Computer Networks

1. Computer Networks Network layer Network layer -- June 2004

2. Design issues Routing Congestion Internetworking Internet Protocols Multimedia or QoS Different networks, protocols? Interconnection styles Internetwork routing Fragmentation Firewalls Network Layer Network layer -- June 2004

3. Internetworking: differences • Different networks will always be around • Installed base is large and growing • Networks get cheaper, so #decision makers  • New technology  new networks  new protocols Network layer -- June 2004

4. Internetworking: differences Network layer -- June 2004

5. Internetworking: differences • Interconnection boxes • Repeaters, hubs • Bridges, switches • Routers • Transport gateways • Application gateways Multifunctional products Network layer -- June 2004

6. Internetworking: styles • Half-[device]s + neutral protocol • Management issue • Cooperation reduced to agreement on protocol Network layer -- June 2004

7. Internetworking: styles • Concatenated virtual circuits • Set-up of a connection • Recognition of remote destination (host, router) and selection of multiprotocol router for first VC • Multiprotocol router extends VC towards … • Data transfer • Same path for all packets • Conversions (packet format, VC numbers,…) in multiprotocol routers • Essential features • Sequence of VCs • Networks should have same/similar properties • Properties: I dentical to single VCs Network layer -- June 2004

8. Internetworking: styles • Concatenated virtual circuits • Set-up of a connection • Recognition of remote destination (host, router) and selection of multiprotocol router for first VC • Multiprotocol router extends VC towards … • Data transfer • Same path for all packets • Conversions (packet format, VC numbers,…) in multiprotocol routers • Essential features • Sequence of VCs • Networks should have same/similar properties • Properties: identical to single VCs Network layer -- June 2004

9. Internetworking: styles • Connectionless internetworking • Datagram approach • Multiple routes • Higher bandwidth • No guarantee for in order delivery • Nearly identical protocols required • Packet conversion • Addressing ( assignment, mapping) • Properties: same as for datagram networks Network layer -- June 2004

10. Internetworking: styles • Connectionless internetworking • Datagram approach • Multiple routes • Higher bandwidth • No guarantee for in order delivery • Nearly identical protocols required • Packet conversion • Addressing ( assignment, mapping) • Properties: same as for datagram networks Network layer -- June 2004

11. Internetworking: styles • Tunneling • Interconnect 2 identical networks using a different one • Behaviour: point-to-point line between multiprotocol routers Network layer -- June 2004

12. Internetworking: routing • Same problem + some complications • 2 levels of routing: • Within a network • Intranetwork routing • Interior gateway protocol • Internetwork routing • Graph construction • Every router can directly access routers on the same network • Packet forwarding + tunneling if necessary • Differences with intranetwork routing • Cross international boundaries adopt national laws • Agreements between operators (transit traffic) • Between networks • Internetwork routing • Exterior gateway protocol Network layer -- June 2004

13. Internetworking: routing • An internetwork: • Router A can communicate with routers B and C Graph of internetwork Network layer -- June 2004

14. Internetworking: fragmentation • Problem: Large packet through network with smaller maximum packet size • Solution: • Break large packet into fragments • Send each fragment as a separate packet • Reassemble: transparent <> non transparent? • Transparent fragmentation • Strategy • Gateway breaks large packet into fragments • Each fragment addressed to same exit gateway • Exit gateway does reassembly Network layer -- June 2004

15. Transparent fragmentation Strategy Gateway breaks large packet into fragments Each fragment addressed to same exit gateway Exit gateway does reassembly Simple, but some problems Gateway must know when it has all pieces Performance loss: all fragments through same gateway Overhead: repeatedly reassemble and refragment Example: ATM segmentation Internetworking: fragmentation Network layer -- June 2004

16. Nontransparent fragmentation Strategy Gateway breaks large packet into fragments Each fragment is forwarded to destination problems Every host must be able to reassembly More headers Example: IP fragmentation Internetworking: fragmentation Network layer -- June 2004

17. Internetworking: fragmentation • Fragment numbering • Hierarchical numbering • Packet 0  packets 0.0, 0.1, 0.2 • Problem: retransmission + different fragmentations • Basic block numbering: in every packet • Original packet number • Sequence number of first block Network layer -- June 2004

