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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
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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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Version 3.0 - Version Date: September 20, 2005
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