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Congestion control

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Abhay Pai
Abhay Pai

This document discusses various techniques for congestion control in computer networks. It describes: 1. The difference between congestion control, which deals with overall traffic levels across a network, and flow control, which regulates traffic between two endpoints. 2. Common congestion control techniques like leaky bucket and token bucket algorithms, which shape traffic to prevent bursts that could cause congestion. 3. Other approaches like choke packets, where routers notify sources to reduce their transmission rates if a link becomes congested, and load shedding as a last resort if congestion cannot be avoided.

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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
Congestion control
Congestion control

More Related Content

Congestion control

  • 1. Congestion Control By: Abhay Nitin Pai (13MCC1032)
  • 2. Index ●Introduction ●Congestion Control ≠ Flow Control ●Solution ●Leaky Bucket ●Token Bucket ●Choke Packets ●Last Resort ●The Trick
  • 3. Introduction What is Congestion Control ?
  • 4. What are the causes for Congestion Control ?
  • 5. 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
  • 6. Solution ●There are two basic approaches for congestion control –Open Loop ●Prevention is better than cure –Closed Loop ●Detect – FeedBack – Correct
  • 7. 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.
  • 9. 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
  • 11. ●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)
  • 12. 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.
  • 13. Last Resort ●Load Shedding ●Packets to drop –Wine Policy The older the better (FTP) –Milk Policy Fresh is better (Multimedia)