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McGraw-Hill The McGraw-Hill Companies, Inc., 2000

23-1

Chapter 23

Simple Network

Management Protocol

(SNMP)

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23-2

Components of Network Management

SNMP is a framework for managing devices in an internet using the TCP/IP

protocol suite. The framework can be used in a heterogeneous environment

made up of devices with various technologies and from various vendors.

Components of TCP/IP Network Management:

ManagementInformation Base

Structure ofManagement

Information

The protocol

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23-3ASN.1

From the networks perspective, application programs send messages to each

other in a form of uninterpreted byte strings. From the application perspectivethese messages contain various kinds of data (arrays of integers, video frames,

lines of text, digital images, sound tracks) that have some meaning. The

problem is how to encode the data so that applications on the both sides of

network are able to see the same data, i.e. understand their meaning. Anotherproblem, is how to make this representation efficient. One way to achieve

efficiency is to remove redundancies from data by compression. In addition, the

data encoding (sometimes called: argument marshalling) must be language and

platform independent. An important aspect of argument marshalling is the issueof byte ordering.

A popular standard for data representation is Abstract Syntax Notation One

(ASN.1)

SMI module is based on ANS.1 (Abstract Syntax Notation One)

and BER (Basic Encoding rules). Therefore the following slides discuss thesetwo standards.

8/12/2019 ch23_SNMP

4/48McGraw-Hill The McGraw-Hill Companies, Inc., 2000

23-4

Intelligent network

GSM

UMTS (3G cell phones)

Voice over IPVideoconference (Microsoft NetMeeting )

Interactive television

Secured electronic transaction: e-commerce, m-commerce

Computer-supported telecommunications applications (CSTA)

SNMP

Some areas in telecom industry that use ANS.1:

ASN.1 (cont.)

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23-5ASN.1 (cont.)

ASN.1 is a formal notation used for describing data transmitted by

telecommunications protocols, regardless of language implementation and

physical representation of these data.

ASN.1 is defined originally by CCITT (now ITU-T) in 1984

ISO (1985) has split the original ASN.1 into:

ASN.1 - Abstract Syntax Notation, and

BER - Basic Encoding Rules

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23-6

Examples of some pre-defined basic types:

INTEGER (integer values),

BOOLEAN (Boolean values),

OBJECT IDENTIFIER(name of an information object)IA5String (string of US ASCII characters ),

UniversalString (character strings)

BIT STRING (bit strings of arbitrary length),

OCTET STRING (bit strings whose length is multiple of 8),NumericString (string of digits and spaces)

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

and aggregated (constructed) types:SEQUENCE (structures),

SEQUENCE OF (arrays, lists),

CHOICE (choice between types)

SET (an unordered collection of variables of different type)SET OF (an unordered collection of variables of the same type)

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

ASN.1 (cont.)In order to describe data ANS.1 uses predefined (simple and aggregated) datatypes, and syntax for construction of new types.

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23-7

AirlineFlight ::= SEQUENCE {

airline IA5String,

flight NumericString,

seats SEQUENCE {

maximum INTEGER,

occupied INTEGER,vacant INTEGER},

airport SEQUENCE {

origin IA5String,

stop1 [0] IA5String OPTIONAL,

stop2 [1] IA5String OPTIONAL,destination IA5String},

crewsize ENUMERATED {

six (6),

eight (8),

ten (10)},cancel BOOLEAN DEFAULT FALSE

}

Example of a type assignment (new type definition):

Example of a value assignment (an instance of typeAirlineFlight):

f1106 ::= {"American","1106",{320, 107, 213},{"BWI","LAX"},10 }

ASN.1 (cont.)

The value for cancel is not

supplied, therefore the

default value (FALSE) isassumed

23 8

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23-8

ASN.1:

PersonalInfo ::= SEQUENCE {

married BOOLEAN,

age INTEGER (123456..124000),

name PrintableString}

Would generate in C:

typedef struct PersonalInfo {

boolean married;int age;

char *name;

} PersonalInfo;

Example of decoding from ANS.1 to C:

ASN.1 (cont.)

