1. 1 Lecture 9: TCP and Congestion Control Slides adapted from: Congestion slides for Computer Networks: A Systems Approach (Peterson and Davis) Chapter 3 slides for Computer Networking: A Top Down Approach Featuring the Internet (Kurose and Ross) ITCS 6166/8166 091 Spring 2007 Jamie Payton Department of Computer Science University of North Carolina at Charlotte February 7, 2007

2. 2 Announcements Homework 2 is available –Due: Feb. 14 Schedule change – Updated on primary course website Additional homework before midterm Lab 1 due date changed

3. 3 Transmission Control Protocol Implementation of sliding window protocol TCP Segment (Packet) Structure TCP uses sliding windows at sender and receiver (buffers)

4. 4 Flow Control What happens if the receiving process is slow and the sending process is fast? TCP provides flow control –sender won’t overflow receiver’s buffer by transmitting too much, too fast

5. 5 Window “Syndromes” Silly Window Syndrome –Sender has data available to send –Flow control reduces effective window to very small size Size of segment dominated by the header information Tinygram Syndrome –Sender doesn’t have much data to send Size of segment dominated by the header information –Nagle’s algorithm combats this problem Coalesces small messages into a single segment if some data is currently unack’d

6. 6 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) A top-10 problem in computer network research!

7. 7 TCP Congestion Control Idea –assumes best-effort network (FIFO or FQ routers) –each source determines network capacity for itself –uses implicit feedback –ACKs pace transmission (self-clocking) Challenges –determining the available capacity in the first place –adjusting to changes in the available capacity

8. 8 TCP Congestion Control Principles Simple idea –Congestion: reduce sending rate –No congestion: increase sending rate How does the source determine whether or not the network is congested? –Lost packets! Timeout signals that a packet was lost Duplicate acknowledgements signal packet was lost (later...) How is rate controlled in TCP? –Control congestion window size –Rate approximately congestion window size / RTT

9. 9 TCP Congestion Control Components Additive Increase/Multiplicative Decrease Slow start Fast Retransmit and Fast Recovery

10. 10 Additive Increase Multiplicative Decrease New state variable per connection: CongWin –Counterpart to flow control’s advertised window –Limits how much data is in transit MaxWin = MIN(CongWin, AdvertisedWindow) EffWin = MaxWin - (LastByteSent - LastByteAcked) Congestion control approach: –increase CongWin when congestion goes down –decrease CongWin when congestion goes up Now EffectiveWindow includes both flow control and congestion control

11. 11 Additive Increase Multiplicative Decrease Approach: –increase transmission rate, probing for usable bandwidth, until loss occurs –additive increase: increase CongWin by 1 MSS every RTT until loss detected –multiplicative decrease: cut CongWin in half after loss Host A one segment RTT Host B time two segments Three segments

12. 12 Slow Start Objective: determine the available capacity at first –increase congestion window rapidly from a cold start Idea: –begin with CongWin = 1 packet –double CongWin each RTT Implemented : increment by 1 packet for each ACK Host A one segment RTT Host B time two segments four segments

13. 13 Refinement 1 Q: When should the exponential increase switch to linear? A: When CongWin gets to 1/2 of its value before timeout. Implementation: Variable Threshold At loss event, Threshold is set to 1/2 of CongWin just before loss event –Initially set to large value

14. 14 Refinement 2 Fast recovery –skip the slow start phase –go directly to threshold half the last successful CongWin Philosophy –Duplicate ACKs also indicate loss –3 duplicate ACKs indicates network capable of delivering some segments –Timeout indicates a “more alarming” congestion scenario

15. 15 TCP Flavors TCP Tahoe –Slow start Switch to AIMD when hit threshold –Handling loss Unconditional reduction of window to 1 MSS TCP Reno –Slow start Switch to AIMD when hit threshold –Handling loss Reduction of window to ½ when triple duplicate ACKS “Fast recovery”: skip slow start, go to threshold

16. 16 Summary: TCP Reno When CongWin is below Threshold, sender in slow-start phase, window grows exponentially. When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS