Re: Thoughts on increasing MTUs on the internet

[Joe Loiacono <jloiacon () csc com>]

Large MTUs enable significant throughput performance enhancements for 
large data transfers over long round-trip times (RTTs.) The original 
question had to do with local subnet to local subnet where the difference 
would not be noticable. But for users transferring large data sets over 
long distances  (e.g. LHC experimental data from CERN in France to 
universities in the US) large MTUs can make a big difference.

For an excellent and detailed (though becoming dated) examination of this 
see:

"Raising the Internet MTU" Matt Mathis, et. al. 

http://www.psc.edu/~mathis/MTU/

Joe




Stephen Wilcox <steve () telecomplete co uk> 
Sent by: owner-nanog () merit edu
04/12/2007 01:45 PM

To
Keegan.Holley () sungard com
cc
NANOG list <nanog () merit edu>
Subject
Re: Thoughts on increasing MTUs on the internet







On Thu, Apr 12, 2007 at 11:34:43AM -0400, Keegan.Holley () sungard com wrote:
>    I think it's a great idea operationally, less work for the routers 
and more
>    efficient use of bandwidth.   It would also be useful to devise some 
way to
>    at least partially reassemble fragmented frames at links capable of 
large
>    MTU's. 

I think you underestimate the memory and cpu required on large links to be 
able to buffer the data that would allow a reassembly by an intermediate 
router

>    Since most PC's are on a subnet with a MTU of 1500 (or 1519) packets
>    would still be limited to 1500B or fragmented before they reach the 
higher
>    speed links.  The problem with bringing this to fruition in the 
internet is
>    going to be cost and effort.  The ATT's and Verizons of the world are 
going
>    to see this as a major upgrade without much benefit or profit.  The 
Cisco's
>    and Junipers are going to say the same thing when they have to write 
this
>    into their code plus interoperability with other vendors 
implementations of
>    it.

I dont think any of the above will throw out any particular objection.. I 
think your problem is in figuring out a way to implement this globally and 
not break stuff which relies so heavily upon 1500 bytes much of which does 
not even cater for the possibility another MTU might be possible.

Steve


>    Iljitsch van Beijnum <iljitsch () muada com>
>    Sent by: owner-nanog () merit edu
>
>    04/12/2007 05:20 AM
>
> To
>
>    NANOG list <nanog () merit edu>
>
> cc
>
> Subject
>
>    Thoughts on increasing MTUs on the internet
>
>    Dear NANOGers,
>    It irks me that today, the effective MTU of the internet is 1500
>    bytes, while more and more equipment can handle bigger packets.
>    What do you guys think about a mechanism that allows hosts and
>    routers on a subnet to automatically discover the MTU they can use
>    towards other systems on the same subnet, so that:
>    1. It's no longer necessary to limit the subnet MTU to that of the
>    least capable system
>    2. It's no longer necessary to manage 1500 byte+ MTUs manually
>    Any additional issues that such a mechanism would have to address?