CHAPTER 6

A web server and a web client use specific protocols and standards in the process of exchanging information to ensure that the messages are received and understood. These protocols cover: application, transport, Internetwork and network access protocols.

Application Protocol

Hypertext Transfer Protocol (HTTP) governs the way that a web server and a web client interact. HTTP defines the format of the requests and responses exchanged between the client and server. HTTP relies on other protocols to govern how the messages are transported between client and server.

Transport Protocol

Transmission Control Protocol (TCP) is the transport protocol that manages the individual conversations between web servers and web clients. TCP formats the HTTP messages into segments to be sent to the destination host. It also provides flow control and acknowledgement of packets exchanged between hosts.

Internetwork Protocol

The most common internetwork protocol is Internet Protocol (IP). IP is responsible for taking the formatted segments from TCP, assigning the logical addressing, and encapsulating them into packets for routing to the destination host.

Network Access Protocols

Network access protocols perform two primary functions:

Data link management protocols take the packets from IP and encapsulate them into the appropriate frame format for the local network. These protocols assign the physical addresses to the frames and prepare them to be transmitted over the network.

The standards and protocols for the physical media govern how the bits are represented on the media, how the signals are sent over the media, and how they are interpreted by the receiving hosts. Network interface cards implement the appropriate protocols for the media that is being used.

TCP

When an application requires acknowledgment that a message is delivered, it uses TCP.

TCP breaks up a message into small pieces known as segments. The segments are numbered in sequence and passed to IP process for assembly into packets. TCP keeps track of the number of segments that have been sent to a specific host from a specific application. If the sender does not receive an acknowledgement within a certain period of time, it assumes that the segments were lost and retransmits them. Only the portion of the message that is lost is resent, not the entire message.

On the receiving host, TCP is responsible for reassembling the message segments and passing them to the application.

FTP and HTTP are examples of applications that use TCP to ensure delivery of data.


The TCP slows down information transfer. In those cases, UDP may be a more appropriate transport protocol.

UDP is a 'best effort' delivery system that does not require acknowledgment of receipt.It is not guaranteed that the letter is received, but the chances are good.

UDP is preferable with applications such as streaming audio, video and voice over IP (VoIP).

An example of an application that uses UDP is Internet radio. If some of the message is lost during its journey over the network, it is not retransmitted. If a few packets are missed, the listener might hear a slight break in the sound. If TCP were used and the lost packets were resent, the transmission would pause to receive them and the disruption would be more noticeable.

PORTS
Is used to keep track of specific conversations and destination services requested. Every message that a host sends contains both a source and destination port.

Destination Port

The client places a destination port number in the segment to tell the destination server what service is being requested.

Source Port

This allows multiple conversations to occur simultaneously. In other words, multiple devices can request HTTP service from a web server at the same time. The separate conversations are tracked based on the source ports.

SOCKET
is used to identify the server and service being requested by the client

Domain Name System (DNS)
Provides a way for hosts to use this name to request the IP address of a specific server. DNS names are registered and organized on the Internet within specific high level groups, or domains. Some of the most common high level domains on the Internet are .com, .edu, and .net.

When the DNS server receives the request, it checks its table to determine the IP address associated with that web server. If the local DNS server does not have an entry for the requested name, it queries another DNS server within the domain. When the DNS server learns the IP address, that information is sent back to the client. If the DNS server cannot determine the IP address, the request will time out and the client will not be able to communicate with the web server.

When a web client receives the IP address of a web server, the client browser uses that IP address and port 80 to request web services. This request is sent to the server using the Hypertext Transfer Protocol (HTTP).

The information content of a web page is encoded using specialized 'mark-up' languages. HTML (Hypertext Mark-up Language) is the most commonly used but others, such as XML and XHTML, are gaining popularity.

The HTTP protocol is not a secure protocol; information could easily be intercepted by other users as it is sent over the network.


The File Transfer Protocol (FTP)
Provides an easy method to transfer files from one computer to another.

FTP server enables a client to
Exchange files between devices
manage files remotely by sending file management commands such as delete or rename It uses two different ports to communicate between client and server

To begin an FTP session it uses port 21. Once the session is opened, the server will change to port 20 to transfer the data files.

EMAILS
Various application protocols used in processing email include SMTP, POP3, IMAP4.


Simple Mail Transfer Protocol (SMTP)

Used by an email client to send messages to its local email server. The local server then decides if the message is destined for a local mailbox or if the message is addressed to a mailbox on another server.
SMTP requests are sent to port 25.


Post Office Protocol (POP3)

A server that supports POP clients receives and stores messages addressed to its users. When the client connects to the email server, the messages are downloaded to the client.Messages are not kept on the server after they have been accessed by the client. Clients contact POP3 servers on port 110.

Internet Message Access Protocol (IMAP4)

It does the same that POP3, but IMAP keeps the messages unless its users errase them.
It requests on port 143.


