1 / 59

TCP Congestion Control and Modern Loss Recovery

This lecture provides an overview of TCP congestion control mechanisms, modern loss recovery techniques, TCP interactions, options, and modeling. It also covers workload changes and header compression.

ritareese
Download Presentation

TCP Congestion Control and Modern Loss Recovery

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 15-441 Computer Networking TCP (cont.)

  2. Overview • TCP congestion control • TCP modern loss recovery • TCP interactions • TCP options • TCP modeling • Workload changes • Header compression Lecture 16: 10-30-01

  3. TCP Congestion Control • Packet loss is seen as sign of congestion and results in a multiplicative rate decrease • Factor of 2 • TCP periodically probes for available bandwidth by increasing its rate Rate Time Lecture 16: 10-30-01

  4. Congestion Avoidance • If loss occurs when cwnd = W • Network can handle 0.5W ~ W segments • Set cwnd to 0.5W (multiplicative decrease) • Upon receiving ACK • Increase cwnd by 1/cwnd • Implements AIMD Lecture 16: 10-30-01

  5. Congestion Avoidance Sequence Plot Sequence No Time Lecture 16: 10-30-01

  6. Congestion Avoidance Behavior Congestion Window Time Cut Congestion Window and Rate Grabbing back Bandwidth Packet loss + Timeout Lecture 16: 10-30-01

  7. Slow Start Packet Pacing • How do we get this clocking behavior to start? • Initialize cwnd = 1 • Upon receipt of every ack, cwnd = cwnd + 1 • Implications • Window actually increases to W in RTT * log2(W) • Can overshoot window and cause packet loss Lecture 16: 10-30-01

  8. One RTT 0R 1 One pkt time 1R 1 2 3 2R 2 3 4 6 5 7 4 5 6 7 3R 8 10 12 14 9 11 13 15 Slow Start Example Lecture 16: 10-30-01

  9. Slow Start Sequence Plot . . . Sequence No Time Lecture 16: 10-30-01

  10. Return to Slow Start • If packet is lost we lose our self clocking as well • Need to implement slow-start and congestion avoidance together • When timeout occurs set ssthresh to 0.5w • If cwnd < ssthresh, use slow start • Else use congestion avoidance Lecture 16: 10-30-01

  11. TCP Saw Tooth Behavior Congestion Window Timeouts may still occur Time Slowstart to pace packets Fast Retransmit and Recovery Initial Slowstart Lecture 16: 10-30-01

  12. Overview • TCP congestion control • TCP modern loss recovery • TCP interactions • TCP options • TCP modeling • Workload changes • Header compression Lecture 16: 10-30-01

  13. TCP Flavors • Tahoe, Reno, Vegas • TCP Tahoe (distributed with 4.3BSD Unix) • Original implementation of Van Jacobson’s mechanisms (VJ paper) • Includes: • Slow start • Congestion avoidance • Fast retransmit Lecture 16: 10-30-01

  14. Fast Retransmit • What are duplicate acks (dupacks)? • Repeated acks for the same sequence • When can duplicate acks occur? • Loss • Packet re-ordering • Window update – advertisement of new flow control window • Assume re-ordering is infrequent and not of large magnitude • Use receipt of 3 or more duplicate acks as indication of loss • Don’t wait for timeout to retransmit packet Lecture 16: 10-30-01

  15. Fast Retransmit Retransmission X Duplicate Acks Sequence No Time Lecture 16: 10-30-01

  16. Multiple Losses X X Now what? X Retransmission X Duplicate Acks Sequence No Time Lecture 16: 10-30-01

  17. Tahoe X X X X Sequence No Time Lecture 16: 10-30-01

  18. TCP Reno (1990) • All mechanisms in Tahoe • Addition of fast-recovery • Opening up congestion window after fast retransmit • Delayed acks • Header prediction • Implementation designed to improve performance • Has common case code inlined • With multiple losses, Reno typically timeouts because it does not see duplicate acknowledgements Lecture 16: 10-30-01

  19. Reno X X X Now what? - timeout X Sequence No Time Lecture 16: 10-30-01

  20. NewReno • The ack that arrives after retransmission (partial ack) could indicate that a second loss occurred • When does NewReno timeout? • When there are fewer than three dupacks for first loss • When partial ack is lost • How fast does it recover losses? • One per RTT Lecture 16: 10-30-01

