06 Router Cong

7/18/2019 06 Router Cong

http://slidepdf.com/reader/full/06-router-cong 1/29

Router Congestion Control:

RED, ECN, and XCP



Where we left off

• Signal of congestion: Packet loss

• Fairness: Cooerating end!hosts using

"#$D % Ne&t lecture: Enforce'ent for (oS, rate,

dela), *itter guarantees

• +ut note: " acket dro is a er) -lunt

indicator of congestion

• Routers know 'ore than the).re telling/



What Would Router Do0

• Congestion Signaling:

% Dro, 'ark, send e&licit 'essages

• +uffer 'anage'ent: % Which ackets to dro0

% When to signal congestion0

• Scheduling % #f 'ultile connections, which one.s ackets to

send at an) gien ti'e0



Congestion Signaling

• Dros 1we.e coered2

• #n!-and 'arking % 3ne -it 1congested or not2: ECN

% $ultile -its 1how congested 4 how 'uchaaila-le2: XCP

• 3ut!of!-and notification

% #P Source (uench• Pro-le': #t sends more ackets when things are

congested/

• Not widel) used5



When to 'ark ackets0

• Dro!tail: % When the -uffer is full

% 6he de!facto 'echanis' toda)

% 7er) eas) to i'le'ent % Causes ackets to -e lost in -ursts

• Can lose 'an) ackets fro' a single flow/

• Can cause s)nchroni8ation of flows

% 9ees aerage ueue length high• ; full5 dela)

% Note relation to F#F3 1first!in!first out2: a schedulingdisciline, N36 a dro olic), -ut the).re often -undled



"ctie (ueue $g't5 w4RED

• E&licitl) tries to kee ueue s'all

% <ow dela), -ut still high throughut under -ursts

% 16his is =ower>: throughut 4 dela)2

• "ssu'es that hosts resond to lost ackets

• 6echniue:

% Rando'i8ation to aoid s)nchroni8ation

• 1Recall that if 'an) flows, don.t need as 'uch -uffer sace?2

% Dro before the ueue is actuall) full



RED algorith'

• #f a @ 'in

% <et all ackets through

• #f a A 'a& % Dro all ackets

• #f a A 'in BB a @ 'a&

% $ark or dro w4ro-a-ilit) a• ow to co'ute a0 ow to co'ute a0



RED 3eration

$in thresh$a& thresh

"erage (ueue <ength

minth maxth

maxP

5

Avg queue length

P(drop)



Co'uting a

• What to use as the ueue occuanc)0 % +alance fast resonse to changes

% With a-ilit) to tolerate transient -urs

% Secial case for idle eriods/• EW$" to the rescue again/

% (a G 1 % w2Ha I w H

• +ut what alue of w0

% +ack of the eneloe: 5J % RED is sensitie to this alue, and it.s one of the things

that 'akes it a -it of a ain in ractice

% See htt:44www5aciri5org4flo)d4red5ht'l



Co'uting a

• P- ia linear interolation

% P- G 'a& H 1a % 'in 4 'a& % 'in2

• $ethod : a G -

% Keo'etric rando' aria-le for inter!arrials -etweendros5

% 6ends to 'ark in -atches 1 S)nc2

• $ethod J:

% Lnifor' r55 X -e unifor' in M, J, / 4-!

% Set a G -41!count H -2

• Count G O un'arked ackets since last 'ark



RED ara'eter sensitiit)

• RED can -e er) sensitie to ara'eters % 6uning the' is a -it of a -lack art?

• 3ne thing: =gentle> RED

% 'a& @G - @G as % 'a&thresh @G a @G JH'a&thresh

% instead of =cliff> effect5 $akes RED 'ore ro-ust tochoice of 'a&thresh, 'a&

•+ut note: Still 'ust choose w, 'inthresh/• RED is not er) widel) delo)ed, -ut testingagainst -oth RED and Dro6ail is er) co''on inresearch, -ecause it could -e5



=$arking>, =Detection>

• RED is =Rando' Earl) Detection”

% Could 'ean 'arking, not droing

•$arking0 % DEC-it: =congestion indication> -inar)

feed-ack sche'e5

% #f ag ueue len Athresh, set the -it

% #f A half of ackets 'arked, e&onential

decrease, otherwise linear increase



$arking J: ECN

• #n #P!land % #nstead of dropping a acket, set a -it

% #f -it set, react the sa'e wa) as if it had -een droed 1-ut )ou don.thae to retrans'it or risk losing "C9 clocking2

• Where does it hel0 % Dela)!sensitie as, articularl) low!-w ones

% S'all window scenarios

• So'e co'le&it): % ow to send in legac) #P ackets 1#P 6oS field2

% Deter'ining ECN suort: two -its 1one =ECN works>, one =congestionor not>

% ow to echo -its to sender 16CP header -it2

• $ore co'le&it): Cheating? % We.ll co'e -ack to this later5 :2



+e)ond congestion indication

• Wh) do we want to do 'ore0

• 6CP doesn.t do so well in a few scenarios: % igh -andwidth!dela) roduct eniron'ents

• "dditie increase w4 acket window

• Could take 'an) R66s to fill u after congestion

• =not a ro-le'> with a single flow with 'assie -uffers 1in theor)2

• a real ro-le' with real routers and -urst) cross!traffic

% Short connections• 6CP neer has a chance to oen its window

% 3ne caeat: " ractical work!around to 'an) of thesero-le's is oening 'ultile 6CP connections5 6heeffects of this are still so'ewhat une&lored with regard tosta-ilit), glo-al fairnes and efficienc), etc5



