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rfc:bcp:bcp56



Internet Engineering Task Force (IETF) M. Nottingham Request for Comments: 9205 June 2022 BCP: 56 Obsoletes: 3205 Category: Best Current Practice ISSN: 2070-1721

                    Building Protocols with HTTP

Abstract

 Applications often use HTTP as a substrate to create HTTP-based APIs.
 This document specifies best practices for writing specifications
 that use HTTP to define new application protocols.  It is written
 primarily to guide IETF efforts to define application protocols using
 HTTP for deployment on the Internet but might be applicable in other
 situations.
 This document obsoletes RFC 3205.

Status of This Memo

 This memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 BCPs is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9205.

Copyright Notice

 Copyright (c) 2022 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1.  Introduction
   1.1.  Notational Conventions
 2.  Is HTTP Being Used?
   2.1.  Non-HTTP Protocols
 3.  What's Important About HTTP
   3.1.  Generic Semantics
   3.2.  Links
   3.3.  Rich Functionality
 4.  Best Practices for Specifying the Use of HTTP
   4.1.  Specifying the Use of HTTP
   4.2.  Specifying Server Behaviour
   4.3.  Specifying Client Behaviour
   4.4.  Specifying URLs
     4.4.1.  Discovering an Application's URLs
     4.4.2.  Considering URI Schemes
     4.4.3.  Choosing Transport Ports
   4.5.  Using HTTP Methods
     4.5.1.  GET
     4.5.2.  OPTIONS
   4.6.  Using HTTP Status Codes
     4.6.1.  Redirection
   4.7.  Specifying HTTP Header Fields
   4.8.  Defining Message Content
   4.9.  Leveraging HTTP Caching
     4.9.1.  Freshness
     4.9.2.  Stale Responses
     4.9.3.  Caching and Application Semantics
     4.9.4.  Varying Content Based Upon the Request
   4.10. Handling Application State
   4.11. Making Multiple Requests
   4.12. Client Authentication
   4.13. Coexisting with Web Browsing
   4.14. Maintaining Application Boundaries
   4.15. Using Server Push
   4.16. Allowing Versioning and Evolution
 5.  IANA Considerations
 6.  Security Considerations
   6.1.  Privacy Considerations
 7.  References
   7.1.  Normative References
   7.2.  Informative References
 Appendix A.  Changes from RFC 3205
 Author's Address

1. Introduction

 Applications other than Web browsing often use HTTP [HTTP] as a
 substrate, a practice sometimes referred to as creating "HTTP-based
 APIs", "REST APIs", or just "HTTP APIs".  This is done for a variety
 of reasons, including:
  • familiarity by implementers, specifiers, administrators,

developers, and users;

  • availability of a variety of client, server, and proxy

implementations;

  • ease of use;
  • availability of Web browsers;
  • reuse of existing mechanisms like authentication and encryption;
  • presence of HTTP servers and clients in target deployments; and
  • its ability to traverse firewalls.
 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
 result, a body of practices and tools has arisen around defining
 HTTP-based APIs that favour these conditions.
 However, when such an application has multiple, separate
 implementations, is deployed on multiple uncoordinated servers, and
 is consumed by diverse clients (as is often the case for HTTP APIs
 defined by standards efforts), tools and practices intended for
 limited deployment can become unsuitable.
 This mismatch is largely because the API's clients and servers will
 implement and evolve at different paces, leading to a need for
 deployments with different features and versions to coexist.  As a
 result, the designers of HTTP-based APIs intended for such
 deployments need to more carefully consider how extensibility of the
 service will be handled and how different deployment requirements
 will be accommodated.
 More generally, an application protocol using HTTP faces a number of
 design decisions, including:
  • Should it define a new URI scheme? Use new ports?
  • Should it use standard HTTP methods and status codes or define new

ones?

  • How can the maximum value be extracted from the use of HTTP?
  • How does it coexist with other uses of HTTP – especially Web

browsing?

  • How can interoperability problems and "protocol dead ends" be

avoided?

 Section 2 defines when this document applies, Section 3 surveys the
 properties of HTTP that are important to preserve, and Section 4
 contains best practices for the specification of applications that
 use HTTP.
 It is written primarily to guide IETF efforts to define application
 protocols using HTTP for deployment on the Internet but might be
 applicable in other situations.  Note that the requirements herein do
 not necessarily apply to the development of generic HTTP extensions.
 This document obsoletes [RFC3205] to reflect the experience and
 developments regarding HTTP in the intervening time.

1.1. Notational Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

2. Is HTTP Being Used?

 Different applications have different goals when using HTTP.  The
 recommendations in this document apply when a specification defines
 an application that:
  • uses the transport port 80 or 443, or
  • uses the URI scheme "http" or "https", or
  • uses an ALPN protocol ID [RFC7301] that generically identifies

HTTP (e.g., "http/1.1", "h2", "h3"), or

  • makes registrations in or overall modifications to the IANA

registries defined for HTTP.

 Additionally, when a specification is using HTTP, all of the
 requirements of the HTTP protocol suite are in force ([HTTP] in
 particular but also other specifications such as the specific version
 of HTTP in use and any extensions in use).
 Note that this document is intended to apply to applications, not
 generic extensions to HTTP.  Furthermore, while it is intended for
 IETF-specified applications, other standards organisations are
 encouraged to adhere to its requirements.

2.1. Non-HTTP Protocols

 An application can rely upon HTTP without meeting the criteria for
 using it as defined above.  For example, an application might wish to
 avoid re-specifying parts of the message format but might change
 other aspects of the protocol's operation, or it might want to use
 application-specific methods.
 Doing so permits more freedom to modify protocol operations, but at
 least a portion of the benefits outlined in Section 3 are lost as
 most HTTP implementations won't be easily adaptable to these changes.
 The benefit of mindshare will also be lost.
 Such specifications MUST NOT use HTTP's URI schemes, transport ports,
 ALPN protocol IDs, or IANA registries; rather, they are encouraged to
 establish their own.

3. What's Important About HTTP

 This section examines the characteristics of HTTP that are important
 to consider when using HTTP to define an application protocol.

