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SNMP Message Field Definitions, General Message Format and Message Sections
(Page 2 of 3)
General Message Format
To understand SNMP messages, it is
important that we first grasp the difference between SNMP messages
and SNMP protocol data units (PDUs). We've seen in looking at
SNMP protocol operations that the two terms are often used interchangeably.
This is because each message carries one PDU, and the PDU is the most
important part of the message.
However, strictly speaking, an SNMP
PDU and an SNMP message are not exactly the same. The PDU is the actual
piece of information that is being communicated between SNMP entities.
It is carried within the SNMP message along with a number
of header fields, which are used to carry identification and security
information. Thus, conceptually, the SNMP message format can be considered
to have two overall sections:
- Message Header: Contains fields used to
control how the message is processed, including fields for implementing
- Message Body (PDU): Contains the main
portion of the message. In this case, the message body is the protocol
data unit (PDU) being transmitted.
The overall SNMP message is sometimes
called a wrapper for the PDU, since it encapsulates the PDU and
precedes it with additional fields. The distinction between the PDU
and the message format as a whole began as a formality in SNMP version
1, but became quite important in later versions. The reason is that
it allows the fields used for basic protocol operations (which are in
the PDU) to be kept separate from fields used to implement security
features. In SNMPv2, the implementation of security became a very big
deal indeed, as we will see, so this flexibility was quite important.
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Version 3.0 - Version Date: September 20, 2005
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