draft-ietf-httpbis-bcp56bis-15.txt   draft-ietf-httpbis-bcp56bis-latest.txt 
HTTP Working Group M. Nottingham HTTP Working Group M. Nottingham
Internet-Draft August 27, 2021 Internet-Draft October 13, 2025
Obsoletes: 3205 (if approved) Obsoletes: 3205 (if approved)
Intended status: Best Current Practice Intended status: Best Current Practice
Expires: February 28, 2022 Expires: April 16, 2026
Building Protocols with HTTP Building Protocols with HTTP
draft-ietf-httpbis-bcp56bis-15 draft-ietf-httpbis-bcp56bis-latest
Abstract Abstract
Applications often use HTTP as a substrate to create HTTP-based APIs. Applications often use HTTP as a substrate to create HTTP-based APIs.
This document specifies best practices for writing specifications This document specifies best practices for writing specifications
that use HTTP to define new application protocols. It is written that use HTTP to define new application protocols. It is written
primarily to guide IETF efforts to define application protocols using primarily to guide IETF efforts to define application protocols using
HTTP for deployment on the Internet, but might be applicable in other HTTP for deployment on the Internet but might be applicable in other
situations. situations.
This document obsoletes [RFC3205]. This document obsoletes RFC3205.
Note to Readers About This Document
_RFC EDITOR: please remove this section before publication_ This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group Status information for this document may be found at
mailing list (ietf-http-wg@w3.org), which is archived at <https://datatracker.ietf.org/doc/draft-ietf-httpbis-bcp56bis/>.
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at http://httpwg.github.io/ Discussion of this document takes place on the HTTP Working Group
[2]; source code and issues list for this draft can be found at mailing list (<mailto:ietf-http-wg@w3.org>), which is archived at
https://github.com/httpwg/http-extensions/labels/bcp56bis [3]. <https://lists.w3.org/Archives/Public/ietf-http-wg/>. Working Group
information can be found at <https://httpwg.org/>.
Source for this draft and an issue tracker can be found at
<https://github.com/httpwg/http-extensions/labels/bcp56bis>.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 28, 2022.
This Internet-Draft will expire on April 16, 2026.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
1. Introduction 1. Introduction
Applications other than Web browsing often use HTTP [HTTP] as a Applications other than Web browsing often use HTTP [HTTP] as a
substrate, a practice sometimes referred to as creating "HTTP-based substrate, a practice sometimes referred to as creating "HTTP-based
APIs", "REST APIs" or just "HTTP APIs". This is done for a variety APIs", "REST APIs", or just "HTTP APIs". This is done for a variety
of reasons, including: of reasons, including:
o familiarity by implementers, specifiers, administrators, o familiarity by implementers, specifiers, administrators,
developers and users, developers, and users;
o availability of a variety of client, server and proxy o availability of a variety of client, server, and proxy
implementations, implementations;
o ease of use, o ease of use;
o availability of Web browsers, o availability of Web browsers;
o reuse of existing mechanisms like authentication and encryption, o reuse of existing mechanisms like authentication and encryption;
o presence of HTTP servers and clients in target deployments, and o presence of HTTP servers and clients in target deployments; and
o its ability to traverse firewalls. o its ability to traverse firewalls.
These protocols are often ad hoc, intended for only deployment by one These protocols are often ad hoc, intended for only deployment by one
or a few servers and consumption by a limited set of clients. As a or a few servers and consumption by a limited set of clients. As a
result, a body of practices and tools has arisen around defining result, a body of practices and tools has arisen around defining
HTTP-based APIs that favours these conditions. HTTP-based APIs that favour these conditions.
However, when such an application has multiple, separate However, when such an application has multiple, separate
implementations, is deployed on multiple uncoordinated servers, and implementations, is deployed on multiple uncoordinated servers, and
is consumed by diverse clients -- as is often the case for HTTP APIs is consumed by diverse clients (as is often the case for HTTP APIs
defined by standards efforts -- tools and practices intended for defined by standards efforts), tools and practices intended for
limited deployment can become unsuitable. limited deployment can become unsuitable.
This mismatch is largely because the API's clients and servers will This mismatch is largely because the API's clients and servers will
implement and evolve at different paces, leading to a need for implement and evolve at different paces, leading to a need for
deployments with different features and versions to co-exist. As a deployments with different features and versions to coexist. As a
result, the designers of HTTP-based APIs intended for such result, the designers of HTTP-based APIs intended for such
deployments need to more carefully consider how extensibility of the deployments need to more carefully consider how extensibility of the
service will be handled and how different deployment requirements service will be handled and how different deployment requirements
will be accommodated. will be accommodated.
More generally, an application protocol using HTTP faces a number of More generally, an application protocol using HTTP faces a number of
design decisions, including: design decisions, including:
o Should it define a new URI scheme? Use new ports? o Should it define a new URI scheme? Use new ports?
o Should it use standard HTTP methods and status codes, or define o Should it use standard HTTP methods and status codes or define new
new ones? ones?
o How can the maximum value be extracted from the use of HTTP? o How can the maximum value be extracted from the use of HTTP?
o How does it coexist with other uses of HTTP -- especially Web o How does it coexist with other uses of HTTP -- especially Web
browsing? browsing?
o How can interoperability problems and "protocol dead ends" be o How can interoperability problems and "protocol dead ends" be
avoided? avoided?
This document contains best current practices for the specification Section 2 defines when this document applies, Section 3 surveys the
of such applications. Section 2 defines when it applies; Section 3 properties of HTTP that are important to preserve, and Section 4
surveys the properties of HTTP that are important to preserve, and contains best practices for the specification of applications that
Section 4 conveys best practices for specifying them. use HTTP.
It is written primarily to guide IETF efforts to define application It is written primarily to guide IETF efforts to define application
protocols using HTTP for deployment on the Internet, but might be protocols using HTTP for deployment on the Internet but might be
applicable in other situations. Note that the requirements herein do applicable in other situations. Note that the requirements herein do
not necessarily apply to the development of generic HTTP extensions. not necessarily apply to the development of generic HTTP extensions.
This document obsoletes [RFC3205], to reflect experience and This document obsoletes [RFC3205] to reflect the experience and
developments regarding HTTP in the intervening time. developments regarding HTTP in the intervening time.
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Is HTTP Being Used? 2. Is HTTP Being Used?
Different applications have different goals when using HTTP. The Different applications have different goals when using HTTP. The
recommendations in this document apply when a specification defines recommendations in this document apply when a specification defines
an application that: an application that:
o uses the transport port 80 or 443, or o uses the transport port 80 or 443, or
o uses the URI scheme "http" or "https", or o uses the URI scheme "http" or "https", or
o uses an ALPN protocol ID [RFC7301] that generically identifies o uses an ALPN protocol ID [RFC7301] that generically identifies
HTTP (e.g., "http/1.1", "h2", "h3"), or HTTP (e.g., "http/1.1", "h2", "h3"), or
o makes registrations in or overall modifications to the IANA o makes registrations in or overall modifications to the IANA
registries defined for HTTP. registries defined for HTTP.
Additionally, when a specification is using HTTP, all of the Additionally, when a specification is using HTTP, all of the
requirements of the HTTP protocol suite are in force (in particular, requirements of the HTTP protocol suite are in force ([HTTP] in
[HTTP], but also other specifications such as the specific version of particular but also other specifications such as the specific version
HTTP in use, and any extensions in use). of HTTP in use and any extensions in use).
Note that this document is intended to apply to applications, not Note that this document is intended to apply to applications, not
generic extensions to HTTP. Furthermore, while it is intended for generic extensions to HTTP. Furthermore, while it is intended for
IETF-specified applications, other standards organisations are IETF-specified applications, other standards organisations are
encouraged to adhere to its requirements. encouraged to adhere to its requirements.
2.1. Non-HTTP Protocols 2.1. Non-HTTP Protocols
An application can rely upon HTTP without meeting the criteria for An application can rely upon HTTP without meeting the criteria for
using it defined above. For example, an application might wish to using it as defined above. For example, an application might wish to
avoid re-specifying parts of the message format, but change other avoid re-specifying parts of the message format but might change
aspects of the protocol's operation; or, it might want to use other aspects of the protocol's operation, or it might want to use
application-specific methods. application-specific methods.
Doing so brings more freedom to modify protocol operations, but loses Doing so permits more freedom to modify protocol operations, but at
at least a portion of the benefits outlined in Section 3, as most least a portion of the benefits outlined in Section 3 are lost as
HTTP implementations won't be easily adaptable to these changes, and most HTTP implementations won't be easily adaptable to these changes.
the benefit of mindshare will be lost. The benefit of mindshare will also be lost.
Such specifications MUST NOT use HTTP's URI schemes, transport ports, Such specifications MUST NOT use HTTP's URI schemes, transport ports,
ALPN protocol IDs or IANA registries; rather, they are encouraged to ALPN protocol IDs, or IANA registries; rather, they are encouraged to
establish their own. establish their own.
3. What's Important About HTTP 3. What's Important About HTTP
This section examines the characteristics of HTTP that are important This section examines the characteristics of HTTP that are important
to consider when using HTTP to define an application protocol. to consider when using HTTP to define an application protocol.
3.1. Generic Semantics 3.1. Generic Semantics
Much of the value of HTTP is in its generic semantics -- that is, the Much of the value of HTTP is in its generic semantics -- that is, the
protocol elements defined by HTTP are potentially applicable to every protocol elements defined by HTTP are potentially applicable to every
resource, not specific to a particular context. Application-specific resource and are not specific to a particular context. Application-
semantics are best expressed in message content and in header fields, specific semantics are best expressed in message content and header
not status codes or methods (although the latter do have generic fields, not status codes or methods (although status codes and
semantics that relate to application state). methods do have generic semantics that relate to application state).
This generic/application-specific split allows a HTTP message to be This split between generic and application-specific semantics allows
handled by common software (e.g., HTTP servers, intermediaries, an HTTP message to be handled by common software (e.g., HTTP servers,
client implementations, and caches) without understanding the intermediaries, client implementations, and caches) without requiring
specific application. It also allows people to leverage their those implementations to understand the application in use. It also
knowledge of HTTP semantics without specialising them for a allows people to leverage their knowledge of HTTP semantics without
particular application. needing specialised knowledge of a particular application.
Therefore, applications that use HTTP MUST NOT re-define, refine or Therefore, applications that use HTTP MUST NOT redefine, refine, or
overlay the semantics of generic protocol elements such as methods, overlay the semantics of generic protocol elements such as methods,
status codes or existing header fields. Instead, they should focus status codes, or existing header fields. Instead, they should focus
their specifications on protocol elements that are specific to that their specifications on protocol elements that are specific to that
application; namely their HTTP resources. application -- namely, their HTTP resources.
When writing a specification, it's often tempting to specify exactly When writing a specification, it's often tempting to specify exactly
how HTTP is to be implemented, supported and used. However, this can how HTTP is to be implemented, supported, and used. However, this
easily lead to an unintended profile of HTTP's behaviour. For can easily lead to an unintended profile of HTTP behaviour. For
example, it's common to see specifications with language like this: example, it's common to see specifications with language like this:
A `POST` request MUST result in a `201 Created` response. A POST request MUST result in a 201 (Created) response.
