Internet Engineering Task Force (IETF) B. Varga, Ed.

Internet Engineering Task Force (IETF) B. Varga, Ed.
Request for Comments: 8938 J. Farkas
Category: Informational Ericsson
ISSN: 2070-1721 L. Berger
LabN Consulting, L.L.C.
A. Malis
Malis Consulting
S. Bryant
Futurewei Technologies
November 2020

Deterministic Networking (DetNet) Data Plane Framework

Abstract

This document provides an overall framework for the Deterministic
Networking (DetNet) data plane. It covers concepts and
considerations that are generally common to any DetNet data plane
specification. It describes related Controller Plane considerations
as well.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8938.

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Table of Contents

1. Introduction
2. Terminology
2.1. Terms Used in This Document
2.2. Abbreviations
3. Overview of the DetNet Data Plane
3.1. Data Plane Characteristics
3.1.1. Data Plane Technology
3.1.2. Encapsulation
3.2. DetNet-Specific Metadata
3.3. DetNet IP Data Plane
3.4. DetNet MPLS Data Plane
3.5. Further DetNet Data Plane Considerations
3.5.1. Functions Provided on a Per-Flow Basis
3.5.2. Service Protection
3.5.3. Aggregation Considerations
3.5.4. End-System-Specific Considerations
3.5.5. Sub-network Considerations
4. Controller Plane (Management and Control) Considerations
4.1. DetNet Controller Plane Requirements
4.2. Generic Controller Plane Considerations
4.2.1. Flow Aggregation Control
4.2.2. Explicit Routes
4.2.3. Contention Loss and Jitter Reduction
4.2.4. Bidirectional Traffic
4.3. Packet Replication, Elimination, and Ordering Functions
(PREOF)
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

DetNet (Deterministic Networking) provides the ability to carry
specified unicast or multicast data flows for real-time applications
with extremely low packet loss rates and assured maximum end-to-end
delivery latency. A description of the general background and
concepts of DetNet can be found in [RFC8655].

This document describes the concepts needed by any DetNet data plane
specification (i.e., the DetNet-specific use of packet header fields)
and provides considerations for any corresponding implementation. It
covers the building blocks that provide the DetNet service, the
DetNet service sub-layer, and the DetNet forwarding sub-layer
functions as described in the DetNet architecture [RFC8655].

The DetNet architecture models the DetNet-related data plane
functions as being decomposed into two sub-layers: a service
sub-layer and a forwarding sub-layer. The service sub-layer is used
to provide DetNet service protection and reordering. The forwarding
sub-layer leverages traffic engineering mechanisms and provides
congestion protection (low loss, assured latency, and limited out-of-
order delivery). A particular forwarding sub-layer may have
capabilities that are not available on other forwarding sub-layers.
DetNet makes use of the existing forwarding sub-layers with their
respective capabilities and does not require 1:1 equivalence between
different forwarding sub-layer capabilities.

As part of the service sub-layer functions, this document describes
typical DetNet node data plane operation. It describes the
functionality and operation of the Packet Replication Function (PRF),
the Packet Elimination Function (PEF), and the Packet Ordering
Function (POF) within the service sub-layer. Furthermore, it
describes the forwarding sub-layer.

As defined in [RFC8655], DetNet flows may be carried over network
technologies that can provide service characteristics required by
DetNet. For example, DetNet MPLS flows can be carried over IEEE
802.1 Time-Sensitive Networking (TSN) sub-networks [IEEE802.1TSNTG].
However, IEEE 802.1 TSN support is not required in DetNet. TSN frame
preemption is an example of a forwarding layer capability that is
typically not replicated in other forwarding technologies. Most of
DetNet's benefits can be gained by running over a data-link layer
that has not been specifically enhanced to support all TSN
capabilities, but for such networks and traffic mixes, delay and
jitter performance may vary due to the forwarding sub-layer's
intrinsic properties.

Different application flows, such as Ethernet or IP, can be mapped on
top of DetNet. DetNet can optionally reuse header information
provided by, or shared with, applications. An example of shared
header fields can be found in [RFC8939].

This document also covers basic concepts related to the Controller
Plane and Operations, Administration, and Maintenance (OAM). Data
plane OAM specifics are out of scope for this document.

2. Terminology

2.1. Terms Used in This Document

This document uses the terminology established in the DetNet
architecture [RFC8655], and it is assumed that the reader is familiar
with that document and its terminology.

2.2. Abbreviations

The following abbreviations are used in this document:

BGP Border Gateway Protocol

CoS Class of Service

d-CW DetNet Control Word

DetNet Deterministic Networking

DN DetNet

GMPLS Generalized Multiprotocol Label Switching

GRE Generic Routing Encapsulation

IPsec IP Security

L2 Layer 2

LSP Label Switched Path

MPLS Multiprotocol Label Switching

OAM Operations, Administration, and Maintenance

PCEP Path Computation Element Communication Protocol

PEF Packet Elimination Function

POF Packet Ordering Function

PREOF Packet Replication, Elimination, and Ordering Functions

PRF Packet Replication Function

PSN Packet Switched Network

QoS Quality of Service

S-Label DetNet "service" label

TDM Time-Division Multiplexing

TSN Time-Sensitive Networking

YANG Yet Another Next Generation

3. Overview of the DetNet Data Plane

This document describes how application flows, or App-flows
[RFC8655], are carried over DetNet networks. The DetNet architecture
[RFC8655] models the DetNet-related data plane functions as
decomposed into two sub-layers: a service sub-layer and a forwarding
sub-layer.

