Download - Network Layer (OSI and TCP/IP)
Network Layer (OSI and TCP/IP)
Lecture 9, May 2, 2003
Data Communications and Networks
Mr. Greg Vogl
Uganda Martyrs University
May 2, 2003 Data Communications and Networks: Lecture 9: Network Layer
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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
Domain name e.g. yahoo.com– Human-friendly name of an Internet location– Used in e-mail and web site addresses
IP number e.g. 207.46.230.229– Logical address of a computer, router, etc.– Set by network administrator
MAC address e.g. 00:00:C0:76:5A:26– Physical address of a computer NIC
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Translating Addresses
Domain Name System (DNS)– Domain name IP number– Type NSLOOKUP at DOS prompt
Address Resolution Protocol (ARP)– Local IP number MAC address– Type ARP -A at DOS prompt
Reverse ARP (RARP)– MAC address local IP number
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Routing
If packet destination is not on local subnet– Forward it to default gateway (router or server)
Routing table in memory of each router– Lists links to other network segments/subnets
Goals– 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
One node is Network Routing Manager– finds over/under use of connections– calculates optimal paths between nodes– makes, sends routing tables to all nodes
Disadvantages– 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
– 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
Static routing– Always use one particular path– If the path is unavailable use an alternative– Rarely used (connections change; congestion)
Weighted routing – Randomly select a path from weighted alternatives
Dynamic or adaptive routing– 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)
Link state routing– Only store table of directly connected links
Assumes routing tables rarely change– Only send update info when link state changes
Routes based on network bandwidth– Reduced traffic; short convergence time
Now more widely used than RIP– 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
Unreliable delivery– delivery, uniqueness, sequence not guaranteed– reliability handled by higher layer (TCP)
Connectionless Delivery– each packet routed, delivered independently
Best Effort Delivery– 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)
IPv4 32-bit addresses are almost all in use– Only 232 (4 billion) unique addresses
Proposed IPv6 uses 128-bit addresses– 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)