Network Working Group R. Fielding Request for Comments: 2068 UC Irvine
Category: Standards Track J. GettysJ. Mogul DEC H. Frystyk T. Berners-Lee MIT/LCS January 1997
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
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".
1 Introduction.............................................7 1.1 Purpose ..............................................7 1.2 Requirements .........................................7 1.3 Terminology ..........................................8 1.4 Overall Operation ...................................11 2 Notational Conventions and Generic Grammar..............13 2.1 Augmented BNF .......................................13 2.2 Basic Rules .........................................15 3 Protocol Parameters.....................................17 3.1 HTTP Version ........................................17
3.2 Uniform Resource Identifiers ........................18 3.2.1 General Syntax ...................................18 3.2.2 http URL .........................................19 3.2.3 URI Comparison ...................................20 3.3 Date/Time Formats ...................................21 3.3.1 Full Date ........................................21 3.3.2 Delta Seconds ....................................22 3.4 Character Sets ......................................22 3.5 Content Codings .....................................23 3.6 Transfer Codings ....................................24 3.7 Media Types .........................................25 3.7.1 Canonicalization and Text Defaults ...............26 3.7.2 Multipart Types ..................................27 3.8 Product Tokens ......................................28 3.9 Quality Values ......................................28 3.10 Language Tags ......................................28 3.11 Entity Tags ........................................29 3.12 Range Units ........................................30 4 HTTP Message............................................30 4.1 Message Types .......................................30 4.2 Message Headers .....................................31 4.3 Message Body ........................................32 4.4 Message Length ......................................32 4.5 General Header Fields ...............................34 5 Request.................................................34 5.1 Request-Line ........................................34 5.1.1 Method ...........................................35 5.1.2 Request-URI ......................................35 5.2 The Resource Identified by a Request ................37 5.3 Request Header Fields ...............................37 6 Response................................................38 6.1 Status-Line .........................................38 6.1.1 Status Code and Reason Phrase ....................39 6.2 Response Header Fields ..............................41 7 Entity..................................................41 7.1 Entity Header Fields ................................41 7.2 Entity Body .........................................42 7.2.1 Type .............................................42 7.2.2 Length ...........................................43 8 Connections.............................................43 8.1 Persistent Connections ..............................43 8.1.1 Purpose ..........................................43 8.1.2 Overall Operation ................................44 8.1.3 Proxy Servers ....................................45 8.1.4 Practical Considerations .........................45 8.2 Message Transmission Requirements ...................46 9 Method Definitions......................................48 9.1 Safe and Idempotent Methods .........................48
9.1.1 Safe Methods .....................................48 9.1.2 Idempotent Methods ...............................49 9.2 OPTIONS .............................................49 9.3 GET .................................................50 9.4 HEAD ................................................50 9.5 POST ................................................51 9.6 PUT .................................................52 9.7 DELETE ..............................................53 9.8 TRACE ...............................................53 10 Status Code Definitions................................53 10.1 Informational 1xx ..................................54 10.1.1 100 Continue ....................................54 10.1.2 101 Switching Protocols .........................54 10.2 Successful 2xx .....................................54 10.2.1 200 OK ..........................................54 10.2.2 201 Created .....................................55 10.2.3 202 Accepted ....................................55 10.2.4 203 Non-Authoritative Information ...............55 10.2.5 204 No Content ..................................55 10.2.6 205 Reset Content ...............................56 10.2.7 206 Partial Content .............................56 10.3 Redirection 3xx ....................................56 10.3.1 300 Multiple Choices ............................57 10.3.2 301 Moved Permanently ...........................57 10.3.3 302 Moved Temporarily ...........................58 10.3.4 303 See Other ...................................58 10.3.5 304 Not Modified ................................58 10.3.6 305 Use Proxy ...................................59 10.4 Client Error 4xx ...................................59 10.4.1 400 Bad Request .................................60 10.4.2 401 Unauthorized ................................60 10.4.3 402 Payment Required ............................60 10.4.4 403 Forbidden ...................................60 10.4.5 404 Not Found ...................................60 10.4.6 405 Method Not Allowed ..........................61 10.4.7 406 Not Acceptable ..............................61 10.4.8 407 Proxy Authentication Required ...............61 10.4.9 408 Request Timeout .............................62 10.4.10 409 Conflict ...................................62 10.4.11 410 Gone .......................................62 10.4.12 411 Length Required ............................63 10.4.13 412 Precondition Failed ........................63 10.4.14 413 Request Entity Too Large ...................63 10.4.15 414 Request-URI Too Long .......................63 10.4.16 415 Unsupported Media Type .....................63 10.5 Server Error 5xx ...................................64 10.5.1 500 Internal Server Error .......................64 10.5.2 501 Not Implemented .............................64
10.5.3 502 Bad Gateway .................................64 10.5.4 503 Service Unavailable .........................64 10.5.5 504 Gateway Timeout .............................64 10.5.6 505 HTTP Version Not Supported ..................65 11 Access Authentication..................................65 11.1 Basic Authentication Scheme ........................66 11.2 Digest Authentication Scheme .......................67 12 Content Negotiation....................................67 12.1 Server-driven Negotiation ..........................68 12.2 Agent-driven Negotiation ...........................69 12.3 Transparent Negotiation ............................70 13 Caching in HTTP........................................70 13.1.1 Cache Correctness ...............................72 13.1.2 Warnings ........................................73 13.1.3 Cache-control Mechanisms ........................74 13.1.4 Explicit User Agent Warnings ....................74 13.1.5 Exceptions to the Rules and Warnings ............75 13.1.6 Client-controlled Behavior ......................75 13.2 Expiration Model ...................................75 13.2.1 Server-Specified Expiration .....................75 13.2.2 Heuristic Expiration ............................76 13.2.3 Age Calculations ................................77 13.2.4 Expiration Calculations .........................79 13.2.5 Disambiguating Expiration Values ................80 13.2.6 Disambiguating Multiple Responses ...............80 13.3 Validation Model ...................................81 13.3.1 Last-modified Dates .............................82 13.3.2 Entity Tag Cache Validators .....................82 13.3.3 Weak and Strong Validators ......................8213.3.4 Rules for When to Use Entity Tags and Last
modified Dates..........................................85 13.3.5 Non-validating Conditionals .....................86 13.4 Response Cachability ...............................86 13.5 Constructing Responses From Caches .................87 13.5.1 End-to-end and Hop-by-hop Headers ...............88 13.5.2 Non-modifiable Headers ..........................88 13.5.3 Combining Headers ...............................89 13.5.4 Combining Byte Ranges ...........................90 13.6 Caching Negotiated Responses .......................90 13.7 Shared and Non-Shared Caches .......................91 13.8 Errors or Incomplete Response Cache Behavior .......91 13.9 Side Effects of GET and HEAD .......................92 13.10 Invalidation After Updates or Deletions ...........92 13.11 Write-Through Mandatory ...........................93 13.12 Cache Replacement .................................93 13.13 History Lists .....................................93 14 Header Field Definitions...............................94 14.1 Accept .............................................95
14.2 Accept-Charset .....................................97 14.3 Accept-Encoding ....................................97 14.4 Accept-Language ....................................98 14.5 Accept-Ranges ......................................99 14.6 Age ................................................99 14.7 Allow .............................................100 14.8 Authorization .....................................100 14.9 Cache-Control .....................................101 14.9.1 What is Cachable ...............................103 14.9.2 What May be Stored by Caches ...................10314.9.3 Modifications of the Basic Expiration Mechanism 104
14.9.4 Cache Revalidation and Reload Controls .........105 14.9.5 No-Transform Directive .........................107 14.9.6 Cache Control Extensions .......................108 14.10 Connection .......................................109 14.11 Content-Base .....................................109 14.12 Content-Encoding .................................110 14.13 Content-Language .................................110 14.14 Content-Length ...................................111 14.15 Content-Location .................................112 14.16 Content-MD5 ......................................113 14.17 Content-Range ....................................114 14.18 Content-Type .....................................116 14.19 Date .............................................116 14.20 ETag .............................................117 14.21 Expires ..........................................117 14.22 From .............................................118 14.23 Host .............................................119 14.24 If-Modified-Since ................................119 14.25 If-Match .........................................121 14.26 If-None-Match ....................................122 14.27 If-Range .........................................123 14.28 If-Unmodified-Since ..............................124 14.29 Last-Modified ....................................124 14.30 Location .........................................125 14.31 Max-Forwards .....................................125 14.32 Pragma ...........................................126 14.33 Proxy-Authenticate ...............................127 14.34 Proxy-Authorization ..............................127 14.35 Public ...........................................127 14.36 Range ............................................128 14.36.1 Byte Ranges ...................................128 14.36.2 Range Retrieval Requests ......................130 14.37 Referer ..........................................131 14.38 Retry-After ......................................131 14.39 Server ...........................................132 14.40 Transfer-Encoding ................................132 14.41 Upgrade ..........................................132
14.42 User-Agent .......................................134 14.43 Vary .............................................134 14.44 Via ..............................................135 14.45 Warning ..........................................137 14.46 WWW-Authenticate .................................139 15 Security Considerations...............................139 15.1 Authentication of Clients .........................139 15.2 Offering a Choice of Authentication Schemes .......140 15.3 Abuse of Server Log Information ...................141 15.4 Transfer of Sensitive Information .................141 15.5 Attacks Based On File and Path Names ..............142 15.6 Personal Information ..............................143 15.7 Privacy Issues Connected to Accept Headers ........143 15.8 DNS Spoofing ......................................144 15.9 Location Headers and Spoofing .....................144 16 Acknowledgments.......................................144 17 References............................................146 18 Authors' Addresses....................................149 19 Appendices............................................150 19.1 Internet Media Type message/http ..................150 19.2 Internet Media Type multipart/byteranges ..........150 19.3 Tolerant Applications .............................15119.4 Differences Between HTTP Entities and
MIME Entities...........................................152 19.4.1 Conversion to Canonical Form ...................152 19.4.2 Conversion of Date Formats .....................153 19.4.3 Introduction of Content-Encoding ...............153 19.4.4 No Content-Transfer-Encoding ...................153 19.4.5 HTTP Header Fields in Multipart Body-Parts .....153 19.4.6 Introduction of Transfer-Encoding ..............154 19.4.7 MIME-Version ...................................154 19.5 Changes from HTTP/1.0 .............................15419.5.1 Changes to Simplify Multi-homed Web Servers and
Conserve IP Addresses .................................155 19.6 Additional Features ...............................156 19.6.1 Additional Request Methods .....................156 19.6.2 Additional Header Field Definitions ............156 19.7 Compatibility with Previous Versions ..............16019.7.1 Compatibility with HTTP/1.0 Persistent
Connections............................................161
1 Introduction
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 that indicate the purpose of a request. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) [3][20], as a location (URL) [4] or name (URN) , 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 as defined by the Multipurpose Internet Mail Extensions (MIME).
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 [16], NNTP [13], FTP [18], Gopher [2], and WAIS [10] protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications.
MUST This word or the adjective "required" means that the item is an absolute requirement of the specification.
SHOULD This word or the adjective "recommended" means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course.
MAY This word or the adjective "optional" means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item.
An implementation is not compliant if it fails to satisfy one or more of the MUST requirements for the protocols it implements. An implementation that satisfies all the MUST and all the SHOULD requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST requirements but not all the SHOULD requirements for its protocols is said to be "conditionally compliant."
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, 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.
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 cachable 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.
cachable A response is cachable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cachability of HTTP responses are defined in section 13. Even if a resource is cachable, 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.
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 cachable, and some requests may contain modifiers which place special requirements on cache behavior. HTTP requirements for cache behavior and cachable 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, 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
name = definitionThe name of a rule is simply the name itself (without any enclosing "<" and ">") and is separated from its definition by the equal "=" character. Whitespace 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 whitespace (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 whitespace (LWS) can be included between any two adjacent words (token or quoted-string), and between adjacent tokens and delimiters (tspecials), without changing the interpretation of a field. At least one delimiter (tspecials) must exist between any two tokens, since they would otherwise be interpreted as a single token.
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 = <any US-ASCII control character(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)>
<"> = <US-ASCII double-quote mark (34)>
HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all protocol elements except the entity-body (see appendix 19.3 for tolerant applications). The end-of-line marker within an entity-body is defined by its associated media type, as described in section 3.7.
HTTP/1.1 headers can be folded onto multiple lines if the continuation line begins with a space or horizontal tab. All linear white space, including folding, has the same semantics as SP.CRLF = CR LF
The TEXT rule is only used for descriptive field contents and values that are not intended to be interpreted by the message parser. Words of *TEXT may contain characters from character sets other than ISO 8859-1 [22] only when encoded according to the rules of RFC 1522 [14].LWS = [CRLF] 1*( SP | HT )
TEXT = <any OCTET except CTLs,but including LWS>
HEX = "A" | "B" | "C" | "D" | "E" | "F"| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
Many HTTP/1.1 header field values consist of words separated by LWS or special characters. These special characters MUST be in a quoted string to be used within a parameter value.
token = 1*<any CHAR except CTLs or tspecials>
tspecials = "(" | ")" | "<" | ">" | "@"| "," | ";" | ":" | "\" | <"> | "/" | "[" | "]" | "?" | "=" | "{" | "}" | SP | HT
Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing "comment" as part of their field value definition. In all other fields, parentheses are considered part of the field value.
comment = "(" *( ctext | comment ) ")"
ctext = <any TEXT excluding "(" and ")">
A string of text is parsed as a single word if it is quoted using double-quote marks.
quoted-string = ( <"> *(qdtext) <"> )
qdtext = <any TEXT except <">>The backslash character ("\") may be used as a single-character quoting mechanism only within quoted-string and comment constructs.
quoted-pair = "\" CHAR3 Protocol Parameters
The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message.
Note that the major and minor numbers MUST be treated as separate integers and that each may be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and MUST NOT be sent.HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Applications sending Request or Response messages, as defined by this specification, MUST include an HTTP-Version of "HTTP/1.1". Use of this version number indicates that the sending application is at least conditionally compliant with this specification.
The HTTP version of an application is the highest HTTP version for which the application is at least conditionally compliant.
Proxy and gateway applications must be careful when forwarding messages in protocol versions different from that of the application. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway MUST never send a message with a version indicator which is greater than its actual version; if a higher version request is received, the proxy/gateway MUST either downgrade the request version, respond with an error, or switch to tunnel behavior. Requests with a version lower than that of the proxy/gateway's version MAY be upgraded before being forwarded; the proxy/gateway's response to that request MUST be in the same major version as the request.
Note: Converting between versions of HTTP may involve modification of header fields required or forbidden by the versions involved.
URI = ( absoluteURI | relativeURI ) [ "#" fragment ]
absoluteURI = scheme ":" *( uchar | reserved )
relativeURI = net_path | abs_path | rel_path
net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
path = fsegment *( "/" segment )
fsegment = 1*pchar
segment = *pchar
params = param *( ";" param )
param = *( pchar | "/" )
scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." )
net_loc = *( pchar | ";" | "?" )
query = *( uchar | reserved )
fragment = *( uchar | reserved )
pchar = uchar | ":" | "@" | "&" | "=" | "+"
uchar = unreserved | escape
unreserved = ALPHA | DIGIT | safe | extra | national
escape = "%" HEX HEX
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
safe = "$" | "-" | "_" | "."
unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">"
national = <any OCTET excluding ALPHA, DIGIT,reserved, extra, safe, and unsafe>
For definitive information on URL syntax and semantics, see RFC 1738 [4] and RFC 1808 [11]. The BNF above includes national characters not allowed in valid URLs as specified by RFC 1738, since HTTP servers are not restricted in the set of unreserved characters allowed to represent the rel_path part of addresses, and HTTP proxies may receive requests for URIs not defined by RFC 1738.
The HTTP protocol does not place any a priori limit on the length of a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return 414 (Request-URI Too Long) status if a URI is longer than the server can handle (see section 10.4.15).
