[ntp:questions] time delta between clients

Kevin Oberman oberman at es.net
Wed Dec 12 00:41:05 UTC 2007


> From: Rick Jones <rick.jones2 at hp.com>
> Date: Wed, 12 Dec 2007 00:21:28 +0000 (UTC)
> Sender: questions-bounces+oberman=es.net at lists.ntp.org
> 
> 
> Some folks have been asking me if it is possible to use netperf to get
> the one-way latency between two systems, while sending a
> unidirectional stream of data from one to the other.  Netperf already
> has a TCP_RR test (ping-pong) that will report the average round-trip
> latency (and optionally a histogram of the individual RTTs).  That
> though is not a unidirectional test.
> 
> So, I am assuming I need synchronized "clocks."  Running ntp on each
> of the two systems on which I would run netperf is not a problem.
> 
> What I'm curious about, and a topic where I would welcome some gentle
> taps with clue bats, is if I can take the difference in offset between
> each client and the time server and ass-u-me that is the difference in
> time between the two clients.  Or do I have to do something ntp-like
> in netperf itself between the two systems?
> 
> My tests will typically only run for oh, O(60) seconds at a time, so
> I'm ass-u-me-ing I can ignore issues of the two client clocks rates
> changing much.
> 
> For example, here is ntpq peers output from a pair of machines where I
> might want to do this:
> 
> Client 1:
> ntpq> peers
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> +oslowest.raj    15.4.81.61       2 u  118  128  377    0.123   -1.602   2.723
> +lart.lart       15.235.160.30    5 u    4  128  377   35.639    4.115   1.508
> *shovlhead.nashu .TRUE.           1 u   54  128  377  101.115   -5.273   8.190
> 
> 
> Client 2:
> 
> ntpq> peers
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> +linger.raj      15.4.81.61       2 u  104  128  377    0.151    0.002   0.976
> +lart.lart       15.235.160.30    5 u   97  128  377   35.682    3.556   1.003
> *shovlhead.nashu .TRUE.           1 u   33  128  377  105.602   -0.839   3.267
> 
> I would take the difference in offset - -5.273 - -0.839 - and take
> that to be the difference in time between Client 1 and 2.
> 
> Thoughts, suggestions, pointers etc most welcome,
> 
> rick jones
> 
> BTW, in this case, I disabled the interrupt coalescing on Client 1's
> NIC, which I believe is the reason for the difference in delay between
> Client 1 and 2 and linger.raj - all three are on the same LAN.  It
> gets lost in the noise talking to the other two time sources.
> 
> -- 
> oxymoron n, commuter in a gas-guzzling luxury SUV with an American flag
> these opinions are mine, all mine; HP might not want them anyway... :)
> feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
> 
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> 

Netperf is not really the best way to go. The appropriate tool for
one-way latency is OWAMP.  http://e2epi.internet2.edu/owamp/

It requires well synchronized time which is where I became seriously
involved in this area. We try to keep our OWAMP server clocks in sync to
<5 microseconds. (And we almost always do.) They are scattered all over
out backbone from coast to coast.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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