1. Computer networks Name: K.SUDHA Designation: Lecturer Department: Electrical and Electronics Engineering Subject code: CS2361 Year: III Unit: II Title: Introduction to network layers

2. Internetworking An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork

4. Types Local-area networks (LANs)enabled multiple users in a relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers and printers. Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity. technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others.

5. ETH

7. IPV4 Packet Header VersionHLenTOSLength IdentFlags Offset TTLProtocolChecksum SourceAddr Destination Addr Options(variable)Pad(variable) Data

8. Datagram Delivery

9. Packet Format

10. IPV4 Packet header

11. Fragmentation and Reassembly

14. (RARP)Reverse Address Resolution Protocol (RARP) is a Link layer networking protocol RARP is described in internet EngineeringTask ForceETF) publication RFC 903 It has been rendered obsolete by the Bootstrap Protocol (BOOTP) and the modern Dynamic Host Configuration Protocol(DHCP) BOOTP configuration server assigns an IP address to each client from a pool of addresses. BOOTP uses the User Datagram Protocol (UDP)

15. Router A router is a device that determines the next network point to which a packet should be forwarded toward its destination Allow different networks to communicate with each other A router creates and maintain a table of the available routes and their conditions and uses this information to determine the best route for a given packet. A packet will travel through a number of network points with routers before arriving at its destination. There can be multiple routes defined. The route with a lower weight/metric will be tried first. Routing

17. Routing Protocols Static Routing Dynamic Routing IGP (Interior Gateway Protocol): Route data within an Autonomous System RIP (Routing Information Protocol) RIP-2 (RIP Version 2) OSPF (Open Shortest Path First) IGRP (Interior Gateway Routing Protocol) EIGRP (Enhanced Interior Gateway Routing Protocol) IS-IS EGP (Exterior Gateway Protocol): Route data between Autonomous Systems BGP (Border Gateway Protocol) Routing

18. Iskra Djonova-Popova The Routing Algorithm u the shortest path tree is contained in the routing table u Calculations are based on the Bellman-Ford algorithm

19. Iskra Djonova-Popova The Centralized Version of the Algorithm CycleNode B C D E Initial(.,  )(.,  )(.,  )(.,  ) 1(1, 1)(2, 2)(3, 1)(4, 2) ABC DE 2 3 4 5 6 1 ABC DE 2 3 4 1

20. Iskra Djonova-Popova The Distributed Version ABC DE 1 2 3 4 5 6 Example of simple network with 5 nodes (routers) and 6 links (interfaces) The cost of all links is assumed to be 1 From A to Link Cost B 1 1 C 1 2 D 3 1 E 1 2 Routing table for A

21. Iskra Djonova-Popova Advantages simple to implement low requirement in processing and memory at the nodes suitable for small networks

22. Iskra Djonova-Popova Disadvantages Slow convergence Bouncing effect Counting to infinity problem

23. Iskra Djonova-Popova Slow Convergence ABC DE 2 3 4 5 6 XXX When a link breaks the routers are supposed to reestablish the routing tables link 1 breaks

24. Iskra Djonova-Popova The Bouncing Effect link 2 breaks and A sends its routing table to B before B sends it to A ABC DE 3 4 5 6 XXX 1

25. Iskra Djonova-Popova Counting to Infinity Problems DE 3 ABC 2 4 5 XXX Links 1 and 6 break. A sends its old routing table before D sends the new routing table

26. Subnets Each organization assigns IP addresses to specific computers on its networks IP addresses are assigned so that all computers on the same LAN have similar addresses Each of these lans is known as a TCP/IP subnet Any portion of the IP address can be designated as a subnet using a subnet mask* * Subnet masks tell computers what part of an IP address is to be used to determine whether a destination is on the same or a different subnet

27. Subnet Addressing Figure 5-6

28. Subnet Addressing Example 1 Suppose that the first two bytes are the subnet indicator with addresses of the form 131.156.x.x Then, 131.156.29.156 and 131.156.34.215 would be on the same subnet. The subnet mask would be 255.255.0.0, which corresponds to 11111111.11111111.00000000.00000000, where 1 indicates that the position is part of the subnet address and a 0 indicates that it is not.

29. Subnet Addressing Example 2 Partial bytes can also be used as subnets. For example, consider the subnet mask 255.255.255.128, which is 11111111.11111111.11111111.10000000. Here, all computers with the same first three bytes and last byte from 128 to 254 would be on the same subnet.

30. Providing Addresses Providing addresses to networked computers – Static addressing – Dynamic addressing

31. Static Addressing Each computer is given an address through a configuration file Stored on individual computers Problems – Moves, changes, adds and deletes – Individuals could change their own IP address – Network renumbered Companies do not have a good way of tracking the addresses

32. Dynamic Addressing Server supplies a network layer address automatically – Each time user logs in – For a specific lease period Two standards for dynamic addressing – Bootstrap protocol (bootp) - developed in 1985 – Dynamic host control* protocol (DHCP), developed in 1993 * some say C = configuration

33. Dynamic Addressing Bootp and DHCP – Software installed on the client instructs the client to contact the server using data link layer addresses – Message asks server to assign the client a unique network layer address – Server runs corresponding software that sends the client its network address and subnet mask

34. Leasing Bootp or DHCP server can either: – Assign the same network layer address to the client each time the client requests it (bootp) – Lease the network address from the next available on a list of authorized addresses for as long as the client is connected or for a specified amount of time -- common with isps and dial-up users (DHCP)

