As we saw extensively in the previous section, the communication of management information is accomplished through the exchange of SNMP messages that contain protocol data units (PDUs). Like the messages of most TCP/IP protocols, these PDUs are designed to use a particular field format, and are created, addressed and transported according to specific protocol rules. SNMP messages include fields that control the operation of the protocol, and carry a “payload” of management information in the form of Management Information Base (MIB) objects.

In this section I describe the details of how messaging is accomplished in the SNMP Protocol. I begin with a general discussion of issues related to message generation, addressing and transport, and a description of how retransmission of messages is handled when necessary. I discuss the way fields are defined in SNMP messages and describe their general format, explaining the difference between the overall message and the PDU it contains. I then examine the message format used in all of the important SNMP versions, showing the structure of each message type and the fields used.