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The following diagram shows the communication in a black box relationship.
Figure 2: Black box relationship
WSUS is depicted in the gray box and is composed of an update client and two update servers. Specifically, the diagram depicts two update servers in the upstream server (USS) and downstream server (DSS) roles and an update client. Communication between an update client and an update server uses the Windows Update Services: Client-Server Protocol (WUSP). Communication between the DSS and USS uses the Windows Update Services: Server-Server Protocol (WSUSSS). Content download by the update client and DSS is accomplished by making requests to the Background Intelligent Transfer Service (BITS), as specified in [MSDN-BITS]. BITS relays the requests over the HTTP protocol.
The abstract data model (ADM) that represents the state of WSUS is persisted in local storage on the update client and update servers.
Update client and update server management tools can interact with WSUS in an implementation-defined manner by altering the state of the ADM elements that are part of the member protocol or by using one of the protocol initialization mechanisms.
The ADM elements for the WUSP server and ways to populate them are specified in [MS-WUSP] section 3.1.1 and section 3.1.1.1. Out of the ADM elements described, the server management tool can be used to alter the state of the Client Computers Table, Target Group Table, and the Deployment Table to express administrative intent.
The various initialization triggers that the server management tool can fire to initiate the Windows Server Update Services: Server-Server Protocol are specified in [MS-WSUSSS] section 3.2.3.
The Group Policy protocols interact with WSUS by altering the state of the ADM element Policy Table as specified in [MS-WUSP] section 3.2.1.