 10.3.4   303 See Other
    • 10.3.5   304 Not Modified
    • 10.3.6   305 Use Proxy
    • 10.3.7   306 (Unused)
    • 10.3.8   307 Temporary Redirect
  • 10.4   Client Error 4xx
    • 10.4.1   400 Bad Request
    • 10.4.2   401 Unauthorized
    • 10.4.3   402 Payment Required
    • 10.4.4   403 Forbidden
    • 10.4.5   404 Not Found
    • 10.4.6   405 Method Not Allowed
    • 10.4.7   406 Not Acceptable
    • 10.4.8   407 Proxy Authentication Required
    • 10.4.9   408 Request Timeout
    • 10.4.10   409 Conflict
    • 10.4.11   410 Gone
    • 10.4.12   411 Length Required
    • 10.4.13   412 Precondition Failed
    • 10.4.14   413 Request Entity Too Large
    • 10.4.15   414 Request-URI Too Long
    • 10.4.16   415 Unsupported Media Type
    • 10.4.17   416 Requested Range Not Satisfiable
    • 10.4.18   417 Expectation Failed
  • 10.5   Server Error 5xx
    • 10.5.1   500 Internal Server Error
    • 10.5.2   501 Not Implemented
    • 10.5.3   502 Bad Gateway
    • 10.5.4   503 Service Unavailable
    • 10.5.5   504 Gateway Timeout
    • 10.5.6   505 HTTP Version Not Supported
• 11.   Access Authentication
• 12.   Content Negotiation
  • 12.1   Server-driven Negotiation
  • 12.2   Agent-driven Negotiation
  • 12.3   Transparent Negotiation
• 13.   Caching in HTTP
  • 13.1   Overview
    • 13.1.1   Cache Correctness
    • 13.1.2   Warnings
    • 13.1.3   Cache-control Mechanisms
    • 13.1.4   Explicit User Agent Warnings
    • 13.1.5   Exceptions to the Rules and Warnings
    • 13.1.6   Client-controlled Behavior
  • 13.2   Expiration Model
    • 13.2.1   Server-Specified Expiration
    • 13.2.2   Heuristic Expiration
    • 13.2.3   Age Calculations
    • 13.2.4   Expiration Calculations
    • 13.2.5   Disambiguating Expiration Values
    • 13.2.6   Disambiguating Multiple Responses
  • 13.3   Validation Model
    • 13.3.1   Last-Modified Dates
    • 13.3.2   Entity Tag Cache Validators
    • 13.3.3   Weak and Strong Validators
    • 13.3.4   Rules for When to Use Entity Tags and Last-Modified Dates
    • 13.3.5   Non-validating Conditionals
  • 13.4   Response Cacheability
  • 13.5   Constructing Responses From Caches
    • 13.5.1   End-to-end and Hop-by-hop Headers
    • 13.5.2   Non-modifiable Headers
    • 13.5.3   Combining Headers
    • 13.5.4   Combining Byte Ranges
  • 13.6   Caching Negotiated Responses
  • 13.7   Shared and Non-Shared Caches
  • 13.8   Errors or Incomplete Response Cache Behavior
  • 13.9   Side Effects of GET and HEAD
  • 13.10   Invalidation After Updates or Deletions
  • 13.11   Write-Through Mandatory
  • 13.12   Cache Replacement
  • 13.13   History Lists
• 14.   Header Field Definitions
  • 14.1   Accept
  • 14.2   Accept-Charset
  • 14.3   Accept-Encoding
  • 14.4   Accept-Language
  • 14.5   Accept-Ranges
  • 14.6   Age
  • 14.7   Allow
  • 14.8   Authorization
  • 14.9   Cache-Control
    • 14.9.1   What is Cacheable
    • 14.9.2   What May be Stored by Caches
    • 14.9.3   Modifications of the Basic Expiration Mechanism
    • 14.9.4   Cache Revalidation and Reload Controls
    • 14.9.5   No-Transform Directive
    • 14.9.6   Cache Control Extensions
  • 14.10   Connection
  • 14.11   Content-Encoding
  • 14.12   Content-Language
  • 14.13   Content-Length
  • 14.14   Content-Location
  • 14.15   Content-MD5
  • 14.16   Content-Range
  • 14.17   Content-Type
  • 14.18   Date
    • 14.18.1   Clockless Origin Server Operation
  • 14.19   ETag
  • 14.20   Expect
  • 14.21   Expires
  • 14.22   From
  • 14.23   Host
  • 14.24   If-Match
  • 14.25   If-Modified-Since
  • 14.26   If-None-Match
  • 14.27   If-Range
  • 14.28   If-Unmodified-Since
  • 14.29   Last-Modified
  • 14.30   Location
  • 14.31   Max-Forwards
  • 14.32   Pragma
  • 14.33   Proxy-Authenticate
  • 14.34   Proxy-Authorization
  • 14.35   Range
    • 14.35.1   Byte Ranges
    • 14.35.2   Range Retrieval Requests
  • 14.36   Referer
  • 14.37   Retry-After
  • 14.38   Server
  • 14.39   TE
  • 14.40   Trailer
  • 14.41   Transfer-Encoding
  • 14.42   Upgrade
  • 14.43   User-Agent
  • 14.44   Vary
  • 14.45   Via
  • 14.46   Warning
  • 14.47   WWW-Authenticate
• 15.   Security Considerations
  • 15.1   Personal Information
    • 15.1.1   Abuse of Server Log Information
    • 15.1.2   Transfer of Sensitive Information
    • 15.1.3   Encoding Sensitive Information in URI's
    • 15.1.4   Privacy Issues Connected to Accept Headers
  • 15.2   Attacks Based On File and Path Names
  • 15.3   DNS Spoofing
  • 15.4   Location Headers and Spoofing
  • 15.5   Content-Disposition Issues
  • 15.6   Authentication Credentials and Idle Clients
  • 15.7   Proxies and Caching
    • 15.7.1   Denial of Service Attacks on Proxies
• 16.   Acknowledgments
  • 16.1   (RFC2616)
  • 16.2   (This Document)
• 17.   References
  • 17.2   Normative References
  • 17.3   Informative References
• Authors' Addresses
• A.   Internet Media Type message/http and application/http
• B.   Internet Media Type multipart/byteranges
• C.   Tolerant Applications
• D.   Differences Between HTTP Entities and RFC 2045 Entities
  • D.1   MIME-Version
  • D.2   Conversion to Canonical Form
  • D.3   Conversion of Date Formats
  • D.4   Introduction of Content-Encoding
  • D.5   No Content-Transfer-Encoding
  • D.6   Introduction of Transfer-Encoding
  • D.7   MHTML and Line Length Limitations
• E.   Additional Features
  • E.1   Content-Disposition
• F.   Compatibility with Previous Versions
  • F.1   Changes from HTTP/1.0
    • F.1.1   Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses
  • F.2   Compatibility with HTTP/1.0 Persistent Connections
  • F.3   Changes from RFC 2068
  • F.4   Changes from RFC 2616
• G.   Change Log (to be removed by RFC Editor before publication)
  • G.1   Since RFC2616
  • G.2   Since draft-lafon-rfc2616bis-00
  • G.3   Since draft-lafon-rfc2616bis-01
  • G.4   Since draft-lafon-rfc2616bis-02
  • G.5   Since draft-lafon-rfc2616bis-03
  • G.6   Since draft-lafon-rfc2616bis-04
• Intellectual Property and Copyright Statements
• Index

