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Chapter 5

SNMPv1 Network Management:

Communication and Functional Models

Last modified: 17/02/2018

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Network Management: Principles and Practice

Objectives

Communication model: Administrative and messages
Administrative structure

Community-based model

Access policy

MIB view

Message PDU
SNMP protocol specifications
SNMP operations
SNMP MIB
SNMP functional model

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

SNMP Architecture

Truly simple network management protocol
Five messages, 3 from manager and 2 from agent
Three goals of the architecture:

1- It should minimize the number and so complexity of management functions realized by the management agent

2- It should be flexible for future expansion (addition of new aspects of operation and management)

3- The architecture should be independent of architecture and mechanisms of particular hosts and gateways

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

SNMP Messages

Get-Request
Get-Next-Request
Set-Request
Get-Response
Trap
Generic trap
Specific trap
Time stamp

Generic trap
coldStart
warmStart
when reinitializing, no SNMP configuration change
linkDown
linkUp
authenticationfailure
egpNeighborLoss
enterpriseSpecific
Specific trap
For special measurements such as statistics
Time stamp: Time since last initialization

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

The get-request will only match exactly the OID (ObjectIDentifier) specified in the request.
It will fail if no such OID exists on the target.
The get-next-request will return the OID that is the "next" OID in the "tree".
The get-next-request can be used to "walk through" the OIDs in the MIB of a certain domain/system

Administrative Model

Based on community profile and policy
SNMP Entities:
SNMP application entities
- Reside in management stations and network elements
- Manager and agent (Both entities are called an SNMP community.)

SNMP protocol entities
- Communication processes (PDU handlers)
- Peer processes that support application entities

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Peer processes, which implement SNMP, and thus support SNMP application entities, they are termed protocol entities

SNMP Community

Multiple SNMP managers communicating with a single SNMP agent.
While an SNMP manager is monitoring traffic on an element, another manager may be configuring some administrative information on it. A third manager can be monitoring it to perform some statistical study.
Security in SNMPv1 is community based

Network Management: Principles and Practice

Notes

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Authentication scheme, which is a filler module, in manager and agent

SNMP Community

Community: Pairing of two application entities
Community name: String of octets
Two applications in the same community communicate with each other
Application could have multiple community names
Communication is not secured in SNMPv1– no encryption

Network Management: Principles and Practice

Notes

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

A community name acts as a password that is shared, typically, by multiple SNMP agents and one or more SNMP managers. You configure the SNMP manager and the computers or devices that it manages as members of a single SNMP community. An SNMP agent only accepts requests from SNMP managers that are on the agent’s list of acceptable community names.

Community Profile

MIB view
An agent is programmed to view only a subset of managed objects of a network element
Access mode
Each community name is assigned an access mode: read-only, read-write, etc.

Community profile: MIB view + SNMP access mode

Operations on an object determined by community profile and the access mode of the object

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

Community Profile

Total of four access privileges

An SNMP agent with READ-WRITE access mode can perform all operations (get, set, and trap) on Object 4. READ-ONLY access mode can only perform get and trap operations on Object 2.

Object 1 has a "not-,accessible" access mode and hence no operation can be performed on it

Some objects, such as table and table entry are non-accessible

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

Administrative Model

SNMP community paired with SNMP community profile is SNMP access policy

Administrative model is SNMP access policy

Parameters:

Community / communities
Agent / Agents
Manager / Managers

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

Access Policy

Manager manages Community 1 and 2 network components via Agents 1 and 2
Agent 1 has only view of Community Profile 1,e.g., Cisco components
Agent 2 has only view of Community Profile 2,e.g., 3Com components
Manager has total view of both Cisco and 3Com components

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

Generalized Administrative Model

Manager 1 manages community 1, manager 2 manages community 2, and manager 3 (MoM) manages both communities 1 and 2

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

Proxy Access Policy

Proxy agent enables non-SNMP community
elements to be managed by an SNMP manager
An SNMP MIB is created to handle the non-SNMP objects
For example, LAN could be a TCP network with SNMP agent
WAN could be an X.25 network which is not an Internet model  can be managed by a proxy agent

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Protocol Entities

Protocol entities are peer processes that support application entities
Communication among protocol entities is accomplished using messages encapsulated in a UDP datagram
Message consists of:
Version identifier
Community name
Protocol Data Unit (PDU)
Message encapsulated and transmitted
SNMP protocol entity is received on port 161
Except trap received on 162

Network Management: Principles and Practice

Notes

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Protocol Entities

The version and community names are added to the data PDU and along with the application header is passed on to the transport layer as SNMP PDU.

