[e2e] Header length field in IP header

Joe Touch touch at ISI.EDU
Thu Sep 23 10:39:19 PDT 2004

Craig Partridge wrote:
> In message <4152F8A9.2090500 at isi.edu>, Joe Touch writes:
>>Error - drop the packet. If the IP version field is 4 (I'm assuming you 
>>mean IPv4), there are required fixed-length fields. As per RFC791, pg 11:
> Joe:
> You're quite right, but realize the point here -- we have found some
> free bits!!
> All we need to do is write a standards document that says values
> 0,1,2,3 and 4 are to be interpreted as "5 + a bit value to be determined"
> and we get 5 more values we can signal in the IP header!  Diff serv
> has more bits to play with!  <Running for cover... :-)>
> Craig

The best part of that is the packets will be dropped by all intervening 
routers, since they're not "unused" but rather "errors to drop", which 
means you'll get the diff serv you deserve ;-)

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