Katz, Stoica F04

EECS 122: Introduction to Computer Networks

Congestion Control

Computer Science Division

Department of Electrical Engineering and Computer Sciences

University of California, Berkeley

Berkeley, CA 94720-1776

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What We Know

We know: How to process packets in a switch How to route packets in the network How to send packets reliably

We don’t know: How fast to send

"As we know, there are known knowns. There are things we know we know. We also know there are known unknowns. That is to say, we know there are some things we do not know. But there are also unknown unknowns, the ones we don't know we don't know." The Zen of Donald Rumsfeld.

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Implications

Send too slow: link is not fully utilized- Wastes time

Send too fast: link is fully utilized but....- Queue builds up in router buffer (delay)

- Overflow buffers in routers

- Overflow buffers in receiving host (ignore)

Why are buffer overflows a problem?- Packet drops (mine and others)

- Interesting history....(Van Jacobson rides to the rescue)

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Abstract View

Ignore internal structure of router and model it as having a single queue for a particular input-output pair

Sending Host Buffer in Router Receiving Host

A B

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Three Congestion Control Problems

Adjusting to bottleneck bandwidth

Adjusting to variations in bandwidth

Sharing bandwidth between flows

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Single Flow, Fixed Bandwidth

Adjust rate to match bottleneck bandwidth- Without any a priori knowledge

- Could be gigabit link, could be a modem

A B100 Mbps

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Single Flow, Varying Bandwidth

Adjust rate to match instantaneous bandwidth- Assuming you have rough idea of bandwidth

A BBW(t)

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Multiple Flows

Two Issues: Adjust total sending rate to match bandwidth Allocation of bandwidth between flows

A2 B2100 Mbps

A1

A3 B3

B1

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Reality

Congestion control is a resource allocation problem involving many flows, many links, and complicated global dynamics

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What’s Really Happening?View from a Single Flow

Knee – point after which - Throughput increases very slow

- Delay increases fast

Cliff – point after which- Throughput starts to decrease

very fast to zero (congestion collapse)

- Delay approaches infinity

Note (in an M/M/1 queue)- Delay = 1/(1 – utilization)

Load

Load

Th

rou

ghp

ut

De

lay

knee cliff

congestioncollapse

packetloss

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General Approaches

Send without care- Many packet drops- Not as stupid as it seems

Reservations- Pre-arrange bandwidth allocations- Requires negotiation before sending packets- Low utilization

Pricing- Don’t drop packets for the high-bidders- Requires payment model

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General Approaches (cont’d)

Dynamic Adjustment- Probe network to test level of congestion

- Speed up when no congestion

- Slow down when congestion

- Suboptimal, messy dynamics, simple to implement

All three techniques have their place- But for generic Internet usage, dynamic adjustment is

the most appropriate

- Due to pricing structure, traffic characteristics, and good citizenship

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TCP Congestion Control

TCP connection has window- Controls number of unacknowledged packets

Sending rate: ~Window/RTT

Vary window size to control sending rate

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Congestion Window (cwnd)

Limits how much data can be in transit Implemented as # of bytes Described as # packets in this lecture

EffectiveWindow = MaxWindow – (LastByteSent – LastByteAcked)

MaxWindow = min(cwnd, AdvertisedWindow)

LastByteAckedLastByteSent

sequence number increases

MaxWindow

EffectiveWindow

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Two Basic Components

Detecting congestion

Rate adjustment algorithm- Depends on congestion or not

- Three subproblems within adjustment problem

• Finding fixed bandwidth

• Adjusting to bandwidth variations

• Sharing bandwidth

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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)

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Rate Adjustment

Basic structure:- Upon receipt of ACK (of new data): increase rate

- Upon detection of loss: decrease rate

But what increase/decrease functions should we use?

- Depends on what problem we are solving

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Problem #1: Single Flow, Fixed BW

Want to get a first-order estimate of the available bandwidth

- Assume bandwidth is fixed- Ignore presence of other flows

Want to start slow, but rapidly increase rate until packet drop occurs (“slow-start”)

Adjustment: - cwnd initially set to 1- cwnd++ upon receipt of ACK

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Slow-Start

cwnd increases exponentially: cwnd doubles every time a full cwnd of packets has been sent

- Each ACK releases two packets

- Slow-start is called “slow” because of starting pointsegment 1cwnd = 1

cwnd = 2 segment 2segment 3

cwnd = 4 segment 4

segment 5segment 6segment 7

cwnd = 8

cwnd = 3

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Problems with Slow-Start

Slow-start can result in many losses- Roughly the size of cwnd ~ BW*RTT

Example:- At some point, cwnd is enough to fill “pipe”

- After another RTT, cwnd is double its previous value

- All the excess packets are dropped!

