Internet Engineering Task Force (IETF) R. Fielding, Ed. Request for Comments: 9112 Adobe STD: 99 M. Nottingham, Ed. Obsoletes: 7230 Fastly Category: Standards Track J. Reschke, Ed. ISSN: 2070-1721 greenbytes June 2022 HTTP/1.1 Abstract The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns. This document obsoletes portions of RFC 7230. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9112. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction 1.1. Requirements Notation 1.2. Syntax Notation 2. Message 2.1. Message Format 2.2. Message Parsing 2.3. HTTP Version 3. Request Line 3.1. Method 3.2. Request Target 3.2.1. origin-form 3.2.2. absolute-form 3.2.3. authority-form 3.2.4. asterisk-form 3.3. Reconstructing the Target URI 4. Status Line 5. Field Syntax 5.1. Field Line Parsing 5.2. Obsolete Line Folding 6. Message Body 6.1. Transfer-Encoding 6.2. Content-Length 6.3. Message Body Length 7. Transfer Codings 7.1. Chunked Transfer Coding 7.1.1. Chunk Extensions 7.1.2. Chunked Trailer Section 7.1.3. Decoding Chunked 7.2. Transfer Codings for Compression 7.3. Transfer Coding Registry 7.4. Negotiating Transfer Codings 8. Handling Incomplete Messages 9. Connection Management 9.1. Establishment 9.2. Associating a Response to a Request 9.3. Persistence 9.3.1. Retrying Requests 9.3.2. Pipelining 9.4. Concurrency 9.5. Failures and Timeouts 9.6. Tear-down 9.7. TLS Connection Initiation 9.8. TLS Connection Closure 10. Enclosing Messages as Data 10.1. Media Type message/http 10.2. Media Type application/http 11. Security Considerations 11.1. Response Splitting 11.2. Request Smuggling 11.3. Message Integrity 11.4. Message Confidentiality 12. IANA Considerations 12.1. Field Name Registration 12.2. Media Type Registration 12.3. Transfer Coding Registration 12.4. ALPN Protocol ID Registration 13. References 13.1. Normative References 13.2. Informative References Appendix A. Collected ABNF Appendix B. Differences between HTTP and MIME B.1. MIME-Version B.2. Conversion to Canonical Form B.3. Conversion of Date Formats B.4. Conversion of Content-Encoding B.5. Conversion of Content-Transfer-Encoding B.6. MHTML and Line Length Limitations Appendix C. Changes from Previous RFCs C.1. Changes from HTTP/0.9 C.2. Changes from HTTP/1.0 C.2.1. Multihomed Web Servers C.2.2. Keep-Alive Connections C.2.3. Introduction of Transfer-Encoding C.3. Changes from RFC 7230 Acknowledgements Index Authors' Addresses 1. Introduction The Hypertext Transfer Protocol (HTTP) is a stateless application- level request/response protocol that uses extensible semantics and self-descriptive messages for flexible interaction with network-based hypertext information systems. HTTP/1.1 is defined by: * This document * "HTTP Semantics" [HTTP] * "HTTP Caching" [CACHING] This document specifies how HTTP semantics are conveyed using the HTTP/1.1 message syntax, framing, and connection management mechanisms. Its goal is to define the complete set of requirements for HTTP/1.1 message parsers and message-forwarding intermediaries. This document obsoletes the portions of RFC 7230 related to HTTP/1.1 messaging and connection management, with the changes being summarized in Appendix C.3. The other parts of RFC 7230 are obsoleted by "HTTP Semantics" [HTTP]. 1.1. Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Conformance criteria and considerations regarding error handling are defined in Section 2 of [HTTP]. 1.2. Syntax Notation This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], extended with the notation for case- sensitivity in strings defined in [RFC7405]. It also uses a list extension, defined in Section 5.6.1 of [HTTP], that allows for compact definition of comma-separated lists using a "#" operator (similar to how the "*" operator indicates repetition). Appendix A shows the collected grammar with all list operators expanded to standard ABNF notation. As a convention, ABNF rule names prefixed with "obs-" denote obsolete grammar rules that appear for historical reasons. 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), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible [USASCII] character). The rules below are defined in [HTTP]: BWS = <BWS, see [HTTP], Section 5.6.3> OWS = <OWS, see [HTTP], Section 5.6.3> RWS = <RWS, see [HTTP], Section 5.6.3> absolute-path = <absolute-path, see [HTTP], Section 4.1> field-name = <field-name, see [HTTP], Section 5.1> field-value = <field-value, see [HTTP], Section 5.5> obs-text = <obs-text, see [HTTP], Section 5.6.4> quoted-string = <quoted-string, see [HTTP], Section 5.6.4> token = <token, see [HTTP], Section 5.6.2> transfer-coding = <transfer-coding, see [HTTP], Section 10.1.4> The rules below are defined in [URI]: absolute-URI = <absolute-URI, see [URI], Section 4.3> authority = <authority, see [URI], Section 3.2> uri-host = <host, see [URI], Section 3.2.2> port = <port, see [URI], Section 3.2.3> query = <query, see [URI], Section 3.4> 2. Message HTTP/1.1 clients and servers communicate by sending messages. See Section 3 of [HTTP] for the general terminology and core concepts of HTTP. 2.1. Message Format An HTTP/1.1 message consists of a start-line followed by a CRLF and a sequence of octets in a format similar to the Internet Message Format [RFC5322]: zero or more header field lines (collectively referred to as the "headers" or the "header section"), an empty line indicating the end of the header section, and an optional message body. HTTP-message = start-line CRLF *( field-line CRLF ) CRLF [ message-body ] A message can be either a request from client to server or a response from server to client. Syntactically, the two types of messages differ only in the start-line, which is either a request-line (for requests) or a status-line (for responses), and in the algorithm for determining the length of the message body (Section 6). start-line = request-line / status-line In theory, a client could receive requests and a server could receive responses, distinguishing them by their different start-line formats. In practice, servers are implemented to only expect a request (a response is interpreted as an unknown or invalid request method), and clients are implemented to only expect a response. HTTP makes use of some protocol elements similar to the Multipurpose Internet Mail Extensions (MIME) [RFC2045]. See Appendix B for the differences between HTTP and MIME messages. 