Hi Pascal,
Sorry for the long delay in responding your mail! I am opening up a new
thread
as you suggested since the topic on retaining or dropping addresses is
really
interesting.
Thanks a lot for the detailed response for my innocuous observation on using
the reverse address for the sake of using a symmetrical path. Never realized
the security implications! Thanks for the reference to P2P RPL.
Here is my further take on the topic.
Having a symmetric path, in principle, certainly offers advantages when
we want
the communicating nodes to use a well-defined path that has been set up
to meet
the QoS requirements of an application. Such a pinned path can be
appropriately
provisioned with required bandwidth resources along the entire path.
I found the P2P RPL very appropriate for the current discussion. What is
interesting is that the Bidirectional Route presented here actually enables
creating a symmetric path between end nodes by passing on the complete path
information in its signaling messages. In this process, routing state is
installed along the path.
As you noted, keeping signaling separate from data is certainly an
elegant way.
There can however be contexts when in-band signaling that uses RH3 as per
RFC6554 data can be more efficient than using a signaling protocol, assuming
the security concerns are addressed as per RFC2460. This in-band approach is
attractive when we want to set up a rapid on-the-fly symmetric path
along with
6TiSCH OTF making bandwidth reservation along the way for transactional
message
exchanges. I am hinting at the possible PCE/NME based solution that (i)
works
with the RPL DODAG, (ii) considers the hop distance between the
communicating
node to assess energy costs, (iii) optimizes network resource
availability, and
finally provides right inputs for the nodes. It may well be that
dropping the
address is the right choice.
I think I am doing too much of hand waving going on by leaving out all the
essential details. Hope I managed to vaguely convey the point.
I tend to feel that distributed, centralized and their combination might
co-exist to optimize network resources, and therefore retaining or
dropping the
address in RH3-6LoRH can be made optional.
Am I making sense ?
Anand
On Saturday 23 January 2016 02:34 PM, Pascal Thubert (pthubert) wrote:
> Hello Anand: > > > > 2 good points , that we need to continue in
different threads if we do. > > > The source routing optimization done
by dropping the addresses on the way > certainly has benefits. However,
there could be loss of a natural "symmetric" > property that one might
want to enforce between communicating end nodes in the > routing path.
By symmetry I mean using the same path in both the directions of >
communication. Policy based routing, centralized routing, for instance,
could > be potential users of this property. May be this does not
represent a common > use case. But nevertheless, we have to be aware of
this side effect which RFC6554 > swapping process does not have. > > > >
Ø Pascal: Yes, Simon made that point yesterday. > > Ø It is generally
not a good idea to reverse a routing header. The RH may have been used
to stay away from the shortest path for some reason that is only valid
on the way in (segment routing). > > Ø P2P RPL reverses a path that is
learnt reactively, so we have a real protocol for doing that sort of
thing as opposed to an echo. > > Ø Reversing a header is discouraged by
RFC 2460 (for RH0) unless it is authenticated (which means AH). We do
not authenticate the RH3, there are a number of reasons for that,
general deprecation of AH, no use of AH in LLNs etc& Note that AH does
not protect the RH on the way, it is just a validation at the receiver
for the sole value of reversing it. > > Ø A RPL domain is usually
protected by L2 security and that secures both RPL itself and the RH in
packets, at every hop > > Ø The benefit of saving energy and lowering
the chances of loss are seen as overwhelming compared to the value of
possibly reversing the header > > Ø Yet we might define a variation
where we do not pop out the first entry as we go. I do not see a
consensus on the value of doing it now. > > > > > > Going further, the
DAO projection proposal > (draft-thubert-roll-dao-projection-02.txt)
will have several virtual roots > inside the RPL domain. The automatic
assumption of a well known root may not apply > when nodes within RPL
domain communicate with each other. I suppose it will > have a bearing
on the RH3-6LoRH performance. > > > > Ø It is not really an assumption,
but something we leverage as we go. The RPI is very much like a context
indicator. If an address shares a lon prefix with a root, adding the RPI
of that root is actually a compression technique. > > > > The above
observations are not serious, but feels good to ponder over. Will be >
happy to receive your comments. > > Thanks Anand! > > > > Pascal > > > >
> > On Monday 18 January 2016 11:24 PM, Pascal Thubert (pthubert)
wrote: > > > > > Dear all > > > > > > > > > > > > The picture below
illustrates how the RH3 6LoRH works with draft 03 in a case like Root ->
A -> B -> C -> leaf > > > > > > > > > > > > > > > > > > The first 6LoRH
is expected to be a full address (128 bits) to set up a reference and
the next 6LoRH are expected to be smaller and just override the
rightmost bits which form the delta from the reference. > > > > > > > >
> > > > Proposal: we could consider that the 128bits source of the IP
header before the RH3 is the reference to start with. > > > > > > > > >
> > > With that even the first hop could be compressed the same way as
the other hops. With RPL, the root is the encapsulator if IP in IP in
used. Good thing, in that case the root is totally elided with the
IP-in-IP 6LoRH. > > > > > > > > > > > > So this simple proposal saves up
to 16 octets (thats in the case with a single subnet and all addresses
differ only by the last 2 bytes). Im willing to add it in the next
revision. > > > > > > > > > > > > Any opposition? > > > > > > > > > > >
> Pascal > > > > > > > > > > > > -- > > > This message has been scanned
for viruses and > > > dangerous content by MailScanner, and is > > >
believed to be clean. > > > > > >
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