content-range-spec = byte-content-range-spec
byte-content-range-spec = bytes-unit SP first-byte-pos "-"
last-byte-pos "/" entity-length
entity-length = 1*DIGIT
Unlike byte-ranges-specifier values, a byte-content-range-spec may
only specify one range, and must contain absolute byte positions for
both the first and last byte of the range.
A byte-content-range-spec whose last-byte-pos value is less than its
first-byte-pos value, or whose entity-length value is less than or
equal to its last-byte-pos value, is invalid. The recipient of an
invalid byte-content-range-spec MUST ignore it and any content
transferred along with it.
Examples of byte-content-range-spec values, assuming that the entity
contains a total of 1234 bytes:
o The first 500 bytes:
bytes 0-499/1234
o The second 500 bytes:
bytes 500-999/1234
o All except for the first 500 bytes:
bytes 500-1233/1234
o The last 500 bytes:
bytes 734-1233/1234
When an HTTP message includes the content of a single range (for
example, a response to a request for a single range, or to a request
for a set of ranges that overlap without any holes), this content is
transmitted with a Content-Range header, and a Content-Length header
showing the number of bytes actually transferred. For example,
HTTP/1.1 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022
Content-Length: 26012
Content-Type: image/gif
When an HTTP message includes the content of multiple ranges (for
example, a response to a request for multiple non-overlapping
ranges), these are transmitted as a multipart MIME message. The
multipart MIME content-type used for this purpose is defined in this
specification to be "multipart/byteranges". See appendix 19.2 for its
definition.
A client that cannot decode a MIME multipart/byteranges message
should not ask for multiple byte-ranges in a single request.
When a client requests multiple byte-ranges in one request, the
server SHOULD return them in the order that they appeared in the
request.
If the server ignores a byte-range-spec because it is invalid, the
server should treat the request as if the invalid Range header field
did not exist. (Normally, this means return a 200 response containing
the full entity). The reason is that the only time a client will make
such an invalid request is when the entity is smaller than the entity
retrieved by a prior request.
14.18 Content-Type
The Content-Type entity-header field indicates the media type of the
entity-body sent to the recipient or, in the case of the HEAD method,
the media type that would have been sent had the request been a GET.
Content-Type = "Content-Type" ":" media-type
Media types are defined in section 3.7. An example of the field is
Content-Type: text/html; charset=ISO-8859-4
Further discussion of methods for identifying the media type of an
entity is provided in section 7.2.1.
14.19 Date
The Date general-header field represents the date and time at which
the message was originated, having the same semantics as orig-date in
RFC822. The field value is an HTTP-date, as described in section
3.3.1.
Date = "Date" ":" HTTP-date
An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT
If a message is received via direct connection with the user agent
(in the case of requests) or the origin server (in the case of
responses), then the date can be assumed to be the current date at
the receiving end. However, since the date--as it is believed by the
origin--is important for evaluating cached responses, origin servers
MUST include a Date header field in all responses. Clients SHOULD
only send a Date header field in messages that include an entity-
body, as in the case of the PUT and POST requests, and even then it
is optional. A received message which does not have a Date header
field SHOULD be assigned one by the recipient if the message will be
cached by that recipient or gatewayed via a protocol which requires a
Date.
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
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 RFC822 (as updated by RFC1123 ):
From = "From" ":" mailbox
An example is:
From: webmaster@w3.org
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.
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
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.
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.
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.
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.
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.
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
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.
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
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.
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
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.
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.
14.39 Server
The Server response-header field contains information about the
software used by the origin server to handle the request. The field
can contain multiple product tokens (section 3.8) and comments
identifying the server and any significant subproducts. The product
tokens are listed in order of their significance for identifying the
application.
Server = "Server" ":" 1*( product | comment )
Example:
Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the proxy
application MUST NOT modify the Server response-header. Instead, it
SHOULD include a Via field (as described in section 14.44).
Note: Revealing the specific software version of the server may
allow the server machine to become more vulnerable to attacks
against software that is known to contain security holes. Server
implementers are encouraged to make this field a configurable
option.
14.40 Transfer-Encoding
The Transfer-Encoding general-header field indicates what (if any)
type of transformation has been applied to the message body in order
to safely transfer it between the sender and the recipient. This
differs from the Content-Encoding in that the transfer coding is a
property of the message, not of the entity.
Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-
coding
Transfer codings are defined in section 3.6. An example is:
Transfer-Encoding: chunked
Many older HTTP/1.0 applications do not understand the Transfer-
Encoding header.
