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Practical Ethernet

Practical Ethernet. What is Ethernet?. Ethernet is the most widely-installed local area network (LAN) technology. Specified in a standard, IEEE 802.3. IEEE = Institute for electical and electronical engineers. Physical interface Medium MAC interface Station Ethernet Frame.

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Practical Ethernet

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  1. Practical Ethernet

  2. What is Ethernet?

  3. Ethernet is the most widely-installed local area network (LAN) technology. Specified in a standard, IEEE802.3 IEEE = Institute for electical and electronical engineers • Physical interface Medium • MAC interface Station • Ethernet Frame

  4. Table of content • Intro • Basic principles • PHY’s • MAC and IP • Sniffing the Ethernet • HUB / Switch / Router / Gateway • LAN to LAN over WAN (Ethernet via SDH) • Future

  5. Terminology • Ethernet, the standard: IEEE 802.3 • Ethernet, the working off.. “CSMA/CD”. Carrier Sense Multiple Access with Collision Detection • Actually “Ethernet” is a proprietary network technology from Xerox (1979), later joined by DEC and Intel. This “Ethernet” was used as the blueprint for IEEE 802.3, first published in 1983. • You can get a free copy of all IEEE 802 standards at http://standards.ieee.org/getieee802

  6. How it all began… • This diagram was reputedly drawn by Dr. Robert M. Metcalfe in 1976 to present Ethernet to the National Computer Conference in June of that year. On the drawing are the original terms for describingEthernet. (source: http://www.ieee802.org/3)

  7. Ethernet, the standaard IEEE802.3 Logical Link Control

  8. Carrier Sense Multiple Access with Collision Detection

  9. Basic Principles – Multiple Access The original Ethernet architecture is a bus: All stations are connected to the same physical medium and compete with each other for “air time”. Clients discard all frames that are not addressed to them.

  10. Basic Principles – Carrier Sense / Collision Detection A host must not initiate transmission of a packet when an other host is transmitting (Carrier Sense). When two hosts start transmitting simultaneously, this is detected (Collision Detection), and both hosts will perform random back-off.

  11. Basic Principles – Full Duplex Twisted Pair (UTP) PHYs and certain optical fiber PHYs can be used for point-to-point links only. Since such a LAN consists of exactly two end stations, a Full Duplex mode can optionally be used. Full Duplex mode is mandatory at speeds above 1000Mbps. Pause Frame

  12. Autonegotiation • Twisted pair PHYs send out regular “link test pulses” to let the peer know they’re still there (see green LED on connector). • Autonegotiation was introduced with the 100Mbps PHY generation. It uses bursts of link pulses to enable different PHYs to negotiate the “best” common mode of operation on a given twisted-pair link: • Negotiation of bit rate: 10Mbps / 100Mbps / 1000Mbps • Negotiation of duplex: half/full • Negotiation of modulation: T, TX, T4, T2 • This seems to be one of the (few) weak points in interoperability between Ethernet equipment vendors.

  13. Basic Principles – Bridged LAN Topology LAN A BRIDGE 2 LAN C BRIDGE 1 LAN B BRIDGE 3 BPDU (Bridge Protocol Data Units)

  14. root path cost bridge LAN Basic Principles – Spanning Trees (cont’d) 0 2 2 11 10 4 4 4 12 13 9 19 19 19 Path cost 10Mbps  100 100Mbps  19 1000Mbps  4 1Gbps  2 4 19 5 1 2 3 4 100 100 100 6 7 8

  15. PHYs

  16. Medium Modifier Bitrate, in Mbps Modulation (uppercase!) PHY Overview – Naming Convention 100BASE-TX

  17. PHY Overview

  18. The 10Mbps Generation • 1990: 10BASE-T • Requires 2 pairs of CAT-3 wiring (very widespread) • Maximum reach approximately 100m (up to 150m on CAT-5) • Uses RJ-45 connector • Manchester encoding • Full Duplex mode is optional • 1993: 10BASE-F • Introduced to enhance and replace FOIRL • 10BASE-FL: Manchester encoding on 2 MMF up to 2000m • 10BASE-FB: only to be used as inter-repeater link, Half Duplex • 10BASE-FP: fiber optic passive star, up to 33 hosts at up to 500m

  19. Modern PHYs • The 100Mbps Generation (“Fast Ethernet”) • The 1000Mbps Generation (“Gigabit Ethernet”) • The 10Gbps Generation (“10 Gigabit Ethernet”) • The Media Independent Interface

