Internetworking End-to-End Requirements Essay

Hence such characteristics as executing, reliableness, scalability, mobility, and QoS of DS atomic number 18 impacted by the underlying interlocking engine room and the OS ? Principles of computer clearing Every vane has ? An architecture or forges of communications protocols ? pile switching for talksue ? Route withdrawion and selective information drift ? Comm Sub arrangements ( communicate technologies rest on) transmittance media wires, cables, type, wireless (sat, IR, RF, mwave) heavy(a)wargon devices routers, switches, bridges, hubs, repeaters, sack styles/card/transceivers. Softw ar comp 1nts protocol stacks, comm handlers/ ragrs, OS primitives, net income-focus APIs ? Hosts The computers and kibosh-devices that practise the comm sub placement Subnet A undivided cluster or charm of nodes, which reach each different on the analogous somatic ordinary and open of routing crush and incoming messages The earnings is a collection of some(prenominal)(prenominal)(prenominal) subnets (or intranets) ? neting issues for distri simplyed systems Initial requirements for DS performances ftp, rlogin, e beam, wisesgroup Subsequent generation of DS applics. online dual-lane imagerys. Current requirements performance, reliability, scalability, mobility, aegis, QoS, multicasting ? Performance key fruit go to assume unit(s) of messages mingled with a pair of inter attached computers/devices point-to-point latency (delay) from displace out of outgoing-buffer and receiving into incoming-buffer. Usu alvirtuosoy entrea sheetle to softwargon package eachwhere forelands, craft load, and path selection entropy transfer/bit rate drive on of information transfer amidst 2 computers (bps). Usually due to somatogenic properties of the medium. ? message trans time = latency + length/bit-rate ? Bandwidth vs. bit-rate. The wide-cut system bandwidth (volume of selective information move and accepted in a u nit time, e. g. , per sec. ) is a measure of its byput Bit rate or transfer rate is restricted to the mediums ability to propagate individual(a) bits/signals in a unit time In most LANs, e. g. , Ethernets, when full transmittal capacity is devoted to messaging (with unretentive or no latency), accordingly bandwidth and bit-rate atomic number 18 same in measure conduceical anesthetic memory vs internet resources ? finishs slantler to divided resources on same ne twainrk normally under millisecond ? Applications access to local anaesthetic memory commonly under msec (1000x faster)?However, for high hie ne twork nett- horde, with caches, the access time is much faster (than local disk access due to hard disk latency) ? Scalability ( network and DSs) Future egression of computation nodes of net profit (hosts, switches) in 109s (100s of 106 hosts alone) Requires substantial changes to routing and portion outing schemes ( more than(prenominal) later ) Curre nt traffic (load) on cyberspace approx. measured by the latencies ( bet www. mids. org), which seem to defend reduced (with advances in medium and protocol display nationals). Future growth and sustainability dep balance on economies of role, charge rate, neck of the woods/placement of shargond resource? dependability Failures ar typically, not due to the physical medium, plainly at the end-end (at host levels) softw be (application-level), therefore, fracture detection/correction is at the level Suggesting that the chat subsystem need not be misapprehension-free (made transp arnt/hidden to exploiter) beca commit reliability is roundwhat stock warrantd at the send/receiving system ends (where errors whitethorn be ca employ by, e. g. , buffer over black market, measure drifts cavictimization premature timeouts) ? Security most(prenominal) intranets atomic number 18 protected from orthogonal ( profits-wide) DSs by firewall. A firewall protects all the resources of an organized from flagitious/malicious access by external designrs, and subdue/ overseeing of use of resources outside(a) the firewall A firewall (bundle of security software and network hardware) plays on a gate itinerary the accession/exit point of the merged intranet A firewall is usually configured ground on corporate security policy, and filters incoming and outgoing messages. To go beyond firewalls, and grant access to world- or lucre-wide resources, lengthways authencetication, privacy, and security (Standards) are needed to allow DSs to character E. g., techniques are Cryptographic and Authentication usually enforced at a level supra the communication subsystem Virtual Private clear (VPN) security concept allows intranet-level certificate of such features/devices as local routers and secure tie in to mobile devices ?Mobility Need wireless to confine portable computers and hand-held devices receiving quite a little cogitate are susceptible to, e. g. , eavesdropping, distortions in medium, out-of-sight/ cuckold transmitters/ bring forthrs Current steering and routing schemes are based on wired technologies, which take in down been adapted and, therefore, not perfect and need extensions?QoS (Quality of gain) Meeting deadlines and user requirements in transmitting/ molding streams of real time multimedia data E. g. , QoS requirements guaranteed bandwidth, timely manner of speaking or bounded latencies, or high-powered packjustments to requirements ? Multicasting approximately transmittances are point-to-point, but several(prenominal) involve one-to- umteen ( all one-to-all broadcast or discriminating broadcast multicast) Simply direct the same message from one node to several destinations is inefficient Multicasting technique allows single transmission to multiple destination (simultaneously) by apply special addressing scheme 3. Multimedia Transmission and net profitworking Heterogeneous Systems ? Types of Networks LANs (confined to littler, typically, 2. 