Routing & RPL · Study deck

Worked Example: Rank and Parent Choice

A new low-power node hears advertisements from two possible RPL parents and must choose a route toward the root.

Packet Pete is your guide for this deck.

dodagworked-examplerank

Packet Pete, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Explain: The important result is not only the final diagram, but the evidence trail: why one parent was selected, why another was rejected, and how the resulting path supports the traffic direction being tested.
Explain: The route with the lower rank may change after a weak link or restart, so the final receiver result must stay attached to the route evidence.
Explain: The link costs in the The example topology gives each node a small set of candidate parents and link-cost labels for hand calculation diagram let the second-hop nodes compare candidate paths to the root instead of choosing a parent by position.
work a full parent-choice example using the objective function

Major section

Calculate the Parent Before Drawing the Tree

The direction of rank is a loop-avoidance constraint, not a geographic distance.

Parent A has path cost 40 and N measures link cost 18 to A, giving (40+18=58).
Parent B has path cost 32 but link cost 31, giving (32+31=63).
Under this additive example, N selects A because 58 is lower than 63.
The apparently better upstream cost at B is outweighed by N's weaker local link.

The example topology gives each node a small set of candidate parents and link-cost labels for hand calculation.

Major section

Start With the Node That Must Join

The route with the lower rank may change after a weak link or restart, so the final receiver result must stay attached to the route evidence.

RPL means the routing method used by low-power and lossy device networks.
A DODAG means the root-oriented, loop-free route shape that its nodes build.
Latency means the elapsed time from a stated start event to a stated finish event.

Major section

Start With the Node That Must Join (continued)

This runway does not predict every field route or delay.

A worked DODAG example becomes real when one node has to join a network already in motion.
It hears candidate parents, compares Rank and metrics, avoids loops, and chooses a place in the graph that affects every later packet.
The important result is not only the final diagram, but the evidence trail: why one parent was selected, why another was rejected, and how the resulting path supports the traffic direction being tested.

Major section

Overview: Parent Choice Is a Calculation, Not a Guess

It lays out how the example topology gives each node a small set of candidate parents and link-cost labels for hand calculation, so the next decision can be tied to an observable state or transition.

This diagram tests Overview: Parent Choice Is a Calculation, Not a Guess.
To make Overview: Parent Choice Is a Calculation, Not a Guess reviewable, pause at the The example topology gives each node a small set of candidate parents and link-cost labels for hand calculation diagram.
The link costs in the The example topology gives each node a small set of candidate parents and link-cost labels for hand calculation diagram let the second-hop nodes compare candidate paths to the root instead of choosing a parent by position.

Major section

Practitioner: Calculate Every Candidate Before Choosing

The arithmetic selects a candidate; a later link failure requires E or F to recalculate and demonstrate a working replacement path.

A lower calculated Rank supports the choice, but the selected parent and forwarded packet still need to agree.
The alternate Rank totals for E and F in the E and F both choose B in this teaching model because B gives the lower resulting candidate Rank diagram expose which candidate parent gives the lower resulting Rank.
In the E and F both choose B in this teaching model because B gives the lower resulting candidate Rank diagram, the visual shows candidate parent comparison for E and F showing resulting Rank values through alternate parents.

E and F both choose B in this teaching model because B gives the lower resulting candidate Rank.

Major section

Under the Hood: Parent Trees and Downward Routes Are Different Evidence

A downward test needs DAO-derived state and a packet from the root to the child, not the Rank diagram alone.

DIOs carry the DODAG ID, version, and Rank; the DODAG Configuration option identifies the Objective Function used to compute Rank.
The teaching offsets begin at zero; with the default MinHopRankIncrease, the actual root advertises Rank 256, and DAO evidence is needed for downward reachability.

The final DODAG separates upward parent choices from DAO evidence needed for downward reachability claims.

Deck summary

Key takeaways

The direction of rank is a loop-avoidance constraint, not a geographic distance.

The route with the lower rank may change after a weak link or restart, so the final receiver result must stay attached to the route evidence.
This runway does not predict every field route or delay.
It lays out how the example topology gives each node a small set of candidate parents and link-cost labels for hand calculation, so the next decision can be tied to an observable state or transition.
The arithmetic selects a candidate; a later link failure requires E or F to recalculate and demonstrate a working replacement path.

Retrieval practice

Recall check 1 of 3

Packet Pete says: answer from memory, then check your reasoning.

Q1What is the safest claim after a worked example calculates the preferred parent for every node?

AThe upward parent choices are explained under the stated Rank model

BRoot-to-node command delivery is proven without DAO route-state evidence

CThe node with the fewest hops must always be selected

DEvery visible neighbor is automatically a valid backup parent

Show answer

Answer: A Parent calculations explain upward choices under the stated model.

Retrieval practice

Recall check 2 of 3

Packet Pete says: answer from memory, then check your reasoning.

Q2Node F can choose B with parent offset 256 and link cost 1.0, or C with parent offset 384 and link cost 1.5. Using candidate_offset = parent_offset + round(256 x link_cost), which parent should F select?

AChoose B: 256 + round(256 × 1.0) gives offset 512.

BChoose C because its parent has a direct root connection.

CChoose C to spread children even if candidate cost is higher.

DChoose either because both candidates appear in the DODAG.

Show answer

Answer: A Through B, F gets candidate offset 512.

Retrieval practice

Recall check 3 of 3

Packet Pete says: answer from memory, then check your reasoning.

Q3After B fails, E, F, H, and I recalculate their Rank values. What evidence is still needed before claiming root-to-H command delivery works?

AFresh DAO or route-state evidence for H's downward reachability after repair

BOnly the fact that H still has a lower-Rank path toward the root

CThe original parent diagram, since it already identifies H's path through the network

DNo extra evidence, because Rank values also prove DAO behavior

Show answer

Answer: A Repaired upward parents do not automatically prove that downward route state has been refreshed.

Print reference

Answers

Answer key.

A · Parent calculations explain upward choices under the stated model.
A · Through B, F gets candidate offset 512.
A · Repaired upward parents do not automatically prove that downward route state has been refreshed.