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Network Working Group                                      L. Qiang, Ed.
Internet-Draft                                                    B. Liu
Intended status: Informational                            T. Eckert, Ed.
Expires: January 3, 2019                                          Huawei
                                                                 L. Geng
                                                                 L. Wang
                                                            China Mobile
                                                            July 2, 2018

                   Large-Scale Deterministic Network


   This document presents the framework and key methods for Large-scale
   Deterministic Networks (LDN).  It achieves scalability for the number
   of supportable deterministic traffic flows via Scalable Deterministic
   Forwarding (SDF) that does not require per-flow state in transit
   nodes and precise time synchronization among nodes.  It achieves
   Scalable Resource Reservation (SRR) by allowing for it to be
   decoupled from the forwarding plane nodes, and aggregating resource
   reservation status in time slots.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 3, 2019.

Copyright Notice

   Copyright (c) 2018 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

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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology & Abbreviations . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Deterministic End-to-End Latency  . . . . . . . . . .   4
       2.2.2.  Hop-by-Hop Delay  . . . . . . . . . . . . . . . . . .   4
       2.2.3.  Cyclic Forwarding . . . . . . . . . . . . . . . . . .   5
       2.2.4.  Co-Existence with Non-Deterministic Traffic . . . . .   5
     2.3.  System Components . . . . . . . . . . . . . . . . . . . .   6
   3.  Scalable Deterministic Forwarding . . . . . . . . . . . . . .   7
     3.1.  Three Queues  . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Cycle Mapping . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  Cycle Identifier Carrying . . . . . . . . . . . . . .   9
   4.  Scalable Resource Reservation . . . . . . . . . . . . . . . .   9
   5.  Performance Analysis  . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Queueing Delay  . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Jitter  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Deploying deterministic service over large-scale network will face
   some technical challenges, such as

   o  massive number of deterministic flows vs. per-flow operation and

   o  long link propagation may bring in significant jitter;

   o  time synchronization is hard to be achieved among numerous
      devices, etc.

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   Motivated by these challenges, this document presents a Large-scale
   Deterministic Network (LDN) system, which consists of Scalable
   Deterministic Forwarding (SDF) at forwarding plane and Scalable
   Resource Reservation (SRR) at control plane.  The technologies of SDF
   and SRR can be used independently.

   As [draft-ietf-detnet-problem-statement] indicates, deterministic
   forwarding can only apply on flows with well-defined traffic
   characteristics.  The traffic characteristics of DetNet flow has been
   discussed in [draft-ietf-detnet-architecture], that could be achieved
   through shaping at Ingress node or up-front commitment by
   application.  This document assumes that DetNet flows follow some
   specific traffic patterns accordingly.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

1.2.  Terminology & Abbreviations

   This document uses the terminology defined in

   TSN: Time Sensitive Network

   CQF: Cyclic Queuing and Forwarding

   LDN: Large-scale Deterministic Network

   SDF: Scalable Deterministic Forwarding

   SRR: Scalable Resource Reservation

   DSCP: Differentiated Services Code Point

   EXP: Experimental

   TC: Traffic Class

   T: the length of a cycle

   H: the number of hops

   K: the size of aggregated resource reservation window

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2.  Overview

2.1.  Summary

   The Large-Scale Deterministic Network solution (LDN) consists of two
   parts: The Scalable Deterministic Forwarding Plane (SDF) as its
   forwarding plane and the Scalable Resource Reservation (SRR) as its
   control plane.  In the SDF, nodes in the network have synchronized
   frequency, and each node forwards packets in a slotted fashion based
   on a cycle identifiers carried in packets.  Ingres nodes or senders
   have a function called gate to shape/condition traffic flows.  Except
   for this gate function, the SDF has no awareness of individual flows.
   The SRR maintains resource reservation states for deterministic
   flows, Ingress nodes maintain per-flow states and core nodes
   aggregate per-flow states in time slots.

2.2.  Background

   This section motivates the design choices taken by the proposed
   solution and gives the necessary background for deterministic delay
   based forwarding plane designs.

