Network Viva Questions And Answers 3x134

           Questions by REVA IT Dept of Information Science & Engg

Viva Questions 1. What are functions of different layers? Functions of each Layer: Physical Layer(Layer 1):This layer defines the physical and electrical characteristics of the network. Helps to strings(data) of ones and zeros down the wire. Device:Hub, NIC, Repeater.. etc Data Link Layer: It defines the strategy for sharing the physical medium. Helps convert the bits into frame. Devices: Bridge, Switch Network Layer: Layer 3 is the Network Layer, providing a means for communicating open systems to establish, maintain and terminate network connections. Device:Router Transport Layer: The main function of this Layer is to ensure data reliability and integrity. Session Layer: It provides two communicating presentation entities to exchange data with eachother. Presentation Layer: Application data is either unpacked or packed only in this layer. Protocol conversions, encryption/decryption and graphics expansion all takes place here. Application Layer: This is where you find your end- and end-application protocols, such as telnet, ftp, and mail(pop3 and smtp). 2. Differentiate between T/IP Layers and OSI Layers The Session layer permits two parties to hold ongoing communications called a session across a network. Not found in T/IP model In T/IP,its characteristics are provided by the T protocol. (Transport Layer) The Presentation Layer handles data format information for networked communications. This is done by converting data into a generic format that could be understood by both sides. Not found in T/IP model In T/IP, this function is provided by the Application Layer. e.g. External Data Representation Standard (XDR) Multipurpose Internet Mail Extensions (MIME)

Answers by S.Raaj Nishanth

1 / 29


The Application Layer is the top layer of the reference model. It provides a set of interfaces for applications to obtain access to networked services as well as access to the kinds of network services that applications directly. OSI - FTAM,VT,MHS,DS,CMIP T/IP - FTP,SMTP,TELNET,DNS,SNMP Although the notion of an application process is common to both, their approaches to constructing application entities is different TRANSPORT LAYER OSI It takes the information to be sent and breaks it into individual packets that are sent and reassembled into a complete message by the Transport Layer at the receiving node Transport Layer protocols include the capability to acknowledge the receipt of a packet; if no acknowledgement is received, the Transport Layer protocol can retransmit the packet or time-out the connection and signal an error T Defines two standard transport protocols: T and UDP T implements a reliable data-stream protocol connection oriented UDP implements an unreliable data-stream connectionless T is responsible for data recovery by providing a sequence number with each packet that it sends T requires ACK (ackowledgement) to ensure correct data is received Packet can be retransmitted if error detected NETWORK LAYER Like all the other OSI Layers, the network layer provides both connectionless and connectionoriented services. As for the T/IP architecture, the internet layer is exclusively connectionless.

3. Why header is required? (answer below) 4. What is the use of adding header and trailer to frames?

IPv4 Packet Header The Internet Protocol (IP) uses a Datagram service to transfer packets of data between end systems using routers.


2 / 29


The IPv4 packet header consists of 20 bytes of data. An option exists within the header that allows further optional bytes to be added, but this is not normally used (with the occasional exception of something called "Router Alert"). The full header is shown below:

The header fields are discussed below: • • •

• • •

• • • •

• • •

Version (always set to the value 4 in the current version of IP) IP Header Length (number of 32 -bit words forming the header, usually five) Type of Service (ToS), now known as Differentiated Services Code Point (DS) (usually set to 0, but may indicate particular Quality of Service needs from the network, the DS defines the way routers should queue packets while they are waiting to be forwarded). Size of Datagram (in bytes, this is the combined length of the header and the data) Identification ( 16-bit number which together with the source address uniquely identifies this packet - used during reassembly of fragmented datagrams) Flags (a sequence of three flags (one of the 4 bits is unused) used to control whether routers are allowed to fragment a packet (i.e. the Don't Fragment, DF, flag), and to indicate the parts of a packet to the receiver) Fragmentation Offset (a byte count from the start of the original sent packet, set by any router which performs IP router fragmentation) Time To Live (Number of hops /links which the packet may be routed over, decremented by most routers - used to prevent accidental routing loops) Protocol (Service Access Point (SAP) which indicates the type of transport packet being carried (e.g. 1 = ICMP; 2= IGMP; 6 = T; 17= UDP). Header Checksum (A 1's complement checksum inserted by the sender and updated whenever the packet header is modified by a router - Used to detect processing errors introduced into the packet inside a router or bridge where the packet is not protected by a link layer cyclic redundancy check. Packets with an invalid checksum are discarded by all nodes in an IP network) Source Address (the IP address of the original sender of the packet) Destination Address (the IP address of the final destination of the packet) Options (not normally used, but, when used, the IP header length will be greater than five 32-bit words to indicate the size of the options field)

5. What is encapsulation? Answers by S.Raaj Nishanth

3 / 29


Encapsulation, closely related to the concept of Protocol Layering, refers to the practice of enclosing data using one protocol within messages of another protocol. To make use of encapsulation, the encapsulating protocol must be open-ended, allowing for arbitrary data to placed in its messages. Another protocol can then be used to define the format of that data. Encapsulation Example For example, consider an Internet host that requests a hypertext page over a dialup serial connection. The following scenario is likely: First, the HyperText Transfer Protocol (HTTP) is used to construct a message requesting the page. The message, the exact format of which is unimportant at this time, is represented as follows:

