Routers MUST support the timestamp option in forwarded packets. A timestamp value MUST follow the rules given [INTRO:2].
If the flags field = 3 (timestamp and prespecified address), the router MUST add its timestamp if the next prespecified address matches any of the router's IP addresses. It is not necessary that the prespecified address be either the address of the interface on which the packet arrived or the address of the interface over which it will be sent.
To maximize the utility of the timestamps contained in the timestamp option, it is suggested that the timestamp inserted be, as nearly as practical, the time at which the packet arrived at the router. For datagrams originated by the router, the timestamp inserted should be, as nearly as practical, the time at which the datagram was passed to the network layer for transmission.
A router MAY provide a configuration option which, if enabled, will cause the router to ignore (i.e., pass through unchanged) Timestamp options in forwarded datagrams when the flag word is set to zero (timestamps only) or one (timestamp and registering IP address). If provided, such an option MUST default to off (that is, the router does not ignore the timestamp). This option should not affect the processing of Timestamp options in datagrams received by the router itself (in particular, a router will insert timestamps into Timestamp options in datagrams received by the router, and Timestamp options in ICMP echo requests will still be processed according to Section [4.3.3.6]).
Like the Record Route option, the Timestamp option can reveal information about a network's topology. Some people consider this to be a security concern.