2.2. Message Parsing The normal procedure for parsing an HTTP message is to read the start-line into a structure, read each header field line 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. A recipient MUST parse an HTTP message as a sequence of octets in an encoding that is a superset of US-ASCII [USASCII]. Parsing an HTTP message as a stream of Unicode characters, without regard for the specific encoding, creates security vulnerabilities due to the varying ways that string processing libraries handle invalid multibyte character sequences that contain the octet LF (%x0A). String-based parsers can only be safely used within protocol elements after the element has been extracted from the message, such as within a header field line value after message parsing has delineated the individual field lines. Although the line terminator for the start-line and fields is the sequence CRLF, a recipient MAY recognize a single LF as a line terminator and ignore any preceding CR. A sender MUST NOT generate a bare CR (a CR character not immediately followed by LF) within any protocol elements other than the content. A recipient of such a bare CR MUST consider that element to be invalid or replace each bare CR with SP before processing the element or forwarding the message. Older HTTP/1.0 user agent implementations might send an extra CRLF after a POST request as a 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 user agent 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 user agent MUST count the terminating CRLF octets as part of the message body length. In the interest of robustness, a server that is expecting to receive and parse a request-line SHOULD ignore at least one empty line (CRLF) received prior to the request-line. A sender MUST NOT send whitespace between the start-line and the first header field. A recipient that receives whitespace between the start-line and the first header field MUST either reject the message as invalid or consume each whitespace-preceded line without further processing of it (i.e., ignore the entire line, along with any subsequent lines preceded by whitespace, until a properly formed header field is received or the header section is terminated). Rejection or removal of invalid whitespace-preceded lines is necessary to prevent their misinterpretation by downstream recipients that might be vulnerable to request smuggling (Section 11.2) or response splitting (Section 11.1) attacks. 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 SHOULD respond with a 400 (Bad Request) response and close the connection. 2.3. HTTP Version HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of the protocol. This specification defines version "1.1". Section 2.5 of [HTTP] specifies the semantics of HTTP version numbers. The version of an HTTP/1.x message is indicated by an HTTP-version field in the start-line. HTTP-version is case-sensitive. HTTP-version = HTTP-name "/" DIGIT "." DIGIT HTTP-name = %s"HTTP" When an HTTP/1.1 message is sent to an HTTP/1.0 recipient [HTTP/1.0] 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 conformant 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. Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as tunnels) MUST send their own HTTP-version in forwarded messages, unless it is purposefully downgraded as a workaround for an upstream issue. In other words, an intermediary is not allowed to blindly forward the start-line without ensuring that the protocol version in that message matches a version to which that intermediary is conformant 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. A server MAY send an HTTP/1.0 response to an HTTP/1.1 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 conform to 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. 3. Request Line A request-line begins with a method token, followed by a single space (SP), the request-target, and another single space (SP), and ends with the protocol version. request-line = method SP request-target SP HTTP-version Although the request-line grammar rule requires that each of the component elements be separated by a single SP octet, recipients MAY instead parse on whitespace-delimited word boundaries and, aside from the CRLF terminator, treat any form of whitespace as the SP separator while ignoring preceding or trailing whitespace; such whitespace includes one or more of the following octets: SP, HTAB, VT (%x0B), FF (%x0C), or bare CR. However, lenient parsing can result in request smuggling security vulnerabilities if there are multiple recipients of the message and each has its own unique interpretation of robustness (see Section 11.2). HTTP does not place a predefined limit on the length of a request- line, as described in Section 2.3 of [HTTP]. A server that receives a method longer than any that it implements SHOULD respond with a 501 (Not Implemented) status code. A server that receives a request- target longer than any URI it wishes to parse MUST respond with a 414 (URI Too Long) status code (see Section 15.5.15 of [HTTP]). Various ad hoc limitations on request-line length are found in practice. It is RECOMMENDED that all HTTP senders and recipients support, at a minimum, request-line lengths of 8000 octets. 