Question

In the Cristian’s algorithm, explain why a client sets its clock to the time t + Tround/2, and with its accuracy of +-(Tround/2), where t is the time value in the reply message sent by the time server and Tround is the round trip time for sending a request message and receiving a reply message. What would happen when the minimum transmission time, min, is known

Answer 1

Cristian’s Algorithm is a clock synchronization algorithm is used to synchronize time with a time server by client processes. This algorithm works well with low-latency networks where Round Trip Time is short as compared to accuracy while redundancy prone distributed systems/applications do not go hand in hand with this algorithm. Here Round Trip Time refers to the time duration between start of a Request and end of corresponding Response.

1) The process on the client machine sends the request for fetching clock time(time at server) to the Clock Server at time .

2) The Clock Server listens to the request made by the client process and returns the response in form of clock server time.

3) The client process fetches the response from the Clock Server at time T1 and calculates the synchronised client clock time using the formula given below.

  

where refers to the synchronised clock time,
refers to the clock time returned by the server,
refers to the time at which request was sent by the client process,
refers to the time at which response was received by the client pro

Using iterative testing over the network, we can define a minimum transfer time using which we can formulate an improved synchronization clock time(less synchronization error).
Here, by defining a minimum transfer time, with a high confidence, we can say that the server time will
always be generated after and the will always be generated before , where is the minimum transfer time which is the minimum value of and during several iterative tests. Here synchronization error can be formulated as follows:

  

Similarily, if and differ by a considerable amount of time, we may substitute by and , where is the minimum observed request time and refers to the minimum observed response time over the network.
The synchronized clock time in this case can be calculated as:

  

So, by just introducing response and request time as separate time latencies, we can improve synchronization of clock time and hence decrease the overall synchronization error. Number of iterative tests to be ran depends on the overall clock drift observed

cess