Network Shaofu. Peng Internet-Draft ZTE Corporation Intended status: Standards Track Zongpeng. Du Expires: 4 January 2025 China Mobile Kashinath. Basu Oxford Brookes University Zuopin. Cheng New H3C Technologies Dong. Yang Beijing Jiaotong University Chang. Liu China Unicom 3 July 2024 Deadline Based Deterministic Forwarding draft-peng-detnet-deadline-based-forwarding-11 Abstract This document describes a deterministic forwarding mechanism to IP/ MPLS network, as well as corresponding resource reservation, admission check, policing, etc, to provide guaranteed latency. Especially, latency compensation with core stateless is discussed to replace reshaping to be suitable for Diff-Serv architecture [RFC2475]. 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 4 January 2025. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. Peng, et al. Expires 4 January 2025 [Page 1] Internet-Draft Deadline Queueing Mechanism July 2024 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 2. EDF Scheduling Overview . . . . . . . . . . . . . . . . . . . 5 2.1. Planned Residence Time of the DetNet Flow . . . . . . . . 6 2.2. Delay Levels Provided by the Network . . . . . . . . . . 7 2.3. Relationship Between Planned Residence Time and Delay Level . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4. Relationship Between Service Burst Interval and Delay Level . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Sorted Queue . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Scheduling Mode for PIFO . . . . . . . . . . . . . . . . 8 3.2. Schedulability Condition for PIFO . . . . . . . . . . . . 9 3.2.1. Conditions for Leaky Bucket Constraint Function . . . 9 3.2.2. Schedulability Condition Analysis for On-time Mode . 11 3.3. Buffer Size Design . . . . . . . . . . . . . . . . . . . 11 4. Rotation Priority Queues . . . . . . . . . . . . . . . . . . 11 4.1. Alternate Queue Allocation Rules . . . . . . . . . . . . 14 4.2. Scheduling Mode for RPQ . . . . . . . . . . . . . . . . . 14 4.3. Schedulability Condition for RPQ . . . . . . . . . . . . 14 4.3.1. Schedulability Condition for Alternate QAR . . . . . 15 4.3.2. Conditions for Leaky Bucket Constraint Function . . . 15 4.3.3. Schedulability Condition Analysis for On-time Mode . 16 4.4. Buffer Size Design . . . . . . . . . . . . . . . . . . . 16 5. Reshaping . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6. Latency Compensation . . . . . . . . . . . . . . . . . . . . 18 6.1. Get Accumulated Residence Time Deviation . . . . . . . . 18 6.2. Get Allowable Queueing Delay . . . . . . . . . . . . . . 19 6.3. Scheduled by Allowable Queueing Delay . . . . . . . . . . 20 7. Option-1: Reshaping plus Sorted Queue . . . . . . . . . . . . 21 8. Option-2: Reshaping plus RPQ . . . . . . . . . . . . . . . . 23 9. Option-3: Latency Compensation plus Sorted Queue . . . . . . 24 9.1. Packet Disorder Considerations . . . . . . . . . . . . . 25 10. Option-4: Latency Compensation plus RPQ . . . . . . . . . . . 27 10.1. Packet Disorder Considerations . . . . . . . . . . . . . 29 11. Jitter Performance by On-time Scheduling . . . . . . . . . . 31 12. Resource Reseravtion . . . . . . . . . . . . . . . . . . . . 34 12.1. Delay Resource Definition . . . . . . . . . . . . . . . 35 Peng, et al. Expires 4 January 2025 [Page 2] Internet-Draft Deadline Queueing Mechanism July 2024 12.2. Traffic Engineering Path Calculation . . . . . . . . . . 37 13. Policing on the Ingress . . . . . . . . . . . . . . . . . . . 37 14. Overprovision Analysis . . . . . . . . . . . . . . . . . . . 40 15. Compatibility Considerations . . . . . . . . . . . . . . . . 40 16. Deployment Considerations . . . . . . . . . . . . . . . . . . 42 17. Evaluations . . . . . . . . . . . . . . . . . . . . . . . . . 43 17.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 45 18. Taxonomy Considerations . . . . . . . . . . . . . . . . . . . 48 19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49 20. Security Considerations . . . . . . . . . . . . . . . . . . . 49 21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50 22. References . . . . . . . . . . . . . . . . . . . . . . . . . 50 22.1. Normative References . . . . . . . . . . . . . . . . . . 50 22.2. Informative References . . . . . . . . . . . . . . . . . 52 Appendix A. Proof of Schedulability Condition for RPQ . . . . . 53 Appendix B. Proof of Schedulability Condition for Alternate QAR of RPQ . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57 1. Introduction [RFC8655] describes the architecture of deterministic network and defines the QoS goals of deterministic forwarding: Minimum and maximum end-to-end latency from source to destination, timely delivery, and bounded jitter (packet delay variation); packet loss ratio under various assumptions as to the operational states of the nodes and links; an upper bound on out-of-order packet delivery. In order to achieve these goals, deterministic networks use resource reservation, explicit routing, service protection and other means. Resource reservation provides dedicated resources (such as bandwidth, buffer space, time slots, etc.) to DetNet flows. Explicit routing ensures the stability of the route and does not change with the real- time change of network topology. Service protection reduces the packet loss by sending multiple DetNet flows along multiple disjoint paths at the same time. Peng, et al. Expires 4 January 2025 [Page 3] Internet-Draft Deadline Queueing Mechanism July 2024 [P802.1DC] described some Quality of Service (QoS) features specified in IEEE Std 802.1Q, such as per-stream filtering and policing, queuing, transmission selection, stream control and preemption, in a network system which is not a bridge. The internal structure of IP/ MPLS routers may also be based on these components to describe the scheduling process of packets. In the presence of admission check, policing, reshaping, a large number of packet scheduling techniques can provide bounded latency. However, many solutions may result in an inefficient use of network resources, or provide an overestimated latency. Currently the underlying scheduling mechanisms in IP/MPLS networks generally use SP (Strict Priority) and WFQ (Weighted Fair Queuing), and manage a small number of priority based queues. They are rate based schedulers. For SP, the highest priority queue can consume the total port bandwidth, while for WFQ scheduler, each queue may be configured with a pre-set rate limit. Both of them can provide the worst-case latency, but evaluation is generally overestimated. In the case where the network core supports reshaping per flow (or optimized reshaping as provided by [IR-Theory]), the worst-case latency of a flow is approximately equal to the aggregated burst of the traffic class divided by the rate limit of that traffic class (note that a rate-based scheduler may refer to [Net-Calculus] to obtain its rate- latency service curve and get a more tighter evaluation). When the network core does not implement reshaping, multiple flows sharing the same priority may form burst cascade, making it more difficult or even impossible to evaluate the worst-case latency of a single flow. [EF-FIFO] discusses the SP scheduling behavior in this core-stateless situation, which requires the overall network utilization level to be limited to a small portion of its link capacity in order to provide an appropriate bounded latency. To address the overestimation issue of rate based scheduling (i.e., if want a low latency, may be forced to allocate a large service rate.), according to [EDF-algorithm], an EDF (earliest-deadline- first) scheduler, which always selects the packet with the shortest deadline for transmission, is an optimal scheduler for a bounded delay service in the sense that it can support the delay bounds for any set of connections that can be supported by some other scheduling method. EDF is a delay-based scheduler, which further distinguishes traffic in terms of time urgency, rather than rough traffic classes. This document introduces EDF scheduling mechanism to IP/MPLS network, as well as corresponding resource reservation, admission control, policing, etc, to provide guaranteed latency, as a supplement to IEEE 802.1 TSN mechanisms (please refer to [I-D.hp-detnet-tsn-queuing-mechanisms-evaluation] for their challenges meeting large scaling requirements). It is a layer-3 Peng, et al. Expires 4 January 2025 [Page 4] Internet-Draft Deadline Queueing Mechanism July 2024 solution and can operate over different types of QoS sensitive layer 2 including TSN but is not an alternative to TSN. Especially, a latency compensation based option is recommonded to replace reshaping to be suitable for Diff-Serv architecture [RFC2475]. This document also discusses two scheduling behaviors: in-time scheduling and on- time scheduling. The former only provide bounded delay, while the latter further provide bounded jitter. 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. EDF Scheduling Overview The EDF scheduler assigns a deadline for each incoming packet, which is equal to the time the packet arrives at the node plus the latency limit, i.e., planned residence time (D), see Section 2.1. The EDF scheduling algorithm always selects the packet with the earliest deadline for transmission. The precondition for EDF to work properly is that any DetNet flow must always satisfy the given traffic constraint function when it reaches a certain EDF scheduler. Therefore, it should generally implement traffic regulation at the network entrance to ensure that the admitted traffic complies with the constraints; And, implement reshaping on each intermediate node to temporarily cache packets to ensure that packets entering the EDF scheduler queue comply with the constraints. However, reshaping per flow is a challenge in large- scaling networks. Some core stateless optimization method need to be considered. Another challenge of EDF scheduling is that queued packets must be sorted and stored according to their deadline, and whenever a new packet arrives at the scheduler, it needs to perform search and insert operations on the corresponding data structure, e.g., a List, a PIFO (put-in first-out) queue, or other type of sorted queue, at line rate. [RPQ] described rotating-priority-queues that approximate EDF scheduling behavior, and do not require deadline based sorting of queued packets, simplifying enqueueing operations. According to the above two challenges and the potential optimization methods, we will obtain four combination solutions. Operators should choose appropriate solutions based on the actual network situation. This document suggests using option-3 or option-4, which are referred Peng, et al. Expires 4 January 2025 [Page 5] Internet-Draft Deadline Queueing Mechanism July 2024 to as CEDF (latency Compensation EDF). CEDF adjusts and sorts the arrival flows through the latency compensation factor carried in the packets, ensuring that the flows arrived at the EDF scheduler always conform to their constraints, avoiding the network core from maintaining the flow states to meet large scaling requirements. * option-1: Reshaping plus sorted queue. * option-2: Reshaping plus RPQ. * option-3: Latency Compensation plus sorted queue. * option-4: Latency Compensation plus RPQ. 2.1. Planned Residence Time of the DetNet Flow The planned residence time (termed as D) of the packet is an offset time, which is based on the arrival time of the packet and represents the maximum time allowed for the packet to stay inside the node. For a deterministic path based on deadline scheduling, the path has deterministic end-to-end delay requirements. The end-to-end delay includes two parts, the accumulated residence time and the accumulated link propagation delay. Subtract the accumulated link propagation delay from end-to-end delay budget to obtain the accumulated residence time. The accumulated residence time may be shared equally by each node along the path to obtain the average planned residence time of each node, or each node may have different planned residence time. Note that the link propagation delay in reality may be not always constant, e.g., due to the affection of temperature, we assume that the tool for detecting the link propagation delay can sense the changes beyond the preset threshold and trigger the recalculation of the deterministic path. There are many ways to indicate the planned residence time of the packet. Peng, et al. Expires 4 January 2025 [Page 6] Internet-Draft Deadline Queueing Mechanism July 2024 * Carried in the packet. The ingress PE node, when encapsulating DetNet flows, can explicitly insert the planned residence time into the packet according to SLA. The transit node, after receiving the packet, can directly obtain the planned residence time from the packet. Generally, only a single planned residence time needs to be carried in the packet, which is applicable to all nodes along the path; Or insert a stack composed of multiple planned residence time, one for each node. [I-D.peng-6man-deadline-option] defined a method to carry the shared planned residence time in the IPv6 packets. [I-D.pb-6man-deterministic-crh], [I-D.p-6man-deterministic-eh] defined methods to carry the stack of planned residence time in the IPv6 packets. * Included in the matched local FIB entry or policy entry. An implementation should support the policy to forcibly override the planned residence time obtained from the packet. 