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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 Messaging and Message Formats

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SNMP Protocol Message Generation, Addressing, Transport and Retransmission
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SNMP Version 1 (SNMPv1) Message Format
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SNMP Message Field Definitions, General Message Format and Message Sections
(Page 3 of 3)

General PDU Format

The fields in each PDU depend on the PDU type, but can again be divided into the following general substructure:

  • PDU Control Fields: A set of fields that describe the PDU and communicate information from one SNMP entity to another.

  • PDU Variable Bindings: A set of descriptions of the MIB objects in the PDU. Each object is described as a “binding” of a name to a value.

Each PDU will follow this general structure, which is shown in Figure 276, differing only in the number of control fields, the number of variable bindings, and how they are used. In theory, each PDU could have a different message format using a distinct set of control fields, but in practice, most PDUs in a particular SNMP version use the same control fields (though there are exceptions.)


Figure 276: SNMP General Message Format

 


Each variable binding describes one MIB object. The binding consists of a pair of subfields, one specifying the name of the object in standard SNMP object identifier notation, and one its value, formatted to match the object's SMI syntax. For example, if the object is of type Integer, the value field would be 4 bytes wide and contain a numeric integer value. Table 211 describes the subfield format for each PDU variable binding.


Table 211: SNMP Variable Binding Format

Subfield Name

Syntax

Size (bytes)

Description

Object Name

Sequence of Integer

Variable

Object Name: The numeric object identifier of the MIB object, specified as a sequence of integers. For example, the object sysLocation has the object identifier 1.3.6.1.2.1.1.6, so it would be specified as “1 3 6 1 2 1 1 6” using ASN.1

Object Value

Variable

Variable

Object Value: In any type of “get” request, this subfield is a “placeholder”; it is structured using the appropriate syntax for the object but has no value (since the “get” request is asking for that value!)

In a “set” request (SetRequest-PDU) or in a reply message carrying requested data (GetResponse-PDU or Response-PDU), the value of the object is placed here.


Key Concept: The general format of SNMP messages consists of a message header and a message body. The body of the message is also called the protocol data unit or PDU, and contains a set of PDU control fields and a number of variable bindings. Each variable binding describes one MIB object and consists of the object’s name and value.



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