DetNet D. Guo
Internet Draft S. Xu
Intended status: Informational New H3C Technologies Co., Ltd
Expires: December 9, 2023 June 9, 2023
Jitter Reduction Mechanism for DetNet
draft-guo-detnet-jitter-reduction-mechanism-00
Abstract
In large-scale deterministic networks (LDN), App-flows need to span
multiple deterministic network domains, and the latency in multiple
domains is added together. The jitter will be increased. In order to
realize the protection service function, App-flows should be
transmitted on multiple paths. The delay difference in data
transmission on different paths is no different from jitter in end-
to-end services. Jitter generated by various factors needs to be
controlled to meet business requirements.
This document describes the end-to-end jitter reduction mechanism in
an LDN. This mechanism can effectively control the end-to-end jitter
to meet specific business needs and make the planning of multiple
paths for service protection more flexible.
Status of this Memo
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Table of Contents
1. Introduction...................................................3
2. Terminology....................................................4
3. Architecture...................................................6
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4. Proposal Description...........................................7
4.1. Data-Plane Overview......................................10
4.2. Control-Plane Overview...................................12
5. Security Considerations.......................................13
6. IANA Considerations...........................................13
7. Acknowledgments...............................................13
8. Contributors..................................................13
9. References....................................................14
9.1. Normative References.....................................14
Authors' Addresses...............................................16
1. Introduction
/----\
/ \
| DetNet3 |
\ /
/ \--------/ \
/-------\/ \/-------\
/ \ / \
| DetNet2| | DetNet4 |
\ / \ /
\-----/ \-----/
| |
---|--- --|---
/ \ / \
| DetNet1 | | DetNet5 |
\ / \ /
\--|--/ \--|--/
| |
Talker Listener
Figure 1 Multiple domains
In deterministic networks, as stated in [I-D.ietf-detnet-scaling-
requirements], end-to-end service may across multiple network
domains and adopt a variety of different queuing mechanisms within
each domain. For end-to-end services spanning multiple domains,
jitter exists in various factors:
o Scheduling and traffic admission control at domain boundaries may
cause jitter;
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o The jitter generated by queuing and forwarding mechanisms in the
DetNet domain, such as the [IEEE 802.1 QCR] asynchronous shaping
method;
o Flow aggregation generates jitter. In [RFC 8938], the DetNet data
plane also allows for the aggregation of DetNet flows, which can
improve scalability by reducing the per-hop state. When the
aggregated flows are scheduled, the jitter of the flows cannot be
precisely controlled.
At the same time, according to [I-D.ietf-detnet-scaling-
requirements], large-scale deterministic networks should support
cross-domain asynchronous clocks; in addition, multiple domains may
be heterogeneous networks (such as TSN, DetNet IP and 5GS). The
factors all determine that it is difficult to reduce jitter between
domains through a mechanism similar to CSQF[I-D.chen-detnet-sr-
based-bounded-latency] or TCQF[I-D.draft-eckert-detnet-tcqf]. While
the jitter generated by various factors in the end-to-end
transmission path is accumulated, it may not meet the applications'
requirements on jitter.
This document describes a jitter reduction mechanism to eliminate
jitter introduced by multiple factors in large-scale deterministic
networks.
2. Terminology
The following terminology is introduced in this document:
Actual Delay (ActD): The actual transmission delay of a
deterministic data packet passing through a specified network domain
is called the Actual Time.
Reference Delay (RefD): The maximum delay for packets of DetNet
flows to pass through the DetNet domain.
Fixed delay (FixD): The approximately constant part of the end-to-
end transmission delay of the data packets of the App-flows through
the DetNet.
Path Reference Delay (PthRefD): The maximum end-to-end transmission
delay of data packets of App-flows through the DetNet.
Path Actual Delay (PthActD): The end-to-end actual transmission
delay of a certain packet of App-flows through the DetNet.
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Compensation Delay (CompD): The delay required to compensate the
actual delay according to the reference delay.
Clock Source: Used as a clock source for subnet time
synchronization.
I-Gw: The ingress gateway of the deterministic subnet.
E-Gw: The egress gateway of the deterministic subnet.
HEAD NODE: The I-Gw of the first DetNet domain accessed by the App-
flows in the DetNet forwarding path is HEAD NODE.
COMPENSATION NODE: This node performs calculation and compensation
for time delay.
INGRESS-RELAY NODE: The I-Gw gateway of each domain except the HEAD
NODE.
EGRESS-RELAY NODE: The E-Gw gateway of each domain except the
COMPENSATION NODE.
Virtual Clock Reference Plane (VCRP): Provides a frequency
synchronization reference for the clock used for DetNet data plane
[DDP] timing.