Figure 1, reproduced from [RFC8655], shows a logical DetNet service
with the two sub-layers.

| packets going | ^ packets coming ^
v down the stack v | up the stack |
+-----------------------+ +-----------------------+
| Source | | Destination |
+-----------------------+ +-----------------------+
| Service sub-layer: | | Service sub-layer: |
| Packet sequencing | | Duplicate elimination |
| Flow replication | | Flow merging |
| Packet encoding | | Packet decoding |
+-----------------------+ +-----------------------+
| Forwarding sub-layer: | | Forwarding sub-layer: |
| Resource allocation | | Resource allocation |
| Explicit routes | | Explicit routes |
+-----------------------+ +-----------------------+
| Lower layers | | Lower layers |
+-----------------------+ +-----------------------+
v ^
\_________________________/

Figure 1: DetNet Data Plane Protocol Stack

The DetNet forwarding sub-layer may be directly provided by the
DetNet service sub-layer -- for example, by IP tunnels or MPLS.
Alternatively, an overlay approach may be used in which the packet is
natively carried between key nodes within the DetNet network (say,
between PREOF nodes), and a sub-layer is used to provide the
information needed to reach the next hop in the overlay.

The forwarding sub-layer provides the QoS-related functions needed by
the DetNet flow. It may do this directly through the use of queuing
techniques and traffic engineering methods, or it may do this through
the assistance of its underlying connectivity. For example, it may
call upon Ethernet TSN capabilities defined in IEEE 802.1 TSN
[IEEE802.1TSNTG]. The forwarding sub-layer uses buffer resources for
packet queuing, as well as reservation and allocation of bandwidth
capacity resources.

The service sub-layer provides additional support beyond the
connectivity function of the forwarding sub-layer. See Section 4.3
regarding PREOF. The POF uses sequence numbers added to packets,
enabling a range of packet order protection from simple ordering and
dropping out-of-order packets to more complex reordering of a fixed
number of out-of-order, minimally delayed packets. Reordering
requires buffer resources and has an impact on the delay and jitter
of packets in the DetNet flow.

The method of instantiating each of the layers is specific to the
particular DetNet data plane method, and more than one approach may
be applicable to a given network type.

3.1. Data Plane Characteristics

The data plane has two major characteristics: the technology and the
encapsulation, as discussed below.

3.1.1. Data Plane Technology

The DetNet data plane is provided by the DetNet service and
forwarding sub-layers. The DetNet service sub-layer generally
provides its functions for the DetNet application flows by using or
applying existing standardized headers and/or encapsulations. The
DetNet forwarding sub-layer may provide capabilities leveraging that
same header or encapsulation technology (e.g., DN IP or DN MPLS), or
it may be achieved via other technologies, as shown in Figure 2
below. DetNet is currently defined for operation over packet-
switched (IP) networks or label-switched (MPLS) networks.

3.1.2. Encapsulation

DetNet encodes specific flow attributes (flow identity and sequence
number) in packets. For example, in DetNet IP, zero encapsulation is
used, and no sequence number is available; in DetNet MPLS, DetNet-
specific information may be added explicitly to the packets in the
form of an S-Label and a d-CW [DetNet-MPLS].

The encapsulation of a DetNet flow allows it to be sent over a data
plane technology other than its native type. DetNet uses header
information to perform traffic classification, i.e., identify DetNet
flows, and provide DetNet service and forwarding functions. As
mentioned above, DetNet may add headers, as is the case for DN MPLS,
or may use headers that are already present, as is the case for DN
IP. Figure 2 illustrates some relationships between the components.

+-----+
| TSN |
+-------+ +-+-----+-+
| DN IP | | DN MPLS |
+--+--+----+----+ +-+---+-----+-+
| TSN | DN MPLS | | TSN | DN IP |
+-----+---------+ +-----+-------+

Figure 2: DetNet Service Examples

The use of encapsulation is also required if additional information
(metadata) is needed by the DetNet data plane and either (1) there is
no ability to include it in the client data packet or (2) the
specification of the client data plane does not permit the
modification of the packet to include additional data. An example of
such metadata is the inclusion of a sequence number required by
PREOF.

Encapsulation may also be used to carry or aggregate flows for
equipment with limited DetNet capability.

3.2. DetNet-Specific Metadata

The DetNet data plane can provide or carry the following metadata:

1. Flow-ID

2. Sequence number

The DetNet data plane framework supports a Flow-ID (for
identification of the flow or aggregate flow) and/or a sequence
number (for PREOF) for each DetNet flow. The Flow-ID is used by both
the service and forwarding sub-layers, but the sequence number is
only used by the service layer. Metadata can also be used for OAM
indications and instrumentation of DetNet data plane operation.

Metadata inclusion can be implicit or explicit. Explicit inclusions
involve a dedicated header field that is used to include metadata in
a DetNet packet. In the implicit method, part of an already-existing
header field is used to encode the metadata.

Explicit inclusion of metadata is possible through the use of IP
options or IP extension headers. New IP options are almost
impossible to get standardized or to deploy in an operational network
and will not be discussed further in this text. IPv6 extension
headers are finding popularity in current IPv6 development work,
particularly in connection with Segment Routing of IPv6 (SRv6) and IP
OAM. The design of a new IPv6 extension header or the modification
of an existing one is a technique available in the toolbox of the
DetNet IP data plane designer.

Explicit inclusion of metadata in an IP packet is also possible
through the inclusion of an MPLS label stack and the MPLS d-CW, using
one of the methods for carrying MPLS over IP
[DetNet-MPLS-over-UDP-IP]. This is described in more detail in
Section 3.5.5.

Implicit metadata in IP can be included through the use of the
network programming paradigm [SRv6-Network-Prog], in which the suffix
of an IPv6 address is used to encode additional information for use
by the network of the receiving host.

