Short Notes: Application Layer Protocols (DNS, SMTP, POP, FTP, HTTP) | Short Notes for Computer Science Engineering - Computer Science Engineering (CSE) PDF Download

 Page 1


An application layer protocol defines how application processes (clients and 
servers), running on different end systems, pass messages to each other. In 
particular, an application layer protocol defines:
• The types of messages, e.g., request messages and response messages.
• The syntax of the various message types, i.e., the fields in the message and 
how the fields are delineated.
• The semantics of the fields, i.e., the meaning of the information that the field 
is supposed to contain;
• Rules for determining when and how a process sends messages and 
responds to messages.
Page 2


An application layer protocol defines how application processes (clients and 
servers), running on different end systems, pass messages to each other. In 
particular, an application layer protocol defines:
• The types of messages, e.g., request messages and response messages.
• The syntax of the various message types, i.e., the fields in the message and 
how the fields are delineated.
• The semantics of the fields, i.e., the meaning of the information that the field 
is supposed to contain;
• Rules for determining when and how a process sends messages and 
responds to messages.
A p p l i c a t i o n T y p e A p p l i c a t i o n - l a y e r p r o t o c o l T r a n s p o r t P r o t o c o l
E le c tr o n ic m ail
S en d : S im p le M a il T r a n s fe r P r o to c o l 
S M T P [R F C 8 2 1 ]
T C P 2 5
R e ceiv e: Post Office Protocol v3 
P O P 3 |R C F 1939]
T C P 110
R e m o te te r m in a l a c c e s s T e ln e t [R F C 854 ] T C P 2 3
W o r ld W id e W e b (W W W ) HyperText transfer Protocol 1.1 
H T T P 1.1 [R F C 2 0 6 8 ]
T C P 80
F ile T r a n s fe r
F ile T r a n s fe r P ro to c o l 
F T P |R F C 959 ]
T C P 21
T r iv ia l F ile T r a n s fe r P ro to c o l 
T F T P [R F C 1 3 5 0 |
L D P 69
R e m o te file serv e r N F S [M c K u silt 1996] L D P o r T C P
S tr e a m in g m u ltim e d ia P r o p r ie ta r y (e .g ., R eal N e tw o r k s) L D P o r T C P
I n te r n e t te le p h o n y P r o p r ie ta r y (e .g ., V o e a lte c ) U s u a lly U D P
SMTP (Simple Mail Transfer Protocol):
• One of the most popular network service is electronic mail (e-mail).
• The TCP/IP protocol that supports electronic mail on the Internet is called 
Simple Mail Transfer Protocol (SMTP).
• SMTP transfers messages from senders' mail servers to the recipients' mail 
servers using TCP connections.
• Users based on e-mail addresses.
• SMTP provides services for mail exchange between users on the same or 
different computers.
• Following the client/server model:
° SMTP has two sides: a client side which executes on a sender's mail 
server, and server side which executes on recipient's mail server.
° Both the client and server sides of SMTP run on every mail server.
° When a mail server sends mail (to other mail servers), it acts as an 
SMTP client.
° When a mail server receives mail (from other mail servers) it acts as an 
SMTP server.
TELNET (Terminal Network):
• TELNET is client-server application that allows a user to log onto remote 
machine and lets the user to access any application program on a remote 
computer.
• TELNET uses the NVT (Network Virtual Terminal) system to encode 
characters on the local system.
• On the server (remote) machine, NVT decodes the characters to a form 
acceptable to the remote machine.
• TELNET is a protocol that provides a general, bi-directional, eight-bit byte 
oriented communications facility.
• Many application protocols are built upon the TELNET protocol
Page 3


