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5. RPL Instance

Within a given LLN, there may be multiple, logically independent RPL Instances. A RPL node may belong to multiple RPL Instances, and it may act as a router in some and as a leaf in others. This document describes how a single instance behaves.

There are two types of RPL Instances: Local and Global. RPL divides the RPLInstanceID space between Global and Local instances to allow for both coordinated and unilateral allocation of RPLInstanceIDs. Global RPL Instances are coordinated, have one or more DODAGs, and are typically long-lived. Local RPL Instances are always a single DODAG whose singular root owns the corresponding DODAGID and allocates the local RPLInstanceID in a unilateral manner. Local RPL Instances can be used, for example, for constructing DODAGs in support of a future on-demand routing solution. The mode of operation of Local RPL Instances is out of scope for this specification and may be described in other companion specifications.

The definition and provisioning of RPL Instances are out of scope for this specification. Guidelines may be application and implementation specific, and they are expected to be elaborated in future companion specifications. Those operations are expected to be such that data packets coming from the outside of the RPL network can unambiguously be associated to at least one RPL Instance and be safely routed over any instance that would match the packet.

Control and data packets within RPL network are tagged to unambiguously identify of which RPL Instance they are a part.

Every RPL control message has a RPLInstanceID field. Some RPL control messages, when referring to a local RPLInstanceID as defined below, may also include a DODAGID.

Data packets that flow within the RPL network expose the RPLInstanceID as part of the RPL Packet Information that RPL requires, as further described in Section 11.2. For data packets coming from outside the RPL network, the ingress router determines the RPLInstanceID and places it into the resulting packet that it injects into the RPL network.

5.1. RPL Instance ID​

A global RPLInstanceID MUST be unique to the whole LLN. Mechanisms for allocating and provisioning global RPLInstanceID are out of scope for this specification. There can be up to 128 Global instance in the whole network. Local instances are always used in conjunction with a DODAGID (which is either given explicitly or implicitly in some cases), and up 64 Local instances per DODAGID can be supported. Local instances are allocated and managed by the node that owns the DODAGID, without any explicit coordination with other nodes, as further detailed below.

A global RPLInstanceID is encoded in a RPLInstanceID field as follows:

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |0|     ID      |  Global RPLInstanceID in 0..127
    +-+-+-+-+-+-+-+-+

      Figure 4: RPLInstanceID Field Format for Global Instances

A local RPLInstanceID is autoconfigured by the node that owns the DODAGID and it MUST be unique for that DODAGID. The DODAGID used to configure the local RPLInstanceID MUST be a reachable IPv6 address of the node, and it MUST be used as an endpoint of all communications within that Local instance.

A local RPLInstanceID is encoded in a RPLInstanceID field as follows:

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |1|D|   ID      |  Local RPLInstanceID in 0..63
    +-+-+-+-+-+-+-+-+

      Figure 5: RPLInstanceID Field Format for Local Instances

The 'D' flag in a local RPLInstanceID is always set to 0 in RPL control messages. It is used in data packets to indicate whether the DODAGID is the source or the destination of the packet. If the 'D' flag is set to 1, then the destination address of the IPv6 packet MUST be the DODAGID. If the 'D' flag is cleared, then the source address of the IPv6 packet MUST be the DODAGID.

For example, consider a Node A that is the DODAG root of a Local RPL Instance, and has allocated a local RPLInstanceID. By definition, all traffic traversing that Local RPL Instance will either originate or terminate at Node A. In this case, the DODAGID will be the reachable IPv6 address of Node A. All traffic will contain the address of Node A, and thus the DODAGID, in either the source or destination address. Thus, the local RPLInstanceID may indicate that the DODAGID is equivalent to either the source address or the destination address by setting the 'D' flag appropriately.

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