| Internet-Draft | AODV-RPL | September 2021 |
| Anamalamudi, et al. | Expires 20 March 2022 | [Page] |
Route discovery for symmetric and asymmetric Peer-to-Peer (P2P) traffic flows is a desirable feature in Low power and Lossy Networks (LLNs). For that purpose, this document specifies a reactive P2P route discovery mechanism for both hop-by-hop routing and source routing: Ad Hoc On-demand Distance Vector Routing (AODV) based RPL protocol (AODV-RPL). Paired Instances are used to construct directional paths, for cases where there are asymmetric links between source and target nodes.¶
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Routing Protocol for Low-Power and Lossy Networks (RPL) [RFC6550] is an IPv6 distance vector routing protocol designed to support multiple traffic flows through a root-based Destination-Oriented Directed Acyclic Graph (DODAG). Typically, a router does not have routing information for most other routers. Consequently, for traffic between routers within the DODAG (i.e., Peer-to-Peer (P2P) traffic) data packets either have to traverse the root in non-storing mode, or traverse a common ancestor in storing mode. Such P2P traffic is thereby likely to traverse longer routes and may suffer severe congestion near the root (for more information see [RFC6997], [RFC6998]). The network environment that is considered in this document is assumed to be the same as described in Section 1 of [RFC6550].¶
The route discovery process in AODV-RPL is modeled on the analogous procedure specified in AODV [RFC3561]. The on-demand nature of AODV route discovery is natural for the needs of peer-to-peer routing in RPL-based LLNs. AODV terminology has been adapted for use with AODV-RPL messages, namely RREQ for Route Request, and RREP for Route Reply. AODV-RPL currently omits some features compared to AODV -- in particular, flagging Route Errors, "blacklisting" unidirectional links ([RFC3561]), multihoming, and handling unnumbered interfaces.¶
AODV-RPL reuses and extends the core RPL functionality to support routes with bidirectional asymmetric links. It retains RPL's DODAG formation, RPL Instance and the associated Objective Function (defined in [RFC6551]), trickle timers, and support for storing and non-storing modes. AODV-RPL adds basic messages RREQ and RREP as part of RPL DODAG Information Object (DIO) control message, which go in separate (paired) RPL instances. AODV-RPL does not utilize the Destination Advertisement Object (DAO) control message of RPL. AODV-RPL specifies a new Mode of Operation (MOP) running in a separate instance dedicated to discover P2P routes, which may differ from routes discoverable by native RPL. AODV-RPL can be operated whether or not native RPL is running otherwise.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
AODV-RPL reuses names for messages and data structures, including Rank, DODAG and DODAGID, as defined in RPL [RFC6550].¶
With AODV-RPL, routes from OrigNode to TargNode within the LLN network are established "on-demand". In other words, the route discovery mechanism in AODV-RPL is invoked reactively when OrigNode has data for delivery to the TargNode but existing routes do not satisfy the application's requirements. AODV-RPL is thus functional without requiring the use of RPL or any other routing protocol.¶
The routes discovered by AODV-RPL are not constrained to traverse a common ancestor. AODV-RPL can enable asymmetric communication paths in networks with bidirectional asymmetric links. For this purpose, AODV-RPL enables discovery of two routes: namely, one from OrigNode to TargNode, and another from TargNode to OrigNode. When possible, AODV-RPL also enables symmetric route discovery along Paired DODAGs (see Section 5).¶
In AODV-RPL, routes are discovered by first forming a temporary DAG rooted at the OrigNode. Paired DODAGs (Instances) are constructed according to the AODV-RPL Mode of Operation (MOP) during route formation between the OrigNode and TargNode. The RREQ-Instance is formed by route control messages from OrigNode to TargNode whereas the RREP-Instance is formed by route control messages from TargNode to OrigNode. Intermediate routers join the Paired DODAGs based on the Rank [RFC6550] as calculated from the DIO message. Henceforth in this document, the RREQ-DIO message means the AODV-RPL mode DIO message from OrigNode to TargNode, containing the RREQ option (see Section 4.1). Similarly, the RREP-DIO message means the AODV-RPL mode DIO message from TargNode to OrigNode, containing the RREP option (see Section 4.2). The route discovered in the RREQ-Instance is used for transmitting data from TargNode to OrigNode, and the route discovered in RREP-Instance is used for transmitting data from OrigNode to TargNode.¶
