rfc9205v1.txt   rfc9205.txt 
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3.1. Generic Semantics 3.1. Generic Semantics
3.2. Links 3.2. Links
3.3. Rich Functionality 3.3. Rich Functionality
4. Best Practices for Specifying the Use of HTTP 4. Best Practices for Specifying the Use of HTTP
4.1. Specifying the Use of HTTP 4.1. Specifying the Use of HTTP
4.2. Specifying Server Behaviour 4.2. Specifying Server Behaviour
4.3. Specifying Client Behaviour 4.3. Specifying Client Behaviour
4.4. Specifying URLs 4.4. Specifying URLs
4.4.1. Discovering an Application's URLs 4.4.1. Discovering an Application's URLs
4.4.2. Considering URI Schemes 4.4.2. Considering URI Schemes
4.4.3. Transport Ports 4.4.3. Choosing Transport Ports
4.5. Using HTTP Methods 4.5. Using HTTP Methods
4.5.1. GET 4.5.1. GET
4.5.2. OPTIONS 4.5.2. OPTIONS
4.6. Using HTTP Status Codes 4.6. Using HTTP Status Codes
4.6.1. Redirection 4.6.1. Redirection
4.7. Specifying HTTP Header Fields 4.7. Specifying HTTP Header Fields
4.8. Defining Message Content 4.8. Defining Message Content
4.9. Leveraging HTTP Caching 4.9. Leveraging HTTP Caching
4.9.1. Freshness 4.9.1. Freshness
4.9.2. Stale Responses 4.9.2. Stale Responses
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ones? ones?
* How can the maximum value be extracted from the use of HTTP? * How can the maximum value be extracted from the use of HTTP?
* How does it coexist with other uses of HTTP -- especially Web * How does it coexist with other uses of HTTP -- especially Web
browsing? browsing?
* How can interoperability problems and "protocol dead ends" be * How can interoperability problems and "protocol dead ends" be
avoided? avoided?
This document contains best current practices for the specification Section 2 defines when this document applies, Section 3 surveys the
of such applications. Section 2 defines when it applies, Section 3 properties of HTTP that are important to preserve, and Section 4
surveys the properties of HTTP that are important to preserve, and contains best practices for the specification of applications that
Section 4 conveys best practices for specifying them. use HTTP.
It is written primarily to guide IETF efforts to define application It is written primarily to guide IETF efforts to define application
protocols using HTTP for deployment on the Internet but might be protocols using HTTP for deployment on the Internet but might be
applicable in other situations. Note that the requirements herein do applicable in other situations. Note that the requirements herein do
not necessarily apply to the development of generic HTTP extensions. not necessarily apply to the development of generic HTTP extensions.
This document obsoletes [RFC3205] to reflect the experience and This document obsoletes [RFC3205] to reflect the experience and
developments regarding HTTP in the intervening time. developments regarding HTTP in the intervening time.
1.1. Notational Conventions 1.1. Notational Conventions
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encouraged to adhere to its requirements. encouraged to adhere to its requirements.
2.1. Non-HTTP Protocols 2.1. Non-HTTP Protocols
An application can rely upon HTTP without meeting the criteria for An application can rely upon HTTP without meeting the criteria for
using it as defined above. For example, an application might wish to using it as defined above. For example, an application might wish to
avoid re-specifying parts of the message format but might change avoid re-specifying parts of the message format but might change
other aspects of the protocol's operation, or it might want to use other aspects of the protocol's operation, or it might want to use
application-specific methods. application-specific methods.
Doing so brings more freedom to modify protocol operations, but loses Doing so permits more freedom to modify protocol operations, but at
at least a portion of the benefits outlined in Section 3, as most least a portion of the benefits outlined in Section 3 are lost as
HTTP implementations won't be easily adaptable to these changes, and most HTTP implementations won't be easily adaptable to these changes.
the benefit of mindshare will be lost. The benefit of mindshare will also be lost.
Such specifications MUST NOT use HTTP's URI schemes, transport ports, Such specifications MUST NOT use HTTP's URI schemes, transport ports,
ALPN protocol IDs, or IANA registries; rather, they are encouraged to ALPN protocol IDs, or IANA registries; rather, they are encouraged to
establish their own. establish their own.
3. What's Important About HTTP 3. What's Important About HTTP
This section examines the characteristics of HTTP that are important This section examines the characteristics of HTTP that are important
to consider when using HTTP to define an application protocol. to consider when using HTTP to define an application protocol.
3.1. Generic Semantics 3.1. Generic Semantics
Much of the value of HTTP is in its generic semantics -- that is, the Much of the value of HTTP is in its generic semantics -- that is, the
protocol elements defined by HTTP are potentially applicable to every protocol elements defined by HTTP are potentially applicable to every
resource and are not specific to a particular context. Application- resource and are not specific to a particular context. Application-
specific semantics are best expressed in message content and in specific semantics are best expressed in message content and header
header fields, not status codes or methods (although the latter do fields, not status codes or methods (although status codes and
have generic semantics that relate to application state). methods do have generic semantics that relate to application state).
This generic/application-specific split allows an HTTP message to be This split between generic and application-specific semantics allows
handled by common software (e.g., HTTP servers, intermediaries, an HTTP message to be handled by common software (e.g., HTTP servers,
client implementations, and caches) without understanding the intermediaries, client implementations, and caches) without requiring
specific application. It also allows people to leverage their those implementations to understand the application in use. It also
knowledge of HTTP semantics without specialising them for a allows people to leverage their knowledge of HTTP semantics without
particular application. needing specialised knowledge of a particular application.
Therefore, applications that use HTTP MUST NOT redefine, refine, or Therefore, applications that use HTTP MUST NOT redefine, refine, or
overlay the semantics of generic protocol elements such as methods, overlay the semantics of generic protocol elements such as methods,
status codes, or existing header fields. Instead, they should focus status codes, or existing header fields. Instead, they should focus
their specifications on protocol elements that are specific to that their specifications on protocol elements that are specific to that
application -- namely, their HTTP resources. application -- namely, their HTTP resources.
