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Versions: 00 01 02 03 04 draft-ietf-detnet-bounded-latency

DetNet                                                           N. Finn
Internet-Draft                               Huawei Technologies Co. Ltd
Intended status: Standards Track                          J-Y. Le Boudec
Expires: April 25, 2019                                  E. Mohammadpour
                                                                    EPFL
                                                                J. Zhang
                                             Huawei Technologies Co. Ltd
                                                                B. Varga
                                                               J. Farkas
                                                                Ericsson
                                                        October 22, 2018


                         DetNet Bounded Latency
                  draft-finn-detnet-bounded-latency-02

Abstract

   This document presents a parameterized timing model for Deterministic
   Networking (DetNet), so that existing and future standards can
   achieve the DetNet quality of service features of bounded latency and
   zero congestion loss.  It defines requirements for resource
   reservation protocols or servers.  It calls out queuing mechanisms,
   defined in other documents, that can provide the DetNet quality of
   service.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 25, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   3.  Terminology and Definitions . . . . . . . . . . . . . . . . .   4
   4.  DetNet bounded latency model  . . . . . . . . . . . . . . . .   4
     4.1.  Flow creation . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Relay system model  . . . . . . . . . . . . . . . . . . .   5
   5.  Computing End-to-end Latency Bounds . . . . . . . . . . . . .   7
     5.1.  Non-queuing delay bound . . . . . . . . . . . . . . . . .   7
     5.2.  Queuing delay bound . . . . . . . . . . . . . . . . . . .   8
       5.2.1.  Per-flow queuing mechanisms . . . . . . . . . . . . .   8
       5.2.2.  Per-class queuing mechanisms  . . . . . . . . . . . .   8
   6.  Achieving zero congestion loss  . . . . . . . . . . . . . . .  10
     6.1.  A General Formula . . . . . . . . . . . . . . . . . . . .  10
   7.  Queuing model . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Queuing data model  . . . . . . . . . . . . . . . . . . .  11
     7.2.  Preemption  . . . . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Time-scheduled queuing  . . . . . . . . . . . . . . . . .  13
     7.4.  Time-Sensitive Networking with Asynchronous Traffic
           Shaping . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.5.  IntServ . . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Time-based DetNet QoS . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Cyclic Queuing and Forwarding . . . . . . . . . . . . . .  19
     8.2.  Time Scheduled Queuing  . . . . . . . . . . . . . . . . .  19
   9.  Parameters for the bounded latency model  . . . . . . . . . .  20
     9.1.  Sender parameters . . . . . . . . . . . . . . . . . . . .  20
     9.2.  Relay system parameters . . . . . . . . . . . . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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)



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   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]

   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.

   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
   network nodes that relay packets that is applicable to all of those
   referenced queuing mechanisms.  The parameters specified in this
   model:

   o  Characterize a DetNet flow in a way that provides externally
      measurable 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 reasonably 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 device, 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 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.

   This document does not specify any resource reservation protocol or
   server.  It does not describe all of the requirements for that



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   protocol or server.  It does describe requirements for such resource
   reservation methods, and for queuing mechanisms that, if met, will
   enable them to work together.

   NOTE: This draft is not yet complete, but it is sufficiently so to
   share with the Working Group and to obtain opinions and direction.
   The present intent of is for this draft to become a normative RFC,
   defining how one SHALL/SHOULD provide the DetNet quality of service.
   There are still a few authors' notes to each other present in this
   draft.

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].

   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 assumes the use of the following paradigm
   for provisioning a particular DetNet flow:

   1.  Perform any configuration 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 not
       tied to any particular flow.

   2.  Characterize the DetNet flow in terms of limitations on the
       sender [Section 9.1] and flow requirements [Section 9.2].

   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.



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   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 to
       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 queuing mechanisms 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 using a central server model.  In
   some situations, backtracking and recursing through this list may be
   necessary.

   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.  Relay system model

   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
      +-------------------------+       +------------------------+
      |              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: Preemption 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



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   [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.

   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] and [IEEE8021Qbu]
      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 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




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      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
   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 subsystem delay (6) calculations,
      these variations need not be visible outside the DetNet relay
      node.

