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EEC-484/584 Computer Networks

This lecture covers Address Resolution Protocol (ARP), Dynamic Host Configuration Protocol (DHCP), Ethernet/MAC Addresses, and the use of hubs and switches in computer networks.

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EEC-484/584 Computer Networks

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

  2. Outline Reminder Lab#5: this Wednesday Quiz#4: 12/8 Wed Peer review due 12/6 Monday (EEC584 only) 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. 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) Question: how to determine MAC address of B knowing B’s IP address? 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. ARP Protocol: Same LAN 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 EEC-484/584: Computer Networks

  8. 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 Addressing: Routing to Another LAN 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 EEC-484/584: Computer Networks

  9. 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 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! 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. 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 32-bit Internet address ARP RARP 48-bit Ethernet 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 dynamically obtain 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 agent can 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. 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 twisted pair hub 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 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 A 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 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? A 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. Switch: Self-Learning 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 Source: A Dest: A’ MAC addr interface TTL 60 1 A A A’ A 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. Self-Learning, Forwarding: Example Frame destination unknown: flood Destination A location known: selective send Source: A Dest: A’ A’ A MAC addr interface TTL 60 60 4 1 A A’ A A’ A A’ A A’ A A’ A A’ A A’ A C’ B 1 2 3 6 4 5 C B’ A’ Switch table (initially empty) EEC-484/584: Computer Networks

  25. Interconnecting Switches Switches can be connected together S4 S3 S2 F I D H G E S1 A C B • Q: sending from A to G - how does S1 know to forward frame destined to G 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: Exercise 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

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