CSS432 Congestion Control Textbook Ch6.1 – 6.4

CSS432: Congestion Control 1

CSS432 Congestion ControlTextbook Ch6.1 – 6.4

Instructor: Joe McCarthy(based on Prof. Fukuda’s slides)


Taxonomy Limited resources in network systems

Link bandwidth Buffer size in routers or switches

Resource allocation & congestion control: 2 sides of same coin Pre-allocate resources so as to avoid congestion Control congestion only when it occurs

Flow control vs. 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


Connectionless Flow Datagrams

Switched independently Typically flow through same set of routers if transmitted from the same source to the same destination

Connectionless Flows

Routers & states: No state: purely connectionless service Hard state: purely connection-oriented service Soft state: allocate resources on a per-flow basis


Queuing Discipline First-In-First-Out (FIFO) w/ Tail Drop

Does not discriminate between traffic sources (flows) Fair Queuing (FQ)

Work conserving: link is never left idle (if data to be sent) Explicitly segregates traffic based on flows Ensures no flow captures more than its share of capacity

If there are n flows sending data, each is allocated 1/n bandwidth Variation: weighted fair queuing (WFQ)

Problem: Variable packet length Flow 1

Flow 2

Flow 3

Flow 4

Round-robinservice

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

[Section 3.2]


Bit-Round Fair Queuing (BRFQ) Algorithm

For each queue, compute the virtual finish time (F) upon arrival of a new packet. Choose a packet with the lowest virtual finish time. No preemption

Pros and Cons Emulates bit-by-bit fair queuing Not perfect: can’t preempt a large packet currently being transmitted

Example of fair queuing in action: (a) packets with earlier finishing timesare sent first; (b) sending of a packet already in progress is completed


TCP Congestion Control Created by Van Jacobson, 1980s,

~8 years after TCP/IP protocol stack became operational Immediately preceding this time, the Internet was

suffering from congestion collapse hosts would send their packets into the Internet as fast

as the advertised window would allow, congestion would occur at some router (causing packets

to be dropped), and the hosts would time out hosts retransmit their packets, resulting in even more

congestion


TCP Congestion Control Concept:

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 # of in-transit packets in response to dynamic

changes in the available capacity


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 deceases Decrease CongestionWindow when

congestion increases

Sending application

LastByteWritten

TCP

LastByteSentLastByteAcked

LastByteSent – LastByteAcked ≤ AdvertisedWindow

EffectiveWindow= AdvertisedWindow – (LastByteSent – LastByteAcked)

y

Options (variable)

Data

Checksum

SrcPort DstPort

HdrLen 0 Flags

UrgPtr

AdvertisedWindow

SequenceNum

Acknowledgment

0 4 10 16 31


AIMD (cont)

Question: how does the source determine whether or not the network is congested?


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


AIMD (cont)

In practice: increment a little for each ACK

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

Algorithm Increment CongestionWindow by 1 packet per RTT (additive increase) Divide CongestionWindow by 2 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

Con

gest

ionW

indo

w S

ize


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

…


Slow Start 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 = INFwhile (true) {

CongestionWindow = 1while ( CongestionWindow < CongestionThreshold )

CongestionWindow *= 2 (based on slow start, exponential growth)while ( ACK returned )

CongestionWindow++ (based on additive increase, linear growth)if timeout occurs,

CongestionThreshold = CongestionWindow / 2Continue

}


Slow Start Trace:

Where: Colored line = value of CongestionWindow Solid bullets at top = timeouts Hash marks = time when each packet is transmitted Vertical bars = time when a packet that was eventually retransmitted (i.e.,

was lost) was first transmitted

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

KB

70

304050

10


Slow Start

http://www.6test.edu.cn/~lujx/linux_networking/0131777203_ch24lev1sec4.html

http://www.6test.edu.cn/~lujx/linux_networking/0131777203_ch24lev1sec4.html


Slow Start 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


Fast Retransmit (TCP Tahoe) Problem: coarse-grained 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 3 ACKs.

Packet 1

Packet 2Packet 3Packet 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


Effect of Fast Retransmit

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0

KB

70

304050

10

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

KB

70

304050

10


Effect of Fast Retransmit

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 allow transmission of more packets

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 flat (no increase)

Coarse-grained timeouts


Fast Recovery (TCP Reno)

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


Congestion


Congestion Avoidance TCP’s strategy: congestion control

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

congestion occurs, and then back off Alternative strategy: congestion avoidance

predict when congestion is about to happen reduce rate before packets start being discarded

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.


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


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


RED Details Detect / respond to long-lived congestion (vs. short bursts)

Low-pass filter 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


RED Details (cont) Two queue length thresholds

if AvgLen <= MinThreshold then enqueue the packet

else if AvgLen >= MaxThreshold then drop arriving packet

else // MinThreshold < AvgLen < MaxThreshold calculate probability P drop arriving pack with probability P


RED Details (cont) Computing probability P

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

P = TempP / (1 - count * TempP)

Drop Probability CurveKeep track of how many newly arrivingpackets have been queued while AvgLenhas remained between the 2 thresholds

Typically 0.02

Typically:• MaxThreshold = MinThreshold * 2• MaxThreshold < MaxBuffer


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)

Exercise 2

TCP uses a host-centric, feedback-based, window-based resource allocation model. How might TCP have been designed to use instead the following models:(a) Host-centric, feedback-based and rate-

based.(b) Router-centric and feedback-based.


Exercise 6 Consider the arrangement of hosts H and routers R

and R1 in Figure 6.27. All links are full-duplex, and all routers are faster than their links. Show that R1 cannot become congested and for any other router R, we can find a traffic pattern that congests that router alone.


Exercise 25 You are an Internet Service Provider; your client hosts

connect directly to your routers. You know some hosts are using experimental TCPs and suspect some may be using a “greedy” TCP with no congestion control.

What measurements might you make at your router to establish that a client was not using a slow start at all?

If a client used slow start on startup but not after a timeout, could you detect that?



27. Consider the TCP trace in Figure 6.28. Identify time intervals representing slow start on startup, slow start after timeout, and linear-increase congestion avoidance. Explain what is going on from T=0.5 to T=1.9. The TCP version that generated this trace includes a feature absent from the TCP that generated Figure 6.11. What is this feature? This trace and the one in Figure 6.13 both lack a feature. What is it?

Figure 6.28

Figure 6.11

Figure 6.13

Exercise 34

Consider a RED gateway with MaxP = 0.01 and with an average queue length halfway between the two thresholdsFind the drop probability Pcount for count = 1 and

count = 100Calculate the probability that none of the first 50

packets is dropped. Note that this is (1 – P1) * … * (1 – P50)


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CSS432 Congestion Control Textbook Ch6.1 – 6.4

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