Deterministic Networking Architecture
draft-finn-detnet-architecture-02
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| Authors | Norman Finn , Pascal Thubert , Michael Johas Teener | ||
| Last updated | 2015-11-01 | ||
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draft-finn-detnet-architecture-02
DetNet N. Finn
Internet-Draft P. Thubert
Intended status: Standards Track Cisco
Expires: May 4, 2016 M. Johas Teener
Broadcom
November 1, 2015
Deterministic Networking Architecture
draft-finn-detnet-architecture-02
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on May 4, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Providing the DetNet Quality of Service . . . . . . . . . . . 5
3.1. Zero Congestion Loss . . . . . . . . . . . . . . . . . . 7
3.2. Pinned-down paths . . . . . . . . . . . . . . . . . . . . 8
3.3. Seamless Redundancy . . . . . . . . . . . . . . . . . . . 8
4. DetNet Architecture . . . . . . . . . . . . . . . . . . . . . 9
4.1. The Application Plane . . . . . . . . . . . . . . . . . . 11
4.2. The Controller Plane . . . . . . . . . . . . . . . . . . 11
4.3. The Network Plane . . . . . . . . . . . . . . . . . . . . 12
4.4. Elements of DetNet Architecture . . . . . . . . . . . . . 13
4.5. DetNet streams . . . . . . . . . . . . . . . . . . . . . 14
4.5.1. Talker guarantees . . . . . . . . . . . . . . . . . . 14
4.5.2. Incomplete Networks . . . . . . . . . . . . . . . . . 16
4.6. Queuing, Shaping, Scheduling, and Preemption . . . . . . 16
4.7. Coexistence with normal traffic . . . . . . . . . . . . . 17
4.8. Fault Mitigation . . . . . . . . . . . . . . . . . . . . 17
4.9. Protocol Stack Model . . . . . . . . . . . . . . . . . . 18
4.10. Advertising resources, capabilities and adjacencies . . . 19
4.11. Provisioning model . . . . . . . . . . . . . . . . . . . 20
4.11.1. Centralized Path Computation and Installation . . . 20
4.11.2. Distributed Path Setup . . . . . . . . . . . . . . . 20
5. Related IETF work . . . . . . . . . . . . . . . . . . . . . . 20
5.1. Deterministic PHB . . . . . . . . . . . . . . . . . . . . 20
5.2. 6TiSCH . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
9. Access to IEEE 802.1 documents . . . . . . . . . . . . . . . 22
10. Informative References . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
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.
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).
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.
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.
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.
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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.
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.
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.
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
Section 4.6 also require time synchronization among relay systems.
The means used to achieve time synchronization are not addressed in
this document.
2. Terminology
The following special terms are used in this document in order to
avoid the assumption that a given element in the architecture does or
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does not have Internet Protocol stack, functions as a router or a
bridge, or otherwise plays a particular role at Layer-3 or higher:
bridge
A Customer Bridge as defined by IEEE 802.1Q
[IEEE802.1Q-2014].
end system
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.
listener
An end system capable of sinking a DetNet stream.
relay system
A router or a bridge.
reservation
A trail of configuration from talker to listener(s) through
relay systems associated with a DetNet stream, required to
deliver the benefits of DetNet.
stream
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.
talker
An end system capable of sourcing a DetNet stream.
3. Providing the DetNet Quality of Service
DetNet Quality of Service is expressed in terms of:
o Minimum and maximum end-to-end latency from talker to listener;
o Probability of loss of a packet, assuming the normal operation of
the relay systems and links;
o Probability of loss of a packet in the event of the failure of a
relay system or link.
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
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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.
Three techniques are employed by DetNet to achieve these QoS
parameters:
a. Zero congestion loss (Section 3.1). 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.
b. Pinned-down paths (Section 3.2). 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.
c. Packet replication and deletion (Section 3.3). 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.
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:
o Pinned-down paths (a) plus packet replication (b) are exactly the
techniques employed by [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.
o Zero congestion loss (a) alone is is offered by IEEE 802.1 Audio
Video bridging [IEEE802.1BA-2011]. As long as the network suffers
no failures, zero congestion loss can be achieved through the use
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of a reservation protocol (MSRP), shapers in every relay system
(bridge), and a bit of network calculus.
o Using all three together gives maximum protection.
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.
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.
3.1. Zero Congestion Loss
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.
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 Section 4.5.2) 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.
The low-level mechanisms described in Section 4.6 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 Section 4.11.
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3.2. Pinned-down paths
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.
