DNS, Email Services and SMTP/POP Protocols

DNS Services and Protokol
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The Transport layer uses an addressing scheme called a port number. Port numbers identify applications and Application layer services that are the source and destination of data. Server programs generally use predefined port numbers that are commonly known by clients. As we examine the different TCP/IP Application layer protocols and services, we will be referring to the TCP and UDP port numbers normally associated with these services. Some of these services are:
Domain Name System (DNS) - TCP/UDP Port 53
Hypertext Transfer Protocol (HTTP) - TCP Port 80
Simple Mail Transfer Protocol (SMTP) - TCP Port 25
Post Office Protocol (POP) - UDP Port 110
Telnet - TCP Port 23
Dynamic Host Configuration Protocol - UDP Port 67
File Transfer Protocol (FTP) - TCP Ports 20 and 21

DNS
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The Domain Name System (DNS) was created for domain name to address resolution for these networks. DNS uses a distributed set of servers to resolve the names associated with these numbered addresses.

The DNS protocol defines an automated service that matches resource names with the required numeric network address. It includes the format for queries, responses, and data formats. DNS protocol communications use a single format called a message. This message format is used for all types of client queries and server responses, error messages, and the transfer of resource record information between servers.

DNS is a client/server service; however, it differs from the other client/server services that we are examining. While other services use a client that is an application (such as web browser, e-mail client), the DNS client runs as a service itself. The DNS client, sometimes called the DNS resolver, supports name resolution for our other network applications and other services that need it.

A DNS server provides the name resolution using the name daemon, which is often called named, (pronounced name-dee).

The DNS server stores different types of resource records used to resolve names. These records contain the name, address, and type of record.

Some of these record types are:
A - an end device address
NS - an authoritative name server
CNAME - the canonical name (or Fully Qualified Domain Name) for an alias; used when multiple services have the single network address but each service has its own entry in DNS
MX - mail exchange record; maps a domain name to a list of mail exchange servers for that domain

At the top of the hierarchy, the root servers maintain records about how to reach the top-level domain servers, which in turn have records that point to the secondary level domain servers and so on.

The different top-level domains represent the either the type of organization or the country or origin. Examples of top-level domains are:
.au - Australia
.co - Colombia
.com - a business or industry
.jp - Japan
.org - a non-profit organization


Email Services and SMTP/POP Protocols
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E-mail, the most popular network service, has revolutionized how people communicate through its simplicity and speed. Yet to run on a computer or other end device, e-mail requires several applications and services. Two example Application layer protocols are
1. Post Office Protocol (POP)
2. Simple Mail Transfer Protocol (SMTP),
shown in the figure. As with HTTP, these protocols define client/server processes.

When people compose e-mail messages, they typically use an application called a Mail User Agent (MUA), or e-mail client. The MUA allows messages to be sent and places received messages into the client's mailbox, both of which are distinct processes.

In order to receive e-mail messages from an e-mail server, the e-mail client can use POP. Sending e-mail from either a client or a server uses message formats and command strings defined by the SMTP protocol. Usually an e-mail client provides the functionality of both protocols within one application.

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E-mail Server Processes - MTA and MDA

The e-mail server operates two separate processes:
Mail Transfer Agent (MTA)
Mail Delivery Agent (MDA)

The Mail Transfer Agent (MTA) process is used to forward e-mail. As shown in the figure, the MTA receives messages from the MUA or from another MTA on another e-mail server. Based on the message header, it determines how a message has to be forwarded to reach its destination. If the mail is addressed to a user whose mailbox is on the local server, the mail is passed to the MDA. If the mail is for a user not on the local server, the MTA routes the e-mail to the MTA on the appropriate server.

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E-mail can use the protocols, POP and SMTP.
POP and POP3 (Post Office Protocol, version 3) are inbound mail delivery protocols and are typical client/server protocols. They deliver e-mail from the e-mail server to the client (MUA). The MDA listens for when a client connects to a server. Once a connection is established, the server can deliver the e-mail to the client.

The Simple Mail Transfer Protocol (SMTP), on the other hand, governs the transfer of outbound e-mail from the sending client to the e-mail server (MDA), as well as the transport of e-mail between e-mail servers (MTA). SMTP enables e-mail to be transported across data networks between different types of server and client software and makes e-mail exchange over the Internet possible.

The SMTP protocol message format uses a rigid set of commands and replies. These commands support the procedures used in SMTP, such as session initiation, mail transaction, forwarding mail, verifying mailbox names, expanding mailing lists, and the opening and closing exchanges.

Some of the commands specified in the SMTP protocol are:
HELO - identifies the SMTP client process to the SMTP server process
EHLO - Is a newer version of HELO, which includes services extensions
MAIL FROM - Identifies the sender
RCPT TO - Identifies the recipient
DATA - Identifies the body of the message

SEMOGA BERMANFAAT

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