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Ethernet Protocols in Computer Networks

Explore Manchester encoding, Ethernet frame structure, MAC sublayer protocol, CSMA with collision detection, backoff algorithms, switched Ethernet, ARP, DHCP, and more. Learn how Ethernet functions and optimizes network performance.

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Ethernet Protocols in Computer Networks

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  1. EEC-484/584Computer Networks Lecture 14 Wenbing Zhao wenbing@ieee.org

  2. Outline • Reminder • Lab#5: Wednesday • Wiki project initial draft due • Manchester Encoding • The Ethernet MAC Sublayer Protocol • The Binary Exponential Backoff Algorithm • Switched Ethernet • ARP and DHCP EEC-484/584: Computer Networks

  3. Manchester Encoding • Binary encoding • Hard to distinguish 0 bit (0-volt) from idle (0-volt) • Requires clocks of all stations synchronized • Manchester encoding and differential Manchester encoding EEC-484/584: Computer Networks

  4. Ethernet Frame Structure • Preamble: for clock synchronization • First 7 bytes with pattern 10101010, last byte with pattern 10101011 • The two consecutive 1’s indicate the start of a frame • How can the receiver tell the end of the frame? • No current on the wire (interesting discussion at http://www.tomshardware.com/forum/19951-42-detecting-length-ethernet-frame) Not considered as part of the header! >= 64 bytes EEC-484/584: Computer Networks

  5. Ethernet Frame Structure • Destination address: 6 bytes (48 bits) • Highest order bit: 0 individual, 1 multicast; all 1’s broadcast • Frames received with non-matching destination address is discarded • Type/Length: type of network layer protocol (or length of payload) • Pad– used to produce valid frame >= 64 bytes • Checksum– 32-bit cyclic redundancy check EEC-484/584: Computer Networks

  6. Ethernet MAC Sublayer Protocol • Uses 1-persistent CSMA/CD • Binary exponential backoff • Provides unreliable connectionless service EEC-484/584: Computer Networks

  7. CSMA with Collision Detection • If two stations start transmitting simultaneously, both detect collision and stop transmitting • Monitor collision while sending • Minimum time to detect collision => minimum frame length EEC-484/584: Computer Networks

  8. Minimum Time to Detect Collision • To ensure the sender can detect collision • All frames must take more than 2t to send so that transmission is still taking place when the noise burst gets back to the sender EEC-484/584: Computer Networks

  9. Randomization and Binary Exponential Backoff • Time divided into slots • Length of slot = 2t = worst-case round-trip propagation time • To accommodate longest path, slot time = 512 bit times = 51.2 msec (10Mbps Ethernet) => min frame length: 51.2 msec X 10 Mbps = 512 b = 64 byte • Binary exponential backoff EEC-484/584: Computer Networks

  10. Randomization and Binary Exponential Backoff • After 1st collision, station picks 0 or 1 at random, waits that number of slots and tries again • After 2nd collision, station picks 0,1,2,3 at random, waits that number of slots and tries again • …. • After i-th collision, station picks 0,1,…,2i-1 at random, … • If 10 <= i < 16, station picks 0,1,…,210-1 at random • If i=16, controller reports failure to computer Why randomization is needed? EEC-484/584: Computer Networks

  11. Ethernet Performance • Binary exponential backoff results in • Low delay when few stations collide • Reasonable delay for collision resolution when many stations collide • When other factors are fixed, channel efficiency decreases when • Network bandwidth increases • Cable length increases • Number of stations increases • Frame length decreases EEC-484/584: Computer Networks

  12. Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times EEC-484/584: Computer Networks

  13. Switched Ethernet • Switch – contains a high-speed backplane and room for typically 4 to 32 plug-in line cards, each containing 1-8 connectors • Possibly each card forms its own collision domain, or • Full-duplex operation if each input port is buffered EEC-484/584: Computer Networks

  14. ARP – Address Resolution Protocol How do IP addresses get mapped onto data link layer addresses, such as Ethernet? EEC-484/584: Computer Networks

  15. ARP Optimization • ARP result is cached (step 5 in figure) • When A wants to communicate with B, A includes its IP-to-Ethernet mapping in the ARP packet so that B knows the mapping right away (step 3 in figure) • Have every machine broadcast its mapping when it boots, so that everyone else knows the mapping • To accommodate changes, entries in the ARP cache time out after a few minutes EEC-484/584: Computer Networks

  16. ARP: How to Handle Remote Traffic • Proxy ARP– A router is configured to answer ARP requests on one of its networks for a host on another network EEC-484/584: Computer Networks

  17. ARP – Exercise • Node 1 wants to send a packet to node 4what 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. Exercise • An IP packet to be transmitted by Ethernet is 60 bytes long. Is padding needed in the Ethernet frame, and if so, how many bytes? EEC-484/584: Computer Networks

  25. Exercise • Consider building a CSMA/CD network running at 1 Gbps over a 1-km cable. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size? EEC-484/584: Computer Networks

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