DetNet Bounded Latency
draft-ietf-detnet-bounded-latency-05
DetNet N. Finn
Internet-Draft Huawei Technologies Co. Ltd
Intended status: Informational J-Y. Le Boudec
Expires: October 17, 2021 E. Mohammadpour
EPFL
J. Zhang
Huawei Technologies Co. Ltd
B. Varga
J. Farkas
Ericsson
April 15, 2021
DetNet Bounded Latency
draft-ietf-detnet-bounded-latency-05
Abstract
This document references specific queuing mechanisms, defined in
other documents, that can be used to control packet transmission at
each output port and achieve the DetNet qualities of service. This
document presents a timing model for sources, destinations, and the
DetNet transit nodes that relay packets that is applicable to all of
those referenced queuing mechanisms. Using the model presented in
this document, it should be possible for an implementor, user, or
standards development organization to select a particular set of
queuing mechanisms for each device in a DetNet network, and to select
a resource reservation algorithm for that network, so that those
elements can work together to provide the DetNet service.
Status of This Memo
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This Internet-Draft will expire on October 17, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 4
3. DetNet bounded latency model . . . . . . . . . . . . . . . . 4
3.1. Flow admission . . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Static latency calculation . . . . . . . . . . . . . 4
3.1.2. Dynamic latency calculation . . . . . . . . . . . . . 5
3.2. Relay node model . . . . . . . . . . . . . . . . . . . . 6
4. Computing End-to-end Delay Bounds . . . . . . . . . . . . . . 8
4.1. Non-queuing delay bound . . . . . . . . . . . . . . . . . 8
4.2. Queuing delay bound . . . . . . . . . . . . . . . . . . . 9
4.2.1. Per-flow queuing mechanisms . . . . . . . . . . . . . 9
4.2.2. Aggregate queuing mechanisms . . . . . . . . . . . . 9
4.3. Ingress considerations . . . . . . . . . . . . . . . . . 10
4.4. Interspersed DetNet-unaware transit nodes . . . . . . . . 11
5. Achieving zero congestion loss . . . . . . . . . . . . . . . 11
6. Queuing techniques . . . . . . . . . . . . . . . . . . . . . 12
6.1. Queuing data model . . . . . . . . . . . . . . . . . . . 13
6.2. Frame Preemption . . . . . . . . . . . . . . . . . . . . 15
6.3. Time Aware Shaper . . . . . . . . . . . . . . . . . . . . 15
6.4. Credit-Based Shaper with Asynchronous Traffic Shaping . . 16
6.4.1. Delay Bound Calculation . . . . . . . . . . . . . . . 18
6.4.2. Flow Admission . . . . . . . . . . . . . . . . . . . 19
6.5. Guaranteed-Service IntServ . . . . . . . . . . . . . . . 20
6.6. Cyclic Queuing and Forwarding . . . . . . . . . . . . . . 21
7. Example application on DetNet IP network . . . . . . . . . . 22
8. Security considerations . . . . . . . . . . . . . . . . . . . 24
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
The ability for IETF Deterministic Networking (DetNet) or IEEE 802.1
Time-Sensitive Networking (TSN, [IEEE8021TSN]) to provide the DetNet
services of bounded latency and zero congestion loss depends upon A)
configuring and allocating network resources for the exclusive use of
DetNet flows; B) identifying, in the data plane, the resources to be
utilized by any given packet, and C) the detailed behavior of those
resources, especially transmission queue selection, so that latency
bounds can be reliably assured.
As explained in [RFC8655], DetNet flows are characterized by 1) a
maximum bandwidth, guaranteed either by the transmitter or by strict
input metering; and 2) a requirement for a guaranteed worst-case end-
to-end latency. That latency guarantee, in turn, provides the
opportunity for the network to supply enough buffer space to
guarantee zero congestion loss.
To be used by the applications identified in [RFC8578], it must be
possible to calculate, before the transmission of a DetNet flow
commences, both the worst-case end-to-end network latency, and the
amount of buffer space required at each hop to ensure against
congestion loss.
This document references specific queuing mechanisms, defined in
[RFC8655], that can be used to control packet transmission at each
output port and achieve the DetNet qualities of service. This
document presents a timing model for sources, destinations, and the
DetNet transit nodes that relay packets that is applicable to all of
those referenced queuing mechanisms. It furthermore provides end-to-
end delay bound and backlog bound computations for such mechanisms
that can be used by the control plane to provide DetNet QoS.
Using the model presented in this document, it should be possible for
an implementor, user, or standards development organization to select
a particular set of queuing mechanisms for each device in a DetNet
network, and to select a resource reservation algorithm for that
network, so that those elements can work together to provide the
DetNet service. Section 7 provides an example application of this
document to a DetNet IP network with combination of different queuing
mechanisms.
This document does not specify any resource reservation protocol or
control plane function. It does not describe all of the requirements
for that protocol or control plane function. It does describe
requirements for such resource reservation methods, and for queuing
mechanisms that, if met, will enable them to work together.
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2. Terminology and Definitions
This document uses the terms defined in [RFC8655].
3. DetNet bounded latency model
3.1. Flow admission
This document assumes that following paradigm is used to admit DetNet
flows:
1. Perform any configuration required by the DetNet transit nodes in
the network for aggregates of DetNet flows. This configuration
is done beforehand, and not tied to any particular DetNet flow.
2. Characterize the new DetNet flow, particularly in terms of
required bandwidth.
3. Establish the path that the DetNet flow will take through the
network from the source to the destination(s). This can be a
point-to-point or a point-to-multipoint path.
4. Compute the worst-case end-to-end latency for the DetNet flow,
using one of the methods, below (Section 3.1.1, Section 3.1.2).
In the process, determine whether sufficient resources are
available for the DetNet flow to guarantee the required latency
and to provide zero congestion loss.
5. Assuming that the resources are available, commit those resources
to the DetNet flow. This may or may not require adjusting the
parameters that control the filtering and/or queuing mechanisms
at each hop along the DetNet flow's path.
This paradigm can be implemented using peer-to-peer protocols or
using a central controller. In some situations, a lack of resources
can require backtracking and recursing through this list.
