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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)

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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

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**LIA (RFC 6356): "Linked-Increases Algorithm"**

adhoc design based on 3 goals improve throughput: total throughput ≥ TCP over best path do not harm: not more aggressive than a TCP over a path balance congestion while meeting the first two goals as also stated in RFC 6356, LIA does not fully satisfy goal 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 examples of performance issues can these problems be fixed in practice?

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**Measurement-based study supported by theory**

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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 MPTCP CAN PENALIZE USERS

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**Scenario A: MPTCP can penalize TCP users**

bottleneck for Type 1 users is at server side high speed connections N1 C1 N1 x1 bottleneck for Type 2 users is at access side

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**Scenario A: MPTCP can penalize TCP users**

bottleneck for Type 1 users is at server side high speed connections N1 C1 x2 N1( x1+x2 ) bottleneck for Type 2 users is at access side

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**Throughput of type 2 users reduced without any benefit for type 1 users**

N1 C1 x2

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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

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**Part of problem is in nature of things, but MPTCP seems to be far from optimal**

N1 C1 x2

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**Scenario B: MPTCP can penalize other MPTCP users**

bottleneck with capacity Cx bottleneck with capacity CT

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**By upgrading red users to MPTCP, the throughput of everybody decreases**

decrease is 3% using optimal algorithm with probing cost 15 blue and 15 red users, CX=27 and CT=36 Mbps

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**CAN THE SUBOPTIMALITY OF MPTCP WITH LIA BE FIXED?**

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**LIA’s design forces tradeoff between responsiveness and load balancing**

provide load balancing be responsive optimal load balacing but not responsive responsive but bad load balancing LIA’s 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

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**Definition: set of best paths and paths with maximum windows**

for a user u, with Ru as set of paths, define set of best paths: set of path with maximum windows: : number of successful transmissions between losses on path r rttr(t): RTT on path r

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**OLIA: "Opportunistic Linked-Increases Algorithm"**

For a user u, on each path r in Ru: increase part: for each ACK on r, increase wr by αr= decrease part: each loss on r, decreases wr by wr/2 optimal load balancing responsiveness

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**An illustrative example of OLIA’s behavior**

MPTCP with OLIA MPTCP with OLIA MPTCP with LIA MPTCP with LIA paths have similar quality, OLIA uses both (non-flappy and responsive) second path is congested, OLIA uses only the first one

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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

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**Scenario A: OLIA performs close to optimal algorithm with probing cost**

N1 C1 x2

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**Scenario B: using OLIA, we observe 3.5% drop in aggregate throughput**

MPTCP with OLIA MPTCP with LIA 15 blue and 15 red users, CX=27 and CT=36 Mbps

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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

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BACKUP SLIDES

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

wifi

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**Scenario C: OLIA achieves much better fairness (solving P2)**

when C1/C2≥1, for any theoretical fairness criterion: (x1+x2)/C1=1 and y/C2=1

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