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10 Simple Network Management Protocol (SNMP)

10.1Overview

SNMP was developed in 1988 as an operating system for the management of the data flow from a series of remote information generators, or Agents, connected to a central computer, or Manager, by way of a network.

The software installed in the Agent (whether it is supplied by Alpha or customer supplied) uses SNMP to translate the data stream into a format that can be readily downloaded by way of the client's network to the Manager computer located at the customer's Central Office used as a hub for the network.

Each Agent collects data from a source, translates that data into the SNMP format and then transmits the translated data, when requested, to the Manager computer.

In the case of a network of Alpha Power Systems, the data stream from an individual power plant is developed through the CXC System Controller, which directly interfaces with that power system. This data stream is referred to as the Management Information Base or MIB, see 0.

An SNMP may have a number of simple messages that can be sent directed towards the retrieval of single or multiple object variables or to establish the value of a single variable.

The Alpha SNMP Agent software employs an event notification called a "Trap" to the management system to identify the occurrence of conditions that exceed a predetermined value such as an alarm event. Trap Protocol Data Units or PDU’s have the following format:

Enterprise – identifies the type of object causing the trap. Agent Address – IP address of agent that sent the trap. Generic Trap ID – the common standard traps.

Specific Trap ID – proprietary or enterprise trap. Time Stamp – when trap occurred in time ticks.

10.1.1Outline of the SNMP Protocol

•Each SNMP managed object belongs to a community.

•Network Management Station (NMS) may belong to multiple communities.

•A community is defined by a community name, which is an Octet String with 0 to 255 octets in length.

•Each SNMP message consists of three components:

1.Version number

2.Community name

3.Data - a sequence of PDU's associated with the request.

10.1.2Underlying Communication Protocols

SNMP assumes that the communication path is a connectionless communication sub-network. In other words, no prearranged communication path is established prior to the transmission of data. As a result, SNMP makes no guarantees about the reliable delivery of the data. Although in practice most messages get through, and those that don't can be retransmitted. The primary protocols that SNMP implements are the User Datagram Protocol (UDP) and the Internet Protocol (IP). SNMP also requires Data Link Layer protocols such as Ethernet or Token Ring to implement the communication channel from the management to the managed agent.

SNMP's simplicity and connectionless communication also produce a degree of robustness. Neither the manager nor the agent relies on the other for its operation. Thus, a manager may continue to function even if a remote agent fails. When the agent resumes functioning, it can send a trap to the manager, notifying it of its change in operational status. The connectionless nature of SNMP leaves the recovery and error detection up to the Manager computer and even up to the Agent.

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10.1.3Typical UDP Transport

•Agent listens on UDP port 161.

•Responses are sent back to the originating NMS port from a dynamic port, although many agents use port 161 also for this target.

•Maximum SNMP message size is limited by maximum UDP message size; i.e. 65507 octets.

•All SNMP implementations have to receive packets at least 484 octets in length.

•Some SNMP implementation will (incorrectly or not) handle packets exceeding 484 octets.

•Asynchronous Traps are received on port 162 of the NMS.

•UDP is more suitable than TCP when dynamic route changes occur often; e.g., when there are problems in the network.

•UDP packets minimize the demands placed on the network (no resource tied up as with connection mode).

•Agent and NMS are responsible for determining error recovery.

Figure 115–UDP transport (trap event)

SNMP is transport independent (although original design was connectionless transport function, which corresponds to the UDP protocol) and can be implemented on other transports as well:

•TCP (Connected approach)

•Direct mapping onto Ethernet MAC level

•Encapsulation onto X25 protocol

•Encapsulation onto an ATM Cell.