Requirements of Layer 3 Deterministic Latency Service
draft-geng-detnet-requirements-bounded-latency-03

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Network Working Group                                            L. Geng
Internet-Draft                                              China Mobile
Intended status: Informational                                 P. Willis
Expires: January 9, 2020                                              BT
                                                                S. Homma
                                                                     NTT
                                                                L. Qiang
                                                                  Huawei
                                                            July 8, 2019

         Requirements of Layer 3 Deterministic Latency Service
           draft-geng-detnet-requirements-bounded-latency-03

Abstract

   This document specifies some technical, operational and management
   requirements of deploying deterministic latency service on layer 3
   networks from the perspective of the service provider.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology & Abbreviations . . . . . . . . . . . . . . .   3
   2.  Technical Requirements  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirement 1: Must support the dynamic creation,
           modification and deletion of deterministic services . . .   3
     2.2.  Requirement 2: Should tolerate a certain degree of time
           variance  . . . . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Sub-requirement 2.1: Should support asynchronous
               clocks across domains . . . . . . . . . . . . . . . .   4
       2.2.2.  Sub-requirement 2.2: Should tolerate clock jitter &
               wander within a clock synchronous domain  . . . . . .   4
     2.3.  Requirement 3: Must support Inter-Continental propagation
           delay . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Operational and Management Requirements . . . . . . . . . . .   6
     3.1.  Requirement 4: Should have self-monitoring capability . .   6
     3.2.  Requirement 5: Should be robust against denial of service
           attacks . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Requirement 6: Must tolerate failures of links or nodes
           and topology changes  . . . . . . . . . . . . . . . . . .   7
     3.4.  Requirement 7: Must be scalable . . . . . . . . . . . . .   7
       3.4.1.  Sub-requirement 7.1: Must be scalable to numerous
               network devices . . . . . . . . . . . . . . . . . . .   7
       3.4.2.  Sub-requirement 7.2: Must be scalable to massive
               traffic flows . . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   DetNet is chartered to provide bounds on latency, jitter (delay
   variation) and packet loss [draft-ietf-detnet-problem-statement].
   Where latency and jitter are two closely related performance
   characteristics, this document refers to bounded latency and bounded
   jitter collectively as deterministic latency.

   This document does not intend to define any specific mechanism, but
   specifies some technical, operational and management requirements

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   that must be considered when deploying deterministic latency service
   at layer 3.

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.

1.2.  Terminology & Abbreviations

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

   TSN: Time Sensitive Network

2.  Technical Requirements

2.1.  Requirement 1: Must support the dynamic creation, modification and
      deletion of deterministic services

   There are at least two modes to provide deterministic service over an
   operator's network: 1) deterministic VPN; 2) point-to-point
   deterministic tunnel.  No matter in which mode, the layer 3
   deterministic latency mechanisms must be able to support the dynamic
   creation, modification and deletion of deterministic services without
   affecting any deterministic services that are already running in the
   network.

   In a local area network such as a factory, the information about when
   a deterministic service will start, how long the service will last,
   can be known in advance, or can even be planned.  Based on this
   information, the local area network can adopt a global programming
   mechanism to calculate the accurate processing behaviors for each
   device, and achieve a global optimal performance.  However, such
   global programming mechanisms are unsuitable for service providers'
   networks.  Many deterministic applications are expected to running on
   a service provider's network simultaneously.  Different deterministic
   applications may have different lifecycles and SLA requirements,
   hence the network state changes dynamically.  If a mechanism relies
   on a stable network state for global computing, any change in network
   state (e.g., new application starts, or an application finishes, or
   SLA requirement changes) will lead to re-computing, even worse if all
   devices need to stop work and install the recomputed results, then
   this mechanism is hard to be deployed on service provider's network.

