Network Shaofu. Peng Internet-Draft ZTE Corporation Intended status: Standards Track Zongpeng. Du Expires: 20 April 2024 China Mobile Kashinath. Basu Oxford Brookes University Zuopin. Cheng New H3C Technologies Dong. Yang Beijing Jiaotong University Chang. Liu China Unicom 18 October 2023 Deadline Based Deterministic Forwarding draft-peng-detnet-deadline-based-forwarding-07 Abstract This document describes a deterministic forwarding mechanism to IP/ MPLS network, as well as corresponding resource reservation, admission control, 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 20 April 2024. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. Peng, et al. Expires 20 April 2024 [Page 1] Internet-Draft Deadline Queueing Mechanism October 2023 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 Service Flow . . . . . . . 6 2.2. Delay Levels Provided by the Network . . . . . . . . . . 6 2.3. Relationship Between Planned Residence Time and Delay Level . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4. Relationship Between Service Burst Interval and Delay Level . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Sorted Queue . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Scheduling Mode for PIFO . . . . . . . . . . . . . . . . 8 3.2. Schedulability Condition for PIFO . . . . . . . . . . . . 8 3.2.1. Conditions for Leaky Bucket Constraint Function . . . 9 3.2.2. Schedulability Condition Analysis for On-time Mode . 10 3.3. Buffer Size Design . . . . . . . . . . . . . . . . . . . 12 4. Rotation Priority Queues . . . . . . . . . . . . . . . . . . 12 4.1. Alternate Queue Allocation Rules . . . . . . . . . . . . 14 4.2. Scheduling Mode for RPQ . . . . . . . . . . . . . . . . . 15 4.3. Schedulability Condition for RPQ . . . . . . . . . . . . 15 4.3.1. Schedulability Conditions for Alternate QAR . . . . . 18 4.3.2. Conditions for Leaky Bucket Constraint Function . . . 19 4.4. Buffer Size Design . . . . . . . . . . . . . . . . . . . 20 5. Reshaping . . . . . . . . . . . . . . . . . . . . . . . . . . 21 6. Latency Compensation . . . . . . . . . . . . . . . . . . . . 22 6.1. Get Existing Accumulated Planned Residence Time . . . . . 22 6.2. Get Existing Accumulated Actual Residence Time . . . . . 22 6.3. Get Existing Accumulated Residence Time Deviation . . . . 23 6.4. Get Allowable Queueing Delay . . . . . . . . . . . . . . 23 6.5. Scheduled by Allowable Queueing Delay . . . . . . . . . . 24 7. Option-1: Reshaping plus Sorted Queue . . . . . . . . . . . . 25 8. Option-2: Reshaping plus RPQ . . . . . . . . . . . . . . . . 25 9. Option-3: Latency Compensation plus Sorted Queue . . . . . . 26 9.1. Packet Disorder Considerations . . . . . . . . . . . . . 26 10. Option-4: Latency Compensation plus RPQ . . . . . . . . . . . 28 10.1. Packet Disorder Considerations . . . . . . . . . . . . . 30 11. Resource Reseravtion . . . . . . . . . . . . . . . . . . . . 32 11.1. Delay Resource Definition . . . . . . . . . . . . . . . 33 Peng, et al. Expires 20 April 2024 [Page 2] Internet-Draft Deadline Queueing Mechanism October 2023 11.2. Traffic Engineering Path Calculation . . . . . . . . . . 34 12. Admission Control on the Ingress . . . . . . . . . . . . . . 35 13. Overprovision Analysis . . . . . . . . . . . . . . . . . . . 37 14. Compatibility Considerations . . . . . . . . . . . . . . . . 38 15. Deployment Considerations . . . . . . . . . . . . . . . . . . 40 16. Evaluations . . . . . . . . . . . . . . . . . . . . . . . . . 41 17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 18. Security Considerations . . . . . . . . . . . . . . . . . . . 42 19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42 20. References . . . . . . . . . . . . . . . . . . . . . . . . . 42 20.1. Normative References . . . . . . . . . . . . . . . . . . 42 20.2. Informative References . . . . . . . . . . . . . . . . . 44 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45 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 refers to the occupation of resources by service traffic, exclusive or shared in a certain proportion, such as dedicated physical link, link bandwidth, queue resources, etc. * Explicit routing means that the transmission path of traffic flow in the network needs to be selected in advance to ensure the stability of the route and does not change with the real-time change of network topology, and based on this, the upper bound of end-to-end delay and delay jitter can be accurately calculated. * Service protection refers to sending multiple service flows along multiple disjoint paths at the same time to reduce the packet loss rate. In general, a deterministic path is a strictly explicit path calculated by a centralized controller, and resources are reserved on the nodes along the path to meet the SLA requirements of deterministic services. [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. In the presence of admission Peng, et al. Expires 20 April 2024 [Page 3] Internet-Draft Deadline Queueing Mechanism October 2023 control, policing, reshaping, a large number of packet scheduling techniques can provide bounded latency. However, many packet schedulers may result in an inefficient use of network resources, or provide an overestimated latency. 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. We assume that when providing deterministic services in such a network, the observed flow always has the highest (or relatively high) priority. 