DetNet N. Finn
Internet-Draft Huawei
Intended status: Standards Track P. Thubert
Expires: March 16, 2019 Cisco
B. Varga
J. Farkas
Ericsson
September 12, 2018
Deterministic Networking Architecture
draft-ietf-detnet-architecture-08
Abstract
This document provides the overall Architecture for Deterministic
Networking (DetNet), which provides a capability to carry specified
unicast or multicast data flows for real-time applications with
extremely low data loss rates and bounded latency. Techniques used
include: 1) reserving data plane resources for individual (or
aggregated) DetNet flows in some or all of the intermediate nodes
(e.g., bridges or routers) along the path of the flow; 2) providing
explicit routes for DetNet flows that do not immediately change with
the network topology; and 3) distributing data from DetNet flow
packets over time and/or space to ensure delivery of each packet's
data in spite of the loss of a path. DetNet operates at the IP layer
and delivers service over sub-network technologies such as MPLS and
IEEE 802.1 TSN.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 16, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Terms used in this document . . . . . . . . . . . . . . . 4
2.2. IEEE 802.1 TSN to DetNet dictionary . . . . . . . . . . . 7
3. Providing the DetNet Quality of Service . . . . . . . . . . . 7
3.1. Primary goals defining the DetNet QoS . . . . . . . . . . 7
3.2. Mechanisms to achieve DetNet QoS . . . . . . . . . . . . 9
3.2.1. Congestion protection . . . . . . . . . . . . . . . . 9
3.2.1.1. Eliminate congestion loss . . . . . . . . . . . . 9
3.2.1.2. Jitter Reduction . . . . . . . . . . . . . . . . 10
3.2.2. Service Protection . . . . . . . . . . . . . . . . . 11
3.2.2.1. In-Order Delivery . . . . . . . . . . . . . . . . 11
3.2.2.2. Packet Replication and Elimination . . . . . . . 11
3.2.2.3. Packet encoding for service protection . . . . . 13
3.2.3. Explicit routes . . . . . . . . . . . . . . . . . . . 13
3.3. Secondary goals for DetNet . . . . . . . . . . . . . . . 14
3.3.1. Coexistence with normal traffic . . . . . . . . . . . 15
3.3.2. Fault Mitigation . . . . . . . . . . . . . . . . . . 15
4. DetNet Architecture . . . . . . . . . . . . . . . . . . . . . 16
4.1. DetNet stack model . . . . . . . . . . . . . . . . . . . 16
4.1.1. Representative Protocol Stack Model . . . . . . . . . 16
4.1.2. DetNet Data Plane Overview . . . . . . . . . . . . . 18
4.1.3. Network reference model . . . . . . . . . . . . . . . 20
4.2. DetNet systems . . . . . . . . . . . . . . . . . . . . . 22
4.2.1. End system . . . . . . . . . . . . . . . . . . . . . 22
4.2.2. DetNet edge, relay, and transit nodes . . . . . . . . 23
4.3. DetNet flows . . . . . . . . . . . . . . . . . . . . . . 23
4.3.1. DetNet flow types . . . . . . . . . . . . . . . . . . 23
4.3.2. Source transmission behavior . . . . . . . . . . . . 24
4.3.3. Incomplete Networks . . . . . . . . . . . . . . . . . 25
4.4. Traffic Engineering for DetNet . . . . . . . . . . . . . 25
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4.4.1. The Application Plane . . . . . . . . . . . . . . . . 26
4.4.2. The Controller Plane . . . . . . . . . . . . . . . . 26
4.4.3. The Network Plane . . . . . . . . . . . . . . . . . . 27
4.5. Queuing, Shaping, Scheduling, and Preemption . . . . . . 28
4.6. Service instance . . . . . . . . . . . . . . . . . . . . 29
4.7. Flow identification at technology borders . . . . . . . . 30
4.7.1. Exporting flow identification . . . . . . . . . . . . 30
4.7.2. Flow attribute mapping between layers . . . . . . . . 32
4.7.3. Flow-ID mapping examples . . . . . . . . . . . . . . 33
4.8. Advertising resources, capabilities and adjacencies . . . 35
4.9. Scaling to larger networks . . . . . . . . . . . . . . . 35
4.10. Compatibility with Layer-2 . . . . . . . . . . . . . . . 35
5. Security Considerations . . . . . . . . . . . . . . . . . . . 36
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 36
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
9. Informative References . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
This document provides the overall Architecture for Deterministic
Networking (DetNet), which provides a capability for the delivery of
data flows with extremely low packet loss rates and bounded end-to-
end delivery latency. DetNet operates at the IP layer and delivers
service over sub-network technologies such as MPLS and IEEE 802.1
TSN. DetNet accomplishes these goals by dedicating network resources
such as link bandwidth and buffer space to DetNet flows and/or
classes of DetNet flows, and by replicating packets along multiple
paths. Unused reserved resources are available to non-DetNet
packets.
The Deterministic Networking Problem Statement
[I-D.ietf-detnet-problem-statement] introduces Deterministic
Networking, and Deterministic Networking Use Cases
[I-D.ietf-detnet-use-cases] summarizes the need for it. See
[I-D.ietf-detnet-dp-sol-mpls] and [I-D.ietf-detnet-dp-sol-ip] for
specific techniques that can be used to identify DetNet flows and
assign them to specific paths through a network.
A goal of DetNet is a converged network in all respects. That is,
the presence of DetNet flows does not preclude non-DetNet flows, and
the benefits offered DetNet flows should not, except in extreme
cases, prevent existing QoS mechanisms from operating in a normal
fashion, subject to the bandwidth required for the DetNet flows. A
single source-destination pair can trade both DetNet and non-DetNet
flows. End systems and applications need not instantiate special
interfaces for DetNet flows. Networks are not restricted to certain
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topologies; connectivity is not restricted. Any application that
generates a data flow that can be usefully characterized as having a
maximum bandwidth should be able to take advantage of DetNet, as long
as the necessary resources can be reserved. Reservations can be made
by the application itself, via network management, by an
application's controller, or by other means, e.g., a dynamic control
plane (e.g., [RFC2205]).
Many applications that are intended to be served by Deterministic
Networking require the ability to synchronize the clocks in end
systems to a sub-microsecond accuracy. Some of the queue control
techniques defined in Section 4.5 also require time synchronization
among relay and transit nodes. The means used to achieve time
synchronization are not addressed in this document. DetNet can
accommodate various time synchronization techniques and profiles that
are defined elsewhere to address the needs of different market
segments.
2. Terminology
2.1. Terms used in this document
The following terms are used in the context of DetNet in this
document:
allocation
Resources are dedicated to support a DetNet flow. Depending
on an implementation, the resource may be reused by non-
DetNet flows when it is not used by the DetNet flow.
App-flow
The native format of a DetNet flow.
DetNet destination
An end system capable of terminating a DetNet flow.
DetNet domain
The portion of a network that is DetNet aware. It includes
end systems and other DetNet nodes.
DetNet flow
A DetNet flow is a sequence of packets to which the DetNet
service is to be provided.
DetNet compound flow and DetNet member flow
A DetNet compound flow is a DetNet flow that has been
separated into multiple duplicate DetNet member flows for
service protection at the DetNet service layer. Member flows
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are merged back into a single DetNet compound flow such that
there are no duplicate packets. "Compound" and "member" are
strictly relative to each other, not absolutes; a DetNet
compound flow comprising multiple DetNet member flows can, in
turn, be a member of a higher-order compound.
DetNet intermediate node
A DetNet relay node or transit node.
DetNet edge node
An instance of a DetNet relay node that acts as a source and/
or destination at the DetNet service layer. For example, it
can include a DetNet service layer proxy function for DetNet
service protection (e.g., the addition or removal of packet
sequencing information) for one or more end systems, or
starts or terminates congestion protection at the DetNet
transport layer, or aggregates DetNet services into new
DetNet flows. It is analogous to a Label Edge Router (LER)
or a Provider Edge (PE) router.
DetNet-UNI
User-to-Network Interface with DetNet specific
functionalities. It is a packet-based reference point and
may provide multiple functions like encapsulation, status,
synchronization, etc.
end system
Commonly called a "host" in IETF documents, and an "end
station" is IEEE 802 documents. End systems of interest to
this document are either sources or destinations of DetNet
flows. And end system may or may not be DetNet transport
layer aware or DetNet service layer aware.
link
A connection between two DetNet nodes. It may be composed of
a physical link or a sub-network technology that can provide
appropriate traffic delivery for DetNet flows.
