Data-link layer is responsible for implementation of point-to-point flow and error control mechanism.
Flow Control
When a data frame (Layer-2 data) is sent from one host to another over a single medium, it is required that the sender and receiver should work at the same speed. That is, sender sends at a speed on which the receiver can process and accept the data. What if the speed (hardware/software) of the sender or receiver differs? If sender is sending too fast the receiver may be overloaded, (swamped) and data may be lost.
Two types of mechanisms can be deployed to control the flow:
This paper focuses on the routing protocols of VANETs between vehicle to vehicle communication, the routing protocol classification and the related research open issues in VANET routing today on the basis of various aspects such as characteristics, quality of services, techniques used, routing algorithms, routing information, network architecture etc. VANET routing is broadly classified into following categories: multicast, broadcast, unicast, topology-based and position-based routing.
- Stop and WaitThis flow control mechanism forces the sender after transmitting a data frame to stop and wait until the acknowledgement of the data-frame sent is received.
- Sliding WindowIn this flow control mechanism, both sender and receiver agree on the number of data-frames after which the acknowledgement should be sent. As we learnt, stop and wait flow control mechanism wastes resources, this protocol tries to make use of underlying resources as much as possible.
Error Control
When data-frame is transmitted, there is a probability that data-frame may be lost in the transit or it is received corrupted. In both cases, the receiver does not receive the correct data-frame and sender does not know anything about any loss.In such case, both sender and receiver are equipped with some protocols which helps them to detect transit errors such as loss of data-frame. Hence, either the sender retransmits the data-frame or the receiver may request to resend the previous data-frame.
Requirements for error control mechanism:
- Error detection - The sender and receiver, either both or any, must ascertain that there is some error in the transit.
- Positive ACK - When the receiver receives a correct frame, it should acknowledge it.
- Negative ACK - When the receiver receives a damaged frame or a duplicate frame, it sends a NACK back to the sender and the sender must retransmit the correct frame.
- Retransmission: The sender maintains a clock and sets a timeout period. If an acknowledgement of a data-frame previously transmitted does not arrive before the timeout the sender retransmits the frame, thinking that the frame or it’s acknowledgement is lost in transit.
There are three types of techniques available which Data-link layer may deploy to control the errors by Automatic Repeat Requests (ARQ):
Stop-and-wait ARQ
The following transition may occur in Stop-and-Wait ARQ:- The sender maintains a timeout counter.
- When a frame is sent, the sender starts the timeout counter.
- If acknowledgement of frame comes in time, the sender transmits the next frame in queue.
- If acknowledgement does not come in time, the sender assumes that either the frame or its acknowledgement is lost in transit. Sender retransmits the frame and starts the timeout counter.
- If a negative acknowledgement is received, the sender retransmits the frame.
Go-Back-N ARQ
Stop and wait ARQ mechanism does not utilize the resources at their best.When the acknowledgement is received, the sender sits idle and does nothing. In Go-Back-N ARQ method, both sender and receiver maintain a window.The sending-window size enables the sender to send multiple frames without receiving the acknowledgement of the previous ones. The receiving-window enables the receiver to receive multiple frames and acknowledge them. The receiver keeps track of incoming frame’s sequence number.When the sender sends all the frames in window, it checks up to what sequence number it has received positive acknowledgement. If all frames are positively acknowledged, the sender sends next set of frames. If sender finds that it has received NACK or has not receive any ACK for a particular frame, it retransmits all the frames after which it does not receive any positive ACK.Selective Repeat ARQ
In Go-back-N ARQ, it is assumed that the receiver does not have any buffer space for its window size and has to process each frame as it comes. This enforces the sender to retransmit all the frames which are not acknowledged.In Selective-Repeat ARQ, the receiver while keeping track of sequence numbers, buffers the frames in memory and sends NACK for only frame which is missing or damaged.The sender in this case, sends only packet for which NACK is received.
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Services and Security Threats in SDN Based VANETs: A Survey
1Department of Computer Science, National University of Computer and Emerging Sciences, Chiniot-Faisalabad Campus, Chiniot 35400, Pakistan
2Department of Computer and Information Communication Engineering, Hongik University, Sejong City 30016, Republic of Korea
2Department of Computer and Information Communication Engineering, Hongik University, Sejong City 30016, Republic of Korea
Correspondence should be addressed to ; rk.ca.kignoh@sbnsj
Received 25 October 2017; Revised 5 March 2018; Accepted 14 March 2018; Published 23 April 2018
Academic Editor: Jianhua He
Copyright © 2018 Hammad Shafiq et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
As the number of vehicles is increasing, the number of road side accidents is also increasing rapidly. The majority of these accidents is caused by the negligence of the driver. For intelligent transportation services, new protocols and architecture are continuously being developed by researchers around the globe. Thus to ensure the safety of drivers many countries are now adopting and investing a lot on vehicular ad hoc network (VANET). On the other perspective, there are many issues related to this field that must be resolved before VANET technology is practically adopted. In the case of no or low-security, several attacks can occur that may affect the efficiency and the reliability of the system. To make VANET systems more efficient software defined networking (SDN) technology is introduced in it. This technique was shortly named as SDN-based VANET. SDN-based VANET system helps us to get rid of the restriction and the challenges that are present in the simple VANET systems. It helps us to decrease the overall load on the network by managing the overall network through a single remote controller. In this survey paper, we will elaborate the concept of SDN-based VANET, its working, applications, services, security threats, and benefits over the previous techniques.
