ent and Datagram Protocol (UDP). future direction for the management of Internet. RFC 1109 was Above level 4 there are several higher level protocols, associated published as a result of this, which confirmed that SNMP had long with various services, some being illustrated in Figure 16.22. SMTP term relevance for network management, which seemed to negate is used for mail transfer, FTP for file management and transfer, and the original aim of moving to OSI. TELNET for terminal sessions. A measure of SNMP's success and ease of implementation, can be gauged by the fact that at the 1990 Interop, only two years after publication of the RFCs, almost fifty individual companies took part APPLICATION in the SNMP solutions showcase, demonstrating an interconnected TELNET and managed network. PRESENTATION SESSION LAYER 16.9.3 The SNMP structure TRANSPORT SNMP has three main elements, and these are defined in the RFCs LAYER issued by the IAB. The Structure of Management Information (SMI) is specified in NETWORK RFC 1065, upgraded to RFC 1155. It provides the administrative LAYER and organisational policies for defining managed objects. For each managed object SMI provides a low level syntax, based on ASN.1, DATA LINK status information on the object, access rights to that object, and guidelines for naming objects. This ensures that a manager can use LAYER a common format when controlling equipment from several ven- dors, provide they support SNMP variables on their product. PHYSICAL PHYSICAL The second element is the Management Information Base for LAYER Network (MIB), published as RFC 1066, upgraded to RFC 1156. This provides a list of the network objects that can be interrogated on any agent, and also defines the information which can be re- trieved. MIB variables can have a status of mandatory or optional. Man- Figure 16.22 A TCP/IP protocol stack datory items need to be supported by everyone, but optional items are usually vendor specific. The MIB document defines a series of objects as given in Table Below level 3 of t