One document matched: draft-finn-detnet-architecture-02.xml
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<front>
<title>Deterministic Networking Architecture</title>
<author initials="N" surname="Finn" fullname="Norman Finn" >
<organization abbrev="Cisco">
Cisco Systems
</organization>
<address>
<postal>
<street>170 W Tasman Dr.</street>
<city>San Jose</city>
<code>95134</code>
<region>California</region>
<country>USA</country>
</postal>
<phone>+1 408 526 4495</phone>
<email>nfinn@cisco.com</email>
</address>
</author>
<author initials="P" surname="Thubert" fullname="Pascal Thubert">
<organization abbrev="Cisco">
Cisco Systems
</organization>
<address>
<postal>
<street>Village d'Entreprises Green Side</street>
<street>400, Avenue de Roumanille</street>
<street>Batiment T3</street>
<city>Biot - Sophia Antipolis</city>
<code>06410</code>
<country>FRANCE</country>
</postal>
<phone>+33 4 97 23 26 34</phone>
<email>pthubert@cisco.com</email>
</address>
</author>
<author initials="M" surname="Johas Teener" fullname="Michael Johas Teener">
<organization abbrev="Broadcom">
Broadcom Corp.
</organization>
<address>
<postal>
<street>3151 Zanker Rd.</street>
<city>San Jose</city>
<code>95134</code>
<region>California</region>
<country>USA</country>
</postal>
<phone>+1 831 824 4228</phone>
<email>MikeJT@broadcom.com</email>
</address>
</author>
<date/>
<area>Internet</area>
<workgroup>DetNet</workgroup>
<abstract>
<t>
Deterministic Networking (DetNet) provides a capability to carry specified unicast or multicast
data streams for real-time applications with extremely low data loss rates and bounded
latency. Techniques used include: 1) reserving data plane resources for individual
(or aggregated) DetNet streams in some or all of the relay systems (bridges or routers) along
the path of the stream; 2) providing fixed paths for DetNet streams that do not
rapidly change with the network topology; and 3) sequentializing, replicating, and eliminating
duplicate packets at various points to ensure the availability of at least one path. The
capabilities can be managed by configuration, or by manual or automatic network management.
</t>
</abstract>
</front>
<middle>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor='introduction' title="Introduction">
<t>
Operational Technology (OT) refers to industrial networks that are typically
used for monitoring systems and supporting control loops, as well as movement
detection systems for use in process control (i.e., process manufacturing) and
factory automation (i.e., discrete manufacturing). Due to its different goals,
OT has evolved in parallel but in a manner that is radically different from
IT/ICT, focusing on highly secure, reliable and deterministic networks, with
limited scalability over a bounded area.
</t> <t>
The convergence of IT and OT technologies, also called the Industrial Internet,
represents a major evolution for both sides. The work has already started;
in particular, the industrial automation space has been developing a number
of Ethernet-based replacements for existing digital control systems, often
not packet-based (fieldbus technologies).
</t> <t>
These replacements are meant to provide similar behavior as the incumbent
protocols, and their common focus is to transport a fully characterized
flow over a well-controlled environment (i.e., a factory floor), with a
bounded latency, extraordinarily low frame loss, and a very narrow jitter.
Examples of such protocols include PROFINET, ODVA Ethernet/IP, and EtherCAT.
</t><t>
In parallel, the need for determinism in professional and home audio/video
markets drove the formation of the Audio/Video Bridging (AVB) standards effort
of IEEE 802.1. With the explosion of demand for connectivity and multimedia in
transportation in general, the Ethernet AVB technology has become one of the
hottest topics, in particular in the automotive connectivity. It is finding
application in all elements of the vehicle from head units, to rear seat
entertainment modules, to amplifiers and camera modules. While aimed at less
critical applications than some industrial networks, AVB networks share the
requirement for extremely low packet loss rates and ensured finite latency
and jitter.
</t><t>
Other instances of in-vehicle deterministic networks have arisen as well for
control networks in cars, trains and buses, as well as avionics, with, for
instance, the mission-critical "Avionics Full-Duplex Switched Ethernet" (AFDX)
that was designed as part of the ARINC 664 standards. Existing automotive
control networks such as the LIN, CAN and FlexRay standards were not designed
to cover these increasing demands in terms of bandwidth and scalability that we
see with various kinds of Driver Assistance Systems (DAS) and new multiplexing
technologies based on Ethernet are now getting traction.
</t><t>
The generalization of the needs for more deterministic networks have led to the
IEEE 802.1 AVB Task Group becoming the Time-Sensitive Networking (TSN)
Task Group (TG), with a much-expanded constituency from the industrial and
vehicular markets. Along with this expansion, the networks in consideration are
becoming larger and structured, requiring deterministic forwarding beyond the
LAN boundaries. For instance, Industrial Automation segregates the network along
the broad lines of the Purdue Enterprise Reference Architecture (PERA), using
different technologies at each level, and public infrastructures such as
Electricity Automation require deterministic properties over the Wide Area. The
realization is now coming that the convergence of IT and OT networks requires
Layer-3, as well as Layer-2, capabilities.
</t><t>
While the initial user base has focused almost entirely on Ethernet physical
media and Ethernet-based bridging protocol (from several Standards Development
Organizations), the need for Layer-3 expressed, above, must not be confined
to Ethernet and Ethernet-like media, and while such media must be encompassed
by any useful DetNet architecture, cooperation between IETF and other
SDOs must not be limited to IEEE or IEEE 802. Furthermore, while the work
completed and ongoing in other SDOs, and in IEEE 802 in particular, provide
an obvious starting point for a DetNet architecture, we must not assume that
these other SDOs' work confines the space in which the DetNet architecture
progresses.
</t><t>
The present architecture is the result of a collaboration of IETF IEEE members,
and describes an abstract model that can be applicable both at Layer-2
and Layer-3, and along segments of different technologies. With this new work,
a path may span, for instance, across a (limited) number of 802.1 bridges and
then a (limited) number of IP routers. In that example, the IEEE 802.1 bridges
may be operating at Layer-2 over Ethernet whereas the IP routers may be 6TiSCH
nodes operating at Layer-2 and/or Layer-3 over the IEEE 802.15.4e MAC.
