Flow & Congestion Control

Transcription

1 Read 7.E & 7.F Flow & Congestion Control Prof. Dina Kat abi Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, and Sam Madden 1

2 This Lecture More about Sliding Window Flow Cont rol Congest ion Cont rol 2

3 Sliding Window Window Size Packets ACK d Out standing Packets OK Un-ack d pkt s to send Packets not OK to send yet The window advances/slides upon the arrival of an ack The sender sends only packets in the window Receiver usually sends cum ulat ive acks i.e., receiver acks the next expected in-order packet 3

4 window = 1-5 Src x Rcvr p1 4

5 window = 2-6 Src Rcvr a2 x p1 6 5

6 window = 2-6 Src x a2 6 a2 Rcvr p1 p3 6

7 window = 2-6 Tim eout Src a2 a2 a2 a2 a2 a7 x Rcvr p1 p3 p4p5 p6 p2 7

8 Tim eout window = 7-11 Src a2 a2 a2 a2 a2 a7 x Rcvr p1 p3 p4p5 p6 p2 In this exam ple, the receiver sent cum ulative acks, but the sam e behavior happens if the receiver acks the received sequence num ber 8

9 This Lecture More about Sliding Window Flow Cont rol Congest ion Cont rol 9

10 What is the right window size? The window limits how fast the sender sends Two mechanisms control the window: Flow control Congest ion cont rol 10

11 Flow Control The receiver may be slow in processing the packets receiver is a bottleneck To prevent the sender form overwhelming the receiver, the receiver tells the sender the maximum number of packets it can buffer fwnd Sender sets W fwnd But, what if the bottleneck is a slow link inside the network Need Congestion Control 11

12 This Lecture More about Sliding Window Flow Cont rol Congest ion Cont rol 12

13 Sharing the Internet How do you manage the resources in a huge system like the Internet, where users with different interests share the same resources? Difficult because of: Size Millions of users, links, routers Het erogeneit y bandwidth: 9.6Kb/s (then modem, now cellular), 10 Tb/s latency: 50us (LAN), 133m s (wired), 1s (satellite), 260s (Mars) 13

14 Congestion S 1 10Mb/s R1 D 2Mb/s S 2 100Mb/s Sources share links, and buffer space Why a problem? Sources are unaware of current stat e of resource Sources are unaware of each other Manifest at ions: Lost packets (buffer overflow at rout ers) Long delays (queuing in router buffers) S 1 S 2 14

15 Congestion Collapse Increase in input traffic leads to decrease in useful work knee Throughput Lat ency cliff Congestion Collapse Input t raffic Causes of Congest ion Collapse Spurious retransmissions of packets that are still in flight Packet consume resources and then they are dropped downst ream 15

16 What can be done? Increase net work resources But dem ands will increase too! Adm ission Cont rol & Scheduling Used in telephone networks Hard in the Internet because can t model traffic well Pricing senders pay more in times of congestion Congestion control: ask the sources to slow down But how? How do the sources learn of congestion? What is the correct window? How to adapt the window as the level of congestion changes? 16

17 How do senders learn of congestion? Potential options: Router sends a Source Quench to the sender Router flags the packets indicating congestion Router drops packets when congestion occurs Sender learns about the drop because it notices the lack of ack Drops are the solution currently used in the Internet 17

18 How do senders learn how m uch to send? Define a congestion control window cwnd Sender s window is set to W = min (fwnd, cwnd) Simple heuristic to find cwnd: Sender increases its cwnd slowly until it sees a drop Upon a drop, sender decreases its cwnd quickly to react to congestion Sender increases again slowly 18

19 TCP Increase/decrease algorithm Additive Increase / Multiplicative Decrease (AIMD) Every RTT: No drop: cwnd = cwnd + 1 drop: cwnd = cwnd /2 19

20 Additive Increase Src cwnd = 1 cwnd + = 1 cwnd = 2 cwnd = 3 cwnd = 4 D A D D A A D D D A A A Rcvr 20

21 TCP AIMD Cw nd Tim eout because of a packet loss desire d cwnd Halv e Cw nd Grab capacity again Tim e Need the queue to absorb these saw-tooth oscillations 21

22 TCP Slow Start How to set the initial cwnd? At the beginning of a connection, increase exponent ially Every RTT, double cwnd Src D A D D A A D D D D A A A A A A A A Rcvr 22

23 Cw nd Tim eout Slow Start + AIMD Addit iv e increas e Slow start Multiplicative decrease Tim e 23