CSE 4213: Computer Networks II Suprakash Datta datta@cs.yorku.ca Office: CSEB 3043 Phone: 416-736-2100 ext 77875 Course page: http://www.cs.yorku.ca/course/4213 4/22/2019 COSC 4213 - S.Datta

TCP congestion control – the problem Roadmap TCP congestion control – the problem TCP congestion control – solutions Performance of TCP congestion control The future of TCP 4/22/2019 COSC 4213 - S.Datta

TCP congestion control: the problem Arguably the most crucial part of TCP Lots of research effort (over 25+ years) Performance Objectives Link utilization Fairness Keep congestion down Q: What is the right notion of fairness? 4/22/2019 COSC 4213 - S.Datta

TCP congestion control: the problem Nodes do not know about other nodes total or available capacity on any link the number of packets in a router buffer So, how does a node sense congestion? delays, losses,…. absence of congestion? lower delays, fewer losses…. 4/22/2019 COSC 4213 - S.Datta

TCP congestion control – the problem Roadmap TCP congestion control – the problem TCP congestion control – solutions Performance of TCP congestion control The future of TCP 4/22/2019 COSC 4213 - S.Datta

TCP congestion control – solutions “Packet loss indicates congestion” Sensing congestion: timeouts, duplicate ACKs Reacting to congestion: drastic decrease of sending rates How/when does a sender recover from congestion? Continuous probing Conservative increases Q: Formula for increase and decrease ? 4/22/2019 COSC 4213 - S.Datta

TCP congestion control – v1 Let’s think in terms of rates: Additive (conservative) increase, multiplicative (drastic) decrease of sending rates (AIMD) Starting rate: High? Low? Transmission rate 24 Kb/s 16 Kb/s 8 Kb/s time 4/22/2019 COSC 4213 - S.Datta

TCP congestion control – v2 TCP Tahoe (1988) Two modes: Aggressive probing (Slow start) and careful probing (Congestion avoidance). Slow(!) start mode: Multiplicative (aggressive) increase, multiplicative decrease of sending rates. Congestion avoidance mode: Additive (conservative) increase, multiplicative (drastic) decrease of sending rates. 4/22/2019 COSC 4213 - S.Datta

TCP Tahoe illustration Assume that ssthresh = 8 14 12 10 ssthresh 8 ssthresh 6 Cwnd (in segments) 4 2 t=0 t=2 t=4 t=6 Rounds 4/22/2019 COSC 4213 - S.Datta

TCP Reno(1990) and Fast recovery A further improvement TCP Reno(1990) and Fast recovery Intuition: Distinguish between loss and 3-dup ACK 4/22/2019 COSC 4213 - S.Datta

TCP uses windows (number of packets sent without waiting for ACKs) Windows vs rates TCP uses windows (number of packets sent without waiting for ACKs) Transmission rate = W * Packet size/RTT RTT = round trip time Self-clocking 4/22/2019 COSC 4213 - S.Datta

TCP congestion control – the problem Roadmap TCP congestion control – the problem TCP congestion control – solutions Performance of TCP congestion control The future of TCP 4/22/2019 COSC 4213 - S.Datta

Performance of TCP congestion control Fairness Link utilization Algorithmic perspective? Note: Many other TCP versions have been proposed. No one version outperforms all others in all situations 4/22/2019 COSC 4213 - S.Datta

TCP Performance – Fairness [Chiu and Jain, 89] Two competing LONG sessions, no arrival or departures, assume rates are real numbers. R equal bandwidth share loss: decrease window by factor of 2 Session 2 throughput congestion avoidance: additive increase Session 1 throughput R 4/22/2019 COSC 4213 - S.Datta

Once the trajectory hits x=y line, it stays there Lessons Caveat: In reality, both sessions may not detect congestion at the same time Once the trajectory hits x=y line, it stays there For real-valued rates, reaches x=y only as t  Will leave optimal point after reaching it (continuous probing) Generalizes to n sessions MIMD, MIAD, AIAD all unfair Can be shown to hold in the presence of arrivals and departures if all sessions are long [Edmonds, Datta, Dymond, 03] 4/22/2019 COSC 4213 - S.Datta

TCP Performance – link utilization At least 25% capacity lost Transmission rate C 3C/4 C/2 time 4/22/2019 COSC 4213 - S.Datta

TCP Performance – algorithmic view How good an algorithm is it? i.e. Optimal? Near-optimal? …. Very difficult question - Does not correspond to clean theoretical models. [Edmonds, Datta, Dymond, 03] TCP “performs well” (is competitive against a limited adversary) when each session is of a minimum specified size… 4/22/2019 COSC 4213 - S.Datta

TCP Performance – problems When/where does TCP not work well? Multimedia networking Non-smooth (sawtooth) bit rate Best-effort: no QoS guarantees Wireless/hybrid networks “Packet loss indicates congestion” ? Congestion and loss are not always correlated Very high-speed networks Fairness issues: parallel sessions, multiple bottlenecks, small sessions… 4/22/2019 COSC 4213 - S.Datta

Co-operative algorithm Policing is hard to do TCP/IP Design issues Co-operative algorithm Policing is hard to do Tracing of malicious hosts/users difficult 4/22/2019 COSC 4213 - S.Datta

A 2-minute introduction to the TCP/IP architecture The role of TCP Roadmap A 2-minute introduction to the TCP/IP architecture The role of TCP TCP congestion control – the problem TCP congestion control – solutions Performance of TCP congestion control The future of TCP 4/22/2019 COSC 4213 - S.Datta

Active queue management schemes The Future of TCP Active queue management schemes Drop packets proactively to reduce congestion RED, REM, BLUE,… TCP for ultra-high speed networks FastTCP, HSTCP,…. Usually use delay for being more responsive to congestion Less draconian TCP for wireless, hybrid networks Treats wireless parts separately 4/22/2019 COSC 4213 - S.Datta