In trying to track down potential performance problems at my ISP, I've
discovered that I'm seeing a fair number of TCP packets (not IP, TCP)
discarded for failing the TCP checksum.  (discovered with netstat -ptcp)
This is most interesting, since I'm seeing it over links where all of the
media layers have checksums of their own (ethernet, PPP).  It might be
source-host dependant (I'm using ftp to create the traffic), but I have
seen at least one connection range from a small number of errors to a
fairly large number.

One very obvious suspect is, of course, the TCP checksum code on NetBSD, so
I am wondering if anyone else is seeing TCP checksum errors, especially in
situations where on-the-wire corruption can be reasonably ruled out.
(Conversely, can anyone think of perfectly natural reasons why I would see these
errors in the normal course of affairs?)

I am connected to the Internet via a PPP link.