BIT 4554 EXAM 1
Web-browser approach
- An ISP provides a special web page that displays messages from a user's mailbox Advantages - Ability to read email from any computer - A user doesn't need to run a special mail interface application
Electronic mail (email) is divided into 2 pieces. What are they?
- An email interface application: a mechanism for a user to compose and edit outgoing messages as well as read and process incoming email - A mail transfer program: acts as a client to send a message to the mail server on the destination computer; the mail server accepts incoming messages and deposits each in the appropriate user's mailbox
Special mail application approach
- Can download an entire mailbox onto a local computer - can process email when the laptop is disconnected from the internet - once internet connectivity is regained, it communicates with the server at the ISP to upload email the user has created and download any new email
File Transfer Protocol (FTP) - one accounted for the most packets on the inetrnet
- Client forms a control connection to send requests - Server forms data connection for each file transferred - Server closes data connection after transfer complete (Using a separate connection allows arbitrary data transfer) (For data connections, the server becomes the client and client becomes the server)
Advantages of frequency domain representation
- Compactness - both small and easy to read - Each sine waves occupy a single point along the x-axis
How does a conceptual piece of communication system work?
- Gather the information - Prepare the information for transmission - transmit the information across the shared physical medium - Extracts the information for the destination and delivers the information
Benefits of clients/ servers multitasking
- Hardware can be shared - A single computer has lower system administration overhead than multiple computer systems - Demand for a server is often sporadic - Significant cost reduction without reducing performance
How does IDNA translate international character?
- IDNA relies on applications to translate the international character - The rules for translating international domain names are complex and use unicode. - Firefox and internet explorer now implement IDNA
What happens if a user enters an abbreviated name
- It will first try to resolve the name - and will receive an error because no such name exists - then it will try appending a suffix and looking up the resulting name Ex- www.amzn.com and www.amazon.com
Why does networking seem complex?
- Many technology exist - Each technology has features that distinguish it from the others - Commercial network products and services often use technologies in new unconventional ways - Technologies can be combined and interconnected in many ways
Facts about DNS
- Names are hierarchical - Each name are divided into segments by period character - Most significant segment is on the right - Rightmost segment is known as top- level domain (TLD)
What are the specifications in network interface layer about?
- Network addresses- MAC address - maximum packet size that a network can support - protocols used to access the underlying medium - and hardware addressing
What are 5 aspects of networking?
- Network applications and network programming - Data communications - Packet switching and networking technologies - Inter networking with TCP/IP - Additional networking concepts and technologies
MIME adds two lines to an email header, why/
- One to declare that MIME has been used to create the message - Another to specify how MIME information is included in the body ex- MIME- version: 1.0 Content-Type: Multipart/mixed; boundary = mime_separator
Characteristics of access protocol
- Provides access to a user's mailbox - Permit a user to view headers, download, delete or send messages - Client runs on user's personal computer - Server runs on a computer that stores user's mailbox - Viewing a list of messages without downloading the message contents is useful (ex- checking email on your phone, look at header and directly send to spam without downloading the content)
What is packet switching?
- Provides the basis of the modern internet - Allows multiple users to share a network - Divides data into small blocks, called packets - Devices throughout the network each have information about how to reach each possible destination.
Difference between public network and private network
- Public network provides services to subscribers (A company that offers service is known as service provider) (Public refers to the general availability of service, not to the data being transferred) - A private network is controlled by one particular group
What are the advantages of resource sharing?
- Resource availability - Concurrency - Possible cost reduction - Possible security enhancements
Domain Name System (DNS)
- Runs at application layer - Important piece of internet infrastructure - Translates human readable names into the binary addresses used by the internet protocol Ex- www.but.vt.edu has its IP address
Physical Layer (Layer 1)
- Specify details about the underlying transmission medium and hardware - All specifications related to electrical properties, radio frequencies, and signals belong in layer 1
Simple mail transfer protocol (SMTP)
- Standard protocol sed by the mail transfer program Can be characterized as: - Follows a stream paradigm - Uses textual control messages - Only transfers text messages - Allows a send to specify recipients name and check each name - Sends one copy of a given message - CAN NOT include graphic images: ONLY TEXT - Can send a single message to multiple recipients: the protocol allows a client to list users and then send a copy of a message for all users on the list
Multipurpose internet mail extensions (MIME)
- Standard protocol set by the mail representation program - Allows email to include attachments such as graphic images or binary files - Specifies how a binary file can be encoded into printable characters, included in a message and decoded by the receiver - Allows a sender to divide a message into several parts and to specify an encoding for each part independently (a use can send a plain text and attach pictures with their own encoding)
RFC mail message format
- Takes its name from IETF standards document RFC - A mail message is represented as a text file and consists of: a header section, a blank line and a body - Header lines each have the form: keyword: information, where the set of keywords is defined to include From, To, Subject, cc etc.
