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HTTP                                                       M. Nottingham
Internet-Draft                                          November 9, 2018
Obsoletes: 3205 (if approved)
Intended status: Best Current Practice
Expires: May 13, 2019


                      Building Protocols with HTTP
                     draft-ietf-httpbis-bcp56bis-08

Abstract

   HTTP is often used as a substrate for other application protocols
   (a.k.a.  HTTP-based APIs).  This document specifies best practices
   for such protocols' use of HTTP when they are defined for diverse
   implementation and broad deployment (e.g., in standards efforts).

Note to Readers

   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/ [1].

   Working Group information can be found at http://httpwg.github.io/
   [2]; source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/bcp56bis [3].

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/.

   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 May 13, 2019.








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Copyright Notice

   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . .   4
   2.  Is HTTP Being Used? . . . . . . . . . . . . . . . . . . . . .   4
   3.  What's Important About HTTP . . . . . . . . . . . . . . . . .   5
     3.1.  Generic Semantics . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Links . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Rich Functionality  . . . . . . . . . . . . . . . . . . .   7
   4.  Best Practices for Using HTTP . . . . . . . . . . . . . . . .   8
     4.1.  Specifying the Use of HTTP  . . . . . . . . . . . . . . .   8
     4.2.  Defining HTTP Resources . . . . . . . . . . . . . . . . .   9
     4.3.  Specifying Client Behaviours  . . . . . . . . . . . . . .   9
     4.4.  HTTP URLs . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.4.1.  Initial URL Discovery . . . . . . . . . . . . . . . .  11
       4.4.2.  URL Schemes . . . . . . . . . . . . . . . . . . . . .  11
       4.4.3.  Transport Ports . . . . . . . . . . . . . . . . . . .  12
     4.5.  HTTP Methods  . . . . . . . . . . . . . . . . . . . . . .  13
       4.5.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.5.2.  OPTIONS . . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  HTTP Status Codes . . . . . . . . . . . . . . . . . . . .  15
       4.6.1.  Redirection . . . . . . . . . . . . . . . . . . . . .  16
     4.7.  HTTP Header Fields  . . . . . . . . . . . . . . . . . . .  17
     4.8.  Defining Message Payloads . . . . . . . . . . . . . . . .  18
     4.9.  HTTP Caching  . . . . . . . . . . . . . . . . . . . . . .  18
     4.10. Application State . . . . . . . . . . . . . . . . . . . .  20
     4.11. Client Authentication . . . . . . . . . . . . . . . . . .  21
     4.12. Co-Existing with Web Browsing . . . . . . . . . . . . . .  21
     4.13. Application Boundaries  . . . . . . . . . . . . . . . . .  23
     4.14. Server Push . . . . . . . . . . . . . . . . . . . . . . .  23
     4.15. Versioning and Evolution  . . . . . . . . . . . . . . . .  24
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  25



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     6.1.  Privacy Considerations  . . . . . . . . . . . . . . . . .  25
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  26
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  27
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   Appendix A.  Changes from RFC 3205  . . . . . . . . . . . . . . .  30
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   HTTP [I-D.ietf-httpbis-semantics] is often used as a substrate for
   applications other than Web browsing; this is sometimes referred to
   as creating "HTTP-based APIs", or just "HTTP APIs".  This is done for
   a variety of reasons, including:

   o  familiarity by implementers, specifiers, administrators,
      developers and users,

   o  availability of a variety of client, server and proxy
      implementations,

   o  ease of use,

   o  availability of Web browsers,

   o  reuse of existing mechanisms like authentication and encryption,

   o  presence of HTTP servers and clients in target deployments, and

   o  its ability to traverse firewalls.

   These protocols are often ad hoc; they are intended for only
   deployment by one or a few servers, and consumption by a limited set
   of clients.  Perhaps because of the factors cited above, a body of
   practices and tools has arisen around defining HTTP-based APIs that
   favours these conditions.

   However, when such an application has multiple, separate
   implementations of the server component, is deployed on multiple
   uncoordinated servers, and is consumed by diverse clients - as is
   often the case for standards efforts to define new HTTP APIs - tools
   and practices intended for limited deployment can become unsuitable.

