Internet DRAFT - draft-liao-mpls-obs
draft-liao-mpls-obs
Internet Engineering Task Force Jia Jia Liao
Internet Draft Ping Zhang
Expires: October 2006 Zheng Bin Li
An Shi Xu
National Laboratory on Local Fiber-Optic Communication Network
& Advanced Optical Communication System
Peking University, China
April 2006
Recovery in Optical Burst Switching Network
draft-liao-mpls-obs-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006). All Rights Reserved.
Abstract
Protection and restoration at optical layer is critical to network
integrity since data is transmitted and switched at a considerably
high speed in optical domain. A few second halt may cause tens to
thousands gigabit loss. Optical burst switching is a promising
technology, bridging optical circuit switching and optical package
switching. Unlike optical circuit switching and time division
multiplexing, OBS is featured with unidirectional reservation and
statistical multiplexing of wavelength resources. The general idea
behind protection and restoration techniques is to utilize redundant
bandwidth resources as backup. The flexibility brought by OBS
provides alternatives for existed protection and restoration schemes
at optical layer.
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Conventions
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC 2119].
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Table of Contents
1. Introduction.....................................................4
1.1. OBS Network Architecture....................................4
1.2. Multi-layer Interoperation..................................5
2. Fault Management.................................................5
2.1. Failure Profiles............................................6
2.1.1. Link Failure..........................................6
2.1.2. Node Failure..........................................6
2.2. Fault Detection.............................................7
2.2.1. Link Fault Detection..................................7
2.2.2. Node Fault Detection..................................7
2.3. Fault Notification..........................................7
2.3.1. Link Fault Notification...............................7
2.3.2. Node Fault Notification...............................8
3. Restoration at OBS Layer.........................................9
3.1. Motivation to Restore at OBS Layer..........................9
3.2. Single Layer Restoration Schemes...........................10
3.2.1. Link Failure Restoration Schemes.....................10
3.2.2. Node Failure Restoration Schemes.....................10
3.3. Multi-layer Restoration Schemes............................11
3.3.1. IP Dynamic Routing...................................11
3.3.2. MPLS Protection Switching............................11
3.3.3. Optical Layer Resilient Schemes......................12
3.3.4. Recovery Scheme Comparison...........................12
3.3.5. Operational Coordination.............................12
4. Acknowledgements................................................13
5. References......................................................13
6. AUTHORS' ADDRESSES..............................................13
7. IPR NOTICE......................................................13
8. FULL COPYRIGHT STATEMENT........................................14
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1. Introduction
The basic difference among Optical Circuit Switching (OCS), Optical
Burst Switching (OBS) and Optical Packet Switching (OPS) is that the
three work at different granularity. OCS, which has already been
widely deployed, aims to switch at wavelength, waveband or even fiber
level. However, in most cases, individual users can hardly afford a
whole wavelength, and thus TDM is applied to provide each channel
with fixed percentage of the total bandwidth by splitting wavelength
into recurring time-slots. This approach has been proved to be less
bandwidth efficient than OPS, which is able to switch packets like IP
network in optical domain. However, some critical technologies
essential to OPS, such as optical random access memory, are far away
from maturity. OBS, supposed to bridge above two mechanisms, is able
to switch bufferlessly at sub-wavelength level. OBS, featured with
unidirectional reservation and statistical multiplexing of wavelength
resources, has brought great flexibility to optical bandwidth
distribution.
1.1. OBS Network Architecture
OBS network is composed of two sub-planes, namely data plane and
control plan, as shown in Figure 1. In data plane, traffic from OBS
client layer (e.g. IP or ATM layer) is aggregated into Data Bursts
(DBs) at ingress edge nodes which perform as an interface to the
upperlayer and local at the edge of OBS layer [IPOWDM]. DBs will be
sent through core nodes to their egress edge nodes without o-e-o
conversion, in which a few optical fiber delay lines may be applied
to reduce overall blocking probability. DBs usually contains tens to
thousands of thousand bits including payload and frame overhead.