</t><t>
Many applications of interest to Deterministic Networking require the ability
to synchronize the clocks in end systems to a sub-microsecond accuracy. Some
of the queue control techniques defined in <xref target="QueuingModels"/> also
require time synchronization among relay systems. The means used to achieve
time synchronization are not addressed in this document.
</t>
</section>
<section title="Terminology">
<t>
The following special terms are used in this document in order to avoid the
assumption that a given element in the architecture does or does not have
Internet Protocol stack, functions as a router or a bridge, or otherwise
plays a particular role at Layer-3 or higher:
<list hangIndent="8" style="hanging">
<t hangText="bridge"><vspace blankLines="0"/>
A Customer Bridge as defined by <xref target="IEEE802.1Q-2014">IEEE 802.1Q</xref>.
</t>
<t hangText="end system"><vspace blankLines="0"/>
Commonly called a "host" in IETF documents, and an "end station" is IEEE 802
documents. End systems of interest to this document are talkers and
listeners.
</t>
<t hangText="listener"><vspace blankLines="0"/>
An end system capable of sinking a DetNet stream.
</t>
<t hangText="relay system"><vspace blankLines="0"/>
A router or a bridge.
</t>
<t hangText="reservation"><vspace blankLines="0"/>
A trail of configuration from talker to listener(s) through relay systems
associated with a DetNet stream, required to deliver the benefits of DetNet.
</t>
<t hangText="stream"><vspace blankLines="0"/>
A DetNet stream is a sequence of packets from a single talker, through some
number of relay systems to one or more listeners, that is
limited by the talker in its maximum packet size and transmission rate,
and can thus be ensured the DetNet Quality of Service (QoS) from the network.
</t>
<t hangText="talker"><vspace blankLines="0"/>
An end system capable of sourcing a DetNet stream.
</t>
</list>
</t>
</section>
<section anchor="ProvidingQoS" title="Providing the DetNet Quality of Service">
<t>
DetNet Quality of Service is expressed in terms of:
<list style="symbols">
<t>
Minimum and maximum end-to-end latency from talker to listener;
</t><t>
Probability of loss of a packet, assuming the normal operation of the
relay systems and links;
</t><t>
Probability of loss of a packet in the event of the failure of a relay system or
link.
</t>
</list>
</t><t>
It is a distinction of DetNet that it is concerned solely with worst-case values
for all of the above parameters. Average, mean, or typical values are of no interest,
because they do not affect the ability of a real-time system to perform its
tasks. For example, in this document, we will often speak of assuring a DetNet
flow a bounded latency. In general, a trivial priority-based queuing scheme will
give better average latency to a flow than DetNet, but of course, the worst-case
latency is essentially unbounded.
</t><t>
Three techniques are employed by DetNet to achieve these QoS parameters:
<list style="letters">
<t>
Zero congestion loss (<xref target="Zero"/>). Network resources such as
link bandwidth, buffers, queues, shapers, and scheduled input/output slots
are assigned in each relay system to the use of a specific DetNet stream or
class of streams. Given a finite amount of buffer space, zero
congestion loss necessarily ensures a bounded end-to-end
latency. Depending on the resources employed, a minimum latency, and thus
bounded jitter, can also be achieved.
</t><t>
Pinned-down paths (<xref target="pinned"/>). Point-to-point paths or point-to-multipoint
trees through the network from a talker to one or more listeners can be established,
and DetNet streams assigned to follow a particular path or tree.
</t><t>
Packet replication and deletion (<xref target="Seamless"/>). End systems and/or
relay systems can number packets sequentially, replicate them, and later eliminate
all but one of the replicants, at
multiple points in the network in order to ensure that one (or more) equipment
failure events still leave at least one path intact for a DetNet stream.
</t>
</list>
</t><t>
These three techniques can be applied independently, giving eight possible combinations,
including none (no DetNet), although some combinations are of wider utility than others.
This separation keeps the protocol stack coherent and maximizes interoperability with
existing and developing standards in this (IETF) and other
Standards Development Organizations. Some examples of typical expected combinations:
<list style="symbols">
<t>
Pinned-down paths (a) plus packet replication (b) are exactly the techniques
employed by <xref target="HSR-PRP"/>. Pinned-down paths are achieved by limiting
the physical topology of the network, and the sequentialization, replication, and
duplicate elimination are facilitated by packet tags added at the front or the end
of Ethernet frames.
</t><t>
Zero congestion loss (a) alone is is offered by IEEE 802.1 Audio Video bridging
<xref target="IEEE802.1BA-2011"/>. As long as the network suffers no failures,
zero congestion loss can be achieved through the use of
a reservation protocol (MSRP), shapers in every relay system (bridge), and a
bit of network calculus.
</t><t>
Using all three together gives maximum protection.
</t>
</list>
</t><t>
There are, of course, simpler methods available (and employed, today) to achieve
levels of latency and packet loss that are satisfactory for many applications.
Prioritization and over-provisioning is one such technique. However, these
methods generally work best in the absence of any significant amount of non-critical
traffic in the network (if, indeed, such traffic is supported at all), or work only if
the critical traffic constitutes only a small portion of the network's theoretical
capacity, or work only if all systems are functioning properly, or in the absence of
actions by end systems that disrupt the network's operations.
</t><t>
There are any number of methods in use, defined, or in progress for accomplishing each
of the above techniques. It is expected that this DetNet Architecture will assist
various vendors, users, and/or "vertical"
Standards Development Organizations (dedicated to a single industry) to make selections
among the available means of implementing DetNet networks.
</t>
<section anchor="Zero" title="Zero Congestion Loss">
<t>
The primary means by which DetNet achieves its QoS assurances is to completely
eliminate congestion at an output port as a cause of packet loss. Given that a
DetNet stream cannot be throttled, this can be achieved only by the provision of
sufficient buffer storage at each hop through the network to ensure that no
packets are dropped due to a lack of buffer storage.