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 accumulated burst of its 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 accurate 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 latency based scheduler, which always selects the packet with the shortest deadline for transmission. EDF 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. Especially, an enhanced option based on latency compensation is discussed to replace reshaping and also achieve low jitter. Peng, et al. Expires 20 April 2024 [Page 4] Internet-Draft Deadline Queueing Mechanism October 2023 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 the traffic of any service 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. * option-1: Reshaping plus sorted queue. * option-2: Reshaping plus RPQ. * option-3: Latency Compensation plus sorted queue. * option-4: Latency Compensation plus RPQ. Peng, et al. Expires 20 April 2024 [Page 5] Internet-Draft Deadline Queueing Mechanism October 2023 2.1. Planned Residence Time of the Service 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 delay includes two parts, one is the accumulated residence delay and the other is the accumulated link propagation delay. The end-to-end delay is subtracted from the accumulated link propagation delay to obtain the accumulated residence delay. A simple method is that the accumulated residence delay is shared equally by each node along the path to obtain the planning residence time of each node. Note that the link propagation delay in reality may be not always fixed, 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. * Carried in the packet. The ingress PE node, when encapsulating the deterministic service flow, 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 deadlines, one for each node. [I-D.peng-6man-deadline-option] defined a method to carry the 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. Peng, et al. Expires 20 April 2024 [Page 6] Internet-Draft Deadline Queueing Mechanism October 2023 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. 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 individual flow, while the delay level (d) is the capability provided by the node. 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. Peng, et al. Expires 20 April 2024 [Page 7] Internet-Draft Deadline Queueing Mechanism October 2023 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 set 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. 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. The test of schedulability conditions needs to be based on the whole network view. When we need to add new traffic to the network, we need to consider which nodes the related path will pass through, and then check in turn whether these nodes will still meet the schedulability conditions after adding new traffic. Peng, et al. Expires 20 April 2024 [Page 8] Internet-Draft Deadline Queueing Mechanism October 2023 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 ... ... 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 deadline 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 service 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, a more common example is that the preset r_i of each level d_i will divide C roughly equally, and the preset b_i is the maximum packet size (such as 2000 bytes). The 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 level, 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. Peng, et al. Expires 20 April 2024 [Page 9] Internet-Draft Deadline Queueing Mechanism October 2023 The delay resource reservation status of each level is independent. For example, in the case that the parameter b_1 is determined, if the required burst of the d_1 service is large, then although only a few d_1 services can be supported, but if r_1 is very small, the network can still support more services of other levels at the same time. 3.2.2. Schedulability Condition Analysis for On-time Mode Compared with the in-time mode, on-time mode is non-work-conserving, which can be considered as the combination of damper and EDF scheduler. The intuitive understanding of the on-time mode is that the on-time forwarding behavior applied on a flow maintains the time interval (regulation interval by the regulator on the ingresss node) between packets of that flow, but not lead to an increase in the bandwidth occupied by that flow. Therefore, the on-time scheduling mode does not cause the arrival curve to violate the traffic constraint function. So that the schedulability condition (i.e., Equation-1) can also be applied to the on-time scheduling mode. However, the on-time scheduling mode explicitly introduces the hold time, the actual departure time of the packet may be after the deadline. Suppose that the selected parameters of the constraint function of each delay level makes the scheduler need to work at full speed (i.e., service rate C), then 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 on-time mode, the busy period may be just begin at its deadline and may make the sending time of the packet really exceed its deadline, but the worst case of the extra delay will not exceed the delay level value. The following Figure 1 shows the difference between on-time scheduling and in-time scheduling. Peng, et al. Expires 20 April 2024 [Page 10] Internet-Draft Deadline Queueing Mechanism October 2023 arrival traffic: 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: #1 #2 #3 #4 #5 $1 $2 $3 $4 ... ... &1 ------+-----------+-----------+-----------+--... ...--+-----------+--- \__ d_1 __/ \________ d_2 ________/ \______________ d_3 ______________/ ... ... \_________________________ d_n ___________________________/ Figure 1: Difference between On-time and In-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 levels of service 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 mode, 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. Note that in-time mode cannot guarantee jitter. While in the on-time mode, 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 Peng, et al. Expires 20 April 2024 [Page 11] Internet-Draft Deadline Queueing Mechanism October 2023 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 (see Section 7 for enqueue rule). 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. 3.3. Buffer Size Design The service rate of the deadline 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 deadline 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. Please refer to Section 15 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), and AT (Authorization Time) that is for sending duration. AT is also the CT difference between two adjacent queues. Note that RTI must be less than or equal to the AT, with AT = N * RTI, where the natural number N >= 1. * The smaller the CT, the higher the priority. At the beginning, all queues have different CT values, i.e., staggered from each other, e.g, one queue has the minimum CT value (termed as MIN_CT), and one queue has the maximum CT value (termed as MAX_CT), and the CT values of all queues increase equally by AT. 