stefano giordano – icc 2005 - seoul, may 16 - 2005 1 on the use of pipesize estimators to improve...

1 Stefano Giordano – ICC 2005 - Seoul, May 16 - 2005

On the Use of Pipesize Estimators to Improve TCP Transient Behavior

Stefano Giordano, Gregorio Procissi, Franco Russo, Raffaello Secchi

Dipartimento di Ingegneria dell’InformazioneUniversita’ di Pisa


Outline

• Introduction and Related Works

• Algorithm rationale and implementation

• Comparisons with TCP NewReno

– NewReno/ESSE performance, Robustness to

Backward Congestion, Sensitivity to Network

Parameters

• Fairness & Friendliness

– Impact on Short Lived Flows

– Impact on NewReno Steady-State Behavior


Short versus Long Lived Connections

• Recent measurement studies (CAIDA) reveal that, though the bulk of traffic volume is carried over long lived connections, the majority of Internet traffic consists of short-lived flows

• Characteristics of Internet stream dimension

Long-Lived (Elephants)• Transmit large number of packets• TCP mechanisms designed for long-

lived flows• Operate primarily in TCP congestion

avoidance

Short-Lived (Mice)• Transmit small number of packets• Operate primarily in slow-start

phase• Example: web browsing traffic

Nevil Brownlee, “Some Observations of Internet Stream Lifetimes”, CAIDA, UC San Diego and New Zealand

Passive and Active Measurements Workshop


TCP Dynamics

• Real-time Bandwidth Estimation techniques shows clear benefits in long term TCP congestion control (e.g. TCP Vegas, TCP Westwood)

• Could we use available bandwidth estimation also to improve the transient behavior of TCP connections (duration, packet drop-rate)?

• TCP mechanism is a loss-driven scheme:– TCP slow-downs (reduces cwnd) when detects a packet loss– cwnd < ssthresh, TCP quickly ramps up to fill up the pipe (Slow-

Start)– cwnd > ssthresh, TCP probes for bandwidth (Congestion Avoidance)

• But, ssthresh is initially set to an arbitrarily large value (e.g. 64KB in many OS implementations)


Improving the TCP Startup Behavior

timetimeout

Early Slow-Start Exit (ESSE)

cwnd

ssthresh

Slow-Start phase

congestion window evolutionpacket

losses

NewReno

Problems:• safe choice of ssthresh (bandwidth-delay product estimation)

• few ACK

observations


Related Works

• J. Hoe, “Improving the Start-up Behavior of a Congestion Control Scheme for TCP”, in proc. of SIGCOMM, 1996

– First time introduced bandwidth estimation in TCP startup phase and an estimation technique proposed

• M. Gerla et al. “Sender-Side TCP Modifications: An Analytical Study”, in proc. of Networking 2002

– An analytical framework to slow-start modification is developed, assessed performance of idealized bandwidth estimators

• M.Gerla et al., “TCP Startup Performance in Large Bandwidth Delay Networks”, in proc. of INFOCOM 2003

– Eligible Rate Estimation (ERE) from ACK streams using an adaptive scheme


How to select the slow-start threshold ?

• The ssthresh setting should be the right tradeoff between:– too low values: TCP connections prematurely switch in congestion

avoidance– too high values: congestion window overfeeds the pipe

• A reasonable choice of ssthresh should be close to the bandwidth-delay product (maximizing overall network performance). It can be generally written as:

spacing

RTT

packet_len

bandwidthRTTpipesize

• ACK spacing reflects time between two packet transmissions sent back to back at the bottleneck link


Bandwidth Estimation Concept

(seconds)

mint

• The shadow region encloses estimation values as:

• Algorithm robustness is investigated by considering extreme cases

pipesize(RTT,Δt)

packet spacin

g

(packets

)

RTTmax

RTTmin

maxt

spacing

RTTpipesize

TCP

cross traffic


Estimator Implementation (1)