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3. Architecture
*------------------------------------------------------------*
/ /
/ /
/ Virtual Clock Reference Plane /
/ (VCRP) /
/ /
*------+----------+----------------------+------------+------*
| | | |
| +----+----+ +----+----+ |
| | | | | |
| | Clock | | Clock | |
| | Source2 | | Source3 | |
| | | | | |
| +---------+ +---------+ |
| / \ / \ |
| / \ / \ |
| +-------+ +-------+ +-------+ +-------+ |
| | I-Gw2 +--+ E-Gw2 +---+ I-Gw3 +--+ E-Gw3 | |
| +---+---+ +-------+ +-------+ +---+---+ |
| | | |
| | | |
| +-------+ +------+ |
| | | |
+---+-----+ | | +----+----+
| | | | | |
| Clock | | | | Clock |
| Source1 | | | | Source4 |
| | | | | |
+---------+ | | +---------+
/ \ | | / \
/ \ | | / \
+-------+ +-------+ | | +-------+ +-------+
| I-Gw1 +--+ E-Gw1 +--+ +----+ I-Gw4 +--+ E-Gw4 |
+---+---+ +-------+ +-------+ +---+---+
| |
| |
Talker Listener
Figure 2 Architecture
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In Figure 2, the Clock Source is a part of VCRP. I-Gw1 is HEAD NODE.
E-Gw1, E-Gw2, E-Gw3 are EGRESS-RELAY NODE. I-Gw2, I-Gw3 are INGRESS-
RELAY NODE. E-Gw4 is COMPENSATION NODE.
The I-Gw gateway of each domain except the HEAD NODE should extract
the packet receiving time, and calculate the actual delay of the
previous domain, and carry it into the packet.
The E-Gw gateway of each domain except the COMPENSATION NODE should
save the sending time information of the domain.
It is difficult to improve the accuracy of time synchronization,
because VCRP may have a large geographical span. Clock source of
VCRP provides time synchronization for each DetNet domain. Time
synchronization is required within each DetNet domain, not between
DetNet domains. Each domain is frequency-synchronized with the clock
source provided by VCRP to avoid excessive deviation caused by each
domain due to the influence of the environment. Because the clock
sources that provide synchronization references for each DetNet
domain in VCRP may not physically connected, it is difficult to
achieve time synchronization in VCRP. On the premise that the
transmission delay in each domain is not large, the frequency
accuracy of the clock used for timing is relatively low. It is
relatively easy for VCRP to provide a stable clock source with a
certain accuracy for each DetNet domain for time synchronization.
4. Proposal Description
Basic idea of the scheme: when establishing a deterministic stream
session, obtain the reference delay of the path, obtain the actual
delay of the data packet during transmission, calculate the
compensation delay according to the reference delay and the actual
transmission delay, and compensate the transmission delay at the
COMPENSATION NODE connected to the Listener. The implementation
principle is described in detail below.
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| | | |
+----------T2------------+ +----------T3------------+
| | | |
+----------T'2-----------+ +----------T'3-----------+
| | | |
| | | |
+--------+--------+------+ +--------+--------+------+
| I-Gw2 | DetNet2| E-Gw2|----+ I-Gw2 | DetNet2| E-Gw2+-+
+----+---+--------+------+ +--------+--------+------+ |
| |
| |
| |
+--------------------+ +---------------------------+
| |
| |
| || || |
+----------T1------------+| |+----------T4------------+
| || || |
+----------T'2-----------+| |+----------T'4-----------+
| || || |
| || || |
+--------+--------+------+| |+--------+--------+------+
| I-Gw2 | DetNet2| E-Gw2++ ++ I-Gw4 | DetNet4| E-Gw4|
+----+---+--------+------+ +--------+--------+---+--+
| |
| |
| |
Talker Listener
Figure 3 The timing model of transmission delay
Figure 3 describes the abstract model of multi-domain transmission
delay segmentation timing, where Tn (n=1~4 in Figure 3) is the
reference delay RefDn of each domain, and this value is obtained
according to the selection of queuing mechanism combined with
engineering applications.
For a specific deterministic path, there is a master clock in the
same domain, and each node in the path will perform time
synchronization with this clock, and finally obtain intra-domain
time synchronization. The reference delay of the end-to-end path is:
PthRefD = FixD + T1 + T2 + T3 + T4.
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If service protection is realized by sending copies of the same data
packet through multiple paths, the reference delay of path i is:
PthRefD_i = FixD_i + T1_i + T2_i + T3_i + T4_i.
Select the reference delay of the path with the largest one as the
common reference delay of all the paths, and the final reference
delay is:
PthRefD = ceil(PthRefD1, PthRefD2, ..., PthRefDn).