Note: Servers should be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations may not properly support these lengths.
http_URL = "http:" "//" host [ ":" port ] [ abs_path ]
host = <A legal Internet host domain nameor IP address (in dotted-decimal form), as defined by Section 2.1 of RFC 1123>
port = *DIGITIf the port is empty or not given, port 80 is assumed. The semantics are that the identified resource is located at the server listening for TCP connections on that port of that host, and the Request-URI for the resource is abs_path. The use of IP addresses in URL's SHOULD be avoided whenever possible (see RFC 1900 [24]). If the abs_path is not present in the URL, it MUST be given as "/" when used as a Request-URI for a resource (section 5.1.2).
o A port that is empty or not given is equivalent to the default port for that URI;
o Comparisons of host names MUST be case-insensitive;
o Comparisons of scheme names MUST be case-insensitive;
o An empty abs_path is equivalent to an abs_path of "/".
Characters other than those in the "reserved" and "unsafe" sets (see section 3.2) are equivalent to their ""%" HEX HEX" encodings.
HTTP applications have historically allowed three different formats for the representation of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123 Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036 Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123 (an update to RFC 822). The second format is in common use, but is based on the obsolete RFC 850 [12] date format and lacks a four-digit year. HTTP/1.1 clients and servers that parse the date value MUST accept all three formats (for compatibility with HTTP/1.0), though they MUST only generate the RFC 1123 format for representing HTTP-date values in header fields.
Note: Recipients of date values are encouraged to be robust in accepting date values that may have been sent by non-HTTP applications, as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.
All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for time zone, and MUST be assumed when reading the asctime format.
HTTP-date = rfc1123-date | rfc850-date | asctime-date
rfc1123-date = wkday "," SP date1 SP time SP "GMT"
rfc850-date = weekday "," SP date2 SP time SP "GMT"
asctime-date = wkday SP date3 SP time SP 4DIGIT
date1 = 2DIGIT SP month SP 4DIGIT; day month year (e.g., 02 Jun 1982)
date2 = 2DIGIT "-" month "-" 2DIGIT; day-month-year (e.g., 02-Jun-82)
date3 = month SP ( 2DIGIT | ( SP 1DIGIT )); month day (e.g., Jun 2)
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT; 00:00:00 - 23:59:59
wkday = "Mon" | "Tue" | "Wed"| "Thu" | "Fri" | "Sat" | "Sun"
weekday = "Monday" | "Tuesday" | "Wednesday"| "Thursday" | "Friday" | "Saturday" | "Sunday"
month = "Jan" | "Feb" | "Mar" | "Apr"| "May" | "Jun" | "Jul" | "Aug" | "Sep" | "Oct" | "Nov" | "Dec"
Note: HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers are not required to use these formats for user presentation, request logging, etc.
delta-seconds = 1*DIGIT
The term "character set" is used in this document to refer to a method used with one or more tables to convert a sequence of octets into a sequence of characters. Note that unconditional conversion in the other direction is not required, in that not all characters may be available in a given character set and a character set may provide more than one sequence of octets to represent a particular character. This definition is intended to allow various kinds of character encodings, from simple single-table mappings such as US- ASCII to complex table switching methods such as those that use ISO 2022's techniques. However, the definition associated with a MIME character set name MUST fully specify the mapping to be performed from octets to characters. In particular, use of external profiling information to determine the exact mapping is not permitted.
Note: This use of the term "character set" is more commonly referred to as a "character encoding." However, since HTTP and MIME share the same registry, it is important that the terminology also be shared.
HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character Set registry [19].
charset = tokenAlthough HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA Character Set registry MUST represent the character set defined by that registry. Applications SHOULD limit their use of character sets to those defined by the IANA registry.
content-coding = tokenAll content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (section 14.3) and Content-Encoding (section 14.12) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding mechanism will be required to remove the encoding.
The Internet Assigned Numbers Authority (IANA) acts as a registry for content-coding value tokens. Initially, the registry contains the following tokens:
gzip An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a LempelZiv coding (LZ77) with a 32 bit CRC.
compress The encoding format produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch coding (LZW).
Note: Use of program names for the identification of encoding formats is not desirable and should be discouraged for future encodings. Their use here is representative of historical practice, not good design. For compatibility with previous implementations of HTTP, applications should consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.
deflate The "zlib" format defined in RFC 1950[31] in combination with the "deflate" compression mechanism described in RFC 1951[29].
New content-coding value tokens should be registered; to allow interoperability between clients and servers, specifications of the content coding algorithms needed to implement a new value should be publicly available and adequate for independent implementation, and conform to the purpose of content coding defined in this section.
transfer-coding = "chunked" | transfer-extension
transfer-extension = tokenAll transfer-coding values are case-insensitive. HTTP/1.1 uses transfer coding values in the Transfer-Encoding header field (section 14.40).
Transfer codings are analogous to the Content-Transfer-Encoding values of MIME , which were designed to enable safe transport of binary data over a 7-bit transport service. However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message-bodies is the difficulty in determining the exact body length (section 7.2.2), or the desire to encrypt data over a shared transport.
The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an optional footer containing entity-header fields. This allows dynamically-produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.
Chunked-Body = *chunk"0" CRLF footer CRLF
chunk = chunk-size [ chunk-ext ] CRLFchunk-data CRLF
hex-no-zero = <HEX excluding "0">
chunk-size = hex-no-zero *HEX
chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
chunk-ext-name = token
chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET)
footer = *entity-headerThe chunked encoding is ended by a zero-sized chunk followed by the footer, which is terminated by an empty line. The purpose of the footer is to provide an efficient way to supply information about an entity that is generated dynamically; applications MUST NOT send header fields in the footer which are not explicitly defined as being appropriate for the footer, such as Content-MD5 or future extensions to HTTP for digital signatures or other facilities.
An example process for decoding a Chunked-Body is presented in appendix 19.4.6.
All HTTP/1.1 applications MUST be able to receive and decode the "chunked" transfer coding, and MUST ignore transfer coding extensions they do not understand. A server which receives an entity-body with a transfer-coding it does not understand SHOULD return 501 (Unimplemented), and close the connection. A server MUST NOT send transfer-codings to an HTTP/1.0 client.
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = tokenParameters may follow the type/subtype in the form of attribute/value pairs.
parameter = attribute "=" value
attribute = token
value = token | quoted-stringThe type, subtype, and parameter attribute names are caseinsensitive. Parameter values may or may not be case-sensitive, depending on the semantics of the parameter name. Linear white space (LWS) MUST NOT be used between the type and subtype, nor between an attribute and its value. User agents that recognize the media-type MUST process (or arrange to be processed by any external applications used to process that type/subtype by the user agent) the parameters for that MIME type as described by that type/subtype definition to the and inform the user of any problems discovered.
Note: some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications, implementations should only use media type parameters when they are required by that type/subtype definition.
Media-type values are registered with the Internet Assigned Number Authority (IANA). The media type registration process is outlined in RFC 2048 [17]. Use of non-registered media types is discouraged.
When in canonical form, media subtypes of the "text" type use CRLF as the text line break. HTTP relaxes this requirement and allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an entire entity-body. HTTP applications MUST accept CRLF, bare CR, and bare LF as being representative of a line break in text media received via HTTP. In addition, if the text is represented in a character set that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character sets, HTTP allows the use of whatever octet sequences are defined by that character set to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the entity-body; a bare CR or LF MUST NOT be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries).
If an entity-body is encoded with a Content-Encoding, the underlying data MUST be in a form defined above prior to being encoded.
The "charset" parameter is used with some media types to define the character set (section 3.4) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or its subsets MUST be labeled with an appropriate charset value.
Some HTTP/1.0 software has interpreted a Content-Type header without charset parameter incorrectly to mean "recipient should guess." Senders wishing to defeat this behavior MAY include a charset parameter even when the charset is ISO-8859-1 and SHOULD do so when it is known that it will not confuse the recipient.
Unfortunately, some older HTTP/1.0 clients did not deal properly with an explicit charset parameter. HTTP/1.1 recipients MUST respect the charset label provided by the sender; and those user agents that have a provision to "guess" a charset MUST use the charset from the content-type field if they support that charset, rather than the recipient's preference, when initially displaying a document.
In HTTP, multipart body-parts MAY contain header fields which are significant to the meaning of that part. A Content-Location header field (section 14.15) SHOULD be included in the body-part of each enclosed entity that can be identified by a URL.
In general, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in RFC 1867 [15].
product = token ["/" product-version]
product-version = token
User-Agent: CERN-LineMode/2.15 libwww/2.17b3 Server: Apache/0.8.4Product tokens should be short and to the point -- use of them for advertising or other non-essential information is explicitly forbidden. Although any token character may appear in a productversion, this token SHOULD only be used for a version identifier (i.e., successive versions of the same product SHOULD only differ in the product-version portion of the product value).
qvalue = ( "0" [ "." 0*3DIGIT ] )| ( "1" [ "." 0*3("0") ] )
"Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.
The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 [1]. In summary, a language tag is composed of 1 or more parts: A primary language tag and a possibly empty series of subtags:
language-tag = primary-tag *( "-" subtag )
primary-tag = 1*8ALPHA
subtag = 1*8ALPHAWhitespace is not allowed within the tag and all tags are caseinsensitive. The name space of language tags is administered by the IANA. Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin
where any two-letter primary-tag is an ISO 639 language abbreviation and any two-letter initial subtag is an ISO 3166 country code. (The last three tags above are not registered tags; all but the last are examples of tags which could be registered in future.)
entity-tag = [ weak ] opaque-tag
weak = "W/"
opaque-tag = quoted-stringA "strong entity tag" may be shared by two entities of a resource only if they are equivalent by octet equality.
A "weak entity tag," indicated by the "W/" prefix, may be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no significant change in semantics. A weak entity tag can only be used for weak comparison.
An entity tag MUST be unique across all versions of all entities associated with a particular resource. A given entity tag value may be used for entities obtained by requests on different URIs without implying anything about the equivalence of those entities.
range-unit = bytes-unit | other-range-unit
bytes-unit = "bytes"
other-range-unit = tokenThe only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 implementations may ignore ranges specified using other units. HTTP/1.1 has been designed to allow implementations of applications that do not depend on knowledge of ranges.
4 HTTP Message
Request (section 5) and Response (section 6) messages use the generic message format of RFC 822 [9] for transferring entities (the payload of the message). Both types of message consist of a start-line, one or more header fields (also known as "headers"), an empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header fields, and an optional message-body.HTTP-message = Request | Response ; HTTP/1.1 messages
generic-message = start-line*message-header CRLF [ message-body ]
start-line = Request-Line | Status-LineIn the interest of robustness, servers SHOULD ignore any empty line(s) received where a Request-Line is expected. In other words, if the server is reading the protocol stream at the beginning of a message and receives a CRLF first, it should ignore the CRLF.
Note: certain buggy HTTP/1.0 client implementations generate an extra CRLF's after a POST request. To restate what is explicitly forbidden by the BNF, an HTTP/1.1 client must not preface or follow a request with an extra CRLF.
message-header = field-name ":" [ field-value ] CRLF
field-name = token
field-value = *( field-content | LWS )
field-content = <the OCTETs making up the field-valueand consisting of either *TEXT or combinations of token, tspecials, and quoted-string>
The order in which header fields with differing field names are received is not significant. However, it is "good practice" to send general-header fields first, followed by request-header or responseheader fields, and ending with the entity-header fields.
Multiple message-header fields with the same field-name may be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.
message-body = entity-body| <entity-body encoded as per Transfer-Encoding>
Transfer-Encoding MUST be used to indicate any transfer codings applied by an application to ensure safe and proper transfer of the message. Transfer-Encoding is a property of the message, not of the entity, and thus can be added or removed by any application along the request/response chain.
The rules for when a message-body is allowed in a message differ for requests and responses.
The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field in the request's message-headers. A message-body MAY be included in a request only when the request method (section 5.1.1) allows an entity-body.
For response messages, whether or not a message-body is included with a message is dependent on both the request method and the response status code (section 6.1.1). All responses to the HEAD request method MUST NOT include a message-body, even though the presence of entityheader fields might lead one to believe they do. All 1xx (informational), 204 (no content), and 304 (not modified) responses MUST NOT include a message-body. All other responses do include a message-body, although it may be of zero length.
1. Any response message which MUST NOT include a message-body (such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated by the first empty line after the header fields, regardless of the entity-header fields present in the message.
2. If a Transfer-Encoding header field (section 14.40) is present and indicates that the "chunked" transfer coding has been applied, then
the length is defined by the chunked encoding (section 3.6).
3. If a Content-Length header field (section 14.14) is present, its value in bytes represents the length of the message-body.
4. If the message uses the media type "multipart/byteranges", which is self-delimiting, then that defines the length. This media type MUST NOT be used unless the sender knows that the recipient can parse it; the presence in a request of a Range header with multiple byte-range specifiers implies that the client can parse multipart/byteranges responses.
5. By the server closing the connection. (Closing the connection cannot be used to indicate the end of a request body, since that would leave no possibility for the server to send back a response.)
For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing a message-body MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. If a request contains a message-body and a Content-Length is not given, the server SHOULD respond with 400 (bad request) if it cannot determine the length of the message, or with 411 (length required) if it wishes to insist on receiving a valid Content-Length.
All HTTP/1.1 applications that receive entities MUST accept the "chunked" transfer coding (section 3.6), thus allowing this mechanism to be used for messages when the message length cannot be determined in advance.
Messages MUST NOT include both a Content-Length header field and the "chunked" transfer coding. If both are received, the Content-Length MUST be ignored.
When a Content-Length is given in a message where a message-body is allowed, its field value MUST exactly match the number of OCTETs in the message-body. HTTP/1.1 user agents MUST notify the user when an invalid length is received and detected.
general-header = Cache-Control ; Section 14.9| Connection ; Section 14.10 | Date ; Section 14.19 | Pragma ; Section 14.32 | Transfer-Encoding ; Section 14.40 | Upgrade ; Section 14.41 | Via ; Section 14.44
General-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields may be given the semantics of general header fields if all parties in the communication recognize them to be general-header fields. Unrecognized header fields are treated as entity-header fields.
Request = Request-Line ; Section 5.1*( general-header ; Section 4.5 | request-header ; Section 5.3 | entity-header ) ; Section 7.1 CRLF [ message-body ] ; Section 7.2
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
Method = "OPTIONS" ; Section 9.2| "GET" ; Section 9.3 | "HEAD" ; Section 9.4 | "POST" ; Section 9.5 | "PUT" ; Section 9.6 | "DELETE" ; Section 9.7 | "TRACE" ; Section 9.8 | extension-method
extension-method = tokenThe list of methods allowed by a resource can be specified in an Allow header field (section 14.7). The return code of the response always notifies the client whether a method is currently allowed on a resource, since the set of allowed methods can change dynamically. Servers SHOULD return the status code 405 (Method Not Allowed) if the method is known by the server but not allowed for the requested resource, and 501 (Not Implemented) if the method is unrecognized or not implemented by the server. The list of methods known by a server can be listed in a Public response-header field (section 14.35).
The methods GET and HEAD MUST be supported by all general-purpose servers. All other methods are optional; however, if the above methods are implemented, they MUST be implemented with the same semantics as those specified in section 9.
The three options for Request-URI are dependent on the nature of the request. The asterisk "*" means that the request does not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily apply to a resource. One example would beRequest-URI = "*" | absoluteURI | abs_path
from a valid cache, and return the response. Note that the proxy MAY forward the request on to another proxy or directly to the server specified by the absoluteURI. In order to avoid request loops, a proxy MUST be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. An example Request-Line would be:
The most common form of Request-URI is that used to identify a resource on an origin server or gateway. In this case the absolute path of the URI MUST be transmitted (see section 3.2.1, abs_path) as the Request-URI, and the network location of the URI (net_loc) MUST be transmitted in a Host header field. For example, a client wishing to retrieve the resource above directly from the origin server would create a TCP connection to port 80 of the host "www.w3.org" and send the lines:
GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.w3.orgfollowed by the remainder of the Request. Note that the absolute path cannot be empty; if none is present in the original URI, it MUST be given as "/" (the server root).
If a proxy receives a request without any path in the Request-URI and the method specified is capable of supporting the asterisk form of request, then the last proxy on the request chain MUST forward the request with "*" as the final Request-URI. For example, the request
OPTIONS * HTTP/1.1
Host: www.ics.uci.edu:8001after connecting to port 8001 of host "www.ics.uci.edu".
