MPTCP is not Pareto-Optimal: Performance Issues and a Possible Solution

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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 currentMPTCP
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

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Scenario A MPTCP can penalize TCP users
bottleneck for Type 1users 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 1users is at server side
high speed connections
N1 C1
x2
N1( x1x2 )
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
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We compare MPTCP with a theoretical baselines

• optimal algorithm with probing costtheoretical
  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
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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, CX27 and CT36 Mbps
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CAN THE SUBOPTIMALITY OF MPTCP WITH LIA BE FIXED?
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LIAs design forces tradeoff between
responsiveness and load balancing
provide load balancing
be responsive
e 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
• ar
• decrease part each loss on r, decreases wr by
  wr/2

optimal load balancing
responsiveness
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An illustrative example of OLIAs 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
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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, CX27 and CT36 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)
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Scenario C OLIA achieves much better fairness
(solving P2)
when C1/C21, for any theoretical fairness
criterion (x1x2)/C11 and y/C21