When writing a specification, it's often tempting to specify exactly When writing a specification, it's often tempting to specify exactly
how HTTP is to be implemented, supported, and used. However, this how HTTP is to be implemented, supported, and used. However, this
can easily lead to an unintended profile of HTTP behaviour. For can easily lead to an unintended profile of HTTP behaviour. For
example, it's common to see specifications with language like this: example, it's common to see specifications with language like this:
A 'POST' request MUST result in a '201 Created' response. | A POST request MUST result in a 201 (Created) response.
This forms an expectation in the client that the response will always This forms an expectation in the client that the response will always
be 201 Created when in fact there are a number of reasons why the be 201 (Created) when in fact there are a number of reasons why the
status code might differ in a real deployment; for example, there status code might differ in a real deployment; for example, there
might be a proxy that requires authentication, or a server-side might be a proxy that requires authentication, or a server-side
error, or a redirection. If the client does not anticipate this, the error, or a redirection. If the client does not anticipate this, the
application's deployment is brittle. application's deployment is brittle.
See Section 4.2 for more details. See Section 4.2 for more details.
3.2. Links 3.2. Links
Another common practice is assuming that the HTTP server's namespace Another common practice is assuming that the HTTP server's namespace
(or a portion thereof) is exclusively for the use of a single (or a portion thereof) is exclusively for the use of a single
application. This effectively overlays special, application-specific application. This effectively overlays special, application-specific
semantics onto that space and precludes other applications from using semantics onto that space and precludes other applications from using
it. it.
As explained in [RFC8820], such "squatting" on a part of the URL As explained in [BCP190], such "squatting" on a part of the URL space
space by a standard usurps the server's authority over its own by a standard usurps the server's authority over its own resources,
resources, can cause deployment issues, and is therefore bad practice can cause deployment issues, and is therefore bad practice in
in standards. standards.
Instead of statically defining URI components like paths, it is Instead of statically defining URI components like paths, it is
RECOMMENDED that applications using HTTP define and use links RECOMMENDED that applications using HTTP define and use links
[WEB-LINKING] to allow flexibility in deployment. [WEB-LINKING] to allow flexibility in deployment.
Using runtime links in this fashion has a number of other benefits -- Using runtime links in this fashion has a number of other benefits --
especially when an application is to have multiple implementations especially when an application is to have multiple implementations
and/or deployments (as is often the case for those that are and/or deployments (as is often the case for those that are
standardised). standardised).
For example, navigating with a link allows a request to be routed to For example, navigating with a link allows a request to be routed to
a different server without the overhead of a redirection, thereby a different server without the overhead of a redirection, thereby
supporting deployment across machines well. supporting deployment across machines well.
It also becomes possible to "mix and match" different applications on By using links, it also becomes possible to "mix and match" different
the same server and offers a natural mechanism for extensibility, applications on the same server. The use of links also offers a
versioning, and capability management since the document containing natural mechanism for extensibility, versioning, and capability
the links can also contain information about their targets. management because the document containing the links can also contain
information about their targets.
Using links also offers a form of cache invalidation that's seen on Using links also offers a form of cache invalidation that's seen on
the Web; when a resource's state changes, the application can change the Web; when a resource's state changes, the application can change
its link to it so that a fresh copy is always fetched. the affected links so that a fresh copy is always fetched.
3.3. Rich Functionality 3.3. Rich Functionality
HTTP offers a number of features to applications, such as: HTTP offers a number of features to applications, such as:
* Message framing * Message framing
* Multiplexing (in HTTP/2 [HTTP2] and HTTP/3 [HTTP3]) * Multiplexing (in HTTP/2 [HTTP2] and HTTP/3 [HTTP3])
* Integration with TLS * Integration with TLS
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and reliability and reliability
* Granularity of access control (through use of a rich space of * Granularity of access control (through use of a rich space of
URLs) URLs)
* Partial content to selectively request part of a response * Partial content to selectively request part of a response
* The ability to interact with the application easily using a Web * The ability to interact with the application easily using a Web
browser browser
Applications that use HTTP are encouraged to utilise the various An application that uses HTTP is encouraged to utilise the various
features that the protocol offers so that their users receive the features that the protocol offers so that its users receive the
maximum benefit from it and to allow it to be deployed in a variety maximum benefit from those features and so that the application can
of situations. This document does not require specific features to be deployed in a variety of situations. This document does not
be used since the appropriate design trade-offs are highly specific require specific features to be used since the appropriate design
to a given situation. However, following the practices in Section 4 trade-offs are highly specific to a given situation. However,
is a good starting point. following the practices in Section 4 is a good starting point.
4. Best Practices for Specifying the Use of HTTP 4. Best Practices for Specifying the Use of HTTP
This section contains best practices for specifying the use of HTTP This section contains best practices for specifying the use of HTTP
by applications, including practices for specific HTTP protocol by applications, including practices for specific HTTP protocol
elements. elements.
4.1. Specifying the Use of HTTP 4.1. Specifying the Use of HTTP
Specifications should use [HTTP] as the primary reference for HTTP; Specifications should use [HTTP] as the primary reference for HTTP;
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* HTTP header fields, per Section 4.7; and * HTTP header fields, per Section 4.7; and
* The behaviour of resources, as identified by link relations * The behaviour of resources, as identified by link relations
[WEB-LINKING]. [WEB-LINKING].
An application can define its operation by composing these protocol An application can define its operation by composing these protocol
elements to define a set of resources that are identified by link elements to define a set of resources that are identified by link
relations and that implement specified behaviours, including: relations and that implement specified behaviours, including:
* retrieval of their state using GET in one or more formats * retrieval of resource state using GET in one or more formats
identified by media type; identified by media type;
* resource creation or update using POST or PUT, with an * resource creation or update using POST or PUT, with an
appropriately identified request content format; appropriately identified request content format;
* data processing using POST and identified request and response * data processing using POST and identified request and response
content format(s); and content format(s); and
* Resource deletion using DELETE. * Resource deletion using DELETE.
