Link layer, LANs: outline (5B)MAC addresses and ARPLAN addresses and ARPLAN addresses (more)ARP: address resolution protocolARP protocol: same LANAddressing: routing to another LANSlide 8Slide 9Slide 10Slide 11Slide 12Link layer, LANs: outlineEthernetEthernet: physical topologyEthernet frame structureEthernet frame structure (more)Ethernet: unreliable, connectionless802.3 Ethernet standards: link & physical layersLink Layer 5-1Link layer, LANs: outline (5B)5.1 introduction, services5.2 error detection, correction 5.3 multiple access protocols5.4 LANsaddressing, ARPEthernetswitchesVLANS5.5 link virtualization: MPLS5.6 data center networking5.7 a day in the life of a web requestLink Layer 5-2MAC addresses and ARP32-bit IP address: network-layer address for interfaceused for layer 3 (network layer) forwardingMAC (or LAN or physical or Ethernet) address: function: used ‘locally” to get frame from one interface to another physically-connected interface (same network, in IP-addressing sense)48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settablee.g.: 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each “number” represents 4 bits)Link Layer 5-3LAN addresses and ARPeach adapter on LAN has unique LAN addressadapter1A-2F-BB-76-09-AD58-23-D7-FA-20-B00C-C4-11-6F-E3-9871-65-F7-2B-08-53 LAN(wired orwireless)Link Layer 5-4LAN addresses (more)MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)analogy:MAC address: like Social Security NumberIP address: like postal address MAC flat address ➜ portability can move LAN card from one LAN to anotherIP hierarchical address not portable address depends on IP subnet to which node is attachedLink Layer 5-5ARP: address resolution protocolARP table: each IP node (host, router) on LAN has tableIP/MAC address mappings for some LAN nodes: < IP address; MAC address; TTL>TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)Question: how to determineinterface’s MAC address, knowing its IP address?1A-2F-BB-76-09-AD58-23-D7-FA-20-B00C-C4-11-6F-E3-9871-65-F7-2B-08-53 LAN137.196.7.23137.196.7.78137.196.7.14137.196.7.88Link Layer 5-6ARP protocol: same LANA wants to send datagram to BB’s MAC address not in A’s ARP table.A broadcasts ARP query packet, containing B's IP address dest MAC address = FF-FF-FF-FF-FF-FFall nodes on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC addressframe sent to A’s MAC address (unicast)A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshedARP is “plug-and-play”:nodes create their ARP tables without intervention from net administratorLink Layer 5-7walkthrough: send datagram from A to B via R focus on addressing – at IP (datagram) and MAC layer (frame) assume A knows B’s IP address assume A knows IP address of first hop router, R (how?) assume A knows R’s MAC address (how?)Addressing: routing to another LANR1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112111.111.111.11174-29-9C-E8-FF-55A222.222.222.22249-BD-D2-C7-56-2A222.222.222.22188-B2-2F-54-1A-0FBR1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112111.111.111.11174-29-9C-E8-FF-55A222.222.222.22249-BD-D2-C7-56-2A222.222.222.22188-B2-2F-54-1A-0FBLink Layer 5-8Addressing: routing to another LANIPEthPhyIP src: 111.111.111.111 IP dest: 222.222.222.222A creates IP datagram with IP source A, destination B A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagramMAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4BR1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112111.111.111.11174-29-9C-E8-FF-55A222.222.222.22249-BD-D2-C7-56-2A222.222.222.22188-B2-2F-54-1A-0FBLink Layer 5-9Addressing: routing to another LANIPEthPhyframe sent from A to RIPEthPhyframe received at R, datagram removed, passed up to IPMAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4BIP src: 111.111.111.111 IP dest: 222.222.222.222IP src: 111.111.111.111 IP dest: 222.222.222.222R1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112111.111.111.11174-29-9C-E8-FF-55A222.222.222.22249-BD-D2-C7-56-2A222.222.222.22188-B2-2F-54-1A-0FBLink Layer 5-10Addressing: routing to another LANIP src: 111.111.111.111 IP dest: 222.222.222.222R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagramMAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2AIPEthPhyIPEthPhyR1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112111.111.111.11174-29-9C-E8-FF-55A222.222.222.22249-BD-D2-C7-56-2A222.222.222.22188-B2-2F-54-1A-0FBLink Layer 5-11Addressing: routing to another LANR forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagramIP src: 111.111.111.111 IP dest: 222.222.222.222MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2AIPEthPhyIPEthPhyR1A-23-F9-CD-06-9B222.222.222.220111.111.111.110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D111.111.111.112111.111.111.11174-29-9C-E8-FF-55A222.222.222.22249-BD-D2-C7-56-2A222.222.222.22188-B2-2F-54-1A-0FBLink Layer 5-12Addressing: routing to another LANR forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagramIP src: 111.111.111.111 IP dest: 222.222.222.222MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2AIPEthPhyLink Layer 5-13Link layer, LANs: outline5.1 introduction, services5.2 error detection, correction 5.3 multiple access protocols5.4 LANsaddressing, ARPEthernetswitchesVLANS5.5 link virtualization: MPLS5.6 data center networking5.7 a day in the life of a web requestLink Layer 5-14Ethernet“dominant” wired LAN technology: cheap $20 for NICfirst widely used LAN technologysimpler, cheaper than token LANs and ATMkept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketchLink Layer 5-15Ethernet: physical topologybus:
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