SNMP Version 2 (SNMPv2) Message Formats
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After SMNP version 1 had been in use for several years, certain issues with it were noticed and areas for improvement identified. This led to the development of the original SNMP version 2, which was intended to enhance SNMPv1 in many areas, including MIB object definitions, protocol operations and security. This last area, security, led to the proliferation of SNMPv2 version variants that I described in the topic on SNMP versions.
Since there are in fact several different SNMPv2s, there are also several message formats for SNMPv2, not just one. This is confusing enough, but would be even worse without the modular nature of SNMP messages coming to the rescue. The protocol operations in SNMPv2 were changed from SNMPv1, which necessitated some modifications to the format of SNMPv2 PDUs. However, the protocol operations are the same for all the SNMPv2 variations. The differences between SNMPv2 variants are in the areas of security implementation. Thus, the result of this is that the PDU format is the same for all the SNMPv2 types, while the overall message format differs for each variant. (This is why the distinction between a PDU and a message is not just an academic one!)
During the SNMPv2 divergence, four variations were defined: the original SNMPv2 (SNMPv2p); community-based SNMPv2 (SNMPv2c), user-based SNMPv2 (SNMPv2u) and SNMPv2 star (SNMPv2*). Of these, the first three were documented in sets of SNMP RFC standards, the fourth was not. The structure of the overall message format for each variant is discussed in an administrative or security standard for the variation in question, which makes reference to the shared SNMPv2 standard for the PDU format (RFC 1905).
The overall message formats for SNMPv2p, SNMPv2c and SNMPv2u are as follows:
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