1. 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)

2. 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

3. 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

4. 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?

5. Measurement-based study supported by theory

6. 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

7. 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

8. 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

9. Throughput of type 2 users reduced without any benefit for type 1 users
N1 C1 x2

10. 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

11. Part of problem is in nature of things, but MPTCP seems to be far from optimal
N1 C1 x2

12. Scenario B: MPTCP can penalize other MPTCP users
bottleneck with capacity Cx
bottleneck with capacity CT

13. 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

14. CAN THE SUBOPTIMALITY OF MPTCP WITH LIA BE FIXED?

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Scenario A: OLIA performs close to optimal algorithm with probing cost
N1 C1 x2

22. 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

23. 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

24. BACKUP SLIDES

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

26. 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