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

Each pair of {es} nodes 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. {es} must wait while the retransmissions
are happening and can only react once the operating system decides to give up.
Users must therefore also wait for a sequence of retransmissions to complete.

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 and even harmful
on the high quality networks used by most {es} installations. When a cluster
detects a node failure it reacts by reallocating lost shards, rerouting
searches, and maybe electing a new master node. Highly available clusters must
be able to detect node failures promptly, which can be achieved by reducing the
permitted number of retransmissions. Connections to
<<modules-remote-clusters,remote clusters>> should also prefer to detect
failures much more quickly than the Linux default allows. 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 other than {es} clusters too. If your
clusters communicate with external systems over a low quality 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 {es} clusters, and must not be
used.

