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POSTECH DP&NM Lab

Simple Network Management Protocol

(SNMP)J. Won-Ki Hong

Dept. of Computer Science and Engineering

POSTECH

Tel: 054-279-2244

Email: jwkhong@postech.ac.kr

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POSTECH DP&NM Lab

Table of Contents

• SNMP Operations

• Protocol Specification

• Transport-Level Support

• Limitations of SNMPv1

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SNMP Operations

• Operations supported in SNMP are the inspection and modification of variables

• GET operation– retrieves management information (values of scalar

objects)

• SET operation– updates management information (values on scalar

objects)

• TRAP operation– sends unsolicited scalar object values to notify

problems

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SNMP Operations (cont’d)

• Not possible to change the structure of a MIB– cannot add or delete object instances

• No explicit action is supported• Access is provided only to leaf objects in the

MIB tree– not possible to access an entire table or a row of a

table with a single atomic action

• These simplify the implementation of SNMP but limit the capability of the NMS

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SNMP Security Concepts• Authentication service

– agent may wish to limit access to the MIB to authorized managers

• Access policy– agent may wish to give different access privileges to different

managers

• Proxy service– agent may act as a proxy to other managed devices– this may require authentication service and access policy for other

managed devices on the proxy

• SNMP provides only a primitive and limited security capability via the concept of community

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SNMP Community• is a relationship between an agent and a set of

managers that defines authentication, access control & proxy characteristics

• a community is locally defined by the agent– each community is given a unique community name– an agent may establish a number of communities– the community name is needed for all get and set operations– the same community name may be used by different agents

• SNMP authentication service– every SNMP message from a manager includes a community

name (used as a password) --- very primitive– most agents only allow GET operations

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SNMP Community (cont’d)

• SNMP Access Policy– an agent can provide different categories of MIB access using the f

ollowing concepts: SNMP MIB View & Access Mode

• SNMP MIB View– a subset of objects within a MIB– different MIB views may be defined for each community– the set of objects in a view need not belong to a single subtree

• SNMP Access Mode– an access mode {READ-ONLY, READ-WRITE} is defined for each

community– the access mode is applied uniformly to all objects in the MIB view

• SNMP Community Profile– a combination of a MIB view and an access mode

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MIB ACCESS Category vs. SNMP Access Mode

SNMP Access ModeMIB ACCESS

Category READ-ONLY READ-WRITE

read-only Available for get and trap operations

read-writeAvailable for get and

trap operations

Available for get, set,

and trap operations

write-only

Available for get and

trap operations, but the

value is

implementation-specific

Available for get, set,

and trap operations, but

the value is

implementation-specific

for get and trap

operations.

not

accessibleUnavailable

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SNMP Administrative Concepts

set of SNMP managers

SNMP Access Policy

SNMP community(community name)

SNMP communityprofile

SNMPagent

SNMP MIB view

SNMPaccess mode

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Object Instance Identification

• SNMP defines two techniques for identifying a specific object instance– Serial access technique (via lexicographic ordering of objects)– Random access technique

• Random access technique– objects in MIB tables are referred to as columnar objects– the object identifier is not sufficient to identify the instance– SNMP convention

• concatenate the scalar object identifier with the values of INDEX objects, listed in the order which the INDEX objects are defined

• see the example in Table 7.2 on page 169

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Lexicographical Ordering

• is used for accessing MIB objects serially• given the tree structure of a MIB, the OID for a par

ticular object may be derived by tracing a path from the root to the object

• lexicographical ordering is also referred to as:– preorder traversal (root, left, right) of a tree– depth-first search

• useful for examining MIBs whose structure is not known to NMS

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Lexicographical OrderingExample

1

1.2

root

2

2.1

2.1.1

1.1

1.2.1

2.1.1.1 2.1.1.2 2.1.1.3

1

1 2

1

2

1

1

12

3

Start End

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Protocol SpecificationSNMP Message Formats

• SNMP manager and agent exchange requests and management information using SNMP messages

• SNMP message includes a version number (e.g., 0 for SNMPv1, 1 for SNMPv2), a community name and one of five types of protocol data units (PDUs)

• PDU Types: GetRequest, GetNext-Request, SetRequest, GetResponse, Trap

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SNMP Message Formats

valueNnameN. . .value2name2value1name1

variablebindingstimestamp

specifictrap

generictrap

agentaddr

enter-prise

PDU type

variablebindingserrorindex

errorstatus

requestid

PDU type

variablebindings00requestid

PDU type

SNMP PDUVersion Community

(a) SNMP message

(b) GetRequest PDU, GetNextRequest PDU, and SetRequest PDU

(c) GetResponse PDU

(d) Trap PDU

(e) variablebindings

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SNMP Message FieldsField Description

version SNMP version(RFC 1157 is version 1.)

community A pairing of an SNMP agent with some arbitrary set of

SNMP application entities (the community name acts as

a password to authenticate the SNMP message)

request-id Used to distinguish among outstanding requests by providing

each request with a unique ID.

error-status Used to indicate that an exception occurred while processing a

request; values are noError (0), tooBig (1), noSuchName (2),

badValue (3), readOnly (4), genErr (5)

error-index When error-status is nonzero, may provide additional information

by indicating which variable in a list caused the exception.

