EEC-484/584 Computer Networks

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