OVS: Enrich UFIDs using ovs-appctl

[amorenoz]

Context: #439

If retis was able to extract UFIDs from the kernel, we would need to talk to OVS to enrich them.

There are currently two interesting ways to enrich UFIDs:

ovs-appctl dpctl/get-flow [dp] ufid:ufid prints the datapath flow, given a UFID
ovs-appctl ofproto/detrace prints the Openflow flows that generated the datapath flow, given a UFID.

This all looks awesome right? What's the catch? Well, the catch is that UFIDs are very volatile. Any change in the OpenFlow flows can make them disappear or if traffic stops matching it, they are expired.

So, the only good way of doing this enrichment is to do it live! That is the challenge: send live requests to OVS without overloading it or delaying event processing too much.

An initial idea (tentative) is:

• Main thread will send UFIDs to an secondary thread.
• The secondary thread will throttle (configurable requests/sec) and cache requests and create metadata events with the enriched information
• sort command will find metadata events associated with (past or future) events