Part 4: Network Layer Part A: Algorithms

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Part 4 Network LayerPart A Algorithms
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Summary

• The Problem
• The Dijkstras Shortest Path Algorithm
• Distance Vector Routing
• Link State Routing
• Hierarchical Routing
• Routing in Ad Hoc Networks

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1. The Problem (1)

• Store-and-Forward Packet Switching
• Services Provided to the Transport Layer
• Connectionless Service
• Connection-Oriented Service

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1. The Problem (2) Packet Switching
fig 5-1
The environment of the network layer protocols.
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1. The Problem (3) Connectionless Service
Routing within a diagram subnet.
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1. The Problem (4) Connection-Oriented Service
Routing within a virtual-circuit subnet.
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1. The Problem (5) Connectionless VS.
Connection-Oriented
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2 Shortest Path Algorithm

• The first 5 steps used in computing the shortest
  path from A to D. The arrows indicate the
  working node.

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3 Distance Vector Routing (1)
(a) A subnet. (b) Input from A, I, H, K, and the
new routing table for J.
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3 Distance Vector Routing (2)
The count-to-infinity problem.
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3 Distance Vector Routing (3) Loop-Breaking
Heuristics

• Set infinity to a limited number, e.g. 16.
• Split horizon
• Split horizon with poison reverse

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3 Example
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Initialization
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Direct Neighbours
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Neighbours of neighbours
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Neighbours of neighbours of neighbours
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Stable convergence
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Good news A new link!
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Direct endpoints know
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Neighbours know
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Neighbours of neighbours know
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A happy and stable network
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Bad news Link crash!!
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Direct endpoints know
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Get help from neighbours
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Routing loop (due to inconsistent state info)
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Counting to infinity
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Stability again
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4 Link State Routing (1)

• Each router must do the following
• Discover its neighbors, learn their network
  address.
• Measure the delay or cost to each of its
  neighbors.
• Construct a packet telling all it has just
  learned.
• Send this packet to all other routers.
• Compute the shortest path to every other router.

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4 Link State Routing (2) Learning about the
Neighbors

1. Nine routers and a LAN.
2. A graph model of (a).

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4 Link State Routing (3) Measuring Line Cost

• A subnet in which the East and West parts are
  connected by two lines.

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4 Link State Routing (4) Building Link State
Packets

• (a) A subnet. (b) The link state packets for
  this subnet.

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4 Link State Routing (5) Distributing the Link
State Packets

• The packet buffer for router B in the previous
  slide (Fig. 5-13).

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5 Hierarchical Routing

• Hierarchical routing.

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6. Routing in Ad Hoc Networks (1)

• Possibilities when the routers are mobile
• Military vehicles on battlefield.
• No infrastructure.
• A fleet of ships at sea.
• All moving all the time
• Emergency works at earthquake .
• The infrastructure destroyed.
• A gathering of people with notebook computers.
• In an area lacking 802.11.

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6. Routing in Ad Hoc Networks (2) Route Discovery

• (a) Range of A's broadcast.
• (b) After B and D have received A's broadcast.
• (c) After C, F, and G have received A's
  broadcast.
• (d) After E, H, and I have received A's
  broadcast.
• Shaded nodes are new recipients. Arrows show
  possible reverse routes.

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6. Routing in Ad Hoc Networks (3) Route Discovery

• Format of a ROUTE REQUEST packet.

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6. Routing in Ad Hoc Networks (4) Route
Discovery

• Format of a ROUTE REPLY packet.

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6. Routing in Ad Hoc Networks (5) Route
Maintenance

• (a) D's routing table before G goes down.
• (b) The graph after G has gone down.