[FS#1088] PPPoE session breaks under heavy WAN load traffic, losing IPv6 connectivity.

[LEDE Bugs]

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

User who did this - forfree (freefor) 

Attached to Project - LEDE Project
Summary - PPPoE session breaks under heavy WAN load traffic, losing IPv6 connectivity.
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - High
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Router: Buffalo wbmr-hp-g300h
Lede version: Reboot (SNAPSHOT, r5106-398edca)
ISP: TIM Italy - ADSL2+
NIC: Realtek PCIe GBE - Driver 10.19.627.2017 (27/06/2017 - Latest)

Notes:
I have set up a new pppoe session(pppoe-wan2) to get IPv6 on all my mobile devices. Without a dedicated pppoe session,
my Android phone could not reach IPv4-only hosts.

My ISP delegates via DHCPv6-PD a single dynamic /64.

Problem:
When I utilize my bandwidth at 100% for a long period of time(downloading or uploading files), my pppoe session breks. 
Sometimes I lose only my IPv6 address but the pppoe session is still up but even after I stop downloading it won't renegotiate an
IPv6 address. At this point, I tried "ifdown wan2" and then "ifup wan2" to reinitialize the pppoe session but I don't get an IPv6 address,
only an IPv4 one. I have to reboot the entire router and it will get an IPv6.

Something must have changes after my last bug report FS#964. At that time I used the build r4786-05c3647d35 and I didn't have this problem.
I don't exclude some problems IPS's side but using a ASUS DSL-N14U I have IPv6 connectivity even under heavy load and the pppoe session is always
up. So, for now, I doubt it is a congestion problem..

I attach here the syslog which includes odhcp debug level=7 and odhcpc verbose mode (/lib/netifd/proto/dhcpv6.sh - -s /lib/netifd/dhcpv6.script -v $opts $iface).
There is also a pcap file with the following tests:
1. I rebooted the router
2. I started capturing traffic on my pppoe-wan2 interface (ip6raonly.pcap file - icmpv6 filtered)
3. UTC 12:44 I start downloading Ubuntu 17.10 ISO file from torrent
4. UTC 12:50 pppoe-wan2 interface goes down completely
5. UTC 12:52 pppoe-wan2 is up again but without IPv6 and it stays like this until I reboot the router
6. UTC 13:24 started capturing again pppoe-wan2 but I can see only this traffic(ip6ra3.pcap file)


As always, let me know what test should I try.
Thank you.

One or more files have been attached.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1088