Root initiated routing state in RPL

The Routing Protocol for Low Power and Lossy Networks (LLN) (RPL) specification defines a generic Distance Vector protocol that is designed for very low energy consumption and adapted to a variety of LLNs. RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs) which root often acts as the Border Router to connect the RPL domain to the Internet. The root is responsible to select the RPL Instance that is used to forward a packet coming from the Internet into the RPL domain and set the related RPL information in the packets. In the non-storing mode (NSM) of operation (MOP), the root also computes routes down the DODAG towards the end device and leverages source routing to get there, while the default route via the root is used for routing upwards within the LLN and to the Internet at large. NSM is the dominant MOP because because networks may get arbitrary large and in Storing Mode, the amount of memory in nodes close to the root may unexpectedly require memory beyond a node's capabilities. But as a network gets deep, the size of the source routing header that the root must add to all the downward packets may also become an issue for far away target devices. In some use cases, a RPL network forms long lines and a limited amount of well-targeted routing state would allow to make the source routing operation loose as opposed to strict, and save packet size. Limiting the packet size is directly beneficial to the energy budget, but, mostly, it reduces the chances of frame loss and/or packet fragmentation, which is highly detrimental to the LLN operation. Because the capability to store a routing state in every node is limited, the decision of which route is installed where can only be optimized with a global knowledge of the system, a knowledge that the root has in non-storing mode. Additionally, RPL storing mode is optimized or Point-to-Multipoint (P2MP), root to leaves and Multipoint-to-Point (MP2P) leaves to root operations, whereby routes are always installed along the RPL DODAG. Transversal Peer to Peer (P2P) routes in a RPL network will generally suffer from some stretch since routing between 2 peers always happens via a common parent. In NSM, all peer-to-peer routes travel all the way to the root, which adds a source routing header and forwards the packet down to the destination, resulting in the longest stretch and overload of the radio bandwidth near the root. A controller, for instance collocated with the RPL root, with enough topological awareness of the connectivity between nodes, would be able to compute more direct routes, avoiding the vicinity of the root whenever possible. The 6TiSCH architecture leverages the Deterministic Networking Architecture as one possible model whereby the device resources and capabilities are exposed to an external controller which installs routing states into the network based on some objective functions that reside in that external entity. Based on heuristics of usage, path length, and knowledge of device capacity and available resources such as battery levels and reservable buffers, a Path Computation Element () with a global visibility on the system could install additional P2P routes that are more optimized for the current needs as expressed by the objective function. This draft enables a RPL root, with optionally the assistance of a PCE, to install and maintain additional storing mode routes within the RPL domain, along a selected set of nodes and for a selected duration, thus providing routes from suitable than those obtained from the distributed operation of RPL in either storing and non-storing modes.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . The Terminology used in this document is consistent with and incorporates that described in `Terminology in Low power And Lossy Networks' and .

Section 6.7 of specifies Control Message Options (CMO) to be placed in RPL messages such as the DAO message. The RPL Target Option and the Transit Information Option (TIO) are such options; the former indicates a node to be reached and the latter specifies a parent that can be used to reach that node. Options may be factorized; one or more contiguous TIOs apply to the one or more contiguous Target options that immediately precede the TIOs in the RPL message. This specification introduces a new Control Message Option, the Via Information option (VIO). Like the TIO, the VIO MUST be preceded by one or more RPL Target options to which it applies. Unlike the TIO, the VIO are not factorized: multiple contiguous Via options indicate an ordered sequence of hops to reach the target(s), presented in the same order as they would appear in a routing header.

The Via Information option MAY be present in DAO messages, and its format is as follows:

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x0A | Option Length | Path Sequence | Path Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + . . . Next-Hop Address . . . + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 0x0A (to be confirmed by IANA) Variable, depending on whether or not Parent Address is present. 8-bit unsigned integer. When a RPL Target option is issued by the root of the DODAG (i.e. in a DAO message), that root sets the Path Sequence and increments the Path Sequence each time it issues a RPL Target option with updated information. The indicated sequence deprecates any state for a given Target that was learned from a previous sequence and adds to any state that was learned for that sequence. 8-bit unsigned integer. The length of time in Lifetime Units (obtained from the Configuration option) that the prefix is valid for route determination. The period starts when a new Path Sequence is seen. A value of all one bits (0xFF) represents infinity. A value of all zero bits (0x00) indicates a loss of reachability. A DAO message that contains a Via Information option with a Path Lifetime of 0x00 for a Target is referred as a No-Path (for that Target) in this document. 8 or 16 bytes. IPv6 Address of the next hop towards the destination(s) indicated in the target option that immediately precede the VIO. The /64 prefix can be elided if it is the same as that of (all of) the target(s). In that case, the Next-Hop Address is expressed as the 8-bytes suffix only, otherwise it is expressed as 16 bytes.

