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Add IGMP documentation

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# IGMP (Internet Group Management Protocol) and MLD (Multicast Listener Discovery)
IGMP (for IPv4) and MLD (for IPv6) are protocols that control the distribution
of Layer-3 Multicast packets on the LAN, which otherwise would be flooded across the
entire network. For this to work, IGMP/MLD messages are sent, in particular
from MC consumers (e.g. the video-player that plays an IP-Multicast stream), but
also Multicast-aware routers to control switching of the IP-MC or underlying
L2-MC packets. The main usage in home networks is IPTV.
The RTL8372/3 SoC supports managing IPv4-MC using either Destination-IP (the IPv4
multicast group address)/Source-IP (typically 0.0.0.0) matching or via controlling
the switching of the underlying L2-MC packets (i.e. packets in 01:00:5e:xx:yy:zz, where
xx:yy:zz are the LSBs of the IPv4-MC address). The DIP/SIP-based switching is
not VLAN-aware, meaning a stream will be available in all VLANs if subscribed to.
This is not a problem in a typical home network, however. The L2-based method
is VLAN aware, but currently not supported in the software.
Although there is hardware support for IPv6/MLD-based Multicast management (i.e. intelligent
management by the switch), the current software does not implement managing IPv6 Multicast.
Instead, all IPv6 Multicast pakets will be flooded to all ports, just as an unmanaged
switch would do.
The current software support works by trapping IGMP packets (only v3 supported, which
is used in the vast majority of today's networks) to the CPU of the switch which will
update the L3 and L2 switching tables to include switch ports in a stream or remove
them. This trapping to the CPU is also called IGMP snooping. While there is support
in the HW to handle IGMP/MLD packets (v3 has only limited support) entirely in hardware
and even send out reports, it is currently not understood
how this works, and instead IGMP is handled entirely in software, which also allows
to fully support IGMPv3 packet which are the standard in present-day networks.
## IP-MC control
The relevant registers for controlling IP-MC switching are:
```
#define RTL837X_IPV4_PORT_MC_LM_ACT 0x4f78
#define RTL837X_IPV6_PORT_MC_LM_ACT 0x4f7c
#define RTL837X_IGMP_PORT_CFG 0x52a0
#define IGMP_MAX_GROUP 0x00ff0000
#define IGMP_PROTOCOL_ENABLE 0x00007c00
#define IGMP_TRAP 0x0000002a
#define IGMP_FLOOD 0x00000015
#define IGMP_ASIC 0x00000000
#define RTL837X_IGMP_ROUTER_PORT 0x529c
#define RTL837X_IPV4_UNKN_MC_FLD_PMSK 0x5368
#define RTL837X_IPV6_UNKN_MC_FLD_PMSK 0x536c
#define RTL837X_IGMP_TRAP_CFG 0x50bc
#define IGMP_TRAP_PRIORITY 0x7
#define IGMP_CPU_PORT 0x00010000
```
`RTL837X_IPV4_PORT_MC_LM_ACT/RTL837X_IPV6_PORT_MC_LM_ACT` control the action when an
IP-MC packet is encountered at a switch port and there is no rule for forwarding in
the forwarding tables. The default action is to flood such Lookup-Miss packets to all
ports. This is the configuration without IGMP/MLD enabled.
When IGMP/MLD is turned on, the Lookup-Miss action will be changed to drop such packets
unless a rule is found in the forwarding tables, which will need to be configured by
IGMP packets.
Switching on IGMP also configures all ports via `RTL837X_IGMP_PORT_CFG` to trap all
incoming IGMP packets to the CPU. `RTL837X_IGMP_TRAP_CFG` then is used to configure
priority and CPU-Port of trapped IGMP/MLD packets.
Configuration of the IP-MC-forwarding to the listening ports is done by managing the
forwarding tables of the switch, see [L2 learning](l2.md).
## IGMP API
The code currently provides the following functions:
```
void igmp_setup(void) __banked;
void igmp_enable(void) __banked;
void igmp_router_port_set(uint16_t pmask) __banked;
void igmp_packet_handler(void) __banked;
void igmp_show(void) __banked;
```c
`igmp_setup()` is called at boot-time and configures flooding of all IP-MC packets by
default, as otherwise no IP-MC would be possible in the network.
`igmp_enable()`starts IGMP which cause IGMP packets to be handled by the CPU and forwarding
of IP-MC packets to be limited to only subscribed ports.
`igmp_router_port_set()`configures forwarding ports for IGMP messages.
`igmp_packet_handler()` implements handling of trapped IGMP packets by the CPU.
`igmp_show()` prints out the IGMP configuration on the CLI.
## IGMP configuration on the Serial Console
For testing the following commands are provided on the serial console:
```
> igmp [on/off]
Enables or disables IGMP
> igmp show
Shows information on IGMP
```
## LAG configuration via the Web Interface
Not implemented, yet!
## A Test with IP-MC streaming using vlc
The following is a simple test verifying the IGMP and IP-MC switching capabilities.
You will need 2 Linux/Windows devices with a GUI plus a switch.
Connect the switch to an MC-aware router (e.g. to your home network). Connect the 2 Linux/Windows
devices to the switch. The connection to the router makes sure that Linux/Windows will send
out IGMP messages on the ports connected to the switch, which they will only do if they are aware
that there is a MC-aware router in the network. Make sure the 2 GUI devices are in the home network
(e.g. via DHCP).
Start streaming on one of the Linux/Windows machines:
```
$ vlc your_video.mp4 --sout="#std{access=udp, mux=ts, dst=239.255.0.1:8090}"
```
At this point you should see all switch ports flickering heavily as the MC stream is switched to all
switch ports, including flooding your home network. If you do not see any packets arriving at the switch,
you can force the output interface of vlc by using `--miface=<ifname>`
Enable IGMP on the switch-CLI:
```
> igmp on
```
The flickering should now stop on all ports except the port where the streaming device is connected:
the switch drops all IP-MC packets as there are no listeners.
Now, on the second Linux/Windows device start listening to the stream:
```
$ vlc udp://@239.255.0.1:8090
```
You should see the port-led of the port the displaying machine is connected to, to start flickering
and after some synchronization, the video should start playing.
Stopping vlc should also switching of the IP-MC frames to the listening device, i.e. the port-leds
should stop flickering.