CSCI Topics: Internet Programming Fall 2008

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1 CSCI Topics: Internet Programming Fall 2008 Transport Layer Derek Leonard Hendrix College October 22, 2008 Original slides copyright J.F Kurose and K.W. Ross 1

2 Chapter 3: Roadmap 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

3 TCP Slow Start (More) Slow start Double CongWin every RTT Done by incrementing CongWin for every ACK received: W = W+1 per ACK RTT Host A Host B one segment two segments four segments (B = B + MSS per ACK) Summary: initial rate is slow but ramps up exponentially fast time

4 Refinement Reno: triple ACK, Tahoe: timeout previous timeout Upon timeout: CongWin is set to 1 MSS Window then grows exponentially (slow start) to a threshold, then grows linearly (congestion avoidance) Introduced in TCP Tahoe After 3 dup ACKs: CongWin is cut in half Window then grows linearly Called Fast Recovery Introduced in TCP Reno window W in pkts Philosophy: Three dup ACKs indicate that network is capable of delivering some segments Timeout before 3 dup ACKs is more alarming

5 Refinement (More) Initial slow start ends when either Loss occurs Initial threshold is reached Initial threshold is usually set to the receiver s advertised window Implementation: Variable threshold ssthresh is the slow start threshold At loss events, ssthresh is set to CongWin/2

6 Summary: TCP Congestion Control When CongWin is below ssthresh, sender is in slow-start, window grows exponentially (both Reno and Tahoe) When CongWin is above ssthresh, sender is in congestion-avoidance, window grows linearly (both Reno and Tahoe) When a triple duplicate ACK occurs, CongWin is set to CongWin/2 (Reno only) When timeout occurs, ssthresh is set to CongWin/2 and CongWin is set to 1 MSS (both Reno and Tahoe)

7 TCP Reno Sender Congestion Control Event State TCP Sender Action Commentary ACK receipt for previously unacked data Slow Start (SS) CongWin += MSS, If (CongWin > ssthresh) { Set state to Congestion Avoidance } Results in a doubling of CongWin every RTT ACK receipt for previously unacked data Congestion Avoidance (CA) CongWin += MSS 2 / CongWin Additive increase, resulting in increase of CongWin by 1 MSS every RTT Loss event detected by triple duplicate ACK SS or CA ssthresh = max(congwin/2, MSS) CongWin = ssthresh Set state to Congestion Avoidance Fast recovery, implementing multiplicative decrease Timeout SS or CA ssthresh = max(congwin/2, MSS) CongWin = MSS Set state to Slow Start Enter slow start Duplicate ACK SS or CA Increment duplicate ACK count for segment being acked CongWin and Threshold not changed

8 TCP Throughput W/2 W What s the average throughout of TCP as a function of max window size W and RTT? Ignore slow start and assume perfect AIMD (no timeouts) Let W be the window size when loss occurs When window is W, throughput is W *MSS/RTT Just after loss, window drops to W/2, throughput to W *MSS /(2RTT) Average rate: 3/4 * W *MSS/RTT

9 TCP Throughput Example: 1500-byte segments, 100 ms RTT, want 10 gb/s average throughput r av Requires max window size W = 111,111 in-flight segments (W av = 83,333 packets) This is huge in terms of buffer space Throughput in terms of loss rate: p is packet loss 1.22 MSS RTT p

10 TCP Throughput (Discussion) Q: what is the max packet loss rate allowed if we want to sustain 10 gb/s average throughput? A: p = 2.1x10-10 wow! Such low rates are not possible (corruption even in wired networks occurs more frequently) Q: in congestion avoidance, how long does it take TCP to go from 5 gb/s to 10 gb/s with RTT = 0.1 sec? A: at 5 gb/s, the window size is 41,666 pkts and at 10 gb/s it is 83,333 pkts Then, we need (83,333-41,666) RTTs to close this gap This is 4,166 seconds = 1 hour 9 minutes

11 TCP Future TCP is slow, but what if most transfers are short? How long before TCP reaches 10 gb/s in slow start? Idea: starting at W = 1 we need to reach W = 83,333 packets at an exponential rate The time needed to reach full capacity is ceil(log 2 (83333))*RTT = 1.7 seconds (17 steps)! How much data can we squeeze in slow start? Total data transmitted (pkt size 1500) ~ 39.3 MB Conclusion: short connects are fine with current TCP

12 TCP Fairness Fairness goal: if K TCP sessions share same bottleneck link of bandwidth C, each should have average rate of C/K TCP connection 1 TCP connection 2 bottleneck router of capacity C

13 Why Is TCP Fair? AIMD allows flows to converge to fairness Intuitive reasoning: during increase, both flows gain bandwidth at the same rate; however, during decrease, the faster flow releases more C equal bandwidth share y loss: decrease window by factor of 2 no loss: increase by MSS/RTT x C

14 Fairness Example AIMD example C = 1544 kb/s, 2 flows Start in the maximally unfair state x = 1544, y = 0 Eventually converge to fairness Caveat: fairness in TCP is achievable only when flows have the same RTT y(t) sending rate time step x(t)

15 Fairness (Final Thoughts) Fairness and UDP Multimedia apps often do not use TCP Do not want rate throttled by congestion control Instead use UDP: Pump audio/video at constant rate, tolerate packet loss Research area: TCP friendly Fairness and parallel TCP connections Nothing prevents app from opening parallel flows between 2 hosts Web browsers do this Example: link of rate C with 10 flows present: New app asks for 1 TCP connection, gets rate C/11 New app asks for 10 TCPs, gets C/2!

16 Chapter 3: Summary Principles behind transport layer services: Multiplexing, demultiplexing Reliable data transfer Flow control Congestion control Instantiation and implementation in the Internet UDP TCP Next: Leaving the network edge (application, transport layers) Into the network core

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