[ core ]
/[ node ]\
/ | \
/ |------| \
/ | -\/- | \
| [ core ]/ /| -/\- |\ \[ core ] |
from |------[ node ]\ / |------| \ /[ node ]------| to
upperlayer| | \/ \/ | |upperlayer
------->| |------| /\ /\ |------| |-------->
------->|------| -\/- |/ \ / \| -\/- |------|-------->
------->| | -/\- |\ \[ core ] /| -/\- | |-------->
traffic | |------| \ [ node ] / |------| | traffic
| \ | / |
Ingress edge node \ |------| / Egress edge node
\| -\/- |/
| -/\- |
|------|
Figure 1: OBS Network Architecture
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Control plane mainly conduct routing and resource reservation by
configuring optical switching fabric according to signalling. An
offset time ahead of DB transmission, Control Packets (CPs) will be
sent through core nodes to establish an available light path from
ingress edge node to the egress one. At each stop at core nodes, CPs
will experience o-e-o conversion and be processed electrically to
trigger switching fabric. As long as every configuration is
successful, DB is able to transmit across the network. However, once
one of the core nodes along light path fails to act, the DB will have
to be discarded and bandwidth that has already been reserved will be
released.
1.2. Multi-layer Interoperation
OBS layer, viewed as an data-link layer, aims to provide reliable
end-to-end path for data transmission. As shown in Figure 2, its
server layer is optical layer with huge physical bandwidth and
its client layer can be network layer (e.g. IP) or others like ATM.
With the increase in volume and importance of IP traffic,
applications based on IP has become dominant. Thus in this draft, we
only consider IP as the client layer, for which OBS acts to provide
available bitpipe.
|+++++++++++++++++++| |+++++++++++++++++++|
| Application Layer |<-------->| Application Layer |
|+++++++++++++++++++| |+++++++++++++++++++|
| IP Layer |<-------->| Network Layer |
|+++++++++++++++++++| |+++++++++++++++++++|
| OBS Layer |<-------->| Data-link Layer |
|+++++++++++++++++++| |+++++++++++++++++++|
| Optical Layer |<-------->| Physical Layer |
|+++++++++++++++++++| |+++++++++++++++++++|
Figure 2: Layered Network
Optical layer, lying under OBS layer, focuses on optical signal
transmission, amplifying, multiplexing and demultiplexing. From the
view of OBS, optical layer offers Optical Channel-Path (OCh-P),
connecting distributed OBS nodes. OCh-P represents the end-to-end
transport of a lightpath across multiple regenerators in the path
[Optical].
OBS nodes are classifies as edge nodes and core nodes. Edge nodes
consist of aggregating queues, CPs' generator and CPs' and DBs'
transmitter or receiver. Core nodes comprise CPs' processor, switch
driver, and switching fabric.
2. Fault Management
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Fault management involves detecting problems in the network and
alerting the management systems appropriately through alarms. If a
certain parameter is being monitored and its value falls outside its
present range, the network equipment generates an alarm [Optical]. In
other cases, alarms could also be triggered by outright failures,
such as the failure of switch driver or other components in the
system. Fault management also includes restoring service in the event
of failure, but we organize the latter as a separate section.
2.1. Failure Profiles
Various failures may occur in a multi-layer network. Some are caused
by OBS network elements; Some are not but may be restored by OBS
layer mechanisms. Providing an exhaustive list of all the possible
failure types is aimless, but it is worth listing the main categories
of failures, namely link failures and node failures, from OBS
perspective.
2.1.1. Link Failures
Several types of failures may result in OBS link failure.
Fiber cut may cause all the OCh-Ps in a physical link to fail. If
optical layer refuses to provide protection, it can be passed on to
OBS layer.
Optical equipment failures such as amplifier failure also belong to
optical layer, but it may affect several OCh-Ps and decrease the
transmission capacity to some degree. Such failure can be protected
at optical layer too.
