• January 20, 2023

Html Accept Header

Accept - HTTP - MDN Web Docs

Accept – HTTP – MDN Web Docs

The Accept request HTTP header indicates which content types, expressed as MIME types, the client is able to understand. The server uses content negotiation to select one of the proposals and informs the client of the choice with the Content-Type response header. Browsers set required values for this header based on the context of the request. For example, a browser uses different values in a request when fetches a CSS stylesheet, image, video, or a script.
Header type
Request header
Forbidden header name
no
CORS-safelisted request header
yes, with the additional restriction that values can’t contain a
CORS-unsafe request header byte: 0x00-0x1F (except 0x09 (HT)),
“():<>? @[\]{}, and 0x7F (DEL).
SyntaxAccept: /
Accept: /*
Accept: */*
// Multiple types, weighted with the quality value syntax:
Accept: text/html, application/xhtml+xml, application/xml;q=0. 9, image/webp, */*;q=0. 8
Directives
/
A single, precise MIME type, like text/html.
/*
A MIME type, but without a subtype. image/* corresponds to image/png, image/svg, image/gif, and other image types.
*/*
Any MIME type;q= (q-factor weighting)
A value used is placed in an order of preference expressed using a relative quality value called the weight.
ExamplesAccept: text/html
Accept: image/*
// General default
// Default for navigation requests
Accept: text/html, application/xhtml+xml, application/xml;q=0. 9, */*;q=0. 8
SpecificationsSpecificationHypertext Transfer Protocol (HTTP/1. 1): Semantics and Content (HTTP/1. 1)# ceptBrowser compatibilityBCD tables only load in the browserSee also
HTTP content negotiation
List of default Accept values
A header with the result of the content negotiation: Content-Type
Other similar headers: TE, Accept-Encoding, Accept-Language
Accept - HTTP - MDN Web Docs

Accept – HTTP – MDN Web Docs

The Accept request HTTP header indicates which content types, expressed as MIME types, the client is able to understand. The server uses content negotiation to select one of the proposals and informs the client of the choice with the Content-Type response header. Browsers set required values for this header based on the context of the request. For example, a browser uses different values in a request when fetches a CSS stylesheet, image, video, or a script.
Header type
Request header
Forbidden header name
no
CORS-safelisted request header
yes, with the additional restriction that values can’t contain a
CORS-unsafe request header byte: 0x00-0x1F (except 0x09 (HT)),
“():<>? @[\]{}, and 0x7F (DEL).
SyntaxAccept: /
Accept: /*
Accept: */*
// Multiple types, weighted with the quality value syntax:
Accept: text/html, application/xhtml+xml, application/xml;q=0. 9, image/webp, */*;q=0. 8
Directives
/
A single, precise MIME type, like text/html.
/*
A MIME type, but without a subtype. image/* corresponds to image/png, image/svg, image/gif, and other image types.
*/*
Any MIME type;q= (q-factor weighting)
A value used is placed in an order of preference expressed using a relative quality value called the weight.
ExamplesAccept: text/html
Accept: image/*
// General default
// Default for navigation requests
Accept: text/html, application/xhtml+xml, application/xml;q=0. 9, */*;q=0. 8
SpecificationsSpecificationHypertext Transfer Protocol (HTTP/1. 1): Semantics and Content (HTTP/1. 1)# ceptBrowser compatibilityBCD tables only load in the browserSee also
HTTP content negotiation
List of default Accept values
A header with the result of the content negotiation: Content-Type
Other similar headers: TE, Accept-Encoding, Accept-Language
HTTP/1.1: Header Field Definitions

