[NTP issue] how long does it take to ntp synchronization?

Ray Han · ‎05-31-2010

Hi all,

I'm testing about ntp synch time with below topology

RT1 ------------ RT2 ------------- RT3

RT1 : ntp client

RT2 : ntp master

RT3 : ntp client

RT1 config

ntp server 1.1.1.1(RT2 ip address)

RT2 config

ntp master

RT3 config

ntp server 1.1.1.1

above condition,

when i change the clock on RT2,

RT1 and RT2 clock will change.

but the time of clock change is different between RT1 and RT3.

RT1's clock was changed 6 mininutes later after clock change of the RT2(ntp master)

but RT3's clock was changed 8 minitues later after clock change of the RT2.

why above synch time difference does occur?

is there anyone knows about this?

Giuseppe Larosa · ‎05-31-2010

Hello Sanghee,

NTP protocol design looks for providing accurate time references but without talking too much (= very few packets one UDP packet every 60 seconds )

So NTP is not designed for fast convergence and as you have noted it may take some time to synchronize and / or to update a time source.

Hope to help

Giuseppe

Ray Han · ‎05-31-2010

Hi Giuseppe,

it's a long time to see you in this forum

i lost my password for account of hsh7603

unwillingly i created a new account of Ray_Han a few day ago.

first thanks for your comment.

my customer has a question about the difference of ntp convergence time

but i couldn't give answer exactly about that.

I'm reading(rfc1305) and testing to find a convergence variety.

as soon as finding it, i'll update this post.

Thanks again.

Ganesh Hariharan · ‎05-31-2010

Hi all,

I'm testing about ntp synch time with below topology

RT1 ------------ RT2 ------------- RT3

RT1 : ntp client

RT2 : ntp master

RT3 : ntp client

RT1 config

ntp server 1.1.1.1(RT2 ip address)

RT2 config

ntp master

RT3 config

ntp server 1.1.1.1

above condition,

when i change the clock on RT2,

RT1 and RT2 clock will change.

but the time of clock change is different between RT1 and RT3.

RT1's clock was changed 6 mininutes later after clock change of the RT2(ntp master)

but RT3's clock was changed 8 minitues later after clock change of the RT2.

why above synch time difference does occur?

is there anyone knows about this?

Hi,

Time sync in ntp client and server generally used to excahage several packet where each exchange is a pair of request and reply. When sending out a request, the client stores its own time (originate timestamp) into the packet being sent. When a server receives such a packet, it will in turn store its own time (receive timestamp) into the packet, and the packet will be returned after putting a transmit timestamp into the packet. When receiving the reply, the receiver will once more log its own receipt time to estimate the travelling time of the packet. The travelling time (delay) is estimated to be half of "the total delay minus remote processing time", assuming symmetrical delays.

Those time differences can be used to estimate the time offset between both machines, as well as the dispersion (maximum offset error). The shorter and more symmetric the round-trip time, the more accurate the estimate of the current time.

Hope to Help !!

Ganesh.H