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Understanding TCP Dynamics in an Integrated Services Internet

Understanding TCP Dynamics in an Integrated Services Internet. Wu-chang Feng, Dilip Kandlur, Debanjan Saha, and Kang Shin. Motivation. Many TCP-based applications can take advantage of guarantees in the network Majority of these applications don’t require strict delay bound guarantees

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Understanding TCP Dynamics in an Integrated Services Internet

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  1. Understanding TCP Dynamics in an Integrated Services Internet Wu-chang Feng, Dilip Kandlur, Debanjan Saha, and Kang Shin NOSSDAV '97

  2. Motivation • Many TCP-based applications can take advantage of guarantees in the network • Majority of these applications don’t require strict delay bound guarantees • Examples • Non-interactive audio and video • Data streaming applications • Elastic applications (ftp, http) NOSSDAV '97

  3. Controlled-load Service • IETF defined service which provides more flexible guarantees to applications than Guaranteed Service • Application provides TSpec • Compliant traffic receives service similar to that in an “unloaded” network • Non-compliant traffic is treated as best-effort NOSSDAV '97

  4. Question Can TCP-based applications take advantage of a network which provides controlled-load service? NOSSDAV '97

  5. System Model • Source • Compliance check done at the source using a token bucket filter derived from TSpec • Compliant packets sent marked • Non-compliant packets sent unmarked • Network • Enhanced Random Early Detection (ERED) NOSSDAV '97

  6. Source Model Sending source TCP Send Compliance Check Network NOSSDAV '97

  7. Network support • RED queues • Random early packet dropping for congestion avoidance • Keep queue lengths small • Avoid synchronization • Remove biases against bursty traffic NOSSDAV '97

  8. Enhanced RED queues (ERED) • Same as RED, but marked packets have a much lower drop probability than unmarked packets • Single queue implementation • Retains FIFO ordering • Does not require per-flow information in the data forwarding path NOSSDAV '97

  9. Example scenario • Reserved connections should get reserved rate and a share of the excess bandwidth • 3 sources with 1Mbs, 2Mbs and 4Mbs policed with token buckets of depth 50ms • 3 best-effort sources • 80KB ERED queues at each router • Simulated using ns-1.1 S D 10Mb 10Mb 10Mb 10Mb 45Mb NOSSDAV '97

  10. TCP with reservations NOSSDAV '97

  11. Problem • TCP uses acknowledgement based triggers to send data • Well-known problem of ACK compression which can cause gaps in ACK stream • Transmission credits build up in token bucket as TCP waits for an ACK • Credits overflow and are lost NOSSDAV '97

  12. TCP losing tokens S D NOSSDAV '97

  13. TCP timer modification After every acknowledgement if (room under cwnd and awnd) if (tokens available > packet size) send packet marked else send packet unmarked After every timer expiry reset timer if (room under awnd) if (tokens available > packet size) send packet marked NOSSDAV '97

  14. TCP timer modification NOSSDAV '97

  15. TCP timer modification NOSSDAV '97

  16. Rate-adaptive windowing Normal Windowing Window Size Time Rate Adaptive Windowing Window Size Time NOSSDAV '97

  17. TCP windowing modification After every new acknowledgement if (cwnd < ssthresh) cwnd = cwnd + (cwnd-rwnd)/cwnd elsecwnd = cwnd + 1/cwnd Upon detection of loss from DUPACKs cwnd = rwnd + (cwnd-rwnd)/2 + ndup ssthresh = rwnd + (cwnd-rwnd)/2 Upon RTO cwnd = rwnd + 1 ssthresh = rwnd + (cwnd-rwnd)/2 NOSSDAV '97

  18. TCP w/ timer and window mods NOSSDAV '97

  19. Additional Experiments • Performance when a subset or when no network routers support service differentiation • Integration into a more elaborate packet scheduling and/or link scheduling experiments • Influence on pricing • Reservations vs. adaptation NOSSDAV '97

  20. Summary • TCP’s ack-clocking and windowing algorithm limit its performance in an integrated services environment • Fine-grained timer and rate-adaptive windowing can solve this problem • Extended version and simulation results at http://www.eecs.umich.edu/~wuchang/ered/ TCP Brooklyn? NOSSDAV '97 (We don’t play chess all day)

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