23 9ASN 1 ( t )

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23-9

Object Identifiers

ASN.1 (cont.)

Object identifiers are globally unique authoritatively assigned names of variables

(objects) that can be remotely referenced (retrieved, changed, monitored). Since

there are millions of such variables used in various areas (standards, internet,

variables recognizable by networking devices, etc.) the names are hierarchicallystructured (similarly as file names in a file system, or domain names in DNS).

According to ASN.1 the names are represented as sequence of integers separated

with dots, for example:

DescriptionObject NameObject Identifier

A counter that shows the total number

of received UDP datagrams for which

there was no application at thedestination port

udpNoPorts1.3.6.1.2.1.7.2

A counter that shows the total number

of input datagrams successfullydelivered to IP user protocols

ipInDelivers1.3.6.1.2.1.4.9

23 10

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23-10

1.3.6.1.2.1.7.1

ISOISO-ident. Org.

US DoD

Internet

udpInDatagramsUDP

MIB2

management

Object Identifiers (cont.)

The numbers in OID represent authorities responsible for assigning the

subsequent numbers and names. For example:

23 11Obj Id ifi ( )

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23-11

iso(1)

org(3)

dod(6)

mgmt(2)

ccitt(0) joint-iso-ccitt(2)

root

directory(1)experimental(3)

internet(1)

1.3.6.1

1.3.6.1.2.1

private(4)

mib(1)

enterprises(1)

system(1)

...interfaces(2)

... at(3)

...

ip(4)

...icmp(5)

...tcp(6)

...

udp(7)

...


The ASN.1 OID hierarchy:

egp(8)

...

snmp(11)

...

23 12

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23-12

mib(1)

udp(7)

udpInDatagrams(1)

udpNoPorts(2)udpOutDatagrams(4)

udpTable(5)

udpEntry(1)

udpLocalAddress(1) udpLocalPort(2)

1.3.6.1.2.1.7

system(1)...

snmp(11)

udpInErrors(3)


1.3.6.1.2.1.7.5

1.3.6.1.2.1.7.5.1.1

23-13BER

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23-13

Basic Encoding Rules (BER)

BER

In order to make the data described by ASN.1 syntax ready for transmission

over the network, they have to be encoded by the rules that can be understoodby both, sender and receiver. BER (which is part of ASN.1) specifies that

each piece of data be encoded in a triplet:

{tag, length, value}

Aggregated data types (structures, arrays) are constructed by nesting of simple

types:

{tag, length, tag, length, value,. . . , tag, length, value}

23-14BER (cont )

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23 14BER (cont.)

50500101000NULL

. . . . . . . . .

30

06

04

02

Tag (Hex)

600110000OBJECT IDENTIFIER

4810000100SEQUENCE

. . . . . . . .. . . . . . .. . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . .

400100000OCTET STRING

200010000INTEGER

Tag (Dec)NumberFormatClassData Type

00 Universal (ANS.1)

01 Application wide (like SMI)10 Context specific (the meaning can change from protocol to protocol)11 Private (vendor specific)

23-15BER (cont )

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23 15

Example: INTEGER 14

Example: ObjectIdentifier 1.3.6.1

Example: OCTET STRING HI

BER (cont.)

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BER (cont.)

Encoding rules are applied at the presentation layer

23-17PER

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PER

Packet Encoding Rules

PER is very compressed encoding based on ANS.1 type information.

Like BER, PER specifies how data should be encoded for transmission,

independently of machine type, programming language, or representationwithin an application program.

Unlike BER, tags are never transmitted, while lengths and values are not

transmitted if known by both peers.

PERs reason for existence is to conserve bandwidth. It is valuable in audio

and video over the Internet, air-ground communication, radio-paging, or

wherever bandwidth is critical.

PER is used in H.323 multimedia standard.

23-18PER (cont )

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Age ::= INTEGER (0..7)

firstGrade Age ::= 6

PER (cont.)

Application 1 Application 202 01 06

Value

Length

Tag

}Total of

24 bits

Application 1 Application 2110

total length of 3 bits

BER

PER

23-19SMI

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SMI

ASN.1 is a huge and complex grammar mechanism. SNMP seeks to simplify to

a smaller set of types/constructs and macros in order to facilitate theinteroperability between managers/agents.

Therefore SMI defines a subset of ASN.1. The retained types are:INTEGER

OCTET STRING

OBJECT IDENTIFIER

NULL

SEQUENCE, SEQUENCE OF

SMI also adds some new types (see next slide for definitions):NetworkAddress

IpAddress

CounterGauge

TimeTicks

Opaque

SMI is recommended in RFC 1155, May 1990

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SMI (cont.)

An address from one of possibly several protocol families.

It is represented as CHOICE. Currently, only one protocol

family, the Internet family, is present in this CHOICE.

NetworkAddress

A non-negative integer, which may increase or decrease,

but which latches at a maximum value. Maximum value is2^32-1 = 4294967295.

Gauge

A non-negative integer which counts the time in 1/100th of

a second since some epoch.TimeTicks

Uninterpreted stringOpaque

Non-negative integer which monotonically increases until

it reaches a maximum value, when it wraps around andstarts increasing again from zero. (maximum value is 2^32-

1 = 4294967295).

Counter

32-bit internet address. It is represented as an OCTET

STRING of length 4, in network byte-order.IpAddress

DescriptionNew Type

Added types (in SMIv.1):

23-21SMI (cont )

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SMI v.2 is adding new simple types:

integer32

unsigned32

counter32

counter64

gauge32

SMI (cont.)

23-22MIB

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MIB

Administrative name of the system, name of the

contact person, the system description, etc..

The type of technology for each interface, the estimate

of current bandwidth, the interface state, statistics

about incoming and outgoing traffic ,

The Address Translation Table, contains the address

mappings (physical address, network address )

deprecated

Configurations and statistics variables relevant to

protocols IP, ICMP, TCP, UDP and EGP

Variables that count incoming and outgoing SNMPmessages (get-request, get-next-request, set-request, get-

response and trap)

MIB is a formal description of a set of network objects that

can be managed using SNMP. Each object in MIB contains aunique identifier, objects type (INTEGER,), objects

access level (read, read/write), size restrictions and range

information. Objects in MIB are divided into several groups.

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MIB (cont.)

sysService

sysLocation

sysName

sysContact

sysUpTime

sysObjectID

sysDescr

System Group (1.3.6.1.2.1.1.x)

7

6

5

4

3

2

1

atIfIndexatPhysAddressIfIndex

atNetAddress

1.1

1

AT Group (1.3.6.1.2.1.3.x)

1.1.3

1.1.21.1.1

atTable

atEntry

23-24MIB (cont.)

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ifAdminStatus

ifOperStatus

ifLastChange

ifInOctets

ifInUcastPkts

ifPhysAddress

ifSpeedifMtu

ifType

ifDescr

ifIndex

Interface Group (1.3.6.1.2.1.2.x)

2.1.11

2.1.10

2.1.9

2.1.8

2.1.7

2.1.6

2.1.52.1.4

2.1.3

2.1.2

2.1.1

ifInNUcastPkts

ifInDiscards 2.1.13

2.1.12

ifInErrors

ifInUnknownProtos

ifOutQLen

ifSpecific

ifOutErrors

ifOutDiscardsifOutNUcastPkts

ifOutUcastPkts

ifOutOctets

2.1.22

2.1.21

2.1.20

2.1.192.1.18

2.1.17

2.1.16

2.1.15

2.1.14

ifNumber

ifTable

ifEntry 2.1

2

1

MIB (cont.)