(A variable is an instance of a managed object.)

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SNMP Message Fields (cont’d)

Field Description

variablebindings A list of variable names and corresponding values (In

some cases, such as Getrequest PDU, the values are null.)

enterprise Type of object generating trap; based on sysObjectID

agent-addr Address of object generating trap

generic-trap Generic trap type; values are coldStart (0), warmStart (1),

linkDown (2), linkUp (3), authentication failure (4),

egpNeighborLoss (5), enterprise Specific (6).

specific-trap Specific trap code

time-stamp Time elapsed between the last (re)initialization of the

network entity and the generation of the trap;

contains the value of sysUpTime.

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Transmission of SNMP Message1. The PDU is constructed using ASN.1

2. This PDU is passed to an authentication service with a community name and source & destination transport addresses passed– the authentication service performs any required transformations

such as encryption or the inclusion of an authentication code

3. The protocol entity then constructs a message, consisting of a version field, the community name, and the result from step 2

4. This new ASN.1 object is then encoded using BER and passed to the transport service

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Receipt of SNMP Message1. The SNMP entity performs basic syntax-check

of the message and discards it if it fails to parse2. It verifies the version number and discards it if

there is a mismatch3. It then passes the community name, the PDU

portion of the message and the source/destination transport address to an authentication service– if authentication fails, the message is discarded– if authentication succeeds, the authentication service returns

a PDU in the form of an ASN.1 object

4. If the PDU passes a basic syntax-check, the appropriate SNMP access policy is selected and the PDU is processed accordingly

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SNMP PDU Sequences

Manager Agent

GetRequest PDU

GetResponse PDU

Manager Agent

SetRequest PDU

GetResponse PDU

Manager Agent

GetNextRequest PDU

GetResponse PDU

Manager Agent

Trap PDU

(a) Get values (b) Get next values

(c) Set values (d) Send trap

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GetRequest PDU

• is issued by an SNMP manager on behalf of NMS to retrieve information from an agent

• includes PDU type, request-id & variablebindings• GetResponse PDU containing the same request-i

d is used for the reply• operation is atomic (all values are returned or none is)

• possible error-status:– noSuchName: object instance cannot be found or it is an aggrega

te type– tooBig: the size of resulting values exceed a local limitation– genErr: may not be able to supply a value for at least one of the o

bjects for some other reason

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GetNextRequest PDU

• is also issued by an SNMP manager on behalf of NMS to retrieve information from an agent

• the PDU is the same as GetRequest PDU except: – In the GetRequest PDU, each variable in the variablebindings list r

efers to an object instance whose value is to be returned

– In the GetNextRequest PDU, for each variable in the variablebindings, the value of the object instance that is next in lexicographic order is returned

• allows NMS to discover the structure of a MIB view dynamically

• provides an efficient mechanism for searching a table whose entries are unknown

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SetRequest PDU

• is issued by an SNMP manager on behalf of NMS to modify information in an agent

• the operation is also atomic– if any one of the values can’t be set, then the whole operation fails

• GetResponse PDU containing the same request-id is used for the reply– if the operation succeeds, a GetResponse PDU is returned with th

e same variablebindings as in the original SetRequest PDU

• possible error-status:– noSuchName, tooBig, genErr plus– badValue: PDU contains at least one pair of variable name and va

lue that is inconsistent

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Trap PDU

• is issued by an SNMP agent to notify NMS of some significant event

• Trap PDU does not require a response and is not acknowledged can get lost

• Generic Trap types:– coldStart (0): unexpected restart due to a crash or major fault– warmStart (1): routine restart– linkDown (2): a communication link is inoperational– linkUp (3): the link is back in operation– authenticationFailure (4): received authentication-failed message– egpNeighborLoss (5): EGP neighbor is down– enterpriseSpecific (6): some enterprise-specific event occurred

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Transport-Level Support

• SNMP requires the use of a transport service for the delivery of SNMP messages.– SNMP makes no assumption about whether the underlying

service is reliable or unreliable, connectionless or connection-oriented

• Most SNMP implementations use UDP• It is possible to use CLTS • UDP

– Unreliable, connectionless transport service in Internet

• CLTS– Unreliable, connectionless transport service in the OSI

architecture

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Issues in using UDP• Since UDP provides unreliable transport servi

ce, SNMP messages can get lost

• What happens if a GetRequest or GetNextRequest message is lost?

• What happens if a SetRequest message is lost?

• What happens if a Trap message is lost?

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Limitations of SNMP• SNMP may not be suitable for the mgmt of truly large netw

orks because of the performance limitations of polling• SNMP is not well suited for retrieving large volumes of dat

a, such as an entire routing table• SNMP traps are unacknowledged & may not be delivered• SNMP provides only trivial authentication• SNMP does not support explict actions• SNMP MIB model is limited (does not support mgmt querie

s based on object types or values)• SNMP does not support manager-to-manager communicat

ions

Many of these problems are addressed in SNMPv2!