ROLL R. Koutsiamanis, Ed.
Internet-Draft G. Papadopoulos
Intended status: Standards Track N. Montavont
Expires: July 21, 2018 IMT Atlantique
P. Thubert
January 17, 2018

RPL DAG Metric Container (MC) Node State and Attribute (NSA) object type extension


Implementing 6TiSCH Packet Replication and Elimination from / to the RPL root requires the ability to forward copies of packets over different paths via different RPL parents. Selecting the appropriate parents to achieve ultra-low latency and jitter requires information about a node's parents. This document details what information needs to be transmitted and how it is encoded within a packet to enable this functionality.

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Table of Contents

1. Introduction

Industrial network applications have stringent requirements on reliability and predictability, and typically leverage 1+1 redundancy, aka Packet Replication and Elimination (PRE) to achieve their goal. In order for wireless networks to be able to be used in such applications, the principles of Deterministic Networking lead to designs that aim at maximizing packet delivery rate and minimizing latency and jitter. Additionally, given that the network nodes often do not have an unlimited power supply, energy consumption needs to be minimized as well.

To meet this goal, IEEE Std. 802.15.4 provides Time-Slotted Channel Hopping (TSCH), a mode of operation which uses a fixed communication schedule to allow deterministic medium access as well as channel hopping to work around radio interference. However, since TSCH uses retransmissions in the event of a failed transmission, end-to-end delay and jitter performance can deteriorate.

The 6TiSCH working group, focusing on IPv6 over IEEE Std. 802.15.4-TSCH, has worked on the issues previously highlighted and produced the "6TiSCH Architecture" to address that case. Building on this architecture, "Exploiting Packet Replication and Elimination in Complex Tracks in 6TiSCH LLNs" leverages PRE to improve the Packet Delivery Ratio (PDR), provide a hard bound to the end-to-end latency, and limit jitter.

PRE achieves a controlled redundancy by laying multiple forwarding paths through the network and using them in parallel for different copies of a same packet. PRE can follow the Destination-Oriented Directed Acyclic Graph (DODAG) formed by RPL from a node to the root. Building a multi-path DODAG can be achieved based on the RPL capability of having multiple parents for each node in a network, a subset of which is used to forward packets. In order for this subset to be defined, a RPL parent subset selection mechanism, which falls within the remit of the RPL Objective Function (OF), needs to have specific path information. The specification of the transmission of this information is the focus of this document.

More concretely, this specification focuses on the extensions to the DAG Metric Container required for providing the PRE mechanism a part of the information it needs to operate. This information is the RPL parent node address set of a node and it must be sent to potential children nodes of the node. The RPL DIO Control Message is the canonical way of broadcasting this kind of information and therefore its DAG Metric Container field is used to append a Node State and Attribute (NSA) object. The node's parent node address set is stored as an optional TLV within the NSA object. This specification defines the type value and structure for this TLV.

2. Terminology

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 [RFC2119].

3. Tracks

3.1. Tracks Overview

The concept of Track is introduced in the "6TiSCH Architecture", defined as a sequence of elements, each consisting of the 3-tuple of a transmitter, a receiver, and a given timeslot expressed as a slotOffset/channelOffset tuple. A simple Track is intended to provide the full resources required to allow the transmission of a single packet from a source 6TiSCH node to a destination 6TiSCH node across a 6TiSCH multihop path.

3.2. Complex Tracks

Similarly to, but as a generalization of a simple Track, a Complex Track is defined in the "6TiSCH Architecture" as a DODAG starting at a source 6TiSCH node and leading to a sink 6TiSCH node in order to support multi-path forwarding. Multiple independent paths may be produced by using techniques for Packet Replication and Elimination (PRE) based on DetNet principles. As an example, a complex Track allows for branching off and rejoining over non-congruent paths.

In the following Section, we will detail Deterministic Networks PRE techniques.

4. Packet Replication and Elimination principles

The idea behind Packet Replication and Elimination (PRE) is to transmit the same data packet through parallel and adjacent paths in a network with the aim of improving reliability and predictability through redundancy.

The process of replication consists of identifying multiple potential paths, selecting a subset to use, and sending copies of a single packet through each path. When receiving packets the process of elimination is required so that multiple copies of the same packet are not replicated again, to avoid an exponential growth in unnecessary traffic. Combined together, these processes enable controlled redundancy which in turn can be used to achieve the previously stated goals of reliability (i.e., ultra-high packet delivery rate) and predictability (i.e., ultra-low end-to-end delay and jitter) in wireless networks. For example, in Figure 1, the source 6TiSCH node S is sending the data packet to its RPL Default Parent (DP) (node A) and Alternative Parent (AP) (node B) in two different timeslots.