OrigNode selects one of its IPv6 addresses and sets it in the DODAGID field of the RREQ-DIO message. Exactly one RREQ option MUST be present in a RREQ-DIO message, otherwise the message MUST be dropped.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Option Length |S|H|X| Compr | L | MaxRank | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Orig SeqNo | | +-+-+-+-+-+-+-+-+ | | | | | | Address Vector (Optional, Variable Length) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OrigNode supplies the following information in the RREQ option:¶
2-bit unsigned integer determining the length of time that a node is able to belong to the RREQ-Instance (a temporary DAG including the OrigNode and the TargNode). Once the time is reached, a node MUST leave the RREQ-Instance and stop sending or receiving any more DIOs for the RREQ-Instance. This naturally depends on the node's ability to keep track of the time. L is independent from the route lifetime, which is defined in the DODAG configuration option.¶
TargNode can join the RREQ instance at a Rank whose integer portion is less than or equal to the MaxRank. Other nodes MUST NOT join a RREQ instance if its own Rank would be equal to or higher than MaxRank. A router MUST discard a received RREQ if the integer part of the advertised Rank equals or exceeds the MaxRank limit.¶
TargNode sets one of its IPv6 addresses in the DODAGID field of the RREP-DIO message. Exactly one RREP option MUST be present in a RREP-DIO message, otherwise the message MUST be dropped. TargNode supplies the following information in the RREP 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |G|H|X| Compr | L | MaxRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shift |X X| |
+-+-+-+-+-+-+-+-+ |
| |
| |
| Address Vector (Optional, Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The AODV-RPL Target (ART) Option is based on the Target Option in core RPL [RFC6550]. The Flags field is replaced by the Destination Sequence Number of the TargNode and the Prefix Length field is reduced to 7 bits so that the value is limited to be no greater than 127.¶
A RREQ-DIO message MUST carry at least one ART Option. A RREP-DIO message MUST carry exactly one ART Option. Otherwise, the message MUST be dropped.¶
OrigNode can include multiple TargNode addresses via multiple AODV-RPL Target Options in the RREQ-DIO, for routes that share the same requirement on metrics. This reduces the cost to building only one DODAG.¶
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Dest SeqNo |X|Prefix Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ |
| Target Prefix / Address (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Links are considered symmetric until indication to the contrary is received. In Figure 4 and Figure 5, BR is the Border Router, O is the OrigNode, each R is an intermediate router, and T is the TargNode. If the RREQ-DIO arrives over an interface that is known to be symmetric, and the S bit is set to 1, then it remains as 1, as illustrated in Figure 4. If an intermediate router sends out RREQ-DIO with the S bit set to 1, then each link en route from the OrigNode O to this router has met the requirements of route discovery, and the route can be used symmetrically.¶
BR
/----+----\
/ | \
/ | \
R R R
_/ \ | / \
/ \ | / \
/ \ | / \
R -------- R --- R ----- R -------- R
/ \ <--S=1--> / \ <--S=1--> / \
<--S=1--> \ / \ / <--S=1-->
/ \ / \ / \
O ---------- R ------ R------ R ----- R ----------- T
/ \ / \ / \ / \
/ \ / \ / \ / \
/ \ / \ / \ / \
R ----- R ----------- R ----- R ----- R ----- R ---- R----- R
>---- RREQ-Instance (Control: O-->T; Data: T-->O) ------->
<---- RREP-Instance (Control: T-->O; Data: O-->T) -------<
Upon receiving a RREQ-DIO with the S bit set to 1, a node determines whether this link can be used symmetrically, i.e., both directions meet the requirements of data transmission. If the RREQ-DIO arrives over an interface that is not known to be symmetric, or is known to be asymmetric, the S bit is set to 0. If the S bit arrives already set to be '0', it is set to be '0' when the RREQ-DIO is propagated (Figure 5). For an asymmetric route, there is at least one hop which doesn't satisfy the Objective Function. Based on the S bit received in RREQ-DIO, TargNode T determines whether or not the route is symmetric before transmitting the RREP-DIO message upstream towards the OrigNode O.¶
