Transcript OMPTCP: an opportunistic Multipath TCP algorithm

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
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
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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?
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Measurement-based study supported by theory
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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
high speed connections
bottleneck for Type 1
users is at server side
N1 C1
N1 x1
bottleneck for Type 2
users is at access side
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Scenario A: MPTCP can penalize TCP users
high speed connections
bottleneck for Type 1
users is at server side
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
x2
N 1 C1
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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
x2
N 1 C1
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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
LIA’s implementation
(RFC 6356)
responsive but
bad load balancing
ε=0
ε=1
ε=2
ε is a design parameter
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
optimal load
balancing
responsiveness
αr=
•decrease part: each loss on r, decreases wr by wr/2
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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
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only the first one
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
x2
N1 C1
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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
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y/C2=1