DetNet Bounded Packet-Delay-Variation
draft-mohammadpour-detnet-bounded-delay-variation-00

Document Type Active Internet-Draft (individual)
Authors Ehsan Mohammadpour  , Jean-Yves Le Boudec 
Last updated 2021-09-10
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DetNet                                                   E. Mohammadpour
Internet-Draft                                            J-Y. Le Boudec
Intended status: Informational                                      EPFL
Expires: 14 March 2022                                 10 September 2021

                 DetNet Bounded Packet-Delay-Variation
          draft-mohammadpour-detnet-bounded-delay-variation-00

Abstract

   Some DetNet use-cases (applications) require guaranteed bounds on
   packet delay-variation, not just on latency.  This document gives a
   methodology to derive guaranteed packet delay-variation bounds in
   DetNet and apply it to a number of proposed mechanisms.  When the
   required packet delay-variations is very low, clock non-idealities
   affect the bounds, even in a synchronized DetNet networks.  This
   document also gives a methodology to account for such an effect.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 14 March 2022.

Copyright Notice

   Copyright (c) 2021 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
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Clock Model . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Computing End-to-end Packet-Delay-Variation Bound . . . . . .   4
     4.1.  DetNet Time Model . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Methodology . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Packet Scheduling Techniques  . . . . . . . . . . . . . . . .   5
     5.1.  Guaranteed-Service IntServ  . . . . . . . . . . . . . . .   5
     5.2.  Differentiated Services . . . . . . . . . . . . . . . . .   5
     5.3.  Credit-Based Shaper with Asynchronous Traffic Shaping . .   5
     5.4.  Cyclic Queuing and Forwarding (CQF) . . . . . . . . . . .   5
     5.5.  Dampers . . . . . . . . . . . . . . . . . . . . . . . . .   5
       5.5.1.  Damper Classification . . . . . . . . . . . . . . . .   7
       5.5.2.  Bound Computations  . . . . . . . . . . . . . . . . .   8
     5.6.  Mechanism XXX . . . . . . . . . . . . . . . . . . . . . .   9
     5.7.  Mechanism XXX . . . . . . . . . . . . . . . . . . . . . .   9
     5.8.  Mechanism XXX . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Example application on DetNet IP network  . . . . . . . . . .   9
   7.  Security considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Some applications that use DetNet networks, such as such as
   industrial Internet of Things [ITU-Y3000] and electrical utilities
   [RFC8578], require not just guaranteed bounds on the worst-case
   packet delay, but also on packet delay variation (PDV), defined as
   the difference between worst-case and best-case delays.

   A general framework to compute latency bounds is presented in
   [I-D.ietf-detnet-bounded-latency].  In this document, we extend this
   framework to compute guaranteed bounds on PDV.

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   When the packet-delay-variation requirement is very low, even in a
   time-synchronized DetNet network, clock non-idealities affect the
   bounds, as seen, e.g., in [MohammadpourDamper].  This document gives
   a methodology, derived from [ThomasTime], to incorporate such effects
   in the computation of packet-delay-variation bounds within DetNet.

   This document also applies the presented framework to compute packet-
   delay-variation bounds on a number of packet scheduling mechanisms,
   some of which are taken from [RFC8655] and
   [I-D.ietf-detnet-bounded-latency], while others are specifically
   targetting low PDV.  Finally, this document gives an application of
   the framework to compute end-to-end packet-delay-variation bounds on
   a sample DetNet network with a combination of various packet
   scheduling mechanisms.

2.  Terminology and Definitions

   This document uses the terms defined in [RFC8655].  This document
   also uses the following terms:.

   PDV
      Packet Delay-Variation as in [RFC3393].  It is also called
      "latency variation" or "jitter" in [RFC8655].

