Daniel Ellard wrote:
> > man ifconfig<cr>
> > /mtu<cr>
>
> The original question asked about the TCP MSS, not the MTU. Looking
> at ifconfig isn't going to help.
Actually, the original question is about how to cause the
creation of fragments, for the purposes of testing. The
MSS question is kind of based on a loose assumption about
where baby frags come from, which isn't quite correct.
> Note that the actual MSS is negotiated; if both ends can't support the
> same value, the smaller is chosen.
This is the problem. The MSS can be negotiated up to the
MTU size from the default, or down, to match an intermediate
hop. The only thing changing the default will do for you is
increase the negotiation time.
> For MTUs, in case that's really what you meant, it's even more strict
> and depending on how the transport layer is implemented, it may be
> impossible (or reckless) to increase the MTU. For example, on
> ethernet many switches simply do not support an MTU larger than 1500.
This is the one thing he *can* hard-code, via ifconfig. MSS
is not hard-codeable.
There's an outside chance that he wanted the MTU larger (e.g.
9k for "jumbograms" on gigabit hardware). Actually, hardware
MSS negotiation doesn't work between some gigabit cards, and
has to be set manually on both ends to get the higher number.
But since he was talking about fargs vs. no frags, it's a good
bet that what he wanted was:
MTU X MTU X/3
host1 <----------------> router <----------------> host 2
So that frags are created when transmitting data from host1 to
host2.
-- Terry
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