  21. NewReno X X X Now what? – partial ack recovery X Sequence No Time Lecture 16: 10-30-01

  22. SACK • Basic problem is that cumulative acks only provide little information • Ack for just the packet received • What if acks are lost?  carry cumulative also • Not used • Bitmask of packets received • Selective acknowledgement (SACK) • How to deal with reordering Lecture 16: 10-30-01

  23. SACK X X X Now what? – send retransmissions as soon as detected X Sequence No Time Lecture 16: 10-30-01

  24. Performance Issues • Timeout >> fast rexmit • Need 3 dupacks/sacks • Not great for small transfers • Don’t have 3 packets outstanding • What are real loss patterns like? • Right edge recovery • Allow packets to be sent on arrival of first and second duplicate ack • Helps recovery for small windows • How to deal with reordering? Lecture 16: 10-30-01

  25. Overview • TCP congestion control • TCP modern loss recovery • TCP interactions • TCP options • TCP modeling • Workload changes • Header compression Lecture 16: 10-30-01

  26. How to Change Window • When a loss occurs have W packets outstanding • New cwnd = 0.5 * cwnd • How to get to new state? Lecture 16: 10-30-01

  27. Fast Recovery • Each duplicate ack notifies sender that single packet has cleared network • When < cwnd packets are outstanding • Allow new packets out with each new duplicate acknowledgement • Behavior • Sender is idle for some time – waiting for ½ cwnd worth of dupacks • Transmits at original rate after wait • Ack clocking rate is same as before loss Lecture 16: 10-30-01

  28. Fast Recovery Sent for each dupack after W/2 dupacks arrive Sequence No X Time Lecture 16: 10-30-01

  29. NewReno Changes • Send a new packet out for each pair of dupacks • Adapt more gradually to new window • Will not halve congestion window again until recovery is completed • Identifies congestion events vs. congestion signals Lecture 16: 10-30-01

  30. Rate Halving Recovery Sent after every other dupack Sequence No X Time Lecture 16: 10-30-01

  31. Delayed Ack Impact • TCP congestion control triggered by acks • If receive half as many acks  window grows half as fast • Slow start with window = 1 • Will trigger delayed ack timer • First exchange will take at least 200ms • Start with > 1 initial window • Bug in BSD, now a “feature”/standard Lecture 16: 10-30-01

  32. Overview • TCP congestion control • TCP modern loss recovery • TCP interactions • TCP options • TCP modeling • Workload changes • Header compression Lecture 16: 10-30-01

  33. TCP Extensions • Implemented using TCP options • Timestamp • Protection from sequence number wraparound • Large windows Lecture 16: 10-30-01

  34. Protection From Wraparound • Wraparound time vs. Link speed • 1.5Mbps: 6.4 hours • 10Mbps: 57 minutes • 45Mbps: 13 minutes • 100Mbps: 6 minutes • 622Mbps: 55 seconds  < MSL! • 1.2Gbps: 28 seconds • Use timestamp to distinguish sequence number wraparound Lecture 16: 10-30-01

  35. RTT Window Size Throughput = Roundtrip Time Window Size vs Throughput Sender Receiver Time Lecture 16: 10-30-01

  36. TCP syn SW? 3 TCP ack SW yes 2 Window Scaling:Example Use of Options • “Large window” option (RFC 1323) • Negotiated by the hosts during connection establishment • Option 3 specifies the number of bits by which to shift the value in the 16 bit window field • Independently set for the two transmit directions • The scaling factor specifies bit shift of the window field in the TCP header • Scaling value of 2 translates into a factor of 4 • Old TCP implementations will simply ignore the option • Definition of an option • Scaling results in a loss of accuracy • Not a big deal • Alternatives? TCP syn,ack SW yes 3 SW? 2 Lecture 16: 10-30-01

  37. Large Windows • Delay-bandwidth product for 100ms delay • 1.5Mbps: 18KB • 10Mbps: 122KB > max 16bit window • 45Mbps: 549KB • 100Mbps: 1.2MB • 622Mbps: 7.4MB • 1.2Gbps: 14.8MB • Scaling factor on advertised window • Specifies how many bits window must be shifted to the left • Scaling factor exchanged during connection setup Lecture 16: 10-30-01

