What are the network protocols that can be used to measure the system clock (time) of a remote server? So far I have:
- Query with NTP
- TCP Timestamps
- HTTP headers
Are there any other?
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There's always the Daytime Protocol
From the RFC:
A useful debugging and measurement tool is a daytime service. A daytime service simply sends a the current date and time as a character string without regard to the input. TCP Based Daytime Service One daytime service is defined as a connection based application on TCP. A server listens for TCP connections on TCP port 13. Once a connection is established the current date and time is sent out the connection as a ascii character string (and any data received is thrown away). The service closes the connection after sending the quote. UDP Based Daytime Service Another daytime service service is defined as a datagram based application on UDP. A server listens for UDP datagrams on UDP port 13. When a datagram is received, an answering datagram is sent containing the current date and time as a ASCII character string (the data in the received datagram is ignored).