= Orphan Mode
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Sometimes an NTP subnet becomes isolated from all UTC sources such as
local reference clocks or Internet time servers. In such cases it may be
necessary that the subnet servers and clients remain synchronized to a
common timescale, not necessarily the UTC timescale. Previously, this
function was provided by the local clock driver to simulate a UTC
source. A server with this driver could be used to synchronize other
hosts in the subnet directly or indirectly.

There are many disadvantages using the local clock driver, primarily
that the subnet is vulnerable to single-point failures and multiple
server redundancy is not possible. Orphan mode is intended to replace
the local clock driver. It provides a single simulated UTC source with
multiple servers and provides seamless switching as servers fail and
recover.

A common configuration for private networks includes one or more core
servers operating at the lowest stratum. Good practice is to configure
each of these servers as backup for the others using symmetric or
broadcast modes. As long as at least one core server can reach a UTC
source, the entire subnet can synchronize to it.

If no UTC sources are available to any core server, one of them can
provide a simulated UTC source for all other hosts in the subnet.
However, only one core server can simulate the UTC source and all direct
dependents, called orphan children, must select the same server, called
the orphan parent.

Hosts sharing the same common subnet, including potential orphan parents
and potential orphan children, can be enabled for orphan mode using the
+orphan stratum+ option of the link:miscopt.html#tos[+tos command+],
where +stratum+ is some stratum less than 16 and greater than any
anticipated stratum that might occur with configured Internet time
servers. However, sufficient headroom should remain so every subnet host
dependent on the orphan children has stratum less than 16. Where no
associations for other servers or reference clocks are configured, the
orphan stratum can be set to 1. These are the same considerations that
guide the local clock driver stratum selection.

In order to avoid premature enabling orphan mode, a holdoff delay occurs
when the daemon is first started and when all sources have been lost
after that. The delay is intended to allow time for other sources to
become reachable and selectable. Only when the delay has expired with no
sources will orphan mode be enabled. By default the delay is 300 s (five
minutes), but this can be changed using the +orphanwait+ option of the
link:miscopt.html#tos[+tos+] command.

A orphan parent with no sources shows reference ID LOOP if operating at
stratum 1 and 127.0.0.1 (IPv4 loopback address) otherwise. While
ordinary NTP clients use a selection metric based on delay and
dispersion, orphan children use a metric computed from the IP address of
each core server. Each orphan child chooses as the orphan parent as the
core server with the smallest metric.  This guarantees that all
orphan children choose the same orphan parent.

For orphan mode to work well, each core server with available sources
should operate at the same stratum. All core servers and orphan children
should include the same tos command in the configuration file. Each
orphan child should include in the configuration file all core servers.

image:pic/peer.gif[]

Figure 1. Orphan Peer Configuration

For example, consider the peer network configuration in Figure 1, where
two or more campus primary or secondary (stratum 2) servers are
configured with reference clocks or public Internet primary servers and
with each other using symmetric modes. With this configuration a server
that loses all sources continues to discipline the system clock using
the other servers as backup. Only the core servers and orphan children
need to be enabled for orphan mode.

image:pic/broad.gif[]

Figure 2. Orphan Broadcast Configuration

For broadcast networks each core server is configured in both broadcast
server and broadcast client modes as shown in Figure 2. Orphan children
operate as broadcast clients of all core servers. As in peer networks,
the core servers back up each other and only they and the orphan
children need to be enabled for orphan mode.

In normal operation subnet hosts operate below stratum 5, so the subnet
is automatically configured as described in the NTP specification. If
all UTC sources are lost, all core servers become orphans and the orphan
children will select the same core server to become the orphan parent.

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