The criteria used to determine whether or not each link is symmetric is beyond the scope of the document. For instance, intermediate routers can use local information (e.g., bit rate, bandwidth, number of cells used in 6tisch [RFC9030]), a priori knowledge (e.g., link quality according to previous communication) or use averaging techniques as appropriate to the application. Other link metric information can be acquired before AODV-RPL operation, by executing evaluation procedures; for instance test traffic can be generated between nodes of the deployed network. During AODV-RPL operation, OAM techniques for evaluating link state (see [RFC7548], [RFC7276], [co-ioam]) MAY be used (at regular intervals appropriate for the LLN). The evaluation procedures are out of scope for AODV-RPL.¶
Appendix A describes an example method using the upstream Expected Number of Transmissions (ETX) and downstream Received Signal Strength Indicator (RSSI) to estimate whether the link is symmetric in terms of link quality using an averaging technique.¶
BR
/----+----\
/ | \
/ | \
R R R
/ \ | / \
/ \ | / \
/ \ | / \
R --------- R --- R ---- R --------- R
/ \ --S=1--> / \ --S=0--> / \
--S=1--> \ / \ / --S=0-->
/ \ / \ / \
O ---------- R ------ R------ R ----- R ----------- T
/ \ / \ / \ / \
/ <--S=0-- / \ / \ / <--S=0--
/ \ / \ / \ / \
R ----- R ----------- R ----- R ----- R ----- R ---- R----- R
<--S=0-- <--S=0-- <--S=0-- <--S=0-- <--S=0--
>---- RREQ-Instance (Control: O-->T; Data: T-->O) ------->
<---- RREP-Instance (Control: T-->O; Data: O-->T) -------<
As illustrated in Figure 5, an intermediate router determines the S bit value that the RREQ-DIO should carry using link asymmetry detection methods as discussed earlier in this section. In many cases the intermediate router has already made the link asymmetry decision by the time RREQ-DIO arrives.¶
The route discovery process is initiated when an application at the OrigNode has data to be transmitted to the TargNode, but does not have a route that satisfies the Objective Function for the target of the data transmission. In this case, the OrigNode builds a local RPLInstance and a DODAG rooted at itself. Then it transmits a DIO message containing exactly one RREQ option (see Section 4.1) via link-local multicast. The DIO MUST contain at least one ART Option (see Section 4.3). The required ART Option indicates the TargNode. The S bit in RREQ-DIO sent out by the OrigNode is set to 1.¶
Each node maintains a sequence number; the operation is specified in section 7.2 of [RFC6550]. When the OrigNode initiates a route discovery process, it MUST increase its own sequence number to avoid conflicts with previously established routes. The sequence number is carried in the Orig SeqNo field of the RREQ option.¶
The address in the ART Option can be a unicast IPv6 address or a prefix. The OrigNode can initiate the route discovery process for multiple targets simultaneously by including multiple ART Options. Within a RREQ-DIO the requirements for the routes to different TargNodes MUST be the same.¶
OrigNode can maintain different RPLInstances to discover routes with different requirements to the same targets. Using the RPLInstanceID pairing mechanism (see Section 6.3.3), route replies (RREP-DIOs) for different RPLInstances can be generated.¶
The transmission of RREQ-DIO obeys the Trickle timer [RFC6206]. If the length of time specified by the L field has elapsed, the OrigNode MUST leave the DODAG and stop sending RREQ-DIOs in the related RPLInstance.¶
Upon receiving a RREQ-DIO, a router goes through the steps below. If the router has not joined the RREQ-Instance, then the maximum useful rank (MaxUseRank) is MaxRank. Otherwise, MaxUseRank is set to be the Rank value that was stored when the router processed the best previous RREQ for the DODAG with the given RREQ-Instance.¶
If the OrigNode tries to reach multiple TargNodes in a single RREQ-Instance, one of the TargNodes can be an intermediate router to the others, therefore it MUST continue sending RREQ-DIO to reach other targets. In this case, before transmitting the RREQ-DIO via link-local multicast, a TargNode MUST delete the Target Option encapsulating its own address, so that downstream routers with higher Rank values do not try to create a route to this TargNode.¶
An intermediate router could receive several RREQ-DIOs from routers with lower Rank values in the same RREQ-Instance with different lists of Target Options. When transmitting the RREQ-DIO, the intersection of all received lists MUST be included. For example, suppose two RREQ-DIOs are received with the same RPLInstance and OrigNode. Suppose further that the first RREQ has (T1, T2) as the targets, and the second one has (T2, T4) as targets. Then only T2 needs to be included in the generated RREQ-DIO. If the intersection is empty, it means that all the targets have been reached, and the router MUST NOT transmit any RREQ-DIO. For the purposes of determining the intersection with previous incoming RREQ-DIOs, the intermediate router maintains a record of the targets that have been requested for a given RREQ-Instance. Any incoming RREQ-DIO message having multiple ART Options coming from a router with higher Rank than the Rank of the stored targets is ignored.¶
