Networking Working Group JP. Vasseur, Ed. Internet-Draft Cisco Systems, Inc Intended status: Standards Track M. Kim, Ed. Expires: June 6, 2010 Future Tech Lab., Korea Telecom K. Pister Dust Networks H. Chong Future Tech Lab., Korea Telecom December 3, 2009 Routing Metrics used for Path Calculation in Low Power and Lossy Networks draft-ietf-roll-routing-metrics-04 Abstract Low power and Lossy Networks (LLNs) have unique characteristics compared with traditional wired and ad-hoc networks that require the specification of new routing metrics and constraints. By contrast with typical Interior Gateway Protocol (IGP) routing metrics using hop counts or link metrics, this document specifies a set of link and node routing metrics and constraints suitable to LLNs. Requirements Language 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 RFC 2119 [RFC2119]. 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), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. Vasseur, et al. Expires June 6, 2010 [Page 1] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on June 6, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. 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 BSD License. Vasseur, et al. Expires June 6, 2010 [Page 2] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Object Formats . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Node Metrics And Constraints Objects . . . . . . . . . . . . . 9 3.1. Node State And Attributes Object . . . . . . . . . . . . . 9 3.2. Node Energy Object . . . . . . . . . . . . . . . . . . . . 11 3.3. Hop-Count Object . . . . . . . . . . . . . . . . . . . . . 14 4. Link Metrics and Constraints Objects . . . . . . . . . . . . . 14 4.1. Throughput . . . . . . . . . . . . . . . . . . . . . . . . 15 4.2. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3. Link reliability . . . . . . . . . . . . . . . . . . . . . 17 4.3.1. The Link Quality Level Reliability Metric . . . . . . 18 4.3.2. The Expected Transmission Count (ETX) Reliability Object . . . . . . . . . . . . . . . . . . . . . . . . 19 4.4. Link Color Object . . . . . . . . . . . . . . . . . . . . 21 4.4.1. Link Color Object Description . . . . . . . . . . . . 21 4.4.2. Mode of Operation . . . . . . . . . . . . . . . . . . 22 5. Computation of Dynamic Metrics and Attributes . . . . . . . . 23 6. Objective Function . . . . . . . . . . . . . . . . . . . . . . 23 7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 24 7.1. Other metric under consideration . . . . . . . . . . . . . 24 8. Metric Consistency . . . . . . . . . . . . . . . . . . . . . . 24 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 9.1. Routing Objects . . . . . . . . . . . . . . . . . . . . . 25 9.2. Routing Object Common Header . . . . . . . . . . . . . . . 25 9.3. NSA Object . . . . . . . . . . . . . . . . . . . . . . . . 26 9.4. Hop-count Object . . . . . . . . . . . . . . . . . . . . . 26 9.5. DAG Information Option (DIO) suboption for the OCP Object . . . . . . . . . . . . . . . . . . . . . . . . . . 27 9.6. Objective Code Point (OCP) Object . . . . . . . . . . . . 27 10. Security Considerations . . . . . . . . . . . . . . . . . . . 27 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 12.1. Normative References . . . . . . . . . . . . . . . . . . . 27 12.2. Informative References . . . . . . . . . . . . . . . . . . 28 12.3. References . . . . . . . . . . . . . . . . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29 Vasseur, et al. Expires June 6, 2010 [Page 3] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 1. Introduction This document makes use of the terminology defined in [I-D.ietf-roll-terminology]. Low power and Lossy Networks (LLNs) have specific routing characteristics compared with traditional wired or ad-hoc networks that have been spelled out in [RFC5548], [RFC5673], [I-D.ietf-roll-home-routing-reqs] and [I-D.ietf-roll-building-routing-reqs]. Historically, IGP such as OSPF ([RFC2328]) and IS-IS ([RFC1195]) have used quantitative static link metrics. Other mechanisms such as Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) (see [RFC2702] and [RFC3209]) make use of other link attributes such as the available reserved bandwidth (dynamic) or link affinities (static) to compute constrained shortest paths for Traffic Engineering Label Switched Paths (TE LSPs). This document specifies routing constraints and metrics to be used in path calculation by the Routing Protocol for Low Power and Lossy Networks (RPL) specified in [I-D.ietf-roll-rpl]. One of the prime objectives of this document is to propose a flexible mechanism for the advertisement of routing metrics and constraints used by RPL. Some RPL implementations may elect to adopt an extremely simple approach based on the use of a single metric with no constraint whereas other implementations may use a larger set of link and node routing metrics and constraints. This specification provides a high degree of flexibility and new routing metrics; new constraints could be defined in the future, as needed. RPL is a distance vector routing protocol that builds a Directed Acyclic Graph (DAG) based on routing metrics and constraints. DAG formation rules are defined in [I-D.ietf-roll-rpl]: o The DAG root may advertise a routing constraint used as a "filter" to prune links and nodes that do not satisfy specific properties. For example, it may be required for the path to only traverse nodes that are main powered or links that have at least a minimum reliability or a specific "color" reflecting a user defined link characteristic (e.g the link layer supports encryption). o A routing metric is a quantitative value that is used to evaluate the path quality, also referred to as the path cost. Link and nodes metrics are usually (but not always) additive: for example, the throughput or the reliability link metric can be used to evaluate the path cost. Vasseur, et al. Expires June 6, 2010 [Page 4] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The best path is the path with the lowest cost with respect to some metrics that satisfies all constraints (if any) and is also called the