[e2e] ICMP & TCP segments with IP ID = 0?

Andi Kleen ak at muc.de
Wed May 16 10:51:58 PDT 2001

On Wed, May 16, 2001 at 07:20:44PM +0200, Greg Minshall wrote:
> > Why are they evil? ipid is only useful for defragmentation, and perhaps 
> > to recover from bugs in IP checksum ...
> i think this is one of those "be conservative in what you send" things.  the 
> conservative thing to do, i would argue, is to obey the principle of least 
> surprise and send unique IDs; this is what systems expect, orthogonal from DF 
> or not DF.  (of course, the rest of the world should obey the second half: "be 
> liberal in what you accept", and so deal with ID == 0.)

The problem is that generating them is relatively costly for various
reasons in current linux (it's a longer story why that is so), and as there
was no good reason found to keep them they were turned off for DF=0. 

Also I generally don't believe IP IDs are very useful for anything anymore 
with current network speeds, the 16bit space is simply far too small and 
wraps in a jiffy on any reasonable network 
(fortunately fragmentation is not too common anymore and mostly only used
on low latency links, otherwise it would be surely a bigger problem). 


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