3.  Clock Model

   We call H_TAI the perfect clock, i.e. the international atomic time
   (Temps Atomique International).  In practice, the local clock of a
   system deviates from the perfect clock [ThomasTime].  In time-
   sensitive networks, clocks can be synchronized or non-synchronized.
   Non-synchronized clocks are independently configured and do not
   interact with each other; this corresponds to the free-running mode
   in Section 4.4.1 of [g810].  When clocks are synchronized, using
   methods like Network Time Protocol (NTP), Precision Time Protocol
   (PTP), WhiteRabbit, Global Positioning System (GPS), the occurrence
   of an event, when measured with different clocks, is bounded by the
   time error bound (~1us or less in PTP, WhiteRabbit, and GPS; ~100ms
   in NTP).

   This document follows the clock model in [ThomasTime], which applies
   to time-sensitive networks.  Consider a clock H_i that is either
   synchronized with time error bound ω, or not synchronized (in
   which case we set ω=+∞).  Let d^H_i [resp. d^H_TAI] be a
   delay measurement done with clock H_i [resp. in TAI], then
   [ThomasTime]:

      d^H_TAI - d^H_i <= min((&#961;-1) * d^H_i + &#951;, 2&#969;),

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      d^H_TAI - d^H_i >= - min((1- 1/&#961;) * d^H_i+ &#951;/&#961;,
      2&#969;),

   where &#961; is the stability bound and &#951; the timing-jitter
   bound of the clock H_i.  Note that this set of bounds is symmetric,
   i.e. we can exchange the roles of H_i and H_TAI in the above
   equation.  We assume that the parameters &#969;, &#961;,&#951; are
   valid for all clocks in the network, i.e. we consider network-wide
   time-error, stability and time-jitter bounds.

4.  Computing End-to-end Packet-Delay-Variation Bound

   Computation of end-to-end PDV bound requires a time model that
   includes all the sources of latency within a flow path.  In this
   document we use the existing time model presented in
   [I-D.ietf-detnet-bounded-latency].

4.1.  DetNet Time Model

   Figure 1 is a breakdown of the per-hop latency experienced by a
   packet passing through a DetNet transit node, taken from
   [I-D.ietf-detnet-bounded-latency].

         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

4.2.  Methodology

   Consider a DetNet flow (or an aggregate of DetNet flows).  The end-
   to-end delay-variation for the flow is defined as difference between
   the end-to-end worst-case and best-case latencies of its packets,
   measured in TAI,

      e2e_PDV = e2e_worst_case_latency - e2e_best_case_latency.

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   "V" is an upper-bound on the end-to-end PDV of the flow if and only
   if for any two packets n and m with end-to-end latencies "d_n" and
   "d_m"

      |d_n - d_m| <= V.

   Then V is computed as:

      V = e2e_latency_upper_bound - e2e_latency_lower_bound.

   A general framework to compute end_to_end_latency_upper_bound is
   described in [I-D.ietf-detnet-bounded-latency].  The same framework
   can be used to compute e2e_latency_lower_bound; in Section 5, we
   provide the bound for a set of queuing mechanisms.

5.  Packet Scheduling Techniques

   This section provides formulas to compute PDV bounds for a number of
   packet scheduling mechanisms within DetNet networks.

5.1.  Guaranteed-Service IntServ

   TBD from [I-D.ietf-detnet-bounded-latency] and [RFC2212].

5.2.  Differentiated Services

   TBD from [RFC2475] and [RFC7657].

5.3.  Credit-Based Shaper with Asynchronous Traffic Shaping

   TBD from [I-D.ietf-detnet-bounded-latency]

5.4.  Cyclic Queuing and Forwarding (CQF)

   TBD from [IEEE8021Q]

5.5.  Dampers

   Dampers are proposed to reduce packet delay-variation in time-
   sensitive networks [VermaJitter],[ZhangRCSP],[CruzScedPlus].  A
   damper delays every DetNet packet by an amount written in a packet
   header field, called damper header, which carries an estimate of the
   earliness of this packet with respect to a known latency upper-bound
   of upstream systems.  This ideally leads to zero PDV; in practice,
   there is still some small residual PDV, due to errors in acquiring
   timestamps and in computing and implementing delays.  As a positive
   side effect, dampers create packet timings that are almost the same
   as at the source, with small errors due to residual PDV, and thus

