Reliable and Available Wireless Architecture
draft-ietf-raw-architecture-10
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draft-ietf-raw-architecture-10
RAW P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Informational 14 November 2022
Expires: 18 May 2023
Reliable and Available Wireless Architecture
draft-ietf-raw-architecture-10
Abstract
Reliable and Available Wireless (RAW) provides for high reliability
and availability for IP connectivity across any combination of wired
and wireless network segments. The RAW Architecture extends the
DetNet Architecture and other standard IETF concepts and mechanisms
to adapt to the specific challenges of the wireless medium, in
particular intermittently lossy connectivity. This document defines
a network control loop that optimizes the use of constrained spectrum
and energy while maintaining the expected connectivity properties,
typically reliability and latency. The loop involves OAM, PCE, and
PREOF extensions, and a new Controller plane Function called the Path
Selection Engine, that dynamically selects the DetNet path for the
next packets to route around local failures.
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-
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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 18 May 2023.
Copyright Notice
Copyright (c) 2022 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
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. ARQ . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2. FEC . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.3. HARQ . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.4. OAM . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.5. OODA . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.6. PAREO . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Link and Direction . . . . . . . . . . . . . . . . . . . 6
2.2.1. Flapping . . . . . . . . . . . . . . . . . . . . . . 6
2.2.2. Uplink . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.3. Downlink . . . . . . . . . . . . . . . . . . . . . . 6
2.2.4. Downstream . . . . . . . . . . . . . . . . . . . . . 6
2.2.5. Upstream . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Path and Tracks . . . . . . . . . . . . . . . . . . . . . 6
2.3.1. Path . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.2. Track . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.3. Segment . . . . . . . . . . . . . . . . . . . . . . . 9
2.4. Deterministic Networking . . . . . . . . . . . . . . . . 9
2.4.1. Flow . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4.2. Deterministic Flow Identifier (L2) . . . . . . . . . 9
2.4.3. Deterministic Flow Identifier (L3) . . . . . . . . . 10
2.4.4. TSN . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.5. Reliability and Availability . . . . . . . . . . . . . . 10
2.5.1. Service Level Agreement . . . . . . . . . . . . . . . 10
2.5.2. Service Level Objective . . . . . . . . . . . . . . . 10
2.5.3. Service Level Indicator . . . . . . . . . . . . . . . 10
2.5.4. Reliability . . . . . . . . . . . . . . . . . . . . . 10
2.5.5. Available . . . . . . . . . . . . . . . . . . . . . . 11
2.5.6. Availability . . . . . . . . . . . . . . . . . . . . 11
2.6. OAM variations . . . . . . . . . . . . . . . . . . . . . 11
2.6.1. Active OAM . . . . . . . . . . . . . . . . . . . . . 11
2.6.2. In-Band OAM . . . . . . . . . . . . . . . . . . . . . 11
2.6.3. Out-of-Band OAM . . . . . . . . . . . . . . . . . . . 11
2.6.4. Limited OAM . . . . . . . . . . . . . . . . . . . . . 11
2.6.5. Upstream OAM . . . . . . . . . . . . . . . . . . . . 12
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2.6.6. Residence Time . . . . . . . . . . . . . . . . . . . 12
2.6.7. Additional References . . . . . . . . . . . . . . . . 12
3. Reliable and Available Wireless . . . . . . . . . . . . . . . 12
3.1. Reliability and Availability . . . . . . . . . . . . . . 12
3.1.1. High Availability Engineering Principles . . . . . . 12
3.1.2. Applying Reliability Concepts to Networking . . . . . 15
3.1.3. Wireless Effects Affecting Reliability . . . . . . . 15
3.2. The RAW problem . . . . . . . . . . . . . . . . . . . . . 17
4. The RAW Conceptual Model . . . . . . . . . . . . . . . . . . 20
4.1. The RAW Planes . . . . . . . . . . . . . . . . . . . . . 20
4.2. RAW vs. Upper and Lower Layers . . . . . . . . . . . . . 22
4.3. RAW and DetNet . . . . . . . . . . . . . . . . . . . . . 23
5. The RAW Control Loop . . . . . . . . . . . . . . . . . . . . 26
5.1. Routing Time Scale vs. Forwarding Time Scale . . . . . . 26
5.2. A OODA Loop . . . . . . . . . . . . . . . . . . . . . . . 28
5.3. Observe: The RAW OAM . . . . . . . . . . . . . . . . . . 29
5.4. Orient: The Path Computation Engine . . . . . . . . . . . 30
5.5. Decide: The Path Selection Engine . . . . . . . . . . . . 31
5.6. Act: DetNet Path Selection and PAREO functions . . . . . 33
6. Security Considerations . . . . . . . . . . . . . . . . . . . 33
6.1. Layer-2 encryption . . . . . . . . . . . . . . . . . . . 34
6.2. Forced Access . . . . . . . . . . . . . . . . . . . . . . 34
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 34
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
10.1. Normative References . . . . . . . . . . . . . . . . . . 35
10.2. Informative References . . . . . . . . . . . . . . . . . 36
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
Deterministic Networking is an attempt to emulate the properties of a
serial link over a switched fabric, by providing a bounded latency
and eliminating congestion loss, even when co-existing with best-
effort traffic. It is getting traction in various industries
including professional A/V, manufacturing, online gaming, and
smartgrid automation, with both cost savings and complexity benefits
(e.g., vs. loads of P2P cables).
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
budgeted volume of data per unit of time that fits the physical
capabilities of the underlying network, and the use of time-shared
resources (bandwidth and buffers) per circuit, and/or by shaping and/
or scheduling the packets at every hop.
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This innovation was initially introduced on wired networks, with IEEE
802.1 Time Sensitive networking (TSN) - for Ethernet LANs - and IETF
DetNet. But the wired and the wireless media are fundamentally
different at the physical level and in the possible abstractions that
can be built for IPv6 [IPv6], more in [IPoWIRELESS]. Nevertheless,
deterministic capabilities are required in a number of wireless use
cases as well [RAW-USE-CASES]. With new scheduled radios such as
TSCH and OFDMA [RAW-TECHNOS] being developed to provide determinism
over wireless links at the lower layers, providing DetNet
capabilities is now becoming possible.
Wireless networks operate on a shared medium where uncontrolled
interference, including the self-induced multipath fading cause
random transmission losses. Fixed and mobile obstacles and
reflectors may block or alter the signal, causing transient and
unpredictable variations of the throughput and packet delivery ratio
(PDR) of a wireless link. This adds new dimensions to the
statistical effects that affect the quality and reliability of the
link.
Reliable and Available Wireless (RAW) takes up the challenge of
providing highly available and reliable end-to-end performances in a
network with scheduled wireless segments. To achieve this, RAW
leverages multiple links and parallel transmissions, providing enough
diversity and redundancy to ensure the timely packet delivery while
preserving energy and optimizing the use of the shared spectrum.
2. Terminology
RAW reuses terminology defined for DetNet in the "Deterministic
Networking Architecture" [RFC8655], e.g., PREOF for Packet
Replication, Elimination and Ordering Functions.
RAW also reuses terminology defined for 6TiSCH in [6TiSCH-ARCHI] such
as the term Track. A Track associates a topological graph with usage
metadata that represent how the paths within the Track are used.
In an quantic analogy, a Track is to a path what an atomic orbital is
to a planetary orbit, in that the electron has a probability of
presence within a known shape as opposed to a deterministic
trajectory.
In a herding gnous analogy, a gnou follows its own path that it marks
with its hooves as it goes; before the herd starts, any point within
the Track has a statistical chance to be marked by one or more
hooves, meaning on path for those gnous; once the herd has passed,
the Track can be observed from above, but it was there in advance as
a potential that the gnous were to follow.
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The concept of Track is agnostic to the underlaying technology and
applies but is not limited to any fully or partially wireless mesh.
RAW specifies strict and loose Tracks depending on whether the path
is fully controlled by RAW or traverses an opaque network where RAW
cannot observe and control the individual hops.
