Internet DRAFT - draft-papadopoulos-paw-pre-reqs
draft-papadopoulos-paw-pre-reqs
PAW G. Papadopoulos, Ed.
Internet-Draft R. Koutsiamanis
Intended status: Standards Track N. Montavont
Expires: September 26, 2019 IMT Atlantique
P. Thubert
Cisco
March 25, 2019
Exploiting Packet Replication and Elimination in Complex Tracks in LLNs
draft-papadopoulos-paw-pre-reqs-01
Abstract
The Packet Replication and Elimination (PRE) mechanism duplicates
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 PRE.
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 September 26, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
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 Propagated Information . . . . . 6
5.3. Requirements Related to Promiscuous Overhearing . . . . . 7
5.4. Requirements Related to Packet Elimination . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Informative references . . . . . . . . . . . . . . . . . 8
8.2. Other Informative References . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 any specific solution or
design for the PAW architecture or protocols. These are the subjects
of other PAW drafts. The PAW use cases are not considered to be
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].
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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 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 [I-D.ietf-6tisch-architecture]. By
employing DetNet [I-D.ietf-detnet-architecture] 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).
Some more details for Deterministic Network PRE techniques are
presented in the following Section.
4. Packet Replication and Elimination principles
In a nutshell, PRE establishes several paths in a network to provide
redundancy and parallel transmissions to bound the end-to-end 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 shown
in Figure 1, many different paths are possible for S to reach R. A
simple way to 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 the
lower level of the path to the upper level.
PRE may also take advantage of the shared properties of the wireless
medium to compensate for the potential loss that is incurred with
radio transmissions. For instance, when the source sends to A, B may
listen also 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 in a
dedicated transmission towards B.
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(A) (C) (E)
source (S) (R) (root)
(B) (D) (F)
Figure 1: A Typical Ladder Shape with Two Parallel Paths Toward the
Destination
The PRE model can be implemented in both centralized and distributed
scheduling approaches. 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, at each node in the paths, a default parent
and alternative parent(s) should be selected to set up complex
tracks.
In the following Subsections, all the required operations defined by
PRE, namely, Alternative Path Selection, Packet Replication, Packet
Elimination and Promiscuous Overhearing, are described.
4.1. Packet Replication
The objective of PRE is to provide deterministic networking
properties: high reliability and bounded latency. To achieve this
goal, determinism in every hop of the forwarding paths MUST be
guaranteed. By employing a Packet Replication procedure, each node
forwards a copy of each data packet to multiple parents: its Default
Parent (DP) and multiple Alternative Parents (APs). To do so, each
node (i.e., source and intermediate node) transmits the data packet
multiple times 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. An
example TSCH schedule is shown in Figure 3. Thus, the packet
eventually obtains parallel paths to the destination.
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===> (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).
Timeslot
+---------++------+------+------+------+------+------+------+
| Channel || 0 | 1 | 2 | 3 | 4 | 5 | 6 |
+---------++======+======+======+======+======+======+======+
| 0 || S->A | S->B | B->C | B->D | C->F | E->R | F->R |
+---------++------+------+------+------+------+------+------+
| 1 || | A->C | A->D | C->E | D->E | D->F | |
+---------++------+------+------+------+------+------+------+
Figure 3: Packet Replication: Sample TSCH schedule
4.2. Packet Elimination
The replication operation increases the traffic load in the network,
due to packet duplications. Thus, a 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 subsequent copies. Because the first copy
that reaches a node is the one that matters, it is the only copy that
will be forwarded upward. Then, once a node performs the Packet
Elimination operation, it will proceed with the 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, a DP and some AP(s) eventually
have more chances to receive the data packets. In Figure 4, 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 corellated 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 and
increases energy efficiency.
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===> (A) ====> (C) ====> (E) ====
// ^ | \\ \\
source (S) | | \\ (R) (root)
\\ | v \\ //
===> (B) ====> (D) ====> (F) ====
Figure 4: 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 Packet 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.