This forms an expectation in the client that the response will always This forms an expectation in the client that the response will always
be "201 Created", when in fact there are a number of reasons why the be 201 (Created) when in fact there are a number of reasons why the
status code might differ in a real deployment; for example, there status code might differ in a real deployment; for example, there
might be a proxy that requires authentication, or a server-side might be a proxy that requires authentication, or a server-side
error, or a redirection. If the client does not anticipate this, the error, or a redirection. If the client does not anticipate this, the
application's deployment is brittle. application's deployment is brittle.
See Section 4.2 for more details. See Section 4.2 for more details.
3.2. Links 3.2. Links
Another common practice is assuming that the HTTP server's name space Another common practice is assuming that the HTTP server's namespace
(or a portion thereof) is exclusively for the use of a single (or a portion thereof) is exclusively for the use of a single
application. This effectively overlays special, application-specific application. This effectively overlays special, application-specific
semantics onto that space, precludes other applications from using semantics onto that space and precludes other applications from using
it. it.
As explained in [RFC8820], such "squatting" on a part of the URL As explained in [BCP190], such "squatting" on a part of the URL space
space by a standard usurps the server's authority over its own by a standard usurps the server's authority over its own resources,
resources, can cause deployment issues, and is therefore bad practice can cause deployment issues, and is therefore bad practice in
in standards. standards.
Instead of statically defining URI components like paths, it is Instead of statically defining URI components like paths, it is
RECOMMENDED that applications using HTTP define and use links RECOMMENDED that applications using HTTP define and use links
[WEB-LINKING] to allow flexibility in deployment. [WEB-LINKING] to allow flexibility in deployment.
Using runtime links in this fashion has a number of other benefits -- Using runtime links in this fashion has a number of other benefits --
especially when an application is to have multiple implementations especially when an application is to have multiple implementations
and/or deployments (as is often the case for those that are and/or deployments (as is often the case for those that are
standardised). standardised).
For example, navigating with a link allows a request to be routed to For example, navigating with a link allows a request to be routed to
a different server without the overhead of a redirection, thereby a different server without the overhead of a redirection, thereby
supporting deployment across machines well. supporting deployment across machines well.
It also becomes possible to "mix and match" different applications on By using links, it also becomes possible to "mix and match" different
the same server, and offers a natural mechanism for extensibility, applications on the same server. The use of links also offers a
versioning and capability management, since the document containing natural mechanism for extensibility, versioning, and capability
the links can also contain information about their targets. management because the document containing the links can also contain
information about their targets.
Using links also offers a form of cache invalidation that's seen on Using links also offers a form of cache invalidation that's seen on
the Web; when a resource's state changes, the application can change the Web; when a resource's state changes, the application can change
its link to it so that a fresh copy is always fetched. the affected links so that a fresh copy is always fetched.
See Section 4.4 for more details.
3.3. Rich Functionality 3.3. Rich Functionality
HTTP offers a number of features to applications, such as: HTTP offers a number of features to applications, such as:
o Message framing o Message framing
o Multiplexing (in HTTP/2 [HTTP2] and HTTP/3 [HTTP3]) o Multiplexing (in HTTP/2 [HTTP2] and HTTP/3 [HTTP3])
o Integration with TLS o Integration with TLS
o Support for intermediaries (proxies, gateways, Content Delivery o Support for intermediaries (proxies, gateways, content delivery
Networks) networks (CDNs))
o Client authentication o Client authentication
o Content negotiation for format, language, and other features o Content negotiation for format, language, and other features
o Caching for server scalability, latency and bandwidth reduction, o Caching for server scalability, latency and bandwidth reduction,
and reliability and reliability
o Granularity of access control (through use of a rich space of o Granularity of access control (through use of a rich space of
URLs) URLs)
o Partial content to selectively request part of a response o Partial content to selectively request part of a response
o The ability to interact with the application easily using a Web o The ability to interact with the application easily using a Web
browser browser
Applications that use HTTP are encouraged to utilise the various An application that uses HTTP is encouraged to utilise the various
features that the protocol offers, so that their users receive the features that the protocol offers so that its users receive the
maximum benefit from it, and to allow it to be deployed in a variety maximum benefit from those features and so that the application can
of situations. This document does not require specific features to be deployed in a variety of situations. This document does not
be used, since the appropriate design tradeoffs are highly specific require specific features to be used since the appropriate design
to a given situation. However, following the practices in Section 4 trade-offs are highly specific to a given situation. However,
is a good starting point. following the practices in Section 4 is a good starting point.
4. Best Practices for Specifying the Use of HTTP 4. Best Practices for Specifying the Use of HTTP
This section contains best practices for specifying the use of HTTP This section contains best practices for specifying the use of HTTP
by applications, including practices for specific HTTP protocol by applications, including practices for specific HTTP protocol
elements. elements.
4.1. Specifying the Use of HTTP 4.1. Specifying the Use of HTTP
Specifications should use [HTTP] as the primary reference for HTTP; Specifications should use [HTTP] as the primary reference for HTTP;
it is not necessary to reference all of the specifications in the it is not necessary to reference all of the specifications in the
HTTP suite unless there are specific reasons to do so (e.g., a HTTP suite unless there are specific reasons to do so (e.g., a
particular feature is called out). particular feature is called out).
Because HTTP is a hop-by-hop protocol, a connection can be handled by Because HTTP is a hop-by-hop protocol, a connection can be handled by
implementations that are not controlled by the application; for implementations that are not controlled by the application; for
example, proxies, CDNs, firewalls and so on. Requiring a particular example, proxies, CDNs, firewalls, and so on. Requiring a particular
version of HTTP makes it difficult to use in these situations, and version of HTTP makes it difficult to use in these situations and
harms interoperability. Therefore, it is NOT RECOMMENDED that harms interoperability. Therefore, it is NOT RECOMMENDED that
applications using HTTP specify a minimum version of HTTP to be used. applications using HTTP specify a minimum version of HTTP to be used.
However, if an application's deployment would benefit from the use of However, if an application's deployment benefits from the use of a
a particular version of HTTP (for example, HTTP/2's multiplexing), particular version of HTTP (for example, HTTP/2's multiplexing), this
this ought be noted. ought be noted.
Applications using HTTP MUST NOT specify a maximum version, to Applications using HTTP MUST NOT specify a maximum version, to
preserve the protocol's ability to evolve. preserve the protocol's ability to evolve.
When specifying examples of protocol interactions, applications When specifying examples of protocol interactions, applications
should document both the request and response messages, with complete should document both the request and response messages with complete
header sections, preferably in HTTP/1.1 format [HTTP11]. For header sections, preferably in HTTP/1.1 format [HTTP11]. For
example: example:
GET /thing HTTP/1.1 GET /thing HTTP/1.1
Host: example.com Host: example.com
Accept: application/things+json Accept: application/things+json
User-Agent: Foo/1.0 User-Agent: Foo/1.0
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/things+json Content-Type: application/things+json
skipping to change at page 8, line 23 skipping to change at page 8, line 28
Server: Bar/2.2 Server: Bar/2.2
[content here] [content here]
4.2. Specifying Server Behaviour 4.2. Specifying Server Behaviour
The server-side behaviours of an application are most effectively The server-side behaviours of an application are most effectively
specified by defining the following protocol elements: specified by defining the following protocol elements:
o Media types [RFC6838], often based upon a format convention such o Media types [RFC6838], often based upon a format convention such
as JSON [JSON], as JSON [JSON];
o HTTP header fields, as per Section 4.7, and o HTTP header fields, per Section 4.7; and
o The behaviour of resources, as identified by link relations o The behaviour of resources, as identified by link relations
[WEB-LINKING]. [WEB-LINKING].
An application can define its operation by composing these protocol An application can define its operation by composing these protocol
elements to define a set of resources that are identified by link elements to define a set of resources that are identified by link
relations and that implement specified behaviours, including: relations and that implement specified behaviours, including:
o retrieval of their state using GET, in one or more formats o retrieval of resource state using GET in one or more formats
identified by media type; identified by media type;
o resource creation or update using POST or PUT, with an o resource creation or update using POST or PUT, with an
appropriately identified request content format; appropriately identified request content format;
o data processing using POST and identified request and response o data processing using POST and identified request and response
content format(s); and content format(s); and
o Resource deletion using DELETE. o Resource deletion using DELETE.
For example, an application might specify: For example, an application might specify:
Resources linked to with the "example-widget" link relation type are Resources linked to with the "example-widget" link relation type
Widgets. The state of a Widget can be fetched in the are Widgets. The state of a Widget can be fetched in the
"application/example-widget+json" format, and can be updated by PUT "application/example-widget+json" format, and can be updated by
to the same link. Widget resources can be deleted. PUT to the same link. Widget resources can be deleted.
The "Example-Count" response header field on Widget representations The Example-Count response header field on Widget representations
indicates how many Widgets are held by the sender. indicates how many Widgets are held by the sender.
The "application/example-widget+json" format is a JSON [RFC8259] The "application/example-widget+json" format is a JSON [RFC8259]
format representing the state of a Widget. It contains links to format representing the state of a Widget. It contains links to
related information in the link indicated by the Link header field related information in the link indicated by the Link header field
value with the "example-other-info" link relation type. value with the "example-other-info" link relation type.
Applications can also specify the use of URI Templates [URI-TEMPLATE] Applications can also specify the use of URI Templates [URI-TEMPLATE]
to allow clients to generate URLs based upon runtime data. to allow clients to generate URLs based upon runtime data.
4.3. Specifying Client Behaviour 4.3. Specifying Client Behaviour
An application's expectations for client behaviour ought to be An application's expectations for client behaviour ought to be
closely aligned with those of Web browsers, to avoid interoperability closely aligned with those of Web browsers to avoid interoperability
issues when they are used. issues when they are used.
One way to do this is to define it in terms of [FETCH], since that is One way to do this is to define it in terms of [FETCH] since that is
the abstraction that browsers use for HTTP. the abstraction that browsers use for HTTP.
Some client behaviours (e.g., automatic redirect handling) and Some client behaviours (e.g., automatic redirect handling) and
extensions (e.g., Cookies) are not required by HTTP, but nevertheless extensions (e.g., Cookies) are not required by HTTP but nevertheless
have become very common. If their use is not explicitly specified by have become very common. If their use is not explicitly specified by
applications using HTTP, there may be confusion and interoperability applications using HTTP, there may be confusion and interoperability
problems. In particular: problems. In particular:
o Redirect handling - Applications need to specify how redirects are Redirect handling: Applications need to specify how redirects are
expected to be handled; see Section 4.6.1. expected to be handled; see Section 4.6.1.
o Cookies - Applications using HTTP should explicitly reference the Cookies: Applications using HTTP should explicitly reference the
Cookie specification [COOKIES] if they are required. Cookie specification [COOKIES] if they are required.
o Certificates - Applications using HTTP should specify that TLS Certificates: Applications using HTTP should specify that TLS
certificates are to be checked according to Section 4.3.4 of certificates are to be checked according to Section 4.3.4 of
[HTTP] when HTTPS is used. [HTTP] when HTTPS is used.
