DetNet Shaofu. Peng Internet-Draft ZTE Intended status: Standards Track Peng. Liu Expires: 23 November 2023 China Mobile Kashinath. Basu Oxford Brookes University Aihua. Liu ZTE Dong. Yang Beijing Jiaotong University 22 May 2023 Generic Packet Timeslot Scheduling Mechanism draft-peng-detnet-packet-timeslot-mechanism-02 Abstract IP/MPLS networks use packet switching (with the feature store-and- forward) and are based on statistical multiplexing. Statistical multiplexing is essentially a variant of time division multiplexing, which refers to the asynchronous and dynamic allocation of link timeslot resources. In this case, the service flow does not occupy a fixed timeslot, and the length of the timeslot is not fixed, but depends on the size of the packet. Statistical multiplexing has certain challenges and complexity in meeting deterministic QoS, and its delay performance is dependent on the the used queueing mechanism. This document further describes a generic time division multiplexing scheme in IP/MPLS networks, which we call packet timeslot scheme. It aims to make it easier for the control plane to calculate the delay performance and more flexible to allocate deterministic resources, and also make it easier for the data plane to create more flexible timeslot mapping. 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." Peng, et al. Expires 23 November 2023 [Page 1]
Internet-Draft Packet Generic Timeslot May 2023 This Internet-Draft will expire on 23 November 2023. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Timeslot Resource Reservation in Control-plane . . . . . 8 3.1.1. Timeslot Mapping Relationship . . . . . . . . . . . . 9 3.1.1.1. Deduced by Single Timeslot Mapping Detection . . 10 3.1.1.2. Deduced by Phase Difference of Orchestration Period . . . . . . . . . . . . . . . . . . . . . . 11 3.1.2. Timeslot Resource Definition . . . . . . . . . . . . 12 3.1.3. Arrival Postion in the Orchestration Period . . . . . 14 3.1.4. Proccess of Each Reservation Sub-task . . . . . . . . 16 3.1.4.1. Resource Reservation on the Ingress Node . . . . 18 3.1.4.2. Resource Reservation on the Transit Node . . . . 19 3.1.4.3. Resource Reservation on the Egress Node . . . . . 20 3.1.4.4. End-to-end Delay and Jitter . . . . . . . . . . . 20 3.2. Timeslot Resource Access in Data-plane . . . . . . . . . 21 3.2.1. Conversion of Timeslot ID . . . . . . . . . . . . . . 22 4. Global Timeslot ID . . . . . . . . . . . . . . . . . . . . . 24 4.1. Fixed Timeslot Mapping Mode . . . . . . . . . . . . . . . 24 4.2. Variable Timeslot Mapping Mode . . . . . . . . . . . . . 27 5. Summary of Timeslot Style . . . . . . . . . . . . . . . . . . 28 6. Queue Design . . . . . . . . . . . . . . . . . . . . . . . . 29 6.1. Full Queues . . . . . . . . . . . . . . . . . . . . . . . 29 6.2. Non-full Queues . . . . . . . . . . . . . . . . . . . . . 30 7. Admission Control on the Headend . . . . . . . . . . . . . . 30 8. Frequency Synchronization . . . . . . . . . . . . . . . . . . 31 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 10. Security Considerations . . . . . . . . . . . . . . . . . . . 31 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 Peng, et al. Expires 23 November 2023 [Page 2]
Internet-Draft Packet Generic Timeslot May 2023 12.1. Normative References . . . . . . . . . . . . . . . . . . 31 12.2. Informative References . . . . . . . . . . . . . . . . . 32 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32 1. Introduction IP/MPLS networks use packet switching (with the feature store-and- forward) and are based on statistical multiplexing. The discussion of supporting multiplexing in the network was first seen in the time division multiplexing (TDM), frequency division multiplexing (FDM) and other technologies of telephone communication network (using circuit switching). Statistical multiplexing is essentially a variant of time division multiplexing, which refers to the asynchronous and dynamic allocation of link resources. In this case, the service flow does not occupy a fixed timeslot, and the length of the timeslot is not fixed, but depends on the size of the packet. In contrast, synchronous time division multiplexing means that a sampling frame (or termed as time frame) includes a fixed number of fixed length timeslots, and the timeslot at a specific position is allocated to a specific service. The utilization rate of link resources in statistical multiplexing is higher than that in synchronous time division multiplexing. However, if we want to provide deterministic end-to-end delay in packet switched networks based on statistical multiplexing, the difficulty is greater than that in synchronous time division multiplexing. The main challenge is to obtain a deterministic upper bound on the queuing delay, which is closely related to the queuing mechanism used in the network. In addition to IP/MPLS network, other packet switched network technologies, such as ATM, also discusses how to provide corresponding transmission quality guarantee for different service types. Before service communication, ATM needs to establish a connection to reserve virtual path/channel resources, and use fixed- length short cells and timeslots. The advantage of short cell is small interference delay, but the disadvantage is low encoding efficiency. The mapping relationship between ATM cells and timeslots is not fixed, so it still depends on a specific cells scheduling mechanism (such as [ATM-LATENCY]) to ensure delay performance. Although the calculation of delay performance based on short and fixed-length cells is more concise than that of IP/MPLS networks based on non-fixed-length packets, they all essentially depend on the queueing mechanism. [CQF] introduce a hybrid of asynchronous and synchronous time- division multiplexing method based on fixed-length cycle in Ethernet LAN. [Multi-CQF] is a further enhancement of the classic CQF to be applicable to IP/MPLS networks. Generally, the service flow is not mapped to a fixed cycle at the network entrance, but dynamically Peng, et al. Expires 23 November 2023 [Page 3]
Internet-Draft Packet Generic Timeslot May 2023 selects an idle cycle, which can be regarded as asynchronous, but at the intermediate nodes it is based on a fixed and inherent cycle mapping, which can be regarded as synchronous. CQF with 2-buffer mode or Mult-CQF with 3-buffer mode only uses a small number of cycles to establish the inherent cycle mapping between a port-pair of two adjacent nodes, which can minimize the residence delay in the node and limit end-to-end jitter at the same time. The inherent cycle mapping is independent of the service and is also irrelevant to the resource reservation of the service. That is, the cycle of CQF/ Mult-CQF is not a resource that is open to the service and can be reserved. [Multi-CQF] describes the deterministic behavior for each cycle-level based on traditional bandwidth resource allocation, with cycle-based admission control. However, overprovisioning (i.e., burst / cycle is larger than service bandwidth) may affect the service scale that it can support. Alternatively, other literatures have also been discussing the cycle-based resource allocation, which mainly affects the sending cycle selected for the service flow on the entry node, but does not change the inherent cycle mapping in the network. The path composed of inherent cycle mapping relationship is like a highway without traffic lights along the way. Traffic control can only be implemented at the entrance of the path, so it may be possible to avoid car conflict of multiple paths at the intermediate nodes, but not always likely. In addition, since the number of cycles used by CQF/Multi-CQF is small, such as 3 or 4, it is not easy to determine whether the cycle conflict of service flows on multiple paths is true or false conflict (True conflict is when two flows used the same slot in the same scheduling period whereas false conflict arise when two flows use the same slot in different scheduling period). In order to improve the service scale supported by the network, this document further discusses a generic time division multiplexing scheduling mechanism using timeslot resource in IP/MPLS networks, which we call Timeslot Queueing and Forwarding (TQF) mechanism. The timeslot resource is the enhancement of bandwidth resource, has the feature of time awareness, is visible and assignable to the service. From the perspective of data plane, the timeslot resource is a time interval that can continuously send packets, while from the control plane, it is the amount of bits in the time interval (e.g, the total amount of bits that is reservable, and the amount of unreserved bits). Based on the timeslot resources, it is easier for the control plane to reserve strictly non-conflicting resources for different paths that can get deterministic delay performance, and the data plane can create more flexible (i.e., not inherent, but based on reservation) timeslot mapping to avoid conflicts. Peng, et al. Expires 23 November 2023 [Page 4]
