there are only a few control messages implemented:

The client can ask the server for the current operating parameters

The server can tell the client about maximum sockbuf sizes, or
  sockbuf sizes that should be force

The server can tell the client which version of the output format to
  use.

The plan is that there will be more in the future: ie., the client
  and server will be able to exchange messages about new socket
  buffer sizes set by client apps, and exchange information about
  sockopts, etc. the client app may have set.

Add support to libnetmon for capping the socket buffer size.