Download Network Layer Pt 1

OSI Network Layer
Network Fundamentals – Chapter 5
Version 4.0
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Objectives

Identify the role of the Network Layer, as it describes
communication from one end device to another end device.

Examine the most common Network Layer protocol, Internet
Protocol (IP), and its features for providing connectionless and
best-effort service.

Understand the principles used to guide the division or grouping
of devices into networks.

Understand the hierarchical addressing of devices and how this
allows communication between networks.

Understand the fundamentals of routes, next hop addresses and
packet forwarding to a destination network.
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Network Layer Protocols and Internet Protocol (IP)
 Layer 3 of OSI
 Receives segments or PDUs from TL
 4 tasks:
• Addressing packets with an IP address
• Encapsulation
• Routing
• Decapsulation
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Network Layer Protocols and Internet Protocol (IP)
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Network Layer Tasks
• Addressing packets with an IP address
• Each sending and receiving device must have unique IP
address
• Device with IP address = hosts
• Sending host = source IP address
• Receiving host = destination IP address
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Network Layer Tasks
• Encapsulation
• IP header – source and destination IP addresses
• Process of adding information = encapsulation
• Encapsulated PDU = packet
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Network Layer Tasks
• Routing
• Routers =device that connect networks
• Routers – understand packets and calculating best path for
packets
• Routing = process perform by routers : receive packets,
analyzing dest add info, select a path and forwarding packets to
the next router
• Each route to next device = hop
• Decapsulation
• Process of removing encapsulation data
• Actually encap and decap happened at all layers of OSI model
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Network Layer Protocols
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IPv4
 Most widely used
 Basic characteristics:
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IPv4
 Connectionless
•No established connection
•IP simply sends packets without informing receiver
•Requires less data to perform required tasks – uses much less
processing power and bandwidth = overhead
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IPv4
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IPv4
 Best Effort
•TCP is reliable
•IP is unreliable
•IP makes a ‘best effort’ to deliver packets
•TCP can be relied on delivery problems
•TCP/IP – TL & NL
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IPv4
 Best Effort
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IPv4
 Media independent
• IP is not concerned with physical medium that carries packet
• Internetwork communication – multimedia journey
• ex. wireless, ethernet cable, fiber optic
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IPv4
 Media independent
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IPv4
 IPv4 encapsulates or packages the TL segment or
datagram as packets
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IPv4 Packet Header
 IP Source Address
32 bits
 IP Destination Address
32 bits
 Time to Live (TTL)
8 bits
Max hops the packet can take before considered
lost/undeliverable
Each router decrements TTL field by at leased 1
If TTL reaches 0 – packet will be dropped
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IPv4 Packet Header
 Type of Service (ToS)
8 bits
Describes level of throughput
Ex – voice data precede streaming music
Quality of Service - QOS
 Protocol
8 bits
Indicate upper layer protocol
TCP, UDP or ICMP
 Flag and Fragment Offset
Packet fragmented – small MTU
Used to reconstruct the packets
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IPv4 Packet Header
 Version
IPv4 or IPv6
 Internet Header Length (IHL)
How long the header - Options may caused different lengths
 Packet Length
Total length of datagram including the header
Min 20 bytes, max 65,535 bytes
 Identification
Help reassemble any fragments
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IPv4 Packet Header
 Header Checksum
Indicate length of header
Checked by each router
If invalid, packet assumed to be corrupted and is dropped
Relation to TTL?
 Options
Special routing services
 Padding
Fill bits when header data does not end on 32 bits boundary
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TBC - Wednesday
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Grouping Devices into Networks and Hierarchical
Addressing
 Networks are communities of computers and other
hosts
 Like human communities
 Small town
Easy to find and communicate, not need large roads &
expensive traffic signals, not many services, trust each other
and considered safer
 Large town
Ex..address
 Same to computer communities
 More planning to address the network so it can be
managed efficiently
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Grouping Devices into Networks and Hierarchical
Addressing
 Grouping devices into sub-networks
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Grouping Devices into Networks and Hierarchical
Addressing
 Large
 Computer networks can be separated into
internetworks
 Departments and groups share computers and servers
into common subnetwork or subnet
 Geographically, Specific Purpose, Ownership
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Grouping Devices into Networks and Hierarchical
Addressing
 Geographically
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Grouping Devices into Networks and Hierarchical
Addressing
 Specific Purpose
Different user – different reasons, different tools, different
requirements
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Grouping Devices into Networks and Hierarchical
Addressing
 Ownership
Main concern – security
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Why Separate Hosts into Networks?
 Performance degradation
 Security issues
 Address management
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Why Separate Hosts into Networks?
 Performance
Hosts can be chatty devices
Broadcast news about themselves
Broadcast = message sent from one host to all other hosts on
the network
Share own information and request information about other
hosts
More broadcast = more bandwidth consumed
Broadcast domain
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Why Separate Hosts into Networks?
 Performance
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Why Separate Hosts into Networks?
 Security
Isolating and shielding devices from public access
Better protection
Local network manager can more easily control outside access
to the smaller network
Router or firewall at the perimeter of the network
Configured – known & trusted data/user to access network
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Why Separate Hosts into Networks?
 Security
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Why Separate Hosts into Networks?
 Address Management
Gateway router – send/receive messages beyond the network
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Why Separate Hosts into Networks?
 Address Management
Hierarchical Addressing
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Dividing Networks from Networks
 IPv4 address = 32 bits
 Two parts
Network = 24 bits - postcode
Host = 8 bits - destination
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Intermediary gateway device allowing devices to
communicate across sub-divided networks
 A host has a default gateway address defined
 Ipconfig command
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 IP packet traverses unchanged via routers from sub
network to sub-network
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Gateway needed to send packet out of the network
 Routers add routes for the connected network to their
routing table
 Routing table stores information about connected and
remote networks
 When configured with IP and subnet mask, the
interface becomes part of the network
 The routing table includes that network as directly
connected network.
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 3 mains features of routes in routing table
Destination network
Next-hop
Metric
• Hop Count
• Delay
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Destination network in routing table entry represents a
range of host addresses or network and host addresses
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Next Hop – where the packet goes next
 Next hop is the address of the device that will process
the packet next
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Steps of IP packets as they are routed through several
gateways from devices on one sub network to devices
on other sub networks
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Static Routing
Manually configured route information on the router
Low router processing overhead, High administrative cost
 Dynamic Routing
Routers can learn about routes automatically from other routers
High router processing overhead, Little administrative cost
 Routing protocols
Are the set of rules by which router dynamically share their
routing information
-Routing Information Protocols (RIP)
- Enhance interior Gateway Protocol (EIGRP)
- Open Shortest Path First (OSPF)
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Static Routing
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Fundamentals of Routes, Next Hop Addresses and
Packet Forwarding
 Dynamic Routing
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Summary
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