HTTP/1.1, part 1: URIs, Connections, and Message Parsing

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

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

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234].¶

The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible [USASCII] character), and WSP (whitespace).¶

As a syntactic convention, ABNF rule names prefixed with "obs-" denote "obsolete" grammar rules that appear for historical reasons.¶

HTTP is a stateless request/response protocol that operates by exchanging messages across a reliable transport or session-layer "connection". An HTTP "client" is a program that establishes a connection to a server for the purpose of sending one or more HTTP requests. An HTTP "server" is a program that accepts connections in order to service HTTP requests by sending HTTP responses.¶

Note that the terms client and server refer only to the roles that these programs perform for a particular connection. The same program might act as a client on some connections and a server on others. We use the term "user agent" to refer to the program that initiates a request, such as a WWW browser, editor, or spider (web-traversing robot), and the term "origin server" to refer to the program that can originate authoritative responses to a request. For general requirements, we use the term "sender" to refer to whichever component sent a given message and the term "recipient" to refer to any component that receives the message.¶

Most HTTP communication consists of a retrieval request (GET) for a representation of some resource identified by a URI. In the simplest case, this might be accomplished via a single bidirectional connection (===) between the user agent (UA) and the origin server (O).¶

A client sends an HTTP request to the server in the form of a request message (Section 4), beginning with a method, URI, and protocol version, followed by MIME-like header fields containing request modifiers, client information, and payload metadata, an empty line to indicate the end of the header section, and finally the payload body (if any).¶

A server responds to the client's request by sending an HTTP response message (Section 5), beginning with a status line that includes the protocol version, a success or error code, and textual reason phrase, followed by MIME-like header fields containing server information, resource metadata, and payload metadata, an empty line to indicate the end of the header section, and finally the payload body (if any).¶

client request:

GET /hello.txt HTTP/1.1
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com
Accept: */*

server response:

HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length: 14
Vary: Accept-Encoding
Content-Type: text/plain

Hello World!

Fundamentally, HTTP is a message-based protocol. Although message bodies can be chunked (Section 6.2.1) and implementations often make parts of a message available progressively, this is not required, and some widely-used implementations only make a message available when it is complete. Furthermore, while most proxies will progressively stream messages, some amount of buffering will take place, and some proxies might buffer messages to perform transformations, check content or provide other services.¶

Therefore, extensions to and uses of HTTP cannot rely on the availability of a partial message, or assume that messages will not be buffered. There are strategies that can be used to test for buffering in a given connection, but it should be understood that behaviors can differ across connections, and between requests and responses.¶

Recipients MUST consider every message in a connection in isolation; because HTTP is a stateless protocol, it cannot be assumed that two requests on the same connection are from the same client or share any other common attributes. In particular, intermediaries might mix requests from different clients into a single server connection. Note that some existing HTTP extensions (e.g., [RFC4559]) violate this requirement, thereby potentially causing interoperability and security problems.¶

HTTP messaging is independent of the underlying transport or session-layer connection protocol(s). HTTP only presumes a reliable transport with in-order delivery of requests and the corresponding in-order delivery of responses. The mapping of HTTP request and response structures onto the data units of the underlying transport protocol is outside the scope of this specification.¶

The specific connection protocols to be used for an interaction are determined by client configuration and the target resource's URI. For example, the "http" URI scheme (Section 2.7.1) indicates a default connection of TCP over IP, with a default TCP port of 80, but the client might be configured to use a proxy via some other connection port or protocol instead of using the defaults.¶

A connection might be used for multiple HTTP request/response exchanges, as defined in Section 7.1.¶

HTTP enables the use of intermediaries to satisfy requests through a chain of connections. There are three common forms of HTTP intermediary: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.¶

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. Some HTTP communication options might 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 might be engaged in multiple, simultaneous communications. For example, B might 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.¶

We use the terms "upstream" and "downstream" to describe various requirements in relation to the directional flow of a message: all messages flow from upstream to downstream. Likewise, we use the terms inbound and outbound to refer to directions in relation to the request path: "inbound" means toward the origin server and "outbound" means toward the user agent.¶

A "proxy" is a message forwarding agent that is selected by the client, usually via local configuration rules, to receive requests for some type(s) of absolute URI and attempt to satisfy those requests via translation through the HTTP interface. Some translations are minimal, such as for proxy requests for "http" URIs, whereas other requests might require translation to and from entirely different application-layer protocols. Proxies are often used to group an organization's HTTP requests through a common intermediary for the sake of security, annotation services, or shared caching.¶

An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed or configured to modify request or response messages in a semantically meaningful way (i.e., modifications, beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder, or an intranet-to-Internet privacy filter. Such transformations are presumed to be desired by the client (or client organization) that selected the proxy and are beyond the scope of this specification. However, when a proxy is not intended to transform a given message, we use the term "non-transforming proxy" to target requirements that preserve HTTP message semantics. See Section 8.2.4 of [Part2] and Section 3.6 of [Part6] for status and warning codes related to transformations.¶

A "gateway" (a.k.a., "reverse proxy") is a receiving agent that acts as a layer above some other server(s) and translates the received requests to the underlying server's protocol. Gateways are often used to encapsulate legacy or untrusted information services, to improve server performance through "accelerator" caching, and to enable partitioning or load-balancing of HTTP services across multiple machines.¶

A gateway behaves as an origin server on its outbound connection and as a user agent on its inbound connection. All HTTP requirements applicable to an origin server also apply to the outbound communication of a gateway. A gateway communicates with inbound servers using any protocol that it desires, including private extensions to HTTP that are outside the scope of this specification. However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party HTTP servers MUST comply with HTTP user agent requirements on the gateway's inbound connection and MUST implement the Connection (Section 9.1) and Via (Section 9.9) header fields for both connections.¶

A "tunnel" acts as a blind relay between two connections without changing the messages. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel might have been initiated by an HTTP request. A tunnel ceases to exist when both ends of the relayed connection are closed. Tunnels are used to extend a virtual connection through an intermediary, such as when transport-layer security is used to establish private communication through a shared firewall proxy.¶

In addition, there may exist network intermediaries that are not considered part of the HTTP communication but nevertheless act as filters or redirecting agents (usually violating HTTP semantics, causing security problems, and otherwise making a mess of things). Such a network intermediary, often referred to as an "interception proxy" [RFC3040], "transparent proxy" [RFC1919], or "captive portal", differs from an HTTP proxy because it has not been selected by the client. Instead, the network intermediary redirects outgoing TCP port 80 packets (and occasionally other common port traffic) to an internal HTTP server. Interception proxies are commonly found on public network access points, as a means of enforcing account subscription prior to allowing use of non-local Internet services, and within corporate firewalls to enforce network usage policies. They are indistinguishable from a man-in-the-middle attack.¶

