Socket Programming

• Sockets facilitate networked programs by acting as endpoints for two-way communication links.

Layers

• Application programmers don't need to be concerned with routing, Ethernet frames, WiFi WPA2 encryption, or reliability implementations when using sockets.
• Application programmers pass data down and focus on the application.
• Lower layers need destination hostname, resolved with gethostbyname(), IP address, and service, indicated by port number.
• After creation, a socket is represented by a file descriptor.
• Unix philosophy states that everything can be seen as a file.
• Sockets are considered a de facto standard for TCP/UDP.
• Replaced NetBIOS/NetBEUI, IPX/SPX.
• They are commonly used by network applications to send messages.
• They are also used in network applications to share data like images, music, and videos.
• They are used for Interprocess Communication (IPC).
• Stream sockets are connection-oriented and use TCP.
• The steps for stream sockets are to first make a connection, transfer data, and then close the connection, as well as ensure sequence and error checking.
• Example: Youtube video.
• Datagram sockets are connectionless and use UDP.
• Datagram sockets transfer data without explicitly making a connection.
• Example: DNS.
• The standard API for connecting processes can be local or networked
• Sockets are widely supported.
• Examples: Linux / UNIX, Windows, and macOS.
• Sockets are low level.
• Because of this, they minimized the amount of data transfer, very little overhead, and are customizable, flexible, and self-defined protocol.
• Sockets are low level and need more efforts.
• This includes the definition of the protocol, message boundaries, data representation, and security.
• Sockets include session control, such as authentication.
• Sockets facilitate connections between a client and server.
• Includes client socket address, server socket address, client, FTP and HTTP Server, client and server host addresses, and connection socket pair.

Overview and Setup

• A server passively waits for a connection, therefore called a passive socket.
• A client initiates the connection, and it is called an active socket.
• Steps for socket connection are to setup, transfer data, and close.
• Setup includes determining where the remote host is and what service.
• Transfer Data: send/receive ~ write()/read().

Socket Connection Overview

• The socket connects a client and server with the following operations: socket, bind, listen, connect, accept write, read, close.

Server: Overview

• Operations for server socket: socket(), bind(), listen(), accept().

Important struct

• Important information for the struct sockaddr_in includes: sin_family, sin_port, sin_addr, sin_zero.
• In the struct in_addr, one important piece is: s_addr.

Server: Setup socket()

• int socket(int domain, int type, int protocol);
• domain: AF_INET (IPv4) or AF_INET6 (IPv6)
• type: SOCK_STREAM (TCP) or SOCK_DGRAM (UDP)
• protocol: 0
• Example
• int sockfd = socket(AF_INET, SOCK_STREAM, 0);

Server: Binding bind()

• int bind(int sockfd, const struct sockaddr *bind_addr, socklen_t addrlen);
• sockfd: file descriptor that socket() returned
• bind_addr: a "struct sockaddr_in" for IPv4
• addrlen: size of the struct pointed by bind_addr

Server: Example for socket() and bind()

• Example code, see image.

Server: Listen to Incoming Connections listen()

• int listen(int sockfd, int backlog);
• sockfd: file descriptor that socket() returned
• backlog: number of pending connections to queue
• Example
• listen(sockfd, 10);

Server: Accept an Incoming Connection accept()

• int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen)
• sockfd: file descriptor that socket() returned
• addr: pointer to store client address
• addrlen: size of addr
• Returns a file descriptor for the connected socket
• Example
• int client = accept(sockfd, (struct sockaddr_in *) &caddr, &clen);

Server: Example for listing() and accept()

• Example code, see image.

Server: Complete Example

• int sockfd, clen, clientfd;
• struct sockaddr_in saddr, caddr;
• unsigned short port = 80;
• Example code, see image.

Practical Work 3: Server Setup

• Write a new program in C
• Name it « 03.practical.work.server.setup.c »
• Write a server that:
• Listens to TCP port 8784 [USTH in a T9 dial pad!]
• Binds to all possible interfacces
• Prints a message when a client connects to it
• Deploy to your shiny VPS
• Test the connection
• Use «telnet» or «nc»
• Push your C program to corresponding forked Github repository

REMINDER: SOCKET OVERVIEW

• The socket connects a client and server with the following operations: socket, bind, listen, connect, accept write, read, close.

Client: Setup socket()

• Similar to server's
• int socket(int domain, int type, int protocol);
• domain: AF_INET (IPv4) or AF_INET6 (IPv6)
• type: SOCK_STREAM (TCP) or SOCK_DGRAM (UDP)
• protocol: 0
• Example: int sockfd = socket(AF_INET, SOCK_STREAM, 0);

Client: Connect to Server connect()

• int connect(int sockfd, const struct sockaddr *saddr, socklen_t addrlen);
• sockfd: file descriptor that socket() returned
• addr: pointer to store server address addrlen: size of addr
• Example connect(sockfd, (struct sockaddr *) &saddr, sizeof(saddr));

Client: Complete Example

• struct sockaddr_in saddr;
• struct hostent *h;
• int sockfd;
• unsigned short port = 80;
• See the image for the rest of the code.

Practical Work 4: Client setup

• Write a new program in C
• Name it « 04.practical.work.client.setup.c
• Write a client that:
• gets server hostname from program arguments
• in case no argument, asks hostname from STDIN
• resolves its IP address, print to STDOUT
• connects to that server, TCP port 8784 [It's USTH]
• prints a message if it connects to server successfully
• Test the connection from your client to your server on VPS
• Push your C program to corresponding forked Github repository