owner-nanog@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@swm.pp.se