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+AT. So the CT attribute of a queue is actually a range [CT, CT+AT). Peng, et al. Expires 20 April 2024 [Page 12] Internet-Draft Deadline Queueing Mechanism October 2023 * For a queue whose CT has been reduced to MIN_CT, after a new round of AT, the CT will return to MAX_CT. The above AT, RTI, MIN_CT and MAX_CT value should be choosed according to the hardware capacity. Each link can independently use different AT. The general principle is that the larger bandwidth, the smaller AT. The AT 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 service flows, so that CT updated per RTI can match the delay requirements of different services. For example, if the delay difference of different traffic flows is several microseconds, RTI can be choosed as 1 us. If the delay difference of different traffic flows is several 10 microseconds, RTI can be choosed as 10 us. A specific example of RPQ with in-time scheduling mode is depicted in Figure 2. +------------------------------+ +------------------------------+ | 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) ######| | queue-7(CT=-10us) ######| | queue-7(CT=-11us) ######| +------------------------------+ +------------------------------+ +------------------------------+ +------------------------------+ | Other Queue Group: | | Other Queue Group: | | queue-8 ############ | | queue-8 ############ | | queue-9 ############ | | queue-9 ############ | | queue-10 ############ | | queue-10 ############ | | ... ... | | ... ... | +------------------------------+ +------------------------------+ -o----------------------------------o--------------------------------> T0 T0+1us time Figure 2: Example of RPQ Groups Peng, et al. Expires 20 April 2024 [Page 13] Internet-Draft Deadline Queueing Mechanism October 2023 In this example, the AT for RPQ group is configured to 10us. Queue-1 ~ queue-7 are members of RPQ group. Each queue has its CT attribute. The MAX_CT is 50us, the MIN_CT is -10us. At the initial time (T0), the CT of all queues are staggered from each other. For example, the CT of queue-1 is 50us, the CT of queue-2 is 40uS, and so on. Suppose the scheduling engine initiates a rotation timer with a time interval of 1us, i.e., AT = 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-7 will return to MAX_CT. Note that the minimum D requested by a service 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 transmission should have Q (i.e., D + E - F) that is not be smaller than d_1, and should never be inserted to a queue with negative CT. However, considering there may be some abnormal case during scheduling, adding a queue with MIN_CT = -AT for in-time mode to ensure that incoming urgent traffic is sent before the queue's CT rolling-over appears harmless. 4.1. Alternate Queue Allocation Rules It may further let a RPQ queue (act as the virutal parent queue) contain multiple sub-queues, each for a delay level. 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 is beneficial because when packets with smaller D facing larger interference delay, it is difficult to have space for latency compensation on downstream nodes, while packets with larger D have larger space for latency compensation. For a virtual parent queue, 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). This alternate queue allocation rule enables on-time mode to provide appropriate jitter performance (i.e., the worst case of exceeding the deadline is the delay level value). However, it can also be uniformly applied to in-time mode. According to different scheduling behavior of in-time mode and on-time mode, MIN_CT may be designed to -AT for in-time mode and -N*AT for on-time mode, where N is the amount of delay levels. Peng, et al. Expires 20 April 2024 [Page 14] Internet-Draft Deadline Queueing Mechanism October 2023 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 deadline queues with in-time scheduling mode. 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+AT-d_i) for all i>=2} <= C*t, if a d_i contains only one D_i. (Equation-2) * sum{A_i(t+AT-d_i) for all i>=1} <= C*t, if d_i contains multiple D_i. (Equation-3) where AT is the CT interval between adjacency queue, RTI is the rotation time interval, C is service rate of the deadline 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. Figure 3 below gives a rough explanation. Peng, et al. Expires 20 April 2024 [Page 15] Internet-Draft Deadline Queueing Mechanism October 2023 ^ | Planned Residence Time | | | |CT_t+AT+2*RTI -> ===== | | | CT_t+AT+RTI -> =====| | CT_t + - - - - - - - - - - - - >+====+ -\ +AT | | | | | | | | | | | > 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 3: Deadline Queues 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+AT. 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 service i with planned residence time D_i meeting CT_t <= D_i < CT_t+AT, 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+AT) 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 service i with planned residence time D_i meeting D_i >= CT_t+AT, the workload is sum{A'_i[t-tau', t-ofst-(D_i-CT_t-AT)]}. Peng, et al. Expires 20 April 2024 [Page 16] Internet-Draft Deadline Queueing Mechanism October 2023 Explanation: although the packets with planned residence time D_i larger than CT_t+AT 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-AT), will become more urgent at time t, and must be sent before the observed packet. * For all service 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+AT-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+AT-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+AT-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+AT-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 20 April 2024 [Page 17] Internet-Draft Deadline Queueing Mechanism October 2023 sum{A'_i(x+AT-d_i) for all D_i belongs to d_i} - C*(x) That is sum{A_i(x+AT-d_i) for all d_i} - C*(x), and let it less than zero, then we get Equation-3. 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 AT factor. Other common considerations are the same as Section 3.2. 