DetNet N. Finn
Internet-Draft Huawei
Intended status: Standards Track P. Thubert
Expires: November 2, 2018 Cisco
B. Varga
J. Farkas
Ericsson
May 1, 2018
Deterministic Networking Architecture
draft-ietf-detnet-architecture-05
Abstract
Deterministic Networking (DetNet) 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 rapidly 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. The capabilities can be managed by
configuration, or by manual or automatic network management.
Status of This Memo
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This Internet-Draft will expire on November 2, 2018.
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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 TSN to DetNet dictionary . . . . . . . . . . . . 6
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
3.2.2. Explicit routes . . . . . . . . . . . . . . . . . . . 9
3.2.3. Jitter Reduction . . . . . . . . . . . . . . . . . . 10
3.2.4. Packet Replication and Elimination . . . . . . . . . 11
3.2.5. Packet encoding for service protection . . . . . . . 12
3.3. Secondary goals for DetNet . . . . . . . . . . . . . . . 13
3.3.1. Coexistence with normal traffic . . . . . . . . . . . 13
3.3.2. Fault Mitigation . . . . . . . . . . . . . . . . . . 13
4. DetNet Architecture . . . . . . . . . . . . . . . . . . . . . 14
4.1. DetNet stack model . . . . . . . . . . . . . . . . . . . 14
4.1.1. Representative Protocol Stack Model . . . . . . . . . 14
4.1.2. DetNet Data Plane Overview . . . . . . . . . . . . . 16
4.1.3. Network reference model . . . . . . . . . . . . . . . 18
4.2. DetNet systems . . . . . . . . . . . . . . . . . . . . . 19
4.2.1. End system . . . . . . . . . . . . . . . . . . . . . 19
4.2.2. DetNet edge, relay, and transit nodes . . . . . . . . 20
4.3. DetNet flows . . . . . . . . . . . . . . . . . . . . . . 21
4.3.1. DetNet flow types . . . . . . . . . . . . . . . . . . 21
4.3.2. Source guarantees . . . . . . . . . . . . . . . . . . 21
4.3.3. Incomplete Networks . . . . . . . . . . . . . . . . . 23
4.4. Traffic Engineering for DetNet . . . . . . . . . . . . . 23
4.4.1. The Application Plane . . . . . . . . . . . . . . . . 23
4.4.2. The Controller Plane . . . . . . . . . . . . . . . . 24
4.4.3. The Network Plane . . . . . . . . . . . . . . . . . . 24
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4.5. Queuing, Shaping, Scheduling, and Preemption . . . . . . 25
4.6. Service instance . . . . . . . . . . . . . . . . . . . . 26
4.7. Flow identification at technology borders . . . . . . . . 27
4.7.1. Exporting flow identification . . . . . . . . . . . . 27
4.7.2. Flow attribute mapping between layers . . . . . . . . 29
4.7.3. Flow-ID mapping examples . . . . . . . . . . . . . . 30
4.8. Advertising resources, capabilities and adjacencies . . . 32
4.9. Provisioning model . . . . . . . . . . . . . . . . . . . 32
4.9.1. Centralized Path Computation and Installation . . . . 32
4.9.2. Distributed Path Setup . . . . . . . . . . . . . . . 32
4.10. Scaling to larger networks . . . . . . . . . . . . . . . 33
4.11. Connected islands vs. networks . . . . . . . . . . . . . 33
4.12. Compatibility with Layer-2 . . . . . . . . . . . . . . . 33
5. Security Considerations . . . . . . . . . . . . . . . . . . . 34
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 34
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
9. Access to IEEE 802.1 documents . . . . . . . . . . . . . . . 35
10. Informative References . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
Deterministic Networking (DetNet) is a service that can be offered by
a network to DetNet flows. DetNet provides these flows extremely low
packet loss rates and assured maximum end-to-end delivery latency.
This is accomplished 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.dt-detnet-dp-alt] for a discussion of 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
topologies; connectivity is not restricted. Any application that
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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 applications
controller, or by other means.
Many applications of interest to 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 should accommodate various
synchronization techniques and profiles that are defined elsewhere to
solve exchange time in different market segments.
