tcp on current network technologies -...

TCP on current networktechnologiesDEA DIF lecture

C. PhamThese slides borrow material from various

sources which are indicated below eachslide when necessary

The congestion phenomenom

From Computer Networks, A. Tanenbaum

When congestion happens?

qTraffic from fast links to abottleneck link, traffic aggregationqDifference in routers speed

10 Mbps

100 Mbps

1.5 Mbps

Load

Load

Thr

ough

put

Del

ay

knee cliff

congestioncollapse

packetloss

Congestion: A Close-up Viewq knee – point after whichq throughput increases

very slowlyq delay increases fast

q cliff – point after whichq throughput starts to

decrease very fast tozero (congestioncollapse)

q delay approachesinfinity

q Note (in an M/M/1 queue)q delay = 1/(1 – utilization)

From Shivkumar Kalyanaraman

LoadT

hrou

ghpu

t knee cliff

congestioncollapse

Congestion Control vs.Congestion Avoidance

qCongestion controlgoalqstay left of cliff

qCongestionavoidance goalqstay left of knee

qRight of cliff:qCongestion collapse


Basic Control Model

qLet’s assume window-based operationqReduce window when congestion is

perceivedqHow is congestion signaled?

• Either mark or drop packets

qWhen is a router congested?• Drop tail queues – when queue is full• Average queue length – at some threshold

qIncrease window otherwiseqProbe for available bandwidth – how?


Simple linear control

qMany different possibilities for reaction tocongestion and methods for probingqExamine simple linear controlsqWindow(t + 1) = a + b Window(t)qDifferent ai/bi for increase and ad/bd for

decreaseqSupports various reaction to signalsqIncrease/decrease additivelyqIncreased/decrease multiplicativelyqWhich of the four combinations is optimal?


Phase plotsq Simple way to visualize behavior of competing flows

over time

q Caveat: assumes 2 flows, synchronized feedback, equalRTT, discrete “rounds” of operation

Efficiency Line x1+x2=C

Fairness Line x1=x2

User 1’s Allocation x1

User 2’sAllocation

x2Optimal point

Overload

Underutilization


Additive Increase/Decrease

T0

T1

Efficiency Line

Fairness Line



x2

q Both X1 and X2 increase/decrease by the same amountover timeq Additive increase improves fairness & increases loadq Additive decrease reduces fairness & decreases load


Multiplicative Increase/Decreaseq Both X1 and X2 increase by the same factor over timeq Fairness unaffected (constant), but load increases (MI) or

decreases (MD)

T0

T1

Efficiency Line

Fairness Line



x2


Additive Increase/Multiplicative Decrease (AIMD)

q Assumption: decrease policy must (at minimum) reverse theload increase over-and-above efficiency lineq Implication: decrease factor should be conservatively set to

account for any congestion detection lags etc

x0

x1

x2

Efficiency Line

Fairness Line



x2


Further readings

qP. Gevros, J. Crowcroft, P. Kirstein,and S. Bhatti, "Congestion controlmechanisms and the best effortservice model," IEEE Network, vol.15, pp. 16 - 26, May/June 2001.

Reliable communications onthe Internet

qTCP (RFC 793) has been proposed asearly as 1974, no congestion controlqSuccessfully tested on the ArpaNet

until first congestion collapse in 1986qJacobson introduces the TCP CC

mechanism (AIMD) ‹ TCP TahoeqNow, 90% of reliable transfers use

TCP Tahoe, Reno or New Reno

TCP, adding reliability to IPHost A

Seq=92, 8 bytes data

ACK=100

loss

timeo

ut

time Lost ACK

Host B

X


ACK=100

Host A


ACK=100

Seq

=92

timeo

uttime Early timeout

Cumulated ACKs

Host B


ACK=120


Seq

=100

tim

eout

ACK=120

TCP (Tahoe) CongestionControl

qMaintains three variables:qcwnd – congestion windowqrcv_win – receiver advertised windowqssthresh – threshold size (used to update cwnd)

• Rough estimate of knee point…

qFor sending use: win = min(rcv_win, cwnd)


TCP congestion control: the big picture

q CongW grows exponentially (slow start), thenlinearly (congestion avoidance),

q If loss, divides threshold by 2 (multiplicativedecrease) and restart with CongW=1 packet

From Computer Networks, A. Tanenbaum

TCP: Slow Start

q Goal: initialize system and discover congestion quicklyq How? Quickly increase cwnd until network congested ‡ get

a rough estimate of the optimal cwndq How do we know when network is congested?q packet loss (TCP)

• over the cliff here ‡ congestion controlq congestion notification (eg: DEC Bit, ECN)

• over knee; before the cliff‡congestion avoidance

q Implications of using loss as congestion indicatorq Late congestion detection if the buffer sizes largerq Higher speed links or large buffers => larger windows => higher

probability of burst lossq Interactions with retransmission algorithm and timeouts


