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)