3.1. Generic Semantics

 Much of the value of HTTP is in its generic semantics -- that is, the
 protocol elements defined by HTTP are potentially applicable to every
 resource and are not specific to a particular context.  Application-
 specific semantics are best expressed in message content and header
 fields, not status codes or methods (although status codes and
 methods do have generic semantics that relate to application state).
 This split between generic and application-specific semantics allows
 an HTTP message to be handled by common software (e.g., HTTP servers,
 intermediaries, client implementations, and caches) without requiring
 those implementations to understand the application in use.  It also
 allows people to leverage their knowledge of HTTP semantics without
 needing specialised knowledge of a particular application.
 Therefore, applications that use HTTP MUST NOT redefine, refine, or
 overlay the semantics of generic protocol elements such as methods,
 status codes, or existing header fields.  Instead, they should focus
 their specifications on protocol elements that are specific to that
 application -- namely, their HTTP resources.
 When writing a specification, it's often tempting to specify exactly
 how HTTP is to be implemented, supported, and used.  However, this
 can easily lead to an unintended profile of HTTP behaviour.  For
 example, it's common to see specifications with language like this:
 |  A POST request MUST result in a 201 (Created) response.
 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
 status code might differ in a real deployment; for example, there
 might be a proxy that requires authentication, or a server-side
 error, or a redirection.  If the client does not anticipate this, the
 application's deployment is brittle.
 See Section 4.2 for more details.

3.2. Links

 Another common practice is assuming that the HTTP server's namespace
 (or a portion thereof) is exclusively for the use of a single
 application.  This effectively overlays special, application-specific
 semantics onto that space and precludes other applications from using
 it.
 As explained in [BCP190], such "squatting" on a part of the URL space
 by a standard usurps the server's authority over its own resources,
 can cause deployment issues, and is therefore bad practice in
 standards.
 Instead of statically defining URI components like paths, it is
 RECOMMENDED that applications using HTTP define and use links
 [WEB-LINKING] to allow flexibility in deployment.
 Using runtime links in this fashion has a number of other benefits --
 especially when an application is to have multiple implementations
 and/or deployments (as is often the case for those that are
 standardised).
 For example, navigating with a link allows a request to be routed to
 a different server without the overhead of a redirection, thereby
 supporting deployment across machines well.
 By using links, it also becomes possible to "mix and match" different
 applications on the same server.  The use of links also offers a
 natural mechanism for extensibility, versioning, and capability
 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
 the Web; when a resource's state changes, the application can change
 the affected links so that a fresh copy is always fetched.
 See Section 4.4 for more details.

3.3. Rich Functionality

 HTTP offers a number of features to applications, such as:
  • Message framing
  • Multiplexing (in HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3])
  • Integration with TLS
  • Support for intermediaries (proxies, gateways, content delivery

networks (CDNs))

  • Client authentication
  • Content negotiation for format, language, and other features
  • Caching for server scalability, latency and bandwidth reduction,

and reliability

  • Granularity of access control (through use of a rich space of

URLs)

  • Partial content to selectively request part of a response
  • The ability to interact with the application easily using a Web

browser

 An application that uses HTTP is encouraged to utilise the various
 features that the protocol offers so that its users receive the
 maximum benefit from those features and so that the application can
 be deployed in a variety of situations.  This document does not
 require specific features to be used since the appropriate design
 trade-offs are highly specific to a given situation.  However,
 following the practices in Section 4 is a good starting point.

4. 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
 elements.

4.1. Specifying the Use of HTTP

 Specifications should use [HTTP] as the primary reference for HTTP;
 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
 particular feature is called out).
 Because HTTP is a hop-by-hop protocol, a connection can be handled by
 implementations that are not controlled by the application; for
 example, proxies, CDNs, firewalls, and so on.  Requiring a particular
 version of HTTP makes it difficult to use in these situations and
 harms interoperability.  Therefore, it is NOT RECOMMENDED that
 applications using HTTP specify a minimum version of HTTP to be used.
 However, if an application's deployment benefits from the use of a
 particular version of HTTP (for example, HTTP/2's multiplexing), this
 ought be noted.
 Applications using HTTP MUST NOT specify a maximum version, to
 preserve the protocol's ability to evolve.
 When specifying examples of protocol interactions, applications
 should document both the request and response messages with complete
 header sections, preferably in HTTP/1.1 format [HTTP/1.1].  For
 example:
 GET /thing HTTP/1.1
 Host: example.com
 Accept: application/things+json
 User-Agent: Foo/1.0
 HTTP/1.1 200 OK
 Content-Type: application/things+json
 Content-Length: 500
 Server: Bar/2.2
 [content here]

4.2. Specifying Server Behaviour

 The server-side behaviours of an application are most effectively
 specified by defining the following protocol elements:
  • Media types [RFC6838], often based upon a format convention such

as JSON [JSON];

  • HTTP header fields, per Section 4.7; and
  • The behaviour of resources, as identified by link relations

[WEB-LINKING].

 An application can define its operation by composing these protocol
 elements to define a set of resources that are identified by link
 relations and that implement specified behaviours, including:
  • retrieval of resource state using GET in one or more formats

identified by media type;

  • resource creation or update using POST or PUT, with an

appropriately identified request content format;

  • data processing using POST and identified request and response

content format(s); and

  • Resource deletion using DELETE.
 For example, an application might specify:
 |  Resources linked to with the "example-widget" link relation type
 |  are Widgets.  The state of a Widget can be fetched in the
 |  "application/example-widget+json" format, and can be updated by
 |  PUT to the same link.  Widget resources can be deleted.
 |  
 |  The Example-Count response header field on Widget representations
 |  indicates how many Widgets are held by the sender.
 |  
 |  The "application/example-widget+json" format is a JSON [RFC8259]
 |  format representing the state of a Widget.  It contains links to
 |  related information in the link indicated by the Link header field
 |  value with the "example-other-info" link relation type.
 Applications can also specify the use of URI Templates [URI-TEMPLATE]
 to allow clients to generate URLs based upon runtime data.

