Re: [Roll] Border router failure detection

[Konrad Iwanicki <[e-mail]>]

Dear all,

The RNFD document expired. I reposted it.

In addition, below I summarize the questions and answers from the last 
interim regarding the protocol. Hopefully, this will allow for 
progressing with the document.

1. What happens when Sentinels (the root's one-hop neighbors that 
monitor its state) don't hear each other? Does the algorithm detect the 
crash of the root?

Yes, the decision is made not only by Sentinels but all nodes. There is 
no requirement for the Sentinels to form a connected graph. In other 
words, if there is any path connecting the sub-DODAGS of the individual 
Sentinels and enough individual Sentinels consider the root as down, the 
entire network will eventually come to a decision that the root has crashed.

2. What if most of the direct links to the root fail but the root is in 
fact alive?

If most of the direct links to the root fail, then Sentinels monitoring 
those links will consider the root as dead. Since the root takes part in 
the communication, it will be aware that the number of such Sentinels 
increases. It will react by initiating a new DODAG version.

3. Is rebuilding the DODAG in such a case desirable?

If the majority of links to the root that have once formed a DODAG are 
currently down, then the DODAG should probably look different than for 
the network with those links up. Rebuilding the DODAG, at least in my 
opinion, makes a lot of sense in such a case. Furthermore, the threshold 
describing how large the majority is is configurable. Depending on 
whether one wants to prioritize speeding up root failure detection or 
slowing down DODAG rebuilding, different values can be chosen.

4. Why can't Sentinels ask the root whether it is dead?

In fact they do. If a Sentinel observes that an increasing number of 
other Sentinels considers the root as dead, it may perform verification 
by trying to contact the root via a direct link.

I hope to have covered all the questions.

Best,
--
- Konrad Iwanicki.