Next, the Transmission Control Protocol (T) is used to provide the connection management and reliable delivery that HTTP requires, but does not provide itself. T defines a message header format, which can be followed by arbitrary data. So, a T message is constructed by attaching a T header to the HTTP message, as follows:

Now T does not provide any facilities for actually relaying a message from one machine to another in order to reach its destination. This feature is provided by the Internet Protocol (IP), which defines its own message header format. An IP message is constructed by attaching an IP header to the combined T/HTTP message:

Finally, although IP can direct messages between machines, it can not actually transmit the message from one machine to the next. This function is dependent on the actual communications hardware. In this example, we're using a dialup modem connection, so it's likely that the first step in transmitting the message will involve the PointtoPoint Protocol (PPP):


4 / 29


5. Why fragmentation required? Every packet-based network has an MTU (Maximum Transmission Unit) size. The MTU is the size of the largest packet which that network can transmit. Packets larger than the allowable MTU must be divided into multiple smaller packets, or fragments, to enable them to traverse the network. 6. What is MTU? In computer networking, the maximum transmission unit (MTU) of a layer of a communications protocol is the size (in bytes) of the largest protocol data unit that it can  onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with pointtopoint serial links). A higher MTU brings greater efficiency because each packet carries more  data while protocol overheads, such as headers or underlying per packet delays remain fixed, and higher efficiency means a slight improvement in bulk protocol throughput. 7. Which layer imposes MTU? Layer 2 (Data Link Layer)

Differentiate between flow control and congestion control. Flow control vs. congestion control:     Flow control mean preventing the source from sending data that the  sink will end up dropping because it runs out of buffer space.     This is fairly easy with a sliding window protocoljust make sure the source's window is no larger than the free space in the sink's  buffer.  T does this by letting the sink  its free buffer     space in the window field of the acks.     Congestion control means preventing (or trying to prevent) the source from sending data that will end up getting dropped by a  router because its queue is full.  This is more complicated, because     packets from different sources travelling different paths can  converge on the same queue. In a connectionoriented network:


5 / 29


    ission control and policers can be used to avoid congestion.  Before a source starts sending data, it sets up a connection, which   requires permission from the routers/switches along the path.  If     the requested resources are unavailable, the connection is not set  up.  Once the connection is set up, policers at the edge of the  network can make sure the source does not send more than it was     allowed to. In a connectionless network:     Congestion is unavoidable, because the routers are not warned ahead of time that a source will be sending packets along some path.     Congestion can, however, be managed, if sources are informed when their packets encounter congestion and they slow down.

8. Differentiate between PointtoPoint Connection and EndtoEnd connections. In networking, the PointtoPoint 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 used over many types of physical networks including serial cable, phone line, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. Most Internet service providers (ISPs) use PPP for customer dialup access to the Internet

The endtoend principle states that, whenever possible, communications protocol operations should be defined to occur at the endpoints of a communications system, or as close as possible to the resource being controlled. According to the endtoend principle, protocol features are only justified in the lower layers of a system if they are a performance optimization. Example : example is that of file transfer. Every reliable file transfer protocol and file transfer program should contain a checksum, which is validated only after everything has been successfully stored on disk. Disk errors, router errors, and file transfer software errors make an endtoend checksum necessary.

9. What are protocols running in different layers? OSI MODEL, LAYERS & PROTOCOLS 7 Application Web Browser, Email, Print Serivces, SIP, SSH and S, NFS, RTSP, Feed, XMPP, Whois, SMB; DNS; FTP; TFTP; BOOTP; SNMP;R; SMTP; MIME; NFS; FINGER; TELNET; N; APPC; AFP; SMB 6 Presentation XDR, ASN.1, SMB, AFP, N, MIDI, HTML, GIF, TIFF, JPEG, ASCII, EBCDIC


6 / 29

           Questions by REVA IT Dept of Information Science & Engg 5 Session TLS, SSH, X.225, RPC, NetBIOS, ASP, Winsock, BSD 4 Transport T, UDP, RTP, SCTP, SPX, ATP Gateway, Advanced Cable Tester, Brouter 3 Network IP, ICMP, IGMP, BGP, OSPF, RIP, IGRP, EIGRP, ARP, RARP, X.25, NETBEUI Brouter, Router, Frame Relay Device, ATM Switch, Advanced Cable Tester, DDP 2 Data Link Ethernet, Token ring, StarLAN, HDLC, Frame relay, ISDN, ATM, 802.11 WiFi, FDDI, PPP, Bridge, Switch, ISDN Router, Intelligent Hub, NIC, Advanced Cable Tester, ARCNET, LocalTalk, FDDI, ATM. NIC Drivers: Open Datalink Interface (ODI), Network Independent Interface Specification (NDIS) 1 Physical NIC, Twisted Pair, Coax, Fiber Optic, Wireless Media, Repeater, Multiplexer, Hubs, (ive/Active), TDR, Oscilloscope, Amplifier, Carrier pigeon T LAYERS 4 Application (OSI - Layers5 through 7) HTTP, FTP, DNS (Routing protocols like BGP and RIP, which for a variety of reasons run over T and UDP respectively, may also be considered part of the Internetwork layer) 3 Transport (OSI - Layers4 and 5) T, UDP, RTP, SCTP (Routing protocols like OSPF, which run over IP, may also be considered part of the Internetwork layer) 2 Internetwork (OSI - Layer 3) For T/IP this is the Internet Protocol (IP) (Required protocols like ICMP and IGMP run over IP, but may still be considered part of the Internetwork layer; ARP does not run over IP) 1 Link (OSI - Layers 1 and 2) Ethernet, Wi-Fi, MPLS, etc.