The Request-URI is transmitted in the format specified in section 3.2.1. The origin server MUST decode the Request-URI in order to properly interpret the request. Servers SHOULD respond to invalid Request-URIs with an appropriate status code.
In requests that they forward, proxies MUST NOT rewrite the "abs_path" part of a Request-URI in any way except as noted above to replace a null abs_path with "*", no matter what the proxy does in its internal implementation.
Note: The "no rewrite" rule prevents the proxy from changing the meaning of the request when the origin server is improperly using a non-reserved URL character for a reserved purpose. Implementers should be aware that some pre-HTTP/1.1 proxies have been known to rewrite the Request-URI.
An origin server that does not allow resources to differ by the requested host MAY ignore the Host header field value. (But see section 19.5.1 for other requirements on Host support in HTTP/1.1.)
An origin server that does differentiate resources based on the host requested (sometimes referred to as virtual hosts or vanity hostnames) MUST use the following rules for determining the requested resource on an HTTP/1.1 request:
1. If Request-URI is an absoluteURI, the host is part of the Request-URI. Any Host header field value in the request MUST be ignored.
2. If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host header field value.
3. If the host as determined by rule 1 or 2 is not a valid host on the server, the response MUST be a 400 (Bad Request) error message.
Recipients of an HTTP/1.0 request that lacks a Host header field MAY attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine what exact resource is being requested.
equivalent to the parameters on a programming language method invocation.
request-header = Accept ; Section 14.1| Accept-Charset ; Section 14.2 | Accept-Encoding ; Section 14.3 | Accept-Language ; Section 14.4 | Authorization ; Section 14.8 | From ; Section 14.22 | Host ; Section 14.23 | If-Modified-Since ; Section 14.24 | If-Match ; Section 14.25 | If-None-Match ; Section 14.26 | If-Range ; Section 14.27 | If-Unmodified-Since ; Section 14.28 | Max-Forwards ; Section 14.31 | Proxy-Authorization ; Section 14.34 | Range ; Section 14.36 | Referer ; Section 14.37 | User-Agent ; Section 14.42
Request-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of requestheader fields if all parties in the communication recognize them to be request-header fields. Unrecognized header fields are treated as entity-header fields.
Response = Status-Line ; Section 6.1*( general-header ; Section 4.5 | response-header ; Section 6.2 | entity-header ) ; Section 7.1 CRLF [ message-body ] ; Section 7.2
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit:
o 1xx: Informational - Request received, continuing process
o 2xx: Success - The action was successfully received, understood, and accepted
o 3xx: Redirection - Further action must be taken in order to complete the request
o 4xx: Client Error - The request contains bad syntax or cannot be fulfilled
o 5xx: Server Error - The server failed to fulfill an apparently valid request
The individual values of the numeric status codes defined for HTTP/1.1, and an example set of corresponding Reason-Phrase's, are presented below. The reason phrases listed here are only recommended -- they may be replaced by local equivalents without affecting the protocol.
Status-Code = "100" ; Continue| "101" ; Switching Protocols | "200" ; OK | "201" ; Created | "202" ; Accepted | "203" ; Non-Authoritative Information | "204" ; No Content | "205" ; Reset Content | "206" ; Partial Content | "300" ; Multiple Choices | "301" ; Moved Permanently | "302" ; Moved Temporarily
| "303" ; See Other | "304" ; Not Modified | "305" ; Use Proxy | "400" ; Bad Request | "401" ; Unauthorized | "402" ; Payment Required | "403" ; Forbidden | "404" ; Not Found | "405" ; Method Not Allowed | "406" ; Not Acceptable | "407" ; Proxy Authentication Required | "408" ; Request Time-out | "409" ; Conflict | "410" ; Gone | "411" ; Length Required | "412" ; Precondition Failed | "413" ; Request Entity Too Large | "414" ; Request-URI Too Large | "415" ; Unsupported Media Type | "500" ; Internal Server Error | "501" ; Not Implemented | "502" ; Bad Gateway | "503" ; Service Unavailable | "504" ; Gateway Time-out | "505" ; HTTP Version not supported | extension-code
extension-code = 3DIGIT
HTTP status codes are extensible. HTTP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications MUST understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class, with the exception that an unrecognized response MUST NOT be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents SHOULD present to the user the entity returned with the response, since that entity is likely to include humanreadable information which will explain the unusual status.Reason-Phrase = *<TEXT, excluding CR, LF>
response-header = Age ; Section 14.6| Location ; Section 14.30 | Proxy-Authenticate ; Section 14.33 | Public ; Section 14.35 | Retry-After ; Section 14.38 | Server ; Section 14.39 | Vary ; Section 14.43 | Warning ; Section 14.45 | WWW-Authenticate ; Section 14.46
Response-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of responseheader fields if all parties in the communication recognize them to be response-header fields. Unrecognized header fields are treated as entity-header fields.
In this section, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity.
entity-header = Allow ; Section 14.7| Content-Base ; Section 14.11 | Content-Encoding ; Section 14.12 | Content-Language ; Section 14.13 | Content-Length ; Section 14.14 | Content-Location ; Section 14.15 | Content-MD5 ; Section 14.16 | Content-Range ; Section 14.17 | Content-Type ; Section 14.18 | ETag ; Section 14.20 | Expires ; Section 14.21 | Last-Modified ; Section 14.29 | extension-header
extension-header = message-headerThe extension-header mechanism allows additional entity-header fields to be defined without changing the protocol, but these fields cannot be assumed to be recognizable by the recipient. Unrecognized header fields SHOULD be ignored by the recipient and forwarded by proxies.
entity-body = *OCTETAn entity-body is only present in a message when a message-body is present, as described in section 4.3. The entity-body is obtained from the message-body by decoding any Transfer-Encoding that may have been applied to ensure safe and proper transfer of the message.
entity-body := Content-Encoding( Content-Type( data ) )Content-Type specifies the media type of the underlying data. Content-Encoding may be used to indicate any additional content codings applied to the data, usually for the purpose of data compression, that are a property of the requested resource. There is no default encoding.
Any HTTP/1.1 message containing an entity-body SHOULD include a Content-Type header field defining the media type of that body. If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URL used to identify the resource. If the media type remains unknown, the recipient SHOULD treat it as type "application/octet-stream".
8 Connections
Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP servers and causing congestion on the Internet. The use of inline images and other associated data often requires a client to make multiple requests of the same server in a short amount of time. Analyses of these performance problems are available [30][27]; analysis and results from a prototype implementation are in [26].
Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling takes place using the Connection header field. Once a close has been signaled, the client MUST not send any more requests on that connection.
An HTTP/1.1 client MAY expect a connection to remain open, but would decide to keep it open based on whether the response from a server contains a Connection header with the connection-token close. In case the client does not want to maintain a connection for more than that request, it SHOULD send a Connection header including the connection-token close.
If either the client or the server sends the close token in the Connection header, that request becomes the last one for the connection.
Clients and servers SHOULD NOT assume that a persistent connection is maintained for HTTP versions less than 1.1 unless it is explicitly signaled. See section 19.7.1 for more information on backwards compatibility with HTTP/1.0 clients.
In order to remain persistent, all messages on the connection must have a self-defined message length (i.e., one not defined by closure of the connection), as described in section 4.4.
Clients which assume persistent connections and pipeline immediately after connection establishment SHOULD be prepared to retry their connection if the first pipelined attempt fails. If a client does such a retry, it MUST NOT pipeline before it knows the connection is persistent. Clients MUST also be prepared to resend their requests if the server closes the connection before sending all of the corresponding responses.
The proxy server MUST signal persistent connections separately with its clients and the origin servers (or other proxy servers) that it connects to. Each persistent connection applies to only one transport link.
A proxy server MUST NOT establish a persistent connection with an HTTP/1.0 client.
When a client or server wishes to time-out it SHOULD issue a graceful close on the transport connection. Clients and servers SHOULD both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does not detect the other side's close promptly it could cause unnecessary resource drain on the network.
A client, server, or proxy MAY close the transport connection at any time. For example, a client MAY have started to send a new request at the same time that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed while it was idle, but from the client's point of view, a request is in progress.
This means that clients, servers, and proxies MUST be able to recover from asynchronous close events. Client software SHOULD reopen the transport connection and retransmit the aborted request without user interaction so long as the request method is idempotent (see section
9.1.2); other methods MUST NOT be automatically retried, although user agents MAY offer a human operator the choice of retrying the request.
However, this automatic retry SHOULD NOT be repeated if the second request fails.
Servers SHOULD always respond to at least one request per connection, if at all possible. Servers SHOULD NOT close a connection in the middle of transmitting a response, unless a network or client failure is suspected.
Clients that use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD maintain AT MOST 2 connections with any server or proxy. A proxy SHOULD use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion of the Internet or other networks.
o HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.
o An HTTP/1.1 (or later) client sending a message-body SHOULD monitor the network connection for an error status while it is transmitting the request. If the client sees an error status, it SHOULD immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (section 3.6), a zero length chunk and empty footer MAY be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header, the client MUST close the connection.
o An HTTP/1.1 (or later) client MUST be prepared to accept a 100 (Continue) status followed by a regular response.
o An HTTP/1.1 (or later) server that receives a request from a HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue) response; it SHOULD either wait for the request to be completed normally (thus avoiding an interrupted request) or close the connection prematurely.
Upon receiving a method subject to these requirements from an HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either respond with 100 (Continue) status and continue to read from the input stream, or respond with an error status. If it responds with an error status, it MAY close the transport (TCP) connection or it MAY continue to read and discard the rest of the request. It MUST NOT perform the requested method if it returns an error status.
Clients SHOULD remember the version number of at least the most recently used server; if an HTTP/1.1 client has seen an HTTP/1.1 or later response from the server, and it sees the connection close before receiving any status from the server, the client SHOULD retry the request without user interaction so long as the request method is idempotent (see section 9.1.2); other methods MUST NOT be automatically retried, although user agents MAY offer a human operator the choice of retrying the request.. If the client does retry the request, the client
o MUST first send the request header fields, and then
o MUST wait for the server to respond with either a 100 (Continue) response, in which case the client should continue, or with an error status.
If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from the server, it should assume that the server implements HTTP/1.0 or older and will not use the 100 (Continue) response. If in this case the client sees the connection close before receiving any status from the server, the client SHOULD retry the request. If the client does retry the request to this HTTP/1.0 server, it should use the following "binary exponential backoff" algorithm to be assured of obtaining a reliable response:
3. Initialize a variable R to the estimated round-trip time to the server (e.g., based on the time it took to establish the connection), or to a constant value of 5 seconds if the round-trip time is not available.
4. Compute T = R * (2**N), where N is the number of previous retriesof this request.
5. Wait either for an error response from the server, or for T seconds (whichever comes first)
6. If no error response is received, after T seconds transmit the body of the request.
7. If client sees that the connection is closed prematurely, repeat from step 1 until the request is accepted, an error response is received, or the user becomes impatient and terminates the retry process.
No matter what the server version, if an error status is received, the client
o MUST NOT continue and
o MUST close the connection if it has not completed sending the message.
An HTTP/1.1 (or later) client that sees the connection close after receiving a 100 (Continue) but before receiving any other status SHOULD retry the request, and need not wait for 100 (Continue) response (but MAY do so if this simplifies the implementation).
The Host request-header field (section 14.23) MUST accompany all HTTP/1.1 requests.
Implementers should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they may take which may have an unexpected significance to themselves or others.
In particular, the convention has been established that the GET and HEAD methods should never have the significance of taking an action other than retrieval. These methods should be considered "safe." This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in
fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.
Unless the server's response is an error, the response MUST NOT include entity information other than what can be considered as communication options (e.g., Allow is appropriate, but Content-Type is not). Responses to this method are not cachable.
If the Request-URI is an asterisk ("*"), the OPTIONS request is intended to apply to the server as a whole. A 200 response SHOULD include any header fields which indicate optional features implemented by the server (e.g., Public), including any extensions not defined by this specification, in addition to any applicable general or response-header fields. As described in section 5.1.2, an "OPTIONS *" request can be applied through a proxy by specifying the destination server in the Request-URI without any path information.
If the Request-URI is not an asterisk, the OPTIONS request applies only to the options that are available when communicating with that resource. A 200 response SHOULD include any header fields which indicate optional features implemented by the server and applicable to that resource (e.g., Allow), including any extensions not defined by this specification, in addition to any applicable general or response-header fields. If the OPTIONS request passes through a proxy, the proxy MUST edit the response to exclude those options which apply to a proxy's capabilities and which are known to be unavailable through that proxy.
The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.
The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred, as described in section 14.36. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client.
The response to a GET request is cachable if and only if it meets the requirements for HTTP caching described in section 13.
The response to a HEAD request may be cachable in the sense that the information contained in the response may be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the cache MUST treat the cache entry as stale.
o Annotation of existing resources;
o Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;
o Providing a block of data, such as the result of submitting a form, to a data-handling process;
o Extending a database through an append operation.
The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.
The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.
If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an entity which describes the status of the request and refers to the new resource, and a Location header (see section 14.30).
Responses to this method are not cachable, unless the response includes appropriate Cache-Control or Expires header fields. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cachable resource.
POST requests must obey the message transmission requirements set out in section 8.2.
If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries should be treated as stale. Responses to this method are not cachable.
The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource may be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server MUST NOT attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI, it MUST send a 301 (Moved Permanently) response; the user agent MAY then make its own decision regarding whether or not to redirect the request.
A single resource MAY be identified by many different URIs. For example, an article may have a URI for identifying "the current version" which is separate from the URI identifying each particular version. In this case, a PUT request on a general URI may result in several other URIs being defined by the origin server.
HTTP/1.1 does not define how a PUT method affects the state of an origin server.
PUT requests must obey the message transmission requirements set out in section 8.2.
A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the response is OK but does not include an entity.
If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries should be treated as stale. Responses to this method are not cachable.
TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (section 14.44) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.
If successful, the response SHOULD contain the entire request message in the entity-body, with a Content-Type of "message/http". Responses to this method MUST NOT be cached.
The protocol should only be switched when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol may be advantageous when delivering resources that use such features.
GET an entity corresponding to the requested resource is sent in the response;
HEAD the entity-header fields corresponding to the requested resource are sent in the response without any message-body;
The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The entity returned with this response SHOULD include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled.
response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view. The response MAY include new metainformation in the form of entity-headers, which SHOULD apply to the document currently in the user agent's active view.
The 204 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.
A cache that does not support the Range and Content-Range headers MUST NOT cache 206 (Partial) responses.
Unless it was a HEAD request, the response SHOULD include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the ContentType header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice may be performed automatically. However, this specification does not define any standard for such automatic selection.
If the server has a preferred choice of representation, it SHOULD include the specific URL for that representation in the Location field; user agents MAY use the Location field value for automatic redirection. This response is cachable unless indicated otherwise.
If the new URI is a location, its URL SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).
If the 301 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.
Note: When automatically redirecting a POST request after receiving a 301 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request.
If the new URI is a location, its URL SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).
If the 302 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.
Note: When automatically redirecting a POST request after receiving a 302 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request.
If the new URI is a location, its URL SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).
o ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request
o Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same variant
If the conditional GET used a strong cache validator (see section 13.3.3), the response SHOULD NOT include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response MUST NOT include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.
If a 304 response indicates an entity not currently cached, then the cache MUST disregard the response and repeat the request without the conditional.
If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.
The 304 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.
Note: If the client is sending data, a server implementation using TCP should be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send a reset packet to the client, which may erase the client's
unacknowledged input buffers before they can be read and interpreted by the HTTP application.
If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.
Unless it was a HEAD request, the response SHOULD include an entity containing a list of available entity characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice may be performed automatically. However, this specification does not define any standard for such automatic selection.
Note: HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept headers sent in the request. In some cases, this may even be preferable to sending a 406 response. User agents are encouraged to inspect the headers of an incoming response to determine if it is acceptable. If the response could be unacceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further actions.
Conflicts are most likely to occur in response to a PUT request. If versioning is being used and the entity being PUT includes changes to a resource which conflict with those made by an earlier (third-party) request, the server MAY use the 409 response to indicate that it can't complete the request. In this case, the response entity SHOULD contain a list of the differences between the two versions in a format defined by the response Content-Type.
