Delay in Packet Switched Networks

Transcription

1 1 Delay in Packe Swiched Neworks Required reading: Kurose 1.5 and 1.6 CSE 4213, Fall 2006 Insrucor: N. Vlajic

2 Delay in Packe-Swiched Neworks 2 Link/Nework Performance Measures: hroughpu and delay Link Throughpu capaciy poenially available for an applicaion generally expressed in bi per second effecive capaciy my be reduced by applicaion muliplexing and proocol overhead (header bis, acknowledgmens, ec.) effecive hroughpu Nework Delay average ime for a block of daa o go from an applicaion on one sysem o corresponding applicaion on anoher sysem four ypes of delay ha conribue o overall delay: d = d + d + d + d oal processing queueing propagaion ransmission

3 Delay in Packe-Swiched Neworks (con.) 3 A ransmission propagaion B nodal processing queueing Processing Delay: ime required o process a packe (a) a source prior o sending, (b) a any inermediae rouer, and (c) a desinaion prior o delivering o applicaion ime o check for bi errors, o deermine oupu link, ec. on he order of 10-6 seconds or less ofen negligible Queueing Delay: ime spen waiing in a queue a any poin along he roue depends on inensiy and naure of raffic arriving a queue(s) on he order of 10-6 seconds o 10-3 seconds

4 Delay in Packe-Swiched Neworks (con.) 4 Propagaion Delay: ime for one bi o propagae from source o desinaion a propagaion speed of he link depends on physical medium of he link on he order of 10-6 seconds negligible for wo rouers on he same LAN; significan for wo geosaionary saellies disance beween source and desinaion [m] dpropagaio n [sec] = d [m] s [m/sec] propagaion speed of medium [m/s] Transmission Delay: ime o send ou / absorb all of he packe bis also known as sore-and-forward delay on he order of 10-6 seconds o 10-3 seconds negligible for ransmission raes 10 Mbps; significan for large packes sen over low-speed links dransmissi on [sec] = L [bi] R [bps] packe size [bis] link ransmission rae [bps]

5 5 ransmission delay propagaion delay Torono Vancouver When will Alice arrive o Vancouver?!

6 Delay in Packe-Swiched Neworks (con.) 6 Example [ nework delay one packe, one hop ] Suppose ha a user a a PC a one end of Canada sends a 1-Mbi file o a remoe server on he oher end over a daa link operaing a 64 kbps. Assume ha we are using a fiber opic link wih a propagaion rae of he speed of ligh, approximaely m/sec, and ha he disance is 4800 km. Ignore any processing or queueing delays. Wha is he overall nework delay, i.e. ime o ransmi he file? d = d + d oal propagaion ransmission R=64 [kbps] d [m] s [m/sec] [m] 3 10 [m/sec] dpropagaio n = = = [sec] A d=4800 [km] B L [bis] = R [bps] 6 10 [bis] [bps] dransmissi on = = [sec] ransmission delay >> propagaion delay A high-speed channel would reduce he overall delay.

7 Delay in Packe-Swiched Neworks (con.) 7 Example [ nework delay one packe, one hop ] For he same problem, now suppose ha we have a 1-Gbps link. Wha is he overall nework delay, i.e. ime o ransmi he file, in his case? d = d + d oal propagaion ransmission Propagaion delay sill he same: d propagaio n = [sec] L [bis] 10 = = R [bps] 10 [bis] [bps] 6 dransmissi on = [sec] d oal = [sec] Increasing daa rae beyond cerain value on a long (e.g saellie) link will no noiceably speed up file delivery.

8 Packe vs. Virual Circui Swiching Delay 8 Example [ nework delay one packe, muliple hops ] A message needs o be ransmied over a pah ha involves wo inermediae swiches. For simpliciy assume ha he propagaion delay and he bi rae of he ransmission lines are he same, and ignore any queueing delay. (1) Wha is he overall end-o-end message delay in case of daagram packe swiching? (2) Wha is he overall delay in case of virual-circui packe swiching? Draw corresponding diagrams. Message Subscriber A Message Message Rouer A d propagaion ime for 1 s bi o ransi from rouer A o rouer B Rouer B d ransmission ime for rouer B o absorb all packe bis Subscriber B Nework nodes

9 Packe vs. Virual Circui Swiching Delay (con.) 9 (1) delay in case of daagram packe swiching 2 inermediae rouers 3 hops beween source and desinaion d oal = 3*d propagaion + 3*d ransmission + 3*d processing ransmission begins (1 s bi leaves source) las bi leaves source las bi reaches Swich 1 packe processed d ransmission Source Swich 1 Swich 1 canno sar processing packe unil i receives all bis Swich 2 d propagaion d processing Desinaion 1-hop delay overall delay

