PAW G. Papadopoulos, Ed.
Internet-Draft N. Montavont
Intended status: Standards Track IMT Atlantique
Expires: August 1, 2019 P. Thubert
January 28, 2019

Exploiting Packet Replication and Elimination in Complex Tracks in LLNs


Packet Replication and Elimination mechanism consists in duplicating data packets into several paths in the network to increase reliability and provide low jitter. Over a wireless medium, this technique can take advantage of communication overhearing, when parallel transmissions over two adjacent paths are scheduled. This document presents the concept and details the required changes to the current specifications that will be necessary to enable this.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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."

This Internet-Draft will expire on August 1, 2019.

Copyright Notice

Copyright (c) 2019 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 ( 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 Simplified BSD License.

Table of Contents

1. Introduction

This draft describes industrial use cases which require deterministic flows over wireless multi-hop paths.

The PAW use cases explicitly do not propose or suggest any specific solution or design for PAW architecture or protocols. These are subjects of other PAW drafts. The PAW use cases are not considered as concrete requirements by the PAW Working Group.

The industrial use cases covered in this draft are professional audio, wireless for industrial applications and amusement parks.

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 6TiSCH architecture introduces the concept of Tracks in 6TiSCH Architecture. A simple track is composed of a sequence of cells (a combination of a transmitter, a receiver and a given channel offset) to ensure the transmission of a single packet from a source node to a destination node across a multihop path.

3.2. Complex Tracks

A Complex Track is designed as a directed acyclic graph from a source node towards a destination node to support multi-path forwarding, as introduced in 6TiSCH Architecture. By employing DetNet Packet Replication and Elimination (PRE) functions, several paths may be computed, and these paths may be more or less independent. For example, a complex Track may branch off and rejoin over non-congruent paths (branches).

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

4. Packet Replication and Elimination principles

In a nutshell, PRE consists in establishing several paths in a network to provide redundancy and parallel transmissions to bound the delay to traverse the network. Optionally, promiscuous listening between paths is possible, such that the nodes on one path may overhear transmissions along the other path. Considering the scenario depicted in Figure 1, many different paths are possible for S to reach R. A simple way to take benefit from this topology could be to use the two independent paths via nodes A, C, E and via B, D, F. But more complex paths are possible by interleaving transmissions from one level of the path to the upper level.

The PRE may also take advantage of the shared properties of the medium to compensate for the potential loss that is incurred with radio transmissions. For instance, when the source sends to A, B may listen and get a second chance to receive the frame without an additional transmission. Note that B would not have to listen if it already received that particular frame at an earlier timeslot.

                 (A)   (C)   (E)   
   source (S)                       (R) (root)
                 (B)   (D)   (F)  


Figure 1: A Typical Ladder Shape with Two Parallel Paths Toward the Destination

PRE model can be implemented in both centralized and distributed scheduling approach. In the centralized approach, a Path Computation Element (PCE) scheduler calculates the routes and schedules the communication among the nodes along a circuit such as a Label switched path. In the distributed approach, each node selects its route to the destination, typically using a source routing header. In both cases, a default parent and alternate parent(s) should be selected to set up complex tracks.

In the following Subsections, detailed description of all required operations defined by PRE, namely, Alternative Path Selection, Packet Replication, Packet Elimination and Promiscuous Overhearing, will be described.

4.1. Packet Replication

The objective of PRE is to provide deterministic networking properties, with high reliability and bounded latency. To achieve this goal, determinism in every hop of the forwarding paths MUST be guaranteed. By employing Packet Replication procedure, each node transmits (i.e., replicates) each data packet to both its Default Parent (DP) and Alternative Parent (AP). To do so, each node (i.e., source and intermediate nodes) transmits the data packet twice in unicast to each parent. For instance, in Figure 2, the source node S is transmitting the packet to both parents, nodes A and B, in two different timeslots within the same TSCH slotframe. Thus, the packet eventually obtains parallel paths to the destination.

