Internet DRAFT - draft-pthubert-raw-architecture
draft-pthubert-raw-architecture
RAW P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Informational G.Z. Papadopoulos
Expires: 8 January 2022 IMT Atlantique
L. Berger
LabN Consulting, L.L.C.
7 July 2021
Reliable and Available Wireless Architecture/Framework
draft-pthubert-raw-architecture-09
Abstract
Reliable and Available Wireless (RAW) provides for high reliability
and availability for IP connectivity over a wireless medium. The
wireless medium presents significant challenges to achieve
deterministic properties such as low packet error rate, bounded
consecutive losses, and bounded latency. This document defines the
RAW Architecture. It builds on the DetNet Architecture and discusses
specific challenges and technology considerations needed to deliver
DetNet service utilizing scheduled wireless segments and other media,
e.g., frequency/time-sharing physical media resources with stochastic
traffic.
Status of This Memo
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This Internet-Draft will expire on 8 January 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The RAW problem . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Reliability and Availability . . . . . . . . . . . . . . 7
2.2.1. High Availability Engineering Principles . . . . . . 8
2.2.2. Applying Reliability Concepts to Networking . . . . . 10
2.2.3. Reliability in the Context of RAW . . . . . . . . . . 11
2.3. Use Cases and Requirements Served . . . . . . . . . . . . 12
2.3.1. Radio Access Protection . . . . . . . . . . . . . . . 13
2.3.2. End-to-End Protection in a Wireless Mesh . . . . . . 13
2.4. Related Work at The IETF . . . . . . . . . . . . . . . . 14
3. The RAW Framework . . . . . . . . . . . . . . . . . . . . . . 15
3.1. Scope and Prerequisites . . . . . . . . . . . . . . . . . 15
3.2. Routing Time Scale vs. Forwarding Time Scale . . . . . . 16
3.3. Wireless Tracks . . . . . . . . . . . . . . . . . . . . . 17
3.4. PAREO Functions . . . . . . . . . . . . . . . . . . . . . 18
3.4.1. Packet Replication . . . . . . . . . . . . . . . . . 19
3.4.2. Packet Elimination . . . . . . . . . . . . . . . . . 20
3.4.3. Promiscuous Overhearing . . . . . . . . . . . . . . . 20
3.4.4. Constructive Interference . . . . . . . . . . . . . . 20
4. The RAW Architecture . . . . . . . . . . . . . . . . . . . . 21
4.1. The RAW Conceptual Model . . . . . . . . . . . . . . . . 21
4.2. The Path Selection Engine . . . . . . . . . . . . . . . . 23
4.3. RAW OAM . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3.1. DetNet OAM . . . . . . . . . . . . . . . . . . . . . 25
4.3.2. RAW Extensions . . . . . . . . . . . . . . . . . . . 26
4.3.3. Observed Metrics . . . . . . . . . . . . . . . . . . 27
4.4. Flow Identification vs. Path Identification . . . . . . . 27
4.5. Source-Routed vs. Distributed Forwarding Decision . . . . 30
4.6. Encapsulation and Decapsulation . . . . . . . . . . . . . 31
5. Security Considerations . . . . . . . . . . . . . . . . . . . 31
5.1. Forced Access . . . . . . . . . . . . . . . . . . . . . . 31
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 31
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . 32
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9.2. Informative References . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
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, enabling cost and performance optimizations
(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
budgetted 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.
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 IP [IPoWIRELESS]. Wireless networks operate on a
shared medium where uncontrolled interference, including the self-
induced multipath fading, cause random transmission losses and add
new dimensions to the statistical effects that affect reachability
and packet delivery.
To defeat those additional causes of transmission delay and loss,
Reliable and Available Wireless (RAW) leverages scheduled
transmissions with redundancy and diversity in the spatial, time,
code, and frequency domains. The challenge is to provide enough
diversity and redundancy to ensure the timely packet delivery while
preserving energy and optimizing the use of the shared spectrum.
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.
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Uncontrolled interference and transmission obstacles may impede the
wireless transmission, causing rapid variations of the throughput and
packet delivery ratio (PDR) of the link. This uncertainty limits the
volume and/or duration of traffic that can be safely transmitted on
the same link while conforming to a RAW Service Level Agreement
(SLA).
This increased complexity explains why the development of
deterministic wireless technologies has been lagging behind the
similar efforts for wired systems, both at the IEEE and the IETF.
But recent progress on scheduled radios such as TSCH and OFDMA
indicates that wireless is finally catching up at the lower layers.
Sitting at the layer above, RAW takes up the challenge of providing
highly available and reliable end-to-end performances in a network
with scheduled wireless segments.
RAW provides 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.
