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http1.1---5

王朝other·作者佚名  2006-01-08
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Fielding, et. al. Standards Track [Page 55]

RFC 2068 HTTP/1.1 January 1997

response is primarily intended to allow input for actions to take

place without causing a change to the user agent's active document

view. The response MAY include new metainformation in the form of

entity-headers, which SHOULD apply to the document currently in the

user agent's active view.

The 204 response MUST NOT include a message-body, and thus is always

terminated by the first empty line after the header fields.

10.2.6 205 Reset Content

The server has fulfilled the request and the user agent SHOULD reset

the document view which caused the request to be sent. This response

is primarily intended to allow input for actions to take place via

user input, followed by a clearing of the form in which the input is

given so that the user can easily initiate another input action. The

response MUST NOT include an entity.

10.2.7 206 Partial Content

The server has fulfilled the partial GET request for the resource.

The request must have included a Range header field (section 14.36)

indicating the desired range. The response MUST include either a

Content-Range header field (section 14.17) indicating the range

included with this response, or a multipart/byteranges Content-Type

including Content-Range fields for each part. If multipart/byteranges

is not used, the Content-Length header field in the response MUST

match the actual number of OCTETs transmitted in the message-body.

A cache that does not support the Range and Content-Range headers

MUST NOT cache 206 (Partial) responses.

10.3 Redirection 3xx

This class of status code indicates that further action needs to be

taken by the user agent in order to fulfill the request. The action

required MAY be carried out by the user agent without interaction

with the user if and only if the method used in the second request is

GET or HEAD. A user agent SHOULD NOT automatically redirect a request

more than 5 times, since such redirections usually indicate an

infinite loop.

Fielding, et. al. Standards Track [Page 56]

RFC 2068 HTTP/1.1 January 1997

10.3.1 300 Multiple Choices

The requested resource corresponds to any one of a set of

representations, each with its own specific location, and agent-

driven negotiation information (section 12) is being provided so that

the user (or user agent) can select a preferred representation and

redirect its request to that location.

Unless it was a HEAD request, the response SHOULD include an entity

containing a list of resource characteristics and location(s) from

which the user or user agent can choose the one most appropriate. The

entity format is specified by the media type given in the Content-

Type header field. Depending upon the format and the capabilities of

the user agent, selection of the most appropriate choice may be

performed automatically. However, this specification does not define

any standard for such automatic selection.

If the server has a preferred choice of representation, it SHOULD

include the specific URL for that representation in the Location

field; user agents MAY use the Location field value for automatic

redirection. This response is cachable unless indicated otherwise.

10.3.2 301 Moved Permanently

The requested resource has been assigned a new permanent URI and any

future references to this resource SHOULD be done using one of the

returned URIs. Clients with link editing capabilities SHOULD

automatically re-link references to the Request-URI to one or more of

the new references returned by the server, where possible. This

response is cachable unless indicated otherwise.

If the new URI is a location, its URL SHOULD be given by the Location

field in the response. Unless the request method was HEAD, the entity

of the response SHOULD contain a short hypertext note with a

hyperlink to the new URI(s).

If the 301 status code is received in response to a request other

than GET or HEAD, the user agent MUST NOT automatically redirect the

request unless it can be confirmed by the user, since this might

change the conditions under which the request was issued.

Note: When automatically redirecting a POST request after receiving

a 301 status code, some existing HTTP/1.0 user agents will

erroneously change it into a GET request.

Fielding, et. al. Standards Track [Page 57]

RFC 2068 HTTP/1.1 January 1997

10.3.3 302 Moved Temporarily

The requested resource resides temporarily under a different URI.

Since the redirection may be altered on occasion, the client SHOULD

continue to use the Request-URI for future requests. This response is

only cachable if indicated by a Cache-Control or Expires header

field.

If the new URI is a location, its URL SHOULD be given by the Location

field in the response. Unless the request method was HEAD, the entity

of the response SHOULD contain a short hypertext note with a

hyperlink to the new URI(s).

If the 302 status code is received in response to a request other

than GET or HEAD, the user agent MUST NOT automatically redirect the

request unless it can be confirmed by the user, since this might

change the conditions under which the request was issued.

Note: When automatically redirecting a POST request after receiving

a 302 status code, some existing HTTP/1.0 user agents will

erroneously change it into a GET request.