7 / 29


10. What is Protocol Stack? A protocol stack (sometimes communications stack) is a particular software implementation of a computer networking protocol suite. The  are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them. In practical implementation, protocol stacks are often divided into three major sections: media, transport, and applications. A particular operating system or platform will often have two well defined software interfaces: one between the media and transport layers, and one between the transport layers and applications.


8 / 29



9 / 29


11. Differentiate between T and UDP.


10 / 29


12. Differentiate between Connectionless and connection oriented connection. Connection Oriented Connection-Oriented means that when devices communicate, they perform handshaking to set up an end-to-end connection. The handshaking process may be as simple as syncrhonization such as in the transport layer protocol T, or as complex as negotiating communications parameters as with a modem. Connection-Oriented systems can only work in bi-directional communications environments. To negotiate a connection, both sides must be able to communicate with each other. This will not work in a unidirectional environment.

Connectionless Connectionless means that no effort is made to set up a dedicated end-to-end connection. Connectionless communication is usually achieved by transmitting information in one direction, from source to destination without checking to see if the destination is still there, or if it is prepared to receive the information. When there is little interferance, and plenty of speed available, these systems work fine. In environments where there is difficulty transmitting to the destination, information may have to be retransmitted several times before the complete message is received. Walkie-talkies, or Citizens Band radios are a good examples of connectionless communication. You speak into the mike, and the radio transmitter sends out your signal. If the person receiving you doesn't understand you, there's nothing his radio can do to correct things, the receiver must send you a message back to repeat your last message. IP, UDP, ICMP, DNS, TFTP and SNMP are examples of connectionless protocols in use on the Internet.


11 / 29


13. Why frame sorting is required?

14. What is meant by subnet? A subnet (short for "subnetwork") is an identifiably separate part of an organization's network. Typically, a subnet may represent all the machines at one geographic location, in one building, or on the same local area network (LAN). Having an organization's network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate subnetworks, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. 15. What is meant by Gateway?    A node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISP that connects the  to the internet. In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway. 16. What is an IP address? An Internet Protocol (IP) address is a numerical label that is assigned to devices participating in a computer network utilizing the Internet Protocol for communication between its nodes.[1] An IP address serves two principal functions in networking: host or network interface identification and location addressing. 17. What is MAC address? In computer networking, a Media Access Control address (MAC address) is a unique identifier assigned to most network adapters or network interface cards (NICs) by the manufacturer for identification, and used in the Media Access Control protocol sublayer. If assigned by the manufacturer, a MAC address usually encodes the manufacturer's ed identification number. It may also be known as an Ethernet Hardware Address (EHA), hardware address, adapter address, or physical address. 18. Why IP address is required when we have MAC address? (refer to above two answers) 19. What is meant by port? An interface on a computer to which you can connect a device. Personal computers have various Answers by S.Raaj Nishanth

12 / 29


types of ports. Internally, there are several ports for connecting disk drives, display screens, and keyboards. Externally, personal computers have ports for connecting modems, printers, mice, and other peripheral devices. Almost all personal computers come with a serial RS232C port or RS422 port for connecting a modem or mouse and a parallel port for connecting a printer. On PCs, the parallel port is a Centronics interface that uses a 25pin connector. SCSI (Small Computer System Interface) ports higher transmission speeds than do conventional ports and enable you to attach up to seven devices to the same port. In T/IP and UDP networks, an endpoint to a logical connection. The port number identifies what type of port it is. For example, port 80 is used for HTTP traffic. 20. What are ephemerical port number and well known port numbers? Ephemeral ports are temporary ports assigned by a machine's IP stack, and are assigned from a designated range of ports for this purpose.  When the connection terminates, the ephemeral port is available for reuse, although most IP stacks won't reuse that port number until the entire pool of ephemeral ports have been used.  So, if the client program reconnects, it will be assigned a different ephemeral port number for its side of the new connection. Wellknown port numbers are assigned to particular services throughout the Internet, by IANA, the Internet Assigned Numbers Authority. The wellknown port numbers are in the range 0 through 1023. File Transfer Protocol (FTP) 21 Telnet 23 Hypertext Transfer Protocol (HTTP) 80 HTTP with Secure Sockets Layer (SSL) 443 CORBA Internet InterORB Protocol (IIOP) 683 CORBA IIOP with SSL 684 21. What is a socket? Sockets is a method for communication between a client program and a server program in a network. A socket is defined as "the endpoint in a connection." Sockets are created and used with a set of programming requests or "function calls" sometimes called the sockets application programming interface (API). The most common sockets API is the Berkeley Unix C interface for sockets. Sockets can also be used for communication between processes within the same computer. 22. What are the parameters of socket()? int socket(int domain, int type, int protocol) 23. Describe bind(), listen(), accept(),connect(), send() and recv(). int bind(int socket, const struct sockaddr *address,socklen_t address_len); bind() function shall assign a local socket address address to a socket identified by descriptor socket that has no local socket address assigned. Sockets created with the socket() function are initially unnamed; they are identified only by their address family.


13 / 29


int listen(int s, int backlog); The listen() function marks a connectionmode socket (for example, those of type SOCK_STREAM), specified by the socket argument s, as accepting connections, and limits the number of outstanding connections in the socket's listen queue to the value specified by the backlog argument. The socket s is put into 'ive' mode where incoming connection requests are acknowledged and queued pending acceptance by the process. int accept(int s, struct sockaddr *addr, socklen_t *addrlen); The accept() function accepts a connection on a socket. An incoming connection is acknowledged and associated with an immediately created socket. The original socket is returned to the listening state. int connect(int socket, const struct sockaddr *address,socklen_t address_len); The connect() function shall attempt to make a connection on a socket. ssize_t send(int socket, const void *buffer, size_t length, int flags); The send() function shall initiate transmission of a message from the specified socket to its peer. The send() function shall send a message only when the socket is connected (including when the peer of a connectionless socket has been set via connect()).     ssize_t recv(int socket, void *buffer, size_t length, int flags); The recv() function shall receive a message from a connectionmode or connectionlessmode socket. It is normally used with connected sockets because it does not permit the application to retrieve the source address of received data. 24. What are system calls? Mention few of them. A system call is a request made by any program to the operating system for performing tasks— picked from a predefined set—which the said program does not have required permissions to execute in its own flow of execution. System calls provide the interface between a process and the operating system.

System calls for low level file I/O <map name="boxmap-p8"> <area shape="RECT" coords="14, 200, 103, 207" href="http://rcm.amazon.com/e/cm/privacy-policy.html?o=1"> <area coords="0,0,10000,10000" href="http://www.amazon.com/exec/obidos/redirecthome/ref=nikolaibezroukov"> Shop at Amazon.com

System calls for low level file I/O o o

creat(name, permissions) open(name, mode)


14 / 29


o o o o o

close(fd) unlink(fd) read(fd, buffer, n_to_read) write(fd, buffer, n_to_write) lseek(fd, offest, whence)

System Calls for process control o o o o o o

fork() wait() execl(), execlp(), execv(), execvp() exit() signal(sig, handler) kill(sig, pid)

System Calls for IPC o o

pipe(fildes) dup(fd)

25. What is IPC? Name three techniques. Interprocess communication (IPC) is a set of techniques for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC techniques are divided into methods for message ing, synchronization, shared memory, and remote procedure calls (RPC). 26. Explain mkfifo(), open(), close() with parameters. int mkfifo(const char *path, mode_t mode); The mkfifo() function creates a new FIFO special file named by the pathname pointed to by path. The file permission bits of the new FIFO are initialised from mode. The file permission bits of the mode argument are modified by the process' file creation mask. int open(const char *path, int oflag, ... ); The open() function shall establish the connection between a file and a file descriptor. It shall create an open file description that refers to a file and a file descriptor that refers to that open file description. The file descriptor is used by other I/O functions to refer to that file. The path argument points to a pathname naming the file. int close(int fildes); The close() function shall deallocate the file descriptor indicated by fildes. To deallocate means to make the file descriptor available for return by subsequent calls to open() or other functions that


15 / 29


allocate file descriptors. All outstanding record locks owned by the process on the file associated with the file descriptor shall be removed (that is, unlocked). 27. What is meant by file descriptor? file descriptor is an index for an entry in a kernelresident data structure containing the details of all open files. In POSIX this data structure is called a file descriptor table, and each process has its own file descriptor table. The  application es the abstract key to the kernel through a system call, and the kernel will access the file on behalf of the application, based on the key. The application itself cannot read or write the file descriptor table directly. In Unixlike systems, file descriptors can refer to files, directories, block or character devices (also called "special files"), sockets, FIFOs (also called named pipes), or unnamed pipes. 28. What is meant by traffic shaping? Traffic shaping (also known as "packet shaping" or ITMPs: Internet Traffic Management Practices) is the control of computer network traffic in order to optimize or guarantee performance, lower latency, and/or increase usable bandwidth by delaying packets that meet certain criteria.[1] More specifically, traffic shaping is any action on a set of packets (often called a stream or a flow) which imposes additional delay on those packets such that they conform to some predetermined constraint (a contract or traffic profile).[2] Traffic shaping provides a means to control the volume of traffic being sent into a network in a specified period (bandwidth throttling), or the maximum rate at which the traffic is sent. Traffic shaping is always achieved by delaying packets. 29. How do you classify congestion control algorithms?     By the type and amount of  received from the network: Loss; delay; singlebit or multi bit explicit signals    By incremental deployability on the current Internet: Only sender needs modification; sender and receiver need modification; only router needs modification; sender, receiver and routers need modification.     By the aspect of performance it aims to improve: high bandwidthdelay product networks; lossy links; fairness; advantage to short flows; variablerate links    By the fairness criterion it uses: maxmin, proportional, "minimum potential delay"


16 / 29


30. Differentiate between Leaky bucket and Token bucket.

31. How do you implement Leaky bucket? The leakybucket implementation is used to control the rate at which traffic is sent to the network A leaky bucket provides a mechanism by which bursty traffic can be shaped to present a steady stream of traffic to the network, as opposed to traffic with erratic bursts of lowvolume and highvolume flows.


17 / 29


32. How do you generate bursty traffic? Bursty traffic refers to an uneven pattern of data transmission: sometime very high data transmission rate while other time it might be very low. 33. What is the polynomial used in CRCCCITT? x16 + x12 + x5 +1 34. What are the other error detection & correction algorithms?

   Parity check

           LRC Longitudinal Redundancy Check          CRC Cyclic Redundancy Check           FC Fire codes          BCH BoseChaudhuriHocquenghem          RS ReedSolomon           HC Hamming codes          Turbo Codes     Answers by S.Raaj Nishanth

18 / 29


     VCC Virerbi Convilutional Coding for Forward error Correction (FEC)

    Golay

35. What is difference between CRC and Hamming code?

36. Why Hamming code is called 7,4 code? encodes 4 bits of data into 7 bits by adding 3 parity bits. 37. What is odd parity and even parity? When using even parity, the parity bit is set to 1 if the number of ones in a given set of bits (not including the parity bit) is odd, making the entire set of bits (including the parity bit) even. When using odd parity, the parity bit is set to 1 if the number of ones in a given set of bits (not including the parity bit) is even, making the entire set of bits (including the parity bit) odd. In other words, an even parity bit will be set to "1" if the number of 1's + 1 is even, and an odd parity bit will be set to "1" if the number of 1's +1 is odd. 38. What is meant by syndrome?

39. What is generator matrix?

40. What is spanning tree? A spanning tree of that graph is a subgraph which is a tree and connects all the vertices together. A single graph can have many different spanning trees. We can also assign a weight to each edge, which is a number representing how unfavorable it is, and use this to assign a weight to a spanning tree by computing the sum of the weights of the edges in that spanning tree. A minimum spanning tree (MST) or minimum weight spanning tree is then a spanning tree with weight less than or equal to the weight of every other spanning tree. More generally, any undirected graph (not necessarily connected) has a minimum spanning forest, which is a union of minimum spanning trees for its connected components. 41. Where Prim’s algorithm does finds its use in Networks?

42. Differentiate between Prim’s and Kruskal’s algorithm. Kruska's builds a minimum spanning tree by adding one edge at a time. The next line is always the shortest (minimum weight) ONLY if it does NOT create a cycle. Answers by S.Raaj Nishanth

19 / 29


Prims builds a mimimum spanning tree by adding one vertex at a time. The next vertex to be added is always the one nearest to a vertex already on the graph. Prim always s a "new" vertex to an "old" vertex, so that every stage is a tree. Kruskal's allows both "new" to "new" and "old" to "old" to get connected, so it risks creating a circuit and must check for them every time. So Kruskal's has a larger complexity than Prim. 43. What are Routing algorithms? Routers use routing algorithms to find the best route to a destination. Routing (or routeing) is the process of selecting paths in a network along which to send network traffic. 44. How do you classify routing algorithms? Give examples for each. View: global or local            global: graph of entire network (routers, links). [link state]. eg. Dijkstra shortest path algorithm.            local: partial knowledge of remote parts of network. [distance vector approach] eg.Bellman Ford routing algorithms and FordFulkerson routing algorithms       Centralized or decentralized            Centralized:one node maintains view, and distributes routes to other nodes           Decetralized:all nodes maintain view      Static or dynamic?       Static:  infrequent route changes,infrequent view update; static link costs (e.g. up/down)      Dynamic: frequent periodic route changes       frequent view update; dynamic link costs (e.g. delay) 45. What are drawbacks in distance vector algorithm? The algorithm does not prevent routing loops from happening and suffers from the counttoinfinity problem. The core of the counttoinfinity problem is that if A tells B that it has a path somewhere, there is no way for B to know if it is on the path. 46. How routers update distances to each of its neighbor? Routers learn about remote networks from neighboring routers or an .  The router then builds a routing table that tells how to get to the remote networks.  Routes are either directly connected, static, or dynamic.  Static routes are entered in by the .  Dynamic routes are learned from neighboring routers using routing protocols.  In dynamic routing, the routers update each other at set intervals.  Changes cause the routers to update all the other routers.  If a routers receives a packet with a destination network not in its routing tables, it will discard the packet.


