6.033 Computer System Engineering

Transcription

1 MIT OpenCourseWare Computer System Engineering Spring 2009 For information about citing these materials or our Terms of Use, visit:

2 L13: Sharing in network systems Dina Katabi Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, Sam Madden, and Robert Morris

3 Where is sharing happening? client stub presentation Layer server stub End-to-end layer RPC Data Header Data Header RPC D H Network Layer D H D H D H D H D H Link Layer

4 This Lecture Problems: Sharing server Sharing network Solution: Set the window size carefully Sharing server: flow control Sharing the network: congestin control

5 Sliding Window Window Size Packets ACK d Outstanding Un-ack d pkts Packets OK 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 cumulative acks i.e., receiver acks the next expected in-order packet

6 window = 1-5 Src x Rcvr p1

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

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

9 window = 2-6 Timeout Src a2 a2 a2 a2 a2 a7 x Rcvr p1 p3 p4 p5 p6 p2

10 In this example, the receiver sent cumulative acks, but the same behavior happens if the receiver acks the received sequence number

11 What is the right window size? The window limits how fast the sender sends Two mechanisms control the window: Flow control Congestion control

12 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

13 How to set fwnd? Multiple applications run on the same machine but use different ports Fwnd = B x RTT Size of queue substitute for B Adapts to RTT changes B changes self-pacing B1 App D B2 App TCP P1 D D P2 H D P1 H P2 network

14 Sharing the network 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 Heterogeneity bandwidth: 9.6Kb/s (then modem, now cellular), 10 Tb/s latency: 50us (LAN), 133ms (wired), 1s (satellite), 260s (Mars)

15 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 state of resource Sources are unaware of each other Manifestations: Lost packets (buffer overflow at routers) Long delays (queuing in router buffers) Long delays may lead to retransmissions, which lead to more packets. S 1 S 2

16 Danger: Congestion Collapse Increase in input traffic leads to decrease in useful work knee Throughput Latency cliff Congestion Collapse Input traffic Causes of Congestion Collapse Retransmissions introduce duplicate packets Duplicate packets consume resources wasting link capacity

17 Example: old TCP implementations Fwnd 1 Fwnd 2 Long haul network (i.e., large RTT) Router drops some of TCP 2 s fwnd packets Each discard packet will result in timeout At timeout TCP 2 resends complete window Cumulative ACK, timeouts fire off at same time Blizzard of retransmissions can result in congestion collapse Insufficiently adaptive timeout algorithm made things worse

18 What can be done in general? Avoid congestion: Increase network resources But demands will increase too! Admission Control & Scheduling Used in telephone networks Hard in the Internet because can t model traffic well Perhaps combined with 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?

19 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 Sender adjusts window

20 Case study: current TCP 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 No changes to protocol necessary!

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

22 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

23 TCP AIMD Cwnd Timeout because of a packet loss desired cwnd Halve Cwnd Grab capacity again Time Need the queue to absorb these saw-tooth oscillations

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

25 Cwnd Timeout Slow Start + AIMD Additive increase Slow start Multiplicative decrease Time

26 Fairness? No! Applications don t have to use TCP Use multiple TCP connections

27 Summary Controlling complexity in network systems Layering Interesting division of labors based on E2E principle Case study: Internet Interesting problems and techniques Packets Protocols Client-server implementation Next: Application-level reliability and security