NTP, Network Time Protocol

Description Glossary RFCs Publications Obsolete RFCs

Description:

Protocol suite: TCP/IP.
Protocol type:Application layer time protocol.
Multicast addresses:224.0.1.1.
Related protocols: SNTP, Simple Network Time Protocol.
Port:123 (UDP).
MIME subtype:
SNMP MIBs:iso.org.dod.internet.mgmt.mib-2.ntpSnmp (1.3.6.1.2.1.197).
Working groups: ntp, Network Time Protocol.
Links: www.ntp.org
www.pool.ntp.org

NTP version 0

NTP version 1

NTP version 2

NTP version 3

NTP version 4
This version adds support for the Autokey security protocol.


MAC header IP header UDP header NTP message

NTP version 3 message:

0001020304050607 0809101112131415 1617181920212223 2425262728293031
LI Version Mode Stratum Poll interval Precision
Root delay
Root dispersion
Reference clock identifier
Reference timestamp
-
Originate timestamp
-
Receive timestamp
-
Transmit timestamp
-
Authenticator
-

LI, Leap Indicator. 2 bits.
This field is used to warn of an impending leap second.

LIDescription
0No warning.
1Last minute has 61 seconds.
2Last minute has 59 seconds.
3Alarm condition, clock not synchronized.

Version. 3 bits.
NTP version number.

Mode. 3 bits.

ModeDescription
0reserved.
1Symmetric active.
2Symmetric passive.
3Client.
4Server.
5Broadcast.
6NTP control message.
7private use.

Stratum. 8 bits.
Stratum level of the local clock.

StratumDescription
0unspecified.
1Primary reference.
2
-
255
Secondary reference.

Poll interval. 8 bits, signed.
The maximum interval between messages in seconds to the nearest power of two.

Precision. 8 bits, signed.
Precision of the local clock in seconds to the nearest power of two.

Root delay. 32 bits, signed fixed point.
The total roundtrip delay to the primary reference source, in seconds with the fraction point between bits 15 and 16. Positive and negative values are valid.

Root dispersion. 32 bits, signed fixed point.
The maximum error relative to the primary reference source in seconds with the fraction point between bits 15 and 16. Only positive values greater than zero are valid.

Reference clock identifier. 32 bits.
Used to identify the particular reference clock.

In the case of stratum 2 and greater (secondary reference) this is the IPv4 address of the primary reference host.

In the case of stratum 0 (unspecified) or stratum 1 (primary reference), this is a four byte, left-justified, zero padded ASCII string. While not enumerated as part of the NTP specification, the following are suggested ASCII identifiers:

StratumIdentifierDescription
0"DCN"DCN routing protocol.
0"NIST"NIST public modem.
0"TSP"TSP time protocol.
0"DTS"Digital Time Service.
1"ATOM"Atomic clock (calibrated).
1"VLF"VLF radio.
1callsignGeneric radio.
1"LORC"LORAN-C.
1"GOES"GOES UHF environment satellite.
1"GPS"GPS UHF positioning satellite.

Reference timestamp. 64 bits.
The local time at which the local clock was last set or corrected.

Originate timestamp. 64 bits.
The local time when the client sent the request.

Receive timestamp. 64 bits.
The local time when the request was received by the server.

Transmit timestamp. 64 bits.
The local time when the reply was sent from the server.

Authenticator. 0 or 96 bits.


Glossary:


RFCs:

[RFC 1128] Measured Performance of the Network Time Protocol in the Internet System.

[RFC 1129] Internet Time Synchronization: the Network Time Protocol.

[RFC 1165] Network Time Protocol (NTP) over the OSI Remote Operations Service.

[RFC 1589] A Kernel Model for Precision Timekeeping.

[RFC 1708] NTP PICS PROFORMA For the Network Time Protocol Version 3.

[RFC 2783] Pulse-Per-Second API for UNIX-like Operating Systems, Version 1.0.

[RFC 5905] Network Time Protocol Version 4: Protocol and Algorithms Specification.

[RFC 5906] Network Time Protocol Version 4: Autokey Specification.

[RFC 5907] Definitions of Managed Objects for Network Time Protocol Version 4 (NTPv4).


Publications:


Obsolete RFCs:

[RFC 958] Network Time Protocol (NTP).

[RFC 1059] Network Time Protocol (Version 1).

[RFC 1119] Network Time Protocol (Version 2) Specification and Implementation.

[RFC 1305] Network Time Protocol (Version 3) Specification, Implementation and Analysis.


Description Glossary RFCs Publications Obsolete RFCs