osi network layer -...

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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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