> Hi Xiaoyan,
>
> The "policy" that I was talking about is just a way to get access to the
> underlying data structure (insert/erase events).
>
> In any case, as it could be a generally useful metric, I decided to
> implement the necessary mechanics myself.   If you're curious, you can
> check commit 8566f458f2aacc25f06683ce00219f7168ee71f5, in particular
> utils/trie/lifetime-stats-policy.h policy file.
>
> As of right now (commit f4a0359ac1dc60390ea41d8347c73a8c24a6789e) you can
> use the following code in order to get samples for lifetime of cached
> entries (copy from http://ndnsim.net/helpers.html#content-store):
>
> --------------------
> void
> CacheEntryRemoved (std::string context, Ptr<const ndn::cs::Entry> entry,
> Time lifetime)
> {
>     std::cout << entry->GetName () << " " << lifetime.ToDouble (Time::S)
> << "s" << std::endl;
> }
> ...
> ndnHelper.SetContentStore ("ns3::ndn::cs::Stats::Lru", "MaxSize", "10000");
> ...
> ndnHelper.Install (nodes);
> // connect to lifetime trace
> Config::Connect ("/NodeList/*/$ns3::ndn::cs::Stats::Lru/WillRemoveEntry",
> MakeCallback (CacheEntryRemoved));
> --------------------
>
> I haven't had time, but you can write a tracer (similar to the one you
> already wrote for CacheHits/CacheMisses), which will make tracing totally
> trivial.
>
> Please note, that you *have to* use a specialized version of content store
> (ns3::ndn::cs::Stats::Lru, ns3::ndn::cs::Stats::Random, or
> ns3::ndn::cs::Stats::Fifo), and
> Config::Connect/Config::ConnectWithoutContext *have to* specify the correct
> content store implementation.
>
> Another note, for some replacement policies, entries may not be put in
> cache at all.  In the current implementation, such entries will be reported
> with lifetime 0.
>