HTTP/1.1
RFC 9112
also known as STD 99
| Document | Type |
RFC
- Internet Standard
(June 2022)
Errata
Obsoletes RFC 7230
|
|
|---|---|---|---|
| Authors | Roy T. Fielding , Mark Nottingham , Julian Reschke | ||
| Last updated | 2024-01-04 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| IESG | Responsible AD | Francesca Palombini | |
| Send notices to | (None) |
RFC 9112
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
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Contributions published or made publicly available before November
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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
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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 8.4.1.1 of [HTTP].
deflate
See Section 8.4.1.2 of [HTTP].
gzip (and x-gzip)
See Section 8.4.1.3 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
to repeat the request in order to determine what action to take next.
A message body that uses the chunked transfer coding 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 coding nor Content-Length is terminated by closure of the
connection and, if the header section was received intact, is
considered complete unless an error was indicated by the underlying
connection (e.g., an "incomplete close" in TLS would leave the
response incomplete, as described in Section 9.8).
9. Connection Management
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 an underlying transport
protocol is outside the scope of this specification.
As described in Section 7.3 of [HTTP], the specific connection
protocols to be used for an HTTP interaction are determined by client
configuration and the target URI. For example, the "http" URI scheme
(Section 4.2.1 of [HTTP]) 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.
HTTP implementations are expected to engage in connection management,
which includes maintaining the state of current connections,
establishing a new connection or reusing an existing connection,
processing messages received on a connection, detecting connection
failures, and closing each connection. Most clients maintain
multiple connections in parallel, including more than one connection
per server endpoint. Most servers are designed to maintain thousands
of concurrent connections, while controlling request queues to enable
fair use and detect denial-of-service attacks.
9.1. Establishment
It is beyond the scope of this specification to describe how
connections are established via various transport- or session-layer
protocols. Each HTTP connection maps to one underlying transport
connection.
9.2. Associating a Response to a Request
HTTP/1.1 does not include a request identifier for associating a
given request message with its corresponding one or more response
messages. Hence, it relies on the order of response arrival to
correspond exactly to the order in which requests are made on the
same connection. More than one response message per request only
occurs when one or more informational responses (1xx; see
Section 15.2 of [HTTP]) precede a final response to the same request.
A client that has more than one outstanding request on a connection
MUST maintain a list of outstanding requests in the order sent and
MUST associate each received response message on that connection to
the first outstanding request that has not yet received a final (non-
1xx) response.
If a client receives data on a connection that doesn't have
outstanding requests, the client MUST NOT consider that data to be a
valid response; the client SHOULD close the connection, since message
delimitation is now ambiguous, unless the data consists only of one
or more CRLF (which can be discarded per Section 2.2).
9.3. Persistence
HTTP/1.1 defaults to the use of "persistent connections", allowing
multiple requests and responses to be carried over a single
connection. HTTP implementations SHOULD support persistent
connections.
A recipient determines whether a connection is persistent or not
based on the protocol version and Connection header field
(Section 7.6.1 of [HTTP]) in the most recently received message, if
any:
* If the "close" connection option is present (Section 9.6), the
connection will not persist after the current response; else,
* If the received protocol is HTTP/1.1 (or later), the connection
will persist after the current response; else,
* If the received protocol is HTTP/1.0, the "keep-alive" connection
option is present, either the recipient is not a proxy or the
message is a response, and the recipient wishes to honor the
HTTP/1.0 "keep-alive" mechanism, the connection will persist after
the current response; otherwise,
* The connection will close after the current response.
A client that does not support persistent connections MUST send the
"close" connection option in every request message.
A server that does not support persistent connections MUST send the
"close" connection option in every response message that does not
have a 1xx (Informational) status code.
A client MAY send additional requests on a persistent connection
until it sends or receives a "close" connection option or receives an
HTTP/1.0 response without a "keep-alive" connection option.
In order to remain persistent, all messages on a connection need to
have a self-defined message length (i.e., one not defined by closure
of the connection), as described in Section 6. A server MUST read
the entire request message body or close the connection after sending
its response; 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.
A proxy server MUST NOT maintain a persistent connection with an
HTTP/1.0 client (see Appendix C.2.2 for information and discussion of
the problems with the Keep-Alive header field implemented by many
HTTP/1.0 clients).
See Appendix C.2.2 for more information on backwards compatibility
with HTTP/1.0 clients.
9.3.1. Retrying Requests
Connections can be closed at any time, with or without intention.
Implementations ought to anticipate the need to recover from
asynchronous close events. The conditions under which a client can
automatically retry a sequence of outstanding requests are defined in
Section 9.2.2 of [HTTP].
