Subject: Re: [Roll] Review of AODV-RPL

From: Charlie Perkins <[email-masked]>
Date: Mon, 26 Feb 2018 16:59:51 -0800

Hello Rahul,

Follow-up below to your comments...

On 1/5/2018 3:42 AM, Rahul Jadhav wrote:
>
> [Remy] I think we have a misunderstanding here. “Bidirectional” talks 
> about the reachability, while “symmetric” talks about the upward and 
> downward routes. The “asymmetric” here signifies the upward and 
> downward routes between the OrigNode and the TargNode are different, 
> while "bidirectional" means control messages can be received in two 
> directions. Both RPL and AODV-RPL use bidirectional links.
>
>
> [RJ] I m not very clear of this text. AODV-RPL cannot depend on 
> bidirectional links to establish asymmetric paths.

We state in the draft that the protocol is applicable to networks that 
have bidirectional but asymmetric links.  We can put in black hole 
detection if that is preferred.  It is well-known how to do this, and 
AODV does do it, but the cost is a small bit of overhead and stateful 
operation.

>
>
>     [Remy] The upstream destination is the OrigNode, which is the root
>     of the DODAG. An intermediate node only needs to maintain a route
>     entry to the OrigNode, including the information of the next hop
>     on the upstream route. The next hop is actually the node’s
>     preferred parent in this DODAG.
>
>
> [RJ] I understand that from the routing table perspective only the 
> OrigNode entry has to be established. My point was in context to 
> identifying/probing adjoining next hop through which this route would 
> be established. For this probing you need to maintain a table which 
> will have all adjoining peers.

I think the protocol will work even without maintaining such a neighbor 
table.  Even when the table is maintained, it is always possible for the 
information in it to grow stale.

>
>     [Remy] The node doesn’t have to probe all its neighbor. Don’t
>     forget that AODV-RPL is an extension of RPL, and the RREP and RREQ
>     are two new DIO options. Thus the DIO transmission, local repair
>     and many other mechanisms to maintain the state are the same as
>     RPL.  Therefore, the control overhead will stay the same.
>
>
> [RJ] I understand that the DIO transmission, and other factors remain 
> the same. Adding RREP/RREQ on top of existing DIO messaging does not 
> achieve asymmetric paths. When OrigNode sends a DIO, its received by 
> another node downstream and the path is established upstream. This is 
> the mechanism RPL follows. Also AODV-RPL follows the same signalling 
> mechanism and this is where the problem starts with the asymmetric 
> path claim.

But, AODV-RPL *does* allow establishment of asymmetric peer-to-peer 
paths, as advertised.  I agree that the signaling mechanism is not 
radically different, and I consider that a good thing.  But it is different.

>
> [RJ]: No. Probing cannot be done the same way. In case of RPL, DIO is 
> sent downstream and the downstream nodes use DIO metrics and messaging 
> signal qualities to establish upstream path. This is ok in case of RPL 
> because RPL says that it depends upon bidirectional connectivity (not 
> necessarily symmetric but still bidirectional for e.g. power levels in 
> both direction might vary but the messaging still works) for it to work.
> Now for AODV-RPL's asymmetric claim to work, the upstream and 
> downstream paths between the same pair of nodes can be different. For 
> asymmetric links, AODV-RPL cannot depend upon RPL's style of DIO 
> messaging.

Can you be more specific about where the AODV-RPL mechanism fails? Maybe 
an example would be helpful.

Regards,
Charlie P.