</t><t>
Ensuring adequate buffering requires, in turn, that the talker, and every relay
system along the path to the listener (or nearly every relay system -- see
<xref target="Incomplete"/>) be careful to regulate its output to not exceed the
data rate for any stream, except for brief periods when making up for
interfering traffic. Any packet sent ahead of its time potentially adds to the
number of buffers required by the next hop, and may thus exceed the resources
allocated for a particular stream.
</t><t>
The low-level mechanisms described in <xref target="QueuingModels"/> provide
the necessary
regulation of transmissions by an edge system or relay system to ensure
zero congestion loss. The reservation of the bandwidth and
buffers for a stream requires the provisioning described in
<xref target="Provisioning"/>.
</t>
</section>
<section anchor="pinned" title="Pinned-down paths">
<t>
In networks controlled by typical peer-to-peer protocols such as IEEE 802.1 ISIS bridged
networks or IETF OSPF routed networks, a network topology event in one part of the network
can impact, at least briefly, the delivery of data in parts of the network remote from the
failure or recovery event. Thus, even redundant paths through a network, if controlled by
the typical peer-to-peer protocols, do not eliminate the chances of brief losses of contact.
</t><t>
Many real-time networks rely on physical rings or chains of two-port devices, with
a relatively simple ring control protocol. This supports redundant paths with a minimum
of wiring. As an additional benefit, ring topologies can often
utilize different topology management protocols than those used for a mesh network, with
a consequent reduction in the response time to topology changes. Of course, this comes
at some cost in terms of increased hop count, and thus latency, for the typical path.
</t><t>
In order to get the advantages of low hop count and still ensure against even very brief
losses
of connectivity, DetNet employs pinned-down paths, where the path taken by a given DetNet stream
does not change, at least immediately, and likely not at all, in response to network
topology events. When combined with seamless redundancy
(<xref target="Seamless"/>), this results in a high likelihood of continuous connectivity.
</t>
</section>
<section anchor="Seamless" title="Seamless Redundancy">
<t>
After congestion loss has been eliminated, the most important causes of packet
loss are random media and/or memory faults, and equipment failures.
</t><t>
Seamless redundancy involves three capabilities:
<list style="symbols">
<t>
Adding sequence numbers, once, to the packets of a DetNet stream.
</t><t>
Replicating these packets and, typically, sending them along at least two
different paths to the listener(s). (Often, the pinned-down paths of
<xref target="pinned"/>.)
</t><t>
Discarding duplicated packets.
</t>
</list>
</t><t>
In the simplest case, this amounts to replicating each packet in a talker that
has two interfaces, and conveying them through the network, along separate paths,
to the
similarly dual-homed listeners, that discard the extras. This ensures that one
path (with zero congestion loss) remains, even if some relay system fails.
</t><t>
Alternatively, relay systems in the network can provide replication and elimination
facilities at various points in the network, so that multiple failures can be
accommodated.
</t><t>
This is shown in the following figure, where the two relay systems
each replicate (R) the DetNet stream on input, sending the stream to both the other
relay system and to the end system, and eliminate duplicates (E) on the output
interface to the right-hand end system. Any one links in the network can
fail, and the Detnet stream can still get through. Furthermore, two links can
fail, as long as they are in different segments of the network.
</t>
<figure align="center" anchor="FigSeamless">
<artwork align="left"><![CDATA[
> > > > > > > > relay > > > > > > > >
> /------------+ R system E +------------\ >
> / v + ^ \ >
end R + v | ^ + E end
system + v | ^ + system
> \ v + ^ / >
> \------------+ R relay E +------------/ >
> > > > > > > > system > > > > > > > >
]]></artwork>
</figure>
<t>
Note that seamless redundancy does not react to and correct failures; it is
entirely passive. Thus, intermittent failures, mistakenly created access control
lists, or misrouted data is handled just the same as the equipment failures
that are detected handled by typical routing and bridging protocols.
</t>
</section>
</section>
<section anchor="arch" title="DetNet Architecture">
<t>
<xref target="TEAS">Traffic Engineering Architecture and Signaling (TEAS)
</xref> defines traffic-engineering architectures for generic applicability
across packet and non-packet networks.
From TEAS perspective, Traffic Engineering (TE) refers to techniques
that enable operators to control how specific traffic flows are treated
within their networks.
</t>
<t>
Because if its very nature of establishing pinned-down optimized paths,
Deterministic Networking can be seen as a new, specialized branch of
Traffic Engineering, and inherits its architecture with a separation
into planes.
</t><t>
The Deterministic Networking architecture is thus composed
of three planes, a (User) Application Plane, a Controller Plane, and a
Network Plane, which echoes that of
<xref target="RFC7426">Software-Defined Networking (SDN):
Layers and Architecture Terminology</xref> which is represented below:
</t>
<figure align="center" anchor="RFC7426archi">
<preamble>SDN Layers and Architecture Terminology per RFC 7426</preamble>
<artwork align="left"><![CDATA[
o--------------------------------o
| |
| +-------------+ +----------+ |
| | Application | | Service | |
| +-------------+ +----------+ |
| Application Plane |
o---------------Y----------------o
|
*-----------------------------Y---------------------------------*
| Network Services Abstraction Layer (NSAL) |
*------Y------------------------------------------------Y-------*
| |
| Service Interface |
| |
o------Y------------------o o---------------------Y------o
| | Control Plane | | Management Plane | |
| +----Y----+ +-----+ | | +-----+ +----Y----+ |
| | Service | | App | | | | App | | Service | |
| +----Y----+ +--Y--+ | | +--Y--+ +----Y----+ |
| | | | | | | |
| *----Y-----------Y----* | | *---Y---------------Y----* |
| | Control Abstraction | | | | Management Abstraction | |
| | Layer (CAL) | | | | Layer (MAL) | |
| *----------Y----------* | | *----------Y-------------* |
| | | | | |
o------------|------------o o------------|---------------o
| |
| CP | MP
| Southbound | Southbound
| Interface | Interface
| |
*------------Y---------------------------------Y----------------*
| Device and resource Abstraction Layer (DAL) |
*------------Y---------------------------------Y----------------*
| | | |
| o-------Y----------o +-----+ o--------Y----------o |
| | Forwarding Plane | | App | | Operational Plane | |
| o------------------o +-----+ o-------------------o |
| Network Device |
+---------------------------------------------------------------+
]]></artwork>
</figure>
<section anchor="appplane" title="The Application Plane">
<t>
Per <xref target="RFC7426"/>,
the Application Plane includes both applications and services. In particular,
the Application Plane incorporates the User Agent, a specialized application
that interacts with the end user / operator and performs requests for
Deterministic Networking services via an abstract Stream Management Entity,
(SME) which may or may not be collocated with (one of) the end systems.
