15-441 Computer Networking

1. 15-441 Computer Networking TCP

2. Overview • TCP timeouts • Congestion sources and collapse • Congestion control basics • TCP congestion control • TCP fast recovery Lecture 15: 10-25-01

3. Reliability Challenges • Like reliability on links • Similar techniques (timeouts, acknowledgements, etc.) • New challenges • Congestion related losses • Variable packet delays • What should the timeout be? • Reordering of packets • Ensure sequences numbers are not reused • How long do packets live? • MSL = 120 seconds based on IP behavior Lecture 15: 10-25-01

4. TCP = Go-Back-N Variant • Receiver can only return a single “ack” sequence number to the sender. • Acknowledges all bytes with a lower sequence number • Starting point for retransmission • But: sender only retransmits a single packet. • Reason??? • Error control is based on byte sequences, not packets. • Retransmitted packet can be different from the original lost packet – why? • Packets can overlap – why? • Sliding window with cumulative acks • Ack field contains last in-order packet received • Duplicate acks sent when out-of-order packet received Lecture 15: 10-25-01

5. Round-trip Time Estimation • Wait at least one RTT before retransmitting • Importance of accurate RTT estimators: • Low RTT  unneeded retransmissions • High RTT  poor throughput • RTT estimator must adapt to change in RTT • But not too fast, or too slow! • Spurious timeouts • “Conservation of packets” principle – more than a window worth of packets in flight Lecture 15: 10-25-01

6. Initial Round-trip Estimator • Round trip times exponentially averaged: • New RTT = a (old RTT) + (1 - a) (new sample) • Recommended value for a: 0.8 - 0.9 • 0.875 for most TCP’s • Retransmit timer set to b RTT, where b = 2 • Every time timer expires, RTO exponentially backed-off • Like Ethernet • Not good at preventing spurious timeouts Lecture 15: 10-25-01

7. Jacobson’s Retransmission Timeout • Key observation: • At high loads round trip variance is high • Solution: • Base RTO on RTT and standard deviation or RRTT • rttvar =  * dev + (1- )rttvar • Dev = linear deviation • Inappropriately named – actually smoothed linear deviation Lecture 15: 10-25-01

8. A B Original transmission RTO ACK Sample RTT retransmission Retransmission Ambiguity A B Original transmission X RTO Sample RTT retransmission ACK Lecture 15: 10-25-01

9. Karn’s RTT Estimator • Accounts for retransmission ambiguity • If a segment has been retransmitted: • Don’t count RTT sample on ACKs for this segment • Keep backed off time-out for next packet • Reuse RTT estimate only after one successful transmission Lecture 15: 10-25-01

10. Timestamp Extension • Used to improve timeout mechanism by more accurate measurement of RTT • When sending a packet, insert current timestamp into option • 4 bytes for seconds, 4 bytes for microseconds • Receiver echoes timestamp in ACK • Actually will echo whatever is in timestamp • Removes retransmission ambiguity • Can get RTT sample on any packet Lecture 15: 10-25-01

11. Timer Granularity • Many TCP implementations set RTO in multiples of 200,500,1000ms • Why? • Avoid spurious timeouts – RTTs can vary quickly due to cross traffic • Make timers interrupts efficient • What happens for the first couple of packets? • Pick a very conservative value (seconds) Lecture 15: 10-25-01

12. Delayed ACKS • Problem: • In request/response programs, you send separate ACK and Data packets for each transaction • Solution: • Don’t ACK data immediately • Wait 200ms (must be less than 500ms – why?) • Must ACK every other packet • Must not delay duplicate ACKs Lecture 15: 10-25-01

13. TCP ACK Generation[RFC 1122, RFC 2581] TCP Receiver action Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Immediately send single cumulative ACK Send duplicate ACK, indicating seq. # of next expected byte Immediate ACK if segment starts at lower end of gap Event In-order segment arrival, No gaps, Everything else already ACKed In-order segment arrival, No gaps, One delayed ACK pending Out-of-order segment arrival Higher-than-expect seq. # Gap detected Arrival of segment that Partially or completely fills gap Lecture 15: 10-25-01

14. Overview • TCP timeouts • Congestion sources and collapse • Congestion control basics • TCP congestion control • TCP fast recovery Lecture 15: 10-25-01

15. 10 Mbps 1.5 Mbps 100 Mbps Congestion • Different sources compete for resources inside network • Why is it 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) • In many situations will result in < 1.5 Mbps of throughput for the above topology (congestion collapse) Lecture 15: 10-25-01

