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CSS432 Congestion Control Textbook Ch6.1 – 6.4

CSS432 Congestion Control Textbook Ch6.1 – 6.4. Prof. Athirai Irissappane http://courses.washington.edu/css432/athirai/ athirai@uw.edu. Taxonomy. Network Congestion: Too many packets in the network (more than its capacity) Too many packets in the switch/router buffer

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CSS432 Congestion Control Textbook Ch6.1 – 6.4

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  1. CSS432 Congestion ControlTextbook Ch6.1 – 6.4 Prof. Athirai Irissappane http://courses.washington.edu/css432/athirai/ athirai@uw.edu CSS432: Congestion Control

  2. Taxonomy • Network Congestion: • Too many packets in the network (more than its capacity) • Too many packets in the switch/router buffer • Congestion control is closely related to resource allocation • How many packets can be sent in the network at a time • Can the switch handle requests from many hosts • Two sides of the same coin • Pre-allocate resources so as to avoid congestion • Control congestion that leads to resource restrictions • Flow control versus congestion control • Flow control: to keep a fast sender from overrunning a slow receiver • Congestion control: to keep a set of senders from sending two much data into the network. CSS432: Congestion Control

  3. Taxonomy of resource allocation mechanisms • Router Centric: • Router makes decisions about resource allocation • When to forward, drop packets • Inform hosts how many packets can be sent to them Host Centric: • Host makes decisions • Observe network conditions and adjust • E.g., advertised window • Reservation-based • Reserve necessary resources prior to transmission • Host can reserve capacity in the network for transmission Feedback-based • No prior reservation • Host send data without reserving then adjust sending rate based on observation • Rate Based Limit Transmission rate Window Based Advertised Window CSS432: Congestion Control

  4. Source 1 Router Destination 1 Router Source 2 Router Destination 2 Source 3 Connectionless Flow • Datagrams • No connection • Switched independently • Flowing through a particular set of routers if transmitted from the same source/destination pair • Connectionless Flows • maintain state information of each flow at router for better resource allocation decisions: e.g., what should be done to the packets of the same flow • Routers • No state if they follow purely connectionless service • Hard state if they follow purely connection-oriented service • Soft state used to make resource allocation decision for each flow – How? CSS432: Congestion Control

  5. Flow 1 Flow 2 Round-robin service Flow 3 Flow 4 Queuing Discipline • First-In-First-Out (FIFO) w/ Tail Drop • Does not discriminate between traffic sources (flows) • Fair Queuing (FQ) • Work conserving: link is never left idle (if data to be sent) • Explicitly segregates traffic based on flows • Ensures no flow captures more than its share of capacity • If there are n flows sending data, each is allocated 1/n bandwidth • Variation: weighted fair queuing (WFQ) • Problem: • Variable packet length CSS432: Congestion Control

  6. Bit-Round Fair Queuing (BRFQ) • Algorithm • For each queue, compute the virtual finish time (F) upon arrival of a new packet. • Choose a packet with the lowest virtual finish time. • No preemption – Cannot interrupt a transmitting packet • A new packet with shorter F than a waiting (large) packet will be transmitted first • F = S (Start time) + P (Packet length) • StartTime = Max(F_prev, Arrival Time) • Clock ticks for one bit transmitted • Pros and Cons • Emulates bit-by-bit fair queuing • Not perfect: can’t preempt a large packet currently being transmitted Example of fair queuing in action: (a) packets with earlier finishing times are sent first; (b) sending of a packet already in progress is completed CSS432: Congestion Control

  7. Source 1 10-Mbps Ethernet Router Destination 1.5-Mbps T1 link 100-Mbps FDDI Source 2 Congestion in Packet-Switched Network • Source cannot directly observe the traffic on the slow network • A congested link could be assigned a large edge weight by the route propagation protocol • All packets may have to flow through the same (bottleneck) router to the destination. FQ or BRFQ drops off overflowed packets. CSS432: Congestion Control

