This patch adds the lcp-rtt-file configuration option, which instructs pppd to add a timestamp to the data section of each LCP echo request frame and then log their round trip time and any detected packet loss to a circular buffer in that file. Other programs then can asynchronously read the file and report statistics about the line. I am attaching as examples a simple program which dumps the content of the log file and a Prometheus exporter which can be run as a CGI. This feature was inspired by a similar one in a commercial PPP implementation, but I have never actually seen that and I do not know how it actually works or it was implemented. Signed-off-by: Marco d'Itri -- ciao, Marco