Re: [v6ops] How do you solve 3GPP issue if neither operator nor handset supports PD?

otroan@employees.org Tue, 24 November 2020 14:50 UTC

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Subject: Re: [v6ops] How do you solve 3GPP issue if neither operator nor handset supports PD?
From: otroan@employees.org
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To: Philip Homburg <pch-ipv6-ietf-6@u-1.phicoh.com>
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Philip,

>> Neither do any implementation I'm aware do that.
>> To be clear. If a link-type has no L2 address / is point to point, then 
>> no implementation I've touched does ND address resolution.
>> It would be happy to respond to a NUD message.
> 
> ND answers two questions, what is the L2 address of an address and is the
> remote node alive.

ND has many functions.
One is address resolution and another is NUD (neighbor unreachability detection).
They use the same messages but have distinct functions.

> So from my point of view it is obvious to just send an NS, mark the neighbor
> cache as incomplete until you get an NA back. Then you know that neighbor
> is alive.

If you have a neighbor cache for neighbors for p2p links.

> Now, I wouldn't mind if other implementations would not send the NS and
> just assume that their neighbors are alive.
> 
> However I found that most sutff just doesn't reply with an NA at all.

Right. While NUD isn't a particularly effective keepalive/link state detection mechanism I would expect it to be supported.

>> The current behaviour of 64share does that (and gets the hack label
>> for that reason).  What I'm saying is that 64share (and what I
>> propose in p2p ethernet) is a lot closer to PD than it is to address
>> assignment.  As soon as you stop thinking that a p2p link has a
>> shared subnet, that's where you end up.
> 
> From a 'bits on the wire' point of view, there is nothing magic to PD.
> It is just a message that says: here is prefix, you can do with it as you
> see fit.
> 
> The devil is in the details. What if the down stream router then hands out
> the prefix using DHCPv6 PD. Do we need to do anything special for homenet?
> (I guess doing PD using RA would actually be the 3rd version of PD, because 
> homenet also does prefix delegation).

no, I can't see that the protocol external to the homenet matters.
homenet does not do prefix delegation. homenet does prefix assignment.

> If we want this to work on ethernet, we may have to say something specific,
> like 'a route will be installed that points to the (link local) source
> address of the RS'. Which I guess implies that an RA with a PDIO can only
> be sent in response to an RS (on ethernet). Then, what do we do on p2p links.

p2p links are simple. my proposal is also to treat Ethernet as a P2P link.
But otherwise, yes, you would have to install the route with the LL as NH.

The biggest difference I see between DHCP PD and something we might do in RA is the coupling between interface state and prefix.
DHCPv6 PD is essentially a fax-replacement. A delegation is independent of the upstream link-state, and in theory also which interface the delegation was received on. Although the realities of routing quickly sets in. If in RA the delegation's lifetime is restricted by the link-state/lifetime of RA, then that will have significant issues for downstream hosts/applications.

Cheers,
Ole