Subject: [Detnet-dp-dt] early text for the architecture (comments welcome)

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Wed, 1 Jun 2016 15:49:03 +0000

4.  DetNet Layers and Services

   The DetNet Quality of Service is obtained by a combination of DetNet
   Service Layer (Det Layer) and DetNet Transport Layer (Net Layer)
   operations.  The Det Layer provides the expected deterministic
   properties and will usually be operated at the very edge of the
   deterministic network or even in the End Systems, whereas the Net
   Layer provides services to the Det Layer and will be operated in
   possibly all the Relay Systems along the path.

      User                -------                     User
      App             /            \                   App
     --------        |   network     -             ---------
      DetNet        |                 |              DetNet
      Service        \             |                Service
      Layer            -----------                    Layer
     -------------------------------------------------------

                  DetNet Transport Layer

     -------------------------------------------------------

                          Figure 2: DetNet Layers

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4.1.  DetNet Service Layer

   The DetNet Service Layer signaling elements are represented below:

   OAM ------------------------------------------> Service Assurance
                                       \
                                        ---------> Traceability

                                        ---------> Duplicate Elimination
                                       /
   Flow  ----- Packet Sequencing -----
   Identification                      \
                                        ---------> In-Order Delivery
                                       /
   TimeStamping ---------------------------------> Jitter Elimination
                                       \
                                        ---------> Obsolete Elimination

                      Figure 3: DetNet Service Layer

   They include:

   OAM:
         Operations, Administration, and Maintenance leverages in-band
         and out-of-and signaling that validates whether the service is
         effectively obtained within QoS constraints.

   Flow Identification:
         Typically a dedicated tagging in the packet, the flow
         identification may also leverage network and/or transport layer
         fields.

   Packet Sequencing:
         Typically a Sequence Number.  There are multiple uses for a
         sequence number in the packets, and some, such as related to
         security, demand that the number never wraps.  For DetNet
         applications, the requirement is to cover the number of packets
         that may be outstanding in the network.

   Time Stamping:
         The precision that is needed for the time stamp depends on the
         application.  In the art, time recovery are usually operated at
         that layer.  A DetNet time stamp should be flexible enough to
         adapt to the requirements of the transported flow.

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   With those signaling elements, the DetNet Service Layer may provide
   the following services:

   Service Assurance:
         This includes the validation of all guaranteed QoS parameters
         in run time.  In some occasions, the validation can only be
         done by piggybacking the instrumentation with the data packets.

   Traceability:
         Specific tagging may be added in the packets for tracing
         implementation or network configuration errors; traceability
         enables to find whether a packet is a replica, which node
         performed the replication, and which segment was intended for
         the replica;

   Duplicate Elimination:
         The level of resilience that is expected from DetNet often
         implies transporting multiple copies of a packet across the
         network.  There comes the need to eliminate the duplicates
         before delivery to the application.  Duplicate Elimination
         typically depends on explicit packet sequencing to recognize
         the copy.  The duplication may also be inferred from other
         information such as the precise time of reception in a
         scheduled network.

   In-Order Delivery:
         In-Order Delivery may be obtained based on either precise time
         stamping or sequence numbering.  Depending on the gap between
         packets, time stamping may require a too high resolution and
         may not be a practical solution, but if it is, then it may
         alleviate the need for having both time and sequence tagged in
         the packet.

   Jitter Elimination:
         Unless the packets are evenly spaced at the source, the jitter
         elimination service requires a precise time stamping on the
         packet.

   Obsolete Elimination:
         In some networks, it is better not to deliver a packet than to
         deliver it with an out-of-bounds latency.  This service also
         requires a precise time stamping on the packet.

4.2.  DetNet Transport Layer

   The DetNet Transport Layer capabilities are represented below.

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                 Clock Synchronization, Precision Time

  ------ - ------ - ------ - ------ - ------ - ------ - ------ - ------

                    -------- Pinned Path ------------> 0 congestion loss
                  /
  Flow Awareness ----------- Duplication  -----------> Remerge
                  \                              /
                    -------- Balancing   -------
                    \
                      ----- Packet Shaping ----------> flow isolation

                     Figure 4: DetNet Transport Layer

   The capabilities related to the DetNet Transport Layer include:

   Clock Synchronization:
         Most of DetNet capabilities require that the nodes share a
         precise sense of time, either by synchronizing clocks on a hop-
         by-hop basis, in a network distributed fashion, or from a
         common time source such as the Global Positioning System (GPS).
         In any case, this specification considers that the clock
         synchronization is actually a service that is external to the
         DetNet Layers.  DetNet should accommodate to various
         synchronization techniques and profiles that are defined to
         solve exchange time in different market segments.

