Zach, I'm wondering if the term 'non-reflexive' in RFC4861 should
have been 'non-symmetric'. Mathematically, an equivalence
relationship is defined as being reflexive, symmetric and
transitive. Symmetric is the property which is untrue on a
unidirectional link not reflexive. Apologies if this is nonsense,
I'm new to this list. Thanks Peter
-----Original Message----- From: [email protected]
[mailto:[email protected]] On Behalf Of Zach Shelby Sent:
2009 May 12 10:38 To: 6lowpan Subject: [6lowpan] Terminology
Hi,
I am working on an updated terminology set for the nd draft. The
terminology is now split with 6LoWPAN general terminology now in
its own section. I would like comments on this updated set (below)
where I have tried to find a solution based on the constructive
discussions we had on the list.
After looking through all the background RFCs in detail, it
actually turns out this is not as hard as we thought. RFC4861
actually does cover the wireless case as it defines assymetric
properties of wireless links including non-transitivity (see
Section 2.2). In fact RFC4861 actually mentions that the protocol
(ND) will presumably be extended in the future to deal with links
that are assymetric (non-reflexive, non-transitive). That is what
we are doing with ND for 6LoWPAN!
Therefore I have now defined link as being non-transitive and
complex NBMA, which can be somewhat overcome using link-layer mesh
techniques or by with IP routing. This greatly simplifies the
definition of a subnet (whew!), as we keep the RFC4291 where subnet
<= link. As we are performing IP routing to overcome the
non-transitive nature, the subnet does exhibit one aspect of
multi-link subnet mentioned in RFC4903.
IP routing has been defined as Alex recommended as it has specific
properties to 6LoWPAN. In the architecture section of nd-03 we will
include LoWPAN IP routing examples including topology and what is
in the table.
General 6LoWPAN Terminology:
This section defines additional general terms related to the
6LoWPAN architecture used in this specification:
IP Routing
The forwarding of datagrams at the IP layer between arbitrary
source-destination pairs, during which the hop limit is
decremented. In the LoWPAN context, IP routing is performed by
LoWPAN Routers on a single interface within the same link to
overcome the non-transient nature of the link. Exact match search
is performed on the dst address of the IP packet to find the next-
hop to the destination. Referred to as routing in this document.
Link
The link is a communication facility or medium over which nodes can
communicate at the link-layer, i.e., the layer directly below IP
([RFC4861]). 6LoWPAN assumes the use of low-power and lossy
wireless links such as IEEE 802.15.4, which is a special type of
link as described in [RFC4861] exhibiting severe assymetric
reachability with both non-reflexive and non-transitive qualities.
Furthermore complex Non-broadcast Multi-Access (NBMA) behaviour is
exhibited as these links do not support native multicast, and
broadcast reaches only a subset of nodes on the link. The use of
link-layer mesh technology (see Mesh Under) emulates transitivity
across the link but still has problems with non-reflexitivity.
Multicast on a link-layer mesh is usually implemented as a
broadcast flood.
Link-local
Standard IPv6 link-local scope as defined in [RFC4291] and
[RFC4861] is supported by the 6LoWPAN link and subnet model.
Link-local scope is achieved by setting the hop limit to 1, using
link-local prefix or link-local multicast scope. If a link is
non-transient then link-local scope includes only a subset of nodes
on the link (the set of nodes within assymetric radio range of a
node). Nodes in the link-local scope of a node are its neighbors,
and this link-local scope may be different for each node on a link.
LoWPAN Host
A node that only sources or sinks IPv6 datagrams. Referred to as a
host in this document.
LoWPAN Node
A node that composes a LoWPAN and is used to refer to both hosts
and routers. Referred to as a node in this document.
LoWPAN Router
A node that forwards datagrams between arbitrary source-
destination pairs using a single 6LoWPAN interface performing IP
routing on that interface.
Mesh Under
A term referring to a configuration where the link-local scope is
defined by the boundaries of the LoWPAN and includes all the
6LoWPAN interfaces within it. Forwarding and multihop routing
functions are achieved at the link layer. In this configuration
the link may still exhibit assymetric behaviour.
Route Over
A term referring to a configuration where the link is non-
transient and the link-local scope reaches only a subset of the
LoWPAN nodes. IP routing is performed by LoWPAN Routers to
overcome to the non-transient nature of the link. This
configuration may consist of both routers nad hosts.
Subnet
A subnet is the collection of interfaces having the same IPv6
subnet prefix on a link, as defined in [RFC4291]. A LoWPAN is made
up of the interfaces of LoWPAN Nodes and Edge Routers sharing the
same subnet prefix. Due to the non-transient nature of 6LoWPAN
links, IP routing may be used on the link to provide transitivity.
This exhibits a multi-link subnet feature with regard to hop limit
as defined in [RFC4903], and thus 6LoWPAN applications should make
no assumptions about the hop limit as it may be decremented in a
LoWPAN.
-- http://www.sensinode.com 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
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