HTTP/1.1: Header Field Definitions

HTTP/1. 1: Header Field Definitions
part of Hypertext Transfer Protocol — HTTP/1. 1
RFC 2616 Fielding, et al.
14 Header Field Definitions
This section defines the syntax and semantics of all standard
HTTP/1. 1 header fields. For entity-header fields, both sender and
recipient refer to either the client or the server, depending on who
sends and who receives the entity.
14. 1 Accept
The Accept request-header field can be used to specify certain media
types which are acceptable for the response. Accept headers can be
used to indicate that the request is specifically limited to a small
set of desired types, as in the case of a request for an in-line
image.
Accept = “Accept” “:”
#( media-range [ accept-params])
media-range = ( “*/*”
| ( type “/” “*”)
| ( type “/” subtype)) *( “;” parameter)
accept-params = “;” “q” “=” qvalue *( accept-extension)
accept-extension = “;” token [ “=” ( token | quoted-string)]
The asterisk “*” character is used to group media types into ranges,
with “*/*” indicating all media types and “type/*” indicating all
subtypes of that type. The media-range MAY include media type
parameters that are applicable to that range.
Each media-range MAY be followed by one or more accept-params,
beginning with the “q” parameter for indicating a relative quality
factor. The first “q” parameter (if any) separates the media-range
parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that
media-range, using the qvalue scale from 0 to 1 (section 3. 9). The
default value is q=1.
Note: Use of the “q” parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
“q” from being used with a media range, such an event is believed
to be unlikely given the lack of any “q” parameters in the IANA
media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from
registering any parameter named “q”.
The example
Accept: audio/*; q=0. 2, audio/basic
SHOULD be interpreted as “I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality. ”
If no Accept header field is present, then it is assumed that the
client accepts all media types. If an Accept header field is present,
and if the server cannot send a response which is acceptable
according to the combined Accept field value, then the server SHOULD
send a 406 (not acceptable) response.
A more elaborate example is
Accept: text/plain; q=0. 5, text/html,
text/x-dvi; q=0. 8, text/x-c
Verbally, this would be interpreted as “text/html and text/x-c are
the preferred media types, but if they do not exist, then send the
text/x-dvi entity, and if that does not exist, send the text/plain
entity. ”
Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a given
type, the most specific reference has precedence. For example,
Accept: text/*, text/html, text/html;level=1, */*
have the following precedence:
1) text/html;level=1
2) text/html
3) text/*
4) */*
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
which matches that type. For example,
Accept: text/*;q=0. 3, text/html;q=0. 7, text/html;level=1,
text/html;level=2;q=0. 4, */*;q=0. 5
would cause the following values to be associated:
text/html;level=1 = 1
text/html = 0. 7
text/plain = 0. 3
image/jpeg = 0. 5
text/html;level=2 = 0. 4
text/html;level=3 = 0. 7
Note: A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is
a closed system which cannot interact with other rendering agents,
this default set ought to be configurable by the user.
14. 2 Accept-Charset
The Accept-Charset request-header field can be used to indicate what
character sets are acceptable for the response. This field allows
clients capable of understanding more comprehensive or special-
purpose character sets to signal that capability to a server which is
capable of representing documents in those character sets.
Accept-Charset = “Accept-Charset” “:”
1#( ( charset | “*”)[ “;” “q” “=” qvalue])
Character set values are described in section 3. 4. Each charset MAY
be given an associated quality value which represents the user’s
preference for that charset. The default value is q=1. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0. 8
The special value “*”, if present in the Accept-Charset field,
matches every character set (including ISO-8859-1) which is not
mentioned elsewhere in the Accept-Charset field. If no “*” is present
in an Accept-Charset field, then all character sets not explicitly
mentioned get a quality value of 0, except for ISO-8859-1, which gets
a quality value of 1 if not explicitly mentioned.
If no Accept-Charset header is present, the default is that any
character set is acceptable. If an Accept-Charset header is present,
according to the Accept-Charset header, then the server SHOULD send
an error response with the 406 (not acceptable) status code, though
the sending of an unacceptable response is also allowed.
14. 3 Accept-Encoding
The Accept-Encoding request-header field is similar to Accept, but
restricts the content-codings (section 3. 5) that are acceptable in
the response.
Accept-Encoding = “Accept-Encoding” “:”
1#( codings [ “;” “q” “=” qvalue])
codings = ( content-coding | “*”)
Examples of its use are:
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0. 5, gzip;q=1. 0
Accept-Encoding: gzip;q=1. 0, identity; q=0. 5, *;q=0
A server tests whether a content-coding is acceptable, according to
