Avoid unnecessary requests to the time sync server

[dsilhavy]

Environment

• Link to playable MPD file: https://livesim.dashif.org/livesim/mup_30/testpic_2s/Manifest.mpd
• Dash.js version: 3.2.1
• Browser name/version: all
• OS name/version: all

Observed behavior

Play the stream and wait for MPD udaptes. After each MPD update a request to the UTC sync server is done: https://time.akamai.com/?iso&ms . Expect for the first request, the response is ignored.

Expected behavior

We should avoid unnecessary requests. DASH-IF 4.3 states:
4.7.3: "Clients should not access the UTCTiming server more frequently than necessary".

Suggestion: Only do a single sync at startup. Behavior would be similar to now since we ignore the response anyways. Clarify with @haudiobe what the best behavior is.

[dsilhavy]

Based on discussion with DASH-IF

1. Make a configurable amount of requests at start of playback (e.g 3 requests)
2. Repeat that request at a manifest load if time since last sync > T minutes. This value can default to 1 and be settable via an API.
3. Remember the delta between system time and reference time. If it drifts more then 20%, then halve T. If it is less than 20%, then double T. T can never be less than segment duration.
4. Force a time sync if there is a 404 or any unexpected playback problems are encountered.

[haudiobe]

DASH-IF call:

• do an initial request once and start
• verify this initial time with 1 or 2 additional ones in the background.
• set the default to T=5
  Should go to IOP guidelines.

[dsilhavy]

Akamai timing source with RTT attribute: http://time.akamai.com/?xml&ms