The "Deterministic Networking Architecture" [RFC8655] is composed of
three planes: the Application (User) Plane, the Controller Plane, and
the Network Plane. The RAW Architecture extends the DetNet Network
Plane, to accomodate one or multiple hops of homogeneous or
heterogeneous wireless technologies, e.g. a Wi-Fi6 Mesh or parallel
CBRS access links federated by a 5G backhaul.
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 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 operates within the Network Plane at the forwarding
time scale on one DetNet flow over a complex path called a Track.
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 covers Network Plane protocol elements such as
Operations, Administration and Maintenance (OAM) to observe some or
all hops along a Track as well as the end-to-end packet delivery, and
in-band control to optimize the use of redundancy to achieve the
required SLA with minimal use of constrained resources.
2. The RAW problem
2.1. 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 as a complex path with associated PAREO
operations. The concept is abstract to the underlaying technology
and applies to any fully or partially wireless mesh, including, e.g.,
a Wi-Fi 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:
PAREO: Packet (hybrid) ARQ, Replication, Elimination and Ordering.
PAREO is a superset Of DetNet's PREOF that includes radio-specific
techniques such as short range broadcast, MUMIMO, constructive
interference and overhearing, which can be leveraged separately or
combined to increase the reliability.
Flow: A collection of consecutive packets that 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 subTrack that is selected for the flow may
change over time under the control of the PSE.
Track: A networking graph that can be used as a "path" to transport
RAW packets with equivalent treatment; as opposed to the usual
understanding of a path (see for instance the definition of "path"
in section 1.1 of [RFC9049]), a Track may fork and rejoin to
enable the PAREO operations.
In DetNet [RFC8655] terms, a Track has the following properties:
* A Track has one Ingress and one Egress nodes, which operate as
DetNet Edge nodes.
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* 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 the Track are DetNet Relay nodes that operate
at the DetNet Service sublayer and provide the PAREO functions.
* The topological edges of the graph are serial sequences of
DetNet Transit nodes that operate at the DetNet Forwarding
sublayer.
SubTrack: A Track within a Track. The RAW PSE selects a subTrack on
a per-packet or a per-collection of packets basis to provide the
desired reliability for the transported flows.
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.
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.
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.
Active OAM: See [RFC7799]. In the context of RAXW, Active OAM is
used to observe a particular Track, subTrack, or Segment of a
Track regardless of whether it is used for traffic at that time.
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.
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.
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Limited OAM: An active OAM packet is a Limited OAM packet when it
observes the RAW operation over a node, a segment, or a subTrack
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 layer replication point) that is being tested.
Reverse OAM: A Reverse 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
subTrack, or Segment of a Track.
[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.
In the context of the RAW work, Reliability and Availability are
defined as follows:
Reliability: Reliability is a measure of the probability that an
item will perform its intended function for a specified interval
under stated conditions. For RAW, the service that is expected is
delivery within a bounded latency and a failure is when the packet
is either lost or delivered too late. RAW expresses reliability
in terms of Mean Time Between Failure (MTBF) and Maximum
Consecutive Failures (MCF). More in [NASA].
Availability: Availability is a measure of the relative amount of
time where a path operates in stated condition, in other words
(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 path that is a lot more complex than what
DetNet typically envisages (a Track).
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.
2.2. Reliability and Availability
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2.2.1. High Availability Engineering Principles
The reliability criteria of a critical system pervade 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.
2.2.1.1. 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 in
the transmissions over the air to combat the additional causes of
transmission loss.
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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 parcimony.
2.2.1.2. 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].
Deterministic flows, on the contrary, are attached to specific paths
where dedicated resources are reserved for each flow. This is why
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.
2.2.1.3. 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.
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"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. But RAW itself operates in the Network Plane at a
faster time scale. To act on the Data Plane, RAW needs live
information from the Operational Plane , e.g., using Bidirectional
Forwarding Detection [BFD] and its variants (bidirectional and remote
BFD) to protect a link, and OAM techniques to protect a path.
2.2.2. Applying Reliability Concepts to Networking
The terms Reliability and Availability are defined for use in RAW in
Section 2.1 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
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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].
2.2.3. Reliability in the Context of RAW
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
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 the something moves that 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.
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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 maximised to optimize the
PDR.
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.
2.3. Use Cases and Requirements Served
In order to focus on real-worlds issues and assert the feasibility of
the proposed capabilities, RAW focuses 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.
"Deterministic Networking Use Cases" [RFC8578] presents a number of
wireless use cases including Wireless, such as application to
Industrial Applications, Pro-Audio, and SmartGrid Automation.
