NTP Working Group	David L. Mills
Technical Report 06-6-1	 University of Delaware
	June 2006	

Network Time Protocol Version 4
Reference and Implementation Guide

Abstract

This document describes the Network Time Protocol Version 4 (NTPv4), which is widely used 
to synchronize the time for Internet hosts, routers and ancillary devices to Coordinated Universal 
Time (UTC) as disseminated by national standards laboratories. It describes the core 
architecture, protocol, state machine, data structures and algorithms. It explains the fundamental 
on-wire protocol used to exchange time values between peers, servers and clients. It summarizes 
the clock offset, roundtrip delay and various other statistics used by the mitigation algorithms to 
calculate the maximum error and nominal error inherent in computing these values. It describes 
several changes from Version 3 of NTP (NTPv3) originally described in RFC 1305, including 
the introduction of a modified protocol header to accommodate Internet Protocol Version 6 and a 
new header extension field to support the Autokey public key authentication scheme.

This document is based on the reference implementation available at www.ntp.org. It is intended 
as a reference and implementation guide, not as a formal standard. The main body of the 
document describes the basic model data structures and algorithms necessary for an 
implementation which can interoperate properly with another implementation faithful to this 
model. This document describes a number of crafted mitigation algorithms which can improve 
the accuracy and stability of the timekeeping function, especially in NTP subnets with many 
servers and clients. It also describes the clock discipline function used to adjust the system clock 
in time and frequency to agree with the available sources of synchronization.

As an implementation aid, a code skeleton for the reference implementation is presented in an 
appendix. It includes most of the data structures and algorithms of that program, but certain 
features, such as the control and monitoring protocol, Autokey public key authentication scheme, 
huff-’n-puff scheme and server discovery schemes are not included. These are discussed in 
companion documents on the Web and in print.

Keywords: network time synchronization, computer time synchronization, time synchronization 
protocol

Sponsored by: Naval Surface Weapons Center (Dahlgren) Contract N00178-04-1-9001.

Table of Contents

1.	Introduction	1

2.	Modes of Operation	2

3.	Definitions	4

4.	Implementation Model	5

5.	Data Types	6

6.	Data Structures	9

6.1	Structure Conventions	9

6.2	Global Parameters	10

6.3	Packet Header Variables	11

7.	On-Wire Protocol	14

8.	Peer Process	16

8.1	Peer Process Variables	16

8.2	Peer Process Operations	18

8.3	Clock Filter Algorithm	22

9.	System Process	24

9.1	System Process Variables	24

9.2	System Process Operations	25

9.2.1	Selection Algorithm	26

9.2.2	Clustering Algorithm	28

9.2.3	Combining Algorithm	29

9.2.4	Clock Discipline Algorithm	31

9.3	Clock Adjust Process	36

10.	Poll Process	36

10.1	Poll Process Variables and Parameters	37

10.2	Poll Process Operations	37

11.	References	39

Appendix A.	NTPv4 Packet Formats	41

A.1	NTP Header Field Format	41

A.2	NTPv4 Extension Field Format	42

Appendix B.	Code Skeleton	44

B.1	Global Definitions	44

B.1.1	Definitions, Constants and Parameters	44

B.1.2	Packet Data Structures	47

B.1.3	Association Data Structures	48

B.1.4	System Data Structures	49

B.1.5	Local Clock Data Structure	50

B.1.6	Function Prototypes	50

B.2	Main Program and Utility Routines	51

B.3	Kernel Input/Output Interface	55

B.4	Kernel System Clock Interface	55

B.5	Peer Process	57

B.5.1	receive()	58

B.5.2	packet()	62

B.5.3	clock_filter()	64

B.5.4	fast_xmit()	65

B.5.5	access()	66

B.6	System Process	66

B.6.1	clock_select()	67

B.6.2	root_dist()	70

B.6.3	accept()	70

B.6.4	clock_update()	71

B.6.5	clock_combine()	73

B.6.6	local_clock()	73

B.6.7	rstclock()	78

B.7	Clock Adjust Process	79

B.7.1	clock_adjust()	79

B.8	Poll Process	80

B.8.1	poll()	80

B.8.2	poll_update()	82

B.8.3	transmit()	83