RAW uses the following terminology and acronyms:
2.1. Acronyms
2.1.1. ARQ
Automatic Repeat Request, enabling an acknowledged transmission and
retries. ARQ is a typical model at Layer-2 on a wireless medium.
ARQ is typically implemented hop-by-hop and not end-to-end in
wireless networks. Else, it introduces excessive indetermination in
latency, but a limited number of retries within a bounded time may be
used within end-to-end constraints.
2.1.2. FEC
Forward Error Correction, adding redundant data to protect against a
partial loss without retries.
2.1.3. HARQ
Hybrid Automatic Repeat Request, combining FEC and ARQ.
2.1.4. OAM
OAM stands for Operations, Administration, and Maintenance, and
covers the processes, activities, tools, and standards involved with
operating, administering, managing and maintaining any system. This
document uses the terms Operations, Administration, and Maintenance,
in conformance with the 'Guidelines for the Use of the "OAM" Acronym
in the IETF' [RFC6291] and the system observed by the RAW OAM is the
Track.
2.1.5. OODA
Observe, Orient, Decide, Act. The OODA Loop is a conceptual cyclic
model developed by USAF Colonel John Boyd, and that is applicable in
multiple domains where agility can provide benefits against brute
force.
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2.1.6. PAREO
Packet (hybrid) ARQ, Replication, Elimination and Ordering. PAREO is
a superset Of DetNet's PREOF that includes leveraging lower-layer
(typically wireless) techniques such as short range broadcast,
MUMIMO, PHY rate and other Modulation Coding Scheme (MCS) adaptation,
constructive interference and overhearing, separately or in
combination, to increase the end-to-end reliability. PAREO functions
that are actuated at the lower layers may be controlled through
abstract interfaces by the RAW extensions within the DetNet Service
sublayer.
2.2. Link and Direction
2.2.1. Flapping
In the context of RAW, a link flaps when the reliability of the
wireless connectivity drops abruptly for a short period of time,
typically of a subsecond to seconds duration.
2.2.2. Uplink
Connection from end-devices to a data communication equipment. In
the context of wireless, uplink refers to the connection between a
station (STA) and a controller (AP) or a User Equipment (UE) to a
Base Station (BS) such as a 3GPP 5G gNodeB (gNb).
2.2.3. Downlink
The reverse direction from uplink.
2.2.4. Downstream
Following the direction of the flow data path along a Track.
2.2.5. Upstream
Against the direction of the flow data path along a Track.
2.3. Path and Tracks
2.3.1. Path
Quoting section 1.1.3 of [INT-ARCHI]:
| At a given moment, all the IP datagrams from a particular source
| host to a particular destination host will typically traverse the
| same sequence of gateways. We use the term "path" for this
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| sequence. Note that a path is uni-directional; it is not unusual
| to have different paths in the two directions between a given host
| pair.
Section 2 of [I-D.irtf-panrg-path-properties] points to a longer,
more modern definition of path, which begins as follows:
| A sequence of adjacent path elements over which a packet can be
| transmitted, starting and ending with a node. A path is
| unidirectional. Paths are time-dependent, i.e., the sequence of
| path elements over which packets are sent from one node to another
| may change. A path is defined between two nodes.
It follows that the general acceptance of a path is a linear sequence
of links and nodes, as opposed to a multi-dimensional graph, defined
by the experience of the packet that went from a node A to a node B.
In the context of this document, a path is observed by following one
copy or one fragment of a packet that conserves its uniqueness and
integrity. For instance, if C replicates to E and F and D eliminates
on the way from A to B, a packet from A to B experiences 2 paths,
A->C->E->D->B and A->C->F->D->B. The adjectives "serial" or "simple"
are used to clarify when dealing with such path.
With DetNet and RAW, a packet may be duplicated, fragmented and
network-coded, and the various byproducts may travel different paths
that are not necessarily end-to-end between A and B; we refer to that
complex experience as a DetNet path. As such, the DetNet path
extends the above description of a path, but it still matches the
experience of a packet that traverses the network.
With RAW, that experience is subject to change from a packet to the
next, but all the possible experiences are all contained within a
finite set. Therefore we introduce below the term of a Track that
coalesces that set and covers the overall topology where the possible
DetNet paths are all contained. As such, the Track coalesces all the
possible paths that a flow may experience, each with its own
statistical probability to be used.
2.3.2. Track
A networking graph that can be followed to transport packets with
equivalent treatment, associated with useage metadata; as opposed to
the definition of a path above, a Track represents not an actual but
a potential, it is not necessarily a linear sequence like a simple
path, and is not necessarily fully traversed (flooded) by all packets
of a flow like a Detnet Path. Still, and as a simplification, the
casual reader may consider that a Track is very much like a DetNet
path, aggregating multiple paths that may overlap, fork and rejoin,
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for instance to enable a protection service by the PREOF operations.
+---------+
| IoT G/W |
+---------+
EGR <=== Elimination at Egress
| |
/------/ \-------\ Wired backbone
| |
+--|--+ +--|--+
| | | Backbone | | | Backbone
| | | Router | | | Router
+--|--+ +--|--+
| |
o \ o / Track branch
o o o---o---o o o o o
\ o / o o o
o o \ / o low power lossy network
\/ o o o
o IN <=== Replication at Track Ingress
|
o <- source device
Figure 1: Example IoT Track to an IoT gateway with 1+1 redundancy
Refining further, a Track is defined as the coalescence of the
collection of all the feasible DetNet Paths that a packet which flow
is assigned to the Track may be forwarded along. A packet that is
assigned to the Track will experience one of the feasible DetNet
Paths based on the current selection by the PSE at the time the
packet traverses the network.
Refining even further, the feasible DetNet Paths within the Track may
or may not be computed in advance, but decided upon the detection of
a change from a clean slate. Furthermore, the PSE decision may be
distributed, which yields a large combination of possible and
dependant decisions, with no node in the network capable of reporting
which is the current DetNet Path within the Track.
In DetNet [RFC8655] terms, a Track has the following properties:
* A Track is a Layer-3 abstraction built upon P2P IP links between
routers. A router may form multiple P2P IP links over a single
radio interface.
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* A Track has one Ingress and one Egress nodes, which operate as
DetNet Edge nodes.
* The graph of a Track is reversible, meaning that packets can be
routed against the flow of data packets, e.g., to carry OAM
measurements or control messages back to the Ingress.
* The vertices of that graph are DetNet Relay nodes that operate at
the DetNet Service sub-layer and provide the PAREO functions.
* The topological edges of the graph are serial sequences of DetNet
Transit nodes that operate at the DetNet Forwarding sub-layer.
2.3.3. Segment
A serial path formed by a topological edge of a Track. East-West
Segments are oriented from Ingress (East) to Egress (West). North/
South Segments can be bidirectional; to avoid loops, measures must be
taken to ensure that a given packet flows either Northwards or
Southwards along a bidirectional Segment, but never bounces back.
2.4. Deterministic Networking
This document reuses the terminology in section 2 of [RFC8557] and
section 4.1.2 of [RFC8655] for deterministic networking and
deterministic networks.
2.4.1. Flow
A collection of consecutive IP packets defined by the upper layers
and signaled by the same 5 or 6-tuple, see section 5.1 of [RFC8939].
Packets of the same flow must be placed on the same Track to receive
an equivalent treatment from Ingress to Egress within the Track.
Multiple flows may be transported along the same Track. The DetNet
Path that is selected for the flow may change over time under the
control of the PSE.
2.4.2. Deterministic Flow Identifier (L2)
A tuple identified by a stream_handle, and provided by a bridge, in
accordance with IEEE 802.1CB. The tuple comprises at least source
MAC, destination MAC, VLAN ID, and L2 priority. Continuous streams
are characterized by bandwidth and max packet size; scheduled streams
are characterized by a repeating pattern of timed transmissions.
