SNMP Protocol Message Generation, Addressing, Transport and Retransmission
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The section on SNMP protocol operations described the way that SNMP messages are employed to communicate network management information. Those discussions focused primarily on the logical processes by which different tasks are accomplished using these messages. Let's now take a closer look at the lower-level details of exactly how these messages are created, addressed and sent by an SNMP device.
Message generation in SNMP is a bit different than the typical TCP/IP client/server model used for most other protocols. There aren't really any formal clients and servers in SNMP, since management information can be obtained from any deviceit is distributed. Most of the message exchanges use a matched pair of request and reply messages. The network management station (NMS) usually acts as the client in these exchanges, sending a particular get or set request to an SNMP agent, which plays the role of server for the information it contains. However, SNMP agents aren't usually considered servers in the conventional sense of the term.
SNMP traps deviate from the normal request/reply model of message generation entirely. When a trap is triggered, an SNMP agent sends a trap message to a network management station on its own, not in reaction to receiving a request. Since trap messages are unconfirmed there is no reply. Note, however, that the SNMPv2/v3 InformRequest-PDU message is confirmed, and a response message is thus sent back to the NMS that generates it.
Once a message has been generated, it is sent using the protocols at the levels below the application layer where SNMP resides. As we saw in the overview of the SNMP Protocol, the current SNMP standard set separates description of protocol operations and PDUs from the methods used to actually send them. Starting with version 2, SNMP has defined several transport mappings that describe how SNMP PDUs can be sent over a variety of internetworking protocol suites, including TCP/IP, OSI, IPX/SPX (Novell) and Appletalk.
Many of the specific details of SNMP messaging depend on the transport mapping that is used in a particular implementation. SNMP is of course primarily used on TCP/IP internetworks, and TCP/IP is where our main interest lies in this part of the Guide. I will therefore continue this discussion by looking at transport issues when SNMP is used over IP.
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