| ROLL | S. Anamalamudi |
| Internet-Draft | Huaiyin Institute of Technology |
| Intended status: Standards Track | M. Zhang |
| Expires: March 13, 2018 | AR. Sangi |
| Huawei Technologies | |
| C. Perkins | |
| Futurewei | |
| S.V.R.Anand | |
| Indian Institute of Science | |
| September 9, 2017 |
Asymmetric AODV-P2P-RPL in Low-Power and Lossy Networks (LLNs)
draft-ietf-roll-aodv-rpl-02
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 hop-by-hop routing (storing mode) based on Ad Hoc On-demand Distance Vector Routing (AODV) based RPL protocol. Two separate Instances are used to construct directional paths in case some of the links between source and target node are asymmetric.
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RPL[RFC6550], the IPv6 distance vector routing protocol for Low-power and Lossy Networks (LLNs), is designed to support multiple traffic flows through a root-based Destination-Oriented Directed Acyclic Graph (DODAG). For traffic flows between routers within the DODAG (i.e., Point-to-Point (P2P) traffic), this means that data packets either have to traverse the root in non-storing mode (source routing), or traverse a common ancestor in storing mode (hop-by-hop routing). Such P2P traffic is thereby likely to flow along sub-optimal routes and may suffer severe traffic congestion near the DAG root [RFC6997], [RFC6998].
To discover optimal paths for P2P traffic flows in RPL, P2P-RPL [RFC6997] specifies a temporary DODAG where the source acts as temporary root. The source initiates "P2P Route Discovery mode (P2P-RDO)" with an address vector for both non-storing mode (H=0) and storing mode (H=1). Subsequently, each intermediate router adds its IP address and multicasts the P2P-RDO message, until the message reaches the target node (TargNode). TargNode sends the "Discovery Reply" option. P2P-RPL is efficient for source routing, but much less efficient for hop-by-hop routing due to the extra address vector overhead. In fact, when the P2P-RDO message is being multicast from the source hop-by-hop, receiving nodes are able to determine a next hop towards the source in symmetric links. When TargNode subsequently replies to the source along the established forward route, receiving nodes can determine the next hop towards TargNode. In other words, it is efficient to use only routing tables for P2P-RDO message instead of "Address vector" for hop-by-hop routes (H=1) in symmetric links.
RPL and P2P-RPL both specify the use of a single DODAG in networks of symmetric links. But, application-specific routing requirements that are defined in IETF ROLL Working Group [RFC5548], [RFC5673], [RFC5826] and [RFC5867] may need routing metrics and constraints enabling use of asymmetric bidirectional links. For this purpose, [I-D.thubert-roll-asymlink] describes bidirectional asymmetric links for RPL [RFC6550] with Paired DODAGs, for which the DAG root (DODAGID) is common for two Instances. This can satisfy application-specific routing requirements for bidirectional asymmetric links in base RPL [RFC6550]. P2P-RPL for Paired DODAGs, on the other hand, requires two DAG roots: one for the source and another for the target node due to temporary DODAG formation. For networks composed of bidirectional asymmetric links (see Section 4), AODV-RPL specifies P2P route discovery, utilizing RPL with a new MoP. AODV-RPL makes use of two multicast messages to discover possibly asymmetric routes. AODV-RPL eliminates the need for address vector control overhead, significantly reducing the control packet size which is important for Constrained LLN networks. Both discovered routes meet the application specific metrics and constraints that are defined in the Objective Function for each Instance [RFC6552].
The route discovery process in AODV-RPL is modeled on the analogous process that has been specified in AODV [RFC6550]. The on-demand nature of AODV route discovery is natural for the needs of peer-to-peer routing as envisioned for RPL-based LLNs. Similar terminology has been adopted for use with the discovery messages, namely RREQ for Route Request, and RREP for Route Reply. AODV-RPL is, at heart, a simpler protocol than AODV, since there are no analogous operations for flagging Route Errors, blacklisting unidirectional links, multihoming, or handling unnumbered interfaces. Some of the simpler features of AODV, on the other hand, have been imported into AODV-RPL -- for instance, prefix advertisement is allowed on RREP and RREQ message where appropriate.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Additionally, this document uses the following terms:
With AODV-RPL, routes from OrigNode to TargNode within the LLN network established are "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. The routes discovered by AODV-RPL are point-to-point; in other words the routes are not constrained to traverse a common ancestor. Unlike base RPL [RFC6550] and P2P-RPL [RFC6997], 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 routing along Paired DODAGs (see Section 4).
In AODV-RPL, route discovery is initiated by forming a temporary DAG rooted at the OrigNode. Paired DODAGs (Instances) are constructed according to a new 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 (as shown in Figure 2). Intermediate routers join the Paired DODAGs based on the rank as calculated from the DIO message. Henceforth in this document, the RREQ-Instance message means the AODV-RPL DIO message from OrigNode to TargNode, containing the RREQ option. Similarly, the RREP-Instance message means the AODV-RPL DIO message from TargNode to OrigNode, containing the RREP option. Subsequently, the RREQ-Instance is used for data transmission from TargNode to OrigNode and RREP-Instance is used for Data transmission 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |Version Number | Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|0| MOP | Prf | DTSN |S| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
Figure 1: DIO modification to support asymmetric route discovery
The AODV-RPL Mode of Operation defines a new bit, the Symmetric bit ('S'), which is added to the base DIO message as illustrated in Figure 1. OrigNode sets the the 'S' bit to 1 in the RREQ-Instance message when initiating route discovery.
A device originating a AODV-RPL message supplies the following information in the DIO header of the message:
BR
/ | \
/ | \
/ | \
R R R
/ \ | / \
/ \ | / \
/ \ | / \
R -------- R --- R ----- R -------- R
/ \ <--s=1--> / \ <--s=1--> / \
<--s=1--> \ / \ / <--s=1-->
/ \ / \ / \
S ---------- R ------ R------ R ----- R ----------- D
/ \ / \ / \ / \
/ \ / \ / \ / \
/ \ / \ / \ / \
R ----- R ----------- R ----- R ----- R ----- R ---- R----- R
>---- RREQ-Instance (Control: S-->D; Data: D-->S) ------->
<---- RREP-Instance (Control: D-->S; Data: S-->D) -------<
Figure 2: AODV-RPL with Symmetric Paired Instances
BR
/ | \
/ | \
/ | \
R R R
/ \ | / \
/ \ | / \
/ \ | / \
R --------- R --- R ---- R --------- R
/ \ --s=1--> / \ --s=0--> / \
--s=1--> \ / \ / --s=0-->
/ \ / \ / \
S ---------- R ------ R------ R ----- R ----------- D
/ \ / \ / \ / \
/ <--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: S-->D; Data: D-->S) ------->
<---- RREP-Instance (Control: D-->S; Data: S-->D) -------<
Figure 3: AODV-RPL with Asymmetric Paired Instances
Figure 2. In Figure 2 and Figure 3, BR is the BorderRouter, S is the OrigNode, R is an intermediate node, and D is the TargNode. Figure 3). Based on the 'S' bit received in RREQ-Instance, the TargNode decides whether or not the route is symmetric before transmitting the RREP-Instance message upstream towards the OrigNode. The metric used to determine symmetry (i.e., set the "S" bit to be "1" (Symmetric) or "0" (asymmetric)) is implementation specific. We used ETX/RSSI to verify the feasibility of the protocol operations in this draft, as discussed in Appendix A.
The 'S' bit is set to mean that the route is symmetric. If the RREQ-Instance arrives over an interface that is known to be symmetric, and the 'S' bit is set to 1, then it remains set at 1, as illustrated in
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Orig SeqNo | Dest SeqNo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| TargNode IPv6 Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: DIO RREQ option format for AODV-RPL MoP
Figure 4) to its one-hop neighbours. In order to enable intermediate nodes R_i to associate a future RREP message to an incoming RREQ message, the InstanceID of RREQ-Instance MUST assign an odd number.
In order to establish the upstream route from TargNode to OrigNode, OrigNode multicasts the RREQ-Instance message (see
Each intermediate node R_i computes the rank for RREQ-Instance and creates a routing table entry for the upstream route towards the source if the routing metrics/constraints are satisfied. For this purpose R_i must use the asymmetric link metric measured in the upstream direction, from R_i to its upstream neighbor that multicasted the RREQ-Instance message.
When an intermediate node R_i receives a RREQ message in storing mode, it MUST store the OrigNode's InstanceID (RREQ-Instance) along with the other routing information needed to establish the route back to the OrigNode. This will enable R_i to determine that a future RREP message (containing a paired InstanceID for the TargNode) must be transmitted back to the OrigNode's IP address.
If the paths to and from TargNode are not known, the intermediate node multicasts the RREQ-Instance message with updated rank to its next-hop neighbors until the message reaches TargNode (Figure 2). Based on the 'S' bit in the received RREQ message, the TargNode will decide whether to unicast or multicast the RREP message back to OrigNode.
As described in Section 7, in certain circumstances R_i MAY unicast a Gratuitous RREP towards OrigNode, thereby helping to minimize multicast overhead during the Route Discovery process.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Dest SeqNo | Prefix Sz |T|G| Rsvd | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | TargNode IPv6 Address (when present) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: DIO RREP option format for AODV-RPL MoP
The TargNode supplies the following information in the RREP message:
In order to reduce the need for the TargNode IPv6 Address to be included with the RREP message, the InstanceID of the RREP-Instance is paired, whenever possible, with the InstanceID from the RREQ message, which is always an odd number. The pairing is accomplished by adding one to the InstanceID from the RREQ message and using that, whenever possible, as the InstanceID for the RREP message. If this is not possible (for instance because the incremented InstanceID is still a valid InstanceID for another route to the TargNode from an earlier Route Discovery operation), then the 'T' bit is set and an alternative even number is chosen for the InstanceID of RREP from TargNode.
