QUESTION: 85

Which of the following OSI layers establishes, manages, and terminates the connections between the local and remote applications?

Data Link layer
Network layer
Application layer
Session layer

Answer(s): D

Explanation:

The session layer of the OSI/RM controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.
Answer option C is incorrect. The Application Layer of TCP/IP model refers to the higher-level protocols used by most applications for network communication. Examples of application layer protocols include the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). Data coded according to application layer protocols are then encapsulated into one or more transport layer protocols, which in turn use lower layer protocols to affect actual data transfer.

Answer option A is incorrect. The Data Link Layer is Layer 2 of the seven-layer OSI model of computer networking. It corresponds to or is part of the link layer of the TCP/IP reference model. The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. The Data Link Layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical Layer. Examples of data link protocols are Ethernet for local area networks (multi-node), the Point-to-Point Protocol (PPP), HDLC, and ADCCP for point-to-point (dual-node) connections.
Answer option B is incorrect. The network layer controls the operation of subnet, deciding which physical path the data should take, based on network conditions, priority of service, and other factors. Routers work on the Network layer of the OSI stack.

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QUESTION: 86

Adam, a malicious hacker, is sniffing an unprotected Wi-FI network located in a local store with Wireshark to capture hotmail e-mail traffic. He knows that lots of people are using their laptops for browsing the Web in the store. Adam wants to sniff their e-mail messages traversing the unprotected Wi-Fi network. Which of the following Wireshark filters will Adam configure to display only the packets with hotmail email messages?

(http = "login.pass.com") && (http contains "SMTP")
(http contains "email") && (http contains "hotmail")
(http contains "hotmail") && (http contains "Reply-To")
(http = "login.passport.com") && (http contains "POP3")

Answer(s): C

Explanation:

Adam will use (http contains "hotmail") && (http contains "Reply-To") filter to display only the packets with hotmail email messages. Each Hotmail message contains the tag Reply-To: and "xxxx-xxx- xxx.xxxx.hotmail.com" in the received tag. Wireshark is a free packet sniffer computer application. It is used for network troubleshooting, analysis, software and communications protocol development, and education.
Wireshark is very similar to tcpdump, but it has a graphical front-end, and many more information sorting and filtering options. It allows the user to see all traffic being passed over the network (usually an Ethernet network but support is being added for others) by putting the network interface into promiscuous mode. Wireshark uses pcap to capture packets, so it can only capture the packets on the networks supported by pcap. It has the following features: Data can be captured "from the wire" from a live network connection or read from a file that records the already-captured packets. Live data can be read from a number of types of network, including Ethernet, IEEE 802.11, PPP, and loopback. Captured network data can be browsed via a GUI, or via the terminal (command line) version of the utility, tshark. Captured files can be programmatically edited or converted via command-line switches to the "editcap" program. Data display can be refined using a display filter. Plugins can be created for dissecting new protocols.
Answer options B, A, and D are incorrect. These are invalid tags.

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QUESTION: 87

Which of the following are the distance-vector routing protocols? Each correct answer represents a complete solution. Choose all that apply.

IS-IS
OSPF
IGRP
RIP

Answer(s): C,D

Explanation:

Following are the two distance-vector routing protocols:
RIP: RIP is a dynamic routing protocol used in local and wide area networks. As such, it is classified as an interior gateway protocol (IGP). It uses the distance-vector routing algorithm. It employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. It implements the split horizon, route poisoning, and hold-down mechanisms to prevent incorrect routing information from being propagated.
IGRP: Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary distance vector Interior Gateway Protocol (IGP). It is used by Cisco routers to exchange routing data within an autonomous system (AS). This is a classful routing protocol and does not support variable length subnet masks (VLSM). IGRP supports multiple metrics for each route, including bandwidth, delay, load, MTU, and reliability.
Answer options B and A are incorrect. OSPF and IS-IS are link state routing protocols.

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QUESTION: 88

With which of the following forms of acknowledgment can the sender be informed by the data receiver about all segments that have arrived successfully?

Block Acknowledgment
Negative Acknowledgment
Cumulative Acknowledgment
Selective Acknowledgment

Answer(s): D

Explanation:

Selective Acknowledgment (SACK) is one of the forms of acknowledgment. With selective acknowledgments, the sender can be informed by a data receiver about all segments that have arrived successfully, so the sender retransmits only those segments that have actually been lost. The selective acknowledgment extension uses two TCP options: The first is an enabling option, "SACK-permitted", which may be sent in a SYN segment to indicate that the SACK option can be used
once the connection is established. The other is the SACK option itself, which can be sent over an established connection once permission has been given by "SACK-permitted".
Answer option A is incorrect. Block Acknowledgment (BA) was initially defined in IEEE 802.11e as an optional scheme to improve the MAC efficiency. IEEE 802.11n capable devices are also referred to as High Throughput (HT) devices. Instead of transmitting an individual ACK for every MPDU, multiple MPDUs can be acknowledged together using a single BA frame. Block Ack (BA) contains bitmap size of 64*16 bits. Each bit of this bitmap represents the status (success/failure) of an MPDU.
Answer option B is incorrect. With Negative Acknowledgment, the receiver explicitly notifies the sender which packets, messages, or segments were received incorrectly that may need to be retransmitted.
Answer option C is incorrect. With Cumulative Acknowledgment, the receiver acknowledges that it has correctly received a packet, message, or segment in a stream which implicitly informs the sender that the previous packets were received correctly. TCP uses cumulative acknowledgment with its TCP sliding window.

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