1. Niveles 4, 5, 6, y 7 del Modelo OSI COMP 316

2. Layer 4 – Transport This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.hosts flow control

3. Host A computer system that is accessed by a user working at a remote location. Typically, the term is used when there are two computer systems connected by modems and telephone lines. The system that contains the data is called the host, while the computer at which the user sits is called the remote terminal.computer systemuserremotecomputermodemssystemdatacomputerterminal A computer that is connected to a TCP/IP network, including the Internet. Each host has a unique IP address. InternetIP address To provide the infrastructure for a computer service. For example, there are many companies that host files, programs, applications or even a Web server for companies and individuals. In the case of a Web server, it means that they provide the hardware, software, and communications lines required by the server, but the content on the server may be controlled by someone else.Web server hardwaresoftware communications

4. Flow Control In communications, the process of adjusting the flow of data from one device to another to ensure that the receiving device can handle all of the incoming data. This is particularly important where the sending device is capable of sending data much faster than the receiving device can receive it.communications There are many flow control mechanisms. One of the most common flow control protocols for asynchronous communication is called xon-xoff. In this case, the receiving device sends a an xoff message to the sending device when its buffer is full. The sending device then stops sending data. When the receiving device is ready to receive more data, it sends an xon signal.protocolsasynchronousbuffer Flow control can be implemented in hardware or software, or a combination of both.software

5. Layer 5- Session This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordinationapplications

6. Applications A program or group of programs designed for end users. Application software can be divided into two general classes: systems software and applications software. Systems software consists of low-level programs that interact with the computer at a very basic level. This includes operating systems, compilers, and utilities for managing computer resources.programend userssoftwaresystems softwaresoftwarecomputeroperating systemscompilersutilitiesresources In contrast, applications software (also called end-user programs) includes database programs, word processors, and spreadsheets. Figuratively speaking, applications software sits on top of systems software because it is unable to run without the operating system and system utilities.databaseword processorsspreadsheets applicationsrunsystem

7. System Software vs. Applications Software

8. Layer 6 - Presentation This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer. encryption network

9. Data encryption The translation of data into a secret code. Encryption is the most effective way to achieve data security. To read an encrypted file, you must have access to a secret key or password that enables you to decrypt it. Unencrypted data is called plain text ; encrypted data is referred to as cipher text.data Encryptionsecurityreadfilekeypassworddecryptplain text cipher text There are two main types of encryption: asymmetric encryption (also called public-key encryption) and symmetric encryption.public-key encryptionsymmetric encryption

10. NameMethodPlaintextCiphertext Transposition Switch the order of characters SOFTWAREOSTFAWER Expansion Insert characters between existing characters USERUYSYEYRY Substitution Replace characters with other characters INFORMATIONWLDIMXQUWIL CompactionRemove characters and store elsewhere ACTIVATIONACIVTIN Some Encryption Methods

11. Public-key encryption A cryptographic system that uses two keys -- a public key known to everyone and a private or secret key known only to the recipient of the message. When John wants to send a secure message to Jane, he uses Jane's public key to encrypt the message. Jane then uses her private key to decrypt it.cryptographickeysencryptdecrypt An important element to the public key system is that the public and private keys are related in such a way that only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them. Moreover, it is virtually impossible to deduce the private key if you know the public key. Public-key systems, such as Pretty Good Privacy (PGP), are becoming popular for transmitting information via the Internet. They are extremely secure and relatively simple to use. The only difficulty with public-key systems is that you need to know the recipient's public key to encrypt a message for him or her. What's needed, therefore, is a global registry of public keys, which is one of the promises of the new LDAP technology.Pretty Good Privacy (PGP)informationInternetLDAP Public key cryptography was invented in 1976 by Whitfield Diffie and Martin Hellman. For this reason, it is sometime called Diffie-Hellman encryption. It is also called asymmetric encryption because it uses two keys instead of one key (symmetric encryption).

12. LDAP Short for Lightweight Directory Access Protocol, a set of protocols for accessing information directories. LDAP is based on the standards contained within the X.500 standard, but is significantly simpler. And unlike X.500, LDAP supports TCP/IP, which is necessary for any type of Internet access. Because it's a simpler version of X.500, LDAP is sometimes called X.500-lite. protocolsinformationX.500TCP/IP Internet Although not yet widely implemented, LDAP should eventually make it possible for almost any application running on virtually any computer platform to obtain directory information, such as email addresses and public keys. Because LDAP is an open protocol, applications need not worry about the type of server hosting the directory.computerpublic keysapplications

13. symmetric encryption A type of encryption where the same key is used to encrypt and decrypt the message. This differs from asymmetric (or public-key) encryption, which uses one key to encrypt a message and another to decrypt the message.encryptionkeyasymmetric (or public-key) encryption

14. Layer 7- Application This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.applicationsyntaxfile transferse-mailnetworksoftwareTelnetFTP

15. Telnet (tel´net) (n.) A terminal emulation program for TCP/IP networks such as the Internet. The Telnet program runs on your computer and connects your PC to a server on the network. You can then enter commands through the Telnet program and they will be executed as if you were entering them directly on the server console. This enables you to control the server and communicate with other servers on the network. To start a Telnet session, you must log in to a server by entering a valid username and password. Telnet is a common way to remotely control Web servers.terminal emulation TCP/IPnetworksInternetcomputerservercommandsserverconsoleserversusernamepasswordremotely controlWeb servers

16. FTP Short for File Transfer Protocol, the protocol for exchanging files over the Internet. FTP works in the same way as HTTP for transferring Web pages from a server to a user's browser and SMTP for transferring electronic mail across the Internet in that, like these technologies, FTP uses the Internet's TCP/IP protocols to enable data transfer.protocolfilesInternetHTTP serverSMTP electronic mailTCP/IP FTP is most commonly used to download a file from a server using the Internet or to upload a file to a server (e.g., uploading a Web page file to a server).downloadupload

17. E-mail Short for electronic mail, the transmission of messages over communications networks. The messages can be notes entered from the keyboard or electronic files stored on disk. Most mainframes, minicomputers, and computer networks have an e-mail system. Some electronic-mail systems are confined to a single computer system or network, but others have gateways to other computer systems, enabling users to send electronic mail anywhere in the world. Companies that are fully computerized make extensive use of e-mail because it is fast, flexible, and reliablecommunicationsnetworkskeyboardfiles storeddiskmainframesminicomputers computersystem computer system gatewaysusers