Winter 2003CS244a Handout 71 CS492B Project #2 TCP Tutorial # Jin Hyun Ju

Winter 2003CS244a Handout 72 CS244a: An Introduction to Computer Networks Handout 7: Congestion Control Nick McKeown Professor of Electrical Engineering and Computer Science, Stanford University

Winter 2003CS244a Handout 73 Main points  Congestion is inevitable  TCP sources detect congestion and, co-operatively, reduce the rate at which they transmit.  The rate is controlled using the TCP window size.  TCP modifies the rate according to “Additive Increase, Multiplicative Decrease (AIMD)”.  To jump start flows, TCP uses a fast restart mechanism (called “slow start”!).

Winter 2003CS244a Handout 74 Congestion H1H1 H2H2 R1 H3H3 A 1 (t) 10Mb/s D(t) 1.5Mb/s A 2 (t) 100Mb/s A1(t)A1(t) A2(t)A2(t) X(t)X(t) D(t)D(t) A1(t)A1(t) A2(t)A2(t) D(t)D(t) X(t)X(t) Cumulative bytes t

Winter 2003CS244a Handout 75 Congestion is unavoidable Arguably it’s good!  We use packet switching because it makes efficient use of the links. Therefore, buffers in the routers are frequently occupied.  If buffers are always empty, delay is low, but our usage of the network is low.  If buffers are always occupied, delay is high, but we are using the network more efficiently.  So how much congestion is too much?

Winter 2003CS244a Handout 76 Detecting Congestion  duplicated ACK  time out  without congestion control…

Winter 2003CS244a Handout 77 TCP Congestion Control  TCP implements host-based, feedback- based, window-based congestion control.  TCP sources attempts to determine how much capacity is available  TCP sends packets, then reacts to observable events (loss).

Winter 2003CS244a Handout 78 TCP Congestion Control  TCP sources change the sending rate by modifying the window size: Window = min{Advertized window, Congestion Window}  In other words, send at the rate of the slowest component: network or receiver.  “cwnd” follows additive increase/multiplicative decrease  On receipt of Ack: cwnd += 1  On packet loss (timeout): cwnd *= 0.5 ReceiverTransmitter (“cwnd”)

Winter 2003CS244a Handout 79 Additive Increase D A DDAADDAADA Src Dest When ACK is received: cwnd += 1/cwnd

Winter 2003CS244a Handout 710 Leads to the TCP “sawtooth” t Rate halved Timeouts Could take a long time to get started!

Winter 2003CS244a Handout 711 “Slow Start” Designed to cold-start connection quickly at startup or if a connection has been halted (e.g. window dropped to zero, or window full, but ACK is lost). How it works: increase cwnd by 1 for each ACK received. D A DDAADD AA D A Src Dest D A

Winter 2003CS244a Handout 712 Fast Recovery  Upon 3 duplicate ACKs, TCP retransmits.  Does not enter slow-start: there are probably ACKs in the pipe that will continue correct AIMD operation.

Winter 2003CS244a Handout 713 TCP Reno Congestion Control halved Timeout Exponential “slow start” t Rate Why is it called slow-start? Because TCP originally had no congestion control mechanism. The source would just start by sending a whole window’s worth of data. Slow start in operation until it reaches half of previous cwnd. Triple duplicated ACK