Add file linked_datasets.txt containing the Trip Updates, Vehicle Positions and Service Alerts URLs

gtfs/spec/en/reference.md

@@ -22,6 +22,7 @@ This document explains the types of files that comprise a GTFS transit feed and
     -   [shapes.txt](#shapestxt)
     -   [frequencies.txt](#frequenciestxt)
     -   [transfers.txt](#transferstxt)
+    -   [linked_datasets.txt](#linkeddatasetstxt)
     -   [feed\_info.txt](#feed_infotxt)
 
 ## Term Definitions
@@ -50,6 +51,7 @@ This specification defines the following files along with their associated conte
 |  [shapes.txt](#shapestxt)  | Optional | Rules for drawing lines on a map to represent a transit organization's routes. |
 |  [frequencies.txt](#frequenciestxt)  | Optional | Headway (time between trips) for routes with variable frequency of service. |
 |  [transfers.txt](#transferstxt)  | Optional | Rules for making connections at transfer points between routes. |
+|  [linked_datasets.txt](#linkeddatasetstxt)  | Optional | URLs to linked datasets: Trip Updates, Vehicle Positions and Service Alerts. |
 |  [feed_info.txt](#feed_infotxt)  | Optional | Additional information about the feed itself, including publisher, version, and expiration information. |
 
 ## File Requirements
@@ -414,6 +416,30 @@ Trip planners normally calculate transfer points based on the relative proximity
 |  min_transfer_time | Optional | When a connection between routes requires an amount of time between arrival and departure (transfer_type=2), the **min_transfer_time** field defines the amount of time that must be available in an itinerary to permit a transfer between routes at these stops. The min_transfer_time must be sufficient to permit a typical rider to move between the two stops, including buffer time to allow for schedule variance on each route. |
 |   |  | The min_transfer_time value must be entered in seconds, and must be a non-negative integer. |
 
+### linked_datasets.txt
+
+File: **Optional**
+
+|  Field Name | Required | Details |
+|  ------ | ------ | ------ |
+|  url | **Required** | The **url** field contains the URL to the linked dataset. The value must be a fully qualified URL that includes a scheme such as **http**:// or **https**://, and any special characters in the URL must be correctly escaped. See http://www.w3.org/Addressing/URL/4_URI_Recommentations.html for a description of how to fully qualified URL values (e.g. https://my.feed.com/trip_updates.pb). |
+|  trip_updates | **Required** | The **trip_updates** field indicates whether the dataset at this URL may contain [a `TripUpdate` entity](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-tripupdate). Valid values for this field are: |
+|   |  | * **0** - The dataset at this URL doesn't contain TripUpdate entities. |
+|   |  | * **1** - The dataset at this URL may contain TripUpdate entities. |
+|  vehicle_positions | **Required** | The **vehicle_positions** field indicates whether the dataset at this URL may contain [a `VehiclePosition` entity](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-vehicleposition). Valid values for this field are: |
+|   |  | * **0** - The dataset at this URL doesn't contain VehiclePosition entities. |
+|   |  | * **1** - The dataset at this URL may contain VehiclePosition entities. |
+|  service_alerts | **Required** | The **service_alerts** field indicates whether the dataset at this URL may contain [an `Alert` entity](https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#message-alert). Valid values for this field are: |
+|   |  | * **0** - The dataset at this URL doesn't contain Alert entities. |
+|   |  | * **1** - The dataset at this URL may contain Alert entities. |
+|  authentication_type | **Required** | The **authentication_type** field defines the type of authentication required to access the URL. Valid values for this field are: |
+|   |  | * **0** or **(empty)** - No authentication required. |
+|   |  | * **1** - Ad-hoc authentication required, visit URL in `authentication_info_url` for more information. |
+|   |  | * **2** - The authentication requires an API key, which should be passed as value of the parameter `api_key_parameter_name` in the URL. Please visit URL in `authentication_info_url` for more information. |
+|  authentication_info_url | **Optional** | If authentication is required, the **authentication_info_url** field contains a URL to a human-readable page describing how the authentication should be performed and how credentials can be created. The value must be a fully qualified URL that includes **http**:// or **https**://, and any special characters in the URL must be correctly escaped. See http://www.w3.org/Addressing/URL/4_URI_Recommentations.html for a description of how to fully qualified URL values. This field is required for `authentication_type` `1` or greater. |
+|  api_key_parameter_name | **Optional** | The **api_key_parameter_name** field defines the name of the parameter to pass in the URL to provide the API key. This field is required for `authentication_type` `2`. |
+
+
 ### feed_info.txt
 
 File: **Optional**