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

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.¶

A response is "cacheable" if a cache is allowed to store a copy of the response message for use in answering subsequent requests. Even when a response is cacheable, there might be additional constraints placed by the client or by the origin server on when that cached response can be used for a particular request. HTTP requirements for cache behavior and cacheable responses are defined in Section 2 of [Part6].¶

There are a wide variety of architectures and configurations of caches and proxies deployed across the World Wide Web and inside large organizations. 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 optical media, and so on.¶

HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of the protocol. This specification defines version "1.1". The protocol version as a whole indicates the sender's compliance with the set of requirements laid out in that version's corresponding specification of HTTP.¶

The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message. HTTP-Version is case-sensitive.¶

The HTTP version number consists of two decimal digits separated by a "." (period or decimal point). The first digit ("major version") indicates the HTTP messaging syntax, whereas the second digit ("minor version") indicates the highest minor version to which the sender is at least conditionally compliant and able to understand for future communication. The minor version advertises the sender's communication capabilities even when the sender is only using a backwards-compatible subset of the protocol, thereby letting the recipient know that more advanced features can be used in response (by servers) or in future requests (by clients).¶

When an HTTP/1.1 message is sent to an HTTP/1.0 recipient [RFC1945] or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0 message if all of the newer features are ignored. This specification places recipient-version requirements on some new features so that a compliant sender will only use compatible features until it has determined, through configuration or the receipt of a message, that the recipient supports HTTP/1.1.¶

The interpretation of an HTTP header field does not change between minor versions of the same major version, though the default behavior of a recipient in the absence of such a field can change. Unless specified otherwise, header fields defined in HTTP/1.1 are defined for all versions of HTTP/1.x. In particular, the Host and Connection header fields ought to be implemented by all HTTP/1.x implementations whether or not they advertise compliance with HTTP/1.1.¶

New header fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation of previously defined header fields. When an implementation receives an unrecognized header field, the recipient MUST ignore that header field for local processing regardless of the message's HTTP version. An unrecognized header field received by a proxy MUST be forwarded downstream unless the header field's field-name is listed in the message's Connection header-field (see Section 9.1). These requirements allow HTTP's functionality to be enhanced without requiring prior update of all compliant intermediaries.¶

Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as a tunnel) MUST send their own HTTP-Version in forwarded messages. In other words, they MUST NOT blindly forward the first line of an HTTP message without ensuring that the protocol version matches what the intermediary understands, and is at least conditionally compliant to, for both the receiving and sending of messages. Forwarding an HTTP message without rewriting the HTTP-Version might result in communication errors when downstream recipients use the message sender's version to determine what features are safe to use for later communication with that sender.¶

An HTTP client SHOULD send a request version equal to the highest version for which the client is at least conditionally compliant and whose major version is no higher than the highest version supported by the server, if this is known. An HTTP client MUST NOT send a version for which it is not at least conditionally compliant.¶

An HTTP client MAY send a lower request version if it is known that the server incorrectly implements the HTTP specification, but only after the client has attempted at least one normal request and determined from the response status or header fields (e.g., Server) that the server improperly handles higher request versions.¶

An HTTP server SHOULD send a response version equal to the highest version for which the server is at least conditionally compliant and whose major version is less than or equal to the one received in the request. An HTTP server MUST NOT send a version for which it is not at least conditionally compliant. A server MAY send a 505 (HTTP Version Not Supported) response if it cannot send a response using the major version used in the client's request.¶

An HTTP server MAY send an HTTP/1.0 response to an HTTP/1.0 request if it is known or suspected that the client incorrectly implements the HTTP specification and is incapable of correctly processing later version responses, such as when a client fails to parse the version number correctly or when an intermediary is known to blindly forward the HTTP-Version even when it doesn't comply with the given minor version of the protocol. Such protocol downgrades SHOULD NOT be performed unless triggered by specific client attributes, such as when one or more of the request header fields (e.g., User-Agent) uniquely match the values sent by a client known to be in error.¶

The intention of HTTP's versioning design is that the major number will only be incremented if an incompatible message syntax is introduced, and that the minor number will only be incremented when changes made to the protocol have the effect of adding to the message semantics or implying additional capabilities of the sender. However, the minor version was not incremented for the changes introduced between [RFC2068] and [RFC2616], and this revision is specifically avoiding any such changes to the protocol.¶

Uniform Resource Identifiers (URIs) [RFC3986] are used throughout HTTP as the means for identifying resources. URI references are used to target requests, indicate redirects, and define relationships. HTTP does not limit what a resource might be; it merely defines an interface that can be used to interact with a resource via HTTP. More information on the scope of URIs and resources can be found in [RFC3986].¶

This specification adopts the definitions of "URI-reference", "absolute-URI", "relative-part", "port", "host", "path-abempty", "path-absolute", "query", and "authority" from the URI generic syntax [RFC3986]. In addition, we define a partial-URI rule for protocol elements that allow a relative URI but not a fragment.¶

Each protocol element in HTTP that allows a URI reference will indicate in its ABNF production whether the element allows any form of reference (URI-reference), only a URI in absolute form (absolute-URI), only the path and optional query components, or some combination of the above. Unless otherwise indicated, URI references are parsed relative to the effective request URI, which defines the default base URI for references in both the request and its corresponding response.¶

The normal procedure for parsing an HTTP message is to read the start-line into a structure, read each header field into a hash table by field name until the empty line, and then use the parsed data to determine if a message-body is expected. If a message-body has been indicated, then it is read as a stream until an amount of octets equal to the message-body length is read or the connection is closed.¶

Care must be taken to parse an HTTP message as a sequence of octets in an encoding that is a superset of US-ASCII. Attempting to parse HTTP as a stream of Unicode characters in a character encoding like UTF-16 might introduce security flaws due to the differing ways that such parsers interpret invalid characters.¶

Older HTTP/1.0 client implementations might send an extra CRLF after a POST request as a lame workaround for some early server applications that failed to read message-body content that was not terminated by a line-ending. An HTTP/1.1 client MUST NOT preface or follow a request with an extra CRLF. If terminating the request message-body with a line-ending is desired, then the client MUST include the terminating CRLF octets as part of the message-body length.¶

In the interest of robustness, servers SHOULD ignore at least one empty line 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. Likewise, although the line terminator for the start-line and header fields is the sequence CRLF, we recommend that recipients recognize a single LF as a line terminator and ignore any CR.¶

