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docs: bridge: add multicast doc

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Add multicast part for bridge document.

Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
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Hangbin Liu 2023-12-01 16:19:47 +08:00 committed by Paolo Abeni
parent 041a6ac4bf
commit 75ceac88ef
1 changed files with 56 additions and 0 deletions
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Documentation/networking/bridge.rst
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@ -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/20210120145203.1109140-1-razor@blackwall.org/>`_
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/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
===