TCP IP Illustrated


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Unformatted text preview: nk of it as a two-dimensional array or table. For example, the UDP listener table is named udpTable and it is a SEQUENCE OF the 2element SEQUENCE (structure) UdpEntry that we just described. Figure 25.5 shows this two-dimensional array. Figure 25.5 UDP listener table (udpTable) as a two-dimensional array in SNMP. The number of rows in these tables is not specified by SNMP, but we'll see that a manager using the get-next operator (Section 25.7) can determine when the final row of a table has been returned. Also, in Section 25.6 we'll see how the manager specifies which row of a table it wants to get or set. 25.4 Object Identifiers An object identifier is a data type specifying an authoritatively named object. By "authoritative" we mean that these identifiers are not assigned randomly, but are allocated by some organization that has responsibility for a group of identifiers. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/snmp_sim.htm (5 of 33) [12/09/2001 14.47.40] Chapter 25. SNMP: Simple Network Management Protocol An object identifier is a sequence of integers separated by decimal points. These integers traverse a tree structure, similar to the DNS (Figure 14.1) or a Unix filesystem. There is an unnamed root at the top of the tree where the object identifiers start. (This is the same direction of tree traversal that's used with a Unix filesystem.) Figure 25.6 shows the structure of this tree when used with SNMP. All variables in the MIB start with the object identifier Each node in the tree is also given a textual name. The name corresponding to the object identifier is These names are for human readability. The names of the MIB variables that are in the packets exchanged between the manager and agent (Figure 25.2) are the numeric object identifiers, all of which begin with Figure 25.6 Object identifiers in the Management Information Base. Besides the mi...
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