1. Introduction
Survivability refers to the ability of the system to recover in time after the occurrence of abnormal events. It is a promising method to solve the problems of faults and various attacks in vehicle communication networks. The goal of survivability research is to analyze and evaluate the survival ability of vehicle networks, in order to find out the hidden dangers and weak links in the network and take a series of effective measures to improve the survival ability of network links.
The effective communication between the vehicle and roadside infrastructure (vehicle to infrastructure, V2I) in vehicle networks is an important factor for the road traffic convenience and safety, where the roadside infrastructure mainly includes a roadside unit (RSU), wireless access point (Wi-Fi), base station (BS, for WIMAX/LTE/4G/5G/6G), and so on. Via the roadside infrastructure, the vehicle personnel can search the optimal path to the destination, understand the real-time traffic information, transmit the current emergency information, download entertainment and news video information timely, and so forth. Thus, the connectivity of each communication link in V2I networks is key to ensure the normal operation of the whole vehicle network, and it is also reflected in the survivability of the vehicle network. It can be observed that, in the actual process of V2I network communication, due to the high-speed mobility of the vehicle, there is various randomness and uncertainty when connecting to infrastructures, meanwhile some abnormal conditions are usually encountered, leading to an interruption or failure of the link connection. These factors include encountering a denial of service (DoS) attack [
1], channel fading, high-level objects, human accidents, natural disasters, and so on. In addition, the frequent switching between V2I links also provides the problem of communication delay or failure. Aiming to solve these problems, it is necessary to analyze the survivability for V2I communications to achieve the maximum transmission performance.
On the other hand, software-defined network (SDN)-based vehicle networking refers to the programmable design and flexible management under the control of the software program center [
2,
3]. This mobile management mechanism can be effectively adapted to the dynamic change of vehicle networks, which is more suitable for future vehicle networks based on 5G and 6G technologies than the traditional fixed management mechanism. Due to the advantage of a centralized control for SDN, the link connectivity can be effectively controlled in vehicle networks. More specifically, a SDN central controller can effectively switch to other idle links to continue the data communication, when the link connected to the vehicle has a fault problem and is waiting to be repaired. In these link-switching processes, switching mechanisms under the same or different access technologies can be used to complete the switching. It is known that the previous switching mechanism uses a transmission control protocol (TCP) and a one-to-one internet protocol (IP) path to switch frequently via a single network interface, resulting in a possible seamless connectivity along with the latency. Recently, a new switching mechanism was investigated by using multi-path transmission control protocol (MPTCP), with advantages that include making no changes to the original roadside infrastructure but simply configuring the vehicle with multiple network interfaces and IP paths, it can satisfy the one-to-many link seamless switching and significantly improve the network performance.
In recent years, many researches related to MPTCP have been applied to emerging networks, such as vehicle networks, future long term evolution (LTE) networks, an 5G networks [
4,
5,
6,
7,
8]. In Reference [
4], the authors described how MPTCP can effectively utilize the current radio technology and switch robustly to keep the vehicle connection and improve the network performance. Due to that, MPTCP is one of the most important candidate protocols to improve end-to-end (E2E) communication reliability, elasticity, and bandwidth efficiency in 5G mobile networks. The authors in Reference [
5] introduced the applications of MPTCP to LTE networks in detail. In Reference [
6], the MPTCP congestion control extension algorithm for LTE networks is proposed with the goal of aggregating the available bandwidth of multiple paths, as well as that it can avoid the common single-path TCP transmission of shared paths attack behavior. In addition, the combination of MPTCP and SDN in V2I networks within the vehicle short-distance communication is proposed in Reference [
7], where the connectivity between the vehicle and Wi-Fi, vehicle and RSU are simulated by using Mininet-WiFi. Moreover, the connectivity performance is also measured from three indexes: packet loss rate, round-trip time, and average throughput, respectively. In Reference [
8], the authors proposed the use of MPTCP to communicate reliably on the Internet of Vehicles (IoV).
It is worth noting that, in this paper, the survivability refers to the service quality of the system to complete the key tasks in time when the software and hardware fail, which not only pays attention to the overall security of the network system but also ensures the timely and effective transmission of the key information. On the basis of this, the study of survivability has been applied in various networks, such as information systems [
9], wavelength division multiplexing (WDM) fiber optic networks [
10,
11], asynchronous transfer mode (ATM) networks [
12], internet [
13], military communications networks [
14], satellite communications networks [
15], wireless sensor networks [
16,
17,
18], and vehicular ad-hoc networks (VANETs) [
19]. Moreover, the research on survivability mainly includes the definition of survivability, the model of survivability, and the method of survivability analysis. The definition of survivability varies according to the application environment [
20,
21,
22,
23,
24,
25]. The present survivability models are mainly the Markov model, Markov reward model, semi-Markov model, and stochastic graph model [
21,
24,
25,
26,
27,
28,
29,
30]. To further improve the communication link survivability when the network fails, some effective modeling methods are introduced in the literature [
31,
32,
33,
34,
35]. In addition, survivability analysis mainly includes quantitative analysis, in which quantitative analysis is presented in a numerical form and the performance is more intuitive.
