Ethernet Frame

In the TCP/IP world , the encapsulaton of IP datagrams is defined in RFC 894 for Ethernet and in RFC 1042 for IEEE 802 networks.

The Host Requirements RFC requires that every Internet host connected to a 10Mbits/sec Ethernet cable:

1. Must be able to send and recieve packets using RFC 894 encapsulation.

2. Should be able to recieve RFC 1042 packets intermixed with RFC 894

3. May be able to send packets using RFC1042 encapsulation. If the host can send both types of packets , the type of packets mustbe configuratble and the configuration option must default to RFC 894    packets.

• The 802 lengh field says how many bytes follow,up to but not including the CRC at end.
• The Ethernet type filed identifies the type of data that follows.
• In the 802 frame the same type filed occurs later int the SNAP(Sub-network Access Protocol)header , Fortunately none of the valid 802 lengh values is the same as Ethernet type values , making the two frame formats  distinguishable.
• In the Ethernet frame the data immediately follows the type filed, while in the 802 frame format 3 Bytes of 802.2LLC and 5 bytes of 802.2 SNAP follow.
• The DSAP(Destination Service Access Point) and SSAP (Source Service Access Point) are both set to 0xAA . the control filed is set to 0×03. The next 3 bytes , the Org code are all o.
• The CRC field is a cyclic redundancy check that detects errors in the rest of the frame ( this is also called the FCS or frame check sequence)
• There is a minimum size for 802.3 and Ethernet frames. The minimun requires that the data portion be at least 38 bytes for 802.3 or 46 bytes for Ethernet