Wired and wireless media differ greatly in a number of ways,
including connectivity possibilities and the reliability of packet
transmission. While some of the techniques described in this
document may be applicable to wireless media, the DetNet architecture
assumes the use of links with characteristics typical of wired, and
not wireless, media.
2. Terminology
2.1. Terms used in this document
The following special terms are used in this document in order to
avoid the assumption that a given element in the architecture does or
does not have Internet Protocol stack, functions as a router, bridge,
firewall, or otherwise plays a particular role at Layer-2 or higher.
App-flow
The native format of a DetNet flow.
destination
An end system capable of receiving 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, which
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are eventually merged back into a single DetNet compound
flow, at the DetNet transport layer. "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 includes either 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, analogous to a Label Edge Router (LER).
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" or "node" 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 node
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
incorporates DetNet transport layer functions as well, in
which case it is collocated with a transit node.
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reservation
A trail of configuration between source to destination(s)
through transit nodes and subnets associated with a DetNet
flow, to provide congestion protection.
DetNet service layer
The layer at which service protection is provided, either
packet sequencing, replication, and elimination
(Section 3.2.4) or packet encoding (Section 3.2.5).
source
An end system capable of sourcing 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. Optionally provides congestion protection
over those paths. An MPLS LSR is an example of a DetNet
transit node.
DetNet transport layer
The layer that optionally provides congestion protection for
DetNet flows over paths provided by the underlying network.
TSN
Time-Sensitive Networking, TSN is a Task Group of the IEEE
802.1 Working Group.
2.2. IEEE 802 TSN to DetNet dictionary
This section also serves as a dictionary for translating from the
terms used by the IEEE 802 Time-Sensitive Networking (TSN) Task Group
to those of the DetNet WG.
Listener
The IEEE 802 term for a destination of a DetNet flow.
relay system
The IEEE 802 term for a DetNet intermediate node.
Stream
The IEEE 802 term for a DetNet flow.
Talker
The IEEE 802 term for the source of a DetNet flow.
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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 jitter avoidance derive from these constraints
o Probability of loss of a packet, under various assumptions as to
the operational states of the nodes and links. A derived property
is whether it is acceptable to deliver a duplicate packet, which
is an inherent risk in highly reliable and/or broadcast
transmissions
It is a distinction of DetNet that it is concerned solely with worst-
case values for the end-to-end latency. Average, mean, or typical
values are of no 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.
Three techniques are used by DetNet to provide these qualities of
service:
o Congestion protection (Section 3.2.1).
o Explicit routes (Section 3.2.2).
o Service protection (Section 3.2.4).
Congestion protection operates by reserving 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.
Congestion protection addresses both 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
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these losses. The mechanisms employed are constrained by the
requirement to meet the users' latency requirements. Packet
replication and elimination (Section 3.2.4) and packet encoding
(Section 3.2.5) are described in this document to provide service
protection; others may be found. 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 cannot suffer temporary interruptions caused by
the reconvergence 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
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 [HSR-PRP]. Explicit routes are achieved by limiting
the physical topology of the network, and the sequentialization,
replication, and duplicate elimination are facilitated by packet
tags added at the front or the end of Ethernet frames.
o Congestion protection alone is is offered by IEEE 802.1 Audio
Video bridging [IEEE802.1BA-2011]. As long as the network suffers
no failures, zero congestion loss can be achieved through the use
of a reservation protocol (MSRP), shapers in every bridge, and a
bit of network calculus.
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
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industry) to make selections among the available means of
implementing DetNet networks.
3.2. Mechanisms to achieve DetNet Qos
3.2.1. Congestion protection
The primary means by which DetNet achieves its QoS assurances is to
reduce, or even completely eliminate, congestion at an output port as
a cause of packet loss. Given that a DetNet flow cannot be
throttled, 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 reservation
of the bandwidth and buffers for a DetNet flow requires the
provisioning described in Section 4.9. 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.
