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Towards Secure and Self-Diagnosable Optical Networks

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Abstract

Given the pivotal role of optical networks in supporting critical societal services, their robustness to deliberate attacks targeting disruption at the physical layer requires advanced approaches for security assurance, diagnostics and response. This paper analyzes the necessary advancements in optical network security needed to achieve secure and self-diagnosable systems. Keywords-optical network security, attack signature, security diagnostics, attack-aware network design, attack detection, attack recovery.
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Towards Secure and Self-Diagnosable
Optical Networks
Marija Furdek
School of Electrical Engineering and Computer Science
KTH Royal Institute of Technology
Stockholm, Sweden
marifur@kth.se
AbstractGiven the pivotal role of optical networks in
supporting critical societal services, their robustness to
deliberate attacks targeting disruption at the physical layer
requires advanced approaches for security assurance,
diagnostics and response. This paper analyzes the necessary
advancements in optical network security needed to achieve
secure and self-diagnosable systems.
Keywordsoptical network security, attack signature,
security diagnostics, attack-aware network design, attack
detection, attack recovery.
I. INTRODUCTION
Optical networks are a critical part of communication
infrastructure responsible for transferring immense amounts
of data generated by services indispensable for a vital and
functional society. Security breaches that violate
confidentiality, integrity or availability of communication at
the optical layer can proliferate to multiple network layers
that reside on top of it, magnifying the detrimental effects
across the entire network stack. In spite of such tremendous
significance, optical communication infrastructure has
traditionally been viewed as secure, and little attention has
been paid to optical network security. In a changing cyber-
security landscape, where malicious attacks are in general
becoming ever larger in size, magnitude and sophistication,
and the deployment of critical optical network infrastructure
is widely exceeding a secure perimeter, optical network
security is receiving increasing attention from researchers in
industry and academia.
Optical-layer security threats are typically categorized
using the well-established division according to the objective
of the attacker into eavesdropping and service disruption
attacks. In an eavesdropping attack, the adversary gains
unauthorized access to the optical signal (e.g., by bending the
fiber and detecting the leaked light) and the unprotected
carried data. The carried information can be protected by
encrypting it at different network layers. More and more
equipment vendors offer devices for ‘in-flight’ data
encryption at the optical layer to protect the aggregate data at
line rates beyond 100/200 Gbit/s, allowing for secure end-to-
end communication between data centers and enterprise
sites. In addition, significant efforts are invested into post-
quantum cryptography and quantum key distribution, which
are out of scope of this study.
In service disruption attacks, which are in the focus of
this paper, the attacker can damage critical optical
components (e.g., cut an important fiber link or disable a
node), insert a harmful signal (e.g., jamming), or tamper with
the properties of the carried signals (e.g., via external
modulation) and degrade their quality [1]. In order to
increase the security of the optical network from such
attacks, substantial, multifaceted efforts are needed in
developing advanced security assurance, diagnostics, and
threat response approaches. Security assurance encompasses
actions aimed at assuring a certain level of security and
managing the related risks. Security diagnostics entail
detection of security breaches and identification of their
cause. Threat response refers to handling of security
incidents and recovery of affected services. The challenges in
realizing the above approaches in optical networks emanate
from several complicating factors. These primarily refer to
the high heterogeneity and complexity of the optical network
infrastructure shared among a multitude of users and
services, the technological and deployment limitations of
optical performance monitoring (OPM) devices, the limited
availability of physical-layer security information to the
network management system, the lack of data analytic
frameworks for attack identification and incident response
approaches, and the immense amount of traffic stressing the
scalability of security approaches. The remainder of this
paper summarizes key challenges and outlines possible
solutions towards enabling secure and self-diagnosable
optical networks.
II. SECURITY ASSURANCE IN OPTICAL NETWORKS
Today’s optical networks are a multi-domain ecosystem
of heterogeneous devices with diverse capabilities, ranging
from legacy equipment with low flexibility to programmable
and agile state-of-the-art components. Moreover, they
aggregate traffic from multiple services with different
security requirements, and carry it across large geographical
areas with different geopolitical circumstances, making
security assurance and risk assessment challenging.
A basic security measure is to minimize the attack
surface, i.e., network exposure to attack, potential intrusion
points, or the extent of potentially incurred damage. At the
optical layer, this can entail, for example, designing the
network topology in a way which aggravates the effort an
attacker needs to invest into causing wide-spread service
disruption. In a data center environment, where replicas of
content are distributed across multiple geographical locations
and users can connect to any network node that hosts the
desired content in an anycast manner, selecting the number
of replicas and their locations plays an important role in
network robustness to targeted link cuts. Approaches for
identification of critical fiber links whose cutting maximizes
the connectivity interruption and reduces accessibility to
content, as well as sparse link addition and content
placement strategies to increase network robustness can be
found in [2] and [3], respectively.
