Talisker Internet Protocol IP Frame Header Image

FDDI headers

When Ethereal or TCPDump analyses FDDI packets they do not account for the FDDI Frame Control byte at the start of the packet.

Below is a selection of Frame Control bytes and their meaning.

Hex	Description
40	Don’t know yet
41	Station Management
4F	Station Management
C2	MAC
C3	MAC
50	LLC, I reckon that any Frame Control byte that begins with 5 defines an LLC header following, after the MACs
51
60	Implementer
70	Reserved
80	Non Restricted Token
C0	Restricted Token

The basic format of an FDDI header:

FC	Destination MAC	Source MAC	DATA	CRC
1	6	6	0 to 4478	4

The digits above are the length of the field in bytes.

FC – The frame control byte contains the decoder for the next header i.e. LLC, this can be used to determine whether the frame was originated on Ethernet or if it is native FDDI.

Destination MAC – speaks for itself
Source MAC – speaks for itself
DATA – can contain user data or embedded headers and bits and pieces
CRC – Cyclic Redundancy Check.