A potential subscriber sends TRACK_STATUS as the first and only message on a
new bidi stream to obtain information about the current status of a given track.
The TRACK_STATUS message format is identical to the SUBSCRIBE message (Section 9.6), but subscriber parameters related to Track delivery (e.g. SUBSCRIBER_PRIORITY) are not included. The parameters that can appear in a TRACK_STATUS are AUTHORIZATION_TOKEN and INCLUDE_PROPERTIES.
The receiver of a TRACK_STATUS message treats it identically as if it had received a SUBSCRIBE message, except it does not create downstream subscription state or send any Objects. If successful, the publisher responds with a TRACK_STATUS_OK with the same parameters and Track Properties it would have set in a SUBSCRIBE_OK. Track Alias is not used. A publisher responds to a failed TRACK_STATUS with an appropriate TRACK_STATUS_ERROR message. The bidi stream is closed with a FIN after TRACK_STATUS_OK or TRACK_STATUS_ERROR are sent.
Relays without an Established subscription MAY forward TRACK_STATUS to one or more
publishers, or MAY initiate a subscription (subject to authorization) as
described in Section 7.6 to determine the response. The publisher
does not send PUBLISH_DONE for this request, and the subscriber cannot send
REQUEST_UPDATE.