Internet-Draft Packet Generic Timeslot May 2023 2. Terminology The following terminology is introduced in this document: Timeslot: The smallest unit of packet timeslot scheduling. It needs to design a reasonable value, such as 10us, to send at least one complete packet. Different nodes can be configured with different length of timeslot. Timeslot Scheduling: The packet is stored in the buffer corresponding to a specific timeslot, then sent in that timeslot. Service Burst Interval: The traffic specification of deterministic services generally follows the principle of generating a specific burst amounts within a specific length of cyclic burst interval. For example, a service generates 1000 bits of burst per 1 ms, where 1 ms is the service burs interval. Orchestration Period: The orchestration Period is adopted by the control plane according to the needs of all deterministic services, including a fixed number (termed as N and numbered from 0 to N-1) of timeslots, for example, the orchestration period include 1000 timeslots and each timeslot length is 10 us. The timeslot resources within the orchestration period can be allocated for services, i.e., which timeslots are occupied by services and how many bits are occupied in timeslots. The orchestration period is the Least Common Multiple of all service burst intervals. It is also a multiple of the scheduling period. It is recommended that all nodes of the network be configured with the same length of orchestration period (note that timeslot length may still be different), because it is service-related and also crucial for establishing a stable timeslot mapping relationship. Ongoing Sending Period: The orchestration period which the ongoing sending timeslot belongs to. Scheduling Period: The period of the packet timeslot scheduling mechanism implemented by the forwarding plane of the network node, including a fixed number (termed as M and numbered from 0 to M-1) of timeslots, for example, the scheduling period include 100 timeslots and each timeslot length is 10 us. Different nodes can be configured with different length of scheduling period. Incoming Timeslot: For an intermediate node in a specific path, the Peng, et al. Expires 23 November 2023 [Page 5]
Internet-Draft Packet Generic Timeslot May 2023 timeslot contained in the packet received from the upstream node (i.e., the outgoing timeslot of the upstream node) is its incoming timeslot. An incoming timeslot is the timeslot number in the orchestration period. Outgoing Timeslot: For an intermediate node in a specific path, when it continues to send packets received from the upstream node to downstream nodes, according to resource reservation or certain rules, it chooses to send packets in the specified timeslot, which is the outgoing timeslot. An outgoing timeslot is the timeslot number in the orchestration period. Ongoing Sending Timeslot: For the headend of the path, packets received from the client side and sent to the downstream node. When the packet reaches the outgoing port, the timeslot currently in the sending state is the ongoing sending timeslot; For intermediate nodes of the path, packets received from the upstream node and sent to the downstream node. When the end of the incoming timeslot to which the packet belongs reaches the outgoing port, the timeslot currently in the sending state is the ongoing sending timeslot. Note that the ongoing sending timeslot is different with the outgoing timeslot. An ongoing sending timeslot is the timeslot number in the orchestration period. 3. Overview This scheme introduces the time-division multiplexing scheduling mechanism based on the fixed length timeslot in the IP/MPLS network. Note that the time-division multiplexing here is a L3 packet-level scheduling mechanism, rather than the TDM port (such as SONET/SDH) implemented in L1. The latter generally involves the time frame and the corresponding framing specification, which is not necessary in this document. Figure 1 shows the packet timeslot scheduling behavior implemented by the intermediate node P through which multiple deterministic paths passes on to the outgoing port (P-PE2). Peng, et al. Expires 23 November 2023 [Page 6]
Internet-Draft Packet Generic Timeslot May 2023 +---+ +---+ +---+ |PE1| --------------- | P | --------------- |PE2| +---+ +---+ +---+ orchestration period +---+---+-+-+---+---------+---+ | 0 | 1 | 2 | 3 | ... ... |N-1| +---+---+---+---+---------+---+ ^ ^ reserve slots: | | reserve slots: a,b,c | | x,y path-1 -------------------------o--|----------------> path-2 -------------------------|--o----------------> | | access slots: | | access slots: a',b',c' v v x',y' / +-------------------+ ___ | | queue-0 @slot 0 | / \ | +-------------------+ | | | | queue-1 @slot 1 | | | Scheduling < +-------------------+ | Period | | ... ... | | ^ | +-------------------+ | | | | queue-n @slot M-1| \___/ \ +-------------------+ Figure 1 Where, both the orchestration period and the scheduling period consist of multiple timeslots, the total amount of bits that can be reserved or send in each timeslot can be preset, generally not exceeding the result of the total bandwidth of the link multiplied by the timeslot length. The orchestration period or scheduling period of all nodes in the network does not need to be synchronized, and phase difference is allowed. However, for each node, the phase of timeslot of orchestration period and the scheduling period are strictly aligned. This is indeed natural because multiple scheduling periods forms an orchestration period. Peng, et al. Expires 23 November 2023 [Page 7]
Internet-Draft Packet Generic Timeslot May 2023 In the figure, path-1 and path-2 allocate timeslot resource from the orchestration period of link P-PE2 respectively. Path-1 reserves timeslot a, b, c from orchestration period, and finally accesses timeslot a', b', c' from scheduling period. Path-2 reserves timeslot x, y from orchestration period, and finally accesses timeslot x', y' from scheduling period. There is a mapping relationship function between the timeslot i of orchestration period and the timeslot i' of scheduling period, i.e., i' = f(i). There are many mapping options, such as a'=a, a'=a+offset, a'=a%M, and a'=random(a), etc. In general, the packet timeslot scheduling mechanism implemented on all nodes in the network may use the same length of timeslot and scheduling period. However, considering the capability differences of each node in the network (for example, the capabilities of the edge nodes are weaker than the core nodes), it is feasible for different nodes to use different length of timeslot and scheduling period. The scheme involves two aspects: the path calculation and timeslot resource reservation in the control plane, and timeslot resource access in the data plane. 3.1. Timeslot Resource Reservation in Control-plane The control plane (centralized controller or distributed protocol) can reserve corresponding timeslot resources along the deterministic path. Note that a path may carry multiple services, then the path will reserve timeslot resources for the combined services, and may reserve the bit resources in multiple timeslots at the same time in the orchestration period. However, it would still be beneficial to distinguish between reservation sub-tasks corresponding to different service flows in the combined reservation task. In this document, we refer to a reservation sub-task as a timeslot resource reservation action related to the service flow. Note that one or more reservation sub-tasks for a specific service flow may be derived based on its TSpec, and each reservation sub-task will allocate corresponding timeslot resource. The intermediate nodes do not maintain the state of service flow and only reserve timeslot resources based on the reservation sub-tasks. Peng, et al. Expires 23 November 2023 [Page 8]
