Chapter 21
Network Management:
SNMP
Objectives
Upon completion you will be able to:
• Understand the SNMP manager and the SNMP agent
• Understand the roles of SMI and MIB in network management
• Be familiar with SMI object attributes and encoding methods
• Know how an MIB variable is accessed
• Be familiar with the SNMP PDU and format
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21.1 CONCEPT
SNMP defines a manager, which is typically a host that controls and
monitors a set of agents, and usually routers.
Note: SNMP has gone thru 3 versions. SNMP v1 was very common.
SNMP v2 is rare. Many installations have skipped v2 and gone straight
to v3.
The topics discussed in this section include:
Managers and Agents
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Figure 21.1
SNMP concept
One or more
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21.2 MANAGEMENT COMPONENTS
SNMP requires the use of two other protocols: Structure of Management
Information (SMI) and Management Information Base (MIB). Network
management on the Internet is done through the cooperation of SNMP,
SMI, and MIB.
The topics discussed in this section include:
Role of SNMP
Role of SMI
Role of MIB
An Analogy
An Overview
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Figure 21.2
Components of network management on the Internet
We will discuss each of these components in turn,
but first …
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Note:
SNMP defines the format of packets
exchanged between a manager and an
agent. It reads and changes the status
(values) of objects (variables) in
SNMP packets.
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Note:
SMI defines the general rules for naming
objects, defining object types (including
range and length), and showing how to
encode objects and values. SMI defines
neither the number of objects an entity
should manage, nor names the objects to
be managed, nor defines the association
between the objects and their values.
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Note:
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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Note:
We can compare the task of network
management to the task of writing a
program.
❏ Both tasks need rules. In network management
this is handled by SMI.
❏ Both tasks need variable declarations. In network
management this is handled by MIB.
❏ Both tasks have actions performed by statements.
In network management this is handled by SNMP.
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Figure 21.3
Management overview
SNMP client
SNMP server
Example: A manager station (SNMP client) wants to send a message to
an agent station (SNMP server) to find the number of UDP datagrams
received by the agent.
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21.3 SMI
SMI is a component used in network management. It names objects,
defines the type of data that can be stored in an object, and shows how
data can be encoded for transmission over the network
The topics discussed in this section include:
Name
Type
Encoding Method
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Figure 21.4
Object attributes
Each object has three attributes: name, type, and
encoding method.
Each managed object (such as a router or a variable
in a router) has to have a unique name.
Use the following hierarchical design to name an object.
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Figure 21.5
Object identifier
The objects that
are used in SMNP
are located under
the mib-2 object,
so their identifiers
always start with
1.3.6.1.2.1.
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Figure 21.6
Data type
The second attribute of an object is the data type.
To define the data type, SMI uses Abstract Syntax Notation
1 (ASN.1).
SMI has two broad categories of data type: simple and
structured.
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Table 21.1 Simple data types
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First 5 are from ASN.1; next 7 are defined by SMI.
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Figure 21.7
Conceptual data types
SMI defines two structured data types: sequence and sequence of.
Sequence is similar to C’s struct or record
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Figure 21.8
Encoding format
SMI uses another standard – Basic Encoding Rules (BER) – to
encode data to be transmitted over the network.
BER specifies that each piece of data be encoded in triplet
format: tag, length, and value.
The Class subfield defines the scope of the data: 00 – universal,
01 – application-wide, 10 – context-specific, 11 – private.
The universal data types are shown in the first 5 rows of the
next table. The application-wide types are the last 5 rows.
Format bit: simple is 0 and structured is 1.
The number field further divides simple or structured into subgroups.
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Table 21.2 Codes for data types
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Figure 21.9
Length format
If length is only 1 byte, first bit = 0
If length is >1 byte, first bit is 1 and remaining seven
bits define how many bytes in the length.
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Example 1
Figure 21.10 shows how to define INTEGER 14.
See Next Slide
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Figure 21.10
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Example 1, INTEGER 14
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Example 2
Figure 21.11 shows how to define the OCTET STRING
“HI.”
See Next Slide
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Figure 21.11 Example 2, OCTET STRING “HI”
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Example 3
Figure 21.12 shows how to define ObjectIdentifier
1.3.6.1 (iso.org.dod.internet).
See Next Slide
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Figure 21.12
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Example 3, ObjectIdentifier 1.3.6.1
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Example 4
Figure 21.13 shows how to define IPAddress
131.21.14.8.
See Next Slide
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Figure 21.13
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Example 4, IPAddress 131.21.14.8
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21.4 MIB
MIB (Management Information Base) is the second component used in
network management. Each agent has its own MIB, a collection of all
the objects that the manager can manage.
The topics discussed in this section include:
Accessing MIB Variables
Lexicographic Ordering
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Figure 21.14
mib-2
The objects in an MIB are categorized under 10 different groups.
sys: general info about the node (system)
if: defines information about all of the interfaces of the node
at: info about the ARP table
ip: defines info related to IP, such as routing table and IP address
icmp: info related to ICMP, such as number of packets sent, total errors
tcp: data such as connection table, time-out values, number of ports
udp: data such as number of ports, number of packets sent and received
snmp: general info related to SNMP itself
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Figure 21.15
udp group
Say you want to reference the variable udpIn Datagrams.
The id of the variable would be 1.3.6.1.2.1.7.1.
To reference udpLocal Address: 1.3.6.1.2.1.7.5.1.1.
But these only reference the variable. If you want to show the
contents of each variable, add a 0 to the end.
So the value of updIn Datagrams is in 1.3.6.1.2.1.7.1.0.
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Figure 21.16
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udp variables and tables
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Figure 21.17
Indexes for udpTable
Note that entries in a table are not referenced by simple integer,
as they are in most programming languages. Table entries are
indexed by local IP address and local port number.
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Figure 21.18
Lexicographic ordering
Data is ordered by columns.
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21.5 SNMP
SNMP v3 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; and 3) an agent to send an alarm
message about an abnormal situation to the manager.
SNMP uses both SMI and MIB to accomplish this.
The topics discussed in this section include:
PDUs
Format
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Figure 21.19
SNMP PDUs
SNMP defines 8 types of packets (or PDUs):
GetRequest – sent from manager (client) to agent (server)
GetNextRequest – mostly used to retrieve values of the entries in a table
GetBulkRequest – to retrieve a large amount of data
SetRequest – sent from manager to agent to set(store) a value
Response – in response to GetRequest or GetNextRequest
Trap – reports an event. For example, if the agent is rebooted, it informs
the manager and reports the time of rebooting.
InformRequest – sent from one manager to another remote manager to
get the values of some variables under the control of the remote manager
Report – to report some types of errors between managers. Not yet in
use.
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Figure 21.19
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SNMP PDUs
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Figure 21.20
SNMP PDU format
Format for the 8 PDUs:
PDU Type – See Table 21.4
Error Status – Used in Response PDUs; see Table 21.3.
Error Index – offset that tells which variable caused error
VarBind List – set of variables the manager wants to retrieve (or set)
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Table 21.3 Types of errors
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21.6 MESSAGES
SNMP does not just send PDUs, it embeds the PDU in a message.
A message in SNMP is made of four elements: version, header, security
parameters, and data (which includes the encoded PDU). (See the next
figure)
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Figure 21.21
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SNMP message
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Table 21.4 Codes for SNMP messages
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Example 5
In this example, a manager station (SNMP client) uses the
GetRequest message to retrieve the number of UDP datagrams
that a router has received. There is only one VarBind entity.
The corresponding MIB variable related to this information is
udpInDatagrams with the object identifier 1.3.6.1.2.1.7.1.0.
The manager wants to retrieve a value (not to store a value), so
the value defines a null entity. Figure 21.22 shows the
conceptual view of the packet showing the hierarchical nature
of sequences. We have used white and color boxes for the
sequence and a gray one for the PDU.
See Next Slide
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Example 5
The VarBind list has only one VarBind. The variable is of type
06 and length 09. The value is of type 05 and length 00. The
whole is a sequence of length 0D (13). The VarBind list is also
a sequence of length 0F (15). The GetRequest PDU is of length
1D (29). Now we have three OCTET STRINGs related to
security parameter, security model, and flags. Then we have
two integers defining maximum size (1024) and message ID
(64). The header is a sequence of length 12, which we left
blank for simplicity. There is one integer, version (version 3).
The whole message is a sequence of 52 bytes. Figure 21.23
shows the actual message sent by the manager station (client)
to the agent (server).
See Next Slide
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Figure 21.22
Example 5
BER notation: 30 means Sequence of, 34 is length (Table 21.2)
Max. size and Msg.ID
Security parameter, security model, flags
Get
Request
PDU
Table 21.4
Request ID, Error Status,
Error Index (Fig 21.20)
object identifier, length 9, udpInDatagrams (1.3.6.1.2.1.7.1.0)
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Figure 21.23
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GetRequest message
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21.7 UDP PORTS
SNMP uses the services of UDP on two well-known ports, 161 and 162.
The well-known port 161 is used by the server (agent), and the wellknown port 162 is used by the client (manager).
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Figure 21.24
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Port numbers for SNMP
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21.8 SECURITY
The main difference between SNMPv3 and SNMPv2 is the enhanced
security. SNMPv3 provides two types of security: general and specific.
SNMPv3 provides message authentication, privacy, and manager
authorization.
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