Most of the present researches on the improvement of the network performance method are mainly to evaluate the network operations from the point of view of a theoretical algorithm and simulation. To the best of our acknowledge, few studies focus on the research and applications of SDN- and MPTCP-based vehicle networks, especially for V2I network communications. Moreover, as one of the evaluation indexes of the network performance, survivability is less applied in the vehicle network, which refers to the ability of the network to still transmit tasks (or continuous service) after the failure and has a certain research value in certain kinds of vehicle networks. Therefore, it is necessary to study vehicle network survivability in the process of a random dynamic change of vehicle network topology and the failure of vehicle network links caused by various uncertain factors. With the above observations, our motivation is to analyze and evaluate the survivability in different vehicle network communication models and propose effective survivability models to provide the vehicle network survivability in abnormal situations. This paper proposes to use the probabilistic checking method to accurately analyze and evaluate the survivability of a SDN-based vehicle network from the point of view of network link failure (including software and hardware failure) and recovery. The contributions of this work can be summarized as follows:
Because the research on the survivability focuses on the direct communication between vehicle to vehicle (V2V) and V2I networks, a MPTCP mechanism for SDN-based vehicle network survivability model is proposed. In the action of the SDN central control and MPTCP, the link between each V2I can be switched seamlessly, leading to that results in the vehicle network still have some survivability under abnormal conditions (i.e., software or hardware is destroyed), and the survivability of the network is improved rapidly for a timely repairing mechanism.
Current survivability research methods normally adopt the mathematical theory derivation and simulation, which consume a lot of time and energy. Therefore, in this work, we propose time-based attributes by using the probability model detection. During analyses, by using probabilistic model detector PRISM [
36,
37,
38], we can not only automatically search all the state space but also use continuous stochastic logic (CSL) to define the properties of the proposed model at various angles, verify the model, and obtain accurate numerical analysis results. The proposed method avoids the complicated mathematical derivation process and carries out the automatic mathematical accurate calculation to the complex survivability model. In the meanwhile, it has a simple and high efficiency characteristic and a certain reference value to the other network survivability research.
SDN-based vehicle networks currently focus more on reliability and security research but not for survivability. This paper proposes a survivability study for a SDN-based vehicle network. Since that survivability pays attention to the overall security performance of the network system, considering whether the system can "survive" under the accident of attack and hardware and software failure. Therefore, this paper comprehensively considers the software and hardware failure problems that may be encountered in the whole V2I network, injects emerging MPTCP protocol into the survivability comprehensive model, and analyzes the survival performance of the proposed model.
Due the use of single form Markov chain modeling for current survivability models, this paper considers two fault types that exist in a SDN-based vehicle network that lead to the failures of V2I link communications, these are, the vehicle node fault and the link fault. In what follows, the survivability model of these two fault types is established separately and the survivability models are considered. From the proposed survivability comprehensive model of the network, the analysis results show that the model satisfies the survivability property definitions.
The remainder of this paper is organized as follows.
Section 2 describes the proposed SDN-based V2I vehicle network.
Section 3 proposes the survivability definition of the V2I vehicle network.
Section 4 provides a probabilistic model checking approach to analyze the survivability of the V2I network.
Section 5 concludes this paper and provides possible future works.
2. SDN-Based V2I Network Communications
The SDN-based vehicle network architecture is shown in
Figure 1, which is divided into three layers: the data layer, control layer, and application layer. For the data layer, there are three elements: the vehicle, infrastructure, and switching route. The switching between communication links in V2I networks is completed through switches and/or routers. For the control layer, the SDN controller serves as a network center management part, separating the network control and the network topology of the vehicle networks, where the original network architecture extricates from the hardware control, instead of a software programmable design to centralize the dynamic management of various network parts. By using the Open Flow protocol at the south interface, it can be connected to various switching or routing devices in the data layer. Particularly, its north interface connects each application service and coordinates and controls each application service program to carry on effectively. On the other hand, for the application service layer, it mainly includes applications of the vehicle network: the security application service of the vehicle network, web service, media service of watching or downloading video, mobile information service of querying the electronic map in real time, real-time communication service, and so on. Each of these applications is controlled by the SDN controller.