5km diameter spread) ? higher(prenominal) speed, single medium for inter touch onion ( depraved pair, decoy, opt), no routing at nooky segments all point-to-point (from hub), inter-segment links via switches/hubs, low latency, low error rate ? E. g. , Ethernet, token ring, slotted ring protocols, wired.(1) Ethernet 1970 with bandwidth of 10Mbps, with wide versions of 100/1000Mbps, lacking latency and bandwidth QoS for DSs (2) cash machine apply frame cells and optical fills the commotion but expensive for LAN, newer high-speed Ethernets digest improvement and cost-effective MANs (confined to extended, regional world, typically, up to 50km spread) ? Based on high-bandwidth copper and fiber optics for multimedia (audio/video/voice), ? E. g. , technologies atmosphere, high-speed Ethernet (IEEE 802. 6 protocols for MANs), DSL (digital subscriber line) apply ATM switches to switch digitized voice over twisted pair 0.25-6Mbps within 1. 5km, cable modem uses coax 1. 5Mpbs apply analog signalise on TV networks and longer distances than DSL WANs (worldwide, lour speeds over sets of varying til nowts of circuits with routers) ? High latency (due to switching and route searching) between 0. 1-0. 5s, signaling speed approximately 3x105km/s (bounds latency) asset propagation delay (round-trip) of about 0. 2s if using satellite/geostationary dishes generally long-playing at 10-100kbps or best 1-2Mbps Wireless (connecting portable, wearable devices using access points) ? uncouth protocol IEEE 802.11 (a, b, and now g) (WaveLAN) 2-11Mbps (11gs bandwidth stuffy 54Mbps) over 150m creating a WLANs, nearly mobiles attached to fixed devices printers, innkeepers, palmtops to create a WPANs (wireless personal area networks) using IR cogitate or low-powered Blue likewiseth intercommunicate network tech 1-2Mbps over 10m. ? or so mobile cell phones use Bluetooth tech. e. g. , European GSM s tandard and US, mostly, analog-based AMP cellular radio network, atop by CDPD cellular digital mailboat data communication system, operational over wider areas at lower speed 9. 6-19. 2kbps.?Tiny screens of mobiles and wearables require a new WAP protocol Internetworks ? Building open, extendible system for DSs, supporting network heterogeneity, multi-protocol system involving LANs, MANs, WLANs, machine-accessible by routers and gateways with shapes of software for data and protocol conversions creating a virtual network using underlying physical networks ? E. g. , the Internet using transmission stop scalawag protocol/IP (over several other(a) physical protocols) Comparisons ? Range of performance characteristics ? frequency and types of failures, when use for DS applics? packet boat speech communication/loss, duplicates (masked at transmission control protocol level to guarantee near reliability and transparency to DSs but whitethorn use UDP faster but less reliable and DS applics responsibility to guarantee reliability) plat 3. 2 Network Principles Packet Transmission Packet transmission superseded ring/telegraph switched network Messages are packetized and packets are queued, buffered (in local storage), and patrimonial when lines are functional using asynchronous transmission protocol Data be adrift Multimedia data female genital organt be packetized due to unpredicted delays.AV data are streamed at higher frequency and bandwidth at continuous flow rate Delivery of multimedia data to its destination is time-critical / low latency requiring end-to-end predefined route E. g. networks ATM, IPv6 ( neighboring generation for take out recrudesce steamed IP packets at network form and use RSVP (resource reserv. protocol) resource/bandwidth prealloc and RTP play-time/time-reqs (real-time transp protocol) at works 3 & 1, respectively) to work ? Switching Schemes 4 Kinds of switching methods typically employ. Broadcast no switchi ng logic, all nodes see signals on circuits/cells (e.g. , Ethernet, wireless networks) Circuit Switching matching segments of circuits via switches/ substitution boxes, e. g. , POTS (Plain Old Telephone System) Packet Switching Developed as compute tech advanced with processors and storage spaces using store-and-forward algorithms and computers as switches. Packets are not displace instantaneously, routed on different links, re casted, whitethorn be lost, high latency (few msec msecs). reference point to switch audio/video data brought integration of digitized data for computer comm. , telephone operate, TV, and radio broadcasting, teleconferencing. Frame Relay PS (not instantaneous, just an illusion ), but FR, which integrates CS and PS techniques, streams smaller packets (53 byte-cells called frames) as bits at processing nodes. E. g. , ATM Protocols Protocols implemented as pairs of software modules in send/receive nodes, watch the sequence of messages for tran smission Specify the format of the data in the messages Protocols Layers form architecture, hobby the OSI entourage packets are communicated as peer-to-peer transmission but effected vertically across layers by encapsulation method over a physical mediumProtocols Suites The 7-layered architecture of the ISO-OSI Each layer shows utility to the layer above it and extends the service provided by the layer beneath it A round set of protocol layers constitute a suite or stack Layering simplifies and generalizes the software interface definitions, but costly overhead due to encapsulations and protocol conversions plat 3. 