Chapter 3

Question 1

What is multiplexing in the transport layer?

Answer 1

Multiplexing is the process of gathering data from multiple application processes, adding transport-layer headers, and sending the data to the network layer.

Question 2

What is demultiplexing in the transport layer?

Answer 2

Demultiplexing is the process of directing incoming transport-layer segments to the correct application process based on the destination port number.

Question 3

What information does the transport layer use to demultiplex incoming segments?

Answer 3

It uses the destination port number to determine which socket/application should receive the data.

Question 4

How does UDP perform multiplexing and demultiplexing?

Answer 4

UDP uses a 2-tuple (Destination IP, Destination Port) to send data to the correct application without requiring a connection.

Question 5

How does TCP perform multiplexing and demultiplexing?

Answer 5

TCP uses a 4-tuple (Source IP, Source Port, Destination IP, Destination Port) to uniquely identify each connection.

Question 6

Why does UDP use a 2-tuple instead of a 4-tuple like TCP?

Answer 6

UDP does not establish connections; it sends each packet independently, only needing a destination IP and port to deliver the data.

Question 7

Why does TCP use a 4-tuple for connection management?

Answer 7

TCP connections must be uniquely identified so that multiple connections between the same hosts can coexist.

Question 8

What are common applications that use UDP multiplexing and demultiplexing?

Answer 8

DNS, VoIP, video streaming, and online gaming.

Question 9

What are common applications that use TCP multiplexing and demultiplexing?

Answer 9

Web browsing (HTTP/HTTPS), file transfers (FTP), and email (SMTP, IMAP, POP3).

Question 10

What is the main advantage of UDP’s multiplexing/demultiplexing?

Answer 10

Low overhead and fast data transmission without connection setup.

Question 11

What is the main advantage of TCP’s multiplexing/demultiplexing?

Answer 11

Reliable, ordered delivery with congestion control.

Question 12

What happens if two UDP packets have different source IPs but the same destination IP and port?

Answer 12

Both packets will be delivered to the same application process at the destination.

Question 13

What happens if two TCP packets have different source ports but the same destination IP and port?

Answer 13

They will be treated as separate connections since TCP differentiates connections using a 4-tuple.

Question 14

Why does DNS use UDP instead of TCP?

Answer 14

UDP is faster and avoids connection setup delays, which is important for quick domain name resolution.

Question 15

Why does HTTP use TCP instead of UDP?

Answer 15

HTTP requires reliable, in-order delivery of web pages and data, which TCP provides.

Question 16

What transport services does UDP provide?

Answer 16

Multiplexing/demultiplexing, error detection (checksum), best-effort delivery, no congestion control, and stateless communication.

Question 17

Why is UDP faster than TCP?

Answer 17

No connection setup, minimal header, no retransmissions, and no congestion control.

Question 18

Why does DNS use UDP instead of TCP?

Answer 18

Because DNS queries need to be fast and avoid connection setup delays.

Question 19

What happens when a UDP packet is lost?

Answer 19

The packet is simply discarded; no retransmission occurs.

Question 20

Why is UDP useful for live video streaming?

Answer 20

Delayed packets are useless in real-time streaming; minor data loss is better than buffering.

Question 21

What is the main challenge of providing reliable transport over an unreliable network?

Answer 21

Packets may be lost, duplicated, delayed, or corrupted.

Question 22

What does a transport protocol use to detect errors?

Answer 22

A checksum.

Question 23

What is the purpose of acknowledgments (ACKs)?

Answer 23

To confirm that a packet was received correctly.

Question 24

How does a sender detect lost packets?

Answer 24

By using a timer and retransmitting if an ACK is not received.

Question 25

What is the purpose of sequence numbers in a reliable transport protocol?

Answer 25

To distinguish between new and duplicate packets.

Question 26

How does rdt3.0 improve reliability over lossy networks?

Answer 26

It introduces timeouts and retransmissions.

Question 27

What is the purpose of sequence numbers in reliable transport?

Answer 27

They differentiate new packets from retransmitted ones and ensure in-order delivery.

Question 28

How does a transport protocol detect errors in received packets?

Answer 28

By using a checksum.

Question 29

What happens if an ACK is lost in the Alternating-Bit Protocol?

Answer 29

The sender retransmits the packet when the timer expires.

Question 30

Why does the receiver resend the last ACK when it receives a duplicate packet?

Answer 30

To let the sender know the last ACK was lost, preventing unnecessary retransmissions.

Question 31

How does rdt3.0 handle packet loss?