A classical RPL implementation in a very constrained LLN uses the non-storing mode of operation whereby a RPL node indicates a parent-child relationship to the root, using a Destination Advertisement Object (DAO) that is unicast from the node directly to the root, and the root builds a path to a destination down the DODAG by concatenating this information.

------+--------- | Internet | +-----+ | | Border Router | | (RPL Root) +-----+ ^ | | | | DAO | ACK | o o o o | | | Strict o o o o o o o o o | | | Source o o o o o o o o o o | | | Route o o o o o o o o o | | | o o o o o o o o | v v o o o o LLN Nodes are not expected to store downward routing state via their children, and the routing operates in strict source routing mode as detailed in An IPv6 Routing Header for Source Routes with RPL This draft proposes an addition whereby the root projects a route through an extended DAO to an arbitrary node down the DODAG, indicating a child or a direct sequence of children via which a certain destination (target) may be reached. The root is expected to use the mechanism optimally and with required parsimony to fit within the device resources, but how the root figures the amount of resources that are available is out of scope.

------+--------- | Internet | +-----+ | | Border Router | | (RPL Root) +-----+ | ^ | | | DAO | ACK | o o o o | | | Loose o o o o o o o o o | ^ | Source o o o o o o o o o o | | DAO | Route o o o o o o o o o | ^ | o o o o o o o o v | DAO v o o o o LLN When a RPL domain operates in non-storing Mode of Operation (NS-MOP), only the root possesses routing information about the whole network. A packet that is generated within the domain first reaches the root, which can then apply a source routing information to reach the destination. Similarly, a packet coming from the outside of the domain for a destination that is expected to be in a RPL domain reaches the root. In NS-MOP, the root, or some associated centralized computation engine, can thus determine the amount of packets that reach a destination in the RPL domain, and thus the amount of energy and bandwidth that is wasted for transmission, between itself and the destination, as well as the risk of fragmentation, any potential delays because of a paths longer than necessary (shorter paths exist that would not traverse the root). Additionally, the DAG root knows the whole DAG topology, so when the source of a packet is also in the RPL domain, the root can determine the common parent that would have been used in storing mode, and thus the list of nodes in the path between the common parent and the destination. For instance in the below diagram, if the source is 41 and the destination 52, the common parent is the node 22.

------+--------- | Internet | +-----+ | | Border Router | | (RPL Root) +-----+ | \ \____ / \ \ o 11 o 12 o 13 / | / \ o 22 o 23 o 24 o 25 / \ | \ \ o 31 o 32 o o o 35 / / | \ | \ o 41 o 42 o o o 45 o 46 | | | | \ | o 51 o 52 o 53 o o 55 o 56 LLN With this draft, the root can install routing states along a segment that is either itself to the destination, or from one or more common parents for a particular source/destination pair towards that destination (in our example, this would be the segment made of nodes 22, 32, 42). The draft expects that the root has enough information about the capability for each node to store a number of routes, which can be discovered for instance using a Network Management System (NMS) and/or the RPL routing extensions specified in Routing for Path Calculation in LLNs. Based on that information, the root computes which segment should be routed and which relevant state should be installed in which nodes. The algorithm is out of scope but it is envisaged that the root could compute the ratio between the optimal path (existing path not traversing the root, and the current path), the application SLA for specific flows that could benefit from shorter paths, the energy wasted in the network, local congestion on various links that would benefit from having flows routed along other paths. This draft introduces a new mode of operation for loose source routing in the LLN, the Non-Storing with Projected routes MOP. With this new MOP, the root sends a unicast DAO message to the last node of the routing segment that must be installed. The DAO message contains the ordered list of hops along the segment as a list of Via Information options that are preceded by one or more RPL Target options to which they relate. Each Via Information option contains a lifetime for which state is to be maintained. The root sends the DAO directly to the last node in the segment, which is expected to be able to route to the targets on its own. The last node in the segment may have another information to reach the target(s), such as a connected route or an already installed projected route. If it does not have such a route then the node should lookup the address on the relevant interfaces. If one of the targets cannot be located, the node MUST answer to the root with a negative DAO-ACK listing the target(s) that could not be located (suggested status 10), and continue the process for those targets that could be located if any. For the targets that could be located, last node in the segment generates a DAO to its loose predecessor in the segment as indicated in the list of Via Information options. The node strips the last Via Information option which corresponds to self, and uses it as source address for the DAO to the predecessor. The address of the predecessor to be used as destination for the DAO message is found in the now last Via Information option. The predecessor is expected to have a route to the address used as source, either connected, installed previously as another DAO, or from other means. The predecessor is expected to have a route to the address used as source and that is his successor. If it does not and cannot locate the successor, the predecessor node MUST answer to the root with a negative DAO-ACK indicating the successor that could not be located. The DAO-ACK contains the list of targets that could not be routed to (suggested status 11). If the predecessor can route to the successor node, then it installs a route to the targets via the successor. If that route is not connected then a recursive lookup will take place to reach the target(s). From there, the node strips the last Via Information option and either answers to the root with a positive DAO-ACK that contains the list of targets that could be routed to, or propagates the DAO to its own predecessor. A NULL lifetime in the Via Information option along the segment is used to clean up the state. In the example below, say that there is a lot of traffic to nodes 55 and 56 and the root decides to reduce the size of routing headers to those destinations. The root can first send a DAO to node 45 indicating target 55 and a Via segment (35, 45), as well as another DAO to node 46 indicating target 56 and a Via segment (35, 46). This will save one entry in the routing header on both sides. The root may then send a DAO to node 35 indicating targets 55 and 56 a Via segment (13, 24, 35) to fully optimize that path. Alternatively, the root may send a DAO to node 45 indicating target 55 and a Via segment (13, 24, 35, 45) and then a DAO to node 46 indicating target 56 and a Via segment (13, 24, 35, 46), indicating the same DAO Sequence.

