Ch 13 WAN Technologies and Routing

1. Ch 13 WAN Technologies and Routing Spring 2003

2. Motivation • How to build a packet switching system that can span a large area • Building blocks • Point-to-point long-distance connections • Packet switches • Key issue that separate MAN/WAN from LAN is scalability • Provide sufficient capacity to permit computers to communicate simultaneously

3. Motivation

4. Packet Switches • A hardware device used to connects to • Other packet switches • Computers

5. Illustration of a Packet Switch • Special-purpose computer system • CPU • Memory • I/O interfaces • Firmware

6. Illustration of a WAN

7. Forming a WAN • Place one or more packet switches at each site • Interconnect switches • LAN technology for local connections • Leased digital circuits for long-distance connections • Interconnections depend on • Estimated traffic (e.g., T1) • Reliability needed

8. Store and Forward • Packets • Sent from source computer • Travels switch-to-switch to destination • Switch • “Stores” packet in buffer • Examines packet’s destination address • “Forwards” packet toward destination • Output-port buffer

9. Physical Addressing in a WAN • Each WAN technology defines the exact frame format • Each device connected to a WAN is assigned a unique physical address • Many WANs use a hierarchical addressing scheme for efficient forwarding • Packet switch number • Port number

10. Illustration of WAN Addressing • An address is encoded as a single binary value • Higher-order bits for switch number • Low-order bits for computer number

11. Next-Hop Forwarding • The process of forwarding a packet to its next hop is known as routing • Routing table for switch 2

12. Forwarding Table Abbreviations • Many entries point to same next hop can be condensed (default) • Only examines the first part of the address • Reduces the size of routing table (scalability) • Improves lookup efficiency

13. Source Independency • Next-hop forwarding does not depends on • Packet’s source address • The path the packet has taken • Next-hop forwarding does depends on • Packet’s destination

14. Relationship of Routing to Graph edge

15. Default Route • Used to eliminate the case of duplication routing

16. Source of Routing Table Information • Static routing (manual) • Table created by hand • Useful in small networks (simplicity and low network overhead) • Useful if routes never change • Dynamic routing (automatic) • Software creates / updates table • Needed in large networks • Changes routes when failures occur

17. Dynamic Routing • Distance Vector (DV) • Switches exchange information in their routing tables • Link-state • Switches exchange link status information • Can have a global view • Both used in practice

18. Distance Vector • Periodic, two-way exchange between neighbors • During exchange, switch sends • List of pairs • Each pair gives (destination, distance) • Receiver • Compares each item in list to local routes • Changes routes if better path exists

19. P V N A X Distance Vector Intuition • If no local route to V or local routed has cost greater than C, install a route with next hop N and cost C • Else ignore pair N Cost=2 A

20. Link-State Routing • Overcomes instabilities in DV • Pair of switches periodically • test link between them • broadcast link status message • Switch • Receives status message • Computes new routes • Uses Dijkstra’s algorithm

21. Link-State Routing

22. Shortest Path Computation in a Graph • Possible distance metric • Geographic distance • Economic cost • Inverse of capacity • Darkened path is minimum for node 4 to node 5

23. Shortest Path Computation in a Graph • Dijkstra’s Algorithms • Routing table for switch 4 is constructed during the computation of the paths • A switch • Only communicates with directly attached neighbors • Must learn route to each destination

24. Early WAN Technologies • ARPANET • Historically important in packet switching • Fast when invented, slow by current standards • X.25 • Early commercial service • Still Used • More popular in Europe

25. Recent WAN Technologies • Frame Relay • Offered by phone companies • Widely used commercial service • SMDS (Switched Multi-megabit Data Service) • Offered by phone companies • Not as popular as Frame Relay • ATM

26. Summary • Wide Area Networks (WANs) • Span long distances • Connect many computers • Built from packet switches • Use store-and-forward • WAN addressing • Two-part address • Switch/computer

27. Summary (continued) • Routing • Each switch contains routing table • Table gives next-hop for destination • Routing tables created • Manually • Automatically • Two basic routing algorithms • Distance vector • Link state

28. Summary (continued) • Example WAN technologies • ARPANET • X.25 • SMDS • Frame Relay • ATM

29. Example of Distance Vector Routing • Consider transmission of one DV message • Node 2 send to 3, 5, and 6 • Node 6 installs cost 8 route to 2 • Later 3 sends update to 6 • 6 changes route to make 3 the next hop for destination 2

30. Dijkstra’s Shortest Path Algorithm • Input • Graph with weighted edges • Node, n • Output • Set of shortest paths from n to each node • Cost of each path • Called Shortest Path First (SPF) algorithm

31. Dijkstra’s Algorithm R[u]

32. Algorithm Intuition • Start with self as source node • Move outward • At each step • Find node u such that it • Has not been considered • Is “closest” to source • Compute • Distance from u to each neighbor v • If distance shorter, make path from u go through v

33. Result of Dijkstra’s Algorithm • Example routes from node 6 • To 3, next hop = 3, cost = 2 • To 2, next hop = 3, cost = 5 • To 5, next hop = 3, cost = 11 • To 4, next hop = 7, cost = 8