Timeslot Queueing and Forwarding Mechanism
draft-peng-detnet-packet-timeslot-mechanism-06
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Shaofu Peng , Peng Liu , Kashinath Basu , Aihua Liu , Dong Yang , Guoyu Peng | ||
| Last updated | 2024-03-04 | ||
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draft-peng-detnet-packet-timeslot-mechanism-06
DetNet Shaofu. Peng
Internet-Draft ZTE
Intended status: Standards Track Peng. Liu
Expires: 5 September 2024 China Mobile
Kashinath. Basu
Oxford Brookes University
Aihua. Liu
ZTE
Dong. Yang
Beijing Jiaotong University
Guoyu. Peng
Beijing University of Posts and Telecommunications
4 March 2024
Timeslot Queueing and Forwarding Mechanism
draft-peng-detnet-packet-timeslot-mechanism-06
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 used queueing mechanism.
This document further describes a generic time division multiplexing
scheme in IP/MPLS networks, which we call timeslot queueing and
forwarding (TQF) mechanism. It aims to bring timeslot resources to
layer-3, to make it easier for the control plane to calculate the
delay performance based on the timeslot resources, and also make it
easier for the data plane to create more flexible timeslot mapping.
The functions of TQF can better meet large scaling requirements.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Timeslot Resource Reservation in Control-plane . . . . . 11
3.1.1. Timeslot Mapping Relationship . . . . . . . . . . . . 12
3.1.1.1. Deduced by BTM . . . . . . . . . . . . . . . . . 12
3.1.1.2. Deduced by BOM . . . . . . . . . . . . . . . . . 15
3.1.2. Timeslot Resource Definition . . . . . . . . . . . . 17
3.1.3. Arrival Postion in the Orchestration Period . . . . . 18
3.1.4. Proccess of Each Reservation Sub-task . . . . . . . . 21
3.1.4.1. Resource Reservation on the Ingress Node . . . . 23
3.1.4.2. Resource Reservation on the Transit Node . . . . 24
3.1.4.3. Resource Reservation on the Egress Node . . . . . 25
3.1.4.4. End-to-end Delay and Jitter . . . . . . . . . . . 26
3.2. Timeslot Resource Access in Data-plane . . . . . . . . . 26
3.2.1. Round Robin Queue: Conversion of Timeslot ID . . . . 27
3.2.2. PIFO: Directly Using Outgoing Timeslots . . . . . . . 29
4. Global Timeslot ID . . . . . . . . . . . . . . . . . . . . . 29
5. Summary of Timeslot Style . . . . . . . . . . . . . . . . . . 32
6. In-time Scheduling . . . . . . . . . . . . . . . . . . . . . 33
7. Queue Design . . . . . . . . . . . . . . . . . . . . . . . . 34
7.1. Round Robin Queues . . . . . . . . . . . . . . . . . . . 34
7.1.1. Full Queues . . . . . . . . . . . . . . . . . . . . . 35
7.1.2. Non-full Queues . . . . . . . . . . . . . . . . . . . 35
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7.2. PIFO Queue . . . . . . . . . . . . . . . . . . . . . . . 35
8. Multiple Orchestration Periods . . . . . . . . . . . . . . . 36
9. Admission Control on the Headend . . . . . . . . . . . . . . 38
10. Frequency Synchronization . . . . . . . . . . . . . . . . . . 39
11. Evaluations . . . . . . . . . . . . . . . . . . . . . . . . . 40
11.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 41
12. Taxonomy Considerations . . . . . . . . . . . . . . . . . . . 44
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
14. Security Considerations . . . . . . . . . . . . . . . . . . . 45
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 45
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
16.1. Normative References . . . . . . . . . . . . . . . . . . 46
16.2. Informative References . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
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 queueing delay, which
is closely related to the queueing 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
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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 variable length packets, they all essentially depend on the
queueing mechanism.
[TAS] introduces a synchronous time-division multiplexing method
based on gate control list (GCL) rotation in Ethernet LAN. Its basic
idea is to calculate when the packets of the service flow arrive at a
certain node, then the node will turn on the green light (i.e., the
transmission state is set to OPEN) for the corresponding queue
inserted by the service flow at that time duration, which is defined
as TimeInterval between two adjacent items in gating cycle. The
TimeInterval is exactly the timeslot resource that can be reserved
for service flow. A set of queues is controlled by the GCL, with
round robin per gating cycle. The gating cycle (e.g, 250 us)
contains a lot of items, and each item is used to set the OPEN/CLOSED
states of all traffic class queues. By strictly controlling the
release time of service flow at the network entry node, multiple
flows always arrive sequentially during each gating cycle at the
intermediate node and are sent during their respective fixed timeslot
to avoid conflicts, with extremely low queueing delay. However, the
GCL state (i.e., items set, and different TimeInterval value between
any two adjacent items) is related with all ordered flows that passes
through the node. Calculating and installing GCL states separately
on each node has scalability issues.
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[CQF] introduces a synchronous time-division multiplexing method
based on fixed-length cycle in Ethernet LAN. [ECQF] is a further
enhancement of the classic CQF and may be applicable to large scaling
networks. CQF with 2-bin mode or ECQF with 3-bin mode only uses a
small number of cycles to establish the cycle mapping between a port-
pair of two adjacent nodes, which is independent of the individual
service flow. The cycle mapping may be maintained on each node and
swaped based on a single cycle id carried in the packet during
forwarding ([I-D.eckert-detnet-tcqf]), or all cycle mappings are
carried in the packet as a cycle stack and read per hop during
forwarding ([I-D.chen-detnet-sr-based-bounded-latency]). According
to [ECQF], how many cycles (i.e., x-bin mode) are required depends on
the proportion of the variation in intra-node forwarding delay
relative to the cycle size. If the proportion is small, 3-bin is
enough, otherwise, more than 3 bins needed. Compared to TAS, CQF/
ECQF no longer maintains GCL on each node, but instead replaces the
large number of variable length of timeslots related to service flows
in GCL with a small number of fixed length cycles unrelated to
service flows. Thus, CQF/ECQF simplifies the data plane, but leaves
the complexity to the control plane, by calculating and controlling
the release time of service flow at the network entry, to guarantee
no conflicts between flows in any cycle on any intermediate nodes.
In order to meet the large scaling requirements, this document
describes a scheduling mechanism for enhancing TAS. Firstly, it
brings timeslot type of resources to layer-3 and construct timeslot
resources on each link within gating cycle, which are advertised in
the network, and opened and reserved for DetNet flows to implement
timeslot orchestration (i.e., flow interleaving). Secondly, it
defines timeslot based queueing mechanism on the data plane with on-
time or in-time behavior. We call this mechanism as Timeslot
Queueing and Forwarding (TQF), as a supplement to IEEE 802.1 TSN TAS.
In TQF, The selected length of gating cycle depends on the length of
the supported service burst interval.
Similar to TAS and CQF/ECQF, TQF is also TDM based scheduling
mechanisms.
* Compared to classic TAS, TQF may use round robin queues
corresponding to the count of timeslots during gating cycle, while
TAS only maintains queues corresponding to the number of traffic
classes and one of them is used for the Scheduled Traffic (i.e.,
DetNet flows). That means TQF need more queues than TAS (i.e.,
multiple timeslot queues vs single traffic class queue). However,
TAS needs to use other complex methods to control the arrival
order of all flows sharing the same traffic class queue to isolate
them (so that each flow faces almost zero queuing delay), while
TQF's timeslot queue naturally isolates flows by timeslot id of
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gating cycle. And, TQF with in-time scheduling mode may use a
single PIFO (put in first out) queue to approximate the ultra-low
delay of TAS.