5.  Computing End-to-end Latency Bounds

5.1.  Non-queuing delay bound

   End-to-end latency bounds can be computed using the delay model in
   Section 4.2.  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



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   DetNet flow.  Then set non_queuing_latency = the sum of per-
   hop_non_queuing_latency[h] over all hops h.

5.2.  Queuing delay bound

   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.

   For several queuing mechanisms, queuing_latency 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) per-class queuing with
   regulators, as explained in Section 5.2.1, Section 5.2.2, and
   Section 7.

   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.

5.2.1.  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 delay.  For every flow the per-node delay bound as well as
   end-to-end delay bound can be computed from the traffic specification
   of this flow at its source and from the values of rates and latencies
   at all nodes along its path.  Details of calculation for IntServ are
   described in Section 7.5.

5.2.2.  Per-class queuing mechanisms

   With such mechanisms, the flows that have the same class share the
   same queue.  A practical example is the queuing mechanism in Time
   Sensitive Networking.  One key issue in this context is how to deal
   with the burstiness cascade: individual flows that share a resource
   dedicated to a class may see their burstiness increase, which may in
   turn cause increased burstiness to other flows downstream of this
   resource.  Computing latency 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.




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   A solution to deal with this issue is to reshape the flows 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 per-class queuing.  An alternative is the interleaved regulator,
   which reshapes individual flows without per-flow queuing
   ([Specht2016UBS], [IEEE8021Qcr]").  With an interleaved 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_boudec_theory_2018].  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, per-class
   queuing mechanism and interleaved regulators as in Figure 1.  An end-
   to-end delay bound for 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 aggregate response time
   of queuing subsystem in node i and interleaved regulator of node j,
   and S4 is a bound on the response time of the queuing subsystem in
   node 4 for flow f.  In fact, using the delay definitions in
   Section 4.2, 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 7.4.

                                   f
                         ----------------------------->
                       +---+   +---+   +---+   +---+   +---+
                       | 1 |---| 2 |---| 3 |---| 4 |---| 5 |
                       +---+   +---+   +---+   +---+   +---+
                         \__C12_/\__C23_/\__C34_/\_S4_/



             Figure 2: End-to-end latency 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
   flow [TSNwithATS].






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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 regulator and queuing subsystem of Figure 1 must be computed
   during resource reservation.  This bound depends on the set of flows
   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.  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.

   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.




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   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

7.1.  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 relay 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 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 general model for the flow of packets through the
   queues of a DetNet relay node.  Packets are assigned to a class of
   service.  The classes of service are mapped to some number of
   regulator queues.  Only DetNet/TSN packets pass through regulators.
   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----------------------------------+
   |                    Class of Service 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                      V              V       V       V
        DetNet/TSN queue       DetNet/TSN queue    non-DetNet/TSN queues

              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 flow is
   misbehaving, and the DetNet QoS does not include "yellow" service for
   packets in excess of committed rate.

   The Class of Service Assignment function can be quite complex, even
   in a bridge [IEEE8021Q], since the introduction of [IEEE802.1Qci].
   In addition to the Layer 2 priority expressed in the 802.1Q VLAN tag,
   a DetNet relay node can utilize any of the following information to
   assign a packet to a particular class of service (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.
      ([I-D.ietf-detnet-dp-sol-ip], [I-D.ietf-detnet-dp-sol-mpls]) (Work
      items are expected to add MPC and other indicators.)

   o  The Class of Service 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.

7.2.  Preemption

   In IEEE Std 802.1Q, 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.  The Class of
   Service (queue) determines which packets are which.  Only one layer
   of preemption is supported.  DetNet flows pass through the
   interrupting MAC.  Only best-effort queues pass through the
   interruptible MAC, and can thus be preempted.

7.3.  Time-scheduled queuing

   In [IEEE8021Qbv], the notion of time-scheduling queue gates were
   introduced.  On below every output queue (the lower row of queues in
   Figure 3) is a gate that permits or denies the queue to present data
   for transmission selection.  The gates are controlled by a rotating
   schedule that can be locked to a clock that is synchronized with
   other relay nodes.  The DetNet class of service can be supplied by
   queuing mechanisms based on time, rather than the regulator model in
   Figure 3.  These queuing mechanisms are discussed in Section 8,
   below.