For example, an application might specify: For example, an application might specify:
Resources linked to with the "example-widget" link relation type are | Resources linked to with the "example-widget" link relation type
Widgets. The state of a Widget can be fetched in the | are Widgets. The state of a Widget can be fetched in the
"application/example-widget+json" format, and can be updated by PUT | "application/example-widget+json" format, and can be updated by
to the same link. Widget resources can be deleted. | PUT to the same link. Widget resources can be deleted.
|
The "Example-Count" response header field on Widget representations | The Example-Count response header field on Widget representations
indicates how many Widgets are held by the sender. | indicates how many Widgets are held by the sender.
|
The "application/example-widget+json" format is a JSON [RFC8259] | The "application/example-widget+json" format is a JSON [RFC8259]
format representing the state of a Widget. It contains links to | format representing the state of a Widget. It contains links to
related information in the link indicated by the Link header field | related information in the link indicated by the Link header field
value with the "example-other-info" link relation type. | value with the "example-other-info" link relation type.
Applications can also specify the use of URI Templates [URI-TEMPLATE] Applications can also specify the use of URI Templates [URI-TEMPLATE]
to allow clients to generate URLs based upon runtime data. to allow clients to generate URLs based upon runtime data.
4.3. Specifying Client Behaviour 4.3. Specifying Client Behaviour
An application's expectations for client behaviour ought to be An application's expectations for client behaviour ought to be
closely aligned with those of Web browsers to avoid interoperability closely aligned with those of Web browsers to avoid interoperability
issues when they are used. issues when they are used.
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Redirect handling: Applications need to specify how redirects are Redirect handling: Applications need to specify how redirects are
expected to be handled; see Section 4.6.1. expected to be handled; see Section 4.6.1.
Cookies: Applications using HTTP should explicitly reference the Cookies: Applications using HTTP should explicitly reference the
Cookie specification [COOKIES] if they are required. Cookie specification [COOKIES] if they are required.
Certificates: Applications using HTTP should specify that TLS Certificates: Applications using HTTP should specify that TLS
certificates are to be checked according to Section 4.3.4 of certificates are to be checked according to Section 4.3.4 of
[HTTP] when HTTPS is used. [HTTP] when HTTPS is used.
Applications using HTTP should not statically require HTTP features Applications using HTTP should not require that clients statically
that are usually negotiated to be supported by clients. For example, support HTTP features that are usually negotiated. For example,
requiring that clients support responses with a certain content requiring that clients support responses with a certain content
coding ([HTTP], Section 8.4.1) instead of negotiating for it ([HTTP], coding ([HTTP], Section 8.4.1) instead of negotiating for it ([HTTP],
Section 12.5.3) means that otherwise conformant clients cannot Section 12.5.3) means that otherwise conformant clients cannot
interoperate with the application. Applications can encourage the interoperate with the application. Applications can encourage the
implementation of such features, though. implementation of such features, though.
4.4. Specifying URLs 4.4. Specifying URLs
In HTTP, the resources that clients interact with are identified with In HTTP, the resources that clients interact with are identified with
URLs [URL]. As [RFC8820] explains, parts of the URL are designed to URLs [URL]. As [BCP190] explains, parts of the URL are designed to
be under the control of the owner (also known as the "authority") of be under the control of the owner (also known as the "authority") of
that server to give them the flexibility in deployment. that server to give them the flexibility in deployment.
This means that in most cases, specifications for applications that This means that in most cases, specifications for applications that
use HTTP won't contain fixed application URLs or paths; while it is use HTTP won't contain fixed application URLs or paths; while it is
common practice for a specification of a single-deployment API to common practice for a specification of a single-deployment API to
specify the path prefix "/app/v1" (for example), doing so in an IETF specify the path prefix "/app/v1" (for example), doing so in an IETF
specification is inappropriate. specification is inappropriate.
Therefore, the specification writer needs some mechanism to allow Therefore, the specification writer needs some mechanism to allow
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The most straightforward mechanism for URL discovery is to configure The most straightforward mechanism for URL discovery is to configure
the client with (or otherwise convey to it) a full URL. This might the client with (or otherwise convey to it) a full URL. This might
be done in a configuration document or through another discovery be done in a configuration document or through another discovery
mechanism. mechanism.
However, if the client only knows the server's hostname and the However, if the client only knows the server's hostname and the
identity of the application, there needs to be some way to derive the identity of the application, there needs to be some way to derive the
initial URL from that information. initial URL from that information.
An application cannot define a fixed prefix for its URL paths; see An application cannot define a fixed prefix for its URL paths; see
[RFC8820]. Instead, a specification for such an application can use [BCP190]. Instead, a specification for such an application can use
one of the following strategies: one of the following strategies:
* Register a well-known URI [WELL-KNOWN-URI] as an entry point for * Register a well-known URI [WELL-KNOWN-URI] as an entry point for
that application. This provides a fixed path on every potential that application. This provides a fixed path on every potential
server that will not collide with other applications. server that will not collide with other applications.
* Enable the server authority to convey a URI Template * Enable the server authority to convey a URI Template
[URI-TEMPLATE] or similar mechanism for generating a URL for an [URI-TEMPLATE] or similar mechanism for generating a URL for an
entry point. For example, this might be done in a configuration entry point. For example, this might be done in a configuration
document or other artefact. document or other artefact.
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might or might not work correctly, depending on how they are might or might not work correctly, depending on how they are
defined and implemented. Generally, they are designed and defined and implemented. Generally, they are designed and
implemented with an assumption that the URL will always be "http" implemented with an assumption that the URL will always be "http"
or "https". or "https".
* Web features that require a secure context [SECCTXT] will likely * Web features that require a secure context [SECCTXT] will likely
treat a new scheme as insecure. treat a new scheme as insecure.
See [RFC7595] for more information about minting new URI schemes. See [RFC7595] for more information about minting new URI schemes.
