Congestion Control and Streaming Flashcards Preview

CS6250 Test 2 - PHIL > Congestion Control and Streaming > Flashcards

Flashcards in Congestion Control and Streaming Deck (43):
1

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

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

2

Congestion Collapse (short definition)

throughput less than bottleneck link

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

3

Congestion Collapse causes

1. spurious retransmission
2. undelivered packets

4

Solution to spurious retransmission

1. better timers
2. TCP congestion control

5

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

Congestion. It will slow down as a consequence

6

What do senders do if no packets are dropped?

Increase sending rate

7

TCP increase algorithm behavior

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

8

TCP decrease algorithm behavior

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

9

RTT = 100 milliseconds

packet size = 1 kb (kilobyte)

window size = 10 packets

What is transmission rate in kbps?

800 kbps

10

Rate Based Approach to Rate Adjustment

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

11

Fairness vs Efficiency

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

12

Where does high 'fan in' occur

between leaves and root of data center

13

data center attributes

1. high ' fan in'

2. high bandwidth, low latency workloads

3. many parallel requests

14

TCP incast problem

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

15

bursty retransmission cause

caused by TCP timeouts in TCP incast problem scenario

16

incast

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

17

barrier synchronization

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

18

solution to idle time in barrier synchronization

granular retransmission timers that operate in microseconds

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

19

basic goal of TCP

prevent congestion collapse

20

challenges of streaming

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)

21

analog to digital audio sampling explained

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

22

video compress techniques

1. Spatial redundancy
2. Temporal redundancy

23

spatial redundancy

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

24

temporal redundancy

compression across images via reference anchor and derived frames

25

reference anchor

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

26

derived frame

"P" frame

27

motion vectors

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

28

how does TCP know when to stop increasing rate?

when sender notices packet drops

29

causes of packet drops OTHER than congestion

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

30

how does TCP send increase sending rate?

by increasing the window size

31

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

efficiency

32

every time multiplicative decrease is applied, what is increasing?

fairness

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

33

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

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

34

Challenges of streaming

- large volume of data

-data volume varies over time

-low tolerance for delay variation (video)

-low tolerance for delay, period (games, voip)

35

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

64kbps

36

playout delay

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

37

why is TCP bad for streaming?

-reliable delivery

-slow down upon loss

-protocol overload (headers, acks)

38

why is UDP good for streaming?

-no retransmission

-no sending rate adaptation

-smaller headers

39

what is delegated to higher layers is UDP is implemented?

-when to transmit

-how to encapsulate

-whether to retransmit

-whether to adapt sending rate

40

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

UDP must be 'TCP friendly'

41

QOS (quality of service) properties

-explicit reservations

-mark certain packet streams as high priority

42

weighted fair queueing

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

43

alternatives to weighted fair queueing

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)