9.3.2. Pipelining
A client that supports persistent connections MAY "pipeline" its
requests (i.e., send multiple requests without waiting for each
response). A server MAY process a sequence of pipelined requests in
parallel if they all have safe methods (Section 9.2.1 of [HTTP]), but
it MUST send the corresponding responses in the same order that the
requests were received.
A client that pipelines requests SHOULD retry unanswered requests if
the connection closes before it receives all of the corresponding
responses. When retrying pipelined requests after a failed
connection (a connection not explicitly closed by the server in its
last complete response), a client MUST NOT pipeline immediately after
connection establishment, since the first remaining request in the
prior pipeline might have caused an error response that can be lost
again if multiple requests are sent on a prematurely closed
connection (see the TCP reset problem described in Section 9.6).
Idempotent methods (Section 9.2.2 of [HTTP]) are significant to
pipelining because they can be automatically retried after a
connection failure. A user agent SHOULD NOT pipeline requests after
a non-idempotent method, until the final response status code for
that method has been received, unless the user agent has a means to
detect and recover from partial failure conditions involving the
pipelined sequence.
An intermediary that receives pipelined requests MAY pipeline those
requests when forwarding them inbound, since it can rely on the
outbound user agent(s) to determine what requests can be safely
pipelined. If the inbound connection fails before receiving a
response, the pipelining intermediary MAY attempt to retry a sequence
of requests that have yet to receive a response if the requests all
have idempotent methods; otherwise, the pipelining intermediary
SHOULD forward any received responses and then close the
corresponding outbound connection(s) so that the outbound user
agent(s) can recover accordingly.
9.4. Concurrency
A client ought to limit the number of simultaneous open connections
that it maintains to a given server.
Previous revisions of HTTP gave a specific number of connections as a
ceiling, but this was found to be impractical for many applications.
As a result, this specification does not mandate a particular maximum
number of connections but, instead, encourages clients to be
conservative when opening multiple connections.
Multiple connections are typically used to avoid the "head-of-line
blocking" problem, wherein a request that takes significant server-
side processing and/or transfers very large content would block
subsequent requests on the same connection. However, each connection
consumes server resources.
Furthermore, using multiple connections can cause undesirable side
effects in congested networks. Using larger numbers of connections
can also cause side effects in otherwise uncongested networks,
because their aggregate and initially synchronized sending behavior
can cause congestion that would not have been present if fewer
parallel connections had been used.
Note that a server might reject traffic that it deems abusive or
characteristic of a denial-of-service attack, such as an excessive
number of open connections from a single client.
9.5. Failures and Timeouts
Servers will usually have some timeout value beyond which they will
no longer maintain an inactive connection. Proxy servers might make
this a higher value since it is likely that the client will be making
more connections through the same proxy server. The use of
persistent connections places no requirements on the length (or
existence) of this timeout for either the client or the server.
A client or server that wishes to time out SHOULD issue a graceful
close on the connection. Implementations SHOULD constantly monitor
open connections for a received closure signal and respond to it as
appropriate, since prompt closure of both sides of a connection
enables allocated system resources to be reclaimed.
A client, server, or proxy MAY close the transport connection at any
time. For example, a client might have started to send a new request
at the same time that the server has decided to close the "idle"
connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a
request is in progress.
A server SHOULD sustain persistent connections, when possible, and
allow the underlying transport's flow-control mechanisms to resolve
temporary overloads rather than terminate connections with the
expectation that clients will retry. The latter technique can
exacerbate network congestion or server load.
A client sending a message body SHOULD monitor the network connection
for an error response while it is transmitting the request. If the
client sees a response that indicates the server does not wish to
receive the message body and is closing the connection, the client
SHOULD immediately cease transmitting the body and close its side of
the connection.
9.6. Tear-down
The "close" connection option is defined as a signal that the sender
will close this connection after completion of the response. A
sender SHOULD send a Connection header field (Section 7.6.1 of
[HTTP]) containing the "close" connection option when it intends to
close a connection. For example,
Connection: close
as a request header field indicates that this is the last request
that the client will send on this connection, while in a response,
the same field indicates that the server is going to close this
connection after the response message is complete.
Note that the field name "Close" is reserved, since using that name
as a header field might conflict with the "close" connection option.
A client that sends a "close" connection option MUST NOT send further
requests on that connection (after the one containing the "close")
and MUST close the connection after reading the final response
message corresponding to this request.
A server that receives a "close" connection option MUST initiate
closure of the connection (see below) after it sends the final
response to the request that contained the "close" connection option.
The server SHOULD send a "close" connection option in its final
response on that connection. The server MUST NOT process any further
requests received on that connection.
