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In theory, the date SHOULD represent the moment just before the
entity is generated. In practice, the date can be generated at any
time during the message origination without affecting its semantic
value.
The format of the Date is an absolute date and time as defined by
HTTP-date in section 3.3; it MUST be sent in RFC1123 [8]-date format.
14.20 ETag
The ETag entity-header field defines the entity tag for the
associated entity. The headers used with entity tags are described in
sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used
for comparison with other entities from the same resource (see
section 13.3.2).
ETag = "ETag" ":" entity-tag
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
14.21 Expires
The Expires entity-header field gives the date/time after which the
response should be considered stale. A stale cache entry may not
normally be returned by a cache (either a proxy cache or an user
agent cache) unless it is first validated with the origin server (or
with an intermediate cache that has a fresh copy of the entity). See
section 13.2 for further discussion of the expiration model.
The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that
time.
The format is an absolute date and time as defined by HTTP-date in
section 3.3; it MUST be in RFC1123-date format:
Expires = "Expires" ":" HTTP-date
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An example of its use is
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Note: if a response includes a Cache-Control field with the max-age
directive, that directive overrides the Expires field.
HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").
To mark a response as "already expired," an origin server should use
an Expires date that is equal to the Date header value. (See the
rules for expiration calculations in section 13.2.4.)
To mark a response as "never expires," an origin server should use an
Expires date approximately one year from the time the response is
sent. HTTP/1.1 servers should not send Expires dates more than one
year in the future.
The presence of an Expires header field with a date value of some
time in the future on an response that otherwise would by default be
non-cacheable indicates that the response is cachable, unless
indicated otherwise by a Cache-Control header field (section 14.9).
14.22 From
The From request-header field, if given, SHOULD contain an Internet
e-mail address for the human user who controls the requesting user
agent. The address SHOULD be machine-usable, as defined by mailbox
in RFC 822 (as updated by RFC 1123 ):
From = "From" ":" mailbox
An example is:
From: [email protected]
This header field MAY be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It SHOULD NOT
be used as an insecure form of access protection. The interpretation
of this field is that the request is being performed on behalf of the
person given, who accepts responsibility for the method performed. In
particular, robot agents SHOULD include this header so that the
person responsible for running the robot can be contacted if problems
occur on the receiving end.
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The Internet e-mail address in this field MAY be separate from the
Internet host which issued the request. For example, when a request
is passed through a proxy the original issuer's address SHOULD be
used.
Note: The client SHOULD not send the From header field without the
user's approval, as it may conflict with the user's privacy
interests or their site's security policy. It is strongly
recommended that the user be able to disable, enable, and modify
the value of this field at any time prior to a request.
14.23 Host
The Host request-header field specifies the Internet host and port
number of the resource being requested, as obtained from the original
URL given by the user or referring resource (generally an HTTP URL,
as described in section 3.2.2). The Host field value MUST represent
the network location of the origin server or gateway given by the
original URL. This allows the origin server or gateway to
differentiate between internally-ambiguous URLs, such as the root "/"
URL of a server for multiple host names on a single IP address.
Host = "Host" ":" host [ ":" port ] ; Section 3.2.2
A "host" without any trailing port information implies the default
port for the service requested (e.g., "80" for an HTTP URL). For
example, a request on the origin server for
<http://www.w3.org/pub/WWW/> MUST include:
GET /pub/WWW/ HTTP/1.1
Host: www.w3.org
A client MUST include a Host header field in all HTTP/1.1 request
messages on the Internet (i.e., on any message corresponding to a
request for a URL which includes an Internet host address for the
service being requested). If the Host field is not already present,
an HTTP/1.1 proxy MUST add a Host field to the request message prior
to forwarding it on the Internet. All Internet-based HTTP/1.1 servers
MUST respond with a 400 status code to any HTTP/1.1 request message
which lacks a Host header field.
See sections 5.2 and 19.5.1 for other requirements relating to Host.
14.24 If-Modified-Since
The If-Modified-Since request-header field is used with the GET
method to make it conditional: if the requested variant has not been
modified since the time specified in this field, an entity will not
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be returned from the server; instead, a 304 (not modified) response
will be returned without any message-body.
If-Modified-Since = "If-Modified-Since" ":" HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since header and no Range header
requests that the identified entity be transferred only if it has
been modified since the date given by the If-Modified-Since header.
