Network Layer (OSI and TCP/IP) Lecture 9, May 2, 2003 Data Communications and Networks Mr. Greg Vogl Uganda Martyrs University

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer2 Sources n BITDCO lectures n Hodson Ch. 12 n IU A247 lectures 5, 6, 8, 9, 11 n Chappell & Tittel, Guide to TCP/IP, Course Technology, 2002

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer3 Functions of OSI Network Layer n Addressing (sender and receiver machines) n Routing (determining end-to-end path) n Network control (sending/receiving status messages used to make routing decisions) n Congestion control (monitor, reduce delays)

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer4 Network Addresses n Domain name e.g. yahoo.com – Human-friendly name of an Internet location – Used in and web site addresses n IP number e.g – Logical address of a computer, router, etc. – Set by network administrator n MAC address e.g. 00:00:C0:76:5A:26 – Physical address of a computer NIC

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer5 Translating Addresses n Domain Name System (DNS) – Domain name  IP number – Type NSLOOKUP at DOS prompt n Address Resolution Protocol (ARP) – Local IP number  MAC address – Type ARP -A at DOS prompt n Reverse ARP (RARP) – MAC address  local IP number

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer6 Routing n If packet destination is not on local subnet – Forward it to default gateway (router or server) n Routing table in memory of each router – Lists links to other network segments/subnets n Goals – Find the most efficient paths; avoid congestion – Convergence: make all routing tables consistent – Avoid routing loops, packets that live forever

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer7 Centralised Routing n One node is Network Routing Manager – finds over/under use of connections – calculates optimal paths between nodes – makes, sends routing tables to all nodes n Disadvantages – delays to communicate with NRM – delays receiving tables --> inconsistencies – NRM performance/reliability, need backup

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer8 Distributed Routing n e.g. Routing Information Protocol n Each node calculates its own routing table n Periodically transmit status to neighbours – Every 60 seconds, broadcast its routing table n Entries can be added, updated or discarded n Avoids NRM bottleneck n Changes take a long time to reach all nodes

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer9 Static vs. Dynamic Routing n Static routing – Always use one particular path – If the path is unavailable use an alternative – Rarely used (connections change; congestion) n Weighted routing – Randomly select a path from weighted alternatives n Dynamic or adaptive routing – Select best current message route using number of hops, speed and type of link, congestion/traffic

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer10 Open Shortest Path First (OSPF) n Link state routing – Only store table of directly connected links n Assumes routing tables rarely change – Only send update info when link state changes n Routes based on network bandwidth – Reduced traffic; short convergence time n Now more widely used than RIP – Better for larger (enterprise) networks

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer11 Internetworking Protocol Suites n TCP/IP (US Defense Dept, UNIX, etc.) n OSI (ISO) n XNS (Xerox, Ungermann-Bass) n SNA/APPC (IBM) n ATP (Apple) n NetBEUI (Microsoft) n IPX/SPX (Novell)

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer12 OSI Model and Real Protocols

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer13 TCP/IP Protocols and Layers

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer14 OSI Model and Internet Protocols

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer15 IP Datagram Delivery n Unreliable delivery – delivery, uniqueness, sequence not guaranteed – reliability handled by higher layer (TCP) n Connectionless Delivery – each packet routed, delivered independently n Best Effort Delivery – drop packets only if no resources (buffer space)

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer16 IP Datagram Structure

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer17 IP Address Classes

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer18 Default Subnet Masks

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer19 IP Version 6 (IPv6 or IPng) n IPv4 32-bit addresses are almost all in use – Only 2 32 (4 billion) unique addresses n Proposed IPv6 uses 128-bit addresses – Many addresses available (2 128 = ) – 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

May 2, 2003Data Communications and Networks: Lecture 9: Network Layer20 Other IP Version 6 features n Registry service with 32 top level registries n Faster routing (addresses, simplified header) n Quality of Service (reserve resources, request high performance for voice/video) n Security (authentication/encryption) n Auto-configuration (automatically choose an address; similar to BOOTP/DHCP) n Mobile uses (cellphone/wireless)