I've seen instructions before to change router mtu setting when there
are problems and I just got advice on changing mtu size in the
registry. It dawned on me to wonder how these settings interact? If the
packet sizes out of windows are bigger than what the router setting
does the router repackage them? Anybody savvy about this?

Thanks,
Tom

Think of it like a ball rolling through a tube; if the ball is larger than
any part of the tube it'll get stuck.

If the Windows TCP stack creates a packet of (say) 1458 bytes, then that
will pass through any router with a typical MTU of 1500.

In fact, you can use a Windows MTU of 1500 with PPPoA systems, since these
don't increase the packet-size at the router. PPPoE routers DO add a few
bytes to the packet, so in this case you must use a Windows MTU of less than
1500. 

If the packet 'gets stuck' then it won't be lost, instead it will be split
into two smaller packets. This involves a time overhead, so
packet-fragmentation will slow the transmission considerably. Thus if you try
gradually increasing the MTU, you'll find that performance increases up to a
certain point where fragmentation starts, at which point the performance
drastically reduces.

Increasing the MTU still further will recover some of the lost performance
(you're now sending two near-full packets instead of one full, one mostly
empty space) until the packet gets split three ways, and so on.

Thanks. I'll have to play with this and see at what points I get better
and worse response.

Tom


Ian wrote:
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