ReviewOverviewLAN technologiesLAN Addresses and ARPSlide 5LAN Address (more)Recall earlier routing discussionARP: Address Resolution ProtocolARP protocolRouting to another LANSlide 11Slide 12Ethernet Frame StructureEthernet Frame Structure (more)Unreliable, connectionless serviceEthernet uses CSMA/CDEthernet CSMA/CD algorithmEthernet’s CSMA/CD (more)CSMA/CD efficiency10BaseT and 100BaseTHubsSlide 22Interconnecting with hubsSwitchForwardingSelf learningFiltering/ForwardingSwitch exampleSlide 29Switch: traffic isolationSwitches: dedicated accessMore on SwitchesInstitutional networkSwitches vs. RoutersSummary comparisonReviewError Detection: CRCMultiple access protocolsSlotted ALOHACSMA/CDHomework 3 outProject 3 out, link state only. Some slides are in courtesy of J. Kurose and K. RossOverviewLAN addresses and ARPEthernetHubs, bridges, and switchesLAN technologiesData link layer so far:services, error detection/correction, multiple access Next: LAN technologiesaddressingEthernethubs, bridges, switchesPPP 802.11LAN Addresses and ARP32-bit IP address: network-layer addressused to get datagram to destination IP network (recall IP network definition)LAN (or MAC or physical or Ethernet) address: used to get datagram from one interface to another physically-connected interface (same network)48 bit MAC address (for most LANs) burned in the adapter ROMLAN Addresses and ARPEach adapter on LAN has unique LAN addressBroadcast address =FF-FF-FF-FF-FF-FF= adapter1A-2F-BB-76-09-AD58-23-D7-FA-20-B00C-C4-11-6F-E3-9871-65-F7-2B-08-53 LAN(wired orwireless)LAN Address (more)MAC address allocation administered by IEEEmanufacturer buys portion of MAC address space (to assure uniqueness)Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address => portability can move LAN card from one LAN to anotherIP hierarchical address NOT portable depends on IP network to which node is attachedRecall earlier routing discussion223.1.1.1223.1.1.2223.1.1.3223.1.1.4223.1.2.9223.1.2.2223.1.2.1223.1.3.2223.1.3.1223.1.3.27ABEStarting at A, given IP datagram addressed to B:look up net. address of B, find B on same net. as Alink layer send datagram to B inside link-layer frame B’s MACaddrA’s MACaddrA’s IPaddrB’s IPaddrIP payloaddatagramframeframe source,dest addressdatagram source,dest addressARP: Address Resolution ProtocolEach IP node (Host, Router) on LAN has ARP tableARP Table: IP/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 determineMAC address of Bknowing B’s IP address?1A-2F-BB-76-09-AD58-23-D7-FA-20-B00C-C4-11-6F-E3-9871-65-F7-2B-08-53 LAN237.196.7.23237.196.7.78237.196.7.14237.196.7.88ARP protocolA wants to send datagram to B, and A knows B’s IP address.Suppose B’s MAC address is not in A’s ARP table.A broadcasts ARP query packet, containing B's IP address all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC addressframe 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 refreshedARP is “plug-and-play”:nodes create their ARP tables without intervention from net administratorRouting to another LANwalkthrough: send datagram from A to B via R assume A know B’s IP addressTwo ARP tables in router R, one for each IP network (LAN)ARBA creates datagram with source A, destination B 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, frame contains A-to-B IP datagramA’s data link layer sends frame R’s data link layer receives frame R removes IP datagram from Ethernet frame, sees its destined to BR uses ARP to get B’s physical layer address R creates frame containing A-to-B IP datagram sends to BARBOverviewLAN addresses and ARPEthernetHubs, bridges, and switchesEthernet Frame StructureSending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet framePreamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock ratesEthernet Frame Structure (more)Addresses: 6 bytesif adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocolotherwise, adapter discards frameType: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)CRC: checked at receiver, if error is detected, the frame is simply droppedUnreliable, connectionless serviceConnectionless: No handshaking between sending and receiving adapter. Unreliable: receiving adapter doesn’t send acks or nacks to sending adapterstream of datagrams passed to network layer can have gapsgaps will be filled if app is using TCPotherwise, app will see the gapsEthernet uses CSMA/CDNo slotsadapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sensetransmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detectionBefore attempting a retransmission, adapter waits a random time, that is, random accessEthernet CSMA/CD algorithm1. Adaptor gets datagram from and creates frame2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame !4. If adapter detects another transmission while transmitting, aborts and sends jam signal5. After aborting, adapter enters exponential backof: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times and returns to Step 2Ethernet’s CSMA/CD (more)Jam Signal: make sure all other transmitters are aware of collision; 48 bits;Bit time: 0.1 microsec for 10 Mbps Ethernet ;for K=1023, wait time is about 50 msec Exponential Backoff: Goal: adapt retransmission attempts to estimated current loadheavy load: random wait will be longerfirst collision: choose K