TCP Probe: A TCP with Built-in Path Capacity Estimation

Anders Persson, Cesar Marcondes, Ling-Jyh Chen, Li Lao,

M. Y. Sanadidi, Mario Gerla

Computer Science Department

University of California, Los Angeles

March 18, 2005 Global Internet 2005 2

Motivation

Knowledge of “narrow link capacity” is important for: Optimizing network utilization via better congestio

n control & adaptive streaming Tracking dynamic changes in capacity due to Vert

ical Handoffs

March 18, 2005 Global Internet 2005 3

Bandwidth Estimation Techniques Active

Send out-of-band packets into the network CapProbe [SIGCOMM04] and Pathrate [INFOCOM01]

Passive Use ongoing data packets without additional over

head to the network TCP Probe – similar to CapProbe technique but uses onl

y “actual” data packets

March 18, 2005 Global Internet 2005 4

CapProbe Recap

Packet pair technique

Compression & expansion due to cross traffic distorts Tb Filter out distorted samples by using samples with minimum

end-to-end delay sum Fast and accurate estimation of bottleneck capacity

TbTaTb

bTLC

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TCP Probe

Idea: Use CapProbe technique but passively within TCP Rely on dispersion of ACK pairs and filter

distorted dispersion based on end-to-end delay sum

Have to ensure that data packet pairs & corresponding ACK pairs produce accurate capacity estimates

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Challenge

TCP implementations try to reduce network overhead by sending one ACK for two data packets, but: We want an ACK for each data packet!

i+1 i

i+1

TCP Data

ReceiverSenderTCP Ack

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Solution

Sender sends inverted data packets Receiver replies a duplicate ACK and an incr

emental ACK

i i+1

i+1i-1

TCP Data

TCP AckReceiverSender

March 18, 2005 Global Internet 2005 8

Packet Size Issue

Probing packet size: forward direction: TCP data 1500 bytes reverse direction: TCP ACK 40 bytes

The newly developed AsymProbe [Networking05] has shown: If , TCP Probe estimates the capacity of

the forward direction link If , TCP Probe estimates the capacity of

the reverse direction link

40

1500

Re

verse

forward

C

C

40

1500

Re

verse

forward

C

C

March 18, 2005 Global Internet 2005 9

Implementation

Implemented in NS-2 and Linux 2.4 Network Stack

Approach: Periodically mark data packet pairs Sender timestamps the data packets and the corr

esponding ACKs (sec granularity)

March 18, 2005 Global Internet 2005 10

Simulation Scenario

NS-2 simulation

Compare TCP Probe capacity estimation with TCPW BE (bandwidth estimation) TCPW BE is based on dispersion of ACK packets

without filtering samples

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TCP Probe vs. TCPWC

ap

aci

ty E

stim

atio

n (

Mb

ps)

TCP Probe TCPW BE

20 flows of the same kind TCP Probe: All flows estimate C=10 Mbps TCPW: Flows estimate BE between 0 to 7 Mbps

March 18, 2005 Global Internet 2005 12

TCP Probe vs. TCPW

1 TCP Probe and 5 TCP New Reno flows 1 TCPW and 5 TCP New Reno flows

TCP Probe TCPW BE

Ca

pa

city

Est

ima

tion

(M

bp

s)

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Internet Measurement

Three different paths: Los Angeles: 3 Mbps (cable modem) China: 45 Mbps Alabama: 100 Mbps

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Cable Modem (3 Mbps)

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China (45 Mbps)

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Alabama (100 Mbps)

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Statistical Analysis

30 measurements to each destination

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Applications

TCP Probe in Vertical Handoff Scenario A vertical handoff involves two different network interfaces

Usually represent different technologies and thus result in a drastic change in link capacity

How to get advantage of the extra capacity when we are in congestion avoidance?

Revised TCP Probe Re-initiate the estimation every n samples (n=50)

Detect a huge increase in the capacity estimation Trigger a new slow start phase (i.e. fast rate adaptation)

March 18, 2005 Global Internet 2005 19

Simulation Scenario

TCP Probe: 1 → 6 Pareto flows: 7 → 10, 8 → 9, 11 → 14, 12 → 13 Capacity between node 1 and 2 increases from 10Mbp

s to 100Mbps at 80 second

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Fast Rate Adaptation

March 18, 2005 Global Internet 2005 21

Conclusions

TCP Probe passively provides to a TCP flow sender an accurate estimate of path capacity Estimated capacity can be used as an upper bound

on TCP sender rate Vertical Handoff applications can benefit from detect

ing capacity change Work in progress

TCP Probe provides improved performance under high loss rate (5%)

Astart [INFOCOM04] can benefit from accurate capacity estimation