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

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

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 algorithm2

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

• adhoc design based on 3 goals1. 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

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

• outline1. examples of performance issues

2. can these problems be fixed in practice?

4

Measurement-based study supported by theory

5

MPTCP CAN PENALIZE USERS

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

6

Scenario A: MPTCP can penalize TCP users

7bottleneck for Type 2

users is at access side

bottleneck for Type 1users is at server side

high speed connections

N1 C1

N1 x1

Scenario A: MPTCP can penalize TCP users

8bottleneck for Type 2

users is at access side

bottleneck for Type 1users is at server side

high speed connections

N1 C1

N1( x1+x2 )

x2

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

9

N1 C1x2

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

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

11

N1 C1x2

Scenario B: MPTCP can penalize other MPTCP users

12

bottleneck with capacity Cx

bottleneck with capacity CT

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

1315 blue and 15 red users, CX=27 and CT=36 Mbps

decrease is 3% using optimal algorithm with probing cost

CAN THE SUBOPTIMALITY OF MPTCP WITH LIA BE FIXED?

14

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

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

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

17

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/218

optimal load balancing

responsiveness

An illustrative example of OLIA’s behavior

19paths 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

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

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

21

N1 C1x2

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

2215 blue and 15 red users, CX=27 and CT=36 Mbps

MPTCP with OLIA

MPTCP with LIA

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

BACKUP SLIDES

24

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

25

wifi

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

26

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