Answer 31

It starts a timer when sending a packet and retransmits if no ACK is received.

Question 32

Why is Stop-and-Wait inefficient?

Answer 32

It waits for an ACK before sending the next packet, wasting bandwidth.

Question 33

What is the main idea behind pipelining in transport protocols?

Answer 33

Sending multiple packets before waiting for ACKs to improve efficiency.

Question 34

How does Go-Back-N improve efficiency over Stop-and-Wait?

Answer 34

It allows multiple in-flight packets and uses cumulative ACKs.

Question 35

What is the key disadvantage of Go-Back-N?

Answer 35

If a packet is lost, all subsequent packets must be retransmitted.

Question 36

How does Selective Repeat improve upon Go-Back-N?

Answer 36

It only retransmits lost packets, making it more efficient.

Question 37

What new challenge does pipelining introduce?

Answer 37

Out-of-order delivery, requiring buffering and larger sequence numbers.

Question 38

What is the purpose of the sliding window in GBN?

Answer 38

It allows the sender to send up to N unacknowledged packets before stopping.

Question 39

What happens when a packet is lost in GBN?

Answer 39

The sender retransmits the lost packet and all subsequent packets after a timeout.

Question 40

What does an ACK(n) in GBN signify?

Answer 40

It acknowledges all packets up to and including packet n.

Question 41

How does GBN improve efficiency compared to Stop-and-Wait?

Answer 41

It allows multiple packets to be in-flight before waiting for an acknowledgment.

Question 42

What is a major inefficiency in GBN?

Answer 42

Unnecessary retransmissions—if a single packet is lost, all later packets must be resent.

Question 43

What is the key improvement of Selective Repeat over Go-Back-N?

Answer 43

SR only retransmits lost or corrupted packets, while GBN retransmits all packets after a lost one.

Question 44

How does the receiver handle out-of-order packets in SR?

Answer 44

It buffers them until missing packets arrive, then delivers them in order.

Question 45

Why does SR require individual ACKs instead of cumulative ACKs?

Answer 45

Because the receiver may accept packets out of order, so it must acknowledge each packet separately.

Question 46

What is the condition to avoid ambiguity in SR?

Answer 46

Window size must be ≤ half the sequence number space (N ≤ (SeqNumSpace/2)).

Question 47

What happens if the SR window size is too large?

Answer 47

The receiver may mistake an old retransmitted packet for a new one, causing errors.

Question 48

What is the purpose of the TCP connection establishment phase?

Answer 48

It ensures that both sender and receiver are ready for communication, synchronizes sequence numbers, and sets up necessary buffers and state variables.

Question 49

What mechanism does TCP use for connection establishment?

Answer 49

The three-way handshake.

Question 50

What are the three steps of the TCP three-way handshake?

Answer 50

SYN – The client sends a SYN segment to the server. SYN-ACK – The server responds with a SYN-ACK segment. ACK – The client acknowledges and the connection is established.

Question 51

What does the SYN flag in a TCP segment indicate?

Answer 51

It signals the start of a TCP connection and synchronizes sequence numbers.

Question 52

What does the ACK flag in a TCP segment indicate?

Answer 52

It acknowledges receipt of a segment.

Question 53

What is an Initial Sequence Number (ISN)?

Answer 53

A randomly chosen number used to track data flow in a TCP connection.

Question 54

What is the purpose of the SYN-ACK segment?

Answer 54

It acknowledges the client’s connection request and provides the server’s ISN.

Question 55

Why does TCP use a three-way handshake instead of a two-way handshake?

Answer 55

To prevent old, delayed SYN segments from being mistaken for new connection requests and to ensure both sides are fully synchronized.

Question 56

What TCP state does the client enter after sending a SYN?

Answer 56

SYN-SENT.

Question 57

What TCP state does the server enter after receiving a SYN and sending a SYN-ACK?

Answer 57

SYN-RECEIVED.

Question 58

What state do both client and server enter after completing the three-way handshake?

Answer 58

ESTABLISHED.

Question 59

What are the consequences of skipping the TCP connection establishment phase?

Answer 59

Data may be lost, out-of-order, or duplicated, and communication may fail due to uninitialized buffers.

Question 60

What does the TCP receive window (rwnd) do?

Answer 60

It controls how much data the sender can transmit before waiting for an acknowledgment.

Question 61

What is a half-open connection in TCP?

Answer 61

A situation where one side believes a connection is open while the other has closed it, leading to potential data loss.

Question 62

Why are TCP sequence numbers important?

Answer 62

They ensure ordered delivery and enable retransmissions of lost packets.