The OrigNode IP address for RREQ-Instance is available as the DODAGID in the DIO base message (see Figure 1). When TargNode receives a RREQ message with the 'S' bit set to 1 (as illustrated in Figure 2), it unicasts the RREP message with the 'S' bit set to 1. In this case, route control messages and application data between OrigNode and TargNode for both RREQ-Instance and RREP-Instance are transmitted along symmetric links. When the 'T' bit is set to "1" in the RREP-Instance, then the TargNode IPv6 Address is transmitted in the RREP option. Otherwise, the TargNode IPv6 Address is elided in the RREP option.
When (as illustrated in Figure 3) the TargNode receives RREQ message with the 'S' bit set to 0, it also multicasts the RREP message with the 'S' bit set to 0. Intermediate nodes create a routing table entry for the path towards the TargNode while processing the RREP message to OrigNode. Once OrigNode receives the RREP message, it starts transmitting the application data to TargNode along the path as discovered through RREP messages. On the other hand, application data from TargNode to OrigNode is transmitted through the path that is discovered from RREQ message.
Under some circumstances, an Intermediate Node that receives a RREQ message MAY transmit a "Gratuitous" RREP message back to OrigNode instead of continuing to multicast the RREQ message towards TargNode. For these circumstances, the 'G' bit of the RREP option is provided to distinguish the Gratuitous RREP sent by the Intermediate node from the RREP sent by TargNode.
When an Intermediate node R receives a RREQ message and has recent information about the cost of an upstream route from TargNode to R, then R MAY unicast the Gratuitous RREP (GRREP) message to OrigNode. R determines whether its information is sufficiently recent by comparing the value it has stored for the Sequence Number of TargNode against the DestSeqno in the incoming RREQ message. R also must have information about the metric information of the upstream route from TargNode. The GRREP message MUST have PrefixSz == 0 and the 'G' bit set to 1. R SHOULD also unicast the RREQ message to TargNode, to make sure that TargNode will have a route to OrigNode.
The trickle timer operation to control RREQ-Instance/RREP-Instance multicast is similar to that in P2P-RPL [RFC6997].
+-------------+---------------+---------------+
| Value | Description | Reference |
+-------------+---------------+---------------+
| TBD1 (5) | AODV-RPL | This document |
+-------------+---------------+---------------+
Figure 6: Mode of Operation
IANA is required to assign a new Mode of Operation, named "AODV-RPL" for Point-to-Point(P2P) hop-by-hop routing under the RPL registry. The value of TBD1 is assigned from the "Mode of Operation" space [RFC6550].
+-------------+---------------------+---------------+
| Value | Meaning | Reference |
+-------------+---------------------+---------------+
| TBD2 (0x0A) | RREQ Option | This document |
+-------------+---------------------+---------------+
| TBD3 (0x0B) | RREP Option | This document |
+-------------+---------------------+---------------+
Figure 7: AODV-RPL Options
Two entries are required for new AODV-RPL options "RREQ-Instance" and "RREQ-Instance", with values of TBD2 (0x0A) and TBD3 (0x0B) from the "RPL Control Message Options" space [RFC6550].
This document does not introduce additional security issues compared to base RPL. For general RPL security considerations, see [RFC6550].
It may become feasible in the future to design a non-storing version of AODV-RPL's route discovery protocol. Under the current assumption of route asymmetry across bidirectional links, the specification is expected to be straightforward. It should be possible to re-use the same methods of incremental construction for source routes within analogous fields within AODV-RPL's RREQ and RREP messages as is currently done for DAO messages -- in other words the RPL messages for DODAG construction.
There has been some discussion about how to determine the initial state of a link after an AODV-RPL-based network has begun operation. The current draft operates as if the links are symmetric until additional metric information is collected. The means for making link metric information is considered out of scope for AODV-RPL. In the future, RREQ and RREP messages could be equipped with new fields for use in verifying link metrics. In particular, it is possible to identify unidirectional links; an RREQ received across a unidirectional link has to be dropped, since the destination node cannot make use of the received DODAG to route packets back to the source node that originated the route discovery operation. This is roughly the same as considering a unidirectional link to present an infinite cost metric that automatically disqualifies it for use in the reverse direction.
| [I-D.thubert-roll-asymlink] | Thubert, P., "RPL adaptation for asymmetrical links", Internet-Draft draft-thubert-roll-asymlink-02, December 2011. |
Source---------->NodeA---------->NodeB------->Destination
Figure 8: Communication link from Source to Destination
We have tested the combination of "RSSI(downstream)" and "ETX (upstream)" to decide whether the link is symmetric or asymmetric at the intermediate nodes. The example of how the ETX and RSSI values are used in conjuction is explained below:
| RSSI at NodeA for NodeB | Expected ETX at NodeA for nodeB->nodeA |
|---|---|
| > -15 | 150 |
| -25 to -15 | 192 |
| -35 to -25 | 226 |
| -45 to -35 | 662 |
| -55 to -45 | 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, say, within 1:3 ratio. This ratio should be taken as a notional metric for deciding link symmetric/asymmetric nature, and precise definition of the ratio is beyond the scope of the draft. In general, NodeA can only 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 "S=0". Later on, the link from NodeA to Destination is asymmetric with "S" bit remains to "0".