2.2. Delay Levels Provided by the Network The network may provide multiple delay levels on the outgoing port, each with its own delay resource pool. For example, some typical delay levels may be 10us, 20us, 30us, etc. In theory, any additional delay level can be added dynamically, as long as the buffer and remaining bandwidth on the data plane allow. The quantification of delay resource pool for each delay level is actually based on the schedulability conditions of EDF. This document introduces two types of resources per delay level: * Burst: represents the amount of bits bound that a delay level provided. * Bandwidth: represents the amount of bandwidth bound that a delay level provided. The bandwidth possessed by a certain delay level is also known as the service rate of that delay level. For more information on the construction of resource pools, please refer to Section 3.2 and Section 4.3. 2.3. Relationship Between Planned Residence Time and Delay Level The planned residence time (D) is the per-hop latency requirement of the flow, while the delay level (d) is the capability provided by the link. Peng, et al. Expires 4 January 2025 [Page 7] Internet-Draft Deadline Queueing Mechanism July 2024 Generally, we only need to design a limited number of delay levels to support a larger number of per-hop latency requirement. For example, there are delay levels such as d_1, d_2, ..., and d_n, In the resource management of the control plane, we assign d_i resources to all D that meet d_i <= D < d_(i+1). 2.4. Relationship Between Service Burst Interval and Delay Level Although we generally prefer to have the service burst interval (SBI) greater than the maximum delay level, there is actually no necessary association between SBI and delay level. Firstly, can a flow with small SBI (such as 10us) request a larger delay level (such as 100us)? Yes. It seems that during a longer residence time caused by delay level, there will be multiple rounds of burst interval packets leading to bursts accumulation. However, these packets can be distinguished and sent in sequence. In fact, we can multiply the original SBI by several times to obtain the expanded SBI (which includes multiple original bursts), with a length greater than the requested delay level, to get the preferred paradigm. Secondly, can a flow with large SBI (such as 1ms) request a smaller delay level (such as 10us)? This is certainly yes. 3. Sorted Queue [PIFO] defined the push-in first-out queue (PIFO), which is a priority queue that maintains the scheduling order or time. A PIFO allows elements to be pushed into an arbitrary position based on an element's rank (the scheduling order or time), but always dequeues elements from the head. 3.1. Scheduling Mode for PIFO A PIFO queue may be configured as either in-time or on-time scheduling mode, but cannot support both modes simultaneously. In the in-time scheduling mode, as long as the queue is not empty, packets always departured from the head of queue (HoQ) for transmission. The actual bandwidth consumed by the scheduler may exceed its preset service rate C. In the on-time scheduling mode, if the queue is not empty and the rank of the HoQ packet is equal to or earlier than the current system time, the HoQ packet will be sent. Otherwise, not. Peng, et al. Expires 4 January 2025 [Page 8] Internet-Draft Deadline Queueing Mechanism July 2024 3.2. Schedulability Condition for PIFO [RPQ] has given the schedulability condition for classic EDF that based on any type of sorted queue with in-time scheduling mode. Suppose for any delay level d_i, the corresponding accumulated constraint function is A_i(t). Let d_i < d_(i+1), then the schedulability condition is: sum{A_i(t-d_i) for all i} <= C*t (Equation-1) where, C is service rate of the EDF scheduler. It should be noted that for a delay level d_i, its residence time is actually contributed by its own flows and all other more urgent delay levels. Based on the schedulability conditions, we can choose the traffic arrival constraint function according to the preset delay level, or we can choose the delay level according to the preset traffic arrival constraint function. When setting up new flows in the network, admission check based on schedulability condition must be excecuted on each link that the flow passes through. Here, A_i(t) defines the upper limit of eligible arrivals of delay level d_i, and should not be treated as the actual arrivals (we mark it as a_i(t) for distinction). As described in this document, a_i(t) may contain ineligible arrivals that need firstly to be converted (or sorted) into eligible arrivals, e.g., by method of regulation (Section 5) or latency compensation (Section 6), and then processed by the EDF scheduler. 3.2.1. Conditions for Leaky Bucket Constraint Function Assume that we want to support n delay levels (d_1, d_2,..., d_n) in the network, and the traffic arrival constraint function of each delay level d_i is the leaky bucket arrival curve A_i(t) = b_i + r_i * t. Equation-1 can be expressed as: b_1 <= C*d_1 - M b_1 + b_2 + r_1*(d_2-d_1) <= C*d_2 - M b_1 + b_2 + b_3 + r_1*(d_3-d_1) + r_2*(d_3-d_2) <= C*d_3 - M ... ... Peng, et al. Expires 4 January 2025 [Page 9] Internet-Draft Deadline Queueing Mechanism July 2024 sum(b_1+...+b_n) + r_1*(d_n-d_1) + r_2*(d_n-d_2) + ... + r_n_1*(d_n-d_n_1) <= C*d_n - M where, C is the service rate of the EDF scheduler, M is the maximum size of the interference packet. Note that the preset value of b_i does not depend on r_i, but r_i generally refers to b_i (and burst interval) for setting. For example, the preset value of r_i may be small, while the value of b_i may be large. Such parameter design is more suitable for transmitting traffic with large service burst interval, large service burst size, but small bandwidth requirements. An extreme example is that the preset r_i of each level d_i is close to 0 (this is because the burst interval of the served flow is too large, e.g., one hour or one day), but the preset b_i is close to the maximum value (e.g., b_1 = C*d_1 - M, note that this also requires that the depth of the leaky bucket used to regulate the traffic is large enough), then when the concurrent flow of all delay levels is scheduled, the time 0~d_1 is all used to send the burst b_1, the time d_1~d_2 is all used to send the burst b_2, the time d_2~d_3 is all used to send the burst b_3, and so on. However, the typical allocation scheme is that the preset r_i of each level d_i will divide C roughly equally. For example, we may firstly pre-allocate b_1 = C*d_1 - M, r_1 = C/n; Then recursively pre- allocate b_2 = C*(d_2-d_1)*(n-1)/n, r_2 = C/n; And so on. The pre- allocated parameters b_i and r_i of each level d_i constitute the delay resources of that level of the link. A path can reserve required burst and bandwidth from delay resources of the specific delay level d_i, and the reservation is successful only if the two resources are successfully reserved at the same time. As long as neither b_i nor r_i is free, the delay resource of level d_i is exhausted. Alternatively, a more tight allocation scheme is to not preset the parameters of A_i(t), but to dynamically accumulate the parameters of A_i(t) based on the actual flows setup demand, and always check whether the schedulability condition is met based on the updated A_i(t) during the flow setup procedure. In this case, it is still necessary to set a resource limit for each delay level to prevent the flows of a certain delay level from consuming all resources. For example, we may set the resource limit of each delay level d_i to b_i_limit = C*(d_i - d_(i-1)) - M, r_i_limit = C/n. In this case, the dynamically updated b_i and r_i should be treated as utilized resources, and participate in schedulability condition checks. Peng, et al. Expires 4 January 2025 [Page 10] Internet-Draft Deadline Queueing Mechanism July 2024 3.2.2. Schedulability Condition Analysis for On-time Mode Compared with in-time mode, on-time mode is non-work-conserving, which can be considered as the combination of damper and EDF scheduler. On-time scheduling mode applied on a flow try to maintain the time interval between any adjacent packets of that flow to be consistent with the regulated interval on the flow entrance node. The maintenance of time intervals does not lead to an increase in the bandwidth occupied by that flow and cause the arrival curve to violate the traffic constraint function. So that the schedulability condition (i.e., Equation-1) can also be applied to on-time scheduling mode. See Section 11 for more information about jitter control. 3.3. Buffer Size Design The service rate of the EDF scheduler, termed as C, can reach the link rate, but generally only needs to be configured as part of the link bandwidth, such as 50%. Allow hierarchical scheduling, for example, the EDF scheduler may participate in higher-level SP scheduling along with other schedulers. If flows are rate-controlled (i.e., reshaping is done inside the network, or on-time mode is applied), the maximum depth of PIFO should be the total amount of burst resource of all delay levels. Otherwise, more buffer is necessary to store the accumulated bursts, that is, the PIFO zone where the distance from HoQ exceeds the largest delay level is just used to store accumulated bursts. Please refer to Section 16 for more considerations. 4. Rotation Priority Queues [RPQ] described rotating priority queues, and the priority granularity of the queue is the same as that of the flows. If the deadline of the flow is used as priority, it requires a lot of priority and corresponding queues, with scalability issues. Therefore, this section provides rotating priority queues with count- down time range whose rotation interval is more refined, with the following characteristics: * Each queue has CT (Count-down Time) that is decreased by RTI (Rotation Time Interval). The CT difference between two adjacent queues is CTI (CT Interval). RTI must be less than or equal to CTI, with CTI = K * RTI, where the natural number K >= 1. * The smaller the CT, the higher the priority. At the beginning, all queues have different initial CT values, i.e., staggered from each other, e.g., one queue has the minimum CT value (termed as Peng, et al. Expires 4 January 2025 [Page 11] Internet-Draft Deadline Queueing Mechanism July 2024 MIN_CT), and one queue has the maximum CT value (termed as MAX_CT), and the CT values of all queues increase equally by CTI.It should be noted that CT is just the countdown of the HoQ, and the countdown of the end of the queue (EoQ) is near CT+CTI. So the CT attribute of a queue is actually a range [CT, CT+CTI). * For a queue whose CT is MIN_CT, after a new round of CTI, its CT will become MIN_CT - CTI and immediately return to MAX_CT. The above CTI, RTI, MIN_CT and MAX_CT value should be choosed according to the hardware capacity. Each link can independently use different CTI. The general principle is that the larger bandwidth, the smaller CTI. The CTI must be designed large enough to include interference delay caused by a single low priority packet with maximum size. The choose of RTI should consider the latency granularity of various DetNet flows, so that CT updated per RTI can match the delay requirements of different flows. One implementation may not choose to let CT be actually updated at the granularity of RTI, but at the granularity of CTI. For example, the elapsed time within CTI can be recorded, and (cur_CT - elapsed_time) can be used as the actual CT of the queue, where cur_CT is the current CT of the queue that has not been updated yet. Although the update of cur_CT is slow, the actual CT is sensitive enough. According to different scheduling mode configured to the RPQ, MIN_CT may be designed to different values. For in-time mode, MIN_CT may be 0. For on-time mode with option E|D decoupling (see Section 11), MIN_CT may also be 0, assuming that the packet allows departure from pre-scheduler, the time it takes to send to post-scheduler can be ignored. For on-time mode with option E+D integration, MIN_CT may be -N*CTI, where N is the amount of delay levels, considering the transmission time to the output link cannot be ignored. A specific example of RPQ configured with in-time scheduling mode is depicted in Figure 1. Peng, et al. Expires 4 January 2025 [Page 12] Internet-Draft Deadline Queueing Mechanism July 2024 +------------------------------+ +------------------------------+ | RPQ Group: | | RPQ Group: | | queue-1(CT=50us) ###### | | queue-1(CT=49us) ###### | | queue-2(CT=40us) ###### | | queue-2(CT=39us) ###### | | queue-3(CT=30us) ###### | | queue-3(CT=29us) ###### | | queue-4(CT=20us) ###### | | queue-4(CT=19us) ###### | | queue-5(CT=10us) ###### | | queue-5(CT= 9us) ###### | | queue-6(CT=0us) ###### | | queue-6(CT=-1us) ###### | +------------------------------+ +------------------------------+ +------------------------------+ +------------------------------+ | Other Queue Group: | | Other Queue Group: | | queue-7 ############ | | queue-7 ############ | | queue-8 ############ | | queue-8 ############ | | queue-9 ############ | | queue-9 ############ | | ... ... | | ... ... | +------------------------------+ +------------------------------+ -o----------------------------------o-------------------------------> T0 T0+1us time Figure 1: Example of RPQ Groups In this example, the CTI for RPQ group is configured to 10us. Queue-1 ~ queue-6 are members of RPQ group. Each queue has its initial CT attribute, and the CT of all queues are staggered from each other. For example, the CT of queue-1 is 50us (MAX_CT), the CT of queue-2 is 40uS, ..., the CT of queue-6 is 0 (MIN_CT). Suppose the scheduling engine initiates a rotation timer with a time interval of 1us, i.e., CTI = 10 * RTI in this case. As shown in the figure, at T0 + 1us, the CT of queue-1 becomes 49us, the CT of queue-2 becomes 39us, etc. At T0 + 10us, the CT of queue-6 will return to 50us (MAX_CT). Note that the minimum D requested by a DetNet flow should not be smaller than d_1+F, where d_1 is the most urgent delay level, F is the intra node forwarding delay. Therefore any packets with in-time scheduling should have Q (i.e., D + E - F) that is not be smaller than d_1, and should never be inserted to a queue whose CT is negative. Peng, et al. Expires 4 January 2025 [Page 13] Internet-Draft Deadline Queueing Mechanism July 2024 4.1. Alternate Queue Allocation Rules There may be extreme scenarios that multiple delay levels of eligible bursts arrive sequentially, with lower priority burst arriving first and higher priority burst arriving later, and then simultaneously releasing flood. In this case, it is necessary to ensure that the higher priority burst is sent first to meet its deadline. Therefore it may further let a RPQ queue (act as the virutal parent queue) contain multiple sub-queues, each for a delay level. The physical sub-queue with small delay level (e.g., 10us) is ranked before the physical sub-queue with large delay level (e.g., 20us). Packets are actually stored in the physical sub-queues. That is, packets with different D are inserted into different sub-queues and protected. In this way, for two packets with the same Q but different D, we can decide to firstly schedule the packet with the smallest D. This alternate queue allocation rule enables eligible arrivals always have a place to store, avoiding conflicts in local positions of the RPQ queue group and causing overflow. 