The algorithm for determining this includes the following cases:
a)If the request would normally result in anything other than a 200
(OK) status, or if the passed If-Modified-Since date is invalid, the
response is exactly the same as for a normal GET. A date which is
later than the server's current time is invalid.
b)If the variant has been modified since the If-Modified-Since date,
the response is exactly the same as for a normal GET.
c)If the variant has not been modified since a valid If-Modified-Since
date, the server MUST return a 304 (Not Modified) response.
The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.
Note that the Range request-header field modifies the meaning of
If-Modified-Since; see section 14.36 for full details.
Note that If-Modified-Since times are interpreted by the server,
whose clock may not be synchronized with the client.
Note that if a client uses an arbitrary date in the If-Modified-Since
header instead of a date taken from the Last-Modified header for the
same request, the client should be aware of the fact that this date
is interpreted in the server's understanding of time. The client
should consider unsynchronized clocks and rounding problems due to
the different encodings of time between the client and server. This
includes the possibility of race conditions if the document has
changed between the time it was first requested and the If-Modified-
Since date of a subsequent request, and the possibility of clock-
skew-related problems if the If-Modified-Since date is derived from
the client's clock without correction to the server's clock.
Corrections for different time bases between client and server are at
best approximate due to network latency.
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14.25 If-Match
The If-Match request-header field is used with a method to make it
conditional. A client that has one or more entities previously
obtained from the resource can verify that one of those entities is
current by including a list of their associated entity tags in the
If-Match header field. The purpose of this feature is to allow
efficient updates of cached information with a minimum amount of
transaction overhead. It is also used, on updating requests, to
prevent inadvertent modification of the wrong version of a resource.
As a special case, the value "*" matches any current entity of the
resource.
If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
If any of the entity tags match the entity tag of the entity that
would have been returned in the response to a similar GET request
(without the If-Match header) on that resource, or if "*" is given
and any current entity exists for that resource, then the server MAY
perform the requested method as if the If-Match header field did not
exist.
A server MUST use the strong comparison function (see section 3.11)
to compare the entity tags in If-Match.
If none of the entity tags match, or if "*" is given and no current
entity exists, the server MUST NOT perform the requested method, and
MUST return a 412 (Precondition Failed) response. This behavior is
most useful when the client wants to prevent an updating method, such
as PUT, from modifying a resource that has changed since the client
last retrieved it.
If the request would, without the If-Match header field, result in
anything other than a 2xx status, then the If-Match header MUST be
ignored.
The meaning of "If-Match: *" is that the method SHOULD be performed
if the representation selected by the origin server (or by a cache,
possibly using the Vary mechanism, see section 14.43) exists, and
MUST NOT be performed if the representation does not exist.
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A request intended to update a resource (e.g., a PUT) MAY include an
If-Match header field to signal that the request method MUST NOT be
applied if the entity corresponding to the If-Match value (a single
entity tag) is no longer a representation of that resource. This
allows the user to indicate that they do not wish the request to be
successful if the resource has been changed without their knowledge.
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
14.26 If-None-Match
The If-None-Match request-header field is used with a method to make
it conditional. A client that has one or more entities previously
obtained from the resource can verify that none of those entities is
current by including a list of their associated entity tags in the
If-None-Match header field. The purpose of this feature is to allow
efficient updates of cached information with a minimum amount of
transaction overhead. It is also used, on updating requests, to
prevent inadvertent modification of a resource which was not known to
exist.
As a special case, the value "*" matches any current entity of the
resource.
If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
If any of the entity tags match the entity tag of the entity that
would have been returned in the response to a similar GET request
(without the If-None-Match header) on that resource, or if "*" is
given and any current entity exists for that resource, then the
server MUST NOT perform the requested method. Instead, if the request
method was GET or HEAD, the server SHOULD respond with a 304 (Not
Modified) response, including the cache-related entity-header fields
(particularly ETag) of one of the entities that matched. For all
other request methods, the server MUST respond with a status of 412
(Precondition Failed).
See section 13.3.3 for rules on how to determine if two entity tags
match. The weak comparison function can only be used with GET or HEAD
requests.
If none of the entity tags match, or if "*" is given and no current
entity exists, then the server MAY perform the requested method as if
the If-None-Match header field did not exist.