The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for any length of time -- that is left to the discretion of the server owner.
If the condition is temporary, the server SHOULD include a RetryAfter header field to indicate that it is temporary and after what time the client may try again.
Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.
auth-scheme = token
auth-param = token "=" quoted-stringThe 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent. This response MUST include a WWW-Authenticate header field containing at least one challenge applicable to the requested resource.
challenge = auth-scheme 1*SP realm *( "," auth-param )
realm = "realm" "=" realm-value
realm-value = quoted-stringThe realm attribute (case-insensitive) is required for all authentication schemes which issue a challenge. The realm value (case-sensitive), in combination with the canonical root URL (see section 5.1.2) of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication scheme.
A user agent that wishes to authenticate itself with a server-- usually, but not necessarily, after receiving a 401 or 411 response-MAY do so by including an Authorization header field with the request. The Authorization field value consists of credentials
containing the authentication information of the user agent for the realm of the resource being requested.
credentials = basic-credentials| auth-scheme #auth-param
The domain over which credentials can be automatically applied by a user agent is determined by the protection space. If a prior request has been authorized, the same credentials MAY be reused for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot extend outside the scope of its server.
If the server does not wish to accept the credentials sent with a request, it SHOULD return a 401 (Unauthorized) response. The response MUST include a WWW-Authenticate header field containing the (possibly new) challenge applicable to the requested resource and an entity explaining the refusal.
The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional mechanisms MAY be used, such as encryption at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, these additional mechanisms are not defined by this specification.
Proxies MUST be completely transparent regarding user agent authentication. That is, they MUST forward the WWW-Authenticate and Authorization headers untouched, and follow the rules found in section 14.8.
HTTP/1.1 allows a client to pass authentication information to and from a proxy via the Proxy-Authenticate and Proxy-Authorization headers.
Upon receipt of an unauthorized request for a URI within the protection space, the server MAY respond with a challenge like the following:
To receive authorization, the client sends the userid and password, separated by a single colon (":") character, within a base64 encoded string in the credentials.
basic-credentials = "Basic" SP basic-cookie
basic-cookie = <base64 [7] encoding of user-pass,except not limited to 76 char/line>
user-pass = userid ":" password
userid = *<TEXT excluding ":">
password = *TEXT
when there are multiple representations available.
Note: This is not called "format negotiation" because the alternate representations may be of the same media type, but use different capabilities of that type, be in different languages, etc.
Any response containing an entity-body MAY be subject to negotiation, including error responses.
There are two kinds of content negotiation which are possible in
HTTP: server-driven and agent-driven negotiation. These two kinds ofnegotiation are orthogonal and thus may be used separately or in combination. One method of combination, referred to as transparent negotiation, occurs when a cache uses the agent-driven negotiation information provided by the origin server in order to provide server-driven negotiation for subsequent requests.
Server-driven negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to the user agent, or when the server desires to send its "best guess" to the client along with the first response (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to improve the server's guess, the user agent MAY include request header fields (Accept, Accept-Language, Accept-Encoding, etc.) which describe its preferences for such a response.
2. Having the user agent describe its capabilities in every request can be both very inefficient (given that only a small percentage of responses have multiple representations) and a potential violation of
the user's privacy.
3. It complicates the implementation of an origin server and the algorithms for generating responses to a request.
4. It may limit a public cache's ability to use the same response for multiple user's requests.
HTTP/1.1 includes the following request-header fields for enabling server-driven negotiation through description of user agent capabilities and user preferences: Accept (section 14.1), AcceptCharset (section 14.2), Accept-Encoding (section 14.3), AcceptLanguage (section 14.4), and User-Agent (section 14.42). However, an origin server is not limited to these dimensions and MAY vary the response based on any aspect of the request, including information outside the request-header fields or within extension header fields not defined by this specification.
HTTP/1.1 origin servers MUST include an appropriate Vary header field (section 14.43) in any cachable response based on server-driven negotiation. The Vary header field describes the dimensions over which the response might vary (i.e. the dimensions over which the origin server picks its "best guess" response from multiple representations).
HTTP/1.1 public caches MUST recognize the Vary header field when it is included in a response and obey the requirements described in section 13.6 that describes the interactions between caching and content negotiation.
Agent-driven negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally when public caches are used to distribute server load and reduce network usage.
Agent-driven negotiation suffers from the disadvantage of needing a second request to obtain the best alternate representation. This second request is only efficient when caching is used. In addition, this specification does not define any mechanism for supporting automatic selection, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1.
HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable) status codes for enabling agent-driven negotiation when the server is unwilling or unable to provide a varying response using server-driven negotiation.
Transparent negotiation has the advantage of distributing the negotiation work that would otherwise be required of the origin server and also removing the second request delay of agent-driven negotiation when the cache is able to correctly guess the right response.
This specification does not define any mechanism for transparent negotiation, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1. An HTTP/1.1 cache performing transparent negotiation MUST include a Vary header field in the response (defining the dimensions of its variance) if it is cachable to ensure correct interoperation with all HTTP/1.1 clients. The agent-driven negotiation information supplied by the origin server SHOULD be included with the transparently negotiated response.
headers, response codes, etc.
Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former reduces the number of network round-trips required for many operations; we use an "expiration" mechanism for this purpose (see section 13.2). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see section 13.3).
Requirements for performance, availability, and disconnected operation require us to be able to relax the goal of semantic transparency. The HTTP/1.1 protocol allows origin servers, caches, and clients to explicitly reduce transparency when necessary. However, because non-transparent operation may confuse non-expert users, and may be incompatible with certain server applications (such as those for ordering merchandise), the protocol requires that transparency be relaxed
o only by an explicit protocol-level request when relaxed by client or origin server
o only with an explicit warning to the end user when relaxed by cache or client
2. Protocol features that allow an origin server or user agent to explicitly request and control non-transparent operation.
3. Protocol features that allow a cache to attach warnings to responses that do not preserve the requested approximation of semantic transparency.
A basic principle is that it must be possible for the clients to detect any potential relaxation of semantic transparency.
Note: The server, cache, or client implementer may be faced with design decisions not explicitly discussed in this specification. If a decision may affect semantic transparency, the implementer ought to err on the side of maintaining transparency unless a careful and complete analysis shows significant benefits in breaking transparency.
1. It has been checked for equivalence with what the origin server would have returned by revalidating the response with the origin server (section 13.3);
2. It is "fresh enough" (see section 13.2). In the default case, this means it meets the least restrictive freshness requirement of the client, server, and cache (see section 14.9); if the origin server so specifies, it is the freshness requirement of the origin server alone.
3. It includes a warning if the freshness demand of the client or the origin server is violated (see section 13.1.5 and 14.45).
4. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or error (4xx or 5xx) response message.
If the cache can not communicate with the origin server, then a correct cache SHOULD respond as above if the response can be correctly served from the cache; if not it MUST return an error or
warning indicating that there was a communication failure.
If a cache receives a response (either an entire response, or a 304 (Not Modified) response) that it would normally forward to the requesting client, and the received response is no longer fresh, the cache SHOULD forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache SHOULD NOT attempt to revalidate a response simply because that response became stale in transit; this might lead to an infinite loop. An user agent that receives a stale response without a Warning MAY display a warning indication to the user.
Warnings may be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish these responses from true failures.
Warnings are always cachable, because they never weaken the transparency of a response. This means that warnings can be passed to HTTP/1.0 caches without danger; such caches will simply pass the warning along as an entity-header in the response.
Warnings are assigned numbers between 0 and 99. This specification defines the code numbers and meanings of each currently assigned warnings, allowing a client or cache to take automated action in some (but not all) cases.
Warnings also carry a warning text. The text may be in any appropriate natural language (perhaps based on the client's Accept headers), and include an optional indication of what character set is used.
Multiple warnings may be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number. For example, a server may provide the same warning with texts in both English and Basque.
When multiple warnings are attached to a response, it may not be practical or reasonable to display all of them to the user. This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but does suggest some heuristics.
The Warning header and the currently defined warnings are described in section 14.45.
The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between header values, the most restrictive interpretation should be applied (that is, the one that is most likely to preserve semantic transparency). However, in some cases, Cache-Control directives are explicitly specified as weakening the approximation of semantic transparency (for example, "max-stale" or "public").
Many user agents make it possible for users to override the basic caching mechanisms. For example, the user agent may allow the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually add "Cache-Control: max-stale=3600" to every request. The user should have to explicitly request either non-transparent behavior, or behavior that results in abnormally ineffective caching.
If the user has overridden the basic caching mechanisms, the user agent should explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other visual indicator.
If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user agent should continually display an indication (for example, a picture of currency in flames) so that the user does not inadvertently consume excess resources or suffer from excessive latency.
It also allows the user agent to take steps to obtain a first-hand or fresh response. For this reason, a cache SHOULD NOT return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply for technical or policy reasons.
A client's request may specify the maximum age it is willing to accept of an unvalidated response; specifying a value of zero forces the cache(s) to revalidate all responses. A client may also specify the minimum time remaining before a response expires. Both of these options increase constraints on the behavior of caches, and so cannot further relax the cache's approximation of semantic transparency.
A client may also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on the caches, and so may violate the origin server's specified constraints on semantic transparency, but may be necessary to support disconnected operation, or high availability in the face of poor connectivity.
HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding requests is for an origin server to provide an explicit expiration time in the future, indicating that a response may be used to satisfy subsequent requests. In other words, a cache can return a fresh
response without first contacting the server.
Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic transparency, as long as the server's expiration times are carefully chosen.
The expiration mechanism applies only to responses taken from a cache and not to first-hand responses forwarded immediately to the requesting client.
If an origin server wishes to force a semantically transparent cache to validate every request, it may assign an explicit expiration time in the past. This means that the response is always stale, and so the cache SHOULD validate it before using it for subsequent requests. See section 14.9.4 for a more restrictive way to force revalidation.
If an origin server wishes to force any HTTP/1.1 cache, no matter how it is configured, to validate every request, it should use the "must-revalidate" Cache-Control directive (see section 14.9).
Servers specify explicit expiration times using either the Expires header, or the max-age directive of the Cache-Control header.
An expiration time cannot be used to force a user agent to refresh its display or reload a resource; its semantics apply only to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource is initiated. See section 13.13 for explanation of the difference between caches and history mechanisms.
In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially hosts running origin servers and caches, should use NTP [28] or some similar protocol to synchronize their clocks to a globally accurate time standard.
Also note that HTTP/1.1 requires origin servers to send a Date header with every response, giving the time at which the response was generated. We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.
HTTP/1.1 uses the Age response-header to help convey age information between caches. The Age header value is the sender's estimate of the amount of time since the response was generated at the origin server. In the case of a cached response that has been revalidated with the origin server, the Age value is based on the time of revalidation, not of the original response.
In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths.
We use the term "age_value" to denote the value of the Age header, in a form appropriate for arithmetic operations.
2. age_value, if all of the caches along the response path implement HTTP/1.1.
Given that we have two independent ways to compute the age of a response when it is received, we can combine these as
corrected_received_age = max(now - date_value, age_value)and as long as we have either nearly synchronized clocks or all-
Because of network-imposed delays, some significant interval may pass from the time that a server generates a response and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low ages.
Because the request that resulted in the returned Age value must have been initiated prior to that Age value's generation, we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age value is received, it MUST be interpreted relative to the time the request was initiated, not the time that the response was received. This algorithm results in conservative behavior no matter how much delay is experienced. So, we compute:
corrected_initial_age = corrected_received_age+ (now - request_time)
where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.
Summary of age calculation algorithm, when a cache receives a response:
/* * age_value * is the value of Age: header received by the cache with * this response. * date_value * is the value of the origin server's Date: header * request_time * is the (local) time when the cache made the request * that resulted in this cached response * response_time * is the (local) time when the cache received the * response * now * is the current (local) time */
apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;When a cache sends a response, it must add to the corrected_initial_age the amount of time that the response was resident locally. It must then transmit this total age, using the Age header, to the next recipient cache.
Note that a client cannot reliably tell that a response is firsthand, but the presence of an Age header indicates that a response is definitely not first-hand. Also, if the Date in a response is earlier than the client's local request time, the response is probably not first-hand (in the absence of serious clock skew).
We use the term "expires_value" to denote the value of the Expires header. We use the term "max_age_value" to denote an appropriate value of the number of seconds carried by the max-age directive of the Cache-Control header in a response (see section 14.10.
The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:
freshness_lifetime = max_age_value
freshness_lifetime = expires_value - date_valueNote that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server.
If neither Expires nor Cache-Control: max-age appears in the response, and the response does not include other restrictions on caching, the cache MAY compute a freshness lifetime using a heuristic. If the value is greater than 24 hours, the cache must attach Warning 13 to any response whose age is more than 24 hours if
such warning has not already been added.
Also, if the response does have a Last-Modified time, the heuristic expiration value SHOULD be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
The calculation to determine if a response has expired is quite simple:
response_is_fresh = (freshness_lifetime > current_age)
If a client performing a retrieval receives a non-first-hand response for a request that was already fresh in its own cache, and the Date header in its existing cache entry is newer than the Date on the new response, then the client MAY ignore the response. If so, it MAY retry the request with a "Cache-Control: max-age=0" directive (see section 14.9), to force a check with the origin server.
If a cache has two fresh responses for the same representation with different validators, it MUST use the one with the more recent Date header. This situation may arise because the cache is pooling responses from other caches, or because a client has asked for a reload or a revalidation of an apparently fresh cache entry.
Neither the entity tag nor the expiration value can impose an ordering on responses, since it is possible that a later response intentionally carries an earlier expiration time. However, the HTTP/1.1 specification requires the transmission of Date headers on every response, and the Date values are ordered to a granularity of one second.
When a client tries to revalidate a cache entry, and the response it receives contains a Date header that appears to be older than the one for the existing entry, then the client SHOULD repeat the request unconditionally, and include
If the Date values are equal, then the client may use either response (or may, if it is being extremely prudent, request a new response). Servers MUST NOT depend on clients being able to choose deterministically between responses generated during the same second, if their expiration times overlap.
The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (user agent or proxy cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated validator in the request.
The server then checks that validator against the current validator for the entity, and, if they match, it responds with a special status code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns a full response (including entity-body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round trip if it does not match.
Note: the comparison functions used to decide if validators match are defined in section 13.3.3.
In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional.
The protocol includes both positive and negative senses of cachevalidating conditions. That is, it is possible to request either that a method be performed if and only if a validator matches or if and only if no validators match.
Note: a response that lacks a validator may still be cached, and served from cache until it expires, unless this is explicitly prohibited by a Cache-Control directive. However, a cache cannot do a conditional retrieval if it does not have a validator for the entity, which means it will not be refreshable after it expires.
Entity Tags are described in section 3.11. The headers used with entity tags are described in sections 14.20, 14.25, 14.26 and 14.43.
However, there may be cases when a server prefers to change the validator only on semantically significant changes, and not when
insignificant aspects of the entity change. A validator that does not always change when the resource changes is a "weak validator."
Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak." One can think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific entity, while a weak validator is part of an identifier for a set of semantically equivalent entities.
Note: One example of a strong validator is an integer that is incremented in stable storage every time an entity is changed.
An entity's modification time, if represented with one-second resolution, could be a weak validator, since it is possible that the resource may be modified twice during a single second.
Support for weak validators is optional; however, weak validators allow for more efficient caching of equivalent objects; for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during that period is likely "good enough" to be equivalent.
A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators.
Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client may end up with an internally inconsistent entity.
The only function that the HTTP/1.1 protocol defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not:
o The strong comparison function: in order to be considered equal, both validators must be identical in every way, and neither may be weak. o The weak comparison function: in order to be considered equal, both validators must be identical in every way, but either or both of them may be tagged as "weak" without affecting the result.
GET requests. The strong comparison function MUST be used in all other cases.
An entity tag is strong unless it is explicitly tagged as weak. Section 3.11 gives the syntax for entity tags.