Applications using HTTP should not statically require HTTP features Applications using HTTP should not require that clients statically
that are usually negotiated to be supported by clients. For example, support HTTP features that are usually negotiated. For example,
requiring that clients support responses with a certain content- requiring that clients support responses with a certain content
coding ([HTTP], Section 8.4.1) instead of negotiating for it ([HTTP], coding ([HTTP], Section 8.4.1) instead of negotiating for it ([HTTP],
Section 12.5.3) means that otherwise conformant clients cannot Section 12.5.3) means that otherwise conformant clients cannot
interoperate with the application. Applications can encourage the interoperate with the application. Applications can encourage the
implementation of such features, though. implementation of such features, though.
4.4. Specifying URLs 4.4. Specifying URLs
In HTTP, the resources that clients interact with are identified with In HTTP, the resources that clients interact with are identified with
URLs [URL]. As [RFC8820] explains, parts of the URL are designed to URLs [URL]. As [BCP190] explains, parts of the URL are designed to
be under the control of the owner (also known as the "authority") of be under the control of the owner (also known as the "authority") of
that server, to give them the flexibility in deployment. that server to give them the flexibility in deployment.
This means that in most cases, specifications for applications that This means that in most cases, specifications for applications that
use HTTP won't contain fixed application URLs or paths; while it is use HTTP won't contain fixed application URLs or paths; while it is
common practice for a specification of a single-deployment API to common practice for a specification of a single-deployment API to
specify the path prefix "/app/v1" (for example), doing so in an IETF specify the path prefix "/app/v1" (for example), doing so in an IETF
specification is inappropriate. specification is inappropriate.
Therefore, the specification writer needs some mechanism to allow Therefore, the specification writer needs some mechanism to allow
clients to discovery an application's URLs. Additionally, they need clients to discover an application's URLs. Additionally, they need
to specify what URL scheme(s) the application should be used with, to specify which URL scheme(s) the application should be used with
and whether to use a dedicated port, or reuse HTTP's port(s). and whether to use a dedicated port or to reuse HTTP's port(s).
4.4.1. Discovering an Application's URLs 4.4.1. Discovering an Application's URLs
Generally, a client will begin interacting with a given application Generally, a client will begin interacting with a given application
server by requesting an initial document that contains information server by requesting an initial document that contains information
about that particular deployment, potentially including links to about that particular deployment, potentially including links to
other relevant resources. Doing so assures that the deployment is as other relevant resources. Doing so ensures that the deployment is as
flexible as possible (potentially spanning multiple servers), allows flexible as possible (potentially spanning multiple servers), allows
evolution, and also gives the application the opportunity to tailor evolution, and also gives the application the opportunity to tailor
the 'discovery document' to the client. the "discovery document" to the client.
There are a few common patterns for discovering that initial URL. There are a few common patterns for discovering that initial URL.
The most straightforward mechanism for URL discovery is to configure The most straightforward mechanism for URL discovery is to configure
the client with (or otherwise convey to it) a full URL. This might the client with (or otherwise convey to it) a full URL. This might
be done in a configuration document, or through another discovery be done in a configuration document or through another discovery
mechanism. mechanism.
However, if the client only knows the server's hostname and the However, if the client only knows the server's hostname and the
identity of the application, there needs to be some way to derive the identity of the application, there needs to be some way to derive the
initial URL from that information. initial URL from that information.
An application cannot define a fixed prefix for its URL paths; see An application cannot define a fixed prefix for its URL paths; see
[RFC8820]. Instead, a specification for such an application can use [BCP190]. Instead, a specification for such an application can use
one of the following strategies: one of the following strategies:
o Register a Well-Known URI [WELL-KNOWN-URI] as an entry point for o Register a well-known URI [WELL-KNOWN-URI] as an entry point for
that application. This provides a fixed path on every potential that application. This provides a fixed path on every potential
server that will not collide with other applications. server that will not collide with other applications.
o Enable the server authority to convey a URI Template o Enable the server authority to convey a URI Template
[URI-TEMPLATE] or similar mechanism for generating a URL for an [URI-TEMPLATE] or similar mechanism for generating a URL for an
entry point. For example, this might be done in a configuration entry point. For example, this might be done in a configuration
document or other artefact. document or other artefact.
Once the discovery document is located, it can be fetched, cached for Once the discovery document is located, it can be fetched, cached for
later reuse (if allowed by its metadata), and used to locate other later reuse (if allowed by its metadata), and used to locate other
resources that are relevant to the application, using full URIs or resources that are relevant to the application using full URIs or URL
URL Templates. Templates.
In some cases, an application may not wish to use such a discovery In some cases, an application may not wish to use such a discovery
document; for example, when communication is very brief, or when the document -- for example, when communication is very brief or when the
latency concerns of doing so precludes the use of a discovery latency concerns of doing so preclude the use of a discovery
document. These situations can be addressed by placing all of the document. These situations can be addressed by placing all of the
application's resources under a well-known location. application's resources under a well-known location.
4.4.2. Considering URI Schemes 4.4.2. Considering URI Schemes
Applications that use HTTP will typically employ the "http" and/or Applications that use HTTP will typically employ the "http" and/or
"https" URI schemes. "https" is RECOMMENDED to provide "https" URI schemes. "https" is RECOMMENDED to provide
authentication, integrity and confidentiality, as well as mitigate authentication, integrity, and confidentiality, as well as to
pervasive monitoring attacks [RFC7258]. mitigate pervasive monitoring attacks [RFC7258].
However, application-specific schemes can also be defined. When However, application-specific schemes can also be defined. When
defining an URI scheme for an application using HTTP, there are a defining a URI scheme for an application using HTTP, there are a
number of tradeoffs and caveats to keep in mind: number of trade-offs and caveats to keep in mind:
o Unmodified Web browsers will not support the new scheme. While it o Unmodified Web browsers will not support the new scheme. While it
is possible to register new URI schemes with Web browsers (e.g. is possible to register new URI schemes with Web browsers (e.g.,
registerProtocolHandler() in [HTML], as well as several registerProtocolHandler() in [HTML], as well as several
proprietary approaches), support for these mechanisms is not proprietary approaches), support for these mechanisms is not
shared by all browsers, and their capabilities vary. shared by all browsers, and their capabilities vary.
o Existing non-browser clients, intermediaries, servers and o Existing non-browser clients, intermediaries, servers, and
associated software will not recognise the new scheme. For associated software will not recognise the new scheme. For
example, a client library might fail to dispatch the request; a example, a client library might fail to dispatch the request, a
cache might refuse to store the response, and a proxy might fail cache might refuse to store the response, and a proxy might fail
to forward the request. to forward the request.
o Because URLs occur in HTTP artefacts commonly, often being o Because URLs commonly occur in HTTP artefacts and are often
generated automatically (e.g., in the "Location" response header generated automatically (e.g., in the Location response header
field), it can be difficult to assure that the new scheme is used field), it can be difficult to ensure that the new scheme is used
consistently. consistently.
o The resources identified by the new scheme will still be available o The resources identified by the new scheme will still be available
using "http" and/or "https" URLs. Those URLs can "leak" into use, using "http" and/or "https" URLs. Those URLs can "leak" into use,
which can present security and operability issues. For example, which can present security and operability issues. For example,
using a new scheme to assure that requests don't get sent to a using a new scheme to ensure that requests don't get sent to a
"normal" Web site is likely to fail. "normal" Web site is likely to fail.
o Features that rely upon the URL's origin [RFC6454], such as the o Features that rely upon the URL's origin [RFC6454], such as the
Web's same-origin policy, will be impacted by a change of scheme. Web's same-origin policy, will be impacted by a change of scheme.
o HTTP-specific features such as cookies [COOKIES], authentication o HTTP-specific features such as cookies [COOKIES], authentication
[HTTP], caching [HTTP-CACHING], HSTS [RFC6797], and CORS [FETCH] [HTTP], caching [HTTP-CACHING], HTTP Strict Transport Security
(HSTS) [RFC6797], and Cross-Origin Resource Sharing (CORS) [FETCH]
might or might not work correctly, depending on how they are might or might not work correctly, depending on how they are
defined and implemented. Generally, they are designed and defined and implemented. Generally, they are designed and
implemented with an assumption that the URL will always be "http" implemented with an assumption that the URL will always be "http"
or "https". or "https".
o Web features that require a secure context [SECCTXT] will likely o Web features that require a secure context [SECCTXT] will likely
treat a new scheme as insecure. treat a new scheme as insecure.
See [RFC7595] for more information about minting new URI schemes. See [RFC7595] for more information about minting new URI schemes.
4.4.3. Transport Ports 4.4.3. Choosing Transport Ports
Applications can use the applicable default port (80 for HTTP, 443 Applications can use the applicable default port (80 for HTTP, 443
for HTTPS), or they can be deployed upon other ports. This decision for HTTPS), or they can be deployed upon other ports. This decision
can be made at deployment time, or might be encouraged by the can be made at deployment time or might be encouraged by the
application's specification (e.g., by registering a port for that application's specification (e.g., by registering a port for that
application). application).
If a non-default port is used, it needs to be reflected in the If a non-default port is used, it needs to be reflected in the
authority of all URLs for that resource; the only mechanism for authority of all URLs for that resource; the only mechanism for
changing a default port is changing the URI scheme (see changing a default port is changing the URI scheme (see
Section 4.4.2). Section 4.4.2).
Using a port other than the default has privacy implications (i.e., Using a port other than the default has privacy implications (i.e.,
the protocol can now be distinguished from other traffic), as well as the protocol can now be distinguished from other traffic), as well as
skipping to change at page 12, line 50 skipping to change at page 12, line 51
Considerations. Considerations.
See [RFC7605] for further guidance. See [RFC7605] for further guidance.
4.5. Using HTTP Methods 4.5. Using HTTP Methods
Applications that use HTTP MUST confine themselves to using Applications that use HTTP MUST confine themselves to using
registered HTTP methods such as GET, POST, PUT, DELETE, and PATCH. registered HTTP methods such as GET, POST, PUT, DELETE, and PATCH.
New HTTP methods are rare; they are required to be registered in the New HTTP methods are rare; they are required to be registered in the
HTTP Method Registry with IETF Review (see [HTTP]), and are also "HTTP Method Registry" with IETF Review (see [HTTP]) and are also
required to be generic. That means that they need to be potentially required to be generic. That means that they need to be potentially
applicable to all resources, not just those of one application. applicable to all resources, not just those of one application.
While historically some applications (e.g., [RFC4791]) have defined While historically some applications (e.g., [RFC4791]) have defined
non-generic methods, [HTTP] now forbids this. application-specific methods, [HTTP] now forbids this.