When a server listening only for HTTP request messages, or processing what appears from the start-line to be an HTTP request message, receives a sequence of octets that does not match the HTTP-message grammar aside from the robustness exceptions listed above, the server MUST respond with an HTTP/1.1 400 (Bad Request) response.¶

Each HTTP header field consists of a case-insensitive field name followed by a colon (":"), optional whitespace, and the field value.¶

The field-name token labels the corresponding field-value as having the semantics defined by that header field. For example, the Date header field is defined in Section 9.3 as containing the origination timestamp for the message in which it appears.¶

HTTP header fields are fully extensible: there is no limit on the introduction of new field names, each presumably defining new semantics, or on the number of header fields used in a given message. Existing fields are defined in each part of this specification and in many other specifications outside the standards process. New header fields can be introduced without changing the protocol version if their defined semantics allow them to be safely ignored by recipients that do not recognize them.¶

New HTTP header fields SHOULD be registered with IANA according to the procedures in Section 10.1. Unrecognized header fields MUST be forwarded by a proxy unless the field-name is listed in the Connection header field (Section 9.1) or the proxy is specifically configured to block or otherwise transform such fields. Unrecognized header fields SHOULD be ignored by other recipients.¶

The order in which header fields with differing field names are received is not significant. However, it is "good practice" to send header fields that contain control data first, such as Host on requests and Date on responses, so that implementations can decide when not to handle a message as early as possible. A server MUST wait until the entire header section is received before interpreting a request message, since later header fields might include conditionals, authentication credentials, or deliberately misleading duplicate header fields that would impact request processing.¶

Multiple header fields with the same field name MUST NOT be sent in a message unless the entire field value for that header field is defined as a comma-separated list [i.e., #(values)]. Multiple header fields with the same field name can be combined into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field value to the combined field value in order, 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; a proxy MUST NOT change the order of these field values when forwarding a message.¶

The message-body (if any) of an HTTP message is used to carry the payload body associated with the request or response.¶

The message-body differs from the payload body only when a transfer-coding has been applied, as indicated by the Transfer-Encoding header field (Section 9.7). If more than one Transfer-Encoding header field is present in a message, the multiple field-values MUST be combined into one field-value, according to the algorithm defined in Section 3.2, before determining the message-body length.¶

When one or more transfer-codings are applied to a payload in order to form the message-body, the Transfer-Encoding header field MUST contain the list of transfer-codings applied. Transfer-Encoding is a property of the message, not of the payload, and thus MAY be added or removed by any implementation along the request/response chain under the constraints found in Section 6.2.¶

If a message is received that has multiple Content-Length header fields (Section 9.2) with field-values consisting of the same decimal value, or a single Content-Length header field with a field value containing a list of identical decimal values (e.g., "Content-Length: 42, 42"), indicating that duplicate Content-Length header fields have been generated or combined by an upstream message processor, then the recipient MUST either reject the message as invalid or replace the duplicated field-values with a single valid Content-Length field containing that decimal value prior to determining the message-body length.¶

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 header fields, even if the request method does not define any use for a message-body. This allows the request message framing algorithm to be independent of method semantics.¶

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 5.1.1). Responses to the HEAD request method never include a message-body because the associated response header fields (e.g., Transfer-Encoding, Content-Length, etc.) only indicate what their values would have been if the request method had been GET. 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 the body MAY be of zero length.¶

1. Any response to a HEAD request and any response with a status code of 100-199, 204, or 304 is always terminated by the first empty line after the header fields, regardless of the header fields present in the message, and thus cannot contain a message-body.

2. If a Transfer-Encoding header field is present and the "chunked" transfer-coding (Section 6.2) is the final encoding, the message-body length is determined by reading and decoding the chunked data until the transfer-coding indicates the data is complete.

   If a Transfer-Encoding header field is present in a response and the "chunked" transfer-coding is not the final encoding, the message-body length is determined by reading the connection until it is closed by the server. If a Transfer-Encoding header field is present in a request and the "chunked" transfer-coding is not the final encoding, the message-body length cannot be determined reliably; the server MUST respond with the 400 (Bad Request) status code and then close the connection.

   If a message is received with both a Transfer-Encoding header field and a Content-Length header field, the Transfer-Encoding overrides the Content-Length. Such a message might indicate an attempt to perform request or response smuggling (bypass of security-related checks on message routing or content) and thus ought to be handled as an error. The provided Content-Length MUST be removed, prior to forwarding the message downstream, or replaced with the real message-body length after the transfer-coding is decoded.

3. If a message is received without Transfer-Encoding and with either multiple Content-Length header fields having differing field-values or a single Content-Length header field having an invalid value, then the message framing is invalid and MUST be treated as an error to prevent request or response smuggling. If this is a request message, the server MUST respond with a 400 (Bad Request) status code and then close the connection. If this is a response message received by a proxy, the proxy MUST discard the received response, send a 502 (Bad Gateway) status code as its downstream response, and then close the connection. If this is a response message received by a user-agent, it MUST be treated as an error by discarding the message and closing the connection.

4. If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the message-body length in octets. If the actual number of octets sent in the message is less than the indicated Content-Length, the recipient MUST consider the message to be incomplete and treat the connection as no longer usable. If the actual number of octets sent in the message is more than the indicated Content-Length, the recipient MUST only process the message-body up to the field value's number of octets; the remainder of the message MUST either be discarded or treated as the next message in a pipeline. For the sake of robustness, a user-agent MAY attempt to detect and correct such an error in message framing if it is parsing the response to the last request on a connection and the connection has been closed by the server.

5. If this is a request message and none of the above are true, then the message-body length is zero (no message-body is present).

6. Otherwise, this is a response message without a declared message-body length, so the message-body length is determined by the number of octets received prior to the server closing the connection.

