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DetNet Bounded Latency
draft-ietf-detnet-bounded-latency-10

Document Type Active Internet-Draft (detnet WG)
Authors Norman Finn , Jean-Yves Le Boudec , Ehsan Mohammadpour , Jiayi Zhang , Balazs Varga
Last updated 2022-06-02 (Latest revision 2022-04-08)
Replaces draft-finn-detnet-bounded-latency
Stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
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OPSDIR Last Call Review Incomplete, due 2022-02-08
Stream WG state Submitted to IESG for Publication
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draft-ietf-detnet-bounded-latency-10
DetNet                                                           N. Finn
Internet-Draft                               Huawei Technologies Co. Ltd
Intended status: Informational                            J-Y. Le Boudec
Expires: 10 October 2022                                 E. Mohammadpour
                                                                    EPFL
                                                                J. Zhang
                                             Huawei Technologies Co. Ltd
                                                                B. Varga
                                                                Ericsson
                                                            8 April 2022

                         DetNet Bounded Latency
                  draft-ietf-detnet-bounded-latency-10

Abstract

   This document presents a timing model for sources, destinations, and
   DetNet transit nodes.  Using the model, it provides a methodology to
   compute end-to-end latency and backlog bounds for various queuing
   methods.  The methodology can be used by the management and control
   planes and by resource reservation algorithms to provide bounded
   latency and zero congestion loss for the DetNet 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 10 October 2022.

Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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  . . . . . . . . . . . . .   5
       3.1.2.  Dynamic latency-calculation . . . . . . . . . . . . .   6
     3.2.  Relay node model  . . . . . . . . . . . . . . . . . . . .   7
   4.  Computing End-to-end Delay Bounds . . . . . . . . . . . . . .   9
     4.1.  Non-queuing delay bound . . . . . . . . . . . . . . . . .   9
     4.2.  Queuing delay bound . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Per-flow queuing mechanisms . . . . . . . . . . . . .  11
       4.2.2.  Aggregate queuing mechanisms  . . . . . . . . . . . .  11
     4.3.  Ingress considerations  . . . . . . . . . . . . . . . . .  12
     4.4.  Interspersed DetNet-unaware transit nodes . . . . . . . .  13
   5.  Achieving zero congestion loss  . . . . . . . . . . . . . . .  13
   6.  Queuing techniques  . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Queuing data model  . . . . . . . . . . . . . . . . . . .  15
     6.2.  Frame Preemption  . . . . . . . . . . . . . . . . . . . .  17
     6.3.  Time-Aware Shaper . . . . . . . . . . . . . . . . . . . .  17
     6.4.  Credit-Based Shaper with Asynchronous Traffic Shaping . .  18
       6.4.1.  Delay Bound Calculation . . . . . . . . . . . . . . .  20
       6.4.2.  Flow Admission  . . . . . . . . . . . . . . . . . . .  21
     6.5.  Guaranteed-Service IntServ  . . . . . . . . . . . . . . .  22
     6.6.  Cyclic Queuing and Forwarding . . . . . . . . . . . . . .  23
   7.  Example application on DetNet IP network  . . . . . . . . . .  24
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  26
   9.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  27
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  27
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  27
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     12.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

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1.  Introduction

   The ability for IETF Deterministic Networking (DetNet) or IEEE 802.1
   Time-Sensitive Networking [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;

      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 notably characterized by

   1.  a maximum bandwidth, guaranteed either by the transmitter or by
       strict input metering;

   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.  It is assumed in this document that the paths of DetNet flows
   are fixed.  Before the transmission of a DetNet flow, it is possible
   to calculate end-to-end latency bounds and the amount of buffer space
   required at each hop to ensure zero congestion loss; this can be used
   by the applications identified in [RFC8578].

