EEC-484/584 Computer Networks

1. EEC-484/584Computer Networks Lecture 14 Wenbing Zhao wenbingz@gmail.com

2. Outline • Reminder • Lab#5: this Wednesday (4/28) • Quiz#4 moved to 5/12 (Wednesday) • Project due today midnight! • ARP and DHCP • Hubs and switches EEC-484/584: Computer Networks

3. Ethernet/MAC Addresses • Ethernet (or MAC or LAN or physical) address: • Function:get frame from one interface to another physically-connected interface (same network) • 48 bit MAC address • Burned in NIC ROM, also sometimes software settable EEC-484/584: Computer Networks

4. MAC Addresses Each adapter on Ethernet has unique MAC address Broadcast address = FF-FF-FF-FF-FF-FF 1A-2F-BB-76-09-AD LAN (wired or wireless) = adapter 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 EEC-484/584: Computer Networks

5. MAC Addresses • MAC address allocation administered by IEEE • Manufacturer buys portion of MAC address space (to assure uniqueness) • 32-bit IP address: • network-layer address • used to get datagram to destination IP subnet • MAC flat address ➜ portability • Can move LAN card from one LAN to another • IP hierarchical address NOT portable • Address depends on IP subnet to which node is attached EEC-484/584: Computer Networks

6. Question: how to determine MAC address of B knowing B’s IP address? ARP: Address Resolution Protocol • Each IP node (host, router) on LAN has ARPtable • ARP table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23 137.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 137.196.7.88 EEC-484/584: Computer Networks

7. A wants to send datagram to B, and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address Dest MAC address = FF-FF-FF-FF-FF-FF All machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address Frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) Soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: Nodes create their ARP tables without intervention from net administrator ARP Protocol: Same LAN EEC-484/584: Computer Networks

8. 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 E6-E9-00-17-BB-4B 222.222.222.221 1A-23-F9-CD-06-9B 111.111.111.111 222.222.222.222 222.222.222.220 111.111.111.110 R 111.111.111.112 49-BD-D2-C7-56-2A CC-49-DE-D0-AB-7D B A Addressing: Routing to Another LAN Walkthrough: send datagram from A to B via R Assume A knows B’s IP address • Two ARP tables in router R, one for each IP network EEC-484/584: Computer Networks

9. 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 E6-E9-00-17-BB-4B 222.222.222.221 1A-23-F9-CD-06-9B 111.111.111.111 222.222.222.222 222.222.222.220 B A 111.111.111.110 R 111.111.111.112 49-BD-D2-C7-56-2A CC-49-DE-D0-AB-7D This is a really important example – make sure you understand! • A creates IP datagram with source A, destination B • A uses ARP to get R’s MAC address for 111.111.111.110 • A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram • A’s NIC sends frame • R’s NIC receives frame • R removes IP datagram from Ethernet frame, sees its destined to B • R uses ARP to get B’s MAC address • R creates frame containing A-to-B IP datagram sends to B EEC-484/584: Computer Networks

10. ARP – Exercise • Node 1 wants to send a packet to node 4, what will be returned by ARP? • Node 1 wants to send a packet to node 2, what will be returned by ARP? EEC-484/584: Computer Networks

11. 32-bit Internet address ARP RARP 48-bit Ethernet address RARP –Reverse Address Resolution Protocol • RARP - Allows a newly-booted diskless-workstation (e.g., X terminal) to broadcast its Ethernet address and ask for its IP address • RARP server responds to a RARP request with the assigned IP address EEC-484/584: Computer Networks

12. Limitations of RARP • RARP uses a link-layer broadcast, RARP requests are not forwarded by routers, therefore, an RARP server must be present on every network • The only thing returned by the RARP server is the IP address EEC-484/584: Computer Networks

13. BOOTP – Bootstrap Protocol • BOOTP – uses UDP • A client broadcasts to 255.255.255.255 • The source IP address is set to 0.0.0.0 if client does not know its own IP address yet • Port number: 67 for server, 68 for client • BOOTP drawbacks • Requires manual configuration of tables mapping IP address to Ethernet address at the BOOTP server • Replaced by DHCP EEC-484/584: Computer Networks

