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RE: PMTU-D: remember, your load balancer is broken
From: rdobbins () netmore net
Date: Wed, 14 Jun 2000 09:29:48 -0700



The 576 value of the MS PPP MTU is merely a default - it can be changed with
a registry hack.  Yes, fragmentation has indeed become a Great Evil due to
the large amounts of data we're pushing, and the time/resources required for
fragmentation/defragmentation.  Forcing excessive
fragmentation/defragmentation is an effective DoS.

As far as increasing the MTU size on your LAN links, you need to exercise a
lot of care when so doing.  I personally have never tried to change the MTU
size on an Ethernet segment of any type (Ethernet_II/1500 has worked
admirably, and I'm unsure of the result if I tried it); on Token Ring, going
up to 4096 has indeed been beneficial in the past when dealing with large
database writes, etc.  Of course, the protocol I was using at the time
supported 4096-byte frame sizes on Token Ring.

I thought the frame-size limits for Gigabit Ethernet were 64-1518/1522
bytes?  And isn't that the limit on most host IP stacks for Ethernet media?
Or am I off in left field, here?

Finally, I would say that on any medium, <100% utilization in and of itself
isn't grounds for fiddling with the MTU.  There are lots of other things to
look at, first.

---------------------------------------------------------------
 Roland Dobbins <rdobbins () netmore net> // 818.535.5024 voice




-----Original Message-----
From: Roeland Meyer (E-mail) [mailto:rmeyer () mhsc com]
Sent: Wednesday, June 14, 2000 9:07 AM
To: Valdis.Kletnieks () vt edu; 'Marc Slemko'
Cc: nanog () merit edu
Subject: RE: PMTU-D: remember, your load balancer is broken 



Valdis.Kletnieks () vt edu: Wednesday, June 14, 2000 8:07 AM

On Tue, 13 Jun 2000 22:36:08 MDT, Marc Slemko said:

It is also a concern that, in my experience, many of the
links with
MTUs <1500 are also the links with greater packet loss, etc.
so
you really don't want fragmentation on them.

The worst part here is that I suspect that most of these
links (just on
sheer numbers of shipped product) are the aformentioned Win98
576-MTU.

I just set my dial PPP ports to MTU=512+40=552, is this wrong?
Where does the MTU=576 number come from?

I seem to remember that the *original* motivation for
slow-start and
all that was Van Jacobson's observation that the most common
cause of
a TCP retransmit was that an *entire* packet had been silently
dropped
due to queueing congestion, and could thus be treated identical
to
an ICMP Source Quench.

Has this changed?  Has "fragmentation" become a Great Evil,
rather than
an annoyance that some links have to deal with?

I'm having some trouble getting full throughput from a GigE pipe.
Even in the 100baseTX/FDX down-stream, I'm not getting full link
utilization (everything on switches, Cat6509 and 3512XLs). I'm
considering increasing MTU sizes to MTU=4096+40, or even larger.
Most of the data transmissions fall into the 5KB-50KB range. The
site can be considered a large portal. What would be the effect
on my upstream? Would it create problems? The only systems that
see the Internet are the web-servers (dual NICs).




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