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A router MAY implement Timestamp and Timestamp Reply. If they are implemented then:

  • The ICMP Timestamp server function MUST return a Timestamp Reply to every Timestamp message that is received. It SHOULD be designed for minimum variability in delay.

  • An ICMP Timestamp Request message to an IP broadcast or IP multicast address MAY be silently discarded.

  • The IP source address in an ICMP Timestamp Reply MUST be the same as the specific-destination address of the corresponding Timestamp Request message.

  • If a Source Route option is received in an ICMP Timestamp Request, the return route MUST be reversed and used as a Source Route option for the Timestamp Reply message, unless the router is aware of policy that would prevent the delivery of the message.

  • If a Record Route and/or Timestamp option is received in a Timestamp Request, this (these) option(s) SHOULD be updated to include the current router and included in the IP header of the Timestamp Reply message.

  • If the router provides an application-layer interface for sending Timestamp Request messages then incoming Timestamp Reply messages MUST be passed up to the ICMP user interface.

The preferred form for a timestamp value (the standard value) is milliseconds since midnight, Universal Time. However, it may be difficult to provide this value with millisecond resolution. For example, many systems use clocks that update only at line frequency, 50 or 60 times per second. Therefore, some latitude is allowed in a standard value:

  1. A standard value MUST be updated at least 16 times per second (i.e., at most the six low-order bits of the value may be undefined).

  2. The accuracy of a standard value MUST approximate that of operator-set CPU clocks, i.e., correct within a few minutes.


To meet the second condition, a router may need to query some time server when the router is booted or restarted. It is recommended that the UDP Time Server Protocol be used for this purpose. A more advanced implementation would use the Network Time Protocol (NTP) to achieve nearly millisecond clock synchronization; however, this is not required.

Next: Address Mask Request/Reply

Connected: An Internet Encyclopedia Timestamp and Timestamp Reply


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