Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 1 Computernetze 1 (CN1) 2.9 Ethernet Frame Prof....
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Transcript of Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 1 Computernetze 1 (CN1) 2.9 Ethernet Frame Prof....
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 1
Computernetze 1 (CN1)
2.9 Ethernet Frame
Prof. Dr. Andreas SteffenInstitute for Internet Technologies and
Applications
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 2
Lesestoff im Ethernet Buch
• Kapitel 2 Ethernet, Seiten 71-832.10 Frameformate bei Ethernet
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 3
DATA
Ethernet Rahmenformate
Bytes 7 1 6 6 2 46 … 1500 4
DATA FCSPreamble SFD DA SA
IEEE 802.3
Ethernet II (DIX)
IFGPT
802.2 LLC802.2 SNAP
Preamble
Preamble SFD DA SA FCS IFG PreambleLEN LLC PAD
MAC Rahmengrösse 64 … 1518 Bytes
• 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
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 4
Felder im Ethernet-Frame
• Preamble 7 Bytes
1-0-1-0 Sequenz zur Bit-Synchronisation• SFD (Start-of-Frame Delimiter)
1 Byte10101011 zur Byte-Synchronisation, Beginn des MAC-Frames
• DA (Destination Address) 6 Bytes
Hardware Adresse des gewünschten Empfängers• SA (Source Address)
6 BytesHardware 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 BytesPadding mit 0x00 garantiert minimale Paketdauer für CSMA/CD
• FCS (Frame Check Sequence) 4 Bytes32 Bit Cyclic-Redundancy-Check (CRC-32)
• IFG (Inter Frame Gap) 12 BytesMinimaler Abstand zwischen zwei Ethernet-Frames
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 5
MAC Adressierung - UNICAST
One-to-One: UNICAST
FrameFrame
00-0C-00-00-00-01 00-0C-00-00-00-01
Sent directly to MACaddress of Host 2
This frame is for me
00-0C-00-12-34-56
00-0C-00-12-34-56
VENDOR Code (OUI) 24 Bits
Unique Number24 Bits
Unique MAC Hardware Address:
00-0C-00-00-00-02 00-0C-00-00-00-02 00-0C-00-00-00-02 00-0C-00-00-00-02
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 6
One-to-All: BROADCAST
To special destination MACAddress (Broadcast Address)
Frame
Frame
Frame
Frame
We MUST all process this frame
!!
- 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
00-0C-00-00-00-01 00-0C-00-00-00-01 FF-FF-FF-FF-FF-FFFF-FF-FF-FF-FF-FF
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 7
One-to-Many: MULTICAST
Frame
Frame
Frame
Frame
I don’t care about that
frame
To special destination MACAddress (group address)
- IP Multicast Addresses map to Ethernet Multicast Addresses- Efficient Mechanism for Group Traffic (Video, Spanning Tree, etc.)
- Usually locally controlled by additional Protocols (CGMP, IGMP, RGMP, GMRP)- Globally controlled by Multicast routing
00-0C-00-00-00-01 00-0C-00-00-00-01 01-80-C2-00-00-0001-80-C2-00-00-00
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 8
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
Manufacturer Code
LSB MSB
LSB MSBLSB MSBLSB MSBLSB MSBLSB MSB
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 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
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 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
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 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
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 12
Computernetze 1 (CN1)
2.10 Logical Link Control
Layer (LLC)
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 13
14 1 1 1..2 43..1497 4 Bytes
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
MAC Header DSAP SSAP Control MAC TrailerDATA
LLC HeaderLayer3Payload
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 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
D D D D D DI/G U
0 1 2 7 0 1 2 7 0 1 2 3 4 5 7
U=0: User definedU=1: IEEE defined
SSAPDSAP
I/G=0: Individual DSAPI/G=1: Group DSAP
C/R=0: CommandC/R=1: Response
C/R U 1 1 M M P/F M M MD D D D D D
Control LLC Type 1
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 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 SAP
station 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)
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 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
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 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)
MAC Header LLC HeaderEthernetType
MAC TrailerDATA
SNAP Header
Layer3Payload
14 3 3 2 38..1492 4 Bytes
OUI