Internet DRAFT - draft-xiong-rtgwg-precise-tn-problem-statement

draft-xiong-rtgwg-precise-tn-problem-statement







RTGWG                                                           Q. Xiong
Internet-Draft                                           ZTE Corporation
Intended status: Standards Track                                  P. Liu
Expires: October 26, 2020                                   China Mobile
                                                          April 24, 2020


         The Problem Statement for Precise Transport Networking
           draft-xiong-rtgwg-precise-tn-problem-statement-00

Abstract

   As described in [xiong-rtgwg-precise-tn-requirements], the
   deterministic networks not only need to offer the Service Level
   Agreements (SLA) guarantees such as low latency and jitter, low
   packet loss and high reliability, but also need to support the
   precise services such as flexible resource allocation and service
   isolation so as to the Precise Transport Networking.  However, under
   the existing IP network architecture with statistical multiplexing
   characteristics, the existing deterministic technologies are facing
   long-distance transmission, queue scheduling, dynamic flows and per-
   flow state maintenance and other controversial issues especially in
   Wide Area Network (WAN) applications.

   This document analyses the problems in existing deterministic
   technologies to provide precise services in various industries such
   as 5G networks.

Status of This Memo

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   This Internet-Draft will expire on October 26, 2020.







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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   4
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Problem with Traffic Scheduling Mechanisms  . . . . . . .   4
     3.2.  Problem with Long-distance Transmission Delay and Jitter    5
     3.3.  Problem with SLA Guarantees of Dynamic Flows  . . . . . .   5
     3.4.  Problem with Service Isolation  . . . . . . . . . . . . .   6
     3.5.  Problem with Precise Resource Allocation  . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

1.1.  Overview

   5G network is oriented to the internet of everything.  In addition to
   the Enhanced Mobile Broadband (eMBB) and Massive Machine Type
   Communications(mMTC) services, it also supports the Ultra-reliable
   Low Latency Communications (uRLLC) services.  The uRLLC services
   cover the industries such as intelligent electrical network,
   intelligent factory, internet of vehicles, industry automation and
   other industrial internet scenarios, which is the key demand of
   digital transformation of vertical domains.  These uRLLC services




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   demand SLA guarantees such as low latency and high reliability and
   other deterministic and precise properties.

   For the intelligent electrical network, there are deterministic
   requirements for communication delay, jitter and packet loss rate.
   For example, in the electrical current difference model, a delay of
   3~10ms and a jitter variation is no more than 100us are required.
   The isolation requirement is also important.  For example, the
   automatic operation, control of a process, isochronous data and low
   priority service need to meet the requirements of hard isolation.  In
   addition to the requirements of delay and jitter, the differential
   protection (DP) service needs to be isolated from other services.

   The industrial internet is the key infrastructure that coordinate
   various units of work over various system components, e.g. people,
   machines and things in the industrial environment including big data,
   cloud computing, Internet of Things (IOT), Augment Reality (AR),
   industrial robots, Artificial Intelligence (AI) and other basic
   technologies.  For example, automation control is one of the basic
   application and the the core is closed-loop control system.  The
   control process cycle is as low as millisecond level, so the system
   communication delay needs to reach millisecond level or even lower to
   ensure the realization of precise control.  There are three levels of
   real-time requirements for industrial interconnection: factory level
   is about 1s, and process level is 10~100ms, and the highest real-time
   requirement is motion control, which requires less than 1ms.

1.2.  Motivation

   The applications in 5G networks demand much more deterministic and
   precise properties.  But traditional Ethernet, IP and MPLS networks
   which is based on statistical multiplexing provides best-effort
   packet service and offers no delivery and SLA guarantee.  The
   deterministic forwarding can only apply to flows with such well-
   defined characteristics as periodicity and burstiness.

   Technologies to provide deterministic service has been proposed to
   provide bounded latency and jitter based on a best-effort packet
   network.  IEEE 802.1 Time-Sensitive Networking (TSN) has been
   proposed to provide bounded latency and jitter in L2 LAN networks.
   According to [RFC8655], Deterministic Networking (DetNet) operates at
   the IP layer and delivers service which provides extremely low data
   loss rates and bounded latency within a network domain.  However, the
   existing mechanisms are not sufficient for precise performance such
   as precise latency, jitter variation, packet loss and more other
   precise and deterministic properties.





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   As described in [xiong-rtgwg-precise-networking-requirements], the
   deterministic networks not only need to offer the Service Level
   Agreements (SLA) guarantees such as low latency and jitter, low
   packet loss and high reliability, but also need to support the
   precise services such as flexible resource allocation and service
   isolation so as to the Precise Transport Networking.  However, under
   the existing IP network architecture with statistical multiplexing
   characteristics, the existing deterministic technologies are facing
   long-distance transmission, traffic scheduling, dynamic flows, per-
   flow state maintenance and other controversial issues especially in
   Wide Area Network (WAN) applications.

   This document analyses the problems in existing deterministic
   technologies to provide precise services in various industries such
   as 5G networks.

