CS3600 SYSTEMS AND NETWORKS
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1 CS3600 SYSTEMS AND NETWORKS NORTHEASTERN UNIVERSITY Lecture 24: Congestion Control Prof. Alan Mislove Slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang
2 Abstract View A Sending Host Buffer in bottleneck Router B Receiving Host We ignore internal structure of network and model it as having a single bottleneck link Alan Mislove amislove at ccs.neu.edu Northeastern University2
3 Three Congestion Control Problems Adjusting to bottleneck bandwidth Adjusting to variations in bandwidth Sharing bandwidth between flows Alan Mislove amislove at ccs.neu.edu Northeastern University3
4 Single Flow, Fixed Bandwidth A 100 Mbps B Adjust rate to match bottleneck bandwidth without any a priori knowledge could be gigabit link, could be a modem Alan Mislove amislove at ccs.neu.edu Northeastern University4
5 Single Flow, Varying Bandwidth A BW(t) B Adjust rate to match instantaneous bandwidth Bottleneck can change because of a routing change Alan Mislove amislove at ccs.neu.edu Northeastern University5
6 Multiple Flows Two Issues: Adjust total sending rate to match bottleneck bandwidth Allocation of bandwidth between flows A1 A2 100 Mbps B1 B2 A3 B3 Alan Mislove amislove at ccs.neu.edu Northeastern University6
7 General Approaches Send without care many packet drops could cause congestion collapse Reservations pre-arrange bandwidth allocations requires negotiation before sending packets Pricing don t drop packets for the high-bidders requires payment model Alan Mislove amislove at ccs.neu.edu Northeastern University7
8 General Approaches (cont d) Dynamic Adjustment (TCP) Every sender probe network to test level of congestion speed up when no congestion slow down when congestion suboptimal, messy dynamics, simple to implement Distributed coordination problem! Alan Mislove amislove at ccs.neu.edu Northeastern University8
9 TCP Congestion Control TCP connection has window controls number of unacknowledged packets Sending rate: ~Window/RTT Vary window size to control sending rate Introduce a new parameter called congestion window (cwnd) at the sender Congestion control is mainly a sender-side operation Alan Mislove amislove at ccs.neu.edu Northeastern University9
10 Congestion Window (cwnd) Limits how much data can be in transit Implemented as # of bytes Described as # packets in this lecture MaxWindow = min(cwnd, AdvertisedWindow) EffectiveWindow = MaxWindow (LastByteSent LastByteAcked) MaxWindow LastByteAcked LastByteSent EffectiveWindow sequence number increases Alan Mislove amislove at ccs.neu.edu Northeastern University10
11 Two Basic Components Detecting congestion Rate adjustment algorithm (change cwnd size) depends on congestion or not Alan Mislove amislove at ccs.neu.edu Northeastern University11
12 Detecting Congestion Alan Mislove amislove at ccs.neu.edu Northeastern University12
13 Detecting Congestion Packet dropping is best sign of congestion delay-based methods are hard and risky How do you detect packet drops? ACKs TCP uses ACKs to signal receipt of data ACK denotes last contiguous byte received actually, ACKs indicate next segment expected Two signs of packet drops No ACK after certain time interval: time-out Several duplicate ACKs (ignore for now) Alan Mislove amislove at ccs.neu.edu Northeastern University12
14 Detecting Congestion Packet dropping is best sign of congestion delay-based methods are hard and risky How do you detect packet drops? ACKs TCP uses ACKs to signal receipt of data ACK denotes last contiguous byte received actually, ACKs indicate next segment expected Two signs of packet drops No ACK after certain time interval: time-out Several duplicate ACKs (ignore for now) Alan Mislove amislove at ccs.neu.edu Northeastern University12
15 Detecting Congestion Packet dropping is best sign of congestion delay-based methods are hard and risky How do you detect packet drops? ACKs TCP uses ACKs to signal receipt of data ACK denotes last contiguous byte received actually, ACKs indicate next segment expected Two signs of packet drops No ACK after certain time interval: time-out Several duplicate ACKs (ignore for now) May not work well for wireless networks, why? Alan Mislove amislove at ccs.neu.edu Northeastern University12
16 Sliding (Congestion) Window Sliding window: each ACK = permission to send a new packet Ex. cwnd = 3 Alan Mislove amislove at ccs.neu.edu Northeastern University13
17 Self-clocking If we have a large window, ACKs self-clock the data to the rate of the bottleneck link Observe: received ACK spacing bottleneck bandwidth P b P r sender receiver A s A b A r Tiny ACK (very thin) Alan Mislove amislove at ccs.neu.edu Northeastern University14
