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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).
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 Model
7 Application Layer Management and Agent APIs
SNMP
6 Presentation Layer
ASN.1 and BER
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Session Layer
RPC and NetBIOS
4 Transport Layer
TCP and UDP
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Network Layer
IP and IPX
2 Data Link Layer
Ethernet, Token Ring, FDDI
1 Physical Layer
Ports & UDP
•SNMP uses User Datagram Protocol (UDP) as the transport mechanism for
SNMP messages
Etherne
t Frame
IP
Packet
SNMP Message
UDP
Datagram
•Like FTP, SNMP uses two well-known ports to operate:
•UDP Port 161 - SNMP Messages
•UDP Port 162 - SNMP Trap Messages
CRC
Encapsulation
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TCP
header
Application-layer data
IP
header
Frame
header
TCP payload
IP payload
Data-link layer payload
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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.
Figure 24.2
Companion of network management on the Internet
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SNMP
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 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.
SMI
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 SMI defines the general rules for naming objects,
defining object types (including range and length),
and showing how to encode objects and values.
MIB
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 MIB creates a collection of named objects, their
types, and their relationships to each other in an
entity to be managed.
Figure 24.3
Comparing computer programming and network management
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Figure 24.4
Management overview
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SNMP packet
Get Request
SNMP packet
Response
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SMI
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 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.
SMI
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SMI data types are divided into three categories:
 Simple types
 Application-wide types
 Simply constructed types.
simple types
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 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.
application-wide types
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 Network address— Represents an address from a particular
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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.
simply constructed types
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 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.
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TCP/IP Protocol Suite
Figure 24.7
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Encoding format
TCP/IP Protocol Suite
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TCP/IP Protocol Suite
Interactions between manager and agent
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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 Push
 SNMP is a “client pull” model
The management system (client) “pulls” data from the agent (server).
• SNMP is a “server push” model
The agent (server) “pushes” out a trap message to a (client)
management system
objects
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 All objects managed by SNMP are given an object
identifier.
The object identifier always starts with 1.3.6.1.2.1.
Figure 24.5
Object identifier
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Figure 24.12
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mib-2
TCP/IP Protocol Suite
Figure 24.13
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udp group
TCP/IP Protocol Suite