nanog mailing list archives
Re: [NANOG] Microsoft.com PMTUD black hole?
From: Bjørn Mork <bjorn () mork no>
Date: Wed, 07 May 2008 10:10:35 +0200
Iljitsch van Beijnum <iljitsch () muada com> writes:
Many years ago I had occasion to terminate dial-up service over L2TP from modem pools operated by a service provider who shall remain nameless to protect the guilty. This service had the unfortunate tendency to drap all packets larger than 576 bytes. So we needed to negotiate a 576-byte MTU over PPP. We then got many complaints from users who dialed in using ISDN routers (yes this was a while ago) because of broken path MTU discovery. The behavior that Microsoft exhibits was EXTREMELY common in those days, and I have no reason to assume it's any less common today. (I also see it regularly with IPv6.) What I did was clear the DF bit on packets going out to the L2TP virtual interfaces so the packets could be fragmented.
Right. I once stumbled across a SOHO-router doing just that. I never understood why, but now you've given at least one explanation how it could appear to be a good idea. I can also provide the reason why we found it to be an extremely bad idea at the time: Some (most? all?) systems won't set both the DF flag and the identification field at the same time. If you clear the DF flag without changing the identification field, you might end up with fragmented packets that are impossible to reassemble. Which was why I stumbled across the DF-clearing SOHO-router in the first place. The random problems it generated were extremely difficult to debug, and when we started we truly believed that we had a problem with a layer 4 load balancing switch. Note: There are solutions that will both clear the DF flag and generate a new id. E.g. http://www.openbsd.org/faq/pf/scrub.html This is the proper way to clear DF, if you must. Never just clear it. Bjørn _______________________________________________ NANOG mailing list NANOG () nanog org http://mailman.nanog.org/mailman/listinfo/nanog
Current thread:
- Re: [NANOG] Microsoft.com PMTUD black hole?, (continued)
- Re: [NANOG] Microsoft.com PMTUD black hole? Iljitsch van Beijnum (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Nathan Anderson/FSR (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Nathan Anderson/FSR (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Iljitsch van Beijnum (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Nathan Anderson/FSR (May 07)
- Message not available
- Re: [NANOG] Microsoft.com PMTUD black hole? Nathan Anderson/FSR (May 07)
- Re: [NANOG] Microsoft.com PMTUD black hole? Tomas L. Byrnes (May 07)
- Re: [NANOG] Microsoft.com PMTUD black hole? Nathan Anderson/FSR (May 07)
- Re: [NANOG] Microsoft.com PMTUD black hole? Matthew Petach (May 12)
- Re: [NANOG] Microsoft.com PMTUD black hole? Iljitsch van Beijnum (May 06)
- Message not available
- Re: [NANOG] Microsoft.com PMTUD black hole? Iljitsch van Beijnum (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Bjørn Mork (May 07)
- Re: [NANOG] Microsoft.com PMTUD black hole? Tomas L. Byrnes (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Marshall Eubanks (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Nathan Anderson/FSR (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Nathan Anderson/FSR (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Randy Bush (May 06)
- Re: [NANOG] Microsoft.com PMTUD black hole? Glen Turner (May 07)
- Re: [NANOG] Microsoft.com PMTUD black hole? Patrick Giagnocavo (May 07)
- Re: [NANOG] Microsoft.com PMTUD black hole? Mark Newton (May 07)