1. CS470 Computer Networking Protocols Huiping Guo Department of Computer Science California State University, Los Angeles 4. Internetworking

2. 4. Internetworking CS470_W12 Outline r Network technologies r Motivation of Internetworking r Internet and routers r IP addressing r ARP 4-2

3. Network technologies r Ethernet m Twisted-pair Ethernet: 10Base-T m Fast Ethernet: 100Base-TX, 100Base-FX m Gigabit Ethernet: 1000baseX, 1000baseT r FDDI m Ring based token network m Run on fiber m FDDI has since been effectively obsolesced by fast Ethernet 4. Internetworking CS470_W12 4-3

4. Network technologies r PPP (point-to-point protocol) m A data link protocol commonly used in establishing a direct connection between two networking nodes m For both router-router leased line connection and the dial-up host-router connection. m Common leased line services DS1(T1), DS3(T3) Over SONET: OC1, OC12, OC24, OC36, OC48 4. Internetworking CS470_W12 4-4

5. 4. Internetworking CS470_W12 Motivation of internetworking r Why internetworking m There are different network technologies Ethernet, Token ring, ATM, Wireless LAN, … m Connect multiple physical networks into a large, uniform communication system r Challenges m Heterogeneity m Scalability 4-5

6. 4. Internetworking CS470_W12 Challenges of internetworking r Heterogeneity m Users on one type of network should be able to communicate with users on other types of networks m Establishing connectivity between hosts on two different networks may require traversing several other different networks m Each network has its own addressing scheme, media access control protocols, service model,… 4-6

7. 4. Internetworking CS470_W12 Challenges of internetworking r Scalability m The Internet has roughly doubled in size each year for 20 years m Routing How to find an efficient path through a network with millions/billions of nodes m Addressing How to provide suitable identifiers for all nodes 4-7

8. 4. Internetworking CS470_W12 Bridges and switches r Can bridges and switches be used to interconnect different networks? m Bridges and switches only know frames m Different networks use incompatible frame format m A frame created for one network technology cannot be transmitted on a network that uses a different technology 4-8

9. 4. Internetworking CS470_W12 Interconnect heterogeneous networks r Hardware m Routers: interconnect physical networks, also called gateways r Software m IP protocol m Provides universal service m Universal service A user on any computer in any networks can talk to any other user 4-9

10. 4. Internetworking CS470_W12 What is an internetwork r Network m Directly connected or switched network m Such a network uses one technology, such as Ethernet, ATM, Token ring m Also called physical network r Internetwork m A logical network built on a collection of physical networks m Routers are the nodes that connect the physical networks 4-10

11. 4. Internetworking CS470_W12 A simple internetwork FDDI Token Ring H5 H4 H6 H3H2H1 H8H7 R2 R1 R3 Ethernet Point-to-Point Link (e.g., ISDN) Ethernet router host 4-11

12. 4. Internetworking CS470_W12 Software r Achieve universal service m Software is needed on computers and routers r Complexity of the software m Deal with different frame format m Routing how to find an efficient (loop free) path through a network with millions/billions of nodes m Addressing how to provide suitable identifiers for all the nodes Does the MAC address work? 4-12

13. 4. Internetworking CS470_W12 MAC addresses don’t work r MAC addresses m Flat structure m Not efficient to forward data based on MAC addresses m Take space to store m Different in different networks r Hierarchical addresses preferred m An address is divided into two parts Part 1: identify the network Part2: identify the computer m Make forwarding efficient 4-13

14. 4. Internetworking CS470_W12 IP addresses r Specified by Internet Protocol r When we address a datagram m Sending host puts destination internet address in datagram m Destination address can be interpreted by any intermediate router m Routers examine address and forward datagram on to the destination r All of these addresses are virtual m they are defined in software, not hardware 4-14

15. 4. Internetworking CS470_W12 IP Addresses r Each host is assigned a 32-bit number m Unique across entire Internet m Dotted decimal notation: 123.144.2.23 r Each IP address is divided into a prefix and a suffix m Prefix identifies network to which computer is attached No two networks can be assigned the same network number m Suffix identifies computer within that network No two computers on the same network can have the same suffix, but computers on different networks can have the same suffix m Address format makes routing efficient 4-15

16. 4. Internetworking CS470_W12 IP addresses and MAC Addresses r 32-bit IP address: m network-layer address m used to get datagram to destination network r MAC (or LAN or physical or Ethernet) address: m used to get frame from one interface to another physically- connected interface (same network) 4-16

