12 ethernet-wifi

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Last lesson of the computer networking course : Ethernet, Spanning Tree, WiFi and IPv4

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  • This is the most widely used format, it is notably used to carry IP packets.
  • The 10 Gbps zoo is much larger than this, see e.g. http://en.wikipedia.org/wiki/10_gigabit_Ethernet
  • See

    [IEEE802Q] "IEEE Standards for Local and Metropolitan Area
    Networks: Virtual Bridged Local Area Networks", Draft Standard, P802.1Q/D9, February 20, 1998.
  • Example

    802.11b channel frequencies
    Channel Lower frequency Central frequency Upper frequency
    1 2.401 2.412 2.423
    2 2.404 2.417 2.428
    3 2.411 2.422 2.433
    4 2.416 2.427 2.438
    5 2.421 2.432 2.443
    6 2.426 2.437 2.448
    7 2.431 2.442 2.453
    8 2.436 2.447 2.458
    9 2.441 2.452 2.463
    10 2.446 2.457 2.468
    11 2.451 2.462 2.473
  • c
  • 12 ethernet-wifi

    1. 1. Week 12 Ethernet WiFi
    2. 2. Agenda • Ethernet • Spanning Tree • WiFi • IP version 4
    3. 3. Ethernet Frames• DIX Format • proposed by Digital, Intel and Xerox Preamble [8 bytes] Destination address Type [2 bytes] CRC [32 bits] Source address Data [46-1500 bytes Used to mark the beginning of the frame Allows the receiver to synchronise its clock to the sender’s clock Indication of the type of packet contained inside the frame Upper layer protocol must ensure that the payload of the Ethernet frame is at least 46 bytes and at most 1500 bytes
    4. 4. The Ethernet zoo 10BASE5 Thick coaxial cable, 500m 10BASE2 Thin coaxial cable, 185m 10BASE-T Two pairs of category 3+ UTP 10BASE-F 10 Mb/s over optical fiber 100BASE-TX Category 5 UTP or STP, 100 m maximum 100BASE-FX Two multimode optical fiber, 2 km maximum 1000BASE-CX Two pairs shielded twisted pair, 25m maximum 1000BASE-SX Two multimode or single mode optical fibers with lasers 10 Gbps optical fiber but also cat 6 twisted pair 40-100 Gbps being developed, standard expected in 2010, 40Gbps one meter long for switch backplanes, 10 meters for copper cable and 100 meters for fiber optics
    5. 5. Ethernet switch • A switch is a relay that operates in the datalink layer Host A Host BSwitch Physical Phys. Phys. Datalink Network Network Datalink Physical
    6. 6. How to favour high- speed links ? Switch 1 Switch 7 Switch 9 Switch 22 Switch 44 Switch 2
    7. 7. Link costs Bandwidth Recommended link cost range Recommended link cost value 10 Mbps 50-600 100 100 Mbps 10-60 19 1000 Mbps 3-10 4
    8. 8. Selection of root • Root priority vectors • Port 1: 8,7+100,9 • Port 2 : 8,9+1,22 • Port 3 : 8,4+10,17 • Port 4: 8,4+10,18 S91 1 2 34 R=8,C=7,T=9 R=8,C=9,T=22 R=8,C=4,T=17R=8,C=4,T=18 • Switch S91's BPDU • R=8, C=10,T=91
    9. 9. Switches and hubs • How should the spanning tree work with hubs ? S1 1 4 Hub1 S3 1 2 Hub2 2
    10. 10. The states of the ports• Root port • Port having the best root priority vector • Only one root port per switch ! • Designated port • Ports where the switch's BPDU is better than best BDPU received • Blocked ports • Ports where the switch's BPDU is worse than best BDPU received
    11. 11. The root switch • What is the state of the ports of the root switch ? • How to influence the selection of the root switch ? S1 1 2 34
    12. 12. Corner cases • Parallel links • Backup links to same LAN S2S3 1 2 3 4 S1 2 4 S1
    13. 13. Spanning tree 1 2 34 1 1 1 1 2 22 2 3 3 3 4 S222S111 S333 S444 S555 1 Gbps, cost =10 10 Gbps, cost =1
    14. 14. BPDU format • Simplified BPDU format BPDU Header Root Id Switch identifier Root path cost Protocol Identifier Protocol version Configuration BPDU or topology change Flags Identifier of the switch sending the BPDU Port identifier : used when a switch has several ports attached to the same LAN Current root identifier Port identifier Message age Max age Hello time Forward delay
    15. 15. Port states and activityReceive BPDUs Transmit BPDUs Blocked yes no Root yes no Designated yes yes Learn Addresses Forward Data Frames Inactive no no Active yes yes
