Subject: [6lowpan] one-interface routers and links (again)

From: Alexandru Petrescu <[email-masked]>
Date: Wed, 29 Apr 2009 18:43:27 +0200

one-interface routers and links (again)

Let me first describe one-interface routers as I understand are proposed 
in 6LoWPAN:

        +------------------------+---------------+
        |                        |               |
    2001:db8:1::1/128 2001:db8:1::2/128 2001:db8:1::3/128
       _|eth0                   _|eth0          _|eth0
      |R1 |                    |R2 |           |R3 |
       ---                      ---             ---

R1 sends an IP packet to R3 but this reaches only R2.  R2 picks the 
packet, looks at the dst address, finds it's not for self, consults 
routing table, finds a host-based route and sends it to R3.  This can 
work ok.

Now let me picture the same thing, but with The Link:

                            The Link
        +------------------------+---------------+
        |                        |               |
    2001:db8:1::1/128 2001:db8:1::2/128 2001:db8:1::3/128
       _|eth0                   _|eth0          _|eth0
      |R1 |                    |R2 |           |R3 |
       ---                      ---             ---

If there is a such link which links together all nodes then R1 would 
communicate directly to R3, by definition, no need to IP route.  Could 
you please clarify why do IP routing then?

And picture the same with 2 links:
               link1                  link2
        +------------------------+---------------+
        |                        |               |
    2001:db8:1::1/128 2001:db8:1::2/128 2001:db8:1::3/128
       _|eth0                   _|eth0          _|eth0
      |R1 |                    |R2 |           |R3 |
       ---                      ---             ---

But in this figure R2 would need to have two MAC addresses on the same 
interface, because it is in two different links.  I'm not aware of such 
interfaces with two MAC addresses.  Could you please clarify?

Alex

---

From: Zach Shelby <[email-masked]>
Date: Wed, 29 Apr 2009 22:51:18 +0300

Alex,

> Let me first describe one-interface routers as I understand are proposed 

Exactly, you pictured this nicely. This is how LoWPAN Routing works. 
I'll get back to the definition of that in your other thread.

> Now let me picture the same thing, but with The Link:

Then obviously, you don't need to perform LoWPAN Routing. This kind of 
(transitive) link occurs when all nodes are in radio range of 
each-other, or if the link technology is forming some kind of mesh 
underneath IP.

In the ND draft this is really all that "mesh-under" means. It means we 
don't need to do LoWPAN Routing, nor do we even need nodes playing a 
Router roll.

> And picture the same with 2 links:

This doesn't exist if links are defined as you have.

> Alex
> 
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Zach Shelby
Head of Research
Sensinode Ltd.
Kidekuja 2
88610 Vuokatti, FINLAND

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---

From: Alexandru Petrescu <[email-masked]>
Date: Wed, 29 Apr 2009 23:12:08 +0200

Zach Shelby a écrit :
>> Let me first describe one-interface routers as I understand are 
>> proposed in 6LoWPAN:
>>

Ok...

If we say the dashed line is The Link then we're in the ND case, any 
node can talk to any other node at link-layer, no IP routing throughout.

But if it is not The Link, and it is not two times The Link - then what 
is it?

What is the link definition needing a LoWPAN single-interface IP router?

Alex

---

From: Zach Shelby <[email-masked]>
Date: Thu, 30 Apr 2009 09:26:07 +0300

Hi,

> What is the link definition needing a LoWPAN single-interface IP router?

The definition used in the other thread for a LoWPAN link, which in a 
wireless network may be non-transient, I think covers this case. Because 
the link is non-transient R1 and R2 can communicate, R2 and R3 can 
communicate, but R1 and R3 can't. So for non-transient wireless links 
you need LoWPAN routing.

Let's get back to this in the terminology thread.

- Zach

Zach Shelby
Head of Research
Sensinode Ltd.
Kidekuja 2
88610 Vuokatti, FINLAND

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Thu, 30 Apr 2009 15:33:48 +0200

>> Let me first describe one-interface routers as I understand are
proposed
>I'll get back to the definition of that in your other thread.

Note that over the Ethernet backbone shown here, if R1 and R3 are edge
routers then R1 will reach R3 directly. This will happen because R1
still has a connected route 2001:db8:1::/64 over the backbone, and it
will use RFC 4861 to locate R3 there. The binding table will be
consulted first but that will fail because R3 is not a 6LoWPAN node
registered to R1. Very much like a Home Agent operation on a Home link
if you wish.

