6TiSCH G. Papadopoulos, Ed. Internet-Draft N. Montavont Intended status: Informational IMT Atlantique Expires: January 3, 2018 P. Thubert Cisco July 2, 2017 Exploiting Packet Replication and Elimination in Complex Tracks in 6TiSCH LLNs draft-papadopoulos-6tisch-pre-reqs-00 Abstract 6TiSCH 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 http://datatracker.ietf.org/drafts/current/. 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 January 3, 2018. Copyright Notice Copyright (c) 2017 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Papadopoulos, et al. Expires January 3, 2018 [Page 1] Internet-Draft Address Protection ND for LLN July 2017 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Tracks Overview . . . . . . . . . . . . . . . . . . . . . 3 3.2. Complex Tracks . . . . . . . . . . . . . . . . . . . . . 3 4. Packet Replication and Elimination principles . . . . . . . . 3 4.1. Packet Replication . . . . . . . . . . . . . . . . . . . 4 4.2. Packet Elimination . . . . . . . . . . . . . . . . . . . 5 4.3. Promiscuous Overhearing . . . . . . . . . . . . . . . . . 5 5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. Requirements Related to Alternative Parent Selection . . 6 5.2. Requirements Related to Promiscuous Overhearing . . . . . 6 5.3. Requirements Related to Cells without ACKs . . . . . . . 7 5.4. Requirements Related to Packet Elimination . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.1. Informative references . . . . . . . . . . . . . . . . . 8 8.2. Other Informative References . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Some applications (such as Wireless Industrial IoT) require robust communications framework that guarantees data packet delivery in a given delay. For example, a periodic process may need to be repeated identically every time. To reach this ambition, the network must not only be reliable but also deterministic. A deterministic network ensures that the transported data packet will be carried out in a pre-defined and in a tight window of time, whatever the quality of the wireless links and the network congestion. The goal of such network is to exhibit ultra-low jitter performance, i.e., close to 0. IEEE std. 802.15.4 [IEEE802154-2015] has provision to provide guarantees for deterministic networks. Time-Slotted Channel Hopping (TSCH) provides transmission schedule to avoid random access to the medium and channel diversity to fight interferences. However, TSCH is prone to retransmissions when the actual transmission was unsuccessful, due to external interference or Papadopoulos, et al. Expires January 3, 2018 [Page 2] Internet-Draft Address Protection ND for LLN July 2017 potential collision and, consequently, it increases the end-to-end delay performance. This document is mainly motivated by the ongoing work in the 6TiSCH working group. The architecture of a 6TiSCH network is detailed in 6TiSCH Architecture [I-D.ietf-6tisch-architecture] draft, which is used for the remainder of this document. In this specification, we focus on Complex Track with Replication and Elimination. 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 [I-D.ietf-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 6TiSCH node to a destination 6TiSCH node across a 6TiSCH multihop path. 3.2. Complex Tracks A Complex Track is designed as a directed acyclic graph from a source 6TiSCH node towards a destination 6TiSCH node to support multi-path forwarding, as introduced in 6TiSCH Architecture [I-D.ietf-6tisch-architecture]. By employing DetNet Packet Replication and Elimination (PRE) techniques, 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. Optionnally, 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 Papadopoulos, et al. Expires January 3, 2018 [Page 3] Internet-Draft Address Protection ND for LLN July 2017 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 in a ship-in-the-night fashion. The 6TiSCH PRE may also take advantage to 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 time slot. (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 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. 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 offer deterministic networking properties, with high reliability and bounded latency. To achieve this goal, determinism in every level of the forwarding path should 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 6TiSCH nodes) transmits the data packet twice in unicast to each parent. For instance, in Figure 2, the source 6TiSCH node S is transmitting the packet to both parents, nodes A and Papadopoulos, et al. Expires January 3, 2018 [Page 4] Internet-Draft Address Protection ND for LLN July 2017 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 DAG 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, and thus will be the only copy that will be forwarded upward. 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. Papadopoulos, et al. Expires January 3, 2018 [Page 5] Internet-Draft Address Protection ND for LLN July 2017 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 6TiSCH node, for each node in the 6TiSCH network. An AP can be selected in many different ways, and is dependent on the implementation. However, control packets should give some metrics to discriminate between different neighbors. Related requirements are: Req1.1: To design such algorithm, RPL DODAG Information Object (DIO) message format SHOULD be extended with an option to allow for a 6TiSCH node to learn additional information for its potential parent and its list of parents. Req1.2: The routing protocol SHOULD be extended to allow for each 6TiSCH node to select AP(s) and duplicate a packet to several next hops. 5.2. Requirements Related to Promiscuous Overhearing As stated previously, to further increase the 6TiSCH network reliability and to achieve deterministic packet deliveries at the destination 6TiSCH node, promiscuous overhearing can be considered. As it is described in BCP 210 [RFC8180], 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 6TiSCH node. Related requirements are: Req2.1: The 6top Protocol [I-D.ietf-6tisch-6top-protocol] SHOULD be extended to allow optionally 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. Req2.2: 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. Papadopoulos, et al. Expires January 3, 2018 [Page 6] Internet-Draft Address Protection ND for LLN July 2017 5.3. Requirements Related to Cells without ACKs As stated in BCP 210 [RFC8180], each date frame is acknowledged by the receiving 6TiSCH 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 [RFC8180], only the DP MUST acknowledge the data packet. Req4.2: Alternatively, to achieve further consistency the overheard transmission need be acknowledged by both parents, i.e., DP and AP. To do so, BCP 210 [RFC8180] SHOULD be extended accordingly. 5.4. Requirements Related to Packet Elimination By employing packet replication operation, the 6TiSCH 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 [I-D.ietf-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 6TiSCH 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 TODO. 7. IANA Considerations This document has no IANA considerations. 8. References Papadopoulos, et al. Expires January 3, 2018 [Page 7] Internet-Draft Address Protection ND for LLN July 2017 8.1. Informative references [I-D.ietf-6tisch-6top-protocol] Wang, Q., Vilajosana, X., and T. Watteyne, "6top Protocol (6P)", draft-ietf-6tisch-6top-protocol-07 (work in progress), June 2017. [I-D.ietf-6tisch-architecture] Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", draft-ietf-6tisch-architecture-11 (work in progress), January 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., Ed., Thubert, P., Ed., 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, . [RFC8180] Vilajosana, X., Ed., 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 Chataigneraie Cesson-Sevigne - Rennes 35510 FRANCE Phone: +33 299 12 70 04 Email: georgios.papadopoulos@imt-atlantique.fr Papadopoulos, et al. Expires January 3, 2018 [Page 8] Internet-Draft Address Protection ND for LLN July 2017 Nicolas Montavont IMT Atlantique Office B00 - 106A 2 Rue de la Chataigneraie Cesson-Sevigne - Rennes 35510 FRANCE Phone: +33 299 12 70 23 Email: nicolas.montavont@imt-atlantique.fr Pascal Thubert Cisco Systems, Inc Building D 45 Allee des Ormes - BP1200 MOUGINS - Sophia Antipolis 06254 FRANCE Phone: +33 497 23 26 34 Email: pthubert@cisco.com Papadopoulos, et al. Expires January 3, 2018 [Page 9]