[RAW-USE-CASES] adds a number of use cases that demonstrate the need
for RAW capabilities for new applications such as Pro-Gaming and
drones. The use cases can be abstracted in two families, Loose
Protection, e.g., protecting the first hop in Radio Access Protection
and Strict Protection, e.g., providing End-to-End Protection in a
wireless mesh.
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2.3.1. Radio Access Protection
To maintain the required SLA at all times, a wireless Host may use
more than one Radio Access Network (RAN) in parallel.
... ..
RAN 1 ----- ... .. ...
/ . .. ....
+--------+ / . .... +-----------+
|Wireless|- . ..... | Service |
| Device |-***-- RAN 2 -- . Internet ....---| / |
|(STA/UE)|- .. ..... |Application|
+--------+ $$$ . ....... +-----------+
\ ... ... .....
RAN n -------- ... .....
*** = flapping at this time $$$ expensive
Figure 1: Radio Access Protection
The RANs may be heterogeneous, e.g., 3GPP 5G [RAW-5G] and Wi-Fi
[RAW-TECHNOS] for high-speed communication, in which case a Layer-3
abstraction becomes useful to select which of the RANs are used at a
particular point of time, and the amount of traffic that is
distributed over each RAN.
The idea is that the rest of the path to the destination(s) is
protected separately (e.g., uses non-congruent paths, leverages
DetNet / TSN, etc...) and is a lot more reliable, e.g., wired. In
that case, RAW observes the reliability of the end-to-end operation
through each of the RANs but only observes and controls the wireless
operation the first hop.
A variation of that use case has a pair of wireless Hosts connected
over a wired core / backbone network. In that case, RAW observes and
controls the Ingress and Egress RANs, while neglecting the hops in
the core. The resulting loose Track may be instantiated, e.g., using
tunneling or loose source routing between the RANs.
2.3.2. End-to-End Protection in a Wireless Mesh
In radio technologies that support mesh networking (e.g., Wi-Fi and
TSCH), a Track is a complex path with distributed PAREO capabilities.
In that case, RAW operates through the multipath and makes decisions
either at the Ingress or at every hop (more in Section 3.3).
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A-------B-------C-----D
/ \ / / \
Ingress ----M-------N--zzzzz--- Egress
\ \ / /
P--zzz--Q-------------R
zzz = flapping now
Figure 2: End-to-End Protection
The Protection may be imposed by the source based on end-to-end OAM,
or performed hop-by-hop, in which case the OAM must enables the
intermediate Nodes to estimate the quality of the rest of the
feasible paths in the remainder of the Track to the destination.
2.4. Related Work at The IETF
RAW intersects with protocols or practices in development at the IETF
as follows:
* 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.
* [detnet] provides an OAM framework with [DetNet-OAM] that applies
within the DetNet dataplane described in [DetNet-DP],which is
typically based on MPLS or IPv6 pseudowires.
* [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 asynchronous
mode considers delivery as success whereas with DetNet and RAW,
the bounded latency can be as important as the delivery itself,
and delivering too late is actually a failure. Note that the BFD
Demand mode with unsolicited notifications may be more suitable
then the Asynchronous BFD mode. The use of the Demand mode in
MPLS is analyzed in [I-D.mirsky-bfd-mpls-demand] and similar
considerations could apply to IP as well.
* [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 meet the required SLA.
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* [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.
* [IPPM] develops and maintains standard metrics that can be applied
to the quality, performance, and reliability of Internet data
delivery services and applications running over transport layer
protocols (e.g. TCP, UDP) over IP.
3. The RAW Framework
3.1. Scope and Prerequisites
A prerequisite to the RAW operation 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. The concept of
Track is specified in the 6TiSCH Architecture [6TiSCH-ARCHI] to
represent that complex sub-topology. Tracks provide a high degree of
redundancy and diversity and enable the DetNet PREOF, network coding,
and possibly RAW specific techniques such as PAREO, leveraging
frequency diversity, time diversity, and possibly other forms of
diversity as well.
How the routing operation (e.g., PCE) in the Controller Plane
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 SLA 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 the Network
Layer forwarding decision (think DLEP from MANET).
How the radio interface manages its lower layers is out of control
and out of scope for RAW. In the same fashion, the non-RAW portion
along a loose Track is by definition out of control and out of scope
for RAW. Whether it is a single hop or a mesh is also unknown and
out of scope.
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3.2. Routing Time Scale vs. Forwarding Time Scale
With DetNet, the Controller Plane Function that handles the routing
computation and maintenance (the PCE) can be centralized and can
reside outside the network. 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.
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.
