Internet Engineering Task Force M. Goyal, Ed. Internet-Draft University of Wisconsin Milwaukee Intended status: Experimental E. Baccelli, Ed. Expires: February 27, 2011 INRIA August 26, 2010 Reactive Discovery of Point-to-Point Routes in Low Power and Lossy Networks draft-ietf-roll-p2p-rpl-00 Abstract Point to point (P2P) communication between arbitrary IPv6 routers and hosts in a Low power and Lossy Network (LLN) is a key requirement for many applications. RPL, the IPv6 Routing Protocol for LLNs, constrains the LLN topology to a Directed Acyclic Graph (DAG) and requires the P2P routing to take place along the DAG links. Such P2P routes may be significantly suboptimal and may lead to traffic congestion near the DAG root. This document describes a P2P route discovery mechanism complementary to RPL base functionality. This mechanism allows an RPL-aware IPv6 router or host to discover and establish on demand one or more routes to another RPL-aware IPv6 router or host in the LLN such that the discovered routes meet the specified cost criteria. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on February 27, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. Goyal & Baccelli Expires February 27, 2011 [Page 1] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Targeted Use Cases . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Functional Overview . . . . . . . . . . . . . . . . . . . . . 6 5. Propagation of Discovery Messages . . . . . . . . . . . . . . 7 5.1. The Route Discovery Option . . . . . . . . . . . . . . . . 8 5.2. Setting a DIO Carrying a Route Discovery Option . . . . . 9 5.3. Joining a Temporary DAG . . . . . . . . . . . . . . . . . 11 5.4. Processing a DIO Carrying a Route Discovery Option . . . . 11 5.5. Additional Processing of a DIO Carrying a Route Discovery Option At An Intermediate Router . . . . . . . . 12 5.6. Additional Processing of a DIO Carrying a Route Discovery Option At The Target Node . . . . . . . . . . . 13 6. Propagation of Discovery Reply Messages . . . . . . . . . . . 13 6.1. The Discovery Reply Object (DRO) . . . . . . . . . . . . . 14 6.1.1. The Source Route Option . . . . . . . . . . . . . . . 16 6.1.2. Processing a DRO At An Intermediate Router . . . . . . 17 6.2. DRO as Acknowledgement for Backward Source Routes . . . . 18 6.3. DRO as Carrier of Forward/Bidirectional Source Routes . . 18 6.4. Establishing Hop-by-hop Routes Via DRO . . . . . . . . . . 18 7. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 19 8. Security Considerations . . . . . . . . . . . . . . . . . . . 20 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 10. Authors and Contributors . . . . . . . . . . . . . . . . . . . 20 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 11.1. Normative References . . . . . . . . . . . . . . . . . . . 20 11.2. Informative References . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 Goyal & Baccelli Expires February 27, 2011 [Page 2] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 1. Introduction RPL [I-D.ietf-roll-rpl] provides multipoint-to-point (MP2P) routes from nodes in a Low power and Lossy Network (LLN) to a sink node by organizing the nodes along a Directed Acyclic Graph (DAG) rooted at the sink. The nodes determine their position in the DAG so as to optimize their routing cost to reach the DAG root. A node advertises its position (the "rank") in the DAG by originating a DODAG Information Object (DIO) message. The DIO message is sent via link- local multicast and also includes information such as the DAG root's identity, the routing metrics/constraints [I-D.ietf-roll-routing-metrics] and the objective function (OF) in use. When a node joins the DAG, it determines its own rank in the DAG based on that advertised by its neighbors and originates its own DIO message. RPL enables point-to-multipoint (P2MP) routing from a node to its descendants in the DAG by allowing a node to send a Destination Advertisement Object (DAO) upwards along the DAG. The DAO carries the potentially aggregated information regarding the descendants (and other local prefixes) reachable through the originating node. RPL also provides mechanisms for point-to-point (P2P) routing between any two nodes in the DAG. If the destination is within the source's "range", the source may directly send packets to the destination. Otherwise, a packet's path from the source to the destination depends on the storing/non-storing operation mode of the DAG. In non-storing mode operation, only the DAG root maintains downward routing information and hence a packet travels all the way to the DAG root, which then sends it towards its destination using a source route. In storing mode operation, if the destination is a DAG descendant and the source maintains "downwards" routing state about this descendant, it can forward the packet along this route. Otherwise, the source sends the packet to a DAG parent, which then applies the same set of rules to forward the packet further. Thus, a packet travels up the DAG until it reaches a node that knows of the downwards route to the destination and then it travels down the DAG towards its destination. A node may or may not maintain routing state about a descendant depending on whether its immediate children send it such information in their DAOs. Thus, in the best case storing mode scenario, the "upwards" segment of the P2P route between a source and a destination