[95974] in North American Network Operators' Group
Re: Thoughts on increasing MTUs on the internet
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Thu Apr 12 13:58:18 2007
In-Reply-To: <20070412160729.GA498@mx.ytti.net>
Cc: NANOG list <nanog@merit.edu>
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Thu, 12 Apr 2007 19:51:27 +0200
To: Saku Ytti <saku+nanog@ytti.fi>
Errors-To: owner-nanog@merit.edu
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%.