</t>
<t>At the Application Plane, a management interface enables the negotiation of streams between end
systems. An abstraction of the stream called a Traffic Specification (TSpec) provides the
representation. This abstraction is used to place a reservation over the (Northbound) Service
Interface and within the Application plane.
It is associated with an abstraction of location, such as IP addresses and DNS
names, to identify the end systems and eventually specify intermediate relay systems.
</t>
</section>
<section anchor="ctrlplane" title="The Controller Plane">
<t>
The Controller Plane corresponds to the aggregation of the Control and
Management Planes in <xref target="RFC7426"/>, though
Common Control and Measurement Plane (CCAMP) <xref target="CCAMP"/>
makes an additional distinction between management and measurement.
When the logical separation of the Control, Measurement and other
Management entities is not relevant, the term Controller Plane is used
for simplicity to represent them all, and the term controller refers to
any device operating in that plane, whether is it a Path Computation
entity or a Network Management entity (NME).
The Path Computation Element (PCE) <xref target="PCE"/> is a core
element of a controller, in charge of computing Deterministic paths
to be applied in the Network Plane.
</t>
<t>
A (Northbound) Service Interface enables applications in the Application
Plane to communicate with the entities in the Controller Plane.
</t>
<t>
One or more PCE(s) collaborate to implement the requests from the SME
as Per-Stream Per-Hop Behaviors installed in the relay systems for
each individual streams. The PCEs
place each stream along a deterministic sequence of relay systems so as
to respect per-stream constraints such as security and
latency, and optimize the overall result for metrics such as an
abstract aggregated cost. The deterministic sequence can typically be
more complex than a direct sequence and include redundancy path, with
one or more packet replication and elimination points.
</t>
</section>
<section anchor="netplane" title="The Network Plane"><t>
The Network Plane represents the network devices and protocols as a
whole, regardless of the Layer at which the network devices operate.
</t>
<t>
The network Plane comprises the Network Interface Cards (NIC) in the
end systems, which are typically IP hosts,
and relay systems, which are typically IP routers and switches.
Network-to-Network Interfaces such as used for Traffic Engineering
path reservation in <xref target="RFC3209"/>,
as well as User-to-Network Interfaces (UNI) such as provided by
the Local Management Interface (LMI) between network and end systems,
are all part of the Network Plane.
</t>
<t>
A Southbound (Network) Interface enables the entities in the Controller
Plane to communicate with devices in the Network Plane. This interface
leverages and extends TEAS to describe the physical topology and
resources in the Network Plane.
</t>
<figure align="center" anchor="NorthSouth">
<preamble>Stream Management Entity</preamble>
<artwork align="left"><![CDATA[
End End
System System
-+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
PCE PCE PCE PCE
-+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Relay Relay Relay Relay
System System System System
NIC NIC
Relay Relay Relay Relay
System System System System
]]></artwork>
</figure>
<t>
The relay systems (and eventually the end systems NIC) expose their capabilities and physical
resources to the controller (the PCE), and update the PCE with their dynamic perception of the
topology, across the Southbound Interface. In return, the PCE(s) set the per-stream
paths up, providing a Stream Characterization that is more tightly coupled to the relay system
Operation than a TSpec.
</t><t>
At the Network plane, relay systems exchange information regarding the state of the paths,
between adjacent systems and eventually with the end systems, and forward packets within
constraints associated to each stream, or, when unable to do so, perform a last resort
operation such as drop or declassify.
</t><t>
This specification focuses on the Southbound interface and the operation of the Network Plane.
</t>
</section>
<section anchor="elements" title="Elements of DetNet Architecture">
<t>
The DetNet architecture has a number of elements, discussed in the following sections.
Note that not every application requires all of these elements.
<list style="letters">
<t>
A model for the definition, identification, and operation of DetNet streams
(<xref target="DetNetStreams"/>), for use by relay systems to classify and
process individual packets following per-stream rules.
</t><t>
A model for the flow of data from an end system or
through a relay system that can be used to
predict the bounds for that system's impact on the QoS of a DetNet stream,
for use by the Controllers to
configure policing and
shaping engines in Network Systems over the Southbound
interface. The model includes:
<list style="numbers">
<t>
A model for queuing, transmission selection, shaping, preemption,
and timing resources that can be used by an end system or relay system to
control the
selection of packets output on an interface. These models must
have sufficiently well-defined
characteristics, both individually and in the aggregate, to give
predictable
results for the QoS for DetNet packets
(<xref target="QueuingModels"/>).
</t><t>
A model for identifying misbehaving DetNet streams and mitigating their impact
on properly functioning streams
(<xref target="FaultMitigation"/>).
</t>
</list>
</t><t>
A model for the relay system to inform the controller(s) of the
information it needs for adequate path computations including:
<list style="numbers">
<t>
Systems' individual capabilities (e.g. can do replication, can do precise time).
</t><t>
Link capabilities and resources (e.g. bandwidth, transmission delay, hardware
deterministic support to the physical layer, ...)
</t><t>
Physical resources (total and available buffers, timers, queues, etc)
</t><t>
Network Adjacencies (neighbors)
</t>
</list>
</t><t>
A model for the provision of a service, by end systems or relay systems,
to replicate and forward a DetNet stream over redundant paths. The model
includes:
<list style="numbers">
<t>
A model for specifying multiple stable paths (circuits) across a network
that can perform packet forwarding at both Layer 3 and at lower layers, to
which specific DetNet streams can be assigned.
</t><t>
A model and data plane format(s) for sequencing and replicating the packets of a DetNet
stream, typically at or near the talker, sending the replicated streams over
different stable paths, merging and/or re-replicating those
packets at other points in the network, and finally eliminating the duplicates,
typically at or near the listener(s), in order to provide high availability
(<xref target="Seamless"/>).
</t>
</list>
</t><t>
The protocol stack model for an end system and/or a relay system should support
the above elements in a manner that maximizes the applicability of existing
standards and protocols to the DetNet problem, and allows for the creation of new
protocols only where needed, thus making DetNet an add-on feature to existing
networks, rather than a new way to do networking. In particular this protocol
stack supports networks in which the path from talker to listener(s) includes
bridges and/or routers in any order
(<xref target="StackModel"/>).
</t><t>
A variety of models for the provisioning of DetNet streams can be envisioned, including
orchestration by a central controller or by a federation of controllers,
provisioning by relay systems and end systems sharing peer-to-peer protocols, by
off-line configuration, or by a combination of these methods. The provisioning
models are similar to existing Layer-2 and Layer-3 models, in order to
minimize the amount of innovation required in this area
(<xref target="Provisioning"/>).
</t>
</list>
</t>
</section>
<section anchor="DetNetStreams" title="DetNet streams">
<section anchor="StreamLimits" title="Talker guarantees">
<t>
DetNet streams can by synchronous or asynchronous.
In synchronous DetNet streams, at least the relay systems (and possibly
the end systems) are closely time
synchronized, typically to better than 1 microsecond. By transmitting
packets from different streams or classes of streams at different times,
using repeating schedules synchronized among the relay systems, resources
such as buffers and link bandwidth can be shared over the time domain
among different streams. There is a tradeoff among techniques for
synchronous streams between the burden of fine-grained scheduling and the
benefit of reducing the required resources, especially buffer space.
</t><t>
In contrast, asynchronous streams are not coordinated with a fine-grained
schedule, so relay and end systems must assume worst-case interference
among streams contending for buffer resources.
Asynchronous DetNet streams are characterized by:
<list style="symbols">
<t>
A maximum packet size;
</t><t>
An observation interval; and
</t><t>
A maximum number of transmissions during that observation interval.
</t>
</list>
</t><t>
These parameters, together with knowledge of the protocol stack used (and thus the
size of the various headers added to a packet), limit the number of bit times per
observation interval that the DetNet stream can occupy the physical medium.
</t><t>
The talker promises that these limits will not be exceeded. If the talker
transmits less data than this limit allows, the unused resources such as link
bandwidth can be made available by the system to non-DetNet packets. However,
making those resources available to DetNet packets in other streams would serve
no purpose. Those other streams have their own dedicated resources, on the
assumption that all DetNet streams can use all of their resources over a long
period of time.
</t><t>
Note that there is no provision in DetNet for throttling streams; the assumption
is that a DetNet stream, to be useful, must be delivered in its entirety. That
is, while any useful application is written to expect a certain number of lost
packets, the real-time applications of interest to DetNet demand that the loss of
data due to the network is extraordinarily infrequent.
</t><t>
Although DetNet strives to minimize the changes required of an application to
allow it to shift from a special-purpose digital network to an Internet Protocol
network, one fundamental shift in
the behavior of network applications is impossible to avoid--the reservation
of resources before the application starts.
In the first place, a network cannot deliver finite latency and practically zero
packet loss to an arbitrarily high offered load. Secondly, achieving
practically zero packet loss for unthrottled (though bandwidth limited) streams
means that bridges and routers have to dedicate buffer resources to specific
streams or to classes of streams. The requirements of each reservation have to be
translated into the parameters that control each system's
queuing, shaping, and scheduling functions and delivered to the hosts, bridges,
and routers.
</t>
</section>
<section anchor="Incomplete" title="Incomplete Networks">
<t>
The presence in the network of relay systems that are not fully capable of offering
DetNet services complicates the ability of the relay systems and/or controller to
allocate resources, as extra buffering, and thus extra latency, must be allocated
at each point that is downstream from the non-DetNet relay system for some
DetNet stream.
</t>
</section>
</section>
<section anchor="QueuingModels" title="Queuing, Shaping, Scheduling, and Preemption">
<t>
As described above, DetNet achieves its aims
by reserving bandwidth and buffer resources at every hop along
the path of the stream.
The reservation itself is not sufficient, however. Implementors and users of a
number of
proprietary and standard real-time networks have found that standards for
specific data plane techniques are required to enable these assurances to be
made in a multi-vendor
network. The fundamental reason is that latency variation in one system results
in the need for extra buffer space in the next-hop system(s), which in turn,
increases the worst-case per-hop latency.
</t><t>
Standard queuing and transmission selection algorithms allow a central controller
to compute the latency contribution
of each relay node to the end-to-end latency, to compute the amount of buffer space
required in each relay system for each incremental flow, and most importantly, to
translate from a flow specification to a set of values for the managed objects that
control each relay or end system. The IEEE 802 has specified (and is
specifying) a set of queuing, shaping, and scheduling algorithms
that enable each relay system (bridge or router), and/or a central controller, to
compute these values. These algorithms include:
<list style="symbols">
<t>
A credit-based shaper <xref target="IEEE802.1Q-2014">IEEE 802.1Q Clause 34</xref>.
</t><t>
Time-gated queues governed by a rotating time schedule, synchronized among all
relay nodes <xref target="IEEE802.1Qbv"/>.
</t><t>
Synchronized double (or triple) buffers driven by synchronized time ticks.
<xref target="IEEE802.1Qch"/>.
</t><t>
Pre-emption of an Ethernet packet in transmission by a packet with a more stringent
latency requirement, followed by the resumption of the preempted packet
<xref target="IEEE802.1Qbu"/>, <xref target="IEEE802.3br"/>.
</t>
</list>
</t><t>
While these techniques are currently embedded in Ethernet and bridging standards,
we can note that they are all, except perhaps for packet preemption, equally applicable
to other media than Ethernet, and to routers as well as bridges.
</t>
</section>
<section anchor="Coexistence" title="Coexistence with normal traffic">
<t>
A DetNet network supports the dedication of a high proportion (e.g. 75%) of the
network bandwidth
to DetNet streams. But, no matter how much is dedicated for DetNet streams, it is
a goal of DetNet to not interfere excessively with existing QoS schemes. It is also
important that non-DetNet traffic not disrupt the DetNet stream, of course (see
<xref target="FaultMitigation"/> and <xref target="SecurityConsiderations"/>).
For these reasons:
<list style="symbols">
<t>
Bandwidth (transmission opportunities) not utilized by a DetNet stream are available
to non-DetNet packets (though not to other DetNet streams).
</t><t>
DetNet streams can be shaped, in order to ensure that the highest-priority non-DetNet
packet also is ensured a worst-case latency (at any given hop).
</t><t>
When transmission opportunities for DetNet streams are scheduled in detail, then
the algorithm constructing the schedule should leave sufficient opportunities for
non-DetNet packets to satisfy the needs of the uses of the network.
</t>
</list>
</t><t>
Ideally, the net effect of the presence of DetNet streams in a network on the non-DetNet
packets is primarily a reduction in the available bandwidth.
</t>
</section>
<section anchor="FaultMitigation" title="Fault Mitigation">
<t>
One key to building robust real-time systems is to reduce the infinite variety of
possible failures to a number that can be analyzed with reasonable confidence. DetNet
aids in the process by providing filters and policers to detect DetNet packets received
on the wrong interface, or at the wrong time, or in too great a volume, and to then take
actions such as discarding the offending packet, shutting down the offending DetNet stream,
or shutting down the offending interface.
</t><t>
It is also essential that filters and service remarking be employed at the network edge
to prevent non-DetNet
packets from being mistaken for DetNet packets, and thus impinging on the resources
allocated to DetNet packets.
</t><t>
There exist techniques, at present and/or in various stages of standardization, that can
perform these fault mitigation tasks that deliver a high probability that misbehaving
systems will have zero impact on well-behaved DetNet streams, except of course, for
the receiving interface(s) immediately downstream of the misbehaving device.
</t>
</section>
<section anchor="StackModel" title="Protocol Stack Model">
<t>
<xref target="IEEE802.1CB"/>, Annex C, offers
a description of the TSN protocol stack. While this standard is a work in progress,
a consensus around the basic architecture has formed. This stack is summarized in
<xref target="ProtStack1"/>.
</t>
<figure align="center" anchor="ProtStack1">
<preamble>DetNet Protocol Stack</preamble>
<artwork align="center"><![CDATA[
+--------------------------------+
| Upper Layers |
+--------------------------------+
| Sequence generation/recovery |
+--------------------------------+
| Sequence encode/decode |
+--------------------------------+
| Stream splitting/merging |
+--------------------------------+
| Stream encode/decode |
+--------------------------------+
| Lower layers |
+--------------------------------+
]]></artwork>
</figure>
<t>
Not all layers are required for any given application, or even for any
given network. The layers are, from top to bottom:
<list hangIndent="8" style="hanging">
<t hangText="Sequence generation/recovery"><vspace blankLines="0"/>
Supplies the sequence number for Seamless Redundancy (<xref target="Seamless"/>)
for packets going down the stack, and discards duplicate packets coming
up the stack.
</t>
<t hangText="Sequence encode/decode"><vspace blankLines="0"/>
Encodes the sequence number into packets going down the stack, and
extracts the sequence number from packets coming up the stack. This
function may or may not be a null transformation of the packet, and for
some protocols, is not explicitly present, being included in the Stream
encode/decode layer, below.
</t>
<t hangText="Stream splitting/merging"><vspace blankLines="0"/>
Replicates packets going down the stack into two streams, and merges
streams together for packets coming up the stack, based on the packet's
stream identifier. Needed for Seamless Redundancy
(<xref target="Seamless"/>).
</t>
<t hangText="Stream encode/decode"><vspace blankLines="0"/>
Encapsulates packets going down the stack, based on the packet's
locally-significant stream identifier, in order to identify to which
stream the packet belongs, and extracts a locally-significant
stream identifier from
packets coming up the stack. This may be a null transformation (e.g.,
for streams identified by IP 5-tuple) or might be an explicit
encapsulation (e.g., for streams identified with an MPLS label).
Stream identification is the basis for Seamless Redundancy, for
assigning per-flow resources (if any) to packets and for defence
against misbehaving systems (<xref target="FaultMitigation"/>).
When streams are assigned to pinned-down paths, this layer can be
indistinguishable from the data forwarding layer(s).
</t>
</list>
</t><t>
The reader is likely to notice that <xref target="ProtStack1"/> does not
specify the relationship between the DetNet layers, the IP layers, and
the link layers. This is intentional, because they can usefully be placed
different places in the stack, and even in mulitple places, depending on
where their peers are placed.
</t>
</section>
<section anchor="Advertising" title="Advertising resources, capabilities and adjacencies">
<t>
There are three classes of information that a central controller needs to
know that can only be obtained from the end systems and/or relay systems
in the network. When using a peer-to-peer control plane, some of this
information may be required by a system's neighbors in the network.
<list style="symbols"><t>
Details of the system's capabilities that are required in order to
accurately allocate that system's resources, as well as other systems'
resources. This includes, for example, which specific queuing and
shaping algorithms are implemented (<xref target="QueuingModels"/>),
the number of buffers dedicated for DetNet allocation, and the worst-case
forwarding delay.
</t><t>
The dynamic state of an end or relay system's DetNet resources.
</t><t>
The identity of the system's neighbors, and the characteristics of the
link(s) between the systems, including the length (in nanoseconds) of
the link(s).
</t></list>
</t>
</section>
<section anchor="Provisioning" title="Provisioning model">
<section anchor="pce" title="Centralized Path Computation and Installation">
<t>
A centralized routing model, such as provided with a PCE (<xref
target="RFC4655">RFC 4655</xref>), enables global and
per-stream optimizations. The model is attractive but a number of issues are
left to be solved.
In particular:
<list style="symbols"> <t>whether and how the path computation can
be installed by 1) an end device or 2) a Network Management entity,
</t><t>
and how
the path is set up, either by installing state at each hop with a direct
interaction between the forwarding device and the PCE, or along a path by
injecting a source-routed request at one end of the path.
</t> </list>
</t>
</section>
<section anchor="dc" title="Distributed Path Setup">
<t> Whether a distributed alternative without a PCE can be valuable should
be studied as well. Such an alternative could for instance inherit from the
<xref target="RFC5127">Resource ReSerVation Protocol</xref> (RSVP) flows.
</t><t>
In a Layer-2 only environment, or as part of a layered approach to a
mixed environment, IEEE 802.1 also has work, either completed
or in progress. <xref target="IEEE802.1Q-2014"/> Clause 35 describes
SRP, a peer-to-peer protocol for Layer-2 roughly analogous to RSVP. Almost
complete is <xref target="IEEE802.1Qca"/>, which defines how ISIS can
provide multiple disjoint paths or distribution trees. Also in progress
is <xref target="IEEE802.1Qcc"/>, which expands the capabilities
of SRP.
</t>
</section>
</section>
</section>
<section anchor="rel" title="Related IETF work">
<section anchor="del" title='Deterministic PHB'>
<t>
<xref target="I-D.svshah-tsvwg-deterministic-forwarding"/>
defines a Differentiated Services Per-Hop-Behavior
(PHB) Group called Deterministic Forwarding (DF). The document
describes the purpose and semantics of this PHB. It also describes
creation and forwarding treatment of the service class. The document
also describes how the code-point can be mapped into one of the
aggregated Diffserv service classes <xref target="RFC5127"/>.
</t>
</section>
<section anchor="sixt" title='6TiSCH'>
<t>
Industrial process control already leverages deterministic
wireless Low power and Lossy Networks (LLNs) to interconnect critical
resource-constrained devices and form wireless mesh networks, with
standards such as <xref target="ISA100.11a"/> and <xref target="WirelessHART"/>.
</t> <t>
These standards rely on variations of the <xref target="IEEE802154e"/>
<xref target="I-D.ietf-6tisch-tsch">timeSlotted Channel Hopping (TSCH)
</xref> Medium Access Control (MAC), and a form of centralized Path
Computation Element (PCE), to deliver deterministic capabilities.
</t> <t>
The TSCH MAC benefits include high reliability against interference, low
power consumption on characterized streams, and Traffic Engineering
capabilities. Typical applications are open and closed control loops,
as well as supervisory control streams and management.
</t> <t>
The 6TiSCH Working Group focuses only on the TSCH mode of the IEEE 802.15.4e
standard. The WG currently defines a framework for managing the TSCH schedule.
Future work will standardize deterministic operations over so-called tracks
as described in <xref target="I-D.ietf-6tisch-architecture"/>.
Tracks are an instance of a deterministic path, and the DetNet work
is a prerequisite to specify track operations and serve process control
applications.
</t><t><xref target="RFC5673"/> and
<xref target="I-D.ietf-roll-rpl-industrial-applicability"/> section 2.1.3.
and next discusses application-layer paradigms, such as Source-sink (SS)
that is a Multipeer to Multipeer (MP2MP) model that is primarily used for
alarms and alerts, Publish-subscribe (PS, or pub/sub) that is typically
used for sensor data, as well as Peer-to-peer (P2P) and Peer-to-multipeer
(P2MP) communications. Additional considerations on Duocast and its N-cast
generalization are also provided for improved reliability.
</t>
</section>
</section>
<section anchor="SecurityConsiderations" title="Security Considerations">
<t>
Security in the context of Deterministic Networking has an added
dimension; the time of delivery of a packet can be just as important
as the contents of the packet, itself. A man-in-the-middle attack,
for example, can impose, and then systematically adjust, additional
delays into a link, and thus disrupt or subvert a real-time
application without having to crack any encryption methods employed.
See <xref target="RFC7384"/> for an
exploration of this issue in a related context.
</t><t>
Furthermore, in a control system where millions of dollars of equipment, or even
human lives, can be lost if the DetNet QoS is not delivered, one must consider
not only simple equipment failures, where the box or wire instantly becomes
perfectly silent, but bizarre errors such as can be caused by software failures.
Because there is essential no limit to the kinds of failures that can occur,
protecting against realistic equipment failures is indistinguishable, in most
cases, from protecting against malicious behavior, whether accidental or intentional.
See also <xref target="FaultMitigation"/>.
</t>
<t>Security must cover:
<list style="symbols"> <t>
the protection of the signaling protocol
</t><t>
the authentication and authorization of the controlling systems
</t><t>
the identification and shaping of the streams
</t> </list>
</t>
</section>
<section title="IANA Considerations">
<t>This document does not require an action from IANA.
</t>
</section>
<section title="Acknowledgements">
<t>The authors wish to thank Jouni Korhonen, Erik Nordmark, George Swallow,
Rudy Klecka, Anca Zamfir, David Black, Thomas Watteyne, Shitanshu Shah,
Craig Gunther, Rodney Cummings, Wilfried Steiner, Marcel Kiessling, Karl Weber,
Ethan Grossman and Pat Thaler,
for their various contribution with this work.</t>
</section>
<section title="Access to IEEE 802.1 documents">
<t>
To access password protected IEEE 802.1 drafts, see the
IETF IEEE 802.1 information page at
https://www.ietf.org/proceedings/52/slides/bridge-0/tsld003.htm.
