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2.4. Extensions to the SMI in Version 2

SMIv2 extends the SMI object tree by adding the snmpV2 branch to the internet subtree, adding several new datatypes, and making a number of other changes. Figure 2-3 shows how the snmpV2 objects fit into the bigger picture; the OID for this new branch is 1.3.6.1.6.3.1.1, or iso.org.dod.internet.snmpV2.snmpModules.snmpMIB.snmpMIBObjects. SMIv2 also defines some new datatypes, which are summarized in Table 2-2.

Figure 2-3

Figure 2-3. SMIv2 registration tree for SNMPv2

Table 2-2. New Datatypes for SMIv2

Datatype

Description

Integer32
Same as an INTEGER.

Counter32
Same as a Counter.

Gauge32
Same as a Gauge.

Unsigned32
Represents decimal values in the range of 0 to 232 - 1 inclusive.

Counter64
Similar to Counter32, but its maximum value is 18,446,744,073,709,551,615. Counter64 is ideal for situations in which a Counter32 may wrap back to 0 in a short amount of time.

BITS
An enumeration of nonnegative named bits.

The definition of an object in SMIv2 has changed slightly from SMIv1. There are some new optional fields, giving you more control over how an object is accessed, allowing you to augment a table by adding more columns, and letting you give better descriptions. Here's the syntax of an object definition for SMIv2. The changed parts are in bold:

<name> OBJECT-TYPE
    SYNTAX <datatype>
    UnitsParts <Optional, see below>
    MAX-ACCESS <See below>
    STATUS <See below>
    DESCRIPTION 
        "Textual description describing this particular managed object."
    AUGMENTS { <name of table> }
    ::= { <Unique OID that defines this object> }
Table 2-3 briefly describes the object definition enhancements made in SMIv2.

Table 2-3. SMIv2 Object Definition Enhancements

Object Definition Enhancement

Description

UnitsParts
A textual description of the units (i.e., seconds, milliseconds, etc.) used to represent the object.

MAX-ACCESS
An OBJECT-TYPE's ACCESS can be MAX-ACCESS in SNMPv2. The valid options for MAX-ACCESS are read-only, read-write, read-create, not-accessible, and accessible-for-notify.

STATUS
This clause has been extended to allow the current, obsolete, and deprecated keywords. current in SNMPv2 is the same as mandatory in an SNMPv1 MIB.

AUGMENTS
In some cases it is useful to add a column to an existing table. The AUGMENTS clause allows you to extend a table by adding one or more columns, represented by some other object. This clause requires the name of the table the object will augment.

SMIv2 defines a new trap type called NOTIFICATION-TYPE, which we will discuss later in Section 2.6.7, "SNMP Notification". SMIv2 also introduces new textual conventions that allow managed objects to be created in more abstract ways. RFC 2579 defines the textual conventions used by SNMPv2, which are listed in Table 2-4.

Table 2-4. Textual Conventions for SMIv2

Textual Convention

Description

DisplayString
A string of NVT ASCII characters. A DisplayString can be no more than 255 characters in length.

PhysAddress
A media- or physical-level address, represented as an OCTET STRING.

MacAddress
Defines the media-access address for IEEE 802 (the standard for local area networks) in canonical[11] order. (In everyday language, this means the Ethernet address.) This address is represented as six octets.

TruthValue
Defines both true and false Boolean values.

TestAndIncr
Used to keep two management stations from modifying the same managed object at the same time.

AutonomousType
An OID used to define a subtree with additional MIB-related definitions.

VariablePointer
A pointer to a particular object instance, such as the ifDescr for interface 3. In this case, the VariablePointer would be the OID ifDescr.3.

RowPointer
A pointer to a row in a table. For example, ifIndex.3 points to the third row in the ifTable.

RowStatus
Used to manage the creation and deletion of rows in a table, since SNMP has no way of doing this via the protocol itself. RowStatus can keep track of the state of a row in a table, as well as receive commands for creation and deletion of rows. This textual convention is designed to promote table integrity when more than one manager is updating rows. The following enumerated types define the commands and state variables: active(1), notInService(2), notReady(3), createAndGo(4), createAndWait(5), anddestroy(6).

TimeStamp
Measures the amount of time elapsed between the device's system uptime and some event or occurrence.

TimeInterval
Measures a period of time in hundredths of a second. TimeInterval can take any integer value from 0-2147483647.

DateAndTime
An OCTET STRING used to represent date-and-time information.

StorageType
Defines the type of memory an agent uses. The possible values are other(1), volatile(2), nonVolatile(3), permanent(4), andreadOnly(5).

TDomain
Denotes a kind of transport service.

TAddress
Denotes the transport service address. TAddress is defined to be from 1-255 octets in length.

[11]Canonical order means that the address should be represented with the least-significant bit first.



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