CCNA 1 v6.0 Chapter 9: Transport Layer: Check your Understanding Answers
1. What are the primary purposes of the transport layer? (Choose three.)
- Determining the best route between source and destination
- Adding the source and destination MAC address to frames
- Tracking the individual communication between applications on the source and destination hosts
- Adding the source and destination IP address
- Segmenting data for and reassembling segmented data into streams of application data at the destination
- Identifying the proper application for each communication stream
Explanation: The transport layer is responsible for segmenting data, adding the port number to identify the proper application, and keeping track of individual conversations. MAC addresses are added by the data link layer, and IP addresses and routing are handled by the network layer.
2. What is assigned by the transport layer to identify an application or service?
- Segment
- Packet
- Port
- MAC address
- IP address
Explanation: Source port addresses are assigned by the transport layer to identify the individual conversation and destination port addresses to identify the service being looked for.
3. Fill in the blank.
_____ is the term used to describe the interleaving of data from multiple users on the same network.
Multiplexing
4. What is a characteristic of UDP?
- Reliable delivery
- Connectionless
- Windowing
- Expectational acknowledgements
- Flow control
Explanation: TCP uses acknowledgements and windowing to offer flow control and reliable delivery. UDP is connectionless and does not resend lost datagrams.
5. Which of the following types of applications would use UDP? (Choose three.)
- Telnet
- VoIP
- FTP
- HTTP
- DHCP
- TFTP
Explanation: Applications that are simple request and reply transactions, those sensitive to delay, and those that are unidirectional in nature make use of UDP. Applications requiring reliable delivery and that can tolerate delay associated with protocol overhead make use of TCP. In this question, Telnet, FTP, and HTTP use TCP, and VoIP, DHCP, and TFTP use UDP.
6. Which TCP header field specifies the number of segments that can be accepted before an acknowledgement is required?
- Acknowledgement number
- Header length
- Window size
- Checksum
Explanation: The acknowledgement number indicates that the data that has been received, the header length indicates the length of the TCP segment header, the window size specifies the number of segments that can be received before an acknowledgement is required, and the checksum is used for error-checking the segment header and data.
7. Fill in the blank.
Both TCP and UDP use ____ to separate multiple communications on the same channel.
port numbers
8. What is an advantage that UDP has over TCP?
- Advanced flow control
- Low overhead
- Reordering of segments
- Reliable delivery
Explanation: UDP does not track conversations and has the advantage of low overhead relative to TCP.
9. What range of ports can either be used by TCP to identify the requested service on the destination device or as a client source port?
- 0 to 1023
- 0 to 49151
- 1024 to 49151
- 49152 to 65535
Explanation: Well-known ports (numbers 0 to 1023) are commonly used to request a connection to that specific port, and its associated service. Registered ports (numbers 1024 to 49151) are assigned to user processes or applications. These processes are primarily individual applications that a user has chosen to install, rather than common applications that would receive a well-known port number. When not used for a server resource, these ports can also be dynamically selected by a client as its source port. Dynamic or private ports (numbers 49152 to 65535) are most often used to identify the client application during communication.
10. Fill in the blank.
A dynamic port in the range of 49152 to 65535 is also known as a(n) ____ port.
ephemeral or private
11. What does UDP do when receiving messages that are more than one datagram in length?
- UDP place the datagrams into the correct order before passing them to the application.
- UDP reassembles that data in the order it was received and passes it to the application.
- UDP requests retransmission of the datagrams in the correct order before passing to the application.
- UDP transmissions are limited to a single datagram.
Explanation: UDP reassembles the data in the order received and passes it to the application. It is up to the application to sort out the data. UDP has no mechanism for reordering of datagrams or requesting retransmission.
12. Which TCP header control bit is set on to terminate a TCP conversation?
- URG
- ACK
- PSH
- RST
- SYN
- FIN
Explanation: The FIN control bit indicates that there is no more data from the sender.
13. If a client sends an ISN of 2 to a server and a server responds with an ISN of 1 to the client, what is the final stage of the TCP three-way handshake?
- Client sends an acknowledgement of 2.
- Server sends an acknowledgement of 2.
- Client sends an acknowledgement of 3.
- Server sends an acknowledgement of 3.
- Client sends a new ISN of 3.
- Server sends a new ISN of 2.
Explanation: The three-way handshake starts when a client device sends an ISN to the server. The server responds with an acknowledgement of the client ISN+1 and its own ISN. The last step in the process is when the client acknowledges the server ISN by responding with a value of the server ISN+1.
14. A client is downloading a large file from a server using FTP. One of the segments is lost during transit. What will happen?
- The FTP session is immediately terminated.
- The FTP client responds to the server with a smaller window size in the TCP header.
- The FTP client responds to the client with an increased window size in the TCP header.
- The FTP server responds to the client with a smaller window size in the TCP header.
- The FTP server responds to the client with an increased window size in the TCP header.
- The FTP session continues, but the result is a corrupt file that must be downloaded again.
- The FTP session continues with no alteration in window size, and the missing segment is requested again after the rest of the file is downloaded.
Explanation: FTP makes use of TCP, so as soon as segments are lost in transit, the receiving device will send a decreased window size in returning segments.
15. Fill in the blank.
TCP will normally retransmit lost data from the last successful acknowledgement. To allow the destination to acknowledge bytes in discontinuous segments and request retransmission of only the missing data, both hosts must be able to support an optional feature called __________.
selective acknowledgements (SACK)