Date: Sat, 20 Jun 1998 01:51:24 -0400 (EDT) From: Open Systems Networking <opsys@mail.webspan.net> To: spork <spork@super-g.com> Cc: Joel Ray Holveck <joelh@gnu.org>, root@bmccane.maxbaud.net, hackers@FreeBSD.ORG Subject: Re: TweakDUN Message-ID: <Pine.BSF.3.95.980620013654.20127D-100000@orion.webspan.net> In-Reply-To: <Pine.BSF.3.96.980619220851.25847A-100000@super-g.inch.com>
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On Fri, 19 Jun 1998, spork wrote:
> I can't think of anywhere this is true. I'll use our dialup pools as an
> example:
>
> modem-> dialup PPP 1500 -> term server -> ethernet 1500 -> router -> T1(s)
> HDLC 1500 -> core router -> fast ethernet 1500 -> upstream's border
> router -> FDDI 40?? -> upstream core router -> ATM/SONET/whatever ?
>
> Generally, one avoids small MTUs on big links, I beleive. ATM's small
> cell size makes *every* packet get fragmented at layer 2, but I'm not sure
> that's even relevant.
>
> Anyone else? I've never heard of the oft quoted "Internet standard MTU of
> 576"...
Me either, and theres a reason. Its braind dead to do so. Im REALLY tired
right now so im not gonna crack open stevens and quote it. Anyone using an MTU
of 576 destroys performance on bulk transfers such as FTP and and the
like. The ONLY good an MTU of 576 is good for is interactive traffic, like
telnet. At least thats how I have always understood it. For an excercise
the reader can change his MTU from 1500 to 576 and test FTP, and telnet
over it, then switch back to 1500 and do the same. If you do the
calculations of an MTU of 576 and figure in latency. On bulk transfers of
an MTU of 1500 winds over 576. Because for each 1500 byte packet sent over
the MTU link with say a latency of 533ms for dialup, it would take 533ms
for a 1500 byte packet. Now if your using an MTU of 576 and transfer the
same 1500 byte packet with latency of 533ms, you have to send 3 packets of
536 bytes @ 533ms latency each. It isnt rocket science to see why for
anything BUT interactive traffic where all your traffic will fit inside
your MTU your gonna loose and loose big. So why in gods name people think
an MTU of 576 is the "internet standard" AND their actually believing it
astonishes me. My math above maybe be incorrect but you get the idea.
Basically I chalk it up to uninformed admins on the loose, who need to be
put back in their cages. :)
Not to mention by fragmenting all your non-interactive traffic your
creating insane ammounts of traffic. 3 packets at an MTU of 576 or 1
packet at 1500. tripple the traffic. Well you can tell im tired cause im
ranting and probably making no sense. So im going to bed.
Ill probably kick myself for what I just wrote when I read it in the
morning but I am sure some of its right.
Chris
--
"Linux... The choice of a GNUtered generation."
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