3.1. Method The method token indicates the request method to be performed on the target resource. The request method is case-sensitive. method = token The request methods defined by this specification can be found in Section 9 of [HTTP], along with information regarding the HTTP method registry and considerations for defining new methods. 3.2. Request Target The request-target identifies the target resource upon which to apply the request. The client derives a request-target from its desired target URI. There are four distinct formats for the request-target, depending on both the method being requested and whether the request is to a proxy. request-target = origin-form / absolute-form / authority-form / asterisk-form No whitespace is allowed in the request-target. Unfortunately, some user agents fail to properly encode or exclude whitespace found in hypertext references, resulting in those disallowed characters being sent as the request-target in a malformed request-line. Recipients of an invalid request-line SHOULD respond with either a 400 (Bad Request) error or a 301 (Moved Permanently) redirect with the request-target properly encoded. A recipient SHOULD NOT attempt to autocorrect and then process the request without a redirect, since the invalid request-line might be deliberately crafted to bypass security filters along the request chain. A client MUST send a Host header field (Section 7.2 of [HTTP]) in all HTTP/1.1 request messages. If the target URI includes an authority component, then a client MUST send a field value for Host that is identical to that authority component, excluding any userinfo subcomponent and its "@" delimiter (Section 4.2 of [HTTP]). If the authority component is missing or undefined for the target URI, then a client MUST send a Host header field with an empty field value. 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 line or a Host header field with an invalid field value. 3.2.1. origin-form The most common form of request-target is the "origin-form". origin-form = absolute-path [ "?" query ] When making a request directly to an origin server, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send only the absolute path and query components of the target URI as the request-target. If the target URI's path component is empty, the client MUST send "/" as the path within the origin-form of request-target. A Host header field is also sent, as defined in Section 7.2 of [HTTP]. For example, a client wishing to retrieve a representation of the resource identified as http://www.example.org/where?q=now directly from the origin server would open (or reuse) a TCP connection to port 80 of the host "www.example.org" and send the lines: GET /where?q=now HTTP/1.1 Host: www.example.org followed by the remainder of the request message. 3.2.2. absolute-form When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client MUST send the target URI in "absolute-form" as the request-target. absolute-form = absolute-URI The proxy is requested to either service that request from a valid cache, if possible, or make the same request on the client's behalf either to the next inbound proxy server or directly to the origin server indicated by the request-target. Requirements on such "forwarding" of messages are defined in Section 7.6 of [HTTP]. An example absolute-form of request-line would be: GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1 A client MUST send a Host header field in an HTTP/1.1 request even if the request-target is in the absolute-form, since this allows the Host information to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host. When a proxy receives a request with an absolute-form of request- target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request- target. A proxy that forwards such a request MUST generate a new Host field value based on the received request-target rather than forward the received Host field value. When an origin server receives a request with an absolute-form of request-target, the origin server MUST ignore the received Host header field (if any) and instead use the host information of the request-target. Note that if the request-target does not have an authority component, an empty Host header field will be sent in this case. A server MUST accept the absolute-form in requests even though most HTTP/1.1 clients will only send the absolute-form to a proxy. 3.2.3. authority-form The "authority-form" of request-target is only used for CONNECT requests (Section 9.3.6 of [HTTP]). It consists of only the uri-host and port number of the tunnel destination, separated by a colon (":"). authority-form = uri-host ":" port When making a CONNECT request to establish a tunnel through one or more proxies, a client MUST send only the host and port of the tunnel destination as the request-target. The client obtains the host and port from the target URI's authority component, except that it sends the scheme's default port if the target URI elides the port. For example, a CONNECT request to "http://www.example.com" looks like the following: CONNECT www.example.com:80 HTTP/1.1 Host: www.example.com 3.2.4. asterisk-form The "asterisk-form" of request-target is only used for a server-wide OPTIONS request (Section 9.3.7 of [HTTP]). asterisk-form = "*" When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server, the client MUST send only "*" (%x2A) as the request-target. For example, OPTIONS * HTTP/1.1 If a proxy receives an OPTIONS request with an absolute-form of request-target in which the URI has an empty path and no query component, then the last proxy on the request chain MUST send a request-target of "*" when it forwards the request to the indicated origin server. For example, the request OPTIONS http://www.example.org:8001 HTTP/1.1 would be forwarded by the final proxy as OPTIONS * HTTP/1.1 Host: www.example.org:8001 after connecting to port 8001 of host "www.example.org". 