[Techtalk] ping -s and mtu size

[James]

On 7/8/07, Carla Schroder <carla at bratgrrl.com> wrote:
> hey all,
>
> Is the ping -s command something that is useful in the real world? The only
> times I've heard of MTU size problems was when a game dev thought he could
> use 10,000 byte packets or something equally nuts, and of course TCP broke
> them up into smaller fragments, which horked game performance.

I've had MTU problems; some years ago, we had a server (running
Bugzilla) which people couldn't access. It was strange: ping was fine,
ssh was fine, web access wasn't! Eventually, I discovered the
hub/switch (one of those dual-speed 10/100 "hubs", IIRC) was eating
any packet larger than a certain size, so normal pings were fine,
anything with more than a tiny amount of data wasn't. As soon as I set
a low MTU on the Ethernet interface, everything worked fine!

> Suppose you're running ping -s and you carefully increase the size until you
> start seeing dropped packets. Then what? how do you figure out what is
> causing the problem?

In this case, there was only one component between the server and
everything else: the rogue hub. (Come to think of it, it could have
been the server's network card, I suppose, with the same result; since
the cure was the same, it didn't matter at the time.) If there had
been more hops involved - say, the server was in another building, or
on another site - I'd have tried large packets to other machines
nearby, and worked my way towards the server until I found the first
point I can't reach. Basically the same process you'd follow if you
had a machine you couldn't reach at all, but confined to large
packets.


James.