Issues list

1. Introduction

1.1 Purpose

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by [RFC1945], improved the protocol by allowing messages to be in the format of MIME-like messages, containing metainformation about the data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration the effects of hierarchical proxies, caching, the need for persistent connections, or virtual hosts. In addition, the proliferation of incompletely-implemented applications calling themselves "HTTP/1.0" has necessitated a protocol version change in order for two communicating applications to determine each other's true capabilities.

This specification defines the protocol referred to as "HTTP/1.1". This protocol includes more stringent requirements than HTTP/1.0 in order to ensure reliable implementation of its features.

Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP allows an open-ended set of methods and headers that indicate the purpose of a request [RFC2324]. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) [RFC1630], as a location (URL) [RFC1738] or name (URN) [RFC1737], for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by Internet mail [RFC2822] as defined by the Multipurpose Internet Mail Extensions (MIME) [RFC2045].

HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems, including those supported by the SMTP [RFC2821], NNTP [RFC3977], FTP [RFC959], Gopher [RFC1436], and WAIS [WAIS] protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications.

1.2 Requirements

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 [RFC2119].

An implementation is not compliant if it fails to satisfy one or more of the MUST or REQUIRED level requirements for the protocols it implements. An implementation that satisfies all the MUST or REQUIRED level and all the SHOULD level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST level requirements but not all the SHOULD level requirements for its protocols is said to be "conditionally compliant."

