L13: Sharing in network systems

1. L13: Sharing in network systems 6.033 Spring 2007 http://web.mit.edu/6.033 Slides from many folks

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

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

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

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

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

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

9. Timeout 7 2 6 a7 a2 a2 a2 a2 p2 p3 p6 p5 p4 window = 7-11 4 3 2 1 5 9 8 11 10 Src a2 x Rcvr p1 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. H H D D D D P2 P1 P2 P1 How to set fwnd? Multiple applications run on the same machine but use different ports B1 B2 • Fwnd = B x RTT • Size of queue substitute for B • Adapts to • RTT changes • B changes • “self-pacing” App App TCP network

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. 10Mb/s S1 R1 2Mb/s D 100Mb/s S2 S1 S2 Congestion • 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. knee cliff Congestion Collapse Danger: Congestion CollapseIncrease in input traffic leads to decrease in useful work Throughput Latency Input traffic • Causes of Congestion Collapse • Retransmissions introduce duplicate packets • Duplicate packets consume resources wasting link capacity

16. 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

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. 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!

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

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

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

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

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

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