tinc/doc/CONNECTIVITY

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This document describes how nodes in a VPN find and connect to eachother and
maintain a stable network.
Copyright 2001-2004 Guus Sliepen <guus@tinc-vpn.org>
Permission is granted to make and distribute verbatim copies of
this documentation provided the copyright notice and this
permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of
this documentation under the conditions for verbatim copying,
provided that the entire resulting derived work is distributed
under the terms of a permission notice identical to this one.
$Id$
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1. Synchronisation
==================
Each tinc daemon has zero or more connections to other tinc daemons. It will
try to keep its own information synchronised with the other tinc daemons. If
one of its peers sends information, the tinc daemon will check if it is new
information. If so, it will update its own information and forward the new
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information to all the other peers.
This scheme will make sure that after a short amount of time all tinc daemons
share the same information. It will also almost completely prevent information
from looping, because "new" information that is already known is ignored and
not forwarded any further. However, since information can also be deleted
there's the possibility of a looping sequence of add/delete messages. This is
resolved by additionaly adding a unique identifier to each broadcasted message.
Messages are dropped if the same message with that identifier has already been
seen.
2. Routing
==========
Every node tells its peers to which other peers it is connected. This way
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every node will eventually know every connection every node has on the VPN.
Each node will use graph algorithms to determine if other nodes are reachable or not and
what the best route is to other nodes.
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Because all nodes share the same information, using a deterministic algorithm
each node will calculate the same minimum spanning tree for the entire VPN.
The MST will be used to send broadcast VPN packets.