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