An application layer protocol defines how application processes (clients and 
servers), running on different end systems, pass messages to each other. In 
particular, an application layer protocol defines:
• The types of messages, e.g., request messages and response messages.
• The syntax of the various message types, i.e., the fields in the message and 
how the fields are delineated.
• The semantics of the fields, i.e., the meaning of the information that the field 
is supposed to contain;
• Rules for determining when and how a process sends messages and 
responds to messages.
A p p l i c a t i o n T y p e A p p l i c a t i o n - l a y e r p r o t o c o l T r a n s p o r t P r o t o c o l
E le c tr o n ic m ail
S en d : S im p le M a il T r a n s fe r P r o to c o l 
S M T P [R F C 8 2 1 ]
T C P 2 5
R e ceiv e: Post Office Protocol v3 
P O P 3 |R C F 1939]
T C P 110
R e m o te te r m in a l a c c e s s T e ln e t [R F C 854 ] T C P 2 3
W o r ld W id e W e b (W W W ) HyperText transfer Protocol 1.1 
H T T P 1.1 [R F C 2 0 6 8 ]
T C P 80
F ile T r a n s fe r
F ile T r a n s fe r P ro to c o l 
F T P |R F C 959 ]
T C P 21
T r iv ia l F ile T r a n s fe r P ro to c o l 
T F T P [R F C 1 3 5 0 |
L D P 69
R e m o te file serv e r N F S [M c K u silt 1996] L D P o r T C P
S tr e a m in g m u ltim e d ia P r o p r ie ta r y (e .g ., R eal N e tw o r k s) L D P o r T C P
I n te r n e t te le p h o n y P r o p r ie ta r y (e .g ., V o e a lte c ) U s u a lly U D P
SMTP (Simple Mail Transfer Protocol):
• One of the most popular network service is electronic mail (e-mail).
• The TCP/IP protocol that supports electronic mail on the Internet is called 
Simple Mail Transfer Protocol (SMTP).
• SMTP transfers messages from senders' mail servers to the recipients' mail 
servers using TCP connections.
• Users based on e-mail addresses.
• SMTP provides services for mail exchange between users on the same or 
different computers.
• Following the client/server model:
° SMTP has two sides: a client side which executes on a sender's mail 
server, and server side which executes on recipient's mail server.
° Both the client and server sides of SMTP run on every mail server.
° When a mail server sends mail (to other mail servers), it acts as an 
SMTP client.
° When a mail server receives mail (from other mail servers) it acts as an 
SMTP server.
TELNET (Terminal Network):
• TELNET is client-server application that allows a user to log onto remote 
machine and lets the user to access any application program on a remote 
computer.
• TELNET uses the NVT (Network Virtual Terminal) system to encode 
characters on the local system.
• On the server (remote) machine, NVT decodes the characters to a form 
acceptable to the remote machine.
• TELNET is a protocol that provides a general, bi-directional, eight-bit byte 
oriented communications facility.
• Many application protocols are built upon the TELNET protocol
• Telnet services are used on PORT 23.
FTP (File Transfer Protocol):
• FTP is the standard mechanism provided by TCP/IP for copying a file from 
one host to another.
• FTP differs form other client-server applications because it establishes 2 
connections between hosts.
• Two connections are: Data Connection and Control Connection.
• Data Connection uses PORT 20 for the purpose and control connection uses 
PORT 21 for the purpose.
• FTP is built on a client-server architecture and uses separate control and data 
connections between the client and the server.
• One connection is used for data transfer, the other for control information 
(commands and responses).
• It transfer data reliably and efficiently.
Multipurpose Internet Mail Extensions (MIME):
• It is an extension of SMTP that allows the transfer of multimedia messages.
• If binary data is included in a message MIME headers are used to inform the 
receiving mail agent:
o Content-Transfer-Encoding: Header alerts the receiving user agent that 
the message body has been ASCII encoded and the type of encoding 
used.
° Content-Type: Header informs the receiving mail agent about the type of 
data included in the message.
POP (Post Office Protocol):
• POP is also called as POP3 protocol.
• This is a protocol used by a mail server in conjunction with SMTP to receive 
and holds mail for hosts.
• POP3 mail server receives e-mails and filters them into the appropriate user 
folders. When a user connects to the mail server to retrieve his mail, the 
messages are downloaded from mail server to the user's hard disk.
HTTP (Hypertext Transfer Protocol):
• This is a protocol used mainly to access data on the World Wide Web (www).
• The Hypertext Transfer Protocol (HTTP) the Web's main application-layer 
protocol although current browsers can access other types of servers
• A respository of information spread all over the world and linked together.
• The HTIP protocol transfer data in the form of plain text, hyper text, audio, 
video and so on.
• HTTP utilizes TCP connections to send client requests and server replies.
• it is a synchronous protocol which works by making both persistent and non 
persistent connections.
Domain Name System (DNS):
Page 4