* Compared to CQF/ECQF, TQF on-time scheduling maintains round robin
queues corresponding to the count of timeslots during gating
cycle, while CQF/ECQF maintains extra tolerating queues depending
on the proportion of the variation in intra-node forwarding delay
relative to the cycle size. There is no gating cycle with its
timeslot resources designed by CQF/ECQF, it needs to use
additional flow interleaving method to control the arrival order
of flows sharing the same cycle queue to isolate flows (or
alternatively tolerate overprovision), while TQF's timeslot queue
naturally isolates flows by timeslot id of gating cycle. This is
also the semantic difference between cycle id and timeslot id,
where the former is used to indicate the NO. of the aggregated
queues such as sending, receiving, or tolerating queue, rather
than indicating the individual timeslot resource within the gating
cycle like the later. That is, after defining timeslot resources
in IP/MPLS, TQF does not limit the implementations of the data
structure type corresponding to timeslot resources on the data
plane, which may be round robin queues, or a single PIFO queue.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Terminology
The following terminology is introduced in this document:
Timeslot: The unit of TQF 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 zone
corresponding to a specific timeslot id, and may be sent before
(in-time mode) or within (on-time mode) that timeslot. The
timeslot id is always a NO. from the orchestration period.
Service Burst Interval: The traffic specification of DetNet flow
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generally follows the principle of generating a specific burst
amounts within a specific length of periodic 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 used to
orchestrate the DetNet flow and depends on the service burst
interval of DetNet flows. It is actually the gating cycle in
TAS, and its length depends on the length of the service burst
interval of all deterministic flows. It contains a fixed count
(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 DetNet flows, i.e.,
which timeslots are occupied by flows and how many bits are
occupied in a timeslot. 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. It is
possible to configure multiple instances of orchestration period
with different lengths, however, nodes communicate with each
other based on the same length of orchestration period.
Ongoing Sending Period: The orchestration period which the ongoing
sending timeslot belongs to.
Scheduling Period: The scheduling period depends on the hardware's
buffer resources that is supported by the device. Its length
reflects the count of the timeslot resources (termed as M and
numbered from 0 to M-1) that is actually instantiated on the data
plane, which is limited by hardware capabilities. Scheduling
period length may be less than or equal to orchestration period
length in the case of on-time mode, or larger than or equal to
orchestration period length in the case of in-time mode.
Different nodes can be configured with different scheduling
period length. When the orchestration period is larger than the
scheduling period, different parts of the orchestration period
can be mapped to a single scheduling period using appropriate
mapping methods.
Incoming Timeslot: For the headend of the path, when the application
flow received from the client side reaches the UNI port, the
corresponding timeslot of the UNI port after traffic policing is
the incoming timeslot of the packet. For an intermediate node in
a specific path, the timeslot contained in the packet received
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from the upstream node (i.e., the outgoing timeslot of the
upstream node) is its incoming timeslot. An incoming timeslot is
the timeslot id in the orchestration period.
Outgoing Timeslot: When sending a packet to the outgoing port,
according to resource reservation or certain rules, it chooses to
send packet in the specified timeslot of that port, which is the
outgoing timeslot. An outgoing timeslot is the timeslot id in
the orchestration period.
Ongoing Sending Timeslot: When the end of the incoming timeslot to
which the packet belongs reaches a specific port, the timeslot
currently in the sending state is the ongoing sending timeslot of
that port. Note that the ongoing sending timeslot is different
with the outgoing timeslot. An ongoing sending timeslot is the
timeslot id 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. The data structure associated with timeslot resources
may be implemented using round robin queues, or a single PIFO queue,
etc.
Figure 1 shows the TQF scheduling behavior implemented by the
intermediate node P through which a deterministic path passes.
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incoming slots:
i,j,k
+---+ +---+ +---+
|PE1| --------------- | P | --------------- |PE2|
+---+ +---+ +---+
orchestration period (OP)
+---+---+-+-+---+---------+---+
| 0 | 1 | 2 | 3 | ... ... |N-1|
+---+---+---+---+---------+---+
^
reserve |
Outgoing slots |
a,b,c @OP |
path -------------------------o------------------->
|\
| \ (rank by a,b,c @OP)
access slots: | \-----------------------+
a',b',c' @SP v |
/ +-------------------+ __ v
| | queue-0 @slot 0 | / \ +---+
| +-------------------+ | | +---+
| | queue-1 @slot 1 | | | +---+
Scheduling < +-------------------+ | +---+
Period (SP) | | ... ... | | ^ +---+
| +-------------------+ | | +---+
| | queue-n @slot M-1| \__/ +---+
\ +-------------------+ +---+
(Round Robin Queue) (PIFO)
Figure 1: Timeslot Based Scheduling Mechanism
Where, both the orchestration period and the scheduling period
consist of multiple timeslots, the number of timeslots supported by
orchestration period is related to the length of the service burst
interval, while the number of timeslots supported by scheduling
period is limited by hardware capabilities, and it may be
instantiated by a Round Robin queue or PIFO.
The total amount of bits that can be reserved or sent in each
timeslot can be preset, generally not exceeding the result of the
service rate multiplied by the timeslot length. The TQF scheduler
may configure a specific service rate, which must not exceed the port
bandwidth.
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The orchestration period of all nodes in the network does not require
phase alignment. However, within 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. In other words, different parts of the
orchestration period share and reuse the same scheduling period.
That is, within a node, no synchronization mechanism is required
between orchestration period and scheduling period.
In the figure, the path allocates timeslots a, b, c from the
orchestration period of the outgoing port (link P-PE2) for incoming
timeslots i, j, k respectively. It finally accesses the mapped
timeslot from scheduling period. There is a mapping relationship
between the timeslot z of orchestration period and the timeslot z' of
scheduling period, i.e., z' = f(z). There are many mapping options,
such as z'=z, z'=z+offset, z'=z%M, and z'=random(z), etc. Which
option to use depends on the data structure instantiated for timeslot
resources and the specific resource reservation method. In this
document, we mainly discuss two mapping option, one is z'=z%M (in the
case of round robin queue), the other is z'=z (in the case of PIFO).
Section 3.2.1 provides more information on option z'=z%M. Note that
option z'=z does not mean that the scheduling period physically
instantiates every timeslot resource of the orchestration period.
In general, TQF 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/links to use
different length of timeslot and scheduling period.
A TQF enabled link may configure multiple instances of orchestration
period with different lengths. Nodes communicate (i.e., control
plane messages, or data plane packets) with each other based on the
same length of orchestration period. A specific orchestration
periold instance constrains the scale of timeslot resources, and is
used for both control plane and data plane. That is, orchestration
periold instance should not be considered as just a management
object.
The scheme involves two aspects: the path calculation and timeslot
resource reservation in the control plane, and timeslot resource
access in the data plane.
EDITOR'S NOTE: according to WG's discussion this document may be
separated to two documents in the future, one focuses on timeslot
resource reservation and the other on timeslot resource based
scheduling.
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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 if a path carries multiple DetNet flows, then the
path may reserve timeslot resources for the aggregated DetNet flow,
and may reserve the burst resources in multiple timeslots in the
orchestration period at the same time. However, it would still be
beneficial to distinguish between reservation sub-tasks corresponding
to different DetNet flows in the combined reservation task. In this
document, we refer to a reservation sub-task as an individual
timeslot resource reservation action related to a DetNet flow. Note
that one or more reservation sub-tasks for a specific DetNet flow may
be derived based on its TSpec, and each reservation sub-task will
allocate corresponding timeslot. The intermediate nodes do not
maintain the state of DetNet flow and only reserve timeslot resources
based on the reservation sub-tasks.
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 loose 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. This is used to select
outgoing timeslot at each node. 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 id 3), (outgoing port b,
outgoing slot id 60)>
* The timeslot mapping relationship created by the sub-task-2:
<(incoming port a, incoming slot id 3), (outgoing port b,
outgoing slot id 61)>
Special care should be taken not to confuse the use of different
mapping relationships. For specific DetNet 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
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headend, then determine the timeslot reserved on the next hop , and
so on. We assume that the DetNet flow has a periodic arrival time
(i.e., the time when the regulated packet reaches the scheduler), and
there is an ideal 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 DetNet 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.
Two methods are provided in the following sub-sections to determine
the mapping relationship between the incoming timeslot and the
ongoing sending timeslot.