DetNet system
A DetNet aware end system, transit node, or relay node.
"DetNet" may be omitted in some text.
DetNet relay node
A DetNet node including a service layer function that
interconnects different DetNet transport layer paths to
provide service protection. A DetNet relay node can be a
bridge, a router, a firewall, or any other system that
participates in the DetNet service layer. It typically
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incorporates DetNet transport layer functions as well, in
which case it is collocated with a transit node.
PEF A Packet Elimination Function (PEF) eliminates duplicate
copies of packets to prevent excess packets flooding the
network or duplicate packets being sent out of the DetNet
domain. PEF can be implemented by an edge node, a relay
node, or an end system.
PRF A Packet Replication Function (PRF) replicates DetNet flow
packets and forwards them to one or more next hops in the
DetNet domain. The number of packet copies sent to each next
hop is a DetNet flow specific parameter at the node doing the
replication. PRF can be implemented by an edge node, a relay
node, or an end system.
PREOF Collective name for Packet Replication, Elimination, and
Ordering Functions.
POF A Packet Ordering Function (POF) re-orders packets within a
DetNet flow that are received out of order. This function
can be implemented by an edge node, a relay node, or an end
system.
reservation
The set of resources allocated between a source and one or
more destinations through transit nodes and subnets
associated with a DetNet flow, to provide the provisioned
DetNet service.
DetNet service layer
The layer at which A DetNet service, e.g., service protection
is provided.
DetNet service proxy
Maps between App-flows and DetNet flows.
DetNet source
An end system capable of originating a DetNet flow.
DetNet transit node
A node operating at the DetNet transport layer, that utilizes
link layer and/or network layer switching across multiple
links and/or sub-networks to provide paths for DetNet service
layer functions. Typically provides congestion protection
over those paths. An MPLS LSR is an example of a DetNet
transit node.
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DetNet transport layer
The layer that optionally provides congestion protection for
DetNet flows over paths provided by the underlying network.
2.2. IEEE 802.1 TSN to DetNet dictionary
This section also serves as a dictionary for translating from the
terms used by the Time-Sensitive Networking (TSN) Task Group
[IEEE802.1TSNTG] of the IEEE 802.1 WG to those of the DetNet WG.
Listener
The IEEE 802.1 term for a destination of a DetNet flow.
relay system
The IEEE 802.1 term for a DetNet intermediate node.
Stream
The IEEE 802.1 term for a DetNet flow.
Talker
The IEEE 802.1 term for the source of a DetNet flow.
3. Providing the DetNet Quality of Service
3.1. Primary goals defining the DetNet QoS
The DetNet Quality of Service can be expressed in terms of:
o Minimum and maximum end-to-end latency from source to destination;
timely delivery, and bounded jitter (packet delay variation)
derived from these constraints.
o Packet loss ratio, under various assumptions as to the operational
states of the nodes and links.
o An upper bound on out-of-order packet delivery. It is worth
noting that some DetNet applications are unable to tolerate any
out-of-order delivery.
It is a distinction of DetNet that it is concerned solely with worst-
case values for the end-to-end latency, jitter, and misordering.
Average, mean, or typical values are of little interest, because they
do not affect the ability of a real-time system to perform its tasks.
In general, a trivial priority-based queuing scheme will give better
average latency to a data flow than DetNet, but of course, the worst-
case latency can be essentially unbounded.
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Three techniques are used by DetNet to provide these qualities of
service:
o Congestion protection (Section 3.2.1).
o Service protection (Section 3.2.2).
o Explicit routes (Section 3.2.3).
Congestion protection operates by allocating resources along the path
of a DetNet flow, e.g., buffer space or link bandwidth. Congestion
protection greatly reduces, or even eliminates entirely, packet loss
due to output packet congestion within the network, but it can only
be supplied to a DetNet flow that is limited at the source to a
maximum packet size and transmission rate. Note that congestion
protection provided via congestion detection and notification is
explicitly excluded from consideration in DetNet, as it serves a
different set of applications.
Congestion protection addresses two of the DetNet QoS requirements:
latency and packet loss. Given that DetNet nodes have a finite
amount of buffer space, congestion protection necessarily results in
a maximum end-to-end latency. It also addresses the largest
contribution to packet loss, which is buffer congestion.
After congestion, the most important contributions to packet loss are
typically from random media errors and equipment failures. Service
protection is the name for the mechanisms used by DetNet to address
these losses. The mechanisms employed are constrained by the
requirement to meet the users' latency requirements. Packet
replication and elimination (Section 3.2.2) and packet encoding
(Section 3.2.2.3) are described in this document to provide service
protection; others may be found. For instance, packet encoding can
be used to provide service protection against random media errors,
packet replication and elimination can be used to provide service
protection against equipment failures. This mechanism distributes
the contents of DetNet flows over multiple paths in time and/or
space, so that the loss of some of the paths does need not cause the
loss of any packets.
The paths are typically (but not necessarily) explicit routes, so
that they do not normally suffer temporary interruptions caused by
the convergence of routing or bridging protocols.
These three techniques can be applied independently, giving eight
possible combinations, including none (no DetNet), although some
combinations are of wider utility than others. This separation keeps
the protocol stack coherent and maximizes interoperability with
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existing and developing standards in this (IETF) and other Standards
Development Organizations. Some examples of typical expected
combinations:
o Explicit routes plus service protection are exactly the techniques
employed by seamless redundancy mechanisms applied on a ring
topology as described, e.g., in [IEC62439-3-2016]. In this
example, explicit routes are achieved by limiting the physical
topology of the network to a ring. Sequentialization,
replication, and duplicate elimination are facilitated by packet
tags added at the front or the end of Ethernet frames. [RFC8227]
provides another example in the context of MPLS.
o Congestion protection alone is offered by IEEE 802.1 Audio Video
bridging [IEEE802.1BA]. As long as the network suffers no
failures, zero congestion loss can be achieved through the use of
a reservation protocol (MSRP [IEEE802.1Q-2018]), shapers in every
bridge, and proper dimensioning.
o Using all three together gives maximum protection.
There are, of course, simpler methods available (and employed, today)
to achieve levels of latency and packet loss that are satisfactory
for many applications. Prioritization and over-provisioning is one
such technique. However, these methods generally work best in the
absence of any significant amount of non-critical traffic in the
network (if, indeed, such traffic is supported at all), or work only
if the critical traffic constitutes only a small portion of the
network's theoretical capacity, or work only if all systems are
functioning properly, or in the absence of actions by end systems
that disrupt the network's operations.
There are any number of methods in use, defined, or in progress for
accomplishing each of the above techniques. It is expected that this
DetNet Architecture will assist various vendors, users, and/or
"vertical" Standards Development Organizations (dedicated to a single
industry) to make selections among the available means of
implementing DetNet networks.
3.2. Mechanisms to achieve DetNet QoS
3.2.1. Congestion protection
3.2.1.1. Eliminate congestion loss
The primary means by which DetNet achieves its QoS assurances is to
reduce, or even completely eliminate, congestion within a node as a
cause of packet loss. Given that a DetNet flow cannot be throttled,
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this can be achieved only by the provision of sufficient buffer
storage at each hop through the network to ensure that no packets are
dropped due to a lack of buffer storage.
Ensuring adequate buffering requires, in turn, that the source, and
every intermediate node along the path to the destination (or nearly
every node, see Section 4.3.3) be careful to regulate its output to
not exceed the data rate for any DetNet flow, except for brief
periods when making up for interfering traffic. Any packet sent
ahead of its time potentially adds to the number of buffers required
by the next hop and may thus exceed the resources allocated for a
particular DetNet flow.
The low-level mechanisms described in Section 4.5 provide the
necessary regulation of transmissions by an end system or
intermediate node to provide congestion protection. The allocation
of the bandwidth and buffers for a DetNet flow requires provisioning
A DetNet node may have other resources requiring allocation and/or
scheduling, that might otherwise be over-subscribed and trigger the
rejection of a reservation.
3.2.1.2. Jitter Reduction
A core objective of DetNet is to enable the convergence of sensitive
non-IP networks onto a common network infrastructure. This requires
the accurate emulation of currently deployed mission-specific
networks, which for example rely on point-to-point analog (e.g.,
4-20mA modulation) and serial-digital cables (or buses) for highly
reliable, synchronized and jitter-free communications. While the
latency of analog transmissions is basically the speed of light,
legacy serial links are usually slow (in the order of Kbps) compared
to, say, GigE, and some latency is usually acceptable. What is not
acceptable is the introduction of excessive jitter, which may, for
instance, affect the stability of control systems.