4.4.3. Transport Ports 4.4.3. Choosing Transport Ports
Applications can use the applicable default port (80 for HTTP, 443 Applications can use the applicable default port (80 for HTTP, 443
for HTTPS), or they can be deployed upon other ports. This decision for HTTPS), or they can be deployed upon other ports. This decision
can be made at deployment time or might be encouraged by the can be made at deployment time or might be encouraged by the
application's specification (e.g., by registering a port for that application's specification (e.g., by registering a port for that
application). application).
If a non-default port is used, it needs to be reflected in the If a non-default port is used, it needs to be reflected in the
authority of all URLs for that resource; the only mechanism for authority of all URLs for that resource; the only mechanism for
changing a default port is changing the URI scheme (see changing a default port is changing the URI scheme (see
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New HTTP methods are rare; they are required to be registered in the New HTTP methods are rare; they are required to be registered in the
"HTTP Method Registry" with IETF Review (see [HTTP]) and are also "HTTP Method Registry" with IETF Review (see [HTTP]) and are also
required to be generic. That means that they need to be potentially required to be generic. That means that they need to be potentially
applicable to all resources, not just those of one application. applicable to all resources, not just those of one application.
While historically some applications (e.g., [RFC4791]) have defined While historically some applications (e.g., [RFC4791]) have defined
non-generic methods, [HTTP] now forbids this. non-generic methods, [HTTP] now forbids this.
When authors believe that a new method is required, they are When authors believe that a new method is required, they are
encouraged to engage with the HTTP community early (e.g., on the encouraged to engage with the HTTP community early (e.g., on the
<ietf-http-wg@w3.org> mailing list) and document their proposal as a <mailto:ietf-http-wg@w3.org> mailing list) and document their
separate HTTP extension rather than as part of an application's proposal as a separate HTTP extension rather than as part of an
specification. application's specification.
4.5.1. GET 4.5.1. GET
GET is the most common and useful HTTP method; its retrieval GET is the most common and useful HTTP method; its retrieval
semantics allow caching and side-effect free linking and underlie semantics allow caching and side-effect free linking and underlie
many of the benefits of using HTTP. many of the benefits of using HTTP.
Queries can be performed with GET, often using the query component of Queries can be performed with GET, often using the query component of
the URL; this is a familiar pattern from Web browsing, and the the URL; this is a familiar pattern from Web browsing, and the
results can be cached, improving the efficiency of an often expensive results can be cached, improving the efficiency of an often expensive
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requests. Additionally, some HTTP implementations limit the size of requests. Additionally, some HTTP implementations limit the size of
URLs they support, although modern HTTP software has much more URLs they support, although modern HTTP software has much more
generous limits than previously (typically, considerably more than generous limits than previously (typically, considerably more than
8000 octets, as required by [HTTP]). 8000 octets, as required by [HTTP]).
In these cases, an application using HTTP might consider using POST In these cases, an application using HTTP might consider using POST
to express queries in the request's content; doing so avoids encoding to express queries in the request's content; doing so avoids encoding
overhead and URL length limits in implementations. However, in doing overhead and URL length limits in implementations. However, in doing
so, it should be noted that the benefits of GET such as caching and so, it should be noted that the benefits of GET such as caching and
linking to query results are lost. Therefore, applications using linking to query results are lost. Therefore, applications using
HTTP that feel a need to allow POST queries ought to consider HTTP that require support for POST queries ought to consider allowing
allowing both methods. both methods.
Processing of GET requests should not change the application state or Processing of GET requests should not change the application's state
have other side effects that might be significant to the client since or have other side effects that might be significant to the client
implementations can and do retry HTTP GET requests that fail and some since implementations can and do retry HTTP GET requests that fail
GET requests protected by TLS early data might be vulnerable to and some GET requests protected by TLS early data might be vulnerable
replay attacks (see [RFC8470]). Note that this does not include to replay attacks (see [RFC8470]). Note that this does not include
logging and similar functions; see [HTTP], Section 9.2.1. logging and similar functions; see [HTTP], Section 9.2.1.
Finally, note that while the generic HTTP syntax allows a GET request Finally, note that while the generic HTTP syntax allows a GET request
message to contain content, the purpose is to allow message parsers message to contain content, the purpose is to allow message parsers
to be generic; per [HTTP], Section 9.3.1, content in a GET is not to be generic; per [HTTP], Section 9.3.1, content in a GET is not
recommended, has no meaning, and will be either ignored or rejected recommended, has no meaning, and will be either ignored or rejected
by generic HTTP software (such as intermediaries, caches, servers, by generic HTTP software (such as intermediaries, caches, servers,
and client libraries). and client libraries).
4.5.2. OPTIONS 4.5.2. OPTIONS
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* It isn't possible to link to the metadata with a simple URL * It isn't possible to link to the metadata with a simple URL
because OPTIONS is not the default method. because OPTIONS is not the default method.
* OPTIONS responses are not cacheable because HTTP caches operate on * OPTIONS responses are not cacheable because HTTP caches operate on
representations of the resource (i.e., GET and HEAD). If OPTIONS representations of the resource (i.e., GET and HEAD). If OPTIONS
responses are cached separately, their interactions with the HTTP responses are cached separately, their interactions with the HTTP
cache expiry, secondary keys, and other mechanisms need to be cache expiry, secondary keys, and other mechanisms need to be
considered. considered.
* OPTIONS is "chatty" -- always separating metadata out into a * OPTIONS is "chatty" -- requesting metadata separately increases
separate request increases the number of requests needed to the number of requests needed to interact with the application.
interact with the application.
* Implementation support for OPTIONS is not universal; some servers * Implementation support for OPTIONS is not universal; some servers
do not expose the ability to respond to OPTIONS requests without do not expose the ability to respond to OPTIONS requests without
significant effort. significant effort.
