Re: [RFC net-next Patch 0/3] net: bridge: mrp: Add support for Media Redundancy Protocol(MRP)

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 09/01/2020 17:06, Horatiu Vultur wrote:
> Media Redundancy Protocol is a data network protocol standardized by
> International Electrotechnical Commission as IEC 62439-2. It allows rings of
> Ethernet switches to overcome any single failure with recovery time faster than
> STP. It is primarily used in Industrial Ethernet applications.
> 
> This is the first proposal of implementing a subset of the standard. It supports
> only 2 roles of an MRP node. It supports only Media Redundancy Manager(MRM) and
> Media Redundancy Client(MRC). In a MRP ring, each node needs to support MRP and
> in a ring can be only one MRM and multiple MRC. It is possible to have multiple
> instances of MRP on a single node. But a port can be part of only one MRP
> instance.
> 
> The MRM is responsible for detecting when there is a loop in the ring. It is
> sending the frame MRP_Test to detect the loops. It would send MRP_Test on both
> ports in the ring and if the frame is received at the other end, then the ring
> is closed. Meaning that there is a loop. In this case it sets the port state to
> BLOCKED, not allowing traffic to pass through except MRP frames. In case it
> stops receiving MRP_Test frames from itself then the MRM will detect that the
> ring is open, therefor it would notify the other nodes of this change and will
> set the state of the port to be FORWARDING.
> 
> The MRC is responsible for forwarding MRP_Test frames between the ring ports
> (and not to flood on other ports) and to listen when there is a change in the
> network to clear the FDB.
> 
> Similar with STP, MRP is implemented on top of the bridge and they can't be
> enable at the same time. While STP runs on all ports of the bridge, MRP needs to
> run only on 2 ports.
> 
> The bridge needs to:
> - notify when the link of one of the ports goes down or up, because MRP instance
>   needs to react to link changes by sending MRP_LinkChange frames.
> - notify when one of the ports are removed from the bridge or when the bridge
>   is destroyed, because if the port is part of the MRP ring then MRP state
>   machine should be stopped.
> - add a handler to allow MRP instance to process MRP frames, if MRP is enabled.
>   This is similar with STP design.
> - add logic for MRP frames inside the bridge. The bridge will just detect MRP
>   frames and it would forward them to the upper layer to allow to process it.
> - update the logic to update non-MRP frames. If MRP is enabled, then look also
>   at the state of the port to decide to forward or not.
> 
> To create a MRP instance on the bridge:
> $ bridge mrp add dev br0 p_port eth0 s_port eth1 ring_role 2 ring_id 1
> 
> Where:
> p_port, s_port: can be any port under the bridge
> ring_role: can have the value 1(MRC - Media Redundancy Client) or
>            2(MRM - Media Redundancy Manager). In a ring can be only one MRM.
> ring_id: unique id for each MRP instance.
> 
> It is possible to create multiple instances. Each instance has to have it's own
> ring_id and a port can't be part of multiple instances:
> $ bridge mrp add dev br0 p_port eth2 s_port eth3 ring_role 1 ring_id 2
> 
> To see current MRP instances and their status:
> $ bridge mrp show
> dev br0 p_port eth2 s_port eth3 ring_role 1 ring_id 2 ring_state 3
> dev br0 p_port eth0 s_port eth1 ring_role 2 ring_id 1 ring_state 4
> 
> If this patch series is well received, the in the future it could be extended
> with the following:
> - add support for Media Redundancy Automanager. This role allows a node to
>   detect if needs to behave as a MRM or MRC. The advantage of this role is that
>   the user doesn't need to configure the nodes each time they are added/removed
>   from a ring and it adds redundancy to the manager.
> - add support for Interconnect rings. This allow to connect multiple rings.
> - add HW offloading. The standard defines 4 recovery times (500, 200, 30 and 10
>   ms). To be able to achieve 30 and 10 it is required by the HW to generate the
>   MRP_Test frames and detect when the ring is open/closed.
> 
> Horatiu Vultur (3):
>   net: bridge: mrp: Add support for Media Redundancy Protocol
>   net: bridge: mrp: Integrate MRP into the bridge
>   net: bridge: mrp: Add netlink support to configure MRP
> 
>  include/uapi/linux/if_bridge.h |   27 +
>  include/uapi/linux/if_ether.h  |    1 +
>  include/uapi/linux/rtnetlink.h |    7 +
>  net/bridge/Kconfig             |   12 +
>  net/bridge/Makefile            |    2 +
>  net/bridge/br.c                |   19 +
>  net/bridge/br_device.c         |    3 +
>  net/bridge/br_forward.c        |    1 +
>  net/bridge/br_if.c             |   10 +
>  net/bridge/br_input.c          |   22 +
>  net/bridge/br_mrp.c            | 1517 ++++++++++++++++++++++++++++++++
>  net/bridge/br_mrp_timer.c      |  227 +++++
>  net/bridge/br_netlink.c        |    9 +
>  net/bridge/br_private.h        |   30 +
>  net/bridge/br_private_mrp.h    |  208 +++++
>  security/selinux/nlmsgtab.c    |    5 +-
>  16 files changed, 2099 insertions(+), 1 deletion(-)
>  create mode 100644 net/bridge/br_mrp.c
>  create mode 100644 net/bridge/br_mrp_timer.c
>  create mode 100644 net/bridge/br_private_mrp.h
> 

Hi all,
I agree with Stephen here, IMO you have to take note of how STP has progressed
and that bringing it in the kernel was a mistake, these days mstpd has an active
community and much better support which is being extended. This looks best implemented
in user-space in my opinion with minimal kernel changes to support it. You could simply
open a packet socket with a filter and work through that, you don't need new netlink
sockets. I'm not familiar with the protocol so can't really be the judge of that, if
you present a good argument for needing a new netlink socket for these packets - then
sure, ok.

If you do decide to continue with the kernel version (which I would again discourage)
a few general points (from a quick scan):
 - the single 1.6+k line patch is just hard to review, please break it into more digestable
   and logical pieces
 - the locking is wrong, also there're a few use-after-free bugs 
 - please re-work the bridge integration code, it can be simplified and tests can be eliminated
 - your netlink helpers usage is generally wrong and needs more work
 - use the already existing port states instead of adding new ones and you can avoid some tests in fast-path
 - perhaps look into using br_afspec() for configuration/retrieval initially ? I don't think you need the new rtm messages yet.
 - I'm sure I can go on, but I really think all of this should be put in user-space -
   in-kernel STP is a great example of how _not_ to do it. :) As a bonus you'll avoid 90% of the
   problems above just by making your own abstractions and using them for it.


Thanks,
 Nik