An MPLS example of explicit metadata is the sequence number used by
PREOF, or even the case where all the essential information is
included in the DetNet-over-MPLS label stack (the d-CW and the DetNet
S-Label).

3.3. DetNet IP Data Plane

An IP data plane may operate natively or through the use of an
encapsulation. Many types of IP encapsulation can satisfy DetNet
requirements, and it is anticipated that more than one encapsulation
may be deployed -- for example, GRE, IPsec.

One method of operating an IP DetNet data plane without encapsulation
is to use 6-tuple-based flow identification, where "6-tuple" refers
to information carried in IP-layer and higher-layer protocol headers.
General background on the use of IP headers and 6-tuples to identify
flows and support QoS can be found in [RFC3670]. The extra field in
the 6-tuple is the DSCP field in the packet. [RFC7657] provides
useful background on differentiated services (Diffserv) and tuple-
based flow identification. DetNet flow aggregation may be enabled
via the use of wildcards, masks, prefixes, and ranges. The operation
of this method is described in detail in [RFC8939].

The DetNet forwarding plane may use explicit route capabilities and
traffic engineering capabilities to provide a forwarding sub-layer
that is responsible for providing resource allocation and explicit
routes. It is possible to include such information in a native IP
packet either explicitly or implicitly.

3.4. DetNet MPLS Data Plane

MPLS provides a forwarding sub-layer for traffic over implicit and
explicit paths to the point in the network where the next DetNet
service sub-layer action needs to take place. It does this through
the use of a stack of one or more labels with various forwarding
semantics.

MPLS also provides the ability to identify a service instance that is
used to process the packet through the use of a label that maps the
packet to a service instance.

In cases where metadata is needed to process an MPLS-encapsulated
packet at the service sub-layer, the d-CW [DetNet-MPLS] can be used.
Although such d-CWs are frequently 32 bits long, there is no
architectural constraint on the size of this structure -- only the
requirement that it be fully understood by all parties operating on
it in the DetNet service sub-layer. The operation of this method is
described in detail in [DetNet-MPLS].

3.5. Further DetNet Data Plane Considerations

This section provides informative considerations related to providing
DetNet service to flows that are identified based on their header
information.

3.5.1. Functions Provided on a Per-Flow Basis

At a high level, the following functions are provided on a per-flow
basis.

3.5.1.1. Reservation and Allocation of Resources

Resources might be reserved in order to make them available for
allocation to specific DetNet flows. This can eliminate packet
contention and packet loss for DetNet traffic. This also can reduce
jitter for DetNet traffic. Resources allocated to a DetNet flow
protect it from other traffic flows. On the other hand, it is
assumed that DetNet flows behave in accordance with the reserved
traffic profile. It must be possible to detect misbehaving DetNet
flows and to ensure that they do not compromise QoS of other flows.
Queuing, policing, and shaping policies can be used to ensure that
the allocation of resources reserved for DetNet is met.

3.5.1.2. Explicit Routes

A flow can be routed over a specific, precomputed path. This allows
control of network delay by steering the packet with the ability to
influence the physical path. Explicit routes complement reservation
by ensuring that a consistent path can be associated with its
resources for the duration of that path. Coupled with the traffic
mechanism, this limits misordering and bounds latency. Explicit
route computation can encompass a wide set of constraints and can
optimize the path for a certain characteristic, e.g., highest
bandwidth or lowest jitter. In these cases, the "best" path for any
set of characteristics may not be a shortest path. The selection of
the path can take into account multiple network metrics. Some of
these metrics are measured and distributed by the routing system as
traffic engineering metrics.

3.5.1.3. Service Protection

Service protection involves the use of multiple packet streams using
multiple paths -- for example, 1+1 or 1:1 linear protection. For
DetNet, this primarily relates to packet replication and elimination
capabilities. MPLS offers a number of protection schemes. MPLS
hitless protection can be used to switch traffic to an already-
established path in order to restore delivery rapidly after a
failure. Path changes, even in the case of failure recovery, can
lead to the out-of-order delivery of data requiring POFs either
within the DetNet service or at a high layer in the application
traffic. Establishment of new paths after a failure is out of scope
for DetNet services.

3.5.1.4. Network Coding

Network Coding [nwcrg], not to be confused with network programming,
comprises several techniques where multiple data flows are encoded.
These resulting flows can then be sent on different paths. The
encoding operation can combine flows and error recovery information.
When the encoded flows are decoded and recombined, the original flows
can be recovered. Note that Network Coding uses an alternative to
packet-by-packet PREOF. Therefore, for certain network topologies
and traffic loads, Network Coding can be used to improve a network's
throughput, efficiency, latency, and scalability, as well as
resilience to partition, attacks, and eavesdropping, as compared to
traditional methods. DetNet could use Network Coding as an
alternative to other means of protection. Network Coding is often
applied in wireless networks and is being explored for other network
types.

3.5.1.5. Load-Sharing

The use of packet-by-packet load-sharing of the same DetNet flow over
multiple paths is not recommended, except for the cases listed above
where PREOF are utilized to improve protection of traffic and
maintain order. Packet-by-packet load-sharing, e.g., via Equal-Cost
Multipath (ECMP) or Unequal-Cost Multipath (UCMP), impacts ordering
and, possibly, jitter.

3.5.1.6. Troubleshooting

DetNet leverages many different forwarding sub-layers, each of which
supports various tools to troubleshoot connectivity -- for example,
identification of misbehaving flows. The DetNet service layer can
leverage existing mechanisms to troubleshoot or monitor flows, such
as those in use by IP and MPLS networks. At the Application layer, a
client of a DetNet service can use existing techniques to detect and
monitor delay and loss.