18. Internetworking: firewalls • Protection needed against • Information leaking out • Trade secrets, product development plans, … • Information leaking in • Viruses, worms, … • Old medieval analogy • Castle + deep moat around it • Single draw bridge • Example firewall • 2 routers for packet filtering • Application gateway Network layer -- June 2004

19. Packet filtering Acceptable sources + destinations Filters on Address: IP Service port (TCP header) both Application gateway Decisions made per application Header fields, Message size content Internetworking: firewalls Network layer -- June 2004

20. Design issues Routing Congestion Internetworking Internet Protocols Multimedia or QoS IP protocol Internet Control Protocols Routing Internet multicasting Mobile IP IPv6 Network Layer Network layer -- June 2004

21. Internet: IP protocol • View on Internet: • Collection of Autonomous Systems (AS) • Glue: IP designed for internetworking Network layer -- June 2004

22. Internet: IP protocol • IP protocol • best effort service • Datagrams: up to 64 Kbytes • IP header • 20 byte fixed part + optional part • Transmitted in big endian order ( l -> r) Network layer -- June 2004

23. Internet: IP header Network layer -- June 2004

24. Internet: IP header • Options: maximum length = 40 bytes • Too small: 40 bytes -> only 10 IP addresses! Network layer -- June 2004

25. Internet: IP addresses • IP address = 32 bits • Network number • Host number (on network) Network layer -- June 2004

26. Internet: IP addresses • Dotted notation: 134.58.47.25 • Assignment: • ICANN Internet Corporation for Assigned Names and Numbers • Arin (American Registry for Internet Numbers) for N & S America • RIPE (Réseaux IP Européennes)for Europe • APNIC (Asia Pacific Network Information Centre) • Special addresses: Network layer -- June 2004

27. Internet: IP subnets • Subnetting: different views on same network • Internal: network split up in different parts • External: a single net • Why? • Avoid use of different C networks for a single organisation • Allow structuring of class A & B networks Network layer -- June 2004

28. Internet: IP subnets • Subnetting: different views on same network • Internal: network split up in different parts • External: a single net • Why? • Avoid use of different C networks for a single organisation • Allow structuring of class A & B networks • Network <> host? • Subnet mask Network layer -- June 2004

29. (network, 0) : how to get distant network (this-network, host) : … to local host (network, 0) : how to get to distant network (this-network, subnet, 0) : …to host on another subnet (this-network, this-subnet, host) : … to local host Internet: IP subnets Routing: table entries No subnets With subnets • Advantages: • Smaller tables • Management of networks easier (not easy!) Network layer -- June 2004

30. Internet: CIDR • Exponential growth of Internet • Running out of addresses • B is too large <> C is too small • Assign many Cs iso a single B • Routing table explosion • Hierarchical routing • No support in IP addressing scheme • CIDR solution: • Allocate blocks of class C addresses • Introduce hierarchy for remaining addresses • Classless routing CIDR: classless InterDomain Routing Network layer -- June 2004

31. Internet: CIDR • CIDR solution: • Allocate blocks of class C addresses • Variable size described by • IP address • Mask indicating meaningful bits in address • Allocation scheme: block of X addresses starts on X-byte boundary • 2048 addresses (8 C classes): 194.24.0.0 to 194.24.7.255 • 4096 addresses (16 C classes): 194.24.16.0 to 194.24.31.255 • Introduce hierarchy for remaining addresses • Classless routing Network layer -- June 2004

32. Internet: CIDR • 3 blocks assigned: • Entries in router tables: • Route 194.24.17.4? or … 0001 0001 0000 0100 • Test: address & mask Network layer -- June 2004

33. Internet: NAT • Network Address translation • Simple solution to the shortage of IP addresses • Examples? • Technique: • non routable addresses inside a domain • Translate address to a routable one when packet leaves domain Reply packets? Network layer -- June 2004

34. Internet: NAT • Use TCP/UDP port number to differentiate between different local computer systems • NAT translation table (local IP address, source port)  (external IP address, unique port) • 1 IP address can be used for up to ~ 64K hosts Network layer -- June 2004