3.1.1.1. Deduced by BTM
Figure 2 shows that there are three nodes U, V, and W in turn along
the path. All nodes are configured with orchestration period of the
same length (termed as OPL), which is crucial for establishing a
fixed timeslot mapping relationship.
* Port_u2 has timeslot length L_u2, and an orchestration period
contains N_u2 timeslots.
* Port_v1 has timeslot length L_v1, and an orchestration period
contains N_v1 timeslots.
* Port_v2 has timeslot length L_v2, and an orchestration period
contains N_v2 timeslots.
Hence, L_u2*N_u2 = L_v1*N_v1 = L_v2*N_v2. 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 L_u# and L_v# is LCM, OPL is also a multiple of LCM.
Node U may send a detection packet from the end (or head, the process
is similar) of an arbitrary timeslot i of port_u2 connected to node
V. After a certain link propagation delay (D_propagation), the
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packet is received by the incoming port of node V, and i is regarded
as the incoming timeslot by V. At this time, the ongoing sending
timeslot of port_v1 is j', and there is time T_ij' left before the
end of the timeslot j'.
This mapping relationship is termed as:
* <instance OPL, port_u2 slot i, port_v1 slot j', T_ij'>
To avoid confusion, we refer to this mapping relationship as the base
timeslot mapping (BTM), as it is independent of the DetNet flows.
Later, we will see the timeslot mapping relationship related to
DetNet flow, which is the mapping relationship between the outgoing
timeslot of port_u2 and the outgoing timeslot of port_v2, which is
based on timeslot resource reservation and termed as the forwarding
timeslot mapping (FTM).
BTM is generally maintained by node V when processing probe message
received from node U. However, node U may also obtain this
information from node V, e.g, by an ACK message. The advantage of
maintaining BTM by node U is that it is consistent with the
unidirectional link from node U to V, so it is more appropriate for
node U (rather than V) to advertise it in the network. How to detect
BTM and then advertise it in the network (including contoller), will
be described in separate documents.
Note that this document does not recommend directly detecting and
maintaining BTM between the outgoing timeslot of port_u2 and the
ongoing sending timeslot of port_v2 (i.e., the outgoing port of
downstream node V), as this is too trivial. In fact, as shown above,
maintaining only BTM between the outgoing timeslot of port_u2 and the
ongoing sending timeslot of port_v1 (i.e., the incoming port of
downstream node V) is sufficient to derive other mapping
relationships.
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| port_u1
Node U
| port_u2
|
| |<---------------------- OP of port_u2 -------------------->|
| +-----------+---+-------------------------------------------+
| | ... ... | i | ... ... |
| +-----------+---+-------------------------------------------+
| (departured from port_u2)
| |
| \ (link delay)
| v
| |<---------------------- OP of port_u2 -------------------->|
| +-----------+---+-------------------------------------------+
| | ... ... | i | ... ... |
| +-----------+---+-------------------------------------------+
| (arrived at port_v1)
| |
| |<-T_ij'->|
| v (i map to j')
| +-----------+-----------+-----------------------------------+
| | ... ... | j' | ... ... |
| +-----------+-----------+-----------------------------------+
| |<--------------------- OP of port_v1 --------------------->|
| port_v1 \
v |
Node V |
| \ (intra-node forwarding delay)
| port_v2 v
| +---------------+-------+-----------------------------------+
| | ... ... | j | ... ... |
| +---------------+-------+-----------------------------------+
| |<--------------------- OP of port_v2 --------------------->|
|
| port_w1
v
Node W
Figure 2: BTM Detection
Based on the above detected BTM, and knowing the intra-node
forwarding delay (F) including parsing, table lookup, internal fabric
exchange, we can derive BTM between any outgoing timelot x of port_u2
and the ongoing timeslot y of port_v2.
Let t is the offset between the end of the timeslot x of port_u2 and
the beginning of the orchestration period of the port_v2.
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* t = ((j'+1)*L_v1 - T_ij' + OPL + (x-i)*L_u2 + F) % OPL
Then,
* y = [t/L_v2]
And the time T_xy left before the end of the timeslot y is:
* T_xy = (y+1)*L_v2 - t
This document recommends that the time of each port within the same
node must be synchronized, that is, all ports of a node share the
same local system time, which is easy to achieve. It is also
recommended that the begin time of the orchestration period for all
ports within the same node be the same or differ by an integer
multiple of OPL, e.g, maintaining a global initial time as the
logical begin time for the first round of orchestration period for
all ports. Whether node restart or port restart, this initial time
should continue to take effect to avoid affecting the timeslot
mapping relationship between each node. Depending on the
implementation, considering that the initial time may be a historical
time that is too far away from the current system time, regular
updates may be made to it (e.g, self increasing k*OPL, where k is a
natural number) to be closer to the current system time.
3.1.1.2. Deduced by BOM
Figure 3 shows that there are three nodes U, V, and W in turn along
the path. Similar to Section 3.1.1.1, it still has L_u2*N_u2 =
L_v1*N_v1 = L_v2*N_v2.
Node U may send a detection packet from the head (or end, the process
is similar) of the orchestration period of port_u2 connected to node
V. After a certain link propagation delay (D_propagation), the
packet is received by the incoming port of node V. At this time,
there is time P_uv left before the end of the ongoing sending period
of port_v1.
This mapping relationship is termed as:
* <instance OPL, port_u2, port_v1, P_uv>
We refer to this mapping relationship as the base orchestration-
period mapping (BOM), which it is independent of the DetNet flows.
BOM is generally maintained by node V when processing probe message
received from node U. However, node U may also obtain this
information from node V, e.g, by an ACK message. The advantage of
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maintaining BOM by node U is that it is consistent with the
unidirectional link from node U to V, so it is more appropriate for
node U (rather than V) to advertise it in the network. How to detect
BOM and then advertise it in the network (including contoller), will
be described in separate documents.
| port_u1
Node U
| port_u2
|
| |<---------------------- OP of port_u2 -------------------->|
| +-----------+---+-------------------------------------------+
| | ... ... | | ... ... |
| +-----------+---+-------------------------------------------+
| (departured from port_u2)
| \
| \ (link delay)
| \
| |<---------------------- OP of port_u2 -------------------->|
| +-----------+---+-------------------------------------------+
| | ... ... | | ... ... |
| +-----------+---+-------------------------------------------+
| (arrived at port_v1)
| |
| |<---------------------- P_uv -------------------------->|
| v
| +-----------+-----------+-----------------------------------+
| | ... ... | | ... ... |
| +-----------+-----------+-----------------------------------+
| |<--------------------- OP of port_v1 --------------------->|
| port_v1
v \
Node V \ (intra-node forwarding delay)
| \
| port_v2 \
| +---------------+-------+-----------------------------------+
| | ... ... | | ... ... |
| +---------------+-------+-----------------------------------+
| |<--------------------- OP of port_v2 --------------------->|
|
| port_w1
v
Node W
Figure 3: BOM Detection
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Based on BOM, and knowing the intra-node forwarding delay (F), we can
derive the mapping relationship between any outgoing timelot x of
port_u2 and the ongoing timeslot y of port_v2.
Let t is the offset between the end of the timeslot x of port_u2 and
the beginning of the orchestration period of the port_v2.
* t = ((x+1)*L_u2 + OPL - P_uv + F) % OPL
Then,
* y = [t/L_v2]
And the time T_xy left before the end of the timeslot y is:
* T_xy = (y+1)*L_v2 - t
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 (TL): Represents the length of the timeslot, in
units of us. Generally, the length of each timeslot included in
the orchestration period is the same.
* Orchestration Period Length (OPL): Represents the length of the
orchestration period, in units of us. The orchestration period
contains N timeslots, numbered sequentially from 0 to N-1. That
is, OPL = N*TL.
* Scheduling Period Length (SPL): Represents the length of the
scheduling period, in units of us. The scheduling period contains
M timeslots, numbered sequentially from 0 to M-1. That is, SPL =
M*TL..