14.41 Upgrade
The Upgrade general-header allows the client to specify what
additional communication protocols it supports and would like to use
if the server finds it appropriate to switch protocols. The server
MUST use the Upgrade header field within a 101 (Switching Protocols)
response to indicate which protocol(s) are being switched.
Upgrade = "Upgrade" ":" 1#product
For example,
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
The Upgrade header field is intended to provide a simple mechanism
for transition from HTTP/1.1 to some other, incompatible protocol. It
does so by allowing the client to advertise its desire to use another
protocol, such as a later version of HTTP with a higher major version
number, even though the current request has been made using HTTP/1.1.
This eases the difficult transition between incompatible protocols by
allowing the client to initiate a request in the more commonly
supported protocol while indicating to the server that it would like
to use a "better" protocol if available (where "better" is determined
by the server, possibly according to the nature of the method and/or
resource being requested).
The Upgrade header field only applies to switching application-layer
protocols upon the existing transport-layer connection. Upgrade
cannot be used to insist on a protocol change; its acceptance and use
by the server is optional. The capabilities and nature of the
application-layer communication after the protocol change is entirely
dependent upon the new protocol chosen, although the first action
after changing the protocol MUST be a response to the initial HTTP
request containing the Upgrade header field.
The Upgrade header field only applies to the immediate connection.
Therefore, the upgrade keyword MUST be supplied within a Connection
header field (section 14.10) whenever Upgrade is present in an
HTTP/1.1 message.
The Upgrade header field cannot be used to indicate a switch to a
protocol on a different connection. For that purpose, it is more
appropriate to use a 301, 302, 303, or 305 redirection response.
This specification only defines the protocol name "HTTP" for use by
the family of Hypertext Transfer Protocols, as defined by the HTTP
version rules of section 3.1 and future updates to this
specification. Any token can be used as a protocol name; however, it
will only be useful if both the client and server associate the name
with the same protocol.
14.42 User-Agent
The User-Agent request-header field contains information about the
user agent originating the request. This is for statistical purposes,
the tracing of protocol violations, and automated recognition of user
agents for the sake of tailoring responses to avoid particular user
agent limitations. User agents SHOULD include this field with
requests. The field can contain multiple product tokens (section 3.8)
and comments identifying the agent and any subproducts which form a
significant part of the user agent. By convention, the product tokens
are listed in order of their significance for identifying the
application.
User-Agent = "User-Agent" ":" 1*( product | comment )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
14.43 Vary
The Vary response-header field is used by a server to signal that the
response entity was selected from the available representations of
the response using server-driven negotiation (section 12). Field-
names listed in Vary headers are those of request-headers. The Vary
field value indicates either that the given set of header fields
encompass the dimensions over which the representation might vary, or
that the dimensions of variance are unspecified ("*") and thus may
vary over any aspect of future requests.
Vary = "Vary" ":" ( "*" | 1#field-name )
An HTTP/1.1 server MUST include an appropriate Vary header field with
any cachable response that is subject to server-driven negotiation.
Doing so allows a cache to properly interpret future requests on that
resource and informs the user agent about the presence of negotiation
on that resource. A server SHOULD include an appropriate Vary header
field with a non-cachable response that is subject to server-driven
negotiation, since this might provide the user agent with useful
information about the dimensions over which the response might vary.
The set of header fields named by the Vary field value is known as
the "selecting" request-headers.
When the cache receives a subsequent request whose Request-URI
specifies one or more cache entries including a Vary header, the
cache MUST NOT use such a cache entry to construct a response to the
new request unless all of the headers named in the cached Vary header
are present in the new request, and all of the stored selecting
request-headers from the previous request match the corresponding
headers in the new request.
The selecting request-headers from two requests are defined to match
if and only if the selecting request-headers in the first request can
be transformed to the selecting request-headers in the second request
by adding or removing linear whitespace (LWS) at places where this is
allowed by the corresponding BNF, and/or combining multiple message-
header fields with the same field name following the rules about
message headers in section 4.2.
A Vary field value of "*" signals that unspecified parameters,
possibly other than the contents of request-header fields (e.g., the
network address of the client), play a role in the selection of the
response representation. Subsequent requests on that resource can
only be properly interpreted by the origin server, and thus a cache
MUST forward a (possibly conditional) request even when it has a
fresh response cached for the resource. See section 13.6 for use of
the Vary header by caches.
