1. L13: Sharing in network systems Dina Katabi 6.033 Spring 2007 http://web.mit.edu/6.033 Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, Sam Madden, and Robert Morris

2. Where is sharing happening? stub Network Layer Link Layer presentation Layer End-to-end layer RPC HD HD HD HDHD HD HeaderDataHeaderData clientserver

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

4. Sliding Window 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 Window Size Outstanding Un-ack’d pkts Packets OK to send Packets not OK to send yet Packets ACK’d

5. Src Rcvr window = 1-5 1 2 3 4 5 p1 x

6. Src Rcvr window = 2-6 1 2 3 4 5 p1 a2 x 6

7. Src window = 2-6 1 2 3 4 5 p1 a2 x 6 p3 Rcvr

8. Src window = 2-6 1 2 3 4 5 p1 a2 x 6 p3 a2 p4 a2 p6 a2 p5 Timeout 2 a7 p2 Rcvr

9. Src window = 7-11 1 2 3 4 5 p1 a2 x 6 p3 a2 p4 a2 p6 a2 p5 Timeout 2 a7 p2 7 8 9 10 11 Rcvr In this example, the receiver sent cumulative acks, but the same behavior happens if the receiver acks the received sequence number

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

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

12. How to set fwnd? App B1B1 B2B2 TCP Multiple applications run on the same machine but use different ports P1DP2D D H P1D H network Fwnd = B x RTT Size of queue substitute for B Adapts to RTT changes B changes “self-pacing”

13. 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)

14. Congestion S1S1 S2S2 R1 D 10Mb/s 2Mb/s 100Mb/s S1S1 S2S2  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….

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

16. Example: old TCP implementations Fwnd 2 Fwnd 1  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

17. 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?

18. 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

19. 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! Case study: current TCP

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

21. Additive Increase D A Src Rcvr cwnd = 1 cwnd += 1 cwnd = 2 DDAA cwnd = 3 DDAADA cwnd = 4

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

23. TCP “Slow Start” D A DDAADD AA D A Src Rcvr D A 1 2 4 8 AAAA  How to set the initial cwnd?  At the beginning of a connection, increase exponentially  Every RTT, double cwnd

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

25. Fairness? No! Applications don’t have to use TCP Use multiple TCP connections

26. 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