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2.4.3. Deterministic Flow Identifier (L3)
See section 3.3 of [DetNet-DP]. The classical IP 5-tuple that
identifies a flow comprises the source IP, destination IP, source
port, destination port, and the upper layer protocol (ULP). DetNet
uses a 6-tuple where the extra field is the DSCP field in the packet.
The IPv6 flow label is not used for that purpose.
2.4.4. TSN
TSN stands for Time Sensitive Networking and denotes the efforts at
IEEE 802 for deterministic networking, originally for use on
Ethernet. Wireless TSN (WTSN) denotes extensions of the TSN work on
wireless media such as the selected RAW technologies [RAW-TECHNOS].
2.5. Reliability and Availability
In the context of the RAW work, Reliability and Availability are
defined as follows:
2.5.1. Service Level Agreement
In the context of RAW, an SLA (service level agreement) is a contract
between a provider, the network, and a client, the application flow,
about measurable metrics such as latency boundaries, consecutive
losses, and packet delivery ratio (PDR).
2.5.2. Service Level Objective
A service level objective (SLO) is one term in the SLA, for which
specific network setting and operations are implemented. For
instance, a dynamic tuning of the packet redundancy will address an
SLO of consecutive losses in a row by augmenting the chances of
delivery of a packet that follows a loss.
2.5.3. Service Level Indicator
A service level indicator (SLI) measures the compliance of an SLO to
the terms of the contract. It can be for instance the statistics of
individual losses and losses in a row as time series.).
2.5.4. Reliability
Reliability is a measure of the probability that an item will perform
its intended function for a specified interval under stated
conditions (SLA). RAW expresses reliability in terms of Mean Time
Between Failure (MTBF) and Maximum Consecutive Failures (MCF). More
in [NASA].).
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2.5.5. Available
That is exempt of unscheduled outage or derivation from the terms of
the SLA. A basic expectation for a RAW network is that the flow is
maintained in the face of any single breakage or flapping.
2.5.6. Availability
Availability is a measure of the relative amount of time where a RAW
Network operates in stated condition (SLA), expressed as
(uptime)/(uptime+downtime). Because a serial wireless path may not
be good enough to provide the required reliability, and even 2
parallel paths may not be over a longer period of time, the RAW
availability implies a journey that is a lot more complex than
following a serial path.
2.6. OAM variations
2.6.1. Active OAM
See [RFC7799]. In the context of RAW, Active OAM is used to observe
a particular Track, DetNet Path, or Segment of a Track regardless of
whether it is used for traffic at that time.
2.6.2. In-Band OAM
An active OAM packet is considered in-band for the monitored Track
when it traverses the same set of links and interfaces and if the OAM
packet receives the same QoS and PAREO treatment as the packets of
the data flows that are injected in the Track.
2.6.3. Out-of-Band OAM
Out-of-band OAM is an active OAM whose path is not topologically
congruent to the Track, or its test packets receive a QoS and/or
PAREO treatment that is different from that of the packets of the
data flows that are injected in the Track, or both.
2.6.4. Limited OAM
An active OAM packet is a Limited OAM packet when it observes the RAW
operation over a node, a segment, or a DetNet Path of the Track,
though not from Ingress to Egress. It is injected in the datapath
and extracted from the datapath around the particular function or
subnetwork (e.g., around a relay providing a Service sublayer
replication point) that is being tested.
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2.6.5. Upstream OAM
An upstream OAM packet is an Out-of-Band OAM packet that traverses
the Track from egress to ingress on the reverse direction, to capture
and report OAM measurements upstream. The collection may capture all
information along the whole Track, or it may only learn select data
across all, or only a particular DetNet Path, or Segment of a Track.
2.6.6. Residence Time
A residence time (RT) is defined as the time period between the
reception of a packet starts and the transmission of the packet
begins. In the context of RAW, RT is useful for a transit node, not
ingress or egress.
2.6.7. Additional References
[DetNet-OAM] provides additional terminology related to OAM in the
context of DetNet and by extension of RAW, whereas [RFC7799] defines
the Active, Passive, and Hybrid OAM methods.
3. Reliable and Available Wireless
3.1. Reliability and Availability
3.1.1. High Availability Engineering Principles
The reliability criteria of a critical system pervades through its
elements, and if the system comprises a data network then the data
network is also subject to the inherited reliability and availability
criteria. It is only natural to consider the art of high
availability engineering and apply it to wireless communications in
the context of RAW.
There are three principles [pillars] of high availability
engineering:
1. elimination of single points of failure
2. reliable crossover
3. prompt detection of failures as they occur.
These principles are common to all high availability systems, not
just ones with Internet technology at the center. Examples of both
non-Internet and Internet are included.
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3.1.1.1. Elimination of Single Points of Failure
Physical and logical components in a system happen to fail, either as
the effect of wear and tear, when used beyond acceptable limits, or
due to a software bug. It is necessary to decouple component failure
from system failure to avoid the latter. This allows failed
components to be restored while the rest of the system continues to
function.
IP Routers leverage routing protocols to compute alternate routes in
case of a failure. There is a rather open-ended issue over alternate
routes -- for example, when links are cabled through the same
conduit, they form a shared risk link group (SRLG), and will share
the same fate if the bundle is cut. The same effect can happen with
virtual links that end up in a same physical transport through the
games of encapsulation. In a same fashion, an interferer or an
obstacle may affect multiple wireless transmissions at the same time,
even between different sets of peers.
Intermediate network Nodes such as routers, switches and APs, wire
bundles and the air medium itself can become single points of
failure. For High Availability, it is thus required to use
physically link- and Node-disjoint paths; in the wireless space, it
is also required to use the highest possible degree of diversity
(time, space, code, frequency, channel width) in the transmissions
over the air to combat the additional causes of transmission loss.
From an economics standpoint, executing this principle properly
generally increases capitalization expense because of the redundant
equipment. In a constrained network where the waste of energy and
bandwidth should be minimized, an excessive use of redundant links
must be avoided; for RAW this means that the extra bandwidth must be
used wisely and with parsimony.
3.1.1.2. Reliable Crossover
Having a backup equipment has a limited value unless it can be
reliably switched into use within the down-time parameters. IP
Routers execute reliable crossover continuously because the routers
will use any alternate routes that are available [RFC0791]. This is
due to the stateless nature of IP datagrams and the dissociation of
the datagrams from the forwarding routes they take. The "IP Fast
Reroute Framework" [FRR] analyzes mechanisms for fast failure
detection and path repair for IP Fast-Reroute, and discusses the case
of multiple failures and SRLG. Examples of FRR techniques include
Remote Loop-Free Alternate [RLFA-FRR] and backup label-switched path
(LSP) tunnels for the local repair of LSP tunnels using RSVP-TE
[RFC4090].
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Deterministic flows, on the contrary, are attached to specific paths
where dedicated resources are reserved for each flow. Therefore each
DetNet path must inherently provide sufficient redundancy to provide
the guaranteed SLA at all times. The DetNet PREOF typically
leverages 1+1 redundancy whereby a packet is sent twice, over non-
congruent paths. This avoids the gap during the fast reroute
operation, but doubles the traffic in the network.
In the case of RAW, the expectation is that multiple transient faults
may happen in overlapping time windows, in which case the 1+1
redundancy with delayed reestablishment of the second path will not
provide the required guarantees. The Data Plane must be configured
with a sufficient degree of redundancy to select an alternate
redundant path immediately upon a fault, without the need for a slow
intervention from the controller plane.
3.1.1.3. Prompt Notification of Failures
The execution of the two above principles is likely to render a
system where the user will rarely see a failure. But someone needs
to in order to direct maintenance.
There are many reasons for system monitoring (FCAPS for fault,
configuration, accounting, performance, security is a handy mental
checklist) but fault monitoring is sufficient reason.
"An Architecture for Describing Simple Network Management Protocol
(SNMP) Management Frameworks" [STD 62] describes how to use SNMP to
observe and correct long-term faults.
"Overview and Principles of Internet Traffic Engineering" [TE]
discusses the importance of measurement for network protection, and
provides abstract an method for network survivability with the
analysis of a traffic matrix as observed by SNMP, probing techniques,
FTP, IGP link state advertisements, and more.
