ARCNET also has a special problem with framing. ARCNET frames consist of a hardware header and a software header. The software header is in the data area of the hardware packet, and includes at least the protocol ID. There are two types of software header. Old-style ARCNET software headers consist entirely of a one or two byte protocol ID. New ARCNET software headers (defined in RFC 1201 and in the paper "ARCNET Packet Header Definition Standard", Novell, Inc., 1989) include more information. They allow more efficient use of ARCNET through data link layer fragmentation and reassembly (ARCNET has a small Maximum Transmission Unit) and allow sending any size packet up to the MTU (rather than requiring that packets of size 253, 254 and 255 be padded to at least 256 bytes). SANA-II device drivers for ARCNET should implement the old ARCNET packet headers. Driver users which wish to interoperate with platforms using the new software headers must add the new fields to the data to be sent and must process it for incoming data. A SANA-II driver which implemented the data link layer fragmentation internally (and advertised a large MTU) could be more efficient than requiring the driver user to do it. This would make driver writing more difficult and reduce interoperability, but if there is ever a demand for that extra performance, a new hardware type may be assigned by Commodore for SANA-II ARCNET device drivers which implement the new framing.
[Back to Amiga Developer Docs]