Instead of OPTIONS, one of these alternative approaches might be more Instead of OPTIONS, one of these alternative approaches might be more
appropriate: appropriate:
* For server-wide metadata, create a well-known URI [WELL-KNOWN-URI] * For server-wide metadata, create a well-known URI [WELL-KNOWN-URI]
or use an already existing one if appropriate (e.g., host-meta or use an already existing one if appropriate (e.g., host-meta
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To distinguish between multiple error conditions that are mapped to To distinguish between multiple error conditions that are mapped to
the same status code and to avoid the misattribution issue outlined the same status code and to avoid the misattribution issue outlined
above, applications using HTTP should convey finer-grained error above, applications using HTTP should convey finer-grained error
information in the response's message content and/or header fields. information in the response's message content and/or header fields.
[PROBLEM-DETAILS] provides one way to do so. [PROBLEM-DETAILS] provides one way to do so.
Because the set of registered HTTP status codes can expand, Because the set of registered HTTP status codes can expand,
applications using HTTP should explicitly point out that clients applications using HTTP should explicitly point out that clients
ought to be able to handle all applicable status codes gracefully ought to be able to handle all applicable status codes gracefully
(i.e., falling back to the generic n00 semantics of a given status (i.e., falling back to the generic n00 semantics of a given status
code; e.g., 499 can be safely handled as 400 by clients that don't code; e.g., 499 can be safely handled as 400 (Bad Request) by clients
recognise it). This is preferable to creating a "laundry list" of that don't recognise it). This is preferable to creating a "laundry
potential status codes since such a list won't be complete in the list" of potential status codes since such a list won't be complete
foreseeable future. in the foreseeable future.
Applications using HTTP MUST NOT re-specify the semantics of HTTP Applications using HTTP MUST NOT re-specify the semantics of HTTP
status codes, even if it is only by copying their definition. It is status codes, even if it is only by copying their definition. It is
NOT RECOMMENDED they require specific reason phrases to be used; the NOT RECOMMENDED they require specific reason phrases to be used; the
reason phrase has no function in HTTP, is not guaranteed to be reason phrase has no function in HTTP, is not guaranteed to be
preserved by implementations, and is not carried at all in the HTTP/2 preserved by implementations, and is not carried at all in the HTTP/2
[HTTP2] message format. [HTTP2] message format.
Applications MUST only use registered HTTP status codes. As with Applications MUST only use registered HTTP status codes. As with
methods, new HTTP status codes are rare and required (by [HTTP]) to methods, new HTTP status codes are rare and required (by [HTTP]) to
be registered with IETF Review. Similarly, HTTP status codes are be registered with IETF Review. Similarly, HTTP status codes are
generic; they are required (by [HTTP]) to be potentially applicable generic; they are required (by [HTTP]) to be potentially applicable
to all resources, not just to those of one application. to all resources, not just to those of one application.
When authors believe that a new status code is required, they are When authors believe that a new status code is required, they are
encouraged to engage with the HTTP community early (e.g., on the encouraged to engage with the HTTP community early (e.g., on the
<ietf-http-wg@w3.org> mailing list) and document their proposal as a <mailto:ietf-http-wg@w3.org> mailing list) and document their
separate HTTP extension, rather than as part of an application's proposal as a separate HTTP extension, rather than as part of an
specification. application's specification.
4.6.1. Redirection 4.6.1. Redirection
The 3xx series of status codes specified in Section 15.4 of [HTTP] The 3xx series of status codes specified in Section 15.4 of [HTTP]
directs the user agent to another resource to satisfy the request. directs the user agent to another resource to satisfy the request.
The most common of these are 301, 302, 307, and 308, all of which use The most common of these are 301, 302, 307, and 308, all of which use
the Location response header field to indicate where the client the Location response header field to indicate where the client
should resend the request. should resend the request.
There are two ways that the members of this group of status codes There are two ways that the members of this group of status codes
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See Section 16.3.2 of [HTTP] for guidelines to consider when minting See Section 16.3.2 of [HTTP] for guidelines to consider when minting
new header fields. [STRUCTURED-FIELDS] provides a common structure new header fields. [STRUCTURED-FIELDS] provides a common structure
for new header fields and avoids many issues in their parsing and for new header fields and avoids many issues in their parsing and
handling; it is RECOMMENDED that new header fields use it. handling; it is RECOMMENDED that new header fields use it.
It is RECOMMENDED that header field names be short (even when field It is RECOMMENDED that header field names be short (even when field
compression is used, there is an overhead) but appropriately compression is used, there is an overhead) but appropriately
specific. In particular, if a header field is specific to an specific. In particular, if a header field is specific to an
application, an identifier for that application can form a prefix to application, an identifier for that application can form a prefix to
the header field name, separated by a "-". the header field name, separated by a hyphen.
For example, if the "example" application needs to create three For example, if the "example" application needs to create three
header fields, they might be called "example-foo", "example-bar", and header fields, they might be called "example-foo", "example-bar", and
"example-baz". Note that the primary motivation here is to avoid "example-baz". Note that the primary motivation here is to avoid
consuming more generic field names, not to reserve a portion of the consuming more generic field names, not to reserve a portion of the
namespace for the application; see [RFC6648] for related namespace for the application; see [RFC6648] for related
considerations. considerations.
The semantics of existing HTTP header fields MUST NOT be redefined The semantics of existing HTTP header fields MUST NOT be redefined
without updating their registration or defining an extension to them without updating their registration or defining an extension to them
(if allowed). For example, an application using HTTP cannot specify (if allowed). For example, an application using HTTP cannot specify
that the Location header field has a special meaning in a certain that the Location header field has a special meaning in a certain
context. context.
See Section 4.9 for the interaction between header fields and HTTP See Section 4.9 for the interaction between header fields and HTTP
caching; in particular, request header fields that are used to caching; in particular, request header fields that are used to choose
"select" a response have impact there and need to be carefully (per Section 4.1 of [HTTP-CACHING]) a response have impact there and
considered. need to be carefully considered.
See Section 4.10 for considerations regarding header fields that See Section 4.10 for considerations regarding header fields that
carry application state (e.g., Cookie). carry application state (e.g., Cookie).
