 Last Class  Resource Allocation  This Class  Chapter 6.3. ~ 6.4.  TCP congestion control

2 How does TCP figure out the right sending rate?  Situation:  Bottleneck link is inside the network  Network does not tell sender nor receiver  TCP must figure it out on its own  Be conservative in increase and aggressive in decrease  Use Additive-Increase, Multiplicative Decrease

3 AIMD SenderReceiver

4 Ramp-Up Time  How long does it take for a TCP sender to exploit the full bandwidth of 10 Mbps?  when RTT = 10 ms  when RTT = 1 sec

5 Be Aggressive At First SenderReceiver

6 What about doubling each time? TIME RTT Cwnd=1Cwnd=2Cwnd=4Cwnd=8 Multiplicative increase

7 More State Variables for TCP  CongestionWindow (cwnd)  determines the sending rate  CongestionThreshold (ssthreshold)  remembers last max CongestionWindow

8 Then Why Call it Slow Start?  Compare it with what we had in Chap  Dump as many bytes as AdvertisedWindow!  Thus slow start

9 When to stop increasing?  Finite world, finite beginning  ssthresh = set to an arbitrary large number (65KB)  Sender rate exceeds AdvWindow  Only transmit min(cwnd, AdvWindow)  Packets start to be dropped at the bottleneck  Sending rate exceeds bottleneck b/w  Another user shares the bottleneck  Contention for shared resource

10 Evolution of TCP cwnd

11 TCP Thruput?

12 Congestion Control  End-to-end approach in detecting congestion  No ack for some time  Duplicate acks  Network-assisted approach  Routers provide explicit feedback  IBM SNA, DEC DECnet, ATM ABR, TCP ECN

13 No ack for some time?  How to estimate RTT at the sender?  Time between data and ACK  It differs from one ack to another due to different levels of congestion on the fw/rv path  R(t) : estimated RTT at time t R(t+1) = (1-α) R(t) + αR  d(t): estimated variation in RTT at time t d(t+1) = (1-β) d(t) + βd  Timer = R(t) + 4d(t)

14 When congestion happens …  ssthresh = ½ of cwnd  In case of TCP Tahoe, cwnd = 1  When timeout happened/triple ACKs arrived  Start from minimum sending rate (slow start)  In case of TCP Reno, cwnd = ssthresh  When triple duplicate ACKs arrive  Sending rate decreased only by half (fast recovery)  When timeout happened, same as in Tahoe.

15 Slow Start and Congestion Avoidance  If cwnd < ssthresh  Do slow start  Otherwise, increase cwnd by 1 per RTT (congestion avoidance)  If triple ACKs arrive  retransmit the next segment w/o waiting for the retransmission timer to expire (fast retransmit)

16 Next Class  HW #7 due  QoS (Chap 6.5.)  End-to-End data (Chap. 7)