Fibre Channel does not recognize any unit of data less than one word long:
• 1 word = 4 bytes • One cannot assume that the data payload for a frame will end on a word boundary. (The ULP is unaware of this Fibre Channel requirement).
• In order to ensure that the data payload ends on a word boundary, Fibre Channel introduces ‘fill bytes’.
• Fibre Channel uses a maximum of 3 fill bytes. This is the maximum needed to ensure that data (which may be byte orientated) can be aligned on a word boundary (4 bytes) to comply with Fibre Channel requirements.