Congestion Avoidance and Control

Van Jacobson

Jonghyun Kim

E-mail :netgod77@hanmail.net

April 1, 2004

Introduction

• To achieve network stability, TCP entity should obey a ‘packet conservation’ principle, which is borrowed from physics.

• What is packet conservation principle? A new packet is not put into the network

until an old packet leaves( i.e. ‘conservation’) while maintaining ‘equilibrium’ in which a connection runs stably by effectively using the bandwidth available on the path.

• Three failures of packet conservation 1. The connection does not get to equilibrium 2. A sender injects a new packet before an old packet has exited 3. The equilibrium can’t be reached because of resource limits along the path

1, 3 : Equilibrium problem 2 : Conservation problem

• To fix those failures, three algorithm are introduced

* Slow-start algorithm for 1. * Retransmit timeout estimation algorithm for 2. * Congestion avoidance algorithm for 3.

Slow-start algorithm

• To understand slow-start algorithm, we should understand ‘self-clocking’ system

• Things for sender to do

1. Sender finds the bandwidth of the slowest link in the path and sends to the receiver the bursts of packets based on that bandwidth (This is the core of slow-start algorithm)

2. After that, if a new packet is sent whenever receiving an ACK (i.e. ‘self-clocking’), this connection will get to equilibrium without wasting the bandwidth.

• How to find the bandwidth of the slowest link in the path

If timeout occurs, we roughly know the bandwidth bandwidth ≈ The last bursts of sent packets/2

• Slow-start algorithm (Pseudo-code based on C)

cwnd : congestion window rwnd : receiver’s advertised window

cwnd = 1; while(1) { send packets of min (cwnd, rwnd); // bursts of packets wait until receiving all ACKs for the previous sent packets if ( timeout occurs ) cwnd = 1; else cwnd = 2*cwnd; }

• Startup behavior of TCP with Slow-start

• Startup behavior of TCP without Slow-start

Retransmit timeout estimation algorithm

• To estimate retransmit timeout correctly, we should know the mean and variance of round trip time

• The algorithm of RFC 793 SRTT = α*SRTT + (1- α)*RTT RTO = β*SRTT -SRTT: Smoothed RTT (mean) -RTT : Recently measured RTT -α : Smoothing factor (=0.9) -β : Delay variance (=2) -RTO : Retransmit timeout interval *The problem of this algorithm Since β is fixed value, RTO is not proportional to the variance

• The algorithm of Jacobson The idea is to make β above roughly proportional to the standard deviatio

n of RTT by using mean deviation Standard deviation (sdev) = Mean deviation (mdev) = mdev ≈ 1.25*sdev SRTT = α*SRTT + (1- α)*RTT SMD = β*SMD +(1- β)*(|SRTT-RTT|) RTO = SRTT + 4*SMD (α = 0.125, β = 0.25) -SMD : smoothed mean deviation - α ,β : smoothing factor

])[( 2SRTTRTTE

|][| SRTTRTTE

• Performance of the algorithm of RFC793

• Performance of the algorithm of Jacobson

D1>D2. It means that Jacobson’s algorithm estimates RTO more flexiblybecause RTO varies proportionally to the mean deviation

Congestion avoidance algorithm

• We always want to use the full bandwidth available on the subnet, keeping stable network. In order to do so, the congestion window size is increased slowly (i.e. additive increase) to probe for more bandwidth becoming available (e.g. when a host closes its connection). However, on any timeout, the congestion window size is decreased fast (i.e. multiplicative decrease) to avoid congestion

On no congestion (to probe for more bandwidth) : cwnd = cwnd + 1 ; On congestion (to avoid congestion) : cwnd = cwnd/2 ;

• Illustration of congestion avoidance algorithmCo

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Transmission number2 3 4 51 7 8 9106 111213 15161718192014 212223 252627282924 303132

Timeout

To probemore bandwidth

To avoid congestion

To find current bandwidth

To probemore bandwidth

To avoid congestion

• Congestion avoidance algorithm (pseudo-code based on C)

cwnd = 1;while(1) { send packets of min (cwnd, rwnd); // burst wait until receiving all ACKs for the previous sent packets slow-start if ( timeout occurs ) break; else cwnd = 2*cwnd;}threshold = cwnd/2; cwnd = 1;while(1){ if ( cwnd < threshold ){ send packets of min (cwnd, rwnd); //burst wait until receiving all ACKs for the previous sent packets slow-start if ( timeout occurs ){ threshold = cwnd/2; cwnd = 1;} else cwnd = 2*cwnd;} if ( cwnd == threshold || cwnd > threshold ) send packets of min (threshold, rwnd); //burst } else if ( cwnd > threshold ){ send a new packet whenever an ACK is received //self-clocking if ( Sender receives all ACKs for the previous sent packets ) {cwnd = cwnd + 1; send a new packet;} //to probe more bandwidth if ( timeout occurs) {threshold = cwnd/2; cwnd = 1;} //to avoid congestion }}

• Multiple conversation test setup

* Experimental Parameters Local bandwidth : 1MB/s P2P link bandwidth : 25KB/s Receiver’s window size : 16KB Packet size : 512 Byte 50-packet size : 25KB

When receiving more than 50 packets/sec,the routers will drop packets

Since four connections can send 64KB (4*16KB) at a time, they will cause congestion easily.

• Multiple, simultaneous TCPs with no congestion avoidance

* Result -4,000 (36%) of 11,000 packets were retransmitted

• Multiple, simultaneous TCPs with congestion avoidance

* Result -89 (1%) of 8281packets were retransmitted

Big retransmission means big congestion occurrence. On the other hand, small retransmission means small congestion occurrence

• Total bandwidth used by old and new TCPs

Thin line : old TCPThick line : new TCP

Bandwidth negotiation among four connection

Reaching equilibrium

Bandwidth renegotiation due to connection one shutting down

• Effective bandwidth of old and new TCPs

Thin line : old TCPThick line : new TCP

Old TCP uses 75% of the link bandwidthNew TCP uses almost 100% of the link bandwidth

Summary

We can think of network system as a linear system. To make network system stable, TCP should conform a ‘packet conservation’ principle.

There exist three failures of packet conservation in the network system. To fix those failures, three algorithms are introduced. (Slow-start, retransmit timeout estimation, congestion avoidance)

This paper introduced only host-centric congestion control, but we also need to learn router-centric congestion control.

Reference

• JACOBSON, V. Congestion avoidance and control. Proceedings of the ACM SIGCOMM conference on applications, technologies, architectures, and protocols for computer communication (Stanford, California, USA), 18, 4, August 1988, 314-329.

• ANDREW S. TANENBAUM, Computer Networks, 2003• RFC 793