  8. TCP Congestion Control • Created by Van Jacobson, 1980s, • ~8 years after TCP/IP protocol stack became operational • Immediately preceding this time, the Internet was suffering from congestion collapse • hosts would send their packets into the Internet as fast as the advertised window would allow, • congestion would occur at some router (causing packets to be dropped), and the hosts would time out • hosts retransmit their packets, resulting in even more congestion CSS432: Congestion Control

  9. TCP Congestion Control • Idea • Determines network capacity at each source host • Uses implicit feedback • Uses ACKs to pace packet transmission (self-clocking) • Challenge • Determining the available capacity in the first place • Adjusting # in-transit packets in response to dynamic changes in the available capacity • Increase in capacity? CSS432: Congestion Control

  10. Sending application TCP LastByteWritten y LastByteSent LastByteAcked LastByteSent – LastByteAcked ≤ AdvertisedWindow EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked) Additive Increase/Multiplicative Decrease (AIMD) • New state variable per TCP connection: CongestionWindow • Limits how much data source can send: • Previously: EffectiveWindow = AdvertisedWindow – (LastbyteSent - LastByteAcked) • Now: EffectiveWindow = Min( CongestionWindow, AdvertisedWindow) – (LastByteSent – LastByteAcked) • Idea: • Increase CongestionWindow when congestion goes down • Decrease CongestionWindow when congestion goes up CSS432: Congestion Control

  11. AIMD (cont) • Question: how does the source determine whether or not the network is congested? • Answer: a timeout occurs • Timeout signals that a packet was lost • Packets are seldom lost due to transmission error • Lost packet implies congestion CSS432: Congestion Control

  12. Source Destination … AIMD (cont) • Algorithm • Divide CongestionWindow by 2 whenever a timeout occurs(halved) (multiplicative decrease) • Increment CongestionWindow by 1 packet per RTT (linear increase) If all packets (of congestion window) sent are ACKed increment window by 1 • In practice: increment a little for each ACK • Increment = MSS * (MSS/CongestionWindow) CongestionWindow += Increment

  13. 70 60 50 40 KB 30 20 10 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Time (seconds) AIMD (cont) • Saw Tooth • Window increases at slower rate and decreases at a faster rate Congestion Window

  14. Source Destination … Slow Start • Objective: reach the available capacity as fast as possible • Idea: • Begin with CongestionWindow = 1 packet • Double CongestionWindow each RTT (increment by 1 packet for each ACK) • When timeout occurs/packet loss: • Set congestionThreashold to CongestionWindow / 2 • Begin with CongestionWindow = 1 packet again CSS432: Congestion Control

  15. Slow Start Linear exponential http://www.6test.edu.cn/~lujx/linux_networking/0131777203_ch24lev1sec4.html CSS432: Congestion Control

  16. Slow Start • Exponential growth, but slower than all at once • Used… • when first starting connection (do not know anything about network) • When Nagle’s algorithm is used and packets are lost, (timeout occurs and the congestion window is already 0) • Final Algorithm: CongestionThreshold = INF while (true) { CongestionWindow = 1 while ( CongestionWindow < CongestionThreshold ) CongestionWindow *= 2 (based on slow start, exponential growth) while ( ACK returned ) CongestionWindow++ (based on additive increase, linear growth) if timeout occurs, CongestionThreshold = CongestionWindow / 2 Continue } CSS432: Congestion Control

  17. 70 60 50 Congestion Window KB 40 30 20 10 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Slow Start (cont) congestion Multiplicative decrement Multiplicative decrement congestion Additive increase Slow start on startup Additive increase Slow start after timeout CSS432: Congestion Control

  18. 70 60 50 KB 40 30 20 10 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Slow Start (cont) • Trace: • Flat out: Packets lost and no ACKs arrive (Timer running) • Problem: lose up to half a CongestionWindow’s worth of data • Capacity 16packets, first time send 16packets and receive ACK • Double congestion window (drop 16 packets) Timeout Packets lost The actual congestion threshold Congestion window CSS432: Congestion Control