ends at the first common ancestor of the source and the destination. In the worst case, the "upwards" segment would extend all the way to the DAG's root. In both storing and non-storing mode operations, if the destination did not originate a DAO, the packet will travel all the way to the DAG's root, where it will be dropped. The P2P routing functionality available in RPL may be inadequate for Goyal & Baccelli Expires February 27, 2011 [Page 3] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 applications in the home and commercial building domains because of the following reasons [I-D.brandt-roll-rpl-applicability-home-building] [RFC5826][RFC5867]: o The need to maintain routes "proactively", i.e. every possible destination in the DAG must originate a DAO. o Depending on the network topology and OF/metrics in use, the constraint to route only along a DAG may potentially cause significantly suboptimal P2P routes and severe traffic congestion near the DAG root. Clearly, there is a need for a mechanism that provides source- initiated discovery of P2P routes that are not along an existing DAG. This document thus describes such a mechanism, complementary to the basic RPL functionality. The specified scheme is based on a reactive on-demand approach, which enables a node to discover one or more "good enough" routes in either direction between itself and another node in the LLN without any constraints regarding the existing DAG-membership of the links that such routes may use. Such routes may be source-routes or hop-by-hop ones. A complementary functionality, necessary to help decide whether to initiate a route discovery, is a mechanism to measure the end-to-end cost of an existing route. Section 7 provides further details on how such functionality, to be described in a separate document, can be used to determine the "good enough" criteria for use in the route discovery mechanism described in this document. 2. Targeted Use Cases The mechanisms described in this document are intended to be employed as complementary to RPL in specific scenarios that need point-to- point (P2P) routes between arbitrary routers. One target use case, common in a home environment, involves a remote control (or a motion sensor) that suddenly needs to communicate with a lamp module, whose network address it knows apriori. In this case, the source of data (the remote control or the motion sensor) must be able to discover a route to the destination (the lamp module) "on demand". Another target use case, common in a large commercial building environment, involves a large LLN deployment where P2P communication along a particular DAG among hundreds (or thousands) of routers creates severe traffic congestion near that DAG's root, and thus routes across this DAG are desirable. Goyal & Baccelli Expires February 27, 2011 [Page 4] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 Targeted use cases also include scenarios where energy or latency constraints are not satisfied by the P2P routes along a DAG because they involve traversing many more intermediate routers than necessary to reach the destination. 3. Terminology 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 [RFC2119]. Additionally, this document uses terminology from [I-D.ietf-roll-terminology] and [I-D.ietf-roll-rpl]. Specifically, the term node refers to an RPL router or an RPL host as defined in [I-D.ietf-roll-rpl]. This document introduces the following terms: Origin Node: The RPL node initiating the route discovery. The origin node acts as one end point of the routes to be discovered. Target Node: The RPL node at the other end point of the routes to be discovered. Intermediate Router: An RPL router that is neither the origin nor the target. Forward Route: A route from the origin node to the target node. Backward Route: A route from the target node to the origin node. Bidirectional Route: A route that can be used in both directions: from the origin node to the target node and vice versa. Source Route: A complete and ordered list of routers that can be used by a packet to travel from a source node to a destination node. Such source routes can be carried by a packet in a proposed Type 4 Routing Header [I-D.ietf-6man-rpl-routing-header]. Hop-by-hop Route: The route characterized by each router on the route using its routing table to determine the next hop on the route. Propagation Constraints: The constraints on aggregated routing metric values, as defined in [I-D.ietf-roll-routing-metrics], that MUST be satisfied before an intermediate router will process the Route Discovery Option (defined in this document) contained inside a DODAG Information Object (DIO). Goyal & Baccelli Expires February 27, 2011 [Page 5] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 Route Constraints: Additional constraints on aggregated routing metric values, as defined in [I-D.ietf-roll-routing-metrics], that MUST be satisfied by a discovered route in order to be considered "good enough". Good Enough Criteria: The propagation constraints and the route constraints together constitute the good enough criteria. 