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MPTCP is not Pareto-Optimal: Performance Issues and a Possible Solution Ramin Khalili (T-Labs/TUB) Nicolas Gast (LCA2-EPFL) Miroslav Popovic (LCA2-EPFL) Utkarsh Upadhyay (LCA2-EPFL) Jean-Yves Le Boudec (LCA2-EPFL) 1

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MPTCP: Multi-path Transport Solution of IETF allows a user to split its traffic across multiple paths; improve reliability and throughput challenge: design of congestion control algorithm 2

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LIA (RFC 6356): "Linked-Increases Algorithm" adhoc design based on 3 goals 1.improve throughput: total throughput TCP over best path 2.do not harm: not more aggressive than a TCP over a path 3.balance congestion while meeting the first two goals as also stated in RFC 6356, LIA does not fully satisfy goal 3 3

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We identified problems with the current MPTCP implementation P1: MPTCP can penalize users P2: MPTCP users are excessively aggressive toward TCP users P1 and P2 are attributed to LIA outline 1.examples of performance issues 2.can these problems be fixed in practice? 4

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Measurement-based study supported by theory 5

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MPTCP CAN PENALIZE USERS upgrading some TCP users to MPTCP can reduce the throughput of others without any benefit to the upgraded users 6

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Scenario A: MPTCP can penalize TCP users 7 bottleneck for Type 2 users is at access side bottleneck for Type 1 users is at server side high speed connections N 1 C 1 N 1 x 1

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Scenario A: MPTCP can penalize TCP users 8 bottleneck for Type 2 users is at access side bottleneck for Type 1 users is at server side high speed connections N 1 C 1 N 1 ( x 1 +x 2 ) x2x2

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Throughput of type 2 users reduced without any benefit for type 1 users 9 N 1 C 1 x2x2

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We compare MPTCP with a theoretical baselines optimal algorithm with probing cost: theoretical optimal load balancing including minimal probing traffic – using a windows-based algorithm, a minimum probing traffic of 1 MSS/RTT is sent over each path 10

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Part of problem is in nature of things, but MPTCP seems to be far from optimal 11 N 1 C 1 x2x2

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Scenario B: MPTCP can penalize other MPTCP users 12 bottleneck with capacity C x bottleneck with capacity C T

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By upgrading red users to MPTCP, the throughput of everybody decreases blue and 15 red users, C X =27 and C T =36 Mbps decrease is 3% using optimal algorithm with probing cost

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CAN THE SUBOPTIMALITY OF MPTCP WITH LIA BE FIXED? 14

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LIAs design forces tradeoff between responsiveness and load balancing provide load balancingbe responsive optimal load balacing but not responsive responsive but bad load balancing LIAs implementation (RFC 6356) ε=0 ε=1 ε=2 ε is a design parameter

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OLIA: an algorithm inspired by utility maximization framework simultaneously provides responsiveness and load balancing an adjustment of optimal algorithm – by adapting windows increases as a function of quality of paths, we make it responsive and non-flappy implemented on the MPTCP Linux kernel 16

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Definition: set of best paths and paths with maximum windows for a user u, with R u as set of paths, define – set of best paths: – set of path with maximum windows: : number of successful transmissions between losses on path r rtt r (t): RTT on path r 17

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OLIA: "Opportunistic Linked-Increases Algorithm" For a user u, on each path r in R u : increase part: for each ACK on r, increase w r by α r = decrease part: each loss on r, decreases w r by w r /2 18 optimal load balancing responsiveness

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An illustrative example of OLIAs behavior 19 paths have similar quality, OLIA uses both (non-flappy and responsive) second path is congested, OLIA uses only the first one MPTCP with LIA MPTCP with OLIA MPTCP with LIA MPTCP with OLIA

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Theoretical results: OLIA solves problems P1 and P2 using a fluid model of OLIA Theorem: OLIA satisfies design goals of LIA (RFC 6356) Theorem: OLIA is Pareto optimal Theorem: when all paths of a user have similar RTTs, OLIA provides optimal load balancing 20

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Scenario A: OLIA performs close to optimal algorithm with probing cost 21 N 1 C 1 x2x2

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Scenario B: using OLIA, we observe 3.5% drop in aggregate throughput blue and 15 red users, C X =27 and C T =36 Mbps MPTCP with OLIA MPTCP with LIA

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Summary MPTCP with LIA suffers from important performance problems these problems can be mitigated in practice – OLIA: inspired by utility maximization framework suggestion: congestion control part of MPTCP should be revisited by the IETF committees 23

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BACKUP SLIDES 24

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Scenario C: MPTCP users could be excessively aggressive towards regular TCP users (P2) 25 wifi

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Scenario C: OLIA achieves much better fairness (solving P2) 26 when C 1 /C 2 1, for any theoretical fairness criterion: (x1+x2)/C 1 =1 and y/C 2 =1

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