Computer Networks

1. Computer Networks Network Layer

2. Where are we?

3. Will Layer 2 Networking Suffice?

4. Motivation • Connect various link technologies to form a larger internetwork • Universal addressing scheme required • General purpose use • Hides underlying technologies from end user • Facilitate communicate between autonomous domains • Able to move packets between any host on the internetwork

5. Connecting Heterogeneous Networks • Computer System used • Special purpose • Dedicated • Works with LAN or WAN technologies • Known as • router • gateway

6. Illustration of a Router • Cloud denotes an arbitrary network • One interface per network

7. Important Idea A router can interconnect networks that use different technologies, including different media and media access techniques, physical addressing schemes or frame formats.

8. The Internet Concept

9. Key Functions of the Network Layer • Global Addressing • Fragmentation • Routing We’ll be primarily concerned with addressing and routing

10. Example Network Layer: Internet Protocol (IP) • Standardized by IETF as RFC 791 • Most popular Layer 3 protocol • Core protocol used on the public Internet • Connectionless protocol • datagrams contain identity of the destination • each datagram sent/handled independently • Of utmost importance for this class!

11. IP Addressing • Provides an abstraction • Independent of hardware (MAC) addressing • Used by • higher layer protocols • Applications Good IP addressing tutorial: http://www.3com.com/nsc/501302.html

12. IP Address • Virtual • only understood by software • Used for all communication across an internetwork • 32-bit integer • Unique value for each host/interface

13. IP Address Assignment An IP address does not identify a specific computer. Instead, each IP address identifies a connection between a computer and a network. A computer with multiple network connections (e.g., a router) must be assigned one IP address for each connection.

14. IP Address Details • Divided into two parts • prefix identifies the network • suffix identifies the host/interface • Global authority assigns unique prefix for the network • Local administrator assigns unique suffix for the host/interface

15. Class of IP Addresses (Historical) • Initial bits determined the class • The class determines the boundary between prefix and suffix

16. Dotted Decimal Notation • Shorthand for IP addresses • Allows humans to avoid binary • Represents each octet in decimal separated by dots • NOT the same as names like www.depaul.edu

17. Examples of Dotted Decimal Notation • Four decimal values per 32-bit address • Each decimal number • represents eight bits • is between 0 and 255 inclusive

18. Class Hierarchy and Network Size (Historical) • Maximum size determined by class of address • Class A large • Class B medium • Class C small

19. Addressing Example

20. Illustration of Router Addresses • Address prefix identifies the network • Need one address per router connection

21. Special Addresses • Network Address not used in packets • Loopback addresses never leave the local computer

22. Getting IP Addresses • IANA has global authority for allocation • Regional registries: ARIN, RIPE, APNIC • RFC 1918 defines private address space • NOT globally unique • 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 • End users obtain address space from their Internet Service Provider (ISP)

23. IP Addressing: Problems with Classes • Internet growth • Routing table size • Exhaustion of addresses • Administration overhead • Misappropriation of addresses

24. IP Addressing: Solutions • Subnetting • Supernetting • Classless InterDomain Routing (CIDR) • Variable Length Subnet Mask (VLSM)

25. Subnetting • Split the suffix into a local network portion and a smaller host id portion

26. Subnet Masks • Cannot determine prefix on first few bits • Need a 'bit mask' that specifies prefix/suffix • Dotted decimal notation used, but... • I told you binary was important! • Examples: • Network: 140.192.9.0 Subnet mask: 255.255.255.0 • Network: 140.192.9.0 Subnet mask: 255.255.255.128 • Network and mask: 140.192.9.0/24

27. More Subnet Examples • 63.85.18.5/22 - What is the network? • 32.152.6.1/26 - How many hosts possible? • 219.52.33.8/20 - What is the directed broadcast address? • How might you allocate a 10.5.0.0/16 block for an organization with 4 offices of 500 users each?