4. Functional Overview This section contains a high level description of the route discovery mechanism proposed in this document. The route discovery begins with the origin node generating a "Discovery" message. The origin node indicates in the message: o The target node; o The relevant routing metrics; o The constraints on how far the Discovery message may travel (henceforth called the propagation constraints); o Additional constraints used to determine if a discovered route is "good enough" (henceforth called the route constraints); o The direction (forward: from the origin node to the target node; backward: from the target node to the origin node; or bidirectional) of the route being discovered; o The desired number of routes; o Whether the route is a source-route or a hop-by-hop one. The Discovery message propagates via IPv6 link-local multicast with a receiving router discarding the message if it does not satisfy the propagation constraints or if hop-by-hop routes are desired and the router cannot store state for such a route. As a copy of the Discovery message travels towards the target node, it accumulates the relevant routing metric values as well as the route it takes. When the target node receives a copy of the Discovery message, it applies both the propagation constraints and the route constraints to determine if the discovered route is good enough. Thus, the good enough discovered routes satisfy both the propagation constraints as well as the route constraints although the propagation of Discovery messages is guided by propagation constraints alone. The propagation constraints and the route constraints together constitute the good Goyal & Baccelli Expires February 27, 2011 [Page 6] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 enough criteria. Using only a subset of the good enough criteria as the propagation constraints simplifies the operation of intermediate routers, an important consideration in many LLN application domains. The route discovery process may result in the discovery of several good enough routes. This document does not specify how does the target node select routes among the good enough ones. Example selection methods include selecting the routes as they are discovered or selecting the best routes discovered over a certain time period. If the origin node had requested the discovery of backward source- routes, the target node caches one or more good enough source-routes it selects. Additionally, the target node sends one or more "Discovery Reply" message to the origin node to acknowledge the discovery of these routes. These acknowledgements allow the origin node to judge the success of the route discovery. If the origin node had requested the discovery of "n" forward source- routes, the target node sends the "n" good enough source-routes it selects to the origin node in one or more Discovery Reply messages. If the origin node had requested the discovery of "n" bidirectional source-routes, the target node caches the "n" good enough source- routes it identifies and also sends these routes to the origin node in one or more Discovery Reply messages. If the origin node had requested the discovery of "n" forward/ backward/bidirectional hop-by-hop routes, the target node sends out a Discovery Reply message to the origin node for each one of the "n" good enough routes it selects. The Discovery Reply message travels towards the origin node along the discovered route. As this message travels towards the origin node, it establishes appropriate forward/ backward routing state in the routers on the path. 5. Propagation of Discovery Messages RPL uses DIO message propagation to build a DAG. The DIO message travels via IPv6 link-local multicast. Each node joining the DAG determines a rank for itself and ignores the subsequent DIO messages received from lower (higher in numerical value) ranked neighbors. Thus, the DIO messages propagate outward from the DAG root rather than return inward towards the DAG root. The DIO message generation at a node is further controlled by a trickle timer that allows a node to avoid generating unnecessary messages [I-D.ietf-roll-trickle]. The link-local multicast based propagation, trickle-controlled generation and the rank-based poisoning of messages traveling in the wrong direction (towards the DAG root) provide powerful incentives to Goyal & Baccelli Expires February 27, 2011 [Page 7] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 use the DIO message as the Discovery message and propagate the DIO/ Discovery message by creating a "temporary" DAG. The routing metrics used for the creation of this temporary DAG SHOULD be same as (or be a subset of) the routing metrics being used for route discovery. Similarly, the objective function, used for rank calculation in the temporary DAG, SHOULD be same as the objective function that determines the aggregated cost of a route when limited to the routing metrics being used for temporary DAG creation. The propagation constraints limit the spread of the temporary DAG. The temporary DAG restricts the network topology within which the route discovery takes place. Thus, all the discovered routes lie within this restricted topology and implicitly satisfy the propagation constraints. Among the discovered routes, the good enough routes are the ones that meet the route constraints. Thus, for successful route discovery, the propagation constraints and the route constraints MUST be compatible. The division of the overall good enough criteria between the two sets of constraints is an implementation specific decision. If desired, an implementation MAY include all constraints in the set of propagation constraints and keep the set of route constraints empty. 