   This document presents a timing model for sources, destinations, and
   the DetNet transit nodes; using this model, it provides a methodology
   to compute end-to-end latency and backlog bounds for various queuing
   mechanisms that can be used by the management and control planes to
   provide DetNet qualities of service.  The methodology used in this
   document account for the possibility of packet reordering within a
   DetNet node.  The bounds on the amount of packet reordering is out of
   the scope of this document and can be found in
   [PacketReorderingBounds].  Moreover, this document references
   specific queuing mechanisms, mentioned in [RFC8655], as proofs of
   concept that can be used to control packet transmission at each
   output port and achieve the DetNet quality of service.

   Using the model presented in this document, it is possible for an
   implementer, user, or standards development organization to select a
   set of queuing mechanisms for each device in a DetNet network, and to
   select a resource reservation algorithm for that network, so that

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   those elements can work together to provide the DetNet service.
   Section 7 provides an example application of the timing model
   introduced in this document on a DetNet IP network with a 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.

2.  Terminology and Definitions

   This document uses the terms defined in [RFC8655].  Moreover, the
   following terms are used in this document:

   T-SPEC
      TrafficSpecification as defined in Section 5.5 of [RFC9016].

   arrival curve
      An arrival curve function alpha(t) is an upper bound on the number
      of bits seen at an observation point within any time interval t.

   CQF
      Cyclic Queuing and Forwarding.

   CBS
      Credit-based Shaper.

   TSN
      Time-Sensitive Networking.

   PREOF
      A collective name for Packet Replication, Elimination, and
      Ordering Functions.

   Packet Ordering Function (POF)
      A function that reorders packets within a DetNet flow that are
      received out of order.  This function can be implemented by a
      DetNet edge node, a DetNet relay node, or an end system.

3.  DetNet bounded latency model

3.1.  Flow admission

   This document assumes that the following paradigm is used to admit
   DetNet flows:

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   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 the above 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
           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 latency 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.

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   This calculation may be more difficult to perform than the dynamic
   calculation (Section 3.1.2), because the DetNet flows passing through
   one port on a DetNet transit node affect each other's 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.

   Dynamic latency-calculation can be done based on the static one
   described in Section 3.1.1; when a new DetNet flow is created or
   deleted, the entire calculation for all DetNet flows is repeated.  If
   an already-established DetNet flow would be pushed beyond its latency
   requirements by the new DetNet flow request, then the new DetNet flow
   request can be refused, or some other suitable action taken.

   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
   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).

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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.

   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

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      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 regulator) as shown in
      Figure 1.  This delay can be variable, and depends on the details
      of the operation of the forwarding node.

   5.  Regulator delay
      A regulator, also known as shaper in [RFC2475], delays some or all
      of the packets in a traffic stream in order to bring the stream
      into compliance with an arrival curve; an arrival curve 'alpha(t)'
      is an upper bound on the number of bits observed within any
      interval t.  The regulator delay 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.

   In this analysis, the measurement is from the point at which a packet
   is selected for output in a node to the point at which it is selected
   for output in the next downstream node (that is the definition of a
   "hop").  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

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   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:

   *  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.

   *  Variations in processing delay (4) require additional output
      buffers in the queues of that same DetNet transit node.  Depending
      on the details of the queuing 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 latency 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 latency bound is less than the sum
   of the per-hop latency bounds.  An end-to-end latency 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
   upper-bound 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
   [RFC9016].  Then set non_queuing_delay_bound = the sum of per-
   hop_non_queuing_delay_bound[h] over all hops h.

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   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 information on the arrival curve 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.  Note that arrival curve of DetNet flow at source is
   immediately specified by the T-SPEC of this flow.  The computation of
   queuing_delay_bound is described in Section 4.2 as a separate
   section.

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.

   The computation of per-hop queuing delay bounds must account for the
   fact that the arrival curve 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.  If a regulator is placed at a
   hop, an arrival curve of a DetNet flow at the entrance of the queuing
   subsystem of this hop is the one configured at the regulator (also
   called shaping curve in [NetCalBook]); otherwise, an arrival curve of
   the flow can be derived using the delay-jitter of the flow from the
   last regulation point (the last regulator in the path of the flow if
   there is any, otherwise the source of the flow) to the ingress of the
   hop; more formally, assume a DetNet flow has arrival curve at the
   last regulation point equal to 'alpha(t)', and the delay-jitter from
   the last regulation point to the ingress of the hop is 'V'.  Then,
   the arrival curve at the ingress of the hop is 'alpha(t+V)'.