OBS node interface failure may occur in signalling channel, as unlike
DBs, CPs have to experience o-e-o conversion and electrical
process at core nodes. However, such type of failure can not be
protected by optical layer. (TBC)
2.1.2. Node Failures
There are multiple possible causes of node failures whose nature has
very different implications.
Power supply outage provokes both a control and switching plane
failure. But in most cases, backup power supplier will take over to
work.
Switching fabric failure at core nodes would cause traffic loss or
mis-forwarding. So measures should be taken to monitor switching
fabric status and report to the control model or even to other nodes.
Software failures make impact on some specific features or software
crash of the node operating system, for example, packets' aggregation
failure at edge nodes or CPs' process failure at core nodes.
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Hence, besides software bug eradication by testing, a modular
architecture may limit software component failures to a controllable
scope and the failed software component may be restarted
independently of the other modules. (TBC)
2.2. Fault Detection
Fault detection is the first phase in recovery cycle and its time
is part of total recovery time. This time may depend, for instance,
on the speed of fault detection in a lower network layer and
notification toward upper layers, on the time it takes for the node
to gather all abnormal information from various signals and derive
the exact fault state from diagnosis and so on.
2.2.1. Link Fault Detection
According to link failure profiles, most link failures can be
detected at optical layer from the perspective of OBS, for example by
optical channel-path trace. This trace can be inserted at the end of
the CPs and monitored at various locations at control plane along the
lightpath. Moreover, Optical receivers at each node can perform as
detectors at data plane. Once the optical signal-to-noise ratio falls
below a threshold, alarms would be triggered. (TBC)
2.2.2. Node Fault Detection
When comes to node failure, in case of switching fabric failure, core
node may provide the function of self supervising at optical layer.
But, unlike link failures, most other node failures can hardly be
detected at optical layer and thus lower layer failure notification
is not suitable here. However, the mechanism based on hello protocol
could be feasible by sending a periodic hello message between two
neighbors. When one of the node stops receiving hello messages for a
configurable period, it concludes that a failure of the link between
them or the objective node itself has failed. (TBC)
2.3. Fault Notification
Once failures have been detected and located, other network elements
in the same domain or network should be informed by fault alarming
and the propagation time is also part of recovery time. OBS network
can be distributedly or centrally controlled. In the former case,
routing function is performed by each edge node, while in the latter
one, a central controller is responsible for scheduling. Here, we
only consider distributed situation.
2.3.1. Link Fault Notification
OCh-Ps are usually unidirectional. Thus any failure at OCh-P, such as
fiber cut or amplifier outage, will be detected by Recovery Tail-End
(RTE) instead of Recovery Head-End (RHE)[Recovery]. On detecting, RTE
immediately starts to notify other nodes by Link State Advertisement
(LSA). LSA, can be broadcasted or transmitted from end to end.
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In OBS network, edge nodes keep route's table, perform routing and
set offset time between CPs and DBs. Thus LSA must be firstly
reported to each edge node and every node in the network should keep
the routes to each edge node. As LSA is critical to DBs' routing,
LSA transmission must be reliable. So each edge node having received
LSA must return an acknowledgement to RTE. Sometimes, existed link
failure may cut off LSA transmission and thus alternative route
should be pre-computed.
In case of core node acting deflection, these core nodes need to be
alarmed too, otherwise deflected DBs would probably blocked by link
failures. (TBC)
2.3.2. Node Fault Notification
According to node failure profiles, switching fabric at core node and
outright crash will be discussed in this paragraph.
Single switch failure may reduce core node switching throughput and
may be resolved by substituting the outage switch with a new one.
However, when most part of switching fabric fails to work, the total
switching node must alarm other nodes about node failure by
broadcasting or transmitting Node State Advertisement (NSA) from end
to end. Similar to link fault propagation, NSA must be firstly sent
to edge node and then to deflective core node, if necessary.
upperlayer
traffic | 2. NSA |------| 1. Hello |------|
<------->| <-------- | core | --------> | core |
<------->|------------| node |---------------| node |
<------->| |------| no response |------|
edge node
(a) node failure
upperlayer
traffic | 2. NSA |------|1. Hello |------|
<------->| <-------- | core |-------->X | core |
<------->|------------| node |---------------| node |
<------->| |------| no response |------|
edge node
(b) link failure
upperlayer
traffic | 3. NSA |------| 1. Hello |------|
<------->| <-------- | core | --------> | core |
<------->|------------| node |---------------| node |
<------->| |------| X<-------- |------|
2. Hello
edge node
(c) link failure
Figure 3: Hello Message Loss
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Power supply outage or software OS crash may lead to traffic
black-holded and the control model of core node is unable to report
failure by it self. Thus, such kind of failure must be discovered and
notified of by neighbor nodes sending NSA. Once a neighbor node does
not receive hello message from the objective node for a given time,
this node should report the failure to others. However, in such case
as shown in Figure 3, the neighbor node can hardly distinguish node
failure from OCh-P failure. Therefore, such a NSA need to cooperate
with a LSA to identify the real problem. (TBC)
3. Restoration at OBS Layer
OBS layer performs as a data-link layer located between optical layer
and network layer (IP layer). The major duties of this layer is to
provide reliable and quick bitpipes for its client layer (network
layer) and to make effective utilization of huge bandwidth of its
server layer.
3.1. Motivation to Restore at OBS Layer
OBS is a promising technology to explore huge optical bandwidth for
upperlayer applications. Besides OCh-P failure, network elements
outage at OBS layer, such as edge node or signalling process model
failure at core node, can be hardly protected by optical layer,
though upperlayer traffic can be partly restored by IP layer itself.
Thus, to restore at OBS layer could at least provides survivability
for OBS network elements.
Restoration at IP layer is so versatile to deal with failures at
lower layer, but the problem is that total recovery process is rather
time-consuming and even can not meet the QoS of real-time traffic.
Recovery time of IP restoration mechanisms usually ranges from
tens of seconds to minutes. However, restoration at OBS layer is more
responsive and faster than its IP's counterpart, as OBS is able to
allocate optical bandwidth directly.
Protection schemes at optical layer is not mature in mesh topology as
compared to ring topology or point-to-point connection. So
restoration at OBS layer may offer alternatives for optical
protection at a finer granularity. For example, some link failure may
be detected at optical layer, but protection mechanisms at the same
layer will not be triggered with failure passed up on to OBS layer.
Then OBS restoration starts immediately to work.
DBs play as containers to carry upper layer packets across OBS
switching nodes. At ingress edge node, packets sharing the same
destination or QoS could be enclosed into DBs. CPs could help to
route according to not only addresses but also service level. IP
layer can also provide differentiated service for each packet, but
obviously OBS is able to carry out it far more efficient as data can
be routed by larger containers, DBs, instead of packets.
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3.2. Single Layer Restoration Schemes
Protection or restoration schemes should meet the need of performance
evaluation criteria, such as recovery time, bandwidth efficiency and
so on. Performance criteria is defined by service level of traffic.
Hence a wide range of recovery mechanisms may exist to supplement
with each other to serve for various traffic.
3.2.1. Link Failure Restoration Schemes
When comes to link failure, traditional optical protection mechanisms
aim to seek abundant wavelength resources to establish a new
lightpath for data transmission. However, as OBS works at
sub-wavelength granularity and is characteristic of statistical
multiplexing, in case of failure, what OBS restoration schemes seek
is time slot at different wavelength, as the multiplexing density of
DBs is controllable, unlike OCS or TDM.
One or two channel in a link failing to work means the decrease of
transmission capacity of that link. Once nodes are equipped with
wavelength conversion, traffic can be easily multiplexed to other
channels in the same link without help of additional wavelength. When
the transmission capacity of a certain link has decreased below a
threshold and become intolerable, traffic that is used to pass this
link will have to be deflected or rerouted. For example, cable cut
can be considered as total channel failure at that link.
