----- Original Message -----
From: "Iljitsch van Beijnum" <iljitsch@xxxxxxxxx>
To: "John Kristoff" <jtk@xxxxxxxxx>
Cc: <ietf@xxxxxxxx>
Sent: Wednesday, September 01, 2010 11:53 AM
> On 31 aug 2010, at 22:04, John Kristoff wrote:
>
> > I'm trying to locate an RFC that spells out the behavioral
> > requirements, expectations or guidelines for NAT handling of the IP ID
> > field, particularly for UDP messages.
>
> > If this is not written down anywhere, do NATs generally rewrite the ID
> > field with or without the MF bit set?
>
> I don't know.
>
> We had a discussion about this in the BEHAVE working group while working on
stateful IPv6-to-IPv4 translation. Unless I missed something, the ID field needs
uniqueness for any combination of source, destination IP addresses and protocol.
Assuming the source address doesn't change, this means an ID counter should be
maintained per destination address + protocol pair, so the maximum number of
packets can be transmitted for each such pair before an ID value is reused. This
would be the optimal host behavior, and NATs should act like hosts in this
regard. Reusing the ID field from the original packet has a much higher chance
of seeing the same ID field for outstanding fragments of a different flow, which
can cause undetected data corruption in 1 in 65535 cases when the TCP/UDP
checksum doesn't catch this.
>
> Note that DF=1 doesn't save you from all of this, as RFC 2402 says:
>
> Mutable (zeroed prior to ICV calculation)
> Type of Service (TOS)
> Flags
>
> 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.
Tom Petch
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