Congestion control

Congestion Control

By:Abhay Nitin Pai (13MCC1032)

Index

●Introduction●Congestion Control ≠ Flow Control●Solution●Leaky Bucket●Token Bucket●Choke Packets●Last Resort●The Trick

Introduction

What is Congestion Control ?

What are the causes for Congestion Control ?

Congestion Control ≠ Flow Control

●Congestion Control :–The mechanism make sure subnet can carry offered traffic–A global issue involving all the hosts and routers–

●Flow Control–It is related to point to point traffic between given sender and receiver–It always involves direct feedback from receiver to sender

Solution

●There are two basic approaches for congestion control–Open Loop●Prevention is better than cure–Closed Loop●Detect – FeedBack – Correct

Leaky Bucket

●It consists of a finite queue–When a packet arrives, If there is a room on the queue it joins the queue. Otherwise, the packet gets discarded.–At every fixed clock tick, one packet is transmitted unless the queue is empty.●It eleminates bursts completely : Packets are transmitted to the subnet at the same rate●Packet loss may occur in case the bucket gets filled.

Leaky Bucket

Token Bucket

●Tokens are added at a constant rate to the bucket●For a packet to be transmitted, it must capture and destroy the token●Token bucket throws away the token when the bucket is full but never discards the packet●Whereas leaky bucket discards the packet when the bucket is full

Token Bucket

●Let–C --> Capacity (bits)–ρ --> Arrival Rate (bps)–M --> Maximum output rate (bps)–S --> Burst Length (s)●During burst length of S (s), tokens generated are ρS (bits) and output burst contains a maximum of C + ρS (bits)●Also output in maximum burst length S (s) would be MS (bits)●Therefore

● C + ρS = MS● S = C / (M - ρ)

Choke Packets

●The Big Idea–Router Checks Status of each output line–If any line is too occupied, send a choke packet to the source●At the source end–Cuts transmission rate by half and listens to no choke packets for a certain period of time–After that period is expired, the host listens for any further choke packets–If a choke packet arrives again, the source again cuts the transmission rate to half. If no packets arrive, the source may increase the transmission rate.

Last Resort

●Load Shedding●Packets to drop–Wine Policy

The older the better (FTP)–Milk Policy

Fresh is better (Multimedia)

The Trick