Data Link Layer Data Link Layer “Taking Turns” MAC protocols “Taking Turns” MAC protocols Token passing: Polling: T control token passed master node from one node to next “invites” slave nodes data sequentially. to transmit in turn poll token message typically used with (nothing master concerns: “dumb” slave devices to send) data T concerns: token overhead polling overhead latency slaves latency single point of single point of failure (token) failure (master) data Data Link Layer Data Link Layer 49 50 Data Link Layer Data Link Layer Summary of MAC protocols Link Layer channel partitioning, by time, frequency 1 Link Layer and 5 Ethernet services Time Division, Frequency Division 6 Link-layer switches random access 2 Error detection 7 Link virtualization: ALOHA, S-ALOHA, CSMA, CSMA/CD and correction Collision detection: easy in some technologies (wire), hard ATM 3 Multiple access in others (wireless) CSMA/CD used in Ethernet protocols CSMA/CA used in 802.11 4 Link-layer taking turns addressing polling from central site, token passing Bluetooth, FDDI, IBM Token Ring Data Link Layer Data Link Layer 51 52 Data Link Layer Data Link Layer MAC Addresses LAN Addresses 32-bit IP address: Each adapter on LAN has unique LAN address network-layer address used to get datagram to destination IP subnet 1A-2F-BB-76-09-AD Broadcast address = MAC (or LAN or physical or Ethernet) FF-FF-FF-FF-FF-FF address: LAN function: get frame from one interface to another = adapter (wired or physically-connected interface (same network) wireless) 71-65-F7-2B-08-53 48 bit MAC address (for most LANs) 58-23-D7-FA-20-B0 • burned in NIC ROM, also sometimes software settable 0C-C4-11-6F-E3-98 Permanent, globally unique Data Link Layer Data Link Layer 53 54 9
Data Link Layer Data Link Layer LAN Address (more) Why both MAC and IP address? Answer: to keep the layers MAC address allocation administered by IEEE manufacturer buys portion of MAC address space independent (to assure uniqueness) (1) The data link layer does not only serve analogy: IP protocol, including many others (a) MAC address: like Social Security Number (b) IP address: like postal address (2) if adapters use network-layer address, then the adapter needs to be reconfigured every time it moves MAC flat address ➜ portability can move LAN card from one LAN to another (3) if not use MAC add at all, then all IP hierarchical address NOT portable received frames should be forwarded to address depends on IP subnet to which node is attached network layer. Great overhead! Data Link Layer Data Link Layer 55 56 Data Link Layer Data Link Layer ARP: Address Resolution Protocol Arp table on each node Each IP node (host, Question: how does A determine router) on LAN has MAC address of B ARP table knowing B’s IP address? ARP table: IP/MAC address mappings for A 137.196.7.78 some LAN nodes 1A-2F-BB-76-09-AD < IP address; MAC address; TTL> 137.196.7.23 137.196.7.14 TTL (Time To Live): time after which address LAN mapping will be forgotten 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 (typically 20 min) 0C-C4-11-6F-E3-98 137.196.7.88 B Data Link Layer Data Link Layer 57 58 Data Link Layer Data Link Layer How ARP works? Case 2: routing to How ARP works? Case 1: Same LAN another LAN walkthrough: send datagram from A to B via R A wants to send datagram A caches (saves) IP-to- to B, and B’s MAC address MAC address pair in its assume A knows B’s IP address not in A’s ARP table. ARP table until information becomes old (times out) A broadcasts ARP query 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 packet, containing B's IP soft state: information A address that times out (goes E6-E9-00-17-BB-4B 222.222.222.221 away) unless refreshed 1A-23-F9-CD-06-9B dest MAC address = 111.111.111.111 FF-FF-FF-FF-FF-FF ARP is “plug -and- 222.222.222.222 222.222.222.220 all machines on LAN play”: B 111.111.111.110 receive ARP query nodes create their ARP 111.111.111.112 R B receives ARP packet, 49-BD-D2-C7-56-2A tables without replies to A with its (B's) CC-49-DE-D0-AB-7D intervention from net MAC address administrator frame sent to A’s MAC two ARP tables in router R, one for each IP network (LAN) address (unicast) Data Link Layer Data Link Layer 59 60 10
