[ntp:questions] Getting NTP to correct only the clock skew

Spoon devnull at ntp.isc.org
Sat Apr 14 10:25:14 UTC 2007

Richard B. gilbert wrote:

> Spoon wrote:
>> Hans Jørgen Jakobsen wrote:
>>> Spoon wrote:
>>>> Consider two systems, A and B.
>>>> A sends ~1000 UDP packets per second to B.
>>>> A timestamps each packet.
>>>> These packets travel over an IP network, and suffer delay and jitter.
>>>> B is supposed to re-send the packets it receives at the rate they
>>>> were originally sent by A.
>>>> B buffers N packets. Then it sends the first packet in the queue, 
>>>> computes the departure time of the next packet using the timestamps 
>>>> provided by A, and sleeps until that departure time.
>>>> If the clocks on A and B do not tick at the same rate, the buffer 
>>>> used by B will either overflow or underflow.
>>> If the problem is to not have overflow or underflow then why not 
>>> steer on how full the buffer are.
>> This is easier said than done. When there is a lot of jitter on the 
>> link between A and B, it is actually quite hard to even tell whether 
>> the buffer is shrinking or expanding. The problem becomes a digital 
>> signal processing exercise. I didn't find an appropriate filter.
> This seems to me to resemble a very simple flow control problem solved 
> many years go by the simple expedient of having B tell A to stop sending 
> or to resume sending depending on the fullness of B's buffer pool; e.g. 
> the Xon/Xoff protocol used in things like Xmodem and Kermit.

Flow control is not an option. A has no control over the rate of the 
data stream:

A receives a CBR data stream from a local device, stuffs the data into 
packets, and sends them over the network.


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