Applications that are designed to operate on serial links usually do
not provide services to recover the jitter, because jitter simply
does not exist there. DetNet flows are generally expected to be
delivered in-order and the precise time of reception influences the
processes. In order to converge such existing applications, there is
a desire to emulate all properties of the serial cable, such as clock
transportation, perfect flow isolation and fixed latency. While
minimal jitter (in the form of specifying minimum, as well as
maximum, end-to-end latency) is supported by DetNet, there are
practical limitations on packet-based networks in this regard. In
general, users are encouraged to use, instead of, "do this when you
get the packet," a combination of:
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o Sub-microsecond time synchronization among all source and
destination end systems, and
o Time-of-execution fields in the application packets.
Jitter reduction is provided by the mechanisms described in
Section 4.5 that also provide congestion protection.
3.2.2. Service Protection
Service protection aims to mitigate or eliminate packet loss due to
equipment failures, random media and/or memory faults. These types
of packet loss can be greatly reduced by spreading the data over
multiple disjoint forwarding paths. Various service protection
methods are described in [RFC6372], e.g., 1+1 linear protection.
This section describes the functional details of an additional method
in Section 3.2.2.2, which can be implemented as described in
Section 3.2.2.3 or as specified in [I-D.ietf-detnet-dp-sol-mpls] in
order to provide 1+n hitless protection. The appropriate service
protection mechanism depends on the scenario and the requirements.
3.2.2.1. In-Order Delivery
Out-of-order packet delivery can be a side effect of service
protection. Packets delivered out-of-order impact the amount of
buffering needed at the destination to properly process the received
data. Such packets also influence the jitter of a flow. The DetNet
service includes maximum allowed misordering as a constraint. Zero
misordering would be a valid service constraint to reflect that the
end system(s) of the flow cannot tolerate any out-of-order delivery.
DetNet Packet Ordering Functionality (POF) (Section 3.2.2.2) can be
used to provide in-order delivery.
3.2.2.2. Packet Replication and Elimination
This section describes a service protection method that sends copies
of the same packets over multiple paths.
The DetNet service layer includes the packet replication (PRF), the
packet elimination (PEF), and the packet ordering functionality (POF)
for use in DetNet edge, relay node, and end system packet processing.
Either of these functions can be enabled in a DetNet edge node, relay
node or end system. The collective name for all three functions is
PREOF. The packet replication and elimination service protection
method altogether involves four capabilities:
o Providing sequencing information to the packets of a DetNet
compound flow. This may be done by adding a sequence number or
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time stamp as part of DetNet, or may be inherent in the packet,
e.g., in a transport protocol, or associated to other physical
properties such as the precise time (and radio channel) of
reception of the packet. This is typically done once, at or near
the source.
o The Packet Replication Function (PRF) replicates these packets
into multiple DetNet member flows and typically sends them along
multiple different paths to the destination(s), e.g., over the
explicit routes of Section 3.2.3. The location within a node, and
the mechanism used for the PRF is implementation specific.
o The Packet Elimination Function (PEF) eliminates duplicate packets
of a DetNet flow based on the sequencing information and a history
of received packets. The output of the PEF is always a single
packet. This may be done at any node along the path to save
network resources further downstream, in particular if multiple
Replication points exist. But the most common case is to perform
this operation at the very edge of the DetNet network, preferably
in or near the receiver. The location within a node, and
mechanism used for the PEF is implementation specific.
o The Packet Ordering Function (POF) uses the sequencing information
to re-order a DetNet flow's packets that are received out of
order.
The order in which a node applies PEF, POF, and PRF to a DetNet flow
is implementation specific.
Some service protection mechanisms rely on switching from one flow to
another when a failure of a flow is detected. Contrarily, packet
replication and elimination combines the DetNet member flows sent
along multiple different paths, and performs a packet-by-packet
selection of which to discard, e.g., based on sequencing information.
In the simplest case, this amounts to replicating each packet in a
source that has two interfaces, and conveying them through the
network, along separate (disjoint non-SRLG) paths, to the similarly
dual-homed destinations, that discard the extras. This ensures that
one path (with zero congestion loss) remains, even if some
intermediate node fails. The sequencing information can also be used
for loss detection and for re-ordering.
DetNet relay nodes in the network can provide replication and
elimination facilities at various points in the network, so that
multiple failures can be accommodated.
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This is shown in Figure 1, where the two relay nodes each replicate
(R) the DetNet flow on input, sending the DetNet member flows to both
the other relay node and to the end system, and eliminate duplicates
(E) on the output interface to the right-hand end system. Any one
link in the network can fail, and the DetNet compound flow can still
get through. Furthermore, two links can fail, as long as they are in
different segments of the network.
> > > > > > > > > relay > > > > > > > >
> /------------+ R node E +------------\ >
> / v + ^ \ >
end R + v | ^ + E end
system + v | ^ + system
> \ v + ^ / >
> \------------+ R relay E +-----------/ >
> > > > > > > > > node > > > > > > > >
Figure 1: Packet replication and elimination
Packet replication and elimination does not react to and correct
failures; it is entirely passive. Thus, intermittent failures,
mistakenly created packet filters, or misrouted data is handled just
the same as the equipment failures that are handled by typical
routing and bridging protocols.
If packet replication and elimination is used over paths providing
congestion protection (Section 3.2.1), and member flows that take
different-length paths through the network are combined, a merge
point may require extra buffering to equalize the delays over the
different paths. This equalization ensures that the resultant
compound flow will not exceed its contracted bandwidth even after one
or the other of the paths is restored after a failure. The extra
buffering can be also used to provide in-order delivery.
3.2.2.3. Packet encoding for service protection
There are methods for using multiple paths to provide service
protection that involve encoding the information in a packet
belonging to a DetNet flow into multiple transmission units,
combining information from multiple packets into any given
transmission unit. Such techniques, also known as "network coding",
can be used as a DetNet service protection technique.
3.2.3. Explicit routes
In networks controlled by typical dynamic control protocols such as
IS-IS or OSPF, a network topology event in one part of the network
can impact, at least briefly, the delivery of data in parts of the
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network remote from the failure or recovery event. Even the use of
redundant paths through a network defined, e.g., as defined by
[RFC6372] do not eliminate the chances of packet loss. Furthermore,
out-of-order packet delivery can be a side effect of route changes.
Many real-time networks rely on physical rings of two-port devices,
with a relatively simple ring control protocol. This supports
redundant paths for service protection with a minimum of wiring. As
an additional benefit, ring topologies can often utilize different
topology management protocols than those used for a mesh network,
with a consequent reduction in the response time to topology changes.
Of course, this comes at some cost in terms of increased hop count,
and thus latency, for the typical path.
In order to get the advantages of low hop count and still ensure
against even very brief losses of connectivity, DetNet employs
explicit routes, where the path taken by a given DetNet flow does not
change, at least immediately, and likely not at all, in response to
network topology events. Service protection (Section 3.2.2 or
Section 3.2.2.3) over explicit routes provides a high likelihood of
continuous connectivity. Explicit routes can be established in
various ways, e.g., with RSVP-TE [RFC3209], with Segment Routing (SR)
[RFC8402], via a Software Defined Networking approach [RFC7426], with
IS-IS [RFC7813], etc. Explicit routes are typically used in MPLS TE
LSPs.
Out-of-order packet delivery can be a side effect of distributing a
single flow over multiple paths especially when there is a change
from one path to another when combining the flow. This is
irrespective of the distribution method used, and also applies to
service protection over explicit routes. As described in
Section 3.2.2.1, out-of-order packets influence the jitter of a flow
and impact the amount of buffering needed to process the data;
therefore, DetNet service includes maximum allowed misordering as a
constraint. The use of explicit routes helps to provide in-order
delivery because there is no immediate route change with the network
topology, but the changes are plannable as they are between the
different explicit routes.
3.3. Secondary goals for DetNet
Many applications require DetNet to provide additional services,
including coexistence with other QoS mechanisms Section 3.3.1 and
protection against misbehaving transmitters Section 3.3.2.
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3.3.1. Coexistence with normal traffic
A DetNet network supports the dedication of a high proportion (e.g.