An application layer protocol defines how application processes (clients and 
servers), running on different end systems, pass messages to each other. In 
particular, an application layer protocol defines:
• The types of messages, e.g., request messages and response messages.
• The syntax of the various message types, i.e., the fields in the message and 
how the fields are delineated.
• The semantics of the fields, i.e., the meaning of the information that the field 
is supposed to contain;
• Rules for determining when and how a process sends messages and 
responds to messages.
A p p l i c a t i o n T y p e A p p l i c a t i o n - l a y e r p r o t o c o l T r a n s p o r t P r o t o c o l
E le c tr o n ic m ail
S en d : S im p le M a il T r a n s fe r P r o to c o l 
S M T P [R F C 8 2 1 ]
T C P 2 5
R e ceiv e: Post Office Protocol v3 
P O P 3 |R C F 1939]
T C P 110
R e m o te te r m in a l a c c e s s T e ln e t [R F C 854 ] T C P 2 3
W o r ld W id e W e b (W W W ) HyperText transfer Protocol 1.1 
H T T P 1.1 [R F C 2 0 6 8 ]
T C P 80
F ile T r a n s fe r
F ile T r a n s fe r P ro to c o l 
F T P |R F C 959 ]
T C P 21
T r iv ia l F ile T r a n s fe r P ro to c o l 
T F T P [R F C 1 3 5 0 |
L D P 69
R e m o te file serv e r N F S [M c K u silt 1996] L D P o r T C P
S tr e a m in g m u ltim e d ia P r o p r ie ta r y (e .g ., R eal N e tw o r k s) L D P o r T C P
I n te r n e t te le p h o n y P r o p r ie ta r y (e .g ., V o e a lte c ) U s u a lly U D P
SMTP (Simple Mail Transfer Protocol):
• One of the most popular network service is electronic mail (e-mail).
• The TCP/IP protocol that supports electronic mail on the Internet is called 
Simple Mail Transfer Protocol (SMTP).
• SMTP transfers messages from senders' mail servers to the recipients' mail 
servers using TCP connections.
• Users based on e-mail addresses.
• SMTP provides services for mail exchange between users on the same or 
different computers.
• Following the client/server model:
° SMTP has two sides: a client side which executes on a sender's mail 
server, and server side which executes on recipient's mail server.
° Both the client and server sides of SMTP run on every mail server.
° When a mail server sends mail (to other mail servers), it acts as an 
SMTP client.
° When a mail server receives mail (from other mail servers) it acts as an 
SMTP server.
TELNET (Terminal Network):
• TELNET is client-server application that allows a user to log onto remote 
machine and lets the user to access any application program on a remote 
computer.
• TELNET uses the NVT (Network Virtual Terminal) system to encode 
characters on the local system.
• On the server (remote) machine, NVT decodes the characters to a form 
acceptable to the remote machine.
• TELNET is a protocol that provides a general, bi-directional, eight-bit byte 
oriented communications facility.
• Many application protocols are built upon the TELNET protocol
• Telnet services are used on PORT 23.
FTP (File Transfer Protocol):
• FTP is the standard mechanism provided by TCP/IP for copying a file from 
one host to another.
• FTP differs form other client-server applications because it establishes 2 
connections between hosts.
• Two connections are: Data Connection and Control Connection.
• Data Connection uses PORT 20 for the purpose and control connection uses 
PORT 21 for the purpose.
• FTP is built on a client-server architecture and uses separate control and data 
connections between the client and the server.
• One connection is used for data transfer, the other for control information 
(commands and responses).
• It transfer data reliably and efficiently.
Multipurpose Internet Mail Extensions (MIME):
• It is an extension of SMTP that allows the transfer of multimedia messages.
• If binary data is included in a message MIME headers are used to inform the 
receiving mail agent:
o Content-Transfer-Encoding: Header alerts the receiving user agent that 
the message body has been ASCII encoded and the type of encoding 
used.
° Content-Type: Header informs the receiving mail agent about the type of 
data included in the message.
POP (Post Office Protocol):
• POP is also called as POP3 protocol.
• This is a protocol used by a mail server in conjunction with SMTP to receive 
and holds mail for hosts.
• POP3 mail server receives e-mails and filters them into the appropriate user 
folders. When a user connects to the mail server to retrieve his mail, the 
messages are downloaded from mail server to the user's hard disk.
HTTP (Hypertext Transfer Protocol):
• This is a protocol used mainly to access data on the World Wide Web (www).
• The Hypertext Transfer Protocol (HTTP) the Web's main application-layer 
protocol although current browsers can access other types of servers
• A respository of information spread all over the world and linked together.
• The HTIP protocol transfer data in the form of plain text, hyper text, audio, 
video and so on.
• HTTP utilizes TCP connections to send client requests and server replies.
• it is a synchronous protocol which works by making both persistent and non 
persistent connections.
Domain Name System (DNS):
• To identify an entity, TCP/IP protocol uses the IP address which uniquely 
identifies the connection of a host to the Internet.
• DNS is a hierarchical system, based on a distributed database, that uses a 
hierarchy of Name Servers to resolve Internet host names into the 
corresponding IP addresses required for packet routing by issuing a DNS 
query to a name server.
• However, people refer to use names instead of address. Therefore, we need a 
system that can map a name to an address and conversely an address to 
name.
• In TCP/IP, this is the domain name system.
• DNS in the Internet: DNS is protocol that can be used in different platforms.
• Domain name space is divided into three categories.
• Generic Domain: The generic domain defines registered hosts according, to 
their generic behaviour. Each node in the tree defines a domain which is an 
index to the domain name space database.
• Country Domain: The country domain section follows the same format as the 
generic domain but uses 2 characters country abbreviations (e.g., US for 
United States) in place of 3 characters.
• Inverse Domain: The inverse domain is used to map an address to a name.
Overview of Services
S e rv ice T y p e D ire c tio n .
D N S U D P
O u t
H TTP/HTTPS T C P O u t
FTP T C P /U D P O u t
T E L N E T T C P /U D P O u t
POP3 T C P
O u t
SM TP T C P O u t
[RCU T C P /U D P
O u t
ID E N T T C P
In
Private File 
Service
T C P /U D P
I n / O u t
N N TP T C P /U D P
O u t
NTP T C P /U D P
O u t
Remote Desktop T C P /U D P I n / O u t