3.5.1.7. Flow Recognition for Analytics

Network analytics can be inherited from the technologies of the
service and forwarding sub-layers. At the DetNet service edge,
packet and bit counters (e.g., sent, received, dropped, out of
sequence) can be maintained.

3.5.1.8. Correlation of Events with Flows

The provider of a DetNet service may provide other capabilities to
monitor flows, such as more detailed loss statistics and timestamping
of events. Details regarding these capabilities are out of scope for
this document.

3.5.2. Service Protection

Service protection allows DetNet services to increase reliability and
maintain a desired level of service assurance in the case of network
congestion or network failure. DetNet relies on the underlying
technology capabilities for various protection schemes. Protection
schemes enable partial or complete coverage of the network paths and
active protection with combinations of the PRF, PEF, and POF.

3.5.2.1. Linear Service Protection

An example DetNet MPLS network fragment and its packet flow are
illustrated in Figure 3.

1 1.1 1.1 1.2.1 1.2.1 1.2.2
CE1----EN1--------R1-------R2-------R3--------EN2-----CE2
\ 1.2.1 / /
\1.2 /------+ /
+------R4------------------------+
1.2.2

Figure 3: Example of Packet Flow Protected by DetNet

In Figure 3, the numbers are used to identify the instance of a
packet. Packet 1 is the original packet. Packets 1.1 and 1.2 are
two first-generation copies of packet 1, packet 1.2.1 is a second-
generation copy of packet 1.2, and so on. Note that these numbers
never appear in the packet and are not to be confused with sequence
numbers, labels, or any other identifiers that appear in the packet.
They simply indicate the generation number of the original packet so
that its passage through the network fragment can be identified for
the reader.

Customer Equipment device CE1 sends a packet into the DetNet-enabled
network. This is packet 1. Edge Node EN1 encapsulates the packet as
a DetNet packet and sends it to Relay Node R1 (packet 1.1). EN1
makes a copy of the packet (1.2), encapsulates it, and sends this
copy to Relay Node R4.

Note that R1 may be directly attached to EN1, or there may be one or
more nodes on the path that, for clarity, are not shown in Figure 3.
The same holds true for any other path between two DetNet entities as
shown in the figure.

Relay Node R4 has been configured to send one copy of the packet to
Relay Node R2 (packet 1.2.1) and one copy to Edge Node EN2 (packet
1.2.2).

R2 receives packet copy 1.2.1 before packet copy 1.1 arrives and,
having been configured to perform packet elimination on this DetNet
flow, forwards packet 1.2.1 to Relay Node R3. Packet copy 1.1 is of
no further use and so is discarded by R2.

Edge Node EN2 receives packet copy 1.2.2 from R4 before it receives
packet copy 1.2.1 from R2 via Relay Node R3. EN2 therefore strips
any DetNet encapsulation from packet copy 1.2.2 and forwards the
packet to CE2. When EN2 receives packet copy 1.2.1 later on, the
copy is discarded.

The above is of course illustrative of many network scenarios that
can be configured.

This example also illustrates a 1:1 protection scheme, meaning there
is traffic over each segment of the end-to-end path. Local DetNet
relay nodes determine which packets are eliminated and which packets
are forwarded. A 1+1 scheme where only one path is used for traffic
at a time could use the same topology. In this case, there is no
PRF, and traffic is switched upon detection of failure. An OAM
scheme that monitors the paths to detect the loss of a path or
traffic is required to initiate the switch. A POF may still be used
in this case to prevent misordering of packets. In both cases, the
protection paths are established and maintained for the duration of
the DetNet service.

3.5.2.2. Path Differential Delay

In the preceding example, proper operation of duplicate elimination
and the reordering of packets are dependent on the number of out-of-
order packets that can be buffered and the difference in delay of the
arriving packets. DetNet uses flow-specific requirements (e.g.,
maximum number of out-of-order packets, maximum latency of the flow)
for configuration of POF-related buffers. If the differential delay
between paths is excessively large or there is excessive misordering
of the packets, then packets may be dropped instead of being
reordered. Likewise, the PEF uses the sequence number to identify
duplicate packets, and large differential delays combined with high
numbers of packets may exceed the PEF's ability to work properly.

3.5.2.3. Ring Service Protection

Ring protection may also be supported if the underlying technology
supports it. Many of the same concepts apply; however, rings are
normally 1+1 protection for data efficiency reasons. [RFC8227]
provides an example of an MPLS Transport Profile (MPLS-TP) data plane
that supports ring protection.

3.5.3. Aggregation Considerations

The DetNet data plane also allows for the aggregation of DetNet
flows, which can improve scalability by reducing the per-hop state.
How this is accomplished is data plane or control plane dependent.
When DetNet flows are aggregated, transit nodes provide service to
the aggregate and not on a per-DetNet-flow basis. When aggregating
DetNet flows, the flows should be compatible, i.e., the same or very
similar QoS and CoS characteristics. In this case, nodes performing
aggregation will ensure that per-flow service requirements are
achieved.

If bandwidth reservations are used, the reservation should be the sum
of all the individual reservations; in other words, the reservations
should not add up to an oversubscription of bandwidth reservation.
If maximum delay bounds are used, the system should ensure that the
aggregate does not exceed the delay bounds of the individual flows.

When an encapsulation is used, the choice of reserving a maximum
resource level and then tracking the services in the aggregated
service or adjusting the aggregated resources as the services are
added is implementation and technology specific.

DetNet flows at edges must be able to handle rejection to an
aggregation group due to lack of resources as well as conditions
where requirements are not satisfied.