After a GET request, the first line of a response header contains a status code. What does that tell you?
- Tells the browser whether the server handled the request - If the request was incorrectly formed or the requested item was not available, the status code pin points the problem 200- OK 400- Bad Request 404- Not Found Additional lines of header give information about its length, when it was modified, and the content type
How does name resolution work
- The resolver becomes a client by contacting DNS server, DNS server returns an answer to the caller. - The resolver forms a DNS request message (sends the message to the local server and waits for the server to send a DNS reply message for the answer. The resolver will be configured to send the request to the local DNS server. (although it cannot answer the request, the server knows to contact the server, which can generate an answer)
What is the complex part of communication system?
- There are various types of sources - Digitized information before sending - Protect channel errors - May need encryption - Multiple streams of information across a shared media
The protocols used for internet email can be divided into three broad categories. What are they?
- Transfer: A protocol used to move a copy of an email message from one computer to another - Access: A protocol that allows a user to access their mailbox and to view or send email messages - Representation: A protocol that specifies the format of an email message when stored on disk
Internet changed in 2 significant ways: what are they?
- communication speeds - New applications (higher communication speeds enabled applications to transfer volume of data quickly) (The advent of powerful, affordable, personal computers provided the computational power needed for complex computation and graphical displays
Sine wave characteristics
- frequency: the number of oscilllations per unit time (usually second) - Amplitude: the difference between the maximum and minimum signal heights - Phase: how far the start of the sine wave is shifted from a reference time - Wavelength: the length of a cycle as a signal propagates across a medium (is determined by the speed with which a signal propagates) ASin(2πBt+Cπ) A = amplitude, B = frequency, C = phase
What does ping summary include?
- numbers of packets sent and received - packet loss - min/mean/max round trip times Sometimes sites are configured to reject ping packets to avoid Denial of Service (DOS) attacks Heavily used as a diagnostic tool
Bandwidth of digital signal
-In theory, it has infinite bandwidth bc of infinite sine waves -But in practice, we can calculate bandwidth by choosing a minimum number of sine functions to calculate bandwidth
DNS provides an interesting example of client- server interaction
-The mapping is not performed by a single server - The naming information is distributed among a large set of servers located at sites across the internet When an application needs to translate a name - The application becomes a client of the naming system - The client sends a request message to a name server - server finds the corresponding address and sends a reply message
What is network programming?
A UNIFIED underlying network that supports all applications
Caching in browsers part 2
A browser saves the last modified date information along with the cached copy - Makes a head request(requesting status information) to the server - compares the last modified date of the server's copy to the last modified data in the cached - If the cached version is stale, the browser downloads the new version Browser does not cache small items
Digital signal and signal levels
A digital signal can represent multiple bits How to calculate how many levels are required? - 2^n combinations possible with n bits ex- how many levels do we need to send 3 bits at a time? 2^3 = 8
Types of DNS entrues
A domain name A record type A value (Each entry in a DNS server has a type) A query sent to a DNS server specifies both a domain name and a type. The principal type maps a domain name to an IP address Type A, type AAAA for FTP, ping or a browser Type MX for SMTP, mail exchanger
Time domain representation
A graph of a signal as a function of time
Frequency domain representation
A graph shows a set of simple sine waves that constitute a composite function. - The y-axis gives amplitude and x-axis gives frequency
Stream transport
A sequence of bytes flows from one application program to another. Internet's mechanism arranges two streams between a pair of communicating apps, one in each direction Ex- web browsing, file transfer
Difference between access and transfer protocol
Access: involves a single user interacting with a single mailbox Transfer: allows a user to send mail to other users
Converting digital to analog
According to Fourier, an arbitrary curve can be represented as a composite of sine waves. - Accurate representation of a digital signal requires an infinite set of sine waves Engineers adopted a compromise - conversion of a signal from digital to analog is approximate - generate analog waves that closely approximate the digital signal - approximation involves building a composite signal from only a few sine waves.
