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Data Center Networking with Multipath TCP

Data Center Networking with Multipath TCP

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Data Center Networking with Multipath TCP

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  1. UCL Data Center Networking with Multipath TCP Costin Raiciu University College London & Universitatea Politehnica Bucuresti Christopher Pluntke, UCL Adam Greenhalgh, UCL Sebastien Barre, Universite Catholique Louvain Damon Wischik. UCL Mark Handley, UCL

  2. Data Center Networking Today Resource Allocation TCP Path Selection Random load balancing Routing OSPF, VLANs, TRILL Topology FatTree, VL2, BCube, multi-rooted tree

  3. Data Center Networking Tomorrow Resource Allocation TCP Multipath TCP Path Selection Random load balancing Routing OSPF, VLANs, TRILL Topology FatTree, VL2, BCube, multi-rooted tree

  4. Data Centers are Important Cloud computing • Economies of scale: networks of tens of thousands of hosts Cool apps • Web search, GFS, BigTable, DryadLINQ, MapReduce • Dense traffic patterns

  5. Flexibility is Important in Data Centers • Apps distributed across thousands of machines. • Flexibility: want any machine to be able to play any role. But: • Traditional data center topologies are tree based. • Don’t cope well with non-local traffic patterns. Many recent proposals for better topologies.

  6. Traditional Data Center Topology Core Switch 10Gbps Aggregation Switches 10Gbps Top of Rack Switches 1Gbps Racks of servers …

  7. Fat Tree Topology [Fares et al., 2008; Clos, 1953] K=4 Aggregation Switches 1Gbps K Pods with K Switches each 1Gbps Racks of servers

  8. VL2 Topology [Greenberg et al, 2009, Clos topology] 10Gbps … 10Gbps 20 hosts

  9. BCube Topology [Guo et al, 2009] BCube (4,1)

  10. How Do We Use this Capacity? • Need to distribute flows across paths. • Basic solution: Random Load Balancing. • Use Equal-Cost Multipath (ECMP) routing. • Hash to a path at random. • Use many differently rooted VLANs. • End-host hashes to a VLAN; determines path.

  11. Collisions 1Gbps 1Gbps Racks of servers

  12. Can MPTCP self-optimize data-center traffic? With Multipath TCP we can explore many paths: • Instead of using one random path, use many random paths • Don’t worry about collisions. • Just don’t send (much) traffic on colliding paths

  13. Simulation Setup • ~8000 hosts • Long-lived flows • Permutation traffic matrix • Each hosts sends and receives from a single other randomly chosen host • Smallest amount of traffic that can fill the network

  14. Multipath TCP in the Fat Tree Topology Throughput Allocation

  15. Performance depends on topology VL2 BCube

  16. Overloaded Fat Tree: better fairness with Multipath TCP

  17. Centralized Scheduling • With RLB, it’s really hard to utilize FatTree. • Hedera [Fares et al.,2010] uses a centralized scheduler and flow switching. • Start by using RLB • Measure all flow throughput periodically. • Any flow using more than 10% of its interface rate is explicitly scheduled onto an unloaded link. How does centralized scheduling compare with MPTCP?

  18. MPTCP vs Centralized Dynamic Scheduling Centralized Scheduling MPTCP Infinite Scheduling Interval

  19. Can’t we just use many TCP connections? Loss rate of MP-TCP (“linked”) vs multiple uncoupled TCP flows Retransmit timeouts with MPTCP (“linked”) vs uncoupled TCP flows

  20. MPTCP Linked Increases in DCs • Better fairness and less aggressive than uncoupled TCP • Improves throughput in dense traffic in BCube (25%)

  21. The bigger picture Resource Allocation Path Selection Multipath TCP Routing OSPF, VLANs, etc. Topology ? FatTree, VL2, Bcube, multi-rooted tree

  22. Multipath TCP can utilize topologies TCP can’t • Requirement: a subset of hosts should be able to communicate at 10Gb/s 1Gb/s 1Gb/s 10Gb/s

  23. Multipath TCP can utilize topologies TCP can’t [2] • Problem • ToR switch failures wipe out tens of hosts • Repair time is on the order of days • Solution: use two ToRs/rack, multi-home servers • Single path TCP • Single flows still get same max throughput • Which interface do I use? • With Multipath TCP • Flows double their maximum throughput • Path selection automatic

  24. Summary • Data center networking offers many paths between end-hosts. Yet: • Random Load Balancing does a poor job of utilizing them • Centralized scheduling is laggy and has inherently limited knowledge • Multipath TCP naturally optimizes data center networks: • Improves throughput • Improves fairness • More robust than centralized scheduling • Question: what topologies does multipath TCP enable?

  25. Backup Slides

  26. Centralized Scheduling: Setting the Threshold Throughput Hope 1Gbps 17% worse than multipath TCP 100Mbps App Limited

  27. Centralized Scheduling: Setting the Threshold Throughput 1Gbps 21% worse than multipath TCP 100Mbps App Limited Hope

  28. Centralized Scheduling: Setting the Threshold Throughput 1Gbps 51% 100Mbps 500Mbps 17% 45% 21%