Since there is no way to distinguish a successfully completed, close-delimited message from a partially-received message interrupted by network failure, implementations SHOULD use encoding or length-delimited messages whenever possible. The close-delimiting feature exists primarily for backwards compatibility with HTTP/1.0.¶

A server MAY reject a request that contains a message-body but not a Content-Length by responding with 411 (Length Required).¶

Unless a transfer-coding other than "chunked" has been applied, a client that sends a request containing a message-body SHOULD use a valid Content-Length header field if the message-body length is known in advance, rather than the "chunked" encoding, since some existing services respond to "chunked" with a 411 (Length Required) status code even though they understand the chunked encoding. This is typically because such services are implemented via a gateway that requires a content-length in advance of being called and the server is unable or unwilling to buffer the entire request before processing.¶

A client that sends a request containing a message-body MUST include a valid Content-Length header field if it does not know the server will handle HTTP/1.1 (or later) requests; such knowledge can be in the form of specific user configuration or by remembering the version of a prior received response.¶

Request messages that are prematurely terminated, possibly due to a cancelled connection or a server-imposed time-out exception, MUST result in closure of the connection; sending an HTTP/1.1 error response prior to closing the connection is OPTIONAL.¶

Response messages that are prematurely terminated, usually by closure of the connection prior to receiving the expected number of octets or by failure to decode a transfer-encoded message-body, MUST be recorded as incomplete. A response that terminates in the middle of the header block (before the empty line is received) cannot be assumed to convey the full semantics of the response and MUST be treated as an error.¶

A message-body that uses the chunked transfer encoding is incomplete if the zero-sized chunk that terminates the encoding has not been received. A message that uses a valid Content-Length is incomplete if the size of the message-body received (in octets) is less than the value given by Content-Length. A response that has neither chunked transfer encoding nor Content-Length is terminated by closure of the connection, and thus is considered complete regardless of the number of message-body octets received, provided that the header block was received intact.¶

A user agent MUST NOT render an incomplete response message-body as if it were complete (i.e., some indication must be given to the user that an error occurred). Cache requirements for incomplete responses are defined in Section 2.1 of [Part6].¶

A server MUST read the entire request message-body or close the connection after sending its response, since otherwise the remaining data on a persistent connection would be misinterpreted as the next request. Likewise, a client MUST read the entire response message-body if it intends to reuse the same connection for a subsequent request. Pipelining multiple requests on a connection is described in Section 7.1.2.2.¶

There are a few header fields which have general applicability for both request and response messages, but which do not apply to the payload being transferred. These header fields apply only to the message being transmitted.¶

The Request-Line begins with a method token, followed by a single space (SP), the request-target, another single space (SP), the protocol version, and ending with CRLF.¶

The exact resource identified by an Internet request is determined by examining both the request-target and the Host header field.¶

An origin server that does not allow resources to differ by the requested host MAY ignore the Host header field value when determining the resource identified by an HTTP/1.1 request. (But see Appendix A.1.1 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 host names) MUST use the following rules for determining the requested resource on an HTTP/1.1 request: ¶

1. If request-target is an absolute-URI, the host is part of the request-target. Any Host header field value in the request MUST be ignored.
2. If the request-target is not an absolute-URI, 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.¶

HTTP requests often do not carry the absolute URI ([RFC3986], Section 4.3) for the target resource; instead, the URI needs to be inferred from the request-target, Host header field, and connection context. The result of this process is called the "effective request URI". The "target resource" is the resource identified by the effective request URI.¶

If the request-target is an absolute-URI, then the effective request URI is the request-target.¶

If the request-target uses the path-absolute form or the asterisk form, and the Host header field is present, then the effective request URI is constructed by concatenating¶

• the scheme name: "http" if the request was received over an insecure TCP connection, or "https" when received over a SSL/TLS-secured TCP connection,
• the octet sequence "://",
• the authority component, as specified in the Host header field (Section 9.4), and
• the request-target obtained from the Request-Line, unless the request-target is just the asterisk "*".

If the request-target uses the path-absolute form or the asterisk form, and the Host header field is not present, then the effective request URI is undefined.¶

Otherwise, when request-target uses the authority form, the effective request URI is undefined.¶

Example 1: the effective request URI for the message

GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.example.org:8080

(received over an insecure TCP connection) is "http", plus "://", plus the authority component "www.example.org:8080", plus the request-target "/pub/WWW/TheProject.html", thus "http://www.example.org:8080/pub/WWW/TheProject.html".

Example 2: the effective request URI for the message

OPTIONS * HTTP/1.1
Host: www.example.org

(received over an SSL/TLS secured TCP connection) is "https", plus "://", plus the authority component "www.example.org", thus "https://www.example.org".

Effective request URIs are compared using the rules described in Section 2.7.3, except that empty path components MUST NOT be treated as equivalent to an absolute path of "/".¶

The first line of a Response message is the Status-Line, consisting of the protocol version, a space (SP), the status code, another space, a possibly-empty textual phrase describing the status code, and ending with CRLF.¶

The other formats are described here only for compatibility with obsolete implementations.¶

HTTP/1.1 clients and servers that parse a 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.¶

All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal to UTC (Coordinated Universal Time). 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 is case sensitive and MUST NOT include additional whitespace beyond that specifically included as SP in the grammar.¶

The semantics of day-name, day, month, year, and time-of-day are the same as those defined for the RFC 5322 constructs with the corresponding name ([RFC5322], Section 3.3).¶

Transfer-coding values are used to indicate an encoding transformation that has been, can be, or might need to be applied to a payload body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer-coding is a property of the message rather than a property of the representation that is being transferred.¶

Parameters are in the form of attribute/value pairs.¶

All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer-coding values in the TE header field (Section 9.5) and in the Transfer-Encoding header field (Section 9.7).¶

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 ([RFC2045], Section 6). 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 message body length (Section 3.3), or the desire to encrypt data over a shared transport.¶

A server that receives a request message with a transfer-coding it does not understand SHOULD respond with 501 (Not Implemented) and then close the connection. A server MUST NOT send transfer-codings to an HTTP/1.0 client.¶

Product tokens are used to allow communicating applications to identify themselves by software name and version. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by whitespace. By convention, the products are listed in order of their significance for identifying the application.¶

Product tokens SHOULD be short and to the point. They MUST NOT be used for advertising or other non-essential information. Although any token octet MAY appear in a product-version, 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).¶

Both transfer codings (TE request header field, Section 9.5) and content negotiation (Section 5 of [Part3]) use short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. If a parameter has a quality value of 0, then content with this parameter is "not acceptable" for the client. HTTP/1.1 applications MUST NOT generate more than three digits after the decimal point. User configuration of these values SHOULD also be limited in this fashion.¶

[TBD-aliases-harmful: describe why aliases like webcal are harmful.] ¶

[TBD-proxy-other: Configured to use HTTP to proxy HTTP or other protocols.] ¶

[TBD-intercept: Interception of HTTP traffic for initiating access control.] ¶

[TBD-profiles: Profiles of HTTP defined by other protocol. Extensions of HTTP like WebDAV.] ¶

[TBD-hypertext: Instructions on composing HTTP requests via hypertext formats.] ¶

The "Connection" header field allows the sender to specify options that are desired only for that particular connection. Such connection options MUST be removed or replaced before the message can be forwarded downstream by a proxy or gateway. This mechanism also allows the sender to indicate which HTTP header fields used in the message are only intended for the immediate recipient ("hop-by-hop"), as opposed to all recipients on the chain ("end-to-end"), enabling the message to be self-descriptive and allowing future connection-specific extensions to be deployed in HTTP without fear that they will be blindly forwarded by previously deployed intermediaries.¶