Advantages and disadvantages of client- based architecture
Advantages- hardware and applications are less expensive, simple architecture Disadvantages- Data must travel back and forth between server and client
Advantages and disadvantages of Tiered client - server architecture
Advantages- load balancing, more scalable Disadvantages- Each tier increases network load, more complex and difficult to develop application
Advantages and Disadvantages of host- based
Advantages- very simple, single point of control Disadvantages- Host (server) can become a bottleneck, upgrades are expensive
A shift in the audio that users send across the internt
Alert sounds --> human voice --> audio clips --> high- fidelity audio
Peer to peer (p2p) architecture
All devices can act as client and server - Avoids placing data on central server - data is distributed equally among a set of N servers - a given server only provides 1/n of the data (the amount of traffic between a server and the internet is 1/n as much as in the single - server architecture) - Server software can run on the same computer as clients
Extensible Markup Language (XML)
Allows a sender to specify the format of data - Resembles HTML in the sense that both languages embed tags into a text document. XML does not assign any meaning to tags. Tag names can be created as needed and are not specified before hand
Two forms of signals
Analog- uses all values in a continuous range Digital- restrict use to a fixed set of values
How have they measured internet growth?
Analyses by automated tools which probe computer addresses stored in larger on-line databases (DNS)
What are network services provided by?
Application softwares
Why is the use of markup language important
Because it allows a browser to adapt the page to underlying display hardware - a page can be formatted for a high resolution or how resolution display, a large screen or a small hand- held device such as an iphone or PDA
Caching in DNS servers
Caching applies to DNS in 2 ways: Spatial- a user tends to look up names of local computers more often than names of remote computers Temporal- a user tends to look up the same set of domain names repeatedly
Caching in browsers
Caching provides an important optimization for web access. - Backgrounds or banners image do not change frequently - A browser can reduce download times significantly
Client based architecture
Client- Presentation logic, application logic, data access logic Server- Data storage [common in 1980s]
Tiered client - server architecture
Client- presentation logic Web server- Application logic Application server- Application logic Database server- data access logic, data storage
Source encoder and decoder
Concepts include data compression and consequences for communications
Simple signals
Consist of a single sine wave that cannot be decomposed further
Two aspects of interaction
Data representation - Syntax of data items that are exchanged, specific form used during transfer, translation of integers, characters, and files between customers Data transfer - Interaction between client and server, message syntax and semantics, valid and invalid exchange error handling, termination of interaction
HTML is classified as declarative and markup language, what does that mean?
Declarative- allows one to specify what is to be done, not how to do it Mark up language- only gives general guidelines for display and does not include detailed formatting instructions; allows a page to specify the level of importance of a heading; does not require the authors to specify exact font, type face, point size or spacing for the heading;
Bits per second
Depends on the number of signal levels and how frequent the system changes level within a second data rate in bits per second = baud * [log2(levels)] ex- if a system that operates at 1000 baud has 4 signal levels, data rate = 1000 * log2(4) = 2000 bits per second
The internet offers users a rich ______________ of services
Diversity Ex- email, web service etc
DNS Hierarchy and server model
Each DNS server contains information that links the server to other domain name servers up and down the hierarchy. - A given server can be replicated, such that multiple physical copies of the server exist
Message Transport
Each message delivered to a receiver corresponds to a message that was transmitted by a sender (The network never delivers part of a message nor does it join multiple messages together) (If a sender places exactly n bytes in an outgoing message, the receiver will find exactly n bytes in the incoming message)
List some examples of network applications
Email, file transfer, web browsing, voice telephone calls, audio/video teleconferencing
DNS incorporates abbreviations. True or false
False. DNS does NOT incorporate abbreviations. a server only responds to a full name
Each protocol should handle a part of communication handled by other protocols. (T or F)
False. Each protocol should handle a part of communication that are NOT handled by other protocols.
