Short Notes: TCP / UDP and Sockets | Short Notes for Computer Science Engineering - Computer Science Engineering (CSE) PDF Download

 Page 1


Transport Layer Protocols
There are two transport layer protocols as given below.
UDP (User Datagram Protocol)
UDP is a connection less protocol. UDP provides a way for application to send 
encapsulate IP datagram and send them without having to establish a connection.
• Datagram oriented
• unreliable, connectionless
• simple
• unicast and multicast
• Useful only for few applications, e.g., multimedia applications
• Used a lot for services: Network management (SNMP), routing (RIP), naming 
(DNS), etc.
UDP transmitted segments consisting of an 8 byte header followed by the payload. 
The two parts serve to identify the end points within the source and destinations 
machine. When UDP packets arrives, its payload is handed to the process attached 
to the destination ports.
• Source Port Address (16 Bits)
Total length of the User Datagram (16 Bits)
• Destination Port Address (16 Bits)
Checksum (used for error detection) (16 Bits 
TCP (Transmission Control Protocol)
Page 2


Transport Layer Protocols
There are two transport layer protocols as given below.
UDP (User Datagram Protocol)
UDP is a connection less protocol. UDP provides a way for application to send 
encapsulate IP datagram and send them without having to establish a connection.
• Datagram oriented
• unreliable, connectionless
• simple
• unicast and multicast
• Useful only for few applications, e.g., multimedia applications
• Used a lot for services: Network management (SNMP), routing (RIP), naming 
(DNS), etc.
UDP transmitted segments consisting of an 8 byte header followed by the payload. 
The two parts serve to identify the end points within the source and destinations 
machine. When UDP packets arrives, its payload is handed to the process attached 
to the destination ports.
• Source Port Address (16 Bits)
Total length of the User Datagram (16 Bits)
• Destination Port Address (16 Bits)
Checksum (used for error detection) (16 Bits 
TCP (Transmission Control Protocol)
TCP provides full transport layer services to applications. TCP is reliable stream 
transport port-to-port protocol. The term stream in this context, means connection- 
oriented, a connection must be established between both ends of transmission 
before either may transmit data. By creating this connection, TCP generates a 
virtual circuit between sender and receiver that is active for the duration of 
transmission.
TCP is a reliable, point-to-point, connection-oriented, full-duplex protocol.
Source port address 
16 Bits
Destination port address 
16 Bits
Sequence number (32 Bits)
Acknowledgement Number (32 Bits)
HLEN
4 Bits
Reserved 
6 Bits
U A P R S F
Window size 
16 Bits
R C S S Y 1
G K H T N N
Checksum 16 Bits Urgent Pointer 16 Bits
Options and Padding
Flag bits
• URG: Urgent pointer is valid If the bit is set, the following bytes contain an 
urgent message in the sequence number range "SeqNo < = urgent message < = 
SeqNo + urgent pointer"
• ACK: Segment carries a valid acknowledgement
• PSH: PUSH Flag, Notification from sender to the receiver that the receiver 
should pass all data that it has to the application. Normally set by sender 
when the sender’s buffer is empty
• RST: Reset the connection, The flag causes the receiver to reset the 
connection. Receiver of a RST terminates the connection and indicates higher 
layer application about the reset
• SYN: Synchronize sequence numbers, Sent in the first packet when initiating a 
connection
• FIN: Sender is finished with sending. Used for closing a connection, and both 
sides of a connection must send a FIN.
TCP segment format
Each machine supporting TCP has a TCP transport entity either a library procedure, 
a user process or port of kernel. In all cases, it manages TCP streams and 
interfaces to the IP layer. A TCP entities accepts the user data stream from local 
processes, breaks them up into pieces not exceeding 64 K bytes and sends each 
piece as separate IP datagrams.
Sockets
A socket is one end of an inter-process communication channel. The two 
processes each establish their own socket. The system calls for establishing a 
connection are somewhat different for the client and the server, but both involve 
the basic construct of a socket.
