netdev - Re: [RFC PATCHv3 net-next 07/10] docs: bridge: add multicast doc

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Date: Mon, 13 Nov 2023 12:00:04 +0200
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org
Cc: "David S . Miller" <davem@...emloft.net>, David Ahern
 <dsahern@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Ido Schimmel <idosch@...sch.org>, Roopa Prabhu <roopa@...dia.com>,
 Stephen Hemminger <stephen@...workplumber.org>,
 Florian Westphal <fw@...len.de>, Andrew Lunn <andrew@...n.ch>,
 Florian Fainelli <f.fainelli@...il.com>, Vladimir Oltean
 <olteanv@...il.com>, Jiri Pirko <jiri@...nulli.us>
Subject: Re: [RFC PATCHv3 net-next 07/10] docs: bridge: add multicast doc

On 11/10/23 12:15, Hangbin Liu wrote:
> Add multicast part for bridge document.
> 
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
> ---
>   Documentation/networking/bridge.rst | 55 +++++++++++++++++++++++++++++
>   1 file changed, 55 insertions(+)
> 
> diff --git a/Documentation/networking/bridge.rst b/Documentation/networking/bridge.rst
> index 88dfc6eb0919..1fe645c9543d 100644
> --- a/Documentation/networking/bridge.rst
> +++ b/Documentation/networking/bridge.rst
> @@ -164,6 +164,61 @@ on bridge is disabled by default. After enabling VLAN
>   filter on bridge, the bridge can handle VLAN-tagged frames and forward them
>   to the appropriate destinations.
>   
> +Multicast
> +=========
> +
> +The Linux bridge driver has multicast support allowing it to process Internet
> +Group Management Protocol (IGMP) or Multicast Listener Discovery (MLD)
> +messages, and to efficiently forward multicast data packets. The bridge
> +driver support IGMPv2/IGMPv3 and MLDv1/MLDv2.
> +
> +Multicast snooping
> +------------------
> +
> +Multicast snooping is a networking technology that allows network switches
> +to intelligently manage multicast traffic within a local area network (LAN).
> +
> +The switch maintains a multicast group table, which records the association
> +between multicast group addresses and the ports where hosts have joined these
> +groups. The group table is dynamically updated based on the IGMP/MLD messages
> +received. With the multicast group information gathered through snooping, the
> +switch optimizes the forwarding of multicast traffic. Instead of blindly
> +broadcasting the multicast traffic to all ports, it sends the multicast
> +traffic based on the destination MAC address only to ports which have joined
> +the respective destination multicast group.
> +
> +When created, the Linux bridge devices have multicast snooping enabled by
> +default. It maintains a Multicast forwarding database (MDB) which keeps track
> +of port and group relationships.
> +
> +IGMPv3/MLDv2 ETH support

s/ETH/EHT/

Explicit Host Tracking is what we use in the bridge. I know
both are correct, but we should be consistent. You can change
it below as well.

> +------------------------
> +
> +The Linux bridge supports IGMPv3/MLDv2 ETH (Explicit Tracking of Hosts), which
> +was added by `474ddb37fa3a ("net: bridge: multicast: add EHT allow/block handling")
> +<https://lore.kernel.org/netdev/20210120145203.1109140-1-razor@blackwall.org/>`_
> +
> +The explicit tracking of hosts enables the device to keep track of each
> +individual host that is joined to a particular group or channel. The main
> +benefit of the explicit tracking of hosts in IGMP is to allow minimal leave
> +latencies when a host leaves a multicast group or channel.
> +
> +The length of time between a host wanting to leave and a device stopping
> +traffic forwarding is called the IGMP leave latency. A device configured
> +with IGMPv3 or MLDv2 and explicit tracking can immediately stop forwarding
> +traffic if the last host to request to receive traffic from the device
> +indicates that it no longer wants to receive traffic. The leave latency
> +is thus bound only by the packet transmission latencies in the multiaccess
> +network and the processing time in the device.
> +
> +Other multicast features
> +------------------------
> +The Linux bridge also supports `per-VLAN multicast snooping
> +<https://lore.kernel.org/netdev/20210719170637.435541-1-razor@blackwall.org/>`_,
> +which is disabled by default but can be enabled. And `Multicast Router Discovery
> +<https://lore.kernel.org/netdev/20190121062628.2710-1-linus.luessing@c0d3.blue/>`_,
> +which help identify the location of multicast routers.
> +
>   FAQ
>   ===
>