16. Two senders, two receivers One router, infinite buffers No retransmission Large delays when congested Maximum achievable throughput Causes & Costs of Congestion: Scenario 1 Lecture 15: 10-25-01

17. One router, finite buffers Sender retransmission of lost packet Causes & Costs of Congestion: Scenario 2 Lecture 15: 10-25-01

18. Always: (goodput) “Perfect” retransmission only when loss: Retransmission of delayed (not lost) packet makes larger (than perfect case) for same l l l > = l l l in in in out out out Causes & Costs of Congestion: Scenario 2 “Costs” of congestion: • More work (retrans) for given “goodput” • Unneeded retransmissions: link carries multiple copies of pkt Lecture 15: 10-25-01

19. Four senders Multihop paths Timeout/retransmit l l in in Causes & Costs of Congestion: Scenario 3 Q:what happens as and increase ? Lecture 15: 10-25-01

20. Causes & Costs of Congestion: Scenario 3 Another “cost” of congestion: • When packet dropped, any “upstream transmission capacity used for that packet was wasted! Lecture 15: 10-25-01

21. Congestion Collapse • Definition: Increase in network load results in decrease of useful work done • Many possible causes • Spurious retransmissions of packets still in flight • Classical congestion collapse • How can this happen with packet conservation • Solution: better timers and TCP congestion control • Undelivered packets • Packets consume resources and are dropped elsewhere in network • Solution: congestion control for ALL traffic Lecture 15: 10-25-01

22. Other Congestion Collapse Causes • Fragments • Mismatch of transmission and retransmission units • Solutions • Make network drop all fragments of a packet (early packet discard in ATM) • Do path MTU discovery • Control traffic • Large percentage of traffic is for control • Headers, routing messages, DNS, etc. • Stale or unwanted packets • Packets that are delayed on long queues • “Push” data that is never used Lecture 15: 10-25-01

23. Congestion Control and Avoidance • A mechanism which: • Uses network resources efficiently • Preserves fair network resource allocation • Prevents or avoids collapse • Congestion collapse is not just a theory • Has been frequently observed in many networks Lecture 15: 10-25-01

24. End-end congestion control: No explicit feedback from network Congestion inferred from end-system observed loss, delay Approach taken by TCP Network-assisted congestion control: Routers provide feedback to end systems Single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) Explicit rate sender should send at Approaches Towards Congestion Control • Two broad approaches towards congestion control: Lecture 15: 10-25-01

25. Example: ATM Rate-based Flow Control • Provide explicit “per flow” feedback. • Host tells the network how fast it is sending • Switches calculate how fast the host should be sending and include this information in explicit flow control packets that are sent periodically • Feedback can be binary or explicit. • binary: source slows down or speeds up • explicit: switch specifies the rate 16 16 16 16 16 7 7 10 16 10 Lecture 15: 10-25-01

26. Example: TCP • Very simple mechanisms in network • FIFO scheduling with shared buffer pool • Feedback through packet drops • TCP interprets packet drops as signs of congestion and slows down • This is an assumption: packet drops are not a sign of congestion in all networks • E.g. wireless networks • Periodically probes the network to check whether more bandwidth has become available. Lecture 15: 10-25-01

27. Tradeoffs in Congestion Control • Explicit schemes that isolate traffic flows seem preferable, but require more support inside the networks • per-flow buffering, weighted fair queuing, policing • scalability??? • determining nature of the explicit feedback in heterogeneous environment • Diversity in networks makes TCP approach a good solution • Dropping packets is universally a natural response to congestion • But many open issues: how to isolate poorly behaved sources, diversity in TCP implementations, ... Lecture 15: 10-25-01

28. Overview • TCP timeouts • Congestion sources and collapse • Congestion control basics • TCP congestion control • TCP fast recovery Lecture 15: 10-25-01

29. Objectives • Simple router behavior • Distributedness • Efficiency: X = Sxi(t) • Fairness: (Sxi)2/n(Sxi2) • Convergence: control system must be stable Lecture 15: 10-25-01

30. Basic Control Model • Reduce speed when congestion is perceived • How is congestion signaled? • Either mark or drop packets • How much to reduce? • Increase speed otherwise • Probe for available bandwidth – how? Lecture 15: 10-25-01

