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Re: NAT behavior for IP ID field

2010-09-14 11:21:06
...



 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.

I'm curious abut RFC 2402, then. Firstly, the host might not implement
PMTUD, and hence setting the DF bit on its behalf could possibly cause
interoperability problems. Secondly, some hosts clear the DF bit if the
advertised MTU in an ICMP "frag needed" is below some specified
threshols. This RFC2402-behavior could cause problems in this scenario, too.

We made the decision ton exclude the DF bit from the ICV computation in 2402, based on what we believed was happening in the net, irrespective of what should have been happening ;-). We retained this behavior in RFC 4202, for the same reason.

Steve
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