   For example, consider a DetNet flow with T-SPEC "Interval: tau,
   MaxPacketsPerInterval: K, MaxPayloadSize: L" at source.  Then, a
   leaky-bucket arrival curve for such flow at source is alpha(t)=r * t+
   b, t>0; alpha(0)=0, where r is the rate and b is the bucket size,
   computed as

      r = K * (L+L') / tau,

      b = K * (L+L').

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   where L' is the size of any added networking technology-specific
   encapsulation (e.g., MPLS label(s), UDP, and IP headers).  Now, if
   the flow has delay-jitter of 'V' from the last regulation point to
   the ingress of a hop, an arrival curve at this point is r * t + b + r
   * V, implying that the burstiness is increased by r*V.  A more
   detailed information on arrival curves is available in [NetCalBook].

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 latency bound
   as well as an end-to-end latency 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.  An
   instance of per-flow queuing is IntServ's Guaranteed-Service, for
   which the details of latency bound calculation are presented 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
   [CharnyDelay][BennettDelay].  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 ([SpechtUBS], [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 [LeBoudecTheory].  Specifically, when an
   interleaved regulator is appended to a FIFO subsystem, it does not
   increase the worst-case delay of the latter; in Figure 1, when the
   order of packets from output of queuing subsystem at node A to the

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   entrance of regulator at node B is preserved, then the regulator does
   not increase the worst-case latency bounds; this is made possible if
   all the systems are FIFO or a DetNet packet-ordering function (POF)
   is implemented just before the regulator.  This property does not
   hold if packet reordering occurs from the output of a queuing
   subsystem to the entrance of next downstream interleaved regulator,
   e.g., at a non-FIFO switching fabric.

   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: If packet reordering does not occur, the end-to-end latency
   bound calculation provided here gives a tighter latency upper-bound
   than would be obtained by adding the latency 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 latency 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, this
   simply requires added 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-unaware 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 latency 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.  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.

   Let

   *  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.

   *  nb_input_ports be the number input ports that send traffic to this
      output port

   *  max_packet_length be the maximum packet size for packets that may
      be sent to this output port.  This is counted in data units.

   *  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)

   The above bound is over the backlog caused by the traffic entering
   the queue from the input ports of a DetNet node.  If the DetNet node
   also generates packets (e.g., creation of new packets, replication of
   arriving packets), the bound must accordingly incorporate the
   introduced backlog.

6.  Queuing techniques

   In this section, we present a general queuing data model as well as
   some examples of queuing mechanisms.  For simplicity of latency bound
   computation, we assume leaky-bucket arrival curve for each DetNet
   flow at source.  Also, at each DetNet transit node, the service for
   each queue is abstracted with a minimum guaranteed rate and a latency
   [NetCalBook].

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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 FIFO.
   They 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) are performed before
   the DetNet packets enter the regulators.  All Packets are assigned to
   a set of queues.  Packets compete for the selection 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:

   *  Discarding packets because a queue is full.

   *  Discarding packets marked "yellow" by a metering function, in
      preference to discarding "green" packets [RFC2697].

   Ideally, neither of these actions are performed on DetNet packets.
   Full queues for DetNet packets occurs 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 the information from the non-exhaustive list below to assign
   a packet to a particular queue:

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   *  Input port.

   *  Selector based on a rotating schedule that starts at regular,
      time-synchronized intervals and has nanosecond precision.

   *  MAC addresses, VLAN ID, IP addresses, Layer 4 port numbers, DSCP
      [RFC8939], [RFC8964].

   *  The queue assignment function can contain metering and policing
      functions.