23-25MIB (cont.)

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ipReasmTimeout

ipReasmReqds

ipInUnknownProtos

ipInDiscards

ipInDelivers

ipOutRequests

ipOutDiscards

ipOutNoRoutes

ipReasmOKs

ipForwDatagrams

ipInAddrErrorsipInHdrErrors

ipInReceives

ipDefaultTTL

ipForwarding

IP Group (1.3.6.1.2.1.4.x)

1514

13

12

11

10

9

8

7

6

54

3

2

1

20ipAddrTable

20.1.1ipAdEntAddr

20.1.2ipAdEntIfIndex

20.1.3ipAdEntNetMask

20.1.4ipAdEntBcastAddr

20.1.5ipAdEntEntReasmMaxSize

19ipFragCreates

18ipFragFails

17ipFragOKs

16ipReasmFails

MIB (cont.)

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22ipNetToMediaTable

22.1.4ipNetToMediaType

23ipRoutingDiscards

22.1.1ipNetToMediaIfIndex

22.1.2ipNetToMediaPhysAddress22.1.3ipNetToMediaNetAddress

IP Group (cont.)

MIB (cont.)

21.1.6ipRouteMetric4



21.1.3ipRouteMetric121.1.2ipRouteIfIndex

21.1.1ipRouteDest

21ipRouteTable

21.1.13ipRouteInfo


21.1.11ipRouteMask

21.1.10ipRouteAge

21.1.9ipRouteProto

21.1.8ipRouteType

21.1.7ipRouteNextHop

ARP table

23-27MIB (cont.)

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TCP Group (1.3.6.1.2.1.6.x)

13.1.2

13.1.1

12

11

10

9

8

7

6

54

3

2

1

tcpConnState

tcpConnLocalAddress

tcpAttemptFails

tcpEstabResets

tcpCurrEstab

tcpInSegs

tcpOutSegs

tcpRetransSets

tcpPassiveOpens

tcpActiveOpenstcpMaxConn

tcpRtoMax

tcpRtoMin

tcpRtoAlgorithm

15tcpOutRsts14tcpInErrs

13.1.5tcpConnRemPort

13.1.4tcpConnRemAddress

13.1.3tcpConnLocalPort

udpLocalPortudpLocalAddress

udpOutDatagrams

udpInErrors

udpNoPorts

udpInDatagrams

UDP Group (1.3.6.1.2.1.7.x)

5.1.25.1.1

4

3

2

1

(co )

13tcpConnTable udpTable 5

23-28MIB (cont.)

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icmpInAddrMaskReps

icmpOutMsgs

icmpInRedirectsicmpInEchos

icmpInEchoReps

icmpInTimestamps

icmpInTimestampsReps

icmpInAddrMasks

icmpInSrcQuenchs

icmpInParmProbs

icmpInTimeExcds

icmpInDestUnreachs

icmpInErrors

icmpInMsgs

14

13

12

11

10

9

87

6

5

4

3

2

1

ICMP Group (1.3.6.1.2.1.5.x)

icmpOutEchosicmpOutEchoReps

icmpOutTimestampsReps

icmpOutTimestampsReps

icmpOutAddrMasks

icmpOutAddrMaskReps

icmpOutRedirects

icmpOutSrcQuenchs

icmpOutParmProbs

icmpOutTimeExcds

icmpOutDestUnreachs

icmpOutErrors

26

25

24

23

2221

20

19

18

17

16

15

( )

23-29MIB (cont.)

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ifOutDiscards OBJECT-TYPE

SYNTAX Counter

ACCESS read-only

STATUS mandatory

DESCRIPTION

"The number of outbound packets which were chosen to be

discarded even though no errors had been detected to

prevent their being transmitted. One possible reason for

discarding such a packet could be to free up buffer space

::= { ifEntry 19 }

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ipReasmReqds OBJECT-TYPE

SYNTAX Counter

ACCESS read-only

STATUS mandatory

DESCRIPTION

"The number of IP fragments received which needed to be

reassembled at this entity."

