Advanced Image Information (3) 画像情報特論 (3) TCP Variants. 情報理工学専攻甲藤二郎

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1 画像情報特論 (3) Advanced Image Information (3) TCP Variants 情報理工学専攻甲藤二郎

2 TCP Variants

3 TCP-Reno (loss-based) cwnd loss loss loss a= buffer b=0.5 0 BDP increase: cwnd = cwnd + /cwnd decrease: cwnd = cwnd / n AIMD: additive increase multiplicative decrease

4 TCP-Vegas (delay-based) cwnd buffer stored packets in a buffer α BDP n 0 e.g. α=, β=3 diff cwnd RTT min cwnd RTT RTT min stored packets in a buffer increase: cwnd cwnd cwnd cwnd diff otherwise diff decrease: cwnd cwnd *0.75

5 TCP problems, 5 years ago broadband wired networks slow window increase (inefficiency) deployment of wireless networks cannot distinguish wireless errors and buffer overflow TCP-Reno (NewReno, SACK) problem Reno expels Vegas (unfriendliness)

6 TCP Variants in the th century Loss-driven (AIMD) TCP-Reno / NewReno / SACK High-Speed TCP (IETF RFC 3649, Dec 003) Scalable TCP (PFLDnet 003) BIC-TCP / CUBIC-TCP (IEEE INFOCOM 004, PFLDnet 005)... Linux H-TCP (PFLDnet 004) TCP-Westwood (ACM MOBICOM 00) Delay-driven (RTT Observation) TCP-Vegas (IEEE JSAC, Oct 995) FAST-TCP (INFOCOM 004) Hybrid Gentle High-Speed TCP (PfHSN 003) TCP-Africa (IEEE INFOCOM 005) Compound TCP (PFLDnet 006)... Windows (proposed by MSR) Adaptive Reno (PFLDnet 006) YeAH-TCP (PFLDnet 007) TCP-Fusion (PFLDnet 007) our lab

7 Loss-based TCPs aggressive adaptive a Variants Increase / Update Decrease TCP-Reno 0.5 HighSpeed TCP (HS-TCP) w a( w) b( w) p( w) b( w) e.g. 70 (0Gbps, 00ms) Scalable TCP (STCP) 0.0 (per every ACK) BIC-TCP CUBIC-TCP additive increase ( fast) binary search ( slow) max probing ( fast) w 3 0.4( t 3 W max ) Wmax H-TCP 0. 5 t TH TCP-Westwood (TCPW) b log( w) log( Wlow ) b( w) ( bhigh 0.5) 0.5 log( W ) log( W ) high low e.g. 0. (0Gbps, 00ms) RTT RTT RE * RTT BE * RTT min max min min B( k ) B( k) for B( k) / PS / PS otherwise ( congested) ( not congested)

8 Delay-based TCPs Variants Update Decrease TCP-Vegas ) ( ) ( ) ( congestion early w stable w congestion no w w 0.75 FAST-TCP w RTT RTT w w w min ) (, min 0.5 (?) a b

9 Hybrid TCP cwnd loss loss loss buffer 0 BDP n RTT increase: loss mode improvement of friendliness no RTT increase: delay mode improvement of efficiency

10 Hybrid TCPs a b simple adaptive Variants Increase Decrease Gentle HS-TCP HS-TCP / Reno HS-TCP TCP-Africa HS-TCP / Reno HS-TCP Compound TCP (CTCP) 0.5 cwnd / Reno 0.5 Adaptive Reno (ARENO) YeAH-TCP TCP-Fusion B /0Mbps / Reno STCP / Reno B * D PS min 0.5 max ( non congestion loss) ( congestion loss) RTTmin RTT, 0.5 / Reno RTTmin max, 0.5 RTT

11 CUBIC-TCP (Linux default)

12 BIC-TCP () Binary Increase Congestion Control パケットロスが発生した cwnd 平衡点 L.Xu et al: Binary Increase Congestion Control (BIC) for Fast Long-Distance Networks, IEEE INFOCOM 004.

13 BIC-TCP () Window Increase concave convex additive increase (linear increase) Wmax: cwnd when a last loss happened Smax: maximum increase rate (e.g. 3) Smin: minimum increase rate (e.g. 0.0) if (cwnd < Wmax ) Winc = (Wmax cwnd) / ; else Winc = (cwnd - Wmax) / ; if (Winc > Smax) Winc = Smax; elseif (Winc < Smin) Winc = Smin; binary search cwnd = cwnd + Winc / cwnd; L.Xu et al: Binary Increase Congestion Control (BIC) for Fast Long-Distance Networks, IEEE INFOCOM 004.

