[29247] in North American Network Operators' Group
Re: PMTU-D: remember, your load balancer is broken
daemon@ATHENA.MIT.EDU (Valdis.Kletnieks@vt.edu)
Thu Jun 15 03:03:13 2000
Message-Id: <200006150624.e5F6OBL21394@black-ice.cc.vt.edu>
To: Andrew Brown <atatat@atatdot.net>
Cc: North American Noise and Off-topic Gripes <nanog@merit.edu>
In-reply-to: Your message of "Thu, 15 Jun 2000 00:14:43 EDT."
<20000615001443.A12597@noc.untraceable.net>
From: Valdis.Kletnieks@vt.edu
Date: Thu, 15 Jun 2000 02:24:11 -0400
Errors-To: owner-nanog-outgoing@merit.edu
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