+----------------+
| Controller |
| [PCE] |
+----------------+
^
|
Slow
|
_-._-._-._-._-._-. | ._-._-._-._-._-._-._-._-._-._-._-._-
_-._-._-._-._-._-._-. | _-._-._-._-._-._-._-._-._-._-._-._-
|
Expensive
|
.... | .......
.... . | . .......
.... v ...
.. A-------B-------C---D ..
... / \ / / \ ..
. I ----M-------N--***-- E ..
.. \ \ / / ...
.. P--***--Q----------R ....
.. ....
. <----- Fast -------> ....
....... ....
.................
*** = flapping at this time
Figure 3: Time Scales
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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
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).
3.3. Wireless Tracks
The "6TiSCH Architecture" [6TiSCH-ARCHI] introduces the concept of
Track. RAW extends the concept to any wireless mesh technology,
including, e.g., Wi-Fi. A simple Track is composed of a direct
sequence of reserved hops to ensure the transmission of a single
packet from a source Node to a destination Node across a multihop
path.
A Complex Track provides multiple N-ECMP forwarding solutions. The
Complex Track enables to support multi-path redundant forwarding by
employing PRE functions [RFC8655] and the ingress and within the
Track. For example, a Complex Track may branch off and rejoin over
non-congruent segments.
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In the context of RAW, some links or segments in the Track may be
reversible, meaning that they can be used in either direction. In
that case, an indication in the packet signals the direction of the
reversible links or segments that the packet traverses and thus
places a constraint that prevents loops from occuring. An indidual
packet follows a destination-oriented directed acyclic graph (DODAG)
towards a destination Node inside the Complex Track.
3.4. PAREO Functions
RAW may control whether and how to use packet replication and
elimination (PRE), Automatic Repeat reQuest (ARQ), Hybrid ARQ (HARQ)
that includes Forward Error Correction (FEC) and coding, and other
wireless-specific techniques such as overhearing and constructive
interferences, in order to increase the reliabiility and availability
of the end-to-end transmission.
Collectively, those function are called PAREO for Packet (hybrid)
ARQ, Replication, Elimination and Ordering. By tuning dynamically
the use of PAREO functions, RAW avoids the waste of critical
resources such as spectrum and energy while providing that the
guaranteed SLA, e.g., by adding redundancy only when a spike of loss
is observed.
In a nutshell, PAREO establishes several paths in a network to
provide redundancy and parallel transmissions to bound the end-to-end
delay to traverse the network. Optionally, promiscuous listening
between paths is possible, such that the Nodes on one path may
overhear transmissions along the other path. Considering the
scenario shown in Figure 4, many different paths are possible for to
traverse the network from ingress to egress. A simple way to benefit
from this topology could be to use the two independent paths via
Nodes A, C, E and via B, D, F. But more complex paths are possible
by interleaving transmissions from the lower level of the path to the
upper level.
(A) -- (C) -- (E)
/ \
Ingress = | | | = Egress
\ /
(B) -- (D) -- (F)
Figure 4: A Ladder Shape with Two Parallel Paths
PAREO may also take advantage of the shared properties of the
wireless medium to compensate for the potential loss that is incurred
with radio transmissions.
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For instance, when the source sends to Node A, Node B may listen
promiscuously and get a second chance to receive the frame without an
additional transmission. Note that B would not have to listen if it
already received that particular frame at an earlier timeslot in a
dedicated transmission towards B.
The PAREO model can be implemented in both centralized and
distributed scheduling approaches. In the centralized approach, a
Path Computation Element (PCE) scheduler calculates a Track and
schedules the communication. In the distributed approach, the Track
is computed within the network, and signaled in the packets, e.g.,
using BIER-TE, Segment Routing, or a Source Routing Header.
3.4.1. Packet Replication
By employing a Packet Replication procedure, a Node forwards a copy
of each data packet to more than one successor. To do so, each Node
(i.e., Ingress and intermediate Node) sends the data packet multiple
times as separate unicast transmissions. For instance, in Figure 5,
the Ingress Node is transmitting the packet to both successors, nodes
A and B, at two different times.
===> (A) => (C) => (E) ===
// \\// \\// \\
Ingress //\\ //\\ Egress
\\ // \\ // \\ //
===> (B) => (D) => (F) ===
Figure 5: Packet Replication
An example schedule is shown in Table 1. This way, the transmission
leverages with the time and spatial forms of diversity.
+=========+======+======+======+======+======+======+======+
| Channel | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
+=========+======+======+======+======+======+======+======+
| 0 | S->A | S->B | B->C | B->D | C->F | E->R | F->R |
+---------+------+------+------+------+------+------+------+
| 1 | | A->C | A->D | C->E | D->E | D->F | |
+---------+------+------+------+------+------+------+------+
Table 1: Packet Replication: Sample schedule
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3.4.2. Packet Elimination
The replication operation increases the traffic load in the network,
due to packet duplications. This may occur at several stages inside
the Track, and to avoid an explosion of the number of copies, a
Packet Elimination procedure must be applied as well. To this aim,
once a Node receives the first copy of a data packet, it discards the
subsequent copies.