      ( R ) root
      ^^ ^^
     //   \\
    //     \\
  ( C )   ( D )                            
  ^^ ^    ^ ^^                             
  ||  \  /  ||                             
  ||   \/   ||                             
  ||   /\   ||                             
  ||  /  \  ||                             
  ( A )   ( B )                            
  ^^      ^                                
  ||     /                                 
  ||    /                                  
  ||   /                                   
  ||  /             ||  Preferred Parent   
  ( S ) source       |  Potential Parent   

Figure 1: Packet Replication: S transmits the same data packet twice: to its DP (A) and to its AP (B).

In "Exploiting Packet Replication and Elimination in Complex Tracks in 6TiSCH LLNs", the concept of PRE is further expanded along with its requirements.

5. Alternative Parent Selection Issue

In the RPL protocol, each node maintains a list of potential parents. For PRE, the DP node is defined as the RPL DODAG preferred parent node. Furthermore, to construct an alternative path toward the root, in addition to the DP node, each 6TiSCH node in the network registers an AP node as well. There are multiple alternative methods of selecting the AP node, functionality which is included in operation of the RPL Objective Function (OF). In "Exploiting Packet Replication and Elimination in Complex Tracks in 6TiSCH LLNs", a scheme which allows the two paths to remain correlated is detailed. More specifically, in this scheme a 6TiSCH node will select an alternative parent node close to its default parent node to allow the operation of overhearing between parents. To do so, the node will check if its Default Grand Parent (DGP), the DP of its DP, is in the set of parents of a potential AP. If multiple potential APs match this condition, the AP with the lowest rank will be registered.

         (  R  ) root          
         ^^ ^^ ^^            
        //  ||  \\           
       //   ||   \\          
      //    ||    \\         
     //     ||     \\        
  ( C )   ( D )    ( E )    
  ^^ ^    ^ ^^     ^        
  ||  \  /  ||    /         
  ||   \/   ||   /          
  ||   /\   ||  /           
  ||  /  \  || /            
  ( A )   ( B )             
  ^^      ^                 
  ||     /                  
  ||    /                   
  ||   /                    
  ||  /                     ||  Preferred Parent
  ( S ) source               |  Potential Parent

Figure 2: Example Parent Selection mechanism

For instance, in Figure 2, source 6TiSCH node S must know its grandparent sets both through node A and through node B. In this scenario, node A has the parent set {C, D} with C as DP and node B has the parent set {C, D, E} with D as DP. Therefore, node S can decide to use node B as its AP node, since the the DGP of S (via node A) is node C, and node C is in the parent set of node B ({C, D, E}).

In order to select their AP node, 6TiSCH nodes need to be aware of their grandparent node sets. Within RPL, the nodes use the DODAG Information Object (DIO) Control Message to broadcast information about themselves to potential children. However, RPL, does not define how to propagate parent set related information, which is what this document addresses.

6. Node State and Attribute (NSA) object type extension

For supporting PRE, nodes need to report their parent node set to their potential children. DIO messages can carry multiple options, out of which the DAG Metric Container option is the most suitable structurally and semantically for the purpose of carrying the parent node set. The DAG Metric Container option itself can carry different nested objects, out of which the Node State and Attribute (NSA) is appropriate for transferring generic node state data. Within the Node State and Attribute it is possible to store optional TLVs representing various node characteristics. As per the Node State and Attribute (NSA) description, no TLV have been defined for use. This document defines one TLV for the purpose of transmitting a node's parent node set.

 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      |     Flags     |   Reserved    |
|                                                               |
+                                                               +
|                                                               |
+                            DODAGID                            +
|                                                               |
+                                                               +
|                                                               |
| DAGMC Type (2)| DAGMC Length  |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
|                                                               |
//                   DAG Metric Container data                 //
|                                                               |

Figure 3: Example DIO Message with a DAG Metric Container option

The structure of the DIO Control Message when a DAG Metric Container option is included is shown in Figure 3. The DAG Metric Container option type (DAGMC Type in Figure 3) has the value 0x02 as per the IANA registry for the RPL Control Message Options, and is defined in [RFC6550]. The DAG Metric Container option length (DAGMC Length in Figure 3) expresses the the DAG Metric Container length in bytes. DAG Metric Container data holds the actual data and is shown further expanded in Figure 4.