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.
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
(Section 3.3), this results in a high likelihood of continuous
connectivity.
3.3. Seamless Redundancy
After congestion loss has been eliminated, the most important causes
of packet loss are random media and/or memory faults, and equipment
failures.
Seamless redundancy involves three capabilities:
o Adding sequence numbers, once, to the packets of a DetNet stream.
o Replicating these packets and, typically, sending them along at
least two different paths to the listener(s). (Often, the pinned-
down paths of Section 3.2.)
o Discarding duplicated packets.
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.
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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.
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.
> > > > > > > > relay > > > > > > > >
> /------------+ R system E +------------\ >
> / v + ^ \ >
end R + v | ^ + E end
system + v | ^ + system
> \ v + ^ / >
> \------------+ R relay E +------------/ >
> > > > > > > > system > > > > > > > >
Figure 1
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.
4. DetNet Architecture
Traffic Engineering Architecture and Signaling (TEAS) [TEAS] 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.
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.
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 Software-Defined Networking (SDN): Layers
and Architecture Terminology [RFC7426] which is represented below:
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SDN Layers and Architecture Terminology per RFC 7426
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 |
+---------------------------------------------------------------+
Figure 2
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4.1. The Application Plane
Per [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.
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.
4.2. The Controller Plane
The Controller Plane corresponds to the aggregation of the Control
and Management Planes in [RFC7426], though Common Control and
Measurement Plane (CCAMP) [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)
[PCE] is a core element of a controller, in charge of computing
Deterministic paths to be applied in the Network Plane.
A (Northbound) Service Interface enables applications in the
Application Plane to communicate with the entities in the Controller
Plane.
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.
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4.3. The Network Plane
The Network Plane represents the network devices and protocols as a
whole, regardless of the Layer at which the network devices operate.
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 [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.
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.
Stream Management Entity
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
Figure 3
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.
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
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each stream, or, when unable to do so, perform a last resort
operation such as drop or declassify.
This specification focuses on the Southbound interface and the
operation of the Network Plane.
4.4. Elements of DetNet Architecture
The DetNet architecture has a number of elements, discussed in the
following sections. Note that not every application requires all of
these elements.
a. A model for the definition, identification, and operation of
DetNet streams (Section 4.5), for use by relay systems to
classify and process individual packets following per-stream
rules.
b. 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:
1. 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 (Section 4.6).
2. A model for identifying misbehaving DetNet streams and
mitigating their impact on properly functioning streams
(Section 4.8).
c. A model for the relay system to inform the controller(s) of the
information it needs for adequate path computations including:
1. Systems' individual capabilities (e.g. can do replication,
can do precise time).
2. Link capabilities and resources (e.g. bandwidth, transmission
delay, hardware deterministic support to the physical layer,
...)
3. Physical resources (total and available buffers, timers,
queues, etc)
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4. Network Adjacencies (neighbors)
d. 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:
1. 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.
2. 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
(Section 3.3).
e. 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 (Section 4.9).
f. 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 (Section 4.11).
4.5. DetNet streams
4.5.1. Talker guarantees
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
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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.
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:
o A maximum packet size;
o An observation interval; and
o A maximum number of transmissions during that observation
interval.
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.
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.
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.
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
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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.
4.5.2. Incomplete Networks
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.
4.6. Queuing, Shaping, Scheduling, and Preemption
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.
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:
o A credit-based shaper IEEE 802.1Q Clause 34 [IEEE802.1Q-2014].
o Time-gated queues governed by a rotating time schedule,
synchronized among all relay nodes [IEEE802.1Qbv].
o Synchronized double (or triple) buffers driven by synchronized
time ticks. [IEEE802.1Qch].
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o 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 [IEEE802.1Qbu], [IEEE802.3br].
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.
4.7. Coexistence with normal traffic
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 Section 4.8 and Section 6). For these reasons:
o Bandwidth (transmission opportunities) not utilized by a DetNet
stream are available to non-DetNet packets (though not to other
DetNet streams).
o 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).
o 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.
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.
4.8. Fault Mitigation
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.
It is also essential that filters and service remarking be employed
at the network edge to prevent non-DetNet packets from being mistaken
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for DetNet packets, and thus impinging on the resources allocated to
DetNet packets.
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.
4.9. Protocol Stack Model
[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
Figure 4.
