![RFC 6298
(2.4) Whenever RTO is computed, if it is less than 1
second, then the RTO SHOULD be rounded up to 1 second.
- in practice, often 200ms
RFC 2581
The delayed ACK algorithm specified in [Bra89] SHOULD be
used by a TCP receiver. When used, a TCP receiver MUST NOT
excessively delay acknowledgments. Specifically, an ACK SHOULD
be generated for at least every second full-sized segment, and
MUST be generated within 500 ms of the arrival of the first
unacknowledged packet.
- in practice, often 40ms](https://image.slidesharecdn.com/computernetwork5-140901161554-phpapp02/85/Computer-network-5-21-320.jpg)
Computer network (5)
- 1. Data Center Networking Stanford CS144 Lecture 17 Philip Levis, 11/30/11
- 3. Low latencies: μs High capacity: GigE, 10 GigE Specialized traffic Centrally managed
- 4. Topology (picture courtesy of Al-Fares et al, “A Scalable, Commodity Data Center Network Architecture”)
- 5. Storage Workload (picture courtesy of Phanishayee et al, “Measurement and Analysis of TCP Throughput Collapse in Cluster-based Storage Systems”)
- 6. Query Workload (picture courtesy of Alizadeh et al., “Data Center TCP (DCTCP)”)
- 7. Problems
- 8. Per-Pair Bandwidth (picture courtesy of Al-Fares et al, “A Scalable, Commodity Data Center Network Architecture”)
- 9. Incast (from Phanishayee et al, “Measurement and Analysis of TCP Throughput Collapse in Cluster-based Storage Systems”)
- 10. Incast Details (from Phanishayee et al, “Measurement and Analysis of TCP Throughput Collapse in Cluster-based Storage Systems”)
- 11. Mixed traffic • Low latency for short flows • High burst tolerance (incast) • High throughput for long flows
- 12. Recent Research • New switching topology: Al-Fares et al. • Fix TCP incast: Vasudevan et al. • Data Center TCP: Alizadeh et al.
- 13. Per-Pair Bandwidth (picture courtesy of Al-Fares et al, “A Scalable, Commodity Data Center Network Architecture”)
- 14. Fat Tree
- 15. Fat Tree (k/2)2 k/2 k/2 k
- 16. Switching Prefix Port 10.2.0.0/24 0 10.2.1.0/24 1 0.0.0.0/0 Suffix Port 0.0.0.2/8 2 0.0.0.3/8 3 TCAM 10.2.0.X 10.2.1.X X.X.X.2 X.X.X.3 Encoder Prefix Next Hop Port 00 10.2.0.1 0 01 10.2.1.1 1 10 10.4.1.1 2 11 10.4.1.2 3
- 17. Not Perfect (k/2)2 k/2 k/2 k
- 18. Fat-Tree Status
- 19. Incast • RTO = SRTT + (4 X RTTVAR)
- 20. Behavior (from Phanishayee et al, “Measurement and Analysis of TCP Throughput Collapse in Cluster-based Storage Systems”)
- 21. RFC 6298 (2.4) Whenever RTO is computed, if it is less than 1 second, then the RTO SHOULD be rounded up to 1 second. - in practice, often 200ms RFC 2581 The delayed ACK algorithm specified in [Bra89] SHOULD be used by a TCP receiver. When used, a TCP receiver MUST NOT excessively delay acknowledgments. Specifically, an ACK SHOULD be generated for at least every second full-sized segment, and MUST be generated within 500 ms of the arrival of the first unacknowledged packet. - in practice, often 40ms
- 22. Solutions • Proposal 1: Adjust RTO (Vasudevan et al.) • Proposal 2: DCTCP (Alizadeh et al.)
- 23. RTT
- 24. RTT 2
- 25. RTO • Make RTOmin 200μs • Timeout = (RTO + (rand(0.5) x RTO))
- 26. Improvement
- 27. Wide Area
- 28. DCTCP • Three goals • Low latency for short flows • High burst tolerance (incast) • High throughput for long flows • Basic approach: keep switch queues short
- 29. Queue Length • RTT measurements are noisy • At high speeds, very small • GigE: 10 packets is 120μs • 10GigE: 10 paciets is 12μs • Use ECN (explicit congestion notification) • RFC 3168
- 30. Setting ECN K Set ECN bit
- 31. Monitoring α • Per RTT, measure F, the fraction of packets sent that had the ECN bit set • DCTCP acks copy the ECN bit of the corresponding data packets into ECN-Echo field • Compute α, EWMA of F
- 32. Adjusting cwnd • cwnd = cwnd x (1 - α/2)
- 33. DCTCP Caveat “We stress that DCTCP is designed for the data center environment. In this paper, we make no claims about suitability of DCTCP for wide area networks.”
- 34. Data Center Networks • Very different than wide area Internet • Tiny RTTs • Different traffic patterns • Single administrative domain • Standards (e.g., IETF) much less important • A lot of very novel network design