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[ih] IPv4 address size debate

that's a red herring. by the time IP and TCP dealt with the headers,  
the ethernet portion was stripped away.

On Nov 12, 2009, at 2:04 PM, Richard Bennett wrote:

> I remember when handling packets at wirespeed was a challenge, but  
> that was solved by hardware. The 48-bit EtherMac address was a much  
> bigger issue than IP addresses, and the size of the Ethernet header  
> (112 bits) guaranteed that the IP header wasn't going to be 32-bit  
> aligned anyhow.
> John Day wrote:
>> You missed the point of my comment.  I am well aware of the coding  
>> issues.  Although, Oran and others have always argued that variable  
>> was not a big deal in hardware.
>> The point was that if you think in terms of a relative  
>> architecture, rather than the traditional fixed flat architecture,  
>> fixed is variable, or was that variable is fixed?  ;-)
>> I was implying that fixed was really all that was necessary, if you  
>> really understood the inherent structure.  But then you knew that,  
>> didn't you?
>> Take care,
>> John
>> At 1:46 -0500 2009/11/12, Craig Partridge wrote:
>>> > Once one understands the bigger picture, one realizes that  
>>> question
>>>> of variable vs fixed is a non-sequitor. But one does have to get  
>>>> free
>>>> of the constraints of a Ptolemaic approach to architecture.
>>> Hi John:
>>> I'm afraid I disagree (at the risk of being lumped in the  
>>> distinguished
>>> company of Ptolemy).
>>> I agree that in much of the networking and distributed systems  
>>> world, variable
>>> vs. fixed is not a big deal and has all the utility of the binary  
>>> vs. ASCII
>>> representations debate (i.e. not much).
>>> But, in routers and encrypters and similar boxes that handle large  
>>> volumes
>>> of data, fixed vs. variable is still a challenge.  The fundamental  
>>> issue is
>>> that while links work in terms of bits and bytes, processors and  
>>> memories
>>> actually work in terms of blocks/chunks.  That's because they use  
>>> parallelism
>>> they use to go fast (and one reason they use parallelism is  
>>> physics -- prop
>>> times across chip boundaries, etc).
>>> So when writing code for routers that has to go fast, you are  
>>> constantly
>>> thinking about those blocks and trying to avoid crossing block  
>>> boundaries
>>> (both in instructions and data accesses) and trying to keep your  
>>> software
>>> using the minimum number of blocks, as touching an additional  
>>> block is
>>> a serious performance hit.   Knowing exactly how your data is laid  
>>> out
>>> is a huge boon here -- it removes the uncertainty of how many  
>>> blocks you'll
>>> have to touch (and how many instructions you have to execute).
>>> And sizing for the max (assuming the variable address is always  
>>> max length)
>>> doesn't help either -- because there are two addresses in a  
>>> header, if the
>>> first one is short then all your plans for the second address are  
>>> undone.
>>> Upleveling my point -- we have a computing abstraction (bytes)  
>>> which doesn't
>>> match how computers, when stressed for performance, actually work  
>>> and that
>>> has implications for packet headers.
>>> Thanks!
>>> Craig
> -- 
> Richard Bennett
> Research Fellow
> Information Technology and Innovation Foundation
> Washington, DC