1. COMPUTER NETWORKS
CS610
Lecture-16
Hammad Khalid Khan

2. Review Lecture 15 Wide Area Networks Packet Switches
Physical Addressing in a WAN
Store and Forward
Next Hop Forwarding

3. Choosing Next Hop
Packet switch doesn't keep complete information about all possible destination
Just keeps next hop
So, for each packet, packet switch looks up destination in table and forwards through connection to next hop

4. Choosing Next Hop

5. Source Independence
Next hop to destination does not depend on source of packet, called Source Independence
Allows fast, efficient routing
Packet switch need not have complete information, just next hop
Reduces total information
Increases dynamic robustness - network can continue to function even if topology changes without notifying entire network

6. Hierarchical Addressing and Routing
The Process of forwarding the packets from one hop to the next is known as Routing

7. Hierarchical Addressing and Routing

8. Hierarchical Addressing and Routing
In particular, all destinations on same switch have same next hop
Thus, routing table can be collapsed:

9. Routing in a WAN More computers == more traffic
Can add capacity to WAN by adding more links and packet switches
Packet switches need not have computers attached
Interior switch - No attached computers
Exterior switch - Attached computers

10. Routing in a WAN Both Interior and Exterior switches: Forward Packets
Need Routing Tables
Must have:
Universal Routing - Next hop for each possible destination
Optimal Routes - Next hop in table must be on shortest path to destination

11. Modeling a WAN Use a graph: Nodes model switches
Edges model direct connections between switches
Captures essence of network, ignoring attached computers

12. Modeling a WAN

13. Route Computation with a Graph
Can represent routing table with edges:
Graph algorithms can be applied to find routes

14. Redundant Routing Information
Notice duplication of information in routing table for node 1:
Switch 1 has only one outgoing connection; all traffic must traverse that connection

15. Default Routes Can collapse routing table entries with a Default Route
If destination does not have an explicit routing table entry, use the default route:

16. Default Routes

17. Summary Source Independence Hierarchical Addressing and Routing
Routing in a WAN
Default Routes