Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks (LLNs)

RPL, the IPv6 distance vector routing protocol for Low-power and Lossy Networks (LLNs), is designed to support multiple traffic flows through a root-based Destination-Oriented Directed Acyclic Graph (DODAG). For traffic flows between routers within the DODAG (i.e., Point-to-Point (P2P)), this means that data packets either have to traverse the root in non-storing mode (source routing), or traverse a common ancestor in storing mode (hop-by-hop routing). Such P2P traffic is thereby likely to flow along sub-optimal routes and may suffer severe traffic congestion near the DAG root , . To discover optimal paths for P2P traffic flows in RPL, P2P-RPL specifies a temporary DODAG where the source acts as temporary root. The source initiates "P2P Route Discovery mode (P2P-RDO)" with an address vector for both non-storing mode (H=0) and storing mode (H=1). Subsequently, each intermediate router adds its IP address and multicasts the P2P-RDO message, until the message reaches the target node (TargNode). TargNode sends the "Discovery Reply" option. P2P-RPL is efficient for source routing, but much less efficient for hop-by-hop routing due to the extra address vector overhead. In fact, when the P2P-RDO message is being multicast from the source hop-by-hop, receiving nodes are able to determine a next hop towards the source in symmetric links. When TargNode subsequently replies to the source along the established forward route, receiving nodes can determine the next hop towards TargNode. In other words, it is efficient to use only routing tables for P2P-RDO message instead of "Address vector" for hop-by-hop routes (H=1) in symmetric links. RPL and P2P-RPL both specify the use of a single DODAG in networks of symmetric links. But, application-specific routing requirements that are defined in IETF ROLL Working Group , , and may need routing metrics and constraints related to asymmetric bidirectional links. For this purpose, describes bidirectional asymmetric links for RPL with Paired DODAGs, for which the DAG root (DODAGID) is common for two Instances. This can satisfy application-specific routing requirements for bidirectional asymmetric links in base RPL . P2P-RPL for Paired DODAGs, on the other hand, requires two DAG roots: one for the source and another for the target node due to temporary DODAG formation. For networks composed of bidirectional asymmetric links (see ), AODV-RPL specifies P2P route discovery, utilizing RPL with a new MoP. AODV-RPL makes use of two multicast messages to discover possibly asymmetric routes. AODV-RPL eliminates the need for address vector control overhead, significantly reducing the control packet size which is important for Constrained LLN networks. Both discovered routes meet the application specific metrics and constraints that are defined in the Objective Function for each Instance .

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 . Additionally, this document uses the following terms: Ad Hoc On-demand Distance Vector Routing. Either the RREQ-Instance or RREP-Instance A link that can be used in both directions but with different link characteristics (see ). AODV Instance built using the RREQ option; used for control transmission from OrigNode to TargNode, thus enabling data transmission from TargNode to OrigNode. AODV Instance built using the RREP option; used for control transmission from TargNode to OrigNode thus enabling data transmission from OrigNode to TargNode. Routing along the direction from OrigNode to TargNode. Routing when each node stores routing information about the next hop. The IPv6 router (Originating Node) initiating the AODV-RPL route discovery to obtain a route to TargNode. Two DODAGs for a single application. Point-to-Point -- in other words, not constrained to traverse the global DODAG root. An AODV-RPL MoP DIO message containing the RREP option An AODV-RPL MoP DIO message containing the RREQ option The mechanism by which the source supplies the complete route towards the target node along with each data packet . The IPv6 router (Target Node) for which OrigNode requires a route and initiates Route Discovery within the LLN network. Routing along the direction from TargNode to OrigNode.

With AODV-RPL, routes from OrigNode to TargNode within the LLN network established are "on-demand". In other words, the route discovery mechanism in AODV-RPL is invoked reactively when OrigNode has data for delivery to the TargNode but existing routes do not satisfy the application's requirements. The routes discovered by AODV-RPL are point-to-point; in other words the routes are not constrained to traverse a global DAG. Unlike base RPL and P2P-RPL , AODV-RPL can enable asymmetric communication paths in networks with bidirectional asymmetric links. For this purpose, AODV-RPL enables discovery of two routes: namely, one from OrigNode to TargNode, and another from TargNode to OrigNode. When possible, AODV-RPL also enables symmetric routing along Paired DODAGs (see ).

In AODV-RPL, route discovery is initiated by forming a temporary DAG rooted at the OrigNode. Paired DODAGs (Instances) are constructed according to a new AODV-RPL Mode of Operation (MoP) during route formation between the OrigNode and TargNode. The RREQ-Instance is formed by route control messages from OrigNode to TargNode whereas the RREP-Instance is formed by route control messages from TargNode to OrigNode (as shown in ). Intermediate routers join the Paired DODAGs based on the rank as calculated from the DIO message. Henceforth in this document, the RREQ-Instance message means the AODV-RPL DIO message from OrigNode to TargNode, containing the RREQ option. Similarly, the RREP-Instance means the AODV-RPL DIO message from TargNode to OrigNode, containing the RREP option. Subsequently, the RREQ-Instance is used for data transmission from TargNode to OrigNode and RREP-Instance is used for Data transmission from OrigNode to TargNode. The AODV-RPL Mode of Operation defines a new bit, the Symmetric bit ('S'), which is added to the base DIO message as illustrated in . OrigNode sets the the 'S' bit to 1 in the RREQ-Instance message when initiating route discovery.