3.2.2. Explicit routes
In networks controlled by typical peer-to-peer protocols such as IEEE
802.1 ISIS bridged networks or IETF OSPF routed networks, a network
topology event in one part of the network can impact, at least
briefly, the delivery of data in parts of the network remote from the
failure or recovery event. Thus, even redundant paths through a
network, if controlled by the typical peer-to-peer protocols, do not
eliminate the chances of brief losses of contact.
Many real-time networks rely on physical rings or chains 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
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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 3.2.4 or
Section 3.2.5) over explicit routes provides a high likelihood of
continuous connectivity. Explicit routes are commonly used in MPLS
TE LSPs.
3.2.3. Jitter Reduction
A core objective of DetNet is to enable the convergence of Non-IP
networks onto a common network infrastructure. This requires the
accurate emulation of currently deployed mission-specific networks,
which typically 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 exists there. Streams of information are 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:
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.
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3.2.4. Packet Replication and Elimination
After congestion loss has been eliminated, the most important causes
of packet loss are random media and/or memory faults, and equipment
failures. Both causes of packet loss can be greatly reduced by
spreading the data in a packet over multiple transmissions. One such
method for service protection is described in this section, which
sends the same packets over multiple paths.
Packet replication and elimination, also known as seamless redundancy
[HSR-PRP], or 1+1 hitless protection, is a function of the DetNet
service layer. It involves three capabilities:
o Providing sequencing information, once, at or near the source, to
the packets of a DetNet compound flow. This may be done by adding
a sequence number or 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. Section 3.2.2.
o Replicating these packets into multiple DetNet member flows and,
typically, sending them along at least two different paths to the
destination(s), e.g. over the explicit routes of
o Eliminating duplicated packets. This may be done at any step
along the path to save network resources further down, 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.
This function is a "hitless" version of, e.g., the 1+1 linear
protection in [RFC6372]. That is, instead of switching from one flow
to the other when a failure of a flow is detected, DetNet combines
both flows, and performs a packet-by-packet selection of which to
discard, based on sequence number.
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 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 sequence numbers 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 the following figure, 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.
Packet replication and elimination
> > > > > > > > > relay > > > > > > > >
> /------------+ R node E +------------\ >
> / v + ^ \ >
end R + v | ^ + E end
system + v | ^ + system
> \ v + ^ / >
> \------------+ R relay E +-----------/ >
> > > > > > > > > node > > > > > > > >
Figure 1
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 detected 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.
3.2.5. 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.
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3.3. Secondary goals for DetNet
Many applications require DetNet to provide additional services,
including coesistence with other QoS mechanisms Section 3.3.1 and
protection against misbehaving transmitters Section 3.3.2.
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 are 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 also is 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
providing 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
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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
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
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.
DetNet data plane protocol stack
| packets going | ^ packets coming ^
v down the stack v | up the stack |
+----------------------+ +-----------------------+
| Source | | Destination |
+----------------------+ +-----------------------+
| Service layer | | Service layer |
| Packet sequencing | | Duplicate elimination |
| Flow duplication | | Flow merging |
| Packet encoding | | Packet decoding |
+----------------------+ +-----------------------+
| Transport layer | | Transport layer |
| Congestion prot. | | Congestion prot. |
+----------------------+ +-----------------------+
| Lower layers | | Lower layers |
+----------------------+ +-----------------------+
v ^
\_________________________/
Figure 2
Not all layers are required for any given application, or even for
any given network. The layers are, from top to bottom:
Application
Shown as "source" and "destination" in the diagram.
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OAM
Operations, Administration, and Maintenance leverages in-band
and out-of-and 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.
Packet sequencing
As part of DetNet service protection, supplies the sequence
number for packet replication and elimination
(Section 3.2.4). 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 duplication.
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 duplication. It can operate on member flows,
compound flows, or both. The duplication 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 duplication
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 duplication can be an explicit duplication
and remarking of packets, or can be performed by, for
example, techniques similar to ordinary multicast
replication. Peers with DetNet flow merging.
Network 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
duplication. DetNet flow merging, together with packet
sequencing, duplicate elimination, and DetNet flow
duplication, performs packet replication and elimination
(Section 3.2.4).