14 BIC-TCP (3) Window Decrease Wmax=target cwnd update loss Wmax,new=0.9*cwnd loss Wmax,new=cwnd if (cwnd < Wmax ) Wmax,new = cwnd * (-) / ; else Wmax,new = cwnd; cwnd = cwnd * (- ); : decrease rate (e.g. 0.) *0.9: give bandwidth to newly-coming flows... Fast Convergence L.Xu et al: Binary Increase Congestion Control (BIC) for Fast Long-Distance Networks, IEEE INFOCOM 004.

15 CUBIC-TCP () Cubic approximation of BIC-TCP S.Ha et al: CUBIC: A New TCP Friendly HighSpeed TCP Variant, ACM SIGOPS Review, 008.

16 CUBIC-TCP () Window Increase W(t) /* cubic function */ Winc = W(t+RTT) cwnd; cwnd = cwnd + Winc / cwnd; K Wmax*(-) W ( t) C *( t K) K W tcp 3 W max W 3 max equivalent to Reno C t ( t) Wmax ( ) 3 RTT /* TCP mode */ if ( Wtcp > cwnd ) cwnd = Wtcp; window decrease is the same as BIC : decrease rate (e.g. 0.) C: constant (e.g. 0.4) S.Ha et al: CUBIC: A New TCP Friendly HighSpeed TCP Variant, ACM SIGOPS Review, 008.

17 CUBIC-TCP (3) CUBIC s cwnd behavior S.Ha et al: CUBIC: A New TCP Friendly HighSpeed TCP Variant, ACM SIGOPS Review, 008.

18 CUBIC-TCP (4) Advantages stability intra-protocol fairness among multiple CUBIC flows Disadvantages heavy buffer occupancy and delay increase ( delay-based) inter-protocol unfairness against other TCP flows Linux beats Windows! (vs. Compound TCP) K.Munir et al: Linux beats Windows! or the Worrying Evolution of TCP..., PFLDNet 007.

19 Hybrid TCPs

20 Hybrid TCP () single flow packet loss clearing buffer (TCPW) cwnd last cwnd last RTT RTT min vacant capacity Hybrid cwnd last / legacy (Reno) BDP n adaptive switching of two modes (loss & delay): constant rate until RTT increases (delay mode) : efficiency and low delay performs as Reno when RTT increases (loss mode) : friendliness

21 Hybrid TCP () two flows packet loss clearing buffer (TCPW) cwnd last cwnd last RTT RTT min vacant capacity Hybrid 3 cwnd last / legacy (Reno) BDP/ n adaptive switching of two modes (loss & delay): fast cwnd increase (delay... efficiency ) mild cwnd decrease (delay... congestion avoidance) 3 performs as Reno when RTT increases (loss... friendliness )

22 Min-Max Fair (ideal case) Min-Max-Fair: allocate maximum bandwidth to a user who has minimum bandwidth 4 3 allocate (remaining) to a flow allocate (fair) /3 to each flow allocate remaining /3 to a flow 3 5 D.Bertsekas and R.Gallager: Data Networks, Prentice Hall.

23 Ideal: TCP s objective bandwidth session start Min-Max Fair another session 時間 TCP Reno bandwidth session start searching for Min-Max Fair another session 時間

24 TCP behavior model () model definition Loss-mode (TCP-Reno) : cwnd += (per RTT round ) cwnd *= / (when a packet loss is detected) Delay-mode : fill a pipe (fully utilize a link) without causing RTT increase Hybrid : works in delay mode when RTT is not increased works in loss mode when RTT is increases (i.e. when packets are buffered) mode selection: cwnd = max( cwnd loss, cwnd delay )

25 TCP behavior model () parameter definition w : cwnd when a packet loss is detected W : cwnd which just fills a pipe ~ BDP p : packet loss rate assumption packet loss due to buffer overflow is equivalent to packet loss due to random error p 8 3w (in case of TCP-Reno)

26 TCP behavior model (3) TCP friendly model cwnd TCP-Reno w w/ R w: cwnd when a packet loss is detected p: packet loss rate RTT: round trip time 0 w/ RTT round n R: TCP equivalent rate # of transmitted packets until a packet loss is detected = area of a trapezoid w w w 3w 8 p R 8 3w PS RTT 3 p

27 TCP behavior model (4) single flow sender bottleneck link receiver single TCP flow

28 TCP behavior model (5) cwnd & RTT behaviors of ideal models (single flow case) cwnd w loss & hybrid w/ W 0 delay w/ buffer BDP n hybrid cwnd loss w hybrid W delay w/ 0 W-w/ w/ loss-driven delay-driven vacant capacity buffer BDP n cwnd delay & hybrid W w w/ 0 w ~PLR W~bandwidth vacant capacity loss buffer BDP n w/ RTT loss & hybrid delay RTT min 0 w/ n RTT loss & hybrid RTT min delay 0 W-w/ w/ n RTT min RTT 0 w/ loss, delay & hybrid n (i) W < w/ (ii) w/<w <w (iii) w <W large buffer, small PLR (always loss-mode) small buffer, medium PLR (delay loss) large PLR, always vacant (always delay-mode)

