Editing Network Time Protocol (section)

== Clock strata ==
[[File:Usno-amc.jpg|thumb|The [[United States Naval Observatory|U.S. Naval Observatory]] Alternate Master Clock at [[Schriever Air Force Base|Schriever AFB (Colorado)]] is a stratum 0 source for NTP]]
[[File:Network Time Protocol servers and clients.svg|thumb|Yellow arrows indicate a direct connection; red arrows indicate a network connection.]]

NTP uses a hierarchical, semi-layered system of time sources. Each level of this hierarchy is termed a ''stratum'' and is assigned a number starting with zero for the reference clock at the top. A server synchronized to a stratum ''n'' server runs at stratum ''n'' + 1. The number represents the distance from the reference clock and is used to prevent cyclical dependencies in the hierarchy. Stratum is not always an indication of quality or reliability; it is common to find stratum 3 time sources that are higher quality than other stratum 2 time sources.{{efn|Telecommunication systems use a different definition for [[Synchronization in telecommunications|clock strata]].}} A brief description of strata 0, 1, 2 and 3 is provided below.

; Stratum 0
: These are high-precision timekeeping devices such as [[atomic clock]]s, [[GNSS]] (including [[GPS]]) or other [[radio clock]]s, or a [[Precision Time Protocol|PTP]]-synchronized clock.<ref>{{cite web |title=Combining PTP with NTP to Get the Best of Both Worlds |url=https://www.redhat.com/en/blog/combining-ptp-ntp-get-best-both-worlds |website=www.redhat.com |language=en |quote=Programs from the linuxptp package can be used in a combination with an NTP daemon. A PTP clock on a NIC is synchronized by ptp4l and is used as a reference clock by chronyd or ntpd for synchronization of the system clock.}}</ref> They generate a very accurate [[pulse per second]] signal that triggers an [[interrupt]] and timestamp on a connected computer. Stratum 0 devices are also known as reference clocks. NTP servers cannot advertise themselves as stratum 0. A stratum field set to 0 in NTP packet indicates an unspecified stratum.{{Ref RFC|5905|rp=21}}
; Stratum 1
: These are computers whose [[system time]] is synchronized to within a few microseconds of their attached stratum 0 devices. Stratum 1 servers may peer with other stratum 1 servers for [[sanity check]] and backup.<ref name="cisco">{{cite web| url=http://www.cisco.com/en/US/tech/tk869/tk769/technologies_white_paper09186a0080117070.shtml| title=Network Time Protocol: Best Practices White Paper| access-date=15 October 2013| archive-url=https://web.archive.org/web/20131001041853/http://www.cisco.com/en/US/tech/tk869/tk769/technologies_white_paper09186a0080117070.shtml| archive-date=1 October 2013| url-status=live}}</ref> They are also referred to as primary time servers.<ref name="summary" /><ref name="faq" />
; Stratum 2
: These are computers that are synchronized over a network to stratum 1 servers. Often a stratum 2 computer queries several stratum 1 servers. Stratum 2 computers may also peer with other stratum 2 computers to provide more stable and robust time for all devices in the peer group.
; Stratum 3
: These are computers that are synchronized to stratum 2 servers. They employ the same algorithms for peering and data sampling as stratum 2, and can themselves act as servers for stratum 4 computers, and so on.

The upper limit for stratum is 15; stratum 16 is used to indicate that a device is unsynchronized. The NTP algorithms on each computer interact to construct a [[Bellman–Ford]] shortest-path [[spanning tree]], to minimize the accumulated round-trip delay to the stratum 1 servers for all the clients.<ref name="Mills2010" />{{rp|20}}

In addition to stratum, the protocol is able to identify the synchronization source for each server in terms of a reference identifier (refid).

{| class="wikitable sortable"
|+ Common time reference identifiers (refid) codes
|-
! Refid<ref name="MIvTF">{{cite web|url=https://nlug.ml1.co.uk/2012/01/ntpq-p-output/831|title='ntpq -p' output|website=NLUG.ML1.co.uk|access-date=2018-11-12|archive-url=https://web.archive.org/web/20181112141516/https://nlug.ml1.co.uk/2012/01/ntpq-p-output/831|archive-date=2018-11-12|url-status=live}}</ref>!! Clock Source
|-
| GOES || [[Geostationary Operational Environmental Satellite]] (described as “Geosynchronous Orbit Environment Satellite” in RFC 5905)
|-
| GPS || [[Global Positioning System]]
|-
| GAL || [[Galileo (satellite navigation)|Galileo]] Positioning System
|-
| PPS || Generic pulse-per-second
|-
| IRIG || Inter-Range Instrumentation Group
|-
| WWVB || LF Radio [[WWVB]] Fort Collins, Colorado 60&nbsp;kHz
|-
| DCF || LF Radio [[DCF77]] Mainflingen, DE 77.5&nbsp;kHz
|-
| HBG || LF Radio [[HBG (time signal)|HBG]] Prangins, HB 75&nbsp;kHz (ceased operation)
|-
| MSF || LF Radio [[Time from NPL (MSF)|MSF]] Anthorn, UK 60&nbsp;kHz
|-
| JJY || LF Radio [[JJY]] Fukushima, JP 40&nbsp;kHz, Saga, JP 60&nbsp;kHz
|-
| LORC || MF Radio [[Loran-C]] station, 100&nbsp;kHz
|-
| TDF || [[TDF time signal|MF Radio Allouis, FR 162 kHz]]
|-
| CHU || HF Radio [[CHU (radio station)|CHU]] Ottawa, Ontario
|-
| WWV || HF Radio [[WWV (radio station)|WWV]] Fort Collins, Colorado
|-
| WWVH || HF Radio [[WWVH]] Kauai, Hawaii
|-
| NIST || [[NIST]] telephone modem
|-
| ACTS || NIST telephone modem
|-
| USNO || USNO telephone modem
|-
| PTB || German PTB time standard telephone modem
|-
| MRS || (Informal) Multi Reference Sources
|-
| GOOG || (Unofficial) Google Refid used by Google NTP servers as time4.google.com
|}

For servers on stratum 2 and below, the refid is an encoded form of the upstream time server's IP address. For IPv4, this is simply the 32-bit address; for IPv6, it would be the first 32 bits of the MD5 hash of the source address. Refids serve to detect and prevent timing loops to the first degree.{{Ref RFC|5905}}

The refid field is filled with status words in the case of kiss-o'-death (KoD) packets, which tell the client to stop sending requests so that the server can rest.{{Ref RFC|5905}} Some examples are INIT (initialization), STEP (step time change), and RATE (client requesting too fast).<ref>{{cite web |title=Event Messages and Status Words |url=https://docs.ntpsec.org/latest/decode.html#kiss |website=docs.ntpsec.org |quote=Refid codes are used in kiss-o'-death (KoD) packets, the reference identifier field in ntpq and ntpmon billboard displays and log messages.}}</ref> The program output may additionally use codes not transmitted in the packet to indicate error, such as XFAC to indicate a network disconnection.<ref name="MIvTF"/>

The IANA maintains a registry for refid source names and KoD codes. Informal assignments can still appear.<ref>{{cite web |title=Network Time Protocol (NTP) Parameters |url=https://www.iana.org/assignments/ntp-parameters/ntp-parameters.xhtml |website=www.iana.org}}</ref>