The logical functions of Replication and Elimination may be
collocated in an intermediate Node, the Node first eliminating the
redundant copies and then sending the packet exactly once to each of
the selected successors.
3.4.3. Promiscuous Overhearing
Considering that the wireless medium is broadcast by nature, any
neighbor of a transmitter may overhear a transmission. By employing
the Promiscuous Overhearing operation, the next hops have additional
opportunities to capture the data packets. In Figure 6, when Node A
is transmitting to its DP (Node C), the AP (Node D) and its sibling
(Node B) may decode this data packet as well. As a result, by
employing corellated paths, a Node may have multiple opportunities to
receive a given data packet.
===> (A) ====> (C) ====> (E) ====
// ^ | \\ \\
Ingress | | \\ Egress
\\ | v \\ //
===> (B) ====> (D) ====> (F) ====
Figure 6: Unicast with Overhearing
3.4.4. Constructive Interference
Constructive Interference can be seen as the reverse of Promiscuous
Overhearing, and refers to the case where two senders transmit the
exact same signal in a fashion that the emitted symbols add up at the
receiver and permit a reception that would not be possible with a
single sender at the same PHY mode and the same power level.
Constructive Interference was proposed on 5G, Wi-Fi7 and even tested
on IEEE Std 802.14.5. The hard piece is to synchronize the senders
to the point that the signals are emitted at slightly different time
to offset the difference of propagation delay that corresponds to the
difference of distance of the transmitters to the receiver at the
speed of light to the point that the symbols are superposed long
enough to be recognizable.
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4. The RAW Architecture
4.1. 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.
A RAW Network Plane may be strict or loose, 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 control 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
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) called the Path Computation Element
(PCE) [RFC4655] 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.
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 actionned in the Network Plane. The Track may be
expressed loosely to enable traversing a non-RAW subnetwork. In that
case, the expectation is that the non-RAW subnetwork can be neglected
in the RAW computation, that is, considered infinitely fast, reliable
and/or available in comparison with the links between RAW nodes.
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CPF CPF CPF CPF
Southbound API
_-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
_-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
RAW --z RAW --z RAW --z RAW
z-- Node z-- Node z-- Node z-- Node --z
Ingress --z / / z-- Egress
End \ \ .. . End
Node ---z / / .. .. . z-- Node
z-- RAW --z RAW ( non-RAW ) -- RAW --z
Node z-- Node --- ( Nodes ) Node
... .
--z wireless wired
z-- link --- link
Figure 7: 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
PHY mode and the number of users 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.
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4.2. The Path Selection Engine
RAW separates the path computation time scale at which a complex path
is recomputed from the path selection time scale at which the
forwarding decision is taken for one or a few packets (more in
Section 3.2). RAW operates at the path selection time scale. The
RAW problem is to decide, 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 counter-part 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 needs to be advertised at a fast rate but only locally,
within the Track. The forwarding decision may also change rapidly,
but wiht 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 needs to 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 2.
+===============+========================+===================+
| | PCE (Not in Scope) | PSE (In Scope) |
+===============+========================+===================+
| Operation | 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 2: PCE vs. PSE
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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 8.
|
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 8: PSE
4.3. RAW OAM
RAW In-situ OAM operation in the Network Plane may observe either a
full Track or subTracks that are being used at this time. Active RAW
OAM may be needed to observe the unused segments and evaluate the
desirability of a rerouting decision. Finally, the RAW Service Layer
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 in 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 9: 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, the Track is
Loose and in that case only the first hop is observed; the rest of
the path is abstracted and considered infinitely reliable.
In the case of the Radio Access Protection, only the first hop is
protected; the loss of a packet that was sent over one of the
possible first hops is attributed to that first hop, even if a
particular loss effectively happens farther down the path.
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.
4.3.1. DetNet OAM
[detnet] provides an OAM framework with [DetNet-OAM] that applies
within the DetNet dataplane described in [DetNet-DP],which is
typically based on MPLS or IPv6 pseudowires. How the framework
applies to IPv6 is detailed in [DetNet-IP-OAM]. Within that
framework, OAM messages follow the same forward path as the data
packets and gather information about their individual treatment at
each hop. When the destination receives an OAM message, it gets a
view on the full path or at least of a segment of the path from the
source of the flow.