  38. Maximum Segment Size (MSS) • Exchanged at connection setup • Typically pick MTU of local link • What all does this effect? • Efficiency • Congestion control • Retransmission • Path MTU discovery • Why should MTU match MSS? Lecture 16: 10-30-01

  39. Overview • TCP congestion control • TCP modern loss recovery • TCP interactions • TCP options • TCP modeling • Workload changes • Header compression Lecture 16: 10-30-01

  40. TCP Modeling • Given the congestion behavior of TCP can we predict what type of performance we should get? • What are the important factors • Loss rate • Affects how often window is reduced • RTT • Affects increase rate and relates BW to window • RTO • Affects performance during loss recovery • MSS • Affects increase rate Lecture 16: 10-30-01

  41. Overall TCP Behavior • Let’s concentrate on steady state behavior with no timeouts and perfect loss recovery Window Time Lecture 16: 10-30-01

  42. Simple TCP Model • Some additional assumptions • Fixed RTT • No delayed ACKs • In steady state, TCP losses packet each time window reaches W packets • Window drops to W/2 packets • Each RTT window increases by 1 packetW/2 * RTT before next loss • BW = MSS * avg window/RTT = MSS * (W + W/2)/(2 * RTT) = .75 * MSS * W / RTT Lecture 16: 10-30-01

  43. Simple Loss Model • What was the loss rate? • Packets transferred = (.75 W/RTT) * (W/2 * RTT) = 3W2/8 • 1 packet lost  loss rate = p = 8/3W2 • W = sqrt( 8 / (3 * loss rate)) • BW = .75 * MSS * W / RTT • BW = MSS / (RTT * sqrt (2/3p)) Lecture 16: 10-30-01

  44. TCP Friendliness • What does it mean to be TCP friendly? • TCP is not going away • Any new congestion control must compete with TCP flows • Should not clobber TCP flows and grab bulk of link • Should also be able to hold its own, i.e. grab its fair share, or it will never become popular • How is this quantified/shown? • Has evolved into evaluating loss/throughput behavior • If it shows 1/sqrt(p) behavior it is ok • But is this really true? Lecture 16: 10-30-01

  45. Overview • TCP congestion control • TCP modern loss recovery • TCP interactions • TCP options • TCP modeling • Workload changes • Header compression Lecture 16: 10-30-01

  46. Changing Workloads • New applications are changing the way TCP is used • 1980’s Internet • Telnet & FTP  long lived flows • Well behaved end hosts • Homogenous end host capabilities • Simple symmetric routing • 2000’s Internet • Web & more Web  large number of short xfers • Wild west – everyone is playing games to get bandwidth • Cell phones and toasters on the Internet • Policy routing Lecture 16: 10-30-01

  47. Internet Problems with Short Concurrent Flows • Compete for resources • N “slow starts” = aggressive • No shared learning = inefficient • Entire life is in slow start • Fast retransmission is rare f1 f2 Server f(n) Client Lecture 16: 10-30-01

  48. Sharing Information • Congestion control • Share a single congestion window across all connections to a destination • Advantages • Applications can’t defeat congestion control by opening multiple connections simultaneously • Overall loss rate of the network drops • Possibly better performance for applications like Web • Disadvantages? • What if you’re the only one doing this?  you get lousy throughput • What about hosts like proxies? Lecture 16: 10-30-01

  49. Short Transfers • Fast retransmission needs at least a window of 4 packets • To detect reordering • Should not be necessary if small outstanding number of packets • Adjust threshold to min(3, cwnd/outstanding) • Some paths have much more reordering than others • Adapt threshold to past reordering • Allow new packets to be transmitted for first few dupacks • Will create new dupacks and force retransmission • Will not reduce goodput in situations of reordering • Follows packet conservation Lecture 16: 10-30-01

  50. Enhanced TCP Loss Recovery 4 Router 6 5 Server Client Router Server 3 3 Client Router 8 7 Server Client Acknowledgments Data Packets Lecture 16: 10-30-01

More Related