Thoughts on increasing MTUs on the internet

Sat Apr 14 20:10:45 UTC 2007

On 14-apr-2007, at 19:22, Douglas Otis wrote:

>> 1500 byte MTUs in fact work. I'm all for 9K MTUs, and would  
>> recommend them. I don't see the point of 65K MTUs.

> Keep in mind that a 9KB MTU still reduces the Ethernet CRC  
> effectiveness by a fair amount.

In the article "Error Characteristics of FDDI" by Raj Jain (see  
http://citeseer.ist.psu.edu/341988.html ) table VII says:

Hamming Distance of FCS Polynomal

Hamming    Max Frame Size
Weight     Octets
3          11454
4            375
5             37

Of course a 9000 byte packets has 6 times the number of bits in it,  
so the chance of having a number of bit errors in the packet that  
exceeds the hamming distance is ~ 6 times greater.

I can't find bit error rate specs for various types of ethernet real  
quick, but if you assume 10^-9 that means that ~ 1 in 10000 11454  
byte packets has one bit error, so around 1 in 10^12 has four bit  
errors and has a _chance_ to defeat the CRC32. The naieve assumption  
that only 1 in 2^32 of those packets with 3 flipped bits will have a  
valid CRC32 is probably incorrect, but the CRC should still catch  
most of those packetss for a fairly large value of "most".

For 1500 byte packets the fraction of packets with three bits flipped  
would be around 1 : 10^15, correcting for the larger number of  
packets per given amount of data, that's a difference of about 1 :  
100. That seems like a lot, but getting better quality fiber easily  
compensates for this. Expressed differently, the average amount of  
data transmitted where you see one packet with three flipped bits is  
around 10 petabytes for 11454 byte packets and some 1.3 exabytes for  
1500 byte packets. For the large packets that would be one packet in  
three years at 1 Gbps, for the small ones one packet in 380 years.