35. Internet: NAT • Objections to NAT • Violates architectural model of IP • IP address uniquely identifies a single computer • Crash of NAT box  all “connections” lost • Connection oriented flavor • Violates fundamental rule of protocol layers • Only works for TCP & UDP • Addresses inside body are not translated • Ugly and temporary hack delays real solution: IPv6 Network layer -- June 2004

36. Design issues Routing Congestion Internetworking Internet Protocols Multimedia or QoS IP protocol Internet Control Protocols ICMP ARP RARP Routing Internet multicasting Mobile IP IPv6 Network Layer Network layer -- June 2004

37. Internet: ICMP • ICMP: Internet Control Message Protocol • Used by routers to report unexpected events • Definition: RFC 792 Network layer -- June 2004

38. Internet: ARP • ARP: address resolution protocol • How do IP addresses get mapped onto data link layer addresses? • Problem: • Solution: configuration filesunsatisfactory Network layer -- June 2004

39. Internet: ARP • Basic ARP protocol • Broadcast: who owns IP address 192.31.65.5? • Host with that IP address should reply with its data link address • Optimisations: cache mappings! • Values in ARP request (every system on net) • Values in ARP reply (sender of ARP request only) • Gratuitous ARP: upon boot host can send ARP request with its own mappinganswer: duplicate IP address in use! Network layer -- June 2004

40. Internet: ARP • How to handle remote hosts? • Proxy ARP: routers serving the net should reply • Sending hostforwards packet to router Network layer -- June 2004

41. Internet: RARP • RARP: Reverse Address Resolution Protocol • Problem: • Given a data link address • What is the corresponding IP address • Why needed? • Allows a newly booted workstations to get its IP address • Solutions: • RARP protocol + RARP server!! • IP address embedded in OS image (different image for every WS) • BOOTP protocol • Limitation of RARP: server needed on each net as broadcast is not forwarded Network layer -- June 2004

42. Internet: BOOTP • Bootstrap protocol • Uses UDP messages • Broadcast to port 67 • Forwarded over routers • Gives additional information • IP address of file server holding the OS • IP address of default router • Subnet mask to use Network layer -- June 2004

43. Internet: DHCP • DHCP: Dynamic Host Configuration Protocol • Special server + relay agents • Static + dynamic assignment of IP addresses (leasing) • Newly booted machine: broadcasts a DHCP Discover packet Network layer -- June 2004

44. Design issues Routing Congestion Internetworking Internet Protocols Multimedia or QoS IP protocol Internet Control Protocols Routing OSPF BGP Internet multicasting Mobile IP IPv6 Network Layer Network layer -- June 2004

45. Internet: routing • History: • First protocol: RIP (distance vector) • Since 1979: replaced by link state • In 1990 new standard: OSPF • Protocols: • Interior gateway protocol: OSPF – Open Shortest Path First • Exterior gateway protocol: BGP – Border gateway protocol Network layer -- June 2004

46. Internet routing: OSPF • Requirements for OSPF: • Algorithm in open literature • Support for various distance metrics • Dynamic algorithm • Support for routing based on type of service • Do load balancing over multiple lines • Support for hierarchical systems • Security to prevent false updates • Support for routers connected through tunnel • OSPF supports as connections and networks • Point-to-point lines between routers • Multi access networks with broadcasts • (multi access) networks without broadcasts Network layer -- June 2004

47. Internet routing: OSPF • Abstract view on network • Directed graph • Node for each router + network • Arcs: • 2 arcs for each point-to-point line • 2 arcs for each network node to the routers connected to the network • Example Network layer -- June 2004

48. Internet routing: OSPF • Abstract view on network • Directed graph • Node for each router + network • Arcs: • 2 arcs for each point-to-point line • 2 arcs for each network node to the routers connected to the network Network layer -- June 2004

49. Internet routing: OSPF • Network divided into areas • Areas do not overlap • Area = set of contiguous networks • Topology of area not visible outside area • Backbone area • Interconnects areas • Router connected to at least 2 areas is part of backbone • Classes of routers • Internal routers (within an area) • Area border routers (interconnect areas) • Backbone routers • AS boundary router • 3 kinds of routes • Intra-area: shortest path • Interarea: from source to backbone to destination • Inter AS Provisions for very large networks Network layer -- June 2004

50. Internet routing: OSPF Provisions for very large networks Network layer -- June 2004