WAN Technologies and Routing

1. WAN Technologies and Routing Computer Networks and Internets (Comer)

2. Networks • Networks are grouped into (fuzzy) categories based on the distances they span and the number of nodes • HAN: Home Area Network • LAN: Local Area Network • CAN: Campus Area Network • MAN: Metropolitan Area Network • WAN: Wide Area Network

3. Scalability • As the number of nodes increases, issues of scalability arise. For a network to “scale” • One should be able to add nodes easily. • The network’s functionality should remain relatively constant when nodes are added. • The physical problems (attenuation & interference) are more readily overcome than the sharing problem (many nodes trying to transmit on a shared connection).

4. Bridges help

5. Bridges Revisited • Bridges help reduce traffic because a bridge “learns” whether it filters or forwards a frame. • Unicast messages do not have to be forwarded to all segments but only to those connecting the source and destination. • This allows different segments to transmit different signals simultaneously (in parallel) giving the network scalability.

6. Router • A router serves a very similar purpose • It separates traffic into local (network) traffic and (internet) traffic that must be sent on. • It sends the latter in the right general direction. • Routers and bridges work in different layers: • A bridge operates at the Data Link Layer using MAC addresses (hardware addresses). • A router operates at the Network Layer using network addresses (software addresses, e.g. IP addresses).

7. That’s Illogical • Bridges use the MAC address. • The problem with MAC addresses is that they are essentially distributed at random over the network. • Two nearby computers can have very different MAC addresses. • Recall that first part of the MAC address is associated with the manufacturer. • Computers with consecutive MAC addresses could be many thousands of miles apart.

8. MAC addressing (random) 414 289 760 045 014 579 774 533 709 301 705 814

9. Network addressing (logical) 201 101 301 302 202 102 203 103 303 204 304 104

10. Hierarchical addressing Desi Donnelly 12 Park Range Victoria Park Manchester M14 5HQ UNITED KINGDOM Name Address Neighborhood City Country

11. Hierarchical addressing (Cont.) • The postal service reads the previous address starting from the bottom. • There’s no need to read past United Kingdom until the letter reaches the UK. • There’s no need to read past Manchester until the letter reaches Manchester. • And so on.

12. Hierarchy of IP • An IP address has a somewhat similar hierarchy. • An IP address is divided into two parts: • The first part specifies a particular network (a neighborhood of nearby computers). • The second part specifies a particular node. • There’s no sense looking at the second part (the node) until you have a match on the first part (the network). • The subnet mask provides the dividing line between the network and node portions of the address.

13. ipconfig /all Subnet mask 255.255.0.0

14. Another Subnet mask Subnet mask 255.255.248.0

15. 25511111111

16. 25511111111

17. 24811111000

18. 24811111000

19. Subnet masks • 255.255.0.0  11111111.11111111.00000000.00000000 • 255.255.248.0  11111111.11111111.11111000.00000000 • Subnet masks usually have the property that in binary they are a string of all 1’s followed by a string of all 0’s.

20. The subnet-mask division • The part of the IP address (expressed in binary) that corresponds to the 1’s portion of the subnet mask is the network portion. • 1’s  network • The part of the IP address (expressed in binary) that corresponds to the 0’s portion of the subnet mask is the node portion. • 0’s  node

21. Example • IP: 139.84.33.18  10001011.01010100. 00100001. 00010010 • Subnet mask: 255.255.248.0  11111111.11111111.11111000.00000000 • Network part:  10001011. 01010100.00100000. 00000000 • Node part:  00000000. 00000000. 00000001. 00010010

22. Comparison • IP addressing is simpler because it is systematic. • A MAC address consists of 48 bits and all of the bits must be looked at by each and every bridge. • An IP(v4) address consists of 32 bits and only the first part must be looked at by a router until a match is found and after that only the second part must be considered.

23. Router • A router connects two or more networks to form an internet. • Sometimes the router connects two or more sub-networks to form a network. • A router may be a specific device or it may be software on a computer. • A router determines where to forward “internet” traffic (messages whose source is on one network and destination is on another network). • It also filters out local traffic.

24. Designing a WAN • Similar to the ideas of dividing a network into “bridged” segments, • an internet can be divided into “routed” networks. • Or a network can be divided into “routed” subnetworks. • Ideally one should study the usage (traffic patterns) and place routers in such a way that filtering is efficient.

25. Store and Forward • The network is designed so that many signals are being transmitted simultaneously (in parallel), thus several packets may reach a router in very rapid succession. • This feature of packet switching is handled by the STORE and FORWARD paradigm. • Packets are queued up as they arrive (“stored”) and then sent on their way (“forwarded”) when the processor catches up. • If the queue is full, the packet is dropped and the router may or may not send a message indicating as much.

