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Computernetze 1 (CN1)

Computernetze 1 (CN1). 2.9 Ethernet Frame. Prof. Dr. Andreas Steffen Institute for Internet Technologies and Applications. Lesestoff im Ethernet Buch. Kapitel 2 Ethernet, Seiten 71-83 2.10 Frameformate bei Ethernet. Ethernet Rahmenformate. Ethernet II (DIX).

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Computernetze 1 (CN1)

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  1. Computernetze 1 (CN1) 2.9 Ethernet Frame Prof. Dr. Andreas Steffen Institute for Internet Technologies and Applications

  2. Lesestoff im Ethernet Buch • Kapitel 2 Ethernet, Seiten 71-83 2.10 Frameformatebei Ethernet

  3. Ethernet Rahmenformate Ethernet II (DIX) MAC Rahmengrösse 64 … 1518 Bytes Preamble SFD DA SA PT DATA FCS IFG Preamble Bytes 7 1 6 6 2 46 … 1500 4 Preamble SFD DA SA LEN LLC DATA PAD FCS IFG Preamble 802.2 LLC 802.2 SNAP IEEE 802.3 • Unterscheidungsmerkmal zwischen DIX und 802.3 • Maximale Payload Länge ist 1500 Bytes (0x05DC) • Ethernet Payload Types (PT) haben Werte grösser als 1500, z.B.IP: 0x0800, ARP: 0x0806, AppleTalk: 0x809B, Novell: 0x8137, 0x8138

  4. Felder im Ethernet-Frame • Preamble 7 Bytes 1-0-1-0 Sequenz zur Bit-Synchronisation • SFD (Start-of-Frame Delimiter) 1 Byte 10101011 zur Byte-Synchronisation, Beginn des MAC-Frames • DA (Destination Address) 6 Bytes Hardware Adresse des gewünschten Empfängers • SA (Source Address) 6 Bytes Hardware Adresse des Senders • LEN (IEEE 802.3 Length) oder PT (DIX Ethertype) 2 Bytes Länge der LLC PDU oder Ethernet Payload Type • DATA / PAD (Padding) 46...1500 Bytes Padding mit 0x00 garantiert minimale Paketdauer für CSMA/CD • FCS (Frame Check Sequence) 4 Bytes 32 Bit Cyclic-Redundancy-Check (CRC-32) • IFG (Inter Frame Gap) 12 Bytes Minimaler Abstand zwischen zwei Ethernet-Frames

  5. Frame Frame MAC Adressierung - UNICAST Unique MAC Hardware Address: 00-0C-00-12-34-56 VENDOR Code (OUI) 24 Bits Unique Number 24 Bits This frame is for me One-to-One: UNICAST Sent directly to MAC address of Host 2 00-0C-00-00-00-01 00-0C-00-00-00-02 00-0C-00-00-00-02

  6. Frame Frame Frame Frame One-to-All: BROADCAST - Must be received by all stations - Can be a severe burden especially to slow systems and appliances- Should be used as little as possible - Main obstacle in large flat networks We MUST all process this frame !! To special destination MAC Address (Broadcast Address) 00-0C-00-00-00-01 FF-FF-FF-FF-FF-FF

  7. Frame Frame Frame Frame One-to-Many: MULTICAST - IP Multicast Addresses map to Ethernet Multicast Addresses - Efficient Mechanism for Group Traffic (Video, Spanning Tree, etc.) I don’t care about that frame - Usually locally controlled by additional Protocols (CGMP, IGMP, RGMP, GMRP) - Globally controlled by Multicast routing To special destination MAC Address (group address) 00-0C-00-00-00-01 01-80-C2-00-00-00

  8. Manufacturer Code Medium Access Control (MAC)- Adressen • Individual/Group Bit (erstes Bit, LSB der DA): • I/G=0: Individual Address • I/G=1: Group Address (Broadcast / Multicast) • Universal/Local Bit: • U/L=0: Universally (globally) Administered Address • U/L=1: Locally Administered Address • Manufacturer Code LSB MSB LSB MSB LSB MSB LSB MSB LSB MSB LSB MSB