4.2. Scheduling Mode for RPQ A RPQ group may be configured as either in-time or on-time scheduling mode, but cannot support both modes simultaneously. In the in-time scheduling mode, in all non empty queues, the packets in each queue are sequentially sent in the order of high priority queue to low priority queue. The actual bandwidth consumed by the scheduler may exceed its set service rate C. In the on-time scheduling mode, only in all non empty queues with CT <= 0, the packets in each queue are sequentially sent in the order of high priority queue to low priority queue. For a virtual parent queue that is allowed to be sent, for the multiple non empty physical sub-queues it contains, packets are sequentially sent from the non empty physical sub-queues along the direction from the physical sub-queues with small delay levels to the physical sub-queues with large delay levels. Only when a physical sub-queue is cleared can the next non empty physical sub-queue be sent. 4.3. Schedulability Condition for RPQ In this section, we discuss the schedulability condition based on RPQ with in-time scheduling mode firstly. Peng, et al. Expires 4 January 2025 [Page 14] Internet-Draft Deadline Queueing Mechanism July 2024 Suppose for any delay level d_i, the corresponding accumulated constraint function is A_i(t), and let d_i < d_(i+1). Suppose for any planned residence time D_i, the the corresponding constraint function is A'_i(t). For simplicity, we take intra node forwarding delay F as 0. Then the schedulability condition is: * A_1(t-d_1) + sum{A_i(t+CTI-d_i) for all i>=2} <= C*t, if a d_i contains only one D_i. (Equation-2) * sum{A_i(t+CTI-d_i) for all i>=1} <= C*t, if d_i contains multiple D_i. (Equation-3) where CTI is the CT interval between adjacency queue, C is service rate of the EDF scheduler. The proof is similar with that in [RPQ], except that the rotation step is fine-grained by RTI and the priority of each queue is CT range. Please refer to Appendix A for the proof. Note that the key difference between the above two conditions (i.e., Equation-2, Equation-3) and one based on sorted queue (i.e., Equation-1) is the CTI factor. Other common considerations are the same as Section 3.2. 4.3.1. Schedulability Condition for Alternate QAR According to Section 4.1, a RPQ queue may further contain multiple sub-queues, each for a delay level. Under the same parent queue, all sub-queues are sorted in descending order of delay level. In this case, the precise workload should exclude packets with higher delay levels than the observed packet. In the case that d_i contains only one D_i, the schedulability condition is Equation-1. In the case that d_i contains multiple D_i, the schedulability condition is still Equation-3. Please refer to Appendix B for the proof. 4.3.2. Conditions for Leaky Bucket Constraint Function Assume that we want to support delay levels (d_1, d_2,..., d_n) in the network, and the traffic arrival constraint function of each delay level d_i is the leaky bucket arrival curve A_i(t) = b_i + r_i * t. Equation-2 can be expressed as: Peng, et al. Expires 4 January 2025 [Page 15] Internet-Draft Deadline Queueing Mechanism July 2024 b_1 <= C*d_1 - M b_1 + b_2 + (r_1+r_2)*CTI <= C*d_2 - M b_1 + b_2 + b_3 + (r_1+r_2)*2*CTI + r_3*CTI <= C*d_3 - M ... ... sum(b_1+...+b_n) + (r_1+r_2)*(n-1)*CTI + r_3*(n-2)*CTI + ... + r_n*CTI <= C*d_n - M where, C is the service rate of the EDF scheduler, M is the maximum size of the interference packet. Equation-3 can be expressed as: b_1 + r_1*CTI <= C*d_1 - M b_1 + b_2 + r_1*2*CTI + r_2*CTI <= C*d_2 - M b_1 + b_2 + b_3 + r_1*3*CTI + r_2*2*CTI + r_3*CTI <= C*d_3 - M ... ... sum(b_1+...+b_n) + r_1*n*CTI + r_2*(n-1)*CTI + ... + r_n*CTI <= C*d_n - M 4.3.3. Schedulability Condition Analysis for On-time Mode Compared with in-time mode, on-time mode is non-work-conserving, which can be considered as the combination of damper and EDF scheduler. On-time scheduling mode applied on a flow try to maintain the time interval between any adjacent packets of that flow to be consistent with the regulated interval on the flow entrance node. The maintenance of time intervals does not lead to an increase in the bandwidth occupied by that flow and cause the arrival curve to violate the traffic constraint function. So that the schedulability condition (i.e., Equation-2/3) can also be applied to on-time scheduling mode. See Section 11 for more information about jitter control. 4.4. Buffer Size Design The buffer size of each physical RPQ queue may be desgined to CTI * C - M, where M is the maximum size of the packet with low priority. An implementation may let all physical queues share the common buffer with the total buffer cost as the sum of burst resources of all delay levels. Peng, et al. Expires 4 January 2025 [Page 16] Internet-Draft Deadline Queueing Mechanism July 2024 Especially if alternet QAR (Section 4.1) is applied, the actual buffer cost of a virtual parent queue is contributed by all the physical sub-queues it contains. The actual buffer cost of each physical sub queue is dynamically allocated based on whether there is a packet inserted. According to Section 4.3, the maximum buffer cost of a physical sub-queue may reach the upper limit of burst resources for the corresponding delay level (such as C * CTI - M). However, all physical sub-queues with the same delay level under all virtual parent queues cannot simultaneously reach the maximum buffer cost, but their sum may reach the maximum buffer cost. If flows are rate-controlled (i.e., reshaping is done inside the network, or on-time scheduling mode is applied), the MAX_CT may be designed as the maximum delay level, and total necessary buffer shared by all queues should be the total amount of burst resource of all delay levels. Otherwise, MAX_CT should be larger than the maximum delay level, and with more necessary buffer, to store the accumulated bursts, that is, all the queues with CT larger than the maximum delay level are just used to store accumulated bursts. Please refer to Section 16 for more considerations. 5. Reshaping Reshaping per flow inside the network, as described in [RFC2212], is done at all heterogeneous source branch points and at all source merge points, to restore (possibly distorted) traffic's shape to conform to the TSpec. Reshaping entails delaying packets until they are within conformance of the TSpec. A network element MUST provide the necessary buffers to ensure that conforming traffic is not lost at the reshaper. Note that while the large buffer makes it appear that reshapers add considerable delay, this is not the case. Given a valid TSpec that accurately describes the traffic, reshaping will cause little extra actual delay at the reshaping point (and will not affect the delay bound at all). Maintaining a dedicated shaping queue per flow can avoid burstiness cascading between different flows with the same traffic class, but this approach goes against the design goal of packet multiplexing networks. [IR-Theory] describes a more concise approach by maintaining a small number of interleaved regulators (per traffic class and incoming port), but still maintaining the state of each flow. With this regulator, packets of multiple flows are processed in one FIFO queue and only the packet at the head of the queue is examined against the regulation constraints of its flow. However, as the number of flows increases, the IR operation may become burdensome as much as the per-flow reshaping. Peng, et al. Expires 4 January 2025 [Page 17] Internet-Draft Deadline Queueing Mechanism July 2024 For any observed EDF scheduler in the network, when the traffic arriving from all incoming ports is always reshaped, then these flows comply with their arrival constraint functions, which is crucial for the schedulability conditions of EDF scheduling. Based on this, it can quantify the delay resource pool which is open and reserved for DetNet flows. 6. Latency Compensation [RFC9320] presents a latency model for DetNet nodes. There are six type of delays that a packet can experience from hop to hop. The processing delay (type-4), the regulator delay (type-5) , the queueing subsystem delay (type-6), and the output delay (type-1) together contribute to the residence time in the node. In this document, the residence time in the node is simplified into two parts: the first part is to lookup the forwarding table when the packet is received from the incoming port (or generated by the control plane) and deliver the packet to the line card where the outgoing port is located; the second part is to store the packet in the queue of the outgoing port for transmission. These two parts contribute to the actual residence time of the packet in the node. The former can be called forwarding delay (termed as F) and the latter can be called queueing delay (termed as Q). The forwarding delay is related to the chip implementation and is generally constant (with a maximum value); The queueing delay is unstable. 6.1. Get Accumulated Residence Time Deviation The accumulated residence time deviation, also termed as latency deviation (E), equals accumulated planned residence time minus accumulated actual residence time. This value can be zero, positive, or negative. The accumulated planned residence time of the packet refers to the sum of the planned residence time of all upstream nodes before the packet is transmitted to the current node. The accumulated actual residence time of the packet, refers to the sum of the actual residence time of all upstream nodes before the packet is transmitted to the current node. In the case of in-time scheduling, E may be a very large positive value. While in the case of on-time scheduling, E may be 0, or a small value close to 0. Peng, et al. Expires 4 January 2025 [Page 18] Internet-Draft Deadline Queueing Mechanism July 2024 The setting of the latency deviation (E) of the packet needs to be friendly to the chip for reading and writing. For example, it should be designed as a fixed position in the packet. The chip may support flexible configuration for that position. [I-D.peng-6man-delay-options] defined the method for carrying the latency deviation (E) in the IPv6 Hop-by-Hop Options Header. [I-D.pb-6man-deterministic-crh], [I-D.p-6man-deterministic-eh] defined methods for carrying the latency deviation (E) in the IPv6 Routing Header. 