HTTP M. Nottingham
Internet-Draft April 11, 2018
Obsoletes: 3205 (if approved)
Intended status: Best Current Practice
Expires: October 13, 2018
On the use of HTTP as a Substrate
draft-ietf-httpbis-bcp56bis-04
Abstract
HTTP is often used as a substrate for other application protocols
(a.k.a. HTTP-based APIs). This document specifies best practices
for these protocols' use of HTTP.
This document obsoletes RFC 3205.
Note to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at http://httpwg.github.io/
[2]; source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/bcp56bis [3].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 13, 2018.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Is HTTP Being Used? . . . . . . . . . . . . . . . . . . . . . 4
3. What's Important About HTTP . . . . . . . . . . . . . . . . . 5
3.1. Generic Semantics . . . . . . . . . . . . . . . . . . . . 5
3.2. Links . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Rich Functionality . . . . . . . . . . . . . . . . . . . 7
4. Best Practices for Using HTTP . . . . . . . . . . . . . . . . 7
4.1. Specifying the Use of HTTP . . . . . . . . . . . . . . . 8
4.2. Defining HTTP Resources . . . . . . . . . . . . . . . . . 8
4.3. Specifying Client Behaviours . . . . . . . . . . . . . . 9
4.4. HTTP URLs . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4.1. Initial URL Discovery . . . . . . . . . . . . . . . . 11
4.4.2. URL Schemes . . . . . . . . . . . . . . . . . . . . . 11
4.4.3. Transport Ports . . . . . . . . . . . . . . . . . . . 12
4.5. HTTP Methods . . . . . . . . . . . . . . . . . . . . . . 12
4.5.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5.2. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 14
4.6. HTTP Status Codes . . . . . . . . . . . . . . . . . . . . 14
4.7. HTTP Header Fields . . . . . . . . . . . . . . . . . . . 16
4.8. Defining Message Payloads . . . . . . . . . . . . . . . . 17
4.9. HTTP Caching . . . . . . . . . . . . . . . . . . . . . . 17
4.10. Application State . . . . . . . . . . . . . . . . . . . . 19
4.11. Client Authentication . . . . . . . . . . . . . . . . . . 19
4.12. Co-Existing with Web Browsing . . . . . . . . . . . . . . 19
4.13. Application Boundaries . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6. Security Considerations . . . . . . . . . . . . . . . . . . . 22
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. Normative References . . . . . . . . . . . . . . . . . . 22
7.2. Informative References . . . . . . . . . . . . . . . . . 24
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7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Appendix A. Changes from RFC 3205 . . . . . . . . . . . . . . . 27
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
HTTP [RFC7230] is often used as a substrate for other application
protocols; this is sometimes referred to as creating "HTTP-based
APIs", or just "HTTP APIs", although the latter is ambiguous. This
is done for a variety of reasons, including:
o familiarity by implementers, specifiers, administrators,
developers and users,
o availability of a variety of client, server and proxy
implementations,
o ease of use,
o ubiquity of Web browsers,
o reuse of existing mechanisms like authentication and encryption,
o presence of HTTP servers and clients in target deployments, and
o its ability to traverse firewalls.
In many cases, these protocols are ad hoc; they are intended for only
deployment on the server side, and consumption by a limited set of
clients. A body of practices and tools has arisen around defining
HTTP-based APIs that favours these conditions.
However, when such a protocol is standarised, it is typically
deployed on multiple servers, implemented a number of times, and
might be consumed by a broader variety of clients. Such diversity
brings a different set of concerns, and tools and practices intended
for a single-server deployment might not be suitable.
In particular, standards-defined HTTP-based APIs need to more
carefully consider how extensibility and evolution will be handled,
how different deployment requirements will be accommodated, and how
clients will evolve with the API.
At the same time, the IETF has a tradition of protocol reuse (e.g.,
[TELNET] as a substrate for [FTP] and [SMTP]; HTTP as a substrate for
[IPP] and [RESTCONF]). Because HTTP is extensible in many ways, a
number of questions arise, such as:
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o Should an application using HTTP define a new URL scheme? Use new
ports?
o Should it use standard HTTP methods and status codes, or define
new ones?
o How can the maximum value be extracted from the use of HTTP?
o How does it coexist with other uses of HTTP - especially Web
browsing?
o How can interoperability problems and "protocol dead ends" be
avoided?