3 4. military service Provider Management On the Internet, a prudence service supplier (MSP) is a company that manages information technology go for other companies.For example, a company could utilize an MSP to configure and administer its business computers and related systems on a continuing basis, obstetrical delivery the company. An MSP is a servi ce provider that offers system and network instruction tools and expertise. An MSP typically has its consume data c preface that runs advanced network management software such as HP OpenView or Tivoli. It uses these tools to actively monitor and provide reports on aspects of its customers networks, including communication links, network bandwidth, servers, and so on. The MSP may host the customers electronic network servers and application servers at its own site.The operate provided by MSPs have been called clear telemetry go. The MSP friendship defines MSPs as follows Management value Providers deliver information technology (IT) infrastructure management serve to multiple customers over a network on a subscription basis. handle Application benefit Providers (ASPs), Management Service Providers deliver operate via networks that are account to their clients on a recurring fee basis. Unlike ASPs, which deliver business applications to end users, MSPs deliver system m anagement services to IT departments and other customers who manage their own technology assets.TriActive is an example of an MSP. It provides management and monitoring of PCs, servers, networks, and mesh sites from its own NOC (network operations center), which is hosted by exodus Communications. Exodus check outs that the NOC has fully redundant power, network connectivity, routing, and switching to ensure maximum reliability and integrity. A microagent interacts with customer systems to provide system management. The agent is lightweight and designed for use over the Internet. It acts as a habitual agent invoking and managing other agents and courses as needful for specific actions.The service is delivered via the Web through a secure Internet admittance that lets customers overtake management information, based on their role in the organization. For example, CIOs stomach view overall management information while help desk technicians can check call queues, escalations, and open ticket status. Systems analysts can bear asset inventories and view virus reporting. accusing Systems Integrators is other management service provider that provides software solutions for unified network, service application, and process management.OSI was recently acquired by Agilent Technologies. A come of other MSPs may be put together at the MSP Association Web site. Web application and infrastructure monitoring and management have suddenly become critical, even the tools to do them are lacking. Management service providers (MSP), the latest addition to the current bargain of service providers, claim to offer products and services that allow oversee your Web operations. Companies staking claims as early MSPs include Candle Corp. in Santa Monica, Calif. InteQ Corp. in Burlington, Mass. and Nuclio Corp. in Skokie, Ill.What makes MSPs distinctive is that their products and services are provided over the Internet on a subscription basis. That means MSPs can come upon economies of scale that companies who license software cannot, utters Christopher Booth, head of technical operations at FreightWise Inc. , a Forth Worth, Texas-based online transportation exchange. Though he declined to say how much his company is nonrecreational to use Nuclios MSP, Booth did say that the service has been very advantageous. The cost nest egg that MSPs can pass on may help them catch on with corporate customers.Gartner Group Inc. in Stamford, Conn., estimates that the $90 gazillion MSP market result balloon to more than $3. 25 billion by 2005. 5. Programmable/cognitive Networks A key strength of the Internet has been to reduce the intelligence within the network to that required placed at strategic places within the network such as at administrative boundaries, or at messs where there is a heroic match between bandwidth, or where the certain location specific services can be used.Others believe that the inviolate architecture should be rethought as a computational environment, in which everything can be classmed, and the entire network becomes active.The research in this area is aimed at discovering how viable it is to open up such elements of the communications architecture as the routing table. A fundamental question raised by both the active service and the active network approaches is how to ensure that the shared resource of the network remains safe and is protected from misbehaving political platforms. Programs can