4.3.1. Schedulability Conditions 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. This is because, in the workload, for all D_i meeting D_i >= CT_t+AT, 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(AT) (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+AT) as the observed packet (with D_p) is: * D_a1~D_am, where D_a1 is closer to CT_t+AT, they are larger than D_p (but smaller than CT_t+AT) 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. Peng, et al. Expires 20 April 2024 [Page 18] Internet-Draft Deadline Queueing Mechanism October 2023 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-AT]}, that is, only before time t-ofst+I-AT the arrived packets of D_a1 will be placed in a more urgent queue-y with [CT_t, CT_t + AT) than queue-x (at this history time its CT is [CT_t+AT, 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-AT+I]}, for D_am, the contributed workload is sum{A'_i[t-tau', t-ofst+I-AT+(m-1)*I]}. 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-AT-(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. 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: b_1 <= C*d_1 - M b_1 + b_2 + (r_1+r_2)*AT <= C*d_2 - M b_1 + b_2 + b_3 + (r_1+r_2)*2*AT + r_3*AT <= C*d_3 - M ... ... sum(b_1+...+b_n) + (r_1+r_2)*(n-1)*AT + r_3*(n-2)*AT + ... + r_n*AT <= C*d_n - M where, C is the service rate of the deadline scheduler, M is the maximum size of the interference packet. Equation-3 can be expressed as: b_1 + r_1*AT <= C*d_1 - M b_1 + b_2 + r_1*2*AT + r_2*AT <= C*d_2 - M Peng, et al. Expires 20 April 2024 [Page 19] Internet-Draft Deadline Queueing Mechanism October 2023 b_1 + b_2 + b_3 + r_1*3*AT + r_2*2*AT + r_3*AT <= C*d_3 - M ... ... sum(b_1+...+b_n) + r_1*n*AT + r_2*(n-1)*AT + ... + r_n*AT <= C*d_n - M 4.4. Buffer Size Design The buffer size of each RPQ queue is AT * C - M, where M is the maximum size of the packet with low priority. If we divide the time by AT (such as 10 us) and observe the RPQ queue with the lowest priority, such as d_100 (i.e., CT=100 us), then in the first AT, the traffic flow with priority d_100 (traffic arrival follows the constraint of A_100(t)) will enter that queue. In the second AT, the traffic flow with priority of d_90 (traffic arrival follows the constraint of A_90(t)) will enter the same queue (i.e., CT=90 us), and so on. It can be seen that the maximum buffer size required for the queue is sum(A_i(AT)) for all delay level i. Since the stability condition of the deadline scheduler must meet sum(A_i(t)) < C*t, so the buffer size of each deadline queue can be set to C*AT - M. When deadline queues and latency compensation are used in combination, a packet that arrives early is penalized and placed in a queue with a larger CT, it will not cause the queue to overflow, because the queue is just where it is located. That is, assuming that the packet does not arrive early but later on time, it will not be penalized, and will still enter the same queue where the CT becomes smaller later. Similarly, when a late arrival packet is rewarded and placed in a queue with a smaller CT, it will not cause the queue overflow, because the queue is just where it is located. That is, assuming that the packet does not arrive late but arrives on time before, it will not be rewarded, and will still enter the same queue with a smaller CT which has not been reduced before. However, an implementation may let all queues share the common buffer with the total buffer cost as the sum of burst resources of all delay levels. 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*AT-M). However, all physical Peng, et al. Expires 20 April 2024 [Page 20] Internet-Draft Deadline Queueing Mechanism October 2023 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 delay level with largest delay bound, 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 largest delay bound, and with more necessary buffer, to store the accumulated bursts. Please refer to Section 15 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. 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 service flows. Peng, et al. Expires 20 April 2024 [Page 21] Internet-Draft Deadline Queueing Mechanism October 2023 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; The queueing delay is unstable. 6.1. Get Existing Accumulated Planned Residence Time The existing 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. This information needs to be carried in the packet. Every time the packet passes through a node, the node accumulates its corresponding planned residence time to the existing accumulated planned residence time field in the packet. [I-D.peng-6man-deadline-option] defined a method to carry existing accumulated planned residence time in the IPv6 packets. The setting of "existing accumulated planned residence time" in 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. 6.2. Get Existing Accumulated Actual Residence Time The existing 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. This information needs to be carried in the packet. Every time the packet passes through a node, the node accumulates its corresponding actual residence time to the existing accumulated actual residence time field in the packet. [I-D.peng-6man-deadline-option] defined a method to carry existing accumulated actual residence time in the IPv6 packets. Peng, et al. Expires 20 April 2024 [Page 22] Internet-Draft Deadline Queueing Mechanism October 2023 The setting of "existing accumulated actual residence time" in 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. The current node can carry the receiving and sending time of the packet in the auxiliary data structure (note that is not packet itself) of the packet, then the actual residence time of the packet in the node can be calculated according to these two value. Although other methods can also be, for example, carrying the absolute system time of receiving and sending in the packet to let the downstream node compute the actual residence time indirectly, that has a low encapsulation efficiency. 