4.8. Defining Message Content 4.8. Defining Message Content
Common syntactic conventions for message contents include JSON Common syntactic conventions for message contents include JSON
[JSON], XML [XML], and Concise Binary Object Representation (CBOR) [JSON], XML [XML], and Concise Binary Object Representation (CBOR)
[RFC8949]. Best practices for their use are out of scope for this [RFC8949]. Best practices for their use are out of scope for this
document. document.
skipping to change at line 910 skipping to change at line 910
Assigning even a short freshness lifetime ([HTTP-CACHING], Assigning even a short freshness lifetime ([HTTP-CACHING],
Section 4.2) -- e.g., 5 seconds -- allows a response to be reused to Section 4.2) -- e.g., 5 seconds -- allows a response to be reused to
satisfy multiple clients and/or a single client making the same satisfy multiple clients and/or a single client making the same
request repeatedly. In general, if it is safe to reuse something, request repeatedly. In general, if it is safe to reuse something,
consider assigning a freshness lifetime. consider assigning a freshness lifetime.
The most common method for specifying freshness is the max-age The most common method for specifying freshness is the max-age
response directive ([HTTP-CACHING], Section 5.2.2.1). The Expires response directive ([HTTP-CACHING], Section 5.2.2.1). The Expires
header field ([HTTP-CACHING], Section 5.3) can also be used, but it header field ([HTTP-CACHING], Section 5.3) can also be used, but it
is not necessary; all modern cache implementations support Cache- is not necessary; all modern cache implementations support the Cache-
Control, and specifying freshness as a delta is usually more Control header field, and specifying freshness as a delta is usually
convenient and less error-prone. more convenient and less error-prone.
It is not necessary to add the "public" response directive It is not necessary to add the public response directive
([HTTP-CACHING], Section 5.2.2.9) to cache most responses; it is only ([HTTP-CACHING], Section 5.2.2.9) to cache most responses; it is only
necessary when it's desirable to store an authenticated response, or necessary when it's desirable to store an authenticated response, or
when the status code isn't understood by the cache and there isn't when the status code isn't understood by the cache and there isn't
explicit freshness information available. explicit freshness information available.
In some situations, responses without explicit cache freshness In some situations, responses without explicit cache freshness
directives will be stored and served using a heuristic freshness directives will be stored and served using a heuristic freshness
lifetime; see [HTTP-CACHING], Section 4.2.2. As the heuristic is not lifetime; see [HTTP-CACHING], Section 4.2.2. As the heuristic is not
under the control of the application, it is generally preferable to under the control of the application, it is generally preferable to
set an explicit freshness lifetime or make the response explicitly set an explicit freshness lifetime or make the response explicitly
uncacheable. uncacheable.
If caching of a response is not desired, the appropriate response If caching of a response is not desired, the appropriate cache
directive is "Cache-Control: no-store". Other directives are not response directive is no-store. Other directives are not necessary,
necessary, and no-store only needs to be sent in situations where the and no-store only needs to be sent in situations where the response
response might be cached; see [HTTP-CACHING], Section 3. Note that might be cached; see [HTTP-CACHING], Section 3. Note that the no-
"Cache-Control: no-cache" allows a response to be stored, just not cache directive allows a response to be stored, just not reused by a
reused by a cache without validation; it does not prevent caching cache without validation; it does not prevent caching (despite its
(despite its name). name).
For example, this response cannot be stored or reused by a cache: For example, this response cannot be stored or reused by a cache:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/example+xml Content-Type: application/example+xml
Cache-Control: no-store Cache-Control: no-store
[content] [content]
4.9.2. Stale Responses 4.9.2. Stale Responses
Authors should understand that stale responses (e.g., with "Cache- Authors should understand that stale responses (e.g., with Cache-
Control: max-age=0") can be reused by caches when disconnected from Control: max-age=0) can be reused by caches when disconnected from
the origin server; this can be useful for handling network issues. the origin server; this can be useful for handling network issues.
If doing so is not suitable for a given response, the origin should If doing so is not suitable for a given response, the origin should
use "Cache-Control: must-revalidate". See Section 4.2.4 of send the must-revalidate cache directive. See Section 4.2.4 of
[HTTP-CACHING] and also [RFC5861] for additional controls over stale [HTTP-CACHING] and also [RFC5861] for additional controls over stale
content. content.
Stale responses can be refreshed by assigning a validator, saving Stale responses can be refreshed by assigning a validator, saving
both transfer bandwidth and latency for large responses; see both transfer bandwidth and latency for large responses; see
Section 13 of [HTTP]. Section 13 of [HTTP].
4.9.3. Caching and Application Semantics 4.9.3. Caching and Application Semantics
When an application has a need to express a lifetime that's separate When an application has a need to express a lifetime that's separate
skipping to change at line 981 skipping to change at line 981
[HTTP-CACHING], Section 4.2.3). [HTTP-CACHING], Section 4.2.3).
One way to address this is to explicitly specify that responses need One way to address this is to explicitly specify that responses need
to be fresh upon use. to be fresh upon use.
4.9.4. Varying Content Based Upon the Request 4.9.4. Varying Content Based Upon the Request
If an application uses a request header field to change the If an application uses a request header field to change the
response's header fields or content, authors should point out that response's header fields or content, authors should point out that
this has implications for caching; in general, such resources need to this has implications for caching; in general, such resources need to
either make their responses uncacheable (e.g., with the "no-store" either make their responses uncacheable (e.g., with the no-store
cache-control directive defined in [HTTP-CACHING], Section 5.2.2.5) cache directive defined in [HTTP-CACHING], Section 5.2.2.5) or send
or send the Vary response header field ([HTTP], Section 12.5.5) on the Vary response header field ([HTTP], Section 12.5.5) on all
all responses from that resource (including the "default" response). responses from that resource (including the "default" response).
