Subject: [Roll] constraints for temporary dag formation and those for route discovery

From: Mukul Goyal <[email-masked]>
Date: Wed, 21 Jul 2010 13:35:22 -0500 (CDT)

Hi all

Here is the text from p2p draft regarding the max rank field of the route discovery option:

Max Rank: A 7-bit unsigned integer that indicates the maximum rank
      allowed in the temporary DAG advertised by the DIO message.  This
      upper limit on the DAG rank serves as the "distance" referred to
      in Section 4 and provides a control over how far the Route
      Discovery option contained in the DODAG Configuration option in
      the DIO message may travel.  A router MUST NOT join the temporary
      DAG if its rank in the DAG will exceed this limit.  Later versions
      of this draft will map the 128 value space available in this field
      to 16-bit long limits on the DAG rank.

So, this is a 7 bit field imposing limits on 16-bit long rank value. There are two ways to use this field:

1) the 7 bit value is the actual limit on the rank in temporary DAG
2) as mentioned above, 7 bit field gives 128 values that can be used as limits on the rank. Now, we will have to specify what these limits are.

In my view, a much cleaner design will be to eliminate this field altogether. Any limits on how much the temporary DAG can grow can be imposed as metric-specific "constraints" inside the metric container. For example, we can say that temporay DAG formation must satisfy the constraint "hop-count <= 5". 

Also, note that the discovered routes automatically satisfy the constraints for temporary DAG formation.  

So, we will have two sets of constraints: first set that both temporary DAG formation and route selection must satisfy and an additional set of constraints (in a second metric container immediately floowing the route discovery option) that the route selection must additionally satisfy.

This change will also eliminate the problem JP pointed out that, in the current design, the criteria for temporary DAG formation and route discovery may be antagonistic to each other.

Comments?

Thanks
Mukul