org.cometd.server.ext
Class TimesyncExtension

java.lang.Object
  org.cometd.server.ext.TimesyncExtension

All Implemented Interfaces:

Extension

public class TimesyncExtension
extends java.lang.Object
implements 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.

The accuracy of the offset and lag may be calculated with tc-now-l-o, which should be zero if the calculated offset and lag are perfectly accurate.

A cometd server that supports timesync, should respond only if the measured accuracy value is greater than accuracy target. The response will be 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

On receipt of the response, the client is able to use current time to determine the total trip time, from which p is subtracted to determine an approximate two way network traversal time. The measured accuracy is used to adjust the assumption that the network is symmetric for traversal time, so:

• lag = (now-tc-p)/2-a
• offset = ts-tc-lag

Constructor Summary

TimesyncExtension()

Method Summary

int	getAccuracyTarget() timesync responses are not set if the measured accuracy is less than the accuracyTarget.
Message	rcv(Client from, Message message)
Message	rcvMeta(Client from, Message message)
Message	send(Client from, Message message)
Message	sendMeta(Client from, Message 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

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

rcv

public Message rcv(Client from,
                   Message message)

Specified by:

rcv in interface Extension

rcvMeta

public Message rcvMeta(Client from,
                       Message message)

Specified by:

rcvMeta in interface Extension

send

public Message send(Client from,
                    Message message)

Specified by:

send in interface Extension

sendMeta

public Message sendMeta(Client from,
                        Message message)

Specified by:

sendMeta in interface Extension