Multipath routing and congestion control

Transcription

1 Multipath outing and congestion contol Fank Kelly Univesity of Cambidge (with wok of Damon Wischik, Mak Handley and Costin Raiciu, Univesity College London) Chinese Univesity of Hong Kong (Institute of Netwok Coding) 2 Mach 2012

2 Outline Congestion contol - TCP Diffeential equation models, stability Multipath outing fom theoy to pactice UCL s woking implementation

3 End-to-end congestion contol sendes eceives Sendes lean (though feedback fom eceives) of congestion at queue, and slow down o speed up accodingly. With cuent TCP, thoughput of a flow is popotional to 1/( T p) T = ound-tip time, p = packet dop pobability. (Jacobson 1988, Mathis, Semke, Mahdavi, Ott 1997, Padhye, Fioiu, Towsley, Kuose 1998, Floyd and Fall 1999)

4 Souce: CAIDA, Young Hyun

5 Souce: CAIDA - Young Hyun, Badley Huffake (displayed at MOMA)

6 Netwok stuctue oute esouce J R j x ( t) µ ( t) j - set of esouces - set of outes - esouce j is on oute - flow ate on oute at time t - ate of congestion indication, at esouce j at time t

7 TCP (Jacobson s congestion avoidance algoithm) Souce maintains a window of sent, but not yet acknowledged, packets - size cwnd cwnd xt cwnd incemented by 1/cwnd on positive ack cwnd decemented by cwnd/2 on congestion change in ate x pe unit time is about 1 cwnd (1 p) p / T cwnd 2 1 = T / cwnd T 2 p 2 x p 2

8 Diffeential equations fo TCP ( ) = = = ) ( ) ( ) ( 1 1 ) ( 2 ) ( ) ( ) ( 1 ) ( : 2 2 t x p t t t t t x T t t x dt d s j s s j j j j µ µ λ λ λ

9 Equilibium point R T x = 2 1/ λ λ check: the equilibium of the dynamical system ecoves the invese squae oot fomula fo TCP note again the ound-tip time bias

10 Moe geneal algoithm On oute, cwnd incemented by a cwnd n on positive acknowledgement cwnd decemented by b cwnd m fo each congestion indication (m>n) a = 1, b = 1/2, m=1, n= -1 coesponds to Jacobson s TCP

11 Diffeential equations with delays ( ) ( ) = = = ) ( ) ( ) ( 1 1 ) ( ) ( ) ) ( ( )) ( (1 ) ) ( (. ) ( ) ( : sj s j s s j j j j j m n T t x p t T t t t T t x b t T t x a T T t x t x dt d µ µ λ λ λ j j T T T = + j

12 Equilibium point R b a T x n m = 1/ 1 1 λ λ choice of a, b can emove ound tip time bias but what is the impact of delays on stability?

13 Delays and stability sendes eceives Might the inevitable delay associated with the ound-tip time induce instability?

14 Theshold = 5 flows: tt = time

15 Theshold = 10 flows: tt = time

16 Theshold = 20 flows: tt = time

17 Delay stability Johai and Tan (1999), Massoulié (2000), Vinnicombe (2000): Equilibium is locally stable if thee exists a global constant β such that xpʹ j ( x) < β p ( x), a ( x T ) < j n π 2 β condition on sensitivity fo each esouce j condition on aggessiveness fo each oute

18 Consequences? Delay stability condition: n a ( cwnd ) < π 2β n = -1 (Jacobson s TCP) instability if congestion windows ae too small, sluggishness if congestion windows ae too big n = 0 (Scalable TCP, Glenn Vinnicombe, Tom Kelly) condition independent of size of congestion window, and choice of b can emove ound-tip time bias

19 destination Routing? Can we extend algoithms to allow dynamic outing? Dange: oute flap. souce oute esouce

20 Notation J R j S s - set of esouces - set of outes - esouce j is on oute - set of souce-destination pais - oute seves s-d pai s y s ( t) = s x ( t T ) - ate of etuning acknowledgements fo s-d pai s at time t

21 Combined ate contol and outing algoithm On oute x (t) inceased by a / T acknowledgement on positive x (t) deceased by b y s() (t) / T fo each congestion indication s = {} coesponds to scalable TCP

22 Delay stability Kelly and Voice, 2005 Han, Shakkottai, Hollot, Sikant and Towsley, 2006 Equilibium is locally stable if thee exists a global constant β such that x pʹ j ( x) < β p ( x), a (1 + β ) < j π 2 condition on sensitivity fo each esouce j condition on aggessiveness of souces

