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Principles of congestion control

Principles of congestion control. Contents. Causes and cost of congestion Three examples How to handle congestion End-to-end Network-assisted TCP congestion control ATM ABR congestion control Summary. What is congestion?. Overflowing of routers buffers Packet retransmission

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Principles of congestion control

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  1. Principles of congestioncontrol

  2. Contents • Causes and cost of congestion • Three examples • How to handle congestion • End-to-end • Network-assisted • TCP congestion control • ATM ABR congestion control • Summary

  3. What is congestion? • Overflowing of routers buffers • Packet retransmission • Delay the problem? • How to treat problem? • Mechanisms to throttle senders

  4. Causes and cost • Example 1 • 2 senders • Router with infinite buffer space • No retransmission • Sending time infinte • Link capacity (R)

  5. Causes and cost Large queuing delays

  6. Causes and cost • Example 2 • 2 senders • Router with finite buffer space • Retransmission • Offered load • Link capacity (R)

  7. Causes and cost • Retransmissiondue to bufferoverflow • Retrassmission of unneededcopies

  8. Causes and cost • Example 3 • 4 senders • 4 routers with finite buffer space • Multihop paths • Link capacity (R) • Offered load

  9. Causes and cost • Throughputgoes to zero in the the limit of heavy traffic • Wasteduse of upstreamlinkscapacity

  10. How to handle congestion • End-to-end congestion control • No explicit support from network • IP layer provides no feedback • Observed network behavior, ex. packet loss or delay • TCP use end-to-end control

  11. TCP congestion control • Additive-increase, multiple-decrease (AIMD) • Slow start (exponential) • Congestion avoidance (linear) • Fast recovery • Timeout • Tripple duplicat ACK’s • TCP Tahoe • TCP Reno (Newer versions of TCP)

  12. How to handle congestion • Network-assisted • Explicit feedback • Direct feedback • Feedback via receiver • Low impact on traffic • ATM (Asynchronous Transfer Mode) • ABR (Available Bit-Rate) • XCP protocol

  13. ATM ABR congestion control • ATM takes a VC approach • ATM is a kind of architecture • Track behavior of individual senders • ABR takes advantage of spare bandwidth • Resource management cells (RM cells) • RM cells convey congestion-related information • Direct feedback or feedback via receiver

  14. ABR • Three mechanisms • Explicit forward congestion indication (EFCI) bit • Congestion indication and No increase (CI and NI) bit • Expclicit rate (ER) field

  15. Summary • Why do we need to handle congestion? • Minimize delays • Make traffic more effective • Maximize use of bandwidth • How is congestion problem solved? • End-to-end congestion control • Network-assisted congestion control

  16. Questions?

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