DetNet                                                      Shaofu. Peng
Internet-Draft                                                       ZTE
Intended status: Standards Track                               Peng. Liu
Expires: 22 December 2024                                   China Mobile
                                                         Kashinath. Basu
                                               Oxford Brookes University
                                                              Aihua. Liu
                                                                     ZTE
                                                              Dong. Yang
                                             Beijing Jiaotong University
                                                             Guoyu. Peng
                      Beijing University of Posts and Telecommunications
                                                            20 June 2024


               Timeslot Queueing and Forwarding Mechanism
             draft-peng-detnet-packet-timeslot-mechanism-07

Abstract

   IP/MPLS networks use packet switching (with the feature store-and-
   forward) and are based on statistical multiplexing.  Statistical
   multiplexing is essentially a variant of time division multiplexing,
   which refers to the asynchronous and dynamic allocation of link
   timeslot resources.  In this case, the service flow does not occupy a
   fixed timeslot, and the length of the timeslot is not fixed, but
   depends on the size of the packet.  Statistical multiplexing has
   certain challenges and complexity in meeting deterministic QoS, and
   its delay performance is dependent on the used queueing mechanism.
   This document further describes a generic time division multiplexing
   scheme in IP/MPLS networks, which we call timeslot queueing and
   forwarding (TQF) mechanism.  TQF is an enhancement based on TSN TAS,
   which defines a cyclic period consisting of multiple timeslots, and a
   flow is assigned to be transmited within one or more dedicated
   timeslots.

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/.






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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   6
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Timeslot Mapping Relationship . . . . . . . . . . . . . . . .   9
     4.1.  Deduced by BTM  . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Deduced by BOM  . . . . . . . . . . . . . . . . . . . . .  13
   5.  Resources Used by TQF . . . . . . . . . . . . . . . . . . . .  15
   6.  Arrival Postion in the Orchestration Period . . . . . . . . .  15
   7.  Residence Delay Evaluation  . . . . . . . . . . . . . . . . .  18
     7.1.  Residence Delay on the Ingress Node . . . . . . . . . . .  18
     7.2.  Residence Delay on the Transit Node . . . . . . . . . . .  19
     7.3.  Residence Delay on the Egress Node  . . . . . . . . . . .  20
     7.4.  End-to-end Delay and Jitter . . . . . . . . . . . . . . .  20
   8.  Flow States in Data-plane . . . . . . . . . . . . . . . . . .  21
   9.  Queue Allocation Rule of Round Robin Queue  . . . . . . . . .  21
   10. Queue Allocation Rule of PIFO Queue . . . . . . . . . . . . .  23
     10.1.  PIFO with On-time Scheduling Mode  . . . . . . . . . . .  23
     10.2.  PIFO with In-time Scheduling Mode  . . . . . . . . . . .  24
   11. Global Timeslot ID  . . . . . . . . . . . . . . . . . . . . .  25
   12. Multiple Orchestration Periods  . . . . . . . . . . . . . . .  28
   13. Admission Control on the Headend  . . . . . . . . . . . . . .  30
   14. Frequency Synchronization . . . . . . . . . . . . . . . . . .  31
   15. Evaluations . . . . . . . . . . . . . . . . . . . . . . . . .  32
     15.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . .  34



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   16. Taxonomy Considerations . . . . . . . . . . . . . . . . . . .  37
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
   18. Security Considerations . . . . . . . . . . . . . . . . . . .  38
   19. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   20. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     20.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     20.2.  Informative References . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

   IP/MPLS networks use packet switching (with the feature store-and-
   forward) and are based on statistical multiplexing.  The discussion
   of supporting multiplexing in the network was first seen in the time
   division multiplexing (TDM), frequency division multiplexing (FDM)
   and other technologies of telephone communication network (using
   circuit switching).  Statistical multiplexing is essentially a
   variant of time division multiplexing, which refers to the
   asynchronous and dynamic allocation of link resources.  In this case,
   the service flow does not occupy a fixed timeslot, and the length of
   the timeslot is not fixed, but depends on the size of the packet.  In
   contrast, synchronous time division multiplexing means that a
   sampling frame (or termed as time frame) includes a fixed number of
   fixed length timeslots, and the timeslot at a specific position is
   allocated to a specific service.  The utilization rate of link
   resources in statistical multiplexing is higher than that in
   synchronous time division multiplexing.  However, if we want to
   provide deterministic end-to-end delay in packet switched networks
   based on statistical multiplexing, the difficulty is greater than
   that in synchronous time division multiplexing.  The main challenge
   is to obtain a deterministic upper bound on the queueing delay, which
   is closely related to the queueing mechanism used in the network.

   In addition to IP/MPLS network, other packet switched network
   technologies, such as ATM, also discusses how to provide
   corresponding transmission quality guarantee for different service
   types.  Before service communication, ATM needs to establish a
   connection to reserve virtual path/channel resources, and use fixed-
   length short cells and timeslots.  The advantage of short cell is
   small interference delay, but the disadvantage is low encoding
   efficiency.  The mapping relationship between ATM cells and timeslots
   is not fixed, so it still depends on a specific cells scheduling
   mechanism (such as [ATM-LATENCY]) to ensure delay performance.
   Although the calculation of delay performance based on short and
   fixed-length cells is more concise than that of IP/MPLS networks
   based on variable length packets, they all essentially depend on the
   queueing mechanism.




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   [TAS] introduces a synchronous time-division multiplexing method
   based on gate control list (GCL) rotation in Ethernet LAN.  Its basic
   idea is to calculate when the packets of the service flow arrive at a
   certain node, then the node will turn on the green light (i.e., the
   transmission state is set to OPEN) for the corresponding queue
   inserted by the service flow at that time duration, which is defined
   as TimeInterval between two adjacent items in gating cycle.  The
   TimeInterval is exactly the timeslot resource that can be reserved
   for service flow.  A set of queues is controlled by the GCL, with
   round robin per gating cycle.  The gating cycle (e.g, 250 us)
   contains a lot of items, and each item is used to set the OPEN/CLOSED
   states of all traffic class queues.  By strictly controlling the
   release time of service flow at the network entry node, multiple
   flows always arrive sequentially during each gating cycle at the
   intermediate node and are sent during their respective fixed timeslot
   to avoid conflicts, with extremely low queueing delay.  However, the
   GCL state (i.e., items set, and different TimeInterval value between
   any two adjacent items) is related with all ordered flows that passes
   through the node.  Calculating and installing GCL states separately
   on each node has scalability issues.

   [CQF] introduces a synchronous time-division multiplexing method
   based on fixed-length cycle in Ethernet LAN.  [ECQF] is a further
   enhancement of the classic CQF.  CQF with 2-buffer mode or ECQF with
   x-bin mode only uses a small number of cycles to establish the cycle
   mapping between a port-pair of two adjacent nodes, which is
   independent of the individual service flow.  The cycle mapping may be
   maintained on each node and swaped based on a single cycle id carried
   in the packet during forwarding ([I-D.eckert-detnet-tcqf]), or all
   cycle mappings are carried in the packet as a cycle stack and read
   per hop during forwarding
   ([I-D.chen-detnet-sr-based-bounded-latency]).  According to [ECQF],
   how many cycles (i.e., x-bin mode) are required depends on the
   proportion of the variation in intra-node forwarding delay relative
   to the cycle size.  If the proportion is small, 3-bin is enough,
   otherwise, more than 3 bins needed.  Compared to TAS, CQF/ECQF no
   longer maintains GCL on each node, but instead replaces the large
   number of variable length of timeslots related to service flows in
   GCL with a small number of fixed length cycles unrelated to service
   flows.  Thus, CQF/ECQF simplifies the data plane, but leaves the
   complexity to the control plane, by calculating and controlling the
   release time of service flow at the network entry, to guarantee no
   conflicts between flows in any cycle on any intermediate nodes.

   In order to meet the large scaling requirements, this document
   describes a scheduling mechanism for enhancing TAS.  It defines a
   cyclic period consisting of multiple timeslots that share limited
   buffer resources, and a flow is assigned to be transmited within one



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   or more dedicated timeslots.  It does not rely on time
   synchronization, but needs to detect and maintain the phase
   difference of cyclic period between adjacent nodes.  It further
   defines two scheduling modes: on-time or in-time mode.  We call this
   mechanism as Timeslot Queueing and Forwarding (TQF), as a supplement
   to IEEE 802.1 TSN TAS.  In TQF, The selected length of the cyclic
   period (i.e., gating cycle of TAS) depends on the length of the
   supported service burst interval.

   Similar to TAS and CQF/ECQF, TQF is also TDM based scheduling
   mechanisms.

   *  Compared to classic TAS, TQF may use round robin queues
      corresponding to the count of timeslots during gating cycle, while
      TAS only maintains queues corresponding to the number of traffic
      classes and one of them is used for the Scheduled Traffic (i.e.,
      DetNet flows).  That means TQF need more queues than TAS (i.e.,
      multiple timeslot queues vs single traffic class queue).  However,
      TAS needs to use other complex methods to control the arrival
      order of all flows sharing the same traffic class queue to isolate
      them (so that each flow faces almost zero queuing delay), while
      TQF's timeslot queue naturally isolates flows by timeslot id of
      gating cycle.  And, TQF with in-time scheduling mode may use a
      single PIFO (put in first out) queue to approximate the ultra-low
      delay of TAS.

