Transcript Niveles 4, 5, 6, y 7 del Modelo OSI

Niveles 4, 5, 6, y 7
del Modelo OSI
COMP 316
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.
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.
• A computer that is connected to a TCP/IP network, including the
Internet. Each host has a unique IP 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.
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.
• 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.
• Flow control can be implemented in hardware or software, or a
combination of both.
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
coordination
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.
• 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.
System Software vs. Applications
Software
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.
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.
• There are two main types of encryption:
asymmetric encryption (also called public-key
encryption) and symmetric encryption.
Some Encryption Methods
Name
Method
Plaintext
Ciphertext
Transposition
Switch the order of
characters
SOFTWARE
OSTFAWER
Expansion
Insert characters between
existing characters
USER
UYSYEYRY
Substitution
Replace characters with
other characters
INFORMATION
WLDIMXQUWIL
Compaction
Remove characters and
store elsewhere
ACTIVATION
ACIVTIN
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.
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.
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).
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.
• 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.
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.
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.
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.
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.
• 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).
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 reliable