10.3.4 303 See Other

The response to the request can be found under a different URI and

SHOULD be retrieved using a GET method on that resource. This method

exists primarily to allow the output of a POST-activated script to

redirect the user agent to a selected resource. The new URI is not a

substitute reference for the originally requested resource. The 303

response is not cachable, but the response to the second (redirected)

request MAY be cachable.

If the new URI is a location, its URL SHOULD be given by the Location

field in the response. Unless the request method was HEAD, the entity

of the response SHOULD contain a short hypertext note with a

hyperlink to the new URI(s).

10.3.5 304 Not Modified

If the client has performed a conditional GET request and access is

allowed, but the document has not been modified, the server SHOULD

respond with this status code. The response MUST NOT contain a

message-body.

Fielding, et. al. Standards Track [Page 58]

RFC 2068 HTTP/1.1 January 1997

The response MUST include the following header fields:

o Date

o ETag and/or Content-Location, if the header would have been sent in

a 200 response to the same request

o Expires, Cache-Control, and/or Vary, if the field-value might

differ from that sent in any previous response for the same variant

If the conditional GET used a strong cache validator (see section

13.3.3), the response SHOULD NOT include other entity-headers.

Otherwise (i.e., the conditional GET used a weak validator), the

response MUST NOT include other entity-headers; this prevents

inconsistencies between cached entity-bodies and updated headers.

If a 304 response indicates an entity not currently cached, then the

cache MUST disregard the response and repeat the request without the

conditional.

If a cache uses a received 304 response to update a cache entry, the

cache MUST update the entry to reflect any new field values given in

the response.

The 304 response MUST NOT include a message-body, and thus is always

terminated by the first empty line after the header fields.

10.3.6 305 Use Proxy

The requested resource MUST be accessed through the proxy given by

the Location field. The Location field gives the URL of the proxy.

The recipient is expected to repeat the request via the proxy.

10.4 Client Error 4xx

The 4xx class of status code is intended for cases in which the

client seems to have erred. Except when responding to a HEAD request,

the server SHOULD include an entity containing an explanation of the

error situation, and whether it is a temporary or permanent

condition. These status codes are applicable to any request method.

User agents SHOULD display any included entity to the user.

Note: If the client is sending data, a server implementation using

TCP should be careful to ensure that the client acknowledges

receipt of the packet(s) containing the response, before the server

closes the input connection. If the client continues sending data

to the server after the close, the server's TCP stack will send a

reset packet to the client, which may erase the client's

Fielding, et. al. Standards Track [Page 59]

RFC 2068 HTTP/1.1 January 1997

unacknowledged input buffers before they can be read and

interpreted by the HTTP application.

10.4.1 400 Bad Request

The request could not be understood by the server due to malformed

syntax. The client SHOULD NOT repeat the request without

modifications.

10.4.2 401 Unauthorized

The request requires user authentication. The response MUST include a

WWW-Authenticate header field (section 14.46) containing a challenge

applicable to the requested resource. The client MAY repeat the

request with a suitable Authorization header field (section 14.8). If

the request already included Authorization credentials, then the 401

response indicates that authorization has been refused for those

credentials. If the 401 response contains the same challenge as the

prior response, and the user agent has already attempted

authentication at least once, then the user SHOULD be presented the

entity that was given in the response, since that entity MAY include

relevant diagnostic information. HTTP access authentication is

explained in section 11.

10.4.3 402 Payment Required

This code is reserved for future use.

10.4.4 403 Forbidden

The server understood the request, but is refusing to fulfill it.

Authorization will not help and the request SHOULD NOT be repeated.

If the request method was not HEAD and the server wishes to make

public why the request has not been fulfilled, it SHOULD describe the

reason for the refusal in the entity. This status code is commonly

used when the server does not wish to reveal exactly why the request

has been refused, or when no other response is applicable.

10.4.5 404 Not Found

The server has not found anything matching the Request-URI. No

indication is given of whether the condition is temporary or

permanent.

Fielding, et. al. Standards Track [Page 60]

RFC 2068 HTTP/1.1 January 1997

If the server does not wish to make this information available to the

client, the status code 403 (Forbidden) can be used instead. The 410

(Gone) status code SHOULD be used if the server knows, through some

internally configurable mechanism, that an old resource is

permanently unavailable and has no forwarding address.

10.4.6 405 Method Not Allowed

The method specified in the Request-Line is not allowed for the

resource identified by the Request-URI. The response MUST include an

Allow header containing a list of valid methods for the requested

resource.

