[PATCHv4,net-next,07/10] docs: bridge: add multicast doc

Commit Message

Hangbin Liu Dec. 1, 2023, 8:19 a.m. UTC

Add multicast part for bridge document.

Acked-by: Nikolay Aleksandrov <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Signed-off-by: Hangbin Liu <[e-mail]>
---
 Documentation/networking/bridge.rst | 56 +++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

Patch

diff --git a/Documentation/networking/bridge.rst b/Documentation/networking/bridge.rst
index 97936b9564fd..863ad2c8d146 100644
--- a/Documentation/networking/bridge.rst
+++ b/Documentation/networking/bridge.rst
@@ -177,6 +177,62 @@  on a bridge is disabled by default. After enabling VLAN filtering on a bridge,
 it will start forwarding frames to appropriate destinations based on their
 destination MAC address and VLAN tag (both must match).
 
+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 supports 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
+subscribed 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 EHT support
+------------------------
+
+The Linux bridge supports IGMPv3/MLDv2 EHT (Explicit Host Tracking), which
+was added by `474ddb37fa3a ("net: bridge: multicast: add EHT allow/block handling")
+<https://lore.kernel.org/netdev/[e-mail]/>`_
+
+The explicit host tracking 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 host tracking 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/[e-mail]/>`_,
+which is disabled by default but can be enabled. And `Multicast Router Discovery
+<https://lore.kernel.org/netdev/[e-mail]/>`_,
+which help identify the location of multicast routers.
+
 FAQ
 ===