[ovs-discuss] OVS custom action

[Luca Mancini]

Hi Ben,
Firstly thank you for the swift response,
In order to get a better understanding of what I was doing (I’ve starting working with openvswitch a week ago) I tried defining an action called OFPAT_RAW_BUFFER (that did absolutely nothing)
just like the FAQ says and my mininet topology did in fact recognize it as a valid action.
Now I was starting the actual implementation of the action and that’s where my problems arise, I know how I’m going to program it I just don’t know where I’m meant to implement it and it’s somewhat frustrating.

Based on what you said :
“You'll need to make the action send packets that need to be buffered to userspace, with a "userspace" kernel action”
How exactly is this done? I believe the kernel receives the packet, and the datapath folder contains the code where the packet is recvd and processed, how exactly is it then sent to userspace, I’m having a tough time understanding how packets work in ovs.
Diving in the code I found the ovs_dp_process_packet should be what I’m looking for, but I’m not quite sure.

I guess a more accurate question for my case would be, where does someone who is new to ovs start from in order to understand the ovs codebase and learn how to implement a new action such as this one? Thanks again!

L

From: Ben Pfaff<mailto:blp at ovn.org>
Sent: Friday, April 24, 2020 18:50
To: Luca Mancini<mailto:luca.mancini at outlook.com>
Cc: ovs-discuss at openvswitch.org<mailto:ovs-discuss at openvswitch.org>
Subject: Re: [ovs-discuss] OVS custom action

On Fri, Apr 24, 2020 at 02:50:43PM +0000, Luca Mancini wrote:
> I am trying to implement a new openflow action on an openvswitch, that upon request by the controller buffers a given packet.
>
> So assume a topology like H1—s1--s2—H2 where h1 sends packets to h2, the controller would specify a BUFFER action to all packets going from h1 to h2
> Making the s1 switch create a buffer where all these packets are stored until the buffer is full and then send the buffer (as a new packet) down the path to H2.
> I have found very little material regarding the implementation of a new openflow action to ovs:
>
> I followed this guide a little bit but I’m finding some things a bit confusing https://mcfelix.me/blog/modifying-ovs/
> Some questions I have are:
> -is this type of action possible?
> -where exactly do I need to code the actual action function?
> -how and where are packets defined in OVS? I’m having trouble understading if the packet I need to be storing is the sk_buff that is passed as a parameter or if it’s something else

This should be possible.  You'll need to make the action send packets
that need to be buffered to userspace, with a "userspace" kernel
action.  Then you can accumulate the packets in userspace until they
should be released.

Since you'll only be dealing with userspace packets, struct dp_packet is
primarily what you need to care about.

The FAQ says this:

Q: How do I add support for a new OpenFlow action?

    A: Add your new action to ``enum ofp_raw_action_type`` in
    ``lib/ofp-actions.c``, following the existing pattern.  Then recompile and
    fix all of the new warnings, implementing new functionality for the new
    action as needed.  (If you configure with ``--enable-Werror``, as described
    in the :doc:`/intro/install/general`, then it is impossible to miss any
    warnings.)

    If you need to add an OpenFlow vendor extension action for a vendor that
    doesn't yet have any extension actions, then you will also need to add the
    vendor to ``vendor_map`` in ``build-aux/extract-ofp-actions``.  Also, you
    will need to add support for the vendor to ``ofpact_decode_raw()`` and
    ``ofpact_put_raw()`` in ``lib/ofp-actions.c``.  (If you have a choice of
    how to design your vendor extension actions, it will be easier if you make
    them resemble the ONF and OVS extension actions.)

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