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SNMP

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SNMP is an Internet protocol developed by the IETF. It is designed to facilitate the exchange of management information between network elements. By transporting information over the SNMP protocol (such as packets per second and network error rates), network administrators can easily manage network performance and accounting, find and solve network problems, and plan for network growth

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SNMP uses the concept of manager and agent. That is, a manager, usually a host, controls and monitors a set of agents, usually routers or servers (see Figure 24.1).

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Figure 24.1 SNMP concept

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SNMP is an application program that allows:

1. A manager to retrieve the value of an object defined in an agent.

2. A manager to store a value in an object defined in an agent.

3. An agent to send an alarm message about an abnormal situation to the manager.

SNMP & The OSI ModelSNMP & The OSI Model

7 Application Layer Management and Agent APIs SNMP

6 Presentation Layer ASN.1 and BER 5 Session Layer RPC and NetBIOS 4 Transport Layer TCP and UDP 3 Network Layer IP and IPX 2 Data Link Layer Ethernet, Token Ring, FDDI 1 Physical Layer

Ports & UDPPorts & UDP

•SNMP uses User Datagram Protocol (UDP) as the transport mechanism for SNMP messages

•UDP Port 161 - SNMP Messages

•UDP Port 162 - SNMP Trap Messages

•Like FTP, SNMP uses two well-known ports to operate:

Ethernet

FrameIP

PacketUDP

Datagram

SNMP Message CRC

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To do management tasks, SNMP uses two other protocols: Structure of Management Information (SMI) and Management Information Base (MIB). In other words, management on the Internet is done through the cooperation of three protocols: SNMP, SMI, and MIB, as shown in Figure 24.2.

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Figure 24.2 Companion of network management on the Internet

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SNMP

SNMP defines the format of packets exchanged between a manager and an agent. It reads and changes the status of objects (values of variables) in SNMP packets.

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SMI

SMI defines the general rules for naming objects, defining object types (including range and length), and showing how to encode objects and values.

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MIB

MIB creates a collection of named objects, their types, and their relationships to each other in an

entity to be managed.

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Figure 24.3 Comparing computer programming and network management

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Figure 24.4 Management overview

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Get RequestSNMP packet

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ResponseSNMP packet

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SMI

The Structure of Management Information is a component for network management. Its functions are:1. To name objects.2. To define the type of data that can be stored in an

object.3. To show how to encode data for transmission

over the network.SMI is a guideline for SNMP. It emphasizes

three attributes to handle an object: name, data type, and encoding method.

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SMI

SMI data types are divided into three categories:

Simple typesApplication-wide typesSimply constructed types.

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simple types

Integer— A unique value that is a positive or negative whole number (including 0).

OctetString— A unique value that is an ordered sequence of zero or more octets, more commonly called bytes.

ObjectID— A unique value from the set of all object identifiers allocated according to the rules specified in ASN.1.

Bits— New in SMIv2, these are zero or more named bits that specify a value.

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application-wide types

Network address— Represents an address from a particular protocol family. Examples are InetAddressType and InetAddress.

Counter— A nonnegative integer that increments by +1 until it reaches a maximum value, when it is reset to 0

Gauge— A nonnegative integer that can increase or decrease but that latches at a maximum value. The interface load is an example of a gauge.

Timetick— Hundredths of a second since an event. The time since an interface entered its current state is an example of a tick.

INTEGER— Represents signed, integer-valued information. Unsigned integer— Represents unsigned integer-valued

information.

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simply constructed types

Row— References a row in a table. Each element of the row can be a simple type or an application-wide type.

Table— References a table of zero or more rows. Each row has the same number of columns.

TCP/IP Protocol Suite 20

TCP/IP Protocol Suite 21

Figure 24.7 Encoding format

TCP/IP Protocol Suite 22

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Interactions between manager and agent

Interactions between the SNMP manager and managed network elements (SNMP agents) can be any of three different types of commands:• Read— To monitor managed network elements, SNMP

managers read MIB variables maintained by the SNMP agents.

• Write— To control managed network elements, SNMP managers write MIB variables stored within the SNMP agents.

• Notification— SNMP agents use traps or informs to asynchronously report certain events to SNMP managers. Traps are unacknowledged notifications, and informs are notifications acknowledged by the SNMP manager.

Client Pull & Server PushClient Pull & Server Push

SNMP is a “client pull” model

• SNMP is a “server push” model

The management system (client) “pulls” data from the agent (server).

The agent (server) “pushes” out a trap message to a (client) management system

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objects

All objects managed by SNMP are given an object identifier.

The object identifier always starts with 1.3.6.1.2.1.

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Figure 24.5 Object identifier

TCP/IP Protocol Suite 27

Figure 24.12 mib-2

TCP/IP Protocol Suite 28

Figure 24.13 udp group