Figure 4 shows the timeslot resource model of the link, with an
orchestration period instance consisting of N timeslots numbered from
0 to N-1. The resource information of each timeslot includes the
following attributes:
* Timeslot ID: Indicates the NO. of the timeslot in the
orchestration period instance. The NO. of the first timeslot is
0, and the NO. of the last timeslot is N-1.
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* 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 service rate
(termed as C) and the length of the timeslot (termed as TL), and
the Maximum Reservable Bursts should be set to a value not
exceeding C*TL. 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 unreserved bits
reservable corresponding to this timeslot, with unit of bits.
#N-1 +-------------------------------------+
| Timeslot Length: TL(n-1) |
| Maximum Reservable Bursts: MRB(n-1) |
| Unreserved Bursts: UB(n-1) |
+-------------------------------------+
... ... ...
... ... ...
#1 +-------------------------------------+
| Timeslot Length: TL(1) |
| Maximum Reservable Bursts: MRB(1) |
| Unreserved Bursts: UB(1) |
+-------------------------------------+
#0 +-------------------------------------+
| Timeslot Length: TL(0) |
| Maximum Reservable Bursts: MRB(0) |
| Unreserved Bursts: UB(0) |
+-------------------------------------+
----------------------------------------------------------->
Timeslot Resources of an OP Instance of the Link
Figure 4: Timeslot Resources Model
The IGP/BGP extensions to advertise the link's capability and
timeslot resource is defined in
[I-D.peng-lsr-deterministic-traffic-engineering].
3.1.3. Arrival Postion in the Orchestration Period
Generally, a DetNet 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
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one or more shaped sub-burst in the service burst interval. There is
an ideal positional relationship between the departure time (when
each sub-burst leaves the regulator) and the orchestration period of
UNI port, that is, each sub-burst corresponds to an ideal incoming
timeslot of UNI port. Based on the ideal incoming timeslot, an ideal
outgoing timeslot of NNI port is reserved for the sub-burst.
For example, if a DetNet flow distributes m sub-bursts during the
orchestration period, the network entry should maintain m states for
that flow:
* <OPL, ideal incoming slot i_1, ideal outgoing slot z_1>
* <OPL, ideal incoming slot i_2, ideal outgoing slot z_2>
* ... ...
* <OPL, ideal incoming slot i_m, ideal outgoing slot z_m>
However, the packets arrived at the network entry are not always
ideal, and the departure time from regulator may not be in a certain
ideal incoming timeslot. Therefore, an important operation that
needs to be performed by the network entry is to determine the ideal
incoming timeslot i based on the actual departure time. This can
first determine the actual incoming timeslot based on the actual
departure time, and then select an ideal incoming timeslot that is
closest to the actual incoming timeslot and not earlier than the
actual incoming timeslot.
Figure 5 shows, for some typical DetNet flows, the relationship
between the service burst interval (SBI) and the length of
orchestration period (OPL) of headend, as well as the possible
timeslot resource reservation results for these DetNet flows.
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|<--------------------- OPL ---------------------->|
+----+----+----+----+----+----+----+----------+----+
| #0 | #1 | #2 | #3 | #4 | #5 | #6 | ... ... |#N-1|
+----+----+----+----+----+----+----+----------+----+
+--+
Flow 1: | |b1| |
+-----+--+-----------------------------------------+
|<------------------- SBI ------------------------>|
+--+ +--+
Flow 2: | |b1| |b2|
+------------+--+------------------------+--+------+
|<------------------- SBI ------------------------>|
+------+
Flow 3: | | b1 |
+---------------------------+------+---------------+
|<------------------- SBI ------------------------>|
+--+ +--+ +--+
Flow 4: | |b1| | |b1| | |b1| |
+----+--+--------+----+--+--------+----+--+--------+
|<----- SBI ---->|<----- SBI ---->|<----- SBI ---->|
Figure 5: Relationship between SBI and OP
As shown in the figure, the length of service burst intervals for
flows 1, 2, 3 is equal to the length of orchestration period, while
the length of the service burst interval for flow 4 is only 1/3 of
the orchestration period.
* Flow 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;
* Flow 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;
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* Flow 3 generates a large single burst amount within its burst
interval but not be really shaped (due to purchasing a larger
burst resource and served by a larger bucket depth), which may
also be split to multiple back-to-back sub-bursts and reserve
multiple timeslots in the orchestration period, such as timeslots
8 and 9.
* The length of the service burst interval for flow 4 is only 1/3 of
the orchestration period. Hence, construct flow 4' with 3
occurrence of the flow 4 within an orchestration period. So flow
4' is similar to flow 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.
Each sub-burst corresponds to a reservation sub-task. For
simplicity, each regulated sub-burst in the service burst interval
always reserves timeslot resources according to the maximum sub-bust
size.
For a specific DetNet 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, multiply it several times to obtain the
expanded service burst interval to get a new flow'.
* 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).
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* Total Residence Budget: It is the sum of the residence delay
allowed by the DetNet flow within all nodes in the path, which is
equal to the end-to-end delay requirement of the DetNet flow minus
the propagation delay of all links included in the path.
* Node Residence Budget: It refers to the residence 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 residence
delay budget as the Node Residence Budget for each node, or use a
specified budget list to specify the residence delay budget for
each node separately.
* Accumulated Node Residence Budget: It refers to the accumulated
residence delay budget of those nodes that have executed resource
reservation.
* Accumulated Node Residence Evaluation: It refers to the
accumulated evaluation value of the residence delay of nodes that
have executed 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.
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* 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.
- 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, each sub-burst corresponds to
an ideal incoming timeslot i of UNI port. After the intra-node
forwarding delay (F), the end of the incoming timeslot i reaches the
outgoing port, the timeslot currently in the sending state (i.e., the
ongoing sending timeslot of NNI port) is j, and there is time T_ij
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 z = (j+o)%N_h2, where N_h2 is the number of
timeslots in the orchestration period of NNI port.
Note that o must be less than M. (where o is the offset and M is the
number of timeslot in the scheduling period as mentioned in
Section 3.1.2)
Thus, on the headend H the residence delay evaluation value obtained
from the reserved outgoing timeslot z is:
Best Node Residence Evaluation = F + T_ij + (o-1)*L_h2
Worst Node Residence Evaluation = F + L_h1 + T_ij + o*L_h2
Average Node Residence Evaluation = F + T_ij + (L_h1 + (2o-
1)*L_h2)/2
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where, L_h1 is the timeslot length of UNI port, L_h2 is the
timeslot length of NNI port.
The Best Node Residence Evaluation occurs when the sub-burst is at
the end of the ideal incoming timeslot i, and sent at the head of
outgoing timeslot z. The Worst Node Residence Evaluation occurs when
the sub-burst is at the head of the ideal incoming timeslot i, and
sent at the end of outgoing timeslot z. The delay jitter within the
headend is (L_h1 + L_h2). 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 z, the two are equal or nearly equal, and the corresponding
Unreserved Burst resources of the outgoing timeslot z 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 TQF is that adjacent nodes on the path will not
simultaneously face the best or worst residency delay.
Note that there is a runtime jitter (i.e., the resource reservation
process on the control plane is not aware of it), as mentioned
earlier, which depends on the deviation between the actual incoming
timeslot i' and the ideal incoming timeslot i. Assuming that i =
(i'+e)%N_h1, where e is the deviation, N_h1 is the number of
timeslots in the orchestration period of UNI port, then the
additional runtime jitter is e*L_h1, that should be carried in the
packet to eliminate jitter at the network egress.
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 i of
port_u2 and the ongoing sending timeslot j of port_v2, and there is
time T_ij left before the end of the timeslot j.
For a specific sub-task, assume that outgoing timeslot i is reserved
for it on port_u2, and the outgoing timeslot z reserved for it on
port_v2 is offset by o (>=1) timeslots after timeslot j, i.e., z =
(j+o)%N_v2, where N_v2 is the number of timeslots in the
orchestration period of port_v2.
Note that o must be less than M.
