



DetNet Working Group                                             Y. Ryoo
Internet-Draft                                                      ETRI
Intended status: Standards Track                                J. Joung
Expires: 7 January 2026                             Sangmyung University
                                                             6 July 2025


On-time Forwarding with Non-work Conserving Stateless Core Fair Queuing
                      draft-ryoo-detnet-nscore-01

Abstract

   This document specifies the framework and operational procedure for
   deterministic networking that guarantees maximum and minimum end-to-
   end latency bounds to flows.  The solution has non-periodic,
   asynchronous, flow-level, non-work conserving, on-time, and rate-
   based functional characteristics, according to the taxonomy suggested
   by [draft-ietf-detnet-dataplane-taxonomy-03].

   The packets are stored in the queue in ascending order of the ideal
   service start time, called Eligible Time (ET), and the ideal service
   completion time, called Finish Time (FT).  The queued packets were
   forwarded between ET and FT in a non-work conserving manner.  The ET
   and FT are calculated at the entrance node according to the packet
   size and rate of the flow.  All subsequent core nodes are stateless
   and asynchronously compute ET and FT based on metadata received via
   packet headers.  This mechanism is called non-work-preserving
   stateless fair queuing, which guarantees both E2E latency upper and
   lower bounds.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 7 January 2026.





Ryoo & Joung             Expires 7 January 2026                 [Page 1]

Internet-Draft                   N-SCORE                       July 2025


Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Symbols Used in This Document . . . . . . . . . . . . . .   3
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   4.  N-SCORE Packet Scheduler Framework  . . . . . . . . . . . . .   4
   5.  E2E latency and jitter bound  . . . . . . . . . . . . . . . .   5
   6.  Operational Procedure . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Operational Procedure in Entrance Node  . . . . . . . . .   6
     6.2.  Operational Procedure in Core Node  . . . . . . . . . . .   7
   7.  Capability Analysis . . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   A class of schedulers called Fair Queuing (FQ) limits interference
   between flows to the degree of the maximum packet size.  In FQ, the
   ideal service completion time, called Finish Time (FT), of a packet
   is obtained from an imaginary system that can provide the ideal flow
   isolation.  Applying this technique, the end-to-end (E2E) latency
   bound of a flow is similar to that of an ideally isolated system.

   Since calculating the FT of the current packet requires the FT of
   previous packets within the flow, this means that nodes must manage
   the state of the flow.  The complexity of managing the state of a
   large number of flows can be a burden, so the proposed framework for
   large-scale deterministic networking is called work conserving



Ryoo & Joung             Expires 7 January 2026                 [Page 2]

Internet-Draft                   N-SCORE                       July 2025


   stateless core fair queuing (C-SCORE), which generates FT for packets
   at the entrance node and marks FT in the packet to operate with
   stateless in core nodes.

   However, C-SCORE is a scheduler of work conserving approach, so it
   has an in-time characteristic.  Therefore, this draft proposes a non-
   work conserving scheduler method by extending C-SCORE to have an on-
   time characteristic, called N-SCORE.  The entrance node additionally
   obtains an ideal service start time, called an eligible time (ET), of
   the current packet based on the FT of the previous packet or the
   arrival time of the current packet.  All of the nodes queued packets
   in ascending order of the ET and FT and forward the packet between ET
   and FT in a non-work conserving approach.  N-SCORE is a method that
   guarantees not only the upper bound but also the lower bound of E2E
   latency by adding ET while using the information managed by the
   entrance node of the existing C-SCORE.

2.  Terminology

2.1.  Symbols Used in This Document

   FQ          fair queuing
   FT          finish time
   ET          eligible time
   Fh(p)       FT of the packet p at the node h
   Eh(p)       ET of the packet p at the node h
   Ah(p)       arrinal time of the packet p at the node h
   dh(p)       maximum delay of the packet p at the node h
   ch(p)       service complition time of packet p at the node h
   r(p)        service rate of the packet p
   L(p)        length of the packet p
   Rh          link capacity of the node h
   Lhmax       maximum packet length of the node h
   PDh         propagation delay of the link h

2.2.  Abbreviations


3.  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.






