Congestion control for Multipath TCP (MPTCP)

1. THE ROYAL SOCIETY Congestion control for Multipath TCP (MPTCP) Damon Wischik CostinRaiciu Adam Greenhalgh Mark Handley

2. Packet switching ‘pools’ circuits. • Multipath ‘pools’ links : it is Packet Switching 2.0. • TCP controls how a link is shared. • How should a pool be shared? What is TCP 2.0? Two circuits A link A poolof links Two separate links

3. In a BCube data center, can we use multipath to get higher throughput? Initially, there is one flow.

4. In a BCube data center, can we use multipath to get higher throughput? Initially, there is one flow. A new flow starts. Its direct route collides with the first flow.

5. In a BCube data center, can we use multipath to get higher throughput? Initially, there is one flow. A new flow starts. Its direct route collides with the first flow. But it also has longer routes available, which don’t collide.

6. How can a wireless device use two channels simultaneously, without hurting other users of the network?How should it balance its traffic, when the channels have very different characteristics? wifi path: high loss, small RTT 3G path: low loss, high RTT

7. What were our design goals for MPTCP congestion control? I will propose design goals for Multipath TCP, and illustrate them in simple scenarios. I will show experimental results for our MPTCP algorithm in these simple scenarios.

8. What is the MPTCP protocol?MPTCP is a replacement for TCP which lets you use multiple paths simultaneously. user space socket API MPTCP The receiver puts the packets in the correct order The sender stripes packets across paths MPTCP TCP IP addr addr1 addr2

9. Design goal 1:Multipath TCP should be fair to regular TCP at shared bottlenecks • To be fair, Multipath TCP should take as much capacity as TCP at a bottleneck link, no matter how many paths it is using. A multipath TCP flow with two subflows Regular TCP Strawman solution:Run “½ TCP” on each path

10. Design goal 2:MPTCP should use efficient paths • Each flow has a choice of a 1-hop and a 2-hop path. • How should split its traffic? 12Mb/s 12Mb/s 12Mb/s

11. Design goal 2:MPTCP should use efficient paths • If each flow split its traffic 1:1 ... 12Mb/s 8Mb/s 12Mb/s 8Mb/s 8Mb/s 12Mb/s

12. Design goal 2:MPTCP should use efficient paths • If each flow split its traffic 2:1 ... 12Mb/s 9Mb/s 12Mb/s 9Mb/s 9Mb/s 12Mb/s

13. Design goal 2:MPTCP should use efficient paths • If each flow split its traffic 4:1 ... 12Mb/s 10Mb/s 12Mb/s 10Mb/s 10Mb/s 12Mb/s

14. Design goal 2:MPTCP should use efficient paths • If each flow split its traffic ∞:1 ... 12Mb/s 12Mb/s 12Mb/s 12Mb/s 12Mb/s 12Mb/s

15. Design goal 2:MPTCP should use efficient paths 12Mb/s 12Mb/s 12Mb/s 12Mb/s 12Mb/s 12Mb/s • Theoretical solution(Kelly+Voice 2005; Han, Towsley et al. 2006) MPTCP should send all its traffic on its least-congested paths. Theorem. This will lead to the most efficient allocation possible, given a network topology and a set of available paths.

16. MPTCP chooses efficient paths in a BCube data center, hence it gets high throughput. MPTCP shifts its traffic away from the congested link. Initially, there is one flow. A new flow starts. Its direct route collides with the first flow. But it also has longer routes available, which don’t collide.

17. MPTCP chooses efficient paths in a BCube data center, hence it gets high throughput. throughput [Mb/s] We ran packet-level simulations of BCube (125 hosts, 25 switches, 100Mb/s links) and measured average throughput, for three traffic matrices. For two of the traffic matrices, MPTCP and ½ TCP (strawman) were as good. For one of the traffic matrices, MPTCP got 19% higher throughput. perm. traffic matrix sparse traffic matrix local traffic matrix

18. Design goal 3:MPTCP should be fair compared to TCP • Design Goal 2 says to send all your traffic on the least congested path, in this case 3G. But this has high RTT, hence it will give low throughput. wifi path: high loss, small RTT c d 3G path: low loss, high RTT Goal 3a.A Multipath TCP user should get at least as much throughput as a single-path TCP would on the best of the available paths. Goal 3b. A Multipath TCP flow should take no more capacity on any link than a single-path TCP would.

19. MPTCP gives fair throughput. wifi through-put [Mb/s] User in his office, using wifi and 3G time [min] 3G through-put [Mb/s] In the kitchen Going downstairs

20. MPTCP gives fair throughput. 3G has lower loss rate. Design Goal 2 says to shift traffic onto 3G ... MPTCP sends 0.4Mb/s over 3G, and the remaining 1.8Mb/s over wifi. wifi through-put [Mb/s] But, today, TCP over 3G was only getting 0.4Mb/s, so don’t take more than that … time [min] But, today, TCP over wifi was getting 2.2Mb/s, so user is entitled to this much… 3G through-put [Mb/s]

21. MPTCP gives fair throughput. We measured throughput , for both ½ TCP(strawman)and MPTCP, in the office. ½ TCP is unfair to the user, and its throughput is 25% worse than MPTCP. wifi through-put [Mb/s] time [min] 3G through-put [Mb/s]

22. Design goals • Goal 1. Be fair to TCP at bottleneck links • Goal 2. Use efficient paths ... • Goal 3. as much as we can, while being fair to TCP • Goal 4. Adapt quickly when congestion changes • Goal 5. Don’t oscillate redundant How does MPTCP achieve all this?

