Dominik,
I would prefer using the RFC4861 terminology exactly, here is the text
from Section 2.2 page 9:
asymmetric reachability
- a link where non-reflexive and/or non-transitive
reachability is part of normal operation. (Non-
reflexive reachability means packets from A reach B,
but packets from B don't reach A. Non-transitive
reachability means packets from A reach B, and
packets from B reach C, but packets from A don't
reach C.) Many radio links exhibit these
properties.
Here is an updated link definition including examples (in brackets):
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 asymmetric
reachability with both non-reflexive (A can reach B, but B can't
reach A) and non-transitive (A can reach B, and B can reach C, but
A can't reach C) 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. Such a wireless link may consist of multiple
overlapping link-local scopes.
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.
- Zach
Dominik Kaspar wrote:
Hi,
Would 'non-reflexive' (better called 'irreflexive') reachability mean
that a node can not reach itself? If so... I think the term still
makes sense, because 6LoWPAN nodes usually cannot send and receive at
the same time (except if they had two physical radio interfaces).
By the way, typo: assymetric -> asymmetric.
Greetings,
Dominik
On Tue, May 12, 2009 at 11:53 AM, Burnett, Peter
<[email protected]> wrote:
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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--
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
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.
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