Hardcoded limit on the number of meta-connections

[Guus Sliepen]

On Tue, Jul 08, 2014 at 01:37:33PM +0100, Etienne Dechamps wrote:

> >>Is there experimental evidence supporting 3 as the hardcoded maximum
> >>number of meta-connections?
> >
> >Yes. Simulations show that 3 meta-connections per node results in a
> >robust graph where the removal of any one node will not cause others
> >to be disconnected from the VPN. So 3 is already providing
> >redundancy, more meta-connections per node will just result in more
> >overhead.
> 
> I'm not sure I agree. That's only true if you can establish a new
> metaconnection immediately after you lost one. In practice that's not
> always the case (e.g. nodes behind NAT, etc.) so it would take an
> extended amount of time to find a node you can actually establish a
> metaconnection to. This means you can very well get into a situation
> where you would lose most metaconnections before you can re-establish
> some of them.

The two most common cases of losing meta-connections are when another
node goes down, or when your Internet connection is broken. In the first
case you lose only one of your three meta-connections, and it's OK if it
takes a little time to get a third one back. In the latter case there is
nothing you can do anyway. Of course, it would be nice if tinc handles
the third case well where you lose all three of your meta-connections
but you can potentially reach other nodes. I think that scenario is
better solved by improving the autoconnection logic to find nodes it
still can connect to quicker (for example, by keeping track of which
nodes it could or could not reach in the past). I do agree that if a
large fraction of the nodes in the VPN is behind a NAT, it can take
quite a time for the current logic to find a node it can connect to.

-- 
Met vriendelijke groet / with kind regards,
     Guus Sliepen <guus at tinc-vpn.org>
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