This document contains best current practices regarding the use of
HTTP by applications other than Web browsing. Section 2 defines what
applications it applies to; Section 3 surveys the properties of HTTP
that are important to preserve, and Section 4 conveys best practices
for those applications that do use HTTP.
It is written primarily to guide IETF efforts to define application
protocols using HTTP for deployment on the Internet, but might be
applicable in other situations. Note that the requirements herein do
not necessarily apply to the development of generic HTTP extensions.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Is HTTP Being Used?
Different applications have different goals when using HTTP. In this
document, we say an application is "using HTTP" when any of the
following conditions are true:
o The transport port in use is 80 or 443,
o The URL scheme "http" or "https" is used,
o The ALPN protocol ID [RFC7301] generically identifies HTTP (e.g.,
"http/1.1", "h2", "h2c"), or
o The IANA registries defined for HTTP are updated or modified.
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When an application is using HTTP, all of the requirements of the
HTTP protocol suite are in force (including but not limited to
[RFC7230], [RFC7231], [RFC7232], [RFC7233], [RFC7234], [RFC7235] and
[RFC7540]).
An application might not be using HTTP according to this definition,
but still relying upon the HTTP specifications in some manner. For
example, an application might wish to avoid re-specifying parts of
the message format, but change others; or, it might want to use a
different set of methods.
Such applications are referred to as "protocols based upon HTTP" in
this document. These have more freedom to modify protocol operation,
but are also likely to lose at least a portion of the benefits
outlined above, as most HTTP implementations won't be easily
adaptable to these changes, and as the protocol diverges from HTTP,
the benefit of mindshare will be lost.
Protocols that are based upon HTTP MUST NOT reuse HTTP's URL schemes,
transport ports, ALPN protocol IDs or IANA registries; rather, they
are encouraged to establish their own.
3. What's Important About HTTP
There are many ways that applications using HTTP are defined and
deployed, and sometimes they are brought to the IETF for
standardisation. In that process, what might be workable for
deployment in a limited fashion isn't appropriate for standardisation
and the corresponding broader deployment.
This section examines the facets of the protocol that are important
to preserve in these situations.
3.1. Generic Semantics
When writing an application's specification, it's often tempting to
specify exactly how HTTP is to be implemented, supported and used.
However, this can easily lead to an unintended profile of HTTP's
behaviour. For example, it's common to see specifications with
language like this:
A `POST` request MUST result in a `201 Created` response.
This forms an expectation in the client that the response will always
be "201 Created", when in fact there are a number of reasons why the
status code might differ in a real deployment. If the client does
not anticipate this, the application's deployment is brittle.
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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 HTTP software
(e.g., HTTP servers, intermediaries, client implementations), and its
handling to be correctly determined. It also allows people to
leverage their knowledge of HTTP semantics without special-casing
them for a particular application.
Therefore, applications that use HTTP MUST NOT re-define, refine or
overlay the semantics of defined protocol elements. Instead, they
should focus their specifications on protocol elements that are
specific to that application; namely their HTTP resources.
See Section 4.2 for details.
3.2. Links
Another common practice is assuming that the HTTP server's name space
(or a portion thereof) is exclusively for the use of a single
application. This effectively overlays special, application-specific
semantics onto that space, precludes other applications from using
it.
As explained in [RFC7320], such "squatting" on a part of the URL
space by a standard usurps the server's authority over its own
resources, can cause deployment issues, and is therefore bad practice
in standards.
Instead of statically defining URL components like paths, it is
RECOMMENDED that applications using HTTP define links in payloads, to
allow flexibility in deployment.
Using runtime links in this fashion has a number of other benefits.
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" different applications on the same
server, and offers a natural mechanism for extensibility, versioning
and capability management.
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3.3. Rich Functionality
The simplest possible use of HTTP is to POST data to a single URL,
thereby effectively tunnelling through the protocol.
