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Course 2 – The Bits and Bytes Of Computer Networking

Week 3: The Transport and Application Layers

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TABLE OF CONTENT

In the third week of this course, we’ll explore the transport and application layers. By the end of this module, you’ll be able to describe TCP ports and sockets, identify the different components of a TCP header, show the difference between connection-oriented and connectionless protocols, and explain how TCP is used to ensure data integrity.

Learning Objectives

  • Describe TCP ports and sockets.
  • Examine the different components of a TCP header.
  • Compare differences between connection-oriented and connectionless protocols.
  • Explain how TCP is used to ensure data integrity.

PRACTICE QUIZ: THE TRANSPORT LAYER

1. What ordering of TCP flags makes up the Three-way Handshake?

  • FIN, FIN/ACK, ACK
  • SYN, ACK, SYN, ACK
  • SYN, ACK, FIN
  • SYN, SYN/ACK, ACK (CORRECT)

Great work! The computer that wants to establish a connection sends a packet with the SYN flag set. Then, the server responds with a packet with both the SYN and ACK flags set. Finally, the original computer sends a packet with just the ACK flag set.

2. Transport layer protocols, like TCP and UDP, introduce the concept of a port. How many bits is a port field?

  • 4 bits
  • 8 bits
  • 16 bits (CORRECT)
  • 32 bits

Nice job! A TCP or UDP port is a 16-bit number, meaning there are theoretically 65,535 possible values it can have.

3. Please select all valid TCP control flags.

  • WAIT
  • URG
  • LISTEN
  • ACK (CORRECT)
  • CLOSE
  • RST (CORRECT)

Nice job! ACK is short for acknowledged and means that the data was received.

You got it! RST is used to reset a connection if something has gone wrong.

4. A device that blocks traffic that meets certain criteria is know as a ________.

  • Hub
  • Switch
  • Firewall (CORRECT)
  • Router

That’s right! A firewall is used to block certain defined types of traffic.

PRACTICE QUIZ: THE APPLICATION LAYER

1. Unlike our five-layer model, the OSI network model adds two more layers on top of the Application Layer. Select examples of these new layers below.  

  • The encryption layer
  • The interconnection layer
  • The compression layer
  • The session layer (CORRECT)
  • The presentation layer

Nice job! The session layer handles delivery of data from the transport layer to applications themselves.

Great work! The presentation layer might handle things like compression or encryption.

2. An example of something that operates at the application layer is:

  • TCP
  • A router
  • A web browser (CORRECT)
  • UDP

Wohoo! Web browsers and server operate at the application layer.

3. What’s the standard number for a TTL field?

  • 8
  • 16
  • 32
  • 64 (CORRECT)

Awesome! While this value can be set to anything from 0 to 255, 64 is the recommended standard.

GRADED QUIZ: THE TRANSPORT AND APPLICATION LAYER

1. If a TCP socket is ready and listening for incoming connections, it’s in the ______ state.

  • ESTABLISHED
  • CLOSE_WAIT
  • SYN_SENT
  • LISTEN (CORRECT)

The LISTEN state means that a port is waiting for something to connect to it.

2. The most common example of a connection-oriented protocol is _____

  • UDP
  • TCP (CORRECT)
  • IP

Great work! Other examples of connection-oriented protocols exist, but TCP is, by far, the most common.

3. A 32-bit number that’s used to keep track of where you are in a sequence of TCP segments is known as a(n) ______ number.

  • acknowledgement
  • TCP
  • address
  • sequence (CORRECT)

Great work! A sequence number is used to keep track of where you are in a series of TCP segments.

4. Application layer data lives in the _____ section of the transport layer protocol.

  • data payload (CORRECT)
  • header
  • footer
  • flags

Awesome! The payload section of one layer contains the content of the layer above it.

5. How many bits are used to direct traffic to specific services running on a networked computer?

  • 12
  • 32
  • 8
  • 16 (CORRECT)

Great work! A port is a 16-bit number that’s used to direct traffic to specific services running on a networked computer.

6. A user requests an unencrypted webpage from a web server running on a computer, listening on the Internet Protocol address 10.1.1.150. What will be the socket address?

  • 10.1.1.150:21
  • 10.1.1.150:80 (CORRECT)
  • 10.1.1.150.21
  • 10.1.1.150.80

You got it! The socket address will be 10.1.1.150:80.  Unencrypted web traffic uses port 80 and ports are normally denoted with a colon after the IP address.

7. A connection has been terminated and no communication is possible. What is the Transmission Control Protocol (TCP) socket state?

