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RFC 868 / TIME PROTOCOL

Keep unknown time out of replies

04

A server that cannot determine its own time has different failure behavior for TCP and UDP. It should not invent a timestamp.

DIAGRAMRead RFC source: Unavailable-time handling · pages 1–2 ↗
RFC 868 · Unavailable-time handling · pages 1–2EXAMPLE
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Keep unknown time out of repliesThree separate unavailable-clock alternatives. TCP refusal stops connection establishment. TCP accepted-then-close sends no time data. UDP request discarded produces no reply. No fabricated zero timestamp,success path or mandatory timeout. These are lowercase should source rules, not capitalizedMUST.UserNo known timeempty UDP requestDiscard request · no replyNo reply is not a zero timestampSeparate cases · lowercase should in RFC 868
03
Discard the UDP request.

When the UDP server cannot determine time, it should make no reply. Do not turn absence into a zero-valued timestamp.

Source for this drawing: RFC 868 · Read RFC source: Unavailable-time handling · pages 1–2 ↗

Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
New to this? Start hereA quick picture + helpful background

Two clocks may disagree.

A device compares timestamps with another clock. Network delay makes that comparison imperfect. The drawing separates the clock reading from the trip the message takes.

Helpful first, if these ideas are unfamiliar

RFC 9293 TCP RFC 768 UDP RFC 5905 NTPv4

These pictures are analogies to help you learn. The linked RFC explains the actual rules.

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KEY POINT

Silence is a failure outcome, not midnight.

  • For TCP, the server should refuse the connection or close it without sending anything when it cannot determine time.
  • For UDP, it should discard the incoming datagram and send no reply in that case.
  • These are lowercase should statements in the 1983 text, not imported RFC 2119 MUST requirements.
DETAILS

The source link opens page 1 for TCP; the UDP rule is on page 2. The two TCP alternatives and the UDP case are separate examples. No reply cannot distinguish an unavailable clock from a network failure, and RFC 868 gives no mandatory timeout.

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