[[system-config-tcpretries]]
=== TCP retransmission timeout

Each pair of nodes in a cluster communicates via a number of TCP connections
which <<long-lived-connections,remain open>> until one of the nodes shuts down
or communication between the nodes is disrupted by a failure in the underlying
infrastructure.

TCP provides reliable communication over occasionally-unreliable networks by
hiding temporary network disruptions from the communicating applications. Your
operating system will retransmit any lost messages a number of times before
informing the sender of any problem. Most Linux distributions default to
retransmitting any lost packets 15 times. Retransmissions back off
exponentially, so these 15 retransmissions take over 900 seconds to complete.
This means it takes Linux many minutes to detect a network partition or a
failed node with this method. Windows defaults to just 5 retransmissions which
corresponds with a timeout of around 6 seconds.

The Linux default allows for communication over networks that may experience
very long periods of packet loss, but this default is excessive for production
networks within a single data centre as is the case for most {es} clusters.
Highly-available clusters must be able to detect node failures quickly so that
they can react promptly by reallocating lost shards, rerouting searches and
perhaps electing a new master node. Linux users should therefore reduce the
maximum number of TCP retransmissions.

You can decrease the maximum number of TCP retransmissions to `5` by running
the following command as `root`. Five retransmissions corresponds with a
timeout of around six seconds.

[source,sh]
-------------------------------------
sysctl -w net.ipv4.tcp_retries2=5
-------------------------------------

To set this value permanently, update the `net.ipv4.tcp_retries2` setting in
`/etc/sysctl.conf`. To verify after rebooting, run
`sysctl net.ipv4.tcp_retries2`.

IMPORTANT: This setting applies to all TCP connections and will affect the
reliability of communication with systems outside your cluster too. If your
cluster communicates with external systems over an unreliable network then you
may need to select a higher value for `net.ipv4.tcp_retries2`. For this reason,
{es} does not adjust this setting automatically.

==== Related configuration

{es} also implements its own internal health checks with timeouts that are much
shorter than the default retransmission timeout on Linux. Since these are
application-level health checks their timeouts must allow for application-level
effects such as garbage collection pauses. You should not reduce any timeouts
related to these application-level health checks.

You must also ensure your network infrastructure does not interfere with the
long-lived connections between nodes, <<long-lived-connections,even if those
connections appear to be idle>>. Devices which drop connections when they reach
a certain age are a common source of problems to Elasticsearch clusters, and
must not be used.