A server that sends a "close" connection option MUST initiate closure
of the connection (see below) after it sends the response containing
the "close" connection option. The server MUST NOT process any
further requests received on that connection.
A client that receives a "close" connection option MUST cease sending
requests on that connection and close the connection after reading
the response message containing the "close" connection option; if
additional pipelined requests had been sent on the connection, the
client SHOULD NOT assume that they will be processed by the server.
If a server performs an immediate close of a TCP connection, there is
a significant risk that the client will not be able to read the last
HTTP response. If the server receives additional data from the
client on a fully closed connection, such as another request sent by
the client before receiving the server's response, the server's TCP
stack will send a reset packet to the client; unfortunately, the
reset packet might erase the client's unacknowledged input buffers
before they can be read and interpreted by the client's HTTP parser.
To avoid the TCP reset problem, servers typically close a connection
in stages. First, the server performs a half-close by closing only
the write side of the read/write connection. The server then
continues to read from the connection until it receives a
corresponding close by the client, or until the server is reasonably
certain that its own TCP stack has received the client's
acknowledgement of the packet(s) containing the server's last
response. Finally, the server fully closes the connection.
It is unknown whether the reset problem is exclusive to TCP or might
also be found in other transport connection protocols.
Note that a TCP connection that is half-closed by the client does not
delimit a request message, nor does it imply that the client is no
longer interested in a response. In general, transport signals
cannot be relied upon to signal edge cases, since HTTP/1.1 is
independent of transport.
9.7. TLS Connection Initiation
Conceptually, HTTP/TLS is simply sending HTTP messages over a
connection secured via TLS [TLS13].
The HTTP client also acts as the TLS client. It initiates a
connection to the server on the appropriate port and sends the TLS
ClientHello to begin the TLS handshake. When the TLS handshake has
finished, the client may then initiate the first HTTP request. All
HTTP data MUST be sent as TLS "application data" but is otherwise
treated like a normal connection for HTTP (including potential reuse
as a persistent connection).
9.8. TLS Connection Closure
TLS uses an exchange of closure alerts prior to (non-error)
connection closure to provide secure connection closure; see
Section 6.1 of [TLS13]. When a valid closure alert is received, an
implementation can be assured that no further data will be received
on that connection.
When an implementation knows that it has sent or received all the
message data that it cares about, typically by detecting HTTP message
boundaries, it might generate an "incomplete close" by sending a
closure alert and then closing the connection without waiting to
receive the corresponding closure alert from its peer.
An incomplete close does not call into question the security of the
data already received, but it could indicate that subsequent data
might have been truncated. As TLS is not directly aware of HTTP
message framing, it is necessary to examine the HTTP data itself to
determine whether messages are complete. Handling of incomplete
messages is defined in Section 8.
When encountering an incomplete close, a client SHOULD treat as
completed all requests for which it has received either
1. as much data as specified in the Content-Length header field or
2. the terminal zero-length chunk (when Transfer-Encoding of chunked
is used).
A response that has neither chunked transfer coding nor Content-
Length is complete only if a valid closure alert has been received.
Treating an incomplete message as complete could expose
implementations to attack.
A client detecting an incomplete close SHOULD recover gracefully.
Clients MUST send a closure alert before closing the connection.
Clients that do not expect to receive any more data MAY choose not to
wait for the server's closure alert and simply close the connection,
thus generating an incomplete close on the server side.
Servers SHOULD be prepared to receive an incomplete close from the
client, since the client can often locate the end of server data.
Servers MUST attempt to initiate an exchange of closure alerts with
the client before closing the connection. Servers MAY close the
connection after sending the closure alert, thus generating an
incomplete close on the client side.
10. Enclosing Messages as Data
10.1. Media Type message/http
The "message/http" media type can be used to enclose a single HTTP
request or response message, provided that it obeys the MIME
restrictions for all "message" types regarding line length and
encodings. Because of the line length limitations, field values
within "message/http" are allowed to use line folding (obs-fold), as
described in Section 5.2, to convey the field value over multiple
lines. A recipient of "message/http" data MUST replace any obsolete
line folding with one or more SP characters when the message is
consumed.
Type name: message
Subtype name: http
Required parameters: N/A
Optional parameters: version, msgtype
version: The HTTP-version number of the enclosed message (e.g.,
"1.1"). If not present, the version can be determined from the
first line of the body.
msgtype: The message type -- "request" or "response". If not
present, the type can be determined from the first line of the
body.
Encoding considerations: only "7bit", "8bit", or "binary" are
permitted
Security considerations: see Section 11
Interoperability considerations: N/A
Published specification: RFC 9112 (see Section 10.1).
