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

2. CSS432: Congestion Control
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
Router Centric
QoS-based service
Reservation-based
Rate-based
Host Centric
Best-effort service
Feedback-based
Window-based
CSS432: Congestion Control

3. CSS432: Congestion Control
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?
Router
Source 2 1 3
Destination
CSS432: Congestion Control

4. 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.
Destination
1.5-Mbps T1 link
Router
Source 2 1
100-Mbps FDDI
10-Mbps Ethernet
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. Additive Increase/Multiplicative Decrease (AIMD)
New state variable per connection: CongestionWindow
Limits how much data source can send:
Previously:
EffectiveWindow
= AdvertisedWindow – (LastbyteSent - LastByteAcked)
Now:
= Min( CongestionWindow, AdvertisedWindow)
– (LastByteSent – LastByteAcked)
Idea:
Increase CongestionWindow when congestion goes down
Decrease CongestionWindow when congestion goes up
Sending application
LastByteWritten
TCP
LastByteSent
LastByteAcked
LastByteSent – LastByteAcked ≤ AdvertisedWindow
EffectiveWindow
= AdvertisedWindow – (LastByteSent – LastByteAcked) y
CSS432: Congestion Control

7. CSS432: Congestion Control
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
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8. CSS432: Congestion Control
AIMD (cont)
Algorithm
Increment CongestionWindow by one packet per CW (linear increase)
Divide CongestionWindow by two whenever a timeout occurs (multiplicative decrease) 60 20
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In practice: increment a little for each ACK
Increment = MSS * (MSS/CongestionWindow)
CongestionWindow += Increment
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9. CSS432: Congestion Control
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
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11. CSS432: Congestion Control
Slow Start
Source
Destination …
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. CSS432: Congestion Control
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 }
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13. CSS432: Congestion Control
Slow Start (cont)
Trace:
Problem: lose up to half a CongestionWindow’s worth of data 60 20
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Timeout
Packets lost
The actual congestion threshold
Congestion window
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14. CSS432: Congestion Control
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
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15. CSS432: Congestion Control
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.
Packet 1
Packet 2
Packet 3
Packet 4
Packet 5
Packet 6
Retransmit
packet 3
ACK 1
ACK 2
ACK 6
Sender
Receiver
Duplicate ACK 1
Duplicate ACK 2
Duplicate ACK 3
CSS432: Congestion Control

16. CSS432: Congestion Control
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
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CSS432: Congestion Control

17. CSS432: Congestion Control
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
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18. CSS432: Congestion Control
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 60 20
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CSS432: Congestion Control

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

20. CSS432: Congestion Control
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
RFC 1122
TCP Tahoe
TCP Reno
TCP Vegas
RTT Estimation X
Karn’s Algorithm
Slow Start
AIMD
Fast Retransmit
Fast Recovery
Throughput-rate congestion control
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. CSS432: Congestion Control
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
MaxThreshold
MinThreshold A
vgLen
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24. CSS432: Congestion Control
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
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25. CSS432: Congestion Control
RED Details (cont)
Computing probability P
TempP = MaxP * (AvgLen - MinThreshold)/ (MaxThreshold - MinThreshold)
P = TempP/(1 - count * TempP)
Drop Probability Curve
Typically 0.02
Keep track of how many newly arriving
packets have been queued while AveLen
has been between the two thresholds
P(drop)
1.0
MaxP
MinThresh
MaxThresh A
vgLen
CSS432: Congestion Control

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

27. CSS432: Congestion Control
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