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Packet encoding
As part of DetNet service protection, as an alternative to
packet sequencing and flow duplication, 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
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, but since
these are can be closely associated with the means of
providing paths for DetNet flows (e.g. MPLS LSPs or {VLAN,
multicast destination MAC address} pairs), the path and the
congestion protection are conflated in this figure.
The packet sequencing and duplication 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 edge node. The
reader must not confuse a DetNet edge function with other kinds of
edge functions, e.g. an Label Edge Router, although the two functions
may be performed together. The DetNet edge function is concerned
with sequencing packets belonging to DetNet flows. The LER with
encapsulating/decapsulating packets for transport, and is considered
part of the network underlying the DetNet transport layer.
4.1.2. DetNet Data Plane Overview
A "Deterministic Network" will be composed of DetNet enabled nodes
i.e., End Systems, Edge Nodes, Relay Nodes and collectively deliver
DetNet services. DetNet enabled nodes are interconnected via Transit
Nodes (i.e., routers) which support DetNet, but are not DetNet
service aware. Transit nodes see DetNet nodes as end points. All
DetNet enabled nodes are connect 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 IEEE 802.1 TSN and OTN.
Of course, multi-layer DetNet systems may also be possible, where one
DetNet appears as a sub-network, and provides service to, a higher
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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 these 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. 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, RTP/(UDP), GRE
+-----------+
| Transport | (UDP)/IPv6, (UDP)/IPv4, MPLS LSPs, BIER
+-----------+
.
.
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. As an example, a CPRI "application's" I/Q data
mapped directly to Ethernet frames may have to be transported over an
MPLS-based packet switched network (PSN).
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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 addressing and headers used by DetNet
end systems. For example, is the basic service a Layer 2 (e.g.,
Ethernet) or Layer 3 (i.e., IP) service. This decision impacts how
DetNet end systems are addressed, and the basic forwarding logic for
the DetNet service layer.
4.1.3. Network reference model
The figure below shows another view of the DetNet service related
reference points and main components (Figure 5).
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)
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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
connection associated to 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
points. 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), therefore not covered
in this document.
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. Grouping of end systems are shown in
Figure 6.
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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: Grouping of end systems
Note some known use cases for end systems:
o DetNet unaware: The classic case requiring network proxies.
o DetNet t-aware: An extant TSN system. It knows about some TSN
functions (e.g., reservation), but not about replication/
elimination.
o DetNet s-aware: An extant IEC 62439-3 system. It supplies
sequence numbers, but doesn't know about zero congestion loss.
o DetNet st-aware: A full functioning DetNet end station, 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.
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In general, if a DetNet flow passes through one or more DetNet-
unaware network node 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 exra 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 during 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 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 replication/elimination feature ensured by the DetNet service
layer.
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 guarantees
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.
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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 promises that these limits will not be exceeded. If the
source transmits less data than this limit allows, the unused
resources 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
the transmission rate via feedback); the assumption is that a DetNet
flow, to be useful, must be delivered in its entirety. That is,
while any useful 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 extraordinarily
infrequent.
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.
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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, and thus extra latency, must be allocated at points
downstream from the non-DetNet intermediate node for a DetNet flow.
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 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].:
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.
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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 refers to any device
operating in that plane, whether is it a Path Computation entity or a
Network Management entity (NME). The Path Computation Element (PCE)
[PCE] 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.
One or more PCE(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 PCEs 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.
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 (control plane) 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.
This interface leverages and extends TEAS to describe the physical
topology and resources in the Network Plane.
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Flow Management Entity
End End
System System
-+-+-+-+-+-+-+ Northbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
PCE PCE PCE PCE
-+-+-+-+-+-+-+ Southbound -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
intermediate intermed. intermed. intermed.
Node Node Node Node
NIC NIC
intermediate intermed. intermed. intermed.
Node Node Node Node
Figure 7
The intermediate nodes (and eventually the end systems NIC) expose
their capabilities and physical resources to the controller (the
PCE), and update the PCE with their dynamic perception of the
topology, across the Southbound Interface. In return, the PCE(s) 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
constraints associated to each flow, or, when unable to do so,
perform a last resort operation such as drop or declassify.
This specification 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.