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Thus, on the transit node V the residence delay evaluation value
obtained from the reserved outgoing timeslot z is:
Best Node Residence Evaluation = F + T_ij + (o-1)*L_v2
Worst Node Residence Evaluation = F + T_ij + L_u2 + o*L_v2
Average Node Residence Evaluation = F + T_ij + (L_u2+(2o-
1)*L_v2)/2
where, L_u2 and L_v2 is the timeslot length of port_u2 and port_v2
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
timeslot z; The Worst Node Residence Evaluation occurs when the
packet is received at the head of incoming timeslot i and sent at the
end of outgoing timeslot z. The delay jitter within the node is
(L_u2 + L_v2). 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 z, the two are equal or nearly equal, and the corresponding
Unreserved Burst resources of the outgoing timeslot z 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 TQF 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 DetNet 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
queueing. However, the outgoing port facing the client side is not
part of the deterministic path. If it is necessary to continue
supporting TQF mechanism on that port, timeslot resources should be
reserved on the higher-level DetNet path (an overlay path) using the
above reservation method. In this case, the underlay DetNet path
will serve as a virtual link of the overlay path, providing a
deterministic delay performance.
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Therefore, for deterministic paths, the residence dalay evaluation
value on the egress node is only contributed by the forwarding delay
(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 timeslot length of the outgoing port 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
o_i, especially on node P1 the timeslot length of UNI is L_h, then
the end to end delay can be evaluted as follows (not including link
propagation delay):
Best E2E Delay = sum(F_i+T_i+o_i*L_i, for 1<=i<=n) - L_n + F_e
Worst E2E Delay = sum(F_i+T_i+o_i*L_i, for 1<=i<=n) + L_h + F_e
+---+ +---+ +---+ +---+ +---+
| P1| --- | P2| --- | P3| --- ... --- | Pn| --- | E |
+---+ +---+ +---+ +---+ +---+
Figure 6: TQF Forwarding Path
The Best E2E Delay occurs when the sub-burst is at the end of the
ideal incoming timeslot and sent at the head of outgoing timeslot of
each node pi. The Worst E2E Delay occurs when the sub-burst is at
the head of the ideal incoming timeslot and sent at the end of
outgoing timeslot of each node Pi. The E2E delay jitter is (L_h +
L_n).
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 DetNet 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 id carried in the packets or indicated by FIB
entries.
Note that the incoming and outgoing timeslots mentioned here are both
timeslot id within the orchestration period.
[I-D.pb-6man-deterministic-crh] defined a method to carry the stack
of timeslot id in the IPv6 packets.
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By default, the following subsections discuss on-time scheduling
behavior.
3.2.1. Round Robin Queue: Conversion of Timeslot ID
Figure 1 shows that the scheduling period implemented on the data
plane is not completely equivalent to the orchestration period of the
control plane. The scheduling period includes M timeslots (from 0 to
M-1), while the orchestration period includes N timeslots (from 0 to
N-1). 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. 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 round
robin queue corresponding to the target timeslot for transmission.
A simple conversion method is:
* target scheduling timeslot = outgoing timeslot % M
This is safe because during resource reservation, o < M is always
followed, and N is an integer multiple of M.
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 conform to 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:
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.
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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.
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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
(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.
3.2.2. PIFO: Directly Using Outgoing Timeslots
Figure 1 also shows that the scheduling period may also be
instantiated by a PIFO queue. The buffer cost of PIFO queue is the
same as that of round robin queues. It can directly use the begin
time of the outgoing timeslot z as the rank of the packet and insert
the packet into the PIFO for transmission.
* rank = z.begin
Here, the outgoing timeslot z refers to the outgoing timeslot z that
is after the arrival time at the scheduler and closest to the arrival
time.
The rule of the on-time scheduling mode is that if the PIFO is not
empty and the rank of the head of queue is equal to or earlier than
the current system time, the head of queue will be sent; otherwise,
not.
4. Global Timeslot ID
The outgoing timeslots we discussed in the previous sections are
local timeslots style for all nodes. This section discusses the
situation based on global timeslot style.
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 DetNet flow. There
is no need to establish FTM on each node or carry FTM 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 BOM between the adjacent nodes.
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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 without overflow. However, as the ideal design
goal is to keep the scheduling period less than the orchestration
period, 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.
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
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.
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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: Global Timeslot Style Example
In this example:
* BTM between the outgoing timeslot of U1 and the ongoing sending
timeslot of V is i -> i, so the reserved outgoing timeslot for the
incoming timeslot i is i+6 (i.e., belongs to next round of
orchestration periold).
* BTM between the outgoing timeslot of U2 and the ongoing sending
timeslot of V is i -> i-1, so the reserved outgoing timeslot for
the incoming timeslot i is i (i.e., belongs to current round of
orchestration periold).
* BTM between 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 (i.e., belongs to next round of
orchestration periold).
It can be seen that packets from U1 and U3 has large residency delay
in the node V, while packets from U2 has small residency delay in the
node V.
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It should be noted that for the orginal mapping relationship i -> i
or i -> i+1, the packets need to be stored in a buffer prior to the
TQF scheduler (such as the buffer on the input port side) for a fixed
latency (such as serveral timeslots) and then released to the
scheduler. Instead, directly inserting the queue may cause queue
overflow. This fixed-latency buffer is only introduced for specific
upstream nodes. It can be determined according to the initial
detection result of BTM between the outgoing timeslot of the upstream
node and the ongoing sending timeslot of this node. If the original
detection result is i -> i or i -> i+1, it needs to be introduced,
otherwise not. After the introduction of fixed-latency buffer, the
new detection result of BTM will no longer be i -> i or i -> i+1.
For the headend, the residence delay is similar to Section 3.1.4.1,
except that determining the offset o is simpler. Suppose that for
the ideal incoming timeslot i (note that at the headend this incoming
timeslot i is not the reserved timeslot resource), the ongoing
sending timeslot of the ougtoing port is j, and the sub-burst reserve
global timeslot z, then, o equals (N+z-j)%N.
For transit nodes, the residence delay is similar to Section 3.1.4.2,
except that determining the offset o is simpler. Suppose that for
the incoming timeslot z, the ongoing sending timeslot of the ougtoing
port is j, and the sub-burst continues to reserve global timeslot z,
then, o equals (N+z-j)%N.
The end-to-end delay equation is similar to Section 3.1.4.4.
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.
+===============+========================+==================+
| 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 | ECQF | [ECQF] |
| (o=1) | | |
+---------------+------------------------+------------------+
Figure 8: Timeslot Styles
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A local policy may be configured in the path computation engine to
use which strategy on different nodes, as long as the calculated E2E
delay meet the flow's requirement. For example, it is possible to
use strategy "constant o=1" on all transit nodes and use strategy
"flexible o" on ingress node.
Based on the same topology, the success rate of path calculation for
strategy "flexible o" applied on transit nodes is higher than that
for strategy "constant o" applied.
6. In-time Scheduling
So far, in the TQF mechanism presented above, both for local timeslot
style and global timeslot style, the goal is to reserve a fixed
outgoing timeslot for the sub-burst in the orchestration period, and
just send the sub-burst in that timeslot. This is on-time
scheduling.
In this section, we discuss another scheduling variant of TQF, i.e.,
in-time scheduling. In this case, timeslot resources are still
reserved based on delay requirement, but in actual forwarding,
packets do not necessarily have to wait until the reserved outgoing
timeslot for sending.
As is known, in-time scheduling may cause burst accumulation, so that
scheduling period implemented with limited amount of round robin
queues is not suitable for this purpose, while PIFO with excess
length is more suitable. [SP-LATENCY] provides guidance for
evaluating excess buffer requirements.
Similar to Section 3.2.2, it can directly use the begin time of the
outgoing timeslot z as the rank of the packet and insert the packet
into the PIFO for transmission. However, due to in-time scheduling
behavior, the outgoing timeslot z may not be the outgoing timeslot z
that is after the arrival time at the scheduler and closest to the
arrival time, instead, it may be an outgoing timeslot z far away from
the arrival time.
A time deviation (E) may be carried in the packet to help determine
the outgoing timeslot z.