A proxy or gateway MUST parse a received 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, and then remove the Connection header field itself or replace it with the sender's own connection options for the forwarded message.¶

A sender MUST NOT include field-names in the Connection header field-value for fields that are defined as expressing constraints for all recipients in the request or response chain, such as the Cache-Control header field (Section 3.2 of [Part6]).¶

The connection options do not have to correspond to a header field present in the message, since a connection-specific header field might not be needed if there are no parameters associated with that connection option. Recipients that trigger certain connection behavior based on the presence of connection options MUST do so based on the presence of the connection-token rather than only the presence of the optional header field. In other words, if the connection option is received as a header field but not indicated within the Connection field-value, then the recipient MUST ignore the connection-specific header field because it has likely been forwarded by an intermediary that is only partially compliant.¶

When defining new connection options, specifications ought to carefully consider existing deployed header fields and ensure that the new connection-token does not share the same name as an unrelated header field that might already be deployed. Defining a new connection-token essentially reserves that potential field-name for carrying additional information related to the connection option, since it would be unwise for senders to use that field-name for anything else.¶

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,¶

in either the request or the response header fields indicates that the connection SHOULD NOT be considered "persistent" (Section 7.1) after the current request/response is complete.¶

An HTTP/1.1 client that does not support persistent connections MUST include the "close" connection option in every request message.¶

An HTTP/1.1 server that does not support persistent connections MUST include the "close" connection option in every response message that does not have a 1xx (Informational) status code.¶

The "Content-Length" header field indicates the size of the message-body, in decimal number of octets, for any message other than a response to a HEAD request or a response with a status code of 304. In the case of a response to a HEAD request, Content-Length indicates the size of the payload body (not including any potential transfer-coding) that would have been sent had the request been a GET. In the case of a 304 (Not Modified) response to a GET request, Content-Length indicates the size of the payload body (not including any potential transfer-coding) that would have been sent in a 200 (OK) response.¶

An example is¶

Implementations SHOULD use this field to indicate the message-body length when no transfer-coding is being applied and the payload's body length can be determined prior to being transferred. Section 3.3 describes how recipients determine the length of a message-body.¶

Any Content-Length greater than or equal to zero is a valid value.¶

Note that the use of this field in HTTP is significantly different from the corresponding definition in MIME, where it is an optional field used within the "message/external-body" content-type.¶

The "Date" header field represents the date and time at which the message was originated, having the same semantics as the Origination Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The field value is an HTTP-date, as described in Section 6.1; it MUST be sent in rfc1123-date format.¶

An example is¶

Origin servers MUST include a Date header field in all responses, except in these cases: ¶

1. If the response status code is 100 (Continue) or 101 (Switching Protocols), the response MAY include a Date header field, at the server's option.
2. If the response status code conveys a server error, e.g., 500 (Internal Server Error) or 503 (Service Unavailable), and it is inconvenient or impossible to generate a valid Date.
3. If the server does not have a clock that can provide a reasonable approximation of the current time, its responses MUST NOT include a Date header field. In this case, the rules in Section 9.3.1 MUST be followed.

A received message that does not have a Date header field MUST be assigned one by the recipient if the message will be cached by that recipient.¶

Clients can use the Date header field as well; in order to keep request messages small, they are advised not to include it when it doesn't convey any useful information (as it is usually the case for requests that do not contain a payload).¶

The HTTP-date sent in a Date header field SHOULD NOT represent a date and time subsequent to the generation of the message. It SHOULD represent the best available approximation of the date and time of message generation, unless the implementation has no means of generating a reasonably accurate date and time. In theory, the date ought to represent the moment just before the payload is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value.¶

The "Host" header field in a request provides the host and port information from the target resource's URI, enabling the origin server to distinguish between resources while servicing requests for multiple host names on a single IP address. Since the Host field-value is critical information for handling a request, it SHOULD be sent as the first header field following the Request-Line.¶

A client MUST send a Host header field in all HTTP/1.1 request messages. If the target resource's URI includes an authority component, then the Host field-value MUST be identical to that authority component after excluding any userinfo (Section 2.7.1). If the authority component is missing or undefined for the target resource's URI, then the Host header field MUST be sent with an empty field-value.¶

The Host header field MUST be sent in an HTTP/1.1 request even if the request-target is in the form of an absolute-URI, since this allows the Host information to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.¶

When an HTTP/1.1 proxy receives a request with a request-target in the form of an absolute-URI, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. When a proxy forwards a request, it MUST generate the Host header field based on the received absolute-URI rather than the received Host.¶

Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it relies on the Host header field value for redirecting requests to internal servers, or for use as a cache key in a shared cache, without first verifying that the intercepted connection is targeting a valid IP address for that host.¶

A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field or a Host header field with an invalid field-value.¶

See Sections 4.2 and A.1.1 for other requirements relating to Host.¶

The "TE" header field indicates what extension transfer-codings it is willing to accept in the response, and whether or not it is willing to accept trailer fields in a chunked transfer-coding.¶

Its value consists of the keyword "trailers" and/or a comma-separated list of extension transfer-coding names with optional accept parameters (as described in Section 6.2).¶

The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer-coding, as defined in Section 6.2.1. This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding.¶

The TE header field only applies to the immediate connection. Therefore, the keyword MUST be supplied within a Connection header field (Section 9.1) whenever TE is present in an HTTP/1.1 message.¶

A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules: ¶

1. The "chunked" transfer-coding is always acceptable. If the keyword "trailers" is listed, the client indicates that it is willing to accept trailer fields in the chunked response on behalf of itself and any downstream clients. The implication is that, if given, the client is stating that either all downstream clients are willing to accept trailer fields in the forwarded response, or that it will attempt to buffer the response on behalf of downstream recipients.

   Note: HTTP/1.1 does not define any means to limit the size of a chunked response such that a client can be assured of buffering the entire response.

2. If the transfer-coding being tested is one of the transfer-codings listed in the TE field, then it is acceptable unless it is accompanied by a qvalue of 0. (As defined in Section 6.4, a qvalue of 0 means "not acceptable".)

3. If multiple transfer-codings are acceptable, then the acceptable transfer-coding with the highest non-zero qvalue is preferred. The "chunked" transfer-coding always has a qvalue of 1.

