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Re: PMTU-D: remember, your load balancer is broken
From: Valdis.Kletnieks () vt edu
Date: Thu, 15 Jun 2000 02:24:11 -0400


On Thu, 15 Jun 2000 00:14:43 EDT, Andrew Brown said:
i think you have two things confused.  raising the mtu will "speed up
the modem" since you get more data for less overhead, however
*lowering* the mtu will increase interactivity, since each packet is
smaller, the transmit time is shorter, so the next one can get in/out
sooner.  you stand a better chance of getting a "word" in edgewise if
the other guy is using smaller "phrases".

No, he's got it right.

The user *percieves* the modem is "speeded up" if he's doing
interactive work and fighting with a file download.  For instance, if
you're using BIG packets on a slower modem (yes, there's still 14.4 and
33.8 users out htere), the network latency on a telnet fighting with an
FTP can be up to a major fraction of a second - if your MTU is 1/3 the
size, then your character echo is 3 times as fast.   The fact that if
you lower the MTU from 1500 to 500 you're taking a 10% performance
hit (2 extra IP/TCP headers amortized over 1500 bytes) pales in
comparison...

Yes, it's actually 10% lower for throughput.  But for the "boot it,
reinstall AGAIN, call tech support and listen when they tell you to
wave a dead chicken over the CPU while re-re-installing drivers"
crowd, it feels 3 times faster...  So it *is* 3 times faster.

                                Valdis Kletnieks
                                Operating Systems Analyst
                                Virginia Tech



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