shortest constrained path (in the presence of constraints). Routing metrics can be classified according to the following set of characteristics: o Link versus Node metrics o Qualitative versus quantitative o Dynamic versus static It must be noted that the use of dynamic metrics is not new and has been experimented in ARPANET 2 [Khanna1989], with moderate success. The use of dynamic metrics is not trivial and very careful care must be given to the use of dynamic metrics since it may lead to potential routing instabilities. As pointed out in various routing requirements documents (see [RFC5673], [I-D.ietf-roll-home-routing-reqs] [RFC5548] and [I-D.ietf-roll-building-routing-reqs]), it must be possible to take into account a variety of node constraints/metrics during path computation. It is also worth mentioning that it is fairly common for links in LLNs to have fast changing node and link characteristics, which must be taken into account when specifying routing metrics. For instance, in addition to the dynamic nature of wireless connectivity, nodes' resources such as residual energy and other link's charatacteristics such as the throughput are changing continuously and may have to be taken into account during the path computation. Similarly, link attributes including throughput and reliability may drastically change over time due to multi-path interference. Very careful attention must be given when using dynamic metrics and attributes that affect routing decisions in order to preserve routing stability. Routing metrics and constraints may either be static or dynamic. When dynamic, a RPL implementation SHOULD make use of a multi-threshold scheme rather than fine granular metric updates so as to avoid constant routing changes. Furthermore, it is a time and energy consuming process to update dynamic metrics and recompute the routing tables on a frequent basis. Therefore, it may be desirable to use a set of discrete values to reduce computational overhead and bandwidth utilization. Of course, this comes with a cost, namely, reduced metric accuracy. Reliability is an example of qualitative parameters which may be used as a Vasseur, et al. Expires June 6, 2010 [Page 5] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 routing metric by RPL. Such qualitative parameters may be transformed to quantitative values. In other cases, a set of flags may be defined to reflect a node state without having to define discrete values. Some link or node characteristics (e.g. link reliability flag, energy remaining on the node) may either be used by RPL as routing constraints or metric. For example, the path may be computed to avoid links that do not provide a sufficient level of reliability (use as a constraint) or as the path offering the maximum number of links with a specified reliability level (use as a metric). It is not required to use all the metrics and attributes specified in this document. The set of routing metrics and constraints used by an RPL implementation is signalled along the Directed Acyclic Graph (DAG) computed by RPL via an Objective Function (OF) as described in [I-D.ietf-roll-rpl]. Indeed, RPL may be used to build DAGs with different characteristics. For example, it may be desirable to build a DAG trying to maximize reliability by using the link reliability metric: in this case, the OF advertised by RPL in the Dag Information Option (DIO) message specifies an Objective Code Point (OCP) indicating that the link reliability metric is the metric used to compute the "best" path. Another example might be to use the energy node characteristic (e.g. main powered versus battery operated) as a node constraint when building the DAG so as to avoid battery powered nodes in the DAG. Links and nodes routing metrics and constraints are not exclusive. The requirements on reporting frequency may differ among metrics, thus different reporting rates may be used for each category. The specification of objective functions used to compute the DAG built by RPL is out of the scope of this document and will be specified in other documents. Some metrics are either aggregated or recorded. In the former case, the metric is adjusted as the DIO message travels along the DAG. For example, if the metric is the link latency, each node updates the latency metric along the DAG. By contrast, metric may be recorded in which case each node adds a sub-object reflecting the local metric. For example, it might be desirable to record the link quality level along the path. In this case, each visited node adds a sub-object reporting the local link quality level. In order to limit the number of sub-objects, the use of a counter may be desirable (e.g. record the number of links with a certain link quality level). Upon receiving the DIO message from a set of parents, a node can decide which node to choose as a parent based on the maximum number of links with a specific link reliability level for example. Vasseur, et al. Expires June 6, 2010 [Page 6] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 Notion of local versus global metric: some routing objects may have a local or a global significance. In the former case, a metric may be transmitted to a neighbor to charaterize a link or a node as opposed to a path. For example, a node may report information about its local energy without the need to propagate the energy level of all nodes along the path. In contrast, other metrics such as link latency metrics are cumulative and global in the sense that they characterize a path cost using the latency as a metric. In this particular example the delay is an aggregated global and cumulative link metric. 2. Object Formats Routing metrics and constraints are carried within the DAG Metric Container object defined in [I-D.ietf-roll-rpl]. 