DetNet Working Group Y. Ryoo
Internet-Draft ETRI
Intended status: Standards Track J. Joung
Expires: 23 July 2026 Sangmyung University
19 January 2026
On-time Forwarding with Non-work Conserving Stateless Core Fair Queuing
draft-ietf-detnet-nscore-00
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
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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 23 July 2026.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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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. Characteristics . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Scaling requirements . . . . . . . . . . . . . . . . . . 8
7.2. Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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
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large number of flows can be a burden, so the proposed framework for
large-scale deterministic networking is called work conserving
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.
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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)
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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:
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∑[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
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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.
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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. Characteristics
7.1. Scaling requirements
The data and controller plane operations described in this document
have the following characteristics for the requirements described in
[I-D.ietf-detnet-scaling-requirements]. The item numbers below
correspond to the numbers of the technical requirements in Section 3
of [I-D.ietf-detnet-scaling-requirements].
1. N-SCORE does not require time synchronization. However, in order
to apply the eligible time and finish time calculated by the
previous node, the time difference between the previous node and
the current node must be known..
2. N-SCORE supports large single-hop propagation delays and does not
impose any restrictions on the amount of propagation delay.
3. N-SCORE supports the accommodation of the higher link speed. It
is considered possible to implement a PIFO queue supporting
speeds of 100 Gbps or more.
4. N-SCORE is scalable to the large number of flows as it does not
require to maintain flow states in a node.
5. N-SCORE is robust against node and link failures and topology
changes, as the PREOF function can be applied.
6. N-SCORE is a fair queuing-based solution that provides the
benefit of near-complete isolation between flows. Therefore, it
effectively prevents flow fluctuations even when different flows
dynamically join or leave the system.
7. The admission condition of N-SCORE depends solely on the service
rates of flows. Therefore, the admission checking process is
simple, and there are no scalability issues with respect to the
number of hops.
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8. N-SCORE uses a dedicated PIFO queue and clearly distinguishes the
algorithm applied to it from that used for the existing FIFO
queue. It supports multiple mechanisms by appropriately mapping
each flow to a queue based on its SLA. Furthermore, it can
support multiple algorithms across multiple domains by
compartmentalizing the end-to-end delay requirements according to
sections divided by differences in domain or link speed, and
applying an appropriate service rate for each section.
7.2. Taxonomy
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)
* time bound (bounded, left-bounded, right-bounded, unbounded)
* service order (rate-based, time-based, arrival-based, priority-
based)
N-SCORE is a non-periodic, asynchronous, flow level, left-bounded,
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
* Flow level rate based left-bounded category
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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
[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-09, 7 September
2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
detnet-scaling-requirements-09>.
Authors' Addresses
Yeoncheol Ryoo
ETRI
Email: dbduscjf@etri.re.kr
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Jinoo Joung
Sangmyung University
Email: jjoung@smu.ac.kr
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