For example, this response: For example, this response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/example+xml Content-Type: application/example+xml
Cache-Control: max-age=60 Cache-Control: max-age=60
ETag: "sa0f8wf20fs0f" ETag: "sa0f8wf20fs0f"
Vary: Accept-Encoding Vary: Accept-Encoding
[content] [content]
skipping to change at line 1032 skipping to change at line 1032
HTTP/2 [HTTP2] and HTTP/3 [HTTP3] offer multiplexing to applications, HTTP/2 [HTTP2] and HTTP/3 [HTTP3] offer multiplexing to applications,
removing the need to use multiple connections. However, application removing the need to use multiple connections. However, application
performance can still be significantly affected by how the server performance can still be significantly affected by how the server
chooses to prioritize responses. Depending on the application, it chooses to prioritize responses. Depending on the application, it
might be best for the server to determine the priority of responses might be best for the server to determine the priority of responses
or for the client to hint its priorities to the server (see, e.g., or for the client to hint its priorities to the server (see, e.g.,
[HTTP-PRIORITY]). [HTTP-PRIORITY]).
In all versions of HTTP, requests are made independently -- you can't In all versions of HTTP, requests are made independently -- you can't
rely on the relative order of two requests to guarantee the rely on the relative order of two requests to guarantee their
processing order. This is because they might be sent over a processing order. This is because they might be sent over a
multiplexed protocol by an intermediary or sent to different origin multiplexed protocol by an intermediary or sent to different origin
servers, or the server might even perform processing in a different servers, or the server might even perform processing in a different
order. If two requests need strict ordering, the only reliable way order. If two requests need strict ordering, the only reliable way
to ensure the outcome is to issue the second request when the final to ensure the outcome is to issue the second request when the final
response to the first has begun. response to the first has begun.
Applications MUST NOT make assumptions about the relationship between Applications MUST NOT make assumptions about the relationship between
separate requests on a single transport connection; doing so breaks separate requests on a single transport connection; doing so breaks
many of the assumptions of HTTP as a stateless protocol and will many of the assumptions of HTTP as a stateless protocol and will
skipping to change at line 1059 skipping to change at line 1059
identify clients. Per [RFC7617], the Basic authentication scheme is identify clients. Per [RFC7617], the Basic authentication scheme is
not suitable for protecting sensitive or valuable information unless not suitable for protecting sensitive or valuable information unless
the channel is secure (e.g., using the "https" URI scheme). the channel is secure (e.g., using the "https" URI scheme).
Likewise, [RFC7616] requires the Digest authentication scheme to be Likewise, [RFC7616] requires the Digest authentication scheme to be
used over a secure channel. used over a secure channel.
With HTTPS, clients might also be authenticated using certificates With HTTPS, clients might also be authenticated using certificates
[RFC8446], but note that such authentication is intrinsically scoped [RFC8446], but note that such authentication is intrinsically scoped
to the underlying transport connection. As a result, a client has no to the underlying transport connection. As a result, a client has no
way of knowing whether the authenticated status was used in preparing way of knowing whether the authenticated status was used in preparing
the response (though "Vary: *" and/or "Cache-Control: private" can the response (though Vary: * and/or the private cache directive can
provide a partial indication), and the only way to obtain a provide a partial indication), and the only way to obtain a
specifically unauthenticated response is to open a new connection. specifically unauthenticated response is to open a new connection.
When used, it is important to carefully specify the scoping and use When used, it is important to carefully specify the scoping and use
of authentication; if the application exposes sensitive data or of authentication; if the application exposes sensitive data or
capabilities (e.g., by acting as an ambient authority; see capabilities (e.g., by acting as an ambient authority; see
Section 8.3 of [RFC6454]), exploits are possible. Mitigations Section 8.3 of [RFC6454]), exploits are possible. Mitigations
include using a request-specific token to ensure the intent of the include using a request-specific token to ensure the intent of the
client. client.
skipping to change at line 1148 skipping to change at line 1148
If an application has browser compatibility as a goal, client If an application has browser compatibility as a goal, client
interaction ought to be defined in terms of [FETCH] since that is the interaction ought to be defined in terms of [FETCH] since that is the
abstraction that browsers use for HTTP; it enforces many of these abstraction that browsers use for HTTP; it enforces many of these
best practices. best practices.
4.14. Maintaining Application Boundaries 4.14. Maintaining Application Boundaries
Because many HTTP capabilities are scoped to the origin [RFC6454], Because many HTTP capabilities are scoped to the origin [RFC6454],
applications also need to consider how deployments might interact applications also need to consider how deployments might interact
with other applications (including Web browsing) with the same with other applications (including Web browsing) that use the same
origin. origin server.
For example, if cookies [COOKIES] are used to carry application For example, if cookies [COOKIES] are used to carry application
state, they will be sent with all requests to the origin by default state, they will be sent with all requests to the origin by default
(unless scoped by path), and the application might receive cookies (unless scoped by path), and the application might receive cookies
from other applications on the origin. This can lead to security from other applications on the origin server. This can lead to
issues as well as collision in cookie names. security issues as well as collision in cookie names.
One solution to these issues is to require a dedicated hostname for One solution to these issues is to require a dedicated hostname for
the application so that it has a unique origin. However, it is often the application so that it has a unique origin. However, it is often
desirable to allow multiple applications to be deployed on a single desirable to allow multiple applications to be deployed on a single
hostname; doing so provides the most deployment flexibility and hostname; doing so provides the most deployment flexibility and
enables them to be "mixed" together (see [RFC8820] for details). enables them to be "mixed" together (see [BCP190] for details).
Therefore, applications using HTTP should strive to allow multiple
applications on an origin.
