Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

The whole site in one document for easy reference!
The TCP/IP Guide

Custom Search

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

Previous Topic/Section
SNMP Protocol Operations
Previous Page
Pages in Current Topic/Section
Next Page
SNMP Protocol Basic Request/Response Information Poll Using GetRequest and (Get)Response Messages
Next Topic/Section

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





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

GetRequest-PDU, GetNextRequest-PDU

GetRequest-PDU, GetNextRequest-PDU,


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




Messages sent in response to a previous request.




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



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.

Previous Topic/Section
SNMP Protocol Operations
Previous Page
Pages in Current Topic/Section
Next Page
SNMP Protocol Basic Request/Response Information Poll Using GetRequest and (Get)Response Messages
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $

Home - Table Of Contents - Contact Us

The TCP/IP Guide (
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.