0 1 2 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 2 3 4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Type=2 | Container Len | DAG Metric data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 1: DAG Metric Container Format Routing metrics and constraints have a common format consisting of one or more 8-bit words with a common header: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Routing-Ob-Type|Res|R|G| A |O|C| Object Length (bytes) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // (Object body) // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Routing Metric/Constraint common header format The object body carries one or more sub-objects. Note that the routing metric objects defined in this document can appear in any order in the DAG Metric Container object with the exception of the Objective Function object that MUST appear first. Routing-Ob-Type: (Routing Object Type - 8 bits): the Routing Object Type field uniquely identifies each Routing object and is managed by Vasseur, et al. Expires June 6, 2010 [Page 7] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 IANA. Res flags (2 bits). Reserved field. This field MUST be set to zero on transmission and MUST be ignored on receipt. o C Flag. When set, this indicates that the routing object refers to a routing constraint. When cleared the routing object refers to a routing metric. o O Flag: The O flag is used exclusively for routing constraints (C flag is set) and has no meaning for routing metrics. When set, this indicates that the constraint is optional. When cleared, the constraint is mandatory. o A Field: The A field is used to indicate whether a routing metric is additive, reports a maximum or a minimum. * A=0x00: The routing metric is additive * A=0x01: The routing metric reports a maximum * A=0x02: The routing metric reports a minimum o G Flag: When set, the routing object is global. When cleared it is local (see details below). o R Fag: The R Flag is only relevant for global routing metric (C=0 and G=1) and MUST be cleared for all other values of C and G. When set, this indicates that the routing metric is recorded along the path. Conversely, when cleared the routing metric is aggregated. The A field has no meaning when the C Flag is set (i.e. the routing object refers to a routing constraint). Example 1: A DAG formed by RPL where all nodes must be main powered and the link metric is the link throughput. In this case the DAG Metric container carries two routing objects: the link metric is the link throughput (C=0, O=0, A=00, G=1, R=0) and the node constraint is power (C=1, O=0, A=00, G=0, R=0). Note that in this example, the link throughput is a global additive aggregated link metric. An RPL implementation may use the metric to report a minimum (A=01). In this case, when the link throughput metric is processed each node updates it is the link throughput is less than the value reported by the link throughput metric. Example 2: A DAG formed by RPL where the link metric is the link quality level and link quality levels must be recorded along the path. In this case the DAG Metric container carries one routing Vasseur, et al. Expires June 6, 2010 [Page 8] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 object: the link quality level (C=0, O=0, A=00, G=1, R=1). A Routing object may also include one or more type-length-value (TLV) encoded data sets. Each Routing TLV has the same structure: Type: 2 bytes Length: 2 bytes Value: variable A Routing metric TLV is comprised of 2 bytes for the type, 2 bytes specifying the TLV length, and a value field. The Length field defines the length of the value portion in bytes. Unrecognized TLVs MUST be ignored. IANA management of the routing metric objects identifier codespace is described in Section 9. 3. Node Metrics And Constraints Objects It is fairly common for LLNs to be made of nodes with heterogeneous attributes and capabilities (e.g. nodes being battery operated or not, amount of memory, etc). More capable and stable nodes may assist the most constrained ones for routing packets, which results in extension of network lifetime and efficient network operations. This is a typical use of constraint-based routing where the computed path may not be the shortest path according to some specified metrics. 3.1. Node State And Attributes Object The Node State and Attribute (NSA) object is used to provide information on the nodes characteristics. The NSA object MAY be present in the DAG Metric Container. There MUST be only one NSA object per DAG Metric Container object. The NSA object may also contain a set of TLVs used to convey various node characteristics. No TLVs are currently defined. The NSA Routing Object Type is to be assigned by IANA (recommended value=1). Vasseur, et al. Expires June 6, 2010 [Page 9] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The format of the NSA object body is as follows: 0 1 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Res | Flags |A|O| Optional TLVs +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 3: NSA Object format Node workload may be hard to determine and express in some scalar form. However, node workload could be a useful metric to consider during path calculation, in particular when queuing delays must be minimized for highly sensitive traffic considering Medium Access Control (MAC) layer delay. Node workload MAY be set upon CPU overload, lack of memory or any other node related conditions. Using a simple 1-bit flag to characterize the node workload provides a sufficient level of granularity, similarly to the "overload" bit used in routing protocols such as IS-IS. Algorithms used to set the overload bit and to compute path to potentially avoid node with their overload bit set are outside the scope of this document. Data Aggregation Attribute: data fusion involves more complicated processing to improve accuracy of the output data while data aggregation mostly aims at reducing the amount of data. This is listed as a requirement in Section 6.2 of [RFC5548]. Some applications may make use of the aggregation node attribute in their routing decision so as to minimize the amount of traffic on the network, thus potentially increasing its life time in battery operated environments. Applications where high directional data flow is expected on a regular basis may take advantage of data aggregation supported routing. The following two bits of the NSA object are defined: o O