TripUpdate

General guidelines for trip cancellations:

When canceling trips over a number of days, producers should provide TripUpdates referencing the given trip_ids and start_dates as CANCELED as well as an Alert with NO_SERVICE referencing the same trip_ids and TimeRange that can be shown to riders explaining the cancellation (e.g., detour).
If no stops in a trip will be visited, the trip should be CANCELED instead of having all stop_time_updates being marked as SKIPPED.

Field Name	Recommendation
`trip`	Refer to message TripDescriptor.
	If separate `VehiclePosition` and `TripUpdate` feeds are provided, TripDescriptor and VehicleDescriptor ID values pairing should match between the two feeds. For example, a `VehiclePosition` entity has `vehicle_id:A` and `trip_id:4`, then the corresponding `TripUpdate` entity should also have `vehicle_id:A` and `trip_id:4`. If any `TripUpdate` entity has `trip_id:4` and any `vehicle_id` other than 4, this is an error.
`vehicle`	Refer to message VehicleDescriptor.
	If separate `VehiclePosition` and `TripUpdate` feeds are provided, TripDescriptor and VehicleDescriptor ID values pairing should match between the two feeds. For example, a `VehiclePosition` entity has `vehicle_id:A` and `trip_id:4`, then the corresponding `TripUpdate` entity should also have `vehicle_id:A` and `trip_id:4`. If any `TripUpdate` entity has `trip_id:4` and any `vehicle_id` other than 4, this is an error.
`stop_time_update`	`stop_time_updates` for a given `trip_id` should be strictly ordered by increasing `stop_sequence` and no `stop_sequence` should be repeated.
	While the trip is in progress, all `TripUpdates` should include at least one `stop_time_update` with a predicted arrival or departure time in the future. Note that the GTFS Realtime spec says that producers should not drop a past `StopTimeUpdate` if it refers to a stop with a scheduled arrival time in the future for the given trip (i.e. the vehicle has passed the stop ahead of schedule), as otherwise it will be concluded that there is no update for this stop.
`timestamp`	Should reflect the time this prediction for this trip was updated.
`delay`	`TripUpdate.delay` should represent schedule deviation, i.e., the observed past value for how ahead/behind schedule the vehicle is. Predictions for future stops should be provided through `StopTimeEvent.delay` or `StopTimeEvent.time`.