Those measurements are needed in the context of RAW to inform the
controller and make the long term reactive decision to rebuild a
complex path based on statistical and aggregated information. RAW
itself operates in the Network Plane at a faster time scale with live
information on speed, state, etc... This live information can be
obtained directly from the lower layer, e.g., using L2 triggers, read
from a protocol such as the Dynamic Link Exchange Protocol (DLEP)
[DLEP], or transported over multiple hops using OAM and reverse OAM,
as illustrated in Figure 9.
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3.1.2. Applying Reliability Concepts to Networking
The terms Reliability and Availability are defined for use in RAW in
Section 2 and the reader is invited to read [NASA] for more details
on the general definition of Reliability. Practically speaking a
number of nines is often used to indicate the reliability of a data
link, e.g., 5 nines indicate a Packet Delivery Ratio (PDR) of
99.999%.
This number is typical in a wired environment where the loss is due
to a random event such as a solar particle that affects the
transmission of a particular frame, but does not affect the previous
or next frame, nor frames transmitted on other links. Note that the
QoS requirements in RAW may include a bounded latency, and a packet
that arrives too late is a fault and not considered as delivered.
For a periodic networking pattern such as an automation control loop,
this number is proportional to the Mean Time Between Failures (MTBF).
When a single fault can have dramatic consequences, the MTBF
expresses the chances that the unwanted fault event occurs. In data
networks, this is rarely the case. Packet loss cannot never be fully
avoided and the systems are built to resist to one loss, e.g., using
redundancy with Retries (HARQ) or Packet Replication and Elimination
(PRE), or, in a typical control loop, by linear interpolation from
the previous measurements.
But the linear interpolation method cannot resist multiple
consecutive losses, and a high MTBF is desired as a guarantee that
this will not happen, IOW that the number of losses-in-a-row can be
bounded. In that case, what is really desired is a Maximum
Consecutive Failures (MCF). If the number of losses in a row passes
the MCF, the control loop has to abort and the system, e.g., the
production line, may need to enter an emergency stop condition.
Engineers that build automated processes may use the network
reliability expressed in nines or as an MTBF as a proxy to indicate
an MCF, e.g., as described in section 7.4 of the "Deterministic
Networking Use Cases" [RFC8578].
3.1.3. Wireless Effects Affecting Reliability
In contrast with wired networks, errors in transmission are the
predominant source of packet loss in wireless networks.
The root cause for the loss may be of multiple origins, calling for
the use of different forms of diversity:
Multipath Fading: A destructive interference by a reflection of the
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original signal.
A radio signal may be received directly (line-of-sight) and/or as
a reflection on a physical structure (echo). The reflections take
a longer path and are delayed by the extra distance divided by the
speed of light in the medium. Depending on the frequency, the
echo lands with a different phase which may add up to
(constructive interference) or cancel the direct signal
(destructive interference).
The affected frequencies depend on the relative position of the
sender, the receiver, and all the reflecting objects in the
environment. A given hop will suffer from multipath fading for
multiple packets in a row till a physical movement changes the
reflection patterns.
Co-channel Interference: Energy in the spectrum used for the
transmission confuses the receiver.
The wireless medium itself is a Shared Risk Link Group (SRLG) for
nearby users of the same spectrum, as an interference may affect
multiple co-channel transmissions between different peers within
the interference domain of the interferer, possibly even when they
use different technologies.
Obstacle in Fresnel Zone: The optimal transmission happens when the
Fresnel Zone between the sender and the receiver is free of
obstacles.
As long as a physical object (e.g., a metallic trolley between
peers) that affects the transmission is not removed, the quality
of the link is affected.
In an environment that is rich of metallic structures and mobile
objects, a single radio link will provide a fuzzy service, meaning
that it cannot be trusted to transport the traffic reliably over a
long period of time.
Transmission losses are typically not independent, and their nature
and duration are unpredictable; as long as a physical object (e.g., a
metallic trolley between peers) that affects the transmission is not
removed, or as long as the interferer (e.g., a radar) keeps
transmitting, a continuous stream of packets will be affected.
The key technique to combat those unpredictable losses is diversity.
Different forms of diversity are necessary to combat different causes
of loss and the use of diversity must be maximized to optimize the
PDR.
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A single packet may be sent at different times (time diversity) over
diverse paths (spatial diversity) that rely on diverse radio channels
(frequency diversity) and diverse PHY technologies, e.g., narrowband
vs. spread spectrum, or diverse codes. Using time diversity will
defeat short-term interferences; spatial diversity combats very local
causes such as multipath fading; narrowband and spread spectrum are
relatively innocuous to one another and can be used for diversity in
the presence of the other.
3.2. The RAW problem
While the generic "Deterministic Networking Problem Statement"
[RFC8557] applies to both the wired and the wireless media, the
methods to achieve RAW must extend those used to support time-
sensitive networking over wires, as a RAW solution has to address
less consistent transmissions, energy conservation and shared
spectrum efficiency.
Operating at the Layer-3, RAW does not change the wireless technology
at the lower layers. OTOH, it can further increase diversity in the
spatial, time, code, and frequency domains by enabling multiple link-
layer wired and wireless technologies in parallel or sequentially,
for a higher resilience and a wider applicability. RAW can also
provide homogeneous services to critical applications beyond the
boundaries of a single subnetwork, e.g., controlling the use of
diverse radio access technologies to optimize the end-to-end
application experience.
RAW improves the DetNet services by providing elements that are
specialized for transporting IP flows over deterministic radios
technologies such as listed in [RAW-TECHNOS]. Conceptually, RAW is
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. Nevertheless, cross-
layer optimizations may take place to ensure proper link awareness
(think, link quality) and packet handling (think, scheduling).
The "Deterministic Networking Architecture" [RFC8655] is composed of
three planes: the Application (User) Plane, the Controller Plane, and
the Network Plane. The DetNet Network Plane is composed of a DetNet
service sub-layer that focuses on flow protection (e.g., using
redundancy) and can be fully operated at Layer-3, and a DetNet
forwarding sub-layer that associates the flows to the paths, ensures
the availability of the necessary resources, and leverages Layer-2
functionalities for timely delivery to the next DetNet system.
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The RAW Architecture extends the DetNet Network Plane, to accommodate
one or multiple hops of homogeneous or heterogeneous wired and
wireless technologies. RAW adds reactivity to the DetNet service
sub-layer to compensate the dynamics for the radio links in terms of
lossiness and bandwidth. This may apply for instance to mesh
networks as illustrated in Figure 2, or diverse radio access networks
as illustrated in Figure 8.
As opposed to wired links, the availability and performance of an
individual wireless link cannot be trusted over the long term; it
will vary with transient service discontinuity, and any serial path
that includes wireless hops is bound to experience service
discontinuity. On the other hand, the wireless medium provides
unique capabilities that cannot be found on wires and that the RAW
Architecture leverages opportunistically to improve the end-to-end
reliability over a collection of links.
Those capabilities include:
Promiscuous Overhearing: Because the medium is broadcast as opposed
to physically point to point like a wire, more than one node in
the forward direction of the packet may hear or overhear a
transmission, and the reception by one may compensate the loss by
another. The concept of path can be revisited in favor multipoint
to multipoint progress in the orward direction and statistical
chances of successful reception of any of the transmissions by any
of the receivers.
L2-aware routing: As the quality and speed of a link variates over
time, the concept of metric must also be revisited. Shortest path
loses its absolute value, and hop count turns into a bad idea as
the link budget drops with the distance. Routing over radio
requires both 1) a new and more dynamic sense of the link, with
new protocols such as DLEP and L2-trigger to maintain L3 up to
date with the link quality and availability, and 2) a new approach
to multipath routing, where non-equal cost multipath becomes the
norm as shortest path loses its meaning with the instability of
the metrics.
