Overview of Subagent Processing

This overview assumes that the subagent communicates with the agent over a TCP connection. Other connection implementations are possible and, in that case, the processing approach may be a bit different.

We also take a simplistic approach in the sense that we will request the agent to send us at most one varBind per DPI packet, so we do not need to loop through a list of varBinds. Potentially, you may gain performance improvements if you allow for multiple varBinds per DPI packet on GET, GETNEXT, SET requests, but to do so, your code will have to loop through the varBind list and so it becomes somewhat more complicated. We assume that the DPI subagent programmer can handle that once you understand the basics of the DPI API.

Here is the dpiSimple MIB definition as it is implemented by the sample code, which follows:

DPISimple-MIB DEFINITIONS ::= BEGIN

   IMPORTS
        MODULE-IDENTITY, OBJECT-TYPE, snmpModules, enterprises
                   FROM SNMPv2-SMI
        DisplayString
                   FROM SNMPv2-TC

   ibm      OBJECT IDENTIFIER ::= { enterprises 2 }
   ibmDPI   OBJECT IDENTIFIER ::= { ibm 2 }
   dpi20MIB OBJECT IDENTIFIER ::= { ibmDPI 1 }


   dpiSimpleMIB OBJECT IDENTIFIER ::= { dpi20MIB 5 }

   dpiSimpleInteger         OBJECT-TYPE
        SYNTAX  INTEGER
        ACCESS  read-only
        STATUS  mandatory
        DESCRIPTION
            "A sample integer32 value"
        ::= { dpiSimpleMIB 1 }

   dpiSimpleString          OBJECT-TYPE
        SYNTAX  DisplayString
        ACCESS  read-write
        STATUS  mandatory
        DESCRIPTION
            "A sample Display String"
        ::= { dpiSimpleMIB 2 }

   dpiSimpleCounter32       OBJECT-TYPE
        SYNTAX  Counter     -- Counter32 is SNMPv2
        ACCESS  read-only
        STATUS  mandatory
        DESCRIPTION
            "A sample 32-bit counter"
        ::= { dpiSimpleMIB 3 }

   dpiSimpleCounter64       OBJECT-TYPE
        SYNTAX  Counter64   -- Counter64 is SNMPv2,
                            -- Not supported by SNMPv1 agents
        ACCESS  read-only
        STATUS  mandatory
        DESCRIPTION
            "A sample 64-bit counter"
        ::= { dpiSimpleMIB 4 }
END

To make the code more readable, we have defined the following names in our dpi_samp.c source file.

#define DPI_SIMPLE_SUBAGENT   "1.3.6.1.4.1.2.2.1.5"
#define DPI_SIMPLE_MIB        "1.3.6.1.4.1.2.2.1.5."
#define DPI_SIMPLE_INTEGER    "1.0"  /* dpiSimpleInteger.0   */
#define DPI_SIMPLE_STRING     "2.0"  /* dpiSimpleString.0    */
#define DPI_SIMPLE_COUNTER32  "3.0"  /* dpiSimpleCounter32.0 */
#define DPI_SIMPLE_COUNTER64  "4.0"  /* dpiSimpleCounter64.0 */

In addition, we have defined the following variables as global variable in our dpi_samp.c source file.

static int handle;                        /* handle has global scope */
static long int      value1      = 5;
#define              value2_p      cur_val_p   /* writable object    */
#define              value2_len    cur_val_len /* writable object    */
static char         *cur_val_p   = (char *)0;
static char         *new_val_p   = (char *)0;
static char         *old_val_p   = (char *)0;
static unsigned long cur_val_len = 0;
static unsigned long new_val_len = 0;
static unsigned long old_val_len = 0;
static unsigned long value3      = 1;
static snmp_dpi_u64  value4      = {0x80000000,1L};