18 Rate Adjustment Basic structure: Upon receipt of ACK (of new data): increase rate Data successfully delivered, perhaps can send faster Upon detection of loss: decrease rate But what increase/decrease functions should we use? Depends on what problem we are solving Alan Mislove amislove at ccs.neu.edu Northeastern University15
19 Fairness? R Connection 2 throughput Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
20 Fairness? R equal bandwidth share Connection 2 throughput Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
21 Fairness? Two competing sessions: R equal bandwidth share Connection 2 throughput Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
22 Fairness? Two competing sessions: Additive increase (AI) gives slope of 1, as throughout increases R equal bandwidth share Connection 2 throughput Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
23 Fairness? Two competing sessions: Additive increase (AI) gives slope of 1, as throughout increases multiplicative decrease (MD) decreases throughput proportionally R equal bandwidth share Connection 2 throughput Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
24 Fairness? Two competing sessions: Additive increase (AI) gives slope of 1, as throughout increases multiplicative decrease (MD) decreases throughput proportionally R equal bandwidth share Connection 2 throughput congestion avoidance: additive increase Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
25 Fairness? Two competing sessions: Additive increase (AI) gives slope of 1, as throughout increases multiplicative decrease (MD) decreases throughput proportionally R equal bandwidth share Connection 2 throughput Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
26 Fairness? Two competing sessions: Additive increase (AI) gives slope of 1, as throughout increases multiplicative decrease (MD) decreases throughput proportionally R equal bandwidth share Connection 2 throughput loss: decrease window by factor of 2 Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
27 Fairness? Two competing sessions: Additive increase (AI) gives slope of 1, as throughout increases multiplicative decrease (MD) decreases throughput proportionally R equal bandwidth share Connection 2 throughput Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
28 Fairness? Two competing sessions: Additive increase (AI) gives slope of 1, as throughout increases multiplicative decrease (MD) decreases throughput proportionally R equal bandwidth share Connection 2 throughput Fair and link fully utilized (rate R) Connection 1 throughput R Alan Mislove amislove at ccs.neu.edu Northeastern University16
29 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
30 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
31 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
32 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
33 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
34 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
35 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
36 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
37 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
38 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
39 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
40 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
41 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
42 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
43 A D AIMD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University17
44 A D AIMD x y C B E Limit rates: x = y Alan Mislove amislove at ccs.neu.edu Northeastern University17
45 Alan Mislove amislove at ccs.neu.edu Northeastern University18 AIMD Sharing Dynamics A B x D E No congestion à rate increases by one packet/rtt every RTT Congestion à decrease rate by factor 2 Rates equalize à fair share y
46 A D AIAD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University19
47 A D AIAD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University19
48 A D AIAD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University19
49 A D AIAD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University19
50 A D AIAD x y C B E Alan Mislove amislove at ccs.neu.edu Northeastern University19
51 A D AIAD x y C B E Limit rates: x and y depend on initial values Alan Mislove amislove at ccs.neu.edu Northeastern University19
52 Alan Mislove amislove at ccs.neu.edu Northeastern University20 AIAD Sharing Dynamics A B x D E No congestion à x increases by one packet/rtt every RTT Congestion à decrease x by y