20 / 29


Dynamic routes adjust to changes within the internetwork environment automatically.  When network changes occur, routers begin to converge by recalculating routes and distributing route updates.  The route update messages spread through the network, which causes other routers to recalculate their routes.  The process continues until all routes have converged.  Uses protocols to find and update routes on a routing table.  It uses U time and consumes bandwidth between links.  The routing protocol defines the rules used by the routers when they communicate with each other. There are two types of routing protocols on internetworks, Interior Gateway Protocol (IGP) and Exterior Gateway Protocol (EGP).  IGP is used in networks in the same istrative domain. EGPs are used to communicate between the domains.

47. How do you overcome count to infinity problem? The BellmanFord algorithm does not prevent routing loops from happening and suffers from the counttoinfinity problem. The core of the counttoinfinity problem is that if A tells B that it has a path somewhere, there is no way for B to know if the path has B as a part of it. To see the problem clearly, imagine a subnet connected like ABCDEF, and let the metric between the routers be "number of jumps". Now suppose that A goes down (out of order). In the vectorupdateprocess B notices that its once very short route of 1 to A is down B does not receive the vector update from A. The problem is, B also gets an update from C, and C is still not aware of the fact that A is down so it tells B that A is only two jumps from it, which is false. This slowly propagates through the network until it reaches infinity (in which case the algorithm corrects itself, due to the "Relax property" of Bellman Ford). Partial solutions RIP uses Split Horizon with Poison Reverse technique to reduce the chance of forming loops and use a maximum number of hops to counter the counttoinfinity problem. These measures avoid the formation of routing loops in some, but not all, cases. The addition of a hold time (refusing route updates for a few minutes after a route retraction) avoids loop formation in virtually all cases, but causes a significant increase in convergence times. A number of loopfree distance vector protocols, such as EIGRP and DSDV, have been developed. These avoid loop formation in all cases, but suffer from increased complexity, and their deployment has been slowed down by the success of linkstate protocols such as OSPF. 48. What is cryptography? Cryptography can be defined as the conversion of data into a scrambled code that can be deciphered and sent across a public or private network. Cryptography uses two main styles or forms of encrypting data; symmetrical and asymmetrical. Symmetric encryptions, or algorithms, use the same key for encryption as they do for decryption. Other names for this type of encryption are secretkey, sharedkey, and privatekey. The encryption key can be loosely related to the decryption key; it does not necessarily need to be an exact copy. Symmetric cryptography is susceptible to plain text attacks and linear cryptanalysis meaning that they are hackable and at times simple to decode. With careful planning of the coding and functions of the cryptographic process these threats can be greatly reduced. Asymmetric cryptography uses Answers by S.Raaj Nishanth

21 / 29


different encryption keys for encryption and decryption. In this case an end  on a network, public or private, has a pair of keys; one for encryption and one for decryption. These keys are labeled or known as a public and a private key; in this instance the private key cannot be derived from the public key. The asymmetrical cryptography method has been proven to be secure against computationally limited intruders. The security is a mathematical definition based upon the application of said encryption. Essentially, asymmetric encryption is as good as its applied use; this is defined by the method in which the data is encrypted and for what use. The most common form of asymmetrical encryption is in the application of sending messages where the sender encodes and the receiving party decodes the message by using a random key generated by the public key of the sender. 49. How do you classify cryptographic algorithms?

(refer above) 50. What is public key?

The key used to encrypt a message is not the same as the key used to decrypt it. Each  has a pair of cryptographic keys — a public key and a private key. The private key is kept secret, whilst the public key may be widely distributed. Messages are encrypted with the recipient's public key and can only be decrypted with the corresponding private key. The keys are related mathematically, but the private key cannot be feasibly (ie, in actual or projected practice) derived from the public key. This is used in asymmetric cryptography. 51. What is private key?

(refer above) 52. What are key, ciphertext and plaintext? Ciphertext is the result of the process (known as encryption) of transforming information (referred to as plaintext) using an algorithm (called cipher) to make it unreadable [1] to anyone except those possessing special knowledge, usually referred to as a key. This result is also known as encrypted information. The process to read ciphertext is known as decryption. 53. What is simulation? A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system. 54. What are advantages of simulation? Normal analytical techniques make use of extensive mathematical models which require assumptions and restrictions to be placed on the model. This can result in an avoidable inaccuracy in the output data. Simulations avoid placing restrictions on the system and also take random processes into ; in fact in some cases simulation is the only practical modelling technique applicable;[1][2] Analysts can study the relationships between components in detail and can simulate the projected consequences of multiple design options before having to implement the outcome in the realworld. [1][2] Answers by S.Raaj Nishanth

22 / 29


It is possible to easily compare alternative designs so as to select the optimal system.[1] The actual process of developing the simulation can itself provide valuable insights into the inner workings of the network which can in turn be used at a later stage. 55. Differentiate between Simulation and Emulation. A simulation mimics the outward appearance An emulation mimics the cause/process. An emulator generally is a piece of hardware used for tests; it is selfcontained, and is able to be hooked to some kind of development environment. A simulator is a piece of software that duplicates as precisely as possible the processor so you can "run" your code to see if it is correct. Usually simulators are developped for new architectures to test them out before it is committed to silicon. Occasionnally, a vendor will let customer have access to the simulator to help speed development for that architecture. (If you want to convince people that watching television gives you stomachaches, you can simulate this by holding your chest/abdomen and moan. You can emulate it by eating a kilo of unripe apples.) 56. What is meant by router? In packetswitched networks such as the Internet, a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway (where one network meets another), including each pointof presence on the Internet. A router is often included as part of a network switch. 57. What is meant by bridge? A network bridge connects multiple network segments at the data link layer (Layer 2) of the OSI model, and the term Layer 2 switch is very often used interchangeably with bridge. Bridges are similar to repeaters or network hubs, devices that connect network segments at the physical layer; however, with bridging, traffic from one network is managed rather than simply rebroadcast to adjacent network segments. Since bridging takes place at the data link layer of the OSI model, a bridge processes the information from each frame of data it receives. In an Ethernet frame, this provides the MAC address of the frame's source and destination. 58. What is meant by switch? A network switch is a small hardware device that s multiple computers together within one local area network (LAN). Technically, network switches operate at layer two (Data Link Layer) of the OSI model. Answers by S.Raaj Nishanth

23 / 29


Network switches appear nearly identical to network hubs, but a switch generally contains more intelligence (and a slightly higher price tag) than a hub. Unlike hubs, network switches are capable of inspecting data packets as they are received, determining the source and destination device of each packet, and forwarding them appropriately. By delivering messages only to the connected device intended, a network switch conserves network bandwidth and offers generally better performance than a hub. 59. What is meant by hub? A network hub or repeater hub is a device for connecting multiple twisted pair or fiber optic Ethernet devices together and thus making them act as a single network segment. Hubs work at the physical layer (layer 1) of the OSI model. The device is thus a form of multiport repeater. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision. 60. Differentiate between route, bridge, switch and hub.

(refer to the previous answers)

61. What is ping and telnet? Ping is a computer network tool used to test whether a particular host is reachable across an IP network; it is also used to self test the network interface card of the computer, or as a latency test. It works by sending ICMP “echo request” packets to the target host and listening for ICMP “echo response” replies. The "echo response" is sometimes called a pong. Ping measures the roundtrip time  [1]  and records any packet loss, and prints when finished a statistical summary of the echo response packets received, the minimum, mean, max and in some versions the standard deviation of the round trip time. Telnet (teletype network) is a network protocol used on the Internet or local area networks to provide a bidirectional interactive communications facility. Telnet is a  command and an underlying T/IP protocol for accessing remote computers. Through Telnet, an  or another  can access someone else's computer remotely. On the Web, HTTP and FTP protocols allow you to request specific files from remote computers, but not to actually be logged on as a   of that computer. With Telnet, you log on as a regular  with whatever privileges you may have been granted to the specific application and data on that computer. 62. What is FTP? File Transfer Protocol (FTP) is a standard network protocol used to exchange and manipulate files over a T/IP based network, such as the Internet. FTP is built on a clientserver architecture and utilizes separate control and data connections between the client and server applications. FTP is also often used as an application component to automatically transfer files for program internal functions.


24 / 29


63. What is BER? The BER is a measure of signal quality, and is a function of a quantity called Eb/N0, the energy per bit to noise power spectral density ratio of the signal for QPSK signal. In an additive white gaussiannoise (AWGN) channel, the BER is given by: BER = 1 / 2erfc(Eb / N0). This formula relates the BER of any signal to its Eb/N0. In telecommunication, an error ratio is the ratio of the number of bits, elements, characters, or blocks incorrectly received to the total number of bits, elements, characters, or blocks sent during a specified time interval. The most commonly encountered ratio is the bit error ratio (BER) also sometimes referred to as bit error rate. 64. What is meant by congestion window? In Transmission Control Protocol (T), the congestion window determines the number of bytes that can be outstanding at any time. This is a means of stopping the link between two places from getting overloaded with too much traffic. The size of this window is calculated by estimating how much congestion there is between the two places. The sender maintains the congestion window. When a connection is set up, the congestion window is set to the maximum segment size (MSS) allowed on that connection. The window keeps growing linearly until a timeout occurs or the receiver reaches its limit. If a timeout occurs, the window size is halved. 65. What is BSS? In the 802.11 specification, an 802.11 network is referred to as a basic service set (BSS). A BSS is made up of two or more stations (STAs) that communicate with each other. Individual BSS networks are established through a unique service set identifier (SSID). There are two types of BSS networks: Ad hoc     An ad hoc network consists of two or more mobile stations that communicate with each other directly. Ad hoc networks are referred to as independent basic service sets (IBSSs) within the 802.11 specification. Infrastructure     An infrastructure network consists of one or more mobile stations that communicate with each other through an access point (AP). In this type of BSS, the AP itself is considered a STA. An infrastructure network that consists of multiple APs within the same BSS is referred to as an extended service set (ESS) within the 802.11 specification.     Each AP within the same BSS must be assigned the same SSID. After a mobile station is assigned an SSID of the BSS to associate with, it can choose any AP within the ESS. After it is associated, the mobile station can also decide to roam to other APs within the ESS. 66. What is incoming throughput and outgoing throughput? In communication networks, such as Ethernet or packet radio, throughput or network throughput is the average rate of successful message delivery over a communication channel. This data may be delivered over a physical or logical link, or  through a certain network node. The throughput is usually measured in bits per second (bit/s or bps), and sometimes in data packets per second or data Answers by S.Raaj Nishanth