    16. 16. Failures • Failure (power-off) of the root switch • A new root needs to be elected • Failure of a designated switch • Another switch should replace • Failure of a link • a disabled link should be enabled • If the network is split we have two separated networks
    17. 17. Dealing with failures• Regular transmission of BPDUs • Default Hello timer is two seconds • BPDUs stored in the switches age and are removed when they timeout • Failure notification mechanism • When switch detects important failure, it sends a topology change BPDU to Root • Upon reception of TC BPDU all switches stop forwarding data frames and recompute ST
    18. 18. Full duplex Ethernet Observations In many networks, Ethernet is a often a point-to-point technology host-to-switch switch to switch Twisted-pairs and fiber-based physical layers allow to send and receive at the same time S1 S2 HUB HUB
    19. 19. Ethernet full duplex No collision is possible on a full duplex Ethernet/FastEthernet/GigabitEthernet link Disable CSMA/CD on such links Advantages Improves bandwidth Both endpoints can transmit frames at the same time CSMA/CD is disabled No constraint on propagation delay anymore Ethernet network can be as large as we want ! No constraint on minimum frame size anymore We do not need the frame extension hack for Gigabit Ethernet!
    20. 20. Full duplex Ethernet (3) Drawback If CSMA/CD is disabled, access control is disabled and congestion can occur How to solve this problem inside Ethernet ? Add buffers to switches but infinite buffers are impossible and useless anyway Cause collisions (e.g. jamming) to force collisions on the inter-switch link and uplink is server is too fast Drawback : interswitch link could be entirely blocked Develop a new flow control mechanism inside MAC layer Pause frame to slowdown transmission S1 S2Server Client FastEthernet (100 Mbps) Ethernet (10 Mbps)
    21. 21. Ethernet flow control PAUSE frame indicates how much time the upstream should wait before transmitting next frame S1 server Client FastEthernet (100 Mbps) Ethernet (10 Mbps) PAUSE [2msec] Frame1 [10000 bits] Frame3 [10000 bits] Frame2 [10000 bits] 100 nsec Frame1 [10000 bits] 1 microsec Frame2 [10000 bits] Sender blocked
    22. 22. Virtual LANs Allows to build several logical networks on top of a single physical network S A B C D F E Each port on each switch is associated to a particular VLAN All the hosts that reside on the same VLAN can exchange Ethernet frames A host on VLAN1 cannot send an Ethernet frame towards another host that belongs to VLAN2 Broadcast and multicast frames are only sent to the members of the VLAN VLAN1 : A,E,F VLAN2 : B,C,D
    23. 23. VLANs in campus networks How to support VLANs in a campus network S1 A B C D F E VLAN1 : A,E,F VLAN2 : B,C,D S2 Possible solutions Place on each switch a table that maps each MAC address on a VLAN id difficult to manage this table Change frame format used on inter- switch links to include a VLAN identifier new header added by first switch new header removed by last switch
    24. 24. VLAN frame format Destination Address Address Identifies the frame as containing VLANtag Tag control information contains two types of information : - VLAN identifier (12 bits) : up to 4094 different VLANs can be defined - Priority (3 bits) : indicates the importance of the frame and can be used by switches to provide a better service for some frames (e.g. Voice) Type CRC [32 bits] Payload VLAN Protocol Id 0x8100 Tag Control Info
    25. 25. Agenda • Ethernet • Spanning Tree • WiFi • IP version 4
    26. 26. The WiFi zoo Standard Frequency Typical throughput Raw bandwidth Range in/out (m) 802 .11 2.4 GHz 0.9 Mbps 2 Mbps 20 / 100 802 .11a 5 GHz 23 Mbps 54 Mbps 35 / 120 802 .11b 2.4 GHz 4.3 Mbps 11 Mbps 38 / 140 802 .11g 2.4 GHz 19 Mbps 54 Mbps 38 / 140 802 .11n 2.4 / 5 GHz 74 Mbps up to 600 Mbps 70 / 250 Source http://en.wikipedia.org/wiki/IEEE_802.11n
    27. 27. Practical issues with WLAN deployments Home environment A WLAN can interfere with the neighbour’s WLAN
    28. 28. Practical issues with WLAN deployments Enterprise networks One access point can interfere with other access points reduces significantly overall available bandwidth