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In-situ OAM (IOAM) adds telemetry information about the experience of
one packet within the packet itself [I-D.ietf-ippm-ioam-data], with
the caveats that the measurement and the consecutive update of the
packet interfere with the operation being observed, e.g., may
increase the latency of the packet for which it is measured and into
which it is stamped.
Note: IOAM and analogous on-path telemetry methods are capable of
facilitating collection of useful telemetry information that
characterizes the state of a system as experienced by the packet.
But because of statistical character of a packet network, these
methods may not be used to monitor the continuity of a path (Track)
or proper connectivity of the Track (no leaking packets across
Tracks).
This effect can be alleviated by measuring on the fly but reporting
later, e.g., by exporting the data as a separate management packet
[I-D.ietf-ippm-ioam-direct-export].
[I-D.mirsky-ippm-hybrid-two-step] proposes an hybrid two-steps method
(HTS) where a trigger message starts the measurement and a follow up
along the Track packet gathers the measured data.
"Error Performance Measurement" [I-D.mirsky-ippm-epm] uses Fault
Management (FM) and Performance Management (PM) OAM mechanisms to
determine availability/unavailability of a path according to
predefined SLA.
4.3.2. RAW Extensions
Classical OAM typically measures information at the transmitter,
e.g., residence time in the node or transmit queue size. With RAW,
there is a need to combine information at the sender (number of
retries) with that at the receiver (LQI, RSSI). This doubles the
operating cost of an IAOM processing that would gather the experience
of a single packet.
The RAW PSE may be centralized at the Track Ingress, or distributed
long the Track. Either way, the PSE needs instant information about
the rest of the way to the destination over the possible next-hop
adjacencies along the Track in order to decide how to perform simple
forwarding, load balancing, and/or replication, as well as
determining how much latency credit is available for ARQ.
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To provide that information timely, it makes sense that the OAM
packets that gather instantaneous values from the radio senders and
receivers at each hop flow on the reverse path and inform the PSE at
the source and/or the PAREO relays about the state of the rest of the
way. This is achieved using Reverse OAM packets that flow along the
Reversed Track, West to East.
Because the quality of transmission over a wireless medium varies
continuously, it is important that RAW OAM captures the state of the
medium across an adjacency over multiple transmission and over a
recent period of time, whether the transmitted packets belong to this
flow or another. Some of the measured information relates to the
medium itself. In other words, the captured information does not
only relate to the experience of one packet as is the case for IOAM,
but also to the medium itself. This makes an approach like HTS more
suitable as it can trigger the capture of multiple measurements over
a short period of time. On the other hand, the PSE needs a
continuous measurement stream where a single trigger is followed by a
periodic follow up capture.
In other words, the best suited OAM method to enable the PSE make
accurate PAREO forwarding decisions is a periodic variation of the
two-steps method flowing along the reverse Track, as a Reverse OAM
technique. [RAW-OAM] provides more information on the RAW OAM
problem and solution approaches.
4.3.3. Observed Metrics
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.
Those lower-layer metrics are aggregated along a multihop segment
into abstract layer 3 information that reflect the instant
reliability and latency of the observed path.
4.4. Flow Identification vs. Path Identification
Section 4.7 of the DetNet Architecture [RFC8655] ties the app-flow
identification which is an appliation layer concept with the network
path identification that depends on the networking technology by
"exporting of flow identification", e.g., to a MPLS label.
With RAW, this exporting operation is injective but not bijective.
e.g., a flow is fully placed within one RAW Track, but not all
packets along that Track are necessarily part of the same flow. For
instance, out-of-band OAM packets must circulate in the exact same
fashion as the flows that they observe. It results that the flow
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identification that maps to to app-flow at the network layer must be
separate from the path identification that is used to forward a
packet.
Section 3.4 of the DetNet data-plane framework [DetNet-DP] indicates
that for a DetNet IP Data Plane, a flow is identified by an IPv6
6-tuple. With RAW, that 6-tuple is not what indicates the Track, in
other words, the flow ID is not the Track ID.
For instance, the 6TiSCH Architecture [6TiSCH-ARCHI] uses a
combination of the address of the Egress End System and an instance
identifier in a Hop-by-hop option to indicate a Track. This way, if
a packet "escapes" the Track, it will reach the Track Egress point
through normal routing and be treated at the service layer through,
say, elimination and reordering.
The RAW service includes forwarding over a subset of the Links that
form the Track (a subTrack). Packets from the same or a different
flow that are routed through the same Track will not necessarily
traverse the same Links. The PSE selects a subTrack for a packet
based on the links that are preferred and those that should be
avoided at this time.
Each packet is forwarded within the subTrack that provides the best
adequation with the SLA of the flow and the energy and bandwidth
constraints of the network.