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   cancel most of the burstiness increase imposed by the network.  The
   residual burstiness increase that remains when dampers are used is
   not influenced by the burstiness of cross-traffic.  Thus, dampers
   solve the burstiness cascade issue [CharnyDelay]: individual flows
   that share a resource dedicated to a class may see their burstiness
   increase, which may in turn increase the burstiness of other
   downstream flows.  Furthermore, dampers are stateless; hence, solving
   the burstiness cascade in a stateless manner makes the dampers of
   interest for DetNet networks.

   We call jitter-compensated system (JCS) any delay element or
   aggregate of delay elements with known latency and PDV bounds, for
   which we want to compensate PDV by means of dampers.  This is
   typically the queuing system on the output port of a switch or router
   used in DetNet transit nodes.  It can also be a switching fabric or
   an input port processing unit, or even a larger system.  For DetNet
   flows, a JCS should be able to time stamp packet arrivals and
   departures using the available local times.  It should also increment
   the damper header field in every DetNet packet (if one is present) by
   an amount equal to an estimate of the earliness of this packet with
   respect to the known latency upper-bound &#948; at the JCS.  If no
   damper header is present, it inserts one, with a value equal to the
   estimated earliness.  The earliness is computed as:

      earliness = &#948; - actual_delay_in_the_JCS.

   When a DetNet flow crosses a JCS, for actual PDV removal to occur,
   there must be a downstream damper on the path of the flow
   [MohammadpourDamper] (e.g. if the JCS is a switch output port, the
   next downstream damper is typically located on the output port of the
   next downstream switch).  The damper also resets the damper header,
   so that the next downstream damper will see only the earliness
   accumulated downstream of this damper.  Designing a stand-alone
   damper is a challenge, because such a damper may need to release a
   large number of packets instantly or within a very short time, which
   might not be feasible.  This is why damper implementations are often
   associated with queuing systems; then, the time at which a damper
   releases a packet is simply the time at which the packet becomes
   visible to the queuing system.

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   It is generally not possible, or required, to remove PDV in all
   network elements, because time stamping and damper-header update come
   with a cost.  Therefore, it is required, for our timing analysis, to
   consider what we call bounded-delay systems (BDSs), defined as any
   delay element or aggregate of delay elements with known latency and
   PDV bounds, and for which we do not compensate PDV.  Constant latency
   elements (e.g. an output link propagation delay), variable delay
   elements with very low jitter (e.g., very high speed backbone
   network) are examples of BDSs.

   We classify and model existing designs of dampers in next subsections
   and give formulas for the computation of PDV and latency bounds,
   taken from [MohammadpourDamper].

5.5.1.  Damper Classification

   An ideal damper delays a packet by exactly the amount required by the
   damping header.  Consider a packet n with damper header H_n that
   arrives at local time Q_n to a damper.  Then an ideal damper releases
   the packet at time E_n:

      E_n = Q_n + H_n.

   Jitter-control Earliest-Deadline-First [VermaJitter] is an ideal
   damper, used in combination with an Earliest-Deadline-First
   scheduler.

   Many other implementations of dampers use some tolerance for the
   packet release times, due to the difficulty of implementing exact
   timings.  We call damper with tolerances &#916;^L,&#916;^U, a damper
   such that the eligibility time E_n, of packet n, in local time,
   satisfies:

      Q_n + H_n - &#916;^L >= E_n <= Q_n + H_n + &#916;^U.

   The tolerances can vary from hundreds of nanosecond to a few
   microsecond based on implementation.  RCSP [ZhangRCSP] is an instance
   of dampers with tolerance.  Since the definition of damper with
   tolerance does not preclude packet misordering, re-sequencing and
   head-of-line dampers avoid packet misordering.

