L5 Transport Layer 2

Question 1

What services does the transport layer provide to applications?

Answer 1

Process-level addressing
Multiplexing and demultiplexing
Integrity checking and error detection
Connection management (TCP only)
Acknowledgments and retransmissions (TCP)
Flow control

Question 2

What are the two transport protocols available in the Internet?

Answer 2

Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)

Question 3

What does a TCP socket consist of?

Answer 3

Source IP address
Source port number
Destination IP address
Destination port number

Question 4

What is the three-way handshake in TCP connection establishment?

Answer 4

1. Client sends SYN segment (SYN=1, random Seq=x)
2. Server responds with SYN-ACK segment (SYN=1, ACK=x+1, Seq=y)
3. Client sends ACK segment (ACK=y+1)

Question 5

What components are created at each host during TCP connection setup?

Answer 5

Send buffer
Receive buffer
State variables

Question 6

What is the purpose of the TCP rwnd (receive window)?

Answer 6

Prevents sender from overflowing the receiver’s buffer
Controls flow rate based on available buffer space
Updated dynamically and communicated via TCP header

Question 7

Describe TCP reliable data transfer.

Answer 7

Data read at the receiver matches the sender’s data exactly
TCP uses timeouts and retransmissions
TCP uses cumulative ACKs
TCP uses duplicate ACKs to trigger fast retransmission

Question 8

What is fast retransmit in TCP?

Answer 8

Triggered after receiving 3 duplicate ACKs
Sender immediately retransmits missing segment without waiting for timeout

Question 9

What is AIMD (Additive Increase Multiplicative Decrease) in TCP congestion control?

Answer 9

Additively increases congestion window (cwnd) by 1 MSS per RTT
Cuts congestion window in half on packet loss

Question 10

What does the TCP state LISTEN mean?

Answer 10

The server is waiting for incoming TCP connection requests from any remote client.

Question 11

What does the TCP state SYN_SENT mean?

Answer 11

The client has sent a SYN segment and is now waiting for a matching SYN-ACK from the server.

Question 12

What does the TCP state SYN_RCVD mean?

Answer 12

The server has received the SYN segment from the client and sent back a SYN-ACK segment. It is now waiting for the client’s final ACK.

Question 13

What does the TCP state ESTAB (Established) mean?

Answer 13

The TCP connection is fully established; data transfer can now occur in both directions.

Question 14

What does the TCP state FIN_WAIT_1 mean?

Answer 14

The client has sent a FIN segment (request to close) and is waiting for an ACK from the server.

Question 15

What does the TCP state FIN_WAIT_2 mean?

Answer 15

The client has received the ACK for its FIN and is now waiting for the server’s FIN segment.

Question 16

What does the TCP state TIMED_WAIT mean?

Answer 16

The client is waiting for a duration (2 × Maximum Segment Lifetime) to ensure delayed packets are flushed before closing completely.

Question 17

What does the TCP state CLOSE_WAIT mean?

Answer 17

The server has received a FIN from the client and acknowledged it but still has data to send before it closes its side.

Question 18

What does the TCP state LAST_ACK mean?

Answer 18

The server has sent its own FIN segment and is waiting for an acknowledgment from the client before closing.

Question 19

What does the TCP state CLOSED mean?

Answer 19

The TCP connection is fully closed, and all resources (buffers, variables) have been deallocated.

Question 20

What does the ‘Spare room’ represent in the TCP receive buffer diagram?

Answer 20

Unused space available to receive new TCP data

Question 21

What does rwnd measure?

Answer 21

Size of the spare room (available buffer space) at the receiver

Question 22

What happens when TCP data fills the buffer?

Answer 22

Sender must stop or slow down transmission to avoid overflow

Question 23

What causes multiple duplicate ACKs to be sent?

Answer 23

Receiver notices a gap (missing data segment) and keeps acknowledging last in-order byte

Question 24

What does the repeated ACK=100 mean?

Answer 24

Receiver is still missing data starting from byte 100

Question 25

What action should the sender take after seeing 3 duplicate ACKs?

Answer 25

**Perform fast retransmit of the missing segment**

Question 26

What happens when the client sends ‘C’ to the server?

Answer 26

**Client sends segment with Seq=42, ACK=79, Data='C'**

Question 27

How does the server acknowledge and echo the character?

Answer 27

**Server sends back segment with Seq=79, ACK=43, Data='C'**

Question 28

What does Seq=43, ACK=80 mean in the final exchange?

Answer 28

**Client has received data up to byte 79 and is now expecting byte 80**

Question 29

What happens when an ACK is lost in TCP?

Answer 29

**Sender retransmits data after timeout expiration**

Question 30

Why is timeout-based retransmission slower?

Answer 30

**Waiting for the timeout period increases end-to-end delay**

Question 31

How are sequence numbers assigned in TCP file transmission?

Answer 31

**Each segment's Seq number is the byte number of the first byte in the segment**

Question 32

What Seq number will the second segment have if MSS=1000 bytes?

Answer 32

**Seq=1000**

Question 33

Name three important fields in the TCP header.

Answer 33

**Source port**, **Destination port**, **Sequence number**, **Acknowledgment number**, **Flags (URG, ACK, PSH, RST, SYN, FIN)**

Question 34

What is the purpose of the options field in a TCP segment?

Answer 34

**Used for MSS negotiation, window scaling, and timestamping**

Question 35

What fields do TCP and UDP segments always have?

Answer 35

**Source port**, **Destination port**, **Application data (payload)**

Question 36

What does 'logical end-to-end transport' mean?

Answer 36

**Applications at the endpoints communicate logically across multiple physical networks**

Question 37

Which layers participate end-to-end in transport?

Answer 37

**Application layer**, **Transport layer**

Question 38

What does the AIMD sawtooth pattern show?

Answer 38

**Window size increases linearly until loss occurs, then drops sharply**

Question 39

Why is TCP congestion control called 'probing for bandwidth'?

Answer 39

**TCP incrementally increases transmission rate until congestion (loss) is detected**

Question 40

What are the client states during TCP connection setup?

Answer 40

**LISTEN → SYN_SENT → ESTAB**

Question 41

What are the server states during TCP connection setup?

Answer 41

**LISTEN → SYN_RCVD → ESTAB**

Question 42

When is the connection considered established?

Answer 42

**After receiving ACK for the SYN-ACK segment**

Question 43

What client states are involved during TCP termination?

Answer 43

**ESTAB → FIN_WAIT_1 → FIN_WAIT_2 → TIMED_WAIT → CLOSED**

Question 44

What server states are involved during TCP termination?

Answer 44

**ESTAB → CLOSE_WAIT → LAST_ACK → CLOSED**

Question 45

Why does TCP enter TIMED_WAIT after closing?

Answer 45

**To ensure delayed or duplicate segments do not cause confusion if connection is reused**