</t>
</section>
</middle>
<back>
<references title='Informative References'>
<?rfc include='reference.I-D.svshah-tsvwg-deterministic-forwarding'?>
<?rfc include='reference.I-D.ietf-roll-rpl-industrial-applicability'?>
<?rfc include='reference.I-D.ietf-6tisch-tsch'?>
<?rfc include='reference.I-D.ietf-6tisch-architecture'?>
<?rfc include='reference.I-D.finn-detnet-problem-statement'?>
<?rfc include='reference.RFC.2205'?>
<?rfc include='reference.RFC.3209'?>
<?rfc include='reference.RFC.4655'?>
<?rfc include='reference.RFC.5127'?>
<?rfc include='reference.RFC.5673'?>
<?rfc include='reference.RFC.7384'?>
<?rfc include='reference.RFC.7426'?>
<reference anchor="IEEE802.1CB"
target="http://www.ieee802.org/1/files/private/cb-drafts/">
<front>
<title>Seamless Redundancy (IEEE Draft P802.1CB)</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2015" />
</front>
</reference>
<reference anchor="IEEE802.1Qca"
target="http://www.ieee802.org/1/files/private/ca-drafts/">
<front>
<title>Path Control and Reservation</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2015" />
</front>
</reference>
<reference anchor="IEEE802.1Qcc"
target="http://www.ieee802.org/1/files/private/cc-drafts/">
<front>
<title>Stream Reservation Protocol (SRP) Enhancements and Performance Improvements</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2015" />
</front>
</reference>
<reference anchor="IEEE802.1Qbu"
target="http://www.ieee802.org/1/files/private/bu-drafts/">
<front>
<title>Frame Preemption</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2015" />
</front>
</reference>
<reference anchor="IEEE802.1Qbv"
target="http://www.ieee802.org/1/files/private/bv-drafts/">
<front>
<title>Enhancements for Scheduled Traffic</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2015" />
</front>
</reference>
<reference anchor="IEEE802.1AS-2011"
target="http://standards.ieee.org/getIEEE802/download/802.1AS-2011.pdf">
<front>
<title>Timing and Synchronizations (IEEE 802.1AS-2011)</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2011" />
</front>
</reference>
<reference anchor="IEEE802.1BA-2011"
target="http://standards.ieee.org/getIEEE802/download/802.1BA-2011.pdf">
<front>
<title>AVB Systems (IEEE 802.1BA-2011)</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2011" />
</front>
</reference>
<reference anchor="IEEE802.1Q-2014"
target="http://standards.ieee.org/getIEEE802/download/802.1Q-2014.pdf">
<front>
<title>MAC Bridges and VLANs (IEEE 802.1Q-2014</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2014" />
</front>
</reference>
<reference anchor="IEEE802.1Qch"
target="https://development.standards.ieee.org/P883200033/par">
<front>
<title>Cyclic Queuing and Forwarding</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2011" />
</front>
</reference>
<reference anchor="IEEE802.3br"
target="http://www.ieee802.org/3/br/">
<front>
<title>Interspersed Express Traffic</title>
<author>
<organization>IEEE</organization>
</author>
<date year="2015" />
</front>
</reference>
<reference anchor="ISA95"
target="https://www.isa.org/isa95/">
<front>
<title>Enterprise-Control System Integration Part 1: Models and Terminology</title>
<author>
<organization>ANSI/ISA</organization>
</author>
<date year="2000" />
</front>
</reference>
<reference anchor="ISA100.11a"
target=" http://www.isa100wci.org/en-US/Documents/PDF/3405-ISA100-WirelessSystems-Future-broch-WEB-ETSI.aspx">
<front>
<title>ISA100.11a, Wireless Systems for Automation, also IEC 62734</title>
<author>
<organization>ISA/IEC</organization>
</author>
<date year="2011" />
</front>
</reference>
<reference anchor="IEEE802.1TSNTG" target="http://www.IEEE802.org/1/pages/avbridges.html">
<front>
<title>IEEE 802.1 Time-Sensitive Networks Task Group</title>
<author>
<organization>IEEE Standards Association</organization>
</author>
<date year="2013" />
</front>
</reference>
<reference anchor="IEEE802154e">
<front>
<title>IEEE std. 802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area Networks (LR-WPANs) Amendment 1: MAC sublayer</title>
<author>
<organization>IEEE standard for Information Technology</organization>
</author>
<date month="April" year="2012"/>
</front>
</reference>
<reference anchor="IEEE802154">
<front>
<title>IEEE std. 802.15.4, Part. 15.4: Wireless Medium Access Control
(MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless
Personal Area Networks</title>
<author>
<organization>IEEE standard for Information Technology</organization>
</author>
<date month="June" year="2011"/>
</front>
</reference>
<reference anchor="WirelessHART">
<front>
<title>Industrial Communication Networks - Wireless Communication
Network and Communication Profiles - WirelessHART - IEC 62591</title>
<author>
<organization>www.hartcomm.org</organization>
</author>
<date year="2010" />
</front>
</reference>
<reference anchor="HART">
<front>
<title>Highway Addressable Remote Transducer, a group of
specifications for industrial process and control devices
administered by the HART Foundation</title>
<author>
<organization>www.hartcomm.org</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="ODVA">
<front>
<title>The organization that supports network technologies built on
the Common Industrial Protocol (CIP) including EtherNet/IP.</title>
<author>
<organization>http://www.odva.org/</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="AVnu">
<front>
<title>The AVnu Alliance tests and certifies devices for
interoperability, providing a simple and reliable networking
solution for AV network implementation based on the Audio
Video Bridging (AVB) standards.</title>
<author>
<organization>http://www.avnu.org/</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="Profinet" target="http://us.profinet.com/technology/profinet/">
<front>
<title>PROFINET is a standard for industrial networking in
automation. </title>
<author>
<organization>http://us.profinet.com/technology/profinet/</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="HSR-PRP" target="http://webstore.iec.ch/webstore/webstore.nsf/artnum/046615!opendocument">
<front>
<title>High availability seamless redundancy (HSR) is a further
development of the PRP approach, although HSR functions primarily
as a protocol for creating media redundancy while PRP, as described
in the previous section, creates network redundancy.
PRP and HSR are both described in the IEC 62439 3 standard.</title>
<author>
<organization>IEC</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="TEAS" target="https://datatracker.ietf.org/doc/charter-ietf-teas/">
<front>
<title>Traffic Engineering Architecture and Signaling</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="PCE" target="https://datatracker.ietf.org/doc/charter-ietf-pce/">
<front>
<title>Path Computation Element</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="CCAMP" target="https://datatracker.ietf.org/doc/charter-ietf-ccamp/">
<front>
<title>Common Control and Measurement Plane</title>
<author>
<organization>IETF</organization>
</author>
<date></date>
</front>
</reference>
</references>
</back>
</rfc>
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