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EEC-484/584 Computer Networks Lecture 8 Wenbing Zhao [email protected] 2 Outline • Reminder: – 3/3 (Monday): Lab #3 – 3/5 (Wednesday): Quiz #2 • • • • • • CSMA protocols Manchester Encoding The Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm Switched Ethernet ARP and DHCP Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 3 Carrier Sense Multiple Access • When station has data to send, listens to channel to see if anyone else is transmitting • If channel is idle, station transmits a frame • Else station waits for it to become idle • If collisions occurs, station waits random amount of time, tries again • Also called 1-persistent CSMA – With probability 1 station will transmit if channel is idle Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao Carrier Sense Multiple Access: Collision Still Possible • After a station starts sending, it takes a while before 2nd station receives 1st station’s signal – 2nd station might start sending before it knows that another station has already been transmitting • If two stations become ready while third station transmitting – Both wait until transmission ends and start transmitting, collision results Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 4 5 p-persistent CSMA: Reduce the Probability of Collision • Sense continuously, but does not always send when channel is idle – Applicable for slotted channels • When ready to send, station senses the channel – If channel idle, station transmits with probability p, defers to next slot with probability q = 1-p – Else (if channel is busy) station waits until next slot tries again – If next slot idle, station transmits with probability p, defers with probability q = 1-p – … Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 6 Non-Persistent CSMA • Does not sense continuously, send if it senses the channel is idle • Before sending, station senses the channel – If channel is idle, station begins sending – Else station does not continuously sense, waits random amount of time, tries again Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 7 Persistent and Nonpersistent CSMA • Improves over ALOHA because they ensure no station to transmit when it senses channel is busy Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 8 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 9 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 Not considered as part of the header! Spring Semester 2008 >= 64 bytes EEC-484/584: Computer Networks Wenbing Zhao 10 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 Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 11 Ethernet MAC Sublayer Protocol • Uses 1-persistent CSMA/CD • Binary exponential backoff • Provides unreliable connectionless service Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 12 CSMA with Collision Detection • If two stations start transmitting simultaneously, both detect collision and stop transmitting • Minimum time to detect collision = time for signal to propagate • Monitor collision while sending – Minimum time to detect collision => minimum frame length Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 13 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 2008 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) • Binary exponential backoff Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 14 15 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 Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 16 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 17 Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 18 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 2008 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 19 20 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 21 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 22 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? Spring Semester 2008 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 23 24 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 25 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 26 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 27 DHCP Client-Server Scenario A B 223.1.1.2 223.1.1.4 223.1.2.9 223.1.1.3 223.1.3.27 223.1.3.1 Spring Semester 2008 223.1.2.1 DHCP server 223.1.1.1 E 223.1.2.2 223.1.3.2 EEC-484/584: Computer Networks arriving DHCP client needs address in this network Wenbing Zhao DHCP Client-Server Scenario DHCP server: 223.1.2.5 DHCP discover arriving client src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 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 time src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs 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 Spring Semester 2008 EEC-484/584: Computer Networks Wenbing Zhao 28 29 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 30 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 31 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 2008 EEC-484/584: Computer Networks Wenbing Zhao 32 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 2008 EEC-484/584: Computer Networks Wenbing Zhao