35. Address Resolution The process of: – Translating an application layer address to a network address (server name resolution) – Translating the server name address to a data link layer address (data link layer address resolution)

36. Address Resolution Server name resolution – Accomplished by the use of domain name service (DNS) – Computers called name servers provide these DNS services Address data base includes: server names and their corresponding IP address

37. Client computer DNS Server netmgr.cso.niu.edu 131.156.1.11 DNS Request LAN Internet DNS Request Root DNS Server for.EDU domain Weber State University Northern Illinois University DNS Request DNS Response DNS Server sol.acs.unt.edu 137.90.2.122 Figure 5-7

38. Address Resolution Data link layer address resolution – Broadcast message is sent to all computers in its subnet “if your IP address is xxx.Yyy.Zzz.Ttt, please send your data link layer address” Uses address resolution protocol (ARP)

39. Network Routing The process of determining the route a message will take through the network – Centralized – Decentralized Static routing Dynamic routing Broadcast or multicast routing – Connectionless – Connection-oriented routing

40. Route and Route Table Computer B Destination RouteAC DAE FE GC A B C G E D F

41. Internet Routes UEN WSUCanada Other destinations West Coast Europe Asia WSU Destination Route UEN Utah Oxford Europe U of Toronto Canada U of Singapore Asia UC Stanford West Coast Other

42. Types of Routing Centralized routing Static routing (decentralized) Dynamic routing (adaptive and decentralized) – Distance vector – Link state Other types – Broadcast routing – Multicast routing

43. Centralized Routing All routing decisions are made by one computer Main routing for star and mesh topologies Routing tables located on each computer – Central computer sends updated tables as needed – Routing table tells the device where to send messages Simplicity - no wasted resources Hardware failures or changing conditions cause table to be out of sync

44. Decentralized Routing Each of the following types of routing fall under the heading of decentralized routing Each device makes its own routing decisions with the use of a formal routing protocol Routing protocols are self-adjusting – Can automatically adapt to changes in the network configuration Drawbacks – Slows down the network with status messages – Requires more processing by each computer

45. Static Routing Routing table developed by the network manager or some type of committee – Initial table sent to each computer which then updates the routing table as needed – Reroutes as needed with down or removed circuits – Updated when new devices announce their presence – Used in relatively static networks that have few routing options

46. Dynamic Routing (Adaptive) Routing messages over the fastest route – Used when there are multiple routes in the network – Improves network performance by selecting the fastest route to avoid bottlenecks or busy circuits – Initial table developed by network manager – Dynamically updated with changing conditions by the devices themselves – Monitors message transmission time or each device reports how busy it is to avoid bottlenecks Disadvantages – Requires more processing by each computer – “Wastes” network capacity

47. Dynamic Routing (Adaptive) Distance vector dynamic routing – The number of hops along a route – Exchange information with the neighboring computers every few minutes Link state dynamic routing – The number of hops along a route – The speed of the circuits on the route – How busy the route is – Exchanges information with other routing devices every 15-30 minutes – Tries to determine the fastest route – Converges reliable routing information more quickly

48. Routing Protocols RIP, IGP, OSPF, EGP, BGP Distance vector routing protocols (RIP, Appletalk,IPX, IGRP) – Routers inform neighboring routers of table – Closest router is used to route packets Link State routing protocols (OSPF) – Routers have at least a partial map of the network – Changes are flooded throughout network – Routes are recomputed

49. Interior and Exterior Routing Interior routing is within an autonomous system (collection of routers under a single administrative control)—RIP, OSPF Exterior routing occurs between autonomous systems Network access protocols operate at Layer 2. – Transport of IP datagrams – IP over point-to-point connections is used by ISP when you dial in

50. Routing Protocols Border Gateway Protocol Internet Control Message Protocol Routing Information Protocol Open Shortest Path First Enhanced Interior Gateway Routing Protocol

51. Routing Protocols Internet protocols – BGP (border gateway protocol) Exchanges information between autonomous systems about the condition of the internet Complex, hard to administer, exterior routing protocol – ICMP (internet control message protocol) Simple, interior routing protocol used with the internet Reports routing errors but is limited in the ability to update – RIP (routing information protocol) Dynamic distance vector interior routing protocol Counts the number of devices on each route Selects the route with the least number of devices

52. Routing Protocols – OSPF (open shortest path first) Link state interior routing protocol used on the internet Counts number of computers, network traffic, network error rates to select the best route Doesn’t broadcast to all devices just to routing devices Preferred TCP/IP, but also used by IPX/SPX – EIGRP (enhanced interior gateway routing protocol) Link state interior routing protocol developed by CISCO Uses route transmission capacity, delay, reliability and load to select best route Stores multiple routing tables – SAP (service advertisement protocol) Netware servers send SAP advertisements Novell’s broadcast protocol

53. Broadcast Routing Sends the message to all computers on the network Only computer with correct address processes the message Used only in bus networks Wastes network bandwidth

54. Multicasting – Similar to broadcasting – Only works within one LAN or subnet – Messages sent from one computer to another on the network is called a unicast message – Messages sent to a group of computers is called a multicast message – Targeting a specific work group – IGMP (internet group management protocol) Sends an IGMP multicast request to the routing computer Assigned a special class D IP address to identify the group The routing computer sets the data link layer address All participating machines will process messages sent to this address Sends a IGMP message notifying of end of session

55. Thank u