Reliable and Available Wireless Problem Statement
Cisco Systems, Inc
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pthubert@cisco.com
IMT Atlantique
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georgios.papadopoulos@imt-atlantique.fr
Routing Area
RAW
Draft
Due to uncontrolled interferences, including the self-induced multipath fading, deterministic networking can only be approached on wireless links. The radio conditions may change -way- faster than a centralized routing can adapt and reprogram, in particular when the controller is distant and connectivity is slow and limited. RAW separates the routing time scale at which a complex path is recomputed from the forwarding time scale at which the forwarding decision is taken for an individual packet. RAW operates at the forwarded time scale. The RAW problem is to decide, within the redundant solutions that are proposed by the routing, which will be used for each individual packet to provide a DetNet service while minimizing the waste of resources.
Introduction
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 technology, and
the use of time-shared resources (bandwidth and buffers) per circuit, and/or
by shaping and/or scheduling the packets at every hop.
Wireless networks operate on a shared medium where uncontrolled interference,
including the self-induced multipath fading, adds another dimension to the
statistical effects that affect the delivery. Scheduling transmissions can
alleviate those effects by leveraging diversity in the spatial, time, code,
and frequency domains, and provide a Reliable and Available service while
preserving energy and optimizing the use of the shared spectrum.
Deterministic Networking is an attempt to mostly eliminate packet loss for a
committed bandwidth with a guaranteed worst-case end-to-end latency, even
when co-existing with best-effort traffic in a shared network.
This innovation is enabled by recent developments in technologies including
IEEE 802.1 TSN (for Ethernet LANs) and IETF DetNet (for wired IP networks).
It is getting traction in various industries including manufacturing, online
gaming, professional A/V, cellular radio and others, making possible
many cost and performance optimizations.
Reliable and Available Wireless (RAW) networking services extend DetNet
to approach end-to-end deterministic performances in a network with
scheduled wireless segments, possibly combined with wired segments, and
possibly sharing physical resources with non-deterministic traffic.
The wireless and wired media are fundamentally different at the physical
level, and while the generic Problem Statement for DetNet applies to the
wired as well as the wireless medium, the methods to achieve RAW will
differ from those used to support time-sensitive networking over wires, as
a RAW solution will need to address less consistent transmissions, energy
conservation and shared spectrum efficiency.
The development of RAW technologies has been lagging behind deterministic
efforts for wired systems both at the IEEE and the IETF. But recent efforts
at the IEEE and 3GPP indicate that wireless is finally catching up at the
lower layer and that it is now possible for the IETF to extend DetNet for
wireless segments that are capable of scheduled wireless transmissions.
The intent for RAW is to provide 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.
Still, in order to focus on real-worlds issues and assert the feasibility of
the proposed capabilities, RAW will focus 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 for more.
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 RAW 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 at the forwarding time scale on one DetNet flow over one path
that is preestablished and installed by means outside of the scope of RAW.
The scope of the RAW WG comprises Network plane protocol elements such as OAM
and in-band control to improve the RAW operation at the Service and at the
forwarding sub-layers, e.g., controlling whether to use packet replication,
Hybrid ARQ and coding, with a constraint to limit the use of redundancy when
it is really needed, e.g., when a spike of loss is observed.
This is discussed in more details in
and the next sections.
Use Cases and Requirements Served
presents a number of wireless use
cases including Wireless for Industrial Applications.
adds a number
of use cases that demonstrate the need for RAW capabilities in Pro-Audio,
gaming and robotics.
Routing Scale vs. Forwarding Scale
RAW extends DetNet to focus on issues that are mostly a concern on wireless
links. See
for more on DetNet.
With DetNet, the end-to-end routing can be centralized and can reside
outside the network. In wireless, and in particular in a wireless mesh, the
path to the controller that performs the route computation and maintenance
may be slow and expensive in terms of critical resources such as air time and
energy.
Reaching to the routing computation can be slow in regards to the speed of
events that affect the forwarding operation at the radio layer. Due to the
cost and latency to perform a route computation, routing is not expected to
be sensitive/reactive to transient changes. The abstraction of a link at the
routing level is expected to use statistical operational metrics that
aggregate the behavior of a link over long periods of time, and represent its
availability as a shade of gray as opposed to either up or down.
In the case of wireless, the changes that affect the forwarding decision can
happen frequently and often for shot 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 such a model, the routing operation
computes a complex Track that enables multiple non-equal cost multipath
(N-ECMP) forwarding solutions, and leaves it to the forwarding plane to make
the per-packet decision of which of these possibilities should be used.
In the case of wires, the concept is known in traffic engineering where 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 routing time scale that is order of magnitude longer than the
forwarding 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 packet speed, and
uses information that must be pertinent at the present time for the current
transmission.
Prerequisites
A prerequisite to the RAW work 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. For 6TiSCH, this is a Track. The concept
of Track is specified in the 6TiSCH Architecture
.
Tracks provide a high degree of redundancy and diversity and enable DetNet
PREOF, end-to-end network coding, and possibly radio-specific abstracted
techniques such as ARQ, overhearing, frequency diversity, time slotting, and
possibly others.
How the routing operation 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 service 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
forwarding decision (think DLEP from MANET). In the layered approach, how the
radio manages its PHY layer is out of control and out of scope. Whether it is
single hop or meshed is also unknown and out of scope.
Related Work at The IETF
RAW intersects with protocols or practices in development at the IETF
as follows:
-
The Dynamic Link Exchange Protocol (DLEP)
from
can be leveraged at each hop
to derive generic radio metrics (e.g., based on LQI, RSSI, queueing delays
and ETX) on individual hops
-
Operations, Administration and Maintenance (OAM) work at
such as
for the case of the IP Data Plane observes the state of DetNet paths,
typically MPLS and IPv6 pseudowires
,
in the direction of the traffic. RAW needs feedback that flows on the
reverse path and gathers instantaneous values from the radio receivers at
each hop to inform back the source and replicating relays so they can make
optimized forwarding decisions.
The work named ICAN may be related as well.
-
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 considers
delivery as success whereas with RAW the bounded latency can be as
important as the delivery itself.
-
and
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
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 be met.
-
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.
Functional Gaps
Within a large routed topology, the routing operation builds a particular
complex Track with one source and one or more destinations; within the Track,
packets may follows 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
forwarding 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 will consider existing tools such as
L2-triggers, DLEP, BFD and in-band and out-of-band OAM.
One possible way of making the RAW forwarding decisions is to make them all
at the ingress and express them in-band in the packet, which requires new
loose or strict Hop-by-hop signaling. To control the RAW forwarding operation
along a Track for the individual packets, RAW may leverage and extend known
techniques such as DetNet tagging, Segment Routing (SRv6) or BIER-TE such as
done with
.
An alternate way is to enable each forwarding node to make 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 within the Track. Information about
the service should be placed in the packet and matched with the forwarding
node's capabilities and policies.
In either case, a per-flow state is installed in all intermediate nodes to
recognize the flow and determine the forwarding policy to be applied.
References
Normative References
Informative References
Mobile Ad hoc Networking
IETF
Deterministic Networking
IETF
Source Packet Routing in Networking
IETF
Bit Indexed Explicit Replication
IETF
Bidirectional Forwarding Detection
IETF
Common Control and Measurement Plane
IETF