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TNM SNMP Varbind Lists

The SNMP protocol always operates on a list of MIB variables. This list is called a varbind list. 

A varbind list is represented as a Tcl list. Each element of the varbind list is itself a Tcl list describing one varbind element. 

A fully specified varbind list element has three elements: an object identifier, which identifies the MIB variable, the data type of the MIB variable and the value of the MIB variable. 

A fully specified varbind list might look like:

{
{1.3.6.1.2.1.1.3.0 TimeTicks 20308}
{1.3.6.1. 2.1.2.2.1.3.1 INTEGER softwareLoopback}
{1.3.6.1.2.1.2.2.1.2.1 {OCTET STRING} lo0}
{1.3.6.1.2.1.2.2.1.8.1 INTEGER up}
}
It is not necessary to use fully specified varbind lists when invoking SNMP retrieval operations. It is possible to omit the type and value fields when retrieving values from MIB variables. 

For example, to retrieve the fully specified varbind list show above, the following much shorter varbind list has been used:

{
1.3.6.1.2.1.1.3.0
1.3.6.1.2.1.2.2.1.3.1
1.3.6.1.2.1.2.2.1.2.1
1.3.6.1.2.1.2.2.1.8.1
}
It is possible to replace OBJECT IDENTIFIER values with labels or names that resolve to an OBJECT IDENTIFIER value. (See Tnm::mib for details about legal names.) This further simplifies the varbind list above to:
{
SNMPv2-MIB!sysUpTime.0
IF-MIB!ifType.1
IF-MIB!ifDescr.1
IF-MIB!ifOperStatus.1
}
In order to change the value of a MIB variable, you have to include the new value in the varbind list. It is however possible to omit the type element if the type can be obtained from a MIB definition. This allows using a list like
{
{SNMPv2-MIB!sysContact.0 “schoenw@ibr.cs.tu-bs.de”}
}
to change the value of the sysContact.0 MIB variable. Note, that the Tnm extension always returns fully specified varbind lists to avoid ambiguities.

SNMPv2c and SNMPv3 varbind lists may contain exceptions (noSuchObject, noSuchInstance, endOfMibView). These exceptions are signaled in the type element of a varbind list element. 

The value of the varbind is set to a null value that conforms to the data type of the object type:
 

{
{1.3.6.1.2.1.2.2.1.3.2 noSuchInstance 0}
{1.3.6.1.2.1.2.2.1.2.2 noSuchInstance {}}
}
It is the responsibility of the application to check for exceptions. An application will not necessarily crash if such a check is missing, but it might get confused while interpreting null values.


 

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