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Lecture 13

Lecture 13. Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012. CS3516: These slides are generated from those made available by the authors of our text.

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Lecture 13

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  1. Lecture 13 Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012 CS3516: These slides are generated from those made available by the authors of our text. Introduction

  2. Get MAC Address (Getmac.exe) Discovers the Media Access Control (MAC) address and lists associated network protocols for all network cards in a computer, either locally or across a network. C:\Users\jb>getmac Physical Address Transport Name ============ ============================= 60-36-DD-AA-13-69 Media disconnected 60-36-DD-AA-13-65 \Device\Tcpip_{437F350E-DFD7-4A86-B063-0B9650BD4404} 60-36-DD-AA-13-66 Media disconnected 60-36-DD-AA-13-66 Media disconnected B8-CA-3A-DC-C6-2B Media disconnected 08-00-27-00-E4-38 \Device\Tcpip_{F551D578-DC71-4760-B91C-B349EAE4238F} Useful Commands Network Layer

  3. IP Configuration Utility (Ipconfig.exe) Displays all current (TCP/IP) network configurations. C:\Users\jb>ipconfig Windows IP Configuration Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : WPI.EDU Link-local IPv6 Address . . . . . : fe80::e591:74d4:a495:7998%16 IPv4 Address. . . . . . . . . . . : 130.215.28.36 Subnet Mask . . . . . . . . . . . : 255.255.248.0 Default Gateway . . . . . . . . . : 130.215.24.1 C:\Users\jb>ipconfig /?  Prints command line options C:\Users\jb>ipconfig /displaydns gives dns info cached on node cs.wpi.edu ---------------------------------------- Record Name . . . . . : cs.wpi.edu Record Type . . . . . : 1 Time To Live . . . . : 73497 Data Length . . . . . : 4 Section . . . . . . . : Answer A (Host) Record . . . : 130.215.28.181 Useful Commands Network Layer

  4. Name Server Lookup (Nslookup.exe) Displays information about Domain Name System records for specific IP addresses and/or host names so that you can troubleshoot DNS problems. C:\Users\jb>nslookup www.google.com Server: a.resolvers.level3.net  this is the name of the default server Address: 4.2.2.1 Non-authoritative answer: Name: www.google.com Addresses: 2607:f8b0:4000:804::1011 74.125.227.179 74.125.227.180 74.125.227.176 74.125.227.177 74.125.227.178 Useful Commands Network Layer

  5. Net services commands (Net.exe) Performs a broad range of network tasks. Type net with no parameters to see a full list of available command-line options. C:\Users\jb>net help The syntax of this command is: Commands available are: NET ACCOUNTS NET HELPMSG NET STATISTICS NET COMPUTER NET LOCALGROUP NET STOP NET CONFIG NET PAUSE NET TIME NET CONTINUE NET SESSION NET USE NET FILE NET SHARE NET USER NET GROUP NET START NET VIEW NET HELP NET HELP NAMES explains different types of names in NET HELP syntax lines. NET HELP SERVICES lists some of the services you can start. NET HELP SYNTAX explains how to read NET HELP syntax lines. NET HELP command | MORE displays Help one screen at a time. Useful Commands Network Layer

  6. Netstat(Netstat.exe) Displays active TCP connections, ports on which the computer is listening, Ethernet statistics, the IP routing table, and IPv4/IPv6 statistics. C:\Users\jb>netstat Proto Local Address Foreign Address State TCP 127.0.0.1:1029 jb-laptop:5354 ESTABLISHED TCP 127.0.0.1:1036 jb-laptop:27015 ESTABLISHED TCP 127.0.0.1:1047 jb-laptop:19872 ESTABLISHED TCP 127.0.0.1:39055 jb-laptop:39054 ESTABLISHED TCP 172.17.168.138:2492 blugro5relay:2492 ESTABLISHED C:\Users\jb>netstat -s IPv4 Statistics Packets Received = 10158258 Received Header Errors = 2848 Received Address Errors = 2192434 Datagrams Forwarded = 0 Unknown Protocols Received = 170614 Received Packets Discarded = 4173788 Received Packets Delivered = 6692404 Useful Commands Network Layer