an Accept-Encoding field, using these rules:
1. If the content-coding is one of the content-codings listed in
the Accept-Encoding field, then it is acceptable, unless it is
accompanied by a qvalue of 0. (As defined in section 3. 9, a
qvalue of 0 means “not acceptable. “)
2. The special “*” symbol in an Accept-Encoding field matches any
available content-coding not explicitly listed in the header
field.
3. If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred.
4. The “identity” content-coding is always acceptable, unless
specifically refused because the Accept-Encoding field includes
“identity;q=0”, or because the field includes “*;q=0” and does
not explicitly include the “identity” content-coding. If the
Accept-Encoding field-value is empty, then only the “identity”
encoding is acceptable.
If an Accept-Encoding field is present in a request, and if the
server cannot send a response which is acceptable according to the
Accept-Encoding header, then the server SHOULD send an error response
with the 406 (Not Acceptable) status code.
If no Accept-Encoding field is present in a request, the server MAY
assume that the client will accept any content coding. In this case,
if “identity” is one of the available content-codings, then the
server SHOULD use the “identity” content-coding, unless it has
additional information that a different content-coding is meaningful
to the client.
Note: If the request does not include an Accept-Encoding field,
and if the “identity” content-coding is unavailable, then
content-codings commonly understood by HTTP/1. 0 clients (i. e.,
“gzip” and “compress”) are preferred; some older clients
improperly display messages sent with other content-codings. The
server might also make this decision based on information about
the particular user-agent or client.
Note: Most HTTP/1. 0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues will not
work and are not permitted with x-gzip or x-compress.
14. 4 Accept-Language
The Accept-Language request-header field is similar to Accept, but
restricts the set of natural languages that are preferred as a
response to the request. Language tags are defined in section 3. 10.
Accept-Language = “Accept-Language” “:”
1#( language-range [ “;” “q” “=” qvalue])
language-range = ( ( 1*8ALPHA *( “-” 1*8ALPHA)) | “*”)
Each language-range MAY be given an associated quality value which
represents an estimate of the user’s preference for the languages
specified by that range. The quality value defaults to “q=1”. For
example,
Accept-Language: da, en-gb;q=0. 8, en;q=0. 7
would mean: “I prefer Danish, but will accept British English and
other types of English. ” A language-range matches a language-tag if
it exactly equals the tag, or if it exactly equals a prefix of the
tag such that the first tag character following the prefix is “-“.
The special range “*”, if present in the Accept-Language field,
matches every tag not matched by any other range present in the
Accept-Language field.
Note: This use of a prefix matching rule does not imply that
language tags are assigned to languages in such a way that it is
always true that if a user understands a language with a certain
tag, then this user will also understand all languages with tags
for which this tag is a prefix. The prefix rule simply allows the
use of prefix tags if this is the case.
The language quality factor assigned to a language-tag by the
Accept-Language field is the quality value of the longest language-
range in the field that matches the language-tag. If no language-
range in the field matches the tag, the language quality factor
assigned is 0. If no Accept-Language header is present in the
request, the server
SHOULD assume that all languages are equally acceptable. If an
Accept-Language header is present, then all languages which are
assigned a quality factor greater than 0 are acceptable.
It might be contrary to the privacy expectations of the user to send
an Accept-Language header with the complete linguistic preferences of
the user in every request. For a discussion of this issue, see
section 15. 1. 4.
As intelligibility is highly dependent on the individual user, it is
recommended that client applications make the choice of linguistic
preference available to the user. If the choice is not made
available, then the Accept-Language header field MUST NOT be given in
the request.
Note: When making the choice of linguistic preference available to
the user, we remind implementors of the fact that users are not
familiar with the details of language matching as described above,
and should provide appropriate guidance. As an example, users
might assume that on selecting “en-gb”, they will be served any
kind of English document if British English is not available. A
user agent might suggest in such a case to add “en” to get the
best matching behavior.
14. 5 Accept-Ranges
The Accept-Ranges response-header field allows the server to
indicate its acceptance of range requests for a resource:
Accept-Ranges = “Accept-Ranges” “:” acceptable-ranges
acceptable-ranges = 1#range-unit | “none”
Origin servers that accept byte-range requests MAY send
Accept-Ranges: bytes
but are not required to do so. Clients MAY generate byte-range
requests without having received this header for the resource