2.2.1.  Deterministic End-to-End Latency

   Bounded delay is delay that has a deterministic upper and lower

   The delay for packets that need to be forwarded with deterministic
   delay needs to be deterministic on every hop.  If any hop in the
   network introduces non-deterministic delay, then the network itself
   can not deliver a deterministic delay service anymore.

2.2.2.  Hop-by-Hop Delay

   Consider a simple example (without picture), where N has 10 receiving
   interfaces and one outgoing interface I all of the same speed.  There
   are 10 deterministic traffic flows, each consuming 5% of a links
   bandwidth, one from each receiving interface to the outgoing

   Node N sends 'only' 50% deterministic traffic to interface I, so
   there is no ongoing congestion, but there is added delay.  If the
   arrival time of packets for these 10 flows into N is uncontrolled,
   then the worst case is for them to all arrive at the same time.  One
   packet has to wait in N until the other 9 packets are sent out on I,
   resulting in a worst case deterministic delay of 9 packets
   serialization time.  On the next hop node N2 downstream from N, this
   problem can become worse.  Assume N2 has 10 upstream nodes like N,

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   the worst case simultaneous burst of packets is now 100 packets, or a
   99 packet serialization delay as the worst case upper bounded delay
   incurred on this hop.

   To avoid the problem of high upper bound end-to-end delay, traffic
   needs to be conditioned/interleaved on every hop.  This allows to
   create solutions where the per-hop-delay is bounded purely by the
   physics of the forwarding plane across the node, but not the
   accumulated characteristics of prior hop traffic profiles.

2.2.3.  Cyclic Forwarding

   The common approach to solve that problem is that of a cyclic hop-by-
   hop forwarding mechanism.  Assume packets forwarded from N1 via N2 to
   N3 as shown in Figure 1.  When N1 sends a packet P to interface I1
   with a Cycle X, it must be guaranteed by the forwarding mechanism
   that N2 will forward P via I2 to N3 in a cycle Y.

   The cycle of a packet can either be deduced by a receiving node from
   the exact time it was received as is done in SDN/TDMA systems, and/or
   it can be indicated in the packet.  This document solution relies on
   such markings because they allow to reduce the need for synchronous
   hop-by-hop transmission timings of packets.

   In a packet marking based slotted forwarding model, node N1 needs to
   send packets for cycle X before the latest possible time that will
   allow for N2 to further forward it in cycle Y to N3.  Because of the
   marking, N1 could even transmit packets for cycle X before all
   packets for the previous cycle (X-1) have been sent, reducing the
   synchronization requirements between across nodes.

                 P sent in          P sent in         P sent in
                cycle(N1,I1,X)    cycle(N2,I2,Y)     cycle(N3,I3,Z)
                +--------+        +--------+         +--------+
                | Node N1|------->| Node N2|-------->| Node N3|------>
                +--------+I1      +--------+I2       +--------+I3

                        Figure 1: Cyclic Forwarding

2.2.4.  Co-Existence with Non-Deterministic Traffic

   Traffic with deterministic delay requirements can co-exist with
   traffic only requiring non-deterministic delay by using packet
   scheduling where the delay incurred by non-deterministic packets is
   deterministic for the deterministic traffic (and low).  If LDN SDF is
   deployed together with such non-deterministic delay traffic than such
   a scheme must be supported by the forwarding plane.  A simple
   approach for the delay incurred on the sending interface of a

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   deterministic node due to non-deterministic traffic is to serve
   deterministic traffic via a strict, highest-priority queue and
   include the worst case delay of a currently serialized non-
   deterministic packet into the deterministic delay budget of the node.
   Similar considerations apply to the internal processing delays in a

2.3.  System Components

   The Figure 2 shows an overview of the components considered in this
   document system and how they interact.

   A network topology of nodes, Ingress, Core and Egress support a
   method for cyclic forwarding to enable Scalable Deterministic
   Forwarding (SDF).  This forwarding requires no per-flow state on the

   Ingress edge nodes may support the (G)ate function to shape traffic
   from sources into the desired traffic characteristics, unless the
   source itself has such function.  Per-flow state is required on the
   ingress edge node.