DetNet Protocol Stack
+--------------------------------+
| Upper Layers |
+--------------------------------+
| Sequence generation/recovery |
+--------------------------------+
| Sequence encode/decode |
+--------------------------------+
| Stream splitting/merging |
+--------------------------------+
| Stream encode/decode |
+--------------------------------+
| Lower layers |
+--------------------------------+
Figure 4
Not all layers are required for any given application, or even for
any given network. The layers are, from top to bottom:
Sequence generation/recovery
Supplies the sequence number for Seamless Redundancy
(Section 3.3) for packets going down the stack, and discards
duplicate packets coming up the stack.
Sequence encode/decode
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
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explicitly present, being included in the Stream encode/
decode layer, below.
Stream splitting/merging
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 (Section 3.3).
Stream encode/decode
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
(Section 4.8). When streams are assigned to pinned-down
paths, this layer can be indistinguishable from the data
forwarding layer(s).
The reader is likely to notice that Figure 4 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.
4.10. Advertising resources, capabilities and adjacencies
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.
o 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 (Section 4.6), the
number of buffers dedicated for DetNet allocation, and the worst-
case forwarding delay.
o The dynamic state of an end or relay system's DetNet resources.
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o 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).
4.11. Provisioning model
4.11.1. Centralized Path Computation and Installation
A centralized routing model, such as provided with a PCE (RFC 4655
[RFC4655]), enables global and per-stream optimizations. The model
is attractive but a number of issues are left to be solved. In
particular:
o whether and how the path computation can be installed by 1) an end
device or 2) a Network Management entity,
o 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.
4.11.2. Distributed Path Setup
Whether a distributed alternative without a PCE can be valuable
should be studied as well. Such an alternative could for instance
inherit from the Resource ReSerVation Protocol [RFC5127] (RSVP)
flows.
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. [IEEE802.1Q-2014] Clause 35 describes SRP, a peer-to-peer
protocol for Layer-2 roughly analogous to RSVP. Almost complete is
[IEEE802.1Qca], which defines how ISIS can provide multiple disjoint
paths or distribution trees. Also in progress is [IEEE802.1Qcc],
which expands the capabilities of SRP.
5. Related IETF work
5.1. Deterministic PHB
[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 [RFC5127].
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5.2. 6TiSCH
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 [ISA100.11a] and [WirelessHART].
These standards rely on variations of the [IEEE802154e] timeSlotted
Channel Hopping (TSCH) [I-D.ietf-6tisch-tsch] Medium Access Control
(MAC), and a form of centralized Path Computation Element (PCE), to
deliver deterministic capabilities.
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.
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
[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.
[RFC5673] and [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.
6. Security Considerations
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 [RFC7384] for an exploration of this issue in a related context.
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
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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 Section 4.8.
Security must cover:
o the protection of the signaling protocol
o the authentication and authorization of the controlling systems
o the identification and shaping of the streams
7. IANA Considerations
This document does not require an action from IANA.
8. Acknowledgements
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.
9. Access to IEEE 802.1 documents
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.
10. Informative References
[AVnu] http://www.avnu.org/, "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.".
[CCAMP] IETF, "Common Control and Measurement Plane",
<https://datatracker.ietf.org/doc/charter-ietf-ccamp/>.
[HART] www.hartcomm.org, "Highway Addressable Remote Transducer,
a group of specifications for industrial process and
control devices administered by the HART Foundation".
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[HSR-PRP] IEC, "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.",
<http://webstore.iec.ch/webstore/webstore.nsf/
artnum/046615!opendocument>.
[I-D.finn-detnet-problem-statement]
Finn, N. and P. Thubert, "Deterministic Networking Problem
Statement", draft-finn-detnet-problem-statement-04 (work
in progress), October 2015.
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-08 (work
in progress), May 2015.
[I-D.ietf-6tisch-tsch]
Watteyne, T., Palattella, M., and L. Grieco, "Using
IEEE802.15.4e TSCH in an IoT context: Overview, Problem
Statement and Goals", draft-ietf-6tisch-tsch-06 (work in
progress), March 2015.
[I-D.ietf-roll-rpl-industrial-applicability]
Phinney, T., Thubert, P., and R. Assimiti, "RPL
applicability in industrial networks", draft-ietf-roll-
rpl-industrial-applicability-02 (work in progress),
October 2013.
[I-D.svshah-tsvwg-deterministic-forwarding]
Shah, S. and P. Thubert, "Deterministic Forwarding PHB",
draft-svshah-tsvwg-deterministic-forwarding-04 (work in
progress), August 2015.
[IEEE802.1AS-2011]
IEEE, "Timing and Synchronizations (IEEE 802.1AS-2011)",
2011, <http://standards.ieee.org/getIEEE802/
download/802.1AS-2011.pdf>.
[IEEE802.1BA-2011]
IEEE, "AVB Systems (IEEE 802.1BA-2011)", 2011,
<http://standards.ieee.org/getIEEE802/
download/802.1BA-2011.pdf>.
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[IEEE802.1CB]
IEEE, "Seamless Redundancy (IEEE Draft P802.1CB)", 2015,
<http://www.ieee802.org/1/files/private/cb-drafts/>.
[IEEE802.1Q-2014]
IEEE, "MAC Bridges and VLANs (IEEE 802.1Q-2014", 2014,
<http://standards.ieee.org/getIEEE802/
download/802.1Q-2014.pdf>.
[IEEE802.1Qbu]
IEEE, "Frame Preemption", 2015,
<http://www.ieee802.org/1/files/private/bu-drafts/>.
[IEEE802.1Qbv]
IEEE, "Enhancements for Scheduled Traffic", 2015,
<http://www.ieee802.org/1/files/private/bv-drafts/>.
[IEEE802.1Qca]
IEEE, "Path Control and Reservation", 2015,
<http://www.ieee802.org/1/files/private/ca-drafts/>.
[IEEE802.1Qcc]
IEEE, "Stream Reservation Protocol (SRP) Enhancements and
Performance Improvements", 2015,
<http://www.ieee802.org/1/files/private/cc-drafts/>.
[IEEE802.1Qch]
IEEE, "Cyclic Queuing and Forwarding", 2011,
<https://development.standards.ieee.org/P883200033/par>.
[IEEE802.1TSNTG]
IEEE Standards Association, "IEEE 802.1 Time-Sensitive
Networks Task Group", 2013,
<http://www.IEEE802.org/1/pages/avbridges.html>.
[IEEE802.3br]
IEEE, "Interspersed Express Traffic", 2015,
<http://www.ieee802.org/3/br/>.
[IEEE802154]
IEEE standard for Information Technology, "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", June 2011.
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[IEEE802154e]
IEEE standard for Information Technology, "IEEE std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendment 1: MAC sublayer", April
2012.
[ISA100.11a]
ISA/IEC, "ISA100.11a, Wireless Systems for Automation,
also IEC 62734", 2011, < http://www.isa100wci.org/en-
US/Documents/PDF/3405-ISA100-WirelessSystems-Future-broch-
WEB-ETSI.aspx>.
[ISA95] ANSI/ISA, "Enterprise-Control System Integration Part 1:
Models and Terminology", 2000, <https://www.isa.org/
isa95/>.
[ODVA] http://www.odva.org/, "The organization that supports
network technologies built on the Common Industrial
Protocol (CIP) including EtherNet/IP.".
[PCE] IETF, "Path Computation Element",
<https://datatracker.ietf.org/doc/charter-ietf-pce/>.
[Profinet]
http://us.profinet.com/technology/profinet/, "PROFINET is
a standard for industrial networking in automation.",
<http://us.profinet.com/technology/profinet/>.
[RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, DOI 10.17487/RFC2205,
September 1997, <http://www.rfc-editor.org/info/rfc2205>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<http://www.rfc-editor.org/info/rfc3209>.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<http://www.rfc-editor.org/info/rfc4655>.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
February 2008, <http://www.rfc-editor.org/info/rfc5127>.
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[RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
Phinney, "Industrial Routing Requirements in Low-Power and
Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October
2009, <http://www.rfc-editor.org/info/rfc5673>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <http://www.rfc-editor.org/info/rfc7384>.
[RFC7426] Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
Defined Networking (SDN): Layers and Architecture
Terminology", RFC 7426, DOI 10.17487/RFC7426, January
2015, <http://www.rfc-editor.org/info/rfc7426>.
[TEAS] IETF, "Traffic Engineering Architecture and Signaling",
<https://datatracker.ietf.org/doc/charter-ietf-teas/>.
[WirelessHART]
www.hartcomm.org, "Industrial Communication Networks -
Wireless Communication Network and Communication Profiles
- WirelessHART - IEC 62591", 2010.
Authors' Addresses
Norman Finn
Cisco Systems
170 W Tasman Dr.
San Jose, California 95134
USA
Phone: +1 408 526 4495
Email: nfinn@cisco.com
Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
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Michael Johas Teener
Broadcom Corp.
3151 Zanker Rd.
San Jose, California 95134
USA
Phone: +1 831 824 4228
Email: MikeJT@broadcom.com
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