   This is largely because implementations (both client and server) will
   implement and evolve at different paces.  As a result, such an HTTP-
   based API will need to more carefully consider how extensibility of
   the service will be handled and how different deployment requirements
   will be accommodated.



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   More generally, application protocols using HTTP face a number of
   design decisions, including:

   o  Should it define a new URL scheme?  Use new ports?

   o  Should it use standard HTTP methods and status codes, or define
      new ones?

   o  How can the maximum value be extracted from the use of HTTP?

   o  How does it coexist with other uses of HTTP - especially Web
      browsing?

   o  How can interoperability problems and "protocol dead ends" be
      avoided?

   This document contains best current practices regarding the use of
   HTTP by applications other than Web browsing.  Section 2 defines what
   applications it applies to; Section 3 surveys the properties of HTTP
   that are important to preserve, and Section 4 conveys best practices
   for those applications that do 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.

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
   requirements in this document apply when any of the following
   conditions are true:

   o  The transport port in use is 80 or 443,

   o  The URL scheme "http" or "https" is used,

   o  The ALPN protocol ID [RFC7301] generically identifies HTTP (e.g.,
      "http/1.1", "h2", "h2c"), or




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   o  The IANA registries defined for HTTP are updated or modified.

   When an application is using HTTP, all of the requirements of the
   HTTP protocol suite are in force (including but not limited to
   [I-D.ietf-httpbis-semantics], [I-D.ietf-httpbis-cache],
   [I-D.ietf-httpbis-messaging], and [RFC7540]).

   An application might not use HTTP according to this definition and
   still rely upon the HTTP specifications in some manner.  For example,
   an application might wish to avoid re-specifying parts of the message
   format, but change others; or, it might want to use a different set
   of methods.

   Such applications are referred to as "protocols based upon HTTP" in
   this document.  These have more freedom to modify protocol
   operations, but are also likely to lose at least a portion of the
   benefits outlined above, as most HTTP implementations won't be easily
   adaptable to these changes, and as the protocol diverges from HTTP,
   the benefit of mindshare will be lost.

   Protocols that are based upon HTTP MUST NOT reuse HTTP's URL schemes,
   transport ports, ALPN protocol IDs or IANA registries; rather, they
   are encouraged to establish their own.

3.  What's Important About HTTP

   Applications using HTTP are defined and deployed in many ways;
   sometimes they are brought to the IETF for standardisation.  What
   might be workable for deployment in a limited fashion isn't
   appropriate for standardisation and the corresponding broader
   deployment.

   This section examines the facets of the protocol that are important
   to preserve in these situations.

3.1.  Generic Semantics

   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



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   status code might differ in a real deployment.  If the client does
   not anticipate this, the application's deployment is brittle.

   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 expressed in the payload; mostly, in the body, but also
   in header fields.

   This allows a HTTP message to be examined by generic software (e.g.,
   HTTP servers, intermediaries, client implementations, and caches) and
   its handling to be correctly determined.  It also allows people to
   leverage their knowledge of HTTP semantics without special-casing
   them for a particular application.

   Therefore, applications that use HTTP MUST NOT re-define, refine or
   overlay the semantics of defined protocol elements.  Instead, they
   should focus their specifications on protocol elements that are
   specific to that application; namely their HTTP resources.

   See Section 4.2 for details.

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 [RFC7320], 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 URL components like paths, it is
   RECOMMENDED that applications using HTTP define links in payloads, 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.




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   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:

   o  Message framing

   o  Multiplexing (in HTTP/2)

   o  Integration with TLS

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

   o  Client authentication

   o  Content negotiation for format, language, and other features

   o  Caching for server scalability, latency and bandwidth reduction,
      and reliability

   o  Granularity of access control (through use of a rich space of
      URLs)

   o  Partial content to selectively request part of a response

   o  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.







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4.  Best Practices for Using HTTP

   This section contains best practices regarding the use of HTTP by
   applications, including practices for specific HTTP protocol
   elements.