Issues such as service preemption of a DetNet flow in favor of
another, when resources are scarce, are not considered, here. Also
not addressed is the question of how to choose the path to be taken
by a DetNet flow.
3.1.1. Static latency calculation
The static problem:
Given a network and a set of DetNet flows, compute an end-to-
end latency bound (if computable) for each DetNet flow, and
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compute the resources, particularly buffer space, required in
each DetNet transit node to achieve zero congestion loss.
In this calculation, all of the DetNet flows are known before the
calculation commences. This problem is of interest to relatively
static networks, or static parts of larger networks. It provides
bounds on delay and buffer size. The calculations can be extended to
provide global optimizations, such as altering the path of one DetNet
flow in order to make resources available to another DetNet flow with
tighter constraints.
The static latency calculation is not limited only to static
networks; the entire calculation for all DetNet flows can be repeated
each time a new DetNet flow is created or deleted. If some already-
established DetNet flow would be pushed beyond its latency
requirements by the new DetNet flow, then the new DetNet flow can be
refused, or some other suitable action taken.
This calculation may be more difficult to perform than that of the
dynamic calculation (Section 3.1.2), because the DetNet flows passing
through one port on a DetNet transit node affect each others'
latency. The effects can even be circular, from a node A to B to C
and back to A. On the other hand, the static calculation can often
accommodate queuing methods, such as transmission selection by strict
priority, that are unsuitable for the dynamic calculation.
3.1.2. Dynamic latency calculation
The dynamic problem:
Given a network whose maximum capacity for DetNet flows is
bounded by a set of static configuration parameters applied
to the DetNet transit nodes, and given just one DetNet flow,
compute the worst-case end-to-end latency that can be
experienced by that flow, no matter what other DetNet flows
(within the network's configured parameters) might be created
or deleted in the future. Also, compute the resources,
particularly buffer space, required in each DetNet transit
node to achieve zero congestion loss.
This calculation is dynamic, in the sense that DetNet flows can be
added or deleted at any time, with a minimum of computation effort,
and without affecting the guarantees already given to other DetNet
flows.
The choice of queuing methods is critical to the applicability of the
dynamic calculation. Some queuing methods (e.g. CQF, Section 6.6)
make it easy to configure bounds on the network's capacity, and to
make independent calculations for each DetNet flow. Some other
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queuing methods (e.g. strict priority with the credit-based shaper
defined in [IEEE8021Q] section 8.6.8.2) can be used for dynamic
DetNet flow creation, but yield poorer latency and buffer space
guarantees than when that same queuing method is used for static
DetNet flow creation (Section 3.1.1).
3.2. Relay node model
A model for the operation of a DetNet transit node is required, in
order to define the latency and buffer calculations. In Figure 1 we
see a breakdown of the per-hop latency experienced by a packet
passing through a DetNet transit node, in terms that are suitable for
computing both hop-by-hop latency and per-hop buffer requirements.
DetNet transit node A DetNet transit node B
+-------------------------+ +------------------------+
| Queuing | | Queuing |
| Regulator subsystem | | Regulator subsystem |
| +-+-+-+-+ +-+-+-+-+ | | +-+-+-+-+ +-+-+-+-+ |
-->+ | | | | | | | | | + +------>+ | | | | | | | | | + +--->
| +-+-+-+-+ +-+-+-+-+ | | +-+-+-+-+ +-+-+-+-+ |
| | | |
+-------------------------+ +------------------------+
|<->|<------>|<------->|<->|<---->|<->|<------>|<------>|<->|<--
2,3 4 5 6 1 2,3 4 5 6 1 2,3
1: Output delay 4: Processing delay
2: Link delay 5: Regulation delay
3: Frame preemption delay 6: Queuing delay
Figure 1: Timing model for DetNet or TSN
In Figure 1, we see two DetNet transit nodes that are connected via a
link. In this model, the only queues, that we deal with explicitly,
are attached to the output port; other queues are modeled as
variations in the other delay times. (E.g., an input queue could be
modeled as either a variation in the link delay (2) or the processing
delay (4).) There are six delays that a packet can experience from
hop to hop.
1. Output delay
The time taken from the selection of a packet for output from a
queue to the transmission of the first bit of the packet on the
physical link. If the queue is directly attached to the physical
port, output delay can be a constant. But, in many
implementations, the queuing mechanism in a forwarding ASIC is
separated from a multi-port MAC/PHY, in a second ASIC, by a
multiplexed connection. This causes variations in the output
delay that are hard for the forwarding node to predict or control.
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2. Link delay
The time taken from the transmission of the first bit of the
packet to the reception of the last bit, assuming that the
transmission is not suspended by a frame preemption event. This
delay has two components, the first-bit-out to first-bit-in delay
and the first-bit-in to last-bit-in delay that varies with packet
size. The former is typically measured by the Precision Time
Protocol and is constant (see [RFC8655]). However, a virtual
"link" could exhibit a variable link delay.
3. Frame preemption delay
If the packet is interrupted in order to transmit another packet
or packets, (e.g. [IEEE8023] clause 99 frame preemption) an
arbitrary delay can result.
4. Processing delay
This delay covers the time from the reception of the last bit of
the packet to the time the packet is enqueued in the regulator
(Queuing subsystem, if there is no regulation). This delay can be
variable, and depends on the details of the operation of the
forwarding node.
5. Regulator delay
This is the time spent from the insertion of the last bit of a
packet into a regulation queue until the time the packet is
declared eligible according to its regulation constraints. We
assume that this time can be calculated based on the details of
regulation policy. If there is no regulation, this time is zero.
6. Queuing subsystem delay
This is the time spent for a packet from being declared eligible
until being selected for output on the next link. We assume that
this time is calculable based on the details of the queuing
mechanism. If there is no regulation, this time is from the
insertion of the packet into a queue until it is selected for
output on the next link.
Not shown in Figure 1 are the other output queues that we presume are
also attached to that same output port as the queue shown, and
against which this shown queue competes for transmission
opportunities.
The initial and final measurement point in this analysis (that is,
the definition of a "hop") is the point at which a packet is selected
for output. In general, any queue selection method that is suitable
for use in a DetNet network includes a detailed specification as to
exactly when packets are selected for transmission. Any variations
in any of the delay times 1-4 result in a need for additional buffers
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in the queue. If all delays 1-4 are constant, then any variation in
the time at which packets are inserted into a queue depends entirely
on the timing of packet selection in the previous node. If the
delays 1-4 are not constant, then additional buffers are required in
the queue to absorb these variations. Thus:
o Variations in output delay (1) require buffers to absorb that
variation in the next hop, so the output delay variations of the
previous hop (on each input port) must be known in order to
calculate the buffer space required on this hop.
o Variations in processing delay (4) require additional output
buffers in the queues of that same DetNet transit node. Depending
on the details of the queueing subsystem delay (6) calculations,
these variations need not be visible outside the DetNet transit
node.
4. Computing End-to-end Delay Bounds
4.1. Non-queuing delay bound
End-to-end delay bounds can be computed using the delay model in
Section 3.2. Here, it is important to be aware that for several
queuing mechanisms, the end-to-end delay bound is less than the sum
of the per-hop delay bounds. An end-to-end delay bound for one
DetNet flow can be computed as
end_to_end_delay_bound = non_queuing_delay_bound +
queuing_delay_bound
The two terms in the above formula are computed as follows.
First, at the h-th hop along the path of this DetNet flow, obtain an
upperbound per-hop_non_queuing_delay_bound[h] on the sum of the
bounds over the delays 1,2,3,4 of Figure 1. These upper bounds are
expected to depend on the specific technology of the DetNet transit
node at the h-th hop but not on the T-SPEC of this DetNet flow. Then
set non_queuing_delay_bound = the sum of per-
hop_non_queuing_delay_bound[h] over all hops h.
Second, compute queuing_delay_bound as an upper bound to the sum of
the queuing delays along the path. The value of queuing_delay_bound
depends on the T-SPEC of this DetNet flow and possibly of other flows
in the network, as well as the specifics of the queuing mechanisms
deployed along the path of this DetNet flow. The computation of
queuing_delay_bound is described in Section 4.2 as a separate
section.
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4.2. Queuing delay bound
For several queuing mechanisms, queuing_delay_bound is less than the
sum of upper bounds on the queuing delays (5,6) at every hop. This
occurs with (1) per-flow queuing, and (2) aggregate queuing with
regulators, as explained in Section 4.2.1, Section 4.2.2, and
Section 6.
For other queuing mechanisms the only available value of
queuing_delay_bound is the sum of the per-hop queuing delay bounds.
In such cases, the computation of per-hop queuing delay bounds must
account for the fact that the T-SPEC of a DetNet flow is no longer
satisfied at the ingress of a hop, since burstiness increases as one
flow traverses one DetNet transit node.
4.2.1. Per-flow queuing mechanisms
With such mechanisms, each flow uses a separate queue inside every
node. The service for each queue is abstracted with a guaranteed
rate and a latency. For every DetNet flow, a per-node delay bound as
well as an end-to-end delay bound can be computed from the traffic
specification of this DetNet flow at its source and from the values
of rates and latencies at all nodes along its path. The per-flow
queuing is used in Guaranteed-Service IntServ. Details of
calculation for Guaranteed-Service IntServ are described in
Section 6.5.
4.2.2. Aggregate queuing mechanisms
With such mechanisms, multiple flows are aggregated into macro-flows
and there is one FIFO queue per macro-flow. A practical example is
the credit-based shaper defined in section 8.6.8.2 of [IEEE8021Q]
where a macro-flow is called a "class". One key issue in this
context is how to deal with the burstiness cascade: individual flows
that share a resource dedicated to a macro-flow may see their
burstiness increase, which may in turn cause increased burstiness to
other flows downstream of this resource. Computing delay upper
bounds for such cases is difficult, and in some conditions impossible
[charny2000delay][bennett2002delay]. Also, when bounds are obtained,
they depend on the complete configuration, and must be recomputed
when one flow is added. (The dynamic calculation, Section 3.1.2.)
A solution to deal with this issue for the DetNet flows is to reshape
them at every hop. This can be done with per-flow regulators (e.g.
leaky bucket shapers), but this requires per-flow queuing and defeats
the purpose of aggregate queuing. An alternative is the interleaved
regulator, which reshapes individual DetNet flows without per-flow
queuing ([Specht2016UBS], [IEEE8021Qcr]). With an interleaved
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regulator, the packet at the head of the queue is regulated based on
its (flow) regulation constraints; it is released at the earliest
time at which this is possible without violating the constraint. One
key feature of per-flow or interleaved regulator is that, it does not
increase worst-case latency bounds [le_boudec2018theory].
Specifically, when an interleaved regulator is appended to a FIFO
subsystem, it does not increase the worst-case delay of the latter.
Figure 2 shows an example of a network with 5 nodes, aggregate
queuing mechanism and interleaved regulators as in Figure 1. An end-
to-end delay bound for DetNet flow f, traversing nodes 1 to 5, is
calculated as follows:
end_to_end_latency_bound_of_flow_f = C12 + C23 + C34 + S4
In the above formula, Cij is a bound on the delay of the queuing
subsystem in node i and interleaved regulator of node j, and S4 is a
bound on the delay of the queuing subsystem in node 4 for DetNet flow
f. In fact, using the delay definitions in Section 3.2, Cij is a
bound on sum of the delays 1,2,3,6 of node i and 4,5 of node j.
Similarly, S4 is a bound on sum of the delays 1,2,3,6 of node 4. A
practical example of queuing model and delay calculation is presented
Section 6.4.
f
----------------------------->
+---+ +---+ +---+ +---+ +---+
| 1 |---| 2 |---| 3 |---| 4 |---| 5 |
+---+ +---+ +---+ +---+ +---+
\__C12_/\__C23_/\__C34_/\_S4_/
Figure 2: End-to-end delay computation example
REMARK: The end-to-end delay bound calculation provided here gives a
much better upper bound in comparison with end-to-end delay bound
computation by adding the delay bounds of each node in the path of a
DetNet flow [TSNwithATS].
4.3. Ingress considerations
A sender can be a DetNet node which uses exactly the same queuing
methods as its adjacent DetNet transit node, so that the delay and
buffer bounds calculations at the first hop are indistinguishable
from those at a later hop within the DetNet domain. On the other
hand, the sender may be DetNet-unaware, in which case some
conditioning of the DetNet flow may be necessary at the ingress
DetNet transit node.
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This ingress conditioning typically consists of a FIFO with an output
regulator that is compatible with the queuing employed by the DetNet
transit node on its output port(s). For some queuing methods, simply
requires added extra buffer space in the queuing subsystem. Ingress
conditioning requirements for different queuing methods are mentioned
in the sections, below, describing those queuing methods.
4.4. Interspersed DetNet-unaware transit nodes
It is sometimes desirable to build a network that has both DetNet-
aware transit nodes and DetNet-uaware transit nodes, and for a DetNet
flow to traverse an island of DetNet-unaware transit nodes, while
still allowing the network to offer delay and congestion loss
guarantees. This is possible under certain conditions.
In general, when passing through a DetNet-unaware island, the island
may cause delay variation in excess of what would be caused by DetNet
nodes. That is, the DetNet flow might be "lumpier" after traversing
the DetNet-unaware island. DetNet guarantees for delay and buffer
requirements can still be calculated and met if and only if the
following are true:
1. The latency variation across the DetNet-unaware island must be
bounded and calculable.
2. An ingress conditioning function (Section 4.3) is required at the
re-entry to the DetNet-aware domain. This will, at least,
require some extra buffering to accommodate the additional delay
variation, and thus further increases the delay bound.
The ingress conditioning is exactly the same problem as that of a
sender at the edge of the DetNet domain. The requirement for bounds
on the latency variation across the DetNet-unaware island is
typically the most difficult to achieve. Without such a bound, it is
obvious that DetNet cannot deliver its guarantees, so a DetNet-
unaware island that cannot offer bounded latency variation cannot be
used to carry a DetNet flow.
5. Achieving zero congestion loss
When the input rate to an output queue exceeds the output rate for a
sufficient length of time, the queue must overflow. This is
congestion loss, and this is what deterministic networking seeks to
avoid.
To avoid congestion losses, an upper bound on the backlog present in
the regulator and queuing subsystem of Figure 1 must be computed
during resource reservation. This bound depends on the set of flows
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that use these queues, the details of the specific queuing mechanism
and an upper bound on the processing delay (4). The queue must
contain the packet in transmission plus all other packets that are
waiting to be selected for output.
A conservative backlog bound, that applies to all systems, can be
derived as follows.
The backlog bound is counted in data units (bytes, or words of
multiple bytes) that are relevant for buffer allocation. Based on
the que For every flow or an aggregate of flows, we need one buffer
space for the packet in transmission, plus space for the packets that
are waiting to be selected for output. Excluding transmission and
frame preemption times, the packets are waiting in the queue since
reception of the last bit, for a duration equal to the processing
delay (4) plus the queuing delays (5,6).
Let
o total_in_rate be the sum of the line rates of all input ports that
send traffic to this output port. The value of total_in_rate is
in data units (e.g. bytes) per second.
o nb_input_ports be the number input ports that send traffic to this
output port
o max_packet_length be the maximum packet size for packets that may
be sent to this output port. This is counted in data units.
o max_delay456 be an upper bound, in seconds, on the sum of the
processing delay (4) and the queuing delays (5,6) for any packet
at this output port.
Then a bound on the backlog of traffic in the queue at this output
port is
backlog_bound = nb_input_ports * max_packet_length +
total_in_rate* max_delay456
6. Queuing techniques
In this section, for simplicity of delay computation, we assume that
the T-SPEC or arrival curve [NetCalBook] for each DetNet flow at
source is leaky bucket. Also, at each Detnet transit node, the
service for each queue is abstracted with a guaranteed rate and a
latency.
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6.1. Queuing data model
Sophisticated queuing mechanisms are available in Layer 3 (L3, see,
e.g., [RFC7806] for an overview). In general, we assume that "Layer
3" queues, shapers, meters, etc., are precisely the "regulators"
shown in Figure 1. The "queuing subsystems" in this figure are not
the province solely of bridges; they are an essential part of any
DetNet transit node. As illustrated by numerous implementation
examples, some of the "Layer 3" mechanisms described in documents
such as [RFC7806] are often integrated, in an implementation, with
the "Layer 2" mechanisms also implemented in the same node. An
integrated model is needed in order to successfully predict the
interactions among the different queuing mechanisms needed in a
network carrying both DetNet flows and non-DetNet flows.
Figure 3 shows the general model for the flow of packets through the
queues of a DetNet transit node. The DetNet packets are mapped to a
number of regulators. Here, we assume that the PREOF (Packet
Replication, Elimination and Ordering Functions) functions are
performed before the DetNet packets enter the regulators. All
Packets are assigned to a set of queues. Queues compete for the
selection of packets to be passed to queues in the queuing subsystem.
Packets again are selected for output from the queuing subsystem.
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|
+--------------------------------V----------------------------------+
| Queue assignment |
+--+------+----------+---------+-----------+-----+-------+-------+--+
| | | | | | | |
+--V-+ +--V-+ +--V--+ +--V--+ +--V--+ | | |
|Flow| |Flow| |Flow | |Flow | |Flow | | | |
| 0 | | 1 | ... | i | | i+1 | ... | n | | | |
| reg| | reg| | reg | | reg | | reg | | | |
+--+-+ +--+-+ +--+--+ +--+--+ +--+--+ | | |
| | | | | | | |
+--V------V----------V--+ +--V-----------V--+ | | |
| Trans. selection | | Trans. select. | | | |
+----------+------------+ +-----+-----------+ | | |
| | | | |
+--V--+ +--V--+ +--V--+ +--V--+ +--V--+
| out | | out | | out | | out | | out |
|queue| |queue| |queue| |queue| |queue|
| 1 | | 2 | | 3 | | 4 | | 5 |
+--+--+ +--+--+ +--+--+ +--+--+ +--+--+
| | | | |
+----------V----------------------V--------------V-------V-------V--+
| Transmission selection |
+---------------------------------+---------------------------------+
|
V
Figure 3: IEEE 802.1Q Queuing Model: Data flow
Some relevant mechanisms are hidden in this figure, and are performed
in the queue boxes:
o Discarding packets because a queue is full.
o Discarding packets marked "yellow" by a metering function, in
preference to discarding "green" packets.
Ideally, neither of these actions are performed on DetNet packets.
Full queues for DetNet packets should occur only when a DetNet flow
is misbehaving, and the DetNet QoS does not include "yellow" service
for packets in excess of committed rate.
The queue assignment function can be quite complex, even in a bridge
[IEEE8021Q], since the introduction of per-stream filtering and
policing ([IEEE8021Q] clause 8.6.5.1). In addition to the Layer 2
priority expressed in the 802.1Q VLAN tag, a DetNet transit node can
utilize any of the following information to assign a packet to a
particular queue:
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o Input port.
o Selector based on a rotating schedule that starts at regular,
time-synchronized intervals and has nanosecond precision.
o MAC addresses, VLAN ID, IP addresses, Layer 4 port numbers, DSCP.
([RFC8939], [RFC8964]) (Work items are expected to add MPC and
other indicators.)
o The queue assignment function can contain metering and policing
functions.
o MPLS and/or pseudowire ([RFC6658]) labels.
The "Transmission selection" function decides which queue is to
transfer its oldest packet to the output port when a transmission
opportunity arises.
6.2. Frame Preemption
In [IEEE8021Q] and [IEEE8023], the transmission of a frame can be
interrupted by one or more "express" frames, and then the interrupted
frame can continue transmission. The frame preemption is modeled as
consisting of two MAC/PHY stacks, one for packets that can be
interrupted, and one for packets that can interrupt the interruptible
packets. Only one layer of frame preemption is supported -- a
transmitter cannot have more than one interrupted frame in progress.
DetNet flows typically pass through the interrupting MAC. For those
DetNet flows with T-SPEC, latency bound can be calculated by the
methods provided in the following sections that accounts for the
affect of frame preemption, according to the specific queuing
mechanism that is used in DetNet nodes. Best-effort queues pass
through the interruptible MAC, and can thus be preempted.
6.3. Time Aware Shaper
In [IEEE8021Q], the notion of time-scheduling queue gates is
described in section 8.6.8.4. On each node, the transmission
selection for packets is controlled by time-synchronized gates; each
output queue is associated with a gate. The gates can be either open
or close. The states of the gates are determined by the gate control
list (GCL). The GCL specifies the opening and closing times of the
gates. The design of GCL should satisfy the requirement of latency
upper bounds of all DetNet flows; therefore, those DetNet flows
traverse a network should have bounded latency, if the traffic and
nodes are conformant.
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It should be noted that scheduled traffic service relies on a
synchronized network and coordinated GCL configuration. Synthesis of
GCL on multiple nodes in network is a scheduling problem considering
all DetNet flows traversing the network, which is a non-deterministic
polynomial-time hard (NP-hard) problem. Also, at this writing,
scheduled traffic service supports no more than eight traffic queues,
typically using up to seven priority queues and at least one best
effort.
6.4. Credit-Based Shaper with Asynchronous Traffic Shaping
In the considered queuing model, we considered the four traffic
classes (Definition 3.268 of [IEEE8021Q]): control-data traffic
(CDT), class A, class B, and best effort (BE) in decreasing order of
priority. Flows of classes A and B are together referred as AVB
flows. This model is a subset of Time-Sensitive Networking as
described next.
Based on the timing model described in Figure 1, the contention
occurs only at the output port of a DetNet transit node; therefore,
the focus of the rest of this subsection is on the regulator and
queuing subsystem in the output port of a DetNet transit node. Then,
the input flows are identified using the information in (Section 5.1
of [RFC8939]). Then they are aggregated into eight macro flows based
on their DetNet flow aggregate. We refer to each macro flow as a
class. The output port performs aggregate scheduling with eight
queues (queuing subsystems): one for CDT, one for class A flows, one
for class B flows, and five for BE traffic denoted as BE0-BE4. The
queuing policy for each queuing subsystem is FIFO. In addition, each
node output port also performs per-flow regulation for AVB flows
using an interleaved regulator (IR), called Asynchronous Traffic
Shaper [IEEE8021Qcr]. Thus, at each output port of a node, there is
one interleaved regulator per-input port and per-class; the
interleaved regulator is mapped to the regulator depicted in
Figure 1. The detailed picture of scheduling and regulation
architecture at a node output port is given by Figure 4. The packets
received at a node input port for a given class are enqueued in the
respective interleaved regulator at the output port. Then, the
packets from all the flows, including CDT and BE flows, are enqueued
in queuing subsytem; there is no regulator for such classes.
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+--+ +--+ +--+ +--+
| | | | | | | |
|IR| |IR| |IR| |IR|
| | | | | | | |
+-++XXX++-+ +-++XXX++-+
| | | |
| | | |
+---+ +-v-XXX-v-+ +-v-XXX-v-+ +-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | |Class| |Class| |Class| |Class| |Class|
|CDT| | Class A | | Class B | | BE4 | | BE3 | | BE2 | | BE1 | | BE0 |
| | | | | | | | | | | | | | | |
+-+-+ +----+----+ +----+----+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+
| | | | | | | |
| +-v-+ +-v-+ | | | | |
| |CBS| |CBS| | | | | |
| +-+-+ +-+-+ | | | | |
| | | | | | | |
+-v--------v-----------v---------v-------V-------v-------v-------v--+
| Strict Priority selection |
+--------------------------------+----------------------------------+
|
V
Figure 4: The architecture of an output port inside a relay node with
interleaved regulators (IRs) and credit-based shaper (CBS)
Each of the queuing subsystems for classes A and B, contains Credit-
Based Shaper (CBS). The CBS serves a packet from a class according
to the available credit for that class. The credit for each class A
or B increases based on the idle slope, and decreases based on the
send slope, both of which are parameters of the CBS (Section 8.6.8.2
of [IEEE8021Q]). The CDT and BE0-BE4 flows are served by separate
queuing subsystems. Then, packets from all flows are served by a
transmission selection subsystem that serves packets from each class
based on its priority. All subsystems are non-preemptive.
Guarantees for AVB traffic can be provided only if CDT traffic is
bounded; it is assumed that the CDT traffic has leaky bucket arrival
curve with two parameters r_h as rate and b_h as bucket size, i.e.,
the amount of bits entering a node within a time interval t is
bounded by r_h t + b_h.
Additionally, it is assumed that the AVB flows are also regulated at
their source according to leaky bucket arrival curve. At the source,
the traffic satisfies its regulation constraint, i.e. the delay due
to interleaved regulator at source is ignored.
At each DetNet transit node implementing an interleaved regulator,
packets of multiple flows are processed in one FIFO queue; the packet
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at the head of the queue is regulated based on its leaky bucket
parameters; it is released at the earliest time at which this is
possible without violating the constraint.
The regulation parameters for a flow (leaky bucket rate and bucket
size) are the same at its source and at all DetNet transit nodes
along its path in the case of that all clocks are perfect. However,
in reality there is clock nonideality thoughout the DetNet domain
even with clock synchronization. This phenomenon causes inaccuracy
in the rates configured at the regulators that may lead to network
instability. To avoid that, when configuring the regulators, the
rates are set as the source rates with some positive margin.
[Thomas2020time] describes and provides solutions to this issue.
6.4.1. Delay Bound Calculation
A delay bound of the queuing subsystem ((4) in Figure 1) for an AVB
flow of classes A or B can be computed if the following condition
holds:
sum of leaky bucket rates of all flows of this class at this
transit node <= R, where R is given below for every class.
If the condition holds, the delay bounds for a flow of class X (A or
B) is d_X and calculated as:
d_X = T_X + (b_t_X-L_min_X)/R_X - L_min_X/c
where L_min_X is the minimum packet lengths of class X (A or B); c is
the output link transmission rate; b_t_X is the sum of the b term
(bucket size) for all the flows of the class X. Parameters R_X and
T_X are calculated as follows for class A and class B, separately:
If the flow is of class A:
R_A = I_A (c-r_h)/ c
T_A = L_nA + b_h + r_h L_n/c)/(c-r_h)
where L_nA is the maximum packet length of class B and BE packets;
L_n is the maximum packet length of classes A,B, and BE.
If the flow is of class B:
R_B = I_B (c-r_h)/ c
T_B = (L_BE + L_A + L_nA I_A/(c_h-I_A) + b_h + r_h L_n/c)/(c-r_h)
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where L_A is the maximum packet length of class A; L_BE is the
maximum packet length of class BE.
Then, an end-to-end delay bound of class X (A or B)is calculated by
the formula Section 4.2.2, where for Cij:
Cij = d_X
More information of delay analysis in such a DetNet transit node is
described in [TSNwithATS].
6.4.2. Flow Admission
The delay bound calculation requires some information about each
node. For each node, it is required to know the idle slope of CBS
for each class A and B (I_A and I_B), as well as the transmission
rate of the output link (c). Besides, it is necessary to have the
information on each class, i.e. maximum packet length of classes A,
B, and BE. Moreover, the leaky bucket parameters of CDT (r_h,b_h)
should be known. To admit a flow/flows of classes A and B, their
delay requirements should be guaranteed not to be violated. As
described in Section 3.1, the two problems, static and dynamic, are
addressed separately. In either of the problems, the rate and delay
should be guaranteed. Thus,
The static admission control:
The leaky bucket parameters of all AVB flows are known,
therefore, for each AVB flow f, a delay bound can be
calculated. The computed delay bound for every AVB flow
should not be more than its delay requirement. Moreover, the
sum of the rate of each flow (r_f) should not be more than
the rate allocated to each class (R). If these two
conditions hold, the configuration is declared admissible.
The dynamic admission control:
For dynamic admission control, we allocate to every node and
class A or B, static value for rate (R) and maximum
burstiness (b_t). In addition, for every node and every
class A and B, two counters are maintained:
R_acc is equal to the sum of the leaky-bucket rates of all
flows of this class already admitted at this node; At all
times, we must have:
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R_acc <=R, (Eq. 1)
b_acc is equal to the sum of the bucket sizes of all flows
of this class already admitted at this node; At all times,
we must have:
b_acc <=b_t. (Eq. 2)
A new AVB flow is admitted at this node, if Eqs. (1) and (2)
continue to be satisfied after adding its leaky bucket rate
and bucket size to R_acc and b_acc. An AVB flow is admitted
in the network, if it is admitted at all nodes along its
path. When this happens, all variables R_acc and b_acc along
its path must be incremented to reflect the addition of the
flow. Similarly, when an AVB flow leaves the network, all
variables R_acc and b_acc along its path must be decremented
to reflect the removal of the flow.
The choice of the static values of R and b_t at all nodes and classes
must be done in a prior configuration phase; R controls the bandwidth
allocated to this class at this node, b_t affects the delay bound and
the buffer requirement. R must satisfy the constraints given in
Annex L.1 of [IEEE8021Q].
6.5. Guaranteed-Service IntServ
Guaranteed-Service Integrated service (IntServ) is an architecture
that specifies the elements to guarantee quality of service (QoS) on
networks [RFC2212].
The flow, at the source, has a leaky bucket arrival curve with two
parameters r as rate and b as bucket size, i.e., the amount of bits
entering a node within a time interval t is bounded by r t + b.
If a resource reservation on a path is applied, a node provides a
guaranteed rate R and maximum service latency of T. This can be
interpreted in a way that the bits might have to wait up to T before
being served with a rate greater or equal to R. The delay bound of
the flow traversing the node is T + b / R.
Consider a Guaranteed-Service IntServ path including a sequence of
nodes, where the i-th node provides a guaranteed rate R_i and maximum
service latency of T_i. Then, the end-to-end delay bound for a flow
on this can be calculated as sum(T_i) + b / min(R_i).
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The provided delay bound is based on a simple case of Guaranteed-
Service IntServ where only a guaranteed rate and maximum service
latency and a leaky bucket arrival curve are available. If more
information about the flow is known, e.g. the peak rate, the delay
bound is more complicated; the detail is available in [RFC2212] and
Section 1.4.1 of [NetCalBook].
6.6. Cyclic Queuing and Forwarding
Annex T of [IEEE8021Q] describes Cyclic Queuing and Forwarding (CQF),
which provides bounded latency and zero congestion loss using the
time-scheduled gates of [IEEE8021Q] section 8.6.8.4. For a given
class of DetNet flows, a set of two or more buffers is provided at
the output queue layer of Figure 3. A cycle time T_c is configured
for each class of DetNet flows c, and all of the buffer sets in a
class of DetNet flows swap buffers simultaneously throughout the
DetNet domain at that cycle rate, all in phase. In such a mechanism,
the regulator, mentioned in Figure 1, is not required.
In the case of two-buffer CQF, each class of DetNet flows c has two
buffers, namely buffer1 and buffer2. In a cycle (i) when buffer1
accumulates received packets from the node's reception ports, buffer2
transmits the already stored packets from the previous cycle (i-1).
In the next cycle (i+1), buffer2 stores the received packets and
buffer1 transmits the packets received in cycle (i). The duration of
each cycle is T_c.
The per-hop latency is trivially determined by the cycle time T_c:
the packet transmitted from a node at a cycle (i), is transmitted
from the next node at cycle (i+1). Hence, the maximum delay
experienced by a given packet is from the beginning of cycle (i) to
the end of cycle (i+1), or 2T_c; also, the minimum delay is from the
end of cycle (i) to the beginning of cycle (i+1), i.e., zero. Then,
if the packet traverses h hops, the maximum delay is:
(h+1) T_c
and the minimum delay is:
(h-1) T_c
which gives a latency variation of 2T_c.
The cycle length T_c should be carefully chosen; it needs to be large
enough to accomodate all the DetNet traffic, plus at least one
maximum interfering packet, that can be received within one cycle.
Also, the value of T_c includes a time interval, called dead time
(DT), which is the sum of the delays 1,2,3,4 defined in Figure 1.
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The value of DT guarantees that the last packet of one cycle in a
node is fully delivered to a buffer of the next node is the same
cycle. A two-buffer CQF is recommended if DT is small compared to
T_c. For a large DT, CQF with more buffers can be used and a cycle
identification label can be added to the packets.
Ingress conditioning (Section 4.3) may be required if the source of a
DetNet flow does not, itself, employ CQF. Since there are no per-
flow parameters in the CQF technique, per-hop configuration is not
required in the CQF forwarding nodes.
7. Example application on DetNet IP network
This section provides an example application of this document on a
DetNet-enabled IP network. Consider Figure 5, taken from Section 3
of [RFC8939], that shows a simple IP network:
o The end-system 1 implements Guaranteed-Service IntServ as in
Section 6.5 between itself and relay node 1.
o Sub-network 1 is a TSN network. The nodes in subnetwork 1
implement credit-based shapers with asynchronous traffic shaping
as in Section 6.4.
o Sub-network 2 is a TSN network. The nodes in subnetwork 2
implement cyclic queuing and forwarding with two buffers as in
Section 6.6.
o The relay nodes 1 and 2 implement credit-based shapers with
asynchronous traffic shaping as in Section 6.4. They also perform
the aggregation and mapping of IP DetNet flows to TSN streams
(Section 4.4 of [I-D.ietf-detnet-ip-over-tsn]).
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DetNet IP Relay Relay DetNet IP
End-System Node 1 Node 2 End-System
1 2
+----------+ +----------+
| Appl. |<------------ End-to-End Service ----------->| Appl. |
+----------+ ............ ........... +----------+
| Service |<-: Service :-- DetNet flow --: Service :->| Service |
+----------+ +----------+ +----------+ +----------+
|Forwarding| |Forwarding| |Forwarding| |Forwarding|
+--------.-+ +-.------.-+ +-.---.----+ +-------.--+
: Link : \ ,-----. / \ ,-----. /
+......+ +----[ Sub- ]----+ +-[ Sub- ]-+
[Network] [Network]
`--1--' `--2--'
|<--------------------- DetNet IP --------------------->|
|<--- d1 --->|<--------------- d2_p --------------->|<-- d3_p -->|
Figure 5: A Simple DetNet-Enabled IP Network, taken from RFC8939
Consider a fully centeralized control plane for the network of
Figure 5 as described in Section 3.2 of
[I-D.ietf-detnet-controller-plane-framework]. Suppose end-system 1
wants to create a DetNet flow with traffic specification destined to
end-system 2 with end-to-end delay bound requirement D. Therefore,
the control plane receives a flow establishment request and
calculates a number of valid paths through the network (Section 3.2
of [I-D.ietf-detnet-controller-plane-framework]). To select a proper
path, the control plane needs to compute an end-to-end delay bound at
every node of each selected path p.
The end-to-end delay bound is d1 + d2_p + d3_p, where d1 is the delay
bound from end-system 1 to the entrance of relay node 1, d2_p is the
delay bound for path p from relay node 1 to entrance of the first
node in sub-network 2, and d3_p the delay bound of path p from the
first node in sub-network 2 to end-system 2. The computation of d1
is explained in Section 6.5. Since the relay node 1, sub-network 1
and relay node 2 implement aggregate queuing, we use the results in
Section 4.2.2 and Section 6.4 to compute d2_p for the path p.
Finally, d3_p is computed using the delay bound computation of
Section 6.6. Any path p such that d1 + d2_p + d3_p <= D satisfies
the delay bound requirement of the flow. If there is no such path,
the control plane may compute new set of valid paths and redo the
delay bound computation or do not admit the DetNet flow.
As soon as the control plane selects a path that satisfies the delay
bound constraint, it allocates and reserves the resources in the path
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for the DetNet flow (Section 4.2
[I-D.ietf-detnet-controller-plane-framework]).
8. Security considerations
Detailed security considerations for DetNet are cataloged in
[I-D.ietf-detnet-security], and more general security considerations
are described in [RFC8655].
Security aspects that are unique to DetNet are those whose aim is to
provide the specific QoS aspects of DetNet, specifically bounded end-
to-end delivery latency and zero congestion loss. Achieving such
loss rates and bounded latency may not be possible in the face of a
highly capable adversary, such as the one envisioned by the Internet
Threat Model of BCP 72 [RFC3552] that can arbitrarily drop or delay
any or all traffic. In order to present meaningful security
considerations, we consider a somewhat weaker attacker who does not
control the physical links of the DetNet domain but may have the
ability to control a network node within the boundary of the DetNet
domain.
A security consideration for this document is to secure the resource
reservation signaling for DetNet flows. Any forge or manipulation of
packets during reservation may lead the flow not to be admitted or
face delay bound violation. Security mitigation for this issue is
describedd in Section 7.6 of [I-D.ietf-detnet-security].
9. IANA considerations
This document has no IANA actions.
10. References
10.1. Normative References
[RFC2212] Shenker, S., Partridge, C., and R. Guerin, "Specification
of Guaranteed Quality of Service", RFC 2212,
DOI 10.17487/RFC2212, September 1997,
<https://www.rfc-editor.org/info/rfc2212>.
[RFC6658] Bryant, S., Ed., Martini, L., Swallow, G., and A. Malis,
"Packet Pseudowire Encapsulation over an MPLS PSN",
RFC 6658, DOI 10.17487/RFC6658, July 2012,
<https://www.rfc-editor.org/info/rfc6658>.
[RFC7806] Baker, F. and R. Pan, "On Queuing, Marking, and Dropping",
RFC 7806, DOI 10.17487/RFC7806, April 2016,
<https://www.rfc-editor.org/info/rfc7806>.
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[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
[RFC8939] Varga, B., Ed., Farkas, J., Berger, L., Fedyk, D., and S.
Bryant, "Deterministic Networking (DetNet) Data Plane:
IP", RFC 8939, DOI 10.17487/RFC8939, November 2020,
<https://www.rfc-editor.org/info/rfc8939>.
[RFC8964] Varga, B., Ed., Farkas, J., Berger, L., Malis, A., Bryant,
S., and J. Korhonen, "Deterministic Networking (DetNet)
Data Plane: MPLS", RFC 8964, DOI 10.17487/RFC8964, January
2021, <https://www.rfc-editor.org/info/rfc8964>.
10.2. Informative References
[bennett2002delay]
J.C.R. Bennett, K. Benson, A. Charny, W.F. Courtney, and
J.-Y. Le Boudec, "Delay Jitter Bounds and Packet Scale
Rate Guarantee for Expedited Forwarding",
<https://dl.acm.org/citation.cfm?id=581870>.
[charny2000delay]
A. Charny and J.-Y. Le Boudec, "Delay Bounds in a Network
with Aggregate Scheduling", <https://link.springer.com/
chapter/10.1007/3-540-39939-9_1>.
[I-D.ietf-detnet-controller-plane-framework]
A. Malis, X. Geng, M. Chen, F. Qin, and B. Varga,
"Deterministic Networking (DetNet) Controller Plane
Framework draft-ietf-detnet-controller-plane-framework-
00", <https://datatracker.ietf.org/doc/html/draft-ietf-
detnet-controller-plane-framework>.
[I-D.ietf-detnet-ip-over-tsn]
B. Varga, J. Farkas, A. Malis, and S. Bryant, "DetNet Data
Plane: IP over IEEE 802.1 Time Sensitive Networking (TSN)
draft-ietf-detnet-ip-over-tsn-07",
<https://datatracker.ietf.org/doc/html/draft-ietf-detnet-
ip-over-tsn-07>.
[I-D.ietf-detnet-security]
E. Grossman, T. Mizrahi, and A. Hacker, "Deterministic
Networking (DetNet) Security Considerations draft-ietf-
detnet-security-16", <https://datatracker.ietf.org/doc/
draft-ietf-detnet-security/>.
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[IEEE8021Q]
IEEE 802.1, "IEEE Std 802.1Q-2018: IEEE Standard for Local
and metropolitan area networks - Bridges and Bridged
Networks", 2018,
<http://ieeexplore.ieee.org/document/8403927>.
[IEEE8021Qcr]
IEEE 802.1, "IEEE P802.1Qcr: IEEE Draft Standard for Local
and metropolitan area networks - Bridges and Bridged
Networks - Amendment: Asynchronous Traffic Shaping", 2017,
<http://www.ieee802.org/1/files/private/cr-drafts/>.
[IEEE8021TSN]
IEEE 802.1, "IEEE 802.1 Time-Sensitive Networking (TSN)
Task Group", <http://www.ieee802.org/1/>.
[IEEE8023]
IEEE 802.3, "IEEE Std 802.3-2018: IEEE Standard for
Ethernet", 2018,
<http://ieeexplore.ieee.org/document/8457469>.
[le_boudec2018theory]
J.-Y. Le Boudec, "A Theory of Traffic Regulators for
Deterministic Networks with Application to Interleaved
Regulators",
<https://ieeexplore.ieee.org/document/8519761>.
[NetCalBook]
J.-Y. Le Boudec and P. Thiran, "Network calculus: a theory
of deterministic queuing systems for the internet", 2001,
<https://ica1www.epfl.ch/PS_files/NetCal.htm>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases",
RFC 8578, DOI 10.17487/RFC8578, May 2019,
<https://www.rfc-editor.org/info/rfc8578>.
[Specht2016UBS]
J. Specht and S. Samii, "Urgency-Based Scheduler for Time-
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Authors' Addresses
Norman Finn
Huawei Technologies Co. Ltd
3101 Rio Way
Spring Valley, California 91977
US
Phone: +1 925 980 6430
Email: nfinn@nfinnconsulting.com
Jean-Yves Le Boudec
EPFL
IC Station 14
Lausanne EPFL 1015
Switzerland
Email: jean-yves.leboudec@epfl.ch
Ehsan Mohammadpour
EPFL
IC Station 14
Lausanne EPFL 1015
Switzerland
Email: ehsan.mohammadpour@epfl.ch
Finn, et al. Expires October 17, 2021 [Page 27]
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Jiayi Zhang
Huawei Technologies Co. Ltd
Q27, No.156 Beiqing Road
Beijing 100095
China
Email: zhangjiayi11@huawei.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
Finn, et al. Expires October 17, 2021 [Page 28]