  20. MAC and IP

  21. IP as a common layer Browser E-Mail Newsgroups File Transfer ….. Everything runs over IP IP (internet Protocol) IP runs over everything LANEthernet . . . Analogue / ISDN(PSTN) Mobile ADSL ATM

  22. LAYER N Application Data LAYER N-1 Header LAYER N-2 Header PhysicalLAYER Header Trailer Data encapsulation ETHERNET Frame

  23. pre- amble SFD DA SA length type P A Y L O A D (46–1500 Bytes) Payload, encapsulated In LLC/SNAP. Frame length or type information Source MAC address (unique device addr.) Destination MAC address (unique device addr.) Fixed sequence to alert the receiver (0x55555555555555D5) MAC Frame FCS Frame Check Sequence, CRC

  24. Types of payload Destination Address (6 bytes) Source Address (6 bytes) FCS (4 bytes) Type (2 bytes) Preamble (7 bytes) 46<=INFO<= 1500 bytes SFD (1 byte) Type 0800 IP Datagram 2 46-1500 Type 0806 ARP request ARP reply PAD 2 28 18 Type 8035 RARP request RARP reply PAD 2 28 18

  25. MAC address

  26. Definition MAC address • 48 bits expressed as 12 hexadecimal digits • E.g. : 00-90-D0-0A-20-1A (Alcatelmodem) • First 6 digits: vendor id • Last 6 digits: interface serial number given by the vendor • First byte always even to indicate interface source address. An odd byte indicates group (multicast) address. • All ones indicates Ethernet broadcast (FF-FF-FF-FF-FF-FF) • Global MAC address is unique in the world • 281,474,976,710,656 addresses. This is more than 56,000 MAC addresses for each person on the planet • For MAC addresses flat addressing scheme is used

  27. Transport Layer (TCP/UDP) Port Number Internet Layer (IP) IP Address Host to Network Layer (PHYS) Physical Address Addressing principles • When a host transmits a packet the different layers contain source & destination addressing information to guarantee proper delivery: • The Physical Address is used to deliver the packet to another host connected to the same physical network (usually the gateway). • The IP address is used to deliver the IP packet to the destination host which can reside at the other side of the world. • The TCP or UDP port number is used to deliver the data to the correct application (window) after delivery to the host. • The combination of the IP address and the Port number makes that the application window itself is uniquely identified (worldwide). This combination is also called a SOCKET.

  28. Host to host communication Best Effort delivery • When host1 wants to send application data to host2 it will encapsulate the data in an Ethernet frame which contains a source & destination physical MAC address • Ethernet is a broadcast network, so each host will receive all frames. Acceptance of the frame is based upon the destination MAC address. • Before delivery to the destination host the sender must retrieve the destination’s MAC address • Connectionless (every packet contains MAC SA/DA ) • Best effort (no datalink layer): reliability provided by TCP/IP Ethernet LAN MAC1 MAC2 IP1 IP2

  29. 1. Create Ethernet frame (IP1 > IP2) IP Header Eth. Header Src: MAC1Dst: ? Src: MAC1Dst: MAC2 Src IP: IP1Dst IP: IP2 Src IP: IP1Dst IP: IP2 3. Send ARP request to retrieve Dest MAC Ethernet Broadcast Src: MAC1Dst: Broadcast Src IP: IP1Dst IP: IP2 4. Receive ARP reply from IP2 Src: MAC2Dst: MAC1 Src IP: IP2Dst IP: IP1 6. Send IP datagram to IP2 Address Resolution Protocol (ARP) ARP Table MAC1 Ethernet frame IP datagram IP2 MAC2 MAC2 Ethernet LAN IP1 IP2 2. IP1 & IP2 belong to the same IP subnet? Yes 5. Create ARP entry

  30. Address Resolution Protocol • To send Ethernet frames from host to host it is necessary to know the destination MAC address • ARP requests asks to translate IP address into MAC address • Since the physical address is not known, ARP is broadcasted • Also Reverse ARP exists • Two possibilities exist: • Dest IP in the same subnet: send ARP using dest. IP • Dest IP in different subnet: send ARP using IP of gateway • LANs interconnected by bridges are within the same subnet. • Remember: Bridges are IP unaware • A bridge is transparent for ARP messages. It is as if the LANs connected to the bridge are one big LAN • Of course the bridge uses SELF LEARNING to reduce traffic between the LANs

  31. Sniffing the Ether

  32. http://www.ethereal.com Free and open source sniffing tool

  33. http://www.ethereal.com Filter Update in realtime Stop after xx seconds

  34. http://www.ethereal.com

  35. HUB/Switch/Router/Gateway

  36. LAN interconnection: Why ? • Why interconnecting LANs ? • The bandwidth and the CSMA/CD limits the number of stations • Limitation on the cable length (see also physical interfaces)E.g.: more than 2.5 km using repeaters for 802.3 • The LANs are geographically spread • Reliability : for example, a defective node that keeps on sending garbage will cripple the LAN (a bridge/router will block this)

