[tforum] Re: [it-managers] Raising the Internet MTU

Joe Breen Joe.Breen@utah.edu
Wed, 18 Apr 2007 11:46:31 -0600

Tom Ammon wrote:
> Has anyone seen any data concerning large frame size's impact on jitter?

As the speed of the link goes up, the impact of larger frame sizes goes 
down unless the frame sizes grow significantly.  What you bring up is a 
valid question, though, because you don't want latency sensitive 
volkswagen beetles stuck behind huge semi-trucks, or latency sensitive 
voice packets trapped in the queues behind large file transfers.  Some 
discussion exists in some of the links off of the main link I sent out. 
  For example, Phil Dykstra's discussion:

I will have to dig up some other links I have seen on the subject. 
There have been some rigorous papers dealing with the question.

In practice, most jitter on voice and multimedia comes from dropped 
packets, varying latencies due to changing paths, routers running under 
high utilization, strange asymmetric routes or host issues.  As long as 
the pipes are fast, packet drop negligible and the routers are truly 
running wire speed, jitter issues tend to be lower.   Buffer sizes on 
hosts, routers and switches can affect jitter.  Host CPU utilization 
also affects jitter.  How well a host is handling interrupts also 
affects jitter.  Many factors affect jitter, especially in the Wide Area 

Large frame sizes are typically not common on the internet due to ISPs 
and end-customer devices not supporting Jumbo frames.   However, in the 
research world, the national and international backbones do support 
jumbo frames as well as very high speed pipes.  Because the pipes are 
now 2.5Gigabit to 10Gigabit and the max Jumbo frame is only 9kbytes.  A 
9kbyte frame on a 10Gig link is actually less of an impact than the 
standard 1500byte frame on a 10Megabit link.


> Joe Breen wrote:
>> For those following network performance issues,
>> As networks have progressed in speed from 10Megabit to 10Gigabit, the 
>> size of the transmission unit has stayed the same.  This fact impacts 
>> the performance of networks.  The following link is a very useful 
>> starting point on Jumbo-frames and larger Maximum Transmission Units.
>> http://www.psc.edu/~mathis/MTU/
>> Matt Mathis of the Pittsburgh SuperComputing Center is one of the 
>> leading investigators into larger packet sizes and their impact on 
>> performance.  He is also one of the co-authors of a new Request for 
>> Comment (RFC) regarding path MTU discovery, the ability for end 
>> machines to discover the maximum transmission unit.
>>     --Joe