The requirements are:
Req1.1: The routing protocol SHOULD be extended to allow for each
node to select AP(s) in addition to the DP. This enables
packet replication to multiple parents.
Req1.2: Considering that the Packet Replication procedure
significantly increases the traffic in a network, when
proposing solutions for Alternative Parent Selection, they
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
For Alternative Parent(s) selection, nodes MAY need additional
information about the network topology. This draft does not
prescribe the information required for AP selcetion or how it is to
be propagated to the nodes that need to select AP(s). TODO: To be
discussed.
The requirement is:
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Req2.1: Nodes MUST have a way of receiving the required information
for efficient Alternative Parent Selection.
As an example, it is possible to use and extend the RPL [RFC6550]
DODAG Information Object (DIO) Control Message to allow nodes to
propagate information about themselves to potential children. For
instance, "RPL DAG Metric Container (MC) Node State and Attribute
(NSA) object type extension" [I-D.ietf-roll-nsa-extension] focuses on
extending the DAG Metric Container [RFC6551] by defining a new type-
length-value (TLV), entitled Parent Set (PS) which can be carried in
the Node State and Attribute (NSA) object.
5.3. Requirements Related to Promiscuous Overhearing
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 [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 node.
Destination address filtering is performed at the Medium Access
Control (MAC) layer. For example, 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. A change is needed to be able to receive packets whose
destination address is neither multicast nor the overhearing node's
MAC address.
The requirements are:
Req3.1: The MAC implementation MUST be able to disable MAC address
filtering to accept the overheard frame.
Req3.2: The 6top Protocol [RFC8480] specification MUST be extended
to indicate disabling MAC filtering in a receiving cell. This
can be achieved by reserving a bit in the 6P CellOptions Bitmap
(Section 6.2.6 [RFC8480]) for this purpose.
Req3.3: The overhearing node can be configured with the timeslot set
to shared reception, thus, there will be no acknowledgement
from it. However, there is the security issue that needs to be
considered. Since the overhearing case implies that it is not
possible to have per-pair keying, there MUST be a key that the
overhearing node will be aware of. Hence, the Minimal Security
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Framework for 6TiSCH [I-D.ietf-6tisch-architecture]
specification should consider such a scenario.
5.4. Requirements Related to Packet Elimination
By employing Packet Replication, the wireless network is expected to
also perform Packet Elimination to restrict the number of the
duplicated packets, i.e., the unnecessary traffic. As per the 6TiSCH
Architecture [I-D.ietf-6tisch-architecture], 6TiSCH has no position
about how the sequence numbers would be tagged in the packet.
The requirement is:
Req4.1: To perform Packet Elimination the packet copies MUST contain
a sequence number which allows identifying the copies.
6. Security Considerations
TODO.
7. IANA Considerations
This document has no IANA considerations.
8. References
8.1. Informative references
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-20 (work
in progress), March 2019.
[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-12 (work in progress), March 2019.
[I-D.ietf-roll-nsa-extension]
Koutsiamanis, R., Papadopoulos, G., Montavont, N., and P.
Thubert, "RPL DAG Metric Container Node State and
Attribute object type extension", draft-ietf-roll-nsa-
extension-01 (work in progress), March 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[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,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., 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,
<https://www.rfc-editor.org/info/rfc6551>.
[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, <https://www.rfc-editor.org/info/rfc8180>.
[RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Protocol (6P)", RFC 8480,
DOI 10.17487/RFC8480, November 2018,
<https://www.rfc-editor.org/info/rfc8480>.
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
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Remous-Aris Koutsiamanis
IMT Atlantique
Office B00 - 126A
2 Rue de la Chataigneraie
Cesson-Sevigne - Rennes 35510
FRANCE
Phone: +33 299 12 70 49
Email: aris@ariskou.com
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
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