This "RPC" style of communication does get some benefit from using
HTTP - namely, message framing and the availability of
implementations - but fails to realise many others when used
exclusively:
o Caching for server scalability, latency and bandwidth reduction,
and reliability;
o Granularity of access control (through use of a rich space of
URLs);
o Partial content to selectively request part of a response;
o Definition of an information space using URLs; and
o The ability to interact with the application easily using a Web
browser.
Using such a high-level protocol to tunnel simple semantics has
downsides too; because of its more advanced capabilities, breadth of
deployment and age, HTTP's complexity can cause interoperability
problems that could be avoided by using a simpler substrate (e.g.,
WebSockets [RFC6455], if browser support is necessary, or TCP
[RFC0793] if not), or making the application be based upon HTTP,
instead of using it (as defined in Section 2).
Applications that use HTTP are encouraged to accommodate the various
features that the protocol offers, so that their users receive the
maximum benefit from it. 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 will help make them available.
4. Best Practices for Using HTTP
This section contains best practices regarding the use of HTTP by
applications, including practices for specific HTTP protocol
elements.
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4.1. Specifying the Use of HTTP
When specifying the use of HTTP, an application SHOULD use [RFC7230]
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 MAY specify a minimum version to be supported
(HTTP/1.1 is suggested), and MUST NOT specify a maximum version, to
preserve the protocol's ability to evolve.
Likewise, applications need not specify what HTTP mechanisms - such
as redirection, caching, authentication, proxy authentication, and so
on - are to be supported. For example, an application can specify
that it uses HTTP like this:
Foo Application uses HTTP [RFC7230]. Implementations MUST support
HTTP/1.1, and MAY support later versions.
When specifying examples of protocol interactions, applications
SHOULD document both the request and response messages, with full
headers, preferably in HTTP/1.1 format. For example:
GET /thing HTTP/1.1
Host: example.com
Accept: application/things+json
User-Agent: Foo/1.0
HTTP/1.1 200 OK
Content-Type: application/things+json
Content-Length: 500
Server: Bar/2.2
[payload here]
4.2. Defining HTTP Resources
Applications that use HTTP should focus on defining the following
application-specific protocol elements:
o Media types [RFC6838], often based upon a format convention such
as JSON [RFC8259],
o HTTP header fields, as per Section 4.7, and
o The behaviour of resources, as identified by link relations
[RFC8288].
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By composing these protocol elements, an application can define a set
of resources, identified by link relations, that implement specified
behaviours, including:
o Retrieval of their state using GET, in one or more formats
identified by media type;
o Resource creation or update using POST or PUT, with an
appropriately identified request body format;
o Data processing using POST and identified request and response
body format(s); and
o Resource deletion using DELETE.
For example, an application might specify:
Resources linked to with the "example-widget" link relation type are
Widgets. The state of a Widget can be fetched in the
"application/example-widget+json" format, and can be updated by PUT
to the same link. Widget resources can be deleted.
The "Example-Count" response header field on Widget representations
indicates how many Widgets are held by the sender.
The "application/example-widget+json" format is a JSON [RFC8259]
format representing the state of a Widget. It contains links to
related information in the link indicated by the Link header field
value with the "example-other-info" link relation type.
4.3. Specifying Client Behaviours
HTTP does not mandate some behaviours that have nevertheless become
very common; if these are not explicitly specified by applications
using HTTP, there may be confusion and interoperability problems.
This section recommends default handling for these mechanisms.
o Redirect handling - Applications using HTTP SHOULD specify that
3xx redirect status codes be followed automatically. See
[RFC7231], Section 6.4.
o Redirect methods - Applications using HTTP SHOULD specify that 301
and 302 redirect status codes rewrite the POST method to GET, in
order to be compatible with browsers. See [RFC7231], Section 6.4.
o Cookies - Applications using HTTP MUST explicitly reference the
Cookie specification [RFC6265] if they are required.
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o Certificates - Applications using HTTP MUST specify that TLS
certificates are to be checked according to [RFC2818] when HTTPS
is used.
