Ben Love on 16 Dec 2009 19:41:59 -0800

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Re: [PLUG] Problem with streaming of youtube videos on linux pc

> There is what Lee said about the default hardware.  It's also possible, 
> if not likely, that you need to tweak your network settings.  Wired or 
> wireless?  What FiOS hardware?

One thing I have seen in the past (though very rarely) is an MTU
problem.  Ethernet (and the Internet at large) generally assumes a 1500
byte MTU.  There are ICMP algorithms for path MTU discovery, but these
are often broken because many routers block ICMP indiscriminantly.

Sometimes FiOS or other providers will tunnel one ethernet connection
through another.  It's not common practice, but it does happen.  This
will shrink the MTU a little bit (to say 1452), and if you remember your
networking class, the smallest MTU of any link in the path is the MTU of
the entire path.  Indeed, it can cause very strange packet fragmenting
problems.  (Here again, some routers refuse to fragment packets and
forward each one.)  This can often manifest itself as paused/timed-out
connections, even to generic web pages.

The solution to an MTU problem like this is to use iptables with the
--clamp-mss-to-pmtu option: (something like this)

$ iptables -t mangle -A POSTROUTING -p tcp --tcp-flags SYN,RST SYN -o
eth0 -j TCPMSS --clamp-mss-to-pmtu

There's a little bit of information about it here:

This is exactly how I solved a very similar problem with my FiOS
connection.  The symptoms included webpages failing to load properly or
partially downloading.  However, I've been told that using PPPoE like I
do is not very common with FiOS anymore.


Ben Love
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