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Thread: Re: Thoughts on increasing MTUs on the internet
Re: Thoughts on increasing MTUs on the internet
country flaguser name
United States		 2007-04-12 15:31:48 

  


  

    

owner-nanogmerit.edu wrote on 04/12/2007 04:05:43
PM:



> 

> On Thu, 12 Apr 2007, Joe Loiacono wrote:

> 

> > Large MTUs enable significant throughput performance enhancements
for

> > large data transfers over long round-trip times (RTTs.) The original


> 

> This is solved by increasing TCP window size, it doesn't depend very
much 

> on MTU.




Window size is of course critical, but it turns out
that MTU also impacts rates (as much as 33%, see below):




        MSS      0.7

Rate = ----- * -------


        RTT    (P)**0.5



MSS = Maximum Segment Size

RTT = Round Trip Time

P   = packet loss




Mathis, et. al. have 'verified the model through both
simulation and live Internet measurements.'



Also (http://www.aarnet.edu.au/engineering/networkdesign/mtu/why.html):




"This is shown to be the case in Anand and Hartner's

"TCP/IP Network Stack Performance in Linux Kernel 2.4 and 2.5"

in Proceedings of the Ottawa Linux Symposium, 2002. Their experience was
that a machine using a 1500 byte MTU could only reach 750Mbps whereas the
same machine configured with 9000 byte MTUs handsomely reached 1Gbps."



AARnet - Australia's Academic and Research Network




> 

> Larger MTU is better for devices that for instance do per-packet 

> interrupting, like most endsystems probably do. It doesn't increase


> long-RTT transfer performance per se (unless you have high packetloss


> because you'll slow-start more efficiently).

> 

> -- 

> Mikael Abrahamsson    email: swmikeswm.pp.se


  

  
  
   
     
    

  

    Re: Thoughts on increasing MTUs on the
internet
  


  

     country flaguser name
Sweden		
     2007-04-12 15:48:09
  


  

    
On Thu, 12 Apr 2007, Joe Loiacono wrote:

> Window size is of course critical, but it turns out
that MTU also impacts
> rates (as much as 33%, see below):
>
>        MSS      0.7
> Rate = ----- * -------
>        RTT    (P)**0.5
>
> MSS = Maximum Segment Size
> RTT = Round Trip Time
> P   = packet loss

So am I to understand that with 0 packetloss I get infinite
rate? And TCP 
window size doesn't affect the rate?

I am quite confused by this statement. Yes, under congestion
larger MSS is 
better, but without congestion I don't see where it would
differ apart 
from the interrupt load I mentioned earlier?

-- 
Mikael Abrahamsson    email: swmikeswm.pp.se



  


  

    
  

  
  
   
     
    






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