4.1.  Specifying the Use of HTTP

   When specifying the use of HTTP, an application SHOULD use
   [I-D.ietf-httpbis-semantics] as the primary reference; 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).

   Applications using HTTP SHOULD NOT specify a minimum version of HTTP
   to be used; because it is a hop-by-hop protocol, a HTTP 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 for little reason (since
   HTTP's semantics are stable between protocol versions).

   However, if an application's deployment would benefit from the use of
   a particular version of HTTP (for example, HTTP/2's multiplexing),
   this SHOULD 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 full
   headers, preferably in HTTP/1.1 format.  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

   [payload here]






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4.2.  Defining HTTP Resources

   Applications that use HTTP should focus on defining the following
   application-specific protocol elements:

   o  Media types [RFC6838], often based upon a format convention such
      as JSON [RFC8259],

   o  HTTP header fields, as per Section 4.7, and

   o  The behaviour of resources, as identified by link relations
      [RFC8288].

   By composing these protocol elements, an application can define a set
   of resources, identified by link relations, that implement specified
   behaviours, including:

   o  Retrieval of their state using GET, in one or more formats
      identified by media type;

   o  Resource creation or update using POST or PUT, with an
      appropriately identified request body format;

   o  Data processing using POST and identified request and response
      body format(s); and

   o  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.

4.3.  Specifying Client Behaviours

   Some behaviours (e.g., automatic redirect handling) and extensions
   (e.g., Cookies) are not required by HTTP, but nevertheless have
   become very common, possibly because they are supported by Web



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   browsers.  If their use is not explicitly specified by applications
   using HTTP, there may be confusion and interoperability problems.
   This section recommends default handling for these mechanisms.

   o  Redirect handling - Applications need to specify how redirects are
      expected to be handled; see Section 4.6.1.

   o  Cookies - Applications using HTTP MUST explicitly reference the
      Cookie specification [I-D.ietf-httpbis-rfc6265bis] if they are
      required.

   o  Certificates - Applications using HTTP MUST specify that TLS
      certificates are to be checked according to [RFC2818] when HTTPS
      is used.

   In general, applications using HTTP ought to align their usage as
   closely as possible with Web browsers, to avoid interoperability
   issues when they are used.  See Section 4.12.

   If an application using HTTP 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.

   Applications using HTTP MUST NOT require HTTP features that are
   usually negotiated to be supported by clients.  For example,
   requiring that clients support responses with a certain content-
   coding ([I-D.ietf-httpbis-semantics], Section 6.2.2) instead of
   negotiating for it ({{?I-D.ietf-httpbis-semantics, Section 8.4.4)
   means that otherwise conformant clients cannot interoperate with the
   application.  Applications MAY encourage the implementation of such
   features, though.

4.4.  HTTP URLs

   In HTTP, URLs are opaque identifiers under the control of the server.
   As outlined in [RFC7320], standards cannot usurp this space, since it
   might conflict with existing resources, and constrain implementation
   and deployment.

   In other words, applications that use HTTP shouldn't associate
   application semantics with specific URL paths on arbitrary servers.
   Doing so inappropriately conflates the identity of the resource (its
   URL) with the capabilities that resource supports, bringing about
   many of the same interoperability problems that [RFC4367] warns of.






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   For example, specifying that a "GET to the URL /foo retrieves a bar
   document" is bad practice.  Likewise, specifying "The widget API is
   at the path /bar" violates [RFC7320].

   Instead, applications are encouraged to ensure that URLs are
   discovered at runtime, allowing HTTP-based services to describe their
   own capabilities.  One way to do this is to use typed links [RFC8288]
   to convey the URIs that are in use, as well as the semantics of the
   resources that they identify.  See Section 4.2 for details.

4.4.1.  Initial URL Discovery

   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.

   Applications are encouraged to allow an arbitrary URL to be used as
   that entry point.  For example, rather than specifying "the initial
   document is at "/foo/v1", they should allow a deployment to use any
   URL as the entry point for the application.

   In cases where doing so is impractical (e.g., it is not possible to
   convey a whole URL, but only a hostname) applications can request a
   well-known URL [I-D.nottingham-rfc5785bis] as an entry point.