On the headend node:
* E initially equals to the begin time of the ideal incoming
timeslot minus the actual departure time from the regulator.
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* Use the result of "departure time + E" (note that it is just the
begin time of the ideal incoming timeslot, and the main purpose
here is to describe how E works) to determine the expected
outgoing timeslot z that is after this result and closest to this
result.
* rank = z.begin
* When the packet leaves the headend, E is updated to z.begin minus
the actual sending time from the PIFO. The updated E will be
carried in the sending packet.
On the transit node:
* Obtain E from the received packet.
* Use the result of "arrival time + E" to determine the expected
outgoing timeslot z that is after this result and closest to this
result. Here, the arrival time is the time that the packet
arrived at the scheduler.
* rank = z.begin
* When the packet leaves the headend, E is updated to z.begin minus
the actual sending time from the PIFO. The updated E will be
carried in the sending packet.
The rule of the in-time scheduling mode is that as long as the PIFO
is not empty, packets are always obtained from the head of queue for
transmission.
In summary, the in-time scheduling with the help of time deviation
(E), can suffer from the uncertainty caused by burst accumulation,
and it is recommended only deployed in small networks, i.e., a
limited domain with a small number of hops, where the burst
accumulation issue is not serious; The on-time scheduling is
recommended to be used in large networks.
7. Queue Design
7.1. Round Robin Queues
Round robin queues operate in on-time scheduling mode by default.
The number of round robin queues should be designed according to the
number of timeslots included in the scheduling period. Each timeslot
corresponds to a separate queue, in which the buffered packets must
be able to be sent within a timeslot.
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The length of the queue, i.e., the total number of bits that can be
reserved or sent for a timeslot, equals to the allocated bandwidth of
the corresponding OP instance (see Section 8) multiplied by the
timeslot length.
7.1.1. Full Queues
Case: 1-to-1 mapping between the orchestration period timeslot and
the scheduling period timeslot.
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 (e.g, the global timeslot style). 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.
For example, if the total 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-DetNet flows, the TQF
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 may be 100~200 ms.
7.1.2. Non-full Queues
Case: Many-to-1 mapping between the orchestration period timeslot and
the scheduling period timeslot.
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.
7.2. PIFO Queue
PIFO can be configured to operate in either in-time or on-time
scheduling mode.
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For on-time mode, the buffer cost is the same as that of round robin
queues. The rank of the packet equals to the begin time of the
outgoing timeslot z, that can be safely obtained a "z" closet to the
arrival time at the scheduler.
For in-time mode, excess buffers are required to cope with burst
accumulation. [SP-LATENCY] provides guidance for evaluating excess
buffer requirements. The rank of the packet equals to the begin time
of the expected outgoing timeslot z, that can be obtained based on a
"z" closet to the result of arrival time at the scheduler plus time
deviation E.
8. Multiple Orchestration Periods
A single orchestration period may not be able to cover a wide range
of service needs, such as some with a burst interval of microseconds,
while others have a burst interval of minutes or even larger. When
using a single orchestration period to simultaneously serve these
services, the timeslot length must be microseconds, but the
orchestration period length is minutes or more, resulting in the need
to include a large number of timeslots in the orchestration period.
The final result is a proportional increase in the number of queues
required for the scheduling period (to avoid the potential timeslot
conflicts).
Multiple orchestration periods each with different length may be
provided by the network. A TQF enabled link can be configured with
multiple TQF scheduling instances each corresponding to specific
orchestration period length. For simplicity, the orchestration
period length itself can be used to identify a specific instance.
For example, one orchestration period length is 300 us, termed as
OPL-300us, which is the LCM of the burst interval of the set of flows
served. Another orchestration period length is 100 ms, termed as
OPL-100ms, which is the LCM of the burst interval of another set of
flows served. Each orchestration period instance has its own
timeslot length. The timeslot length of a long orchestration period
instance should be longer than that of a short orchestration period
instance, and the former is an integer multiple of the latter. But
the long orchestration period itself may not necessarily be an
integer multiple of the short orchestration period.
As shown in Figure 9, both link-a and link-b are configured with n
orchestration period instances, with the corresponding orchestration
period lengths OPL_1, OPL_2, ..., OPL_n in descending order. For
each orchestration period length OPL_i, the bandwidth resource
allocated is BW_U_i for node U (or BW_V_i for node V), and the
timeslot length is TL_U_i for node U (or TL_V_i for node V). For
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each TQF enabled link, the sum of bandwidth resources allocated to
all orchestration period instances must not exceed the total
bandwidth of the link.
+---+ link-a +---+ link-b +---+
| U | -------------------- | V | -------------------- | W |
+---+ +---+ +---+
OPL_1: OPL_1:
TL_U_1 TL_V_1
BW_U_1 BW_V_1
OPL_2: OPL_2:
TL_U_2 TL_V_2
BW_U_2 BW_V_2
... ... ... ...
OPL_n: OPL_n:
TL_U_n TL_V_n
BW_U_n BW_V_n
Figure 9: Multiple OP Instances
Due to the fact that long orchestration periods serve DetNet flows
with large burst intervals, for a given burst size, the larger the
burst interval, the less bandwidth consumed by the DetNet flow.
Therefore, it is recommended that the bandwidth resources allocated
to long orchestration period instances are less than those allocated
to short orchestration period instances, which is also beneficial for
reducing the queue length required for long orchestration period
instances.
Interworking between different nodes is based on the same
orchestration period instance. That means that the timeslot mapping
described in Section 3.1.1 should be maintained in the context of the
specific orchestration period instance, and the timeslot resource
reservation along the path for a sub-task should also be in the
context of the specific orchestration period instance. The
orchestration period length should be carried in the forwarding
packets to let the DetNet flow to access the timeslot resources
corresponding to that orchestration period instance.
If round robin queues are used, each orchestration period instance
has its own separate queue set. Time division multiplexing
scheduling is based on the granularity of the minimum timeslot length
of all instances. Within each time unit of this granularity, the
queues in the sending state of all instances are always scheduled in
the order of OPL_1, OPL_2, ..., OPL_n.
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If PIFO queue is used, all orchestration period instances may share a
single PIFO queue.
9. Admission Control on the Headend
On the network entry, traffic regulation must be performed on the
incoming port, so that the DetNet 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 DetNet flow. A DetNet 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 DetNet flows on the network entry. It is
expected that the distribution of sub-bursts (after regulation) of
the DetNet flow will always appear in an ideal position within the
orchestration period of UNI port. Based on this ideal position, any
packets of the DetNet flow will be matched to the sub-burst
forwarding state that contains the ideal incoming timeslot and
corresponding reserved outgoing timeslot. Note that the network
entry may maintain multiple sub-burst forwarding states for a single
DetNet flow, due to many sub-bursts within the service burst
interval.
For example, the network entry may maintain up to 3 sub-burst
forwarding states for a flow. Ideally, all packets of this flow are
split into 3 sub-bursts after regulation, each sub-burst matching one
of the states. Here, 3 is the maximum sub-bursts for this flow, and
it does not always contain so many bursts within the burst interval
during actual sending.
For a specific sub-burst, some amount of deviation (i.e., the
deviation between the actual incoming timeslot and the ideal incoming
timeslot) is permitted. Generally, the headend will select an ideal
incoming timeslot closet to the actual incoming timeslot for the
packet.
For on-time scheduling, the position deviation should not exceed o-1
for late arrival case, or M-o-1 for early arrival case, where o is
the offset between the reserved outgoing timeslot and ongoing sending
timeslot as mentioned above. Intuitively, large o can tolerate large
late arrival deviations, while small o (or large M even for large o)
can tolerate large early arrival deviations.
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This position deviation limitation is beneficial for on-time
scheduling, to achieve the ideal design goal that scheduling period
is smaller than the orchestration period, and packets can always be
successfully inserted into the scheduling queue without conflicts.
For example, there may contain one or more scheduling periods between
the departure time from the regulator and the choosed ideal incoming
timeslot, and therefore there is an overflow risk when inserting
packets into the queue based on the corresponding ideal outgoing
timeslot z at the departue time.