75%) of the network bandwidth to DetNet flows. But, no matter how
much is dedicated for DetNet flows, it is a goal of DetNet to coexist
with existing Class of Service schemes (e.g., DiffServ). It is also
important that non-DetNet traffic not disrupt the DetNet flow, of
course (see Section 3.3.2 and Section 5). For these reasons:
o Bandwidth (transmission opportunities) not utilized by a DetNet
flow is available to non-DetNet packets (though not to other
DetNet flows).
o DetNet flows can be shaped or scheduled, in order to ensure that
the highest-priority non-DetNet packet is also ensured a worst-
case latency (at any given hop).
o When transmission opportunities for DetNet flows are scheduled in
detail, then the algorithm constructing the schedule should leave
sufficient opportunities for non-DetNet packets to satisfy the
needs of the users of the network. Detailed scheduling can also
permit the time-shared use of buffer resources by different DetNet
flows.
Ideally, the net effect of the presence of DetNet flows in a network
on the non-DetNet packets is primarily a reduction in the available
bandwidth.
3.3.2. Fault Mitigation
One key to building robust real-time systems is to reduce the
infinite variety of possible failures to a number that can be
analyzed with reasonable confidence. DetNet aids in the process by
allowing for filters and policers to detect DetNet packets received
on the wrong interface, or at the wrong time, or in too great a
volume, and to then take actions such as discarding the offending
packet, shutting down the offending DetNet flow, or shutting down the
offending interface.
It is also essential that filters and service remarking be employed
at the network edge to prevent non-DetNet packets from being mistaken
for DetNet packets, and thus impinging on the resources allocated to
DetNet packets.
There exist techniques, at present and/or in various stages of
standardization, that can perform these fault mitigation tasks that
deliver a high probability that misbehaving systems will have zero
impact on well-behaved DetNet flows, except of course, for the
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receiving interface(s) immediately downstream of the misbehaving
device. Examples of such techniques include traffic policing
functions (e.g. [RFC2475]) and separating flows into per-flow rate-
limited queues.
4. DetNet Architecture
4.1. DetNet stack model
DetNet functionality (Section 3) is implemented in two adjacent
layers in the protocol stack: the DetNet service layer and the DetNet
transport layer. The DetNet service layer provides DetNet service,
e.g., service protection, to higher layers in the protocol stack and
applications. The DetNet transport layer supports DetNet service in
the underlying network, e.g., by providing explicit routes and
congestion protection to DetNet flows.
4.1.1. Representative Protocol Stack Model
Figure 2 illustrates a conceptual DetNet data plane layering model.
One may compare it to that in [IEEE802.1CB], Annex C.
| packets going | ^ packets coming ^
v down the stack v | up the stack |
+----------------------+ +-----------------------+
| Source | | Destination |
+----------------------+ +-----------------------+
| Service layer: | | Service layer: |
| Packet sequencing | | Duplicate elimination |
| Flow replication | | Flow merging |
| Packet encoding | | Packet decoding |
+----------------------+ +-----------------------+
| Transport layer: | | Transport layer: |
| Congestion prot. | | Congestion prot. |
| Explicit routes | | Explicit routes |
+----------------------+ +-----------------------+
| Lower layers | | Lower layers |
+----------------------+ +-----------------------+
v ^
\_________________________/
Figure 2: DetNet data plane protocol stack
Not all layers are required for any given application, or even for
any given network. The functionality shown in Figure 2 is:
Application
Shown as "source" and "destination" in the diagram.
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Packet sequencing
As part of DetNet service protection, supplies the sequence
number for packet replication and elimination
(Section 3.2.2). Peers with Duplicate elimination. This
layer is not needed if a higher-layer transport protocol is
expected to perform any packet sequencing and duplicate
elimination required by the DetNet flow replication.
Duplicate elimination
As part of the DetNet service layer, based on the sequenced
number supplied by its peer, packet sequencing, Duplicate
elimination discards any duplicate packets generated by
DetNet flow replication. It can operate on member flows,
compound flows, or both. The replication may also be
inferred from other information such as the precise time of
reception in a scheduled network. The duplicate elimination
layer may also perform resequencing of packets to restore
packet order in a flow that was disrupted by the loss of
packets on one or another of the multiple paths taken.
Flow replication
As part of DetNet service protection, packets that belong to
a DetNet compound flow are replicated into two or more DetNet
member flows. This function is separate from packet
sequencing. Flow replication can be an explicit replication
and remarking of packets, or can be performed by, for
example, techniques similar to ordinary multicast
replication, albeit with resource allocation implications.
Peers with DetNet flow merging.
Flow merging
As part of DetNet service protection, merges DetNet member
flows together for packets coming up the stack belonging to a
specific DetNet compound flow. Peers with DetNet flow
replication. DetNet flow merging, together with packet
sequencing, duplicate elimination, and DetNet flow
replication perform packet replication and elimination
(Section 3.2.2).
Packet encoding
As part of DetNet service protection, as an alternative to
packet sequencing and flow replication, packet encoding
combines the information in multiple DetNet packets, perhaps
from different DetNet compound flows, and transmits that
information in packets on different DetNet member Flows.
Peers with Packet decoding.
Packet decoding
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As part of DetNet service protection, as an alternative to
flow merging and duplicate elimination, packet decoding takes
packets from different DetNet member flows, and computes from
those packets the original DetNet packets from the compound
flows input to packet encoding. Peers with Packet encoding.
Congestion protection
The DetNet transport layer provides congestion protection.
See Section 4.5. The actual queuing and shaping mechanisms
are typically provided by underlying subnet layers, these can
be closely associated with the means of providing paths for
DetNet flows, the path and the congestion protection are
conflated in this figure.
Explicit routes
The DetNet transport layer provides mechanisms to ensure that
fixed paths are provided for DetNet flows. These explicit
paths avoid the impact of network convergence.
Operations, Administration, and Maintenance (OAM) leverages in-band
and out-of-band signaling that validates whether the service is
effectively obtained within QoS constraints. OAM is not shown in
Figure 2; it may reside in any number of the layers. OAM can involve
specific tagging added in the packets for tracing implementation or
network configuration errors; traceability enables to find whether a
packet is a replica, which relay node performed the replication, and
which segment was intended for the replica. Active and hybrid OAM
methods require additional bandwidth to perform fault management and
performance monitoring of the DetNet domain. OAM may, for instance,
generate special test probes or add OAM information into the data
packet.
The packet sequencing and replication elimination functions at the
source and destination ends of a DetNet compound flow may be
performed either in the end system or in a DetNet relay node.
4.1.2. DetNet Data Plane Overview
A "Deterministic Network" will be composed of DetNet enabled end
systems and nodes, i.e., edge nodes, relay nodes and collectively
deliver DetNet services. DetNet enabled nodes are interconnected via
transit nodes (e.g., LSRs) which support DetNet, but are not DetNet
service aware. All DetNet enabled nodes are connected to sub-
networks, where a point-to-point link is also considered as a simple
sub-network. These sub-networks will provide DetNet compatible
service for support of DetNet traffic. Examples of sub-networks
include MPLS TE, IEEE 802.1 TSN and OTN. Of course, multi-layer
DetNet systems may also be possible, where one DetNet appears as a
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sub-network, and provides service to, a higher layer DetNet system.
A simple DetNet concept network is shown in Figure 3.
TSN Edge Transit Relay DetNet
End System Node Node Node End System
+---------+ +.........+ +---------+
| Appl. |<--:Svc Proxy:-- End to End Service ---------->| Appl. |
+---------+ +---------+ +---------+ +---------+
| TSN | |TSN| |Svc|<-- DetNet flow ---: Service :-->| Service |
+---------+ +---+ +---+ +---------+ +---------+ +---------+
|Transport| |Trp| |Trp| |Transport| |Trp| |Trp| |Transport|
+-------.-+ +-.-+ +-.-+ +--.----.-+ +-.-+ +-.-+ +---.-----+
: Link : / ,-----. \ : Link : / ,-----. \
+.......+ +-[ Sub ]-+ +........+ +-[ Sub ]-+
[Network] [Network]
`-----' `-----'
Figure 3: A Simple DetNet Enabled Network
Distinguishing the function of two DetNet data plane layers, the
DetNet service layer and the DetNet transport layer, helps to explore
and evaluate various combinations of the data plane solutions
available, some are illustrated in Figure 4. This separation of
DetNet layers, while helpful, should not be considered as formal
requirement. For example, some technologies may violate these strict
layers and still be able to deliver a DetNet service.
.
.
+-----------+
| Service | PW, UDP, GRE
+-----------+
| Transport | IPv6, IPv4, MPLS TE LSPs, MPLS SR
+-----------+
.
.
Figure 4: DetNet adaptation to data plane
In some networking scenarios, the end system initially provides a
DetNet flow encapsulation, which contains all information needed by
DetNet nodes (e.g., Real-time Transport Protocol (RTP) [RFC3550]
based DetNet flow transported over a native UDP/IP network or
PseudoWire). In other scenarios, the encapsulation formats might
differ significantly.
