Subject: Re: [6tisch] draft-papadopoulos-6tisch-pre-reqs review

From: "Georgios Z. Papadopoulos" <[email-masked]>
Date: Mon, 25 Mar 2019 12:05:08 +0100

Hello Xavi,

Once again, many thanks for your valuable comments.
Please see my responses below.

Note that the review was done over the an older version (draft-papadopoulos-6tisch-pre-reqs-00), while (draft-papadopoulos-6tisch-pre-reqs-01) was already online.
I hope that the latest renamed version (draft-papadopoulos-paw-pre-reqs-01) will be much more clear.

Thanks again,
Georgios

-------------------------

Dear George and all

as agreed during the last call meeting I did a review to the draft-papadopoulos-6tisch-pre-reqs. Find inline my comments tagged with [XV].

regards
X
-------------------------

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

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   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

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   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).
   
[XV] this is not clear. What are the requirements to compute that paths? 
are they feasible given the restrictions of a 6TiSCH network. Could you detail more how this paths may be computed? what is the information that is required and how this information is made available to the nodes in the network? Is this a L4 task?  or is handled at the L2.5? 
[GP] please allow me to reply to this comment in the new version. We have referred to these requirements in the "Requirements Related to Cell Reservation" section in the new version. We also address this a bit further down in this version in section 4 "PRE model can be implemented in ...".

   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

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   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.

[xv] Not clear what in a ship-in-the-night fashion means here
[GP] "ship-in-the-night" removed.

   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.
[xv] This is assuming some sort of implicit knowledge in B. Not sure if this is 
possible without specific signaling.  
[GP] Yes, we may need additional 6P transactions from the source to B, or 
multicast 6P transaction from the source to both A and B. However, this 
need to be discussed and defined. We have referred to these requirements in 
the "5.3 Requirements Related to Promiscuous Overhearing" section in the new version.

                               (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

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   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).

[XV] here you are only considering duplication at the source. While would be better IMHO to duplicate before those links that show lower performance. Would you also consider duplication at inner hops? 
[GP] Yes, definitely, there is replication (duplication) at inner hops. 
For instance, A will transmit both to its default parent C and alternative D.
The Figure 2 and the text in 4.1 is updated.
As far as only duplicating at lower performance links, this is a very good point and a nice optimization target. This needs to be discussed further.

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.
   
[xv] Will this node duplicate it again?
[GP] Yes, it will replicate when it will forward it. Text is updated.

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.

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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.
   
[XV] What information? a node receives DIOs from different neighbors. Could that information be used without modifying the DIO?
[GP] Please check the new 5.2 subsection.

   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.

[XV] This is implementation no? In their neighbor set nodes can be select possible parents by rank given a destination (upstream). 
Usually querying RPL returns the preferred parent given a destination address.  Extending this to return the N best connected parents
seems implementaiton specific no? 
Proposed answer:
[GP] I agree that the core routing protocol already supports selecting the N best parents. This part does not require changing.
However, constraining the paths for higher efficiency requires a more complicated way of ranking the parents.
For example, if you just take the 2 best parents based on ETX, the paths typically diverge (i.e. disjoint paths), resulting in:
a) flooding, because Packet Elimination is not possible,
b) compromising determinism, by not controlling the number of hops to the destination (i.e. risking low jitter performance).
Thus, I think that we will need to define a new RPL Objective Function for PRE.

[XV] what is specific in duplicating a packet that needs to be known at L3? Does L3 need to know this is a duplicate packet? or this is something that 
is handled at L4-7? 
[GP] L3 does need to know because that is where packet elimination is also performed at, and in order to perform it a sequence number will need to be available at L3 as well. See the new section 5.4 for more.

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.

[XV] this is an interesting requirement. The destination address filtering is performed by the MAC layer,  usually a node receiving a packet with a destination address different than its own and different to 0xFF  discards the packet immediately. Note that this functionality can even be automatically performed by hardware in some MCUs.
[GP] Good point. A new Req3.1 is added in Section 5.3 in the new version, many thanks.

[XV] If we assume that a 15.4 implementation can bypass this filtering, either by using an anycast/multicast address as  destination or by programmatically forcing to accept such a frame, then we can talk about what 6P should provide to support it.  
[GP] Unfortunately, we cannot use an anycast/multicast destination address if we want an ACK (from the DP), in 802.15.4 networks.

"""
IEEE 802.15.4-2015 TSCH: Section 6.7.4. Use of acknowledgments and retransmissions
A Data frame or MAC command shall be sent with the AR field set appropriately for the frame. A Beacon
frame or Ack frame shall always be sent with the AR field set to indicate no acknowledgment requested.
Similarly, any frame that is broadcast or has a group address as the extended destination address, as defined
in IEEE Std 802, shall be sent with its AR field set to indicate no acknowledgment requested.
"""

[XV] Given that consideration and assuming that the MAC Layer is the responsible of handling the reception of a non-matching destination address, 6P can simply support that traffic pattern through the use of a TX cell at the transmitter side and a RX cell at the reception side (in both DP and AP) right ? 
Proposed answer:
[GP] For the case where a unicast destination address is used, 6P already supports creating the correct cell type in the AP (shared RX). However, we also need a way to disable MAC address filtering. A new bit to indicate this can be added to the 6P CellOptions Bitmap (RFC8480 Section 6.2.6.).
   
   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.

[XV] Sending it twice is fine. Not need to complicate 6P with multicast.
[GP] For a small number of APs (1-2) the overhead should be OK. We might want something more intelligent for more APs than that, though. We can leave this unaddressed for now though.

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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.
   
[XV] which may happen as the AP will detect that the destination address does not match its address and hence does not ACK. 
   this is an implementation decision in my opinion. 
[GP] Yes, I agree. This has been removed.

   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.
   
[XV] This will require major changes in the MAC layer operation. I see it unfeasible
[GP] I agree that it will require changes to the MAC layer, which will be very hard to do at this point. I have removed it.

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.

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-- 
Dr. Xavier Vilajosana
Wireless Networks Lab
Internet Interdisciplinary Institute (IN3)
Professor
(+34) 646 633 681
[email-masked]
http://xvilajosana.org
http://wine.rdi.uoc.edu
Parc Mediterrani de la Tecnologia 
Av Carl Friedrich Gauss 5, B3 Building
08860 Castelldefels (Barcelona). Catalonia. Spain
Universitat Oberta de Catalunya  
_______________________________________________
6tisch mailing list
[email-masked]
https://www.ietf.org/mailman/listinfo/6tisch

> On Feb 6, 2018, at 12:48, Xavi Vilajosana Guillen <[email-masked]> wrote:
> 
> Dear George and all
> 
> as agreed during the last call meeting I did a review to the draft-papadopoulos-6tisch-pre-reqs. Find inline my comments tagged with [XV].
> 
> regards
> X
> -------------------------
> 
> 
> 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/ <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 <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
> 
> 
> 
> 
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>    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).
>    
>  [XV] this is not clear. What are the requirements to compute that paths? 
>  are they feasible given the restrictions of a 6TiSCH network. Could you detail more how this paths may be computed? what is the information that is required and how this information is made available to the nodes in the network? Is this a L4 task?  or is handled at the L2.5? 
> 
>    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
> 
> 
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>    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.
> 
> [xv] Not clear what in a ship-in-the-night fashion means here
> 
>    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.
> [xv] This is assuming some sort of implicit knowledge in B. Not sure if this is 
> possible without specific signaling.      
> 
> 
>                                (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
> 
> 
> 
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>    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).
> 
> [XV] here you are only considering duplication at the source. While would be better IMHO to duplicate before those links that show lower performance. Would you also consider duplication at inner hops? 					 
> 					 
> 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.
>    
> [xv] Will this node duplicate it again?    
> 
> 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.
> 				   
> 					   
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> 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.
>    
> [XV] What information? a node receives DIOs from different neighbors. Could that information be used without modyfing the DIO?   
> 
>    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.
> 
> [XV] This is implementation no? In their neighbor set nodes can be select possible parents by rank given a destination (upstream). 
> Usually querying RPL returns the preferred parent given a destination address.  Extending this to return the N best connected parents
> seems implementaiton specific no? 
> [XV] what is specific in duplicating a packet that needs to be known at L3? Does L3 need to know this is a duplicate packet? or this is something that 
> is handled at L4-7? 
>    
> 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.
> 
> [XV] this is an interesting requirement. The destination address filtering is performed by the MAC layer,  usually a node receiving a packet with a destination address different than its own and different to 0xFF  discards the packet immediatelly. Note that this functionality can even be automatically performed by hardware in some MCUs.
> 
> If we assume that a 15.4 implementation can baypass this filtering, either by using an anycast/multicast address as  destination or by programmatically forcing to accept such a frame, then we can talk about what 6P should provide to support it.  
> 
> Given that consideration and assuming that the MAC Layer is the responsible of handling the reception of a non-matching destination address, 6P  can simply support that traffic pattern through the use of a TX cell at the transmitter side and a RX cell at the reception side (in both DP and AP) right ?  
>    
>    
>    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.
> 
> [XV] Sending it twice is fine. Not need to complicate 6P with multicast.
> 
> 
> 
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> 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.
>    
>    [XV] which may happen as the AP will detect that the destination address does not match its address and hence does not ACK. 
>    this is an implementation decision in my opinion. 
>    
> 
>    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.
>    
>    [XV] This will require major changes in the MAC layer operation. I see it unfeasible
> 
> 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.
> 
> 
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> 
> -- 
> Dr. Xavier Vilajosana
> Wireless Networks Lab
> Internet Interdisciplinary Institute (IN3)
> Professor
> (+34) 646 633 681
> [email-masked] <mailto:[email-masked]>
> http://xvilajosana.org <http://xvilajosana.org/>
> http://wine.rdi.uoc.edu <http://wine.rdi.uoc.edu/>
> Parc Mediterrani de la Tecnologia 
> Av Carl Friedrich Gauss 5, B3 Building
> 08860 Castelldefels (Barcelona). Catalonia. Spain
>   
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