Re: Thoughts on increasing MTUs on the internet

[Iljitsch van Beijnum <iljitsch () muada com>]

On 12-apr-2007, at 18:07, Saku Ytti wrote:

> > > I agree. The throughput gains are small. You're talking about a
> > > difference between a 4% header overhead versus a 1% header overhead
> > > (for TCP).

> > 6% including ethernet overhead and assuming the very common TCP
> > timestamp option.

> Out of curiosity how is this calculated?

8 bytes preamble
14 bytes ethernet II header
20 bytes IP header
20 bytes TCP header
12 bytes timestamp option
4 bytes FCS/CRC
12 bytes equivalent inter frame gap

90 bytes total overhead, 52 deducted from the ethernet payload, 38  
added to it.

90 / (1500 - 52 = 1448) * 100 = 6.21

90 / (9000 - 52 = 8948) * 100 = 1

Also note that the real overhead is much bigger because for every two  
full size TCP packets an ACK is sent so that adds 90 bytes per 2 data  
packets, or increases the overhead to 9% / 1.5%.