 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-MC-Type|Res Flags|P|C|O|R| A   |  Prec | Length (bytes)| |=>MC
|     Res       |  Flags    |A|O|    PNS type   |   PNS Length  | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |=>NSA
| PNS IPv6 address(es) ...                                        |

Figure 4: DAG Metric Container (MC) data with Node State and Attribute (NSA) object body and a TLV

The structure of the DAG Metric Container data in the form of a Node State and Attribute (NSA) object with a TLV in the NSA Optional TLVs field is shown in Figure 4. The first 32 bits comprise the DAG Metric Container header and all the following bits are part of the Node State and Attribute object body, as defined in [RFC6551]. This document defines a new TLV, which CAN be carried in the Node State and Attribute (NSA) object Optional TLVs field. The TLV is named Parent Node Set and is abbreviated as PNS in Figure 4.

PNS type:
The type of the Parent Node Set TLV. The value is TBD1.
PNS Length:
The total length of the TLV value field (PNS IPv6 address(es)) in bytes.
PNS IPv6 address(es):
A sequence of zero or more IPv6 addresses belonging to a node's parent set. Each address requires 16 bytes. The order of the parents in the parent set is in decreasing preference based on the Objective Function used by the node.

6.1. Usage

The PNS SHOULD be used in the process of parent selection, and especially in alternative parent selection, since it can help the alternative path from significantly deviating from the preferred path. The Parent Node Set is information local to the node that broadcasts it. It does not make sense for this information to be aggregated due to the scalability issue created by the space required for many IPv6 addresses. Therefore, the PNS MUST NOT be aggregated.

6.1.1. DAG Metric Container fields

Given the intended usage, when using the PNS, the NSA object it is contained in MUST be used as a constraint in the DAG Metric Container. More specifically, using the PNS places the following requirements on the DAG Metric Container header fields:

6.1.2. Node State and Attribute fields

For reasons of clarity, the usage of the PNS places no additional restrictions on the NSA flags ('A' and 'O'), which can be used as normally defined in [RFC6550].

6.2. Compression

The PNS IPv6 address(es) field in the Parent Node Set TLV MAY be compressed using any compression method available to conserve space.

7. Security Considerations


8. IANA Considerations


9. References

9.1. Informative references

[I-D.ietf-6tisch-architecture] Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-13, November 2017.
[I-D.ietf-detnet-architecture] Finn, N., Thubert, P., Varga, B. and J. Farkas, "Deterministic Networking Architecture", Internet-Draft draft-ietf-detnet-architecture-04, October 2017.
[I-D.papadopoulos-6tisch-pre-reqs] Papadopoulos, G., Montavont, N. and P. Thubert, "Exploiting Packet Replication and Elimination in Complex Tracks in 6TiSCH LLNs", Internet-Draft draft-papadopoulos-6tisch-pre-reqs-01, December 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP. and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, March 2012.
[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N. and D. Barthel, "Routing Metrics Used for Path Calculation in Low-Power and Lossy Networks", RFC 6551, DOI 10.17487/RFC6551, March 2012.

9.2. Other Informative References

[IEEE802154-2015] IEEE standard for Information Technology, "IEEE Std 802.15.4-2015 Standard for Low-Rate Wireless Personal Area Networks (WPANs)", December 2015.

Authors' Addresses

Remous-Aris Koutsiamanis (editor) IMT Atlantique Office B00 - 126A 2 Rue de la Chataigneraie Cesson-Sevigne - Rennes, 35510 FRANCE Phone: +33 299 12 70 49 EMail:
Georgios Papadopoulos IMT Atlantique Office B00 - 114A 2 Rue de la Chataigneraie Cesson-Sevigne - Rennes, 35510 FRANCE Phone: +33 299 12 70 04 EMail:
Nicolas Montavont IMT Atlantique Office B00 - 106A 2 Rue de la Chataigneraie Cesson-Sevigne - Rennes, 35510 FRANCE Phone: +33 299 12 70 23 EMail:
Pascal Thubert Cisco Systems, Inc Building D 45 Allee des Ormes - BP1200 MOUGINS - Sophia Antipolis, 06254 FRANCE Phone: +33 497 23 26 34 EMail: