Congestion Control Tanenbaum 5.3, 6.5. 6/12/2015Congestion Control (A Loss Based Technique: TCP)2...

Congestion Control

Tanenbaum 5.3, 6.5

04/18/23 Congestion Control (A Loss Based Technique: TCP) 2

What? Why?

• Congestion occurs when – there is no reservation– Demand exceeds Resources

• If congestion persists, resources must be increased

• If congestion happens in short bursts, buffers can avoid packet losses to some extent.


Congestion Control, How?

• Network Layer– Congestion control point to point

• Transport Layer– Congestion control end-to-end

PacketsDelivered

Packets Sent

Maximum capacity of bottleneck link

ObjectiveCongestion

Congestion Collapse


Network As Black Box

Network/Network/Network ElementNetwork Element

Load

State variable u

What can U be?

Round trip time (e2e)

Packet loss (e2e or p2p)

Link utilization (p2p)

Queue size (p2p)

Throughput (e2e)


Congestion Control in Packet Switched Networks

• Monitor some system state variable u

• u may be link utilization (p2p), loss rate (p2p or e2e)

• Instantaneous unow

• Average value u computed as:– unew = .ulast + (1- ) unow

– 0 <= <= 1

• Use of thresholds


Congestion Control:Strategies

• Open loop– Prevent congestion by design– E.g., resource reservation, admission control

• Closed loop– Monitor some characteristics of system– Inform key elements that can take action– Adjust system operation whenever needed

End-to-end Congestion Control


Network As Black Box

NetworkNetworkWindow Size

OrRate

Packet Loss,Round trip time,

or Throughput

Packet Loss BasedCongestion Control

TCP Congestion Control(Read Van Jacobson, “Congestion Avoidance and Control,” Sigcomm’88)


System (Connection) In Equilibrium

Time

Bandwidth

High Speed LANs Slow long-haul link


Keeping A System in Equilibrium

• Suppose that a system is in equilibrium, how to keep it there?– If there are no losses, easy: just send a

packet whenever an ack is received.– If losses occur, we need to distinguish

between delayed packets (link capacity shortage) and lost packets need an accurate retransmission timer.


How to Get A System To Equilibrium?

• At the beginning, no packet is sent yet.

• Propose a strategy. Let us discuss some strategies:

Strategy 1: Blast a guessed number of packets.

Strategy 2: Start with sending one packet (WS=1) and wait1. when ack is received, send two packets (WS=2)

2. For each ack, send one packet, plus one packet when all window is acked (WS=3)


How to Get A System To Equilibrium?

• Strategy 2 may be inefficient

• Consider a download of a 21 KB file using a 1 KB segment size over a 45 Mbps link with 25ms one way latency.

• Compute the efficiency of such a transfer


How to Get A System To Equilibrium? (Strategy 2)

Time WS Segments Delivered Throughput Efficiency

RTT 1 1 160 Kbps 0.3%

2 RTT 2 3 320 Kbps 0.7%

3 RTT 3 6 480 Kbps 1.0%

4 RTT 4 10 640 Kbps 1.4%

5 RTT 5 15 800 Kbps 1.7%

6 RTT 6 21 960 Kbps 2.1%


A Faster Strategy: Slow Start

• For each acked packet, increase window by 1


Slow-Start to Get A System To Equilibrium? (Strategy 2)

Time WS Segments Delivered Throughput Efficiency

RTT 1 1 160 Kbps 0.3%

2 RTT 2 3 320 Kbps 0.5%

3 RTT 4 7 640 Kbps 1.4%

4 RTT 8 15 1.28 Mbps 2.8%

5 RTT 16 21 ----- ----

6 RTT


A System in Equilibrium MayUNDERUTILIZE the Link Capacity

• If losses do not happen, this is an indication that link capacity may not be fully utilized

• What to do?– Slightly INCREASE the window size after a

complete window of data made it safely through the network (this happens every round trip time.) Congestion avoidance phase


TCP Congestion Avoidance and Control

• Start with congestion window=1 (CWND)

• Use slow start until CWND reaches the threshold sstresh (sstresh is half the congestion window vale reached when last loss occured)

• From CWND=sstresh, use congestion avoidance


Example

Round trip time

CWND


Code

When a packet is acked if cwnd < ssthresh* {

/* Slow-Start: increment cwnd every ack*/cwnd = cwnd + 1;

} else {/* Congestion avoidance: increment cwnd every RTT */cwnd = cwnd + 1/cwnd

}

When a packet is lost (timeout) ssthresh = cwnd / 2; cwnd = init_cwnd;

* ssthresh may be dynamically adjusted at start-up (see Janey Hoe work)


Fast Recovery/ Fast Retransmit

• When 3rd dupack received:– Fast Recovery:

• ssthresh = cwnd / 2;• cwnd = ssthresh + 3;

– Fast Retransmit:• Retransmit missing packet without waiting for

timeout


Example

Round trip time

CWND

24

8

16


TCP Performance: A Simple Derivation

• Mathis et. Al: "The Macroscopic Behavior of the TCP

Congestion Avoidance Algorithm”

• Assume – a very simple loss model: periodical loss with

loss rate p– No timeout– Neglect the start-up behavior


TCP Performance: A Simple Derivation (2)cwnd

W

Startup Before

Stabiliza.

0loss

(RTT)W/2loss

Wloss

3W/2loss

W/2


TCP Performance (3)

• Facts:– A loss happens whenever packets are sent

(periodical loss model).– A loss happens every RTTs– The total number of packets sent every cycle (time

between consecutive 2 losses) is the area under the curve.

– The area is

p1

2W

2832

2212

2 WWW


TCP Performance (4)

• Facts (cont’d):– The number of packets delivered per cycle is – Solving,

– Throughput = # packets received per cycle/cycle

duration

– Throughput = (packets per second)

pW 38

p1

pRTT 231


Taking Into Account Timeout• Jitendra Padhye et. Al “Modeling TCP Reno Performance: A Simple Model

and Its Empirical Validation” .

• Wmax : Max window size (flow control)• RTT: round trip time• b = # of packets received by an ack.• p: probability of packet loss• To= Duration of first timeout


Shortcomings/Limits of TCP

• TCP is not suited for streams => how to exercise TCP like congestion control on streams (UDP)?

• TCP, as is, will not scale well to very high speeds => How to adapt TCP to very high speed networks?


Impact of Policies on Congestion Control

• Link Layer policies– Retransmission– Out-of-order caching– Acknowledgement– Flow control


Impact of Policies on Congestion Control (2)

• Network Layer policies– Virtual circuit/datagram– Packet scheduling– Queue management– Routing algorithm– Packet lifetime management


Impact of Policies on Congestion Control (3)

• Transport Layer policies– Retransmission– Out-of-order caching– Acknowledgement– Flow control– Timeout determination


Congestion Control in Packet Switched Networks

• Monitor some system state variable u

• u may be link utilization, loss rate

• Instantaneous unow

• Average value u computed as:– unew = .ulast + (1- ) unow

– 0 <= <= 1

• Use of thresholds


Techniques at Link Layer

• DECBit (Warning bit)

• Source Quench packets (choke packets)

• Hop-by-hop choke packets

• Load shedding

• RED

Congestion Control Tanenbaum 5.3, 6.5. 6/12/2015Congestion Control (A Loss Based Technique: TCP)2...

Documents

Transcript of Congestion Control Tanenbaum 5.3, 6.5. 6/12/2015Congestion Control (A Loss Based Technique: TCP)2...