HTTP Date标头背后的理由是什么？ [英] What's the rationale behind the HTTP Date header?

问题描述

我已阅读 RFC 2616 ，但仍然我不知道，Date字段是什么。最后修改的字段，除了仅仅提供元数据，即缓存（If-Modified-Since）之外，其实还有一个意义。

I have read RFC 2616, but still I wonder, what the Date field is for. There is the Last-Modified field, that actually has a meaning besides just serving metadata, that is, for caching ('If-Modified-Since').

但是使用它在单独的日期标题中将信息加倍？

But what use has it to double the info in a separate Date header?

推荐答案

Per 规格，它用于年龄计算。如果您不知道服务器什么时候认为是这样，您将无法计算资源的年龄。以下是规范的相关文本：

Per the spec, it is used in age calculations. If you don't know what time the server thinks it is, you won't be able to calculate the "age" of a resource. Here's the relevant text from the spec:

当缓存收到响应时，年龄计算算法摘要：

Summary of age calculation algorithm, when a cache receives a response:

age_value

是由$ b缓存接收的Age： $ b此回复。

age_value
is the value of Age: header received by the cache with this response.

date_value

是原始服务器的价值日期：标题

request_time

是当缓存提出请求
导致缓存响应的（本地）时间

request_time
is the (local) time when the cache made the request that resulted in this cached response

response_time

是缓存收到
响应的（本地）时间

response_time
is the (local) time when the cache received the response

现在

是当前（本地）时间

now
is the current (local) time

apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age   = corrected_initial_age + resident_time;

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