To analyze and study the survivability of SDN-based V2I vehicle networks, we establish a V2I network communication model based on the vehicle network architecture, which is shown in
Figure 2. In this V2I network communication model, each vehicle equips three V2I communication links in three directions, namely the vehicle-to-RSU connection, vehicle-to-Wi-Fi connection, and vehicle-to-BS (such as LTE/4G/5G/6G) connection. In addition, the connection between the vehicle and RSU is carried out through the IEEE 802.11P protocol, which guarantees short-range communication between vehicle and vehicle. Moreover, vehicles and Wi-Fi IEEE802.11 standards are adopted to connect the vehicle network interface via the wireless access point (AP). For the MPTCP application, to improve the quality of network services, it is assumed that each vehicle involves multiple network interfaces. Each of them switches through MPTCP ports. The transfer between the TCP1, TCP2, and TCP3 interfaces in the MPTCP is controlled and switched by the Open Flow protocol in SDN central controller. In the
Figure 2, due to the high speed movement of vehicles, there are some uncertain abnormal factors, such as the malicious vehicle intrusion or denial of service attack, channel fading, man-made accidents, natural disasters, and so on. All of these events can possibly lead to a breakdown for the V2I network and communication link. When one of the V2I links fails, MPTCP immediately responds by switching to one of the other two idle links to continue communication until the fault link is repaired in time. If either of the other two links fail, multiple repairs should be performed until the data is restored for transmission. For our proposed analysis and study of the survivability of V2I networks, we consider the fault state, fault type, and ability of the network to continue to complete the transmission task when the failure is repaired in time. The fault state includes three V2I link faults (vehicle and RSU, vehicle and base station, and vehicle and Wi-Fi) and the failure of the vehicle node. We consider that the fault types include link failures and node failures. In addition, in order to further study the ability of data transmission after the network failure, we also define the V2I network survivability according to the vehicle network communication model and then establish the survivability mode of the V2I network.
3. V2I Network Survivability Definition
According to the above analysis of the SDN-based V2I vehicle network communication models, the survivability of our proposed scheme can be defined as:
Definition: The SDN-based V2I vehicle network survivability model (VSNSM) contains a quaternion, i.e., VSNSM = {E, R, P, F}, where the definition details of each element can be summarized as follows:
E is the vehicle network communication application environment, the specific network communication structure is shown in
Figure 2, which is based on SDN vehicle networks. The vehicle serves as a network node and communicates with each infrastructure. The inter-link switching between them controls the MPTCP through the SDN central control section.
R is the specific parameter setting in the vehicle network. There are two types of faults in the vehicle network, which are defined as internal and external failures. For these two faults, the internal fault refers to the failure of the vehicle node itself, and the external fault refers to the V2I link fault. We set the V2I link failure rate as λ
1 and the vehicle node failure rate as λ
2. Their corresponding repair rates are represented by
µ1 and
µ2, respectively. It is further assumed that the number of vehicles in the vehicle network is
N, and the number of possible failures is
n (
n ≤
N). Since each vehicle has three corresponding V2I links, the maximum number of possible link failures is set to be 3
N. The details of specific parameters are concluded in
Table 1.
P denotes the series of probability distributions of parameters set in the R under certain attribute conditions, i.e., the time boundary conditions.
F represents the survivability model as finite state machines of a continuous time Markov chain (CTMC), which is a quaternion as M = {S, S0, L, T}. The parameters are described as follows:
S = {S0, S1, S2, …, Sn, …, SN} or S = {S0, S1, S2, …, Sn, …, S3N}, where N is a finite positive integer. Two different state sets correspond to the vehicle node fault and V2I link fault, respectively. The state transition is related to the parameters in R;
S0 stands for the initial state of a finite state machine;
L: S → 2AP represents the label function of atomic propositions (AP) which are true in S;
T: denotes state transfer matrix.
4. V2I Network Survivability Model
According to the V2I definition of vehicle network survivability in
Section 3, we now establish the survivability model of the vehicle under the V2I link failure with the specific design shown in
Figure 3. To simplify the state diagram, we set “
S0,
S1,
S2...
Sn...
S3N” states in the diagram to represent the state “0, 1, 2, 3...
n...3
N” in the definition of survivability, respectively. The model is a CTMC with
N vehicles on the road and their failure probability.
