MTU path discovery and IPSec

Joe Maimon jmaimon at ttec.com
Fri Dec 5 01:37:57 UTC 2003



Crist Clark wrote:

>Joe Maimon wrote:
>  
>
>>Tony Rall wrote:
>>
>>    
>>
>>>On Wednesday, 2003-12-03 at 09:38 PST, David Sinn <dsinn at dsinn.com> wrote:
>>>
>>>
>>>
>>>
>>>      
>>>
>><snipped>
>>
>>    
>>
>>>(And note that frag 1 often is not the first fragment to arrive at
>>>downstream nodes.  In my example in (1), frequently frag 2 will reach
>>>places before frag 1 does (if any router along the path reorders its
>>>transmit queue based on packet size).)
>>>
>>>
>>>
>>>      
>>>
>>I agree with all I have snipped.
>>I was wondering would it not be wiser for fraggers to frag in half
>>instead of just the overflow?
>>
>>For instance, suppose router has to fragment 1500 byte packet to go over
>>1476 GRE. Instead of having  a big packet/little fragment why not just
>>divide in half?
>>This would give them more equal buffer treatment, but an even bigger
>>potential win is to avoid perhaps a second (maybe ipsec?) fragmenting
>>later on down the pipe.
>>
>>Once you are going to do it, do it right. It is not as if your
>>decreasing header overhead by producing small fragment packets. And I am
>>assuming the whole packet is already in buffer when it comes time to
>>fragment it.
>>    
>>
>
>Programmers are lazy.
>
>Excerise for the reader:
>
>Devise an algorthm that will take an arbitrarily sized packet 20-65535
>octets and an arbitrarily sized MTU, > 576 octets, and split the 
>packet into the minimum number of "n" fragments where each fragment is
>(1) less than the MTU, (2) no two fragments differ by more than 8 octets,
>and the fragments obey the IP fragmentation rules, (3) data payload must
>end on an 8-octet boundary for all but the last fragment and (4) each
>fragment has an exact copy of the original header except for differences
>in the fragmentation fields and checksum.
>
>Compare to the algorithm of cutting the data in to "m" (mtu - ip_hl)-
>chunks and putting the leftovers into the final fragment.
>  
>
How about only going to the bother if 'n' would only be 2 in either 
algorithm? That should keep things nice and simple for all the lazy 
programmers.

And we wonder why there are so many security holes.

As for the rest, I do not see the real difference. And now I will shut 
ip about implementation until/when(if ever) I could write some.






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