Module 9: The Transport Layer Quiz Answers

1. What are two roles of the transport layer in data communication on a network? (Choose two.)

  • tracking the individual communication between applications on the source and destination hosts
  • performing a cyclic redundancy check on the frame for errors
  • identifying the proper application for each communication stream
  • providing the interface between applications and the underlying network over which messages are transmitted
  • providing frame delimiting to identify bits making up a frame

Explanation: The transport layer has several responsibilities. The primary responsibilities include the following:

  • Tracking the individual communication streams between applications on the source and destination hosts
  • Segmenting data at the source and reassembling the data at the destination
  • Identifying the proper application for each communication stream through the use of port numbers

2. During a TCP session, a destination device sends an acknowledgment number to the source device. What does the acknowledgment number represent?

  • the last sequence number that was sent by the source
  • one number more than the sequence number
  • the total number of bytes that have been received
  • the next byte that the destination expects to receive

Explanation: The window size determines the number of bytes that will be sent before expecting an acknowledgement. The acknowledgement number is the number of the next expected byte. For example, if a host has received 3140 bytes, the host would respond with an acknowledgement number of 3141.

3. Which two services or protocols use the preferred UDP protocol for fast transmission and low overhead? (Choose two)

  • DNS
  • HTTP
  • FTP
  • POP3
  • VoIP

Explanation: Both DNS and VoIP use UDP to provide low overhead services within a network implementation.​

4. Which transport layer feature is used to guarantee session establishment?

  • TCP port number
  • UDP ACK flag
  • TCP 3-way handshake
  • UDP sequence number

Explanation: TCP uses the 3-way handshake. UDP does not use this feature. The 3-way handshake ensures there is connectivity between the source and destination devices before transmission occurs.

5. Data is being sent from a source PC to a destination server. Which three statements correctly describe the function of TCP or UDP in this situation? (Choose three.)

  • The TCP source port number identifies the sending host on the network.
  • The source port field identifies the running application or service that will handle data returning to the PC.
  • UDP segments are encapsulated within IP packets for transport across the network.
  • The UDP destination port number identifies the application or service on the server which will handle the data.
  • TCP is the preferred protocol when a function requires lower network overhead.
  • The TCP process running on the PC randomly selects the destination port when establishing a session with the server.

Explanation: Layer 4 port numbers identify the application or service which will handle the data. The source port number is added by the sending device and will be the destination port number when the requested information is returned. Layer 4 segments are encapsulated within IP packets. UDP, not TCP, is used when low overhead is needed. A source IP address, not a TCP source port number, identifies the sending host on the network. Destination port numbers are specific ports that a server application or service monitors for requests.

6. What is the purpose of the TCP sliding window?

  • to end communication when data transmission is complete
  • to request that a source decrease the rate at which it transmits data
  • to inform a source to retransmit data from a specific point forward
  • to ensure that segments arrive in order at the destination

Explanation: The TCP sliding window allows a destination device to inform a source to slow down the rate of transmission. To do this, the destination device reduces the value contained in the window field of the segment. It is acknowledgment numbers that are used to specify retransmission from a specific point forward. It is sequence numbers that are used to ensure segments arrive in order. Finally, it is a FIN control bit that is used to end a communication session.

7. What happens if part of an FTP message is not delivered to the destination?

  • The message is lost because FTP does not use a reliable delivery method.
  • The part of the FTP message that was lost is re-sent.
  • The entire FTP message is re-sent.
  • The FTP source host sends a query to the destination host.

Explanation: Because FTP uses TCP as its transport layer protocol, sequence and acknowledgment numbers will identify the missing segments, which will be re-sent to complete the message.

8. Which two flags in the TCP header are used in a TCP three-way handshake to establish connectivity between two network devices? (Choose two.)

  • FIN
  • SYN
  • ACK
  • PSH
  • RST
  • URG

Explanation: TCP uses the SYN and ACK flags in order to establish connectivity between two network devices.

9. Which tool is used to provide a list of open ports on network devices?

  • Tracert
  • Nmap
  • Whois
  • Ping

Explanation: The Nmap tool is a port scanner that is used to determine which ports are open on a particular network device. A port scanner is used before launching an attack.

10. Which two fields are included in the TCP header but not in the UDP header? (Choose two.)

  • destination port
  • checksum
  • sequence number
  • window
  • source port

Explanation: The sequence number and window fields are included in the TCP header but not in the UDP header.

11. Refer to the exhibit. Which three lines represent the TCP three-way handshake?

Module 9: The Transport Layer Quiz Answers 1

  • lines 6, 7, and 8
  • lines 2, 3, and 4
  • lines 2, 8, and 9
  • lines 1, 2, and 3
  • lines 4, 5, and 6

Explanation: A three-way handshake is recognizable by the SYN flag being set first, then the SYN, ACK response, followed by the final ACK flag being sent in a packet.

12. What is a characteristic of a TCP server process?

  • A host running two different applications can have both configured to use the same server port.
  • There can be many ports open simultaneously on a server, one for each active server application.
  • An individual server can have two services assigned to the same port number within the same transport layer services.
  • Every application process running on the server has to be configured to use a dynamic port number.

Explanation: Each application process running on the server is configured to use a port number, either by default or manually, by a system administrator. An individual server cannot have two services assigned to the same port number within the same transport layer services. A host running a web server application and a file transfer application cannot have both configured to use the same server port. There can be many ports open simultaneously on a server, one for each active server application.


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