53 Adapting cwin So far: sliding window + self-clocking of ACKs How to know the best cwnd (and best transmission rate)? Phases of TCP congestion control 1.Slow start (getting to equilibrium) 1. Want to find this very very fast and not waste time 2.Congestion Avoidance Additive increase - gradually probing for additional bandwidth Multiplicative decrease - decreasing cwnd upon loss/timeout Alan Mislove amislove at ccs.neu.edu Northeastern University21
54 Phases of Congestion Control Congestion Window (cwnd) Initial value is 1 MSS (=maximum segment size) counted as bytes Slow-start threshold Value (ss_thresh) Initial value is the advertised window size slow start (cwnd < ssthresh) congestion avoidance (cwnd >= ssthresh) Alan Mislove amislove at ccs.neu.edu Northeastern University22
55 TCP: Slow Start Goal: discover roughly the proper sending rate quickly Whenever starting traffic on a new connection, or whenever increasing traffic after congestion was experienced: Intialize cwnd =1 Each time a segment is acknowledged, increment cwnd by one (cwnd++). Continue until Reach ss_thresh Packet loss Alan Mislove amislove at ccs.neu.edu Northeastern University23
56 The congestion window size grows very rapidly Slow Start Illustration TCP slows down the increase of cwnd when cwnd >= ss_thresh Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! Alan Mislove amislove at ccs.neu.edu Northeastern University24
57 Slow Start Illustration The congestion window size grows very rapidly cwnd = 1 segment 1 TCP slows down the increase of cwnd when cwnd >= ss_thresh Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! Alan Mislove amislove at ccs.neu.edu Northeastern University24
58 Slow Start Illustration The congestion window size grows very rapidly cwnd = 1 segment 1 ACK for segment 1 TCP slows down the increase of cwnd when cwnd >= ss_thresh Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! Alan Mislove amislove at ccs.neu.edu Northeastern University24
59 Slow Start Illustration The congestion window size grows very rapidly TCP slows down the increase of cwnd when cwnd >= ss_thresh cwnd = 1 cwnd = 2 segment 1 ACK for segment 1 Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! Alan Mislove amislove at ccs.neu.edu Northeastern University24
60 Slow Start Illustration The congestion window size grows very rapidly TCP slows down the increase of cwnd when cwnd >= ss_thresh cwnd = 1 cwnd = 2 segment 1 ACK for segment 1 segment 2 segment 3 Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! Alan Mislove amislove at ccs.neu.edu Northeastern University24
61 Slow Start Illustration The congestion window size grows very rapidly TCP slows down the increase of cwnd when cwnd >= ss_thresh cwnd = 1 cwnd = 2 segment 1 ACK for segment 1 segment 2 segment 3 ACK for segments Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! Alan Mislove amislove at ccs.neu.edu Northeastern University24
62 Slow Start Illustration The congestion window size grows very rapidly TCP slows down the increase of cwnd when cwnd >= ss_thresh cwnd = 1 cwnd = 2 cwnd = 4 segment 1 ACK for segment 1 segment 2 segment 3 ACK for segments Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! Alan Mislove amislove at ccs.neu.edu Northeastern University24
63 Slow Start Illustration The congestion window size grows very rapidly TCP slows down the increase of cwnd when cwnd >= ss_thresh Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! cwnd = 1 cwnd = 2 cwnd = 4 segment 1 ACK for segment 1 segment 2 segment 3 ACK for segments segment 4 segment 5 segment 6 segment 7 Alan Mislove amislove at ccs.neu.edu Northeastern University24
64 Slow Start Illustration The congestion window size grows very rapidly TCP slows down the increase of cwnd when cwnd >= ss_thresh Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! cwnd = 1 cwnd = 2 cwnd = 4 segment 1 ACK for segment 1 segment 2 segment 3 ACK for segments segment 4 segment 5 segment 6 segment 7 ACK for segments Alan Mislove amislove at ccs.neu.edu Northeastern University24
65 Slow Start Illustration The congestion window size grows very rapidly TCP slows down the increase of cwnd when cwnd >= ss_thresh Observe: Each ACK generates two packets slow start increases rate exponentially fast (doubled every RTT)! cwnd = 1 cwnd = 2 cwnd = 4 cwnd = 8 segment 1 ACK for segment 1 segment 2 segment 3 ACK for segments segment 4 segment 5 segment 6 segment 7 ACK for segments Alan Mislove amislove at ccs.neu.edu Northeastern University24
66 Congestion Avoidance (After Slow Start) Slow Start figures out roughly the rate at which the network starts getting congested Congestion Avoidance continues to react to network condition Probes for more bandwidth, increase cwnd if more bandwidth available If congestion detected, aggressive cut back cwnd Alan Mislove amislove at ccs.neu.edu Northeastern University25
