Note that IETF standards-track documents have status of "proposed", "draft", "full", "experimental", or "historic". Note that at this time only the SNMPv1 protocol has widespread usage and is an Internet (full) standard.
There currently exists the following versions of the SNMP protocol:
The SNMPv1, SNMPv2c, SNMPv2u, and SNMPv3 protocol messages have a common form, which is an ASN.1 sequence containing a message version field, followed by version dependent fields.
The SNMPsec, SNMPv2p, and SNMPv2* protocol messages have a common form, which is a tagged ASN.1 context specific sequence containing message dependent fields.
The SMI defines the format for defining managed objects that are accesses via the SNMP protocol, and contains a few administrative assignments. There are currently two versions of the SMI, which are:
SMIv2 is a backward compatible update of SMIv1, in all cases except for data type Counter64. That is, it is possible to mechanically create a definition of managed objects in the SMIv1 format from a definition in the SMIv2 format except for objects whose data type is Counter64. There is no complete mechanical conversion from definitions of managed objects in the SMIv1 format to the SMIv2 format, since the SMIv2 format contains fields for additional information that must be provided by the designer of the definitions. Also, the SMIv2 format contains contructs to define requrement specifications and to define implementation specifications, not found in the SMIv1 format.
The definition of managed objects is independent of the protocol to access them except for objects with data type of Counter64. That data type does not exist in the SNMPv1 and SNMPsec protocols. A conforming SNMPv1/SNMPsec entity will generate an ASN.1 parse error when parsing a message containing containing a Counter64 data type. RFC 2089 defines the behavior of a conforming bi-lingual agent that has access to objects with Counter64 data type.
At this time there is widespead use and support of both versions of the SMI. This is due in part to the policy in the IETF that new versions of RFCs must specify MIBs in the SMIv2 format.
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