  19. Fast Retransmit Sender Receiver Packet 1 Packet 2 ACK for 1 Packet 3 • Problem: coarse-grain TCP timeouts lead to idle periods • If no retransmit: • Say window full after Packet 6, When ACK 2 arrives, no action is taken, wait for time out and retransmit Packet 3 (idle period) ACK for 2 Packet 4 Duplicate ACK for 2 Packet 5 Packet 6 Duplicate ACK for 2 ACK for 2 Duplicate Time Out Retransmit packet 3 ACK for 6 CSS432: Congestion Control

  20. Fast Retransmit Sender Receiver Packet 1 Packet 2 ACK 1 Packet 3 • Problem: coarse-grain (use timer) TCP timeouts lead to idle periods. • Fast retransmit: use duplicate ACKs to trigger retransmission • The receiver sends back the same ACK as the last packet received in the correct sequential order. • The sender retransmits the packet whose ID is one larger than this duplicate ACK, upon receiving three ACKs. ACK 2 Packet 4 Duplicate ACK 1 ACK 2 Packet 5 Packet 6 Duplicate ACK 2 ACK 2 Duplicate ACK 3 ACK 2 Time Out Retransmit packet 3 ACK 6 CSS432: Congestion Control

  21. Results A packet lost Duplicate Acks allowing to keep transmitting more packet CongestionWindow is divided in a half upon retransmits rather than timeouts Too many packets sent A half of them dropped off No ACKs returned CongestionWindow stays in flat Coarse-grained timeouts 70 60 50 40 KB 30 20 10 1.0 2.0 3.0 4.0 5.0 6.0 7.0 CSS432: Congestion Control

  22. Fast Recovery • Fast recovery • skip the slow start phase • go directly to half the last successful CongestionWindow (threshold) • Slow start • Only at beginning • Coarse timeout occurs CSS432: Congestion Control

  23. Congestion Avoidance • TCP’s strategy • control congestion once it happens • repeatedly increase load in an effort to find the point at which congestion occurs, and then back off • Alternative strategy • predict when congestion is about to happen • reduce rate before packets start being discarded • call this congestion avoidance, instead of congestion control • Two possibilities • router-centric: RED Gateways • Explanation in the following slides • host-centric: TCP Vegas • Compare measured and expected throughput rate, and shrink congestion window if the measured rate is smaller. CSS432: Congestion Control

  24. Random Early Detection (RED) • Job of congestion control - router • Router notifies host • Notification is implicit • just drop the packet (TCP will timeout) • could make explicit by marking the packet • Early random drop • rather than wait for queue to become full, drop each arriving packet with some drop probability whenever the queue length exceeds some drop level • Congestion avoidance • Global synchronization avoidance CSS432: Congestion Control

  25. MaxThreshold MinThreshold A vgLen RED Details • Compute average queue length AvgLen = (1 - Weight) * AvgLen + Weight * SampleLen 0 < Weight < 1 (usually 0.002) SampleLen is queue length each time a packet arrives CSS432: Congestion Control

  26. RED Details (cont) • Two queue length thresholds if AvgLen <= MinThreshold then enqueue the packet if MinThreshold < AvgLen < MaxThreshold then calculate probability P drop arriving packet with probability P if MaxThreshold <= AvgLen then drop arriving packet CSS432: Congestion Control

  27. P(drop) 1.0 MaxP A vgLen MinThresh MaxThresh RED Details (cont) Typically 0.02 • Computing probability P TempP = MaxP * (AvgLen - MinThreshold)/ (MaxThreshold - MinThreshold) P = TempP/(1 - count * TempP) • Drop Probability Curve • P increases between min thresholg and max Threshold • Sharply increases to 1 for MaxThreshold Keep track of how many newly arriving packets have been queued while AveLen has been between the two thresholds

  28. Summary of TCP Versions CSS432: Congestion Control

  29. Reviews • Queuing disciplines: FIFO FQ • TCP congestion control: AIMD, cold/slow start, and fast retransmit/fast recovery • Congestion avoidance: RED and TCP vegas • Exercises in Chapter 6 • Ex. 2 (Avoidance) • Ex. 6 (Router congestions) • Ex. 25(Slow start) • Ex. 27 (AIMD, slow start) • Ex. 34 (RED) CSS432: Congestion Control

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