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Flow 1 (6-tuple) ----+
|
Flow 2 (6-tuple) ---+ |
| |
OAM -----------+ | |
| | |
| | |
| | | | |
| v v v |
| |
+---------+---------+
|
|
Track i (Ingress IP Address, RPLinstanceId)
|
|
|
+---------+-----+--....-------+
| | |
| | |
subTrack 1 subTrack 2 subTrack n
| | |
| | |
V V V
+-----------------------------------+
| |
| Destination |
| |
+-----------------------------------+
Figure 10: Flow Injection
With 6TiSCH, packets are tagged with the same (destination address,
instance ID) will experience the same RAW service regardless of the
IPv6 6-tuple that indicates the flow. The forwarding does not depend
on whether the packets transport application flows or OAM. In the
generic case, the Track or the subTrack can be signaled in the packet
through other means, e.g., encoded in the suffix of the destination
address as a Segment Routing Service Instruction [SR-ARCHI], or
leveraging Bit Index Explicit Replication [BIER] Traffic Engineering
[BIER-TE].
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4.5. Source-Routed vs. Distributed Forwarding Decision
Within a large routed topology, the route-over mesh operation builds
a particular complex Track with one source and one or more
destinations; within the Track, packets may follow 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.
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 Data 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 considers existing tools such as L2-triggers,
DLEP, BFD and leverages in-band and out-of-band OAM to capture and
report that information to the PSE.
One possible way of making the RAW forwarding decisions within a
Track is to position a unique PSE at the Ingress and express its
decision in-band in the packet, which requires the explicit signaling
of the subTrack within the Track. In that case, the RAW forwarding
operation along the Track is encoded by the source, e.g., by
indicating the subTrack in the Segment Routing (SRv6) Service
Instruction, or by leveraging BIER-TE such as done with [BIER-PREF].
The alternate way is to operate the PSE in each forwarding Node,
which makes 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 based on OAM. Information about
the desired service should be placed in the packet and matched with
the forwarding Node's capabilities and policies.
In either case, a per-track/subTrack state is installed in all the
intermediate Nodes to recognize the packets that are following a
Track and determine the forwarding operation to be applied.
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4.6. Encapsulation and Decapsulation
In the generic case where the Track Ingress Node is not the source of
the Packet, the Ingress Node needs to encapsulate IP-in-IP to ensure
that the Destination IP Address is that of the Egress Node and that
the necessary Headers (Routing Header, Segment Routing Header and/or
Hop-By-Hop Header) can be added to the packet to signal the Track or
the subTrack, conforming [IPv6] that discourages the insertion of a
Header on the fly.
In the specific case where the Ingress Node is the source of the
packet, the encapsulation can be avoided, provided that the source
adds the necessary headers and that the destination is set to the
Egress Node. Forwarding to a final destination beyond the Egress
Node is possible, e.g., with a Segment Routing Header that signals
the rest of the way. In that case a Hop-by-Hop Header is not
recommmended since its validity is within the Track only.
5. 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.
5.1. 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.
6. IANA Considerations
This document has no IANA actions.
7. Contributors
The editor wishes to thank:
Xavi Vilajosana: Wireless Networks Research Lab, Universitat Oberta
de Catalunya
Remous-Aris Koutsiamanis: IMT Atlantique
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Nicolas Montavont: IMT Atlantique
Rex Buddenberg: Individual contributor
Greg Mirsky: ZTE
for their contributions to the text and ideas exposed in this
document.
8. Acknowledgments
TBD
9. References
9.1. 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>.
[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-01, 19 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-raw-
technologies-01>.
[RAW-USE-CASES]
Papadopoulos, G. Z., Thubert, P., Theoleyre, F., and C. J.
Bernardos, "RAW use cases", Work in Progress, Internet-
Draft, draft-ietf-raw-use-cases-01, 21 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-raw-use-
cases-01>.
[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>.
[BFD] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
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[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>.
[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>.
[SR-ARCHI] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[BIER] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[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>.
[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>.
[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>.
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9.2. 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>.
[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>.
[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://datatracker.ietf.org/doc/html/draft-
thubert-bier-replication-elimination-03>.
[DetNet-IP-OAM]
Mirsky, G., Chen, M., and D. Black, "Operations,
Administration and Maintenance (OAM) for Deterministic
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Networks (DetNet) with IP Data Plane", Work in Progress,
Internet-Draft, draft-ietf-detnet-ip-oam-02, 30 March
2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
detnet-ip-oam-02>.
