tailieunhanh - Internetworking with TCP/IP- P62

Internetworking with TCP/IP- P62: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. SNMP Message Format 569 GetRequest-PDU 0 IMPLICIT SEQUENCE request-id Integer32 error-status INTEGER error-index INTEGER variable-bindings VarBindList Figure The definition of a get-request message. Formally the message is defined to be a GetRequest-PDU. Further definitions in the standard specify the remaining undefined terms. Both error-status and error-index are single octet integers which contain the value zero in a request. If an error occurs the values sent in a response identify the cause of the error. Finally VarBindList contains a list of object identifiers for which the client seeks values. In terms the definitions specify that VarBindList is a sequence of pairs of object name and value. represents the pairs as a sequence of two items. Thus in the simplest possible request VarBindList is a sequence of two items a name and a null. Example Encoded SNMP Message The encoded form of uses variable-length fields to represent items. In general each field begins with a header that specifies the type of object and its length in bytes. For example each SEQUENCE begins with an octet containing 30 hexadecimal the next octet specifies the number of following octets that comprise the sequence. Figure contains an example SNMP message that illustrates how values are encoded into octets. The message is a get-request that specifies data item sysDescr numeric object identifier . Because the example shows an actual message it includes many details. In particular the message contains a msgSecurityParameters section which has not been discussed above. This particular message uses the UsmSecurityParameters form of security parameters. It should be possible however to correlate other sections of the message with the definitions above. 570 Applications Internet Management SNMP Chap. 30 30 SEQUENCE 67 02 01 03 len 103 INTEGER len l vers 3 30 SEQUENCE 0D len 13 02 01 2A INTEGER len l msgID 42