• The four considered worst-cases can be approximated at real-time:

kNkRTTRTT

maxmax

kNkRTTRTT

minmin

kNk

tt

minmin

kNk

kNk

RTTRTTRTTRTTt

minmaxminmaxmax

The number of ACK (N=5) for the estimation is a critical parameter;

it should be chosen as a compromise between speed and accuracy

when no ACK compression is present


Estimator Implementation (2)

name estimator

MinMin

MaxMin

MinMed

MaxMed

k

k

t

RTT

min

max

mint

pipesize

packet spacing

(packets

)

(seconds)

RTTmin

maxt

RTTmax

k

k

t

RTT

min

min

kk

k

RTTRTT

RTT

minmax

min

kk

k

RTTRTT

RTT

minmax

max

MinMin

MaxMin

MaxMed

MinMed


Sender Side Algorithm

• Implementation issues:– The ACK counter is set to zero at the beginning

of connection and when a timeout expires – The ssthresh is maintained larger than two– Both packet of pair must be together in flight for

ACK spacing to be considered

ACK receivedACK received

DupAck?no

t estimationk = k+1

k = N?

yes

ssthresh = pipesize

no

yesslow start

ACK1

ACK2NEWRENOACK3

ACK4


Simulative Reference Topology

R1 R2

S1

Sn

S2

H1

Hm

H2Bottleneck link1Mbps, 20ms

Access Links10Mbps, 5ms

• New TCP connections are scheduled according to a Poisson process

• Disturb Traffic (along reverse path) is a Poissonan Arrival of Bursts process (LRD process)

Drop-Tail, RED

buffer

TC

P s

ou

rces

TC

P r

ece

ivers

conn. length 40pkt/conn

packet size 1000B

Buffer size 20pkts

RTT 60ms


Performance Evaluation (1)

Loss Probability at Bottleneck Average Completion Time

No disturb traffic along reverse bottleneck

Reducing drastically the drop-rate at the bottleneck, ESSE shows a lower aggressiveness in control mechanism


Characteristics of Pipesize Estimators

maxmin

minmin

maxmed

minmedconservative

estimation

pip

esiz

e

mint maxt

minmin

maxmin

maxmed

minmed

spacing


Performance Evaluation (2)

Loss Probability at Bottleneck Average Completion Time

The reverse bottleneck buffer is congested (50% total link capacity)


Sensitivity to Network Parameters

Round Trip Delay Buffer

• For small pipesize (buffesize) NR aggressively saturate network resources resulting in poor performance


Fairness & Friendliness (1)

2

2

i

i

xn

xF

Jain’s Fairness Index: xi = i-th connection duration

fair

unfair



Traffic MixMaxMin completion time

MaxMed completion time

MinMin completion time

MinMed completion time

ESSE NR ESSE NR ESSE NR ESSE NR ESSE NR

80% 20% 1.79 1.83 1.57 1.65 1.68 1.70 1.52 1.67

60% 40% 1.79 1.96 1.59 1.78 1.79 1.76 1.65 1.78

40% 60% 1.84 1.90 1.79 1.89 1.90 1.93 1.77 1.90

20% 80% 1.96 1.94 1.89 1.97 1.91 1.94 1.92 1.97

ESSE impact on mice traffic (70% loaded bottleneck) in a mix NewReno and ESSE-modified connections

• The presence of ESSE modified traffic has a beneficial impact on the average completion time of TCP NR connections



• ESSE impact on TCP steady-state (elephant connections)– Single connection throughput when TCP shares with homogeneous

Newreno and ESSE modified flows.


Conclusions

• Primary focus on TCP startup mechanism (wide-area Internet connection is the superposition of many short-lived flows)

• The design of pipesize estimator (defined as the ratio between RTT and packet spacing) consider the worst cases

• Results: speeding up TCP connections, drastically reducing the drop-rate at the bottleneck

• ESSE modified TCP shows fairness among homogeneous connection and friendliness towards (NewReno) heterogeneous flows.

stefano giordano – icc 2005 - seoul, may 16 - 2005 1 on the use of pipesize estimators to improve...

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