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Network Working Group                                            L. Geng
Internet-Draft                                              China Mobile
Intended status: Informational                                  L. Qiang
Expires: September 9, 2019                           Huawei Technologies
                                                               T. Eckert
                                                                  Huawei
                                                           March 8, 2019


          Technical Requirements of Bounded Latency Forwarding
           draft-geng-detnet-requirements-bounded-latency-01

Abstract

   This document analyses the technical requirements that Layer 3
   bounded latency forwarding scheme should satisfy.

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 September 9, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Terminology & Abbreviations . . . . . . . . . . . . . . .   3
   2.  Tolerance of Time Deviation . . . . . . . . . . . . . . . . .   3
   3.  Long Link Propagation Delay . . . . . . . . . . . . . . . . .   3
   4.  Massive Dynamic Flows . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   DetNet is chartered to provide deterministic forwarding over Layer 3.
   Deterministic forwarding means packet forwarding with bounded
   latency, loss and delay variation
   [draft-ietf-detnet-problem-statement].  In current DetNet's
   discussion, low packet loss is mainly achieved through PREOF (Packet
   Replication, Elimination, and Ordering
   Functions)[draft-ietf-detnet-architecture].  This document focuses on
   bounded latency.

   Common IP/MPLS forwarding has long tail effect that couldn't
   guarantee bounded latency.  DetNet has to have an approach to
   identify DetNet flows, and divert them into a DetNet forwarding plane
   in which some schemes are adapted to guarantee bounded latency.
   There are several schemes are proposed for bounded latency forwarding
   such as dedicated tunnel, light load with per-flow per-hop shaping,
   Time Aware Shaping[IEEE802.1Qbv], Cyclic Queuing and
   Forwarding[IEEE802.1Qch], Scalable Deterministic
   Forwarding[draft-qiang-detnet-large-scale-detnet], and SR based
   bounded latency[draft-chen-detnet-sr-based-bounded-latency].  This
   document is not going to compare and analyze these schemes, but to
   propose some factors (Layer 3 specific) worth to be considered when
   selecting Layer 3 bounded latency forwarding scheme.

1.1.  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.





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1.2.  Terminology & Abbreviations

   This document uses the terminology defined in
   [draft-ietf-detnet-architecture].

   TSN: Time Sensitive Network

2.  Tolerance of Time Deviation

   One of DetNet's objectives is to stitch TSN domains together as shown
   in Figure 1.  We know that devices inside a TSN domain are time-
   synchronized, and most of TSN forwarding schemes rely on precise time
   synchronization.  However two TSN domains have a high probability of
   being asynchronous, while DetNet needs to connect them together and
   provide end-to-end deterministic forwarding.  Therefore, it is worthy
   of have a DetNet forwarding plane which can keep the bounded latency
   even under an unsynchronized situation.  Otherwise, buffer is needed
   to absorb the time deviation, and end-users have to bear the latency
   and cost increase introduced by buffer.

   Moreover, there are

       +--------------+                             +--------------+
       |              |      DetNet Forwarding      |              |
       | TSN Domain I +-----------------------------+ TSN Domain II|
       |              |                             |              |
       +--------------+                             +--------------+

                         Figure 1: DetNet Scenario

3.  Long Link Propagation Delay

   In contrast to Layer 2 TSN that deployed in LAN, Layer 3 DetNet is
   expected to be deployed in larger scale network that has longer link
   propagation delay.  Long link propagation delay can cause some
   troubles to simple cyclic forwarding schemes, like[IEEE802.1Qch].
   IEEE 802.1 Qch works on the basis of time synchronization, and one
   hop forwarding (include packet sending, packet transmission on link,
   and packet receiving three operations) is required to be finished
   within one cycle as shown in Figure 2.  Long links whose latency
   exceed the cycle time will make IEEE 802.1 Qch doesn't work.  One
   possible solution is to set cycle time to be a bigger value, in order
   to absorb long link propagation delay.  However, this solution will
   lead to larger jitter.  The reason is that cyclic forwarding schemes
   only try to ensure the packet arrives at a node within a certain
   cycle, and packet's arrival time may vary within that cycle.





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                   /--\        "           "
                  |    |       " Send      "
                   \--/        "  *        "
               Upstream Node   "   \       "
                               "    \      "
                               " Short     "
                               "  Link\    "
                               "       \   "
                   /--\        "        v  "
                  |    |       "         * "
                   \--/        "    Receive"
             Downstream Node   "           "
                               "           "
                               |           |           |
                Time Line  ----+-----------+-----------+---->
                               0           1           2

                    Figure 2: One Hop Forwarding in Qch

   [Jitter on link is TBD]

4.  Massive Dynamic Flows

   Considering the features of TSN applications we can speculate that
   the number of TSN flows will not dramatically change with time.
   While DetNet targets at larger-scale deployment.  There are more
   time-sensitive applications such as VR communication, they may
   require establishment or tear-down of the DetNet connections
   frequently.  Meanwhile, layer 3 device may serve millions of traffic
   flows simultaneously.  Hence those schemes that need complex
   calculations may not be applicable in DetNet.  More importantly,
   forwarding schemes need to avoid impact on those in-transit flows
   when new flows are added (or old flows are removed).

5.  IANA Considerations

   This document makes no request of IANA.

6.  Security Considerations

   This document will not introduce new security problems.

7.  Acknowledgements

   The Authors would like to thank David Black for his review,
   suggestion and comments to this document.





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8.  Normative References

   [draft-chen-detnet-sr-based-bounded-latency]
              "SR based Bounded Latency",
              <https://datatracker.ietf.org/doc/
              draft-chen-detnet-sr-based-bounded-latency/>.

   [draft-ietf-detnet-architecture]
              "DetNet Architecture", <https://datatracker.ietf.org/doc/
              draft-ietf-detnet-architecture/>.

   [draft-ietf-detnet-problem-statement]
              "DetNet Problem Statement",
              <https://datatracker.ietf.org/doc/
              draft-ietf-detnet-problem-statement/>.

   [draft-qiang-detnet-large-scale-detnet]
              "Large-Scale Deterministic Network",
              <https://datatracker.ietf.org/doc/
              draft-qiang-detnet-large-scale-detnet/>.

   [IEEE802.1Qbv]
              "Enhancements for Scheduled Traffic", 2016.

   [IEEE802.1Qch]
              "Cyclic Queuing and Forwarding", 2016.

   [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>.

   [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>.

Authors' Addresses

   Liang Geng
   China Mobile
   Beijing
   China

   Email: gengliang@chinamobile.com







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   Li Qiang
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: qiangli3@huawei.com


   Toerless Eckert
   Huawei USA - Futurewei Technologies Inc.
   2330 Central Expy
   Santa Clara  95050
   USA

   Email: tte+ietf@cs.fau.de



































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