involved. Range units are defined in section 3. 12.
Servers that do not accept any kind of range request for a
resource MAY send
Accept-Ranges: none
to advise the client not to attempt a range request.
14. 6 Age
The Age response-header field conveys the sender’s estimate of the
amount of time since the response (or its revalidation) was
generated at the origin server. A cached response is “fresh” if
its age does not exceed its freshness lifetime. Age values are
calculated as specified in section 13. 2. 3.
Age = “Age” “:” age-value
age-value = delta-seconds
Age values are non-negative decimal integers, representing time in
seconds.
If a cache receives a value larger than the largest positive
integer it can represent, or if any of its age calculations
overflows, it MUST transmit an Age header with a value of
2147483648 (2^31). An HTTP/1. 1 server that includes a cache MUST
include an Age header field in every response generated from its
own cache. Caches SHOULD use an arithmetic type of at least 31
bits of range.
14. 7 Allow
The Allow entity-header field lists the set of methods supported
by the resource identified by the Request-URI. The purpose of this
field is strictly to inform the recipient of valid methods
associated with the resource. An Allow header field MUST be
present in a 405 (Method Not Allowed) response.
Allow = “Allow” “:” #Method
Example of use:
Allow: GET, HEAD, PUT
This field cannot prevent a client from trying other methods.
However, the indications given by the Allow header field value
SHOULD be followed. The actual set of allowed methods is defined
by the origin server at the time of each request.
The Allow header field MAY be provided with a PUT request to
recommend the methods to be supported by the new or modified
resource. The server is not required to support these methods and
SHOULD include an Allow header in the response giving the actual
supported methods.
A proxy MUST NOT modify the Allow header field even if it does not
understand all the methods specified, since the user agent might
have other means of communicating with the origin server.
14. 8 Authorization
A user agent that wishes to authenticate itself with a server–
usually, but not necessarily, after receiving a 401 response–does
so by including an Authorization request-header field with the
request. The Authorization field value consists of credentials
containing the authentication information of the user agent for
the realm of the resource being requested.
Authorization = “Authorization” “:” credentials
HTTP access authentication is described in “HTTP Authentication:
Basic and Digest Access Authentication” [43]. If a request is
authenticated and a realm specified, the same credentials SHOULD
be valid for all other requests within this realm (assuming that
the authentication scheme itself does not require otherwise, such
as credentials that vary according to a challenge value or using
synchronized clocks).
When a shared cache (see section 13. 7) receives a request
containing an Authorization field, it MUST NOT return the
corresponding response as a reply to any other request, unless one
of the following specific exceptions holds:
1. If the response includes the “s-maxage” cache-control
directive, the cache MAY use that response in replying to a
subsequent request. But (if the specified maximum age has
passed) a proxy cache MUST first revalidate it with the origin
server, using the request-headers from the new request to allow
the origin server to authenticate the new request. (This is the
defined behavior for s-maxage. ) If the response includes “s-
maxage=0”, the proxy MUST always revalidate it before re-using
it.
2. If the response includes the “must-revalidate” cache-control
subsequent request. But if the response is stale, all caches
MUST first revalidate it with the origin server, using the
request-headers from the new request to allow the origin server
to authenticate the new request.
3. If the response includes the “public” cache-control directive,
it MAY be returned in reply to any subsequent request.
14. 9 Cache-Control
The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response chain. The directives specify behavior intended to
prevent caches from adversely interfering with the request or
response. These directives typically override the default caching
algorithms. Cache directives are unidirectional in that the presence
of a directive in a request does not imply that the same directive is
to be given in the response.
Note that HTTP/1. 0 caches might not implement Cache-Control and
might only implement Pragma: no-cache (see section 14. 32).
Cache directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives might be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-
directive for a specific cache.
Cache-Control = “Cache-Control” “:” 1#cache-directive
cache-directive = cache-request-directive
| cache-response-directive
cache-request-directive =
“no-cache”; Section 14. 9. 1
| “no-store”; Section 14. 2
| “max-age” “=” delta-seconds; Section 14. 3, 14. 4
| “max-stale” [ “=” delta-seconds]; Section 14. 3
| “min-fresh” “=” delta-seconds; Section 14. 3
| “no-transform”; Section 14. 5
| “only-if-cached”; Section 14. 4
| cache-extension; Section 14. 6
