Re: laptop fails to "hear" replies

[Dave Johnson]

Lloyd Kvam writes:
> When my laptop boots, it is configured to use a dhcp server.  For some
> time now, that processing has failed.  I can run 
> tcpdump -n ether host 
> and see the packets going out, but not the replies.
> 
> At the dhcp server, the same tcpdump command shows the requests and
> replies.
> 
> I can get the laptop network stack to function by manually assigning a
> valid IP address and then repeatedly pinging other computers on the
> network.  The arp table is empty, showing incompletes for the mac
> addresses.  Finally a ping will work and the arp table gets filled in
> and the networking functions are OK from then on.  I can restart the
> network service and now dhcp works.  (This behavior occurs at other
> sites, so it is not a switch or cable problem.)
> 
> Since everything eventually works OK, I figure the hardware is good and
> I've fouled up some configuration item.  I've tried enabling and
> disabling NetworkManager, but it does not seem to have any impact.
> Stopping iptables also has no effect.
> 
> I'm hoping someone can suggest a debugging approach or possibly stuff I
> misconfigured to create this problem.

Sounds like your switch(es) are running STP, LACP, and/or EAPoL.
Any of these can disable your ethernet port on the switch side
temporarially after link up.  Those protocols will give up after 30-40
seconds and enable your port.

LACP has a state where a newly link-up port can send into the network
but not recieve from the network, however I think this only applies
when a port is entereing into a LAG (this is a staging state called
the 'collecting state' that is used to allow a new port to join into a
pre-existing group without causing packet loss).

How long is dhcp trying before giving up?  You can control this in isc
dhcp3 with the 'timeout' configurable in /etc/dhcp3/dhclient.conf

you can see if your switches are causing problems by manually
configuring the laptop, then bounce the link on the cable and see if
you immediately recover or it takes time after each link bounce.

-- 
Dave
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Re: mythtv and digital tv

[Dave Johnson]

Derek Atkins writes:
> The biggest difference is that cable companies can choose to
> encrypt their QAM, which means you either need a cablebox or a
> device with a cablecard.  As far as I know no device usable by
> mythtv directly can use a cablecard without having yet another
> Digital-Analog-Digital conversion.

The 2nd biggest difference is cable companies tend to re-compress the
heck out of their digital channels to squeeze as many channels into as
little bandwidth as possible.

Still, just 1-2 HDTV channels on cable is on the order of what is
devoted to Internet bandwidth (10-12mbps).  Imagine if the cable
companies dropped all TV (both analog and digital) and did only
broadband Internet over coax. Throw in some 20mbps+ IPTV steams
via multicast and it'd give FIOS some competition.

As mentioned before comcast at least does the broadcast channels
QAM unencrypted in both SD and HD which my pcHDTV HD3000 grabs
bit-for-bit off the wire just fine.  Way better quality then having
the SD cable box decode the QAM MPEG2 sent over horrible composite
video/RCA audio (stupid cable box doesn't even have svideo) into my
PVR250 which then re-encodes them back into MPEG2.

-- 
Dave
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