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

SNMP Protocol Message Generation, Addressing, Transport and Retransmission 1 2 3 SNMP Version 1 (SNMPv1) Message Format

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 SNMP security.
  
  
• 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.

SNMP Protocol Message Generation, Addressing, Transport and Retransmission 1 2 3 SNMP Version 1 (SNMPv1) Message Format