Following up to my own note, I see that I misread your response. Let
me see if we can get this straight.
First, restricting attention to the PPP interface, are you seeing
"TCPv4 checksum" errors in the kernel logs, or are you seeing framing
errors in "ifconfig" or "cat /proc/net/dev" output? The latter are
not, strictly speaking, checksum errors; they are CRC errors at the
PPP frame level, and one possible cause is lost interrupts resulting
in dropped characters. They can occur with or without VJ compression.
Second, if VJ compression is truly disabled, you should never see a
TCP checksum error caused by packet corruption on the PPP link. This
is because the PPP CRC check should fail, so the kernel will never see
the corrupted packet. In this case, there will be no TCP checksum
error, but there *will* be a framing error. If you do see TCP
checksum errors but do *not* see framing errors, this is probably
because some host or router is sending you corrupt packets.
If you do see TCP checksum errors and you also see framing errors in
roughly equal numbers then this is almost certainly because you have
VJ compression enabled and the PPP link is corrupting packets. This
is how VJ compression error recovery works: if there is a framing
error, it will normally be followed by a TCP checksum error on the
next packet. In this case, the advice I've given previously still
holds; disabling VJ compression will mask the symptom, but you'll have
to go interrupt hunting to cure the disease.
Third, you should *certainly* not be seeing framing or other errors on
the loopback device. Be warned that old versions of "ifconfig"
misalign the columns while reading device information from newer
kernels. They can print the number of packets as the number of
errors. I suspect this is your problem with the loopback and network
card devices. Check the output of "cat /proc/net/dev" instead.
Kevin <buhr@stat.wisc.edu>
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