Deflection is a local method, in which a new route will be selected
by RTE. This new route may be pre-computed or computed on-the-fly.
Then RTE transmits LSAs with the new route to each edge nodes by
broadcasting or end-to-end transmission. In case of core node able to
deflect, these core nodes need to be alarmed too, otherwise deflected
DBs would probably blocked by link failures.
Rerouting is a global and more radical method by edge nodes selecting
a new path. Once a link failure occurs, RTE simply reports the
location of the failure to each edge node, which will figure out a
new path by pre-computing or computing on-the-fly.
Deflecting method is usually faster than rerouting, but rerouting can
be more bandwidth effective as it focuses on the global resources
[Restoration]. So in practice, two methods may work sequencially or
integratedly to collaborate with each other. (TBC)
3.2.2. Node Failure Restoration Schemes
Node failure is much severer than link failure, which can obviously
cause all the links connected to it to fail and even change the
topology of network. So a backup node and backup switching fabric is
indispensable. Once failures occur, all traffic could be switched to
the backup one. In case of planned node failure, resulted from
hardware and software of switching node updating, traffic may be
gracefully rerouted to the backup node.
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If no backup node is available, recovery schemes for node failure at
OBS layer can only restore traffics forwarded by the fault core node.
Traffics that begin or end with the fault node can not be restored.
And as well edge nodes can not be restored either.
With respect to core nodes, there are two different kinds, namely
ingress/egress switching node and intermediate switching node
according to JIT protocol. Ingress switching node is the outlet of
traffic from edge node and egress switching node is the destination,
so any failure at above two nodes is similar to that at edge nodes.
Only traffic forwarded by fault intermediate switching node can be
restored by deflecting or rerouting. (TBC)
3.3. Multi-layer Restoration Schemes
OBS layer is an intermediate layer between optical layer and IP
layer. In realistic network, each of them has its own recovery
mechanisms. However, not every failure in a particular network layer
can be resolved by recovery mechanism in that same layer. Upon
detection of a fault, more than one layers could initiate recovery
actions. If these recovery mechanisms are merely triggered by
detection of a fault, an uncoordinated and inefficient action may
result.
3.3.1. IP Dynamic Routing
Restoration at IP layer is mainly accomplished by exchanging, between
adjacent routers, control messages that are used to update the
routers' tables, thus enabling IP packets to be dynamically rerouted
around link and node failures. However, it is usually slow, from
several to hundreds seconds, and its behavior is unpredictable.
Some enhancements of the protocol have been proposed to overcome its
drawbacks. One approach is equal cost multi-path forwarding, in which
the router relies on more than one path for transmitting packets
sharing a common destination by maintaining multiple next-hop entries
for the same destination within each router's routing table. Another
approach partitions the network into multiple areas, as defined in
hierarchical link state routing protocols such as OSPF and IS-IS.
3.3.2. MPLS Protection Switching
MPLS protection switching is an alternative approach to circumvent
the latency drawback of dynamic routing. MPLS protection entities can
be set up either dynamically or in a prenegotiated way. Protection
entities, dynamically set up, restore traffic based on failure
information, bandwidth allocation, and optimized reroute assignment.
Prenegotiated protection consists of working LSPs that have
preestablished protection paths. In general, dynamic protection
increase resource utilization but requires longer restoration times
than preestablished protection.
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3.3.3. Optical Layer Resilient Schemes
Both the optical channel (OCh) section and optical multiplex section
(OMS) feature dynamic restoration and preplanned protection. The main
difference between OCh and OMS resilient schemes is represented by
the granularity at which layers operate. OCh resilient schemes
protect individual lightpaths, thus allowing selective recovery of
optical line terminal failures. OMS resilient schemes work at the
aggregated signal level, thus recovering all lightpaths present on
the failed line concurrently.
