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Congestion Control

Computer Networks

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Where are we?

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Recall

Data Link Layer Link level specific transmission

Network Layer End-to-End host addressing and routing

Transport Layer End-to-End application multiplexing and

message flow-control

An expert: Sally Floyd <http://www.aciri.org/floyd/>An expert: Van Jacobson

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Note

Flow control is a subset of congestion control. The former attempts to properly match the rate of the

sender with that of the network and receiver. The later deals with the

sustained overload of intermediate network elements such as

internetwork routers.

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Congestion Collapse

As the network load increases, packet drops and thus packet retransmissions increase

Fragments dropped are especially annoying, the remaining fragments get sent, but cannot be used

As retransmissions increase, less actual work gets done

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Some Congestion Fixes

When congestion increases, slow down! Additive Increase, Multiplicative Decrease is

used in TCP Setup reservations or service classes

Packets failing to adhere to their class or reservation are simply discarded or put onto a low priority queue/link

Discover end-to-end MTU if fragments are getting dropped

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Fairness

Equal share bandwidth to end stations Fair share based on application Fair share based on timeliness of data Fair share based on value of data Fair share based on price paid ...and so on

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Active Congestion Control Mechanisms

Eligible discard Queue management Network Signaling and Notification End station avoidance Class of service signaling Quality of service reservations

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Eligible Discard

Frames, cells or packets are marked according to a drop priority

Source or edge intermediate device may mark based on some policy watermark/threshold reached data type source destination cost

Usually implemented at data link or network layer

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Eligible Discard Illustrated

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Queue Management

First in, first dropped (FIFO) Tail drop (LIFO)

Leaky bucket Token bucket

Random early detection (RED) Weighted Fair Queueing

Usually implemented in intermediate devices such as routers and switches

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First In, First Out Illustrated

Queue pointers need to be updated Sender learns of drop sooner

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Last In, First Out Illustrated

Simple - no queue pointers to update Source cannot react as quick

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Leaky Bucket Illustrated

From Tanenbaum Figure 5-24, graphic will print to a Postscript printer

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Token Bucket Illustrated

From Tanenbaum Figure 5-26, graphic will print to a Postscript printer

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RED Illustrated

Probability marking applied to each packet based on queue length, packet being dropped

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Weighted Fair Queueing

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Network Signaling and Notification

Also called choke packets In Frame Relay

Forward Explicit Congestion Notification (FECN) Backward Explicit Congestion Notification (BECN) Bit in frame set

Experimental Internet mechanism Explicit Congestion Notification (ECN) Bits set in packets to hosts

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End Station Avoidance

Also called end-to-end control TCP

Slow start Congestion avoidance Fast Retransmit Fast Recovery

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Class of Service Signaling

Packets marked to a particular traffic class

IEEE 802.1p Differentiated Services (DiffServ) Re-defines IP Type of Service (ToS)

bit fields Asynchronous Transfer Mode

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Quality of Service Reservations

Resource ReSerVation Protocol Reserve resources in routers Requires stateful path

Asynchronous Transfer Mode (ATM)