3.5.3.1. IP Aggregation

IP aggregation has both data plane and Controller Plane aspects. For
the data plane, flows may be aggregated for treatment based on shared
characteristics such as 6-tuple [RFC8939]. Alternatively, an IP
encapsulation may be used to tunnel an aggregate number of DetNet
flows between relay nodes.

3.5.3.2. MPLS Aggregation

MPLS aggregation also has data plane and Controller Plane aspects.
MPLS flows are often tunneled in a forwarding sub-layer, under the
reservation associated with that MPLS tunnel.

3.5.4. End-System-Specific Considerations

Data flows requiring DetNet service are generated and terminated on
end systems. Encapsulation depends on the application and its
preferences. For example, in a DetNet MPLS domain, the sub-layer
functions use the d-CWs, S-Labels, and F-Labels [DetNet-MPLS] to
provide DetNet services. However, an application may exchange
further flow-related parameters (e.g., timestamps) that are not
provided by DetNet functions.

As a general rule, DetNet domains are capable of forwarding any
DetNet flows, and the DetNet domain does not mandate the
encapsulation format for end systems or edge nodes. Unless some form
of proxy is present, end systems peer with similar end systems using
the same application encapsulation format. For example, as shown in
Figure 4, IP applications peer with IP applications, and Ethernet
applications peer with Ethernet applications.

+-----+
| X | +-----+
+-----+ | X |
| Eth | ________ +-----+
+-----+ _____ / \ | Eth |
\ / \__/ \___ +-----+
\ / \ /
0======== tunnel-1 ========0_
| \
\ |
0========= tunnel-2 =========0
/ \ __/ \
+-----+ \__ DetNet MPLS domain / \
| X | \ __ / +-----+
+-----+ \_______/ \_____/ | X |
| IP | +-----+
+-----+ | IP |
+-----+

Figure 4: End Systems and the DetNet MPLS Domain

3.5.5. Sub-network Considerations

Any of the DetNet service types may be transported by another DetNet
service. MPLS nodes may be interconnected by different sub-network
technologies, which may include point-to-point links. Each of these
sub-network technologies needs to provide appropriate service to
DetNet flows. In some cases, e.g., on dedicated point-to-point links
or TDM technologies, all that is required is for a DetNet node to
appropriately queue its output traffic. In other cases, DetNet nodes
will need to map DetNet flows to the flow semantics (i.e.,
identifiers) and mechanisms used by an underlying sub-network
technology. Figure 5 shows several examples of sub-network
encapsulations that can be used to carry DetNet MPLS flows over
different sub-network technologies. L2 represents a generic Layer 2
encapsulation that might be used on a point-to-point link. TSN
represents the encapsulation used on an IEEE 802.1 TSN network, as
described in [DetNet-MPLS-over-TSN]. UDP/IP represents the
encapsulation used on a DetNet IP PSN, as described in
[DetNet-MPLS-over-UDP-IP].

+------+ +------+ +------+
App-flow | X | | X | | X |
+-----+======+--+======+--+======+-----+
DetNet-MPLS | d-CW | | d-CW | | d-CW |
+------+ +------+ +------+
|Labels| |Labels| |Labels|
+-----+======+--+======+--+======+-----+
Sub-network | L2 | | TSN | | UDP |
+------+ +------+ +------+
| IP |
+------+
| L2 |
+------+

Figure 5: Example DetNet MPLS Encapsulations in Sub-networks

4. Controller Plane (Management and Control) Considerations

4.1. DetNet Controller Plane Requirements

The Controller Plane corresponds to the aggregation of the Control
and Management Planes discussed in [RFC7426] and [RFC8655]. While
more details regarding any DetNet Controller Plane are out of scope
for this document, there are particular considerations and
requirements for the Controller Plane that result from the unique
characteristics of the DetNet architecture and data plane as defined
herein.

The primary requirements of the DetNet Controller Plane are that it
must be able to:

* Instantiate DetNet flows in a DetNet domain (which may, for
example, include some or all of the following: explicit path
determination, link bandwidth reservations, restricting flows to
IEEE 802.1 TSN links, node buffer and other resource reservations,
specification of required queuing disciplines along the path,
ability to manage bidirectional flows, etc.) as needed for a flow.

* In the case of MPLS, manage DetNet S-Label and F-Label allocation
and distribution. In cases where the DetNet MPLS encapsulation is
being used, see [DetNet-MPLS].

* Support DetNet flow aggregation.

* Advertise static and dynamic node and link resources such as
capabilities and adjacencies to other network nodes (for dynamic
signaling approaches) or to network controllers (for centralized
approaches).

* Scale to handle the number of DetNet flows expected in a domain
(which may require per-flow signaling or provisioning).

* Provision flow identification information at each of the nodes
along the path. Flow identification may differ, depending on the
location in the network and the DetNet functionality (e.g.,
transit node vs. relay node).

These requirements, as stated earlier, could be satisfied using
distributed control protocol signaling (such as RSVP-TE), centralized
network management provisioning mechanisms (BGP, PCEP, YANG,
[DetNet-Flow-Info], etc.), or hybrid combinations of the two, and
could also make use of MPLS-based segment routing.

In the abstract, the results of either distributed signaling or
centralized provisioning are equivalent from a DetNet data plane
perspective -- flows are instantiated, explicit routes are
determined, resources are reserved, and packets are forwarded through
the domain using the DetNet data plane.

However, from a practical and implementation standpoint, Controller
Plane alternatives are not equivalent at all. Some approaches are
more scalable than others in terms of signaling load on the network.
Some alternatives can take advantage of global tracking of resources
in the DetNet domain for better overall network resource
optimization. Some solutions are more resilient than others if link,
node, or management equipment failures occur. While a detailed
analysis of the control plane alternatives is out of scope for this
document, the requirements from this document can be used as the
basis of a future analysis of the alternatives.

4.2. Generic Controller Plane Considerations

This section covers control plane considerations that are independent
of the data plane technology used for DetNet service delivery.

While the management plane and the control plane are traditionally
considered separately, from a data plane perspective, there is no
practical difference based on the origin of flow-provisioning
information, and the DetNet architecture [RFC8655] refers to these
collectively as the "Controller Plane". This document therefore does
not distinguish between information provided by distributed control
plane protocols (e.g., RSVP-TE [RFC3209] [RFC3473]) or centralized
network management mechanisms (e.g., RESTCONF [RFC8040], YANG
[RFC7950], PCEP [PCECC]), or any combination thereof. Specific
considerations and requirements for the DetNet Controller Plane are
discussed in Section 4.1.

Each respective data plane document also covers the control plane
considerations for that technology. For example, [RFC8939] also
covers IP control plane normative considerations, and [DetNet-MPLS]
also covers MPLS control plane normative considerations.

4.2.1. Flow Aggregation Control

Flow aggregation means that multiple App-flows are served by a single
new DetNet flow. There are many techniques to achieve aggregation.
For example, in the case of IP, IP flows that share 6-tuple
attributes or flow identifiers at the DetNet sub-layer can be
grouped. Another example includes aggregation accomplished through
the use of hierarchical LSPs in MPLS and tunnels.

Control of aggregation involves a set of procedures listed here.
Aggregation may use some or all of these capabilities, and the order
may vary:

Traffic engineering resource collection and distribution:
Available resources are tracked through control plane or
management plane databases and distributed amongst controllers or
nodes that can manage resources.

Path computation and resource allocation:
When DetNet services are provisioned or requested, one or more
paths meeting the requirements are selected and the resources
verified and recorded.

Resource assignment and data plane coordination:
The assignment of resources along the path depends on the
technology and includes assignment of specific links, coordination
of queuing, and other traffic management capabilities such as
policing and shaping.

Assigned resource recording and updating:
Depending on the specific technology, the assigned resources are
updated and distributed in the databases, preventing
oversubscription.

4.2.2. Explicit Routes

Explicit routes are used to ensure that packets are routed through
the resources that have been reserved for them and hence provide the
DetNet application with the required service. A requirement for the
DetNet Controller Plane will be the ability to assign a particular
identified DetNet IP flow to a path through the DetNet domain that
has been assigned the required per-node resources. This provides the
appropriate traffic treatment for the flow and also includes
particular links as a part of the path that are able to support the
DetNet flow. For example, by using IEEE 802.1 TSN links (as
discussed in [DetNet-MPLS-over-TSN]), DetNet parameters can be
maintained. Further considerations and requirements for the DetNet
Controller Plane are discussed in Section 4.1.

Whether configuring, calculating, and instantiating these routes is a
single-stage or multi-stage process, or is performed in a centralized
or distributed manner, is out of scope for this document.

There are several approaches that could be used to provide explicit
routes and resource allocation in the DetNet forwarding sub-layer.
For example:

* The path could be explicitly set up by a controller that
calculates the path and explicitly configures each node along that
path with the appropriate forwarding and resource allocation
information.

* The path could use a distributed control plane such as RSVP
[RFC2205] or RSVP-TE [RFC3473] extended to support DetNet IP
flows.

* The path could be implemented using IPv6-based segment routing
when extended to support resource allocation.

See Section 4.1 for further discussion of these alternatives. In
addition, [RFC2386] contains useful background information on QoS-
based routing, and [RFC5575] (which will be updated by
[Flow-Spec-Rules]) discusses a specific mechanism used by BGP for
traffic flow specification and policy-based routing.

4.2.3. Contention Loss and Jitter Reduction

This document does not specify the mechanisms needed to eliminate
packet contention or packet loss or to reduce jitter for DetNet flows
at the DetNet forwarding sub-layer. The ability to manage node and
link resources to be able to provide these functions is a necessary
part of the DetNet Controller Plane. It is also necessary to be able
to control the required queuing mechanisms used to provide these
functions along a flow's path through the network. See [RFC8939] and
Section 4.1 for further discussion of these requirements. Some forms
of protection may minimize packet loss or change jitter
characteristics in the cases where packets are reordered when out-of-
order packets are received at the service sub-layer.

4.2.4. Bidirectional Traffic

In many cases, DetNet flows can be considered unidirectional and
independent. However, there are cases where the DetNet service
requires bidirectional traffic from a DetNet application service
perspective. IP and MPLS typically treat each direction separately
and do not force interdependence of each direction. The IETF MPLS
Working Group has studied bidirectional traffic requirements. The
definitions provided in [RFC5654] are useful to illustrate terms such
as associated bidirectional flows and co-routed bidirectional flows.
MPLS defines a point-to-point associated bidirectional LSP as
consisting of two unidirectional point-to-point LSPs, one from A to B
and the other from B to A, which are regarded as providing a single
logical bidirectional forwarding path. This is analogous to standard
IP routing. MPLS defines a point-to-point co-routed bidirectional
LSP as an associated bidirectional LSP that satisfies the additional
constraint that its two unidirectional component LSPs follow the same
path (in terms of both nodes and links) in both directions. An
important property of co-routed bidirectional LSPs is that their
unidirectional component LSPs share fate. In both types of
bidirectional LSPs, resource reservations may differ in each
direction. The concepts of associated bidirectional flows and
co-routed bidirectional flows can also be applied to DetNet IP flows.

