CSS432: Congestion Control 1

CSS432 Congestion ControlTextbook Ch6.1 – 6.4

Professor: Munehiro Fukuda

CSS432: Congestion Control 2

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

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

Source2

Source1

Source3

Router

Router

Destination2

Destination1

CSS432: Congestion Control 4

Queuing Discipline First-In-First-Out (FIFO)

Does not discriminate between traffic sources Fair Queuing (FQ)

Work conserving: link is never left idle Explicitly segregates traffic based on flows Ensures no flow captures more than its share of capacity

If there are n flows sending data, 1/nth bandwidth is at most given Variation: weighted fair queuing (WFQ)

Problem? Flow 1

Flow 2

Flow 3

Flow 4

Round-robinservice

CSS432: Congestion Control 5

Bit-Round Fair Queuing (BRFQ) Algorithm

For each queue, compute the virtual finish time upon the arrival of a new packet. Choose a packet with the lowest virtual finish time.

Pros and Cons Emulate bit-by-bit fair queuing Not perfect: can’t preempt the current large packet.

Packet 1 Packet 2 Packet 1 Packet 2 Packet 1

Real arrival time ti 0 2 1 2 3

Transmission time Pi

3 1 1 4 2

Virtual start time Si 0 3 1 2 3

Virtual finish time Fi

3 4 2 6 5

Queue A Queue B Queue C

Time Queue A Queue B Queue C

0

1

2

3

4

5

6

7

8

9

10

11

12

Real arrivalFQBRFQ

Fqi=Sq

i + Pqi

Sqi = max[Fq

i-1, R(tqi)]

R(t) = R(t + 1) + 1/#rounds

max( )

CSS432: Congestion Control 6

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.

Destination1.5-Mbps T1 link

Router

Source2

Source1

100-Mbps FDDI

10-Mbps Ethernet

FQ or BRFQ drops off overflowed packets.

CSS432: Congestion Control 7

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 8

Additive Increase/Multiplicative Decrease (AIMD)

New state variable per connection: CongestionWindow Limits how much data source can send: Previously:

EffectiveWindow= AdvertisedWindow – (LastbyteSent - LastByteAcked)

Now:EffectiveWindow= Min( CongestionWindow, AdvertisedWindow) – (LastByteSent – LastByteAcked)

Idea: Increase CongestionWindow when

congestion goes down Decrease CongestionWindow when

congestion goes up

Sending application

LastByteWritten

TCP

LastByteSentLastByteAcked

LastByteSent – LastByteAcked ≤ AdvertisedWindow

EffectiveWindow= AdvertisedWindow – (LastByteSent – LastByteAcked)

y

CSS432: Congestion Control 9

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

CSS432: Congestion Control 10

AIMD (cont)

In practice: increment a little for each ACK

Increment = MSS * (MSS/CongestionWindow)CongestionWindow += Increment

Algorithm Increment CongestionWindow by one packet per RTT (linear increase) Divide CongestionWindow by two whenever a timeout occurs

(multiplicative decrease)

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

KB

Time (seconds)

70

304050

10

10.0

CSS432: Congestion Control 11

Slow Start

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.

Source Destination

…

CSS432: Congestion Control 12

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

}

CSS432: Congestion Control 13

Slow Start (cont) Trace:

Problem: lose up to half a CongestionWindow’s worth of data

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

KB

70

304050

10

The actual congestion threshold Congestion windowPackets lost

Timeout

CSS432: Congestion Control 14

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

Retransmitpacket 3

ACK 1

ACK 2

ACK 2

ACK 2

ACK 6

ACK 2

Sender Receiver

Duplicate ACK 1

Duplicate ACK 2

Duplicate ACK 3

CSS432: Congestion Control 15

Results

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0

KB

70

304050

10

A packet lost

Duplicate Acks allowing to keep transmitting more packet

CongestionWindow is divided in a half upon retransmits rather than timeouts

Too many packets sent

A half of them dropped off

No ACKs returned

CongestionWindow stays in flat

Coarse-grained timeouts

CSS432: Congestion Control 16

Fast Recovery

Fast recoveryskip the slow start phasego directly to half the last successful CongestionWindow (ssthresh)

CSS432: Congestion Control 17

Congestion Avoidance TCP’s strategy

control congestion once it happens repeatedly increase load in an effort to find the point at which

congestion occurs, and then back off

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

Compare measured and expected throughput rate, and shrink congestion window if the measured rate is smaller.

CSS432: Congestion Control 18

Summary of TCP VersionsRFC 1122 TCP Tahoe TCP Reno TCP Vegas

RTT Estimation X X X X

Karn’s Algorithm X X X X

Slow Start X X X X

AIMD X X X X

Fast Retransmit X X X

Fast Recovery X X

Throughput-rate congestion control

X

CSS432: Congestion Control 19

Random Early Detection (RED)

Notification is implicit just drop the packet (TCP will timeout)could make explicit by marking the packet

Early random droprather 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 20

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

AvgLen

CSS432: Congestion Control 21

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

CSS432: Congestion Control 22

RED Details (cont) Computing probability P

TempP = MaxP * (AvgLen - MinThreshold)/ (MaxThreshold - MinThreshold)

P = TempP/(1 - count * TempP)

Drop Probability CurveP(drop)

1.0

MaxP

MinThresh MaxThresh

AvgLen

Keep track of how many newly arrivingpackets have been queued while AveLenhas been between the two thresholds

Typically 0.02

CSS432: Congestion Control 23

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)