Flag: When set, this indicates that the node is overloaded and may not be able to process traffic. o A Flag: When set, this indicates that the node can act as a traffic aggregator. An implementation MAY decide to add optional TLVs (not currently defined) to further describe the node traffic aggregator functionality. The Flag field of the NSA Routing object is managed by IANA. Unassigned bits are considered as reserved. They MUST be set to zero on transmission and MUST be ignored on receipt. Vasseur, et al. Expires June 6, 2010 [Page 10] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 3.2. Node Energy Object Whenever possible, a node with low residual energy should not be selected as a router, thus the support for constrained-based routing is needed. In such cases, the routing protocol engine may compute a longer path (constraint based) for some traffic in order to increase the network life duration. The routing engine may prefer a "longer" path that traverses mains- powered nodes or nodes equipped with energy scavenging, rather than a "shorter" path through battery operated nodes. Power and energy are clearly critical resources in LLNs. As yet there are no simple abstractions which adequately cover the broad range of power sources and energy storage devices used in existing LLN nodes. These include line-power, primary batteries, energy- scavengers, and a variety of secondary storage mechanisms. Scavengers may provide a reliable low level of power, such as might be available from a 4-20mA loop; a reliable but periodic stream of power, such as provided by a well-positioned solar cell; or unpredictable power, such as might be provided by a vibrational energy scavenger on an intermittently powered pump. Routes which are viable when the sun is shining may disappear at night. A pump turning on may connect two previously disconnected sections of a network. Storage systems like rechargeable batteries often suffer substantial degradation if regularly used to full discharge, leading to different residual energy numbers for regular versus emergency operation. A route for emergency traffic may have a different optimum than one for regular reporting. Batteries used in LLNs often degrade substantially if their average current consumption exceeds a small fraction of the peak current that they can deliver. It is not uncommon for LLN nodes to have a combination of primary storage, energy scavenging, and secondary storage, leading to three different values for acceptable average current depending on the time frame being considered, e.g. milliseconds, seconds, and hours/years. Raw power and energy values are meaningless without knowledge of the energy cost of sending and receiving packets, and lifetime estimates have no value without some higher-level constraint on the lifetime required of a device. In some cases the path that exhausts the battery of a node on the bed table in a month may be preferable to a route that reduces the lifetime of a node in the wall to a decade. Given the complexity of trying to address such a broad collection of Vasseur, et al. Expires June 6, 2010 [Page 11] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 constraints, this document defines three levels of fidelity in the solution. The simplest solution relies on a 2-bit field encoding three types of power sources: "powered", "battery", "scavenger". This simple approach may be sufficient for many applications. The mid-complexity solution is a single parameter that can be used to encode the energetic happiness of both battery powered and scavenging nodes. For scavenging nodes, the 8 bit quantity is the power provided by the scavenger divided by the power consumed by the application, H=P_in/P_out, in units of percent. Nodes which are scavenging more power than they are consuming will register above 100. The time period for averaging power in this calculation is out of the scope of this document but something related to the discharge time of the energy storage device on the node is probably appropriate. For battery powered devices, H is the current expected lifetime divided by the desired minimum lifetime. The estimation of remaining battery energy and actual power consumption can be difficult, and the specifics of this calculation are out of scope of this document, but two examples are presented. If the node can measure its average power consumption, then H can be calculated as the ratio of desired max power (initial energy E_0 divided by desired lifetime T) to actual power H=P_max/P_now. Alternatively, if the energy in the battery E_bat can be estimated, and the total elapsed lifetime, t, is available, then H can be calculated as the total stored energy remaining versus the target energy remaining: H= E_bat / [E_0 (T-t)/T]. An example OF is max(min(H)) for all battery operated nodes along the route, subject to the constraint that H>=100 for all scavengers along the route. The Node Energy (NE) object is used to provide information related to node energy and may be used as a metric or as constraint. The NE object MAY be present in the DAG Metric Container. There MUST be only one NE object per DAG Metric Container object. The NE Routing Object Type is to be assigned by IANA (recommended value=2). Vasseur, et al. Expires June 6, 2010 [Page 12] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The format of the NE object body is as follows: 0 1 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | NE Sub-objects +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 4: NE Object format When used as a constraint, the NE object comprises only one NE sub- object. The format of the NE sub-object body is as follows: 0 1 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Flags |I| T |E| E-E | Optional TLVs +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 5: NE sub-object format The NE sub-object may also contain a set of TLVs used to convey various nodes' characteristics. The following flags are currently defined: o T (node Type): 2-bit field indicating the node type. When E=0x00, the node is main powered. When E=0x01 is battery powered. When E=0x02 the node is powered by a scavenger. o I (Included): the I bit is only relevant when the node type is used as a constraint. For example, the OF may indicate that the path must only traversed main powered node. Conversely, the OF may indicate that battery operated node must be excluded. The I bit is used to stipulate inclusion versus exclusion. When set, this indicates that nodes of type specified in the node type field MUST be included. Conversely, when cleared, this indicates that nodes of type specified in the node type field MUST be excluded. o E (Estimation): when set, the estimated percentage of remaining energy is indicated in the E-E 8-bit field. When cleared, the estimated percentage of remaining energy is not provided. E-E (Estimated-Energy): 8-bit field indicating the estimated percentage of remaining energy on the node. The E-E field is only relevant when the E flag is set, and MUST be set to 0 when the E flag is cleared. Vasseur, et al. Expires June 6, 2010 [Page 13] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 No TLVs are currently defined. The most complex solution involves a half dozen TLV parameters representing energy storage, consumption, and generation capabilities of the node, as well as desired lifetime, and will appear in the next version of this document. 3.3. Hop-Count Object The Hop-Count (HP) object is used to report the number of traversed nodes along the path. The HP object MAY be present in the DAG Metric Container. There MUST be only one HP object per DAG Metric Container object. The HP object may also contain a set of TLVs used to convey various node characteristics. No TLVs are currently defined. The HP routing metric object Type is to be assigned by IANA (recommended value=3) The HP routing metric object is a global routing object that characterizes a path. The format of the Hop Count object body is as follows: 0 1 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Res | Flags | Hop Count | Optional TLVs +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 6: Hop Count Object format No Flags are currently defined. The HP object may be used as a constraint or a metric. When used as a constraint, the DAG root indicates the maximum number of hops that a path may traverse. When that number is reached no other node can join that path. When used as a metric each visited node simply increments the Hop Count field. 4. Link Metrics and Constraints Objects Vasseur, et al. Expires June 6, 2010 [Page 14] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 4.1. Throughput Many LLNs support a wide range of throughputs, measured either in bits per second or packets per second. For some links, this may be due to variable coding. For the deeply duty-cycled links found in many LLNs, the variability comes as a result of trading power consumption for bit rate. There are several MAC sub-layer protocols which allow the effective bit rate and power consumption of a link to vary over more than three orders of magnitude, with a corresponding change in power consumption. For efficient operation, it may be desirable for nodes to report the range of throughput that their links can handle in addition to the currently available throughput. The Throughput object MAY be present in the DAG Metric Container. There MUST be only one Throughput object per DAG Metric Container object. The Throughput object is made of throughput sub-objects and MUST as least comprise one Throughput sub-object. Each Throughput sub-object has a fixed length of 4 bytes. The Throughput object does not contain any additional TLV. The Throughput Object Type is to be assigned by IANA (recommended value=4) The Throughput Object is a global metric. Vasseur, et al. Expires June 6, 2010 [Page 15] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The format of the Throughput object body is as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (sub-object) ..... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Throughput Object Body Format 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Throughput | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Throughput Sub-object format Throughput: 32 bits. The Throughput is encoded in 32 bits in IEEE floating point format (see [IEEE.754.1985]), expressed in bytes per second. Refer to Section 3.1.2 of [RFC3471] for a table of commonly used values. 4.2. Latency Similarly to throughput, the latency of many LLN MAC sub-layers can be varied over many orders of magnitude, again with a corresponding change in current consumption. Some LLN MAC link layers will allow the latency to be adjusted globally on the subnet, or on a link-by- link basis, or not at all. Some will insist that it be fixed for a given link, but allow it to be variable from link to link. The Latency object MAY be present in the DAG Metric Container. There MUST be only one Latency object per DAG Metric Container object. The Latency object is made of Latency sub-objects and MUST as least comprise one Latency sub-object. Each Latency sub-object has a fixed length of 4 bytes. The Latency object does not contain any additional TLV. The Latency object Type is to be assigned by IANA (recommended value=5) The Latency Object is a global metric or constraint. Vasseur, et al. Expires June 6, 2010 [Page 16] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The format of the Latency object body is as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (sub-object) ..... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Latency Object Body Format 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Latency | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Latency Sub-object format Latency: 32 bits. The Latency is encoded in 32 bits in IEEE floating point format (see [IEEE.754.1985]), expressed in milliseconds. Refer to Section 3.1.2 of [RFC3471] for a table of commonly used values. The Latency object may be used as a constraint or a path metric. For example, an Objective Function may indicate that the latency must not exceed some values. In this case, the latency object common header indicates that the value relates to a constraint with a maximum value. In another example, the latency object may be used as an aggregated additive metric where the value is updated along the path to reflect to path latency. 