cache-response-directive =
“public”; Section 14. 1
| “private” [ “=” <"> 1#field-name <">]; Section 14. 1
| “no-cache” [ “=” <"> 1#field-name <">]; Section 14. 1
| “must-revalidate”; Section 14. 4
| “proxy-revalidate”; Section 14. 4
| “max-age” “=” delta-seconds; Section 14. 3
| “s-maxage” “=” delta-seconds; Section 14. 3
cache-extension = token [ “=” ( token | quoted-string)]
When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or
response. This mechanism supports extensibility; implementations of
future versions of the HTTP protocol might apply these directives to
header fields not defined in HTTP/1. 1.
The cache-control directives can be broken down into these general
categories:
– Restrictions on what are cacheable; these may only be imposed by
the origin server.
– Restrictions on what may be stored by a cache; these may be
imposed by either the origin server or the user agent.
– Modifications of the basic expiration mechanism; these may be
– Controls over cache revalidation and reload; these may only be
imposed by a user agent.
– Control over transformation of entities.
– Extensions to the caching system.
14. 1 What is Cacheable
By default, a response is cacheable if the requirements of the
request method, request header fields, and the response status
indicate that it is cacheable. Section 13. 4 summarizes these defaults
for cacheability. The following Cache-Control response directives
allow an origin server to override the default cacheability of a
response:
public
Indicates that the response MAY be cached by any cache, even if it
would normally be non-cacheable or cacheable only within a non-
shared cache. (See also Authorization, section 14. 8, for
additional details. )
private
Indicates that all or part of the response message is intended for
a single user and MUST NOT be cached by a shared cache. This
allows an origin server to state that the specified parts of the
response are intended for only one user and are not a valid
response for requests by other users. A private (non-shared) cache
MAY cache the response.
Note: This usage of the word private only controls where the
response may be cached, and cannot ensure the privacy of the
message content.
no-cache
If the no-cache directive does not specify a field-name, then a
cache MUST NOT use the response to satisfy a subsequent request
without successful revalidation with the origin server. This
allows an origin server to prevent caching even by caches that
have been configured to return stale responses to client requests.
If the no-cache directive does specify one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, the
specified field-name(s) MUST NOT be sent in the response to a
subsequent request without successful revalidation with the origin
server. This allows an origin server to prevent the re-use of
certain header fields in a response, while still allowing caching
of the rest of the response.
Note: Most HTTP/1. 0 caches will not recognize or obey this
directive.
14. 2 What May be Stored by Caches
no-store
The purpose of the no-store directive is to prevent the
inadvertent release or retention of sensitive information (for
example, on backup tapes). The no-store directive applies to the
entire message, and MAY be sent either in a response or in a
request. If sent in a request, a cache MUST NOT store any part of
either this request or any response to it. If sent in a response,
a cache MUST NOT store any part of either this response or the
request that elicited it. This directive applies to both non-
shared and shared caches. “MUST NOT store” in this context means
that the cache MUST NOT intentionally store the information in
non-volatile storage, and MUST make a best-effort attempt to
remove the information from volatile storage as promptly as
possible after forwarding it.
Even when this directive is associated with a response, users
might explicitly store such a response outside of the caching
system (e. g., with a “Save As” dialog). History buffers MAY store
such responses as part of their normal operation.
The purpose of this directive is to meet the stated requirements
of certain users and service authors who are concerned about
accidental releases of information via unanticipated accesses to
cache data structures. While the use of this directive might
improve privacy in some cases, we caution that it is NOT in any
way a reliable or sufficient mechanism for ensuring privacy. In
particular, malicious or compromised caches might not recognize or
obey this directive, and communications networks might be
vulnerable to eavesdropping.
14. 3 Modifications of the Basic Expiration Mechanism
The expiration time of an entity MAY be specified by the origin
server using the Expires header (see section 14. 21). Alternatively,
it MAY be specified using the max-age directive in a response. When
the max-age cache-control directive is present in a cached response,
the response is stale if its current age is greater than the age
value given (in seconds) at the time of a new request for that
resource. The max-age directive on a response implies that the
response is cacheable (i. e., “public”) unless some other, more
restrictive cache directive is also present.
If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires header, even
