Mid Term Exam Results

Transcription

1 Mid Term Exam Results v Grade Count Percentage v % v % v % v % v % Please hand the paper back to me after this class since we have to update the system (university requirement) by tomorrow Otherwise, you won t have mid term exam credit Transport Layer 3-1

2 Chapter 3 Transport Layer Lec 11: Congestion Control Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Transport Layer 3-2

3 Chapter 3 outline 3.1 transport-layer services 3.2 multiplexing and demultiplexing 3.3 connectionless transport: UDP 3.4 principles of reliable data transfer 3.5 connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 principles of congestion control 3.7 TCP congestion control Transport Layer 3-3

4 Principles of congestion control congestion: v informally: too many sources sending too much data too fast for network to handle v different from flow control! v manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) v a top-10 problem! Transport Layer 3-4

5 Approaches towards congestion control two broad approaches towards congestion control: end-end congestion control: v no explicit feedback from network v congestion inferred from end-system observed loss, delay v approach taken by TCP network-assisted congestion control: v routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate for sender to send at Transport Layer 3-5

6 Chapter 3 outline 3.1 transport-layer services 3.2 multiplexing and demultiplexing 3.3 connectionless transport: UDP 3.4 principles of reliable data transfer 3.5 connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 principles of congestion control 3.7 TCP congestion control Transport Layer 3-6

7 Three Qs for TCP Con Ctrl v How does sender perceive congestion occurs? v How does sender limit sending rate? v What algorithm used to do congestion control? Transport Layer 3-7

8 Guiding Principles v A lost segment implies congestion Sender decreases sending rate v A new ack segment indicates available bandwidth Sender increases sending rate v Bandwidth probing Back off if congestion happens Move forward if bandwidth available Note: No explicit signaling of congestion state by the network ACKs and loss events serve as implicit signals Transport Layer 3-8

9 TCP congestion control: additive increase multiplicative decrease v approach: sender increases transmission rate (window size), probing for usable bandwidth, until loss occurs additive increase: increase cwnd by 1 MSS (Max Segment Size) every RTT until loss detected multiplicative decrease: cut cwnd in half after loss AIMD saw tooth behavior: probing for bandwidth cwnd: TCP sender congestion window size additively increase window size. until loss occurs (then cut window in half) time Transport Layer 3-9

10 TCP Congestion Control: details sender sequence number space cwnd last byte ACKed sent, notyet ACKed ( inflight ) v sender limits transmission: LastByteSent- LastByteAcked last byte sent <= Min(cwnd, rwnd) v cwnd is dynamic, function of perceived network congestion TCP sending rate: v roughly: send cwnd bytes, wait RTT for ACKS, then send more bytes rate ~ cwnd RTT bytes/sec Transport Layer 3-10

11 TCP congestion-control algorithm v Slow start (Required) v Congestion avoidance (CA) (Required) v Fast recovery (Recommended ) Transport Layer 3-11

12 TCP Slow Start v when connection begins, increase rate exponentially until first loss event: initially cwnd = 1 MSS Increment cwnd by 1 MSS for every ACK received Result in doubling cwnd every RTT v summary: initial rate is slow but ramps up exponentially fast Host A RTT one segment two segments four segments Host B time Transport Layer 3-12

13 TCP: detecting, reacting to loss v loss indicated by timeout: cwnd set to 1 MSS; window then grows exponentially (as in slow start) to threshold, then grows linearly More serious v loss indicated by 3 duplicate ACKs (Fast Recovery) dup ACKs indicate network capable of delivering some segments cwnd is cut in half window then grows linearly in Reno In old Tahoe version, cwnd is set to 1MSS and begin with slow start Transport Layer 3-13

14 TCP: switching from slow start to CA Q: when should the exponential increase switch to linear? A: when cwnd gets to 1/2 of its value before timeout. Fast Recovery Implementation: v variable ssthresh v on loss event, ssthresh is set to 1/2 of cwnd just before loss event Slow start CA Old version: TCP Tahole Transport Layer 3-14

15 Summary: TCP Congestion Control Λ cwnd = 1 MSS ssthresh = 64 KB dupackcount = 0 timeout ssthresh = cwnd/2 cwnd = 1 MSS dupackcount = 0 retransmit missing segment dupackcount == 3 ssthresh= cwnd/2 cwnd = ssthresh + 3 retransmit missing segment duplicate ACK dupackcount++ slow start New ACK! new ACK cwnd = cwnd+mss dupackcount = 0 transmit new segment(s), as allowed cwnd > ssthresh Λ timeout ssthresh = cwnd/2 cwnd = 1 MSS dupackcount = 0 retransmit missing segment timeout ssthresh = cwnd/2 cwnd = 1 dupackcount = 0 retransmit missing segment fast recovery new ACK cwnd = cwnd + MSS (MSS/cwnd) dupackcount = 0 transmit new segment(s), as allowed duplicate ACK congestion avoidance New ACK! New ACK cwnd = ssthresh dupackcount = 0 cwnd = cwnd + MSS transmit new segment(s), as allowed. New ACK! duplicate ACK dupackcount++ dupackcount == 3 ssthresh= cwnd/2 cwnd = ssthresh + 3 retransmit missing segment Transport Layer 3-15

16 TCP Congestion Control v When cwnd is below ssthresh, sender in slow-start phase, window grows exponentially. v When cwnd is above ssthresh, sender is in congestion-avoidance phase, window grows linearly. v When a triple duplicate ACK occurs, Threshold set to cwnd/2 and cwnd set to Threshold. v When timeout occurs, Threshold set to cwnd/2 and cwnd is set to 1 MSS. Transport Layer 3-16

17 Chapter 3: summary v principles behind transport layer services: multiplexing, demultiplexing reliable data transfer flow control congestion control v instantiation, implementation in the Internet UDP TCP next: v leaving the network edge (application, transport layers) v into the network core Transport Layer 3-17