   *  Compared to CQF/ECQF, TQF on-time scheduling maintains round robin
      queues corresponding to the count of timeslots during gating
      cycle, while CQF/ECQF maintains extra tolerating queues depending
      on the proportion of the variation in intra-node forwarding delay
      relative to the cycle size.  There is no gating cycle with its
      timeslot resources designed by CQF/ECQF, it needs to use
      additional flow interleaving method to control the arrival order
      of flows sharing the same cycle queue to isolate flows (or
      alternatively tolerate overprovision), while TQF's timeslot queue
      naturally isolates flows by timeslot id of gating cycle.  This is
      also the semantic difference between cycle id and timeslot id,
      where the former is used to indicate the NO. of the aggregated
      queues such as sending, receiving, or tolerating queue, rather
      than indicating the individual timeslot resource within the gating
      cycle like the later.  That is, after defining timeslot resources
      in IP/MPLS, TQF does not limit the implementations of the data
      structure type corresponding to timeslot resources on the data
      plane, which may be round robin queues, or a single PIFO queue.







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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology

   The following terminology is introduced in this document:

   Timeslot:  The unit of TQF scheduling.  It needs to design a
       reasonable value, such as 10us, to send at least one complete
       packet.  Different nodes can be configured with different length
       of timeslot.

   Timeslot Scheduling:  The packet is stored in the buffer zone
       corresponding to a specific timeslot id, and may be sent before
       (in-time mode) or within (on-time mode) that timeslot.  The
       timeslot id is always a NO. from the orchestration period.

   Service Burst Interval:  The traffic specification of DetNet flow
       generally follows the principle of generating a specific burst
       amounts within a specific length of periodic burst interval.  For
       example, a service generates 1000 bits of burst per 1 ms, where 1
       ms is the service burs interval.

   Orchestration Period:  The orchestration period is a cyclic period
       and used to instantiate timeslot resources on the link.  The
       selection of orchestration period length depends on the service
       burst interval of DetNet flows, e.g., the Least Common Multiple
       of service burst intervals of all flows.  It is actually the
       gating cycle in TAS, just with different queue allocation rules.
       It contains a fixed count (termed as N and numbered from 0 to
       N-1) of timeslots.  For example, the orchestration period include
       1000 timeslots and each timeslot length is 10 us.  Multiple
       orchestration period length may be enabled on the link, but all
       nodes included in the DetNet path must interoperate based on the
       same orchestration period length.  A specific orchestration
       period length can be used to indicate the corresponding TQF
       forwarding instance.

   Ongoing Sending Period:  The orchestration period which the ongoing
       sending timeslot belongs to.

   Scheduling Period:  The scheduling period of a TQF forwarding




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       instance depends on the hardware's buffer resources that is
       supported by the device.  Its length reflects the count of the
       timeslot resources (termed as M and numbered from 0 to M-1) with
       related buffer resources that is actually instantiated on the
       data plane, which is limited by hardware capabilities.
       Scheduling period length may be less than or equal to
       orchestration period length in the case of on-time mode, or
       larger than or equal to orchestration period length in the case
       of in-time mode.  Packets belonging to a specific timeslot (in
       orchestration period) will be mapped and stored in the buffer
       zone of the corresponding timeslot of the scheduling period.

   Incoming Timeslot:  For the headend of the DetNet path, the current
       timeslot of UNI at which a flow arriving and after being policing
       is the incoming timeslot.  For any intermediate node of the
       DetNet path, the timeslot contained in the packet received from
       the upstream node (i.e., the outgoing timeslot of the upstream
       node) is the incoming timeslot.  An incoming timeslot id is the
       timeslot in the context of the orchestration period.

   Outgoing Timeslot:  When sending a packet to the outgoing port,
       according to resource reservation or certain rules, it chooses to
       send packet in the specified timeslot of that port, which is the
       outgoing timeslot.  An outgoing timeslot id is the timeslot in
       the context of the orchestration period.

   Ongoing Sending Timeslot:  When the end of the incoming timeslot to
       which the packet belongs reaches a specific port, the timeslot
       currently in the sending state is the ongoing sending timeslot of
       that port.  Note that the ongoing sending timeslot is different
       with the outgoing timeslot.  An ongoing sending timeslot id is
       the timeslot in the context of the orchestration period.

3.  Overview

   This scheme introduces the time-division multiplexing scheduling
   mechanism based on the fixed length timeslot in the IP/MPLS network.
   Note that the time-division multiplexing here is a L3 packet-level
   scheduling mechanism, rather than the TDM port (such as SONET/SDH)
   implemented in L1.  The latter generally involves the time frame and
   the corresponding framing specification, which is not necessary in
   this document.  The data structure associated with timeslot resources
   may be implemented using round robin queues, or a single PIFO queue,
   etc.

   Figure 1 shows the TQF scheduling behavior implemented by the
   intermediate node P through which a deterministic path passes.




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             incoming slots:
                      i,j,k
       +---+                 +---+                 +---+
       |PE1| --------------- | P | --------------- |PE2|
       +---+                 +---+                 +---+

                                orchestration period (OP)
                            +---+---+-+-+---+---------+---+
                            | 0 | 1 | 2 | 3 | ... ... |N-1|
                            +---+---+---+---+---------+---+
                                   ^
                    Outgoing slots |
                    a,b,c @OP      |
     DetNet path ------------------o------------------->
                                   |\
                                   | \    (rank by a,b,c @OP)
                     access slots: |  \-----------------------+
                     a',b',c' @SP  v                          |
                        /  +-------------------+    __        v
                        |  |  queue-0 @slot 0  |   /  \     +---+
                        |  +-------------------+  |    |    +---+
                        |  |  queue-1 @slot 1  |  |    |    +---+
            Scheduling <   +-------------------+  |         +---+
            Period (SP) |  |  ... ...          |  |    ^    +---+
                        |  +-------------------+  |    |    +---+
                        |  |  queue-n @slot M-1|   \__/     +---+
                        \  +-------------------+            +---+
                            (Round Robin Queue)             (PIFO)


               Figure 1: Timeslot Based Scheduling Mechanism

   Where, both the orchestration period and the scheduling period
   consist of multiple timeslots, the number of timeslots supported by
   orchestration period is related to the length of the service burst
   interval, while the number of timeslots supported by scheduling
   period is limited by hardware capabilities, and it may be
   instantiated by a Round Robin queue or PIFO.

   A TQF enabled link may configure multiple TQF scheduling instances
   each with specific orchestration period length.  Nodes communicate
   with each other based on the same instance.









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   Each TQF scheduling instance related to a specific orchestration
   period length may configure its service rate, and the sum of service
   rates of all instances must not exceed the port bandwidth.  For a TQF
   scheduling instance, the total amount of bits that can be consumed in
   each timeslot is generally not exceeding the result of the service
   rate multiplied by the timeslot length.

   For each orchestration period length, all nodes in the network does
   not require phase alignment.  The phase difference of orchestration
   period between adjacent nodes should be detected.

   For a specific orchestration period configured on different links in
   the network, these links may configure different timeslot lengths
   because their capabilities vary, for example, the link capability of
   the edge nodes is weaker than that of core nodes.

   In Figure 1, a DetNet path consumes timeslots i, j, k from the
   orchestration period of the link PE1-P, and a, b, c from the
   orchestration period of the link P-PE2 respectively.  From node P's
   perspective, i, j, k are incoming timeslots, while a, b, c are
   outgoing timeslots.  The cross connection between an incoming
   timeslot and an outgoing timeslot will result in the corresponding
   residence delay, which depends on the offset between the incoming and
   outgoing timeslots based on the phase difference of the orchestration
   periods of link PE1-P and P-PE2.

   An outgoing timeslot in the orchestration period will finally access
   the mapped timeslot in the scheduling period.  There is a mapping
   function from the timeslot z in the orchestration period to the
   timeslot z' in the scheduling period, i.e., z' = f(z).  For example,
   the mapping function may be z' = z, z' = z + offset, z' = z % M, and
   z' = random(z), etc.  Which function to use depends on the queue
   allocation rule and data structure instantiated for timeslot
   resources.  In this document, we mainly discuss two mapping function,
   z' = z % M (in the case of round robin queue), and z' = z (in the
   case of PIFO).

   How to calculate a DetNet path that meets the flow requirements is
   not within the scope of this document.

4.  Timeslot Mapping Relationship

   In order to determine the offset between the incoming timeslot and
   the outgoing timeslot in the context of specific TQF scheduling
   instance that is identified by orchestration period length, it is
   necessary to first determine the ongoing sending timeslot that the
   incoming timeslot falls into, i.e., the mapping relationship between
   the incoming timeslot and the ongoing sending timeslot.



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   Two methods are provided in the following sub-sections to determine
   the mapping relationship between the incoming timeslot and the
   ongoing sending timeslot.

4.1.  Deduced by BTM

   Figure 2 shows that there are three nodes U, V, and W in turn along
   the path.  All nodes are configured with the same orchestration
   period length (termed as OPL), which is crucial for establishing a
   fixed timeslot mapping relationship between the adjacent nodes.

   *  Port_u2 has timeslot length L_u2, and an orchestration period
      contains N_u2 timeslots.

   *  Port_v1 has timeslot length L_v1, and an orchestration period
      contains N_v1 timeslots.

   *  Port_v2 has timeslot length L_v2, and an orchestration period
      contains N_v2 timeslots.