if the Expires header is more restrictive. This rule allows an origin
server to provide, for a given response, a longer expiration time to
an HTTP/1. 1 (or later) cache than to an HTTP/1. 0 cache. This might be
useful if certain HTTP/1. 0 caches improperly calculate ages or
expiration times, perhaps due to desynchronized clocks.
Many HTTP/1. 0 cache implementations will treat an Expires value that
is less than or equal to the response Date value as being equivalent
to the Cache-Control response directive “no-cache”. If an HTTP/1. 1
cache receives such a response, and the response does not include a
Cache-Control header field, it SHOULD consider the response to be
non-cacheable in order to retain compatibility with HTTP/1. 0 servers.
Note: An origin server might wish to use a relatively new HTTP
cache control feature, such as the “private” directive, on a
network including older caches that do not understand that
feature. The origin server will need to combine the new feature
with an Expires field whose value is less than or equal to the
Date value. This will prevent older caches from improperly
caching the response.
s-maxage
If a response includes an s-maxage directive, then for a shared
cache (but not for a private cache), the maximum age specified by
this directive overrides the maximum age specified by either the
max-age directive or the Expires header. The s-maxage directive
also implies the semantics of the proxy-revalidate directive (see
section 14. 4), i. e., that the shared cache must not use the
entry after it becomes stale to respond to a subsequent request
without first revalidating it with the origin server. The s-
maxage directive is always ignored by a private cache.
Note that most older caches, not compliant with this specification,
do not implement any cache-control directives. An origin server
wishing to use a cache-control directive that restricts, but does not
prevent, caching by an HTTP/1. 1-compliant cache MAY exploit the
requirement that the max-age directive overrides the Expires header,
and the fact that pre-HTTP/1. 1-compliant caches do not observe the
max-age directive.
Other directives allow a user agent to modify the basic expiration
mechanism. These directives MAY be specified on a request:
max-age
Indicates that the client is willing to accept a response whose
age is no greater than the specified time in seconds. Unless max-
stale directive is also included, the client is not willing to
accept a stale response.
min-fresh
freshness lifetime is no less than its current age plus the
specified time in seconds. That is, the client wants a response
that will still be fresh for at least the specified number of
max-stale
Indicates that the client is willing to accept a response that has
exceeded its expiration time. If max-stale is assigned a value,
then the client is willing to accept a response that has exceeded
its expiration time by no more than the specified number of
seconds. If no value is assigned to max-stale, then the client is
willing to accept a stale response of any age.
If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to
override the expiration time of a response, the cache MUST attach a
Warning header to the stale response, using Warning 110 (Response is
stale).
A cache MAY be configured to return stale responses without
validation, but only if this does not conflict with any “MUST”-level
requirements concerning cache validation (e. g., a “must-revalidate”
cache-control directive).
If both the new request and the cached entry include “max-age”
directives, then the lesser of the two values is used for determining
the freshness of the cached entry for that request.
14. 4 Cache Revalidation and Reload Controls
Sometimes a user agent might want or need to insist that a cache
revalidate its cache entry with the origin server (and not just with
the next cache along the path to the origin server), or to reload its
cache entry from the origin server. End-to-end revalidation might be
necessary if either the cache or the origin server has overestimated
the expiration time of the cached response. End-to-end reload may be
necessary if the cache entry has become corrupted for some reason.
End-to-end revalidation may be requested either when the client does
not have its own local cached copy, in which case we call it
“unspecified end-to-end revalidation”, or when the client does have a
local cached copy, in which case we call it “specific end-to-end
revalidation. ”
The client can specify these three kinds of action using Cache-
Control request directives:
End-to-end reload
The request includes a “no-cache” cache-control directive or, for
compatibility with HTTP/1. 0 clients, “Pragma: no-cache”. Field
names MUST NOT be included with the no-cache directive in a
request. The server MUST NOT use a cached copy when responding to
such a request.
Specific end-to-end revalidation
The request includes a “max-age=0” cache-control directive, which
forces each cache along the path to the origin server to
revalidate its own entry, if any, with the next cache or server.
The initial request includes a cache-validating conditional with
the client’s current validator.
Unspecified end-to-end revalidation
The request includes “max-age=0” cache-control directive, which