  • FINISHED
  • CLOSE_WAIT
  • CLOSED (CORRECT)
  • FIN_WAIT

Woohoo! The TCP socket will be in the CLOSED state when the connection has been fully terminated and no further communication is possible.

8. A device receives a Transmission Control Protocol (TCP) packet. The device understands where the actual data payload begins. Which portion of the TCP header provides this information?

  • Checksum
  • Data offset (CORRECT)
  • Sequence number
  • Acknowledgement number

Great work! The data offset field communicates how long the TCP header for a segment is.   This causes the receiving network device to understand where the actual data payload begins.

9. Which field in a Transmission Control Protocol (TCP) header is not typically used in modern networking?

  • Sequence number
  • Checksum
  • Acknowledgement number
  • Urgent pointer (CORRECT)

Right on! The urgent pointer field is not typically used in modern networking.  This field points out particular segments that might be more important than others, but is a feature of TCP that hasn’t really ever been adopted.

10. How many bits are in the checksum field in a Transmission Control Protocol (TCP) header?

  • 4
  • 8
  • 16 (CORRECT)
  • 12

Great work! The checksum field is a 16-bit field and is used to make sure no data was lost or corrupted during transmission.

11. Connection-oriented protocols protect against dropped data by forming connections and using what type of constant stream?

  • Acknowledgements (CORRECT)
  • Checks
  • Approvals
  • Verifiers

Woohoo! Connection-oriented protocols protect against dropped data with a constant stream of acknowledgements.

12. In which scenario should you use the User Datagram Protocol (UDP)?

  • When you make a phone call
  • When you are using instant messaging with a co-worker
  • When you are sending an email
  • When you are streaming a video (CORRECT)

Great work! Streaming a video through a connectionless protocol, such as UDP, will require less traffic, which will provide a faster connection.

13. You are sending a very fount of information that you need the listening program to respond to immediately. Which Transmission Control Protocol (TCP) flag will be used?

  • RST
  • PSH (CORRECT)
  • ACK
  • URG

You nailed it! The PSH flag will be used to push the information immediately.

14. A device involved in a Transmission Control Protocol (TCP) connection is ready to close the connection. The other device in the connection agrees. What has occured?

  • Three-way handshake
  • Four-way handshake (CORRECT)
  • Handshake
  • Two-way handshake

Awesome! A four-way handshake occurs when the session is closed.

15. Ports 1024-49151 are known as ______ ports.

  • system
  • registered (CORRECT)
  • destination
  • source

Yep! Registered ports are used by less common applications.

16. If the transmitting device would like for the receiving device to push currently buffered data to the application on the receiving end immediately, it would set the ____ flag.

  • SYN
  • ACK
  • PSH (CORRECT)
  • FIN

You nailed it! The PSH flag ensures that the receiving end doesn’t wait for any further data.

17. The sequence of SYN, SYN/ACK, and ACK packets is known as the _________.

  • three-way handshake  (CORRECT)
  • four-way handshake
  • two-way handshake
  • high five

Nice work! That series of flags is used to establish every TCP connection.

18. Which field in a Transmission Control Protocol (TCP) header provides the next expected segment?

  • Acknowledgement number (CORRECT)
  • Checksum
  • Sequence number
  • Data offset

Well done! The acknowledgement number is the number of the next expected segment.

19. A communication between two devices is over the maximum limit of an ethernet frame size. The Transmission Control Protocol (TCP) splits up the data into segments. Which field in the header helps keep track of the many segments?

  • Sequence number (CORRECT)
  • Checksum
  • Acknowledgement number
  • Urgent pointer

Nice job! The sequence number is used to keep track of where in a sequence of TCP segments that the packet is expected to be.

20. A communication sent through Transmission Control Protocol (TCP) arrives out of order. What allows the data to be put back together in the correct order?

  • Preamble
  • Acknowledgement number
  • Sequence numbers (CORRECT)
  • Checksum

Well done! Sequence numbers allow the data to be put back together in the correct order.

21. How many Transmission Control Protocol (TCP) control flags are there?

  • 5
  • 7
  • 8
  • 6 (CORRECT)

You got it! There are 6 TCP control flags.

22. What does a value of one in an ACK control flag represent?

  • There is one more packet to be transmitted.
  • The acknowledgement number field should be examined. (CORRECT)
  • It is the first transmission.
  • There is one packet to deliver.

Woohoo! A value of one in the ACK flag field means that the acknowledgement number field should be examined.

23. A Transmission Control Protocol (TCP) connection is established and two devices ensure that they’re speaking the same protocol. What has occurred?

