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The OSI Reference ModelThe OSI Reference Model

Network Layer

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The The SubnetSubnetA point-to-point network interconnecting fast specialized computers(routers or switches ) dedicated to packet routing using very high bandwidth links forming the backbone of the entire network.

Operation of the subnet is the concern of the network layer.

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The Network LayerThe Network Layer• Concerned with getting packets from the source all the way to the

destination: Routing through the subnet, load balancing, congestioncontrol.

• Protocol Data Unit (PDU) for network layer protocols = packet

• Types of network services to the transport layer:

– Connectionless: Each packet carries full destination address.

– Connection-oriented:• Connection is set up between network layer processes on the

sending and receiving sides.• The connection is given a special identifier until all data has been

sent.

• Internal organization of the network layer (in the subnet):– Datagram: Packets are sent and routed independently with each

carrying the full destination address (TCP/IP).– Virtual circuit: A virtual circuit is set up to the destination using

a circuit number stored in tables in routers along the way. Packetsonly carry the virtual circuit number. All packets follow the sameroute (cells in the case of ATM networks).

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Virtual CircuitVs.

Datagram Subnets

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Service AndService And Subnet SubnetStructure CombinationsStructure Combinations

Or TCP Over IP Over ATM

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Placement of Network LayerPlacement of Network LayerRouting ProcessRouting Process

Two adjacent subnet routers shown.

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Routing AlgorithmsRouting Algorithms To decide which output line an incoming packet should be

transmitted on to reach its destination.

• Static Routing (Non-adaptive algorithms):– Shortest path routing:

• Build a graph of the subnet with each node representing a routerand each arc representing a communication link.

• The weight on the arcs represents: a function of distance,bandwidth, communication costs, mean queue length, and otherperformance factors.

• Several algorithms exist including Dijkstra’s shortest pathalgorithm.

– Selective flooding: Send the packet on all output lines going in theright direction to the destination.

– Flow-based routing: Based on known capacity and link loads.

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Static Routing: Shortest Path RoutingStatic Routing: Shortest Path Routing• First five steps of an example using Dijktra’s algorithm

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Static Routing: Flow-Based RoutingStatic Routing: Flow-Based Routing• A routing matrix is constructed; used when the mean data

flow in network links is known and stable.

• Given: Capacity matrix Cij, Traffic matrix Fij,

• Mean delay at each line T= 1/(µC -λ)

µC line capacity packet/sec

λ traffic packet/sec

A subnet with linkcapacities given in kbps The traffic in packets/sec

and routing table

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Flow-Based Routing Calculation ExampleFlow-Based Routing Calculation Example

Flow-based routing analysis example for network on previous page, assuming:Mean packet size = 800 bits.Reverse traffic is the same as forward trafficweighti = flow in link i / total flow in the subnet

Mean delay per packet = ∑ weighti x Ti = 86 msec in above example

Goal find a flow with minimum mean delay per packet.

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Dynamic Routing: Distance VectorDynamic Routing: Distance Vector• Each router maintains a table with one

entry for each router in the subnetincluding the preferred outgoing link,and an estimate of time or distance tothe destination router.

• Neighbor router table entries aregathered by sending ECHO packetswhich are sent back with a time stamp.

• Each router exchanges its table ofestimates with its neighbors; the bestestimate is chosen.

Used in ARPANET until 1979

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Distance VectorDistance VectorRouting:Routing:

Count-to-InfinityCount-to-InfinityProblemProblem

Link AB Initially down

Link AB Initially Up

This is the main reason DistanceVector Routing has been mostlyabandoned.

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Dynamic Routing: Link State RoutingDynamic Routing: Link State Routing• Resolves Count-to-Infinity Problem present in Distance Vector Routing.• Variants of link state routing are widely used.• Each router using Link State Routing must:

– Discover its neighbors and know their network address.– Measure the delay or cost to each of the neighbors using ECHO packets.– Construct a link state packet to include what it learned about its

neighbors including the age of the information.

– Send the link state packet to all other routers in the subnet.– Compute the shortest path to every other router based on information

gathered from all link state packets (Dijksra’a algorithm may be usedhere).

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Hierarchical RoutingHierarchical Routing• The subnet is divided into several regions of routers.• Each router maintains a table of routing information to routers in its region only.• Packets destined to another region are routed to a designated router in that

region.

A two-level Hierarchical Routing Example:

Optimum number of hierarchy levels: For a subnet of N routers: Optimum # of levels = ln N Total # of table entries = e ln N