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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  TCP/IP Network Management Framework and Protocols (SNMP and RMON)
                9  TCP/IP Simple Network Management Protocol (SNMP) Protocol
                     9  SNMP Protocol Operations

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SNMP Protocol Operations
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SNMP Protocol Basic Request/Response Information Poll Using GetRequest and (Get)Response Messages
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SNMP Protocol General Operation, Communication Methods and Message Classes
(Page 2 of 2)

SNMP PDU Classes

SNMPv1 originally defined six PDUs. The number of PDUs was expanded and some changes made to their name and use in SNMPv2 and SNMPv3. The current SNMP Framework categorizes the PDUs into different classes. These classes describe both the function of each message type and the kind of communication they use to perform their task (polling versus interrupting).

Table 210 shows the main SNMPv2/SNMPv3 PDU classes, describes them, and shows which PDUs are in each class in SNMPv2/SNMPv3. These classes were not used in SNMPv1 but for clarity I also show which messages from SNMPv1 fall into the classes conceptually:


Table 210: SNMP PDU (Message) Classes

SNMPv3 PDU Class

Description

SNMPv1 PDUs

SNMPv2/SNMPv3 PDUs

Read

Messages that read management information from a managed device using a polling mechanism.

GetRequest-PDU, GetNextRequest-PDU

GetRequest-PDU, GetNextRequest-PDU,
GetBulkRequest-PDU

Write

Messages that change management information on a managed device to affect the device's operation.

SetRequest-PDU

SetRequest-PDU

Response

Messages sent in response to a previous request.

GetResponse-PDU

Response-PDU

Notification

Messages used by a device to send an interrupt-like notification to an SNMP manager.

Trap-PDU

Trapv2-PDU,
InformRequest-PDU


The GetBulkRequest-PDU and InformRequest-PDU messages are new in SNMPv2/v3. The GetResponse-PDU message was renamed just Response-PDU (since it is in fact a response and not a message that “gets” anything), and the new Trapv2-PDU replaces Trap-PDU.

There are three other “special” classes defined by the current SNMP Framework that are of less interest to us because they don't define actively-used messages, but which I should mention for completeness. The Internal class contains a special message called Report-PDU defined for internal SNMP communication. The SNMP standards also provide two classes called Confirmed and Unconfirmed, used to categorize the messages in my table above based on whether or not they are acknowledged. The Report-PDU, Trapv2-PDU, and Response-PDU messages are considered Unconfirmed and the rest are Confirmed.

The next few topics will show how the major message types in the four main classes are used. Note that in general terms, all protocol exchanges in SNMP are described in terms of one SNMP entity sending messages to another. Most commonly, the entity sending requests is an SNMP manager and the one responding an SNMP agent, except for traps, which are sent by agents. For greater clarity, I try to use these more specific terms (manager or agent) when possible, rather than just “entity”.

Key Concept: SNMP messages consist of a set of fields wrapped around a data element called a protocol data unit or PDU. In some cases, the terms “message” and “PDU” are used interchangeably, though they are technically not the same. SNMP PDUs are arranged into classes based on their function.



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SNMP Protocol Basic Request/Response Information Poll Using GetRequest and (Get)Response Messages
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Version 3.0 - Version Date: September 20, 2005

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