1.3 Terminology

This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.

connection

A transport layer virtual circuit established between two programs for the purpose of communication.

message

The basic unit of HTTP communication, consisting of a structured sequence of octets matching the syntax defined in Section 4 and transmitted via the connection.

request

An HTTP request message, as defined in Section 5.

response

An HTTP response message, as defined in Section 6.

resource

A network data object or service that can be identified by a URI, as defined in Section 3.2. Resources may be available in multiple representations (e.g. multiple languages, data formats, size, and resolutions) or vary in other ways.

entity

The information transferred as the payload of a request or response. An entity consists of metainformation in the form of entity-header fields and content in the form of an entity-body, as described in Section 7.

representation

An entity included with a response that is subject to content negotiation, as described in Section 12. There may exist multiple representations associated with a particular response status.

content negotiation

The mechanism for selecting the appropriate representation when servicing a request, as described in Section 12. The representation of entities in any response can be negotiated (including error responses).

variant

A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations is termed a `variant'. Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation.

client

A program that establishes connections for the purpose of sending requests.

user agent

The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user tools.

server

An application program that accepts connections in order to service requests by sending back responses. Any given program may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the program's capabilities in general. Likewise, any server may act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.

origin server

The server on which a given resource resides or is to be created.

proxy

An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy MUST implement both the client and server requirements of this specification. A "transparent proxy" is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. A "non-transparent proxy" is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies.

gateway

A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway.

tunnel

An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist when both ends of the relayed connections are closed.

cache

A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.

cacheable

A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cacheability of HTTP responses are defined in Section 13. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular request.

first-hand

A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more proxies. A response is also first-hand if its validity has just been checked directly with the origin server.

explicit expiration time

The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.

heuristic expiration time

An expiration time assigned by a cache when no explicit expiration time is available.

age

The age of a response is the time since it was sent by, or successfully validated with, the origin server.

freshness lifetime

The length of time between the generation of a response and its expiration time.

fresh

A response is fresh if its age has not yet exceeded its freshness lifetime.

stale

A response is stale if its age has passed its freshness lifetime.

semantically transparent

A cache behaves in a "semantically transparent" manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except for hop-by-hop headers) that it would have received had its request been handled directly by the origin server.

validator

A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent copy of an entity.

upstream/downstream

Upstream and downstream describe the flow of a message: all messages flow from upstream to downstream.

inbound/outbound

Inbound and outbound refer to the request and response paths for messages: "inbound" means "traveling toward the origin server", and "outbound" means "traveling toward the user agent"

1.4 Overall Operation

The HTTP protocol is a request/response protocol. A client sends a request to the server in the form of a request method, URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a connection with a server. The server responds with a status line, including the message's protocol version and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible entity-body content. The relationship between HTTP and MIME is described in Appendix D.

Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin server (O).

       request chain ------------------------>
    UA -------------------v------------------- O
       <----------------------- response chain

A more complicated situation occurs when one or more intermediaries are present in the request/response chain. There are three common forms of intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent, receiving requests for a URI in its absolute form, rewriting all or part of the message, and forwarding the reformatted request toward the server identified by the URI. A gateway is a receiving agent, acting as a layer above some other server(s) and, if necessary, translating the requests to the underlying server's protocol. A tunnel acts as a relay point between two connections without changing the messages; tunnels are used when the communication needs to pass through an intermediary (such as a firewall) even when the intermediary cannot understand the contents of the messages.

       request chain -------------------------------------->
    UA -----v----- A -----v----- B -----v----- C -----v----- O
       <------------------------------------- response chain

The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response message that travels the whole chain will pass through four separate connections. This distinction is important because some HTTP communication options may apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along the chain. Although the diagram is linear, each participant may be engaged in multiple, simultaneous communications. For example, B may be receiving requests from many clients other than A, and/or forwarding requests to servers other than C, at the same time that it is handling A's request.

Any party to the communication which is not acting as a tunnel may employ an internal cache for handling requests. The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from O (via C) for a request which has not been cached by UA or A.

          request chain ---------->
       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
          <--------- response chain

Not all responses are usefully cacheable, and some requests may contain modifiers which place special requirements on cache behavior. HTTP requirements for cache behavior and cacheable responses are defined in Section 13.

In fact, there are a wide variety of architectures and configurations of caches and proxies currently being experimented with or deployed across the World Wide Web. These systems include national hierarchies of proxy caches to save transoceanic bandwidth, systems that broadcast or multicast cache entries, organizations that distribute subsets of cached data via CD-ROM, and so on. HTTP systems are used in corporate intranets over high-bandwidth links, and for access via PDAs with low-power radio links and intermittent connectivity. The goal of HTTP/1.1 is to support the wide diversity of configurations already deployed while introducing protocol constructs that meet the needs of those who build web applications that require high reliability and, failing that, at least reliable indications of failure.

HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80 (<http://www.iana.org/assignments/port-numbers>), but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification.

