Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. THE EVOLUTION OF NETWORK TECHNOLOGY: DISTRIBUTED COMPUTING AND THE CONVERGENCE OF NETWORKS 12 NETWORK PROCESSORS When the network is running normally, SNMP collects and logs detailed statistics about numerous variables and returns them in easy-to-interpret displays and reports. All network-connected devices supporting SNMP contain and maintain a set of management information bases (MIBs) with network statistics. In order to provide the network manager with meaningful information, the SNMP management station queries the MIBs of the network-attached devices. Based on the answers it obtains, it compiles a well-rounded more or less real-time picture of how the network behaves. SNMP is structured in a client-server model. The client model (also known as the network manager) establishes a virtual con- nection with a server program (also known as the SNMP agent), which runs on a remote network device. The local database maintained by the SNMP agent is known as the SNMP MIB. It contains a standardized set of statistics and values of specific control variables. Commands from the network manager (client) consist of identifiers of SNMP variables (also known as MIB object identifiers or MIB variables) along with instructions to either get the value of the corresponding identifier or set the identifier value to a new value. The network manager obtains the relevant information through queries issued to the agent's MIB. This is the traditional technique of polling. An alternative technique is used when unsolicited responses from the network-attached devices are sent to the SNMP management sta- tion. We are referring to "traps" that the agent is throwing at the manager to signal that something unusual has happened. Beyond the standardized MIBs, network equipment vendors have also created private MIBs, which al