Transport Layer

Question 1

What is the transport layer?

Answer 1

end o end connection between 2 hosts

Question 2

What are the services of the transport layer?

Answer 2

• communication between application processes running on 2 different hosts

Question 3

What does the transport layer sender do?

Answer 3

break application messages into segments and pass to network layer

Question 4

What does the transport layer receiver do?

Answer 4

reassemble segments into messages and pass to application layer

Question 5

What are the 2 transport layer protocols?

Answer 5

TCP
UDP

Question 6

What is UDP?

Answer 6

• unreliable
• connectionless
• demultiplexes IP packets
• error checks data

Question 7

What is TCP?

Answer 7

• connection oriented
• full duplex reliable mechanisms
• recovery mechanism for packets
• flow and congestion control

Question 8

What are the differences between TCP and UDP segment headers?

Answer 8

TCP much more complex as has more services
UDP has checksum

Question 9

What are reliable data transfer mechanisms?

Answer 9

stop and wait
sliding window: go back n, selective repeat

Question 10

How does TCP work?

Answer 10

initiates connection
send acks
closes connection

Question 11

What does TCP do?

Answer 11

reliable data transfer over unreliable service

Question 12

What triggers TCP retransmissions?

Answer 12

timeouts
duplicate acks

Question 13

How can you implement TCP flow control?

Answer 13

buffer
- when buffer full, segments arriving don’t get acks

Question 14

What is end to end congestion control?

Answer 14

used in TCP
- no explicit feedback
- congestion inferred from observed loss and delay

Question 15

What is the equation for timeout interval?

Answer 15

timeout interval = estimated RTT + 4*DevRTT

• DevRTT = 4% safety margin

Question 16

What is additive increase multiplicative decrease (AIMD) for TCP?

Answer 16

additively increase window size until loss occurs
then cut window in half

Question 17

What is slow start for TCP?

Answer 17

when connection begins increase rate exponentially until first loss
initially congestion window = 1
double every RTT
increment for every ack received

Question 18

How to switch from slow start to congestion avoidance for TCP?

Answer 18

variable slow start threshold
on event of loss, slow start threshold set to half of window size just before loss event

Question 19

How is TCP fair?

Answer 19

divide capacity by number of flows so equal flow for each session?

capacity/no. sessions