TCP Westwood for Wireless - cs.hac.ac.ilcs.hac.ac.il/staff/martin/seminar/westwood.pdf · Seminar...

Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 1

TCPWestwood

for Wirelessתוכן

מבוא1.

TCP - בקרת עומס ב—רקע טכני 2.

.3TCPעל קשר אלחוטי

TCP Westwoodשיפור תפוקה עם פרוטוקול 4.

סיכום5.


מבואמתאר בקצרה

מקומו של הנושא בתחום התקשורת

הבעיות שהנושא פותר

המצב שהיה לפני קיומו

כיצד הנושא מקדם את המדע


TCP Over WirelessWireless internet

Widely usedRoutine workflowRequires TCP/IP

ProblemTCP handles packet loss as congestion

Lost packet ⇒ bit error or congestionTCP designed for cable (BER < 10-9)Congestion ⇒ slow packet transmission

Wireless BER ≈ 10-3

Many random errors without congestionNo need (or efficacy) in lower packet rate

Westwood Modified TCP SEQ/ACK mechanismImproved treatment of random errors http://cdn.androidpolice.com/wp-content/uploads/

2011/10/SC20111017-101459.png

http://www.quora.com/Public-Transport/What-did-passengers-do-on-the-metro-trains-before-the-proliferation-of-smart-phones


הרקע התיאורטימתאר בקצרה

הפיתוח /המערכת/תיאור הת פקוד של הטכנולוגיה

המדעי

ברמה הדרושה להבנת הנושא על ידי בעל תואר ראשון

במדעי המחשב

שאין לו מומחיות מיוחדת בתחום הנדון

בסגנון של מאמר מק צועי או כתב התקן


Congestion ControlFlow control

Avoid overflow in TCP receiver bufferCongestion control

Avoid overflow in router buffers

Flow Control TCPBuffer

RouterBuffer


Slow-StartCongestion window (cwnd)

Source windowMaximum number of "unACKed" bytes

Initial cwnd = 1 MSS (maximum segment size)Data rate = 1 MSS / RTT

Exponential growthOn each ACK

cwnd ← cwnd + size of data ACKedif (cwnd > maximum cwnd = destination window)

cwnd ← max cwndif (ACK timeout)

Segment size threshold (ssthresh) ← last cwndcwnd ← initial cwnd = 1 MSS

Sender Receiver

RTT

Timeout

ACK 1 MSS

ACK 2 MSS

ACK 3 MSS


RTT and Buffer ErrorsRound Trip Time

RTT = data transmit time + send buffer times+ ACK transmit time + ACK buffer times

Buffer time ~ typical service time × buffer levelRTT = random variable (rise / fall sharply)

Buffer time in TCPTimeout

Buffer level ⎯→ RTT > RTO (retransmit timeout)Packet considered lost

Out-of-order packetBuffer level ⎯→ RTT(packet k) > RTT(packet k+1)Receiver will send cumulative ACK if OOO packet not lost

Sender Receiver

SEQRTT

ACK


RTT and CongestionBuffer error condition

Buffer level ⎯→ RTT > RTO for time Terror

Isolated errorTerror < time between packetsBuffer empties before next packetNo need to lower transmission rate

CongestionTerror > time between packets Multiple buffer errorsLower transmission rate to prevent buffer errors

Sender Receiver

SEQRTT

ACK


Congestion Avoidance

Slow start phaseOn (ACK && cwnd < ssthresh)

cwnd ← cwnd + size of data ACKed

On (ACK timeout)

ssthresh ← cwnd

cwnd ← initial cwnd = 1 MSS

RTO ← 2 * RTO

Congestion avoidance phaseOn (ACK && cwnd > ssthresh)

cwnd ← cwnd + 1 MSS

Fast retransmit with fast recoveryOn (3 dupACKs)

ssthresh ← cwnd

cwnd ← cwnd / 2

TCP Reno protocol

Retransmit lost packetWait 1 RTT → continue sending

For > 3 dupACKscwnd++ on each new dupACK


Error-Free Transmission

0

20

40

60

80

100

0 1 2 3

ACKSEQcwnd

Slow Start CollisionAvoidance

Latency = 2.77Utilization = 34.2%goodput = 34.2%


Congestion ⎯→ Longer RTT

0

20

40

60

80

100

0 1 2 3 4

ACK (no congestion)SEQ (no congestion)cwnd (no congestion)ACKSEQcwnd

Latency = 3.70Latency (lossless) = 2.77Excess latency = 33.57%Utilization = 25.6%goodput = 25.6%


