© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1 Version 4.0 OSI Network Layer Network Fundamentals – Chapter 5
© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1 Version 4.0
OSI Network Layer
Network Fundamentals – Chapter 5
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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
• iPrivacyTools.com
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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
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Network Layer Tasks • Decapsulation
• Process of removing encapsulation data
• Actually encap and decap happened at all layers of OSI model
• When the IP packet is decapsulated, the information in the packet is handed up to the upper layers for delivery and processing.
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Network Layer Protocols IP is the most common network layer protocol
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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 (Chapter 4)
•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? –Each Hop….
Options
Special routing services
Padding
Fill bits when header data does not end on 32 bits boundary
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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
The default route is used when the destination network is not represented by any other route in the rout-ing table.
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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
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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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