31. Linear Control • Many different possibilities for reaction to congestion and probing • Examine simple linear controls • Window(t + 1) = a + b Window(t) • Different ai/bi for increase and ad/bd for decrease • Supports various reaction to signals • Increase/decrease additively • Increased/decrease multiplicatively • Which of the four combinations is optimal? Lecture 15: 10-25-01

32. Phase Plots • Simple way to visualize behavior of competing connections over time User 2’s Allocation x2 User 1’s Allocation x1 Lecture 15: 10-25-01

33. Phase Plots • What are desirable properties? • What if flows are not equal? Fairness Line Overload User 2’s Allocation x2 Optimal point Underutilization Efficiency Line User 1’s Allocation x1 Lecture 15: 10-25-01

34. Additive Increase/Decrease • Both X1 and X2 increase/ decrease by the same amount over time • Additive increase improves fairness and additive decrease reduces fairness Fairness Line T1 User 2’s Allocation x2 T0 Efficiency Line User 1’s Allocation x1 Lecture 15: 10-25-01

35. Muliplicative Increase/Decrease • Both X1 and X2 increase by the same factor over time • Extension from origin – constant fairness Fairness Line T1 User 2’s Allocation x2 T0 Efficiency Line User 1’s Allocation x1 Lecture 15: 10-25-01

36. Convergence to Efficiency Fairness Line xH User 2’s Allocation x2 Efficiency Line User 1’s Allocation x1 Lecture 15: 10-25-01

37. a=0 b=1 Fairness Line xH User 2’s Allocation x2 Efficiency Line User 1’s Allocation x1 Distributed Convergence to Efficiency Lecture 15: 10-25-01

38. Convergence to Fairness Fairness Line xH User 2’s Allocation x2 xH’ Efficiency Line User 1’s Allocation x1 Lecture 15: 10-25-01

39. Convergence to Efficiency & Fairness Fairness Line xH User 2’s Allocation x2 xH’ Efficiency Line User 1’s Allocation x1 Lecture 15: 10-25-01

40. Fairness Line x1 x0 User 2’s Allocation x2 x2 Efficiency Line User 1’s Allocation x1 What is the Right Choice? • Constraints limit us to AIMD • Can have multiplicative term in increase • AIMD moves towards optimal point Lecture 15: 10-25-01

41. Overview • TCP timeouts • Congestion sources and collapse • Congestion control basics • TCP congestion control • TCP fast recovery Lecture 15: 10-25-01

42. TCP Congestion Control • Motivated by ARPANET congestion collapse • Underlying design principle: packet conservation • At equilibrium, inject packet into network only when one is removed • Basis for stability of physical systems • Why was this not working? • Connection doesn’t reach equilibrium • Spurious retransmissions • Resource limitations prevent equilibrium Lecture 15: 10-25-01

43. TCP Congestion Control - Solutions • Reaching equilibrium • Slow start • Eliminates spurious retransmissions • Accurate RTO estimation • Fast retransmit • Adapting to resource availability • Congestion avoidance Lecture 15: 10-25-01

44. 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 15: 10-25-01

45. TCP Congestion Control Open Questions • How can this be implemented? • Operating system timers are very coarse – how do you accurately calculate the transmission rate? • How does TCP know what is a good initial rate to start with? • Should work both for a CDPD (10s of Kbs or less) and for supercomputer links (2.4 Gbs and growing) Lecture 15: 10-25-01

46. TCP Congestion ControlImplementation • Implemented using a congestion window that limits how much data can be in the network. • TCP also keeps track of how much data is in transit • Data can only be sent when the amount of outstanding data is less than the congestion window. • The amount of outstanding data is increased on a “send” and decreased on “ack” • (last sent – last acked) < congestion window • Window limited by both congestion and buffering • Sender’s maximum window = Min (advertised window, cwnd) Lecture 15: 10-25-01

47. TCP Packet Pacing • Congestion window helps to “pace” the transmission of data packets • In steady state, a packet is sent when an ack is received • Data transmission remains smooth, once it is smooth • Self-clocking behavior Pb Pr Sender Receiver As Ar Ab Lecture 15: 10-25-01

48. 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 15: 10-25-01

49. Congestion Avoidance Sequence Plot Sequence No Time Lecture 15: 10-25-01

50. Congestion Avoidance Behavior Congestion Window Time Cut Congestion Window and Rate Grabbing back Bandwidth Packet loss + Timeout Lecture 15: 10-25-01