4.3. Link reliability In LLNs, link reliability is degraded by external interference and multi-path interference. Multipath typically affects both directions on the link equally, whereas external interference is sometimes uni- directional. Time scales vary from milliseconds to days, and are often periodic and linked to human activity. Packet error rates can generally be measured directly, and other metrics (e.g. bit error rate, mean time between failures) are typically derived from that. A change in link quality can affect network connectivity, thus, link quality may be taken into account as a critical routing metric. Link quality metric should be applied to each directional link unless bi- directionality is one of routing metrics. A number of link reliability metrics could be defined reflecting several reliability aspects. Two link reliability metrics are Vasseur, et al. Expires June 6, 2010 [Page 17] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 defined in this document: the Link Quality Level (LQL) and the Expected Transmission count Metric (ETX). Note that an RPL implementation MAY either use the LQL, the ETX or both. 4.3.1. The Link Quality Level Reliability Metric The Link Quality Level (LQL) object is used to quantify the link reliability using a discrete value, from 0 to 3 where 0 indicates that the link quality level is unknown and 1 reports the highest link quality level. The mechanisms and algorithms used to compute the LQL is implementation specific and outside of the scope of this document. The LQL is global and can either be used as a metric or a constraint. When used as a metric, the LQL metric can be recorded or aggregated. For example, the DAG may require to record the LQL for all traversed links. Each node can then use the LQL to select the parent based on user defined rules (e.g. "select the path with the maximum number of links reporting a LQL value of 3"). By contrast the LQL link metric may be aggregated in which case, the sum of all LQL may be reported (additive metric) or the minimum value may be reported along the path. When used as a recorded metric, a counter is used to compress the information where the number of links for each LQL is reported. The LQL object MAY be present in the DAG Metric Container. There MUST be only one LQL object with at least one LQL sub-object per DAG Metric Container object. The LQL object contains one or more sub-object used to report the number of links along with their LQL. The LQL object Type is to be assigned by IANA (recommended value=6) The LQL object is a global object that characterizes the path reliability. Vasseur, et al. Expires June 6, 2010 [Page 18] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The format of the LQL object body is as follows: 0 1 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Res | LQL Sub-object +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 11: LQL Object format When the LQL metric is recorded, the LQL object body comprises one or more LQL Type 1 sub-object. The format of the LQL Type 1 sub-object is as follows 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |Val| Counter | +-+-+-+-+-+-+-+-+ Figure 12: LQL Type 1 sub-Object format Val: LQL value from 0 to 3 where 0 means undetermined and 1 indicates the lowest link quality. Counter: number of links. When the LQL metric is aggregated, the LQL object body comprises one LQL Type 2 sub-object: The format of the LQL Type 2 sub-object is as follows 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Aggregated LQL Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13: LQL Type 2 sub-Object format Aggregated LQL Value: when used an an additive metric (A=0x00), the aggregated LQL value reports the sum of all the LQL values for all links along the path. When used to report a minimum (A=0x02), the field reports the minimum LQL value of all links along the path. 4.3.2. The Expected Transmission Count (ETX) Reliability Object The Expected Transmission Count (ETX) metric is the number of transmissions a node expects to make to a destination in order to Vasseur, et al. Expires June 6, 2010 [Page 19] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 successfully deliver a packet. For example, an implementation may use the following formula: ETX= 1 / (Df * Dr) where Df is the measured probability that a packet is received by the neighbor and Dr is the measured probability that the acknowledgment packet is successfully received. This document does not mandate the use of a specific formula to comput the ETX value. The ETX object MAY be present in the DAG Metric Container. There MUST be only one ETX object per DAG Metric Container object. The ETX object is made of ETX sub-objects and MUST as least comprise one ETX sub-object. Each ETX sub-object has a fixed length of 8 bits. The ETX object does not contain any additional TLV. The ETX object Type is to be assigned by IANA (recommended value=7) The ETX object is a global metric or constraint. The format of the Latency object body is as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (sub-object) ..... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 14: ETX Object Body Format 0 1 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | ETX | +-+-+-+-+-+-+-+-+ Figure 15: ETX Sub-object format ETX: 8 bits. The ETX is encoded in 8 bits in IEEE floating point format (see [IEEE.754.1985]). Refer to Section 3.1.2 of [RFC3471] for a table of commonly used values. The ETX object may be used as a constraint or a path metric. For example, an Objective Function may indicate that the ETX must not be below some specified value. In this case, the ETX object common header indicates that the value relates to a constraint with a Vasseur, et al. Expires June 6, 2010 [Page 20] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 minimum value. In another example, the ETX object may be used as an aggregated additive metric where the value is updated along the path to reflect to path quality. 