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Add UDP discovery mechanism.

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This adds a new mechanism by which tinc can determine if a node is
reachable via UDP. The new mechanism is currently redundant with the
PMTU discovery mechanism - that will be fixed in a future commit.

Conceptually, the UDP discovery mechanism works similarly to PMTU
discovery: it sends UDP probes (of minmtu size, to make sure the tunnel
is fully usable), and assumes UDP is usable if it gets replies. It
assumes UDP is broken if too much time has passed since the last reply.

The big difference with the current PMTU discovery mechanism, however,
is that UDP discovery probes are only triggered as part of the
packet TX path (through try_tx()). This is quite interesting, because
it means tinc will never send UDP pings more often than normal packets,
and most importantly, it will automatically stop sending pings as soon
as packets stop flowing, thereby nicely reducing network chatter.

Of course, there are small drawbacks in some edge cases: for example,
if a node only sends one packet every minute to another node, these
packets will only be sent over TCP, because the interval between packets
is too long for tinc to maintain the UDP tunnel. I consider this a
feature, not a bug: I believe it is appropriate to use TCP in scenarios
where traffic is negligible, so that we don't pollute the network with
pings just to maintain a UDP tunnel that's seeing negligible usage.
This commit is contained in:
Etienne Dechamps 2014-12-29 10:34:39 +00:00
parent 5d6478b9fb
commit 7939ee1283
10 changed files with 96 additions and 14 deletions
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12
doc/tinc.conf.5.in
View file

@ -485,6 +485,18 @@ and will only allow connections with nodes for which host config files are prese
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/
directory.
Setting this options also implicitly sets StrictSubnets.
.It Va UDPDiscovery Li = yes | no Po yes Pc
When this option is enabled tinc will try to establish UDP connectivity to nodes,
using TCP while it determines if a node is reachable over UDP. If it is disabled,
tinc always assumes a node is reachable over UDP.
Note that tinc will never use UDP with nodes that have
.Va TCPOnly
enabled.
.It Va UDPDiscoveryInterval Li = Ar seconds Pq 9
The minimum amount of time between sending UDP ping datagrams to test UDP connectivity.
.It Va UDPDiscoveryTimeout Li = Ar seconds Pq 30
If tinc doesn't receive any UDP ping replies over the specified interval,
it will assume UDP communication is broken and will fall back to TCP.
.It Va UDPRcvBuf Li = Ar bytes Pq OS default
Sets the socket receive buffer size for the UDP socket, in bytes.
If unset, the default buffer size will be used by the operating system.

16
doc/tinc.texi
View file

@ -1232,6 +1232,22 @@ and will only allow connections with nodes for which host config files are prese
@file{@value{sysconfdir}/tinc/@var{netname}/hosts/} directory.
Setting this options also implicitly sets StrictSubnets.
@cindex UDPDiscovey
@item UDPDiscovery = <yes|no> (yes)
When this option is enabled tinc will try to establish UDP connectivity to nodes,
using TCP while it determines if a node is reachable over UDP. If it is disabled,
tinc always assumes a node is reachable over UDP.
Note that tinc will never use UDP with nodes that have TCPOnly enabled.
@cindex UDPDiscoveryInterval
@item UDPDiscoveryInterval = <seconds> (9)
The minimum amount of time between sending UDP ping datagrams to test UDP connectivity.
@cindex UDPDiscoveryTimeout
@item UDPDiscoveryTimeout = <seconds> (30)
If tinc doesn't receive any UDP ping replies over the specified interval,
it will assume UDP communication is broken and will fall back to TCP.
@cindex UDPRcvBuf
@item UDPRcvBuf = <bytes> (OS default)
Sets the socket receive buffer size for the UDP socket, in bytes.