Lesson 2 - Transport and Application Layers

Question 1

Describe the transport layer

Answer 1

The transport layer provides an end-to-end connection between two applications that are running on different hosts.

Question 2

What is the relationship between the network and the transport layer

Answer 2

Network layer only delivers data

Transport layer guarantees integrity and delivery of data.

Question 3

What are the 2 main protocols within the transport layer

Answer 3

User datagram protocol (UDP) and the Transmission Control Protocol (TCP)

Question 4

What is connectionless multiplexing/demultiplexing?

Answer 4

requires the “IP Address” and “destination port #” whether it be TCP or UDP. It requires only that two-tuple to identify where to send the datagram.
UDP is connectionless.

Question 5

What is connection-oriented multiplexing/demultiplexing?

Answer 5

is only TCP viable and requires a four-tuple, meaning in order for the datagram to know where to be sent it needs (source IP address, source port number, destination IP address, destination port number).

Question 6

What are the differences between UDP & TCP?

Answer 6

UDP is connctionless protocol

TCP is connection oriented protocol

Question 7

Explain TCP three-way handshake.

Answer 7

1. client initiates handshake with sequence
2. server response (SYNACK) includes SYN bit set to 1 & client sequence +1.
3. Client receives SYNACK segment and acknowledges with SYN bit set to 0.

Question 8

Explain TCP connection teardown

Answer 8

1. Client ends by sending segment with FIN set to 1.
2. Server acknowledges and start to close connection.
3. Server sends segment with FIN set to 1, indicating closed.
4. Client sends ACK

Question 9

What is Automatic Repeat Request or ARQ (TCP)?

Answer 9

method of using acknowledgements and timeouts to deliver and repeat requests.

Question 10

What is Stop and Wait ARQ (TCP)?

Answer 10

Sender sends a packet and waits for its acknowledgement from the receiver.

Question 11

What is Go-back-N (TCP)?

Answer 11

receiver ACK last received in-order packet. sender sends all packets from the most recently received. May cause the receiver to discard out of order packets.

Question 12

What is selective ACKing (TCP)?

Answer 12

During data exchange, the sender retransmits only those packets that it suspects were received in error.

Question 13

What is fast retransmit (TCP)?

Answer 13

When the sender receives 3 duplicate ACKs for a packet, it considers the packet to be lost and will retransmit it instead of waiting for the timeout.

(EX. packet 7 was missed, when 8, 9, & 10 are received the receiver sends an ACK for the last in-order packet (6) for each packet received. Once the receiver receives the 3 ACKs for packet 6, it would resend 7 without waiting for a timeout)

Question 14

What is transmission control and why do we need to control it?

Answer 14

transmission control is the process of limiting transmission rate. It is utilized to enable a fair use of the bandwidth available (congestion control) and protect the receiver’s buffer from overflowing (flow control)

Question 15

What is flow control and why do we need to control it?

Answer 15

Flow control is utilized to throttle the transmission rate of the sender to protect the receiver’s buffer from overflowing.

Question 16

What is the receive window?

Answer 16

The receive window (rwnd) is the spare unused space in the receiver’s buffer. The receiver advertises this value of rwnd in every segment/ACK it sends back to the sender.

Question 17

What is congestion control?

Answer 17

a mechanism to control the transmission rate at the sender in order to avoid congestion in the network.

Question 18

What are the goals of congestion control?

Answer 18

Efficiency - network utilization should be high
Fairness
Low delay
Fast convergence - determine what is “fair” quickly.

Question 19

What is an E2E congestion control?

Answer 19

The hosts (ends) infer congestion from the network behavior and adapt the transmission rate.

Question 20

What is network-assisted control?

Answer 20

The network layer to provides explicit feedback to the sender about congestion in the network.

Question 21

How does a host infer congestion?

Answer 21

Packet delay - round-trip time can be based on ACKs

Packet loss - TCP’s earliest implementation utilized this because it was already in use to provide reliability.

Question 22

How does a TCP sender limit the sending rate?

Answer 22

TCP uses a congestion window which is similar to the receive window used for flow control.
Under regular conditions, TCP increases the congestion window trying to achieve the available throughput. Once it detects congestion then the congestion window is decreased.

Question 23

What is additive increase (TCP)?

Answer 23

Every time the sending host successfully sends a congestion window (cwnd) number of packets it increases the window by 1.

Question 24

What is Multiplicative Decrease?

Answer 24

When the TCP sender detects that a timeout occurred, then it sets the Congestion Window (cwnd) to half of its previous value.

Question 25

What is a slow start in TCP?

Answer 25

The congestion window is increased exponentially instead of linearly as in the case of AIMD. It’s termed “slow start” because the increase starts off slowly only adding 1 packet to the congestion window, but builds quickly.

Question 26

Is TCP fair in case 2 connections have the same RTT? Explain why.

Answer 26

Yes. Both will increase/decrease their congestion window similarly and approach an equal bandwidth share.

Question 27

Is TCP fair in case 2 connections have different RTTs? Explain why.

Answer 27

connections with smaller RTT values would increase their congestion window faster than the ones with longer RTT values. This leads to an unequal sharing of the bandwidth.

Question 28

Explain how TCP cubic works

Answer 28

TCP CUBIC uses a cubic function for congestion window growth so that the window grows aggressively at first, but slow near the transmission rate of the last lost packet (Wmax).

Question 29

Explain TCP throughput calculation

Answer 29

On a graph of packets per round trip time, the area under the curve is the number of packets sent per cycle. (Transmission rate = packets per cycle / seconds per cycle)
Or for an increase rate of 1 packet per cycle, the packets per cycle can be calculated as = 3/8 (packets)^2