Configuration option for RFC 8138

It is RECOMMENDED to only deploy nodes that support in a network where the compression is turned on. A node that does not support MUST only be used as a leaf in that network. states that "Nodes other than the DODAG root MUST NOT modify this information when propagating the DODAG Configuration option". In other words, the configuration option is a way for the root to configure the LLN nodes but it cannot be used by a parent to advertise its capabilities down the DODAG. It results whether a parent supports RFC 8138 is not known by the child with the current level of specifications, and a child cannot favor a parent based on a particular support. Sections 8.5 and 9.2 of also suggests that a RPL-aware node may attach to a DODAG as a leaf node only, e.g., when a node does not support the Mode of Operation of a RPL Instance, the Objective Function (OF) as indicated by the Objective Code Point (OCP) or some other parameters in the configuration option. But the node is also free to refrain from joining an Instance when a parameter is not suitable. This means that changing the OCP in a DODAG can be used to force nodes that do not support a particular feature to join as leaf only. This specification reiterates that a node that is configured to operate in an Instance but does not support a value for a known parameter that is mandatory for routing MUST NOT operate as a router but MAY still joins as a leaf. Note that a legacy node will not recognize when a reserved field is now used and will not turn to a leaf when that happens. A node that supports but not this specification can only be used in an homogeneous network and an upgrade requires a "flag day" where all nodes are updated and then the network is rebooted with implicitely RFC 8138 compression turned on with the "T" flag set on. A node that supports this specification can work in a network with RFC 8138 compression turned on or off with the "T" flag set accordingly and in a network in transition from off to on or on to off (see ). A node that does not support can interoperate with a node that supports this specification in a network with RFC 8138 compression turned off. But it cannot forward compressed packets and therefore it cannot act as a router in a network with RFC 8138 compression turned on. It may remain connected to that network as a leaf and generate uncompressed packets as long as imcoming packets are decapsulated by the parent and delivered in uncompressed form. The intent for this specification is to perform a migration once and for all without the need for a flag day. In particular it is not the intention to undo the setting of the "T" flag, and though it is possible to roll back (see ), adding nodes that do not support after a roll back may be problematic if the roll back is not fully complete (see caveats in ).

When the "T" flag is turned on in the configuration option by the root, the information slowly percolates through the DODAG as the DIO gets propagated. Some nodes will see the flag and start sourcing packets in the compressed form while other nodes in the same instance are still not aware of it. Conversely, in non-storing mode, the root will start using RFC 8138 with a SRH-6LoRH that routes all the way to the last router or possibly to the leaf, if the leaf supports RFC 8138. This is why it is required that all the routers in the Instance support at the time of the switch, and all nodes that do not support only operate as leaves. Setting the "T" flag is ultimately the responsibility of the network administrator. In a case of upgrading a network to turn the compression on, the network SHOULD be operated with the "T" flag reset until all targeted nodes are upgraded to support this specification. and provide possible transition scenarios where this can be enforced.

In a single instance scenario, nodes that support RFC 8138 are configured with a new OCP, that may use the same OF operation or a variation of it. when it finally sets the "T" flag, the root also migrates to the new OCP. As a result, nodes that do not support RFC 8138 join as leaves and do not forward packets anymore. The leaves generate packets without compression. The parents - which supports RFC 8138 - may encapsulate the packets using RFC 8138 if needed. The other way around, the root encapsulates packets to the leaves all the way to the parent, which decapsulates and distribute the uncompresses inner packet to the leaf, as illustrated in Section 4.3 of This scenario presents a number of caveats: The method consumes an extra OCP. It also requires a means to signal the capabilities of the leaf, e.g., using "RPL Mode of Operation extension". If an implementation does not move to a leaf mode when the OCP is changed to an unknown one, then the node may be stalled. If the only possible parents of a node are nodes that do not support RFC 8138, then that node will loose all its parent at the time of the migration and it will be stalled until a parent is deployed with the new capability. Nodes that only support RFC8138 for forwarding may not parse the RPI in native form. If such nodes are present, the parent needs to encapsulate with RFC8138.

An alternate to the Single Instance Scenario is to deploy an additional Instance for the nodes that support . The two instances operate as ships-in-the-night as specified in . The preexisting Instance that does not use , whereas the new Instance does. This is signaled by the "T" flag which is only set in the configuration option in DIO messages in the new Instance. The legacy nodes would not be configured to participate to the second instance, and islands that are only connected via legacy nodes would not be reachable over the second instance. Nodes that support RFC 8138 participate to both Instances but favor the new Instance for the traffic that they source. On the other hand, nodes that only support the uncompressed format would either not be configured for the new instance, or would be configured to join it as leaves only. This method eliminates the risks of nodes being stalled that are described in but requires implementations to support at least two RPL Instances and demands management capabilities to introduce new Instances and deprecate old ones.

After downgrading a network to turn the compression off, the administrator SHOULD make sure that all nodes have converged to the "T" flag reset before allowing nodes that do not support the compression in the network (see caveats in ). This also requires a means to signal the current state of the setting of the logic that controls the compression in the node, also using .