Data Link Layer Data Link Layer A creates IP datagram with source A, destination B Illustration A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, A creates IP datagram with IP source A, destination B frame contains A-to-B IP datagram A creates link-layer frame with R's MAC address as dest, A’s NIC sends frame frame contains A-to-B IP datagram R’s NIC receives frame MAC src: 74-29-9C-E8-FF-55 R removes IP datagram from Ethernet frame, sees its MAC dest: E6-E9-00-17-BB-4B destined to B IP src: 111.111.111.111 IP dest: 222.222.222.222 R uses ARP to get B’s MAC address IP R creates frame containing A-to-B IP datagram sends to B Eth Phy 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 B A A R E6-E9-00-17-BB-4B 222.222.222.221 1A-23-F9-CD-06-9B 111.111.111.111 111.111.111.111 222.222.222.222 74-29-9C-E8-FF-55 49-BD-D2-C7-56-2A 222.222.222.222 222.222.222.220 222.222.222.220 B 1A-23-F9-CD-06-9B 111.111.111.110 R 111.111.111.112 111.111.111.110 222.222.222.221 111.111.111.112 49-BD-D2-C7-56-2A E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D CC-49-DE-D0-AB-7D Data Link Layer 61 Link Layer 5-62 Data Link Layer Data Link Layer Illustration Illustration frame sent from A to R R forwards datagram with IP source A, destination B frame received at R, datagram removed, passed up to IP R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram MAC src: 74-29-9C-E8-FF-55 MAC src: 1A-23-F9-CD-06-9B MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP dest: 222.222.222.222 IP IP Eth IP IP Eth Phy Eth Eth Phy Phy Phy B B A A R R 111.111.111.111 111.111.111.111 222.222.222.222 222.222.222.222 74-29-9C-E8-FF-55 74-29-9C-E8-FF-55 49-BD-D2-C7-56-2A 49-BD-D2-C7-56-2A 222.222.222.220 222.222.222.220 1A-23-F9-CD-06-9B 1A-23-F9-CD-06-9B 111.111.111.110 222.222.222.221 111.111.111.110 222.222.222.221 111.111.111.112 111.111.111.112 E6-E9-00-17-BB-4B E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D CC-49-DE-D0-AB-7D Link Layer 5-63 Link Layer 5-64 Data Link Layer Data Link Layer Illustration Illustration R forwards datagram with IP source A, destination B R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram frame contains A-to-B IP datagram MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A MAC src: 1A-23-F9-CD-06-9B IP src: 111.111.111.111 MAC dest: 49-BD-D2-C7-56-2A IP dest: 222.222.222.222 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP IP Eth Eth IP Phy Phy Eth Phy B B A A R R 111.111.111.111 111.111.111.111 222.222.222.222 222.222.222.222 74-29-9C-E8-FF-55 74-29-9C-E8-FF-55 49-BD-D2-C7-56-2A 49-BD-D2-C7-56-2A 222.222.222.220 222.222.222.220 1A-23-F9-CD-06-9B 1A-23-F9-CD-06-9B 111.111.111.110 111.111.111.110 222.222.222.221 222.222.222.221 111.111.111.112 111.111.111.112 E6-E9-00-17-BB-4B E6-E9-00-17-BB-4B 88-B2-2F-54-1A-0F 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D CC-49-DE-D0-AB-7D Link Layer 5-65 Link Layer 5-66 11
Data Link Layer Data Link Layer Link Layer Mobile network 1 Introduction and 5 Ethernet Global ISP Data Link Layer services 6 Link-layer switches (Part B) 2 Error detection Home network 7 Link virtualization: and correction Regional ISP ATM 3 Multiple access Yanmin Zhu protocols Institutional network Department of Computer 4 Link-layer Science and Engineering Addressing COMNETS @CSE COMNETS @CSE 1 2 Data Link Layer Data Link Layer Ethernet Inventors of Ethernet “dominant” wired LAN technology: Robert Metcalfe cheap $20 for NIC PhD Harvard, first widely used LAN technology 1973 simpler, cheaper than token LANs and ATM David Boggs kept up with speed race: 10 Mbps – 10 Gbps PhD Stanford 1982 Mr Metcalfe generating the ideas Metcalfe’s Ethernet Mr Boggs figuring sketch out how to build Robert Metcalfe David Boggs the system COMNETS @CSE COMNETS @CSE 3 4 Data Link Layer Data Link Layer Xerox PARC Metcalfe's law Ethernet Value of Laser Printing a telecommunications GUI network is proportional Object-oriented Programming to the square of the (SmallTalk) number of connected WYSIWYG users of the system …… ( n 2 ). COMNETS @CSE COMNETS @CSE 5 6 1
Recommend
More recommend