6.3. Get Existing Accumulated Residence Time Deviation The existing accumulated residence time deviation (termed as E) equals existing accumulated planned residence time minus existing accumulated actual residence time. This value can be zero, positive, or negative. If the existing accumulated planned residence time and the existing accumulated actual residence time are carried in the packet, it is not necessary to carry the existing accumulated residence time deviation. Otherwise, it is necessary. The advantage of the former is that it can be applied to more scenarios, while the later has less packaging overhead. In the case of in-time scheduling mode, E may be a very large positive value. While in the case of on-time scheduling mode, E may be 0, or a small value close to 0. 6.4. Get Allowable Queueing Delay When a node receives a packet from the upstream node, it can first get the existing accumulated residence time deviation (E), and then add it to the planned residence time (D) of the packet at this node to obtain the adjustment residence value, and then deduct the forwarding delay (F) of the packet in the node, to obtain the allowable queueing delay (Q) for that packet. Q = D + E - F Peng, et al. Expires 20 April 2024 [Page 23] Internet-Draft Deadline Queueing Mechanism October 2023 In detailed, assume that the current node in a deterministic path is i, all upstream nodes are from 1 to i-1. Let the planned residence time be D, the actual residence time be R, the forwarding delay intra-node be F, then the allowable queueing delay (Q) of the packet on the current node i is calculated as follows: E(i-1) = sum(D(1), ..., D(i-1)) - sum(R(1), ..., R(i-1)) Q(i) = D(i) + E(i-1) - F(i) 6.5. 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 core stateless latency compensation can achieve the effect of reshaping per flow. Q can be used to identify ineligibility arrvials of one delay level and prevent it from interferring with the scheduling of eligibility arrvials of other delay levels. Firstly, at network entry, all packets (after regulation) of the same flow will be released to the EDF scheduler one after another at different time (termed as begin time), but with the same allowable queueing delay (Q), with initial E = 0. Then, the ideal departure time of each packet should be its begin time plus Q. If all packets has the ideal departure time (i.e., the updated E is still 0), then the arrived traffic faced by the next hop also obey its arrival constraint function. If all packets of all delay levels released by all sources have the ideal departure time, all concurrent flows received by a transit node will comply with their arrival constraints. However, taking the in-time mode as an example, packets may have an advanced departure time (i.e., the updated E is larger than 0), instead of the ideal time. Therefore, the arrived traffic faced by the downstream node may violate its arrival constraint function. In this case, the downstream node may punish the ineligibility arrving packets based on E, i.e. obtain appropriate Q to restore eligibility arrvials. 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 ineligibility bursts, which will then enter the queueing subsystem. While if latency compensation is used, this ineligibility 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. Peng, et al. Expires 20 April 2024 [Page 24] Internet-Draft Deadline Queueing Mechanism October 2023 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 along multiple paths with different propagation lengths, even if these packets are all eligibility packets, bursts accumulation may still form and cannot even be punished. 7. Option-1: Reshaping plus Sorted Queue A receivd packet is inserted to the PIFO queue according to rank = A + D + E, where, A is the time that packet arrived at the incoming interface. Note that E is always 0 and not updated. 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. 8. Option-2: Reshaping plus RPQ A receivd packet is inserted to the appropriate RPQ queue according to Q = D - F. That is, E is always 0 and not updated. Enqueue rule: Peng, et al. Expires 20 April 2024 [Page 25] Internet-Draft Deadline Queueing Mechanism October 2023 * For a packet with Q, select the target RPQ queue (i.e., the virtual parent queue) with corresponding CT, that meet CT <= Q < CT+AT. * 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: * 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. 9. Option-3: Latency Compensation plus Sorted Queue A receivd packet is inserted to the PIFO queue according to rank = A + D + E, where, A is the time that packet arrived at the incoming interface. Note that E is generally not 0 and updated per hop. The planned residence time (D) and accumulated residence time deviation (E) should be carried in the packet. The enqueue and dequeue operations are the same as Section 7. 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. Peng, et al. Expires 20 April 2024 [Page 26] Internet-Draft Deadline Queueing Mechanism October 2023 As shown in Figure 4, P1 and P2 are two back-to-back packets belonging to the same burst. 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. packets arrived later packets arrived earlier | | | V V --------+-----------------------------------------------+--------- ... ... | .P1.................P2....................... | ... ... --------+-----------------------------------------------+--------- P1.Q=40us P2.Q=39us | | --------o---------------------o---------------------------------> T1 T2 (=T1+0.9us) | ___________________| | | v v PIFO ############################################################## top Figure 4: Packets queueing based on Latency Compensation DetNet architecture [RFC8655] provides Packet Ordering Function (POF), that can be used to solve the above disorder problem caused by the latency compensation. Alternatively, if the POF is not enabled, we can also maintain states for service flows to record the last queueing information to address this issue. For example, one ore more OGOs (order guarantee object) are maintained per delay level and incoming port, on each outgoing port. An OGO records the rank (i.e., arrival time at the incoming interface plus D) of the last inserted packet mapped to this OGO. Peng, et al. Expires 20 April 2024 [Page 27] Internet-Draft Deadline Queueing Mechanism October 2023 When a packet arrives at the scheduler, it is first mapped to its OGO, and get the rank of OGO, and put behind that rank. 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, 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. 10. Option-4: Latency Compensation plus RPQ A receivd packet is inserted to the appropriate RPQ queue according to Q = D + E - F. The planned residence time (D) and accumulated residence time deviation (E) should be carried in the packet. The enqueue and dequeue operations are the same as Section 8. Figure 5 depicts an example of packets inserted to the RPQ queues. Peng, et al. Expires 20 April 2024 [Page 28] Internet-Draft Deadline Queueing Mechanism October 2023 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--------------------------------> receiving-time base +F time Figure 5: Time Sensitive Packets Inserted to RPQ As shown in Figure 5, 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 in the node (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 20 April 2024 [Page 29] Internet-Draft Deadline Queueing Mechanism October 2023 * 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 eligibility 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 AT 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 + AT even if it is at the end of the queue, then according to D = Q - E, i.e., after time E (the penalty time), we have CT_x - E <= Q - E < CT_x - E + AT That is CT_y <= D < CT_y + AT So, in essence, it is still equivalent to an eligibility 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+AT), 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 6, P1 and P2 are two packets belonging to the same burst. The arrival time when they are received on the scheduler is shown in the figure. Suppose that the AT of the deadline queue is Peng, et al. Expires 20 April 2024 [Page 30] Internet-Draft Deadline Queueing Mechanism October 2023 10us, the decreasing step 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 6: Packets queueing based on Latency Compensation DetNet architecture [RFC8655] provides Packet Ordering Function (POF), that can be used to solve the above disorder problem caused by the latency compensation. Alternatively, if the POF is not enabled, we can also maintain states for service flows to record the last queueing information to address this issue. For example, one ore more OGOs (order guarantee object) are maintained per delay level and incoming port, on each outgoing port. An OGO records the queueing information which is the queue that all the packets mapped to this OGO was inserted recently. For simplicity, a count-down time (CT), which is copied from the recent inserted deadline queue, can be recorded in OGO. Note that the CT of OGO needs to decrease synchronously with that of other deadline queues, with the same decreasing step RTI. If the CT of OGO decreases to 0, it will remain at 0. Peng, et al. Expires 20 April 2024 [Page 31] Internet-Draft Deadline Queueing Mechanism October 2023 When a packet arrives at the deadline scheduler at the outgoing port , it is first mapped to its OGO, and get the CT of OGO, termed as OGO.CT. Then, according to the above queueing rule (CT <= Q < CT+AT), get the CT of a preliminarily selected queue, termed as preliminary CT. * Let Q' is MAX{OGO.CT, preliminary CT}, and put the packet in the target queue according to CT <= Q' < CT+AT * Update the value of OGO.CT to the CT of target queue. 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. 11. Resource Reseravtion Generally, a path may carry multiple service 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, and r_i_j) from the resource quota (b_i, and r_i) of the link's delay level d_i. A service flow k that carried in path j, may use resources (b_i_j_k, and 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 services that can be supported. The following expression exists. * b_i_j >= sum(b_i_j_k), for all service k over the path j. * r_i_j >= sum(r_i_j_k), for all service k over the path j. * b_i >= sum(b_i_j), for all path j through the specific link. * r_i >= sum(r_i_j), for all path j through the specific link. Peng, et al. Expires 20 April 2024 [Page 32] Internet-Draft Deadline Queueing Mechanism October 2023 11.1. 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 7 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- configuration value based on the schedulability condition. * Unreserved Bursts: Refers to the amount of bits reservable (i.e., free amount) corresponding to this delay level. * Maximum Reservable Bandwidth: Refers to the maximum amount bandwidth corresponding to this delay level. It is a pre- configuration value based on the schedulability condition. * Unreserved Bandwidth: Refers to the amount of bandwidth reservable (i.e., free amount) corresponding to this delay level. Peng, et al. Expires 20 April 2024 [Page 33] Internet-Draft Deadline Queueing Mechanism October 2023 d_n +----------------------------------------+ | Maximum Reservable Bursts: MRBu_n | | Unreserved Bursts: UBu_n | | Maximum Reservable Bandwidth: MRB_n | | Unreserved Bandwidth: UB_n | +----------------------------------------+ ... ... ... ... ... ... d_2 +----------------------------------------+ | Maximum Reservable Bursts: MRBu_2 | | Unreserved Bursts: UBu_2 | | Maximum Reservable Bandwidth: MRB_2 | | Unreserved Bandwidth: UB_2 | +----------------------------------------+ d_1 +----------------------------------------+ | Maximum Reservable Bursts: MRBu_1 | | Unreserved Bursts: UBu_1 | | Maximum Reservable Bandwidth: MRB_1 | | Unreserved Bandwidth: UB_1 | +----------------------------------------+ -----------------------------------------------------------> Delay Resource of the Link Figure 7 The IGP/BGP extensions to advertise the link's capability and delay resource is defined in [I-D.peng-lsr-deterministic-traffic-engineering]. 11.2. 