Package org.cometd.server.ext

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

      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