While the DetNet IP data plane must support bidirectional DetNet
flows, there are no special bidirectional features with respect to
the data plane other than the need for the two directions of a
co-routed bidirectional flow to take the same path. That is to say,
bidirectional DetNet flows are solely represented at the management
plane and control plane levels, without specific support or knowledge
within the DetNet data plane. Fate-sharing and associated or
co-routed bidirectional flows can be managed at the control level.

DetNet's use of PREOF may increase the complexity of using co-routing
bidirectional flows, because if PREOF are used, the replication
points in one direction would have to match the elimination points in
the other direction, and vice versa. In such cases, the optimal
points for these functions in one direction may not match the optimal
points in the other, due to network and traffic constraints.
Furthermore, due to the per-packet service protection nature,
bidirectional forwarding may not be ensured. The first packet of
received member flows is selected by the elimination function
independently of which path it has taken through the network.

Control and management mechanisms need to support bidirectional
flows, but the specification of such mechanisms is out of scope for
this document. Example control plane solutions for MPLS can be found
in [RFC3473], [RFC6387], and [RFC7551]. These requirements are
included in Section 4.1.

4.3. Packet Replication, Elimination, and Ordering Functions (PREOF)

The Controller Plane protocol solution required for managing the
processing of PREOF is outside the scope of this document. That
said, it should be noted that the ability to determine, for a
particular flow, optimal packet replication and elimination points in
the DetNet domain requires explicit support. There may be existing
capabilities that can be used or extended -- for example, GMPLS end-
to-end recovery [RFC4872] and GMPLS segment recovery [RFC4873].

5. Security Considerations

Security considerations for DetNet are described in detail in
[DetNet-Security]. General security considerations for the DetNet
architecture are described in [RFC8655]. This section considers
architecture-level DetNet security considerations applicable to all
data planes.

Part of what makes DetNet unique is its ability to provide specific
and reliable QoS (delivering data flows with extremely low packet
loss rates and bounded end-to-end delivery latency), and the
security-related aspects of protecting that QoS are similarly unique.

As for all communications protocols, the primary consideration for
the data plane is to maintain integrity of data and delivery of the
associated DetNet service traversing the DetNet network. Application
flows can be protected through whatever means is provided by the
underlying technology. For example, encryption may be used, such as
that provided by IPsec [RFC4301] for IP flows and/or by an underlying
sub-network using MACsec [IEEE802.1AE-2018] for Ethernet (Layer 2)
flows.

At the management and control levels, DetNet flows are identified on
a per-flow basis, which may provide Controller Plane attackers with
additional information about the data flows (when compared to
Controller Planes that do not include per-flow identification). This
is an inherent property of DetNet that has security implications that
should be considered when determining if DetNet is a suitable
technology for any given use case.

To provide uninterrupted availability of the DetNet service,
provisions can be made against DoS attacks and delay attacks. To
protect against DoS attacks, excess traffic due to malicious or
malfunctioning devices can be prevented or mitigated -- for example,
through the use of existing mechanisms such as policing and shaping
applied at the input of a DetNet domain. To prevent DetNet packets
from being delayed by an entity external to a DetNet domain, DetNet
technology definitions can allow for the mitigation of man-in-the-
middle attacks -- for example, through the use of authentication and
authorization of devices within the DetNet domain.

In order to prevent or mitigate DetNet attacks on other networks via
flow escape, edge devices can, for example, use existing mechanisms
such as policing and shaping applied at the output of a DetNet
domain.

6. IANA Considerations

This document has no IANA actions.

7. References

7.1. Normative References

[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>.

7.2. Informative References

[DetNet-Flow-Info]
Varga, B., Farkas, J., Cummings, R., Jiang, Y., and D.
Fedyk, "DetNet Flow Information Model", Work in Progress,
Internet-Draft, draft-ietf-detnet-flow-information-model-
11, 21 October 2020, <https://tools.ietf.org/html/draft-
ietf-detnet-flow-information-model-11>.

[DetNet-MPLS]
Varga, B., Ed., Farkas, J., Berger, L., Malis, A., Bryant,
S., and J. Korhonen, "DetNet Data Plane: MPLS", Work in
Progress, Internet-Draft, draft-ietf-detnet-mpls-13, 11
October 2020,
<https://tools.ietf.org/html/draft-ietf-detnet-mpls-13>.

[DetNet-MPLS-over-TSN]
Varga, B., Ed., Farkas, J., Malis, A., and S. Bryant,
"DetNet Data Plane: MPLS over IEEE 802.1 Time Sensitive
Networking (TSN)", Work in Progress, Internet-Draft,
draft-ietf-detnet-mpls-over-tsn-04, 2 November 2020,
<https://tools.ietf.org/html/draft-ietf-detnet-mpls-over-
tsn-04>.

[DetNet-MPLS-over-UDP-IP]
Varga, B., Ed., Farkas, J., Berger, L., Malis, A., and S.
Bryant, "DetNet Data Plane: MPLS over UDP/IP", Work in
Progress, Internet-Draft, draft-ietf-detnet-mpls-over-udp-
ip-07, 11 October 2020, <https://tools.ietf.org/html/
draft-ietf-detnet-mpls-over-udp-ip-07>.

[DetNet-Security]
Grossman, E., Ed., Mizrahi, T., and A. Hacker,
"Deterministic Networking (DetNet) Security
Considerations", Work in Progress, Internet-Draft, draft-
ietf-detnet-security-12, 2 October 2020,
<https://tools.ietf.org/html/draft-ietf-detnet-security-
12>.

[Flow-Spec-Rules]
Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
Bacher, "Dissemination of Flow Specification Rules", Work
in Progress, Internet-Draft, draft-ietf-idr-rfc5575bis-27,
15 October 2020, <https://tools.ietf.org/html/draft-ietf-
idr-rfc5575bis-27>.