When H=1 in the incoming RREQ, the TargNode MUST NOT generate a RREP if one of its addresses is present in the Address Vector. If the implementation selects the symmetric route, and the L field is not 0, the TargNode MAY delay transmitting the RREP-DIO for duration RREP_WAIT_TIME to await a route with a lower Rank. The value of RREP_WAIT_TIME is set by default to 1/4 of the duration determined by the L field. For L == 0, RREP_WAIT_TIME is set by default to 0. Depending upon the application, RREP_WAIT_TIME may be set to other values. Smaller values enable quicker formation for the P2P route. Larger values enable formation of P2P routes with better Rank values.¶
If a RREQ-DIO arrives at TargNode with the S bit set to 1, there is a symmetric route both of whose directions satisfy the Objective Function. Other RREQ-DIOs might later provide better upward routes. The method of selection between a qualified symmetric route and an asymmetric route that might have better performance is implementation-specific and out of scope.¶
For a symmetric route, the RREP-DIO message is unicast to the next hop according to the accumulated address vector (H=0) or the route entry (H=1). Thus the DODAG in RREP-Instance does not need to be built. The RPLInstanceID in the RREP-Instance is paired as defined in Section 6.3.3. In case the H bit is set to 0, the address vector received in the RREQ-DIO MUST be included in the RREP-DIO. TargNode increments its current sequence number and uses the incremented result in the Dest SeqNo in the ART option of the RREQ-DIO. The address of the OrigNode MUST be encapsulated in the ART Option and included in this RREP-DIO message.¶
When a RREQ-DIO arrives at a TargNode with the S bit set to 0, the TargNode MUST build a DODAG in the RREP-Instance corresponding to the RREQ-DIO, rooted at itself in order to discover the downstream route from the OrigNode to the TargNode. The RREP-DIO message MUST be transmitted via link-local multicast until the OrigNode is reached or MaxRank is exceeded.¶
The settings of the fields in RREP option and ART option are the same as for the symmetric route, except for the value of the S bit associated with the RREP-instance.¶
Since the RPLInstanceID is assigned locally (i.e., there is no coordination between routers in the assignment of RPLInstanceID), the tuple (OrigNode, TargNode, RPLInstanceID) is needed to uniquely identify a discovered route. It is possible that multiple route discoveries with dissimilar Objective Functions are initiated simultaneously. Thus between the same pair of OrigNode and TargNode, there can be multiple AODV-RPL route discovery instances. To avoid any mismatch, the RREQ-Instance and the RREP-Instance in the same route discovery MUST be paired using the RPLInstanceID.¶
When preparing the RREP-DIO, a TargNode could find the RPLInstanceID candidate for the RREP-Instance is already occupied by another RPL Instance from an earlier route discovery operation which is still active. This unlikely case might happen if two distinct OrigNodes need routes to the same TargNode, and they happen to use the same RPLInstanceID for RREQ-Instance. In such cases, the original RPLInstanceID of an already active RREP-Instance MUST NOT be used again for assigning RPLInstanceID for the later RREP-Instance. Reusing the same RPLInstanceID for two distinct DODAGs originated with the same DODAGID (TargNode address) would prevent intermediate routers to distinguish between these DODAGs (and their associated Objective Functions). Instead, the RPLInstanceID MUST be replaced by another value so that the two RREP-instances can be distinguished. In RREP-DIO option, the Shift field of the RREP-DIO message(Figure 2) indicates the shift to be applied to original RPLInstanceID to obtain the replacement RPLInstanceID. When the new RPLInstanceID after shifting exceeds 255, it rolls over starting at 0. For example, if the original RPLInstanceID is 252, and shifted by 6, the new RPLInstanceID will be 2. Related operations can be found in Section 6.4. RPLInstanceID collisions do not occur across RREQ-DIOs; the DODAGID equals the OrigNode address and is sufficient to disambiguate between DODAGs.¶
