Re: [6lowpan] Whiteboards

["Colin O'Flynn" <[e-mail]>]

Hello,

> What would it be used for and how?

The initial version of 6lowpan-nd added this idea in. See
http://tools.ietf.org/html/draft-hui-6lowpan-nd-00 

> You mean the "EUI-64 Identifier"?  (it's not an address, 
> it's an
> identifier).  Is there such a thing as a 64bit MAC 

The 802.15.4 Address (aka: MAC address) is guaranteed to be unique by IEEE
802.15.4-2006, at least everywhere within the PAN. 

Nodes can use either 64-bit or 16-bit addressing. When nodes power up they
are preprogrammed with a 64-bit IEEE MAC address they use to communicate.
They then may be assigned a shorter 16-bit address, which they can use to
save some bytes. However the 64-bit address still remains valid, and the
node can be addresses by it.

Nor do you need to do this 16-bit addressing. You can keep just using 64-bit
addressing and skip assigning 16-bit addresses. 

The idea is almost very node in the world could be programmed with a
different 64-bit MAC address. Hence you could bring two nodes together and
be sure there was no address collision. But with 16-bit MAC addressing you
are much more likely to get a collision, hence why the 16-bit addresses are
assigned dynamically by the network. The network assures no two nodes are
assigned the same 16-bit address.

> Sounds as forming an IPv6 link-local address in order to 
> do DHCP dance
> prior to obtain a global IPv6 address.  

Basically yes! The node has to do this anyway, as a node using 6LoWPAN will
bring up minimum of two valid unicast addresses:

Link-local based on 64-bit 802.15.4 Address
Global based on 64-bit 802.15.4 Address

If the node can use short addressing, you add two more:

Link-local based on 16-bit 802.15.4 Address
Global based on 16-bit 802.15.4 Address

>Sorry can't follow this.  Which prefix?  Which PAN-ID?

I mean when the node joins a network, it knows that network's PAN-ID. Let's
say it's running on 0xBAAD. From the RS/RA it also knows the prefix, or
maybe even DHCP tells the node. Say it's dead:beef:cafe:0::/64 . Hence if
the node is assigned the following address:

Dead:Beef:cafe:0:b8ad:00ff:fe00:0005

The node can derive that that address becomes 6LoWPAN compressible if it has
the 802.15.4 short address '5'. 

> Do IP nodes care whether their MAC address (the short MAC > address
> included) is unique?  

Well they have to be, as specified by IEEE 802.15.4. How you decide they are
unique is part of the discussion. Sure you could run a full 802.15.4 MAC,
followed by a full 6LoWPAN layer, followed by a full IPv6 stack. It would
work and make a nice block diagram for a paper.

But you'd be wasting tons of code space and transmission time.

>6LoWPAN deal with - is it IP?  Is it MAC?

Both isn't it? I mean I think the whole point is that you need to 'break'
the nice clean divisions between layers when it comes to constrained
devices. After all:
"Transmission of IPv6 Packets over IEEE 802.15.4 Networks"

Regards,

 -Colin


-----Original Message-----
From: Alexandru Petrescu [mailto:[e-mail]] 
Sent: December 17, 2009 6:41 PM
To: Colin O'Flynn
Cc: 'Alexandru Petrescu'; [e-mail]; [e-mail]
Subject: Re: [6lowpan] Whiteboards

Colin O'Flynn a écrit :
> Hello,
> 
>> Excuse me, sorry, but are the other non-16bit addresses (48bit)
>> ever assigned by DHCP?
> 
> There's at least two options I know of for this:
> 
> #1 Add another 6lowpan-nd specific extension to DHCPv6 to add a
> simple link-layer address option

Hmm... makes some sense, what would it look like? (rfc4944 defines such
an option for ND).

What would it be used for and how?

> #2 Nodes could assign their own short addresses. Nodes power on and 
> autoconfigure a IPv6 address based on the 64-bit MAC address.

You mean the "EUI-64 Identifier"?  (it's not an address, it's an
identifier).  Is there such a thing as a 64bit MAC address?

> With this they then do the DHCP dance, and get another address.

Sounds as forming an IPv6 link-local address in order to do DHCP dance
prior to obtain a global IPv6 address.  (yes, in IPv4 one could say that
the node uses its MAC address while doing the DHCP initial dance and the
unspecified IPv4 address 0.0.0.0, but in IPv6 the unspecified addresses
(search " :: " in rfc3315) are not used DHCPv6 in the initial DHCPv6
dance, but the link-local addresses are).

Forming an IPv6 link-local address based on the 16bit short MAC address
- ok, using on RFC4944.  And then do DAD on this IPv6 link-local
address, because the EUI-64 identifier based on the 16bit short MAC
address is not guaranteed global, has that g bit unset.

> Nodes check if this address can be used to assign their own short
> address.

Yes, usually they do DAD to make sure their IPv6 link-local address is
unique within that link.

But don't rely on the IP address uniqueness to conclude that the MAC
address corresponding to that IP address is unique.

> Aka: the IPv6 address it assigns matches the prefix, the PAN-ID, etc.
> 
Sorry can't follow this.  Which prefix?  Which PAN-ID?

> Thus nodes 'figure out' if a certain short address will make their
> IPv6 address compressible.

Do IP nodes care whether their MAC address (the short MAC address
included) is unique?  I don't think so.  I think IP nodes care only
whether their IP address is unique.

Also, it is completely unclear to me what kind of layer does 6LoWPAN
deal with - is it IP?  Is it MAC?

Alex


> 
> Regards,
> 
> -Colin
> 
> 
> 
> -----Original Message----- From: [e-mail]
> [mailto:[e-mail]] On Behalf Of Alexandru Petrescu 
> Sent: December 16, 2009 9:14 PM To: [e-mail] Cc:
> [e-mail] Subject: Re: [6lowpan] Whiteboards
> 
> Daniel Gavelle a écrit :
>> I agree with the recent proposal to remove the mandatory
>> requirement for a whiteboard and duplicate address detection.
>> 
>> However, 16 bit 802.15.4 addresses are a very useful optimisation.
>>  Assigning these in a standard way is important in the absence of a
>>  whiteboard.  One option may be to use DHCPv6.  However, the DHCPv6
>>  packet sizes are quite large and so some sort of DHCPv6 message 
>> compression would be useful. Extended LowPANs would also be useful
>> in some applications.
>> 
>> If the whiteboard and DAD are removed, I would like the issues of
>> 16 bit address assignment and extended LowPANs
> 
> Excuse me, sorry, but are the other non-16bit addresses (48bit) ever
>  assigned by DHCP?
> 
> I doubt IETF could spec a means to assign MAC addresses...
> 
> Thanks,
> 
> Alex
> 
> 
> to still be addressed by an RFC
>> within the IETF 6LowPAN group, rather than having several different
>> non interoperable implementations.
>> 
>> 
> 
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