0

20

40

60

80

100

0 2 4 6 8

ACKSEQcwnd

1 Lost Packet — Early

Packet 3 lostTimeout



Steady State

0

20

40

60

80

100

0 2 4 6

ACKSEQcwnd

Packet 9 lost3 dupACKs






1 —> ACK 1 cwd —> 22,3 —> ACK 3 cwd —> 4

4 – 7 —> ACK 7 cwd —> 88 – 15, 9 lost —> ACK 8, 3 dupACK 8

(on 10, 11, 12)cwd —> 4 + resend 8


The Trouble with Wireless — 1Variations in transmission medium

Multiple correlated packet losses + bit errors TCP interprets as serious congestion — timeouts + slow start

Fading channelsRefractionReflectionAbsorptionMultipath

EMI (electromagnetic interference)Other usersOther radio equipmentOther radiating equipment

absorption

reflection

refraction

medium

station

station

station

station


The Trouble with Wireless — 2Mobility drops

User moves between wireless domainsAdds delays + buffers + dupACKs + timeouts

MSC

RNC-1 RNC-2

1 2 3 4

MSC

RNC-1 RNC-2

1 2 3 4Node-B Cells

Clusters

TCP response in cell 4TCP request in cell 1


The Trouble with Wireless — 3Link asymmetry

Upstream channel slower than downstream channelLarger buffer ⇒ longer buffer delay ⇒ lower average B/s

ACK compression

Base Mobile

ACKs delayed in upstream buffer RTT ↑ ⇒ B/s ↓

All ACKs arrive together

cwnd ↑ sharplySender floods forward

channel buffer


The Trouble with Wireless — 4MACA in WiFi

Required to prevent hidden node problem

MACA overheadRTS+CTS+ACKMAC time ≈ TCP ACK timeTCP WiFi ACK delay = 2 × wired TCP ACK delay ⇒ RTT ↑Timeouts ⇒ cwnd ↓

RTS

CTS

DATA

ACKMAC

A B C D

RTS CTS

E F

cwnd segment sizeB/s

RTT

RTT and cwnd B/s

×=

⇒↑ ↓ ↓


דיון ממו קדחומר המ קצועי הבסיסי שהוסכם עליו עם המנחה

תיאור של החומר שנקרא ומטרתו

סקירה על תוכן החומר שנלמד

דוגמאות מקוריות להמחשת ה נושא


TCP WestwoodReference

Saverio Mascolo, Claudio Casetti, Mario Gerla, M. Y. Sanadidi and RenWang, TCP Westwood: Bandwidth Estimation for Enhanced Transport over Wireless Links (2001) ACM SIGMOBILE.

Modifies TCP senderNot dependent on negotiation with TCP senderNot dependent on support in router or receiver

Estimates available bandwidthCounts dupACKs as successful traffic

On packet loss set cwnd = available bandwidthImproves on Reno cwnd = cwnd / 2


Average Transmission Rate in RenoSenders must measure bandwidth estimate R

Know cwndMeasure <RTT>

TCP average RTT (simplified)R'= last RTT measurement

SRTT ← (1 - α) * SRTT + α * R'

RTO ← max(1 sec, SRTT)

α = 1/8

= =packets sent cwnd

Rtime to ACK RTT


Noise Filtering in WestwoodModel bandwidth as noisy signal

BWmeas (time) = BW (time) + noise (time)

Filter out noise with low-pass filter

( ) ( )

( ) ( ) ( )

( )

2

2

20

0

1

1

Fourier transform input signal

Filter in frequency domain

Low-pass filter

Output signal "cut off" at frequency

ift

ift

x t X f e df

y t H f X f e df

H fff

f

π

π

∞

−∞

∞

−∞

=

=

=⎛ ⎞

+ ⎜ ⎟⎝ ⎠

∫

∫


Low Pass Filter Example

( ) ( ) ( )

( ) ( ) ( )2 2

1sin 2 sin 20

101 1 1

sin 2 sin 20101 10

1 12 2

x t t t

y t t t

π π

π π

= +

= +⎛ ⎞ ⎛ ⎞+ +⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

Filter input

Filter output

( )( )

x t

y t

( ) 02

0

12

1

, H f ff

f

= =⎛ ⎞

+ ⎜ ⎟⎝ ⎠

( )X f

f1 10


DSP — Low Pass Filter for Sampled Signals

1

1

, 0,1, 2,...

, 0,1, 2,...

