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congestion-control

congestion-control - 1 Csci 232 Computer Networks TCP...

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Unformatted text preview: 1 Csci 232: Computer Networks TCP Congestion Control 1 Transport Layer: TCP Congestion Control & Buffer Management ¡ Congestion Control ¡ What is congestion? Impact of Congestion ¡ Approaches to congestion control ¡ TCP Congestion Control ¡ End-to-end based: implicit congestion inference/notification ¡ Two Phases: slow start and congestion avoidance ¡ CongWin, theshold, AIMD, triple duplicates and fast recovery ¡ TCP Performance and Modeling; TCP Fairness Issues ¡ Router-Assisted Congestion Control and Buffer Management ¡ RED (random early detection) ¡ Fair queueing Readings: Sections 6.1-6.4 Csci 232: Computer Networks TCP Congestion Control 2 What is Congestion? • Informally: “too many sources sending too much data too fast for network to handle” • Different from flow control! • Manifestations: – Lost packets (buffer overflow at routers) – Long delays (queuing in router buffers) Csci 232: Computer Networks TCP Congestion Control 3 Effects of Retransmission on Congestion • Ideal case – Every packet delivered successfully until capacity – Beyond capacity: deliver packets at capacity rate • Realistically – As offered load increases, more packets lost • More retransmissions Æ more traffic Æ more losses … – In face of loss, or long end-end delay • Retransmissions can make things worse • In other words, no new packets get sent! – Decreasing rate of transmission in face of congestion • Increases overall throughput (or rather “goodput”) ! Csci 232: Computer Networks TCP Congestion Control 4 Congestion: Moral of the Story • When losses occur – Back off, don’t aggressively retransmit i.e., be a nice guy! • Issue of fairness – “Social” versus “individual” good – What about greedy senders who don’t back off? Csci 232: Computer Networks TCP Congestion Control 5 Approaches towards Congestion Control End-end congestion control: • no explicit feedback from network • congestion inferred from end-system observed as loss, delay • approach taken by TCP Network-assisted congestion control: • routers provide feedback to end systems – single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) – explicit rate sender should send at Two broad approaches towards congestion control: Csci 232: Computer Networks TCP Congestion Control 6 TCP Approach • End to End congestion control: – How to limit, - How to predict, - What algorithm? • Basic Ideas: – Each source “determines” network capacity for itself – Uses implicit feedback, adaptive congestion window • Packet loss is regarded as indication of network congestion! – ACKs pace transmission (“self-clocking”) • Challenges – Determining available capacity in the first place – Adjusting to changes in the available capacity • Available capacity depends on # of users and their traffic, which vary over time! 2 Csci 232: Computer Networks TCP Congestion Control 7 TCP Congestion Control • Changes to TCP motivated by...
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