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
