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CSS432 Congestion Control Textbook Ch6.1 – 6.4

CSS432 Congestion Control Textbook Ch6.1 – 6.4. Professor: Munehiro Fukuda (Augmented by Rob Nash). Taxonomy. Resource in network systems Link bandwidth Buffer size in routers or switches Two sides of the same coin P re-allocate resources so as to avoid congestion

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CSS432 Congestion Control Textbook Ch6.1 – 6.4

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  1. CSS432 Congestion ControlTextbook Ch6.1 – 6.4 Professor: Munehiro Fukuda (Augmented by Rob Nash) CSS432: Congestion Control

  2. Taxonomy • Resource in network systems • Link bandwidth • Buffer size in routers or switches • 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. • Two points of implementation CSS432: Congestion Control

  3. Source 1 Router Destination 1 Router Source 2 Router Destination 2 Source 3 Connectionless Flow • Datagrams • Switched independently • Flowing through a particular set of routers if transmitted from the same source/destination pair • Connectionless Flows • 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

  4. 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. drops off overflowed packets. CSS432: Congestion Control

  5. TCP Congestion Control • Idea • Assumes best-effort network (FIFO or FQ routers) • 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 CSS432: Congestion Control

  6. Sending application TCP LastByteWritten y LastByteSent LastByteAcked LastByteSent – LastByteAcked ≤ AdvertisedWindow EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked) Additive Increase/Multiplicative Decrease (AIMD) • New state variable per 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

  7. 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

  8. 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 T ime (seconds) AIMD (cont) • Algorithm • Increment CongestionWindow by one packet per CW (linear increase) • Divide CongestionWindow by two whenever a timeout occurs (multiplicative decrease) • In practice: increment a little for each ACK Increment = MSS * (MSS/CongestionWindow) CongestionWindow += Increment CSS432: Congestion Control

  9. AI in AIMD too Slow? • Connections just starting up ramp up too slowly and waste resources • Not doing effective probing, either • We need something faster, that sounds faster: • Quick Ramp Up? • Fast Start? CSS432: Congestion Control

  10. Slow Start Overview (p481) • Avoids “out of the gate” bursts of windowSize • Quicker than a linear increase • Begin with CongestionWindow = 1 • On timeout, set a threshold == CW/2 • This is the multiplicative decrease • Set CongestionWindow to 1 and slow start • Then switch to linear once CW >= threshold CSS432: Congestion Control

  11. 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: • Set congestionThreashold to CongestionWindow / 2 • Begin with CongestionWindow = 1 packet again • Observe slow start with tcpdump in assignment 3. CSS432: Congestion Control

  12. Slow Start (cont) • Exponential growth, but slower than all at once • Used… • when first starting connection • 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 lincrease, linear growth) • If timeout occurs, • CongestionThreshold = CongestionWindow / 2 • Continue • } CSS432: Congestion Control

  13. 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: • Problem: lose up to half a CongestionWindow’s worth of data Timeout Packets lost The actual congestion threshold Congestion window CSS432: Congestion Control

  14. Slow Start Notes • Only used at the beginning of a connection or whenever a timeout occurs • Slow Start augments the AI in AIMD • Less time to use more bandwidth after a course-grained timeout • So, our ramp up is steeper • Fast Retransmit deals with cumulative ACKS • Avoid waiting for long timeouts • Fast Recovery skips Slow Start • avoids “resetting to 1” (the slow start approach) and just uses CW/2 CSS432: Congestion Control

  15. Sender Receiver Packet 1 Packet 2 ACK 1 Packet 3 ACK 2 Packet 4 ACK 2 Packet 5 Packet 6 ACK 2 ACK 2 Retransmit packet 3 ACK 6 Fast Retransmit • Problem: coarse-grain 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. Duplicate ACK 1 Duplicate ACK 2 Duplicate ACK 3 CSS432: Congestion Control

  16. Results Too many packets sent A half of them dropped off No ACKs returned CongestionWindow stays in flat Coarse-grained timeouts A packet lost Duplicate Acks allowing to keep transmitting more packet CongestionWindow is divided in a half upon retransmits rather than 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

  17. Implications • Congestion Threshold is divided in a half upon retransmits and timeouts • CW = 1 • We use ACKS and DUP ACKS as our timers • Since these move faster than our internal clocks • A retransmission indicates congestion and we don’t need to wait forever (course timeout) to notice this CSS432: Congestion Control

  18. 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 T ime (seconds) Fast Recovery • Fast recovery • skip the slow start phase • go directly to half the last successful CongestionWindow (ssthresh) • Below lacks the initial slow start & cg timeouts CSS432: Congestion Control

  19. Chapter 6, Problem 27 Consider the TCP traces… CSS432: Congestion Control

  20. Congestion Avoidance • TCP’s strategy is Congestion Control, not Avoidance • control congestion once it happens • repeatedly increase load in an effort to find the point at which congestion occurs, and then back off • TCP needs to observe losses to find the sweet spot in the bandwidth • 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 (guess who?) • Compare measured and expected throughput rate, and shrink congestion window if the measured rate is smaller. CSS432: Congestion Control

  21. Summary of TCP Versions CSS432: Congestion Control

  22. Random Early Detection (RED) • 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

  23. 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

  24. 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

  25. 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 Keep track of how many newly arriving packets have been queued while AveLen has been between the two thresholds CSS432: Congestion Control

  26. Chapter 6, Problem 33 DECbit V.S. RED Explicit Congestion Avoidance V.S. Implicit Congestion Avoidance CSS432: Congestion Control

  27. 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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