The DPI GETNEXT packet contains the object(s) on which the GETNEXT operation must be performed. For this operation, the subagent is to return the name, type, length, and value of the next variable it supports whose (ASN.1) name lexicographically follows the one passed in the group ID (sub-tree) and instance ID.
In this case, the instance ID may not be present (NULL) in the incoming DPI packet implying that the NEXT object must be the first instance of the first object in the sub-tree that was registered.
It is important to realize that a given subagent may support several discontiguous sections of the MIB tree. In that situation, it would be incorrect to jump from one section to another. This problem is correctly handled by examining the group ID in the DPI packet. This group ID represents the "reason" why the subagent is being called. It holds the prefix of the tree that the subagent had indicated it supported (registered).
If the next variable supported by the subagent does not begin with that prefix, the subagent must return the same object instance as in the request, for example the group ID and instance ID with a value of SNMP_TYPE_endOfMibView (implied NULL value). This endOfMibView is not considered an error, so the error_code and error_index should be zero. If required, the SNMP agent will call upon the subagent again, but pass it a different group ID (prefix). This is illustrated in the discussion below.
Assume there are two subagents. The first subagent registers two distinct sections of the tree: A and C. In reality, the subagent supports variables A.1 and A.2, but it correctly registers the minimal prefix required to uniquely identify the variable class it supports.
The second subagent registers section B, which appears between the two sections registered by the first agent.
If a management station begins browsing the MIB, starting from A, the following sequence of queries of the form get-next(group ID,instance ID) would be performed:
Subagent 1 gets called:
get-next(A,none) = A.1
get-next(A,1) = A.2
get-next(A,2) = endOfMibView
Subagent 2 is then called:
get-next(B,none) = B.1
get-next(B,1) = endOfMibView
Subagent 1 gets called again:
get-next(C,none) = C.1
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