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.
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This Internet-Draft will expire on January 3, 2018.
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carefully, as they describe your rights and restrictions with respect
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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 protected] <[email protected]>
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
[image: Universitat Oberta de Catalunya]
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