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Advanced TCP Congestion Control: New Reno, SACK, Vegas, and RED Methods

This document discusses various methods for managing congestion in TCP, focusing on enhancements to TCP Reno, such as New Reno and Selective Acknowledgments (SACK). It explains how TCP New Reno addresses issues caused by burst losses in packets during transmission. The SACK mechanism informs senders about received segments, allowing for efficient retransmission of missing data. Additionally, the text covers TCP Vegas, which predicts potential congestion through monitoring round-trip times, and RED (Random Early Detection) which actively signals congestion before packet loss occurs, ensuring optimal network performance.

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Advanced TCP Congestion Control: New Reno, SACK, Vegas, and RED Methods

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Presentation Transcript

    1. CS 640 1 Other Methods of Dealing with Congestion Outline New Reno SACK TCP Vegas RED

    2. CS 640 2 Motivation for improving RENO In real networks, packets area often transmitted in bursts Thus, losses also often happen in bursts Due to FIFO (drop tail) queues in routers Problem with TCP RENO: If new ACK after Fast Retransmit is not for ALL data transferred since dup ACK, then DO NOT do Fast Recovery Since only mechanism for recognizing other drops is a timeout

    3. CS 640 3 TCP New Reno Simple observation Recognized that losses happen in bursts Upon receipt of a partial ACK (ACK for some but not all outstanding data), retransmit next data packet in sequence Same congestion control mechanism as RENO But, do not avoid Fast Recovery on multiple drops Enables recovery from multiple packet losses in sequence without timeout Timeouts are still used

    4. CS 640 4 TCP SACK Guess what? Losses do not always happen in bursts sometimes things get a little crazy How about telling the sender what has arrived (!) Selective Acknowledgements (SACK) Same congestion control mechanisms as TCP RENO Uses TCP options fields When out of order data arrives, tell sender (via SACK block) which block of data has been received Enables sender to maintain image of receivers queue Sender then resends all missing segments without waiting to timeout Doesnt send beyond CWND When no old data needs to be resent, then send new data Timeouts are still used

    5. CS 640 5 Congestion Avoidance TCPs strategy control congestion once it happens repeatedly increase load in an effort to find the point at which congestion occurs, and then back off Alternative strategy predict when congestion is about to happen reduce rate before packets start being discarded call this congestion avoidance, instead of congestion control Two possibilities host-centric: TCP Vegas router-centric: RED Gateways

    6. CS 640 6 TCP Vegas Idea: source watches for some sign that routers queue is building up and congestion will happen too; e.g., RTT grows sending rate flattens

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