Ryoo & Joung             Expires 7 January 2026                 [Page 3]

Internet-Draft                   N-SCORE                       July 2025


4.  N-SCORE Packet Scheduler Framework

   Utilizing the concept of virtual clock (VC) scheduler, C-SCORE
   defines FT for packet p as

   F(p) = max{F(p-1), A(p)} + L(p)/r(p). (1)

   Where (p-1) and p are consecutive packets of the flow being observed,
   F(p-1) is the finish time of p-1, A(p) is the arrival time of p, L(p)
   is the length of p, and r(p) is the flow service rate.  Flow
   exponents are omitted.

   In C-SCORE, the entrance node manages F(p-1) and obtains F(p) by
   comparing it with A(p).  Then, it calculates F(p) of the next node
   and marks it in the packet header.  The service period of packet p in
   each node is defined as (A(p), F(p)].  Assuming the link propagation
   delay is zero, an example of the packet service period at the
   entrance node and core node with the C-SCORE scheduler is illustrated
   as follows:


   A1(1)           A1(2)A1(3)A1(4)
   |               |    |    |
   V               V    V    V
   <----1---->     <----2---->F1(2)                               node 1
   |       F1(1)   |    <-------3------->F1(3)
   |               |    |    <----------4---------->F1(4)
   |               |    |    |
   A2(1)           A2(2)A2(3)A2(4)
   |               |    |    |
   V               V    V    V
   <----------1--------->F2(1)                                    node 2
                   <---------2---------->F2(2)
                        <-------------3------------>F2(3)
                             <-----------------4-------------->F2(4)


             Figure 1: C-SCORE packet scheduler service period

   The proposed N-SCORE framework introduces an additional parameter, ET
   (Eligible Time), which is used as the earliest possible packet
   service start time.  Without requiring additional state management
   for ET, N-SCORE utilizes the information already managed by the
   entrance node in the existing C-SCORE to obtain ET and FT as follows:

   E(p) = max{F(p-1), A(p)} (2)

   F(p) = E(p) + L(p)/r (3)



Ryoo & Joung             Expires 7 January 2026                 [Page 4]

Internet-Draft                   N-SCORE                       July 2025


   A packet can join the output link scheduler immediately after its ET.
   If no other packet is present in the scheduler, the packet is served
   right away.  Otherwise, the packet joins the queue.  Packets in the
   queue are served in ascending order of their ET and FT.  Since the FT
   of N-SCORE is identical to that of C-SCORE, packets in N-SCORE follow
   the same service order as in C-SCORE.  The only difference between
   the two systems is the existence of ET.  However, in N-SCORE, due to
   the presence of the ET, the service period of packet p, while
   maintaining the same service order, is defined as (E(p), F(p)].
   Here, E(p) and F(p) are ET and FT of packet p, respectively.
   Consequently, N-SCORE forwards packets in a non-work-preserving
   manner, maintaining a constant interval between E(p) and F(p) in all
   nodes.  The service periods of packets within the same flow do not
   overlap at each node.  Assuming zero link propagation delay, the
   packet service period at the entrance and core nodes with the N-SCORE
   scheduler is illustrated as follows:


   A1(1)           A1(2)A1(3)A1(4)
   |               |    |    |
   V               V    V    V
   <----1---->     <----2----><----3----><----4---->              node 1
   E1(1)  F1(1)    E1(2) F1(2)=E1(3) F1(3)=E1(4) F1(4)
   |               |          |          |
   A2(1)           A2(2)      A2(3)      A2(4)
   |               |          |          |
   V               V          V          V
   ...........<----1---->.....<----2----><----3----><----4---->   node 2
              E2(1)  F2(1)    E2(2) F2(3)=E2(3) F2(3)=E2(4) F2(4)


             Figure 2: N-SCORE packet scheduler service period

5.  E2E latency and jitter bound

   The end-to-end (E2E) latency of N-SCORE is upper-bounded by:

   (B-L)/r+∑[h=0,H]{L/r + Lhmax/Rh} (4)

   which is the same as that of C-SCORE, which operates based on FT.
   Here, B, L, and r represent the maximum burst size, maximum packet
   length, and service rate of the observed flow, respectively.  The
   link propagation delay is omitted.