4.3. Specifying Client Behaviour

 An application's expectations for client behaviour ought to be
 closely aligned with those of Web browsers to avoid interoperability
 issues when they are used.
 One way to do this is to define it in terms of [FETCH] since that is
 the abstraction that browsers use for HTTP.
 Some client behaviours (e.g., automatic redirect handling) and
 extensions (e.g., cookies) are not required by HTTP but nevertheless
 have become very common.  If their use is not explicitly specified by
 applications using HTTP, there may be confusion and interoperability
 problems.  In particular:
 Redirect handling:  Applications need to specify how redirects are
    expected to be handled; see Section 4.6.1.
 Cookies:  Applications using HTTP should explicitly reference the
    Cookie specification [COOKIES] if they are required.
 Certificates:  Applications using HTTP should specify that TLS
    certificates are to be checked according to Section 4.3.4 of
    [HTTP] when HTTPS is used.
 Applications using HTTP should not require that clients statically
 support HTTP features that are usually negotiated.  For example,
 requiring that clients support responses with a certain content
 coding ([HTTP], Section 8.4.1) instead of negotiating for it ([HTTP],
 Section 12.5.3) means that otherwise conformant clients cannot
 interoperate with the application.  Applications can encourage the
 implementation of such features, though.

4.4. Specifying URLs

 In HTTP, the resources that clients interact with are identified with
 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
 that server to give them the flexibility in deployment.
 This means that in most cases, specifications for applications that
 use HTTP won't contain fixed application URLs or paths; while it is
 common practice for a specification of a single-deployment API to
 specify the path prefix "/app/v1" (for example), doing so in an IETF
 specification is inappropriate.
 Therefore, the specification writer needs some mechanism to allow
 clients to discover an application's URLs.  Additionally, they need
 to specify which URL scheme(s) the application should be used with
 and whether to use a dedicated port or to reuse HTTP's port(s).

4.4.1. Discovering an Application's URLs

 Generally, a client will begin interacting with a given application
 server by requesting an initial document that contains information
 about that particular deployment, potentially including links to
 other relevant resources.  Doing so ensures that the deployment is as
 flexible as possible (potentially spanning multiple servers), allows
 evolution, and also gives the application the opportunity to tailor
 the "discovery document" to the client.
 There are a few common patterns for discovering that initial URL.
 The most straightforward mechanism for URL discovery is to configure
 the client with (or otherwise convey to it) a full URL.  This might
 be done in a configuration document or through another discovery
 mechanism.
 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
 initial URL from that information.
 An application cannot define a fixed prefix for its URL paths; see
 [BCP190].  Instead, a specification for such an application can use
 one of the following strategies:
  • Register a well-known URI [WELL-KNOWN-URI] as an entry point for

that application. This provides a fixed path on every potential

    server that will not collide with other applications.
  • Enable the server authority to convey a URI Template

[URI-TEMPLATE] or similar mechanism for generating a URL for an

    entry point.  For example, this might be done in a configuration
    document or other artefact.
 Once the discovery document is located, it can be fetched, cached for
 later reuse (if allowed by its metadata), and used to locate other
 resources that are relevant to the application using full URIs or URL
 Templates.
 In some cases, an application may not wish to use such a discovery
 document -- for example, when communication is very brief or when the
 latency concerns of doing so preclude the use of a discovery
 document.  These situations can be addressed by placing all of the
 application's resources under a well-known location.

4.4.2. Considering URI Schemes

 Applications that use HTTP will typically employ the "http" and/or
 "https" URI schemes. "https" is RECOMMENDED to provide
 authentication, integrity, and confidentiality, as well as to
 mitigate pervasive monitoring attacks [RFC7258].
 However, application-specific schemes can also be defined.  When
 defining a URI scheme for an application using HTTP, there are a
 number of trade-offs and caveats to keep in mind:
  • Unmodified Web browsers will not support the new scheme. While it

is possible to register new URI schemes with Web browsers (e.g.,

    registerProtocolHandler() in [HTML], as well as several
    proprietary approaches), support for these mechanisms is not
    shared by all browsers, and their capabilities vary.
  • Existing non-browser clients, intermediaries, servers, and

associated software will not recognise the new scheme. For

    example, a client library might fail to dispatch the request, a
    cache might refuse to store the response, and a proxy might fail
    to forward the request.
  • Because URLs commonly occur in HTTP artefacts and are often

generated automatically (e.g., in the Location response header

    field), it can be difficult to ensure that the new scheme is used
    consistently.
  • The resources identified by the new scheme will still be available

using "http" and/or "https" URLs. Those URLs can "leak" into use,

    which can present security and operability issues.  For example,
    using a new scheme to ensure that requests don't get sent to a
    "normal" Web site is likely to fail.
  • 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.

  • HTTP-specific features such as cookies [COOKIES], authentication

[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
    defined and implemented.  Generally, they are designed and
    implemented with an assumption that the URL will always be "http"
    or "https".
  • Web features that require a secure context [SECCTXT] will likely

treat a new scheme as insecure.

 See [RFC7595] for more information about minting new URI schemes.

4.4.3. Choosing Transport Ports

 Applications can use the applicable default port (80 for HTTP, 443
 for HTTPS), or they can be deployed upon other ports.  This decision
 can be made at deployment time or might be encouraged by the
 application's specification (e.g., by registering a port for that
 application).
 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
 changing a default port is changing the URI scheme (see
 Section 4.4.2).
 Using a port other than the default has privacy implications (i.e.,
 the protocol can now be distinguished from other traffic), as well as
 operability concerns (as some networks might block or otherwise
 interfere with it).  Privacy implications (including those stemming
 from this distinguishability) should be documented in Security
 Considerations.
 See [RFC7605] for further guidance.

4.5. Using HTTP Methods

 Applications that use HTTP MUST confine themselves to using
 registered HTTP methods such as GET, POST, PUT, DELETE, and PATCH.
 New HTTP methods are rare; they are required to be registered in the
 "HTTP Method Registry" with IETF Review (see [HTTP]) and are also
 required to be generic.  That means that they need to be potentially
 applicable to all resources, not just those of one application.
 While historically some applications (e.g., [RFC4791]) have defined
 application-specific methods, [HTTP] now forbids this.
 When authors believe that a new method is required, they are
 encouraged to engage with the HTTP community early (e.g., on the
 <mailto:ietf-http-wg@w3.org> mailing list) and document their
 proposal as a separate HTTP extension rather than as part of an
 application's specification.

4.5.1. GET

 GET is the most common and useful HTTP method; its retrieval
 semantics allow caching and side-effect free linking and underlie
 many of the benefits of using HTTP.
 Queries can be performed with GET, often using the query component of
 the URL; this is a familiar pattern from Web browsing, and the
 results can be cached, improving the efficiency of an often expensive
 process.  In some cases, however, GET might be unwieldy for
 expressing queries because of the limited syntax of the URI; in
 particular, if binary data forms part of the query terms, it needs to
 be encoded to conform to the URI syntax.
 While this is not an issue for short queries, it can become one for
 larger query terms or those that need to sustain a high rate of
 requests.  Additionally, some HTTP implementations limit the size of
 URLs they support, although modern HTTP software has much more
 generous limits than previously (typically, considerably more than
 8000 octets, as required by [HTTP]).
 In these cases, an application using HTTP might consider using POST
 to express queries in the request's content; doing so avoids encoding
 overhead and URL length limits in implementations.  However, in doing
 so, it should be noted that the benefits of GET such as caching and
 linking to query results are lost.  Therefore, applications using
 HTTP that require support for POST queries ought to consider allowing
 both methods.
 Processing of GET requests should not change the application's state
 or have other side effects that might be significant to the client
 since implementations can and do retry HTTP GET requests that fail.
 Furthermore, some GET requests protected by TLS early data might be
 vulnerable to replay attacks (see [RFC8470]).  Note that this does
 not include logging and similar functions; see [HTTP], Section 9.2.1.
 Finally, note that while the generic HTTP syntax allows a GET request
 message to contain content, the purpose is to allow message parsers
 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
 by generic HTTP software (such as intermediaries, caches, servers,
 and client libraries).

4.5.2. OPTIONS

 The OPTIONS method was defined for metadata retrieval and is used
 both by Web Distributed Authoring and Versioning (WebDAV) [RFC4918]
 and CORS [FETCH].  Because HTTP-based APIs often need to retrieve
 metadata about resources, it is often considered for their use.
 However, OPTIONS does have significant limitations:
  • It isn't possible to link to the metadata with a simple URL

because OPTIONS is not the default method.

  • OPTIONS responses are not cacheable because HTTP caches operate on

representations of the resource (i.e., GET and HEAD). If OPTIONS

    responses are cached separately, their interactions with the HTTP
    cache expiry, secondary keys, and other mechanisms need to be
    considered.
  • OPTIONS is "chatty" – requesting metadata separately increases

the number of requests needed to interact with the application.

  • Implementation support for OPTIONS is not universal; some servers

do not expose the ability to respond to OPTIONS requests without

    significant effort.
 Instead of OPTIONS, one of these alternative approaches might be more
 appropriate:
  • For server-wide metadata, create a well-known URI [WELL-KNOWN-URI]

or use an already existing one if appropriate (e.g., host-meta

    [RFC6415]).
  • For metadata about a specific resource, create a separate resource

and link to it using a Link response header field or a link

    serialised into the response's content.  See [WEB-LINKING].  Note
    that the Link header field is available on HEAD responses, which
    is useful if the client wants to discover a resource's
    capabilities before they interact with it.

4.6. Using HTTP Status Codes

 HTTP status codes convey semantics both for the benefit of generic
 HTTP components -- such as caches, intermediaries, and clients -- and
 applications themselves.  However, applications can encounter a
 number of pitfalls in their use.
 First, status codes are often generated by components other than the
 application itself.  This can happen, for example, when network
 errors are encountered; when a captive portal, proxy, or content
 delivery network is present; or when a server is overloaded or thinks
 it is under attack.  They can even be generated by generic client
 software when certain error conditions are encountered.  As a result,
 if an application assigns specific semantics to one of these status
 codes, a client can be misled about its state because the status code
 was generated by a generic component, not the application itself.
 Furthermore, mapping application errors to individual HTTP status
 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
 number of bad practices -- including minting new, application-
 specific status codes or using existing status codes even though the
 link between their semantics and the application's is tenuous at
 best.
 Instead, applications using HTTP should define their errors to use
 the most applicable status code, making generous use of the general
 status codes (200, 400, and 500) when in doubt.  Importantly, they
 should not specify a one-to-one relationship between status codes and
 application errors, thereby avoiding the exhaustion issue outlined
 above.
 To distinguish between multiple error conditions that are mapped to
 the same status code and to avoid the misattribution issue outlined
 above, applications using HTTP should convey finer-grained error
 information in the response's message content and/or header fields.
 [PROBLEM-DETAILS] provides one way to do so.
 Because the set of registered HTTP status codes can expand,
 applications using HTTP should explicitly point out that clients
 ought to be able to handle all applicable status codes gracefully
 (i.e., falling back to the generic n00 semantics of a given status
 code; e.g., 499 can be safely handled as 400 (Bad Request) by clients
 that don't recognise it).  This is preferable to creating a "laundry
 list" of potential status codes since such a list won't be complete
 in the foreseeable future.
 Applications using HTTP MUST NOT re-specify the semantics of HTTP
 status codes, even if it is only by copying their definition.  It is
 NOT RECOMMENDED they require specific reason phrases to be used; the
 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
 [HTTP/2] message format.
 Applications MUST only use registered HTTP status codes.  As with
 methods, new HTTP status codes are rare and required (by [HTTP]) to
 be registered with IETF Review.  Similarly, HTTP status codes are
 generic; they are required (by [HTTP]) to be potentially applicable
 to all resources, not just to those of one application.
 When authors believe that a new status code is required, they are
 encouraged to engage with the HTTP community early (e.g., on the
 <mailto:ietf-http-wg@w3.org> mailing list) and document their
 proposal as a separate HTTP extension, rather than as part of an
 application's specification.

4.6.1. Redirection

 The 3xx series of status codes specified in Section 15.4 of [HTTP]
 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 Location response header field to indicate where the client
 should resend the request.
 There are two ways that the members of this group of status codes
 differ:
  • Whether they are permanent or temporary. Permanent redirects can

be used to update links stored in the client (e.g., bookmarks),

    whereas temporary ones cannot.  Note that this has no effect on
    HTTP caching; it is completely separate.
  • Whether they allow the redirected request to change the request