7.4.  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




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   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
   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 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 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 4: Architecture of a 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



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   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.

   [[ EM: THE FOLLOWING PARAGRAPH SHOULD BE ALIGNED WITH Section 9.2. ]]

   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.

   Details of end-to-end delay bound calculation in such a system is
   described in [TSNwithATS].

7.5.  IntServ

   In this section, a worst-case queuing latency calculating method is
   provided.  In deterministic network, the traffic of a flow is
   constrained by arrival curve.  Queuing mechanisms in a DetNet node
   can be characterized and constrained by service curve.  By using
   arrival curve and service curve with Network Calculus theory
   [NetCalBook], a tight worst-case queuing latency can be calculated.

   Considering a DetNet flow at output port, R(s) is the cumulative
   arrival data until time s.  For any time period t, the incremental
   arrival data is constrained by an arrival curve a(t)

      R(s+t)-R(s) <= a(t), \any s>=0, t>=0



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   The scheduling that a relay node performs to a DetNet flow can be
   abstracted as service curve.  It describes the minimal service the
   network can offer.  The service curve b(t) of a node is defined as
   below, if the accumulative input data R and output data R_out of the
   node satisfies

      R_out(t) >= inf(R(s) + b(t-s) ), \any s <=t

   where the operator "inf" calculates the greatest lower bound in
   period t.

   By calculating the maximum vertical deviation between arrival curve
   a(t) and service curve b(t), one can obtain the backlog bound in data
   unit

      Backlog_bound = sup_t(a(t) - b(t) )

   where operator "sup_t" calculates the minimum upper bound with
   respect to t.  The buffer space at a node should be no less than the
   backlog bound to achieve zero congestion loss.

   NOTE: Section 6.1 gives a general formula for computing the buffer
   requirements.  This is an alternative calculation based on the
   arrival curve and service curve.

   By calculating the maximum horizontal deviation between arrival curve
   a(t) and service curve b(t), one can obtain the delay bound as below

      Delay_bound = sup_s( inf_t( t>=0 | a(s) <= b(s+t) )

   where the operator " inf_t" calculates the maximum lower bound with
   respect to t, the operator "sup_s" calculates the minimum upper bound
   with respect to s.  Figure 5 shows an example of arival curve,
   service curve, backlog bound h, and delay bound v.

















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                    + bit              .        *
                    |                 .     *
                    |                .  *
                    |               *
                    |           *  .
                    |       *     .
                    |   *   |    .        ..  Service curve
                    *-----h-|---.         **  Arrival curve
                    |       v  .           h  Delay_bound
                    |       | .            v  Backlog_bound
                    |       |.
                    +-------.--------------------+ time

    Figure 5: Computation of backlog bound and delay bound.  Note that
        arrival and service curves are not necessary to be linear.

   Note that in the formula of Delay_bound, the service curve b(t) can
   describe either per-hop scheduling that a DetNet node offers to a
   flow, or concatenation of multiple nodes that represents end-to-end
   scheduling that DetNet path offers to a flow.  In the latter case,
   the obtained delay bound is end-to-end worst case delay.  To
   calculate this, we should at first derive the concatenated service
   curve.

   Consider a flow traverse two DetNet nodes, which offer service curve
   b1(t) and b2(t) sequentially.  Then concatenation of the two nodes
   offers a service curve b_concatenated as below

      b_concatenated(t) =inf_s (b1(s) + b2(t-s) ) , \any 0 <=s <=t

   The concatenation of service curve can be directly generalized to
   include more than two nodes.

      a_out(t) = sup_u( a(t+u) - b(u) ), \any u>=0

   In DetNet, the arrival curve and service curve can be characterized
   by a group of parameters, which will be defined in Section 8.

   Integrated service (IntServ) is an architecture that specifies the
   elements to guarantee quality of service (QoS) on networks.  To
   satisfied guaranteed service, a flow must conform to a traffic
   specification (T-spec), and reservation is made along a path, only if
   routers are able to guarantee the required bandwidth and buffer.