Otherwise, for randomly arriving DetNet flows, it can be supported by
taking a large M (or even M = N) (option-1) to accommodate random
arrival, 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).
* 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 orchestration
period.
For in-time scheduling, the position deviation should not exceed o-1
for late arrival case. We only focus on late arrivals here, as in-
time scheduling naturally handles early arrivals. If the late
arrival exceed the above limitation, the sub-burst may need to be
sent during the next orchestration period in the worst case, or may
be lucky to be scheduled immediately.
Note that the position deviation is a runtime latency during
forwarding, and the resource reservation process on the control plane
is not aware of it. It should be carried in the packet to eliminate
jitter at the network egress on demand. Please refer to
[I-D.peng-detnet-policing-jitter-control] for the elimination of
jitter caused by policing delay on the network entry node. The
runtime position deviation should be considered as a part of policing
delay.
10. Frequency Synchronization
The basic explanation for frequency synchronization is that the
crystal frequency of the hardware is consistent, 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 IEEE 1588-2008 Precision Time Protocol (PTP) [IEEE-1588] and
synchronous Ethernet (syncE) [syncE], is not within the scope of this
document.
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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.
11. Evaluations
This section gives the evaluation results of the TQF mechanism based
on the requirements that is defined in
[I-D.ietf-detnet-scaling-requirements].
+======================+============+===============================+
| Requirements | Evaluation | Notes |
+======================+============+===============================+
| 3.1 Tolerate Time | Yes | No time sync needed, only need|
| Asynchrony | | frequency sync (3.1.3). |
+----------------------+------------+-------------------------------+
| 3.2 Support Large | | The timeslot mapping covers |
| Single-hop | Yes | any value of link propagation |
| Propagation | | delay. |
| Latency | | |
+----------------------+------------+-------------------------------+
| 3.3 Accommodate the | | The higher the service rate, |
| Higher Link | Partial | the more buffer needed for the|
| Speed | | same timeslot length. |
+----------------------+------------+-------------------------------+
| 3.4 Be Scalable to | | Multiple OPL instance, each |
| the Large Number | | for a set of serivce flows, |
| of Flows and | | without overprovision. |
| Tolerate High | | Utilization may reach 100% |
| Utilization | Yes | link bandwidth. |
| | | The unused bandwidth of the |
| | | timeslot can be used by |
| | | best-effot flows. |
| | | Calculating paths is NP-hard. |
+----------------------+------------+-------------------------------+
| 3.5 Tolerate Failures| | Independent of queueing |
| of Links or Nodes| N/A | mechanism. |
| and Topology | | |
| Changes | | |
+----------------------+------------+-------------------------------+
| 3.6 Prevent Flow | | Flows are permitted based on |
| Fluctuation | Yes | timeslot reservation, isolated|
| | | from each other through |
| | | timeslots. |
+----------------------+------------+-------------------------------+
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| 3.7 Be scalable to a | | E2E latency is liner with hops|
| Large Number of | | , from ultra-low to low |
| Hops with Complex| Yes | latency by multiple OPL. |
| Topology | | E2E jitter is low by on-time |
| | | mode. |
| | | Calculating paths is NP-hard. |
+----------------------+------------+-------------------------------+
| 3.8 Support Multi- | | Independent of queueing |
| Mechanisms in | N/A | mechanism. |
| Single Domain and| | |
| Multi-Domains | | |
+----------------------+------------+-------------------------------+
Figure 10: Evaluation for Large Scaling Requirements
11.1. Examples
This section will describe the example of how the TQF mechanism
supports DetNet flows with different latency requirements. As shown
in Figure 11:
* Network transmission capacity: each link has rate 10 Gbps.
Assuming the service rate of TQF scheduler allocate the total port
bandwidth.
* TSpec of each flow, maybe:
- burst size 1000 bits, SBI 1 ms, and average arrival rate 1
Mbps.
- or, burst size 1000 bits, SBI 100 us, and average arrival rate
10 Mbps.
- or, burst size 1000 bits, SBI 100 us, and average arrival rate
100 Mbps.
- or, burst size 10000 bits, SBI 10 ms, and average arrival rate
1 Mbps.
- or, burst size 10000 bits, SBI 1 ms, and average arrival rate
10 Mbps.
- or, burst size 10000 bits, SBI 100 us, and average arrival rate
100 Mbps.
* RSpec of each flow, maybe:
- E2E latency 100us, and E2E jitter less than 10us or 100us.
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- or, E2E latency 200us, and E2E jitter less than 20us or 200us.
- or, E2E latency 300us, and E2E jitter less than 30us or 300us.
- or, E2E latency 400us, and E2E jitter less than 40us or 400us.
- or, E2E latency 500us, and E2E jitter less than 50us or 500us.
- or, E2E latency 600us, and E2E jitter less than 60us or 600us.
- or, E2E latency 700us, and E2E jitter less than 70us or 700us.
- or, E2E latency 800us, and E2E jitter less than 80us or 800us.
- or, E2E latency 900us, and E2E jitter less than 90us or 900us.
- or, E2E latency 1000us, and E2E jitter less than 100us or 1ms.
@ # $
v v v
+---+ @@@ +---+ ### +---+ $$$ &&& +---+
src ---> | 0 | --- | 1 | --- | 2 | --- ... ... --- | 9 | ---> dest
(flow i:*)+---+ *** +---+ *** +---+ *** *** +---+ ***
| |@ |# |&
| |v |v |v
+---+ +---+ +---+ +---+
--- | | --- | | --- | | --- ... ... --- | | ---
+---+ +---+ +---+ +---+
| | | |
| | | |
... ... ... ... ... ... ... ...
Figure 11: Common Topology Example
For the observed flow i (marked with *), its TSpec and RSpec may be
any of the above. Assuming that the path calculated by the
controller for the flow i passes through 10 nodes (i.e., node 0~9).
Especially, at each hop, flow i may conflict with other deterministic
flows, also with similar TSpec and RSpec as above, originated from
other sources, e.g, conflicts with flow-set "@" at node 0, conflicts
with flow-set "#" at node 1, etc.
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For each link along the path, it may configure OPL-10ms instance with
allocated bandwidth 10 Gbps, containing 1000 timeslots each with
length 10us. Assuming no link propagation delay and intra node
forwarding delay, if flow i reserve outgoing timeslot by o=1, it can
ensure an E2E latency of 100us (i.e., o * TL * 10 hops), and jitter
of 20us(on-time mode) or 100us (in-time mode). The reservation by
other o values is similar.
The table below shows the possible supported service scales. As
flows arrived synchronously, the reservation for each timeslot in the
orchestration period may be caused by any value of o. For example,
if the ideal incoming timeslots of all flows are perfectly
interleaved, then they can all reserve timeslots by o=1 to get per-
hop latency 10us, or all reserve timeslots by o=2 to get per-hop
latency 20us, etc. However, due to the fixed length of OPL, after
all timeslot resources are exhausted by specific o value, it means
that there are no timeslot resources to be reserved by other o
values. Another example is that the ideal incoming timeslots of all
flows are the same, then some of them reserve timeslots by o=1, some
reserve timeslots by o=2, and so on. In either case, the total
service scale is OPL * C / burst_size, that is composed of sum(s_i),
where s_i is the service scale for o=i. The table provides the total
scale and the average scale corresponding to each o value.
Note that in the table each column only shows the data where all
flows served based on all o values have the same TSpec (e.g, in the
first colunm, TSpec per flow is burst size 1000 bits and arrival rate
1 Mbps), while in reality, flows served based on different o values
generally have different TSpec. It is easy to add colunms to
describe various combinations.