When authors believe that a new method is required, they are When authors believe that a new method is required, they are
encouraged to engage with the HTTP community early (e.g., on the encouraged to engage with the HTTP community early (e.g., on the
ietf-http-wg@w3.org mailing list), and document their proposal as a <mailto:ietf-http-wg@w3.org> mailing list) and document their
separate HTTP extension, rather than as part of an application's proposal as a separate HTTP extension rather than as part of an
specification. application's specification.
4.5.1. GET 4.5.1. GET
GET is the most common and useful HTTP method; its retrieval GET is the most common and useful HTTP method; its retrieval
semantics allow caching, side-effect free linking and underlies many semantics allow caching and side-effect free linking and underlie
of the benefits of using HTTP. many of the benefits of using HTTP.
Queries can be performed with GET, often using the query component of Queries can be performed with GET, often using the query component of
the URL; this is a familiar pattern from Web browsing, and the the URL; this is a familiar pattern from Web browsing, and the
results can be cached, improving efficiency of an often expensive results can be cached, improving the efficiency of an often expensive
process. In some cases, however, GET might be unwieldy for process. In some cases, however, GET might be unwieldy for
expressing queries, because of the limited syntax of the URI; in expressing queries because of the limited syntax of the URI; in
particular, if binary data forms part of the query terms, it needs to particular, if binary data forms part of the query terms, it needs to
be encoded to conform to URI syntax. be encoded to conform to the URI syntax.
While this is not an issue for short queries, it can become one for While this is not an issue for short queries, it can become one for
larger query terms, or ones which need to sustain a high rate of larger query terms or those that need to sustain a high rate of
requests. Additionally, some HTTP implementations limit the size of requests. Additionally, some HTTP implementations limit the size of
URLs they support -- although modern HTTP software has much more URLs they support, although modern HTTP software has much more
generous limits than previously (typically, considerably more than generous limits than previously (typically, considerably more than
8000 octets, as required by [HTTP]). 8000 octets, as required by [HTTP]).
In these cases, an application using HTTP might consider using POST In these cases, an application using HTTP might consider using POST
to express queries in the request's content; doing so avoids encoding to express queries in the request's content; doing so avoids encoding
overhead and URL length limits in implementations. However, in doing overhead and URL length limits in implementations. However, in doing
so it should be noted that the benefits of GET such as caching and so, it should be noted that the benefits of GET such as caching and
linking to query results are lost. Therefore, applications using linking to query results are lost. Therefore, applications using
HTTP that feel a need to allow POST queries ought to consider HTTP that require support for POST queries ought to consider allowing
allowing both methods. both methods.
Processing of GET requests should not change application state or Processing of GET requests should not change the application's state
have other side effects that might be significant to the client, or have other side effects that might be significant to the client
since implementations can and do retry HTTP GET requests that fail, since implementations can and do retry HTTP GET requests that fail.
and some GET requests protected by TLS Early Data might be vulnerable Furthermore, some GET requests protected by TLS early data might be
to replay attacks (see [RFC8470]). Note that this does not include vulnerable to replay attacks (see [RFC8470]). Note that this does
logging and similar functions; see [HTTP], Section 9.2.1. not include logging and similar functions; see [HTTP], Section 9.2.1.
Finally, note that while the generic HTTP syntax allows a GET request Finally, note that while the generic HTTP syntax allows a GET request
message to contain content, the purpose is to allow message parsers message to contain content, the purpose is to allow message parsers
to be generic; as per [HTTP], Section 9.3.1, content on a GET is not to be generic; per [HTTP], Section 9.3.1, content in a GET is not
recommended, has no meaning, and will be either ignored or rejected recommended, has no meaning, and will be either ignored or rejected
by generic HTTP software (such as intermediaries, caches, servers, by generic HTTP software (such as intermediaries, caches, servers,
and client libraries). and client libraries).
4.5.2. OPTIONS 4.5.2. OPTIONS
The OPTIONS method was defined for metadata retrieval, and is used The OPTIONS method was defined for metadata retrieval and is used
both by WebDAV [RFC4918] and CORS [FETCH]. Because HTTP-based APIs both by Web Distributed Authoring and Versioning (WebDAV) [RFC4918]
often need to retrieve metadata about resources, it is often and CORS [FETCH]. Because HTTP-based APIs often need to retrieve
considered for their use. metadata about resources, it is often considered for their use.
However, OPTIONS does have significant limitations: However, OPTIONS does have significant limitations:
o It isn't possible to link to the metadata with a simple URL, o It isn't possible to link to the metadata with a simple URL
because OPTIONS is not the default method. because OPTIONS is not the default method.
o OPTIONS responses are not cacheable, because HTTP caches operate o OPTIONS responses are not cacheable because HTTP caches operate on
on representations of the resource (i.e., GET and HEAD). If representations of the resource (i.e., GET and HEAD). If OPTIONS
OPTIONS responses are cached separately, their interaction with responses are cached separately, their interactions with the HTTP
HTTP cache expiry, secondary keys and other mechanisms needs to be cache expiry, secondary keys, and other mechanisms need to be
considered. considered.
o OPTIONS is "chatty" - always separating metadata out into a o OPTIONS is "chatty" -- requesting metadata separately increases
separate request increases the number of requests needed to the number of requests needed to interact with the application.
interact with the application.
o Implementation support for OPTIONS is not universal; some servers o Implementation support for OPTIONS is not universal; some servers
do not expose the ability to respond to OPTIONS requests without do not expose the ability to respond to OPTIONS requests without
significant effort. significant effort.
Instead of OPTIONS, one of these alternative approaches might be more Instead of OPTIONS, one of these alternative approaches might be more
appropriate: appropriate:
o For server-wide metadata, create a well-known URI o For server-wide metadata, create a well-known URI [WELL-KNOWN-URI]
[WELL-KNOWN-URI], or use an already existing one if appropriate or use an already existing one if appropriate (e.g., host-meta
(e.g., HostMeta [RFC6415]). [RFC6415]).
o For metadata about a specific resource, create a separate resource o For metadata about a specific resource, create a separate resource
and link to it using a Link response header field or a link and link to it using a Link response header field or a link
serialised into the response's content. See [WEB-LINKING]. Note serialised into the response's content. See [WEB-LINKING]. Note
that the Link header field is available on HEAD responses, which that the Link header field is available on HEAD responses, which
is useful if the client wants to discover a resource's is useful if the client wants to discover a resource's
capabilities before they interact with it. capabilities before they interact with it.
4.6. Using HTTP Status Codes 4.6. Using HTTP Status Codes
HTTP status codes convey semantics both for the benefit of generic HTTP status codes convey semantics both for the benefit of generic
HTTP components -- such as caches, intermediaries, and clients -- and HTTP components -- such as caches, intermediaries, and clients -- and
applications themselves. However, applications can encounter a applications themselves. However, applications can encounter a
number of pitfalls in their use. number of pitfalls in their use.
First, status codes are often generated by components other than the First, status codes are often generated by components other than the
application itself. This can happen, for example, when network application itself. This can happen, for example, when network
errors are encountered, a captive portal, proxy or Content Delivery errors are encountered; when a captive portal, proxy, or content
Network is present, when a server is overloaded, or it thinks it is delivery network is present; or when a server is overloaded or thinks
under attack. They can even be generated by generic client software it is under attack. They can even be generated by generic client
when certain error conditions are encountered. As a result, if an software when certain error conditions are encountered. As a result,
application assigns specific semantics to one of these status codes, if an application assigns specific semantics to one of these status
a client can be misled about its state, because the status code was codes, a client can be misled about its state because the status code
generated by a generic component, not the application itself. was generated by a generic component, not the application itself.
Furthermore, mapping application errors to individual HTTP status Furthermore, mapping application errors to individual HTTP status
codes one-to-one often leads to a situation where the finite space of codes one-to-one often leads to a situation where the finite space of
applicable HTTP status codes is exhausted. This, in turn, leads to a applicable HTTP status codes is exhausted. This, in turn, leads to a
number of bad practices -- including minting new, application- number of bad practices -- including minting new, application-
specific status codes, or using existing status codes even though the specific status codes or using existing status codes even though the
link between their semantics and the application's is tenuous at link between their semantics and the application's is tenuous at
best. best.
Instead, applications using HTTP should define their errors to use Instead, applications using HTTP should define their errors to use
the most applicable status code, making generous use of the general the most applicable status code, making generous use of the general
status codes (200, 400 and 500) when in doubt. Importantly, they status codes (200, 400, and 500) when in doubt. Importantly, they
should not specify a one-to-one relationship between status codes and should not specify a one-to-one relationship between status codes and
application errors, thereby avoiding the exhaustion issue outlined application errors, thereby avoiding the exhaustion issue outlined
above. above.
To distinguish between multiple error conditions that are mapped to To distinguish between multiple error conditions that are mapped to
the same status code, and to avoid the misattribution issue outlined the same status code and to avoid the misattribution issue outlined
above, applications using HTTP should convey finer-grained error above, applications using HTTP should convey finer-grained error
information in the response's message content and/or header fields. information in the response's message content and/or header fields.
[PROBLEM-DETAILS] provides one way to do so. [PROBLEM-DETAILS] provides one way to do so.
Because the set of registered HTTP status codes can expand, Because the set of registered HTTP status codes can expand,
applications using HTTP should explicitly point out that clients applications using HTTP should explicitly point out that clients
ought to be able to handle all applicable status codes gracefully ought to be able to handle all applicable status codes gracefully
(i.e., falling back to the generic "n00" semantics of a given status (i.e., falling back to the generic n00 semantics of a given status
code; e.g., "499" can be safely handled as "400" by clients that code; e.g., 499 can be safely handled as 400 (Bad Request) by clients
don't recognise it). This is preferable to creating a "laundry list" that don't recognise it). This is preferable to creating a "laundry
of potential status codes, since such a list won't be complete in the list" of potential status codes since such a list won't be complete
foreseeable future. in the foreseeable future.
Applications using HTTP MUST NOT re-specify the semantics of HTTP Applications using HTTP MUST NOT re-specify the semantics of HTTP
status codes, even if it is only by copying their definition. It is status codes, even if it is only by copying their definition. It is
NOT RECOMMENDED they require specific reason phrases to be used; the NOT RECOMMENDED they require specific reason phrases to be used; the
reason phrase has no function in HTTP, is not guaranteed to be reason phrase has no function in HTTP, is not guaranteed to be
preserved by implementations, and is not carried at all in the HTTP/2 preserved by implementations, and is not carried at all in the HTTP/2
HTTP2 message format. [HTTP2] message format.
Applications MUST only use registered HTTP status codes. As with Applications MUST only use registered HTTP status codes. As with
methods, new HTTP status codes are rare, and required (by [HTTP]) to methods, new HTTP status codes are rare and required (by [HTTP]) to
be registered with IETF Review. Similarly, HTTP status codes are be registered with IETF Review. Similarly, HTTP status codes are
generic; they are required (by [HTTP]) to be potentially applicable generic; they are required (by [HTTP]) to be potentially applicable
to all resources, not just to those of one application. to all resources, not just to those of one application.
When authors believe that a new status code is required, they are When authors believe that a new status code is required, they are
encouraged to engage with the HTTP community early (e.g., on the encouraged to engage with the HTTP community early (e.g., on the
ietf-http-wg@w3.org mailing list), and document their proposal as a <mailto:ietf-http-wg@w3.org> mailing list) and document their
separate HTTP extension, rather than as part of an application's proposal as a separate HTTP extension, rather than as part of an
specification. application's specification.
4.6.1. Redirection 4.6.1. Redirection
The 3xx series of status codes specified in Section 15.4 of [HTTP] The 3xx series of status codes specified in Section 15.4 of [HTTP]
direct the user agent to another resource to satisfy the request. directs the user agent to another resource to satisfy the request.
The most common of these are 301, 302, 307 and 308, all of which use The most common of these are 301, 302, 307, and 308, all of which use
the Location response header field to indicate where the client the Location response header field to indicate where the client
should resend the request. should resend the request.
There are two ways that the members of this group of status codes There are two ways that the members of this group of status codes
differ: differ:
o Whether they are permanent or temporary. Permanent redirects can o Whether they are permanent or temporary. Permanent redirects can
be used to update links stored in the client (e.g., bookmarks), be used to update links stored in the client (e.g., bookmarks),
whereas temporary ones cannot. Note that this has no effect on whereas temporary ones cannot. Note that this has no effect on
HTTP caching; it is completely separate. HTTP caching; it is completely separate.
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o Whether they allow the redirected request to change the request o Whether they allow the redirected request to change the request
method from POST to GET. Web browsers generally do change POST to method from POST to GET. Web browsers generally do change POST to
GET for 301 and 302; therefore, 308 and 307 were created to allow GET for 301 and 302; therefore, 308 and 307 were created to allow
redirection without changing the method. redirection without changing the method.
This table summarises their relationships: This table summarises their relationships:
+-------------------------------------------+-----------+-----------+ +-------------------------------------------+-----------+-----------+
| | Permanent | Temporary | | | Permanent | Temporary |
+-------------------------------------------+-----------+-----------+ +-------------------------------------------+-----------+-----------+
| Allows changing the request method from | 301 | 302 | | Allows change of the request method from | 301 | 302 |
| POST to GET | | | | POST to GET | | |
| | | | | | | |
| Does not allow changing the request | 308 | 307 | | Does not allow change of the request | 308 | 307 |
| method | | | | method | | |
+-------------------------------------------+-----------+-----------+ +-------------------------------------------+-----------+-----------+
The 303 See Other status code can be used to inform the client that The 303 (See Other) status code can be used to inform the client that
the result of an operation is available at a different location using the result of an operation is available at a different location using
GET. GET.
As noted in [HTTP], a user agent is allowed to automatically follow a As noted in [HTTP], a user agent is allowed to automatically follow a
3xx redirect that has a Location response header field, even if they 3xx redirect that has a Location response header field, even if they
don't understand the semantics of the specific status code. However, don't understand the semantics of the specific status code. However,
they aren't required to do so; therefore, if an application using they aren't required to do so; therefore, if an application using
HTTP desires redirects to be automatically followed, it needs to HTTP desires redirects to be automatically followed, it needs to
explicitly specify the circumstances when this is required. explicitly specify the circumstances when this is required.
Redirects can be cached (when appropriate cache directives are Redirects can be cached (when appropriate cache directives are
present), but beyond that they are not 'sticky' -- i.e., redirection present), but beyond that, they are not "sticky" -- i.e., redirection
of a URI will not result in the client assuming that similar URIs of a URI will not result in the client assuming that similar URIs
(e.g., with different query parameters) will also be redirected. (e.g., with different query parameters) will also be redirected.
Applications using HTTP are encouraged to specify that 301 and 302 Applications using HTTP are encouraged to specify that 301 and 302
responses change the subsequent request method from POST (but no responses change the subsequent request method from POST (but no
other method) to GET, to be compatible with browsers. Generally, other method) to GET to be compatible with browsers. Generally, when
when a redirected request is made, its header fields are copied from a redirected request is made, its header fields are copied from the
the original request's. However, they can be modified by various original request. However, they can be modified by various
mechanisms; e.g., sent Authorization ([HTTP], Section 11) and Cookie mechanisms; e.g., sent Authorization ([HTTP], Section 11) and Cookie
([COOKIES]) header fields will change if the origin (and sometimes ([COOKIES]) header fields will change if the origin (and sometimes
path) of the request changes. An application using HTTP should path) of the request changes. An application using HTTP should
specify if any request header fields that it defines need to be specify if any request header fields that it defines need to be
modified or removed upon a redirect; however, this behaviour cannot modified or removed upon a redirect; however, this behaviour cannot
be relied upon, since a generic client (like a browser) will be be relied upon since a generic client (like a browser) will be
unaware of such requirements. unaware of such requirements.
4.7. Specifying HTTP Header Fields 4.7. Specifying HTTP Header Fields
Applications often define new HTTP header fields. Typically, using Applications often define new HTTP header fields. Typically, using
HTTP header fields is appropriate in a few different situations: HTTP header fields is appropriate in a few different situations:
o The field is useful to intermediaries (who often wish to avoid o The field is useful to intermediaries (who often wish to avoid
parsing message content), and/or parsing message content), and/or
o The field is useful to generic HTTP software (e.g., clients, o The field is useful to generic HTTP software (e.g., clients,
servers), and/or servers), and/or
o It is not possible to include their values in the message content o It is not possible to include their values in the message content
(usually because a format does not allow it). (usually because a format does not allow it).
When the conditions above are not met, it is usually better to convey When the conditions above are not met, it is usually better to convey
application-specific information in other places; e.g., the message application-specific information in other places -- e.g., the message
content or the URL query string. content or the URL query string.
New header fields MUST be registered, as per Section 16.3 of [HTTP]. New header fields MUST be registered, per Section 16.3 of [HTTP].
See Section 16.3.2 of [HTTP] for guidelines to consider when minting See Section 16.3.2 of [HTTP] for guidelines to consider when minting
new header fields. [STRUCTURED-FIELDS] provides a common structure new header fields. [STRUCTURED-FIELDS] provides a common structure
for new header fields, and avoids many issues in their parsing and for new header fields and avoids many issues in their parsing and
handling; it is RECOMMENDED that new header fields use it. handling; it is RECOMMENDED that new header fields use it.
It is RECOMMENDED that header field names be short (even when field It is RECOMMENDED that header field names be short (even when field
compression is used, there is an overhead) but appropriately compression is used, there is an overhead) but appropriately
specific. In particular, if a header field is specific to an specific. In particular, if a header field is specific to an
application, an identifier for that application can form a prefix to application, an identifier for that application can form a prefix to
the header field name, separated by a "-". the header field name, separated by a hyphen.
For example, if the "example" application needs to create three For example, if the "example" application needs to create three
header fields, they might be called "example-foo", "example-bar" and header fields, they might be called "example-foo", "example-bar", and
"example-baz". Note that the primary motivation here is to avoid "example-baz". Note that the primary motivation here is to avoid
consuming more generic field names, not to reserve a portion of the consuming more generic field names, not to reserve a portion of the
namespace for the application; see [RFC6648] for related namespace for the application; see [RFC6648] for related
considerations. considerations.
The semantics of existing HTTP header fields MUST NOT be re-defined The semantics of existing HTTP header fields MUST NOT be redefined
without updating their registration or defining an extension to them without updating their registration or defining an extension to them
(if allowed). For example, an application using HTTP cannot specify (if allowed). For example, an application using HTTP cannot specify
that the "Location" header field has a special meaning in a certain that the Location header field has a special meaning in a certain
context. context.
See Section 4.9 for the interaction between header fields and HTTP See Section 4.9 for the interaction between header fields and HTTP
caching; in particular, request header fields that are used to caching; in particular, request header fields that are used to choose
"select" a response have impact there, and need to be carefully (per Section 4.1 of [HTTP-CACHING]) a response have impact there and
considered. need to be carefully considered.
See Section 4.10 for considerations regarding header fields that See Section 4.10 for considerations regarding header fields that
carry application state (e.g., Cookie). carry application state (e.g., Cookie).
4.8. Defining Message Content 4.8. Defining Message Content
Common syntactic conventions for message contents include JSON Common syntactic conventions for message contents include JSON
[JSON], XML [XML], and CBOR [RFC8949]. Best practices for their use [JSON], XML [XML], and Concise Binary Object Representation (CBOR)
are out of scope for this document. [RFC8949]. Best practices for their use are out of scope for this
document.
Applications should register distinct media types for each format Applications should register distinct media types for each format
they define; this makes it possible to identify them unambiguously they define; this makes it possible to identify them unambiguously
and negotiate for their use. See [RFC6838] for more information. and negotiate for their use. See [RFC6838] for more information.
4.9. Leveraging HTTP Caching 4.9. Leveraging HTTP Caching
HTTP caching [HTTP-CACHING] is one of the primary benefits of using HTTP caching [HTTP-CACHING] is one of the primary benefits of using
HTTP for applications; it provides scalability, reduces latency and HTTP for applications; it provides scalability, reduces latency, and
improves reliability. Furthermore, HTTP caches are readily available improves reliability. Furthermore, HTTP caches are readily available
in browsers and other clients, networks as forward and reverse in browsers and other clients, networks as forward and reverse
proxies, Content Delivery Networks and as part of server software. proxies, content delivery networks, and as part of server software.
Even when an application using HTTP isn't designed to take advantage Even when an application using HTTP isn't designed to take advantage
of caching, it needs to consider how caches will handle its of caching, it needs to consider how caches will handle its responses
responses, to preserve correct behaviour when one is interposed to preserve correct behaviour when one is interposed (whether in the
(whether in the network, server, client, or intervening network, server, client, or intervening infrastructure).
infrastructure).
4.9.1. Freshness 4.9.1. Freshness
Assigning even a short freshness lifetime ([HTTP-CACHING], Assigning even a short freshness lifetime ([HTTP-CACHING],
Section 4.2) -- e.g., 5 seconds -- allows a response to be reused to Section 4.2) -- e.g., 5 seconds -- allows a response to be reused to
satisfy multiple clients, and/or a single client making the same satisfy multiple clients and/or a single client making the same
request repeatedly. In general, if it is safe to reuse something, request repeatedly. In general, if it is safe to reuse something,
consider assigning a freshness lifetime. consider assigning a freshness lifetime.
The most common method for specifying freshness is the max-age The most common method for specifying freshness is the max-age
response directive ([HTTP-CACHING], Section 5.2.2.1). The Expires response directive ([HTTP-CACHING], Section 5.2.2.1). The Expires
header field ([HTTP-CACHING], Section 5.3) can also be used, but it header field ([HTTP-CACHING], Section 5.3) can also be used, but it
is not necessary; all modern cache implementations support Cache- is not necessary; all modern cache implementations support the Cache-
Control, and specifying freshness as a delta is usually more Control header field, and specifying freshness as a delta is usually
convenient and less error-prone. more convenient and less error-prone.
It is not necessary to add the "public" response directive It is not necessary to add the public response directive
([HTTP-CACHING], Section 5.2.2.9) to cache most responses; it is only ([HTTP-CACHING], Section 5.2.2.9) to cache most responses; it is only
necessary when it's desirable to store an authenticated response, or necessary when it's desirable to store an authenticated response, or
when the status code isn't understood by the cache and there isn't when the status code isn't understood by the cache and there isn't
explicit freshness information available. explicit freshness information available.
In some situations, responses without explicit cache freshness In some situations, responses without explicit cache freshness
directives will be stored and served using a heuristic freshness directives will be stored and served using a heuristic freshness
lifetime; see [HTTP-CACHING], Section 4.2.2. As the heuristic is not lifetime; see [HTTP-CACHING], Section 4.2.2. As the heuristic is not
under control of the application, it is generally preferable to set under the control of the application, it is generally preferable to
an explicit freshness lifetime, or make the response explicitly set an explicit freshness lifetime or make the response explicitly
uncacheable. uncacheable.
If caching of a response is not desired, the appropriate response If caching of a response is not desired, the appropriate cache
directive is "Cache-Control: no-store". Other directives are not response directive is no-store. Other directives are not necessary,
necessary, and no-store only need be sent in situations where the and no-store only needs to be sent in situations where the response
response might be cached; see [HTTP-CACHING], Section 3. Note that might be cached; see [HTTP-CACHING], Section 3. Note that the no-
"Cache-Control: no-cache" allows a response to be stored, just not cache directive allows a response to be stored, just not reused by a
reused by a cache without validation; it does not prevent caching cache without validation; it does not prevent caching (despite its
(despite its name). name).
For example, this response cannot be stored or reused by a cache: For example, this response cannot be stored or reused by a cache:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/example+xml Content-Type: application/example+xml
Cache-Control: no-store Cache-Control: no-store
[content] [content]
4.9.2. Stale Responses 4.9.2. Stale Responses
Authors should understand that stale responses (e.g., with "Cache- Authors should understand that stale responses (e.g., with Cache-
Control: max-age=0") can be reused by caches when disconnected from Control: max-age=0) can be reused by caches when disconnected from
the origin server; this can be useful for handling network issues. the origin server; this can be useful for handling network issues.
If doing so is not suitable for a given response, the origin should If doing so is not suitable for a given response, the origin should
use "Cache-Control: must-revalidate". See Section 4.2.4 of send the must-revalidate cache directive. See Section 4.2.4 of
[HTTP-CACHING], and also [RFC5861] for additional controls over stale [HTTP-CACHING] and also [RFC5861] for additional controls over stale
content. content.
Stale responses can be refreshed by assigning a validator, saving Stale responses can be refreshed by assigning a validator, saving
both transfer bandwidth and latency for large responses; see both transfer bandwidth and latency for large responses; see
Section 13 of [HTTP]. Section 13 of [HTTP].
4.9.3. Caching and Application Semantics 4.9.3. Caching and Application Semantics
When an application has a need to express a lifetime that's separate When an application has a need to express a lifetime that's separate
from the freshness lifetime, this should be conveyed separately, from the freshness lifetime, this should be conveyed separately,
either in the response's content or in a separate header field. When either in the response's content or in a separate header field. When
this happens, the relationship between HTTP caching and that lifetime this happens, the relationship between HTTP caching and that lifetime
needs to be carefully considered, since the response will be used as needs to be carefully considered since the response will be used as
long as it is considered fresh. long as it is considered fresh.
In particular, application authors need to consider how responses In particular, application authors need to consider how responses
that are not freshly obtained from the origin server should be that are not freshly obtained from the origin server should be
handled; if they have a concept like a validity period, this will handled; if they have a concept like a validity period, this will
need to be calculated considering the age of the response (see need to be calculated considering the age of the response (see
[HTTP-CACHING], Section 4.2.3). [HTTP-CACHING], Section 4.2.3).
One way to address this is to explicitly specify that responses need One way to address this is to explicitly specify that responses need
to be fresh upon use. to be fresh upon use.
4.9.4. Varying Content Based Upon the Request 4.9.4. Varying Content Based Upon the Request
If an application uses a request header field to change the If an application uses a request header field to change the
response's header fields or content, authors should point out that response's header fields or content, authors should point out that
this has implications for caching; in general, such resources need to this has implications for caching; in general, such resources need to
either make their responses uncacheable (e.g., with the "no-store" either make their responses uncacheable (e.g., with the no-store
cache-control directive defined in [HTTP-CACHING], Section 5.2.2.5) cache directive defined in [HTTP-CACHING], Section 5.2.2.5) or send
or send the Vary response header field ([HTTP], Section 12.5.5) on the Vary response header field ([HTTP], Section 12.5.5) on all
all responses from that resource (including the "default" response). responses from that resource (including the "default" response).
For example, this response: For example, this response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/example+xml Content-Type: application/example+xml
Cache-Control: max-age=60 Cache-Control: max-age=60
ETag: "sa0f8wf20fs0f" ETag: "sa0f8wf20fs0f"
Vary: Accept-Encoding Vary: Accept-Encoding
[content] [content]
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request header field. request header field.
4.10. Handling Application State 4.10. Handling Application State
Applications can use stateful cookies [COOKIES] to identify a client Applications can use stateful cookies [COOKIES] to identify a client
and/or store client-specific data to contextualise requests. and/or store client-specific data to contextualise requests.
When used, it is important to carefully specify the scoping and use When used, it is important to carefully specify the scoping and use
of cookies; if the application exposes sensitive data or capabilities of cookies; if the application exposes sensitive data or capabilities
(e.g., by acting as an ambient authority), exploits are possible. (e.g., by acting as an ambient authority), exploits are possible.
Mitigations include using a request-specific token to assure the Mitigations include using a request-specific token to ensure the
intent of the client. intent of the client.
4.11. Making Multiple Requests 4.11. Making Multiple Requests
Clients often need to send multiple requests to perform a task. Clients often need to send multiple requests to perform a task.
In HTTP/1 [HTTP11], parallel requests are most often supported by In HTTP/1 [HTTP11], parallel requests are most often supported by
opening multiple connections. Application performance can be opening multiple connections. Application performance can be
impacted when too many simultaneous connections are used, because impacted when too many simultaneous connections are used because
connections' congestion control will not be coordinated. connections' congestion control will not be coordinated.
Furthermore, it can be difficult for applications to decide when to Furthermore, it can be difficult for applications to decide when to
issue and which connection to use for a given request, further issue and which connection to use for a given request, further
impacting performance. impacting performance.
HTTP/2 [HTTP2] and HTTP/3 [HTTP3] offer multiplexing to applications, HTTP/2 [HTTP2] and HTTP/3 [HTTP3] offer multiplexing to applications,
removing the need to use multiple connections. However, application removing the need to use multiple connections. However, application
performance can still be significantly affected by how the server performance can still be significantly affected by how the server
chooses to prioritize responses. Depending on the application, it chooses to prioritize responses. Depending on the application, it
might be best for the server to determine the priority of responses, might be best for the server to determine the priority of responses
or for the client to hint its priorities to the server (see, e.g., or for the client to hint its priorities to the server (see, e.g.,
[HTTP-PRIORITY]). [HTTP-PRIORITY]).
In all versions of HTTP, requests are made independently -- you can't In all versions of HTTP, requests are made independently -- you can't
rely on the relative order of two requests to guarantee processing rely on the relative order of two requests to guarantee their
order. This is because they might be sent over a multiplexed processing order. This is because they might be sent over a
protocol by an intermediary, sent to different origin servers, or the multiplexed protocol by an intermediary or sent to different origin
server might even perform processing in a different order. If two servers, or the server might even perform processing in a different
requests need strict ordering, the only reliable way to assure the order. If two requests need strict ordering, the only reliable way
outcome is to issue the second request when the final response to the to ensure the outcome is to issue the second request when the final
first has begun. response to the first has begun.
Applications MUST NOT make assumptions about the relationship between Applications MUST NOT make assumptions about the relationship between
separate requests on a single transport connection; doing so breaks separate requests on a single transport connection; doing so breaks
many of the assumptions of HTTP as a stateless protocol, and will many of the assumptions of HTTP as a stateless protocol and will
cause problems in interoperability, security, operability and cause problems in interoperability, security, operability, and
evolution. evolution.
4.12. Client Authentication 4.12. Client Authentication
Applications can use HTTP authentication Section 11 of [HTTP] to Applications can use HTTP authentication (Section 11 of [HTTP]) to
identify clients. As per [RFC7617], the Basic authentication scheme identify clients. Per [RFC7617], the Basic authentication scheme is
is not suitable for protecting sensitive or valuable information not suitable for protecting sensitive or valuable information unless
unless the channel is secure (e.g., using the "HTTPS" URI scheme). the channel is secure (e.g., using the "https" URI scheme).
Likewise, [RFC7616] requires the Digest authentication scheme to be Likewise, [RFC7616] requires the Digest authentication scheme to be
used over a secure channel. used over a secure channel.
With HTTPS, clients might also be authenticated using certificates With HTTPS, clients might also be authenticated using certificates
[RFC8446], but note that such authentication is intrinsically scoped [RFC8446], but note that such authentication is intrinsically scoped
to the underlying transport connection. As a result, a client has no to the underlying transport connection. As a result, a client has no
way of knowing whether the authenticated status was used in preparing way of knowing whether the authenticated status was used in preparing
the response (though "Vary: *" and/or "Cache-Control: private" can the response (though Vary: * and/or the private cache directive can
provide a partial indication), and the only way to obtain a provide a partial indication), and the only way to obtain a
specifically unauthenticated response is to open a new connection. specifically unauthenticated response is to open a new connection.
When used, it is important to carefully specify the scoping and use When used, it is important to carefully specify the scoping and use
of authentication; if the application exposes sensitive data or of authentication; if the application exposes sensitive data or
capabilities (e.g., by acting as an ambient authority; see capabilities (e.g., by acting as an ambient authority; see
Section 8.3 of [RFC6454]), exploits are possible. Mitigations Section 8.3 of [RFC6454]), exploits are possible. Mitigations
include using a request-specific token to assure the intent of the include using a request-specific token to ensure the intent of the
client. client.
4.13. Co-Existing with Web Browsing 4.13. Coexisting with Web Browsing
Even if there is not an intent for an application to be used with a Even if there is not an intent for an application to be used with a
Web browser, its resources will remain available to browsers and Web browser, its resources will remain available to browsers and
other HTTP clients. This means that all such applications that use other HTTP clients. This means that all such applications that use
HTTP need to consider how browsers will interact with them, HTTP need to consider how browsers will interact with them,
particularly regarding security. particularly regarding security.
For example, if an application's state can be changed using a POST For example, if an application's state can be changed using a POST
request, a Web browser can easily be coaxed into cross-site request request, a Web browser can easily be coaxed into cross-site request
forgery (CSRF) from arbitrary Web sites. forgery (CSRF) from arbitrary Web sites.
skipping to change at page 23, line 38 skipping to change at page 23, line 38
using HTTP must consider. Generally, the best approach is to using HTTP must consider. Generally, the best approach is to
actually consider the application as a Web application, and to follow actually consider the application as a Web application, and to follow
best practices for their secure development. best practices for their secure development.
A complete enumeration of such practices is out of scope for this A complete enumeration of such practices is out of scope for this
document, but some considerations include: document, but some considerations include:
o Using an application-specific media type in the Content-Type o Using an application-specific media type in the Content-Type
header field, and requiring clients to fail if it is not used. header field, and requiring clients to fail if it is not used.
o Using X-Content-Type-Options: nosniff [FETCH] to assure that o Using X-Content-Type-Options: nosniff [FETCH] to ensure that
content under attacker control can't be coaxed into a form that is content under attacker control can't be coaxed into a form that is
interpreted as active content by a Web browser. interpreted as active content by a Web browser.
o Using Content-Security-Policy [CSP] to constrain the capabilities o Using Content-Security-Policy [CSP] to constrain the capabilities
of active content (i.e., that which can execute scripts, such as of active content (i.e., that which can execute scripts, such as
HTML [HTML] and PDF), thereby mitigating Cross-Site Scripting HTML [HTML] and PDF), thereby mitigating XSS attacks.
attacks.
o Using Referrer-Policy [REFERRER-POLICY] to prevent sensitive data o Using Referrer-Policy [REFERRER-POLICY] to prevent sensitive data
in URLs from being leaked in the Referer request header field. in URLs from being leaked in the Referer request header field.
o Using the 'HttpOnly' flag on Cookies to assure that cookies are o Using the 'HttpOnly' flag on Cookies to ensure that cookies are
not exposed to browser scripting languages [COOKIES]. not exposed to browser scripting languages [COOKIES].
o Avoiding use of compression on any sensitive information (e.g., o Avoiding use of compression on any sensitive information (e.g.,
authentication tokens, passwords), as the scripting environment authentication tokens, passwords), as the scripting environment
offered by Web browsers allows an attacker to repeatedly probe the offered by Web browsers allows an attacker to repeatedly probe the
compression space; if the attacker has access to the path of the compression space; if the attacker has access to the network path
communication, they can use this capability to recover that of the communication, they can use this capability to recover that
information. information.
Depending on how they are intended to be deployed, specifications for Depending on how they are intended to be deployed, specifications for
applications using HTTP might require the use of these mechanisms in applications using HTTP might require the use of these mechanisms in
specific ways, or might merely point them out in Security specific ways or might merely point them out in Security
Considerations. Considerations.
An example of a HTTP response from an application that does not An example of an HTTP response from an application that does not
intend for its content to be treated as active by browsers might look intend for its content to be treated as active by browsers might look
like this: like this:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/example+json Content-Type: application/example+json
X-Content-Type-Options: nosniff X-Content-Type-Options: nosniff
Content-Security-Policy: default-src 'none' Content-Security-Policy: default-src 'none'
Cache-Control: max-age=3600 Cache-Control: max-age=3600
Referrer-Policy: no-referrer Referrer-Policy: no-referrer
[content] [content]
If an application has browser compatibility as a goal, client If an application has browser compatibility as a goal, client
interaction ought to be defined in terms of [FETCH], since that is interaction ought to be defined in terms of [FETCH] since that is the
the abstraction that browsers use for HTTP; it enforces many of these abstraction that browsers use for HTTP; it enforces many of these
best practices. best practices.
4.14. Maintaining Application Boundaries 4.14. Maintaining Application Boundaries
Because many HTTP capabilities are scoped to the origin [RFC6454], Because many HTTP capabilities are scoped to the origin [RFC6454],
applications also need to consider how deployments might interact applications also need to consider how deployments might interact
with other applications (including Web browsing) on the same origin. with other applications (including Web browsing) that use the same
origin server.
For example, if Cookies [COOKIES] are used to carry application For example, if cookies [COOKIES] are used to carry application
state, they will be sent with all requests to the origin by default state, they will be sent with all requests to the origin by default
(unless scoped by path), and the application might receive cookies (unless scoped by path), and the application might receive cookies
from other applications on the origin. This can lead to security from other applications on the origin server. This can lead to
issues, as well as collision in cookie names. security issues as well as collision in cookie names.
One solution to these issues is to require a dedicated hostname for One solution to these issues is to require a dedicated hostname for
the application, so that it has a unique origin. However, it is the application so that it has a unique origin. However, it is often
often desirable to allow multiple applications to be deployed on a desirable to allow multiple applications to be deployed on a single
single hostname; doing so provides the most deployment flexibility hostname; doing so provides the most deployment flexibility and
and enables them to be "mixed" together (See [RFC8820] for details). enables them to be "mixed" together (see [BCP190] for details).
Therefore, applications using HTTP should strive to allow multiple Therefore, applications using HTTP should strive to allow multiple
applications on an origin. applications on an origin. Specifically, when specifying the use of
cookies, HTTP authentication realms [HTTP], or other origin-wide HTTP
To enable this, when specifying the use of Cookies, HTTP mechanisms, applications using HTTP should not mandate the use of a
authentication realms [HTTP], or other origin-wide HTTP mechanisms, particular name but instead let deployments configure them.
applications using HTTP should not mandate the use of a particular Consideration should be given to scoping them to part of the origin,
name, but instead let deployments configure them. Consideration using their specified mechanisms for doing so.
should be given to scoping them to part of the origin, using their
specified mechanisms for doing so.
Modern Web browsers constrain the ability of content from one origin Modern Web browsers constrain the ability of content from one origin
to access resources from another, to avoid leaking private to access resources from another to avoid leaking private
information. As a result, applications that wish to expose cross- information. As a result, applications that wish to expose cross-
origin data to browsers will need to implement the CORS protocol; see origin data to browsers will need to implement the CORS protocol; see
[FETCH]. [FETCH].
4.15. Using Server Push 4.15. Using Server Push
HTTP/2 added the ability for servers to "push" request/response pairs HTTP/2 added the ability for servers to "push" request/response pairs
to clients in [HTTP2], Section 8.4. While server push seems like a to clients in [HTTP2], Section 8.4. While server push seems like a
natural fit for many common application semantics (e.g., "fanout" and natural fit for many common application semantics (e.g., "fanout" and
publish/subscribe), a few caveats should be noted: publish/subscribe), a few caveats should be noted:
o Server push is hop-by-hop; that is, it is not automatically o Server push is hop by hop; that is, it is not automatically
forwarded by intermediaries. As a result, it might not work forwarded by intermediaries. As a result, it might not work
easily (or at all) with proxies, reverse proxies, and Content easily (or at all) with proxies, reverse proxies, and content
Delivery Networks. delivery networks.
o Server push can have negative performance impact on HTTP when used o Server push can have a negative performance impact on HTTP when
incorrectly; in particular, if there is contention with resources used incorrectly, particularly if there is contention with
that have actually been requested by the client. resources that have actually been requested by the client.
o Server push is implemented differently in different clients, o Server push is implemented differently in different clients,
especially regarding interaction with HTTP caching, and especially regarding interaction with HTTP caching, and
capabilities might vary. capabilities might vary.
o APIs for server push are currently unavailable in some o APIs for server push are currently unavailable in some
implementations, and vary widely in others. In particular, there implementations and vary widely in others. In particular, there
is no current browser API for it. is no current browser API for it.
o Server push is not supported in HTTP/1.1 or HTTP/1.0. o Server push is not supported in HTTP/1.1 or HTTP/1.0.
o Server push does not form part of the "core" semantics of HTTP, o Server push does not form part of the "core" semantics of HTTP and
and therefore might not be supported by future versions of the therefore might not be supported by future versions of the
protocol. protocol.
Applications wishing to optimise cases where the client can perform Applications wishing to optimise cases where the client can perform
work related to requests before the full response is available (e.g., work related to requests before the full response is available (e.g.,
fetching links for things likely to be contained within) might fetching links for things likely to be contained within) might
benefit from using the 103 (Early Hints) status code; see [RFC8297]. benefit from using the 103 (Early Hints) status code; see [RFC8297].
Applications using server push directly need to enforce the Applications using server push directly need to enforce the
requirements regarding authority in [HTTP2], Section 8.4, to avoid requirements regarding authority in [HTTP2], Section 8.4 to avoid
cross-origin push attacks. cross-origin push attacks.
4.16. Allowing Versioning and Evolution 4.16. Allowing Versioning and Evolution
It's often necessary to introduce new features into application It's often necessary to introduce new features into application
protocols, and change existing ones. protocols and change existing ones.
In HTTP, backwards-incompatible changes can be made using mechanisms In HTTP, backwards-incompatible changes can be made using mechanisms
such as: such as:
o Using a distinct link relation type [WEB-LINKING] to identify a o Using a distinct link relation type [WEB-LINKING] to identify a
URL for a resource that implements the new functionality. URL for a resource that implements the new functionality.
o Using a distinct media type [RFC6838] to identify formats that o Using a distinct media type [RFC6838] to identify formats that
enable the new functionality. enable the new functionality.
o Using a distinct HTTP header field to implement new functionality o Using a distinct HTTP header field to implement new functionality
outside the message content. outside the message content.
5. IANA Considerations 5. IANA Considerations
This document has no requirements for IANA. This document has no IANA actions.
6. Security Considerations 6. Security Considerations
Applications using HTTP are subject to the security considerations of Applications using HTTP are subject to the security considerations of
HTTP itself and any extensions used; [HTTP], [HTTP-CACHING], and HTTP itself and any extensions used; [HTTP], [HTTP-CACHING], and
[WEB-LINKING] are often relevant, amongst others. [WEB-LINKING] are often relevant, amongst others.
Section 4.4.2 recommends support for 'https' URLs, and discourages Section 4.4.2 recommends support for "https" URLs and discourages the
the use of 'http' URLs, to provide authentication, integrity and use of "http" URLs to provide authentication, integrity, and
confidentiality, as well as mitigate pervasive monitoring attacks. confidentiality, as well as to mitigate pervasive monitoring attacks.
Many applications using HTTP perform authentication and authorization Many applications using HTTP perform authentication and authorization
with bearer tokens (e.g., in session cookies). If the transport is with bearer tokens (e.g., in session cookies). If the transport is
unencrypted, an attacker that can eavesdrop upon or modify HTTP unencrypted, an attacker that can eavesdrop upon or modify HTTP
communications can often escalate their privilege to perform communications can often escalate their privilege to perform
operations on resources. operations on resources.
Section 4.9.3 highlights the potential for mismatch between HTTP Section 4.9.3 highlights the potential for mismatch between HTTP
caching and application-specific storage of responses or information caching and application-specific storage of responses or information
therein. therein.
Section 4.10 discusses the impact of using stateful mechanisms in the Section 4.10 discusses the impact of using stateful mechanisms in the
protocol as ambient authority, and suggests a mitigation. protocol as ambient authority and suggests a mitigation.
Section 4.13 highlights the implications of Web browsers' Section 4.13 highlights the implications of Web browsers'
capabilities on applications that use HTTP. capabilities on applications that use HTTP.