[RAW-5G] Farkas, J., Dudda, T., Shapin, A., and S. Sandberg, "5G -
Ultra-Reliable Wireless Technology with Low Latency", Work
in Progress, Internet-Draft, draft-farkas-raw-5g-00, 1
April 2020, <https://datatracker.ietf.org/doc/html/draft-
farkas-raw-5g-00>.
[BIER-TE] Eckert, T., Cauchie, G., and M. Menth, "Tree Engineering
for Bit Index Explicit Replication (BIER-TE)", Work in
Progress, Internet-Draft, draft-ietf-bier-te-arch-09, 30
October 2020, <https://datatracker.ietf.org/doc/html/
draft-ietf-bier-te-arch-09>.
[IPoWIRELESS]
Thubert, P., "IPv6 Neighbor Discovery on Wireless
Networks", Work in Progress, Internet-Draft, draft-
thubert-6man-ipv6-over-wireless-09, 17 May 2021,
<https://datatracker.ietf.org/doc/html/draft-thubert-6man-
ipv6-over-wireless-09>.
[RAW-OAM] Theoleyre, F., Papadopoulos, G. Z., Mirsky, G., and C. J.
Bernardos, "Operations, Administration and Maintenance
(OAM) features for RAW", Work in Progress, Internet-Draft,
draft-ietf-raw-oam-support-02, 3 June 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-raw-oam-
support-02>.
[I-D.ietf-ippm-ioam-direct-export]
Song, H., Gafni, B., Zhou, T., Li, Z., Brockners, F.,
Bhandari, S., Sivakolundu, R., and T. Mizrahi, "In-situ
OAM Direct Exporting", Work in Progress, Internet-Draft,
draft-ietf-ippm-ioam-direct-export-03, 17 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-ippm-
ioam-direct-export-03>.
[DetNet-OAM]
Mirsky, G., Theoleyre, F., Papadopoulos, G. Z., and C. J.
Bernardos, "Framework of Operations, Administration and
Maintenance (OAM) for Deterministic Networking (DetNet)",
Work in Progress, Internet-Draft, draft-ietf-detnet-oam-
framework-01, 19 May 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-detnet-
oam-framework-01>.
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[I-D.mirsky-ippm-hybrid-two-step]
Mirsky, G., Lingqiang, W., Zhui, G., and H. Song, "Hybrid
Two-Step Performance Measurement Method", Work in
Progress, Internet-Draft, draft-mirsky-ippm-hybrid-two-
step-09, 30 March 2021,
<https://datatracker.ietf.org/doc/html/draft-mirsky-ippm-
hybrid-two-step-09>.
[I-D.mirsky-ippm-epm]
Mirsky, G., Min, X., and L. Han, "Error Performance
Measurement in Packet-switched Networks", Work in
Progress, Internet-Draft, draft-mirsky-ippm-epm-03, 26
March 2021, <https://datatracker.ietf.org/doc/html/draft-
mirsky-ippm-epm-03>.
[I-D.mirsky-bfd-mpls-demand]
Mirsky, G., "BFD in Demand Mode over Point-to-Point MPLS
LSP", Work in Progress, Internet-Draft, draft-mirsky-bfd-
mpls-demand-09, 30 March 2021,
<https://datatracker.ietf.org/doc/html/draft-mirsky-bfd-
mpls-demand-09>.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
for In-situ OAM", Work in Progress, Internet-Draft, draft-
ietf-ippm-ioam-data-12, 21 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-ippm-
ioam-data-12>.
[NASA] Adams, T., "RELIABILITY: Definition & Quantitative
Illustration", <https://kscddms.ksc.nasa.gov/Reliability/
Documents/150814-3bWhatIsReliability.pdf>.
[MANET] IETF, "Mobile Ad hoc Networking",
<https://dataTracker.ietf.org/doc/charter-ietf-manet/>.
[detnet] IETF, "Deterministic Networking",
<https://dataTracker.ietf.org/doc/charter-ietf-detnet/>.
[SPRING] IETF, "Source Packet Routing in Networking",
<https://dataTracker.ietf.org/doc/charter-ietf-spring/>.
[BIER] IETF, "Bit Indexed Explicit Replication",
<https://dataTracker.ietf.org/doc/charter-ietf-bier/>.
[BFD] IETF, "Bidirectional Forwarding Detection",
<https://dataTracker.ietf.org/doc/charter-ietf-bfd/>.
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[CCAMP] IETF, "Common Control and Measurement Plane",
<https://dataTracker.ietf.org/doc/charter-ietf-ccamp/>.
[IPPM] IETF, "IP Performance Measurement",
<https://dataTracker.ietf.org/doc/charter-ietf-ippm/>.
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
Lou Berger
LabN Consulting, L.L.C.
United States of America
Email: lberger@labn.net
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