ARQ, FEC and codes: Though feasible on any technology, proactive
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(forward) and reactive (ARQ) error correction are typical to the
wireless media. Bounded latency can still be obtained on a
wireless link while operating those technologies, provided that
the extra transmission happens within the budget allocated to that
hop, or that the introduced delay is compensated along the path.
In the case of coded fragments and retries, it makes sense to
variate all the possible physical properties of the transmission
to reduce the chances that the same effect causes the loss of both
original and redundant transimissions.
Relay Coordination and constructive interference: Though it can be
difficult to achieve at high speed, a fine time synchronization
and a precise sense of phase allows the energy from multiple
coordinated senders to add up at the receiver and actually improve
the signal quality, compensating for either distance or physical
objects in the Fresnel zone that would reduce the link budget.
RAW and DetNet route application flows that require a special
treatment along the paths that will provide that treatment. This may
be seen as a form of Path Aware Networking and may be subject to
impediments documented in [RFC9049].
The establishment of a path is not in-scope for RAW. It may be the
product of a centralized Controller Plane Function like a Path
computation Element (PCE) [RFC4655] or a distributed routing
protocol. For the most part, the remainder of the document mentions
centralized control and PCE, but conceptually, the same issues and
needs would arise for a distributed protocol that would attempt to
allocate constrained resources and optimize globally, and the
distributed approach is considered in scope too.
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 (see
Section 2.3) and to leave it to the Network Plane to select which of
those forwarding solutions are to be used for a given packet based on
the current conditions.
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 Network Plane operations happen at the forwarding time
scale on one DetNet flow over a complex path delineated by a Track
(see Section 2.3.2). The Track is preestablished and installed by
means outside of the scope of RAW; it may be strict or loose
depending on whether each or just a subset of the hops are observed
and controlled by RAW.
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The RAW Architecture is based on an abstract OODA Loop (Observe,
Orient, Decide, Act). The generic concept involves:
1. Network Plane measurement protocols for Operations,
Administration and Maintenance (OAM) to Observe some or all hops
along a Track as well as the end-to-end packet delivery
2. Optional Controller plane elements that report the links
statistics to be used to compute and install the Tracks, and
provides meta data to Orient the routing decision, e.g., by a PCE
in a centralized controller
3. A Runtime distributed Path Selection Engine (PSE) that Decides
which DetNet Path to use for the next packet(s) that are routed
along the Track
4. Packet (hybrid) ARQ, Replication, Elimination and Ordering
Dataplane actions that operate at the DetNet Service sublayer to
increase the reliability of the end-to-end transmissions. The
RAW architecture also covers in-situ signaling when the decision
is Acted by a node that down the Track from the PSE.
The overall OODA Loop optimizes the use of redundancy to achieve the
required reliability and availability Service Level Agreement (SLA)
while minimizing the use of constrained resources such as spectrum
and battery.
This document presents the RAW problem and associated terminology in
Section 3.2, and elaborates in Section 5.2 on the OODA loop based on
the RAW conceptual model presented in Section 4.
4. The RAW Conceptual Model
RAW inherits the conceptual model described in section 4 of the
DetNet Architecture [RFC8655]. RAW extends the DetNet service layer
to provide additional agility against transmission loss.
4.1. The RAW Planes
A RAW Network Plane may be strict (as illustrated in Figure 4 or
loose (as illustrated in Figure 5, depending on whether RAW observes
and takes actions on all hops or not. For instance, the packets
between two wireless entities may be relayed over a wired
infrastructure such as a Wi-Fi extended service set (ESS) or a 5G
Core; in that case, RAW observes and controls the transmission over
the wireless first and last hops, as well as end-to-end metrics such
as latency, jitter, and delivery ratio. This operation is loose
since the structure and properties of the wired infrastructure are
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ignored, and may be either controlled by other means such as DetNet/
TSN, or neglected in the face of the wireless hops.
A Controller Plane Function (CPF) such as a PCE interacts with RAW
Nodes over a Southbound API. The RAW Nodes are DetNet relays that
are capable of additional diversity mechanisms and measurement
functions related to the radio interface, in particular the PAREO
diversity mechanisms. RAW leverages a CPF that operates inside the
RAW Nodes (typically the Ingress Edge Nodes) to dynamically adapt the
path of the packets and optimizes the resource usage.
The PCE defines a complex Track between an Ingress End System and an
Egress End System, and indicates to the RAW Nodes where the PAREO
operations may be actioned in the Network Plane. The Track may be
strict, meaning that the DetNet forwarding sublayer operations are
enforced end-to-end The Track may be expressed loosely to enable
traversing a non-RAW subnetwork as in Figure 5. In that case, RAW
can not leverage end-to-end DetNet and cannot provide latency
guarantees. The non-RAW subnetwork is neglected in the RAW
computation, that is, considered jitterless, and infinitely reliable
and/or available in comparison with the links between RAW nodes, so
loss and jitter that is measured end-to-end is attributed to the RAW
hops (typically an access link).
CPF CPF CPF CPF
Southbound API
_-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
_-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
RAW --/ RAW --/ RAW --/ RAW
/-- Node /-- Node /-- Node /-- Node --/
Ingress --/ / / /-- Egress
End / / .. . End
Node ---/ / / .. .. . /-- Node
/-- RAW --/ RAW ( non-RAW ) -- RAW --/
Node /-- Node --- ( Nodes ) Node
... .
--/ wireless wired
/-- link --- link
Figure 2: RAW Nodes
The Link-Layer metrics are reported to the PCE in a time-aggregated,
e.g., statistical fashion. Example Link-Layer metrics include
typical Link bandwidth (the medium speed depends dynamically on the
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PHY mode), number of flows (bandwidth that can be reserved for a flow
depends on the number and size of flows sharing the spectrum) and
average and mean squared deviation of availability and reliability
figures such as Packet Delivery Ratio (PDR) over long periods of
time.
Based on those metrics, the PCE installs the Track with enough
redundant forwarding solutions to ensure that the Network Plane can
reliably deliver the packets within a System Level Agreement (SLA)
associated to the flows that it transports. The SLA defines end-to-
end reliability and availability requirements, where reliability may
be expressed as a successful delivery in order and within a bounded
delay of at least one copy of a packet.
Depending on the use case and the SLA, the Track may comprise non-RAW
segments, either interleaved inside the Track, or all the way to the
Egress End Node (e.g., a server in the Internet). RAW observes the
Lower-Layer Links between RAW nodes (typically, radio links) and the
end-to-end Network Layer operation to decide at all times which of
the PAREO diversity schemes is actioned by which RAW Nodes.
Once a Track is established, per-segment and end-to-end reliability
and availability statistics are periodically reported to the PCE to
assure that the SLA can be met or have it recompute the Track if not.
4.2. RAW vs. Upper and Lower Layers
RAW improves the reliability of transmissions and the availability of
the communication resources, but does not provide scheduling and
shaping, so RAW itself does not provide guarantees such as latency
for the application payload. Rather, it should be seen as a dynamic
optimization of the use of redundancy to maintain it within certain
boundaries. For instance, ARQ, which is part of the PAREO
capabilities (see Section 5.6) is operated by the lower layers and
RAW will only abstract the concept and hint the lower layers on the
desired outcome, as opposed to performing the retries at Layer-3.
Guarantees such as bounded latency depend on the upper layers
(Transport or Application) to provide the payload in volumes and at
times that match the contract with the DetNet sublayers and the
layers below. Excess of incoming traffic at the DetNet Ingress will
cause either dropping, queueing, or reclassification of the packets,
and entail loss, latency, or jitter, and moot the guarantees that are
provided inside the DetNet Network.
When the traffic from upper layers matches the expectation of the
lower layers, RAW still depends on the lower layers to provide the
timing and physical resources guarantees that are needed to match the
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traffic SLA. When the availability of the physical resource varies,
RAW will act on the distribution of the traffic to leverage
alternates within a finite set of potential resources.
4.3. RAW and DetNet
RAW leverages the DetNet Forwarding sub-layer and requires the
support of in-situ OAM in DetNet Transit Nodes (see fig 3 of
[RFC8655] for the dynamic acquisition of link capacity and state to
maintain a strict RAW service, end-to-end, over a DetNet Network.