   *  MPLS and/or pseudo-wire labels [RFC6658].

   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 bounds can be calculated by the
   methods provided in the following sections that account for the
   effect 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 closed.  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 must satisfy the requirement of
   latency upper bounds of all DetNet flows; therefore, those DetNet
   flows that traverse a network that uses this kind of shaper must have
   bounded latency, if the traffic and nodes are conformant.

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   Note 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 [Sch8021Qbv].  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 this queuing model, it is assumed that the DetNet nodes are FIFO.
   We consider 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 DetNet flows that are less critical than CDT (such as studio
   audio and video traffic, as in IEEE 802.1BA Audio-Video-Bridging).
   This model is a subset of Time-Sensitive Networking as described
   next.

   Based on the timing model described in Figure 1, 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.  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 service requirements; 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 class A and B
   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 subsystem; there is no regulator for CDT and BE flows.

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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 a
   Credit-Based Shaper (CBS).  The CBS serves a packet from a class
   according to the available credit for that class.  As described in
   Section 8.6.8.2 and Annex L.1 of [IEEE8021Q], the credit for each
   class A or B increases based on the idle slope (as guaranteed rate),
   and decreases based on the sendslope (typically equal to the
   difference between the guaranteed and the output link rates), both of
   which are parameters of the CBS.  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 classes A and B traffic can be provided
   only if CDT traffic is bounded; it is assumed that the CDT traffic
   has a 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 classes A and B flows are also
   regulated at their source according to a leaky bucket arrival curve.
   At the source, the traffic satisfies its regulation constraint, i.e.,
   the delay due to interleaved regulator at the source is ignored.

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   At each DetNet transit node 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 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 where all clocks are perfect.  However, in
   reality there is clock non-ideality throughout 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.
   [ThomasTime] describes and provides solutions to this issue.

6.4.1.  Delay Bound Calculation

   A delay bound of the queuing subsystem ((4) in Figure 1) of a given
   DetNet node for a 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 I_A is the idle slope for class A; 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; r_h is the rate and b_h is the bucket size of
   CDT traffic leaky bucket arrival curve.

   If the flow is of class B:

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      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)

   where I_B is the idle slope for class B; L_A is the maximum packet
   length of class A; L_BE is the maximum packet length of class BE.

   Then, as discussed in Section 4.2.2; an interleaved regulator does
   not increase the delay bound of the upstream queuing subsystem;
   therefore an end-to-end delay bound for a DetNet flow of class X (A
   or B) is the sum of d_X_i for all node i in the path the flow, where
   d_X_i is the delay bound of queuing subsystem in node i which is
   computed as above.  According to the notation in Section 4.2.2, the
   delay bound of queuing subsystem in a node i and interleaved
   regulator in node j, i.e., Cij, is:

      Cij = d_X_i

   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)
   must be known.  To admit a flow/flows of classes A and B, their delay
   requirements must 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 must be
   guaranteed.  Thus,

   The static admission control:
           The leaky bucket parameters of all class A or B flows are
           known, therefore, for each class A or B flow f, a delay bound
           can be calculated.  The computed delay bound for every class
           A or B flow must not be more than its delay requirement.
           Moreover, the sum of the rate of each flow (r_f) must not be
           more than the rate allocated to each class (R).  If these two
           conditions hold, the configuration is declared admissible.

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   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 bucket
           size (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:

              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 class A or B 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.  A class A or
           B 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 a class A or B 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.  The value of R must be set such that

      R <= I_X*(c-r_h)/c

   where I_X is the idleslope of credit-based shaper for class X={A,B},
   c is the transmission rate of the output link and r_h is the leaky-
   bucket rate of the CDT class.

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].

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   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).

   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 details are 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 cycle time T_c must be carefully chosen; it needs to be large
   enough to accommodate all the DetNet traffic, plus at least one
   maximum packet (or fragment) size from lower priority queues, which
   might be received within a 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.  The value of DT guarantees that the

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   last packet of one cycle in a node is fully delivered to a buffer of
   the next node in 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.