   Hence, L_u2*N_u2 = L_v1*N_v1 = L_v2*N_v2.  In general, the link
   bandwidth of edge nodes is small, and they will be configured with a
   larger timeslot length than the aggregated/backbone nodes.

   It has been mathematically proven that if the least common multiple
   of L_u# and L_v# is LCM, OPL is also a multiple of LCM.

   Node U may send a detection packet from the end (or head, the process
   is similar) of an arbitrary timeslot i of port_u2 connected to node
   V.  After a certain link propagation delay (D_propagation), the
   packet is received by the incoming port of node V, and i is regarded
   as the incoming timeslot by V.  At this time, the ongoing sending
   timeslot of port_v1 is j, and there is time T_ij left before the end
   of the timeslot j.

   This mapping relationship is termed as:

   *  <instance OPL, port_u2 slot i, port_v1 slot j, T_ij>

   To avoid confusion, we refer to this mapping relationship as the base
   timeslot mapping (BTM), as it is independent of the DetNet flows.
   Later, we will see the timeslot mapping relationship related to
   DetNet flow, which is the mapping relationship between the outgoing
   timeslot of port_u2 and the outgoing timeslot of port_v2, which is
   based on timeslot resource reservation and termed as the forwarding
   timeslot mapping (FTM).





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   BTM is generally maintained by node V when processing probe message
   received from node U.  However, node U may also obtain this
   information from node V, e.g, by an ACK message.  The advantage of
   maintaining BTM by node U is that it is consistent with the
   unidirectional link from node U to V, so it is more appropriate for
   node U (rather than V) to advertise it in the network.  A BTM
   detection method can be found in [I-D.xp-ippm-detnet-stamp], and the
   advertisement method can be found in
   [I-D.peng-lsr-deterministic-traffic-engineering].

   Note that this document does not recommend directly detecting and
   maintaining BTM between the outgoing timeslot of port_u2 and the
   ongoing sending timeslot of port_v2 (i.e., the outgoing port of
   downstream node V), as this is too trivial.  In fact, as shown above,
   maintaining only BTM between the outgoing timeslot of port_u2 and the
   ongoing sending timeslot of port_v1 (i.e., the incoming port of
   downstream node V) is sufficient to derive other mapping
   relationships.

































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     | port_u1
   Node U
     | port_u2
     |
     | |<---------------------- OP of port_u2 -------------------->|
     | +-----------+---+-------------------------------------------+
     | |  ... ...  | i |                ... ...                    |
     | +-----------+---+-------------------------------------------+
     | (departured from port_u2)
     |                 |
     |                  \ (link delay)
     |                   v
     |   |<---------------------- OP of port_u2 -------------------->|
     |   +-----------+---+-------------------------------------------+
     |   |  ... ...  | i |                 ... ...                   |
     |   +-----------+---+-------------------------------------------+
     |    (arrived at port_v1)
     |                   |
     |                   |<-T_ij-->|
     |                   v (i map to j)
     |     +-----------+-----------+-----------------------------------+
     |     |  ... ...  |     j     |              ... ...              |
     |     +-----------+-----------+-----------------------------------+
     |     |<--------------------- OP of port_v1 --------------------->|
     | port_v1            \
     v                     |
   Node V                  |
     |                      \ (intra-node forwarding delay)
     | port_v2               v
     |     +---------------+-------+-----------------------------------+
     |     |    ... ...    |   j'  |              ... ...              |
     |     +---------------+-------+-----------------------------------+
     |     |<--------------------- OP of port_v2 --------------------->|
     |
     | port_w1
     v
   Node W

                          Figure 2: BTM Detection

   Based on the above detected BTM, and knowing the intra-node
   forwarding delay (F) including parsing, table lookup, internal fabric
   exchange, we can derive BTM between any outgoing timelot x of port_u2
   and the ongoing timeslot y of port_v2.

   Let t is the offset between the end of the timeslot x of port_u2 and
   the beginning of the orchestration period of the port_v2.




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   *  t = ((j'+1)*L_v1 - T_ij' + OPL + (x-i)*L_u2 + F) % OPL

   Then,

   *  y = [t/L_v2]

   And the time T_xy left before the end of the timeslot y is:

   *  T_xy = (y+1)*L_v2 - t

   This document recommends that the time of each port within the same
   node must be synchronized, that is, all ports of a node share the
   same local system time, which is easy to achieve.  It is also
   recommended that the begin time of the orchestration period for all
   ports within the same node be the same or differ by an integer
   multiple of OPL, e.g, maintaining a global initial time as the
   logical begin time for the first round of orchestration period for
   all ports.  Whether node restart or port restart, this initial time
   should continue to take effect to avoid affecting the timeslot
   mapping relationship between each node.  Depending on the
   implementation, considering that the initial time may be a historical
   time that is too far away from the current system time, regular
   updates may be made to it (e.g, self increasing k*OPL, where k is a
   natural number) to be closer to the current system time.

4.2.  Deduced by BOM

   Figure 3 shows that there are three nodes U, V, and W in turn along
   the path.  Similar to Section 4.1, it still has L_u2*N_u2 = L_v1*N_v1
   = L_v2*N_v2.

   Node U may send a detection packet from the head (or end, the process
   is similar) of the orchestration period of port_u2 connected to node
   V.  After a certain link propagation delay (D_propagation), the
   packet is received by the incoming port of node V.  At this time,
   there is time P_uv left before the end of the ongoing sending period
   of port_v1.

   This mapping relationship is termed as:

   *  <instance OPL, port_u2, port_v1, P_uv>

   We refer to this mapping relationship as the base orchestration-
   period mapping (BOM), which it is independent of the DetNet flows.

   BOM is generally maintained by node V when processing probe message
   received from node U.  However, node U may also obtain this
   information from node V, e.g, by an ACK message.  The advantage of



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   maintaining BOM by node U is that it is consistent with the
   unidirectional link from node U to V, so it is more appropriate for
   node U (rather than V) to advertise it in the network.  A BOM
   detection method can be found in [I-D.xp-ippm-detnet-stamp], and the
   advertisement method can be found in
   [I-D.peng-lsr-deterministic-traffic-engineering].

     | port_u1
   Node U
     | port_u2
     |
     | |<---------------------- OP of port_u2 -------------------->|
     | +-----------+---+-------------------------------------------+
     | |  ... ...  |   |                  ... ...                  |
     | +-----------+---+-------------------------------------------+
     | (departured from port_u2)
     |  \
     |   \ (link delay)
     |    \
     |     |<---------------------- OP of port_u2 -------------------->|
     |     +-----------+---+-------------------------------------------+
     |     |  ... ...  |   |                  ... ...                  |
     |     +-----------+---+-------------------------------------------+
     |     (arrived at port_v1)
     |     |
     |     |<---------------------- P_uv -------------------------->|
     |     v
     |  +-----------+-----------+-----------------------------------+
     |  |  ... ...  |           |              ... ...              |
     |  +-----------+-----------+-----------------------------------+
     |  |<--------------------- OP of port_v1 --------------------->|
     | port_v1
     v      \
   Node V    \ (intra-node forwarding delay)
     |        \
     | port_v2 \
     |  +---------------+-------+-----------------------------------+
     |  |    ... ...    |       |              ... ...              |
     |  +---------------+-------+-----------------------------------+
     |  |<--------------------- OP of port_v2 --------------------->|
     |
     | port_w1
     v
   Node W


                          Figure 3: BOM Detection




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   Based on BOM, and knowing the intra-node forwarding delay (F), we can
   derive the mapping relationship between any outgoing timelot x of
   port_u2 and the ongoing timeslot y of port_v2.

   Let t is the offset between the end of the timeslot x of port_u2 and
   the beginning of the orchestration period of the port_v2.

   *  t = ((x+1)*L_u2 + OPL - P_uv + F) % OPL

   Then,

   *  y = [t/L_v2]

   And the time T_xy left before the end of the timeslot y is:

   *  T_xy = (y+1)*L_v2 - t

5.  Resources Used by TQF

   The operation of TQF scheduling mechanism will consume two types of
   resources:

   *  Bandwidth: Each TQF scheduling instance may configure its service
      rate that is a dedicated bandwidth resource from the outgoing
      port, and the sum of service rates of all instances must not
      exceed the port bandwidth.

   *  Burst: The burst resources of a specific TQF scheduling instance
      can be represented as the corresponding bit amounts of all
      timeslots included in the orchestration period.  The total amount
      of bits that can be consumed by flows in each timeslot is
      generally not exceeding the result of the service rate multiplied
      by the timeslot length.

6.  Arrival Postion in the Orchestration Period

   Generally, a DetNet flow has its TSpec, such as periodically
   generating traffic of a specific burst size within a specific length
   of burst interval, which regularly reaches the network entry.  The
   headend executes traffic regulation (e.g, setting appropriate
   parameters for leaky bucket shaping), which generally make packets
   evenly distributed within the service burst interval, i.e, there are
   one or more shaped sub-burst in the service burst interval.  There is
   an ideal positional relationship between the departure time (when
   each sub-burst leaves the regulator) and the orchestration period of
   UNI port, that is, each sub-burst corresponds to an ideal incoming
   timeslot of UNI port.  Based on the ideal incoming timeslot, an ideal
   outgoing timeslot of NNI port may be selected and consumed by the



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   sub-burst.

