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Re: ntp config tech note

  • From: Michael Sinatra
  • Date: Fri May 21 19:06:18 2004

Hi John:

John Kristoff wrote:
On Thu, 20 May 2004 21:08:43 -0700
Michael Sinatra <> wrote:

I run two stratum-1 servers and a few stratum-2s and I provide time via multicast (, but I don't use it for my servers, except for

Presumably you meant
Yep, sorry.

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.
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.)

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 probably comment.

Except in the extreme case such as's unfortunate experience,
does multicast buy much?  Traffic loads for properly running clients
and distributed servers tend to be relatively low in my experience.
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.

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 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 SNTP.

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 servers.)

Any thoughts or comments on the advantages and disadvantages of the above techniques are welcome, as well as corrections.


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