Mode of Operation extension

RPL specifies a proactive distance-vector based routing scheme. The protocol creates a DAG-like structure that operates with a given "Mode of Operation" (MOP) determining the minimum and mandatory set of primitives to be supported by all the participating nodes. MOP as per is a 3-bit value carried in DIO messages and is specific to the RPL Instance. The recipient of the DIO message can join the specified network as a router only when it can support the primitives as required by the mode of operation value. For example, in the case of MOP=3 (Storing MOP with multicast support), the nodes can join the network as routers only when they can handle the DAO advertisements from the peers and manage routing tables. The 3-bit value is fast depleting and requires replenishment. This document introduces a mechanism to extend the mode of operation values. This document further extends the RPL Control Option syntax to handle generic flags. The primary aim of these flags is to define the behavior of a node not supporting the given control type. If a node does not support a given RPL Control Option, there are following possibilities: Strip off the option Copy the option as-is Ignore the message containing this option Let the node join in only as a leaf to this parent

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. MOP: Mode of Operation. Identifies the mode of operation of the RPL Instance as administratively provisioned at and distributed by the DODAG root. MOPex: Extended MOP: This document extends the MOP values over a bigger range. This extension of MOP is called MOPex. DAO: Destination Advertisement Object (see Section 6.4 of ) DIO: DODAG Information Object (see Section 6.3 of ) This document uses the terminology described in . For the sake of readability, some of the known relevant terms are repeated in this section.

The following are the requirements considered for this document: MOP extension. The 3-bits MOP as defined in [RFC6550] is fast depleting. A MOP extension needs to extend the possibility of adding new MOPs in the future. Backwards compatibility. The new options and new fields in the DIO message should be backward compatible i.e. if there are nodes that support old MOPs they could still operate in their RPL Instances.

This document assigns MOP value 7 to be used as an extender . A DIO message with a MOP value of 7 indicates that the MOP for RPL instance is contained in the Extended MOP (MOPex) option.

The Option Length value MUST be less than or equal to 2. An Option Length value of zero is invalid and the implementation MUST silently ignore the DIO on receiving a value of zero. Section 6.7.1 of explains how to interpret Option Length and subsequent Option Data (which is MOPex-value in this context).

The MOPex option MUST be used only if the DIO MOP is 7. If the DIO MOP is 7 and if the MOPex option is not present or invalid then the DIO MUST be silently ignored. If the DIO MOP is less than 7 then MOPex MUST NOT be used. In case the base MOP is 7 and if the MOPex option is present, then the implementation MUST use the MOPex value. Note that allows a node that does not support the received MOP to still join the network as a leaf node. This semantics continues to be true even in the case of MOPex. All the general assumptions that are applicable in the context of MOP are applicable in the context of MOPex as well.

The MOPex option should also be used for existing MOP values 0-6. The use of current MOPs (values 0 to 6) in MOPex indicates that the MOP might be supported with an extended set of semantics e.g., the capability options .

Section 6.7.1 of describes the RPL Control Message Option Generic Format. This document extends the format as follows:

The new fields in the Extended RPL Option are specified as follows: Option Type: 8-bit identifier of the type of option. The Option Type values are assigned by IANA (see Section 20.4 of ). 'X' bit in Option Type: Value 1 indicates that this is an extended option. If the 'X' flag is set, a 1-byte Option Flags field follows the Option Length field. Option Length: 8-bit unsigned integer, representing the length in octets of the option, not including the Option Type and Length fields. Option Flags and variable length Option Data fields are included in the length. Option Flags: 8-bit unsigned integer, representing flags in the context of the Extended RPL Control Option. This document defines three flags 'J', 'C', and 'I'. These flags only apply if the Option Type is unknown or unsupported.

'J' (Join) flag: If set, the node MAY only join the network as a leaf. 'C' (Copy) flag: If set, the node MUST copy this Option Type in the DIO message it generates. If unset, the node MUST strip off the Option and process the message. 'I' (Ignore) flag: If set, the node MUST ignore the whole message regardless of the setting of the J and C flags. Note that this format does not deprecate the previous format, it simply extends it. The new format is applicable only when the most significant bit (MSB), 'X' flag, of the Option Type is set. Option Type 0x80 to 0xFF are thus applicable only as extended options. 'J' bit 'C' bit Handling 0 0 Strip off the option, and the node can join as RPL router 0 1 Copy the option, and the node can join as RPL router 1 NA Join as leaf

In , it was possible to discard an unsupported MOP just by inspecting the DIO base object. With the extensions in this document, the MOPex is in a control message option and thus discarding of the DIO message can only happen after inspecting the message options. An implementation should carefully set the Option Flags considering implications of nodes not supporting the corresponding Option Type. Unsetting the 'J' flag means that a node receiving an unsupported Option Type would be allowed to join as a RPL router. Thus the implementation should carefully consider the implications of such a node joining the network as a RPL router. Setting the 'C' (Copy) bit should be carefully considered since the node would copy the Option of its preferred parent whose DIO it has accepted from the set of parent nodes.

Thank you Dominique Barthel for important review/feedback on extending Control Options. Thanks to Alvaro Retana for shaping the final version with detailed review comments.

IANA is requested to assign a new Mode of Operation, named "MOPex" for MOP extension under the RPL registry. The value of 7 is to be assigned from the "Mode of Operation" space . Value Description Reference 7 MOPex This document This document updated to remove the reservation on Mode of Operation value 7. IANA is requested to assign the Mode of Operation value 7 to MOPex, as shown in Table 2. As shown there, all other references related to value 7 are to be removed. IANA is also requested to replace the reference to in the overall registry with a reference to this document.

IANA is requested to assign a value from the RPL Control Message Options registry for the Extended MOP (MOPex) option as shown in Table 3. Value Meaning Reference TBD1 Extended MOP (MOPex) This document

IANA is requested to create a registry for the MOPex-value . This registry should be located in the Routing Protocol for Low Power and Lossy Networks (RPL) group. New MOPex values may be allocated only by an IETF review. Each value is tracked with the following qualities: MOPex value Description Reference MOPex Value Description Reference 0 to 6 MOP [RFC6550]

IANA is requested to modify the RPL Control Message Options registry to include an Extended Control Options range as shown in . IANA is also requested to add [this document] as a reference for this updated registry. Range Option Type Reference 0x00 to 0x7f Base Options [RFC6550] 0x80 to 0xFf Extended Options [this document]

The options defined in this document are carried in the base message objects as defined in . The RPL control message options are protected by the same security mechanisms that protect the base messages. As such, the Security Consideration in [RFC6550] apply. The use of MOP 7 can reveal that the node has been upgraded or is running a old feature set. This document assumes that the base messages that carry these options are protected by RPL security mechanisms and thus are not visible to a malicious node.