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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. The IEEE 802 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 [IEEE802.1Q-2014] Clause 34.
o Time-gated queues governed by a rotating time schedule,
synchronized among all transit nodes [IEEE802.1Qbv].
o Synchronized double (or triple) buffers driven by synchronized
time ticks. [IEEE802.1Qch].
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], [IEEE802.3br].
While these techniques are currently embedded in Ethernet 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.
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 reference model is shown in Figure 8 for a DetNet-
Service scenario (i.e. between two DetNet-UNIs). In this figure, the
end systems ("A" and "B") are connected directly to the edge nodes of
the IP/MPLS network ("PE1" and "PE2"). End-systems participating
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.
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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 routing or bridging
function depending on what connectivity the participating end systems
require (L3 or L2). 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.
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 reference model
The tunnel between the service instances may have some special
characteristics. For example, in case of a "packet PW" based tunnel,
there are differences in the usage of the packet PW for DetNet
traffic compared to the network model described in [RFC6658]. In the
DetNet scenario, the packet PW is 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 equipments 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".
4.7. Flow identification at technology borders
4.7.1. Exporting flow identification
An interesting feature of DetNet, and one that invites
implementations that can be accused of "layering violations", is the
need for lower layers to be aware of specific flows at higher layers,
in order to provide specific queuing and shaping services for
specific flows. For example:
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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.
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 (LERs) 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-level DetNet 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 at least somewhat 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 {VLAN, multicast
destination MAC address} pair 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 multicast MAC address.
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 used as a carrier for multiple DetNet sub-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 sub-flows.
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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
the essential complexity that DetNet brings to existing control plane
models.
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 main forwarding methods considered for deterministic
networking are:
o IP routing
o MPLS label switching
o Ethernet bridging
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,
so that it must be unique inside the 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 (pushed).
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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 stream, 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 stream, 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 identifcation 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
decentralized control plane needs to know that can only be obtained
from the end systems and/or transit 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.
o The dynamic state of an end or transit 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. Provisioning model
4.9.1. Centralized Path Computation and Installation
A centralized routing model, such as provided with a PCE (RFC 4655
[RFC4655]), enables global and per-flow optimizations. (See
Section 4.4.) The model is attractive but a number of issues are
left to be solved. In particular:
o Whether and how the path computation can be installed by 1) an end
device or 2) a Network Management entity,
o And how the path is set up, either by installing state at each hop
with a direct interaction between the forwarding device and the
PCE, or along a path by injecting a source-routed request at one
end of the path.
4.9.2. Distributed Path Setup
Significant work on distributed path setup can be leveraged from MPLS
Traffic Engineering, in both its GMPLS and non-GMPLS forms. The
protocols within scope are Resource ReSerVation Protocol [RFC3209]
[RFC3473](RSVP-TE), OSPF-TE [RFC4203] [RFC5392] and ISIS-TE [RFC5307]
[RFC5316]. These should be viewed as starting points as there are
feature specific extensions defined that may be applicable to DetNet.
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In a Layer-2 only environment, or as part of a layered approach to a
mixed environment, IEEE 802.1 also has work, either completed or in
progress. [IEEE802.1Q-2014] Clause 35 describes SRP, a peer-to-peer
protocol for Layer-2 roughly analogous to RSVP [RFC2205].
[IEEE802.1Qca] defines how ISIS can provide multiple disjoint paths
or distribution trees. Also in progress is [IEEE802.1Qcc], which
expands the capabilities of SRP.
The integration/interaction of the DetNet control layer with an
underlying IEEE 802.1 sub-network control layer will need to be
defined.
4.10. 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 into tunnels, which themselves can be
viewed by the transit nodes' data planes largely as individual DetNet
flows. Without such aggregation, the per-relay system may limit the
scale of DetNet networks.
4.11. Connected islands vs. networks
Given that users have deployed examples of the IEEE 802.1 TSN TG
standards, which provide capabilities similar to DetNet, it is
obvious to ask whether the IETF DetNet effort can be limited to
providing Layer-2 connections (VPNs) between islands of bridged TSN
networks. While this capability is certainly useful to some
applications, and must not be precluded by DetNet, tunneling alone is
not a sufficient goal for the DetNet WG. As shown in the
Deterministic Networking Use Cases draft [I-D.ietf-detnet-use-cases],
there are already deployments of Layer-2 TSN networks that are
encountering the well-known problems of over-large broadcast domains.
