------+---------                           
                 |          Internet               
                 |                                     
              +-----+                                  
              |     | Border Router   
              |     |  (RPL Root)     
              +-----+                 
               | \  \____                
              /   \       \
            o 11   o 12     o  13   
           /       |       /  \
         o 22      o 23   o 24  o 25
        /  \       | \      \
      o 31   o 32  o   o     o 35 
     /      /      |    \    |    \
    o 41   o 42    o     o   o 45   o 46   
    |      |       |     |    \     |
    o 51   o 52    o 53  o     o 55 o 56
                       LLN
       

Figure 3: Non-Storing with Projected routes

With this draft, the root can install routing states along a segment that is either itself to the destination, or from one or more common parents for a particular source/destination pair towards that destination (in our example, this would be the segment made of nodes 22, 32, 42).

The draft expects that the root has enough information about the capability for each node to store a number of routes, which can be discovered for instance using a Network Management System (NMS) and/or the RPL routing extensions specified in Routing for Path Calculation in LLNs [RFC6551]. Based on that information, the root computes which segment should be routed and which relevant state should be installed in which nodes. The algorithm is out of scope but it is envisaged that the root could compute the ratio between the optimal path (existing path not traversing the root, and the current path), the application SLA for specific flows that could benefit from shorter paths, the energy wasted in the network, local congestion on various links that would benefit from having flows routed along other paths.

This draft introduces a new mode of operation for loose source routing in the LLN, the Non-Storing with Projected routes MOP. With this new MOP, the root sends a unicast DAO message to the last node of the routing segment that must be installed. The DAO message contains the ordered list of hops along the segment as a list of Via Information options that are preceded by one or more RPL Target options to which they relate. Each Via Information option contains a lifetime for which state is to be maintained.

The root sends the DAO directly to the last node in the segment, which is expected to be able to route to the targets on its own.

The last node in the segment may have another information to reach the target(s), such as a connected route or an already installed projected route. If it does not have such a route then the node should lookup the address on the relevant interfaces. If one of the targets cannot be located, the node MUST answer to the root with a negative DAO-ACK listing the target(s) that could not be located (suggested status 10), and continue the process for those targets that could be located if any.

For the targets that could be located, last node in the segment generates a DAO to its loose predecessor in the segment as indicated in the list of Via Information options.

The node strips the last Via Information option which corresponds to self, and uses it as source address for the DAO to the predecessor. The address of the predecessor to be used as destination for the DAO message is found in the now last Via Information option. The predecessor is expected to have a route to the address used as source, either connected, installed previously as another DAO, or from other means.

The predecessor is expected to have a route to the address used as source and that is his successor. If it does not and cannot locate the successor, the predecessor node MUST answer to the root with a negative DAO-ACK indicating the successor that could not be located. The DAO-ACK contains the list of targets that could not be routed to (suggested status 11).

If the predecessor can route to the successor node, then it installs a route to the targets via the successor. If that route is not connected then a recursive lookup will take place to reach the target(s). From there, the node strips the last Via Information option and either answers to the root with a positive DAO-ACK that contains the list of targets that could be routed to, or propagates the DAO to its own predecessor.

A NULL lifetime in the Via Information option along the segment is used to clean up the state.

In the example below, say that there is a lot of traffic to nodes 55 and 56 and the root decides to reduce the size of routing headers to those destinations. The root can first send a DAO to node 45 indicating target 55 and a Via segment (35, 45), as well as another DAO to node 46 indicating target 56 and a Via segment (35, 46). This will save one entry in the routing header on both sides. The root may then send a DAO to node 35 indicating targets 55 and 56 a Via segment (13, 24, 35) to fully optimize that path.

Alternatively, the root may send a DAO to node 45 indicating target 55 and a Via segment (13, 24, 35, 45) and then a DAO to node 46 indicating target 56 and a Via segment (13, 24, 35, 46), indicating the same DAO Sequence.

5. Centralized Computation of Optimized Peer-to-Peer Routes

With the initial specifications of RPL [RFC6550], the P2P path from a source to a destination is often stretched, as illustrated in [RFC6550]:

• - in non-storing mode, all packets routed within the DODAG flow all the way up to the root of the DODAG. If the destination is in the same DODAG, the root must encapsulate the packet to place a Routing Header that has the strict source route information down the DODAG to the destination. This will be the case even if the destination is relatively close to the source and the root is relatively far off.
• - in storing mode, unless the destination is a child of the source, the packets will follow the default route up the DODAG as well. If the destination is in the same DODAG, they will eventually reach a common parent that has a DAO route to the destination; at worse, the common parent may also be the root. From that common parent, the packet will follow a path down the DODAG that is optimized for the Objective Function that was used to build the DODAG.

It results that it is often beneficial to enable additional P2P routes, either if the RPL route present a stretch from shortest path, or if the new route is engineered with a different objective.

                   ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border Router
                 |     |  (RPL Root)
                 +-----+                     
                    X                    
              ^    v   o    o                
          ^ o   o  v   o  o  o o   o          
         ^  o o  o v    o   o   o  o  o      
         ^   o    o  v     o  o    o  o  o   
        S  o   o  o   D         o   o o       
        o          o             o     o
                          LLN
       

              ------+---------
                    |          Internet
                    |
                 +-----+
                 |     | Border Router
                 |     |  (RPL Root)
                 +-----+                     
                    |                    
              o    o   o    o                
          o o   o  o   o  o  o o   o          
         o  o o  o o    o   o   o  o  o      
         o   o    o  o     o  o    o  o  o   
        S>>o>>>o>>o>>>D         o   o o       
        o          o             o     o
                          LLN