3.3. Reconstructing the Target URI The target URI is the request-target when the request-target is in absolute-form. In that case, a server will parse the URI into its generic components for further evaluation. Otherwise, the server reconstructs the target URI from the connection context and various parts of the request message in order to identify the target resource (Section 7.1 of [HTTP]): * If the server's configuration provides for a fixed URI scheme, or a scheme is provided by a trusted outbound gateway, that scheme is used for the target URI. This is common in large-scale deployments because a gateway server will receive the client's connection context and replace that with their own connection to the inbound server. Otherwise, if the request is received over a secured connection, the target URI's scheme is "https"; if not, the scheme is "http". * If the request-target is in authority-form, the target URI's authority component is the request-target. Otherwise, the target URI's authority component is the field value of the Host header field. If there is no Host header field or if its field value is empty or invalid, the target URI's authority component is empty. * If the request-target is in authority-form or asterisk-form, the target URI's combined path and query component is empty. Otherwise, the target URI's combined path and query component is the request-target. * The components of a reconstructed target URI, once determined as above, can be recombined into absolute-URI form by concatenating the scheme, "://", authority, and combined path and query component. Example 1: The following message received over a secure connection GET /pub/WWW/TheProject.html HTTP/1.1 Host: www.example.org has a target URI of https://www.example.org/pub/WWW/TheProject.html Example 2: The following message received over an insecure connection OPTIONS * HTTP/1.1 Host: www.example.org:8080 has a target URI of http://www.example.org:8080 If the target URI's authority component is empty and its URI scheme requires a non-empty authority (as is the case for "http" and "https"), the server can reject the request or determine whether a configured default applies that is consistent with the incoming connection's context. Context might include connection details like address and port, what security has been applied, and locally defined information specific to that server's configuration. An empty authority is replaced with the configured default before further processing of the request. Supplying a default name for authority within the context of a secured connection is inherently unsafe if there is any chance that the user agent's intended authority might differ from the default. A server that can uniquely identify an authority from the request context MAY use that identity as a default without this risk. Alternatively, it might be better to redirect the request to a safe resource that explains how to obtain a new client. Note that reconstructing the client's target URI is only half of the process for identifying a target resource. The other half is determining whether that target URI identifies a resource for which the server is willing and able to send a response, as defined in Section 7.4 of [HTTP]. 4. Status Line The first line of a response message is the status-line, consisting of the protocol version, a space (SP), the status code, and another space and ending with an OPTIONAL textual phrase describing the status code. status-line = HTTP-version SP status-code SP [ reason-phrase ] Although the status-line grammar rule requires that each of the component elements be separated by a single SP octet, recipients MAY instead parse on whitespace-delimited word boundaries and, aside from the line terminator, treat any form of whitespace as the SP separator while ignoring preceding or trailing whitespace; such whitespace includes one or more of the following octets: SP, HTAB, VT (%x0B), FF (%x0C), or bare CR. However, lenient parsing can result in response splitting security vulnerabilities if there are multiple recipients of the message and each has its own unique interpretation of robustness (see Section 11.1). The status-code element is a 3-digit integer code describing the result of the server's attempt to understand and satisfy the client's corresponding request. A recipient parses and interprets the remainder of the response message in light of the semantics defined for that status code, if the status code is recognized by that recipient, or in accordance with the class of that status code when the specific code is unrecognized. status-code = 3DIGIT HTTP's core status codes are defined in Section 15 of [HTTP], along with the classes of status codes, considerations for the definition of new status codes, and the IANA registry for collecting such definitions. The reason-phrase element exists for the sole purpose of providing a textual description associated with the numeric status code, mostly out of deference to earlier Internet application protocols that were more frequently used with interactive text clients. reason-phrase = 1*( HTAB / SP / VCHAR / obs-text ) A client SHOULD ignore the reason-phrase content because it is not a reliable channel for information (it might be translated for a given locale, overwritten by intermediaries, or discarded when the message is forwarded via other versions of HTTP). A server MUST send the space that separates the status-code from the reason-phrase even when the reason-phrase is absent (i.e., the status-line would end with the space). 