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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
   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.  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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