Reliable and Available Wireless Problem Statement
draft-pthubert-raw-problem-statement-03
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| Authors | Pascal Thubert , Georgios Z. Papadopoulos | ||
| Last updated | 2019-10-08 (Latest revision 2019-10-03) | ||
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draft-pthubert-raw-problem-statement-03
RAW P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Informational G.Z. Papadopoulos
Expires: 10 April 2020 IMT Atlantique
8 October 2019
Reliable and Available Wireless Problem Statement
draft-pthubert-raw-problem-statement-03
Abstract
Due to uncontrolled interferences, including the self-induced
multipath fading, deterministic networking can only be approached on
wireless links. The radio conditions may change -way- faster than a
centralized routing can adapt and reprogram, in particular when the
controller is distant and connectivity is slow and limited. RAW
separates the routing time scale at which a complex path is
recomputed from the forwarding time scale at which the forwarding
decision is taken for an individual packet. RAW operates at the
forwarded time scale. The RAW problem is to decide, within the
redundant solutions that are proposed by the routing, which will be
used for each individual packet to provide a DetNet service while
minimizing the waste of resources.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 10 April 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Use Cases and Requirements Served . . . . . . . . . . . . . . 4
3. Routing Scale vs. Forwarding Scale . . . . . . . . . . . . . 4
4. Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Related Work at The IETF . . . . . . . . . . . . . . . . . . 5
6. Functional Gaps . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Bringing determinism in a packet network means eliminating the
statistical effects of multiplexing that result in probabilistic
jitter and loss. This can be approached with a tight control of the
physical resources to maintain the amount of traffic within a
budgetted volume of data per unit of time that fits the physical
capabilities of the underlying technology, and the use of time-shared
resources (bandwidth and buffers) per circuit, and/or by shaping and/
or scheduling the packets at every hop.
Wireless networks operate on a shared medium where uncontrolled
interference, including the self-induced multipath fading, adds
another dimension to the statistical effects that affect the
delivery. Scheduling transmissions can alleviate those effects by
leveraging diversity in the spatial, time, code, and frequency
domains, and provide a Reliable and Available service while
preserving energy and optimizing the use of the shared spectrum.
Deterministic Networking is an attempt to mostly eliminate packet
loss for a committed bandwidth with a guaranteed worst-case end-to-
end latency, even when co-existing with best-effort traffic in a
shared network. This innovation is enabled by recent developments in
technologies including IEEE 802.1 TSN (for Ethernet LANs) and IETF
DetNet (for wired IP networks). It is getting traction in various
industries including manufacturing, online gaming, professional A/V,
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cellular radio and others, making possible many cost and performance
optimizations.
Reliable and Available Wireless (RAW) networking services extend
DetNet to approach end-to-end deterministic performances in a network
with scheduled wireless segments, possibly combined with wired
segments, and possibly sharing physical resources with non-
deterministic traffic. The wireless and wired media are
fundamentally different at the physical level, and while the generic
Problem Statement for DetNet applies to the wired as well as the
wireless medium, the methods to achieve RAW will differ from those
used to support time-sensitive networking over wires, as a RAW
solution will need to address less consistent transmissions, energy
conservation and shared spectrum efficiency.
The development of RAW technologies has been lagging behind
deterministic efforts for wired systems both at the IEEE and the
IETF. But recent efforts at the IEEE and 3GPP indicate that wireless
is finally catching up at the lower layer and that it is now possible
for the IETF to extend DetNet for wireless segments that are capable
of scheduled wireless transmissions.
The intent for RAW is to provide DetNet elements that are specialized
for short range radios. From this inheritance, RAW stays agnostic to
the radio layer underneath though the capability to schedule
transmissions is assumed. How the PHY is programmed to do so, and
whether the radio is single-hop or meshed, are unknown at the IP
layer and not part of the RAW abstraction.
Still, in order to focus on real-worlds issues and assert the
feasibility of the proposed capabilities, RAW will focus on selected
technologies that can be scheduled at the lower layers: IEEE Std.
