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Computer Networks

by: Miss Alden Shanahan

Computer Networks CS 4263

Miss Alden Shanahan

GPA 3.9

Abdullah Faruque

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Abdullah Faruque
Class Notes
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This 4 page Class Notes was uploaded by Miss Alden Shanahan on Tuesday October 20, 2015. The Class Notes belongs to CS 4263 at Southern Polytechnic State University taught by Abdullah Faruque in Fall. Since its upload, it has received 29 views. For similar materials see /class/225456/cs-4263-southern-polytechnic-state-university in ComputerScienence at Southern Polytechnic State University.

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Date Created: 10/20/15
Chapter 1 Review Questions N E 4 V39 0 gt1 9 gt0 There is no difference Throughout this text the words host and end system are used interchangeably End systems include PCs workstations Web servers mail servers Intemetconnected PDAs WebTVs etc Suppose Alice an ambassador of country A wants to invite Bob an ambassador of country B over for dinner Alice doesn t simply just call Bob on the phone and say come to our dinner table now Instead she calls Bob and suggests a date and time Bob may respond by saying he s not available that particular date but he is available another date Alice and Bob continue to send messages back and forth until they agree on a date and time Bob then shows up at the embassy on the agreed date hopefully not more than 15 minutes before or after the agreed time Diplomatic protocols also allow for either Alice or Bob to politely cancel the engagement if they have reasonable excuses A networking program usually has two programs each running on a different host communicating with each other The program that initiates the communication is the client Typically the client program requests and receives services from the server program 1 Dialup modem over telephone line residential 2 DSL over telephone line residential or small office 3 Cable to HFC residential 4 100 Mbps switched Etherent company 5 Wireless LAN mobile 6 Cellular mobile access for example WAP mobile HFC bandwidth is shared among the users On the downstream channel all packets emanate from a single source namely the head end Thus there are no collisions in the downstream channel Current possibilities include dialup DSL cable modem fibertothehome Ethernet LANs have transmission rates of 10 Mbps 100 Mbps 1 Gbps and 10 Gbps For an X Mbps Ethernet where X 10 100 1000 or 10000 a user can continuously transmit at the rate X Mbps if that user is the only person sending data If there are more than one active user then each user cannot continuously transmit at X Mbps Ethernet most commonly runs over twistedpair copper wire and thin coaxial cable It also can run over fibers optic links and thick coaxial cable Dial up modems up to 56 Kbps bandwidth is dedicated ISDN up to 128 kbps bandwidth is dedicated ADSL downstream channel is 58 Mbps upstream channel is up to 1 Mbps bandwidth is dedicated HFC downstream channel is 1030 Mbps and upstream channel is usually less than a few Mbps bandwidth is shared 10 There are two most popular wireless Internet access technologies today a Wireless LAN In a wireless LAN wireless users transmitreceive packets tofrom a base station wireless access point within a radius of few tens of meters The base station is typically connected to the wired Internet and thus serves to connect wireless users to the wired network b Widearea wireless access network In these systems packets are transmitted over the same wireless infrastructure used for cellular telephony with the base station thus being managed by a telecommunications provider This provides wireless access to users within a radius of tens of kilometers of the base station A circuitswitched network can guarantee a certain amount of endtoend bandwidth for the duration of a call Most packetswitched networks today including the Internet cannot make any endtoend guarantees for bandwidth In a packet switched network the packets from different sources owing on a link do not follow any fixed predef1ned pattern In TDM circuit switching each host gets the same slot in a revolving TDM frame At time to the sending host begins to transmit At time I LR 1 the sending host completes transmission and the entire packet is received at the router no propagation delay Because the router has the entire packet at time t it can begin to transmit the packet to the receiving host at time t1 At time t2 I LR2 the router completes transmission and the entire packet is received at the receiving host again no propagation delay Thus the endtoend delay is LRJ LR2 A tierl ISP connects to all other tierl ISPs a tier2 ISP connects to only a few of the tierl ISPs Also a tier2 ISP is a customer of one or more tierl a 2 users can be supported because each user requires half of the link bandwidth b Since each user requires leps when transmitting if two or fewer users transmit simultaneously a maximum of 2Mbps will be required Since the available bandwidth of the shared link is 2Mbps there will be no queuing delay before the link Whereas if three users transmit simultaneously the bandwidth required will be 3Mbps which is more than the available bandwidth of the shared link In this case there will be queuing delay before the link c Probability that a given user is transmitting 02 3 3 3 d Probability that all three users are transmitting simultaneously 3 P3 139 P 16 00 O N O N N 4 023 0008 Since the queue grows when all the users are transmitting the fraction of time during which the queue grows which is equal to the probability that all three users are transmitting simultaneously is 0008 The delay components are processing delays transmission delays propagation delays and queuing delays All of these delays are xed except for the queuing delays which are variable Java Applet 10msec ds no no a 500 kbps b 64 seconds c 100kbps 320 seconds End system A breaks the large le into chunks To each chunk it adds header generating multiple packets from the le The header in each packet includes the address of the destination end system B The packet switch uses the destination address to determine the outgoing link Asking which road to take is analogous to a packet asking which outgoing link it should be forwarded on given the packet s address Java Applet Five generic tasks are error control ow control segmentation and reassembly multiplexing and connection setup Yes these tasks can be duplicated at different layers For example error control is often provided at more than one layer The ve layers in the Internet protocol stack are 7 from top to bottom 7 the application layer the transport layer the network layer the link layer and the physical layer The principal responsibilities are outlined in Section 151 Applicationlayer message data which an application wants to send and passed onto the transport layer transportlayer segment generated by the transport layer and encapsulates applicationlayer message with transport layer header networklayer datagram encapsulates transportlayer segment with a networklayer header link layer frame encapsulates networklayer datagram with a linklayer header Routers process layers 1 through 3 This is a little bit of a white lie as modern routers sometimes act as rewalls or caching components and process layer four as well Link layer switches process layers 1 through 2 Hosts process all ve layers a Virus Requires some form of human interaction to spread Classic example Email viruses N bWorms No user replication needed Worm in infected host scans IP addresses and port numbers looking for vulnerable processes to infect c Trojan horse Hidden devious part of some otherwise useful software Creation of a botnet requires an attacker to find vulnerability in some application or system eg exploiting the buffer over ow vulnerability that might exist in an application After finding the vulnerability the attacker needs to scan for hosts that are vulnerable The target is basically to compromise a series of systems by exploiting that particular vulnerability Any system that is part of the botnet can automatically scan its environment and propagate by exploiting the vulnerability An important property of such botnets is that the originator of the botnet can remotely control and issue commands to all the nodes in the botnet Hence it becomes possible for the attacker to issue a command to all the nodes that target a single node for example all nodes in the botnet might be commanded by the attacker to send a TCP SYN message to the target which might result in a TCP SYN ood attack at the target Trudy can pretend to be Bob to Alice and viceversa and partially or completely modify the messages being sent from Bob to Alice For example she can easily change the phrase Alice I owe you 1000 to Alice I owe you 10000 Furthermore Trudy can even drop the packets that are being sent by Bob to Alice and viseversa even if the packets from Bob to Alice are encrypted


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