Thoughts on increasing MTUs on the internet

Thu Apr 12 20:31:48 UTC 2007

owner-nanog at merit.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: swmike at swm.pp.se
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