Class TimesyncExtension

  • All Implemented Interfaces:
    org.cometd.bayeux.server.BayeuxServer.Extension

    public class TimesyncExtension
    extends java.lang.Object
    implements org.cometd.bayeux.server.BayeuxServer.Extension

    Timesync extension (server side).

    With each handshake or connect, the extension sends timestamps within the ext field like: {ext:{timesync:{tc:12345567890,l:23,o:4567},...},...} where:

    • tc is the client timestamp in ms since 1970 of when the message was sent.
    • l is the network lag that the client has calculated.
    • o is the clock offset that the client has calculated.

    A CometD server that supports timesync, can respond with an ext field like: {ext:{timesync:{tc:12345567890,ts:1234567900,p:123,a:3},...},...} where:

    • tc is the client timestamp of when the message was sent,
    • ts is the server timestamp of when the message was received
    • p is the poll duration in ms - ie the time the server took before sending the response.
    • a is the measured accuracy of the calculated offset and lag sent by the client

    The relationship between tc, ts, o and l on the server is given by ts=tc+o+l (the time the server received the message is the client time plus the offset plus the network lag). Thus the accuracy of the o and l settings can be determined with a=tc+o+l-ts.

    When the client has received the response, it can make a more accurate estimate of the lag as l2=(now-tc-p)/2 (assuming symmetric lag). A new offset can then be calculated with the relationship on the client that ts=tc+o2+l2, thus o2=ts-tc-l2.

    Since the client also receives the a value calculated on the server, it should be possible to analyse this and compensate for some asymmetry in the lag. But the current client does not do this.

    • Nested Class Summary

      • Nested classes/interfaces inherited from interface org.cometd.bayeux.server.BayeuxServer.Extension

        org.cometd.bayeux.server.BayeuxServer.Extension.Adapter
    • Field Summary

      Fields 
      Modifier and Type Field Description
      static java.lang.String LAG_ATTRIBUTE  
    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      int getAccuracyTarget()
      timesync responses are not set if the measured accuracy is less than the accuracyTarget.
      boolean rcvMeta​(org.cometd.bayeux.server.ServerSession from, org.cometd.bayeux.server.ServerMessage.Mutable message)  
      boolean sendMeta​(org.cometd.bayeux.server.ServerSession session, org.cometd.bayeux.server.ServerMessage.Mutable message)  
      void setAccuracyTarget​(int target)
      timesync responses are not set if the measured accuracy is less than the accuracyTarget.
      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
      • Methods inherited from interface org.cometd.bayeux.server.BayeuxServer.Extension

        incoming, outgoing, rcv, send
    • Constructor Detail

      • TimesyncExtension

        public TimesyncExtension()
    • Method Detail

      • getAccuracyTarget

        public int getAccuracyTarget()
        timesync responses are not set if the measured accuracy is less than the accuracyTarget.
        Returns:
        accuracy target in ms (default 25ms)
      • setAccuracyTarget

        public void setAccuracyTarget​(int target)
        timesync responses are not set if the measured accuracy is less than the accuracyTarget.
        Parameters:
        target - accuracy target in ms
      • rcvMeta

        public boolean rcvMeta​(org.cometd.bayeux.server.ServerSession from,
                               org.cometd.bayeux.server.ServerMessage.Mutable message)
        Specified by:
        rcvMeta in interface org.cometd.bayeux.server.BayeuxServer.Extension
      • sendMeta

        public boolean sendMeta​(org.cometd.bayeux.server.ServerSession session,
                                org.cometd.bayeux.server.ServerMessage.Mutable message)
        Specified by:
        sendMeta in interface org.cometd.bayeux.server.BayeuxServer.Extension