2. COMP 1121: Computers and Computer Networks Session 5: Network Topologies and Routing

3. Session Objectives By the time you have finished this teaching/learning session you should be able to: –List and identify the main network topologies –Explain the basic principles of packet routing by IP address –Explain how ALOHA works as a means of resolving contention between packet streams –Relate IEEE802 standards to topologies and contention resolution types

4. Network Topologies Star - central computer with cables radiating out Ring - ring of cable with computers linked to the ring Bus - end-to-end cable (backbone) with computers linked to the backbone Mesh - computers each linked (with cable?) to several other computers

5. Star Topology Central server(s) or hub Remote client

6. Star Topology from http://www.linuxdoc.com/howto/intro_to_networking/index.lxp?lxpwrap=c5434.htm

7. Star Topology Used since the very early days of computer networking Similar to system of connecting a mainframe or midrange computer to dumb terminals –note: the mainframe/dumb terminal scenario is actually a multi-user system, and not a network, because there is only one intelligent device - the central computer

8. Star Topology (Hubs) hub server

9. Star Topology (Hubs) All nodes connected to a common central location, known as a hub Hubs can link to other secondary hubs, creating subnets All packets of data transferred through the network go through the main hub –Therefore it must have a high bandwidth

10. Star Topology (Hubs) Quite fault tolerant because: –If one network link fails… –only a single node loses connection However, if the hub itself fails then the whole subnet/network fails!

11. Ring Topology no central hub data from one node: –passes into a “ring” of cabling –travels round the ring Mechanism: –every node receives all network data –takes the data that is addressed to it –passes the rest back to the ring

12. Ring Topology from http://www.courtonline.co.uk/ringtop.htm

13. Ring Topology Evaluation –If a node fails or a cable breaks, the whole ring could fail –“monitor stations” are therefore used to quickly detect failures –If data does not find its destination node, it will pass right round the ring until it returns to its originator

14. Ring wiring image from http://www.delmar.edu/Courses/ITNW2313/access.htm Not always a physical ring… –a logical ring may be actually wired as a physical star (opp) –or can even use a physical bus arrangement (next page)

15. Bus Topology Central Server Remote client Terminator

16. Linear bus topology from http://fcit.usf.edu/network/chap5/chap5.htm

17. Bus Topology Once Popular technology for small (<20 computers) networks T-pieces electrically connect computers to the backbone Bus carries all network data All nodes are connected to a single common cable known as the backbone The backbone is end-to-end, and a terminator is needed at each end to prevent signal bounce and to complete the circuit

18. Bus Topology If one node fails, other nodes are not affected If the backbone cable breaks, the whole network may halt Removing either of the terminator(s) prevents to flow of data and stops the network Unfastening a T-piece will disconnect a computer from the network, but the rest of the network will still function

19. Mesh topology Each computer on the network is attached to several other computers Only suitable for peer-peer networks Uses a lot of cable Very good for fault tolerance The Internet is a mesh network

20. The Internet is a large Mesh Network Over 750 million Server computers! Source: ISC Domain name Survey, online at http://www.isc.org/solutions/survey

21. Full mesh topology from http://www.cisco.com/warp/public/759/ipj_3-4/ipj_3-4_routers.html

22. Partial mesh topology from http://www.cisco.com/warp/public/759/ipj_3-4/ipj_3-4_routers.html

23. Mesh Topology (3) How many links are needed to connect together 3 nodes in a full mesh? 4 nodes? 5 nodes? 10 nodes? 1000 nodes?

24. Routing Problem: how to get a message from one computer to another Needs a means of identifying the destination –IP address, or similar And a means of finding that destination –calculating/choosing a pathway to the destination through the network

25. Types of routing 1: Broadcast Routing (e.g. Ethernet) Message is sent to all computers in the subnet Only the computer to which it is addressed bothers to read its contents Uses MAC addresses (unique identifier of the NIC, function at OSI layer 2) Useful on small local networks Uses too much bandwidth for large networks

26. Types of routing 2: Addressed routing Uses IP addresses or something similar Each packet has destination address information in its header Packet is passed from router to router towards its destination –and eventually arrives… Used on the Internet and other large networks