RAW enhances DetNet to improve the protection against link errors
such as transient flapping that are far more common in wireless
links. Nevertheless, the RAW methods are for the most part
applicable to wired links as well, e.g., when energy savings are
desirable and the available path diversity exceeds 1+1 linear
redundancy.
RAW extends the DetNet Stack (see fig 4 of [RFC8655]) with additional
functionality at the DetNet Service sub-layer for the PSE operation.
Layer-3 in general and DetNet in particular operates on abstractions
of the lower layers and through APIs to control those abstractions.
For instance, DetNet already leverages lower layers for time-
sensitive operations such as time synchronization and traffic
shapers. Because the performances of the radio layers are subject to
rapid changes, so RAW needs more dynamic gauges and knobs. To that
effect, the DetNet PREOF is extended with the PAREO capabilities (see
Section 5.6) and the RAW PAREO Actuator manages dynamically the PAREO
operations, which may be performed either within the DetNet sublayers
or at a lower layer, using a common radio abstraction and APIs in the
latter case. In particular, PAREO needs the capability to push
reliability and timing hints like suggest X retries (min, max) within
a time window, or send unicast (one next hop) or multicast (for
overhearing). The other way around RAW needs hints about the radio
conditions like L2 triggers (RSSI, LQI, ETX...) over all the wireless
hops. This information is useful in the controller plane for both
the PCE and PSE.
The RAW Service sub-layer also adds the OAM Propagator that
(re)generates the OAM information as it is formed and propagated In-
Band or Out-of-Band. The RAW Service sub-layer may be present in
DetNet Edge and Relay Nodes, though the PAREO Actuator has no
operation in the Egress Edge Node.
RAW also adds a Control sub-layer that operates in the DetNet
Controller Plane. The RAW Control sub-layer typically runs only in
the DetNet Ingress Edge Node or End System, though it may also run in
DetNet Relay Nodes when the RAW Control sub-layer is distributed
along the Track. The RAW Control sub-layer functionality includes
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the PSE that decides the DetNet Path for the next packets of a flows
and controls the PAREO Actuators along the DetNet Path through
specific signaling, and the OAM Supervisor that triggers, and learns
from, OAM observations, and feeds the PSE for its next decision.
+------------------------------+ +--------------------------------+
| | | |
.....................................................................
| | | |
| +----------+ +------------+ | | .-.-.-.-.-.--. .-.-.-.-.-.--. |
| | PSE | | OAM | | | | Distr. PSE | | Distr. OAM | |
| | | | Supervisor | | | | | | Supervisor | |
| +----------+ +------------+ | | .-.-.-.-.-.--. .-.-.-.-.-.--. |
| | | optional optional |
RAW Control sub-layer
.....................................................................
DetNet Service sub-layer
| | | |
| +----------+ +------------+ | | +------------+ +------------+ |
| | PAREO | | OAM | | | | PAREO | | OAM | |
| | Actuator | | Observer | | | | Actuator | | Observer | |
| +----------+ +------------+ | | +------------+ +------------+ |
| | | |
DetNet Service sub-layer
.....................................................................
DetNet Forwarding sub-layer
| | | |
| +------------+ | | +------------+ |
| |In-Situ OAM | | | |In-Situ OAM | |
| +------------+ | | +------------+ |
| | | |
+------------------------------+ +--------------------------------+
End System or Relay
Ingress Edge Node Node
Figure 3: RAW functional posture within DetNet sublayers
There are 2 main proposed models to deploy RAW and DetNet. In the
first model (strict) illustrated in Figure 4, RAW operates over a
continuous DetNet Service end-to-end between the Ingress and the
Egress Edge Nodes or End Systems.
A minimal Forwarding sub-layer service is provided at all DetNet
Nodes to ensure that the OAM information flows. Relay Nodes may or
may not support RAW services, and the Edge nodes do support RAW.
DetNet guarantees such as latency are provided end-to-end, and RAW
supports the DetNet Service to optimize the use of resources.
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--------------------Flow Direction---------------------------------->
+---------+
| RAW |
| Control |
+---------+ +---------+ +---------+
| RAW + | | RAW + | | RAW + |
| DetNet | | DetNet | | DetNet |
| Service | | Service | | Service |
+---------+---------------------------+---------+--------+---------+
| DetNet |
| Forwarding |
+------------------------------------------------------------------+
Ingress Transit Relay Egress
Edge ... Nodes ... Nodes ... Edge
Node Node
<--------------------Full Guarantees------------------------------->
Figure 4: (Strict) RAW over DetNet
In the second model (loose), illustrated in Figure 5, RAW operates
over a partial DetNet Service where typically only the Ingress and
the Egress End Systems support RAW. The DetNet Domain may extend
beyond the Ingress node, or there may be a DetNet domain starting at
an Ingress Edge Node at the first hop after the End System.
In the loose model, RAW cannot observe the hops in network, and the
path beyond the first hop is opaque; RAW can still observe the end-
to-end behavior and use Layer-3 measurements to decide whether to
replicate a packet and select the first hop interface(s).
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--------------------Flow Direction---------------------------------->
+---------+
| RAW |
| Control |
+---------+ +---------+ +---------+
| RAW + | | DetNet | | RAW + |
| DetNet | | Only | | DetNet |
| Service | | Service | | Service |
+---------+----------------------+---+ +---+---------+
| DetNet | | DetNet |
| Forwarding | | Forwarding |
+------------------------------------+ +-------------+
Ingress Transit Relay Internet Egress
End ... Nodes ... Nodes ... ... End
System System
<----------------------No Guarantee-------------------------------->
Figure 5: Loose RAW
5. The RAW Control Loop
5.1. Routing Time Scale vs. Forwarding Time Scale
With DetNet, the Controller Plane Function (CPF) handles the routing
computation and maintenance. With RAW, the CPF also performs the PSE
orientation, proposing DetNet Paths to use in response to network
events. The CPF can be can be centralized in a PCE, and can reside
outside the network. This is how the remainder of this document
depicts it, though the CPF could be implemented otherwise without
affecting the architecture. In a wireless mesh, the path to the PCE
can be expensive and slow, possibly going across the whole mesh and
back. Reaching to the PCE can also be slow in regards to the speed
of events that affect the forwarding operation at the radio layer.
In the same fashion, a distributed routing protocol may also take
time and consume excessive wireless resources to reconverge to a new
optimized state.
Due to that cost and latency, the Controller Plane is not expected to
be sensitive/reactive to transient changes. The abstraction of a
link at the routing level is expected to use statistical metrics that
aggregate the behavior of a link over long periods of time, and
represent its properties as shades of gray as opposed to numerical
values such as a link quality indicator, or a boolean value for
either up or down.
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+----------------+
| Controller |
| [PCE] |
+----------------+
^
|
Slow
|
_-._-._-._-._-._-. | ._-._-._-._-._-._-._-._-._-._-._-._-
_-._-._-._-._-._-._-. | _-._-._-._-._-._-._-._-._-._-._-._-
|
Expensive
|
.... | .......
.... . | . .......
.... v ...
.. A-------B-------C---D ..
... / \ / \ ..
. I ----M-------N--***-- E ..
.. \ / / ...
.. P--***--Q-----M---R ....
.. ....
. <----- Fast -------> ....
....... ....
.................
*** = flapping at this time
Figure 6: Time Scales
In the case of wireless, the changes that affect the forwarding
decision can happen frequently and often for short 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 that model, the
routing operation computes a complex Track that enables multiple Non-
Equal Cost Multi-Path (N-ECMP) forwarding solutions, and leaves it to
the Data Plane to make the per-packet decision of which of these
possibilities should be used.
In the wired world, and more specifically in the context of Traffic
Engineering (TE), 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
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collection of SD-WAN tunnels. RAW formalizes a forwarding time scale
that is an order(s) of magnitude shorter than the controller plane
routing 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 the scale of the packet rate, and uses
information that must be pertinent at the present time for the
current transmission(s).