   The per-hop latency is determined by the cycle time T_c: a packet
   transmitted from a node at a cycle (i), is transmitted from the next
   node at cycle (i+1).  Then, if the packet traverses h hops, the
   maximum latency experienced by the packet is from the beginning of
   cycle (i) to the end of cycle (i+h); also, the minimum latency is
   from the end of cycle (i) before the DT, to the beginning of cycle
   (i+h).  Then, the maximum latency is:

      (h+1) T_c

   and the minimum latency is:

      (h-1) T_c + DT.

   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 the timing model
   presented in this document to control the admission of a DetNet flow
   on a DetNet-enabled IP network.  Consider Figure 5, taken from
   Section 3 of [RFC8939], that shows a simple IP network:

   *  The end-system 1 implements Guaranteed-Service IntServ as in
      Section 6.5 between itself and relay node 1.

   *  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.

   *  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.

   *  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 [RFC9023]).

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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 centralized 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 reject the DetNet flow.

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   As soon as the control plane selects a path that satisfies the delay
   bound constraint, it allocates and reserves the resources in the path
   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
   [RFC9055], 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 or change the behavior of some resources within
   the boundary of the DetNet domain.

   Latency bound calculations use parameters that reflect physical
   quantities.  If an attacker finds a way to change the physical
   quantities, unknown to the control and management planes, the latency
   calculations fail and may result in latency violation and/or
   congestion losses.  An example of such attacks is to make some
   traffic sources under the control of the attacker send more traffic
   than their assumed T-SPECs.  This type of attack is typically avoided
   by ingress conditioning at the edge of a DetNet domain.  However, it
   must be insured that such ingress conditioning is done per-flow and
   that the buffers are segregated such that if one flow exceeds its
   T-SPEC, it does not cause buffer overflow for other flows.

   Some queuing mechanisms require time synchronization and operate
   correctly only if the time synchronization works correctly.  In the
   case of CQF, the correct alignments of cycles can fail if an attack
   against time synchronization fools a node into having an incorrect
   offset.  Some of these attacks can be prevented by cryptographic
   authentication as in Annex K of [IEEE1588] for the Precision Time
   Protocol (PTP).  However, the attacks that change the physical
   latency of the links used by the time synchronization protocol are
   still possible even if the time synchronization protocol is protected
   by authentication and cryptography [DelayAttack].  Such attacks can
   be detected only by their effects on latency bound violations and
   congestion losses, which do not occur in normal DetNet operation.

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9.  IANA considerations

   This document has no IANA actions.

10.  Acknowledgement

   We would like to thank Lou Berger, Tony Przygienda, John Scudder,
   Watson Ladd, Yoshifumi Nishida, Ralf Weber, Robert Sparks, Gyan
   Mishra, Martin Duke, Eric Vyncke, Lars Eggert, Roman Danyliw, and
   Paul Wouters for their useful feedback on this document.

11.  Contributors

   RFC 7322 limits the number of authors listed on the front page to a
   maximum of 5.  The editor wishes to thank and acknowledge the
   following author for contributing text to this document

      Janos Farkas
      Ericsson
      Email: janos.farkas@ericsson.com

12.  References

12.1.  Normative References

   [IEEE8021Q]
              IEEE 802.1, "IEEE Std 802.1Q-2018: IEEE Standard for Local
              and metropolitan area networks - Bridges and Bridged
              Networks", 2018,
              <https://ieeexplore.ieee.org/document/8403927>.

   [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>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [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>.

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   [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>.

   [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>.

   [RFC9016]  Varga, B., Farkas, J., Cummings, R., Jiang, Y., and D.
              Fedyk, "Flow and Service Information Model for
              Deterministic Networking (DetNet)", RFC 9016,
              DOI 10.17487/RFC9016, March 2021,
              <https://www.rfc-editor.org/info/rfc9016>.

12.2.  Informative References

   [BennettDelay]
              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>.

