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Network Layer COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021

Network Layer
Connecting different networks (internetworking)
Framing, error and flow control, MAC
Different transmission media, signals, modulation …
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Outline
 Network layer in the Internet
 Types of services
 Internetworking
 Tunneling
 Fragmentation
 Path MTU discovery
 Internet Protocol  Addressing
 Subnetting
 Routing algorithms
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Network Layer in the Internet (1)
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4G SONET ADSL
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Network Layer in the Internet (2)
 Internet is a collection of many networks that is interconnected by IP
 Provides a best-effort service to route datagrams from source host to destination host
 These hosts may be  On the same network  On different networks
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Network Layer in the Internet (3)
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Network Layer in the Internet (4)
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Store-and-Forward Packet Switching
 Hosts generate packets and inject into the network  Router routes packets through the network
 Routers treat packets as messages, receive/store them and then forward them based on how the message is addressed
ISP’s equipment
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Services Provided to the Transport Layer
 Design goals:
 Services should be independent of router
technologies
 Transport layer should be shielded from the number, type and topology of routers
 Network addressing should use a uniform numbering plan (network identifier)
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Types of Services
 Connectionless: Packets are injected into
subnet individually and routed independently
to the destination
 Flow and error control done by other layers
 Internet: move packets in a potentially unreliable subnet; QoS is not easily implemented
 Connection-oriented: Packets travelling to the destination following the same route
 Telecommunication: guarantee reliability; QoS is important
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Routing within a
 Connectionless – post office model: packets are routed individually based on destination addresses in them
 Packetscantakedifferentpaths
 e.g.,P1sendsalongmessagetoP2
Routing table (can be fixed or change over time)
Routing algorithm – manages the routing table
A’s table (initially)
A’s table (later)
C’s Table
E’s Table
ISP’s equipment
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Routing within a Virtual-Circuit Subnet
 Connection-oriented – telephone network model: packets are routed through virtual circuits based on connection id in them.
 Packetstakethesamepathtoavoidhavingtochooseanewpathfor every packet
 e.g.,MultiProtocolLabelSwitchingNetwork
ISP’s equipment
connection identifier
A’s table C’s Table E’s Table
in out
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Datagram vs. Virtual-Circuit Subnets
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Compromises in VC and s (1)
 Setup time vs. address parsing time
 VC: requires setup time and resources, but packet transmission
is very fast after that
 Datagram: more complicated lookup procedure
 Memory of router
 VC: requires entry per virtual circuit
 Datagram: requires large tables of every possible destination route
 Bandwidth
 VC: saves potential overhead in full addressing of each packet
and computation of path. Still needs them during setup
 Datagram: full destination address in every packet
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Compromises in VC and s (2)
 QoS and congestion avoidance
 VC: easier to provide QoS, able to reserve CPU, bandwidth and
buffer in advance  Longevity
 VC: can be setup for repeating and long-running uses e.g. Permanent VC’s
 Vulnerability
 VC: particularly vulnerable to hardware/software crashes, all VC’s
aborted and no traffic until they are rebuilt  Datagram: can use an alternative route
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Different Networks
 Service offered: connectionless vs. connection-oriented  Packet size: different max
 Addressing: different sizes, flat or hierarchical
 Quality of service: present or absent
 Reliability: different levels of loss  Security: privacy rules, encryption  Parameters: different timeouts
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Outline
 Network layer in the Internet
 Types of services
 Internetworking
 Tunneling
 Fragmentation
 Path MTU discovery
 Internet Protocol  Addressing
 Subnetting
 Routing algorithms
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Internetworking
 Internetworking joins multiple, different networks into a single larger network
 Issues when connecting networks:
 Different network types and protocols
 Different motivations for network choices
 Different technologies at both hardware and software levels
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How Different Networks are Connected
 Internetworking based on a common network layer – IP
Common protocol (IP) carried all the way
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Tunneling
 Tunneling is a special case used when the source and destination are on the same network, but there is a different network in between.
 Source packets are encapsulated in packets, travelling through connecting network
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Tunneling IPv6 Packets through IPv4
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