   For example, if a DetNet flow distributes m sub-bursts during the
   orchestration period, the network entry should maintain m states for
   that flow:

   *  <OPL, ideal incoming slot i_1, ideal outgoing slot z_1>

   *  <OPL, ideal incoming slot i_2, ideal outgoing slot z_2>

   *  ... ...

   *  <OPL, ideal incoming slot i_m, ideal outgoing slot z_m>

   However, the packets arrived at the network entry are not always
   ideal, and the departure time from regulator may not be in a certain
   ideal incoming timeslot.  Therefore, an important operation that
   needs to be performed by the network entry is to determine the ideal
   incoming timeslot i based on the actual departure time.  This can
   first determine the actual incoming timeslot based on the actual
   departure time, and then select an ideal incoming timeslot that is
   closest to the actual incoming timeslot and not earlier than the
   actual incoming timeslot.

   Figure 4 shows, for some typical DetNet flows, the ideal incoming
   timeslots in the orchestration period of UNI, as well as the ideal
   outgoing timeslots of NNI consumed by these DetNet flows.
























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               |<--------------------- OPL ---------------------->|
               +----+----+----+----+----+----+----+----------+----+
               | #0 | #1 | #2 | #3 | #4 | #5 | #6 |  ... ... |#N-1|
               +----+----+----+----+----+----+----+----------+----+

                     +--+
    Flow 1:    |     |b1|                                         |
               +-----+--+-----------------------------------------+
               |<------------------- SBI ------------------------>|


                            +--+                        +--+
    Flow 2:    |            |b1|                        |b2|
               +------------+--+------------------------+--+------+
               |<------------------- SBI ------------------------>|


                                           +------+
    Flow 3:    |                           |  b1  |
               +---------------------------+------+---------------+
               |<------------------- SBI ------------------------>|


                    +--+             +--+             +--+
    Flow 4:    |    |b1|        |    |b1|        |    |b1|        |
               +----+--+--------+----+--+--------+----+--+--------+
               |<----- SBI ---->|<----- SBI ---->|<----- SBI ---->|

                 Figure 4: Relationship between SBI and OP

   As shown in the figure, the service burst interval of flows 1, 2, 3
   is equal to the orchestration period length, while the service burst
   interval of flow 4 is only 1/3 of the orchestration period length.

   *  Flow 1 generates a small single burst within its burst interval,
      which may consume timeslot 2 or other subsequent timeslot of NNI;

   *  Flow 2 generates two small discrete sub-bursts within its burst
      interval and also be shaped, which may respectively consume slots
      4 and N-1 of NNI;

   *  Flow 3 generates a large single burst within its burst interval
      but not be really shaped (due to purchasing a larger burst
      resource and served by a larger bucket depth), which may also be
      split to multiple back-to-back sub-bursts and consume multiple
      consecutive timeslots, such as 8 and 9 of NNI.





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   *  The service burst interval of flow 4 is only 1/3 of the
      orchestration period.  Hence, construct flow 4' with 3 occurrence
      of the flow 4 within an orchestration period.  So flow 4' is
      similar to flow 2, generating three separate sub-bursts within its
      burst interval.  It may consume timeslots 3, 7, and N-1 of NNI.

7.  Residence Delay Evaluation

7.1.  Residence Delay on the Ingress Node

   On the headend H, the received flow corresponds to an ideal incoming
   timeslot i of UNI port.  Although there is actually no timeslot
   mapping relationship established between the end-system and the
   headend, we can still assume that the end-system applies the same
   orchestration period as UNI, and the BOM with phase aligned is
   detected.  Then, according to Section 4, for the above incoming
   timeslot i, the ongoing sending timeslot j of NNI port, as well as
   the remaining time T_ij of timeslot j , can be deduced.

   An outgoing timeslot z of NNI, which offset o (>=1) timeslots from j,
   can be selected for the flow.  That is, z = (j+o)%N_h2, where N_h2 is
   the number of timeslots in the orchestration period of NNI.

   Thus, on the headend H the residence delay obtained from the outgoing
   timeslot z is:

      Best Residence Delay = F + T_ij + (o-1)*L_h2

      Worst Residence Delay = F + L_h1 + T_ij + o*L_h2

      Average Residence Delay = F + T_ij + (L_h1 + (2o-1)*L_h2)/2

      where, L_h1 is the timeslot length of UNI port, L_h2 is the
      timeslot length of NNI port.

   The best residence delay occurs when the flow arrived at the end of
   the ideal incoming timeslot i, and sent at the head of the outgoing
   timeslot z.

   The worst residence delay occurs when the flow arrived at the head of
   the ideal incoming timeslot i, and sent at the end of the outgoing
   timeslot z.

   The delay jitter within the headend is (L_h1 + L_h2).  However, the
   jitter of the entire path is not the sum of the jitters of all nodes.






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   Note that there is a runtime jitter, as mentioned earlier, which
   depends on the deviation between the actual incoming timeslot i' and
   the ideal incoming timeslot i.  Assuming that i = (i'+e)%N_h1, where
   e is the deviation, N_h1 is the number of timeslots in the
   orchestration period of UNI port, then the additional runtime jitter
   is e*L_h1, that should be carried in the packet to eliminate jitter
   at the network egress.

7.2.  Residence Delay on the Transit Node

   On the transit node V, according to Section 4, for any given incoming
   timeslot i, the ongoing sending timeslot j of the outgoing port
   (port_v2), as well as the remaining time T_ij of timeslot j, can be
   deduced.

   An outgoing timeslot z of the outgoing port, which offset o (>=1)
   timeslots from j, can be selected for the flow.  That is, z =
   (j+o)%N_v2, where N_v2 is the number of timeslots in the
   orchestration period of port_v2.

   Thus, on the transit node V the residence delay obtained from the
   outgoing timeslot z is:

      Best Residence Delay = F + T_ij + (o-1)*L_v2

      Worst Residence Delay = F + T_ij + L_u2 + o*L_v2

      Average Residence Delay = F + T_ij + (L_u2+(2o-1)*L_v2)/2

      where, L_u2 and L_v2 is the timeslot length of port_u2 and port_v2
      respectively.

   The best residence delay occurs when the flow is received at the end
   of the incoming timeslot i and sent at the head of the outgoing
   timeslot z.

   The worst residence delay occurs when the flow is received at the
   head of the incoming timeslot i and sent at the end of the outgoing
   timeslot z.

   The delay jitter within the node is (L_u2 + L_v2).  However, the
   jitter of the entire path is not the sum of the jitters of all nodes.









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7.3.  Residence Delay on the Egress Node

   Generally, for the deterministic path carrying the DetNet flow, the
   flow needs to continue forwarding from the outgoing port of the
   egress node to the client side, and also faces the issues of
   queueing.  However, the outgoing port facing the client side is the
   part of the overlay routing.  It is possible to continue supporting
   TQF mechanism on that port.  In this case, the underlay DetNet path
   will serve as a virtual link of the overlay path, providing a
   deterministic delay performance.

   Therefore, for the underlay deterministic paths, the residence dalay
   on the egress node is only contributed by the forwarding delay (F)
   including parsing, table lookup, internal fabric exchange, etc.

7.4.  End-to-end Delay and Jitter

   Figure 5 shows that a path from headend P1 to endpoint E, for each
   node Pi, the timeslot length of the outgoing port is L_i, the intra-
   node forwarding delay is F_i, the remaining time of the mapped
   ongoing sending timeslot is T_i, the number of timeslots offset by
   the outgoing timeslot relative to the ongoing sending timeslot is
   o_i, especially on node P1 the timeslot length of UNI is L_h, then
   the end to end delay can be evaluted as follows (not including link
   propagation delay):

      Best E2E Delay = sum(F_i+T_i+o_i*L_i, for 1<=i<=n) - L_n + F_e

      Worst E2E Delay = sum(F_i+T_i+o_i*L_i, for 1<=i<=n) + L_h + F_e

       +---+     +---+     +---+             +---+     +---+
       | P1| --- | P2| --- | P3| --- ... --- | Pn| --- | E |
       +---+     +---+     +---+             +---+     +---+


                       Figure 5: TQF Forwarding Path

   The best E2E delay occurs when the flow arrived at the end of the
   ideal incoming timeslot and sent at the head of the outgoing timeslot
   of each node pi.  The worst E2E delay occurs when the flow arrived at
   the head of the ideal incoming timeslot and sent at the end of the
   outgoing timeslot of each node Pi.  The E2E delay jitter is (L_h +
   L_n).








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8.  Flow States in Data-plane

   The headend of the path needs to maintain the timeslot resource
   information with the granularity of sub-burst of each flow, so that
   each sub-burst of the DetNet flow can access the mapped timeslot
   resources.  However, the intermediate node does not need to maintain
   states per flow, but only access the timeslot resources based on the
   timeslot id carried in the packets.

   [I-D.pb-6man-deterministic-crh], [I-D.p-6man-deterministic-eh]
   defined methods to carry the stack of timeslot id in the IPv6
   packets.

9.  Queue Allocation Rule of Round Robin Queue

   TQF may organize its buffer resources in the form of fixed number of
   round robin queues.  In this case, only on-time scheduling mode is
   considered.  In-time scheduling mode may cause urgent and non urgent
   packets to be stored in the same queue.

   The number of round robin queues should be designed according to the
   number of timeslots included in the scheduling period.  Each timeslot
   corresponds to a separate queue, in which the buffered packets must
   be able to be sent within a timeslot.

