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Re: Strange public traceroutes return private RFC1918 addresses

  • From: Leo Bicknell
  • Date: Tue Feb 03 11:08:32 2004

In a message written on Tue, Feb 03, 2004 at 08:15:13AM -0600, Terry Baranski wrote:
> The performance gain achieved by using jumbo frames outside of very
> specific LAN scenarios is highly questionable, and they're still not
> standardized.  Are "jumbo" Internet MTUs seen as a pressing issue by
> ISPs and vendors these days?

While the rate of request is still very low, I would say we get
more and more requests for jumbo frames everyday.  The pressing
application today is "larger" frames; that is don't think two hosts
talking 9000 MTU frames to each other, but rather think IPSec or
other tunneling boxes talking 1600 byte packets to each other so
they don't have to split 1500 byte Ethernet packets in half.  Since
most POS is 4470, adding a jumbo frame GigE edge makes this application
work much more efficiently, even if it doesn't enable jumbo (9k)
frames end to end.  The interesting thing here is it means there 
absolutely is a PMTU issue, a 9K edge with a 4470 core.

There is also a lot of work going on in academic networks that uses
jumbo frames.  I suspect in a few more years this will make it into
more common applications.

In a message written on Tue, Feb 03, 2004 at 04:40:15PM +0200, Petri Helenius wrote:
> Me wonders why people ask for 40 byte packets at linerate if the mtu is 
> supposedly
> larger?

This is a problem that is going to get worse.  I support IP you
have to support a 40 byte packet.  As long as that exists, DDOS
tools will use 40 byte packets, knowing more lookups are harder on
the software/hardware in routers.  At the same time I suspect software
is going to continue to slowly move to larger and larger packets,
because at the higher data rates (eg 40 gige) it makes a huge difference
in host usage.  You can fit 6 times in the data in a 9K packet that
you can in a 1500 byte packet, which means 1/6th the interrupts, DMA
transfers, ACL checks, etc, etc, etc.

       Leo Bicknell - [email protected] - CCIE 3440
        PGP keys at
Read TMBG List - [email protected],

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