TCP (Transmission Control Protocol) is a connection-oriented protocol in which the data can be transferred bidirectionally.

UDP (User Datagram Protocol) is a connectionless protocol in which data is sent in datagrams.

Let’s talk about the differences between them and how to choose them correctly according to scenarios.

1. The differences

1.1 Protocol

TCP is a connection-oriented protocol, which means before peers need to establish a connection before they begin to exchange data. This procedure is called TCP 3-way handshake.

During the whole process of data transmission, the connection’s state needs to be maintained until the termination. The peers go through a process of TCP 4-way handshake to disconnect the connection.

UDP is a datagram oriented protocol. It is much more lightweight than TCP. UDP is designed as close as to plain IP as possible. There is no connection during data transmission. Because of this, one peer could send data to multiple targets at one time.

1.2 Transmission reliability

TCP make checks for errors and try to fix them. TCP is designed to provide a
**”reliable stream”**. There is an acknowledgment for every segment transferred. The sender will retry to deliver it if there is any segment without a response. There is a strategy of flow control to control the sending speed, make sure the receiver could process received data properly.

UDP have a checksum for the data but there is no guarantee of delivery, ordering, or duplicate protection. The application layer track lost packets and to recover from losses.

1.3 Performance

Because UDP does not need to maintain the states of connection, the transmission speed is better than TCP ?

The answer depends on whether there are packet losses.

When a packet is lost during the transmission, TCP retransmission timeouts start to introduce delays and make a difference in the time performance.

Since UDP ignore losses, many parts of time could be saved.

1.4 Header Size

TCP header: 20 Bytes, UDP header: 8 Bytes.

2. When to use TCP?

Most networking applications use TCP, such as Web browser, Email, FTP. It’s better to choose TCP whenever data losses can not be accepted.

3. When to use UDP?

In real-world applications, UDP is adopted in limited scenarios. It is OK to use UDP when application accept some packet loss, reordering, errors or duplication.

Typically, Domain Name System (DNS), Dynamic Host Configuration Protocol (DHCP), voice and video applications use UDP.

If you need the reliability, there are some open-source projects(such as rudp) provide a reliable transmission on UDP.

Others

In Portable Operating System Interface (POSIX), TCP and UDP will pass different for function of socket.

 // int socket(int domain, int type, int protocol)

 // Creating socket file descriptor for TCP
 if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) { 
     perror("UDP socket creation failed"); 
     exit(EXIT_FAILURE); 
 } 

/////////////////////////////////////////////////////////////

 // Creating socket file descriptor for UDP
 if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 ) { 
     perror("TCP socket creation failed"); 
     exit(EXIT_FAILURE); 
 } 

Here are simple demo implementations for UDP Server-Client and TCP Server-Client.

References

TCP wiki

UDP wiki

Great TCP-vs-UDP Debate