a received message that does not have a date header field must be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires a date. an http implementation without a clock must not cache responses without revalidating them on every use. an http cache,David_Tai_情刚_新浪博客,www.ld6899.com,捷省牌JT通用省电器,省电工程改造和咨询,捷省牌JL路灯省电器 - 宁波市捷能电器有限公司网站, especially a shared cache, should use a mechanism,Internet Explorer 8 for WinXP 简体中文官方版 下载 - 天空软件站 - 浏览工具 - 网络软件, such as ntp [28],八旬助学老人拟宴请200名四川震区孩子, to synchronize its clock with a reliable external standard.

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,赛马会, and even then it is optional. a client without a clock must not send a date header field in a request.

the http-date sent in a date header should not represent a date and time subsequent to the generation of the message. it should represent the best available approximation of the date and time of message generation,菲律宾太阳城, unless the implementation has no means of generating a reasonably accurate date and time. in theory,全讯网, the date ought to 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.

some origin server implementations might not have a clock available. an origin server without a clock must not assign expires or last- modified values to a response, unless these values were associated with the resource by a system or user with a reliable clock. it may assign an expires value that is known,六合彩资料, at or before server configuration time,广西建设职业技术学院,高清电视棒, to be in the past (this allows "pre-expiration" of responses without storing separate expires values for each resource).

the etag response-header field provides the current value of the entity tag for the requested variant. the headers used with entity tags are described in sections 14.24, 14.26 and 14.44. the entity tag may be used for comparison with other entities from the same resource (see section 13.3.3).