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Unformatted text preview: e variables maintained by the agent. We describe
the database of information maintained by the agent (the MIB), looking at the groups that we've
described in this text: IP, UDP, TCP, and so on. We show examples at each point along the way,
tying network management back to the protocol concepts from earlier chapters. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/snmp_sim.htm (1 of 33) [12/09/2001 14.47.40] Chapter 25. SNMP: Simple Network Management Protocol 25.2 Protocol
SNMP defines only five types of messages that are exchanged between the manager and agent.
1. Fetch the value of one or more variables: the get-request operator.
2. Fetch the next variable after one or more specified variables: the get-next-request
operator. (We describe what we mean by "next" later in this chapter.)
3. Set the value of one or more variables: the set-request operator.
4. Return the value of one or more variables: the get-response operator. This is the
message returned by the agent to the manager in response to the get-request, getnext-request, and set-request operators.
5. Notify the manager when something happens on the agent: the trap operator.
The first three messages are sent from the manager to the agent, and the last two are from the agent
to the manager. (We'll refer to the first three as the get, get-next, and set operators.) Figure
25.1 summarizes these five operators.
Since four of the five SNMP messages are simple request-reply protocols (the manager sends a
request, the agent sends back a reply) SNMP uses UDP. This means that a request from the manager
may not arrive at the agent, and the agent's reply may not make it back to the manager. The manager
probably wants to implement a timeout and retransmission. Figure 25.1 Summary of the five SNMP operators.
The manager sends its three requests to UDP port 161. The agent sends traps to UDP port 162. By
using two different port numbers, a single system can easily run both a manager and an agent. (See
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