Upon receiving a RREP-DIO, a router performs the following steps:¶
Upon receiving a RREP-DIO, a router which already belongs to the RREP-Instance SHOULD drop the RREP-DIO.¶
In some cases, an Intermediate router that receives a RREQ-DIO message MAY transmit a "Gratuitous" RREP-DIO message back to OrigNode instead of continuing to multicast the RREQ-DIO towards TargNode. The intermediate router effectively builds the RREP-Instance on behalf of the actual TargNode. The G bit of the RREP option is provided to distinguish the Gratuitous RREP-DIO (G=1) sent by the Intermediate node from the RREP-DIO sent by TargNode (G=0).¶
The gratuitous RREP-DIO can be sent out when an intermediate router receives a RREQ-DIO for a TargNode, and the router has a more recent (larger destination sequence number) pair of downward and upward routes to the TargNode which also satisfy the Objective Function.¶
In case of source routing, the intermediate router MUST unicast the received RREQ-DIO to TargNode including the address vector between the OrigNode and the router. Thus the TargNode can have a complete upward route address vector from itself to the OrigNode. Then the router MUST transmit the gratuitous RREP-DIO including the address vector from the router itself to the TargNode.¶
In case of hop-by-hop routing, the intermediate router MUST unicast the received RREQ-DIO to the Next Hop on the route. The Next Hop router along the route MUST build new route entries with the related RPLInstanceID and DODAGID in the downward direction. The above process will happen recursively until the RREQ-DIO arrives at the TargNode. Then the TargNode MUST unicast recursively the RREP-DIO hop-by-hop to the intermediate router, and the routers along the route SHOULD build new route entries in the upward direction. Upon receiving the unicast RREP-DIO, the intermediate router sends the gratuitous RREP-DIO to the OrigNode as defined in Section 6.3.¶
The trickle timer operation to control RREQ-Instance/RREP-Instance multicast uses [RFC6206] to control RREQ-DIO and RREP-DIO transmissions. The Trickle control of these DIO transmissions follow the procedures described in the Section 8.3 of [RFC6550] entitled "DIO Transmission".¶
Note to RFC editor:¶
The sentences "The parenthesized number 5 is only a suggestion." and "The parenthesized numbers are only suggestions." are to be removed prior publication.¶
A Subregistry in this section refers to a named sub-registry of the "Routing Protocol for Low Power and Lossy Networks (RPL)" registry.¶
IANA is asked to assign a new Mode of Operation, named "AODV-RPL" for peer-to-peer hop-by-hop routing from the "Mode of Operation" Subregistry. The parenthesized number 5 is only a suggestion.¶
+-------------+---------------+---------------+ | Value | Description | Reference | +-------------+---------------+---------------+ | TBD1 (5) | AODV-RPL | This document | +-------------+---------------+---------------+
IANA is asked to assign three new AODV-RPL options "RREQ", "RREP" and "ART", as described in Figure 7 from the "RPL Control Message Options" Subregistry. The parenthesized numbers are only suggestions.¶
+-------------+------------------------+---------------+ | Value | Meaning | Reference | +-------------+------------------------+---------------+ | TBD2 (0x0B) | RREQ Option | This document | +-------------+------------------------+---------------+ | TBD3 (0x0C) | RREP Option | This document | +-------------+------------------------+---------------+ | TBD4 (0x0D) | ART Option | This document | +-------------+------------------------+---------------+
The security considerations for the operation of AODV-RPL are similar to those for the operation of RPL (as described in Section 19 of the RPL specification [RFC6550]). Sections 6.1 and 10 of [RFC6550] describe RPL's optional security framework, which AODV-RPL relies on to provide data confidentiality, authentication, replay protection, and delay protection services. Additional analysis for the security threats to RPL can be found in [RFC7416].¶
A router can join a temporary DAG created for a secure AODV-RPL route discovery only if it can support the security configuration in use (see Section 6.1 of [RFC6550]), which also specifies the key in use. It does not matter whether the key is preinstalled or dynamically acquired. The router must have the key in use before it can join the DAG being created for secure route discovery.¶