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
    redirection without changing the method.
 This table summarises their relationships:
       +==============================+===========+===========+
       |                              | Permanent | Temporary |
       +==============================+===========+===========+
       | Allows change of the request | 301       | 302       |
       | method from POST to GET      |           |           |
       +------------------------------+-----------+-----------+
       | Does not allow change of the | 308       | 307       |
       | request method               |           |           |
       +------------------------------+-----------+-----------+
                               Table 1
 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
 GET.
 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
 don't understand the semantics of the specific status code.  However,
 they aren't required to do so; therefore, if an application using
 HTTP desires redirects to be automatically followed, it needs to
 explicitly specify the circumstances when this is required.
 Redirects can be cached (when appropriate cache directives are
 present), but beyond that, they are not "sticky" -- i.e., redirection
 of a URI will not result in the client assuming that similar URIs
 (e.g., with different query parameters) will also be redirected.
 Applications using HTTP are encouraged to specify that 301 and 302
 responses change the subsequent request method from POST (but no
 other method) to GET to be compatible with browsers.  Generally, when
 a redirected request is made, its header fields are copied from the
 original request.  However, they can be modified by various
 mechanisms; e.g., sent Authorization ([HTTP], Section 11) and Cookie
 ([COOKIES]) header fields will change if the origin (and sometimes
 path) of the request changes.  An application using HTTP should
 specify if any request header fields that it defines need to be
 modified or removed upon a redirect; however, this behaviour cannot
 be relied upon since a generic client (like a browser) will be
 unaware of such requirements.

4.7. Specifying HTTP Header Fields

 Applications often define new HTTP header fields.  Typically, using
 HTTP header fields is appropriate in a few different situations:
  • The field is useful to intermediaries (who often wish to avoid

parsing message content), and/or

  • The field is useful to generic HTTP software (e.g., clients,

servers), and/or

  • It is not possible to include their values in the message content

(usually because a format does not allow it).

 When the conditions above are not met, it is usually better to convey
 application-specific information in other places -- e.g., the message
 content or the URL query string.
 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
 new header fields.  [STRUCTURED-FIELDS] provides a common structure
 for new header fields and avoids many issues in their parsing and
 handling; it is RECOMMENDED that new header fields use it.
 It is RECOMMENDED that header field names be short (even when field
 compression is used, there is an overhead) but appropriately
 specific.  In particular, if a header field is specific to an
 application, an identifier for that application can form a prefix to
 the header field name, separated by a hyphen.
 For example, if the "example" application needs to create three
 header fields, they might be called "example-foo", "example-bar", and
 "example-baz".  Note that the primary motivation here is to avoid
 consuming more generic field names, not to reserve a portion of the
 namespace for the application; see [RFC6648] for related
 considerations.
 The semantics of existing HTTP header fields MUST NOT be redefined
 without updating their registration or defining an extension to them
 (if allowed).  For example, an application using HTTP cannot specify
 that the Location header field has a special meaning in a certain
 context.
 See Section 4.9 for the interaction between header fields and HTTP
 caching; in particular, request header fields that are used to choose
 (per Section 4.1 of [HTTP-CACHING]) a response have impact there and
 need to be carefully considered.
 See Section 4.10 for considerations regarding header fields that
 carry application state (e.g., Cookie).

4.8. Defining Message Content

 Common syntactic conventions for message contents include JSON
 [JSON], XML [XML], and Concise Binary Object Representation (CBOR)
 [RFC8949].  Best practices for their use are out of scope for this
 document.
 Applications should register distinct media types for each format
 they define; this makes it possible to identify them unambiguously
 and negotiate for their use.  See [RFC6838] for more information.

4.9. Leveraging HTTP Caching

 HTTP caching [HTTP-CACHING] is one of the primary benefits of using
 HTTP for applications; it provides scalability, reduces latency, and
 improves reliability.  Furthermore, HTTP caches are readily available
 in browsers and other clients, networks as forward and reverse
 proxies, content delivery networks, and as part of server software.
 Even when an application using HTTP isn't designed to take advantage
 of caching, it needs to consider how caches will handle its responses
 to preserve correct behaviour when one is interposed (whether in the
 network, server, client, or intervening infrastructure).

4.9.1. Freshness

 Assigning even a short freshness lifetime ([HTTP-CACHING],
 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
 request repeatedly.  In general, if it is safe to reuse something,
 consider assigning a freshness lifetime.
 The most common method for specifying freshness is the max-age
 response directive ([HTTP-CACHING], Section 5.2.2.1).  The Expires
 header field ([HTTP-CACHING], Section 5.3) can also be used, but it
 is not necessary; all modern cache implementations support the Cache-
 Control header field, and specifying freshness as a delta is usually
 more convenient and less error-prone.
 It is not necessary to add the public response directive
 ([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
 when the status code isn't understood by the cache and there isn't
 explicit freshness information available.
 In some situations, responses without explicit cache freshness
 directives will be stored and served using a heuristic freshness
 lifetime; see [HTTP-CACHING], Section 4.2.2.  As the heuristic is not
 under the control of the application, it is generally preferable to
 set an explicit freshness lifetime or make the response explicitly
 uncacheable.
 If caching of a response is not desired, the appropriate cache
 response directive is no-store.  Other directives are not necessary,
 and no-store only needs to be sent in situations where the response
 might be cached; see [HTTP-CACHING], Section 3.  Note that the no-
 cache directive allows a response to be stored, just not reused by a
 cache without validation; it does not prevent caching (despite its
 name).
 For example, this response cannot be stored or reused by a cache:
 HTTP/1.1 200 OK
 Content-Type: application/example+xml
 Cache-Control: no-store
 [content]

4.9.2. Stale Responses

 Authors should understand that stale responses (e.g., with Cache-
 Control: max-age=0) can be reused by caches when disconnected from
 the origin server; this can be useful for handling network issues.
 If doing so is not suitable for a given response, the origin should
 send the must-revalidate cache directive.  See Section 4.2.4 of
 [HTTP-CACHING] and also [RFC5861] for additional controls over stale
 content.
 Stale responses can be refreshed by assigning a validator, saving
 both transfer bandwidth and latency for large responses; see
 Section 13 of [HTTP].

4.9.3. Caching and Application Semantics

 When an application has a need to express a lifetime that's separate
 from the freshness lifetime, this should be conveyed separately,
 either in the response's content or in a separate header field.  When
 this happens, the relationship between HTTP caching and that lifetime
 needs to be carefully considered since the response will be used as
 long as it is considered fresh.
 In particular, application authors need to consider how responses
 that are not freshly obtained from the origin server should be
 handled; if they have a concept like a validity period, this will
 need to be calculated considering the age of the response (see
 [HTTP-CACHING], Section 4.2.3).
 One way to address this is to explicitly specify that responses need
 to be fresh upon use.

4.9.4. Varying Content Based Upon the Request

 If an application uses a request header field to change the
 response's header fields or content, authors should point out that
 this has implications for caching; in general, such resources need to
 either make their responses uncacheable (e.g., with the no-store
 cache directive defined in [HTTP-CACHING], Section 5.2.2.5) or send
 the Vary response header field ([HTTP], Section 12.5.5) on all
 responses from that resource (including the "default" response).
 For example, this response:
 HTTP/1.1 200 OK
 Content-Type: application/example+xml
 Cache-Control: max-age=60
 ETag: "sa0f8wf20fs0f"
 Vary: Accept-Encoding
 [content]
 can be stored for 60 seconds by both private and shared caches, can
 be revalidated with If-None-Match, and varies on the Accept-Encoding
 request header field.

4.10. Handling Application State

 Applications can use stateful cookies [COOKIES] to identify a client
 and/or store client-specific data to contextualise requests.
 When used, it is important to carefully specify the scoping and use
 of cookies; if the application exposes sensitive data or capabilities
 (e.g., by acting as an ambient authority), exploits are possible.
 Mitigations include using a request-specific token to ensure the
 intent of the client.

4.11. Making Multiple Requests

 Clients often need to send multiple requests to perform a task.
 In HTTP/1 [HTTP/1.1], parallel requests are most often supported by
 opening multiple connections.  Application performance can be
 impacted when too many simultaneous connections are used because
 connections' congestion control will not be coordinated.
 Furthermore, it can be difficult for applications to decide when to
 issue and which connection to use for a given request, further
 impacting performance.
 HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3] offer multiplexing to
 applications, removing the need to use multiple connections.
 However, application performance can still be significantly affected
 by how the server chooses to prioritize responses.  Depending on the
 application, it 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., [HTTP-PRIORITY]).
 In all versions of HTTP, requests are made independently -- you can't
 rely on the relative order of two requests to guarantee their
 processing order.  This is because they might be sent over a
 multiplexed protocol by an intermediary or sent to different origin
 servers, or the server might even perform processing in a different
 order.  If two requests need strict ordering, the only reliable way
 to ensure the outcome is to issue the second request when the final
 response to the first has begun.
 Applications MUST NOT make assumptions about the relationship between
 separate requests on a single transport connection; doing so breaks
 many of the assumptions of HTTP as a stateless protocol and will
 cause problems in interoperability, security, operability, and
 evolution.

4.12. Client Authentication

 Applications can use HTTP authentication (Section 11 of [HTTP]) to
 identify clients.  Per [RFC7617], the Basic authentication scheme is
 not suitable for protecting sensitive or valuable information unless
 the channel is secure (e.g., using the "https" URI scheme).
 Likewise, [RFC7616] requires the Digest authentication scheme to be
 used over a secure channel.
 With HTTPS, clients might also be authenticated using certificates
 [RFC8446], but note that such authentication is intrinsically scoped
 to the underlying transport connection.  As a result, a client has no
 way of knowing whether the authenticated status was used in preparing
 the response (though Vary: * and/or the private cache directive can
 provide a partial indication), and the only way to obtain a
 specifically unauthenticated response is to open a new connection.
 When used, it is important to carefully specify the scoping and use
 of authentication; if the application exposes sensitive data or
 capabilities (e.g., by acting as an ambient authority; see
 Section 8.3 of [RFC6454]), exploits are possible.  Mitigations
 include using a request-specific token to ensure the intent of the
 client.

4.13. Coexisting with Web Browsing

 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
 other HTTP clients.  This means that all such applications that use
 HTTP need to consider how browsers will interact with them,
 particularly regarding security.
 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
 forgery (CSRF) from arbitrary Web sites.
 Or, if an attacker gains control of content returned from the
 application's resources (for example, part of the request is
 reflected in the response, or the response contains external
 information that the attacker can change), they can inject code into
 the browser and access data and capabilities as if they were the
 origin -- a technique known as a cross-site scripting (XSS) attack.
 This is only a small sample of the kinds of issues that applications
 using HTTP must consider.  Generally, the best approach is to
 actually consider the application as a Web application, and to follow
 best practices for their secure development.
 A complete enumeration of such practices is out of scope for this
 document, but some considerations include:
  • Using an application-specific media type in the Content-Type

header field, and requiring clients to fail if it is not used.

  • Using X-Content-Type-Options: nosniff [FETCH] to ensure that

content under attacker control can't be coaxed into a form that is

    interpreted as active content by a Web browser.
  • Using Content-Security-Policy [CSP] to constrain the capabilities

of active content (i.e., that which can execute scripts, such as

    HTML [HTML] and PDF), thereby mitigating XSS attacks.
  • Using Referrer-Policy [REFERRER-POLICY] to prevent sensitive data

in URLs from being leaked in the Referer request header field.

  • Using the 'HttpOnly' flag on Cookies to ensure that cookies are

not exposed to browser scripting languages [COOKIES].

  • Avoiding use of compression on any sensitive information (e.g.,

authentication tokens, passwords), as the scripting environment

    offered by Web browsers allows an attacker to repeatedly probe the
    compression space; if the attacker has access to the network path
    of the communication, they can use this capability to recover that
    information.
 Depending on how they are intended to be deployed, specifications for
 applications using HTTP might require the use of these mechanisms in
 specific ways or might merely point them out in Security
 Considerations.
 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
 like this:
 HTTP/1.1 200 OK
 Content-Type: application/example+json
 X-Content-Type-Options: nosniff
 Content-Security-Policy: default-src 'none'
 Cache-Control: max-age=3600
 Referrer-Policy: no-referrer
 [content]
 If an application has browser compatibility as a goal, client
 interaction ought to be defined in terms of [FETCH] since that is the
 abstraction that browsers use for HTTP; it enforces many of these
 best practices.

