Date: Tue, 1 Jun 1999 12:24:50 +0000 (GMT) From: slava <sl@zeus.dnt.md> To: "Bruce A. Mah" <bmah@CA.Sandia.GOV> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: [off-topic] DF bit and IP Message-ID: <Pine.BSF.4.05.9906011223050.13274-100000@zeus.dnt.md> In-Reply-To: <199905311851.LAA24784@stennis.ca.sandia.gov>
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Why is MTU 576 so popular? Is it because any device will have no less? Can anyone explain? On Mon, 31 May 1999, Bruce A. Mah wrote: > If memory serves me right, slava wrote: > > > Does TCP always gets encapsulated in an IP header with DF bit set? > > I know this is needed for path MTU discovery to make tcp more efficient > > but is this implemented on all OSes? > > Not all OSs implement PMTU discovery, but I admit that I don't recall > off the top of my head who does and who doesn't. > > > What if something in the way is blocking the icmp packet-too-big type > > to the initiator of a TCP connection and it never finds out about a small > > MTU in the path? > > As far as the sending TCP is concerned, the TCP segments that would > have generated those ICMP packets just vanished. It then tries again > (very likely with the same packet sizes), and the packets vanish again, > until it finally concludes the receiving host is dead and gives up. > This is Very Bad (TM). > > > Will it retry with a smaller MTU itself? > > Not in most cases, no, because TCP can't tell the difference between > "packet lost due to congestion" and "packet apparently lost because the > ICMP packet-too-big message was blocked". > > Some ISPs think they're being smart by blocking all ICMP packets, but > doing so plays heck with PMTU discovery. At one point, someone sent me > the pointer below, which has some more explanation: > > http://www.freelabs.com/~whitis/isp_mistakes.html > > (I dimly recall, however, a survey of PMTU sizes along various paths > which showed that a high fraction of paths supported sending of > Ethernet-MTU-sized packets.) > > Bruce. > > > > > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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