From: (no name) (no email)
Date: Wed Jun 14 2000 - 12:29:48 EDT
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 <> // 818.535.5024 voice
-----Original Message-----
From: Roeland Meyer (E-mail) [mailto:]
Sent: Wednesday, June 14, 2000 9:07 AM
To: ; 'Marc Slemko'
Cc:
Subject: RE: PMTU-D: remember, your load balancer is broken
> : 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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