A Vary field value consisting of a list of field-names signals that
the representation selected for the response is based on a selection
algorithm which considers ONLY the listed request-header field values
in selecting the most appropriate representation. A cache MAY assume
that the same selection will be made for future requests with the
same values for the listed field names, for the duration of time in
which the response is fresh.
The field-names given are not limited to the set of standard
request-header fields defined by this specification. Field names are
case-insensitive.
14.44 Via
The Via general-header field MUST be used by gateways and proxies to
indicate the intermediate protocols and recipients between the user
agent and the server on requests, and between the origin server and
the client on responses. It is analogous to the "Received" field of
RFC822 and is intended to be used for tracking message forwards,
avoiding request loops, and identifying the protocol capabilities of
all senders along the request/response chain.
Via = "Via" ":" 1#( received-protocol received-by [ comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
protocol-name = token
protocol-version = token
received-by = ( host [ ":" port ] ) | pseudonym
pseudonym = token
The received-protocol indicates the protocol version of the message
received by the server or client along each segment of the
request/response chain. The received-protocol version is appended to
the Via field value when the message is forwarded so that information
about the protocol capabilities of upstream applications remains
visible to all recipients.
The protocol-name is optional if and only if it would be "HTTP". The
received-by field is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message.
However, if the real host is considered to be sensitive information,
it MAY be replaced by a pseudonym. If the port is not given, it MAY
be assumed to be the default port of the received-protocol.
Multiple Via field values represent each proxy or gateway that has
forwarded the message. Each recipient MUST append its information
such that the end result is ordered according to the sequence of
forwarding applications.
Comments MAY be used in the Via header field to identify the software
of the recipient proxy or gateway, analogous to the User-Agent and
Server header fields. However, all comments in the Via field are
optional and MAY be removed by any recipient prior to forwarding the
message.
For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at nowhere.com, which completes
the request by forwarding it to the origin server at www.ics.uci.edu.
The request received by www.ics.uci.edu would then have the following
Via header field:
Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
Proxies and gateways used as a portal through a network firewall
SHOULD NOT, by default, forward the names and ports of hosts within
the firewall region. This information SHOULD only be propagated if
explicitly enabled. If not enabled, the received-by host of any host
behind the firewall SHOULD be replaced by an appropriate pseudonym
for that host.
For organizations that have strong privacy requirements for hiding
internal structures, a proxy MAY combine an ordered subsequence of
Via header field entries with identical received-protocol values into
a single such entry. For example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
Applications SHOULD NOT combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. Applications MUST NOT combine entries which
have different received-protocol values.
14.45 Warning
The Warning response-header field is used to carry additional
information about the status of a response which may not be reflected
by the response status code. This information is typically, though
not exclusively, used to warn about a possible lack of semantic
transparency from caching operations.
Warning headers are sent with responses using:
Warning = "Warning" ":" 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
warn-code = 2DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding
; the Warning header, for use in debugging
warn-text = quoted-string
A response may carry more than one Warning header.
The warn-text should be in a natural language and character set that
is most likely to be intelligible to the human user receiving the
response. This decision may be based on any available knowledge,
such as the location of the cache or user, the Accept-Language field
in a request, the Content-Language field in a response, etc. The
default language is English and the default character set is ISO-
8859-1.
If a character set other than ISO-8859-1 is used, it MUST be encoded
in the warn-text using the method described in RFC1522 [14].
Any server or cache may add Warning headers to a response. New
Warning headers should be added after any existing Warning headers. A
cache MUST NOT delete any Warning header that it received with a
response. However, if a cache successfully validates a cache entry,
it SHOULD remove any Warning headers previously attached to that
entry except as specified for specific Warning codes. It MUST then
add any Warning headers received in the validating response. In other
words, Warning headers are those that would be attached to the most
recent relevant response.
When multiple Warning headers are attached to a response, the user
agent SHOULD display as many of them as possible, in the order that
they appear in the response. If it is not possible to display all of
the warnings, the user agent should follow these heuristics:
o Warnings that appear early in the response take priority over those
appearing later in the response.
o Warnings in the user's preferred character set take priority over
warnings in other character sets but with identical warn-codes and
warn-agents.
Systems that generate multiple Warning headers should order them with
this user agent behavior in mind.
This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.
10 Response is stale
MUST be included whenever the returned response is stale. A cache may
add this warning to any response, but may never remove it until the