4.4.2.  URL Schemes

   Applications that use HTTP will typically employ the "http" and/or
   "https" URL 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 URL scheme for an application using HTTP, there are a
   number of tradeoffs and caveats to keep in mind:

   o  Unmodified Web browsers will not support the new scheme.  While it
      is possible to register new URL schemes with Web browsers (e.g.
      registerProtocolHandler() in [HTML5], as well as several
      proprietary approaches), support for these mechanisms is not
      shared by all browsers, and their capabilities vary.

   o  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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   o  Because URLs occur in HTTP artefacts commonly, often being
      generated automatically (e.g., in the "Location" response header),
      it can be difficult to assure that the new scheme is used
      consistently.

   o  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.

   o  Features that rely upon the URL's origin [RFC6454], such as the
      Web's same-origin policy, will be impacted by a change of scheme.

   o  HTTP-specific features such as cookies
      [I-D.ietf-httpbis-rfc6265bis], authentication
      [I-D.ietf-httpbis-semantics], caching [I-D.ietf-httpbis-cache],
      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".

   o  Web features that require a secure context [SECCTXT] will likely
      treat a new scheme as insecure.

   See [RFC7595] for more information about minting new URL 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 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 should be documented in
   Security Considerations.

   See [RFC7605] for further guidance.





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4.5.  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
   [I-D.ietf-httpbis-semantics]), 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, [I-D.ietf-httpbis-semantics] now forbids this.

   When authors believe that a new method is required, they are
   encouraged to engage with the HTTP community early, and document
   their proposal as a separate HTTP extension, rather than as part of
   an application's specification.

4.5.1.  GET

   GET is one of the most common and useful HTTP methods; its retrieval
   semantics allow caching, side-effect free linking and underlies many
   of the benefits of using HTTP.

   A common use of GET is to perform queries, 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 URL; in particular, if binary
   data forms part of the query terms, it needs to be encoded to conform
   to URL 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 [I-D.ietf-httpbis-semantics].

   In these cases, an application using HTTP might consider using POST
   to express queries in the request body; 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




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

   Applications SHOULD NOT define GET requests to have side effects,
   since implementations can and do retry HTTP GET requests that fail.

   Finally, note that while HTTP allows GET requests to have a body
   syntactically, this is done only to allow parsers to be generic; as
   per [I-D.ietf-httpbis-semantics], Section 7.3.1, a body on a GET has
   no meaning, and will be either ignored or rejected by generic HTTP
   software.

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:

   o  It isn't possible to link to the metadata with a simple URL,
      because OPTIONS is not the default GET method.

   o  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.

   o  OPTIONS is "chatty" - always separating metadata out into a
      separate request increases the number of requests needed to
      interact with the application.

   o  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:

   o  For server-wide metadata, create a well-known URI
      [I-D.nottingham-rfc5785bis], or using an already existing one if
      it's appropriate (e.g., HostMeta [RFC6415]).

   o  For metadata about a specific resource, create a separate resource
      and link to it using a Link response header or a link serialised
      into the representation's body.  See [RFC8288].  Note that the



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      Link header 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.  HTTP Status Codes

   The primary function of a HTTP status code is to convey semantics for
   the benefit of generic HTTP software, not to convey application-
   specific semantics.

   Status codes are often generated or overwritten by intermediaries, as
   well as server and client implementations.  This can happen, for
   example, when network errors are encountered, a captive portal is
   present, when an implementation is overloaded, or it thinks it is
   under attack.  As a result, the status code that a server-side
   application generates and the one that the client software receives
   often differ.

   This means that status codes are not a reliable way to carry
   application-specific signals.  Specifying that a particular status
   code has a specific meaning in the context of an application can have
   unintended side effects; if that status code is generated by a
   generic HTTP component can lead clients to believe that the
   application is in a state that wasn't intended.

   Instead, applications using HTTP should specify the implications of
   general classes of responses (e.g., "successful response" for 2xx;
   "client error" for 4xx and "server error" for 5xx), conveying any
   application-specific information in the message body and/or HTTP
   header fields, not the status code.  [RFC7807] provides one way for
   applications using HTTP to do so for error conditions.

