Re: [Bridge] Getting + maintaining a consistent MDB view in userspace?

["Linus Lüssing" <linus.luessing@c0d3.blue>]

On Thu, Aug 10, 2023 at 12:56:50AM +0200, Linus Lüssing wrote:
> [...]
> When setting a bridge's "multicast_mld_version" to 2 it looks to me like
> there is quite a bit of redundancy in both "bridge mdb show"
> and "bridge monitor mdb" output. And addition and deletion notifications
> don't look that symmetric to me then anymore. There doesn't
> happen to exist some documentation about these (netlink specific)
> additions that came with the IGMPv3/MLDv2 implementation, or does
> it?

And just a few examples which confuse me a bit:

1) ASM, single listener:

$ ./mcjoin -d -i veth0 "[ff12::125]

bridge monitor mdb:

```
dev br0 port veth1 grp ff12::125 temp filter_mode include proto kernel
dev br0 port veth1 grp ff12::125 temp filter_mode exclude proto kernel
```

-> first line seems to be superfluous / no-op?
   Can I always ignore such an event with:
   "n->nlmsg_type == RTM_NEWMDB && mode == MCAST_INCLUDE &&
    !tb[MDBA_MDB_EATTR_SOURCE] && !tb[MDBA_MDB_EATTR_SRC_LIST]" ?

Leave only creates one event:

$ kill <mcjoin ...>

```
Deleted dev br0 port veth1 grp ff12::125 temp filter_mode include proto kernel
```

2) SSM, two listeners:

$ ./mcjoin -d -i veth0 "[fe80::130],[ff12::130]"

```
dev br0 port veth1 grp ff12::130 temp filter_mode include proto kernel
dev br0 port veth1 grp ff12::130 src fe80::130 temp filter_mode include proto kernel
dev br0 port veth1 grp ff12::130 temp filter_mode include source_list fe80::130/260.00 proto kernel
```

-> again first line superfluous / no-op?
   and second + third line identical?

Adding a second join:

$ ./mcjoin -d -i veth0 "[fe80::131],[ff12::130]"

```
dev br0 port veth1 grp ff12::130 src fe80::131 temp filter_mode include proto kernel
dev br0 port veth1 grp ff12::130 temp filter_mode include source_list fe80::131/260.00,fe80::130/247.86 proto kernel
```

-> so looks like the source_list line is actually always a (superfluous /
   redundant?) summary of all sources for this group+port?

And deleting here creates three events:

$ kill <mcjoin -d -i veth0 "[fe80::131],[ff12::130]">

```
Deleted dev br0 port veth1 grp ff12::130 src fe80::131 temp filter_mode include proto kernel
```

$ kill <mcjoin -d -i veth0 "[fe80::130],[ff12::130]">

```
Deleted dev br0 port veth1 grp ff12::130 src fe80::130 temp filter_mode include proto kernel
Deleted dev br0 port veth1 grp ff12::130 temp filter_mode include proto kernel
```

And here the last line seems to be superfluous / redundant?
And the behaviour seems to differ from 1) / ASM, where
I would always only get one event when leaving?


----------

So this currently confuses me a bit regarding which of these netlink
notifications I might be able to ignore safely and which not. And
how to interpret them, which ones are supposed to be a singular update
I would need to add and which ones are supposed to be summaries
that are supposed to potentially update/replace multiple entries.

Regards, Linus