27. Media Access (OSI layers 1 & 2) All network topologies require that network devices use a common carrier medium to access network resources –e.g. copper cabling, optical fibre, radio waves If packets from different sources arrive at the carrier simultaneously, there will be big problems… This basic media access issue is known as contention –particular problem with broadcast networks

28. Collisions Transmission of a data packet takes a finite time Travel of the whole packet to the furthest point on the network takes a further finite time If, at any time during this period, another host on the network starts to transmit, there will be other hosts that receive parts of both packets simultaneously Result: neither packet can be received properly

29. Collision animation from http://handsonhowto.com/lan102.html Transmitter A Transmitter B

30. Resolving contention Protocols for human communication How do we decide who speaks? What rules do we use? In groups – 5 minutes From http://www.nuskin.com/corp/opportunity/index.shtml

31. Resolving Contention A number of possible solutions have been devised to ensure packets do not collide: –Polling –Token Passing –ALOHA

32. Polling Central controller interrogates each node in turn and asks if it has a message to send If a message is waiting to be sent, it is delivered If no message, controller passes on to the next node

33. Token passing Software ‘token’ is passed from node to node in a predetermined order Nodes can only send messages when they are in possession of the token Token is passed to the next node in sequence when it is no longer needed –if no message is to be sent, or when the message has been sent

34. Aloha (1) Named after Hawaiian greeting First used to connect several computers by radio in Hawaii –hard (cabled) links not practicable between islands Rule: –each station transmits as and when needed

35. Aloha (2) If a collision occurs, messages are retransmitted after a random wait –If both stations retransmitted immediately, or after a fixed interval, a further collision would occur OK for light traffic (and few collisions) Maximum efficiency possible about 18% due to collisions

36. Commercial Aloha – “Ethernet” Name derived from “the ether” –a hypothetical invisible substance that was once thought to exist in space and allowed radio waves to be conducted through it (!) Ethernet name invented by Dr Graham Metcalfe, of Xerox –wanted to connect 100 computers together over a 1km cable –used a system based on pure Aloha to resolve contention

37. Ethernet & Network Contention Pure ALOHA allows nodes to attempt to broadcast at any time –Then manages the attempted broadcasts to ensure that collisions are detected and dealt with by resending OK for lightly used network But means that many workstations all working concurrently on the same network segment can create a lot of collisions Result can be a severely reduced response time

38. Ethernet – reduction in collisions using CSMA (1) Carrier Sense Multiple Access (CSMA) Strategy used in broadcast systems to reduce the likelihood of collisions Each node listens before transmitting –if the medium is busy then the node waits –otherwise it transmits

39. Ethernet – reduction in collisions using CSMA (2) Problem: signals travel at a finite speed –So two nodes could begin to transmit almost simultaneously –and not detect each other’s transmission before starting their own message Solution: each node listens while transmitting to be sure that no other node starts transmitting

40. Ethernet and CSMA (3) If a collision is detected: –both nodes stop transmitting immediately It would be a waste of time to finish message that is corrupted… – then wait a random time and retry (binary exponential backoff)

41. Ethernet and CSMA (4) Called unslotted or non-persistent CSMA based on and replaces pure Aloha Unslotted because any node can transmit at any time –Some protocols only allow transmissions to start at certain time – starts of ‘time ‘slots’ Non-persistent because nodes wait before retrying –if they both tried to transmit again immediately, they would cause another collision!

42. Full Ethernet model (CSMA/CD) (1) Pre-dated the OSI model! Provided an algorithm for the protocol for sending data that could be represented in the following way: Carrier Sense –The node “listens” to the carrier medium to see if anyone is transmitting –If another node is transmitting, wait until it has finished

43. Full Ethernet model (CSMA/CD) (2) Multiple Access –Many nodes are connected to the same carrier medium Collision Detection –Each node listens during transmission –If a collision is detected during transmission, both transmitting nodes will stop transmitting immediately and then retry after a random wait –If collisions continue to occur, nodes extend their wait time

44. Collision detection? from http://www.dawndreamer.ca/photos_to_ponder.htm

45. Full Ethernet model (CSMA/CD) (3) With good design, this method can deliver 90% efficiency of bandwidth Main advantage is that time is not wasted in transmitting whole packets when a collision occurs Most bus networks now use CSMA/CD Collisions can still cause problems if too many workstations are connected to a single network segment –no means of prioritising

