One document matched: draft-finn-detnet-architecture-03.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 flows 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 flows in some or all of the relay systems (bridges or routers) along
the path of the flow; 2) providing fixed paths for DetNet flows 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>
Deterministic Networking (DetNet) is a service that can be offered by a
network to data flows
(DetNet flows) that that are limited, at their source, to a maximum data
rate specified by that source. DetNet provides these flows extremely
low packet loss rates and assured maximum end-to-end delivery latency.
This is accomplished by dedicating network resources such as link bandwidth
and buffer space to DetNet flows and/or
classes of DetNet flows. Unused reserved resources are available to non-DetNet
packets.
</t><t>
The <xref target="I-D.finn-detnet-problem-statement">Deterministic Networking
Problem Statement</xref> introduces Deterministic Networking, and
<xref target="I-D.grossman-detnet-use-cases">Deterministic Networking
Use Cases</xref> summarizes the need for it.
</t><t>
A goal of DetNet is a converged network in all respects. That is, the presence
of DetNet flows does not preclude non-DetNet flows, and the benefits offered
DetNet flows should not, except in extreme cases, prevent existing QoS
mechanisms from operating in a
normal fashion, subject to the bandwidth required for the DetNet flows. A
single source-destination pair can trade both DetNet and non-DetNet flows.
End systems and applications need not instantiate special interfaces for DetNet flows.
Networks are not restricted to certain topologies; connectivity is not restricted.
Any application that generates a data flow that can be usefully
characterized as having a maximum bandwidth should be able to take advantage
of DetNet, as long as the necessary resources can be reserved. Reservations
can be made by the application itself, via network management, by an
applications controller, or by other means.
</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><t>
The present document is an individual contribution, intended by the authors
for eventual adoption by the DetNet working group. As such, it expresses the
only the opinions of the authors.
</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, bridge, firewall, or otherwise
plays a particular role at Layer-2 or higher. This section also serves as a
dictionary for translating between IEEE 802 and DetNet terminology.
<list hangIndent="8" style="hanging">
<t hangText="destination"><vspace blankLines="0"/>
An end system capable of sinking a DetNet flow.
</t>
<t hangText="DetNet flow"><vspace blankLines="0"/>
A DetNet flow is a sequence of packets from a single source, through some
number of relay systems to one or more destinations, that is
limited by the source 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="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 either sources or
destinations.
</t>
<t hangText="listener"><vspace blankLines="0"/>
The IEEE 802 term for a destination of a DetNet flow.
</t>
<t hangText="relay system"><vspace blankLines="0"/>
A router, bridge, Label Switch Router (LSR), firewall, or any other system
that forwards packets from one interface to another.
</t>
<t hangText="reservation"><vspace blankLines="0"/>
A trail of configuration from source to destination(s) through relay systems
associated with a DetNet flow, required to deliver the benefits of DetNet.
</t>
<t hangText="source"><vspace blankLines="0"/>
An end system capable of sourcing a DetNet flow.
</t>
<t hangText="stream"><vspace blankLines="0"/>
The IEEE 802 term for a DetNet flow.
</t>
<t hangText="talker"><vspace blankLines="0"/>
The IEEE 802 term for the source of a DetNet flow.
</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 source to destination;
</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 data 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 flow or
class of DetNet flows. 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 paths (<xref target="pinned"/>). Point-to-point paths or point-to-multipoint
trees through the network from a source to one or more destinations can be established,
and DetNet flows 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 flow.
</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 paths (a) plus packet replication (b) are exactly the techniques
employed by <xref target="HSR-PRP"/>. Pinned 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 flow 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 source, and every relay
system along the path to the destination (or nearly every relay system -- see
<xref target="Incomplete"/>) be careful to regulate its output to not exceed the
data rate for any DetNet flow, 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 DetNet flow.
</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 DetNet flow requires the provisioning described in
<xref target="Provisioning"/>.
</t>
</section>
<section anchor="pinned" title="Pinned 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 paths, where the path taken by a given DetNet flow
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 flow.
</t><t>
Replicating these packets and, typically, sending them along at least two
different paths to the destination(s). (Often, the pinned 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 source that
has two interfaces, and conveying them through the network, along separate paths,
to the
similarly dual-homed destinations, 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 flow on input, sending the DetNet flow 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 link in the network can
fail, and the Detnet flow 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">
<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 flows
(<xref target="DetNetFlows"/>), for use by relay systems to classify and
process individual packets following per-flow rules.
</t><t>
A model for the flow of data out of 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 flow,
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 flows and mitigating their impact
on properly functioning DetNet flows
(<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 (<xref target="te"/>) 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 flow over redundant paths. The model
includes:
<list style="numbers">
<t>
A model for specifying multiple stable paths across a network
that can perform packet forwarding at both Layer 3 and at lower layers, to
which specific DetNet flows can be assigned (<xref target="te"/>).
