Generic Packet Timeslot Scheduling Mechanism
draft-peng-detnet-packet-timeslot-mechanism-02
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draft-peng-detnet-packet-timeslot-mechanism-02
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
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provisions of BCP 78 and BCP 79.
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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
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Please review these documents carefully, as they describe your rights
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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
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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
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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.
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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
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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).
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+---+ +---+ +---+
|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.
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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.
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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.
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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
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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 --------------------->|
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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.
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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
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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.
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|<--------------------- 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.
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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.
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* 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.
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- 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.
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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
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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
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+---+ +---+ +---+ +---+ +---+
| 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.
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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:
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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
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(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
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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
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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.
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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
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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.
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+===============+========================+==================+
| 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.
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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).
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* 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>.
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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
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