| Internet-Draft | DODAG size | December 2020 |
| She, et al. | Expires 21 June 2021 | [Page] |
This document extends [RFC6551] by defining a new DODAG metric called DODAG size, which can be used for DODAG selection in Low-Power and Lossy Networks (LLNs). DODAG size is an important metric for nodes to decide which DODAG to join, or which DODAG to migrate. This document proposes methods to disseminate DODAG size from the Root to all nodes in the DODAG, so that the DODAG size can be advertised to new joining nodes.¶
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Low-power and Lossy Networks (LLNs) typically consist of large number of nodes connected by lossy and unstable links. Such networks are typically compised of nodes that are constrained in CPU power, memory, and energy.¶
RPL, the "Routing Protocol for LLNs" [RFC6550], is an IPv6 routing procotol with specific optimizations for such networks. RPL builds routes proactively but maintains them on-demand based on their utilization. Point-to-multipoint (P2MP) and multipoint-to-point (MP2P) routes to and from the Root are optimized, but other point-to-point (P2P) routes are stretched to minimize the control traffic and the state in every node.¶
When used in conjunction with IEEE Std. 802.15.4 [IEEE802154], RPL can be used to form a Personal Area Network (PAN) composed by a 6LoWPAN Border Router (6LBR) that is typically collocated with the DODAG Root, and multiple 6LoWPAN Nodes (6LN), that can be RPL routers of leaves.¶
The PAN formation process starts from a DODAG Root. Before a node joins a PAN, it has no information regarding available neighbors or PANs. To discover available PANs, a joining node transmits PAN Advertisement Solicits and listens for PAN Advertisements from either the Root or other joined nodes.¶
The PAN Advertisements contain minimum information (such as network name, DODAG size, etc.) for a node to select an appropriate PAN to join or migrate. The DODAG size is the number of nodes in the DODAG and communicating through the Root. The DODAG size thus is an important metric for a node to decide which PAN to join. Therefore, it is essential to ensure the value of DODAG size advertised is up-to-date.¶
At this early stage, this document propose two methods to disserminates the DODAG size to the PAN.¶
The terminology used in this document is consistent with and incorporates that described in "Terms Used in Routing for Low-Power and Lossy Networks (LLNs)" [RFC7102]. Other terms in use in LLNs are found in "Terminology for Constrained-Node Networks" [RFC7228].¶
"RPL", the "RPL Packet Information" (RPI), and "RPL Instance" (indexed by a RPLInstanceID) are defined in "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks" [RFC6550]. The RPI is the abstract information that RPL defines to be placed in data packets, e.g., as the RPL Option [RFC6551] within the IPv6 Hop-By-Hop Header. By extension the term "RPI" is often used to refer to the RPL Option itself. The DODAG Information Solicitation (DIS), Destination Advertisement Object (DAO) and DODAG Information Object (DIO) messages are also specified in [RFC6550].¶
This document often uses the following acronyms:¶
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 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The value of DODAG size is collected by the Root, and disseminated to all nodes in the PAN. To ensure timely delivering of DODAG size, it has to be contained in periodic PAN-wide messages that can reach every node in the PAN.¶
The DODAG size is defined by the DODAG Size Object and MAY be present in the DODAG Metric Container option [RFC6551].¶
[RFC6551] specifies a set of link and node routing metrics and constraints suitable to LLNs. This document extends [RFC6551] by defining a new DODAG metric called DODAG size.¶
The DODAG size object MAY be present in the DODAG Metric Container. There MUST NOT be more than one DODAG size object as a metric per DODAG Metric Container.¶
The DODAG size object is made of DODAG size fields and MUST at least comprise one DODAG size field. Each DODAG size field has a fixed length of 16 bits.¶
The DODAG size object does not contain any additional TLVs.¶
The DODAG size object Type has been assigned value TBD by IANA.¶
The format of the ETX object body is as follows:¶
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (field) ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DODAG Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DODAG Size: 16 bits. It is encoded using 16 bits in unsigned integer format.¶
According to [RFC6550], the DIO message is periodically sent to the PAN, and it MAY carry an option called DODAG Metric Container. The DODAG size object can be present in this option. Through the DIO message, the DODAG size is gradually disseminated to nodes in the PAN¶
The DAO-ACK message [RFC6550] is sent as unicast packet by the DODAG Root in response to a unicast DAO message.¶
It MAY carry the DODAG Metric Container option [RFC6550]. The DODAG size MAY be present in the DODAG Metric Container option.¶
The nodes in a PAN might be able to get the DODAG size timely through the DAO-ACK messsage.. Compared with the DIO message, the DAO-ACK message is tyipcally sent more frequently. Moreover, nodes deep in the DODAG can get the DODAG size more quickly since the DAO-ACK is directly sent by the Root in unicast.¶
This specification updates the "Routing Metric/Constraint Type" subregistry of the "Routing Protocol for Low Power and Lossy Networks (RPL) Routing Metric/Constraint" Registry that was created for [RFC6551].¶
IANA is thereby requested to allocate one new value as follows:¶
| Value | Description | Reference |
| 9 (suggested) | DODAG size | This document |
It is worth noting that in RPL [RFC6550], every node in the LLN that is RPL-aware and has access to the RPL domain can inject any RPL-based attack in the network, more in [RFC7416]. This document applies typically to an existing deployment and does not change its security requirements and operations. It is assumed that the security mechanisms as defined for RPL are followed.¶
The authors wish to thank TBD.¶