>My first thought is that creating overflows on the other site by filling
>16 bytes at once shouldn't be possible. The other side should use a driver
>that uses a buffer of the right size and a well defined threshold to handle
>the extra bytes whenever it has asked the other end to stop transmitting.
>
>Of course, it's alway possible to have lots of data `on the line' but
>16 more bytes should never matter.

I'd like to say that I've observed that it does indeed matter.

I have a modem with flashable firmware.  You basically send an AT command
and then a Motorola S-record file.  It uses hardware flow control to
regulate the data.

With the old serial drivers, I could upgrate the firmware from NetBSD;
with the new ones, I cannot.  I couldn't figure it out, but this sounds
like the most likely explanation (but I will say that I am not 100% sure
this is the exact problem).

--Ken