UTCTiming---Clock-synchronization.md

+## Description
+
+During playback of dynamic presentations, a wall wall clock is used as the timing reference for DASH client decisions. This is a synchronized clock shared by the DASH client and service [1].
+
+It is critical to synchronize the clocks of the DASH client and service when using a dynamic presentation because the MPD timeline of a dynamic presentation is mapped to wall clock time and many playback decisions are clock driven and assume a common understanding of time by the DASH client and service [1].
+
+Clock synchronization mechanisms are described by UTCTiming elements in the MPD ([DASH] 5.8.4.11) [1].
+
+For further information please check the References [1] and [2].
+
+## Clock synchronization in dash.js
+dash.js supports multiple schemeIdUri and value combinations for clock synchronization:
+
+```javascript
+    'urn:mpeg:dash:utc:http-head:2014': _httpHeadHandler,
+    'urn:mpeg:dash:utc:http-xsdate:2014': _httpHandler.bind(null, _xsdatetimeDecoder),
+    'urn:mpeg:dash:utc:http-iso:2014': _httpHandler.bind(null, _iso8601Decoder),
+    'urn:mpeg:dash:utc:direct:2014': _directHandler,
+
+    // some specs referencing early ISO23009-1 drafts incorrectly use
+    // 2012 in the URI, rather than 2014. support these for now.
+    'urn:mpeg:dash:utc:http-head:2012': _httpHeadHandler,
+    'urn:mpeg:dash:utc:http-xsdate:2012': _httpHandler.bind(null, _xsdatetimeDecoder),
+    'urn:mpeg:dash:utc:http-iso:2012': _httpHandler.bind(null, _iso8601Decoder),
+    'urn:mpeg:dash:utc:direct:2012': _directHandler,
+```
+
+The default timing source in dash.js uses the following schemeIdUri/value combination:
+```javascript
+ defaultTimingSource: {
+   scheme: 'urn:mpeg:dash:utc:http-xsdate:2014',
+   value: 'http://time.akamai.com/?iso&ms'
+}
+```
+`UTCTiming` elements in the MPD take precedence over the default timing source specified in the settings.
+
+### Default synchronization
+By default, dash.js performs a clock synchronization at playback start and after each MPD update. 
+
+#### Startup synchronization
+At playback start an initial request to the timing server is issued. The offset between the client and the server clock is calculated as described in Section _Offset calculation_.
+
+In addition, dash.js performs a predefined number of background requests to verify the initially calculated offset. The number of background attempts can be adjusted in the settings:
+
+```javascript
+player.updateSettings({
+  streaming: {
+    utcSynchronization: {
+      backgroundAttempts: 2
+    }
+  }
+})
+```
+
+For further details on background synchronization check the Section _Background synchronization_
+
+### Error synchronization
+
+### Background synchronization
+
+### Offset calculation
+
+### Complete Configuration
+
+## References
+* [1] [DASH-IF Implementation Guidelines](https://dashif-documents.azurewebsites.net/Guidelines-TimingModel/master/Guidelines-TimingModel.html#clock-sync)
+* [2] [DASH-IF IOP Guidelines](https://dash-industry-forum.github.io/docs/DASH-IF-IOP-v4.3.pdf)
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