Routed solutions, and combinations routed/bridged solutions, are both
required.
4.12. 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. It is the intention of the authors (and
hopefully, as this draft progresses, of the DetNet Working Group)
that IETF and IEEE 802 will coordinate their work, via the
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participation of common individuals, liaisons, and other means, to
maximize the compatibility of their outputs.
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 802.1TSN 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.
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 bizarre
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.
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7. IANA Considerations
This document does not require an action from IANA.
8. Acknowledgements
The authors wish to thank Jouni Korhonen, Erik Nordmark, George
Swallow, Rudy Klecka, Anca Zamfir, David Black, Thomas Watteyne,
Shitanshu Shah, Craig Gunther, Rodney Cummings, Balazs Varga,
Wilfried Steiner, Marcel Kiessling, Karl Weber, Janos Farkas, Ethan
Grossman, Pat Thaler, Lou Berger, and especially Michael Johas
Teener, for their various contribution with this work.
9. Access to IEEE 802.1 documents
To access password protected IEEE 802.1 drafts, see the IETF IEEE
802.1 information page at https://www.ietf.org/proceedings/52/slides/
bridge-0/tsld003.htm.
10. Informative References
[AVnu] http://www.avnu.org/, "The AVnu Alliance tests and
certifies devices for interoperability, providing a simple
and reliable networking solution for AV network
implementation based on the Audio Video Bridging (AVB)
standards.".
[CCAMP] IETF, "Common Control and Measurement Plane",
<https://datatracker.ietf.org/doc/charter-ietf-ccamp/>.
[HSR-PRP] IEC, "High availability seamless redundancy (HSR) is a
further development of the PRP approach, although HSR
functions primarily as a protocol for creating media
redundancy while PRP, as described in the previous
section, creates network redundancy. PRP and HSR are both
described in the IEC 62439 3 standard.",
<http://webstore.iec.ch/webstore/webstore.nsf/
artnum/046615!opendocument>.
[I-D.dt-detnet-dp-alt]
Korhonen, J., Farkas, J., Mirsky, G., Thubert, P.,
Zhuangyan, Z., and L. Berger, "DetNet Data Plane Protocol
and Solution Alternatives", draft-dt-detnet-dp-alt-04
(work in progress), September 2016.
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[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-13 (work
in progress), November 2017.
[I-D.ietf-6tisch-tsch]
Watteyne, T., Palattella, M., and L. Grieco, "Using
IEEE802.15.4e TSCH in an IoT context: Overview, Problem
Statement and Goals", draft-ietf-6tisch-tsch-06 (work in
progress), March 2015.
[I-D.ietf-detnet-problem-statement]
Finn, N. and P. Thubert, "Deterministic Networking Problem
Statement", draft-ietf-detnet-problem-statement-03 (work
in progress), March 2018.
[I-D.ietf-detnet-use-cases]
Grossman, E., "Deterministic Networking Use Cases", draft-
ietf-detnet-use-cases-15 (work in progress), April 2018.
[I-D.ietf-roll-rpl-industrial-applicability]
Phinney, T., Thubert, P., and R. Assimiti, "RPL
applicability in industrial networks", draft-ietf-roll-
rpl-industrial-applicability-02 (work in progress),
October 2013.
[I-D.svshah-tsvwg-deterministic-forwarding]
Shah, S. and P. Thubert, "Deterministic Forwarding PHB",
draft-svshah-tsvwg-deterministic-forwarding-04 (work in
progress), August 2015.
[I-D.varga-detnet-service-model]
Varga, B. and J. Farkas, "DetNet Service Model", draft-
varga-detnet-service-model-02 (work in progress), May
2017.
[IEEE802.1AS-2011]
IEEE, "IEEE Std 802.1AS Timing and Synchronization for
Time-Sensitive Applications in Bridged Local Area
Networks", 2011,
<http://ieeexplore.ieee.org/document/5741898/>.
