-------------------------------------------+
       Multicast DIS                             |
      ----------------------+--------------------+
       |      N=1, T=0      |      N=1, T=1      |
      -+--------------------+--------------------+
       | multicast DIO,     | unicast DIO,       |
       | don't              | don't              |
       | reset Trickle timer| reset Trickle timer|
      -+--------------------+--------------------+
       |                    |                    |
       | do nothing         | do nothing         |
       |                    |                    |
       |                    |                    |
      -+--------------------+--------------------+
       | multicast DIO,     | unicast DIO,       |
       | don't              | don't              |
       | reset Trickle timer| reset Trickle timer|
       |                    |                    |
      -+--------------------+--------------------+            

Figure 4: Overall DIS behavior, part 2

For the sake of completeness, let's remind here that a specific, one-shot DIO generated in response to a DIS must contain a Configuration option and that its transmission is delayed according to the Delay Spreading option of the DIS, if one such option is present.

6. IANA Considerations

6.1. DIS Flags

IANA is requested to allocate bits 6 and 7 of the DIS Flag Field to become the "No Inconsistency" and "DIO Type" bits, the functionality of which is described in Section 3 of this document.

Value	Meaning	Reference
6	No Inconsistency	This document
7	DIO Type	This document

6.2. RPL Control Message Options

IANA is requested to allocate a new code point in the "RPL Control Message Options" registry for the "Response Spreading" option, the behavior of which is described in Section 4.2.

Value	Meaning	Reference
0x0A	Response Spreading	This document

RPL Control Message Options

7. Security Considerations

TBA

8. Acknowledgements

A lot of text in this document originates from now-expired [I-D.goyal-roll-dis-modifications] co-authored with M. Goyal. The requirements and solutions also draw from now-expired [I-D.dejean-roll-selective-dis] co-authored with N. Dejean. Their contribution is deeply acknowledged.

We also thank (TBA) for their useful feedback and discussion.

9. References

9.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6550]	Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP. and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, March 2012.
[RFC6551]	Vasseur, JP., Kim, M., Pister, K., Dejean, N. and D. Barthel, "Routing Metrics Used for Path Calculation in Low-Power and Lossy Networks", RFC 6551, DOI 10.17487/RFC6551, March 2012.

9.2. Informative References

[RFC4861]	Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007.
[RFC5184]	Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R. and K. Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3 (L3)-Driven Fast Handover", RFC 5184, DOI 10.17487/RFC5184, May 2008.
[RFC5881]	Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI 10.17487/RFC5881, June 2010.
[RFC6206]	Levis, P., Clausen, T., Hui, J., Gnawali, O. and J. Ko, "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206, March 2011.

Appendix A. Applications

This section details two example mechanisms that use the DIS flags and options defined in this document. The first mechanism describes how a leaf node may join a desired DAG in an energy efficient manner. The second mechanism details how a node may identify defunct DAGs for which it still maintains state.

A.1. A Leaf Node Joining a DAG

A new leaf node that joins an established LLN runs an iterative algorithm in which it requests (using multicast DIS) DIOs from routers belonging to the desired DAG.

The DIS message has the "No Inconsistency" flag set to prevent resetting of Trickle timer in responding routers, thereby keeping the aggregated number of transmissions low. It also has the "DIO Type" flag set to make responding routers send unicast DIOs back, thereby not triggering full reception in nearby nodes that have state-of-the-art radio receivers with hardware-based address filtering.

The DIS message can include a Response Spreading option prescribing a suitable spreading interval based on the expected density of nearby routers and on the expected Layer 2 technology.

The DIS will likely include a Metric Container listing the routing constraints that the responding routers must satisfy in order to be allowed to respond.

At each iteration, the node multicasts such a DIS and waits for forthcoming DIOs. After a time equal to the spreading interval, the node considers the current iteration to be unsuccessful. The node consequently relaxes the routing constraints somewhat and proceeds to the next iteration.

The cycle repeats until the node receives one or more DIOs or until it has relaxed the constraints to the lowest acceptable values.

This algorithm has been proven in the field to be extremely energy-efficient, especially when routers have a wide communication range.