5.1. The Route Discovery Option 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 = 9 | Option Length | D |H| N | L |O| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Target Address | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OCP | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Format of the Route Discovery Option In order to be used as a Discovery message, a DIO MUST carry a "Route Discovery" option illustrated in Figure 1. A DIO MUST NOT carry more than one Route Discovery options. A router MUST ignore the second and subsequent Route Discovery options carried by a DIO. A Route Discovery option consists of the following fields: o Option Type = 0x09 (to be confirmed by IANA). Goyal & Baccelli Expires February 27, 2011 [Page 8] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 o Option Length = 20 or 22 octets depending on whether the OCP field is included or not. o D: A 2-bit field that indicates the direction of the desired routes: * D = 0x00: Forward; * D = 0x01: Backward; * D = 0x02: Bidirectional. o H: This flag, when set, indicates if hop-by-hop routes are desired. The flag is cleared if source routes are desired. o N: A 3-bit unsigned integer indicating the number of routes desired. o L: A 2-bit field indicating the minimum "Life Time" of the temporary DAG, i.e., the minimum duration a router joining the temporary DAG must maintain its membership in the DAG: * L = 0x00: Minimum life time is 5 seconds; * L = 0x01: Minimum life time is 10 seconds; * L = 0x02: Minimum life time is 1 minute; * L = 0x03: Minimum life time is 10 minutes. o O: This flag, when set, indicates that an OCP field is present in the Route Discovery option. o Target Address: The IPv6 address of the target node. o OCP: 16 bit unsigned integer. An optional field, present only if the O flag is set, This field indicates the objective function that MAY be used by the target node to compare two good enough routes. 5.2. Setting a DIO Carrying a Route Discovery Option A DIO message that carries a Route Discovery option MUST set the Base Object, described in [I-D.ietf-roll-rpl], in the following manner: o RPLInstanceID: RPLInstanceID MUST be a local value as described in Section 4.1 of [I-D.ietf-roll-rpl]. Goyal & Baccelli Expires February 27, 2011 [Page 9] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 o Grounded (G) Flag: MUST be cleared since the objective of DAG formation is the propagation of Route Discovery option. This DAG is temporary in nature and is not used for routing purpose. o Destination Advertisement Supported (A) Flag: MUST be cleared for same reasons as described above. o Destination Advertisement Trigger (T) Flag: MUST be cleared. o Mode of Operation (MOP): This document suggests a new value (0x04) for this field (to be confirmed by IANA). o DODAGPreference (Prf): TBD o Destination Advertisement Trigger Sequence Number (DTSN): TBD o DODAGID: IPv6 address of the origin node. The other fields in the Base Object are set as per the rules described in [I-D.ietf-roll-rpl]. The DODAG Configuration option, carried in the DIO message, specifies the parameters for the trickle timer operation that governs the generation of DIO messages by the routers joining the temporary DAG. The future versions of this document will specify the default values to be used for these parameters. The other fields defined in the DODAG Configuration option are set as follows: o The MaxRankIncrease field MUST be set to 0 to disable local repair of the temporary DAG. o This document RECOMMENDS a value 1 for the MinHopRankInc field. o Objective Code Point (OCP): The OCP to be used for temporary DAG formation. This document RECOMMENDS RPL Objective Function 0, as defined in [I-D.ietf-roll-of0], for use as the objective function for the formation of the temporary DAG. The objective function used for temporary DAG formation SHOULD be compatible with the objective function to determine the aggregated cost of a discovered route. A DIO, that contains a Route Discovery option, MUST specify the propagation constraints in one or more Metric Container options placed before the Route Discovery option and the route constraints in the Metric Container options placed after the Route Discovery option inside the DIO. The routing metrics being used for temporary DAG formation SHOULD be same as or a subset of the routing metrics being used for route discovery. These routing metrics MUST be placed in Goyal & Baccelli Expires February 27, 2011 [Page 10] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 the Metric Container options placed before the Route Discovery option. A DIO, carrying a Route Discovery option, MUST NOT carry any Route Information or Prefix Information options described in [I-D.ietf-roll-rpl]. 