| ROLL | S. Anamalamudi |
| Internet-Draft | SRM University-AP |
| Intended status: Standards Track | M. Zhang |
| Expires: November 8, 2020 | Huawei Technologies |
| C. Perkins | |
| Deep Blue Sky Networks | |
| S.V.R.Anand | |
| Indian Institute of Science | |
| B. Liu | |
| Huawei Technologies | |
| May 7, 2020 |
AODV based RPL Extensions for Supporting Asymmetric P2P Links in Low-Power and Lossy Networks
draft-ietf-roll-aodv-rpl-08
Route discovery for symmetric and asymmetric Point-to-Point (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, in case some of the links between source and target node are asymmetric.
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Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
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RPL [RFC6550] (Routing Protocol for Low-Power and Lossy Networks) 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., Point-to-Point (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 DAG root (for more information see [RFC6997], [RFC6998]).
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, multihoming, and handling unnumbered interfaces.
AODV-RPL reuses and provides a natural extension to the core RPL functionality to support routes with birectional asymmetric links. It retains RPL's DODAG formation, RPL Instance and the associated Objective Function, trickle timers, and support for storing and non-storing modes. AODV adds basic messages RREQ and RREP as part of RPL DIO (DODAG Information Object) control messages, and does not utilize the DAO message of RPL. AODV-RPL specifies a new MOP running in a seperate instance dedicating to discover P2P routes, which may differ from the P2MP 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.
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 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.
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) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format for AODV-RPL RREQ Option
OrigNode sets its IPv6 address in the DODAGID field of the RREQ-DIO message. A RREQ-DIO message MUST carry exactly one RREQ option, otherwise it SHOULD be dropped.
TargNode can join the RREQ instance at a Rank whose integer portion is 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.
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 |Rsv| |
+-+-+-+-+-+-+-+-+ |
| |
| |
| Address Vector (Optional, Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format for AODV-RPL RREP option
TargNode sets its IPv6 address in the DODAGID field of the RREP-DIO message. A RREP-DIO message MUST carry exactly one RREP option, otherwise the message SHOULD be dropped. TargNode supplies the following information in the RREP option:
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 SHOULD 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 |r|Prefix Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ |
| Target Prefix / Address (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Target option format for AODV-RPL MOP
In Figure 4 and Figure 5, BR is the Border Router, O is the OrigNode, 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 all the one-hop links on the route from the OrigNode O to this router meet 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) -------<
Figure 4: AODV-RPL with Symmetric Paired Instances
Upon receiving a RREQ-DIO with the S bit set to 1, a node determines whether this one-hop link can be used symmetrically, i.e., both the two 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' on retransmission (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, and may be implementation-specific. For instance, intermediate routers can use local information (e.g., bit rate, bandwidth, number of cells used in 6tisch), a priori knowledge (e.g. link quality according to previous communication) or use averaging techniques as appropriate to the application.
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) -------<
Figure 5: AODV-RPL with Asymmetric Paired Instances
Appendix A describes an example method using the ETX and RSSI to estimate whether the link is symmetric in terms of link quality is given in using an averaging technique.
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 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, and 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 InstanceID pairing mechanism (see Section 6.3.3), route replies (RREP-DIOs) for different RPLInstances can be distinguished.
The transmission of RREQ-DIO obeys the Trickle timer [RFC6206]. If the duration specified by the L bit 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 does not belong to 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 rebroadcasting the RREQ-DIO, 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 TargetNode.
An intermediate router could receive several RREQ-DIOs from routers with lower Rank values in the same RREQ-Instance but have different lists of Target Options. When rebroadcasting the RREQ-DIO, the intersection of these 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 send out 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 associated with the RREQ-Instance. Any RREQ-DIO message with different ART Options coming from a router with higher Rank is ignored.
If a RREQ-DIO arrives at TargNode with the S bit set to 1, there is a symmetric route along which both directions satisfy the Objective Function. Other RREQ-DIOs might later provide asymmetric upward routes (i.e. S=0). Selection between a qualified symmetric route and an asymmetric route that might have better performance is implementation-specific and out of scope. If the implementation selects the symmetric route, and the L bit is not 0, the TargNode MAY delay transmitting the RREP-DIO for duration RREP_WAIT_TIME to await a symmetric route with a lower Rank. The value of RREP_WAIT_TIME is set by default to 1/4 of the time duration determined by the L bit.