1. Introduction
Because travel modes change over time, new techniques and mechanisms are introduced to increase their efficiency. At present, cars and other vehicles are used for traveling, and drivers are interested in having their own vehicles. However, as the number of private vehicles increases, the ratio of road accidents also increases. There are many reasons for road accidents, such as the negligence of the driver and mechanical faults in the vehicle. In the modern world, safe means of travel are required in order to prevent accidents that may cause serious damage to human lives.
To resolve these problems, the researcher investigated the wireless network domain. The wireless network provides the convenience of a wire-free environment to its hosts, so they have the ability to move freely, which leads to dynamic topologies. The main idea is to include the vehicles in a specific area in a communication network. However, this dynamic arrangement of nodes also causes unpredictability in network topologies. To achieve our goal, we use a mobile ad hoc network (MANET), which is related to the wireless ad hoc network [1]. MANETs are wireless ad hoc networks in which every device is independent and moves freely in any direction. MANET is a self-configuring and infrastructure-less network that supports the mobility of devices. Each device frequently changes its links from one device to the other devices, which results in a highly dynamic and autonomous topology. Each device in this network plays the dual role of participant and router. The mobile ad hoc network is constructed as soon as the devices are connected to each other. Hence, the node that forwards data depends on the network connectivity, whereas, in wired networks, a fixed router performs the routing tasks. This system also differs from a wired infrastructure because in wireless networks, a specific node can act as an access point [2].
Within the field of MANET, scientists have discovered a new field known as the vehicle ad hoc network (VANET). It uses the same technology as MANET uses, that is, each node in VANET is free to move in any direction, which leads to frequent changes in the links between them. The nodes forward data to other nodes, which are then vehicles. The devices on the nodes must send data continuously to maintain the connection and the proper flow of data. These devices consist of efficient hardware that sends correct data without any delay and may be connected to the internet. At any point in time, many devices may be connected to a single node that is a device on the vehicle. For the purpose of communication, all the components of VANET communicate wirelessly, which regulates the various aspects of the communication, such as security, latency, and the data transmission range. In wireless communication, different routing protocols are used. Protocols are the combination of rules for communication between two or more entries using any physical medium. Moreover, protocols are rules that are used to define the syntax of the communication and recovery method in case of any fault. Protocols are implemented in software and hardware and may be a combination of both. Several routing protocols are similar in MANETs and VANETs, but when they are compared according to behavior and characteristics, many of these protocols are dissimilar. A key difference between VANET and MANET is the production cost. The network topology of VANET is fast and highly dynamic because of the high speed of cars, whereas MANET is slow. In addition, more network bandwidth is required in VANET than in MANET [3]. Delivering accurate data is now considered a key task because of frequent signal disruption, contact opportunities in VANET, and the fast changes in topology. However, in VANET, the transmission of the signals is a main challenge because as the vehicle changes its position, the possibility of disconnection increases. Other challenges are the failed transmission of data because of the inadequate quality of the wireless links between different nodes and the absence of a link between the source and the destination. To address the problem of disconnection between nodes, dedicated short-range communication (DSRC) protocols are used for communication in VANET. DSRC provides a high data transmission rate and is used in safety applications. Through this form of communication, vehicles can share secure information to prevent any mishap as well as in postaccident investigation [4]. Information that is not relevant is discarded, which saves processing time. The aim of sharing the information is to deliver an alert message to the driver about the expected risk, which decreases the possibility of accidents while driving.
VANET has attracted researchers in different fields to develop protocols, applications, and simulation tools. However, researchers and developers still face several challenges. Peoples from different countries are contributing to get rid of these challenges by developing new communication protocols, advanced hardware, data security, and privacy techniques [5, 6].
To increase the efficiency of the entire network, a software defined network (SDN) technique was introduced in the field of VANET system. The SDN is innovative in the network field, and it is now considered a new, alternative technique for controlling the flow of the entire network in a programmable and systematic way. The SDN system also defines the data plane in data forwarding and the network control plane in controlling the entire network. Previously, all applications related to SDN-based systems used wires, such as Facebook, Google, and Cisco. However, because of its potential flexibility, the SDN system is considered an alternative method for mobile wireless networks. The separation of the control and data planes makes this network simple and easy to manage even when the number of nodes in the network increases rapidly.
This new SDN system has been introduced in VANET to manage the communication throughout the network. In a simple VANET system, all communication is controlled with the help of routing protocols. However, because of the increase in the demand for the VANET system, it has become difficult for the routing protocols to manage the communication, privacy, and security of the system. Nevertheless, this problem can be solved easily with the help of the SDN system. Collaboration is developed between a simple VANET and an SDN-based system to control the overall communication in the system. The introduction of SDN in VANET has simplified the management of the overall behavior of the network.
This paper is organized as follows. Section 2 describes the background and related work, Section 3 presents the SDN-based VANET, and Section 4 presents the services of SDN-based VANET. Section 5 describes the security threats and challenges related to the SDN-based VANET. Section 6 describes the evolution of the SDN controller architecture for VANET, and Section 7 presents the applications of the SDN-based VANET. The final section concludes the paper.
2. Background and Related Work
Several devices are used for communication in computer networks, such as routers and switches. Network operators, which are responsible for the events that occur in the system configure these devices. Previously, network operators manually installed policies in the devices, which sometimes required frequent changes. Hence, it became very difficult to manage communication in the network, which could lead to its failure. Furthermore, internet-based applications and services have become increasingly complex, and it has become necessary to resolve many issues to make them work efficiently. To resolve this problem, a programmable network was introduced, which was further defined as the SDN [7].
2.1. Software Defined Network
The core concept of the SDN system is the separation of the data plane and the control plane as shown in Figure 1.