In general, applications using HTTP ought to align their usage as
closely as possible with Web browsers, to avoid interoperability
issues when they are used. See Section 4.12.
If an application using HTTP has browser compatibility as a goal,
client interaction ought to be defined in terms of [FETCH], since
that is the abstraction that browsers use for HTTP; it enforces many
of these best practices.
Applications using HTTP MUST NOT require HTTP features that are
usually negotiated to be supported. For example, requiring that
clients support responses with a certain content-encoding ([RFC7231],
Section 3.1.2.2) instead of negotiating for it ([RFC7231],
Section 5.3.4) means that otherwise conformant clients cannot
interoperate with the application. Applications MAY encourage the
implementation of such features, though.
4.4. HTTP URLs
In HTTP, URLs are opaque identifiers under the control of the server.
As outlined in [RFC7320], standards cannot usurp this space, since it
might conflict with existing resources, and constrain implementation
and deployment.
In other words, applications that use HTTP shouldn't associate
application semantics with specific URL paths on arbitrary servers.
Doing so inappropriately conflates the identity of the resource (its
URL) with the capabilities that resource supports, bringing about
many of the same interoperability problems that [RFC4367] warns of.
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 that use HTTP 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.
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4.4.1. Initial URL Discovery
Generally, a client will begin interacting with a given application
server by requesting an initial document that contains information
about that particular deployment, potentially including links to
other relevant resources.
Applications that use HTTP are encouraged to allow an arbitrary URL
to be used as that entry point. For example, rather than specifying
"the initial document is at "/foo/v1", they should allow a deployment
to use any URL as the entry point for the application.
In cases where doing so is impractical (e.g., it is not possible to
convey a whole URL, but only a hostname) standard applications that
use HTTP can request a well-known URL [RFC5785] as an entry point.
4.4.2. URL Schemes
Applications that use HTTP will typically employ the "http" and/or
"https" URL schemes. "https" is RECOMMENDED to provide
authentication, integrity and confidentiality, as well as mitigate
pervasive monitoring attacks [RFC7258].
However, application-specific schemes can be defined as well.
When defining an URL scheme for an application using HTTP, there are
a number of tradeoffs and caveats to keep in mind:
o Unmodified Web browsers will not support the new scheme. While it
is possible to register new URL schemes with Web browsers (e.g.
registerProtocolHandler() in [HTML5], as well as several
proprietary approaches), support for these mechanisms is not
shared by all browsers, and their capabilities vary.
o Existing non-browser clients, intermediaries, servers and
associated software will not recognise the new scheme. For
example, a client library might fail to dispatch the request; a
cache might refuse to store the response, and a proxy might fail
to forward the request.
o Because URLs occur in and are generated in HTTP artefacts
commonly, often without human intervention (e.g., in the
"Location" response header), it can be difficult to assure that
the new scheme is used consistently.
o The resources identified by the new scheme will still be available
using "http" and/or "https" URLs. Those URLs can "leak" into use,
which can present security and operability issues. For example,
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using a new scheme to assure that requests don't get sent to a
"normal" Web site is likely to fail.
o Features that rely upon the URL's origin [RFC6454], such as the
Web's same-origin policy, will be impacted by a change of scheme.
o HTTP-specific features such as cookies [RFC6265], authentication
[RFC7235], caching [RFC7234], and CORS [FETCH] might or might not
work correctly, depending on how they are defined and implemented.
Generally, they are designed and implemented with an assumption
that the URL will always be "http" or "https".
o Web features that require a secure context [SECCTXT] will likely
treat a new scheme as insecure.
See [RFC7595] for more information about minting new URL schemes.
4.4.3. Transport Ports
Applications that use HTTP can use the applicable default port (80
for HTTP, 443 for HTTPS), or they can be deployed upon other ports.
This decision can be made at deployment time, or might be encouraged
by the application's specification (e.g., by registering a port for
that application).
In either case, non-default ports will need to be reflected in the
authority of all URLs for that resource; the only mechanism for
changing a default port is changing the scheme (see Section 4.4.2).
Using a port other than the default has privacy implications (i.e.,
the protocol can now be distinguished from other traffic), as well as
operability concerns (as some networks might block or otherwise
interfere with it). Privacy implications should be documented in
Security Considerations.