10 Packe vs. Virual Circui Swiching Delay (con.) 10 (2) delay in case of VC packe swiching 2 inermediae rouers 3 hops beween source and desinaion d oal = 3*d CR + 3*d CC + 3*d propagaion + 3*d ransmission + 3*d processing 0 Source Swich 1 Connec Reques d ransmission CC Swich 2 CR CC d propagaion Desinaion CR Connec Confirm Delay

11 Delay Comparison Muliple Packes 11 Example [ nework delay muliple packes, muliple hops ]

12 Delay Comparison Muliple Packes (con.) 12 Example [ nework delay muliple packes, muliple hops ] Assume: N = number of hops L = message lengh [bis] R = daa rae [bps] P = packe size [bis] (payload + header) H = packe overhead S = seup ime [sec] d propagaion = propagaion delay per hop [sec] Compue end-o-end delay for circui, daagram and virual circui swiching, assuming: N=4, L=3200, R=9600, P=1024, H=16, S=0.2, d propagaion = Ignore any queueing or processing delay. Message Message message lengh Message Subscriber A Message header Nework nodes Subscriber B packe size

13 Delay Comparison Muliple Packes (con.) 13 (1) delay in case of circui swiching S d ransmission R1 R2 R3 D d propagaion Delay d = C1 + C2, where C1 = Call Seup Time C2 = Message Delivery Time = Propagaion Delay + Transmission Time C1 = S = 0.2 C2 = N x d propagaion + L/R = 4 x /9600 = d = = sec

14 Delay Comparison Muliple Packes (con.) 14 (2) delay in case of daagram packe swiching There are P H = = 1008 daa bis per packe. A message of 3200 bis require four packes: Np = 4. (3200 bis/1008 bis/packe = 3.17 packes which we round up o 4 packes.) S d ransmission R1 R1 d propagaion P 1 P 2 P 3 P 4 R2 P 1 P 2 P 3 P 4 R3 P 1 P 2 P 3 P 4 D P 1 P 2 P 3 P 4 Delay d = D1 + D2 + D3 + D4, where D1 = Time o propagae 1 s bi over all hops + ime o ransmi enire 1 s packe D2 = Time o ransmi enire 2 nd packe D3 = Time o ransmi enire 3 rd packe D4 = Time o ransmi enire 4 h packe ime o absorb he res of he message

15 Delay Comparison Muliple Packes (con.) 15 Le: T = ransmission ime for one packe = P/R D1 = N*d propagaion + 4*T = = 4*d propagaion + 4*P/R = = 4 x *1024/9600 = = D2 = D3 = D4 = T = = (P/R) = = (1024/9600) = = d = *0.107 = = sec

16 Delay Comparison Muliple Packes (con.) 16 (3) delay in case of virual-circui packe swiching S R1 R2 R3 d ransmission d propagaion P 1 P 2 P 3 P 4 P 1 P 2 P 3 P 4 P 1 P 2 P 3 P 4 D Delay P 1 P 2 P 3 P 4 d = V1 + V2, where V1 = Call Seup Time V2 = Daagram Packe Swiching Time = d d = S = = sec

17 Delay Comparison Muliple Packes (con.) 17 Now, assuming non-negligible packe processing delay (Pd): Pd daagram 0, Pd virual_circui 0, and Pd daagram > Pd virual_circui (1) delay in case of circui swiching: d circui = S + N x d propagaion + L/R (2) delay in case of daagram packe swiching: d daagram = N*(T+d propagaion ) + (Np 1)*T + N*Np*Pd daagram (3) delay in case of virual-circui packe swiching: d virual_circui = S + [d daagram -N*Np*Pd daagram + N*Np*Pd circui_swiching ]= = d daagram + S - N*Np*[Pd daagram -Pd circui_swiching ]

18 Delay Comparison Muliple Packes (con.) 18 Relaionship beween Packe Size and Transmission Time by breaking message ino smaller packes, nodes can begin ransmiing 1 s packe as soon as i has arrived, wihou waiing for 2 nd packe his overlap in packe ransmission can resul in considerably shorer overall delay smaller packe will sooner be received / absorbed a he desinaion however, process of using more and smaller packes evenually resuls in increased raher han reduced delay, since each packe mus conain a header Possible disadvanages of using small packes? in conras o ransmission delay, processing and queueing delays always increase when more packes are handled for a single message packe-swiched nework designers mus consider all facors when aemping o find an opimum packe size

19 Queueing Delay and Packe Loss 19 Queueing Delay mos complex componen of nework delay unlike he oher ree delays (d processing, d ransmission, d propagaion ) queueing delay can vary from packe o packe! example: 10 packes arrive a an empy queue a he same ime 1 s packe ransmied will suffer no queueing delay, las packe ransmied will suffer relaively large queueing delay when characerizing queueing delay, one ypically uses probabilisic and saisical measures, such as: average queueing delay, average queue size, probabiliy ha queue exceeds some specific size, ec. queue Calculaion of queueing delay (for known λ and w) discussed earlier!!!