The initial request does not include a cache-validating
conditional; the first cache along the path (if any) that holds a
cache entry for this resource includes a cache-validating
conditional with its current validator.
When an intermediate cache is forced, by means of a max-age=0
directive, to revalidate its own cache entry, and the client has
supplied its own validator in the request, the supplied validator
might differ from the validator currently stored with the cache
entry. In this case, the cache MAY use either validator in making
its own request without affecting semantic transparency.
However, the choice of validator might affect performance. The
best approach is for the intermediate cache to use its own
validator when making its request. If the server replies with 304
(Not Modified), then the cache can return its now validated copy
to the client with a 200 (OK) response. If the server replies with
a new entity and cache validator, however, the intermediate cache
can compare the returned validator with the one provided in the
client’s request, using the strong comparison function. If the
client’s validator is equal to the origin server’s, then the
intermediate cache simply returns 304 (Not Modified). Otherwise,
it returns the new entity with a 200 (OK) response.
If a request includes the no-cache directive, it SHOULD NOT
include min-fresh, max-stale, or max-age.
only-if-cached
In some cases, such as times of extremely poor network
connectivity, a client may want a cache to return only those
responses that it currently has stored, and not to reload or
revalidate with the origin server. To do this, the client may
include the only-if-cached directive in a request. If it receives
this directive, a cache SHOULD either respond using a cached entry
that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status. However, if a group
of caches is being operated as a unified system with good internal
connectivity, such a request MAY be forwarded within that group of
caches.
must-revalidate
Because a cache MAY be configured to ignore a server’s specified
expiration time, and because a client request MAY include a max-
stale directive (which has a similar effect), the protocol also
includes a mechanism for the origin server to require revalidation
of a cache entry on any subsequent use. When the must-revalidate
directive is present in a response received by a cache, that cache
MUST NOT use the entry after it becomes stale to respond to a
subsequent request without first revalidating it with the origin
server. (I. e., the cache MUST do an end-to-end revalidation every
time, if, based solely on the origin server’s Expires or max-age
value, the cached response is stale. )
The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances an
HTTP/1. 1 cache MUST obey the must-revalidate directive; in
particular, if the cache cannot reach the origin server for any
reason, it MUST generate a 504 (Gateway Timeout) response.
Servers SHOULD send the must-revalidate directive if and only if
failure to revalidate a request on the entity could result in
incorrect operation, such as a silently unexecuted financial
transaction. Recipients MUST NOT take any automated action that
violates this directive, and MUST NOT automatically provide an
unvalidated copy of the entity if revalidation fails.
Although this is not recommended, user agents operating under
severe connectivity constraints MAY violate this directive but, if
so, MUST explicitly warn the user that an unvalidated response has
been provided. The warning MUST be provided on each unvalidated
access, and SHOULD require explicit user confirmation.
proxy-revalidate
The proxy-revalidate directive has the same meaning as the must-
revalidate directive, except that it does not apply to non-shared
user agent caches. It can be used on a response to an
authenticated request to permit the user’s cache to store and
later return the response without needing to revalidate it (since
it has already been authenticated once by that user), while still
requiring proxies that service many users to revalidate each time
(in order to make sure that each user has been authenticated).
Note that such authenticated responses also need the public cache
control directive in order to allow them to be cached at all.
14. 5 No-Transform Directive
no-transform
Implementors of intermediate caches (proxies) have found it useful
to convert the med

Frequently Asked Questions about html accept header

What is accept * * in header means?

22. The accept: header defines the content type the client accepts, or expects to be returned by the server. Depending on the situation this can be text/css, text/html, image/png, .. etc. – just some mime type. The * character is considered the wildcard.Feb 8, 2013

What is the use of Accept header?

Accept headers can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image. The asterisk “*” character is used to group media types into ranges, with “*/*” indicating all media types and “type/*” indicating all subtypes of that type.

Is HTTP Accept header mandatory?

Accept isn’t mandatory; the server can (and often does) either not implement it, or decides to return something else.Nov 22, 2015

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