4.14. Maintaining Application Boundaries

 Because many HTTP capabilities are scoped to the origin [RFC6454],
 applications also need to consider how deployments might interact
 with other applications (including Web browsing) that use the same
 origin server.
 For example, if cookies [COOKIES] are used to carry application
 state, they will be sent with all requests to the origin by default
 (unless scoped by path), and the application might receive cookies
 from other applications on the origin server.  This can lead to
 security issues as well as collision in cookie names.
 One solution to these issues is to require a dedicated hostname for
 the application so that it has a unique origin.  However, it is often
 desirable to allow multiple applications to be deployed on a single
 hostname; doing so provides the most deployment flexibility and
 enables them to be "mixed" together (see [BCP190] for details).
 Therefore, applications using HTTP should strive to allow multiple
 applications on an origin.  Specifically, when specifying the use of
 cookies, HTTP authentication realms [HTTP], or other origin-wide HTTP
 mechanisms, applications using HTTP should not mandate the use of a
 particular name but instead let deployments configure them.
 Consideration 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
 to access resources from another to avoid leaking private
 information.  As a result, applications that wish to expose cross-
 origin data to browsers will need to implement the CORS protocol; see
 [FETCH].

4.15. Using Server Push

 HTTP/2 added the ability for servers to "push" request/response pairs
 to clients in [HTTP/2], Section 8.4.  While server push seems like a
 natural fit for many common application semantics (e.g., "fanout" and
 publish/subscribe), a few caveats should be noted:
  • Server push is hop by hop; that is, it is not automatically

forwarded by intermediaries. As a result, it might not work

    easily (or at all) with proxies, reverse proxies, and content
    delivery networks.
  • Server push can have a negative performance impact on HTTP when

used incorrectly, particularly if there is contention with

    resources that have actually been requested by the client.
  • Server push is implemented differently in different clients,

especially regarding interaction with HTTP caching, and

    capabilities might vary.
  • APIs for server push are currently unavailable in some

implementations and vary widely in others. In particular, there

    is no current browser API for it.
  • Server push is not supported in HTTP/1.1 or HTTP/1.0.
  • Server push does not form part of the "core" semantics of HTTP and

therefore might not be supported by future versions of the

    protocol.
 Applications wishing to optimise cases where the client can perform
 work related to requests before the full response is available (e.g.,
 fetching links for things likely to be contained within) might
 benefit from using the 103 (Early Hints) status code; see [RFC8297].
 Applications using server push directly need to enforce the
 requirements regarding authority in [HTTP/2], Section 8.4 to avoid
 cross-origin push attacks.

4.16. Allowing Versioning and Evolution

 It's often necessary to introduce new features into application
 protocols and change existing ones.
 In HTTP, backwards-incompatible changes can be made using mechanisms
 such as:
  • Using a distinct link relation type [WEB-LINKING] to identify a

URL for a resource that implements the new functionality.

  • Using a distinct media type [RFC6838] to identify formats that

enable the new functionality.

  • Using a distinct HTTP header field to implement new functionality

outside the message content.

5. IANA Considerations

 This document has no IANA actions.

6. Security Considerations

 Applications using HTTP are subject to the security considerations of
 HTTP itself and any extensions used; [HTTP], [HTTP-CACHING], and
 [WEB-LINKING] are often relevant, amongst others.
 Section 4.4.2 recommends support for "https" URLs and discourages the
 use of "http" URLs to provide authentication, integrity, and
 confidentiality, as well as to mitigate pervasive monitoring attacks.
 Many applications using HTTP perform authentication and authorization
 with bearer tokens (e.g., in session cookies).  If the transport is
 unencrypted, an attacker that can eavesdrop upon or modify HTTP
 communications can often escalate their privilege to perform
 operations on resources.
 Section 4.9.3 highlights the potential for mismatch between HTTP
 caching and application-specific storage of responses or information
 therein.
 Section 4.10 discusses the impact of using stateful mechanisms in the
 protocol as ambient authority and suggests a mitigation.
 Section 4.13 highlights the implications of Web browsers'
 capabilities on applications that use HTTP.
 Section 4.14 discusses the issues that arise when applications are
 deployed on the same origin as websites (and other applications).
 Section 4.15 highlights risks of using HTTP/2 server push in a manner
 other than that specified.
 Applications that use HTTP in a manner that involves modification of
 implementations -- for example, requiring support for a new URI
 scheme or a non-standard method -- risk having those implementations
 "fork" from their parent HTTP implementations, with the possible
 result that they do not benefit from patches and other security
 improvements incorporated upstream.

6.1. Privacy Considerations

 HTTP clients can expose a variety of information to servers.  Besides
 information that's explicitly sent as part of an application's
 operation (for example, names and other user-entered data) and "on
 the wire" (which is one of the reasons "https" is recommended in
 Section 4.4.2), other information can be gathered through less
 obvious means -- often by connecting activities of a user over time.
 This includes session information, tracking the client through
 fingerprinting, and code execution.
 Session information includes things like the IP address of the
 client, TLS session tickets, Cookies, ETags stored in the client's
 cache, and other stateful mechanisms.  Applications are advised to
 avoid using session mechanisms unless they are unavoidable or
 necessary for operation, in which case these risks need to be
 documented.  When they are used, implementations should be encouraged
 to allow clearing such state.
 Fingerprinting uses unique aspects of a client's messages and
 behaviours to connect disparate requests and connections.  For
 example, the User-Agent request header field conveys specific
 information about the implementation; the Accept-Language request
 header field conveys the users' preferred language.  In combination,
 a number of these markers can be used to uniquely identify a client,
 impacting its control over its data.  As a result, applications are
 advised to specify that clients should only emit the information they
 need to function in requests.
 Finally, if an application exposes the ability to execute code, great
 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
 obtain and manipulate private data (including session information).
 For example, access to high-resolution timers (even indirectly) can
 be used to profile the underlying hardware, creating a unique
 identifier for the system.  Applications are advised to avoid
 allowing the use of mobile code where possible; when it cannot be
 avoided, the resulting system's security properties need be carefully
 scrutinised.