   A Scalable Resource Reservation (SRR) works as control plane.  It
   records reserved resources for deterministic flows.  Per-flow state
   is maintained on the ingress edge node, and aggregated state is
   maintained on core node.

                per-flow   time-based aggregated
                 status           status

        /--\.     +--+        +--+      +--+        +--+.     /--\
       | (G)+-----+GS+--------+ S+------+ S+--------+ S+-----+    |
        \--/      +--+        +--+      +--+        +--+      \--/

       Sender    Ingress      Core      Core       Egress   Receiver
                Edge Node     Node      Node     Edge Node

      Forwarding     high link delay propagation tolerant
      Plane:SDF          cycle-based forwarding

                         Figure 2: System Overview

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3.  Scalable Deterministic Forwarding

   DetNet aims at providing deterministic service over large scale
   network.  In such large scale network, it is difficulty to get
   precise time synchronization among numerous devices.  To reduce
   requirements, the forwarding mechanism described in this document
   assumes only frequency synchronization but not time synchronization
   across nodes: nodes maintain the same clock frequency 1/T, but do not
   require the same time as shown in Figure 3.

         <-----T----->                       <-----T----->
         |           |           |           |           |           |
 Node A  +-----------+-----------+   Node A  +-----------+-----------+
                     T0                                  T0

         |           |           |              |           |           |
 Node B  +-----------+-----------+   Node B     +-----------+-----------+
                     T0                                     T0

     (i) time synchronization           (ii) frequency synchronization

     T: length of a cycle
     T0: timestamp

          Figure 3: Time Synchronization & Clock Synchronization

   IEEE 802.1 CQF is an efficient forwarding mechanism in TSN that
   guarantees bounded end-to-end latency.  CQF is designed for limited
   scale networks.  Time synchronization is required, and the link
   propagation delay is required to be smaller than a cycle length T.
   Considering the large scale network deployment, the proposed Scalable
   Deterministic Forwarding (SDF) permits frequency synchronization and
   link propagation delay may exceed T.  Besides these two points, CQF
   and the asynchronous forwarding of SDF are very similar.

   Figure 4 compares CQF and SDF through an example.  Suppose Node A is
   the upstream node of Node B.  In CQF, packets sent from Node A at
   cycle x, will be received by Node B at the same cycle, then further
   be sent to downstream node by Node B at cycle x+1.  Due to long link
   propagation delay and frequency synchronization, Node B will receive
   packets from Node A at different cycle denoted by y in the SDF, and
   Node B swaps the cycles carried in packets with y+1, then sends out
   those packets at cycle y+1.  This cycle mapping (e.g., x --> y+1)
   exists between any pair of neighbor nodes.  With this mapping, the
   receiving node can easily figure out when the received packets should
   be send out, the only requirement is to carry the cycle identifier of
   sending node in the packets.

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       |  cycle x  | cycle x+1 |             |  cycle x  | cycle x+1 |
Node A +-----------+-----------+      Node A +-----------+-----------+
          \                                         \
           \packet                                   \packet
            \receiving                                \receiving
             \                                         \
       |      V    | cycle x+1 |                 |      V    |  cycle y+1|
Node B +-----------+-----------+      Node B     +-----------+-----------+
          cycle x      \packet                      cycle y      \packet
                        \sending                                  \sending
                         \                                         \
                          \                                         \
                           V                                         V

              (i) CQF                                (ii) SDF

                            Figure 4: CQF & SDF

3.1.  Three Queues

   In CQF each port needs to maintain 2 (or 3) queues: one is used to
   buffer newly received packets, another one is used to store the
   packets that are going to be sent out, one more queue may be needed
   to avoid output starvation [scheduled-queues].  In SDF, at least 3
   queues are needed.

   As Figure 5 illustrated, a node may receive packets sent at two
   different cycles from a single upstream node due to the absence of
   time synchronization.  Following the cycle mapping (i.e., x --> y+1),
   packets that carry cycle identifier x should be sent out by Node B at
   cycle y+1, and packets that carry cycle identifier x+1 should be sent
   out by Node B at cycle y+2.  Therefore, two queues are needed to
   store the newly received packets, as well as one queue to store the
   sending packets.  In order to absorb more link delay variation (such
   as on radio interface), more queues may be necessary.

