SNMP Manager Programmer's Guide

Introduction to Manager Services
SNMP Manager Programmer’s Guide–134249
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SNMP Packets
As the SYNTAX clause indicates, any value assigned to this object must be an integer,
specifically 0 or 1 in this case. The ACCESS clause assigns an access attribute of
read-write to this object, indicating that managers can both retrieve and change the value
of the object. A manager can send a request packet to retrieve information when the
access attribute is read-only or read-write. A manager can send a request packet to
change information when the access attribute is read-write or write-only.
Some objects, like zhrmDynamRefreshNow, are called scalar objects, because they can
have only one instance. When a MIB object can have multiple instances, as when the
object is the name of a printer and a managed system has multiple printers, the MIB
defines a table for describing the object.
A table consists of one or more entries, each entry representing a row in the table. An
entry can contain one object or multiple objects. Individual objects occurring in table
rows are uniquely identified by an index, whose value is derived from the value of one
or more of the objects in an entry. Packets describing table entries use numeric
identifiers that include an index value identifying a specific object instance.
SNMP Packets
An SNMP packet, which must be structured as described in RFC 1157, has the general
format shown in Figure 1-2.
The version field identifies the version of SNMP whose information and packet protocol
is being used; currently, managers, agents, and subagents that run as NonStop Kernel
processes support version 1 of SNMP. The community field identifies the name by
which the manager involved in the message exchange is known to the agent with which
it is communicating; defined by managers, this name is used by agents for
authentication. The PDU field is actually a collection of fields, which vary slightly with
the message type:
•
Get, GetNext, and Set PDUs are requests to retrieve a value, retrieve the next value,
or change the value of a managed resource attribute, respectively. Packets
containing these PDUs originate from a manager and are sent to an agent. These
messages describe the type of operation and provide one or more variable bindings
to identify the target MIB objects.
•
Response PDUs are replies to request messages. Packets containing these PDUs are
sent by an agent to a manager on behalf of the agent or a subagent. Response PDUs
indicate whether the requested operation succeeded and provide the results of the
operation.
•
Trap PDUs are unsolicited notifications for managers that receive no reply. Packets
containing these PDUs usually originate from an agent or subagent, but may
originate from a manager. Trap PDUs contain the physical address of the entity
Figure 1-2. General SNMP Packet Format
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version community Protocol Data Unit (PDU)