RANDOM EARLY DETECTION GATEWAYS FOR CONGESTION AVOIDANCE

Jinyoung YouCS540, Network Architect

Contents

1. Introduction of the Problem2. Previous works3. Design goals of the RED gateway4. The RED algorithm5. Simulation Results6. Calculation7. Parameter sensitivity8. Conclusions

Problem

In high-speed networks Previously, the TCP detects congestion only after a

packet has been dropped at the gateway It occurs to have large queues Large maximum queues accommodates transient con-

gestion Therefore, it’s necessary to keep throughput high but

average queue sizes low

Previous work

Without explicit feedback from Gateway Estimating bottleneck service time from changes in

throughput, end-to-end delay. Informed by packet drops Limitation

By the timescales of the connection By the traffic pattern of the connection By the lack of knowledge of the number of congested gateways By the possibilities of routing changes By distinguishing propagation delay from persistent queueing

delay

Previous work

Detection by gateway itself Can distinguish between propagation delay and persis-

tent queueing delay Has a unified view of the queueing behavior over time Gateway scheduling mechanisms with per-connection

gateway mechanisms Fair Queueing Hop-by-hop flow control schemes Limited by circumstances where it can be used

Previous work

Drop tail If the queue is full, then the gateway drops packets ar-

riving later than others. When the queue overflows, packets are often dropped

from several connections, and these connections de-crease their windows at the same time.

That state refers to “Global Synchronization” Global Synchronization occurs decrement of through-

put Have a bias against bursty traffic

Previous work

Random Drop With Random Drop gateways, when a packet arrives at

the gateway and the queue is full, the gateway ran-domly chooses a packet from the gateway queue to drop

Able to reduce Global Synchronization rather than Drop tail

However, both Drop tail and Random Drop could not keep average queue sizes low, effectively

Have a bias against bursty traffic

Previous work

Early Random Drop gateways If the queue length exceeds a certain drop level, Then the gateway drops each packet arriving at the

gateway with a fixed or dynamic drop probability Problem

Not successful in controlling misbehaving users

Previous work

The DECbit congestion avoidance scheme Uses a congestion-indication bit in packet headers to pro-

vide feedback about congestion in the network When a packet arrives at the gateway, the gateway calcu-

lates the average queue length When the average queue length exceeds one, then the

gateway sets the congestion-indication bit in the packet header of arriving packets

If at least half of the packets in the last window had the congestion indication bit set, then the window is decreased exponentially. Otherwise, the window is increased linearly.

Previous work

Differences between the DECbit and the RED gate-ways Computing the average queue size. The method for choosing connections to notify of conges-

tion; notified connection decreases the windows size Adaptive window schemes

The source nodes increase or decrease their windows ac-cording to feedback concerning the queue lengths at the gateways

Design goals of the RED gateway

Main goals Provide congestion avoidance by controlling the average

queue size Additional goals

The avoidance of global synchronization The avoidance of a bias against bursty traffic Maintain an upper bound on the average queue size with-

out cooperation from transport-layer protocols

Design goals of the RED gateway

The gateway should detect incipient congestion and to notify of this congestion Because the gateway can monitor the size of the queue

and has a unified view of the various sources contributing to this congestion, it appropriates to support that

The gateway should decide which connections to no-tify of congestion at the gateway. We will refer these works, dropping and notifying as Mark-

ing and Notification

Design goals of the RED gateway

The gateway could avoid a bias against bursty traffic The gateways such as Drop Tail and Random Drop gateways

have a bias against bursty traffic It could make the gateway queue will overflow

The gateway should avoid the global synchronization, when deciding which connections to notify of conges-tion It results in loss of throughput in the network

Design goals of the RED gateway

To solve the bursty traffic and the global synchronization Gateways can use distinct algorithms for congestion detection

and for deciding which connections to notify of this conges-tion

The RED gateway uses randomization in choosing which arriving packets to mark

By using the probability of marking a packet from a particular connection is roughly proportional to that connection’s share of the bandwidth through the gateway

Design goals of the RED gateway

The last goal is that the gateway have the ability to control the average queue size even in the absence of cooperating sources This can be done if the gateway drops arriving packets

when the average queue size exceeds some maximum threshold

The RED algorithm

Simulation Results

Simulation Results

The x-axis shows the time in secondsThe y-axis shows the size of queueTwo straight lines are min and max threshholds

Simulation Results

The x-axis shows the time in secondsThe y-axis shows the packets of each nodeEach ‘X’ shows a packet dropped

Simulation Results

Simulation Results

Calculation

Calculating the average queue length

If wq is too large, then the averaging procedure will not fil-ter out transient congestion at the gateway.

If wq is set too low, then avg responds too slowly to changes in the actual queue size. In this case, the gateway is unable to detect the initial stages of congestion.

Calculating the packet-marking probability

Parameter sensitivity

Ensure adequate calculation of the average queue size

Set minth sufficiently high to maximize network power

Make maxth−minth sufficiently large to avoid global synchronization

Conclusions

Congestion avoidance Guarantees that the calculated average queue size

Appropriate time scales By notifying a connection of congestion

No global synchronization By marking packets at as low a rate as possible

Simplicity Using simple algorithm

Maximizing global power Simulations shows high link utilization

Conclusions

Fairness The fraction of marked packets for each connection is

roughly proportional to that connection’s share of the bandwidth. Because of randomly choosing packets to be marked during

congestion, it easily identifies misbehaving users, which have large share of the bandwidth

In addition, it doesn’t have a bias against bursty traffic, which is traffic from a connection where the amount of data transmitted in one roundtrip time.

Appropriate for a wide range of environments

Questions and Discussion