  7. Network Command Shell (Netsh.exe) Displays or modifies the network configuration of a local or remote computer that is currently running. This command-line scripting utility has a huge number of options, which are fully detailed in Help. TCP/IP Route (Route.exe) Displays and modifies entries in the local IP routing table. C:\Users\jb>route print Interface List 13...60 36 ddaa 13 65 ......Intel(R) Centrino(R) Wireless-N 2230 12...60 36 ddaa 13 69 ......Bluetooth Device (Personal Area Network) 31...08 00 27 00 e4 38 ......VirtualBox Host-Only Ethernet Adapter IPv4 Route Table Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 172.17.1.1 172.17.168.138 25 127.0.0.0 255.0.0.0 On-link 127.0.0.1 306 127.0.0.1 255.255.255.255 On-link 127.0.0.1 306 127.255.255.255 255.255.255.255 On-link 127.0.0.1 306 169.254.0.0 255.255.0.0 On-link 169.254.40.182 276 169.254.40.182 255.255.255.255 On-link 169.254.40.182 276 169.254.255.255 255.255.255.255 On-link 169.254.40.182 276 172.17.0.0 255.255.0.0 On-link 172.17.168.138 281 172.17.168.138 255.255.255.255 On-link 172.17.168.138 281 172.17.255.255 255.255.255.255 On-link 172.17.168.138 281 224.0.0.0 240.0.0.0 On-link 169.254.40.182 276 Useful Commands Network Layer

  8. (Arp.exe) Displays current ARP entries by interrogating the current protocol data. If inet_addr is specified, the IP and Physical addresses for only the specified computer are displayed. If more than one network interface uses ARP, entries for each ARP table are displayed. C:\Users\jb>arp -a Interface: 130.215.28.36 --- 0x10 Internet Address Physical Address Type 130.215.24.1 00-00-5e-00-01-01 dynamic 130.215.24.2 00-23-9c-94-97-f0 dynamic 130.215.27.252 f0-1f-af-2f-e1-27 dynamic 130.215.28.63 00-16-3e-c5-01-25 dynamic 130.215.29.165 00-24-e8-32-32-1d dynamic 130.215.31.255 ff-ff-ff-ff-ff-ff static Useful Commands Network Layer

  9. 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4LANs addressing, ARP Ethernet switches VLANS Link layer, LANs: outline Link Layer

  10. terminology: hosts and routers: nodes communication channels that connect adjacent nodes along communication path: links wired links wireless links LANs layer-2 packet: frame,encapsulates datagram Link layer: introduction global ISP data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link Link Layer

  11. datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link each link protocol provides different services e.g., may or may not provide rdt over link Link layer: context • framing, link access: • encapsulate datagram into frame, adding header, trailer • channel access if shared medium • “MAC” addresses used in frame headers to identify source, dest • different from IP address! • reliable delivery between adjacent nodes • we learned how to do this already – Transport layer • seldom used on low bit-error link (fiber, some twisted pair) • wireless links: high error rates • Q: why both link-level and end-end reliability? Link Layer

  12. Link layer services (more) • flow control: • pacing between adjacent sending and receiving nodes • error detection: • errors caused by signal attenuation, noise. • receiver detects presence of errors: • signals sender for retransmission or drops frame • error correction: • receiver identifies and corrects bit error(s) without resorting to retransmission • half-duplex and full-duplex • with half duplex, nodes at both ends of link can transmit, but not at same time Link Layer

  13. in each and every host link layer implemented in “adaptor” (aka network interface card NIC) or on a chip Ethernet card, 802.11 card; Ethernet chipset implements link, physical layer attaches into host’s system buses combination of hardware, software, firmware application transport network link link physical Where is the link layer implemented? cpu memory host bus (e.g., PCI) controller physical transmission network adapter card Link Layer

  14. 5.1introduction, services 5.2error detection, correction  SKIPPED 5.3 multiple access protocols 5.4LANs addressing, ARP Ethernet switches VLANS Link layer, LANs: outline Link Layer