20 Queueing Delay and Packe Loss (con.) 20 Traffic Inensiy (Rouer Uilizaion) plays criical role in esimaing exen of queueing delay raffic inensiy = La R aka server uilizaion L = packe size [bis/packe] a = average packe arrival rae [packe/sec] R = oupu ransmission rae [bps] arrival bi rae - λ deparure bi rae - μ ρ = La/R 0: average queueing delay small ρ = La/R 1: delay becomes large ρ = La/R > 1: average rae a which bis arrive exceeds he rae a which bis can be ransmied (a) infinie queue qeueing delay (b) finie queue rouer drops packes La [bps] R[bps]

21 Queueing Delay and Packe Loss (con.) 21 Example [ average queueing delay ] Assume La/R=1 (average arrival bi rae = average deparure bi rae). Deermine he average queueing delay in he following wo cases: (a) Packes arrive periodically one packe of size L every L/R seconds. (b) Packes arrive in burss, bu periodically N packe every (L/R)*N seconds. (a) Every packe will arrive a an empy queue no queueing delay. (b) 1 s packe: no queueing delay 2 nd packe: delay = L/R [sec] 3 rd packe: delay = 2*L/R [sec] N h packe: delay = (N-1)*L/R [sec] average delay =??? Typically, arrivals do no follow any paern and packes are spaced apar by random amoun of ime La/R is no usually sufficien o fully characerize delay saisics sics!

22 The Inerne 22 Inernework The Inerne wo or more inerconneced neworks mos noable daagram packe-swiching inernework evolved from he ARPANET (nework of compuers operaed by several universiies doing miliary research) iniially developed in 1969 componen neworks differ in erms of heir underlying echnology and operaion spread over 200 counries made up of 100,000s of inerconneced neworks, 10,000,000s of inerconneced hoss, and 100,000,000s of users sill grows exponenially Inerne Service Provider (ISP) Inerne connecion poin allows users or oher neworks o connec o he Inerne 3-iered hierarchy of inerconneced ISPs keeps he Inerne ogeher lower-ier ISPs provide access o home users hrough dial-up, cable, DSL, high-speed LANs, ec. upper-ier ISPs provide access o lower-ier ISPs hey consis of high-speed rouers and high- -speed fiber-opic links

23 The Inerne (con.) 23 Example [ number of Inerne hoss ]

24 The Inerne (con.) 24 3-iered Hierarchy of ISPs Tier-1: Inernaional / Naional ISPs Tier-2: Regional ISPs Tier-3: Local ISPs local ISP Tier 3 ISP Tier-2 ISP local ISP Tier 1 P local ISP Tier-2 ISP NAP local ISP local ISP Tier-2 ISP Tier 1 ISP local ISP Tier-2 ISP local ISP Tier 1 Tier-2 ISP local ISP

25 The Inerne (con.) 25 Inernaional ISP Inerne backbone nework one of he neworks a he op of ISP-hierarchy has inernaional coverage (e.g. MCI, Sprin, AT&T, UUNe) look similar o any oher nework (links + rouers), bu link speed = 600 Mbps 10 Gbps range, rouers mus be able o forward packes a exremely high raes direcly connec o each of he oher ier-1 ISPs; also connec o a large number of ier-2 ISPs and oher cusomer neworks NAP se of high-speed rouers hrough which rouers from differen ier-1 ISP can exchange raffic NAP can be owned and operaed by a hird-pary elecom company or by an Inerne backbone provider ier-1 providers inerconnec (peer) privaely hrough privae swiching saions called peering poins Tier 1 Tier 1 NAP Tier 1 ier-1 providers also inerconnec a public nework access poins (NAPs)

26 The Inerne (con.) 26 Example [ Sprin US backbone nework ]

27 The Inerne (con.) 27 Regional ISP smaller ISP ha connecs o one or more ier-1 ISPs and possibly oher ier-2 ISPs o reach he global Inerne, a ier-2 ISP needs o connec o and roue raffic hrough one of he ier-1 ISP ier-2 ISP is cusomer, ier-1 ISP is provider provider charges cusomer a fee a ier-2 ISP can also connec direcly o oher ier-2 nework wihou having o pass hrough a ier-1 nework some ier-1 are also ier-2 providers, selling Inerne access direcly o end users (e.g. Sprin, AT&T, ) Tier-2 ISP pays ier-1 ISP for conneciviy o res of Inerne Tier-2 ISP Tier 1 Tier 1 Tier-2 ISP NAP Tier 1 Tier-2 ISP Tier-2 ISPs also peer privaely wih each oher, inerconnec a NAP Tier-2 ISP Tier-2 ISP