Internet-Draft Packet Generic Timeslot May 2023 During resource reservation, it is necessary to distinguish the requirements between low latency service and non-low latency service . For low latency service requirements, the physical offset between the reserved outgoing timeslot and the incoming timeslot is small; while for non-low latency service requirements, this physical offset can be large. It is necessary to maintain the end-to-end total residence delay budget for each reservation sub-task, used to select outgoing timeslot, as long as the sum of residence delays caused by all nodes should not exceed the total residence delay budget. Multiple reservation sub-tasks may generate different incoming/ outgoing timeslot mapping relationships on node P. For example: * The timeslot mapping relationship created by the sub-task-1: <(incoming port a, incoming slot number 3), (outgoing port b, outgoing slot number 60)> * The timeslot mapping relationship created by the sub-task-2: <(incoming port a, incoming slot number 3), (outgoing port b, outgoing slot number 61)> Special care should be taken not to confuse the use of different mapping relationships. For specific service flows, P need to explicitly use specific timeslot mapping relationships. It is recommended, but not mandatory, to reserve timeslot resources on the outgoing port of each hop from the headend of the path to the endpoint, that is, first determine the timeslot reserved on the headend, then determine the timeslot reserved on the next hop , and so on. We assume that the service flow has a periodic arrival time, and there is a fixed position relationship between the arrival time and the orchestration period of the headend, so selecting the outgoing timeslot closed to the arrival time or within the expected offset range in the orchestration period can minimize the residency delay of the packet on the headend. However, sometimes it is necessary to get a larger residence delay on the headend and a smaller residence delay on other nodes to ensure successful path calculation. 3.1.1. Timeslot Mapping Relationship In order to reserve outgoing timeslot resources for the service flow , it is necessary to first determine the ongoing sending timeslot that the incoming timeslot falls into, i.e., the mapping relationship between the incoming timeslot and the ongoing sending timeslot. Peng, et al. Expires 23 November 2023 [Page 9]
Internet-Draft Packet Generic Timeslot May 2023 Suppose a path contains three nodes U, V, and W in turn along the path. All nodes are configured with orchestration period of the same length (termed as LOP), which is crucial for establishing a fixed timeslot mapping relationship. Node U config timeslot length L_u, and an orchestration period contains N_u timeslots. Node V config timeslot length L_v, and an orchestration period contains N_v timeslots. In general, the link bandwidth of edge nodes is small, and they will be configured with a larger timeslot length than the aggregated/backbone nodes. It has been mathematically proven that if the least common multiple of Lu and Lv is LCM, LOP is also a multiple of LCM. Two methods are provided in the following sub-sections to determine the timeslot mapping relationship. 3.1.1.1. Deduced by Single Timeslot Mapping Detection Figure 2 shows that Node U sends a detection packet from the end (or head, the process is similar) of an arbitrary timeslot i on the outgoing port connected to node V. After a certain link propagation delay (D_propagation), the packet is received by the incoming port of node V, and i is regarded as the incoming timeslot by V. The packet finally arrives at the outgoing port connected to node W after the intra-node forwarding delay (D_forwarding) including parsing, table lookup, internal fabric exchange, etc. At this time, the ongoing sending timeslot is j, and there is time T_ij left before the end of the timeslot j. |<------------------------ LOP ---------------------------->| +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ U | | | | i | | | | | | x | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | |<--T_ij->| |<--T_xy->| v v +-----------+-----------+-----------+-----------+-----------+ V | | j | ... ... | y | | +-----------+-----------+-----------+-----------+-----------+ |<------------------------ LOP ---------------------------->| Figure 2 Peng, et al. Expires 23 November 2023 [Page 10]
Internet-Draft Packet Generic Timeslot May 2023 Then, based one the detection result of the mapping relationship between incomming timeslot i and ongoing sending timeslot j, for any other outgoing timeslot x of node U, the mapped ongoing sending timeslot y of node V is: * y = (j + ((N_u+x-i)*L_u-T_ij)/L_v + 1) % N_v And the time Txy left before the end of the timeslot y is: * T_xy = L_v - ((N_u+x-i)*L_u-T_ij)%L_v Note that the detection message used to get the mapping relationship i->j does not really need to be sent to the outgoing port, i.e., the mapping relationship cannot be obtained only on the outgoing port, but on the incoming port side. Assuming that the orchestration period of all ports within a node are strictly synchronized, this is easy to achieve. On the incoming port, upon receiving the detection message, immediately determine the ongoing sending timeslot j' that the incoming timeslot falls into and the corresponding T_ij', and then based on a fixed forwarding delay evaluation value (but not less than the actual forwarding delay D_forwarding) to estimate the timeslot j that the incoming timeslot falls into and the corresponding T_ij. 3.1.1.2. Deduced by Phase Difference of Orchestration Period Figure 3 shows that Node U sends a detection packet from the end (or head, the process is similar) of the orchestration period on the outgoing port connected to node V. After a certain link propagation delay (D_propagation), the packet is received by the incoming port of node V and finally arrives at the outgoing port connected to node W after the intra-node forwarding delay (D_forwarding). At this time, there is time P_uv left before the end of the ongoing sending period. |<------------------- LOP --------------------->| +---+---+---+---+---+---+---+---+---+---+---+---+ U | | | | | | | x | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+ | | |<----Puv---->| |<--T_xy->| | v +-----------+-----------+-----------+-----------+ V | | y | | | +-----------+-----------+-----------+-----------+ |<------------------- LOP --------------------->| Peng, et al. Expires 23 November 2023 [Page 11]
Internet-Draft Packet Generic Timeslot May 2023 Figure 3 Then, based one the phase difference of orchestration period, for any outgoing timeslot x of node U, the mapped ongoing sending timeslot y of node V is: * y = ((LOP+(x+1)*L_u-P_uv)/L_v) % N_v And the time Txy left before the end of the timeslot y is: * T_xy = Lv - (LOP+(x+1)*L_u-P_uv)%Lv Note that the detection message used to get the phase difference of orchestration period does not really need to be sent to the outgoing port, i.e., the phase difference cannot be obtained only on the outgoing port, but on the incoming port side. Assuming again that the orchestration period of all ports within a node are strictly synchronized, on the incoming port, upon receiving the detection message, immediately determine the phase difference P_uv', and then based on a fixed forwarding delay evaluation value (but not less than the actual forwarding delay D_forwarding) to estimate the phase difference P_uv. Note that in Section 3.1.1.1, the phase difference of orchestration period may also be derived firstly by the mapping relationship of i->j, and then get the mapping relationship for other timeslots according to the above formula. 3.1.2. Timeslot Resource Definition The timeslot resources of a link can be represented as the corresponding bit amounts of all timeslots included in an orchestration period. Basically, the link capability should contain the following information: * Timeslot Length (L_T): Represents the length of the timeslot, in units of us. Generally, the length of each timeslot included in the orchestration period is the same. * Length of Orchestration Period (LOP): Indicates the number of timeslots (N) included in the orchestration period, numbered sequentially from 0 to N-1. * Length of Scheduling Period (LSP): Indicates the number of timeslots (M) included in the scheduling period, numbered sequentially from 0 to M-1. Peng, et al. Expires 23 November 2023 [Page 12]