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2.2.  Requirement 2: Should tolerate a certain degree of time variance

2.2.1.  Sub-requirement 2.1: Should support asynchronous clocks across
        domains

   One of DetNet's objectives is to stitch TSN islands together.  All
   devices inside a TSN domain are time-synchronized, and most of TSN
   technologies rely on precise time synchronization.  However,
   different TSN islands may have different clocks which are not
   synchronized as shown in Figure 1, where the time difference of two
   TSN domain is D.  DetNet needs to connect these two TSN domains
   together and provide end-to-end deterministic latency service.  The
   mechanism adopted by DetNet should be able to support the interaction
   across time domains by using a fine controlled buffer (the time
   difference 'D' may be us-level) to absorb the time difference, or
   relying on the mechanism itself to implement cross domain time
   mapping.

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

                        |        |        |        |        |
        Clock of TSN    +--------+--------+--------+--------+
        Domain I        =
                        =
                        =       |        |        |        |        |
        Clock of TSN    =       +--------+--------+--------+--------+
        Domain II       =       =
                        =<==D==>=
                        =       =

                   Figure 1: TSN islands interconnecting

2.2.2.  Sub-requirement 2.2: Should tolerate clock jitter & wander
        within a clock synchronous domain

   DetNet domain itself can be time synchronous or asynchronous,
   depending on actual deployment, application, use cases, etc.  Even
   within a time synchronous domain, the synchronized clocks may also
   experience jitter & wander.  Different areas have different clock
   accuracy, for example the crystal oscillator in Ethernet is specified
   at 100 ppm [Fast-Ethernet-MII-clock], SyncE can achieve 50 ppb
   [G.8262], and more precise time synchronization [G.8273] is expected
   in 5G mobile backhaul.  Hence the mechanisms adopted by DetNet should

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   be able to tolerate a certain degree of clock jitter & wander
   accordingly.

2.3.  Requirement 3: Must support Inter-Continental propagation delay

   In contrast to Layer 2 TSN that is deployed on a LAN [IEEE-TSN],
   DetNet is expected to be deployed in a larger scale carrier networks
   that have long link propagation delay which means that DetNet must
   work on network links that have inter-continental propagation delays.
   Long link propagation delay can cause some troubles to cyclic
   forwarding type mechanisms.  In order to guarantee deterministic
   latency, cyclic forwarding type mechanisms usually require a packet
   be sent out (or received) at a particular cycle, rather than be sent
   out (or received) randomly.  There is a mapping between the sending
   cycle of upstream node and the receiving cycle of downstream node.
   In a local area network that with short link propagation delay, the
   cycle mapping relationship could be very simple.

   As an example shown in Figure 2, where the length of a cycle is 10
   us, and the sending cycle of upstream node X and the receiving cycle
   of downstream node Y correspond to the same period of time (e.g.,
   0~10 us).  Packets sent from X at sending cycle will arrive at
   downstream node Y at receiving cycle, and the link propagation delay
   between X and Y is smaller than the length of a cycle (i.e., 10 us).

   Suppose a large scale network wants to keep using this simple cycle
   mapping relationship, however the link distance between two nodes is
   longer.  Moreover, a downstream node may have many upstream nodes
   each with different link propagation delays (e.g., 9 us, 10 us, 11
   us, 15 us, 20 us).  In order to absorb the longest link propagation
   delay, then the length of cycle must be set to at least 20 us.
   However since packet's arrival time varies within the receiving
   cycle, larger cycle length means larger delay variance.

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             Upstream Node X |sending cycle  |            |
                             +--"------------+------------+
                             =  "\           =            =
                             =  " \          =            =
                             =  "  \         =            =
                             =  "   \        =            =
                             =  "    V       =            =
           Downstream Node Y |receiving cycle|            |
                             +--"----"-------+----\-------+
                             =  "    "       =     \      =
                             =  "    "       =      V resent out
                             =  "    "       =            =
                Time Line   -=--"----"-------=------------=----->
                (in us)      0   Link        10           20
                                 Propagation
                                 Delay

                   Figure 2: An example of limited link

3.  Operational and Management Requirements

   [Authors note: more explanations for each requirement need to be
   added in later versions.]