See [RFC7605] for further guidance.
4.5. HTTP Methods
Applications that use HTTP MUST confine themselves to using
registered HTTP methods such as GET, POST, PUT, DELETE, and PATCH.
New HTTP methods are rare; they are required to be registered with
IETF Review (see [RFC7232]), 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.
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While historically some applications (e.g., [RFC4791]) have defined
non-generic methods, [RFC7231] now forbids this.
When authors believe that a new method is required, they are
encouraged to engage with the HTTP community early, and document
their proposal as a separate HTTP extension, rather than as part of
an application's specification.
4.5.1. GET
GET is one of the most common and useful HTTP methods; its retrieval
semantics allow caching, side-effect free linking and forms the basis
of many of the benefits of using HTTP.
A common use of GET is to perform queries, often using the query
component of the URL; this is this a familiar pattern from Web
browsing, and the results can be cached, improving efficiency of an
often expensive process.
In some cases, however, GET might be unwieldy for expressing queries,
because of the limited syntax of the URL; in particular, if binary
data forms part of the query terms, it needs to be encoded to conform
to URL syntax.
While this is not an issue for short queries, it can become one for
larger query terms, or ones which need to sustain a high rate of
requests. Additionally, some HTTP implementations limit the size of
URLs they support - although modern HTTP software has much more
generous limits than previously (typically, considerably more than
8000 octets, as required by [RFC7230], Section 3.1.1).
In these cases, an application using HTTP might consider using POST
to express queries in the request body; doing so avoids encoding
overhead and URL length limits in implementations. However, in doing
so it should be noted that the benefits of GET such as caching and
linking to query results are lost. Therefore, applications using
HTTP that feel a need to allow POST queries ought consider allowing
both methods.
Applications that use HTTP SHOULD NOT define GET requests to have
side effects, since implementations can and do retry HTTP GET
requests that fail.
Finally, note that while HTTP allows GET requests to have a body
syntactically, this is done only to allow parsers to be generic; as
per [RFC7231], Section 4.3.1, a body on a GET has no meaning, and
will be either ignored or rejected by generic HTTP software.
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4.5.2. OPTIONS
The OPTIONS method was defined for metadata retrieval, and is used
both by WebDAV [RFC4918] and CORS [FETCH]. Because HTTP-based APIs
often need to retrieve metadata about resources, it is often
considered for their use.
However, OPTIONS does have significant limitations:
o It isn't possible to link to the metadata with a simple URL,
because OPTIONS is not the default GET method.
o OPTIONS responses are not cacheable, because HTTP caches operate
on representations of the resource (i.e., GET and HEAD). If
OPTIONS responses are cached separately, their interaction with
HTTP cache expiry, secondary keys and other mechanisms needs to be
considered.
o OPTIONS is "chatty" - always separating metadata out into a
separate request increases the number of requests needed to
interact with the application.
o Implementation support for OPTIONS is not universal; some servers
do not expose the ability to respond to OPTIONS requests without
significant effort.
Instead of OPTIONS, one of these alternative approaches might be more
appropriate:
o For server-wide metadata, create a well-known URI [RFC5785], or
using an already existing one if it's appropriate (e.g., HostMeta
[RFC6415]).
o For metadata about a specific resource, use a Link response
header, or a link in the representation format for that resource.
See [RFC8288]. Note that the Link header is available on HEAD
responses, which is useful if the client wants to discover a
resource's capabilities before they interact with it.
4.6. HTTP Status Codes
The primary function of a HTTP status code is to convey semantics for
the benefit of generic HTTP software, not to convey application-
specific semantics.
In particular, status codes are often generated or overwritten by
intermediaries, as well as server and client implementations; for
example, when network errors are encountered, a captive portal is
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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, and is not guaranteed to be preserved
by implementations, and the reason phrase is not carried at all in
the [RFC7540] message format.
Applications that use HTTP MUST only use registered HTTP status
codes. As with methods, new HTTP status codes are rare, and required
(by [RFC7231]) to be registered with IETF review. Similarly, HTTP
status codes are generic; they are required (by [RFC7231]) to be
potentially applicable to all resources, not just to those of one
application.
