Simple Network Management Protocol Cont...

Follow the Basic Encoding Rules when laying out the bytes of an SNMP message :
- The most fundamental rule states that each field is encoded in three parts: Type, Length, and Data.
- Type specifies the data type of the field using a single byte identifier.
- Length specifies the length in bytes of the following Data section.
- Data is the actual value communicated (the number, string, OID, etc).
- Rule applies when encoding the first two numbers in the OID. According to BER, the first two numbers of any OID (x.y) are encoded as one value using the formula (40*x)+y. The first two numbers in an SNMP OID are always 1.3. Therefore, the first two numbers of an SNMP OID are encoded as 43 or 0x2B, because (40*1)+3 = 43. After the first two numbers are encoded, the subsequent numbers in the OID are each encoded as a byte.
- The rule for large numbers states that only the lower 7 bits in the byte are used for holding the value (0-127). The highest order bit is used as a flag to let the recipient know that this number spans more than one byte. Therefore, any number over 127 must be encoded using more than one byte. According to this rule, the number 2680 must be encoded 0x94 0x78.

SNMP Primitives

SNMP has three control primitives that initiate data flow from the requester which is usually the Manager. These would be get, get-next and set. The manager uses the get primitive to get a single piece of information from an agent. You would use get-next if you had more than one item. You can use set when you want to set a particular value.

SNMP Operation

SNMP design is pretty simple. There are two main players in SNMP. The manager and the agent. The manager is generally the ‘main’ station such as HP Openview. The agent would be the SNMP software running on a client system you are trying to monitor.

Simple Network Management Protocol Architecture - SNMP

SNMP architectural model is a collection of network management stations and network elements. Network management stations execute management applications which monitor and control network elements. The Simple Network Management Protocol (SNMP) is used to communicate management information between the network management stations and the agents in the network elements.

Goals of SNMP architecture

The SNMP explicitly minimizes the number and complexity of management functions realized by the management agent itself.
- The development cost for management agent software necessary to support the protocol is accordingly reduced.
- The degree of management function that is remotely supported is accordingly increased, thereby admitting fullest use of internet resources in the management task.
- The degree of management function that is remotely supported is accordingly increased, thereby imposing the fewest possible restrictions on the form and sophistication of management tools.
- Simplified sets of management functions are easily understood and used by developers of network management tools.

A second goal of the protocol is that the functional paradigm for monitoring and control be sufficiently extensible to accommodate additional, possibly unanticipated aspects of network operation and management.

A third goal is that the architecture be, as much as possible, independent of the architecture and mechanisms of particular hosts or particular gateways.

Elements of the Architecture

- Scope of the management information communicated by the protocol.
- Representation of the management information communicated by the protocol.
- Operations on management information supported by the protocol.
- The form and meaning of exchanges among management entities.
- The definition of administrative relationships among management entities.
- The form and meaning of references to management information.

Overview of Simple Network Management Protocol - SNMP

- The Simple Network Management Protocol (SNMP) is an application layer protocol that facilitates the exchange of management information between network devices.
- It is part of the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite.
- SNMP enables network administrators to manage network performance, find and solve network problems, and plan for network growth.
- SNMP is a popular protocol for network management.
- SNMP can collect information such as a server’s CPU level, server chassis Temperature etc.
- SNMP is the protocol that allows an SNMP manager (the controller) to control an SNMP agent (the controlee) by exchanging SNMP messages.
- The SNMP protocol was designed to provide a "simple" method of centralizing the management of TCP/IP-based networks.

SNMP Basic Components

SNMP consists of three key components: managed devices, agents, and network-- management systems (NMSs).
- A managed device is a network node that contains an SNMP agent and that resides on a managed network.
- An agent is a network-management software module that resides in a managed device. An agent has local knowledge of management information and translates that information into a form compatible with SNMP.
- An NMS executes applications that monitor and control managed devices.

SNMP Commands

- The read command is used by an NMS to monitor managed devices. The NMS examines different variables that are maintained by managed devices.
- The write command is used by an NMS to control managed devices. The NMS changes the values of variables stored within managed devices.
- The trap command is used by managed devices to asynchronously report events to the NMS. When certain types of events occur, a managed device sends a trap to the NMS.

SNMP itself does not define which information (which variables) a managed system should offer. Rather, SNMP uses an extensible design, where the available information is defined by management information bases (MIBs). MIBs describe the structure of the management data of a device subsystem; they use a hierarchical namespace containing object identifiers (OID). Each OID identifies a variable that can be read or set via SNMP.