               ===> (A) => (C) => (E) === 
             //        \\//   \\//       \\
   source (S)          //\\   //\\         (R) (root)
             \\       //  \\ //  \\      //
               ===> (B) => (D) => (F) ===


Figure 2: Packet Replication: S transmits twice the same data packet, to its DP (A) and to its AP (B).

4.2. Packet Elimination

The replication operation increases the traffic load in the network, due to packet duplications. Thus, Packet Elimination operation should be applied at each RPL DODAG level to reduce the unnecessary traffic. To this aim, once a node receives the first copy of a data packet, it discards the following copies. Because the first copy that reaches a node is the one that counts, it is the only copy that will be forwarded upward. Then, once a node performed the Packet Elimination operation, it will proceed with Packet Replication operation to forward the packet toward the RPL DODAG Root.

4.3. Promiscuous Overhearing

Considering that the wireless medium is broadcast by nature, any neighbor of a transmitter may overhear a transmission. By employing the Promiscuous Overhearing operation, DP and AP eventually have more chances to receive the data packets. In Figure 3, when node A is transmitting to its DP (node C), the AP (node D) and its Sibling (node B) may decode this data packet as well. As a result, by employing correlated paths, a node may have multiple opportunities to receive a given data packet. This feature not only enhances the end-to-end reliability but also it reduces the end-to-end delay.

               ===> (A) ====> (C) ====> (E) ==== 
             //     ^ | \\            	        \\
   source (S)       | |   \\                      (R) (root)
             \\     | v     \\                  //
               ===> (B) ====> (D) ====> (F) ====

Figure 3: Unicast to DP with Overhearing: by employing Promiscuous Overhearing, DP, AP and the Sibling nodes have more opportunities to receive the same data packet.

5. Requirements

5.1. Requirements Related to Alternative Parent Selection

To perform the Replication procedure, it is necessary to define the Alternative Parent(s) and, consequently, the path to the destination node, for each node in the wireless network. An AP can be selected in many different ways, and is dependent on the implementation.

Related requirements are:

Req1.1: The routing protocol SHOULD be extended to allow for each node to select AP(s) in addition to DP. Thus, packet duplication (i.e., replication) to multiple parents could be possible.

Req1.2: Considering that the Replication procedure significantly increases the traffic in a network, when proposing solutions for Alternative Parent Selection, it should be efficient enough to mitigate the potential uncontrolled packet duplications.

Req1.3: The topology SHOULD be defined when proposing solutions for Alternative Parent Selection. For instance, the ladder topology should be defined explicitly e.g., number of parallel paths, density.

5.2. Requirements Related to Propagated Information

To select an Alternative Parent, nodes MUST be aware of their grandparent node sets. Thus, it is necessary nodes to propagate such information to their neighbors. RPL defines DODAG Information Object (DIO) Control Message to allow nodes to propagate information about themselves to potential children. In Figure 4, DIO control message with a DAG Metric Container option is illustrated. However, RPL, does not indicates how to propagate parent set related information.

 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 4: Example DIO Message with a DAG Metric Container option

Related requirements are:

Req2.1: DIO control messages can include multiple options. DAG Metric Container option is structurally suitable for transferring parent node set information. Therefore, to enable PRE, nodes MUST broadcast their parent node set to their potential children through the extended DIO control message. For instance, "RPL DAG Metric Container (MC) Node State and Attribute (NSA) object type extension" focuses on extending the DAG Metric Container by defining new type-length-value (TLV), entitled Parent Node Set (PNS) which CAN be carried in the Node State and Attribute (NSA) object.

5.3. Requirements Related to Cell Reservation

As stated previously, to further increase the network reliability and to achieve deterministic packet deliveries at the destination node, Promiscuous Overhearing can be considered.

As it is described in BCP 210, in TSCH mode, the data frames are transmitted in unicast mode and are acknowledged by the receiving neighbor. To perform the promiscuous overhearing procedure, there SHOULD be an option for the transmitted frames, i.e., in unicast, to be overheard by the potential neighborhood node.

