ipv6 MTU bug/problem

[Chrysalis]

For full details of the problem read this.

http://community.plus.net/forum/index.php/topic,135485.0.html

The short version is that when a non default MTU is specified in the WAN config such as 1492 for PPPoE, the value is not automatically put in radvd.conf and as such ipv6 traffic can get mtu issues, Windows PC's seem to handle it fine but android does not, and I have had issues on googleplay and youtube from my android phone.

The fix is simple.

Add a line such as this in the interface section of radvd.conf

AdvLinkMTU 1492;

The value would match the value configured for the WAN MTU.

This is probably an ASUS bug but I am reporting here so John and Merlin can fix in both firmwares, thanks.

 

[Chrysalis]

I actually suggest setting it to 20bytes below the WAN configured value.

My PPP MTU is 1492 but the highest ipv6 MTU I can use without issues is 1472, so -20bytes is probably the safe value. Either way it does need fixing as it currently ignores the MTU configured in WAN.

 

[john9527]

The fix is simple.

Add a line such as this in the interface section of radvd.conf

AdvLinkMTU 1492;

Can you post a copy of your radvd.conf to give me some keywords to make sure I'm in the right place? (I'm still not IPv6)

Edit: Nevermind.....got it.....

 

[Chrysalis]

If you still need the config I will send to you in PM as it includes my prefix.

For now I am overriding it using /jffs/configs.

 

[john9527]

Could you verify the values for a couple of nvram vars for me? Just want to double check....

ipv6_ifdev
ipv6_mtu
wan_pppoe_mtu

Thanks.

 

[RMerlin]

For full details of the problem read this.

http://community.plus.net/forum/index.php/topic,135485.0.html

The short version is that when a non default MTU is specified in the WAN config such as 1492 for PPPoE, the value is not automatically put in radvd.conf and as such ipv6 traffic can get mtu issues, Windows PC's seem to handle it fine but android does not, and I have had issues on googleplay and youtube from my android phone.

The fix is simple.

Add a line such as this in the interface section of radvd.conf

AdvLinkMTU 1492;

The value would match the value configured for the WAN MTU.

This is probably an ASUS bug but I am reporting here so John and Merlin can fix in both firmwares, thanks.

The radvd code path is no longer in use, as Asus moved to dnsmasq for router advertisement.

 

[Chrysalis]

Could you verify the values for a couple of nvram vars for me? Just want to double check....

ipv6_ifdev
ipv6_mtu
wan_pppoe_mtu

Thanks.

no problem.

admin@RT-AC66U:/jffs/configs# nvram get ipv6_ifdev
ppp
admin@RT-AC66U:/jffs/configs# nvram get ipv6_mtu  
admin@RT-AC66U:/jffs/configs# nvram get wan_pppoe_mtu
1492

Yes the 2nd value is blank.

 

[john9527]

Thanks.....that's what I expected.

Here's a quick summary of what I found

Latest Merlin build
- radvd code is still there even though it's not used
- the start_radvd function was changed from the fork to include some special cases, including PPP connections
- The AdvLinkMTU value for PPP connections was hard coded to 1492

- I don't know enough about the new IPv6 implementation to see what, if anything, may need to be done there

Fork Code
- no differentiation for PPP connections
- FIX: ported the special cases including PPP connections.
The AdvLinkMTU value is set to the PPP MTU - 20 bytes

Will pick it up in the next release.