Chi-Cheng Lin, Winona State University

CS 313 Introduction to Computer Networking &

Telecommunication

Chapter 5 Network Layer

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Topics

Design Issues Routing Algorithms Congestion Control Internetworking

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Congestion Control

CongestionToo many packets present in the

subnet Effects

Performance degradedPacket lost

Congestion Control Algorithms (2)

When too much traffic is offered, congestion sets in and performance degrades sharply.

(Could be achieved bycongestion control)

(Without congestion control)

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Causes of Congestion Causes

Too many packets need an output line queuingProblem: not enough memory packets droppedSolution(?): adding more memoryNew problem: timeout and retransmit worse

Slow processorsLow bandwidth lines

Congestion tends to feed upon itself and become worse

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Congestion Control

Congestion = (Load > Resources) Solutions

Increase resourcesDecrease load

Approaches to Congestion Control

Timescales of approaches to congestion control

Traffic-Aware Routing

A network in which the East and West parts are connected by two links.

Admission Control

(a) A congested network. (b) The portion of the network that is not congested. A virtual circuit from A to B is also shown.

Traffic Throttling

Explicit congestion notification

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Choke Packets

ApproachEach router monitors output line utilizationThreshold for "warning state"A receiving router

Checks packet to see if output line in warning state

If yes thensend a "choke packet" back to

source hostoriginal packet tagged and

forwarded

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Choke Packet

Source, upon receiving a choke packetReduces traffic by a percentage after

receiving choke packetChoke packet referred to same destination

is ignored for a fixed time intervalAfter time interval expired, listens

If choke packet received then goto the step of reducing traffic

else increase traffic

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Choke Packet

TypicallyFirst choke packet causes data rate

reduced to 50%, then 25%, … Traffic is increased in smaller

increments Why?

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Hop-by-Hop Choke Packets

Problem in high speed and long distance slow reaction

SolutionHop-by-hop choke packets

Buffers needed in routers Effects:

Quick relief at the price of more buffers

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Load Shedding

Discard whatever cannot be handled

Which packets to drop?Application-dependentPriorities

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Load Shedding

StrategiesWine or milkPriority

Priority classesCoupled with traffic shaping token bucket

Packet without token sent with lowest priority

Allowing VC set up with exceeding specification Contingent on low priority

Header field needed Example: ATM CLP field (1-bit, 0 means high priority)

Rule of thumbDiscard as early as possible!

Choke Packets

A choke packet that affects only the source.

Choke Packets

A choke packet that affects each hop it passes through.

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Internetworking

A collection of interconnected networks.

How Networks Differ

Some of the many ways networks can differ

How Networks Can Be Connected

(a)A packet crossing different networks. (b)Network and link layer protocol processing.

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Tunneling

Encapsulating packets of a protocol in the payload of packets of another protocol

Useful inInternetworkingVPNIPv4 to IPv6 transition…

Tunneling

Tunneling a packet from Paris to London.

Tunneling

Tunneling a car from France to England