TCP Congestion Control. Congestion Avoidance

Transcription

1 TCP Congstion Control TCP sourcs chang th snding rat by modifying th window siz: Window = min {Advrtisd window, Congstion Window} Rcivr Transmittr ( cwnd ) In othr words, snd at th rat of th slowst componnt: ntwork or rcivr. cwnd follows additiv incras/multiplicativ dcras (AIMD) On rcipt of Ack: cwnd += 1 On packt loss (timout): cwnd *= 0.5 Congstion Avoidanc TCP s stratgy control congstion onc it happns rpatdly incras load in an ffort to find th point at which congstion occurs, and thn back off Altrnativ stratgy prdict whn congstion is about to happn rduc rat bfor packts start bing discardd call this congstion avoidanc, instad of congstion control Two possibilitis routr-cntric: RED Gatways host-cntric: TCP Vgas 1

2 RED Gatways Routr can hlp with mor information than ndusrs Schdulr FIFO WFQ (Wightd Fair Quuing) managmnt Drop-tail Global synchronization Lock out Random arly drop Random Early Drop (RED) Routr should signal congstion whn th quu first starts building up (by dropping a packt) Early: don t wait for quu to ovrflow Random: don t drop packts in burst, but spac drops out Avoid synchronization and lock-out Giv flows tim to rduc thir snding rats Drop packts basd on th avrag quu siz 4 2

3 RED Probabilistically discard (or mark) packts Probability is computd as a function of avrag quu lngth Drop Probability 1 0 min_th max_th Avrag Quu Lngth Dsign Goal of RED High throughput Low quuing dlay Accommodat small burst 3

4 Basic Architctural Componnts of an IP Routr Routing Protocols Routing Tabl Control Plan Forwarding Tabl Switching Datapath pr-packt procssing 7 Gnric Routr Architctur Hadr Procssing Lookup IP Addrss Updat Hadr Managr Addrss Tabl Mmory Hadr Procssing Lookup IP Addrss Updat Hadr Managr Addrss Tabl Mmory Hadr Procssing Lookup IP Addrss Updat Hadr Managr Addrss Tabl Mmory 8 4

5 TCP Vgas Ida: sourc watchs for som sign that routr s quu is building up and congstion will happn too;.g., RTT grows snding rat flattns Congstion window Avg. sourc snd rat spac at routr In shadd rgion w xpct throughput to incras but it cannot incras byond availabl bandwidth B 40 K s p B K g 1100 n i r 900 d 700 t n 500 u o 300 S r 100 n i z i s u u Q Vgas Algorithm Vgas tris not to snd at a rat that causs buffrs to fill Lt BasRTT b th minimum of all masurd RTTs If not ovrflowing th connction, thn ExpctRat = CongstionWindow/BasRTT Assum CWND = numbr of byts in transit Sourc calculats snding rat (ActualRat) onc pr RTT Pick on packt pr RTT, timstamp snd/ackrciv tim and divids by numbr of byts in transit Sourc compars ActualRat with ExpctRat Diff = ExpctdRat ActualRat if Diff < α incras CongstionWindow linarly ls if Diff > β dcras CongstionWindow linarly Els lav CongstionWindow unchangd 5

6 Algorithm (cont) Paramtrs α = 1 buffr β = 3 buffrs B 40 K Black lin = actual rat Grn lin = xpctd rat Shadd = rgion btwn α and β s p B240 K M120 A 80 C Not: Linar dcras in Vgas dos not violat AIMD sinc it Happns bfor packts loss TCP Modling Givn th congstion bhavior of TCP can w prdict what typ of prformanc w should gt? What ar th important factors MSS: Affcts incras rat Loss rat: Affcts how oftn window is rducd RTT: Affcts incras rat and rlats BW to window (RTO: Affcts prformanc during loss rcovry) 6

7 Ovrall TCP Bhavior Lt s concntrat on stady stat bhavior with no timouts and prfct loss rcovry Packts transfrrd = ara undr curv Window Tim Simpl TCP Modl Som additional assumptions Fixd RTT No dlayd ACKs In stady stat, TCP losss packt ach tim window rachs W packts Window drops to W/2 packts Each RTT window incrass by 1 packt W/2 * RTT bfor nxt loss 7

8 Transmission Rat What is ara undr curv? W = pkts/rtt, T = RTTs A = avg window * tim = ¾ W * T What was bandwidth? BW = A / T = ¾ W In packts pr RTT Nd to convrt to byts pr scond BW = ¾ W * MSS / RTT What is W? Dpnds on loss rat W W/2 Tim Simpl Loss Modl What was th loss rat? Packts transfrrd = (¾ W/RTT) * (W/2 * RTT) = 3W 2 /8 1 packt lost loss rat = p = 8/3W 2 BW = ¾ * W * MSS / RTT W = W = 8 3p 8 4 = 3p 3 MSS BW = RTT 2 p p 8

9 TCP Fairnss BW proportional to 1/RTT? Do flows sharing a bottlnck gt th sam bandwidth? NO! TCP is RTT fair If flows shar a bottlnck and hav th sam RTTs thn thy gt sam bandwidth Othrwis, in invrs proportion to th RTT TCP Frindlinss What dos it man to b TCP frindly? TCP is not going away Any nw congstion control must compt with TCP flows Should not clobbr TCP flows and grab bulk of link Should also b abl to hold its own, i.. grab its fair shar, or it will nvr bcom popular How is this quantifid/shown? Has volvd into valuating loss/throughput bhavior If it shows 1/sqrt(p) bhavior it is ok 9