   The length of the queue, i.e., the total number of bits that can be
   sent for a timeslot, equals to the allocated bandwidth of the
   corresponding TQF instance (see Section 12) multiplied by the
   timeslot length.

   Figure 1 shows that the scheduling period actually instantiated on
   the data plane is not completely equivalent to the orchestration
   period.  The scheduling period includes M timeslots (from 0 to M-1),
   while the orchestration period includes N timeslots (from 0 to N-1).
   N is an integer multiple of M.  In the orchestration period, from
   timeslot 0 to M-1 is the first scheduling period, from timeslot M to
   slot 2M-1 is the second scheduling period, and so on.  Therefore, it
   is necessary to convert the outgoing timeslot of the orchestration
   period to the target timeslot of the scheduling period, and insert
   the packet to the round robin queue corresponding to the target
   timeslot for transmission.

   A simple conversion method is:

   *  target scheduling timeslot = outgoing timeslot % M






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   This is safe when o < M is always followed, where o is the number of
   offset timeslots between the outgoing timeslot z and the ongoing
   sending timeslot j (please refer to Section 7).

   Next, we briefly demonstrate that the sub-burst that arrives at the
   outgoing port during the ongoing sending timeslot (j) can be safely
   inserted into the corresponding queue in the scheduling period mapped
   by the outgoing timeslot z, and that queue will not overflow.

   Assuming that each timeslot in the orchestration period has a virtual
   queue, for example, termed the virtual queue corresponding to the
   outgoing timeslot z as queue-z, the packets that can be inserted into
   queue-z may only come from the following flows:

      During the ongoing sending timeslot j = (z-M+1+N)%N, the flows
      that arrive at the outgoing port, that is, these flows may consume
      the outgoing timeslot (z) according to o = M-1.

      During the ongoing sending timeslot j = (z-M+2+N)%N, the flows
      that arrive at the outgoing port, that is, these flows may consume
      the outgoing timeslot (z) according to o = M-2.

      ... ...

      During the ongoing sending timeslot j = (z-1+N)%N, the flows that
      arrive at the outgoing port, that is, these flows may consume the
      outgoing timeslot (z) according to o = 1;

      The total consumed burst resources of all these flows does not
      exceed the burst resource of the outgoing timeslot (z).

   Then, when the ongoing sending timeslot changes to z, queue-z will be
   sent and cleared.  In the following time, starting from timeslot z+1
   to the last timeslot N-1, there are no longer any packets inserted
   into queue-z.  Obviously, this virtual queue is a great waste of
   queue resources.  In fact, queue-z can be reused by the subsequent
   outgoing timeslot (z+M)%N.  Namely:

      During the ongoing sending timeslot j = (z+1)%N, the flows that
      arrive at the outgoing port, that is, these flows may consume the
      outgoing timeslot (z+M)%N according to o = M-1.

      During the ongoing sending timeslot j = (z+2)%N, the flows that
      arrive at the outgoing port, that is, these flows may consume the
      outgoing timeslot (z+M)%N according to o = M-2.

      ... ...




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      During the ongoing sending timeslot j = (z+M-1)%N, the flows that
      arrive at the outgoing port, that is, these flows may consume the
      outgoing timeslot (z+M)%N according to o = 1.

      The total consumed burst resources of all these flows does not
      exceed the burst resource of the outgoing timeslot (z+M)%N.

   It can be seen that queue-z can be used by any outgoing timeslot
   (z+k*M)%N, where k is a non negative integer.  By observing
   (z+k*M)%N, it can be seen that the minimum z satisfies 0<= z< M, that
   is, the entire orchestration period actually only requires M queues
   to store packets, which are the queues corresponding to M timeslots
   in the scheduling period.  That is to say, the minimum z is the
   timeslot id in the scheduling period, while the outgoing timeslot
   (z+k*M)% N is the timeslot id in the orchestration period.  The
   latter obtains the former by moduling M, which can then access the
   queue corresponding to the former.  In short, the reason why a queue
   can store packets from multiple outgoing timeslots without being
   overflowed is that the packets stored in the queue earlier (more than
   M timeslots ago) have already been sent.

   For example, if the total length of all queues supported by the
   hardware is 4G bytes, the queue length corresponding to a timeslot of
   10us at a port rate of 100G bps is 1M bits, then a maximum of 32K
   timeslot queues can be provided, and TQF scheduler can use some of
   the queue resources, e.g., M may be 1K queues to construct a
   scheduling periold with 10 ms, and the corresponding orchestration
   period may be several 10ms.

10.  Queue Allocation Rule of PIFO Queue

   TQF may also organize its buffer resources in the form of PIFO queue.
   In this case, both in-time and on-time scheduling mode can be easily
   supported, because packets with different rank always ensure
   scheduling order.

10.1.  PIFO with On-time Scheduling Mode

   In the case of on-time mode, the buffer cost of PIFO queue is the
   same as that of round robin queues.  It can directly use the begin
   time of the outgoing timeslot z as the rank of the packet and insert
   the packet into the PIFO for transmission.

   *  rank = z.begin

   Here, the outgoing timeslot z refers to the outgoing timeslot z that
   is after the arrival time at the scheduler and closest to the arrival
   time.



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   The rule of the on-time scheduling mode is that if the PIFO is not
   empty and the rank of the head of queue is equal to or earlier than
   the current system time, the head of queue will be sent; otherwise,
   not.

10.2.  PIFO with In-time Scheduling Mode

   In the case of in-time mode, the buffer cost of PIFO queue is
   generally larger than that of on-time mode due to burst accumulation.
   [SP-LATENCY] provides guidance for evaluating excess buffer
   requirements.

   Similar to Section 10.1, it can directly use the begin time of the
   outgoing timeslot z as the rank of the packet and insert the packet
   into the PIFO for transmission.  However, due to in-time scheduling
   behavior, the outgoing timeslot z may not be the outgoing timeslot z
   that is after the arrival time at the scheduler and closest to the
   arrival time, instead, it may be an outgoing timeslot z far away from
   the arrival time.

   A time deviation (E) may be carried in the packet to help determine
   the outgoing timeslot z.

   On the headend node:

   *  E initially equals to the begin time of the ideal incoming
      timeslot minus the actual departure time from the regulator.

   *  Use the result of "departure time + E" (note that it is just the
      begin time of the ideal incoming timeslot, and the main purpose
      here is to describe how E works) to determine the expected
      outgoing timeslot z that is after this result and closest to this
      result.

   *  rank = z.begin

   *  When the packet leaves the headend, E is updated to z.begin minus
      the actual sending time from the PIFO.  The updated E will be
      carried in the sending packet.

   On the transit node:

   *  Obtain E from the received packet.

   *  Use the result of "arrival time + E" to determine the expected
      outgoing timeslot z that is after this result and closest to this
      result.  Here, the arrival time is the time that the packet
      arrived at the scheduler.



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   *  rank = z.begin

   *  When the packet leaves the headend, E is updated to z.begin minus
      the actual sending time from the PIFO.  The updated E will be
      carried in the sending packet.

   The rule of the in-time scheduling mode is that as long as the PIFO
   is not empty, packets are always obtained from the head of queue for
   transmission.

   In summary, the in-time scheduling with the help of time deviation
   (E), can suffer from the uncertainty caused by burst accumulation,
   and it is recommended only deployed in small networks, i.e., a
   limited domain with a small number of hops, where the burst
   accumulation issue is not serious; The on-time scheduling is
   recommended to be used in large networks.

11.  Global Timeslot ID

   The outgoing timeslots discussed in the previous sections are local
   timeslots style for all nodes.  This section discusses the situation
   based on global timeslot style.

   Global timeslot style refers to that all nodes in the path are
   identified with the same timeslot id, which of course requires all
   nodes to use the same timeslot length.  There is no need to establish
   FTM for the DetNet flow on each node or carry FTM in packets.  The
   packet only needs to carry the unique global timeslot id.  However,
   the disadvantage is that the latency performance of the path may be
   large, which depends on BOM between the adjacent nodes.  Another
   disadvantage is that the success rate of finding a path that matches
   the service requirements is not as high as local timeslot style.

   Global timeslot style requires that the orchestration period is equal
   to the scheduling period, mainly considering that arrival packets
   with any global timeslot id can be successfully inserted into the
   corresponding queue without overflow.  However, as the ideal design
   goal is to keep the scheduling period less than the orchestration
   period, further research is needed on other methods (such as
   basically aligning orchestration period between nodes), to ensure
   that packets with any global timeslot id can queue normally when the
   scheduling period is less than the orchestration period.

   Compared to the local timeslot style, global timeslot style means
   that the incoming timeslot i must map to the outgoing timeslot i too.






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   As the example shown in Figure 6, each orchestration period contains
   6 timeslots.  Node V has three connected upstream nodes U1, U2, and
   U3.  During each hop forwarding, the packet accesses the outgoing
   timeslot corresponding to the global timeslot id and forwards to the
   downstream node with the global timeslot id unchanged.  For example,
   U1 sends some packets with global slot-id 0, termed as g0, in the
   outgoing timeslot 0.  The packets with other global slot-id 1~5 are
   similarly termed as g1~g5 respectively.  The figure shows the
   scheduling results of these 6 batches of packets sent by upstream
   nodes when node V continues to send them.

