MTU path discovery and IPSec

Crist Clark crist.clark at globalstar.com
Fri Dec 5 01:22:23 UTC 2003


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.
-- 
Crist J. Clark                               crist.clark at globalstar.com
Globalstar Communications                                (408) 933-4387

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