Internet-Draft DODAG size December 2020
She, et al. Expires 21 June 2021 [Page]
Intended Status:
Standards Track
H. She
Cisco Systems
L. Zhao
Cisco Systems
P. Thubert
Cisco Systems

A DODAG Metric Used for DODAG Selection in Low-Power and Lossy Networks


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.

Status of This Memo

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This Internet-Draft will expire on 21 June 2021.

Table of Contents

1. Introduction

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.

2. Terminology

2.1. References

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].

2.2. Glossary

This document often uses the following acronyms:

IPv6 over Low-Power Wireless Personal Area Network
6LoWPAN Routing Header
DODAG Information Object (a RPL message)
Directed Acyclic Graph
Destination-Oriented Directed Acyclic Graph
Low-Power and Lossy Network
Personal Area Network
IPv6 Routing Protocol for Low-Power and Lossy Networks

2.3. Requirements Language

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.

3. Disseminating DODAG size

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].

3.1. DODAG Size Object

[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) .....       |
Figure 1: DODAG Size Object Body Format

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   |            DODAG Size           |

Figure 2: DODAG Size field Format

DODAG Size: 16 bits. It is encoded using 16 bits in unsigned integer format.

3.2. Disserminating DODAG size through DIO

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

3.3. Disserminating DODAG size through DAO-ACK

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.

4. IANA Considerations

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:

Table 1: New DODAG Metric Object Type
Value Description Reference
9 (suggested) DODAG size This document

5. Security Considerations

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.

6. Acknowledgments

The authors wish to thank TBD.

7. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP., and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, , <>.
Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N., and D. Barthel, "Routing Metrics Used for Path Calculation in Low-Power and Lossy Networks", RFC 6551, DOI 10.17487/RFC6551, , <>.
Vasseur, JP., "Terms Used in Routing for Low-Power and Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, , <>.

8. Informative References

IEEE standard for Information Technology, "IEEE Std. 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks".
Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, , <>.
Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A., and M. Richardson, Ed., "A Security Threat Analysis for the Routing Protocol for Low-Power and Lossy Networks (RPLs)", RFC 7416, DOI 10.17487/RFC7416, , <>.

Authors' Addresses

Huimin She
Cisco Systems, Inc
Xinsi Building
No. 926 Yishan Road, Xuhui District
Li Zhao
Cisco Systems, Inc
Xinsi Building
No. 926 Yishan Road, Xuhui District
Pascal Thubert
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 MOUGINS - Sophia Antipolis