** Plug queueing disciplines **

  The 'plug' qdisc type is not documented. It is even not supported
  by traffic shaping tools like 'tc' from iproute2 package.

  Such qdiscs have already been used by Yelp engineers but outside
  of haproxy with libnl-utils tools (especially nl-qdisc-* tools)
  to implement a workaround and make haproxy reloads work.

  Indeed with such plug qdiscs coupled with iptables configurations
  we are able to temporarily bufferize IP packets and to release them as
  needed. So, they may be very useful to "synchronize" TCP sessions
  or at higher level to put network applications in states approaching
  the ones suspected to occur during bugs. Furthermore to be sure
  to produce a correct bug fix, it may be useful to reproduce
  as mush as needed such painful bugs. This is where plug qdiscs
  may be useful.

  To have an idea about how to use plug qdisc on the command line I highly recommend to
  read Willy Tarreau blog here:

  https://www.haproxy.com/blog/truly-seamless-reloads-with-haproxy-no-more-hacks/

  which refers to this other one from Yelp:

  https://engineeringblog.yelp.com/2015/04/true-zero-downtime-haproxy-reloads.html

  The code found in plug_qdisc.c file already helped in fixing a painful bug hard to
  fix because hard to reproduce. To use the API it exports this is quite easy:

    - First your program must call plug_disc_attach() to create if not already created
      a plug qdisc and use it (must be done during your application own already existing
      initializations).
      Note that this function calls plug_qdisc_release_indefinite_buffer() so that to
      release already buffered packets before you start your application,

    - then call plug_qdisc_plug_buffer() to start buffering packets incoming to your
      plug qdisc. So they won't be delivered to your application,

    - then call plug_qdisc_release_indefinite_buffer() to stop buffering the packets
      incoming to your plug qdisc and release those already buffered.
      So, that to be deliver them to your application.

  This code is short and simple. But uses several libraries especially libnl-route module
  part of libnl library. To compile haproxy and make it use the plug_qdisc.c code we had
  to link it against several libnl3 library modules like that:

     -lnl-genl-3 -lnl-route-3 -lnl-3 -lnl-cli-3


  - Some references:
    Libnl API documentation may be found here:
    https://www.infradead.org/~tgr/libnl/doc/api/index.html

    Kernel sources:
    http://elixir.free-electrons.com/linux/latest/source/net/sched/sch_plug.c

    Nice website about traffic shaping with queuing disciplines:
    http://wiki.linuxwall.info/doku.php/en:ressources:dossiers:networking:traffic_control