Let Tn (n=1~4 in Figure 3) be the actual transmission delay ActD of
a certain data packet in each domain, and it depends on the time
entering the domain and the time of exiting the domain when the data
packet is actually delivered. The residence time in domain can also
be obtained after the data packet is transmitted. Therefore, the
actual transmission delay of this packet is:
PthActD = FixD + T1 + T2 + T3 + T4.
The delay of the packet needs to be compensated at CompD:
CompD = PthRefD - PthActD = (PthRefD - FixD) - (T1 + T2 + T3+
T4).
If service protection is achieved by sending copies of the same data
packet through multiple paths, the actual delay of path i is
PthActD_i = FixD_i + T1_i + T2_i + T3_i + T4_i.
The delay of the packet needs to be compensated after transmission
in path i is CompD:
CompD = (PthRefD - FixD_i) - (T1_i + T2_i + T3_i + T4_i)
The formula can be simplified as:
CompD = Cap - (T1 + T2 + T3 + T4) (Formula 1),
or
CompD = Cap_i - (T1_i + T2_i + T3_i + T4_i) (Formula 2),
where (PthRefD - FixD) is Cap, and (PthRefD - FixD_i) is Cap_i. Cap
or Cap_i can be obtained directly or indirectly before deploying a
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deterministic streaming session. The specific obtaining method will
be added in subsequent updates. Compensation is performed on E-Gw4
according to Formula 1 or Formula 2 when the DetNet data packet is
transmitted.
4.1. Data-Plane Overview
In the data plane, it is necessary to obtain the reference delay
from the HEAD NODE to the COMPENSATION NODE. During actual
transmission, collect the actual transmission delay in each domain,
and compensate the delay at the COMPENSATION NODE based on the
reference delay.
For delay collection, two methods provide the relevant information
required by each gateway:
Method 1: Specify the operation of the subsequent gateway in the
HEAD NODE. After the HEAD NODE identifies the flow, it encapsulates
the relevant information into a data packet, and the subsequent
gateway node performs corresponding operations according to the
information in the data packet. The advantage of this method is that
it simplifies the subsequent gateway configuration, but the
disadvantage is that the overhead is large.
Method 2: Configure the relevant information in the edge gateway
along the path, and the edge gateway node will identify the DetNet
flow and then obtain the information from the configuration for
corresponding operations. The advantage of this method is that the
overhead is small, and the disadvantage is that subsequent gateways
need flow-by-flow configuration.
The specific operation will be supplemented in subsequent updates.
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+-------------+ +-------------+ +-------------+
| HEAD +----+EGRESS RELAY +----+INGRESS RELAY+-+
+-------------+ +-------------+ +-------------+ |
|
+-------------------------------------------------------+
|
| +-------------+ +-------------+ +-------------+
+-+EGRESS RELAY +----|INGRESS RELAY+----+COMPENSATION |
+-------------+ +-------------+ +-------------+
Figure 4 Data Flow
The process required for the different roles of the gateway that
DetNet flows pass through is described as follows.
When receiving a packet, the HEAD NODE performs the following
process:
1. Identify the DetNet flow and obtain the cross-domain information
provided by the control plane. This cross-domain information
includes the actions of the subsequent gateways, which can be
encapsulated in the packet or configured by the control plane to
the subsequent gateways.
2. Obtain the time when this node receives the packet.
3. Fill the cross-domain information and the time that this node
receives the packet into the packet.
When receiving a packet, the INGRESS-RELAY NODE performs the
following process:
1. Identify the DetNet flow and obtain the cross-domain information
provided by the control plane from the packet or from the
configuration. The gateway takes actions according to the
information.
The main procedure is:
a) Obtain the time when this node receives the packet.
b) Calculate the residence delay of the previous domain.
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2. Fill the residence delay of the previous domain and the time when
this node receives the packet into the packet.
When receiving a packet, the EGRESS-RELAY NODE performs the
following process:
1. Identify the DetNet flow, obtain the cross-domain information
provided by the control plane from the packet, and obtain the
required actions from the configuration.
The main procedure is:
a) Add the time when the packet is sent to the specified position.
When receiving a packet, the COMPENSATION NODE performs the
following process:
1. Identify the DetNet flow, obtain the cross-domain information
provided by the control plane from the packet, and obtain the
required actions from the configuration.
The main procedure is:
a) Obtain the time when this node receives the packet and calculate
the residence delay of this domain.
b) Accumulate the residence delays of each domain.
c) Calculate the compensation delay.
2. Send out the packet after completing the compensation.
4.2. Control-Plane Overview
The control plane needs to cooperate with the data plane to complete
the following transactions:
1. Define gateway operations along the path and configure data plane
gateways.
