SNTP, Simple Network Time Protocol

Description Glossary RFCs Publications Obsolete RFCs

Description:

Protocol suite: TCP/IP.
Protocol type:Application layer time protocol.
Base protocol: NTP, Network Time Protocol.
Port:123 (UDP).
MIME subtype:
SNMP MIBs:
Working groups: ntp, Network Time Protocol.
Links:

MAC header IP header UDP header SNTP message

SNTP message, version 4:

0001020304050607 0809101112131415 1617181920212223 2425262728293031
LI VN Mode Stratum Poll Precision
Root Delay
Root Dispersion
Reference Identifier
Reference Timestamp
-
Originate Timestamp
-
Receive Timestamp
-
Transmit Timestamp
-
Key Identifier
Message Digest
-
-
-

LI, Leap Indicator. 2 bits.
This code warns of an impending leap second to be inserted/deleted in the last minute of the current day.

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

VN, Version Number. 3 bits.
The NTP/SNTP version number. If necessary to distinguish between IPv4, IPv6 and OSI, the encapsulating context must be inspected.

VNDescription
0
1
2
3IPv4 only.
4IPv4, IPv6 and OSI.
5
6
7

Mode. 3 bits.

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

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

StratumDescription
0Unspecified or unavailable.
1Primary reference (e.g., radio clock).
2
-
15
Secondary reference (via NTP or SNTP).
16
-
255
reserved.

Poll. 8 bits, signed.
The maximum interval between successive messages, in seconds to the nearest power of two. The values that can appear in this field presently range from 4 (16 s) to 14 (16284 s); however, most applications use only the sub-range 6 (64 s) to 10 (1024 s).

Precision. 8 bits.
The precision of the local clock, in seconds to the nearest power of two. The values that normally appear in this field range from -6 for mains-frequency clocks to -20 for microsecond clocks found in some workstations.

Root Delay. 32 bits, signed fixed point.
The total roundtrip delay to the primary reference source, in seconds with fraction point between bits 15 and 16. Note that this variable can take on both positive and negative values, depending on the relative time and frequency offsets. The values that normally appear in this field range from negative values of a few milliseconds to positive values of several hundred milliseconds.

Root Dispersion. 32 bits, unsigned fixed point.
The nominal error relative to the primary reference source, in seconds with fraction point between bits 15 and 16. The values that normally appear in this field range from 0 to several hundred milliseconds.

Reference Identifier. 32 bits.
The reference source. In the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or stratum-1 (primary) servers, this is a four-character ASCII string, left justified and zero padded to 32 bits. In NTP Version 3 secondary servers, this is the 32-bit IPv4 address of the reference source. In NTP Version 4 secondary servers, this is the low order 32 bits of the latest transmit timestamp of the reference source. NTP primary (stratum 1) servers should set this field to a code identifying the external reference source according to the following list. If the external reference is one of those listed, the associated code should be used. Codes for sources not listed can be contrived as appropriate.

CodeDescription
ACTSNIST dialup modem service.
CHUOttawa (Canada) Radio 3330, 7335, 14670 kHz.
DCF Mainflingen (Germany) Radio 77.5 kHz.
GOES Geostationary Orbit Environment Satellite.
GPS Global Positioning Service.
LOCL Uncalibrated local clock used as a primary reference for a subnet without external means of synchronization.
LORC LORAN-C radionavigation system.
MSF Rugby (UK) Radio 60 kHz.
OMEG OMEGA radionavigation system.
PPS Atomic clock or other pulse-per-second source individually calibrated to national standards.
PTBPTB (Germany) modem service.
TDFAllouis (France) Radio 164 kHz.
USNOUSNO modem service.
WWVFt. Collins (US) Radio 2.5, 5, 10, 15, 20 MHz.
WWVBBoulder (US) Radio 60 kHz.
WWVHKaui Hawaii (US) Radio 2.5, 5, 10, 15 MHz.

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

Originate Timestamp. 64 bits.
The time at which the request departed the client for the server.

Receive Timestamp. 64 bits.
The time at which the request arrived at the server.

Transmit Timestamp. 64 bits.
The time at which the reply departed the server for the client.

Key Identifier. 32 bits.
Optional.

Message Digest. 128 bits.
Optional.


Glossary:


RFCs:

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


Publications:


Obsolete RFCs:

[RFC 1361] Simple Network Time Protocol (SNTP).

[RFC 1769] Simple Network Time Protocol (SNTP).

[RFC 2030] Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI.

[RFC 4330] Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI.


Description Glossary RFCs Publications Obsolete RFCs