In section 5.3.4.2 Channel of the Notification for the Subscription, the IG states that:
NDH SHALL support the channels.

• rest-hook
• websocket

The Backport Subscription IG overview of using Websocket appears to indicate that the operation $get-ws-binding-token is necessary to establish a websocket subscription, but it is not mentioned in the IG and it is not part of the Exchange CapabilityStatement.

On a related note, is there a reason that websocket is part of the IG given the challenges and the stated purpose for using websockets listed in the Subscription Backport specification? What are the anticipated clients/use cases
that would require websocket instead of rest-hook?