Week 13 - TCP (transport layer protocol)

Question 1

Is TCP simplex, half-duplex, or full-duplex

Answer 1

full-duplex

Segment sequence numbers are independent in each direction.

Question 2

In a TCP segment, what does the sequence number refer to?

Answer 2

The first byte of data in the segment.

Question 3

What is the checksum for in a TCP segment?

Answer 3

Ensures entire TCP segment is not corrupt.

Calculated only at receiver unlike network layer IP datagram.

Question 4

How is TCP flow control implemented?

Answer 4

TCP sender has receive window (rwnd) variable

Question 5

Is TCP connection or connectionless?

Answer 5

connection - reliability

Question 6

How is TCP error control implemented? (4 mechanisms)

Answer 6

Acknowledgements
Sequence numbers
Retransmission
Checksum

Question 7

What does the value in the acknowledgment field in a TCP segment define?

Answer 7

The byte number of the next byte a party expects to receive.

The ACK number is cumulative

Question 8

What are the 2 types of retransmission in TCP?

Answer 8

Timeout

Three Duplicate ACK segments

Question 9

How does TCP detect congestion?

Answer 9

By observing the cause of retransmission.

1. Timeout indicates strong congestion
2. Three duplicate ACKS indicates weak congestion

Question 10

What is the TCP congestion window?

Answer 10

It’s similar to rwnd but this time it’s not about the receiver buffer but rather the congestion perceived by the sender based on retransmission.

Question 11

What is MSS?

Answer 11

Maximum TCP segment payload size. It’s similar to MTU which is correlated to data-link layer frame.

Question 12

How does the TCP congestion control algorithm work?

AKA how is the cwnd adjusted (MSS)?

Answer 12

Uses 2 components:

1. Slow start (exponential) doubles per RTT and continues until ssthresh reached. Whenever a timeout or 3dupACKs occurs, the ssthresh is half of the current cwnd
2. cwnd increases linearly beyond the ssthresh.