4.4. Link Color Object 4.4.1. Link Color Object Description The Link Color (LC) object is an administrative 10-bit static link constraint used to avoid or attract specific links for specific traffic types. The LC object can either be used as a metric or as a constraint. When used as a metric, the LC metric can only be recorded. For example, the DAG may require recording the link colors for all traversed links. Each node can then use the LC to select the parent based on user defined rules (e.g. "select the path with the maximum number of links having their first bit set 1 (e.g. encrypted links)"). The LC object may also be used as a constraint. When used as a recorded metric, a counter is used to compress the information where the number of links for each Link Color is reported. The Link Color (LC) object MAY be present in the DAG Metric Container. There MUST be only one LC object with at least one LC sub-object per DAG Metric Container object. The LC routing object does not contain any additional TLV. The LC routing object Type is to be assigned by IANA (recommended value=8) The LC object may either be local or global. The format of the LC object body is as follows: 0 1 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Res | LC Sub-objects +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 16: LC Object format When the LC object is used as a global recorded metric, the LC object body comprises one or more LC Type 1 sub-objects. Vasseur, et al. Expires June 6, 2010 [Page 21] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 The format of the LC Type 1 sub-object body is as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Color | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 17: LC Type 1 sub-object format When the LC object is used as a constraint, the LC object body comprises one or more LC Type 2 sub-objects. The format of the LC Type 2 sub-object body is as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Color |I| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 18: LC Type 2 sub-object format I Bit: When cleared, this indicates that links with the specified color must be included. When set, this indicates that links with the specified color must be excluded. The use of the LC object is outside of the scope of this document. 4.4.2. Mode of Operation The Objective Function may use the link color as a constraint or a metric. o When used as global constraint, the LC object may be inserted in the DAG Container metric object to indicate that links with a specific color should be included or excluded from the computed path. o When used as global recorded metric, each node along the path may insert a LC object in the DAG container metric to report the color of the local link. If there is already a LC object reported a similar color, the node MUST NOT add another identical LC sub- object and MUST increment the counter field. Vasseur, et al. Expires June 6, 2010 [Page 22] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 5. Computation of Dynamic Metrics and Attributes As already pointed out, dynamically calculated metrics are of the utmost importance in many circumstances in LLNs. This is mainly because a variety of metrics change on a frequent basis, thus implying the need to adapt the routing decisions. That being said, care must be given to the pace at which changes are reported in the network. The attributes will change according to their own time scales. RPL controls the reporting rate. To minimize metric updates, multi-threshold algorithms MAY be used to determine when updates should be sent. When practical, a low-pass filter should be used to avoid rapid fluctuations of these values. Finally, although the specification of path computation algorithms using dynamic metrics are out the scope of this document, the objective function should be designed carefully to avoid too frequent computation of new routes upon metric values changes. Controlled adaptation of the routing metrics and rate at which paths are computed are critical to avoid undesirable routing instabilities resulting in increased latencies and packet loss because of temporary micro-loops. Furthermore, excessive route changes will impact the traffic and power consumption in the network adversely. 6. Objective Function The Objective Function (OF) is used by RPL to specify how the routing metric and constraints should be used to reach specific objectives. For example, the OF may specify that the objective is to find the constrained shortest path where the constraint is related to the node power mode and the metric is the ETX (e.g. "Avoid battery operated links and compute the path that optimizes reliability"). As specified in [I-D.ietf-roll-rpl], the OF is used by a node to select its parent during the DAG building construction process. The OCP object is used to specify the OF and is carried as a sub- option within the DIO Base Option object defined in [I-D.ietf-roll-rpl]. There MUST be a single instance of the OCP object within the sub- option field of the DIO Base option object. Vasseur, et al. Expires June 6, 2010 [Page 23] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | OCP | (TLVs) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Figure 19: OCP Object Format The OCP object suboption type is 5 (to be confirmed by IANA). The OCP object body may carry optional TLVs. No TLVs are currently defined. OCP (Objective Code Point - 16 bits): the OCP field identifies the OF and is managed by IANA. 7. Open Issues Other items to be addressed in further revisions of this document include: 7.1. Other metric under consideration Metrics related to security (e.g. capability to avoid a node that has not been authorized or authenticated). 8. Metric Consistency Since a set of metrics and constraints will be used for links and nodes in LLN, it is particularly critical to ensure the use of consistent metric calculation mechanisms for all links and nodes in the network. 