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Unformatted text preview: igure 25.2 shows the format of the five SNMP messages, encapsulated in a UDP datagram. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/snmp_sim.htm (2 of 33) [12/09/2001 14.47.40] Chapter 25. SNMP: Simple Network Management Protocol Figure 25.2 Format of the five SNMP messages.
In this figure we specify the size in bytes of the IP and UDP headers only. This is because the
encoding used for the SNMP message-called ASN.1 and BER, which we describe later in this
chapter-varies depending on the type of variable and its value.
The version is 0. This value is really the version number minus one, as the version of SNMP that we
describe is called SNMPv1.
Figure 25.3 shows the values for the PDU type. (PDU stands for Protocol Data Unit, a fancy word
PDU type Name 0 get-request 1 get-next-request 2 set-request 3 get-response 4 trap Figure 25.3 PDU types for SNMP messages.
The community is a character string that is a cleartext password between the manager and agent. A
common value is the 6-character string public.
For the get, get-next, and set operators, the request ID is set by the manager, and returned by
the agent in the get-response message. We've seen this type of variable with other UDP
applications. (Recall the DNS identification field in Figure 14.3, and the transaction ID field in
Figure 16.2.) It lets the client (the manager in this case) match the responses from the server (the
agent) to the queries that the client issued. "This field also allows the manager to issue multiple
requests to one or more agents, and then be able to sort out the returned replies. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/snmp_sim.htm (3 of 33) [12/09/2001 14.47.40] Chapter 25. SNMP: Simple Network Management Protocol The error status is an integer returned by the agent specifying an error. Figure 25.4 shows the
values, names, and descriptions.
error status Name Description 0 noError all is...
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