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Adaptive Multi-level Explicit Congestion

Notification

Deepak Ghosh (deepak.ghosh@rediffmail.com)Dept. of Computer Science & Information Technology

!harati "idyapeeth College of #ngineering $um%ai ni'ersity

%stract

In this paper e present to you an e*tended $ulti+le'el #C, a ne TC- congestion

scheme a concept proposed %y $ukundan Sridharan ran Durresi and /a 0ain of The 1hioState ni'ersity Colum%us 1hio. The $ulti+le'el #*plicit Congestion ,otification ($#C,)

algorithm allos netork operators to achie'e high throughput ith corresponding lo

delays. !ut $#C, a'erage 2ueue is sensiti'e to its parameter settings and its le'el ofcongestion hence no guarantees can %e gi'en a%out delay. Delay %eing a maor component

of the 2uality of ser'ice netork operators ould naturally like to ha'e a rough estimate of

the a'erage delays in their congested routers. To achie'e predicta%le a'erage delays ith$#C, ould re2uire constant tuning of the parameters to adust to current traffic

conditions. The goal of this paper is to present a solution to the parameter tuning pro%lem

of the $#C,. 3e compare the performance of the dapti'e $#C, system ith the

dapti'e /#D system using simulations on the ,S+4 simulator. !ased on simulations efind that dapti'e $#C, performs %etter than dapti'e /#D.

I. I,T/1DCTI1,#nd+to+end congestion control schemes continue to %e one of the main determinants of the

ro%ustness of the Internet. Congestion remains the main o%stacle to 5uality of Ser'ice (5oS) on

the Internet. lthough a num%er of schemes ha'e %een proposed for netork congestion control

the search for ne schemes continues. 678 gi'es a sur'ey of different congestion control schemes.!ut the inner for the time %eing seems to %e /#D9#C, class of algorithms. #C, as made a

standard %y the I#T: in 4;;< 6=8. >ence it %ecomes imperati'e that the possi%ilities of utili?ing

the #C, frameork to the fullest are e*plored. 6

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performance of $#C, ith /#D and $#C, and sho that it performs %etter than the

other schemes. In Section II e gi'e a %rief introduction to the $#C, protocol. In Section

III e present to you the concept of 3ireless $C, (3$C,). In Section I" e introduce thedapti'e $ultile'el #C, protocol and site some guidelines on setting the parameters. In Section

" e present the conclusions.

II. !/I#:I,T/1DCTI1, T1$#C,

A. Marking bits at the router

$#C, 6

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The $#C, source reaction can %e summari?ed as

3hen there is a packet+drop the cnd is reduced %y !H7;. This done for to reasons

:irst a packet drop means se'ere congestion and %uffer o'erflo and some se'ere actions need

to %e taken. Second to maintain %ackard compati%ility ith routers those donJt implement

#C,. :or other le'els of congestion such a drastic step as reducing the cnd %y half is not

necessary and might make the flo less 'igorous. 3hen there is no congestion the cnd is

alloed to gro additi'ely as usual. 3hen the marking is

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Congestion Feedback Meaning Weights

01 No Congestion 0

10 Mild Congestion 1

11 Severe Congestion 2

Fig. 1.2 Congestion Bits, their meaning and weights

B. Filtering Context and !ource esponse

The reflection of the congestion %its at the recei'er is done in the same ay as in $#C,. s in

$#C, the congestion feed%ack %its can ha'e three 'alues A;

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In all the a%o'e four conditions the packet hich as lost is retransmitted. gain in all the a%o'e

conditions AcndA is decreased only if it had not %een decreased once %efore in the same /TT elsethe packets are ust retransmitted and the indo is not reduced. #ach time a packet loss is

identified as a Aireless lossA it is treated as a congestion feed%ack ith a A;ence the scheme is highly accurate and this scheme performs %etter than all other

schemes hich doesnAt make any assumptions a%out the netork.

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I". D-TI"#$#C,

A. Motivation

In dapti'e $#C, the o%ecti'e is to maintain the 2ueue near the targetqueue. If the a'erage

2ueue does not 'ary and remains constant at targetqueue the pro%a%ility of packet drop9markill remain fi*ed. Ket the pro%a%ility %e -target. 3e set the targetqueueto %e in %eteen minth

and midth. >ence only the first pro%a%ility cur'e ill %e acti'e in this region. >ence the

pro%a%ility -target is gi'en %y

-target H (-ma*9(ma*th+minth))N(averagequeue+minth)OOOOOO...O. (ence if Averagequeue is greater thantarget2ueue at any instant e need to increase -ma* hich ould decrease the

Averagequeue so that it %ecomes e2ual to the targetqueueand if the Averagequeue is less

than targetqueue at any instant e need to decrease -ma* to allo 2ueue to gro hichould gi'e a %etter throughput. Thus to keep a constant 2ueue e need to adopt the -ma*.

lso e need to set other parameters like 32 ma*th minth and midth automatically. Thea%o'e discussion leads to the conclusion of the re2uirement of the $#C, algorithmP

dapt -ma* in response to measured 2ueue lengths and set 32 ma*th midth and minth

automatically %ased on link speed and target 2ueue.

B. Algorithm

=

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The o'erall dapti'e $#C, hich as implemented has the folloing features

-ma* is adapted to keep the a'erage 2ueue si?e ith a target range half ay %eteen minth

and ma*th.

-ma* is adapted sloly o'er time scales greater than a typical /TT and in small steps. The

time scale is generally 7+ence it is %elie'ed that and ! should also %e adapted according to the positionof the a'erage 2ueue ith respect to the target 2ueue. So the 'alue of and ! are also

recalculated e'ery ;.7 seconds hen the -ma* calculation is done. Taking the recommendation

form 648 that !Q;.E the 'alue of ! is scaled from ;.E to

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Thus use the formula gi'en %elo to adapt !

! H

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". C1,CKSI1,S

In this paper e presented the dapti'e $ulti+Ke'el #*plicit Congestion ,otification scheme

hich adapts the $#C, parameter -ma* and automatically sets the $#C, parameters 32

midth minth and ma*th. The $#C, maintains a %uffer 2ueue hich is set according to thedelay re2uirements of the users. The choice of the target 2ueue si?e is a tradeoff %eteen the

link utili?ation and delay. 3e sho using simulations that $#C, has %etter delay and

throughput performances than dapti'e /#D.

"I. /#:#/#,C#S

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