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. By default, select the appropriate delay level d_i (d_i <= D-F) closest to the average planned residence time (D), and then reserve resources from delay level d_i on each hop. However, a local policy may allow more larger D to consume resources with smaller delay levels. Note that it is D, not d_i, carried in the forwarding packets. Peng, et al. Expires 20 April 2024 [Page 34] Internet-Draft Deadline Queueing Mechanism October 2023 12. Admission Control on the Ingress On the ingress PE node, traffic regulation must be performed on the incoming port, so that the service traffic does not exceed its T-SPEC. This kind of regulation is usually the shaping using leaky bucket combined with the incoming queue that receives service traffic. A service may generate discrete multiple bursts within its periodic service burst interval. According to [RFC9016], the values of Burst Interval, MaxPacketsPerInterval, MaxPayloadSize of the service 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 arrival rate that can be calculated. The leaky bucket shaping will essentially make all the bursts within the service burst interval evenly distributed within the service burst interval, which may be inconsistent with the original arrival curve of the service flow. Therefore, some bursts within the service burst interval may face more shaping delay. For example, on the head of the service burst interval, it contains two discrete bursts with the same size, but the bandwidth reserved by the service is very small (i.e., total burst size/burst interval). 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 service burst interval. Although the shaped curve and the original arrival curve can be as consistent as possible by increasing the bucket depth, to minimize the shaping delay of each burst, but this means that the service 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 spend more money to buy a larger burst. On the entry node, for the burst that faces the shaping delay, its 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 shaping result and violation of the arrival constraint function. Then, the regulated traffic arrives at the deadline 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. Peng, et al. Expires 20 April 2024 [Page 35] Internet-Draft Deadline Queueing Mechanism October 2023 Note that the flow sent from the deadine scheduler of the headend to the next hop still follows the arrival constraint function of the path after reshaping or latency compensation on the next hop. Then on the next hop, when concurrent flows received from multiple paths are aggregated to the same outgoing port for transmission, within any d_1 duration, the aggregated d_1 traffic will not exceed the burst resources of delay level d_1 reserved by these paths on the outgoing port, and each aggregated d_i traffic will not exceed the bandwidth resources of delay level d_i reserved by these paths on the outgoing port; Similarly, within any d_2 duration, the aggregated d_2 traffic will not exceed the burst resources of level d_2 reserved by these paths on the outgoing port, and each aggregated d_i traffic will not exceed the bandwidth resources of level d_i reserved by these paths on the outoging port, and so on. Figure 8 depicts an example of deadline based traffic regulated and scheduled on the ingress PE node in the case of option-4. In the figure, the shaping delay caused by the previous burst is termed as S#, and forwarding delay termed as F. 1st burst | received v +-+ +-+ +----+ +-+ +--+ +------+ |1| |2| | 3 | |4| |5 | | 6 | <= burst sequence +-+ +-+ +----+ +-+ +--+ +------+ | | | | | | ~+0 ~+S1 ~+0 ~+S3~+S4 ~+0 ~+F ~+F ~+F ~+F ~+F ~+F | | | | | | UNI v v v v v v ingr-PE -+--------+--------+--------+--------+--------+--------+----> NNI | Auth | Auth | Auth | Auth | Auth | Auth | time | time | time | time | time | time | time | 1,2 in 3 in 4 in 5 in 6 in Queue-A Queue-B Queue-C Queue-D Queue-E (CT<=Q) (CT<=Q) (CT<=Q) (CT<=Q) (CT<=Q) | | | | | ~+Q ~+Q ~+Q ~+Q ~+Q | | | | | sending v v v v v +-+ +-+ +----+ +-+ +--+ +------+ |1| |2| | 3 | |4| |5 | | 6 | +-+ +-+ +----+ +-+ +--+ +------+ Peng, et al. Expires 20 April 2024 [Page 36] Internet-Draft Deadline Queueing Mechanism October 2023 Figure 8: Deadline Based Packets Orchestrating There are 6 bursts received from the client. The burst-2, 4, 5 has regulation delay S1, S3, S4 that caused by previous burst respectively. While burst-1, 3, 6 has zero regulation delay because the number of tokens is sufficient. The regulation makes 6 bursts roughly distributed within the service burst interval. Suppose that each burst passes through the same intra-node forwarding delay F, and when they arrive at the deadline scheduler in turn. In the case of latency compensation plus RPQ, they will have the same allowable queueing delay (Q), regardless of whether they have experienced shaping delay before. When the packets of burst-1, 2 arrive at the scheduler, according to CT <= Q < CT+AT, 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. 13. 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 services 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 modified 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 services 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 services of delay level d_i is b_i. If the burst of each service of level d_i is b_k, then the number service can be supported is b_i/b_k, which is the worst case considering the concurrent arrival of these service 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 deterministic services, as can be seen from the schedulability condition equation. Peng, et al. Expires 20 April 2024 [Page 37] Internet-Draft Deadline Queueing Mechanism October 2023 14. 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 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 priority based scheduling mechanism, and ignore the possible deadline information carried in the packet, thus the intra node 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 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 service with relatively loose time sensitive. Figure 9 shows an example of upgrading only several network border nodes. In the figure, only R1, R2, R3 and R4 are upgraded to support deadline 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 strict priority based forwarding mechanism. The encoding of the packet sent by R1 includes the planned residence time and the accumulated residence time deviation. 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 during the resource reservation procedure. 