   Re-sequencing damper with tolerances &#916;^L,&#916;^U is a system
   that behaves as the concatenation of a damper with same tolerances
   and a re-sequencing buffer that, if needed, re-orders packets based
   on the packet order at the entrance of the damper.  The packet order
   is with respect to a flow of interest.  SCED+ [CruzScedPlus] is an
   instance of re-sequencing dampers.

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   Head-of-line damper is introduced in [GrigorjewJCATS] and is
   implemented as a FIFO queue.  It has tolerance parameters
   &#916;^L,&#916;^U as well as processing bounds &#966;^min,&#966;^max.
   A head-of-line damper behaves as re-sequencing damper with with
   tolerances &#916;^L,&#916;^U followed by a single-server FIFO queue
   with processing bounds &#966;^min,&#966;^max.  When a packet arrives,
   its arrival time is collected and the packet is stored at the tail of
   the queue.  Only the packet at the head of the queue is examined; if
   its eligibility time is passed, it is immediately released, otherwise
   it is delayed and released at its eligibility time.  When the head
   packet is released, it is removed from the damper queue and the next
   packet (if any) becomes the head of the queue and is examined.  When
   an arriving packet finds an empty queue, it is immediately examined.
   By construction, packet ordering is preserved.

5.5.2.  Bound Computations

   In this subsection, we provide latency and PDV bounds for a simple
   case (general case is available in [MohammadpourDamper]) where we
   want to compensate the PDV imposed by the queuing delay (6) in
   Figure 1 by the mean of dampers.  Therefore the queuing subsystem is
   a JCS with a known latency upper-bound (the latency upper-bound
   includes the delay from first-bit-in to last-bit-in hidden in the
   link delay (2) of Figure 1).  A damper is placed before the queuing
   subsystems (replaced the regulator in Figure 1).  The delays (1), (2)
   only the first-bit-out to first-bit-in, (3), (4) in Figure 1 are
   assumed to be the BDSs.

   Assume that the queuing subsystem has latency upper-bound &#948; and
   PDV J, and the latency upper-bound, lower-bound and PDV bound on the
   BDSs are &#960;, &#960;' and v.  Also, assume that a damper with
   tolerances &#916;^L,&#916;^U is placed before the queuing subsystem
   in the DetNet transit node B.  Then, the latency lower-bound, upper-
   bound and the PDV bound from the entrance of queuing subsystem in
   transit node A to the entrance of queuing subsytem in transit node B,
   in TAI, are computed as follows.

   If damper with tolerances &#916;^L,&#916;^U is used:

      latency_lower-bound = &#948; + &#960;' - &#916;^L - &#949; -
      &#968;' ,

      latency_upper-bound = &#948; + &#960; + &#916;^U + &#949; + &#968;
      ,

      PDV_bound = v + 2 * &#949; + &#968; + &#968;' ,

   where

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      &#968;' = min((&#961;-1) * (&#948; + &#916;^U + &#949;) + 2 *
      &#951; , 4 * &#969;),

      &#968; = min((1-1\&#961;) * (&#948; - &#916;^L + -&#949;) ) + 2 *
      &#951;/&#961; , 4 * &#969;).

   If re-sequencing damper with tolerances &#916;^L,&#916;^U is used and
   all the other elements are FIFO, the same bounds as mentioned above
   is obtained.

   If head-of-line damper with tolerances &#916;^L,&#916;^U and
   processing bounds &#966;^min,&#966;^max is used and all the other
   elements are FIFO, the latency_upper-bound and PDV_bound are
   increased by &#952; where

      &#952; = ((b + r * PDV_bound) * &#966;^max ; if &#966;^max <= 1/r,

      &#952; = +&#8734; ; if &#966;^max > 1/r,

   where the DetNet flow has per-packet leaky-bucket arrival curve at
   the entrance of queuing subsystem at DetNet transit node A, i.e., the
   number of packets that can be emitted by the flow within any period
   of time t is not larger than r * t + b where r is the rate of packets
   and b is bucket size in packets.

