[CrOS Tether] Add tether network properties (battery percentage, carrier, and signal strength) to t…

components/onc/docs/onc_spec.md

 # Open Network Configuration
 
 [TOC]
 
 ## Objective
 
 We would like to create a simple, open, but complete format to describe
 multiple network configurations for WiFi, Ethernet, Cellular,
 Bluetooth/WiFi-Direct, and VPN connections in a single file format, in order
 to simplify and automate network configuration for users.
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 * **WiMAX**
     * (required if **Type** is *WiMAX*, otherwise ignored) -
       [WiMAX](#WiMAX-type)
     * WiMAX settings.
 
 * **Cellular**
     * (required if **Type** is *Cellular*, otherwise ignored) -
       [Cellular](#Cellular-type)
     * Cellular settings.
 
+* **Tether**
+    * (required if **Type** is *Tether*, otherwise ignored) -
+      [Tether](#Tether-type)
+    * Tether settings.
+
 * **Type**
     * (required if **Remove** is *false*, otherwise ignored) - **string**
     * `Allowed values are` *Cellular*,
         *Ethernet*, *WiFi*,
         *WiMAX* and *VPN*.
     * Indicates which kind of connection this is.
 
 * **ConnectionState**
     * (optional, read-only) - **string**
     * The current connection state for this network, provided by the system.
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 * **RetriesLeft**
     * (optional) - **integer**
     * If **LockType** is empty
       or *sim-pin*, then this property represents
       the number of attempts remaining to supply a correct PIN before the
       PIN becomes blocked, at which point a PUK provided by the carrier would
       be necessary to unlock the SIM (and **LockType**
       changes to *sim-puk*).
 
 
+## Tether Networks
+
+For Tether connections, **Type** must be set to *Tether* and the
+field **Tether** must be set to an object of type [Tether](#Tether-type).
+
+Used for representing a tether hotspot provided by an external device, e.g.
+a phone.
+
+### Tether type
+
+* **BatteryPercentage**
+    * (optional, read-only) - **integer**
+    * The battery percentage of the device providing the tether hotspot in the
+      range [0, 100].
+
+* **Carrier**
+    * (optional, read-only) - **string**
+    * The name of the cellular carrier when the hotspot is provided by a
+      cellular connection.
+
+* **SignalStrength**
+    * (optional, read-only) - **integer**
+    * The current signal strength for the hotspot's connection in the range
+      [0, 100]. Note that this value refers to the strength of the signal
+      between the external device and its data provider, not the strength of the
+      signal between the current device and the external device.
+
+
 ## Bluetooth / WiFi Direct Networks
 
 This format will eventually also cover configuration of Bluetooth and WiFi
 Direct network technologies, however they are currently not supported.
 
 
 ## Certificates
 
 Certificate data is stored in a separate section. Each certificate may be
 referenced from within the NetworkConfigurations array using a certificate
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 is transmitted or saved to disk should be secure. On client device, when
 user names for connections that are user-specific are persisted to disk,
 they should be stored in a location that is encrypted. Users can also opt in
 these cases to not save their user credentials in the config file and will
 instead be prompted when they are needed.
 
 ## Authors
 
 * pneubeck@chromium.org
 * stevenjb@chromium.org