Re: [PATCH net-next v2 0/3] Allow more than 255 IPv4 multicast interfaces

[David Miller]

From: Paul Davey <paul.davey@xxxxxxxxxxxxxxxxxxx>
Date: Tue,  8 Sep 2020 10:04:05 +1200

> Currently it is not possible to use more than 255 multicast interfaces
> for IPv4 due to the format of the igmpmsg header which only has 8 bits
> available for the VIF ID.  There is space available in the igmpmsg
> header to store the full VIF ID in the form of an unused byte following
> the VIF ID field.  There is also enough space for the full VIF ID in
> the Netlink cache notifications, however the value is currently taken
> directly from the igmpmsg header and has thus already been truncated.
> 
> Adding the high byte of the VIF ID into the unused3 byte of igmpmsg
> allows use of more than 255 IPv4 multicast interfaces. The full VIF ID
> is  also available in the Netlink notification by assembling it from
> both bytes from the igmpmsg.
> 
> Additionally this reveals a deficiency in the Netlink cache report
> notifications, they lack any means for differentiating cache reports
> relating to different multicast routing tables.  This is easily
> resolved by adding the multicast route table ID to the cache reports.
> 
> changes in v2:
>  - Added high byte of VIF ID to igmpmsg struct replacing unused3
>    member.
>  - Assemble VIF ID in Netlink notification from both bytes in igmpmsg
>    header.

Series applied, thank you.