The locally arbitrary but unique identifier associated
with this protocolDir entry.
The value for each supported protocol must remain constant at
least from one re-initialization of the entity's network
management system to the next re-initialization, except that
if a protocol is deleted and re-created, it must be re-created
with a new value that has not been used since the last
re-initialization.
The specific value is meaningful only within a given SNMP
entity. A protocolDirLocalIndex must not be re-used until the
next agent restart in the event that the protocol directory
entry is deleted.