5.2. A OODA Loop
OODA (Observe, Orient, Decide, Act) is a generic formalism to
represent the operational steps in a Control Loop. The RAW
Architecture applies that generic model to continuously optimize the
spectrum and energy used to forward packets within a Track,
instantiating the OODA steps as follows:
Observe: Network Plane measurement protocols for Operations,
Administration and Maintenance (OAM) to Observe some or all hops
along a Track as well as the end-to-end packet delivery, more in
Section 5.3;
Orient: Controller plane elements to report the links statistics to
a distributed or centralized control function such as a Path
Computation Element (PCE), that computes and installs the Tracks,
and provides meta data to Orient the routing decision, more in
Section 5.4;
Decide: A Runtime distributed Path Selection Engine (PSE) thar
Decides which DetNet Path to use for the next packet(s) that are
routed along the Track, more in Section 5.5;
Act: Packet (hybrid) ARQ, Replication, Elimination and Ordering
Dataplane actions are controlled from the DetNet Service sublayer
to increase the reliability of the end-to-end transmission. The
RAW architecture also covers in-situ signaling when the decision
is Acted by a node that down the Track from the PSE, more in
Section 5.6.
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+-------> Orient (PCE) --------+
| link stats, |
| pre-trained model |
| ... |
| v
Observe (OAM) Decide (PSE)
^ |
| |
| |
+-------- Act (PAREO) <--------+
At DetNet
Service sub-layer
Figure 7: The RAW OODA Loop
The overall OODA Loop optimizes the use of redundancy to achieve the
required reliability and availability Service Level Agreement (SLA)
while minimizing the use of constrained resources such as spectrum
and battery.
5.3. Observe: The RAW OAM
RAW In-situ OAM operation in the Network Plane may observe either a
full Track or DetNet Paths that are being used at this time. As
packets may be load balanced, replicated, eliminated, and / or
fragmented for Network Coding (NC) forward error correction (FEC),
the RAW In-situ operation needs to be able to signal which operation
occured to an individual packet.
Active RAW OAM may be needed to observe the unused segments and
evaluate the desirability of a rerouting decision.
Finally, the RAW Service sublayer Assurance may observe the
individual PAREO operation of a relay node to ensure that it is
conforming; this might require injecting an OAM packet at an upstream
point inside the Track and extracting that packet at another point
downstream before it reaches the egress.
This observation feeds the RAW PSE that makes the decision on which
PAREO function is actioned at which RAW Node, for one a small
continuous series of packets.
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... ..
RAN 1 ----- ... .. ...
/ . .. ....
+-------+ / . .. .... +------+
|Ingress|- . ..... |Egress|
| End |------ RAN 2 -- . Internet ....---| End |
|System |- .. ..... |System|
+-------+ \ . ...... +------+
\ ... ... .....
RAN n -------- ... .....
<------------------> <-------------------->
Observed by OAM Opaque to OAM
Figure 8: Observed Links in Radio Access Protection
In the case of a End-to-End Protection in a Wireless Mesh, the Track
is strict and congruent with the path so all links are observed.
Conversely, in the case of Radio Access Protection illustrated in
Figure 8, the Track is Loose and only the first hop is observed; the
rest of the path is abstracted and considered infinitely reliable.
The loss if a packet is attributed to the first hop Radio Access
Network (RAN), even if a particular loss effectively happens farther
down the path. In that case, RAW enables technology diversity (e.g.
Wi-Fi and 5G) which in turn improves the diversity in spectrum usage.
The Links that are not observed by OAM are opaque to it, meaning that
the OAM information is carried across and possibly echoed as data,
but there is no information capture in intermediate nodes. In the
example above, the Internet is opaque and not controlled by RAW;
still the RAW OAM measures the end-to-end latency and delivery ratio
for packets sent via each if RAN 1, RAN 2 and RAN 3, and determines
whether a packet should be sent over either or a collection of those
access links.
5.4. Orient: The Path Computation Engine
RAW separates the long time scale at which a Track is elaborated and
installed, from the short time scale at which the forwarding decision
is taken for one or a few packets (see in Section 5.1) that will
experience the same path until the network conditions evolve and
another path is selected within the same Track.
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The Track computation is out of scope, but RAW expects that the
Controller plane protocol that installs the Track also provides
related knowledge in the form of meta data about the links, segments
and possible DetNet Paths. That meta data can be a pre-digested
statistical model, and may include prediction of future flaps and
packet loss, as well as recommended actions when that happens.
The meta data may include:
* Pre-Determined DetNet Paths to match predictable error profiles
* Pre-Trained models
* Link Quality Statistics and their projected evolution
The Track is installed with measurable objectives that are computed
by the PCE to achieve the RAW SLA. The objectives can be expressed
as any of maximum number of packet lost in a row, bounded latency,
maximal jitter, maximum number of interleaved out of order packets,
average number of copies received at the elimination point, and
maximal delay between the first and the last received copy of the
same packet.
5.5. Decide: The Path Selection Engine
The RAW OODA Loop operates at the path selection time scale to
provide agility vs. the brute force approach of flooding the whole
Track. The OODA Loop controls, within the redundant solutions that
are proposed by the PCE, which will be used for each packet to
provide a Reliable and Available service while minimizing the waste
of constrained resources.
To that effect, RAW defines the Path Selection Engine (PSE) that is
the counterpart of the PCE to perform rapid local adjustments of the
forwarding tables within the diversity that the PCE has selected for
the Track. The PSE enables to exploit the richer forwarding
capabilities with PAREO and scheduled transmissions at a faster time
scale over the smaller domain that is the Track, in either a loose or
a strict fashion.
Compared to the PCE, the PSE operates on metrics that evolve faster,
but that need to be advertised at a fast rate but only locally,
within the Track. The forwarding decision may also change rapidly,
but with a scope that is also contained within the Track, with no
visibility to the other Tracks and flows in the network. This is as
opposed to the PCE that must observe the whole network and optimize
all the Tracks globally, which can only be done at a slow pace and
using long-term statistical metrics, as presented in Table 1.
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+===============+========================+===================+
| | PCE (Not in Scope) | PSE (In Scope) |
+===============+========================+===================+
| Operation | Typically Centralized | Source-Routed or |
| | | Distributed |
+---------------+------------------------+-------------------+
| Communication | Slow, expensive | Fast, local |
+---------------+------------------------+-------------------+
| Time Scale | hours and above | seconds and below |
+---------------+------------------------+-------------------+
| Network Size | Large, many Tracks to | Small, within one |
| | optimize globally | Track |
+---------------+------------------------+-------------------+
| Considered | Averaged, Statistical, | Instant values / |
| Metrics | Shade of grey | boolean condition |
+---------------+------------------------+-------------------+
Table 1: PCE vs. PSE
The PSE sits in the DetNet Service sub-Layer of Edge and Relay Nodes.
On the one hand, it operates on the packet flow, learning the Track
and path selection information from the packet, possibly making local
decision and retagging the packet to indicate so. On the other hand,
the PSE interacts with the lower layers and with its peers to obtain
up-to-date information about its radio links and the quality of the
overall Track, respectively, as illustrated in Figure 9.
|
packet | going
down the | stack
+==========v==========+=====================+=====================+
| (iOAM + iCTRL) | (L2 Triggers, DLEP) | (oOAM) |
+==========v==========+=====================+=====================+
| Learn from Learn from |
| packet tagging Maintain end-to-end |
+----------v----------+ Forwarding OAM packets |
| Forwarding decision < State +---------^-----------|
+----------v----------+ | Enrich or |
+ Retag Packet | Learn abstracted > Regenerate |
| and Forward | metrics about Links | OAM packets |
+..........v..........+..........^..........+.........^.v.........+
| Lower layers |
+..........v.....................^....................^.v.........+
frame | sent Frame | L2 Ack oOAM | | packet
over | wireless In | In | | and out
v | | v
Figure 9: PSE
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5.6. Act: DetNet Path Selection and PAREO functions
The main action by the PSE is the swapping of the DetNet Path within
the Track for the next series of packets. The candidate DetNet Paths
represent different energy and spectrum profiles, and provide
protection against different failures.