802.15.4 timeslotted channel hopping (TSCH), 3GPP 5G ultra-reliable
low latency communications (URLLC), IEEE 802.11ax/be where 802.11be
is extreme high throughput (EHT), and L-band Digital Aeronautical
Communications System (LDACS). See [RAW-TECHNOS] for more.
The establishment of a path is not in-scope for RAW. It may be the
product of a centralized Controller Plane as described for DetNet.
As opposed to wired networks, the action of installing a path over a
set of wireless links may be very slow relative to the speed at which
the radio conditions vary, and it makes sense in the wireless case to
provide redundant forwarding solutions along a complex path and to
leave it to the RAW Network Plane to select which of those forwarding
solutions are to be used for a given packet based on the current
conditions.
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RAW distinguishes the longer time scale at which routes are computed
from the the shorter forwarding time scale where per-packet decisions
are made. RAW operates at the forwarding time scale on one DetNet
flow over one path that is preestablished and installed by means
outside of the scope of RAW. The scope of the RAW WG comprises
Network plane protocol elements such as OAM and in-band control to
improve the RAW operation at the Service and at the forwarding sub-
layers, e.g., controlling whether to use packet replication, Hybrid
ARQ and coding, with a constraint to limit the use of redundancy when
it is really needed, e.g., when a spike of loss is observed. This is
discussed in more details in Section 3 and the next sections.
2. Use Cases and Requirements Served
[RFC8578] presents a number of wireless use cases including Wireless
for Industrial Applications. [RAW-USE-CASES] adds a number of use
cases that demonstrate the need for RAW capabilities in Pro-Audio,
gaming and robotics.
3. Routing Scale vs. Forwarding Scale
RAW extends DetNet to focus on issues that are mostly a concern on
wireless links. See [DetNet-ARCH] for more on DetNet. With DetNet,
the end-to-end routing can be centralized and can reside outside the
network. In wireless, and in particular in a wireless mesh, the path
to the controller that performs the route computation and maintenance
may be slow and expensive in terms of critical resources such as air
time and energy.
Reaching to the routing computation can be slow in regards to the
speed of events that affect the forwarding operation at the radio
layer. Due to the cost and latency to perform a route computation,
routing is not expected to be sensitive/reactive to transient
changes. The abstraction of a link at the routing level is expected
to use statistical operational metrics that aggregate the behavior of
a link over long periods of time, and represent its availability as a
shade of gray as opposed to either up or down.
In the case of wireless, the changes that affect the forwarding
decision can happen frequently and often for shot durations, e.g., a
mobile object moves between a transmitter and a receiver, and will
cancel the line of sight transmission for a few seconds, or a radar
measures the depth of a pool and interferes on a particular channel
for a split second.
There is thus a desire to separate the long term computation of the
route and the short term forwarding decision. In such a model, the
routing operation computes a complex Track that enables multiple non-
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equal cost multipath (N-ECMP) forwarding solutions, and leaves it to
the forwarding plane to make the per-packet decision of which of
these possibilities should be used.
In the case of wires, the concept is known in traffic engineering
where an alternate path can be used upon the detection of a failure
in the main path, e.g., using OAM in MPLS-TP or BFD over a collection
of SD-WAN tunnels. RAW formalizes a routing time scale that is order
of magnitude longer than the forwarding time scale, and separates the
protocols and metrics that are used at both scales. Routing can
operate on long term statistics such as delivery ratio over minutes
to hours, but as a first approximation can ignore flapping. On the
other hand, the RAW forwarding decision is made at packet speed, and
uses information that must be pertinent at the present time for the
current transmission.
4. Prerequisites
A prerequisite to the RAW work is that an end-to-end routing function
computes a complex sub-topology along which forwarding can happen
between a source and one or more destinations. For 6TiSCH, this is a
Track. The concept of Track is specified in the 6TiSCH Architecture
[6TiSCH-ARCH]. Tracks provide a high degree of redundancy and
diversity and enable DetNet PREOF, end-to-end network coding, and
possibly radio-specific abstracted techniques such as ARQ,
overhearing, frequency diversity, time slotting, and possibly others.
