netfilter: conntrack: add conntrack event timestamp

netfilter: conntrack: add conntrack event timestamp

Nadia Pinaeva writes:
  I am working on a tool that allows collecting network performance
  metrics by using conntrack events.
  Start time of a conntrack entry is used to evaluate seen_reply
  latency, therefore the sooner it is timestamped, the better the
  precision is.
  In particular, when using this tool to compare the performance of the
  same feature implemented using iptables/nftables/OVS it is crucial
  to have the entry timestamped earlier to see any difference.

At this time, conntrack events can only get timestamped at recv time in
userspace, so there can be some delay between the event being generated
and the userspace process consuming the message.

There is sys/net/netfilter/nf_conntrack_timestamp, which adds a
64bit timestamp (ns resolution) that records start and stop times,
but its not suited for this either, start time is the 'hashtable insertion
time', not 'conntrack allocation time'.

There is concern that moving the start-time moment to conntrack
allocation will add overhead in case of flooding, where conntrack
entries are allocated and released right away without getting inserted
into the hashtable.

Also, even if this was changed it would not with events other than
new (start time) and destroy (stop time).

Pablo suggested to add new CTA_TIMESTAMP_EVENT, this adds this feature.
The timestamp is recorded in case both events are requested and the
sys/net/netfilter/nf_conntrack_timestamp toggle is enabled.

Reported-by: Nadia Pinaeva <n.m.pinaeva@gmail.com>
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>