Dynamic Latency Guarantee
draft-liu-detnet-dynamic-latency-guarantee-00
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| Authors | Peng Liu , Liang Geng | ||
| Last updated | 2019-03-11 | ||
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draft-liu-detnet-dynamic-latency-guarantee-00
Deterministic Networking Working Group P. Liu
Internet-Draft L. Geng
Intended status: Informational China Mobile
Expires: September 11, 2019 March 10, 2019
Dynamic Latency Guarantee
draft-liu-detnet-dynamic-latency-guarantee-00
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 11, 2019.
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
(https://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Core Technology of Bounded Latency . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . 4
3.1. Problems . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Requirments . . . . . . . . . . . . . . . . . . . . . . . 5
4. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Dynamic Latency Guarantee . . . . . . . . . . . . . . . . 6
4.2. Feedback System . . . . . . . . . . . . . . . . . . . . . 7
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
With the vigorous development of 5G and industrial Internet, network
technology has become more and more important to support new types of
services, such as AR/VR, V2X, industrial motion control, etc., which
have stringent requirements for latency and stability. In order to
meet the requirements of the above applications, new network
technologies such as time-sensitive network TSN, deterministic
network DETNET, etc., have proposed corresponding technical means to
provide network bearers with deterministic latency and packet loss
rate and guarantee the user's business experience.
TSN includes a set of standards developed by the IEEE 802.1 Working
Group's. The TSN task group inherited from the previous Audio/Video
Bridging working group and has expanded its applications to in-
vehicle, industrial, and mobile networks. Deterministic network
(DETNET) is based on the mechanism of TSN. The difference is that
TSN is applied to the data link layer and below. DETNET is committed
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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. Core Technology of Bounded Latency
Based on time synchronization, TSN has a range of bounded latency
technologies.
2.1. IEEE 802.1Qav Forwarding and Queuing Enhancements for Time-
Sensitive Streams
IEEE 802.1Qav Forwarding and Queuing Enhancements for Time-Sensitive
Streams 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.
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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
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
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.
For example, there are currently two queues are transmited following
the gate control list which assuming is the following table. In the
table, T00, T01, T02... represent the order of each time slice and
switching. The "01" and "10" in the right represent whether the two
queue can be transmitted in the current period. Assuming that 0
indicates that the gate is closed and the corresponding queue cannot
be transmitted. while 1 indicates the gate is open and corresponding
queue can be transferred. Then the following two cases may occur:
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+-----------------------+
| Gate Control List |
+-----------------------+
| T00 | 01 |
+-----------------------+
| T01 | 10 |
+-----------------------+
| T02 | 01 |
+-----------------------+
| T03 | 10 |
+-----------------------+
| T.. | .. |
+-----------------------+
Gate Control List
Case 1, The preemptable frame is interrupted many times before the
transmission is completed, which causes a high transmission delay of
the preemptable frame.
Case 2, the preemptable frame cannot be transmitted after once being
interrupted.
+-------------+---------+-------------+---------+---+-------------+
| Part 1 of | Express | Part 2 of | Express | | Part n of |
| Preemptable | | Preemptable | |...| Preemptable |
| Frame | Frame A | Frame | Frame B | | Frame |
+-------------+---------+-------------+---------+---+-------------+
Case 1 of Preemption
+-------------+---------+---------+----------+-----+----------+
| Part 1 of | Express | Express | Express | | Express |
| Preemptable | | | | ... | |
| Frame | Frame A | Frame B | Frame C | | Frame N |
+-------------+---------+---------+----------+-----+----------+
Case 2 of Preemption
3.2. Requirments
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
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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
multiple streams' transmission. Worst of all, the gate control list
is not properly calculated, the network may not transmit or failure.
4.1. Dynamic Latency Guarantee
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.
In the transmission process, the priority of data is based on the
"Traffic Class" in IEEE802.1 Qav. that is, the priority of data
frames is converted into traffic class according to the mapping
table. If the data frame is preempted once, the corresponding
traffic class is increased according to certain functional rules.
Functional rules can be defined as needed, for example, by assuming:
T = Time of preempted
M = Lifting coefficient
and F(tm)=Increased traffic class
That is to say, with the increase of preemption times, the
preemptable frames will gradually increase their priority (the
corresponding traffic class). When it is greater than or equal to
the Traffic Class of express frames, the preemptable frames could
complete the transmission.
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The lifting factor M can be either a constant or a variable varying
with T, depending on the network requirements of specific business
application scenarios, which will not been discussed in detail in
this document.
4.2. Feedback 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.
6. Security Considerations
TBD.
7. IANA Considerations
TBD.
8. References
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-02 (work in progress), October
2018.
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[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-11 (work in progress), February 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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