The following are typical entries when configuring an email client:

POP3 or IMAP4 Server name
SMTP Server name
Username
User password
SPAM and Virus filters

A call to a regular telephone (land line or cell phone) requires the use of a gateway to access the Public Switched Telephone Network (PSTN).

PORTS
Help servers to know which service is being requested by a client.

Are assigned and managed by the Internet Corporation for Assigned Names and Numbers (ICANN).

Well-Known Ports

Destination ports that are associated with common network applications. These ports are in the range of 1 to 1023.

Registered Ports

Ports 1024 through 49151 can be used as either source or destination ports. These can be used by organizations to register specific applications such as IM applications.

Private Ports
Ports 49152 through 65535, often used as source ports. These ports can be used by any application.


layered model
Depicts the operation of the protocols occurring within each layer, as well as the interaction with the layers above and below it.

Benefits:

*Assists in protocol design.
*Fosters competition because products from different vendors can work together.
*Prevents technology or capability changes in one layer from affecting other layers above and and below.
*Fosters competition because products from different vendors can work together.
*Prevents technology or capability changes in one layer from affecting other layers above and below.
*Provides a common language to describe networking functions and capabilities.

TCP/IP MODEL
It specify the interaction of each protocol

OSI MODEL
Includes all functions, or tasks, associated with Inter-network communications, not just those related to the TCP/IP protocols. Compared to the TCP/IP model, which only has four layers, the OSI model organizes the tasks into seven more specific groups. A task, or group of tasks, is then assigned to each of the seven OSI layers.

APPLICATION LAYER:
define interfaces between application software and network communication functions.
provides standardized services such as file transfer between systems.

PRESENTATION LAYER:
standardizes user data formats for use between different types of systems.
encodes and decodes user data; encrypts and decrypts data; compresses and and decompresses data.

SESSION LAYER:
manages user sessions and dialogues.
mantains logical links between systems.

TRANSPORT LAYER:
manages end-to-end message delivery over the network.
can provide reliable and sequential packet delivery through error recovery and flow control mechanisms.

NETWORK LAYER:
routes packets according to unique network devices addresses.

DATA LINK LAYER:
defines procedures for operating the communications links.
detects and corrects frame transmit errors.

PHYSICAL LAYER:
defines physcial means of sending data over network devices.
interfaces between network medium and devices.
defines optical, electrical and mechanical characteristics.














The Packet Tracer (PT)allows viewing of simulated data being transmitted between two hosts. It uses Protocol Data Units (PDUs) to represent network traffic frames and displays protocol stack information at the appropriate layers of the OSI model.

Layer 1 (Physical): Fast Ethernet port

Layer 2 (Data Link): Ethernet Mac addresses

Layer 3 (Network): IP addresses

Layer 4 (Transport): TCP port numbers

CHAPTER 5

A host needs an IP address to participate on the Internet.

IP address

Is a logical network address that identifies a particular host. It must be properly configured and unique in order to communicate with other devices on the Internet.

Is assigned to the Network interface connection for a host.
This connection is usually a network interface card (NIC) installed in the device.

Examples:
workstations, servers, network printers and IP phones.
Some servers can have more than one NIC and each of these has its own IP address.
Router interfaces that provide connections to an IP network will also have an IP address.

Is simply a series of 32 binary bits (ones and zeros)

The 32 bits are grouped into four 8-bit bytes called octets.

The 32-bit IP address is defined with IP version 4 (IPv4) and is currently the most common form of IP address on the Internet.

If all 8 bits are 1s, 11111111 the value of the octet is 255 (128+64+32+16+8+4+2+1).

If the 8 bits are mixed, such as the example

Hierarchical addressing

The logical 32-bit IP address is made up of two parts. The first part identifies the network and the second part identifies a host on that network. Both parts are required in an IP address.

Routers only need to know how to reach each network.

subnet mask
signifies which part of the IP address is network and which part is host.

The 1s in the subnet mask represent the network; the 0s represent the host.

When a host sends a packet, it compares its subnet mask to its own IP address and the destination IP address. If the network bits match, both the source and destination host are on the same network and the packet can be delivered locally.

IP addresses classes

Classes A, B and C are commercial addresses and are assigned to hosts. Class D is reserved for multicast use and Class E is for experimental use.

Class C addresses have three octets for the network portion and one for the hosts. Usually assigned to small networks.

Class B addresses have two octets to represent the network portion and two for the hosts. Typically used for medium-sized networks.

Class A addresses have only one octet to represent the network portion and three to represent the hosts. Typically assigned to large organizations.

Unique public IP address
All hosts that connect directly to the Internet needs it.There is a risk of running out of IP addresses.

One solution to this problem was to reserve some private addresses for use exclusively inside an organization. This allows hosts within an organization to communicate with one another without the need of a unique public IP address.

RFC 1918
Is a standard that reserves several ranges of addresses within each of the classes A, B and C.



Private addressesAre not routed on the Internet
The use of private addresses can provide a measure of security since they are only visible on the local network.
Loopback addressThere are also private Addresses that can be used for the diagnostic testing of devices.
The class A, 127.0.0.0 network, is reserved for it.

we also categorize IP addresses as
UNICAST
For a unicast packet to be sent and received, a destination IP address must be in the IP packet header. A corresponding destination MAC address must also be present in the Ethernet frame header. The IP address and MAC address combine to deliver data to one specific destination host.

BROADCAST

This means that all hosts on that local network (broadcast domain) will receive and look at the packet. Many network protocols, such as ARP and DHCP use broadcasts.

A broadcast IP address for a network needs a corresponding broadcast MAC address in the Ethernet frame.

MULTICAST

As with a unicast or broadcast address, multicast IP addresses need a corresponding multicast MAC address to actually deliver frames on a local network. The multicast MAC address is a special value that begins with 01-00-5E in hexadecimal. The value ends by converting the lower 23 bits of the IP multicast group address into the remaining 6 hexadecimal characters of the Ethernet address.

IP addresses can be assigned either statically or dynamically


STATIC

With a static assignment, the network administrator must manually configure the network information for a host.
This includes:
the host IP address, subnet mask and default gateway.

Advantages:
Useful for printers, servers and other networking devices that need to be accessible to clients on the network.
If hosts normally access a server at a particular IP address, it would not be good if that address changed.

Provide increased control of network resources, but it can be time consuming to enter the information on each host. Therefore, errors are more likely to occur.

It is important to maintain an accurate list of which IP addresses are assigned to which devices.These are permanent addresses and are not normally reused.


DYNAMIC

On local networks it is often the case that the user population changes frequently. New users arrive with laptops and need a connection. Others have new workstations that need to be connected. Rather than have the network administrator assign IP addresses for each workstation, it is easier to have IP addresses assigned automatically. This is done using a protocol known as Dynamic Host Configuration Protocol (DHCP).

DHCP provides a mechanism for the automatic assignment of addressing information such as IP address, subnet mask, default gateway, and other configuration information.

Benefit of DHCP:
an address is not permanently assigned to a host but is only leased for a period of time.

The router provides a GATEWAY through which hosts on one network can communicate with hosts on different networks. Each interface on a router is connected to a separate network.


Every host on a network must use the router as a gateway to other networks. Therefore, each host must know the IP address of the router interface connected to the network where the host is attached. This address is known as the default gateway address It can be either statically configured on the host, or received dynamically by DHCP.

When an integrated router is configured as a DHCP server, it provides its own internal IP address as the default gateway to DHCP clients. It also provides them with their respective IP address and subnet mask.


The integrated router acts as a DHCP server for all local hosts attached to it, either by Ethernet cable or wirelessly.The default IP address configured on the local integrated router interface is usually a private Class C address.

When an integrated router is connected to the ISP, it acts like a DHCP client to receive the correct external network IP address for the Internet interface. ISPs usually provide an Internet-routable address, which enables hosts connected to the integrated router to have access to the Internet.


WAYS OF CONNECTING A HOST

Direct Connection
Some customers have just a single computer with a direct connection from the ISP through a modem. In this case, the public address from the ISP DHCP server is assigned to the single host.

Connection Through an Integrated Router

When there is more than one host that needs access to the Internet, the ISP modem can be attached directly to an integrated router instead of directly to a single computer. This enables the creation of a home or small business network. The integrated router receives the public address from the ISP. Internal hosts receive private addresses from the integrated router.

Connection Through a Gateway Device
Gateway devices combine an integrated router and a modem in one unit, and connect directly to the ISP service. As with integrated routers, the gateway device receives a public address from the ISP and internal PCs will receive private addresses from the gateway device.

TRANSLATION NAT
The integrated router is able to translate many internal IP addresses to the same public address, by using it.Only packets destined for other networks need to be translated.

apunte quark xpress: capas

Las capas son componentes para documentos de diseño
cada uno de los elementos que integran nuestro trabajo. en quark xpress una capa o layer se define como una rebanada de lay out que contiene elementos especificos, cada capa puede ocultarse, bloquearse, traer al frente o enviar hacia atras, imprimir o borrar. la ventaja de usar capas es el poder manipular cada una de las partes de un trabajo por separado, para asi no modificar o alterar algun otro elemento.

Menu color

existen diversos tipos o bases de colores con los cuales podemos crear fotolitos o colores, entre ellos estan:
cmyk: tinta de colores estandar utilizando en el proceso de impresion a 4 colores. Tambien llamado tinta de procesos. El proceso de impresion a 4 colores utiliza la combinacion cmyk sobrepuesta para lograr un efecto deseado al contrario, el spot color utiliza solamente una tinta para su impresion.

los colores websafe se utilizan para el diseño de paginas web, los cuales estan en un formato exadecimal y tienen una basa rgp(rojo verde azul) que es un modelo de color con el cual se obtiene un tono diferente a partir de la suma de distintas cantidades de los tres colores primarios. Es el modelo mas utilizado para visualizar y trabajar con imagenes digitales en una pantalla

Calibracion de color

su finalidad es medir o ajustar la respuesta del color de un dispositivo, para establcer un modelo del color estandar