6.2. Get Allowable Queueing Delay When an EDF scheduler receives a packet, it can calculate allowable queueing delay (Q) for the packet. Specifically, it can first get the latency deviation (E), and add it to the planned residence time (D) of the packet at this node to obtain the adjustment residence time, and then deduct the actual forwarding delay (F) of the packet in the node, to obtain the allowable queueing delay (Q) for that packet. * Q = D + E - F The scheduler selects a buffer position (e.g., queue-id, or rank) for the packet based on Q. Note that one implementation may calculate Q at incoming port and determine the buffer position of the outgoing port. In this case, Q = D + E, and a buffer position indication may be notified from the incoming port to the outgoing port. In detailed, assume that the current node in a deterministic path is h, all upstream nodes are from 1 to h-1. For any node i, denote the planned residence time as D[i], the actual residence time as R[i], the input latency deviation (contributed by all upstream nodes) as E[i], the forwarding delay intra-node as F[i], then the allowable queueing delay (Q) of the packet on node h is: Q[h] = D[h] + E[h] - F[h] E[h] = D[h-1] + E[h-1] - R[h-1] D[0], E[0], R[0] = 0 Peng, et al. Expires 4 January 2025 [Page 19] Internet-Draft Deadline Queueing Mechanism July 2024 6.3. Scheduled by Allowable Queueing Delay The packet will be sheduled based on its Q, that is, the packet is scheduled based on latency compensation contributed by E, instead of only D. The earliest literature similar to the idea of latency compensation based on E can be found in [Jitter-EDF]. The core stateless latency compensation can achieve the effect of reshaping per flow to get the eligible arrivals pattern. Q can be used to sort ineligible arrvials of one delay level and prevent them from interferring with the scheduling of eligible arrvials of other delay levels. Firstly, at the flow (e.g., flow i) entrance node, all packets (after regulation) of flow i will be released to the EDF scheduler one after another at different time (termed as ideal arrival time), but with the same allowable queueing delay (Q), with initial E = 0, i.e., Q = D, assuming no link propagation delay and intra-node forwarding delay for simplicity. We denote this arrival pattern faced by the shceduler on the flow entrance node as arrival_pattern_0, which contains a sequence of packets with varaint of intervals between adjacent packets. We say that arrival_pattern_0 is eligible arrivals because its arrival curve is less than the constraint function A_i(t). For any packet p in arrival_pattern_0, assuming its ideal arrival time is t_p_0. Then, we can get arrival_pattern_1 = arrival_pattern_0 + D that is also eligible arrivals, where, arrival_pattern_0 + D means that the ideal arrival time (at the scheduler of flow entrance node) of each packet in arrival_pattern_0 is added with D. In fact, arrival_pattern_1 is the eligible arrivals on the second node. That is, the second node may recover the eligible arrivals arrival_pattern_1 from the actual arrivals with the help of latency compensation, and then to schedule based on arrival_pattern_1. How did arrival_pattern_1 recover? For any packet p, assuming it experiences an actual queuing delay q on the flow enrance node, and will actually arrive at the second node at time t_p_0 + q, with E = D - q carried in the sending packet. The second node will recover the eligible arrival time of packet p by, eligible arrival time = actual arrival time + E = t_p_0 + q + D - q = t_p_0 + D. Therefore arrival_pattern_1 is recovered. Similarly, the third node may recover arrival_pattern_2 = arrival_pattern_0 + 2*D, and the fourth node may recover arrival_pattern_3 = arrival_pattern_0 + 3*D, and so on. On any node h, packet p will be sorted in the scheduler queue based on its eligible arrival time plus D, i.e., ideal departure time, for scheduling. Peng, et al. Expires 4 January 2025 [Page 20] Internet-Draft Deadline Queueing Mechanism July 2024 Because the scheduler always schedules based on eligible arrivals, its scheduling power will not be overwhelmed by actual arrivals that may include burst accumulation. We may think the packets sorted in the queue with ideal departure time as a virtual regulation, because the rank distance between the adjacent packets of the flow i is exactly maintained consistently with the corresponding regulated interval between these two adjacent packets on the flow entrance node. There is no need to require that this virtual regulation and a real regulation component must have exactly the same pattern, as long as each pattern is less than the arrival constraint function A_i(t). Although, lantency compensation has the effect of reshaping, but it is not equivalent to reshaping. Considering an accumulated bursts that violates the traffic constraint function and arrives at a node, if reshaping is used, it will substantially introduce shaping delay for the ineligible bursts, which will then enter the queueing subsystem. While if latency compensation is used, this ineligible bursts will only be penalized with a larger Q and tolerated to be placed in the queueing sub-system, and in the case of in-time mode it may be immediately sent if higher priority queues are empty. Note that the premise of latency compensation is that a flow must be based on a fixed explicit path. If multiple packets from the same flow arrive at the intermediate node via multiple paths with different propagation lengths, even if these packets are all eligible, bursts accumulation may still form and cannot even be punished. 7. Option-1: Reshaping plus Sorted Queue As shown in Figure 2, a receivd packet is inserted to the PIFO queue according to rank = A + D - F, where, A is the time that packet arrived at the scheduler, i.e., arrive_time_S in the figure. Depending on the situation of the accumulated burst arrived at the input port, different packets may face different shaping delays. The shaper will convert the input ineligible arrivals pattern (if possible) into an eligible arrivals pattern. Here, D - F may be denoted as the allowable queueing delay Q. Peng, et al. Expires 4 January 2025 [Page 21] Internet-Draft Deadline Queueing Mechanism July 2024 +---------------------------------------------------+ | +---+ +-+ +--------+ +-----------+ | | | | |X| | | | Scheduler | | Input | | S | |X| | | | (PIFO) | | Output port -O -> | & | -> |X| -> | Shaper | -> | top->[==] | -> O- port | | F | |X| | | | [==] | | | | | |X| | | |rank->[==] | | | | | |X| | | | [==] | | | +---+ +-+ +--------+ +-----------+ | +---------------------------------------------------+ | | ------o----------------------------------o-------------------> arrive_time_I arrive_time_S time |<-------- F ------->|<---- S ---->|<----- Q ------>| Figure 2: Reshaping plus Sorted Queue Enqueue rule: * For two packets with different rank, the packet with a smaller rank is closer to the head of the queue. * For two packets with the same rank, the packet with a smaller D is closer to the head of the queue. * For two packets with the same rank and D, the packet that arrive at the scheduler first is closer to the head of the queue. The planned residence time (D) should be carried in the packet. The scheduling mode (in-time or on-time) should also be carried in the packet, and used to insert packet into PIFO with the corresponding scheduling mode. Dequeue rule: * As mentioned in Section 3.1, for a PIFO with in-time scheduling mode, as long as the queue is not empty, packets are always departured from the HoQ for transmission; while for PIFO with on- time scheduling mode, only if the queue is not empty and the rank of the HoQ packet is equal to or earlier than the current system time, the HoQ packet can be sent. However, in this option the dequeue rule of on-time mode can not guarantee jitter, due to lack of factor E to absorb jitter per hop. The dequeue rule of on-time mode only controls the starting time when packets are allowed to be scheduled, but cannot guarantee that different packets have the same queuing delay. Peng, et al. Expires 4 January 2025 [Page 22] Internet-Draft Deadline Queueing Mechanism July 2024 8. Option-2: Reshaping plus RPQ As shown in Figure 3, a receivd packet is inserted to the appropriate RPQ queue with specific CT to meet CT <= Q < CT+CTI when the packet arrived at the scheduler, where Q = D - F. Depending on the situation of the accumulated burst arrived at the input port, different packets may face different shaping delays. The shaper will convert the input ineligible arrivals pattern (if possible) into an eligible arrivals pattern. +----------------------------------------------------+ | +---+ +-+ +--------+ +------------+ | | | | |X| | | | Scheduler | | Input | | S | |X| | | | (RPQ) | | Output port -O -> | & | -> |X| -> | Shaper | -> | CT1 #### | -> O- port | | F | |X| | | | CT2 #### | | | | | |X| | | |Q->CT3 #### | | | | | |X| | | | CT4 #### | | | +---+ +-+ +--------+ +------------+ | +----------------------------------------------------+ | | ------o----------------------------------o--------------------> arrive_time_I arrive_time_S time |<-------- F ------->|<---- S ---->|<------ Q ------>| Figure 3: Reshaping plus RPQ Enqueue rule: * For a packet with Q, select the target RPQ queue (i.e., the virtual parent queue) with corresponding CT, that meet CT <= Q < CT+CTI. * Under the selected virtual parent queue, select the target physical sub-queue with corresponding delay level d_i, which is closest to D-F and not greater than D-F. The planned residence time (D) should be carried in the packet. The scheduling mode (in-time or on-time) should also be carried in the packet, and used to insert packet into RPQ with the corresponding scheduling mode. Dequeue rule: Peng, et al. Expires 4 January 2025 [Page 23] Internet-Draft Deadline Queueing Mechanism July 2024 * As mentioned in Section 4.2, for a RPQ group with in-time scheduling mode, in all non empty queues, the packets in each queue are sequentially sent in the order of high priority queue to low priority queue; while for a RPQ group with on-time scheduling mode, only in all non empty queues with CT <= 0, the packets in each queue are sequentially sent in the order of high priority queue to low priority queue. However, in this option the dequeue rule of on-time mode can not guarantee jitter, due to lack of factor E to absorb jitter per hop. The dequeue rule of on-time mode only controls the starting time when packets are allowed to be scheduled, but cannot guarantee that different packets have the same queuing delay. 9. Option-3: Latency Compensation plus Sorted Queue As shown in Figure 4, a receivd packet is inserted to the PIFO queue according to rank = A1 + E + D, or rank = A2 + E + D - F, where, A1 is the time that packet arrived at the input port (i.e., arrive_time_I in the figure), A2 is the time that packet arrived at the scheduler (i.e., arrive_time_S in the figure). Note that E is initially 0 on the flow entrance node, and generally not 0 on other nodes and will update per hop. Depending on the situation of the accumulated burst arrived at the input port, different packets may have different input latency deviation E. Latency compensation will convert the input ineligible arrivals pattern (if possible) into an eligible arrivals pattern. Here, E + D - F may be denoted as the allowable queueing delay Q. +-----------------------------------------+ | +---+ +-+ +-----------+ | | | | |X| | Scheduler | | Input | | S | |X| | (PIFO) | | Output port -O --> | & | --> |X| --> | top->[==] | --> O- port | | F | |X| | [==] | | | | | |X| |rank->[==] | | | | | |X| | [==] | | | +---+ +-+ +-----------+ | +-----------------------------------------+ | | ------o-----------------------o-------------------> arrive_time_I arrive_time_S time |<--------- F --------->|<------ Q ------>| Figure 4: Latency Compensation plus Sorted Queue Peng, et al. Expires 4 January 2025 [Page 24] Internet-Draft Deadline Queueing Mechanism July 2024 The planned residence time (D) and latency deviation (E) should be carried in the packet. The enqueue and dequeue operations are the same as Section 7. In this option the dequeue rule of on-time mode can guarantee jitter with the help of factor E to absorb jitter per hop. See Section 11 for more information. 9.1. Packet Disorder Considerations Suppose that two packets, P1, P2, are generated instantaneously from a specific flow at the source, and the two packets have the same planned residence time. P1 may face less interference delay than P2 in their journey. When they arrive at an intermediate node in turn, P2 will have less allowable queueing delay (Q) than P1 to try to stay close to P1 again. It should be noted that to compary who is ealier is based on the time arriving at the scheduler plus packet's Q. The time difference between the arrival of two packets at the scheduler may not be consistent with the difference between their Q. It is possible to get an unexpected comparision result. As shown in Figure 5, P1 and P2 are two back-to-back packets belonging to the same flow. The arrival time when they are received on the scheduler is shown in the figure. Suppose that the Q values of two adjacent packets P1 and P2 are 40us and 39us, and arrive at the scheduler at time T1 and T2 respectively. P1 will be sorted based on T1 + 40us, while P2 will be sorted based on T2 + 39us. Ideally, T2 should be T1 + 1us. However, this may be not the case. For example, it is possible that T2 = T1 + 0.9us, Q1 = 40, Q2 = 39.1, but just because the calculation accuracy of Q1 and Q2 is microseconds, so they are, e.g., with half-adjust, approximately 40 us and 39 us, respectively. This means that P2 will be sorted before P1 in the PIFO, resulting in disorder. Peng, et al. Expires 4 January 2025 [Page 25] Internet-Draft Deadline Queueing Mechanism July 2024 packets arrived later packets arrived earlier | | | V V --------+-----------------------------------------------+--------- ... ... | .P1.................P2....................... | ... ... --------+-----------------------------------------------+--------- P1.Q=40us P2.Q=39us | | --------o---------------------o---------------------------------> T1 T2 (=T1+0.9us) time | ___________________| | | v v PIFO ############################################################## top Figure 5: Disorder Illustration of PIFO DetNet architecture [RFC8655] provides Packet Ordering Function (POF), that can be used to solve the above disorder problem caused by the latency compensation. Alternatively, Section 11 provides E|D decoupling method to firstly absorb latency deviation E by the pre-scheduler which may maintain FIFO queue per incoming port plus delay level. In this case, packets from the same flow will only determine the damping delay, but not the position, in the FIFO based on latency deviation E, to avoid disorder. Latency deviation E no longer works in the post-scheduler. Note that in practical situations, two back-to-back packets of the same flow are generally evenly distributed within the burst interval by regulation, which means that the distance between these two packets is generally much greater than the calculation accuracy mentioned above, meaning that the disordered phenomenon will not really occur. For example, the regulated result meets a Length Rate Quotient (LRQ) constraint, and the time interval between two consecutive packets of size l_i and l_j should be at least l_i/r, where r is the flow rate (i.e., the reserved