5.3. Joining a Temporary DAG When a node joins a temporary DAG advertized by a DIO carrying the Route Discovery option, it MUST maintain its membership in the DAG for the Minimum Life Time duration listed in the Route Discovery option. Maintaining membership in the DAG implies remembering: o The RPLInstanceID, the DODAGID and the DODAGVersionNumber for the temporary DAG; o The node's rank in the temporary DAG as well as the address of at least one DAG parent; o The propagation and the route constraints being used; o In case of intermediate routers, the values for the routing metrics, along with the associated source route from the origin node till this node (carried in a Record Route IPv6 Extension Header proposed in [I-D.thubert-6man-reverse-routing-header]), contained in the best DIO (as per the OCP specified in the DODAG Configuration option) received so far. Although the main purpose of a temporary DAG's existence is to facilitate the propagation of the Route Discovery option, the temporary DAG MAY also be used for the Discovery Reply Object (defined in Section 6.1 to travel from the target node to the origin node. Hence, a node in a temporary DAG SHOULD also remember the address of at least one DAG parent that provides, as per the node's knowledge, the best end-to-end route back to the origin node. A node SHOULD delete information about a temporary DAG once the duration of its membership in the DAG has exceeded the DAG's minimum life time. 5.4. Processing a DIO Carrying a Route Discovery Option The rules for DIO processing and transmission, described in Section 7 of RPL [I-D.ietf-roll-rpl], apply to DIOs carrying a Route Discovery option as well except as modified in this document. The following rules for processing a DIO carrying a Route Discovery Option apply to both intermediate routers and the target nodes. Goyal & Baccelli Expires February 27, 2011 [Page 11] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 A node MUST discard the DIO with no further processing and optionally log an error if any of the following conditions are true: o The node does not support the OCP specified in the DODAG Configuration option. o The node does not support one or more of the metrics contained in the Metric Container options in the DIO. o The node does not have sufficient information to calculate the values of these routing metrics. A node MUST discard the DIO with no further processing and optionally log an error if any of the following conditions are found to be true while processing a Route Discovery option contained in the received DIO: o The H field is set, i.e. hop-by-hop routes are desired, but the node cannot participate in a hop-by-hop route. o The node cannot maintain its membership in the temporary DAG for the minimum life time duration mentioned in the Route Discovery option. 5.5. Additional Processing of a DIO Carrying a Route Discovery Option At An Intermediate Router After executing the steps listed in Section 5.4, an intermediate router processes a received DIO carrying a Route Discovery option in the following manner. The router updates the routing metric values contained in all the Metric Container options inside the DIO. The router MUST discard the DIO with no further processing and optionally log an error if the aggregated values of the routing metrics do not meet every propagation constraint listed in the DIO. The router MAY optionally check the route constraints listed in the DIO and discard the DIO with no further processing if these constraints are not met. The router determines if this DIO is the best it has received so far for this temporary DAG (as per the OCP in the DODAG Configuration object). If yes, the router makes a copy of the routing metric values contained in this DIO along with the route travelled by the DIO so far. The router also resets the trickle timer and, at the expiry of the timer, generates a new DIO for this temporary DAG carrying the Route Discovery option, the best metric values it knows and the source route associated with these values (in a Record Route IPv6 extension header proposed in Goyal & Baccelli Expires February 27, 2011 [Page 12] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 [I-D.thubert-6man-reverse-routing-header]). 5.6. Additional Processing of a DIO Carrying a Route Discovery Option At The Target Node After executing the steps listed in Section 5.4, a target node processes a received DIO carrying a Route Discovery option in the following manner. The target node updates the routing metric values contained in all the Metric Container options inside the DIO. The target node MUST discard the DIO with no further processing and optionally log an error if the aggregated values of the routing metrics do not meet every propagation and route constraint listed in the Metric Container options in the DIO. Otherwise, the target node considers the source route accumulated by the received DIO as good enough and MAY select it as one of the discovered routes. This document does not prescribe a particular method for selecting routes among the good enough ones. Suppose the Route Discovery option requires the target node to select "n" good enough routes. The target node may select these "n" routes in any manner it desires. Example selection methods include selecting the first "n" good enough routes it discovers or selecting the "n" best good enough routes (using the OCP specified in the Route Discovery option to do the comparison) discovered over a certain time period. If the target node selects at least one good enough route, it MUST send one or more RPL Control Messages carrying a Discovery Reply Object (defined in the next section) back to the origin node (identified by the DODAGID field in the DIO Base Object) as discussed in the following sections. A node MUST NOT forward a DIO carrying a Route Discovery option that lists one of its own addresses as the Target Address. 