Internet-Draft Packet Generic Timeslot May 2023 Figure 4 shows the timeslot resource model of the link, with an orchestration period consisting of N timeslots numbered from 0 to N-1. The resource information of each timeslot includes the following attributes: * Sequence Number: Indicates the number of the timeslot in the orchestration period. The number of the first timeslot is 0, and the number of the last timeslot is N-1. * Maximum Reservable Bursts (MRB): Refers to the maximum amount of bit quota corresponding to this timeslot, with unit of bits. It is a configurable preset value that is related to the total bandwidth of the link (termed as C) and the length of the timeslot (termed as L_T), then the Maximum Reservable Bursts should be set to a value not exceeding C*L_T, considering the transmission needs of other non-deterministic service flows in the network. Generally, the Maximum Reservable Bursts of each timeslot included in the orchestration period are all the same. * Unreserved Bursts (UB): Refers to the amount of bits reservable corresponding to this timeslot, with unit of bits. #N-1 +-------------------------------------+ | Timeslot Length: L_T(n-1) | | Maximum Reservable Bursts: MRB(n-1) | | Unreserved Bursts: UB(n-1) | +-------------------------------------+ ... ... ... ... ... ... #1 +-------------------------------------+ | Timeslot Length: L_T(1) | | Maximum Reservable Bursts: MRB(1) | | Unreserved Bursts: UB(1) | +-------------------------------------+ #0 +-------------------------------------+ | Timeslot Length: L_T(0) | | Maximum Reservable Bursts: MRB(0) | | Unreserved Bursts: UB(0) | +-------------------------------------+ -----------------------------------------------------------> Timeslot Resource of the Link Figure 4 Peng, et al. Expires 23 November 2023 [Page 13]
Internet-Draft Packet Generic Timeslot May 2023 The IGP/BGP extensions to advertise the link's capability and timeslot resource will be specified in seperate documents. 3.1.3. Arrival Postion in the Orchestration Period Generally, a deterministic service flow has its TSpec, such as periodically generating traffic of a specific burst size within a specific length of burst interval, which regularly reaches the network entry. The headend executes traffic regulation (e.g, setting appropriate parameters for leaky bucket shaping), which generally make packets evenly distributed within the service burst interval, i.e, there are one or more shaped sub-burst in the service burst interval. There is a fixed positional relationship between the departure time when each sub-burst leaves the regulator and the orchestration period, based on that a specific outgoing time slot is reserved for the sub-burst. Note that there may be jitter when a sub-burst departure from the regulator occurs, that is, there may be jitter in the positional relationship between it and the orchestration period. Therefore, when reserving the outgoing timeslot, this jitter should be included (i.e., selecting a later outgoing timeslot). Figure 5 shows, for some typical service flows, the relationship between the service burst interval (SBI) and the length of orchestration period (LOP) of headend, as well as the possible timeslot resource reservation results for these service flows. Peng, et al. Expires 23 November 2023 [Page 14]
Internet-Draft Packet Generic Timeslot May 2023 |<--------------------- LOP ---------------------->| +----+----+----+----+----+----+----+----------+----+ | #0 | #1 | #2 | #3 | #4 | #5 | #6 | ... ... |#N-1| +----+----+----+----+----+----+----+----------+----+ +--+ Service 1: | |b1| | +-----+--+-----------------------------------------+ |<------------------- SBI ------------------------>| +--+ +--+ Service 2: | |b1| |b2| +------------+--+------------------------+--+------+ |<------------------- SBI ------------------------>| +------+ Service 3: | | b1 | +---------------------------+------+---------------+ |<------------------- SBI ------------------------>| +--+ +--+ +--+ Service 4: | |b1| | |b1| | |b1| | +----+--+--------+----+--+--------+----+--+--------+ |<----- SBI ---->|<----- SBI ---->|<----- SBI ---->| Figure 5 As shown in the figure, the length of service burst intervals for services 1, 2, 3 is equal to the length of orchestration period. Service 1 generates a very small single burst amounts within its burst interval, which may reserve timeslot 2 or other subsequent timeslot in the orchestration period; Service 2 generates two small discrete sub-bursts within its burst interval and also be shaped, which may reserve slots 4 and N-1 in the orchestration period for each sub-burst respectively; Service 3 generates a large single burst amount within its burst interval but not be shaped (due to purchasing a larger bucket depth), which may also be split to multiple sub- bursts and reserve multiple timeslots in the orchestration period, such as timeslots 8 and 9. The length of the service burst interval for service 4 is only 1/3 of the orchestration cycle, then first construct service 4' whose burst interval is equal to the length of orchestration period and contains three times of service 4. So service 4' is similar to service 2, generating a small amount of three separate sub-bursts within its burst interval. It may reserve timeslots 3, 7, and N-1 in the orchestration period. Peng, et al. Expires 23 November 2023 [Page 15]
Internet-Draft Packet Generic Timeslot May 2023 Each sub-burst corresponds to a reservation sub-task. For a specific service flow, to determine how many reservation sub- tasks are required, can be summarized as: * First, align the service burst interval with the Orchestration Period of the headend to ensure that the two are of equal length. If the service burst interval is only a fraction of the Orchestration Period, then multiply it several times to obtain the expanded service burst interval to get a new service'. * Check how many discrete sub-bursts will be generated during the orchestration Period, and for each sub-burst: - If the proportion of the sub-burst size to the MRB of a single timeslot does not exceed a specific value, then the sub-burst corresponds to a reservation sub-task; - Otherwise, continue to split the sub-burst into multiple sub- sub-bursts (note that each sub-sub-burst must contain a complete packet), so that the proportion of each sub-sub-burst size to the MRB of a single timeslot does not exceed the specific value, and each sub-sub-burst corresponds to a reservation sub-task. 3.1.4. Proccess of Each Reservation Sub-task Each reservation sub-task contains a separate parameter set, which is used in the process of timeslot resource reservation. Note that this set may be a local information for the path compuation engine (e.g, a controller), or may signal between nodes (e.g, RSVP-TE). * Total Residence Budget: It is the sum of the residence delay allowed by the service flow within all nodes in the path, which is equal to the end-to-end delay requirement of the service flow minus the propagation delay of all links included in the path. * Node Residence Budget: It refers to the resident delay budget of the current node traversed during the process of reserving timeslot resources on each node along the path in sequence. A simple way is to divide the Total Residence Budget by the number of nodes included in the path to obtain the average resident delay budget as the Node Residence Budget for each node, or use a specified budget list to specify the resident delay budget for each node separately. Peng, et al. Expires 23 November 2023 [Page 16]