Pascal

---

From: Alexandru Petrescu <[email-masked]>
Date: Fri, 01 May 2009 02:17:05 +0200

Pascal Thubert (pthubert) a écrit :
>>> +------------------------+---------------+ |
>>> |               | 2001:db8:1::1/128 2001:db8:1::2/128
>>> 2001:db8:1::3/128 _|eth0                   _|eth0          _|eth0
>>>  |R1 |                    |R2 |           |R3 | ---
>>> ---             ---
>>> 
>>> R1 sends an IP packet to R3 but this reaches only R2.  R2 picks
>>> the packet, looks at the dst address, finds it's not for self,
>>> consults routing table, finds a host-based route and sends it to
>>> R3.  This can work ok.
>> Exactly, you pictured this nicely. This is how LoWPAN Routing
>> works. I'll get back to the definition of that in your other
>> thread.
> 
> Note that over the Ethernet backbone shown here, if R1 and R3 are
> edge routers then R1 will reach R3 directly.

The dashed lines are actually the wireless link(s) not the backbone 
wired link...

> This will happen because R1 still has a connected route
> 2001:db8:1::/64 over the backbone, and it will use RFC 4861 to locate
> R3 there. The binding table will be consulted first but that will
> fail because R3 is not a 6LoWPAN node registered to R1. Very much
> like a Home Agent operation on a Home link if you wish.

Seems as a distinguishing point which should be mentioned in the Edge 
Router discussion... if I udnerstand it correctly.

Alex

---

From: Alexandru Petrescu <[email-masked]>
Date: Fri, 01 May 2009 02:31:09 +0200

Zach Shelby a écrit :
>>>> 2001:db8:1::3/128 _|eth0                   _|eth0
>>>> _|eth0 |R1 |                    |R2 |           |R3 | ---
>>>> ---             ---
>>>> 
>>>> consults routing table, finds a host-based route and sends it
>>>> to R3.  This can work ok.
>>> 
>> If we say the dashed line is The Link then we're in the ND case,
>> any node can talk to any other node at link-layer, no IP routing
>> throughout.
>> 
>> But if it is not The Link, and it is not two times The Link - then
>>  what is it?
>> 
>  wireless network may be non-transient, I think covers this case.

The definition in the thread, inheriting from rfc4861 and other rfcs, is
_not_ a non-transitive link.  That link is clearly defined as linking
all nodes in the medium: all nodes communicate at link-layer, 
one-by-one, any node to any node:

>    link       -  a communication facility or medium over which
>                  nodes can communicate at the link layer, i.e.,
>                  the layer immediately below IP (each node can
>                  communicate to each other in this medium).
> 
>                  Examples are Ethernets (simple or bridged), PPP
>                  links, X.25, Frame Relay, wireless links or ATM
>                  networks as well as Internet-layer (or
>                  higher-layer) "tunnels", such as tunnels over
>                  IPv4 or IPv6 itself.
> 
>                  This is a slightly modified definition of the link
>                  defined in RFC4861, in order to cover also the wireless
>                  links.  Wireless links may be non-transitive (node A
>                  communicates at link layer to both B and C yet B and C
>                  are not on the same link).  Hidden terminal problem in
>                  wireless communications is described in [reference to
>                  individual draft in AUTOCONF]
>                  draft-baccelli-multi-hop-wireless-communication-02 

This says that a wireless link is also a link.  The fact that "wireless 
link_s_ may be non-transitive" is alleviated by the fact that "link - 
... each node can communicate to each other in this medium".

Maybe we shouldn't use "Wireless links" above but "Wireless media" 
because "link" is the term being defined.

> Because the link is non-transient R1 and R2 can communicate, R2 and
> R3 can communicate, but R1 and R3 can't.

A 'non-transitive' link is different from the Link definition we 
mentioned above, because nodes on that link can all communicate to each 
other at link-layer.  That Link is not non-transitive, it is transitive.

How would one define a 'non-transitive' link?  As two serially connected 
Links?  This implies the middle router has two interfaces - which we 
don't want.

So, what is the definition of a 'non-transitive' Link?

(I'm not sure how to explain this better, but we don't seem to agree).

Alex