Chapter 3- Part3

Question 1

What’s the typical MSS (Maximum segment size) and what does it include?

Answer 1

1460 Bytes

refers to the PAYLOAD of the segment. NOT the entire segment

Question 2

Key points of TCP?

Answer 2

1. Point-to-point: 1 sender,1 receiver
2. in order
3. bidirectional data flow
4. cumulative ACKs
5. pipelining (using window size)
6. connection oriented
7. flow controlled

Question 3

What does a TCP Segment structure include?

Answer 3

• Source and dest port
• Seq Number
• ACK Number
• Length
• Receiver window
• Checksum
• Urgent data pointer
• Options
• App Data (Variable Length)

Question 4

What does a UDP Segment structure include?

Answer 4

• Source and dest port
• Length
• Checksum
• App Data

Question 5

A bit = 0 vs A bit = 1

Answer 5

0: ignore this

1: this segment is a valid ACK number

Question 6

What is the sequence number in TCP?

Answer 6

Points to the BYTE NUMBER not the segment number.

Number of the first byte in segment’s data

Question 7

What is the ACK number?

Answer 7

seq # of the next byte expected

Question 8

How does the receiver handle out-of-order segments for TCP?

Answer 8

TCP spec doesn’t say. It’s up to the implementor

Question 9

Ideal Timeout value?

Answer 9

Longer than RTT.

Question 10

How does TCP send ACKs?

Answer 10

Delayed ack. Wait up to 500ms for the next segment.

Question 11

What is the TCP Fast Retransmit?

Answer 11

If sender receives 3 additional ACKS for the same data (3 DUPLICATES, 4 total). Resend the unACKed segment with the smallest seq. number

Question 12

How does flow control occur in TCP?

Answer 12

Receiver controls the sender through the receive window (rwnd). Indicates # of bytes receiver is willing to acceptr.

Sender can’t overflow the receiver’s buffer

In TCP this is two-way so they both have a rcvBuffer size.

Question 13

What are the RST, SYN, and FIN bits?

Answer 13

RST - reset bit

SYN - synchronize bit (for establishing a TCP connection)

FIN - finish bit (for closing a TCP connection) . Set to 1 when closing a connection

Question 14

3 Way Handshake for TCP?

Answer 14

1. Sender sends TCP SYN message (SynBit = 1) and Initial Seq Number [NO DATA SENT]
2. Receiver sends TCP SYNACK message and Initial Seq Number [NO DATA SENT]
3. Sender received SYNACK

Closing Connection: Both sides set FIN=1

Question 15

Explain TCP AIMD?

Answer 15

AIMD means: Additive Increase, Multiplicative Decrease

Additive Increase: incease sending rate by 1 MSS every RTT until loss detected

Multiplicative Decrease: cut sending rate in half at each loss event

Question 16

What kind of congestion control does TCP have?

Answer 16

End-to-end. Only based on observed network behavior seen by sender.

Question 17

What is the cwnd window?

Answer 17

Congestion Window Size. THe max amount of bytes we can send.

LastByteSent - LastByteAcked <= cwnd

Cwnd can be dynamically adjusted

Question 18

Explain TCP Slow Start

Answer 18

Increases the rate of sending exponentially until first loss event.

Double the cwnd every RTT by:
- Incrementing the cwnd by 1 for each ACK received. So it doubles the packets sent for each ACK received.

Question 19

How does the ssthresh work in TCP connections?

Answer 19

ssthresh is used to determine if TCP should send exponential or linear.

The rule is:
1. increase congestion window exponentially until it reaches ssthresh

2. increase congestion window linearly when it reaches ssthresh

Question 20

2 types of loss events?

Answer 20

1. TIMEOUT
2. TRIPLE Duplicate ACK

Question 21

What happens to cwnd on loss events?

Answer 21

Timeout: Cut to 1 MSS

For TCP Reno:
Triple Duplicate ACK: Half + 3

Question 22

How is congestion detected by the TCP

Answer 22

Congestion is determined by the sender

Question 23

What happens if you increase TCP sending rate with a congested bottleneck

Answer 23

• it won’t increase end-to-end throughput
• it will increase measured RTT

Question 24

Explain the delay-based TCP congestion control

Answer 24

keep sender-to-receiver “just full enough but no fuller”

= cwnd/RTT

Case 1: throughput close to uncongested throughput: increase cwnd linearly

Case 2: throughput less than uncongested throughput: decrease cwnd linearly

Different from AIMD because it doesn’t induce/force loss

Question 25

What is explicit congestion notification? (ECN)

Answer 25

• Routers in between source and dest are alerted of congestion.
• Router alerts destination -> destination alerts source
• ECN = 11 to notify congestion