Can't post to Forums when on FTTN; ok via Sonic VPN?

[blakers]

test post; no vpn

[williamt]

Seems to be working!

[Guest]

test post; no vpn

Did you/Sonic do anything to make this happen?

[blakers]

WilliamT @ Sonic made some changes/fixes to their MTU path negotiation to/through the Uverse modem.

We'd 1st figured out that if I manually dropped my MTU = 1472, I could log in & post with no VPN.

The DHCP server in the modem is, apparently, serving up a MTU = 1500; I needed to override it.

After changes @Sonic, I can simply allow the Modem to assign what it'll assign, i.e. MTU = 1500, and the link now "just works" for posting at Sonic forums (uplloading to Webmail also turnned out to have been a problem) without VPN.

[Guest]

WilliamT @ Sonic made some changes/fixes to their MTU path negotiation to/through the Uverse modem.

That's great. Here's some food for thought. http://rachelbythebay.com/w/2015/05/15/pmtud. I used to block it on my firewall, too. But nowadays I tend to leave it on so Sonic could get valuable statistics on their end. I don't think they monitor Fusion FTTN connections but I still do it so whoever is responsible for the last mile can still troubleshoot.

[blakers]

I know about pmtud. I did not have it enabled in this case. I assumed that the modem's getting the correct value from upstream via DHCP client, and turning around and serving that same correct value to my router's DHCP client.

What I do NOT know, is whether it'd find the best/right MTU in this ^^ case -- where the assigned MTU seemed fine with any/all sites, except for specific function (post) on a specific site (Sonic). Not sure if or how the pmtud auto-discovery would get (re)triggered.


EDIT

Digging around on this I find

How do I resolve timeouts and connectivity issues when IPv6 is enabled on my RG?
https://www.dslreports.com/faq/17822

It refers to issues only with IPv6, but I suspect similar issues here on IPv4 only (since IPv6 is DOA ...)

As mentioned above, MTU=1472 worked prior to the changes @Sonic; =1490 did not. I didn't test @ = 1480 ...

EDIT2

This reply

Re: What are the req'd MTUs for FTTN wan- & lan-side ?
viewtopic.php?f=13&t=2890&p=19313#p19312

provides some heuristic evidence that MTU=1500 is the right setting for FTTN -- at least on 1x VDSL2.

Which suggests that WilliamT's correction on the @Sonic-side, rather than something on the user's end, is the right solution.

[kgc]

There is no conceivable reason to disable PMTUD. Without it, you will have trouble communicating with any endpoint that has a smaller MTU that your endpoint.

[blakers]

I did not disable it.

I did not have it explicitly enabled.

What the default is -- enabled or disabled -- I do not know.

Here, anyway, it's enabled explicitly now.

In any case, per your last post in the referred thread, it appears to be have been unrelated to this issue.