26. Hop • A hop is the transmission of a packet from one router (or other intermediate point) to another. • In TCP/IP protocol, the number of hops a packet is allowed to make is kept in the packet header (in the TTLtime-to-live field). Each router reduces the number by one. When the TTL reaches 0, the packet is discarded. • This is another difference between a bridge and a router: a router can make (small) changes to the packet.

27. Next-Hop Forwarding • A router has information about the next place (hop) to forward the packet so that it will eventually reach its destination. • The next-hop information is put into a table (called a routing table). • It tells the packet which interface to use as it passes through the router.

28. Fig. 13-6 (Comer)

29. Fig. 13-6 (Comer)

30. Packets don’t look back • Next-hop switching has “source independence” and more generally “history independence.” • The next-hop only depends on the packet’s destination, not on where it started or where it’s been. • It makes the forwarding mechanism more compact and efficient.

31. Routing • All destination addresses with the same first part will be forwarded (routed) to the same packet switch. • That’s not exactly true. A given network destination (as opposed to node destination) may be listed on the table a few times. • This allows the router to “re-route” packets if previous packets have not gotten through, in case a connection goes down or is jammed with traffic.

32. Routing • Using only this first part of the address (the network portion) to route the packet results in • A shorter table: an entry per network as opposed to an entry per computer • A shorter computation time for routing as the table is shorter and better organized • think of searching a sorted array versus searching an unsorted array

33. Interior/Exterior • Some routers do not connect to a network (group of end-user computers) but only to other routers or gateways. They are said to be “interior.” • Those connected to computers are called “exterior.” • Analogy: interior gateways are like two major highways meeting without any cities nearby

34. The best of all possible routes • Routing tables should have an entry for each possible destination, this is called Universal Routing. • Routing tables should have the “shortest” path for the destinations, this is called Optimal Routing.

35. Default Routes • One way to deal with the competing needs of having a hop for each destination and having small tables is to use default routes. • It’s analogous to the default case in a switch statement, any case not explicitly found elsewhere in the table follows the default hop. • If one hop is very popular, making it the default shortens the table.

36. Routing Table Computation • Static Routing • A program computes and installs a packet switch when it boots and the routes do not change. • Routing is simplistic and low network overhead, but the routing is inflexible.

37. Routing Table Computation • Dynamic • A program builds an initial routing table when router boots and alters the table as the conditions in the network change. • The network can handle problems automatically. • Used by most large networks. They are designed with redundant hardware to handle the occasional failures. The dynamic computation allows the network to recover easily.

38. Bridges/Routers • Recall that bridges could not be used simultaneously if there was a loop. • Routers can have loops and remain active; they are more “intelligent” and can avoid the infinite loop problem. • A connection does not have to be down for a router to reroute packets, heavy traffic may be enough.

39. Dijkstra’s algorithm

40. Building a routing table • Each router sends packets to the routers it connects to (its “nearest neighbors”). • From the responses to these packets, the router gathers information on the “cost” to transmit to each neighbor. • Cost is based on bandwidth, traffic, etc. • This information is compiled into the “link state packet” (LSP). • The LSP is transmitted to the neighbors and beyond.

41. Building a routing table (Cont.) • The router now has information on • all of the nodes in the networks • the links connecting them • the cost thereof • The router wants to find the least costly path to all of the other nodes.

42. Math terminology • Abstractly, the network is represented by what mathematicians call a graph. • A graph is made up of two sets • The vertices (our nodes) • The edges (our transmission lines) which connect two vertices • A graph is said to be “connected” if there is at least one path along the edges connecting every pair of vertices.

43. Math terminology (Cont.) • A graph is called “simply connected” if for each pair of vertices, there is only one path connecting them. • A simply connected graph has no loops. • A tree is a simply connected graph. • Networks are typically not simply connected (they are not trees), so there is often more than one path between nodes.

44. Dijkstra’s algorithm • Dijkstra’s algorithm is a way for a router to select the least costly path to another node (router). • One constructs a “tree” that spans the graph. • The root of the tree is the router for which we are building the table.

45. Dijkstra Demo

46. Dijkstra Demo (the root)

47. Nearest Neighbors • Next one includes the branches to the root’s nearest neighbors. • One selects the least costly one of these to continue the procedure. • This is shown in red in the demo screen captures.

48. Dijkstra Demo

49. Dijkstra Demo Ikeda is the cheapest at this stage.

50. Competing paths • Examine the nearest neighbors of that least costly path. • The cost of a path is the sum of the costs of the links connecting the source (root) to a node. • If a path takes us to a node which already has been reached, then we compare the costs of the two paths and keep the lower.