   Flow Awareness:
         The DetNet Transport Layer must be capable to recognize the
         Service Layer tagging that indexes the flows and enables the
         traceability; the tag(s) enables to select the local state that
         indicates the Per-Hop behavior that is expected for each flow;
         DetNet should accommodate to the various flow tagging that are
         defined to signal flows at Layer 2, 3 and 4

   Packet Shaping:
         DetNet generally expects constant bit rate (CBR) flows for
         which precise reservations are place on the network.  Excess or
         malformed traffic may impeach the proper network operation and
         shaping and policing must be performed at ingress to protect
         from chatty sources, as well as along the path to protect from
         misbehaving relays.  DetNet should accommodate to various types
         of shapers that are defined to solve real-world problems in
         different market segments.

   Provided the capabilities above, the DetNet Service Layer may provide
   the following services:

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   Pinned Path:
         Hard guarantees can be provided with the precise knowledge of
         the physical capabilities of the relays in such terms as
         latency, throughput, buffers, timing precision, and queues, and
         if reservations can be placed on these resources to make sure
         they are available when needed.  This implies that the packets
         are routed along a path were the resources are reserved.
         Pinned paths (Section 3.2) are Point-to-point paths or point-
         to-multipoint trees through the network from a source to one or
         more destinations can be established, and DetNet flows assigned
         to follow a particular path or tree.

   Zero Congestion Loss:
         Zero congestion loss (Section 3.1).  Network resources such as
         link bandwidth, buffers, queues, shapers, and scheduled input/
         output slots are assigned in each relay system to the use of a
         specific DetNet flow or class of DetNet flows.  Given a finite
         amount of buffer space, zero congestion loss necessarily
         ensures a bounded end-to-end latency.  Depending on the
         resources employed, a minimum latency, and thus bounded jitter,
         can also be achieved.

   Duplication:
         Packet replication and deletion (Section 3.4) leverages the
         capability at the Net Layer to duplicate packets.  With this
         functionality, End systems and/or relay systems can number
         packets sequentially, replicate them, and later eliminate all
         but one of the replication nodes, at multiple points in the
         network in order to ensure that one (or more) equipment failure
         events still leave at least one path intact for a DetNet flow.
         This function is a "hitless" version of, e.g., the 1+1 linear
         protection in [RFC6372].  That is, instead of switching from
         one flow to the other when a failure of a flow is detected,
         DetNet combines both flows, and performs a packet-by-packet
         selection of which to discard, based on sequence number.

   Balancing:
         A strict usage of pinned paths per reservation may end up
         fragmenting the network to a point that new reservations can
         not be accepted in the network without moving existing
         reservation, which may be disruptive to the application.  An
         alternate option consists in splitting the flows and load
         balancing over multiple paths, in a fashion that is similar to
         Link Aggregation.  The balancing should be sensitive to the
         latencies and throughput of the parallel paths, and even so,
         The Net Layer may deliver packets out of order, in which case
         it may be required that the order is recovered at the DetNet
         Service Layer prior to delivery to the application.  This

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         function may have negative impacts such as end-to-end latency ,
         burstiness, and loss, and it may fail if there is an
         exaggerated separation between the merged flows to a point that
         buffering at the Det Layer cannot compensate.

   Remerge:
         When a flow is split across multiple paths, whether it is for
         reason of Replication or Balancing, there is a need to
         recombine the flows at some point in the network before
         delivery.  The DetNet Service Layer may operate reordering of
         jitter compensation at that point and/or further down the path,
         typically at the edge or in the End node.  The functions of
         Remerge and Elimination may be coupled and operated at multiple
         points of a lossy network.

   Flow Isolation:
         A deterministic flow must traverse the network with a
         predictable pattern regardless of the load of the network.
         Isolation protects the network against misbehaving DetNet
         flows, and it protects any individual DetNet flow against
         variations of network load.  Classical techniques can prevent
         flows from leaking into one others, but DetNet leverages a new
         generation of shapers to guarantee that there can be no
         observable influence from other network activities onto any
         given deterministic flow.