UDP Server-Client implementation in C++

There are two primary transport layer protocols to communicate between hosts: TCP and UDP. Creating TCP Server/Client was discussed in a previous post. 

Prerequisite: Creating TCP Server/Client

Theory: In UDP, the client does not form a connection with the server like in TCP and instead sends a datagram. Similarly, the server need not accept a connection and just waits for datagrams to arrive. Datagrams upon arrival contain the address of the sender which the server uses to send data to the correct client. 

👁 Image

The entire process can be broken down into the following steps : 

UDP Server :

1. Create a UDP socket.
2. Bind the socket to the server address.
3. Wait until the datagram packet arrives from the client.
4. Process the datagram packet and send a reply to the client.
5. Go back to Step 3.

UDP Client :

1. Create a UDP socket.
2. Send a message to the server.
3. Wait until a response from the server is received.
4. Process the reply and go back to step 2, if necessary.
5. Close socket descriptor and exit.

Necessary Functions :

int socket(int domain, int type, int protocol)
Creates an unbound socket in the specified domain.
Returns socket file descriptor.

Arguments : 
domain - Specifies the communication 
domain ( AF_INET for IPv4/ AF_INET6 for IPv6 ) 
type - Type of socket to be created 
( SOCK_STREAM for TCP / SOCK_DGRAM for UDP ) 
protocol - Protocol to be used by the socket. 
0 means using the default protocol for the address family.  

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen)
Assigns address to the unbound socket.

Arguments : 
sockfd - File descriptor of a socket to be bonded 
addr - Structure in which the address to be bound to is specified 
addrlen - Size of addr structure  

ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
 const struct sockaddr *dest_addr, socklen_t addrlen)
Send a message on the socket

Arguments : 
sockfd - File descriptor of the socket 
buf - Application buffer containing the data to be sent 
len - Size of buf application buffer 
flags - Bitwise OR of flags to modify socket behavior 
dest_addr - Structure containing the address of the destination 
addrlen - Size of dest_addr structure  

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
 struct sockaddr *src_addr, socklen_t *addrlen)
Receive a message from the socket.

Arguments : 
sockfd - File descriptor of the socket 
buf - Application buffer is a pointer which receives the data 
len - Size of buf application buffer 
flags - Bitwise OR of flags to modify socket behavior 
src_addr - Structure containing source address is returned 
addrlen - Variable in which the size of src_addr structure is returned 

int close(int fd)
Close a file descriptor

Arguments:

 fd - File descriptor

In the below code, the exchange of one hello message between the server and client is shown to demonstrate the model.

Filename: UDPServer.c++

Filename: UDPClient.c++

Output : 

$ ./server
Client : Hello from client
Hello message sent.

$ ./client
Hello message sent.
Server : Hello from server