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If the request would, without the If-None-Match header field, result
in anything other than a 2xx status, then the If-None-Match header
MUST be ignored.
The meaning of "If-None-Match: *" is that the method MUST NOT be
performed if the representation selected by the origin server (or by
a cache, possibly using the Vary mechanism, see section 14.43)
exists, and SHOULD be performed if the representation does not exist.
This feature may be useful in preventing races between PUT
operations.
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
14.27 If-Range
If a client has a partial copy of an entity in its cache, and wishes
to have an up-to-date copy of the entire entity in its cache, it
could use the Range request-header with a conditional GET (using
either or both of If-Unmodified-Since and If-Match.) However, if the
condition fails because the entity has been modified, the client
would then have to make a second request to obtain the entire current
entity-body.
The If-Range header allows a client to "short-circuit" the second
request. Informally, its meaning is `if the entity is unchanged, send
me the part(s) that I am missing; otherwise, send me the entire new
entity.'
If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
If the client has no entity tag for an entity, but does have a Last-
Modified date, it may use that date in a If-Range header. (The server
can distinguish between a valid HTTP-date and any form of entity-tag
by examining no more than two characters.) The If-Range header should
only be used together with a Range header, and must be ignored if the
request does not include a Range header, or if the server does not
support the sub-range operation.
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If the entity tag given in the If-Range header matches the current
entity tag for the entity, then the server should provide the
specified sub-range of the entity using a 206 (Partial content)
response. If the entity tag does not match, then the server should
return the entire entity using a 200 (OK) response.
14.28 If-Unmodified-Since
The If-Unmodified-Since request-header field is used with a method to
make it conditional. If the requested resource has not been modified
since the time specified in this field, the server should perform the
requested operation as if the If-Unmodified-Since header were not
present.
If the requested variant has been modified since the specified time,
the server MUST NOT perform the requested operation, and MUST return
a 412 (Precondition Failed).
If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally (i.e., without the If-Unmodified-Since
header) would result in anything other than a 2xx status, the If-
Unmodified-Since header should be ignored.
If the specified date is invalid, the header is ignored.
14.29 Last-Modified
The Last-Modified entity-header field indicates the date and time at
which the origin server believes the variant was last modified.
Last-Modified = "Last-Modified" ":" HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The exact meaning of this header field depends on the implementation
of the origin server and the nature of the original resource. For
files, it may be just the file system last-modified time. For
entities with dynamically included parts, it may be the most recent
of the set of last-modify times for its component parts. For database
gateways, it may be the last-update time stamp of the record. For
virtual objects, it may be the last time the internal state changed.
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An origin server MUST NOT send a Last-Modified date which is later
than the server's time of message origination. In such cases, where
the resource's last modification would indicate some time in the
future, the server MUST replace that date with the message
origination date.
An origin server should obtain the Last-Modified value of the entity
as close as possible to the time that it generates the Date value of
its response. This allows a recipient to make an accurate assessment
of the entity's modification time, especially if the entity changes
near the time that the response is generated.
HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
14.30 Location
The Location response-header field is used to redirect the recipient
to a location other than the Request-URI for completion of the
request or identification of a new resource. For 201 (Created)
responses, the Location is that of the new resource which was created
by the request. For 3xx responses, the location SHOULD indicate the
server's preferred URL for automatic redirection to the resource. The
field value consists of a single absolute URL.
Location = "Location" ":" absoluteURI
An example is
Location: http://www.w3.org/pub/WWW/People.html
Note: The Content-Location header field (section 14.15) differs
from Location in that the Content-Location identifies the original
location of the entity enclosed in the request. It is therefore
possible for a response to contain header fields for both Location
and Content-Location. Also see section 13.10 for cache requirements
of some methods.
14.31 Max-Forwards
The Max-Forwards request-header field may be used with the TRACE
method (section 14.31) to limit the number of proxies or gateways
that can forward the request to the next inbound server. This can be
useful when the client is attempting to trace a request chain which
appears to be failing or looping in mid-chain.
Max-Forwards = "Max-Forwards" ":" 1*DIGIT
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The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message may be forwarded.