If the TE field-value is empty or if no TE field is present, the only transfer-coding is "chunked". A message with no transfer-coding is always acceptable.¶

The "Trailer" header field indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer-coding.¶

An HTTP/1.1 message SHOULD include a Trailer header field in a message using chunked transfer-coding with a non-empty trailer. Doing so allows the recipient to know which header fields to expect in the trailer.¶

If no Trailer header field is present, the trailer SHOULD NOT include any header fields. See Section 6.2.1 for restrictions on the use of trailer fields in a "chunked" transfer-coding.¶

Message header fields listed in the Trailer header field MUST NOT include the following header fields: ¶

• Transfer-Encoding
• Content-Length
• Trailer

The "Transfer-Encoding" header field indicates what transfer-codings (if any) have been applied to the message body. It differs from Content-Encoding (Section 2.2 of [Part3]) in that transfer-codings are a property of the message (and therefore are removed by intermediaries), whereas content-codings are not.¶

If multiple encodings have been applied to a representation, the transfer-codings MUST be listed in the order in which they were applied. Additional information about the encoding parameters MAY be provided by other header fields not defined by this specification.¶

Many older HTTP/1.0 applications do not understand the Transfer-Encoding header field.¶

The "Upgrade" header field allows the client to specify what additional communication protocols it would like to use, if the server chooses to switch protocols. Servers can use it to indicate what protocols they are willing to switch to.¶

For example,¶

The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by the server, possibly according to the nature of the request method or target resource).¶

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 9.1) 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 3xx redirection response (Section 8.3 of [Part2]).¶

Servers MUST include the "Upgrade" header field in 101 (Switching Protocols) responses to indicate which protocol(s) are being switched to, and MUST include it in 426 (Upgrade Required) responses to indicate acceptable protocols to upgrade to. Servers MAY include it in any other response to indicate that they are willing to upgrade to one of the specified protocols.¶

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 2.6 and future updates to this specification. Additional tokens can be registered with IANA using the registration procedure defined below.¶

The "Via" header field MUST be sent by a proxy or gateway to indicate the intermediate protocols and recipients between the user agent and the server on requests, and between the origin server and the client on responses. It is analogous to the "Received" field used by email systems (Section 3.6.7 of [RFC5322]) and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain.¶

The 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 excluded 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 each recipient, 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.¶

A proxy or gateway used as a portal through a network firewall SHOULD NOT forward the names and ports of hosts within the firewall region unless it is explicitly enabled to do so. If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host.¶

For organizations that have strong privacy requirements for hiding internal structures, a proxy or gateway MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. For example,¶

could be collapsed to¶

Senders SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. Senders MUST NOT combine entries which have different received-protocol values.¶

Furthermore, the header field name "Close" shall be registered as "reserved", as its use as HTTP header field would be in conflict with the use of the "close" connection option for the "Connection" header field (Section 9.1).¶

The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".¶

The entries for the "http" and "https" URI Schemes in the registry located at <http://www.iana.org/assignments/uri-schemes.html> shall be updated to point to Sections 2.7.1 and 2.7.2 of this document (see [RFC4395]).¶

This document serves as the specification for the Internet media types "message/http" and "application/http". The following is to be registered with IANA (see [RFC4288]).¶

The registration procedure for HTTP Transfer Codings is now defined by Section 6.2.3 of this document.¶

The registration procedure for HTTP Upgrade Tokens — previously defined in Section 7.2 of [RFC2817] — is now defined by Section 9.8.1 of this document.¶

HTTP clients are often privy to large amounts of personal information (e.g., the user's name, location, mail address, passwords, encryption keys, etc.), and SHOULD be very careful to prevent unintentional leakage of this information. We very strongly recommend that a convenient interface be provided for the user to control dissemination of such information, and that designers and implementors be particularly careful in this area. History shows that errors in this area often create serious security and/or privacy problems and generate highly adverse publicity for the implementor's company.¶

A server is in the position to save personal data about a user's requests which might identify their reading patterns or subjects of interest. This information is clearly confidential in nature and its handling can be constrained by law in certain countries. People using HTTP to provide data are responsible for ensuring that such material is not distributed without the permission of any individuals that are identifiable by the published results.¶

Implementations of HTTP origin servers SHOULD be careful to restrict the documents returned by HTTP requests to be only those that were intended by the server administrators. If an HTTP server translates HTTP URIs directly into file system calls, the server MUST take special care not to serve files that were not intended to be delivered to HTTP clients. For example, UNIX, Microsoft Windows, and other operating systems use ".." as a path component to indicate a directory level above the current one. On such a system, an HTTP server MUST disallow any such construct in the request-target if it would otherwise allow access to a resource outside those intended to be accessible via the HTTP server. Similarly, files intended for reference only internally to the server (such as access control files, configuration files, and script code) MUST be protected from inappropriate retrieval, since they might contain sensitive information. Experience has shown that minor bugs in such HTTP server implementations have turned into security risks.¶

HTTP clients rely heavily on the Domain Name Service (DNS), and are thus generally prone to security attacks based on the deliberate misassociation of IP addresses and DNS names not protected by DNSSec. Clients need to be cautious in assuming the validity of an IP number/DNS name association unless the response is protected by DNSSec ([RFC4033]).¶

By their very nature, HTTP proxies are men-in-the-middle, and represent an opportunity for man-in-the-middle attacks. Compromise of the systems on which the proxies run can result in serious security and privacy problems. Proxies have access to security-related information, personal information about individual users and organizations, and proprietary information belonging to users and content providers. A compromised proxy, or a proxy implemented or configured without regard to security and privacy considerations, might be used in the commission of a wide range of potential attacks.¶

Proxy operators need to protect the systems on which proxies run as they would protect any system that contains or transports sensitive information. In particular, log information gathered at proxies often contains highly sensitive personal information, and/or information about organizations. Log information needs to be carefully guarded, and appropriate guidelines for use need to be developed and followed. (Section 11.2).¶

Proxy implementors need to consider the privacy and security implications of their design and coding decisions, and of the configuration options they provide to proxy operators (especially the default configuration).¶

Users of a proxy need to be aware that proxies are no trustworthier than the people who run them; HTTP itself cannot solve this problem.¶

The judicious use of cryptography, when appropriate, might suffice to protect against a broad range of security and privacy attacks. Such cryptography is beyond the scope of the HTTP/1.1 specification.¶

Because HTTP uses mostly textual, character-delimited fields, attackers can overflow buffers in implementations, and/or perform a Denial of Service against implementations that accept fields with unlimited lengths.¶

To promote interoperability, this specification makes specific recommendations for size limits on request-targets (Section 4.1.2) and blocks of header fields (Section 3.2). These are minimum recommendations, chosen to be supportable even by implementations with limited resources; it is expected that most implementations will choose substantially higher limits.¶

This specification also provides a way for servers to reject messages that have request-targets that are too long (Section 8.4.15 of [Part2]) or request entities that are too large (Section 8.4 of [Part2]).¶

Other fields (including but not limited to request methods, response status phrases, header field-names, and body chunks) SHOULD be limited by implementations carefully, so as to not impede interoperability.¶

They exist. They are hard to defend against. Research continues. Beware.¶

13.1. Normative References

[ISO-8859-1]

International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1”, ISO/IEC 8859-1:1998, 1998.