7. References

7.1. Normative References

 [BCP190]   Nottingham, M., "URI Design and Ownership", BCP 190,
            RFC 8820, DOI 10.17487/RFC8820, June 2020,
            <https://www.rfc-editor.org/rfc/rfc8820>.
 [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP Semantics", STD 97, RFC 9110,
            DOI 10.17487/RFC9110, June 2022,
            <https://www.rfc-editor.org/info/rfc9110>.
 [HTTP-CACHING]
            Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP Caching", STD 98, RFC 9111,
            DOI 10.17487/RFC9111, June 2022,
            <https://www.rfc-editor.org/info/rfc9111>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
            DOI 10.17487/RFC6454, December 2011,
            <https://www.rfc-editor.org/info/rfc6454>.
 [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
            "Deprecating the "X-" Prefix and Similar Constructs in
            Application Protocols", BCP 178, RFC 6648,
            DOI 10.17487/RFC6648, June 2012,
            <https://www.rfc-editor.org/info/rfc6648>.
 [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
            Specifications and Registration Procedures", BCP 13,
            RFC 6838, DOI 10.17487/RFC6838, January 2013,
            <https://www.rfc-editor.org/info/rfc6838>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [STRUCTURED-FIELDS]
            Nottingham, M. and P-H. Kamp, "Structured Field Values for
            HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
            <https://www.rfc-editor.org/info/rfc8941>.
 [URL]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
            <https://www.rfc-editor.org/info/rfc3986>.
 [WEB-LINKING]
            Nottingham, M., "Web Linking", RFC 8288,
            DOI 10.17487/RFC8288, October 2017,
            <https://www.rfc-editor.org/info/rfc8288>.
 [WELL-KNOWN-URI]
            Nottingham, M., "Well-Known Uniform Resource Identifiers
            (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
            <https://www.rfc-editor.org/info/rfc8615>.

7.2. Informative References

 [COOKIES]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
            DOI 10.17487/RFC6265, April 2011,
            <https://www.rfc-editor.org/info/rfc6265>.
 [CSP]      West, M., "Content Security Policy Level 3", W3C Working
            Draft, June 2021,
            <https://www.w3.org/TR/2021/WD-CSP3-20210629>.
 [FETCH]    WHATWG, "Fetch - Living Standard",
            <https://fetch.spec.whatwg.org>.
 [HTML]     WHATWG, "HTML - Living Standard",
            <https://html.spec.whatwg.org>.
 [HTTP-PRIORITY]
            奥 一穂 (Oku, K.) and L. Pardue, "Extensible Prioritization
            Scheme for HTTP", RFC 9218, DOI 10.17487/RFC9218, June
            2022, <https://www.rfc-editor.org/info/rfc9218>.
 [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
            June 2022, <https://www.rfc-editor.org/info/rfc9112>.
 [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
            DOI 10.17487/RFC9113, June 2022,
            <https://www.rfc-editor.org/info/rfc9113>.
 [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
            June 2022, <https://www.rfc-editor.org/info/rfc9114>.
 [JSON]     Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
            Interchange Format", STD 90, RFC 8259,
            DOI 10.17487/RFC8259, December 2017,
            <https://www.rfc-editor.org/info/rfc8259>.
 [PROBLEM-DETAILS]
            Nottingham, M. and E. Wilde, "Problem Details for HTTP
            APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
            <https://www.rfc-editor.org/info/rfc7807>.
 [REFERRER-POLICY]
            Eisinger, J. and E. Stark, "Referrer Policy", W3C
            Candidate Recommendation CR-referrer-policy-20170126,
            January 2017,
            <https://www.w3.org/TR/2017/CR-referrer-policy-20170126>.
 [RFC3205]  Moore, K., "On the use of HTTP as a Substrate", BCP 56,
            RFC 3205, DOI 10.17487/RFC3205, February 2002,
            <https://www.rfc-editor.org/info/rfc3205>.
 [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
            "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
            DOI 10.17487/RFC4791, March 2007,
            <https://www.rfc-editor.org/info/rfc4791>.
 [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
            Authoring and Versioning (WebDAV)", RFC 4918,
            DOI 10.17487/RFC4918, June 2007,
            <https://www.rfc-editor.org/info/rfc4918>.
 [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
            Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
            <https://www.rfc-editor.org/info/rfc5861>.
 [RFC6415]  Hammer-Lahav, E., Ed. and B. Cook, "Web Host Metadata",
            RFC 6415, DOI 10.17487/RFC6415, October 2011,
            <https://www.rfc-editor.org/info/rfc6415>.
 [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
            Transport Security (HSTS)", RFC 6797,
            DOI 10.17487/RFC6797, November 2012,
            <https://www.rfc-editor.org/info/rfc6797>.
 [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
            Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
            2014, <https://www.rfc-editor.org/info/rfc7258>.
 [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
            "Transport Layer Security (TLS) Application-Layer Protocol
            Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
            July 2014, <https://www.rfc-editor.org/info/rfc7301>.
 [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
            and Registration Procedures for URI Schemes", BCP 35,
            RFC 7595, DOI 10.17487/RFC7595, June 2015,
            <https://www.rfc-editor.org/info/rfc7595>.
 [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
            Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
            August 2015, <https://www.rfc-editor.org/info/rfc7605>.
 [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
            Digest Access Authentication", RFC 7616,
            DOI 10.17487/RFC7616, September 2015,
            <https://www.rfc-editor.org/info/rfc7616>.
 [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
            RFC 7617, DOI 10.17487/RFC7617, September 2015,
            <https://www.rfc-editor.org/info/rfc7617>.
 [RFC8297]  Oku, K., "An HTTP Status Code for Indicating Hints",
            RFC 8297, DOI 10.17487/RFC8297, December 2017,
            <https://www.rfc-editor.org/info/rfc8297>.
 [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
            Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
            <https://www.rfc-editor.org/info/rfc8446>.
 [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
            Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
            2018, <https://www.rfc-editor.org/info/rfc8470>.
 [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
            Representation (CBOR)", STD 94, RFC 8949,
            DOI 10.17487/RFC8949, December 2020,
            <https://www.rfc-editor.org/info/rfc8949>.
 [SECCTXT]  West, M., "Secure Contexts", W3C Candidate Recommendation,
            September 2021,
            <https://www.w3.org/TR/2021/CRD-secure-contexts-20210918>.
 [URI-TEMPLATE]
            Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
            and D. Orchard, "URI Template", RFC 6570,
            DOI 10.17487/RFC6570, March 2012,
            <https://www.rfc-editor.org/info/rfc6570>.
 [XML]      Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
            F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
            Edition)", W3C Recommendation REC-xml-20081126, November
            2008, <https://www.w3.org/TR/2008/REC-xml-20081126>.

Appendix A. Changes from RFC 3205

 [RFC3205] captured the Best Current Practice in the early 2000s based
 on the concerns facing protocol designers at the time.  Use of HTTP
 has changed considerably since then; as a result, this document is
 substantially different.  Consequently, the changes are too numerous
 to list individually.

Author's Address

 Mark Nottingham
 Prahran
 Australia
 Email: mnot@mnot.net
 URI:   https://www.mnot.net/
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