Connect to receive the TCP time
02The user opens a TCP connection to the server on port 37. Establishment triggers the time reply; no application request string is specified.
The user receives the time and closes; the server also closes after sending. No detailed TCP close-packet ordering is implied.
Source for this drawing: RFC 868 · Read RFC source: TCP service · page 1 ↗
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
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
The TCP connection triggers one time value.
- The server listens on TCP port 37, which the RFC also writes as octal 45.
- After connection establishment the server sends its 32-bit time and closes the connection.
- The user receives the time and closes its connection. The picture shows application events, not TCP handshake or FIN packets.
TCP is a byte stream: this guide does not promise that one receive call returns the whole value. Transport reliability is separate from the accuracy or authenticity of the server clock.
Read the RFC section ↗