Giao thức cũng có thể hoạt động ngược lại: máy chủ Cache có thể không có lời mời, hãy nói cho các máy chủ khác để thay đổi lưu trữ của họ. Các tin nhắn để làm điều đó được thiết lập và clr. Như con số 5,31 cho thấy, ngay cả một máy chủ Web nguồn gốc có thể sử dụng htcp để giữ cho các máy chủ bộ nhớ cache hỗ trợ nó cho đến nay. | Accelerating HTTP 221 Table continued Field Meaning IDENTITY The object in the local cache that changed. METHOD The HTTP method used to access the object. URI The object s Uniform Resource Identifier. VERSION The HTTP version used to access the object. REQ-HDRS The HTTP headers included in the request for the object. RESP-HDRS The HTTP headers included in the re- sponse to the request. ENTITY-HDRS HTTP headers applying to the object. CACHE-HDRS Cache information about the object. The htcp mon exchange allows a cache server to ask for updates to another s cache. The protocol can also operate in reverse Cache servers can without invitation tell other servers to modify their caches. The messages to do that are set and clr. As figure shows even an origin Web server can use HTCP to keep cache servers supporting it up to date. The SET and CLR messages are tools that the origin server could use to do so. A SET message updates the headers corresponding to an object including for example its expiration time. A Internet Figure Origin servers may use HTCP to proactively update cache servers telling them for example when HTTP headers corresponding to a cached object have changed. 222 HTTP Essentials clr message asks a cache server to remove the object from its cache entirely. Because the set and clr messages allow an external system to modify the contents of a server s cache it is important to be able to verify the identity of the system that sends them. To provide that verification htcp defines a mechanism for authenticating system identity. The approach is very similar to that of the Network Element Control Protocol. The communicating systems must first share a secret value. A sending system adds the contents of the message to the secret key computes a cryptographic digest of the combination and appends that digest result to the message. The receiving system performs the same computation and makes sure that the digest results match. If they don t match the .