14. Dynamic Host Configuration Protocol • Allow host to dynamicallyobtain its IP address from network server when it joins network • IP address assignment is lease-based (to cope with client failure, also enables reuse of addresses) • Can renew its lease on address in use • DHCP overview (UDP is used for communication) • Host broadcasts “DHCP discover” msg • DHCP server responds with “DHCP offer” msg • Host requests IP address: “DHCP request” msg • DHCP server sends address: “DHCP ack” msg EEC-484/584: Computer Networks

15. DHCP Replay • A DHCP relay agentcan be configured on each LAN • The agent stores the IP address of the DHCP server and forward the request to the server EEC-484/584: Computer Networks

16. DHCP with Replay Agent • To find its IP address, a newly-booted machine broadcasts a DHCP Discover packet • The DHCP relay agent on its LAN receives all DHCP broadcasts • On receiving a DHCP Discover packet, the agent sends the packet as a unicast packet to the DHCP server, possibly on a distant network EEC-484/584: Computer Networks

17. Link Layer Devices • Hubs • Switches EEC-484/584: Computer Networks

18. twisted pair hub Hubs … physical-layer (“dumb”) repeaters: • Bits coming in one link go out all other links at same rate • All nodes connected to hub can collide with one another • No frame buffering • No CSMA/CD at hub: host NICs detect collisions EEC-484/584: Computer Networks

19. Switch • Link-layer device: smarter than hubs, take active role • Store, forward Ethernet frames • Examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment • Transparent • Hosts are unaware of presence of switches • Plug-and-play, self-learning • Switches do not need to be configured EEC-484/584: Computer Networks

20. Switch: Allows Multiple Simultaneous Transmissions A • Hosts have dedicated, direct connection to switch • Switches buffer packets • Ethernet protocol used on each incoming link, but no collisions; full duplex • Each link is its own collision domain • Switching:a-to-a’ and b-to-b’ simultaneously, without collisions • Not possible with dumb hub C’ B 1 2 3 6 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6) EEC-484/584: Computer Networks

21. Switch Table A • Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? • A:each switch has a switchtable,each entry: • (MAC address of host, interface to reach host, time stamp) • Looks like a routing table! • Q: how are entries created, maintained in switch table? • Something like a routing protocol? C’ B 1 2 3 6 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6) EEC-484/584: Computer Networks

22. Source: A Dest: A’ MAC addr interface TTL 60 1 A A A’ Switch: Self-Learning A • Switchlearns which hosts can be reached through which interfaces • When frame received, switch “learns” location of sender: incoming LAN segment • Records sender/location pair in switch table C’ B 1 2 3 6 4 5 C B’ A’ Switch table (initially empty) EEC-484/584: Computer Networks

23. Switch: Frame Filtering/Forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destinationthen { if dest on segment from which frame arrivedthen drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived EEC-484/584: Computer Networks

24. Source: A Dest: A’ A’ A MAC addr interface TTL 60 60 1 4 A’ A A A’ A A’ A A’ A A’ A A’ A A’ Self-Learning, Forwarding: Example A • Frame destination unknown: flood • Destination A location known: selective send C’ B 1 2 3 6 4 5 C B’ A’ Switch table (initially empty) EEC-484/584: Computer Networks

25. S4 S3 S2 F I D H G E Interconnecting Switches • Switches can be connected together S1 A C B • Q: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3? • A: self learning! (works exactly the same as in single-switch case!) EEC-484/584: Computer Networks

26. Self-Learning Multi-Switch Example Suppose C sends frame to I, I responds to C S4 1 S1 2 S3 S2 A F I D C B H G E • Q: show switch tables and packet forwarding in S1, S2, S3, S4 EEC-484/584: Computer Networks

27. Institutional Network EEC-484/584: Computer Networks

28. Switches vs. Routers • Both store-and-forward devices • Routers: network layer devices (examine network layer headers) • Switches are link layer devices • Routers maintain routing tables, implement routing algorithms • Switches maintain switch tables, implement filtering, learning algorithms EEC-484/584: Computer Networks