A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is strong, using the following rules:
o The validator is being compared by an origin server to the actual current validator for the entity and, o That origin server reliably knows that the associated entity did not change twice during the second covered by the presented validator. or
o The validator is about to be used by a client in an If-ModifiedSince or If-Unmodified-Since header, because the client has a cache entry for the associated entity, and o That cache entry includes a Date value, which gives the time when the origin server sent the original response, and o The presented Last-Modified time is at least 60 seconds before the Date value. or
o The validator is being compared by an intermediate cache to the validator stored in its cache entry for the entity, and o That cache entry includes a Date value, which gives the time when the origin server sent the original response, and o The presented Last-Modified time is at least 60 seconds before the Date value.
This method relies on the fact that if two different responses were sent by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60- second limit guards against the possibility that the Date and LastModified values are generated from different clocks, or at somewhat different times during the preparation of the response. An implementation may use a value larger than 60 seconds, if it is believed that 60 seconds is too short.
If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, it may do this only if the Last-Modified time is strong in the sense described here.
A cache or origin server receiving a cache-conditional request, other than a full-body GET request, MUST use the strong comparison function to evaluate the condition.
These rules allow HTTP/1.1 caches and clients to safely perform subrange retrievals on values that have been obtained from HTTP/1.0 servers.
In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity tag and a Last-Modified value.
In order to be legal, a strong entity tag MUST change whenever the associated entity value changes in any way. A weak entity tag SHOULD change whenever the associated entity changes in a semantically significant way.
Note: in order to provide semantically transparent caching, an origin server must avoid reusing a specific strong entity tag value for two different entities, or reusing a specific weak entity tag value for two semantically different entities. Cache entries may persist for arbitrarily long periods, regardless of expiration times, so it may be inappropriate to expect that a cache will never again attempt to validate an entry using a validator that it obtained at some point in the past.
o If only a Last-Modified value has been provided by the origin server, SHOULD use that value in non-subrange cache-conditional requests (using If-Modified-Since). o If only a Last-Modified value has been provided by an HTTP/1.0 origin server, MAY use that value in subrange cache-conditional requests (using If-Unmodified-Since:). The user agent should provide a way to disable this, in case of difficulty. o If both an entity tag and a Last-Modified value have been provided by the origin server, SHOULD use both validators in cache-conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
An HTTP/1.1 cache, upon receiving a request, MUST use the most restrictive validator when deciding whether the client's cache entry matches the cache's own cache entry. This is only an issue when the request contains both an entity tag and a last-modified-date validator (If-Modified-Since or If-Unmodified-Since).
A note on rationale: The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much nonredundant information as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative assumptions about the validators they receive.
HTTP/1.0 clients and caches will ignore entity tags. Generally, last-modified values received or used by these systems will support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then HTTP/1.1 origin servers should not provide one.
there is neither a cache validator nor an explicit expiration time associated with a response, we do not expect it to be cached, but certain caches may violate this expectation (for example, when little or no network connectivity is available). A client can usually detect that such a response was taken from a cache by comparing the Date header to the current time.
Note that some HTTP/1.0 caches are known to violate this expectation without providing any Warning.
However, in some cases it may be inappropriate for a cache to retain an entity, or to return it in response to a subsequent request. This may be because absolute semantic transparency is deemed necessary by the service author, or because of security or privacy considerations. Certain Cache-Control directives are therefore provided so that the server can indicate that certain resource entities, or portions thereof, may not be cached regardless of other considerations.
Note that section 14.8 normally prevents a shared cache from saving and returning a response to a previous request if that request included an Authorization header.
A response received with a status code of 200, 203, 206, 300, 301 or 410 may be stored by a cache and used in reply to a subsequent request, subject to the expiration mechanism, unless a Cache-Control directive prohibits caching. However, a cache that does not support the Range and Content-Range headers MUST NOT cache 206 (Partial Content) responses.
A response received with any other status code MUST NOT be returned in a reply to a subsequent request unless there are Cache-Control directives or another header(s) that explicitly allow it. For example, these include the following: an Expires header (section 14.21); a "max-age", "must-revalidate", "proxy-revalidate", "public" or "private" Cache-Control directive (section 14.9).
o End-to-end headers, which must be transmitted to the ultimate recipient of a request or response. End-to-end headers in responses must be stored as part of a cache entry and transmitted in any response formed from a cache entry. o Hop-by-hop headers, which are meaningful only for a single transport-level connection, and are not stored by caches or forwarded by proxies.
A cache or non-caching proxy MUST NOT modify any of the following fields in a request or response, nor may it add any of these fields if not already present:
o Content-Location o ETag o Expires o Last-Modified
A cache or non-caching proxy MUST NOT modify or add any of the following fields in a response that contains the no-transform CacheControl directive, or in any request:
o Content-Encoding o Content-Length o Content-Range o Content-Type
A cache or non-caching proxy MAY modify or add these fields in a response that does not include no-transform, but if it does so, it MUST add a Warning 14 (Transformation applied) if one does not already appear in the response.
Warning: unnecessary modification of end-to-end headers may causeauthentication failures if stronger authentication mechanisms are introduced in later versions of HTTP. Such authentication mechanisms may rely on the values of header fields not listed here.
In other words, the set of end-to-end headers received in the incoming response overrides all corresponding end-to-end headers stored with the cache entry. The cache may add Warning headers (see section 14.45) to this set.
If a header field-name in the incoming response matches more than one header in the cache entry, all such old headers are replaced.
Note: this rule allows an origin server to use a 304 (Not Modified) response to update any header associated with a previous response for the same entity, although it might not always be meaningful or correct to do so. This rule does not allow an origin server to use a 304 (not Modified) response to entirely delete a header that it had provided with a previous response.
If a cache has a stored non-empty set of subranges for an entity, and an incoming response transfers another subrange, the cache MAY combine the new subrange with the existing set if both the following conditions are met:
o Both the incoming response and the cache entry must have a cache validator. o The two cache validators must match using the strong comparison function (see section 13.3.3).
If either requirement is not meant, the cache must use only the most recent partial response (based on the Date values transmitted with every response, and using the incoming response if these values are equal or missing), and must discard the other partial information.
A server MUST use the Vary header field (section 14.43) to inform a cache of what header field dimensions are used to select among multiple representations of a cachable response. A cache may use the selected representation (the entity included with that particular response) for replying to subsequent requests on that resource only when the subsequent requests have the same or equivalent values for all header fields specified in the Vary response-header. Requests with a different value for one or more of those header fields would be forwarded toward the origin server.
If an entity tag was assigned to the representation, the forwarded request SHOULD be conditional and include the entity tags in an IfNone-Match header field from all its cache entries for the RequestURI. This conveys to the server the set of entities currently held by the cache, so that if any one of these entities matches the requested entity, the server can use the ETag header in its 304 (Not Modified) response to tell the cache which entry is appropriate. If the entity-tag of the new response matches that of an existing entry, the
new response SHOULD be used to update the header fields of the existing entry, and the result MUST be returned to the client.
The Vary header field may also inform the cache that the representation was selected using criteria not limited to the request-headers; in this case, a cache MUST NOT use the response in a reply to a subsequent request unless the cache relays the new request to the origin server in a conditional request and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates which entity should be used.
If any of the existing cache entries contains only partial content for the associated entity, its entity-tag SHOULD NOT be included in the If-None-Match header unless the request is for a range that would be fully satisfied by that entry.
If a cache receives a successful response whose Content-Location field matches that of an existing cache entry for the same RequestURI, whose entity-tag differs from that of the existing entry, and whose Date is more recent than that of the existing entry, the existing entry SHOULD NOT be returned in response to future requests, and should be deleted from the cache.
If a cache receives a 5xx response while attempting to revalidate an entry, it may either forward this response to the requesting client,
or act as if the server failed to respond. In the latter case, it MAY return a previously received response unless the cached entry includes the "must-revalidate" Cache-Control directive (see section 14.9).
We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those containing a "?" in the rel_path part) to perform operations with significant side effects, caches MUST NOT treat responses to such URLs as fresh unless the server provides an explicit expiration time. This specifically means that responses from HTTP/1.0 servers for such URIs should not be taken from a cache. See section 9.1.1 for related information.
There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request that caused the change at the origin server may not have gone through the proxy where a cache entry is stored. However, several rules help reduce the likelihood of erroneous behavior.
In this section, the phrase "invalidate an entity" means that the cache should either remove all instances of that entity from its storage, or should mark these as "invalid" and in need of a mandatory revalidation before they can be returned in response to a subsequent request.
Some HTTP methods may invalidate an entity. This is either the entity referred to by the Request-URI, or by the Location or ContentLocation response-headers (if present). These methods are:
o PUT o DELETE o POST
In order to prevent denial of service attacks, an invalidation based on the URI in a Location or Content-Location header MUST only be performed if the host part is the same as in the Request-URI.
The alternative (known as "write-back" or "copy-back" caching) is not allowed in HTTP/1.1, due to the difficulty of providing consistent updates and the problems arising from server, cache, or network failure prior to write-back.
Note: a new response that has an older Date header value than existing cached responses is not cachable.
History mechanisms and caches are different. In particular history mechanisms SHOULD NOT try to show a semantically transparent view of the current state of a resource. Rather, a history mechanism is meant to show exactly what the user saw at the time when the resource was retrieved.
By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism should display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history documents.
This should not be construed to prohibit the history mechanism from telling the user that a view may be stale.
Note: if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider it important that users not be presented with error messages or warning messages when they use navigation controls (such as BACK) to view previously fetched resources. Even though sometimes such resources ought not to cached, or ought to expire quickly, user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects of improperly functioning history mechanisms.
Accept = "Accept" ":"#( media-range [ accept-params ] )
media-range = ( "*/*"| ( type "/" "*" ) | ( type "/" subtype ) ) *( ";" parameter )
accept-params = ";" "q" "=" qvalue *( accept-extension )
accept-extension = ";" token [ "=" ( token | quoted-string ) ]The asterisk "*" character is used to group media types into ranges, with "*/*" indicating all media types and "type/*" indicating all subtypes of that type. The media-range MAY include media type parameters that are applicable to that range.
Each media-range MAY be followed by one or more accept-params, beginning with the "q" parameter for indicating a relative quality factor. The first "q" parameter (if any) separates the media-range parameter(s) from the accept-params. Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the qvalue scale from 0 to 1 (section 3.9). The default value is q=1.
Note: Use of the "q" parameter name to separate media type parameters from Accept extension parameters is due to historical practice. Although this prevents any media type parameter named "q" from being used with a media range, such an event is believed to be unlikely given the lack of any "q" parameters in the IANA media type registry and the rare usage of any media type parameters in Accept. Future media types should be discouraged from registering any parameter named "q".
Accept: audio/*; q=0.2, audio/basicSHOULD be interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality."
If no Accept header field is present, then it is assumed that the client accepts all media types. If an Accept header field is present, and if the server cannot send a response which is acceptable according to the combined Accept field value, then the server SHOULD send a 406 (not acceptable) response.
Accept: text/plain; q=0.5, text/html,text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are the preferred media types, but if they do not exist, then send the text/x-dvi entity, and if that does not exist, send the text/plain entity."
Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example,
1) text/html;level=1 2) text/html 3) text/* 4) */*
The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type. For example,
Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,text/html;level=2;q=0.4, */*;q=0.5
would cause the following values to be associated:
text/html;level=1 = 1
text/html = 0.7
text/plain = 0.3
image/jpeg = 0.5
text/html;level=2 = 0.4
text/html;level=3 = 0.7Note: A user agent may be provided with a default set of quality values for certain media ranges. However, unless the user agent is a closed system which cannot interact with other rendering agents,
this default set should be configurable by the user.
Accept-Charset = "Accept-Charset" ":"1#( charset [ ";" "q" "=" qvalue ] )
Character set values are described in section 3.4. Each charset may be given an associated quality value which represents the user's preference for that charset. The default value is q=1. An example is
Accept-Encoding = "Accept-Encoding" ":"#( content-coding )
Accept-Encoding: compress, gzipIf no Accept-Encoding header is present in a request, the server MAY assume that the client will accept any content coding. If an AcceptEncoding header is present, and if the server cannot send a response which is acceptable according to the Accept-Encoding header, then the server SHOULD send an error response with the 406 (Not Acceptable) status code.
The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request.
Accept-Language = "Accept-Language" ":"1#( language-range [ ";" "q" "=" qvalue ] )
language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )Each language-range MAY be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range. The quality value defaults to "q=1". For example,
Note: This use of a prefix matching rule does not imply that language tags are assigned to languages in such a way that it is always true that if a user understands a language with a certain tag, then this user will also understand all languages with tags for which this tag is a prefix. The prefix rule simply allows the use of prefix tags if this is the case.
The language quality factor assigned to a language-tag by the Accept-Language field is the quality value of the longest languagerange in the field that matches the language-tag. If no languagerange in the field matches the tag, the language quality factor assigned is 0. If no Accept-Language header is present in the request, the server SHOULD assume that all languages are equally acceptable. If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are acceptable.
It may be contrary to the privacy expectations of the user to send an Accept-Language header with the complete linguistic preferences of the user in every request. For a discussion of this issue, see
section 15.7.
Note: As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user. If the choice is not made available, then the Accept-Language header field must not be given in the request.
Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges
acceptable-ranges = 1#range-unit | "none"
Accept-Ranges: bytesbut are not required to do so. Clients MAY generate byte-range requests without having received this header for the resource involved.
Servers that do not accept any kind of range request for a resource MAY send
Age = "Age" ":" age-value
age-value = delta-secondsAge values are non-negative decimal integers, representing time in seconds.
If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows, it MUST transmit an Age header with a value of 2147483648 (2^31). HTTP/1.1 caches MUST send an Age header in every response. Caches SHOULD use an arithmetic type of at least 31 bits of range.
Example of use:Allow = "Allow" ":" 1#method
The Allow header field MAY be provided with a PUT request to recommend the methods to be supported by the new or modified resource. The server is not required to support these methods and SHOULD include an Allow header in the response giving the actual supported methods.
A proxy MUST NOT modify the Allow header field even if it does not understand all the methods specified, since the user agent MAY have other means of communicating with the origin server.
The Allow header field does not indicate what methods are implemented at the server level. Servers MAY use the Public response-header field (section 14.35) to describe what methods are implemented on the server as a whole.
HTTP access authentication is described in section 11. If a request is authenticated and a realm specified, the same credentials SHOULD be valid for all other requests within this realm.Authorization = "Authorization" ":" credentials
When a shared cache (see section 13.7) receives a request containing an Authorization field, it MUST NOT return the corresponding response as a reply to any other request, unless one of the following specific exceptions holds:
1. If the response includes the "proxy-revalidate" Cache-Control directive, the cache MAY use that response in replying to a subsequent request, but a proxy cache MUST first revalidate it with the origin server, using the request-headers from the new request to allow the origin server to authenticate the new request. 2. If the response includes the "must-revalidate" Cache-Control directive, the cache MAY use that response in replying to a subsequent request, but all caches MUST first revalidate it with the origin server, using the request-headers from the new request to allow the origin server to authenticate the new request. 3. If the response includes the "public" Cache-Control directive, it may be returned in reply to any subsequent request.
Note that HTTP/1.0 caches may not implement Cache-Control and may only implement Pragma: no-cache (see section 14.32).
Cache directives must be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a cachedirective for a specific cache.
Cache-Control = "Cache-Control" ":" 1#cache-directive
cache-directive = cache-request-directive| cache-response-directive
cache-request-directive = "no-cache" [ "=" <"> 1#field-name <"> ] | "no-store" | "max-age" "=" delta-seconds | "max-stale" [ "=" delta-seconds ] | "min-fresh" "=" delta-seconds | "only-if-cached" | cache-extension
cache-response-directive = "public" | "private" [ "=" <"> 1#field-name <"> ] | "no-cache" [ "=" <"> 1#field-name <"> ] | "no-store" | "no-transform" | "must-revalidate" | "proxy-revalidate" | "max-age" "=" delta-seconds | cache-extension
cache-extension = token [ "=" ( token | quoted-string ) ]When a directive appears without any 1#field-name parameter, the directive applies to the entire request or response. When such a directive appears with a 1#field-name parameter, it applies only to the named field or fields, and not to the rest of the request or response. This mechanism supports extensibility; implementations of future versions of the HTTP protocol may apply these directives to header fields not defined in HTTP/1.1.