                         |  cycle x  | cycle x+1 |
                 Node A  +-----------+-----------+
                                  \      \
                                   \      \packet
                                    \      \receiving
                                   | V      V  |           |
                        Node B     +-----------+-----------+
                                      cycle y    cycle y+1

                       Figure 5: Three Queues in SDF

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3.2.  Cycle Mapping

   When this packet is received by Node B, some methods are possible how
   the forwarding plane could operate.  In one method, Node B has a
   mapping determined by the control plane.  Packets from (the link
   from) Node A indicating cycle x are mapping into cycle y+1.  This
   mapping is necessary, because all the packets from one cycle of the
   sending node need to get into one cycle of the receiving node.  This
   is called "configured cycle mapping".

   Instead of configuring an explicit cycle mapping such as cycle x ->
   cycle y+1, the receiving Node B could also have the intelligence in
   the forwarding plane to recognize the first packet from (the link
   from) Node A that has a new cycle x number, and map this cycle x to
   the next cycle after the current cycle y, aka: cycle y+1.  We call
   this option "self synchronized cycle mapping".

3.2.1.  Cycle Identifier Carrying

   In self synchronized cycle mapping, cycle identifier needs to be
   carried in the SDF packets, so that an appropriate queue can be
   selected accordingly.  That means 2 bits are needed in the three
   queues model of SDF, in order to identify different cycles between a
   pair of neighboring nodes.  There are several ways to carry this 2
   bits cycle identifier.  This document does not yet aim to propose
   one, but gives an (incomplete) list of ideas:

   o  DSCP of IPv4 Header

   o  Traffic Class of IPv6 Header

   o  TC of MPLS Header (used to be EXP)

   o  EtherType of Ethernet Header

   o  IPv6 Extension Header

   o  TLV of SRv6

   o  TC of MPLS-SR Header (used to be EXP)

   o  Three labels/adjacency SIDs for MPLS-SR

4.  Scalable Resource Reservation

   SDF must work with some resource reservation mechanisms, that can
   fulfill the role of the Scalable Resource Reservation (SRR).  This
   resource reservation guarantees the necessary network resources when

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   deterministic flows are scheduled including the slots through which
   the traffic travels hop-by-hop.  Network nodes have to record how
   many network resources are reserved for a specific flow from when it
   starts to when it ends (e.g., <flow_identifier, reserved_resource,
   start_time, end_time>).  Maintaining per-flow resource reservation
   state may be acceptable to edge nodes, but un-acceptable to core
   nodes.  [draft-ietf-detnet-architecture] pointed out that aggregation
   must be supported for scalability.

   SRR aggregates per-flow resource reservation states for each time

   1.  Dividing time into time slots.  Then the per-flow resource
       reservation states can be expressed as <flow_identifier,
       reserved_resource, start_time_slot, end_time_slot> accordingly.
       Note that time slot here is irrelevant to the cycle in SDF.

   2.  Edge node still maintains per-flow resource reservation states.
       While core node calculates and maintains the sum of
       reserved_resources (or remaining resources) of each time slot.
       That is a core node just needs to maintain a variable for each
       time slot.  Suppose that a core node can maintain K time slots'
       results, i.e., the aggregated resource reservation window of a
       core node is K.

   3.  New resource reservation request succeed only if there are
       sufficient resources along the path.  Resource is reserved in
       unit of time slot, and at most K time slots.  If more than K time
       slots' resources are needed, edge node/host can send renewal
       request before the expiration of K time slots.  Edge node/host
       also can active teardown the resource reservation along the path.

   4.  Core nodes refresh their aggregated resource reservation windows
       according to the per-flow resource reservation states maintained
       by edge nodes.

5.  Performance Analysis

5.1.  Queueing Delay

   We consider forwarding from an LDN node A via an LDN node B to an LDN
   node C and call the single-hop LDN delay the time between a packet
   being sent by A and the time it is re-sent by B.  This single-hop
   delay is composed from the A->B propagation delay and the single-hop
   queuing delay A->B.