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When authors believe that a new status code is required, they are
encouraged to engage with the HTTP community early, and document
their proposal as a separate HTTP extension, rather than as part of
an application's specification.
4.7. HTTP Header Fields
Applications that use HTTP MAY define new HTTP header fields.
Typically, using HTTP header fields is appropriate in a few different
situations:
o Their content is useful to intermediaries (who often wish to avoid
parsing the body), and/or
o Their content is useful to generic HTTP software (e.g., clients,
servers), and/or
o It is not possible to include their content in the message body
(usually because a format does not allow it).
New header fields MUST be registered, as per [RFC7231] and [RFC3864].
See [RFC7231], Section 8.3.1 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.
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See Section 4.9 for the interaction between headers and HTTP caching;
in particular, request headers that are used to "select" a response
have impact there, and need to be carefully considered.
See Section 4.10 for considerations regarding header fields that
carry application state (e.g., Cookie).
4.8. Defining Message Payloads
There are many potential formats for payloads; for example, JSON
[RFC8259], XML [XML], and CBOR [RFC7049]. Best practices for their
use are out of scope for this document.
Applications SHOULD register distinct media types for each format
they define; this makes it possible to identify them unambiguously
and negotiate for their use. See [RFC6838] for more information.
4.9. HTTP Caching
HTTP caching [RFC7234] 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 ([RFC7234], 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.
Understand that stale responses (e.g., one 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
[RFC7234], 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
[RFC7232].
If an application defines a request header field that might be used
by a server 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 [RFC7234],
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Section 5.2.2.3) or consistently send the Vary response header
([RFC7231], Section 7.1.4).
For example, this response:
HTTP/1.1 200 OK
Content-Type: application/example+xml
Cache-Control: max-age=60
ETag: "sa0f8wf20fs0f"
Vary: Accept-Encoding
[content]
can be stored for 60 seconds by both private and shared caches, can
be revalidated with If-None-Match, and varies on the Accept-Encoding
request header field.
In some situations, responses without explicit cache directives
(e.g., Cache-Control or Expires) will be stored and served using a
heuristic freshness lifetime; see [RFC7234], 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 [RFC7234],
Section 3. Note that "Cache-Control: no-cache" allows a response to
be stored, just not reused by a cache; it does not prevent caching
(despite its name).
For example, this response cannot be stored or reused by a cache:
HTTP/1.1 200 OK
Content-Type: application/example+xml
Cache-Control: no-store
[content]
When an application has a need to express a lifetime that's separate
from the freshness lifetime, this should be expressed separately,
either in the response's body or in a separate header field. When
this happens, the relationship between HTTP caching and that lifetime
need to be carefully considered, since the response will be used as
long as it is considered fresh.
Like other functions, HTTP caching is generic; it does not have
knowledge of the application in use. Therefore, caching extensions
need to be backwards-compatible, as per [RFC7234], Section 5.2.3.
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4.10. Application State
Applications that use HTTP MAY use stateful cookies [RFC6265] 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.
4.11. Client Authentication
Applications that use HTTP MAY use HTTP authentication [RFC7235] 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".
When used, it is important to carefully specify the scoping and use
of authentication; if the application exposes sensitive data or
capabilities (e.g., by acting as an ambient authority), exploits are
possible. Mitigations include using a request-specific token to
assure the intent of the client.
4.12. Co-Existing with Web Browsing
Even if there is not an intent for an application that uses HTTP to
be used with a Web browser, its resources will remain available to
browsers and other HTTP clients.
This means that all such applications 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 making that request
by a HTML form on an arbitrary Web site.
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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
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 _as_ a Web application, and to follow best
practices for their secure development.
