Building Protocols with HTTP
draft-ietf-httpbis-bcp56bis-15

Document Type Active Internet-Draft (httpbis WG)
Author Mark Nottingham 
Last updated 2021-11-19 (latest revision 2021-08-27)
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HTTP                                                       M. Nottingham
Internet-Draft                                            27 August 2021
Obsoletes: 3205 (if approved)                                           
Intended status: Best Current Practice                                  
Expires: 28 February 2022

                      Building Protocols with HTTP
                     draft-ietf-httpbis-bcp56bis-15

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 [RFC3205].

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/
   (https://lists.w3.org/Archives/Public/ietf-http-wg/).

   Working Group information can be found at http://httpwg.github.io/
   (http://httpwg.github.io/); source code and issues list for this
   draft can be found at https://github.com/httpwg/http-
   extensions/labels/bcp56bis (https://github.com/httpwg/http-
   extensions/labels/bcp56bis).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 28 February 2022.

Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   2.  Is HTTP Being Used? . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Non-HTTP Protocols  . . . . . . . . . . . . . . . . . . .   5
   3.  What's Important About HTTP . . . . . . . . . . . . . . . . .   6
     3.1.  Generic Semantics . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Links . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Rich Functionality  . . . . . . . . . . . . . . . . . . .   7
   4.  Best Practices for Specifying the Use of HTTP . . . . . . . .   8
     4.1.  Specifying the Use of HTTP  . . . . . . . . . . . . . . .   8
     4.2.  Specifying Server Behaviour . . . . . . . . . . . . . . .   9
     4.3.  Specifying Client Behaviour . . . . . . . . . . . . . . .  10
     4.4.  Specifying URLs . . . . . . . . . . . . . . . . . . . . .  11
       4.4.1.  Discovering an Application's URLs . . . . . . . . . .  11
       4.4.2.  Considering URI Schemes . . . . . . . . . . . . . . .  12
       4.4.3.  Transport Ports . . . . . . . . . . . . . . . . . . .  13
     4.5.  Using HTTP Methods  . . . . . . . . . . . . . . . . . . .  14
       4.5.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  14
       4.5.2.  OPTIONS . . . . . . . . . . . . . . . . . . . . . . .  15
     4.6.  Using HTTP Status Codes . . . . . . . . . . . . . . . . .  16
       4.6.1.  Redirection . . . . . . . . . . . . . . . . . . . . .  17
     4.7.  Specifying HTTP Header Fields . . . . . . . . . . . . . .  18
     4.8.  Defining Message Content  . . . . . . . . . . . . . . . .  20
     4.9.  Leveraging HTTP Caching . . . . . . . . . . . . . . . . .  20
       4.9.1.  Freshness . . . . . . . . . . . . . . . . . . . . . .  20

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       4.9.2.  Stale Responses . . . . . . . . . . . . . . . . . . .  21
       4.9.3.  Caching and Application Semantics . . . . . . . . . .  21
       4.9.4.  Varying Content Based Upon the Request  . . . . . . .  22
     4.10. Handling Application State  . . . . . . . . . . . . . . .  22
     4.11. Making Multiple Requests  . . . . . . . . . . . . . . . .  22
     4.12. Client Authentication . . . . . . . . . . . . . . . . . .  23
     4.13. Co-Existing with Web Browsing . . . . . . . . . . . . . .  24
     4.14. Maintaining Application Boundaries  . . . . . . . . . . .  25
     4.15. Using Server Push . . . . . . . . . . . . . . . . . . . .  26
     4.16. Allowing Versioning and Evolution . . . . . . . . . . . .  27
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
     6.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .  28
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  29
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Appendix A.  Changes from RFC 3205  . . . . . . . . . . . . . . .  33
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  33

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 favours these conditions.

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   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 co-exist.  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?

   This document contains best current practices for the specification
   of such applications.  Section 2 defines when it applies; Section 3
   surveys the properties of HTTP that are important to preserve, and
   Section 4 conveys best practices for specifying them.

   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 experience and
   developments regarding HTTP in the intervening time.

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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 (in particular,
   [HTTP], 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 defined above.  For example, an application might wish to
   avoid re-specifying parts of the message format, but change other
   aspects of the protocol's operation; or, it might want to use
   application-specific methods.

