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Packets & Routing

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Packets & Routing

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  1. Packets & Routing • Lower OSI layers (1-3) concerned with packets and the network • Packets carry data independently through the network, and into other networks… • Only know where to go because Routers (and Routing Switches) tell them where to go next • use header information on each packet • Contains source & destination IP addresses header data (payload)

  2. Internet Routing • Uses IP address (OSI level 3) • Compare: MAC addresses used at OSI level 2 for local network navigation via switches • OSI model? link • Each router reads the IP destination address of each packet • then decides which other router to forward the packet on to…

  3. Packet Creation… • https://www.youtube.com/watch?v=R3CMVw7tM2Y

  4. Packet Switching • Resulted from… UK scientific brilliance… • Donald Davies • developed “packet switching” to get digital data in packets through a network efficiently • Ideas drained to the US… http://news.bbc.co.uk/1/hi/technology/8499501.stm • https://www.youtube.com/watch?v=tT4AaelwvV4

  5. Routing Packets: ‘Store and Forward’ (1) • May come to router from several different connections • May not arrive regularly • arrivals may be ‘bursty’ • Router may not be able to process all packets immediately on arrival • e.g. 2 packets arriving almost simultaneously on different connections

  6. ‘Store and Forward’ (2) • As packets arrive, added to a “buffer” • area of memory where packets are stored temporarily until they can be processed • Processed in order of arrival • ‘first in, first out’ (FIFO) stack • Note: if the buffer gets full, further packets are discarded and lost

  7. Routing decisions • Router uses the destination IP address in the packet IP header to decide what to do with the packet • Maybe send to a local network to which the router is connected • router must know the IP addresses relevant to its own local network(s) • Maybe forward on to another router • But which one?

  8. Routing tables • Each router has a database of IP addresses (or starts of IP addresses) • For each IP address, there is a link that a packet for that IP address should be forwarded to • Router reads the destination IP address, consults the routing table, and forwards the packet through the appropriate link

  9. Routing tablefrom http://www.answers.com/topic/routing-protocol-1?cat=technology

  10. Construction of routing tables • Routing tables can be set up when the router is first booted • But network conditions may change • a route that was once optimal may become congested • a link or router may malfunction or be removed • a new link may be added to the network • Dynamic updates needed

  11. Updating routing tables • Routers send each other information about network conditions • Broken links • New links • Routers going off-line • Congestion problems • Each router updates its routing tables accordingly

  12. Router B Router C Recipient Router A Sender Normal route of message

  13. Router B Router C Recipient Router A Sender Disaster strikes: link is broken Message cannot be delivered

  14. What happens • Router A finds out about the broken link • No traffic received from router B • Regular updates from router B don’t arrive • Router A knows enough about the local network topology to use an alternative route • Routing table adjusted accordingly

  15. Router B Router C Recipient Router A Sender Solution: a new route is found

  16. Decisions between multiple possible routes • Factors to be considered by the router: • Total number of ‘hops’ in the path • Traffic loading on each hop • Financial costs of using certain hops • Algorithms for finding best routes: • Bellman-Ford (distance-vector algorithm) • Dijkstra (link-state algorithm)

  17. Check out“Routing Algorithms”these give a router a real headache!https://www.youtube.com/watch?v=L8VpthhRaEg

  18. e.g. Border Gateway Protocol • Protocol used by Internet routers • Application layer protocol • TCP port 179 • First defined in RFC 1105 (June 1989) • Updated to version 4 in RFC 1774 (March 1995) • Latest update to version 4: RFC 4271 (January 2008)

  19. Firewalls • Try out Windows Firewall now…

  20. VPNs • What are they? • How can they be implemented? • From the TWIT network… (excuse the adverts) • https://www.youtube.com/watch?v=sGYIsJUIxH8