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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