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RFC 868 / TIME PROTOCOL

Send an empty UDP request

03

The UDP user sends an empty datagram to port 37. The server sends back a datagram containing the 32-bit time.

DIAGRAMRead RFC source: UDP service · page 2 ↗
RFC 868 · UDP service · page 2EXAMPLE
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Send an empty UDP requestThree UDP states:zero-application-byte request inside transport envelope,sending empty datagram toUDP37,and separate server reply with32bit time to originating source. No TCP connection or close. Reply time is binary; no auth,era,transaction ID or retry timer is defined inRFC868.UserTime serverUDP port 37empty UDP datagramNo connection establishmentreply: 32-bit timeReply to the originating sourceSeparate datagrams; no TCP close exchange
03
Return one time datagram.

The reply carries 32 bits of time to the originating source, independently of the TCP service.

Source for this drawing: RFC 868 · Read RFC source: UDP service · page 2 ↗

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Two clocks may disagree.

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RFC 9293 TCP RFC 768 UDP RFC 5905 NTPv4

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KEY POINT

An empty request still has a transport envelope.

  • The request has zero application-data bytes; UDP/IP headers still exist.
  • The server listens on UDP port 37 and replies to the originating source.
  • The reply contains the time as a 32-bit binary number; no TCP connection or close exchange occurs.
DETAILS

RFC 868 defines no application transaction identifier, retry timer or authentication field. UDP delivery guarantees come from neither this request nor the small size of the reply. A missing reply is not a time value.

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