Internet-Draft Packet Generic Timeslot May 2023 * Accumulated Node Residence Budget: It refers to the cumulative residence delay budget of those nodes that have executed resource reservation. * Accumulated Node Residence Evaluation: It refers to the cumulative evaluation value of the residence delay of nodes that have executed timeslot resource reservation. The residence delay evaluation value of a node refers to the residence delay evaluation value calculated based on the delay formula (see below) when the node actually reserves a certain outgoing timeslot for the reservation sub-task. Generally, if a node is able to reserve the expected outgoing timeslot according to its residence delay budget, the residence delay evaluation value does not differ from the residence delay budget. However, in some cases, due to insufficient resources in the expected timeslot, resources have to be reserved in the timeslot adjacent to the expected timeslot, which can lead to a difference between the residence delay evaluation value and the budget value. * Accumulated Node Residence Deviation: It is equal to the Accumulated Node Residence Budget minus the Accumulated Node Residence Evaluation. * Node Residence Budget Adjustment: It is equal to the Node Residence Budget plus the Accumulated Node Residence Deviation. The usage for the above parameter set is: * For specific reservation sub-task, determine the Node Residence Budget for each node in the path, which can be taken from the average residence delay budget per node or the specified budget list. * From the headend to the endpoint, on each node's outgoing port in sequence, reserve outgoing timeslot resources based on the Node Residence Budget Adjustment, to let the residence delay evaluation value of the node obtained from the reserved outgoing timeslot be equal to or close to the Node Residence Budget Adjustment. - On the headend, the Accumulated Node Residence Deviation is the initial value of 0. Therefore, the Node Residence Budget Adjustment is equal to the Node Residence Budget. Peng, et al. Expires 23 November 2023 [Page 17]
Internet-Draft Packet Generic Timeslot May 2023 - On any other nodes, the Accumulated Node Residence Deviation is generally not 0. If the residence delay evaluation value of the node obtained from the reserved outgoing timeslot be equal to the Node Residence Budget Adjustment, it will cause the Accumulated Node Residence Deviation faced by the downstream node in the path to be 0 again. Note that the above parameter set is only an implementation choice and is not mandatory. There may be more intelligent path calculation methods available. 3.1.4.1. Resource Reservation on the Ingress Node On the headend H, as mentioned above, there is a fixed positional relationship (with possible jitter) between the departure time when the sub-burst leaves the regulator and the orchestration period. From the departure time when the sub-burst leaves the regulator, after the intra-node forwarding delay (d_f) including parsing, table lookup, internal fabric exchange, etc, the sub-burst finally arrives at the ougoing port, and at this time the ongoing sending timeslot is j, and there is time T_j left before the end of the timeslot j. The outgoing timeslot reserved for the sub-burst by the headend is offset by o (>=1) timeslots after timeslot j, which means the outgoing timeslot is (j+o)%N_h, where N_h is the number of timeslots in the orchestration period for node H. Note that o must be less than M. Thus, on the headend H the residence delay evaluation value obtained from the reserved outgoing timeslot (j+o)%N_h is: Best Node Residence Evaluation = d_f + T_j + (o-1)*L_h Worst Node Residence Evaluation = d_f + T_j + o*L_h Average Node Residence Evaluation = d_f + T_j + (2o-1)*L_h/2 where, L_h is the length of timeslot for node H. The Best Node Residence Evaluation occurs when the sub-burst is sent at the head of outgoing timeslot j+o. The Worst Node Residence Evaluation occurs when the sub-burst is sent at the end of outgoing timeslot j+o. The delay jitter within the node is L_h. However, the jitter of the entire path is not the sum of the jitters of all nodes. Peng, et al. Expires 23 November 2023 [Page 18]
Internet-Draft Packet Generic Timeslot May 2023 Depending on the implementation, the above Best Node Residence Evaluation, Worst Node Residence Evaluation, or Average Node Residence Evaluation can be used to compare with the Node Residence Budget Adjustment, so that when selecting the appropriate outgoing timeslot (j+o)%N_h, the two are equal or nearly equal, and the corresponding Unreserved Burst resources of the outgoing timeslot (j+o)%N_h meet the burst demand of the sub-burst. However, this document suggests using the Average Node Residence Evaluation to compare with the Node Residence Budget Adjustment, because the characteristic of the forwarding behavior based on packet timeslot is that adjacent nodes on the path will not simultaneously face the best or worst residency delay. 3.1.4.2. Resource Reservation on the Transit Node On the transit node V, as described in Section 3.1.1, there is a timeslot mapping relationship between the outgoing timeslot of the upstream node U and the ongoing sending timeslot of node V. For a specific sub-task, assume that an outgoing timeslot i is reserved for it on the outgoing port of the upstream node U, and after the intra-node forwarding delay (d_f) then mapped to the ongoing sending timeslot j of node V, and there is time T_ij left before the end of the timeslot j. The outgoing timeslot reserved for the sub-task by node V is offset by o (>=1) timeslots after timeslot j, which means the outgoing timeslot is (j+o)%N_v, where N_v is the number of timeslots in the orchestration period of node V. Note that o must be less than M. Thus, on the transit node V the residence delay evaluation value obtained from the reserved outgoing timeslot (j+o)%N_v is: Best Node Residence Evaluation = d_f + T_ij + (o-1)*L_v Worst Node Residence Evaluation = d_f + T_ij + L_u + o*L_v Average Node Residence Evaluation = d_f + T_ij + (L_u+(2o- 1)*L_v)/2 where, L_u and L_v is the length of timeslot for node U and V respectively. The Best Node Residence Evaluation occurs when the packet is received at the end of incoming timeslot i and sent at the head of outgoing slot j+o; The Worst Node Residence Evaluation occurs when t he packet Peng, et al. Expires 23 November 2023 [Page 19]
Internet-Draft Packet Generic Timeslot May 2023 is received at the head of incoming timeslot i and sent at the end of outgoing timeslot j+o. The delay jitter within the node is (L_u + L_v). However, the jitter of the entire path is not the sum of the jitters of all nodes. Depending on the implementation, the above Best Node Residence Evaluation, Worst Node Residence Evaluation, or Average Node Residence Evaluation can be used to compare with the Node Residence Budget Adjustment, so that when selecting the appropriate outgoing timeslot (j+o)%N_v, the two are equal or nearly equal, and the corresponding Unreserved Burst resources of the outgoing timeslot (j+o)%N_v meet the burst demand of the sub-burst. However, this document suggests using the Average Node Residence Evaluation to compare with the Node Residence Budget Adjustment, because the characteristic of the forwarding behavior based on packet timeslot is that adjacent nodes on the path will not simultaneously face the best or worst residency delay. 3.1.4.3. Resource Reservation on the Egress Node Generally, for the deterministic path carrying the service flow, the flow needs to continue forwarding from the outgoing port of the egress node to the client side, and also faces the issues of queuing. However, the outgoing port facing the client side is not part of the deterministic path. If it is necessary to continue supporting packet timeslot scheduling mechanism on that port, timeslot resources should be reserved on the higher-level service path (an overlay path) using the above reservation method. In this case, the deterministic path will serve as a virtual link of the overlay path, providing a deterministic delay performance. Therefore, for deterministic paths, the residence dalay evaluation value on the egress node is only contributed by the forwarding delay (d_f) including parsing, table lookup, internal fabric exchange, etc. 3.1.4.4. End-to-end Delay and Jitter Figure 6 shows that a path from headend P1 to endpoint E, for each node Pi, the length of timeslot is L_i, the intra-node forwarding delay is F_i, the remaining time from the end of the mapped ongoing sending timeslot is T_i, the number of timeslots offset by outgoing timeslot relative to ongoing sending timeslot is X_i, then the end to end delay can be evaluted as follows: Best E2E Delay = sum(F_i+T_i+X_i*L_i, for 1<=i<=n) - L_n + F_e Worst E2E Delay = sum(F_i+T_i+X_i*L_i, for 1<=i<=n) + F_e Peng, et al. Expires 23 November 2023 [Page 20]
