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Re: ntp config tech note
- From: Michael Sinatra
- Date: Fri May 21 19:06:18 2004
John Kristoff wrote:
On Thu, 20 May 2004 21:08:43 -0700
Michael Sinatra <email@example.com> wrote:
I run two stratum-1 servers and a few stratum-2s and I provide time via
multicast (184.108.40.206), but I don't use it for my servers, except for
Presumably you meant 220.127.116.11.
Yes it is. Session breakage doesn't seem to be a problem, as long as
you're per-flow traffic sharing for equal-cost routes across your
backbone. I can see where per-packet switching would make a mess of
things, but that's true with any anycast service. Also, if the flow
table entry ages out before the next NTP poll, I can see how a client
running a full-blown ntpd would probably perceive an otherwise
transparent switch back and forth between two servers as excess jitter.
If you're worried about this, then one way around it would be to
adjust the costs of your injected routes so that one server is always
preferred over another. In that case, you're not buying
load-distribution across servers, but backup for clients where ease of
configuration is more important that accuracy, but where reliability
(ability to poll at least one server all the time) is still important,
and multicast may or may not provide the level of reliability that you
need. (I am a fan of multicast, BTW.)
testing and verification. I am also providing anycast ntp, and, if the
belt and suspenders weren't enough, I am experimenting with manycast.
Noting that NTP uses more than a reply response message exchange. No
concerns about session breakage? SNTP would certainly be a very
viable candidate for anycast.
In our case it's useful, as you note, for pointing an increasing number
of SNTP clients (including network equipment) to one address that's
reliable and redundant.
I know there are others who have experience in doing NTP anycast, at
least within an enterprise, and perhaps as service provider, who can
Yes. The main "buy" is ease of configuration, and that holds for
multicast, manycast, and anycast. I get a lot of requests for providing
NTP in such a way that sysadmins and users can use a really simple
bootstrap configuration in clients that they can replicate across their
enterprise. I'll also note that some OS vendors ship a stock multicast
config for their (x)ntpd.
Except in the extreme case such as wisc.edu's unfortunate experience,
does multicast buy much? Traffic loads for properly running clients
and distributed servers tend to be relatively low in my experience.
I have found that load can get higher when you start hammering servers
with thousands of SNTP clients; it can be something of an issue
considering that NTP servers tend to be hand-me-down hardware. (BTW: Is
rackety.udel.edu still a Sun IPC?) So, my "problem" is, how can I
properly distribute the load across servers, increase reliability, and
still give users simplicity in their configuration? Each of the three
solutions has its own set of pros and cons:
manycast - probably the best solution; solves authentication and
possible session issues with anycast, more accurate than multicast,
since true associations are established with (hopefully) several
participating servers. Downside is that it's only supported in v4,
which not all vendor OSes have as their stock daemon; doesn't work with
anycast - probably best solution for SNTP clients, may also be useful
for not v4 clients that can't do manycast. May break authentication,
depending on how keys are managed, but I haven't actually tested this.
(If all anycast servers have the same key pair, will authentication
break if a client switches servers due to a routing change? I haven't
tested this yet.)
multicast - Still a good solution for v3 clients that want simple
configuration. A lot of people still *ask* about multicast NTP, since
that's the config some OS vendors ship.
Then of course, there's the good old configure-4+-ntp-servers-manually,
which is good for important boxes that really need good time. (I have
one box that does all three of the above, plus has manually configured
Any thoughts or comments on the advantages and disadvantages of the
above techniques are welcome, as well as corrections.