WAN technologies and routing

1. WAN technologies and routing • Packet switches and store and forward • Hierarchical addresses, routing and routing tables • Routing table computation • Example WAN technologies

2. Categories of network technology • Local Area Network (LAN) • Metropolitan Area Network (MAN) • Wide Area network (WAN) • Key distinguishing feature is scale: • geographic distance AND • number of connected computers

3. Packet Switches • In order to grow, WANs use many switches • Basic component is the packet switch that can connect to local computers and to other packet switches

4. WAN topology • Chosen to accommodate expected traffic and to provide redundancy

5. Store and forward • Each switch receives packets, queues them in its memory and then sends them out when possible (i.e., when the destination is available) • Many computers can send packets simultaneously

6. Physical addressing in a WAN • A WAN defines a frame format and assigns physical addresses to its computers • Hierarchical addressing, e.g., • first part identifies packet switch • second part identifies computer on this switch [2,2] [1,2] A C address [1,5] B [2,6] D switch 2 switch 1

7. Next hop forwarding • A packet switch determines the destination for each packet from the destination address • local computer or • another packet switch • Only has information about how to reach the next switch - next hop • This is held in a routing table

8. Example routing table [3,2] [1,2] Routing table for S2 Destination Next Hop [1,2] interface 1 [1,5] interface 1 [3,2] interface 4 [3,5] interface 4 [2,1] computer E [2,6] computer F C A S3 S1 B D [1,5] [3,5] S2 F E [2,6] [2,1]

9. Source independence • The next hop depends upon the destination address but not on the source address

10. Hierarchical addresses and routing • Routing is the process of forwarding a packet to its next hop • Hierarchical addresses simplify routing • smaller routing tables • more efficient look up

11. Routing in a WAN • Large WANs use interior switches and exterior switches • Their routing tables must guarantee that • Universal routing - there must be a next hop for every possible destination • Optimal routes - the next hop must point to the “shortest path” to the destination

12. Default routes • A routing table may be simplified by including (at most) one default route • Switch 1 • Destn next hop • - • * interface 3 • Switch 2 • Destn next hop • - • interface 4 • * interface 3 • Switch 3 • Destn next hop • 3 - • interface 2 • interface 3 • 4 interface 4

13. Routing table computation • Manual computation is not feasible for large networks • Static routing - program computes and installs routes when a switch boots; these routes do not change. • Dynamic routing - program builds an initial table when the switch boots; this alters as conditions change.

14. WANs and graphs • A Wan can be modelled as a graph • Nodes are switches: 1, 2, 3, 4 • Edges are connections between switches: (1,3) (2,3) (2,4) (3,4) • Weights are ‘distance’ along a connection

15. Computing the shortest path • Dijkstra’s algorithm - find the distance from a source node to each other node in a graph • Run this for each switch and create next-hop routing table as part of the process • “Distance” represented by weights on edges

16. Example of the shortest path

17. Dijkstra’s algorithm • S = set of nodes, labelled with current distance from source, but for which minimum distance is not yet known • Initialise S to everything but the source • Iterate: • select the node, u, from S that has smallest current distance from source • examine each neighbour of u and if distance to the neighbour from source is less via u than is currently recorded in S then update

18. Implementation • Data structure to store information about the graph (nodes and edges) • Integer labels for the nodes [1..n] • Three data structures: • current distance to each node - array - D[1..n] • next hop destination for each node array - R[1..n] • set S of remaining nodes - linked list - S • weight(i, j) function (infinity if no edge)

19. Given: a graph with nonnegative weight assigned to each edge and a designated source node Compute: the shortest distance from the source node to each other node and a next-hop routing table Method: Initialise the set S to contain all nodes except the source Initialise D so that D[u] = weight(source, u) Initialise R so that R[v] = v if there is an edge from the source to v and zero otherwise

20. while ( set S is not empty ) { choose a node u from S such that D[u] is minimum if ( D[u] is infinity ) { error: no path exists to nodes in S; quit } delete u from set S for each node v such that (u, v) is an edge { if v still in S { c = D[u] + weight (u,v); if c < D[v] { R[v] = R[u] D[v] = c; } } } }

21. Example S = { 1, 2, 3, 4, 5, 6, 7 } Distance D 1 2 3 4 5 6 7 Next hop route R 1 2 3 4 5 6 7

22. Example S = source = 6 S = { 1, 2, 3, 4, 5, 6, 7 } Distance D 1 2 3 4 5 6 7 Next hop route R 1 2 3 4 5 6 7 ∞ 8 2 ∞ ∞ - 5 X 5 X 13 0 2 3 0 0 - 7 0 3 3 3 0 - 7

23. Distributed route computation • Dijkstra’s algorithm requires each switch to hold a complete description of the network • In distributed route computation computation, each switch periodically computes a new local table and sends it to its neighbours • After a while, the switches learn shortest routes or adapt to changes in the network

24. Given: a local routing table, a weight for each link that connects to another switch and an incoming routing message Compute: an updated routing table Method: maintain a distance field in each routing table entry initialise routing table with a single entry that has the destination equal to the local packet switch, the next hop unused and the distance set to sero

25. Repeat forever { wait for the next routing message to arrive over the network; let the sender be switch N for each entry in the message { let V be the destination in the entry and D the distance compute C as D plus the weight of the link over which the message arrived examine and update the local routing table { if ( no route exists to V ) { add an entry to the local table for V with next hop N and distance C } else if ( a route exists with next hop N ) { replace existing distance with C } else if (route exists with distance > C ) change next hop to N and distance to C } } }

26. Link state routing • Each switch periodically broadcasts state of specific links to other switches • Switches collect these messages and then apply Dijkstra’s algorithm to their version of the state of the network