In HTTP/1.0, most implementations used a new connection for each request/response exchange. In HTTP/1.1, a connection may be used for one or more request/response exchanges, although connections may be closed for a variety of reasons (see Section 8.1).

2. Notational Conventions and Generic Grammar

2.1 Augmented BNF

All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar to that used by [RFC822ABNF]. Implementors will need to be familiar with the notation in order to understand this specification. The augmented BNF includes the following constructs:

name = definition

The name of a rule is simply the name itself (without any enclosing "<" and ">") and is separated from its definition by the equal "=" character. White space is only significant in that indentation of continuation lines is used to indicate a rule definition that spans more than one line. Certain basic rules are in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used within definitions whenever their presence will facilitate discerning the use of rule names.

"literal"

Quotation marks surround literal text. Unless stated otherwise, the text is case-insensitive.

rule1 | rule2

Elements separated by a bar ("|") are alternatives, e.g., "yes | no" will accept yes or no.

(rule1 rule2)

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".

*rule

The character "*" preceding an element indicates repetition. The full form is "<n>*<m>element" indicating at least <n> 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.

[rule]

Square brackets enclose optional elements; "[foo bar]" is equivalent to "*1(foo bar)".

N rule

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.

#rule

A construct "#" is defined, similar to "*", for defining lists of elements. The full form is "<n>#<m>element" indicating at least <n> and at most <m> elements, each separated by one or more commas (",") and OPTIONAL linear white space (LWS). This makes the usual form of lists very easy; a rule such as

   ( *LWS element *( *LWS "," *LWS 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 element 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.

; comment

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.

implied *LWS

The grammar described by this specification is word-based. Except where noted otherwise, linear white space (LWS) can be included between any two adjacent words (token or quoted-string), and between adjacent words and separators, without changing the interpretation of a field. At least one delimiter (LWS and/or separators) MUST exist between any two tokens (for the definition of "token" below), since they would otherwise be interpreted as a single token.

2.2 Basic Rules

The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII coded character set is defined by ANSI X3.4-1986 [USASCII].

 I ↓  OCTET          = <any 8-bit sequence of data>
  CHAR           = <any US-ASCII character (octets 0 - 127)>
  UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
  LOALPHA        = <any US-ASCII lowercase letter "a".."z">
  ALPHA          = UPALPHA | LOALPHA
  DIGIT          = <any US-ASCII digit "0".."9">
  CTL            =  I ↓<any US-ASCII control character
                   (octets 0 - 31) and DEL (127)>%x00-1F | %x7F
                   ; (octets 0 - 31) and DEL (127)
  CR             = <US-ASCII CR, carriage return (13)>
  LF             = <US-ASCII LF, linefeed (10)>
  SP             = <US-ASCII SP, space (32)>
  HT             = <US-ASCII HT, horizontal-tab (9)>
  ↑ I ↓<">   DQUOTE         = <US-ASCII double-quote mark (34)>

  CRLF           = CR LF

  LWS            = [CRLF] 1*( SP | HT )

  TEXT           = ↑ I ↓<any OCTET except CTLs,
                   but including LWS>%x20-7E | %x80-FF | LWS
                   ; any OCTET except CTLs, but including LWS

A CRLF is allowed in the definition of TEXT only as part of a header field continuation. It is expected that the folding LWS will be replaced with a single SP before interpretation of the TEXT value.

  HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                 | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

↑ I ↓  ↑ I ↓token          = 1*<any CHAR except CTLs or separators>
  separators     = "(" | ")" | "<" | ">" | "@"
                 | "," | ";" | ":" | "\" | ↑ I ↓<">DQUOTE
                 | "/" | "[" | "]" | "?" | "="
                 | "{" | "}" | SP | HT↑ I ↓
                 
  tchar          = "!" | "#" | "$" | "%" | "&" | "'" | "*" | "+" | "-"
                 | "." | "^" | "_" | "`" | "|" | "~" | DIGIT | ALPHA
                   ; any CHAR except CTLs or separators
  token          = 1*tchar

  comment        = "(" *( ctext | quoted-pair | comment ) ")"
  ctext          = ↑ I ↓<any TEXT excluding "(" and ")">%x20-27 | %x2A-7E | %x80-FF | LWS
                   ; any TEXT excluding "(" and ")"

  quoted-string  = ( ↑ I ↓<">DQUOTE *(qdtext | quoted-pair ) ↑ I ↓<">DQUOTE )
  qdtext         = ↑ I ↓<any TEXT excluding ↑ I ↓<">DQUOTE and "\">%x20-21 | %x23-5B | %x5D-7E | %x80-FF | LWS
                   ; any TEXT excluding ↑ I ↓<">DQUOTE and "\"