How the routing operation computes the Track is out of scope for RAW.
The scope of the RAW operation is one Track, and the goal of the RAW
operation is to optimize the use of the Track at the forwarding
timescale to maintain the expected service while optimizing the usage
of constrained resources such as energy and spectrum.
Another prerequisite is that an IP link can be established over the
radio with some guarantees in terms of service reliability, e.g., it
can be relied upon to transmit a packet within a bounded latency and
provides a guaranteed BER/PDR outside rare but existing transient
outage windows that can last from split seconds to minutes. The
radio layer can be programmed with abstract parameters, and can
return an abstract view of the state of the Link to help forwarding
decision (think DLEP from MANET). In the layered approach, how the
radio manages its PHY layer is out of control and out of scope.
Whether it is single hop or meshed is also unknown and out of scope.
5. Related Work at The IETF
RAW intersects with protocols or practices in development at the IETF
as follows:
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* The Dynamic Link Exchange Protocol (DLEP) [RFC8175] from [MANET]
can be leveraged at each hop to derive generic radio metrics
(e.g., based on LQI, RSSI, queueing delays and ETX) on individual
hops
* Operations, Administration and Maintenance (OAM) work at [DetNet]
such as [DetNet-IP-OAM] for the case of the IP Data Plane observes
the state of DetNet paths, typically MPLS and IPv6 pseudowires
[DetNet-DP-FW], in the direction of the traffic. RAW needs
feedback that flows on the reverse path and gathers instantaneous
values from the radio receivers at each hop to inform back the
source and replicating relays so they can make optimized
forwarding decisions. The work named ICAN may be related as well.
* [BFD] detect faults in the path between an ingress and an egress
forwarding engines, but is unaware of the complexity of a path
with replication, and expects bidirectionality. BFD considers
delivery as success whereas with RAW the bounded latency can be as
important as the delivery itself.
* [SPRING] and [BIER] define in-band signaling that influences the
routing when decided at the head-end on the path. There's already
one RAW-related draft at BIER [BIER-PREF] more may follow. RAW
will need new in-band signaling when the decision is distributed,
e.g., required chances of reliable delivery to destination within
latency. This signaling enables relays to tune retries and
replication to be met.
* [CCAMP] defines protocol-independent metrics and parameters
(measurement attributes) for describing links and paths that are
required for routing and signaling in technology-specific
networks. RAW would be a source of requirements for CCAMP to
define metrics that are significant to the focus radios.
6. Functional Gaps
Within a large routed topology, the routing operation builds a
particular complex Track with one source and one or more
destinations; within the Track, packets may follows different paths
and may be subject to RAW forwarding operations that include
replication, elimination, retries, overhearing and reordering.
The RAW forwarding decisions include the selection of points of
replication and elimination, how many retries can take place, and a
limit of validity for the packet beyond which the packet should be
destroyed rather than forwarded uselessly further down the Track.
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The decision to apply the RAW techniques must be done quickly, and
depends on a very recent and precise knowledge of the forwarding
conditions within the complex Track. There is a need for an
observation method to provide the RAW forwarding plane with the
specific knowledge of the state of the Track for the type of flow of
interest (e.g., for a QoS level of interest). To observe the whole
Track in quasi real time, RAW will consider existing tools such as
L2-triggers, DLEP, BFD and in-band and out-of-band OAM.
One possible way of making the RAW forwarding decisions is to make
them all at the ingress and express them in-band in the packet, which
requires new loose or strict Hop-by-hop signaling. To control the
RAW forwarding operation along a Track for the individual packets,
RAW may leverage and extend known techniques such as DetNet tagging,
Segment Routing (SRv6) or BIER-TE such as done with [BIER-PREF].
An alternate way is to enable each forwarding node to make the RAW
forwarding decisions for a packet on its own, based on its knowledge
of the expectation (timeliness and reliability) for that packet and a
recent observation of the rest of the way across the possible paths
within the Track. Information about the service should be placed in
the packet and matched with the forwarding node's capabilities and
policies.
