Hi James,
I run the both ends of the pppd with the record flag. It shows that the
sending and receiving files differ and the receiving file includes frames
discarted due to either the BAD FCS residue error or the frame larger
than the MRU, both of which do point into the problems under the
ppp layer.
I appreciate very much your comments and prompt responses.
One comment that I have is that it would be useful to be able to retrieve
the actual cause of the errors with the pppstats or some other way from
the ppp driver. When they show up in the ifconfig it is not clear what's
causing them and it requires some work to get to the actual cause.
Thanks again,
Swavek
On Wed, 12 Jan 2011, James Carlson wrote:
On 01/12/11 13:40, Slawomir Skret wrote:
I added an instrumental printk statement in the ppp_receive_frame to
print a message before the ppp_receive_error(), rebuilt the kernel, and
got these prints for every error count reported. What does that mean? Is
there anything I can do/turn on/instrument/etc to tell what's wrong with
these frames?
It means that your hardware is destroying the data in flight. There's
not much that PPP can do about that except detect the errors (using the
Frame Check Sequence -- a CRC-16 scheme) and discard the packets that
are affected.
You could use the "record filename" option to capture the actual raw
data. That interposes a pty pair, so it's not completely transparent.
To do transparent analysis, you'll need an external serial analyzer.
Several vendors (such as HP) make such machines. They're by no means
cheap, but if you're doing real hardware development, there's no
substitute for having good test gear.
Reasonable operation of PPP assumes a lower layer that has only a modest
-- and hopefully not traffic-sensitive -- error rate. It doesn't have
to be completely error free, but even a small error rate will have a
relatively large effect on usability and performance of the link. (And
systematic errors, such as [say] always discarding the 256th byte of
packets with 256 or more bytes, will make the link effectively useless
for ordinary networking purposes.)
(This isn't really a characteristic of PPP itself, but rather of any
interface intended for datagram networking purposes. Error rates other
than "a little" are bad news.)
Also, I built the pppstats and run it:
/var # ./pppstats
IN PACK VJCOMP VJUNC VJERR | OUT PACK VJCOMP VJUNC
NON-VJ
2351664 1587 0 0 0 | 78364 1465 0 0 1465
but it does not report these errors.
That indicates that there's no significant errors above the framing
level. That's good, as it likely means there are no difficult software
problems to solve.
It's merely a matter of inadequate hardware.
--
James Carlson 42.703N 71.076W <carlsonj@xxxxxxxxxxxxxxx>
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