   There are limited exceptions to this; for example, applications might
   use 201 (Created) or 404 (Not Found) to convey application semantics
   that are compatible with the generic HTTP semantics of those status
   codes.  In general, though, applications should resist the temptation
   to map their semantics into fine-grained status codes.

   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" by clients that
   don't recognise it).  This is preferable to creating a "laundry list"
   of potential status codes, since such a list is never complete.

   Applications using HTTP MUST NOT re-specify the semantics of HTTP
   status codes, even if it is only by copying their definition.  They



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   MUST NOT require specific reason phrases to be used; the reason
   phrase has no function in HTTP, is not guaranteed to be preserved by
   implementations, and the reason phrase is not carried at all in the
   HTTP/2 [RFC7540] message format.

   Applications MUST only use registered HTTP status codes.  As with
   methods, new HTTP status codes are rare, and required (by
   [I-D.ietf-httpbis-semantics]) to be registered with IETF Review.
   Similarly, HTTP status codes are generic; they are required (by
   [I-D.ietf-httpbis-semantics]) 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, 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
   [I-D.ietf-httpbis-semantics], Section 9.4 direct the user agent to
   another resource to satisfy the request.  The most common of these
   are 301, 302, 307 and 308 ([RFC7538]), all of which use the Location
   response header field to indicate where the client should send the
   request to.

   There are two ways that this group of status codes differ:

   o  Whether they are permanent or temporary.  Permanent redirects can
      be used to update links stored in the client (e.g., bookmarks),
      whereas temporary ones can not.  Note that this has no effect on
      HTTP caching; it is completely separate.

   o  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 changing the request method from   | 301       | 302       |
   | POST to GET                               |           |           |
   | Does not allow changing the request       | 308       | 307       |
   | method                                    |           |           |
   +-------------------------------------------+-----------+-----------+



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   As noted in [I-D.ietf-httpbis-semantics], 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.

   Applications using HTTP SHOULD 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
   ([I-D.ietf-httpbis-semantics]) and Cookie
   ([I-D.ietf-httpbis-rfc6265bis]) headers will change if the origin
   (and sometimes path) of the request changes.  Applications using HTTP
   SHOULD specify if any request headers 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.  HTTP Header Fields

   Applications MAY define new HTTP header fields.  Typically, using
   HTTP header fields is appropriate in a few different situations:

   o  Their content is useful to intermediaries (who often wish to avoid
      parsing the body), and/or

   o  Their content is useful to generic HTTP software (e.g., clients,
      servers), and/or

   o  It is not possible to include their content in the message body
      (usually because a format does not allow it).

   New header fields MUST be registered, as per
   [I-D.ietf-httpbis-semantics].

   See [I-D.ietf-httpbis-semantics], Section 4.1.3 for guidelines to
   consider when minting new header fields.
   [I-D.ietf-httpbis-header-structure] 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 HTTP/2
   header compression is in effect, there is an overhead) but



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   appropriately specific.  In particular, if a header field is specific
   to an application, an identifier for that application SHOULD form a
   prefix to the header field name, separated by a "-".

   For example, if the "example" application needs to create three
   headers, they might be called "example-foo", "example-bar" and
   "example-baz".  Note that the primary motivation here is to avoid
   consuming more generic header 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 has a special meaning in a certain
   context.

   See Section 4.9 for the interaction between headers and HTTP caching;
   in particular, request headers 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).

4.8.  Defining Message Payloads

   There are many potential formats for payloads; for example, JSON
   [RFC8259], XML [XML], and CBOR [RFC7049].  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.  HTTP Caching

   HTTP caching [I-D.ietf-httpbis-cache] 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.

   Assigning even a short freshness lifetime ([I-D.ietf-httpbis-cache],
   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; cache implementations take



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   active measures to remove content intelligently when they are out of
   space, so "it will fill up the cache" is not a valid concern.

