Simple Network Management Protocol

JARINGAN KOMPUTER LANJUT

Simple Network Management Protocol

   NAMA               :FITRI ARIYANTI

NIM                  :A12.2011.04364

UNIVERSITAS DIAN NUSWANTORO

SISTEM INFORMASI

KOMPUTER

Simple Network Management Protocol

Background

The Simple Network Management Protocol (SNMP) is an application layer protocol that facilitates theexchange of management information between network devices. It is part of the Transmission ControlProtocol/Internet Protocol (TCP/IP) protocol suite. SNMP enables network administrators to managenetwork performance, find and solve network problems, and plan for network growth.Two versions of SNMP exist: SNMP version 1 (SNMPv1) and SNMP version 2 (SNMPv2). Bothversions have a number of features in common, but SNMPv2 offers enhancements, such as additionalprotocol operations. Standardization of yet another version of SNMP—SNMP Version 3 (SNMPv3)—ispending. This chapter provides descriptions of the SNMPv1 and SNMPv2 protocol operations. Figure56-1 illustrates a basic network managed by SNMP.

SNMP Version 1

SNMP version 1 (SNMPv1) is the initial implementation of the SNMP protocol. It is described in RequestFor Comments (RFC) 1157 and functions within the specifications of the Structure of ManagementInformation (SMI). SNMPv1 operates over protocols such as User Datagram Protocol (UDP), InternetProtocol (IP), OSI Connectionless Network Service (CLNS), AppleTalk Datagram-Delivery Protocol(DDP), and Novell Internet Packet Exchange (IPX). SNMPv1 is widely used and is the de factonetwork-management protocol in the Internet community.

SNMPv1 and Structure of Management Information

The Structure of Management Information (SMI) defines the rules for describing managementinformation, using Abstract Syntax Notation One (ASN.1). The SNMPv1 SMI is defined in RFC 1155.The SMI makes three key specifications: ASN.1 data types, SMI-specific data types, and SNMP MIBtables.

SNMPv1 Protocol Operations

SNMP is a simple request/response protocol. The network-management system issues a request, andmanaged devices return responses. This behavior is implemented by using one of four protocoloperations: Get, GetNext, Set, and Trap. The Get operation is used by the NMS to retrieve the value ofone or more object instances from an agent. If the agent responding to the Get operation cannot providevalues for all the object instances in a list, it does not provide any values. The GetNext operation is usedby the NMS to retrieve the value of the next object instance in a table or a list within an agent. The Setoperation is used by the NMS to set the values of object instances within an agent. The Trap operationis used by agents to asynchronously inform the NMS of a significant event.

SNMP Version 2

SNMP version 2 (SNMPv2) is an evolution of the initial version, SNMPv1. Originally, SNMPv2 waspublished as a set of proposed Internet standards in 1993; currently, it is a draft standard. As withSNMPv1, SNMPv2 functions within the specifications of the Structure of Management Information(SMI). In theory, SNMPv2 offers a number of improvements to SNMPv1, including additional protocol

operations.

SNMPv2 and Structure of Management Information

The Structure of Management Information (SMI) defines the rules for describing managementinformation, using ASN.1.The SNMPv2 SMI is described in RFC 1902. It makes certain additions and enhancements to theSNMPv1 SMI-specific data types, such as including bit strings, network addresses, and counters. Bitstrings are defined only in SNMPv2 and comprise zero or more named bits that specify a value. Networkaddresses represent an address from a particular protocol family. SNMPv1 supports only 32-bit IP

addresses, but SNMPv2 can support other types of addresses as well. Counters are non-negative integersthat increase until they reach a maximum value and then return to zero. In SNMPv1, a 32-bit counter

size is specified. In SNMPv2, 32-bit and 64-bit counters are defined.

SNMPv2 Protocol Operations

The Get, GetNext, and Set operations used in SNMPv1 are exactly the same as those used in SNMPv2.However, SNMPv2 adds and enhances some protocol operations. The SNMPv2 Trap operation, forexample, serves the same function as that used in SNMPv1, but it uses a different message format andis designed to replace the SNMPv1 Trap.

SNMPv2 also defines two new protocol operations: GetBulk and Inform. The GetBulk operation is usedby the NMS to efficiently retrieve large blocks of data, such as multiple rows in a table. GetBulk fills aresponse message with as much of the requested data as will fit. The Inform operation allows one NMSto send trap information to another NMS and to then receive a response. In SNMPv2, if the agentresponding to GetBulk operations cannot provide values for all the variables in a list, it provides partialresults.

SNMP Management

SNMP is a distributed-management protocol. A system can operate exclusively as either an NMS or anagent, or it can perform the functions of both. When a system operates as both an NMS and an agent,

SNMP Security

SNMP lacks any authentication capabilities, which results in vulnerability to a variety of securitythreats. These include masquerading occurrences, modification of information, message sequence andtiming modifications, and disclosure.

SNMP InteroperabilityRFC 1908 defines two possible SNMPv1/v2 coexistence strategies: proxy agents andbilingual network-management systems.

Proxy Agents

An SNMPv2 agent can act as a proxy agent on behalf of SNMPv1 managed devices, as follows:

An SNMPv2 NMS issues a command intended for an SNMPv1 agent.

• The NMS sends the SNMP message to the SNMPv2 proxy agent.

• The proxy agent forwards Get, GetNext, and Set messages to the SNMPv1 agent unchanged.

• GetBulk messages are converted by the proxy agent to GetNext messages and then are forwarded tothe SNMPv1 agent.

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Bilingual Network-Management System

Bilingual SNMPv2 network-management systems support both SNMPv1 and SNMPv2. To support thisdual-management environment, a management application in the bilingual NMS must contact an agent.The NMS then examines information stored in a local database to determine whether the agent supports

SNMPv1 or SNMPv2. Based on the information in the database, the NMS communicates with the agentusing the appropriate version of SNMP.

SNMP Reference: SNMPv1 Message Formats

SNMPv1 messages contain two parts: a message header and a protocol data unit (PDU). Figure 56-4illustrates the basic format of an SNMPv1 message.

Figure 56-4 An SNVPv1 Message Consists of a Header and a PDU

SNMPv1 Message Header

SNMPv1 message headers contain two fields: Version Number and Community Name.

The following descriptions summarize these fields:

• Version number

• Community name

SNMPv1 Protocol Data Unit

SNMPv1 PDUs contain a specific command (Get, Set, and so on) and operands that indicate the objectinstances involved in the transaction. SNMPv1 PDU fields are variable in length, as prescribed byASN.1. Figure 56-5 illustrates the fields of the SNMPv1 Get, GetNext, Response, and Set PDUstransactions

SNMP Reference: SNMPv2 Message Format

SNMPv2 messages consist of a header and a PDU. Figure 56-7 illustrates the basic format of anSNMPv2 message.

Figure 56-7 SNMPv2 Messages Also Consist of a Header and a PDU

SNMPv2 Message Header

SNMPv2 message headers contain two fields: Version Number and Community Name.

The following descriptions summarize these fields:

• Version number

• Community name

SNMPv2 Protocol Data Unit

SNMPv2 specifies two PDU formats, depending on the SNMP protocol operation. SNMPv2 PDU fieldsare variable in length, as prescribed by Abstract Syntax Notation One (ASN.1).

Figure 56-8 illustrates the fields of the SNMPv2 Get, GetNext, Inform, Response, Set, and Trap PDUs.