Section 4.14 discusses the issues that arise when applications are Section 4.14 discusses the issues that arise when applications are
deployed on the same origin as Web sites (and other applications). deployed on the same origin as websites (and other applications).
Section 4.15 highlights risks of using HTTP/2 server push in a manner Section 4.15 highlights risks of using HTTP/2 server push in a manner
other than specified. other than that specified.
Applications that use HTTP in a manner that involves modification of Applications that use HTTP in a manner that involves modification of
implementations -- for example, requiring support for a new URI implementations -- for example, requiring support for a new URI
scheme, or a non-standard method -- risk having those implementations scheme or a non-standard method -- risk having those implementations
"fork" from their parent HTTP implementations, with the possible "fork" from their parent HTTP implementations, with the possible
result that they do not benefit from patches and other security result that they do not benefit from patches and other security
improvements incorporated upstream. improvements incorporated upstream.
6.1. Privacy Considerations 6.1. Privacy Considerations
HTTP clients can expose a variety of information to servers. Besides HTTP clients can expose a variety of information to servers. Besides
information that's explicitly sent as part of an application's information that's explicitly sent as part of an application's
operation (for example, names and other user-entered data), and "on operation (for example, names and other user-entered data) and "on
the wire" (which is one of the reasons https is recommended in the wire" (which is one of the reasons "https" is recommended in
Section 4.4.2), other information can be gathered through less Section 4.4.2), other information can be gathered through less
obvious means -- often by connecting activities of a user over time. obvious means -- often by connecting activities of a user over time.
This includes session information, tracking the client through This includes session information, tracking the client through
fingerprinting, and code execution. fingerprinting, and code execution.
Session information includes things like the IP address of the Session information includes things like the IP address of the
client, TLS session tickets, Cookies, ETags stored in the client's client, TLS session tickets, Cookies, ETags stored in the client's
cache, and other stateful mechanisms. Applications are advised to cache, and other stateful mechanisms. Applications are advised to
avoid using session mechanisms unless they are unavoidable or avoid using session mechanisms unless they are unavoidable or
necessary for operation, in which case these risks needs to be necessary for operation, in which case these risks need to be
documented. When they are used, implementations should be encouraged documented. When they are used, implementations should be encouraged
to allow clearing such state. to allow clearing such state.
Fingerprinting uses unique aspects of a client's messages and Fingerprinting uses unique aspects of a client's messages and
behaviours to connect disparate requests and connections. For behaviours to connect disparate requests and connections. For
example, the User-Agent request header field conveys specific example, the User-Agent request header field conveys specific
information about the implementation; the Accept-Language request information about the implementation; the Accept-Language request
header field conveys the users' preferred language. In combination, header field conveys the users' preferred language. In combination,
a number of these markers can be used to uniquely identify a client, a number of these markers can be used to uniquely identify a client,
impacting its control over its data. As a result, applications are impacting its control over its data. As a result, applications are
advised to specify that clients should only emit the information they advised to specify that clients should only emit the information they
need to function in requests. need to function in requests.
Finally, if an application exposes the ability to execute code, great Finally, if an application exposes the ability to execute code, great
care needs to be taken, since any ability to observe its environment care needs to be taken since any ability to observe its environment
can be used as an opportunity to both fingerprint the client and to can be used as an opportunity to both fingerprint the client and to
obtain and manipulate private data (including session information). obtain and manipulate private data (including session information).
For example, access to high-resolution timers (even indirectly) can For example, access to high-resolution timers (even indirectly) can
be used to profile the underlying hardware, creating a unique be used to profile the underlying hardware, creating a unique
identifier for the system. Applications are advised to avoid identifier for the system. Applications are advised to avoid
allowing the use of mobile code where possible; when it cannot be allowing the use of mobile code where possible; when it cannot be
avoided, the resulting system's security properties need be carefully avoided, the resulting system's security properties need be carefully
scrutinised. scrutinised.
7. References 7. References
7.1. Normative References 7.1. Normative References
[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP [BCP190] Consisting of: [RFC8820],
Semantics", draft-ietf-httpbis-semantics-18 (work in <https://www.rfc-editor.org/info/bcp190>.
progress), August 2021.
[HTTP] Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Semantics", draft-ietf-httpbis-semantics-19 (work in
progress), September 2021.
[HTTP-CACHING] [HTTP-CACHING]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Caching", draft-ietf-httpbis-cache-18 (work in progress), Caching", draft-ietf-httpbis-cache-19 (work in progress),
August 2021. September 2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011, DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>. <https://www.rfc-editor.org/info/rfc6454>.
skipping to change at page 29, line 30 skipping to change at page 29, line 30
Nottingham, M., "Well-Known Uniform Resource Identifiers Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019, (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>. <https://www.rfc-editor.org/info/rfc8615>.
7.2. Informative References 7.2. Informative References
[COOKIES] Barth, A., "HTTP State Management Mechanism", RFC 6265, [COOKIES] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011, DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>. <https://www.rfc-editor.org/info/rfc6265>.
[CSP] West, M., "Content Security Policy Level 3", World Wide [CSP] "Content Security Policy Level 3", W3C WD WD-
Web Consortium WD WD-CSP3-20160913, September 2016, CSP3-20160913, W3C WD-CSP3-20160913, September 2016,
<https://www.w3.org/TR/2016/WD-CSP3-20160913>. <https://www.w3.org/TR/2016/WD-CSP3-20160913/>.
[FETCH] WHATWG, "Fetch - Living Standard", n.d., [FETCH] WHATWG, "Fetch - Living Standard", n.d.,
<https://fetch.spec.whatwg.org>. <https://fetch.spec.whatwg.org>.
[HTML] WHATWG, "HTML - Living Standard", n.d., [HTML] WHATWG, "HTML - Living Standard", n.d.,
<https://html.spec.whatwg.org>. <https://html.spec.whatwg.org>.
[HTTP-PRIORITY] [HTTP-PRIORITY]
Oku, K. and L. Pardue, "Extensible Prioritization Scheme Oku, K. and L. Pardue, "Extensible Prioritization Scheme
for HTTP", draft-ietf-httpbis-priority-04 (work in for HTTP", draft-ietf-httpbis-priority-12 (work in
progress), July 2021. progress), January 2022.
[HTTP11] Fielding, R. T., Nottingham, M., and J. Reschke, [HTTP11] Fielding, R., Nottingham, M., and J. Reschke, "HTTP/1.1",
"HTTP/1.1", draft-ietf-httpbis-messaging-18 (work in draft-ietf-httpbis-messaging-19 (work in progress),
progress), August 2021. September 2021.
[HTTP2] Thomson, M. and C. Benfield, "Hypertext Transfer Protocol [HTTP2] Thomson, M. and C. Benfield, "HTTP/2", draft-ietf-httpbis-
Version 2 (HTTP/2)", draft-ietf-httpbis-http2bis-03 (work http2bis-07 (work in progress), January 2022.
in progress), July 2021.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3 [HTTP3] Bishop, M., "HTTP/3", draft-ietf-quic-http-34 (work in
(HTTP/3)", draft-ietf-quic-http-34 (work in progress), progress), February 2021.
February 2021.
[JSON] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data [JSON] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259, Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017, DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>. <https://www.rfc-editor.org/info/rfc8259>.
[PROBLEM-DETAILS] [PROBLEM-DETAILS]
Nottingham, M. and E. Wilde, "Problem Details for HTTP Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<https://www.rfc-editor.org/info/rfc7807>. <https://www.rfc-editor.org/info/rfc7807>.
[REFERRER-POLICY] [REFERRER-POLICY]
Eisinger, J. and E. Stark, "Referrer Policy", World Wide Stark, E., Ed. and J. Eisinger, Ed., "Referrer Policy",
Web Consortium CR CR-referrer-policy-20170126, January W3C CR CR-referrer-policy-20170126, W3C CR-referrer-
2017, policy-20170126, January 2017,
<https://www.w3.org/TR/2017/CR-referrer-policy-20170126>. <https://www.w3.org/TR/2017/CR-referrer-policy-20170126/>.
[RFC3205] Moore, K., "On the use of HTTP as a Substrate", BCP 56, [RFC3205] Moore, K., "On the use of HTTP as a Substrate", RFC 3205,
RFC 3205, DOI 10.17487/RFC3205, February 2002, DOI 10.17487/RFC3205, February 2002,
<https://www.rfc-editor.org/info/rfc3205>. <https://www.rfc-editor.org/info/rfc3205>.
[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault, [RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791, "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
DOI 10.17487/RFC4791, March 2007, DOI 10.17487/RFC4791, March 2007,
<https://www.rfc-editor.org/info/rfc4791>. <https://www.rfc-editor.org/info/rfc4791>.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, Authoring and Versioning (WebDAV)", RFC 4918,
DOI 10.17487/RFC4918, June 2007, DOI 10.17487/RFC4918, June 2007,
skipping to change at page 31, line 49 skipping to change at page 31, line 49
[RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early [RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>. 2018, <https://www.rfc-editor.org/info/rfc8470>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949, Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020, DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>. <https://www.rfc-editor.org/info/rfc8949>.
[SECCTXT] West, M., "Secure Contexts", World Wide Web Consortium CR [SECCTXT] West, M., Ed., "Secure Contexts", W3C CR CR-secure-
CR-secure-contexts-20160915, September 2016, contexts-20160915, W3C CR-secure-contexts-20160915,
<https://www.w3.org/TR/2016/CR-secure-contexts-20160915>. September 2016,
<https://www.w3.org/TR/2016/CR-secure-contexts-20160915/>.
[STRUCTURED-FIELDS] [STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>. <https://www.rfc-editor.org/info/rfc8941>.
[URI-TEMPLATE] [URI-TEMPLATE]
Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570, and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012, DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>. <https://www.rfc-editor.org/info/rfc6570>.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and [XML] Maler, E., Ed., Yergeau, F., Ed., Paoli, J., Ed.,
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth Sperberg-McQueen, M., Ed., and T. Bray, Ed., "Extensible
Edition)", World Wide Web Consortium Recommendation REC- Markup Language (XML) 1.0 (Fifth Edition)", W3C REC REC-
xml-20081126, November 2008, xml-20081126, W3C REC-xml-20081126, November 2008,
<https://www.w3.org/TR/2008/REC-xml-20081126>. <https://www.w3.org/TR/2008/REC-xml-20081126/>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/bcp56bis
Appendix A. Changes from RFC 3205 Appendix A. Changes from RFC 3205
[RFC3205] captured the Best Current Practice in the early 2000's, [RFC3205] captured the Best Current Practice in the early 2000s based
based on the concerns facing protocol designers at the time. Use of on the concerns facing protocol designers at the time. Use of HTTP
HTTP has changed considerably since then, and as a result this has changed considerably since then; as a result, this document is
document is substantially different. As a result, the changes are substantially different. Consequently, the changes are too numerous
too numerous to list individually. to list individually.
Author's Address Author's Address
Mark Nottingham Mark Nottingham
Prahran Prahran
Australia Australia
Email: mnot@mnot.net Email: mnot@mnot.net
URI: https://www.mnot.net/ URI: https://www.mnot.net/
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