5. Field Syntax Each field line consists of a case-insensitive field name followed by a colon (":"), optional leading whitespace, the field line value, and optional trailing whitespace. field-line = field-name ":" OWS field-value OWS Rules for parsing within field values are defined in Section 5.5 of [HTTP]. This section covers the generic syntax for header field inclusion within, and extraction from, HTTP/1.1 messages. 5.1. Field Line Parsing Messages are parsed using a generic algorithm, independent of the individual field names. The contents within a given field line value are not parsed until a later stage of message interpretation (usually after the message's entire field section has been processed). No whitespace is allowed between the field name and colon. In the past, differences in the handling of such whitespace have led to security vulnerabilities in request routing and response handling. A server MUST reject, with a response status code of 400 (Bad Request), any received request message that contains whitespace between a header field name and colon. A proxy MUST remove any such whitespace from a response message before forwarding the message downstream. A field line value might be preceded and/or followed by optional whitespace (OWS); a single SP preceding the field line value is preferred for consistent readability by humans. The field line value does not include that leading or trailing whitespace: OWS occurring before the first non-whitespace octet of the field line value, or after the last non-whitespace octet of the field line value, is excluded by parsers when extracting the field line value from a field line. 5.2. Obsolete Line Folding Historically, HTTP/1.x field values could be extended over multiple lines by preceding each extra line with at least one space or horizontal tab (obs-fold). This specification deprecates such line folding except within the "message/http" media type (Section 10.1). obs-fold = OWS CRLF RWS ; obsolete line folding A sender MUST NOT generate a message that includes line folding (i.e., that has any field line value that contains a match to the obs-fold rule) unless the message is intended for packaging within the "message/http" media type. A server that receives an obs-fold in a request message that is not within a "message/http" container MUST either reject the message by sending a 400 (Bad Request), preferably with a representation explaining that obsolete line folding is unacceptable, or replace each received obs-fold with one or more SP octets prior to interpreting the field value or forwarding the message downstream. A proxy or gateway that receives an obs-fold in a response message that is not within a "message/http" container MUST either discard the message and replace it with a 502 (Bad Gateway) response, preferably with a representation explaining that unacceptable line folding was received, or replace each received obs-fold with one or more SP octets prior to interpreting the field value or forwarding the message downstream. A user agent that receives an obs-fold in a response message that is not within a "message/http" container MUST replace each received obs-fold with one or more SP octets prior to interpreting the field value. 6. Message Body The message body (if any) of an HTTP/1.1 message is used to carry content (Section 6.4 of [HTTP]) for the request or response. The message body is identical to the content unless a transfer coding has been applied, as described in Section 6.1. message-body = *OCTET The rules for determining when a message body is present in an HTTP/1.1 message differ for requests and responses. The presence of a message body in a request is signaled by a Content-Length or Transfer-Encoding header field. Request message framing is independent of method semantics. The presence of a message body in a response, as detailed in Section 6.3, depends on both the request method to which it is responding and the response status code. This corresponds to when response content is allowed by HTTP semantics (Section 6.4.1 of [HTTP]). 6.1. Transfer-Encoding The Transfer-Encoding header field lists the transfer coding names corresponding to the sequence of transfer codings that have been (or will be) applied to the content in order to form the message body. Transfer codings are defined in Section 7. Transfer-Encoding = #transfer-coding ; defined in [HTTP], Section 10.1.4 Transfer-Encoding is analogous to the Content-Transfer-Encoding field of MIME, which was 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's case, Transfer-Encoding is primarily intended to accurately delimit dynamically generated content. It also serves to distinguish encodings that are only applied in transit from the encodings that are a characteristic of the selected representation. A recipient MUST be able to parse the chunked transfer coding (Section 7.1) because it plays a crucial role in framing messages when the content size is not known in advance. A sender MUST NOT apply the chunked transfer coding more than once to a message body (i.e., chunking an already chunked message is not allowed). If any transfer coding other than chunked is applied to a request's content, the sender MUST apply chunked as the final transfer coding to ensure that the message is properly framed. If any transfer coding other than chunked is applied to a response's content, the sender MUST either apply chunked as the final transfer coding or terminate the message by closing the connection. For example, Transfer-Encoding: gzip, chunked indicates that the content has been compressed using the gzip coding and then