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Notes of Internet and Intranet [CT 754]

Protocols and Client-Server Applications


Standard Protocols

1. Electronic Mail:

- Mail server is the computer system that is responsible to forward mails towards its intended recipient.
- Every email that is sent passes through a series of mail servers before reaching recipient.
- Without the series of mail servers, it would be possible to send emails within same domain only.
- The control message format is documented in RFC22.
- Each control message is embedded in appropriate message structure.
- It is shown in given figure:


Components of Mail System

1. Mail User Agent:
- Writes email to MTA using SMTP.
- Reads email delivered by MDA or retrieved by MRA.

2. Mail Retrieval Agent:
- Retrieves email from MAA.
- Makes mail available to MUA.

3. Mail Access Agent:
- It authenticates MUA.
- Reads email from mailbox and makes it available to MUA.

4. Mail Submission Agent:
- It accepts email from MUA, prepares and delivers to MTA.

5. Mail Transfer Agent:
- It routes incoming mails and determines which MDA to send mail to.

6. Mail Delivery Agent:
- It accepts mail from MTA and delivers mail to mailbox or other MTA.

Process of sending email

- The email client connects to the domain’s SMTP server.
- The email client communicate with SMTP and provides your email address, recipient email address and message body.
- The SMTP server processes the recipient’s email address. If domain of sender and receiver is same, the message is routed directly over to domain’s POP3 or IMAP server. Otherwise, SMTP server communicates with other domain’s server.
- SMTP server communicates with DNS to find recipient’s server and the DNS provides IP address.
- The SMTP server can connect to recipient SMTP server by routing messages along a series of unrelated SMTP servers.
- The recipient SMTP server scans incoming message and forwards to domain’s POP3 or IMAP server if it recognizes the sender’s domain and username.

2. SMTP:

- SMTP stands for Simple Mail Transfer Protocol.
- It is an email transmit text based protocol which moves the email on and across networks using a process called ‘store and forward’.
- It works with Main Transfer Agent to send communication to the right computer.
- It provides a set of codes that simply communicate email messages between email servers.
- When you send out a message, it is turned into strings of text separated by code words that identify the purpose of each section.
- It provides those codes to servers.
- Email server software helps to understand their meaning.
- As message travels towards destination, it passes through a no of computers.
- Each computer stores it before moving on to next computer in the path.
- It is able to transfer text only.

Support for images in SMTP

- Multipurpose Internet Mail Extensions can be used to support image messages in SMTP.
- MIME encodes the non-text content into plain text, which can be transmitted via SMTP.
- It consists of MIME header.
- MIME header includes:
a) MIME-Version = indicates MIME formatted messages
b) Content-Type = indicates media type of message content. For image; image/png
c) Content-Disposition = specify presentation styles of mail messages.
d) Content-Transfer-Encoding = indicates whether or not binary to text encoding scheme is used.
- The non ASCII data uses MIME encoded word syntax.
- The syntax uses string of ASCII characters indicating both original character encoding (charset) and content-transfer-encoding used to map bytes of charset into ASCII characters.

3. POP:

- POP stands for Post Office Protocol.
- It is the standard client/server protocol for receiving emails.
- Email is received and held for the user by the Internet server.
- Periodically, the user check their mail box on the server and download any mail.
- As soon as the user downloaded the mail, POP3 deletes the mail on the server.
- It is a kind of ‘store and forward’ service.

4. IMAP:

- IMAP stands for Internet Message Access Protocol.
- It is the standard protocol for receiving emails in which the stored messages on the mail server can be viewed and manipulated by the end users as though they are stored locally.
- Users can organize messages into folders on the server.
- It is a kind of remote file server.
- It also supports multiple logins.

5. PGP:

- PGP stands for Pretty Good Privacy.
- It helps to secure e-mails.
- It is a program used to encrypt and decrypt email over the Internet as well as authenticate messages with digital signatures.
- Each user has encryption key and private key.
- Message is encrypted and send to someone using their encryption key.
- It uses faster encryption algorithm to encrypt message.
- The receiver private key is used to decrypt to short key; which is the key used to decrypt the message.

6. HTTP:

- HTTP stands for Hyper Text Transfer Protocol.
- HTTP is an application protocol for distributed, collaborative and hypermedia information system used for data communication in WWW.
- It acts as a request-response protocol in client-server computing model.
- A request message consists of request line, request header field, empty line and optional message body.
- A response message consists of status line, response header field, empty line and optional message body.
Example Communication over HTTP
Consider that a http client be web browser which requests

Client request:
- A client sends a request in request message as:
GET / HTTP/1.1 (Request line)
Host: (Request header field)
(Empty line)

Server response:
HTTP/1.1 200 OK
Date: Mon, 23 May 2016 22:38:45 GMT
Content-Type: text/html; charset = UTF-8
Content-Encoding: UTF-8
Content-Length: 138
Last Modified: Wed, 11 April 2015 11:22:32 GMT
Server: Apache/
Etag: “3f80f-1b6-3e1cb03b”
Accept-Ranges: bytes
Connection: close

7. FTP:

- FTP stands for File Transfer Protocol.
- FTP is a standard network protocol used for transfer of files from a server to a client using client-server architecture on a network.