The steps involved in establishing a socket on the client side are as follows:
1. Create a socket with the socket () system call
2. Connect the socket to the address of the server using the connect () system 
call
Page 3


Transport Layer Protocols
There are two transport layer protocols as given below.
UDP (User Datagram Protocol)
UDP is a connection less protocol. UDP provides a way for application to send 
encapsulate IP datagram and send them without having to establish a connection.
• Datagram oriented
• unreliable, connectionless
• simple
• unicast and multicast
• Useful only for few applications, e.g., multimedia applications
• Used a lot for services: Network management (SNMP), routing (RIP), naming 
(DNS), etc.
UDP transmitted segments consisting of an 8 byte header followed by the payload. 
The two parts serve to identify the end points within the source and destinations 
machine. When UDP packets arrives, its payload is handed to the process attached 
to the destination ports.
• Source Port Address (16 Bits)
Total length of the User Datagram (16 Bits)
• Destination Port Address (16 Bits)
Checksum (used for error detection) (16 Bits 
TCP (Transmission Control Protocol)
TCP provides full transport layer services to applications. TCP is reliable stream 
transport port-to-port protocol. The term stream in this context, means connection- 
oriented, a connection must be established between both ends of transmission 
before either may transmit data. By creating this connection, TCP generates a 
virtual circuit between sender and receiver that is active for the duration of 
transmission.
TCP is a reliable, point-to-point, connection-oriented, full-duplex protocol.
Source port address 
16 Bits
Destination port address 
16 Bits
Sequence number (32 Bits)
Acknowledgement Number (32 Bits)
HLEN
4 Bits
Reserved 
6 Bits
U A P R S F
Window size 
16 Bits
R C S S Y 1
G K H T N N
Checksum 16 Bits Urgent Pointer 16 Bits
Options and Padding
Flag bits
• URG: Urgent pointer is valid If the bit is set, the following bytes contain an 
urgent message in the sequence number range "SeqNo < = urgent message < = 
SeqNo + urgent pointer"
• ACK: Segment carries a valid acknowledgement
• PSH: PUSH Flag, Notification from sender to the receiver that the receiver 
should pass all data that it has to the application. Normally set by sender 
when the sender’s buffer is empty
• RST: Reset the connection, The flag causes the receiver to reset the 
connection. Receiver of a RST terminates the connection and indicates higher 
layer application about the reset
• SYN: Synchronize sequence numbers, Sent in the first packet when initiating a 
connection
• FIN: Sender is finished with sending. Used for closing a connection, and both 
sides of a connection must send a FIN.
TCP segment format
Each machine supporting TCP has a TCP transport entity either a library procedure, 
a user process or port of kernel. In all cases, it manages TCP streams and 
interfaces to the IP layer. A TCP entities accepts the user data stream from local 
processes, breaks them up into pieces not exceeding 64 K bytes and sends each 
piece as separate IP datagrams.
Sockets
A socket is one end of an inter-process communication channel. The two 
processes each establish their own socket. The system calls for establishing a 
connection are somewhat different for the client and the server, but both involve 
the basic construct of a socket.
The steps involved in establishing a socket on the client side are as follows:
1. Create a socket with the socket () system call
2. Connect the socket to the address of the server using the connect () system 
call
3. Send and receive data. There are a number of ways to do this, but the 
simplest is to use the read () and write () system calls.
The steps involved in establishing a socket on the server side are as follows:
1. Create a socket with the socket () system call
2. Bind the socket to an address using the bind () system call. For a server 
socket on the Internet, an address consists of a port number on the host 
machine.
3. Listen for connections with the listen () system call
4. Accept a connection with the accept () system call. This call typically blocks 
until a client connects with the server.
5. Send and receive data
When a socket is created, the program has to specify the address domain and the 
socket type.
Two processes can communicate with each other only if their sockets are of the 
same type and in the same domain.
There are two widely used address domains, the unix domain, in which two 
processes which share a common file system communicate, and the Internet 
domain, in which two processes running on any two hosts on the Internet 
communicate. Each of these has its own address format.
The address of a socket in the Unix domain is a character string which is basically 
an entry in the file system.
The address of a socket in the Internet domain consists of the Internet address of 
the host machine (every computer on the Internet has a unique 32 bit address, 
often referred to as its IP address). In addition, each socket needs a port number 
on that host. Port numbers are 16 bit unsigned integers. The lower numbers are 
reserved in Unix for standard services.
For example, the port number for the FTP server is 21. It is important that standard 
services be at the same port on all computers so that clients will know their 
addresses. However, port numbers above 2000 are generally available.
Socket Types
There are two widely used socket types, stream sockets, and datagram sockets. 
Stream sockets treat communications as a continuous stream of characters, while 
datagram sockets have to read entire messages at once. Each uses its own 
communications protocol. Stream sockets use TCP (Transmission Control 
Protocol), which is a reliable, stream oriented protocol, and datagram sockets use 
UDP (Unix Datagram Protocol), which is unreliable and message oriented. A second 
type of connection is a datagram socket. You might want to use a datagram socket 
in cases where there is only one message being sent from the client to the server, 
and only one message being sent back. There are several differences between a 
datagram socket and a stream socket.
1. Datagrams are unreliable, which means that if a packet of information gets 
lost somewhere in the Internet, the sender is not told (and of course the 
receiver does not know about the existence of the message). In contrast, with 
a stream socket, the underlying TCP protocol will detect that a message was
Page 4


Transport Layer Protocols
There are two transport layer protocols as given below.
UDP (User Datagram Protocol)
UDP is a connection less protocol. UDP provides a way for application to send 
encapsulate IP datagram and send them without having to establish a connection.
• Datagram oriented
• unreliable, connectionless
• simple
• unicast and multicast
• Useful only for few applications, e.g., multimedia applications
• Used a lot for services: Network management (SNMP), routing (RIP), naming 
(DNS), etc.
UDP transmitted segments consisting of an 8 byte header followed by the payload. 
The two parts serve to identify the end points within the source and destinations 
machine. When UDP packets arrives, its payload is handed to the process attached 
to the destination ports.
• Source Port Address (16 Bits)
Total length of the User Datagram (16 Bits)
• Destination Port Address (16 Bits)
Checksum (used for error detection) (16 Bits 
TCP (Transmission Control Protocol)
TCP provides full transport layer services to applications. TCP is reliable stream 
transport port-to-port protocol. The term stream in this context, means connection- 
oriented, a connection must be established between both ends of transmission 
before either may transmit data. By creating this connection, TCP generates a 
virtual circuit between sender and receiver that is active for the duration of 
transmission.
TCP is a reliable, point-to-point, connection-oriented, full-duplex protocol.
Source port address 
16 Bits
Destination port address 
16 Bits
Sequence number (32 Bits)
Acknowledgement Number (32 Bits)
HLEN
4 Bits
Reserved 
6 Bits
U A P R S F
Window size 
16 Bits
R C S S Y 1
G K H T N N
Checksum 16 Bits Urgent Pointer 16 Bits
Options and Padding
Flag bits
• URG: Urgent pointer is valid If the bit is set, the following bytes contain an 
urgent message in the sequence number range "SeqNo < = urgent message < = 
SeqNo + urgent pointer"
• ACK: Segment carries a valid acknowledgement
• PSH: PUSH Flag, Notification from sender to the receiver that the receiver 
should pass all data that it has to the application. Normally set by sender 
when the sender’s buffer is empty
• RST: Reset the connection, The flag causes the receiver to reset the 
connection. Receiver of a RST terminates the connection and indicates higher 
layer application about the reset
• SYN: Synchronize sequence numbers, Sent in the first packet when initiating a 
connection
• FIN: Sender is finished with sending. Used for closing a connection, and both 
sides of a connection must send a FIN.