TCP: Slow Start

qWhenever starting traffic on a newconnection, or whenever increasing trafficafter congestion was experienced:

• Set cwnd =1• Each time a segment is acknowledged increment cwnd

by one (cwnd++).

qDoes Slow Start increment slowly? Notreally. In fact, the increase of cwnd isexponential!! Window increases to W inRTT * log2(W)


ACK for segment 1

segment 1cwnd = 1

cwnd = 2 segment 2segment 3

ACK for segments 2 + 3

cwnd = 4 segment 4segment 5segment 6segment 7

ACK for segments 4+5+6+7

cwnd = 8

Slow Start Exampleq The congestion

window increasesquite rapidly infact


Round Trip Time

1

One RTT

One pkt time

0R

2

1R

3

4

2R

567

83R

91011

1213

1415

1

2 3

4 5 6 7


Slow Start Sequence Plot

Time

Sequence No

.

.

.

CongW double everyRTT


Congestion Avoidance

qGoal: maintain operating point at the leftof the cliff:qHow?qadditive increase: starting from the rough

estimate (ssthresh), slowly increase cwnd toprobe for additional available bandwidthqmultiplicative decrease: cut congestion window

size aggressively if a loss is detected.


Congestion Avoidance

Purpose: Slow down “Slow Start”

If cwnd > ssthresh theneach time a segment is acknowledgedincrement cwnd by 1/cwnd‹cwnd += 1/cwnd.

(So cwnd is increased by one only if allsegments have been acknowledged)


Congestion AvoidanceSequence Plot

Time

Sequence No Window growsby 1 every round


From Guy Leduc, RHDM 2002

From Chandi Barakat, PhD defense, 2001

Putting it together

TCP evolution

1975 1980 1985 1990

1982TCP & IP

RFC 793 & 791

1974TCP described by

Vint Cerf and Bob KahnIn IEEE Trans Comm

1983BSD Unix 4.2

supports TCP/IP

1984Nagel’s algorithmto reduce overhead

of small packets;predicts congestion

collapse

1987Karn’s algorithmto better estimate

round-trip time

1986Congestion

collapseobserved

1988Van Jacobson’s

algorithmscongestion avoidanceand congestion control(most implemented in

4.3BSD Tahoe)

19904.3BSD Renofast retransmitdelayed ACK’s

1975Three-way handshake

Raymond TomlinsonIn SIGCOMM 75


TCP in the 90s

1993 1994 1996

1994ECN

(Floyd)Explicit

CongestionNotification

1993TCP Vegas

(Brakmo et al)real congestion

avoidance

1994T/TCP

(Braden)Transaction

TCP

1996SACK TCP(Floyd et al)

SelectiveAcknowledgement

1996Hoe

Improving TCPstartup

1996FACK TCP

(Mathis et al)extension to SACK


Further readings

q V. Jacobson, «!Congestion avoidance and control!»,ACM SIGCOMM, 1988.

q L. Brakmo and L. Peterson, «!TCP Vegas: End toend congestion avoidance on a global internet!»,IEEE Journal on Selected Areas inCommunications, 13(8), October 1995.

q J. Widmer, R. Denda, and M. Mauve, "A survey onTCP-friendly congestion control," IEEE Network,vol. 15, pp. 28 - 37, May/June 2001.

q http://www.cnaf.infn.it/~ferrari/tcp.html for alist of papers on TCP analysis

TCP Modeling

q Given the congestion behavior of TCP can wepredict what type of performance we should get?

qWhat are the important factorsq Loss rate

• Affects how often window is reducedq RTT

• Affects increase rate and relates BW to windowq RTO

• Affects performance during loss recoveryqMSS

• Affects increase rate



(N/2)2+1/2(N/2)2

, from (N+N/2)/2

Further readings

q The first «!square root!» TCP formulaq Matthew Mathis, Jeffrey Semke, Jamshid Mahdavi,

Teunis Ott, «!The Macroscopic Behavior of the TCPCongestion Avoidance Algorithm!», ComputerCommunications Review, Vol. 27(3), July 1997

qMore accurate TCP modellingq Padhye et al., «!Modeling TCP Throughput: A Simple

Model and its Empirical Validation!», SIGCOMM 98.

q Even more accurate TCP modellingq Chadi Barakat, «!TCP modeling and validation!», IEEE

Networks, vol. 15, no. 3, pp. 38-47, May 2001.

Some results

From Padhye, SIGCOMM98

Modeling means considering

qTimouts and retransmit strategiesqHow the window size increasesqHow the RTTs are distributedqHow the losses are correlatedqHow congestions are distributedqAnd…what have not been discovered

yet!