Replications are useful for barely used servers, because they provide information about top level domains. (True or false)
False. Replications are useful for HEAVILY used servers such as root servers that provide information about top-level domains
4 major HTTP request types
GET- Requests a document; server responds by sending status information followed by a copy of the document HEAD- requests status information; server responds by sending information, but doesn't send a copy of the document POST- Sends data to a server; the server appends the data to a specified item (a message is appended to a list) PUT- Sends data to a server; the server uses the data to completely replace the specified item (overwrites the previous data)
A GET request has the following form:
GET/item version CRLF - item gives the URL for the item being requested - Version specifies a version of the protocol (HTTP/1.0) - CRLF denotes two ASCII characters Version information is important because it allows the protocol to change and yet remain backward compatible
The success of TCP/IP lies in its tolerance of .............
Heterogeneity
Baud
How many times the signal can change per second. Ex- if a system requires the signal to remain at a given level for .01 seconds, the system operates at 1/0.01 = 100 baud
How does a client identify a server?
Identifier for the computer (Internet protocol address and domain name system (DNS)) Server identification (16 bit port number)
Encryptor and Decryptor
Information can be encrypted (i.e scrambled) and decrypted - concepts include cryptographic techniques and algorithms
How to guarantee that protocol works well together?
Instead of creating each protocol in isolation, protocols are designed in complete. cooperative sets called suites or families
In 1960s, Department of Defense for Advanced Research Projects Agency (ARPA) became interested in finding ways to share resources. They needed to:
Interconnect all computer Allow a researcher to use the best suitable computer to perform a given task
Each application offers a specific service with its own form of user ................., but all applications can communicate over a single, ............... network
Interface, shared
What is the most revolutionary invention in the 70s- 80s
Internet
Top level Domain (TLD) is controlled by
Internet Corporation for Assigned Names and Numbers (ICANN)
How do you calculate frequency?
Inverse of the time required for one cycle (which is known as the period) Period = T seconds Frequency = 1/T Hertz Typical communication systems use high frequencies
Data communications
Involves the transmission of information over physical media - The subject draws on ideas about electric current, light, and other forms of electromagnetic radiation - Uses mathematics and includes various forms of analysis to design and build transmission systems
The DNS offers a CNAME. What does that mean?
It is analogous to a symbolic link in a file system; the entry provides an alias for another DNS entry.
Internet stream service is connection oriented, what does that mean?
It operates analogous to a telephone call: - Two applications requests a connection - The applications send data in either direction - When finish, connection is terminated
DNS uses the ASCII character set. What problem did it cause
Languages such as Russian, Greek, Chinese each contains characters fir which no ASCII representation exists. IETF debated modifications and extensions of the DNS to accommodate international domain names. IETF chose an approach known as Internationalizing Domain Names in Applications (IDNA) IDNA uses ASCII to store all names; if a domain name contains a non ASCII character, IDNA translates the name into a sequence of ASCII character
Pulse code modulation (PCM)
Level of an analog signal is measured repeatedly at fixed time intervals and converted to digital form. 3 steps taken during conversion - sampling (each measurement is known as a sample) - A sample is quantized (by converting it into a small integer value; the range of the signal from min to max levels is divided into a set of slots, typically a power of 2) - Then encoded into a specific format
What is data communication
Low-level mechanisms and technologies used to send information across a physical communication medium. Ex- wire, radio wave, or light beam Provides a foundation of concepts on which the rest of networking is built
Converting analog to digital
Many sources of information are analog - They must be converted to digital form for further processing (and encrypted) 2 basic approaches 1. Pulse code modulation 2. Delta modulation
Modulator and demodulator
Modulator refers to the way electromagnetic radiation is used to send information
Composite signals
Multiple sine waves added together - Can be decomposed into a set of simple sine waves - Corresponds to combining multiple signals
Multiplexor and demultiplexor
Multiplexor refers to the way information from multiple sources is combined for transmission across a shared medium Demultiplexor is separating them back
TCP/IP defines a .................... and a network-independent identification scheme.
Network- independent packet
How many samples should be taken per second?