  9. Manufacturer Codes 00-00-0C Cisco 00-02-55 IBM 00-09-7B Cisco 00-AA-00 Intel 00-0B-5D Fujitsu (Broadcom Gigabit Ethernet NIC) 00-20-AF 3COM Corporation 00-0D-88 D-Link 08-00-09 Hewlett-Packard 08-00-20 Sun What is your PC‘s OUI (Organizationally Unique Identifier)? Look it up at: http://standards.ieee.org/regauth/oui/index.shtml Each OUI allows to uniquely address 224 = 16‘777‘216 NICs

  10. Multicast Adressen 01-80-C2-00-00-00 Spanning tree (for bridges), Ethertype: -802- 01-80-C2-00-00-10 Bridge Management, Ethertype: -802- 01-00-5E-00-00-00 DoD Internet Multicast (RFC 1112), Ethertype: 0800 CF-00-00-00-00-00 Ethernet Configuration Test protocol (Loopback), Ethertype: 9000

  11. Bestimmung der MAC-Adresse • Windows:ipconfig • Unix/Linux: ifconfig • Beispiel: satay# ifconfig eth0 eth0 Link encap:Ethernet HWaddr 00:0D:88:3C:30:F9 inet addr:80.219.192.127 Bcast:255.255.255.255 Mask:255.255.248.0 inet6 addr: fe80::20d:88ff:fe3c:30f9/64

  12. Computernetze 1 (CN1) 2.10 Logical Link ControlLayer (LLC)

  13. LLC Header Format • LLC header is prepended to higher layer data • DSAP (Destination Service Access Point), 8 bits • SSAP (Source Service Access Point), 8 bits • Control Field, 8 or 16 bits • LLC PDU • is encapsulated in MAC frame Layer3Payload LLC Header MAC Header DSAP SSAP Control DATA MAC Trailer 14 1 1 1..2 43..1497 4 Bytes

  14. DSAP and SSAP • Destination Service Access Point and Source SAP • identify the higher level protocol, which is the destination and the source of the given frame • are a kind of protocol type or protocol stack identifier 0 1 2 7 0 1 2 7 0 1 2 3 4 5 7 I/G U D D D D D D C/R U D D D D D D 1 1 M M P/F M M M DSAP SSAP Control LLC Type 1 I/G=0: Individual DSAP I/G=1: Group DSAP U=0: User defined U=1: IEEE defined C/R=0: Command C/R=1: Response

  15. Values for DSAP / SSAP Field • 128 values possible for I/G = 0 • 63 are reserved for IEEE protocols (U =1) • 63 for vendor specific protocols and for free application usage (U = 0) • SAP Examples: 0x00 Null SAPstation with running LLC software always responds to a frame destined to the Null SAP -> LLC Ping can be implemented 0x03 LLC sub-layer management (U=1) 0x06 DoD IP (U=1) 0x42 802.1d Spanning Tree Protocol (U=1) 0xAA SNAP sub-layer (U=1) [0 1 0 1 0 1 0 1] 0xFE ISO Network Layer (U=1)

  16. Values for LLC Control Field • LLC Control field and protocol procedures are similar to HDLC • HDLC procedures allow connection-less and connection-oriented services on a layer 2 link • Connection-less mode is used by • IP, IPX, AppleTalk, etc. over LLC Type 1 • Connection-oriented mode is used by • SNA over LLC Type 2 • NetBIOS over LLC Type 2 (NetBEUI)e.g. in Microsoft Peer-to-Peer Network

  17. Sub Net Access Protocol (SNAP) • SNAP header is prepended to higher layer data • OUI (Organizational Unit Identifier), 3 bytes • Ethernet Type, 2 bytes • In an IP-based Network • OUI: 00-00-00 • Ethertype: 0x0800 (IP), 0x0806 (ARP) Layer3Payload SNAP Header MAC Header LLC Header OUI EthernetType DATA MAC Trailer 14 3 3 2 38..1492 4 Bytes

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