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There are many valid options to create a data plane solution for
DetNet traffic by selecting a technology approach for the DetNet
service layer and also selecting a technology approach for the DetNet
transport layer. There are a high number of valid combinations.
One of the most fundamental differences between different potential
data plane options is the basic headers used by DetNet nodes. For
example, the basic service can be delivered based on an MPLS label or
an IP header. This decision impacts the basic forwarding logic for
the DetNet service layer. Note that in both cases, IP addresses are
used to address DetNet nodes. The selected DetNet transport layer
technology also needs to be mapped to the sub-net technology used to
interconnect DetNet nodes. For example, DetNet flows will need to be
mapped to TSN Streams.
4.1.3. Network reference model
Figure 5 shows another view of the DetNet service related reference
points and main components.
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DetNet DetNet
end system end system
_ _
/ \ +----DetNet-UNI (U) / \
/App\ | /App\
/-----\ | /-----\
| NIC | v ________ | NIC |
+--+--+ _____ / \ DetNet-UNI (U) --+ +--+--+
| / \__/ \ | |
| / +----+ +----+ \_____ | |
| / | | | | \_______ | |
+------U PE +----+ P +----+ \ _ v |
| | | | | | | ___/ \ |
| +--+-+ +----+ | +----+ | / \_ |
\ | | | | | / \ |
\ | +----+ +--+-+ +--+PE |------ U-----+
\ | | | | | | | | | \_ _/
\ +---+ P +----+ P +--+ +----+ | \____/
\___ | | | | /
\ +----+__ +----+ DetNet-1 DetNet-2
| \_____/ \___________/ |
| |
| | End-to-End service | | | |
<------------------------------------------------------------->
| | DetNet service | | | |
| <------------------------------------------------> |
| | | | | |
Figure 5: DetNet Service Reference Model (multi-domain)
DetNet-UNIs ("U" in Figure 5) are assumed in this document to be
packet-based reference points and provide connectivity over the
packet network. A DetNet-UNI may provide multiple functions, e.g.,
it may add networking technology specific encapsulation to the DetNet
flows if necessary; it may provide status of the availability of the
resources associated with a reservation; it may provide a
synchronization service for the end system; it may carry enough
signaling to place the reservation in a network without a controller,
or if the controller only deals with the network but not the end
systems. Internal reference points of end systems (between the
application and the NIC) are more challenging from control
perspective and they may have extra requirements (e.g., in-order
delivery is expected in end system internal reference points, whereas
it is considered optional over the DetNet-UNI).
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4.2. DetNet systems
4.2.1. End system
The native data flow between the source/destination end systems is
referred to as application-flow (App-flow). The traffic
characteristics of an App-flow can be CBR (constant bit rate) or VBR
(variable bit rate) and can have L1 or L2 or L3 encapsulation (e.g.,
TDM (time-division multiplexing), Ethernet, IP). These
characteristics are considered as input for resource reservation and
might be simplified to ensure determinism during transport (e.g.,
making reservations for the peak rate of VBR traffic, etc.).
An end system may or may not be DetNet transport layer aware or
DetNet service layer aware. That is, an end system may or may not
contain DetNet specific functionality. End systems with DetNet
functionalities may have the same or different transport layer as the
connected DetNet domain. Categorization of end systems are shown in
Figure 6.
End system
|
|
| DetNet aware ?
/ \
+------< >------+
NO | \ / | YES
| v |
DetNet unaware |
End system |
| Service/
| Transport
/ \ aware ?
+--------< >-------------+
t-aware | \ / | s-aware
| v |
| | both |
| | |
DetNet t-aware | DetNet s-aware
End system | End system
v
DetNet st-aware
End system
Figure 6: Categorization of end systems
Note some known use case examples for end systems:
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o DetNet unaware: The classic case requiring service proxies.
o DetNet t-aware: A DetNet transport-aware system. It knows about
some TSN functions (e.g., reservation), but not about service
protection.
o DetNet s-aware: A DetNet service-aware system. It supplies
sequence numbers, but doesn't know about zero congestion loss.
o DetNet st-aware: A full functioning DetNet end system, it has
DetNet functionalities and usually the same forwarding paradigm as
the connected DetNet domain. It can be treated as an integral
part of the DetNet domain.
4.2.2. DetNet edge, relay, and transit nodes
As shown in Figure 3, DetNet edge nodes providing proxy service and
DetNet relay nodes providing the DetNet service layer are DetNet-
aware, and DetNet transit nodes need only be aware of the DetNet
transport layer.
In general, if a DetNet flow passes through one or more DetNet-
unaware network nodes between two DetNet nodes providing the DetNet
transport layer for that flow, there is a potential for disruption or
failure of the DetNet QoS. A network administrator needs to ensure
that the DetNet-unaware network nodes are configured to minimize the
chances of packet loss and delay, and provision enough extra buffer
space in the DetNet transit node following the DetNet-unaware network
nodes to absorb the induced latency variations.
4.3. DetNet flows
4.3.1. DetNet flow types
A DetNet flow can have different formats while it is transported
between the peer end systems. Therefore, the following possible
types / formats of a DetNet flow are distinguished in this document:
o App-flow: native format of the data carried over a DetNet flow.
It does not contain any DetNet related attributes.
o DetNet-t-flow: specific format of a DetNet flow. Only requires
the congestion / latency features provided by the DetNet transport
layer.
o DetNet-s-flow: specific format of a DetNet flow. Only requires
the service protection feature ensured by the DetNet service
layer.
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o DetNet-st-flow: specific format of a DetNet flow. It requires
both DetNet service layer and DetNet transport layer functions
during forwarding.
4.3.2. Source transmission behavior
For the purposes of congestion protection, DetNet flows can be
synchronous or asynchronous. In synchronous DetNet flows, at least
the intermediate nodes (and possibly the end systems) are closely
time synchronized, typically to better than 1 microsecond. By
transmitting packets from different DetNet flows or classes of DetNet
flows at different times, using repeating schedules synchronized
among the intermediate nodes, resources such as buffers and link
bandwidth can be shared over the time domain among different DetNet
flows. There is a tradeoff among techniques for synchronous DetNet
flows between the burden of fine-grained scheduling and the benefit
of reducing the required resources, especially buffer space.
In contrast, asynchronous DetNet flows are not coordinated with a
fine-grained schedule, so relay and end systems must assume worst-
case interference among DetNet flows contending for buffer resources.
Asynchronous DetNet flows are characterized by:
o A maximum packet size;
o An observation interval; and
o A maximum number of transmissions during that observation
interval.
These parameters, together with knowledge of the protocol stack used
(and thus the size of the various headers added to a packet), limit
the number of bit times per observation interval that the DetNet flow
can occupy the physical medium.
The source is required not to exceed these limits in order to obtain
DetNet service. If the source transmits less data than this limit
allows, the unused resource such as link bandwidth can be made
available by the system to non-DetNet packets. However, making those
resources available to DetNet packets in other DetNet flows would
serve no purpose. Those other DetNet flows have their own dedicated
resources, on the assumption that all DetNet flows can use all of
their resources over a long period of time.
There is no provision in DetNet for throttling DetNet flows (reducing
end-to-end transmission rate via any explicit congestion
notification); the assumption is that a DetNet flow, to be useful,
must be delivered in its entirety. That is, while any useful
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application is written to expect a certain number of lost packets,
the real-time applications of interest to DetNet demand that the loss
of data due to the network is an extraordinarily event.
Although DetNet strives to minimize the changes required of an
application to allow it to shift from a special-purpose digital
network to an Internet Protocol network, one fundamental shift in the
behavior of network applications is impossible to avoid: the
reservation of resources before the application starts. In the first
place, a network cannot deliver finite latency and practically zero
packet loss to an arbitrarily high offered load. Secondly, achieving
practically zero packet loss for unthrottled (though bandwidth
limited) DetNet flows means that bridges and routers have to dedicate
buffer resources to specific DetNet flows or to classes of DetNet
flows. The requirements of each reservation have to be translated
into the parameters that control each system's queuing, shaping, and
scheduling functions and delivered to the hosts, bridges, and
routers.
4.3.3. Incomplete Networks
The presence in the network of transit nodes or subnets that are not
fully capable of offering DetNet services complicates the ability of
the intermediate nodes and/or controller to allocate resources, as
extra buffering must be allocated at points downstream from the non-
DetNet intermediate node for a DetNet flow. This extra buffering may
increase latency and/or jitter.