ˆ ˆ

number of bytes ACKedtime of ACK time of previous ACK

k

k k k

k kk

k k k

k k k

t k

t t t

d db k

t t t

b b

−

−

=Δ = −

= = = =Δ − −

= α

Sample input at times

Sample stream from input

Output stream (Tustin approximation)

( ) 11

0

12

21 , 2

2

1

parameter related to Nyquist sampling theorem

k kk

kk k

k

b b

tt

t

f

−−

++ − α

τ − Δα = < τ > Δ

τ + Δ

τ =

Cutoff frequency


Bandwidth Sampling in Westwind Bandwidth sample

1

1

2

0 2

number of bytes ACKedtime of ACK time of previous ACK

No ACK for time > insert "virtual ACK"

Insert at time

k kk

k k k

k k k

d db

t t t

d b t τ

−

−

= = =Δ − −

τ ⇒= ⇒ = +


Westwood Packet Counting ExamplePacket transmission times

Time ACKArrived at Receiver

Packets for BWE

Counted

dupACKBWE

t1 1 1 1 0 1 / (t1 – t0)

t2 3 2 + 3 2 0 2 / (t2 – t1)

t3 4 4 1 0 1 / (t3 – t2)

t4 4 6 1 1 1 / (t4 – t3)

t5 4 7 1 2 1 / (t5 – t4)

t6 4 8 1 3 1 / (t6 – t5)

t7 9 5 + 9 2 0 2 / (t7 – t6)

1 2 3 4 5 6 7 8 5 9

t0 t1 t2 t3 t6

ACK jumps to 9 from 4 = 5 but 3 ACKs (6, 7, 8) already counted as dupACKs


Westwood ACK Counter (for BWE)

newACK = ACK – prevACK ; Packets ACKed by new ACK

// if (newACK = 1) do nothing No error condition — report 1 ACK

if (newACK = 0)

count++ ;

newACK = 1 ;

ACK "stuck" on old value (dupACK)

Increment dupACK counter

Count 1 dupACK

if (newACK > 1) ACK advances

if (count >= newACK)

count = count – newACK + 1 ;

newACK = 1 ;

Not all arrived packets ACKed

Remove ACKed from dupACK count

Count as additional dupACK

else if (count < newACK)

newACK = newACK - count ;

count = 0;

All packets ACKed in order

Report ACKed – counted (new ACKs)

Zero dupACK counter

prevACK = ACK ;

return(newACK);


Westwood Scenario 1 — No ErrorsReceive 1 — 2,3 — 4,5,6,7 — 8,9,10,11,12,13,14,15ACK 1 — 3 — 7 — 11 — 15 ⎯→ newACK = 1 — 2 — 4 — 4 — 4

newACK = ACK – prevACK ; 1 2 4 4 4

if (newACK = 0)

count++ ;

newACK = 1 ;

— — — — —

if (newACK > 1)

—



newACK = 1 ;

— — — —



count = 0;

20

40

40

40

prevACK = ACK ; 1 3 7 11 15

return(newACK); 1 2 4 4 4Total = 15


Westwood Scenario 2 — 1 Packet Out-of-OrderReceive 1 — 2,3 — 5,6,7,4 — 8,9,10,11,12,13,14,15ACK 1 — 3 — 3 — 3 — 3 — 7 — 11 — 15

newACK = ACK – prevACK ; 1 2 0 0 0 4 4 4

if (newACK = 0)

count++ ;

newACK = 1 ;

— — 11

21

31

— — —

if (newACK > 1)

— — — —



newACK = 1 ;

— — — —



count = 0;

20

10

40

40

prevACK = ACK ; 1 3 3 3 3 7 11 15

return(newACK); 1 2 1 1 1 1 4 4Total = 15


Westwind Congestion ControlReno slow start

On (ACK && cwnd < ssthresh)

cwnd ← cwnd + size of data ACKed

Reno congestion avoidanceOn (ACK && cwnd > ssthresh)

cwnd ← cwnd + 1

Modified fast recoveryOn 3 dupACKs

ssthresh = BWE * RTT_min / segment_size

if (cwnd > ssthresh) cwnd = ssthresh

Modified timeoutssthresh = BWE * RTT_min / segment_size

if (ssthresh < 2) ssthresh = 2

cwnd = 1


Westwood Performance — Throughput


SummaryTCP over wireless

TCP designed for wired systemsDoes not expect isolated bit errors

Treats all lost packets as congestion

Congestion ⇒ TCP lowers transmission rate

Many more isolated errors over wireless No need to lower transmission rate

TCP over wireless wastes bandwidth

WestwoodImproves error handling at senderCounts dupACKs as traffic arriving successfully after lost packet

Estimated available bandwidth including dupACKsCounts real ACKs and dupACKs separately

Higher average throughput than standard TCPFuture prospects — not implemented since 2001 research paper

TCP Westwood for Wireless - cs.hac.ac.ilcs.hac.ac.il/staff/martin/seminar/westwood.pdf · Seminar...

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