Each proxy or gateway recipient of a TRACE request containing a Max-
Forwards header field SHOULD check and update its value prior to
forwarding the request. If the received value is zero (0), the
recipient SHOULD NOT forward the request; instead, it SHOULD respond
as the final recipient with a 200 (OK) response containing the
received request message as the response entity-body (as described in
section 9.8). If the received Max-Forwards value is greater than
zero, then the forwarded message SHOULD contain an updated Max-
Forwards field with a value decremented by one (1).
The Max-Forwards header field SHOULD be ignored for all other methods
defined by this specification and for any extension methods for which
it is not explicitly referred to as part of that method definition.
14.32 Pragma
The Pragma general-header field is used to include implementation-
specific directives that may apply to any recipient along the
request/response chain. All pragma directives specify optional
behavior from the viewpoint of the protocol; however, some systems
MAY require that behavior be consistent with the directives.
Pragma = "Pragma" ":" 1#pragma-directive
pragma-directive = "no-cache" | extension-pragma
extension-pragma = token [ "=" ( token | quoted-string ) ]
When the no-cache directive is present in a request message, an
application SHOULD forward the request toward the origin server even
if it has a cached copy of what is being requested. This pragma
directive has the same semantics as the no-cache cache-directive (see
section 14.9) and is defined here for backwards compatibility with
HTTP/1.0. Clients SHOULD include both header fields when a no-cache
request is sent to a server not known to be HTTP/1.1 compliant.
Pragma directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a pragma for a
specific recipient; however, any pragma directive not relevant to a
recipient SHOULD be ignored by that recipient.
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HTTP/1.1 clients SHOULD NOT send the Pragma request-header. HTTP/1.1
caches SHOULD treat "Pragma: no-cache" as if the client had sent
"Cache-Control: no-cache". No new Pragma directives will be defined
in HTTP.
14.33 Proxy-Authenticate
The Proxy-Authenticate response-header field MUST be included as part
of a 407 (Proxy Authentication Required) response. The field value
consists of a challenge that indicates the authentication scheme and
parameters applicable to the proxy for this Request-URI.
Proxy-Authenticate = "Proxy-Authenticate" ":" challenge
The HTTP access authentication process is described in section 11.
Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
only to the current connection and SHOULD NOT be passed on to
downstream clients. However, an intermediate proxy may need to obtain
its own credentials by requesting them from the downstream client,
which in some circumstances will appear as if the proxy is forwarding
the Proxy-Authenticate header field.
14.34 Proxy-Authorization
The Proxy-Authorization request-header field allows the client to
identify itself (or its user) to a proxy which requires
authentication. The Proxy-Authorization field value consists of
credentials containing the authentication information of the user
agent for the proxy and/or realm of the resource being requested.
Proxy-Authorization = "Proxy-Authorization" ":" credentials
The HTTP access authentication process is described in section 11.
Unlike Authorization, the Proxy-Authorization header field applies
only to the next outbound proxy that demanded authentication using
the Proxy-Authenticate field. When multiple proxies are used in a
chain, the Proxy-Authorization header field is consumed by the first
outbound proxy that was expecting to receive credentials. A proxy MAY
relay the credentials from the client request to the next proxy if
that is the mechanism by which the proxies cooperatively authenticate
a given request.
14.35 Public
The Public response-header field lists the set of methods supported
by the server. The purpose of this field is strictly to inform the
recipient of the capabilities of the server regarding unusual
methods. The methods listed may or may not be applicable to the
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Request-URI; the Allow header field (section 14.7) MAY be used to
indicate methods allowed for a particular URI.
Public = "Public" ":" 1#method
Example of use:
Public: OPTIONS, MGET, MHEAD, GET, HEAD
This header field applies only to the server directly connected to
the client (i.e., the nearest neighbor in a chain of connections). If
the response passes through a proxy, the proxy MUST either remove the
Public header field or replace it with one applicable to its own
capabilities.
14.36 Range
14.36.1 Byte Ranges
Since all HTTP entities are represented in HTTP messages as sequences
of bytes, the concept of a byte range is meaningful for any HTTP
entity. (However, not all clients and servers need to support byte-
range operations.)
Byte range specifications in HTTP apply to the sequence of bytes in
the entity-body (not necessarily the same as the message-body).