An example illustrating the impact of replica placement
on the content accessibility under link cut attacks is shown in
Fig. 1a). In an effort to maximize the disruption, the two
links with the highest centrality (5-7 and 6-8) are cut by an
attacker. In the top solution, the two most central nodes
(nodes 5 and 6) host the content and the attack disconnects
five network nodes (nodes 7 to 11) from it. The measure of
Average Content Accessibility (ACA) [3] quantifies the
This article is based upon work from Celtic-Plus project SENDATE-
EXTEND and COST Action 15127 RECODIS.
portion of nodes which lose access to a replica in the
considered attack scenario. In the bottom solution, where
nodes 3 and 9 host the content, the considered attack scenario
does not disconnect any nodes, and it takes much greater
effort to prevent network nodes from accessing content.
In order to enhance security from attacks that rely on
insertion of harmful signals, careful assignment of physical
routes and spectrum to optical connections can reduce the
extent of damage caused by anticipated attacks and confine it
to a subset of known connections [4]-[6]. Moreover,
specialized devices for monitoring [7] and/or thwarting [8]
harmful signals can be placed at strategic network locations
to further enhance the security from the corresponding
service disruption attacks.
III. DETECTION OF OPTICAL-LAYER SECURITY BREACHES
Real-time tracking of optical network performance is a
fundamental prerequisite for quick and accurate attack
detection. Optical performance monitoring (OPM)
encompasses estimation and acquisition of different physical
parameters of transmitted signals [9]. A major obstacle in
detection of physical-layer threats is the inability of current
optical networks to acquire extensive real-time information
about the signals’ physical states. Namely, OPM is costly
and there is no standard set of OPM parameters supported by
the monitoring devices (typical OPM parameters include
average and peak optical signal power, optical spectrum, eye
diagram, polarization state, dispersion, and optical/carrier
phase characteristics). Instead, it greatly depends on the
details of the physical network design and OPM device
deployment, which is cost-constrained and, consequently,
sparse. Therefore, efficient detection of security breaches
calls for intelligent approaches for analyzing the security
status of individual connections, and for correlating such
status across the network in order to meet two main
challenges:
i. How to distinguish when a particular connection is
affected by an attack?
ii. How to identify which network element (e.g., link or
node) is the ground zero of a physical-layer breach?
Since the existing physical-layer attack models are
scarce, attack detection approaches are very limited as well.
A major step forward in achieving situational awareness in
optical networks is enabled by the introduction of machine
learning (ML) techniques. ML-based techniques can achieve
great benefits in solving problems where explicit statistical
characterization is difficult or unknown. When physical-layer
security is considered, it is extremely difficult to explicitly
model the consequences of a variety of known and
forthcoming attack methods, making security situation
assessment an ideal ground for the application of machine
learning methods. An ML-based approach for detection of
power jamming attacks of different intensities was recently
proposed in [10], by analyzing the resulting degradation of
the considered optical channel. The artificial neural network
approach proposed therein analyzes 25 different OPM
parameters provided by the coherent receiver at the
connection destination, and is able to identify an attack and
its intensity with 93% average accuracy.
Being able to identify when a connection is affected by
an attack by analyzing the rich OPM data set at its
destination, where the signal must be detected in any case,
realizes an important prerequisite for discovering attack
patterns at the network level. Existing approaches for
detection of high-power signal insertion attacks [11][12]
track power surges associated with an attack along each
connection, requiring knowledge of real-time status of all
connections at the input and the output ports of all nodes in
the network. Aside from their limited applicability to only
attacks which cause power surges, these solutions are costly
and unscalable in the current networks that support tens or
hundreds of nodes and hundreds of high-capacity
connections, possibly causing alarm storms and putting a
tremendous burden of message parsing on the control plane.
A promising approach to attack source identification is
correlating the security status of different connections and,
based on their affected subset, deducing the insertion point
and/or the propagation path of the attack. Fig. 2 illustrates a
promising approach for identifying the attacking connection
that carries a harmful signal based on attack syndromes [13].
An attack syndrome is a binary word that describes the scope
of a considered attack scenario, and is defined for each
potential source of attack. Each bit in the syndrome
represents one connection, and its value set to 1’ signifies
that the corresponding connection is identified as degraded
by an attack (‘0’ otherwise). The attack syndrome is then
compared to the known syndromes in the database to identify
the source of the breach. In this way, the optical network acts
as a sensor for physical-layer attacks and enables security
self-diagnostic abilities.
To provide a meaningful diagnostic insight, attack
syndromes of each potential source of attack must be unique.