chunked using the chunked coding while forming the message body. Unlike Content-Encoding (Section 8.4.1 of [HTTP]), Transfer-Encoding is a property of the message, not of the representation. Any recipient along the request/response chain MAY decode the received transfer coding(s) or apply additional transfer coding(s) to the message body, assuming that corresponding changes are made to the Transfer-Encoding field value. Additional information about the encoding parameters can be provided by other header fields not defined by this specification. Transfer-Encoding MAY be sent in a response to a HEAD request or in a 304 (Not Modified) response (Section 15.4.5 of [HTTP]) to a GET request, neither of which includes a message body, to indicate that the origin server would have applied a transfer coding to the message body if the request had been an unconditional GET. This indication is not required, however, because any recipient on the response chain (including the origin server) can remove transfer codings when they are not needed. A server MUST NOT send a Transfer-Encoding header field in any response with a status code of 1xx (Informational) or 204 (No Content). A server MUST NOT send a Transfer-Encoding header field in any 2xx (Successful) response to a CONNECT request (Section 9.3.6 of [HTTP]). A server that receives a request message with a transfer coding it does not understand SHOULD respond with 501 (Not Implemented). Transfer-Encoding was added in HTTP/1.1. It is generally assumed that implementations advertising only HTTP/1.0 support will not understand how to process transfer-encoded content, and that an HTTP/1.0 message received with a Transfer-Encoding is likely to have been forwarded without proper handling of the chunked transfer coding in transit. A client MUST NOT send a request containing Transfer-Encoding unless it knows the server will handle HTTP/1.1 requests (or later minor revisions); such knowledge might be in the form of specific user configuration or by remembering the version of a prior received response. A server MUST NOT send a response containing Transfer- Encoding unless the corresponding request indicates HTTP/1.1 (or later minor revisions). Early implementations of Transfer-Encoding would occasionally send both a chunked transfer coding for message framing and an estimated Content-Length header field for use by progress bars. This is why Transfer-Encoding is defined as overriding Content-Length, as opposed to them being mutually incompatible. Unfortunately, forwarding such a message can lead to vulnerabilities regarding request smuggling (Section 11.2) or response splitting (Section 11.1) attacks if any downstream recipient fails to parse the message according to this specification, particularly when a downstream recipient only implements HTTP/1.0. A server MAY reject a request that contains both Content-Length and Transfer-Encoding or process such a request in accordance with the Transfer-Encoding alone. Regardless, the server MUST close the connection after responding to such a request to avoid the potential attacks. A server or client that receives an HTTP/1.0 message containing a Transfer-Encoding header field MUST treat the message as if the framing is faulty, even if a Content-Length is present, and close the connection after processing the message. The message sender might have retained a portion of the message, in buffer, that could be misinterpreted by further use of the connection. 6.2. Content-Length When a message does not have a Transfer-Encoding header field, a Content-Length header field (Section 8.6 of [HTTP]) can provide the anticipated size, as a decimal number of octets, for potential content. For messages that do include content, the Content-Length field value provides the framing information necessary for determining where the data (and message) ends. For messages that do not include content, the Content-Length indicates the size of the selected representation (Section 8.6 of [HTTP]). A sender MUST NOT send a Content-Length header field in any message that contains a Transfer-Encoding header field. | *Note:* HTTP's use of Content-Length for message framing | differs significantly from the same field's use in MIME, where | it is an optional field used only within the "message/external- | body" media-type. 6.3. Message Body Length The length of a message body is determined by one of the following (in order of precedence): 1. Any response to a HEAD request and any response with a 1xx (Informational), 204 (No Content), or 304 (Not Modified) status code 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 or trailer section. 2. Any 2xx (Successful) response to a CONNECT request implies that the connection will become a tunnel immediately after the empty line that concludes the header fields. A client MUST ignore any Content-Length or Transfer-Encoding header fields received in such a message. 3. If a message is received with both a Transfer-Encoding and a Content-Length header field, the Transfer-Encoding overrides the Content-Length. Such a message might indicate an attempt to perform request smuggling (Section 11.2) or response splitting (Section 11.1) and ought to be handled as an error. An intermediary that chooses to forward the message MUST first remove the received Content-Length field and process the Transfer-Encoding (as described below) prior to forwarding the message downstream. 4. If a Transfer-Encoding header field is present and the chunked transfer coding (Section 7.1) 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. 