10.4.7 406 Not Acceptable

The resource identified by the request is only capable of generating

response entities which have content characteristics not acceptable

according to the accept headers sent in the request.

Unless it was a HEAD request, the response SHOULD include an entity

containing a list of available entity characteristics and location(s)

from which the user or user agent can choose the one most

appropriate. The entity format is specified by the media type given

in the Content-Type header field. Depending upon the format and the

capabilities of the user agent, selection of the most appropriate

choice may be performed automatically. However, this specification

does not define any standard for such automatic selection.

Note: HTTP/1.1 servers are allowed to return responses which are

not acceptable according to the accept headers sent in the request.

In some cases, this may even be preferable to sending a 406

response. User agents are encouraged to inspect the headers of an

incoming response to determine if it is acceptable. If the response

could be unacceptable, a user agent SHOULD temporarily stop receipt

of more data and query the user for a decision on further actions.

10.4.8 407 Proxy Authentication Required

This code is similar to 401 (Unauthorized), but indicates that the

client MUST first authenticate itself with the proxy. The proxy MUST

return a Proxy-Authenticate header field (section 14.33) containing a

challenge applicable to the proxy for the requested resource. The

client MAY repeat the request with a suitable Proxy-Authorization

header field (section 14.34). HTTP access authentication is explained

in section 11.

Fielding, et. al. Standards Track [Page 61]

RFC 2068 HTTP/1.1 January 1997

10.4.9 408 Request Timeout

The client did not produce a request within the time that the server

was prepared to wait. The client MAY repeat the request without

modifications at any later time.

10.4.10 409 Conflict

The request could not be completed due to a conflict with the current

state of the resource. This code is only allowed in situations where

it is expected that the user might be able to resolve the conflict

and resubmit the request. The response body SHOULD include enough

information for the user to recognize the source of the conflict.

Ideally, the response entity would include enough information for the

user or user agent to fix the problem; however, that may not be

possible and is not required.

Conflicts are most likely to occur in response to a PUT request. If

versioning is being used and the entity being PUT includes changes to

a resource which conflict with those made by an earlier (third-party)

request, the server MAY use the 409 response to indicate that it

can't complete the request. In this case, the response entity SHOULD

contain a list of the differences between the two versions in a

format defined by the response Content-Type.

10.4.11 410 Gone

The requested resource is no longer available at the server and no

forwarding address is known. This condition SHOULD be considered

permanent. Clients with link editing capabilities SHOULD delete

references to the Request-URI after user approval. If the server does

not know, or has no facility to determine, whether or not the

condition is permanent, the status code 404 (Not Found) SHOULD be

used instead. This response is cachable unless indicated otherwise.

The 410 response is primarily intended to assist the task of web

maintenance by notifying the recipient that the resource is

intentionally unavailable and that the server owners desire that

remote links to that resource be removed. Such an event is common for

limited-time, promotional services and for resources belonging to

individuals no longer working at the server's site. It is not

necessary to mark all permanently unavailable resources as "gone" or

to keep the mark for any length of time -- that is left to the

discretion of the server owner.

Fielding, et. al. Standards Track [Page 62]

RFC 2068 HTTP/1.1 January 1997

10.4.12 411 Length Required

The server refuses to accept the request without a defined Content-

Length. The client MAY repeat the request if it adds a valid

Content-Length header field containing the length of the message-body

in the request message.

10.4.13 412 Precondition Failed

The precondition given in one or more of the request-header fields

evaluated to false when it was tested on the server. This response

code allows the client to place preconditions on the current resource

metainformation (header field data) and thus prevent the requested

method from being applied to a resource other than the one intended.

10.4.14 413 Request Entity Too Large

The server is refusing to process a request because the request

entity is larger than the server is willing or able to process. The

server may close the connection to prevent the client from continuing

the request.

If the condition is temporary, the server SHOULD include a Retry-

After header field to indicate that it is temporary and after what

time the client may try again.

10.4.15 414 Request-URI Too Long

The server is refusing to service the request because the Request-URI

is longer than the server is willing to interpret. This rare

condition is only likely to occur when a client has improperly

converted a POST request to a GET request with long query

information, when the client has descended into a URL "black hole" of

redirection (e.g., a redirected URL prefix that points to a suffix of

itself), or when the server is under attack by a client attempting to

exploit security holes present in some servers using fixed-length

buffers for reading or manipulating the Request-URI.

10.4.16 415 Unsupported Media Type

The server is refusing to service the request because the entity of

the request is in a format not supported by the requested resource

for the requested method.

