Re: [Roll] New Version Notification - draft-ietf-roll-minrank-hysteresis-of-07.txt

[Matteo Paris <[e-mail]>]

Hi Phil,

my question boils down to whether RPL requires members of the parent set to have lower DAGRank, or merely lower rank.  My reading of the RPL draft is that all parents must have lower DAGRank.

I get this from two clauses in the RPL draft that I quoted below.  Unfortunately both are confusingly worded.

RPL Section 3.5.1 says: "A node A has a rank less than the rank of a node B if DAGRank(A) is less than DAGRank(B)."  I believe the intention here was to say "if and only if", otherwise the clause is pointless (trivially true).

RPL Section 3.5.2 says "[F]or a node N, all parents in the DODAG parent set must be of rank less than DAGRank(N)."  Here, a rank is being compared with a DAGRank, which is nonsensical, unless "rank" actually means "DAGRank".  It would be much clearer if it said so explicitly, ie "must be of DAGRank less than DAGRank(N)."  

What is your interpretation -- are parents required to have lower rank or lower DAGRank? Perhaps the confusion is a result of some text accidentally left over from an older version of the draft.

Thanks,
Matteo

PS My emails have not been posting to the list. I'm working on it.

On Mar 16, 2012, at 5:29 PM, Philip Levis wrote:

> 
> On Mar 16, 2012, at 1:44 PM, Matteo Paris wrote:
> 
>> 
>> Hi Phil.  These changes were helpful.  But I am still unclear on the rank computation in section 3.3.  From the RPL draft:
>> 
>> RPL Section 3.5.1:  "DAGRank(rank) = floor(rank/MinHopRankIncrease)"
>> RPL Section 3.5.1:  "A node A has a rank less than the rank of a node B if DAGRank(A) is less than DAGRank(B)."
>> RPL Section 3.5.2:  "[F]or a node N, all parents in the DODAG parent set must be of rank less than DAGRank(N)."
>> RPL Section 8.2.1:  "A node's rank MUST be greater than all elements of its DODAG parent set."
>> 
>> The rank computation in MRHOF section 3.3 can result in parents P for which DAGRank(P) is not less than DAGRank(N),
>> which would seem to violate the third and fourth clauses above, given the definitions in the first two clauses.
> 
> How? I intended case 2 in 3.3 to cover this. Can you explain the edge case you see where it does not?
> 
>> 
>> One other minor question.  In the sentence, "The second case covers requirement 4..." of MRHOF section 3.3, do you mean requirement 5?
> 
> Yes! It should be requirement 5. I will fix this in the next revision. Thank you for catching it.
> 
> Phil