Re: [DNA] Issue about multiply connected to the same link and DNAbeing per-interface or not (was Re: WG LC comments ondraft-ietf-dna-goals-01.txt)

[Erik Nordmark <erik.nordmark@sun.com>]

Pekka Nikander wrote:

>>> For example, if a host has earlier been connected twice to the
>>> same link through two different interfaces, and then it detects
>>> that one of the interfaces has undergone a link change, IMHO that
>>> is a strong indication that there may have been a link change
>>> at the other interface, too.

> The specific example, though simplistic, that I've had in my mind
> is my home setting with my laptop.  I have a bridged Ethernet and
> WLAN.  When my laptop notices that I've disconnected it from the
> Ethernet, I may have just moving it around in our house, and it
> is still connected to the same link through WLAN.  On the other
> hand, I may have suspended it and moved it e.g. to the office,
> in which case it is now attached to a different WLAN.

Yes, but your neighborhood ISP might provide metro Ethernet including
your house and your office. Taking GPRS and 802.11 as the interfaces 
illustrates this better - detecting attachment to a new link on one of
them doesn't say much about the other link.

In your example the more likely hint is that the laptop has been 
suspeded for some time, hence reverifying things makes a lot of sense.

  > Another example that I could imagine is a large operator network
> that from an IPv6 point of view acts as a single, large "LAN"
> or NBMA link.  (I know one operator that has plans like that.)
> The operator might offer different radio access technologies to
> access that "large LAN".  When one radio interface undergoes a
> change, it may make sense to quickly verify whether the other
> interfaces still are connected to the "large LAN".

But I still don't see why there would be a correlation since the 
technologies and operators might be different.

> In general, if two interfaces are connected to the same subnet,
> and one of the undergoes a link change, it often makes sense to
> check if the other one has also went through a link change.

I still don't understanding which assumption you are making.

A counter example would be that I have wired Ethernet and 802.11 in the 
house. Just because I unplug from the wire to walk around, or untangle 
some other wires on the floor in my office, doesn't mean I'm likely
to have changed the 802.11 AP. In any case, if I change the 802.11 AP
the device driver will associate with another AP hence can provide a 
hint to DNA.

> I was mostly objecting the notion that we should keep DNA on
> separate interfaces _completely_ independent.  To me, it looks
> like that there are cases when they are not.  I am oblivious
> how to handle this, through link hints, relaxing the language
> in the goals draft, or whatever.

Yes, at least across interfaces with compatible technology it makes 
sense to share retained information. Imagine a host which remembers DNA 
info (prefixes, default routers) and binds them with some L2 key (the 
MAC address of the Ethernet bridge or AP).
If the host has multiple Ethernets interfaces (or multiple 802.11 
interfaces) it should be able to find this cached information and make 
use of it; we shouldn't require that the same Ethernet wire be plugged 
in as the last time.

    Erik