17. 4. Internetworking CS470_W12 Resolving Addresses r Hardware only recognizes MAC addresses r IP only uses IP addresses r Consequence: software needed to perform translation m Known as address resolution 4-17

18. 4. Internetworking CS470_W12 Address Resolution Protocol r Layer 2 protocol r Given m A locally-connected network, N m IP address C of computer on N r Find m Hardware address for C r Technique m Address Resolution Protocol 4-18

19. 4. Internetworking CS470_W12 ARP is local to a network r One computer can resolve the address of another computer only if both computers attach to the same network r A computer never resolves the address of a computer on a remote network! 4-19

20. 4. Internetworking CS470_W12 ARP is local to a network (cont.) r A sends data to B m A uses B’s IP address m A resolves B’s IP address to B’s hardware address m A sends the frame to B directly 4-20

21. 4. Internetworking CS470_W12 ARP is local to a network (cont.) r A sends data to F m A uses F’s IP address m A determines that the data must travel through R1 m A resolves R1’s hardware address m A sends the frame to R1 directly m It’s the responsibility of R1 to send the frame to F 4-21

22. 4. Internetworking CS470_W12 Address Resolution Protocol (ARP) r Keep mappings in table r Table entry contains pair of addresses for one computer m IP address m Hardware address r Build table automatically as needed 4-22

23. 4. Internetworking CS470_W12 ARP Table Lookup r Use a simple list containing IP address and hardware address for each host on net r Search on IP address and extract corresponding hardware address Note that all IP addresses have same prefix; can save space by dropping prefix 4-23

24. 4. Internetworking CS470_W12 r Sequential search may be prohibitively expensive (O(n)) r Can use indexing or hashing for O(1) lookup m Indexing - use hostid part of IP address as list (array) index m Hashing - use hashing function on hostid to generate list index ARP Table Lookup 4-24

25. 4. Internetworking CS470_W12 ARP on your computer r Open a command window m Start  Run  cmd r Display the arp table on your compuer m Under MS-DOS, type arp –a r Clear the arp table m Under MS-DOS, type arp –d * 4-25

26. 4. Internetworking CS470_W12 ARP Table r How to get the table in the first place? r Address resolution with message exchange m Each computer on the network participates in address resolution by agreeing to answer the resolution requests for its address m A computer that needs to resolve an address sends a broadcast request on the network. m All machines receive the request and examine the requested address m If the request matches a computer’s address, the computer responds 4-26

27. 4. Internetworking CS470_W12 ARP Message Exchange Example W sends a broadcast message asking for Y’s MAC address Everyone hears the message Only Y responds with a Unicast message (still broadcast on bus) 4-27

28. 4. Internetworking CS470_W12 ARP protocol example 1 (in a LAN) 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN 237.196.7.23 237.196.7.78 237.196.7.14 237.196.7.88 r A wants to send datagram to B r B’s MAC address not in A’s ARP table. A B 4-28

29. 4. Internetworking CS470_W12 ARP protocol example 1 (in a LAN) r A broadcasts ARP query datagram, containing B's IP address m Dest MAC address = FF-FF-FF-FF-FF-FF m all machines on LAN receive ARP query r B receives ARP datagram, replies to A with its (B's) MAC address m frame sent to A’s MAC address (unicast) r A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) r ARP is “plug-and-play”: m nodes create their ARP tables without intervention from net administrator 4-29

30. 4. Internetworking CS470_W12 ARP protocol example 2 (to another LAN) A R B A sends a datagram from A to B via R, assume A knows B’s IP address 4-30

31. 4. Internetworking CS470_W12 ARP protocol example 2 (to another LAN) r Two types of nodes m Host: one adapter, IP address and one ARP table m Router: one adapter, one IP Address and one ARP table for each interface r Router has m Two interfaces m Two IP addresses m Two adapters m Two ARP tables 4-31

32. 4. Internetworking CS470_W12 ARP protocol example 2 (to another LAN) r A creates datagram with source A, destination B r A uses ARP to get R’s MAC address for 111.111.111.110 r A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram r A’s adapter sends frame r R’s adapter receives frame r R removes IP datagram from Ethernet frame, sees its destined to B r R uses ARP to get B’s MAC address r R creates frame containing A-to-B IP datagram sends to B 4-32