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[ethernet|LLC]

Ethernet


Ethernet is a widely used data communications network standard developed by DEC, Intel, and Xerox. It uses a bus topology and CMSA/CD access method. The terms Ethernet and the IEEE 802.3 standard are often used interchangeably.

The Ethernet header structure is shown in the illustration below.

Destination Source Len Data unit + pad FCS
(6 bytes) 6 bytes) (2) (46-1500 bytes) (4 bytes)
Table: Ethernet header structure

Destination address
The address structure is as follows:

I/G U/L Address bits
Table: Ethernet destination address structure

FieldDescription
I/GIndividual / group address may be:
0: Individual address.
1: Group address.
U/L Universal /local address may be:
0:Universally administered.
1:Locally administered.

Source address
The address structure is as follows:

0 U/L Address bits
Table: Ethernet source address structure

FieldDescription
0The first bit is always 0.
U/LUniversal/local address may be:
0: Universally administered.
1:Locally administered.

Length/type
In the Ethernet protocol, the value (³ 0x0600 Hex) of this field is Ethernet List, indicating the protocol inside.

In the 802.3 protocol, the value (46-1500 Dec) is the length of the inner protocol, which is the LLC encapsulated inner protocol. (The LLC header indicates the inner protocol type.)

Data unit + pad
LLC protocol.

FCS
Frame check sequence.

LLC


The IEEE 802.2 Logical Link Control (LLC) protocol provides a link mechanism for upper layer protocols. LLC type I service provides a datalink connectionless mode service, while LLC type II provides a connection-oriented service at the datalink layer.

The LLC header structure is shown in the illustration below.

DSAP SSAP Control LLC information
1 byte 1 bytes 1 or 2 bytes
Table: LLC header structure

DSAP
The destination service access point structure is as follows:

I/G Address bits
Table: LLC DSAP structure


FieldDescription
I/G Individual/group address may be:
0:Individual DSAP.
1:Group DSAP.

SSAP
The source service access point structure is as follows:

C/R Address bits
Table: LLC SSAP structure

FieldDescription
C/RCommand/response:
0:Command.
1:Response.

Control
The structure of the control field is as follows:

  1 8 9 16 bits
Information 0 N(S) P/F N(R)
Supervisory 1 0 SS + XXXX P/F N(R)
Unnumbered 1 1 MM + ( P/F + MMM)
Table: LLC control field structure

FieldDescription
N(S) Transmitter send sequence number.
N(R)Transmitter receive sequence number.
P/FPoll/final bit. Command LLC PDU transmission/response LLC PDU transmission.
S: Supervisory function bits:
00:RR (receive ready).
01REJ (reject).
10RNR (receive not ready).
XReserved and set to zero.
MModifier function bits.

LLC information
LLC data or higher layer protocols.

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