Re: [Roll] [6tisch] Proposed improvement in RH3-6LoRH : To drop or not to

["S.V.R.Anand" <[e-mail]>]

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. > > > > > > 
_______________________________________________ > > > 6tisch mailing 
list > > > [e-mail] > > > 
https://www.ietf.org/mailman/listinfo/6tisch > > > > -- > This message 
has been scanned for viruses and > dangerous content by MailScanner, and 
is > believed to be clean. > > > 
_______________________________________________ > 6tisch mailing list > 
[e-mail] > https://www.ietf.org/mailman/listinfo/6tisch