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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

SNMP Protocol Operations 1 2 SNMP Protocol Basic Request/Response Information Poll Using GetRequest and (Get)Response Messages

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:

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”.

SNMP Protocol Operations 1 2 SNMP Protocol Basic Request/Response Information Poll Using GetRequest and (Get)Response Messages