Applications that use this media type: N/A
Fragment identifier considerations: N/A
Additional information: Magic number(s): N/A
Deprecated alias names for this type: N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person and email address to contact for further information: See Aut
hors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: IESG
10.2. Media Type application/http
The "application/http" media type can be used to enclose a pipeline
of one or more HTTP request or response messages (not intermixed).
Type name: application
Subtype name: http
Required parameters: N/A
Optional parameters: version, msgtype
version: The HTTP-version number of the enclosed messages (e.g.,
"1.1"). If not present, the version can be determined from the
first line of the body.
msgtype: The message type -- "request" or "response". If not
present, the type can be determined from the first line of the
body.
Encoding considerations: HTTP messages enclosed by this type are in
"binary" format; use of an appropriate Content-Transfer-Encoding
is required when transmitted via email.
Security considerations: see Section 11
Interoperability considerations: N/A
Published specification: RFC 9112 (see Section 10.2).
Applications that use this media type: N/A
Fragment identifier considerations: N/A
Additional information: Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person and email address to contact for further information: See Aut
hors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: IESG
11. Security Considerations
This section is meant to inform developers, information providers,
and users about known security considerations relevant to HTTP
message syntax and parsing. Security considerations about HTTP
semantics, content, and routing are addressed in [HTTP].
11.1. Response Splitting
Response splitting (a.k.a. CRLF injection) is a common technique,
used in various attacks on Web usage, that exploits the line-based
nature of HTTP message framing and the ordered association of
requests to responses on persistent connections [Klein]. This
technique can be particularly damaging when the requests pass through
a shared cache.
Response splitting exploits a vulnerability in servers (usually
within an application server) where an attacker can send encoded data
within some parameter of the request that is later decoded and echoed
within any of the response header fields of the response. If the
decoded data is crafted to look like the response has ended and a
subsequent response has begun, the response has been split, and the
content within the apparent second response is controlled by the
attacker. The attacker can then make any other request on the same
persistent connection and trick the recipients (including
intermediaries) into believing that the second half of the split is
an authoritative answer to the second request.
For example, a parameter within the request-target might be read by
an application server and reused within a redirect, resulting in the
same parameter being echoed in the Location header field of the
response. If the parameter is decoded by the application and not
properly encoded when placed in the response field, the attacker can
send encoded CRLF octets and other content that will make the
application's single response look like two or more responses.
A common defense against response splitting is to filter requests for
data that looks like encoded CR and LF (e.g., "%0D" and "%0A").
However, that assumes the application server is only performing URI
decoding rather than more obscure data transformations like charset
transcoding, XML entity translation, base64 decoding, sprintf
reformatting, etc. A more effective mitigation is to prevent
anything other than the server's core protocol libraries from sending
a CR or LF within the header section, which means restricting the
output of header fields to APIs that filter for bad octets and not
allowing application servers to write directly to the protocol
stream.
11.2. Request Smuggling
Request smuggling ([Linhart]) is a technique that exploits
differences in protocol parsing among various recipients to hide
additional requests (which might otherwise be blocked or disabled by
policy) within an apparently harmless request. Like response
splitting, request smuggling can lead to a variety of attacks on HTTP
usage.
This specification has introduced new requirements on request
parsing, particularly with regard to message framing in Section 6.3,
to reduce the effectiveness of request smuggling.
11.3. Message Integrity
HTTP does not define a specific mechanism for ensuring message
integrity, instead relying on the error-detection ability of
underlying transport protocols and the use of length or chunk-
delimited framing to detect completeness. Historically, the lack of
a single integrity mechanism has been justified by the informal
nature of most HTTP communication. However, the prevalence of HTTP
as an information access mechanism has resulted in its increasing use
within environments where verification of message integrity is
crucial.
The mechanisms provided with the "https" scheme, such as
authenticated encryption, provide protection against modification of
messages. Care is needed, however, to ensure that connection closure
cannot be used to truncate messages (see Section 9.8). User agents
might refuse to accept incomplete messages or treat them specially.
For example, a browser being used to view medical history or drug
interaction information needs to indicate to the user when such
information is detected by the protocol to be incomplete, expired, or
corrupted during transfer. Such mechanisms might be selectively
enabled via user agent extensions or the presence of message
integrity metadata in a response.
The "http" scheme provides no protection against accidental or
malicious modification of messages.
Extensions to the protocol might be used to mitigate the risk of
unwanted modification of messages by intermediaries, even when the
"https" scheme is used. Integrity might be assured by using message
authentication codes or digital signatures that are selectively added
to messages via extensible metadata fields.
