(6LBR)
        |
        |
        |
       (A)
       / \
      /   \
     /     \
   (G)     (H)
    |       |
    |       |
    |       |
   (B)     (C)
     \      ;
      \    ;
       \  ;
        (D)
        / \
       /   \
      /     \
    (E)     (F)
        

Figure 1: Sample topology

Node (D) is connected via preferred parent (B). (D) has an alternate path via (C) towards the BR. Node (A) is the common ancestor for (D) for paths through (B)-(G) and (C)-(H). When (D) switches from (B) to (C), [RFC6550] suggests sending No-Path DAO to (B) and regular DAO to (C).

1.3. Cases when No-Path DAO may be used

There are following cases in which a node switches its parent and may employ No-Path DAO messaging:

Case I: Current parent becomes unavailable because of transient or permanent link or parent node failure.

Case II: The node finds a better parent node i.e. the metrics of another parent is better than its current parent.

Case III: The node switches to a new parent whom it "thinks" has a better metric but does not in reality.

The usual steps of operation when the node switches the parent is that the node sends a No-Path DAO message via its current parent to invalidate its current route and subsequently it tries to establish a new routing path by sending a new DAO via its new parent.

1.4. Why No-Path DAO is important?

Nodes in LLNs may be resource constrained. There is limited memory available and routing entry records are the one of the primary elements occupying dynamic memory in the nodes. Route invalidation helps 6LR nodes to decide which entries could be discarded to better achieve resource utilization in case of contention. Thus it becomes necessary to have efficient route invalidation mechanism. Also note that a single parent switch may result in a "sub-tree" switching from one parent to another. Thus the route invalidation needs to be done on behalf of the sub-tree and not the switching node alone. In the above example, when Node (D) switches parent, the route invalidation needs to be done for (D), (E) and (F). Thus without efficient route invalidation, a 6LR may have to hold a lot of unwanted route entries.

2. Problems with current No-Path DAO messaging

2.1. Lost NP-DAO due to link break to the previous parent

When a node switches its parent, the NPDAO is to be sent via its previous parent and a regular DAO via its new parent. In cases where the node switches its parent because of transient or permanent parent link/node failure then the NPDAO message is bound to fail. RPL assumes communication link with the previous parent for No-Path DAO messaging.

RPL allows use of route lifetime to remove unwanted routes in case the routes could not be refreshed. But route lifetimes in case of LLNs could be substantially high and thus the route entries would be stuck for long.

2.2. Invalidate routes to dependent nodes of the switching node

No-path DAO is sent by the node who has switched the parent but it does not work for the dependent child nodes below it. The specification does not specify how route invalidation will work for sub-childs, resulting in stale routing entries on behalf of the sub-childs on the previous route. The only way for 6LR to invalidate the route entries for dependent nodes would be to use route lifetime expiry which could be substantially high for LLNs.

In the example topology, when Node (D) switches its parent, Node (D) generates an NPDAO on its behalf. Post switching, Node (D) transmits a DIO with incremented DTSN so that child nodes, node (E) and (F), generate DAOs to trigger route update on the new path for themselves. There is no NPDAO generated by these child nodes through the previous path resulting in stale entries on nodes (B) and (G) for nodes (E) and (F).

2.3. Route downtime caused by asynchronous operation of NPDAO and DAO

A switching node may generate both an NPDAO and DAO via two different paths at almost the same time. There is a possibility that an NPDAO generated may invalidate the previous route and the regular DAO sent via the new path gets lost on the way. This may result in route downtime thus impacting downward traffic for the switching node. In the example topology, consider Node (D) switches from parent (B) to (C) because the metrics of the path via (C) are better. Note that the previous path via (B) may still be available (albeit at relatively bad metrics). An NPDAO sent from previous route may invalidate the existing route whereas there is no way to determine whether the new DAO has successfully updated the route entries on the new path.

An implementation technique to avoid this problem is to further delay the route invalidation by a fixed time interval after receiving an NPDAO, considering the time taken for the new path to be established. Coming up with such a time interval is tricky since the new route may also not be available and it may subsequently require more parent switches to establish a new path.

3. Requirements for the No-Path DAO Optimization

3.1. Req#1: Tolerant to the link failures to the previous parents

When the switching node send the NP-DAO message to the previous parent, it is normal that the link to the previous parent is prone to failure. Therefore, it is required that the NP-DAO message MUST be tolerant to the link failure during the switching.

3.2. Req#2: Dependent nodes route invalidation on parent switching

While switching the parent node and sending NP-DAO message, it is required that the routing entries to the dependent nodes of the switching node will be updated accordingly on the previous parents and other relevant upstream nodes.

3.3. Req#3: No impact on traffic while NP-DAO operation in progress

While sending the NP-DAO and DAO messages, it is possible that the NP-DAO successfully invalidates the previous path, while the newly sent DAO gets lost (new path not set up successfully). This will result into downstream unreachability to the current switching node. Therefore, it is desirable that the NP-DAO is synchronized with the DAO to avoid the risk of route downtime.

4. Proposed changes to NPDAO signaling

4.1. Change in NPDAO semantics

As described in Section 1.2, currently the NPDAO originates at the node switching the parent and traverses upstream towards the root. In order to solve the problems as mentioned in Section 2, the draft proposes to change the way NPDAO originates and traverses the network. The proposed NPDAO originates at a common ancestor node between the new and old path. The trigger for the common ancestor node to generate this NPDAO is the change in the next hop for the node on reception of an update message in the form of regular DAO for the target.

In the Figure 1, when node D decides to switch the path from B to C, it sends a regular DAO to node C with reachability information containing target as address of D and a incremented path sequence number. Node C will update the routing table based on the reachability information in DAO and in turn generate another DAO with the same reachability information and forward it to H. Node H also follows the same procedure as Node C and forwards it to node A. When node A receives the regular DAO, it finds that it already has a routing table entry on behalf of the target address of node D. It finds however that the next hop information for reaching node D has changed i.e. the node D has decided to change the paths. In this case, Node A which is the common ancestor node for node D along the two paths (previous and new), may generate an NPDAO which traverses downwards in the network. The document in the subsequent section will explain the message format changes to handle this downward flow of NPDAO.

4.2. DAO message format changes

Every RPL message is divided into base message fields and additional Options. The base fields apply to the message as a whole and options are appended to add message/use-case specific attributes. As an example, a DAO message may be attributed by one or more "RPL Target" options which specifies the reachability information for the given targets. Similarly, a Transit Information option may be associated with a set of RPL Target options.

The draft proposes a change in DAO message to contain "Invalidate previous route" (I) bit. This I-bit which is carried in regular DAO message, signals the common ancestor node to generate a downstream NPDAO on behalf of the target node. The I-bit is carried in the transit container option which augments the reachability information for a given set of RPL Target(s).

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Type = 0x06 | Option Length |E|I|  Flags    | Path Control  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Sequence | Path Lifetime |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
+                                                               +
|                                                               |
+                        Parent Address*                        +
|                                                               |
+                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+			
			

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |K|D|R| Flags   |   Reserved    | DAOSequence   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                            DODAGID*                           +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Option(s)...
+-+-+-+-+-+-+-+-+