   Consider the traffic model which conforms to token bucket regulator
   (r, b), with

   o  Token bucket depth (b).



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   o  Token bucket rate (r).

   The traffic specification can be described as an arrival curve a(t)

      alpha(t) = b + rt

   This token bucket regulator requires that, during any time window of
   width t, the number of bit for the flow is limited by alpha(t) = b +
   rt.

   If resource reservation on a path is applied, IntServ model on a
   router can be described as a rate-latency service curve beta(t).

      beta(t) = max(0, R(t-T))

   It describes that bits might have to wait up to T before being served
   with a rate greater or equal to R.

   It should be noted that, the guaranteed service rate R is a share of
   link's bandwidth.  The choice of R is related to the specification of
   flows which will transmit on this node.  For example, in strict
   priority policy, considering a flow with priority j, its share of
   bandwidth may be R=c-sum(r_i), i<j, where c is the link bandwidth,
   r_i is the token bucket rate for the flows with priority higher than
   j.  The choice of T is also related to the specification of all the
   flows traversing this node.  For example, in a generalized processor
   sharing (GPS) node, T = L / R + L_max/c, where L is the maximum
   packet size for the flow, L_max is the maximum packet size in the
   node across all flows.  Other choice of R and T are also supported,
   according to the specific scheduling of the node and flows traversing
   this node.

   As mentioned previously in this section, delay bound and backlog
   bound can be easily obtained by comparing arrival curve and service
   curve.  Backlog bound, or buffer bound, is the maximum vertical
   derivation between curves alpha(t) and beta(t), which is x=b+rT.
   Delay bound is the maximum horizontal derivation between curves
   alpha(t) and beta(t), which is d = T+b/R.  Graphical illustration of
   the IntServ model is shown in Figure 5.

   The output bound, or the next-hop arrival curve, is alpha_out(t) = b
   + rT + rt, where burstiness of the flow is increased by rT, compared
   with the arrival curve.

   We can calculate the end-to-end delay bound, for a path including N
   nodes, among which the i-th node offers service curve beta_i(t),

      beta_i(t) = max(0, R_i(t-T_i)), i=1,...,N



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   According to [Section 5.1], by concatenating those IntServ nodes, an
   end-to-end service curve can be computed as

      beta_e2e (t) = max(0, R_e2e(t-T_e2e) )

   where

      R_e2e = min(R_1,..., R_N)

      T_e2e = T_1 + ... + T_N

   Similarly, delay bound, backlog bound and output bound can be
   computed by using the original arrival curve alpha(t) and
   concatenated service curve beta_e2e(t).

8.  Time-based DetNet QoS

8.1.  Cyclic Queuing and Forwarding

   [IEEE802.1Qci] and [IEEE802.1Qch] describe Cyclic Queuing and
   Forwarding (CQF), which provide the bounded latency and zero
   congestion loss using the time-scheduled gates of [IEEE8021Qbv].  For
   each different DetNet class of service, a set of two or three buffers
   is provided at the out queue layer of Figure 3.  A cycle time is
   configured for each class, and all of the buffer sets in a class swap
   buffers simultaneously throughout the DetNet domain at that cycle
   rate.  The choice of using two or three buffers depends on the link
   lengths and forwarding delay times; two buffers can be used if the
   delay from hop to hop is nearly an integral number of cycle times,
   and three are required if not.  Flows are assigned to a class of
   service only until the amount of data to be transmitted in one cycle
   would exceed the cycle time for some interface.  Every packet dwells
   either two or three cycles at each hop, so the calculation of worst-
   case latency and latency variation is trivial.

8.2.  Time Scheduled Queuing

   [IEEE8021Qbv] specifies a time-aware queue-draining procedure for
   transmission selection at egress port of a relay node, which supports
   up to eight traffic classes.  Each traffic class has a separate
   queue, frame transmission from each queue is allowed or prevented by
   a time gate.  This time gate controlled scheduling allows time-
   sensitive traffic classes to transmit on dedicate time slots.  Within
   the time slots, the transmitting flows can be granted exclusive use
   of the transmission medium.  Generally, this time-aware scheduling is
   a layer 2 time division multiplexing (TDM) technique.