Nyquist Sampling rate = 2 * F(max) where f(max) is the highest frequency in the composite signal
ISPs
Offers email services that help people manage an configure an email server (provides a mailbox for each user) - Email access follows one of two forms --- Special purpose email interface (outlook) --- A web browser that accesses an email web page (gmail)
Domain registration
Organization - applies under a specific top-level domain - can choose an internal hierarchy - assigns each computer a name Geographic registration is possible - Corporation for national research initiatives (CNRI)- cnri.reston.va.us Some countries impose conventions - universities in Great Britain register under ac.uk
Two mail access protocols are;
POP3 (Post office protocol version 3) IMAP (Internet Mail access protocol)
What are protocol suites and layering models?
Physical, network interface, internet, transport, application
TCP/IP Protocol Suites
Physical (Layer 1) Network Interface (Layer 2) Internet (Layer 3) Transport (Layer 4) Application (Layer 5)
Data communication is associated with _________ layer
Physical. It touches on physics, mathematics, engineering It includes transmission of signals, encoding data and modulation and multiplexing Data communications theory concentrates on low-level communications systems - information sources can include microphones, sensors and measuring devices such as thermometers and scales - destinations can include audio output devices such as earphones and loud speakers as well as devices such as LEDs that emit light
Examples of simple network probing tools:
Ping- sends message that is echoed by some remote computer Traceroute- reports path to remote computer
Types of telecommunication
Point to point: Over telephone lines Broadcast: Through radio broadcasts Multiplex system: Multiple transmitters and multiple receivers
HyperText Transfer Protocol (HTTP)
Primary transfer protocol that a browser uses to interact with a web server - A browser is a client that extracts a server name from a URL and contacts the server - More URLs contain an explicit protocol reference of http:// or omit the protocol altogether (HTTP is assumed) - HTTP cam be characterized as follows: (Uses textual control messages, transfers binary data files, can download or upload data, incorporates caching)
Two broad types of application- layer protocol
Private communication Standardized service - For standardized service: - application protocol must be documents independent of implementation - The specification must be precise and umambiguous
Transport Layer (Layer 4)
Provides for communication from an application program on one computer to an application program on another Includes specification on -controlling the maximum rate a receiver can accept data - mechanisms to avoid network congestion techniques to insure that all data is received in the correct order
Delta modulation (DM)
Quantization value gives the difference between the previous value and the current value Takes samples like PCM, but - no quantization for each sample, - sends one quantization value followed by a string of values that give the difference between the previous value and current value - Requires fewer bits to transmit differences than sending full values. - Main tradeoff (if any item in the sequence is lost or damaged, all successive values will be interpreted; communication systems that expect data values to be lost or changed during transmission usually use PCM)
Two important email representations standards:
RFC (request for comments) Multi-purpose internet mail extensions (MIME)
Hypertext markup language (HTML)
Representation standard that specifies the syntax of a webpage. HTML has following characteristics - Uses textual description - Describes pages that contain multimedia - Follows a declarative rather than procedural paradigm - Provides markup specifications instead of formatting - Permits a hyperlink to be embedded in an arbitrary object - Allows a document to include metadata Allows a programmer to specify a complex web page that contains graphics, audio, and video, as well as text
What is the most important motivation of creating network?
Resource sharing
Communication involves at least two entities, what are those?
Sender and receiver - All entities in a network must agree on how information will be represented and communicated - All communicating parties agree on details and follow the same set of rules, an exact set of specifications
Analog signals- sine waves
Sin waves are important in information sources. (natural phenomena produce sine waves- audible tones, radio waves, light energy)
Host- based architecture
Server contains all components (server based) [ 1960s- common with wireframes and terminals] Client- no component Server- All 4
Client- server model
Server- (Starts first and passively waits for contact. No need to know which client will contact it. Send/ receives data / Stays running) Client- (Starts second and initiates the connection. Must know which server to contact. Sends/ receives data. May terminate after communication completes.)
How does DNS exploit spatial and temporal locality
Spatial- a name resolver contacts a local server first Temporal- a DNS server caches all lookups
Application Layer (Layer 5)
Specifies how a pair of applications interact when they communicate Specifies details about - The format and the meaning of messages that application can exchange - the procedures that need to be followed Examples: email exchange, file transfer, web browsing, telephone services and video teleconferencing
Internet Layer (Layer 3)
Specify communication across the internet
Network Interface Layer / Data Link (Layer 2)
Specify details about communication between higher layers of protocols (Implemented in SW) and the underlying network (implemented in hardware)
Internet supports 2 communication paradigms:
Stream Transport in the internet Message Transport in the internet
Browser architecture
Structure is complex - Must understand HTTP - Provides support for other protocols - Must contain client code for each of the protocols used - Must know how to interact with a server and how to interpret responses - Must know how to access the FTP service
What does network application define?