3.1.  Requirement 4: Should have self-monitoring capability

   Both network operators and end-users need to be able to measure if
   the deterministic latency service is working correctly and meeting
   its SLA.  Once the monitored results exceed the expected bounds,
   network operators and end-users should be notified, and some service
   protection mechanisms should also be triggered accordingly.  In
   addition, network operators can input the collected results into a
   reporting system and produce the latency (and jitter) distribution
   over a period of time, which would be helpful for operators in
   understanding their networks performance.

   However, such fine-granularity monitoring is costly.  Hence although
   the self-monitoring is an important capability to both operators and
   customers, it is not recommended as a mandatory requirement until the
   use cases are clear.

3.2.  Requirement 5: Should be robust against denial of service attacks

   To protect the services requiring deterministic latency, the
   mechanisms implemented by DetNet should be robust to denial of
   service attacks.  This includes robustness against attacks on the
   mechanisms to generate clock synchronization.
   [draft-ietf-detnet-architecture] has discussed using the traffic

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   admission control at the ingress or through the fault mitigation
   methods, to prevent (or mitigate) the excess traffic caused by
   malicious or malfunction devices.  DetNet also considers using
   authentication and authorization to mitigate man-in-the middle
   attacks

3.3.  Requirement 6: Must tolerate failures of links or nodes and
      topology changes

   A step change in latency due to a single network topology change is
   inevitable.  However if the topology keeps changing many times, then
   DetNet may not deliver on its SLA.  The topology changes alone may
   not be sufficient on a traditional IP network to raise any alarms,
   but the mechanism's self-monitoring should detect this, and keep
   working in flapping network topologies.

3.4.  Requirement 7: Must be scalable

   Further to the requirement to work on inter-continental links, the
   deterministic latency forwarding mechanisms must scale to networks of
   significant size with numerous network devices and massive traffic
   flows.

3.4.1.  Sub-requirement 7.1: Must be scalable to numerous network
        devices

   A simple use case to understand is ultra-low-latency (public) 5G
   transport networks, which would require DetNet extend to every 5G
   base station.  For some network operators, their network may need to
   connect to ~100 K base stations (serving multiple MNOs'), and this
   number will only increase with 5G.

3.4.2.  Sub-requirement 7.2: Must be scalable to massive traffic flows

   If each ultra-low-latency slice or MNO is treated as a separate
   deterministic latency traffic flow (or tunnel), then even if each
   base station has a limited number of ultra-low latency slices or MNOs
   (e.g. ~10), there will still be a lot of, ~1M, deterministic latency
   traffic flows on one network simultaneously.

4.  IANA Considerations

   This document makes no request of IANA.

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5.  Security Considerations

   This document will not introduce new security problems.

6.  Acknowledgements

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

7.  Normative References

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

   [draft-ietf-detnet-problem-statement]
              G.8262 : Timing characteristics of a synchronous Ethernet
              equipment slave clockG.8262 : Timing characteristics of a
              synchronous Ethernet equipment slave clock, "DetNet
              Problem Statement", <https://datatracker.ietf.org/doc/
              draft-ietf-detnet-problem-statement/>.

   [Fast-Ethernet-MII-clock]
              "Fast Ethernet MII clock",
              <http://www.ti.com/lit/ds/symlink/dp83865.pdf>.

   [G.8262]   "G.8262 : Timing characteristics of a synchronous Ethernet
              equipment slave clock",
              <https://www.itu.int/rec/T-REC-G.8262>.

   [G.8273]   "G.8273: Framework of phase and time clocks",
              <https://www.itu.int/rec/T-REC-G.8273/en>.

   [IEEE-TSN]
              "IEEE TSN Task Group",
              <http://www.ieee802.org/1/pages/tsn.html>.

   [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>.

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Authors' Addresses

   Liang Geng
   China Mobile
   Beijing
   China

   Email: gengliang@chinamobile.com

   Peter Willis
   BT
   BT Adastral Park
   Ipswich IP5 3RE
   UK

   Email: peter.j.willis@bt.com

   Shunsuke Homma
   NTT
   Tokyo
   Japan

   Email: shunsuke.homma.fp@hco.ntt.co.jp

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

   Email: qiangli3@huawei.com

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