bandwidth of the flow). This can be done by LRQ based regulation, or enhanced leaky bucket based regulation, depending on implementation. Peng, et al. Expires 4 January 2025 [Page 26] Internet-Draft Deadline Queueing Mechanism July 2024 10. Option-4: Latency Compensation plus RPQ As shown in Figure 6, a receivd packet is inserted to the appropriate RPQ queue with specific CT to meet CT <= Q < CT+CTI when the packet arrived at the scheduler, where Q = D + E - F. Depending on the situation of the accumulated burst arrived at the input port, different packets may have different input latency deviation E. Latency compensation will convert the input ineligible arrivals pattern (if possible) into an eligible arrivals pattern. +-------------------------------------------+ | +---+ +-+ +-------------+ | | | | |X| | Scheduler | | Input | | S | |X| | (RPQ) | | Output port -O --> | & | --> |X| --> | CT1 #### | --> O- port | | F | |X| | CT2 #### | | | | | |X| |Q-> CT3 #### | | | | | |X| | CT4 #### | | | +---+ +-+ +-------------+ | +-------------------------------------------+ | | ------o-----------------------o----------------------> arrive_time_I arrive_time_S time |<--------- F --------->|<------ Q ------>| Figure 6: Latency Compensation plus RPQ The planned residence time (D) and latency deviation (E) should be carried in the packet. The enqueue and dequeue operations are the same as Section 8. In this option the dequeue rule of on-time mode can guarantee jitter with the help of factor E to absorb jitter per hop. See Section 11 for more information. Figure 7 depicts an example of packets inserted to the RPQ queues. Peng, et al. Expires 4 January 2025 [Page 27] Internet-Draft Deadline Queueing Mechanism July 2024 P2 P1 +------------------------------+ +--------+ +--------+ | RPQ Group: | | D=20us | | D=30us | | queue-1(CT=45us) ###### | | E=15us | | E=-8us | +--+ | queue-2(CT=35us) ###### | +--------+ +--------+ |\/| | queue-3(CT=25us) ###### | ------incoming port-1------> |/\| | queue-4(CT=15us) ###### | |\/| | queue-5(CT=5us) ###### | P4 P3 |/\| | queue-6(CT=-5us) ###### | +--------+ +--------+ |\/| | queue-7(CT=-15us)###### | | | | D=30us | |/\| | ... ... | +--------+ | E=-30us| |\/| +------------------------------+ +--------+ |/\| ------incoming port-2------> |\/| +------------------------------+ |/\| | Other Queue Group: | P6 P5 |\/| | queue-8 ############ | +--------+ +--------+ |/\| | queue-9 ############ | | | | D=40us | |\/| | queue-10 ############ | +--------+ | E=40us | |/\| | ... ... | +--------+ +--+ +------------------------------+ ------incoming port-3------> ----------outgoing port---------> -o----------------------------------o------------------------------> arrival time +F time Figure 7: Time Sensitive Packets Inserted to RPQ As shown in Figure 7, the node successively receives six packets from three incoming ports, among which packet 1, 2, 3 and 5 have corresponding deadline information, while packet 4 and 6 are best- effort packets. These packets need to be forwarded to the same outgoing port. It is assumed that they arrive at the line card where the outgoing port is located at almost the same time after the forwarding delay (F = 5us). At this time, the queue status of the outgoing port is shown in the figure. Then: * The allowable queueing delay (Q) of packet 1 is 30 - 8 - 5 = 17us, and it will be put into queue-4 (its CT is 15us), meeting the condition that Q is in the range [15, 25). * The allowable queueing delay (Q) of packet 2 is 20 + 15 - 5 = 30us, and it will be put into queue-3 (its CT is 25us), meeting the condition that Q is in the range [25, 35). * The allowable queueing delay (Q) of packet 3 is 30 - 30 - 5 = -5us, and it will be put into queue-6 (its CT is -5us), meeting the condition that Q is in the range [-5, 5). Peng, et al. Expires 4 January 2025 [Page 28] Internet-Draft Deadline Queueing Mechanism July 2024 * The allowable queueing delay (Q) of packet 5 in the node is 40 + 40 - 5 = 75us, and the queue it is placed on is not shown in the figure (such as a hierarchical queue). * Packets 4 and 6 will be put into the non-deadline queue in the traditional way. According to Section 4.3, An eligible packet (i.e., E = 0) from a specific delay level, even at the end of the inserted queue, can ensure that it does not exceed its deadline, which is the key role of the CTI factor in the condition equation. Now, assuming that a packet is penalized to a lower priority queue based on its positive E, this penalty will not result in more than expected delay, apart from potential delay E. For example, when a packet is inserted queue based on CT_x <= Q < CT_x + CTI even if it is at the end of the queue, according to D = Q - E, i.e., after time E (the penalty time), we have CT_x - E <= Q - E < CT_x - E + CTI That is CT_y <= D < CT_y + CTI So, in essence, it is still equivalent to an eligible packet entering the corresponding queue based on its delay level, and apply the schedulability condition. 10.1. Packet Disorder Considerations Suppose that two packets, P1, P2, are generated instantaneously from a specific flow at the source, and the two packets have the same planned residence time. P1 may face less interference delay than P2 in their journey. When they arrive at an intermediate node in turn, P2 will have less allowable queueing delay (Q) than P1 to try to stay close to P1 again. It should be noted that to compary who is ealier is based on queue's CT and packet's Q, according to the above queueing rule (CT <= Q < CT+CTI), and the CT of the queue is not changed in real-time, but gradually with the decreasing step RTI. It is possible to get an unexpected comparision result. As shown in Figure 8, P1 and P2 are two packets belonging to the same flow. The arrival time when they are received on the scheduler is shown in the figure. Suppose that CTI is 10us, the decreasing step Peng, et al. Expires 4 January 2025 [Page 29] Internet-Draft Deadline Queueing Mechanism July 2024 RTI is 1us, and the transmission time of each packet is 0.01us. Also suppose that the Q values of two adjacent packets P1 and P2 are 40us and 39us respectively, and they are both received in the window from T0 to T0+1us. P1 will enter queue-B with CT range [40, 50), while P2 will enter queue-A with CT range [30, 40) just before the rotation event occurred. This means that P2 will be scheduled before P1, resulting in disorder. packets arrived later packets arrived earlier | | | V V --------+-----------------------------------------------+--------- ... ... | .P1.................P2....................... | ... ... --------+-----------------------------------------------+--------- P1.Q=40us P2.Q=39us | | | --------o---------------------o---------------------o-----------> T0 T0+1us T0+2us time queue-A.CT[30,40) queue-A.CT[29,39) queue-B.CT[40,50) queue-B.CT[39,49) queue-C.CT[50,60) queue-C.CT[49,59) Figure 8: Disorder Illustration of RPQ DetNet architecture [RFC8655] provides Packet Ordering Function (POF), that can be used to solve the above disorder problem caused by the latency compensation. Alternatively, Section 11 provides E|D decoupling method to firstly absorb latency deviation E by the pre-scheduler which may maintain FIFO queue per incoming port plus delay level. In this case, packets from the same flow will only determine the damping delay, but not the position, in the FIFO based on latency deviation E, to avoid disorder. Latency deviation E no longer works in the post-scheduler. Note that in practical situations, two back-to-back packets of the same flow are generally evenly distributed within the burst interval by policing, which means that the distance between these two packets is generally much greater than the calculation accuracy mentioned above, meaning that the disordered phenomenon will not really occur. For example, the regulated result meets a Length Rate Quotient (LRQ) constraint. Peng, et al. Expires 4 January 2025 [Page 30] Internet-Draft Deadline Queueing Mechanism July 2024 11. Jitter Performance by On-time Scheduling The enqueue and dequeue rule of on-time mode described in Section 9 and Section 10 will absorb latency deivation E on each hop, and archieve a low jitter. The ultimate E2E jitter depends on the delay experienced on the last node of the flow, which may be from 0 to the delay bound, i.e., the corresponding delay level d_i. Depending on different methods of absorbing E, there are slight differences in scheduling behavior. * E+D integration: E will be added to D to get the adjusted D'; The packet is scheduled by the EDF scheduler configured with on-time mode based on D'. * E|D decoupling: There are 2-tier schedulers; The packet is scheduled by pre-scheduler configured with on-time mode based on E, then scheduled by post-scheduler configured with in-time mode based on D. In the case of E+D integration, it may explicitly introduce the mandatory hold time, and cause that the actual departure time of the packet may be after its deadline. Asumming that the eligible arrivals pattern of all delay levels cause the scheduler to work at full speed (i.e., service rate C), for in-time mode, the worst case is that there may be a packet of a specific delay level to be sent just before its deadline during the busy period; While for E+D integration case, the busy period may just start at its deadline and cause the sending time of the packet to exceed its deadline. However, as mentioned above, the worst case of this exceeding value will not exceed the delay level value, which is intuitive because it is equivalent to the situation where the observed packet arrives asynchronously after the delay level value. Note that this exceeding deadline does not accumulate with the number of hops. The E2E latency is in the range [D*hops, D*hops+d_i]. In the case of E|D decoupling, the explicitly mandatory hold time is only contributed by E ensured by the pre-scheduler (configured with on-time mode), and the actual departure time (from the post- scheduler) of the packet will always be before its deadline. Asumming that the eligible arrivals pattern of all delay levels cause the post-scheduler (configured with in-time mode) to work at full speed, for E|D decoupling case, the worst case is that there may be a packet of a specific delay level to be sent just before its deadline during the busy period. The E2E latency is in the range [D*(hops-1), D*(hops-1)+d_i]. Note that the pre-scheduler may maintain a PIFO, an RPQ, or serveral FIFO queues each for particular incoming port plus delay level. Figure 9 shows the functional entities inside the node. Peng, et al. Expires 4 January 2025 [Page 31] Internet-Draft Deadline Queueing Mechanism July 2024 +--------------------------------------------------+ | +---+ +-+ +---------+ +---------+ | | | | |X| |Pre- | |Post- | | Input | | S | |X| |Scheduler| |Scheduler| | Output port -O -> | & | -> |X| -> | (by E) | -> | (by D) | -> O- port | | F | |X| |[*]PIFO | |[*]PIFO | | | | | |X| |[*]RPQ | |[*]RPQ | | | | | |X| |[*]FIFO | | | | | +---+ +-+ +---------+ +---------+ | +--------------------------------------------------+ | | ------o-----------------------------------o-------------------> arrive_time_I arrive_time_S time |<-------- F ------->|<---- E ----->|<---- D-F --->| Figure 9: E|D decoupling with 2-tier Schedulers The following Figure 10 shows the difference between on-time scheduling and in-time scheduling. Peng, et al. Expires 4 January 2025 [Page 32] Internet-Draft Deadline Queueing Mechanism July 2024 arrival flows: A_1: #1 #2 #3 #4 #5 ... A_2: $1 $2 $3 $4 $5 ... ... A_5: &1 &2 &3 &4 &5 ... | v In-time Scheduling: #1$1...&1 #2$2...&2 #3$3...&3 #4$4...&4 #5$5...&5 ... On-time Scheduling (E+D integration): #1 #2 #3 #4 #5 $1 $2 $3 $4 ... ... &1 On-time Scheduling (E|D decoupling): #1$1...&1 #2$2...&2 #3$3...&3 #4$4...&4 #5$5...&5 ... ------+---------+---------+---------+--... ...--+---------+----> \_ d_1 _/ time \______ d_2 ______/ \___________ d_3 ___________/ ... ... \_____________________ d_n _______________________/ Figure 10: Difference between In-time and On-time Scheduling As shown in the figure, each burst of A_1 (corresponding to delay level d_1) is termed as #num, each burst of A_2 (corresponding to delay level d_2) as $num, and each burst of A_5 (corresponding to delay level d_5) as &num. A single burst may contain multiple packets. For example, burst #1 may contain several packets, and the actual time interval between #1 and #2 may be small. Although the figure shows the example that the burst interval of multiple flows is the same and the phase is aligned, the actual situation is far from that. However, this example depicts the typical scheduling behavior. In the in-time scheduling, all concurrent traffic of multiple levels will be scheduled as soon as possible according to priority, to construct a busy period. For example, in the duration d_1, in addition to the burst #1 that must be sent, the burst $1~&1 may also be sent, but the latter is not necessarily scheduled to be sent before the burst #2 as shown in the figure. Here we clearly see that in-time scheduling cannot guarantee jitter. Peng, et al. Expires 4 January 2025 [Page 33] Internet-Draft Deadline Queueing Mechanism July 2024 While in the case of on-time scheduling with E+D integration option, each burst is scheduled at its deadline, which may just be the begin of the busy period. Because of the scheduling delay, the transmission of the burst will exceed its deadline. The last packet of the burst will face more delay than the first packet. For example, when burst #5 enters the PIFO, it may have the same deadline with bursts from $4 of A_2 to &1 of A_5. When the deadlines of multiple packets are the same, use planned residence time (D) as tiebreaker, i.e., the smaller the D, the smaller the rank. So, #5 send first and may exceed the deadline by one d_1; Then send $4 and may exceed the deadline by one d_2; ...; Finally, send &1 and may exceed the deadline by one d_5. In the case of on-time scheduling with E|D decoupling, assuming that the latency deviation E for each burst is 0 in the above figure, all concurrent traffic of multiple levels, similar to in-time, will also be scheduled as soon as possible according to priority, to construct a busy period. For example, in the duration d_1, in addition to the burst #1 that must be sent, the burst $1~&1 may also be sent. However, $1~&1 will receive punishment based on their E on the next node (not shown in the figure). 