If a rogue router knows the key for the security configuration in use, it can join the secure AODV-RPL route discovery and cause various types of damage. Such a rogue router could advertise false information in its DIOs in order to include itself in the discovered route(s). It could generate bogus RREQ-DIO, and RREP-DIO messages carrying bad routes or maliciously modify genuine RREP-DIO messages it receives. A rogue router acting as the OrigNode could launch denial-of-service attacks against the LLN deployment by initiating fake AODV-RPL route discoveries. When rogue routers might be present, RPL's preinstalled mode of operation, where the key to use for route discovery is preinstalled, SHOULD be used.¶
When a RREQ-DIO message uses the source routing option by setting the H bit to 0, a rogue router may populate the Address Vector field with a set of addresses that may result in the RREP-DIO traveling in a routing loop.¶
If a rogue router is able to forge a gratuitous RREP, significant damage might result.¶
The authors thank Pascal Thubert, Rahul Jadhav, and Lijo Thomas for their support and valuable inputs. The authors specially thank Lavanya H.M for implementing AODV-RPl in Contiki and conducting extensive simulation studies.¶
The authors would like to acknowledge the review, feedback and comments from the following people, in alphabetical order: Roman Danyliw, Lars Eggert, Benjamin Kaduk, Tero Kivinen, Erik Kline, Murray Kucherawy, Warren Kumari, Francesca Palombini, Alvaro Retana, Ines Robles, John Scudder, Meral Shirazipour, Peter Van der Stok, Eric Vyncke, and Robert Wilton.¶
The combination of Received Signal Strength Indication(downstream) (RSSI) and Expected Number of Transmissions(upstream) (ETX) has been tested to determine whether a link is symmetric or asymmetric at intermediate nodes. We present two methods to obtain an ETX value from RSSI measurement.¶
In the first method, we constructed a table measuring RSSI vs ETX using the Cooja simulation [cooja] setup in the Contiki OS environment[contiki]. We used Contiki-2.7 running 6LoWPAN/RPL protocol stack for the simulations. For approximating the number of packet drops based on the RSSI values, we implemented simple logic that drops transmitted packets with certain pre-defined ratios before handing over the packets to the receiver. The packet drop ratio is implemented as a table lookup of RSSI ranges mapping to different packet drop ratios with lower RSSI ranges resulting in higher values. While this table has been defined for the purpose of capturing the overall link behavior, it is highly recommended to conduct physical radio measurement experiments, in general. By keeping the receiving node at different distances, we let the packets experience different packet drops as per the described method. The ETX value computation is done by another module which is part of RPL Objective Function implementation. Since ETX value is reflective of the extent of packet drops, it allowed us to prepare a useful ETX vs RSSI table. ETX versus RSSI values obtained in this way may be used as explained below:¶
Source---------->NodeA---------->NodeB------->Destination
| RSSI at NodeA for NodeB | Expected ETX at NodeA for NodeB->NodeA |
|---|---|
| > -60 | 150 |
| -70 to -60 | 192 |
| -80 to -70 | 226 |
| -90 to -80 | 662 |
| -100 to -90 | 3840 |
We tested the operations in this specification by making the following experiment, using the above parameters. In our experiment, a communication link is considered as symmetric if the ETX value of NodeA->NodeB and NodeB->NodeA (see Figure 8) are within, say, a 1:3 ratio. This ratio should be understood as determining the link's symmetric/asymmetric nature. NodeA can typically know the ETX value in the direction of NodeA -> NodeB but it has no direct way of knowing the value of ETX from NodeB->NodeA. Using physical testbed experiments and realistic wireless channel propagation models, one can determine a relationship between RSSI and ETX representable as an expression or a mapping table. Such a relationship in turn can be used to estimate ETX value at nodeA for link NodeB--->NodeA from the received RSSI from NodeB. Whenever nodeA determines that the link towards the nodeB is bi-directional asymmetric then the S bit is set to 0. Afterwards, the link from NodeA to Destination remains designated as asymmetric and the S bit remains set to 0.¶
Note to the RFC Editor: please remove this section before publication.¶
Abdur Rashid Sangi¶
Huaiyin Institute of Technology¶
No.89 North Beijing Road, Qinghe District¶
Huaian 223001¶
P.R. China¶
Email: sangi_bahrian@yahoo.com¶
Malati Hegde¶
Indian Institute of Science¶
Bangalore 560012¶
India¶
Email: malati@iisc.ac.in¶
Mingui Zhang¶
Huawei Technologies¶
No. 156 Beiqing Rd. Haidian District¶
Beijing 100095¶
P.R. China¶
Email: zhangmingui@huawei.com¶