| Rfc | 8578 |
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| Title | Deterministic Networking Use Cases |
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| Author | E. Grossman, Ed. |
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| Date | May 2019 |
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| Format: | TXT, HTML |
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| Status: | INFORMATIONAL |
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|
Internet Engineering Task Force (IETF) E. Grossman, Ed.
Request for Comments: 8578 DOLBY
Category: Informational May 2019
ISSN: 2070-1721
Deterministic Networking Use Cases
Abstract
This document presents use cases for diverse industries that have in
common a need for "deterministic flows". "Deterministic" in this
context means that such flows provide guaranteed bandwidth, bounded
latency, and other properties germane to the transport of time-
sensitive data. These use cases differ notably in their network
topologies and specific desired behavior, providing as a group broad
industry context for Deterministic Networking (DetNet). For each use
case, this document will identify the use case, identify
representative solutions used today, and describe potential
improvements that DetNet can enable.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8578.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................6
2. Pro Audio and Video .............................................7
2.1. Use Case Description .......................................7
2.1.1. Uninterrupted Stream Playback .......................8
2.1.2. Synchronized Stream Playback ........................9
2.1.3. Sound Reinforcement .................................9
2.1.4. Secure Transmission ................................10
2.1.4.1. Safety ....................................10
2.2. Pro Audio Today ...........................................10
2.3. Pro Audio in the Future ...................................10
2.3.1. Layer 3 Interconnecting Layer 2 Islands ............10
2.3.2. High-Reliability Stream Paths ......................11
2.3.3. Integration of Reserved Streams into IT Networks ...11
2.3.4. Use of Unused Reservations by Best-Effort Traffic ..11
2.3.5. Traffic Segregation ................................11
2.3.5.1. Packet-Forwarding Rules, VLANs,
and Subnets ...............................12
2.3.5.2. Multicast Addressing (IPv4 and IPv6) ......12
2.3.6. Latency Optimization by a Central Controller .......12
2.3.7. Reduced Device Costs due to Reduced Buffer Memory ..13
2.4. Pro Audio Requests to the IETF ............................13
3. Electrical Utilities ...........................................14
3.1. Use Case Description ......................................14
3.1.1. Transmission Use Cases .............................14
3.1.1.1. Protection ................................14
3.1.1.2. Intra-substation Process Bus
Communications ............................21
3.1.1.3. Wide-Area Monitoring and Control Systems ..23
3.1.1.4. WAN Engineering Guidelines
Requirement Classification ................25
11.1.4. Layer 2 and Layer 3 Integration ...................76
11.1.5. IPv4 Considerations ...............................76
11.1.6. Guaranteed End-to-End Delivery ....................77
11.1.7. Replacement for Multiple Proprietary
Deterministic Networks ............................77
11.1.8. Mix of Deterministic and Best-Effort Traffic ......77
11.1.9. Unused Reserved Bandwidth to Be Available
to Best-Effort Traffic ............................77
11.1.10. Lower-Cost, Multi-Vendor Solutions ...............77
11.2. Scalable Size ............................................78
11.2.1. Scalable Number of Flows ..........................78
11.3. Scalable Timing Parameters and Accuracy ..................78
11.3.1. Bounded Latency ...................................78
11.3.2. Low Latency .......................................78
11.3.3. Bounded Jitter (Latency Variation) ................79
11.3.4. Symmetrical Path Delays ...........................79
11.4. High Reliability and Availability ........................79
11.5. Security .................................................79
11.6. Deterministic Flows ......................................79
12. Security Considerations .......................................80
13. IANA Considerations ...........................................80
14. Informative References ........................................80
Appendix A. Use Cases Explicitly Out of Scope for DetNet ..........90
A.1. DetNet Scope Limitations ...................................90
A.2. Internet-Based Applications ................................90
A.2.1. Use Case Description ...................................91
A.2.1.1. Media Content Delivery .............................91
A.2.1.2. Online Gaming ......................................91
A.2.1.3. Virtual Reality ....................................91
A.2.2. Internet-Based Applications Today ......................91
A.2.3. Internet-Based Applications in the Future ..............91
A.2.4. Internet-Based Applications Requests to the IETF .......92
A.3. Pro Audio and Video - Digital Rights Management (DRM) ......92
A.4. Pro Audio and Video - Link Aggregation .....................92
A.5. Pro Audio and Video - Deterministic Time to Establish
Streaming ..................................................93
Acknowledgments ...................................................93
Contributors ......................................................95
Author's Address ..................................................97
1. Introduction
This memo documents use cases for diverse industries that require
deterministic flows over multi-hop paths. Deterministic Networking
(DetNet) flows can be established from either a Layer 2 or Layer 3
(IP) interface, and such flows can coexist on an IP network with
best-effort traffic. DetNet also provides for highly reliable flows
through provision for redundant paths.
The DetNet use cases explicitly do not suggest any specific design
for DetNet architecture or protocols; these are topics for other
DetNet documents.
The DetNet use cases, as originally submitted, explicitly were not
considered by the DetNet Working Group (WG) to be concrete
requirements. The DetNet WG and Design Team considered these use
cases, identifying which of their elements could be feasibly
implemented within the charter of DetNet; as a result, certain
originally submitted use cases (or elements thereof) were moved to
Appendix A ("Use Cases Explicitly Out of Scope for DetNet") of this
document.
This document provides context regarding DetNet design decisions. It
also serves a long-lived purpose of helping those learning (or new
to) DetNet understand the types of applications that can be supported
by DetNet. It also allows those WG contributors who are users to
ensure that their concerns are addressed by the WG; for them, this
document (1) covers their contributions and (2) provides a long-term
reference regarding the problems that they expect will be served by
the technology, in terms of the short-term deliverables and also as
the technology evolves in the future.
This document has served as a "yardstick" against which proposed
DetNet designs can be measured, answering the question "To what
extent does a proposed design satisfy these various use cases?"
The industries covered by the use cases in this document are
o professional audio and video (Section 2)
o electrical utilities (Section 3)
o building automation systems (BASs) (Section 4)
o wireless for industrial applications (Section 5)
o cellular radio (Section 6)
o industrial machine to machine (M2M) (Section 7)
o mining (Section 8)
o private blockchain (Section 9)
o network slicing (Section 10)
For each use case, the following questions are answered:
o What is the use case?
o How is it addressed today?
o How should it be addressed in the future?
o What should the IETF deliver to enable this use case?
The level of detail in each use case is intended to be sufficient to
express the relevant elements of the use case but no more than that.
DetNet does not directly address clock distribution or time
synchronization; these are considered to be part of the overall
design and implementation of a time-sensitive network, using existing
(or future) time-specific protocols (such as [IEEE-8021AS] and/or
[RFC5905]).
Section 11 enumerates the set of common properties implied by these
use cases.
2. Pro Audio and Video
2.1. Use Case Description
The professional audio and video industry ("ProAV") includes:
o Music and film content creation
o Broadcast
o Cinema
o Live sound
o Public address, media, and emergency systems at large venues
(e.g., airports, stadiums, churches, theme parks)
These industries have already transitioned audio and video signals
from analog to digital. However, the digital interconnect systems
remain primarily point to point, with a single signal or a small
number of signals per link, interconnected with purpose-built
hardware.
These industries are now transitioning to packet-based
infrastructures to reduce cost, increase routing flexibility, and
integrate with existing IT infrastructures.
Today, ProAV applications have no way to establish deterministic
flows from a standards-based Layer 3 (IP) interface; this is a
fundamental limitation of the use cases described here. Today,
deterministic flows can be created within standards-based Layer 2
LANs (e.g., using IEEE 802.1 TSN ("TSN" stands for "Time-Sensitive
Networking")); however, these flows are not routable via IP and thus
are not effective for distribution over wider areas (for example,
broadcast events that span wide geographical areas).
It would be highly desirable if such flows could be routed over the
open Internet; however, solutions of more-limited scope (e.g.,
enterprise networks) would still provide substantial improvements.
The following sections describe specific ProAV use cases.
2.1.1. Uninterrupted Stream Playback
Transmitting audio and video streams for live playback is unlike
common file transfer in that uninterrupted stream playback in the
presence of network errors cannot be achieved by retrying the
transmission; by the time the missing or corrupt packet has been
identified, it is too late to execute a retry operation. Buffering
can be used to provide enough delay to allow time for one or more
retries; however, this is not an effective solution in applications
where large delays (latencies) are not acceptable (as discussed
below).
Streams with guaranteed bandwidth can eliminate congestion on the
network as a cause of transmission errors that would lead to playback
interruption. The use of redundant paths can further mitigate
transmission errors and thereby provide greater stream reliability.
Additional techniques, such as Forward Error Correction (FEC), can
also be used to improve stream reliability.
2.1.2. Synchronized Stream Playback
Latency in this context is the time between when a signal is
initially sent over a stream and when it is received. A common
example in ProAV is time-synchronizing audio and video when they take
separate paths through the playback system. In this case, the
latency of both the audio stream and the video stream must be bounded
and consistent if the sound is to remain matched to the movement in
the video. A common tolerance for audio/video synchronization is one
National Television System Committee (NTSC) video frame (about
33 ms); to maintain the audience's perception of correct lip-sync,
the latency needs to be consistent within some reasonable tolerance
-- for example, 10%.
A common architecture for synchronizing multiple streams that have
different paths through the network (and thus potentially different
latencies) enables measurement of the latency of each path and has
the data sinks (for example, speakers) delay (buffer) all packets on
all but the slowest path. Each packet of each stream is assigned a
presentation time that is based on the longest required delay. This
implies that all sinks must maintain a common time reference of
sufficient accuracy, which can be achieved by various techniques.
This type of architecture is commonly implemented using a central
controller that determines path delays and arbitrates buffering
delays.
2.1.3. Sound Reinforcement
Consider the latency (delay) between the time when a person speaks
into a microphone and when their voice emerges from the speaker. If
this delay is longer than about 10-15 ms, it is noticeable and can
make a sound-reinforcement system unusable (see slide 6 of
[SRP_LATENCY]). (If you have ever tried to speak in the presence of
a delayed echo of your voice, you might be familiar with this
experience.)
Note that the 15 ms latency bound includes all parts of the signal
path -- not just the network -- so the network latency must be
significantly less than 15 ms.
In some cases, local performers must perform in synchrony with a
remote broadcast. In such cases, the latencies of the broadcast
stream and the local performer must be adjusted to match each other,
with a worst case of one video frame (33 ms for NTSC video).
In cases where audio phase is a consideration -- for example,
beam-forming using multiple speakers -- latency can be in the 10 us
range (one audio sample at 96 kHz).
2.1.4. Secure Transmission
2.1.4.1. Safety
Professional audio systems can include amplifiers that are capable of
generating hundreds or thousands of watts of audio power. If used
incorrectly, such amplifiers can cause hearing damage to those in the
vicinity. Apart from the usual care required by the systems
operators to prevent such incidents, the network traffic that
controls these devices must be secured (as with any sensitive
application traffic).
2.2. Pro Audio Today
Some proprietary systems have been created that enable deterministic
streams at Layer 3; however, they are "engineered networks" that
require careful configuration to operate and often require that the
system be over-provisioned. Also, it is implied that all devices on
the network voluntarily play by the rules of that network. To enable
these industries to successfully transition to an interoperable
multi-vendor packet-based infrastructure requires effective open
standards. Establishing relevant IETF standards is a crucial factor.
2.3. Pro Audio in the Future
2.3.1. Layer 3 Interconnecting Layer 2 Islands
It would be valuable to enable IP to connect multiple Layer 2 LANs.
As an example, ESPN constructed a state-of-the-art 194,000 sq. ft.,
$125-million broadcast studio called "Digital Center 2" (DC2). The
DC2 network is capable of handling 46 Tbps of throughput with 60,000
simultaneous signals. Inside the facility are 1,100 miles of fiber
feeding four audio control rooms (see [ESPN_DC2]).
In designing DC2, they replaced as much point-to-point technology as
they could with packet-based technology. They constructed seven
individual studios using Layer 2 LANs (using IEEE 802.1 TSN) that
were entirely effective at routing audio within the LANs. However,
to interconnect these Layer 2 LAN islands together, they ended up
using dedicated paths in a custom SDN (Software-Defined Networking)
router because there is no standards-based routing solution
available.
2.3.2. High-Reliability Stream Paths
On-air and other live media streams are often backed up with
redundant links that seamlessly act to deliver the content when the
primary link fails for any reason. In point-to-point systems, this
redundancy is provided by an additional point-to-point link; the
analogous requirement in a packet-based system is to provide an
alternate path through the network such that no individual link can
bring down the system.
2.3.3. Integration of Reserved Streams into IT Networks
A commonly cited goal of moving to a packet-based media
infrastructure is that costs can be reduced by using off-the-shelf,
commodity-network hardware. In addition, economy of scale can be
realized by combining media infrastructure with IT infrastructure.
In keeping with these goals, stream-reservation technology should be
compatible with existing protocols and should not compromise the use
of the network for best-effort (non-time-sensitive) traffic.
2.3.4. Use of Unused Reservations by Best-Effort Traffic
In cases where stream bandwidth is reserved but not currently used
(or is underutilized), that bandwidth must be available to
best-effort (i.e., non-time-sensitive) traffic. For example, a
single stream may be "nailed up" (reserved) for specific media
content that needs to be presented at different times of the day,
ensuring timely delivery of that content, yet in between those times
the full bandwidth of the network can be utilized for best-effort
tasks such as file transfers.
This also addresses a concern of IT network administrators that are
considering adding reserved-bandwidth traffic to their networks that
"users will reserve large quantities of bandwidth and then never
unreserve it even though they are not using it, and soon the network
will have no bandwidth left."
2.3.5. Traffic Segregation
Sink devices may be low-cost devices with limited processing power.
In order to not overwhelm the CPUs in these devices, it is important
to limit the amount of traffic that these devices must process.
As an example, consider the use of individual seat speakers in a
cinema. These speakers are typically required to be cost reduced,
since the quantities in a single theater can reach hundreds of seats.
Discovery protocols alone in a 1,000-seat theater can generate enough
broadcast traffic to overwhelm a low-powered CPU. Thus, an
installation like this will benefit greatly from some type of traffic
segregation that can define groups of seats to reduce traffic within
each group. All seats in the theater must still be able to
communicate with a central controller.
There are many techniques that can be used to support this feature,
including (but not limited to) the following examples.
2.3.5.1. Packet-Forwarding Rules, VLANs, and Subnets
Packet-forwarding rules can be used to eliminate some extraneous
streaming traffic from reaching potentially low-powered sink devices;
however, there may be other types of broadcast traffic that should be
eliminated via other means -- for example, VLANs or IP subnets.
2.3.5.2. Multicast Addressing (IPv4 and IPv6)
Multicast addressing is commonly used to keep bandwidth utilization
of shared links to a minimum.
Because Layer 2 bridges by design forward Media Access Control (MAC)
addresses, it is important that a multicast MAC address only be
associated with one stream. This will prevent reservations from
forwarding packets from one stream down a path that has no interested
sinks simply because there is another stream on that same path that
shares the same multicast MAC address.
In other words, since each multicast MAC address can represent 32
different IPv4 multicast addresses, there must be a process in place
to make sure that any given multicast MAC address is only associated
with exactly one IPv4 multicast address. Requiring the use of IPv6
addresses could help in this regard, due to the much larger address
range of IPv6; however, due to the continued prevalence of IPv4
installations, solutions that are effective for IPv4 installations
would be practical in many more use cases.
2.3.6. Latency Optimization by a Central Controller
A central network controller might also perform optimizations based
on the individual path delays; for example, sinks that are closer to
the source can inform the controller that they can accept greater
latency, since they will be buffering packets to match presentation
times of sinks that are farther away. The controller might then move
a stream reservation on a short path to a longer path in order to
free up bandwidth for other critical streams on that short path. See
slides 3-5 of [SRP_LATENCY].
Additional optimization can be achieved in cases where sinks have
differing latency requirements; for example, at a live outdoor
concert, the speaker sinks have stricter latency requirements than
the recording-hardware sinks. See slide 7 of [SRP_LATENCY].
2.3.7. Reduced Device Costs due to Reduced Buffer Memory
Device costs can be reduced in a system with guaranteed reservations
with a small bounded latency due to the reduced requirements for
buffering (i.e., memory) on sink devices. For example, a theme park
might broadcast a live event across the globe via a Layer 3 protocol.
In such cases, the size of the buffers required is defined by the
worst-case latency and jitter values of the worst-case segment of the
end-to-end network path. For example, on today's open Internet, the
latency is typically unacceptable for audio and video streaming
without many seconds of buffering. In such scenarios, a single
gateway device at the local network that receives the feed from the
remote site would provide the expensive buffering required to mask
the latency and jitter issues associated with long-distance delivery.