67 Congestion Avoidance: Additive Increase After exiting slow start, slowly increase cwnd to probe for additional available bandwidth Competing flows may end transmission May have been unlucky with an early drop If cwnd > ss_thresh then each time a segment is acknowledged increment cwnd by 1/cwnd (cwnd += 1/cwnd). cwnd is increased by one only if all segments have been acknowledged Increases by 1 per RTT, vs. doubling per RTT Alan Mislove amislove at ccs.neu.edu Northeastern University26
68 Example of Slow Start + Congestion Avoidance Assume that ss_thresh = 8 15 Cwnd (in segments) ssthresh 0 t=0 t=1 t=2 t=3 Roundtrip times t=4 t=5 t=6 t=7 Alan Mislove amislove at ccs.neu.edu Northeastern University27
69 Detecting Congestion via Timeout If there is a packet loss, the ACK for that packet will not be received The packet will eventually timeout No ack is seen as a sign of congestion Alan Mislove amislove at ccs.neu.edu Northeastern University28
70 Congestion Avoidance: Multiplicative Decrease Timeout = congestion Each time when congestion occurs, ss_thresh is set to half the current size of the congestion window: ss_thresh = cwnd / 2 cwnd is reset to one: cwnd = 1 and slow-start is entered Alan Mislove amislove at ccs.neu.edu Northeastern University29
71 TCP illustration cwnd ss_thresh Timeout Slow Start Congestion Avoidance ss_thresh Time Alan Mislove amislove at ccs.neu.edu Northeastern University30
72 Responses to Congestion (Loss) There are algorithms developed for TCP to respond to congestion TCP Tahoe - the basic algorithm (discussed previously) TCP Reno - Tahoe + fast retransmit & fast recovery Most end hosts today implement TCP Reno and many more: TCP Vegas (research: use timing of ACKs to avoid loss) TCP SACK (future deployment: selective ACK) Alan Mislove amislove at ccs.neu.edu Northeastern University31
73 TCP Reno Problem with Tahoe: If a segment is lost, there is a long wait until timeout Reno adds a fast retransmit and fast recovery mechanism Upon receiving 3 duplicate ACKs, retransmit the presumed lost segment ( fast retransmit ) But do not enter slow-start. Instead enter congestion avoidance ( fast recovery ) Alan Mislove amislove at ccs.neu.edu Northeastern University32
74 Fast Retransmit Resend a segment after 3 duplicate ACKs remember a duplicate ACK means that an out-of sequence segment was received ACK-n means packets 1,, n all received cwnd = 1 cwnd = 2 ACK 1 ACK 2 segment 1 segment 2 segment 3 Notes: duplicate ACKs due to packet reordering! if window is small don t get duplicate ACKs! 3 duplicate ACKs cwnd = 4 ACK 3 ACK 3 ACK 3 ACK 3 segment 4 segment 5 segment 6 segment 7 Alan Mislove amislove at ccs.neu.edu Northeastern University33
75 Fast Recovery After a fast-retransmit cwnd = cwnd/2 (vs. 1 in Tahoe) ss_thresh = cwnd i.e. starts congestion avoidance at new cwnd Not slow start from cwnd = 1 After a timeout ss_thresh = cwnd/2 cwnd = 1 Do slow start Same as Tahoe Alan Mislove amislove at ccs.neu.edu Northeastern University34
76 Fast Retransmit and Fast Recovery cwnd Slow Start Congestion Avoidance Time Retransmit after 3 duplicate ACKs prevent expensive timeouts Slow start only once per session (if no timeouts) In steady state, cwnd oscillates around the ideal window size. Alan Mislove amislove at ccs.neu.edu Northeastern University35
77 TCP Congestion Control Summary Measure available bandwidth slow start: fast, hard on network AIMD: slow, gentle on network Detecting congestion timeout based on RTT robust, causes low throughput Fast Retransmit: avoids timeouts when few packets lost can be fooled, maintains high throughput Recovering from loss Fast recovery: don t set cwnd=1 with fast retransmits Alan Mislove amislove at ccs.neu.edu Northeastern University36
78 TCP Reno Quick Review Slow-Start if cwnd < ss_thresh cwnd++ upon every new ACK (exponential growth) Timeout: ss_thresh = cwnd/2 and cwnd = 1 Congestion avoidance if cwnd >= ss_thresh Additive Increase Multiplicative Decrease (AIMD) ACK: cwnd = cwnd + 1/cwnd Timeout: ss_thresh = cwnd/2 and cwnd = 1 Fast Retransmit & Recovery 3 duplicate ACKS (interpret as packet loss) Retransmit lost packet cwnd=cwnd/2, ss_thresh = cwnd Alan Mislove amislove at ccs.neu.edu Northeastern University37
79 TCP Reno Saw Tooth Behavior Congestion Window Timeouts may still occur Initial Slowstart Slowstart to pace packets Fast Retransmit and Recovery Time Alan Mislove amislove at ccs.neu.edu Northeastern University38
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