NTP message format
NTP uses two types of messages, clock synchronization messages and NTP control messages. An NTP
control message is used in environments where network management is needed. Because it is not
essential for clock synchronization, it is not described in this document.
NOTE:
All NTP messages mentioned in this document refer to NTP clock synchronization messages.
A clock synchronization message is encapsulated in a UDP message, in the format shown in Figure 18.
Figure 18 Clock synchronization message format
LI VN Mode Stratum Poll Precision
0 7 15 23 31
Root delay (32 bits)
Root dispersion (32 bits)
Reference identifier (32 bits)
Receive timestamp (64 bits)
Transmit timestamp (64 bits)
Authenticator (optional 96 bits)
Reference timestamp (64 bits)
Originate timestamp (64 bits)
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The following main fields are described below:
• LI (Leap Indicator)—A 2-bit leap indicator. When set to 11, it warns of an alarm condition (clock
unsynchronized). When set to any other value, it is not to be processed by NTP.
• VN (Version Number) —A 3-bit version number that indicates the version of NTP. The latest version
is version 3.
• Mode—A 3-bit code that indicates the work mode of NTP. This field can be set to these values:
{ 0 — reserved
{ 1 — symmetric active
{ 2 — symmetric passive
{ 3 — client
{ 4 — server
{ 5 — broadcast or multicast
{ 6 — NTP control message
{ 7 — reserved for private use.
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