RE: CAN bus netboot

["Martin Atkins" <martin@cs.york.ac.uk>]

Hi all,
Markus writes:
> Thomas Wuensche writes:
>  > Of course it is possible to put a TCP/IP protocol stack onto CAN,
>  > however this is not used very much - CAN is used in the automotive
>  > and industrial automation field, and the applications taking
>  > benefit from CAN mostly use other protocols.
> 

I did some development work using CAN in a previous incarnation,
and developed a higher-level protocol on top of the basic CAN protocol.

> Can you give a brief description of how TCP/IP is implemented on CAN?
> I always thought the the main advantage of CAN was the automatic
> collision detection/resolution that was implemented in hardware. To a
> certain extent, this also buys you some guarantees about the available

Indeed, it is possible (we did it!) to give absolute guarantees on the 
time it will take to propagate some information. (In the absence of
communication errors :-)

> bandwidth for each different type of service. My understanding is,
> that this requires specially formated device IDs at the beginning of
> each datagram. Do IP packets get encapsulated within CAN packets or do
> people forgo the advantages of the CAN protocol and directly send IP
> frames (or more likely, ethernet frames) onto the wire?

It is not possible to put other kinds of frames on to the wire (and still
call it CAN!). The hardware provides arbitrated, single-writer, 
multiple-reader reliable multicast, and as such is not really the
infrastructure which TCP/IP was designed for. Adding TCP/IP's acknowledges,
etc, loses many of the benefits (and bandwidth!) of CAN. However it could
be done by simply getting each node to use it's own (unique) identifier
as the CAN identifier of the messages it transmits. All nodes would then
have to check the first bytes of every message to see if a packet was
addressed to them - the max 8 byte packet size would be a significant
problem though!

I don't think this is now very relevent to netboot - so maybe any further
communication should be off the list?

Martin


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