TCP segment format
Each machine supporting TCP has a TCP transport entity either a library procedure, 
a user process or port of kernel. In all cases, it manages TCP streams and 
interfaces to the IP layer. A TCP entities accepts the user data stream from local 
processes, breaks them up into pieces not exceeding 64 K bytes and sends each 
piece as separate IP datagrams.
Sockets
A socket is one end of an inter-process communication channel. The two 
processes each establish their own socket. The system calls for establishing a 
connection are somewhat different for the client and the server, but both involve 
the basic construct of a socket.
The steps involved in establishing a socket on the client side are as follows:
1. Create a socket with the socket () system call
2. Connect the socket to the address of the server using the connect () system 
call
3. Send and receive data. There are a number of ways to do this, but the 
simplest is to use the read () and write () system calls.
The steps involved in establishing a socket on the server side are as follows:
1. Create a socket with the socket () system call
2. Bind the socket to an address using the bind () system call. For a server 
socket on the Internet, an address consists of a port number on the host 
machine.
3. Listen for connections with the listen () system call
4. Accept a connection with the accept () system call. This call typically blocks 
until a client connects with the server.
5. Send and receive data
When a socket is created, the program has to specify the address domain and the 
socket type.
Two processes can communicate with each other only if their sockets are of the 
same type and in the same domain.
There are two widely used address domains, the unix domain, in which two 
processes which share a common file system communicate, and the Internet 
domain, in which two processes running on any two hosts on the Internet 
communicate. Each of these has its own address format.
The address of a socket in the Unix domain is a character string which is basically 
an entry in the file system.
The address of a socket in the Internet domain consists of the Internet address of 
the host machine (every computer on the Internet has a unique 32 bit address, 
often referred to as its IP address). In addition, each socket needs a port number 
on that host. Port numbers are 16 bit unsigned integers. The lower numbers are 
reserved in Unix for standard services.
For example, the port number for the FTP server is 21. It is important that standard 
services be at the same port on all computers so that clients will know their 
addresses. However, port numbers above 2000 are generally available.
Socket Types
There are two widely used socket types, stream sockets, and datagram sockets. 
Stream sockets treat communications as a continuous stream of characters, while 
datagram sockets have to read entire messages at once. Each uses its own 
communications protocol. Stream sockets use TCP (Transmission Control 
Protocol), which is a reliable, stream oriented protocol, and datagram sockets use 
UDP (Unix Datagram Protocol), which is unreliable and message oriented. A second 
type of connection is a datagram socket. You might want to use a datagram socket 
in cases where there is only one message being sent from the client to the server, 
and only one message being sent back. There are several differences between a 
datagram socket and a stream socket.
1. Datagrams are unreliable, which means that if a packet of information gets 
lost somewhere in the Internet, the sender is not told (and of course the 
receiver does not know about the existence of the message). In contrast, with 
a stream socket, the underlying TCP protocol will detect that a message was
lost because it was not acknowledged, and it will be retransmitted without thg 
process at either end knowing about this.
2. Message boundaries are preserved in datagram sockets. If the sender sends a 
datagram of 100 bytes, the receiver must read all 100 bytes at once. This can 
be contrasted with a stream socket, where if the sender wrote a 100 byte 
message, the receiver could read it in two chunks of 50 bytes or 100 chunks 
of one byte.
3. The communication is done using special system calls sendto () and 
receivefrom() rather than the more generic read () and write ().
4. There is a lot less overhead associated with a datagram socket because 
connections do not need to be established and broken down, and packets do 
not need to be acknowledged. This is why datagram sockets are often used 
when the service to be provided is short, such as a time-of-day service.
State Transition Diagram at Transport Layer:
client server
s o c k e t.b in d , lis t e n 
LISTEN (passive open)
wri te 
read (blocks)
ESTABLISHED 
a ccep t returns 
read (blocks)
<.<cnvr procnw*
w r it e
read (blocks)
c lo s e
(active dose) HN_WA1T_1
CLOSE.WAIT (passive close) 
read returns 0
close
LA5T_ACK
CLOSED