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Re: MTU of the Internet?

  • From: Tony Torzillo
  • Date: Wed Feb 04 14:08:31 1998

576 isn't a magic number.

>From "Internetworking with TCP/IP by Douglas E. Comer, Third Edition":

"The IP specification states that routers must accept datagrams of up to
576 octets.  (Hosts are also required to accept, and reassemble if
necessary, datagrams of at least 576 octets.)

On Wed, 4 Feb 1998, Forrest W. Christian wrote:

]On Wed, 4 Feb 1998, Perry E. Metzger wrote:
]> > Several people have noted to the Microsoft Support and Product groups that
]> > they want the Windows 95 PPP MTU to be set to 576 (down from 1500).  this
]> > change is in Windows 98.
]> accept a large MTU, no matter what Windows would like. The entire
]> story sounds, to say the least, fishy.
]I think what is really going on is that people tweaking on the MTU setting
]have discovered that for some unknown reason 576 just plain works better
]over a dialup PPP connection than ~1500 or any other value for that
]My guess would be that it is in some way related to the packet latency
]generated by clocking in 1500 bytes over a ppp link (~500 ms PLUS the
]V.whatever overhead)  A ~500 byte packet would be more like ~166 ms.  I
]just did a real-world check of these numbers, and they seem pretty close
]to reality for a 28.8 connection.   They're a little high for a 33.6.
](real 33.6 numbers were 250 (total clock+v.?) and 412 for 500 and 1500
]Now it's been a while since I looked at latency vs transfer rates, so
]maybe someone who works on this on an everyday basis would like to comment
]on what ~200 more ms of latency on a 28.8 link would do to throughput
]end-to-end across the net (totals of something like 350 and 512 ms
]I'm pretty certain though, that the common "myth" that 576 works better
]because of end-to-end MTU's is just that - a myth.   My network is all
]1500 or better - not sure if I've EVER seen anything less than 1500 or
]not in the last few years, but I doubt it.
]- Forrest W. Christian ([email protected]) 
]iMach, Ltd., P.O. Box 5749, Helena, MT 59604
]Solutions for your high-tech problems.                  (406)-442-6648