abuse the network by generating packet explosions and can abuse the shared processor by using all the memory and the processor cycles. Worse, they may tip over the working of correct programs so that they too break.If network programmability is going to be available to the application designers, we need to ensure that they do not break things by accident, let alone by intention. Traditional systems approaches to protection are based upon what a program should be able to do, then using runtime checks to ensure that the program doesnt happen these bounds. This leads to the sandbox model of protection, as used in Java and enhanced to provide protection for Active Networks. However, there are major problems with this approach. First, each runtime check reduces the performance of the system, increasing the overhead of each use of system resources.Second, it is very difficult to ensure that the protection mechanisms are correct, and cannot be subverted in any way. An alternative approach is to use compile time checks upon what the program is doing. This uses the type system to represent predicates about program functionality and if a program is well-typed, then it proves the program to obey the policies implemented in the type system. This approach has been used to allow users to run programs within the kernel as in Spin, and in protecting access to router functionality in the Switchware project. To provide a network computer programming language based on Internet best effort communication. To provide scaleable high-ranking communication based on outback(a) spawn from which other communication can be built. To make use of types as safety properties, to ensure that the safety and security policies of the network are maintained. To rapidly ideal tools such as compilers and simulators in order to drive the development of the language by examples. Best-effort distributed programming In the Internet, an application transmits a packet, which is sent to the next router on the way to the destination.At this router, the arrival of the packet causes label to run, which calls other recruit dependent upon the palm in the header of the packet. This code may access and modify local earth stored in the router and then copy or create one or more packets to be sent out from the router. These packets are then routed on output links depending upon the destination for each packet, and so on until the packets reach their destination, or are undo within the network for whatever reason. In our programming model, we have attempted to double this basic structure of packet transmission.In the Internet, the arrival of a packet initiates some vagabond of control which uses the data within the packet to decide upon the disposition of the packet. In our model, a packet becomes a move in of control, carrying the code to be run and the chance upon calling or values of any data referenced within that code. When a thread arrives at a Safetynet-aware router or end system, the thread code is instantiated within the runtime and runs within a de_ned scheduling class. The thread of control may call other code to be run on its behalf.The other code is encapsulated within classes, which are either present in the router, or are dynamically loaded from elsewhere. Threads can spawn other threads, either topically or on the next jump to some destination. 6. Design of an Internetwork We will debate about the networking especially the internetworking. In this case we will n eed to discuss some topics related with the Internet Infrastructure, Internet routing, orbital cavity name and address effect, internet protocol and the applications. Internet Infrastructure The Internet rachis is made up of many large networks which join with each other.These large networks are receiven as Network Service Providers or Naps. some of the large Naps are UUNet, Cerf Net, IBM, BBN Planet, Sprint Net, PSINet, as well as others. These networks peer with each other to exchange packet traffic. Each NSP is required to connect to three Network Access Points or NAPs. At the NAPs, packet traffic may jump from one NSPs gumption to another(prenominal) NSPs prickle. NSPs as well interconnect at Metropolitan Area Exchanges or MAEs. MAEs serve the same purpose as the NAPs but are privately owned. NAPs were the schoolmaster Internet interconnects points. Both NAPs and MAEs are referred to as Internet Exchange Points or IXs.NSPs also sell bandwidth to smaller networks, such as ISPs and smaller bandwidth providers. Below is a exhibit showing this hierarchical infrastructure. Diagram 4 This is not a current imitation of an actual piece of the Internet. Diagram 4 is only meant to demonstrate how the NSPs could interconnect with each other and smaller ISPs. None of the physical network components are shown in Diagram 4 as they are in Diagram 3. This is because a single NSPs backbone infrastructure is a colonial drawing by itself. Most NSPs let go of maps of their network infrastructure on their weave sites and can be found easily.To draw an actual map of the Internet would be nearly impossible due to its size, complexity, and dynamic structure. The Internet Routing Working It is general phenomenon that No computer knows where any of the other computers are, and packets do not get sent to every computer. The information used to get packets to their destinations is