Robert,
I agree 100%, and this is the approach we have taken already for a long
time in the working group. I propose we define this as a "Wireless
Link", which has a different definition rfc4861 link. The link
definition in the current drafts did need improvement, and we should put
it in perspective of the current rfc4861 link assumptions.
These architectural and terminology definitions need to go into their
own "6LoWPAN Architecture" draft ASAP. This way there is a single place
to work on these... they are currently in the ND draft for lack of a
better place.
- Zach
Robert Assimiti wrote:
Hello Alex,
Thanks for the very constructive discussion.
It seems that we are trying to reconcile legacy IETF link definitions with the
realities of the wireless media.
Well, I am afraid that we will need a new definition.
Looking at the definitions of a link in most wireless standards (802.15.4,
Zigbee, ISA100.11a, etc) you will find that a link in the wireless context is
quite different that a link over wired media.
In you depiction of the three nodes, R1 communicates with R3 over 2 wireless
links (or at least what is regarded as a link in most wireless standards).
A wireless link is characterized typically by:
1. Direct connectivity between two wireless nodes
2. Characterized by various quality indicators
3. Uni-directional (R1 -> R2 is one link, and R2 -> R1 is 1 link). Uni-directionality
is typically imposed by inherent variance of quality indicators in the wireless world. Just
because R1 -> R2 have a certain quality indicator, it does not mean that R2 ->R1 will
be characterized by the same indicator.
When it comes to the definition of a link, a one-glove-fits-all approach cannot
reconcile the differences (physics) between wired and wireless medias.
"The nice thing about standards is that there are so many to choose from." - Andrew S. Tanenbaum
Robert Assimiti
Executive Staff Engineer
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-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Alexandru Petrescu
Sent: Thursday, April 30, 2009 8:31 PM
To: Zach Shelby
Cc: 6lowpan
Subject: Re: [6lowpan] non-transitive links and one-interface routers
Zach Shelby a écrit :
Hi,
Alexandru Petrescu wrote:
Zach Shelby a écrit :
Alex,
Alexandru Petrescu wrote:
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.
Exactly, you pictured this nicely. This is how LoWPAN Routing
works. I'll get back to the definition of that in your other
thread.
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?
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.
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
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