Sink devices in the local location would have no additional buffering
requirements, and thus no additional costs, beyond those required for
delivery of local content. The sink device would be receiving
packets identical to those sent by the source and would be unaware of
any latency or jitter issues along the path.
2.4. Pro Audio Requests to the IETF
o Layer 3 routing on top of Audio Video Bridging (AVB) (and/or other
high-QoS (Quality of Service) networks)
o Content delivery with bounded, lowest possible latency
o IntServ and DiffServ integration with AVB (where practical)
o Single network for A/V and IT traffic
o Standards-based, interoperable, multi-vendor solutions
o IT-department-friendly networks
o Enterprise-wide networks (e.g., the size of San Francisco but not
the whole Internet (yet...))
3. Electrical Utilities
3.1. Use Case Description
Many systems that an electrical utility deploys today rely on high
availability and deterministic behavior of the underlying networks.
Presented here are use cases for transmission, generation, and
distribution, including key timing and reliability metrics. In
addition, security issues and industry trends that affect the
architecture of next-generation utility networks are discussed.
3.1.1. Transmission Use Cases
3.1.1.1. Protection
"Protection" means not only the protection of human operators but
also the protection of the electrical equipment and the preservation
of the stability and frequency of the grid. If a fault occurs in the
transmission or distribution of electricity, then severe damage can
occur to human operators, electrical equipment, and the grid itself,
leading to blackouts.
Communication links, in conjunction with protection relays, are used
to selectively isolate faults on high-voltage lines, transformers,
reactors, and other important electrical equipment. The role of the
teleprotection system is to selectively disconnect a faulty part by
transferring command signals within the shortest possible time.
3.1.1.1.1. Key Criteria
The key criteria for measuring teleprotection performance are command
transmission time, dependability, and security. These criteria are
defined by International Electrotechnical Commission (IEC)
Standard 60834 [IEC-60834] as follows:
o Transmission time (speed): The time between the moment when a
state change occurs at the transmitter input and the moment of the
corresponding change at the receiver output, including propagation
delay. The overall operating time for a teleprotection system is
the sum of (1) the time required to initiate the command at the
transmitting end, (2) the propagation delay over the network
(including equipment), and (3) the time required to make the
necessary selections and decisions at the receiving end, including
any additional delay due to a noisy environment.
o Dependability: The ability to issue and receive valid commands in
the presence of interference and/or noise, by minimizing the
Probability of Missing Commands (PMC). Dependability targets are
typically set for a specific Bit Error Rate (BER) level.
o Security: The ability to prevent false tripping due to a noisy
environment, by minimizing the Probability of Unwanted Commands
(PUC). Security targets are also set for a specific BER level.
Additional elements of the teleprotection system that impact its
performance include:
o Network bandwidth
o Failure recovery capacity (aka resiliency)
3.1.1.1.2. Fault Detection and Clearance Timing
Most power-line equipment can tolerate short circuits or faults for
up to approximately five power cycles before sustaining irreversible
damage or affecting other segments in the network. This translates
to a total fault clearance time of 100 ms. As a safety precaution,
however, the actual operation time of protection systems is limited
to 70-80% of this period, including fault recognition time, command
transmission time, and line breaker switching time.
Some system components, such as large electromechanical switches,
require a particularly long time to operate and take up the majority
of the total clearance time, leaving only a 10 ms window for the
telecommunications part of the protection scheme, independent of the
distance of travel. Given the sensitivity of the issue, new
networks impose requirements that are even more stringent: IEC
Standard 61850-5:2013 [IEC-61850-5:2013] limits the transfer time for
protection messages to 1/4-1/2 cycle or 4-8 ms (for 60 Hz lines) for
messages considered the most critical.
3.1.1.1.3. Symmetric Channel Delay
Teleprotection channels that are differential must be synchronous;
this means that any delays on the transmit and receive paths must
match each other. Ideally, teleprotection systems support zero
asymmetric delay; typical legacy relays can tolerate delay
discrepancies of up to 750 us.
Some tools available for lowering delay variation below this
threshold are as follows:
o For legacy systems using Time-Division Multiplexing (TDM), jitter
buffers at the multiplexers on each end of the line can be used to
offset delay variation by queuing sent and received packets. The
length of the queues must balance the need to regulate the rate of
transmission with the need to limit overall delay, as larger
buffers result in increased latency.
o For jitter-prone IP networks, traffic management tools can ensure
that the teleprotection signals receive the highest transmission
priority to minimize jitter.
o Standard packet-based synchronization technologies, such as the
IEEE 1588-2008 Precision Time Protocol (PTP) [IEEE-1588] and
synchronous Ethernet (syncE) [syncE], can help keep networks
stable by maintaining a highly accurate clock source on the
various network devices.
3.1.1.1.4. Teleprotection Network Requirements
Table 1 captures the main network metrics. (These metrics are based
on IEC Standard 61850-5:2013 [IEC-61850-5:2013].)
+---------------------------------+---------------------------------+
| Teleprotection Requirement | Attribute |
+---------------------------------+---------------------------------+
| One-way maximum delay | 4-10 ms |
| | |
| Asymmetric delay required | Yes |
| | |
| Maximum jitter | Less than 250 us (750 us for |
| | legacy IEDs) |
| | |
| Topology | Point to point, point to |
| | multipoint |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Less than 50 ms - hitless |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 0.1% to 1% |
+---------------------------------+---------------------------------+
Table 1: Teleprotection Network Requirements
3.1.1.1.5. Inter-trip Protection Scheme
"Inter-tripping" is the signal-controlled tripping of a circuit
breaker to complete the isolation of a circuit or piece of apparatus
in concert with the tripping of other circuit breakers.
+---------------------------------+---------------------------------+
| Inter-trip Protection | Attribute |
| Requirement | |
+---------------------------------+---------------------------------+
| One-way maximum delay | 5 ms |
| | |
| Asymmetric delay required | No |
| | |
| Maximum jitter | Not critical |
| | |
| Topology | Point to point, point to |
| | multipoint |
| | |
| Bandwidth | 64 kbps |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Less than 50 ms - hitless |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 0.1% |
+---------------------------------+---------------------------------+
Table 2: Inter-trip Protection Network Requirements
3.1.1.1.6. Current Differential Protection Scheme
Current differential protection is commonly used for line protection
and is typically used to protect parallel circuits. At both ends of
the lines, the current is measured by the differential relays; both
relays will trip the circuit breaker if the current going into the
line does not equal the current going out of the line. This type of
protection scheme assumes that some form of communication is present
between the relays at both ends of the line, to allow both relays to
compare measured current values. Line differential protection
schemes assume that the telecommunications delay between both relays
is very low -- often as low as 5 ms. Moreover, as those systems are
often not time-synchronized, they also assume that the delay over
symmetric telecommunications paths is constant; this allows the
comparison of current measurement values taken at exactly the
same time.
+---------------------------------+---------------------------------+
| Current Differential Protection | Attribute |
| Requirement | |
+---------------------------------+---------------------------------+
| One-way maximum delay | 5 ms |
| | |
| Asymmetric delay required | Yes |
| | |
| Maximum jitter | Less than 250 us (750 us for |
| | legacy IEDs) |
| | |
| Topology | Point to point, point to |
| | multipoint |
| | |
| Bandwidth | 64 kbps |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Less than 50 ms - hitless |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 0.1% |
+---------------------------------+---------------------------------+
Table 3: Current Differential Protection Metrics
3.1.1.1.7. Distance Protection Scheme
The distance (impedance relay) protection scheme is based on voltage
and current measurements. The network metrics are similar (but not
identical) to the metrics for current differential protection.
+---------------------------------+---------------------------------+
| Distance Protection Requirement | Attribute |
+---------------------------------+---------------------------------+
| One-way maximum delay | 5 ms |
| | |
| Asymmetric delay required | No |
| | |
| Maximum jitter | Not critical |
| | |
| Topology | Point to point, point to |
| | multipoint |
| | |
| Bandwidth | 64 kbps |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Less than 50 ms - hitless |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 0.1% |
+---------------------------------+---------------------------------+
Table 4: Distance Protection Requirements
3.1.1.1.8. Inter-substation Protection Signaling
This use case describes the exchange of sampled values and/or GOOSE
(Generic Object Oriented Substation Events) messages between
Intelligent Electronic Devices (IEDs) in two substations for
protection and tripping coordination. The two IEDs are in
master-slave mode.
The Current Transformer or Voltage Transformer (CT/VT) in one
substation sends the sampled analog voltage or current value to the
Merging Unit (MU) over hard wire. The MU sends the time-synchronized
sampled values (as specified by IEC 61850-9-2:2011
[IEC-61850-9-2:2011]) to the slave IED. The slave IED forwards the
information to the master IED in the other substation. The master
IED makes the determination (for example, based on sampled value
differentials) to send a trip command to the originating IED. Once
the slave IED/relay receives the GOOSE message containing the command
to trip the breaker, it opens the breaker. It then sends a
confirmation message back to the master. All data exchanges between
IEDs are through sampled values and/or GOOSE messages.
+---------------------------------+---------------------------------+
| Inter-substation Protection | Attribute |
| Requirement | |
+---------------------------------+---------------------------------+
| One-way maximum delay | 5 ms |
| | |
| Asymmetric delay required | No |
| | |
| Maximum jitter | Not critical |
| | |
| Topology | Point to point, point to |
| | multipoint |
| | |
| Bandwidth | 64 kbps |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Less than 50 ms - hitless |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 1% |
+---------------------------------+---------------------------------+
Table 5: Inter-substation Protection Requirements
3.1.1.2. Intra-substation Process Bus Communications
This use case describes the data flow from the CT/VT to the IEDs in
the substation via the MU. The CT/VT in the substation sends the
analog voltage or current values to the MU over hard wire. The MU
converts the analog values into digital format (typically
time-synchronized sampled values as specified by IEC 61850-9-2:2011
[IEC-61850-9-2:2011]) and sends them to the IEDs in the substation.
The Global Positioning System (GPS) Master Clock can send 1PPS or
IRIG-B format to the MU through a serial port or IEEE 1588 protocol
via a network. 1PPS (One Pulse Per Second) is an electrical signal
that has a width of less than 1 second and a sharply rising or
abruptly falling edge that accurately repeats once per second. 1PPS
signals are output by radio beacons, frequency standards, other types
of precision oscillators, and some GPS receivers. IRIG (Inter-Range
Instrumentation Group) time codes are standard formats for
transferring timing information. Atomic frequency standards and GPS
receivers designed for precision timing are often equipped with an
IRIG output. Process bus communication using IEC 61850-9-2:2011
[IEC-61850-9-2:2011] simplifies connectivity within the substation,
removes the requirement for multiple serial connections, and removes
the slow serial-bus architectures that are typically used. This also
ensures increased flexibility and increased speed with the use of
multicast messaging between multiple devices.
+---------------------------------+---------------------------------+
| Intra-substation Protection | Attribute |
| Requirement | |
+---------------------------------+---------------------------------+
| One-way maximum delay | 5 ms |
| | |
| Asymmetric delay required | No |
| | |
| Maximum jitter | Not critical |
| | |
| Topology | Point to point, point to |
| | multipoint |
| | |
| Bandwidth | 64 kbps |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Less than 50 ms - hitless |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes or No |
| | |
| Packet loss | 0.1% |
+---------------------------------+---------------------------------+
Table 6: Intra-substation Protection Requirements
3.1.1.3. Wide-Area Monitoring and Control Systems
The application of synchrophasor measurement data from Phasor
Measurement Units (PMUs) to wide-area monitoring and control systems
promises to provide important new capabilities for improving system
stability. Access to PMU data enables more-timely situational
awareness over larger portions of the grid than what has been
possible historically with normal SCADA (Supervisory Control and Data
Acquisition) data. Handling the volume and the real-time nature of
synchrophasor data presents unique challenges for existing
application architectures. The Wide-Area Management System (WAMS)
makes it possible for the condition of the bulk power system to be
observed and understood in real time so that protective,
preventative, or corrective action can be taken. Because of the very
high sampling rate of measurements and the strict requirement for
time synchronization of the samples, the WAMS has stringent
telecommunications requirements in an IP network, as captured in
Table 7:
+---------------------------------+---------------------------------+
| WAMS Requirement | Attribute |
+---------------------------------+---------------------------------+
| One-way maximum delay | 50 ms |
| | |
| Asymmetric delay required | No |
| | |
| Maximum jitter | Not critical |
| | |
| Topology | Point to point, point to |
| | multipoint, multipoint to |
| | multipoint |
| | |
| Bandwidth | 100 kbps |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Less than 50 ms - hitless |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 1% |
| | |
| Consecutive packet loss | At least one packet per |
| | application cycle must be |
| | received. |
+---------------------------------+---------------------------------+
Table 7: WAMS Special Communication Requirements
3.1.1.4. WAN Engineering Guidelines Requirement Classification
The IEC has published a technical report (TR) that offers guidelines
on how to define and deploy Wide-Area Networks (WANs) for the
interconnection of electric substations, generation plants, and SCADA
operation centers. IEC TR 61850-90-12:2015 [IEC-61850-90-12:2015]
provides four classes of WAN communication requirements, as
summarized in Table 8:
+----------------+-----------+----------+----------+----------------+
| WAN | Class WA | Class WB | Class WC | Class WD |
| Requirement | | | | |
+----------------+-----------+----------+----------+----------------+
| Application | EHV | HV (High | MV | General- |
| field | (Extra- | Voltage) | (Medium | purpose |
| | High | | Voltage) | |
| | Voltage) | | | |
| | | | | |
| Latency | 5 ms | 10 ms | 100 ms | >100 ms |
| | | | | |
| Jitter | 10 us | 100 us | 1 ms | 10 ms |
| | | | | |
| Latency | 100 us | 1 ms | 10 ms | 100 ms |
| asymmetry | | | | |
| | | | | |
| Time accuracy | 1 us | 10 us | 100 us | 10 to 100 ms |
| | | | | |
| BER | 10^-7 to | 10^-5 to | 10^-3 | |
| | 10^-6 | 10^-4 | | |
| | | | | |
| Unavailability | 10^-7 to | 10^-5 to | 10^-3 | |
| | 10^-6 | 10^-4 | | |
| | | | | |
| Recovery delay | Zero | 50 ms | 5 s | 50 s |
| | | | | |
| Cybersecurity | Extremely | High | Medium | Medium |
| | high | | | |
+----------------+-----------+----------+----------+----------------+
Table 8: Communication Requirements (Courtesy of
IEC TR 61850-90-12:2015)
3.1.2. Generation Use Case
Energy generation systems are complex infrastructures that require
control of both the generated power and the generation
infrastructure.
3.1.2.1. Control of the Generated Power
The electrical power generation frequency must be maintained within a
very narrow band. Deviations from the acceptable frequency range are
detected, and the required signals are sent to the power plants for
frequency regulation.
Automatic Generation Control (AGC) is a system for adjusting the
power output of generators at different power plants, in response to
changes in the load.
+---------------------------------+---------------------------------+
| FCAG (Frequency Control | Attribute |
| Automatic Generation) | |
| Requirement | |
+---------------------------------+---------------------------------+
| One-way maximum delay | 500 ms |
| | |
| Asymmetric delay required | No |
| | |
| Maximum jitter | Not critical |
| | |
| Topology | Point to point |
| | |
| Bandwidth | 20 kbps |
| | |
| Availability | 99.999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | N/A |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 1% |
+---------------------------------+---------------------------------+
Table 9: FCAG Communication Requirements
3.1.2.2. Control of the Generation Infrastructure
The control of the generation infrastructure combines requirements
from industrial automation systems and energy generation systems.
This section describes the use case for control of the generation
infrastructure of a wind turbine.
Figure 1 presents the subsystems that operate a wind turbine.
|
|
| +-----------------+
| | +----+ |
| | |WTRM| WGEN |
WROT x==|===| | |
| | +----+ WCNV|
| |WNAC |
| +---+---WYAW---+--+
| | |
| | | +----+
|WTRF | |WMET|
| | | |
Wind Turbine | +--+-+
Controller | |
WTUR | | |
WREP | | |
WSLG | | |
WALG | WTOW | |
Figure 1: Wind Turbine Control Network
The subsystems shown in Figure 1 include the following:
o WROT (rotor control)
o WNAC (nacelle control) (nacelle: housing containing the generator)
o WTRM (transmission control)
o WGEN (generator)
o WYAW (yaw controller) (of the tower head)
o WCNV (in-turbine power converter)
o WTRF (wind turbine transformer information)
o WMET (external meteorological station providing real-time
information to the tower's controllers)
o WTUR (wind turbine general information)
o WREP (wind turbine report information)
o WSLG (wind turbine state log information)
o WALG (wind turbine analog log information)
o WTOW (wind turbine tower information)
Traffic characteristics relevant to the network planning and
dimensioning process in a wind turbine scenario are listed below.