::= { ip 14 }. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Detailed description of MIB-2 is given in RFC 1213 (1991).

Excerpt from RFC 1213:

23-30MIB (cont.)

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Every variable listed in the MIB must be identified when SNMP is making a

reference to it (to fetch or set its value).

Only leaf nodes are referenced. SNMP does not manipulate entire rows or

columns of tables.

Referencing Simple Variables

Simple variables are referenced by appending 0 to the variables OID. For

example variable udpInDatagrams is referenced as:

udpInDatagrams.0, or

1.3.6.1.2.1.7.1.0, or

iso.org.dod.internet.mgmt.mib.udp.udpInDatagrams.0

1.3.6.1.2.1.7.1 defines the variable

1.3.6.1.2.1.7.1.0 defines the instance of the variable

( )

23-31MIB (cont.)

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Referencing TablesIn order to reference a variable in a particular row an index should be

appended to the variable OID. MIB doesnt use conventional indices

(subscripts) to reference a row of table, it rather uses the associative

memory approach. Therefore the index for a desired row is replaced with

the value(s) of the variable(s) in that row:

variable OID.value.value.

It can use a single value or values of several fields separated by dots. For

example the second field in the first row is referenced as:

( )

1.3.6.1.2.1.7.5.1.2.181.23.45.14.23

Variable OID Value of the

first field

Value of the

second field

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Samevariable,

differentinstances181.23.45.14

230.20.5.24

192.13.5.10

23

161

212

Values

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Indexing of table entries

23-34

L i hi d i

MIB (cont.)

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Lexicographic ordering

The OIDs in MIB, including the instance identifiers, are in lexicographic order.

Tables are ordered according to column-row rules. This will help inGetNextRequest command as shown later.

23-35MIB (cont.)

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NOTICE:

MIB is not the actual database itself. It only contains a collection of

definitions which define properties of managed objects, but doesn't contain

actual variable values. Therefore the MIB is sometimes called virtualinformation base.

The SNMP client puts the SMI (ANS.1) identifier for the MIB variable it

wants to get into request message, and it sends this message to the server.The server then maps this identifier into local variable (i.e. into a memory

location where the value for this variable is stored), retrieves the current

value held in this variable, and uses BER to encode the value it sends back

to the client.

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23-37SNMP (cont.)

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Infrastructure for network management

agent

MIB

agent

MIB

agent

MIB

agent

MIB

managed device

managed device

manager MIB

Networkmanagement

protocol

managed devicemanaged device

Managers host runs

SNMP client

Agents runSNMP server

data

data

data

data

23-38

SNMP (cont.)

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Devices that dont support SNMP can be managed viaproxy agents. Proxy agent

translates protocol interactions it receives from manager into whateverinteractions are supported by foreign devices.

data

data

realagent data

proxyagent

MIB

managed device

managed device

manager MIB

managed device

SNMP

Non SNMP

manageddevices

realagent

realagent

Proprietary

messages

23-39SNMP (cont.)

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agent MIB

Managed device

Manager

response

agentMIB

Managed device

trap msg

request

request/response mode trap mode

There are two ways to convey MIB info, commands:

Manager

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SNMP Messages (v.3)

23-41SNMP (cont.)

SNMP M

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GetRequest

GetNextRequest

GetBulkRequest

Mgr-to-agent: get me data

(instance, next in list, block)

Message type Function

InformRequest Mgr-to-Mgr: heres MIB value

SetRequest Mgr-to-agent: set MIB value

Response

Agent-to-mgr: value, response to

request

Trap Agent-to-mgr: inform manager

of exceptional event

SNMP Messages

NOTICE: Messages are in SNMP jargon called PDUs

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SNMP Messages (notices):

The GetNextRequest command is used for reading subsequent table

entries. The retrieved value is the value of the object following (in lexicographic

order) the defined OID in the message. The command is useful in case when the

manager doesn't know the table indices. However, for each piece of data aseparate request has to be made, which results in longer time and increased

traffic.

