OSI Network Layer. Network Fundamentals Chapter 5. Version Cisco Systems, Inc. All rights reserved. Cisco Public 1

Transcription

1 OSI Network Layer Network Fundamentals Chapter 5 Version 4.0 1

2 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. 2

3 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 3

4 Network Layer Protocols and Internet Protocol (IP) 4

5 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 5

6 Network Layer Tasks Encapsulation IP header source and destination IP addresses Process of adding information = encapsulation Encapsulated PDU = packet 6

7 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 7

8 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. 8

9 Network Layer Protocols IP is the most common network layer protocol 9

10 IPv4 Most widely used Basic characteristics: 10

11 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 11

12 IPv4 12

13 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 13

14 IPv4 Best Effort 14

15 IPv4 Media independent IP is not concerned with physical medium that carries packet Internetwork communication multimedia journey ex. wireless, ethernet cable, fiber optic 15

16 IPv4 Media independent 16

17 IPv4 IPv4 encapsulates or packages the TL segment or datagram as packets 17

18 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 18

19 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 19

20 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 20

21 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 21

22 Dividing Networks from Networks IPv4 address = 32 bits Two parts Network = 24 bits - postcode Host = 8 bits - destination 22

23 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 23

24 Fundamentals of Routes, Next Hop Addresses and Packet Forwarding IP packet traverses unchanged via routers from sub network to sub-network 24

25 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. 25

26 Fundamentals of Routes, Next Hop Addresses and Packet Forwarding 26

27 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 27

28 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 28

29 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. 29

30 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 30

31 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 31

32 Fundamentals of Routes, Next Hop Addresses and Packet Forwarding Static Routing 32

33 Fundamentals of Routes, Next Hop Addresses and Packet Forwarding Dynamic Routing 33

34 Summary 34

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