RINA - scott whaps at the nanog hornets nest :-)

Brielle Bruns bruns at 2mbit.com
Sun Nov 7 01:34:54 CST 2010

So, question I don't want to forget between now and when I wake up (since its late in my neck of the woods)...

Has any work been done with >1500 mtu on 802.11 links?

Is it feasable, or even possible?

I'm in the middle of rolling out a wisp in an area, and it dawned on me I never even considered this aspect of the mtu issue.

Brielle Bruns
http://www.sosdg.org  /  http://www.ahbl.org

-----Original Message-----
From: "George Bonser" <gbonser at seven.com>
Date: Sun, 7 Nov 2010 00:19:03 
To: <nanog at nanog.org>
Subject: RE: RINA - scott whaps at the nanog hornets nest :-)

> Also, if we're going to go for bigger MTUs, going from 1500 to 9000 is
> basically worthless, if we really want to do something, we should go
> for
> 64k or even bigger.

I agree but we need to work with what we have.  Practically everything
currently appearing at a peering point will support 9000.  Getting
equipment that would support 64000 would be more difficult.

> About 1500 MTU degrading performance, that's a TCP implementation
> issue,
> not really a network issue. 

True, but TCP is what we are stuck with for right now.  Different
protocols could be developed to handle the small packets better.

> Interrupt performance in end systems for
> high-speed transfers isn't really a general problem, and not until you
> reach speeds of several gigabit/s. Routers handle PPS just fine, this
> was
> "solved" long ago after we stopped using regular CPUs in them.

We are starting to move to 10Gig + peering connections.  I have two 10G
peering ports currently on order.  "several gigabits/sec" is here today.

> Increasing MTU on the Internet is not something driven by the end-
> users,
> so it's not going to happen in the near future. 

It depends on what those end users are doing.  If they are loading a web
page, you are probably correct.  If they are an enterprise user
transferring log files from Hong Kong to New York, it makes a huge
difference, particularly the moment a packet gets lost somewhere.  At
some point it becomes faster to put data on disks and fly them across
the ocean than to try to transmit it by TCP with 1500 byte MTU.  Trying
to explain to someone that they are not going to get any better
performance on that daily data transfer from the far East by upgrading
from 100 Mb to GigE is hard for them to understand as it is a bit
counter-intuitive.  They believe 1G is "faster" than 100Meg, when it
isn't.  If you tell them they could get a faster file transfer rate by
using a an OC-3 with MTU 4000 than they would get by upgrading from
100Mb ethernet to GigE with MTU 1500, they just don't get it.  In fact,
telling them that they won't get one iota of improvement going from
100Mb to GigE doesn't make sense to them because they believe GigE is
"faster".  It isn't faster, it is fatter.   And yes, TCP is the limiting
factor but we are stuck with it for now.  I can't change what protocol
is being used but I can change the MTU of the existing protocol.  I
believe the demand for such high-bandwidth streams is going to greatly
increase.  This is particularly true as people move out of academic
environments where they are used to working on Abilene (Inet2) and move
into industry and the programs they built won't work because it takes
two days to send one day's worth of data. 

There are end users and there are end users.  It depends on the sort of
end user you are talking about and what they are doing.  If they are
watching TV, they might want a higher MTU.  If they are on Twitter, they
don't care.  Industry end users will have different requirements from
residential end users.

> They are just fine with
> 1500 MTU. Higher MTU is a nice to have, not something that is
> hindering performance on the Internet as it is today or in the next
> tens of years.

I disagree with that statement because I believe that the next few years
will see an increased demand for high-bandwidth traffic that needs to be
delivered quickly (HDTV from Tokyo to London, for example). 

One of the reasons people aren't interested is because they don't know.
They are ignorant in most cases.  They just know "bandwidth".  They
believe that if they get a fatter pipe, it will improve their viewing of
that Australian porn.  Then they pay for the upgrade and it doesn't
change a thing.  It doesn't change the data transfer rate at all.  They
go from a 10Meg to a 100Meg pipe and that file *still* transfers at
3Meg/sec.  If they could increase the MTU to 9000, they might get

But you are correct, going to an even higher MTU is what is really
needed but going for what is attainable is the first step.  Everyone can
physically do 9000 at the peering points (or at least as far as I am
aware they can) and the only thing that is preventing that is just not
wanting to because they don't fully appreciate the benefit and believe
it might break something.  Increasing MTU never breaks PMTUD.  PMTUD is
only needed because something in the path has a *smaller* MTU than the
end points.  The end points don't care of the path in between has a
larger MTU.

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