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
A
Sending Host

B
Buffer in Router
Receiving Host
Ignore internal structure of router and model it as
having a single queue for a particular inputoutput pair
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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
A

100 Mbps
B
Adjust rate to match bottleneck bandwidth
- Without any a priori knowledge
- Could be gigabit link, could be a modem
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Single Flow, Varying Bandwidth
A

BW(t)
B
Adjust rate to match instantaneous bandwidth
- Assuming you have rough idea of bandwidth
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Multiple Flows
Two Issues:
 Adjust total sending rate to match bandwidth
 Allocation of bandwidth between flows
A1
A2
A3
B1
100 Mbps
B2
B3
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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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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

Delay

Throughput
What’s Really Happening?
View from a Single Flow
knee
packet
loss
cliff
congestion
collapse
Load
Note (in an M/M/1 queue)
- Delay = 1/(1 – utilization)
Load
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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
MaxWindow = min(cwnd, AdvertisedWindow)
EffectiveWindow = MaxWindow – (LastByteSent – LastByteAcked)
MaxWindow
LastByteAcked
LastByteSent
EffectiveWindow
sequence number increases
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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 point
cwnd = 1
cwnd = 2
cwnd = 3
cwnd = 4
cwnd =
8
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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
A
Rate (pkts/RTT)
60
50
40
30
Backlog in router (pkts)
Congested if > 20
20
10
487
460
433
406
379
352
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
C = 50 pkts/RTT
No congestion  x increases by one packet/RTT every RTT
Congestion  decrease x by factor 2
1

B
x
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AIMD Sharing Dynamics
x
y
A
D
60
Rates equalize  fair share
50
40
30
20
10
487
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433
406
379
352
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
E
No congestion  rate increases by one packet/RTT every RTT
Congestion  decrease rate by factor 2
1

B
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AIAD Sharing Dynamics
x
A
y
D
60
50
40
30
20
10
487
460
433
406
379
352
325
298
271
244
217
190
163
136
109
82
55
0
28

E
No congestion  x increases by one packet/RTT every RTT
Congestion  decrease x by 1
1

B
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Efficient Allocation

Too slow
2 user example
- 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

overload
Efficiency
line
underload
Optimal:
- xi=Xgoal

User 2: x2

Efficiency = 1 - distance from
efficiency line
User 1: x1
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AIMD
A
D
C
C
x
B
y
E
y
Limit rates:
x=y
x
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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)
cwnd
Timeout
AIMD
Timeout
AIMD
ssthresh
Slow
Start
Slow
Start
Slow
Start
Time
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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
cwnd = 1
cwnd = 2
cwnd = 4

Notes:
- ACKs are for next
expected packet
3 duplicate
- Packet reordering can
ACKs
cause duplicate ACKs
- Window may be too small
to get enough duplicate
ACKs
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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
cwnd
AI/MD
Slow Start
Fast retransmit

Time
Retransmit after 3 duplicated acks
- Prevent expensive timeouts


Reduce slow starts
At steady state, cwnd oscillates around the
optimal window size
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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
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