Fielding, et. al. Standards Track [Page 63]

RFC 2068 HTTP/1.1 January 1997

10.5 Server Error 5xx

Response status codes beginning with the digit "5" indicate cases in

which the server is aware that it has erred or is incapable of

performing the request. Except when responding to a HEAD request, the

server SHOULD include an entity containing an explanation of the

error situation, and whether it is a temporary or permanent

condition. User agents SHOULD display any included entity to the

user. These response codes are applicable to any request method.

10.5.1 500 Internal Server Error

The server encountered an unexpected condition which prevented it

from fulfilling the request.

10.5.2 501 Not Implemented

The server does not support the functionality required to fulfill the

request. This is the appropriate response when the server does not

recognize the request method and is not capable of supporting it for

any resource.

10.5.3 502 Bad Gateway

The server, while acting as a gateway or proxy, received an invalid

response from the upstream server it accessed in attempting to

fulfill the request.

10.5.4 503 Service Unavailable

The server is currently unable to handle the request due to a

temporary overloading or maintenance of the server. The implication

is that this is a temporary condition which will be alleviated after

some delay. If known, the length of the delay may be indicated in a

Retry-After header. If no Retry-After is given, the client SHOULD

handle the response as it would for a 500 response.

Note: The existence of the 503 status code does not imply that a

server must use it when becoming overloaded. Some servers may wish

to simply refuse the connection.

10.5.5 504 Gateway Timeout

The server, while acting as a gateway or proxy, did not receive a

timely response from the upstream server it accessed in attempting to

complete the request.

Fielding, et. al. Standards Track [Page 64]

RFC 2068 HTTP/1.1 January 1997

10.5.6 505 HTTP Version Not Supported

The server does not support, or refuses to support, the HTTP protocol

version that was used in the request message. The server is

indicating that it is unable or unwilling to complete the request

using the same major version as the client, as described in section

3.1, other than with this error message. The response SHOULD contain

an entity describing why that version is not supported and what other

protocols are supported by that server.

11 Access Authentication

HTTP provides a simple challenge-response authentication mechanism

which MAY be used by a server to challenge a client request and by a

client to provide authentication information. It uses an extensible,

case-insensitive token to identify the authentication scheme,

followed by a comma-separated list of attribute-value pairs which

carry the parameters necessary for achieving authentication via that

scheme.

auth-scheme = token

auth-param = token "=" quoted-string

The 401 (Unauthorized) response message is used by an origin server

to challenge the authorization of a user agent. This response MUST

include a WWW-Authenticate header field containing at least one

challenge applicable to the requested resource.

challenge = auth-scheme 1*SP realm *( "," auth-param )

realm = "realm" "=" realm-value

realm-value = quoted-string

The realm attribute (case-insensitive) is required for all

authentication schemes which issue a challenge. The realm value

(case-sensitive), in combination with the canonical root URL (see

section 5.1.2) of the server being accessed, defines the protection

space. These realms allow the protected resources on a server to be

partitioned into a set of protection spaces, each with its own

authentication scheme and/or authorization database. The realm value

is a string, generally assigned by the origin server, which may have

additional semantics specific to the authentication scheme.

A user agent that wishes to authenticate itself with a server--

usually, but not necessarily, after receiving a 401 or 411 response-

-MAY do so by including an Authorization header field with the

request. The Authorization field value consists of credentials

Fielding, et. al. Standards Track [Page 65]

RFC 2068 HTTP/1.1 January 1997

containing the authentication information of the user agent for the

realm of the resource being requested.

credentials = basic-credentials

| auth-scheme #auth-param

The domain over which credentials can be automatically applied by a

user agent is determined by the protection space. If a prior request

has been authorized, the same credentials MAY be reused for all other

requests within that protection space for a period of time determined

by the authentication scheme, parameters, and/or user preference.

Unless otherwise defined by the authentication scheme, a single

protection space cannot extend outside the scope of its server.

If the server does not wish to accept the credentials sent with a

request, it SHOULD return a 401 (Unauthorized) response. The response

MUST include a WWW-Authenticate header field containing the (possibly

new) challenge applicable to the requested resource and an entity

explaining the refusal.

The HTTP protocol does not restrict applications to this simple

challenge-response mechanism for access authentication. Additional

mechanisms MAY be used, such as encryption at the transport level or

via message encapsulation, and with additional header fields

specifying authentication information. However, these additional

mechanisms are not defined by this specification.

