Lecture 21: Congestion ControlCSE 123 –Lecture 21: Congestion control 17! Goal: quickly find the equilibrium sending rate! Quickly increase sending rate until congestion detected

CSE 123: Computer NetworksAaron Schulman

Lecture 21:Congestion Control

! How fast should a sending host transmit data?u Not to fast, not to slow, just right…

! Should not be faster than the sender’s shareu Bandwidth allocation

! Should not be faster than the network can processu Congestion control

! Congestion control & bandwidth allocation are separate ideas, but frequently combined

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CC Overview

CSE 123 – Lecture 21: Congestion Control

! How much bandwidth should each flow from a source to a destination receive when they compete for resources?

u What is a “fair” allocation?

Router

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Bandwidth Allocation


! Buffer intended to absorb bursts when input rate > output! But if sending rate is persistently > drain rate, queue builds! Dropped packets represent wasted work; goodput < throughput

Destination1.5-Mbps T1 link

Router

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100-Mbps FDDI

10-Mbps Ethernet

Packets dropped here

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Congestion


Network Load

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Congestion collapse

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Drop-Tail QueuingLoss due toCongestion


! Rough definition: “When an increase in network load produces a decrease in useful work”

! Why does it happen?u Sender sends faster than bottleneck link speedu Packets queue until droppedu In response to packets being dropped, sender retransmitsu All hosts repeat in steady state…

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Congestion Collapse


! Increase network resourcesu More buffers for queuingu Increase link speedu Pros/Cons of these approaches?

! Reduce network loadu Send data more slowly u How much more slowly?u When to slow down?

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Mitigation Options


! Open loopu Explicitly reserve bandwidth in the network in advance

! Closed loopu Respond to feedback and adjust bandwidth allocation

! Network-basedu Network implements and enforces bandwidth allocation

! Host-basedu Hosts are responsible for controlling their sending rate

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Designing a Control


Network Load

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“Knee”“Cliff”

Congestion collapse

! Congestion avoidance: try to stay to the left of the knee! Congestion control: try to stay to the left of the cliff

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Proactive vs. Reactive

Loss due toCongestion


! How to detect congestion?

! How to limit sending data rate?

! How fast to send?

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Challenges to Address


! Explicit congestion signalingu Source Quench: ICMP message from router to senderu DECBit / Explicit Congestion Notification (ECN):

» Router marks packet based on queue occupancy (i.e. indication that packet encountered congestion along the way)

» Receiver tells sender if queues are getting too full

! Implicit congestion signalingu Packet loss

» Assume congestion is primary source of packet loss» Lost packets indicate congestion

u Packet delay» Round-trip time increases as packets queue» Packet inter-arrival time is a function of bottleneck link

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Detecting Congestion


! Window-based (TCP)u Constrain number of outstanding packets allowed in networku Increase window to send faster; decrease to send sloweru Pro: Cheap to implement (already have windowing),

good failure propertiesu Con: Creates traffic bursts (requires bigger buffers)

! Rate-based (many streaming media protocols)u Two parameters (period, packets)u Allow sending of x packets in period yu Pro: smooth trafficu Con: fine-grained per-connection timers, what if receiver fails?

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Throttling Options


! Ideally: Keep equilibrium at “knee” of power curveu Find “knee” somehowu Keep number of packets “in flight” the sameu Don’t send a new packet into the network until you know one

has left (I.e. by receiving an ACK)u What if you guess wrong, or if bandwidth availability changes?

! Compromise: adaptive approximation u If congestion signaled, reduce sending rate by xu If data delivered successfully, increase sending rate by yu How to relate x and y? Most choices don’t converge…

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Choosing a Send Rate


! Each source independently probes the network to determine how much bandwidth is availableu Changes over time, since everyone does this

! Assume that packet loss implies congestionu Since errors are rare; also, requires no support from routers

Sink45 Mbps T3 link

RouterSource100 Mbps Ethernet

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TCP’s Probing Approach


! Window-based congestion controlu Unified congestion control and flow control mechanismu rwin: advertised flow control window from receiveru cwnd: congestion control window

» Estimate of how much outstanding data network can deliver in a round-trip time

u Sender can only send MIN(rwin,cwnd) at any time

! Idea: decrease cwnd when congestion is encountered; increase cwnd otherwiseu Question: how much to adjust?