The values in this section are based mainly on the relevant
references [Ahm14] and [Spe09]. Each logical node (Figure 1) is a
part of the metering network and produces analog measurements and
status information that must comply with their respective data-rate
constraints.
+-----------+--------+----------+-----------+-----------+-----------+
| Subsystem | Sensor | Analog | Data Rate | Status | Data Rate |
| | Count | Sample | (bytes/s) | Sample | (bytes/s) |
| | | Count | | Count | |
+-----------+--------+----------+-----------+-----------+-----------+
| WROT | 14 | 9 | 642 | 5 | 10 |
| | | | | | |
| WTRM | 18 | 10 | 2828 | 8 | 16 |
| | | | | | |
| WGEN | 14 | 12 | 73764 | 2 | 4 |
| | | | | | |
| WCNV | 14 | 12 | 74060 | 2 | 4 |
| | | | | | |
| WTRF | 12 | 5 | 73740 | 2 | 4 |
| | | | | | |
| WNAC | 12 | 9 | 112 | 3 | 6 |
| | | | | | |
| WYAW | 7 | 8 | 220 | 4 | 8 |
| | | | | | |
| WTOW | 4 | 1 | 8 | 3 | 6 |
| | | | | | |
| WMET | 7 | 7 | 228 | - | - |
+-----------+--------+----------+-----------+-----------+-----------+
Table 10: Wind Turbine Data-Rate Constraints
QoS constraints for different services are presented in Table 11.
These constraints are defined by IEEE Standard 1646 [IEEE-1646] and
IEC Standard 61400 Part 25 [IEC-61400-25].
+---------------------+---------+-------------+---------------------+
| Service | Latency | Reliability | Packet Loss Rate |
+---------------------+---------+-------------+---------------------+
| Analog measurement | 16 ms | 99.99% | <10^-6 |
| | | | |
| Status information | 16 ms | 99.99% | <10^-6 |
| | | | |
| Protection traffic | 4 ms | 100.00% | <10^-9 |
| | | | |
| Reporting and | 1 s | 99.99% | <10^-6 |
| logging | | | |
| | | | |
| Video surveillance | 1 s | 99.00% | No specific |
| | | | requirement |
| | | | |
| Internet connection | 60 min | 99.00% | No specific |
| | | | requirement |
| | | | |
| Control traffic | 16 ms | 100.00% | <10^-9 |
| | | | |
| Data polling | 16 ms | 99.99% | <10^-6 |
+---------------------+---------+-------------+---------------------+
Table 11: Wind Turbine Reliability and Latency Constraints
3.1.2.2.1. Intra-domain Network Considerations
A wind turbine is composed of a large set of subsystems, including
sensors and actuators that require time-critical operation. The
reliability and latency constraints of these different subsystems are
shown in Table 11. These subsystems are connected to an intra-domain
network that is used to monitor and control the operation of the
turbine and connect it to the SCADA subsystems. The different
components are interconnected using fiber optics, industrial buses,
industrial Ethernet, EtherCAT [EtherCAT], or a combination thereof.
Industrial signaling and control protocols such as Modbus [MODBUS],
PROFIBUS [PROFIBUS], PROFINET [PROFINET], and EtherCAT are used
directly on top of the Layer 2 transport or encapsulated over TCP/IP.
The data collected from the sensors and condition-monitoring systems
is multiplexed onto fiber cables for transmission to the base of the
tower and to remote control centers. The turbine controller
continuously monitors the condition of the wind turbine and collects
statistics on its operation. This controller also manages a large
number of switches, hydraulic pumps, valves, and motors within the
wind turbine.
There is usually a controller at the bottom of the tower and also in
the nacelle. The communication between these two controllers usually
takes place using fiber optics instead of copper links. Sometimes, a
third controller is installed in the hub of the rotor and manages the
pitch of the blades. That unit usually communicates with the nacelle
unit using serial communications.
3.1.2.2.2. Inter-domain Network Considerations
A remote control center belonging to a grid operator regulates the
power output, enables remote actuation, and monitors the health of
one or more wind parks in tandem. It connects to the local control
center in a wind park over the Internet (Figure 2) via firewalls at
both ends. The Autonomous System (AS) path between the local control
center and the wind park typically involves several ISPs at different
tiers. For example, a remote control center in Denmark can regulate
a wind park in Greece over the normal public AS path between the two
locations.
+--------------+
| |
| |
| Wind Park #1 +----+
| | | XXXXXX
| | | X XXXXXXXX +----------------+
+--------------+ | XXXX X XXXXX | |
+---+ XXX | Remote Control |
XXX Internet +----+ Center |
+----+X XXX | |
+--------------+ | XXXXXXX XX | |
| | | XX XXXXXXX +----------------+
| | | XXXXX
| Wind Park #2 +----+
| |
| |
+--------------+
Figure 2: Wind Turbine Control via Internet
The remote control center is part of the SCADA system, setting the
desired power output to the wind park and reading back the result
once the new power output level has been set. Traffic between the
remote control center and the wind park typically consists of
protocols like IEC 60870-5-104 [IEC-60870-5-104], OPC XML-Data Access
(XML-DA) [OPCXML], Modbus [MODBUS], and SNMP [RFC3411]. At the time
of this writing, traffic flows between the remote control center and
the wind park are best effort. QoS requirements are not strict, so
no Service Level Agreements (SLAs) or service-provisioning mechanisms
(e.g., VPNs) are employed. In the case of such events as equipment
failure, tolerance for alarm delay is on the order of minutes, due to
redundant systems already in place.
Future use cases will require bounded latency, bounded jitter, and
extraordinarily low packet loss for inter-domain traffic flows due to
the softwarization and virtualization of core wind-park equipment
(e.g., switches, firewalls, and SCADA server components). These
factors will create opportunities for service providers to install
new services and dynamically manage them from remote locations. For
example, to enable failover of a local SCADA server, a SCADA server
in another wind-park site (under the administrative control of the
same operator) could be utilized temporarily (Figure 3). In that
case, local traffic would be forwarded to the remote SCADA server,
and existing intra-domain QoS and timing parameters would have to be
met for inter-domain traffic flows.
+--------------+
| |
| |
| Wind Park #1 +----+
| | | XXXXXX
| | | X XXXXXXXX +----------------+
+--------------+ | XXXX XXXXX | |
+---+ Operator- XXX | Remote Control |
XXX Administered +----+ Center |
+----+X WAN XXX | |
+--------------+ | XXXXXXX XX | |
| | | XX XXXXXXX +----------------+
| | | XXXXX
| Wind Park #2 +----+
| |
| |
+--------------+
Figure 3: Wind Turbine Control via Operator-Administered WAN
3.1.3. Distribution Use Case
3.1.3.1. Fault Location, Isolation, and Service Restoration (FLISR)
"Fault Location, Isolation, and Service Restoration (FLISR)" refers
to the ability to automatically locate the fault, isolate the fault,
and restore service in the distribution network. This will likely
be the first widespread application of distributed intelligence in
the grid.
The static power-switch status (open/closed) in the network dictates
the power flow to secondary substations. Reconfiguring the network
in the event of a fault is typically done manually on site to
energize/de-energize alternate paths. Automating the operation of
substation switchgear allows the flow of power to be altered
automatically under fault conditions.
FLISR can be managed centrally from a Distribution Management System
(DMS) or executed locally through distributed control via intelligent
switches and fault sensors.
+---------------------------------+---------------------------------+
| FLISR Requirement | Attribute |
+---------------------------------+---------------------------------+
| One-way maximum delay | 80 ms |
| | |
| Asymmetric delay required | No |
| | |
| Maximum jitter | 40 ms |
| | |
| Topology | Point to point, point to |
| | multipoint, multipoint to |
| | multipoint |
| | |
| Bandwidth | 64 kbps |
| | |
| Availability | 99.9999% |
| | |
| Precise timing required | Yes |
| | |
| Recovery time on node failure | Depends on customer impact |
| | |
| Performance management | Yes; mandatory |
| | |
| Redundancy | Yes |
| | |
| Packet loss | 0.1% |
+---------------------------------+---------------------------------+
Table 12: FLISR Communication Requirements
3.2. Electrical Utilities Today
Many utilities still rely on complex environments consisting of
multiple application-specific proprietary networks, including TDM
networks.
In this kind of environment, there is no mixing of Operation
Technology (OT) and IT applications on the same network, and
information is siloed between operational areas.
Specific calibration of the full chain is required; this is costly.
This kind of environment prevents utility operations from realizing
operational efficiency benefits, visibility, and functional
integration of operational information across grid applications and
data networks.
In addition, there are many security-related issues, as discussed in
the following section.
3.2.1. Current Security Practices and Their Limitations
Grid-monitoring and control devices are already targets for cyber
attacks, and legacy telecommunications protocols have many intrinsic
network-related vulnerabilities. For example, the Distributed
Network Protocol (DNP3) [IEEE-1815], Modbus, PROFIBUS/PROFINET, and
other protocols are designed around a common paradigm of "request and
respond". Each protocol is designed for a master device such as an
HMI (Human-Machine Interface) system to send commands to subordinate
slave devices to perform data retrieval (reading inputs) or control
functions (writing to outputs). Because many of these protocols lack
authentication, encryption, or other basic security measures, they
are prone to network-based attacks, allowing a malicious actor or
attacker to utilize the request-and-respond system as a mechanism for
functionality similar to command and control. Specific security
concerns common to most industrial-control protocols (including
utility telecommunications protocols) include the following:
o Network or transport errors (e.g., malformed packets or excessive
latency) can cause protocol failure.
o Protocol commands may be available that are capable of forcing
slave devices into inoperable states, including powering devices
off, forcing them into a listen-only state, or disabling alarming.
o Protocol commands may be available that are capable of
interrupting processes (e.g., restarting communications).
o Protocol commands may be available that are capable of clearing,
erasing, or resetting diagnostic information such as counters and
diagnostic registers.
o Protocol commands may be available that are capable of requesting
sensitive information about the controllers, their configurations,
or other need-to-know information.
o Most protocols are application-layer protocols transported over
TCP; it is therefore easy to transport commands over non-standard
ports or inject commands into authorized traffic flows.
o Protocol commands may be available that are capable of
broadcasting messages to many devices at once (i.e., a
potential DoS).
o Protocol commands may be available that will query the device
network to obtain defined points and their values (i.e., perform a
configuration scan).
o Protocol commands may be available that will list all available
function codes (i.e., perform a function scan).
These inherent vulnerabilities, along with increasing connectivity
between IT and OT networks, make network-based attacks very feasible.
By injecting malicious protocol commands, an attacker could take
control over the target process. Altering legitimate protocol
traffic can also alter information about a process and disrupt the
legitimate controls that are in place over that process. A
man-in-the-middle attack could result in (1) improper control over a
process and (2) misrepresentation of data that is sent back to
operator consoles.
3.3. Electrical Utilities in the Future
The business and technology trends that are sweeping the utility
industry will drastically transform the utility business from the way
it has been for many decades. At the core of many of these changes
is a drive to modernize the electrical grid with an integrated
telecommunications infrastructure. However, interoperability
concerns, legacy networks, disparate tools, and stringent security
requirements all add complexity to the grid's transformation. Given
the range and diversity of the requirements that should be addressed
by the next-generation telecommunications infrastructure, utilities
need to adopt a holistic architectural approach to integrate the
electrical grid with digital telecommunications across the entire
power delivery chain.
The key to modernizing grid telecommunications is to provide a
common, adaptable, multi-service network infrastructure for the
entire utility organization. Such a network serves as the platform
for current capabilities while enabling future expansion of the
network to accommodate new applications and services.
To meet this diverse set of requirements both today and in the
future, the next-generation utility telecommunications network will
be based on an open-standards-based IP architecture. An end-to-end
IP architecture takes advantage of nearly three decades of IP
technology development, facilitating interoperability and device
management across disparate networks and devices, as has already been
demonstrated in many mission-critical and highly secure networks.
IPv6 is seen as a future telecommunications technology for the smart
grid; the IEC and different national committees have mandated a
specific ad hoc group (AHG8) to define the strategy for migration to
IPv6 for all the IEC Technical Committee 57 (TC 57) power automation
standards. The AHG8 has finalized its work on the migration
strategy, and IEC TR 62357-200:2015 [IEC-62357-200:2015] has been
issued.
Cloud-based SCADA systems will control and monitor the critical and
non-critical subsystems of generation systems -- for example, wind
parks.
3.3.1. Migration to Packet-Switched Networks
Throughout the world, utilities are increasingly planning for a
future based on smart-grid applications requiring advanced
telecommunications systems. Many of these applications utilize
packet connectivity for communicating information and control signals
across the utility's WAN, made possible by technologies such as
Multiprotocol Label Switching (MPLS). The data that traverses the
utility WAN includes:
o Grid monitoring, control, and protection data
o Non-control grid data (e.g., asset data for condition monitoring)
o Data (e.g., voice and video) related to physical safety and
security
o Remote worker access to corporate applications (voice, maps,
schematics, etc.)
o Field area network Backhaul for smart metering
o Distribution-grid management
o Enterprise traffic (email, collaboration tools, business
applications)
WANs support this wide variety of traffic to and from substations,
the transmission and distribution grid, and generation sites; between
control centers; and between work locations and data centers. To
maintain this rapidly expanding set of applications, many utilities
are taking steps to evolve present TDM-based and frame relay
infrastructures to packet systems. Packet-based networks are
designed to provide greater functionalities and higher levels of
service for applications, while continuing to deliver reliability and
deterministic (real-time) traffic support.
3.3.2. Telecommunications Trends
These general telecommunications topics are provided in addition to
the use cases that have been addressed so far. These include both
current and future telecommunications-related topics that should be
factored into the network architecture and design.
3.3.2.1. General Telecommunications Requirements
o IP connectivity everywhere
o Monitoring services everywhere, and from different remote centers
o Moving services to a virtual data center
o Unified access to applications/information from the corporate
network
o Unified services
o Unified communications solutions
o Mix of fiber and microwave technologies - obsolescence of the
Synchronous Optical Network / Synchronous Digital Hierarchy
(SONET/SDH) or TDM
o Standardizing grid telecommunications protocols to open standards,
to ensure interoperability
o Reliable telecommunications for transmission and distribution
substations
o IEEE 1588 time-synchronization client/server capabilities
o Integration of multicast design
o Mapping of QoS requirements
o Enabling future network expansion
o Substation network resilience
o Fast convergence design
o Scalable headend design
o Defining SLAs and enabling SLA monitoring
o Integration of 3G/4G technologies and future technologies
o Ethernet connectivity for station bus architecture
o Ethernet connectivity for process bus architecture
o Protection, teleprotection, and PMUs on IP
3.3.2.2. Specific Network Topologies of Smart-Grid Applications
Utilities often have very large private telecommunications networks
that can cover an entire territory/country. Until now, the main
purposes of these networks have been to (1) support transmission
network monitoring, control, and automation, (2) support remote
control of generation sites, and (3) provide FCAPS (Fault,
Configuration, Accounting, Performance, and Security) services from
centralized network operation centers.
Going forward, one network will support the operation and maintenance
of electrical networks (generation, transmission, and distribution),
voice and data services for tens of thousands of employees and for
exchanges with neighboring interconnections, and administrative
services. To meet those requirements, a utility may deploy several
physical networks leveraging different technologies across the
country -- for instance, an optical network and a microwave network.
Each protection and automation system between two points has two
telecommunications circuits, one on each network. Path diversity
between two substations is key. Regardless of the event type
(hurricane, ice storm, etc.), one path needs to stay available so the
system can still operate.
In the optical network, signals are transmitted over more than tens
of thousands of circuits using fiber optic links, microwave links,
and telephone cables. This network is the nervous system of the
utility's power transmission operations. The optical network
represents tens of thousands of kilometers of cable deployed along
the power lines, with individual runs as long as 280 km.
3.3.2.3. Precision Time Protocol
Some utilities do not use GPS clocks in generation substations. One
of the main reasons is that some of the generation plants are 30 to
50 meters deep underground and the GPS signal can be weak and
unreliable. Instead, atomic clocks are used. Clocks are
synchronized amongst each other. Rubidium clocks provide clock and
1 ms timestamps for IRIG-B.
Some companies plan to transition to PTP [IEEE-1588], distributing
the synchronization signal over the IP/MPLS network. PTP provides a
mechanism for synchronizing the clocks of participating nodes to a
high degree of accuracy and precision.
PTP operates based on the following assumptions:
o The network eliminates cyclic forwarding of PTP messages within
each communication path (e.g., by using a spanning tree protocol).
o PTP is tolerant of an occasional missed message, duplicated
message, or message that arrived out of order. However, PTP
assumes that such impairments are relatively rare.
o As designed, PTP expects a multicast communication model; however,
PTP also supports a unicast communication model as long as the
behavior of the protocol is preserved.
o Like all message-based time transfer protocols, PTP time accuracy
is degraded by delay asymmetry in the paths taken by event
messages. PTP cannot detect asymmetry, but if such delays are
known a priori, time values can be adjusted to correct for
asymmetry.
