PPPOE MSS claimping problem

[meyergru]

That means you obviously cannot use more than that, which is quite uncommon. Somehow it seems your ISP limits you to that. So you will have to limit the WAN MTU to that value at most. It should work then. Usually, you can at least get 1488 with these kind of setups.

With that lower MTU, you should be good even for sites that do not work with PMTU. IDK if you LAN clients will automatically adapt to that lower MTU, so you probably need MSS clamping for the lower MTU, as well. I never bothered to make such adjustments, because I can use 1500 bytes.

[mbk2]

@meyergru: your suggestion or how-to is mainly about maximizing the MTU size to avoid fragmentation, which makes sense. But @Deltorek112 and I are dealing with a more basic issue around MSS clamping.

Up until now, neither of us had ever set any MTU or MSS values manually and everything just worked. That's no longer the case, and now we need to adjust things by hand to get stable connections.

As I mentioned in post #20, there's a small difference — exactly 8 bytes — with MSS clamping before and after setting MTU and MSS manually. I think it might be related to the 8-byte overhead from the PPPoE interface. For some reason, the automatic MSS clamping doesn't seem to work properly anymore.

Just guessing here, but: there was an MTU-related change issued here. Could that be connected to the problems we're seeing now?

[Deltorek112]

So, I reinstalled 25.1.12, and restored settings(from before upgrade). And it works fine, no need to set anything.
So I would say something changed from this version to 25.7 and it broke MSS clamping or something associated with that.

[meyergru]

@mbk2: I know, I just wanted to present a fix where you do not have to rely on the MSS clamping, which in the past did not work for me, as well. Of course, that fix only works if your ISP allows it.