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Basic TCP Algorithm


! Goal: Adapt to changes in available bandwidth

! Additive Increase, Multiplicative Decrease (AIMD)u Increase sending rate by a constant (e.g. MSS)u Decrease sending rate by a linear factor (e.g. divide by 2)

! Rough intuition for why this worksu Let Li be queue length at time iu In steady state: Li = N, where N is a constantu During congestion, Li = N + yLi-1, where y > 0u Consequence: queue size increases multiplicatively

» Must reduce sending rate multiplicatively as well

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Congestion Avoidance


TCP Bandwidth Probing! TCP uses AIMD to adjust congestion window

u Converges to fair share of bottleneck linku Increases modestly in good timesu Cuts drastically in bad times

! But what rate should a TCP flow use initially?u Need some initial congestion windowu We’d like to TCP to work on all manner of linksu Need to span 6+ orders of magnitude, e.g., 10 K to 10 Gbps.u Starting too fast is catastrophic!

17CSE 123 – Lecture 21: Congestion control

! Goal: quickly find the equilibrium sending rate

! Quickly increase sending rate until congestion detectedu Remember last rate that worked and don’t overshoot it

! TCP Reno Algorithm: u On new connection, or after timeout, set cwnd=1 MSSu For each segment acknowledged, increment cwnd by 1 MSSu If timeout then divide cwnd by 2, and set ssthresh = cwndu If cwnd >= ssthresh then exit slow start

! Why called slow? Its exponential after all…

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Slow Start

CSE 123 – Lecture 21: Congestion control

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Slow Start Example

CSE 123 – Lecture 20: Scheduling & QoS

Slow Start + Congestion Avoidance

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Basic Mechanisms


! Fast retransmitu Timeouts are slow (default often 200 ms or 1 second)u When packet is lost, receiver still ACKs last in-order packetu Use 3 duplicate ACKs to indicate a loss; detect losses quickly

» Why 3? When wouldn’t this work?

! Fast recoveryu Goal: avoid stalling after loss u If there are still ACKs coming in, then no need for slow startu If a packet has made it through -> we can send another oneu Divide cwnd by 2 after fast retransmitu Increment cwnd by 1 MSS for each additional duplicate ACK

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Fast Retransmit & Recovery


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Fast retransmit

Fast recovery(increase cwnd by 1)

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Fast Retransmit Example


Slow Start + Congestion Avoidance +Fast Retransmit + Fast Recovery

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More Sophistication


! Short connection: only contains a few pkts! How do short connections and Slow-Start interact?

u What happens when a packet is lost during Slow-Start?u What happens when the SYN is dropped?

! Bottom line: Which packet gets dropped matters a lotu SYNu Slow-Startu Congestion avoidance

! Do you think most flows are short or long?! Do you think most traffic is in short flows or long flows?

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Short Connections


! TCP is designed around the premise of cooperationu What happens to TCP if it competes with a UDP flow?u What if we divide cwnd by 3 instead of 2 after a loss?

! There are a bunch of magic numbersu Decrease by 2x, increase by 1/cwnd, 3 duplicate acks, initial

timeout = 3 seconds, etc.

! But overall it works really well!u Still being constantly tweaked…

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Open Issues


! TCP Probes the network for bandwidth, assuming thatloss signals congestion

! The congestion window is managed with an additive increase/multiplicative decrease policyu It took fast retransmit and fast recovery to get thereu Fast recovery keeps pipe “full” while recovering from a loss

! Slow start is used to avoid lengthy initial delaysu Ramp up to near target rate, then switch to AIMD

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TCP CC Summary


For next time…

! RP&D 6.5

! Keep going on Project 2

27CSE 123 – Lecture 19: Congestion Control

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Lecture 21: Congestion ControlCSE 123 –Lecture 21: Congestion control 17! Goal: quickly find the equilibrium sending rate! Quickly increase sending rate until congestion detected