11.4. Message Confidentiality
HTTP relies on underlying transport protocols to provide message
confidentiality when that is desired. HTTP has been specifically
designed to be independent of the transport protocol, such that it
can be used over many forms of encrypted connection, with the
selection of such transports being identified by the choice of URI
scheme or within user agent configuration.
The "https" scheme can be used to identify resources that require a
confidential connection, as described in Section 4.2.2 of [HTTP].
12. IANA Considerations
The change controller for the following registrations is: "IETF
(iesg@ietf.org) - Internet Engineering Task Force".
12.1. Field Name Registration
IANA has added the following field names to the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" at
<https://www.iana.org/assignments/http-fields>, as described in
Section 18.4 of [HTTP].
+===================+===========+=========+============+
| Field Name | Status | Section | Comments |
+===================+===========+=========+============+
| Close | permanent | 9.6 | (reserved) |
+-------------------+-----------+---------+------------+
| MIME-Version | permanent | B.1 | |
+-------------------+-----------+---------+------------+
| Transfer-Encoding | permanent | 6.1 | |
+-------------------+-----------+---------+------------+
Table 1
12.2. Media Type Registration
IANA has updated the "Media Types" registry at
<https://www.iana.org/assignments/media-types> with the registration
information in Sections 10.1 and 10.2 for the media types "message/
http" and "application/http", respectively.
12.3. Transfer Coding Registration
IANA has updated the "HTTP Transfer Coding Registry" at
<https://www.iana.org/assignments/http-parameters/> with the
registration procedure of Section 7.3 and the content coding names
summarized in the table below.
+============+===========================================+=========+
| Name | Description | Section |
+============+===========================================+=========+
| chunked | Transfer in a series of chunks | 7.1 |
+------------+-------------------------------------------+---------+
| compress | UNIX "compress" data format [Welch] | 7.2 |
+------------+-------------------------------------------+---------+
| deflate | "deflate" compressed data ([RFC1951]) | 7.2 |
| | inside the "zlib" data format ([RFC1950]) | |
+------------+-------------------------------------------+---------+
| gzip | GZIP file format [RFC1952] | 7.2 |
+------------+-------------------------------------------+---------+
| trailers | (reserved) | 12.3 |
+------------+-------------------------------------------+---------+
| x-compress | Deprecated (alias for compress) | 7.2 |
+------------+-------------------------------------------+---------+
| x-gzip | Deprecated (alias for gzip) | 7.2 |
+------------+-------------------------------------------+---------+
Table 2
| *Note:* the coding name "trailers" is reserved because its use
| would conflict with the keyword "trailers" in the TE header
| field (Section 10.1.4 of [HTTP]).
12.4. ALPN Protocol ID Registration
IANA has updated the "TLS Application-Layer Protocol Negotiation
(ALPN) Protocol IDs" registry at <https://www.iana.org/assignments/
tls-extensiontype-values/> with the registration below:
+==========+=============================+===========+
| Protocol | Identification Sequence | Reference |
+==========+=============================+===========+
| HTTP/1.1 | 0x68 0x74 0x74 0x70 0x2f | RFC 9112 |
| | 0x31 0x2e 0x31 ("http/1.1") | |
+----------+-----------------------------+-----------+
Table 3
13. References
13.1. Normative References
[CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/info/rfc9111>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950,
DOI 10.17487/RFC1950, May 1996,
<https://www.rfc-editor.org/info/rfc1950>.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
<https://www.rfc-editor.org/info/rfc1951>.
[RFC1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[Welch] Welch, T., "A Technique for High-Performance Data
Compression", IEEE Computer 17(6),
DOI 10.1109/MC.1984.1659158, June 1984,
<https://ieeexplore.ieee.org/document/1659158/>.
13.2. Informative References
[HTTP/1.0] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945,
DOI 10.17487/RFC1945, May 1996,
<https://www.rfc-editor.org/info/rfc1945>.
[Klein] Klein, A., "Divide and Conquer - HTTP Response Splitting,
Web Cache Poisoning Attacks, and Related Topics", March
2004, <https://packetstormsecurity.com/papers/general/
whitepaper_httpresponse.pdf>.
[Linhart] Linhart, C., Klein, A., Heled, R., and S. Orrin, "HTTP
Request Smuggling", June 2005,
<https://www.cgisecurity.com/lib/HTTP-Request-
Smuggling.pdf>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996,
<https://www.rfc-editor.org/info/rfc2049>.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, DOI 10.17487/RFC2068, January 1997,
<https://www.rfc-editor.org/info/rfc2068>.
[RFC2557] Palme, J., Hopmann, A., and N. Shelness, "MIME
Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,
<https://www.rfc-editor.org/info/rfc2557>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Collected ABNF
In the collected ABNF below, list rules are expanded per
Section 5.6.1 of [HTTP].