Internet-Draft Packet Generic Timeslot May 2023 +---+ +---+ +---+ +---+ +---+ | P1| --- | P2| --- | P3| --- ... --- | Pn| --- | E | +---+ +---+ +---+ +---+ +---+ Figure 6 The Best E2E Delay occurs when the sub-burst is sent at the head of outgoing timeslot of node Pn. The Worst E2E Delay occurs when the sub-burst is sent at the end of outgoing timeslot of node Pn. The delay jitter is L_n. Note that at the headend P1, regardless of whether it has the best or worst residence latency, it will be aligned to the worst latency on the downstream node; Every hop is like this, except for the last one. 3.2. Timeslot Resource Access in Data-plane The headend of the path needs to maintain the timeslot resource information with the granularity of sub-burst, so that each sub-burst of the service flow can access the mapped timeslot resources. However, the intermediate node does not need to maintain this mapping state. The intermediate node only access the timeslot resources based on the timeslot number carried in the packets or indicated by FIB entries. The entry node determines the appropriate outgoing timeslot and sends the packet according to the periodic arrival time of the sub-burst, and the maintained mapping relationship between the sub-burst of service flow and the outgoing timeslot. The relationship between the incoming timeslot and the outgoing timeslot can be installed on the intermediate node or carried in the packet, so that the packet can access the corresponding outgoing timeslot on the intermediate node. Note that the incoming and outgoing timeslots mentioned here are both timeslot numbers within the orchestration period. It should be noted that the forwarding outgoing port for the service flow is still determined according to the traditional routing entries (e.g, Segment Routing), but the outgoing timeslot used by the packet is determined by the timeslot resource reservation information. Peng, et al. Expires 23 November 2023 [Page 21]
Internet-Draft Packet Generic Timeslot May 2023 3.2.1. Conversion of Timeslot ID Figure 1 shows that the scheduling period implemented on the forwarding plane is not completely equivalent to the orchestration period of the control plane. The scheduling period includes timeslot numbers from 0 to M-1, while the orchestration period includes timeslot numbers from 0 to N-1. Therefore, it is necessary to convert the outgoing timeslot of the orchestration period to the target timeslot of the scheduling period, and insert the packet to the queue corresponding to the target timeslot for transmission. A simple conversion method is: * target timeslot = outgoing timeslot % M This is safe because during resource reservation, o < M is always followed, and N is an integer multiple of M. In the orchestration period, from timeslot 0 to M-1 is the first scheduling period, from timeslot M to slot 2M-1 is the second scheduling period, and so on. From timeslot N-M to slot N-1 is the N/M scheduling period. Each timeslot in the scheduling period corresponds to an associated queue, which is used to store packets for sending in the corresponding timeslot. According to the timeslot resource reservation process mentioned above, when the sub-burst corresponding to any outgoing timeslot (e.g, z) arrived at the outgoing port of any node of the path, the ongoing sending timeslot (e.g, j) in the orchestration period of the outgoing port must be offset by o before the outgoing timeslot (z), and meet o < M, which means that the sub-burst does not randomly arrive at this node, but strictly abide by the time so that when it reaches the outgoing port, it will definitely fall into the ongoing sending timeslot (j). Next, we briefly demonstrate that the sub-burst that arrives at the outgoing port during the ongoing sending timeslot (j) can be safely inserted into the corresponding queue in the scheduling period, and that queue will not overflow. Assuming that each timeslot in the orchestration period has a virtual queue, the length of the virtual queue is the MRB of that timeslot. For example, termed the virtual queue corresponding to the outgoing timeslot (z) as queue-z, the packets that can be inserted into queue-z may only come from the following bursts: Peng, et al. Expires 23 November 2023 [Page 22]
Internet-Draft Packet Generic Timeslot May 2023 During the ongoing sending timeslot j = (z-M+1+N)%N, the bursts that arrive at the outgoing port, that is, these bursts may reserve the outgoing timeslot (z) according to o = M-1. During the ongoing sending timeslot j = (z-M+2+N)%N, the bursts that arrive at the outgoing port, that is, these bursts may reserve the outgoing timeslot (z) according to o = M-2. ... ... During the ongoing sending timeslot j = (z-1+N)%N, the bursts that arrive at the outgoing port, that is, these bursts may reserve the outgoing timeslot (z) according to o = 1; The total reserved amount of all these bursts does not exceed the MRB of the outgoing timeslot (z). Then, when the ongoing sending timeslot changes to z, queue-z will be sent and cleared. In the following time, starting from timeslot z+1 to the last timeslot N-1 in the orchestration period, there are no longer any packets inserted into queue-z. Obviously, this virtual queue is a great waste of queue resources. In fact, queue-z can be reused by the subsequent outgoing timeslot (z+M)%N. Namely: During the ongoing sending timeslot j = (z+1)%N, the bursts that arrive at the outgoing port, that is, these bursts may reserve the outgoing timeslot (z+M)%N according to o = M-1. During the ongoing sending timeslot j = (z+2)%N, the bursts that arrive at the outgoing port, that is, these bursts may reserve the outgoing timeslot (z+M)%N according to o = M-2. ... ... During the ongoing sending timeslot j = (z+M-1)%N, the bursts that arrive at the outgoing port, that is, these bursts may reserve the outgoing timeslot (z+M)%N according to o = 1. The total reserved amount of all these bursts does not exceed the MRB of the outgoing timeslot (z+M)%N. It can be seen that queue-z can be used by any outgoing timeslot (z+k*M)%N, where k is a non negative integer. By observing (z+k*M)%N, it can be seen that the minimum z satisfies 0<= z< M, that is, the entire orchestration period actually only requires M queues to store packets, which are the queues corresponding to M timeslots in the scheduling period. That is to say, the minimum z is the timeslot id in the scheduling period, while the outgoing timeslot Peng, et al. Expires 23 November 2023 [Page 23]
Internet-Draft Packet Generic Timeslot May 2023 (z+k*M)% N is the timeslot id in the orchestration period. The latter obtains the former by moduling M, which can then access the queue corresponding to the former. In short, the reason why a queue can store packets from multiple outgoing timeslots without being overflowed is that the packets stored in the queue earlier (more than M timeslots ago) have already been sent. 4. Global Timeslot ID The outgoing timeslots we discussed in the previous sections are all local timeslots for nodes. This section discusses the situation based on global timeslot. Global timeslot style refers to that all nodes in the path are identified with the same timeslot id, which of course requires all nodes to use the same timeslot length. The advantages are that the resource reservation based on global timeslots is simple, always reserving a specified outgoing timeslot for the service flow. There is no need to establish a local timeslot mapping relationship on each node or carry this mapping relationship in packets. The packet only needs to carry the unique global timeslot id. However, the disadvantage is that the latency performance of the path may be large, which depends on the phase difference between the inherent orchestration periods between the adjacent nodes. Another disadvantage is that the success rate of finding a path that matches the service requirements is not as high as local timeslot style. Global timeslot style requires that the orchestration period is equal to the scheduling period, mainly considering that arrival packets with any global timeslot id can be successfully inserted into the corresponding queue. However, as the scheduling period is less than the orchestration period is the ideal design goal, further research is needed on other methods(such as basically aligning orchestration period between nodes), to ensure that packets with any global timeslot id can queue normally when the scheduling period is less than the orchestration period. 4.1. Fixed Timeslot Mapping Mode So far, the packet timeslot scheduling scheme presented above is to reserve a fixed outgoing timeslot for services in the orchestration period, even if global slot-id is used. Compared to the local timeslot style, global timeslot style means that the incoming timeslot i must map to the outgoing timeslot i too. As the example shown in Figure 7, each orchestration period contains 6 timeslots. Node V has three connected upstream nodes U1, U2, and U3. During each hop forwarding, the packet accesses the outgoing Peng, et al. Expires 23 November 2023 [Page 24]
