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

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Message-ID: <20231110101548.1900519-8-liuhangbin@gmail.com>
Date: Fri, 10 Nov 2023 18:15:44 +0800
From: Hangbin Liu <liuhangbin@...il.com>
To: 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>,
	Nikolay Aleksandrov <razor@...ckwall.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>,
	Hangbin Liu <liuhangbin@...il.com>
Subject: [RFC PATCHv3 net-next 07/10] docs: bridge: add multicast doc

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
+------------------------
+
+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
 ===
 
-- 
2.41.0