  37. LAN interconnection: How ? • How interconnecting LANs ? • Repeaters • Bridges • Routers • Gateways

  38. Basic Principles – Switching & Routing • Bridged Ethernet LANs are switched topologies: frames are forwarded on the basis of an exact match of the 6-byte DA in the forwarding table. • If no match can be found, the frame is flooded to all ports. • MAC addresses are allocated to pieces of hardware. There is no underlying hierarchy. Ethernet is really peer-to-peer. • The IP network commonly running over an Ethernet uses routing mechanisms to forward packets on the basis of a best match of the 4-byte IP-address in the routing table. • IP addresses are inherently hierarchical (scalability). • An IP-aware device uses an “arp table” to associate IP addresses with MAC addresses.

  39. L2 L7 L6 L5 L4 L3 L1 L7 L1 L5 L4 L3 L2 L1 L1 L6 Repeater / HUB • A repeater acts at the physical level (amplify and reshape) • Allows to increase the distance between nodes • Not aware of packets or frames • NO traffic reduction End node End node Repeater

  40. Ethernet hub (repeater) • Ethernet frames are always transmitted to all stations • Also the sending station receives its own data transmitted. This allows to execute the collision detection

  41. L3 L1 L1 L2 L1 L2 L3 L5 L6 L4 L1 L2 L2 L4 L5 L6 L7 L7 Bridge End node End node Bridge MAC address • A Bridge acts at the MAC layer (L2) • Forwarding or not based upon MAC address • Not aware of higher layers (IP, IPX, …) • Self learning (power on and it works!) • Traffic reduction

  42. Increased Network capacity using bridging • As long as traffic remains within LANx you can use the double bandwidth compared to one large LAN • A bridge will buffer a frame from LANx to LANy because LANy could be temporary busy

  43. Bridging Principle • A bridge monitors the traffic on all ports and remembers for each source MAC address on which port it resides. This is called SELF LEARNING. • If the destination MAC address is not known, the frame is forwarded to all interfaces: ‘If you do not know, send it to everybody’ • If the destination MAC address is known as a result of the self learning, the frame is forwarded to the indicated interface • A bridge can be : • self-learning • forwarding • blocking

  44. L4 L2 L1 L2 L3 L4 L5 L6 L1 L3 L7 L2 L5 L6 L7 L1 L1 L2 L3 L3 Router End node End node Router IP address • A Router acts at the Internet layer (L3) • Routing based upon IP address • Not aware of higher layers (TCP segments, applications) • Self learning based upon routing protocols • Traffic reduction

  45. L1 L1 L4 L3 L2 L7 L6 L6 L5 L4 L3 L2 L5 L7 L1 L6 L5 L4 L7 L6 L7 L4 L5 L2 L3 L2 L1 L3 Gateways End node Gateway End node • A Gateway acts at all layers (including applications) • Decisions based upon packet content • Packets can be rerouted, discarted, change content, ...

  46. LAN to LAN over WAN

  47. Ethernet LAN MAC2 IP2 LAN interconnect using SDH CORPORATE LAN LAN switch or Router MAC1 IP1 VC12, VC3 or VC4 connection Eth LAN to LAN board(physical connection so no bridging or routing, so no traffic reduction) SDH Ethernet LAN Eth Eth MAC3 IP3

  48. BRIDGEor ROUTER BRIDGEor ROUTER Decrease Traffic using Bridge or Router • Without the bridge or router all the intra-LAN traffic of one LAN is sent over the WAN (SDH) to the other LAN • Unnecessary load on the WAN • Each LAN looses bandwidth due to other intra-LAN traffic • A router will send only the inter-LAN traffic over the WAN which in best case doubles the bandwidth capabilities. • A bridge will initially broadcast all traffic over the WAN, but after selflearning only the inter-LAN traffic over the WAN Ethernet LAN Ethernet LAN ISA ISA SDH IP3 IP2

  49. Future?

  50. DTE Power via MDI (802.3af) Power over Ethernet • IEEE802.3af defines a way to provide electrical power to data terminal equipment over a 10BASE-T,100BASE-TX or 1000BASE-T link. • It uses a previously unused pair to convey up to 12.95W, which can be used to power… • palm/laptop computers • Ethernet telephones • wireless LAN access points • webcams • … • Claims to be the first worldwide power distribution standard!

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