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Understanding RED in TCP Congestion Control

This article discusses the need for Random Early Detection (RED) in computer networks and its key design concepts. It explores the benefits of dropping or marking packets to detect and resolve congestion, while considering issues such as fairness and underutilization. The discussion also touches upon the use of RED in different protocols and the determination of optimum average queue size for maximum throughput and minimum delay.

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Understanding RED in TCP Congestion Control

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  1. Random Early Detection (RED)15-744 Computer Networks Gunhee Kim September 24, 2010

  2. Why We Need it? • We learned TCP congestion control last week! • Sent, ACKed, congestion window, AIMD, etc. • Issues • TCP needs to create losses to find the available bandwidth of the connection. • Who’s the best to detect congestion and resolve it? • Full queue of a gateway significantly increases the average delay. • Congestion avoidance by Gateway !

  3. Key Design Concepts of RED (1/2) • How to know? • Average queue sizes • Effect of a low-pass filter • Busty traffic vs persistent traffic • How to let them know? • Drop a packet or Mark a packet probabilistically. • Dropping requires end hosts to follow the TCP rule • Marking requires end hosts to understand the special bits (Peterson)

  4. Key Design Concepts of RED (2/2) • Avoid global synchronization • Prevents underutilization • Fairness • More share of the bandwidth, more likely to be dropped • Randomly drop or mark a packet with prob p!

  5. Discussion • Based on other literature and Class Discussion Pages • Dropping or Marking a packet ? • Understandable in the TCP context, may not be in other protocols or to ignorant users. • In ATM network, if one cell is dropped, then all the others are useless. • Queue? • Largely rely on FIFO • How to determine optimum average queue size for max throughput and min delay for various configurations? • Scheduling or multi-level queue?

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