[IEEE802.1BA-2011]
IEEE, "IEEE Std 802.1BA Audio Video Bridging (AVB)
Systems", 2011,
<http://ieeexplore.ieee.org/document/6032690/>.
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[IEEE802.1CB]
IEEE, "IEEE Std 802.1CB Frame Replication and Elimination
for Reliability", 2017,
<http://www.ieee802.org/1/files/private/cb-drafts/>.
[IEEE802.1Q-2014]
IEEE, "IEEE Std 802.1Q Bridges and Bridged Networks",
2014, <http://ieeexplore.ieee.org/document/6991462/>.
[IEEE802.1Qbu]
IEEE, "IEEEE Std 802.1Qbu Bridges and Bridged Networks -
Amendment 26: Frame Preemption", 2016,
<http://ieeexplore.ieee.org/document/7553415/>.
[IEEE802.1Qbv]
IEEE, "IEEEE Std 802.1Qbu Bridges and Bridged Networks -
Amendment 25: Enhancements for Scheduled Traffic", 2015,
<http://ieeexplore.ieee.org/document/7572858/>.
[IEEE802.1Qca]
IEEE, "IEEE Std 802.1Qca Bridges and Bridged Networks -
Amendment 24: Path Control and Reservation", June 2015,
<http://ieeexplore.ieee.org/document/7565435/>.
[IEEE802.1Qcc]
IEEE, "Stream Reservation Protocol (SRP) Enhancements and
Performance Improvements (IEEE Draft P802.1Qcc)", 2017,
<http://www.ieee802.org/1/files/private/cc-drafts/>.
[IEEE802.1Qch]
IEEE, "Cyclic Queuing and Forwarding (IEEE Draft
P802.1Qch)", 2017,
<http://www.ieee802.org/1/files/private/ch-drafts/>.
[IEEE802.1TSNTG]
IEEE Standards Association, "IEEE 802.1 Time-Sensitive
Networks Task Group", 2013,
<http://www.IEEE802.org/1/pages/avbridges.html>.
[IEEE802.3-2015]
IEEE, "IEEE Std 802.3 Standard for Ethernet", 2015,
<http://ieeexplore.ieee.org/document/7428776/>.
[IEEE802.3br]
IEEE, "IEEE Std 802.3br Standard for Ethernet Amendment 5:
Specification and Management Parameters for Interspersing
Express Traffic", 2016,
<http://ieeexplore.ieee.org/document/7900321/>.
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[ISA95] ANSI/ISA, "Enterprise-Control System Integration Part 1:
Models and Terminology", 2000,
<https://www.isa.org/isa95/>.
[ODVA] http://www.odva.org/, "The organization that supports
network technologies built on the Common Industrial
Protocol (CIP) including EtherNet/IP.".
[PCE] IETF, "Path Computation Element",
<https://datatracker.ietf.org/doc/charter-ietf-pce/>.
[Profinet]
http://us.profinet.com/technology/profinet/, "PROFINET is
a standard for industrial networking in automation.",
<http://us.profinet.com/technology/profinet/>.
[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>.
[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>.
[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>.
[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.
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[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>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.
[RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
December 2008, <https://www.rfc-editor.org/info/rfc5316>.
[RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392,
January 2009, <https://www.rfc-editor.org/info/rfc5392>.
[RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
Phinney, "Industrial Routing Requirements in Low-Power and
Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October
2009, <https://www.rfc-editor.org/info/rfc5673>.
[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>.
[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>.
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[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>.
[TEAS] IETF, "Traffic Engineering Architecture and Signaling",
<https://datatracker.ietf.org/doc/charter-ietf-teas/>.
Authors' Addresses
Norman Finn
Huawei
3755 Avocado Blvd.
PMB 436
La Mesa, California 91941
US
Phone: +1 925 980 6430
Email: norman.finn@mail01.huawei.com
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
Konyves Kalman krt. 11/B
Budapest 1097
Hungary
Email: balazs.a.varga@ericsson.com
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Janos Farkas
Ericsson
Konyves Kalman krt. 11/B
Budapest 1097
Hungary
Email: janos.farkas@ericsson.com
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