EEC-484/584 Computer Networks Lecture 14 Wenbing Zhao (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer Networking book)

2 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Outline Reminder: –Wiki project#2 due: 4/16 Wednesday –Quiz 4: 4/21 Monday TCP –Reliable data transfer (ack generation) –Flow control –Congestion control

3 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP ACK Generation Event at Receiver Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. #. Gap detected Arrival of segment that partially or completely fills gap TCP Receiver action Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Immediately send single cumulative ACK, ACKing both in-order segments Immediately send duplicate ACK, indicating seq. # of next expected byte Immediate send ACK, provided that segment starts at lower end of gap

4 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Flow Control Receive side of TCP connection has a receive buffer: Speed-matching service: matching the send rate to the receiving app’s drain rate App process may be slow at reading from buffer Flow control: sender won’t overflow receiver’s buffer by transmitting too much, too fast

5 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Flow Control (Suppose TCP receiver discards out-of-order segments) Spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] Rcvr advertises spare room by including value of RcvWindow in segments Sender limits unACKed data to RcvWindow –guarantees receive buffer doesn’t overflow

6 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Principles of Congestion Control Congestion: Informally: “too many sources sending too much data too fast for network to handle” Different from flow control! Manifestations: –lost packets (buffer overflow at routers) –long delays (queueing in router buffers)

7 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Approaches towards Congestion Control End-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end systems –single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) –explicit rate sender should send at Two broad approaches towards congestion control

8 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Congestion Control: Additive Increase, Multiplicative Decrease Approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs –Additive increase: increase cwnd every RTT until loss detected –Multiplicative decrease: cut cwnd after loss Saw tooth behavior: probing for bandwidth

9 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Congestion Control Sender limits transmission: LastByteSent-LastByteAcked  cwnd Roughly, cwnd is dynamic, function of perceived network congestion How does sender perceive congestion? loss event = timeout or 3 duplicate acks TCP sender reduces rate ( cwnd ) after loss event rate = cwnd RTT Bytes/sec

10 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Slow Start When connection begins, cwnd = 1 MSS –Example: MSS = 500 bytes & RTT = 200 msec –Initial rate = 25 kBps Available bandwidth may be >> MSS/RTT –Desirable to quickly ramp up to respectable rate When connection begins, increase rate exponentially fast until first loss event

11 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Slow Start When connection begins, increase rate exponentially until first loss event: –Double cwnd every RTT –Done by incrementing cwnd for every ACK received Summary: initial rate is slow but ramps up exponentially fast Host A one segment RTT Host B time two segments four segments

12 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Congestion Avoidance Q: When should the exponential increase switch to linear? A: When cwnd gets to 1/2 of its value before timeout Implementation: Variable Threshold At loss event, Threshold is set to 1/2 of cwnd just before loss event How to increase cwnd linearly: cwnd (new) = cwnd + mss*mss/cwnd

13 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Congestion Control After 3 duplicated ACKs: –cwnd is cut in half –window then grows linearly But after timeout event: –cwnd instead set to 1 MSS –window then grows exponentially –to a threshold, then grows linearly  3 dup ACKs indicates network capable of delivering some segments  timeout indicates a “more alarming” congestion scenario Philosophy:

14 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Summary: TCP Congestion Control When cwnd is below Threshold, sender in slow- start phase, window grows exponentially When cwnd is above Threshold, sender is in congestion-avoidance phase, window grows linearly When a triple duplicate ACK occurs, Threshold set to cwnd/2 and cwnd set to Threshold When timeout occurs, Threshold set to cwnd /2 and cwnd is set to 1 MSS

15 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Sender Congestion Control StateEventTCP Sender ActionCommentary Slow Start (SS) ACK receipt for previously unacked data CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance” Resulting in a doubling of CongWin every RTT Congestion Avoidance (CA) ACK receipt for previously unacked data CongWin = CongWin+ MSS * (MSS/CongWin) Additive increase, resulting in increase of CongWin by 1 MSS every RTT

16 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Sender Congestion Control StateEventTCP Sender ActionCommentary SS or CALoss event detected by triple duplicate ACK Threshold = CongWin/2, CongWin = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS. SS or CATimeoutThreshold = CongWin/2, CongWin = 1 MSS, Set state to “Slow Start” Enter slow start SS or CADuplicate ACK Increment duplicate ACK count for segment being acked CongWin and Threshold not changed

17 Spring Semester 2008EEC-484/584: Computer NetworksWenbing Zhao TCP Congestion Control Segment lost Repeated acks Slow start