B/ Network Dialog; C/ Working With Several Servers - Gorgy Timing LEDI NETWORK IN Mode D'emploi

1. INTRODUCTION

B/ Network dialog

The real drift and offset of the LEDI Network IN clock related to the NTP server are es-
timated from a measured offset. This offset is computed by subtracting the client clock
date from the server clock date.
Periodically, the client estimate current offset from the server. This measure is done
using a message-passing mechanism as shown on following figure:
client side server side
Network
Query server date
Return current server date
Receive reply date
On a LAN, it takes some milliseconds between the time when the client asks for the
date and the time when it receives the reply. This time is due to transmission delay
on the network. It can vary because of network load or NTP server load. Worse, the
transmission time from client to server is generally not the same that the one from the
server to client.
Actually, on a large period of time, the difference of sum of client-server and server-client
transmission delays is zero. The NTP client algorithm implemented in the LEDI Network
IN is designed to compensate transmission delay and to average the measures.

C/ Working with several servers

The NTP client software has the ability to work with several NTP servers (up to 3 ser-
vers). When not enough reply is received from a server or this server doesn't reply or
reply too slowly, the NTP client software uses an other one.
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