Mention zero-window state in networking docs (#124969) (#124970)

Clarify that it is expected sometimes to see inter-node connections
sending zero-window advertisements as part of the usual TCP backpressure
mechanism.

Backport of #124967 to 8.x
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David Turner 2025-03-16 20:43:14 +00:00 committed by GitHub
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@ -195,6 +195,15 @@ setting `transport.ping_schedule` if keepalives cannot be configured. Devices
which drop connections when they reach a certain age are a common source of which drop connections when they reach a certain age are a common source of
problems to {es} clusters, and must not be used. problems to {es} clusters, and must not be used.
If an {es} node is temporarily unable to handle network traffic it may stop
reading data from the network and advertise a zero-length TCP window to its
peers so that they pause the transmission of data to the unavailable node. This
is the standard backpressure mechanism built into TCP. When the node becomes
available again, it will resume reading from the network. Configure your
network to permit TCP connections to exist in this paused state without
disruption. Do not impose any limit on the length of time that a connection may
remain in this paused state.
For information about troubleshooting unexpected network disconnections, see For information about troubleshooting unexpected network disconnections, see
<<troubleshooting-unstable-cluster-network>>. <<troubleshooting-unstable-cluster-network>>.