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Re: Thoughts on increasing MTUs on the internet

  • From: Iljitsch van Beijnum
  • Date: Thu Apr 12 14:04:32 2007


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%.