1 TCP over ATM Prot ocol Archit et ure Typical prot ocol st ack - - PDF document

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 1

TCP over ATM - Cont ent s

• General
• What t o r esear ch
• P

r ot ocol Archit ect ur e

• TCP
• ver UBR
• TCP
• ver ABR

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 2

TCP over ATM

• TCP

/ I P has t radit ionally been r unned over net wor ks wit h relat ively f ew f eat ur es f or congest ion cont rol and qualit y of ser vice (QoS)

• ATM net wor ks

– Capable of complex QoS f unct ions – Have a wide varit y of congest ion and t r af f ic cont rol f acilit ies

• Gr owing use of TCP
• ver ATM
• I ncr eased r esear ch on t he perf or mance of TCP
• ver ATM

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 3

TCP over ATM - Resear ch

• What t o invest igat e?

– How t o best manage TCPs:

• Segment size
• Window management
• Congest ion cont r ol policies

– How t o best manage ATMs:

• QoS
• Tr af f ic cont r ol policies

– To achieve:

• High t hr oughput f or TCP t r af f ic
• Fair allocat ion among var ious TCP connect ions
• Ef f icient use of t he under lying ATM net wor k

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 4

TCP over ATM – Prot ocol Archit et ure

• Typical prot ocol st ack

– TCP – I P – AAL5

• CS
• SAR

– ATM

• Fig 10.15
• Each CS PDU car r ies exact ly one I P

dat agram

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 5

TCP over ATM - Cases

• Two dif f erent cases

– TCP over UBR

• For delay-t oler ant applicat ions (f ile t r ansf er , mail)

– TCP over ABR

• For applicat ions wher e delay is a concer n (on-line sessions)
• Main dif f erence

– ABR provides congest ion f eed-back, UBR does not

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 6

TCP over ATM – TCP over UBR

• To guarant ee t hat no segment get lost

– Each ATM swit ch needs t o have a buf f er capacit y equal t o t he sum of t he TCP receive windows f or all act ive TCP connect ions over t he net work

• Buf f er capacit y at ATM swit ches is a cr it ical

par amet er in assessing TCP t hr oughput per f or mance

• Fig 10.16a,b

– When one cell of a I P dat agram must be dropped, t he

• t her cells will st ill be f orwarded

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 7

TCP over ATM – TCP over UBR (2)

• P

ar amet er s

– Swit ch buf f er size – TCP segment size – TCP receive window – Number of TCP connect ion

• Solut ion

– I ncrease swit ch buf f er size – Part ial packet discard (PPD) – Early packet discard (EPD)

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 8

TCP over ATM – Part ial Packet Discard

• I f a cell is dropped, dr op t he subsequent cells in

t hat I P dat agram as well, but not t he last cell wit h SDU t ype bit set t o one

• Fig 10.16c

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 9

TCP over ATM – Early Packet Discard

• When a swit ch buf f er r eaches a t hr eshold level,

but bef ore it is act ually requir ed t o discar d any cells, an ent ir e I P dat agram is dropped

• Fig 10.16d

– Thr eshold is set t o half t he t ot al buf f er size

• St ill one problem: Fair ness

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 10

TCP over ATM – EPD wit h FBA

• EP

D has a bias against connect ions wit h shor t er I P dat agr ams

• EP

D has a bias against connect ions t hat pass t hrough mult iple congest ed swit ches

• Solut ion: Fair Buf f er Allocat ion (FBA)

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 11

TCP over ATM – EPD wit h FBA

• The swit ch will choose t o discar d f r om t he VC

t hat is using mor e t han it s f air shar e of t he swit ch’s buf f er

• Fig 10.17
• W(i) = N(i) / (N / V)
• Select ive drop

– I f N > R AND W(i) > Z t hen drop packet

• FBA

– I f N > R AND W(i) > Z x ((B – R) / (N – R)) t hen drop packet – More aggr essive

• Table 10.2

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 12

TCP over ATM – TCP over ABR

• Since good perf or mance of TCP
• ver UBR can be

achieved, t here has been less resear ch on using t he mor e complex and mor e expensive ABR ser vice

• But ABR is now f ully def ined and is likely t o be

increasingly implement ed by ATM swit ch vendor s

• TCP
• ver ABR is mor e complex and t her eby mor e

dif f icult t o make general conclusions

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20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 13

TCP over ATM - ABR

• Minimum cell r at e (MCR)
• P

eak cell r at e (P CR)

• ABR f low cont r ol consit s of t wo modes of
• perat ion

– Binary mode

• A swit ch signals beginning / end of congest ion

– Explicit rat e mode

• A swit ch sends explicit r at e guidance t o each sour ce

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 14

TCP over ATM – TCP over ABR (2)

• TCP
• ver ATM t raf f ic operat es in t wo dif f erent

modes

– Window-limit ed mode

• At st ar t : Slow St art

– Rat e-limit ed mode

• Congest ion in ATM swit ches
• P

er f or mance

– Paramet er set t ings is cr it ical – ATM swit ch buf f er size is import ant – More t o be done...

20 March 2001 Computer Networks, advanced course Jonny Pettersson, UmU 15

TCP over ATM - Summary

• What t o r easearch
• TCP
• ver UBR

– Plain UBR – Part ial packet discard – Early packet discard – Early packet discard wit h f air buf f er allocat ion

• TCP
• ver ABR