tcp_info::tcpi_last_ack_recv becomes "negative" sometimes. -10 msec or 1 jiffy in my case. Probably due to lack of synchronization in the kernel.

struct tcp_info tinfo = {0};
socklen_t len = sizeof(tinfo);
if (getsockopt(thesocket, SOL_TCP, TCP_INFO, (void*)&tinfo, &len) == 0)
    printf("%u\n", tinfo.tcpi_last_ack_recv);