NTP Server

Cutler James R james.cutler at consultant.com
Mon Oct 25 02:39:33 UTC 2010


Routers are not a good choice for time servers as it complicates configuration and, to some extent, constrains deployment methodology for routers to be effective with time service. We don't run DNS on routers, it is a service. Time service via NTP is a service as well. The NTP daemon in a router is not implemented in hardware and requires CPU resources better dedicated to RIB management.

In my experience, a reliable NTP peer group can be implemented on the same set of boxes as DNS (bind, etc.) with little or no impact on DNS performance. If you can count to four or more, you can make a reliable peer group of time servers.




On Oct 24, 2010, at 8:15 PM, Brandon Kim wrote:

> 
> Hi Sean:
> 
> By local I meant in-house, on-site in our datacenter. As far as what applications could use our NTP service, I would
> leave that up to each client and what they are running. For my own personal purposes, it would just be for log purposes. 
> (error logs, syslogs, etc etc)
> 
> I have heard that routers don't make good NTP servers since they weren't designed to keep track of time. This, I have read
> from a Cisco source. Can't remember where though. Or maybe they were just referring to older less powerful routers like 2500 series...
> 
> Brandon
> 
> 
> 
> 
> 
> 
>> Date: Sun, 24 Oct 2010 14:42:24 -0400
>> From: sean at donelan.com
>> To: nanog at nanog.org
>> Subject: Re: NTP Server
>> 
>> On Sun, 24 Oct 2010, Brandon Kim wrote:
>>> 1) How necessary do you believe in local NTP servers? Do you really 
>>> need the logs to be perfectly accurate?
>>> 2) If you do have a local NTP server, is it only for local internal 
>>> use, or do you provide this NTP server to your clients as an added 
>>> service?
>>> 3) If you do have a local NTP server, do you have a standby local NTP 
>>> server or do you use the internet as your standby server?
>> 
>> First terminology.  What do you mean by a local NTP server?
>> 
>> Almost any Cisco/Juniper router, Unix server and some recent Windows 
>> servers have NTP server software and can synchronize clocks in your 
>> network.  So you may already have a NTP server capable device.  You just 
>> need to configure it, and give it a good source of time.  It would be a 
>> Stratum 2 or greater NTP server because the good source of time is 
>> another NTP server.  Left to itself, NTP is pretty good at keeping clocks 
>> in arbitrary networks synchronized with each other. But most people are 
>> also interested in synchronizing clocks with some official time source.
>> 
>> The Network Time Protocol doesn't really have the notion of a "standby" 
>> server.  It uses multiple time sources together, and works best with about 
>> four time sources.  But for many end-systems, the Simple Network Time 
>> Protocol with a single time source may be sufficient.
>> 
>> If you are in a regulated industry (stock broker, electric utility, 9-1-1 
>> answering point, etc) there are specific time and frequency standards you 
>> must follow.
>> 
>> On the other hand, are you are asking about a local clock receiver (radio, 
>> satellite, etc) for a stratum 1 NTP server?  Clock receivers are getting 
>> cheaper, the problem is usually the antenna location.
>> 
>> Or on the third hand, are you asking about local primary reference clock 
>> (caesium, rubium, etc) for a stratum 1 NTP server?  These are still 
>> relatively expensive up to extremely expensive.
>> 
>> Or on the fourth hand, are you a time scientist working to improve 
>> international time standards.  If you are one of these folks, you already
>> know.
>> 
>> 
>> Most major ISPs use NTP across their router backbone, and incidently 
>> provide it to their customers. The local ISP router connected to your 
>> circuit probably has NTP enabled.
>> 
>> Required accuracy is in the eye of the beholder. NASDAQ requires brokers 
>> to have their clocks synchronized within 3 seconds of UTC(NIST).  9-1-1 
>> centers are required to have their clocks synchronized within 0.5 seconds 
>> of UTC.  Kerberos/Active Directory requires clocks to be synchronized 
>> within 5 minutes of each other.
>> 
>> If your log files have a resolution of 1 second, you probably won't see 
>> much benefit of sub-second clock precision or accuracy.  If you are 
>> conducting distributed measurements with sub-microsecond resolution, you
>> probably will want something more.
>> 
>> 
>> 
>                     =

James R. Cutler
james.cutler at consultant.com




More information about the NANOG mailing list