   Unlike C-SCORE, which has no lower bound for E2E latency, the E2E
   latency of N-SCORE, which operates based on both ET and FT, is lower-
   bounded by:




Ryoo & Joung             Expires 7 January 2026                 [Page 5]

Internet-Draft                   N-SCORE                       July 2025


   ∑[h=0, H-1]{L/r+(Lhmax)/Rh} +Lmin/RH (5)

   where L, Lmin, and r denote the maximum packet length, minimum packet
   length, and service rate of the observed flow, respectively.  The
   link propagation delay is omitted.

   Therefore, unlike C-SCORE, which exhibits high jitter ranging from 0
   to the E2E maximum delay, the E2E jitter of N-SCORE is bounded by:

   B/r+(LHmax)/RH - Lmin/RH (6)

6.  Operational Procedure

   The N-SCORE scheduler in all nodes has a deterministic service period
   of ( E(p), F(p)] for packet p.  Packets are queued in a priority
   queue in ascending order of ET and FT and can be dequeued after ET in
   a non-work-conserving manner.  It operates at a constant interval
   that depends on the packet size and the service rate.

   N-SCORE manages per-flow state to calculate ET and FT at the entrance
   node.  However, core nodes do not maintain state to accommodate
   large-scale networks.  As a result, N-SCORE calculates and applies ET
   and FT differently at the entrance node and subsequent core nodes.

   Whenever a packet arrives, the entrance node calculates its ET and FT
   based on the managed per-flow state, updates the state using the
   calculated FT, and appends ET and FT as metadata to the packet
   header.  Subsequent core nodes retrieve ET and FT from the metadata
   without maintaining state separately.  At the same time, they
   calculate new ET and FT for the next node and update the metadata
   accordingly.

6.1.  Operational Procedure in Entrance Node

   The entrance node manages the per-flow state, including the FT of the
   previous packet, F(p−1), and the service rate assigned to the flow,
   r(p).  When a packet arrives at the entrance node, its ET, E(p), is
   determined as max{F(p−1), A(p)}.  The entrance node compares each
   packet's arrival time, A(p), with the managed F(p−1) and sets the
   later time as E(p).  The FT of the arriving packet, F(p), is
   calculated as E(p)+L(p)/r(p), and the FT of the previous packet is
   updated with the newly obtained F(p).  Packets are stored in a
   priority queue in ascending order of E(p) and F(p) and can be
   dequeued after E(p) in a non-work-conserving manner.

   When the packet arrival interval is greater than the service rate, as
   seen with the first and second packets in Figure 2, the arrival times
   of these packets at node 1, A1(1) and A1(2), are later than the FT of



Ryoo & Joung             Expires 7 January 2026                 [Page 6]

Internet-Draft                   N-SCORE                       July 2025


   the previous packet managed by the entrance node, F1(0) and F1 (1),
   respectively.  Therefore, the ET of the first and second packets at
   node 1, E1(1) and E1(2), are set as A1(1) and A1(2), respectively.
   In this case, the service period is (A(p), A(p)+L(p)/r(p)], which
   matches the service period of C-SCORE.