25 / 29


packets per time slot. (Use this to frame the answer for incoming and outgoing). 67. What is collision? In a half duplex Ethernet network, a collision is the result of two devices on the same Ethernet network attempting to transmit data at exactly the same time. The network detects the "collision" of the two transmitted packets and discards them both. Collisions are a natural occurrence on Ethernets. Ethernet uses Carrier Sense Multiple Access/ Collision Detect (CSMA/CD) as its method of allowing devices to "take turns" using the signal carrier line. When a device wants to transmit, it checks the signal level of the line to determine whether someone else is already using it. If it is already in use, the device waits and retries, perhaps in a few seconds. If it isn't in use, the device transmits. However, two devices can transmit at the same time in which case a collision occurs and both devices detect it. Each device then waits a random amount of time and retries until successful in getting the transmission sent. The best remedy for collisions is to upgrade to a full duplex switched environment. 68. How do you generate multiple traffics across different senderreceiver pairs?

(steps in using NCTUns) 69. How do you setup Ethernet LAN? (steps in using NCTUns)

70. What is meant by mobile host? Client (host) who is connected to the network by wireless means to allow maximum mobility. 71. What is meant by NCTUns? National Chiao Tung University network simulator. It is a network simulator for linux. 72. What are dispatcher, coordinator and nctunsclient? 73. Name few other Network simulators NS 2.0(linux, requires knowledge of TCL/TK scripting language) , Netsim(linux), OPNET(windows) 74. Differentiate between logical and physical address.

Dispatcher

NCTUns provides a flexible simulation architecture, by which the GUI program and the simulation engine program can be run on different machines. In NCTUns, the GUI program need not find a simulation server to run the simulation engine program for a simulation. Instead, it sends the Dispatcher program an inquiry message to know which simulation server is currently available. (The details of the handshake protocol used by these components are explained later.) The Dispatcher program is responsible for monitoring the statuses of the simulation servers that it manages Answers by S.Raaj Nishanth

26 / 29


and selecting an available simulation server (if one exists) to serve the simulation request issued from the GUI program.

Coordinator The Coordinator program has the following four tasks: 1) processing the commands sent from Dispatcher; 2) forking (creating) a simulation engine process to perform a simulation; 3) reporting the status of the created simulation engine process to the Dispatcher program; and 4) collecting the simulation results produced by its created simulation engine process and sending them to the GUI program. Before starting any simulation on a simulation server, one should first run up a Coordinator program on it.

nctunsclient NCTUns provides a frontend GUI program (called “nctunsclient” in its package), which provides useful facilities for s to efficiently create simulation and emulation cases. According to s’ common needs, it groups the operations of generating a simulation/emulation case into four modes, which are briefly introduced here. a) The “Draw Topology” mode: In this mode, one can insert network nodes, create network links, and specify the locations and moving paths of mobile nodes. In addition, the GUI program provides a complete tool kit for s to construct road networks, which is fundamental to wireless vehicular network simulations, where many P2P researchers are proposing to run P2P applications. b) The “Edit Property” mode: In this mode, one can doubleclick the icon of a network node to configure its properties (e.g., the network protocol stack used in this node, the applications to be run on this node during simulation, and other parameters). c) The “Run Simulation” mode: In this mode, the GUI program provides s with a complete set of commands to start/pause/stop a simulation. One can easily control the progress of a simulation by simply pressing a button on the GUI control . d) The “Play Back” mode: After a simulation is finished, the GUI program will automatically switch itself into the “Play Back” mode and read the packet trace file generated during the simulation. In this mode, one can use the GUI program to replay a node’s packet transmission/reception operations in an animated manner. 75. Which address gets affected if a system moves from one place to another place?

ipaddress

76. What is ICMP? What are uses of ICMP? Name few.


27 / 29


The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is chiefly used by networked computers' operating systems to send error messages— indicating, for instance, that a requested service is not available or that a host or router could not be reached. ICMP[1] relies on IP to perform its tasks, and it is an integral part of IP. It differs in purpose from transport protocols such as T and UDP in that it is typically not used to send and receive data between end systems. It is usually not used directly by  network applications, with some notable exceptions being the ping tool and traceroute.

79. Which layer implements security for data? Transport Layer: The main function of this Layer is to ensure data reliability and integrity. Presentation Layer: Application data is either unpacked or packed only in this layer. Protocol conversions, encryption/decryption and graphics expansion all takes place here.


28 / 29



29 / 29

Network Viva Questions And Answers 3x134

Overview 3e4r5l

More details w3441

Related Documents 3m3m1z

Network Viva Questions And Answers 3x134

Microprocessor Viva Questions And Answers 255x1d

Microcontroller Viva Questions And Answers 231s2q

Network Theory Questions And Answers 4f173e