BWS = <BWS, see [HTTP], Section 5.6.3>
HTTP-message = start-line CRLF *( field-line CRLF ) CRLF [
message-body ]
HTTP-name = %x48.54.54.50 ; HTTP
HTTP-version = HTTP-name "/" DIGIT "." DIGIT
OWS = <OWS, see [HTTP], Section 5.6.3>
RWS = <RWS, see [HTTP], Section 5.6.3>
Transfer-Encoding = [ transfer-coding *( OWS "," OWS transfer-coding
) ]
absolute-URI = <absolute-URI, see [URI], Section 4.3>
absolute-form = absolute-URI
absolute-path = <absolute-path, see [HTTP], Section 4.1>
asterisk-form = "*"
authority = <authority, see [URI], Section 3.2>
authority-form = uri-host ":" port
chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
chunk-data = 1*OCTET
chunk-ext = *( BWS ";" BWS chunk-ext-name [ BWS "=" BWS chunk-ext-val
] )
chunk-ext-name = token
chunk-ext-val = token / quoted-string
chunk-size = 1*HEXDIG
chunked-body = *chunk last-chunk trailer-section CRLF
field-line = field-name ":" OWS field-value OWS
field-name = <field-name, see [HTTP], Section 5.1>
field-value = <field-value, see [HTTP], Section 5.5>
last-chunk = 1*"0" [ chunk-ext ] CRLF
message-body = *OCTET
method = token
obs-fold = OWS CRLF RWS
obs-text = <obs-text, see [HTTP], Section 5.6.4>
origin-form = absolute-path [ "?" query ]
port = <port, see [URI], Section 3.2.3>
query = <query, see [URI], Section 3.4>
quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
reason-phrase = 1*( HTAB / SP / VCHAR / obs-text )
request-line = method SP request-target SP HTTP-version
request-target = origin-form / absolute-form / authority-form /
asterisk-form
start-line = request-line / status-line
status-code = 3DIGIT
status-line = HTTP-version SP status-code SP [ reason-phrase ]
token = <token, see [HTTP], Section 5.6.2>
trailer-section = *( field-line CRLF )
transfer-coding = <transfer-coding, see [HTTP], Section 10.1.4>
uri-host = <host, see [URI], Section 3.2.2>
Appendix B. Differences between HTTP and MIME
HTTP/1.1 uses many of the constructs defined for the Internet Message
Format [RFC5322] and Multipurpose Internet Mail Extensions (MIME)
[RFC2045] to allow a message body to be transmitted in an open
variety of representations and with extensible fields. However, some
of these constructs have been reinterpreted to better fit the needs
of interactive communication, leading to some differences in how MIME
constructs are used within HTTP. These differences were carefully
chosen to optimize performance over binary connections, allow greater
freedom in the use of new media types, ease date comparisons, and
accommodate common implementations.
This appendix describes specific areas where HTTP differs from MIME.
Proxies and gateways to and from strict MIME environments need to be
aware of these differences and provide the appropriate conversions
where necessary.
B.1. MIME-Version
HTTP is not a MIME-compliant protocol. However, messages can include
a single MIME-Version header field to indicate what version of the
MIME protocol was used to construct the message. Use of the MIME-
Version header field indicates that the message is in full
conformance with the MIME protocol (as defined in [RFC2045]).
Senders are responsible for ensuring full conformance (where
possible) when exporting HTTP messages to strict MIME environments.
B.2. Conversion to Canonical Form
MIME requires that an Internet mail body part be converted to
canonical form prior to being transferred, as described in Section 4
of [RFC2049], and that content with a type of "text" represents line
breaks as CRLF, forbidding the use of CR or LF outside of line break
sequences [RFC2046]. In contrast, HTTP does not care whether CRLF,
bare CR, or bare LF are used to indicate a line break within content.
A proxy or gateway from HTTP to a strict MIME environment ought to
translate all line breaks within text media types to the RFC 2049
canonical form of CRLF. Note, however, this might be complicated by
the presence of a Content-Encoding and by the fact that HTTP allows
the use of some charsets that do not use octets 13 and 10 to
represent CR and LF, respectively.
Conversion will break any cryptographic checksums applied to the
original content unless the original content is already in canonical
form. Therefore, the canonical form is recommended for any content
that uses such checksums in HTTP.
B.3. Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (Section 5.6.7 of
[HTTP]) to simplify the process of date comparison. Proxies and
gateways from other protocols ought to ensure that any Date header
field present in a message conforms to one of the HTTP/1.1 formats
and rewrite the date if necessary.