         0   1   2   3   4   5   0   1   2   3   4   5   0   1   2
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   U1  | g0| g1| g2|   |   |   |   |   |   |   |   |   |   |   |   |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


          1   2   3   4   5   0   1   2   3   4   5   0   1   2   3
        +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   U2   |   |   | g3| g4|   |   |   |   |   |   |   |   |   |   |   |
        +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


         5   0   1   2   3   4   5   0   1   2   3   4   5   0   1
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   U3  | g5|   |   |   |   |   |   |   |   |   |   |   |   |   |   |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


           0   1   2   3   4   5   0   1   2   3   4   5   0   1   2
         +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   V     |   |   |   | g3| g4| g5| g0| g1| g2|   |   |   |   |   |   |
         +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


                  Figure 6: Global Timeslot Style Example

   In this example:

   *  BTM between the outgoing timeslot of U1 and the ongoing sending
      timeslot of V is i -> i, so the global outgoing timeslot for the
      incoming timeslot i is i+6 (i.e., belongs to next round of
      orchestration periold).

   *  BTM between the outgoing timeslot of U2 and the ongoing sending
      timeslot of V is i -> i-1, so the global outgoing timeslot for the
      incoming timeslot i is i (i.e., belongs to current round of
      orchestration periold).



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   *  BTM between the outgoing timeslot from U3 and the ongoing sending
      timeslot of V is i -> i+1, so the global outgoing timeslot for the
      incoming timeslot i is i+6 (i.e., belongs to next round of
      orchestration periold).

   It can be seen that packets from U1 and U3 has large residency delay
   in the node V, while packets from U2 has small residency delay in the
   node V.

   It should be noted that if round robin queue is used, for the orginal
   BTM i -> i (example of U1), or i -> i+1 (example of U3), the packets
   need to be stored in a buffer prior to the TQF scheduler (such as the
   buffer on the input port side) for a fixed latency (such as serveral
   timeslots) and then released to the scheduler.  Otherwise, directly
   inserting the queue may cause jitter, i.e., part of the packets
   belonging to the same incoming timeslot i can be sent in the outgoing
   timeslot i, while the other part of the packets has to be delayed to
   be sent in the next round of timeslot i.  This fixed-latency buffer
   is only introduced for specific upstream nodes.  It can be determined
   according to the initial detection result of BTM between the adjacent
   nodes.  If the original BTM is i -> i or i -> i+1, it needed,
   otherwise not.  After the introduction of fixed-latency buffer, the
   new detection result of BTM will no longer be i -> i, or i -> i+1.

   If PIFO queue is used, there is no need to introduce a fixed-latency
   buffer because in this case, rank = i.begin + OPL, and it will not be
   scheduled to be sent in the current outgoing timeslot i, but in the
   next round.  However, in this case the PIFO itself serves as a fixed-
   latency buffer.

   For the headend, the residence delay is similar to Section 7.1.  For
   a flow which has the ideal incoming timeslot i, it may select a
   global outgoing timeslot z based on the BTM i -> j, where, j is the
   ongoing sending timeslot of the ougtoing port, and the timeslot
   offset o equals (N+z-j)%N.

   For transit nodes, the residence delay is similar to Section 7.2, in
   addition to considering possible latency contributed by the above
   fixed-latency buffer.  For a flow with the global incoming timeslot
   z, it still select the global outgoing timeslot z based on the BTM z
   -> j, where, j is the ongoing sending timeslot of the ougtoing port,
   and the timeslot offset o equals (N+z-j)%N.

   The end-to-end delay equation is similar to Section 7.4, in addition
   to considering possible cumulated latency contributed by the above
   fixed-latency buffer.





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12.  Multiple Orchestration Periods

   A single orchestration period may not be able to cover a wide range
   of service needs, such as some with a burst interval of microseconds,
   while others have a burst interval of minutes or even larger.  When
   using a single orchestration period to simultaneously serve these
   services, the timeslot length must be microseconds, but the
   orchestration period length is minutes or more, resulting in the need
   to include a large number of timeslots in the orchestration period.
   The final result is a proportional increase in the buffer size
   required for the scheduling period.

   Multiple orchestration periods each with different length may be
   provided by the network.  A TQF enabled link can be configured with
   multiple TQF scheduling instances each corresponding to specific
   orchestration period length.  For simplicity, the orchestration
   period length itself can be used to identify a specific instance.

   For example, one orchestration period length is 300 us, termed as
   OPL-300us, which is the LCM of the burst interval of the set of flows
   served.  Another orchestration period length is 100 ms, termed as
   OPL-100ms, which is the LCM of the burst interval of another set of
   flows served.  Each orchestration period instance has its own
   timeslot length.  The timeslot length of a long orchestration period
   instance should be longer than that of a short orchestration period
   instance, and the former is an integer multiple of the latter.  But
   the long orchestration period itself may not necessarily be an
   integer multiple of the short orchestration period.

   As shown in Figure 7, both link-a and link-b are configured with n
   orchestration period instances, with the corresponding orchestration
   period lengths OPL_1, OPL_2, ..., OPL_n in ascending order.  For each
   orchestration period length OPL_i, the dedicated bandwidth resource
   is BW_U_i for node U (or BW_V_i for node V), and the timeslot length
   is TL_U_i for node U (or TL_V_i for node V).  For each TQF enabled
   link, the sum of dedicated bandwidth resources of all TQF scheduling
   instances must not exceed the total bandwidth of the link.














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       +---+    link-a            +---+    link-b            +---+
       | U | -------------------- | V | -------------------- | W |
       +---+                      +---+                      +---+
              OPL_1:                    OPL_1:
                     TL_U_1                    TL_V_1
                     BW_U_1                    BW_V_1
              OPL_2:                    OPL_2:
                     TL_U_2                    TL_V_2
                     BW_U_2                    BW_V_2

              ... ...                   ... ...

              OPL_n:                    OPL_n:
                     TL_U_n                    TL_V_n
                     BW_U_n                    BW_V_n

                      Figure 7: Multiple TQF Instances

   Due to the fact that long orchestration periods serve DetNet flows
   with large burst intervals, for a given burst size, the larger the
   burst interval, the less bandwidth consumed by the DetNet flow.
   Therefore, it is recommended that the bandwidth resources of the long
   orchestration period is less than that of the short orchestration
   period, which is beneficial for reducing the buffer required for long
   orchestration period.

   Interworking between different nodes is based on the same
   orchestration period.  That means that the timeslot mapping described
   in Section 4 should be maintained in the context of the specific
   orchestration period.  The orchestration period length should be
   carried in the forwarding packets to let the DetNet flow to consume
   the timeslot resources corresponding to the TQF scheduling instance.

   If round robin queues are used, each TQF scheduling instance has its
   own separate queue set.  Time division multiplexing scheduling is
   based on the granularity of the minimum timeslot length of all
   instances.  Within each time unit of this granularity, the queues in
   the sending state of all instances are always scheduled in the order
   of OPL_1, OPL_2, ..., OPL_n.

   If PIFO queue is used, all TQF scheduling instances may share a
   single PIFO queue.  An implementation may use rank (i.e., the
   beginning of the outgoing timeslot z) plus timeslot length to
   determine the insertion order of two packets from different
   instances, so that the packet from the short orchstration period
   inserted at the front.





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13.  Admission Control on the Headend

   On the network entry, traffic regulation must be performed on the
   incoming port, so that the DetNet flow does not exceed its T-SPEC
   such as burst interval, burst size, maximum packet size, etc.  This
   kind of regulation is usually the shaping using leaky bucket combined
   with the incoming queue that receives DetNet flow.  A DetNet flow may
   contain discrete multiple sub-bursts within its periodic burst
   interval.  The leaky bucket depth should be larger than the maximum
   packet size, and should be consistent with the reserved burst
   resources required for the maximum sub-burst.

   The scheduling mechanism described in this document has a requirement
   on the arrival time of DetNet flows on the network entry.  It is
   expected that the distribution of sub-bursts (after regulation) of
   the DetNet flow will always appear in an ideal incoming timeslot of
   UNI port.  Based on this ideal position, an ideal outgoing timeslot
   is selected.  For a single DetNet flow, the network entry may
   maintain multiple forwarding states each containing <ideal incoming
   timeslot, ideal outgoing timeslot>, due to many sub-bursts within the
   service burst interval.

   For example, the network entry may maintain up to 3 sub-burst
   forwarding states for a flow.  Ideally, all packets of this flow are
   split into 3 sub-bursts after regulation, each sub-burst matching one
   of the states.  Here, 3 is the maximum sub-bursts for this flow, and
   it does not always contain so many bursts within the burst interval
   during actual sending.

   For a specific sub-burst, some amount of deviation (i.e., the
   deviation between the actual incoming timeslot and the ideal incoming
   timeslot) is permitted.  Generally, the headend will select an ideal
   incoming timeslot closet to the actual incoming timeslot for the
   packet.

   For on-time scheduling, the position deviation should not exceed o-1
   for late arrival case, or M-o-1 for early arrival case, where o is
   the offset between the outgoing timeslot and ongoing sending timeslot
   as mentioned above.  Intuitively, large o can tolerate large late
   arrival deviations, while small o (or large M even for large o) can
   tolerate large early arrival deviations.

   This position deviation limitation is beneficial for on-time
   scheduling, to achieve the ideal design goal that scheduling period
   is smaller than the orchestration period, and packets can always be
   successfully inserted into the scheduling queue (RR or PIFO) without
   overflow.  For example, there may contain one or more scheduling
   periods between the departure time (from the regulator) and the



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   choosed ideal incoming timeslot, and therefore there is an overflow
   risk when inserting packets into the queue based on the corresponding
   ideal outgoing timeslot z at the departue time.

