RE: about violation of standards

["Pascal Thubert (pthubert)" <[e-mail]>]

> > Some functions in the router are complex to implement because same value
> for a link local address appears on multiple interfaces.
> 
> Like what?

Like structuring the tables where you maintain the address binding.

Because a link local is only unique within the link / broadcast domain where DAD occurs, you have to add the sense of that link / broadcast domain as an additional key to your binding table, whereas for the GUA the prefix gives you that from within the address, and the address is sufficient as a key.

Turns out nothing is too particularly neat for the role. On Ethernet, a VLAN does not guarantee a unique global value though at least it is a local sense of the broadcast domain and you can use it as a node-local key. But you cannot get a global sense of where the link local address is in your network by just looking at it. 

If there was an encoding of that global sense of the broadcast domain in the link local address, one could log in or even send a message to any router in that domain and ask if the link local is live by making it ping or what. And one could generate that request using the link local address only, as opposed to encoding a address%interface that is locally significant to the particular router or host that I'd use to ping the link local address, always a hassle.

This would extend the value of the link local without changing its scope, and enable common APIs, tables and GUIs, understanding that what we'd encode represents a physical domain that is not necessarily mapped to a logical subnet. 


> > It would be useful to be able to encode an abstract interface ID somewhere
> in the /64. Legacy 00 would mean unspecified...
> 
> Sounds like you need subnet-id election?

Not of a subnet, rather an ID for a link or a broadcast domain, a bit like what DNA was after. On an Ethernet domain it is easy to confuse those things because the shared wiring defines at the same time the physical link, the broadcast domain and the subnet that we map over it. But the difference shows on legacy NBMA like ATM or FR, on shared links and on newer types of NBMA such as radio and composite radio-wires networks. 

In radios, the broadcast domain is defined differently by each transmitter as opposed to defined commonly by a shared wire. 
In mesh-under (e.g., a Wi-Fi mesh or IEEE 802.15.10) the link extends beyond the broadcast domain. In route-over (e.g., 6TiSCH with RPL), the subnet extends beyond the link.
Note that a Wi-Fi BSS is an exception whereby the broadcast domain of the AP emulates the common wire, but that takes a particular L2 emulation that is not present in other types of WPAN/WLAN/LPWAN radio links.

On radios without a BSS, links between peers come and go as their individual broadcast domains meet physically. The ND DAD operation cannot provide once and for all guarantees on the broadcast domain defined by one radio transmitter if that transmitter keeps meeting new peers on the go. The nodes may need to form new LLAs to talk to one another and the scope where LLA uniqueness can be dynamically checked is really that pair of nodes. As long as there's no conflict a node may use the same LLA with multiple peers but it has to revalidate DAD very time. So it's like if each pair of nodes defines a temporary P2P link, like a sub-interface of the radio interface. In that case, we could encode something about that P2P link in the LLA, like something derived from MAC addresses, while keeping the same IID. 

All the best,

Pascal

> -----Original Message-----
> From: ipv6 <[e-mail]> On Behalf Of Ole Troan
> Sent: mardi 23 avril 2019 09:33
> To: Pascal Thubert (pthubert) <[e-mail]>
> Cc: Fernando Gont <[e-mail]>; Alexandre Petrescu
> <[e-mail]>; 6man WG <[e-mail]>; 神明達哉
> <[e-mail]>
> Subject: Re: about violation of standards
> 
> > Some functions in the router are complex to implement because same value
> for a link local address appears on multiple interfaces.
> 
> Like what?
> 
> > It would be useful to be able to encode an abstract interface ID somewhere
> in the /64. Legacy 00 would mean unspecified...
> 
> Sounds like you need subnet-id election?
> 
> Ole
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