A byte range operation may specify a single range of bytes, or a set
of ranges within a single entity.
ranges-specifier = byte-ranges-specifier
byte-ranges-specifier = bytes-unit "=" byte-range-set
byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec )
byte-range-spec = first-byte-pos "-" [last-byte-pos]
first-byte-pos = 1*DIGIT
last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the byte-offset
of the first byte in a range. The last-byte-pos value gives the
byte-offset of the last byte in the range; that is, the byte
positions specified are inclusive. Byte offsets start at zero.
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If the last-byte-pos value is present, it must be greater than or
equal to the first-byte-pos in that byte-range-spec, or the byte-
range-spec is invalid. The recipient of an invalid byte-range-spec
must ignore it.
If the last-byte-pos value is absent, or if the value is greater than
or equal to the current length of the entity-body, last-byte-pos is
taken to be equal to one less than the current length of the entity-
body in bytes.
By its choice of last-byte-pos, a client can limit the number of
bytes retrieved without knowing the size of the entity.
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
A suffix-byte-range-spec is used to specify the suffix of the
entity-body, of a length given by the suffix-length value. (That is,
this form specifies the last N bytes of an entity-body.) If the
entity is shorter than the specified suffix-length, the entire
entity-body is used.
Examples of byte-ranges-specifier values (assuming an entity-body of
length 10000):
o The first 500 bytes (byte offsets 0-499, inclusive):
bytes=0-499
o The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999
o The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500
o Or
bytes=9500-
o The first and last bytes only (bytes 0 and 9999):
bytes=0-0,-1
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o Several legal but not canonical specifications of the second
500 bytes (byte offsets 500-999, inclusive):
bytes=500-600,601-999
bytes=500-700,601-999
14.36.2 Range Retrieval Requests
HTTP retrieval requests using conditional or unconditional GET
methods may request one or more sub-ranges of the entity, instead of
the entire entity, using the Range request header, which applies to
the entity returned as the result of the request:
Range = "Range" ":" ranges-specifier
A server MAY ignore the Range header. However, HTTP/1.1 origin
servers and intermediate caches SHOULD support byte ranges when
possible, since Range supports efficient recovery from partially
failed transfers, and supports efficient partial retrieval of large
entities.
If the server supports the Range header and the specified range or
ranges are appropriate for the entity:
o The presence of a Range header in an unconditional GET modifies
what is returned if the GET is otherwise successful. In other
words, the response carries a status code of 206 (Partial
Content) instead of 200 (OK).
o The presence of a Range header in a conditional GET (a request
using one or both of If-Modified-Since and If-None-Match, or
one or both of If-Unmodified-Since and If-Match) modifies what
is returned if the GET is otherwise successful and the condition
is true. It does not affect the 304 (Not Modified) response
returned if the conditional is false.
In some cases, it may be more appropriate to use the If-Range header
(see section 14.27) in addition to the Range header.
If a proxy that supports ranges receives a Range request, forwards
the request to an inbound server, and receives an entire entity in
reply, it SHOULD only return the requested range to its client. It
SHOULD store the entire received response in its cache, if that is
consistent with its cache allocation policies.
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14.37 Referer
The Referer[sic] request-header field allows the client to specify,
for the server's benefit, the address (URI) of the resource from
which the Request-URI was obtained (the "referrer", although the
header field is misspelled.) The Referer request-header allows a
server to generate lists of back-links to resources for interest,
logging, optimized caching, etc. It also allows obsolete or mistyped
links to be traced for maintenance. The Referer field MUST NOT be
sent if the Request-URI was obtained from a source that does not have
its own URI, such as input from the user keyboard.
Referer = "Referer" ":" ( absoluteURI | relativeURI )
Example:
Referer: http://www.w3.org/hypertext/DataSources/Overview.html
If the field value is a partial URI, it SHOULD be interpreted
relative to the Request-URI. The URI MUST NOT include a fragment.
Note: Because the source of a link may be private information or
may reveal an otherwise private information source, it is strongly
recommended that the user be able to select whether or not the
Referer field is sent. For example, a browser client could have a
toggle switch for browsing openly/anonymously, which would
respectively enable/disable the sending of Referer and From
information.
14.38 Retry-After
The Retry-After response-header field can be used with a 503 (Service
Unavailable) response to indicate how long the service is expected to
be unavailable to the requesting client. The value of this field can
be either an HTTP-date or an integer number of seconds (in decimal)
after the time of the response.
Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
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