5. The RAW Conceptual Model
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
5. The RAW Conceptual Model
RAW extends the conceptual model described in Section 4 of
"Deterministic Networking Architecture" [DetNet-ARCH] with the PLR at
the Service sub-layer, as illustrated in Figure 3. The PLR (see
Section 6.5) provides additional agility against transmission loss.
For example, the PLR can act based on indications from the lower
layer or based on OAM.
| packets going | ^ packets coming ^
v down the stack v | up the stack |
+-----------------------+ +-----------------------+
| Source | | Destination |
+-----------------------+ +-----------------------+
| Service sub-layer: | | Service sub-layer: |
| Packet sequencing | | Duplicate elimination |
| Flow replication | | Flow merging |
| Packet encoding | | Packet decoding |
| Point of Local Repair | | |
+-----------------------+ +-----------------------+
| Forwarding sub-layer: | | Forwarding sub-layer: |
| Resource allocation | | Resource allocation |
| Explicit routes | | Explicit routes |
+-----------------------+ +-----------------------+
| Lower layers | | Lower layers |
+-----------------------+ +-----------------------+
v ^
\_________________________/
Figure 3: Extended DetNet Data Plane Protocol Stack
5.1. The RAW Planes
The RAW nodes are DetNet relay nodes that operate in the RAW Network
Plane and are capable of additional diversity mechanisms and
measurement functions related to the radio interface. RAW leverages
an Operational Plane orientation function (that typically operates
inside the ingress edge nodes) to dynamically adapt the path of the
packets and optimize the resource usage.
In the case of centralized routing operations, the RAW Controller
Plane Function (CPF) interacts with RAW nodes over a Southbound API.
It consumes data and information from the network and generates
knowledge and wisdom to help steer the traffic optimally inside a
recovery graph.
DetNet Routing
CPF CPF CPF CPF
Southbound API
_-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
_-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
___ RAW ___ RAW ___ RAW ___ RAW __
/ Node Node Node Node \
Ingress __/ / \ / \ \____Egress
End __ / \ / .- -- . \ ___ End
Node \ / \ / .-( ). \ / Node
\_ RAW ___ RAW ___(Non-RAW Nodes)__ RAW _/
Node Node (___.______.____) Node
Figure 4: RAW Nodes (Centralized Routing Case)
When a new flow is defined, the routing function uses its current
knowledge of the network to build a new recovery graph between an
ingress End System and an egress End System for that flow; it
indicates to the RAW nodes where the PREOF and/or radio diversity and
reliability operations may be actioned in the Network Plane.
* The recovery graph may be strict, meaning that the DetNet
forwarding sub-layer operations are enforced end to end.
* The recovery graph may be expressed loosely to enable traversing a
non-RAW subnetwork as in Figure 7. In that case, RAW cannot
leverage end-to-end DetNet and cannot provide latency guarantees.
The information that the orientation function reports to the routing
function may be time aggregated (e.g., statistical), to match the
longer-term operation of the routing function. Example information
includes link-layer metrics such as link bandwidth (the medium speed
depends dynamically on the mode of the PHY layer), number of flows
(bandwidth that can be reserved for a flow depends on the number and
size of flows sharing the spectrum), and the average and mean squared
deviation of availability and reliability metrics (such as PDR) over
long periods of time. It may also report an aggregated Expected
Transmission Count (ETX) or a variation of it.
Based on those metrics, the routing function installs the recovery
graph with enough redundant forwarding solutions to ensure that the
Network Plane can reliably deliver the packets within an SLA
associated with the flows that it transports. The SLA defines end-
to-end reliability and availability requirements, in which
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 recovery graph may
comprise non-RAW segments, either interleaved inside the recovery
graph (e.g., over tunnels) or all the way to the egress End node
(e.g., a server in the local wired domain). 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
diversity schemes is actioned by which RAW nodes.
Once a recovery graph is established, per-segment and end-to-end
reliability and availability statistics are periodically reported to
the routing function to ensure that the SLA can be met; if not, then
the recovery graph is recomputed.
5.2. RAW Versus Upper and Lower Layers
RAW builds on DetNet-provided features such as scheduling and
shaping. In particular, RAW inherits the DetNet guarantees on end-
to-end latency, which can be tuned to ensure that DetNet and RAW
reliability mechanisms have no side effect on upper layers, e.g., on
transport-layer packet recovery. RAW operations include possible
rerouting, which in turn may affect the ordering of a burst of
packets. RAW also inherits PREOF from DetNet, which can be used to
reorder packets before delivery to the upper layers. As a result,
DetNet in general and RAW in particular offer a smoother transport
experience for the upper layers than the Internet at large, with
ultra-low jitter and loss.
RAW improves the reliability of transmissions and the availability of
the communication resources and should be seen as a dynamic
optimization of the use of redundancy to maintain reliability and
availability metrics within certain boundaries. For instance, ARQ
(which provides one-hop reliability through acknowledgements and
retries) and FEC codes (such as turbo codes which reduce the PER) are
typically operated at Layer 2 and Layer 1, respectively. In both
cases, redundant transmissions improve the one-hop reliability at the
expense of energy and latency, which are the resources that RAW must
control. In order to achieve its goals, RAW may leverage the lower-
layer operations by abstracting the concept and providing hints to
the lower layers on the desired outcome (e.g., in terms of
reliability and timeliness), as opposed to performing the actual
operations at Layer 3.
Guarantees such as bounded latency depend on the upper layers
(transport or application) to provide the payload in volumes and at
times that match the contract with the DetNet sub-layers and the
layers below. An excess of incoming traffic at the DetNet ingress
may result in dropping or queueing of packets and can entail loss,
latency, or jitter; this violates the guarantees that are provided
inside the DetNet Network.
When the traffic from upper layers matches the expectation of the
lower layers, RAW still depends on DetNet mechanisms and the lower
layers to provide the timing and physical resource guarantees that
are needed to match the traffic SLA. When the availability of the
physical resource varies, RAW acts on the distribution of the traffic
to leverage alternates within a finite set of potential resources.
The Operational Plane elements (routing and OAM control) may gather
aggregated information from lower layers (e.g., information about
link quality), via measurement or communication with the lower layer.
This information may be obtained from inside the device using
specialized APIs (e.g., L2 triggers) via monitoring and measurement
protocols such as Bidirectional Forwarding Detection (BFD) [RFC5880]
and Simple Two-way Active Measurement Protocol (STAMP) [RFC8762],
respectively, or via a control protocol exchange with the lower layer
(e.g., DLEP [DLEP]). It may then be processed and exported through
OAM messaging or via a YANG data model and exposed to the Controller
Plane.
5.3. RAW and DetNet
RAW leverages the DetNet forwarding sub-layer and requires the
support of OAM in DetNet transit nodes (see Figure 3 of
[DetNet-ARCH]) for the dynamic acquisition of link capacity and state
to maintain a strict RAW service end to end over a DetNet Network.
In turn, DetNet and thus RAW may benefit from or leverage
functionality such as that provided by TSN at the lower layers.
RAW extends DetNet to improve the protection against link errors such
as transient flapping that are far more common in wireless links.
Nevertheless, for the most part, the RAW methods are applicable to
wired links as well, e.g., when energy savings are desirable and the
available path diversity exceeds 1+1 linear redundancy.
RAW adds sub-layer functions that operate in the DetNet Operational
Plane, which is part of the Network Plane. The RAW orientation
function may run only in the DetNet edge nodes (ingress edge node or
End System), or it can also run in DetNet relay nodes when the RAW
operations are distributed along the recovery graph. The RAW Service
sub-layer includes the PLR, which decides the DetNet path for the
future packets of a flow along the DetNet path, Maintenance End
Points (MEPs) on edge nodes, and Maintenance Intermediate Points
(MIPs) within. The MEPs trigger, and learn from, OAM observations
and feed the PLR for its next decision.
As illustrated in Figure 5, RAW extends the DetNet Stack (see
Figure 4 of [DetNet-ARCH] and Figure 3) with additional functionality
at the DetNet Service sub-layer for the actuation of PREOF based on
the PLR decision. DetNet operates at Layer 3, leveraging
abstractions of the lower layers and APIs that control those
abstractions. For instance, DetNet already leverages lower layers
for time-sensitive operations such as time synchronization and
traffic shapers. As the performances of the radio layers are subject
to rapid changes, RAW needs more dynamic gauges and knobs. To that
effect, the LL API provides an abstraction to the DetNet layer that
can be used to push reliability and timing hints, like suggesting X
retries (min, max) within a time window or sending unicast (one next
hop) or multicast (for overhearing). In the other direction up the
stack, the RAW PLR needs hints about the radio conditions such as L2
triggers (e.g., RSSI, LQI, or ETX) over all the wireless hops.
RAW uses various OAM functionalities at the different layers. For
instance, the OAM function in the DetNet Service sub-layer may
perform Active and/or Hybrid OAM to estimate the link and path
availability, either end to end or limited to a segment. The RAW
functions may be present in the Service sub-layer in DetNet edge and
relay nodes.
+-----------------+ +-------------------+
| Routing | | OAM Control |
+-----------------+ +-------------------+
Controller Plane
+-+-+-+-+-+-+-+-+ Southbound Interface -+-+-+-+-+-+-+-+-+-+-+-+
Network Plane
|
Operational Plane . Data Plane
|
+-----------------+ .
| Orientation | |
+-----------------+ .
|
+-----------------+ +-------------------+ .
| Point of Local | | OAM Maintenance | |
| Repair (PLR) | | End Point (MEP) | .
+-----------------+ +-------------------+ |
.
|
Figure 5: RAW Function Placement (Centralized Routing Case)
There are two main proposed models to deploy RAW and DetNet: strict
(Figure 6) and loose (Figure 7). In the strict model, illustrated in
Figure 6, RAW operates over a continuous DetNet service end to end
between the ingress and the egress edge nodes or End Systems.
In the loose model, illustrated in Figure 7, RAW may traverse a
section of the network that is not serviced by DetNet. RAW OAM may
observe the end-to-end traffic and make the best of the available
resources, but it may not expect the DetNet guarantees over all
paths. For instance, the packets between two wireless entities may
be relayed over a wired infrastructure, in which case RAW observes
and controls the transmission over the wireless first and last hops,
as well as end-to-end metrics such as latency, jitter, and delivery
ratio. This operation is loose since the structure and properties of
the wired infrastructure are ignored and may be either controlled by
other means such as DetNet/TSN or neglected in the face of the
wireless hops.
A minimal forwarding sub-layer service is provided at all DetNet
nodes to ensure that the OAM information flows. DetNet relay nodes
may or may not support RAW services, whereas the DetNet edge nodes
are required to support RAW in any case. DetNet guarantees, such as
bounded latency, are provided end to end. RAW extends the DetNet
Service sub-layer to optimize the use of resources.
--------------------Flow Direction---------------------------------->
+---------+
| RAW |
| Control |
+---------+ +---------+ +---------+
| RAW + | | RAW + | | RAW + |
| DetNet | | DetNet | | DetNet |
| Service | | Service | | Service |
+---------+---------------------------+---------+--------+---------+
| DetNet |
| Forwarding |
+------------------------------------------------------------------+
Ingress Transit Relay Egress
Edge ... Nodes ... Nodes ... Edge
Node Node
<------------------End-to-End DetNet Service----------------------->
Figure 6: RAW over DetNet (Strict Model)
In the loose model (illustrated in Figure 7), RAW operates over a
partial DetNet service where typically only the ingress and the
egress End Systems support RAW. The DetNet domain may extend beyond
the ingress node, or there may be a DetNet domain starting at an
ingress edge node at the first hop after the End System.
In the loose model, RAW cannot observe the hops in the network, and
the path beyond the first hop is opaque; RAW can still observe the
end-to-end behavior and use Layer 3 measurements to decide whether to
replicate a packet and select the first-hop interface(s).
--------------------Flow Direction---------------------------------->
+---------+
| RAW |
| Control |
+---------+ +---------+ +---------+
| RAW + | | DetNet | | RAW + |
| DetNet | | Only | | DetNet |
| Service | | Service | | Service |
+---------+----------------------+---+ +---+---------+
| DetNet |_______________| DetNet |
| Forwarding _______________ Forwarding |
+------------------------------------+ +-------------+
Ingress Transit Relay Tunnel Egress
End ... Nodes ... Nodes ... ... End
System System
<---------------Partitioned DetNet Service------------------------->
Figure 7: RAW over DetNet (Loose Model)