The cache-control directives can be broken down into these general categories:
o Restrictions on what is cachable; these may only be imposed by the origin server. o Restrictions on what may be stored by a cache; these may be imposed by either the origin server or the user agent. o Modifications of the basic expiration mechanism; these may be imposed by either the origin server or the user agent. o Controls over cache revalidation and reload; these may only be imposed by a user agent. o Control over transformation of entities. o Extensions to the caching system.
public Indicates that the response is cachable by any cache, even if it would normally be non-cachable or cachable only within a non-shared cache. (See also Authorization, section 14.8, for additional details.)
private Indicates that all or part of the response message is intended for a single user and MUST NOT be cached by a shared cache. This allows an origin server to state that the specified parts of the response are intended for only one user and are not a valid response for requests by other users. A private (non-shared) cache may cache the response.
Note: This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content.
no-cache Indicates that all or part of the response message MUST NOT be cached anywhere. This allows an origin server to prevent caching even by caches that have been configured to return stale responses to client requests.
Note: Most HTTP/1.0 caches will not recognize or obey this directive.
Even when this directive is associated with a response, users may explicitly store such a response outside of the caching system (e.g., with a "Save As" dialog). History buffers may store such responses as part of their normal operation.
The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive may improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches may not recognize or obey this directive; and communications networks may be vulnerable to eavesdropping.
If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This may be useful if certain HTTP/1.0 caches improperly calculate ages or expiration times, perhaps due to desynchronized clocks.
Note: most older caches, not compliant with this specification, do not implement any Cache-Control directives. An origin server wishing to use a Cache-Control directive that restricts, but does not prevent, caching by an HTTP/1.1-compliant cache may exploit the requirement that the max-age directive overrides the Expires header, and the fact that non-HTTP/1.1-compliant caches do not observe the max-age directive.
Other directives allow an user agent to modify the basic expiration mechanism. These directives may be specified on a request:
max-age Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless max-stale directive is also included, the client is not willing to accept a stale response.
min-fresh Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the
specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number of seconds.
max-stale Indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age.
If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured to override the expiration time of a response, the cache MUST attach a Warning header to the stale response, using Warning 10 (Response is stale).
End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it "specific end-to-end revalidation."
The client can specify these three kinds of action using CacheControl request directives:
End-to-end reload The request includes a "no-cache" Cache-Control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache". No field names may be included with the no-cache directive in a request. The server MUST NOT use a cached copy when responding to such a request.
Specific end-to-end revalidation The request includes a "max-age=0" Cache-Control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial
request includes a cache-validating conditional with the client's current validator.
Unspecified end-to-end revalidation The request includes "max-age=0" Cache-Control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional; the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional with its current validator.
When an intermediate cache is forced, by means of a max-age=0 directive, to revalidate its own cache entry, and the client has supplied its own validator in the request, the supplied validator may differ from the validator currently stored with the cache entry. In this case, the cache may use either validator in making its own request without affecting semantic transparency.
However, the choice of validator may affect performance. The best approach is for the intermediate cache to use its own validator when making its request. If the server replies with 304 (Not Modified), then the cache should return its now validated copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate cache should compare the returned validator with the one provided in the client's request, using the strong comparison function. If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified). Otherwise, it returns the new entity with a 200 (OK) response.
If a request includes the no-cache directive, it should not include min-fresh, max-stale, or max-age.
In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses that it currently has stored, and not to reload or revalidate with the origin server. To do this, the client may include the only-if-cached directive in a request. If it receives this directive, a cache SHOULD either respond using a cached entry that is consistent with the other constraints of the request, or respond with a 504 (Gateway Timeout) status. However, if a group of caches is being operated as a unified system with good internal connectivity, such a request MAY be forwarded within that group of caches.
Because a cache may be configured to ignore a server's specified expiration time, and because a client request may include a max-stale directive (which has a similar effect), the protocol also includes a
mechanism for the origin server to require revalidation of a cache entry on any subsequent use. When the must-revalidate directive is present in a response received by a cache, that cache MUST NOT use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server. (I.e., the cache must do an end-to-end revalidation every time, if, based solely on the origin server's Expires or max-age value, the cached response is stale.)
The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey the must-revalidate directive; in particular, if the cache cannot reach the origin server for any reason, it MUST generate a 504 (Gateway Timeout) response.
Servers should send the must-revalidate directive if and only if failure to revalidate a request on the entity could result in incorrect operation, such as a silently unexecuted financial transaction. Recipients MUST NOT take any automated action that violates this directive, and MUST NOT automatically provide an unvalidated copy of the entity if revalidation fails.
Although this is not recommended, user agents operating under severe connectivity constraints may violate this directive but, if so, MUST explicitly warn the user that an unvalidated response has been provided. The warning MUST be provided on each unvalidated access, and SHOULD require explicit user confirmation.
The proxy-revalidate directive has the same meaning as the mustrevalidate directive, except that it does not apply to non-shared user agent caches. It can be used on a response to an authenticated request to permit the user's cache to store and later return the response without needing to revalidate it (since it has already been authenticated once by that user), while still requiring proxies that service many users to revalidate each time (in order to make sure that each user has been authenticated). Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at all.
Serious operational problems have already occurred, however, when these transformations have been applied to entity bodies intended for certain kinds of applications. For example, applications for medical imaging, scientific data analysis and those using end-to-end authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body.
Therefore, if a response includes the no-transform directive, an intermediate cache or proxy MUST NOT change those headers that are listed in section 13.5.2 as being subject to the no-transform directive. This implies that the cache or proxy must not change any aspect of the entity-body that is specified by these headers.
This extension mechanism depends on a HTTP cache obeying all of the cache-control directives defined for its native HTTP-version, obeying certain extensions, and ignoring all directives that it does not understand.
For example, consider a hypothetical new response directive called "community" which acts as a modifier to the "private" directive. We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members of the community named within its value may cache the response. An origin server wishing to allow the "UCI" community to use an otherwise private response in their shared cache(s) may do so by including
Unrecognized cache-directives MUST be ignored; it is assumed that any cache-directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard directives (or the response's default cachability) such that the cache behavior will remain minimally correct even if the cache does not understand the extension(s).
Connection-header = "Connection" ":" 1#(connection-token)
connection-token = tokenHTTP/1.1 proxies MUST parse the Connection header field before a message is forwarded and, for each connection-token in this field, remove any header field(s) from the message with the same name as the connection-token. Connection options are signaled by the presence of a connection-token in the Connection header field, not by any corresponding additional header field(s), since the additional header field may not be sent if there are no parameters associated with that connection option. HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion of the response. For example,
HTTP/1.1 applications that do not support persistent connections MUST include the "close" connection option in every message.
If no Content-Base field is present, the base URI of an entity is defined either by its Content-Location (if that Content-Location URI is an absolute URI) or the URI used to initiate the request, in thatContent-Base = "Content-Base" ":" absoluteURI
order of precedence. Note, however, that the base URI of the contents within the entity-body may be redefined within that entity-body.
Content codings are defined in section 3.5. An example of its use isContent-Encoding = "Content-Encoding" ":" 1#content-coding
If multiple encodings have been applied to an entity, the content codings MUST be listed in the order in which they were applied.
Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification.
Language tags are defined in section 3.10. The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user's own preferred language. Thus, if the body content is intended only for a Danish-literate audience, the appropriate field isContent-Language = "Content-Language" ":" 1#language-tag
does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi," presented simultaneously in the original Maori and English versions, would call for
Content-Language may be applied to any media type -- it is not limited to textual documents.
An example isContent-Length = "Content-Length" ":" 1*DIGIT
Any Content-Length greater than or equal to zero is a valid value. Section 4.4 describes how to determine the length of a message-body if a Content-Length is not given.
Note: The meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the "message/external-body" content-type. In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being transferred.
Content-Location = "Content-Location" ":"( absoluteURI | relativeURI )
If no Content-Base header field is present, the value of ContentLocation also defines the base URL for the entity (see section 14.11).
The Content-Location value is not a replacement for the original requested URI; it is only a statement of the location of the resource corresponding to this particular entity at the time of the request. Future requests MAY use the Content-Location URI if the desire is to identify the source of that particular entity.
A cache cannot assume that an entity with a Content-Location different from the URI used to retrieve it can be used to respond to later requests on that Content-Location URI. However, the ContentLocation can be used to differentiate between multiple entities retrieved from a single requested resource, as described in section 13.6.
If the Content-Location is a relative URI, the URI is interpreted relative to any Content-Base URI provided in the response. If no Content-Base is provided, the relative URI is interpreted relative to the Request-URI.
Content-MD5 = "Content-MD5" ":" md5-digest
md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864>The Content-MD5 header field may be generated by an origin server to function as an integrity check of the entity-body. Only origin servers may generate the Content-MD5 header field; proxies and gateways MUST NOT generate it, as this would defeat its value as an end-to-end integrity check. Any recipient of the entity-body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received.
The MD5 digest is computed based on the content of the entity-body, including any Content-Encoding that has been applied, but not including any Transfer-Encoding that may have been applied to the message-body. If the message is received with a Transfer-Encoding, that encoding must be removed prior to checking the Content-MD5 value against the received entity.
This has the result that the digest is computed on the octets of the entity-body exactly as, and in the order that, they would be sent if no Transfer-Encoding were being applied.
HTTP extends RFC 1864 to permit the digest to be computed for MIME composite media-types (e.g., multipart/* and message/rfc822), but this does not change how the digest is computed as defined in the preceding paragraph.
Note: There are several consequences of this. The entity-body for composite types may contain many body-parts, each with its own MIME and HTTP headers (including Content-MD5, Content-Transfer-Encoding, and Content-Encoding headers). If a body-part has a ContentTransfer-Encoding or Content-Encoding header, it is assumed that the content of the body-part has had the encoding applied, and the body-part is included in the Content-MD5 digest as is -- i.e., after the application. The Transfer-Encoding header field is not allowed within body-parts.
Note: while the definition of Content-MD5 is exactly the same for HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
in which the application of Content-MD5 to HTTP entity-bodies differs from its application to MIME entity-bodies. One is that HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does use Transfer-Encoding and Content-Encoding. Another is that HTTP more frequently uses binary content types than MIME, so it is worth noting that, in such cases, the byte order used to compute the digest is the transmission byte order defined for the type. Lastly, HTTP allows transmission of text types with any of several line break conventions and not just the canonical form using CRLF. Conversion of all line breaks to CRLF should not be done before computing or checking the digest: the line break convention used in the text actually transmitted should be left unaltered when computing the digest.
Content-Range = "Content-Range" ":" content-range-spec
content-range-spec = byte-content-range-spec
byte-content-range-spec = bytes-unit SP first-byte-pos "-"last-byte-pos "/" entity-length
entity-length = 1*DIGITUnlike byte-ranges-specifier values, a byte-content-range-spec may only specify one range, and must contain absolute byte positions for both the first and last byte of the range.
A byte-content-range-spec whose last-byte-pos value is less than its first-byte-pos value, or whose entity-length value is less than or equal to its last-byte-pos value, is invalid. The recipient of an invalid byte-content-range-spec MUST ignore it and any content transferred along with it.
Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes:
o The first 500 bytes:
o The second 500 bytes:
o All except for the first 500 bytes:
o The last 500 bytes:
When an HTTP message includes the content of a single range (for example, a response to a request for a single range, or to a request for a set of ranges that overlap without any holes), this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred. For example,
HTTP/1.1 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT Last-modified: Wed, 15 Nov 1995 04:58:08 GMT Content-Range: bytes 21010-47021/47022 Content-Length: 26012 Content-Type: image/gifWhen an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart MIME message. The multipart MIME content-type used for this purpose is defined in this specification to be "multipart/byteranges". See appendix 19.2 for its definition.
A client that cannot decode a MIME multipart/byteranges message should not ask for multiple byte-ranges in a single request.
When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request.
If the server ignores a byte-range-spec because it is invalid, the server should treat the request as if the invalid Range header field
did not exist. (Normally, this means return a 200 response containing the full entity). The reason is that the only time a client will make such an invalid request is when the entity is smaller than the entity retrieved by a prior request.
Content-Type = "Content-Type" ":" media-typeMedia types are defined in section 3.7. An example of the field is
An example isDate = "Date" ":" HTTP-date
In theory, the date SHOULD represent the moment just before the entity is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value.
The format of the Date is an absolute date and time as defined by HTTP-date in section 3.3; it MUST be sent in RFC1123 [8]-date format.
Examples:ETag = "ETag" ":" entity-tag
ETag: "xyzzy" ETag: W/"xyzzy" ETag: ""
The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time.
The format is an absolute date and time as defined by HTTP-date in section 3.3; it MUST be in RFC1123-date format:
Expires = "Expires" ":" HTTP-date
Expires: Thu, 01 Dec 1994 16:00:00 GMTNote: if a response includes a Cache-Control field with the max-age directive, that directive overrides the Expires field.
HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
To mark a response as "already expired," an origin server should use an Expires date that is equal to the Date header value. (See the rules for expiration calculations in section 13.2.4.)
To mark a response as "never expires," an origin server should use an Expires date approximately one year from the time the response is sent. HTTP/1.1 servers should not send Expires dates more than one year in the future.
The presence of an Expires header field with a date value of some time in the future on an response that otherwise would by default be non-cacheable indicates that the response is cachable, unless indicated otherwise by a Cache-Control header field (section 14.9).
An example is:From = "From" ":" mailbox
The Internet e-mail address in this field MAY be separate from the Internet host which issued the request. For example, when a request is passed through a proxy the original issuer's address SHOULD be used.
Note: The client SHOULD not send the From header field without the user's approval, as it may conflict with the user's privacy interests or their site's security policy. It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any time prior to a request.
A "host" without any trailing port information implies the default port for the service requested (e.g., "80" for an HTTP URL). For example, a request on the origin server for <http://www.w3.org/pub/WWW/> MUST include:Host = "Host" ":" host [ ":" port ] ; Section 3.2.2
GET /pub/WWW/ HTTP/1.1
Host: www.w3.orgA client MUST include a Host header field in all HTTP/1.1 request messages on the Internet (i.e., on any message corresponding to a request for a URL which includes an Internet host address for the service being requested). If the Host field is not already present, an HTTP/1.1 proxy MUST add a Host field to the request message prior to forwarding it on the Internet. All Internet-based HTTP/1.1 servers MUST respond with a 400 status code to any HTTP/1.1 request message which lacks a Host header field.
The If-Modified-Since request-header field is used with the GET method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not
be returned from the server; instead, a 304 (not modified) response will be returned without any message-body.
An example of the field is:If-Modified-Since = "If-Modified-Since" ":" HTTP-date
a)If the request would normally result in anything other than a 200 (OK) status, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.
b)If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.
c)If the variant has not been modified since a valid If-Modified-Since date, the server MUST return a 304 (Not Modified) response.
The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.
Note that the Range request-header field modifies the meaning of If-Modified-Since; see section 14.36 for full details.
Note that If-Modified-Since times are interpreted by the server, whose clock may not be synchronized with the client.
Note that if a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between the client and server. This includes the possibility of race conditions if the document has changed between the time it was first requested and the If-ModifiedSince date of a subsequent request, and the possibility of clockskew-related problems if the If-Modified-Since date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between client and server are at best approximate due to network latency.
If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist.If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
A server MUST use the strong comparison function (see section 3.11) to compare the entity tags in If-Match.
If none of the entity tags match, or if "*" is given and no current entity exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it.
If the request would, without the If-Match header field, result in anything other than a 2xx status, then the If-Match header MUST be ignored.
The meaning of "If-Match: *" is that the method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.43) exists, and MUST NOT be performed if the representation does not exist.
A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. Examples:
If-Match: "xyzzy" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: *
As a special case, the value "*" matches any current entity of the resource.
If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-None-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MUST NOT perform the requested method. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache-related entity-header fields (particularly ETag) of one of the entities that matched. For all other request methods, the server MUST respond with a status of 412 (Precondition Failed).If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
See section 13.3.3 for rules on how to determine if two entity tags match. The weak comparison function can only be used with GET or HEAD requests.