B.4. Conversion of Content-Encoding
MIME does not include any concept equivalent to HTTP's Content-
Encoding header field. Since this acts as a modifier on the media
type, proxies and gateways from HTTP to MIME-compliant protocols
ought to either change the value of the Content-Type header field or
decode the representation before forwarding the message. (Some
experimental applications of Content-Type for Internet mail have used
a media-type parameter of ";conversions=<content-coding>" to perform
a function equivalent to Content-Encoding. However, this parameter
is not part of the MIME standards.)
B.5. Conversion of Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding field of MIME.
Proxies and gateways from MIME-compliant protocols to HTTP need to
remove any Content-Transfer-Encoding prior to delivering the response
message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format
and encoding for safe transport on that protocol, where "safe
transport" is defined by the limitations of the protocol being used.
Such a proxy or gateway ought to transform and label the data with an
appropriate Content-Transfer-Encoding if doing so will improve the
likelihood of safe transport over the destination protocol.
B.6. MHTML and Line Length Limitations
HTTP implementations that share code with MHTML [RFC2557]
implementations need to be aware of MIME line length limitations.
Since HTTP does not have this limitation, HTTP does not fold long
lines. MHTML messages being transported by HTTP follow all
conventions of MHTML, including line length limitations and folding,
canonicalization, etc., since HTTP transfers message-bodies without
modification and, aside from the "multipart/byteranges" type
(Section 14.6 of [HTTP]), does not interpret the content or any MIME
header lines that might be contained therein.
Appendix C. Changes from Previous RFCs
C.1. Changes from HTTP/0.9
Since HTTP/0.9 did not support header fields in a request, there is
no mechanism for it to support name-based virtual hosts (selection of
resource by inspection of the Host header field). Any server that
implements name-based virtual hosts ought to disable support for
HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in fact,
badly constructed HTTP/1.x requests caused by a client failing to
properly encode the request-target.
C.2. Changes from HTTP/1.0
C.2.1. Multihomed Web Servers
The requirements that clients and servers support the Host header
field (Section 7.2 of [HTTP]), report an error if it is missing from
an HTTP/1.1 request, and accept absolute URIs (Section 3.2) are among
the most important changes defined by HTTP/1.1.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP
addresses and servers; there was no established mechanism for
distinguishing the intended server of a request other than the IP
address to which that request was directed. The Host header field
was introduced during the development of HTTP/1.1 and, though it was
quickly implemented by most HTTP/1.0 browsers, additional
requirements were placed on all HTTP/1.1 requests in order to ensure
complete adoption. At the time of this writing, most HTTP-based
services are dependent upon the Host header field for targeting
requests.
C.2.2. Keep-Alive Connections
In HTTP/1.0, each connection is established by the client prior to
the request and closed by the server after sending the response.
However, some implementations implement the explicitly negotiated
("Keep-Alive") version of persistent connections described in
Section 19.7.1 of [RFC2068].
Some clients and servers might wish to be compatible with these
previous approaches to persistent connections, by explicitly
negotiating for them with a "Connection: keep-alive" request header
field. However, some experimental implementations of HTTP/1.0
persistent connections are faulty; for example, if an HTTP/1.0 proxy
server doesn't understand Connection, it will erroneously forward
that header field to the next inbound server, which would result in a
hung connection.
One attempted solution was the introduction of a Proxy-Connection
header field, targeted specifically at proxies. In practice, this
was also unworkable, because proxies are often deployed in multiple
layers, bringing about the same problem discussed above.
As a result, clients are encouraged not to send the Proxy-Connection
header field in any requests.
Clients are also encouraged to consider the use of "Connection: keep-
alive" in requests carefully; while they can enable persistent
connections with HTTP/1.0 servers, clients using them will need to
monitor the connection for "hung" requests (which indicate that the
client ought to stop sending the header field), and this mechanism
ought not be used by clients at all when a proxy is being used.
C.2.3. Introduction of Transfer-Encoding
HTTP/1.1 introduces the Transfer-Encoding header field (Section 6.1).
Transfer codings need to be decoded prior to forwarding an HTTP
message over a MIME-compliant protocol.
C.3. Changes from RFC 7230
Most of the sections introducing HTTP's design goals, history,
architecture, conformance criteria, protocol versioning, URIs,
message routing, and header fields have been moved to [HTTP]. This
document has been reduced to just the messaging syntax and connection
management requirements specific to HTTP/1.1.
Bare CRs have been prohibited outside of content. (Section 2.2)
The ABNF definition of authority-form has changed from the more
general authority component of a URI (in which port is optional) to
the specific host:port format that is required by CONNECT.