    29. 29. The WiFi channel frequencies WiFi standards operate on several frequencies called channels Usually about a dozen channels Why multiple channels ? Some channels my be affected by interference and have a lower performance Some frequencies are reserved for specific usage in some countries Allows frequency reuse when there are multiple WiFi networks in the same area Unfortunately, many home access points operate by default on the same factory set channel which causes interference and reduced bandwidth
    30. 30. WLAN in enterprise environments What could be done to improve the performance of WLANs ? Reduce interference as much as possible Tune channel frequencies Reduce transmission power Similar to techniques used in GSM networks Recent deployments rely on centralized controllers and thin access points
    31. 31. 802.11 frame format Frame control [2 bytes] Duration/Id [2 bytes] Address 2 [6 bytes] Address 1 [6 bytes] Standard header - Protocol version [2 bits] : current version 0 - Type [2 bits] : control / data / management frame - Subtype [2 bits] : specific subtype of frame - to DS [1 bit] : frame is sent to distribution system - from DS [1 bit] : frame is from distribution system - more fragment [1 bit] : used when packets are fragmented - Retry [1 bit] : retransmission - Power Management [1 bit] : used for power management fct - More data [1 bit] : indicates that there are other frames for this station at the access point - WEP [1 bit] : 1 if frame has been encrypted with WEP - order [1 bit] : for strictly ordered class Address 3 [6 bytes] Sequence control [2 bytes] Frame body [0-2312 bytes] Frame Check Sequence Sequence number - 12 bits frame sequence number - 4 bits fragment number
    32. 32. Some 802.11 control frames Frame control [2 bytes] Duration [2 bytes] Receiver address [6 bytes] Frame Check Sequence Frame control [2 bytes] Duration [2 bytes] Receiver address [6 bytes] Frame Check Sequence Transmitter address [6 bytes] Frame control [2 bytes] Duration [2 bytes] Receiver address [6 bytes] Frame Check Sequence ACK frame RTS frame CTS frame
    33. 33. IP over 802.11 Frame control Duration/Id [2 bytes] Address 2 [6 bytes] Address 1 [6 bytes] Address 3 [6 bytes] Sequence control [2 bytes] IP packet Frame Check Sequence LLC/SNAP 0x800 LLC/SNAP - 4 bytes header EtherType - 0x800 for IP, 0x86DD for IPv6
    34. 34. Agenda • Ethernet • Spanning Tree • WiFi • IP version 4
    35. 35. IP version 4 Ver IHL DS Total length Payload 32 bits ChecksumTTL Protocol Flags FragmentOffset 20 bytes Source IP address Identification Destination IP address Differentiated Services Byte used to specify Quality of Service expected for this packet IP version used to encode header - current version is 4 - IP version 6 Header length (default 20 bytes) Maximum : 64 bytes for entire header including options Binary flags More Don't Fragment : Packet cannot be fragmented by intermediate routers Allows to identify the “user” above the IP layer (e.g. UDP, TPC, ...) Plays similar role to TCP port numbers Packet identification used for fragmentation and reassembly Options Optional header extension Time to Live
    36. 36. IPv4 addresses • 32 bits long, one address per interface • Example Notation 138.48.26.1/23 or 138.48.26.1 255.255.254.0 • All hosts that belong to the same subnetwork can directly exchange frames through datalink layer
    37. 37. ARP : Address Resolution Protocol IP: 10.0.1.9 Eth : B IP: 10.0.1.8 Eth : C IP: 10.0.1.11 Eth : E IP: 10.0.1.22 Eth : A 10.0.1.22 needs to send an IP packet to 10.0.1.8 1 IP: 10.0.1.9 Eth : B IP: 10.0.1.8 Eth : C IP: 10.0.1.11 Eth : E IP: 10.0.1.22 Eth : A ARP : broadcast frame Addr Eth 10.0.1.8 ? 2 IP: 10.0.1.9 Eth : B IP: 10.0.1.8 Eth : C IP: 10.0.1.11 Eth : E IP: 10.0.1.22 Eth : A 10.0.1.8 replies in an Ethernet frame and A knows the MAC address to send its IP packet 3
    38. 38. ARP : frame format Preamble [7 bytes] Delimiter[1byte] Destination Address Type: 0x806 CRC [32 bits] Source Address MAC address of the sender Broadcast : 111...111 Header Sender MAC Sender IP Target MAC Common header for all ARP frames - Hardware type Ethernet is 1 - Protocol type , IP is 0x0800.- Hardware length : length of MAC address - Protocol length : length of network layer address - Operation : 1 for request, 2 for reply, 3 for RARP request, and 4 for RARP reply. Target IP
