Congestion Control & Streaming

Question 1

Congestion

Answer 1

1. Different sources compete for resources
2. Sources are unaware of each other
3. Sources are unaware of the state of the resource

Question 2

Congestion Collapse

Answer 2

Increase in load leads to decrease in useful work done

Question 3

Congestion Collapse: Causes

Answer 3

1. Spurious retransmission

2. undelivered packets

Question 4

Spurious retransmission

Answer 4

Data was sent again that the receiver had already acknowledged. Also called “needless retransmission”

Question 5

Spurious retransmission: solution

Answer 5

1. Have better timers

2. Use TCP congestion control

Question 6

Undelivered packets: solution

Answer 6

1. Use TCP congestion control on all traffic

Question 7

Congestion Control: Goals

Answer 7

1. Efficiency: Use network resources efficiently
2. Fairness: Preserve fair allocation of resources
3. Avoid congestion collapse

Question 8

Congestion Control: Approaches

Answer 8

1. end 2 end congestion control

2. network assisted congestion control

Question 9

End 2 End congestion control

Answer 9

This is the approach taken by TCP congestion control

1. No feedback from network
2. Congestion inferred by loss and delay

Question 10

Network Assisted Congestion Control

Answer 10

1. Routers provide feedback

Question 11

TCP Congestion Control

Answer 11

1. Increase algorithm: Senders increase rate until packets are dropped. This is because the routers buffer fills up.
2. Decrease algorithm: TCP interprets packets loss as congestion. Slows down.

Question 12

Windowing

Answer 12

1. Increased window size increases rate

Question 13

AIMD

Answer 13

Additive Increase: 1 packet increase per successful round trip

Multiplicative decrease: Window size reduced by 1/2 per failure.

Question 14

Fairness: contributing mechanism

Answer 14

Multiplicative decrease

Question 15

Efficiency: contributing mechanism

Answer 15

Additive increase

Question 16

TCP in-cast

Answer 16

drastic reduction in application throughput that results when servers (all using TCP) request data at the same time.

1. Result in bursty retransmissions
2. Caused by TCP timeouts

Question 17

Barrier synchronization: solutions

Answer 17

1. microsecond granularity retransmission

2. ACK for every other packet

Question 18

Digitizing Audio

Answer 18

1. Sample audio signal at fixed intervals

2. Represent the amplitude as a number of bits

Question 19

Video Compression

Answer 19

1. Spacial redundancy

2. Temporal redundancy

Question 20

Spacial Redundancy

Answer 20

Compressing an individual image to exploit aspects that are not noticeable to humans

Question 21

Temporal Redundancy

Answer 21

I frame (aka reference or anchor frame) is divided into blocks

The p frame (aka derived frame) is the combo of the original I frame blocks + a few motion vectors

Question 22

Playout Buffer

Answer 22

Client stores video/audio packets from stream.

Plays them in order by using timestamp

Question 23

Downsides of TCP for streaming

Answer 23

1. Reliable delivery. TCP retransmitted packets might not always be useful.
2. Slows down sending rate after packet loss.
3. Protocol overhead. The header is too large for audio streaming. Also sending ACK for every other packet is too much.

Question 24

Advantages of UDP for streaming

Answer 24

1. Does not re-transmit lost packets
2. Does not automatically adapt the sending rate
3. Smaller header

Question 25

QoS techniques

Answer 25

1. Marking and Policing

2. Scheduling