Need a more gentle adjustment algorithm once have rough estimate of bandwidth

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Problem #2: Single Flow, Varying BW

Want to be able to track available bandwidth, oscillating around its current value

Possible variations: (in terms of RTTs)- Multiplicative increase or decrease: cwnd a*cwnd- Additive increase or decrease: cwnd cwnd + b

Four alternatives:- AIAD: gentle increase, gentle decrease- AIMD: gentle increase, drastic decrease- MIAD: drastic increase, gentle decrease (too many losses)- MIMD: drastic increase and decrease

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Problem #3: Multiple Flows

Want steady state to be “fair”

Many notions of fairness, but here all we require is that two identical flows end up with the same bandwidth

This eliminates MIMD and AIAD

AIMD is the only remaining solution!

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Buffer and Window Dynamics

No congestion x increases by one packet/RTT every RTT Congestion decrease x by factor 2

A BC = 50 pkts/RTT

0

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Backlog in router (pkts)Congested if > 20

Rate (pkts/RTT)

x

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AIMD Sharing Dynamics

A Bx

D E

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No congestion rate increases by one packet/RTT every RTT Congestion decrease rate by factor 2

Rates equalize fair share

y

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AIAD Sharing Dynamics

A Bx

D E No congestion x increases by one packet/RTT every RTT Congestion decrease x by 1

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Efficient Allocation

Too slow- Fail to take advantage of

available bandwidth underload

Too fast- Overshoot knee overload,

high delay, loss Everyone’s doing it

- May all under/over shoot large oscillations

Optimal:

xi=Xgoal Efficiency = 1 - distance from

efficiency line

User 1: x1U

ser

2: x

2

Efficiencyline

2 user example

overload

underload

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AIMD

C

x

y

A Bx

C

D Ey

Limit rates:x = y

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Implementing AIMD

After each ACK- Increment cwnd by 1/cwnd (cwnd += 1/cwnd)

- As a result, cwnd is increased by one only if all segments in a cwnd have been acknowledged

But need to decide when to leave slow-start and enter AIMD Use ssthresh variable

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Slow Start/AIMD Pseudocode

Initially:cwnd = 1;ssthresh = infinite;

New ack received:if (cwnd < ssthresh) /* Slow Start*/ cwnd = cwnd + 1;else /* Congestion Avoidance */ cwnd = cwnd + 1/cwnd;

Timeout:/* Multiplicative decrease */ssthresh = cwnd/2;cwnd = 1;

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The big picture (with timeouts)

Time

cwnd

Timeout

SlowStart

AIMD

ssthresh

Timeout

SlowStart

SlowStart

AIMD

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Congestion Detection Revisited

Wait for Retransmission Time Out (RTO)- RTO kills throughput

In BSD TCP implementations, RTO is usually more than 500ms

- The granularity of RTT estimate is 500 ms

- Retransmission timeout is RTT + 4 * mean_deviation

Solution: Don’t wait for RTO to expire

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Fast Retransmits

Resend a segment after 3 duplicate ACKs

- Duplicate ACK means that an out-of sequence segment was received

Notes:- ACKs are for next

expected packet - Packet reordering can

cause duplicate ACKs- Window may be too small

to get enough duplicate ACKs

ACK 2

segment 1cwnd = 1

cwnd = 2 segment 2segment 3

ACK 4cwnd = 4 segment 4

segment 5segment 6segment 7

ACK 3

3 duplicateACKs

ACK 4

ACK 4

ACK 4

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Fast Recovery: After a Fast Retransmit

ssthresh = cwnd / 2 cwnd = ssthresh

- instead of setting cwnd to 1, cut cwnd in half (multiplicative decrease)

For each dup ack arrival- dupack++- MaxWindow = min(cwnd + dupack, AdvWin) - indicates packet left network, so we may be able to send more

Receive ack for new data (beyond initial dup ack)- dupack = 0- exit fast recovery

But when RTO expires still do cwnd = 1

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Fast Retransmit and Fast Recovery

Retransmit after 3 duplicated acks- Prevent expensive timeouts

Reduce slow starts At steady state, cwnd oscillates around the

optimal window size

Time

cwnd

Slow Start

AI/MD

Fast retransmit

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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