A.2. Identifying A Defunct DAG

A RPL node may remove a neighbor from its parent set for a DAG for a number of reasons:

• The neighbor is no longer reachable, as determined using a mechanism such as Neighbor Unreachanility Detection (NUD) [RFC4861], Bidirectional Forwarding Detection (BFD) [RFC5881] or L2 triggers [RFC5184]; or
• The neighbor advertises an infinite rank in the DAG; or
• Keeping the neighbor as a parent would required the node to increase its rank beyond L + DAGMaxRankIncrease, where L is the minimum rank the node has had in this DAG; or
• The neighbor advertises membership in a different DAG within the same RPL Instance, where a different DAG is recognised by a different DODAGID or a different DODAGVersionNumber.

Even if the conditions listed above exist, a RPL node may fail to remove a neighbor from its parent set because:

• The node may fail to receive the neighbor's DIOs advertising an increased rank or the neighbor's membership in a different DAG;
• The node may not check, and hence may not detect, the neighbor's unreachability for a long time. For example, the node may not have any data to send to this neighbor and hence may not encounter any event (such as failure to send data to this neighbor) that would trigger a check for the neighbor's reachability.

In such cases, a node would continue to consider itself attached to a DAG even if all its parents in the DAG are unreachable or have moved to different DAGs. Such a DAG can be characterized as being defunct from the node's perspective. If the node maintains state about a large number of defunct DAGs, such state may prevent a considerable portion of the total memory in the node from being available for more useful purposes.

To alleviate the problem described above, a RPL node may invoke the following procedure to identify a defunct DAG and delete the state it maintains for this DAG. Note that, given the proactive nature of RPL protocol, the lack of data traffic using a DAG can not be considered a reliable indication of the DAG's defunction. Further, the Trickle timer based control of DIO transmissions means the possibility of an indefinite delay in the receipt of a new DIO from a functional DAG parent. Hence, the mechanism described here is based on the use of a DIS message to solicit DIOs about a DAG suspected of defunction. Further, a multicast DIS is used so as to avoid the need to query each parent individually and also to discover other neighbor routers that may serve as the node's new parents in the DAG.

When a RPL node has not received a DIO from any of its parents in a DAG for more than a locally configured time duration:

• The node generates a multicast DIS message with:
  • the "No Inconsistency" flag set so that the responding routers do not reset their Trickle timers.
  • the "DIO Type" flag not set so that the responding routers send multicast DIOs and other nodes in the vicinity do not need to invoke this procedure.
  • a Solicited Information option to identify the DAG in question. This option must have the I and D flags set and the RPLInstanceID/DODAGID fields must be set to values identifying the DAG. The V flag inside the Solicited Information option should not be set so as to allow the neighbors to send DIOs advertising the latest version of the DAG.
  • a Response Spreading option specifying a suitable time interval over which the DIO responses may arrive.
• After sending the DIS, the node waits for the duration specified inside the Response Spreading option to receive the DIOs generated by its neighbors. At the conclusion of the wait duration:
  • If the node has received one or more DIOs advertising newer version(s) of the DAG, it joins the latest version of the DAG, selects a new parent set among the neighbors advertising the latest DAG version and marks the DAG status as functional.
  • Otherwise, if the node has not received a DIO advertising the current version of the DAG from a neighbor in the parent set, it removes that neighbor from the parent set. As a result, if the node has no parent left in the DAG, it marks the DAG as defunct and schedule the deletion of the state it has maintained for the DAG after a locally configured "hold" duration. (This is because, as per RPL specification, when a node no longer has any parents left in a DAG, it is still required to remember the DAG's identity (RPLInstanceID, DODAGID, DODAGVersionNumber), the lowest rank (L) it has had in this DAG and the DAGMaxRankIncrease value for the DAG for a certain time interval to ensure that the node does not join an earlier version of the DAG and does not rejoin the current version of the DAG at a rank higher than L + DAGMaxRankIncrease.)

Appendix B. Experimental data

The effectiveness of these flags and options has been measured on real industrial hardware.

Data to be added

Authors' Addresses