19.4.1 Decoding and Encoding SNMP Packets

The SnmpPduFactory hook provided by Java DMK 5.1 involves the following Java classes:

com.sun.management.snmp.SnmpPduFactory

Defines the interface of the object in charge of encoding and decoding SNMP packets.

com.sun.jdmk.snmp.SnmpPduFactoryBER

The default implementation of the SnmpPduFactory interface.

com.sun.management.snmp.SnmpPdu

The fully decoded representation of an SNMP packet.

com.sun.management.snmp.SnmpMsg

A partially decoded representation of an SNMP packet, containing the information stored in any SNMPv1, SNMPv2 or SNMPv3 message.

After receiving an SNMP packet, Java DMK 5.1 performs the following steps:

1. The received bytes are translated into an SnmpMsg object by the message processing subsystem. If the SNMP protocol version of the original request was either v1 or v2, this step simply involves the BER decoding of the ASN.1 Message sequence as defined in RFC 1901. If the SNMP protocol version of the original request was v3, the message processing subsystem will in addition invoke the security subsystem to authenticate and decrypt the message.

2. The SnmpMsg object is then translated into an SnmpPdu object.

3. The SnmpPdu is analyzed and the corresponding operation is performed.

Before sending an SNMP packet, Java DMK 5.1 performs the following steps:

1. An SnmpPdu object is initialized according to the requested operation. This could be either an SnmpPduPacketfor SNMPv1or v2, or an SnmpScopedPduPacket for SNMPv3.

2. The SnmpPdu object is translated into an SnmpMsg.

3. The SnmpMsg is then passed to the message processing subsystem, and translated into bytes. If SNMPv1 or SNMPv2 is being used, this step simply involves the BER encoding of the ASN.1 message sequence as defined in RFC 1901. If SNMPv3 is being used, the message processing subsystem also invokes the security subsystem to sign and encrypt the message.

The SnmpPdu object is the fully decoded description of the SNMP request. In particular, it includes the operation type (get, set, and so on), the list of variables to be operated upon, the request identifier, and the protocol version, as shown in Example 19–12.

Example 19–12 Using the SnmpPdu Class

abstract class SnmpPdu {
             ...
             public int version ;
             public int type ;
             public int requestId ;
             public SnmpVarBind[] varBindList ;
             ...
     }

The use of the SnmpMsg class is shown in Example 19–13. The SnmpMsg class is a partially decoded representation of the SNMP request. Only the protocol version and security parameters are decoded. All the other parameters remain encoded.

The SnmpMsg class is the base class derived from the message syntax from RFC 1157 and RFC 1901, and SNMPv3Message from RFC 2572. The SnmpMessage class that was present in releases of Java DMK before 5.0 derives from SnmpMsg and represents an SNMPv1 or SNMPv2 message. Because SNMPv3 introduces additional security parameters, the SnmpMessage class can only be used for SNMPv1 or SNMPv2 messages. SnmpPduFactory implementations that make direct use of SnmpMessage will therefore need to be updated if they are to be imported into a Java DMK 5.1 SNMPv3 protocol adaptor. However, they do not need to be changed as long if the old SnmpAdaptorServer is used instead of SnmpV3AdaptorServer.

Example 19–13 Using the SnmpMsg Class

abstract class SnmpMsg {
             ...
             public int version ;
             ...
     }

     class SnmpMessage extends SnmpMsg {
             ...
             public byte[] community ;
             public byte[] data ;
             ...
     }