   The most common method for specifying freshness is the max-age
   response directive ([I-D.ietf-httpbis-cache], Section 5.2.2.8).  The
   Expires header ([I-D.ietf-httpbis-cache], Section 5.3) can also be
   used, but it is not necessary to specify it; all modern cache
   implementations support Cache-Control, and specifying freshness as a
   delta is usually more convenient and always less error-prone.

   Understand that stale responses (e.g., with "Cache-Control: max-
   age=0") can be reused when the cache is disconnected from the origin
   server; this can be useful for handling network issues.  See
   [I-D.ietf-httpbis-cache], Section 4.2.4, 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
   [I-D.ietf-httpbis-semantics].

   If an application uses a request header field to change the
   response's headers or body, 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 [I-D.ietf-httpbis-cache],
   Section 5.2.2.3) or send the Vary response header
   ([I-D.ietf-httpbis-semantics], Section 10.1.4) 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.

   In some situations, responses without explicit cache directives
   (e.g., Cache-Control or Expires) will be stored and served using a
   heuristic freshness lifetime; see [I-D.ietf-httpbis-cache],
   Section 4.2.2.  As the heuristic is not under control of the




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   application, it is generally preferable to set an explicit freshness
   lifetime.

   If caching of a response is not desired, the appropriate response
   directive is "Cache-Control: no-store".  This only need be sent in
   situations where the response might be cached; see
   [I-D.ietf-httpbis-cache], Section 3.  Note that "Cache-Control: no-
   cache" allows a response to be stored, just not reused by a cache; 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]

   When an application has a need to express a lifetime that's separate
   from the freshness lifetime, this should be expressed separately,
   either in the response's body or in a separate header field.  When
   this happens, the relationship between HTTP caching and that lifetime
   need to be carefully considered, since the response will be used as
   long as it is considered fresh.

   Like other functions, HTTP caching is generic; it does not have
   knowledge of the application in use.  Therefore, caching extensions
   need to be backwards-compatible, as per [I-D.ietf-httpbis-cache],
   Section 5.2.3.

4.10.  Application State

   Applications MAY use stateful cookies [I-D.ietf-httpbis-rfc6265bis]
   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.

   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.




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4.11.  Client Authentication

   Applications MAY use HTTP authentication [I-D.ietf-httpbis-semantics]
   to identify clients.  The Basic authentication scheme [RFC7617] MUST
   NOT be used unless the underlying transport is authenticated,
   integrity-protected and confidential (e.g., as provided the "HTTPS"
   URL scheme, or another using TLS).  The Digest scheme [RFC7616] MUST
   NOT be used unless the underlying transport is similarly secure, or
   the chosen hash algorithm is not "MD5".

   With HTTPS, clients might also be authenticated using certificates
   [RFC5246].

   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), exploits are
   possible.  Mitigations include using a request-specific token to
   assure the intent of the client.

4.12.  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 content returned from the application's resources is under
   control of an attacker (for example, part of the request is reflected
   in the response, or the response contains external information that
   might be under the control of the attacker), a cross-site scripting
   (XSS) attack is possible, whereby an attacker can inject code into
   the browser and access data and capabilities on that origin.

   This is only a small sample of the kinds of issues that applications
   using HTTP must consider.  Generally, the best approach is to
   consider the application actually 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:




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   o  Using an application-specific media type in the Content-Type
      header, and requiring clients to fail if it is not used

   o  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

   o  Using Content-Security-Policy [CSP] to constrain the capabilities
      of active content (such as HTML [HTML5]), thereby mitigating
      Cross-Site Scripting attacks

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

   o  Using the 'HttpOnly' flag on Cookies to assure that cookies are
      not exposed to browser scripting languages
      [I-D.ietf-httpbis-rfc6265bis]

   o  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.




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4.13.  Application Boundaries

   Because the origin [RFC6454] is how many HTTP capabilities are
   scoped, applications also need to consider how deployments might
   interact with other applications (including Web browsing) on the same
   origin.

   For example, if Cookies [I-D.ietf-httpbis-rfc6265bis] 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
   single hostname; doing so provides the most deployment flexibility
   and enables them to be "mixed" together (See [RFC7320] 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 [I-D.ietf-httpbis-semantics], 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].