Communication in FTP
- FTP may run in active or passive mode, which determines how data connection is established.
- In active mode, client starts listening for incoming data connections from server on port M. It sends FTP command PORT M to inform server on which port it is listening. The server then initiates data channel to the client from its port 20.
- In passive mode, the client uses control connection to send PASV command to the server and recieves server IP address and server port no from the server. The client then uses to open a data connection from an arbitary client port to server IP address and server port no received.


N-tiered Client/Server Architecture

N-Tiered Client Server Architecture

- A client server system is the one in which the server provides some kind of services that is used by multiple clients.
- A tier is a layer.
- N-Tier client-server architecture is a client-server system with N layers.
- A two tiered client-server architecture consists of presentation tier (client) and application tier (server).
- A three tiered client-server architecture consists of presentation tier (client), application tier (server) and database tier.
- Presentation tier deals with interaction with the users. It processes user inputs, sends request to users and shows the result of these requests to the user.
- Application tier processes the requests of all the users.
- Database tier contains database management system that manages all the persistent data.

Challenges of N-Tiered Architecture:

1. Communication and distribution is handled by third party middleware like CORBA, EJB, etc.
2. Software becomes heterogeneous and parallel.
3. It is necessary to learn a lot of new technologies.
4. The design of truly reusable objects is difficult.
5. Load balancing is difficult.
6. General distributed object protocols are slow.

Tiered ISP Network:

- The Internet backbone is connected to the users for Internet access through the 3-tiered ISP network.
- Tier 1 ISPs are large national or international ISPs. They are directly connected to the Internet backbone and can be considered part of the backbone itself. They have the highest speed connections and very reliable networks. Their customers are either lower-tiered ISPs or large companies that are looking for a very reliable and fast access to the Internet. A major advantage of purchasing service from a tier 1 ISP is if there is a problem with access, only one company is involved, so solving the problem is that much easier. Sprint is a tier 1 ISP.

- Tier 2 ISPs purchase their Internet service from a tier 1 ISP. Tier 2 ISPs tend to cover a specific region. They focus on business customers and have lower quality networks and slower access than tier 1 ISPs.

- Tier 3 ISPs also purchase their Internet service from tier 1 ISPs. Tier 3 ISPs tend to focus on the retail market, and they also tend to cover a specific region. Network quality and access speed are relatively low. Prices are much lower than for tier 2 or tier 1 ISPs.


Universal Internet Browsing

- Internet browser or web browser is the program that is used to access Internet and view web pages in the computer.
- The main purpose of Internet browser is to translate or render the code that the websites are designed in into the text, graphics and other features of the web pages.
- Example : Internet Explorer, Google Chrome, Moxilla Firefox, Safari and so on.

Working of Browser:

1. You type a website's URL into your browser's address bar; "" is an example of a URL.
2. The browser locates and requests that page's information from a web server.
3. The browser receives a file in a computer code like HTML or Javascript, which includes instructions about how to display the information on that page.
4. The browser interprets that file and displays the page for you to read and interact with. And it does all of this in just a few seconds, usually.

Multiprotocol Support

- Multiprotocol support means existence of multiple protocols to be followed while providing a service.
- For a generic request/reply protocol, there are some basic requirements to be met.
- But, a single protocol may not have all the requirements.
- So, different protocols are layered on top of other protocol to meet all the requirements.

Multi Protocol Label Switching:

- MPLS is a scalable, protocol independent transport technique.
- In MPLS network, data packets are assigned labels.
- Packet forwarding decisions are based on the assigned labels.
- It helps in creation of end-to-end circuits across any type of transport medium using any protocol.
- It operates at a layer between data link layer and network layer of OSI reference model.
- MPLS provides connection oriented QoS Support, Traffic Engineering, VPN Support and Multi Protocol Support.
- QoS support guarantees the fixed capacity for specific applications.
- Traffic engineering is the ability to dynamically define routes for load management and optimization of network usage.

Operation of MPLS:

- MPLS works by prefixing packets with the MPLS header, containing one or more labels.
- Each label consists of four fields:
1. 20 bit label value
2. 3 bit Traffic class field for QoS priority and ECN.
3. 1 bit bottom of stack flag (When set, represents current label is last entry in the stack)
4. 8 bit TTL field.
- MPLS router is required that helps MPLS labeled packets to be switched after a label lookup.


Path Setup:
- Labeled switched path is established before routing and delivery of packets.
- QoS parameters are established along the path.

Packet Handling:
- Packet enters into the domain through edge label switching router (LSR).
- Label Switching Router assigns packet to Forward Equivalence Class (FEC) and then Label Switched Path (LSP).
- Label is appended to the packet and then it is forwarded.
- Within the domain, the Label Switching Router gets the packet, remove the incoming label, attach the outgoing label and then forwarded to next label switching router.
- The final LSR within the domain strips the label, reads the IP and forwards the packet.
- MPLS packet forwarding is explained in given figure:


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