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. Peng, et al. Expires 20 April 2024 [Page 38] Internet-Draft Deadline Queueing Mechanism October 2023 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 according to deadline information other than traffic class. Therefore, when analyzing the latency of EDF scheduling, the latency is not evaluated just according to the order in which the packets arrive at the scheduler, but also according to the deadline of the packets. 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, e.g, the existing acumulated residence time deviation (E), of the packet can avoid always overestimating worst-case latency on all hops as SP. According to schedulability condition, the worst-case latancy per hop is d_i. When the border node (e.g, R2) receives the deterministic traffic, it will obtain its rank according to the existing accumulated residence time deviation information carried in the packet, and always sent as soon as possible. For a specific deterministic 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. If the traffic experiences less latency within the SP domain, on-time mode may work on the egress node to achieve the end-to-end jitter target. _____ ___ _____ ___ _____ ___ / \/ \___ / \/ \___ / \/ \___ / \ / \ / \ +--+ +--+ +--+ +--+ |R1| Strict Priority |R2| Strict Priority |R3| Strict Priority |R4| +--+ domian 1 +--+ domian 2 +--+ domian 3 +--+ \____ __/ \____ __/ \____ __/ \_______/ \_______/ \_______/ Figure 9: Example of partial upgrade Peng, et al. Expires 20 April 2024 [Page 39] Internet-Draft Deadline Queueing Mechanism October 2023 15. Deployment Considerations According to the above schedulability conditions, the delay levels (e.g, d_i) that can be provided in the network is related to the entire deployed service flows. Each delay level d_i has independent delay resources, and the smaller d_i, the more valuable it is. The operator needs to match the corresponding d_i for each service. When option-3 with in-time mode is choosed, PIFO needs to be designed with a large depth to store accumulated bursts. Similarly, when option-4 with in-time mode is choosed, more deadline queues are needed to store accumulated bursts. The accumulated bursts on a intermediate node consists of multiple rounds of burst interval flows, for example, the flow 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 flow 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 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. Operators may also use on-time scheduling mode to simplify the design of buffers. On-time scheduling mode 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 egress node there are full traffic contributed by all delay levels, that are discharging floodwater at the same time, however, in reality, the service flow of the output port facing the destination customer side may only involve one delay level, then the jitter may be only one AT (e.g, 10us)). Peng, et al. Expires 20 April 2024 [Page 40] Internet-Draft Deadline Queueing Mechanism October 2023 16. Evaluations This section gives the evaluation results of the Deadline mechanism based on the requirements that is defined in [I-D.ietf-detnet-scaling-requirements]. +======================+============+===============================+ | requiremens | Evaluation | Notes | +======================+============+===============================+ | 3.1 Tolerate Time | Partial | No time synchronization needed| | Asynchrony | | , but need frequency sync. | +----------------------+------------+-------------------------------+ | 3.2 Support Large | | The eligibility 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 | | | | 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. | +----------------------+------------+-------------------------------+ | 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 | | | | mode. | Peng, et al. Expires 20 April 2024 [Page 41] Internet-Draft Deadline Queueing Mechanism October 2023 +----------------------+------------+-------------------------------+ | 3.8 Support Multi- | | Independent of queueing | | Mechanisms in | N/A | mechanism. | | Single Domain and| | | | Multi-Domains | | | +----------------------+------------+-------------------------------+ Figure 10 17. IANA Considerations There is no IANA requestion for this document. 18. Security Considerations Security considerations for DetNet are described in detail in [RFC9055]. General security considerations for the DetNet architecture are described in [RFC8655]. Considerations specific to the DetNet data plane are summarized in [RFC8938]. Adequate admission control policies should be configured in the edge of the DetNet domain to control access to specific 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. 19. Acknowledgements TBD 20. References 20.1. Normative References [I-D.ietf-detnet-scaling-requirements] Liu, P., Li, Y., Eckert, T. T., Xiong, Q., Ryoo, J., zhushiyin, and X. Geng, "Requirements for Scaling Deterministic Networks", Work in Progress, Internet-Draft, draft-ietf-detnet-scaling-requirements-03, 7 July 2023, . [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, . Peng, et al. Expires 20 April 2024 [Page 42] Internet-Draft Deadline Queueing Mechanism October 2023 [I-D.peng-lsr-deterministic-traffic-engineering] Peng, S., "IGP Extensions for Deterministic Traffic Engineering", Work in Progress, Internet-Draft, draft- peng-lsr-deterministic-traffic-engineering-01, 4 July 2023, . [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, . [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, . Peng, et al. Expires 20 April 2024 [Page 43] Internet-Draft Deadline Queueing Mechanism October 2023 [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, . 20.2. 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, . [IR-Theory] "A Theory of Traffic Regulators for Deterministic Networks with Application to Interleaved Regulators", 2018, . [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, . [UBS] "Urgency-Based Scheduler for Time-Sensitive Switched Ethernet Networks", 2016, . Peng, et al. Expires 20 April 2024 [Page 44] Internet-Draft Deadline Queueing Mechanism October 2023 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 20 April 2024 [Page 45]