   When an element is not FIFO in Figure 1, comparing to dampers with
   tolerance, using a re-sequencing damper worsens the PDV bound by J
   and a head-of-line damper worsens the PDV bound by 2 * J; further
   discussion is available in [MohammadpourDamper].

5.6.  Mechanism XXX

   TBD

5.7.  Mechanism XXX

   TBD

5.8.  Mechanism XXX

   TBD

6.  Example application on DetNet IP network

   TBD

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7.  Security considerations

   Detailed security considerations for DetNet are cataloged in
   [RFC9055], and more general security considerations are described in
   [RFC8655].

8.  IANA considerations

   This document has no IANA actions.

9.  References

9.1.  Normative References

   [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>.

9.2.  Informative References

   [CharnyDelay]
              A. Charny and J.-Y. Le Boudec, "Delay Bounds in a Network
              with Aggregate Scheduling", 2002,
              <https://link.springer.com/
              chapter/10.1007/3-540-39939-9_1>.

   [CruzScedPlus]
              R. L. Cruz, "SCED+: efficient management of quality of
              service guarantees", 1998,
              <https://ieeexplore.ieee.org/document/665083>.

   [g810]     L. Thomas and J.-Y. Le Boudec, "G.810 : Definitions and
              terminology for synchronization networks",
              <https://www.itu.int/rec/T-REC-G.810-199608-I/en>.

   [GrigorjewJCATS]
              A. Grigorjew, F. Metzger, T. Hossfeld, J. Specht, F.-J.
              Goetz, F. Chen, and J. Schmitt, "Asynchronous Traffic
              Shaping with Jitter Control", 2020,
              <https://opus.bibliothek.uni-
              wuerzburg.de/frontdoor/index/index/docId/20582>.

   [I-D.ietf-detnet-bounded-latency]
              N. Finn, J-Y. Le Boudec, E. Mohammadpour, J. Zhang, B.
              Varga, and J. Farkas, "DetNet Bounded Latency",
              <https://www.ietf.org/archive/id/draft-ietf-detnet-
              bounded-latency-07.html>.

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

   [ITU-Y3000]
              ITU-T, "ITU-T Y.3000-series - Representative use cases and
              key network requirements for Network 2030", 2020,
              <https://www.itu.int/rec/T-REC-Y.Sup67-202007-I>.

   [MohammadpourDamper]
              E. Mohammadpour and J.-Y. Le Boudec, "Analysis of Dampers
              in Time-Sensitive Networks with Non-ideal Clocks", 2021,
              <https://arxiv.org/abs/2109.02757>.

   [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>.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,
              <https://www.rfc-editor.org/info/rfc3393>.

   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,
              <https://www.rfc-editor.org/info/rfc7657>.

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,
              <https://www.rfc-editor.org/info/rfc8578>.

   [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>.

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   [ThomasTime]
              L. Thomas and J.-Y. Le Boudec, "On Time Synchronization
              Issues in Time-Sensitive Networks with Regulators and
              Nonideal Clocks", 2020,
              <https://dl.acm.org/doi/10.1145/3393691.3394206>.

   [VermaJitter]
              D.C. Verma, H. Zhang, and D. Ferrari, "Delay jitter
              control for real-time communication in a packet switching
              network", 1991,
              <https://ieeexplore.ieee.org/abstract/document/152873>.

   [ZhangRCSP]
              H. Zhang and D. Ferrari, "Rate-controlled static-priority
              queueing", 1993,
              <https://ieeexplore.ieee.org/abstract/document/253355>.

Authors' Addresses

   Ehsan Mohammadpour
   EPFL
   IC Station 14
   CH-1015 Lausanne EPFL
   Switzerland

   Email: ehsan.mohammadpour@epfl.ch

   Jean-Yves Le Boudec
   EPFL
   IC Station 14
   CH-1015 Lausanne EPFL
   Switzerland

   Email: jean-yves.leboudec@epfl.ch

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