Internet-Draft Packet Generic Timeslot May 2023 timeslot corresponding to the global timeslot id and forwards to the downstream node with the global timeslot id unchanged. For example, U1 sends some packets with global slot-id 0, termed as g0, in the outgoing timeslot 0. The packets with other global slot-id 1~5 are similarly termed as g1~g5 respectively. The figure shows the scheduling results of these 6 batches of packets sent by upstream nodes when node V continues to send them. 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ U1 | g0| g1| g2| | | | | | | | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ U2 | | | g3| g4| | | | | | | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ U3 | g5| | | | | | | | | | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ V | | | | g3| g4| g5| g0| g1| g2| | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Figure 7 In this example, the mapping relationship of the outgoing timeslot from U1 and the ongoing sending timeslot of V is i -> i, so the reserved outgoing timeslot for the incoming timeslot i is i+6. The mapping relationship of the outgoing timeslot from U2 and the ongoing sending timeslot of V is i -> i-1, so the reserved outgoing timeslot for the incoming timeslot i is i. And, the mapping relationship of the outgoing timeslot from U3 and the ongoing sending timeslot of V is i -> i+1, so the reserved outgoing timeslot for the incoming timeslot i is i+6-1. For the headend, the residence delay depends on the arrival time when the sub-burst arrives at the scheduler and specified global timeslot. Suppose that the ongoing sending timeslot is j at the arrival time when the sub-burst arrives at the scheduler, and there is time T_j Peng, et al. Expires 23 November 2023 [Page 25]
Internet-Draft Packet Generic Timeslot May 2023 left before the end of the timeslot j, and the sub-burst is specified to use global timeslot i, then, the reserved outgoing timeslot is (j+o)%N, where o equals (N+i-j)%N. The residence delay equation for headend is similar to Section 3.1.4.1. For any other nodes, the residence delay depends on the phase difference of the orchestration period between upstream node (U) and this node (V), i.e., the difference between the orchestration period of the upstream node (U) and the ongoing sending period of this node (V). Suppose that the incoming timeslot i mapped to the ongoing sending timeslot j, and there is time T_ij left before the end of the timeslot j. Then, the reserved outgoing timeslot is (j+o)%N, where o equals (N+i-j)%N. The residence delay equation for intermediate node is similar to Section 3.1.4.2. For example, the packets g3 sent by upstream node U2 falls into the ongoing sending timeslot 2 of node V, it can be sent in outgoing global timeslot 3. In this case, the residency delay in the node V is small. While, the packets g5 sent by upstream node U3 falls into the ongoing sending timeslot 0 of node V, so it needs to wait for timeslot 0, 1, 2, 3, 4 to be sent in global outgoing timeslot 5. In this case, the residency delay in the node V is large. For example, the packets g0 sent by upstream node U1 fall into the ongoing sending timeslot 0 of node V, the packets need to wait for the end of the ongoing sending period to be sent in the global outgoing timeslot 0 in the next round of orchestration period, which will introduce a large node residency delay. It should be noted that in this case, the packets g0, when they fall into the ongoing sending timeslot 0, cannot be placed in the buffer corresponding to timeslot 0. Instead, it needs to be stored in a buffer prior to the packet timeslot scheduler (such as the buffer on the input port side) for a fixed latency (such as a fixed timeslot) and then released to the timeslot scheduler. This fixed-latency buffer is only created for specific upstream nodes. It can be determined according to the initial detection result of the mapping relationship between the outgoing timeslot of the upstream node and the ongoing sending timeslot of this node. If the initial detection result is slot-id i -> slot-id i, it needs to be introduced, otherwise it is unnecessary. After the introduction of fixed-latency buffer, the new detection result will no longer be i -> i. The end-to-end delay equation for intermediate node is similar to Section 3.1.4.4. Peng, et al. Expires 23 November 2023 [Page 26]
Internet-Draft Packet Generic Timeslot May 2023 4.2. Variable Timeslot Mapping Mode In variable timeslot mapping Mode, timeslot resources are still reserved based on global timeslot id on the control plane, but in actual forwarding, packets do not necessarily have to wait until the outgoing time slot before being sent. Variable timeslot mapping is similar to cell scheduling in ATM. During each hop forwarding, the packets dynamically maps to an idle local outgoing timeslot according to the global slot-id, according to the principle of minimum offset (or expected offset range) between the global slot-id and local slot-id, but the sending packets still carry the global slot-id without changed. In this case, the delay performance is related to the mapping algorithm (i.e., the scheduling algorithm) adopted. The suggested scheduling algorithm will be discussed in later versions. As the example shown in Figure 8, each orchestration period contains 6 timeslots. Node V has three connected upstream nodes U1, U2, and U3. Node U1 dynamically maps the packets with global slot-id 0,1,2 to the outgoing timeslot 3,4,5 respectively, node U2 dynamically maps the packets with global slot-id 3,4,5 to the outgoing timeslot 4,5,0 respectively, and node V dynamically map the packets with global slot-id 0~5 to the outgoing timeslot 4,5,1,0,2,3 respectively. 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ U1 | | | | g0| g1| g2| | | | | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ U2 | | | | | g3| g4| g5| | | | | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ V | | | | | g0| g1| g3| g2| g4| g5| | | | | | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Figure 8 Because the service flow arrived at the network entry node is periodic, each entry node should maintain state about a fixed mapping relationship between global slot-id and the actual outgoing slot-id Peng, et al. Expires 23 November 2023 [Page 27]