   [CharnyDelay]
              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>.

   [DelayAttack]
              S. Barreto, A. Suresh, and J.-Y. Le Boudec, "Cyber-attack
              on packet-based time synchronization protocols: The
              undetectable Delay Box",
              <https://ieeexplore.ieee.org/document/7520408>.

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   [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-
              01", <https://datatracker.ietf.org/doc/html/draft-ietf-
              detnet-controller-plane-framework>.

   [IEEE1588] IEEE Std 1588-2008, "IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems", 2008,
              <https://ieeexplore.ieee.org/document/4579760>.

   [IEEE8021Qcr]
              IEEE 802.1, "IEEE P802.1Qcr: Bridges and Bridged Networks
              - Amendment: Asynchronous Traffic Shaping", 2017,
              <https://1.ieee802.org/tsn/802-1qcr/>.

   [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>.

   [LeBoudecTheory]
              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://leboudec.github.io/netcal/>.

   [PacketReorderingBounds]
              E. Mohammadpour, and J.-Y. Le Boudec, "On Packet
              Reordering in Time-Sensitive Networks",
              <https://ieeexplore.ieee.org/document/9640523>.

   [RFC2697]  Heinanen, J. and R. Guerin, "A Single Rate Three Color
              Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999,
              <https://www.rfc-editor.org/info/rfc2697>.

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   [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>.

   [RFC9023]  Varga, B., Ed., Farkas, J., Malis, A., and S. Bryant,
              "Deterministic Networking (DetNet) Data Plane: IP over
              IEEE 802.1 Time-Sensitive Networking (TSN)", RFC 9023,
              DOI 10.17487/RFC9023, June 2021,
              <https://www.rfc-editor.org/info/rfc9023>.

   [RFC9055]  Grossman, E., Ed., Mizrahi, T., and A. Hacker,
              "Deterministic Networking (DetNet) Security
              Considerations", RFC 9055, DOI 10.17487/RFC9055, June
              2021, <https://www.rfc-editor.org/info/rfc9055>.

   [Sch8021Qbv]
              S. Craciunas, R. Oliver, M. Chmelik, and W. Steiner,
              "Scheduling Real-Time Communication in IEEE 802.1Qbv Time
              Sensitive Networks",
              <https://dl.acm.org/doi/10.1145/2997465.2997470>.

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

   [ThomasTime]
              L. Thomas and J.-Y. Le Boudec, "On Time Synchronization
              Issues in Time-Sensitive Networks with Regulators and
              Nonideal Clocks",
              <https://dl.acm.org/doi/10.1145/3393691.3394206>.

   [TSNwithATS]
              E. Mohammadpour, E. Stai, M. Mohiuddin, and J.-Y. Le
              Boudec, "Latency and Backlog Bounds in Time-Sensitive
              Networking with Credit Based Shapers and Asynchronous
              Traffic Shaping",
              <https://ieeexplore.ieee.org/document/8493026>.

Authors' Addresses

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   Norman Finn
   Huawei Technologies Co. Ltd
   3101 Rio Way
   Spring Valley, California 91977
   United States of America
   Phone: +1 925 980 6430
   Email: nfinn@nfinnconsulting.com

   Jean-Yves Le Boudec
   EPFL
   IC Station 14
   CH-1015 Lausanne EPFL
   Switzerland
   Email: jean-yves.leboudec@epfl.ch

   Ehsan Mohammadpour
   EPFL
   IC Station 14
   CH-1015 Lausanne EPFL
   Switzerland
   Email: ehsan.mohammadpour@epfl.ch

   Jiayi Zhang
   Huawei Technologies Co. Ltd
   Q27, No.156 Beiqing Road
   Beijing
   100095
   China
   Email: zhangjiayi11@huawei.com

   Balázs Varga
   Ericsson
   Budapest
   Konyves Kálmán krt. 11/B
   1097
   Hungary
   Email: balazs.a.varga@ericsson.com

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