   However, when the packet arrival interval is smaller than the service
   rate, as seen with the third and fourth packets in Figure 2, the
   arrival times of these packets at node 1, A1(3) and A1(4), are
   earlier than the FT of the previous packet managed by the entrance
   node, F1(2) and F1(3), respectively.  Consequently, the ET of the
   third and fourth packets at node 1, E1(3) and E1(4), are set as F1(2)
   and F1(3), respectively.  In this case, unlike C-SCORE’s service
   period of (A(p), F(p−1) + L(p)/r(p)], the N-SCORE service period is
   (F(p−1), F(p−1) + L(p)/r(p)].  N-SCORE regulates packet transmission
   based on the service rate, ensuring a deterministic and non-
   overlapping service period for all packets.

   A packet is dequeued after E(p), and before leaving, the entrance
   node marks metadata in the packet header, including L(p)/r(p), as
   well as the ET and FT for the next node.  The subsequent core nodes
   then use this metadata to determine their ET and FT.

6.2.  Operational Procedure in Core Node

   When the ET and FT of a packet are determined at the entrance node,
   the ET and FT of all subsequent nodes are determined based on the
   previous node's ET and FT as follows:

   Eligible Time for the next node:

   Eh+1(p) = Eh(p) + dh(p) (7)

   Finish Time for the next node:

   Fh+1(p) = Fh(p) + dh(p) (8)

   Here, dh(p) represents the maximum delay within node h, which is
   calculated as:

   dh(p) = L(p)/r(p) + Lhmax/Rh (9)

   The term Lhmax/Rh accounts for delay factors at node h, where Lhmax
   is the max packet length at node h across all flows transmitted
   through the observed output port, and Rh is the link capacity of node
   h.





Ryoo & Joung             Expires 7 January 2026                 [Page 7]

Internet-Draft                   N-SCORE                       July 2025


   The entrance node delivers the metadata, including L(p)/r(p), ET, and
   FT, through the packet header.  Subsequent core nodes obtain their ET
   and FT from the metadata without per-flow state management.  Based on
   its delay factors and L(p)/r(p) value in the metadata, each core node
   computes dh(p), determines the ET and FT for the next node, and
   updates the metadata accordingly.

   Packets are stored in a priority queue in ascending order of E(p) and
   F(p), as derived from the metadata, and can be dequeued after E(p) in
   a non-work conserving manner.

7.  Capability Analysis

   Based on the draft of the taxonomy, latency-bound solutions are
   classified according to functional characteristics such as

   *  periodicity (periodic, non-periodic)

   *  network synchronization (phase and frequency synchronous,
      asynchronous)

   *  traffic granularity (flow level, flow aggregate level, class
      level)

   *  work conserving (work conserving, non-work conserving)

   *  target transmission time (in-time, on-time)

   *  service order (rate-based, time-based, arrival-based, priority-
      based)

   N-SCORE is a non-periodic, asynchronous, flow level, non-work
   conserving, on-time, rate-based solution.

   The draft of the taxonomy also defines seven suitable categories for
   deterministic networking as follows.

   *  Right-bounded category

   *  Flow level periodic bounded category

   *  Class level periodic bounded category

   *  Flow level non-periodic bounded category

   *  Class level non-periodic bounded category

   *  Flow level rate based unbounded category



Ryoo & Joung             Expires 7 January 2026                 [Page 8]

Internet-Draft                   N-SCORE                       July 2025


   *  Flow level rate based left-bounded category

   N-SCORE belongs to the "Flow level rate based left-bounded category",
   which is an on-time solution with rate-based service order
   characteristic that can handle a large number of dynamic flows with
   simple admission control.  Additionally, it has flow-level traffic
   granularity characteristics that can minimize the effects of other
   flows' bursts.

8.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

9.  Security Considerations

   TBD

10.  References

10.1.  Normative References

   [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>.

   [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>.

10.2.  Informative References

Authors' Addresses

   Yeoncheol Ryoo
   ETRI
   Email: dbduscjf@etri.re.kr


   Jinoo Joung
   Sangmyung University
   Email: jjoung@smu.ac.kr






Ryoo & Joung             Expires 7 January 2026                 [Page 9]