23 Delay stability xpʹ ( x) j < β p ( x), a (1 + β ) < j Kelly and Voice, 2005 Han, Shakkottai, Hollot, Sikant and Towsley, 2006 Equilibium is locally stable if thee exists a global constant β such that impact of outing π 2 condition on sensitivity fo each esouce j condition on aggessiveness of souces

24 Conclusion? It may be possible to achieve stable, scalable load shaing acoss paths, based on end-to end measuements, on the same time-scale as ate contol Fo dynamic outing, the key constaint on the esponsiveness of each oute is the ound-tip time of that oute, infomation which is natually available at souces

25 Mul$path TCP Mak Handley Cos/n Raiciu Damon Wischik UCL - fom h<p://

26 We have a woking implementa/on of mul/path tanspot laptop with multipath TCP wifi 3G seve with multipath TCP

27 We have a woking implementa/on of mul/path tanspot wifi laptop with multipath TCP 3G seve with multipath TCP wifi thoughput [Mb/s] 3G thoughput [Mb/s] time [min]

28 This use clealy benefits fom mul/path. But is it safe fo the netwok and othe uses? O does it cause instability, oute flap, unfainess, disaste?

29 I. Resouce pooling as a design pinciple Resouce pooling means making a collec/on of esouces behave like a single pooled esouce. It has been a design goal of the Intenet fom the beginning. A single link, split into two cicuits Packet switching pools the two cicuits Multipath pools the two links

30 Resouce pooling means the netwok is be<e able to accommodate a suge in taffic by shiping taffic and theeby diffusing conges/on acoss the netwok. o a loss of capacity

31 The Intenet aleady has esouce pooling, in the fom of mul/- homing, BGP, etc.

32 The Intenet aleady has esouce pooling, in the fom of mul/- homing, BGP, etc.

33 The Intenet aleady has esouce pooling, in the fom of mul/- homing, BGP, etc.

34 The Intenet aleady has esouce pooling, in the fom of mul/- homing, BGP, etc. We think esouce pooling should be achieved by end- system mul/path. This would haness the apid esponsiveness of end systems.

35 Thee is a lage body of wok on fluid models of conges/on contol: wite down a netwok u/lity maximiza/on poblem, wite down a system of diffeen/al equa/ons, show that the (unique) fixed point solves the u/lity maximiza/on, and intepet it as a discete conges/on contol algoithm. Mul/path conges/on contol theoy has been developed by Kelly and Voice (2005), and by Han, Shakko<ai, Hollot, Sikant, Towsley (2006). Intepeta/on Incease x by a constant, evey /me you get an acknowledgement on path Decease x by an amount popo/onal to y s() if you detect a dop on path

36 How we expect the fluid model to behave:

37 How they behave in simula/on: When thee ae many flows, then each flow will flip independently, and the aggegate will behave how the fluid models pedict.

38 the naïve coupled congestion contolle, inspied by Kelly+Voice un independent TCP contol on each path static netwok φ=0 good at esouce pooling: even though the links have unequal capacities, congestion is balanced pefectly φ=2 bad at esouce pooling: the low-capacity link is highly congested dynamic netwok bad at esouce pooling: shifts too enthusiastically to the less loaded link, and is slow to lean when the othe link impoves good at esouce pooling: constantly pobes both links, so leans quickly when congestion levels change

39 the naïve coupled congestion contolle, inspied by Kelly+Voice un independent TCP contol on each path static netwok φ=0 good at esouce pooling: even though the links have unequal capacities, congestion is balanced pefectly φ=2 bad at esouce pooling: the low-capacity link is highly congested dynamic netwok bad at esouce pooling: shifts too enthusiastically to the less loaded link, and is slow to lean when the othe link impoves good at esouce pooling: constantly pobes both links, so leans quickly when congestion levels change

40 SUMMARY. We have a woking implementa/on of mul/path tanspot. It achieves a easonable degee of load balancing. This means that the netwok achieves some degee of esouce pooling (subject to having good enough outes). It maintains a easonable degee of equipoise. This means it adapts sensibly to fluctua/ng conges/on. It is guaanteed to be fai compaed to TCP. The algoithm is eady fo deployment. It is an expeimental RFC in the mptcp woking goup at the IETF.

41 Mul$path TCP Mak Handley Cos/n Raiciu Damon Wischik UCL - fom h<p://