[FS#253] Multicast over wireless ceases to work properly after a while on ath9k with many clients connected

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Michael Marley (mamarley) 

Attached to Project - LEDE Project
Summary - Multicast over wireless ceases to work properly after a while on ath9k with many clients connected
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - High
Priority - Very Low
Reported Version - All
Due in Version - Undecided
Due Date - Undecided
Details - I am having a problem on a UBNT UAP-LR where, if many clients stay connected the router for a long time, multicast packets will stop being reliably sent to the wireless clients.  I can reproduce this with LEDE r1953, but it goes back at least as far as OpenWRT 15.05.1 with kernel 3.18.  I can not, however, reproduce it on another network with the same WAP but with only one or two wireless clients.

This originally manifested as IPv6 ceasing to work since the (multicasted) RA packets were not reaching the wireless clients.  I used tcpdump on the WAP and determined that the RAs were arriving through the wired interface and leaving through the wireless interface as they should, but tcpdump on the client indicates that a vast majority of the RAs are never received.  (Please note that the IPv6 issue is not reproducible with the router running OpenWRT/LEDE and odhcpd, since odhcpd unicasts RAs sent as a response to an RS instead of multicasting them, while it is reproducible when the router is running radvd, for example as pfSense does.)

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=253