With the initial specifications of RPL , the P2P path from a source to a destination is often stretched, as illustrated in : - in non-storing mode, all packets routed within the DODAG flow all the way up to the root of the DODAG. If the destination is in the same DODAG, the root must encapsulate the packet to place a Routing Header that has the strict source route information down the DODAG to the destination. This will be the case even if the destination is relatively close to the source and the root is relatively far off. - in storing mode, unless the destination is a child of the source, the packets will follow the default route up the DODAG as well. If the destination is in the same DODAG, they will eventually reach a common parent that has a DAO route to the destination; at worse, the common parent may also be the root. From that common parent, the packet will follow a path down the DODAG that is optimized for the Objective Function that was used to build the DODAG. It results that it is often beneficial to enable additional P2P routes, either if the RPL route present a stretch from shortest path, or if the new route is engineered with a different objective.

------+--------- | Internet | +-----+ | | Border Router | | (RPL Root) +-----+ X ^ v o o ^ o o v o o o o o ^ o o o v o o o o o ^ o o v o o o o o S o o o D o o o o o o o LLN For that reason, earlier work at the IETF introduced the Reactive Discovery of Point-to-Point Routes in Low Power and Lossy Networks, which specifies a distributed method for establishing optimized P2P routes. This draft proposes an alternate based on a centralized route computation. It must be noted that RPL has a concept of instance but does not have a concept of an administrative distance, which exists in certain proprietary implementations to sort out conflicts between multiple sources. This draft conforms the instance model as follows: - if the PCE needs to influence a particular instance to add better routes in conformance with the routing objectives in that instance, it may do so. When the PCE modifies an existing instance then the added routes must not create a loop in that instance. This is achieved by always preferring a route obtained from the PCE over a route that is learned via RPL. - If the PCE installs a more specific (Traffic Engineering) route between a particular pair of nodes then it should use a Local Instance from the ingress node of that path. Only packets associated with that instance will be routed along that path. In all cases, the path is indicated by VIA options, and the flow is similar to the flow used to obtain loose source routing. The root sends the DAO with the target option and the Via Option to the lest router in the path; the last router removes the last Via Option and passes the DAO to the previous hop.

------+--------- | Internet | +-----+ | | Border Router | | (RPL Root) +-----+ ^ Projected DAO-ACK from S / o o o / o o o o o o o | o o o o o o o o o | o o o o o o o o S A B C D o o o o o o o LLN The process recurses till the destination which sends a DAO-ACK to the root. In the example above, for target D, the list of via options is S, A, B and C. The projected DAO is sent by the root to

This draft uses messages that are already present in with optional secured versions. The same secured versions may be used with this draft, and whatever security is deployed for a given network also applies to the flows in this draft.

This document updates the IANA registry for the Mode of Operation (MOP) 4: Non-Storing with Projected routes [this] This document updates IANA registry for the RPL Control Message Options 0x0A: Via descriptor [this]

The authors wish to acknowledge JP Vasseur and Patrick Wetterwald for their contributions to the ideas developed here.