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===================================================
| o=1| o=2| o=3| o=4| o=5| o=6| o=7| o=8| o=9|o=10|
===================================================================
|TSpec: | total = 10000 |
| 1000 bits |----+----+----+----+----+----+----+----+----+----|
| SBI 1 ms |1000|1000|1000|1000|1000|1000|1000|1000|1000|1000|
===================================================================
|TSpec: | total = 1000 |
| 1000 bits |----+----+----+----+----+----+----+----+----+----|
| SBI 100 us | 100| 100| 100| 100| 100| 100| 100| 100| 100| 100|
===================================================================
|TSpec: | total = 100 |
| 1000 bits |----+----+----+----+----+----+----+----+----+----|
| SBI 10 us | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 |
===================================================================
|TSpec: | total = 10000 |
| 10000 bits |----+----+----+----+----+----+----+----+----+----|
| SBI 10 ms |1000|1000|1000|1000|1000|1000|1000|1000|1000|1000|
===================================================================
|TSpec: | total = 1000 |
| 10000 bits |----+----+----+----+----+----+----+----+----+----|
| SBI 1 ms | 100| 100| 100| 100| 100| 100| 100| 100| 100| 100|
===================================================================
|TSpec: | total = 100 |
| 10000 bits |----+----+----+----+----+----+----+----+----+----|
| SBI 100 us | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 |
===================================================================
Figure 12: Timeslot Reservation and Service Scale Example
12. Taxonomy Considerations
[I-D.joung-detnet-taxonomy-dataplane] provides criteria for
classifying data plane solutions. TQF is a periodic, frequency
synchronous, class level, work-conserving/non-work-conserving
configurable, in-time/on-time configurable, time based solution.
* Periodic: Periodicity of TQF contains two characteristics, the
first is that there is a time period P (i.e., orchestration
periold) containing multiple time slots, and the second is that a
flow is assigned repeatly to a particular set of time slots in the
period.
* Frequency synchronous: TQF requires frequency synchronization
(i.e., crystal frequency of the hardware) so that all nodes in the
network have the same time lapse rate. TQF does not require
different nodes to use the same timeslot length.
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* Class level: DetNet Flows may be grouped by similar service
requirements, i.e., timeslot id(s), on the network entrance.
Packets will be provided TQF service based on timeslot id(s),
without checking flow characteristic.
* Work-conserving/non-work-conserving configurable: The TQF
scheduler configured with in-time scheduling mode is work-
conserving (i.e., to send the packet as soon as possible before
its outgoing timeslot), while the TQF scheduler configured with
on-time scheduling mode is non work-conserving (i.e., to ensure
that the packet can always be sent within its outgoing timeslot).
* In-time/on-time configurable: The TQF scheduler configured with
in-time scheduling mode is in-time to get bounded end-to-end
latency, while the TQF scheduler configured with on-time
scheduling mode is on-time to get bounded end-to-end delay jitter.
* Time based: A DetNet flow is scheduled based on its expected
outgoing timeslot(s). All DetNet flows are interleaved and
arranged in different timeslots to obtain the maximum number of
admission flows.
In addition, the per hop latency dominant factor of TQF is the offset
between incoming timeslot and outgoing timeslot that is reserved to
the flow.
13. IANA Considerations
TBD.
14. Security Considerations
Security considerations for DetNet are described in detail in
[RFC9055]. General security considerations for the DetNet
architecture are described in [RFC8655]. Considerations specific to
the DetNet data plane are summarized in [RFC8938].
Adequate admission control policies should be configured in the edge
of the DetNet domain to control access to specific timeslot
resources. Access to classification and mapping tables must be
controlled to prevent misbehaviors, e.g, an unauthorized entity may
modify the table to map traffic to an unallowed timeslot resource,
and competes and interferes with normal traffic.
15. Acknowledgements
TBD.
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16. References
16.1. Normative References
[I-D.chen-detnet-sr-based-bounded-latency]
Chen, M., Geng, X., Li, Z., Joung, J., and J. Ryoo,
"Segment Routing (SR) Based Bounded Latency", Work in
Progress, Internet-Draft, draft-chen-detnet-sr-based-
bounded-latency-03, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-chen-detnet-
sr-based-bounded-latency-03>.
[I-D.eckert-detnet-tcqf]
Eckert, T. T., Li, Y., Bryant, S., Malis, A. G., Ryoo, J.,
Liu, P., Li, G., Ren, S., and F. Yang, "Deterministic
Networking (DetNet) Data Plane - Tagged Cyclic Queuing and
Forwarding (TCQF) for bounded latency with low jitter in
large scale DetNets", Work in Progress, Internet-Draft,
draft-eckert-detnet-tcqf-05, 5 January 2024,
<https://datatracker.ietf.org/doc/html/draft-eckert-
detnet-tcqf-05>.
[I-D.ietf-detnet-scaling-requirements]
Liu, P., Li, Y., Eckert, T. T., Xiong, Q., Ryoo, J.,
zhushiyin, and X. Geng, "Requirements for Scaling
Deterministic Networks", Work in Progress, Internet-Draft,
draft-ietf-detnet-scaling-requirements-05, 20 November
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
detnet-scaling-requirements-05>.
[I-D.joung-detnet-taxonomy-dataplane]
Joung, J., Geng, X., Peng, S., and T. T. Eckert,
"Dataplane Enhancement Taxonomy", Work in Progress,
Internet-Draft, draft-joung-detnet-taxonomy-dataplane-01,
25 February 2024, <https://datatracker.ietf.org/doc/html/
draft-joung-detnet-taxonomy-dataplane-01>.
[I-D.pb-6man-deterministic-crh]
Peng, S. and R. Bonica, "Deterministic Routing Header",
Work in Progress, Internet-Draft, draft-pb-6man-
deterministic-crh-00, 1 March 2024,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
pb-6man-deterministic-crh/>.
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[I-D.peng-detnet-policing-jitter-control]
Peng, S., Liu, P., and K. Basu, "Policing Caused Jitter
Control Mechanism", Work in Progress, Internet-Draft,
draft-peng-detnet-policing-jitter-control-00, 18 January
2024, <https://datatracker.ietf.org/doc/html/draft-peng-
detnet-policing-jitter-control-00>.
[I-D.peng-lsr-deterministic-traffic-engineering]
Peng, S., "IGP Extensions for Deterministic Traffic
Engineering", Work in Progress, Internet-Draft, draft-
peng-lsr-deterministic-traffic-engineering-01, 4 July
2023, <https://datatracker.ietf.org/doc/html/draft-peng-
lsr-deterministic-traffic-engineering-01>.
[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>.
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
[RFC8938] Varga, B., Ed., Farkas, J., Berger, L., Malis, A., and S.
Bryant, "Deterministic Networking (DetNet) Data Plane
Framework", RFC 8938, DOI 10.17487/RFC8938, November 2020,
<https://www.rfc-editor.org/info/rfc8938>.
[RFC9055] Grossman, E., Ed., Mizrahi, T., and A. Hacker,
"Deterministic Networking (DetNet) Security
Considerations", RFC 9055, DOI 10.17487/RFC9055, June
2021, <https://www.rfc-editor.org/info/rfc9055>.
16.2. Informative References
[ATM-LATENCY]
"Bounded Latency Scheduling Scheme for ATM Cells", 1999,
<https://ieeexplore.ieee.org/document/780828/>.
[CQF] "Cyclic queueing and Forwarding", 2017,
<https://ieeexplore.ieee.org/document/7961303>.
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[ECQF] "Enhancements to Cyclic Queuing and Forwarding", 2023,
<https://1.ieee802.org/tsn/802-1qdv/>.
[IEEE-1588]
"IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
2008, <https://standards.ieee.org/findstds/
standard/1588-2008.html>.
[SP-LATENCY]
"Guaranteed Latency with SP", 2020,
<https://ieeexplore.ieee.org/document/9249224>.
[syncE] "Timing and synchronization aspects in packet networks",
2013, <https://www.itu.int/rec/T-REC-G.8261>.
[TAS] "Time-Aware Shaper", 2015,
<https://standards.ieee.org/ieee/802.1Qbv/6068/>.
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
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Email: dyang@bjtu.edu.cn
Guoyu Peng
Beijing University of Posts and Telecommunications
China
Email: guoyupeng@bupt.edu.cn
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