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EEC-484/584 Computer Networks Lecture 14 Wenbing Zhao [email protected] 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao Ethernet Frame Structure 4 • 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! Spring Semester 2009 >= 64 bytes EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 6 Ethernet MAC Sublayer Protocol • Uses 1-persistent CSMA/CD • Binary exponential backoff • Provides unreliable connectionless service Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 9 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? Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 12 Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao ARP – Address Resolution Protocol How do IP addresses get mapped onto data link layer addresses, such as Ethernet? Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 14 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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? Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao RARP – Reverse Address Resolution Protocol 32-bit Internet address ARP RARP 48-bit Ethernet address • RARP - Allows a newly-booted disklessworkstation (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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 18 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 21 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 22 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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 Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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? Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao 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? Spring Semester 2009 EEC-484/584: Computer Networks Wenbing Zhao