29 TCP behavior model (6) formulation TCP CA round (i) W < w/ (ii) w/ W < w (iii) w W Loss transmitted packets elapsed time 3 w 8 w RTTmin (3w 4wW ) 8 PS B 3 3 w w 8 8 w RTTmin ( w W) PS B w RTT min Delay transmitted packets ww ww w W elapsed time w RTT min w RTT min w RTT min Hybrid transmitted packets 3 w 8 ww ( w W ) w W elapsed time w RTTmin (3w 4wW ) 8 PS B w RTTmin ( w W ) PS B w RTT min PS: Packet size, B: Link bandwidth

30 TCP behavior model (7) abstraction of actual hybrids Hybrids Window increase Window decrease Compound TCP 0.5*cwnd 0.75 / ARENO B/0Mbps /~ YeAH-TCP Scalable TCP (.0) /, RTT min /RTT, 7/8 TCP-Fusion B*D min /(N*PS) RTT min /RTT D min : timer resolution, N: # of flows

31 TCP behavior model (8) evaluation by models and simulations throughput 0 Throughput (Mbps) Case Case Case 3 loss-driven delay & hybrid Reno (sim) FAST (sim) CTCP (sim) ARENO (sim) YeAH (sim) Fusion (sim) Loss (model) Delay (model) Hybrid/Fusion (model) CTCP (model) ARENO (model) YeAH (model) Packet Loss Rate (0^n) loss rate Gbps Gbps 00Mbps RTT=40ms buffer size = BDP (constant) Packet loss rate : variable when PLR is large (w/<w), throughputs of delay & hybrid are much larger than that of loss-mode (i.e. efficiency) degradation of Compound & YeAH is due to fixed window decrease

32 TCP behavior model (9) two flows (competing) loss-based TCP flow bottleneck link senders receivers loss-based or hybrid TCP flow

33 TCP behavior model (0) cwnd behavior of ideal models (two flow case) cwnd loss-driven hybrid total (loss + hybrid) w ~PLR W~bandwidth w W w/ 0 total loss & hybrid w/ buffer BDP n cwnd W w W/ w/ 0 total hybrid loss (W-w)/ w/ buffer BDP n cwnd W W/ w w/ 0 total hybrid loss w/ buffer BDP n (i) W < w (low PLR) (ii) w <W < *w (medium PLR) (iii) *w <W (high PLR) always buffered (loss mode) vacant buffered (delay loss) always vacant (delay mode) large buffer, small PLR small buffer, medium PLR large PLR, always vacant

34 TCP behavior model () formulation TCP CA round (i) W < w (ii) w W < w (iii) w W Loss Hybrid (common) transmitted packets transmitted packets elapsed time 3 w w w ( W w) w RTTmin w(3w W ) 4 PS B 3 3 w w 8 8 w RTTmin (w W) 4 PS B 3 w W w 8 w RTT min PS: Packet size, B: Link bandwidth

35 TCP behavior model () evaluation by models and simulations throughput Throughput (Mbps) Case Case Case 3 delay & hybrid loss-driven Reno (sim) FAST (sim) CTCP (sim) ARENO (sim) YeAH (sim) Fusion (sim) Loss (model) Hybrid (model) CTCP (,model) ARENO (model) YeAH (model) Fusion (model) Gbps Gbps 00Mbps RTT=40ms buffer size = BDP (constant) Packet loss rate : variable when PLR is large (w<w), throughputs of delay & hybrid are much larger than that of loss-mode (efficiency) Packet Loss Rate (0^n) loss rate when PLR is low (w>w), hybrid behaves similar to loss-mode (friendliness)

36 TCP behavior model (3) Advantages of Hybrid TCP when vacant capacity exists (or PLR is large), throughput efficiency is greatly improved (advantage of delay-mode) when no vacant capacity exists (or buffer size is large), friendliness to legacy TCP (i.e. Reno) is achieved (advantage of loss-mode) Disadvantages of Hybrid TCP when buffer size is large, delay-mode is never activated

37 Summary of Hybrid TCP Efficiency, Friendliness and Low delay can be applied to real-time streaming and large file download might be effective in wireless networks friendliness to CUBIC-TCP or Compound-TCP CUBIC-TCP: Linux default Compound-TCP: Windows other metrics RTT fairness, mice/elephant (short-lived or longlived), convergence speed, etc efficiency is brought by delay-mode

38 Summary

39 Summary of TCP versions CUBIC-TCP provides efficiency, but tends to increase latency because router buffers are filled up Compound-TCP provides low delay thanks to its delay mode, but suffers from unfriendliness against CUBIC-TCP Some community discusses redesign of TCP