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                        |cycle x |
                 Node A +-------\+
                                   |\ cycle y|cycle y+1|
                 Node B            +V--------+--------\+
                                   :                   \
                                   :   Queueing Delay  :\
                                   :...=2*T ............ V

                    Figure 6: Single-Hop Queueing Delay

   As Figure 6 shows, cycle x of Node A will be mapped into cycle y+1 of
   Node B as long as the last packet sent from A->B is received within
   the cycle y.  If the last packet is re-sent out by B at the end of
   cycle y+1, then the largest single-hop queueing delay is 2*T.
   Therefore the end-to-end queueing delay's upper bound is 2*T*H, where
   H is the number of hops.

   If A did not forward the LDN packet from a prior LDN forwarder but is
   the actual traffic source, then the packet may have been delayed by a
   gate function before it was sent to B.  The delay of this function is
   outside of scope for the LDN delay considerations.  If B is not
   forwarding the LDN packet but the final receiver, then the packet may
   not need to be queued and released in the same fashion to the
   receiver as it would be queued/released to a downstream LDN node, so
   if a path has one source followed by N LDN forwarders followed by one
   receivers, this should be considered to be a path with N-1 LDN hops
   for the purpose of latency and jitter calculations.

5.2.  Jitter

   Considering the simplest scenario one hop forwarding at first,
   suppose Node A is the upstream node of Node B, the packet sent from
   Node A at cycle x will be received by Node B at cycle y as Figure 7

      - The best situation is Node A sends packet at the end of cycle x,
      and Node B receives packet at the beginning of cycle y, then the
      delay is denoted by w;

      - The worst situation is Node A sends packet at the beginning of
      cycle x, and Node B receives packet at the end of cycle y, then
      the delay= w + length of cycle x + length of cycle y= w+2*T;

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      - Hence the jitter's upper bound of this simplest scenario= worst
      case-best case=2*T.

             |cycle x |                        |cycle x |
      Node A +-------\+                 Node A +\-------+
                     :\                           \     :
                     : \                           -------------\
                     :  \                               :        \
                     :w |\        |                     :w|       \ |
      Node B         :  +V--------+     Node B          : +--------V+
                          cycle y                           cycle y

          (a) best situation               (b) worst situation

             Figure 7: Jitter Analysis for One Hop Forwarding

   Next considering two hops forwarding as Figure 8 shows.

      - The best situation is Node A sends packet at the end of cycle x,
      and Node C receives packet at the beginning of cycle z, then the
      delay is denoted by w';

      - The worst situation is Node A sends packet at the beginning of
      cycle x, and Node C receives packet at the end of cycle z, then
      the delay= w' + length of cycle x + length of cycle z= w'+2*T;

      - Hence the jitter's upper bound = worst case-best case=2*T.

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                        |cycle x |
                 Node A +-------\+
                                 :\| cycle y |
                 Node B          : \---------+
                                 :  \
                                 :   \--------\
                                 :             \         |
                 Node C          ......w'......+V--------+
                                                 cycle z

                            (a) best situation

                         |cycle x |
                  Node A +\-------+
                           \      :
                            \     : | cycle y |
                  Node B     \    : +---------+
                              \   :
                                  :             |       \ |
                  Node C          :......w'.....+--------V+
                                                  cycle z

                             (b) worst situation

             Figure 8: Jitter Analysis for Two Hops Forwarding

   And so on.  For multi-hop forwarding, the end-to-end delay will
   increase as the number of hops increases, while the delay variation
   (jitter) still does not exceed 2*T.

6.  IANA Considerations

   This document makes no request of IANA.

7.  Security Considerations

   Security issues have been carefully considered in
   [draft-ietf-detnet-security].  More discussion is TBD.

8.  Acknowledgements


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9.  Normative References

              "DetNet Architecture", <https://datatracker.ietf.org/doc/

              "DetNet Data Plane Encapsulation",

              "DetNet Problem Statement",

              "DetNet Security Considerations",

              "DetNet Use Cases", <https://datatracker.ietf.org/doc/

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

              "Scheduled queues, UBS, CQF, and Input Gates",

Authors' Addresses

   Li Qiang (editor)

   Email: qiangli3@huawei.com

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   Bingyang Liu

   Email: liubingyang@huawei.com

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

   Email: tte+ietf@cs.fau.de

   Liang Geng
   China Mobile

   Email: gengliang@chinamobile.com

   Lei Wang
   China Mobile

   Email: wangleiyjy@chinamobile.com

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