Backported from X.org:

 commit db1089eafc1c5371fa0030202de588d2e2b4f8e5
 Author: Keith Packard <keithp@keithp.com>
 Date:   Mon Sep 21 07:16:17 2015 +0100

    os/xdmcp: Just send XDMCP keepalive packets once every three minutes

    There was a complicated scheme to increase the time between keepalives
    from 3 minutes up to as much as 24 hours in an attempt to reduce
    network traffic from idle X terminals. X terminals receiving X
    traffic, or receiving user input would use the 3 minute value; X
    terminals without any network traffic would use a longer value.

    However, this was actually broken -- any activity in the X server,
    either client requests or user input, would end up resetting the
    keepalive timeout, so a user mashing on the keyboard would never
    discover that the XDMCP master had disappeared and have the session
    terminated, which was precisely the design goal of the XDMCP keepalive
    mechanism.

    Instead of attempting to fix this, accept the cost of a pair of XDMCP
    packets once every three minutes and just perform keepalives
    regularly.

    This will also make reworking the block and wakeup handler APIs to
    eliminate select masks easier.
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
Backported-to-NX-by: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>