Internet DRAFT - draft-liu-detnet-dynamic-latency-guarantee
draft-liu-detnet-dynamic-latency-guarantee
Deterministic Networking Working Group P. Liu
Internet-Draft L. Geng
Intended status: Informational China Mobile
Expires: January 9, 2020 July 08, 2019
Dynamic Latency Guarantee
draft-liu-detnet-dynamic-latency-guarantee-01
Abstract
Aiming at the deterministic demand for network latency in future
vertical industry applications, this document analyzes the existing
latency control methods for data transmission, points out the
possible shortcomings, and proposes some directions for optimizing
the latency control method. .
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 9, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Technologies of Latency Control . . . . . . . . . . . . . . . 2
2.1. IEEE 802.1Qav Forwarding and Queuing Enhancements for
Time-Sensitive Streams . . . . . . . . . . . . . . . . . 3
2.2. IEEE 802.1Qbv Enhancements for Scheduled Traffic . . . . 3
2.3. IEEE 802.1Qbu Frame Preemption . . . . . . . . . . . . . 3
3. Problems and Requirments . . . . . . . . . . . . . . . . . . 3
3.1. Problems in Bounded Latency . . . . . . . . . . . . . . . 4
3.2. Requirments of Deterministic latency . . . . . . . . . . 4
4. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
New types of services such as AR/VR, V2X, industrial motion control,
etc. have stringent requirements for latency and stability. In order
to meet those requirements, some network technologies such as time-
sensitive network, deterministic network, etc., have proposed
corresponding technical means to provide network bearers with
deterministic latency and packet loss rate to guarantee the service
experience. TSN includes a set of standards developed by the IEEE
802.1 Working Group's. Deterministic network (DETNET) is based on
the mechanism of TSN and committed to applying the method to the IP
layer to provide more reliable and stable network transmission. This
document will present some problems when applying TSN in DETNET, and
try to propose reference methods to solve the corresponding problems.
2. Technologies of Latency Control
Based on time synchronization, TSN has a range of bounded latency
technologies.
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2.1. IEEE 802.1Qav Forwarding and Queuing Enhancements for Time-
Sensitive Streams
IEEE 802.1Qav inherited from the AVB, including priority mapping
algorithms and Credit-based Traffic Shaping algorithms. The priority
mapping algorithms is to mapping the priority to 'traffic class',
which represents whether the stream is time sensitive or not.
Credit-based Traffic Shaping algorithms provide the method to
allocate bandwidth of different streams.
2.2. IEEE 802.1Qbv Enhancements for Scheduled Traffic
In IEEE 802.1Qbv, the gate control list is created according to the
actual stream and timescale. It contains the transmission sequence
of all streams, and controls whether the data stream of each priority
is sent at the current time or not. All streams will be transmitted
strictly according to the current list. More Than This, IEEE
802.1Qbv also defines the guard band mechanism and spares part of the
time to guarantee the transmission of high priority data frames at
the beginning of the next time slice.
2.3. IEEE 802.1Qbu Frame Preemption
In the preemption mechanism, high-priority frames can interrupt the
transmission of low-priority data frames unless low-priority data
frames can no longer be fragmented. This standard fully guarantees
the transmission delay of the highest priority data frame, and also
reduces the guard band in IEEE 802.1Qbv to 127 bytes. The frame
preemption mechanism changes the transmission rules of the ethernet
frame and is used in conjunction with the IEEE 802.3Qbr .
In addition to these, there are also other standards to guarantee the
sequence of receiving data streams, which are fine-grained traffic
scheduling technology and the key technologies of TSN in bounded
latency.
3. Problems and Requirments
DETNET refers to the bounded latency mechanism of TSN, so it needs to
pay attention to some problems in the bounded latency mechanism.
There are several standards refers to bounded latency. Users can
decide whether to use a specific standard or not, which depends on
the requirments of network and business. Some TSN testbeds have been
established these years whose basic concept is realizing 802.1Qbv to
ensure the deterministic transmission of time sensitive stream.
Though it realized ignoring the interfere of background stream, the
testbed was too simple. In fact, networking is complicated. There
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will be more than two kind of streams being transmitted. So it is
not that easily to apply those mechanisms on real networks.
3.1. Problems in Bounded Latency
Because of the complicated of real networks, there may be some
situations that the preemptable data frame transmission delay is too
large or cannot be transmitted. Thoes might occur when both
Enhancements for Scheduled Traffic and Frame Preemption are enabled.
