On 2 sep 2010, at 10:04, t.petch wrote:
So it is legal to rewrite the DF bit from 1 to 0. I also know that this
happens in the wild because I used to do this at one time.
Curious; RFC2402 says
" Flags -- This field is excluded since an intermediate router might
set the DF bit, even if the source did not select it."
which is a licence to set the bit but I had not thought to reset the bit.
RFC791, RFC1122 and RFC1812 would appear to be silent on this.
Ah, I did't read that far. Not sure why a router would set the DF bit, though,
that creates a PMTUD black hole.
I agree that there is no explicit permission to modify the DF bit in transit,
but RFC 2402 certainly recognizes that this may happen in practice. It's a
pretty effective way of getting rid of PMTUD black holes that you run into when
there is an MTU smaller than 1500 in the middle of the network. Most people
just rewrite the MSS option in TCP SYNs (which are certainly NOT defined as
mutable in transit), though.
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