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Understanding Ethernet and Networking Concepts in Computer Networks

This lecture covers key concepts in computer networking, including Manchester Encoding, the Ethernet MAC Sublayer Protocol, and the Binary Exponential Backoff Algorithm. Students will learn about Ethernet frame structure, minimum frame length requirements for collision detection, and the dynamics of switched Ethernet networks. The session also emphasizes the Address Resolution Protocol (ARP) and its optimizations. A reminder for upcoming quizzes is included. Emphasis is placed on practical scenarios and the implications of network performance with various configurations.

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Understanding Ethernet and Networking Concepts in Computer Networks

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  1. EEC-484/584Computer Networks Lecture 10 Wenbing Zhao wenbing@ieee.org (Part of the slides are based on materials supplied by Dr. Louise Moser at UCSB and Prentice-Hall)

  2. Outline • Manchester Encoding • The Ethernet MAC Sublayer Protocol • The Binary Exponential Backoff Algorithm • Switched Ethernet • ARP and DHCP • Reminder: Quiz 2 • Oct 19 Thursday 4-6pm (TTh Session) • Oct 23 Monday 2-4pm (MW Session) 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 >= 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: type of network layer protocol • Pad– used to produce valid frame >= 64 bytes • Checksum– 32-bit cyclic redundancy check EEC-484/584: Computer Networks

  6. Minimum Frame Length • 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

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

  8. 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) • Binary exponential backoff EEC-484/584: Computer Networks

  9. 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 EEC-484/584: Computer Networks

  10. 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

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

  12. 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

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

  14. 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

  15. 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

  16. ARP – Exercise • Node 1 wants to send a packet to node 3, 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

  17. 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

  18. 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

  19. 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

  20. 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: • 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

  21. E B A DHCP Client-Server Scenario 223.1.2.1 DHCP 223.1.1.1 server 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 arriving DHCP client needs address in this network 223.1.1.3 223.1.3.27 223.1.3.2 223.1.3.1 EEC-484/584: Computer Networks

  22. DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP Client-Server Scenario arriving client DHCP server: 223.1.2.5 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs time DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs EEC-484/584: Computer Networks

  23. DHCP Replay • A DHCP relay agentis 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

  24. 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

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

  26. 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

  27. Exercise • A switch designed for use with fast Ethernet has a backplane that can move 10 Gbps. How many frames/sec can it handle? Assume there is an endless stream of 64-byte (512-bit) frames. EEC-484/584: Computer Networks

  28. Exercise • How long does a station, s, have to wait in the worst case before it can start transmitting its frame over a LAN that uses the basic bit-map protocol? (Assume N stations, each frame is d bits) EEC-484/584: Computer Networks

  29. Exercise • Six stations, A through F, communicate using the MACA protocol. Is it possible that two transmissions take place simultaneously? EEC-484/584: Computer Networks

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