4.4. Traffic Engineering for DetNet
Traffic Engineering Architecture and Signaling (TEAS) [TEAS] defines
traffic-engineering architectures for generic applicability across
packet and non-packet networks. From a TEAS perspective, Traffic
Engineering (TE) refers to techniques that enable operators to
control how specific traffic flows are treated within their networks.
Because if its very nature of establishing explicit optimized paths,
Deterministic Networking can be seen as a new, specialized branch of
Traffic Engineering, and inherits its architecture with a separation
into planes.
The Deterministic Networking architecture is thus composed of three
planes, a (User) Application Plane, a Controller Plane, and a Network
Plane, which echoes that of Figure 1 of Software-Defined Networking
(SDN): Layers and Architecture Terminology [RFC7426].:
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4.4.1. The Application Plane
Per [RFC7426], the Application Plane includes both applications and
services. In particular, the Application Plane incorporates the User
Agent, a specialized application that interacts with the end user /
operator and performs requests for Deterministic Networking services
via an abstract Flow Management Entity, (FME) which may or may not be
collocated with (one of) the end systems.
At the Application Plane, a management interface enables the
negotiation of flows between end systems. An abstraction of the flow
called a Traffic Specification (TSpec) provides the representation.
This abstraction is used to place a reservation over the (Northbound)
Service Interface and within the Application plane. It is associated
with an abstraction of location, such as IP addresses and DNS names,
to identify the end systems and eventually specify intermediate
nodes.
4.4.2. The Controller Plane
The Controller Plane corresponds to the aggregation of the Control
and Management Planes in [RFC7426], though Common Control and
Measurement Plane (CCAMP) [CCAMP] makes an additional distinction
between management and measurement. When the logical separation of
the Control, Measurement and other Management entities is not
relevant, the term Controller Plane is used for simplicity to
represent them all, and the term Controller Plane Function (CPF)
refers to any device operating in that plane, whether is it a Path
Computation Element (PCE) [RFC4655], or a Network Management entity
(NME), or a distributed control plane. The CPF is a core element of
a controller, in charge of computing Deterministic paths to be
applied in the Network Plane.
A (Northbound) Service Interface enables applications in the
Application Plane to communicate with the entities in the Controller
Plane as illustrated in Figure 7.
One or more CPF(s) collaborate to implement the requests from the FME
as Per-Flow Per-Hop Behaviors installed in the intermediate nodes for
each individual flow. The CPFs place each flow along a deterministic
sequence of intermediate nodes so as to respect per-flow constraints
such as security and latency, and optimize the overall result for
metrics such as an abstract aggregated cost. The deterministic
sequence can typically be more complex than a direct sequence and
include redundancy path, with one or more packet replication and
elimination points.
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4.4.3. The Network Plane
The Network Plane represents the network devices and protocols as a
whole, regardless of the Layer at which the network devices operate.
It includes Forwarding Plane (data plane), Application, and
Operational Plane (e.g., OAM) aspects.
The network Plane comprises the Network Interface Cards (NIC) in the
end systems, which are typically IP hosts, and intermediate nodes,
which are typically IP routers and switches. Network-to-Network
Interfaces such as used for Traffic Engineering path reservation in
[RFC5921], as well as User-to-Network Interfaces (UNI) such as
provided by the Local Management Interface (LMI) between network and
end systems, are both part of the Network Plane, both in the control
plane and the data plane.
A Southbound (Network) Interface enables the entities in the
Controller Plane to communicate with devices in the Network Plane as
illustrated in Figure 7. This interface leverages and extends TEAS
to describe the physical topology and resources in the Network Plane.
End End
System System
-+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
CPF CPF CPF CPF
-+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
intermediate intermed. intermed. intermed.
Node Node Node Node
NIC NIC
intermediate intermed. intermed. intermed.
Node Node Node Node
Figure 7: Northbound and Southbound interfaces
The intermediate nodes (and eventually the end systems NIC) expose
their capabilities and physical resources to the controller (the
CPF), and update the CPFs with their dynamic perception of the
topology, across the Southbound Interface. In return, the CPFs set
the per-flow paths up, providing a Flow Characterization that is more
tightly coupled to the intermediate node Operation than a TSpec.
At the Network plane, intermediate nodes may exchange information
regarding the state of the paths, between adjacent systems and
eventually with the end systems, and forward packets within
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constraints associated to each flow, or, when unable to do so,
perform a last resort operation such as drop or declassify.
This document focuses on the Southbound interface and the operation
of the Network Plane.
4.5. Queuing, Shaping, Scheduling, and Preemption
DetNet achieves congestion protection and bounded delivery latency by
reserving bandwidth and buffer resources at every hop along the path
of the DetNet flow. The reservation itself is not sufficient,
however. Implementors and users of a number of proprietary and
standard real-time networks have found that standards for specific
data plane techniques are required to enable these assurances to be
made in a multi-vendor network. The fundamental reason is that
latency variation in one system results in the need for extra buffer
space in the next-hop system(s), which in turn, increases the worst-
case per-hop latency.
Standard queuing and transmission selection algorithms allow a
central controller to compute the latency contribution of each
transit node to the end-to-end latency, to compute the amount of
buffer space required in each transit node for each incremental
DetNet flow, and most importantly, to translate from a flow
specification to a set of values for the managed objects that control
each relay or end system. For example, the IEEE 802.1 WG has
specified (and is specifying) a set of queuing, shaping, and
scheduling algorithms that enable each transit node (bridge or
router), and/or a central controller, to compute these values. These
algorithms include:
o A credit-based shaper [IEEE802.1Qav] (superseded by
[IEEE802.1Q-2018]).
o Time-gated queues governed by a rotating time schedule,
synchronized among all transit nodes [IEEE802.1Qbv] (superseded by
[IEEE802.1Q-2018]).
o Synchronized double (or triple) buffers driven by synchronized
time ticks. [IEEE802.1Qch] (superseded by [IEEE802.1Q-2018]).
o Pre-emption of an Ethernet packet in transmission by a packet with
a more stringent latency requirement, followed by the resumption
of the preempted packet [IEEE802.1Qbu] (superseded by
[IEEE802.1Q-2018]), [IEEE802.3br] (superseded by
[IEEE802.3-2018]).
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While these techniques are currently embedded in Ethernet
[IEEE802.3-2018] and bridging standards, we can note that they are
all, except perhaps for packet preemption, equally applicable to
other media than Ethernet, and to routers as well as bridges. Other
media may have its own methods, see, e.g.,
[I-D.ietf-6tisch-architecture], [RFC7554]. DetNet may include such
definitions in the future, or may define how these techniques can be
used by DetNet nodes.
4.6. Service instance
A Service instance represents all the functions required on a node to
allow the end-to-end service between the UNIs.
The DetNet network general reference model is shown in Figure 8 for a
DetNet service scenario (i.e., between two DetNet-UNIs). In this
figure, end systems ("A" and "B") are connected directly to the edge
nodes of an IP/MPLS network ("PE1" and "PE2"). End systems
participating in DetNet communication may require connectivity before
setting up an App-flow that requires the DetNet service. Such a
connectivity related service instance and the one dedicated for
DetNet service share the same access. Packets belonging to a DetNet
flow are selected by a filter configured on the access ("F1" and
"F2"). As a result, data flow specific access ("access-A + F1" and
"access-B + F2") are terminated in the flow specific service instance
("SI-1" and "SI-2"). A tunnel is used to provide connectivity
between the service instances.
The tunnel is used to transport exclusively the packets of the DetNet
flow between "SI-1" and "SI-2". The service instances are configured
to implement DetNet functions and a flow specific DetNet transport.
The service instance and the tunnel may or may not be shared by
multiple DetNet flows. Sharing the service instance by multiple
DetNet flows requires properly populated forwarding tables of the
service instance.
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access-A access-B
<-----> <-------- tunnel ----------> <----->
+---------+ ___ _ +---------+
End system | +----+ | / \/ \_ | +----+ | End system
"A" -------F1+ | | / \ | | +F2----- "B"
| | +========+ IP/MPLS +=======+ | |
| |SI-1| | \__ Net._/ | |SI-2| |
| +----+ | \____/ | +----+ |
|PE1 | | PE2|
+---------+ +---------+
Figure 8: DetNet network general reference model
The tunnel between the service instances may have some special
characteristics. For example, in case of a DetNet L3 service, there
are differences in the usage of the PW for DetNet traffic compared to
the network model described in [RFC6658]. In the DetNet scenario,
the PW is likely to be used exclusively by the DetNet flow, whereas
[RFC6658] states: "The packet PW appears as a single point-to-point
link to the client layer. Network-layer adjacency formation and
maintenance between the client equipment will follow the normal
practice needed to support the required relationship in the client
layer ... This packet PseudoWire is used to transport all of the
required Layer-2 and Layer-3 protocols between LSR1 and LSR2".