Related requirements are:

Req3.1: The destination address filtering is performed at the MAC layer. According to IEEE std. 802.15.4 [IEEE802154-2015], a node receiving a packet with a destination address different than its own and different to 0xFF discards the packet. Thus, IEEE std. 802.15.4 implementation SHOULD bypass this filtering either by configuration forcing to accept such the receiving frame or by using anycast/multicast address as destination.

Req3.2: The 6top Protocol SHOULD be extended to possibly allow a cell reservation with two receivers, i.e., DP and AP. Considering that each frame may be transmitted twice in unicast to each parent, then depending the transmission, either DP will acknowledge the frame or AP will.

Req3.3: Next, to request the overhearing cells, the 6P ADD Request Format SHOULD be transmitted either twice to each parent, i.e., DP and AP, or once in multicast to both parents. This procedure SHOULD be considered in 6top Protocol specification.

5.4. Requirements Related to Cells without ACKs

As stated in BCP 210, each date frame is acknowledged by the receiving node. However, by employing promiscuous overhearing operation, particular attention should be given to who will acknowledge a transmission, i.e., the DP, and / or one of the AP(s)

Related requirements are:

Req4.1: To avoid the ACK collision, the TSCH Schedule as per BCP 210, only the destination node of a packet MUST acknowledge the data packet.

Req4.2: The overhearing node can be configured with the timeslot set to shared, thus, there will be no acknowledgement from it. However, there is the security issue that needs to be considered. Since, the overhearing case imply that it is not possible to have per-pair keying, thus, there MUST be a key that the overhearing node will be aware of. Hence, Minimal Security Framework for 6TiSCH specification should consider such scenario.

Req4.3: Optionally, to achieve further consistency the overheard transmission need be acknowledged by both parents, i.e., DP and AP. To do so, MAC layer operation MUST be extended accordingly.

5.5. Requirements Related to Packet Elimination

By employing packet replication operation, the wireless network expects to perform the packet elimination operation along a complex Track to bound the number of the duplicated packets, i.e., the unnecessary traffic.

Related requirements are:

Req5.1: As per 6TiSCH Architecture, 6TiSCH has no position about how the sequence numbers would be tagged in the packet. However, it comes with Tagging Packets for Flow Identification. More specifically, a wireless network expects that timeslots corresponding to copies of a same frame along a complex Track are correlated by configuration and, thus, does not need to process the sequence numbers.

6. Security Considerations


7. IANA Considerations

This document has no IANA considerations.

8. References

8.1. Informative references

[I-D.ietf-6tisch-6top-protocol] Wang, Q., Vilajosana, X. and T. Watteyne, "6TiSCH Operation Sublayer Protocol (6P)", Internet-Draft draft-ietf-6tisch-6top-protocol-12, June 2018.
[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-19, December 2018.
[I-D.ietf-detnet-architecture] Finn, N., Thubert, P., Varga, B. and J. Farkas, "Deterministic Networking Architecture", Internet-Draft draft-ietf-detnet-architecture-10, December 2018.
[I-D.koutsiamanis-roll-nsa-extension] Koutsiamanis, R., Papadopoulos, G., Montavont, N. and P. Thubert, "RPL DAG Metric Container Node State and Attribute object type extension", Internet-Draft draft-koutsiamanis-roll-nsa-extension-04, November 2018.
[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.
[RFC8180] Vilajosana, X., Pister, K. and T. Watteyne, "Minimal IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180, May 2017.

8.2. Other Informative References

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

Authors' Addresses

Georgios Papadopoulos (editor) IMT Atlantique Office B00 - 102A 2 Rue de la Châtaigneraie Cesson-Sévigné - Rennes, 35510 FRANCE Phone: +33 299 12 70 04 EMail:
Nicolas Montavont IMT Atlantique Office B00 - 106A 2 Rue de la Châtaigneraie Cesson-Sévigné - 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: