DetNet N. Finn
Internet-Draft Huawei Technologies Co. Ltd
Intended status: Standards Track J-Y. Le Boudec
Expires: January 3, 2019 E. Mohammadpour
EPFL
B. Varga
J. Farkas
Ericsson
July 2, 2018
DetNet Bounded Latency
draft-finn-detnet-bounded-latency-01
Abstract
This document presents a parameterized timing model for Deterministic
Networking so that existing and future standards can achieve bounded
latency and zero congestion loss.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 4
4. DetNet bounded latency model . . . . . . . . . . . . . . . . 4
4.1. Flow creation . . . . . . . . . . . . . . . . . . . . . . 4
4.2. End-to-end model . . . . . . . . . . . . . . . . . . . . 5
4.3. Relay system model . . . . . . . . . . . . . . . . . . . 5
5. Computing End-to-end Latency Bounds . . . . . . . . . . . . . 7
5.1. Examples of Computations . . . . . . . . . . . . . . . . 8
5.1.1. Per-flow queuing . . . . . . . . . . . . . . . . . . 8
5.1.2. Time-Sensitive Networking with Asynchronous Traffic
Shaping . . . . . . . . . . . . . . . . . . . . . . . 8
6. Achieving zero congestion loss . . . . . . . . . . . . . . . 9
6.1. A General Formula . . . . . . . . . . . . . . . . . . . . 9
7. Queuing model . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Queuing data model . . . . . . . . . . . . . . . . . . . 10
7.2. IEEE 802.1 Queuing Model . . . . . . . . . . . . . . . . 12
7.2.1. Queuing Data Model with Preemption . . . . . . . . . 12
7.2.2. Transmission Selection Model . . . . . . . . . . . . 13
7.3. Time-Sensitive Networking with Asynchronous Traffic
Shaping . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.4. Other queuing models, e.g. IntServ . . . . . . . . . . . 17
8. Parameters for the bounded latency model . . . . . . . . . . 17
8.1. Sender parameters . . . . . . . . . . . . . . . . . . . . 17
8.2. Relay system parameters . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
The ability for IETF Deterministic Networking (DetNet) or IEEE 802.1
Time-Sensitive Networking (TSN) 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/TSN
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. Thus, DetNet is an example of an
INTSERV Guaranteed Quality of Service [RFC2212]
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As explained in [I-D.ietf-detnet-architecture], 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 of use to the
applications identified in [I-D.ietf-detnet-use-cases], 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.
Rather than defining, in great detail, specific mechanisms to be used
to control packet transmission at each output port, this document
presents a timing model for sources, destinations, and the network
nodes that relay packets. The parameters specified in this model:
o Characterize a DetNet flow in a way that provides externally
measureable verification that the sender is conforming to its
promised maximum, can be implemented reasonably easily by a
sending device, and does not require excessive over-allocation of
resources by the network.
o Enable resonably accurate computation of worst-case end-to-end
latency, in a way that requires as little detailed knowledge as
possible of the behavior of the Quality of Service (QoS)
algorithms implemented in each devince, including queuing,
shaping, metering, policing, and transmission selection
techniques.
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 QoS algorithms 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.
This document does not specify any resource reservation protocol or
server. It does not describe all of the requirements for that
protocol or server. It does describe a set of requirements for
resource reservation algorithms and for QoS algorithms that, if met,
will enable them to work together.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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The lowercase forms with an initial capital "Must", "Must Not",
"Shall", "Shall Not", "Should", "Should Not", "May", and "Optional"
in this document are to be interpreted in the sense defined in
[RFC2119], but are used where the normative behavior is defined in
documents published by SDOs other than the IETF.
3. Terminology and Definitions
This document uses the terms defined in
[I-D.ietf-detnet-architecture].
4. DetNet bounded latency model
4.1. Flow creation
The bounded latency model assusmes the use of the following paradigm
for provisioning a particular DetNet flow:
1. Perform any onfiguration required by the relay systems in the
network for the classes of service to be offered, including one
or more classes of DetNet service. This configuration is
general; it is not tied to any particular flow.
2. Characterize the DetNet flow in terms of limitations on the
sender Section 8.1 and flow requirements Section 8.2.
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. Select one of the DetNet classes of service for the DetNet flow.
5. Compute the worst-case end-to-end latency for the DetNet flow.
In the process, determine whether sufficient resources are
available for that flow to guarantee the required latency and
provide zero congestion loss.
6. Assuming that the resources are available, commit those resources
to the flow. This may or may not require adjusting the
parameters that control the QoS algorithms at each hop along the
flow's path.
This paradigm can be static and/or dynamic, and can be implemented
using peer-to-peer protocols or with a central server model. In some
situations, backtracking and recursing through this list may be
necessary.
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Issues such as un-provisioning a DetNet flow in favor of another when
resources are scarce are not considered. How the path to be taken by
a DetNet flow is chosen is not considered in this document.
4.2. End-to-end model
[Suggestion: This is the introduction to network calculus. The
starting point is a model in which a relay system is a black box.]
4.3. Relay system model
[NWF I think that at least some of this will be useful. We won't
know until we see what J-Y has to say in Section 4.2. I'm especially
interested in whether J-Y thinks that the "output delay" in Figure 1
is useful in determining the number of buffers needed in the next
hop. It is possible that we can define the parameters we need
without this section.]
In Figure 1 we see a breakdown of the per-hop latency experienced by
a packet passing through a relay system, in terms that are suitable
for computing both hop-by-hop latency and per-hop buffer
requirements.
DetNet relay node A DetNet relay node B
+-------------------+ +-------------------+
| Reg. Queue | | Reg. Queue |
| +-+-+ +-+-+-+ | | +-+-+ +-+-+-+ |
-->+ | | | | | | + +------->+ | | | | | | + +--->
| +-+-+ +-+-+-+ | | +-+-+ +-+-+-+ |
| | | |
+-------------------+ +-------------------+
|<->|<-->|<---->|<->|<------>|<->|<-->|<---->|<->|<--
2,3 4 5 6 1 2,3 4 5 6 1 2,3
1: Output delay 3: Preemption delay
2: Link delay 4: Processing delay
5: Regulation delay 6: Queuing delay.
Figure 1: Timing model for DetNet or TSN
In Figure 1, we see two DetNet relay nodes (typically, bridges or
routers), with a wired link between them. In this model, the only
queues 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 five delays that a
packet can experience from hop to hop.
1. Output delay
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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.
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 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 [I-D.ietf-detnet-architecture]).
However, a virtual "link" could exhibit a variable link delay.
3. Preemption delay
If the packet is interrupted (e.g. [IEEE8023br] preemption) in
order to transmit another packet or packets, an arbitrary delay
can result.
4. Processing delay
This delay covers the time from the reception of the last bit of
the packet to that packet being eligible, if there were no other
packets in the queue, for selection for output. This delay can be
variable, and depends on the details of the operation of the
forwarding node.
5. Regulation delay
This is the time spent from the insertion of the 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 delay
This is the time spent for a packet from being declared eligibile
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
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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
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 relay node. Depending
on the details of the queueing delay (6) calculations, these
variations need not be visible outside the DetNet relay node.
5. Computing End-to-end Latency Bounds
End-to-end latency bounds can be computed using the delay model in
Section 4.3. Here it is important to be aware that for several
queuing mechanisms, the worst-case end-to-end delay is less than the
sum of the per-hop worst-case delays. An end-to-end latency bound
for one detnet flow can be computed as
end_to_end_latency_bound = non_queuing_latency + queuing_latency
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 upper
bound per-hop_non_queuing_latency[h] on the sum of delays 1,2,3,4 of
Figure 1. These upper-bounds are expected to depend on the specific
technology of the node at the h-th hop but not on the T-SPEC of this
detnet flow. Then set non_queuing_latency = the sum of per-
hop_non_queuing_latency[h] over all hops h.
Second, compute queuing_latency as an upper bound to the sum of the
queuing delays along the path. The value of queuing_latency depends
on the T-SPEC of this flow and possibly of other flows in the
network, as well as the specifics of the queuing mechanisms deployed
along the path of this flow.
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For several queuing mechanisms, queuing_latency is less than the sum
of upper bounds on the queuing delays (5,6) at every hop.
Section 5.1 gives such practical computation examples.
For other queuing mechanisms the only available value of
queuing_latency 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 node.
5.1. Examples of Computations
5.1.1. Per-flow queuing
[[ JYLB: THIS IS WHERE DETAILS OF END-TO-END LATENCY COMPUTATION ARE
GIVEN FOR PER-FLOW QUEUING]]
5.1.2. Time-Sensitive Networking with Asynchronous Traffic Shaping
Figure 2 shows an example of a network with 5 nodes, which have the
queuing model as Section 7.3. An end-to-end delay bound for flow f
of a given AVB class (A or B), traversing from node 1 to 5, is
calculated as following:
end_to_end_latency_bound_of_flow_f = C12 + C23 + C34 + S4
In the above formula, Cij is a bound on the aggregate response time
of the AVB FIFO queue with CBS (Credit Based Shaper) in node i and
interleaved regulator of node j, and S4 is a bound on the response
time of the AVB FIFO queue with CBS in node 4 for flow f. In fact,
using the delay definitions in Section 4.3, 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. The detail of
calculation for the these response time bounds can be found in
[TSNwithATS].
f
----------------------------->
+---+ +---+ +---+ +---+ +---+
| 1 |---| 2 |---| 3 |---| 4 |---| 5 |
+---+ +---+ +---+ +---+ +---+
\__C12_/\__C23_/\__C34_/\_S4_/
Figure 2: End-to-end latency computation example
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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
flow [TSNwithATS].
6. 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.
6.1. A General Formula
To avoid congestion losses, an upper bound on the backlog present in
the queue of Figure 1 must be computed during path computation. This
bound depends on the set of flows that use this queue, 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. For every
class 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 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 nb_classes be the number of classes of traffic that may use this
output port
o total_in_rate be the sum of the line rates of all input ports that
send traffic of any class 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 of any
class to this output port
o max_packet_length be the maximum packet size for packets of any
class that may be sent to this output port. This is counted in
data units.
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o max_delay45 be an upper bound, in seconds, on the sum of the
processing delay (4) and the queuing delays (5,6) for a packet of
any class at this ouput port.
Then a bound on the backlog of traffic of all classes in the queue at
this output port is
backlog_bound = ( nb_classes + nb_input_ports ) *
max_packet_length + total_in_rate* max_delay45
7. Queuing model
[[ JYLB: THIS IS WHERE DETAILS OF END-TO-END LATENCY COMPUTATION ARE
GIVEN FOR PER-FLOW QUEUING AND FOR TSN WITH ATS]]
7.1. Queuing data model
Sophisticated QoS 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 instantiated hierarchically
above the "Layer 2" queuing mechanisms, among which packets compete
for opportunities to be transmitted on a physical (or sometimes,
logical) medium. These "Layer 2 queuing mechanisms" are not the
province solely of bridges; they are an essential part of any DetNet
relay node. As illustrated by numerous implementation examples, the
"Layer 3" some of mechanisms described in documents such as [RFC7806]
are often integrated, in an implementation, with the "Layer 2"
mechanisms also implemented in the same system. 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 (very simple) model for the flow of packets
through the queues of an IEEE 802.1Q bridge. Packets are assigned to
a class of service. The classes of service are mapped to some number
of physical FIFO queues. IEEE 802.1Q allows a maximum of 8 classes
of service, but it is more common to implement 2 or 4 queues on most
ports.
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|
+--------------V---------------+
| Class of Service Assignment |
+--+-------+---------------+---+
| | |
+--V--+ +--V--+ +--V--+
|Class| |Class| |Class|
| 0 | | 1 | . . . | n |
|queue| |queue| |queue|
+--+--+ +--+--+ +--+--+
| | |
+--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 "Class n queue" box:
o Discarding packets because a queue is full.
o Discarding packets marked "yellow" by a metering function, in
preference to discarding "green" packets.
The Class of Service Assignment function can be quite complex, since
the introduction of [IEEE802.1Qci]. In addition to the Layer 2
priority expressed in the 802.1Q VLAN tag, a bridge can utilize any
of the following information to assign a packet to a particular class
of service (queue):
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.
(Work items expected to add MPC and other indicators.)
o The Class of Service Assignment function can contain metering and
policing functions.
The "Transmission selection" function decides which queue is to
transfer its oldest packet to the output port when a transmission
opportunity arises.
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7.2. IEEE 802.1 Queuing Model
7.2.1. Queuing Data Model with Preemption
Figure 3 must be modified if the output port supports preemption
([IEEE8021Qbu] and [IEEE8023br]). This modification is shown in
Figure 4.
|
+------------------------------V------------------------------+
| Class of Service Assignment |
+--+-------+-------+-------+-------+-------+-------+-------+--+
| | | | | | | |
+--V--+ +--V--+ +--V--+ +--V--+ +--V--+ +--V--+ +--V--+ +--V--+
|Class| |Class| |Class| |Class| |Class| |Class| |Class| |Class|
| a | | b | | c | | d | | e | | f | | g | | h |
|queue| |queue| |queue| |queue| |queue| |queue| |queue| |queue|
+--+--+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+
| | | +-+ | | | |
| | | | | | | |
+--V-------V-------V------+ +V-----V-------V-------V-------V--+
| Interrupted xmit select | | Preempting xmit select | 802.1
+-------------+-----------+ +----------------+----------------+
| | ======
+-------------V-----------+ +----------------V----------------+
| Preemptible MAC | | Express MAC | 802.3
+--------+----------------+ +----------------+----------------+
| |
+--------V-----------------------------------V----------------+
| MAC merge sublayer |
+--------------------------+----------------------------------+
|
+--------------------------V----------------------------------+
| PHY (unaware of preemption) |
+--------------------------+----------------------------------+
|
V
Figure 4: IEEE 802.1Q Queuing Model: Data flow with preemption
From Figure 4, we can see that, in the IEEE 802 model, the preemption
feature 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. The Class of Service (queue)
determines which packets are which. In Figure 4, the classes of
service are marked "a, b, ..." instead of with numbers, in order to
avoid any implication about which numeric Layer 2 priority values
correspond to preemptible or preempting queues. Although it shows
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three queues going to the preemptible MAC/PHY, any assignment is
possible.
7.2.2. Transmission Selection Model
In Figure 5, we expand the "Transmission selection" function of
Figure 4.
Figure 5 does NOT show the data path. It shows an example of a
configuration of the IEEE 802.1Q transmission selection box shown in
Figure 3 and Figure 4. Each queue m presents a "Class m Ready"
signal. These signals go through various logic, filters, and state
machines, until a single queue's "not empty" signal is chosen for
presentation to the underlying MAC/PHY. When the MAC/PHY is ready to
take another output packet, then a packet is selected from the one
queue (if any) whose signal manages to pass all the way through the
transmission selection function.
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+-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
|Class| |Class| |Class| |Class| |Class| |Class| |Class| |Class|
| 1 | | 0 | | 4 | | 5 | | 6 | | 7 | | 2 | | 3 |
|Ready| |Ready| |Ready| |Ready| |Ready| |Ready| |Ready| |Ready|
+--+--+ +--+--+ +--+--+ +-XXX-+ +--+--+ +--+--+ +--+--+ +--+--+
| | | | | | |
| +--V--+ +--V--+ +--+--+ +--V--+ | +--V--+ +--V--+
| |Prio.| |Prio.| |Prio.| |Prio.| | |Sha- | |Sha- |
| | 0 | | 4 | | 5 | | 6 | | | per| | per|
| | PFC | | PFC | | PFC | | PFC | | | A | | B |
| +--+--+ +--+--+ +-XXX-+ +-XXX-+ | +--+--+ +-XXX-+
| | | | |
+--V--+ +--V--+ +--V--+ +--+--+ +--+--+ +--V--+ +--V--+ +--+--+
|Time | |Time | |Time | |Time | |Time | |Time | |Time | |Time |
| Gate| | Gate| | Gate| | Gate| | Gate| | Gate| | Gate| | Gate|
| 1 | | 0 | | 4 | | 5 | | 6 | | 7 | | 2 | | 3 |
+--+--+ +-XXX-+ +--+--+ +--+--+ +-XXX-+ +--+--+ +-XXX-+ +--+--+
| | |
+--V-------+-------V-------+--+ |
|802.1Q Enhanced Transmission | |
| Selection (ETS) = Weighted | |
| Fair Queuing (WFQ) | |
+--+-------+------XXX------+--+ |
| |
+--V-------+-------+-------+-------+-------V-------+-------+--+
| Strict Priority selection (rightmost first) |
+-XXX------+-------+-------+-------+-------+-------+-------+--+
|
V
Figure 5: 802.1Q Transmission Selection
The following explanatory notes apply to Figure 5
o The numbers in the "Class n Ready" boxes are the values of the
Layer 2 priority that are assigned to that Class of Service in
this example. The rightmost CoS is the most important, the
leftmost the least. Classes 2 and 3 are made the most important,
because they carry DetNet flows. It is all right to make them
more important than the priority 7 queue, which typically carries
critical network control protocols such as spanning tree or IS-IS,
because the shaper ensures that the highest priority best-effort
queue (7) will get reasonable access to the MAC/PHY. Note that
Class 5 has no Ready signal, indicating that that queue is empty.
o Below the Class Ready signals are shown the Priority Flow Control
gates (IEEE Std 802.1Qbb-2011 Priority-based Flow Control, now
[IEEE8021Q] clause 36) on Classes of Service 1, 0, 4, and 5, and
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two 802.1Q shapers, A and B. Perhaps shaper A conforms to the
IEEE Std 802.1Qav-2009 (now [IEEE8021Q] clause 34) credit-based
shaper, and shaper B conforms to [IEEE8021Qcr] Asynchronous
Traffic Shaper. Any given Class of Service can have either a PFC
function or a shaper, but not both.
o Next are the IEEE Std 802.1Qbv time gates ([IEEE8021Qbv]). Each
one of the 8 Classes of Service has a time gate. The gates are
controlled by a repeating schedule that restarts periodically, and
can be programmed to turn any combination of gates on or off with
nanosecond precision. (Although the implementation is not
necessarily that accurate.)
o Following the time gates, any number of Classes of Service can be
linked to one ore more instances of the Enhanced Transmission
Selection function. This does weighted fair queuing among the
members of its group.
o A final selection of the one queue to be selected for output is
made by strict priority. Note that the priority is determined not
by the Layer 2 priority, but by the Class of Service.
o An "XXX" in the lower margin of a box (e.g. "Prio. 5 PFC"
indicates that the box has blocked the "Class n Ready" signal.
o IEEE 802.1Qch Cyclic Queuing and Forwarding [IEEE802.1Qch] is
accomplished using two or three queues (e.g. 2 and 3 in the
figure), using sophisticated time-based schedules in the Class of
Service Assignment function, and using the IEEE 802.1Qbv time
gates [IEEE8021Qbv] to swap between the output buffers.
7.3. Time-Sensitive Networking with Asynchronous Traffic Shaping
Consider a network with a set of nodes (switches and hosts) along
with a set of flows between hosts. Hosts are sources or destinations
of flows. There are four types of flows, namely, 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 to
as AVB flows. It is assumed a subset of TSN functions as described
next.
It is also assumed that contention occurs only at the output port of
a TSN node. Each node output port performs per-class scheduling with
eight classes: one for CDT, one for class A traffic, one for class B
traffic, and five for BE traffic denoted as BE0-BE4 (according to TSN
standard). In addition, each node output port also performs per-flow
regulation for AVB flows using an interleaved regulator (IR), called
Asynchronous Traffic Shaper (ATS) in TSN. Thus, at each output port
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of a node, there is one interleaved regulator per-input port and per-
class. The detailed picture of scheduling and regulation
architecture at a node output port is given by Figure 6. 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 a class based FIFO system (CBFS) [TSNwithATS].
+--+ +--+ +--+ +--+
| | | | | | | |
|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 6: Architecture of one TSN node output port with interleaved
regulators (IRs)
The CBFS includes two CBS subsystems, one for each class A and B.
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. The CDT and BE0-BE4 flows in the
CBFS are served by separate FIFO 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 an
affine arrival curve r t + b in each node, i.e. the amount of bits
entering a node within a time interval t is bounded by r t + b.
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[[ EM: THE FOLLOWING PARAGRAPH SHOULD BE ALIGNED WITH Section 8.2. ]]
Additionally, it is assumed that flows are regulated at their source,
according to either leaky bucket (LB) or length rate quotient (LRQ).
The LB-type regulation forces flow f to conform to an arrival curve
r_f t+b_f . The LRQ-type regulation with rate r_f ensures that the
time separation between two consecutive packets of sizes l_n and
l_n+1 is at least l_n/r_f. Note that if flow f is LRQ-regulated, it
satisfies an arrival curve constraint r_f t + L_f where L_f is its
maximum packet size (but the converse may not hold). For an LRQ
regulated flow, b_f = L_f. At the source hosts, the traffic
satisfies its regulation constraint, i.e. the delay due to
interleaved regulator at hosts is ignored.
At each switch implementing an interleaved regulator, packets of
multiple flows are processed in one FIFO queue; the packet at the
head of the queue is regulated based on its regulation constraints;
it is released at the earliest time at which this is possible without
violating the constraint. The regulation type and parameters for a
flow are the same at its source and at all switches along its path.
7.4. Other queuing models, e.g. IntServ
[[NWF More sections that discuss specific models]]
8. Parameters for the bounded latency model
8.1. Sender parameters
8.2. Relay system parameters
[[NWF This section talks about the paramters that must be passed hop-
by-hop (T-SPEC? F-SPEC?) by a resoure reservation protocol.]]
9. References
9.1. Normative References
[I-D.ietf-detnet-architecture]
Finn, N. and P. Thubert, "Deterministic Networking
Architecture", draft-ietf-detnet-architecture-00 (work in
progress), September 2016.
[I-D.ietf-detnet-dp-alt]
Korhonen, J., Farkas, J., Mirsky, G., Thubert, P.,
Zhuangyan, Z., and L. Berger, "DetNet Data Plane Protocol
and Solution Alternatives", draft-ietf-detnet-dp-alt-00
(work in progress), October 2016.
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[I-D.ietf-detnet-use-cases]
Grossman, E., "Deterministic Networking Use Cases", draft-
ietf-detnet-use-cases-16 (work in progress), May 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[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>.
9.2. Informative References
[IEEE802.1Qch]
IEEE, "IEEE Std 802.1Qch-2017 IEEE Standard for Local and
metropolitan area networks - Bridges and Bridged Networks
Amendment 29: Cyclic Queuing and Forwarding (amendment to
802.1Q-2014)", 2017,
<http://www.ieee802.org/1/files/private/ch-drafts/>.
[IEEE802.1Qci]
IEEE, "IEEE Std 802.1Qci-2017 IEEE Standard for Local and
metropolitan area networks - Bridges and Bridged Networks
- Amendment 30: Per-Stream Filtering and Policing", 2017,
<http://www.ieee802.org/1/files/private/ci-drafts/>.
[IEEE8021Q]
IEEE 802.1, "IEEE Std 802.1Q-2014: IEEE Standard for Local
and metropolitan area networks - Bridges and Bridged
Networks", 2014, <http://standards.ieee.org/getieee802/
download/802-1Q-2014.pdf>.
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[IEEE8021Qbu]
IEEE, "IEEE Std 802.1Qbu-2016 IEEE Standard for Local and
metropolitan area networks - Bridges and Bridged Networks
- Amendment 26: Frame Preemption", 2016,
<http://standards.ieee.org/getieee802/
download/802.1Qbu-2016.zip>.
[IEEE8021Qbv]
IEEE 802.1, "IEEE Std 802.1Qbv-2015: IEEE Standard for
Local and metropolitan area networks - Bridges and Bridged
Networks - Amendment 25: Enhancements for Scheduled
Traffic", 2015, <http://standards.ieee.org/getieee802/
download/802.1Qbv-2015.zip>.
[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 802.3-2015: IEEE Standard for Local
and metropolitan area networks - Ethernet", 2015,
<http://standards.ieee.org/getieee802/
download/802.3-2015.zip>.
[IEEE8023br]
IEEE 802.3, "IEEE Std 802.3br-2016: IEEE Standard for
Local and metropolitan area networks - Ethernet -
Amendment 5: Specification and Management Parameters for
Interspersing Express Traffic", 2016,
<http://standards.ieee.org/getieee802/
download/802.3br-2016.pdf>.
[TSNwithATS]
E. Mohammadpour, E. Stai, M. Mohiuddin, and J.-Y. Le
Boudec, "End-to-end Latency and Backlog Bounds in Time-
Sensitive Networking with Credit Based Shapers and
Asynchronous Traffic Shaping",
<https://arxiv.org/abs/1804.10608/>.
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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: norman.finn@mail01.huawei.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
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
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