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SubjectRe: "Greased TCPenguin"
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Richard Gooch <rgooch@atnf.CSIRO.AU> writes:

> Andrea Arcangeli writes:
> > On Tue, 24 Mar 1998, Richard Gooch wrote:
> >
> > >Something still not right:
> > >
> > >TCPv4 bad checksum from 130.155.198.47:03ff to 130.155.191.26:03fd, len=32/32/52
> > >TCPv4 bad checksum from 130.155.198.47:03ff to 130.155.191.26:03fd, len=32/32/52
> > >TCPv4 bad checksum from 130.155.198.47:0448 to 130.155.191.26:1770, len=32/32/52
> > >TCPv4 bad checksum from 130.155.198.47:0201 to 130.155.191.26:03ff, len=20/20/4
> >
> > I read messages such us this a lot of times but I never related it to a
> > possible TCP bug, I always think that the ppp/slip is a poor connection
> > and can miss something (we use checksum for that cases no?).
>
> My modem link has compression and error checking, so in theory I have
^^^^^^^^^
(but your serial cable between computer and modem hasn't), also note
that theory and practice often differ. Another possibility is that
the packets got corrupted further down the link.

> a perfect link. I also didn't get these messages with earlier versions
> of the kernel (it seems 2.1.86 is the first time I got it, according
> to my kernel logs).

I assume that is because 2.1.91-pre stresses your PPP link much more
than earlier 2.1 kernels. Note that 2.0 doesn't look these messages,
because they're for debugging only (and actually forbidden by RFC1122
- which specifies that these packets should be "silently discarded").

You can check if there are checksum errors in 2.0 too by checking
the InErr field of TCP and the InErrors field of UDP in /proc/net/snmp
[or use newer nettools that support the -s switch].

To state it again - some checksum errors are perfectly ok. One should
only worry when they happen extensively.

-A.

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