Hi,
I may be easily confused but I think this thread might confuse the more
robust thinker too. Maybe thinking in terms of context will help.
Within 802.15.4 in the context of the PHY layer, "link" is used to describe
a data path between two PHY entities - thus we know for, example in the
definition of Link Quality Indicator (LQI) is as Zach interprets it because
LQI is defined in the PHY specification clause. In the MAC specification
clause, "link" is used in the context of connecting MAC layer entities.
With 15.4 this is simple since the MAC "peer" is a direct neighbor, as the
MAC provides no forwarding or other multi-hop mechanisms. The term "logical
link" is used to describe the data path the PHY+MAC provides to the logical
link control layer (LLC) above the MAC.
In the 802 architecture, the OSI/RM Data Link layer maps to the LLC and the
part of the MAC; the Physical maps to the PHY and part of the MAC, and 802
scope covers only Physical and Data Link layers (IEEE 802-2001 "Overview and
Architecture" Figure 1). Some 802 standards provide MAC layer mechanisms
supporting multi-hop (forwarding, bridging, etc). The path between two LLC
layer entities may traverse multiple lower layer links. The OSI/RM is more
general but consistent that a "logical link" provides the path between two
network layer entities, which in the real world might encompass many
Physical paths. I believe this general usage of logical link is consistent
with RFC-4861 ("the layer directly below IP") but I might be wrong.
Hope that helps.
-Ben
----- Original Message -----
From: "Zach Shelby" <[email protected]>
To: "Robert Assimiti" <[email protected]>
Cc: "6lowpan" <[email protected]>
Sent: Saturday, May 02, 2009 3:29 AM
Subject: Re: [6lowpan] non-transitive links and one-interface routers
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
Office: [678]-202-6859
Mobile: [404]-578-0205
[email protected]
-----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
_______________________________________________
6lowpan mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/6lowpan
This e-mail (including any attachments to it) is confidential,
proprietary, legally privileged, subject to copyright and is sent for the
personal attention of the intended recipient only. If you have received
this e-mail in error, please reply to advise us immediately, delete it and
destroy any printed copies of it. You are notified that reading,
disclosing, copying, distributing or taking any action in reliance on the
contents of this information is strictly prohibited. No employee is
authorized to conclude any binding agreement on behalf of NIVIS LLC with
another party by e-mail without express written confirmation by an officer
of the company. Although we have taken reasonable precautions to ensure no
viruses are present in this e-mail, we cannot accept responsibility for
any loss or damage arising from the viruses in this e-mail or attachments.
_______________________________________________
6lowpan mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/6lowpan
--
http://zachshelby.org - My blog “On the Internet of Things”
Mobile: +358 40 7796297
Zach Shelby
Head of Research
Sensinode Ltd.
Kidekuja 2
88610 Vuokatti, FINLAND
This e-mail and all attached material are confidential and may contain
legally privileged information. If you are not the intended recipient,
please contact the sender and delete the e-mail from your system without
producing, distributing or retaining copies thereof.
_______________________________________________
6lowpan mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/6lowpan
_______________________________________________
6lowpan mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/6lowpan