In either case, a per-flow state is installed in all intermediate
nodes to recognize the flow and determine the forwarding policy to be
applied.
7. References
7.1. Normative References
[6TiSCH-ARCH]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", Work in Progress, Internet-Draft,
draft-ietf-6tisch-architecture-26, 27 August 2019,
<https://tools.ietf.org/html/draft-ietf-6tisch-
architecture-26>.
[DetNet-ARCH]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", Work in Progress,
Internet-Draft, draft-ietf-detnet-architecture-13, 6 May
2019, <https://tools.ietf.org/html/draft-ietf-detnet-
architecture-13>.
[RAW-TECHNOS]
Thubert, P., Cavalcanti, D., Vilajosana, X., and C.
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Schmitt, "Reliable and Available Wireless Technologies",
Work in Progress, Internet-Draft, draft-thubert-raw-
technologies-03, 1 July 2019,
<https://tools.ietf.org/html/draft-thubert-raw-
technologies-03>.
[RAW-USE-CASES]
Papadopoulos, G., Thubert, P., Theoleyre, F., and C.
Bernardos, "RAW use cases", Work in Progress, Internet-
Draft, draft-bernardos-raw-use-cases-00, 5 July 2019,
<https://tools.ietf.org/html/draft-bernardos-raw-use-
cases-00>.
[RFC8175] Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
DOI 10.17487/RFC8175, June 2017,
<https://www.rfc-editor.org/info/rfc8175>.
[RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases",
RFC 8578, DOI 10.17487/RFC8578, May 2019,
<https://www.rfc-editor.org/info/rfc8578>.
7.2. Informative References
[BFD] IETF, "Bidirectional Forwarding Detection", October 2019,
<https://dataTracker.ietf.org/doc/charter-ietf-bfd/>.
[BIER] IETF, "Bit Indexed Explicit Replication", October 2019,
<https://dataTracker.ietf.org/doc/charter-ietf-bier/>.
[BIER-PREF]
Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
TE extensions for Packet Replication and Elimination
Function (PREF) and OAM", Work in Progress, Internet-
Draft, draft-thubert-bier-replication-elimination-03, 3
March 2018,
<https://tools.ietf.org/html/draft-thubert-bier-
replication-elimination-03>.
[CCAMP] IETF, "Common Control and Measurement Plane", October
2019,
<https://dataTracker.ietf.org/doc/charter-ietf-ccamp/>.
[DetNet] IETF, "Deterministic Networking", October 2019,
<https://dataTracker.ietf.org/doc/charter-ietf-detnet/>.
[DetNet-DP-FW]
Varga, B., Farkas, J., Berger, L., Fedyk, D., Malis, A.,
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Bryant, S., and J. Korhonen, "DetNet Data Plane
Framework", Work in Progress, Internet-Draft, draft-ietf-
detnet-data-plane-framework-02, 13 September 2019,
<https://tools.ietf.org/html/draft-ietf-detnet-data-plane-
framework-02>.
[DetNet-IP-OAM]
Mirsky, G. and M. Chen, "Operations, Administration and
Maintenance (OAM) for Deterministic Networks (DetNet) with
IP Data Plane", Work in Progress, Internet-Draft, draft-
mirsky-detnet-ip-oam-00, 8 July 2019,
<https://tools.ietf.org/html/draft-mirsky-detnet-ip-oam-
00>.
[MANET] IETF, "Mobile Ad hoc Networking", October 2019,
<https://dataTracker.ietf.org/doc/charter-ietf-manet/>.
[SPRING] IETF, "Source Packet Routing in Networking", October 2019,
<https://dataTracker.ietf.org/doc/charter-ietf-spring/>.
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems, Inc
Building D, 45 Allee des Ormes - BP1200
06254 MOUGINS - Sophia Antipolis
France
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Georgios Z. Papadopoulos
IMT Atlantique
Office B00 - 114A, 2 Rue de la Chataigneraie
35510 Cesson-Sevigne - Rennes
France
Phone: +33 299 12 70 04
Email: georgios.papadopoulos@imt-atlantique.fr
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