The UDP header is added at the transport layer, which then forms the transport PDU for the network layer.
The addition of the IP header to the Transport PDU forms the Network PDU for the data link layer (DLC).
The network or DLC header is added before the frame is transmitted on to the physical medium.

Network Management: Principles and Practice

Notes

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Get and Set PDU

PDU Types: enumerated INTEGER

VarBindList: multiple instances of VarBind pairs
RequesID: is used to track a message with the expected response or for loss of a message.
VarBind: the pairing of the variable (MO) and its value

Network Management: Principles and Practice

Notes

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

A managed object is represented in SNMP by a scalar variable and is simply called a variable.
Associated with the variable is its value. The pairing of the variable and value is called variable binding or VarBind.
The data PDU in the message contains a VarBind pair. For efficiency sake, a list of VarBind pairs can be sent in a message.

PDUs ::=

CHOICE {

get-request GetRequest-PDU,

get-next-request GetNextRequest-PDU,

get-response GetResponse-PDU,

set-request SetRequest-PDU,

trap Trap-PDU

}

get-request [0]

get-next-request [1]

set-request [2]

get-response [3]

trap [4]

Notes

Error in Response

Error Index: No. of VarBind that the first error occurred
Ex: 1 if the error occurred in the first varBind, 5 if the fifth varBind had an error and so on.

Network Management: Principles and Practice

A non zero ErrorStatus is used to indicate that an error occurred
Errorlndex is used to provide additional information on the error

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

ErrorStatus ::=

INTEGER {

noError(0)

tooBig(1)

noSuchName(2)

bad value(3)

readOnly(4)

genErr(5)

}

Trap PDU

Enterprise and agent address pertain to the system generating the trap
Seven generic traps specified by enumerated INTEGER
Specific trap is a trap not covered by enterprise specific trap
Timestamp indicates elapsed time since last re-initialization

Network Management: Principles and Practice

Notes

Figure 5.9 Trap PDUs

Table 5.1 Generic Traps

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

The agent is reporting that the peer relationship between an External Gateway Protocol (EGP) neighbor and an EGP peer no longer exists.

Generic Trap Type	Description (brief)
coldStart(0)	Sending protocol entity is reinitializing itself; agent's configuration or protocol entity implementation may be altered
warmStart(1)	Sending protocol entity is reinitializing itself; agent configuration or protocol entity implementation not altered
linkDown(2)	Failure of one of the communication links
linkUp(3)	One of the links has come up
authenticationFailure(4)	Authentication failure
egpNeighborLoss(5)	Loss of EGP neighbor
enterpriseSpecific(6)	Enterprise-specific trap

SNMP Operations

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

The agent here is integrated in the NE since in the GetRequest, no MO is specified.

Figure 5.10 shows a sequence of operations in retrieving the values

of objects in a System group. It starts with the get-request operation using a GetRequest PDU from a

manager process to an agent process and the get-response from the agent with a GetResponse PDU.

SNMP operations	messages
Manager to Agent	Get-request, get-next-request, and set-request messages
Agent to Manager	Get-response and trap messages

.0= indicqtes q scqlqr vqlure ?

MIB for Get-Next-Request

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

MIB for Get-Next-Request

Network Management: Principles and Practice

Notes

The first two objects A and B are single-valued scalar objects. They are followed by an aggregate object represented by the table T with an entry E and two rows of three columnar objects, T.E.1.1. through T.E.3.2. The MIB group ends with a scalar object Z.