   Otherwise, for randomly arriving DetNet flows, it can be supported by
   taking a large M (or even M = N) (option-1) to accommodate random
   arrival, or it can be supported by introducing an explicit buffer put
   before the scheduler on the network entry to let the arrival time
   always meet the position deviation limitation (option-2).

   *  Note that due to randomness of arrival time, the packet may just
      miss the scheduling (or arrive too earlier) and need to wait in
      the scheduling queue (in the case of option-1) or the explicit
      buffer (in the case of option-2) for the next orchestration
      period.

   For in-time scheduling, the position deviation should not exceed o-1
   for late arrival case.  We only focus on late arrivals here, as in-
   time scheduling naturally handles early arrivals.  If the late
   arrival exceed the above limitation, the sub-burst may need to be
   sent during the next orchestration period in the worst case, or may
   be lucky to be scheduled immediately.

   Note that the position deviation is a runtime latency during
   forwarding, whether using PIFO or RR.  It should be carried in the
   packet to eliminate jitter at the network egress on demand.  Please
   refer to [I-D.peng-detnet-policing-jitter-control] for the
   elimination of jitter caused by policing delay on the network
   entrance node.  The runtime position deviation should be considered
   as a part of policing delay.

14.  Frequency Synchronization

   The basic explanation for frequency synchronization is that the
   crystal frequency of the hardware is consistent, which enables all
   nodes in the network to be in the same inertial frame and have the
   same time lapse rate.  This is a prerequisite for TQF mechanism.  The
   related frequency synchronization mechanisms, such as IEEE 1588-2008
   Precision Time Protocol (PTP) [IEEE-1588] and synchronous Ethernet
   (syncE) [syncE], are not within the scope of this document.

   Sometimes, people also refer to the frequency asynchrony as the
   timeslot rotation frequency difference caused by different node
   configurations with different timeslot lengths.  This document
   supports the interconnection between nodes with this type of
   frequency asynchrony.





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15.  Evaluations

   This section gives the evaluation results of the TQF mechanism based
   on the requirements that is defined in
   [I-D.ietf-detnet-scaling-requirements].














































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   +======================+============+===============================+
   |     Requirements     | Evaluation |              Notes            |
   +======================+============+===============================+
   | 3.1 Tolerate Time    |    Yes     | No time sync needed, only need|
   |     Asynchrony       |            | frequency sync (3.1.3).       |
   +----------------------+------------+-------------------------------+
   | 3.2 Support Large    |            | The timeslot mapping covers   |
   |     Single-hop       |    Yes     | any value of link propagation |
   |     Propagation      |            | delay.                        |
   |     Latency          |            |                               |
   +----------------------+------------+-------------------------------+
   | 3.3 Accommodate the  |            | The higher the service rate,  |
   |     Higher Link      |  Partial   | the more buffer needed for the|
   |     Speed            |            | same timeslot length.         |
   +----------------------+------------+-------------------------------+
   | 3.4 Be Scalable to   |            | Multiple OPL instance, each   |
   |     the Large Number |            | for a set of serivce flows,   |
   |     of Flows and     |            | without overprovision.        |
   |     Tolerate High    |            | Utilization may reach 100%    |
   |     Utilization      |    Yes     | link bandwidth.               |
   |                      |            | The unused bandwidth of the   |
   |                      |            | timeslot can be used by       |
   |                      |            | best-effot flows.             |
   |                      |            | Calculating paths is NP-hard. |
   +----------------------+------------+-------------------------------+
   | 3.5 Tolerate Failures|            | Independent of queueing       |
   |     of Links or Nodes|    N/A     | mechanism.                    |
   |     and Topology     |            |                               |
   |     Changes          |            |                               |
   +----------------------+------------+-------------------------------+
   | 3.6 Prevent Flow     |            | Flows are isolated from each  |
   |     Fluctuation      |    Yes     | other through timeslots.      |
   +----------------------+------------+-------------------------------+
   | 3.7 Be scalable to a |            | E2E latency is liner with hops|
   |     Large Number of  |            | , from ultra-low to low       |
   |     Hops with Complex|    Yes     | latency by multiple OPL.      |
   |     Topology         |            | E2E jitter is low by on-time  |
   |                      |            | mode.                         |
   |                      |            | Calculating paths is NP-hard. |
   +----------------------+------------+-------------------------------+
   | 3.8 Support Multi-   |            | Independent of queueing       |
   |     Mechanisms in    |    N/A     | mechanism.                    |
   |     Single Domain and|            |                               |
   |     Multi-Domains    |            |                               |
   +----------------------+------------+-------------------------------+

            Figure 8: Evaluation for Large Scaling Requirements




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15.1.  Examples

   This section will describe the example of how the TQF mechanism
   supports DetNet flows with different latency requirements.  As shown
   in Figure 9:

   *  Network transmission capacity: each link has rate 10 Gbps.
      Assuming the service rate of TQF scheduler allocate the total port
      bandwidth.

   *  TSpec of each flow, maybe:

      -  burst size 1000 bits, SBI 1 ms, and average arrival rate 1
         Mbps.

      -  or, burst size 1000 bits, SBI 100 us, and average arrival rate
         10 Mbps.

      -  or, burst size 1000 bits, SBI 100 us, and average arrival rate
         100 Mbps.

      -  or, burst size 10000 bits, SBI 10 ms, and average arrival rate
         1 Mbps.

      -  or, burst size 10000 bits, SBI 1 ms, and average arrival rate
         10 Mbps.

      -  or, burst size 10000 bits, SBI 100 us, and average arrival rate
         100 Mbps.

   *  RSpec of each flow, maybe:

      -  E2E latency 100us, and E2E jitter less than 10us or 100us.

      -  or, E2E latency 200us, and E2E jitter less than 20us or 200us.

      -  or, E2E latency 300us, and E2E jitter less than 30us or 300us.

      -  or, E2E latency 400us, and E2E jitter less than 40us or 400us.

      -  or, E2E latency 500us, and E2E jitter less than 50us or 500us.

      -  or, E2E latency 600us, and E2E jitter less than 60us or 600us.

      -  or, E2E latency 700us, and E2E jitter less than 70us or 700us.

      -  or, E2E latency 800us, and E2E jitter less than 80us or 800us.




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      -  or, E2E latency 900us, and E2E jitter less than 90us or 900us.

      -  or, E2E latency 1000us, and E2E jitter less than 100us or 1ms.


               @         #         $
               @         #         $
               v         v         v
             +---+ @@@ +---+ ### +---+ $$$         &&& +---+
    src ---> | 0 o-----| 1 o-----| 2 o---- ... ... ----| 9 o----> dest
   (flow i:*)+---+ *** +---+ *** +---+ ***         *** +---+ ***
               |         |@        |#                    |&
               |         |@        |#                    |&
               |         |v        |v                    |v
             +---+     +---+     +---+                 +---+
         --- |   | --- |   | --- |   | --- ... ... --- |   | ---
             +---+     +---+     +---+                 +---+
               |         |         |                     |
               |         |         |                     |
            ... ...   ... ...   ... ...               ... ...

                     Figure 9: Common Topology Example

   For the observed flow i (marked with *), its TSpec and RSpec may be
   any of the above.  Assuming that the path calculated by the
   controller for the flow i passes through 10 nodes (i.e., node 0~9).
   Especially, at each hop, flow i may conflict with other competitive
   flows, also with similar TSpec and RSpec as above, originated from
   other sources, e.g, competing with flow-set "@" at node 0, competing
   with flow-set "#" at node 1, etc.

   For each link along the path, it may configure OPL-10ms instance with
   dedicated bandwidth 10 Gbps, containing 1000 timeslots each with
   length 10us.  Assuming no link propagation delay and intra node
   forwarding delay, if flow i consumes outgoing timeslot by o=1, it can
   ensure an E2E latency of 100us (i.e., o * TL * 10 hops), and jitter
   of 20us(on-time mode) or 100us (in-time mode).  The consumption by
   other o values is similar.

   The table below shows the possible supported service scales.  As
   flows arrived synchronously, the consumption of each timeslot in the
   orchestration period may be caused by any value of o.  For example,
   if the ideal incoming timeslots of all flows are perfectly
   interleaved, then they can all consume timeslots by o=1 to get per-
   hop latency 10us, or all consume timeslots by o=2 to get per-hop
   latency 20us, etc.  However, due to the fixed length of OPL, after
   all timeslot resources are exhausted by specific o value, it means
   that there are no timeslot resources used by other o values.  Another



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   example is that the ideal incoming timeslots of all flows are the
   same, then some of them consume timeslots by o=1, some consume
   timeslots by o=2, and so on.  In either case, the total service scale
   is OPL * C / burst_size, that is composed of sum(s_i), where s_i is
   the service scale for o = i.  The table provides the total scale and
   the average scale corresponding to each o value.

   Note that in the table each column only shows the data where all
   flows served based on all o values have the same TSpec (e.g, in the
   first colunm, TSpec per flow is burst size 1000 bits and arrival rate
   1 Mbps), while in reality, flows served based on different o values
   generally have different TSpec.  It is easy to add colunms to
   describe various combinations.