12. Resource Reseravtion Generally, a path may carry multiple DetNet flows with different delay levels. For a certain delay level d_i, the path will reserve some resources from the delay resource pool of the link. The delay resource pool here, as leaky bucket constraint function shown in Section 3.2.1 or Section 4.3.2, is a set of preset parameters that meet the schedulability conditions. For example, the level d_1 has a burst upper limit of b_1 and a bandwidth upper limit of r_1. A path j may allocate partial resources (b_i_j, r_i_j) from the resource pool (b_i, r_i) of the link's delay level d_i. A DetNet flow k that carried in path j, may use resources (b_i_j_k, r_i_j_k) according to its T_SPEC. It can be seen that the values of b_i_j and r_i_j determine the scale of the number of paths that can be supported, while the values of b_i_j_k and r_i_j_k determine the scale of the number of flows that can be supported. The following expression exists. * sum(b_i_j_k) <= b_i_j, for all flow k over the path j. * sum(r_i_j_k) <= r_i_j, for all flow k over the path j. * sum(b_i_j) <= b_i, for all path j through the specific link. * sum(r_i_j) <= r_i, for all path j through the specific link. Peng, et al. Expires 4 January 2025 [Page 34] Internet-Draft Deadline Queueing Mechanism July 2024 12.1. Delay Resource Definition The delay resources of a link can be represented as the corresponding burst and bandwidth resources for each delay level. Basically, what delay levels (e.g., 10us, 20us, 30us, etc) are supported by a link should be included in the link capability. Figure 11 shows the delay resource model of the link. The resource information of each delay level includes the following attributes: * Delay Bound: Refers to the delay bound intra node corresponding to this delay level. It is a pre-configuration value. * Maximum Reservable Bursts: Refers to the maximum amount of bit quota corresponding to this delay level. It is a pre-allocated value or resource limit set based on the schedulability condition. * Utilized Bursts: Refers to the burst utilization of this delay level. * Maximum Reservable Bandwidth: Refers to the maximum amount bandwidth corresponding to this delay level. It is a pre- allocated value or resource limit set based on the schedulability condition. * Utilized Bandwidth: Refers to the bandwidth utilization of this delay level. Peng, et al. Expires 4 January 2025 [Page 35] Internet-Draft Deadline Queueing Mechanism July 2024 d_n +----------------------------------------+ | Maximum Reservable Bursts: MRBur_n | | Utilized Bursts: UBur_n | | Maximum Reservable Bandwidth: MRBan_n | | Utilized Bandwidth: UBan_n | +----------------------------------------+ ... ... ... ... ... ... d_2 +----------------------------------------+ | Maximum Reservable Bursts: MRBur_2 | | Utilized Bursts: UBur_2 | | Maximum Reservable Bandwidth: MRBan_2 | | Utilized Bandwidth: UBan_2 | +----------------------------------------+ d_1 +----------------------------------------+ | Maximum Reservable Bursts: MRBur_1 | | Utilized Bursts: UBur_1 | | Maximum Reservable Bandwidth: MRBan_1 | | Utilized Bandwidth: UBan_1 | +----------------------------------------+ -----------------------------------------------------------> Unidirectional Link Figure 11: Delay Resource of the Link For a specific link: * If Maximum Reservable Bursts and Maximum Reservable Bandwidth are used to participate schedulability condition checking, they need to set reasonable values at the beginning to meet the schedulability condition, and in the future, there is no need to excecute schedulability condition checking during the setup procedure of any flow passing through this link, but only need to check that the aggregated burst and bandwidth of all flows belonging to the same delay level do not exceed Maximum Reservable Bursts and Maximum Reservable Bandwidth, respectively. * If Utilized Bursts and Utilized Bandwidth are used to participate schedulability condition checking, there is necessary to excecute schedulability condition checking during the setup procedure of any new flows passing through this link. The IGP/BGP extensions to advertise the link's capability and delay resource is defined in [I-D.peng-lsr-deterministic-traffic-engineering]. Peng, et al. Expires 4 January 2025 [Page 36] Internet-Draft Deadline Queueing Mechanism July 2024 12.2. Traffic Engineering Path Calculation A candidate path may be selected according to the end-to-end delay requirement of the flow. Subtract the accumulated link propagation delay from the end-to-end delay requirement, and then divide it by the number of hops to obtain the average planned residence time (D) for each node. Or, different nodes may have different planned residence time (D). By default, select the appropriate delay level d_i (d_i <= D-F) closest to the planned residence time (D), and then reserve resources from delay level d_i on each hop. A local policy may allow more larger D to consume resources with smaller delay levels. Note that it is planned residence time (D), not delay level (d_i), carried in the forwarding packets. 13. Policing on the Ingress On the ingress PE node, policing must be performed on the incoming port, so that DetNet flow does not exceed its T-SPEC. This kind of traffic regulation is usually the shaping using leaky bucket combined with the incoming queue that receives flows. A DetNet flow may generate discrete multiple bursts within its periodic service burst interval (SBI). According to [RFC9016], the values of Burst Interval, MaxPacketsPerInterval, MaxPayloadSize of the DetNet flow will be written in the SLA between the customer and the network provider, and the network entry node will set the corresponding bucket depth according to MaxPayloadSize to forcibly delay the excess bursts. The entry node also sets the corresponding bucket rate according to the promised arrival rate. The leaky bucket shaping for the DetNet flow will essentially make all the bursts within the SBI evenly distributed within the SBI, which may be inconsistent with the original arrival pattern of that flow. Therefore, some bursts within the SBI may face more shaping delay. For example, at the beginning of the SBI, it contains two discrete bursts with the same size, but the bandwidth reserved by the flow is very small (i.e., total burst size / SBI). Assuming that the bucket depth is the size of a single burst, the shaping delay faced by the second burst is approximately half of the SBI. Peng, et al. Expires 4 January 2025 [Page 37] Internet-Draft Deadline Queueing Mechanism July 2024 Although the shaped pattern and the original arrival pattern can be as consistent as possible by increasing the bucket depth, to minimize the shaping delay of each burst, but this means that the flow will occupy more burst resources, and reduce the service scale that the network can support according to the schedulability conditions. Unless, customers are willing to pay for a larger burst resource. On the network entrance node, for the burst with applied shaping delay, shaping delay cannot be included in the latency compensation equation, otherwise, it will make that burst catch up with the previous burst, resulting in damage to the policing result and violation of the arrival constraint function. Please refer to [I-D.peng-detnet-policing-jitter-control] for the elimination of jitter caused by shaping delay on the network entrance node. Then, the regulated traffic arrives at the EDF scheduler on the outgoing port. Since the traffic of each delay level meets the leaky bucket arrival constraint function and the parameters of the shaping curve do not exceed the limits of the parameters provided by the schedulability conditions, the traffic can be successfully scheduled based on deadline. Note that the flow arrived at the next hop, after reshaping or latency compensation, will still follow the arrival constraint function of that flow. When this flow is aggregated with other flows and sent to the same outgoing port, within any duration d_i, the aggregated d_i traffic will not exceed the burst and bandwidth resources of delay level d_i reserved by these flows on the outgoing port. Figure 12 depicts an example of policing and deadline based scheduling on the ingress PE node in the case of option-4 with on- time mode. In the figure, the shaping delay caused by the previous burst is termed as S#. Peng, et al. Expires 4 January 2025 [Page 38] Internet-Draft Deadline Queueing Mechanism July 2024 1st burst | received v +-+ +-+ +----+ +-+ +--+ +------+ |1| |2| | 3 | |4| |5 | | 6 | <= burst +-+ +-+ +----+ +-+ +--+ +------+ sequence | | | | | | ~+0 ~+S1 ~+0 ~+S3~+S4 ~+0 <= shaping | \ | | \_ | delay | | | | | | UNI v v v v v v ingr-PE -+--------+--------+--------+--------+--------+--------+----> NNI | CTI | CTI | CTI | CTI | CTI | CTI | time 1,2 in 3 in 4 in 5 in 6 in queue-A queue-B queue-C queue-D queue-E A.CT<=Q B.CT<=Q C.CT<=Q D.CT<=Q E.CT<=Q | | | | | ~+Q ~+Q ~+Q ~+Q ~+Q <= e.g., Q = CTI \_____ \_____ \_____ \_____ \_____ | | | | | sending v(que-A) v(que-B) v(que-C) v(que-D) v(que-E) +-+-+ +----+ +-+ +--+ +------+ |1|2| | 3 | |4| |5 | | 6 | +-+-+ +----+ +-+ +--+ +------+ Figure 12: Deadline Based Packets Orchestrating There are 6 bursts received from the client. The burst-2, 4, 5 has policing delay S1, S3, S4 that caused by previous burst respectively. While burst-1, 3, 6 has zero policing delay because the number of tokens is sufficient. The policing makes 6 bursts roughly distributed within the service burst interval. Assuming that the forwarding delay F experienced by all bursts is 0. In the case of latency compensation plus RPQ, they will have the same allowable queueing delay (Q), regardless of whether they have experienced policing delay before. When the packets of burst-1, 2 arrive at the scheduler, according to CT <= Q < CT+CTI, they will be placed in Queue-A with matched CT and waiting to be sent. Similarly, when the packets of burst-3/4/5/6 arrive at the scheduler, they will be placed in Queue-B/C/D/E respectively and waiting to be sent according to the de-queue rules of on-time mode. Note that each sending burst may get a latency deviation E, especially for burst-2, which is sent closely adjacent to burst-1 in the sending pattern. Peng, et al. Expires 4 January 2025 [Page 39] Internet-Draft Deadline Queueing Mechanism July 2024 14. Overprovision Analysis For each delay level d_i, the delay resource of the specific link is (b_i, r_i). A path j may allocate partial resources (b_i_j, r_i_j) from the resource pool (b_i, r_i). In order to support more d_i flows in the network, it is necessary to set larger b_i and r_i. However, as mentioned earlier, the values of b_i and r_i are set according to schedulability conditions and cannot be set at will. Therefore, the meaningful analysis is the service scale that the network can support under the premise of determined b_i and r_i. For bandwidth resource reservation case, the upper limit of the total bandwidth that can be reserved for all aggregated flows of delay level d_i is r_i, which is the same as the behavior of traditional bandwidth resource reservation. There is no special requirement for the measurement interval of calculating bandwidth value. For the burst resource reservation case, the upper limit of the total burst that can be reserved for all aggregated flows of delay level d_i is b_i. If the burst of each flow of level d_i is b_k, then the number of flows can be supported is b_i/b_k, which is the worst case considering the concurrent arrival of these flows. However, the burst resource reservation is independent of bandwidth resource, i.e., it does not take the calculation result of b_k/d_i to get an overprovision bandwidth and then to affect the reservable bandwidth resources. By providing multiple delay levels, we can allocate 100% of the link bandwidth to DetNet flows, as can be seen from the schedulability condition equation. 