Protection schemes, namely Dedicated Path Protection (DPP), Shared
Path Protection (SPP), Optical Unidirectional Path Switched Ring
(OUPSR), Optical Bidirectional Path Switched Ring (OBPSR) and so on,
guarantee service restoration completion times of hundreds, tens and
even fractions of milliseconds. However, restoration schemes at
optical layer are slower and less mature than protection schemes.
3.3.4. Recovery Scheme Comparison
From the view of rerouting, restoration schemes at OBS layer is
similar to MPLS protection switching. But MPLS is processing and
routing in electronic domain, while DBs in OBS network is switched in
optical domain and CPs need o-e-o conversion. In MPLS protection
schemes, labels are followed closely by payload and distributed by
LDP. However, OBS CPs are set out an offset time prior to BDs and the
route is computed at edge nodes according to Dijsktra algorithm.
OBS enables optical network to become more flexible and intelligent
by enhancing signalling at control plan. DBs with proper size is
multiplexed statistically onto a wavelength, which can lead to more
efficient utilization of wavelength bandwidth than TDM. So
restoration schemes at OBS layer may provide alternatives for
traditional optical protection mechanisms, for example, deflection
according to DBs' quality of service.
3.3.5. Operational Coordination
Coordination between resilient schemes, at distinct layers, is
required to avoid multiple schemes concurrently activated upon a
single network fault. Two kinds of coordinating strategies are
sequential approach and integrated approach [Recovery]. In the former
scheme, the server layer may start recovery immediately, whereas the
recovery mechanism in the client layer has a build-in hold-off time
before initiating the client recovery process. The latter one
combines several mechanisms into one integrated multi-layer recovery
schemes coordinated by management plane.
However, the sequential approach is easier to apply than the
integrated one as the latter may cause great complexity to control
and management planes. When comes to restoration at OBS layer, as the
sequential approach requested, the recovery time should be shorter
than that of IP layer and near to that of optical layer, for example
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total recovery time ranging from tens milliseconds to several seconds.
4. Acknowledgements
This research is funded by the National High Technology Research and
Development Program of China (863 Program).
The authors are grateful to other colleagues for their work and
useful suggestions.
5. References
[Recovery] J. Vasseur et al., "Network Recovery". Morgan Kaufmann
Publishers, 2004.
[Optical] R. Ramaswami and K. N. Sivarajan, "Optical Networks".
Morgan Kaufmann Publishers, 2004.
[IPOWDM] S. Dixit, "IP OVER WDM: Building the Next-Generation
Optical Internet". WILEY-INTERSCIENCE, 2002.
[Restoration] Y. Xin et al., "Fault Management with Fast Restoration for
Optical Burst Switched Networks". BROADNETS'04.
6. AUTHORS' ADDRESSES
Jia Jia Liao
National Laboratory on Local Fiber-Optic Communication Network
& Advanced Optical Communication System, Peking University, 100871
P.R. China
Email: jjliao@ele.pku.edu.cn
Ping Zhang
National Laboratory on Local Fiber-Optic Communication Network
& Advanced Optical Communication System, Peking University, 100871
P.R. China
Email: zhangping@pku.edu.cn
Zheng Bin Li
National Laboratory on Local Fiber-Optic Communication Network
& Advanced Optical Communication System, Peking University, 100871
P.R. China
Email: lizhengbin@pku.edu.cn
An Shi Xu
National Laboratory on Local Fiber-Optic Communication Network
& Advanced Optical Communication System, Peking University, 100871
P.R. China
Email: lyrxas@pku.edu.cn
7. IPR NOTICE
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Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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might or might not be available; nor does it represent that it has
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Copies of IPR disclosures made to the IETF Secretariat and any
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The IETF invites any interested party to bring to its attention any
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8. FULL COPYRIGHT STATEMENT
Copyright (C) The Internet Society (2006). This document is subject
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This document and translations of it may be copied and furnished to
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