6. Propagation of Discovery Reply Messages Goyal & Baccelli Expires February 27, 2011 [Page 13] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 6.1. The Discovery Reply Object (DRO) 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RPLInstanceID | Version | D |H| N | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | DODAGID(*) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Target Address(*) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option(s)... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+... Figure 2: Format of the Discovery Reply Object (DRO) This document defines a new RPL Control Message type, the Discovery Reply Object (DRO) with code 0x04 (to be confirmed by IANA), that serves the following functions: o An acknowledgement from the target node to the origin node regarding the successful discovery of backward source routes; o Carries one or more forward/bidirectional source routes from the target to the origin node; o Establishes a hop-by-hop forward/backward/bidirectional route as it travels from the target to the origin node. The format for a Discovery Reply Object (DRO) is shown in Figure 2. A DRO consists of the following fields: o RPLInstanceID: The RPLInstanceID of the temporary DAG used for route discovery. o Version: The Version of the temporary DAG used for route discovery. o D: A 2-bit field that indicates the direction of the discovered routes: Goyal & Baccelli Expires February 27, 2011 [Page 14] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 * D = 0x00: Forward; * D = 0x01: Backward; * D = 0x02: Bidirectional. This field has the same value as the corresponding field in the Route Discovery option. o H: A flag that is set if the Discovery Reply Object is establishing an hop-by-hop route. If this flag is set, the Discovery Reply Object also includes: * The DODAGID and Target Address fields; and * One Source Route option (defined in Section 6.1.1) that contains the remaining routers on the hop-by-hop route being established. This flag is clear if the Discovery Reply Object carries (or is an acknowledgement for the discovery of) one or more source routes contained in the Source Route options. o N: A 3-bit field that indicates the number of source routes carried or acknowledged in the Discovery Reply Object. This field MUST have value 1 if the Discovery Reply Object is establishing a hop-by-hop route. o Reserved: These bits are reserved for future use. These bits MUST be set to zero on transmission and MUST be ignored on reception. o DODAGID: The DODAGID of the temporary DAG used for route discovery. The DODAGID also identifies the origin node. This is an optional field that MUST be present in the Discovery Reply Object if H flag is set. The RPLInstanceID, the Version and the DODAGID together uniquely identify the temporary DAG used for route discovery and can be copied from the Base Object of the DIO advertizing the temporary DAG. o Target Address: The IPv6 address of the target node originating the Discovery Reply Object. This is an optional field that MUST be present in the Discovery Reply Object if H flag is set. o Options: The Discovery Reply Object MAY carry up to N Source Route options (defined in the next section) with each such option carrying a source route and optionally followed by a Metric Container option that lists the aggregated values for the routing metrics for the source route. Goyal & Baccelli Expires February 27, 2011 [Page 15] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 6.1.1. The Source Route Option 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 = 10 | Option Length | Compr | Pad | D | Resvd | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . Address[1..n] . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Format of the Source Route Option The Source Route option, illustrated in Figure 3, carries a source route. A Source Route option MAY be a part of the Discovery Reply Object. When a Source Route option carries a complete source route between the origin and the target node, it MAY be immediately followed by a Metric Container option that contains the aggregated values of the routing metrics for this source route. A Source Route option consists of the following fields: o Option Type = 0x0A (to be confirmed by IANA). o Option Length = Variable, depending on the size of the Addresses vector. o Compr: 4-bit unsigned integer indicating the number of prefix octets that are elided from each address. For example, Compr value will be 0 if full IPv6 addresses are carried in the Addresses vector. o Pad: 4-bit unsigned integer. Number of octets that are used for padding between Address[n] and the end of the Source Route option. o D: A 2-bit field that indicates the direction of the source route: * D = 0x00: Forward, i.e. from the origin node to the target node; * D = 0x01: Backward i. e., from the target node to the origin node; * D = 0x02: Bidirectional. Goyal & Baccelli Expires February 27, 2011 [Page 16] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 Note that the D field in a Source Route option is independent from the D field in the DRO containing the Source Route option. o Resvd: These bits are reserved for future use. These bits MUST be set to zero on transmission and MUST be ignored on reception. o Address[1..n]: Vector of addresses, numbered 1 to n. Each vector element has size (16 - Compr) octets. Note that the format of the Source Route option is very similar to that of proposed Type 4 Routing Header [I-D.ietf-6man-rpl-routing-header]. A common network configuration for an RPL