cho phép một khách hàng rõ ràng cho thấy những loại nội dung có thể chấp nhận trong cơ thể tin nhắn trả lời của máy chủ, cũng như sở thích tương đối của nó đối với từng loại nội dung. Dưới đây là một ví dụ vềiêu đề hy vọng Ngày hết hạn từ máy chủ Nếu | HTTP Messages OGET URI If-Modified Thu 11 Oct 2000. GET URI Client Proxy Origin Server 89 Figure A proxy server can use the If-Modified header to ask for an object only if it has changed. In this example the object has not changed so the server returns a 304 status. 304 Not Modified Last-Modified Thu 11 Oct 2000. may have saved considerable network bandwidth and delay because the object doesn t have to travel from the origin server to the proxy a second time. Clients can use the If-Modified-Since header not only for standard requests but also for partial requests with the Range header. In such cases the If-Modified-Since value applies to the object as a whole not just to the requested part of the object. Servers receiving requests with If-Modified-Since headers should honor that header only if they would otherwise return a 200 OK status. Also if the date in an If-Modified-Since header is invalid either because it is in the wrong format or because it is later than the server s current time then the server should ignore the header and return the resource. Clients that use the If-Modified-Since header should take into account two problems with many deployed servers. First some servers compare the If-Modified-Since value for an exact match with the resource s Last-Modified value. Even if the If-Modified-Since value is later than the Last-Modified value those servers will return the full entity. Clients that want to accommodate this behavior should use only values from Last-Modified headers. The second issue is one of clock synchronization. Clients should be aware that server clocks may not always be correct they are 90 HTTP Essentials subject both to inaccuracies in timing and to human errors . in setting the wrong time zone . Again the best way for clients to accommodate such problems is to use only values from the servers Last-Modified headers. If-None-Match The If-None-Match header is the complement of the If-Match header it has the exact opposite .
