2. The RAW Problem
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
2. The RAW Problem
While the generic "Deterministic Networking Problem Statement"
[RFC8557] applies to both wired and wireless media, the
"Deterministic Networking Architecture" [DetNet-ARCH] must be
extended to address less consistent transmissions, energy
conservation, and shared spectrum efficiency.
Operating at Layer 3, RAW does not change the wireless technology at
the lower layers. On the other hand, 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.,
using diverse radio access technologies to optimize the end-to-end
application experience.
RAW extends the DetNet services by providing elements that are
specialized for transporting IP flows over deterministic radio
technologies such as those listed in [RAW-TECHNOS]. Conceptually,
RAW is agnostic to the lower layer, though the capability to control
latency is assumed to ensure the DetNet services that RAW extends.
How the lower layers are operated to do so (and whether a radio
network is single hop or meshed, for example) are opaque to the IP
layer and not part of the RAW abstraction. Nevertheless, cross-layer
optimizations may take place to ensure proper link awareness (such as
link quality) and packet handling (such as scheduling).
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 forwarding
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 4 or diverse radio access networks
as illustrated in Figure 10.
As opposed to wired links, the availability and performance of an
individual wireless link cannot be trusted over the long term; it
varies with transient service discontinuity, and any path that
includes wireless hops is bound to face short periods of high loss.
On the other hand, being broadcast in nature, the wireless medium
provides capabilities that are atypical on modern wired links and
that the RAW architecture can leverage opportunistically to improve
the end-to-end reliability over a collection of paths.
Those capabilities include:
Promiscuous overhearing: Some wired and wireless technologies allow
for multiple lower-layer attached nodes to receive the same packet
sent by another node. This differs from a lower-layer network
that is physically point-to-point, like a wire. With overhearing,
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 of multipoint-to-multipoint progress in the
forward 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 varies over
time, the concept of metric must also be revisited. Shortest-path
cost loses its absolute value, and hop count turns into a bad idea
as the link budget drops with the physical distance. Routing over
radio requires both:
1. a new and more dynamic sense of link metrics, with new
protocols such as the Dynamic Link Exchange Protocol (DLEP)
and Layer 2 (L2) triggers to keep Layer 3 (L3) up to date with
the link quality and availability, and
2. an approach to multipath routing, where multiple link metrics
are considered since simple shortest-path cost loses its
meaning with the instability of the metrics.
Redundant transmissions: Though feasible on any technology,
proactive (forward) and reactive (acknowledgment-based) error
correction is typical for wireless media. Bounded latency can
still be obtained on a wireless link while operating those
technologies, provided that link latency used in path selection
allows for the extra transmission or the introduced delay is
compensated along the path. In the case of coded fragments and
retries, it makes sense to vary all the possible physical
properties of the transmission to reduce the chances that the same
effect causes the loss of both original and redundant
transmissions.
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.
From a DetNet perspective, this may translate to taking into
account how transmission from one node may interfere with the
transmission of another node attached to the same wireless sub-
layer network.
RAW and DetNet enable application flows that require a special
treatment along paths that can provide that treatment. This may be
seen as a form of Path Aware networking and may be subject to
impediments documented in [RFC9049].
The mechanism used to establish a path is not unique to, or
necessarily impacted by, RAW. The mechanism is expected to be the
product of the DetNet Controller Plane [DetNet-PLANE]; it may use a
Path Computation Element (PCE) [RFC4655] or the DetNet YANG data
model [RFC9633], or it may be computed in a distributed fashion as
per the Resource ReSerVation Protocol (RSVP) [RFC2205]. Either way,
the assumption is that it is slow relative to local forwarding
operations along the path. To react fast enough to transient changes
in the radio transmissions, RAW leverages DetNet Network Plane
enhancements to optimize the use of the paths and match the quality
of the transmissions over time.
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; thus, in the wireless case, it makes sense
to provide redundant forwarding solutions along alternate paths (see
Section 3.3) 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. The RAW Network Plane operations happen
within the scope of a recovery graph (see Section 3.3.2) that is pre-
established and installed by means outside of the scope of RAW. A
recovery graph may be strict or loose depending on whether each hop
or just a subset of the hops is observed and controlled by RAW.
RAW distinguishes the longer timescale at which routes are computed
from the shorter timescale where forwarding decisions are made (see
Section 6.1). The RAW Network Plane operations happen at a timescale
that sits timewise between the routing and the forwarding timescales.
Within the resources delineated by a recovery graph, their goal is to
dynamically select the protection path(s) that the upcoming packets
of a DetNet flow shall follow. As they influence the path for the
entirety of the flows or a portion of them, the RAW Network Plane
operations may affect the metrics used in their rerouting decisions,
which could potentially lead to oscillations; such effects must be
avoided or dampened.