 covering all the User Datagram SNMP document defines the means for getting information between Protocol Counters management stations and managed objects. SNMP usually uses the This is the exterior gateway protocol and is User Datagram Protocol (UDP/IP) instead of TCP/IP. used for Internet core gateways TCP checks for error, whilst UDP assumes a reliable transmission path and does not carry out checks. SNMP messages have the Statistical information on SNMP traffic potential to be carried over any transport mechanism that sup- ports bi-directional flow and addressability. Figure 16.24 shows a communication architecture within SNMP. The messages, de- scribed in the next section, are packaged using ASN.1 and sent over the UDP/IP services. SNMP uses a simple polling access process and no sessions are iso (1) ccitt (2) iso/ccitt (3) maintained between the manager and its managed objects. The rate of polling can be varied, which provides a means for trading off between the response time and the amount of bandwidth used. A org (3) single packet is employed for polling, and a relatively small number of packets would be expected in reply. Figure 16.25 shows the way in which a SNMP message is con- dod (6) tained in a typical frame structure. The packet lengths vary, depend- ing on the transport mechanism being used. If this mechanism internet (1) allows a very small data size, such as 128 bytes for X.25, then the datagrams are broken into smaller sections by the sender and reas- directory (1) mgmt (2) experimental (3) private (4) sembled by the receiver. SNMP is basically a request-response protocol having five mess- enterprise (1) age types. mib (1) GET-REQUEST is used by the manager to ask for one or more interfaces (2) network management variables from an agent MIB. GET-NEXT-REQUEST has a similar function to GET- RE- ifTable (2) QUEST, but it enables portions of the MIB to be stepped through, and the next variable to be asked for. The command is therefore used for traversing tables, SO avoiding the overhead of a separat