TCP in the Internet jungle!

qHigh Speed NetworksqLong distance, Large bandwidth, Low BERqDelay.bandwidth is high (memory)

qAsymetric networksqBandwidth, latency (cable, xDSL)

qWireless networksqHigh BER on lossy linksqSatellite, high latencies

qHow good is the old design of TCP?

TCP’s parameters onperformances

qTransmission window sizeqCongestion window sizeqBuffer sizeqTimersq?

High Speed Networks

Asymetric networks

Conclusions

The most obvious problems

qProblem with TCP headerqRwnd is on 16-bit only…q…solve by Window Scale OptionqSeqNum is 32-bit only…q…solve by Protect Against Wrapped

Sequence Number (PAWS)

From 1st PFLDnet Workshop, Sally Floyd

I

N

D

I

A

N

A

U

N

I

V

E

R

S

I

T

Y

Motivation (1)

• Basic assumption of TCP:– Packet loss is due to network congestion

• TCP thus reacts to packet loss withexponential backoff

• After backoff, transmission speed growsonly linearly

From 1st PFLDnet Workshop, Steven Wallace

I

N

D

I

A

N

A

U

N

I

V

E

R

S

I

T

Y

Motivation (2)

• What about high-speed research networks?– Packet loss is usually not due to congestion

– Loss comes from equipment, cabling, etc.

– This loss cannot necessarily be avoided

• TCP will collapse even though plenty ofcapacity is still available

From 1st PFLDnet Workshop, Steven Wallace

High Speed TCP (S. Floyd)

qModify the response function to allow for morelink utilization in current high-speed networkswhere the loss rate is smaller than that of thenetworks TCP was design for (at most 10-2)

TCP Throughput (Mbps) RTTs Between Losses W P --------------------- ------------------- ---- ----- 1 5.5 8.3 0.02 10 55.5 83.3 0.0002 100 555.5 833.3 0.000002 1000 5555.5 8333.3 0.00000002 10000 55555.5 83333.3 0.0000000002

Table 1: RTTs Between Congestion Events for Standard TCP, for 1500-Byte Packets and a Round-Trip Time of 0.1 Seconds.

From draft-ietf-tsvwg-highspeed-01.txt

Modifying the response Packet Drop Rate P Congestion Window W RTTs Between Losses ------------------ ------------------- ------------------- 10^-2 12 8 10^-3 38 25 10^-4 120 80 10^-5 379 252 10^-6 1200 800 10^-7 3795 2530 10^-8 12000 8000 10^-9 37948 25298 10^-10 120000 80000

Table 2: TCP Response Function for Standard TCP. The average congestion window W in MSS-sized segments is given as a function of the packet drop rate P.

To specify a modified responsefunction for HighSpeed TCP, weuse three parameters, Low_Window,High_Window, and High_P. ToEnsure TCP compatibility, theHighSpeed response function usesthe same response function asStandard TCP when the currentcongestion window is at mostLow_Window, and uses the HighSpeedresponse function when the currentcongestion window is greater thanLow_Window. In this document weset Low_Window to 38 MSS-sizedsegments, corresponding to a packetdrop rate of 10^-3 for TCP.

Packet Drop Rate P Congestion Window W RTTs Between Losses ------------------ ------------------- ------------------- 10^-2 12 8 10^-3 38 25 10^-4 263 38 10^-5 1795 57 10^-6 12279 83 10^-7 83981 123 10^-8 574356 180 10^-9 3928088 264 10^-10 26864653 388

Table 3: TCP Response Function for HighSpeed TCP. The average congestion window W in MSS-sized segments is given as a function of the packet drop rate P.

From draft-ietf-tsvwg-highspeed-01.txt

See it in image


Some simulationsFr

om 1

st PF

LDne

t Wor

ksho

p, S

ally

Flo

yd

Adding Limited Slow-Start


Further readings

qhttp://www.ietf.org/internet-drafts/draft-ietf-tsvwg-highspeed-01.txtqhttp://www.ietf.org/internet-

drafts/draft-ietf-tsvwg-slowstart-00.txtqhttp://www.icir.org/floyd/hstcp.html

Scalable TCP

qTCP’s overview (steady state)qcwnd=cwnd+1/cwndqCwnd=cwnd-1/2cwnd

From 1st PFLDnet Workshop, Tom Kelly

STCP generalized algorithm


STCP in images


Fairness


Some results


Further readings

q http://www-lce.eng.cam.ac.uk/~ctk21/scalable/

q And much more…q Tsunamiq E2E lightpathq Fast TCPq UDP Blastq XFTPq …

High Speed Networks

Asymetric networks

Conclusions

Research on TCP means

qSeveral possible research objectivesqRemain compatible with currently deployed

versions of TCP (mainly Reno and New Reno)• On better understanding the behavior of TCP (self-

similarity, bottleneck)• On proposing small variations on the current TCP

(ECN, HSTCP…)

qOn proposing completely new solutions• For very high speed, lightpath…• For changing the congestion control part

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