For the routing example shown in the top of Fig. 2,
connections C1, C2 and C3 have matching syndromes, as do
connections C4 and C5. The shown syndromes are formed for
1
2
3
4
5
6
7
8
9
10
11
1
2
3
4
5
6
7
8
9
10
11
Content-hosting node
Average Content
Accessibility:
|{7,8,9,10,11}|/11 = 45%
Average Content
Accessibility: 100%
Fig. 1. The impact of content placement on the accessibility of content
when two most central links are deliberately cut.
1
2
3
4
5
6
C1C2
C4
C3
C5
C1 C2 C3 C4 C5
C1 1 1 1 0 0
C2 1 1 1 0 0
C3 1 1 1 0 0
C4 0 0 0 1 1
C5 0 0 0 1 1
1
2
3
4
5
6
C1C2
C4
C3
C5
C1 C2 C3 C4 C5 P
C1 1 1 0 0 0 1
C2 1 1 0 0 0 0
C3 0 0 1 0 0 0
C4 0 0 0 1 1 1
C5 0 0 0 1 1 0
P
Attack syndromes:
Ambiguous
syndromes
Ambiguous
syndromes
Unique
syndromes
Fig. 2. Localization of the harmful connection through security status
correlation by means of attack syndromes. Tailored routing and strategic
addition of attack monitoring probes reduces syndrome ambiguity.
a jamming attack scenario, by assuming that any connection
can carry a jamming signal inserted at its source node,
propagating to the destination node, and affecting all
connections that share a common link with it. Syndrome
ambiguity can be resolved through tailored routing aimed at
reducing the number of generated matching attack
syndromes, as well as sparsely adding attack monitoring
probes. A solution with slightly different connection routing
and an added monitoring probe P, shown in the bottom part
of Fig. 2, is capable of unambiguously identifying the
harmful connection by means of unique attack syndromes.
Note that this solution also reduces attack surface, i.e., the
number of connections which can be affected if C1, C2 or C3
carries a harmful signal is lower than in the upper figure.
IV. RECOVERY FROM SECURITY BREACHES
Recovery from security breaches implies an appropriate,
automated reaction of the network management system to
compensate for the degradation induced by an attack and
restore operability of the affected services. Depending on the
attack method, connections can be recovered by changing
their allocated spectrum to a band unaffected by the attack.
Changing the higher-order modulation format to a lower-
order one with smaller sensitivity to physical-layer
impairments (e.g., crosstalk) aggravated by the harmful
signal can also reduce the effect of some service disruption
attacks, particularly those of lower intensity. Such approach
could leverage on the well-known distance-adaptive
modulation format assignment [14] and the field-trialed
approaches for recovery from soft failures via modulation
format adaptation [15].
In more severe attack scenarios, the affected connections
should be rerouted to an alternative path. In doing so, it is
important to ensure that the backup resources are not within
reach of the same attack, which would render the rerouting
futile. An approach for modeling the attack scope of the
working and the backup path of a connection by means of
attack groups can be found in [16]. Therein, protection from
jamming attacks is ensured by guaranteeing that the working
and the backup path of a connection do not share common
elements in their attack groups, i.e., they are not subject to
the same attack.
In data center networks, content placement, assignment
of working and backup data centers, and routing of the
respective paths can also play a significant role in recovery
from service disruption attacks, as illustrated on a simple
example in Fig. 3. In the solution shown in the left side of the
figure, the harmful signal affects both the working and the
backup path of both connections on their common links 3-5
and 6-8. The solution to the right side uses slightly different
content placement and path routing, and is not affected by
the considered attack scenario.
V. CONCLUSIONS
This paper focused on progress and issues in designing
and operating secure and self-diagnosable optical networks
in the presence of physical-layer breaches, primarily those
aimed at service disruption. While the related efforts are
picking up pace, a number of challenges remain to be
addressed. These primarily refer to a predominant lack of
security monitoring and response systems which would
collect OPM data in real time, analyze it to identify
signatures of various threats, share with relevant subsystems
and stakeholders, and autonomously decide on fast recovery
actions.
ACKNOWLEDGMENTS
The author gratefully acknowledges the contributions of Carlos
Natalino and Prof. Lena Wosinska (KTH), Prof. Vincent Chan
(MIT), Marco Schiano and Andrea Di Giglio (Telecom Italia),
Federico Pederzolli and Domenico Siracusa (Fondazione Bruno
Kessler), to the presented security diagnostics concepts and
approaches.
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Content-hosting node
Working path Backup path Harmful signal
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... Management of opticallayer security has been in focus of substantial research efforts worldwide. It can broadly be classified into [12]: (i) security assurance through modeling of attack consequences and attack surface minimization [13]- [16]; (ii) security assessment through monitoring and detection of attacks, which is the primary goal of this work; and (iii) attack recovery through re-configuration of affected connections, attack source neutralization and network adaptation [17], [18]. ...
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