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requests, replies with Get-Response message. On some special situations, like network
device ports are on Up/Down status or the network topology changes, Agents can send
Trap messages to NMS to inform the abnormal events. Besides, NMS can also be set to
alert to some abnormal events by enabling RMON function. When alert events are
triggered, Agents will send Trap messages or log the event according to the settings.
Inform-Request is mainly used for inter-NMS communication in the layered network
management.
USM ensures the transfer security by well-designed encryption and authentication.
USM encrypts the messages according to the user typed password. This mechanism
ensures that the messages can’t be viewed on transmission. And USM authentication
ensures that the messages can’t be changed on transmission. USM employs DES-CBC
cryptography. And HMAC-MD5 and HMAC-SHA are used for authentication.
VACM is used to classify the users’ access permission. It puts the users with the
same access permission in the same group. Users can’t conduct the operation which is
not authorized.
2.4.2 Introduction to MIB
The network management information accessed by NMS is well defined and
organized in a Management Information Base (MIB). MIB is pre-defined information which
can be accessed by network management protocols. It is in layered and structured form.
The pre-defined management information can be obtained from monitored network
devices. ISO ASN.1 defines a tree structure for MID. Each MIB organizes all the available
information with this tree structure. And each node on this tree contains an OID (Object
Identifier) and a brief description about the node. OID is a set of integers divided by
periods. It identifies the node and can be used to locate the node in a MID tree structure,
shown in the figure below: