Steve:
Many thanks for that link to your paper on NTP
http://leapsecond.com/ntp/NTP_Paper_Sommars_PTTI2017.pdf
I read the entire paper, not just figs 7 and 8, and it explains everything.
I was beginning to doubt my own sanity because several authorities that I
had consulted told me quite categorically that The Network would have no
business looking at source port addresses within packets, and would have no
reason to treat packets from some port addresses differently from packets
from other port addresses. I had in particular consulted a named person at
one organisation which operates ntp servers in Colorado and Maryland. I had
spotted this effect first on ALL their servers and had not, at that time,
seen it on any others, so I had good reason to ask them whether there could
be something at their end which might be responsible. "No there isn't - our
servers are OK" I was told, but nothing more.
I could still make a valid point which could be of concern to writers of ntp
client software. If a client used a fixed, constant, local port address, we
can assume that routers in the internet (or at least those which do
implement this load-spreading technique) would route all ntp traffic - from
client to server and back - via the same path each time, so there would be
no spread in offset times seen by the client. There might, of course, be
some asymetry in the paths. By contrast, a client that used a different
local port address for each query might see a different route each time and
there would be a spread in offset times of the same order as the spread in
propagation times of the routes in use.
For example, I have seen, when querying servers in Colorado and Maryland
from here (the UK), standard deviations in the derived offset, of about
100usec when I use a fixed local port address, and about 3 msec when I use a
port address which either increments by 1 for each query or is chosen as a
random number between 49152 and 65535 each time. I am not sure of the
statistics, but I am not sure that randomizing the local port address has
any useful advantages.
Just for amusement I implemented a process whereby my client program starts
with a randomized port address, but keeps a record of the port number of the
query which gives the lowest round-trip-time during this "search" phase,
then switches to this fixed port address thereafter. The offset thus
achieved was always "better" than with any other random choice of fixed port
address. Maybe this is going too far!
Steve: Thanks again. VERY useful.
Peter
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