15. Compatibility Considerations Deadline is suitable for end-to-end and interconnection between different networks. A large-scale network may span multiple networks, and one of the goals of DetNet is to connect each network domain to provide end-to-end deterministic delay service. The adoption techniques and capabilities of each network are different, and the corresponding topology models are either piecewise or nested. For a particular path, if only some nodes in the path upgrade support the deadline based mechanism defined in this document, the end-to-end deterministic delay/jitter target will only be partially achieved. Those legacy devices may adopt the existing SP or WFQ mechanisms, and ignore the possible deadline information carried in the packet, thus the residence delay produced by them cannot be perceived by the adjacent upgraded node. The more upgraded nodes included in the path, the closer to the delay/jitter target. Although, the legacy Peng, et al. Expires 4 January 2025 [Page 40] Internet-Draft Deadline Queueing Mechanism July 2024 devices may not support the data plane mechanism described in this document, but they can be freely programmed (such as P4 language) to measure and insert the deadline information into packets, in this case the delay/jitter target may be achieved. Only a few key nodes are upgraded to support deadline mechanism, which is low-cost, but can meet a flow with relatively loose time sensitive. Figure 13 shows an example of upgrading only several network border nodes. In the figure, only R1, R2, R3 and R4 are upgraded to support deadline based mechanism. A deterministic path across domain 1, 2, and 3 is established, which contains nodes R1, R2, R3, and R4, as well as explicit nodes in each domain. Domain 1, 2 and 3 use the traditional SP mechanism. The encoding of the packet sent by R1 includes the planned residence time and the latency deviation E. Especially, DS filed in IP header ([RFC2474]) are also set to appropriate values. The basic principle of setting is that the less the planned residence time, the higher the priority. In order to avoid the interference of non deterministic flow to deterministic flow, the priority of deterministic flow should be set as high as possible. The delay analysis based on strict priority without re-shaping in each domain can be found in [SP-LATENCY], which gives the equation to evaluate the worst-case delay of each hop. The worst-case delay per hop depends on the number of hops and the burst size of interference flows that may be faced on each hop. [EF-FIFO] also shows that, for FIFO packet scheduling be used to support the EF (expedited forwarding) per-hop behavior (PHB), if the network utilization level alpha < l/(H-l), the worst-case delay bound is inversely proportional to 1-alpha*(H-1), where H is the number of hops in the longest path of the network. Although the EDF scheduling with in-time mode, the SP scheduling and EF FIFO scheduling are all work-conserving, the EDF scheduling can further distinguish between urgent and non urgent packets according to deadline information other than traffic class. An intuitive phenomenon is that if a packet unfortunately faces more interference delays at the upstream nodes, it will become more urgent at the downstream node, and will not always be unfortunate. This operation of dynamically modifying the key fields, i.e., the latency deviation (E), of the packet can avoid always overestimating worst-case latency on all hops just like SP. For a specific DetNet flow, if it experiences too much latency in the SP domain (due to unreasonable setting of DS field and the inability to distinguish between deterministic and non deterministic flows), even if the boundary node accelerates the transmission, it may not be able to achieve the target of low E2E latency strictly, but rather a Peng, et al. Expires 4 January 2025 [Page 41] Internet-Draft Deadline Queueing Mechanism July 2024 loose result. If the traffic experiences less latency within the SP domain, the on-time scheduling mode applied on the border node can help achieve the end-to-end jitter target. _____ ___ _____ ___ _____ ___ / \/ \___ / \/ \___ / \/ \___ / \ / \ / \ +--+ +--+ +--+ +--+ |R1| Strict Priority |R2| Strict Priority |R3| Strict Priority |R4| +--+ domian 1 +--+ domian 2 +--+ domian 3 +--+ \____ __/ \____ __/ \____ __/ \_______/ \_______/ \_______/ Figure 13: Example of partial upgrade 16. Deployment Considerations According to the above schedulability conditions, each delay level d_i has dedicated delay resources, and the smaller d_i, the more valuable it is. The operator needs to match the corresponding d_i for each flow. It should be noted that the per-hop latency provided by the deadline based mechanism for the flow is based on flow's RSpec, not TSpec. In the case of option-3 and 4 with in-time scheduling behavior, more buffer is required to store accumulated bursts. For a specific flow, the accumulated bursts on a intermediate node consists of multiple rounds of burst interval. For example, the packets generated by the source within the first round of burst interval (always experiencing the worst case delay along the path) is caught up by the packets generated within the second round of burst interval (always experiencing the best case delay along the path). For delay level d_i, the worst case delay is d_i, the best case delay is l/R, where l is the smallest packet size of the flow, R is the port rate. For simplicity to get the estimate size of accumulated bursts, here we just take the best case delay as 0. Drawing on the method provided in [SP-LATENCY], the accumulated bursts of d_i is: * ACC_BUR_i = ((d_i * h) / burst_interval) * b_i Peng, et al. Expires 4 January 2025 [Page 42] Internet-Draft Deadline Queueing Mechanism July 2024 For example, d_i is 10 us, burst_interval is 250 us, this means that within the 25th hop, there will only be one b_10 burst in the queue. If it exceeds 25 hops and is within 50 hops, there may be two b_10 burst simutaneously in the queue. The accumulated bursts of other delay levels can be similarly estimated. Operators need to evaluate the required buffer size based on network hops and the supported delay levels. The benefit of in- time scheduling is to obtain an E2E latency of no more than D*hops as small as possible. Operators may also apply on-time scheduling to simplify the design of buffers. On-time scheduling absorbed latency deviation E on each hop and can get a jitter for each delay level to the value of delay level in theory (i.e., the worst case is that on the last node there are full traffic contributed by all delay levels that are discharging floodwater at the same time, however, in reality, the DetNet flow of the output port facing the destination customer side may only involve one delay level, then the jitter may be only one CTI (e.g., 10us)). In summary, the in-time scheduling with latency compensation, 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. 17. Evaluations Now we summarize how the deadline based mechanism ensures bounded latency as below: 1) Partition delay resource for each delay level on the outgoing port according to the schedulability condition, i.e., preset parameters of the arrival constraint function. 2) Execute delay resource reservation on each port of each calculated path on the control plane. This step is also known as admission condition check. 3) Execute policing on the network entry, to let the admited traffic obey its constraint function (i.e., TSpec). 4) Execute reshaping or latency compensation (recommended) in the network core for each flow, to convert the ineligible arrivals to eligible arrivals that still obey the constraint function of each flow. Peng, et al. Expires 4 January 2025 [Page 43] Internet-Draft Deadline Queueing Mechanism July 2024 5) Guarantee bounded delay by in-time scheduling mode; Guarantee bounded delay and jitter by on-time scheduling mode. The following table is the evaluation results of the Deadline mechanism based on the requirements that is defined in [I-D.ietf-detnet-scaling-requirements]. Note that all asynchronous mechanisms (such as EDF, ATS) do not require complete synchronization of crystal oscillator frequencies between devices, and the latency error caused by the deviations of clocks from their nominal rates (e.g., +100ppm) is generally in the nanosecond range and can be ignored. +======================+============+===============================+ | requiremens | Evaluation | Notes | +======================+============+===============================+ | 3.1 Tolerate Time | Yes | No time sync needed; | | Asynchrony | | No frequency sync needed. | +----------------------+------------+-------------------------------+ | 3.2 Support Large | | The eligible arrival of | | Single-hop | Yes | flows is independent with the | | Propagation | | link propagation delay. | | Latency | | | +----------------------+------------+-------------------------------+ | 3.3 Accommodate the | | The higher service rate, the | | Higher Link | Partial | more buffer needed for each | | Speed | | delay level. And, extra | | | | instructions to calculate E. | +----------------------+------------+-------------------------------+ | 3.4 Be Scalable to | | Multiple delay levels, each | | the Large Number | | with limited delay resources, | | of Flows and | | can support lots of flows, | | Tolerate High | Yes | without overprovision. | | Utilization | | Utilization may reach 100% | | | | link bandwidth. | | | | The unused bandwidth of the | | | | high delay level can be used | | | | by the low levels or BE flows.| +----------------------+------------+-------------------------------+ | 3.5 Tolerate Failures| | Independent of queueing | | of Links or Nodes| N/A | mechanism. | | and Topology | | | | Changes | | | +----------------------+------------+-------------------------------+ | 3.6 Prevent Flow | | Flows are permitted based on | | Fluctuation | Yes | the resources reservation of | | | | delay levels, and isolated | | | | from each other. | Peng, et al. Expires 4 January 2025 [Page 44] Internet-Draft Deadline Queueing Mechanism July 2024 +----------------------+------------+-------------------------------+ | 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 delay | | Topology | | levels. | | | | E2E jitter is low by on-time | | | | scheduling. | +----------------------+------------+-------------------------------+ | 3.8 Support Multi- | | Independent of queueing | | Mechanisms in | N/A | mechanism. | | Single Domain and| | | | Multi-Domains | | | +----------------------+------------+-------------------------------+ Figure 14: Evaluation for Large Scaling Requirements 17.1. Examples This section will describe the example of how the deadline mechanism supports service flows with different latency requirements. As shown in Figure 15: * Network transmission capacity: each link has rate 10 Gbps. Assuming the service rate of EDF scheduler allocate the total port bandwidth. * TSpec of each flow, maybe: - burst size 1000 bits, and average arrival rate 1 Mbps. - or, burst size 1000 bits, and average arrival rate 10 Mbps. - or, burst size 1000 bits, and average arrival rate 100 Mbps. - or, burst size 10000 bits, and average arrival rate 1 Mbps. - or, burst size 10000 bits, and average arrival rate 10 Mbps. - or, burst size 10000 bits, 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. Peng, et al. Expires 4 January 2025 [Page 45] Internet-Draft Deadline Queueing Mechanism July 2024 - 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. - or, E2E latency 900us, and E2E jitter less than 90us or 900us. - or, E2E latency 1ms, 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 15: 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 deterministic flows, also with similar TSpec and RSpec as above, originated from other sources, e.g., conflicts with flow-set "@" at node 0, conflicts with flow-set "#" at node 1, and so on. For each link along the path, it may provide delay resources with multiple delay levels, e.g., d1 (10us), d2 (20us), ..., d10 (100us) for any flows passing through it to consume. Assuming no link propagation delay and intra node forwarding delay, if flow i uses d1 resources, it can ensure an E2E latency of 100us (i.e., d1 * 10 hops), and jitter of 10us(on-time mode) or 100us (in-time mode). The results of using resources of other delay levels are similar. Peng, et al. Expires 4 January 2025 [Page 46] Internet-Draft Deadline Queueing Mechanism July 2024 The table below shows the possible tight allocation of delay resources on each link based on Equation-1, as well as the corresponding service scale supported, where, b = utilized burst resource (K bits), r = utilized bandwidth resource (Mbps), s = service scale (number), assuming that the resource limit of each delay level is b_limit = 100000 bits, r_limit = 1 Gbps. Note that in the table each column only shows the data where all flows served by all delay levels have the same TSpec (e.g., in colunm-1, TSpec per flow is burst size 1000 bits and arrival rate 1 Mbps), while in reality, flows served by different delay levels generally have different TSpec. It is easy to add colunms to describe various combinations. Peng, et al. Expires 4 January 2025 [Page 47] Internet-Draft Deadline Queueing Mechanism July 2024 =================================================== | d1 | d2 | d3 | d4 | d5 | d6 | d7 | d8 | d9 | d10| =================================================================== | column-1 | b | 100| 99 | 98 | 97 | 96 | 95 | 94 | 93 | 92 | 91 | | |---+----+----+----+----+----+----+----+----+----+----| |TSpec: | r | 100| 99 | 98 | 97 | 96 | 95 | 94 | 93 | 92 | 91 | | 1000 bits |---+----+----+----+----+----+----+----+----+----+----| | 1 Mbps | s | 100| 99 | 98 | 97 | 96 | 95 | 94 | 93 | 92 | 91 | =================================================================== | column-2 | b | 100| 90 | 81 | 73 | 66 | 60 | 53 | 48 | 43 | 39 | | |---+----+----+----+----+----+----+----+----+----+----| |TSpec: | r |1000| 900| 810| 729| 656| 590| 531| 478| 430| 387| | 1000 bits |---+----+----+----+----+----+----+----+----+----+----| | 10 Mbps | s | 100| 90 | 81 | 72 | 65 | 59 | 53 | 47 | 43 | 38 | =================================================================== | column-3 | b | 100| 90 | 80 | 70 | 60 | 50 | 40 | 30 | 20 | 10 | | |---+----+----+----+----+----+----+----+----+----+----| |TSpec: | r |1000|1000|1000|1000|1000|1000|1000|1000|1000|1000| | 1000 bits |---+----+----+----+----+----+----+----+----+----+----| | 100 Mbps | s | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | =================================================================== | column-4 | b | 100| 100| 