  15. Multiple access links, protocols two types of “links”: • point-to-point  NO Collisions • PPP for dial-up access • point-to-point link between Ethernet switch, host • broadcast (shared wire or medium)  Collisions • old-fashioned Ethernet • upstream HFC • 802.11 wireless LAN • algorithm that determines how nodes share channel shared RF (e.g., 802.11 WiFi) shared RF (satellite) shared wire (e.g., cabled Ethernet) humans at a cocktail party (shared air, acoustical) Link Layer

  16. MAC protocols: taxonomy three broad classes of sharing: • channel partitioning • divide channel into smaller “pieces” (time slots, frequency, code) • allocate piece to node for exclusive use • Subdividing the capacity – TDM, FDM • random access • channel not divided, allow collisions • “recover” from collisions • “taking turns” • nodes take turns, but nodes with more to send can take longer turns Link Layer

  17. Random access protocols • when node has packet to send • transmit at full channel data rate R. • no a priori coordination among nodes • two or more transmitting nodes ➜“collision”, • random access MAC protocol specifies: • how to detect collisions • how to recover from collisions (e.g., via delayed retransmissions) • examples of random access MAC protocols: • slotted ALOHA • ALOHA • CSMA, CSMA/CD, CSMA/CA Link Layer

  18. assumptions: all frames same size time divided into equal size slots (time to transmit 1 frame) nodes start to transmit only slot beginning nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision operation: when node obtains fresh frame, transmits in next slot if no collision: node can send new frame in next slot if collision: node retransmits frame in each subsequent slot with prob. p until success Slotted ALOHA Link Layer

  19. Pros: single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons: collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization 1 1 1 1 node 1 2 2 2 node 2 node 3 3 3 3 C E S C S E C E S Slotted ALOHA Link Layer

  20. suppose:N nodes with many frames to send, each transmits in slot with probability p prob that given node has success in a slot = p(1-p)N-1 prob that any node has a success = Np(1-p)N-1 max efficiency: find p* that maximizes Np(1-p)N-1 for many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives: max efficiency = 1/e = .37 Slotted ALOHA: efficiency efficiency: long-run fraction of successful slots (many nodes, all with many frames to send) ! at best: channel used for useful transmissions 37% of time! Link Layer

  21. max efficiency: find p* that maximizes F(p) = Np(1-p)N-1 max efficiency when F’(p) = 0 dF/dp = d (Np(1-p)N-1 ) /dp = N(1-p)N-1 + Np(N-1)(-1)(1-p)N-2 N(1-p)N-1 = Np(N-1)(1-p)N-1/ (1 – p) 1 = p(N – 1) / ( 1 – p) ( 1 – p ) = p ( N – 1) = pN - p 1 = pN p = 1 / N F(max) = N(1/N)(1-(1/N))N-1 = ( 1 – 1/N )N-1 As N goes to infinity F(max) = 1 / e = 0.37 Slotted ALOHA: efficiency Link Layer

  22. Pure (unslotted) ALOHA • unslotted Aloha: simpler, no synchronization • when frame first arrives • transmit immediately • collision probability increases: • frame sent at t0 collides with other frames sent in [t0-1,t0+1] • Efficiency of only 0.18! Link Layer

  23. CSMA (carrier sense multiple access) CSMA: listen before transmit: if channel sensed idle: transmit entire frame • if channel sensed busy, defer transmission • human analogy: don’t interrupt others! Link Layer

  24. collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted distance & propagation delay play role in determining collision probability CSMA collisions spatial layout of nodes Link Layer

  25. CSMA/CD (collision detection) CSMA/CD:carrier sensing, deferral as in CSMA • collisions detected within short time • colliding transmissions aborted, reducing channel wastage • collision detection: • easy in wired LANs: measure signal strengths, compare transmitted, received signals • difficult in wireless LANs: received signal strength overwhelmed by local transmission strength • human analogy: the polite conversationalist Link Layer