Further details are network technology specific and can be found in
[I-D.ietf-detnet-dp-sol-mpls] and [I-D.ietf-detnet-dp-sol-ip].
4.7. Flow identification at technology borders
4.7.1. Exporting flow identification
A DetNet node may need to map specific flows to lower layer flows (or
Streams) in order to provide specific queuing and shaping services
for specific flows. For example:
o A non-IP, strictly L2 source end system X may be sending multiple
flows to the same L2 destination end system Y. Those flows may
include DetNet flows with different QoS requirements, and may
include non-DetNet flows.
o A router may be sending any number of flows to another router.
Again, those flows may include DetNet flows with different QoS
requirements, and may include non-DetNet flows.
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o Two routers may be separated by bridges. For these bridges to
perform any required per-flow queuing and shaping, they must be
able to identify the individual flows.
o A Label Edge Router (LER) may have a Label Switched Path (LSP) set
up for handling traffic destined for a particular IP address
carrying only non-DetNet flows. If a DetNet flow to that same
address is requested, a separate LSP may be needed, in order that
all of the Label Switch Routers (LSRs) along the path to the
destination give that flow special queuing and shaping.
The need for a lower-layer node to be aware of individual higher-
layer flows is not unique to DetNet. But, given the endless
complexity of layering and relayering over tunnels that is available
to network designers, DetNet needs to provide a model for flow
identification that is better than packet inspection. That is not to
say that packet inspection to layer 4 or 5 addresses will not be
used, or the capability standardized; but, there are alternatives.
A DetNet relay node can connect DetNet flows on different paths using
different flow identification methods. For example:
o A single unicast DetNet flow passing from router A through a
bridged network to router B may be assigned a TSN Stream
identifier that is unique within that bridged network. The
bridges can then identify the flow without accessing higher-layer
headers. Of course, the receiving router must recognize and
accept that TSN Stream.
o A DetNet flow passing from LSR A to LSR B may be assigned a
different label than that used for other flows to the same IP
destination.
In any of the above cases, it is possible that an existing DetNet
flow can be an aggregate carrying multiple other DetNet flows. (Not
to be confused with DetNet compound vs. member flows.) Of course,
this requires that the aggregate DetNet flow be provisioned properly
to carry the aggregated flows.
Thus, rather than packet inspection, there is the option to export
higher-layer information to the lower layer. The requirement to
support one or the other method for flow identification (or both) is
a complexity that is part of DetNet control models.
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4.7.2. Flow attribute mapping between layers
Transport of DetNet flows over multiple technology domains may
require that lower layers are aware of specific flows of higher
layers. Such an "exporting of flow identification" is needed each
time when the forwarding paradigm is changed on the transport path
(e.g., two LSRs are interconnected by a L2 bridged domain, etc.).
The three representative forwarding methods considered for
deterministic networking are:
o IP routing
o MPLS label switching
o Ethernet bridging
A packet with corresponding Flow-IDs is illustrated in Figure 9,
which also indicates where each Flow-ID can be added or removed.
add/remove add/remove
Eth Flow-ID IP Flow-ID
| |
v v
+-----------------------------------------------------------+
| | | | |
| Eth | MPLS | IP | Application data |
| | | | |
+-----------------------------------------------------------+
^
|
add/remove
MPLS Flow-ID
Figure 9: Packet with multiple Flow-IDs
The additional (domain specific) Flow-ID can be
o created by a domain specific function or
o derived from the Flow-ID added to the App-flow.
The Flow-ID must be unique inside a given domain. Note that the
Flow-ID added to the App-flow is still present in the packet, but
transport nodes may lack the function to recognize it; that's why the
additional Flow-ID is added.
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4.7.3. Flow-ID mapping examples
IP nodes and MPLS nodes are assumed to be configured to push such an
additional (domain specific) Flow-ID when sending traffic to an
Ethernet switch (as shown in the examples below).
Figure 10 shows a scenario where an IP end system ("IP-A") is
connected via two Ethernet switches ("ETH-n") to an IP router ("IP-
1").
IP domain
<-----------------------------------------------
+======+ +======+
|L3-ID | |L3-ID |
+======+ /\ +-----+ +======+
/ \ Forward as | |
/IP-A\ per ETH-ID |IP-1 | Recognize
Push ------> +-+----+ | +---+-+ <----- ETH-ID
ETH-ID | +----+-----+ |
| v v |
| +-----+ +-----+ |
+------+ | | +---------+
+......+ |ETH-1+----+ETH-2| +======+
.L3-ID . +-----+ +-----+ |L3-ID |
+======+ +......+ +======+
|ETH-ID| .L3-ID . |ETH-ID|
+======+ +======+ +------+
|ETH-ID|
+======+
Ethernet domain
<---------------->
Figure 10: IP nodes interconnected by an Ethernet domain
End system "IP-A" uses the original App-flow specific ID ("L3-ID"),
but as it is connected to an Ethernet domain it has to push an
Ethernet-domain specific flow-ID ("VID + multicast MAC address",
referred as "ETH-ID") before sending the packet to "ETH-1" node.
Ethernet switch "ETH-1" can recognize the data flow based on the
"ETH-ID" and it does forwarding toward "ETH-2". "ETH-2" switches the
packet toward the IP router. "IP-1" must be configured to receive
the Ethernet Flow-ID specific multicast flow, but (as it is an L3
node) it decodes the data flow ID based on the "L3-ID" fields of the
received packet.
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Figure 11 shows a scenario where MPLS domain nodes ("PE-n" and "P-m")
are connected via two Ethernet switches ("ETH-n").
MPLS domain
<----------------------------------------------->
+=======+ +=======+
|MPLS-ID| |MPLS-ID|
+=======+ +-----+ +-----+ +=======+ +-----+
| | Forward as | | | |
|PE-1 | per ETH-ID | P-2 +-----------+ PE-2|
Push -----> +-+---+ | +---+-+ +-----+
ETH-ID | +-----+----+ | \ Recognize
| v v | +-- ETH-ID
| +-----+ +-----+ |
+---+ | | +----+
+.......+ |ETH-1+----+ETH-2| +=======+
.MPLS-ID. +-----+ +-----+ |MPLS-ID|
+=======+ +=======+
|ETH-ID | +.......+ |ETH-ID |
+=======+ .MPLS-ID. +-------+
+=======+
|ETH-ID |
+=======+
Ethernet domain
<---------------->
Figure 11: MPLS nodes interconnected by an Ethernet domain
"PE-1" uses the MPLS specific ID ("MPLS-ID"), but as it is connected
to an Ethernet domain it has to push an Ethernet-domain specific
flow-ID ("VID + multicast MAC address", referred as "ETH-ID") before
sending the packet to "ETH-1". Ethernet switch "ETH-1" can recognize
the data flow based on the "ETH-ID" and it does forwarding toward
"ETH-2". "ETH-2" switches the packet toward the MPLS node ("P-2").
"P-2" must be configured to receive the Ethernet Flow-ID specific
multicast flow, but (as it is an MPLS node) it decodes the data flow
ID based on the "MPLS-ID" fields of the received packet.
One can appreciate from the above example that, when the means used
for DetNet flow identification is altered or exported, the means for
encoding the sequence number information must similarly be altered or
exported.
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4.8. Advertising resources, capabilities and adjacencies
There are three classes of information that a central controller or
distributed control plane needs to know that can only be obtained
from the end systems and/or nodes in the network. When using a peer-
to-peer control plane, some of this information may be required by a
system's neighbors in the network.
o Details of the system's capabilities that are required in order to
accurately allocate that system's resources, as well as other
systems' resources. This includes, for example, which specific
queuing and shaping algorithms are implemented (Section 4.5), the
number of buffers dedicated for DetNet allocation, and the worst-
case forwarding delay and misordering.
o The dynamic state of a node's DetNet resources.
o The identity of the system's neighbors, and the characteristics of
the link(s) between the systems, including the length (in
nanoseconds) of the link(s).
4.9. Scaling to larger networks
Reservations for individual DetNet flows require considerable state
information in each transit node, especially when adequate fault
mitigation (Section 3.3.2) is required. The DetNet data plane, in
order to support larger numbers of DetNet flows, must support the
aggregation of DetNet flows. Such aggregated flows can be viewed by
the transit nodes' data plane largely as individual DetNet flows.