[Part2]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 2: Message Semantics”, Internet-Draft draft-ietf-httpbis-p2-semantics-16 (work in progress), August 2011.

[Part3]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 3: Message Payload and Content Negotiation”, Internet-Draft draft-ietf-httpbis-p3-payload-16 (work in progress), August 2011.

[Part6]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-16 (work in progress), August 2011.

[RFC1950]

Deutsch, L. and J-L. Gailly, “ZLIB Compressed Data Format Specification version 3.3”, RFC 1950, May 1996.
RFC 1950 is an Informational RFC, thus it might be less stable than this specification. On the other hand, this downward reference was present since the publication of RFC 2068 in 1997 ([RFC2068]), therefore it is unlikely to cause problems in practice. See also [BCP97].

[RFC1951]

Deutsch, P., “DEFLATE Compressed Data Format Specification version 1.3”, RFC 1951, May 1996.
RFC 1951 is an Informational RFC, thus it might be less stable than this specification. On the other hand, this downward reference was present since the publication of RFC 2068 in 1997 ([RFC2068]), therefore it is unlikely to cause problems in practice. See also [BCP97].

[RFC1952]

Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. Randers-Pehrson, “GZIP file format specification version 4.3”, RFC 1952, May 1996.
RFC 1952 is an Informational RFC, thus it might be less stable than this specification. On the other hand, this downward reference was present since the publication of RFC 2068 in 1997 ([RFC2068]), therefore it is unlikely to cause problems in practice. See also [BCP97].

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

[RFC3986]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.

[RFC5234]

Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

[USASCII]

American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.

13.2. Informative References

[BCP97]

Klensin, J. and S. Hartman, “Handling Normative References to Standards-Track Documents”, BCP 97, RFC 4897, June 2007.

[Kri2001]

Kristol, D., “HTTP Cookies: Standards, Privacy, and Politics”, ACM Transactions on Internet Technology Vol. 1, #2, November 2001, <http://arxiv.org/abs/cs.SE/0105018>.

[Nie1997]

Frystyk, H., Gettys, J., Prud'hommeaux, E., Lie, H., and C. Lilley, “Network Performance Effects of HTTP/1.1, CSS1, and PNG”, ACM Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97, September 1997, <http://doi.acm.org/10.1145/263105.263157>.

[Pad1995]

Padmanabhan, V. and J. Mogul, “Improving HTTP Latency”, Computer Networks and ISDN Systems v. 28, pp. 25-35, December 1995, <http://portal.acm.org/citation.cfm?id=219094>.

[RFC1123]

Braden, R., “Requirements for Internet Hosts - Application and Support”, STD 3, RFC 1123, October 1989.

[RFC1919]

Chatel, M., “Classical versus Transparent IP Proxies”, RFC 1919, March 1996.

[RFC1945]

Berners-Lee, T., Fielding, R., and H. Nielsen, “Hypertext Transfer Protocol -- HTTP/1.0”, RFC 1945, May 1996.

[RFC2045]

Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies”, RFC 2045, November 1996.

[RFC2047]

Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text”, RFC 2047, November 1996.

[RFC2068]

Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.

[RFC2145]

Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, “Use and Interpretation of HTTP Version Numbers”, RFC 2145, May 1997.

[RFC2616]

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.

[RFC2817]

Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1”, RFC 2817, May 2000.

[RFC2818]

Rescorla, E., “HTTP Over TLS”, RFC 2818, May 2000.

[RFC2965]

Kristol, D. and L. Montulli, “HTTP State Management Mechanism”, RFC 2965, October 2000.

[RFC3040]

Cooper, I., Melve, I., and G. Tomlinson, “Internet Web Replication and Caching Taxonomy”, RFC 3040, January 2001.

[RFC3864]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

[RFC4033]

Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements”, RFC 4033, March 2005.

[RFC4288]

Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 4288, December 2005.

[RFC4395]

Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes”, BCP 115, RFC 4395, February 2006.

[RFC4559]

Jaganathan, K., Zhu, L., and J. Brezak, “SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows”, RFC 4559, June 2006.

[RFC5226]

Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

[RFC5322]

Resnick, P., “Internet Message Format”, RFC 5322, October 2008.

[RFC6265]

Barth, A., “HTTP State Management Mechanism”, RFC 6265, April 2011.

[Spe]

Spero, S., “Analysis of HTTP Performance Problems”, <http://sunsite.unc.edu/mdma-release/http-prob.html>.

[Tou1998]

Touch, J., Heidemann, J., and K. Obraczka, “Analysis of HTTP Performance”, ISI Research Report ISI/RR-98-463, Aug 1998, <http://www.isi.edu/touch/pubs/http-perf96/>.
(original report dated Aug. 1996)

This section summarizes major differences between versions HTTP/1.0 and HTTP/1.1.¶

Empty list elements in list productions have been deprecated. (Section 1.2.1)¶

Rules about implicit linear whitespace between certain grammar productions have been removed; now it's only allowed when specifically pointed out in the ABNF. (Section 1.2.2)¶

Clarify that the string "HTTP" in the HTTP-Version ABFN production is case sensitive. Restrict the version numbers to be single digits due to the fact that implementations are known to handle multi-digit version numbers incorrectly. (Section 2.6)¶

Require that invalid whitespace around field-names be rejected. (Section 3.2)¶

The NUL octet is no longer allowed in comment and quoted-string text. The quoted-pair rule no longer allows escaping control characters other than HTAB. Non-ASCII content in header fields and reason phrase has been obsoleted and made opaque (the TEXT rule was removed). (Section 3.2.3)¶

Require recipients to handle bogus Content-Length header fields as errors. (Section 3.3)¶

Remove reference to non-existent identity transfer-coding value tokens. (Sections 3.3 and 6.2)¶

Update use of abs_path production from RFC 1808 to the path-absolute + query components of RFC 3986. State that the asterisk form is allowed for the OPTIONS request method only. (Section 4.1.2)¶

Clarification that the chunk length does not include the count of the octets in the chunk header and trailer. Furthermore disallowed line folding in chunk extensions. (Section 6.2.1)¶

Remove hard limit of two connections per server. (Section 7.1.4)¶

Change ABNF productions for header fields to only define the field value. (Section 9)¶

