Building Protocols with HTTP

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.¶

When writing a specification, it’s often tempting to specify exactly how HTTP is to be implemented, supported and used.¶

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. 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.¶

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.¶

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.¶

• Message framing
• Multiplexing (in HTTP/2)
• Integration with TLS
• Support for intermediaries (proxies, gateways, Content Delivery Networks)
• 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.¶

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.¶

• Media types [RFC6838], often based upon a format convention such as JSON [RFC8259],
• HTTP header fields, as per Section 4.7, and
• The behaviour of resources, as identified by link relations [RFC8288].

• 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 body format;
• Data processing using POST and identified request and response body format(s); and
• Resource deletion using DELETE.

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 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.¶

• Redirect handling - Applications need to specify how redirects are expected to be handled; see Section 4.6.1.
• Cookies - Applications using HTTP MUST explicitly reference the Cookie specification [I-D.ietf-httpbis-rfc6265bis] if they are required.
• 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.¶

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.¶

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.¶

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.¶

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 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.¶

• Their content is useful to intermediaries (who often wish to avoid parsing the body), and/or
• Their content is useful to generic HTTP software (e.g., clients, servers), and/or
• 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 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).¶

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.¶

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 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).¶

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 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).¶

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.¶

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.¶

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.¶

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.¶

• Using an application-specific media type in the Content-Type header, 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 (such as HTML [HTML5]), 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
• Using the ‘HttpOnly’ flag on Cookies to assure that cookies are not exposed to browser scripting languages [I-D.ietf-httpbis-rfc6265bis]
• 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.¶

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.¶

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].¶

• 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.

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.¶

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

• Using a distinct link relation type [RFC8288] 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 body

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.¶

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.¶

7.1. Normative References

[I-D.ietf-httpbis-cache]

Fielding, R., Nottingham, M., and J. Reschke, “HTTP Caching”, Internet-Draft 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”, Internet-Draft draft-ietf-httpbis-messaging-03 (work in progress), October 2018.

[I-D.ietf-httpbis-semantics]

Fielding, R., Nottingham, M., and J. Reschke, “HTTP Semantics”, Internet-Draft 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>.

[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>.

[I-D.ietf-httpbis-header-structure]

Nottingham, M. and P. Kamp, “Structured Headers for HTTP”, Internet-Draft draft-ietf-httpbis-header-structure-08 (work in progress), October 2018.

[I-D.ietf-httpbis-rfc6265bis]

Barth, A. and M. West, “Cookies: HTTP State Management Mechanism”, Internet-Draft draft-ietf-httpbis-rfc6265bis-02 (work in progress), August 2017.

[I-D.nottingham-rfc5785bis]

Nottingham, M., “Well-Known Uniform Resource Identifiers (URIs)”, Internet-Draft 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>.

[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>.

[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>.