</t><t>
A model and data plane format(s) for sequencing and replicating the packets of a DetNet
flow, typically at or near the source, sending the replicated DetNet flows 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 destination(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 source to destination(s) includes
bridges and/or routers in any order
(<xref target="StackModel"/>).
</t><t>
A variety of models for the provisioning of DetNet flows 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="te" title="Traffic Engineering for DetNet">
<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 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 Flow Management Entity,
(FME) 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 flows between end
systems. An abstraction of the flow 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 FME
as Per-fFlow Per-Hop Behaviors installed in the relay systems for
each individual flow. The PCEs
place each flow along a deterministic sequence of relay systems so as
to respect per-flow 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>Flow 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-flow
paths up, providing a Flow 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 flow, 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>
<section anchor="DetNetFlows" title="DetNet flows">
<section anchor="FlowLimits" title="Source guarantees">
<t>
DetNet flows can by synchronous or asynchronous.
In synchronous DetNet flows, 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 DetNet flows or classes of DetNet flows 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 DetNet flows. There is a tradeoff among techniques for
synchronous DetNet flows between the burden of fine-grained scheduling and the
benefit of reducing the required resources, especially buffer space.
</t><t>
In contrast, asynchronous DetNet flows are not coordinated with a fine-grained
schedule, so relay and end systems must assume worst-case interference
among DetNet flows contending for buffer resources.
Asynchronous DetNet flows 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 flow can occupy the physical medium.
</t><t>
The source promises that these limits will not be exceeded. If the source
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 DetNet flows would serve
no purpose. Those other DetNet flows have their own dedicated resources, on the
assumption that all DetNet flows can use all of their resources over a long
period of time.
</t><t>
Note that there is no provision in DetNet for throttling DetNet flows
(reducing the transmission rate via feedback); the assumption
is that a DetNet flow, 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) DetNet flows
means that bridges and routers have to dedicate buffer resources to specific
DetNet flows or to classes of DetNet flows. 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 points downstream from the non-DetNet relay system for a DetNet flow.
</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 DetNet flow.
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 DetNet 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"/> Clause 34.
</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 flows. But, no matter how much is dedicated for DetNet flows, 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 flow, 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 flow are available
to non-DetNet packets (though not to other DetNet flows).
</t><t>
DetNet flows 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 flows 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 flows 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 flow,
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 flows, 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 |
+--------------------------------+
| DetNet flow splitting/merging |
+--------------------------------+
| Individual flow checking |
+--------------------------------+
| Sequence encode/decode |
+--------------------------------+
| DetNet flow 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="DetNet flow splitting/merging"><vspace blankLines="0"/>
Replicates packets going down the stack into two DetNet flows, and merges
DetNet flows together for packets coming up the stack, based on the packet's
DetNet flow identifier. Needed for Seamless Redundancy
(<xref target="Seamless"/>).
</t>
<t hangText="Individual flow checking"><vspace blankLines="0"/>
Examines packets belonging to individual flows, discards duplicate
packets coming up the stack, and performs checks to detect contract
violations.
</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 DetNet flow
encode/decode layer, below.
</t>
<t hangText="DetNet flow encode/decode"><vspace blankLines="0"/>
Encapsulates packets going down the stack, based on the packet's
locally-significant DetNet flow identifier, in order to identify to which
DetNet flow the packet belongs, and extracts a locally-significant
DetNet flow identifier from
packets coming up the stack. This may be a null transformation (e.g.,
for DetNet flows identified by IP 5-tuple) or might be an explicit
encapsulation (e.g., for DetNet flows identified with an MPLS label).
DetNet flow identification is the basis for Seamless Redundancy, for
assigning per-flow resources (if any) to packets and for defense
against misbehaving systems (<xref target="FaultMitigation"/>).
When DetNet flows are assigned to pinned 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-flow optimizations. (See <xref target="te"/>.)
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 anchor="Scaling" title="Scaling to larger networks">
<t>
Reservations for individual DetNet flows require considerable state information in
each relay system, especially when adequate fault mitigation
(<xref target="FaultMitigation"/>) is required. The DetNet data plane, in order to
support larger numbers of DetNet flows, must support the aggregation of DetNet flows
into tunnels, which themselves can be viewed by the relay systems' data planes
largely as individual DetNet flows.
</t>
</section>
<section anchor="Islands" title="Connected islands vs. networks">
<t>
Given that users have deployed examples of the IEEE 802.1 TSN TG standards, which
provide capabilities similar to DetNet, it is obvious to ask whether the IETF
DetNet effort can be limited to providing Layer-2 tunnels between islands of
bridged TSN networks. While this capability is certainly useful to some
applications, and must not be precluded by DetNet, tunneling alone is not a
sufficient goal for the DetNet WG. As shown in the
<xref target="I-D.grossman-detnet-use-cases">Deterministic Networking Use Cases draft</xref>,
there are already deployments of Layer-2 TSN networks that are encountering
the well-known problems of over-large broadcast domains. Routed solutions, and
combinations routed/bridged solutions, are both required.