   Doing so brings more freedom to modify protocol operations, but loses
   at least a portion of the benefits outlined in Section 3, as most
   HTTP implementations won't be easily adaptable to these changes, and
   the benefit of mindshare will be lost.

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   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, not specific to a particular context.  Application-specific
   semantics are best expressed in message content and in header fields,
   not status codes or methods (although the latter do have generic
   semantics that relate to application state).

   This generic/application-specific split allows a HTTP message to be
   handled by common software (e.g., HTTP servers, intermediaries,
   client implementations, and caches) without understanding the
   specific application.  It also allows people to leverage their
   knowledge of HTTP semantics without specialising them for a
   particular application.

   Therefore, applications that use HTTP MUST NOT re-define, 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's 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.

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3.2.  Links

   Another common practice is assuming that the HTTP server's name space
   (or a portion thereof) is exclusively for the use of a single
   application.  This effectively overlays special, application-specific
   semantics onto that space, precludes other applications from using
   it.

   As explained in [RFC8820], 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.

   It also becomes possible to "mix and match" different applications on
   the same server, and offers a natural mechanism for extensibility,
   versioning and capability management, since 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
   its link to it so that a fresh copy is always fetched.

3.3.  Rich Functionality

   HTTP offers a number of features to applications, such as:

   *  Message framing

   *  Multiplexing (in HTTP/2 [HTTP2] and HTTP/3 [HTTP3])

   *  Integration with TLS

   *  Support for intermediaries (proxies, gateways, Content Delivery
      Networks)

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   *  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

   Applications that use HTTP are encouraged to utilise the various
   features that the protocol offers, so that their users receive the
   maximum benefit from it, and to allow it to be deployed in a variety
   of situations.  This document does not require specific features to
   be used, since the appropriate design tradeoffs 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 would benefit from the use of
   a particular version of HTTP (for example, HTTP/2's multiplexing),
   this ought be noted.

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   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 [HTTP11].  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, as 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 their 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.

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   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.

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   Applications using HTTP should not statically require HTTP features
   that are usually negotiated to be supported by clients.  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 [RFC8820] 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 discovery an application's URLs.  Additionally, they need
   to specify what URL scheme(s) the application should be used with,
   and whether to use a dedicated port, or 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 assures 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.

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   An application cannot define a fixed prefix for its URL paths; see
   [RFC8820].  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 precludes 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 mitigate
   pervasive monitoring attacks [RFC7258].

   However, application-specific schemes can also be defined.  When
   defining an URI scheme for an application using HTTP, there are a
   number of tradeoffs 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.

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   *  Because URLs occur in HTTP artefacts commonly, often being
      generated automatically (e.g., in the "Location" response header
      field), it can be difficult to assure 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 assure 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], HSTS [RFC6797], and 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.  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.

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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
   non-generic 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
   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, side-effect free linking and underlies 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 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 URI syntax.

   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
   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 feel a need to allow POST queries ought to consider
   allowing both methods.

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   Processing of GET requests should not change application state or
   have other side effects that might be significant to the client,
   since implementations can and do retry HTTP GET requests that fail,
   and 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; as per [HTTP], Section 9.3.1, content on 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 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 interaction with
      HTTP cache expiry, secondary keys and other mechanisms needs to be
      considered.

   *  OPTIONS is "chatty" - always separating metadata out into a
      separate request 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., HostMeta [RFC6415]).

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   *  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, a captive portal, proxy or Content Delivery
   Network is present, when a server is overloaded, or it 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

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   (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
   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
   HTTP2 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
   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]
   direct 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:

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          +=============================+===========+===========+
          |                             | Permanent | Temporary |
          +=============================+===========+===========+
          | Allows changing the request | 301       | 302       |
          | method from POST to GET     |           |           |
          +-----------------------------+-----------+-----------+
          | Does not allow changing 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's.  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

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   *  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, as 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 "-".

   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 re-defined
   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
   "select" 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).

