Problems accessing with non-1500 mtu connection

Fabian Bläse fabian at
Sat May 4 21:21:47 BST 2019


On 04.05.19 09:52, Ben Cooksley wrote:
> Okay. This is a rather strange issue as we've not seen any other
> reports of people having issues accessing our sites, which I would
> expect to receive if people were having issues with IPv4. My own
> ability to do local testing is limited unfortunately, but I can
> confirm my connection's MTU for IPv4 is most definitely less than 1500
> and has no issue accessing the affected sites.
For IPv4 this might be hidden by the fact, that mss clamping (dirty hack, only works for TCP) is done pretty much everywhere with smaller-than 1500 mtu.
Even some really big companies like ubiquiti (who even make routers..) screw up the behaviour with icmp packet too big messages with IPv4 on their websites.

With IPv6 Path MTU Discovery gets far more important however, because it does not allow fragmentation on router level.

> Would you mind testing against our providers site,
I can't verify the behaviour for that site, because it isn't even connected to the (non-legacy, ipv6) internet.. (doesn't have an AAAA record)
Jokes aside, it probably has the same issues.
But because, as you already noticed, pings with big payload get dropped, I can't investigate this further (because, due to broken PMTUD on many sites, we have mss clamping active for IPv4)

> As you're an ISP it might be easier to put you in direct contact with
> them as chances are their entire network is affected.
If you want I can contact them directly.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: OpenPGP digital signature
URL: <>

More information about the kde-www mailing list