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…

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…

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 table from http://www. answers. com/topic/routing-protocol-1

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

13. Disaster strikes: link is broken
Recipient
Router B
Router C
Router A
Sender
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. Solution: a new route is found
Recipient
Router B
Router C
Router A
Sender

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

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)