If none of the entity tags match, or if "*" is given and no current entity exists, then the server MAY perform the requested method as if the If-None-Match header field did not exist.
If the request would, without the If-None-Match header field, result in anything other than a 2xx status, then the If-None-Match header MUST be ignored.
The meaning of "If-None-Match: *" is that the method MUST NOT be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.43) exists, and SHOULD be performed if the representation does not exist. This feature may be useful in preventing races between PUT operations.
If-None-Match: "xyzzy" If-None-Match: W/"xyzzy" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-None-Match: *
The If-Range header allows a client to "short-circuit" the second request. Informally, its meaning is `if the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity.'
If the client has no entity tag for an entity, but does have a LastModified date, it may use that date in a If-Range header. (The server can distinguish between a valid HTTP-date and any form of entity-tag by examining no more than two characters.) The If-Range header should only be used together with a Range header, and must be ignored if the request does not include a Range header, or if the server does not support the sub-range operation.If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server should provide the specified sub-range of the entity using a 206 (Partial content) response. If the entity tag does not match, then the server should return the entire entity using a 200 (OK) response.
If the requested variant has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition Failed).
An example of the field is:If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified.
An example of its use isLast-Modified = "Last-Modified" ":" HTTP-date
An origin server MUST NOT send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace that date with the message origination date.
An origin server should obtain the Last-Modified value of the entity as close as possible to the time that it generates the Date value of its response. This allows a recipient to make an accurate assessment of the entity's modification time, especially if the entity changes near the time that the response is generated.
The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource. For 201 (Created) responses, the Location is that of the new resource which was created by the request. For 3xx responses, the location SHOULD indicate the server's preferred URL for automatic redirection to the resource. The field value consists of a single absolute URL.
An example isLocation = "Location" ":" absoluteURI
Max-Forwards = "Max-Forwards" ":" 1*DIGIT
The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded.
Each proxy or gateway recipient of a TRACE request containing a MaxForwards header field SHOULD check and update its value prior to forwarding the request. If the received value is zero (0), the recipient SHOULD NOT forward the request; instead, it SHOULD respond as the final recipient with a 200 (OK) response containing the received request message as the response entity-body (as described in section 9.8). If the received Max-Forwards value is greater than zero, then the forwarded message SHOULD contain an updated MaxForwards field with a value decremented by one (1).
The Max-Forwards header field SHOULD be ignored for all other methods defined by this specification and for any extension methods for which it is not explicitly referred to as part of that method definition.
Pragma = "Pragma" ":" 1#pragma-directive
pragma-directive = "no-cache" | extension-pragma
extension-pragma = token [ "=" ( token | quoted-string ) ]When the no-cache directive is present in a request message, an application SHOULD forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive has the same semantics as the no-cache cache-directive (see section 14.9) and is defined here for backwards compatibility with HTTP/1.0. Clients SHOULD include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant.
Pragma directives MUST be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific recipient; however, any pragma directive not relevant to a recipient SHOULD be ignored by that recipient.
HTTP/1.1 clients SHOULD NOT send the Pragma request-header. HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in HTTP.
The HTTP access authentication process is described in section 11. Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the current connection and SHOULD NOT be passed on to downstream clients. However, an intermediate proxy may need to obtain its own credentials by requesting them from the downstream client, which in some circumstances will appear as if the proxy is forwarding the Proxy-Authenticate header field.Proxy-Authenticate = "Proxy-Authenticate" ":" challenge
The HTTP access authentication process is described in section 11. Unlike Authorization, the Proxy-Authorization header field applies only to the next outbound proxy that demanded authentication using the Proxy-Authenticate field. When multiple proxies are used in a chain, the Proxy-Authorization header field is consumed by the first outbound proxy that was expecting to receive credentials. A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request.Proxy-Authorization = "Proxy-Authorization" ":" credentials
Request-URI; the Allow header field (section 14.7) MAY be used to indicate methods allowed for a particular URI.
Example of use:Public = "Public" ":" 1#method
Since all HTTP entities are represented in HTTP messages as sequences of bytes, the concept of a byte range is meaningful for any HTTP entity. (However, not all clients and servers need to support byterange operations.)
Byte range specifications in HTTP apply to the sequence of bytes in the entity-body (not necessarily the same as the message-body).
A byte range operation may specify a single range of bytes, or a set of ranges within a single entity.
ranges-specifier = byte-ranges-specifier
byte-ranges-specifier = bytes-unit "=" byte-range-set
byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec )
byte-range-spec = first-byte-pos "-" [last-byte-pos]
first-byte-pos = 1*DIGIT
last-byte-pos = 1*DIGITThe first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.
If the last-byte-pos value is present, it must be greater than or equal to the first-byte-pos in that byte-range-spec, or the byterange-spec is invalid. The recipient of an invalid byte-range-spec must ignore it.
If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the entity-body, last-byte-pos is taken to be equal to one less than the current length of the entitybody in bytes.
By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the entity.
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGITA suffix-byte-range-spec is used to specify the suffix of the entity-body, of a length given by the suffix-length value. (That is, this form specifies the last N bytes of an entity-body.) If the entity is shorter than the specified suffix-length, the entire entity-body is used.
Examples of byte-ranges-specifier values (assuming an entity-body of length 10000):
o The first 500 bytes (byte offsets 0-499, inclusive):
bytes=0-499
o The second 500 bytes (byte offsets 500-999, inclusive):
o The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500
o Or
bytes=9500-
o The first and last bytes only (bytes 0 and 9999):
bytes=0-0,-1
o Several legal but not canonical specifications of the second 500 bytes (byte offsets 500-999, inclusive):
A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches SHOULD support byte ranges when possible, since Range supports efficient recovery from partially failed transfers, and supports efficient partial retrieval of large entities.Range = "Range" ":" ranges-specifier
If the server supports the Range header and the specified range or ranges are appropriate for the entity:
o The presence of a Range header in an unconditional GET modifies what is returned if the GET is otherwise successful. In other words, the response carries a status code of 206 (Partial Content) instead of 200 (OK).
o The presence of a Range header in a conditional GET (a request using one or both of If-Modified-Since and If-None-Match, or one or both of If-Unmodified-Since and If-Match) modifies what is returned if the GET is otherwise successful and the condition is true. It does not affect the 304 (Not Modified) response returned if the conditional is false.
In some cases, it may be more appropriate to use the If-Range header (see section 14.27) in addition to the Range header.
If a proxy that supports ranges receives a Range request, forwards the request to an inbound server, and receives an entire entity in reply, it SHOULD only return the requested range to its client. It SHOULD store the entire received response in its cache, if that is consistent with its cache allocation policies.
Example:Referer = "Referer" ":" ( absoluteURI | relativeURI )
Note: Because the source of a link may be private information or may reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not the Referer field is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending of Referer and From information.
Two examples of its use areRetry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT Retry-After: 120
Example:Server = "Server" ":" 1*( product | comment )
Note: Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Server implementers are encouraged to make this field a configurable option.
Transfer-Encoding = "Transfer-Encoding" ":" 1#transfercoding
Transfer-Encoding: chunkedMany older HTTP/1.0 applications do not understand the TransferEncoding header.
MUST use the Upgrade header field within a 101 (Switching Protocols) response to indicate which protocol(s) are being switched.
For example,Upgrade = "Upgrade" ":" 1#product
The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection. Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial HTTP request containing the Upgrade header field.
The Upgrade header field only applies to the immediate connection. Therefore, the upgrade keyword MUST be supplied within a Connection header field (section 14.10) whenever Upgrade is present in an HTTP/1.1 message.
The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it is more appropriate to use a 301, 302, 303, or 305 redirection response.
This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of section 3.1 and future updates to this specification. Any token can be used as a protocol name; however, it will only be useful if both the client and server associate the name with the same protocol.
Example:User-Agent = "User-Agent" ":" 1*( product | comment )
The Vary response-header field is used by a server to signal that the response entity was selected from the available representations of the response using server-driven negotiation (section 12). Fieldnames listed in Vary headers are those of request-headers. The Vary field value indicates either that the given set of header fields encompass the dimensions over which the representation might vary, or that the dimensions of variance are unspecified ("*") and thus may vary over any aspect of future requests.
An HTTP/1.1 server MUST include an appropriate Vary header field with any cachable response that is subject to server-driven negotiation. Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that resource. A server SHOULD include an appropriate Vary header field with a non-cachable response that is subject to server-driven negotiation, since this might provide the user agent with useful information about the dimensions over which the response might vary.Vary = "Vary" ":" ( "*" | 1#field-name )
The set of header fields named by the Vary field value is known as the "selecting" request-headers.
When the cache receives a subsequent request whose Request-URI specifies one or more cache entries including a Vary header, the cache MUST NOT use such a cache entry to construct a response to the new request unless all of the headers named in the cached Vary header
are present in the new request, and all of the stored selecting request-headers from the previous request match the corresponding headers in the new request.
The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first request can be transformed to the selecting request-headers in the second request by adding or removing linear whitespace (LWS) at places where this is allowed by the corresponding BNF, and/or combining multiple messageheader fields with the same field name following the rules about message headers in section 4.2.
A Vary field value of "*" signals that unspecified parameters, possibly other than the contents of request-header fields (e.g., the network address of the client), play a role in the selection of the response representation. Subsequent requests on that resource can only be properly interpreted by the origin server, and thus a cache MUST forward a (possibly conditional) request even when it has a fresh response cached for the resource. See section 13.6 for use of the Vary header by caches.
A Vary field value consisting of a list of field-names signals that the representation selected for the response is based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation. A cache MAY assume that the same selection will be made for future requests with the same values for the listed field names, for the duration of time in which the response is fresh.
The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names are case-insensitive.
Via = "Via" ":" 1#( received-protocol received-by [ comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
protocol-name = token
protocol-version = token
received-by = ( host [ ":" port ] ) | pseudonym
pseudonym = tokenThe received-protocol indicates the protocol version of the message received by the server or client along each segment of the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients.
The protocol-name is optional if and only if it would be "HTTP". The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. If the port is not given, it MAY be assumed to be the default port of the received-protocol.
Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applications.
Comments MAY be used in the Via header field to identify the software of the recipient proxy or gateway, analogous to the User-Agent and Server header fields. However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message.
For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by forwarding it to the origin server at www.ics.uci.edu. The request received by www.ics.uci.edu would then have the following Via header field:
For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. For example,
Warning = "Warning" ":" 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
warn-code = 2DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym; the name or pseudonym of the server adding ; the Warning header, for use in debugging
warn-text = quoted-string
If a character set other than ISO-8859-1 is used, it MUST be encoded in the warn-text using the method described in RFC 1522 [14].
Any server or cache may add Warning headers to a response. New Warning headers should be added after any existing Warning headers. A cache MUST NOT delete any Warning header that it received with a response. However, if a cache successfully validates a cache entry, it SHOULD remove any Warning headers previously attached to that entry except as specified for specific Warning codes. It MUST then add any Warning headers received in the validating response. In other words, Warning headers are those that would be attached to the most recent relevant response.
When multiple Warning headers are attached to a response, the user agent SHOULD display as many of them as possible, in the order that they appear in the response. If it is not possible to display all of the warnings, the user agent should follow these heuristics:
o Warnings that appear early in the response take priority over those appearing later in the response. o Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes and warn-agents.
Systems that generate multiple Warning headers should order them with this user agent behavior in mind.
This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning.
10 Response is stale MUST be included whenever the returned response is stale. A cache may add this warning to any response, but may never remove it until the response is known to be fresh.
11 Revalidation failed MUST be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability to reach the server. A cache may add this warning to any response, but may never remove it until the response is successfully revalidated.
12 Disconnected operation SHOULD be included if the cache is intentionally disconnected from the rest of the network for a period of time.
13 Heuristic expiration MUST be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.
14 Transformation applied MUST be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, unless this Warning code already appears in the response. MUST NOT be deleted from a response even after revalidation.
99 Miscellaneous warning The warning text may include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action.
The HTTP access authentication process is described in section 11. User agents MUST take special care in parsing the WWW-Authenticate field value if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents of a challenge may itself contain a comma-separated list of authentication parameters.WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge
The most serious flaw in Basic authentication is that it results in the essentially clear text transmission of the user's password over the physical network. It is this problem which Digest Authentication attempts to address.
Because Basic authentication involves the clear text transmission of passwords it SHOULD never be used (without enhancements) to protect sensitive or valuable information.
A common use of Basic authentication is for identification purposes -- requiring the user to provide a user name and password as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this way it is tempting to think that there is no danger in its use if illicit access to the protected documents is not a major concern. This is only correct if the server issues both user name and password to the users and in particular does not allow the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid the task of maintaining multiple passwords.
If a server permits users to select their own passwords, then the threat is not only illicit access to documents on the server but also illicit access to the accounts of all users who have chosen to use their account password. If users are allowed to choose their own password that also means the server must maintain files containing the (presumably encrypted) passwords. Many of these may be the account passwords of users perhaps at distant sites. The owner or administrator of such a system could conceivably incur liability if this information is not maintained in a secure fashion.
Basic Authentication is also vulnerable to spoofing by counterfeit servers. If a user can be led to believe that he is connecting to a host containing information protected by basic authentication when in fact he is connecting to a hostile server or gateway then the attacker can request a password, store it for later use, and feign an error. This type of attack is not possible with Digest Authentication [32]. Server implementers SHOULD guard against the possibility of this sort of counterfeiting by gateways or CGI scripts. In particular it is very dangerous for a server to simply turn over a connection to a gateway since that gateway can then use the persistent connection mechanism to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable by the client.
scheme. The order of the challenges returned to the user agent is in the order that the server would prefer they be chosen. The server should order its challenges with the "most secure" authentication scheme first. A user agent should choose as the challenge to be made to the user the first one that the user agent understands.
When the server offers choices of authentication schemes using the WWW-Authenticate header, the "security" of the authentication is only as malicious user could capture the set of challenges and try to authenticate him/herself using the weakest of the authentication schemes. Thus, the ordering serves more to protect the user's credentials than the server's information.
A possible man-in-the-middle (MITM) attack would be to add a weak authentication scheme to the set of choices, hoping that the client will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest scheme that it understands from the choices accepted.
An even better MITM attack would be to remove all offered choices, and to insert a challenge that requests Basic authentication. For this reason, user agents that are concerned about this kind of attack could remember the strongest authentication scheme ever requested by a server and produce a warning message that requires user confirmation before using a weaker one. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy caching service to gullible users.
Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Implementers SHOULD make the Server header field a configurable option.
Proxies which serve as a portal through a network firewall SHOULD take special precautions regarding the transfer of header information that identifies the hosts behind the firewall. In particular, they SHOULD remove, or replace with sanitized versions, any Via fields generated behind the firewall.
The Referer field allows reading patterns to be studied and reverse links drawn. Although it can be very useful, its power can be abused if user details are not separated from the information contained in the Referer. Even when the personal information has been removed, the Referer field may indicate a private document's URI whose publication would be inappropriate.
The information sent in the From field might conflict with the user's privacy interests or their site's security policy, and hence it SHOULD NOT be transmitted without the user being able to disable, enable, and modify the contents of the field. The user MUST be able to set the contents of this field within a user preference or application defaults configuration.
We suggest, though do not require, that a convenient toggle interface be provided for the user to enable or disable the sending of From and Referer information.
An approach that limits the loss of privacy would be for a user agent to omit the sending of Accept-Language headers by default, and to ask the user whether it should start sending Accept-Language headers to a server if it detects, by looking for any Vary response-header fields generated by the server, that such sending could improve the quality of service.
Elaborate user-customized accept header fields sent in every request, in particular if these include quality values, can be used by servers as relatively reliable and long-lived user identifiers. Such user identifiers would allow content providers to do click-trail tracking, and would allow collaborating content providers to match cross-server click-trails or form submissions of individual users. Note that for many users not behind a proxy, the network address of the host running the user agent will also serve as a long-lived user identifier. In environments where proxies are used to enhance privacy, user agents should be conservative in offering accept header configuration options to end users. As an extreme privacy measure, proxies could filter the accept headers in relayed requests. General purpose user agents which provide a high degree of header configurability should warn users about the loss of privacy which can be involved.