100| 100| 100| 100| 99 | 99 | 99 | 99 | | |---+----+----+----+----+----+----+----+----+----+----| |TSpec: | r | 10 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | | 10000 bits|---+----+----+----+----+----+----+----+----+----+----| | 1 Mbps | s | 10 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | =================================================================== | column-5 | b | 100| 99 | 98 | 97 | 96 | 95 | 94 | 93 | 92 | 91 | | |---+----+----+----+----+----+----+----+----+----+----| |TSpec: | r | 100| 99 | 98 | 97 | 96 | 95 | 94 | 93 | 92 | 91 | | 10000 bits|---+----+----+----+----+----+----+----+----+----+----| | 10 Mbps | s | 10 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | 9 | =================================================================== | column-6 | b | 100| 90 | 81 | 73 | 66 | 59 | 53 | 48 | 43 | 39 | | |---+----+----+----+----+----+----+----+----+----+----| |TSpec: | r |1000| 900| 810| 729| 656| 590| 531| 478| 430| 387| | 10000 bits|---+----+----+----+----+----+----+----+----+----+----| | 100 Mbps | s | 10 | 9 | 8 | 7 | 6 | 5 | 5 | 4 | 4 | 3 | =================================================================== Figure 16: Delay Resource Pool and Service Scale Example 18. Taxonomy Considerations [I-D.ietf-detnet-dataplane-taxonomy] provides criteria for classifying data plane solutions. CEDF is a non-periodic, asynchronous, class level, work-conserving/non-work-conserving configurable, in-time/on-time configurable, delay based solution. Peng, et al. Expires 4 January 2025 [Page 48] Internet-Draft Deadline Queueing Mechanism July 2024 * Non-periodic: The scheduling power of an EDF is measured over an arbitrarily long non repetitive time range, scheduling in an orderly manner according to the urgency of the packets, and there is no defined periodic quantification unit of scheduling power. * Asynchronous: The EDF scheduler always assumes that all DetNet flows arrive asynchronously and does not require these flows to be interleaved with each other. The EDF schedulers in the network do not need to synchronize their states (e.g., busy periold) with each other, but work independently. * Class level: DetNet Flows may be grouped by similar service requirements, i.e., delay levels, on the network entrance node. Packets will be provided EDF service based on delay level, without checking flow characteristic. * Work-conserving/non-work-conserving configurable: The EDF scheduler configured with in-time scheduling mode is work- conserving (i.e., to send packets as soon as possible), while the EDF scheduler configured with on-time scheduling mode is non work- conserving (i.e., to ensure that the packet always experiences the worst-case delay per hop). * In-time/on-time configurable: The EDF scheduler configured with in-time scheduling mode is in-time to get bounded end-to-end latency, while the EDF scheduler configured with on-time scheduling mode is on-time to get bounded end-to-end delay jitter. * Delay based: A DetNet flow is scheduled based on its expected delay level, but not on its reserved bandwidth (i.e., rate), to fit well the case of low bandwidth but urgent flows. In addition, the per hop latency dominant factor of CEDF is simply delay levels that is defined according to schedulability condition. 19. IANA Considerations There is no IANA requestion for this document. 20. 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]. Peng, et al. Expires 4 January 2025 [Page 49] Internet-Draft Deadline Queueing Mechanism July 2024 Adequate admission control policies should be configured in the edge of the DetNet domain to control access to specific delay 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 expensive delay resource, and competes and interferes with normal traffic. 21. Acknowledgements TBD 22. References 22.1. Normative References [I-D.hp-detnet-tsn-queuing-mechanisms-evaluation] Yan, J., Han, Y., Peng, S., and Y. Gao, "Analysis and Evaluation for TSN Queuing Mechanisms", Work in Progress, Internet-Draft, draft-hp-detnet-tsn-queuing-mechanisms- evaluation-01, 20 December 2023, . [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, . [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, . [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, . Peng, et al. Expires 4 January 2025 [Page 50] Internet-Draft Deadline Queueing Mechanism July 2024 [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, . [I-D.peng-6man-deadline-option] Peng, S., Tan, B., and P. Liu, "Deadline Option", Work in Progress, Internet-Draft, draft-peng-6man-deadline-option- 01, 11 July 2022, . [I-D.peng-6man-delay-options] Peng, S., "Delay Options", Work in Progress, Internet- Draft, draft-peng-6man-delay-options-00, 18 January 2024, . [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, . [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-02, 24 June 2024, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2212] Shenker, S., Partridge, C., and R. Guerin, "Specification of Guaranteed Quality of Service", RFC 2212, DOI 10.17487/RFC2212, September 1997, . [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, December 1998, . Peng, et al. Expires 4 January 2025 [Page 51] Internet-Draft Deadline Queueing Mechanism July 2024 [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas, "Deterministic Networking Architecture", RFC 8655, DOI 10.17487/RFC8655, October 2019, . [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, . [RFC9016] Varga, B., Farkas, J., Cummings, R., Jiang, Y., and D. Fedyk, "Flow and Service Information Model for Deterministic Networking (DetNet)", RFC 9016, DOI 10.17487/RFC9016, March 2021, . [RFC9055] Grossman, E., Ed., Mizrahi, T., and A. Hacker, "Deterministic Networking (DetNet) Security Considerations", RFC 9055, DOI 10.17487/RFC9055, June 2021, . [RFC9320] Finn, N., Le Boudec, J.-Y., Mohammadpour, E., Zhang, J., and B. Varga, "Deterministic Networking (DetNet) Bounded Latency", RFC 9320, DOI 10.17487/RFC9320, November 2022, . 22.2. Informative References [EDF-algorithm] "A framework for achieving inter-application isolation in multiprogrammed, hard real-time environments", 1996, . [EF-FIFO] "Fundamental Trade-Offs in Aggregate Packet Scheduling", 2001, . Peng, et al. Expires 4 January 2025 [Page 52] Internet-Draft Deadline Queueing Mechanism July 2024 [IR-Theory] "A Theory of Traffic Regulators for Deterministic Networks with Application to Interleaved Regulators", 2018, . [Jitter-EDF] "Delay Jitter Control for Real-Time Communication in a Packet Switching Network", 1991, . [Net-Calculus] "Network Calculus: A Theory of Deterministic Queuing Systems for the Internet", 2001, . [P802.1DC] "Quality of Service Provision by Network Systems", 2023, . [PIFO] "Programmable Packet Scheduling at Line Rate", 2016, . [RPQ] "Exact Admission Control for Networks with a Bounded Delay Service", 1996, . [SP-LATENCY] "Guaranteed Latency with SP", 2020, . Appendix A. Proof of Schedulability Condition for RPQ Figure 17 below gives the proof of schedulability condition for RPQ. Peng, et al. Expires 4 January 2025 [Page 53] Internet-Draft Deadline Queueing Mechanism July 2024 ^ | Planned Residence Time | | | |CT_t+CTI+2*RTI -> ===== | | | CT_t+CTI+RTI -> =====| | CT_t + - - - - - - - - - - - - >+====+ -\ +CTI | | | | | | | | | | | > Phycical queue-x D_p + - - - - - - - - - - - - >| | | | | | | CT_t + - - - - - - - - - - - - >+====+ -/ | | |===== <- CT_t-RTI | | | ===== <- CT_t-2*RTI | | | | | RTI| ... ... | RTI| RTI| RTI| RTI| RTI| RTI| ---+----+-----------+----+----+----+----+----+----+---------> 0 ^ ^ ^ ^ time | | | | t-tau' t-ofst t t+tau (busy period begin) (arrival) (departure) Figure 17: RPQ Based Scheduling Suppose that the observed packet, with planned residence time D_p, arrives at the scheduler at time t and leaves the scheduler at time t+tau. It will be inserted to physical queue-x with count-down time CT_t at the current timer interval RTI with starting time t-ofst and end time t-ofst+RTI. According to the above packet queueing rules, we have CT_t <= D_p < CT_t+CTI. Also suppose that t-tau' is the beginning of the busy period closest to t. Then, we can get the amount of packets within time interval [t-tau', t+tau] that must be scheduled before the observed packet. In detailed: * For all flow i with planned residence time D_i meeting CT_t <= D_i < CT_t+CTI, the workload is sum{A'_i[t-tau', t]}. Explanation: since the packets with planned residence time D_i in the range [CT_t, CT_t+CTI) arrived at time t will be sent before the observed packet, the packets with the same D_i before time t will become more urgent at time t, and must also be sent before the observed packet. * For all flow i with planned residence time D_i meeting D_i >= CT_t+CTI, the workload is sum{A'_i[t-tau', t-ofst-(D_i-CT_t- CTI)]}. Peng, et al. Expires 4 January 2025 [Page 54] Internet-Draft Deadline Queueing Mechanism July 2024 Explanation: although the packets with planned residence time D_i larger than CT_t+CTI arrived at time t will be sent after the observed packet, but the packets with the same D_i before time t, especially before time t-ofst-(D_i-CT_t-CTI), will become more urgent at time t, and must be sent before the observed packet. * For all flow i with planned residence time D_i meeting D_i < CT_t, the workload is sum{A'_i[t-tau', t+(CT_t-D_i)]}. Explanation: the packets with planned residence time D_i less than CT_t at time t will certainly be sent before the observed packet, at a future time t+(CT_t-D_i) the packets with the same D_i will still be urgent than the observed packet (even the observed packet also become urgent), and must be sent before the observed packet. * Then deduct the traffic that has been sent during the busy period, i.e., C*(tau+tau'). Let tau as D_p, and remember that CT_t <= D_p, the above workload is less than sum{A'_i(tau'+CT_t+CTI-D_i) for all D_i >= CT_t} + sum{A'_i(tau'+CT_t-D_i) for all D_i < CT_t} - C*(tau'+D_p) It is further less than sum{A'_i(tau'+D_p+CTI-D_i) for all D_i >= D_2} + A'_1(tau'+D_p- D_1) - C*(tau'+D_p) Then, denote x as tau'+D_p, we have sum{A'_i(x+CTI-D_i) for all D_i >= D_2} + A'_1(x-D_1) - C*(x) In the case that d_i contains only one D_i, we have A_i = A'_i, d_i = D_i, so the above workload is sum{A_i(x+CTI-d_i) for all d_i >= d_2} + A_1(x-d_1) - C*(x) Let the above workload be less than zero, then we get Equation-2. In the case that d_i contains multiple D_i, e.g., d_1 is the minimum delay level with 10us, D_1 ~ D_10 is 10 ~ 19us respectively, d_2 is 20us, D_11 ~ D_20 iS 20 ~29us respectively, etc. Let D_1 ~ D_10 consume the resources of d_1, and D_11 ~ D_20 consume the resources of d_2, etc. Then, the above workload is less than Peng, et al. Expires 4 January 2025 [Page 55] Internet-Draft Deadline Queueing Mechanism July 2024 sum{A'_i(x+CTI-d_i) for all D_i belongs to d_i} - C*(x) That is sum{A_i(x+CTI-d_i) for all d_i} - C*(x), and let it less than zero, then we get Equation-3. Appendix B. Proof of Schedulability Condition for Alternate QAR of RPQ In the case that d_i contains only one D_i, the schedulability condition is Equation-1. This is because, in the workload, for all D_i meeting D_i >= CT_t+CTI, their contributed workload is changed to sum{A'_i[t-tau', t-ofst-(D_i-CT_t)]} based on the analysis of Equation-2, that is, the amount of workload A'_i(CTI) (that is placed in queue-x) is excluded. In the case that d_i contains multiple D_i, the schedulability condition is still Equation-3. This is because multiple D_i may belong to the same delay level as D_p. Assuming that within time zone [t-ofst, t-ofst+I] the list of all arrived D_i in the same parent queue-x with [CT_t, CT_t+CTI) as the observed packet (with D_p) is: * D_a1~D_am, where D_a1 is closer to CT_t+CTI, they are larger than D_p (but smaller than CT_t+CTI) and belongs to a larger delay level than d_p (corresponding delay level of D_p). * D_b1~D_bm, they are larger than D_p and belongs to the same delay level as d_p. * D_p. * D_c1~D_cm, they are smaller than D_p, and may belongs to the same delay level as d_p or a lower delay level than d_p. So that both D_b1~D_bm and D_c1~D_cm should be scheduled before the observed packet. This is also true for these set of packets that have arrived in history. Strictly, for D_a1, the contributed workload is sum{A'_i[t-tau', t-ofst+I-CTI]}, that is, only before time t-ofst+I-CTI the arrived packets of D_a1 will be placed in a more urgent queue-y with [CT_t, CT_t + CTI) than queue-x (at this history time its CT is [CT_t+CTI, CT_t + 2AT)) and should be shceduled before the observed packet. Similarly, for D_a2, the contributed workload is sum{A'_i[t-tau', t-ofst+I-CTI+I]}, for D_am, the contributed workload is sum{A'_i[t-tau', t-ofst+I-CTI+(m-1)*I]}. Peng, et al. Expires 4 January 2025 [Page 56] Internet-Draft Deadline Queueing Mechanism July 2024 Note that queue-x also contains packets with D_i (e.g., D_a0, larger than D_a1) that have arrived in history. For D_a0, the contributed workload is sum{A'_i[t-tau', t-ofst+I-CTI-(D_a0-D_a1)]}. However, the number of m is not fixed. For safety, we can appropriately overestimate workload time zone of D_a1~D_am to time instant t and regard that they need to be scheduled before the observed packet. Based on this, we can get the Equation-3. Authors' Addresses Shaofu Peng ZTE Corporation China Email: peng.shaofu@zte.com.cn Zongpeng Du China Mobile China Email: duzongpeng@foxmail.com Kashinath Basu Oxford Brookes University United Kingdom Email: kbasu@brookes.ac.uk Zuopin Cheng New H3C Technologies China Email: czp@h3c.com Dong Yang Beijing Jiaotong University China Email: dyang@bjtu.edu.cn Chang Liu China Unicom China Email: liuc131@chinaunicom.cn Peng, et al. Expires 4 January 2025 [Page 57]