Syntax and semantic of messages Who initiates interaction How to handle error When to terminate communication
The availability of high quality _________________ systems is significant for business
Teleconferencing
Shift in the type of data users send across the internet
Text --> graphic images --> video clips --> full - motion video
The most common form of interaction begins with the browser requesting a page from the server
The browser (client) sends a GET request over The server responds by sending a header, a blank-line and the requested document
Whats a potential problem with a single server
The network connection between the server and the internet can become a bottleneck
Application Program interface (API)
The interface an application uses to specify communication. Details of API depends on the OS Socket API or sockets has emerged as the de facto standard for software that communicates over the internet,
How did TCP/IP internet protocol suite start?
The started interconnecting many packet switching technologies into a functional whole
What is name resolution?
The translation of domain name into an address. (The name is said to be resolved to an address and software to perform the translation is known as a name resolver.
Application architecture
The way the functions of the application layer are spread out across the client and server: 4 components - Presentation logic - Application logic - Data access logic - Data storage
MIME is backward compatible with email systems that do NOT understand the MIME standard or encoding. How?
They treat the body as a single block of text
Client server architecture
Thick- client architecture (more functionality) ---- client: presentation logic, application logic ---- server: data access logic, data storage Thin-client architecture (easier to manage) ---- client: presentation logic ---- server: Application logic, data access logic, data storage ex- web service: server- webserver (IIS, Apache) client- web browser (mozilla)
Why should a set of protocols be constructed?
To ensure that the resulting communication system is complete and efficient
Telecommunication
Transmission of information that allows for communication between people/devices that are separated by distance
A small organization that only has a few computers can contract with an ISP to run a DNS server. But, an organization that chooses to run its own server can choose to place all names for the organization in a single physical server or divide its name among multiple servers. True or false
True
APRANET is the prototype for the eventual internet. (T or F)
True
Growth of computer networking has changed business communication. (T or F)
True
HTML tags are case insensitive. (True or false)
True
Is it possible to design network applications, and write codes that communicates over a network, without understanding the hardware / software technologies
True
It is possible for the corporation to divide the workload between separate computers by mapping type A lookups to one computer and type MX lookups to another. (T or F)
True
VT operates a server for names ending in vt.edu (T or F)
True
Uniform resource locators
URLs are used to specify a web page. The general form of a URL is... protocol, computer_name, port, document_name, % ex- http://www.netbook.cs.purdue.edu/toc/toc01.html sometimes a user can omit many of the parts. www.bit.edu [this url omits the protocol (http is assumed), the port (80 is assumed), document name and parameters
One of the most interesting aspects of the internet arises from the way that internet applications change even thought the _______________ essentially remains the same
Underlying technology
Message paradigm allows delivery in different forms:
Unicast- message sent from an application on one computer directly to an application on another, 1- to - 1 Multicast- message sent to some of the computers on a network, 1 to many Broadcast- message broadcast to all computers on a given network, 1 to all Messages may be lost ( never delivered), duplicated (more than one copy arrives), delivered out of order
Chanel encoder and decoder
Used to detect and correct transmission errors
The _________ __________ _____ is one of the most widely used services in the internet
World wide web (www)
Can there be multiple clients and multiple servers?
Yes. A computer can run - A single client - A single server - Multiple copies of a client that contact a given server - Multiple clients that each contact a particular server - Multiple servers, each for a particular service Ex- users can have 3 windows open simultaneously running 3 applications
Example of top-level domains:
aero- air transport industry asia- for or about asia edu- educational institutions etc
Bandwidth of an analog signal
difference between the highest and lowest frequency from a frequency domain plot
Most organizations choose a host name to match the service a computer offers, but
host name doesn't necessarily specify which servers are running ex- a computer named mail could run a web server
Aliases can be very useful because
it permits an organization to change the computer used for a particular service without changing the names or addresses - The use of aliases allows an organization to associate multiple aliases with a single computer
The internet has grown from an early research ..................... connecting a handful of sites to a ............... communication system
prototype, global
