CS 557 RED Random Early Detection Gateways for Congestion Avoidance - - PowerPoint PPT Presentation

CS 557 RED

Random Early Detection Gateways for Congestion Avoidance

• S. Floyd and V. Jacobson, 1993

Spring 2013

Network layer: Addressing, Fragmentation, Dynamic Routing, Best Effort Forwarding Transport layer: End to End communication, Multiplexing, Reliability, Congestion control, Flow control,

The Story So Far….

Data Layer: richly connected network (many paths) with many types of unreliable links Some Essential Apps: DNS (naming) and NTP (time).

Congestion Control Options

• Control Congestion at Source

– Typically adjust congestion windows – But can’t assume everyone will do this

• Faulty implementation of the control algorithm
• Intentional ignorance of the control algorithm
• Dynamic Routing

– Route packets around congestion – Has problems with jitter, convergence, etc.

• Managing the Queues at the Routers

– Topic of this paper

RED Main Points

• Objective:

– Technique to notify senders of congestion and achieve high throughput and average queue sizes

• Approach:

– Gateways mark or drop packets – Monitor average queue size – Randomly mark/drop some packets above a given min threshold, – Mark/drop all packets above a max threshold

• Contributions:

– Well known technique and implemented in production routers

Congestion at Gateways

• End system can only infer congestion

– Lack a global understanding of traffic – Lack a sufficiently long time period

• Gateway best suited to signal congestion

– But should also tolerate bursty traffic

• How to do this?

The Basic Idea

• Gateway signals congestions by

– Marking some packets – Dropping some packets

• Maintain an average queue value

– Want to tolerate bursts – Don’t overreact to transient congestion

• Drop/Mark some packets when average queue

exceeds a min threshold (minthresh)

• Drop/Mark all packets when average queue

exceeds a max threshold

RED Algorithm

• For each packet arrival

calculate average queue size = avg if minth < avg < maxth calculate probability pa with probability pa, mark packet else if avg > maxth mark packet

Calculating the Average

• Weight average function

– avg = (1-wq)avg + wq*q

• How to Select wq?

– If wq is too large, will not filter out transient congestion – To allow burst of L packets, Want avg < minth L + 1 ((1-wq)^(L+1) - 1)/wq < minth ex: minth = 5, L = 50, => wq < 0.0042 – If wq is too small, avg doesn’t reflect actual queue size

Calculating the Drop Probability

• Marking probability

– pb = maxp ( avg - minth) / (maxth - minth)

• Method 1:

– Each packet marked with probability = pb – Result Prob[X=n] = pb * (1-pb)^(n-1)

• Method 2:

– Keep count of packets since last mark – Each packet marked with probability = pb/(1- count*pb)

Why is Randomness Important

• Need to avoid global synchronization

– Don’t want every connection to slow down at the same time.

• Fraction of marked packets should be roughly

proportional to connections share of the bandwidth

The Impact of RED on Queuing

• Note the average queue value (avg) in the above

– The queue size is bursty – The average tolerates transient bursts of packets – The average also tracks the general trend in queue size

Comparing Red and Drop-Tail

RED Summary

• Effective In Helping Manage Queue Sizes

– Does not assume a particular transport protocol, – But well suited to TCP style backoff

• Tolerates Transient Bursts

– Uses a weighted average of queue size rather than the current queue state

• Avoids Global Synchronization

– Probability of drops increase with congestion

• High Degree of Operational Flexibility

– Can select appropriate wq, maxp, minth, maxth, etc. – Some guidelines for selecting values

• Though optimal values not known