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. Large queuing delays Causes and cost

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

7. Causes and cost Retransmission due to buffer overflow Retrassmission of unneeded copies

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 Throughput goes to zero in the the limit of heavy traffic Wasted use of upstream links capacity

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?