4.14.  Server Push

   HTTP/2 adds the ability for servers to "push" request/response pairs
   to clients in [RFC7540], Section 8.2.  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:

   o  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.






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   o  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.

   o  Server push is implemented differently in different clients,
      especially regarding interaction with HTTP caching, and
      capabilities might vary.

   o  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.

   o  Server push is not supported in HTTP/1.1 or HTTP/1.0.

   o  Server push does not form part of the "core" semantics of HTTP,
      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 [RFC7540], Section 8.2, to avoid
   cross-origin push attacks.

4.15.  Versioning and Evolution

   It's often necessary to introduce new features into application
   protocols, and change existing ones.

   In HTTP, backwards-incompatible changes are possible using a number
   of mechanisms:

   o  Using a distinct link relation type [RFC8288] to identify a URL
      for a resource that implements the new functionality

   o  Using a distinct media type [RFC6838] to identify formats that
      enable the new functionality

   o  Using a distinct HTTP header field to implement new functionality
      outside the message body








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5.  IANA Considerations

   This document has no requirements for IANA.

6.  Security Considerations

   Section 4.10 discusses the impact of using stateful mechanisms in the
   protocol as ambient authority, and suggests a mitigation.

   Section 4.4.2 requires support for 'https' URLs, and discourages the
   use of 'http' URLs, to provide authentication, integrity and
   confidentiality, as well as mitigate pervasive monitoring attacks.

   Section 4.12 highlights the implications of Web browsers'
   capabilities on applications that use HTTP.

   Section 4.13 discusses the issues that arise when applications are
   deployed on the same origin as Web sites (and other applications).

   Section 4.14 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 URL
   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 mobile code.

   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.



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   Fingerprinting uses unique aspects of a client's messages and
   behaviours to connect disparate requests and connections.  For
   example, the User-Agent request header conveys specific information
   about the implementation; the Accept-Language request header 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 run mobile 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 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

   [I-D.ietf-httpbis-cache]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Caching", draft-ietf-httpbis-cache-03 (work in progress),
              October 2018.

   [I-D.ietf-httpbis-messaging]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP/1.1
              Messaging", draft-ietf-httpbis-messaging-03 (work in
              progress), October 2018.

   [I-D.ietf-httpbis-semantics]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", draft-ietf-httpbis-semantics-03 (work in
              progress), October 2018.

   [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>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.



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   [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>.

   [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>.

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,
              <https://www.rfc-editor.org/info/rfc7320>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [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>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

7.2.  Informative References

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

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

   [HTML5]    WHATWG, "HTML - Living Standard", n.d.,
              <https://html.spec.whatwg.org>.



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   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-07 (work in progress),
              July 2018.

   [I-D.ietf-httpbis-rfc6265bis]
              Barth, A. and M. West, "Cookies: HTTP State Management
              Mechanism", draft-ietf-httpbis-rfc6265bis-02 (work in
              progress), August 2017.

   [I-D.nottingham-rfc5785bis]
              Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", draft-nottingham-rfc5785bis-08 (work in
              progress), October 2018.

   [REFERRER-POLICY]
              Eisinger, J. and E. Stark, "Referrer Policy", World Wide
              Web Consortium CR 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>.

   [RFC4367]  Rosenberg, J., Ed. and IAB, "What's in a Name: False
              Assumptions about DNS Names", RFC 4367,
              DOI 10.17487/RFC4367, February 2006,
              <https://www.rfc-editor.org/info/rfc4367>.

   [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>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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>.



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   [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>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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>.

   [RFC7538]  Reschke, J., "The Hypertext Transfer Protocol Status Code
              308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,
              April 2015, <https://www.rfc-editor.org/info/rfc7538>.

   [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>.

   [RFC7807]  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>.

   [RFC8259]  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>.




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   [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>.

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

   [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, November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126>.

7.3.  URIs

   [1] https://lists.w3.org/Archives/Public/ietf-http-wg/

   [2] http://httpwg.github.io/

   [3] https://github.com/httpwg/http-extensions/labels/bcp56bis

Appendix A.  Changes from RFC 3205

   [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

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/















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