GetBulkRequest is a newer PDU that can retrieve many data items in asingle request. More efficient than GetNextRequest. Saves time and traffic.

The Trap PDU is sent from the agent to the manager to report an event. For

example, if the agent is rebooted, it informs the manager and reports the time of

rebooting. Trap is an asynchronous notification of some significant event.

InformRequest is sent from a manager for passing information to an

application running in another manager. Response PDU is used to

acknowledge the request. Used in hierarchical or distributed management wheremultiple managers are involved

23-43

(d) variable-bindings

SNMP (cont.)GetRequestGetNextRequest

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variable-bindings00requestid

PDUtype

variable-bindingserrorindexerrorstatusrequestidPDUtype

variable-bindingsmax-repetitionsnon-repeatersrequestidPDUtype

valueNnameN. . .value2name2value1name1

( ) g

SNMP PDU Formats

Sequence IDto matchresponse withthe request

noErrortooBignoSuchNamebadValuereadOnlygenErr

GetBulkRequest

Response

GetNextRequestSetRequestInformRequest

Trap

Offset, tellswhichvariablecaused theerror

Variable bindings

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GetBulkRequest Example

Client Agent

(e.g, router)

Agent returns single value for X, Y, and six rows of table :

GetBulkRequest(non-repeaters=2,max-repeaters=6,X,Y,TA,TB,TC)

Manager issues request with six variable names;for the first two variable (non-repeaters=2),a single value is requested; for the remaining variablessix successive values (max-repeaters=6) are requested.

Response [X, Y, TA(1), TB(1), TC(1),

TA(2), TB(2), TC(2),

TA(3), TB(3), TC(3),

TA(4), TB(4), TC(4),

TA(5), TB(5), TC(5),

TA(6), TB(6), TC(6) ]

x Y

Table

TA TB TC

23-45SNMP (cont.)

SNMP PDU i b dd d i SNMP

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SNMP PDU is embedded into an SNMP message.

SNMP v.3 = 3

Unique message ID,Max message size that sender can handle,Message flags (report/authentication/privacy)

Message security model (SNMPv1,v2c,USM

Used to create a message digest

Used only if PDU is encrypted

23-46

Example of an SNMP v3 message:Network manager wants to know the number of UDP

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NULL

SEQUENCE

Length=31

udpInDatagrams

(1.3.6.1.2.1.7.1.0)

Value=0

gdatagrams a router has received

GetRequest

OBJECT IDENTIFIER

Length=9

BER encoding

has been used here

23-47

S 230 34

Example of an SNMP v3 message (cont.)

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OCTET STRING, length=1, all flags=004 01 00

NULL, length=005 00

OBJECT IDENTIFIER, length=9, udpInDatagram

(1.3.6.1.2.1.7.1.0)

06 09 01 03 06 01

02 01 07 01 00

SEQUENCE, length 1330 0D

SEQUENCE, length 1530 0FINTEGER, length=1, Error Index=002 01 00

INTEGER, length=1, Error Status=002 01 00

INTEGER, length=4, Request ID=0x0001061102 04 00 01 06 11

GetRequest, length=29 (no encryption)A0 1D

SEQUENC, length=31 (Data)30 1F

OCTET STRING, length=0, no security parameter04 00

OCTET STRING, length=0, no security model02 00

INTEGER, length=2, max size=102402 02 04 00

INTEGER, length=1, message ID=6402 01 40

SEQUENCE, length=12 (Header)30 0C

INTEGER, length=1, version=302 01 03

Sequence length = 5230 34

23-48Port numbers for SNMP

8/12/2019 ch23_SNMP

48/48


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ch23_SNMP

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Transcript of ch23_SNMP