5. If a message is received without Transfer-Encoding and with an invalid Content-Length header field, then the message framing is invalid and the recipient MUST treat it as an unrecoverable error, unless the field value can be successfully parsed as a comma-separated list (Section 5.6.1 of [HTTP]), all values in the list are valid, and all values in the list are the same (in which case, the message is processed with that single value used as the Content-Length field value). If the unrecoverable error is in a request message, the server MUST respond with a 400 (Bad Request) status code and then close the connection. If it is in a response message received by a proxy, the proxy MUST close the connection to the server, discard the received response, and send a 502 (Bad Gateway) response to the client. If it is in a response message received by a user agent, the user agent MUST close the connection to the server and discard the received response. 6. If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the expected message body length in octets. If the sender closes the connection or the recipient times out before the indicated number of octets are received, the recipient MUST consider the message to be incomplete and close the connection. 7. 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). 8. 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 response message from a partially received message interrupted by network failure, a server SHOULD generate encoding or length-delimited messages whenever possible. The close-delimiting feature exists primarily for backwards compatibility with HTTP/1.0. | *Note:* Request messages are never close-delimited because they | are always explicitly framed by length or transfer coding, with | the absence of both implying the request ends immediately after | the header section. 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 transfer coding, since some existing services respond to chunked with a 411 (Length Required) status code even though they understand the chunked transfer coding. 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 user agent that sends a request that contains a message body MUST send either a valid Content-Length header field or use the chunked transfer coding. A client MUST NOT use the chunked transfer coding unless it knows 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. If the final response to the last request on a connection has been completely received and there remains additional data to read, a user agent MAY discard the remaining data or attempt to determine if that data belongs as part of the prior message body, which might be the case if the prior message's Content-Length value is incorrect. A client MUST NOT process, cache, or forward such extra data as a separate response, since such behavior would be vulnerable to cache poisoning. 7. Transfer Codings Transfer coding names are used to indicate an encoding transformation that has been, can be, or might need to be applied to a message's content 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. All transfer-coding names are case-insensitive and ought to be registered within the HTTP Transfer Coding registry, as defined in Section 7.3. They are used in the Transfer-Encoding (Section 6.1) and TE (Section 10.1.4 of [HTTP]) header fields (the latter also defining the "transfer-coding" grammar). 7.1. Chunked Transfer Coding The chunked transfer coding wraps content in order to transfer it as a series of chunks, each with its own size indicator, followed by an OPTIONAL trailer section containing trailer fields. Chunked enables content streams of unknown size to be transferred as a sequence of length-delimited buffers, which enables the sender to retain connection persistence and the recipient to know when it has received the entire message. chunked-body = *chunk last-chunk trailer-section CRLF chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF chunk-size = 1*HEXDIG last-chunk = 1*("0") [ chunk-ext ] CRLF chunk-data = 1*OCTET ; a sequence of chunk-size octets The chunk-size field is a string of hex digits indicating the size of the chunk-data in octets. The chunked transfer coding is complete when a chunk with a chunk-size of zero is received, possibly followed by a trailer section, and finally terminated by an empty line. A recipient MUST be able to parse and decode the chunked transfer coding. HTTP/1.1 does not define any means to limit the size of a chunked response such that an intermediary can be assured of buffering the entire response. Additionally, very large chunk sizes may cause overflows or loss of precision if their values are not represented accurately in a receiving implementation. Therefore, recipients MUST anticipate potentially large hexadecimal numerals and prevent parsing errors due to integer conversion overflows or precision loss due to integer representation. The chunked coding does not define any parameters. Their presence SHOULD be treated as an error. 7.1.1. Chunk Extensions The chunked coding allows each chunk to include zero or more chunk extensions, immediately following the chunk-size, for the sake of supplying per-chunk metadata (such as a signature or hash), mid- message control information, or randomization of message body size. chunk-ext = *( BWS ";" BWS chunk-ext-name [ BWS "=" BWS chunk-ext-val ] ) chunk-ext-name = token chunk-ext-val = token / quoted-string The chunked coding is specific to each connection and is likely to be removed or recoded by each recipient (including intermediaries) before any higher-level application would have a chance to inspect the extensions. Hence, the use of chunk extensions is generally limited to specialized HTTP services such as "long polling" (where client and server can have shared expectations regarding the use of chunk extensions) or for padding within an end-to-end secured connection. A recipient MUST ignore unrecognized chunk extensions. A server ought to limit the total length of chunk extensions received in a request to an amount reasonable for the services provided, in the same way that it applies length limitations and timeouts for other parts of a message, and generate an appropriate 4xx (Client Error) response if that amount is exceeded. 