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4.8.  Defining Message Content

   Common syntactic conventions for message contents include JSON
   [JSON], XML [XML], and 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 Cache-
   Control, 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

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   under 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 response
   directive is "Cache-Control: no-store".  Other directives are not
   necessary, and no-store only need be sent in situations where the
   response might be cached; see [HTTP-CACHING], Section 3.  Note that
   "Cache-Control: no-cache" 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
   use "Cache-Control: must-revalidate".  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.

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   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-control 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 assure the
   intent of the client.

4.11.  Making Multiple Requests

   Clients often need to send multiple requests to perform a task.

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   In HTTP/1 [HTTP11], 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 [HTTP2] and HTTP/3 [HTTP3] 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 processing
   order.  This is because they might be sent over a multiplexed
   protocol by an intermediary, 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 assure 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.  As 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 "Cache-Control: private" can
   provide a partial indication), and the only way to obtain a
   specifically unauthenticated response is to open a new connection.

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   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 assure the intent of the
   client.

4.13.  Co-Existing 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 assure 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 Cross-Site Scripting
      attacks.

   *  Using Referrer-Policy [REFERRER-POLICY] to prevent sensitive data
      in URLs from being leaked in the Referer request header field.

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   *  Using the 'HttpOnly' flag on Cookies to assure 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 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 a 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) on the same origin.

   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.  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

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   single hostname; doing so provides the most deployment flexibility
   and enables them to be "mixed" together (See [RFC8820] for details).
   Therefore, applications using HTTP should strive to allow multiple
   applications on an origin.

   To enable this, 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 [HTTP2], 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 negative performance impact on HTTP when used
      incorrectly; in particular, 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.

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   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 [HTTP2], 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 requirements for IANA.

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 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.

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   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 Web sites (and other applications).

   Section 4.15 highlights risks of using HTTP/2 server push in a manner
   other than 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 needs 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.

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   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

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-18, 18 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-18>.

   [HTTP-CACHING]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-18, 18 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              cache-18>.

   [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/rfc/rfc2119>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc6838>.

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   [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/rfc/rfc8174>.

   [RFC8820]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 8820, DOI 10.17487/RFC8820, June 2020,
              <https://www.rfc-editor.org/rfc/rfc8820>.

   [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/rfc/rfc3986>.

   [WEB-LINKING]
              Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc6265>.

   [CSP]      West, M., "Content Security Policy Level 3", World Wide
              Web Consortium WD WD-CSP3-20160913, 13 September 2016,
              <https://www.w3.org/TR/2016/WD-CSP3-20160913>.

   [FETCH]    WHATWG, "Fetch - Living Standard", n.d.,
              <https://fetch.spec.whatwg.org>.

   [HTML]     WHATWG, "HTML - Living Standard", n.d.,
              <https://html.spec.whatwg.org>.

   [HTTP-PRIORITY]
              Oku, K. and L. Pardue, "Extensible Prioritization Scheme
              for HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-priority-04, 11 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              priority-04>.

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   [HTTP11]   Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-18, 18 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              messaging-18>.

   [HTTP2]    Thomson, M. and C. Benfield, "Hypertext Transfer Protocol
              Version 2 (HTTP/2)", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-http2bis-03, 12 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              http2bis-03>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [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/rfc/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/rfc/rfc7807>.

   [REFERRER-POLICY]
              Eisinger, J. and E. Stark, "Referrer Policy", World Wide
              Web Consortium CR CR-referrer-policy-20170126, 26 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/rfc/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/rfc/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/rfc/rfc4918>.

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   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
              <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/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/rfc/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/rfc/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/rfc/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/rfc/rfc7616>.

   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7617>.

   [RFC8297]  Oku, K., "An HTTP Status Code for Indicating Hints",
              RFC 8297, DOI 10.17487/RFC8297, December 2017,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc8446>.

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   [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/rfc/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/rfc/rfc8949>.

   [SECCTXT]  West, M., "Secure Contexts", World Wide Web Consortium CR 
              CR-secure-contexts-20160915, 15 September 2016,
              <https://www.w3.org/TR/2016/CR-secure-contexts-20160915>.

   [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/rfc/rfc8941>.

   [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/rfc/rfc6570>.

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, 26 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 2000's,
   based on the concerns facing protocol designers at the time.  Use of
   HTTP has changed considerably since then, and as a result this
   document is substantially different.  As a result, 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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