Date   : Sat, 20 Nov 2010 10:54:13 +0000
From   : mfirth@... (Michael Firth)
Subject: Rockin' out with awServer :-)

  On 20/11/2010 04:24, Rick Murray wrote:
> On 20/11/2010 01:35, Mark Usher wrote:
>
>> bytes. If we are talking about inter connectivity to Ethernet, we need to be
>> keeping the packet size below the Ethernet MTU to keep things simple and not
>> having to implement stupid size buffers.
> Isn't the MTU something weird like 1460?
>
>
> Ah, here we go.
> My Livebox claims its MTU is 1492 bytes. That doesn't divide by a power
> of two. Odd size...
Virtually nothing on Ethernet uses straight "power of two" maths, I 
don't really know why:

Ethernet frame size: 1518 bytes - 14 byte header, 1500 byte payload, 4 
byte checksum
IP Header: 20 bytes
TCP Header: 20 bytes
UDP Header: 8 bytes

It means that if you are doing power of two addressing to store multiple 
Ethernet frames, you need to waste a lot of bytes, reserving 2048 for 
something that can be a maximum of 1518.

In the video world of your PVR things aren't any better. The RTP header 
that would often be on video over Ethernet is 12 bytes, and the video 
packets are then 188 bytes, which give a maximum of 7 video packets per 
Ethernet packet.

You'd never guess I do this sort of stuff for a day job :-)

Regards

Michael