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Unformatted text preview: 1 Csci 232: Computer Networks TCP Congestion Control 1 Transport Layer: TCP Congestion Control & Buffer Management Âˇ Congestion Control Âˇ What is congestion? Impact of Congestion Âˇ Approaches to congestion control Âˇ TCP Congestion Control Âˇ End-to-end based: implicit congestion inference/notification Âˇ Two Phases: slow start and congestion avoidance Âˇ CongWin, theshold, AIMD, triple duplicates and fast recovery Âˇ TCP Performance and Modeling; TCP Fairness Issues Âˇ Router-Assisted Congestion Control and Buffer Management Âˇ RED (random early detection) Âˇ Fair queueing Readings: Sections 6.1-6.4 Csci 232: Computer Networks TCP Congestion Control 2 What is Congestion? â€˘ Informally: â€śtoo many sources sending too much data too fast for network to handleâ€ť â€˘ Different from flow control! â€˘ Manifestations: â€“ Lost packets (buffer overflow at routers) â€“ Long delays (queuing in router buffers) Csci 232: Computer Networks TCP Congestion Control 3 Effects of Retransmission on Congestion â€˘ Ideal case â€“ Every packet delivered successfully until capacity â€“ Beyond capacity: deliver packets at capacity rate â€˘ Realistically â€“ As offered load increases, more packets lost â€˘ More retransmissions Ă† more traffic Ă† more losses â€¦ â€“ In face of loss, or long end-end delay â€˘ Retransmissions can make things worse â€˘ In other words, no new packets get sent! â€“ Decreasing rate of transmission in face of congestion â€˘ Increases overall throughput (or rather â€śgoodputâ€ť) ! Csci 232: Computer Networks TCP Congestion Control 4 Congestion: Moral of the Story â€˘ When losses occur â€“ Back off, donâ€™t aggressively retransmit i.e., be a nice guy! â€˘ Issue of fairness â€“ â€śSocialâ€ť versus â€śindividualâ€ť good â€“ What about greedy senders who donâ€™t back off? Csci 232: Computer Networks TCP Congestion Control 5 Approaches towards Congestion Control End-end congestion control: â€˘ no explicit feedback from network â€˘ congestion inferred from end-system observed as loss, delay â€˘ approach taken by TCP Network-assisted congestion control: â€˘ routers provide feedback to end systems â€“ single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) â€“ explicit rate sender should send at Two broad approaches towards congestion control: Csci 232: Computer Networks TCP Congestion Control 6 TCP Approach â€˘ End to End congestion control: â€“ How to limit, - How to predict, - What algorithm? â€˘ Basic Ideas: â€“ Each source â€śdeterminesâ€ť network capacity for itself â€“ Uses implicit feedback, adaptive congestion window â€˘ Packet loss is regarded as indication of network congestion! â€“ ACKs pace transmission (â€śself-clockingâ€ť) â€˘ Challenges â€“ Determining available capacity in the first place â€“ Adjusting to changes in the available capacity â€˘ 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 â€˘ Changes to TCP motivated by...
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- Fall '09
- Transmission Control Protocol, congestion control, TCP Vegas