The proposed resolution here is to implement this functionality, within the
basic framework established in Issue 30.

In detail:

GIMPS messages which manipulate routing state (Q/R/C) messages contain extra
information along with the routing state lifetime, i.e. the TTL. Note that it
may be needed in the Confirm (in case of query being sent in the upstream
direction, the confirm is the first chance to report the measured TTL from the
response). In particular, it must be possible to carry it in C-mode messages
(for reporting).

In a downstream message, the sender inserts the original TTL of the IP datagram,
or 0 if measurement will not be possible (e.g. if the encapsulation makes it
meaningless or the implementation cannot access the information at all). The
receiver should compare this with the TTL in the received datagram and store the
result as part of the routing state.

In an upstream message, the sender inserts the current value of its measured TTL
(taken from the routing state), or 0 if there is no measured value available.
The receiver should record this value in its routing state.