To enable this, when specifying the use of cookies, HTTP Therefore, applications using HTTP should strive to allow multiple
authentication realms [HTTP], or other origin-wide HTTP mechanisms, applications on an origin. Specifically, when specifying the use of
applications using HTTP should not mandate the use of a particular cookies, HTTP authentication realms [HTTP], or other origin-wide HTTP
name but instead let deployments configure them. Consideration mechanisms, applications using HTTP should not mandate the use of a
should be given to scoping them to part of the origin, using their particular name but instead let deployments configure them.
specified mechanisms for doing so. 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 Modern Web browsers constrain the ability of content from one origin
to access resources from another to avoid leaking private to access resources from another to avoid leaking private
information. As a result, applications that wish to expose cross- information. As a result, applications that wish to expose cross-
origin data to browsers will need to implement the CORS protocol; see origin data to browsers will need to implement the CORS protocol; see
[FETCH]. [FETCH].
4.15. Using Server Push 4.15. Using Server Push
HTTP/2 added the ability for servers to "push" request/response pairs HTTP/2 added the ability for servers to "push" request/response pairs
skipping to change at line 1321 skipping to change at line 1320
be used to profile the underlying hardware, creating a unique be used to profile the underlying hardware, creating a unique
identifier for the system. Applications are advised to avoid identifier for the system. Applications are advised to avoid
allowing the use of mobile code where possible; when it cannot be allowing the use of mobile code where possible; when it cannot be
avoided, the resulting system's security properties need be carefully avoided, the resulting system's security properties need be carefully
scrutinised. scrutinised.
7. References 7. References
7.1. Normative References 7.1. Normative References
[BCP190] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 8820, DOI 10.17487/RFC8820, June 2020,
<https://www.rfc-editor.org/rfc/rfc8820>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", RFC 9110, DOI 10.17487/RFC9110, Ed., "HTTP Semantics", RFC 9110, DOI 10.17487/RFC9110,
February 2022, <https://www.rfc-editor.org/info/rfc9110>. February 2022, <https://www.rfc-editor.org/info/rfc9110>.
[HTTP-CACHING] [HTTP-CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", RFC 9111, DOI 10.17487/RFC9111, Ed., "HTTP Caching", RFC 9111, DOI 10.17487/RFC9111,
February 2022, <https://www.rfc-editor.org/info/rfc9111>. February 2022, <https://www.rfc-editor.org/info/rfc9111>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
skipping to change at line 1354 skipping to change at line 1357
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013, RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>. <https://www.rfc-editor.org/info/rfc6838>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190, [STRUCTURED-FIELDS]
RFC 8820, DOI 10.17487/RFC8820, June 2020, Nottingham, M. and P-H. Kamp, "Structured Field Values for
<https://www.rfc-editor.org/info/rfc8820>. HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
[URL] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [URL] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005, RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>. <https://www.rfc-editor.org/info/rfc3986>.
[WEB-LINKING] [WEB-LINKING]
Nottingham, M., "Web Linking", RFC 8288, Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017, DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>. <https://www.rfc-editor.org/info/rfc8288>.
skipping to change at line 1380 skipping to change at line 1384
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019, (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>. <https://www.rfc-editor.org/info/rfc8615>.
7.2. Informative References 7.2. Informative References
[COOKIES] Barth, A., "HTTP State Management Mechanism", RFC 6265, [COOKIES] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011, DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>. <https://www.rfc-editor.org/info/rfc6265>.
[CSP] West, M., "Content Security Policy Level 3", W3C Working [CSP] West, M., "Content Security Policy Level 3", W3C Working
Draft WD-CSP3-20160913, 13 September 2016, Draft, June 2021,
<https://www.w3.org/TR/2016/WD-CSP3-20160913>. <https://www.w3.org/TR/2021/WD-CSP3-20210629>.
[FETCH] WHATWG, "Fetch - Living Standard", [FETCH] WHATWG, "Fetch - Living Standard",
<https://fetch.spec.whatwg.org>. <https://fetch.spec.whatwg.org>.
[HTML] WHATWG, "HTML - Living Standard", [HTML] WHATWG, "HTML - Living Standard",
<https://html.spec.whatwg.org>. <https://html.spec.whatwg.org>.
[HTTP-PRIORITY] [HTTP-PRIORITY]
Oku, K. and L. Pardue, "Extensible Prioritization Scheme Oku, K. and L. Pardue, "Extensible Prioritization Scheme
for HTTP", Work in Progress, Internet-Draft, draft-ietf- for HTTP", Work in Progress, Internet-Draft, draft-ietf-
skipping to change at line 1496 skipping to change at line 1500
[RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early [RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>. 2018, <https://www.rfc-editor.org/info/rfc8470>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949, Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020, DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>. <https://www.rfc-editor.org/info/rfc8949>.
[SECCTXT] West, M., "Secure Contexts", W3C Candidate Recommendation [SECCTXT] West, M., "Secure Contexts", W3C Candidate Recommendation,
CR-secure-contexts-20160915, 15 September 2016, September 2021,
<https://www.w3.org/TR/2016/CR-secure-contexts-20160915>. <https://www.w3.org/TR/2021/CRD-secure-contexts-20210918>.
[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
[URI-TEMPLATE] [URI-TEMPLATE]
Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570, and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012, DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>. <https://www.rfc-editor.org/info/rfc6570>.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and [XML] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", W3C Recommendation REC-xml-20081126, November Edition)", W3C Recommendation REC-xml-20081126, November
2008, <https://www.w3.org/TR/2008/REC-xml-20081126>. 2008, <https://www.w3.org/TR/2008/REC-xml-20081126>.
Appendix A. Changes from RFC 3205 Appendix A. Changes from RFC 3205
[RFC3205] captured the Best Current Practice in the early 2000s based [RFC3205] captured the Best Current Practice in the early 2000s based
on the concerns facing protocol designers at the time. Use of HTTP on the concerns facing protocol designers at the time. Use of HTTP
has changed considerably since then; as a result, this document is has changed considerably since then; as a result, this document is
substantially different. As a result, the changes are too numerous substantially different. Consequently, the changes are too numerous
to list individually. to list individually.
Author's Address Author's Address
Mark Nottingham Mark Nottingham
Prahran Prahran
Australia Australia
Email: mnot@mnot.net Email: mnot@mnot.net
URI: https://www.mnot.net/ URI: https://www.mnot.net/
 End of changes. 42 change blocks. 
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