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Destination address (DA) — The DA field is used to identify which
network device(s) should receive the packet. It is a unique address.
Please see the section of MAC addressing.
Source addresses (SA) — The SA field indicates the source node. The SA is
always an individual address and the left-most bit in the SA field is always 0.
Length/Type — This field indicates either the number of the data bytes
contained in the data field of the frame, or the Ethernet type of data. If the value of
first two bytes is less than or equal to 1500 in decimal, the number of bytes in the
data field is equal to the Length/Type value, i.e. this field acts as Length indicator at
this moment. When this field acts as Length, the frame has optional fields for
802.3/802.2 SNAP encapsulation, 802.3/802.2 encapsulation and Netware 802.3
RAW encapsulation. Each of them has different fields following the Length field.
If the Length/Type value is greater than 1500, it means the
Length/Type acts as Type. Different type value means the frames
with different protocols running over Ethernet being sent or received.
For example,
0x0800 IP datagram
0x0806 ARP
0x0835 RARP
0x8137 IPX datagram
0x86DD IPv6
Data — Less than or equal to 1500 bytes and greater or equal to 46 bytes. If
data is less than 46 bytes, the MAC will automatically extend the padding bits and
have the payload be equal to 46 bytes. The length of data field must equal the value
of the Length field when the Length/Type acts as Length.
Frame check sequence (FCS) — This field contains a 32-bit cyclic redundancy
check (CRC) value, and is a check sum computed with DA, SA, through the end of
the data field with the following polynomial.
It is created by the sending MAC and recalculated by the receiving MAC to
check if the packet is damaged or not.
Publication date: January, 2005
Revision A1
28