contained in routing tables kept by each router connected to the Internet. The Routers are called the packet switches. A router is usually connected between networks to route packets between them. Each router knows about its sub-networks and which IP addresses they use.The router usually doesnt know what IP addresses are above it. Examine Diagram 5 below. The black boxes connecting the backbones are routers. The larger NSP backbones at the top are connected at a NAP. Under them are several sub-networks, and under them, more sub-networks. At the bottom are two local area networks with computers attached. Diagram 5 When a packet arrives at a router, the router examines the IP address put there by the IP protocol layer on the originating computer. The router checks its routing table. If the network containing the IP address is found, the packet is sent to that network.If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone pecking order to the next router. Hopefully the next router will know whe re to send the packet. If it does not, once again the packet is routed upwards until it reaches a NSP backbone. The routers connected to the NSP backbones hold the largest routing tables and here the packet will be routed to the correct backbone, where it will get great deal its journey downward through smaller and smaller networks until it fall upons its destination. country remarks and deal Resolution.But what if you dont know the IP address of the computer you call for to connect to? What if the you need to access a weather vane server referred to as www. anothercomputer. com? How does your entanglement tissue browser know where on the Internet this computer lives? The answer to all these questions is the Domain Name Service or DNS. The DNS is a distributed database which play alongs track of computers label and their tally IP addresses on the Internet. many an(prenominal) computers connected to the Internet host part of the DNS database and the software that allow s others to access it. These computers are cognise as DNS servers.No DNS server contains the entire database they only contain a subset of it. If a DNS server does not contain the farming name prayered by another computer, the DNS server re-directs the pick uping computer to another DNS server. Diagram 6 The Domain Name Service is structured as a pecking order similar to the IP routing hierarchy. The computer takeing a name resolution will be re-directed up the hierarchy until a DNS server is found that can resoluteness the demesne name in the request. Figure 6 illustrates a portion of the hierarchy. At the top of the tree are the range roots.Some of the older, more common domains are seen near the top. What is not shown are the multitude of DNS servers around the world which form the rest of the hierarchy? When an Internet connective is setup (e. g. for a LAN or Dial-Up Networking in Windows), one elemental and one or more secondary DNS servers are usually condition as p art of the installation. This way, any Internet applications that need domain name resolution will be able to function correctly. For example, when you enter a web address into your web browser, the browser first connects to your primary DNS server.After obtaining the IP address for the domain name you entered, the browser then connects to the aim computer and requests the web scalawag you wanted. The hamper DNS in Windows If youre using Windows 95/NT and access the Internet, you may view your DNS server(s) and even disable them. If you use Dial-Up Networking Open your Dial-Up Networking window (which can be found in Windows Explorer under your CD-ROM drive and above Network Neighborhood). Right break down on your Internet connection and polish off Properties. Near the bottom of the connection properties window press the transmission control protocol/IP Settings button.If you have a permanent connection to the Internet Right click on Network Neighborhood and click Properties. Click transmission control protocol/IP Properties. Select the DNS Configuration tab at the top. You should now be expression at your DNS servers IP addresses. Here you may disable DNS or set your DNS servers to 0. 0. 0. 0. (Write down your DNS servers IP addresses first. You will plausibly have to restart Windows as well. ) Now enter an address into your web browser. The browser wont be able to resolve the domain name and you will probably get a nasty dialog box explaining that a DNS server couldnt be found.However, if you enter the corresponding IP address instead of the domain name, the browser will be able to detect the desired web page. (Use ping to get the IP address prior to disqualifying DNS. ) Other Microsoft operating systems are similar. Internet protocols As hinted to earlier in the subdivision about protocol stacks, one may surmise that there are many protocols that are used on the Internet. This is true there are many communication protocols required for the Inte rnet to function. These include the TCP and IP protocols, routing protocols, medium access control protocols, application level protocols, etceteraThe following sections describe some of the more classic and commonly used protocols on the Internet. Higher-level protocols are discussed first, followed by lower level protocols. Application Protocols HTTP and the World full Web unmatched of the most commonly used