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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


   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",
   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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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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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
   (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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

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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

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

6.  Security Considerations


7.  IANA Considerations


8.  References

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8.1.  Normative References

              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.

              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,

8.2.  Informative References

              IEEE, "Forwarding and Queuing Enhancements for Time-
              Sensitive Streams (IEEE 802.1Qav)", 2009.

              IEEE, "Frame Preemption", 2015.

              IEEE, "Cyclic Queuing and Forwarding", 2015.

              IEEE, "Enhancements for Scheduled Traffic", 2016.

Authors' Addresses

   Peng Liu
   China Mobile
   Beijing  100053

   Email: liupengyjy@chinamobile.com

   Liang Geng
   China Mobile
   Beijing  100053

   Email: gengliang@chinamobile.com

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