domain is that all routers within an LLN share a common prefix. The Source Route option uses the Compr field to allow compaction of the Address[1..n] vector when all entries share the same prefix as the DODAGID or the Target Address of the encapsulating Discovery Reply Object. The shared prefix octets are not carried within the Source Route option and each entry in Address[1..n] has size (16 - Compr) octets. When Compr is non-zero, there may exist unused octets between the last entry, Address[n], and the end of the Source Route option. The Pad field indicates the number of unused octets that are used for padding. Note that when Compr is 0, Pad MUST be null and carry a value 0. The Source Route option MUST NOT specify a path that visits a router more than once. When generating a Source Route option, the target node may not know the mapping between IPv6 addresses and routers. Minimally, the target node MUST ensure that: o The IPv6 Addresses do not appear more than once; o The IPv6 addresses of the origin and the target nodes do not appear in the Address vector. Multicast addresses MUST NOT appear in a Source Route option. 6.1.2. Processing a DRO At An Intermediate Router When an intermediate router receives a DRO with a clear H flag, it MUST forward the DRO to a parent node in the temporary DAG. When an intermediate router receives a DRO that has H flag set and contains multiple Source Route options, the router MUST drop the DRO with no further processing and optionally log an error message. When an intermediate router receives a DRO that has H flag set and contains a single Source Route option, the router processes the DRO Goyal & Baccelli Expires February 27, 2011 [Page 17] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 as described in Section 6.4. 6.2. DRO as Acknowledgement for Backward Source Routes After selecting one or more backward source routes, a target node MUST send a DRO message to the origin node as an acknowledgement for the discovered routes. Such an acknowledgement helps the origin node determine the success of route discovery. A DRO, serving as an acknowledgement for backward source route discovery, has its D field set to 0x01 (indicating backward) while the H flag is cleared (indicating source route). The N field is set to indicate the number of discovered backward source routes being acknowledged. Such a DRO message MUST NOT contain any option. The target node MAY unicast this DRO message to the origin node or it MAY forward the DRO message to a parent in the temporary DAG. The target node should take in consideration the minimum life time of the temporary DAG when deciding to use it to send the DRO to the origin node. 6.3. DRO as Carrier of Forward/Bidirectional Source Routes The target node conveys the discovered forward/bidirectional source routes to the origin node via the Source Route options inside one or more DRO messages. Such a DRO message MUST have its D field set to 0x00 (if it carries forward routes) or 0x02 (if its carries bidirectional routes). Also, the H flag MUST be cleared and the N field MUST indicate the number of Source Route options in the DRO. Each Source Route option inside the DRO MAY immediately be followed by a Metric Container option that carries the aggregated values of the relevant routing metrics for this source route. The target node MAY unicast this DRO message to the origin node or it MAY forward the DRO message to a parent in the temporary DAG. The target node should take in consideration the minimum life time of the temporary DAG when deciding to use it to send the DRO to the origin node. 6.4. Establishing Hop-by-hop Routes Via DRO In order to establish a hop-by-hop route, the target node sends a DRO message along the discovered route, which is specified in a Source Route option. The D field in the DRO is set to reflect the direction of the discovered route. The H bit in the DRO MUST be set and the DRO MUST include the DODAGID and Target Address fields. The N field in the DRO MUST be set to 1. The target node forwards the DRO to the next hop along the discovered route and includes the discovered Goyal & Baccelli Expires February 27, 2011 [Page 18] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 route, excluding itself and the origin node, inside the Source Route option in backward direction. Thus, the D field in the Source Route option MUST be 0x01. A router receiving a DRO message MUST drop the DRO and optionally log an error if the router cannot establish the hop-by-hop state for the route or if its own address does not lie as the first element in the Address vector inside the Source Route option. Otherwise, the router MUST establish the hop-by-hop state in the direction specified in the D field in the DRO. The hop-by-hop state in the forward direction includes the RPLInstanceID, the DODAGID and the target node's address. The hop-by-hop state in the backward direction includes the RPLInstanceID, the DODAGID and the origin node's address. After establishing the hop-by-hop state, the router MUST remove its own address from the route contained in the Source Route option and forward the DRO to the next hop (Address[0] in the Source Route option). 