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   Consider the static configuration of a deterministic network.  To
   provide end-to-end latency guaranteed service, network nodes can
   support time-based behavior, which is determined by gate control list
   (GCL).  GCL defines the gate operation, in open or closed state, with
   associated timing for each traffic class queue.  A time slice with
   gate state "open" is called transmission window.  The time-based
   traffic scheduling must be coordinated among the relay nodes along
   the path from sender to receiver, to control the transmission of
   time-sensitive traffic.

   Ideally all network devices are time synchronized and static GCL
   configurations on all devices along the routed path are coordinated
   to ensure that length of transmission window fits the assigned
   frames, and no two time windows for DetNet traffic on the same port
   overlap.  (DetNet flows' windows can overlap with best-effort
   windows, so that unused DetNet bandwidth is available to best-effort
   traffic.)  The processing delay, link delay and output delay in
   transmitting are considered in GCL computation.  Transmission window
   for a certain flow may require that a time offset on consecutive hops
   be selected to reduce queueing delay as much as possible.  In this
   case, TSN/DetNet frames transmit at the assigned transmission window
   at every node through the routed path, with zero congestion loss and
   bounded end-to-end latency.  Then, the worst-case end-to-end latency
   of flow can be derived from GCL configuration.  For a TSN or DetNet
   frame, denote the transmission window on last hop closes at
   gate_close_time_last_hop.  Assuming talker supports scheduled traffic
   behavior, it starts the transmission at gate_open_time_on_talker.
   Then worst case end-to-end delay of this flow is bounded by
   gate_close_time_last_hop - gate_open_time_on_talker +
   link_delay_last_hop.

   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 TSN/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 classes, typically using up to seven priority classes and at
   least one best effort class.

9.  Parameters for the bounded latency model

9.1.  Sender parameters








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9.2.  Relay system parameters

   [[NWF This section talks about the parameters that must be used hop-
   by-hop by a resource reservation protocol.]]

10.  References

10.1.  Normative References

   [I-D.ietf-detnet-architecture]
              Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", draft-ietf-
              detnet-architecture-08 (work in progress), September 2018.

   [I-D.ietf-detnet-dp-sol-ip]
              Korhonen, J. and B. Varga, "DetNet IP Data Plane
              Encapsulation", draft-ietf-detnet-dp-sol-ip-00 (work in
              progress), July 2018.

   [I-D.ietf-detnet-dp-sol-mpls]
              Korhonen, J. and B. Varga, "DetNet MPLS Data Plane
              Encapsulation", draft-ietf-detnet-dp-sol-mpls-00 (work in
              progress), July 2018.

   [I-D.ietf-detnet-use-cases]
              Grossman, E., "Deterministic Networking Use Cases", draft-
              ietf-detnet-use-cases-19 (work in progress), October 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>.

   [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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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>.

   [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>.

   [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>.






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   [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/>.

   [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>.

   [le_boudec_theory_2018]
              J.-Y. Le Boudec, "A Theory of Traffic Regulators for
              Deterministic Networks with Application to Interleaved
              Regulators", <http://arxiv.org/abs/1801.08477/>.

   [NetCalBook]
              Le Boudec, Jean-Yves, and Patrick Thiran, "Network
              calculus: a theory of deterministic queuing systems for
              the internet", 2001, <https://arxiv.org/abs/1804.10608/>.

   [Specht2016UBS]
              J. Specht and S. Samii, "Urgency-Based Scheduler for Time-
              Sensitive Switched Ethernet Networks",
              <https://ieeexplore.ieee.org/abstract/document/7557870>.

   [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/>.

Authors' Addresses










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   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


   Jiayi Zhang
   Huawei Technologies Co. Ltd
   Q22, 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







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   Janos Farkas
   Ericsson
   Konyves Kalman krt. 11/B
   Budapest  1097
   Hungary

   Email: janos.farkas@ericsson.com












































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