Clarify exactly when close connection options must be sent. (Section 9.1)¶

Define the semantics of the "Upgrade" header field in responses other than 101 (this was incorporated from [RFC2817]). (Section 9.8)¶

Extracted relevant partitions from [RFC2616].¶

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/1>: "HTTP Version should be case sensitive" (<http://purl.org/NET/http-errata#verscase>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/2>: "'unsafe' characters" (<http://purl.org/NET/http-errata#unsafe-uri>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/3>: "Chunk Size Definition" (<http://purl.org/NET/http-errata#chunk-size>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/4>: "Message Length" (<http://purl.org/NET/http-errata#msg-len-chars>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/8>: "Media Type Registrations" (<http://purl.org/NET/http-errata#media-reg>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/11>: "URI includes query" (<http://purl.org/NET/http-errata#uriquery>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/15>: "No close on 1xx responses" (<http://purl.org/NET/http-errata#noclose1xx>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/16>: "Remove 'identity' token references" (<http://purl.org/NET/http-errata#identity>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/26>: "Import query BNF"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/31>: "qdtext BNF"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/42>: "RFC2606 Compliance"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/45>: "RFC977 reference"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/46>: "RFC1700 references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/47>: "inconsistency in date format explanation"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference typo"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1 Reference"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-to-date references"

Other changes: ¶

• Update media type registrations to use RFC4288 template.
• Use names of RFC4234 core rules DQUOTE and WSP, fix broken ABNF for chunk-data (work in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/19>: "Bodies on GET (and other) requests"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/55>: "Updating to RFC4288"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/57>: "Status Code and Reason Phrase"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not used"

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" -> "trailer-part").
• Avoid underscore character in rule names ("http_URL" -> "http-URL", "abs_path" -> "path-absolute").
• Add rules for terms imported from URI spec ("absoluteURI", "authority", "path-absolute", "port", "query", "relativeURI", "host) — these will have to be updated when switching over to RFC3986.
• Synchronize core rules with RFC5234.
• Get rid of prose rules that span multiple lines.
• Get rid of unused rules LOALPHA and UPALPHA.
• Move "Product Tokens" section (back) into Part 1, as "token" is used in the definition of the Upgrade header field.
• Add explicit references to BNF syntax and rules imported from other parts of the specification.
• Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/51>: "HTTP-date vs. rfc1123-date"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/64>: "WS in quoted-pair"

Ongoing work on IANA Message Header Field Registration (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>): ¶

• Reference RFC 3984, and update header field registrations for headers defined in this document.

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Replace string literals when the string really is case-sensitive (HTTP-Version).

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/28>: "Connection closing"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/97>: "Move registrations and registry information to IANA Considerations"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/120>: "need new URL for PAD1995 reference"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/127>: "IANA Considerations: update HTTP URI scheme registration"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/128>: "Cite HTTPS URI scheme definition"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/129>: "List-type headers vs Set-Cookie"

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Replace string literals when the string really is case-sensitive (HTTP-Date).
• Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/34>: "Out-of-date reference for URIs"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/132>: "RFC 2822 is updated by RFC 5322"

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Use "/" instead of "|" for alternatives.
• Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
• Only reference RFC 5234's core rules.
• Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").
• Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/30>: "Header LWS"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/52>: "Sort 1.3 Terminology"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/63>: "RFC2047 encoded words"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/74>: "Character Encodings in TEXT"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/77>: "Line Folding"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/83>: "OPTIONS * and proxies"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/94>: "Reason-Phrase BNF"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/111>: "Use of TEXT"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/118>: "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/134>: "RFC822 reference left in discussion of date formats"

Final work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>): ¶

• Rewrite definition of list rules, deprecate empty list elements.
• Add appendix containing collected and expanded ABNF.

Other changes: ¶

• Rewrite introduction; add mostly new Architecture Section.
• Move definition of quality values from Part 3 into Part 1; make TE request header field grammar independent of accept-params (defined in Part 3).

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for numeric protocol elements"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/162>: "comment ABNF"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/88>: "205 Bodies" (took out language that implied that there might be methods for which a request body MUST NOT be included)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/163>: "editorial improvements around HTTP-date"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/93>: "Repeating single-value headers"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/131>: "increase connection limit"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/157>: "IP addresses in URLs"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/172>: "take over HTTP Upgrade Token Registry"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/173>: "CR and LF in chunk extension values"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/184>: "HTTP/0.9 support"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/188>: "pick IANA policy (RFC5226) for Transfer Coding / Content Coding"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/189>: "move definitions of gzip/deflate/compress to part 1"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/194>: "disallow control characters in quoted-pair"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/148>: "update IANA requirements wrt Transfer-Coding values" (add the IANA Considerations subsection)

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/201>: "header parsing, treatment of leading and trailing OWS"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of 13.5.1 and 13.5.2"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/200>: "use of term "word" when talking about header structure"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/73>: "Clarification of the term 'deflate'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/83>: "OPTIONS * and proxies"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/122>: "MIME-Version not listed in P1, general header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/143>: "IANA registry for content/transfer encodings"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/165>: "Case-sensitivity of HTTP-date"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/200>: "use of term "word" when talking about header structure"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the requested resource's URI"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/28>: "Connection Closing"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/90>: "Delimiting messages with multipart/byteranges"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/95>: "Handling multiple Content-Length headers"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify entity / representation / variant terminology"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider removing the 'changes from 2068' sections"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/159>: "HTTP(s) URI scheme definitions"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/193>: "Trailer requirements"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/204>: "Text about clock requirement for caches belongs in p6"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/221>: "effective request URI: handling of missing host in HTTP/1.0"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/248>: "confusing Date requirements for clients"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/95>: "Handling multiple Content-Length headers"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/75>: "RFC2145 Normative"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/159>: "HTTP(s) URI scheme definitions" (tune the requirements on userinfo)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/210>: "define 'transparent' proxy"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header Classification"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/233>: "Is * usable as a request-uri for new methods?"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/240>: "Migrate Upgrade details from RFC2817"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/279>: "update RFC 2109 reference"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/53>: "Allow is not in 13.5.2"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/95>: "Handling multiple Content-Length headers"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/286>: "Content-Length ABNF broken"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/273>: "HTTP-Version should be redefined as fixed length pair of DIGIT . DIGIT"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/282>: "Recommend minimum sizes for protocol elements"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/283>: "Set expectations around buffering"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/288>: "Considering messages in isolation"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/100>: "DNS Spoofing / DNS Binding advice"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/254>: "move RFCs 2145, 2616, 2817 to Historic status"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/270>: "\-escaping in quoted strings"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/305>: "'Close' should be reserved in the HTTP header field registry"