Subject: Re: [Roll] A simple loop avoidance mechanism for use in P2P DV routing not using DAGs

From: Mukul Goyal <[email-masked]>
Date: Thu, 16 Jul 2009 21:00:46 -0500 (CDT)

Yes. While the node is in the "loop avoidance" state, it does not change its next hop(s). So, the fate of the packets it forwards during this transient state indeed depends on the quality of its current next hop(s).

I guess the performance under the transient states may not be good under RPL as well when a node detaches from a DAG and is waiting to reattach at a higher depth or when it wants to jump to a different DAG. Further, reattachment of a node to a DAG may potentially take a long time since the root needs to generate a new sequence number which need to percolate all the way to the waiting node before it can rejoin the DAG. 

Thanks
Mukul

----- Original Message -----
From: "Philip Levis" <[email-masked]>
To: "Mukul Goyal" <[email-masked]>
Cc: "Omprakash Gnawali" <[email-masked]>, "roll" <[email-masked]>
Sent: Thursday, July 16, 2009 8:17:36 PM GMT -06:00 US/Canada Central
Subject: Re: [Roll] A simple loop avoidance mechanism for use in P2P DV routing not using DAGs

On Jul 16, 2009, at 6:13 PM, Mukul Goyal wrote:

> Om,
>
> This proposal does not "guarantee" loop-free routing. It is a loop  
> "avoidance" strategy rather than a loop "prevention" mechanism  
> (RPL's DAG seems to me as a loop "prevention" mechanism). There is  
> an obvious dependence on the advertisements not getting lost. This  
> dependence can be lessened by allowing a node to repeat its  
> _marked_ advertisement during its sojourn in the "loop avoidance"  
> state.
>
> Thanks
> Mukul

I think Om's question is more about what happens to all of the data  
packets a node is asked to forward while it "waits for some time."  
Note that these devices have very limited queue sizes. In this  
scheme, any time a node wants to choose a new parent, it will  
possibly drop a burst of dropped packets as well as cause a big jump  
in RTT. Both of these pose issues for TCP.

Phil