    39. 39. ICMP version 4 Ver IHL DS IP header ChecksumTTL Protocol Source IP address Identification Destination IP address Data Type Code Ver IHL DS Total length ChecksumTTL Protocol Flags FragmentOffset Source IP address Identification Destination IP address First 64 bits of payload Flags FragmentOffset Protocol=1 for ICMP covers entire ICMP message Additional information about error, type of error Total length Checksum 32 bits ICMP header Type and Code indicate the type of error detected l Destination unreachable lnetwork unreachable lhost unreachable lprotocol unreachable lport unreachable lfragmentation needed lsource route failed lRedirect lParameter problem lTime exceeded lTTL exceeded lreassembly time exceeded lEcho requEast et Echo reply
    40. 40. IP over Ethernet Detailed example Examples IP packet from 10.0.1.22 to 10.0.3.11 IP packet from 10.0.2.9 to 10.0.1.22 IP packet from 10.0.3.11 to 10.0.1.22 IP: 10.0.2.9/24 Eth : B 10.0.1.0/24 via 10.0.2.1 10.0.3.0/24 via 10.0.2.2 ARP table Empty IP: 10.0.1.8/24 R default: 10.0.1.1 Eth : C ARP table Empty IP: 10.0.1.22/24 R default: 10.0.1.1 Eth : A ARP table Empty R1 IP: 10.0.1.1/24 IP: 10.0.2.1/24 Eth : R1-West Eth : R1-East ARP table Empty H1 S2 R2 Hub Router Switch Router IP: 10.0.3.11/24 Eth : F R default:10.0.3.2 ARP table Empty IP: 10.0.2.2/24 IP: 10.0.3.2/24 Eth : R2-West Eth : R2-East ARP table Empty
    41. 41. The Internet architecture that students learn Physical Datalink Network Transport Application O. Bonaventure, Computer networking : Principles, Protocols and Practice, open ebook, http://inl.info.ucl.ac.be/cnp3 Physical Physical Datalink Physical Datalink Network
    42. 42. A typical "academic" network Physical Datalink Network Transport Application Physical Datalink Network Transport Application Physical Datalink Network Physical Datalink
    43. 43. The end-to-end principle Physical Datalink Network Transport Application Physical Datalink Network Transport Application Physical Datalink Network Physical Datalink TCP
    44. 44. In reality – almost as many middleboxes as routers – various types of middleboxes are deployed Sherry, Justine, et al. "Making middleboxes someone else's problem: Network processing as a cloud service." Proceedings of the ACM SIGCOMM 2012 conference. ACM, 2012.
    45. 45. A middlebox zoo http://www.cisco.com/web/about/ac50/ac47/2.html Web Security Appliance NAC Appliance ACE XML Gateway Streamer VPN Concentrator SSL Terminator Cisco IOS Firewall IP Telephony Router PIX Firewall Right and Left Voice GatewayVVVV Content Engine NAT
    46. 46. How to model those middleboxes ? • In the official architecture, they do not exist • In reality... Physical Datalink Network Transport Application Physical Datalink Network Transport Application Physical Datalink Network TCP Physical Datalink Network Transport Application
    47. 47. TCP segments processed by a router Source port Destination port Checksum Urgent pointer THL Reserved Flags Acknowledgment number Sequence number Window Ver IHL ToS Total length ChecksumTTL Protocol Flags Frag. Offset Source IP address Identification Destination IP address Payload Options Source port Destination port Checksum Urgent pointer THL Reserved Flags Acknowledgment number Sequence number Window Ver IHL ToS Total length ChecksumTTL Protocol Flags Frag. Offset Source IP address Identification Destination IP address Payload Options IP TCP
    48. 48. Network Address Translators • Preserves IP addresses by using private addresses in LAN – Packets's addresses are rewritten by NAT Private addresses Public addresses
    49. 49. TCP segments processed by a NAT Source port Destination port Checksum Urgent pointer THL Reserved Flags Acknowledgment number Sequence number Window Ver IHL ToS Total length ChecksumTTL Protocol Flags Frag. Offset Source IP address Identification Destination IP address Payload Options Source port Destination port Checksum Urgent pointer THL Reserved Flags Acknowledgment number Sequence number Window Ver IHL ToS Total length ChecksumTTL Protocol Flags Frag. Offset Source IP address Identification Destination IP address Payload Options

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