services on the Internet is the World Wide Web (WWW). The application protocol that makes the web work is Hypertext Transfer Protocol or HTTP. Do not confuse this with the Hypertext Markup oral communication (HTML). HTML is the language used to keep open web pages. HTTP is the protocol that web browsers and web servers use to communicate with each other over the Internet.It is an application level protocol because it sits on top of the TCP layer in the protocol stack and is used by specific applications to talk to one another. In this case the applications are web browse rs and web servers. HTTP is a connectionless text based protocol. Clients (web browsers) send requests to web servers for web elements such as web pages and images. After the request is serviced by a server, the connection between client and server across the Internet is disconnected. A new connection mustiness be made for each request. Most protocols are connection oriented.This means that the two computers communicating with each other keep the connection open over the Internet. HTTP does not however. Before an HTTP request can be made by a client, a new connection must be made to the server. When you type a uniform resource locator into a web browser, this is what happens If the URL contains a domain name, the browser first connects to a domain name server and retrieves the corresponding IP address for the web server. The web browser connects to the web server and sends an HTTP request (via the protocol stack) for the desired web page. The web server receives the request and ch ecks for the desired page.If the page exists, the web server sends it. If the server cannot stick the requested page, it will send an HTTP 404 error message. (404 mean paginate Not put in as anyone who has rangeed the web probably knows. ) The web browser receives the page back and the connection is closed. The browser then parses through the page and looks for other page elements it needs to complete the web page. These usually include images, applets, etc. For each element needed, the browser makes surplus connections and HTTP requests to the server for each element.When the browser has ruined loading all images, applets, etc.the page will be completely loaded in the browser window. Retrieving a Web Page Using HTTP Telnet is a remote terminal service used on the Internet. Its use has declined lately, but it is a very expedient tool to study the Internet. In Windows find the default telnet program. It may be fit(p) in the Windows directory named telnet. exe.When opened, cl eave down the pole menu and select Preferences. In the preferences window, check Local Echo. (This is so you can see your HTTP request when you type it. ) Now pull down the Connection menu and select Remote System. Enter www. google. com for the Host Name and 80 for the Port.(Web servers usually listen on port 80 by default. ) weigh Connect. Now type GET / HTTP/1. 0 And press Enter twice. This is a dewy-eyed HTTP request to a web server for its root page. You should see a web page flash by and then a dialog box should papa up to tell you the connection was lost. If youd like to save the retrieved page, turn on logging in the Telnet program. You may then browse through the web page and see the HTML that was used to carry through it. Most Internet protocols are stipulate by Internet documents known as a Request for Comments or RFCs. RFCs may be found at several locations on the Internet.See the Resources section below for appropriate URLs. HTTP version 1. 0 is specified by RFC 19 45. Application Protocols SMTP and electronic Mail Another commonly used Internet service is electronic mail. netmail uses an application level protocol called impartial Mail Transfer Protocol or SMTP.SMTP is also a text-based protocol, but hostile HTTP, SMTP is connection oriented. SMTP is also more involved than HTTP. There are many more look acrosss and considerations in SMTP than there are in HTTP. When you open your mail client to read your e-mail, this is what typically happens The mail client (Netscape Mail, lotus Notes, Microsoft Outlook, etc.) opens a connection to its default mail server.The mail servers IP address or domain name is typically setup when the mail client is installed. The mail server will unendingly transmit the first message to cite itself. The client will send an SMTP HELO command to which the server will respond with a 250 OK message. Depending on whether the client is checking mail, sending mail, etc. the appropriate SMTP commands will be sent to the server, which will respond accordingly. This request/response transaction will get across until the client sends an SMTP QUIT command. The server will then say goodbye and the connection will be closed.Transmission carry Protocol Under the application layer in the protocol stack is the TCP layer. When applications open a connection to another computer on the Internet, the messages they send (using a specific application layer protocol) get passed down the stack to the TCP layer. TCP is responsible for routing application protocols to the correct application on the destination computer. To accomplish this, port numbers are used. Ports can be thought of as separate transmit on each computer. For example, you can surf the web while reading e-mail. This is because these two applications (the web browser and the mail clien.