Ben,
I understand that a link at the PHY is defined a limited to a single
hop between two direct neighbors.
If the DLL / LLC does not provide any forwarding capability then a link
would seem to be the same - a single hop between two direct neighbors.
What is a link if the DLL or LLC provides a multi-hop forwarding
capability?
geoff
If the On Sat, 2009-05-02 at 09:55 -0700, Benjamin A. Rolfe wrote:
> 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
>
_______________________________________________
6lowpan mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/6lowpan
