[ntp:questions] Problems with huffpuff

Rick Jones rick.jones2 at hp.com
Tue Feb 27 20:31:15 UTC 2007


Eric <nospam-01 at jensenresearch.com> wrote:
> Any ideas about whether 2uS is even a possible ntp delay value on a
> 100mb lan

If 2uS is two microseconds, 2uS is not possible even for a single-byte
netperf TCP_RR test over a 100Mb LAN so I suspect the same holds true
for NTP.

A minimum sized Ethernet Frame is 60 bytes, or 480 bits.  At 100
Mbit/s that takes 4.8 microseconds just to toggle onto the wire.
Given the need to have two of those, and adding-in host delays and
such, and that just won't be anywhere near two microseconds.

Here is a single-byte netperf TCP_RR between a pair of fast systems
connected via a _gigabit_ link:

hpcpc107:~# netperf -t TCP_RR -H 192.168.9.108
TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.9.108 (192.168.9.108) port 0 AF_INET : first burst 0
Local /Remote
Socket Size   Request  Resp.   Elapsed  Trans.
Send   Recv   Size     Size    Time     Rate         
bytes  Bytes  bytes    bytes   secs.    per sec   

16384  87380  1        1       10.00    14644.22   
16384  87380 

Invert the transactions/s and that is just shy of 70 microseconds
round-trip.  Given that gigabit would toggle those bits 10X faster
than 100BT, we can ass-u-me :) that most of the delay is actually
somewhere other than the wire in the test above, and then that the
dleay for 100BT would be roughly the same.  (of course, as the request
and response sizes increase, link bitrate will become a component of
the delay greater than epsilon)

rick jones
-- 
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