23. How does TCP congestion control work? • Maintain a congestion window w. • Increase w for each ACK, by 1/w • Decrease w for each drop, by w/2

24. How does MPTCP congestion control work? • Maintain a congestion window wr, one window for each path, where r ∊ R ranges over the set of available paths. • Increase wr for each ACK on path r, by • Decrease wr for each drop on path r, by wr/2

25. How does MPTCP congestion control work? • Maintain a congestion window wr, one window for each path, where r ∊ R ranges over the set of available paths. • Increase wr for each ACK on path r, by • Decrease wr for each drop on path r, by wr/2 Design goal 3: At any potential bottleneck S that path r might be in, look at the best that a single-path TCP could get, and compare to what I’m getting.

26. How does MPTCP congestion control work? • Maintain a congestion window wr, one window for each path, where r ∊ R ranges over the set of available paths. • Increase wr for each ACK on path r, by • Decrease wr for each drop on path r, by wr/2 Design goal 2: We want to shift traffic away from congestion. To achieve this, we increase windows in proportion to their size.

27. MPTCP is a control plane for multipath transport. What problem is it trying to solve?

28. At a multihomed web server, MPTCP tries to share the ‘pooled access capacity’ fairly. 2 TCPs @ 50Mb/s 100Mb/s 100Mb/s 4 TCPs@ 25Mb/s

29. At a multihomed web server, MPTCP tries to share the ‘pooled access capacity’ fairly. 2 TCPs @ 33Mb/s 100Mb/s 1 MPTCP @ 33Mb/s 100Mb/s 4 TCPs@ 25Mb/s

30. At a multihomed web server, MPTCP tries to share the ‘pooled access capacity’ fairly. The total capacity, 200Mb/s, is shared out evenly between all 8 flows. 2 TCPs @ 25Mb/s 100Mb/s 2 MPTCPs @ 25Mb/s 100Mb/s 4 TCPs@ 25Mb/s

31. At a multihomed web server, MPTCP tries to share the ‘pooled access capacity’ fairly. 2 TCPs @ 22Mb/s 100Mb/s 3 MPTCPs @ 22Mb/s 100Mb/s 4 TCPs@ 22Mb/s The total capacity, 200Mb/s, is shared out evenly between all 9 flows. It’s as if they were all sharing a single 200Mb/s link. The two links can be said to form a 200Mb/s pool.

32. At a multihomed web server, MPTCP tries to share the ‘pooled access capacity’ fairly. 2 TCPs @ 20Mb/s 100Mb/s 4 MPTCPs @ 20Mb/s 100Mb/s 4 TCPs@ 20Mb/s The total capacity, 200Mb/s, is shared out evenly between all 10 flows. It’s as if they were all sharing a single 200Mb/s link. The two links can be said to form a 200Mb/s pool.

33. At a multihomed web server, MPTCP tries to share the ‘pooled access capacity’ fairly. 5 TCPs 100Mb/s First 0, then 10 MPTCPs 100Mb/s throughput per flow [Mb/s] 15 TCPs We confirmed in experiments that MPTCP nearly manages to pool the capacity of the two access links. Setup: two 100Mb/s access links, 10ms delay, first 20 flows, then 30. time [min]

34. At a multihomed web server, MPTCP tries to share the ‘pooled access capacity’ fairly. 5 TCPs 100Mb/s First 0, then 10 MPTCPs 100Mb/s 15 TCPs MPTCP makes a collection of links behave like a single large pool of capacity — i.e. if the total capacity is C, and there are n flows, each flow gets throughput C/n.

35. Open question:Can we make a data center behave like a simple ‘capacity pool’? What topologies and path choices would make it behave like a simple capacity pool? What is the pooled capacity?

36. Further questions How much of the Internet can be pooled? What are the implications for network operators? How should we fit multipath congestion control to CompoundTCP or CubicTCP? Is it worth using multipath for small flows?

37. Conclusion • Multipath is Packet Switching 2.0It lets you share capacity between links. • MPTCP is TCP 2.0It is a control plane to harness the flexibility of multipath.It is traffic engineering, done by end-systems, and it works in milliseconds. • We formulated design goals and test scenarios for how multipath congestion control should behave. • We designed, implemented and evaluated MPTCP, a TCP-like congestion control algorithm that achieves these goals.

38. Related work on multipath congestion controlpTCP , CMT over SCTP, and M/TCP that meets goal 1 (fairness at shared bottleneck)mTCP, ≈ R-MTP and goal 2 (choosing efficient paths)Honda et al. (2009), ≈ Tsao and Sivakumar (2009) and goal 5 (non-oscillation)Kelly and Voice (2005), Han et al. (2006) and goal 3 (fairness)and goal 4 (rapid adjustment)(none)