2.  Conventions used in this document

2.1.  Terminology

   The terminology is defined as [RFC8655] and [xiong-rtgwg-precise-
   networking-requirements].

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Problem Statement

3.1.  Problem with Traffic Scheduling Mechanisms

   As described in [RFC8655], the primary means by which DetNet achieves
   its QoS assurances in IP networks is to eliminate the latency and
   packet loss by the provision of sufficient resource at each node.
   But only the resource itself is not sufficient, the traffic
   scheduling mechanisms such as queuing, shaping, and scheduling
   functions must be applied in L3 networks.

   The congestion control, queue scheduling and other traffic mechanisms
   which have been proposed in IEEE 802.1 TSN.  But most of them are
   based on the time synchronization and time cycle, such as
   IEEE802.1Qbv, IEEE802.1Qch and so on.  It will be difficult to
   achieve precise time synchronization with all network nodes due to
   deployment and cost reasons.  And the shaping and queuing methods



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   which are not based on time synchronization such as IEEE802.1Qav and
   IEEE802.1Qcr might not be suitable or available for some L3 networks
   such as WAN application where multiple dynamic traffic flows may be
   existed.

   Moreover, the requirements of the all nodes in WAN networks to apply
   the time synchronization and traffic scheduling mechanisms will also
   lead to the difficulty of network scalability and deployment.

3.2.  Problem with Long-distance Transmission Delay and Jitter

   In WAN application, long-distance transmission will lead to
   uncertainties, such as increasing transmission delay, jitter and
   loss.  The link delay of transmission is variable and can not be
   ignored, and it must be considered in the end-to-end deterministic
   forwarding mechanisms which are based on time synchronous.  So the
   following problems should be considered.

   Precise measurement of the link delay.

   The symmetry of bidirectional forwarding of the link delay.

   Time cycle alignment in flows aggregation scenario.

3.3.  Problem with SLA Guarantees of Dynamic Flows

   As described in [RFC8557], deterministic forwarding can only apply to
   flows with such well-defined characteristics as periodicity and
   burstiness.  As defined in DetNet architecture [RFC8655], the traffic
   characteristics of an App-flow can be CBR (constant bit rate) or VBR
   (variable bit rate) of L1, L2 and L3 layers (VBR takes the maximum
   value when reserving resources).  But the current scenarios and
   technical solutions only consider CBR flow, without considering the
   coexistence of VBR and CBR, the burst and aperiodicity of flows.  The
   operations such as shaping or scheduling have not been specified.
   Even TSN mechanisms are based on a constant and forecastable traffic
   characteristics.

   It will be more complicated in WAN applications where much more flows
   coexist and the traffic characteristics is more dynamic.  It is
   required to offer reliable delivery and SLA guarantee for dynamic
   flows.  For example, periodic flow and aperiodic flow (including
   micro burst flow, etc.), CBR and VBR flow, flow with different
   periods or phases, etc.  When the network needs to forward these
   deterministic flows at the same time, it must solve the problems of
   time window selection, queue processing and aggregation of multiple
   flows.




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   Moreover, the existing solutions do not consider the characteristics
   analysis of service requirements, including the impact of dynamic
   characteristics analysis on the network, mainly about how to ensure
   the certainty in the case of dynamic flows.

3.4.  Problem with Service Isolation

   In some scenarios, such as intelligent electrical network, the
   isolation requirements are very important.  For example, the
   automatic operation or control of a process or isochronous data and
   low priority service need to meet the requirements of hard isolation.
   In addition to the requirements of delay and jitter, the differential
   protection (DP) service needs to be isolated from other services and
   hard isolated tunnel is required.

   The resource reservation in DetNet can only ensure the statistical
   reuse of bandwidth resources, but it can not guarantee the precise
   isolation and control of instantaneous burst and can not realize the
   hard isolation of each flow.  The existing solutions cannot achieve
   the requirements of service isolation.

3.5.  Problem with Precise Resource Allocation

   As described in [RFC8655], the primary means by which DetNet achieves
   its QoS assurances is to reduce, or even completely eliminate, packet
   loss by the provision of sufficient buffer storage at each node.  But
   it can not be achieved by not enough resource which can be allocated
   due to practical and cost reason.  The existing solutions can not
   achieve the precise resource allocation.

4.  Security Considerations

   TBA

5.  Acknowledgements

   TBA

6.  IANA Considerations

   TBA

7.  Normative References

   [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>.



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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8557]  Finn, N. and P. Thubert, "Deterministic Networking Problem
              Statement", RFC 8557, DOI 10.17487/RFC8557, May 2019,
              <https://www.rfc-editor.org/info/rfc8557>.

   [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>.

Authors' Addresses

   Quan Xiong
   ZTE Corporation
   No.6 Huashi Park Rd
   Wuhan, Hubei  430223
   China

   Email: xiong.quan@zte.com.cn


   Peng Liu
   China Mobile
   Beijing  100053
   China

   Email: liupengyjy@chinamobile.com





















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