GetRequest(A), GetResponse(A), B, T.E.1.1, T.E.1.2, …, T.E.3.1, T.E.3.2, Z
GetRequest(T.E.1.3)  receive error message
It is useful to use get-next-request

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Notes

Lexicographic Order

Procedure for ordering:
Start with leftmost digit as first position
Before increasing the order in the first position, select the lowest digit in the second position
Continue the process till the lowest digit in the last position is captured
Increase the order in the last position until all the digits in the last position are captured
Move back to the last but one position and repeat the process
Continue advancing to the first position until all the numbers are ordered
Tree structure for the above process

Network Management: Principles and Practice

Table 5.2 Lexicographic-Order Number Example

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

13 numbers ordered according to their numerical values then ordered according to lexicographic order

Numerical Order

Lexicographic order

1118

115

126

115

250

126

2509

250

321

1118

2509

MIB Lexicographic Order

A 3.1

B 3.2

T Z

1.1

1.2

2.1

2.2

Network Management: Principles and Practice

Notes

Figure 5.12 MIB for Operation Examples in Figures 5.13 and 5.15

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

The nodes can be ordered Lexicographically

Figure 5.12 shows an MIB group. It is a generalized case of a conceptual MIB comprising three scalar objects and a table is shown in Figure 5.12. contains two objects A and B which arc single- valued scalar objects. They are followed by an aggregate object represented by the table T with an entry E and two rows of three columnar objects. Objects of the table go from T.E.1.l. through T.E.3.2.

The MIB group ends with a scalar object Z.

A More Complex MIB Example

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Notes

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

1.1

1.1.5

1.1.18

1.2

1.2.6

2.2

2.10

2.10.9

3.4

3.21

Get-Next-Request Operation

Network Management: Principles and Practice

Notes

Advantages of using get-next-request:
We do not need to know the object identifier for the next entity
In the case of an aggregate object, the number of rows is dynamically changing. Thus, we do not know how many rows exist in the table. The get-next-request resolves this problem.
We can use this to build a MIB tree by repeating the request from any node to any node. This is called MIB walk, and is used by a MIB browser in NMS implementation.

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Get-Next-Request Operation

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

The objects AtIfIndex and AtNetAddress are the indices that uniquely identify a row
In this method, the manager has to know the columnar objects of the table
The first get-next-request does not contain any operand value
IpForwarding is the next group in the Internet MIB  the manager has to recognize this and terminate the process

In this example, instead of issuing ten get-next-requests with a single VarBind in the message, the manager generates four GetNextRequest PDUs with a list of two VarBind fields.

Sniffer Data

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

14:03:36.788270 noc3.btc.gatech.edu.164 > noc1.btc.gatech.edu.snmp:

Community = public

GetRequest(111)

Request ID = 4

system.sysDescr.0

system.sysObjectID.0

system.sysUpTime.0

system.sysContact.0

system.sysName.0

system.sysLocation.0

system.sysServices.0

Figure 5.17(a) Get-Request Message from Manager-to-Agent

14:03:36.798269 noc1.btc.gatech.edu.snmp > noc3.btc.gatech.edu.164:

Community = public

GetResponse(196)

Request ID = 4

system.sysDescr.0 = "SunOS noc1 5.5.1 Generic_103640-08 sun4u"

system.sysObjectID.0 = E:hp.2.3.10.1.2

system.sysUpTime.0 = 247396453

system.sysContact.0 = "Brandon Rhodes"

system.sysName.0 = "noc1"

system.sysLocation.0 = "BTC NM Lab"

system.sysServices.0 = 72

Figure 5.17(b) Get-Response Message from Agent-to-Manager (After)

Sniffer Data

Network Management: Principles and Practice

Sniffer tool is a management tool that can capture packets going across a transmission medium.
The seven object IDs from System.sysDescr.0 to System.sysServices.0 all end with zero to indicate that they are single-valued scalar objects.
The community name is public
The request number identified in the GetResponse PDU is the same as the one in the GetRequest PDU.

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

SNMP MIB

Note: Most of the MIB objects were not used and hence deprecated in SNMPv2

Network Management: Principles and Practice

Notes

SNMPv1 MIB has too many objects that are not used
SNMPv2 obsoleted a large number of them

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

( Application functions)

Fault management
Configuration management
Accounting management
Performance management
Security management