7. Applicability The route discovery mechanism described in this document may be invoked by an origin node when no route exists between itself and the target node or when the existing routes do not satisfy the desired performance requirements. The mechanism is designed to discover one or more "good enough" routes in either direction between an origin and a target node. In some application contexts, the good enough criteria is intrinsically known. For example, an origin node that expects a target node to be less than 5 hops away may use "hop-count < 5" as the good enough criteria. In other application contexts, the origin node may need to measure the cost of an existing route to the target node to determine the good enough criteria. For example, an origin node that measures the total ETX of its along-DAG route to the target node to be 20 may use "ETX < x*20", where x is a fraction that the origin node decides, as the good enough criteria. The functionality required to measure the cost of an existing route between the origin and the target node will be described in a separate document. In case, there is no existing route between the origin and target nodes or the cost measurement for the existing route fails, the origin node will have to guess the good enough criteria for the initial route discovery. Once, the initial route discovery succeeds or fails, the origin node will have a better estimate for the good enough criteria to be used in the subsequent route discovery. This document describes an on-demand discovery mechanism for P2P routes that is complimentary to the proactive routes offered by RPL base functionality. The mechanism described in this document may Goyal & Baccelli Expires February 27, 2011 [Page 19] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 result in discovery of better P2P routes than the ones available along a DAG designed to optimize routing cost to the DAG's root. The improvement in route quality depends on a number of factors including the network topology, the routing metrics in use and the prevalent conditions in the network. A network designer may take in consideration both the benefits (potentially better routes; no need to maintain routes proactively) and costs (control messages generated during the route discovery process) when using this mechanism. 8. Security Considerations TBA 9. IANA Considerations TBA 10. Authors and Contributors In addition to the editors, the authors of this document include the following individuals (listed in alphabetical order). Anders Brandt, Sigma Designs, Emdrupvej 26A, 1., Copenhagen, Dk-2100, Denmark. Phone: +45 29609501; Email: abr@sdesigns.dk Robert Cragie, Gridmerge Ltd, 89 Greenfield Crescent, Wakefieldm WF4 4WA, UK. Phone: +44 1924910888; Email: robert.cragie@gridmerge.com Jerald Martocci, Johnson Controls, Milwaukee, WI 53202, USA. Phone: +1 414 524 4010; Email:jerald.p.martocci@jci.com Charles Perkins, Tellabs Inc., USA. Email:charliep@computer.org Authors gratefully acknowledge the contributions of Richard Kelsey and Zach Shelby in the development of this document. 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Goyal & Baccelli Expires February 27, 2011 [Page 20] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 11.2. Informative References [I-D.brandt-roll-rpl-applicability-home-building] Brandt, A., Baccelli, E., and R. Cragie, "Applicability Statement: The use of RPL in Building and Home Environments", draft-brandt-roll-rpl-applicability-home-building-00 (work in progress), April 2010. [I-D.ietf-6man-rpl-option] Hui, J. and J. Vasseur, "RPL Option for Carrying RPL Information in Data-Plane Datagrams", draft-ietf-6man-rpl-option-00 (work in progress), July 2010. [I-D.ietf-6man-rpl-routing-header] Hui, J., Vasseur, J., and D. Culler, "An IPv6 Routing Header for Source Routes with RPL", draft-ietf-6man-rpl-routing-header-00 (work in progress), July 2010. [I-D.ietf-roll-of0] Thubert, P., "RPL Objective Function 0", draft-ietf-roll-of0-03 (work in progress), July 2010. [I-D.ietf-roll-routing-metrics] Vasseur, J., Kim, M., Networks, D., Dejean, N., and D. Barthel, "Routing Metrics used for Path Calculation in Low Power and Lossy Networks", draft-ietf-roll-routing-metrics-08 (work in progress), July 2010. [I-D.ietf-roll-rpl] Winter, T., Thubert, P., and R. Team, "RPL: IPv6 Routing Protocol for Low power and Lossy Networks", draft-ietf-roll-rpl-11 (work in progress), July 2010. [I-D.ietf-roll-terminology] Vasseur, J., "Terminology in Low power And Lossy Networks", draft-ietf-roll-terminology-03 (work in progress), March 2010. [I-D.ietf-roll-trickle] Levis, P., Gnawali, O., Clausen, T., Hui, J., and J. Ko, "The Trickle Algorithm", draft-ietf-roll-trickle-02 (work in progress), July 2010. [I-D.thubert-6man-reverse-routing-header] Goyal & Baccelli Expires February 27, 2011 [Page 21] Internet-Draft draft-ietf-roll-p2p-rpl-00 August 2010 Thubert, P., "Reverse Routing Header", draft-thubert-6man-reverse-routing-header-00 (work in progress), June 2010. [RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation Routing Requirements in Low-Power and Lossy Networks", RFC 5826, April 2010. [RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen, "Building Automation Routing Requirements in Low-Power and Lossy Networks", RFC 5867, June 2010. Authors' Addresses Mukul Goyal (editor) University of Wisconsin Milwaukee 3200 N Cramer St Milwaukee, WI 53211 USA Phone: +1 414 2295001 Email: mukul@uwm.edu Emmanuel Baccelli (editor) INRIA Phone: +33-169-335-511 Email: Emmanuel.Baccelli@inria.fr URI: http://www.emmanuelbaccelli.org/ Goyal & Baccelli Expires February 27, 2011 [Page 22]