28. Supernetting • Combine multiple smaller address classes into a larger block • Class B was too big • Class C was too small • Combine contiguous Class C addresses • e.g. 199.242.64.0 to 199.242.67.255

29. Classless InterDomain Routing (CIDR) • Employ supernetting style information in IP routers • Advertise smaller CIDR blocks • Decreases the routing table size Advertise 199.242.64.0/22 instead of 199.242.64.0,199.242.65.0, 199.242.66.0 and 199.242.67.0 The CIDR Report: http://www.employees.org/~tbates/cidr-report.html

30. Variable Length Subnet Masks (VLSM) • Ability to use multiple subnet sizes in a single autonomous system • Allows more efficient use of addresses • Routers must support subnets masks • e.g. RIPv1 did not support this! • For example: • May use /24 in most places, but may have a small office with only 10 users! May want to use a /28 for that network.

31. IP Packet (datagram) Format

32. IP Datagrams • Can be delayed • Duplicated • Delivered out of order • Lost • Can change routes from packet to packet • Are connectionless

33. Address Resolution Protocol (ARP) • Resolves IP address to Layer 2 (MAC) address • Node sends MAC broadcast looking for another node • IP src: 140.192.23.1 MAC src: 0x00:80:05:1A:F0 • IP dst: 140.192.23.23 MAC dst: 0xFF:FF:FF:FF:FF • Node with that IP dst address replies with its MAC • 140.192.23.23 replies with 0x00:60:0A:34:AA:3C • ARP Table: contains records of learned relationships.

34. Dynamic Host Configuration Protocol (DHCP) • Standardized in RFC 1531 • Allows hosts to obtain IP address information upon startup from a server • Eliminates cumbersome manual configuration • Grants IP addresses based on a predefined "lease" period

35. IP Routing • Performed by routers • Table-driven • Forwarding on a hop-by-hop basis • Destination address used for route determination

36. Example IP Routing Table • Table (b) is for center router in (a)

37. Routing Table Size Since each destination in a routing table corresponds to a network, the number of entries in a routing table is proportional to the number of networks in the internetwork. Caveat: you can use a "default" route to forward to when route is unknown or when no route specific information is available.

38. Routing/Forwarding Overview • Given a datagram • Extract destination address field, D • Look up D in the routing table • Find next hop address, N • Send datagram to N

39. Key Concept The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the address of the next hop does not appear in the datagram header.

40. Routing/Forwarding Overview • Strip off layer 2 information • Extract destination IP address field • Look up IP address in the routing table • Find next hop address to forward to • Send datagram to the next hop • Add on necessary layer 2 information

41. Routing Protocol Requirements • Efficient routing table size • Efficient routing control messages • Robustness and reliability • prevent loops • avoid black holes • reconvergence time is short

42. Source of Route Table Information • Manual • Table created by hand • Useful in small networks • Useful if routes never change • Automatic • software creates/updates tables • Needed in large networks • Changes routes when failures occur

43. Compute Shortest/Best Path • Possible metric • geographic distance • economic cost • capacity

44. Algorithms for Computing Shortest Path • Distance Vector • Exchange routing tables with neighboring routers • e.g., RIP, RIPv2 • Link State • Routers exchange link status information • e.g., OSPF, IS-IS

45. Distance Vector • Routers periodically advertise and learn about IP networks • Cost of the route is based on hops to the network (number of routers to pass) • Recalculation occurs when links fail

46. Count to Infinity Problem • What happens when link 1<->5 goes down? • Does 5 think it can get to 1 through 2?

47. Solving the Count to Infinity Problem • Hold down • Wait for a period of time before switching paths. Advertise route cost as infinity. Based on timers. • Report the entire path • Guarantees no loops, but expensive. • Split horizon • Do not advertise routes to neighbors if the route was received from that neighbor. Not foolproof.

48. Other Distance Vector Improvements • Triggered updates • Advertise changes as soon as you learn of them. May help convergence time. May create routing instability for flapping routes. • Poison reverse • Used with split horizon. Report infinity rather than nothing at all. • Diffusing Update ALgorithm (DUAL) • Somewhat like hold down, but routers are alerted of broken paths. Complex. Not popular.

49. Example Distance Vector Protocol: RIP • Standardized in RFC 1058 and 2453 • An interior gateway protocol (IGP) • Simple • RIPv2 includes subnet mask in updates • Hop count based (> 15 = unreachable) • Widely used in small to medium sized organizations

50. Link State • Routers distribute link cost and topology information to all other routers in its area. • All routers have complete information about the network. • Each router computes its own optimal path to destinations. • Ensures loop free environments.