[IEEE802.1AE-2018]
IEEE, "IEEE Standard for Local and metropolitan area
networks-Media Access Control (MAC) Security", IEEE Std
802.1AE-2018, DOI 10.1109/IEEESTD.2018.8585421, December
2018, <https://ieeexplore.ieee.org/document/8585421>.

[IEEE802.1TSNTG]
IEEE, "Time-Sensitive Networking (TSN) Task Group",
<https://1.ieee802.org/tsn/>.

[nwcrg] IRTF, "Coding for efficient NetWork Communications
Research Group (nwcrg)",
<https://datatracker.ietf.org/rg/nwcrg/about>.

[PCECC] Li, Z., Peng, S., Negi, M. S., Zhao, Q., and C. Zhou,
"PCEP Procedures and Protocol Extensions for Using PCE as
a Central Controller (PCECC) of LSPs", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-extension-for-pce-
controller-08, 1 November 2020,
<https://tools.ietf.org/html/draft-ietf-pce-pcep-
extension-for-pce-controller-08>.

[RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, DOI 10.17487/RFC2205,
September 1997, <https://www.rfc-editor.org/info/rfc2205>.

[RFC2386] Crawley, E., Nair, R., Rajagopalan, B., and H. Sandick, "A
Framework for QoS-based Routing in the Internet",
RFC 2386, DOI 10.17487/RFC2386, August 1998,
<https://www.rfc-editor.org/info/rfc2386>.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.

[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
DOI 10.17487/RFC3473, January 2003,
<https://www.rfc-editor.org/info/rfc3473>.

[RFC3670] Moore, B., Durham, D., Strassner, J., Westerinen, A., and
W. Weiss, "Information Model for Describing Network Device
QoS Datapath Mechanisms", RFC 3670, DOI 10.17487/RFC3670,
January 2004, <https://www.rfc-editor.org/info/rfc3670>.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.

[RFC4872] Lang, J.P., Ed., Rekhter, Y., Ed., and D. Papadimitriou,
Ed., "RSVP-TE Extensions in Support of End-to-End
Generalized Multi-Protocol Label Switching (GMPLS)
Recovery", RFC 4872, DOI 10.17487/RFC4872, May 2007,
<https://www.rfc-editor.org/info/rfc4872>.

[RFC4873] Berger, L., Bryskin, I., Papadimitriou, D., and A. Farrel,
"GMPLS Segment Recovery", RFC 4873, DOI 10.17487/RFC4873,
May 2007, <https://www.rfc-editor.org/info/rfc4873>.

[RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
and D. McPherson, "Dissemination of Flow Specification
Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,
<https://www.rfc-editor.org/info/rfc5575>.

[RFC5654] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654, DOI 10.17487/RFC5654,
September 2009, <https://www.rfc-editor.org/info/rfc5654>.

[RFC6387] Takacs, A., Berger, L., Caviglia, D., Fedyk, D., and J.
Meuric, "GMPLS Asymmetric Bandwidth Bidirectional Label
Switched Paths (LSPs)", RFC 6387, DOI 10.17487/RFC6387,
September 2011, <https://www.rfc-editor.org/info/rfc6387>.

[RFC7426] Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
Defined Networking (SDN): Layers and Architecture
Terminology", RFC 7426, DOI 10.17487/RFC7426, January
2015, <https://www.rfc-editor.org/info/rfc7426>.

[RFC7551] Zhang, F., Ed., Jing, R., and R. Gandhi, Ed., "RSVP-TE
Extensions for Associated Bidirectional Label Switched
Paths (LSPs)", RFC 7551, DOI 10.17487/RFC7551, May 2015,
<https://www.rfc-editor.org/info/rfc7551>.

[RFC7657] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015,
<https://www.rfc-editor.org/info/rfc7657>.

[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.

[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.

[RFC8227] Cheng, W., Wang, L., Li, H., van Helvoort, H., and J.
Dong, "MPLS-TP Shared-Ring Protection (MSRP) Mechanism for
Ring Topology", RFC 8227, DOI 10.17487/RFC8227, August
2017, <https://www.rfc-editor.org/info/rfc8227>.

[RFC8939] Varga, B., Ed., Farkas, J., Berger, L., Fedyk, D., and S.
Bryant, "Deterministic Networking (DetNet) Data Plane:
IP", RFC 8939, DOI 10.17487/RFC8939, November 2020,
<https://www.rfc-editor.org/info/rfc8939>.

[SRv6-Network-Prog]
Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "SRv6 Network Programming",
Work in Progress, Internet-Draft, draft-ietf-spring-srv6-
network-programming-26, 26 November 2020,
<https://tools.ietf.org/html/draft-ietf-spring-srv6-
network-programming-26>.

Acknowledgements

The authors wish to thank Pat Thaler, Norman Finn, Loa Andersson,
David Black, Rodney Cummings, Ethan Grossman, Tal Mizrahi, David
Mozes, Craig Gunther, George Swallow, Yuanlong Jiang, and Carlos
J. Bernardos for their various contributions to this work.

Contributors

The following people contributed substantially to the content of this
document:

Don Fedyk
Jouni Korhonen

Authors' Addresses

Balázs Varga (editor)
Ericsson
Budapest
Magyar Tudosok krt. 11.
1117
Hungary

Email: [email protected]

János Farkas
Ericsson
Budapest
Magyar Tudosok krt. 11.
1117
Hungary

Email: [email protected]

Lou Berger
LabN Consulting, L.L.C.

Email: [email protected]

Andrew G. Malis
Malis Consulting

Email: [email protected]

Stewart Bryant
Futurewei Technologies

Email: [email protected]