Network Management: Principles and Practice

Chapter 5 SNMPv1 Network Management: Communication and Functional Models

Functional Model

SNMP Manager

Application

Get-Response

Get-Request

GetNext-Request

Set-Request

Trap

SNMP Manager

SNMP

UDP

DLC

PHY

SNMP Agent

Application

Get-Response

Trap

SNMP Agent

SNMP

UDP

DLC

PHY

Physical Medium

Figure 4.9 SNMP Network Management Architecture

Manage-

ment

Data

Get-Request

GetNext-Request

Set-Request

SNMP Manager

Authentication Scheme

SNMP Manager

Authentication Scheme

SNMP Manager

Authentication Scheme

SNMP Agent

Authentication Scheme

Authentic Messages

Figure 5.1 SNMP Community

Figure 5.2 SNMP Community Profile

SNMP Agent

Object 2

read-only

READ-

ONLY

READ-

WRITE

SNMP Access Mode

SNMP MIB View

MIB Access

Object 3

write-only

Object 1

not-accessible

Object 4

read-write

Community

Community Profile 1

Community Profile 2

Agent 2

Agent 1

Manager

Community 1

Community Profile 1

Community Profile 2

Manager 3

(Community 1, Community 2)

Agent 2

Agent 1

Community 2

Community Profile 3

Community Profile 4

Agent 4

Agent 3

Manager 2

(Community 2)

Manager 1

(Community 1)

Figure 5.3 SNMP Access Policy

SNMP Community

non-SNMP

Community

Proxy Agent

SNMP

Agent

SNMP Manager

(Community 1)

Figure 5.4 SNMP Proxy Access Policy

Application

Header

UDP

Header

Version Community SNMP PDU

Application PDU

DLC

Header

Transport

PDU

Network PDU

Application

PDU

Transport

PDU

Network

PDU

Data Link

PDU

Figure 5.5 Encapsulated SNMP Message

Data

SNMP

PDU

Type

RequestID

Error

Status

Error

Index

Figure 5.8 Get and Set Type PDUs

VarBind 1

name

VarBind 1

value

...

VarBind n

name

VarBind n

value

PDUs ::=

CHOICE {

get-request GetRequest-PDU,

get-next-request GetNextRequest-PDU,

get-response GetResponse-PDU,

set-request SetRequest-PDU,

trap Trap-PDU

}

get-request

[0]

get-next-request

[1]

set-request

[2]

get-response

[3]

trap

[4]

ErrorStatus ::=

INTEGER {

noError(0)

tooBig(1)

noSuchName(2)

bad value(3)

readOnly(4)

genErr(5)

}

VarBind 1

name

VarBind 1

value

PDU

Type

Enterprise

Agent

Address

...

VarBind n

name

VarBind n

value

Generic

Trap Type

Specific

Trap Type

Timestamp

Generic Trap Type Description (brief)

coldStart(0) Sending protocol entity is reinitializing itself;

agent's configuration or protocol entity

implementation may be altered

warmStart(1) Sending protocol entity is reinitializing itself;

agent configuration or protocol entity

implementation not altered

linkDown(2) Failure of one of the communication links

linkUp(3) One of the links has come up

authenticationFailu

re(4)

Authentication failure

egpNeighborLoss(

Loss of EGP neighbor

enterpriseSpecific(

Enterprise-specific trap

Figure 5.10 Get-Request Operation for System Group

GetRequest (sysDescr.0)

GetResponse (sysDescr .0= "SunOS" )

GetRequest (sysObjectID.0)

GetResponse ( sysObjectID.0=enterprises.11.2.3.10.1.2 )

GetRequest (sysUpTime.0)

GetResponse (sysUpTime.0=2247349530)

GetRequest (sysContact.0)

GetResponse (sysContact.0=" ")

GetRequest (sysName.0)

GetResponse (sysName.0="noc1 ")

GetRequest (sysLocation.0)

GetResponse (sysLocation.0=" ")

GetRequest (sysServices.0)

GetResponse (sysServices.0=72)

Manager

Process

Agent

Process

1.1

Figure 5.12 MIB for Operation Examples in Figures 5.13 and 5.15

2.1

3.1

1.2

2.2

3.2

Numerical Order

Lexicographic order

1118

115

126

115

250

126

2509

250

321

1118

2509

1.1

2.1

3.1

1.2

2.2

3.2

Figure 5.14 MIB Example for Lexicographic Ordering

1.1

1.1.5

1.1.18

1.2

1.2.6

2.2

2.10

2.10.9

3.4

3.21

T.E.1.1

Figure 5.15 Get-Next-Request Operation for MIB in Figure 5.12

T.E.2.1

T.E.3.1

T.E.1.2

T.E.2.2

T.E.3.2

GetRequest ( A )