RAW also extends the DetNet protection services (typically, PREOF) to
possibly control lower layer one-hop reliability functions that are
more typical to wireless than wires, including Automatic Repeat
reQuest (ARQ), Forward Error Correction (FEC), Hybrid ARQ (HARQ) that
includes both, and other techniques such as overhearing and
constructive interferences. Because RAW may be leveraged on wired
links, e.g., to save power, it is not expected that all lower layers
support all those capabilities.
RAW manipulates abstractions of the lower layer services to hint on
the desired outcome, and the lower layer acts on those hints to
provide the best approximation of that outcome, e.g., a level of
reliability for one-hop transmission within a bounded budget of time
and/or energy. The term PAREO is coined to represent both that the
set of PREOF reliability functions is extended and the fact that some
extensions are only controlled from Layer-3 using an abstract
interface, while they are really operated at the lower layers.
The RAW Path Selection can be implemented in both centralized and
distributed scheduling approaches. In the centralized approach, the
PSE may obtain a set of pre-computed DetNet paths matching a set of
expected failures, and apply the appropriate DetNet paths for the
current state of the wireless links. In the distributed approach,
the signaling in the packet may be more abstract than an explicit
Path, and the PSE decision might be revised along the select DetNet
Path based on a better knowledge of the rest of the way.
The dynamic DetNet Path selection in RAW avoids the waste of critical
resources such as spectrum and energy while providing for the
guaranteed SLA, e.g., by rerouting and/or adding redundancy only when
a spike of loss is observed.
6. Security Considerations
RAW uses all forms of diversity including radio technology and
physical path to increase the reliability and availability in the
face of unpredictable conditions. While this is not done
specifically to defeat an attacker, the amount of diversity used in
RAW makes an attack harder to achieve.
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6.1. Layer-2 encryption
Radio networks typically encrypt at the MAC layer to protect the
transmission. If the encryption is per pair of peers, then certain
RAW operations like promiscuous overhearing become impossible.
6.2. Forced Access
RAW will typically select the cheapest collection of links that
matches the requested SLA, for instance, leverage free WI-Fi vs. paid
3GPP access. By defeating the cheap connectivity (e.g., PHY-layer
interference) the attacker can force an End System to use the paid
access and increase the cost of the transmission for the user.
7. IANA Considerations
This document has no IANA actions.
8. Contributors
The editor wishes to thank the document co-authors:
Lou Berger: Lab N
Xavi Vilajosana: Wireless Networks Research Lab, Universitat Oberta
de Catalunya
Geogios Papadopolous: IMT Atlantique
Remous-Aris Koutsiamanis: IMT Atlantique
Rex Buddenberg: Individual contributor
Greg Mirsky: ZTE
for their contributions to the text and ideas exposed in this
document.
9. Acknowledgments
This architecture could never have been completed without the support
and recommendations from the DetNet Chairs Janos Farkas and Lou
Berger. Many thanks to both.
The authors wish to thank Balazs Varga, Dave Cavalcanti, Don Fedyk,
Nicolas Montavont, and Fabrice Theoleyre for their in-depth reviews
during the development of this document.
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10. References
10.1. Normative References
[6TiSCH-ARCHI]
Thubert, P., Ed., "An Architecture for IPv6 over the Time-
Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)",
RFC 9030, DOI 10.17487/RFC9030, May 2021,
<https://www.rfc-editor.org/info/rfc9030>.
[INT-ARCHI]
Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RAW-TECHNOS]
Thubert, P., Cavalcanti, D., Vilajosana, X., Schmitt, C.,
and J. Farkas, "Reliable and Available Wireless
Technologies", Work in Progress, Internet-Draft, draft-
ietf-raw-technologies-05, 2 February 2022,
<https://www.ietf.org/archive/id/draft-ietf-raw-
technologies-05.txt>.
[RAW-USE-CASES]
Bernardos, C. J., Papadopoulos, G. Z., Thubert, P., and F.
Theoleyre, "RAW Use-Cases", Work in Progress, Internet-
Draft, draft-ietf-raw-use-cases-08, 22 October 2022,
<https://www.ietf.org/archive/id/draft-ietf-raw-use-cases-
08.txt>.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
[RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "Guidelines for the Use of the "OAM"
Acronym in the IETF", BCP 161, RFC 6291,
DOI 10.17487/RFC6291, June 2011,
<https://www.rfc-editor.org/info/rfc6291>.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with
Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
May 2016, <https://www.rfc-editor.org/info/rfc7799>.
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[RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases",
RFC 8578, DOI 10.17487/RFC8578, May 2019,
<https://www.rfc-editor.org/info/rfc8578>.
[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8557] Finn, N. and P. Thubert, "Deterministic Networking Problem
Statement", RFC 8557, DOI 10.17487/RFC8557, May 2019,
<https://www.rfc-editor.org/info/rfc8557>.
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
[RFC8939] Varga, B., Ed., Farkas, J., Berger, L., Fedyk, D., and S.
Bryant, "Deterministic Networking (DetNet) Data Plane:
IP", RFC 8939, DOI 10.17487/RFC8939, November 2020,
<https://www.rfc-editor.org/info/rfc8939>.
[RFC9049] Dawkins, S., Ed., "Path Aware Networking: Obstacles to
Deployment (A Bestiary of Roads Not Taken)", RFC 9049,
DOI 10.17487/RFC9049, June 2021,
<https://www.rfc-editor.org/info/rfc9049>.
10.2. Informative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[TE] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X.
Xiao, "Overview and Principles of Internet Traffic
Engineering", RFC 3272, DOI 10.17487/RFC3272, May 2002,
<https://www.rfc-editor.org/info/rfc3272>.
[STD 62] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
DOI 10.17487/RFC3411, December 2002,
<https://www.rfc-editor.org/info/rfc3411>.
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[RFC4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
DOI 10.17487/RFC4090, May 2005,
<https://www.rfc-editor.org/info/rfc4090>.
[FRR] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.
[RLFA-FRR] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<https://www.rfc-editor.org/info/rfc7490>.
[DetNet-DP]
Varga, B., Ed., Farkas, J., Berger, L., Malis, A., and S.
Bryant, "Deterministic Networking (DetNet) Data Plane
Framework", RFC 8938, DOI 10.17487/RFC8938, November 2020,
<https://www.rfc-editor.org/info/rfc8938>.
[DLEP] 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>.
[I-D.irtf-panrg-path-properties]
Enghardt, R. and C. Krähenbühl, "A Vocabulary of Path
Properties", Work in Progress, Internet-Draft, draft-irtf-
panrg-path-properties-06, 22 September 2022,
<https://www.ietf.org/archive/id/draft-irtf-panrg-path-
properties-06.txt>.
[IPoWIRELESS]
Thubert, P., "IPv6 Neighbor Discovery on Wireless
Networks", Work in Progress, Internet-Draft, draft-
thubert-6man-ipv6-over-wireless-12, 11 October 2022,
<https://www.ietf.org/archive/id/draft-thubert-6man-ipv6-
over-wireless-12.txt>.
[DetNet-OAM]
Mirsky, G., Theoleyre, F., Papadopoulos, G. Z., Bernardos,
C. J., Varga, B., and J. Farkas, "Framework of Operations,
Administration and Maintenance (OAM) for Deterministic
Networking (DetNet)", Work in Progress, Internet-Draft,
draft-ietf-detnet-oam-framework-07, 6 October 2022,
<https://www.ietf.org/archive/id/draft-ietf-detnet-oam-
framework-07.txt>.
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[NASA] Adams, T., "RELIABILITY: Definition & Quantitative
Illustration", <https://kscddms.ksc.nasa.gov/Reliability/
Documents/150814-3bWhatIsReliability.pdf>.
Author's Address
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
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