lecture 22: congestion control cont. and intro to links...

Lecture 22:Congestion Control cont. andIntro to Links and Signaling

CSE 123: Computer NetworksAaron Schulman

Project 3 due Thur 5/20

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Lecture 22 Overview! Finish congestion control

! Signalingu Channel characteristicsu Types of physical media

! Modulationu Narrowband vs. Broadbandu Encoding schemes

! Warning: A lot of this material is not in the book

CSE 123 – Lecture 21: Links and Signaling

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!

3CSE 123 – Lecture 22: 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 22: Congestion Control

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2Ack 2

3Ack 3

4567

cwnd=1

cwnd=2

cwnd=4

cwnd=8

Sender Receiver

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cwnd

Ack 4

Ack 5Ack 6Ack 7Ack 8

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Slow Start Example


Slow Start + Congestion Avoidance

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Timeout

ssthresh

Slow start

Congestionavoidance

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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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Sender Receiver

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Ack 4Ack 4Ack 4Ack 4

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3 Dup Acks

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 that loss 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


Underneath it all: Sending bits! A three-step process

u Take an input stream of bits (digital data)u Modulate some physical media to send data (analog)u Demodulate the signal to retrieve bits (digital again)u Anybody heard of a modem (Modulator-demodulator)?

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digital data(a string of symbols)

digital data(a string of symbols)a signal

modulation demodulation

0101100100100 0101100100100

CSE 123 – Lecture 22: Links and Signaling

A Simple Signaling System

21CSE 123 – Lecture 22: Links and Signaling

Morse Code


Morse Code Message


For Next Class

! Read 2.2


lecture 22: congestion control cont. and intro to links...

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