Transport Layer ll

Question 1

What is TCP?

Answer 1

A reliable, connection-based way to send data — in order and with flow control

Question 2

How is a TCP connection identified?

Answer 2

By 4 values: source IP, source port, destination IP, destination port

Question 3

What are key fields in a TCP header?

Answer 3

• Sequence/Ack numbers: Tracks and confirms data
• 16-bit receive window: Controls how much data to send
• Flags (SYN, ACK, FIN, RST): Connection management
• Checksum: Error detection

Question 4

How is the MSS determined?

Answer 4

MSS = MTU (link layer frame size) - TCP/IP header (40 bytes)

Question 5

Describe the 3-way handshake

Answer 5

1. Client -> Server: SYN (seq=client_isn)
2. Server -> Client: SYN-ACK (seq=server_isn, ack=client_isn+1)
3. Client -> Server: ACK (ack=server_isn+1)

Question 6

How is a TCP connection terminated?

Answer 6

1. One host sends FIN, gets ACKs
2. Other host sends FIN, gets ACKs
3. Wait for timeout (2*MSL) to ensure all segments die

Question 7

How does TCP ensure reliability?

Answer 7

• Sequence numbers: Track bytes
• Cumulative ACKs: Confirm delivery
• Retransmissions: If data is missing

Question 8

What triggers fast retransmit?

Answer 8

Receiving 3 duplicate ACKs for the same segment

Question 9

What is the receive window (rwnd)?

Answer 9

How much space is left in the receiver’s buffer: Free space = total buffer - what’s already received

Question 10

How does the sender avoid overflow?

Answer 10

Sends only up to min(cwnd, rwnd) unacknowledged bytes

Question 11

How does TCP detect congestion?

Answer 11

Packet loss (timeout/duplicate ACKs) or delayed ACKs

Question 12

What is AIMD?

Answer 12

Additive Increase: Increase cwnd by 1 MSS/RTT
Multiplicative Decrease: Halve cwnd if there’s loss

Question 13

When to use UDP over TCP?

Answer 13

For real-time apps (low latency, tolerant to loss), no connection overhead or many clients

Question 14

Key trade-offs

Answer 14

• UDP: No reliability/flow control, low overhead
• TCP: Reliable but slower (handshake, congestion control)