The main function of the SNMP Protocol is to allow management information, in the form of Management Information Base (MIB) objects, to be communicated between SNMP-capable devices. The protocol operations of the SNMP Protocol are what describe how this communication is performed. Before looking at these operations individually in detail, it is instructive to take an overall look at information exchange methods used in SNMP.

For SNMP to be useful in enabling the management of a network, it must allow a network administrator using a network management station (NMS) to easily check the status of SNMP agents in managed devices. In data communications, there are two general techniques that are used in a situation where one entity needs to be kept informed about activity or occurrences on another:

• Poll-Driven: This term refers to the general technique of having the one who wants the information ask for it; just like someone might conduct a political poll. In SNMP, the NMS would poll SNMP agents for information. A common “real life” example of polling is the model used by the regular mail service; every day you go to check your mailbox to see if you have any mail.
  
  
• Interrupt-Driven: This term refers to having a device with information that another needs to know decide to send the information of its own “volition”. In SNMP, this would refer to an SNMP agent sending information to an NMS without being asked. This is the model used by that most famous of “interrupters”, the telephone.

Which is “better”? My quotation marks give me away again. The usual answer applies here: neither is better nor worse universally, which is why both options exist. Due to the obvious strengths and weaknesses of these models, The SNMP Protocol is designed to use both. Polling is used for the periodic gathering of routine information, such as checking the usage statistics and general status of a device. Interrupts are used in the form of traps that a network administrator can set on a managed device. These traps cause an SNMP agent to interrupt an NMS when an event of import occurs.

The actual communication of information in the SNMP Protocol is performed in a manner similar to most other protocols, through the exchange of SNMP messages. These messages are sometimes called protocol data units or PDUs. This is a term you may have heard used in other protocols, and is also part of the formal definition of data encapsulation in the OSI Reference Model. A message is, of course, a “data unit” used by the “protocol”. SNMP messages all have “-PDU” at the ends of their names to identify them.

Some consider “protocol data unit” to be analogous to the military using “oblong metallic-headed manually-operated fastener acceleration device” to refer to a hammer. To be fair though, strictly speaking, in SMNP a PDU and a message are not exactly the same. The PDU is the higher-layer data that SNMP encapsulates, as described by the OSI model. The SNMP message format is a wrapper that encapsulates a PDU along with header fields, as I describe in the section on SNMP messaging. However, the point of a message is to send a PDU, so the two are “close enough”, and the terms are sometimes used interchangeably.