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 Switch 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) Link-state Both used in practice
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. 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 A N X V P
Dest.
Next
Dist. V P 2 N A
Cost=2
Dest.
Next
Dist. V X 6
Dest.
Next
Dist. V N 4

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)
Not as popular as Frame Relay
ATM

26. Summary Wide Area Networks (WANs) WAN addressing 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 Routing tables created
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 1 2 3 4 5 7 D R
S={1,2,3,4,5,7}  8
S={1,2,4,5,7} 13
S={1,4,5,7} 11
S={1,4,5}
S={1,5}
S={1} 20
S={}