Without such aggregation, the per-relay system may limit the scale of
DetNet networks. Example techniques that may be used include MPLS
hierarchy and IP DiffServ Code Points (DSCPs).
4.10. Compatibility with Layer-2
Standards providing similar capabilities for bridged networks (only)
have been and are being generated in the IEEE 802 LAN/MAN Standards
Committee. The present architecture describes an abstract model that
can be applicable both at Layer-2 and Layer-3, and over links not
defined by IEEE 802.
DetNet enabled end systems and intermediate nodes can be
interconnected by sub-networks, i.e., Layer-2 technologies. These
sub-networks will provide DetNet compatible service for support of
DetNet traffic. Examples of sub-networks include MPLS TE, 802.1 TSN,
and a point-to-point OTN link. Of course, multi-layer DetNet systems
may be possible too, where one DetNet appears as a sub-network, and
provides service to, a higher layer DetNet system.
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5. Security Considerations
Security in the context of Deterministic Networking has an added
dimension; the time of delivery of a packet can be just as important
as the contents of the packet, itself. A man-in-the-middle attack,
for example, can impose, and then systematically adjust, additional
delays into a link, and thus disrupt or subvert a real-time
application without having to crack any encryption methods employed.
See [RFC7384] for an exploration of this issue in a related context.
Furthermore, in a control system where millions of dollars of
equipment, or even human lives, can be lost if the DetNet QoS is not
delivered, one must consider not only simple equipment failures,
where the box or wire instantly becomes perfectly silent, but complex
errors such as can be caused by software failures. Because there is
essential no limit to the kinds of failures that can occur,
protecting against realistic equipment failures is indistinguishable,
in most cases, from protecting against malicious behavior, whether
accidental or intentional. See also Section 3.3.2.
Security must cover:
o the protection of the signaling protocol
o the authentication and authorization of the controlling systems
o the identification and shaping of the DetNet flows
6. Privacy Considerations
DetNet is provides a Quality of Service (QoS), and as such, does not
directly raise any new privacy considerations.
However, the requirement for every (or almost every) node along the
path of a DetNet flow to identify DetNet flows may present an
additional attack surface for privacy, should the DetNet paradigm be
found useful in broader environments.
7. IANA Considerations
This document does not require an action from IANA.
8. Acknowledgements
The authors wish to thank Lou Berger, David Black, Stewart Bryant,
Rodney Cummings, Ethan Grossman, Craig Gunther, Marcel Kiessling,
Rudy Klecka, Jouni Korhonen, Erik Nordmark, Shitanshu Shah, Wilfried
Steiner, George Swallow, Michael Johas Teener, Pat Thaler, Thomas
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Watteyne, Patrick Wetterwald, Karl Weber, Anca Zamfir, for their
various contribution with this work.
9. Informative References
[CCAMP] IETF, "Common Control and Measurement Plane Working
Group",
<https://datatracker.ietf.org/doc/charter-ietf-ccamp/>.
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-14 (work
in progress), April 2018.
[I-D.ietf-detnet-dp-sol-ip]
Korhonen, J. and B. Varga, "DetNet IP Data Plane
Encapsulation", draft-ietf-detnet-dp-sol-ip-00 (work in
progress), July 2018.
[I-D.ietf-detnet-dp-sol-mpls]
Korhonen, J. and B. Varga, "DetNet MPLS Data Plane
Encapsulation", draft-ietf-detnet-dp-sol-mpls-00 (work in
progress), July 2018.
[I-D.ietf-detnet-problem-statement]
Finn, N. and P. Thubert, "Deterministic Networking Problem
Statement", draft-ietf-detnet-problem-statement-06 (work
in progress), July 2018.
[I-D.ietf-detnet-use-cases]
Grossman, E., "Deterministic Networking Use Cases", draft-
ietf-detnet-use-cases-17 (work in progress), June 2018.
[IEC62439-3-2016]
International Electrotechnical Commission (IEC) TC 65/SC
65C - Industrial networks, "IEC 62439-3:2016 Industrial
communication networks - High availability automation
networks - Part 3: Parallel Redundancy Protocol (PRP) and
High-availability Seamless Redundancy (HSR)", 2016,
<https://webstore.iec.ch/publication/24447>.
[IEEE802.1BA]
IEEE Standards Association, "IEEE Std 802.1BA-2011 Audio
Video Bridging (AVB) Systems", 2011,
<https://ieeexplore.ieee.org/document/6032690/>.
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[IEEE802.1CB]
IEEE Standards Association, "IEEE Std 802.1CB-2017 Frame
Replication and Elimination for Reliability", 2017,
<https://ieeexplore.ieee.org/document/8091139/>.
[IEEE802.1Q-2018]
IEEE Standards Association, "IEEE Std 802.1Q-2018 Bridges
and Bridged Networks", 2018,
<https://standards.ieee.org/findstds/
standard/802.1Q-2018.html>.
[IEEE802.1Qav]
IEEE Standards Association, "IEEE Std 802.1Qav-2009
Bridges and Bridged Networks - Amendment 12: Forwarding
and Queuing Enhancements for Time-Sensitive Streams",
2009, <https://ieeexplore.ieee.org/document/5375704/>.
[IEEE802.1Qbu]
IEEE Standards Association, "IEEE Std 802.1Qbu-2016
Bridges and Bridged Networks - Amendment 26: Frame
Preemption", 2016,
<https://ieeexplore.ieee.org/document/7553415/>.
[IEEE802.1Qbv]
IEEE Standards Association, "IEEE Std 802.1Qbv-2015
Bridges and Bridged Networks - Amendment 25: Enhancements
for Scheduled Traffic", 2015,
<https://ieeexplore.ieee.org/document/7572858/>.
[IEEE802.1Qch]
IEEE Standards Association, "IEEE Std 802.1Qch-2017
Bridges and Bridged Networks - Amendment 29: Cyclic
Queuing and Forwarding", 2017,
<https://ieeexplore.ieee.org/document/7961303/>.
[IEEE802.1TSNTG]
IEEE Standards Association, "IEEE 802.1 Time-Sensitive
Networking Task Group", 2013,
<http://www.ieee802.org/1/tsn>.
[IEEE802.3-2018]
IEEE Standards Association, "IEEE Std 802.3-2018 Standard
for Ethernet", 2018, <http://standards.ieee.org/findstds/
standard/802.3-2018.html>.
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[IEEE802.3br]
IEEE Standards Association, "IEEE Std 802.3br-2016
Standard for Ethernet Amendment 5: Specification and
Management Parameters for Interspersing Express Traffic",
2016, <http://ieeexplore.ieee.org/document/7900321/>.
[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>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[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>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
[RFC5921] Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau,
L., and L. Berger, "A Framework for MPLS in Transport
Networks", RFC 5921, DOI 10.17487/RFC5921, July 2010,
<https://www.rfc-editor.org/info/rfc5921>.
[RFC6372] Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
Profile (MPLS-TP) Survivability Framework", RFC 6372,
DOI 10.17487/RFC6372, September 2011,
<https://www.rfc-editor.org/info/rfc6372>.
[RFC6658] Bryant, S., Ed., Martini, L., Swallow, G., and A. Malis,
"Packet Pseudowire Encapsulation over an MPLS PSN",
RFC 6658, DOI 10.17487/RFC6658, July 2012,
<https://www.rfc-editor.org/info/rfc6658>.
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[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[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>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.
[RFC7813] Farkas, J., Ed., Bragg, N., Unbehagen, P., Parsons, G.,
Ashwood-Smith, P., and C. Bowers, "IS-IS Path Control and
Reservation", RFC 7813, DOI 10.17487/RFC7813, June 2016,
<https://www.rfc-editor.org/info/rfc7813>.
[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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[TEAS] IETF, "Traffic Engineering Architecture and Signaling
Working Group",
<https://datatracker.ietf.org/doc/charter-ietf-teas/>.
Authors' Addresses
Norman Finn
Huawei
3101 Rio Way
Spring Valley, California 91977
US
Phone: +1 925 980 6430
Email: norman.finn@mail01.huawei.com
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Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
Balazs Varga
Ericsson
Magyar tudosok korutja 11
Budapest 1117
Hungary
Email: balazs.a.varga@ericsson.com
Janos Farkas
Ericsson
Magyar tudosok korutja 11
Budapest 1117
Hungary
Email: janos.farkas@ericsson.com
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