2.9 ethernet frame
TRANSCRIPT
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 1
Computernetze 1 (CN1)
2.9 Ethernet Frame
Prof. Dr. Andreas Steffen
Institute for Internet Technologies and Applications
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 2
Lesestoff im Ethernet Buch
• Kapitel 2 Ethernet, Seiten 71-83
2.10 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 FCS Preamble SFD DA SA
IEEE 802.3
Ethernet II (DIX)
IFG PT
802.2 LLC 802.2 SNAP
Preamble
Preamble SFD DA SA FCS IFG Preamble LEN 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
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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
Steffen/Stettler, 26.09.2013, 2b-Ethernet_Frame.ppt 5
MAC Adressierung - UNICAST
One-to-One: UNICAST
Frame
Frame
00-0C-00-00-00-01
Sent directly to MAC address of Host 2
This frame is for me
00-0C-00-12-34-56
VENDOR Code (OUI) 24 Bits
Unique Number 24 Bits
Unique MAC Hardware Address:
00-0C-00-00-00-02 00-0C-00-00-00-02
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One-to-All: BROADCAST
To special destination MAC Address (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 FF-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 MAC Address (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 01-80-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 MSB LSB MSB LSB MSB LSB MSB LSB MSB
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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 Trailer DATA
LLC Header Layer3 Payload
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 D I/G U
0 1 2 7 0 1 2 7 0 1 2 3 4 5 7
U=0: User defined U=1: IEEE defined
SSAP DSAP
I/G=0: Individual DSAP I/G=1: Group DSAP
C/R=0: Command C/R=1: Response
C/R U 1 1 M M P/F M M M D D D D D D
Control LLC Type 1
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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 Header Ethernet
Type MAC Trailer DATA
SNAP Header
Layer3 Payload
14 3 3 2 38..1492 4 Bytes
OUI