</t>
</section>
</section>
<section anchor="Compatibility" title="Compatibility with Layer-2">
<t>
Standards providing similar
capabilities for bridged networks (only) have been and are being generated in the
IEEE 802 LAN/MAN Standards Committee. The present architecture
describes an abstract model that can be applicable both at Layer-2
and Layer-3, and over links not defined by IEEE 802. It is the intention
of the authors (and hopefully, as this draft progresses, of the DetNet
Working Group) that IETF and IEEE 802 will coordinate their work, via
the participation of common individuals, liaisons, and other means,
to maximize the compatibility of their outputs.
</t>
</section>
<section anchor="Questions" title="Open Questions">
<t>
There are a number of architectural questions that will have to be resolved
before this document can be submitted for publication. Aside from the obvious
fact that this present draft is subject to change, there are specific questions
to which the authors wish to direct the readers' attention.
</t>
<section anchor="Shapers" title="Data plane shapers and schedulers">
<t>
A number of techniques have been defined and are being defined by IEEE 802
for queuing, shaping, and scheduling transmissions on EtherNet media, most
of which are directly applicable to any other medium. Specific selections
of supported techniques are required, because minimizing, and even
eliminating, congestion losses depends strongly on the details of the per-hop
behavior of sources and relay systems.
</t><t>
The present authors expect that, at least, the IEEE 802 mechanisms will be
supported.
</t>
</section>
<section anchor="FlowIdSeq" title="DetNet flow identification and sequencing">
<t>
The techniques to be used for DetNet flow identification must be settled.
The following paragraphs provide a snapshot of the authors' opinions at
the time of writing. These authors anticipate the submission of drafts in
the near future on this subject.
</t><t>
IEEE 802.1 TSN streams are identified by giving each stream (DetNet flow) a
{VLAN identifier, destination MAC address} pair that is unique in the
bridged network, and that the MAC address must be a multicast address.
If a source is generating, for example, two unicast UDP
flows to the same destination, one DetNet and one not, the DetNet flow's
packets must be transformed at some point to have a multicast
destination MAC address, and perhaps, a different VLAN than the non-DetNet
flow's packets.
</t><t>
A similar provision would apply to DetNet packets that are identified by
MPLS labels; any bridges between the LSRs need a {VLAN identifier, destination MAC address} pair
uniquely identifying the DetNet flow in the bridged network.
</t><t>
Provision is made in current draft of <xref target="IEEE802.1CB"/> to
make these transformations either in a Layer-2 shim in the source end system,
on the output side of a router or LSR, or in a proxy function in the
first-hop bridge. It remains to be seen whether this provision is
adequate and/or acceptable to the IETF DetNet WG.
</t><t>
There are also questions regarding the sequentialization of packets for use
with Seamless Redundancy (<xref target="Seamless"/>). <xref target="IEEE802.1CB"/>
defines an EtherNet tag carrying a sequence number. If MPLS Pseudowires
are used with a control word containing a sequence number, the relationship
and interworking between these two formats must be defined.
</t>
</section>
<section anchor="FlatControl" title="Flat vs. hierarchical control">
<t>
Boxes that are solely routers or solely bridges are rare in today's market.
In a multi-tenant data center, multiple users' virtual Layer-2/Layer-3 topologies
exist simultaneously, implemented on a network whose physical topology bears
only accidental resemblance to the virtual topologies.
</t><t>
While the forwarding topology (the bridges and routers) are an important
consideration for a DetNet Flow Management Entity (<xref target="appplane"/>),
so is the purely physical topology. Ultimately, the model used by the
management entities is based on boxes, queues, and links. The authors
hope that the work of the TEAS WG will help to clarify exactly what model
parameters need to be traded between the relay systems and the controller(s).
</t>
</section>
<section anchor="PeerPeerProt" title="Peer-to-peer reservation protocol">
<t>
As described in <xref target="dc"/>, the DetNet WG needs to decide whether
to support a peer-to-peer protocol for a source and a destination
to reserve resources for a DetNet stream. Assuming that enabling the
involvement of the source and/or destination is desirable (see
<xref target="I-D.grossman-detnet-use-cases">Deterministic Networking Use Cases</xref>),
it remains to decide whether the DetNet WG will make it possible to deploy at least some
DetNet capabilities in a network using only a peer-to-peer protocol, without
a central controller.
</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 DetNet flows
</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, Pat Thaler, and Lou Berger
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.I-D.grossman-detnet-use-cases'?>
<?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'?>
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</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>
| PAFTECH AB 2003-2026 | 2026-04-23 05:33:44 |