The document provides an overview of network layer protocols, focusing on algorithms such as Dijkstra's shortest path, distance vector routing, link state routing, and hierarchical routing. It discusses routing challenges including connectionless vs. connection-oriented services, and scenarios in ad hoc networks where routers are mobile with no fixed infrastructure. Additionally, it covers the mechanics of distance vector routing and link state approaches, their advantages, and issues like counting to infinity and loop-breaking heuristics.

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Part 4: Network Layer
Part A: Algorithms

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Summary
1) The Problem
2) The Dijkstra’s Shortest Path Algorithm
3) Distance Vector Routing
4) Link State Routing
5) Hierarchical Routing
6) 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
The environment of the network layer protocols.
fig 5-1

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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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Initialization
3 Example:

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Direct
Neighbours

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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
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Neighbours
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know

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A happy and
stable network

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Bad news:
Link crash!!

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Direct
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Get help
from
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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
(a) Nine routers and a LAN.
(b) 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.