Re: [Roll] [manet] Understanding RPL [was: Working group re-charter ideas]

[Cenk Gündogan <[e-mail]>]

Hello Juliusz,

On 03.04.2015 01:41, Juliusz Chroboczek wrote:
>> Local repair is done by either poisoning or detaching, sections
>> 8.2.2.5.
> I don't see how that can be implemented.
>
> Suppose A is B's parent.  A advertises INFINITE_RANK.  A's retraction
> ("poison") is lost.  B has no reason to change parents, so B has a parent
> with infinite rank.  Oops, B is just violating the MUST NOT in 8.2.2.5.2.
>
> Are you assuming the network is reliable?  Or am I missing some mechanism
> that prevents that from happening?

I also did struggle with this scenario for quite some time.
However, there are several places in the RFC that specifically deal 
(directly or indirectly) with this issue.


          8.2.2.5 <https://tools.ietf.org/html/rfc6550#section-8.2.2.5>.
          Poisoning



    1.  A node poisons routes by advertising a Rank of INFINITE_RANK.

    2.  A node MUST NOT have any nodes with a Rank of INFINITE_RANK in
        its parent set.


My interpretation of these bullet points is that DIOs advertising 
INFINITE_RANK are sent out periodically
(probably in a decaying fashion via Trickle), in order to reduce the 
chance that these DIOs stay unnoticed.
However, this might not be a complete solution, as all DIOs from A to B 
can become lost thus leading to the same problem,
but then the RFC states that links should be guaranteed to be symmetric 
anyways, e.g. by using NUD [1] or other alternatives.

Something else I want to mention is that RPL utilizes a few lifetime 
counters (e.g. for the dodag or route),
which would also help in this scenario (We wouldn't refresh the lifetime 
of a node with INFINITE_RANK, would we?),
but admittedly, depending on the specified lifetime values, in a very 
sluggish way.

At last, it is important to remember that RPL uses data-path validation 
to reduce the chattiness of control packets,
described in [2] and most likely realized by using [3].
In that case A can reset its Trickle timer when encountering 
inconsistencies and start e.g. another poisoning routine
by sending out another set of INFINITE_RANK DIOs.

Best,
Cenk

[1] Neighbor Unreachability Detection 
(https://tools.ietf.org/html/rfc2461#page-66)
[2] https://tools.ietf.org/html/rfc6550#section-3.2.7
[3] https://tools.ietf.org/html/rfc6553