Lecture 9 - Congestion Control

Congestion Control is implemented at the ____ Layer

Transport

Congestion Control measures capacity and demand in…

bps

When demand exceeds supply…

Que fills up

Packets are dropped

Traffic Jam

Too many packets on the network, wait in que, eventually transmit.

Bucket with a Leak Example

Hole (link) can only handle so much (capacity), fill up bucket (queue), overflow bucket (drop).

End-To-End Principle

Have “intelligent” end hosts and “dumb” core network

The senders manage…

End to end connections and congestion

Network provides

Acknowledgement (ACK) packets, recall CSMA

Under TCP, each sender keeps a congestion window

Limit on the number of unacknowledged packets a device can have. When ACK received, “slide” and receive

Sliding Windows are similar to Netflix DVD policy because…

Both allow you fixed number of m ovies at one time, pay for larger “window”

Under no congestion, cwnd should…

Grow, as it efficiently utilizes the network and TCP increases it linearly

Increase cwnd

By 1, if all outstanding packets received, by 1/cwnd for every packet that comes back

Inferring Congestion 1

If a sender waits a long time with no ACK, packet was probably lostfR

Round Trip Time (RTT)

Time to receiver and back

Inferring Congestion Assumption 2

IIf sender received ACK for many future packets, packet was probably lost. Think of them going sequentially.

TCP Congestion Control: Loss-based

All or nothing approach.

Problem: often too late by the time losses are occurring

Delay-based: (how late/early was the packet)

Loss Based Congestion Inference

TCP Reno - Widely used in Windows OS.

Every RTT: Have all packets been received properly?

• Yes: Increases cwnd by 1

• No: Cut cwnd in half

Distributed Power Control

Objective: Match measured to desired SIRs, can reach equilibrium among transmitters.

Distributed Congestion Control

Objective: Match measured to desired transmission rates.