Protocol Layering Concepts and TCP/IP Layering ICSSCADA Practice Questions (Page 3)

Questions 11–15

1. 11application · medium

   A field engineer captures traffic on an Ethernet link between a PLC and an engineering workstation. The engineer sees a frame whose payload contains an IP packet, whose payload contains a TCP segment, whose payload contains a Modbus TCP request. The engineer wants to document the exact sequence of headers added by each device as the request was sent. Which statement accurately describes the encapsulation process for this capture?

   • AThe Ethernet switch added the IP header and TCP header to the Modbus TCP payload before forwarding the frame.
   • BThe engineering workstation added the Modbus TCP payload, then the TCP header, then the IP header, then the Ethernet header.
   • CThe PLC added the Ethernet header first, then the IP header, then the TCP header, then the Modbus TCP payload.
   • DThe capture shows the same header order as the original transmission because encapsulation occurs only at the receiving device.
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2. 12foundation · easy

   In the OSI reference model, which layer is responsible for routing packets across multiple networks by determining the best path based on logical addresses?

   • ATransport layer
   • BSession layer
   • CData Link layer
   • DNetwork layer
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4. 13application · medium

   An engineer is troubleshooting why a DNP3 message sent from a master station to an RTU over a TCP/IP network is not being acknowledged. The engineer verifies that the application layer is generating the DNP3 message correctly. Which layer is responsible for ensuring the DNP3 message is delivered reliably to the RTU?

   • ANetwork layer
   • BApplication layer
   • CData link layer
   • DTransport layer
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5. 14application · medium

   A field engineer captures traffic on an Ethernet link between a PLC and an HMI. The engineer sees a frame whose payload contains an IP packet, and inside that packet is a TCP segment carrying a Modbus/TCP request. The engineer needs to explain to a trainee why the Modbus request is wrapped in three different headers. Which statement correctly describes the encapsulation process that produced this frame?

   • AThe TCP segment is the outermost PDU, and the Ethernet frame and IP packet are both carried inside it as payload.
   • BThe Ethernet frame is created first, then the IP packet is placed inside it, and finally the TCP segment is added at the top of the stack.
   • CThe IP packet is added at the transport layer, and the Ethernet frame is added at the network layer, with the TCP segment remaining unchanged.
   • DEach layer adds its own header as data moves down the stack, so the TCP segment is encapsulated in an IP packet, which is then encapsulated in an Ethernet frame.
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6. 15foundation · easy

   Which of the following correctly pairs each protocol layer with its Protocol Data Unit (PDU) name?

   • ATransport – frame; Network – packet; Data Link – segment
   • BTransport – segment; Network – frame; Data Link – packet
   • CTransport – packet; Network – segment; Data Link – frame
   • DTransport – segment; Network – packet; Data Link – frame
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