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FAST TCP. Speaker: Ray Veune: Room 1026 Date: 25 th October, 2003 Time: 10:00am. Motivation. Demand for ultrascale networking HENP (High Energy and Nuclear Physics) Data volumes of tens of Petabytes (10 15 ) to Exabytes (10 18 ) Require Terabit/sec (10 15 bit/sec or 1000Gbit/sec)

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Fast tcp

FAST TCP

Speaker: Ray

Veune: Room 1026

Date: 25th October, 2003

Time: 10:00am


Motivation
Motivation

  • Demand for ultrascale networking

    • HENP (High Energy and Nuclear Physics)

    • Data volumes of tens of Petabytes (1015) to Exabytes (1018)

    • Require Terabit/sec (1015 bit/sec or 1000Gbit/sec)

  • Scalability problem of TCP

    • Losses must be extremely rare

    • TCP must induce loss

    • Underutilization and oscillation


Scalability problem of tcp extremely loss packet loss possibility
Scalability problem of TCP extremely loss packet loss possibility

  • Rate = 1.3 * MTU / (RTT * sqrt(Loss))

  • MTU = 1500bytes, RTT = 10ms


Scalability problem of tcp inevitable packet loss
Scalability problem of TCP inevitable packet loss

  • TCP needs to create losses

  • Single bit network feedback signal


Scalability problem of tcp underutilization and oscillation
Scalability problem of TCP Underutilization and oscillation

  • AIMD (1, 0.5)

  • At large window size (in excess of 10,000 pkts):

    • Halving window on loss event is too drastic

    • Increasing window by one packet per RTT is too conservative


Fast tcp achievement
FAST TCPAchievement

  • CERN (European Organization for Nuclear Research) sent 1.1 Terabytes of data at 5.44 Gbps

  • Full-length DVD film in 7 seconds !!


Fast tcp1
FAST TCP

  • Flow based vs Packet based

  • Network delay vs Packet loss

  • TCP-Vegas vs TCP-Reno

  • Stabilized Vegas


Tcp vegas techniques
TCP VegasTechniques

  • New Retransmission Mechanism

  • Congestion Avoidance Mechanism

  • Modified Slow-Start Mechanism


Tcp vegas new retransmission mechanism
TCP VegasNew Retransmission Mechanism

  • Timeout

  • n duplicate ACKs


Tcp vegas new retransmission mechanism1
TCP VegasNew Retransmission Mechanism

  • Check time record for the first duplicate packet

  • non-duplicate ACKs first or second after retransmission

    • Catch other segment lost previous to retransmission


Tcp vegas congestion avoidance mechanism

Source

Router

Dest.

TCP VegasCongestion Avoidance Mechanism

  • Detect network delay by monitoring RTT

  • BaseRTT and ActualRTT


Tcp vegas congestion avoidance mechanism1

Diff

α

β

TCP VegasCongestion Avoidance Mechanism

  • Expected = WindowSize / BaseRTT

  • Diff = Expected - Actual

    • Diff >> 0, decrease sending rate

    • Diff = 0, increase sending rate

  • α< Diff < β


Tcp vegas congestion avoidance mechanism2
TCP VegasCongestion Avoidance Mechanism

  • Extra buffers occupied

  • BaseRTT: 100ms, segment size: 1KB

    Expected = WindowSize / BaseRTT

    α = 30KB/s, β=60KB/s

    α=> 30KBps * 100ms / 1KB = 3

    β=> 60KBps * 100ms / 1KB = 6


Tcp vegas congestion avoidance mechanism3
TCP VegasCongestion Avoidance Mechanism

  • α=β

  • Diff <α

    • increase one segment per RTT

  • Diff =α

    • no change in windows size

  • Diff >α

    • decrease one segment per RTT


Tcp vegas slow start mechanism
TCP VegasSlow-Start Mechanism

  • TCP-reno

    • Send two segment for each ACK received

    • Exponential growth every RTT

  • TCP-Vegas

    • Exponential growth every alternative RTT

    • γthreshold

  • Diff >γ

    • Changes from slow-start mode to linear I/D mode


Stability of tcp vegas network model
Stability of TCP Vegas Network model

  • Set of L links with finite capacities c

    • c = (cl , l  L)

  • N sources indexed by r

  • Each source r uses a set of link defined by the LN routing matrix

    Rlr = {

  • if source r uses link l

  • 0 otherwise


Stability of tcp vegas network model1

lr

lr

Stability of TCP VegasNetwork model

  • For each link l, the congestion measure pl(t) is call price

  • For each source r, it maintains a rate xr(t) in packets/sec

  • Equilibrium forward delay from source r to link l : 

  • Equilibrium backward delay from link l to source r : 


Stability of tcp vegas network model2

lr

lr

Stability of TCP VegasNetwork model

  • Aggregate price source r observes in its path

    • qr(t) =  Rlr pl (t -  )

  • Aggregate source rate link l observes

    • yl(t) =  Rlr xr(t -  )

x1(t)

p1(t)

p3(t)

p4(t)

x2(t)

p2(t)

l

r


Stability of tcp vegas network model3

lr

lr

Stability of TCP VegasNetwork model

  • Tr denote equilibrium RTT

     +  = Tr,  l  L

  • Dynamical system of TCP Vegas

    pl (t) = ( yl ( t ) –cl ) / clif pl (t) > 0

    ( yl ( t ) –cl ) / cl )+if pl (t) = 0

    xr (t) = 1/Tr2(t) sgn( 1 –xr(t)qr(t) / rdr )

    Tr (t) = dr + qr( t )

    Where sgn(z) = 1 if z > 0, -1 if z < 0 and 0 if z = 0

    (z)+ = max { 0 , z }


Stability of tcp vegas approximate model
Stability of TCP Vegas Approximate model

  • xr (t) = 1/Tr2(t) sgn( 1 –xr(t)qr(t) / rdr )

  • sgn(z)  2/ tan-1 (z)

    •   

  • xr (t) = (2/Tr2(t))tan-1(1 –xr(t)qr(t) / rdr )


Stability of tcp vegas approximate model1
Stability of TCP Vegas Approximate model

  • In equilibrium, the source rate xr* and aggregate price qr* satisfy

    xr* qr*= r dr


Stability of tcp vegas theorem 1
Stability of TCP Vegas Theorem 1

  • Suppose for all r, k0Tr maxrTr for some k0.