Proxies MUST be completely transparent regarding user agent

authentication. That is, they MUST forward the WWW-Authenticate and

Authorization headers untouched, and follow the rules found in

section 14.8.

HTTP/1.1 allows a client to pass authentication information to and

from a proxy via the Proxy-Authenticate and Proxy-Authorization

headers.

11.1 Basic Authentication Scheme

The "basic" authentication scheme is based on the model that the user

agent must authenticate itself with a user-ID and a password for each

realm. The realm value should be considered an opaque string which

can only be compared for equality with other realms on that server.

The server will service the request only if it can validate the

user-ID and password for the protection space of the Request-URI.

There are no optional authentication parameters.

Fielding, et. al. Standards Track [Page 66]

RFC 2068 HTTP/1.1 January 1997

Upon receipt of an unauthorized request for a URI within the

protection space, the server MAY respond with a challenge like the

following:

WWW-Authenticate: Basic realm="WallyWorld"

where "WallyWorld" is the string assigned by the server to identify

the protection space of the Request-URI.

To receive authorization, the client sends the userid and password,

separated by a single colon (":") character, within a base64 encoded

string in the credentials.

basic-credentials = "Basic" SP basic-cookie

basic-cookie = <base64 [7] encoding of user-pass,

except not limited to 76 char/line>

user-pass = userid ":" password

userid = *<TEXT excluding ":">

password = *TEXT

Userids might be case sensitive.

If the user agent wishes to send the userid "Aladdin" and password

"open sesame", it would use the following header field:

Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

See section 15 for security considerations associated with Basic

authentication.

11.2 Digest Authentication Scheme

A digest authentication for HTTP is specified in RFC 2069 [32].

12 Content Negotiation

Most HTTP responses include an entity which contains information for

interpretation by a human user. Naturally, it is desirable to supply

the user with the "best available" entity corresponding to the

request. Unfortunately for servers and caches, not all users have

the same preferences for what is "best," and not all user agents are

equally capable of rendering all entity types. For that reason, HTTP

has provisions for several mechanisms for "content negotiation" --

the process of selecting the best representation for a given response

Fielding, et. al. Standards Track [Page 67]

RFC 2068 HTTP/1.1 January 1997

when there are multiple representations available.

Note: This is not called "format negotiation" because the alternate

representations may be of the same media type, but use different

capabilities of that type, be in different languages, etc.

Any response containing an entity-body MAY be subject to negotiation,

including error responses.

There are two kinds of content negotiation which are possible in

HTTP: server-driven and agent-driven negotiation. These two kinds of

negotiation are orthogonal and thus may be used separately or in

combination. One method of combination, referred to as transparent

negotiation, occurs when a cache uses the agent-driven negotiation

information provided by the origin server in order to provide

server-driven negotiation for subsequent requests.

12.1 Server-driven Negotiation

If the selection of the best representation for a response is made by

an algorithm located at the server, it is called server-driven

negotiation. Selection is based on the available representations of

the response (the dimensions over which it can vary; e.g. language,

content-coding, etc.) and the contents of particular header fields in

the request message or on other information pertaining to the request

(such as the network address of the client).

Server-driven negotiation is advantageous when the algorithm for

selecting from among the available representations is difficult to

describe to the user agent, or when the server desires to send its

"best guess" to the client along with the first response (hoping to

avoid the round-trip delay of a subsequent request if the "best

guess" is good enough for the user). In order to improve the server's

guess, the user agent MAY include request header fields (Accept,

Accept-Language, Accept-Encoding, etc.) which describe its

preferences for such a response.

Server-driven negotiation has disadvantages:

1. It is impossible for the server to accurately determine what might be

"best" for any given user, since that would require complete

knowledge of both the capabilities of the user agent and the intended

use for the response (e.g., does the user want to view it on screen

or print it on paper?).

2. Having the user agent describe its capabilities in every request can

be both very inefficient (given that only a small percentage of

responses have multiple representations) and a potential violation of

Fielding, et. al. Standards Track [Page 68]

RFC 2068 HTTP/1.1 January 1997

the user's privacy.

3. It complicates the implementation of an origin server and the

algorithms for generating responses to a request.

4. It may limit a public cache's ability to use the same response for

multiple user's requests.

