Congestion Avoidance: Understanding DECbit and Random Early Detection (RED) Schemes, Slides of Computer Science

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Congestion Avoidance

Announcements

 OHT 2 results out

 Programming Assignment 2 queries

 Results (excluding Written Assignment 2)

uploaded to group today

 How many submitted Prog Asgn 1?

 Viva for ENT portion of Prog Asgn on

Thursday (tomorrow) in ENT Lab

Congestion Avoidance Schemes

 Router-based Congestion Avoidance:

 DECbit:

 Routers explicitly notify sources about congestion.

 Random Early Detection (RED):

 Routers implicitly notify sources by dropping packets.  RED drops packets at random, and as a function of the level of congestion.

 Host-based Congestion Avoidance

 Source monitors changes in RTT to detect onset of

congestion.

 A variety of algorithms are described in Section 6.4.3.

DECbit

 Each packet has a “Congestion Notification” bit called the

DECbit in its header.

 If any router on the path is congested, it sets the DECbit.

 Set if average queue length >= 1 packet, averaged since the start of the previous busy cycle.

 To notify the source, the destination copies DECbit into

ACK packets.

 Source adjusts rate to avoid congestion.

 Counts fraction of DECbits set in each window.  If <50% set, increase rate additively.  If >=50% set, decrease rate multiplicatively.

Time

Queue Length at router

Averaging period

RED Drop Probabilities

D(t) A(t)

maxP

minTh maxTh

AvgLen

:

ˆ

ˆ Pr( ) 1 ˆ

If

Drop Packet

AvgLen

AvgLen AvgLen

minTh AvgLen maxTh AvgLen minTh p maxP maxTh minTh p count p

      (^)     

  

counts how long we've been in since we last dropped a packet. i.e. drops are spaced out in time, reducing likelihood of re-entering slow-start.

count minTh  AvgLen  maxTh

pAvgLen

Properties of RED

 Drops packets before queue is full, in the hope of

reducing the rates of some flows.

 Drops packet for each flow roughly in proportion to

its rate. Why?

 Drops are spaced out in time.

 Because it uses average queue length, RED is

tolerant of bursts.

 Random drops hopefully desynchronize TCP sources.

Synchronization of sources

Aggregate Flow

N * RTT

A

B

C

D

RTT

Avg

Desynchronized sources

Source A

A

B

C

D

RTT

N  RTT