The document describes essential application layer protocols, focusing on the Domain Name System (DNS) and File Transfer Protocol (FTP). DNS functions as a distributed internet directory service, facilitating the translation of domain names to IP addresses, while FTP enables file sharing between hosts through TCP connections. Key features of FTP include promoting file sharing, indirect use of remote computers, and efficiently transferring data.

1. Application Layer protocols

2. DNS: Domain Name System (Service)
protocol
• Domain Name System (DNS) is a distributed
Internet directory service.
• DNS is used mostly to translate between domain
names and IP addresses and to control Internet
email delivery.
• Most Internet services rely on DNS to work, and if
DNS fails, web sites cannot be located and email
delivery stalls.
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3. DNS has two independent aspects:
1. IT SPECIFIES THE NAME SYNTAX AND RULES FOR
DELEGATING AUTHORITY OVER NAMES. THE
BASIC SYNTAX IS: LOCAL.GROUP.SITE
2. IT SPECIFIES THE IMPLEMENTATION OF A
DISTRIBUTED COMPUTING SYSTEM THAT
EFFICIENTLY MAPS NAMES TO ADDRESSES.

4. Protocol Structure
ID 16-bit field used to correlate queries
and responses.
Q 1-bit field that identifies the
message as a query or response
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5. FTP: File Transfer Protocol
Protocol Description:
File Transfer Protocol (FTP) enables file sharing between hosts.
FTP uses TCP to create a virtual connection for control
information and then creates a separate TCP connection for data
transfers.
The control connection uses an image of the TELNET protocol to
exchange commands and messages between hosts.
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6. The key functions of FTP are:
1) to promote sharing of files (computer
programs and/or data);
2) 2) to encourage indirect or implicit (via
programs) use of remote computers;
3) 3) to shield a user from variations in file
storage systems among hosts; and
4) 4) to transfer data reliably and efficiently.
FTP, though usable directly by a user at a terminal, is
designed mainly for use by programs. FTP control frames
are TELNET exchanges and can contain TELNET
commands and option negotiation. However, most FTP
control frames are simple ASCII text and can be classified
as FTP commands or FTP messages. FTP messages are
responses to FTP commands and consist of a response
code followed by explanatory text.

7. Protocol Structure
Command Description
ABOR Abort data connection process.
ACCT<account> Account for system privileges.
ALLO <bytes> Allocate bytes for file storage on server.
APPE <filename> Append file to file of same name on
server.
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8. thank you
NAME : MAYAR MAGDY
AHMED
ID: 20206030