4. Reliable and Available Wireless
This section preserves the RFC text for the RAW architecture, including RAW, DetNet, TSN, OAM, PREOF, PLR, PSE, PCE, PDR, SLA, SLO, SLI, recovery graphs, protection paths, LL API, diagrams, tables, and security considerations.
Original RFC Text
4. Reliable and Available Wireless
4.1. High Availability Engineering Principles
The reliability criteria of a critical system pervade 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 each single point 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. Both non-Internet
and Internet examples are included.
4.1.1. 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 reroute to alternate routes
in case of a failure. When links are cabled through the same
conduit, they form a Shared Risk Link Group (SRLG) and share the same
fate if the conduit is cut, making the reroute operation ineffective.
The same effect can happen with virtual links that end up in the same
physical transport through the intricacies of nested encapsulation.
In the 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. Thus, for high availability, the use of physically link-
disjoint and node-disjoint paths is required; in the wireless space,
the use of the highest possible degree of diversity (time, space,
code, frequency, and channel width) in the transmissions over the air
is also required to combat the additional causes of transmission
loss.
From an economics standpoint, executing this principle properly
generally increases capital 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 efficiently.
4.1.2. Reliable Crossover
Backup equipment has limited value unless it can be reliably switched
into use within the downtime parameters. IP routers execute reliable
crossover continuously because the routers 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. "IP Fast Reroute Framework" [FRR]
analyzes mechanisms for fast failure detection and path repair for IP
Fast Reroute (FRR) 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. Therefore,
each DetNet path must inherently provide sufficient redundancy to
provide the assured SLOs 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 FRR 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 does 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.
4.1.3. Prompt Notification of Failures
The execution of the two above principles is likely to render a
system where the end user rarely sees a failure. However, a failure
that occurs must still be detected in order to direct maintenance.
There are many reasons for system monitoring (Fault, Configuration,
Accounting, Performance, and Security (FCAPS) is a handy mental
checklist), but fault monitoring is a sufficient reason.
"Overview and Principles of Internet Traffic Engineering" [TE]
discusses the importance of measurement for network protection and
provides an abstract method for network survivability with the
analysis of a traffic matrix as observed via a network management
YANG data model, probing techniques, file transfers, 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
recovery graph based on statistical and aggregated information. RAW
itself operates in the DetNet Network Plane at a faster timescale
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 DLEP, or transported over
multiple hops using OAM and reverse OAM, as illustrated in Figure 11.
4.2. Applying Reliability Concepts to Networking
The terms "reliability" and "availability" are defined for use in RAW
in Section 3, and the reader is invited to read [NASA1] and [NASA2]
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 packet but does not affect the previous
packet, the next packet, or packets 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 be fully
avoided, and the systems are built to resist some loss. This can be
done by using redundancy with retries (as in HARQ), Packet
Replication and Elimination (PRE) FEC, and Network Coding (e.g.,
using FEC with Static Context Header Compression (SCHC) [RFC8724]
fragments). Also, in a typical control loop, linear interpolation
from the previous measurements can be used.
However, the linear interpolation method cannot resist multiple
consecutive losses, and a high MTBF is desired as a guarantee that
the number of losses in a row is bounded. In this case, what is
really desired is a Maximum Consecutive Loss (MCL). If the number of
losses in a row passes the MCL, 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 as the MTBF and as a proxy to indicate
an MCL, e.g., as described in Section 7.4 of "Deterministic
Networking Use Cases" [RFC8578].
4.3. 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
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 either add up to
(constructive interference) or cancel (destructive interference)
the direct signal.
The affected frequencies depend on the relative position of the
sender, the receiver, and all the reflecting objects in the
environment. A given hop suffers from multipath fading for
multiple packets in a row until 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 Fresnel zone is an elliptical region
of space between and around the transmit and receive antennas in a
point-to-point wireless communication. The optimal transmission
happens when it is free of obstacles.
In an environment that is rich in metallic structures and mobile
objects, a single radio link provides 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 in the ISM band)
keeps transmitting, a continuous stream of packets are 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.
A single packet may be sent at different times (time diversity) over
diverse paths (spatial diversity) that rely on diverse radio channels
(frequency diversity) leveraging diverse PHY technologies (e.g.,
narrowband versus spread spectrum or diverse codes). Using time
diversity defeats short-term interferences; spatial diversity combats
very local causes of interference 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.