Download Design and performance evaluation of an improved TCP congestion

指導教授：林仁勇 老師
學生：吳忠融
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 Author
 Chan, Y.-C.
 Chan, C.-T.
 Chen, Y.-C.
 Source
 IEE Proceedings of Communications,
Volume 151, Issue 1, Feb 2004 Page(s):107 - 111
Digital Object Identifier 10.1049/ip-com:20040229
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 Introduction
 Previous work
 TCP RoVegas
 Performance evaluation
 Conclusions
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 The transmission control protocol (TCP) is currently
the most popular End-to-End transport protocol on
the Internet, and it is implemented in several versions
(i.e. Tahoe, Reno, Vegas…) all of which aim to improve
the network utilization.
 Vegas can achieve a much higher throughput than that
of the other versions.
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 TCP Vegas attempts to control and avoid congestion by
monitoring the difference between the measured and
expected throughputs.
 It uses the congestion window size and measured
round-trip time (RTT) to estimate the amount of data
in the network pipe and maintain extra data between
the lower threshold (α) and the upper threshold (β).
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 If the network congestion occurs in the direction of
ACK packets (backward path), it may underestimate
the actual rate and cause an unnecessary decrease in
the congestion window size.
 Some current networking technologies with
asymmetry network characteristics, such as
asymmetric digital subscriber line (ADSL), cable
modem, and satellite-based networks, greatly increase
the possibility of backward path congestion.
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 We now propose a router-based congestion avoidance
scheme for TCP Vegas (abbreviated as RoVegas
hereafter).
 By judging the direction along which the congestion
occurs, RoVegas significantly reduces the impact and
improves the throughput when the backward path is
congested.
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 Elloumi et al. proposed a modified algorithm for TCP
Vegas.
 It divides a RTT into a forward trip time and a
backward trip time in order to remove the effects of
backward path congestion.
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 Fu and Liew employ an end-to-end method to
measure the actual flow rate on the forward path at a
source of TCP Vegas.
 The source adjusts the congestion window size
depending on the differences between the expected
rate and the actual flow rate on the forward path.
 However, the TCP traffic has a bursty nature that
makes it difficult to decide when to measure the actual
flow rate.
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 TCP Vegas estimates a suitable amount of extra data
（value of diff or delta） to be kept in the network
pipe and controls the congestion window size
accordingly.
 The amount of extra data is between two thresholds α
and β as shown in the following:
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 When backward congestion occurs, the increased
backward queuing time will affect, the actual
throughput and enlarge the difference between the
expected throughput and the actual throughput.
 This results in a decrease in the congestion window
size.
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 A measured RTT can be divided into four parts:
 The forward fixed delay (i.e. propagation delay and
packet processing time).
 Forward queuing time.
 The backward fixed delay.
 The backward queuing time.
 To utilize the network bandwidth efficiently, we
redefine the actual throughput as:
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 we define a new IP option named AQT (accumulated
queuing time) to collect the queuing time along the
path.
 According to the general format of IP options , the
fields of an AQT option are created as in Fig 1.
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 A probing packet is a normal TCP packet (data or ACK)
with an AQT option in its IP header.
 When a RoVegas source sends out a probing packet, it
sets the AQT field to zero.
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 Whenever a RoVegas destination acknowledges a
probing packet, it inserts an AQT option into the ACK.
 The AQT field is set to zero, and the AQT-echo field is
set to the value of the AQT field of the received packet.
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 A RoVegas source is able to obtain both the forward
queuing time (the value of the AQT-echo field) and
backward queuing time (the value of the AQT field)
from the received probing packet.
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 For each ACK packet received by a RoVegas source, the
BaseRTT can be measured based on the following
pseudo-code:
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 The rule for congestion window adjustment is as
follows:
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 We perform the simulations using the network
simulator ns-2.1b9a to compare the throughput
between Vegas and proposed RoVegas.
 Two network topologies have been created as shown in
Figs. 2 and 3 for our performance evaluations.
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 Several variable-bit-rate (VBR) sources are used to
generate the backward traffic.
 Here α=1 ,β=3 and without loss of generality, the data
packet size is set at 1 kbyte.
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6.75倍
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 RoVegas provides a more realistic and effective way to
improve the connection throughput of TCP Vegas
when the backward path is congested.
 Nevertheless, there is still some bandwidth left on the
forward path when the backward congestion occurs.
 The question of how to advance the utilization of the
forward path in such a situation will be the subject of
our further work.
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