GetResponse ( A )

GetNextRequest ( A )

GetResponse ( B )

GetNextRequest ( B )

GetResponse ( T.E.1.1 )

GetNextRequest (T.E.1.1 )

GetResponse ( T.E.1.2 )

GetNextRequest (T.E.1.2 )

GetResponse ( T.E.2.1 )

GetNextRequest (T.E.2.1 )

GetResponse ( T.E.2.2 )

GetNextRequest (T.E.2.2 )

GetResponse ( T.E.3.1 )

GetNextRequest (T.E.3.1 )

GetResponse ( T.E.3.2 )

GetNextRequest (T.E.3.2 )

GetResponse ( Z )

GetNextRequest ( Z )

GetResponse ( noSuchName

)

Manager

Process

Agent

Process

atIfIndex

atPhysAddress

0000000C3920B4

0000000C3920AC

0000000C3920AF

atNetAddress

192.168.3.1

172.16.46.1

172.16.49.1

GetNextRequest (sysObjId,

atPhysAddress)

GetResponse( (sysUpTime.0 = "315131795"),

(atPhysAddress.13.172.16.46.1 = "0000000C3920AC"))

GetNextRequest (sysObjId,

atPhysAddress.13.172.16.46.1)

GetResponse( (sysUpTime.0 = "315131800"),

(atPhysAddress.16.172.16.49.1 = "0000000C3920AF") )

GetNextRequest (sysObjId,

atPhysAddress.16.172.16.49.1)

GetResponse( (sysUpTime.0 = "315131805"),

(atPhysAddress.23.192.168.3.1 = "0000000C3920B4") )

GetNextRequest (sysObjId,

atPhysAddress.23.192.168.3.1)

GetResponse( (sysUpTime.0 = "315131810"),

(ipForwarding.0 = "1") )

Figure 5.16 GetNextRequest Example with Indices

Agent

Process

Manager

Process

14:03:36.788270 noc3.btc.gatech.edu.164 >

noc1.btc.gatech.edu.snmp:

Community = public

GetRequest

(111)

Request ID = 4

system.sysDescr.0

system.sysObjectID.0

system.sysUpTime.0

system.sysContact.0

system.sysName.0

system.sysLocation.0

system.sysServices.0

Figure 5.17(a) Get-Request Message from Manager-to-Agent

14:03:36.798269 noc1.btc.gatech.edu.snmp >

noc3.btc.gatech.edu.164:

Community = public

GetResponse

(196)

Request ID = 4

system.sysDescr.0 = "SunOS noc1 5.5.1 Generic_103640-08

sun4u"

system.sysObjectID.0 = E:hp.2.3.10.1.2

system.sysUpTime.0 = 247396453

system.sysContact.0 = "Brandon Rhodes"

system.sysName.0 = "noc1"

system.sysLocation.0 = "BTC NM Lab"

system.sysServices.0 = 72

Figure 5.17(b) Get-Response Message from Agent-to-

Manager (After)

snmp

(mib-2 11)

snmpInPkts(1)

snmpOutPkts (2)

snmpInBadVersions (3)

snmpInCommunityNames (4)

snmpInBadCommunityUses (5)

snmpInASNParseErrors (6)

-- not used (7)

snmpInTooBigs (8)

snmpInNoSuchNames (9)

snmpInBadValues (10)

snmpInReadOnlys (11)

snmpEnableAuthenTraps (30)

snmpOutTraps (29)

snmpOutGetResponses (28)

snmpOutSetRequests (27)

snmpOutGetNexts (26)

snmpOutGetRequests (25)

snmpOutGenErrs (24)

-- not used (23)

snmpOutBadValues (22)

snmpOutNoSuchNames (21)

snmpOutTooBigs (20)

snmpInGenErrs (12)

snmpInTotalReqVars (13)

snmpInTotalSetVars (14)

snmpInGetRequests (15)

snmpInTraps (19)

snmpInGetResponses

(18)

snmpInSetRequests (17)

snmpInGetNexts (16)

Figure 5.21 SNMP Group