At 4:33 PM +0200 2005-09-11, Brad Knowles wrote:
I've done technical reviews of books on this subject, I've done
consulting in this area (including consulting on behalf of vendors),
I've done invited talks on this subject, I've done seminars on this
subject, and I'm now writing a book on it, yes.
I've always hated to call myself an "expert" on any subject,
because I've always been able to point to people who were more
"expert" on it than I was. I've always preferred to use the term
"specialist" instead.
But maybe I have to start changing my attitude on that....
Within the purview of NTP, there is a much simpler way to do this --
use "manycast" instead. The client sends out a multicast query with a
packet TTL of zero, to see if there are any servers on the local segment
which are configured to listen to that address and provide NTP service.
If none, then it tries again with a TTL of 1. And it continues
incrementing the TTL and retransmitting until such time as it finds
enough servers to meet its configuration requirements.
Sorry, one thing I should have made clear, but didn't -- each
time the packet crosses a router, the TTL is decremented by one, and
a router won't forward a multicast packet if the TTL is zero. In
this way, using multicast ensures that you always find the servers
that are closest to you in a topological sense. Which is precisely
what we want within NTP.
No, that problem is actually a lot harder than you think it is. This
might get you 80% of the way there, but I don't think that even an 80%
solution is adequate here.
Moreover, DNS latency is already such that you really, really
don't want to be doing queries such as shown at
<http://www.google.com/search?q=ip+address+geocoding> before you can
create your reply.
In addition, if you did this, you would be likely to create
situations where a client is directed to one or more servers that is
geographically close to it, but which are not close to it
topologically. Even if you did geocoding of the reply, this would be
no better than having the client select servers that were in nearby
countries, hoping that they would be topologically close.
IMO, this is a worse idea than just continuing the existing use
of country-code.pool.ntp.org.
--
Brad Knowles, <[EMAIL PROTECTED]>
"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."
-- Benjamin Franklin (1706-1790), reply of the Pennsylvania
Assembly to the Governor, November 11, 1755
SAGE member since 1995. See <http://www.sage.org/> for more info.
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