1. CSEE W4140 Networking Laboratory Lecture 7: TCP congestion control Jong Yul Kim 03.04.2009

2. Announcements  Midterm next week in class(1:10~2:25 pm) Problems from the lab exercises, quiz Subnets Network protocols  Similar to ARP exercises we did in class  For example, learn how TCP sequence numbers, ACK numbers work Problems about debugging networks  Lab 5 part 2 after Spring Break Lab report for lab 5 part 1 is due after spring break

3. Short review of TCP from last time  Characteristics of the IP network Delivers packets from host to host May lose packets (discarded by routers) Doesn’t care about packet order  TCP is a reliable, in-order, byte-stream service (delivers data from application to application)

4. Short review of TCP from last time  TCP is a reliable, in-order, byte-stream service The beauty of TCP is that it works without explicit support from the network Each end of the connection cooperate to make sure packets are delivered reliably and in order  Techniques used by TCP Sequence numbers Acknowledgements (and numbers) Retransmissions Timer

5. Today’s lecture  TCP Flow Control Throttling the rate of sender so that the receiver’s buffer does not overflow  TCP Congestion Control Throttling the rate of sender in the face of network congestion

6. TCP Flow Control (Receive Window)  When a connection is established, the receiver allocates a receive buffer.  Incoming packets are stored in the buffer so that the application can read data from the buffer.

7. TCP Flow Control (Receiver behavior)  The receiver lets the sender know: how much space is left in the buffer = RcvWindow by placing that value in the window size field in every segment that it sends to the sender

8. TCP Flow Control (Sender behavior)  Sender can fill up the spare room in the receiver’s buffer by sending more data  Sender maintains the size of data that has already been sent but unacknowledged = bytes_unACKed  Makes sure that bytes_unACKed ≤ RcvWindow

9. Network Congestion  Why does congestion occur? Too many senders sending at high rate Routers dropping packets due to overflowing buffers  What are the symptoms? Packet loss Packet queuing delay More retransmission  more packet loss Link bandwidth wasted on retransmissions

10. TCP Congestion Control  Remember: TCP has no support from the network about congestions Need to use end-to-end congestion control  TCP relies on perceived network congestion and throttles the sending rate accordingly

11. How does TCP know there is congestion in the network?  Answer: packet loss Timeout Three duplicate ACKs shown in diagram on right

12. How does TCP limit sending rate?  Using a variable called congestion window = CongWin  Size of unacknowledged data must be less than CongWin bytes_unACKed ≤ CongWin  Sending rate is roughly CongWin/RTT bytes/sec We can throttle the sending rate by controlling CongWin

13. TCP Congestion Control Algorithm  Is an algorithm that controls CongWin  Simply stated: Packet loss (=congestion)  decrease CongWin All is well  increase CongWin  Three main parts Additive-Increase, Multiplicative Decrease Slow Start Reaction to timeout events

14. Additive Increase, Multiplicative Decrease (AIMD)  Additive Increase Increase CongWin by 1 MSS every RTT while there is no packet loss  Multiplicative Decrease Decrease CongWin by half when packet is lost

15. Additive Increase, Multiplicative Decrease (AIMD) Congestion Avoidance phase

16. Slow Start (SS)  When connection is established CongWin is set to 1 MSS Increase CongWin by 1 MSS every ACK  Different from Additive Increase Until a loss occurs Result is an exponentially fast growth in sending rate

17. Slow Start (SS)

18. Reaction to Timeouts  We already discussed that TCP perceives congestion through packet loss  Packet is considered lost when: Timeout occurs Three duplicate ACKs are received  But timeouts and three duplicate ACKs are different. What do they tell about the severity of congestion?

19. Reaction to Timeouts  Let’s do something different for timeout. Reset CongWin to 1 MSS Begin again from Slow Start Slow start (exponential increase) until when?  New variable called Threshold Threshold is set to ½ CongWin After Threshold, do AIMD instead of SS

20. Three-Way Handshake Why is two-way handshake not enough?

21. Main Points of Lab 5 Parts 5~8  More about TCP Interactive applications over TCP Data transfer applications over TCP Retransmissions Congestion Control  Parts of Lab 5 may not be done because of the use of serial WAN link