SNTP: Simple Network Time Protocol

The Simple Network Time Protocol (SNTP) Version 4 is an adaptation of the Network Time Protocol (NTP) used to synchronize computer clocks in the Internet. SNTP can be used when the ultimate performance of the full NTP implementation is not needed or justified. When operating with current and previous NTP and SNTP versions, SNTP Version 4 involves no changes to the NTP specification or known implementations, but rather a clarification of certain design features of NTP which allow operation in a simple, stateless remote-procedure call (RPC) mode with accuracy and reliability expectations similar to the UDP/TIME protocol.

It is strongly recommended that SNTP be used only at the extremities of the synchronization subnet. SNTP clients should operate only at the leaves (highest stratum) of the subnet and in configurations where no NTP or SNTP client is dependent on another SNTP client for synchronization. SNTP servers should operate only at the root (stratum 1) of the subnet and then only in configurations where no other source of synchronization other than a reliable radio or modem time service is available. The full degree of reliability ordinarily expected of primary servers is possible only using the redundant sources, diverse subnet paths and crafted algorithms of a full NTP implementation. This extends to the primary source of synchronization itself in the form of multiple radio or modem sources and backup paths to other primary servers should all sources fail or the majority deliver incorrect time. Therefore, the use of SNTP rather than NTP in primary servers should be carefully considered.

The only significant protocol change in SNTP Version 4 over previous versions of NTP and SNTP is a modified header interpretation to accommodate Internet Protocol Version 6 (IPv6) and OSI addressing. However, SNTP Version 4 includes certain optional extensions to the basic Version 3 model, including an anycast mode and an authentication scheme designed specifically for multicast and anycast modes.

• LI - Leap Indicatorwarning of impending leap-second to be inserted at the end of the last day of the current month.
• VN - Version number indicating the version number.
• Mode - The mode: This field can contain the following values:

• 0	Reserved.
  1	Symmetric active.
  3	Client.
  4	Server.
  5	Broadcast.
  6	NTP control message.

• Stratum - An integer identifying the stratum level of the local clock.
• Poll - Signed integer indicating the maximum interval between successive messages, in seconds to the nearest power of 2.
• Precision - Signed integer indicating the precision of the local clock, in seconds to the nearest power of 2.
• Root Delay: signed fixed-point number indicating the total roundtrip delay to the primary reference source, in seconds with fraction point between bits 15 and 16.
• Root Dispersion: Unsigned fixed-point number indicating the nominal error relative to the primary reference source, in seconds with fraction point between bits 15 and 16.
• Reference Identifier: Identifying the particular reference source.
• Originate Timestamp: This is the time at which the request departed the client for the server, in 64-bit timestamp format.
• Receive Timestamp: This is the time at which the request arrived at the server, in 64-bit timestamp format.
• Transmit Timestamp: This is the time at which the reply departed the server for the client, in 64-bit timestamp format.
• Authenticator (optional): When the NTP authentication scheme is implemented, the Key Identifier and Message Digest fields contain the message authentication code (MAC) information defined.