Internet-Draft Packet Generic Timeslot May 2023 for each flow, so that it is more likely that a fixed runtime mapping relationship will appear on each intermediate node to avoid jitter. However, the characteristics of variable timeslot mapping determine that this fixed runtime mapping relationship is not always guaranteed. For example, with the addition or deletion of service, the mapping status of global slot-id to the actual outgoing slot-id may have to be updated on the entry node, which will correspondingly lead to changes in the runtime mapping relationship on the intermediate node. The main purpose of global slot-id is used in the timeslots resource allocation. Within the resource planning of the controller, the timeslot resources identified by each global slot-id are allocated for multiple limited service flows without conflict. Intuitively, if all service flows access the outgoing timeslot according to the fixed timeslot mapping mode, there is no timeslot conflict, that is, the total timeslot resources can meet all limited service requirements; Variable timeslot mapping mode is to dynamically access the nearby idle outgoing timeslots without introducing timeslot conflicts, and it will not lead to the result that the total timeslot resources are not enough. How to predict whether the nearest outgoing timeslot is idle is the focus of the selected scheduling algorithm. Assume that the packets with global slot-id i accessing the outgoing timeslot (j+o)%N nearby do not make no resources available when the packets with global timeslot (j+o)%N arrive now or soon, the delay performance of variable timeslot mapping mode is better than fixed timeslot mapping mode, because o may be less than (N+i-j)%N. However, it will introduce jitter. A possible implementation of variable timeslot mapping mode is to schedule packets with work-conserving behavior. That is, the sub- burst is still inserted into the queue corresponding to the global timeslot id, but that queue can be sent in advance, as long as the queues corresponding to all timeslots from the ongoing sending timeslot to the previous timeslot of this global timeslot are empty. It should be noted that this behavior of sending in advance cannot disrupt the basic timeslot rotation (round robin). 5. Summary of Timeslot Style Depending on the strategy of reserving timeslot resources, different timeslot styles will be presented, as shown in the table below. Peng, et al. Expires 23 November 2023 [Page 28]
Internet-Draft Packet Generic Timeslot May 2023 +===============+========================+==================+ | Strategy | Timeslot Style | Referrence | +===============+========================+==================+ | Flexible o | Local timeslot style | section 3.1.4 | | (1<=o<M) | | | +---------------+------------------------+------------------+ | Constant o | Global timeslot style | section 4 | | (o=(N+i-j)%N) | | | +---------------+------------------------+------------------+ | Constant o | Multi-CQF | [Multi-CQF] | | (o=1) | | | +---------------+------------------------+------------------+ Figure 9 6. Queue Design The number of timeslot queues should be designed according to the number of timeslots included in the scheduling period. Each timeslot corresponds to a separate queue (or queue group), in which the buffered packets must be able to be sent within a timeslot. The length of the queue, i.e., the total number of bits that can be reserved or sent for a timeslot, does not have to be set to be exactly equal to the link rate multiplied by the timeslot length. This is because the bandwidth requirements of other non-deterministic services and protocols running in the network should also be considered. 6.1. Full Queues When the scheduling period length is equal to the orchestration period length, the node will implement full queues. The advantage is that the actual forwarding resources are the same view as the resources used for reservation, so that the resource reservation process is simple. However, the disadvantage is that because the scheduling period is generally large to cover all services requirements, the number of queues maintained by the node will be large. Peng, et al. Expires 23 November 2023 [Page 29]
Internet-Draft Packet Generic Timeslot May 2023 For example, if the accumulated length of all queues supported by the hardware is 4G bytes, the queue length corresponding to a timeslot of 10us at a port rate of 100G bps is 1M bits, then a maximum of 32K timeslot queues can be provided, and the maximum length of the orchestration period supported is 320ms. However, considering the queue resource requirements of other non-deterministic services, the packet timeslot function can only use some of the queue resources, such as 10K~20K queues. In this case, the length of the orchestration period supported by the node is 100~200 ms. 6.2. Non-full Queues When the length of the scheduling period is less than the length of the orchestration period, the node will implement a non-full queues. The advantages and disadvantages are opposite to the full queues option. The actual forwarding resources are inconsistent with the view of the resources reservation. But the number of queues maintained by the node is small. 7. Admission Control on the Headend On the network entry, traffic regulation must be performed on the incoming port, so that the service flow does not exceed its T-SPEC such as burst interval, burst size, maximum packet size, etc. This kind of regulation is usually the shaping using leaky bucket combined with the incoming queue that receives service flow. A service flow may contain discrete multiple sub-bursts within its periodic burst interval. The leaky bucket depth should be larger than the maximum packet size, and should be consistent with the reserved burst resources required for the maximum sub-burst. The scheduling mechanism described in this document has a requirement on the arrival time of service flows on the network entry. It is hoped that the distribution of sub-bursts (after regulation) of the service flow will always appear in a fixed position within the orchestration period. Some amount of jitter is permited for this position, but the jitter cannot reach a scheduling cycle. Intuitively, small o (i.e., the offset between the reserved outgoing timeslog and ongoing sending timeslot) can tolerate large early arrival deviations, while large o can tolerate large late arrival deviations. Only in this way can we achieve the ideal design goal that scheduling period is smaller than the orchestration period. Otherwise, for randomly arriving service flows, it can be supported by taking the scheduling period equal to the orchestration period (optoin-1), or it can be supported by introducing an explicit buffer put before the scheduler on the network entry to let the arrival time always meet the fixed position (option-2). Peng, et al. Expires 23 November 2023 [Page 30]
Internet-Draft Packet Generic Timeslot May 2023 * Note that due to randomness of arrival time, the packet may just miss the scheduling (or arrive too earlier) and need to wait in the scheduling queue (in the case of option-1) or the explicit buffer (in the case of option-2) for the next scheduling period. From this perspective, we suggest that it is best for service flows to strictly obey their arrival time, which should be the ideal admission control for all scheduling mechanisms that attempt to forward service flows in the specified time windown. 8. Frequency Synchronization The basic explanation for frequency synchronization is that the crystal frequency of the hardware is equal, which enables all nodes in the network to be in the same inertial frame and have the same time lapse rate. This is a prerequisite for all latency based scheduling mechanisms. This frequency synchronization mechanism (such as syncE) is not within the scope of this document. Sometimes, people also refer to the frequency asynchrony as the timeslot rotation frequency difference caused by different node configurations with different timeslot lengths. This document supports the interconnection between nodes with this type of frequency asynchrony. 9. IANA Considerations TBD. 10. Security Considerations TBD. 11. Acknowledgements TBD. 12. References 12.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. Peng, et al. Expires 23 November 2023 [Page 31]
Internet-Draft Packet Generic Timeslot May 2023 12.2. Informative References [ATM-LATENCY] "Bounded Latency Scheduling Scheme for ATM Cells", 1999, <https://ieeexplore.ieee.org/document/780828/>. [CQF] "Cyclic Queuing and Forwarding", 2017, <https://ieeexplore.ieee.org/document/7961303>. [Multi-CQF] "Multiple Cyclic Queuing and Forwarding", 2021, <https://www.ieee802.org/1/files/public/docs2021/new-finn- multiple-CQF-0921-v02.pdf>. Authors' Addresses Shaofu Peng ZTE China Email: peng.shaofu@zte.com.cn Peng Liu China Mobile China Email: liupengyjy@chinamobile.com Kashinath Basu Oxford Brookes University United Kingdom Email: kbasu@brookes.ac.uk Aihua Liu ZTE China Email: liu.aihua@zte.com.cn Dong Yang Beijing Jiaotong University China Email: dyang@bjtu.edu.cn Peng, et al. Expires 23 November 2023 [Page 32]