Congestion Control and Streaming

Question 1

How does problem of lack of knowledge of shared downstream bottleneck manifest itself?

Answer 1

1. lost packets
2. long delays
3. congestion collapse

Question 2

Congestion Collapse (short definition)

Answer 2

throughput less than bottleneck link

packets consume network resources only to get dropped later at a downstream link

Question 3

Congestion Collapse causes

Answer 3

1. spurious retransmission

2. undelivered packets

Question 4

Solution to spurious retransmission

Answer 4

1. better timers

2. TCP congestion control

Question 5

How does TCP interpret packet loss? What does it do as a result?

Answer 5

Congestion. It will slow down as a consequence

Question 6

What do senders do if no packets are dropped?

Answer 6

Increase sending rate

Question 7

TCP increase algorithm behavior

Answer 7

Sender tests network to determine if network can sustain higher sending rate

Question 8

TCP decrease algorithm behavior

Answer 8

Senders react to to congestion to achieve optimal loss rates, delays, and sending rates

Question 9

RTT = 100 milliseconds

packet size = 1 kb (kilobyte)

window size = 10 packets

What is transmission rate in kbps?

Answer 9

800 kbps

Question 10

Rate Based Approach to Rate Adjustment

Answer 10

1. Sender monitors loss rate
2. sender uses timer to modulate
   (less common method)

Question 11

Fairness vs Efficiency

Answer 11

Fairness is everyone getting ‘fair share’ and efficiency is when network resources are used well.

Question 12

Where does high ‘fan in’ occur

Answer 12

between leaves and root of data center

Question 13

data center attributes

Answer 13

1. high ‘ fan in’
2. high bandwidth, low latency workloads
3. many parallel requests

Question 14

TCP incast problem

Answer 14

throughput collapse resultant from many parallel requests in data center. Switches overflow buffers, causing underutilization of network.

This is a many to one issue

Causes bursty retransmission due to TCP timeouts

Question 15

bursty retransmission cause

Answer 15

caused by TCP timeouts in TCP incast problem scenario

Question 16

incast

Answer 16

drastic reduction in application throughput caused when servers all simultaneously request data

Question 17

barrier synchronization

Answer 17

client/app may have many parallel threads and no forward progress can be made until all the responses for those threads are satisfied.

Question 18

solution to idle time in barrier synchronization

Answer 18

granular retransmission timers that operate in microseconds

another option is for client to acknowledge very other packet (not main solution)

Question 19

basic goal of TCP

Answer 19

prevent congestion collapse

Question 20

challenges of streaming

Answer 20

1. Large volume of data
2. Data volume varies over time
3. Low tolerance for delay variation (video)
4. Low tolerance for delay, period (games, VOIP)

Question 21

analog to digital audio sampling explained

Answer 21

samples taken of audio at fixed intervals, with each sample being a fixed size in bits

Question 22

video compress techniques

Answer 22

1. Spatial redundancy

2. Temporal redundancy

Question 23

spatial redundancy

Answer 23

video compression method which exploits visual aspects humans tend not to notice

Question 24

temporal redundancy

Answer 24

compression across images via reference anchor and derived frames

Question 25

reference anchor

Answer 25

“I” frame. Used as reference frame in video compression. Divided into grid.

Question 26

derived frame

Answer 26

“P” frame

Question 27

motion vectors

Answer 27

difference between the I frame blocks and the P frame blocks in video compression

Question 28

how does TCP know when to stop increasing rate?

Answer 28

when sender notices packet drops

Question 29

causes of packet drops OTHER than congestion

Answer 29

in wireless networks, wireless interference may corrupt packet and result in dropping of packet

Question 30

how does TCP send increase sending rate?

Answer 30

by increasing the window size

Question 31

every time additive increase is applied, what is increasing (that isn’t the window size)

Answer 31

efficiency

Question 32

every time multiplicative decrease is applied, what is increasing?

Answer 32

fairness

This is because you get closer to the x1=x2 fairness line in the phase plot

Question 33

throughput collapse cause (and what example was used in class?)

Answer 33

causes by switch buffer overflow. (exampled used is the barrier synchronization problem)

Question 34

Challenges of streaming

Answer 34

• large volume of data
• data volume varies over time
• low tolerance for delay variation (video)
• low tolerance for delay, period (games, voip)

Question 35

8,000 samples/sec
8 bits/sample
…what is sampling rate?

Answer 35

64kbps

Question 36

playout delay

Answer 36

acceptable delay at beginning of stream when waiting for initial packets to fill a playout buffer

Question 37

why is TCP bad for streaming?

Answer 37

• reliable delivery
• slow down upon loss
• protocol overload (headers, acks)

Question 38

why is UDP good for streaming?

Answer 38

• no retransmission
• no sending rate adaptation
• smaller headers

Question 39

what is delegated to higher layers is UDP is implemented?

Answer 39

• when to transmit
• how to encapsulate
• whether to retransmit
• whether to adapt sending rate

Question 40

what property must UDP have when sharing data through a link?

Answer 40

UDP must be ‘TCP friendly’

Question 41

QOS (quality of service) properties

Answer 41

• explicit reservations

- mark certain packet streams as high priority

Question 42

weighted fair queueing

Answer 42

in network, there are multiple queues, and the queues with the with higher priority are services more frequent;y

Question 43

alternatives to weighted fair queueing

Answer 43

fixed bandwith per app ( bad because this is inefficient from a network utilization perspective)

admission control where app declares its needs in advance and network blocks contending traffic to accomodate (analogous to having a busy signal in a telephone call)