The use of PTP for power automation is defined in
IEC/IEEE 61850-9-3:2016 [IEC-IEEE-61850-9-3:2016]. It is based on
Annex B of IEC 62439-3:2016 [IEC-62439-3:2016], which offers the
support of redundant attachment of clocks to Parallel Redundancy
Protocol (PRP) and High-availability Seamless Redundancy (HSR)
networks.
3.3.3. Security Trends in Utility Networks
Although advanced telecommunications networks can assist in
transforming the energy industry by playing a critical role in
maintaining high levels of reliability, performance, and
manageability, they also introduce the need for an integrated
security infrastructure. Many of the technologies being deployed to
support smart-grid projects such as smart meters and sensors can
increase the vulnerability of the grid to attack. Top security
concerns for utilities migrating to an intelligent smart-grid
telecommunications platform center on the following trends:
o Integration of distributed energy resources
o Proliferation of digital devices to enable management, automation,
protection, and control
o Regulatory mandates to comply with standards for critical
infrastructure protection
o Migration to new systems for outage management, distribution
automation, condition-based maintenance, load forecasting, and
smart metering
o Demand for new levels of customer service and energy management
This development of a diverse set of networks to support the
integration of microgrids, open-access energy competition, and the
use of network-controlled devices is driving the need for a converged
security infrastructure for all participants in the smart grid,
including utilities, energy service providers, large commercial and
industrial customers, and residential customers. Securing the assets
of electric power delivery systems (from the control center to the
substation, to the feeders and down to customer meters) requires an
end-to-end security infrastructure that protects the myriad of
telecommunications assets used to operate, monitor, and control power
flow and measurement.
"Cybersecurity" refers to all the security issues in automation and
telecommunications that affect any functions related to the operation
of the electric power systems. Specifically, it involves the
concepts of:
o Integrity: data cannot be altered undetectably
o Authenticity (data origin authentication): the telecommunications
parties involved must be validated as genuine
o Authorization: only requests and commands from authorized users
can be accepted by the system
o Confidentiality: data must not be accessible to any
unauthenticated users
When designing and deploying new smart-grid devices and
telecommunications systems, it is imperative to understand the
various impacts of these new components under a variety of attack
situations on the power grid. The consequences of a cyber attack on
the grid telecommunications network can be catastrophic. This is why
security for the smart grid is not just an ad hoc feature or product;
it's a complete framework integrating both physical and cybersecurity
requirements and covering the entire smart-grid networks from
generation to distribution. Security has therefore become one of the
main foundations of the utility telecom network architecture and must
be considered at every layer with a defense-in-depth approach.
Migrating to IP-based protocols is key to addressing these challenges
for two reasons:
o IP enables a rich set of features and capabilities to enhance the
security posture.
o IP is based on open standards; this allows interoperability
between different vendors and products, driving down the costs
associated with implementing security solutions in OT networks.
Securing OT telecommunications over packet-switched IP networks
follows the same principles that are foundational for securing the IT
infrastructure, i.e., consideration must be given to (1) enforcing
electronic access control for both person-to-machine and machine-to-
machine communications and (2) providing the appropriate levels of
data privacy, device and platform integrity, and threat detection and
mitigation.
3.4. Electrical Utilities Requests to the IETF
o Mixed Layer 2 and Layer 3 topologies
o Deterministic behavior
o Bounded latency and jitter
o Tight feedback intervals
o High availability, low recovery time
o Redundancy, low packet loss
o Precise timing
o Centralized computing of deterministic paths
o Distributed configuration (may also be useful)
4. Building Automation Systems (BASs)
4.1. Use Case Description
A BAS manages equipment and sensors in a building for improving
residents' comfort, reducing energy consumption, and responding to
failures and emergencies. For example, the BAS measures the
temperature of a room using sensors and then controls the HVAC
(heating, ventilating, and air conditioning) to maintain a set
temperature and minimize energy consumption.
A BAS primarily performs the following functions:
o Periodically measures states of devices -- for example, humidity
and illuminance of rooms, open/close state of doors, fan speed.
o Stores the measured data.
o Provides the measured data to BAS operators.
o Generates alarms for abnormal state of devices.
o Controls devices (e.g., turns room lights off at 10:00 PM).
4.2. BASs Today
4.2.1. BAS Architecture
A typical present-day BAS architecture is shown in Figure 4.
+----------------------------+
| |
| BMS HMI |
| | | |
| +----------------------+ |
| | Management Network | |
| +----------------------+ |
| | | |
| LC LC |
| | | |
| +----------------------+ |
| | Field Network | |
| +----------------------+ |
| | | | | |
| Dev Dev Dev Dev |
| |
+----------------------------+
BMS: Building Management Server
HMI: Human-Machine Interface
LC: Local Controller
Figure 4: BAS Architecture
There are typically two layers of a network in a BAS. The upper
layer is called the management network, and the lower layer is called
the field network. In management networks, an IP-based communication
protocol is used, while in field networks, non-IP-based communication
protocols ("field protocols") are mainly used. Field networks have
specific timing requirements, whereas management networks can be best
effort.
An HMI is typically a desktop PC used by operators to monitor and
display device states, send device control commands to Local
Controllers (LCs), and configure building schedules (for example,
"turn off all room lights in the building at 10:00 PM").
A building management server (BMS) performs the following operations.
o Collects and stores device states from LCs at regular intervals.
o Sends control values to LCs according to a building schedule.
o Sends an alarm signal to operators if it detects abnormal device
states.
The BMS and HMI communicate with LCs via IP-based "management
protocols" (see standards [BACnet-IP] and [KNX]).
An LC is typically a Programmable Logic Controller (PLC) that is
connected to several tens or hundreds of devices using "field
protocols". An LC performs the following kinds of operations:
o Measures device states and provides the information to a BMS
or HMI.
o Sends control values to devices, unilaterally or as part of a
feedback control loop.
At the time of this writing, many field protocols are in use; some
are standards-based protocols, and others are proprietary (see
standards [LonTalk], [MODBUS], [PROFIBUS], and [FL-net]). The result
is that BASs have multiple MAC/PHY modules and interfaces. This
makes BASs more expensive and slower to develop and can result in
"vendor lock-in" with multiple types of management applications.
4.2.2. BAS Deployment Model
An example BAS for medium or large buildings is shown in Figure 5.
The physical layout spans multiple floors and includes a monitoring
room where the BAS management entities are located. Each floor will
have one or more LCs, depending on the number of devices connected to
the field network.
+--------------------------------------------------+
| Floor 3 |
| +----LC~~~~+~~~~~+~~~~~+ |
| | | | | |
| | Dev Dev Dev |
| | |
|--- | ------------------------------------------|
| | Floor 2 |
| +----LC~~~~+~~~~~+~~~~~+ Field Network |
| | | | | |
| | Dev Dev Dev |
| | |
|--- | ------------------------------------------|
| | Floor 1 |
| +----LC~~~~+~~~~~+~~~~~+ +-----------------|
| | | | | | Monitoring Room |
| | Dev Dev Dev | |
| | | BMS HMI |
| | Management Network | | | |
| +--------------------------------+-----+ |
| | |
+--------------------------------------------------+
Figure 5: BAS Deployment Model for Medium/Large Buildings
Each LC is connected to the monitoring room via the management
network, and the management functions are performed within the
building. In most cases, Fast Ethernet (e.g., 100BASE-T) is used for
the management network. Since the management network is not a
real-time network, the use of Ethernet without QoS is sufficient for
today's deployments.
Many physical interfaces used in field networks have specific timing
requirements -- for example, RS232C and RS485. Thus, if a field
network is to be replaced with an Ethernet or wireless network, such
networks must support time-critical deterministic flows.
Figure 6 shows another deployment model, in which the management
system is hosted remotely. This model is becoming popular for small
offices and residential buildings, in which a standalone monitoring
system is not cost effective.
+---------------+
| Remote Center |
| |
| BMS HMI |
+------------------------------------+ | | | |
| Floor 2 | | +---+---+ |
| +----LC~~~~+~~~~~+ Field Network| | | |
| | | | | | Router |
| | Dev Dev | +-------|-------+
| | | |
|--- | ------------------------------| |
| | Floor 1 | |
| +----LC~~~~+~~~~~+ | |
| | | | | |
| | Dev Dev | |
| | | |
| | Management Network | WAN |
| +------------------------Router-------------+
| |
+------------------------------------+
Figure 6: Deployment Model for Small Buildings
Some interoperability is possible in today's management networks but
is not possible in today's field networks due to their non-IP-based
design.
4.2.3. Use Cases for Field Networks
Below are use cases for environmental monitoring, fire detection, and
feedback control, and their implications for field network
performance.
4.2.3.1. Environmental Monitoring
The BMS polls each LC at a maximum measurement interval of 100 ms
(for example, to draw a historical chart of 1-second granularity with
a 10x sampling interval) and then performs the operations as
specified by the operator. Each LC needs to measure each of its
several hundred sensors once per measurement interval. Latency is
not critical in this scenario as long as all sensor value
measurements are completed within the measurement interval.
Availability is expected to be 99.999%.
4.2.3.2. Fire Detection
On detection of a fire, the BMS must stop the HVAC, close the fire
shutters, turn on the fire sprinklers, send an alarm, etc. There are
typically tens of fire sensors per LC that the BMS needs to manage.
In this scenario, the measurement interval is 10-50 ms, the
communication delay is 10 ms, and the availability must be 99.9999%.
4.2.3.3. Feedback Control
BASs utilize feedback control in various ways; the most time-critical
is control of DC motors, which require a short feedback interval
(1-5 ms) with low communication delay (10 ms) and jitter (1 ms). The
feedback interval depends on the characteristics of the device and on
the requirements for the control values. There are typically tens of
feedback sensors per LC.
Communication delay is expected to be less than 10 ms and jitter less
than 1 ms, while the availability must be 99.9999%.
4.2.4. BAS Security Considerations
When BAS field networks were developed, it was assumed that the field
networks would always be physically isolated from external networks;
therefore, security was not a concern. In today's world, many BASs
are managed remotely and are thus connected to shared IP networks;
therefore, security is a definite concern. Note, however, that
security features are not currently available in the majority of BAS
field network deployments.
The management network, being an IP-based network, has the protocols
available to enable network security, but in practice many BASs do
not implement even such available security features as device
authentication or encryption for data in transit.
4.3. BASs in the Future
In the future, lower energy consumption and environmental monitoring
that is more fine-grained will emerge; these will require more
sensors and devices, thus requiring larger and more-complex building
networks.
Building networks will be connected to or converged with other
networks (enterprise networks, home networks, and the Internet).
Therefore, better facilities for network management, control,
reliability, and security are critical in order to improve resident
and operator convenience and comfort. For example, the ability to
monitor and control building devices via the Internet would enable
(for example) control of room lights or HVAC from a resident's
desktop PC or phone application.
4.4. BAS Requests to the IETF
The community would like to see an interoperable protocol
specification that can satisfy the timing, security, availability,
and QoS constraints described above, such that the resulting
converged network can replace the disparate field networks. Ideally,
this connectivity could extend to the open Internet.
This would imply an architecture that can guarantee
o Low communication delays (from <10 ms to 100 ms in a network of
several hundred devices)
o Low jitter (<1 ms)
o Tight feedback intervals (1-10 ms)
o High network availability (up to 99.9999%)
o Availability of network data in disaster scenarios
o Authentication between management devices and field devices (both
local and remote)
o Integrity and data origin authentication of communication data
between management devices and field devices
o Confidentiality of data when communicated to a remote device
5. Wireless for Industrial Applications
5.1. Use Case Description
Wireless networks are useful for industrial applications -- for
example, (1) when portable, fast-moving, or rotating objects are
involved and (2) for the resource-constrained devices found in the
Internet of Things (IoT).
Such network-connected sensors, actuators, control loops, etc.
typically require that the underlying network support real-time QoS,
as well as such specific network properties as reliability,
redundancy, and security.
These networks may also contain very large numbers of devices -- for
example, for factories, "big data" acquisition, and the IoT. Given
the large numbers of devices installed and the potential
pervasiveness of the IoT, this is a huge and very cost-sensitive
market such that small cost reductions can save large amounts of
money.
5.1.1. Network Convergence Using 6TiSCH
Some wireless network technologies support real-time QoS and are thus
useful for these kinds of networks, but others do not.
This use case focuses on one specific wireless network technology
that provides the required deterministic QoS: "IPv6 over the TSCH
mode of IEEE 802.15.4e" (6TiSCH, where "TSCH" stands for
"Time-Slotted Channel Hopping"; see [Arch-for-6TiSCH], [IEEE-802154],
and [RFC7554]).
There are other deterministic wireless buses and networks available
today; however, they are incompatible with each other and with IP
traffic (for example, see [ISA100] and [WirelessHART]).
Thus, the primary goal of this use case is to apply 6TiSCH as a
converged IP-based and standards-based wireless network for
industrial applications, i.e., to replace multiple proprietary and/or
incompatible wireless networking and wireless network management
standards.
5.1.2. Common Protocol Development for 6TiSCH
Today, there are a number of protocols required by 6TiSCH that are
still in development. Another goal of this use case is to highlight
the ways in which these "missing" protocols share goals in common
with DetNet. Thus, it is possible that some of the protocol
technology developed for DetNet will also be applicable to 6TiSCH.
These protocol goals are identified here, along with their
relationship to DetNet. It is likely that ultimately the resulting
protocols will not be identical but will share design principles that
contribute to the efficiency of enabling both DetNet and 6TiSCH.
One such commonality is that -- although on a different time scale --
in both TSN [IEEE-8021TSNTG] and TSCH, a packet that crosses the
network from node to node follows a precise schedule, as does a train
that leaves intermediate stations at precise times along its path.
This kind of operation reduces collisions, saves energy, and enables
engineering of the network for deterministic properties.
Another commonality is remote monitoring and scheduling management of
a TSCH network by a Path Computation Element (PCE) and Network
Management Entity (NME). The PCE and NME manage timeslots and device
resources in a manner that minimizes the interaction with, and the
load placed on, resource-constrained devices. For example, a tiny
IoT device may have just enough buffers to store one or a few IPv6
packets; it will have limited bandwidth between peers such that it
can maintain only a small amount of peer information, and it will not
be able to store many packets waiting to be forwarded. It is
advantageous, then, for the IoT device to only be required to carry
out the specific behavior assigned to it by the PCE and NME (as
opposed to maintaining its own IP stack, for example).
It is possible that there will be some peer-to-peer communication;
for example, the PCE may communicate only indirectly with some
devices in order to enable hierarchical configuration of the system.
6TiSCH depends on [PCE] and [DetNet-Arch].
6TiSCH also depends on the fact that DetNet will maintain consistency
with [IEEE-8021TSNTG].
5.2. Wireless Industrial Today
Today, industrial wireless technology ("wireless industrial") is
accomplished using multiple deterministic wireless networks that are
incompatible with each other and with IP traffic.
6TiSCH is not yet fully specified, so it cannot be used in today's
applications.
5.3. Wireless Industrial in the Future
5.3.1. Unified Wireless Networks and Management
DetNet and 6TiSCH together can enable converged transport of
deterministic and best-effort traffic flows between real-time
industrial devices and WANs via IP routing. A high-level view of
this type of basic network is shown in Figure 7.
---+-------- ............ ------------
| External Network |
| +-----+
+-----+ | NME |
| | LLN Border | |
| | Router +-----+
+-----+
o o o
o o o o
o o LLN o o o
o o o o
o
LLN: Low-Power and Lossy Network
Figure 7: Basic 6TiSCH Network
Figure 8 shows a backbone router federating multiple synchronized
6TiSCH subnets into a single subnet connected to the external
network.
---+-------- ............ ------------
| External Network |
| +-----+
| +-----+ | NME |
+-----+ | +-----+ | |
| | Router | | PCE | +-----+
| | +--| |
+-----+ +-----+
| |
| Subnet Backbone |
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | Backbone | | Backbone | | Backbone
o | | Router | | Router | | Router
+-----+ +-----+ +-----+
o o o o o
o o o o o o o o o o o
o o o LLN o o o o
o o o o o o o o o o o o
Figure 8: Extended 6TiSCH Network
The backbone router must ensure end-to-end deterministic behavior
between the LLN and the backbone. This should be accomplished in
conformance with the work done in [DetNet-Arch] with respect to
Layer 3 aspects of deterministic networks that span multiple Layer 2
domains.
The PCE must compute a deterministic path end to end across the TSCH
network and IEEE 802.1 TSN Ethernet backbone, and DetNet protocols
are expected to enable end-to-end deterministic forwarding.
5.3.1.1. PCE and 6TiSCH ARQ Retries
6TiSCH uses the Automatic Repeat reQuest (ARQ) mechanism
[IEEE-802154] to provide higher reliability of packet delivery. ARQ
is related to Packet Replication and Elimination (PRE) because there
are two independent paths for packets to arrive at the destination.
If an expected packet does not arrive on one path, then it checks for
the packet on the second path.
Although to date this mechanism is only used by wireless networks,
this technique might be appropriate for DetNet, and aspects of the
enabling protocol could therefore be co-developed.
For example, in Figure 9, a track is laid out from a field device in
a 6TiSCH network to an IoT gateway that is located on an IEEE 802.1
TSN backbone.
+-----+
| IoT |
| G/W |
+-----+
^ <---- Elimination
| |
Track Branch | |
+-------+ +--------+ Subnet Backbone
| |
+--|--+ +--|--+
| | | Backbone | | | Backbone
o | | | Router | | | Router
+--/--+ +--|--+
o / o o---o----/ o
o o---o--/ o o o o o
o \ / o o LLN o
o v <---- Replication
o
Figure 9: 6TiSCH Network with PRE
In ARQ, the replication function in the field device sends a copy of
each packet over two different branches, and the PCE schedules each
hop of both branches so that the two copies arrive in due time at the
gateway. In the case of a loss on one branch, one hopes that the
other copy of the packet will still arrive within the allocated time.
If two copies make it to the IoT gateway, the elimination function in
the gateway ignores the extra packet and presents only one copy to
upper layers.
At each 6TiSCH hop along the track, the PCE may schedule more than
one timeslot for a packet, so as to support Layer 2 retries (ARQ).
At the time of this writing, a deployment's TSCH track does not
necessarily support PRE but is systematically multipath. This means
that a track is scheduled so as to ensure that each hop has at least
two forwarding solutions. The forwarding decision will be to try the
preferred solution and use the other solution in the case of Layer 2
transmission failure as detected by ARQ.
5.3.2. Schedule Management by a PCE
A common feature of 6TiSCH and DetNet is actions taken by a PCE when
configuring paths through the network. Specifically, what is needed
is a protocol and data model that the PCE will use to get/set the
relevant configuration from/to the devices, as well as perform
operations on the devices. Specifically, both DetNet and 6TiSCH need
to develop a protocol (and associated data model) that the PCE can
use to (1) get/set the relevant configuration from/to the devices and
(2) perform operations on the devices. These could be initially
developed by DetNet, with consideration for their reuse by 6TiSCH.
The remainder of this section provides a bit more context from the
6TiSCH side.
5.3.2.1. PCE Commands and 6TiSCH CoAP Requests
The 6TiSCH device does not expect to place the request for bandwidth
between itself and another device in the network. Rather, an
operation control system invoked through a human interface specifies
the traffic requirements and the end nodes (in terms of latency and
reliability). Based on this information, the PCE must compute a path
between the end nodes and provision the network with per-flow state
that describes the per-hop operation for a given packet, the
corresponding timeslots, the flow identification that enables
recognizing that a certain packet belongs to a certain path, etc.
For a static configuration that serves a certain purpose for a long
period of time, it is expected that a node will be provisioned in one
shot with a full schedule, i.e., a schedule that defines the behavior
of the node with respect to all data flows through that node. 6TiSCH
expects that the programming of the schedule will be done over the
Constrained Application Protocol (CoAP) as discussed in
[CoAP-6TiSCH].
6TiSCH expects that the PCE commands will be mapped back and forth
into CoAP by a gateway function at the edge of the 6TiSCH network.
For instance, it is possible that a mapping entity on the backbone
transforms a non-CoAP protocol such as the Path Computation Element
Communication Protocol (PCEP) into the RESTful interfaces that the
6TiSCH devices support. This architecture will be refined to comply
with DetNet [DetNet-Arch] when the work is formalized. Related
information about 6TiSCH can be found in [Interface-6TiSCH-6top] and
[RFC6550] ("RPL: IPv6 Routing Protocol for Low-Power and Lossy
Networks").
A protocol may be used to update the state in the devices during
runtime -- for example, if it appears that a path through the network
has ceased to perform as expected, but in 6TiSCH that flow was not
designed and no protocol was selected. DetNet should define the
appropriate end-to-end protocols to be used in that case. The
implication is that these state updates take place once the system is
configured and running, i.e., they are not limited to the initial
communication of the configuration of the system.
A "slotFrame" is the base object that a PCE would manipulate to
program a schedule into an LLN node [Arch-for-6TiSCH].
The PCE should read energy data from devices and compute paths that
will implement policies on how energy in devices is consumed -- for
instance, to ensure that the spent energy does not exceed the
available energy over a period of time. Note that this statement
implies that an extensible protocol for communicating device
information to the PCE and enabling the PCE to act on it will be part
of the DetNet architecture; however, for subnets with specific
protocols (e.g., CoAP), a gateway may be required.
6TiSCH devices can discover their neighbors over the radio using a
mechanism such as beacons, but even though the neighbor information
is available in the 6TiSCH interface data model, 6TiSCH does not
describe a protocol to proactively push the neighbor information to a
PCE. DetNet should define such a protocol; one possible design
alternative is that it could operate over CoAP. Alternatively, it
could be converted to/from CoAP by a gateway. Such a protocol could
carry multiple metrics -- for example, metrics similar to those used
for RPL operations [RFC6551].
5.3.2.2. 6TiSCH IP Interface
Protocol translation between the TSCH MAC layer and IP is
accomplished via the "6top" sublayer [Sublayer-6TiSCH-6top]. The
6top data model and management interfaces are further discussed in
[Interface-6TiSCH-6top] and [CoAP-6TiSCH].
An IP packet that is sent along a 6TiSCH path uses a differentiated
services Per-Hop Behavior Group (PHB) called "deterministic
forwarding", as described in [Det-Fwd-PHB].
5.3.3. 6TiSCH Security Considerations
In addition to the classical requirements for protection of control
signaling, it must be noted that 6TiSCH networks operate on limited
resources that can be depleted rapidly in a DoS attack on the system
-- for instance, by placing a rogue device in the network or by
obtaining management control and setting up unexpected additional
paths.
5.4. Wireless Industrial Requests to the IETF
6TiSCH depends on DetNet to define:
o Configuration (state) and operations for deterministic paths
o End-to-end protocols for deterministic forwarding (tagging, IP)
o A protocol for PRE
6. Cellular Radio
6.1. Use Case Description
This use case describes the application of deterministic networking
in the context of cellular telecom transport networks. Important
elements include time synchronization, clock distribution, and ways
to establish time-sensitive streams for both Layer 2 and Layer 3
user-plane traffic.
6.1.1. Network Architecture
Figure 10 illustrates a 3GPP-defined cellular network architecture
typical at the time of this writing. The architecture includes
"Fronthaul", "Midhaul", and "Backhaul" network segments. The
"Fronthaul" is the network connecting base stations (Baseband Units
(BBUs)) to the Remote Radio Heads (RRHs) (also referred to here as
"antennas"). The "Midhaul" is the network that interconnects base
stations (or small-cell sites). The "Backhaul" is the network or
links connecting the radio base station sites to the network
controller/gateway sites (i.e., the core of the 3GPP cellular
network).
Y (RRHs (antennas))
\
Y__ \.--. .--. +------+
\_( `. +---+ _( `. | 3GPP |
Y------( Front- )----|eNB|----( Back- )------| core |
( ` .haul ) +---+ ( ` .haul) ) | netw |
/`--(___.-' \ `--(___.-' +------+
Y_/ / \.--. \
Y_/ _(Mid-`. \
( haul ) \
( ` . ) ) \
`--(___.-'\_____+---+ (small-cell sites)
\ |SCe|__Y
+---+ +---+
Y__|eNB|__Y
+---+
Y_/ \_Y ("local" radios)
Figure 10: Generic 3GPP-Based Cellular Network Architecture
In Figure 10, "eNB" ("E-UTRAN Node B") is the hardware that is
connected to the mobile phone network and enables the mobile phone
network to communicate with mobile handsets [TS36300]. ("E-UTRAN"
stands for "Evolved Universal Terrestrial Radio Access Network".)
6.1.2. Delay Constraints
The available processing time for Fronthaul networking overhead is
limited to the available time after the baseband processing of the
radio frame has completed. For example, in Long Term Evolution (LTE)
radio, 3 ms is allocated for the processing of a radio frame, but
typically the baseband processing uses most of it, allowing only a
small fraction to be used by the Fronthaul network. In this example,
out of 3 ms, the maximum time allocated to the Fronthaul network for
one-way delay is 250 us, and the existing specification [NGMN-Fronth]
specifies a maximum delay of only 100 us. This ultimately determines
the distance the RRHs can be located from the base stations (e.g.,
100 us equals roughly 20 km of optical fiber-based transport).
Allocation options regarding the available time budget between
processing and transport are currently undergoing heavy discussion in
the mobile industry.
For packet-based transport, the allocated transport time between the
RRH and the BBU is consumed by node processing, buffering, and
distance-incurred delay. An example of the allocated transport time
is 100 us (from the Common Public Radio Interface [CPRI]).
The baseband processing time and the available "delay budget" for the
Fronthaul is likely to change in the forthcoming "5G" due to reduced
radio round-trip times and other architectural and service
requirements [NGMN].
The transport time budget, as noted above, places limitations on the
distance that RRHs can be located from base stations (i.e., the link
length). In the above analysis, it is assumed that the entire
transport time budget is available for link propagation delay.
However, the transport time budget can be broken down into three
components: scheduling/queuing delay, transmission delay, and link
propagation delay. Using today's Fronthaul networking technology,
the queuing, scheduling, and transmission components might become the
dominant factors in the total transport time, rather than the link
propagation delay. This is especially true in cases where the
Fronthaul link is relatively short and is shared among multiple
Fronthaul flows -- for example, in indoor and small-cell networks,
massive Multiple Input Multiple Output (MIMO) antenna networks, and
split Fronthaul architectures.
DetNet technology can improve Fronthaul networks by controlling and
reducing the time required for the queuing, scheduling, and
transmission operations by properly assigning network resources, thus
(1) leaving more of the transport time budget available for link
propagation and (2) enabling longer link lengths. However, link
length is usually a predetermined parameter and is not a controllable
network parameter, since RRH and BBU sites are usually located in
predetermined locations. However, the number of antennas in an RRH
site might increase -- for example, by adding more antennas,
increasing the MIMO capability of the network, or adding support for
massive MIMO. This means increasing the number of Fronthaul flows
sharing the same Fronthaul link. DetNet can now control the
bandwidth assignment of the Fronthaul link and the scheduling of
Fronthaul packets over this link and can provide adequate buffer
provisioning for each flow to reduce the packet loss rate.
Another way in which DetNet technology can aid Fronthaul networks is
by providing effective isolation between flows -- for example,
between flows originating in different slices within a network-sliced
5G network. Note, however, that this isolation applies to DetNet
flows for which resources have been preallocated, i.e., it does not
apply to best-effort flows within a DetNet. DetNet technology can
also dynamically control the bandwidth-assignment, scheduling, and
packet-forwarding decisions, as well as the buffer provisioning of
the Fronthaul flows to guarantee the end-to-end delay of the
Fronthaul packets and minimize the packet loss rate.
[METIS] documents the fundamental challenges as well as overall
technical goals of the future 5G mobile and wireless systems as the
starting point. These future systems should support much higher data
volumes and rates and significantly lower end-to-end latency for 100x
more connected devices (at cost and energy-consumption levels similar
to today's systems).
For Midhaul connections, delay constraints are driven by inter-site
radio functions such as Coordinated Multi-Point (CoMP) processing
(see [CoMP]). CoMP reception and transmission constitute a framework
in which multiple geographically distributed antenna nodes cooperate
to improve performance for the users served in the common cooperation
area. The design principle of CoMP is to extend single-cell-to-
multi-UE (User Equipment) transmission to a multi-cell-to-multi-UE
transmission via cooperation among base stations.
CoMP has delay-sensitive performance parameters: "Midhaul latency"
and "CSI (Channel State Information) reporting and accuracy". The
essential feature of CoMP is signaling between eNBs, so Midhaul
latency is the dominating limitation of CoMP performance. Generally,
CoMP can benefit from coordinated scheduling (either distributed or
centralized) of different cells if the signaling delay between eNBs
is within 1-10 ms. This delay requirement is both rigid and
absolute, because any uncertainty in delay will degrade performance
significantly.
Inter-site CoMP is one of the key requirements for 5G and is also a
goal for 4.5G network architectures.
6.1.3. Time-Synchronization Constraints
Fronthaul time-synchronization requirements are given by [TS25104],
[TS36104], [TS36211], and [TS36133]. These can be summarized for the
3GPP LTE-based networks as:
Delay accuracy:
+-8 ns (i.e., +-1/32 Tc, where Tc is the Universal Mobile
Telecommunications System (UMTS) Chip time of 1/3.84 MHz),
resulting in a round-trip accuracy of +-16 ns. The value is this
low in order to meet the 3GPP Timing Alignment Error (TAE)
measurement requirements. Note that performance guarantees of
low-nanosecond values such as these are considered to be below the
DetNet layer -- it is assumed that the underlying implementation
(e.g., the hardware) will provide sufficient support (e.g.,
buffering) to enable this level of accuracy. These values are
maintained in the use case to give an indication of the overall
application.
TAE:
TAE is problematic for Fronthaul networks and must be minimized.
If the transport network cannot guarantee TAE levels that are low
enough, then additional buffering has to be introduced at the
edges of the network to buffer out the jitter. Buffering is not
desirable, as it reduces the total available delay budget.
Packet Delay Variation (PDV) requirements can be derived from TAE
measurements for packet-based Fronthaul networks.
* For MIMO or TX diversity transmissions, at each carrier
frequency, TAE measurements shall not exceed 65 ns (i.e.,
1/4 Tc).
* For intra-band contiguous carrier aggregation, with or without
MIMO or TX diversity, TAE measurements shall not exceed 130 ns
(i.e., 1/2 Tc).
* For intra-band non-contiguous carrier aggregation, with or
without MIMO or TX diversity, TAE measurements shall not exceed
260 ns (i.e., 1 Tc).
* For inter-band carrier aggregation, with or without MIMO or TX
diversity, TAE measurements shall not exceed 260 ns.
Transport link contribution to radio frequency errors:
+-2 PPB. This value is considered to be "available" for the
Fronthaul link out of the total 50 PPB budget reserved for the
radio interface. Note that the transport link contributes to
radio frequency errors for the following reason: at the time of
this writing, Fronthaul communication is direct communication from
the radio unit to the RRH. The RRH is essentially a passive
device (e.g., without buffering). The transport drives the
antenna directly by feeding it with samples, and everything the
transport adds will be introduced to the radio "as is". So, if
the transport causes any additional frequency errors, the errors
will show up immediately on the radio as well. Note that
performance guarantees of low-nanosecond values such as these are
considered to be below the DetNet layer -- it is assumed that the
underlying implementation (e.g., the hardware) will provide
sufficient support to enable this level of performance. These
values are maintained in the use case to give an indication of the
overall application.
The above-listed time-synchronization requirements are difficult to
meet with point-to-point connected networks and are more difficult to
meet when the network includes multiple hops. It is expected that
networks must include buffering at the ends of the connections as
imposed by the jitter requirements, since trying to meet the jitter
requirements in every intermediate node is likely to be too costly.
However, every measure to reduce jitter and delay on the path makes
it easier to meet the end-to-end requirements.
In order to meet the timing requirements, both senders and receivers
must remain time synchronized, demanding very accurate clock
distribution -- for example, support for IEEE 1588 transparent clocks
or boundary clocks in every intermediate node.
In cellular networks from the LTE radio era onward, phase
synchronization is needed in addition to frequency synchronization
[TS36300] [TS23401]. Time constraints are also important due to
their impact on packet loss. If a packet is delivered too late, then
the packet may be dropped by the host.
6.1.4. Transport-Loss Constraints
Fronthaul and Midhaul networks assume that transport is almost
error free. Errors can cause a reset of the radio interfaces, in
turn causing reduced throughput or broken radio connectivity for
mobile customers.
For packetized Fronthaul and Midhaul connections, packet loss may be
caused by BER, congestion, or network failure scenarios. Different
Fronthaul "functional splits" are being considered by 3GPP, requiring
strict Frame Loss Ratio (FLR) guarantees. As one example (referring
to the legacy CPRI split, which is option 8 in 3GPP), lower-layer
splits may imply an FLR of less than 10^-7 for data traffic and less
than 10^-6 for control and management traffic.
Many of the tools available for eliminating packet loss for Fronthaul
and Midhaul networks have serious challenges; for example,
retransmitting lost packets or using FEC to circumvent bit errors (or
both) is practically impossible, due to the additional delay
incurred. Using redundant streams for better guarantees of delivery
is also practically impossible in many cases, due to high bandwidth
requirements for Fronthaul and Midhaul networks. Protection
switching is also a candidate, but at the time of this writing,
available technologies for the path switch are too slow to avoid a
reset of mobile interfaces.
It is assumed that Fronthaul links are symmetric. All Fronthaul
streams (i.e., those carrying radio data) have equal priority and
cannot delay or preempt each other.
All of this implies that it is up to the network to guarantee that
each time-sensitive flow meets its schedule.
6.1.5. Cellular Radio Network Security Considerations
Establishing time-sensitive streams in the network entails reserving
networking resources for long periods of time. It is important that
these reservation requests be authenticated to prevent malicious
reservation attempts from hostile nodes (or accidental
misconfiguration). This is particularly important in the case where
the reservation requests span administrative domains. Furthermore,
the reservation information itself should be digitally signed to
reduce the risk of a legitimate node pushing a stale or hostile
configuration into another networking node.
Note: This is considered important for the security policy of the
network but does not affect the core DetNet architecture and design.
6.2. Cellular Radio Networks Today
6.2.1. Fronthaul
Today's Fronthaul networks typically consist of:
o Dedicated point-to-point fiber connection (common)
o Proprietary protocols and framings
o Custom equipment and no real networking
At the time of this writing, solutions for Fronthaul are direct
optical cables or Wavelength-Division Multiplexing (WDM) connections.
6.2.2. Midhaul and Backhaul
Today's Midhaul and Backhaul networks typically consist of:
o Mostly normal IP networks, MPLS-TP, etc.
o Clock distribution and synchronization using IEEE 1588 and syncE
Telecommunications networks in the Midhaul and Backhaul are already
heading towards transport networks where precise time-synchronization
support is one of the basic building blocks. In order to meet
bandwidth and cost requirements, most transport networks have already
transitioned to all-IP packet-based networks; however, highly
accurate clock distribution has become a challenge.
In the past, Midhaul and Backhaul connections were typically based on
TDM and provided frequency-synchronization capabilities as a part of
the transport media. More recently, other technologies such as GPS
or syncE [syncE] have been used.
Ethernet, IP/MPLS [RFC3031], and pseudowires (as described in
[RFC3985] ("Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture")
for legacy transport support)) have become popular tools for building
and managing new all-IP Radio Access Networks (RANs)
[SR-IP-RAN-Use-Case]. Although various timing and synchronization
optimizations have already been proposed and implemented, including
PTP enhancements [IEEE-1588] (see also [Timing-over-MPLS] and
[RFC8169]), these solutions are not necessarily sufficient for the
forthcoming RAN architectures, nor do they guarantee the more
stringent time-synchronization requirements such as [CPRI].
Existing solutions for TDM over IP include those discussed in
[RFC4553], [RFC5086], and [RFC5087]; [MEF8] addresses TDM over
Ethernet transports.
6.3. Cellular Radio Networks in the Future
Future cellular radio networks will be based on a mix of different
xHaul networks (xHaul = Fronthaul, Midhaul, and Backhaul), and future
transport networks should be able to support all of them
simultaneously. It is already envisioned today that:
o Not all "cellular radio network" traffic will be IP; for example,
some will remain at Layer 2 (e.g., Ethernet based). DetNet
solutions must address all traffic types (Layer 2 and Layer 3)
with the same tools and allow their transport simultaneously.
o All types of xHaul networks will need some types of DetNet
solutions. For example, with the advent of 5G, some Backhaul
traffic will also have DetNet requirements (for example, traffic
belonging to time-critical 5G applications).
o Different functional splits between the base stations and the
on-site units could coexist on the same Fronthaul and Backhaul
network.
Future cellular radio networks should contain the following:
o Unified standards-based transport protocols and standard
networking equipment that can make use of underlying deterministic
link-layer services
o Unified and standards-based network management systems and
protocols in all parts of the network (including Fronthaul)
New RAN deployment models and architectures may require TSN services
with strict requirements on other parts of the network that
previously were not considered to be packetized at all. Time and
synchronization support are already topical for Backhaul and Midhaul
packet networks [MEF22.1.1] and are also becoming a real issue for
Fronthaul networks. Specifically, in Fronthaul networks, the timing
and synchronization requirements can be extreme for packet-based
technologies -- for example, on the order of a PDV of +-20 ns or less
and frequency accuracy of +-0.002 PPM [Fronthaul].
The actual transport protocols and/or solutions for establishing
required transport "circuits" (pinned-down paths) for Fronthaul
traffic are still undefined. Those protocols are likely to include
(but are not limited to) solutions directly over Ethernet, over IP,
and using MPLS/pseudowire transport.
Interesting and important work for TSN has been done for Ethernet
[IEEE-8021TSNTG]; this work specifies the use of PTP [IEEE-1588] in
the context of IEEE 802.1D and IEEE 802.1Q. [IEEE-8021AS] specifies
a Layer 2 time-synchronizing service, and other specifications such
as IEEE 1722 [IEEE-1722] specify Ethernet-based Layer 2 transport for
time-sensitive streams.
However, even these Ethernet TSN features may not be sufficient for
Fronthaul traffic. Therefore, having specific profiles that take
Fronthaul requirements into account is desirable [IEEE-8021CM].
New promising work seeks to enable the transport of time-sensitive
Fronthaul streams in Ethernet bridged networks [IEEE-8021CM].
Analogous to IEEE 1722, standardization efforts in the IEEE 1914.3
Task Force [IEEE-19143] to define the Layer 2 transport encapsulation
format for transporting Radio over Ethernet (RoE) are ongoing.
As mentioned in Section 6.1.2, 5G communications will provide one of
the most challenging cases for delay-sensitive networking. In order
to meet the challenges of ultra-low latency and ultra-high
throughput, 3GPP has studied various functional splits for 5G, i.e.,
physical decomposition of the 5G "gNodeB" base station and deployment
of its functional blocks in different locations [TR38801].
These splits are numbered from split option 1 (dual connectivity, a
split in which the radio resource control is centralized and other
radio stack layers are in distributed units) to split option 8 (a
PHY-RF split in which RF functionality is in a distributed unit and
the rest of the radio stack is in the centralized unit), with each
intermediate split having its own data-rate and delay requirements.
Packetized versions of different splits have been proposed, including
enhanced CPRI (eCPRI) [eCPRI] and RoE (as previously noted). Both
provide Ethernet encapsulations, and eCPRI is also capable of IP
encapsulation.
All-IP RANs and xHaul networks would benefit from time
synchronization and time-sensitive transport services. Although
Ethernet appears to be the unifying technology for the transport,
there is still a disconnect when it comes to providing Layer 3
services. The protocol stack typically has a number of layers below
Ethernet Layer 2 that might be "visible" to Layer 3. In a fairly
common scenario, on top of the lowest-layer (optical) transport is
the first (lowest) Ethernet layer, then one or more layers of MPLS,
pseudowires, and/or other tunneling protocols, and finally one or
more Ethernet layers that are visible to Layer 3.
Although there exist technologies for establishing circuits through
the routed and switched networks (especially in the MPLS/PWE space),
there is still no way to signal the time-synchronization and
time-sensitive stream requirements/reservations for Layer 3 flows in
a way that addresses the entire transport stack, including the
Ethernet layers that need to be configured.
Furthermore, not all "user-plane" traffic will be IP. Therefore, the
solution in question also must address the use cases where the
user-plane traffic is on a different layer (for example, Ethernet
frames).
6.4. Cellular Radio Networks Requests to the IETF
A standard for data-plane transport specifications that is:
o Unified among all xHauls (meaning that different flows with
diverse DetNet requirements can coexist in the same network and
traverse the same nodes without interfering with each other)
o Deployed in a highly deterministic network environment
o Capable of supporting multiple functional splits simultaneously,
including existing Backhaul and CPRI Fronthaul, and (potentially)
new modes as defined, for example, in 3GPP; these goals can be
supported by the existing DetNet use case "common themes"
(Section 11); of special note are Sections 11.1.8 ("Mix of
Deterministic and Best-Effort Traffic"), 11.3.1 ("Bounded
Latency"), 11.3.2 ("Low Latency"), 11.3.4 ("Symmetrical Path
Delays"), and 11.6 ("Deterministic Flows")
o Capable of supporting network slicing and multi-tenancy; these
goals can be supported by the same DetNet themes noted above
o Capable of transporting both in-band and out-of-band control
traffic (e.g., Operations, Administration, and Maintenance (OAM)
information)
o Deployable over multiple data-link technologies (e.g., IEEE 802.3,
mmWave)
A standard for data-flow information models that is:
o Aware of the time sensitivity and constraints of the target
networking environment
o Aware of underlying deterministic networking services (e.g., on
the Ethernet layer)
7. Industrial Machine to Machine (M2M)
7.1. Use Case Description
"Industrial automation" in general refers to automation of
manufacturing, quality control, and material processing. This M2M
use case focuses on machine units on a plant floor that periodically
exchange data with upstream or downstream machine modules and/or a
supervisory controller within a LAN.
PLCs are the "actors" in M2M communications. Communication between
PLCs, and between PLCs and the supervisory PLC (S-PLC), is achieved
via critical control/data streams (Figure 11).
S (Sensor)
\ +-----+
PLC__ \.--. .--. ---| MES |
\_( `. _( `./ +-----+
A------( Local )-------------( L2 )
( Net ) ( Net ) +-------+
/`--(___.-' `--(___.-' ----| S-PLC |
S_/ / PLC .--. / +-------+
A_/ \_( `.
(Actuator) ( Local )
( Net )
/`--(___.-'\
/ \ A
S A
Figure 11: Current Generic Industrial M2M Network Architecture
This use case focuses on PLC-related communications; communication to
Manufacturing Execution Systems (MESs) are not addressed.
This use case covers only critical control/data streams; non-critical
traffic between industrial automation applications (such as
communication of state, configuration, setup, and database
communication) is adequately served by prioritizing techniques
available at the time of this writing. Such traffic can use up to
80% of the total bandwidth required. There is also a subset of
non-time-critical traffic that must be reliable even though it is not
time sensitive.
In this use case, deterministic networking is primarily needed to
provide end-to-end delivery of M2M messages within specific timing
constraints -- for example, in closed-loop automation control.
Today, this level of determinism is provided by proprietary
networking technologies. In addition, standard networking
technologies are used to connect the local network to remote
industrial automation sites, e.g., over an enterprise or metro
network that also carries other types of traffic. Therefore, flows
that should be forwarded with deterministic guarantees need to be
sustained, regardless of the amount of other flows in those networks.
7.2. Industrial M2M Communications Today
Today, proprietary networks fulfill the needed timing and
availability for M2M networks.
The network topologies used today by industrial automation are
similar to those used by telecom networks: daisy chain, ring,
hub-and-spoke, and "comb" (a subset of daisy chain).
PLC-related control/data streams are transmitted periodically and
carry either a preconfigured payload or a payload configured during
runtime.
Some industrial applications require time synchronization at the end
nodes. For such time-coordinated PLCs, accuracy of 1 us is required.
Even in the case of "non-time-coordinated" PLCs, time synchronization
may be needed, e.g., for timestamping of sensor data.
Industrial-network scenarios require advanced security solutions. At
the time of this writing, many industrial production networks are
physically separated. Filtering policies that are typically enforced
in firewalls are used to prevent critical flows from being leaked
outside a domain.
7.2.1. Transport Parameters
The cycle time defines the frequency of message(s) between industrial
actors. The cycle time is application dependent, in the range of
1-100 ms for critical control/data streams.
Because industrial applications assume that deterministic transport
will be used for critical control-data-stream parameters (instead of
having to define latency and delay-variation parameters), it is
sufficient to fulfill requirements regarding the upper bound of
latency (maximum latency). The underlying networking infrastructure
must ensure a maximum end-to-end message delivery time in the range
of 100 us to 50 ms, depending on the control-loop application.
The bandwidth requirements of control/data streams are usually
calculated directly from the bytes-per-cycle parameter of the control
loop. For PLC-to-PLC communication, one can expect 2-32 streams with
packet sizes in the range of 100-700 bytes. For S-PLC-to-PLC
communication, the number of streams is higher -- up to 256 streams.
Usually, no more than 20% of available bandwidth is used for
critical control/data streams. In today's networks, 1 Gbps links
are commonly used.
Most PLC control loops are rather tolerant of packet loss; however,
critical control/data streams accept a loss of no more than one
packet per consecutive communication cycle (i.e., if a packet gets
lost in cycle "n", then the next cycle ("n+1") must be lossless).
After the loss of two or more consecutive packets, the network may be
considered to be "down" by the application.
As network downtime may impact the whole production system, the
required network availability is rather high (99.999%).
Based on the above parameters, some form of redundancy will be
required for M2M communications; however, any individual solution
depends on several parameters, including cycle time and
delivery time.
7.2.2. Stream Creation and Destruction
In an industrial environment, critical control/data streams are
created rather infrequently, on the order of ~10 times per
day/week/month. Most of these critical control/data streams get
created at machine startup; however, flexibility is also needed
during runtime -- for example, when adding or removing a machine. As
production systems become more flexible going forward, there will be
a significant increase in the rate at which streams are created,
changed, and destroyed.
7.3. Industrial M2M in the Future
We foresee a converged IP-standards-based network with deterministic
properties that can satisfy the timing, security, and reliability
constraints described above. Today's proprietary networks could then
be interfaced to such a network via gateways; alternatively, in the
case of new installations, devices could be connected directly to the
converged network.
For this use case, time-synchronization accuracy on the order of 1 us
is expected.
7.4. Industrial M2M Requests to the IETF
o Converged IP-based network
o Deterministic behavior (bounded latency and jitter)
o High availability (presumably through redundancy) (99.999%)
o Low message delivery time (100 us to 50 ms)
o Low packet loss (with a bounded number of consecutive lost
packets)
o Security (e.g., preventing critical flows from being leaked
between physically separated networks)
8. Mining Industry
8.1. Use Case Description
The mining industry is highly dependent on networks to monitor and
control their systems, in both open-pit and underground extraction as
well as in transport and refining processes. In order to reduce
risks and increase operational efficiency in mining operations, the
location of operators has been relocated (as much as possible) from
the extraction site to remote control and monitoring sites.
In the case of open-pit mining, autonomous trucks are used to
transport the raw materials from the open pit to the refining factory
where the final product (e.g., copper) is obtained. Although the
operation is autonomous, the tracks are remotely monitored from a
central facility.
In pit mines, the monitoring of the tailings or mine dumps is
critical in order to minimize environmental pollution. In the past,
monitoring was conducted through manual inspection of preinstalled
dataloggers. Cabling is not typically used in such scenarios, due to
its high cost and complex deployment requirements. At the time of
this writing, wireless technologies are being employed to monitor
these cases permanently. Slopes are also monitored in order to
anticipate possible mine collapse. Due to the unstable terrain,
cable maintenance is costly and complex; hence, wireless technologies
are employed.
In the case of underground monitoring, autonomous vehicles with
extraction tools travel independently through the tunnels, but their
operational tasks (such as excavation, stone-breaking, and transport)
are controlled remotely from a central facility. This generates
upstream video and feedback traffic plus downstream actuator-control
traffic.
8.2. Mining Industry Today
At the time of this writing, the mining industry uses a
packet-switched architecture supported by high-speed Ethernet.
However, in order to comply with requirements regarding delay and
packet loss, the network bandwidth is overestimated. This results in
very low efficiency in terms of resource usage.
QoS is implemented at the routers to separate video, management,
monitoring, and process-control traffic for each stream.
Since mobility is involved in this process, the connections between
the backbone and the mobile devices (e.g., trucks, trains, and
excavators) are implemented using a wireless link. These links are
based on IEEE 802.11 [IEEE-80211] for open-pit mining and "leaky
feeder" communications for underground mining. (A "leaky feeder"
communication system consists of a coaxial cable, run along tunnels,
that emits and receives radio waves, functioning as an extended
antenna. The cable is "leaky" in that it has gaps or slots in its
outer conductor to allow the radio signal to leak into or out of the
cable along its entire length.)
Lately, in pit mines the use of Low-Power WAN (LPWAN) technologies
has been extended: tailings, slopes, and mine dumps are monitored by
battery-powered dataloggers that make use of robust long-range radio
technologies. Reliability is usually ensured through retransmissions
at Layer 2. Gateways or concentrators act as bridges, forwarding the
data to the backbone Ethernet network. Deterministic requirements
are biased towards reliability rather than latency, as events are
triggered slowly or can be anticipated in advance.
At the mineral-processing stage, conveyor belts and refining
processes are controlled by a SCADA system that provides an
in-factory delay-constrained networking environment.
At the time of this writing, voice communications are served by a
redundant trunking infrastructure, independent from data networks.
8.3. Mining Industry in the Future
Mining operations and management are converging towards a combination
of autonomous operation and teleoperation of transport and extraction
machines. This means that video, audio, monitoring, and process-
control traffic will increase dramatically. Ideally, all activities
at the mine will rely on network infrastructure.
Wireless for open-pit mining is already a reality with LPWAN
technologies; it is expected to evolve to more-advanced LPWAN
technologies, such as those based on LTE, to increase last-hop
reliability or novel LPWAN flavors with deterministic access.
One area in which DetNet can improve this use case is in the wired
networks that make up the "backbone network" of the system. These
networks connect many wireless Access Points (APs) together. The
mobile machines (which are connected to the network via wireless)
transition from one AP to the next as they move about. A
deterministic, reliable, low-latency backbone can enable these
transitions to be more reliable.
Connections that extend all the way from the base stations to the
machinery via a mix of wired and wireless hops would also be
beneficial -- for example, to improve the responsiveness of digging
machines to remote control. However, to guarantee deterministic
performance of a DetNet, the end-to-end underlying network must be
deterministic. Thus, for this use case, if a deterministic wireless
transport is integrated with a wire-based DetNet network, it could
create the desired wired plus wireless end-to-end deterministic
network.
8.4. Mining Industry Requests to the IETF
o Improved bandwidth efficiency
o Very low delay, to enable machine teleoperation
o Dedicated bandwidth usage for high-resolution video streams
o Predictable delay, to enable real-time monitoring
o Potential for constructing a unified DetNet network over a
combination of wired and deterministic wireless links
9. Private Blockchain
9.1. Use Case Description
Blockchain was created with Bitcoin as a "public" blockchain on the
open Internet; however, blockchain has also spread far beyond its
original host into various industries, such as smart manufacturing,
logistics, security, legal rights, and others. In these industries,
blockchain runs in designated and carefully managed networks in which
deterministic networking requirements could be addressed by DetNet.
Such implementations are referred to as "private" blockchain.
The sole distinction between public and private blockchain is defined
by who is allowed to participate in the network, execute the
consensus protocol, and maintain the shared ledger.
Today's networks manage the traffic from blockchain on a best-effort
basis, but blockchain operation could be made much more efficient if
deterministic networking services were available to minimize latency
and packet loss in the network.
9.1.1. Blockchain Operation
A "block" runs as a container of a batch of primary items (e.g.,
transactions, property records). The blocks are chained in such a
way that the hash of the previous block works as the pointer to the
header of the new block. Confirmation of each block requires a
consensus mechanism. When an item arrives at a blockchain node, the
latter broadcasts this item to the rest of the nodes, which receive
it, verify it, and put it in the ongoing block. The block
confirmation process begins as the number of items reaches the
predefined block capacity, at which time the node broadcasts its
proved block to the rest of the nodes, to be verified and chained.
The result is that block N+1 of each chain transitively vouches for
blocks N and previous of that chain.
9.1.2. Blockchain Network Architecture
Blockchain node communication and coordination are achieved mainly
through frequent point-to-multipoint communication; however,
persistent point-to-point connections are used to transport both the
items and the blocks to the other nodes. For example, consider the
following implementation.
When a node is initiated, it first requests the other nodes'
addresses from a specific entity, such as DNS. The node then creates
persistent connections with each of the other nodes. If a node
confirms an item, it sends the item to the other nodes via these
persistent connections.
As a new block in a node is completed and is proven by the
surrounding nodes, it propagates towards its neighbor nodes. When
node A receives a block, it verifies it and then sends an invite
message to its neighbor B. Neighbor B checks to see if the
designated block is available and responds to A if it is unavailable;
A then sends the complete block to B. B repeats the process (as was
done by A) to start the next round of block propagation.
The challenge of blockchain network operation is not overall data
rates, since the volume from both the block and the item stays
between hundreds of bytes and a couple of megabytes per second;
rather, the challenge is in transporting the blocks with minimum
latency to maximize the efficiency of the blockchain consensus
process. The efficiency of differing implementations of the
consensus process may be affected to a differing degree by the
latency (and variation of latency) of the network.
9.1.3. Blockchain Security Considerations
Security is crucial to blockchain applications; at the time of this
writing, blockchain systems address security issues mainly at the
application level, where cryptography as well as hash-based consensus
play a leading role in preventing both double-spending and malicious
service attacks. However, there is concern that in the proposed use
case for a private blockchain network that is dependent on
deterministic properties the network could be vulnerable to delays
and other specific attacks against determinism, as these delays and
attacks could interrupt service.
9.2. Private Blockchain Today
Today, private blockchain runs in Layer 2 or Layer 3 VPNs, generally
without guaranteed determinism. The industry players are starting to
realize that improving determinism in their blockchain networks could
improve the performance of their service, but at present these goals
are not being met.
9.3. Private Blockchain in the Future
Blockchain system performance can be greatly improved through
deterministic networking services, primarily because low latency
would accelerate the consensus process. It would be valuable to be
able to design a private blockchain network with the following
properties:
o Transport of point-to-multipoint traffic in a coordinated network
architecture rather than at the application layer (which typically
uses point-to-point connections)
o Guaranteed transport latency
o Reduced packet loss (to the point where delay incurred by packet
retransmissions would be negligible)
9.4. Private Blockchain Requests to the IETF
o Layer 2 and Layer 3 multicast of blockchain traffic
o Item and block delivery with bounded, low latency and negligible
packet loss
o Coexistence of blockchain and IT traffic in a single network
o Ability to scale the network by distributing the centralized
control of the network across multiple control entities
10. Network Slicing
10.1. Use Case Description
Network slicing divides one physical network infrastructure into
multiple logical networks. Each slice, which corresponds to a
logical network, uses resources and network functions independently
from each other. Network slicing provides flexibility of resource
allocation and service quality customization.
Future services will demand network performance with a wide variety
of characteristics such as high data rate, low latency, low loss
rate, security, and many other parameters. Ideally, every service
would have its own physical network satisfying its particular
performance requirements; however, that would be prohibitively
expensive. Network slicing can provide a customized slice for a
single service, and multiple slices can share the same physical
network. This method can optimize performance for the service at
lower cost, and the flexibility of setting up and releasing the
slices also allows the user to allocate network resources
dynamically.
Unlike the other use cases presented here, network slicing is not a
specific application that depends on specific deterministic
properties; rather, it is introduced as an area of networking to
which DetNet might be applicable.
10.2. DetNet Applied to Network Slicing
10.2.1. Resource Isolation across Slices
One of the requirements discussed for network slicing is the "hard"
separation of various users' deterministic performance. That is, it
should be impossible for activity, lack of activity, or changes in
activity of one or more users to have any appreciable effect on the
deterministic performance parameters of any other slices. Typical
techniques used today, which share a physical network among users, do
not offer this level of isolation. DetNet can supply point-to-point
or point-to-multipoint paths that offer a user bandwidth and latency
guarantees that cannot be affected by other users' data traffic.
Thus, DetNet is a powerful tool when reliability and low latency are
required in network slicing.
10.2.2. Deterministic Services within Slices
Slices may need to provide services with DetNet-type performance
guarantees; note, however, that a system can be implemented to
provide such services in more than one way. For example, the slice
itself might be implemented using DetNet, and thus the slice can
provide service guarantees and isolation to its users without any
particular DetNet awareness on the part of the users' applications.
Alternatively, a "non-DetNet-aware" slice may host an application
that itself implements DetNet services and thus can enjoy similar
service guarantees.
10.3. A Network Slicing Use Case Example - 5G Bearer Network
Network slicing is a core feature of 5G as defined in 3GPP. The
system architecture for 5G is under development at the time of this
writing [TS23501]. A network slice in a mobile network is a complete
logical network, including RANs and Core Networks (CNs). It provides
telecommunications services and network capabilities, which may vary
from slice to slice. A 5G bearer network is a typical use case for
network slicing; for example, consider three 5G service scenarios:
eMBB, URLLC, and mMTC.
o eMBB (Enhanced Mobile Broadband) focuses on services characterized
by high data rates, such as high-definition video, Virtual Reality
(VR), augmented reality, and fixed mobile convergence.
o URLLC (Ultra-Reliable and Low Latency Communications) focuses on
latency-sensitive services, such as self-driving vehicles, remote
surgery, or drone control.
o mMTC (massive Machine Type Communications) focuses on services
that have high connection-density requirements, such as those
typically used in smart-city and smart-agriculture scenarios.
A 5G bearer network could use DetNet to provide hard resource
isolation across slices and within a given slice. For example,
consider Slice-A and Slice-B, with DetNet used to transit services
URLLC-A and URLLC-B over them. Without DetNet, URLLC-A and URLLC-B
would compete for bandwidth resources, and latency and reliability
requirements would not be guaranteed. With DetNet, URLLC-A and
URLLC-B have separate bandwidth reservations; there is no resource
conflict between them, as though they were in different physical
networks.
10.4. Non-5G Applications of Network Slicing
Although the operation of services not related to 5G is not part of
the 5G network slicing definition and scope, network slicing is
likely to become a preferred approach for providing various services
across a shared physical infrastructure. Examples include providing
services for electrical utilities and pro audio via slices. Use
cases like these could become more common once the work for the 5G CN
evolves to include wired as well as wireless access.
10.5. Limitations of DetNet in Network Slicing
DetNet cannot cover every network slicing use case. One issue is
that DetNet is a point-to-point or point-to-multipoint technology;
however, network slicing ultimately needs multipoint-to-multipoint
guarantees. Another issue is that the number of flows that can be
carried by DetNet is limited by DetNet scalability; flow aggregation
and queuing management modification may help address this issue.
Additional work and discussion are needed to address these topics.
10.6. Network Slicing Today and in the Future
Network slicing has promise in terms of satisfying many requirements
of future network deployment scenarios, but it is still a collection
of ideas and analyses without a specific technical solution. DetNet
is one of various technologies that could potentially be used in
network slicing, along with, for example, Flex-E and segment routing.
For more information, please see the IETF 99 Network Slicing BoF
session agenda and materials as provided in [IETF99-netslicing-BoF].
10.7. Network Slicing Requests to the IETF
o Isolation from other flows through queuing management
o Service quality customization and guarantees
o Security
11. Use Case Common Themes
This section summarizes the expected properties of a DetNet network,
based on the use cases as described in this document.
11.1. Unified, Standards-Based Networks
11.1.1. Extensions to Ethernet
A DetNet network is not "a new kind of network" -- it is based on
extensions to existing Ethernet standards, including elements of
IEEE 802.1 TSN and related standards. Presumably, it will be
possible to run DetNet over other underlying transports besides
Ethernet, but Ethernet is explicitly supported.
11.1.2. Centrally Administered Networks
In general, a DetNet network is not expected to be "plug and play";
rather, some type of centralized network configuration and control
system is expected. Such a system may be in a single central
location, or it may be distributed across multiple control entities
that function together as a unified control system for the network.
However, the ability to "hot swap" components (e.g., due to
malfunction) is similar enough to "plug and play" that this kind of
behavior may be expected in DetNet networks, depending on the
implementation.
11.1.3. Standardized Data-Flow Information Models
Data-flow information models to be used with DetNet networks are to
be specified by DetNet.
11.1.4. Layer 2 and Layer 3 Integration
A DetNet network is intended to integrate between Layer 2 (bridged)
network(s) (e.g., an AVB/TSN LAN) and Layer 3 (routed) network(s)
(e.g., using IP-based protocols). One example of this is making
AVB/TSN-type deterministic performance available from Layer 3
applications, e.g., using RTP. Another example is connecting two
AVB/TSN LANs ("islands") together through a standard router.
11.1.5. IPv4 Considerations
This document explicitly does not specify any particular
implementation or protocol; however, it has been observed that
various use cases (and their associated industries) described herein
are explicitly based on IPv4 (as opposed to IPv6), and it is not
considered practical to expect such implementations to migrate to
IPv6 in order to use DetNet. Thus, the expectation is that even if
not every feature of DetNet is available in an IPv4 context, at least
some of the significant benefits (such as guaranteed end-to-end
delivery and low latency) will be available.
11.1.6. Guaranteed End-to-End Delivery
Packets in a DetNet flow are guaranteed not to be dropped by the
network due to congestion. However, the network may drop packets for
intended reasons, e.g., per security measures. Similarly,
best-effort traffic on a DetNet is subject to being dropped (as on a
non-DetNet IP network). Also note that this guarantee applies to
actions taken by DetNet protocol software and does not provide any
guarantee against lower-level errors such as media errors or checksum
errors.
11.1.7. Replacement for Multiple Proprietary Deterministic Networks
There are many proprietary non-interoperable deterministic Ethernet-
based networks available; DetNet is intended to provide an
open-standards-based alternative to such networks.
11.1.8. Mix of Deterministic and Best-Effort Traffic
DetNet is intended to support the coexistence of time-sensitive
operational (OT) traffic and informational (IT) traffic on the same
("unified") network.
11.1.9. Unused Reserved Bandwidth to Be Available to Best-Effort
Traffic
If bandwidth reservations are made for a stream but the associated
bandwidth is not used at any point in time, that bandwidth is made
available on the network for best-effort traffic. If the owner of
the reserved stream then starts transmitting again, the bandwidth is
no longer available for best-effort traffic; this occurs on a
moment-to-moment basis. Note that such "temporarily available"
bandwidth is not available for time-sensitive traffic, which must
have its own reservation.
11.1.10. Lower-Cost, Multi-Vendor Solutions
The DetNet network specifications are intended to enable an ecosystem
in which multiple vendors can create interoperable products, thus
promoting device diversity and potentially higher numbers of each
device manufactured, promoting cost reduction and cost competition
among vendors. In other words, vendors should be able to create
DetNet networks at lower cost and with greater diversity of available
devices than existing proprietary networks.
11.2. Scalable Size
DetNet networks range in size from very small (e.g., inside a single
industrial machine) to very large (e.g., a utility-grid network
spanning a whole country and involving many "hops" over various kinds
of links -- for example, radio repeaters, microwave links, or fiber
optic links). However, recall that the scope of DetNet is confined
to networks that are centrally administered and thereby explicitly
excludes unbounded decentralized networks such as the Internet.
11.2.1. Scalable Number of Flows
The number of flows in a given network application can potentially be
large and can potentially grow faster than the number of nodes and
hops, so the network should provide a sufficient (perhaps
configurable) maximum number of flows for any given application.
11.3. Scalable Timing Parameters and Accuracy
11.3.1. Bounded Latency
DetNet data-flow information models are expected to provide means to
configure the network that include parameters for querying network
path latency, requesting bounded latency for a given stream,
requesting worst-case maximum and/or minimum latency for a given path
or stream, and so on. It is expected that the network may not be
able to provide a given requested service level; if this is indeed
the case, the network control system should reply that the requested
services are not available (as opposed to accepting the parameter but
then not delivering the desired behavior).
11.3.2. Low Latency
Various applications may state that they require "extremely low
latency"; however, depending on the application, "extremely low" may
imply very different latency bounds. For example, "low latency"
across a utility-grid network is a different order of magnitude of
latency values compared to "low latency" in a motor control loop in a
small machine. It is intended that the mechanisms for specifying
desired latency include wide ranges and that architecturally there is
nothing to prevent arbitrarily low latencies from being implemented
in a given network.
11.3.3. Bounded Jitter (Latency Variation)
As with the other latency-related elements noted above, parameters
that can determine or request permitted variations in latency should
be available.
11.3.4. Symmetrical Path Delays
Some applications would like to specify that the transit delay time
values be equal for both the transmit path and the return path.
11.4. High Reliability and Availability
Reliability is of critical importance to many DetNet applications,
because the consequences of failure can be extraordinarily high in
terms of cost and even human life. DetNet-based systems are expected
to be implemented with essentially arbitrarily high availability --
for example, 99.9999% uptime (where 99.9999 means "six nines") or
even 12 nines. DetNet designs should not make any assumptions about
the level of reliability and availability that may be required of a
given system and should define parameters for communicating these
kinds of metrics within the network.
A strategy used by DetNet for providing such extraordinarily high
levels of reliability is to provide redundant paths so that a system
can seamlessly switch between the paths while maintaining its
required level of performance.
11.5. Security
Security is of critical importance to many DetNet applications. A
DetNet network must have the ability to be made secure against device
failures, attackers, misbehaving devices, and so on. In a DetNet
network, the data traffic is expected to be time sensitive; thus, in
addition to arriving with the data content as intended, the data must
also arrive at the expected time. This may present "new" security
challenges to implementers and must be addressed accordingly. There
are other security implications, including (but not limited to) the
change in attack surface presented by PRE.
11.6. Deterministic Flows
Reserved-bandwidth data flows must be isolated from each other and
from best-effort traffic, so that even if the network is saturated
with best-effort (and/or reserved-bandwidth) traffic, the configured
flows are not adversely affected.
12. Security Considerations
This document covers a number of representative applications and
network scenarios that are expected to make use of DetNet
technologies. Each of the potential DetNet use cases will have
security considerations from both the use-specific perspective and
the DetNet technology perspective. While some use-specific security
considerations are discussed above, a more comprehensive discussion
of such considerations is captured in [DetNet-Security]
("Deterministic Networking (DetNet) Security Considerations").
Readers are encouraged to review [DetNet-Security] to gain a more
complete understanding of DetNet-related security considerations.
13. IANA Considerations
This document has no IANA actions.
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[PROFINET] "PROFINET Technology",
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[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
DOI 10.17487/RFC3411, December 2002,
<https://www.rfc-editor.org/info/rfc3411>.
[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985,
DOI 10.17487/RFC3985, March 2005,
<https://www.rfc-editor.org/info/rfc3985>.
[RFC4553] Vainshtein, A., Ed. and YJ. Stein, Ed., "Structure-
Agnostic Time Division Multiplexing (TDM) over Packet
(SAToP)", RFC 4553, DOI 10.17487/RFC4553, June 2006,
<https://www.rfc-editor.org/info/rfc4553>.
[RFC5086] Vainshtein, A., Ed., Sasson, I., Metz, E., Frost, T., and
P. Pate, "Structure-Aware Time Division Multiplexed (TDM)
Circuit Emulation Service over Packet Switched Network
(CESoPSN)", RFC 5086, DOI 10.17487/RFC5086, December 2007,
<https://www.rfc-editor.org/info/rfc5086>.
[RFC5087] Stein, Y(J)., Shashoua, R., Insler, R., and M. Anavi,
"Time Division Multiplexing over IP (TDMoIP)", RFC 5087,
DOI 10.17487/RFC5087, December 2007,
<https://www.rfc-editor.org/info/rfc5087>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
<https://www.rfc-editor.org/info/rfc6551>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.
[RFC8169] Mirsky, G., Ruffini, S., Gray, E., Drake, J., Bryant, S.,
and A. Vainshtein, "Residence Time Measurement in MPLS
Networks", RFC 8169, DOI 10.17487/RFC8169, May 2017,
<https://www.rfc-editor.org/info/rfc8169>.
[Spe09] Barbosa, R., Sadre, R., and A. Pras, "A First Look into
SCADA Network Traffic", IP Network Operations and
Management Symposium, DOI 10.1109/NOMS.2012.6211945,
June 2012, <https://ieeexplore.ieee.org/document/6211945>.
[SR-IP-RAN-Use-Case]
Khasnabish, B., Hu, F., and L. Contreras, "Segment
Routing in IP RAN use case", Work in Progress,
draft-kh-spring-ip-ran-use-case-02, November 2014.
[SRP_LATENCY]
Gunther, C., "Specifying SRP Acceptable Latency",
March 2014, <http://www.ieee802.org/1/files/public/
docs2014/cc-cgunther-acceptable-latency-0314-v01.pdf>.
[Sublayer-6TiSCH-6top]
Wang, Q., Ed. and X. Vilajosana, "6TiSCH Operation
Sublayer (6top)", Work in Progress,
draft-wang-6tisch-6top-sublayer-04, November 2015.
[syncE] International Telecommunication Union, "Timing and
synchronization aspects in packet networks", ITU-T
Recommendation G.8261, August 2013,
<https://www.itu.int/rec/T-REC-G.8261>.
[Timing-over-MPLS]
Davari, S., Oren, A., Bhatia, M., Roberts, P., and L.
Montini, "Transporting Timing messages over MPLS
Networks", Work in Progress,
draft-ietf-tictoc-1588overmpls-07, October 2015.
[TR38801] 3GPP, "Study on new radio access technology: Radio access
architecture and interfaces (Release 14)", 3GPP TR 38.801,
April 2017,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=3056>.
[TS23401] 3GPP, "General Packet Radio Service (GPRS) enhancements
for Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access (Release 16)", 3GPP TS 23.401,
March 2019, <https://portal.3gpp.org/
desktopmodules/ Specifications/
SpecificationDetails.aspx?specificationId=849>.
[TS23501] 3GPP, "System architecture for the 5G System (5GS)
(Release 15)", 3GPP TS 23.501, March 2019,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=3144>.
[TS25104] 3GPP, "Base Station (BS) radio transmission and reception
(FDD) (Release 16)", 3GPP TS 25.104, January 2019,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=1154>.
[TS36104] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Base Station (BS) radio transmission and
reception (Release 16)", 3GPP TS 36.104, January 2019,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=2412>.
[TS36133] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Requirements for support of radio resource
management (Release 16)", 3GPP TS 36.133, January 2019,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=2420>.
[TS36211] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Physical channels and modulation (Release 15)",
3GPP TS 36.211, January 2019,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=2425>.
[TS36300] 3GPP, "Evolved Universal Terrestrial Radio Access (E-UTRA)
and Evolved Universal Terrestrial Radio Access Network
(E-UTRAN); Overall description; Stage 2 (Release 15)",
3GPP TS 36.300, January 2019,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=2430>.
[WirelessHART]
International Electrotechnical Commission, "Industrial
networks - Wireless communication network and
communication profiles - WirelessHART(TM)",
IEC 62591:2016, March 2016.
Appendix A. Use Cases Explicitly Out of Scope for DetNet
This appendix contains text regarding use cases that have been
determined to be outside the scope of the present DetNet work.
A.1. DetNet Scope Limitations
The scope of DetNet is deliberately limited to specific use cases
that are consistent with the WG charter, subject to the
interpretation of the WG. At the time that the DetNet use cases were
solicited and provided by the authors, the scope of DetNet was not
clearly defined. As the scope has been clarified, certain use cases
have been determined to be outside the scope of the present DetNet
work. Text regarding these use cases was moved to this appendix to
clarify that they will not be supported by the DetNet work.
The text was moved to this appendix based on the following
"exclusion" principles. Please note that as an alternative to moving
all such text to this appendix some text has been modified in situ to
reflect these same principles.
The following principles have been established to clarify the scope
of the present DetNet work.
o The scope of networks addressed by DetNet is limited to networks
that can be centrally controlled, i.e., an "enterprise" (aka
"corporate") network. This explicitly excludes "the open
Internet".
o Maintaining time synchronization across a DetNet network is
crucial to its operation; however, DetNet assumes that time is to
be maintained using other means. One example would be PTP
[IEEE-1588]. A use case may state the accuracy and reliability
that it expects from the DetNet network as part of a whole system;
however, it is understood that such timing properties are not
guaranteed by DetNet itself. At the time of this writing, two
open questions remain: (1) whether DetNet protocols will include a
way for an application to communicate expectations regarding such
timing properties to the network and (2) if so, whether those
properties would likely have a material effect on network
performance as a result.
A.2. Internet-Based Applications
There are many applications that communicate over the open Internet
that could benefit from guaranteed delivery and bounded latency.
However, as noted above, all such applications, when run over the
open Internet, are out of scope for DetNet. These same applications
may be in scope when run in constrained environments, i.e., within a
centrally controlled DetNet network. The following are some examples
of such applications.
A.2.1. Use Case Description
A.2.1.1. Media Content Delivery
Media content delivery continues to be an important use of the
Internet, yet users often experience poor-quality audio and video due
to the delay and jitter inherent in today's Internet.
A.2.1.2. Online Gaming
Online gaming is a significant part of the gaming market; however,
latency can degrade the end user's experience. For example, "First
Person Shooter" (FPS) games are highly delay sensitive.
A.2.1.3. Virtual Reality
VR has many commercial applications, including real estate
presentations, remote medical procedures, and so on. Low latency is
critical to interacting with the virtual world, because perceptual
delays can cause motion sickness.
A.2.2. Internet-Based Applications Today
Internet service today is by definition "best effort", with no
guarantees regarding delivery or bandwidth.
A.2.3. Internet-Based Applications in the Future
One should be able to play Internet videos without glitches and play
Internet games without lag.
For online gaming, the desired maximum allowance for round-trip delay
is typically 100 ms. However, it may be less for specific types of
games; for example, for FPS games, the maximum delay should be 50 ms.
Transport delay is the dominant part, with a budget of 5-20 ms.
For VR, a maximum delay of 1-10 ms is needed; if doing remote VR, the
total network delay budget is 1-5 ms.
Flow identification can be used for gaming and VR, i.e., it can
recognize a critical flow and provide appropriate latency bounds.
A.2.4. Internet-Based Applications Requests to the IETF
o Unified control and management protocols that handle time-critical
data flows
o An application-aware flow-filtering mechanism that recognizes
time-critical flows without doing 5-tuple matching
o A unified control plane that provides low-latency service on
Layer 3 without changing the data plane
o An OAM system and protocols that can help provide service
provisioning that is sensitive to end-to-end delays
A.3. Pro Audio and Video - Digital Rights Management (DRM)
The following text was moved to this appendix because this
information is considered a link-layer topic for which DetNet is not
directly responsible.
Digital Rights Management (DRM) is very important to the audio and
video industries. Whenever protected content is introduced into a
network, there are DRM concerns that must be taken into account (see
[Content_Protection]). Many aspects of DRM are outside the scope of
network technology; however, there are cases when a secure link
supporting authentication and encryption is required by content
owners to carry their audio or video content when it is outside their
own secure environment (for example, see [DCI]).
As an example, two such techniques are Digital Transmission Content
Protection (DTCP) and High-bandwidth Digital Content Protection
(HDCP). HDCP content is not approved for retransmission within any
other type of DRM, while DTCP content may be retransmitted under
HDCP. Therefore, if the source of a stream is outside of the network
and it uses HDCP, it is only allowed to be placed on the network with
that same type of protection (i.e., HDCP).
A.4. Pro Audio and Video - Link Aggregation
Note: The term "link aggregation" is used here as defined by the text
in the following paragraph, i.e., not following a more common
network-industry definition.
For transmitting streams that require more bandwidth than a single
link in the target network can support, link aggregation is a
technique for combining (aggregating) the bandwidth available on
multiple physical links to create a single logical link that provides
the required bandwidth. However, if aggregation is to be used, the
network controller (or equivalent) must be able to determine the
maximum latency of any path through the aggregate link.
A.5. Pro Audio and Video - Deterministic Time to Establish Streaming
The DetNet WG decided that guidelines for establishing a
deterministic time to establish stream startup are not within the
scope of DetNet. If the bounded timing for establishing or
re-establishing streams is required in a given use case, it is up to
the application/system to achieve it.
Acknowledgments
Pro audio (Section 2)
As also acknowledged in [DetNet-Audio-Reqs], the editor would like
to acknowledge the help of the following individuals and the
companies they represent.
Jeff Koftinoff, Meyer Sound
Jouni Korhonen, Associate Technical Director, Broadcom
Pascal Thubert, CTAO, Cisco
Kieran Tyrrell, Sienda New Media Technologies GmbH
Utility telecom (Section 3)
Information regarding utility telecom was derived from
[DetNet-Util-Reqs]. As in that document, the following
individuals are acknowledged here.
Faramarz Maghsoodlou, Ph.D., IoT Connected Industries
and Energy Practice, Cisco
Pascal Thubert, CTAO, Cisco
The wind power generation use case has been extracted from the
study of wind parks conducted within the 5GPPP VirtuWind Project.
The project is funded by the European Union's Horizon 2020
research and innovation programme under grant agreement No. 671648
(VirtuWind).
Building automation systems (Section 4)
Please see [BAS-DetNet].
Wireless for industrial applications (Section 5)
See [DetNet-6TiSCH].
[DetNet-6TiSCH] derives from the 6TiSCH architecture, which is the
result of multiple interactions -- in particular, during the
6TiSCH (bi)weekly interim call, relayed through the 6TiSCH mailing
list at the IETF [MailingList-6TiSCH].
As also acknowledged in [DetNet-6TiSCH], the editor wishes to
thank Kris Pister, Thomas Watteyne, Xavier Vilajosana, Qin Wang,
Tom Phinney, Robert Assimiti, Michael Richardson, Zhuo Chen,
Malisa Vucinic, Alfredo Grieco, Martin Turon, Dominique Barthel,
Elvis Vogli, Guillaume Gaillard, Herman Storey, Maria Rita
Palattella, Nicola Accettura, Patrick Wetterwald, Pouria Zand,
Raghuram Sudhaakar, and Shitanshu Shah for their participation and
various contributions.
Cellular radio (Section 6)
See [DetNet-RAN].
Internet applications and CoMP (Section 6)
As also acknowledged in [DetNet-Mobile], authored by Yiyong Zha,
the editor would like to thank the following people for their
reviews, suggestions, comments, and proposed text: Jing Huang,
Junru Lin, Lehong Niu, and Oliver Huang.
Industrial Machine to Machine (M2M) (Section 7)
The editor would like to thank Feng Chen and Marcel Kiessling for
their comments and suggestions.
Mining industry (Section 8)
This text was written by Diego Dujovne, who worked in conjunction
with Xavier Vilajosana.
Private blockchain (Section 9)
This text was written by Daniel Huang.
Network slicing (Section 10)
This text was written by Xuesong Geng, who would like to
acknowledge Norm Finn and Mach Chen for their useful comments.
Contributors
RFC 7322 ("RFC Style Guide") generally limits the number of authors
listed on the front page of a document to five individuals -- far
fewer than the 19 individuals listed below, who also made important
contributions to this document. The editor wishes to thank and
acknowledge each of the following authors for contributing text to
this document. See also the Acknowledgments section.
Craig Gunther (Harman International)
10653 South River Front Parkway
South Jordan, UT 84095
United States of America
Phone: +1 801 568 7675
Email: craig.gunther@harman.com
Pascal Thubert (Cisco Systems, Inc.)
Building D, 45 Allee des Ormes - BP1200
Mougins - Sophia Antipolis 06254
France
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
Patrick Wetterwald (Cisco Systems)
45 Allee des Ormes
Mougins 06250
France
Phone: +33 4 97 23 26 36
Email: pwetterw@cisco.com
Jean Raymond (Hydro-Quebec)
1500 University
Montreal, Quebec H3A 3S7
Canada
Phone: +1 514 840 3000
Email: raymond.jean@hydro.qc.ca
Jouni Korhonen (Broadcom Corporation)
3151 Zanker Road
San Jose, CA 95134
United States of America
Email: jouni.nospam@gmail.com
Yu Kaneko (Toshiba)
1 Komukai-Toshiba-cho
Saiwai-ku, Kasasaki-shi, Kanagawa
Japan
Email: yu1.kaneko@toshiba.co.jp
Subir Das (Vencore Labs)
150 Mount Airy Road
Basking Ridge, NJ 07920
United States of America
Email: sdas@appcomsci.com
Balazs Varga (Ericsson)
Konyves Kalman krt. 11/B
Budapest 1097
Hungary
Email: balazs.a.varga@ericsson.com
Janos Farkas (Ericsson)
Konyves Kalman krt. 11/B
Budapest 1097
Hungary
Email: janos.farkas@ericsson.com
Franz-Josef Goetz (Siemens)
Gleiwitzerstr. 555
Nurnberg 90475
Germany
Email: franz-josef.goetz@siemens.com
Juergen Schmitt (Siemens)
Gleiwitzerstr. 555
Nurnberg 90475
Germany
Email: juergen.jues.schmitt@siemens.com
Xavier Vilajosana (Worldsensing)
483 Arago
Barcelona, Catalonia 08013
Spain
Email: xvilajosana@worldsensing.com
Toktam Mahmoodi (King's College London)
Strand, London WC2R 2LS
United Kingdom
Email: toktam.mahmoodi@kcl.ac.uk
Spiros Spirou (Intracom Telecom)
19.7 km Markopoulou Ave.
Peania, Attiki 19002
Greece
Email: spiros.spirou@gmail.com
Petra Vizarreta (Technical University of Munich)
Maxvorstadt, Arcisstrasse 21
Munich 80333
Germany
Email: petra.stojsavljevic@tum.de
Daniel Huang (ZTE Corporation, Inc.)
No. 50 Software Avenue
Nanjing, Jiangsu 210012
China
Email: huang.guangping@zte.com.cn
Xuesong Geng (Huawei Technologies)
Email: gengxuesong@huawei.com
Diego Dujovne (Universidad Diego Portales)
Email: diego.dujovne@mail.udp.cl
Maik Seewald (Cisco Systems)
Email: maseewal@cisco.com
Author's Address
Ethan Grossman (editor)
Dolby Laboratories, Inc.
1275 Market Street
San Francisco, CA 94103
United States of America
Phone: +1 415 645 4726
Email: ethan.grossman@dolby.com
URI: http://www.dolby.com