2. Cooperate with the data plane to complete the measurement and
calculate the reference delay.
3. Configure the reference delay information to the HEAD NODE or
COMPENSATION NODE.
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4. Maintenance during runtime: periodically collect the reference
delay of each domain and calculate the reference delay of the
whole path, and refresh the reference delay to the HEAD NODE or
COMPENSATION NODE.
For delay collection, the control plane has two methods to cooperate
with the data plane to supply the relevant information required by
each gateway:
Method 1: The control plane globally distributes the gateway ID, and
configures the ID to each edge gateway. The control plane configures
the collected delay information to the HEAD NODE.
Method 2: The control plane configures flow-by-flow operations to
the domain edge gateways along the path.
5. Security Considerations
TBD
6. IANA Considerations
TBD
7. Acknowledgments
The authors express their appreciation and gratitude to Min Liu, Lei
Zhou for the review and helpful comments.
8. Contributors
The editor wishes to thank and acknowledge the following
contributors for contributing text to this document.
Rubing Liu
New H3C Technologies Co., Ltd
100094
Email: liurubing@h3c.com
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Ning Pan
New H3C Technologies Co., Ltd
100094
Email: panning@h3c.com
Xusheng Chen
New H3C Technologies Co., Ltd
100094
Email: cxs@h3c.com
Wei Wang
New H3C Technologies Co., Ltd
100094
Email: david_wang@h3c.com
Steven Yoe
New H3C Technologies Co., Ltd
100094
Email: yoe@h3c.com
9. References
9.1. Normative References
[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>.
[I-D.ietf-detnet-scaling-requirements] Liu, P., Li, Y., Eckert, T.,
Xiong, Q., and J. Ryoo, "Requirements for Large-Scale
Deterministic Networks", draft-liu-detnet-large-scale-
requirements-05 (work in progress), October 2022.
[IEEE802.1Qcr] IEEE, "IEEE Standard for Local and Metropolitan Area
Networks -- Bridges and Bridged Networks - Amendment 34:
Asynchronous Traffic Shaping", IEEE 802.1Qcr-2020, DOI
10.1109/IEEESTD.2020.9253013, 6 November 2020,
<https://doi.org/10.1109/IEEESTD.2020.9253013>.
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[I-D.chen-detnet-sr-based-bounded-latency] Chen, M., Geng, X., and Z.
Li, "Segment Routing (SR) Based Bounded Latency", Work in
Progress, Internet-Draft, draft-chen-detnet-sr-based-
bounded-latency, 3 March 2023,
<https://datatracker.ietf.org/doc/draft-chen-detnet-sr-
based-bounded-latency/>
[I-D.eckert-detnet-tcqf] Eckert, T. , Bryant, S., and A. G. Malis,
"Deterministic Networking (DetNet) Data Plane - Tagged
Cyclic Queuing and Forwarding (TCQF) for bounded latency
with low jitter in large scale DetNets"
<https://datatracker.ietf.org/doc/draft-eckert-detnet-
tcqf/>
[IEEE802.1AS] IEEE Time-Sensitive Networking (TSN) Task Group.,
"IEEE Std 802.1AS-2020: IEEE Standard for Local and
Metropolitan Area Networks - Timing and Synchronization
for Time Sensitive Applications ", 2020.
[I-D.ietf-detnet-mpls-over-ip-preof] Varga, B., Farkas, J., Malis,
A., "Deterministic Networking(DetNet): DetNet PREOF via
MPLS over UDP/IP", Work in Progress, Internet-Draft,
draft-ietf-detnet-mpls-over-ip-preof-02, 6 November 2022,
< https://www.ietf.org/archive/id/draft-ietf-detnet-mpls-
over-ip-preof-02.txt>.
[IEEE802.1Qci] IEEE Time-Sensitive Networking (TSN) Task Group.,
"IEEE Std 802.1Qci-2017: IEEE Standard for Local and
Metropolitan Area Networks - Bridges and Bridged Networks-
Amendment 28: Per-Stream Filtering and Policing", 2017.
[I-D.ietf-detnet-controller-plane-framework] Malis, A., Geng, X.,
Chen, M., Qin, F., arga, B., "Deterministic Networking
(DetNet) Controller Plane Framework" , Work in Progress,
Internet-Draft, draft-ietf-detnet-controller-plane-
framework-02, 29 June 2022,
<https://www.ietf.org/archive/id/draft-ietf-detnet-
controller-plane-framework-02.txt>.
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Authors' Addresses
Daorong Guo
New H3C Technologies Co., Ltd
Beijing
100094
China
Email: guodaorong@h3c.com
Shenchao Xu
New H3C Technologies Co., Ltd
Hangzhou
310052
China
Email: xushenchao@h3c.com
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