  26. CSMA/CD (collision detection) spatial layout of nodes Link Layer

  27. 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits until channel idle, then transmits. 3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame ! 4. If NIC detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, NIC enters binary (exponential) backoff: after mth collision, NIC chooses K at random from {0,1,2, …, 2m-1}. NIC waits K·512 bit times, returns to Step 2 longer backoff interval with more collisions Ethernet CSMA/CD algorithm Link Layer

  28. CSMA/CD efficiency • Tprop = max prop delay between 2 nodes in LAN • ttrans = time to transmit max-size frame • efficiency goes to 1 • as tprop goes to 0 • as ttrans goes to infinity • better performance than ALOHA: and simple, cheap, decentralized! Link Layer

  29. “Taking turns” MAC protocols token passing: • control tokenpassed from one node to next sequentially. • token message • concerns: • token overhead • latency • single point of failure (token) T (nothing to send) T data Link Layer

  30. ISP Cable access network Internet frames,TV channels, control transmitted downstream at different frequencies … cable headend cable modem splitter CMTS … cable modem termination system upstream Internet frames, TV control, transmitted upstream at different frequencies in time slots • multiple40Mbps downstream (broadcast) channels • single CMTS transmits into channels • multiple30 Mbps upstream channels • multiple access: all users contend for certain upstream channel time slots (others assigned)

  31. Cable access network cable headend MAP frame for Interval [t1, t2] • DOCSIS: data over cable service interface spec • FDM over upstream, downstream frequency channels • TDM upstream: some slots assigned, some have contention • downstream MAP frame: assigns upstream slots • request for upstream slots (and data) transmitted random access (binary backoff) in selected slots Downstream channel i CMTS Upstream channel j t1 t2 Residences with cable modems Minislots containing minislots request frames Assigned minislots containing cable modem upstream data frames Link Layer

  32. 5.1introduction, services 5.2error detection, correction 5.3multiple access protocols 5.4 LANs addressing, ARP Ethernet switches VLANS 5.5 link virtualization: MPLS 5.6 data center networking 5.7a day in the life of a web request Link layer, LANs: outline Link Layer

  33. MAC addresses and ARP • 32-bit IP address: • network-layer address for interface • used for layer 3 (network layer) forwarding • MAC (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 settable • e.g.: 1A-2F-BB-76-09-AD hexadecimal (base 16) notation (each “number” represents 4 bits) Link Layer

  34. LAN addresses and ARP each adapter on LAN has unique LAN address 1A-2F-BB-76-09-AD LAN (wired or wireless) adapter 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 Link Layer

  35. LAN addresses (more) • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • analogy: • MAC address: like Social Security Number • IP address: like postal address • MAC flat address ➜ portability • can move LAN card from one LAN to another • IP hierarchical address not portable • address depends on IP subnet to which node is attached Link Layer

  36. Question: how to determine interface’s MAC address, knowing its IP address? ARP: address resolution protocol ARP table: each IP node (host, router) on LAN has 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) 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23 137.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 137.196.7.88 Link Layer

  37. Question: how to determine interface’s MAC address, knowing its IP address? ARP: address resolution protocol ipconfig /all Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : WPI.EDU Description . . . . . . . . . . . : RealtekPCIe FE Controller Physical Address. . . . . . . . . : B8-CA-3A-DC-C6-2B DHCP Enabled. . . . . . . . . . . : Yes Autoconfiguration Enabled . . . . : Yes IPv4 Address. . . . . . . . . . . : 130.215.28.36(Preferred) Subnet Mask . . . . . . . . . . . : 255.255.248.0 Lease Obtained. . . . . . . . . . : Monday,December16, 2013 11:06:43 AM Lease Expires . . . . . . . . . . : Monday,December16, 2013 5:06:43 PM Default Gateway . . . . . . . . . : 130.215.24.1 DHCP Server . . . . . . . . . . . : 130.215.39.18 DNS Servers . . . . . . . . . . . : 130.215.32.18 130.215.39.18 130.215.5.18 NetBIOS over Tcpip. . . . . . . . : Enabled Link Layer

