Week16 lec1
- 1. Chapter 5 Link Layer Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007.
- 2. Last Lecture Link Layer Switches
- 3. Todays’ Lecture Address Resolution Protocol (ARP)
- 4. Switch Link-layer Device Store, forward Ethernet frames Examine incoming frame’s MAC address Selectively forward frame to the outgoing link Switch itself is transparent to the nodes Nodes are unaware of presence of switches Plug-and-Play Switches do not need to be configured Two important functions: Filtering • Determines whether a frame should be forwarded to some interface or should be dropped. Forwarding • Determine the interface to which a frame should be directed
- 5. Switch Table Q: How does switch know that A’ reachable via interface 4, B’ reachable via interface 5? A C’ B A: Each switch has a switch table, each entry: MAC address of a host Interface that leads towards the host Time at which the entry for the node as placed in the table How is the table created? 6 1 5 2 3 4 C B’ A’ switch with six interfaces (1,2,3,4,5,6)
- 6. Switch: self-learning switch learns which hosts can be reached through which interfaces Source: A Dest: A’ A A A’ C’ when frame received, switch “learns” location of sender: incoming LAN segment records sender/location pair in switch table B 1 6 5 2 3 4 C B’ A’ MAC addr interface TTL A 1 60 Switch table (initially empty)
- 7. Self-Learning,Forwarding:Example Source: A Dest: A’ A A A’ C’ B frame destination unknown: flood 6 A A’ 1 2 4 5 destination A location known: selective send C A’ A B’ 3 A’ MAC addr interface TTL A A’ 1 4 60 60 Switch table (initially empty)
- 8. Interconnecting Switches switches can be connected together S4 S1 S2 A B S3 C F D E I G H 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!)
- 9. Switch: Advantages Elimination of Collisions: Switches buffer frames and never transmit more than one frame on any segment at one time Modern switches are Full Duplex Heterogeneous Links Switch isolates one link from another Different links can operate at different speeds and over different media Difference between Hubs and Switches? Difference between Switches and Routers? (Home Assignment)
- 10. MAC Addresses 32-bit IP address: network-layer address used to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address: A node also has a link layer address • Nodes adapter 6 Bytes (48 bit) MAC address (for most LANs) • Expressed in Hexadecimal notation No two adapters have the same MAC address IEEE manages the MAC address space MAC address does not change no matter where the adapter goes MAC address: like NIC number IP address: like postal address
- 11. MAC Addresses Each adapter on LAN has unique LAN address 1A-2F-BB-76-09-AD 71-65-F7-2B-08-53 LAN (wired or wireless) Broadcast address = FF-FF-FF-FF-FF-FF = adapter 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98
- 12. ARP: Address Resolution Protocol How does the sending node A with IP 222.222.222.220 determine the MAC address for the destination node B with IP address say 222.222.222.222? Job of Address Resolution Protocol (ARP) ARP module in the sending node • Takes IP address on the same LAN as input • Return corresponding MAC address Each node has an ARP table Contains mapping of IP addresses to MAC addresses The ARP table contains a time-to-live (TTL) value • Indicates when each mapping will be deleted from the table • Typical expiration time is 20 minutes
- 13. ARP: Same LAN (network) 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 Destination MAC address = FF-FF-FF-FFFF-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) ARP is “plug-and-play”: nodes create their ARP tables without intervention from network administrator
- 14. Addressing: Routing to another LAN Send datagram from A to B via Router (R) assume A knows B’s IP address 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 A 111.111.111.111 E6-E9-00-17-BB-4B 1A-23-F9-CD-06-9B 222.222.222.220 111.111.111.110 111.111.111.112 R 222.222.222.221 222.222.222.222 B 49-BD-D2-C7-56-2A CC-49-DE-D0-AB-7D Router has two interfaces, two IP addresses, two ARP modules and two adapters
- 15. A creates IP datagram with destination IP of B A uses ARP to get MAC address of R (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 adapter sends frame R’s adapter receives frame R removes IP datagram from frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram and sends to B 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 A E6-E9-00-17-BB-4B 111.111.111.111 222.222.222.220 111.111.111.110 111.111.111.112 CC-49-DE-D0-AB-7D 222.222.222.221 1A-23-F9-CD-06-9B R 222.222.222.222 B 49-BD-D2-C7-56-2A
- 16. Announcements Lab Final on Wednesday, 1st January 2014 Every one has to be present at 2pm Objective paper: 25 min at 2pm Hands-on Exam: 45 min For Hands-on Exam: Class divided into two groups On the Spot Marking of Hands-on Exam
- 17. Announcements Research Presentations/Demos Thursday 2nd January, 2014 • 12pm-1pm and 3pm-5pm Friday 3rd January, 2014 • 11am-1pm and 2pm- 4pm Bring hardcopy of your allocated Paper and report Viva/Report for programming project Paper Showing on 15th January, 2014 after exam of Software Engineering at 1pm. Venue will be notified via email