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7.4 Verifying that the Agent Transmitted, and the Manager Received, a Trap PDU

The example in Section 7.3 showed how to verify that a Set command accomplished what it was meant to do despite UDP’s connectionless transport. This case study builds on that example by verifying that another device received the agent-generated Trap message.

In this example (see Figure 7-4), the Trap transmitter is the SynOptics intelligent hub, and the manager receives the trap. An X-Windows client workstation acts as the manager’s display console. A configuration table within the agent’s SNMP parameters defines the IP address of the receiver. When a trap is generated, it is automatically displayed in one of the workstation’s windows.


Figure 7-4.  Verifying a trap

As noted previously, SNMP messages are transmitted using connectionless transport via UDP, which provides minimal error control. However, some applications, such as the X-Windows system, demand more rigorous error control and require guaranteed delivery of every octet of information, in sequence, at the proper time. The TCP Transport layer protocol addresses these requirements. Frames 1 and 2 of Trace 7.4a show the X Client and the manager exchanging information using TCP. Within the TCP header (and shown in the trace) are the destination and source port numbers (D and S, respectively), an acknowledgment number (ACK), a sequence number (SEQ), a length (LEN), and a window advertisement (WIN).

In Frame 3, the manager sends a Set command to the hub agent; the agent responds with a trap (Authentication failure) in Frame 6. The manager then sends the trap information to the X Client for display within the alarm window (Frame 8). Upon receiving the trap, the X Client sends a TCP acknowledgment in Frame 9 and displays the trap.

Trace 7.4a. Verifying trap reception (summary)

  Sniffer Network Analyzer data 23-Sep  at 08:50:38, file SNMP75.ENC, Pg 1

  SUMMARY  Delta T  Destination  Source     Summary
    1               Manager      X Client   TCP D=4255 S=6000
                                            ACK=584967877
                                            SEQ=1899543417
                                            LEN=32 WIN=2920
    2      0.0903   X Client     Manager    TCP D=6000 S=4255
                                            ACK=1899543449
                                            WIN=16384
    3      0.4468   Hub Agent    Manager    SNMP Set
                                            SynOptics.1.3.2.4.20.1.
                                            3.XXX.YYY.170.20 = trap1
    6      0.0276   Manager      Hub Agent  SNMP Trap - v1
                                            Authentication failure
    8      3.5293   X Client     Manager    XWIN C (3) Poly Text8’s
                                            at 27,20 "[23-Sep-1992
                                            08:43:23]
                                            Alarm Type[nms_trapd] from
                                            Object[snmpmg.sim.XX.COM]";
                                            at 27,38 "TRAP [authent ...
    9      0.0714   Manager      X Client   TCP D=4255 S=6000
                                            ACK=584968457 WIN=2920

Trace 7.4b. Verifying trap reception (details)

  Sniffer Network Analyzer data 23-Sep at 08:50:38, file SNMP75.ENC, Pg 1

  ------------------------------- Frame 1 -------------------------------
  TCP: ----- TCP header -----
  TCP:
  TCP: Source port = 6000 (X Windows)
  TCP: Destination port = 4255
  TCP: Sequence number = 1899543417
  TCP: Acknowledgment number = 584967877
  TCP: Data offset = 20 bytes
  TCP: Flags = 18
  TCP: ..0. .... = (No urgent pointer)
  TCP: ...1 .... = Acknowledgment
  TCP: .... 1... = Push
  TCP: .... .0.. = (No reset)
  TCP: .... ..0. = (No SYN)
  TCP: .... ...0 = (No FIN)
  TCP: Window = 2920
  TCP: Checksum = 8A63 (correct)
  TCP: No TCP options
  TCP: [32 byte(s) of data]
  TCP:

  ------------------------------- Frame 2 -------------------------------
  TCP: ----- TCP header -----
  TCP:
  TCP: Source port = 4255
  TCP: Destination port = 6000 (X Windows)
  TCP: Sequence number = 584967877
  TCP: Acknowledgment number = 1899543449
  TCP: Data offset = 20 bytes
  TCP: Flags = 10
  TCP: ..0. .... = (No urgent pointer)
  TCP: ...1 .... = Acknowledgment
  TCP: .... 0... = (No push)
  TCP: .... .0.. = (No reset)
  TCP: .... ..0. = (No SYN)
  TCP: .... ...0 = (No FIN)
  TCP: Window = 16384
  TCP: Checksum = B7AC (correct)
  TCP: No TCP options
  TCP:

  -------------------------- Frame 3 ----------------------------
  SNMP: ----- Simple Network Management Protocol (Version 1)-----
  SNMP:
  SNMP: Version = 0
  SNMP: Community = public
  SNMP: Command = Set request
  SNMP: Request ID = 717289096
  SNMP: Error status = 0 (No error)
  SNMP: Error index = 0
  SNMP:
  SNMP: Object = {1.3.6.1.4.1.45.1.3.2.4.20.1.3.XXX.YYY.170.20}
                 (SynOptics.1.3.2.4.20.1.3.XXX.YYY ...
  SNMP: Value = trap1
  SNMP:

  ------------------------------- Frame 6 -------------------------------
  SNMP: ----- Simple Network Management Protocol (Version 1) -----
  SNMP:
  SNMP: Version = 0
  SNMP: Community = trap1
  SNMP: Command = Trap
  SNMP: Enterprise = {1.3.6.1.4.1.45.3.8.1}
  SNMP: Network address = [XXX.YYY.170.53]
  SNMP: Generic trap = 4 (Authentication failure)
  SNMP: Specific trap = 0
  SNMP: Time ticks = 57685810
  SNMP:

  ------------------------------- Frame 8 -------------------------------
  XWIN: ----- X Windows -----
  XWIN:
  XWIN: Request opcode = 74 (Poly Text8)
  XWIN: Drawable = 01D00034, Graphics context = 01D0001E
  XWIN: X = 27, Y = 20
  XWIN: Delta = 0
  XWIN: String = "[23-Sep-1992 08:43:23] Alarm Type[nms_trapd]
                 from Object[snmpmg.sim.XX.COM]"
  XWIN:
  XWIN: Request opcode = 74 (Poly Text8)
  XWIN: Drawable = 01D00034, Graphics context = 01D0001E
  XWIN: X = 27, Y = 38
  XWIN: Delta = 0
  XWIN: String = "TRAP [authentication failure] received, agent =
                 XXX.YYY.170.53(syn2.sim.XX.COM) ...
  XWIN:
  XWIN: Request opcode = 74 (Poly Text8)
  XWIN: Drawable = 01D00034, Graphics context = 01D0001E
  XWIN: X = 27, Y = 56
  XWIN: Delta = 0
  XWIN: String = " specific-trap = 0."
  XWIN:

  -------------------------- Frame 9 ----------------------------
  TCP: ----- TCP header -----
  TCP:
  TCP: Source port = 6000 (X Windows)
  TCP: Destination port = 4255
  TCP: Sequence number = 1899543609
  TCP: Acknowledgment number = 584968457
  TCP: Data offset = 20 bytes
  TCP: Flags = 10
  TCP: ..0. .... = (No urgent pointer)
  TCP: ...1 .... = Acknowledgment
  TCP: .... 0... = (No push)
  TCP: .... .0.. = (No reset)
  TCP: .... ..0. = (No SYN)
  TCP: .... ...0 = (No FIN)
  TCP: Window = 2920
  TCP: Checksum = E960 (correct)
  TCP: No TCP options
  TCP:


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