When any one of the V2I links fails, through the SDN control center and the MPTCP protocol, these three links of the vehicle node can be seamlessly switched to continue transmitting data. Moreover, we further define that there are 3N + 1 states in the survivability model of the whole vehicle network, in which the state “0” means no V2I link fault, “1” means that a V2I link fault occurs, and the subsequent digital state analogizes the corresponding V2I link fault number in turn, i.e., the maximum number of link failures in the whole network is 3N. At the same time, with the occurrence of a V2I link failure, in order to ensure the continuous operation of the V2I network, the network will respond to the corresponding timely repair function. According to the survival definition, the repair rate of a V2I link failure is set as µ1. The repair rate after two V2I link failures is 2µ1. Similarly, the repair rate after n V2I link failure is nµ1, and the repair rate after 3N V2I link failures is 3Nµ1. In this model, the number of failures corresponding to different states is repaired to different degrees.
The transient probability of the survivability model is the probability at any t time. It reflects the short-term survivability of vehicle networks. It is expressed as
Pi(
t). When the initial time
t = 0, we have the probability
Pi(
t) =
Pi. According to the state transition, the following transient probability equations can be obtained from:
where
t→∞, and they will tend to steady state. However, the steady state probability
Pn =
Pn(
t) is an important measure reflecting the quality of the network operation. Let the steady state probability be
Pi in the
i-th state, it can be obtained from:
where
, by calculating the above Equations (4)–(6), the steady state probability in the
i state can be obtained as follows:
where
, if the expected number of failures in the time range from 0 to
t is calculated as follows:
The above discussions are related to the survivability model of direct failure of the V2I link, however, in the actual process, the internal fault cause of the vehicle node itself may also indirectly lead to the V2I link failure. For this situation, we establish a V2I survivability model based on vehicle node failure, as shown in
Figure 4. It involves
n states, where
n indicates that the vehicle has no failure number, and the total number of failures at the vehicle nodes is not greater than the total number of vehicles. According to the survivability definition in
Section 3, we set the failure rate of each vehicle node as λ
2, meanwhile, for n vehicles, the possible failure rate is
nλ
2. For
n − 1 vehicles, the possible failure rate is (
n − 1) λ
2, and so on. The corresponding failure rate for each state until the “0” state is λ
2. At the same time, the repair rate of timely recovery after the vehicle failure is
μ2. Let the transient probability of any
t time of the survivability model be
Pi (
t). Then, according to the state transition, the transient probability equations of each state can be expressed as:
Hence, when the time
t tends to infinity, transient states tend to be stationary. The steady state probability is
. Let the steady state probability be
Pi in the
i-th state, we have:
where
. After calculating the above equations, the steady state probability of the survival model
i state
Pi can be obtained from:
If there is
k number of faults in the V2I network, for a period of time between 0 and
t, the expected survivability rate can be calculated as follows:
For practical applications, V2I vehicle networks sometimes have both internal and external faults, that is, a link failure between the vehicle and the infrastructure and a failure of the vehicle node itself. In view of this problem, we can combine these two survivability models to establish a more perfect comprehensive survivability model, as shown in
Figure 5. In this model, we take into account the survivability of the above two fault situations when they occur alone but also the survivability for the case in which these two faults occur simultaneously. In
Figure 5, the set state is represented by (
n,
j), and the parameter n indicates a failure-free number of vehicle nodes. At the same time, the parameter
n is less than or equal to the maximum number
N of vehicles. The parameter
j represents the number of link failures for
j ≤ 3
N. An initial state is represented by (
n, 0). The digital characters “
n” and “0” in this state indicate that the vehicle node and V2I link are in normal condition (no failure), respectively. The state (
n,
j) to state (
n,
j + 1) indicates that the link fails, and the failure rate is λ
1, correspondingly, the link repair rate is
μ1 times the number of fault links; the state (
n,
j) to state (
n − 1,
j) indicates that there is a failure of a vehicle node, the failure rate is λ
2 times the number of normal vehicles before the failure occurs, and its repair rate is set to
μ2; the state (
n,
j) to state (
n − 1,
j − 1) indicates that the vehicle node fault and link fault occur simultaneously. The original link fault is repaired from the
j fault number to
j − 1 fault number. The normal number of vehicle nodes changes from the
n original number to
n − 1 vehicle failure nodes. In this process, one link is recovered and one of the vehicles fails. The failure rate is set to the node failure rate λ
2, which is recovered by the repair rate
μ2. According to the state transition relation of the model, we get the steady state and transient probability of the survivability of the model. Assuming that the steady state probability of the model is
Psu, then the probability has the following relation with the probability of each state:
for the initial state (
n − 1,
j), the transient probability for
t = 0 is
Pn−1, j(0) =
Pn−1,j. Hence, we have:
where
, let the transient probability of the model in [0,
t] time be
Psu (
t), then we can get:
On the basis of the above survivability model, the following sections will focus on the probability model checking and validation of the survivability with PRISM. Moreover, according to the state transition and arrival situation, the relevant survival attribute formula is defined by CSL, and the survivability probability analysis and comparison are provided under various time boundary conditions.