RE: [v6ops] RFC7084

["Manfredi, Albert E" <[e-mail]>]

On 10/12/2013 16:51, Ted Lemon wrote:
> On Dec 10, 2013, at 6:08 AM, Sander Steffann <[e-mail]> wrote:
>> As far as I know the M flag is linked only to IA_NA. As far as I can see IA_PD is not linked to the M flag at all.
> 
> There is no agreement on what the M and O bits do.   Some people think M means stateful address management; some think M means IA_NA and O means IA_PD.   RFC 4861 is actually fairly clear, at least to my reading of it, that "M" means IA_NA and IA_PD, and that "O" means stateless DHCPv6, but I've heard people argue vehemently that "M" means _only_ IA_NA, and that "O" means IA_PD, because prefixes aren't addresses.
> 
> So if you believe my reading of RFC 4861, you would set 'M' and expect the HG to get both IA_NA and IA_PD; RFC 7084 makes it clear that _either_ the 'M' or 'O' bit being set triggers the HG to do prefix delegation.
> 
> So if you want to do prefix delegation and not stateful address assignment, set the 'O' bit and _not_ the 'M' bit, even though that contradicts what RFC 4861 says.

This discussion is most enlightening. But I don't see any contradiction with RFC 4861.

It seems to me, RFC 4861 sets M=1 when a host can get the entire address ("it indicates that addresses are available"), from DHCPv6, emulating DHCPv4. At the same time, M=1 does not preclude any other information from being available, so of course prefixes may also be available. RFC 4861: "If the M flag is set, the O flag is redundant and can be ignored because DHCPv6 will return all available configuration information."

Presumably, that would include a list of prefixes.

But RFC 4861 does not limit what "other" information is available when the O bit is set. One would certainly not expect full addresses to be available, since the M bit exists for that purpose, but RFC 4861 only gives a couple of examples of info available with O=1: "Examples of such information are DNS-related information or information on other servers within the network." This does not say that prefixes aren't available.

I don't see any contradiction here. If M=0 and O=1, the full address assignment isn't available in the DHCPv6 server. However, prefixes may well be.

I may want to prevent any host from sending packets through my edge router, unless the host uses IP addresses I assign. For that, I'd want to set M=1. If I don't care about specific addresses, if all I need is for clients to configure a routable address, then I set M=0 and O=1, and let the host choose the IID.

Take the case of a peer-peer network, sans DNS. In that case, I need for hosts to use only a set of prescribed IP addresses, which I distribute via DHCPv6 and the Client Identifier Option. I'd set M=1 for that. The clients may choose to auto-configure their own routable addresses, and I would feel free to NOT route their packets.

Bert