Re: What flexibility do 6to4 NAT have with address formats?

On 2009-10-14 03:38, Christian Huitema wrote:
...
> 
> First, we wonder about the importance of the 64 bit boundary.
> Addressing documents specify that the global address is
> essentially formed of a 64 bit subnet prefix and a 64 bit
> host identifier, with the host identifier compatible with
> IEEE 802 identifiers. Does that mean that the "routing"
> requirement of stateless translation can only be addressed if
> the IPv4 bytes are entirely contained within the subnet
> prefix? Different authors have different opinions, so WG
> input would be beneficial.

As I understand it, routers should never treat /64 as special
or as any kind of limit. It's just a convention that /64 is
considered the longest normal subnet prefix, which happens to
be compatible with SLAAC. So I can't see any fundamental
reason why the IPv4 address bits can't straddle the /64
boundary. However, they should clearly skip over the UG bits
so the resulting 64-bit IID is consistent with the IID rules.
That will break byte alignment.

> 
> Second, we wonder about the constraints of host identifiers.
> A first question is whether an all null identifier would be
> legitimate and practical. There is some evidence that it
> works with most stacks. But there is also a statement in the
> addressing document that the all null address is reserved for
> the subnet anycast address. Do stacks actually implement the
> subnet anycast function? Should the specification be removed
> from the addressing RFC? Can we just ignore it? If we cannot
> ignore it, we will have to specify some value different from
> zero for the suffix. A "checksum neutrality" field might do
> that, but please consider the second question.

If you do straddle the /64 boundary, you will not have a null
IID so the issue goes away. If not, using null feels wrong to
me. Firstly, it conflicts with the current (harmless and
possibly implemented) spec. Secondly, specifying a value is no
big deal as far as I can see.

> 
> The second question regards the uniqueness of host
> identifiers. Suppose we define the address used for stateless
> translation as: 32 bit "provider" prefix, 32 bit IPv4
> address, and a constant identifier, either 0 or the "checksum
> neutrality" value, which is only a function of the provider
> prefix. Suppose now that for some reason there are two "IPv4
> addressed" hosts on the same link, e.g. because many servers
> are located in the same server room. The two hosts will have
> different addresses, in different 64 bit subnets, but they
> will also have different host identifiers. Is that OK?

Why wouldn't it be OK? I can't see why it's a question.
The normal expectation is that different hosts have different
IIDs so I am curious why this matters.

   Brian

> 
> -- Christian Huitema
> 
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