In particular, HTTP clients SHOULD rely on their name resolver for confirmation of an IP number/DNS name association, rather than caching the result of previous host name lookups. Many platforms already can cache host name lookups locally when appropriate, and they SHOULD be configured to do so. These lookups should be cached, however, only when the TTL (Time To Live) information reported by the name server makes it likely that the cached information will remain useful.
If HTTP clients cache the results of host name lookups in order to achieve a performance improvement, they MUST observe the TTL information reported by DNS.
If HTTP clients do not observe this rule, they could be spoofed when a previously-accessed server's IP address changes. As network renumbering is expected to become increasingly common, the possibility of this form of attack will grow. Observing this requirement thus reduces this potential security vulnerability.
This requirement also improves the load-balancing behavior of clients for replicated servers using the same DNS name and reduces the likelihood of a user's experiencing failure in accessing sites which use that strategy.
The HTTP protocol has evolved considerably over the past four years. It has benefited from a large and active developer community--the many people who have participated on the www-talk mailing list--and it is that community which has been most responsible for the success of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special recognition for their efforts in defining early aspects of the protocol.
This document has benefited greatly from the comments of all those participating in the HTTP-WG. In addition to those already mentioned, the following individuals have contributed to this specification:
Gary Adams Albert Lunde Harald Tveit Alvestrand John C. Mallery Keith Ball Jean-Philippe Martin-Flatin Brian Behlendorf Larry Masinter Paul Burchard Mitra Maurizio Codogno David Morris Mike Cowlishaw Gavin Nicol Roman Czyborra Bill Perry Michael A. Dolan Jeffrey Perry David J. Fiander Scott Powers Alan Freier Owen Rees Marc Hedlund Luigi Rizzo Greg Herlihy David Robinson Koen Holtman Marc Salomon Alex Hopmann Rich Salz Bob Jernigan Allan M. Schiffman Shel Kaphan Jim Seidman Rohit Khare Chuck Shotton John Klensin Eric W. Sink Martijn Koster Simon E. Spero Alexei Kosut Richard N. Taylor David M. Kristol Robert S. Thau Daniel LaLiberte Bill (BearHeart) Weinman Ben Laurie Francois Yergeau Paul J. Leach Mary Ellen Zurko Daniel DuBois
Much of the content and presentation of the caching design is due to suggestions and comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
Most of the specification of ranges is based on work originally done by Ari Luotonen and John Franks, with additional input from Steve Zilles.
[2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey, D., and B. Alberti. "The Internet Gopher Protocol: (a distributed document search and retrieval protocol)", RFC 1436, University of Minnesota, March 1993.
[3] Berners-Lee, T., "Universal Resource Identifiers in WWW", A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web", RFC 1630, CERN, June 1994.
[4] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota, December 1994.
[5] Berners-Lee, T., and D. Connolly, "HyperText Markup Language Specification - 2.0", RFC 1866, MIT/LCS, November 1995.
[6] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0.", RFC 1945 MIT/LCS, UC Irvine, May 1996.
[7] Freed, N., and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, Innosoft, First Virtual, November 1996.
[8] Braden, R., "Requirements for Internet hosts - application and support", STD 3, RFC 1123, IETF, October 1989.
[9] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822, UDEL, August 1982.
[10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R., Sui, J., and M. Grinbaum. "WAIS Interface Protocol Prototype Functional Specification", (v1.5), Thinking Machines Corporation, April 1990.
[11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, UC Irvine, June 1995.
[12] Horton, M., and R. Adams. "Standard for interchange of USENET messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic Studies, December 1987.
[13] Kantor, B., and P. Lapsley. "Network News Transfer Protocol." A Proposed Standard for the Stream-Based Transmission of News", RFC 977, UC San Diego, UC Berkeley, February 1986.
[14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047,University of Tennessee, November 1996.
[15] Nebel, E., and L. Masinter. "Form-based File Upload in HTML", RFC 1867, Xerox Corporation, November 1995.
[16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, USC/ISI, August 1982.
[17] Postel, J., "Media Type Registration Procedure", RFC 2048, USC/ISI, November 1996.
[18] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD 9, RFC 959, USC/ISI, October 1985.
[19] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700, USC/ISI, October 1994.
[20] Sollins, K., and L. Masinter, "Functional Requirements for Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation, December 1994.
[21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
[22] ISO-8859. International Standard -- Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2, ISO 8859-2, 1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987. Part 7: Latin/Greek alphabet, ISO 8859-7, 1987. Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.
[23] Meyers, J., and M. Rose "The Content-MD5 Header Field", RFC 1864, Carnegie Mellon, Dover Beach Consulting, October, 1995.
[24] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work", RFC 1900, IAB, February 1996.
[25] Deutsch, P., "GZIP file format specification version 4.3." RFC 1952, Aladdin Enterprises, May 1996.
[26] Venkata N. Padmanabhan and Jeffrey C. Mogul. Improving HTTP Latency. Computer Networks and ISDN Systems, v. 28, pp. 25-35, Dec. 1995. Slightly revised version of paper in Proc. 2nd International WWW Conf. '94: Mosaic and the Web, Oct. 1994, which is available at http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/ HTTPLatency.html.
[27] Joe Touch, John Heidemann, and Katia Obraczka, "Analysis of HTTP Performance", <URL: http://www.isi.edu/lsam/ib/http-perf/>, USC/Information Sciences Institute, June 1996
[28] Mills, D., "Network Time Protocol, Version 3, Specification, Implementation and Analysis", RFC 1305, University of Delaware, March 1992.
[29] Deutsch, P., "DEFLATE Compressed Data Format Specification version 1.3." RFC 1951, Aladdin Enterprises, May 1996.
[30] Spero, S., "Analysis of HTTP Performance Problems" <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>.
[31] Deutsch, P., and J-L. Gailly, "ZLIB Compressed Data Format Specification version 3.3", RFC 1950, Aladdin Enterprises, Info-ZIP, May 1996.
[32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., Luotonen, A., Sink, E., and L. Stewart, "An Extension to HTTP : Digest Access Authentication", RFC 2069, January 1997.
Fax: +1 (714) 824-4056 EMail: fielding@ics.uci.eduJim Gettys MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682 EMail: jg@w3.orgJeffrey C. Mogul Western Research Laboratory Digital Equipment Corporation 250 University Avenue Palo Alto, California, 94305, USA
Henrik Frystyk Nielsen W3 Consortium MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682 EMail: frystyk@w3.orgTim Berners-Lee Director, W3 Consortium MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682 EMail: timbl@w3.org
In addition to defining the HTTP/1.1 protocol, this document serves as the specification for the Internet media type "message/http". The following is to be registered with IANA.
Media Type name: message Media subtype name: http Required parameters: none Optional parameters: version, msgtype
version: The HTTP-Version number of the enclosed message (e.g., "1.1"). If not present, the version can be determined from the first line of the body.
msgtype: The message type -- "request" or "response". If not present, the type can be determined from the first line of the body.
Encoding considerations: only "7bit", "8bit", or "binary" are permitted
When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart MIME message. The multipart media type for this purpose is called "multipart/byteranges".
The multipart/byteranges media type includes two or more parts, each with its own Content-Type and Content-Range fields. The parts are separated using a MIME boundary parameter.
Media Type name: multipart Media subtype name: byteranges Required parameters: boundary Optional parameters: none
Encoding considerations: only "7bit", "8bit", or "binary" are permitted
Date: Wed, 15 Nov 1995 06:25:24 GMT Last-modified: Wed, 15 Nov 1995 04:58:08 GMT Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES--THIS_STRING_SEPARATES
Content-type: application/pdf Content-range: bytes 500-999/8000--THIS_STRING_SEPARATES...the first range...
Content-type: application/pdf Content-range: bytes 7000-7999/8000--THIS_STRING_SEPARATES--...the second range
Clients SHOULD be tolerant in parsing the Status-Line and servers tolerant when parsing the Request-Line. In particular, they SHOULD accept any amount of SP or HT characters between fields, even though only a single SP is required.
The line terminator for message-header fields is the sequence CRLF. However, we recommend that applications, when parsing such headers, recognize a single LF as a line terminator and ignore the leading CR.
The character set of an entity-body should be labeled as the lowest common denominator of the character codes used within that body, with the exception that no label is preferred over the labels US-ASCII or ISO-8859-1.
Additional rules for requirements on parsing and encoding of dates and other potential problems with date encodings include:
o HTTP/1.1 clients and caches should assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve the "year 2000" problem).
o An HTTP/1.1 implementation may internally represent a parsed Expires date as earlier than the proper value, but MUST NOT internally represent a parsed Expires date as later than the proper value.
o All expiration-related calculations must be done in GMT. The local time zone MUST NOT influence the calculation or comparison of an age or expiration time.
o If an HTTP header incorrectly carries a date value with a time zone other than GMT, it must be converted into GMT using the most conservative possible conversion.
This appendix describes specific areas where HTTP differs from MIME. Proxies and gateways to strict MIME environments SHOULD be aware of these differences and provide the appropriate conversions where necessary. Proxies and gateways from MIME environments to HTTP also need to be aware of the differences because some conversions may be required.
Where it is possible, a proxy or gateway from HTTP to a strict MIME environment SHOULD translate all line breaks within the text media types described in section 3.7.1 of this document to the MIME canonical form of CRLF. Note, however, that this may be complicated by the presence of a Content-Encoding and by the fact that HTTP
allows the use of some character sets which do not use octets 13 and 10 to represent CR and LF, as is the case for some multi-byte character sets.
Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where "safe transport" is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.
A process for decoding the "chunked" transfer coding (section 3.6) can be represented in pseudo-code as:
length := 0read chunk-size, chunk-ext (if any) and CRLF while (chunk-size > 0) { read chunk-data and CRLF append chunk-data to entity-body
length := length + chunk-sizeread chunk-size and CRLF } read entity-header while (entity-header not empty) { append entity-header to existing header fields read entity-header }
Content-Length := lengthRemove "chunked" from Transfer-Encoding
MIME version "1.0" is the default for use in HTTP/1.1. However, HTTP/1.1 message parsing and semantics are defined by this document and not the MIME specification.MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
19.5.1 Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses
The requirements that clients and servers support the Host requestheader, report an error if the Host request-header (section 14.23) is missing from an HTTP/1.1 request, and accept absolute URIs (section 5.1.2) are among the most important changes defined by this specification.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP addresses and servers; there was no other established mechanism for distinguishing the intended server of a request than the IP address to which that request was directed. The changes outlined above will allow the Internet, once older HTTP clients are no longer common, to support multiple Web sites from a single IP address, greatly simplifying large operational Web servers, where allocation of many IP addresses to a single host has created serious problems. The Internet will also be able to recover the IP addresses that have been allocated for the sole purpose of allowing special-purpose domain names to be used in root-level HTTP URLs. Given the rate of growth of the Web, and the number of servers already deployed, it is extremely important that all implementations of HTTP (including updates to existing HTTP/1.0 applications) correctly implement these requirements:
o Both clients and servers MUST support the Host request-header.
o Host request-headers are required in HTTP/1.1 requests.
o Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 request does not include a Host request-header.
o Servers MUST accept absolute URIs.
The PATCH method is similar to PUT except that the entity contains a list of differences between the original version of the resource identified by the Request-URI and the desired content of the resource after the PATCH action has been applied. The list of differences is in a format defined by the media type of the entity (e.g., "application/diff") and MUST include sufficient information to allow the server to recreate the changes necessary to convert the original version of the resource to the desired version.
If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.
The actual method for determining how the patched resource is placed, and what happens to its predecessor, is defined entirely by the origin server. If the original version of the resource being patched included a Content-Version header field, the request entity MUST include a Derived-From header field corresponding to the value of the original Content-Version header field. Applications are encouraged to use these fields for constructing versioning relationships and resolving version conflicts.
PATCH requests must obey the message transmission requirements set out in section 8.2.
Caches that implement PATCH should invalidate cached responses as defined in section 13.10 for PUT.
allowing links to be established between resources is that the LINK method does not allow any message-body to be sent in the request and does not directly result in the creation of new resources.
If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.
Caches that implement LINK should invalidate cached responses as defined in section 13.10 for PUT.
If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.
Caches that implement UNLINK should invalidate cached responses as defined in section 13.10 for PUT.
The Alternates response-header field has been proposed as a means for the origin server to inform the client about other available representations of the requested resource, along with their distinguishing attributes, and thus providing a more reliable means for a user agent to perform subsequent selection of another representation which better fits the desires of its user (described as agent-driven negotiation in section 12).
The Alternates header field is orthogonal to the Vary header field in that both may coexist in a message without affecting the interpretation of the response or the available representations. It is expected that Alternates will provide a significant improvement over the server-driven negotiation provided by the Vary field for those resources that vary over common dimensions like type and language.
The Alternates header field will be defined in a future specification.
Examples of the Content-Version field include:Content-Version = "Content-Version" ":" quoted-string
Content-Version: "2.1.2" Content-Version: "Fred 19950116-12:26:48" Content-Version: "2.5a4-omega7"
An example use of the field is:Derived-From = "Derived-From" ":" quoted-string
Link = "Link" ":" #("<" URI ">" *( ";" link-param )
link-param = ( ( "rel" "=" relationship )| ( "rev" "=" relationship ) | ( "title" "=" quoted-string ) | ( "anchor" "=" <"> URI <"> ) | ( link-extension ) )
link-extension = token [ "=" ( token | quoted-string ) ]
relationship = sgml-name| ( <"> sgml-name *( SP sgml-name) <"> )
sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" )Relationship values are case-insensitive and MAY be extended within the constraints of the sgml-name syntax. The title parameter MAY be used to label the destination of a link such that it can be used as identification within a human-readable menu. The anchor parameter MAY be used to indicate a source anchor other than the entire current resource, such as a fragment of this resource or a third resource.
Link: <http://www.cern.ch/TheBook/chapter2> rel="Previous"
field (above). Its primary purpose has been to include a list of additional URIs for the resource, including names and mirror locations. However, it has become clear that the combination of many different functions within this single field has been a barrier to consistently and correctly implementing any of those functions. Furthermore, we believe that the identification of names and mirror locations would be better performed via the Link header field. The URI header field is therefore deprecated in favor of those other fields.
19.7 Compatibility with Previous VersionsURI-header = "URI" ":" 1#( "<" URI ">" )
It is beyond the scope of a protocol specification to mandate compliance with previous versions. HTTP/1.1 was deliberately designed, however, to make supporting previous versions easy. It is worth noting that at the time of composing this specification, we would expect commercial HTTP/1.1 servers to:
o recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 requests;
o understand any valid request in the format of HTTP/0.9, 1.0, or 1.1;
o respond appropriately with a message in the same major version used by the client.
o understand any valid response in the format of HTTP/0.9, 1.0, or 1.1.
For most implementations of HTTP/1.0, each connection is established by the client prior to the request and closed by the server after sending the response. A few implementations implement the Keep-Alive version of persistent connections described in section 19.7.1.1.
However, talking to proxies is the most important use of persistent connections, so that prohibition is clearly unacceptable. Therefore, we need some other mechanism for indicating a persistent connection is desired, which is safe to use even when talking to an old proxy that ignores Connection. Persistent connections are the default for HTTP/1.1 messages; we introduce a new keyword (Connection: close) for declaring non-persistence.
The following describes the original HTTP/1.0 form of persistent connections.
When it connects to an origin server, an HTTP client MAY send the Keep-Alive connection-token in addition to the Persist connectiontoken:
An HTTP/1.1 server may also establish persistent connections with HTTP/1.0 clients upon receipt of a Keep-Alive connection token. However, a persistent connection with an HTTP/1.0 client cannot make use of the chunked transfer-coding, and therefore MUST use a Content-Length for marking the ending boundary of each message.
A client MUST NOT send the Keep-Alive connection token to a proxy server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 for parsing the Connection header field.
Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param
keepalive-param = param-name "=" valueThe Keep-Alive header itself is optional, and is used only if a parameter is being sent. HTTP/1.1 does not define any parameters.
If the Keep-Alive header is sent, the corresponding connection token MUST be transmitted. The Keep-Alive header MUST be ignored if received without the connection token.