A complete enumeration of such practices is out of scope for this
document, but some considerations include:
o Using an application-specific media type in the Content-Type
header, and requiring clients to fail if it is not used
o Using X-Content-Type-Options: nosniff [FETCH]} to assure that
content under attacker control can't be coaxed into a form that is
interpreted as active content by a Web browser
o Using Content-Security-Policy [CSP] to constrain the capabilities
of active content (such as HTML [HTML5]), thereby mitigating
Cross-Site Scripting attacks
o Using Referrer-Policy [REFERRER-POLICY] to prevent sensitive data
in URLs from being leaked in the Referer request header
o Using the 'HttpOnly' flag on Cookies to assure that cookies are
not exposed to browser scripting languages [RFC6265]
o Avoiding use of compression on any sensitive information (e.g.,
authentication tokens, passwords), as the scripting environment
offered by Web browsers allows an attacker to repeatedly probe the
compression space; if the attacker has access to the path of the
communication, they can use this capability to recover that
information.
Depending on how they are intended to be deployed, specifications for
applications using HTTP might require the use of these mechanisms in
specific ways, or might merely point them out in Security
Considerations.
An example of a HTTP response from an application that does not
intend for its content to be treated as active by browsers might look
like this:
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HTTP/1.1 200 OK
Content-Type: application/example+json
X-Content-Type-Options: nosniff
Content-Security-Policy: default-src 'none'
Cache-Control: max-age=3600
Referrer-Policy: no-referrer
[content]
If an application 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.
4.13. Application Boundaries
Because the origin [RFC6454] is how many HTTP capabilities are
scoped, applications also need to consider how deployments might
interact with other applications (including Web browsing) on the same
origin.
For example, if Cookies [RFC6265] 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 [RFC7235], or other origin-wide HTTP
mechanisms, applications using HTTP SHOULD NOT mandate the use of a
particular identifier, 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].
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5. IANA Considerations
This document has no requirements for IANA.
6. Security Considerations
Section 4.10 discusses the impact of using stateful mechanisms in the
protocol as ambient authority, and suggests a mitigation.
Section 4.4.2 requires support for 'https' URLs, and discourages the
use of 'http' URLs, to provide authentication, integrity and
confidentiality, as well as mitigate pervasive monitoring attacks.
Section 4.12 highlights the implications of Web browsers'
capabilities on applications that use HTTP.
Section 4.13 discusses the issues that arise when applications are
deployed on the same origin as Web sites (and other applications).
Applications that use HTTP in a manner that involves modification of
implementations - for example, requiring support for a new URL
scheme, or a non-standard method - risk having those implementations
"fork" from their parent HTTP implementations, with the possible
result that they do not benefit from patches and other security
improvements incorporated upstream.
7. References
7.1. Normative References
[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>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
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[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>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<https://www.rfc-editor.org/info/rfc7232>.
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, DOI 10.17487/RFC7233, June 2014,
<https://www.rfc-editor.org/info/rfc7233>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/info/rfc7235>.
[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>.
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[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>.
[FTP] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985,
<https://www.rfc-editor.org/info/rfc959>.
[HTML5] WHATWG, "HTML - Living Standard", n.d.,
<https://html.spec.whatwg.org>.
[]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-04 (work in progress),
March 2018.
[IPP] Sweet, M. and I. McDonald, "Internet Printing
Protocol/1.1: Model and Semantics", RFC 8011,
DOI 10.17487/RFC8011, January 2017,
<https://www.rfc-editor.org/info/rfc8011>.
[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>.
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[RESTCONF]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<https://www.rfc-editor.org/info/rfc5785>.
[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>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[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>.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, DOI 10.17487/RFC6455, December 2011,
<https://www.rfc-editor.org/info/rfc6455>.
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[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>.
[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>.
[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>.
[SMTP] Klensin, J., Ed., "Simple Mail Transfer Protocol",
RFC 2821, DOI 10.17487/RFC2821, April 2001,
<https://www.rfc-editor.org/info/rfc2821>.
[TELNET] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
1983, <https://www.rfc-editor.org/info/rfc854>.
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[XML] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/bcp56bis
Appendix A. Changes from RFC 3205
[RFC3205] captured the Best Current Practice in the early 2000's,
based on the concerns facing protocol designers at the time. Use of
HTTP has changed considerably since then, and as a result this
document is substantially different. As a result, the changes are
too numerous to list individually.
Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: https://www.mnot.net/
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