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 rooted at itself in order to discover the downstream route from the OrigNode to the TargNode. The RREP-DIO message MUST be re-transmitted via link-local multicast until the OrigNode is reached or MaxRank is exceeded. 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 time duration determined by the L bit.
The settings of the fields in RREP option and ART option are the same as for the symmetric route, except for the S bit.
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 to be used for the RREP-Instance is already occupied by another RPL Instance from an earlier route discovery operation which is still active. In other words, it might happen that two distinct OrigNodes need routes to the same TargNode, and they happen to use the same RPLInstanceID for RREQ-Instance. In this case, the occupied RPLInstanceID MUST NOT be used again. Then the second RPLInstanceID MUST be shifted into another integer so that the two RREP-instances can be distinguished. In RREP option, the Shift field indicates the shift to be applied to original RPLInstanceID. When the new InstanceID after shifting exceeds 63, it rolls over starting at 0. For example, the original InstanceID is 60, and shifted by 6, the new InstanceID will be 2. Related operations can be found in Section 6.4.
Upon receiving a RREP-DIO, a router which does not belong to the RREQ-Instance goes through the following steps:
Upon receiving a RREP-DIO, a router which already belongs to the RREQ-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 send out 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".
+-------------+---------------+---------------+
| Value | Description | Reference |
+-------------+---------------+---------------+
| TBD1 (5) | AODV-RPL | This document |
+-------------+---------------+---------------+
Figure 6: Mode of Operation
IANA is asked to assign a new Mode of Operation, named "AODV-RPL" for Point-to-Point(P2P) hop-by-hop routing from the "Mode of Operation" Registry [RFC6550].
+-------------+------------------------+---------------+
| Value | Meaning | Reference |
+-------------+------------------------+---------------+
| TBD2 (0x0A) | RREQ Option | This document |
+-------------+------------------------+---------------+
| TBD3 (0x0B) | RREP Option | This document |
+-------------+------------------------+---------------+
| TBD4 (0x0C) | ART Option | This document |
+-------------+------------------------+---------------+
Figure 7: AODV-RPL Options
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" Registry [RFC6550].
In general, 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 security framework, which provides data confidentiality, authentication, replay protection, and delay protection services.
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, 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 a secure P2P-RPL 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. In this type of scenario, RPL's authenticated mode of operation, where a node can obtain the key to use for a P2P-RPL route discovery only after proper authentication, SHOULD be used.
When RREQ-DIO message uses source routing option with 'H' set to 0, some of the security concerns that led to the deprecation of Type 0 routing headers [RFC5095] may apply. To avoid the possibility of a RREP-DIO message traveling in a routing loop, if one of its addresses are present as part of the Source Route listed inside the message, the Intermediate Router MUST NOT forward the message.
This document specifies that links are considered symmetric until additional information is collected. 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.
| [co-ioam] | Ballamajalu, Rashmi., S.V.R., Anand. and Malati. Hegde, "Co-iOAM: In-situ Telemetry Metadata Transport for Resource Constrained Networks within IETF Standards Framework", 2018 10th International Conference on Communication Systems & Networks (COMSNETS) pp.573-576, Jan 2018. |
| [RFC6997] | Goyal, M., Baccelli, E., Philipp, M., Brandt, A. and J. Martocci, "Reactive Discovery of Point-to-Point Routes in Low-Power and Lossy Networks", RFC 6997, DOI 10.17487/RFC6997, August 2013. |
| [RFC7276] | Mizrahi, T., Sprecher, N., Bellagamba, E. and Y. Weingarten, "An Overview of Operations, Administration, and Maintenance (OAM) Tools", RFC 7276, DOI 10.17487/RFC7276, June 2014. |
| [RFC7548] | Ersue, M., Romascanu, D., Schoenwaelder, J. and A. Sehgal, "Management of Networks with Constrained Devices: Use Cases", RFC 7548, DOI 10.17487/RFC7548, May 2015. |
Source---------->NodeA---------->NodeB------->Destination
Figure 8: Communication link from Source to Destination
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. ETX and RSSI values may be used in conjunction as explained below:
| 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 | 993 |
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. Later on, the link from NodeA to Destination is asymmetric with S bit remains set to 0.
Note to the RFC Editor: please remove this section before publication.