1. Principles of Congestion Control Chapter 3.6 Computer Networking: A top-down approach

2. Principles of Congestion Control Congestion: informally: “too many sources sending too much data too fast for network to handle” manifestations: o lost packets (buffer overflow at routers) o long delays (queuing in router buffers) different from flow control! a top-10 problem! 3 examples of cause and costs of congestion control Transport Layer3-2

3. Causes/costs of congestion: scenario 1 two senders, two receivers one router, infinite buffers no retransmission Transport Layer3-3 large delays when congested maximum achievable throughput unlimited shared output link buffers Host A in : original data Host B out Host C Host D

4. Causes/costs of congestion: scenario 2 one router, finite buffers sender retransmission of lost packet Transport Layer3-4 finite shared output link buffers Host A in : original data Host B out ' in : original data, plus retransmitted data Host C

5. Causes/costs of congestion: scenario 2 always: (goodput) “perfect” retransmission only when loss: retransmission of delayed (not lost) packet makes larger (than perfect case) for same Transport Layer3-5 in out = in out > in out “costs” of congestion: r more work needed for given “goodput” (retransmission) r unneeded retransmissions: link carries multiple copies of packets C/2 in out b. C/2 in out a. C/2 in out c. C/4 C/3

6. Causes/costs of congestion: scenario 3 four senders multihop paths timeout/retransmit Transport Layer3-6 finite shared output link buffers Host A in : original data Host D out ' in : original data, plus retransmitted data Host C R1 R2 Host B

7. Causes/costs of congestion: scenario 3 Transport Layer3-7 Another “cost” of congestion: r when packet dropped, any “upstream transmission capacity used for that packet was wasted! Host A Host B out

8. Approaches towards congestion control End-to-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Transport Layer3-8 Network-assisted congestion control: routers provide feedback to end systems o single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) o explicit rate sender should send at Two broad approaches towards congestion control:

9. Network-assisted congestion control Feedback in two ways o Direct feedback – network router  sender (choke packet) o Network feedback via receiver – Router marks a packet and receiver notify sender