28 The Inerne (con.) 28 Local ISP small ISP ha connecs o he Inerne via one or more ier-2 ISPs and provides access o end users local ISP can be (1) company ha jus provides Inerne service (2) corporaion ha supplies service o is own employees (3) college or universiy ha runs is own nework ier-3 ISPs are cusomers of higher ier ISPs Connecing hem o res of Inerne local ISP local ISP Tier 3 ISP Tier-2 ISP Tier 1 Tier-2 ISP local ISP local ISP Tier 1 local ISP Tier-2 ISP NAP Tier 1 Tier-2 ISP local ISP local ISP Tier-2 ISP local ISP

29 Delay and Roues in he Inerne 29 Traceroue uiliy (ool) ha races packe from hos_1 o hos_2, showing number of hops beween hoss and how long each hop akes works by sending UDP packes wih low TTL fields TTL specifies how many hops packe is allowed o pass before being discarded (1) sender firs sends a UDP daagram wih TTL=1 as well as an invalid por number o desinaion hos (2) 1 s rouer o see daagram ses TTL=0, discards daagram, and sends an ICMP Time Exceeded message o sender his info enables sender o idenify 1 s machine in roue and associaed roundrip delay (3) raceroue coninues o idenify remaining machines by sending daagrams wih successively larger TTLs raceroue repeas above experimen 3 imes source acually sends 3*N packes o desinaion (N=number of hops) IP(A) 3 probes 3 probes IP(B) IP(B) TTL=1 3 probes

30 Delay and Roues in he Inerne (con.) 30 Traceroue Origin raceroue is a UNIX uiliy, bu nearly all plaforms have somehing similar Windows includes a raceroue uiliy called racer you can run racer from MS-Dos Window, by enering racer followed by domain name, e.g. racer racer implemenaion is differen from raceroue!!! Traceroue Use raceroue is generally used: (1) as nework debugging ool by pinpoining nework conneciviy problems (2) for idenifying IP addresses of inermediae rouers Example [ raceroue ] If you are visiing a Web sie and pages are appearing slowly, you can use raceroue o figure ou where he longes delay(s) are occurring.

31 Delay and Roues in he Inerne (con.) 31 Example [ raceroue: York Universiy o BBC ]

32 Delay and Roues in he Inerne (con.) 32 Example [ Cogen Nework Map ] Cogen's inernaional IP nework is he larges in he world and is scalable o a oal capaciy of over one Terabi.

33 Delay and Roues in he Inerne (con.) 33 Example [ raceroue ] raceroue: gaia.cs.umass.edu o 1 cs-gw ( ) 1 ms 1 ms 2 ms 2 border1-r-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms 3 ch-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms 4 jn1-a wor.vbns.ne ( ) 16 ms 11 ms 13 ms 5 jn1-so wae.vbns.ne ( ) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms ( ) 104 ms 109 ms 106 ms 9 de2-1.de1.de.gean.ne ( ) 109 ms 102 ms 104 ms 10 de.fr1.fr.gean.ne ( ) 113 ms 121 ms 114 ms 11 renaer-gw.fr1.fr.gean.ne ( ) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renaer.fr ( ) 111 ms 114 ms 116 ms 13 nice.cssi.renaer.fr ( ) 123 ms 125 ms 124 ms 14 r32-nice.cssi.renaer.fr ( ) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r32.f.ne ( ) 135 ms 128 ms 133 ms ( ) 126 ms 128 ms 126 ms 17 * * * 18 * * * Three delay measuremens from gaia.cs.umass.edu o cs-gw.cs.umass.edu rans-oceanic link * means no response (probe los, rouer no replying)

34 Delay and Roues in he Inerne (con.) 34 VisualRoue for Inerne Performance: hp:// Quickly locae he source of Inerne delays! VisualRoue provides an auomaed analysis and quick idenificaion of where Inerne conneciviy problems occur. Sop geing he run around from your ISP! VisualRoue enables you o easily provide domain and nework adminisraors proof of Inerne conneciviy problems. Find ou where a Web sie or IP address is locaed! VisualRoue idenifies he acual ciy and counry locaion, in addiion o he WHOIS locaion, and he nework being used. VisualRoue shows your connecion pah and where bolenecks occur. Many people accessing he Web or heir corporae Inrane encouner problems connecing o he desired sie, or find ha he daa or ransacion moves oo slowly. Deermining he source of he problem is a difficul ask for he vas majoriy of users, who ofen ge caugh in he 'blame game' beween various paries. VisualRoue grealy simplifies he process of finding and reporing he problem.

35 Delay and Roues in he Inerne (con.) 35

36 Exercise 36 Check he following apples provided hrough he exbook web-sie: hp://wps.aw.com/aw_kurose_nework_3/0,9212, ,00.hml 1) Transmission versus Propagaion Delay Apple 2) Queuing and Loss Apple 3) Message Segmenaion