7.1.2. Chunked Trailer Section A trailer section allows the sender to include additional fields at the end of a chunked message in order to supply metadata that might be dynamically generated while the content is sent, such as a message integrity check, digital signature, or post-processing status. The proper use and limitations of trailer fields are defined in Section 6.5 of [HTTP]. trailer-section = *( field-line CRLF ) A recipient that removes the chunked coding from a message MAY selectively retain or discard the received trailer fields. A recipient that retains a received trailer field MUST either store/ forward the trailer field separately from the received header fields or merge the received trailer field into the header section. A recipient MUST NOT merge a received trailer field into the header section unless its corresponding header field definition explicitly permits and instructs how the trailer field value can be safely merged. 7.1.3. Decoding Chunked A process for decoding the chunked transfer coding can be represented in pseudo-code as: length := 0 read chunk-size, chunk-ext (if any), and CRLF while (chunk-size > 0) { read chunk-data and CRLF append chunk-data to content length := length + chunk-size read chunk-size, chunk-ext (if any), and CRLF } read trailer field while (trailer field is not empty) { if (trailer fields are stored/forwarded separately) { append trailer field to existing trailer fields } else if (trailer field is understood and defined as mergeable) { merge trailer field with existing header fields } else { discard trailer field } read trailer field } Content-Length := length Remove "chunked" from Transfer-Encoding 7.2. Transfer Codings for Compression The following transfer coding names for compression are defined by the same algorithm as their corresponding content coding: compress (and x-compress) See Section +1-563-711-0698 of [HTTP]. deflate See Section +1-563-711-0698 of [HTTP]. gzip (and x-gzip) See Section +1-563-711-0698 of [HTTP]. The compression codings do not define any parameters. The presence of parameters with any of these compression codings SHOULD be treated as an error. 7.3. Transfer Coding Registry The "HTTP Transfer Coding Registry" defines the namespace for transfer coding names. It is maintained at <https://www.iana.org/assignments/http-parameters>. Registrations MUST include the following fields: * Name * Description * Pointer to specification text Names of transfer codings MUST NOT overlap with names of content codings (Section 8.4.1 of [HTTP]) unless the encoding transformation is identical, as is the case for the compression codings defined in Section 7.2. The TE header field (Section 10.1.4 of [HTTP]) uses a pseudo- parameter named "q" as the rank value when multiple transfer codings are acceptable. Future registrations of transfer codings SHOULD NOT define parameters called "q" (case-insensitively) in order to avoid ambiguities. Values to be added to this namespace require IETF Review (see Section 4.8 of [RFC8126]) and MUST conform to the purpose of transfer coding defined in this specification. Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings. 7.4. Negotiating Transfer Codings The TE field (Section 10.1.4 of [HTTP]) is used in HTTP/1.1 to indicate what transfer codings, besides chunked, the client is willing to accept in the response and whether the client is willing to preserve trailer fields in a chunked transfer coding. A client MUST NOT send the chunked transfer coding name in TE; chunked is always acceptable for HTTP/1.1 recipients. Three examples of TE use are below. TE: deflate TE: TE: trailers, deflate;q=0.5 When multiple transfer codings are acceptable, the client MAY rank the codings by preference using a case-insensitive "q" parameter (similar to the qvalues used in content negotiation fields; see Section 12.4.2 of [HTTP]). The rank value is a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred; a value of 0 means "not acceptable". If the TE field value is empty or if no TE field is present, the only acceptable transfer coding is chunked. A message with no transfer coding is always acceptable. The keyword "trailers" indicates that the sender will not discard trailer fields, as described in Section 6.5 of [HTTP]. Since the TE header field only applies to the immediate connection, a sender of TE MUST also send a "TE" connection option within the Connection header field (Section 7.6.1 of [HTTP]) in order to prevent the TE header field from being forwarded by intermediaries that do not support its semantics. 8. Handling Incomplete Messages A server that receives an incomplete request message, usually due to a canceled request or a triggered timeout exception, MAY send an error response prior to closing the connection. A client that receives an incomplete response message, which can occur when a connection is closed prematurely or when decoding a supposedly chunked transfer coding fails, MUST record the message as incomplete. Cache requirements for incomplete responses are defined in Section 3.3 of [CACHING]. If a response terminates in the middle of the header section (before the empty line is received) and the status code might rely on header fields to convey the full meaning of the response, then the client cannot assume that meaning has been conveyed; the client might need
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