[6tisch] Questions on RPL Settings in RFC 8180

[Yasuyuki Tanaka <[e-mail]>]

Hi all,

I have a few questions on the RPL settings described in RFC
8180. Could anyone help me find answers...?

  https://tools.ietf.org/html/rfc8180#section-5

---

(1) Rank Computation

RFC 8180 says:

rfc8180> 5.1.1.  Rank Computation
rfc8180> (...)
rfc8180> Sp SHOULD be calculated as (3*ETX)-2.  The minimum value of
rfc8180> Sp (MINIMUM_STEP_OF_RANK) indicates a good quality link.  The
rfc8180> maximum value of Sp (MAXIMUM_STEP_OF_RANK) indicates a poor
rfc8180> quality link.  The default value of Sp (DEFAULT_STEP_OF_RANK)
rfc8180> indicates an average quality link.  Candidate parents with
rfc8180> ETX greater than 3 SHOULD NOT be selected.

  https://tools.ietf.org/html/rfc8180#section-5.1.1

MAXIMUM_STEP_OF_RANK is defined to 9. Why?

Sp is calculated as (3 * ETX) - 2 and the worst acceptable ETX is
3. It looks like 7 is the possible largest value of Sp...

(2) Trickle Timer

RFC 8180 says:

rfc8180> 5.3.  Trickle Timer
rfc8180> (...)
rfc8180> For this specification, the Trickle timer MUST be used with the
rfc8180> RPL-defined default values (see Section 8.3.1 of [RFC6550]).

  https://tools.ietf.org/html/rfc8180#section-5.3

So, Imin for DIO Trickle timer starts with 8 ms, which looks too short
for the minimal TSCH schedule where one shared cell in a slotframe of
1.01s. This setting could cause congestion by DIO traffic...

Why is this default value (DEFAULT_DIO_INTERVAL_MIN) reasonable for
the 6TiSCH minimal configuration...?

rfc6550> 17.  RPL Constants and Variables
rfc6550> (...)
rfc6550> DEFAULT_DIO_INTERVAL_MIN: This is the default value used to
rfc6550>      configure Imin for the DIO Trickle timer.
rfc6550>      DEFAULT_DIO_INTERVAL_MIN has a value of 3.  This
rfc6550>      configuration results in Imin of 8 ms.

  https://tools.ietf.org/html/rfc6550#section-17

---

There are some email threads on RPL configuration found in 6TiSCH ML
archive. But, I could not find any discussion about above mentioned
points...

- [6tisch] On 2*ETX in Minimal
  https://mailarchive.ietf.org/arch/msg/6tisch/oWmlpcq2evOlf4gH90moG2gyxzg

- [6tisch] 6TiSCH Minimal: step_of_rank
  https://mailarchive.ietf.org/arch/msg/6tisch/BXbfFtysPHJTekkeQXScv1GVxCM

- [6tsch] Zero Objective Function discussion
  https://mailarchive.ietf.org/arch/msg/6tisch/McGiW7IPnhITB74Vz2y2SKXot80

Thank you!
Yatch