                   ===================================================
                   | o=1| o=2| o=3| o=4| o=5| o=6| o=7| o=8| o=9|o=10|
   ===================================================================
   |TSpec:         |                  total = 10000                  |
   |  1000 bits    |----+----+----+----+----+----+----+----+----+----|
   |  SBI 1 ms     |1000|1000|1000|1000|1000|1000|1000|1000|1000|1000|
   ===================================================================
   |TSpec:         |                  total = 1000                   |
   |  1000 bits    |----+----+----+----+----+----+----+----+----+----|
   |  SBI 100 us   | 100| 100| 100| 100| 100| 100| 100| 100| 100| 100|
   ===================================================================
   |TSpec:         |                  total = 100                    |
   |  1000 bits    |----+----+----+----+----+----+----+----+----+----|
   |  SBI 10 us    | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 |
   ===================================================================
   |TSpec:         |                  total = 10000                  |
   |  10000 bits   |----+----+----+----+----+----+----+----+----+----|
   |  SBI 10 ms    |1000|1000|1000|1000|1000|1000|1000|1000|1000|1000|
   ===================================================================
   |TSpec:         |                  total = 1000                   |
   |  10000 bits   |----+----+----+----+----+----+----+----+----+----|
   |  SBI 1 ms     | 100| 100| 100| 100| 100| 100| 100| 100| 100| 100|
   ===================================================================
   |TSpec:         |                  total = 100                    |
   |  10000 bits   |----+----+----+----+----+----+----+----+----+----|
   |  SBI 100 us   | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 |
   ===================================================================

         Figure 10: Timeslot Reservation and Service Scale Example







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16.  Taxonomy Considerations

   [I-D.ietf-detnet-dataplane-taxonomy] provides criteria for
   classifying data plane solutions.  TQF is a periodic, frequency
   synchronous, class level, work-conserving/non-work-conserving
   configurable, in-time/on-time configurable, time based solution.

   *  Periodic: Periodicity of TQF contains two characteristics, the
      first is that there is a time period P (i.e., orchestration
      periold) containing multiple time slots, and the second is that a
      flow is assigned repeatly to a particular set of time slots in the
      period.

   *  Frequency synchronous: TQF requires frequency synchronization
      (i.e., crystal frequency of the hardware) so that all nodes in the
      network have the same time lapse rate.  TQF does not require
      different nodes to use the same timeslot length.

   *  Class level: DetNet Flows may be grouped by similar service
      requirements, i.e., timeslot id(s), on the network entrance.
      Packets will be provided TQF service based on timeslot id(s),
      without checking flow characteristic.

   *  Work-conserving/non-work-conserving configurable: The TQF
      scheduler configured with in-time scheduling mode is work-
      conserving (i.e., to send the packet as soon as possible before
      its outgoing timeslot), while the TQF scheduler configured with
      on-time scheduling mode is non work-conserving (i.e., to ensure
      that the packet can always be sent within its outgoing timeslot).

   *  In-time/on-time configurable: The TQF scheduler configured with
      in-time scheduling mode is in-time to get bounded end-to-end
      latency, while the TQF scheduler configured with on-time
      scheduling mode is on-time to get bounded end-to-end delay jitter.

   *  Time based: A DetNet flow is scheduled based on its expected
      outgoing timeslot(s).  All DetNet flows are interleaved and
      arranged in different timeslots to obtain the maximum number of
      admission flows.

   In addition, the per hop latency dominant factor of TQF is the offset
   between incoming timeslot and outgoing timeslot that is asigned to
   the flow.

17.  IANA Considerations

   TBD.




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18.  Security Considerations

   Security considerations for DetNet are described in detail in
   [RFC9055].  General security considerations for the DetNet
   architecture are described in [RFC8655].  Considerations specific to
   the DetNet data plane are summarized in [RFC8938].

   Adequate admission control policies should be configured in the edge
   of the DetNet domain to control access to specific timeslot
   resources.  Access to classification and mapping tables must be
   controlled to prevent misbehaviors, e.g, an unauthorized entity may
   modify the table to map traffic to an unallowed timeslot resource,
   and competes and interferes with normal traffic.

19.  Acknowledgements

   TBD.

20.  References

20.1.  Normative References

   [I-D.chen-detnet-sr-based-bounded-latency]
              Chen, M., Geng, X., Li, Z., Joung, J., and J. Ryoo,
              "Segment Routing (SR) Based Bounded Latency", Work in
              Progress, Internet-Draft, draft-chen-detnet-sr-based-
              bounded-latency-03, 7 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-chen-detnet-
              sr-based-bounded-latency-03>.

   [I-D.eckert-detnet-tcqf]
              Eckert, T. T., Li, Y., Bryant, S., Malis, A. G., Ryoo, J.,
              Liu, P., Li, G., Ren, S., and F. Yang, "Deterministic
              Networking (DetNet) Data Plane - Tagged Cyclic Queuing and
              Forwarding (TCQF) for bounded latency with low jitter in
              large scale DetNets", Work in Progress, Internet-Draft,
              draft-eckert-detnet-tcqf-05, 5 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-eckert-
              detnet-tcqf-05>.

   [I-D.ietf-detnet-dataplane-taxonomy]
              Joung, J., Geng, X., Peng, S., and T. T. Eckert,
              "Dataplane Enhancement Taxonomy", Work in Progress,
              Internet-Draft, draft-ietf-detnet-dataplane-taxonomy-00,
              24 May 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-detnet-dataplane-taxonomy-00>.





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   [I-D.ietf-detnet-scaling-requirements]
              Liu, P., Li, Y., Eckert, T. T., Xiong, Q., Ryoo, J.,
              zhushiyin, and X. Geng, "Requirements for Scaling
              Deterministic Networks", Work in Progress, Internet-Draft,
              draft-ietf-detnet-scaling-requirements-06, 22 May 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-detnet-
              scaling-requirements-06>.

   [I-D.p-6man-deterministic-eh]
              Peng, S., "Deterministic Source Route Header", Work in
              Progress, Internet-Draft, draft-p-6man-deterministic-eh-
              00, 20 June 2024,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-p-
              6man-deterministic-eh/>.

   [I-D.pb-6man-deterministic-crh]
              Peng, S. and R. Bonica, "Deterministic Routing Header",
              Work in Progress, Internet-Draft, draft-pb-6man-
              deterministic-crh-00, 29 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-pb-6man-
              deterministic-crh-00>.

   [I-D.peng-detnet-policing-jitter-control]
              Peng, S., Liu, P., and K. Basu, "Policing Caused Jitter
              Control Mechanism", Work in Progress, Internet-Draft,
              draft-peng-detnet-policing-jitter-control-00, 18 January
              2024, <https://datatracker.ietf.org/doc/html/draft-peng-
              detnet-policing-jitter-control-00>.

   [I-D.peng-lsr-deterministic-traffic-engineering]
              Peng, S., "IGP Extensions for Deterministic Traffic
              Engineering", Work in Progress, Internet-Draft, draft-
              peng-lsr-deterministic-traffic-engineering-01, 4 July
              2023, <https://datatracker.ietf.org/doc/html/draft-peng-
              lsr-deterministic-traffic-engineering-01>.

   [I-D.xp-ippm-detnet-stamp]
              Min, X., Peng, S., and X. He, "STAMP Extensions for
              DetNet", Work in Progress, Internet-Draft, draft-xp-ippm-
              detnet-stamp-00, 19 June 2024,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              xp-ippm-detnet-stamp/>.

   [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>.




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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

   [RFC8938]  Varga, B., Ed., Farkas, J., Berger, L., Malis, A., and S.
              Bryant, "Deterministic Networking (DetNet) Data Plane
              Framework", RFC 8938, DOI 10.17487/RFC8938, November 2020,
              <https://www.rfc-editor.org/info/rfc8938>.

   [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>.

20.2.  Informative References

   [ATM-LATENCY]
              "Bounded Latency Scheduling Scheme for ATM Cells", 1999,
              <https://ieeexplore.ieee.org/document/780828/>.

   [CQF]      "Cyclic queueing and Forwarding", 2017,
              <https://ieeexplore.ieee.org/document/7961303>.

   [ECQF]     "Enhancements to Cyclic Queuing and Forwarding", 2023,
              <https://1.ieee802.org/tsn/802-1qdv/>.

   [IEEE-1588]
              "IEEE Standard for a Precision Clock Synchronization
              Protocol for Networked Measurement and Control Systems",
              2008, <https://standards.ieee.org/findstds/
              standard/1588-2008.html>.

   [SP-LATENCY]
              "Guaranteed Latency with SP", 2020,
              <https://ieeexplore.ieee.org/document/9249224>.

   [syncE]    "Timing and synchronization aspects in packet networks",
              2013, <https://www.itu.int/rec/T-REC-G.8261>.

   [TAS]      "Time-Aware Shaper", 2015,
              <https://standards.ieee.org/ieee/802.1Qbv/6068/>.





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Authors' Addresses

   Shaofu Peng
   ZTE
   China
   Email: peng.shaofu@zte.com.cn


   Peng Liu
   China Mobile
   China
   Email: liupengyjy@chinamobile.com


   Kashinath Basu
   Oxford Brookes University
   United Kingdom
   Email: kbasu@brookes.ac.uk


   Aihua Liu
   ZTE
   China
   Email: liu.aihua@zte.com.cn


   Dong Yang
   Beijing Jiaotong University
   China
   Email: dyang@bjtu.edu.cn


   Guoyu Peng
   Beijing University of Posts and Telecommunications
   China
   Email: guoyupeng@bupt.edu.cn















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