A device originating a AODV-RPL message supplies the following information in the DIO header of the message: Symmetric bit in the DIO base object MOP operation in the DIO object MUST be set to "5(TBD1)" for AODV-RPL DIO messages RPLInstanceID in the DIO object MUST be the InstanceID of AODV-Instance. DODAGID in the DIO object MUST be the IPv6 address of the device that initiates the AODV-Instance. Rank in the DIO object MUST be the the rank of the AODV-Instance AODV-Instance messages MAY carry one or more Metric Container options to indicate the relevant routing metrics The 'S' bit is set to mean that the route is symmetric. If the RREQ-Instance arrives over an interface that is known to be symmetric, and the 'S' bit is set to 1, then it remains set at 1, as illustrated in .

|<--S=1-->|<--S=1-->| | | | | <--S=1--> | | <--S=1--> | | | | | | | | S---------R---------R---------R---------R---------R---------D <--S=1-->| | | |<--S=1-->|<--S=1-->| | | | | | | | | | | | | R---------R---------R---------R---------R---------R >---- RREQ-Instance (Control: S-->D; Data: D-->S) -------> <---- RREP-Instance (Control: D-->S; Data: S-->D) -------< ]]> If the RREQ-Instance arrives over an interface that is not known to be symmetric, or is known to be asymmetric, the 'S' bit is set to be 0. Moreover, if the 'S' bit arrives already set to be '0', it is set to be '0' on retransmission (). Based on the 'S' bit received in RREQ-Instance, the TargNode decides whether or not the route is symmetric before transmitting the RREP-Instance message upstream towards the OrigNode.

|--S=1-->|--S=0-->| | | | | --S=1--> | | --S=0--> | | | | --S=1-->| | | | S--------R---------R--------R--------R--------R---------D <--S=0--| | | |--S=0-->| --S=0-->| | | | | | | <--S=0-- | | | | <--S=0-- | | | | | | | <--S=0--|<--S=0--|<--S=0--|<--S=0--|<--S=0-- | R---------R--------R--------R--------R---------R >---- RREQ-Instance (Control: S-->D; Data: D-->S) -------> <---- RREP-Instance (Control: D-->S; Data: S-->D) -------< ]]>

OrigNode supplies the following information in the RREQ option of the RREQ-Instance message: The type of the RREQ option (see ) Sequence Number of OrigNode. If nonzero, the last known Sequence Number for TargNode for which a route is desired. IPv6 address of the TargNode that receives RREQ-Instance message. This address MUST be in the RREQ option (see ) of AODV-RPL. In order to establish the upstream route from TargNode to OrigNode, OrigNode multicasts the RREQ-Instance message (see ) to its one-hop neighbours. Each intermediate node R_i computes the rank for RREQ-Instance and creates a routing table entry for the upstream route towards the source if the routing metrics/constraints are satisfied. For this purpose R_i must use the asymmetric link metric measured in the upstream direction, from R_i to its upstream neighbor that multicasted the RREP-Instance message. If the path towards TargNode is not known, the intermediate node multicasts the RREQ-Instance message with updated rank to its next-hop neighbors until the message reaches to TargNode (). Based on the 'S' bit in the received RREQ message, the TargNode will decide whether to unicast or multicast the RREP message back to OrigNode. As described in , in certain circumstances R_i MAY unicast a Gratuitous RREP towards OrigNode, thereby helping to minimize multicast overhead during the Route Discovery process.

The TargNode supplies the following information in the RREP message:

The type of the RREP option (see ) The Sequence Number for the TargNode for which a route is established. The size of the prefix which the route to the TargNode is available. This allows routing to other nodes on the same subnet as the TargNode. (see ) The OrigNode IP address is available as the DODAGID in the DIO base message (see ). When TargNode receives a RREQ message with the 'S' bit set to 1 (as illustrated in ), it unicasts the RREP message with the 'S' bit set to 1. In this case, route control messages and application data between OrigNode and TargNode for both RREQ-Instance and RREP-Instance are transmitted along symmetric links. When (as illustrated in ) the TargNode receives RREQ message with the 'S' bit set to 0, it also multicasts the RREP message with the 'S' bit set to 0. Intermediate nodes create a routing table entry for the path towards the TargNode while processing the RREP message to OrigNode. Once OrigNode receives the RREP message, it starts transmitting application data to TargNode along the path as discovered through RREP messages. Similarly, application data from TargNode to OrigNode is transmitted through the path that is discovered from RREQ message.

Under some circumstances, an Intermediate Node that receives a RREQ message MAY transmit a "Gratuitous" RREP message back to OrigNode instead of continuing to multicast the RREQ message towards TargNode. For these circumstances, the 'G' bit of the RREP option is provided to distinguish the Gratuitous RREP sent by the Intermediate node from the RREP sent by TargNode. When an Intermediate node R receives a RREQ message and has recent information about the cost of an upstream route from TargNode to R, then R MAY unicast the Gratuitous RREP (GRREP) message to OrigNode. R determines whether its information is sufficiently recent by comparing the value it has stored for the Sequence Number of TargNode against the DestSeqno in the incoming RREQ message. R also must have information about the metric information of the upstream route from TargNode. The GRREP message MUST have PrefixSz == 0 and the 'G' bit set to 1. R SHOULD also unicast the RREQ message to TargNode, to make sure that TargNode will have a route to OrigNode.

The trickle timer operation to control RREQ-Instance/RREP-Instance multicast is similar to that in P2P-RPL .

IANA is required to assign a new Mode of Operation, named "AODV-RPL" for Point-to-Point(P2P) hop-by-hop routing under the RPL registry. The value of TBD1 is assigned from the "Mode of Operation" space .

Two entries are required for new AODV-RPL options "RREQ-Instance" and "RREQ-Instance", with values of TBD2 (0x0a) and TBD3 (0x0b) from the "RPL Control Message Options" space .

This document does not introduce additional security issues compared to base RPL. For general RPL security considerations, see .