Except for the highest priority, the others may be preempted by the
time slice to wait for transmission. In the actual scenario, the
preemptable data frame is not necessarily a completely non-time
sensitive frame, so it also need to guarantee the transmission of
some preemptable frame. However, Under the current mechanism, there
may be multiple preemption to cause a very large transmission delay
or no transmission of preemptable frame, depending on the size of the
express frame and the period of the timescale. In an actual
scenario, a data frame with a Secondary high priority may also be a
time-sensitive. If it cannot be transmitted or the transmission
delay is large, the service cannot be operated.
3.2. Requirments of Deterministic latency
Deterministic network includes deterministic latency and
deterministic packet loss. We need to think how to apply the bounded
latency mechanism effectively. Before using the bounded latency
mechanism, network manager needs to know enough about the network and
applications. For example, which kind of stream is time sensitive?
How about the frame's transceiver frequency of thoes stream? How
much bandwidth does it need? ... When you have a clear understanding
of the real-time state of the network, you can configure a delay-
limited algorithm for the network.
However, the transmission state of the network is not invariable.
Some transfer table might make corresponding adjustments according to
the current network situation. So the parameters that have been
configured before should also be changed. More than this, the
bounded latency mechanism also need a feedback system to receive
current network status and adjust/reconfigure the network.
4. Solutions
The implementation of the mechanism to guarantee latency requires
sophisticated calculation, including timescale and gate control tist
. When the stream in the network becomes diverse, it will consume a
lot of computing resources to schedule each stream. Therefore, a
single transmission rule may not be able to meet the problem of
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multiple streams' transmission. Worst of all, the gate control list
is not properly calculated, the network may not transmit or failure.
Dynamic latency guarantee is a way of thinking based on the latency
guarantee of the whole network. that is, to dynamically adjust the
priority through the current network condition and the transmission
of data stream, and a feedback system is needed to optimize the
system. One of the reasons for this situation is that the prediction
or mastery of the transmission of frames in the network is not
accurate, so a feedback system is needed to tell the network to
centrally configure the system. So it could help to optimize the
gate control list to avoid the frequent occurring of this problems.
The most basic case is that once there are multiple preemption
occured, the switch need to report it to the Centralized
Configuration System. It represent that there might be some
unjustified configurations need to be reconfiguration. For example,
distribute more bandwidth to the corresponding traffic class.
It should be noted that all devices in the network share the same
gate control list. However, due to the difference in time of the
transmission path, it is necessary to keep all devices in the network
"asynchronous" to execute the gate control list. For example, when
the data frame is received by the device A, it is queued to be
transmited first in the currently divided time slice. When the frame
is received by the device B, the time t1 has elapsed. So the gate
control list of device B needs to perform the time difference of t1
with the A device, which can ensure that this frame arrives at every
device with a first-transmiting in current time slice.
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--------------------------------------
| Optimize Configuration |
V |
+-------------+------------+ |
| Centralized |------------------------
| Configuration |
| System |------------------------
+-------------+------------+ |
| Feedback Data|
| of Preemption|
----------------------|------------------------ |
| | | |
V V V |
+---------+ +----------+ +---------+ |
| Switch A|-----------| Switch B |-------------| Switch C|--------
+---------+ t1 +----------+ t2 +---------+
Gate Control Gate Control Gate Control
List List List
Feedback System
5. Conclusion
This draft described the existing mechanism of bounded latency and
point out some problems when using them. It also proposed some
reference methods to solve them. In the process of network
evolution, there might also be more problems need to be noticed and
disscuss. For example, it also needs to consider whether the bounded
latency mechanism of layer 2 can guarantee the deterministic
processing of whole stack. There may be that deterministic
forwarding mechanism is used in Layer 2, but due to the TCP/IP or
other protocol in higher layer, data packets can not be processed in
deterministic order in the queue, which leads to the uncertainty of
latency.
6. Security Considerations
TBD.
7. IANA Considerations
TBD.
8. References
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8.1. Normative References
[I-D.finn-detnet-bounded-latency]
Finn, N., Boudec, J., Mohammadpour, E., Zhang, J., Varga,
B., and J. Farkas, "DetNet Bounded Latency", draft-finn-
detnet-bounded-latency-04 (work in progress), June 2019.
[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-13 (work in progress), May 2019.
[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>.
8.2. Informative References
[IEEE802.1Qav]
IEEE, "Forwarding and Queuing Enhancements for Time-
Sensitive Streams (IEEE 802.1Qav)", 2009.
[IEEE802.1Qbu]
IEEE, "Frame Preemption", 2015.
[IEEE802.1Qch]
IEEE, "Cyclic Queuing and Forwarding", 2015.
[IIEEE802.1Qbv]
IEEE, "Enhancements for Scheduled Traffic", 2016.
Authors' Addresses
Peng Liu
China Mobile
Beijing 100053
China
Email: liupengyjy@chinamobile.com
Liang Geng
China Mobile
Beijing 100053
China
Email: gengliang@chinamobile.com
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