9. IANA Considerations IANA is requested to establish a new top-level registry to contain all Routing Objects codepoints and sub-registries. The allocation policy for each new registry is by IETF Consensus: new values are assigned through the IETF consensus process (see [RFC5226]). Specifically, new assignments are made via RFCs approved by the IESG. Typically, the IESG will seek input on prospective assignments from appropriate persons (e.g., a relevant Working Group if one exists). Vasseur, et al. Expires June 6, 2010 [Page 24] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 9.1. Routing Objects IANA is requested to create a registry for Routing objects. Each Routing object has a Routing object type value. Value Meaning Reference 1 Node State and Attribute This document 2 Node Energy This document 3 Hop Count This document 4 Link Throughput This document 5 Link Latency This document 6 Link Quality Level This document 7 Link ETX This document 8 Link Color This document 9.2. Routing Object Common Header IANA is requested to create a registry to manage the codespace of A field and the Flag field of the Routing Common header. Codespace of the A field (NSA Object) Value Meaning Reference 0 Routing metric is additive This document 2 Routing metric reports a maximum This document 3 Routing metric reports a minimum This document IANA is requested to create a registry to manage the Flag field of the Routing metric common header. New bit numbers may be allocated only by an IETF Consensus action. Each bit should be tracked with the following qualities: o Bit number o Capability Description o Defining RFC Several bits are defined for the Routing metric common header in this document. The following values have been assigned: Vasseur, et al. Expires June 6, 2010 [Page 25] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 Codespace of the Flag field (Routing common header) Bit Description Reference 8 Constraint/metric This document 7 Optional Constraint This document 5-6 Additive/Max/Min This document 4 Global/Local This document 3 Recorded/Aggregated This document 9.3. NSA Object IANA is requested to create a registry to manage the codespace of the Flag field of the NSA Object. New bit numbers may be allocated only by an IETF Consensus action. Each bit should be tracked with the following qualities: o Bit number o Capability Description o Defining RFC Several bits are defined for the NSA Object flag field in this document. The following values have been assigned: Codespace of the Flag field (NSA Object) Bit Description Reference 14 Aggregator This document 15 Overloaded This document 9.4. Hop-count Object IANA is requested to create a registry to manage the codespace of the Flag field of the Hop-count Object. New bit numbers may be allocated only by an IETF Consensus action. Each bit should be tracked with the following qualities: o Bit number o Capability Description o Defining RFC No Flags are currently defined. Vasseur, et al. Expires June 6, 2010 [Page 26] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 9.5. DAG Information Option (DIO) suboption for the OCP Object A new DAG Information Option (DIO) suboption is defined for the OCP object. DIO suboption for the OCP Object Value Description Reference 5 OCP Value This document 9.6. Objective Code Point (OCP) Object IANA is requested to create a registry to manage the codespace of the OCP field of the OCP object. No OCP codepoints are defined in this specification. 10. Security Considerations Routing metrics should be handled in a secure and trustful manner. For instance, a malicious node can not advertise falsely that it has good metrics for routing and belong to the established path to have a chance to intercept packets. 11. Acknowledgements The authors would like to acknowledge the contributions of Young Jae Kim, David Meyer, Mischa Dohler, Anders Brandt, Philip Levis, Pascal Thubert, Richard Kelsey, Jonathan Hui, Phil Levis and Tim Winter for their review and comments. 12. References 12.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. Vasseur, et al. Expires June 6, 2010 [Page 27] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 12.2. Informative References [I-D.ietf-roll-building-routing-reqs] Martocci, J., Riou, N., Mil, P., and W. Vermeylen, "Building Automation Routing Requirements in Low Power and Lossy Networks", draft-ietf-roll-building-routing-reqs-07 (work in progress), September 2009. [I-D.ietf-roll-home-routing-reqs] Brandt, A. and J. Buron, "Home Automation Routing Requirements in Low Power and Lossy Networks", draft-ietf-roll-home-routing-reqs-09 (work in progress), November 2009. [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-04 (work in progress), October 2009. [I-D.ietf-roll-terminology] Vasseur, J., "Terminology in Low power And Lossy Networks", draft-ietf-roll-terminology-02 (work in progress), October 2009. [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J. McManus, "Requirements for Traffic Engineering Over MPLS", RFC 2702, September 1999. [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209, December 2001. [RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching (GMPLS) Signaling Functional Description", RFC 3471, January 2003. [RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel, "Routing Requirements for Urban Low-Power and Lossy Networks", RFC 5548, May 2009. [RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney, "Industrial Routing Requirements in Low-Power and Lossy Networks", RFC 5673, October 2009. Vasseur, et al. Expires June 6, 2010 [Page 28] Internet-Draft draft-ietf-roll-routing-metrics-04 December 2009 12.3. References [IEEE.754.1985] IEEE Standard 754, "Standard for Binary Floating-Point Arithmetic", August 1985. Authors' Addresses JP Vasseur (editor) Cisco Systems, Inc 11, Rue Camille Desmoulins Issy Les Moulineaux, 92782 France Email: jpv@cisco.com Mijeom (editor) Future Tech Lab., Korea Telecom 17 Woomyeon-dong, Seocho-gu Seoul, 137-792 Korea Email: mjkim@kt.com Kris Dust Networks 30695 Huntwood Ave. Hayward, CA 95544 USA Email: kpister@dustnetworks.com Hakjin Future Tech Lab., Korea Telecom 17 Woomyeon-dong, Seocho-gu Seoul, 137-792 Korea Email: hjchong@kt.com Vasseur, et al. Expires June 6, 2010 [Page 29]