Man-in-the-middle attacks occur when an attacker successfully inserts an intermediary software or program between two communicating hosts. The intermediary software or program allows attackers to listen to and modify the communication packets passing between the two hosts. The software intercepts the communication packets and then sends the information to the receiving host. The receiving host responds to the software, presuming it to be the legitimate client. Answer option B is incorrect. A Denial-of-Service (DoS) attack is mounted with the objective of causing a negative impact on the performance of a computer or network. It is also known as a network saturation attack or bandwidth consumption attack. Attackers perform DoS attacks by sending a large number of protocol packets to the network. The effects of a DoS attack are as follows: Saturates network resources Disrupts connections between two computers, thereby preventing communications between services Disrupts services to a specific computer Causes failure to access a Web site Results in an increase in the amount of spam A Denial-of-Service attack is very common on the Internet because it is much easier to accomplish. Most of the DoS attacks rely on the weaknesses in the TCP/IP protocol. Answer option D is incorrect. A buffer-overflow attack is performed when a hacker fills a field, typically an address bar, with more characters than it can accommodate. The excess characters can be run as executable code, effectively giving the hacker control of the computer and overriding any security measures set. There are two main types of buffer overflow attacks: stack-based buffer overflow attack: Stack-based buffer overflow attack uses a memory object known as a stack. The hacker develops the code which reserves a specific amount of space for the stack. If the input of user is longer than the amount of space reserved for it within the stack, then the stack will overflow. heap-based buffer overflow attack: Heap-based overflow attack floods the memory space reserved for the programs. Answer option A is incorrect. Session hijacking refers to the exploitation of a valid computer session to gain unauthorized access to information or services in a computer system. In particular, it is used to refer to the theft of a magic cookie used to authenticate a user to a remote server. It has particular relevance to Web developers, as the HTTP cookies used to maintain a session on many Web sites can be easily stolen by an attacker using an intermediary computer or with access to the saved cookies on the victim's computer (see HTTP cookie theft). TCP session hijacking is when a hacker takes over a TCP session between two machines. Since most authentication only occurs at the start of a TCP session, this allows the hacker to gain access to a machine.

Frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN). Frame relay puts data in a variable-size unit called a frame. It checks for lesser errors as compared to other traditional forms of packet switching and hence speeds up data transmission. When an error is detected in a frame, it is simply dropped. The end points are responsible for detecting and retransmitting dropped frames. Answer option C is incorrect. Integrated Services Digital Network (ISDN) is a digital telephone/telecommunication network that carries voice, data, and video over an existing telephone network infrastructure. It requires an ISDN modem at both the ends of a transmission. ISDN is designed to provide a single interface for hooking up a telephone, fax machine, computer, etc.ISDN has two levels of service, i.e., Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Answer option A is incorrect. The Point-to-Point Protocol, or PPP, is a data link protocol commonly used to establish a direct connection between two networking nodes. It can provide connection authentication, transmission encryption privacy, and compression. PPP is commonly used as a data link layer protocol for connection over synchronous and asynchronous circuits, where it has largely superseded the older, non-standard Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25 protocol suite). PPP was designed to work with numerous network layer protocols, including Internet Protocol (IP), Novell's Internetwork Packet Exchange (IPX), NBF, and AppleTalk. Answer option D is incorrect. The X.25 protocol, adopted as a standard by the Consultative Committee for International Telegraph and Telephone (CCITT), is a commonly-used network protocol. The X.25 protocol allows computers on different public networks (such as CompuServe, Tymnet, or a TCP/IP network) to communicate through an intermediary computer at the network layer level. X.25's protocols correspond closely to the data-link and physical-layer protocols defined in the Open Systems Interconnection (OSI) communication model.