[AusNOG] Upstream PMTUD broken? Packets blackhole

[s.s.o.n.i.k+a.u.s.n.o.g at gmail.com]

Yeah, that's the kind of problem I'm up against. No changes to any configuration, and all of a sudden there's trouble all over the place. Something happened in NSW, and that's the location of the international link used most heavily. Europe - no trouble, provided there's no NSW hops.

Since originally posting here, something upstream has been happening. Tests from New York work for larger packet sizes than before, even though expected ICMP messages are still AWOL. I hope someone accidentally fixes the problem while making changes for their large clients, or peering networks.

Since seeing decreased blackhole window size, I've ramped MTU's back up to 1492 and 1500 for PPPoE and Ethernet clients respectively, and clamped MSS to appropriate sizes to keep packets from disappearing. It still doesn't help anything other than TCP, but that's the rub.

Kind Regards,

-Dave



-----Original Message-----
From: AusNOG [mailto:ausnog-bounces at lists.ausnog.net] On Behalf Of Tim Warnock
Sent: Thursday, 15 September 2016 11:22 AM
To: ausnog at lists.ausnog.net
Subject: Re: [AusNOG] Upstream PMTUD broken? Packets blackhole

Just to add to the case:

I had two dsl services and an nbn service where we had to drop the dialer ip mtu to 1460 (the services had worked forever(tm) on the old settings) because we don't know what changed.

Things that worked and that had worked forever suddenly didn't and that was the fix.

Go figure.

Extra info: 2x ADSL2+ on TW DSLAM, 1x NBN via AAPT NWB.