, identifying the specific
version of the returned resource. The user should be able to check for
staleness in their subsequent requests by supplying the value in the
If-None-Match header.在所有的请求中带上 ETag 头 ， 用于识别特定版本的返回资源。用户可以在随后的请求中通过提供 If-None-Match头的值来检查内容是否过期。