(Section 3.2.3)
Recipients are required to avoid smuggling/splitting attacks when
processing an ambiguous message framing. (Section 6.1)
In the ABNF for chunked extensions, (bad) whitespace around ";" and
"=" has been reintroduced. Whitespace was removed in [RFC7230], but
that change was found to break existing implementations.
(Section 7.1.1)
Trailer field semantics now transcend the specifics of chunked
transfer coding. The decoding algorithm for chunked (Section 7.1.3)
has been updated to encourage storage/forwarding of trailer fields
separately from the header section, to only allow merging into the
header section if the recipient knows the corresponding field
definition permits and defines how to merge, and otherwise to discard
the trailer fields instead of merging. The trailer part is now
called the trailer section to be more consistent with the header
section and more distinct from a body part. (Section 7.1.2)
Transfer coding parameters called "q" are disallowed in order to
avoid conflicts with the use of ranks in the TE header field.
(Section 7.3)
Acknowledgements
See Appendix "Acknowledgements" of [HTTP], which applies to this
document as well.
Index
A C D F G H M O R T X
A
absolute-form (of request-target) Section 3.2.2
application/http Media Type *_Section 10.2_*
asterisk-form (of request-target) Section 3.2.4
authority-form (of request-target) Section 3.2.3
C
chunked (Coding Format) Section 6.1; Section 6.3
chunked (transfer coding) *_Section 7.1_*
close Section 9.3; *_Section 9.6_*
compress (transfer coding) *_Section 7.2_*
Connection header field Section 9.6
Content-Length header field Section 6.2
Content-Transfer-Encoding header field Appendix B.5
D
deflate (transfer coding) *_Section 7.2_*
F
Fields
Close *_Section 9.6, Paragraph 4_*
MIME-Version *_Appendix B.1_*
Transfer-Encoding *_Section 6.1_*
G
Grammar
ALPHA *_Section 1.2_*
CR *_Section 1.2_*
CRLF *_Section 1.2_*
CTL *_Section 1.2_*
DIGIT *_Section 1.2_*
DQUOTE *_Section 1.2_*
HEXDIG *_Section 1.2_*
HTAB *_Section 1.2_*
HTTP-message *_Section 2.1_*
HTTP-name *_Section 2.3_*
HTTP-version *_Section 2.3_*
LF *_Section 1.2_*
OCTET *_Section 1.2_*
SP *_Section 1.2_*
Transfer-Encoding *_Section 6.1_*
VCHAR *_Section 1.2_*
absolute-form Section 3.2; *_Section 3.2.2_*
asterisk-form Section 3.2; *_Section 3.2.4_*
authority-form Section 3.2; *_Section 3.2.3_*
chunk *_Section 7.1_*
chunk-data *_Section 7.1_*
chunk-ext Section 7.1; *_Section 7.1.1_*
chunk-ext-name *_Section 7.1.1_*
chunk-ext-val *_Section 7.1.1_*
chunk-size *_Section 7.1_*
chunked-body *_Section 7.1_*
field-line *_Section 5_*; Section 7.1.2
field-name Section 5
field-value Section 5
last-chunk *_Section 7.1_*
message-body *_Section 6_*
method *_Section 3.1_*
obs-fold *_Section 5.2_*
origin-form Section 3.2; *_Section 3.2.1_*
reason-phrase *_Section 4_*
request-line *_Section 3_*
request-target *_Section 3.2_*
start-line *_Section 2.1_*
status-code *_Section 4_*
status-line *_Section 4_*
trailer-section Section 7.1; *_Section 7.1.2_*
gzip (transfer coding) *_Section 7.2_*
H
Header Fields
MIME-Version *_Appendix B.1_*
Transfer-Encoding *_Section 6.1_*
header line Section 2.1
header section Section 2.1
headers Section 2.1
M
Media Type
application/http *_Section 10.2_*
message/http *_Section 10.1_*
message/http Media Type *_Section 10.1_*
method *_Section 3.1_*
MIME-Version header field *_Appendix B.1_*
O
origin-form (of request-target) Section 3.2.1
R
request-target *_Section 3.2_*
T
Transfer-Encoding header field *_Section 6.1_*
X
x-compress (transfer coding) *_Section 7.2_*
x-gzip (transfer coding) *_Section 7.2_*
Authors' Addresses
Roy T. Fielding (editor)
Adobe
345 Park Ave
San Jose, CA 95110
United States of America
Email: fielding@gbiv.com
URI: https://roy.gbiv.com/
Mark Nottingham (editor)
Fastly
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
Julian Reschke (editor)
greenbytes GmbH
Hafenweg 16
48155 Münster
Germany
Email: julian.reschke@greenbytes.de
URI: https://greenbytes.de/tech/webdav/