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

TCP over ATM - Cont ent s • General • What t o r esear ch • P r ot ocol Archit ect ur e • TCP over UBR • TCP over ABR 20 March 2001 Computer Networks, advanced course 1 Jonny Pettersson, UmU 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 over ATM • I ncr eased r esear ch on t he perf or mance of TCP over ATM 20 March 2001 Computer Networks, advanced course 2 Jonny Pettersson, UmU 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 20 March 2001 Computer Networks, advanced course 3 Jonny Pettersson, UmU 1

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 4 Jonny Pettersson, UmU 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 5 Jonny Pettersson, UmU 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 ot her cells will st ill be f orwarded 20 March 2001 Computer Networks, advanced course 6 Jonny Pettersson, UmU 2

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 7 Jonny Pettersson, UmU 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 8 Jonny Pettersson, UmU 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 20 March 2001 Computer Networks, advanced course 9 Jonny Pettersson, UmU 3

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 10 Jonny Pettersson, UmU 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 11 Jonny Pettersson, UmU TCP over ATM – TCP over ABR • Since good perf or mance of TCP over 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 over ABR is mor e complex and t her eby mor e dif f icult t o make general conclusions 20 March 2001 Computer Networks, advanced course 12 Jonny Pettersson, UmU 4

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 operat 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 13 Jonny Pettersson, UmU TCP over ATM – TCP over ABR (2) • TCP over 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 14 Jonny Pettersson, UmU TCP over ATM - Summary • What t o r easearch • TCP over UBR – Plain UBR – Part ial packet discard – Early packet discard – Early packet discard wit h f air buf f er allocat ion • TCP over ABR 20 March 2001 Computer Networks, advanced course 15 Jonny Pettersson, UmU 5