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Network Layer (OSI and TCP/IP)
Lecture 9, May 2, 2003
Data Communications and Networks
Mr. Greg Vogl
Uganda Martyrs University
Sources
BITDCO lectures 18-20
 Hodson Ch. 12
 IU A247 lectures 5, 6, 8, 9, 11
 Chappell & Tittel, Guide to TCP/IP, Course
Technology, 2002
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Functions of OSI Network Layer
Addressing (sender and receiver machines)
 Routing (determining end-to-end path)
 Network control (sending/receiving status
messages used to make routing decisions)
 Congestion control (monitor, reduce delays)
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Network Addresses
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Domain name e.g. yahoo.com
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IP number e.g. 207.46.230.229
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Human-friendly name of an Internet location
Used in e-mail and web site addresses
Logical address of a computer, router, etc.
Set by network administrator
MAC address e.g. 00:00:C0:76:5A:26
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Physical address of a computer NIC
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Translating Addresses
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Domain Name System (DNS)
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Address Resolution Protocol (ARP)
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Domain name  IP number
Type NSLOOKUP at DOS prompt
Local IP number  MAC address
Type ARP -A at DOS prompt
Reverse ARP (RARP)
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MAC address  local IP number
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Routing
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If packet destination is not on local subnet
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Routing table in memory of each router
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Forward it to default gateway (router or server)
Lists links to other network segments/subnets
Goals
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Find the most efficient paths; avoid congestion
Convergence: make all routing tables consistent
Avoid routing loops, packets that live forever
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Centralised Routing
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One node is Network Routing Manager
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finds over/under use of connections
calculates optimal paths between nodes
makes, sends routing tables to all nodes
Disadvantages
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delays to communicate with NRM
delays receiving tables --> inconsistencies
NRM performance/reliability, need backup
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Distributed Routing
e.g. Routing Information Protocol
 Each node calculates its own routing table
 Periodically transmit status to neighbours
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Every 60 seconds, broadcast its routing table
Entries can be added, updated or discarded
 Avoids NRM bottleneck
 Changes take a long time to reach all nodes
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Static vs. Dynamic Routing
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Static routing
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Weighted routing
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Always use one particular path
If the path is unavailable use an alternative
Rarely used (connections change; congestion)
Randomly select a path from weighted alternatives
Dynamic or adaptive routing
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Select best current message route using number of
hops, speed and type of link, congestion/traffic
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Open Shortest Path First (OSPF)
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Link state routing
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Assumes routing tables rarely change
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Only send update info when link state changes
Routes based on network bandwidth
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Only store table of directly connected links
Reduced traffic; short convergence time
Now more widely used than RIP
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Better for larger (enterprise) networks
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Internetworking Protocol Suites
TCP/IP (US Defense Dept, UNIX, etc.)
 OSI (ISO)
 XNS (Xerox, Ungermann-Bass)
 SNA/APPC (IBM)
 ATP (Apple)
 NetBEUI (Microsoft)
 IPX/SPX (Novell)
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OSI Model and Real Protocols
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TCP/IP Protocols and Layers
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OSI Model and Internet Protocols
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IP Datagram Delivery
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Unreliable delivery
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Connectionless Delivery
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delivery, uniqueness, sequence not guaranteed
reliability handled by higher layer (TCP)
each packet routed, delivered independently
Best Effort Delivery
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drop packets only if no resources (buffer space)
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IP Datagram Structure
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IP Address Classes
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Default Subnet Masks
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IP Version 6 (IPv6 or IPng)
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IPv4 32-bit addresses are almost all in use
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Only 232 (4 billion) unique addresses
Proposed IPv6 uses 128-bit addresses
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Many addresses available (2128 = 1038)
Not easily memorised like IPv4 addresses
Displayed in hexadecimal like MAC addresses
Can contain IPv4 and MAC addresses
Some addresses reserved for uni/multi/anycast
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Other IP Version 6 features
Registry service with 32 top level registries
 Faster routing (addresses, simplified header)
 Quality of Service (reserve resources,
request high performance for voice/video)
 Security (authentication/encryption)
 Auto-configuration (automatically choose
an address; similar to BOOTP/DHCP)
 Mobile uses (cellphone/wireless)
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