congestion control.pptx

7/27/2019 congestion control.pptx

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Outline

What is Congestion? Factors that cause Congestion.

Congestion Control Mechanisms andPolicies. Congestion Prevention Policies. Congestion Control In Virtual Circuits.

Congestion Control Taxonomy.


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An important issue in a packet-switched network iscongestion.

Congestion in a network may occur if the load on the

network

the number of packets sent to the network is greater than the capacity of the network thenumber of packets a network can handle.

Congestion control refers to the mechanisms and

techniques to control the congestion and keep theload below the capacity.

Main reason of congestion is more number of packetsinto the network than it can handle.


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Packet arrival rate exceeds the outgoing linkcapacity.

Insufficient memory to storearriving packets

Bursty traffic

Slow processor


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Congestion control refers to the mechanisms andtechniques used to control congestion and keep thetraffic below the capacity of the network.

Congestion control techniques can be broadlyclassified two broad categories:

Open loop: Protocols to prevent or avoidcongestion, ensuring that the system (or network

under consideration) never enters a Congested State.

Close loop: Protocols that allow system to entercongested state, detect it, and remove it


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Open Loop Good design.

Static in nature.

Decision are madewithout taking intoconsideration thecurrent state of the

network.

Close Loop

Based on concept offeedback.

Dynamic in nature.

Divided into 3 steps:1. Detect congestion.

2. Pass information to

the place whereaction could betaken.

3. Make operations to

correct problem.


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Open Loop

RetransmissionPolicy

Window Policy

AcknowledgementPolicy

Discarding Policy

Admission Policy

CongestionControl

Closed Loop

Back Pressure

Choke Packet

Implicit Signalling

Explicit Signalling


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Retransmission Policy

Retransmission is sometime unavoidable.

If the sender feels that a sent packet is lost or corrupted,the packet needs to be transmitted.

Retransmission may increase congestion in the network.

Window Policy

Policy that the window is using for transmission of data.

Go-Back-N.

Selective repeat window.


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Acknowledgement Policy The ACK policy imposed by the receiver may also affect

congestion.

If the receiver does not ACK every packet it receives, it

may slow down the sender and help prevent congestion. Several approaches are used in this case.

A receiver may send an ACK only if it has a packet to be

sent or a special timer expires. A receiver may decide to ACK only N packet at a time.

Sending fewer ACK means imposing less load onnetwork.


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Discarding Policy

A good discarding policy by the routers may prevent congestionand at the same time may not hard the integrity of the transmission.

Packet lifetime management deals with how long a packet may livebefore being discarded.

Admission Policy

An admission policy, which is a quality-of-service mechanism, canalso prevent congestion in virtual circuit networks.

Switches in a flow first check the resource requirement of a flowbefore admitting it to the network.

A router can deny establishing a virtual circuit connection if thereis congestion in the network or if there is a possibility of futurecongestion.


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Back Pressure It is a node-to-node congestion control mechanism

in which a congested node stops receiving datafrom the immediate upstream node.

It can only be applied to virtual circuit network, asupstream node are known here.

Propagates in opposite direction of data flow.


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Choke Packets

A more direct way of telling the source to slow down. A choke packet is a control packet generated at a congested

node and transmitted to restrict traffic flow.

The source, on receiving the choke packet must reduce itstransmission rate by a certain percentage.

Choke packet scheme is a close loop mechanism where eachlink is monitored to examine how much utilization is takingplace.

If the utilization goes beyond a certain threshold limit, the

link goes to a warning and a special packet, called chokepacket is sent to the source.

On receiving the choke packet, the source reduced the trafficin order to avoid congestion.


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Implicit Signaling

There is no communication between the congested node andthe source.

The source guesses that there is a congestion somewhere inthe network from other symptoms and slow down itself.

Ex:- No ACK received or delay in receiving ACK

Explicit Signaling

Occur in either forward or backward direction

Backward Signaling:- A bit can be set in a packet moving inthe direction opposite to the congestion. This bit can warn thesource that there is congestion and that it needs to slow down.

Forward Signaling:- A bit can be set in a packet moving in thedirection of the congestion. This bit can warn the destination

that there is a congestion. Receiver can slow down the ACK.


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The basic principle is obvious: When setting up avirtual circuit, make sure that congestion can beavoided.

Admission control: Once congestion has been

signaled, no more new virtual circuits can be set upuntil the problem has gone away.

Select alternative routes to avoid part of thenetwork that is overloaded, i.e. temporarily rebuild

your view of network. Negotiate quality of connection in advance, so

that network provider can reserve buffers and other

resources, guaranteed to be there


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Router-Centric The internal network routers take responsibility for:

Which packets to forward

Which packets to drop or mark

The nature of congestion notification to the hosts.

Host-Centric The end hosts adjust their behavior based on observations

of network conditions. (e.g., TCP Congestion Control Mechanisms)


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Reservation-Basedthe hosts attempt to reservenetwork capacity when the flow is established.

The routers allocate resources to satisfy reservations orthe flow is rejected.

The reservation can be receiver-based (e.g., RSVP) orsender-based.

Window-Based - The receiver sends an advertisedwindow to the sender or a window advertisement can be

used to reserve buffer space in routers.

Rate-Based The senders rate is controlled by thereceiver indicating the bits per second it can absorb.


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Feedback-Based - The transmission rate is adjusted (via windowsize) according to feedback received from the sub network.

Explicit feedback FECN, BECN, ECN

Implicit feedback router packet drops.


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congestion control.pptx

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