  • Let M be an upper bound on the number of links in the path of any source, M  maxrlRlr.

  • The Vegas model is locally asymptotically stable around the equilibrium point (xr* , yl* , pl* , qr* ) if

    maxr xr*Tr sinc  (ň / xr*Tr ) <  / Mk02

  • ň = 2/

  • Let (a) be the unique solution in ( 0, /2) of  tan = a as a strictly increasing function of a

  • sinc = sin /


Stability of tcp vegas theorem 11
Stability of TCP Vegas Theorem 1

  • maxr xr*Tr sinc  (ň / xr*Tr ) <  / Mk02

  • () is strictly increasing

  • sinc() is strictly decreasing

  • LHS is strictly increasing in windows size xr* Tr

  • Theorem 1: Stability condition impose a limit on max windows size


Stability of tcp vegas corollary 2

qr* /Tr

minr > Mk02

ň

.

sinc  ( )

qr*

Tr

Stability of TCP Vegas Corollary 2

  • maxr xr*Tr sinc  (ň / xr*Tr ) <  / Mk02

  • All source has the same target queue length, r dr = for all r

  • Corollary 2: LHS is strictly increasing in qr* / Tr , implying a lower bound on queueing delay


Stability of tcp vegas corollary 3
Stability of TCP Vegas Corollary 3

  • Since () <  / 2, sinc () > 2 / , k0  1

  • Corollary 3: minrqr* / Tr > 2M / 

  • If M 2, then RHS bigger than 1, since Tr = dr + qr*

  • M = 1

  • The stability condition cannot be satisfied if a source has more than one link

  • Sufficient in multilink case

  • Necessary and sufficient in single-link-homogeneous-source case


Stability of tcp vegas single link with homogeneous source
Stability of TCP Vegas Single link with homogeneous source

  • A single link of capacity c,

  • Shared by N homogeneous source,

  • with round trip propagation delay d


Stability of tcp vegas single link with homogeneous source1
Stability of TCP Vegas Single link with homogeneous source

  • From corollary 3: qr* / Tr > 2 /  for all r

  • Tr / qr*< /2, since Tr = d + qr*

  • d / qr* < (/2 – 1) => d < (/2 – 1) qr*

  • Since qr* =  /xr* => ( N)/c

  • cd < (/2 – 1)  N

  • Conclusion: bandwidth delay product should be small


Stabilized vegas
Stabilized Vegas

  • xr (t) = (2/Tr2(t))tan-1(1 –xr(t)qr(t) / rdr )

  • xr (t) = (w/Tr2(t))tan-1r(t)(1 –xr(t)qr(t)/rdr -r(t) qr(t))

  • 1 –xr( t ) qr( t ) /r dr

  • 1 –xr( t ) qr( t ) /r dr -r( t ) qr( t )

  • r( t ) = (1 / ) (Tr( t ) / qr( t ) )

  • r( t ) = (  / w ) (xr( t ) Tr( t ) )


Stabilized vegas1
Stabilized Vegas

  • The gain r( t ) serves as a normalization to qr( t )

  • Additional differential term r( t ) qr( t ) anticipates the future of qr( t )

  • Without: xr( t ) will increase if xr(t)qr(t)< rdr

  • Even xr(t)qr(t)/rdr is small, xr( t ) may decrease if prices are rapidly growing


Stabilized vegas2



2 + 2a2

cd < ( - 1 ) N



2 + a2

Stabilized Vegas

  • Stability condition for stabilized Vegas

    where  = tan-1( (2)/(1-) )

  • Stabilized Vegas can choose a small

    ( a>0, (0,1) ) such that RHS can be larger for better stability of the original Vegas cd < (/2 – 1)  N


Simulation results
Simulation Results

One-on-One (300KB and 1MB) Transfers

c = 200KB/s

50ms delay


Simulation results1
Simulation Results

  •  = 20

  • N = 100 flows

  • Fixed packet size of 1KB

  • FIFO /w Droptail, queue capacity = 20000

  • ( a ,  ) = ( 0.5 , 0.015 )


Simulation results c 100 pkts s and d 10ms
Simulation Resultsc = 100 pkts/s and d = 10ms


Simulation results c 1000 pkts s and d 10ms
Simulation Resultsc = 1000 pkts/s and d = 10ms


Simulation results c 100 pkts s and d 100ms
Simulation Resultsc = 100 pkts/s and d = 100ms



Experimental results
Experimental Results

  • FAST TCP was first demonstrated publicly in during the SuperComputing Conference (SC2002) in Baltimore, MD, in November 16–22 2002

  • Caltech-SLAC research team

    • CERN

    • DataTAG

    • StarLight

    • TeraGrid

    • Cisco

    • Level(3).



Experimental results throughput and utilization
Experimental ResultsThroughput and utilization

  • SC2002 FAST experimental result

  • Current TCP implementation in Linux v2.4.18 on Jan 27-28, 2003


Fast tcp conclusion
FAST TCPConclusion

  • Problem of current TCP

    • Equilibrium

    • Dynamic problem

  • FAST TCP

    • Equation-based control with queuing delay

    • TCP Vegas

    • Stabilized Vegas




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