On Mon, 26 Feb 2001, Craig Milo Rogers wrote:
> > > I have a whole 40 bytes (+/-) to share... Now although I don't see
> > > anything explicitly prohibiting the use of unused IP Header option
..
> > > in between.. Has anyone seen any RFC that explicitly says I MUST NOT?
> >
> >Not to my knowledge. Routers already change the time to live field,
> >so I see no reason why they can't do smart things with special IP
> >options either (besides efficiency concerns :-).
I know they 'rewrite/extend' existing options, but have never seen a case
where a router adds an option to a packet beyond those based on what the
original sender set..
> I've forgotten how the Stream ID option was implemented, but I
> won't be surprised if a router inserted it on the fly (but it was
> probably inserted by end systems). On the other hand, there was also
Hmm, have to look at a little history..
> a competing philosophy that said that the IP checksum must be
> recomputed incrementally at routers to catch hardware problems in the
> routers, and an incremental recomputation when changing the size of
> the header would be more work.
ah.. we do recalculate IP Checksums now.. when we update any of the
timestamp rr options etc..
> The one thing I would worry about is unleashing mutant IP
> packets upon the world at large. I hope the proposed experiments have
> a very good firewall. It would be very nice to attempt to acquire an
> officially blessed IP option number for such experiments before
> unleashing these packets upon an unprepared world.
>
> Craig Milo Rogers
Ah, we better have a good firewall <wink> No, if this goes past concept
phase, we will try for de official bless.
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>> a competing philosophy that said that the IP checksum must be
>> recomputed incrementally at routers to catch hardware problems in the
...
>ah.. we do recalculate IP Checksums now.. when we update any of the
>timestamp rr options etc..
But, do you do it incrementally? By which I mean: subtract
(appropriately) the old value of the octet from the existing checksum,
field in the packet then add (appropriately) the new value of the
octet to the checksum? Simply recalculating the IP checksum from
scratch can generate a "correct" checksum for a packet that was
damaged*** while waiting around in memory.
I don't know if people worry about this now, but 20 years ago
there was a fuss about it. Further discussion offline, please.
Craig Milo Rogers
*** Maybe by hardware trouble, or maybe because someone followed a bad
pointer and stomped on part of the header.