  • Four-way handshake
  • Three-way handshake (CORRECT)
  • Two-way handshake
  • Handshake

You nailed it! A three-way handshake involves segments that have SYN, SYN/ACK, and ACK which happens every time a TCP connection is established.

24. The checksum doesn’t compute for a packet sent at the Internet Protocol (IP) level. What will happen to the data?

  • It will be sent, but may be out of order.
  • The data will be sent back to the sending node with an error.
  • The data will be discarded. (CORRECT)
  • The data will be resent.

Correct! At the IP or ethernet level, if a checksum doesn’t compute, all of the data is just discarded. It’s up to TCP to determine when to resend this data.

25. HTTP is an example of a(n) ______ layer protocol.

  • transport
  • data-link
  • application (CORRECT)
  • network

You got it! There are lots of application layer protocols, but HTTP is one of the most common ones.

26. The OSI network model has _____ layers.

  • three
  • five
  • six
  • seven (CORRECT)

Good! Unlike our model, which focuses on five layers, the OSI model has seven layers.

27. The concept of taking traffic that’s all aimed at the same node and delivering it to the proper receiving service is known as _________.

  • multiplexing
  • demultiplexing (CORRECT)
  • routing
  • encapsulation

You got it! Demultiplexing allows traffic intended for many different services to be delivered to the same node.

28. Which field in a Transmission Control Protocol (TCP) header is chosen from ephemeral ports?

  • Destination port
  • Source port (CORRECT)
  • Sequence number
  • Acknowledgement number

You nailed it! A source port is a high-numbered port chosen from a special section of ports known as ephemeral ports.

29. How many bits are reserved for the Transmission Control Protocol (TCP) flags?

  • 10
  • 6 (CORRECT)
  • 8
  • 4

Correct! 6 bits are reserved for the 6 TCP control flags.

30. Which Transmission Control Protocol (TCP) flag is used to make sure the receiving end knows how to examine the sequence number field?

  • URG
  • ACK
  • PSH
  • SYN (CORRECT)

Correct! The SYN flag is used to make sure the receiving end knows how to examine the sequence number field.

31. Ports 1 – 1023 are known as ______ ports.

  • system (CORRECT)
  • registered
  • destination
  • source

Yep! System ports are used for very well-known services.

32. The transport layer handles multiplexing and demultiplexing through what type of device?

  • Hubs
  • Switches
  • Routers
  • Ports (CORRECT)

Great! The transport layer handles multiplexing and demultiplexing through ports.

33. What port does the File Transfer Protocol (FTP) typically listen on?

  • 443
  • 21 (CORRECT)
  • 25
  • 80

Well done! FTP typically listens on port 21.

34. The instantiation of an endpoint in a potential TCP connection is known as a ______.

  • socket (CORRECT)
  • port
  • sequence number
  • TCP segment

Great work! A socket connects the networking stack of an operating system to applications.

35. A Transmission Control Protocol (TCP) connection is in working order and both sides can send each other data. What is the TCP socket state?

  • SYN_RECEIVED
  • ESTABLISHED (CORRECT)
  • LISTEN
  • SYN_SENT

Yep! The ESTABLISHED state means that the TCP connection is in working order and both sides are free to send each other data.

36. The concept of taking traffic that’s all aimed at the same node and delivering it to the proper receiving service is known as _________.

  • multiplexing
  • demultiplexing (CORRECT) 
  • routing
  • encapsulation

Awesome! Demultiplexing allows traffic intended for many different services to be delivered to the same node.

37. One side in a Transmission Control Protocol (TCP) connection has not been able to properly recover from a series of malformed segments. Which Transmission Control Protocol (TCP) flag will be used?

  • FIN
  • SYN (CORRECT)
  • RST
  • PSH

Great work!  The SYN flag is used to make sure the receiving end knows how to examine the sequence number field.

38. A connection, at which layer, implies that every segment of data sent is acknowledged?

  • Data link
  • Transport (CORRECT)
  • Network
  • Application

Great! A connection at the transport layer implies that every segment of data sent is acknowledged.

39. Ports that are generally used to establish outbound connections are known as ______ ports.

  • System
  • reserved
  • ephemeral (CORRECT)
  • registered

Nice job! The exact ports used are different, depending on the operating system, but they’re all known as ephemeral ports.

40. A network has the ability to direct traffic toward all of the receiving services. What provides this ability in the transport layer?

  • Socket address
  • File Transfer
  • Multiplexing (CORRECT)
  • Demultiplexing

Right on! Multiplexing in the transport layer means that nodes on a network have the ability to direct traffic toward many different receiving services.

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