HTTP/1.1 includes the following request-header fields for enabling

server-driven negotiation through description of user agent

capabilities and user preferences: Accept (section 14.1), Accept-

Charset (section 14.2), Accept-Encoding (section 14.3), Accept-

Language (section 14.4), and User-Agent (section 14.42). However, an

origin server is not limited to these dimensions and MAY vary the

response based on any aspect of the request, including information

outside the request-header fields or within extension header fields

not defined by this specification.

HTTP/1.1 origin servers MUST include an appropriate Vary header field

(section 14.43) in any cachable response based on server-driven

negotiation. The Vary header field describes the dimensions over

which the response might vary (i.e. the dimensions over which the

origin server picks its "best guess" response from multiple

representations).

HTTP/1.1 public caches MUST recognize the Vary header field when it

is included in a response and obey the requirements described in

section 13.6 that describes the interactions between caching and

content negotiation.

12.2 Agent-driven Negotiation

With agent-driven negotiation, selection of the best representation

for a response is performed by the user agent after receiving an

initial response from the origin server. Selection is based on a list

of the available representations of the response included within the

header fields (this specification reserves the field-name Alternates,

as described in appendix 19.6.2.1) or entity-body of the initial

response, with each representation identified by its own URI.

Selection from among the representations may be performed

automatically (if the user agent is capable of doing so) or manually

by the user selecting from a generated (possibly hypertext) menu.

Agent-driven negotiation is advantageous when the response would vary

over commonly-used dimensions (such as type, language, or encoding),

when the origin server is unable to determine a user agent's

capabilities from examining the request, and generally when public

caches are used to distribute server load and reduce network usage.

Fielding, et. al. Standards Track [Page 69]

RFC 2068 HTTP/1.1 January 1997

Agent-driven negotiation suffers from the disadvantage of needing a

second request to obtain the best alternate representation. This

second request is only efficient when caching is used. In addition,

this specification does not define any mechanism for supporting

automatic selection, though it also does not prevent any such

mechanism from being developed as an extension and used within

HTTP/1.1.

HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)

status codes for enabling agent-driven negotiation when the server is

unwilling or unable to provide a varying response using server-driven

negotiation.

12.3 Transparent Negotiation

Transparent negotiation is a combination of both server-driven and

agent-driven negotiation. When a cache is supplied with a form of the

list of available representations of the response (as in agent-driven

negotiation) and the dimensions of variance are completely understood

by the cache, then the cache becomes capable of performing server-

driven negotiation on behalf of the origin server for subsequent

requests on that resource.

Transparent negotiation has the advantage of distributing the

negotiation work that would otherwise be required of the origin

server and also removing the second request delay of agent-driven

negotiation when the cache is able to correctly guess the right

response.

This specification does not define any mechanism for transparent

negotiation, though it also does not prevent any such mechanism from

being developed as an extension and used within HTTP/1.1. An HTTP/1.1

cache performing transparent negotiation MUST include a Vary header

field in the response (defining the dimensions of its variance) if it

is cachable to ensure correct interoperation with all HTTP/1.1

clients. The agent-driven negotiation information supplied by the

origin server SHOULD be included with the transparently negotiated

response.

13 Caching in HTTP

HTTP is typically used for distributed information systems, where

performance can be improved by the use of response caches. The

HTTP/1.1 protocol includes a number of elements intended to make

caching work as well as possible. Because these elements are

inextricable from other aspects of the protocol, and because they

interact with each other, it is useful to describe the basic caching

design of HTTP separately from the detailed descriptions of methods,

Fielding, et. al. Standards Track [Page 70]

RFC 2068 HTTP/1.1 January 1997

headers, response codes, etc.

Caching would be useless if it did not significantly improve

performance. The goal of caching in HTTP/1.1 is to eliminate the need

to send requests in many cases, and to eliminate the need to send

full responses in many other cases. The former reduces the number of

network round-trips required for many operations; we use an

"expiration" mechanism for this purpose (see section 13.2). The

latter reduces network bandwidth requirements; we use a "validation"

mechanism for this purpose (see section 13.3).

Requirements for performance, availability, and disconnected

operation require us to be able to relax the goal of semantic

transparency. The HTTP/1.1 protocol allows origin servers, caches,

and clients to explicitly reduce transparency when necessary.

However, because non-transparent operation may confuse non-expert

users, and may be incompatible with certain server applications (such

as those for ordering merchandise), the protocol requires that

transparency be relaxed

o only by an explicit protocol-level request when relaxed by client

or origin server

o only with an explicit warning to the end user when relaxed by cache

or client

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