Tested Helium SU2

Steps
I followed https://wiki.opendaylight.org/view/OpenDaylight_Controller:MD-SAL:Restconf:Change_event_notification_subscription

I subscribed to

"http://<host>:8181/opendaylight-inventory:nodes/datastore=CONFIGURATION/scope=BASE"

In the wiki I couldn't initially find the URI to point the websocket client at because I expected a WS:// protocol as described in section 3 http://tools.ietf.org/html/rfc6455
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3. WebSocket URIs

This specification defines two URI schemes, using the ABNF syntax
defined in RFC 5234 [RFC5234], and terminology and ABNF productions
defined by the URI specification RFC 3986 [RFC3986].

ws-URI = "ws:" "//" host [ ":" port ] path [ "?" query ]
wss-URI = "wss:" "//" host [ ":" port ] path [ "?" query ]

host = <host, defined in [RFC3986], Section 3.2.2>
port = <port, defined in [RFC3986], Section 3.2.3>
path = <path-abempty, defined in [RFC3986], Section 3.3>
query = <query, defined in [RFC3986], Section 3.4>

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To investigate the effect of using http:// I used the python websocket client
autobahn which refused the http:// URI as a wrong protocol and firefox which failed to connect.