46. Collision animation (again) from http://handsonhowto.com/lan102.html Why isn’t this an accurate representation of a collision in an Ethernet LAN? Transmitter B Transmitter A

47. Ethernet Xerox formed an alliance with Digital and Intel (DIX – Digital, Intel, Xerox)) Together, these organisations formulated the original DIX “Blue Book” Ethernet standard in February 1980 Subsequently rewritten to the agreed IEEE standards 802.x that complemented the OSI model

48. Ethernet and OSI (1) It was agreed that the Ethernet solution: –must be addressed through the networking software –should fit into the lower layers of the OSI model Standards set by the IEEE 802 committee (802.3 – 1983 onwards) and D.I.X. Encompasses OSI layers 1 & 2: –an access method to the network –a set of specifications with regard to cable and packet format

49. Ethernet and OSI (2) In 1982 original Blue Book standard evolved into Ethernet II IEEE 802 model provides specifications for: –physical network components, e.g. cable –layout of data within a packet, as “frames” –restrictions caused by network segments

50. IEEE 802 standards and Access Methods (1) IEEE 802.2 standard subdivided the Data Link Layer into two sub-layers, to enable network interface cards to talk to the upper layers of the OSI stack –Logical Link Control (LLC) sublayer presents a uniform interface to the Network layer –Media Access Control (MAC) sublayer interfaces with the physical transmission medium

51. IEEE 802 standards and Access Methods (2) IEEE 802.3 standard refers to CSMA/CD (Ethernet) –similar to Ethernet II –adopted as the Ethernet standard by most manufacturers IEEE 802.4 standard refers to Token Bus networks IEEE 802.5 refers to Token Ring networks

52. Internet Routing Uses IP address (OSI level 3) –Compare: MAC addresses used at level 2 for Ethernet Each router reads the IP destination address of each packet –And then decides what to do with the packet –i.e. which other router to forward the packet on to

53. ‘Store and Forward’ (1) Packets arriving at a router may come from several different connections May not arrive regularly –Arrivals may be ‘bursty’ Router may not be able to process all packets immediately on arrival –e.g. 2 packets arriving almost simultaneously on different connections

54. ‘Store and Forward’ (2) As packets arrive, they are added to a “buffer” –area of memory where packets are stored temporarily until they can be processed Packets are processed in order of arrival –‘first in, first out’ (FIFO) stack Note: if the buffer gets full, further packets are discarded and lost

55. Routing decisions Router uses the destination IP address in the packet IP header to decide what to do with the packet –Maybe send to a local network to which the router is connected router must know the IP addresses relevant to its own local network(s) –Maybe forward on to another router But which one?

56. Routing tables Each router has a database of IP addresses (or starts of IP addresses) For each IP address, there is a link that a packet for that IP address should be forwarded on Router reads the destination IP address, consults the routing table, and forwards the packet on the appropriate link

57. Routing table from http://www.answers.com/topic/routing-protocol-1?cat=technology

58. Construction of routing tables Routing tables can be set up when the router is first booted But network conditions may change –a route that was once optimal may become congested –a link or router may malfunction or be removed –a new link may be added to the network Dynamic updates needed

59. Updating routing tables Routers send each other information about network conditions –Broken links –New links –Routers going off-line –Congestion problems Each router updates its routing tables accordingly

60. Normal route of message Recipient Router B Router C Router A Sender

61. Disaster strikes: link is broken Recipient Router B Router C Router A Sender Message cannot be delivered

62. What happens Router A finds out about the broken link –No traffic received from router B –Regular updates from router B don’t arrive Router A knows enough about the local network topology to use an alternative route Routing table adjusted accordingly

63. Solution: a new route is found Recipient Router B Router C Router A Sender

64. Decisions between multiple possible routes Factors to be considered by the router: –Total number of ‘hops’ in the path –Traffic loading on each hop –Financial costs of using certain hops Algorithms for finding best routes: –Bellman-Ford (distance-vector algorithm) –Dijkstra (link-state algorithm)

65. Border Gateway Protocol Protocol used by Internet routers Application layer protocol –TCP port 179 First defined in RFC 1105 (June 1989) Updated to version 4 in RFC 1774 (March 1995) –Latest update to version 4: RFC 4271 (January 2008)

66. End