  38. A wants to send datagram to B B’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-FF all nodes 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 administrator ARP protocol: same LAN Link Layer

  39. 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.222 49-BD-D2-C7-56-2A Addressing: routing to another LAN walkthrough: 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?) B A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 222.222.222.221 111.111.111.112 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D Link Layer

  40. Addressing: routing to another LAN getmac Physical Address Transport Name =================== ==================== B8-CA-3A-DC-C6-2B \Device\Tcpip_  Wired 08-00-27-00-E4-38 \Device\Tcpip_  Wireless ipconfig /all Physical Address. . . . . . . . . : B8-CA-3A-DC-C6-2B IPv4 Address. . . . . . . . . . . : 130.215.28.36(Preferred) Subnet Mask . . . . . . . . . . . : 255.255.248.0 arp-a Interface: 130.215.28.36 --- 0x10 Internet Address Physical Address Type 130.215.24.1 00-00-5e-00-01-01 dynamic 130.215.24.2 00-23-9c-94-97-f0 dynamic 130.215.27.230 f0-1f-af-2f-e1-3f dynamic 130.215.29.193 04-7d-7b-b0-b1-44 dynamic route print Active Routes: Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 130.215.24.1 130.215.28.36 20 127.255.255.255 255.255.255.255 On-link 127.0.0.1 306 130.215.24.0 255.255.248.0 On-link 130.215.28.36 276 130.215.28.36 255.255.255.255 On-link 130.215.28.36 276 130.215.31.255 255.255.255.255 On-link 130.215.28.36 276 Link Layer

  41. MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.222 49-BD-D2-C7-56-2A Addressing: routing to another LAN • A 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 datagram B A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 222.222.222.221 111.111.111.112 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D Link Layer

  42. MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.222 49-BD-D2-C7-56-2A Addressing: routing to another LAN • frame sent from A to R • frame received at R, datagram removed, passed up to IP B A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 222.222.222.221 111.111.111.112 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D Link Layer

  43. IP Eth Phy MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP Eth Phy IP src: 111.111.111.111 IP dest: 222.222.222.222 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.222 49-BD-D2-C7-56-2A Addressing: routing to another LAN • R 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 datagram B A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 222.222.222.221 111.111.111.112 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D Link Layer

  44. IP Eth Phy MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP Eth Phy IP src: 111.111.111.111 IP dest: 222.222.222.222 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.222 49-BD-D2-C7-56-2A Addressing: routing to another LAN • R 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 datagram B A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 222.222.222.221 111.111.111.112 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D Link Layer

  45. IP Eth Phy MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.222 49-BD-D2-C7-56-2A Addressing: routing to another LAN • R 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 datagram B A R 111.111.111.111 74-29-9C-E8-FF-55 222.222.222.220 1A-23-F9-CD-06-9B 222.222.222.221 111.111.111.112 88-B2-2F-54-1A-0F CC-49-DE-D0-AB-7D Link Layer

  46. 5.1introduction, services 5.2error detection, correction 5.3multiple access protocols 5.4 LANs addressing, ARP Ethernet switches VLANS 5.5 link virtualization: MPLS 5.6 data center networking 5.7a day in the life of a web request Link layer, LANs: outline Link Layer

  47. Ethernet “dominant” wired LAN technology: • cheap $20 for NIC • first widely used LAN technology • simpler, cheaper than token LANs and ATM • kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch Link Layer

  48. Ethernet: physical topology • bus: popular through mid 90s • all nodes in same collision domain (can collide with each other) • star: prevails today • active switchin center • each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) switch star bus: coaxial cable Link Layer

  49. Ethernet frame structure sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame preamble: • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 • used to synchronize receiver, sender clock rates type dest. address source address data (payload) CRC preamble Link Layer

  50. Ethernet frame structure (more) • addresses: 6 byte source, destination MAC addresses • if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol • otherwise, adapter discards frame • type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) • CRC: cyclic redundancy check at receiver • error detected: frame is dropped type dest. address source address data (payload) CRC preamble Link Layer

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