XCP



Feed-ack G

I 5 acket

Round 6ri 6i'e

Congestion Window

Feed-ack G

! 5 acket

ow does XCP Work0



ow Does an XCP Router Co'ute

the Feed-ack0

Congestion Controller Fairness ControllerGoal: Divides ∆ betweenflows to converge to fairness

Looks at a flow’s state inCongestion Header

Algorithm:

If ∆ > ⇒ Divide ∆ e!"all#between flows

If ∆ $ ⇒ Divide ∆ betweenflows %ro%ortionall# to theirc"rrent rates

&I&D AI&D

Goal: &atches in%"t traffic tolink ca%acit# ' drains the !"e"e

Looks at aggregate traffic '!"e"e

Algorithm:

Aggregate traffic changes b# ∆

∆ ( )%are *andwidth

∆ ( + ,"e"e )i-e

)o. ∆ / α d avg )%are + β ,"e"e



∆ / α d avg )%are + β ,"e"e

2

24

0 2α β

π α =<< ανδ

0heorem: )#stem convergesto o%timal "tili-ation 1i2e2.stable3 for an# link bandwidth.dela#. n"mber of so"rces if:

14roof based on 5#!"istCriterion3

Ketting the deil out of the details /

Congestion ControllerFairness Controller

5o 4arameter 0"ning

5o 4arameter 0"ning

Algorithm:

If ∆ > ⇒ Divide ∆ e!"all# between flows

If ∆ $ ⇒ Divide ∆ between flows%ro%ortionall# to their c"rrent rates

5eed to estimate n"mber offlows 5

∑ ×

=

T in pkts

pkt pkt RTT Cwnd T N

)/(

1

600 %kt : 6o"nd 0ri% 0ime in header

Cwnd %kt : Congestion 7indow in header

0 : Co"nting Interval

5o 4er+Flow )tate

5o 4er+Flow )tate



"ortioning feed-ack

• 6rick) -it: Router sees ueue si8es andthroughutsQ hosts deal in cwnd5 $ustconert5

• Ne&t trick) -it: Router sees acketsQhost.s resonse is the sum of feed-ackreceied across its ackets5 $ust

aortion feed-ack onto ackets5• Reuire'ent: No er!flow state at router



XCP: Positie Feed-ack

• sare -4w to allocate

• N flows

• er!flow: ∆ roto rtt % <arger R66 needs 'ore cwnd increase to add

sa'e a'ount of -4w

•er!acket: % O ackets o-sered in ti'e d cwnd4rtt

% co'-ining the': i sare4N H rttJ 4 cwnd



+ut 'ust allocate to a flow

• ow 'an) ackets does flow # send in ti'e 60 % 6 H cwnd# 4 R664#

• So to count O of flows

% counter IG 4 16 H cwndkt 4 R66kt2 % eer) ti'e )ou receie a acket

• So: er!flow increase sare 4 counter

• 6his is a cute trick for statelessl) counting the O

of flows5• Si'ilar to tricks used in CSF( 1Core Stateless

Fair (ueueing2, which we.ll -e hitting ne&t ti'e



XCP decrease

• $ultilicatie Decrease % cwnd G -eta H cwndold 1sa'e -eta for all

flows2

% 6his is like the reerse of the slow!start'echanis'

• Slow start: Each "C9, increase cwnd -) % Results in e&onential increase

• XCP decrease: Each acket, decrease cwnd• +L6: $ust account for rtt# ?G ag R66, so

nor'ali8e % ni G total decrease H 1rtt# 4 agrtt2



XCP -enefits B issues

• Reuires =olicers> at edge if )ou don.t trust hosts toreort cwnd4rtt correctl) % $uch like CSF(/

• Doesn.t roide 'uch -enefit in today’s co''on case % +ut 'a) -e er) significant for tomorrow’s5

% igh -wHrtt eniron'ents 1KigE co'ing to a deskto near)ou/2

% Short flows, highl) d)na'ic workloads

• Cool insight: Decouled fairness and congestioncontrol

• Prett) -ig architectural change



+e)ond RED

• What if )ou want to use RED to tr) to

enforce fairness0



C39e

• C3se and 9ee49ill 1#nfoco' J2

% E&isting sche'es to enali8e unresonsie flows

1FRED4enalt) -o&2 introduce additional co'le&it)

% Si'le, stateless sche'e• During congested eriods

% Co'are new acket with rando' kt in ueue

% #f fro' sa'e flow, dro -oth

% #f not, use RED to decide fate of new acket



C39e

• Can i'roe -ehaior -) selecting 'ore

than one co'arison acket

% Needed when 'ore than one 'is-ehaing

flow

• Does not co'letel) sole ro-le'

% "ggressie flows are unished -ut not li'ited

to fair share % Not good for low degree of 'ultile&ing

wh)0



Stochastic Fair +lue

• Sa'e o-*ectie as RED Penalt) +o&

% #dentif) and enali8e 'is-ehaing flows

• Create < hashes with N -ins each

% Each -in kees track of searate 'arking rate 1'2

% Rate is udated using standard techniue and a -in si8e

% Flow uses 'ini'u' ' of all < -ins it -elongs to

% Non!'is-ehaing flows hoefull) -elong to at least one -in

without a -ad flow

• <arge nu'-ers of -ad flows 'a) cause false osities



Stochastic Fair +lue

• False osities can continuousl) enali8e

sa'e flow

• Solution: 'oing hash function oer ti'e

% +ad flow no longer shares -in with sa'e flows

% #s histor) resetdoes -ad flow get to 'ake

trou-le until detected again0

• No, can erfor' hash war'u in -ackground



"cknowledge'ents

• Seeral of the XCP slides are fro' Dina

9ata-i. S#KC3$$ resentation slides5

• htt:44www5ana5lcs5'it5edu4dina4XCP4

Documents

06 Router Cong